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

Glycine is a transmitter in the human and chimpanzee cochlear nuclei

Introduction

Auditory information is relayed from the cochlea via the eighth cranial nerve to the dorsal and ventral cochlear nuclei (DCN, VCN). The organization, neurochemistry and circuitry of the cochlear nuclei (CN) have been studied in many species. It is well-established that glycine is an inhibitory transmitter in the CN of rodents and cats, with glycinergic cells in the DCN and VCN. There are, however, major differences in the laminar and cellular organization of the DCN between humans (and other primates) and rodents and cats. We therefore asked whether there might also be differences in glycinergic neurotransmission in the CN.

Methods

We studied brainstem sections from humans, chimpanzees, and cats. We used antibodies to glycine receptors (GLYR) to identify neurons receiving glycinergic input, and antibodies to the neuronal glycine transporter (GLYT2) to immunolabel glycinergic axons and terminals. We also examined archival sections immunostained for calretinin (CR) and nonphosphorylated neurofilament protein (NPNFP) to try to locate the octopus cell area (OCA), a region in the VCN that rodents has minimal glycinergic input.

Results

In humans and chimpanzees we found widespread immunolabel for glycine receptors in DCN and in the posterior (PVCN) and anterior (AVCN) divisions of the VCN. We found a parallel distribution of GLYT2-immunolabeled fibers and puncta. The data also suggest that, as in rodents, a region containing octopus cells in cats, humans and chimpanzees has little glycinergic input.

Discussion

Our results show that glycine is a major transmitter in the human and chimpanzee CN, despite the species differences in DCN organization. The sources of the glycinergic input to the CN in humans and chimpanzees are not known.

Startle Disease: New Molecular Insights into an Old Neurological Disorder

Startle disease (SD) is characterized by enhanced startle responses, generalized muscle stiffness, unexpected falling, and fatal apnea episodes due to disturbed feedback inhibition in the spinal cord and brainstem of affected individuals. Mutations within the glycine receptor (GlyR) subunit and glycine transporter 2 (GlyT2) genes have been identified in individuals with SD. Impaired inhibitory neurotransmission in SD is due to pre- and/or postsynaptic GlyR or presynaptic GlyT2 dysfunctions. Previous research has focused on mutated GlyRs and GlyT2 that impair ion channel/transporter function or trafficking. With insights provided by recently solved cryo-electron microscopy and X-ray structures of GlyRs, a detailed picture of structural transitions important for receptor gating has emerged, allowing a deeper understanding of SD at the molecular level. Moreover, studies on novel SD mutations have demonstrated a higher complexity of SD, with identification of additional clinical signs and symptoms and interaction partners representing key players for fine-tuning synaptic processes. Although our knowledge has steadily improved during the last years, changes in synaptic localization and GlyR or GlyT2 homeostasis under disease conditions are not yet completely understood. Combined proteomics, interactomics, and high-resolution microscopy techniques are required to reveal alterations in receptor dynamics at the synaptic level under disease conditions.

Conformational transitions and allosteric modulation in a heteromeric glycine receptor

Glycine Receptors (GlyRs) provide inhibitory neuronal input in the spinal cord and brainstem, which is critical for muscle coordination and sensory perception. Synaptic GlyRs are a heteromeric assembly of α and β subunits. Here we present cryo-EM structures of full-length zebrafish α1βBGlyR in the presence of an antagonist (strychnine), agonist (glycine), or agonist with a positive allosteric modulator (glycine/ivermectin). Each structure shows a distinct pore conformation with varying degrees of asymmetry. Molecular dynamic simulations found the structures were in a closed (strychnine) and desensitized states (glycine and glycine/ivermectin). Ivermectin binds at all five interfaces, but in a distinct binding pose at the β-α interface. Subunit-specific features were sufficient to solve structures without a fiduciary marker and to confirm the 4α:1β stoichiometry recently observed. We also report features of the extracellular and intracellular domains. Together, our results show distinct compositional and conformational properties of α1βGlyR and provide a framework for further study of this physiologically important channel.

Spinal cord synaptic plasticity by GlyRβ release from receptor fields and syndapin I-dependent uptake

Glycine receptor-mediated inhibitory neurotransmission is key for spinal cord function. Recent observations suggested that by largely elusive mechanisms also glycinergic synapses display synaptic plasticity. We imaged receptor fields at ultra-high resolution at freeze-fractured membranes, tracked surface and internalized glycine receptors (GlyR) and studied differential regulations of GlyRβ interactions with the scaffold protein gephyrin and the F-BAR domain protein syndapin I and thereby reveal key principles of this process. S403 phosphorylation of GlyRβ, known to be triggered by synaptic signaling, caused a decoupling from gephyrin scaffolds but simultaneously promoted association of syndapin I with GlyRβ. In line, kainate-treatments used to trigger rearrangements of glycine receptors in murine syndapin I KO spinal cords (mixed sex) showed even more severe receptor field fragmentation than already observed in untreated syndapin I KO spinal cords. Syndapin I KO furthermore resulted in more dispersed receptors and increased receptor mobility also pointing out an important contribution of syndapin I in the organization of GlyRβ fields. Strikingly, syndapin I KO also led to a complete disruption of kainate-induced GlyRβ internalization. Accompanying quantitative ultra-high resolution studies in dissociated spinal cord neurons strongly suggested that the observed defects in GlyR internalization observed in syndapin I KO spinal cords are directly caused by syndapin I deficiency within murine spinal cord neurons. Together our results unveiled important mechanisms organizing and altering glycine receptor fields during both steady-state and particularly upon kainate-induced synaptic rearrangement - principles organizing and fine-tuning synaptic efficacy and plasticity of glycinergic synapses in the spinal cord.SIGNIFICANCE STATEMENTInitial observations suggested that also glycinergic synapses - key for spinal cord and brain stem functions - may display some form of synaptic plasticity. Imaging receptor fields at ultra-high resolution at freeze-fractured membranes, tracking surface and internalized glycine receptors (GlyR) and studying regulations of GlyRβ interactions we here reveal key principles of these kainate-inducible adaptations. A switch from gephyrin-mediated receptor scaffolding to syndapin I-mediated GlyRβ scaffolding and internalization allows for modulating synaptic receptor availability. In line, kainate-induced GlyRβ internalization was completely disrupted and GlyRβ receptor fields were distorted upon syndapin I KO. These results unveiled important mechanisms during both steady-state and kainate-induced alterations of synaptic GlyR fields - principles underlying synaptic efficacy and plasticity of synapses in the spinal cord.

Loss, Gain and Altered Function of GlyR α2 Subunit Mutations in Neurodevelopmental Disorders

Glycine receptors (GlyRs) containing the α2 subunit govern cell fate, neuronal migration and synaptogenesis in the developing cortex and spinal cord. Rare missense variants and microdeletions in the X-linked GlyR α2 subunit gene (GLRA2) have been associated with human autism spectrum disorder (ASD), where they typically cause a loss-of-function via protein truncation, reduced cell-surface trafficking and/or reduced glycine sensitivity (e.g., GLRA2Δex8-9 and extracellular domain variants p.N109S and p.R126Q). However, the GlyR α2 missense variant p.R323L in the intracellular M3-M4 domain results in a gain-of-function characterized by slower synaptic decay times, longer duration active periods and increases in channel conductance. This study reports the functional characterization of four missense variants in GLRA2 associated with ASD or developmental disorders (p.V-22L, p.N38K, p.K213E, p.T269M) using a combination of bioinformatics, molecular dynamics simulations, cellular models of GlyR trafficking and electrophysiology in artificial synapses. The GlyR α2^V–22L variant resulted in altered predicted signal peptide cleavage and a reduction in cell-surface expression, suggestive of a partial loss-of-function. Similarly, GlyR α2^N38K homomers showed reduced cell-surface expression, a reduced affinity for glycine and a reduced magnitude of IPSCs in artificial synapses. By contrast, GlyR α2^K213E homomers showed a slight reduction in cell-surface expression, but IPSCs were larger, with faster rise/decay times, suggesting a gain-of-function. Lastly, GlyR α2^T269M homomers exhibited a high glycine sensitivity accompanied by a substantial leak current, suggestive of an altered function that could dramatically enhance glycinergic signaling. These results may explain the heterogeneity of clinical phenotypes associated with GLRA2 mutations and reveal that missense variants can result in a loss, gain or alteration of GlyR α2 function. In turn, these GlyR α2 missense variants are likely to either negatively or positively deregulate cortical progenitor homeostasis and neuronal migration in the developing brain, leading to changes in cognition, learning, and memory.

PACSIN proteins in vivo: Roles in development and physiology

Protein kinase C and casein kinase substrate in neurons (PACSINs), or syndapins (synaptic dynamin‐associated proteins), are a family of proteins involved in the regulation of cell cytoskeleton, intracellular trafficking and signalling. Over the last twenty years, PACSINs have been mostly studied in the in vitro and ex vivo settings, and only in the last decade reports on their function in vivo have emerged. We first summarize the identification, structure and cellular functions of PACSINs, and then focus on the relevance of PACSINs in vivo. During development in various model organisms, PACSINs participate in diverse processes, such as neural crest cell development, gastrulation, laterality development and neuromuscular junction formation. In mouse, PACSIN2 regulates angiogenesis during retinal development and in human, PACSIN2 associates with monosomy and embryonic implantation. In adulthood, PACSIN1 has been extensively studied in the brain and shown to regulate neuromorphogenesis, receptor trafficking and synaptic plasticity. Several genetic studies suggest a role for PACSIN1 in the development of schizophrenia, which is also supported by the phenotype of mice depleted of PACSIN1. PACSIN2 plays an essential role in the maintenance of intestinal homeostasis and participates in kidney repair processes after injury. PACSIN3 is abundant in muscle tissue and necessary for caveolar biogenesis to create membrane reservoirs, thus controlling muscle function, and has been linked to certain genetic muscular disorders. The above examples illustrate the importance of PACSINs in diverse physiological or tissue repair processes in various organs, and associations to diseases when their functions are disturbed.

Rescue of two trafficking-defective variants of the neuronal glycine transporter GlyT2 associated to hyperekplexia

Hyperekplexia is a rare sensorimotor syndrome characterized by pathological startle reflex in response to unexpected trivial stimuli for which there is no specific treatment. Neonates suffer from hypertonia and are at high risk of sudden death due to apnea episodes. Mutations in the human SLC6A5 gene encoding the neuronal glycine transporter GlyT2 may disrupt the inhibitory glycinergic neurotransmission and cause a presynaptic form of the disease. The phenotype of missense mutations giving rise to protein misfolding but maintaining residual activity could be rescued by facilitating folding or intracellular trafficking. In this report, we characterized the trafficking properties of two mutants associated with hyperekplexia (A277T and Y707C, rat numbering). Transporter molecules were partially retained in the endoplasmic reticulum showing increased interaction with the endoplasmic reticulum chaperone calnexin. One transporter variant had export difficulties and increased ubiquitination levels, suggestive of enhanced endoplasmic reticulum-associated degradation. However, the two mutant transporters were amenable to correction by calnexin overexpression. Within the search for compounds capable of rescuing mutant phenotypes, we found that the arachidonic acid derivative N-arachidonoyl glycine can rescue the trafficking defects of the two variants in heterologous cells and rat brain cortical neurons. N-arachidonoyl glycine improves the endoplasmic reticulum output by reducing the interaction transporter/calnexin, increasing membrane expression and improving transport activity in a comparable way as the well-established chemical chaperone 4-phenyl-butyrate. This work identifies N-arachidonoyl glycine as a promising compound with potential for hyperekplexia therapy.

Expanding GABAAR pharmacology via receptor-associated proteins

Drugs directly targeting γ-aminobutyric acid type A receptors (GABA_ARs), the major mediators of fast synaptic inhibition, contribute significantly to today's neuropharmacology. Emerging evidence establishes intracellularly GABA_AR-associated proteins as the central players in determining cellular and subcellular GABAergic input sites, thereby providing pharmacological opportunities to affect distinct receptor populations and address discrete neuronal dysfunctions. Here, we report on recently studied GABA_AR-associated proteins and highlight challenges and newly available methods for their functional and physical mapping. We anticipate these efforts to contribute to decipher the complexity of GABAergic signalling in the brain and eventually enable therapeutic avenues for, so far, untreatable neuronal disorders and diseases.

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.