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

GPR39 regulated spinal glycinergic inhibition and mechanical inflammatory pain

G protein-coupled receptor 39 (GPR39) senses the change of extracellular divalent zinc ion and signals through multiple G proteins to a broad spectrum of downstream effectors. Here, we found that GPR39 was prevalent at inhibitory synapses of spinal cord somatostatin-positive (SOM+) interneurons, a mechanosensitive subpopulation that is critical for the conveyance of mechanical pain. GPR39 complexed specifically with inhibitory glycine receptors (GlyRs) and helped maintain glycinergic transmission in a manner independent of G protein signalings. Targeted knockdown of GPR39 in SOM+ interneurons reduced the glycinergic inhibition and facilitated the excitatory output from SOM+ interneurons to spinoparabrachial neurons that engaged superspinal neural circuits encoding both the sensory discriminative and affective motivational domains of pain experience. Our data showed that pharmacological activation of GPR39 or augmenting GPR39 interaction with GlyRs at the spinal level effectively alleviated the sensory and affective pain induced by complete Freund's adjuvant and implicated GPR39 as a promising therapeutic target for the treatment of inflammatory mechanical pain.

Two residues determine nicotinic acetylcholine receptor requirement for RIC-3

Nicotinic acetylcholine receptors (N-AChRs) mediate fast synaptic signaling and are members of the pentameric ligand-gated ion channel (pLGIC) family. They rely on a network of accessory proteins in vivo for correct formation and transport to the cell surface. Resistance to cholinesterase 3 (RIC-3) is an endoplasmic reticulum protein that physically interacts with nascent pLGIC subunits and promotes their oligomerization. It is not known why some N-AChRs require RIC-3 in heterologous expression systems, whereas others do not. Previously we reported that the ACR-16 N-AChR from the parasitic nematode Dracunculus medinensis does not require RIC-3 in Xenopus laevis oocytes. This is unusual because all other nematode ACR-16, like the closely related Ascaris suum ACR-16, require RIC-3. Their high sequence similarity limits the number of amino acids that may be responsible, and the goal of this study was to identify them. A series of chimeras and point mutations between A. suum and D. medinensis ACR-16, followed by functional characterization with electrophysiology, identified two residues that account for a majority of the receptor requirement for RIC-3. ACR-16 with R/K159 in the cys-loop and I504 in the C-terminal tail did not require RIC-3 for functional expression. Mutating either of these to R/K159E or I504T, residues found in other nematode ACR-16, conferred a RIC-3 requirement. Our results agree with previous studies showing that these regions interact and are involved in receptor synthesis. Although it is currently unclear what precise mechanism they regulate, these residues may be critical during specific subunit folding and/or assembly cascades that RIC-3 may promote.

Speculation on How RIC-3 and Other Chaperones Facilitate α7 Nicotinic Receptor Folding and Assembly

The process of how multimeric transmembrane proteins fold and assemble in the endoplasmic reticulum is not well understood. The alpha7 nicotinic receptor (α7 nAChR) is a good model for multimeric protein assembly since it has at least two independent and specialized chaperones: Resistance to Inhibitors of Cholinesterase 3 (RIC-3) and Nicotinic Acetylcholine Receptor Regulator (NACHO). Recent cryo-EM and NMR data revealed structural features of α7 nAChRs. A ser-ala-pro (SAP) motif precedes a structurally important but unique "latch" helix in α7 nAChRs. A sampling of α7 sequences suggests the SAP motif is conserved from C. elegans to humans, but the latch sequence is only conserved in vertebrates. How RIC-3 and NACHO facilitate receptor subunits folding into their final pentameric configuration is not known. The artificial intelligence program AlphaFold2 recently predicted structures for NACHO and RIC-3. NACHO is highly conserved in sequence and structure across species, but RIC-3 is not. This review ponders how different intrinsically disordered RIC-3 isoforms from C. elegans to humans interact with α7 nAChR subunits despite having little sequence homology across RIC-3 species. Two models from the literature about how RIC-3 assists α7 nAChR assembly are evaluated considering recent structural information about the receptor and its chaperones.

Discovery and functional characterization of N-(thiazol-2-yl)-benzamide analogs as the first class of selective antagonists of the Zinc-Activated Channel (ZAC)

The Zinc-Activated Channel (ZAC) is an atypical member of the Cys-loop receptor (CLR) superfamily of pentameric ligand-gated ion channels, with its very different endogenous agonists and signalling properties. In this study, a compound library screening at ZAC resulted in the identification of 2-(5-bromo-2-chlorobenzamido)-4-methylthiazole-5-methyl ester (1) as a novel ZAC antagonist. The structural determinants for ZAC activity in 1 were investigated by functional characterization of 61 analogs at ZAC expressed in Xenopus oocytes by two-electrode voltage clamp electrophysiology, and couple of analogs exerting more potent ZAC inhibition than 1 were identified (IC₅₀ values: 1-3 μM). 1 and N-(4-(tert-butyl)thiazol-2-yl)-3-fluorobenzamide (5a, TTFB) were next applied in studies of the functional properties and the mode of action of this novel class of ZAC antagonists. TTFB was a roughly equipotent antagonist of Zn⁺- and H⁺-evoked ZAC signaling and of spontaneous ZAC activity, and the slow on-set of its channel block suggested that its ZAC inhibition is state-dependent. TTFB was found to be a selective ZAC antagonist, exhibiting no significant agonist, antagonist or modulatory activity at 5-HT₃A, α₃β₄ nicotinic acetylcholine, α₁β₂γ_2S GABA_A or α₁ glycine receptors at 30 μM. 1 displayed largely non-competitive antagonism of Zn²⁺-induced ZAC signalling, and TTFB was demonstrated to target the transmembrane and/or intracellular domains of the receptor, which collectively suggests that the N-(thiazol-2-yl)-benzamide analog acts a negative allosteric modulator of ZAC. We propose that this first class of selective ZAC antagonists could constitute useful pharmacological tools in future explorations of the presently poorly elucidated physiological functions governed by this CLR.

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

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

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

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

The intracellular domain of homomeric glycine receptors modulates agonist efficacy

Like other pentameric ligand-gated channels, glycine receptors (GlyRs) contain long intracellular domains (ICDs) between transmembrane helices 3 and 4. Structurally characterized GlyRs are generally engineered to have a very short ICD. We show here that for one such construct, zebrafish GlyREM, the agonists glycine, β-alanine, taurine, and GABA have high efficacy and produce maximum single-channel open probabilities greater than 0.9. In contrast, for full-length human α1 GlyR, taurine and GABA were clearly partial agonists, with maximum open probabilities of 0.46 and 0.09, respectively. We found that the elevated open probabilities in GlyREM are not due to the limited sequence differences between the human and zebrafish orthologs, but rather to replacement of the native ICD with a short tripeptide ICD. Consistent with this interpretation, shortening the ICD in the human GlyR increased the maximum open probability produced by taurine and GABA to 0.90 and 0.70, respectively, but further engineering it to resemble GlyREM (by introducing the zebrafish transmembrane helix 4 and C terminus) had no effect. Furthermore, reinstating the native ICD to GlyREM converted taurine and GABA to partial agonists, with maximum open probabilities of 0.66 and 0.40, respectively. Structural comparison of transmembrane helices 3 and 4 in short- and long-ICD GlyR subunits revealed that ICD shortening does not distort the orientation of these helices within each subunit. This suggests that the effects of shortening the ICD stem from removing a modulatory effect of the native ICD on GlyR gating, revealing a new role for the ICD in pentameric ligand-gated channels.

A proline-rich motif in the large intracellular loop of the glycine receptor α1 subunit interacts with the Pleckstrin homology domain of collybistin

Introduction

The inhibitory glycine receptor (GlyR), a mediator of fast synaptic inhibition, is located and held at neuronal synapses through the anchoring proteins gephyrin and collybistin. Stable localization of neurotransmitter receptors is essential for synaptic function. In case of GlyRs, only beta subunits were known until now to mediate synaptic anchoring.

Objectives

We identified a poly-proline II helix (PPII) in position 365–373 of the intra-cellular TM3-4 loop of the human GlyRα1 subunit as a novel potential synaptic anchoring site. The potential role of the PPII helix as synaptic anchoring site was tested.

Methods

Glycine receptors and collybistin variants were generated and recombinantly expressed in HEK293 cells and cultured neurons. Receptor function was assessed using patch-clamp electrophysiology, protein-protein interaction was studied using co-immuno-precipitation and pulldown experiments.

Results

Recombinantly expressed collybistin bound to isolated GlyRα1 TM3-4 loops in GST-pulldown assays. When the five proline residues P365A, P366A, P367A, P369A, P373A (GlyRα1^P1-5A) located in the GlyRα1-PPII helix were replaced by alanines, the PPII secondary structure was disrupted. Recombinant GlyRα1^P1-5A mutant subunits displayed normal cell surface expression and wildtype-like ion channel function, but binding to collybistin was abolished. The GlyRα1-collybistin interaction was independently confirmed by o-immunoprecipitation assays using full-length GlyRα1 subunits. Surprisingly, the interaction was not mediated by the SH3 domain of collybistin, but by its Pleckstrin homology (PH) domain. The mutation GlyRα1^P366L, identified in a hyperekplexia patient, is also disrupting the PPII helix, and caused reduced collybistin binding.

Conclusion

Our data suggest a novel interaction between α1 GlyR subunits and collybistin, which is physiologically relevant in vitro and in vivo and may contribute to postsynaptic anchoring of glycine receptors.

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

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

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

News about non-secretory exocytosis: mechanisms, properties, and functions

The fusion by exocytosis of many vesicles to the plasma membrane induces the discharge to the extracellular space of their abundant luminal cargoes. Other exocytic vesicles, however, do not contain cargoes, and thus, their fusion is not followed by secretion. Therefore, two distinct processes of exocytosis exist, one secretory and the other non-secretory. The present review deals with the knowledge of non-secretory exocytosis developed during recent years. Among such developments are the dual generation of the exocytic vesicles, initially released either from the trans-Golgi network or by endocytosis; their traffic with activation of receptors, channels, pumps, and transporters; the identification of their tethering and soluble N-ethylmaleimide-sensitive factor attachment protein receptor complexes that govern membrane fusions; the growth of axons and the membrane repair. Examples of potential relevance of these processes for pathology and medicine are also reported. The developments presented here offer interesting chances for future progress in the field.

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 Glycine Receptor Variant with Startle Disease Affects Syndapin I and Glycinergic Inhibition

Glycine receptors (GlyRs) are the major mediators of fast synaptic inhibition in the adult human spinal cord and brainstem. Hereditary mutations to GlyRs can lead to the rare, but potentially fatal, neuromotor disorder hyperekplexia. Most mutations located in the large intracellular domain (TM3-4 loop) of the GlyRα1 impair surface expression levels of the receptors. The novel GLRA1 mutation P366L, located in the TM3-4 loop, showed normal surface expression but reduced chloride currents, and accelerated whole-cell desensitization observed in whole-cell recordings. At the single-channel level, we observed reduced unitary conductance accompanied by spontaneous opening events in the absence of extracellular glycine. Using peptide microarrays and tandem MS-based analysis methods, we show that the proline-rich stretch surrounding P366 mediates binding to syndapin I, an F-BAR domain protein involved in membrane remodeling. The disruption of the noncanonical Src homology 3 recognition motif by P366L reduces syndapin I binding. These data suggest that the GlyRα1 subunit interacts with intracellular binding partners and may therefore play a role in receptor trafficking or synaptic anchoring, a function thus far only ascribed to the GlyRβ subunit. Hence, the P366L GlyRα1 variant exhibits a unique set of properties that cumulatively affect GlyR functionality and thus might explain the neuropathological mechanism underlying hyperekplexia in the mutant carriers. P366L is the first dominant GLRA1 mutation identified within the GlyRα1 TM3-4 loop that affects GlyR physiology without altering protein expression at the whole-cell and surface levels.SIGNIFICANCE STATEMENT We show that the intracellular domain of the inhibitory glycine receptor α1 subunit contributes to trafficking and synaptic anchoring. A proline-rich stretch in this receptor domain forms a noncanonical recognition motif important for the interaction with syndapin I (PACSIN1). The disruption of this motif, as present in a human patient with hyperekplexia led to impaired syndapin I binding. Functional analysis revealed that the altered proline-rich stretch determines several functional physiological parameters of the ion channel (e.g., faster whole-cell desensitization) reduced unitary conductance and spontaneous opening events. Thus, the proline-rich stretch from the glycine receptor α1 subunit represents a multifunctional intracellular protein motif.

The effects of insecticides on two splice variants of the glutamate-gated chloride channel receptor of the major malaria vector, Anopheles gambiae

BACKGROUND AND PURPOSE

Between half to 1 million people die annually from malaria. Anopheles gambiae mosquitoes are major malaria vectors. Unfortunately, resistance has emerged to the agents currently used to control A. gambiae, creating a demand for novel control measures. The pentameric glutamate-gated chloride channel (GluCl) expressed in the muscle and nerve cells of these organisms are a potentially important biological target for malaria control. The pharmacological properties of Anophiline GluCl receptors are, however, largely unknown. Accordingly, we compared the efficacy of four insecticides (lindane, fipronil, picrotoxin, and ivermectin) on two A. gambiae GluCl receptor splice variants with the aim of providing a molecular basis for designing novel anti-malaria treatments.

EXPERIMENTAL APPROACH

The A. gambiae GluCl receptor b1 and c splice variants were expressed homomerically in Xenopus laevis oocytes and studied with electrophysiological techniques, using two-electrode voltage-clamp.

KEY RESULTS

The b1 and c GluCl receptors were activated with similar potencies by glutamate and ivermectin. Fipronil was more potent than picrotoxin and lindane at inhibiting glutamate- and ivermectin-gated currents. Importantly, b1 GluCl receptors exhibited reduced sensitivity to picrotoxin and lindane. They also recovered from these effects to a greater extent than c GluCl receptors CONCLUSIONS AND IMPLICATIONS: The two splice variant subunits exhibited differential sensitivities to multiple, structurally divergent insecticides, without accompanying changes in the sensitivity to the endogenous neurotransmitter, glutamate, implying that drug resistance may be caused by alterations in relative subunit expression levels, without affecting physiological function. Our results strongly suggest that it should be feasible to develop novel subunit-specific pharmacological agents.

Large Intracellular Domain-Dependent Effects of Positive Allosteric Modulators on Glycine Receptors

Glycine receptors (GlyRs) are members of the pentameric ligand-gated ionic channel family (pLGICs) and mediate fast inhibitory neurotransmission in the brain stem and spinal cord. The function of GlyRs can be modulated by positive allosteric modulators (PAMs). So far, it is largely accepted that both the extracellular (ECD) and transmembrane (TMD) domains constitute the primary target for many of these PAMs. On the other hand, the contribution of the intracellular domain (ICD) to the PAM effects on GlyRs remains poorly understood. To gain insight about the role of the ICD in the pharmacology of GlyRs, we examined the contribution of each domain using a chimeric receptor. Two chimeras were generated, one consisting of the ECD of the prokaryotic homologue Gloeobacter violaceus ligand-gated ion channel (GLIC) fused to the TMD of the human α₁GlyR lacking the ICD (Lily) and a second with the ICD (Lily-ICD). The sensitivity to PAMs of both chimeric receptors was studied using electrophysiological techniques. The Lily receptor showed a significant decrease in the sensitivity to four recognized PAMs. Remarkably, the incorporation of the ICD into the Lily background was sufficient to restore the wild-type α₁GlyR sensitivity to these PAMs. Based on these data, we can suggest that the ICD is necessary to form a pLGIC having full sensitivity to positive allosteric modulators.

The dual-gate model for pentameric ligand-gated ion channels activation and desensitization

Pentameric ligand-gated ion channels (pLGICs) mediate fast neurotransmission in the nervous system. Their dysfunction is associated with psychiatric, neurological and neurodegenerative disorders such as schizophrenia, epilepsy and Alzheimer's disease. Understanding their biophysical and pharmacological properties, at both the functional and the structural level, thus holds many therapeutic promises. In addition to their agonist-elicited activation, most pLGICs display another key allosteric property, namely desensitization, in which they enter a shut state refractory to activation upon sustained agonist binding. While the activation mechanisms of several pLGICs have been revealed at near-atomic resolution, the structural foundation of desensitization has long remained elusive. Recent structural and functional data now suggest that the activation and desensitization gates are distinct, and are located at both sides of the ion channel. Such a 'dual gate mechanism' accounts for the marked allosteric effects of channel blockers, a feature illustrated herein by theoretical kinetics simulations. Comparison with other classes of ligand- and voltage-gated ion channels shows that this dual gate mechanism emerges as a common theme for the desensitization and inactivation properties of structurally unrelated ion channels.

PKA and PKC Modulators Affect Ion Channel Function and Internalization of Recombinant Alpha1 and Alpha1-Beta Glycine Receptors

Glycine receptors (GlyRs) are important mediators of fast inhibitory neurotransmission in the mammalian central nervous system. Their function is controlled by multiple cellular mechanisms, including intracellular regulatory processes. Modulation of GlyR function by protein kinases has been reported for many cell types, involving different techniques, and often yielding contradictory results. Here, we studied the effects of protein kinase C (PKC) and cAMP-dependent protein kinase A (PKA) on glycine induced currents in HEK293 cells expressing human homomeric α1 and heteromeric α1-β GlyRs using whole-cell patch clamp techniques as well as internalization assays. In whole-cell patch-clamp measurements, modulators were applied in the intracellular buffer at concentrations between 0.1 μM and 0.5 μM. EC₅₀ of glycine increased upon application of the protein kinase activators Forskolin and phorbol-12-myristate-13-acetate (PMA) but decreased in the presence of the PKC inhibitor Staurosporine aglycon and the PKA inhibitor H-89. Desensitization of recombinant α1 receptors was significantly increased in the presence of Forskolin. Staurosporine aglycon, on the other hand decreased desensitization of heteromeric α1-β GlyRs. The time course of receptor activation was determined for homomeric α1 receptors and revealed two simultaneous effects: cells showed a decrease of EC₅₀ after 3–6 min of establishing whole-cell configuration. This effect was independent of protein kinase modulators. All modulators of PKA and PKC, however, produced an additional shift of EC₅₀, which overlay and eventually exceeded the cells intrinsic variation of EC₅₀. The effect of kinase activators was abolished if the corresponding inhibitors were co-applied, consistent with PKA and PKC directly mediating the modulation of GlyR function. Direct effects of PKA- and PKC-modulators on receptor expression on transfected HEK cells were monitored within 15 min of drug application, showing a significant increase of receptor internalization with PKA and PKC activators, while the corresponding inhibitors had no significant effect on receptor surface expression or internalization. Our results confirm the observation that phosphorylation via PKA and PKC has a direct effect on the GlyR ion channel complex and plays an important role in the fine-tuning of glycinergic signaling.

Structure-Function Analysis of the GlyR α2 Subunit Autism Mutation p.R323L Reveals a Gain-of-Function

Glycine receptors (GlyRs) containing the α2 subunit regulate cortical interneuron migration. Disruption of the GlyR α2 subunit gene (Glra2) in mice leads to disrupted dorsal cortical progenitor homeostasis, leading to a depletion of projection neurons and moderate microcephaly in newborn mice. In humans, rare variants in GLRA2, which is located on the X chromosome, are associated with autism spectrum disorder (ASD) in the hemizygous state in males. These include a microdeletion (GLRA2∆ex8-9) and missense mutations in GLRA2 (p.N109S and p.R126Q) that impair cell-surface expression of GlyR α2, and either abolish or markedly reduce sensitivity to glycine. We report the functional characterization of a third missense variant in GLRA2 (p.R323L), associated with autism, macrocephaly, epilepsy and hypothyroidism in a female proband. Using heterosynapse and macroscopic current recording techniques, we reveal that GlyR α2^R323L exhibits reduced glycine sensitivity, but significantly increased inhibitory postsynaptic current (IPSC) rise and decay times. Site-directed mutagenesis revealed that the nature of the amino acid switch at position 323 is critical for impairment of GlyR function. Single-channel recordings revealed that the conductance of α2^R323Lβ channels was higher than α2β channels. Longer mean opening durations induced by p.R323L may be due to a change in the gating pathway that enhances the stability of the GlyR open state. The slower synaptic decay times, longer duration active periods and increase in conductance demonstrates that the GlyR α2 p.R323L mutation results in an overall gain of function, and that GlyR α2 mutations can be pathogenic in the heterozygous state in females.

The role of charged residues in independent glycine receptor folding domains for intermolecular interactions and ion channel function

Glycine receptor (GlyR) truncations in the intracellular TM3-4 loop, documented in patients suffering from hyperekplexia and in the mouse mutant oscillator, lead to non-functionality of GlyRs. The missing part that contains the TM3-4 loop, TM4 and C-terminal sequences is essential for pentameric receptor arrangements. In vitro co-expressions of GlyRα1-truncated N-domains and C-domains were able to restore ion channel function. An ionic interaction between both domains was hypothesized as the underlying mechanism. Here, we analysed the proposed ionic interaction between GlyR N- and C-domains using C-terminal constructs with either positively or negatively charged N-termini. Charged residues at the N-terminus of the C-domain did interfere with receptor surface expression and ion channel function. In particular, presence of negatively charged residues at the N-terminus led to significantly decreased ion channel function. Presence of positive charges resulted in reduced maximal currents possibly as a result of repulsion of both domains. If the C-domain was tagged by a myc-epitope, low maximal current amplitudes were detected. Intrinsic charges of the myc-epitope and charged N-terminal ends of the C-domain most probably induce intramolecular interactions. These interactions might hinder the close proximity of C-domains and N-domains, which is a prerequisite for functional ion channel configurations. The remaining basic subdomains close to TM3 and 4 were sufficient for domain complementation and functional ion channel formation. Thus, these basic subdomains forming α-helical elements or an intracellular portal represent attractants for incoming negatively charged chloride ions and interact with the phospholipids thereby stabilizing the GlyR in a conformation that allows ion channel opening.

Impaired "Glycine"-mia in Type 2 Diabetes and Potential Mechanisms Contributing to Glucose Homeostasis

The onset and/or progression of type 2 diabetes (T2D) can be prevented if intervention is early enough. As such, much effort has been placed on the search for indicators predictive of prediabetes and disease onset or progression. An increasing body of evidence suggests that changes in plasma glycine may be one such biomarker. Circulating glycine levels are consistently low in patients with T2D. Levels of this nonessential amino acid correlate negatively with obesity and insulin resistance. Plasma glycine correlates positively with glucose disposal, and rises with interventions such as exercise and bariatric surgery that improve glucose homeostasis. A role for glycine in the regulation of glucose, beyond being a potential biomarker, is less clear, however. Dietary glycine supplementation increases insulin, reduces systemic inflammation, and improves glucose tolerance. Emerging evidence suggests that glycine, a neurotransmitter, also acts directly on target tissues that include the endocrine pancreas and the brain via glycine receptors and as a coligand for N-methyl-d-aspartate glutamate receptors to control insulin secretion and liver glucose output, respectively. Here, we review the current evidence supporting a role for glycine in glucose homeostasis via its central and peripheral actions and changes that occur in T2D.

Mutational Analysis at Intersubunit Interfaces of an Anionic Glutamate Receptor Reveals a Key Interaction Important for Channel Gating by Ivermectin

The broad-spectrum anthelmintic drug ivermectin (IVM) activates and stabilizes an open-channel conformation of invertebrate chloride-selective glutamate receptors (GluClRs), thereby causing a continuous inflow of chloride ions and sustained membrane hyperpolarization. These effects suppress nervous impulses and vital physiological processes in parasitic nematodes. The GluClRs are pentamers. Homopentameric receptors assembled from the Caenorhabditis elegans (C. elegans) GluClα (GLC-1) subunit can inherently respond to IVM but not to glutamate (the neurotransmitter). In contrast, heteromeric GluClα/β (GLC-1/GLC-2) assemblies respond to both ligands, independently of each other. Glutamate and IVM bind at the interface between adjacent subunits, far away from each other; glutamate in the extracellular ligand-binding domain, and IVM in the ion-channel pore periphery. To understand the importance of putative intersubunit contacts located outside the glutamate and IVM binding sites, we introduced mutations at intersubunit interfaces, between these two binding-site types. Then, we determined the effect of these mutations on the activation of the heteromeric mutant receptors by glutamate and IVM. Amongst these mutations, we characterized an α-subunit point mutation located close to the putative IVM-binding pocket, in the extracellular end of the first transmembrane helix (M1). This mutation (αF276A) moderately reduced the sensitivity of the heteromeric GluClαF276A/βWT receptor to glutamate, and slightly decreased the receptor subunits’ cooperativity in response to glutamate. In contrast, the αF276A mutation drastically reduced the sensitivity of the receptor to IVM and significantly increased the receptor subunits’ cooperativity in response to IVM. We suggest that this mutation reduces the efficacy of channel gating, and impairs the integrity of the IVM-binding pocket, likely by disrupting important interactions between the tip of M1 and the M2-M3 loop of an adjacent subunit. We hypothesize that this physical contact between M1 and the M2-M3 loop tunes the relative orientation of the ion-channel transmembrane helices M1, M2 and M3 to optimize pore opening. Interestingly, pre-exposure of the GluClαF276A/βWT mutant receptor to subthreshold IVM concentration recovered the receptor sensitivity to glutamate. We infer that IVM likely retained its positive modulation activity by constraining the transmembrane helices in a preopen orientation sensitive to glutamate, with no need for the aforementioned disrupted interactions between M1 and the M2-M3 loop.