The atypical M2 segment of the beta subunit confers picrotoxinin resistance to inhibitory glycine receptor channels

Glycine Receptor β-Targeting Autoantibodies Contribute to the Pathology of Autoimmune Diseases

BACKGROUND AND OBJECTIVES

Stiff-person syndrome (SPS) and progressive encephalomyelitis with rigidity and myoclonus (PERM) are rare neurologic disorders of the CNS. Until now, exclusive GlyRα subunit-binding autoantibodies with subsequent changes in function and surface numbers were reported. GlyR autoantibodies have also been described in patients with focal epilepsy. Autoimmune reactivity against the GlyRβ subunits has not yet been shown. Autoantibodies against GlyRα1 target the large extracellular N-terminal domain. This domain shares a high degree of sequence homology with GlyRβ making it not unlikely that GlyRβ-specific autoantibody (aAb) exist and contribute to the disease pathology.

METHODS

In this study, we investigated serum samples from 58 patients for aAb specifically detecting GlyRβ. Studies in microarray format, cell-based assays, and primary spinal cord neurons and spinal cord tissue immunohistochemistry were performed to determine specific GlyRβ binding and define aAb binding to distinct protein regions. Preadsorption approaches of aAbs using living cells and the purified extracellular receptor domain were further used. Finally, functional consequences for inhibitory neurotransmission upon GlyRβ aAb binding were resolved by whole-cell patch-clamp recordings.

RESULTS

Among 58 samples investigated, cell-based assays, tissue analysis, and preadsorption approaches revealed 2 patients with high specificity for GlyRβ aAb. Quantitative protein cluster analysis demonstrated aAb binding to synaptic GlyRβ colocalized with the scaffold protein gephyrin independent of the presence of GlyRα1. At the functional level, binding of GlyRβ aAb from both patients to its target impair glycine efficacy.

DISCUSSION

Our study establishes GlyRβ as novel target of aAb in patients with SPS/PERM. In contrast to exclusively GlyRα1-positive sera, which alter glycine potency, aAbs against GlyRβ impair receptor efficacy for the neurotransmitter glycine. Imaging and functional analyses showed that GlyRβ aAbs antagonize inhibitory neurotransmission by affecting receptor function rather than localization.

An Anatomical and Physiological Basis for Coincidence Detection Across Time Scales in the Auditory System

Coincidence detection is a common neural computation that identifies co-occurring stimuli by integration of inputs. In the auditory system, octopus cells act as coincidence detectors for complex sounds that include both synchronous and sequenced combinations of frequencies. Octopus cells must detect coincidence on both the millisecond and submillisecond time scale, unlike the average neuron, which integrates inputs over time on the order of tens of milliseconds. Here, we show that octopus cell computations in the cell body are shaped by inhibition in the dendrites, which adjusts the strength and timing of incoming signals to achieve submillisecond acuity. This mechanism is crucial for the fundamental process of integrating the synchronized frequencies of natural auditory signals over time.

Downregulation of Ribosomal Protein Genes Is Revealed in a Model of Rat Hippocampal Neuronal Culture Activation with GABA(A)R/GlyRa2 Antagonist Picrotoxin

Long-read transcriptome sequencing provides us with a convenient tool for the thorough study of biological processes such as neuronal plasticity. Here, we aimed to perform transcriptional profiling of rat hippocampal primary neuron cultures after stimulation with picrotoxin (PTX) to further understand molecular mechanisms of neuronal activation. To overcome the limitations of short-read RNA-Seq approaches, we performed an Oxford Nanopore Technologies MinION-based long-read sequencing and transcriptome assembly of rat primary hippocampal culture mRNA at three time points after the PTX activation. We used a specific approach to exclude uncapped mRNAs during sample preparation. Overall, we found 23,652 novel transcripts in comparison to reference annotations, out of which ~6000 were entirely novel and mostly transposon-derived loci. Analysis of differentially expressed genes (DEG) showed that 3046 genes were differentially expressed, of which 2037 were upregulated and 1009 were downregulated at 30 min after the PTX application, with only 446 and 13 genes differentially expressed at 1 h and 5 h time points, respectively. Most notably, multiple genes encoding ribosomal proteins, with a high basal expression level, were downregulated after 30 min incubation with PTX; we suggest that this indicates redistribution of transcriptional resources towards activity-induced genes. Novel loci and isoforms observed in this study may help us further understand the functional mRNA repertoire in neuronal plasticity processes. Together with other NGS techniques, differential gene expression analysis of sequencing data obtained using MinION platform might provide a simple method to optimize further study of neuronal plasticity.

Role of the Glycine Receptor β Subunit in Synaptic Localization and Pathogenicity in Severe Startle Disease

Startle disease is due to the disruption of recurrent inhibition in the spinal cord. Most common causes are genetic variants in genes (GLRA1, GLRB) encoding inhibitory glycine receptor (GlyR) subunits. The adult GlyR is a heteropentameric complex composed of α1 and β subunits that localizes at postsynaptic sites and replaces embryonically expressed GlyRα2 homomers. The human GlyR variants of GLRA1 and GLRB, dominant and recessive, have been intensively studied in vitro. However, the role of unaffected GlyRβ, essential for synaptic GlyR localization, in the presence of mutated GlyRα1 in vivo is not fully understood. Here, we used knock-in mice expressing endogenous mEos4b-tagged GlyRβ that were crossed with mouse Glra1 startle disease mutants. We explored the role of GlyRβ under disease conditions in mice carrying a missense mutation (shaky) or resulting from the loss of GlyRα1 (oscillator). Interestingly, synaptic targeting of GlyRβ was largely unaffected in both mouse mutants. While synaptic morphology appears unaltered in shaky animals, synapses were notably smaller in homozygous oscillator animals. Hence, GlyRβ enables transport of functionally impaired GlyRα1 missense variants to synaptic sites in shaky animals, which has an impact on the efficacy of possible compensatory mechanisms. The observed enhanced GlyRα2 expression in oscillator animals points to a compensation by other GlyRα subunits. However, trafficking of GlyRα2β complexes to synaptic sites remains functionally insufficient, and homozygous oscillator mice still die at 3 weeks after birth. Thus, both functional and structural deficits can affect glycinergic neurotransmission in severe startle disease, eliciting different compensatory mechanisms in vivo.

Asymmetric gating of a human hetero-pentameric glycine receptor

Hetero-pentameric Cys-loop receptors constitute a major type of neurotransmitter receptors that enable signal transmission and processing in the nervous system. Despite intense investigations into their working mechanism and pharmaceutical potentials, how neurotransmitters activate these receptors remains unclear due to the lack of high-resolution structural information in the activated open state. Here we report near-atomic resolution structures resolved in digitonin consistent with all principle functional states of the human α1β GlyR, which is a major Cys-loop receptor that mediates inhibitory neurotransmission in the central nervous system of adults. Glycine binding induces cooperative and symmetric structural rearrangements in the neurotransmitter-binding extracellular domain but asymmetrical pore dilation in the transmembrane domain. Symmetric response in the extracellular domain is consistent with electrophysiological data showing cooperative glycine activation and contribution from both α1 and β subunits. A set of functionally essential but differentially charged amino acid residues in the transmembrane domain of the α1 and β subunits explains asymmetric activation. These findings provide a foundation for understanding how the gating of the Cys-loop receptor family members diverges to accommodate specific physiological environments.

Asymmetric gating of a human hetero-pentameric glycine receptor

Hetero-pentameric Cys-loop receptors constitute a major type of neurotransmitter receptors that enable signal transmission and processing in the nervous system. Despite intense investigations in their working mechanism and pharmaceutical potentials, how neurotransmitters activate these receptors remain unclear due to the lack of high-resolution structural information in the activated open state. Here we report near-atomic resolution structures in all principle functional states of the human α1β GlyR, which is a major Cys-loop receptor that mediates inhibitory neurotransmission in the central nervous system of adults. Glycine binding induced cooperative and symmetric structural rearrangements in the neurotransmitter-binding extracellular domain, but asymmetrical pore dilation in the transmembrane domain. Symmetric response in the extracellular domain is consistent with electrophysiological data showing similar contribution to activation from all the α1 and β subunits. A set of functionally essential but differentially charged amino-acid residues in the transmembrane domain of the α1 and β subunits explains asymmetric activation. These findings point to a gating mechanism that is distinct from homomeric receptors but more compatible with heteromeric GlyRs being clustered at synapses through β subunit-scaffolding protein interactions. Such mechanism provides foundation for understanding how gating of the Cys-loop receptor members diverge to accommodate specific physiological environment.

Glycine encephalopathy

Inherited neurotransmitter diseases are a subset of rare neurometabolic disorders characterized by hereditary deficiencies in neurotransmitter metabolism or transport. Non-ketotic hyperglycinaemia (NKH), called glycine encephalopathy, is an autosomal recessive glycine metabolism disorder characterized by an abnormal accumulation of glycine in all bodily tissues, including the CNS. The SLC6A9 gene, which codes for the GLYT1 protein, a biochemical abnormality in the GCS, and dihydrolipoamide dehydrogenase enzymes, which function as a GCS component, are responsible for the neonatal form’s symptoms, which include progressive encephalopathy, hypotonia, seizures, and occasionally mortality in the first few days of life. By changing the MAPK signalling pathways, glycine deprivation in the brain damages neurons by increasing NMDA receptor activation, increasing intracellular Ca levels, and leading to DNA breakage and cell death in the neuron region. In addition to the previously mentioned clinical diagnosis, NKH or GE would be determined by MLPA and 13C glycine breath tests. Pediatricians, surgeons, neurologists, and geneticists treat NKH and GE at the newborn period; there is no cure for either condition.

Picrotoxane sesquiterpenoids: chemistry, chemo- and bio-syntheses and biological activities

Covering: up to December 2021Picrotoxane sesquiterpenoids are a special category of natural products known to have a picrotoxane skeleton and are characterised by a highly oxidised cis-hydrindene core, lactone rings, and epoxide functionalities. Ever since the first picrotoxane was isolated from Menispermum cocculus in the early 19^th century, these compounds have long attracted the attention of natural product chemists, synthetic chemists, and pharmacologists for their particular structures and powerful biological activities. This review extensively summarizes a total of 132 naturally occurring picrotoxane sesquiterpenoids, taking into account their distributions, structural classifications, chemical and bio-synthetic researches, and bioactivities. It provides a comprehensive and in-depth perspective for further investigation on picrotoxane sesquiterpenoids.

Nodulisporic acid produces direct activation and positive allosteric modulation of AVR-14B, a glutamate-gated chloride channel from adult Brugia malayi

Glutamate-gated chloride channels (GluCls) are unique to invertebrates and are targeted by macrocyclic lactones. In this study, we cloned an AVR-14B GluCl subunit from adult Brugia malayi, a causative agent of lymphatic filariasis in humans. To elucidate this channel's pharmacological properties, we used Xenopus laevis oocytes for expression and performed two-electrode voltage-clamp electrophysiology. The receptor was gated by the natural ligand L-glutamate (effective concentration, 50% [EC50] = 0.4 mM) and ivermectin (IVM; EC50 = 1.8 nM). We also characterized the effects of nodulisporic acid (NA) on Bma-AVR-14B and NA-produced dual effects on the receptor as an agonist and a type II positive allosteric modulator. Here we report characterization of the complex activity of NA on a nematode GluCl. Bma-AVR-14B demonstrated some unique pharmacological characteristics. IVM did not produce potentiation of L-glutamate-mediated responses but instead, reduced the channel's sensitivity for the ligand. Further electrophysiological exploration showed that IVM (at a moderate concentration of 0.1 nM) functioned as an inhibitor of both agonist and positive allosteric modulatory effects of NA. This suggests that IVM and NA share a complex interaction. The pharmacological properties of Bma-AVR-14B indicate that the channel is an important target of IVM and NA. In addition, the unique electrophysiological characteristics of Bma-AVR-14B could explain the observed variation in drug sensitivities of various nematode parasites. We have also shown the inhibitory effects of IVM and NA on adult worm motility using Worminator. RNA interference (RNAi) knockdown suggests that AVR-14 plays a role in influencing locomotion in B. malayi.

Structure and Mechanism of Glycine Receptor Elucidated by Cryo-Electron Microscopy

Glycine receptors (GlyRs) are pentameric ion channels that mediate fast inhibitory neurotransmission. GlyRs are found in the central nervous system including the spinal cord, brain stem, and cerebellum, as well as in the retina, sperm, macrophages, hippocampus, cochlea, and liver. Due to their crucial roles in counter-balancing excitatory signals and pain signal transmission, GlyR dysfunction can lead to severe diseases, and as a result, compounds that modify GlyR activity may have tremendous therapeutic potential. Despite this potential, the development of GlyR-specific small-molecule ligands is lacking. Over the past few years, high-resolution structures of both homomeric and heteromeric GlyRs structures in various conformations have provided unprecedented details defining the pharmacology of ligand binding, subunit composition, and mechanisms of channel gating. These high-quality structures will undoubtedly help with the development of GlyR-targeted therapies.

Clinical, genetic, and functional characterization of the glycine receptor β-subunit A455P variant in a family affected by hyperekplexia syndrome

Hyperekplexia is a rare neurological disorder characterized by exaggerated startle responses affecting newborns with the hallmark characteristics of hypertonia, apnea, and noise or touch-induced nonepileptic seizures. The genetic causes of the disease can vary, and several associated genes and mutations have been reported to affect glycine receptors (GlyRs); however, the mechanistic links between GlyRs and hyperekplexia are not yet understood. Here, we describe a patient with hyperekplexia from a consanguineous family. Extensive genetic screening using exome sequencing coupled with autozygome analysis and iterative filtering supplemented by in silico prediction identified that the patient carries the homozygous missense mutation A455P in GLRB, which encodes the GlyR β-subunit. To unravel the physiological and molecular effects of A455P on GlyRs, we used electrophysiology in a heterologous system as well as immunocytochemistry, confocal microscopy, and cellular biochemistry. We found a reduction in glycine-evoked currents in N2A cells expressing the mutation compared to WT cells. Western blot analysis also revealed a reduced amount of GlyR β protein both in cell lysates and isolated membrane fractions. In line with the above observations, coimmunoprecipitation assays suggested that the GlyR α1-subunit retained coassembly with βA455P to form membrane-bound heteromeric receptors. Finally, structural modeling showed that the A455P mutation affected the interaction between the GlyR β-subunit transmembrane domain 4 and the other helices of the subunit. Taken together, our study identifies and validates a novel loss-of-function mutation in GlyRs whose pathogenicity is likely to cause hyperekplexia in the affected individual.

Strychnine and its mono- and dimeric analogues: a pharmaco-chemical perspective

Covering: up to November 2021Since its isolation in 1818, strychnine has attracted the attention of a plethora of chemists and pharmacologists who have established its structure, developed total syntheses, and examined its complex pharmacology. While numerous reviews on structure elucidation and total synthesis of strychnine are available, reports on structure-activity relationships (SARs) of this fascinating alkaloid are rare. In this review, we present and discuss structures, synthetic approaches, metabolic transformations, and the diverse pharmacological actions of strychnine and its mono- and dimeric analogues. Particular attention is given to its SARs at glycine receptors (GlyRs) in light of recently published high-resolution structures of strychnine-GlyR complexes. Other pharmacological actions of strychnine and its derivatives, such as their antagonistic properties at nicotinic acetylcholine receptors (nAChRs), allosteric modulation of muscarinic acetylcholine receptors as well as anti-cancer and anti-plasmodial effects are also critically reviewed, and possible future developments in the field are discussed.

Exposure to GABAA Receptor Antagonist Picrotoxin in Pregnant Mice Causes Autism-Like Behaviors and Aberrant Gene Expression in Offspring

Autism spectrum disorder (ASD) is a neurodevelopmental disorder that is characterized by impairments in social interaction and restricted/repetitive behaviors. The neurotransmitter γ-aminobutyric acid (GABA) through GABA_A receptor signaling in the immature brain plays a key role in the development of neuronal circuits. Excitatory/inhibitory imbalance in the mature brain has been investigated as a pathophysiological mechanism of ASD. However, whether and how disturbances of GABA signaling in embryos that are caused by GABA_A receptor inhibitors cause ASD-like pathophysiology are poorly understood. The present study examined whether exposure to the GABA_A receptor antagonist picrotoxin causes ASD-like pathophysiology in offspring by conducting behavioral tests from the juvenile period to adulthood and performing gene expression analyses in mature mouse brains. Here, we found that male mice that were prenatally exposed to picrotoxin exhibited a reduction of active interaction time in the social interaction test in both adolescence and adulthood. The gene expression analyses showed that picrotoxin-exposed male mice exhibited a significant increase in the gene expression of odorant receptors. Weighted gene co-expression network analysis showed a strong correlation between social interaction and enrichment of the "odorant binding" pathway gene module. Our findings suggest that exposure to a GABA_A receptor inhibitor during the embryonic period induces ASD-like behavior, and impairments in odorant function may contribute to social deficits in offspring.

Contribution of GlyR α3 Subunits to the Sensitivity and Effect of Ethanol in the Nucleus Accumbens

The glycine receptor (GlyR), a ligand-gated ion channel, is critical for inhibitory neurotransmission in brainstem, spinal cord, and in supraspinal regions. Recent data from several laboratories have shown that GlyRs are expressed in the brain reward circuitry and that α1 and α2 are the principal subunits expressed in the nucleus accumbens (nAc). In the present study, we studied the sensitivity to ethanol of homomeric and heteromeric α3 GlyR subunits in HEK293 cells and dissociated neurons from the nAc. Finally, we explored ethanol-related behaviors in a Glra3 knockout mouse (Glra3^–/–). Studies in HEK293 cells showed that while homomeric α3 GlyR subunits were insensitive to ethanol, heteromeric α3β GlyR subunits showed higher sensitivity to ethanol. Additionally, using electrophysiological recordings in dissociated accumbal neurons, we found that the glycine current density increased in Glra3^–/– mice and the GlyRs were less affected by ethanol and picrotoxin. We also examined the effect of ethanol on sedation and drinking behavior in Glra3^–/– mice and found that the duration in the loss of righting reflex (LORR) was unchanged compared to wild-type (WT) mice. On the other hand, using the drinking in the dark (DID) paradigm, we found that Glra3^–/– mice have a larger ethanol consumption compared to WT mice, and that this was already high during the first days of exposure to ethanol. Our results support the conclusion that heteromeric α3β, but not homomeric α3, GlyRs are potentiated by ethanol. Also, the increase in GlyR and GABA_AR mediated current densities in accumbal neurons in the KO mice support the presence of compensatory changes to α3 knock out. The increase in ethanol drinking in the Glra3^–/– mice might be associated to the reduction in β and compensatory changes in other subunits in the receptor arrangement.

Novel Functional Properties of Missense Mutations in the Glycine Receptor β Subunit in Startle Disease

Startle disease is a rare disorder associated with mutations in GLRA1 and GLRB, encoding glycine receptor (GlyR) α1 and β subunits, which enable fast synaptic inhibitory transmission in the spinal cord and brainstem. The GlyR β subunit is important for synaptic localization via interactions with gephyrin and contributes to agonist binding and ion channel conductance. Here, we have studied three GLRB missense mutations, Y252S, S321F, and A455P, identified in startle disease patients. For Y252S in M1 a disrupted stacking interaction with surrounding aromatic residues in M3 and M4 is suggested which is accompanied by an increased EC50 value. By contrast, S321F in M3 might stabilize stacking interactions with aromatic residues in M1 and M4. No significant differences in glycine potency or efficacy were observed for S321F. The A455P variant was not predicted to impact on subunit folding but surprisingly displayed increased maximal currents which were not accompanied by enhanced surface expression, suggesting that A455P is a gain-of-function mutation. All three GlyR β variants are trafficked effectively with the α1 subunit through intracellular compartments and inserted into the cellular membrane. In vivo, the GlyR β subunit is transported together with α1 and the scaffolding protein gephyrin to synaptic sites. The interaction of these proteins was studied using eGFP-gephyrin, forming cytosolic aggregates in non-neuronal cells. eGFP-gephyrin and β subunit co-expression resulted in the recruitment of both wild-type and mutant GlyR β subunits to gephyrin aggregates. However, a significantly lower number of GlyR β aggregates was observed for Y252S, while for mutants S321F and A455P, the area and the perimeter of GlyR β subunit aggregates was increased in comparison to wild-type β. Transfection of hippocampal neurons confirmed differences in GlyR-gephyrin clustering with Y252S and A455P, leading to a significant reduction in GlyR β-positive synapses. Although none of the mutations studied is directly located within the gephyrin-binding motif in the GlyR β M3-M4 loop, we suggest that structural changes within the GlyR β subunit result in differences in GlyR β-gephyrin interactions. Hence, we conclude that loss- or gain-of-function, or alterations in synaptic GlyR clustering may underlie disease pathology in startle disease patients carrying GLRB mutations.

Characterization of the subunit composition and structure of adult human glycine receptors

The strychnine-sensitive pentameric glycine receptor (GlyR) mediates fast inhibitory neurotransmission in the mammalian nervous system. Only heteromeric GlyRs mediate synaptic transmission, as they contain the β subunit that permits clustering at the synapse through its interaction with scaffolding proteins. Here, we show that α2 and β subunits assemble with an unexpected 4:1 stoichiometry to produce GlyR with native electrophysiological properties. We determined structures in multiple functional states at 3.6-3.8 Å resolutions and show how 4:1 stoichiometry is consistent with the structural features of α2β GlyR. Furthermore, we show that one single β subunit in each GlyR gives rise to the characteristic electrophysiological properties of heteromeric GlyR, while more β subunits render GlyR non-conductive. A single β subunit ensures a univalent GlyR-scaffold linkage, which means the scaffold alone regulates the cluster properties.

Presence of ethanol sensitive and insensitive glycine receptors in the ventral tegmental area and prefrontal cortex in mice

BACKGROUND AND PURPOSE

Previous studies showed that glycine receptors (GlyRs) composed of α1 and β subunits are primarily found in spinal cord and brainstem and are potentiated by ethanol (10-100 mM). However, much less is known about the presence, composition, and ethanol sensitivity of GlyRs in higher CNS regions. In the present study, we examined two regions of the brain reward system, the ventral tegmental area (VTA) and the prefrontal cortex (PFC), to determine their GlyR subunit composition and sensitivity to ethanol.

EXPERIMENTAL APPROACH

To achieve these aims, we used Western blot, immunohistochemistry and electrophysiological techniques in three different models: Wild-type C57BL/6, GlyR α1 knock-in and GlyR α2 knockout mice.

KEY RESULTS

Similar levels of α and β GlyR subunits were detected in both brain regions, and electrophysiological recordings demonstrated the presence of glycine-activated currents in both areas. The sensitivity of GlyRs to glycine was lower in the PFC compared to VTA. Picrotoxin blocked the glycine-activated current in the PFC and VTA only partially, indicating that both regions express heteromeric αβ receptors. Interestingly, GlyRs in VTA neurons, but not in PFC neurons, were potentiated by ethanol.

CONCLUSION AND IMPLICATIONS

GlyRs in VTA neurons from WT and α2 KO mice were potentiated by ethanol, but not in neurons from the α1 KI mice, supporting the conclusion that α1 GlyRs are predominantly expressed in the VTA. By contrast, GlyRs in PFC neurons were not potentiated in any of the mouse models studied, suggesting the presence of either α2/α3/α4 rather than α1 GlyR subunits.

Modulation of recombinant human alpha 1 glycine receptor by flavonoids and gingerols

The inhibitory glycine receptor (GlyR) is a principal mediator of fast synaptic inhibition in mammalian spinal cord, brainstem, and higher brain centres. Flavonoids are secondary plant metabolites that exhibit many beneficial physiological effects, including modulatory action on neuronal receptors. Using whole-cell current recordings from recombinant human α1 GlyRs, expressed in HEK293 cells, we compared the flavonols kaempferol and quercetin, the flavanone naringenin, the flavones apigenin and nobiletin, the isoflavone genistein, and two gingerols, 6-gingerol and 8-gingerol for their modulation of receptor currents. All compounds were inhibitors of the GlyR with IC₅₀ values ranging between 9.3 ± 2.6 µM (kaempferol) and 46.7 ± 6.5 µM (genistein), following a mixed mode of inhibition. Co-application of two inhibitors revealed distinct binding sites for flavonoids and gingerols. Pore-lining mutants T258A and T258S were strongly inhibited by quercetin and naringenin, but not by 6-gingerol, confirming the existence of distinct binding sites for flavonoids and gingerols. Apigenin, kaempferol, nobiletin, naringenin and 6-gingerol showed biphasic action, potentiating glycine-induced currents at low concentration of both, modulator and glycine, and inhibiting at higher concentrations. Identification of distinct modulatory sites for flavonoids and related compounds may present pharmacological target sites and aid the discovery of novel glycinergic drugs.

The anticonvulsant zonisamide positively modulates recombinant and native glycine receptors at clinically relevant concentrations

GABA_A and glycine receptors mediate fast synaptic inhibitory neurotransmission. Despite studies showing that activation of cerebral glycine receptors could be a potential strategy in the treatment of epilepsy, few studies have assessed the effects of existing anticonvulsant therapies on recombinant or native glycine receptors. We, therefore, evaluated the actions of a series of anticonvulsants at recombinant human homo-oligomeric glycine receptor α1, α2 and α3 subtypes expressed in Xenopus oocytes using two-electrode voltage-clamp methods, and then assessed the most effective drug at native glycine receptors from entorhinal cortex neurons using whole-cell voltage-clamp recordings. Ganaxolone, tiagabine and zonisamide positively modulated glycine induced currents at recombinant homomeric glycine receptors. Of these, zonisamide was the most efficacious and exhibited an EC₅₀ value ranging between 450 and 560 μM at α1, α2 and α3 subtypes. These values were not significantly different indicating a non-selective modulation of glycine receptors. Using a therapeutic concentration of zonisamide (100 μM), the potency of glycine was significantly shifted from 106 to 56 μM at α1, 185 to 112 μM at α2, and 245 to 91 μM at α3 receptors. Furthermore, zonisamide (100 μM) potentiated exogenous homomeric and heteromeric glycine mediated currents from layer II pyramidal cells of the lateral or medial entorhinal cortex. As therapeutic concentrations of zonisamide positively modulate recombinant and native glycine receptors, we propose that the anticonvulsant effects of zonisamide may, at least in part, be mediated via this action.

Identification of the hypertension drug niflumic acid as a glycine receptor inhibitor

Glycine is one of the major neurotransmitters in the brainstem and the spinal cord. Glycine binds to and activates glycine receptors (GlyRs), increasing Cl^- conductance at postsynaptic sites. This glycinergic synaptic transmission contributes to the generation of respiratory rhythm and motor patterns. Strychnine inhibits GlyR by binding to glycine-binding site, while picrotoxin blocks GlyR by binding to the channel pore. We have previously reported that bath application of strychnine to zebrafish embryos causes bilateral muscle contractions in response to tactile stimulation. To explore the drug-mediated inhibition of GlyRs, we screened a chemical library of ~ 1,000 approved drugs and pharmacologically active molecules by observing touch-evoked response of zebrafish embryos in the presence of drugs. We found that exposure of zebrafish embryos to nifedipine (an inhibitor of voltage-gated calcium channel) or niflumic acid (an inhibitor of cyclooxygenase 2) caused bilateral muscle contractions just like strychnine-treated embryos showed. We then assayed strychnine, picrotoxin, nifedipine, and niflumic acid for concentration-dependent inhibition of glycine-mediated currents of GlyRs in oocytes and calculated IC₅₀s. The results indicate that all of them concentration-dependently inhibit GlyR in the order of strychnine > picrotoxin > nifedipine > niflumic acid.

Mechanisms of activation and desensitization of full-length glycine receptor in lipid nanodiscs

Glycinergic synapses play a central role in motor control and pain processing in the central nervous system. Glycine receptors (GlyRs) are key players in mediating fast inhibitory neurotransmission at these synapses. While previous high-resolution structures have provided insights into the molecular architecture of GlyR, several mechanistic questions pertaining to channel function are still unanswered. Here, we present Cryo-EM structures of the full-length GlyR protein complex reconstituted into lipid nanodiscs that are captured in the unliganded (closed), glycine-bound (open and desensitized), and allosteric modulator-bound conformations. A comparison of these states reveals global conformational changes underlying GlyR channel gating and modulation. The functional state assignments were validated by molecular dynamics simulations, and the observed permeation events are in agreement with the anion selectivity and conductance of GlyR. These studies provide the structural basis for gating, ion selectivity, and single-channel conductance properties of GlyR in a lipid environment.

Glycinergic synapses play a central role in motor control and pain processing in the central nervous system. Here, authors present cryo-EM structures of the full-length glycine receptors (GlyRs) reconstituted into lipid nanodiscs in the unliganded, glycine-bound and allosteric modulator-bound conformations and reveal global conformational changes underlying GlyR channel gating and modulation.

Sarcophine and (7S, 8R)-dihydroxydeepoxysarcophine from the Red Sea soft coral Sarcophyton glaucum as in vitro and in vivo modulators of glycine receptors

The inhibitory glycine receptor (GlyR) is a key mediator of synaptic signalling in spinal cord, brain stem, and higher centres of the central nervous system. We examined the glycinergic activity of sarcophine (SN), a marine terpenoid known for its various biological activities, and its trans-diol derivative (7S, 8R)-dihydroxy-deepoxysarcophine (DSN). SN was isolated from the Red Sea soft coral Sacrophyton glaucum, DSN was semisynthesized by hydrolysis of the epoxide ring. In cytotoxicity tests against HEK293 cells, SN and DSN had LD₅₀ values of 29.3 ± 3.0 mM and 123.5 ± 13.0 mM, respectively. Both compounds were tested against recombinant human α1 glycine receptors in HEK293 cells using whole-cell recording techniques. Both, SN and DSN were shown for the first time to be inhibitors of recombinant glycine receptors, with K_Ivalues of 2.1 ± 0.3 μM for SN, and 109 ± 9 μM for DSN. Receptor inhibition was also studied in vivo in a mouse model of strychnine toxicity. Surprisingly, in mouse experiments strychnine inhibition was not augmented by either terpenoid. While DSN had no significant effect on strychnine toxicity, SN even delayed strychnine effects. This could be accounted for by assuming that strychnine and sarcophine derivatives compete for the same binding site on the receptor, so the less toxic sarcophine can prevent strychnine from binding. The combination of modulatory activity and low level of toxicity makes sarcophines attractive structures for novel glycinergic drugs.

Modulators of the Inhibitory Glycine Receptor

The inhibitory glycine receptor is a member of the Cys-loop superfamily of ligand-gated ion channels. It is the principal mediator of rapid synaptic inhibition in the spinal cord and brainstem and plays an important role in the modulation of higher brain functions including vision, hearing, and pain signaling. Glycine receptor function is controlled by only a few agonists, while the number of antagonists and positive or biphasic modulators is steadily increasing. These modulators are important for the study of receptor activation and regulation and have found clinical interest as potential analgesics and anticonvulsants. High-resolution structures of the receptor have become available recently, adding to our understanding of structure-function relationships and revealing agonistic, inhibitory, and modulatory sites on the receptor protein. This Review presents an overview of compounds that activate, inhibit, or modulate glycine receptor function in vitro and in vivo.

Potentiation of the Glycine Response by Bisphenol A, an Endocrine Disrupter, on the Substantia Gelatinosa Neurons of the Trigeminal Subnucleus Caudalis in Mice

Lamina II, also called the substantia gelatinosa (SG) of the medullary dorsal horn (the trigeminal subnucleus caudalis, Vc), is thought to play an essential role in the control of orofacial nociception because it receives the nociceptive signals from primary afferents, including thin myelinated Aδ- and unmyelinated C-fibers. Glycine, the main inhibitory neurotransmitter in the central nervous system, plays an essential role in the transference of nociceptive messages from the periphery to higher brain regions. Bisphenol A (BPA) is reported to alter the morphological and functional characteristics of neuronal cells and to be an effector of a great number of ion channels in the central nervous system. However, the electrophysiological effects of BPA on the glycine receptors of SG neurons in the Vc have not been well studied. Therefore, in this study, we used the whole-cell patch-clamp technique to determine the effect of BPA on the glycine response in SG neurons of the Vc in male mice. We demonstrated that in early neonatal mice (0-3 postnatal day mice), BPA did not affect the glycine-induced inward current. However, in the juvenile and adult groups, BPA enhanced the glycine-mediated responses. Heteromeric glycine receptors were involved in the modulation by BPA. The interaction between BPA and glycine appears to have a significant role in regulating transmission in the nociceptive pathway.

Intraperitoneal injection of ginkgolide B, a major active compound of Ginkgo biloba, dose-dependently increases the amount of wake and decreases non-rapid eye movement sleep in C57BL/6 mice

The terpene lactones of Ginkgo biloba extract, namely ginkgolides (A, B, and C) and bilobalide, possess antioxidant, anti-inflammatory, and neuroprotective effects. They are widely prescribed for the treatment of cerebral dysfunctions and neurological impairments. In addition, they demonstrate antagonistic action at the gamma-aminobutyric acid type A and glycine receptors, which are members of the ligand-gated ion channel superfamily. In the present study, the effects of ginkgolides (A, B, and C) and bilobalide on sleep in C57BL/6 mice were investigated. Ginkgolide B was found to dose-dependently increase the amount of wake and decrease that of non-rapid eye movement sleep without changes in the electroencephalography power density of each sleep/wake stage, core body temperature and locomotor activity for the first 6 h after intraperitoneal injection. Of note, the amount of wake after injection of 5 mg/kg of ginkgolide B showed a significant increase (14.9 %) compared with that of vehicle (P = 0.005). In contrast, there were no significant differences in the amount of sleep, core body temperature, and locomotor activity in the mice injected with ginkgolide A and C. Bilobalide briefly induced a decrease in locomotor activity but did not exert significant effects on the amounts of sleep and wake. The modes of action of the wake-enhancing effects of ginkgolide B are unknown. However, it may act through the antagonism of gamma-aminobutyric acid type A and glycine receptors because it is established that these inhibitory amino acids mediate sleep and sleep-related physiology. It is of interest to further evaluate the stimulant and awaking actions of ginkgolide B on the central nervous system in clinical and basic research studies.

Human Hyperekplexic Mutations in Glycine Receptors Disinhibit the Brainstem by Hijacking GABAA Receptors

Hyperekplexia disease is usually caused by naturally occurring point mutations in glycine receptors (GlyRs). However, the γ-aminobutyric acid type A receptor (GABA_AR) seems to be also involved regarding the therapeutic basis for hyperekplexia using benzodiazepines, which target GABA_ARs but not GlyRs. Here, we show that the function of GABA_ARs was significantly impaired in the hypoglossal nucleus of hyperekplexic transgenic mice. Such impairment appeared to be mediated by interaction between GABA_AR and mutant GlyR. The GABA_AR dysfunction was caused only by mutant GlyR consisting of homomeric α₁ subunits, which locate primarily at pre- and extra-synaptic sites. In addition, the rescue effects of diazepam were attenuated by Xli-093, which specifically blocked diazepam-induced potentiation on α₅-containing GABA_AR, a major form of pre- and extra-synaptic GABA_AR in the brainstem. Thus, our results suggest that the pre- and extra-synaptic GABA_ARs could be a potential therapeutic target for hyperekplexia disease caused by GlyR mutations.

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Hyperekplexic mutant GlyRs interact with GABA_ARs and disrupt the GABA_AR function

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Pre- and extra-synaptic GABA_ARs are deficient in the hyperekplexia disease

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α₅-Containing GABA_AR is a potential therapeutic target for the hyperekplexia disease

The role of tonic glycinergic conductance in cerebellar granule cell signalling and the effect of gain-of-function mutation

KEY POINTS

A T258F mutation of the glycine receptor increases the receptor affinity to endogenous agonists, modifies single-channel conductance and shapes response decay kinetics. Glycine receptors of cerebellar granule cells play their functional role not continuously, but when the granule cell layer starts receiving a high amount of excitatory inputs. Despite their relative scarcity, tonically active glycine receptors of cerebellar granule cells make a significant impact on action potential generation and inter-neuronal crosstalk, and modulate synaptic plasticity in neural networks; extracellular glycine increases probability of postsynaptic response occurrence acting at NMDA receptors and decreases this probability acting at glycine receptors. Tonic conductance through glycine receptors of cerebellar granule cells is a yet undiscovered element of the biphasic mechanism that regulates processing of sensory inputs in the cerebellum. A T258F point mutation disrupts this biphasic mechanism, thus illustrating the possible role of the gain-of-function mutations of the glycine receptor in development of neural pathologies.

ABSTRACT

Functional glycine receptors (GlyRs) have been repeatedly detected in cerebellar granule cells (CGCs), where they deliver exclusively tonic inhibitory signals. The functional role of this signalling, however, remains unclear. Apart from that, there is accumulating evidence of the important role of GlyRs in cerebellar structures in development of neural pathologies such as hyperekplexia, which can be triggered by GlyR gain-of-function mutations. In this research we initially tested functional properties of GlyRs, carrying the yet understudied T258F gain-of-function mutation, and found that this mutation makes significant modifications in GlyR response to endogenous agonists. Next, we clarified the role of tonic GlyR conductance in neuronal signalling generated by single CGCs and by neural networks in cell cultures and in living cerebellar tissue of C57Bl-6J mice. We found that GlyRs of CGCs deliver a significant amount of tonic inhibition not continuously, but when the cerebellar granule layer starts receiving substantial excitatory input. Under these conditions tonically active GlyRs become a part of neural signalling machinery allowing generation of action potential (AP) bursts of limited length in response to sensory-evoked signals. GlyRs of CGCs support a biphasic modulatory mechanism which enhances AP firing when excitatory input intensity is low, but suppresses it when excitatory input rises to a certain critical level. This enables one of the key functions of the CGC layer: formation of sensory representations and their translation into motor output. Finally, we have demonstrated that the T258F mutation in CGC GlyRs modifies single-cell and neural network signalling, and breaks a biphasic modulation of the AP-generating machinery.

Characterization of the Zebrafish Glycine Receptor Family Reveals Insights Into Glycine Receptor Structure Function and Stoichiometry

To study characterization of zebrafish glycine receptors (zGlyRs), we assessed expression and function of five α- and two ß-subunit encoding GlyR in zebrafish. Our qPCR analysis revealed variable expression during development, while in situ hybridizations uncovered expression in the hindbrain and spinal cord; a finding consistent with the reported expression of GlyR subunits in these tissues from other organisms. Electrophysiological recordings using Xenopus oocytes revealed that all five α subunits form homomeric receptors activated by glycine, and inhibited by strychnine and picrotoxin. In contrast, ß subunits only formed functional heteromeric receptors when co-expressed with α subunits. Curiously, the second transmembranes of both ß subunits were found to lack a phenylalanine at the sixth position that is commonly associated with conferring picrotoxin resistance to heteromeric receptors. Consistent with the absence of phenylalanines at the sixth position, heteromeric zGlyRs often lacked significant picrotoxin resistance. Subsequent efforts revealed that resistance to picrotoxin in both zebrafish and human heteromeric GlyRs involves known residues within transmembrane 2, as well as previously unknown residues within transmembrane 3. We also found that a dominant mutation in human GlyRα1 that gives rise to hyperekplexia, and recessive mutations in zebrafish GlyRßb that underlie the bandoneon family of motor mutants, result in reduced receptor function. Lastly, through the use of a concatenated construct we demonstrate that zebrafish heteromeric receptors assemble with a stoichiometry of 3α:2ß. Collectively, our findings have furthered our knowledge regarding the assembly of heteromeric receptors, and the molecular basis of ß subunit-conferred picrotoxin resistance. These results should aid in future investigations of glycinergic signaling in zebrafish and mammals.

Plasminogen binding inhibitors demonstrate unwanted activities on GABAA and glycine receptors in human iPSC derived neurons

Plasminogen binding inhibitors (PBIs) reduce the risk of bleeding in hemorrhagic conditions. However, generic PBIs are also associated with an increased risk of seizures, an adverse effect linked to unwanted activities towards inhibitory neuronal receptors. Development of novel PBIs serve to remove compounds with such properties, but progress is limited by a lack of higher throughput methods with human translatability. Herein we apply human induced pluripotent stem cell (hiPSC) derived neurons in combination with dynamic mass redistribution (DMR) technology to demonstrate robust and reproducible modulation of both GABA_A and glycine receptors. These cells respond to GABA (EC₅₀ 0.33 ± 0.18 μM), glycine (EC₅₀ 11.0 ± 3.7 μM) and additional ligands in line with previous reports from patch clamp technologies. Additionally, we identify and characterize a competitive antagonistic behavior of the prototype inhibitor and drug tranexamic acid (TXA). Finally, we demonstrate proof of concept for effective counter-screening of lead series compounds towards unwanted GABA_A receptor activities. No activity was observed for a previously identified PBI candidate drug, AZD6564, whereas a discontinued analog, AZ13267257, could be characterized as a potent GABA_A receptor agonist.

GABAA receptor: Positive and negative allosteric modulators

gamma-Aminobutyric acid (GABA)-mediated inhibitory neurotransmission and the gene products involved were discovered during the mid-twentieth century. Historically, myriad existing nervous system drugs act as positive and negative allosteric modulators of these proteins, making GABA a major component of modern neuropharmacology, and suggesting that many potential drugs will be found that share these targets. Although some of these drugs act on proteins involved in synthesis, degradation, and membrane transport of GABA, the GABA receptors Type A (GABAAR) and Type B (GABABR) are the targets of the great majority of GABAergic drugs. This discovery is due in no small part to Professor Norman Bowery. Whereas the topic of GABABR is appropriately emphasized in this special issue, Norman Bowery also made many insights into GABAAR pharmacology, the topic of this article. GABAAR are members of the ligand-gated ion channel receptor superfamily, a chloride channel family of a dozen or more heteropentameric subtypes containing 19 possible different subunits. These subtypes show different brain regional and subcellular localization, age-dependent expression, and potential for plastic changes with experience including drug exposure. Not only are GABAAR the targets of agonist depressants and antagonist convulsants, but most GABAAR drugs act at other (allosteric) binding sites on the GABAAR proteins. Some anxiolytic and sedative drugs, like benzodiazepine and related drugs, act on GABAAR subtype-dependent extracellular domain sites. General anesthetics including alcohols and neurosteroids act at GABAAR subunit-interface trans-membrane sites. Ethanol at high anesthetic doses acts on GABAAR subtype-dependent trans-membrane domain sites. Ethanol at low intoxicating doses acts at GABAAR subtype-dependent extracellular domain sites. Thus GABAAR subtypes possess pharmacologically specific receptor binding sites for a large group of different chemical classes of clinically important neuropharmacological agents. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Lung-injury depresses glutamatergic synaptic transmission in the nucleus tractus solitarii via discrete age-dependent mechanisms in neonatal rats

Transition periods (TPs) are brief stages in CNS development where neural circuits can exhibit heightened vulnerability to pathologic conditions such as injury or infection. This susceptibility is due in part to specialized mechanisms of synaptic plasticity, which may become activated by inflammatory mediators released under pathologic conditions. Thus, we hypothesized that the immune response to lung injury (LI) mediated synaptic changes through plasticity-like mechanisms that depended on whether LI occurred just before or after a TP. We studied the impact of LI on brainstem 2nd-order viscerosensory neurons located in the nucleus tractus solitarii (nTS) during a TP for respiratory control spanning (postnatal day (P) 11-15). We injured the lungs of Sprague-Dawley rats by intratracheal instillation of Bleomycin (or saline) just before (P9-11) or after (P17-19) the TP. A week later, we prepared horizontal slices of the medulla and recorded spontaneous and evoked excitatory postsynaptic currents (sEPSCs/eEPSCs) in vitro from neurons in the nTS that received monosynaptic glutamatergic input from the tractus solitarii (TS). In rats injured before the TP (pre-TP), neurons exhibited blunted sEPSCs and TS-eEPSCs compared to controls. The decreased TS-eEPSCs were mediated by differences in postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid receptors (AMPAR). Specifically, compared to controls, LI rats had more Ca2+-impermeable AMPARs (CI-AMPARs) as indicated by: 1) the absence of current-rectification, 2) decreased sensitivity to polyamine, 1-Naphthyl-acetyl-spermine-trihydrochloride (NASPM) and 3) augmented immunoreactive staining for the CI-AMPAR GluA2. Thus, pre-TP-LI acts postsynaptically to blunt glutamatergic transmission. The neuroimmune response to pre-TP-LI included microglia hyper-ramification throughout the nTS. Daily intraperitoneal administration of minocycline, an inhibitor of microglial/macrophage function prevented hyper-ramification and abolished the pre-TP-LI evoked synaptic changes. In contrast, rat-pups injured after the TP (post-TP) exhibited microglia hypo-ramification in the nTS and had increased sEPSC amplitudes/frequencies, and decreased TS-eEPSC amplitudes compared to controls. These synaptic changes were not associated with changes in CI-AMPARs, and instead involved greater TS-evoked use-dependent depression (reduced paired pulse ratio), which is a hallmark of presynaptic plasticity. Thus we conclude that LI regulates the efficacy of TS → nTS synapses through discrete plasticity-like mechanisms that are immune-mediated and depend on whether the injury occurs before or after the TP for respiratory control.

The Glycine Receptor-A Functionally Important Primary Brain Target of Ethanol

Identification of ethanol's (EtOH) primary molecular brain targets and determination of their functional role is an ongoing, important quest. Pentameric ligand-gated ion channels, that is, the nicotinic acetylcholine receptor, the γ-aminobutyric acid type A receptor, the 5-hydroxytryptamine₃ , and the glycine receptor (GlyR), are such targets. Here, aspects of the structure and function of these receptors and EtOH's interaction with them are briefly reviewed, with special emphasis on the GlyR and the importance of this receptor and its ligands for EtOH pharmacology. It is suggested that GlyRs are involved in (i) the dopamine-activating effect of EtOH, (ii) regulating EtOH intake, and (iii) the relapse preventing effect of acamprosate. Exploration of the GlyR subtypes involved and efforts to develop subtype specific agonists or antagonists may offer new pharmacotherapies for alcohol use disorders.

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.

GABA-ρ receptors: distinctive functions and molecular pharmacology

The homomeric GABA-ρ ligand-gated ion channels (also known as GABA_C or GABA_A -ρ receptors) are similar to heteromeric GABA_A receptors in structure, function and mechanism of action. However, their distinctive pharmacological properties and distribution make them of special interest. This review focuses on GABA-ρ ion channel structure, ligand selectivity toward ρ receptors over heteromeric GABA_A receptor sub-types and selectivity between different homomeric ρ sub-type receptors. Several GABA analogues show selectivity at homomeric GABA-ρ receptors over heteromeric GABA_A receptors. More recently, some synthetic ligands have been found to show selectivity at receptors formed from one ρ subtype over others. The unique pharmacological profiles of these agents are discussed in this review. The classical binding site of GABA within the orthosteric site of GABA-ρ homomeric receptors is discussed in detail regarding the loops and residues that constitute the binding site. The ligand-residue interactions in this classical binding and those of mutant receptors are discussed. The structure and conformations of GABA are discussed in regard to its flexibility and molecular properties. Although the binding mode of GABA is difficult to predict, several interactions between GABA and the receptor assist in predicting its potential conformation and mode of action. The structure-activity relationships of GABA and structurally key ligands at ρ receptors are described and discussed.

A noninvasive optical approach for assessing chloride extrusion activity of the K-Cl cotransporter KCC2 in neuronal cells

BACKGROUND

Cation-chloride cotransporters (CCCs) are indispensable for maintaining chloride homeostasis in multiple cell types, but K-Cl cotransporter KCC2 is the only CCC member with an exclusively neuronal expression in mammals. KCC2 is critical for rendering fast hyperpolarizing responses of ionotropic γ-aminobutyric acid and glycine receptors in adult neurons, for neuronal migration in the developing central nervous system, and for the formation and maintenance of small dendritic protrusions-dendritic spines. Deficit in KCC2 expression and/or activity is associated with epilepsy and neuropathic pain, and effective strategies are required to search for novel drugs augmenting KCC2 function.

RESULTS

We revised current methods to develop a noninvasive optical approach for assessing KCC2 transport activity using a previously characterized genetically encoded chloride sensor. Our protocol directly assesses dynamics of KCC2-mediated chloride efflux and allows measuring genuine KCC2 activity with good spatial and temporal resolution. As a proof of concept, we used this approach to compare transport activities of the two known KCC2 splice isoforms, KCC2a and KCC2b, in mouse neuronal Neuro-2a cells.

CONCLUSIONS

Our noninvasive optical protocol proved to be efficient for assessment of furosemide-sensitive chloride fluxes. Transport activities of the N-terminal splice isoforms KCC2a and KCC2b obtained by the novel approach matched to those reported previously using standard methods for measuring chloride fluxes.

Glycinergic dysfunction in a subpopulation of dorsal horn interneurons in a rat model of neuropathic pain

The development of neuropathic pain involves persistent changes in signalling within pain pathways. Reduced inhibitory signalling in the spinal cord following nerve-injury has been used to explain sensory signs of neuropathic pain but specific circuits that lose inhibitory input have not been identified. This study shows a specific population of spinal cord interneurons, radial neurons, lose glycinergic inhibitory input in a rat partial sciatic nerve ligation (PNL) model of neuropathic pain. Radial neurons are excitatory neurons located in lamina II of the dorsal horn, and are readily identified by their morphology. The amplitude of electrically-evoked glycinergic inhibitory post-synaptic currents (eIPSCs) was greatly reduced in radial neurons following nerve-injury associated with increased paired-pulse ratio. There was also a reduction in frequency of spontaneous IPSCs (sIPSCs) and miniature IPSCs (mIPSC) in radial neurons without significantly affecting mIPSC amplitude. A subtype selective receptor antagonist and western blots established reversion to expression of the immature glycine receptor subunit GlyRα2 in radial neurons after PNL, consistent with slowed decay times of IPSCs. This study has important implications as it identifies a glycinergic synaptic connection in a specific population of dorsal horn neurons where loss of inhibitory signalling may contribute to signs of neuropathic pain.

Effects of hypotaurine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis in immature mice

To understand the action and mechanism of hypotaurine, an immediate precursor of taurine, on orofacial nociceptive processing, we examined the direct effects and receptor types involved in hypotaurine-induced responses using the whole-cell patch clamp technique in the substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis (Vc) of immature mice. Under the condition of high-chloride pipette solution, hypotaurine elicited inward currents or upward deflections of membrane potential, which increased in a concentration-dependent manner (30-3000 μM) with the EC₅₀ of 663.8 and 337.6 μM, respectively. The responses to 300 µM hypotaurine were reproducible and recovered upon washout. The 300 µM hypotaurine-induced currents were maintained in the presence of TTX, CNQX, and AP5, indicating direct postsynaptic action of hypotaurine on SG neurons. Responses to both low (300 µM) and high (1 or 3 mM) concentrations of hypotaurine were completely and reversibly blocked by the glycine receptor antagonist strychnine (2 µM), but unaffected by the GABA_A receptor antagonist gabazine (3 µM) which blocks synaptic GABA_A receptors at low concentration. Furthermore, responses to 300 µM hypotaurine and a maximal concentration of glycine (3 mM) were not additive, indicating that hypotaurine and glycine act on the same receptor. Hypotaurine-induced currents were partially antagonized by picrotoxin (50 µM) which blocks homomeric glycine receptors and by bicuculline (10 µM) which is an antagonist of α2 subunit-containing glycine receptors. These results suggest that hypotaurine-induced responses were mediated by glycine receptor activation in the SG neurons and hypotaurine might be used as an effective therapeutics for orofacial pain.

Identification of Positive Allosteric Modulators of Glycine Receptors from a High-Throughput Screen Using a Fluorescent Membrane Potential Assay

Glycine receptor 3 (GlyRα3) is a ligand-gated ion channel of the cys-loop family that plays a key role in mediating inhibitory neurotransmission and regulation of pain signaling in the dorsal horn. Potentiation of GlyRα3 function is therefore of interest as a putative analgesic mechanism with which to target new therapeutics. However, to date, positive allosteric modulators (PAMs) of this receptor with sufficient selectivity to enable target validation studies have not been described. To address this lack of pharmacological tools, we developed a suite of in vitro assays comprising a high-throughput fluorescent membrane potential screen and a medium-throughput electrophysiology assay using IonFlux HT together with conventional manual patch clamp. Using these assays, we conducted a primary screening campaign and report the structures of hit compounds identified as GlyR PAMs. Our functional characterization data reveal a hit compound with high efficacy relative to current known potentiators and selectivity over GABA_AR, another major class of inhibitory neurotransmission receptors of importance to pain. These small-molecule GlyR PAMs have high potential both as early tool compounds to enable pharmacological studies of GlyR inhibitory neurotransmission and as a starting point for the development of potent, selective GlyRα3 PAMs as novel analgesics.

The binding mode of picrotoxinin in GABAA-ρ receptors: Insight into the subunit's selectivity in the transmembrane domain

The channel blocker picrotoxinin has been studied with GABA_A-ρ1 and GABA_A-ρ2 homology models based on the GluCl crystal structure. Picrotoxinin is tenfold more potent for GABA_A-ρ2 than for GABA_A-ρ1 homomeric channels. This intra-subunit selectivity arises from the unconserved residues at the 2' sites, which are the essential molecular basis for both the binding and potency of picrotoxinin. The serine residues at the 2' positions of the ρ2 channel are predicted to form multiple hydrogen bonds and hydrophobic interactions with picrotoxinin, whereas the proline residues in the 2' positions of ρ1 channels are predicted to form only hydrophobic contacts with picrotoxinin. However, although the studied ρ1 P2'G, A, and V models form no hydrogen bonds with picrotoxinin, they may participate in several hydrophobic interactions, and the ligand may have distinctive binding modes with GABA_A-ρ mutant channels. Picrotoxinin has a lower Emodel value with ρ2 than ρ1 homomeric models (-47Kcal/mol and -36Kcal/mol, respectively), suggesting that picrotoxin blocks the pores of the ρ2 channels more effectively.

Defects of the Glycinergic Synapse in Zebrafish

Glycine mediates fast inhibitory synaptic transmission. Physiological importance of the glycinergic synapse is well established in the brainstem and the spinal cord. In humans, the loss of glycinergic function in the spinal cord and brainstem leads to hyperekplexia, which is characterized by an excess startle reflex to sudden acoustic or tactile stimulation. In addition, glycinergic synapses in this region are also involved in the regulation of respiration and locomotion, and in the nociceptive processing. The importance of the glycinergic synapse is conserved across vertebrate species. A teleost fish, the zebrafish, offers several advantages as a vertebrate model for research of glycinergic synapse. Mutagenesis screens in zebrafish have isolated two motor defective mutants that have pathogenic mutations in glycinergic synaptic transmission: bandoneon (beo) and shocked (sho). Beo mutants have a loss-of-function mutation of glycine receptor (GlyR) β-subunit b, alternatively, sho mutant is a glycinergic transporter 1 (GlyT1) defective mutant. These mutants are useful animal models for understanding of glycinergic synaptic transmission and for identification of novel therapeutic agents for human diseases arising from defect in glycinergic transmission, such as hyperekplexia or glycine encephalopathy. Recent advances in techniques for genome editing and for imaging and manipulating of a molecule or a physiological process make zebrafish more attractive model. In this review, we describe the glycinergic defective zebrafish mutants and the technical advances in both forward and reverse genetic approaches as well as in vivo visualization and manipulation approaches for the study of the glycinergic synapse in zebrafish.

Review : Glycine Receptors: Molecular Heterogeneity and Implications for Disease

The inhibitory glycine receptor is a ligand-gated chloride channel that exists in developmentally regulated isoforms. These oligomeric transmembrane proteins are composed of variants of the ligand binding α subunit and structural β polypeptides. The agonist and antagonist sites of the α subunits are formed by discontinuous sequence motifs. In the murine genome, the genes encoding the α1 ( Glra1), α3 ( Glra3), and β ( Glyrb) subunit are autosomally located, whereas the α2 ( Glra2) and α4 ( Glra4) genes reside on the X-chromosome. Mutations of glycine receptor genes have been found to underly hypertonic motor disorders in mice and humans. The mouse mutants spasmodic (spd) and oscillator ( spdot) carry recessive mutations of the Glra 1 gene. In the phenotypically similar mouse mutant spastic ( spa), the intronic insertion of a LINE-1 transposable element into the Gyrb gene results in the aberrant splicing and a consecutive loss of glycine receptors. The human neurological disorder hyperekplexia (startle disease, stiff baby syndrome) is caused by point mutations within the α1 subunit gene ( GLRA1) localized in the human chromosomal region 5q31.3. The Neuroscientist 1:130- 141,1995

A Recombinant Human Pluripotent Stem Cell Line Stably Expressing Halide-Sensitive YFP-I152L for GABAAR and GlyR-Targeted High-Throughput Drug Screening and Toxicity Testing

GABA_ARs and GlyRs are considered attractive drug targets for therapeutic intervention and are also increasingly recognized in the context of in vitro neurotoxicity (NT) and developmental neurotoxicity (DNT) testing. However, systematic human-specific GABA_AR and GlyR-targeted drug screening and toxicity testing is hampered due to lack of appropriate in vitro models that express native GABA_ARs and GlyRs. We have established a human pluripotent stem cell line (NT2) stably expressing YFP-I152L, a halide-sensitive variant of yellow fluorescent protein (YFP), allowing for fluorescence-based functional analysis of chloride channels. Upon stimulation with retinoic acid, NT2 cells undergo neuronal differentiation and allow pharmacological and toxicological evaluation of native GABA_ARs and GlyRs at different stages of brain maturation. We applied the cell line in concentration-response experiments with the neurotransmitters GABA and glycine as well as with the drugs strychnine, picrotoxin, fipronil, lindane, bicuculline, and zinc and demonstrate that the established in vitro model is applicable to GABA_AR and GlyR-targeted pharmacological and toxicological profiling. We quantified the proportion of GABA_AR and GlyR-sensitive cells, respectively, and identified percentages of approximately 20% each within the overall populations, rendering the cells a suitable model for systematic in vitro GABA_AR and GlyR-targeted screening in the context of drug development and NT/DNT testing.

Murine startle mutant Nmf11 affects the structural stability of the glycine receptor and increases deactivation

Key points

Hyperekplexia or startle disease is a serious neurological condition affecting newborn children and usually involves dysfunctional glycinergic neurotransmission.

Glycine receptors (GlyRs) are major mediators of inhibition in the spinal cord and brainstem.

A missense mutation, replacing asparagine (N) with lysine (K), at position 46 in the GlyR α1 subunit induced hyperekplexia following a reduction in the potency of the transmitter glycine; this resulted from a rapid deactivation of the agonist current at mutant GlyRs.

These effects of N46K were rescued by mutating a juxtaposed residue, N61 on binding Loop D, suggesting these two asparagines may interact.

Asparagine 46 is considered to be important for the structural stability of the subunit interface and glycine binding site, and its mutation represents a new mechanism by which GlyR dysfunction induces startle disease.

Abstract

Dysfunctional glycinergic inhibitory transmission underlies the debilitating neurological condition, hyperekplexia, which is characterised by exaggerated startle reflexes, muscle hypertonia and apnoea. Here we investigated the N46K missense mutation in the GlyR α1 subunit gene found in the ethylnitrosourea (ENU) murine mutant, Nmf11, which causes reduced body size, evoked tremor, seizures, muscle stiffness, and morbidity by postnatal day 21. Introducing the N46K mutation into recombinant GlyR α1 homomeric receptors, expressed in HEK cells, reduced the potencies of glycine, β‐alanine and taurine by 9‐, 6‐ and 3‐fold respectively, and that of the competitive antagonist strychnine by 15‐fold. Replacing N46 with hydrophobic, charged or polar residues revealed that the amide moiety of asparagine was crucial for GlyR activation. Co‐mutating N61, located on a neighbouring β loop to N46, rescued the wild‐type phenotype depending on the amino acid charge. Single‐channel recording identified that burst length for the N46K mutant was reduced and fast agonist application revealed faster glycine deactivation times for the N46K mutant compared with the WT receptor. Overall, these data are consistent with N46 ensuring correct alignment of the α1 subunit interface by interaction with juxtaposed residues to preserve the structural integrity of the glycine binding site. This represents a new mechanism by which GlyR dysfunction induces startle disease.

Hyperekplexia or startle disease is a serious neurological condition affecting newborn children and usually involves dysfunctional glycinergic neurotransmission.

Glycine receptors (GlyRs) are major mediators of inhibition in the spinal cord and brainstem.

A missense mutation, replacing asparagine (N) with lysine (K), at position 46 in the GlyR α1 subunit induced hyperekplexia following a reduction in the potency of the transmitter glycine; this resulted from a rapid deactivation of the agonist current at mutant GlyRs.

These effects of N46K were rescued by mutating a juxtaposed residue, N61 on binding Loop D, suggesting these two asparagines may interact.

Asparagine 46 is considered to be important for the structural stability of the subunit interface and glycine binding site, and its mutation represents a new mechanism by which GlyR dysfunction induces startle disease.

Selective potentiation of alpha 1 glycine receptors by ginkgolic acid

Glycine receptors (GlyRs) belong to the superfamily of pentameric cys-loop receptor-operated channels and are involved in numerous physiological functions, including movement, vision, and pain. In search for compounds performing subunit-specific modulation of GlyRs we studied action of ginkgolic acid, an abundant Ginkgo biloba product. Using patch-clamp recordings, we analyzed the effects of ginkgolic acid in concentrations from 30 nM to 25 μM on α1–α3 and α1/β, α2/β configurations of GlyR and on GABA_ARs expressed in cultured CHO-K1 cells and mouse neuroblastoma (N2a) cells. Ginkgolic acid caused an increase in the amplitude of currents mediated by homomeric α1 and heteromeric α1/β GlyRs and provoked a left-shift of the concentration-dependent curves for glycine. Even at high concentrations (10–25 μM) ginkgolic acid was not able to augment ionic currents mediated by α2, α2/β, and α3 GlyRs, or by GABA_AR consisting of α1/β2/γ2 subunits. Mutation of three residues (T59A/A261G/A303S) in the α2 GlyR subunit to the corresponding ones from the α1 converted the action of ginkgolic acid to potentiation with a distinct decrease in EC₅₀ for glycine, suggesting an important role for these residues in modulation by ginkgolic acid. Our results suggest that ginkgolic acid is a novel selective enhancer of α1 GlyRs.

Activation of synaptic and extrasynaptic glycine receptors by taurine in preoptic hypothalamic neurons

Taurine is an essential amino-sulfonic acid having a fundamental function in the brain, participating in both cell volume regulation and neurotransmission. Using a whole cell voltage patch clamp technique, the taurine-activated neurotransmitter receptors in the preoptic hypothalamic area (PHA) neurons were investigated. In the first set of experiments, different concentrations of taurine were applied on PHA neurons. Taurine-induced responses were concentration-dependent. Taurine-induced currents were action potential-independent and sensitive to strychnine, suggesting the involvement of glycine receptors. In addition, taurine activated not only α-homomeric, but also αβ-heteromeric glycine receptors in PHA neurons. Interestingly, a low concentration of taurine (0.5mM) activated glycine receptors, whereas a higher concentration (3mM) activated both glycine and gamma-aminobutyric acid A (GABAA) receptors in PHA neurons. These results suggest that PHA neurons are influenced by taurine and respond via glycine and GABAA receptors.