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

Pleiotropic neurotransmitters: neurotransmitter-receptor crosstalk regulates excitation-inhibition balance in social brain functions and pathologies

Neuronal excitation-inhibition (E/I) balance is essential for maintaining neuronal stability and proper brain functioning. Disruptions in this balance are implicated in various neurological disorders, including autism spectrum disorder, schizophrenia and epilepsy. The E/I balance is thought to be primarily mediated by intrinsic excitability, governed by an array of voltage-gated ion channels, and extrinsic excitability, maintained through a counterbalance between excitatory synaptic transmission primarily mediated by excitatory transmitter glutamate acting on excitatory ion-tropic glutamate receptors and inhibitory synaptic transmissions chiefly mediated by GABA or glycine acting on their respective inhibitory ion-tropic receptors. However, recent studies reveal that neurotransmitters can exhibit interactions that extend beyond their traditional targets, leading to a phenomenon called neurotransmitter-receptor crosstalk. Examples of such crosstalks include earlier discovery of inhibitory glycine functioning as co-transmitter gating on the NMDA subtype of excitatory glutamate receptor, and the most recent demonstration that shows the excitatory glutamate transmitter binds to the inhibitory GABAA receptor, thereby allosterically potentiating its inhibitory function. These studies demonstrate structurally and physiologically important crosstalk between excitatory and inhibitory synaptic transmission, blurring the distinction between the concepts of classic excitatory and inhibitory synaptic transmission. In this article, evidence supporting the forms of excitatory and inhibitory crosstalks will be briefly summarized and their underlying mechanisms will be discussed. Furthermore, this review will discuss the implications of these crosstalks in maintaining the E/I balance, as well as their potential involvement in synaptic plasticity and cognition in the context of social conditions.

The emerging role of glycine receptor α2 subunit defects in neurodevelopmental disorders

Rare neurodevelopmental disorders (NDDs) are one of the most significant unmet challenges in healthcare due to their lifelong nature, high management costs, and recurrence within families. This review will focus on newly-emerging genetic forms of NDDs resulting from variants in the glycine receptor (GlyR) α2 subunit gene. Studies using Glra2 knockout mice have convincingly demonstrated that GlyR α2 is essential for cortical interneuron migration and progenitor homeostasis. Genetic inactivation of GlyR α2 impairs the capacity of apical progenitors to generate basal progenitors, resulting in an overall reduction of projection neurons in the cerebral cortex. As a result, microcephaly is observed in newborn Glra2 knockout mice, as well as defects in neuronal morphology, increased susceptibility to seizures, and defects in novel object recognition, motor memory consolidation, righting reflexes, novelty-induced locomotion in the open field test, and motivational reward tasks. Consistent with these findings, we and others have identified missense variants and microdeletions in the human GlyR α2 subunit gene (GLRA2) in individuals with autism spectrum disorder (ASD), developmental delay (DD) and/or intellectual disability (ID), often accompanied by microcephaly, language delay and epilepsy. In this review, we highlight the critical role of the GlyR α2 subunit revealed by knockout mice and our current understanding of GlyR α2 pathomechanisms in human NDDs. Finally, we will consider the current gaps in our knowledge, which include: (i) Limited functional validation for GlyR α2 missense variants associated with human NDDs; (ii) The lack of gain-of-function GlyR α2 mouse models; (iii) Our limited knowledge of GlyR α2 interacting proteins. We also highlight potential future developments in the field, including routes to personalized medicines for individuals with GlyR α2 mutations.

A Pharmacophore for Drugs Targeting the α4α4 Binding Site of the (α4)3(β2)2 Nicotinic Acetylcholine Receptor

Neuronal nicotinic acetylcholine receptors (nAChRs) have an established role in pain pathways and devastating neurodegenerative diseases; however, few drugs have been successfully developed to target them. The most abundant nAChR in the brain, the α4β2 nAChR, is assembled from five subunits in a 3α:2β stoichiometry-(α4)3(β2)2. This receptor contains a unique agonist-binding site at the α4α4 interface in addition to two classical agonist-binding sites at α4β2 interfaces. Most known agonists target both α4α4 and α4β2 sites, however, a few compounds with selectivity for the α4α4 site have been identified. These α4α4 selective compounds have a modulator-like effect akin to benzodiazepines in the γ-aminobutyric acid type A receptor, which is desirable from a drug development perspective. The two most well characterised α4α4 selective compounds are CMPI and NS9283. Both are structurally very different from classical agonists, and it is puzzling how they occupy the same binding site. In the search for a common pharmacophore, we conducted extensive molecular dynamics simulations with both classical agonists and site-selective non-classical compounds. Analyses of the simulations revealed that the α4α4 binding site contains a unique pocket not found in the α4β2 binding site. CMPI and NS9283 were observed to bind in this pocket, thereby explaining why they are selective for the α4α4 binding site. The proposed binding mode featured a closed-loop C conformation, which is strongly correlated with agonism in nAChRs and explained key site-directed mutagenesis data for both compounds. Based on this binding mode, we proposed a pharmacophore for drugs targeting the α4α4 binding site. The proposed pharmacophore captures the essence of the original model, that is, nicotinic agonists act as a bridge between protein subunits. The pharmacophore model we propose is unique to the α4α4 binding site and provides a template for developing new site-selective therapeutic agents.

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.

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.

Whole exome sequencing revealed variants in four genes underlying X-linked intellectual disability in four Iranian families: novel deleterious variants and clinical features with the review of literature

AIM AND OBJECTIVE

Intellectual disability (ID) is a heterogeneous condition affecting brain development, function, and/or structure. The X-linked mode of inheritance of ID (X-linked intellectual disability; XLID) has a prevalence of 1 out of 600 to 1000 males. In the last decades, exome sequencing technology has revolutionized the process of disease-causing gene discovery in XLIDs. Nevertheless, so many of them still remain with unknown etiology. This study investigated four families with severe XLID to identify deleterious variants for possible diagnostics and prevention aims.

METHODS

Nine male patients belonging to four pedigrees were included in this study. The patients were studied genetically for Fragile X syndrome, followed by whole exome sequencing and analysis of intellectual disability-related genes variants. Sanger sequencing, co-segregation analysis, structural modeling, and in silico analysis were done to verify the causative variants. In addition, we collected data from previous studies to compare and situate our work with existing knowledge.

RESULTS

In three of four families, novel deleterious variants have been identified in three different genes, including ZDHHC9 (p. Leu189Pro), ATP2B3 (p. Asp847Glu), and GLRA2 (p. Arg350Cys) and also with new clinical features and in another one family, a reported pathogenic variant in the L1CAM (p. Glu309Lys) gene has been identified related to new clinical findings.

CONCLUSION

The current study's findings expand the existing knowledge of variants of the genes implicated in XLID and broaden the spectrum of phenotypes associated with the related conditions. The data have implications for genetic diagnosis and counseling.

Age-Related microRNA Overexpression in Lafora Disease Male Mice Provides Links between Neuroinflammation and Oxidative Stress

Lafora disease is a rare, fatal form of progressive myoclonus epilepsy characterized by continuous neurodegeneration with epileptic seizures, characterized by the intracellular accumulation of aberrant polyglucosan granules called Lafora bodies. Several works have provided numerous evidence of molecular and cellular alterations in neural tissue from experimental mouse models deficient in either laforin or malin, two proteins related to the disease. Oxidative stress, alterations in proteostasis, and deregulation of inflammatory signals are some of the molecular alterations underlying this condition in both KO animal models. Lafora bodies appear early in the animal's life, but many of the aforementioned molecular aberrant processes and the consequent neurological symptoms ensue only as animals age. Here, using small RNA-seq and quantitative PCR on brain extracts from laforin and malin KO male mice of different ages, we show that two different microRNA species, miR-155 and miR-146a, are overexpressed in an age-dependent manner. We also observed altered expression of putative target genes for each of the microRNAs studied in brain extracts. These results open the path for a detailed dissection of the molecular consequences of laforin and malin deficiency in brain tissue, as well as the potential role of miR-155 and miR-146a as specific biomarkers of disease progression in LD.

Systematic analysis and prediction of genes associated with monogenic disorders on human chromosome X

Disease gene discovery on chromosome (chr) X is challenging owing to its unique modes of inheritance. We undertook a systematic analysis of human chrX genes. We observe a higher proportion of disorder-associated genes and an enrichment of genes involved in cognition, language, and seizures on chrX compared to autosomes. We analyze gene constraints, exon and promoter conservation, expression, and paralogues, and report 127 genes sharing one or more attributes with known chrX disorder genes. Using machine learning classifiers trained to distinguish disease-associated from dispensable genes, we classify 247 genes, including 115 of the 127, as having high probability of being disease-associated. We provide evidence of an excess of variants in predicted genes in existing databases. Finally, we report damaging variants in CDK16 and TRPC5 in patients with intellectual disability or autism spectrum disorders. This study predicts large-scale gene-disease associations that could be used for prioritization of X-linked pathogenic variants.

Glycine neurotransmission: Its role in development

The accurate function of the central nervous system (CNS) depends of the consonance of multiple genetic programs and external signals during the ontogenesis. A variety of molecules including neurotransmitters, have been implied in the regulation of proliferation, survival, and cell-fate of neurons and glial cells. Among these, neurotransmitters may play a central role since functional ligand-gated ionic channel receptors have been described before the establishment of synapses. This review argues on the function of glycine during development, and show evidence indicating it regulates morphogenetic events by means of their transporters and receptors, emphasizing the role of glycinergic activity in the balance of excitatory and inhibitory signals during development. Understanding the mechanisms involved in these processes would help us to know the etiology of cognitive dysfunctions and lead to improve brain repair strategies.

"Virtual patch clamp analysis" for predicting the functional significance of pathogenic variants in sodium channels

OBJECTIVE

Opening of voltage-gated sodium channels is crucial for neuronal depolarization. Proper channel opening and influx of Na⁺ through the ion pore, is dependent upon binding of Na⁺ ion to a specific amino-acid motif (DEKA) within the pore. In this study we used molecular dynamic simulations, an advanced bioinformatic tool, to research the dysfunction caused by pathogenic variants in SCN1a, SCN2a and SCN8a genes.

METHOD

Molecular dynamic simulations were performed in six patients: three patients with Dravet syndrome (p.Gly177Ala,p.Ser259Arg and p.Met1267Ile, SCN1a), two patients with early onset drug resistant epilepsy(p.Ala263Val, SCN2a and p.Ile251Arg, SCN8a), and a patient with autism (p.Thr155Ala, SCN2a). After predicting the 3D-structure of mutated proteins by homology modeling, time dependent molecular dynamic simulations were performed, using the Schrödinger algorithm. The opening of the sodium channel, including the detachment of the sodium ion to the DEKA motif and pore diameter were assessed. Results were compared to the existent patch clamp analysis in four patients, and consistency with clinical phenotype was noted.

RESULTS

The Na⁺ ion remained attached to DEKA filter longer when compared to wild type in the p.Gly177Ala, p.Ser259Arg,SCN1a, and p.Thr155Ala, SCN2a variants, consistent with loss-of-function. In contrast, it detached quicker from DEKA than wild type in the p.Ala263Val,SCN2a variant, consistent with gain-of-function. In the p.Met1267Ile,SCN1a variant, detachment from DEKA was quicker, but pore diameter decreased, suggesting partial loss-of-function. In the p.Leu251Arg,SCN8a variant, the pore remained opened longer when compared to wild type, consistent with a gain-of-function. The molecular dynamic simulation results were consistent with the existing patch-clamp analysis studies, as well as the clinical phenotype.

SIGNIFICANCE

Molecular dynamic simulation can be useful in predicting pathogenicity of variants and the disease phenotype, and selecting targeted treatment based on channel dysfunction. Further development of these bioinformatic tools may lead to "virtual patch-clamp analysis".