The novel hyperekplexia allele GLRA1(S267N) affects the ethanol site of the glycine receptor

Fine-Tuning of Neuronal Ion Channels-Mapping of Residues Involved in Glucose Sensitivity of Recombinant Human Glycine Receptors

The inhibitory glycine receptor (GlyR) mediates synaptic inhibition in the spinal cord, brain stem, and other regions of the mammalian central nervous system. Glucose was shown to potentiate α1 GlyRs by interacting with K143. Here, additional amino acids involved in glucose modulation were identified using a structure-based approach of site-directed mutagenesis followed by whole-cell patch-clamp analysis. We identified two additional lysine residues in the α1 GlyR extracellular domain, K16 and K281, that were involved in glucose modulation. Mutation of either residue to alanine abolished glucose potentiation. Residue K281 is located in the same pocket as K143 and could thus contribute to glucose binding. The double mutant K143A-K281A showed a 6-fold increase of EC₅₀, while EC₅₀ of both single mutants K143A and K281A was only slightly increased (1.7- and 1.3-fold, respectively). K16 is located at an analgesic binding site that is distant from the agonist or glucose sites, and the K16A mutation may generate a receptor species that is not potentiated. GlyR position α1-S267 is close to the postulated glucose binding site and known for interactions with ethanol and anesthetics. In the presence of glucose, GlyR α1 mutants S267A, S267I, and S267R showed potentiation, no effect, and reduction of current responses, respectively. This pattern follows that of ethanol modulation and suggests that the interaction sites of glucose and ethanol are identical or located close to each other. Our results support the presence of a distinct binding site for glucose on the glycine receptor, overlapping with the ivermectin/ethanol binding pocket near the transmembrane region and the TM2-3 loop.

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.

A novel nonsense autosomal dominant mutation in the GLRA1 gene causing hyperekplexia

We present a family with two members affected by hyperekplexia and two unaffected members. All exons in the glycine receptor alpha 1 subunit gene (GLRA1) were sequenced in all four family members. Our index patient harbored a novel nonsense mutation (p.Trp314*; rs867618642) in the transmembrane domain three of the GLRA1 and a novel missense variant in the NH₂-terminal part (p.Val67Met; rs142888296). After development of tolerance for the effective treatment with clobazam a drug holiday led to a sustained restoration of the treatment response.

Impaired Glycine Receptor Trafficking in Neurological Diseases

Ionotropic glycine receptors (GlyRs) enable fast synaptic neurotransmission in the adult spinal cord and brainstem. The inhibitory GlyR is a transmembrane glycine-gated chloride channel. The immature GlyR protein undergoes various processing steps, e.g., folding, assembly, and maturation while traveling from the endoplasmic reticulum to and through the Golgi apparatus, where post-translational modifications, e.g., glycosylation occur. The mature receptors are forward transported via microtubules to the cellular surface and inserted into neuronal membranes followed by synaptic clustering. The normal life cycle of a receptor protein includes further processes like internalization, recycling, and degradation. Defects in GlyR life cycle, e.g., impaired protein maturation and degradation have been demonstrated to underlie pathological mechanisms of various neurological diseases. The neurological disorder startle disease is caused by glycinergic dysfunction mainly due to missense mutations in genes encoding GlyR subunits (GLRA1 and GLRB). In vitro studies have shown that most recessive forms of startle disease are associated with impaired receptor biogenesis. Another neurological disease with a phenotype similar to startle disease is a special form of stiff-person syndrome (SPS), which is most probably due to the development of GlyR autoantibodies. Binding of GlyR autoantibodies leads to enhanced receptor internalization. Here we focus on the normal life cycle of GlyRs concentrating on assembly and maturation, receptor trafficking, post-synaptic integration and clustering, and GlyR internalization/recycling/degradation. Furthermore, this review highlights findings on impairment of these processes under disease conditions such as disturbed neuronal ER-Golgi trafficking as the major pathomechanism for recessive forms of human startle disease. In SPS, enhanced receptor internalization upon autoantibody binding to the GlyR has been shown to underlie the human pathology. In addition, we discuss how the existing mouse models of startle disease increased our current knowledge of GlyR trafficking routes and function. This review further illuminates receptor trafficking of GlyR variants originally identified in startle disease patients and explains changes in the life cycle of GlyRs in patients with SPS with respect to structural and functional consequences at the receptor level.

Modulation of Recombinant Human α1 Glycine Receptors by Mono- and Disaccharides: A Kinetic Study

Glycine receptors (GlyRs) mediate fast synaptic inhibition in spinal cord, brainstem, and higher brain centers. Recently, glucose was identified as a positive modulator of GlyR-mediated currents. Here, we investigated extent and kinetics of the positive modulation of recombinant human α1 glycine receptors by different mono- and disaccharides and sorbitol using patch-clamp recording techniques. Glucose and fructose augmented glycine-mediated whole-cell currents with an EC50 of 6-7 mM. At concentrations > 10 mM, the maximum of current enhancement was reached within ∼30 min. Kinetics of GlyR modulation resemble those of protein glycation. On-rates were <0.5 h for saturating concentrations of monosaccharides and ∼1.5 h for disaccharides. Off-rates were considerably slower (>24 h). Galactose, the C4-epimer of glucose, and the sugar alcohol sorbitol had no effect on GlyR currents. Recent cryoelectron microscopy structures were used to identify a potential binding site for saccharides near the ivermectin binding pocket with lysine 143 as possible attachment site. The GlyR mutant α1(K143A) was not potentiated by glucose, suggesting an involvement of this residue in glycine receptor modulation by saccharides.

The Intracellular Loop of the Glycine Receptor: It's not all about the Size

The family of Cys-loop receptors (CLRs) shares a high degree of homology and sequence identity. The overall structural elements are highly conserved with a large extracellular domain (ECD) harboring an α-helix and 10 β-sheets. Following the ECD, four transmembrane domains (TMD) are connected by intracellular and extracellular loop structures. Except the TM3–4 loop, their length comprises 7–14 residues. The TM3–4 loop forms the largest part of the intracellular domain (ICD) and exhibits the most variable region between all CLRs. The ICD is defined by the TM3–4 loop together with the TM1–2 loop preceding the ion channel pore. During the last decade, crystallization approaches were successful for some members of the CLR family. To allow crystallization, the intracellular loop was in most structures replaced by a short linker present in prokaryotic CLRs. Therefore, no structural information about the large TM3–4 loop of CLRs including the glycine receptors (GlyRs) is available except for some basic stretches close to TM3 and TM4. The intracellular loop has been intensively studied with regard to functional aspects including desensitization, modulation of channel physiology by pharmacological substances, posttranslational modifications, and motifs important for trafficking. Furthermore, the ICD interacts with scaffold proteins enabling inhibitory synapse formation. This review focuses on attempts to define structural and functional elements within the ICD of GlyRs discussed with the background of protein-protein interactions and functional channel formation in the absence of the TM3–4 loop.

[GLYCINE RECEPTOR: MOLECULAR ORGANIZATION AND PATHOLOGY]

Glycine receptor is the anion-selective channel, providing fast synaptic transmission in the central nervous system of vertebrates. Together with the nicotinic acetylcholine, GABA and serotonin (5-HT3R) receptors, it belongs to the superfamily of pentameric cys-loop receptors. In this review we briefly describe main functions of these transmembrane proteins, their distribution and molecular architecture. Special attention is paid to recent studies on the molecular physiology of these receptors, as well as on presenting of molecular domains responsible for their dysfunction.

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.

The impact of human hyperekplexia mutations on glycine receptor structure and function

Hyperekplexia is a rare neurological disorder characterized by neonatal hypertonia, exaggerated startle responses to unexpected stimuli and a variable incidence of apnoea, intellectual disability and delays in speech acquisition. The majority of motor defects are successfully treated by clonazepam. Hyperekplexia is caused by hereditary mutations that disrupt the functioning of inhibitory glycinergic synapses in neuromotor pathways of the spinal cord and brainstem. The human glycine receptor α1 and β subunits, which predominate at these synapses, are the major targets of mutations. International genetic screening programs, that together have analysed several hundred probands, have recently generated a clear picture of genotype-phenotype correlations and the prevalence of different categories of hyperekplexia mutations. Focusing largely on this new information, this review seeks to summarise the effects of mutations on glycine receptor structure and function and how these functional alterations lead to hyperekplexia.

Glycine receptor mutants of the mouse: what are possible routes of inhibitory compensation?

Defects in glycinergic inhibition result in a complex neuromotor disorder in humans known as hyperekplexia (OMIM 149400) with similar phenotypes in rodents characterized by an exaggerated startle reflex and hypertonia. Analogous to genetic defects in humans single point mutations, microdeletions, or insertions in the Glra1 gene but also in the Glrb gene underlie the pathology in mice. The mutations either localized in the α (spasmodic, oscillator, cincinnati, Nmf11) or the β (spastic) subunit of the glycine receptor (GlyR) are much less tolerated in mice than in humans, leaving the question for the existence of different regulatory elements of the pathomechanisms in humans and rodents. In addition to the spontaneous mutations, new insights into understanding of the regulatory pathways in hyperekplexia or glycine encephalopathy arose from the constantly increasing number of knock-out as well as knock-in mutants of GlyRs. Over the last five years, various efforts using in vivo whole cell recordings provided a detailed analysis of the kinetic parameters underlying glycinergic dysfunction. Presynaptic compensation as well as postsynaptic compensatory mechanisms in these mice by other GlyR subunits or GABA_A receptors, and the role of extra-synaptic GlyRs is still a matter of debate. A recent study on the mouse mutant oscillator displayed a novel aspect for compensation of functionality by complementation of receptor domains that fold independently. This review focuses on defects in glycinergic neurotransmission in mice discussed with the background of human hyperekplexia en route to strategies of compensation.

4-Chloropropofol enhances chloride currents in human hyperekplexic and artificial mutated glycine receptors

BACKGROUND

The mammalian neurological disorder hereditary hyperekplexia can be attributed to various mutations of strychnine sensitive glycine receptors. The clinical symptoms of "startle disease" predominantly occur in the newborn leading to convulsive hypertonia and an exaggerated startle response to unexpected mild stimuli. Amongst others, point mutations R271Q and R271L in the α1-subunit of strychnine sensitive glycine receptors show reduced glycine sensitivity and cause the clinical symptoms of hyperekplexia.Halogenation has been shown to be a crucial structural determinant for the potency of a phenolic compound to positively modulate glycine receptor function.The aim of this in vitro study was to characterize the effects of 4-chloropropofol (4-chloro-2,6-dimethylphenol) at four glycine receptor mutations.

METHODS

Glycine receptor subunits were expressed in HEK 293 cells and experiments were performed using the whole-cell patch-clamp technique.

RESULTS

4-chloropropofol exerted a positive allosteric modulatory effect in a low sub-nanomolar concentration range at the wild type receptor (EC50 value of 0.08 ± 0.02 nM) and in a micromolar concentration range at the mutations (1.3 ± 0.6 μM, 0.1 ± 0.2 μM, 6.0 ± 2.3 μM and 55 ± 28 μM for R271Q, L, K and S267I, respectively).

CONCLUSIONS

4-chloropropofol might be an effective compound for the activation of mutated glycine receptors in experimental models of startle disease.

Characterization of two mutations, M287L and Q266I, in the α1 glycine receptor subunit that modify sensitivity to alcohols

Glycine receptors (GlyRs) are inhibitory ligand-gated ion channels. Ethanol potentiates glycine activation of the GlyR, and putative binding sites for alcohol are located in the transmembrane (TM) domains between and within subunits. To alter alcohol sensitivity of GlyR, we introduced two mutations in the GlyR α1 subunit, M287L (TM3) and Q266I (TM2). After expression in Xenopus laevis oocytes, both mutants showed a reduction in glycine sensitivity and glycine-induced maximal currents. Activation by taurine, another endogenous agonist, was almost abolished in the M287L GlyR. The ethanol potentiation of glycine currents was reduced in the M287L GlyR and eliminated in Q266I. Physiological levels of zinc (100 nM) potentiate glycine responses in wild-type GlyR and also enhance the ethanol potentiation of glycine responses. Although zinc potentiation of glycine responses was unchanged in both mutants, zinc enhancement of ethanol potentiation of glycine responses was absent in M287L GlyRs. The Q266I mutation decreased conductance but increased mean open time (effects not seen in M287L). Two lines of knockin mice bearing these mutations were developed. Survival of homozygous knockin mice was impaired, probably as a consequence of impaired glycinergic transmission. Glycine showed a decreased capacity for displacing strychnine binding in heterozygous knockin mice. Electrophysiology in isolated neurons of brain stem showed decreased glycine-mediated currents and decreased ethanol potentiation in homozygous knockin mice. Molecular models of the wild-type and mutant GlyRs show a smaller water-filled cavity within the TM domains of the Q266I α1 subunit. The behavioral characterization of these knockin mice is presented in a companion article (J Pharmacol Exp Ther 340:317-329, 2012).

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.

Functional complementation of Glra1(spd-ot), a glycine receptor subunit mutant, by independently expressed C-terminal domains

The oscillator mouse (Glra1(spd-ot)) carries a 9 bp microdeletion plus a 2 bp microinsertion in the glycine receptor alpha1 subunit gene, resulting in the absence of functional alpha1 polypeptides from the CNS and lethality 3 weeks after birth. Depending on differential use of two splice acceptor sites in exon 9 of the Glra1 gene, the mutant allele encodes either a truncated alpha1 subunit (spd(ot)-trc) or a polypeptide with a C-terminal missense sequence (spd(ot)-elg). During recombinant expression, both splice variants fail to form ion channels. In complementation studies, a tail construct, encoding the deleted C-terminal sequence, was coexpressed with both mutants. Coexpression with spd(ot)-trc produced glycine-gated ion channels. Rescue efficiency was increased by inclusion of the wild-type motif RRKRRH. In cultured spinal cord neurons from oscillator homozygotes, viral infection with recombinant C-terminal tail constructs resulted in appearance of endogenous alpha1 antigen. The functional rescue of alpha1 mutants by the C-terminal tail polypeptides argues for a modular subunit architecture of members of the Cys-loop receptor family.