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Rauschenberger V, Piro I, Kasaragod VB, Hörlin V, Eckes AL, Kluck CJ, Schindelin H, Meinck HM, Wickel J, Geis C, Tüzün E, Doppler K, Sommer C, Villmann C. Glycine receptor autoantibody binding to the extracellular domain is independent from receptor glycosylation. Front Mol Neurosci 2023; 16:1089101. [PMID: 36860666 PMCID: PMC9969106 DOI: 10.3389/fnmol.2023.1089101] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2022] [Accepted: 01/19/2023] [Indexed: 02/17/2023] Open
Abstract
Glycine receptor (GlyR) autoantibodies are associated with stiff-person syndrome and the life-threatening progressive encephalomyelitis with rigidity and myoclonus in children and adults. Patient histories show variability in symptoms and responses to therapeutic treatments. A better understanding of the autoantibody pathology is required to develop improved therapeutic strategies. So far, the underlying molecular pathomechanisms include enhanced receptor internalization and direct receptor blocking altering GlyR function. A common epitope of autoantibodies against the GlyRα1 has been previously defined to residues 1A-33G at the N-terminus of the mature GlyR extracellular domain. However, if other autoantibody binding sites exist or additional GlyR residues are involved in autoantibody binding is yet unknown. The present study investigates the importance of receptor glycosylation for binding of anti-GlyR autoantibodies. The glycine receptor α1 harbors only one glycosylation site at the amino acid residue asparagine 38 localized in close vicinity to the identified common autoantibody epitope. First, non-glycosylated GlyRs were characterized using protein biochemical approaches as well as electrophysiological recordings and molecular modeling. Molecular modeling of non-glycosylated GlyRα1 did not show major structural alterations. Moreover, non-glycosylation of the GlyRα1N38Q did not prevent the receptor from surface expression. At the functional level, the non-glycosylated GlyR demonstrated reduced glycine potency, but patient GlyR autoantibodies still bound to the surface-expressed non-glycosylated receptor protein in living cells. Efficient adsorption of GlyR autoantibodies from patient samples was possible by binding to native glycosylated and non-glycosylated GlyRα1 expressed in living not fixed transfected HEK293 cells. Binding of patient-derived GlyR autoantibodies to the non-glycosylated GlyRα1 offered the possibility to use purified non-glycosylated GlyR extracellular domain constructs coated on ELISA plates and use them as a fast screening readout for the presence of GlyR autoantibodies in patient serum samples. Following successful adsorption of patient autoantibodies by GlyR ECDs, binding to primary motoneurons and transfected cells was absent. Our results indicate that the glycine receptor autoantibody binding is independent of the receptor's glycosylation state. Purified non-glycosylated receptor domains harbouring the autoantibody epitope thus provide, an additional reliable experimental tool besides binding to native receptors in cell-based assays for detection of autoantibody presence in patient sera.
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Affiliation(s)
- Vera Rauschenberger
- Institute of Clinical Neurobiology, University Hospital, Julius-Maximilians-University of Würzburg, Würzburg, Germany
| | - Inken Piro
- Department of Neurology, University Hospital Würzburg, Würzburg, Germany
| | - Vikram Babu Kasaragod
- Rudolf Virchow Centre for Integrative and Translational Bioimaging, Julius-Maximilians-University of Würzburg, Würzburg, Germany
| | - Verena Hörlin
- Institute of Clinical Neurobiology, University Hospital, Julius-Maximilians-University of Würzburg, Würzburg, Germany
| | - Anna-Lena Eckes
- Institute of Clinical Neurobiology, University Hospital, Julius-Maximilians-University of Würzburg, Würzburg, Germany
| | - Christoph J. Kluck
- Institute of Biochemistry, Emil-Fischer-Center, FAU Erlangen-Nürnberg, Erlangen, Germany
| | - Hermann Schindelin
- Rudolf Virchow Centre for Integrative and Translational Bioimaging, Julius-Maximilians-University of Würzburg, Würzburg, Germany
| | - Hans-Michael Meinck
- Department of Neurology, University Hospital Heidelberg, Heidelberg, Germany
| | - Jonathan Wickel
- Section Translational Neuroimmunology, Department of Neurology, Jena University Hospital, Jena, Germany
| | - Christian Geis
- Section Translational Neuroimmunology, Department of Neurology, Jena University Hospital, Jena, Germany
| | - Erdem Tüzün
- Institute of Experimental Medicine, Istanbul University, Istanbul, Türkiye
| | - Kathrin Doppler
- Department of Neurology, University Hospital Würzburg, Würzburg, Germany
| | - Claudia Sommer
- Department of Neurology, University Hospital Würzburg, Würzburg, Germany
| | - Carmen Villmann
- Institute of Clinical Neurobiology, University Hospital, Julius-Maximilians-University of Würzburg, Würzburg, Germany,*Correspondence: Carmen Villmann, ✉
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Piro I, Eckes AL, Kasaragod VB, Sommer C, Harvey RJ, Schaefer N, Villmann C. Novel Functional Properties of Missense Mutations in the Glycine Receptor β Subunit in Startle Disease. Front Mol Neurosci 2021; 14:745275. [PMID: 34630038 PMCID: PMC8498107 DOI: 10.3389/fnmol.2021.745275] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2021] [Accepted: 09/03/2021] [Indexed: 11/13/2022] Open
Abstract
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.
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Affiliation(s)
- Inken Piro
- Department of Neurology, University Hospital Würzburg, Würzburg, Germany
| | - Anna-Lena Eckes
- Institute for Clinical Neurobiology, University Hospital, Julius-Maximilians-University Würzburg, Würzburg, Germany
| | - Vikram Babu Kasaragod
- Neurobiology Division, MRC Laboratory of Molecular Biology, Cambridge, United Kingdom
| | - Claudia Sommer
- Department of Neurology, University Hospital Würzburg, Würzburg, Germany
| | - Robert J. Harvey
- School of Health and Behavioural Sciences, University of the Sunshine Coast, Maroochydore, QLD, Australia
- Sunshine Coast Health Institute, Birtinya, QLD, Australia
| | - Natascha Schaefer
- Institute for Clinical Neurobiology, University Hospital, Julius-Maximilians-University Würzburg, Würzburg, Germany
| | - Carmen Villmann
- Institute for Clinical Neurobiology, University Hospital, Julius-Maximilians-University Würzburg, Würzburg, Germany
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Grüner J, Stengel H, Werner C, Appeltshauser L, Sommer C, Villmann C, Doppler K. Anti-contactin-1 Antibodies Affect Surface Expression and Sodium Currents in Dorsal Root Ganglia. NEUROLOGY-NEUROIMMUNOLOGY & NEUROINFLAMMATION 2021; 8:8/5/e1056. [PMID: 34429341 PMCID: PMC8407150 DOI: 10.1212/nxi.0000000000001056] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/12/2020] [Accepted: 05/17/2021] [Indexed: 01/13/2023]
Abstract
Background and Objectives As autoantibodies to contactin-1 from patients with chronic inflammatory demyelinating polyradiculoneuropathy not only bind to the paranodes where they are supposed to cause conduction failure but also bind to other neuronal cell types, we aimed to investigate the effect of anti–contactin-1 autoantibodies on contactin-1 surface expression in cerebellar granule neurons, dorsal root ganglion neurons, and contactin-1–transfected human embryonic kidney 293 cells. Methods Immunocytochemistry including structured illumination microscopy and immunoblotting was used to determine expression levels of contactin-1 and/or sodium channels after long-term exposure to autoantibodies from 3 seropositive patients. For functional analysis of sodium channels, whole-cell recordings of sodium currents were performed on dorsal root ganglion neurons incubated with anti–contactin-1 autoantibodies. Results We found a reduction in contactin-1 expression levels on dorsal root ganglion neurons, cerebellar granule neurons, and contactin-1–transfected human embryonic kidney 293 cells and decreased dorsal root ganglion sodium currents after long-term exposure to anti–contactin-1 autoantibodies. Sodium channel density did not decrease. Discussion Our results demonstrate a direct effect of anti–contactin-1 autoantibodies on the surface expression of contactin-1 and sodium currents in dorsal root ganglion neurons. This may be the pathophysiologic correlate of sensory ataxia reported in these patients.
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Affiliation(s)
- Julia Grüner
- From the Department of Neurology (J.G., H.S., L.A., C.S., K.D.), University Hospital Würzburg, Germany; Department of Biotechnology and Biophysics (C.W.), Julius-Maximilians-University of Würzburg; and Institute of Clinical Neurobiology (C.V.), University Hospital, Julius-Maximilians-University of Würzburg, Germany
| | - Helena Stengel
- From the Department of Neurology (J.G., H.S., L.A., C.S., K.D.), University Hospital Würzburg, Germany; Department of Biotechnology and Biophysics (C.W.), Julius-Maximilians-University of Würzburg; and Institute of Clinical Neurobiology (C.V.), University Hospital, Julius-Maximilians-University of Würzburg, Germany
| | - Christian Werner
- From the Department of Neurology (J.G., H.S., L.A., C.S., K.D.), University Hospital Würzburg, Germany; Department of Biotechnology and Biophysics (C.W.), Julius-Maximilians-University of Würzburg; and Institute of Clinical Neurobiology (C.V.), University Hospital, Julius-Maximilians-University of Würzburg, Germany
| | - Luise Appeltshauser
- From the Department of Neurology (J.G., H.S., L.A., C.S., K.D.), University Hospital Würzburg, Germany; Department of Biotechnology and Biophysics (C.W.), Julius-Maximilians-University of Würzburg; and Institute of Clinical Neurobiology (C.V.), University Hospital, Julius-Maximilians-University of Würzburg, Germany
| | - Claudia Sommer
- From the Department of Neurology (J.G., H.S., L.A., C.S., K.D.), University Hospital Würzburg, Germany; Department of Biotechnology and Biophysics (C.W.), Julius-Maximilians-University of Würzburg; and Institute of Clinical Neurobiology (C.V.), University Hospital, Julius-Maximilians-University of Würzburg, Germany
| | - Carmen Villmann
- From the Department of Neurology (J.G., H.S., L.A., C.S., K.D.), University Hospital Würzburg, Germany; Department of Biotechnology and Biophysics (C.W.), Julius-Maximilians-University of Würzburg; and Institute of Clinical Neurobiology (C.V.), University Hospital, Julius-Maximilians-University of Würzburg, Germany
| | - Kathrin Doppler
- From the Department of Neurology (J.G., H.S., L.A., C.S., K.D.), University Hospital Würzburg, Germany; Department of Biotechnology and Biophysics (C.W.), Julius-Maximilians-University of Würzburg; and Institute of Clinical Neurobiology (C.V.), University Hospital, Julius-Maximilians-University of Würzburg, Germany.
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Rauschenberger V, von Wardenburg N, Schaefer N, Ogino K, Hirata H, Lillesaar C, Kluck CJ, Meinck H, Borrmann M, Weishaupt A, Doppler K, Wickel J, Geis C, Sommer C, Villmann C. Glycine Receptor
Autoantibodies Impair Receptor Function and Induce Motor Dysfunction. Ann Neurol 2020; 88:544-561. [DOI: 10.1002/ana.25832] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2019] [Revised: 06/23/2020] [Accepted: 06/23/2020] [Indexed: 12/22/2022]
Affiliation(s)
- Vera Rauschenberger
- Institute for Clinical NeurobiologyUniversity Hospital, Julius Maximilian University of Würzburg Würzburg Germany
| | - Niels von Wardenburg
- Institute for Clinical NeurobiologyUniversity Hospital, Julius Maximilian University of Würzburg Würzburg Germany
| | - Natascha Schaefer
- Institute for Clinical NeurobiologyUniversity Hospital, Julius Maximilian University of Würzburg Würzburg Germany
| | - Kazutoyo Ogino
- Department of Chemistry and Biological ScienceCollege of Science and Engineering, Aoyama Gakuin University Tokyo Japan
| | - Hiromi Hirata
- Department of Chemistry and Biological ScienceCollege of Science and Engineering, Aoyama Gakuin University Tokyo Japan
| | - Christina Lillesaar
- Department of Child and Adolescent PsychiatryCenter of Mental Health, University Hospital of Würzburg Würzburg Germany
| | - Christoph J. Kluck
- Institute of Biochemistry, Emil Fischer Center, Friedrich Alexander University Erlangen–Nürnberg Erlangen Germany
| | | | - Marc Borrmann
- WittenHelios University Hospital Wuppertal, Department of Nephrology and Rheumatology, Witten/Herdecke University Germany
| | - Andreas Weishaupt
- Department of NeurologyUniversity Hospital Würzburg Würzburg Germany
| | - Kathrin Doppler
- Department of NeurologyUniversity Hospital Würzburg Würzburg Germany
| | - Jonathan Wickel
- Section of Translational Neuroimmunology, Department of NeurologyJena University Hospital Jena Germany
| | - Christian Geis
- Section of Translational Neuroimmunology, Department of NeurologyJena University Hospital Jena Germany
| | - Claudia Sommer
- Department of NeurologyUniversity Hospital Würzburg Würzburg Germany
| | - Carmen Villmann
- Institute for Clinical NeurobiologyUniversity Hospital, Julius Maximilian University of Würzburg Würzburg Germany
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A Novel Glycine Receptor Variant with Startle Disease Affects Syndapin I and Glycinergic Inhibition. J Neurosci 2020; 40:4954-4969. [PMID: 32354853 DOI: 10.1523/jneurosci.2490-19.2020] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2019] [Revised: 03/24/2020] [Accepted: 04/24/2020] [Indexed: 11/21/2022] Open
Abstract
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.
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Schaefer N, Roemer V, Janzen D, Villmann C. Impaired Glycine Receptor Trafficking in Neurological Diseases. Front Mol Neurosci 2018; 11:291. [PMID: 30186111 PMCID: PMC6110938 DOI: 10.3389/fnmol.2018.00291] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2018] [Accepted: 08/02/2018] [Indexed: 12/21/2022] Open
Abstract
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.
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Affiliation(s)
- Natascha Schaefer
- Institute for Clinical Neurobiology, Julius-Maximilians-University Würzburg, Würzburg, Germany
| | - Vera Roemer
- Institute for Clinical Neurobiology, Julius-Maximilians-University Würzburg, Würzburg, Germany
| | - Dieter Janzen
- Institute for Clinical Neurobiology, Julius-Maximilians-University Würzburg, Würzburg, Germany
| | - Carmen Villmann
- Institute for Clinical Neurobiology, Julius-Maximilians-University Würzburg, Würzburg, Germany
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Janzen D, Schaefer N, Delto C, Schindelin H, Villmann C. The GlyR Extracellular β8-β9 Loop - A Functional Determinant of Agonist Potency. Front Mol Neurosci 2017; 10:322. [PMID: 29062270 PMCID: PMC5640878 DOI: 10.3389/fnmol.2017.00322] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2017] [Accepted: 09/22/2017] [Indexed: 01/07/2023] Open
Abstract
Ligand-binding of Cys-loop receptors results in rearrangements of extracellular loop structures which are further translated into the tilting of membrane spanning helices, and finally opening of the ion channels. The cryo-EM structure of the homopentameric α1 glycine receptor (GlyR) demonstrated an involvement of the extracellular β8–β9 loop in the transition from ligand-bound receptors to the open channel state. Recently, we identified a functional role of the β8–β9 loop in a novel startle disease mouse model shaky. The mutation of residue GlyRα1Q177 to lysine present in shaky mice resulted in reduced glycine potency, reduced synaptic expression, and a disrupted hydrogen network at the structural level around position GlyRα1Q177. Here, we investigated the role of amino acid volume, side chain length, and charge at position Q177 to get deeper insights into the functional role of the β8–β9 loop. We used a combined approach of in vitro expression analysis, functional electrophysiological recordings, and GlyR modeling to describe the role of Q177 for GlyR ion channel function. GlyRα1Q177 variants do not disturb ion channel transport to the cellular surface of transfected cells, neither in homomeric nor in heteromeric GlyR configurations. The EC50 values were increased for all GlyRα1Q177 variants in comparison to the wild type. The largest decrease in glycine potency was observed for the variant GlyRα1Q177R. Potencies of the partial agonists β-alanine and taurine were also reduced. Our data are further supported by homology modeling. The GlyRα1Q177R variant does not form hydrogen bonds with the surrounding network of residue Q177 similar to the substitution with a basic lysine present in the mouse mutant shaky. Among all investigated Q177 mutants, the neutral exchange of glutamine to asparagine as well as the introduction of the closely related amino acid glutamic acid preserve the hydrogen bond network. Introduction of amino acids with small side chains or larger volume resulted in a loss of their hydrogen bonds to neighboring residues. The β8–β9 loop is thus an important structural and functional determinant of the inhibitory GlyR.
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Affiliation(s)
- Dieter Janzen
- Institute for Clinical Neurobiology, University of Würzburg, Würzburg, Germany
| | - Natascha Schaefer
- Institute for Clinical Neurobiology, University of Würzburg, Würzburg, Germany
| | - Carolyn Delto
- Rudolf Virchow Center for Experimental Biomedicine, University of Würzburg, Würzburg, Germany
| | - Hermann Schindelin
- Rudolf Virchow Center for Experimental Biomedicine, University of Würzburg, Würzburg, Germany
| | - Carmen Villmann
- Institute for Clinical Neurobiology, University of Würzburg, Würzburg, Germany
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Schaefer N, Berger A, van Brederode J, Zheng F, Zhang Y, Leacock S, Littau L, Jablonka S, Malhotra S, Topf M, Winter F, Davydova D, Lynch JW, Paige CJ, Alzheimer C, Harvey RJ, Villmann C. Disruption of a Structurally Important Extracellular Element in the Glycine Receptor Leads to Decreased Synaptic Integration and Signaling Resulting in Severe Startle Disease. J Neurosci 2017; 37:7948-7961. [PMID: 28724750 PMCID: PMC5559766 DOI: 10.1523/jneurosci.0009-17.2017] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2017] [Revised: 06/15/2017] [Accepted: 06/24/2017] [Indexed: 01/09/2023] Open
Abstract
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.
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Affiliation(s)
- Natascha Schaefer
- Institute of Clinical Neurobiology, Julius-Maximilians-University of Würzburg, 97078 Würzburg, Germany
| | - Alexandra Berger
- Princess Margaret Cancer Centre, University Health Network, University of Toronto, Toronto, Ontario M5G 2M9, Canada
| | | | - Fang Zheng
- Institute of Physiology and Pathophysiology, and
| | - Yan Zhang
- Queensland Brain Institute, University of Queensland, Brisbane 4072, Australia
| | - Sophie Leacock
- Department of Pharmacology, UCL School of Pharmacy, London WC1N 1AX, United Kingdom
| | - Laura Littau
- Institute of Clinical Neurobiology, Julius-Maximilians-University of Würzburg, 97078 Würzburg, Germany
| | - Sibylle Jablonka
- Institute of Clinical Neurobiology, Julius-Maximilians-University of Würzburg, 97078 Würzburg, Germany
| | - Sony Malhotra
- Department of Biochemistry, University of Cambridge, Cambridge CB2 1QW, United Kingdom, and
| | - Maya Topf
- Institute of Structural and Molecular Biology, UCL Birkbeck College, London WC1E 7HX, United Kingdom
| | - Friederike Winter
- Institute of Biochemistry, Friedrich-Alexander-University Erlangen-Nürnberg, 91054 Erlangen-Nürnberg, Germany
| | - Daria Davydova
- Institute of Clinical Neurobiology, Julius-Maximilians-University of Würzburg, 97078 Würzburg, Germany
| | - Joseph W Lynch
- Queensland Brain Institute, University of Queensland, Brisbane 4072, Australia
| | - Christopher J Paige
- Princess Margaret Cancer Centre, University Health Network, University of Toronto, Toronto, Ontario M5G 2M9, Canada
| | | | - Robert J Harvey
- Department of Pharmacology, UCL School of Pharmacy, London WC1N 1AX, United Kingdom
| | - Carmen Villmann
- Institute of Clinical Neurobiology, Julius-Maximilians-University of Würzburg, 97078 Würzburg, Germany,
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Langlhofer G, Villmann C. The role of charged residues in independent glycine receptor folding domains for intermolecular interactions and ion channel function. J Neurochem 2017; 142:41-55. [PMID: 28429370 DOI: 10.1111/jnc.14049] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2016] [Revised: 03/18/2017] [Accepted: 04/12/2017] [Indexed: 01/03/2023]
Abstract
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.
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Affiliation(s)
- Georg Langlhofer
- Institute for Clinical Neurobiology, Julius-Maximilians-University of Würzburg, Würzburg, Germany
| | - Carmen Villmann
- Institute for Clinical Neurobiology, Julius-Maximilians-University of Würzburg, Würzburg, Germany
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