A frameshift mutation in the mouse alpha 1 glycine receptor gene (Glra1) results in progressive neurological symptoms and juvenile death

Milestone Review: Unlocking the Proteomics of Glycine Receptor Complexes

Glycine receptors (GlyRs) are typically known for mediating inhibitory synaptic transmission within the spinal cord and brainstem, but they also have key roles in embryonic brain development, learning/memory, inflammatory pain sensitization, and rhythmic breathing. GlyR dysfunction has been implicated in multiple neurological disease states, including startle disease (GlyR α1β) and neurodevelopmental disorders (NDDs) including autism spectrum disorder (ASD), intellectual disability (ID), developmental delay (DD) and epilepsy (GlyR α2). However, GlyRs do not operate in isolation but depend upon stable and transient protein-protein interactions (PPIs) that influence synaptic localization, homeostasis, signaling pathways, and receptor function. Despite the affinity purification of GlyRs using the antagonist strychnine over four decades ago, we still have much to learn about native GlyR stoichiometry and accessory proteins. In contrast to other neurotransmitter receptors, < 20 potential GlyR interactors have been identified to date. These include some well-known proteins that are vital to inhibitory synapse function, such as the postsynaptic scaffolding protein gephyrin and the RhoGEF collybistin. However, the majority of known interactors either bind to the GlyR α1 and β subunits, or the binding partner in the GlyR complex is unknown. Several potential GlyR interactors are not found at inhibitory synapses and/or have no clear functional role. Moreover, other GlyR interactors are secondary interactors that bind indirectly, for example, via gephyrin. In this review, we provide a critical evaluation of known GlyR interacting proteins and methodological limitations to date. We also provide a road map for the use of innovative and emerging interaction proteomic techniques that will unlock the GlyR interactome. With the emergence of disease-associated missense mutations in the α1, α2 and β subunit intracellular domains in startle disease and NDDs, understanding the identity and roles of GlyR accessory proteins is vital in understanding GlyR function and dysfunction in health and disease.

The mRNA expression profile of glycine receptor subunits alpha 1, alpha 2, alpha 4 and beta in female and male mice

Glycine receptors are ligand-gated chloride-selective channels that control excitability in the central nervous system (CNS). Herein, we have investigated the mRNA expression of the glycine receptor alpha 1 (Glra1), alpha 2 (Glra2), alpha 4 (Glra4) and the beta (Glrb) subunits, in adult female and male mice. Single-cell RNA sequencing data re-analysis of the Zeisel et al. (2018) dataset indicated widespread expression of Glra1, Glra2 and Glrb in the CNS, while only a few cells in the cortex, striatum, thalamus, midbrain and the spinal cord expressed Glra4. Highest occurrence of Glra1, Glra2 and Glrb were found in the brainstem. Moreover, Glra1 and Glrb were revealed to have the highest occurrences in the spinal cord of the investigated subunits. However, both Glra2 and Glrb had a more widespread expression in the CNS compared with Glra1 and Glra4. Bulk quantitative real-time-PCR (qRT-PCR) analysis revealed Glra1 expression in the hypothalamus, thalamus, brainstem and the spinal cord, and widespread, but low, Glra2 and Glrb expression in the CNS. Moreover, Glrb could be detected in a few visceral organs. Additionally, females and males were found to express Glra1, Glra2 and Glrb differently in certain brain areas such as the brainstem. Expression levels of Glra4 were too low to be detected using qRT-PCR. Lastly, RNAscope spatially validated the expression of Glra1, Glra2 and Glrb in the areas indicated by the single-cell and bulk analyses, and further revealed that Glra4 can be detected in the cortex, amygdala, hypothalamus, thalamus, brainstem, especially the cochlear nucleus, and in the spinal cord.

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.

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.

Alcohol Withdrawal and the Associated Mood Disorders-A Review

Recreational use of alcohol is a social norm in many communities worldwide. Alcohol use in moderation brings pleasure and may protect the cardiovascular system. However, excessive alcohol consumption or alcohol abuse are detrimental to one's health. Three million deaths due to excessive alcohol consumption were reported by the World Health Organization. Emerging evidence also revealed the danger of moderate consumption, which includes the increased risk to cancer. Alcohol abuse and periods of withdrawal have been linked to depression and anxiety. Here, we present the effects of alcohol consumption (acute and chronic) on important brain structures-the frontal lobe, the temporal lobe, the limbic system, and the cerebellum. Apart from this, we also present the link between alcohol abuse and withdrawal and mood disorders in this review, thus drawing a link to oxidative stress. In addition, we also discuss the positive impacts of some pharmacotherapies used. Due to the ever-rising demands of life, the cycle between alcohol abuse, withdrawal, and mood disorders may be a never-ending cycle of destruction. Hence, through this review, we hope that we can emphasise the importance and urgency of managing this issue with the appropriate approaches.

Positive Allosteric Modulators of Glycine Receptors and Their Potential Use in Pain Therapies

Glycine receptors are ligand-gated ion channels that mediate synaptic inhibition throughout the mammalian spinal cord, brainstem, and higher brain regions. They have recently emerged as promising targets for novel pain therapies due to their ability to produce antinociception by inhibiting nociceptive signals within the dorsal horn of the spinal cord. This has greatly enhanced the interest in developing positive allosteric modulators of glycine receptors. Several pharmaceutical companies and research facilities have attempted to identify new therapeutic leads by conducting large-scale screens of compound libraries, screening new derivatives from natural sources, or synthesizing novel compounds that mimic endogenous compounds with antinociceptive activity. Advances in structural techniques have also led to the publication of multiple high-resolution structures of the receptor, highlighting novel allosteric binding sites and providing additional information for previously identified binding sites. This has greatly enhanced our understanding of the functional properties of glycine receptors and expanded the structure activity relationships of novel pharmacophores. Despite this, glycine receptors are yet to be used as drug targets due to the difficulties in obtaining potent, selective modulators with favorable pharmacokinetic profiles that are devoid of side effects. This review presents a summary of the structural basis for how current compounds cause positive allosteric modulation of glycine receptors and discusses their therapeutic potential as analgesics. SIGNIFICANCE STATEMENT: Chronic pain is a major cause of disability, and in Western societies, this will only increase as the population ages. Despite the high level of prevalence and enormous socioeconomic burden incurred, treatment of chronic pain remains limited as it is often refractory to current analgesics, such as opioids. The National Institute for Drug Abuse has set finding effective, safe, nonaddictive strategies to manage chronic pain as their top priority. Positive allosteric modulators of glycine receptors may provide a therapeutic option.

Glycinergic Modulation of Pain in Behavioral Animal Models

Animal models of human pain conditions allow for detailed interrogation of known and hypothesized mechanisms of pain physiology in awake, behaving organisms. The importance of the glycinergic system for pain modulation is well known; however, manipulation of this system to treat and alleviate pain has not yet reached the sophistication required for the clinic. Here, we review the current literature on what animal behavioral studies have allowed us to elucidate about glycinergic pain modulation, and the progress toward clinical treatments so far. First, we outline the animal pain models that have been used, such as nerve injury models for neuropathic pain, chemogenic pain models for acute and inflammatory pain, and other models that mimic painful human pathologies such as diabetic neuropathy. We then discuss the genetic approaches to animal models that have identified the crucial glycinergic machinery involved in neuropathic and inflammatory pain. Specifically, two glycine receptor (GlyR) subtypes, GlyRα1(β) and GlyRα3(β), and the two glycine transporters (GlyT), GlyT1 and GlyT2. Finally, we review the different pharmacological approaches to manipulating the glycinergic system for pain management in animal models, such as partial vs. full agonism, reversibility, and multi-target approaches. We discuss the benefits and pitfalls of using animal models in drug development broadly, as well as the progress of glycinergic treatments from preclinical to clinical trials.

Nociception in the Glycine Receptor Deficient Mutant Mouse Spastic

Glycine receptors (GlyRs) are the primary mediators of fast inhibitory transmission in the mammalian spinal cord, where they modulate sensory and motor signaling. Mutations in GlyR genes as well as some other genes underlie the hereditary disorder hyperekplexia, characterized by episodic muscle stiffness and exaggerated startle responses. Here, we have investigated pain-related behavior and GlyR expression in the spinal cord of the GlyR deficient mutant mouse spastic (spa). In spastic mice, the GlyR number is reduced due to a β subunit gene (Glrb) mutation resulting in aberrant splicing of GlyRβ transcripts. Via direct physical interaction with the GlyR anchoring protein gephyrin, this subunit is crucially involved in the postsynaptic clustering of heteromeric GlyRs. We show that the mutation differentially affects aspects of the pain-related behavior of homozygous Glrb^spa/Glrb^spa mice. While response latencies to noxious heat were unchanged, chemically induced pain-related behavior revealed a reduction of the licking time and an increase in flinching in spastic homozygotes during both phases of the formalin test. Mechanically induced nocifensive behavior was reduced in spastic mice, although hind paw inflammation (by zymosan) resulted in allodynia comparable to wild-type mice. Immunohistochemical staining of the spinal cord revealed a massive reduction of dotted GlyRα subunit immunoreactivity in both ventral and dorsal horns, suggesting a reduction of clustered receptors at synaptic sites. Transcripts for all GlyRα subunit variants, however, were not reduced throughout the dorsal horn of spastic mice. These findings suggest that the loss of functional GlyRβ subunits and hence synaptically localized GlyRs compromises sensory processing differentially, depending on stimulus modality.

Identification of a stereotypic molecular arrangement of endogenous glycine receptors at spinal cord synapses

Precise quantitative information about the molecular architecture of synapses is essential to understanding the functional specificity and downstream signaling processes at specific populations of synapses. Glycine receptors (GlyRs) are the primary fast inhibitory neurotransmitter receptors in the spinal cord and brainstem. These inhibitory glycinergic networks crucially regulate motor and sensory processes. Thus far, the nanoscale organization of GlyRs underlying the different network specificities has not been defined. Here, we have quantitatively characterized the molecular arrangement and ultra-structure of glycinergic synapses in spinal cord tissue using quantitative super-resolution correlative light and electron microscopy. We show that endogenous GlyRs exhibit equal receptor-scaffold occupancy and constant packing densities of about 2000 GlyRs µm^-2 at synapses across the spinal cord and throughout adulthood, even though ventral horn synapses have twice the total copy numbers, larger postsynaptic domains, and more convoluted morphologies than dorsal horn synapses. We demonstrate that this stereotypic molecular arrangement is maintained at glycinergic synapses in the oscillator mouse model of the neuromotor disease hyperekplexia despite a decrease in synapse size, indicating that the molecular organization of GlyRs is preserved in this hypomorph. We thus conclude that the morphology and size of inhibitory postsynaptic specializations rather than differences in GlyR packing determine the postsynaptic strength of glycinergic neurotransmission in motor and sensory spinal cord networks.

Individual knock out of glycine receptor alpha subunits identifies a specific requirement of glra1 for motor function in zebrafish

Glycine receptors (GlyRs) are ligand-gated chloride channels mediating inhibitory neurotransmission in the brain stem and spinal cord. They function as pentamers composed of alpha and beta subunits for which 5 genes have been identified in human (GLRA1, GLRA2, GLRA3, GLRA4, GLRB). Several in vitro studies showed that the pentameric subtype composition as well as its stoichiometry influence the distribution and the molecular function of the receptor. Moreover, mutations in some of these genes are involved in different human conditions ranging from tinnitus to epilepsy and hyperekplexia, suggesting distinct functions of the different subunits. Although the beta subunit is essential for synaptic clustering of the receptor, the specific role of each alpha subtype is still puzzling in vivo. The zebrafish genome encodes for five glycine receptor alpha subunits (glra1, glra2, glra3, glra4a, glra4b) thus offering a model of choice to investigate the respective role of each subtype on general motor behaviour. After establishing a phylogeny of GlyR subunit evolution between human and zebrafish, we checked the temporal expression pattern of these transcripts during embryo development. Interestingly, we found that glra1 is the only maternally transmitted alpha subunit. We also showed that the expression of the different GlyR subunits starts at different time points during development. Lastly, in order to decipher the role of each alpha subunit on the general motor behaviour of the fish, we knocked out individually each alpha subunit by CRISPR/Cas9-targeted mutagenesis. Surprisingly, we found that knocking out any of the alpha2, 3, a4a or a4b subunit did not lead to any obvious developmental or motor phenotype. However, glra1-/- (hitch) embryos depicted a strong motor dysfunction from 3 days, making them incapable to swim and thus leading to their premature death. Our results infer a strong functional redundancy between alpha subunits and confirm the central role played by glra1 for proper inhibitory neurotransmission controlling locomotion. The genetic tools we developed here will be of general interest for further studies aiming at dissecting the role of GlyRs in glycinergic transmission in vivo and the hitch mutant (hic) is of specific relevance as a new model of hyperekplexia.

The postnatal development of ultrasonic vocalization-associated breathing is altered in glycine transporter 2-deficient mice

KEY POINTS

Newborn mice produce ultrasonic vocalization to communicate with their mother. The neuronal glycine transporter (GlyT2) is required for efficient loading of synaptic vesicles in glycinergic neurons. Mice lacking GlyT2 develop a phenotype that resembles human hyperekplexia and the mice die in the second postnatal week. In the present study, we show that GlyT2-knockout mice do not acquire adult ultrasonic vocalization-associated breathing patterns. Despite the strong impairment of glycinergic inhibition, they can produce sufficient expiratory airflow to produce ultrasonic vocalization. Because mouse ultrasonic vocalization is a valuable read-out in translational research, these data are highly relevant for a broad range of research fields.

ABSTRACT

Mouse models are instrumental with respect to determining the genetic basis and neural foundations of breathing regulation. To test the hypothesis that glycinergic synaptic inhibition is required for normal breathing and proper post-inspiratory activity, we analysed breathing and ultrasonic vocalization (USV) patterns in neonatal mice lacking the neuronal glycine transporter (GlyT2). GlyT2-knockout (KO) mice have a profound reduction of glycinergic synaptic currents already at birth, develop a severe motor phenotype and survive only until the second postnatal week. At this stage, GlyT2-KO mice are smaller, have a reduced respiratory rate and still display a neonatal breathing pattern with active expiration for the production of USV. By contrast, wild-type mice acquire different USV-associated breathing patterns that depend on post-inspiratory control of air flow. Nonetheless, USVs per se remain largely indistinguishable between both genotypes. We conclude that GlyT2-KO mice, despite the strong impairment of glycinergic inhibition, can produce sufficient expiratory airflow to produce ultrasonic vocalization.

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.

Presence of Inhibitory Glycinergic Transmission in Medium Spiny Neurons in the Nucleus Accumbens

It is believed that the rewarding actions of drugs are mediated by dysregulation of the mesolimbic dopaminergic system leading to increased levels of dopamine in the nucleus accumbens (nAc). It is widely recognized that GABAergic transmission is critical for neuronal inhibition within nAc. However, it is currently unknown if medium spiny neurons (MSNs) also receive inhibition by means of glycinergic synaptic inputs. We used a combination of proteomic and electrophysiology studies to characterize the presence of glycinergic input into MSNs from nAc demonstrating the presence of glycine transmission into nAc. In D1 MSNs, we found low frequency glycinergic miniature inhibitory postsynaptic currents (mIPSCs) which were blocked by 1 μM strychnine (STN), insensitive to low (10, 50 mM) and high (100 mM) ethanol (EtOH) concentrations, but sensitive to 30 μM propofol. Optogenetic experiments confirmed the existence of STN-sensitive glycinergic IPSCs and suggest a contribution of GABA and glycine neurotransmitters to the IPSCs in nAc. The study reveals the presence of glycinergic transmission in a non-spinal region and opens the possibility of a novel mechanism for the regulation of the reward pathway.

Functional Consequences of the Postnatal Switch From Neonatal to Mutant Adult Glycine Receptor α1 Subunits in the Shaky Mouse Model of Startle Disease

Mutations in GlyR α1 or β subunit genes in humans and rodents lead to severe startle disease characterized by rigidity, massive stiffness and excessive startle responses upon unexpected tactile or acoustic stimuli. The recently characterized startle disease mouse mutant shaky carries a missense mutation (Q177K) in the β8-β9 loop within the large extracellular N-terminal domain of the GlyR α1 subunit. This results in a disrupted hydrogen bond network around K177 and faster GlyR decay times. Symptoms in mice start at postnatal day 14 and increase until premature death of homozygous shaky mice around 4–6 weeks after birth. Here we investigate the in vivo functional effects of the Q177K mutation using behavioral analysis coupled to protein biochemistry and functional assays. Western blot analysis revealed GlyR α1 subunit expression in wild-type and shaky animals around postnatal day 7, a week before symptoms in mutant mice become obvious. Before 2 weeks of age, homozygous shaky mice appeared healthy and showed no changes in body weight. However, analysis of gait and hind-limb clasping revealed that motor coordination was already impaired. Motor coordination and the activity pattern at P28 improved significantly upon diazepam treatment, a pharmacotherapy used in human startle disease. To investigate whether functional deficits in glycinergic neurotransmission are present prior to phenotypic onset, we performed whole-cell recordings from hypoglossal motoneurons (HMs) in brain stem slices from wild-type and shaky mice at different postnatal stages. Shaky homozygotes showed a decline in mIPSC amplitude and frequency at P9-P13, progressing to significant reductions in mIPSC amplitude and decay time at P18-24 compared to wild-type littermates. Extrasynaptic GlyRs recorded by bath-application of glycine also revealed reduced current amplitudes in shaky mice compared to wild-type neurons, suggesting that presynaptic GlyR function is also impaired. Thus, a distinct, but behaviorally ineffective impairment of glycinergic synapses precedes the symptoms onset in shaky mice. These findings extend our current knowledge on startle disease in the shaky mouse model in that they demonstrate how the progression of GlyR dysfunction causes, with a delay of about 1 week, the appearance of disease symptoms.

Glycine receptors and glycine transporters: targets for novel analgesics?

Glycinergic neurotransmission has long been known for its role in spinal motor control. During the last two decades, additional functions have become increasingly recognized-among them is a critical contribution to spinal pain processing. Studies in rodent pain models provide proof-of-concept evidence that enhancing inhibitory glycinergic neurotransmission reduces chronic pain symptoms. Apparent strategies for pharmacological intervention include positive allosteric modulators of glycine receptors and modulators or inhibitors of the glial and neuronal glycine transporters GlyT1 and GlyT2. These prospects have led to drug discovery efforts in academia and in industry aiming at compounds that target glycinergic neurotransmission with high specificity. Available data show promising analgesic efficacy. Less is currently known about potential unwanted effects but the presence of glycinergic innervation in CNS areas outside the nociceptive system prompts for a careful evaluation not only of motor function, but also of potential respiratory impairment and addictive properties.

Disruption of a Structurally Important Extracellular Element in the Glycine Receptor Leads to Decreased Synaptic Integration and Signaling Resulting in Severe Startle Disease

Functional impairments or trafficking defects of inhibitory glycine receptors (GlyRs) have been linked to human hyperekplexia/startle disease and autism spectrum disorders. We found that a lack of synaptic integration of GlyRs, together with disrupted receptor function, is responsible for a lethal startle phenotype in a novel spontaneous mouse mutant shaky, caused by a missense mutation, Q177K, located in the extracellular β8-β9 loop of the GlyR α1 subunit. Recently, structural data provided evidence that the flexibility of the β8-β9 loop is crucial for conformational transitions during opening and closing of the ion channel and represents a novel allosteric binding site in Cys-loop receptors. We identified the underlying neuropathological mechanisms in male and female shaky mice through a combination of protein biochemistry, immunocytochemistry, and both in vivo and in vitro electrophysiology. Increased expression of the mutant GlyR α1Q177K subunit in vivo was not sufficient to compensate for a decrease in synaptic integration of α1Q177Kβ GlyRs. The remaining synaptic heteromeric α1Q177Kβ GlyRs had decreased current amplitudes with significantly faster decay times. This functional disruption reveals an important role for the GlyR α1 subunit β8-β9 loop in initiating rearrangements within the extracellular-transmembrane GlyR interface and that this structural element is vital for inhibitory GlyR function, signaling, and synaptic clustering.SIGNIFICANCE STATEMENT GlyR dysfunction underlies neuromotor deficits in startle disease and autism spectrum disorders. We describe an extracellular GlyR α1 subunit mutation (Q177K) in a novel mouse startle disease mutant shaky Structural data suggest that during signal transduction, large transitions of the β8-β9 loop occur in response to neurotransmitter binding. Disruption of the β8-β9 loop by the Q177K mutation results in a disruption of hydrogen bonds between Q177 and the ligand-binding residue R65. Functionally, the Q177K change resulted in decreased current amplitudes, altered desensitization decay time constants, and reduced GlyR clustering and synaptic strength. The GlyR β8-β9 loop is therefore an essential regulator of conformational rearrangements during ion channel opening and closing.

Synaptopathies: synaptic dysfunction in neurological disorders - A review from students to students

Synapses are essential components of neurons and allow information to travel coordinately throughout the nervous system to adjust behavior to environmental stimuli and to control body functions, memories, and emotions. Thus, optimal synaptic communication is required for proper brain physiology, and slight perturbations of synapse function can lead to brain disorders. In fact, increasing evidence has demonstrated the relevance of synapse dysfunction as a major determinant of many neurological diseases. This notion has led to the concept of synaptopathies as brain diseases with synapse defects as shared pathogenic features. In this review, which was initiated at the 13th International Society for Neurochemistry Advanced School, we discuss basic concepts of synapse structure and function, and provide a critical view of how aberrant synapse physiology may contribute to neurodevelopmental disorders (autism, Down syndrome, startle disease, and epilepsy) as well as neurodegenerative disorders (Alzheimer and Parkinson disease). We finally discuss the appropriateness and potential implications of gathering synapse diseases under a single term. Understanding common causes and intrinsic differences in disease-associated synaptic dysfunction could offer novel clues toward synapse-based therapeutic intervention for neurological and neuropsychiatric disorders. In this Review, which was initiated at the 13th International Society for Neurochemistry (ISN) Advanced School, we discuss basic concepts of synapse structure and function, and provide a critical view of how aberrant synapse physiology may contribute to neurodevelopmental (autism, Down syndrome, startle disease, and epilepsy) as well as neurodegenerative disorders (Alzheimer's and Parkinson's diseases), gathered together under the term of synaptopathies. Read the Editorial Highlight for this article on page 783.

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

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.

Ethanol effects on glycinergic transmission: From molecular pharmacology to behavior responses

It is well accepted that ethanol is able to produce major health and economic problems associated to its abuse. Because of its intoxicating and addictive properties, it is necessary to analyze its effect in the central nervous system. However, we are only now learning about the mechanisms controlling the modification of important membrane proteins such as ligand-activated ion channels by ethanol. Furthermore, only recently are these effects being correlated to behavioral changes. Current studies show that the glycine receptor (GlyR) is a susceptible target for low concentrations of ethanol (5-40mM). GlyRs are relevant for the effects of ethanol because they are found in the spinal cord and brain stem where they primarily express the α1 subunit. More recently, the presence of GlyRs was described in higher regions, such as the hippocampus and nucleus accumbens, with a prevalence of α2/α3 subunits. Here, we review data on the following aspects of ethanol effects on GlyRs: (1) direct interaction of ethanol with amino acids in the extracellular or transmembrane domains, and indirect mechanisms through the activation of signal transduction pathways; (2) analysis of α2 and α3 subunits having different sensitivities to ethanol which allows the identification of structural requirements for ethanol modulation present in the intracellular domain and C-terminal region; (3) Genetically modified knock-in mice for α1 GlyRs that have an impaired interaction with G protein and demonstrate reduced ethanol sensitivity without changes in glycinergic transmission; and (4) GlyRs as potential therapeutic targets.

Single expressed glycine receptor domains reconstitute functional ion channels without subunit-specific desensitization behavior

Cys loop receptors are pentameric arrangements of independent subunits that assemble into functional ion channels. Each subunit shows a domain architecture. Functional ion channels can be reconstituted even from independent, nonfunctional subunit domains, as shown previously for GlyRα1 receptors. Here, we demonstrate that this reconstitution is not restricted to α1 but can be transferred to other members of the Cys loop receptor family. A nonfunctional GlyR subunit, truncated at the intracellular TM3-4 loop by a premature stop codon, can be complemented by co-expression of the missing tail portion of the receptor. Compared with α1 subunits, rescue by domain complementation was less efficient when GlyRα3 or the GABAA/C subunit ρ1 was used. If truncation disrupted an alternative splicing cassette within the intracellular TM3-4 loop of α3 subunits, which also regulates receptor desensitization, functional rescue was not possible. When α3 receptors were restored by complementation using domains with and without the spliced insert, no difference in desensitization was found. In contrast, desensitization properties could even be transferred between α1/α3 receptor chimeras harboring or lacking the α3 splice cassette proving that functional rescue depends on the integrity of the alternative splicing cassette in α3. Thus, an intact α3 splicing cassette in the TM3-4 loop environment is indispensable for functional rescue, and the quality of receptor restoration can be assessed from desensitization properties.

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.

Intrinsic and synaptic homeostatic plasticity in motoneurons from mice with glycine receptor mutations

Inhibitory synaptic inputs to hypoglossal motoneurons (HMs) are important for modulating excitability in brainstem circuits. Here we ask whether reduced inhibition, as occurs in three murine mutants with distinct naturally occurring mutations in the glycine receptor (GlyR), leads to intrinsic and/or synaptic homeostatic plasticity. Whole cell recordings were obtained from HMs in transverse brainstem slices from wild-type (wt), spasmodic (spd), spastic (spa), and oscillator (ot) mice (C57Bl/6, approximately postnatal day 21). Passive and action potential (AP) properties in spd and ot HMs were similar to wt. In contrast, spa HMs had lower input resistances, more depolarized resting membrane potentials, higher rheobase currents, smaller AP amplitudes, and slower afterhyperpolarization current decay times. The excitability of HMs, assessed by "gain" in injected current/firing-frequency plots, was similar in all strains whereas the incidence of rebound spiking was increased in spd. The difference between recruitment and derecruitment current (i.e., ΔI) for AP discharge during ramp current injection was more negative in spa and ot. GABAA miniature inhibitory postsynaptic current (mIPSC) amplitude was increased in spa and ot but not spd, suggesting diminished glycinergic drive leads to compensatory adjustments in the other major fast inhibitory synaptic transmitter system in these mutants. Overall, our data suggest long-term reduction in glycinergic drive to HMs results in changes in intrinsic and synaptic properties that are consistent with homeostatic plasticity in spa and ot but not in spd. We propose such plasticity is an attempt to stabilize HM output, which succeeds in spa but fails in ot.

The GLRA1 missense mutation W170S associates lack of Zn2+ potentiation with human hyperekplexia

Hyperekplexia is a neurological disorder associated primarily with mutations in the α1 subunit of glycine receptors (GlyRs) that lead to dysfunction of glycinergic inhibitory transmission. To date, most of the identified mutations result in disruption of surface expression or altered channel properties of α1-containing GlyRs. Little evidence has emerged to support an involvement of allosteric GlyR modulation in human hyperekplexia. Here, we report that recombinant human GlyRs containing α1 or α1β subunits with a missense mutation in the α1 subunit (W170S), previously identified from familial hyperekplexia, caused remarkably reduced potentiation and enhanced inhibition by Zn(2+). Interestingly, mutant α1(W170S)β GlyRs displayed no significant changes in potency or maximum response to glycine, taurine, or β-alanine. By temporally separating the potentiating and the inhibitory effects of Zn(2+), we found that the enhancement of Zn(2+) inhibition resulted from a loss of Zn(2+)-mediated potentiation. The W170S mutation on the background of H107N, which was previously reported to selectively disrupt Zn(2+) inhibition, showed remarkable attenuation of Zn(2+)-mediated potentiation and thus indicated that W170 is an important residue for the Zn(2+)-mediated GlyR potentiation. Moreover, overexpressing the α1(W170S) subunit in cultured rat neurons confirmed the results from heterologous expression. Together, our results reveal a new zinc potentiation site on α1 GlyRs and a strong link between Zn(2+) modulation and human disease.

Postnatal development of glycine receptor subunits α1, α2, α3, and β immunoreactivity in multiple brain stem respiratory-related nuclear groups of the rat

The respiratory system is immature at birth and significant development occurs postnatally. A critical period of respiratory development occurs in rats around postnatal days 12-13, when enhanced inhibition dominates over suppressed excitation. The mechanisms underlying the heightened inhibition are not fully understood. The present study tested our hypothesis that the inhibition is marked by a switch in glycine receptor subunits from neonatal to adult form around the critical period. An in-depth immunohistochemical and single neuron optical densitometric study was undertaken on four respiratory-related nuclear groups (the pre-Bötzinger complex, nucleus ambiguus, hypoglossal nucleus, and ventrolateral subnucleus of solitary tract nucleus), and a non-respiratory cuneate nucleus in P2-21 rats. Our data revealed that in the respiratory-related nuclear groups: (1) the expressions of GlyRα2 and GlyRα3 were relatively high at P2, but declined after 1-1½ weeks to their lowest levels at P21; (2) the expression of GlyRα1 increased with age and reached significance at P12; and (3) the expression of GlyRβ rose from P2 to P12 followed by a slight decline until P21. No distinct increase in GlyRα1 at P12 was noted in the cuneate nucleus. Thus, there is a switch in dominance of expression from neonatal GlyRα2/α3 to the adult GlyRα1 and a heightened expression of GlyRα1 around the critical period in all respiratory-related nuclear groups, thereby supporting enhanced inhibition at that time. The rise in the expression of GlyRβ around P12 indicates that it plays an important role in forming the mature heteropentameric glycine receptors in these brain stem nuclear groups.

Distinct phenotypes in zebrafish models of human startle disease

Startle disease is an inherited neurological disorder that causes affected individuals to suffer noise- or touch-induced non-epileptic seizures, excessive muscle stiffness and neonatal apnea episodes. Mutations known to cause startle disease have been identified in glycine receptor subunit (GLRA1 and GLRB) and glycine transporter (SLC6A5) genes, which serve essential functions at glycinergic synapses. Despite the significant successes in identifying startle disease mutations, many idiopathic cases remain unresolved. Exome sequencing in these individuals will identify new candidate genes. To validate these candidate disease genes, zebrafish is an ideal choice due to rapid knockdown strategies, accessible embryonic stages, and stereotyped behaviors. The only existing zebrafish model of startle disease, bandoneon (beo), harbors point mutations in glrbb (one of two zebrafish orthologs of human GLRB) that cause compromised glycinergic transmission and touch-induced bilateral muscle contractions. In order to further develop zebrafish as a model for startle disease, we sought to identify common phenotypic outcomes of knocking down zebrafish orthologs of two known startle disease genes, GLRA1 and GLRB, using splice site-targeted morpholinos. Although both morphants were expected to result in phenotypes similar to the zebrafish beo mutant, our direct comparison demonstrated that while both glra1 and glrbb morphants exhibited embryonic spasticity, only glrbb morphants exhibited bilateral contractions characteristic of beo mutants. Likewise, zebrafish over-expressing a dominant startle disease mutation (GlyR α1(R271Q)) exhibited spasticity but not bilateral contractions. Since GlyR βb can interact with GlyR α subunits 2-4 in addition to GlyR α1, loss of the GlyR βb subunit may produce more severe phenotypes by affecting multiple GlyR subtypes. Indeed, immunohistochemistry of glra1 morphants suggests that in zebrafish, alternate GlyR α subunits can compensate for the loss of the GlyR α1 subunit. To address the potential for interplay among GlyR subunits during development, we quantified the expression time-course for genes known to be critical to glycinergic synapse function. We found that GlyR α2, α3 and α4a are expressed in the correct temporal pattern and could compensate for the loss of the GlyR α1 subunit. Based on our findings, future studies that aim to model candidate startle disease genes in zebrafish should include measures of spasticity and synaptic development.

Novel missense mutations in the glycine receptor β subunit gene (GLRB) in startle disease

Startle disease is a rare, potentially fatal neuromotor disorder characterized by exaggerated startle reflexes and hypertonia in response to sudden unexpected auditory, visual or tactile stimuli. Mutations in the GlyR α(1) subunit gene (GLRA1) are the major cause of this disorder, since remarkably few individuals with mutations in the GlyR β subunit gene (GLRB) have been found to date. Systematic DNA sequencing of GLRB in individuals with hyperekplexia revealed new missense mutations in GLRB, resulting in M177R, L285R and W310C substitutions. The recessive mutation M177R results in the insertion of a positively-charged residue into a hydrophobic pocket in the extracellular domain, resulting in an increased EC(50) and decreased maximal responses of α(1)β GlyRs. The de novo mutation L285R results in the insertion of a positively-charged side chain into the pore-lining 9' position. Mutations at this site are known to destabilize the channel closed state and produce spontaneously active channels. Consistent with this, we identified a leak conductance associated with spontaneous GlyR activity in cells expressing α(1)β(L285R) GlyRs. Peak currents were also reduced for α(1)β(L285R) GlyRs although glycine sensitivity was normal. W310C was predicted to interfere with hydrophobic side-chain stacking between M1, M2 and M3. We found that W310C had no effect on glycine sensitivity, but reduced maximal currents in α(1)β GlyRs in both homozygous (α(1)β(W310C)) and heterozygous (α(1)ββ(W310C)) stoichiometries. Since mild startle symptoms were reported in W310C carriers, this may represent an example of incomplete dominance in startle disease, providing a potential genetic explanation for the 'minor' form of hyperekplexia.

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.

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.

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).

Allosteric modulation of glycine receptors

Inhibitory (or strychnine sensitive) glycine receptors (GlyRs) are anion-selective transmitter-gated ion channels of the cys-loop superfamily, which includes among others also the inhibitory γ-aminobutyric acid receptors (GABA(A) receptors). While GABA mediates fast inhibitory neurotransmission throughout the CNS, the action of glycine as a fast inhibitory neurotransmitter is more restricted. This probably explains why GABA(A) receptors constitute a group of extremely successful drug targets in the treatment of a wide variety of CNS diseases, including anxiety, sleep disorders and epilepsy, while drugs specifically targeting GlyRs are virtually lacking. However, the spatially more restricted distribution of glycinergic inhibition may be advantageous in situations when a more localized enhancement of inhibition is sought. Inhibitory GlyRs are particularly relevant for the control of excitability in the mammalian spinal cord, brain stem and a few selected brain areas, such as the cerebellum and the retina. At these sites, GlyRs regulate important physiological functions, including respiratory rhythms, motor control, muscle tone and sensory as well as pain processing. In the hippocampus, RNA-edited high affinity extrasynaptic GlyRs may contribute to the pathology of temporal lobe epilepsy. Although specific modulators have not yet been identified, GlyRs still possess sites for allosteric modulation by a number of structurally diverse molecules, including alcohols, neurosteroids, cannabinoids, tropeines, general anaesthetics, certain neurotransmitters and cations. This review summarizes the present knowledge about this modulation and the molecular bases of the interactions involved.

The biological role of the glycinergic synapse in early zebrafish motility

Glycine mediates fast inhibitory neurotransmission in the spinal cord, brainstem and retina. Loss of synaptic glycinergic transmission in vertebrates leads to a severe locomotion defect characterized by an exaggerated startle response accompanied by transient muscle rigidity in response to sudden acoustic or tactile stimuli. Several molecular components of the glycinergic synapse have been characterized as an outcome of genetic and physiological analyses of synaptogenesis in mammals. Recently, the glycinergic synapse has been studied using a forward genetic approach in zebrafish. This review aims to discuss molecular components of the glycinergic synapse, such as glycine receptor subunits, gephyrin, gephyrin-binding proteins and glycine transporters, as well as recent studies relevant to the genetic analysis of the glycinergic synapse in zebrafish.

Duplicated gephyrin genes showing distinct tissue distribution and alternative splicing patterns mediate molybdenum cofactor biosynthesis, glycine receptor clustering, and escape behavior in zebrafish

Gephyrin mediates the postsynaptic clustering of glycine receptors (GlyRs) and GABA(A) receptors at inhibitory synapses and molybdenum-dependent enzyme (molybdoenzyme) activity in non-neuronal tissues. Gephyrin knock-out mice show a phenotype resembling both defective glycinergic transmission and molybdenum cofactor (Moco) deficiency and die within 1 day of birth due to starvation and dyspnea resulting from deficits in motor and respiratory networks, respectively. To address whether gephyrin function is conserved among vertebrates and whether gephyrin deficiency affects molybdoenzyme activity and motor development, we cloned and characterized zebrafish gephyrin genes. We report here that zebrafish have two gephyrin genes, gphna and gphnb. The former is expressed in all tissues and has both C3 and C4 cassette exons, and the latter is expressed predominantly in the brain and spinal cord and harbors only C4 cassette exons. We confirmed that all of the gphna and gphnb splicing isoforms have Moco synthetic activity. Antisense morpholino knockdown of either gphna or gphnb alone did not disturb synaptic clusters of GlyRs in the spinal cord and did not affect touch-evoked escape behaviors. However, on knockdown of both gphna and gphnb, embryos showed impairments in GlyR clustering in the spinal cord and, as a consequence, demonstrated touch-evoked startle response behavior by contracting antagonistic muscles simultaneously, instead of displaying early coiling and late swimming behaviors, which are executed by side-to-side muscle contractions. These data indicate that duplicated gephyrin genes mediate Moco biosynthesis and control postsynaptic clustering of GlyRs, thereby mediating key escape behaviors in zebrafish.

Pathophysiological mechanisms of dominant and recessive GLRA1 mutations in hyperekplexia

Hyperekplexia is a rare, but potentially fatal, neuromotor disorder characterized by exaggerated startle reflexes and hypertonia in response to sudden, unexpected auditory or tactile stimuli. This disorder is primarily caused by inherited mutations in the genes encoding the glycine receptor (GlyR) alpha1 subunit (GLRA1) and the presynaptic glycine transporter GlyT2 (SLC6A5). In this study, systematic DNA sequencing of GLRA1 in 88 new unrelated human hyperekplexia patients revealed 19 sequence variants in 30 index cases, of which 21 cases were inherited in recessive or compound heterozygote modes. This indicates that recessive hyperekplexia is far more prevalent than previous estimates. From the 19 GLRA1 sequence variants, we have investigated the functional effects of 11 novel and 2 recurrent mutations. The expression levels and functional properties of these hyperekplexia mutants were analyzed using a high-content imaging system and patch-clamp electrophysiology. When expressed in HEK293 cells, either as homomeric alpha1 or heteromeric alpha1beta GlyRs, subcellular localization defects were the major mechanism underlying recessive mutations. However, mutants without trafficking defects typically showed alterations in the glycine sensitivity suggestive of disrupted receptor function. This study also reports the first hyperekplexia mutation associated with a GlyR leak conductance, suggesting tonic channel opening as a new mechanism in neuronal ligand-gated ion channels.

Attenuated glycine receptor function reduces excitability of mouse medial vestibular nucleus neurons

Spontaneous activity in medial vestibular nucleus (MVN) neurons is modulated by synaptic inputs. These inputs are crucial for maintaining gaze and posture and contribute to vestibular compensation after lesions of peripheral vestibular organs. We investigated how chronically attenuated glycinergic input affects excitability of MVN neurons. To this end we used three mouse strains (spastic, spasmodic, and oscillator), with well-characterized naturally occurring mutations in the inhibitory glycine receptor (GlyR). First, using whole-cell patch-clamp recordings, we demonstrated that the amplitude of the response to rapidly applied glycine was dramatically reduced by 25 to 90% in MVN neurons from mutant mice. We next determined how reduced GlyR function affected MVN neuron output. Neurons were classified using two schemas: (1) the shape of their action potential afterhyperpolarization (AHP); and (2) responses to hyperpolarizing current injection. In the first schema, neurons were classified as types A, B and C. The prevalence of type C neurons in the mutant strains was significantly increased. In the second schema, the proportion of neurons lacking post inhibitory rebound firing (PRF-deficient) was increased. In both schemas an increase in AHP amplitude was a common feature of the augmented neuron group (type C, PRF-deficient) in the mutant strains. We suggest increased AHP amplitude reduces overall excitability in the MVN and thus maintains network function in an environment of reduced glycinergic input.

The glycinergic system in human startle disease: a genetic screening approach

Human startle disease, also known as hyperekplexia (OMIM 149400), is a paroxysmal neurological disorder caused by defects in glycinergic neurotransmission. Hyperekplexia is characterised by an exaggerated startle reflex in response to tactile or acoustic stimuli which first presents as neonatal hypertonia, followed in some with episodes of life-threatening infantile apnoea. Genetic screening studies have demonstrated that hyperekplexia is genetically heterogeneous with several missense and nonsense mutations in the postsynaptic glycine receptor (GlyR) alpha1 subunit gene (GLRA1) as the primary cause. More recently, missense, nonsense and frameshift mutations have also been identified in the glycine transporter GlyT2 gene, SLC6A5, demonstrating a presynaptic component to this disease. Further mutations, albeit rare, have been identified in the genes encoding the GlyR beta subunit (GLRB), collybistin (ARHGEF9) and gephyrin (GPHN) - all of which are postsynaptic proteins involved in orchestrating glycinergic neurotransmission. In this review, we describe the clinical ascertainment aspects, phenotypic considerations and the downstream molecular genetic tools utilised to analyse both presynaptic and postsynaptic components of this heterogeneous human neurological disorder. Moreover, we will describe how the ancient startle response is the preserve of glycinergic neurotransmission and how animal models and human hyperekplexia patients have provided synergistic evidence that implicates this inhibitory system in the control of startle reflexes.

Defective glycinergic synaptic transmission in zebrafish motility mutants

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem. Recently, in vivo analysis of glycinergic synaptic transmission has been pursued in zebrafish using molecular genetics. An ENU mutagenesis screen identified two behavioral mutants that are defective in glycinergic synaptic transmission. Zebrafish bandoneon (beo) mutants have a defect in glrbb, one of the duplicated glycine receptor (GlyR) beta subunit genes. These mutants exhibit a loss of glycinergic synaptic transmission due to a lack of synaptic aggregation of GlyRs. Due to the consequent loss of reciprocal inhibition of motor circuits between the two sides of the spinal cord, motor neurons activate simultaneously on both sides resulting in bilateral contraction of axial muscles of beo mutants, eliciting the so-called 'accordion' phenotype. Similar defects in GlyR subunit genes have been observed in several mammals and are the basis for human hyperekplexia/startle disease. By contrast, zebrafish shocked (sho) mutants have a defect in slc6a9, encoding GlyT1, a glycine transporter that is expressed by astroglial cells surrounding the glycinergic synapse in the hindbrain and spinal cord. GlyT1 mediates rapid uptake of glycine from the synaptic cleft, terminating synaptic transmission. In zebrafish sho mutants, there appears to be elevated extracellular glycine resulting in persistent inhibition of postsynaptic neurons and subsequent reduced motility, causing the 'twitch-once' phenotype. We review current knowledge regarding zebrafish 'accordion' and 'twitch-once' mutants, including beo and sho, and report the identification of a new alpha2 subunit that revises the phylogeny of zebrafish GlyRs.

Glycinergic transmission in the Mammalian retina

Glycine and γ-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the retina. Approximately half of the amacrine cells release glycine at their synapses with bipolar, other amacrine, and ganglion cells. Glycinergic amacrine cells are small-field amacrine cells with vertically oriented dendrites and comprise more than 10 different morphological types. The retinal distributions of glycine receptor (GlyR) α1, α2, α3 and α4 subtypes have been mapped with subunit-specific antibodies. GlyRs were clustered at postsynaptic hot spots which showed selective distributions for the different subunits. As a rule, only one α subunit was expressed at a given postsynaptic site. The kinetic properties of GlyRs were measured by recording spontaneous inhibitory postsynaptic currents (sIPSCs) from identified retinal neurons in wild-type, Glra1^spd-ot, Glra2 and Glra3 knockout mice. From observed differences of sIPSCs in wild-type and mutant mice, the cell-type specific subunit composition of GlyRs could be defined. OFF-cone bipolar cells and A-type ganglion cells receive prominent glycinergic input with fast kinetics that is mainly mediated by α1β GlyRs (decay time constant τ ∼ 5 ms). By contrast, AII amacrine cells express α3β GlyRs with medium fast kinetics (τ ∼ 11 ms). Narrow-field (NF) and wide-field amacrine cells contain predominantly α2β GlyRs with slow kinetics (τ ∼ 27 ms). Lastly, ON-starburst, narrow-field and wide-field amacrine cells in Glra2 knockout mice express α4β GlyRs with very slow kinetics (τ ∼ 70 ms).

Glycinergic input of widefield, displaced amacrine cells of the mouse retina

Glycine receptors (GlyRs) of displaced amacrine cells of the mouse retina were analysed using whole cell recordings and immunocytochemical staining with subunit-specific antibodies. During the recordings the cells were filled with a fluorescent tracer and 11 different morphological types could be identified. The studies were performed in wild-type mice and in mutant mice deficient in the GlyRalpha1 (Glra1(spd-ot), 'oscillator' mouse), the GlyRalpha2 (Glra2(-/-)) and the GlyRalpha3 subunit (Glra3(-/-)). Based on their responses to the application of exogenous glycine in the retinas of wild-type and mutant mice, the cells were grouped into three major classes: group I cells (comprising the morphological types MA-S5, MA-S1, MA-S1/S5, A17, PA-S1, PA-S5 and WA-S1), group II cells (comprising the morphological types PA-S4, WA-S3 and WA-multi) and ON-starburst cells. For further analysis, spontaneous inhibitory postsynaptic currents (sIPSCs) were measured both in wild-type and mutant mouse retinas. Glycinergic sIPSCs and glycine induced currents of group I cells remained unaltered across wild-type and the three mutant mice (mean decay time constant of sIPSCs, tau approximately 25 ms). Group II cells showed glycinergic sIPSCs and glycine induced currents in wild-type, Glra1(spd-ot) and Glra3(-/-) mice (tau approximately 25 ms); however, glycinergic currents were absent in group II cells of Glra2(-/-) mice. Glycine induced currents and sIPSCs recorded from ON-starburst amacrine cells did not differ significantly between wild-type and the mutant mouse retinas (tau approximately 50-70 ms). We propose that GlyRs of group II cells are dominated by the alpha2 subunit; GlyRs of ON-starburst amacrine cells appear to be dominated by the alpha4 subunit.

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.

The genetics of hyperekplexia: more than startle!

Hyperekplexia is characterised by neonatal hypertonia and an exaggerated startle reflex in response to acoustic or tactile stimuli. Genetic analysis of this disorder has revealed mutations in genes for several postsynaptic proteins involved in glycinergic neurotransmission, including the glycine receptor (GlyR) alpha1 and beta subunits, gephyrin and collybistin. However, new research suggests that mutations in the gene encoding the presynaptic glycine transporter GlyT2 are a second major cause of human hyperekplexia, as well as congenital muscular dystonia type 2 (CMD2) in cattle. These findings raise the intriguing possibility that both presynaptic and postsynaptic causes of disease might also exist in related disorders, such as idiopathic generalised epilepsies, where mutations in inhibitory GABA(A) receptor subunit genes have already been identified.

Native glycine receptor subtypes and their physiological roles

The glycine receptor chloride channel (GlyR), a member of the pentameric Cys-loop ion channel receptor family, mediates inhibitory neurotransmission in the spinal cord, brainstem and retina. They are also found presynaptically, where they modulate neurotransmitter release. Functional GlyRs are formed from a total of five subunits (alpha1-alpha4, beta). Although alpha subunits efficiently form homomeric GlyRs in recombinant expression systems, homomeric alpha1, alpha3 and alpha4 GlyRs are weakly expressed in adult neurons. In contrast, alpha2 homomeric GlyRs are abundantly expressed in embryonic neurons, although their numbers decline sharply by adulthood. Numerous lines of biochemical, biophysical, pharmacological and genetic evidence suggest the majority of glycinergic neurotransmission in adults is mediated by heteromeric alpha1beta GlyRs. Immunocytochemical co-localisation experiments suggest the presence of alpha2beta, alpha3beta and alpha4beta GlyRs at synapses in the adult mouse retina. Immunocytochemical and electrophysiological evidence also implicates alpha3beta GlyRs as important mediators of glycinergic inhibitory neurotransmission in nociceptive sensory neuronal circuits in peripheral laminae of the spinal cord dorsal horn. It is yet to be determined why multiple GlyR synaptic subtypes are differentially distributed in these and possibly other locations. The development of pharmacological agents that can discriminate strongly between different beta subunit-containing GlyR isoforms will help to address this issue, and thereby provide important insights into a variety of central nervous system functions including retinal signal processing and spinal pain mechanisms. Finally, agents that selectively potentiate different GlyR isoforms may be useful as therapeutic lead compounds for peripheral inflammatory pain and movement disorders such as spasticity.

A critical role for glycine transporters in hyperexcitability disorders

Defects in mammalian glycinergic neurotransmission result in a complex motor disorder characterized by neonatal hypertonia and an exaggerated startle reflex, known as hyperekplexia (OMIM 149400). This affects newborn children and is characterized by noise or touch-induced seizures that result in muscle stiffness and breath-holding episodes. Although rare, this disorder can have serious consequences, including brain damage and/or sudden infant death. The primary cause of hyperekplexia is missense and non-sense mutations in the glycine receptor (GlyR) alpha1 subunit gene (GLRA1) on chromosome 5q33.1, although we have also discovered rare mutations in the genes encoding the GlyR beta subunit (GLRB) and the GlyR clustering proteins gephyrin (GPNH) and collybistin (ARHGEF9). Recent studies of the Na(+)/Cl(-)-dependent glycine transporters GlyT1 and GlyT2 using mouse knockout models and human genetics have revealed that mutations in GlyT2 are a second major cause of hyperekplexia, while the phenotype of the GlyT1 knockout mouse resembles a devastating neurological disorder known as glycine encephalopathy (OMIM 605899). These findings highlight the importance of these transporters in regulating the levels of synaptic glycine.

A substance in broad beans (Vicia faba) is protective against experimentally induced convulsions in mice

Vicia faba (VF, broad beans) constitutes a major food item for the River Nile populations. Contrary to tropical Africa, the prevalence rate of epilepsy (0.9-1 per 1000) among schoolchildren of Khartoum Province, Sudan, is lower compared with the rates in Europe and North America. To explore whether broad beans contain any anticonvulsant that can explain this observation, Balb/c mice were either treated with VF extract (0.01 mL/g) or kept as a control. Various doses of strychnine and picrotoxin were used to explore the effect of VF extract on strychnine-sensitive glycine receptors and GABA(A) receptors. Diazepam (DIZ) was used as anticonvulsant. Thin-layer chromatography was run for the extract against phenobarbital, DIZ, and/or glycine. VF extract demonstrated a clear protective effect against strychnine-induced convulsions and death in Balb/c mice. Diazepam (20 mg/kg, intraperitoneally), administered 20 minutes prior to strychnine (0.112 mg/kg, intraperitoneally), increased the survival rate to 66.7% and, when given with VF extract (0.01 mL/g), to 100%. Various doses of DIZ protected against picrotoxin-induced convulsions (40 mg/kg, intraperitoneally) and deaths. Pretreatment of mice with VF extract was not protective. On chromatography, VF extract separated in a manner similar to the glycine spot, and revealed one peak coinciding with, but not identical to that of glycine. The extract of broad beans (VF) protects against convulsions probably through the inhibitory glycine receptors, and may contain a substance that is intimately related to glycine. Further research is needed to substantiate this by directly assessing the binding of the VF extract to the glycine receptors or a change in receptor physiology.

Glycine receptors of A-type ganglion cells of the mouse retina

A-type ganglion cells of the mouse retina represent the visual channel that transfers temporal changes of the outside world very fast and with high fidelity. In this study we combined anatomical and physiological methods in order to study the glycinergic, inhibitory input of A-type ganglion cells. Immunocytochemical studies were performed in a transgenic mouse line whose ganglion cells express green fluorescent protein (GFP). The cells were double labeled for GFP and the four alpha subunits of the glycine receptor (GlyR). It was found that most of the glycinergic input of A-type cells is through fast, alpha1-expressing synapses. Whole-cell currents were recorded from A-type ganglion cells in retinal whole mounts. The response to exogenous application of glycine and spontaneous inhibitory postsynaptic currents (sIPSCs) were measured. By comparing glycinergic currents recorded in wildtype mice and in mice with specific deletions of GlyRalpha subunits (Glra1spd-ot, Glra2-/-, Glra3-/-), the subunit composition of GlyRs of A-type ganglion cells could be further defined. Glycinergic sIPSCs of A-type ganglion cells have fast kinetics (decay time constant tau = 3.9 +/- 2.5 ms, mean +/- SD). Glycinergic sIPSCs recorded in Glra2-/- and Glra3-/- mice did not differ from those of wildtype mice. However, the number of glycinergic sIPSCs was significantly reduced in Glra1spd-ot mice and the remaining sIPSCs had slower kinetics than in wildtype mice. The results show that A-type ganglion cells receive preferentially kinetically fast glycinergic inputs, mediated by GlyRs composed of alpha1 and beta subunits.

Diversity of glycine receptors in the mouse retina: localization of the alpha4 subunit

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the retina. Approximately half of the amacrine cells release glycine at their synapses with bipolar, other amacrine, and ganglion cells. Whereas the retinal distributions of glycine receptor (GlyR) subunits alpha1, alpha2, and alpha3 have been mapped, the role of the alpha4 subunit in retinal circuitry remains unclear. A rabbit polyclonal antiserum was raised against a peptide that comprises the C-terminal 14 amino acids of the mouse GlyR alpha4 subunit. Using immunocytochemistry, we localized the alpha4 subunit in the inner plexiform layer (IPL) in brightly fluorescent puncta, which represent postsynaptically clustered GlyRs. This was shown by double-labeling sections for GlyR alpha4 and synaptic markers (bassoon, gephyrin). Double-labeling sections for GlyR alpha4 and the other GlyR alpha subunits shows that they are mostly clustered at different synapses; however, approximately 30% of the alpha4-containing synapses also express the alpha2 subunit. We also studied the pre- and postsynaptic partners at GlyR alpha4-containing synapses and found that displaced (ON-) cholinergic amacrine cells prominently expressed the alpha4 subunit. The density of GlyR alpha4-expressing synapses in wildtype, Glra1(ot/ot), and Glra3(-/-) mouse retinas did not differ significantly. Thus, there is no apparent compensation of the loss of alpha1 or alpha3 subunits by an upregulation of alpha4 subunit gene expression; however, the alpha2 subunit is moderately upregulated.

De novo exon duplication in a new allele of mouse Glra1 (spasmodic)

The novel neurological mutant Cincinatti arose by genomic duplication of exon 5 in the glycine receptor gene Glra1. The mutant transcript results in premature protein truncation. A direct repeat of the pentamer GGGGC is present adjacent to the breakpoints and may have mediated the duplication event by a replication slippage mechanism.

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

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