Startle disease mutations reduce the agonist sensitivity of the human inhibitory glycine receptor

The importance of ligand gated ion channels in sleep and sleep disorders

On average, humans spend about 26 years of their life sleeping. Increased sleep duration and quality has been linked to reduced disease risk; however, the cellular and molecular underpinnings of sleep remain open questions. It has been known for some time that pharmacological modulation of neurotransmission in the brain can promote either sleep or wakefulness thereby providing some clues about the molecular mechanisms at play. However, the field of sleep research has developed an increasingly detailed understanding of the requisite neuronal circuitry and key neurotransmitter receptor subtypes, suggesting that it may be possible to identify next generation pharmacological interventions to treat sleep disorders within this same space. The aim of this work is to examine the latest physiological and pharmacological findings highlighting the contribution of ligand gated ion channels including the inhibitory GABA_A and glycine receptors and excitatory nicotinic acetylcholine receptors and glutamate receptors in the sleep-wake cycle regulation. Overall, a better understanding of ligand gated ion channels in sleep will help determine if these highly druggable targets could facilitate a better night's sleep.

Structural basis for cannabinoid-induced potentiation of alpha1-glycine receptors in lipid nanodiscs

Nociception and motor coordination are critically governed by glycine receptor (GlyR) function at inhibitory synapses. Consequentially, GlyRs are attractive targets in the management of chronic pain and in the treatment of several neurological disorders. High-resolution mechanistic details of GlyR function and its modulation are just emerging. While it has been known that cannabinoids such as Δ9-tetrahydrocannabinol (THC), the principal psychoactive constituent in marijuana, potentiate GlyR in the therapeutically relevant concentration range, the molecular mechanism underlying this effect is still not understood. Here, we present Cryo-EM structures of full-length GlyR reconstituted into lipid nanodisc in complex with THC under varying concentrations of glycine. The GlyR-THC complexes are captured in multiple conformational states that reveal the basis for THC-mediated potentiation, manifested as different extents of opening at the level of the channel pore. Taken together, these structural findings, combined with molecular dynamics simulations and functional analysis, provide insights into the potential THC binding site and the allosteric coupling to the channel pore.

Different Behaviors of a Glycine Receptor Channel Pore Residue between Wild-Type-Mimicking and Disease-Type-Mimicking Formats

The glycine receptor (GlyR) is a neurotransmitter-gated chloride channel that mediates fast inhibitory neurotransmission, predominantly in the spinal cord and brain stem. Mutations of the GlyR are the major cause of hereditary hyperekplexia. Site-specific cysteine substitution followed by labeling with a fluorophore has previously been used to explore the behaviors of the hyperekplexia-related 271 (19') residue of the GlyR. However, this manipulation dramatically compromises sensitivity toward the agonist glycine and alters the pharmacological effects of various agents in manners similar to those of the hyperekplexia-causing R19'Q/L mutations, raising the question whether what is reported by the substituted and modified residue faithfully reflects what actually happens to the wild-type (WT) residue. In this study, a mechanism-rescuing second-site mutation was introduced to create a WT-mimicking GlyR (with the 19' residue cysteine substitution and modification still in place), in which the sensitivity toward glycine and pharmacological effects of various agents were restored. Further experiments revealed stark differences in the behaviors upon the various pharmacological treatments and consequently the underlying mechanisms of the 19' residue between this WT-mimicking GlyR and the GlyR without the mechanism rescue, which is correspondingly defined as the disease-type (DT)-mimicking GlyR. The data presented in this study warn generally that caution is required when attempting to deduce the behaviors of a WT residue from data based on substituted or modified residues that alter protein structure and function. Extra measures, such as rescuing mechanisms via alternative means as presented in this study, are needed to mitigate this challenge.

Charged residues at the pore extracellular half of the glycine receptor facilitate channel gating: a potential role played by electrostatic repulsion

KEY POINTS

The Arg271Gln mutation of the glycine receptor (GlyR) causes hereditary hyperekplexia. This mutation dramatically compromises GlyR function; however, the underlying mechanism is not yet known. This study, by employing function and computation methods, proposes that charged residues (including the Arg residue) at the pore extracellular half from each of the five subunits of the homomeric α1 GlyR, create an electrostatic repulsive potential to widen the pore, thereby facilitating channel opening. This mechanism explains how the Arg271Gln mutation, in which the positively charged Arg residue is substituted by the neutral Gln residue, compromises GlyR function. This study furthers our understanding of the biophysical mechanism underlying the Arg271Gln mutation compromising GlyR function.

ABSTRACT

The R271(19')Q mutation in the α1 subunit of the glycine receptor (GlyR) chloride channel causes hereditary hyperekplexia. This mutation dramatically compromises channel function; however, the underlying mechanism is not yet known. The R271 residue is located at the extracellular half of the channel pore. In this study, an Arg-scanning mutagenesis was performed at the pore extracellular half from the 262(10') to the 272(20') position on the background of the α1 GlyR carrying the hyperekplexia-causing mutation R271(19')Q. It was found that the placement of the Arg residue rescued channel function to an extent inversely correlated with the distance between the residue and the pore central axis (perpendicular to the plane of the lipid bilayer). Accordingly, it was hypothesized that the placed Arg residues from each of the five subunits of the homomeric α1 GlyR create an electrostatic repulsive potential to widen the pore, thereby facilitating channel opening. This hypothesis was quantitatively verified by theoretical computation via exploiting basic laws of electrostatics and thermodynamics, and further supported by more experimental findings that the placement of another positively charged Lys residue or even a negatively charged Asp residue also rescued channel function in the same manner. This study provides a novel mechanism via which charged residues in the pore region facilitate channel gating, not only for the disease-causing 19'R residue in the GlyR, but also potentially for charged residues in the same region of other ion channels.

GABA A Receptor Coupling Junction and Pore GABRB3 Mutations are Linked to Early-Onset Epileptic Encephalopathy

GABA_A receptors are brain inhibitory chloride ion channels. Here we show functional analyses and structural simulations for three de novo missense mutations in the GABA_A receptor β3 subunit gene (GABRB3) identified in patients with early-onset epileptic encephalopathy (EOEE) and profound developmental delay. We sought to obtain insights into the molecular mechanisms that might link defects in GABA_A receptor biophysics and biogenesis to patients with EOEE. The mutant residues are part of conserved structural domains such as the Cys-loop (L170R) and M2-M3 loop (A305V) that form the GABA binding/channel gating coupling junction and the channel pore (T288N), which are functionally coupled during receptor activation. The mutant coupling junction residues caused rearrangements and formation of new hydrogen bonds in the open state, while the mutant pore residue reshaped the pore cavity. Whereas mutant coupling junction residues uncoupled during activation and caused gain of function, the mutant pore residue favoured low conductance receptors and differential sensitivity to diazepam and loss of function. These data reveal novel molecular mechanisms by which EOEE-linked mutations affect GABA_A receptor function.

Structural mechanisms of activation and desensitization in neurotransmitter-gated ion channels

Ion channels gated by neurotransmitters are present across metazoans, in which they are essential for brain function, sensation and locomotion; closely related homologs are also found in bacteria. Structures of eukaryotic pentameric cysteine-loop (Cys-loop) receptors and tetrameric ionotropic glutamate receptors in multiple functional states have recently become available. Here, I describe how these studies relate to established ideas regarding receptor activation and how they have enabled decades' worth of functional work to be pieced together, thus allowing previously puzzling aspects of receptor activity to be understood.

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

Allosteric and hyperekplexic mutant phenotypes investigated on an α1 glycine receptor transmembrane structure

The glycine receptor (GlyR) is a pentameric ligand-gated ion channel (pLGIC) mediating inhibitory transmission in the nervous system. Its transmembrane domain (TMD) is the target of allosteric modulators such as general anesthetics and ethanol and is a major locus for hyperekplexic congenital mutations altering the allosteric transitions of activation or desensitization. We previously showed that the TMD of the human α1GlyR could be fused to the extracellular domain of GLIC, a bacterial pLGIC, to form a functional chimera called Lily. Here, we overexpress Lily in Schneider 2 insect cells and solve its structure by X-ray crystallography at 3.5 Å resolution. The TMD of the α1GlyR adopts a closed-channel conformation involving a single ring of hydrophobic residues at the center of the pore. Electrophysiological recordings show that the phenotypes of key allosteric mutations of the α1GlyR, scattered all along the pore, are qualitatively preserved in this chimera, including those that confer decreased sensitivity to agonists, constitutive activity, decreased activation kinetics, or increased desensitization kinetics. Combined structural and functional data indicate a pore-opening mechanism for the α1GlyR, suggesting a structural explanation for the effect of some key hyperekplexic allosteric mutations. The first X-ray structure of the TMD of the α1GlyR solved here using GLIC as a scaffold paves the way for mechanistic investigation and design of allosteric modulators of a human receptor.

Glycine receptor mouse mutants: model systems for human hyperekplexia

Human hyperekplexia is a neuromotor disorder caused by disturbances in inhibitory glycine-mediated neurotransmission. Mutations in genes encoding for glycine receptor subunits or associated proteins, such as GLRA1, GLRB, GPHN and ARHGEF9, have been detected in patients suffering from hyperekplexia. Classical symptoms are exaggerated startle attacks upon unexpected acoustic or tactile stimuli, massive tremor, loss of postural control during startle and apnoea. Usually patients are treated with clonazepam, this helps to dampen the severe symptoms most probably by up-regulating GABAergic responses. However, the mechanism is not completely understood. Similar neuromotor phenotypes have been observed in mouse models that carry glycine receptor mutations. These mouse models serve as excellent tools for analysing the underlying pathomechanisms. Yet, studies in mutant mice looking for postsynaptic compensation of glycinergic dysfunction via an up-regulation in GABAA receptor numbers have failed, as expression levels were similar to those in wild-type mice. However, presynaptic adaptation mechanisms with an unusual switch from mixed GABA/glycinergic to GABAergic presynaptic terminals have been observed. Whether this presynaptic adaptation explains the improvement in symptoms or other compensation mechanisms exist is still under investigation. With the help of spontaneous glycine receptor mouse mutants, knock-in and knock-out studies, it is possible to associate behavioural changes with pharmacological differences in glycinergic inhibition. This review focuses on the structural and functional characteristics of the various mouse models used to elucidate the underlying signal transduction pathways and adaptation processes and describes a novel route that uses gene-therapeutic modulation of mutated receptors to overcome loss of function mutations.

New hyperekplexia mutations provide insight into glycine receptor assembly, trafficking, and activation mechanisms

Hyperekplexia is a syndrome of readily provoked startle responses, alongside episodic and generalized hypertonia, that presents within the first month of life. Inhibitory glycine receptors are pentameric ligand-gated ion channels with a definitive and clinically well stratified linkage to hyperekplexia. Most hyperekplexia cases are caused by mutations in the α1 subunit of the human glycine receptor (hGlyR) gene (GLRA1). Here we analyzed 68 new unrelated hyperekplexia probands for GLRA1 mutations and identified 19 mutations, of which 9 were novel. Electrophysiological analysis demonstrated that the dominant mutations p.Q226E, p.V280M, and p.R414H induced spontaneous channel activity, indicating that this is a recurring mechanism in hGlyR pathophysiology. p.Q226E, at the top of TM1, most likely induced tonic activation via an enhanced electrostatic attraction to p.R271 at the top of TM2, suggesting a structural mechanism for channel activation. Receptors incorporating p.P230S (which is heterozygous with p.R65W) desensitized much faster than wild type receptors and represent a new TM1 site capable of modulating desensitization. The recessive mutations p.R72C, p.R218W, p.L291P, p.D388A, and p.E375X precluded cell surface expression unless co-expressed with α1 wild type subunits. The recessive p.E375X mutation resulted in subunit truncation upstream of the TM4 domain. Surprisingly, on the basis of three independent assays, we were able to infer that p.E375X truncated subunits are incorporated into functional hGlyRs together with unmutated α1 or α1 plus β subunits. These aberrant receptors exhibit significantly reduced glycine sensitivity. To our knowledge, this is the first suggestion that subunits lacking TM4 domains might be incorporated into functional pentameric ligand-gated ion channel receptors.

Analysis of hyperekplexia mutations identifies transmembrane domain rearrangements that mediate glycine receptor activation

Pentameric ligand-gated ion channels (pLGICs) mediate numerous physiological processes and are therapeutic targets for a wide range of clinical indications. Elucidating the structural differences between their closed and open states may help in designing improved drugs that bias receptors toward the desired conformational state. We recently showed that two new hyperekplexia mutations, Q226E and V280M, induced spontaneous activity in α1 glycine receptors. Gln-226, located near the top of transmembrane (TM) 1, is closely apposed to Arg-271 at the top of TM2 in the neighboring subunit. Using mutant cycle analysis, we inferred that Q226E induces activation via an enhanced electrostatic attraction to Arg-271. This would tilt the top of TM2 toward TM1 and hence away from the pore axis to open the channel. We also concluded that the increased side chain volume of V280M, in the TM2-TM3 loop, exerts a steric repulsion against Ile-225 at the top of TM1 in the neighboring subunit. We infer that this steric repulsion would tilt the top of TM3 radially outwards against the stationary TM1 and thus provide space for TM2 to relax away from the pore axis to create an open channel. Because the transmembrane domain movements inferred from this functional analysis are consistent with the structural differences evident in the x-ray atomic structures of closed and open state bacterial pLGICs, we propyose that the model of pLGIC activation as outlined here may be broadly applicable across the eukaryotic pLGIC receptor family.

Inhibitory glycine receptors: an update

Strychnine-sensitive glycine receptors (GlyRs) mediate synaptic inhibition in the spinal cord, brainstem, and other regions of the mammalian central nervous system. In this minireview, we summarize our current view of the structure, ligand-binding sites, and chloride channel of these receptors and discuss recently emerging functions of distinct GlyR isoforms. GlyRs not only regulate the excitability of motor and afferent sensory neurons, including pain fibers, but also are involved in the processing of visual and auditory signals. Hence, GlyRs constitute promising targets for the development of therapeutically useful compounds.

Function of hyperekplexia-causing α1R271Q/L glycine receptors is restored by shifting the affected residue out of the allosteric signalling pathway

BACKGROUND AND PURPOSE Glycine receptor α1 subunit R271Q and R271L (α1R271Q/L) mutations cause the neuromotor disorder, hereditary hyperekplexia. Studies suggest that the 271 residue is located within the allosteric signalling pathway linking the agonist binding site to the channel gate. The present study aimed to investigate a possible mechanism for restoring the function of the α1R271Q/L glycine receptor. EXPERIMENTAL APPROACH A 12-amino-acid segment incorporating the 271 residue on the glycine receptor α1271Q/L subunit was replaced by the homologous segment from the glycine receptor β subunit (α1(Ch) 271Q/L). The function of the α1(Ch) 271Q/L glycine receptor was examined by whole-cell patch-clamp recording and voltage-clamp fluorometry techniques. KEY RESULTS The function of the α1(Ch) 271Q/L glycine receptor was restored to the level of the wild-type (WT) α1 glycine receptor. Moreover, in the α1(Ch) glycine receptor, in contrast to the α1 glycine receptor, the channel function was not sensitive to various substitutions of the 271 residue, and the conformational change in the vicinity of the 271 residue was uncoupled from the channel gating. CONCLUSIONS AND IMPLICATIONS The 271 residue is shifted out of the allosteric signalling pathway in the α1(Ch) glycine receptor. We propose that this mechanism provides a novel drug design strategy not only for glycine receptor α1R271Q/L-caused hereditary hyperekplexia, but also for any pathological condition that is caused by missense mutation- or covalent modification-induced disorders involving residues in allosteric signalling pathways. Such a strategy makes it possible to design an ideal drug, which only corrects the function of the mutant or modified protein without affecting the WT or naive protein.

Milestones in myoclonus

This review examines some of the advances in understanding myoclonus over the last 25 years. The classification of myoclonus into cortical, brainstem, and spinal forms has been consolidated, each with distinctive clinical characteristics and physiological mechanisms. New genetic causes of myoclonus have been identified, and the molecular basis of several of these conditions has been discovered. It is increasingly apparent that disease of the cerebellum is particularly important in the genesis of cortical reflex myoclonus. However, the precise mechanism and origin of myoclonus in many situations remain uncertain. Effective treatment of myoclonus remains limited, and the challenge lies ahead to develop more therapeutic options.

Contributions of conserved residues at the gating interface of glycine receptors

Glycine receptors (GlyRs) are chloride channels that mediate fast inhibitory neurotransmission and are members of the pentameric ligand-gated ion channel (pLGIC) family. The interface between the ligand binding domain and the transmembrane domain of pLGICs has been proposed to be crucial for channel gating and is lined by a number of charged and aromatic side chains that are highly conserved among different pLGICs. However, little is known about specific interactions between these residues that are likely to be important for gating in α1 GlyRs. Here we use the introduction of cysteine pairs and the in vivo nonsense suppression method to incorporate unnatural amino acids to probe the electrostatic and hydrophobic contributions of five highly conserved side chains near the interface, Glu-53, Phe-145, Asp-148, Phe-187, and Arg-218. Our results suggest a salt bridge between Asp-148 in loop 7 and Arg-218 in the pre-M1 domain that is crucial for channel gating. We further propose that Phe-145 and Phe-187 play important roles in stabilizing this interaction by providing a hydrophobic environment. In contrast to the equivalent residues in loop 2 of other pLGICs, the negative charge at Glu-53 α1 GlyRs is not crucial for normal channel function. These findings help decipher the GlyR gating pathway and show that distinct residue interaction patterns exist in different pLGICs. Furthermore, a salt bridge between Asp-148 and Arg-218 would provide a possible mechanistic explanation for the pathophysiologically relevant hyperekplexia, or startle disease, mutant Arg-218 → Gln.

Fine architecture and mutation mapping of human brain inhibitory system ligand gated ion channels by high-throughput homology modeling

The common architecture of the brain inhibitory system ligand-gated ion-channels was examined at the level of each of the subunits and in their assembled pentameric arrangements. Structural modeling of the GABAA receptor, GlyR1, and the serotonin receptor, 5HTR3A, was carried out on a multi-homolog basis employing a high-throughput homology modeling pipeline. The locations of all the known mutations of each of the subunits of the receptor subfamily were mapped upon their computed structures and structural relationships between patterns of mutations in different subunits were identified, resulting in the zoning of mutations to four specific regions of the common subunit structure. These classifications may be of value in discerning probable molecular mechanisms and functional manifestations of emerging mutations and polymorphisms, providing the foundation for a family-specific predictive algorithm that may allow researchers to focus experimental effort on the most probable molecular indicators of compromised receptor function and disease mechanism.

Inverse effects on gating and modulation caused by a mutation in the M2-M3 Linker of the GABA(A) receptor gamma subunit

M2-M3 linkers are receptor subunit domains known to be critical for the normal function of cysteine-loop ligand-gated ion channels. Previous studies of alpha and beta subunits of type "A" GABA receptors suggest that these linkers couple extracellular elements involved in GABA binding to the transmembrane segments that control the opening of the ion channel. To study the importance of the gamma subunit M2-M3 linker, we examined the macroscopic and single-channel effects of an engineered gamma2(L287A) mutation on GABA activation and propofol modulation. In the macroscopic analysis, we found that the gamma2(L287A) mutation decreased GABA potency but increased the ability of propofol to enhance both GABA potency and efficacy compared with wild-type receptors. Indeed, although propofol had significant effects on GABA potency in wild-type receptors, we found that propofol produced no corresponding increase in GABA efficacy. At the single-channel level, mutant receptors showed a loss in the longest of three open-time components compared with wild-type receptors under GABA activation. Furthermore, propofol reduced the duration of one closed-time component, increased the duration of two open-time components, and generated a third open component with a longer lifetime in mutant compared with wild-type receptors. Taken together, we conclude that although the gamma subunit is not required for the binding of GABA or propofol, the M2-M3 linker of this subunit plays a critical role in channel gating by GABA and allosteric modulation by propofol. Our results also suggest that in wild-type receptors, propofol exerts its enhancing effects by mechanisms extrinsic to channel gating.

Gating mechanisms in Cys-loop receptors

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

A novel GLRA1 mutation associated with an atypical hyperekplexia phenotype

Hyperekplexia (MIM #149400) is a rare neurological disorder characterized by an exaggerated startle response, infantile hypertonia and hyperreflexia without spasticity, a hesitant gait that usually improves by 3 years of age, and nocturnal myoclonus. Familial hyperekplexia is usually autosomal dominant resulting from mutations in the inhibitory glycine receptor subunit alpha 1 (GLRA1) gene on chromosome 5q. We identified a 3-generation family with progressively severe phenotypes of hyperekplexia. All affected family members were found to be heterozygous for a novel arginine271proline mutation in GLRA1. Long-term follow-up of the affected members of the third generation, now aged 6 and 7 years, reveals enhanced startle responses and persistent hypertonia of the extremities without clonus or a catch, tight heel cords and abnormal toe-walking gait, and plantar flexor reflexes. The 7-year-old child recently reponded well to a benzodiazepine. Future studies are warranted to examine whether this new missense mutation is solely responsible for this atypical phenotype.

Mice with mitochondrial complex I deficiency develop a fatal encephalomyopathy

To study effects of mitochondrial complex I (CI, NADH:ubiquinone oxidoreductase) deficiency, we inactivated the Ndufs4 gene, which encodes an 18 kDa subunit of the 45-protein CI complex. Although small, Ndufs4 knockout (KO) mice appeared healthy until approximately 5 weeks of age, when ataxic signs began, progressing to death at approximately 7 weeks. KO mice manifested encephalomyopathy including a retarded growth rate, lethargy, loss of motor skill, blindness, and elevated serum lactate. CI activity in submitochondrial particles from KO mice was undetectable by spectrophotometric assays. However, CI-driven oxygen consumption by intact tissue was about half that of controls. Native gel electrophoresis revealed reduced levels of intact CI. These data suggest that CI fails to assemble properly or is unstable without NDUFS4. KO muscle has normal morphology but low NADH dehydrogenase activity and subsarcolemmal aggregates of mitochondria. Nonetheless, total oxygen consumption and muscle ATP and phosphocreatine concentrations measured in vivo were within normal parameters.

Two Japanese families with hyperekplexia who have a Arg271Gln mutation in the glycine receptor alpha 1 subunit gene

We report two Japanese patients from two families with hyperekplexia who have a Arg271Gln mutation in the glycine receptor alpha 1 subunit gene. The clinical course of both patients was typical for hyperekplexia, characterized by neonatal hypertonia and exaggerated startle response, and which improved gradually with age. One was associated with umbilical hernia and hip dislocation, diagnosed at 11 months, while the other was diagnosed at 1 month. Both showed positive head retraction reflex. Four Japanese families have been reported as having hyperekplexia including our cases, of which three have shown the same missense Arg271Gln mutation, most frequently found in patients from Northern Europe and the United States.

Hereditary hyperekplexia caused by novel mutations of GLRA1 in Turkish families

BACKGROUND

Hyperekplexia, also known as startle disease or stiff-person syndrome, is a neurological condition characterized by neonatal hypertonia and a highly exaggerated startle reflex. Genetic studies have linked mutations in the gene encoding glycine receptor alpha1 (GLRA1) with hereditary hyperekplexia.

METHODS

We analyzed four Turkish families with a history of hyperekplexia. Genomic DNA was obtained from members of these families, and the entire coding sequence of GLRA1 was amplified by PCR followed by the sequencing of PCR products. DNA sequences were analyzed by direct observation using an electropherogram and compared with a published reference sequence.

RESULTS

We identified three novel mutations in GLRA1. These included a large deletion removing the first 7 of 9 exons, a single-base deletion in exon 8 that results in protein truncation immediately after the deletion, and a missense mutation in exon 7 causing a tryptophan-to-cysteine change in the first transmembrane domain (M1). These mutant alleles have some distinct features as compared to previously identified GLRA1 mutations. Our data provides further evidence for mutational heterogeneity in GLRA1. The new mutant alleles reported here should advance our understanding of the etiology of hyperekplexia.

Characterization of newly cloned variant of rat glycine receptor α1 subunit

Responses to glycine, a major inhibitory neurotransmitter within the nervous system, are mediated by glycine receptors (GlyRs). Here, we report the cloning and analysis of a novel splicing variant of the GlyRalpha1 subunit. This variant, named GlyRalpha1del, has a truncated cytoplasmic region between transmembrane domains (TM)3 and TM4, and compared to other variants, the truncation is contributed by a different acceptor site in exon 9. We transfected GlyRalpha1 or GlyRalpha1del into HEK293 cells, and then examined the glycine-activated currents using a whole-cell patch-clamp recording technique. Maximal currents and current-voltage relationships showed no clear difference between GlyRalpha1del and GlyRalpha1. Moreover, dose-response curves indicated that the EC50 values for glycine differed significantly between the two GlyRalpha1 derivatives, although their Hill coefficients were similar. When present with other isoforms, GlyRalpha1del might alter the response to glycine or to other agonists, as this variant expands the potential heterogeneity among glycine receptors.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

Charged Residues in the β2 Subunit Involved in GABAA Receptor Activation

Fast synaptic inhibition in the mammalian central nervous system is mediated primarily via activation of the gamma-aminobutyric acid type A receptor (GABAA-R). Upon agonist binding, the receptor undergoes a structural transition from the closed to the open state. This transition, known as gating, is thought to be associated with a sequence of conformational changes originating at the agonist-binding site, ultimately resulting in opening of the channel. Using site-directed mutagenesis and several different GABAA-R agonists, we identified a number of highly conserved charged residues in the GABAA-R beta2 subunit that appear to be involved in receptor activation. We then used charge reversal double mutants and disulfide trapping to investigate the interactions between these flexible loops within the beta2 subunit. The results suggest that interactions between an acidic residue in loop 7 (Asp146) and a basic residue in pre-transmembrane domain-1 (Lys215) are involved in coupling agonist binding to channel gating.

Glycinergic inputs to cardiac vagal neurons in the nucleus ambiguus are inhibited by nociceptin and mu-selective opioids

Most parasympathetic regulation of heart rate originates from preganglionic cardiac vagal neurons within the nucleus ambiguus. Little is known regarding the modulation of glycinergic transmission to these neurons. However, the presence of mu-opioid receptors and opioid-receptor-like (ORL1) receptors within the ambiguus, together with the presence of endogenous ligands for both receptor types in the same area, suggests opioids may modulate synaptic transmission to cardiac vagal neurons. This study therefore examined the effects of endomorphin-1 and endomorphin-2 (the mu-selective endogenous peptides), DAMGO (a synthetic, mu-selective agonist), and nociceptin (the ORL1-selective endogenous peptide) on spontaneous glycinergic inhibitory postsynaptic currents (IPSCs) in rat cardiac parasympathetic neurons. All four of the opioids used in this study decreased spontaneous IPSCs. At concentrations of 100 microM, the amplitude of the IPSCs was reduced significantly by nociceptin (-56.6%), DAMGO (-46.5%), endomorphin-1 (-45.1%), and endomorphin-2 (-26%). IPSC frequency was also significantly reduced by nociceptin (-61.1%), DAMGO (-69.9%), and endomorphin-1 (-40.8%) but not endomorphin-2. Lower concentrations of nociceptin and DAMGO (10-30 microM) also effectively decreased IPSC amplitude and frequency. The inhibitory effects of DAMGO were blocked by d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2 (C-TOP; 10 microM), a selective mu-receptor antagonist. Neither nociceptin nor DAMGO inhibited the postsynaptic responses evoked by exogenous application of glycine or affected TTX-insensitive glycinergic mini-IPSCs. These results indicate that mu-selective opioids and nociceptin act on preceding neurons to decrease glycinergic inputs to cardiac vagal neurons in the nucleus ambiguus. The resulting decrease in glycinergic transmission would increase parasympathetic activity to the heart and may be a mechanism by which opioids induce bradycardia.

Asymmetric contribution of alpha and beta subunits to the activation of alphabeta heteromeric glycine receptors

This study investigated the role of beta subunits in the activation of alphabeta heteromeric glycine receptor (GlyR) chloride channels recombinantly expressed in HEK293 cells. The approach involved incorporating mutations into corresponding positions in alpha and beta subunits and comparing their effects on receptor function. Although cysteine-substitution mutations to residues in the N-terminal half of the alpha subunit M2-M3 loop dramatically impaired the gating efficacy, the same mutations exerted little effect when incorporated into corresponding positions of the beta subunit. Furthermore, although the alpha subunit M2-M3 loop cysteines were modified by a cysteine-specific reagent, the corresponding beta subunit cysteines showed no evidence of reactivity. These observations suggest structural or functional differences between alpha and beta subunit M2-M3 loops. In addition, a threonine-->leucine mutation at the 9' position in the beta subunit M2 pore-lining domain dramatically increased the glycine sensitivity. By analogy with the effects of the same mutation in other ligand-gated ion channels, it was concluded that the mutation affected the GlyR activation mechanism. This supports the idea that the GlyR beta subunit is involved in receptor gating. In conclusion, this study demonstrates that beta subunits contribute to the activation of the GlyR, but that their involvement in this process is significantly different to that of the alpha subunit.

Modulation of glycine receptor function: a novel approach for therapeutic intervention at inhibitory synapses?

Transmitter-gated ion channels mediate rapid synaptic transmission in the CNS and constitute important targets for many neuroactive drugs. Inhibitory glycine receptors (GlyRs) are members of the nicotinic acetylcholine receptor superfamily and inhibit neuronal firing by opening Cl(-) channels following agonist binding. In this article, we discuss recent developments in GlyR pharmacology, delineate the receptor domains that are involved in binding of agonists and allosteric modulators, and present a molecular model of the extracellular architecture of the receptor. The recent discovery of compounds that act preferentially on specific GlyR isoforms and the differential expression of these isoforms in distinct regions of the developing and adult CNS show considerable promise towards the development of drugs that act in defined glycine-mediated pathways. In particular, compounds that can potentiate GlyR function should provide leads for novel muscle relaxants in addition to sedative and analgesic agents.

Binding site stoichiometry and the effects of phosphorylation on human alpha1 homomeric glycine receptors

The kinetic properties of the human alpha1 homomeric glycine receptor were investigated. Receptors were expressed in HEK 293 cells, and glycine was applied to outside-out membrane patches with sub-millisecond solution exchange. The activation time course of the glycine response was used to investigate receptor stoichiometry. The unbinding of three strychnine molecules and the cooperative binding of two glycine molecules were required to activate the channel. The effects of phosphorylation on glycine receptor kinetics were investigated by pretreating cells with phosphorylators or with phosphatases. Phosphorylation accelerated desensitisation, but slowed deactivation and recovery from desensitisation. A chemical-kinetic model was developed that reproduced the experimental observations. The model suggests that only three binding sites on the glycine channel are functional, while the remaining two binding sites are 'silent', possibly due to strong negative cooperativity.

Hyperekplexia: a treatable neurogenetic disease

Hyperekplexia is primarily an autosomal dominant disease characterized by exaggerated startle reflex and neonatal hypertonia. It can be associated with, if untreated, sudden infant death from apnea or aspiration pneumonia and serious injuries and loss of ambulation from frequent falls. Different mutations in the alpha1 subunit of inhibitory glycine receptor (GLRA1) gene have been identified in many affected families. The most common mutation is Arg271 reported in at least 12 independent families. These mutations uncouple the ligand binding and chloride channel function of inhibitory glycine receptor and result in increased excitability in pontomedullary reticular neurons and abnormal spinal reciprocal inhibition. Three mouse models from spontaneous mutations in GLRA1 and beta subunit of inhibitory glycine receptor (GLRB) genes and two transgenic mouse models are valuable for the study of the pathophysiology and the genotype-phenotype correlation of the disease. The disease caused by mutation in GLRB in mice supports the notion that human hyperekplexia with no detectable mutations in GLRA1 may harbor mutations in GLRB. Clonazepam, a gamma aminobutyric acid (GABA) receptor agonist, is highly effective and is the drug of choice. It enhances the GABA-gated chloride channel function and presumably compensates for the defective glycine-gated chloride channel in hyperekplexia. Recognition of the disease will lead to appropriate treatment and genetic counseling.

Functional characterization of compound heterozygosity for GlyRalpha1 mutations in the startle disease hyperekplexia

The human disease hyperekplexia is characterized by excessive startle reactions to auditory and cutaneous stimuli. In its familial form, hyperekplexia has been associated with both dominant and recessive mutations of the GLRA1 gene encoding the glycine receptor alpha1 subunit (GlyRalpha1), which mediates inhibitory transmission in the spinal cord and brainstem. Here we have examined the functional consequences of two amino acid substitutions found in a compound heterozygous family, R252H and R392H, to investigate the mechanisms determining this inheritance pattern. When expressed in Xenopus laevis oocytes, both mutations were non-functional. Neither mutant affected the electrophysiological properties of wild type GlyRalpha1 when co-expressed. We introduced a green fluorescent protein tag to mutant subunits and found that both mutant proteins were detectable. Evidence that subcellular localization differed from wild type was significant for one of the mutants. Thus, an effective loss of functional GlyRalpha1-mediated current underlies hyperekplexia in this family, whereas a partial loss is asymptomatic.

Cation-selective mutations in the M2 domain of the inhibitory glycine receptor channel reveal determinants of ion-charge selectivity

Ligand-gated ion channel receptors mediate neuronal inhibition or excitation depending on their ion charge selectivity. An investigation into the determinants of ion charge selectivity of the anion-selective alpha1 homomeric glycine receptor (alpha1 glycine receptor [GlyR]) was undertaken using point mutations to residues lining the extra- and intracellular ends of the ion channel. Five mutant GlyRs were studied. A single substitution at the intracellular mouth of the channel (A-1'E GlyR) was sufficient to convert the channels to select cations over anions with P(Cl)/P(Na) = 0.34. This result delimits the selectivity filter and provides evidence that electrostatic interactions between permeating ions and pore residues are a critical factor in ion charge selectivity. The P-2'Delta mutant GlyR retained its anion selectivity (P(Cl)/P(Na) = 3.81), but it was much reduced compared with the wild-type (WT) GlyR (P(Cl)/P(Na) = 27.9). When the A-1'E and the P-2'Delta mutations were combined (selectivity double mutant [SDM] GlyR), the relative cation permeability was enhanced (P(Cl)/P(Na) = 0.13). The SDM GlyR was also Ca(2+) permeable (P(Ca)/P(Na) = 0.29). Neutralizing the extracellular mouth of the SDM GlyR ion channel (SDM+R19'A GlyR) produced a more Ca(2+)-permeable channel (P(Ca)/P(Na) = 0.73), without drastically altering monovalent charge selectivity (P(Cl)/P(Na) = 0.23). The SDM+R19'E GlyR, which introduces a negatively charged ring at the extracellular mouth of the channel, further enhanced Ca(2+) permeability (P(Ca)/P(Na) = 0.92), with little effect on monovalent selectivity (P(Cl)/P(Na) = 0.19). Estimates of the minimum pore diameter of the A-1'E, SDM, SDM+R19'A, and SDM+R19'E GlyRs revealed that these pores are larger than the alpha1 GlyR, with the SDM-based GlyRs being comparable in diameter to the cation-selective nicotinic acetylcholine receptors. This result provides evidence that the diameter of the ion channel is also an important factor in ion charge selectivity.

Openings of the rat recombinant alpha 1 homomeric glycine receptor as a function of the number of agonist molecules bound

The functional properties of rat homomeric alpha 1 glycine receptors were investigated using whole-cell and outside-out recording from human embryonic kidney cells transfected with rat alpha1 subunit cDNA. Whole-cell dose-response curves gave EC(50) estimates between 30 and 120 microM and a Hill slope of approximately 3.3. Single channel recordings were obtained by steady-state application of glycine (0.3, 1, or 10 microM) to outside-out patches. Single channel conductances were mostly 60-90 pS, but smaller conductances of approximately 40 pS were also seen (10% of the events) with a relative frequency that did not depend on agonist concentration. The time constants of the apparent open time distributions did not vary with agonist concentration, but short events were more frequent at low glycine concentrations. There was also evidence of a previously missed short-lived open state that was more common at lower glycine concentrations. The time constants for the different components of the burst length distributions were found to have similar values at different concentrations. Nevertheless, the mean burst length increased with increasing glycine. This was because the relative area of each burst-length component was concentration dependent and short bursts were favored at lower glycine concentrations. Durations of adjacent open and shut times were found to be strongly (negatively) correlated. Additionally, long bursts were made up of longer than average openings separated by short gaps, whereas short bursts usually consisted of single isolated short openings. The most plausible explanation for these findings is that long bursts are generated when a higher proportion of the five potential agonist binding sites on the receptor is occupied by glycine. On the basis of the concentration dependence and the intraburst structure we provide a preliminary kinetic scheme for the activation of the homomeric glycine receptor, in which any number of glycine molecules from one to five can open the channel, although not with equal efficiency.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Characterization of a glycine receptor domain that controls the binding and gating mechanisms of the beta-amino acid agonist, taurine

The beta-amino acid, taurine, is a full agonist of the human glycine receptor alpha1 subunit when recombinantly expressed in a mammalian (HEK293) cell line, but a partial agonist of the same receptor when expressed in Xenopus oocytes. Several residues in the Ala101-Thr112 domain have previously been identified as determinants of beta-amino acid binding and gating mechanisms in Xenopus oocyte-expressed receptors. The present study used the substituted cysteine accessibility method to investigate the role of this domain in controlling taurine-specific binding and gating mechanisms of glycine receptors recombinantly expressed in mammalian cells. Asn102 and Glu103 are identified as taurine and glycine binding sites, whereas Ala101 is eliminated as a possible binding site. The N102C mutation also abolished the antagonistic actions of taurine, indicating that this site does not discriminate between the putative agonist- and antagonist-bound conformations of beta-amino acids. The effects of mutations from Lys104-Thr112 indicate that the mechanism by which this domain controls beta-amino acid-specific binding and gating processes differs substantially depending on whether the receptor is expressed in mammalian cells or Xenopus oocytes. Thr112 is the only domain element in mammalian cell-expressed GlyRs which was demonstrated to discriminate between glycine and taurine.

A mutation (V260M) in the middle of the M2 pore-lining domain of the glycine receptor causes hereditary hyperekplexia

We investigated the molecular basis of hyperekplexia (STHE), an inherited neurological disorder characterised by neonatal hypertonia and an exaggerated startle response, in a kindred and identified a novel missense mutation in the pore-lining M2 domain of the alpha1 subunit of the glycine receptor (GLRA1). Sequencing analysis of all exons of the GLRA1 gene revealed a G1158A base transition in affected, heterozygous patients. The base transition results in a valine to methionine substitution at codon 260 in the middle of the M2 transmembrane domain. The location within the M2 domain suggests for this substitution a likely role in altering ion channel properties.

Mutagenesis of the GABA(A) receptor alpha1 subunit reveals a domain that affects sensitivity to GABA and benzodiazepine-site ligands

We have mutated several amino acids in the region of the GABA(A) receptor alpha1 subunit predicted to form a small extracellular loop between transmembrane domains two and three to investigate its possible role in ligand sensitivity. The mutations were S275T, L276A, P277A, V279A, A280S and Y281F. Mutant alpha1 subunits were co-expressed with beta2 and gamma2 subunits in tsA201 cells or Xenopus oocytes. Binding studies revealed that the only mutation that significantly affected [3H]Ro15-4513 binding was the V279A substitution which reduced the affinity for this ligand. Electrophysiological examination of mutant receptors revealed that L276A, P277A and V279A displayed rightward shifts of their GABA concentration-response curves, the largest occurring with the L276A mutant. The impact of these mutations on allosteric modulation by benzodiazepine-site ligands was examined. V279A reduced the potency of both flunitrazepam and Ro15-4513 but, in each case, their efficacy was enhanced. A280S resulted in a decrease in flunitrazepam efficacy without affecting its potency. Additionally, P277A and A280S resulted in Ro15-4513 losing its inverse agonist effect at these receptors. These results suggest that a domain within this small extracellular loop between TMII-TMIII plays a role in determining the sensitivity of GABA(A) receptors to both GABA and benzodiazepine-site ligands.

Expression of glycine receptors in rat sensory neurons vs. HEK293 cells yields different functional properties

Many structure-function studies of the glycine receptor (GlyR), and other ligand-gated ion channels, use somatic cell lines or Xenopus oocytes as expression systems. Using a polyethylenimine-based technique, we transfected GlyR cDNA into primary cultures of rat dorsal root ganglion (DRG) neurons. We then compared the functional properties of wildtype and a mutant GlyR expressed in DRG neurons with HEK 293 cells. The glycine sensitivity of the wildtype GlyR was nearly identical for the two cell types. The mutant GlyR has an arginine for glutamine substitution at position 271 (R271Q), which results in low glycine sensitivity relative to wildtype receptors expressed in HEK cells. This point mutation is associated with startle disease (hyperekplexia) in humans. Mutant GlyR expression in DRG neurons resulted in a significantly lower glycine sensitivity than was seen in HEK cells. This supports the idea that neuron-specific post-translational modifications may be important for determining receptor function.

Ivermectin, an unconventional agonist of the glycine receptor chloride channel

The effects of the antihelmintic, ivermectin, were investigated in recombinantly expressed human alpha(1) homomeric and alpha(1)beta heteromeric glycine receptors (GlyRs). At low (0.03 microm) concentrations ivermectin potentiated the response to sub-saturating glycine concentrations, and at higher (> or =0.03 microm) concentrations it irreversibly activated both alpha(1) homomeric and alpha(1)beta heteromeric GlyRs. Relative to glycine-gated currents, ivermectin-gated currents exhibited a dramatically reduced sensitivity to inhibition by strychnine, picrotoxin, and zinc. The insensitivity to strychnine could not be explained by ivermectin preventing the access of strychnine to its binding site. Furthermore, the elimination of a known glycine- and strychnine-binding site by site-directed mutagenesis had little effect on ivermectin sensitivity, demonstrating that the ivermectin- and glycine-binding sites were not identical. Ivermectin strongly and irreversibly activated a fast-desensitizing mutant GlyR after it had been completely desensitized by a saturating concentration of glycine. Finally, a mutation known to impair dramatically the glycine signal transduction mechanism had little effect on the apparent affinity or efficacy of ivermectin. Together, these findings indicate that ivermectin activates the GlyR by a novel mechanism.

Molecular ontogeny of major neurotransmitter receptor systems in the mammalian central nervous system: norepinephrine, dopamine, serotonin, acetylcholine, and glycine

Neurotransmitter receptors are critical elements in intercellular signaling within the central nervous system and are divided into two major types based on their molecular structure and biophysical properties. The first are ionotropic receptors--ligand-gated ion channels that directly affect the membrane potential via passage of permeant ions (such as sodium and calcium) and mediate fast synaptic transmission. The second type are slower metabotropic receptors that are also ligand gated but depend on an interaction with guanine nucleotide-binding proteins and mediate signal transduction by activating second-messenger systems within the cell. In the past two decades, a wealth of information has emerged regarding the molecular biology and pharmacology of classic neurotransmitter receptors (including adrenergic, dopaminergic, serotonergic, cholinergic, glycine, gamma-aminobutyric acid [GABA(A)], and glutamate receptors). Further, the distribution of subunits comprising these receptors has been extensively studied. This review focuses on the molecular ontogeny of several of the major neurotransmitter receptor systems in the mammalian central nervous system, highlighting the role that some of these may play during brain development and in certain pathologic states.

M2 pore mutations convert the glycine receptor channel from being anion- to cation-selective

Three mutations in the M2 transmembrane domains of the chloride-conducting alpha1 homomeric glycine receptor (P250Delta, A251E, and T265V), which normally mediate fast inhibitory neurotransmission, produced a cation-selective channel with P(Cl)/P(Na), = 0.27 (wild-type P(Cl)/P(Na) = 25), a permeability sequence P(Cs) > P(K) > P(Na) > P(Li), an impermeability to Ca(2+), and a reduced glycine sensitivity. Outside-out patch measurements indicated reversed and accentuated rectification with extremely low mean single channel conductances of 3 pS (inward current) and 11 pS (outward current). The three inverse mutations, to those analyzed in this study, have previously been shown to make the alpha7 acetylcholine receptor channel anion-selective, indicating a common location for determinants of charge selectivity of inhibitory and excitatory ligand-gated ion channels.

Direct measurement of specific membrane capacitance in neurons

The specific membrane capacitance (C(m)) of a neuron influences synaptic efficacy and determines the speed with which electrical signals propagate along dendrites and unmyelinated axons. The value of this important parameter remains controversial. In this study, C(m) was estimated for the somatic membrane of cortical pyramidal neurons, spinal cord neurons, and hippocampal neurons. A nucleated patch was pulled and a voltage-clamp step was applied. The exponential decay of the capacitative charging current was analyzed to give the total membrane capacitance, which was then divided by the observed surface area of the patch. C(m) was 0.9 microF/cm(2) for each class of neuron. To test the possibility that membrane proteins may alter C(m), embryonic kidney cells (HEK-293) were studied before and after transfection with a plasmid coding for glycine receptor/channels. The value of C(m) was indistinguishable in untransfected cells and in transfected cells expressing a high level of glycine channels, indicating that differences in transmembrane protein content do not significantly affect C(m). Thus, to a first approximation, C(m) may be treated as a "biological constant" across many classes of neuron.

A new photolabile precursor of glycine with improved properties: A tool for chemical kinetic investigations of the glycine receptor

The synthesis and characterization of a new photolabile precursor of glycine (caged glycine) is described. The alpha-carboxyl group of glycine is covalently coupled to the alpha-carboxy-2-nitrobenzyl (alphaCNB) protecting group. Photolysis of the caged glycine with UV light produces free glycine. At 308 nm, the compound photolyzes with a quantum yield of 0.38. The absorption spectrum and the pH dependence of a transient absorption produced after laser-flash illumination are typical for aci-nitro intermediates of alphaCNB-protected compounds. The time constant for the major component of the aci-nitro intermediate decay ( approximately 84% of the total aci-nitro absorbance) was determined to be 7 micros at physiological pH. A minor component ( approximately 16%) decays with a rate constant of 170 micros. The compound does not activate or inhibit the alpha(1)-homomeric glycine receptor transiently expressed in HEK293 cells. After photolysis with a 10 ns pulse of 325 nm laser light, the glycine released from the caged compound activates glycine-mediated whole-cell currents in the same cells. The rise of these currents can be measured in a time-resolved fashion and occurs on a millisecond to sub-millisecond time scale. It can be described with a single-exponential function over >85% of the total current. The rate constant of the current rise is about 2 orders of magnitude slower than the rate constant of caged glycine photolysis. Thermal hydrolysis of the alphaCNB-caged glycine takes place with a half-life of 15.6 h at physiological pH. The new caged glycine is the first in a series of photoprotected glycine derivatives that has the required properties for use with chemical kinetic methods for investigation of glycine-activated cell surface receptors. Photolysis is rapid and efficient with respect to the receptor reactions to be studied; hydrolysis in aqueous solution is sufficiently slow, and the compound is biologically inert. It will, therefore, be a useful tool for investigation of the processes leading to channel opening of glycine receptor channels and the effects of mutations of the glycine receptor and of inhibitors on these processes.