Two molecular pathways (NMD and ERAD) contribute to a genetic epilepsy associated with the GABA(A) receptor GABRA1 PTC mutation, 975delC, S326fs328X

Modulating Endoplasmic Reticulum Chaperones and Mutant Protein Degradation in GABRG2(Q390X) Associated with Genetic Epilepsy with Febrile Seizures Plus and Dravet Syndrome

A significant number of patients with genetic epilepsy do not obtain seizure freedom, despite developments in new antiseizure drugs, suggesting a need for novel therapeutic approaches. Many genetic epilepsies are associated with misfolded mutant proteins, including GABRG2(Q390X)-associated Dravet syndrome, which we have previously shown to result in intracellular accumulation of mutant GABAA receptor γ2(Q390X) subunit protein. Thus, a potentially promising therapeutic approach is modulation of proteostasis, such as increasing endoplasmic reticulum (ER)-associated degradation (ERAD). To that end, we have here identified an ERAD-associated E3 ubiquitin ligase, HRD1, among other ubiquitin ligases, as a strong modulator of wildtype and mutant γ2 subunit expression. Overexpressing HRD1 or knockdown of HRD1 dose-dependently reduced the γ2(Q390X) subunit. Additionally, we show that zonisamide (ZNS)-an antiseizure drug reported to upregulate HRD1-reduces seizures in the Gabrg2+/Q390X mouse. We propose that a possible mechanism for this effect is a partial rescue of surface trafficking of GABAA receptors, which are otherwise sequestered in the ER due to the dominant-negative effect of the γ2(Q390X) subunit. Furthermore, this partial rescue was not due to changes in ER chaperones BiP and calnexin, as total expression of these chaperones was unchanged in γ2(Q390X) models. Our results here suggest that leveraging the endogenous ERAD pathway may present a potential method to degrade neurotoxic mutant proteins like the γ2(Q390X) subunit. We also demonstrate a pharmacological means of regulating proteostasis, as ZNS alters protein trafficking, providing further support for the use of proteostasis regulators for the treatment of genetic epilepsies.

Astrocytic GABA transporter 1 deficit in novel SLC6A1 variants mediated epilepsy: Connected from protein destabilization to seizures in mice and humans

OBJECTIVE

Mutations in γ-aminobutyric acid (GABA) transporter 1 (GAT-1)-encoding SLC6A1 have been associated with myoclonic atonic epilepsy and other phenotypes. We determined the patho-mechanisms of the mutant GAT-1, in order to identify treatment targets.

METHODS

We conducted whole-exome sequencing of patients with myoclonic atonic epilepsy (MAE) and characterized the seizure phenotypes and EEG patterns. We studied the protein stability and structural changes with homology modeling and machine learning tools. We characterized the function and trafficking of the mutant GAT-1 with 3H radioactive GABA uptake assay and confocal microscopy. We utilized different models including a knockin mouse and human astrocytes derived from induced pluripotent stem cells (iPSCs). We focused on astrocytes because of their direct impact of astrocytic GAT-1 in seizures.

RESULTS

We identified four novel SLC6A1 variants associated with MAE and 2 to 4 Hz spike-wave discharges as a common EEG feature. Machine learning tools predicted that the variant proteins are destabilized. The variant protein had reduced expression and reduced GABA uptake due to endoplasmic reticular retention. The consistent observation was made in cortical and thalamic astrocytes from variant-knockin mice and human iPSC-derived astrocytes. The Slc6a+/A288V mouse, representative of MAE, had increased 5-7 Hz spike-wave discharges and absence seizures.

INTERPRETATION

SLC6A1 variants in various locations of the protein peptides can cause MAE with similar seizure phenotypes and EEG features. Reduced GABA uptake is due to decreased functional GAT-1, which, in thalamic astrocytes, could result in increased extracellular GABA accumulation and enhanced tonic inhibition, leading to seizures and abnormal EEGs.

4-Phenylbutyrate restored γ-aminobutyric acid uptake and reduced seizures in SLC6A1 patient variant-bearing cell and mouse models

We have studied the molecular mechanisms of variants in solute carrier Family 6 Member 1 associated with neurodevelopmental disorders, including various epilepsy syndromes, autism and intellectual disability. Based on functional assays of solute carrier Family 6 Member 1 variants, we conclude that partial or complete loss of γ-amino butyric acid uptake due to reduced membrane γ-amino butyric acid transporter 1 trafficking is the primary aetiology. Importantly, we identified common patterns of the mutant γ-amino butyric acid transporter 1 protein trafficking from biogenesis, oligomerization, glycosylation and translocation to the cell membrane across variants in different cell types such as astrocytes and neurons. We hypothesize that therapeutic approaches to facilitate membrane trafficking would increase γ-amino butyric acid transporter 1 protein membrane expression and function. 4-Phenylbutyrate is a Food and Drug Administration-approved drug for paediatric use and is orally bioavailable. 4-Phenylbutyrate shows promise in the treatment of cystic fibrosis. The common cellular mechanisms shared by the mutant γ-amino butyric acid transporter 1 and cystic fibrosis transmembrane conductance regulator led us to hypothesize that 4-phenylbutyrate could be a potential treatment option for solute carrier Family 6 Member 1 mutations. We examined the impact of 4-phenylbutyrate across a library of variants in cell and knockin mouse models. Because γ-amino butyric acid transporter 1 is expressed in both neurons and astrocytes, and γ-amino butyric acid transporter 1 deficiency in astrocytes has been hypothesized to underlie seizure generation, we tested the effect of 4-phenylbutyrate in both neurons and astrocytes with a focus on astrocytes. We demonstrated existence of the mutant γ-amino butyric acid transporter 1 retaining wildtype γ-amino butyric acid transporter 1, suggesting the mutant protein causes aberrant protein oligomerization and trafficking. 4-Phenylbutyrate increased γ-amino butyric acid uptake in both mouse and human astrocytes and neurons bearing the variants. Importantly, 4-phenylbutyrate alone increased γ-amino butyric acid transporter 1 expression and suppressed spike wave discharges in heterozygous knockin mice. Although the mechanisms of action for 4-phenylbutyrate are still unclear, with multiple possibly being involved, it is likely that 4-phenylbutyrate can facilitate the forward trafficking of the wildtype γ-amino butyric acid transporter 1 regardless of rescuing the mutant γ-amino butyric acid transporter 1, thus increasing γ-amino butyric acid uptake. All patients with solute carrier Family 6 Member 1 variants are heterozygous and carry one wildtype allele, suggesting a great opportunity for treatment development leveraging wildtype protein trafficking. The study opens a novel avenue of treatment development for genetic epilepsy via drug repurposing.

The Physiological Roles of the Exon Junction Complex in Development and Diseases

The exon junction complex (EJC) becomes an increasingly important regulator of early gene expression in the central nervous system (CNS) and other tissues. The EJC is comprised of three core proteins: RNA-binding motif 8A (RBM8A), Mago homolog (MAGOH), eukaryotic initiation factor 4A3 (EIF4A3), and a peripheral EJC factor, metastatic lymph node 51 (MLN51), together with various auxiliary factors. The EJC is assembled specifically at exon-exon junctions on mRNAs, hence the name of the complex. The EJC regulates multiple levels of gene expression, from splicing to translation and mRNA degradation. The functional roles of the EJC have been established as crucial to the normal progress of embryonic and neurological development, with wide ranging implications on molecular, cellular, and organism level function. Dysfunction of the EJC has been implicated in multiple developmental and neurological diseases. In this review, we discuss recent progress on the EJC’s physiological roles.

Nonsense-mediated mRNA decay uses complementary mechanisms to suppress mRNA and protein accumulation

Nonsense-mediated mRNA decay (NMD) is an essential, highly conserved quality control pathway that detects and degrades mRNAs containing premature termination codons. Although the essentiality of NMD is frequently ascribed to its prevention of truncated protein accumulation, the extent to which NMD actually suppresses proteins encoded by NMD-sensitive transcripts is less well-understood than NMD-mediated suppression of mRNA. Here, we describe a reporter system that permits accurate quantification of both mRNA and protein levels via stable integration of paired reporters encoding NMD-sensitive and NMD-insensitive transcripts into the AAVS1 safe harbor loci in human cells. We use this system to demonstrate that NMD suppresses proteins encoded by NMD-sensitive transcripts by up to eightfold more than the mRNA itself. Our data indicate that NMD limits the accumulation of proteins encoded by NMD substrates by mechanisms beyond mRNA degradation, such that even when NMD-sensitive mRNAs escape destruction, their encoded proteins are still effectively suppressed.

Cellular and molecular mechanisms underlying autism spectrum disorders and associated comorbidities: A pathophysiological review

Autism spectrum disorders (ASD) are a group of neurodevelopmental disorders that develop in early life due to interaction between several genetic and environmental factors and lead to alterations in brain function and structure. During the last decades, several mechanisms have been placed to explain the pathogenesis of autism. Unfortunately, these are reported in several studies and reviews which make it difficult to follow by the reader. In addition, some recent molecular mechanisms related to ASD have been unrevealed. This paper revises and highlights the major common molecular mechanisms responsible for the clinical symptoms seen in people with ASD, including the roles of common genetic factors and disorders, neuroinflammation, GABAergic signaling, and alterations in Ca⁺² signaling. Besides, it covers the major molecular mechanisms and signaling pathways involved in initiating the epileptic seizure, including the alterations in the GABAergic and glutamate signaling, vitamin and mineral deficiency, disorders of metabolism, and autoimmunity. Finally, this review also discusses sleep disorder patterns and the molecular mechanisms underlying them.

Common molecular mechanisms of SLC6A1 variant-mediated neurodevelopmental disorders in astrocytes and neurons

Solute carrier family 6 member 1 (SLC6A1) is abundantly expressed in the developing brain even before the CNS is formed. Its encoded GABA transporter 1 (GAT-1) is responsible for the reuptake of GABA into presynaptic neurons and glia, thereby modulating neurotransmission. GAT-1 is expressed globally in the brain, in both astrocytes and neurons. The GABA uptake function of GAT-1 in neurons cannot be compensated for by other GABA transporters, while the function in glia can be partially replaced by GABA transporter 3. Recently, many variants in SLC6A1 have been associated with a spectrum of epilepsy syndromes and neurodevelopmental disorders, including myoclonic atonic epilepsy, childhood absence epilepsy, autism, and intellectual disability, but the pathomechanisms associated with these phenotypes remain unclear. The presence of GAT-1 in both neurons and astrocytes further obscures the role of abnormal GAT-1 in the heterogeneous disease phenotype manifestations. Here we examine the impact on transporter trafficking and function of 22 SLC6A1 variants identified in patients with a broad spectrum of phenotypes. We also evaluate changes in protein expression and subcellular localization of the variant GAT-1 in various cell types, including neurons and astrocytes derived from human patient induced pluripotent stem cells. We found that a partial or complete loss-of-function represents a common disease mechanism, although the extent of GABA uptake reduction is variable. The reduced GABA uptake appears to be due to reduced cell surface expression of the variant transporter caused by variant protein misfolding, endoplasmic reticulum retention, and subsequent degradation. Although the extent of reduction of the total protein, surface protein, and the GABA uptake level of the variant transporters is variable, the loss of GABA uptake function and endoplasmic reticulum retention is consistent across induced pluripotent stem cell-derived cell types, including astrocytes and neurons, for the surveyed variants. Interestingly, we did not find a clear correlation of GABA uptake function and the disease phenotypes, such as myoclonic atonic epilepsy versus developmental delay, in this study. Together, our study suggests that impaired transporter protein trafficking and surface expression are the major disease-associated mechanisms associated with pathogenic SLC6A1 variants. Our results resemble findings from pathogenic variants in other genes affecting the GABA pathway, such as GABAA receptors. This study provides critical insight into therapeutic developments for SLC6A1 variant-mediated disorders and implicates that boosting transporter function by either genetic or pharmacological approaches would be beneficial.

Genetic mosaicism, intrafamilial phenotypic heterogeneity, and molecular defects of a novel missense SLC6A1 mutation associated with epilepsy and ADHD

BACKGROUND

Mutations in SLC6A1, encoding γ-aminobutyric acid (GABA) transporter 1 (GAT-1), have been recently associated with a spectrum of neurodevelopmental disorders ranging from variable epilepsy syndromes, intellectual disability (ID), autism and others. To date, most identified mutations are de novo. We here report a pedigree of two siblings associated with myoclonic astatic epilepsy, attention deficit hyperactivity disorder (ADHD), and ID.

METHODS

Next-generation sequencing identified a missense mutation in the SLC6A1 gene (c.373G > A(p.Val125Met)) in the sisters but not in their shared mother who is also asymptomatic, suggesting gonadal mosaicism. We have thoroughly characterized the clinical phenotypes: EEG recordings identified features for absence seizures and prominent bursts of occipital intermittent rhythmic delta activity (OIRDA). The molecular pathophysiology underlying the clinical phenotypes was assessed using a multidisciplinary approach including machine learning, confocal microscopy, and high-throughput ³H radio-labeled GABA uptake assays in mouse astrocytes and neurons.

RESULTS

The GAT-1(Val125Met) mutation destabilizes the global protein conformation and reduces transporter protein expression at total and cell surface. The mutant transporter protein was localized intracellularly inside the endoplasmic reticulum (ER) in both HEK293T cells and astrocytes which may directly contribute to seizures in patients. Radioactive ³H-labeled GABA uptake assay indicated the mutation reduced the function of the mutant GAT-1(Val125Met) to ~30% of the wildtype.

CONCLUSIONS

The seizure phenotypes, ADHD, and impaired cognition are likely caused by a partial loss-of-function of GAT-1 due to protein destabilization resulting from the mutation. Reduced GAT-1 function in astrocytes and neurons may consequently alter brain network activities such as increased seizures and reduced attention.

Endoplasmic reticulum stress increases inflammatory cytokines in an epilepsy mouse model Gabrg2+/Q390X knockin: A link between genetic and acquired epilepsy?

OBJECTIVE

Neuroinflammation is a major theme in epilepsy, which has been characterized in acquired epilepsy but is poorly understood in genetic epilepsy. γ-Aminobutyric acid type A receptor subunit gene mutations are significant causes of epilepsy, and we have studied the pathophysiology directly resulting from defective receptor channels. Here, we determined the proinflammatory factors in a genetic mouse model, the Gabrg2^+/Q390X knockin (KI). We have identified increased cytokines in multiple brain regions of the KI mouse throughout different developmental stages and propose that accumulation of the trafficking-deficient mutant protein may increase neuroinflammation, which would be a novel mechanism for genetic epilepsy.

METHODS

We used enzyme-linked immunosorbent assay, immunoprecipitation, nuclei purification, immunoblot, immunohistochemistry, and confocal microscopy to characterize increased neuroinflammation and its potential causes in a Gabrg2^+/Q390X KI mouse and a Gabrg2^+/- knockout (KO) mouse, each associated with a different epilepsy syndrome with different severities.

RESULTS

We found that proinflammatory cytokines such as tumor necrosis factor alpha, interleukin 1-beta (IL-1β), and IL-6 were increased in the KI mice but not in the KO mice. A major underlying basis for the discrepancy in cytokine expression between the two mouse models is likely chronic mutant protein accumulation and endoplasmic reticulum (ER) stress. The presence of mutant protein dampened cytokine induction upon further cellular stimulation or external stress such as elevated temperature. Pharmacological induction of ER stress upregulated cytokine expression in the wild-type and KO but not in the KI mice. The increased cytokine expression was independent of seizure occurrence, because it was upregulated in both mice and cultured neurons.

SIGNIFICANCE

Together, these data demonstrate a novel pathophysiology for genetic epilepsy, increased neuroinflammation, which is a common mechanism for acquired epilepsy. The findings thus provide the first link of neuroinflammation between genetic epilepsy associated with an ion channel gene mutation and acquired epilepsy.

Endoplasmic reticulum retention and degradation of a mutation in SLC6A1 associated with epilepsy and autism

Mutations in SLC6A1, encoding γ-aminobutyric acid (GABA) transporter 1 (GAT-1), have been recently associated with a spectrum of epilepsy syndromes, intellectual disability and autism in clinic. However, the pathophysiology of the gene mutations is far from clear. Here we report a novel SLC6A1 missense mutation in a patient with epilepsy and autism spectrum disorder and characterized the molecular defects of the mutant GAT-1, from transporter protein trafficking to GABA uptake function in heterologous cells and neurons. The heterozygous missense mutation (c1081C to A (P361T)) in SLC6A1 was identified by exome sequencing. We have thoroughly characterized the molecular pathophysiology underlying the clinical phenotypes. We performed EEG recordings and autism diagnostic interview. The patient had neurodevelopmental delay, absence epilepsy, generalized epilepsy, and 2.5–3 Hz generalized spike and slow waves on EEG recordings. The impact of the mutation on GAT-1 function and trafficking was evaluated by ³H GABA uptake, structural simulation with machine learning tools, live cell confocal microscopy and protein expression in mouse neurons and nonneuronal cells. We demonstrated that the GAT-1(P361T) mutation destabilizes the global protein conformation and reduces total protein expression. The mutant transporter protein was localized intracellularly inside the endoplasmic reticulum (ER) with a pattern of expression very similar to the cells treated with tunicamycin, an ER stress inducer. Radioactive ³H-labeled GABA uptake assay indicated the mutation reduced the function of the mutant GAT-1(P361T), to a level that is similar to the cells treated with GAT-1 inhibitors. In summary, this mutation destabilizes the mutant transporter protein, which results in retention of the mutant protein inside cells and reduction of total transporter expression, likely via excessive endoplasmic reticulum associated degradation. This thus likely causes reduced functional transporter number on the cell surface, which then could cause the observed reduced GABA uptake function. Consequently, malfunctioning GABA signaling may cause altered neurodevelopment and neurotransmission, such as enhanced tonic inhibition and altered cell proliferation in vivo. The pathophysiology due to severely impaired GAT-1 function may give rise to a wide spectrum of neurodevelopmental phenotypes including autism and epilepsy.

Synaptic clustering differences due to different GABRB3 mutations cause variable epilepsy syndromes

GABRB3 is highly expressed early in the developing brain, and its encoded β3 subunit is critical for GABAA receptor assembly and trafficking as well as stem cell differentiation in embryonic brain. To date, over 400 mutations or variants have been identified in GABRB3. Mutations in GABRB3 have been increasingly recognized as a major cause for severe paediatric epilepsy syndromes such as Lennox-Gastaut syndrome, Dravet syndrome and infantile spasms with intellectual disability as well as relatively mild epilepsy syndromes such as childhood absence epilepsy. There is no plausible molecular pathology for disease phenotypic heterogeneity. Here we used a very high-throughput flow cytometry assay to evaluate the impact of multiple human mutations in GABRB3 on receptor trafficking. In this study we found that surface expression of mutant β3 subunits is variable. However, it was consistent that surface expression of partnering γ2 subunits was lower when co-expressed with mutant than with wild-type subunits. Because γ2 subunits are critical for synaptic GABAA receptor clustering, this provides an important clue for understanding the pathophysiology of GABRB3 mutations. To validate our findings further, we obtained an in-depth comparison of two novel mutations [GABRB3 (N328D) and GABRB3 (E357K)] associated with epilepsy with different severities of epilepsy phenotype. GABRB3 (N328D) is associated with the relatively severe Lennox-Gastaut syndrome, and GABRB3 (E357K) is associated with the relatively mild juvenile absence epilepsy syndrome. With functional characterizations in both heterologous cells and rodent cortical neurons by patch-clamp recordings, confocal microscopy and immunoblotting, we found that both the GABRB3 (N328D) and GABRB3 (E357K) mutations reduced total subunit expression in neurons but not in HEK293T cells. Both mutant subunits, however, were reduced on the cell surface and in synapses, but the Lennox-Gastaut syndrome mutant β3 (N328D) subunit was more reduced than the juvenile absence epilepsy mutant β3 (E357K) subunit. Interestingly, both mutant β3 subunits impaired postsynaptic clustering of wild-type GABAA receptor γ2 subunits and prevented γ2 subunits from incorporating into GABAA receptors at synapses, although by different cellular mechanisms. Importantly, wild-type γ2 subunits were reduced and less clustered at inhibitory synapses in Gabrb3+/- knockout mice. This suggests that impaired receptor localization to synapses is a common pathophysiological mechanism for GABRB3 mutations, although the extent of impairment may be different among mutant subunits. The study thus identifies the novel mechanism of impaired targeting of receptors containing mutant β3 subunits and provides critical insights into understanding how GABRB3 mutations produce severe epilepsy syndromes and epilepsy phenotypic heterogeneity.

Chaperoning Endoplasmic Reticulum-Associated Degradation (ERAD) and Protein Conformational Diseases

Misfolded proteins compromise cellular homeostasis. This is especially problematic in the endoplasmic reticulum (ER), which is a high-capacity protein-folding compartment and whose function requires stringent protein quality-control systems. Multiprotein complexes in the ER are able to identify, remove, ubiquitinate, and deliver misfolded proteins to the 26S proteasome for degradation in the cytosol, and these events are collectively termed ER-associated degradation, or ERAD. Several steps in the ERAD pathway are facilitated by molecular chaperone networks, and the importance of ERAD is highlighted by the fact that this pathway is linked to numerous protein conformational diseases. In this review, we discuss the factors that constitute the ERAD machinery and detail how each step in the pathway occurs. We then highlight the underlying pathophysiology of protein conformational diseases associated with ERAD.

A missense mutation in SLC6A1 associated with Lennox-Gastaut syndrome impairs GABA transporter 1 protein trafficking and function

BACKGROUND

Mutations in SLC6A1 have been associated mainly with myoclonic atonic epilepsy (MAE) and intellectual disability. We identified a novel missense mutation in a patient with Lennox-Gastaut syndrome (LGS) characterized by severe seizures and developmental delay.

METHODS

Exome Sequencing was performed in an epilepsy patient cohort. The impact of the mutation was evaluated by ³H γ-aminobutyric acid (GABA) uptake, structural modeling, live cell microscopy, cell surface biotinylation and a high-throughput assay flow cytometry in both neurons and non neuronal cells.

RESULTS

We discovered a heterozygous missense mutation (c700G to A [pG234S) in the SLC6A1 encoding GABA transporter 1 (GAT-1). Structural modeling suggests the mutation destabilizes the global protein conformation. With transient expression of enhanced yellow fluorescence protein (YFP) tagged rat GAT-1 cDNAs, we demonstrated that the mutant GAT-1(G234S) transporter had reduced total protein expression in both rat cortical neurons and HEK 293 T cells. With a high-throughput flow cytometry assay and live cell surface biotinylation, we demonstrated that the mutant GAT-1(G234S) had reduced cell surface expression. ³H radioactive labeling GABA uptake assay in HeLa cells indicated a reduced function of the mutant GAT-1(G234S).

CONCLUSIONS

This mutation caused instability of the mutant transporter protein, which resulted in reduced cell surface and total protein levels. The mutation also caused reduced GABA uptake in addition to reduced protein expression, leading to reduced GABA clearance, and altered GABAergic signaling in the brain. The impaired trafficking and reduced GABA uptake function may explain the epilepsy phenotype in the patient.

Capitalizing on the heterogeneous effects of CFTR nonsense and frameshift variants to inform therapeutic strategy for cystic fibrosis

CFTR modulators have revolutionized the treatment of individuals with cystic fibrosis (CF) by improving the function of existing protein. Unfortunately, almost half of the disease-causing variants in CFTR are predicted to introduce premature termination codons (PTC) thereby causing absence of full-length CFTR protein. We hypothesized that a subset of nonsense and frameshift variants in CFTR allow expression of truncated protein that might respond to FDA-approved CFTR modulators. To address this concept, we selected 26 PTC-generating variants from four regions of CFTR and determined their consequences on CFTR mRNA, protein and function using intron-containing minigenes expressed in 3 cell lines (HEK293, MDCK and CFBE41o-) and patient-derived conditionally reprogrammed primary nasal epithelial cells. The PTC-generating variants fell into five groups based on RNA and protein effects. Group A (reduced mRNA, immature (core glycosylated) protein, function <1% (n = 5)) and Group B (normal mRNA, immature protein, function <1% (n = 10)) variants were unresponsive to modulator treatment. However, Group C (normal mRNA, mature (fully glycosylated) protein, function >1% (n = 5)), Group D (reduced mRNA, mature protein, function >1% (n = 5)) and Group E (aberrant RNA splicing, mature protein, function > 1% (n = 1)) variants responded to modulators. Increasing mRNA level by inhibition of NMD led to a significant amplification of modulator effect upon a Group D variant while response of a Group A variant was unaltered. Our work shows that PTC-generating variants should not be generalized as genetic 'nulls' as some may allow generation of protein that can be targeted to achieve clinical benefit.

Prefrontal cortex-dependent innate behaviors are altered by selective knockdown of Gad1 in neuropeptide Y interneurons

GABAergic dysfunction has been implicated in a variety of neurological and psychiatric disorders, including anxiety disorders. Anxiety disorders are the most common type of psychiatric disorder during adolescence. There is a deficiency of GABAergic transmission in anxiety, and enhancement of GABA transmission through pharmacological means reduces anxiety behaviors. GAD67—the enzyme responsible for GABA production–has been linked to anxiety disorders. One class of GABAergic interneurons, Neuropeptide Y (NPY) expressing cells, is abundantly found in brain regions associated with anxiety and fear learning, including prefrontal cortex, hippocampus and amygdala. Additionally, NPY itself has been shown to have anxiolytic effects, and loss of NPY+ interneurons enhances anxiety behaviors. A previous study showed that knockdown of Gad1 from NPY+ cells led to reduced anxiety behaviors in adult mice. However, the role of GABA release from NPY+ interneurons in adolescent anxiety is unclear. Here we used a transgenic mouse that reduces GAD67 in NPY+ cells (NPYGAD1-TG) through Gad1 knockdown and tested for effects on behavior in adolescent mice. Adolescent NPYGAD1-TG mice showed enhanced anxiety-like behavior and sex-dependent changes in locomotor activity. We also found enhancement in two other innate behavioral tasks, nesting construction and social dominance. In contrast, fear learning was unchanged. Because we saw changes in behavioral tasks dependent upon prefrontal cortex and hippocampus, we investigated the extent of GAD67 knockdown in these regions. Immunohistochemistry revealed a 40% decrease in GAD67 in NPY+ cells in prefrontal cortex, indicating a significant but incomplete knockdown of GAD67. In contrast, there was no decrease in GAD67 in NPY+ cells in hippocampus. Consistent with this, there was no change in inhibitory synaptic transmission in hippocampus. Our results show the behavioral impact of cell-specific interneuron dysfunction and suggest that GABA release by NPY+ cells is important for regulating innate prefrontal cortex-dependent behavior in adolescents.

Isolation and characterization of porcine PILRB gene and its alternative splicing variants

Paired immunoglobulin-like type 2 receptor (PILR)β regulates inflammatory responses to pathogen infection, and therefore plays an important role in host disease resistance/susceptibility. However porcine PILRβ remains poorly characterized. In this study, we obtained the cDNA (V1) of its encoding gene, PILRB, and three alternative splicing (AS) variants (V2-4). The complete coding sequence of V1 was 621 bp long encoding a polypeptide of 206 aa. Compared with V1, V2 and V3 were formed by exon-skipping in the 3'-untranslated region (UTR), while V4 was formed by alternative 3' splice site of exon 3, resulting in a premature termination codon, combined with exon skipping in the 3'-UTR. Expression profile analysis showed that all the isoforms were most abundant in the spleen, and V1 was strongly induced by poly(I:C). Furthermore, the transcription of V1 altered with the increasing age and differed between species. Exon skipping in the 3'-UTR of V2 and V3 down-regulated expression of the luciferase reporter gene, and hence presumably of the PILRB gene, while V4 was subjected to nonsense-mediated mRNA decay. Additionally, five novel splicing patterns were detected using the minigene approach, indicating complex AS of porcine PILRB. These results will help to reveal the role of PILRβ in the host immune response using pig models, and will facilitate the breeding of pigs resistant to viral diseases through molecular breeding methods.

SRR intronic variation inhibits expression of its neighbouring SMG6 gene and protects against temporal lobe epilepsy

D‐serine is a predominant N‐methyl‐D‐aspartate receptor co‐agonist with glutamate, and excessive activation of the receptor plays a substantial role in epileptic seizures. Serine racemase (SRR) is responsible for transforming L‐serine to D‐serine. In this study, we aimed to investigate the genetic roles of SRR and a neighbouring gene, nonsense‐mediated mRNA decay factor (SMG6), in temporal lobe epilepsy (TLE). Here, a total of 496 TLE patients and 528 healthy individuals were successfully genotyped for three SRR tag single nucleotide polymorphisms. The frequencies of the GG genotype at rs4523957 T > G were reduced in the TLE cases in the initial cohort (cohort 1) and were confirmed in the independent cohort (cohort 2). An analysis of all TLE cases in cohort 1 + 2 revealed that the seizure frequency and drug‐resistant incidence were significantly decreased in carriers of the GG genotype at rs4523957. Intriguingly, the activity of the SMG6 promoter with the mutant allele at rs4523957 decreased by 22% in the dual‐luciferase assay, and up‐regulated expression of SMG6 was observed in an epilepsy rat model. This study provides the first demonstration that the GG genotype is a protective marker against TLE. In particular, variation at rs4523957 likely inhibits SMG6 transcription and plays a key role against susceptibility to and severity of TLE. The significance of SMG6 hyperfunction in epileptic seizures deserves to be investigated in future studies.

Beyond Epilepsy and Autism: Disruption of GABRB3 Causes Ocular Hypopigmentation

Reduced ocular pigmentation is common in Angelman syndrome (AS) and Prader-Willi syndrome (PWS) and is long thought to be caused by OCA2 deletion. GABRB3 is located in the 15q11-13 region flanked by UBE3A, GABRA5, GABRG3, and OCA2. Mutations in GABRB3 have frequently been associated with epilepsy and autism, consistent with its role in neurodevelopment. We report here a robust phenotype in the mouse in which deletion of Gabrb3 alone causes nearly complete loss of retinal pigmentation due to atrophied melanosomes, as evidenced by electron microscopy. Using exome and RNA sequencing, we confirmed that only the Gabrb3 gene was disrupted while the Oca2 gene was intact. However, mRNA abundance of Oca2 and other genes adjacent to Gabrb3 is substantially reduced in Gabrb3^-/- mice, suggesting complex transcriptional regulation in this region. These results suggest that impairment in GABRB3 downregulates OCA2 and indirectly causes ocular hypopigmentation and visual defects in AS and PWS.

mRNAs containing NMD-competent premature termination codons are stabilized and translated under UPF1 depletion

mRNAs containing premature termination codons (PTCs) are rapidly degraded through nonsense-mediated mRNA decay (NMD). However, some PTC-containing mRNAs evade NMD, and might generate mutant proteins responsible for various diseases, including cancers. Using PTC-containing human genomic β-globin constructs, we show that a fraction (~30%) of PTC-containing mRNAs expressed from NMD-competent PTC-containing constructs were as stable as their PTC-free counterparts in a steady state. These PTC-containing mRNAs were monosome-enriched and rarely contributed to expression of mutant proteins. Expression of trace amounts of mutant proteins from NMD-competent PTC-containing constructs was not affected by inhibition of eIF4E-dependent translation and such expression was dependent on a continuous influx of newly synthesized PTC-containing mRNAs, indicating that truncated mutant proteins originated primarily in the pioneer round of translation. The generation of mutant proteins was promoted by UPF1 depletion, which induced polysome association of PTC-containing mRNAs, increased eIF4E-bound PTC-containing mRNA levels, and subsequent eIF4E-dependent translation. Our findings suggest that PTC-containing mRNAs are potent and regulatable sources of mutant protein generation.

[Research advances in hereditary epilepsy and precision drug therapy]

Epilepsy is a common nervous system disease. It has been found that the pathogenesis of epilepsy is associated mutations in various genes, including genes encoding voltage-dependent ion channel, genes encoding ligand-gated ion channel, and solute carrier family genes. Different types of epilepsy caused by different mutations have different responses to drugs, and therefore, diagnosis and medication guidance based on genes are new thoughts for developing therapies. With the application of next-generation sequencing technology, more and more genes will be determined, which helps to further study the pathogenic mechanism of mutant genes and provides a basis for precision drug therapy for epilepsy.

[Research advances in hereditary epilepsy and precision drug therapy]

Epilepsy is a common nervous system disease. It has been found that the pathogenesis of epilepsy is associated mutations in various genes, including genes encoding voltage-dependent ion channel, genes encoding ligand-gated ion channel, and solute carrier family genes. Different types of epilepsy caused by different mutations have different responses to drugs, and therefore, diagnosis and medication guidance based on genes are new thoughts for developing therapies. With the application of next-generation sequencing technology, more and more genes will be determined, which helps to further study the pathogenic mechanism of mutant genes and provides a basis for precision drug therapy for epilepsy.

RNA sequencing of synaptic and cytoplasmic Upf1-bound transcripts supports contribution of nonsense-mediated decay to epileptogenesis

The nonsense mediated decay (NMD) pathway is a critical surveillance mechanism for identifying aberrant mRNA transcripts. It is unknown, however, whether the NMD system is affected by seizures in vivo and whether changes confer beneficial or maladaptive responses that influence long-term outcomes such the network alterations that produce spontaneous recurrent seizures. Here we explored the responses of the NMD pathway to prolonged seizures (status epilepticus) and investigated the effects of NMD inhibition on epilepsy in mice. Status epilepticus led to increased protein levels of Up-frameshift suppressor 1 homolog (Upf1) within the mouse hippocampus. Upf1 protein levels were also higher in resected hippocampus from patients with intractable temporal lobe epilepsy. Immunoprecipitation of Upf1-bound RNA from the cytoplasmic and synaptosomal compartments followed by RNA sequencing identified unique populations of NMD-associated transcripts and altered levels after status epilepticus, including known substrates such as Arc as well as novel targets including Inhba and Npas4. Finally, long-term video-EEG recordings determined that pharmacologic interference in the NMD pathway after status epilepticus reduced the later occurrence of spontaneous seizures in mice. These findings suggest compartment-specific recruitment and differential loading of transcripts by NMD pathway components may contribute to the process of epileptogenesis.

Endoplasmic Reticulum-Targeted Subunit Toxins Provide a New Approach to Rescue Misfolded Mutant Proteins and Revert Cell Models of Genetic Diseases

Many germ line diseases stem from a relatively minor disturbance in mutant protein endoplasmic reticulum (ER) 3D assembly. Chaperones are recruited which, on failure to correct folding, sort the mutant for retrotranslocation and cytosolic proteasomal degradation (ER-associated degradation-ERAD), to initiate/exacerbate deficiency-disease symptoms. Several bacterial (and plant) subunit toxins, retrograde transport to the ER after initial cell surface receptor binding/internalization. The A subunit has evolved to mimic a misfolded protein and hijack the ERAD membrane translocon (dislocon), to effect cytosolic access and cytopathology. We show such toxins compete for ERAD to rescue endogenous misfolded proteins. Cholera toxin or verotoxin (Shiga toxin) containing genetically inactivated (± an N-terminal polyleucine tail) A subunit can, within 2–4 hrs, temporarily increase F508delCFTR protein, the major cystic fibrosis (CF) mutant (5-10x), F508delCFTR Golgi maturation (<10x), cell surface expression (20x) and chloride transport (2x) in F508del CFTR transfected cells and patient-derived F508delCFTR bronchiolar epithelia, without apparent cytopathology. These toxoids also increase glucocerobrosidase (GCC) in N370SGCC Gaucher Disease fibroblasts (3x), another ERAD–exacerbated misfiling disease. We identify a new, potentially benign approach to the treatment of certain genetic protein misfolding diseases.

Differential protein structural disturbances and suppression of assembly partners produced by nonsense GABRG2 epilepsy mutations: implications for disease phenotypic heterogeneity

Mutations in GABA_A receptor subunit genes are frequently associated with epilepsy, and nonsense mutations in GABRG2 are associated with several epilepsy syndromes including childhood absence epilepsy, generalized tonic clonic seizures and the epileptic encephalopathy, Dravet syndrome. The molecular basis for the phenotypic heterogeneity of mutations is unclear. Here we focused on three nonsense mutations in GABRG2 (GABRG2(R136*), GABRG2(Q390*) and GABRG2(W429*)) associated with epilepsies of different severities. Structural modeling and structure-based analysis indicated that the surface of the wild-type γ2 subunit was naturally hydrophobic, which is suitable to be buried in the cell membrane. Different mutant γ2 subunits had different stabilities and different interactions with their wild-type subunit binding partners because they adopted different conformations and had different surface hydrophobicities and different tendency to dimerize. We utilized flow cytometry and biochemical approaches in combination with lifted whole cell patch-clamp recordings. We demonstrated that the truncated subunits had no to minimal surface expression and unchanged or reduced surface expression of wild-type partnering subunits. The amplitudes of GABA-evoked currents from the mutant α1β2γ2(R136*), α1β2γ2(Q390*) and α1β2γ2(W429*) receptors were reduced compared to the currents from α1β2γ2 receptors but with differentially reduced levels. This thus suggests differential protein structure disturbances are correlated with disease severity.

Differential molecular and behavioural alterations in mouse models of GABRG2 haploinsufficiency versus dominant negative mutations associated with human epilepsy

Genetic epilepsy is a common disorder with phenotypic variation, but the basis for the variation is unknown. Comparing the molecular pathophysiology of mutations in the same epilepsy gene may provide mechanistic insights into the phenotypic heterogeneity. GABRG2 is an established epilepsy gene, and mutations in it produce epilepsy syndromes with varying severities. The disease phenotype in some cases may be caused by simple loss of subunit function (functional haploinsufficiency), while others may be caused by loss-of-function plus dominant negative suppression and other cellular toxicity. Detailed molecular defects and the corresponding seizures and related comorbidities resulting from haploinsufficiency and dominant negative mutations, however, have not been compared. Here we compared two mouse models of GABRG2 loss-of-function mutations associated with epilepsy with different severities, Gabrg2+/Q390X knockin (KI) and Gabrg2+/- knockout (KO) mice. Heterozygous Gabrg2+/Q390X KI mice are associated with a severe epileptic encephalopathy due to a dominant negative effect of the mutation, while heterozygous Gabrg2+/- KO mice are associated with mild absence epilepsy due to simple haploinsufficiency. Unchanged at the transcriptional level, KI mice with severe epilepsy had neuronal accumulation of mutant γ2 subunits, reduced remaining functional wild-type subunits in dendrites and synapses, while KO mice with mild epilepsy had no intracellular accumulation of the mutant subunits and unaffected biogenesis of the remaining wild-type subunits. Consequently, KI mice with dominant negative mutations had much less wild-type receptor expression, more severe seizures and behavioural comorbidities than KO mice. This work provides insights into the pathophysiology of epilepsy syndrome heterogeneity and designing mechanism-based therapies.

Neuropathological Mechanisms of Seizures in Autism Spectrum Disorder

This manuscript reviews biological abnormalities shared by autism spectrum disorder (ASD) and epilepsy. Two neuropathological findings are shared by ASD and epilepsy: abnormalities in minicolumn architecture and γ-aminobutyric acid (GABA) neurotransmission. The peripheral neuropil, which is the region that contains the inhibition circuits of the minicolumns, has been found to be decreased in the post-mortem ASD brain. ASD and epilepsy are associated with inhibitory GABA neurotransmission abnormalities including reduced GABA_A and GABA_B subunit expression. These abnormalities can elevate the excitation-to-inhibition balance, resulting in hyperexcitablity of the cortex and, in turn, increase the risk of seizures. Medical abnormalities associated with both epilepsy and ASD are discussed. These include specific genetic syndromes, specific metabolic disorders including disorders of energy metabolism and GABA and glutamate neurotransmission, mineral and vitamin deficiencies, heavy metal exposures and immune dysfunction. Many of these medical abnormalities can result in an elevation of the excitatory-to-inhibitory balance. Fragile X is linked to dysfunction of the mGluR5 receptor and Fragile X, Angelman and Rett syndromes are linked to a reduction in GABA_A receptor expression. Defects in energy metabolism can reduce GABA interneuron function. Both pyridoxine dependent seizures and succinic semialdehyde dehydrogenase deficiency cause GABA deficiencies while urea cycle defects and phenylketonuria cause abnormalities in glutamate neurotransmission. Mineral deficiencies can cause glutamate and GABA neurotransmission abnormalities and heavy metals can cause mitochondrial dysfunction which disrupts GABA metabolism. Thus, both ASD and epilepsy are associated with similar abnormalities that may alter the excitatory-to-inhibitory balance of the cortex. These parallels may explain the high prevalence of epilepsy in ASD and the elevated prevalence of ASD features in individuals with epilepsy.

The developmental evolution of the seizure phenotype and cortical inhibition in mouse models of juvenile myoclonic epilepsy

The GABA(A) receptor (GABA(A)R) α1 subunit mutation, A322D, causes autosomal dominant juvenile myoclonic epilepsy (JME). Previous in vitro studies demonstrated that A322D elicits α1(A322D) protein degradation and that the residual mutant protein causes a dominant-negative effect on wild type GABA(A)Rs. Here, we determined the effects of heterozygous A322D knockin (Het(α1)AD) and deletion (Het(α1)KO) on seizures, GABA(A)R expression, and motor cortex (M1) miniature inhibitory postsynaptic currents (mIPSCs) at two developmental time-points, P35 and P120. Both Het(α1)AD and Het(α1)KO mice experience absence seizures at P35 that persist at P120, but have substantially more frequent spontaneous and evoked polyspike wave discharges and myoclonic seizures at P120. Both mutant mice have increased total and synaptic α3 subunit expression at both time-points and decreased α1 subunit expression at P35, but not P120. There are proportional reductions in α3, β2, and γ2 subunit expression between P35 and P120 in wild type and mutant mice. In M1, mutants have decreased mIPSC peak amplitudes and prolonged decay constants compared with wild type, and the Het(α1)AD mice have reduced mIPSC frequency and smaller amplitudes than Het(α1)KO mice. Wild type and mutants exhibit proportional increases in mIPSC amplitudes between P35 and P120. We conclude that Het(α1)KO and Het(α1)AD mice model the JME subsyndrome, childhood absence epilepsy persisting and evolving into JME. Both mutants alter GABA(A)R composition and motor cortex physiology in a manner expected to increase neuronal synchrony and excitability to produce seizures. However, developmental changes in M1 GABA(A)Rs do not explain the worsened phenotype at P120 in mutant mice.

Mutant GABA(A) receptor subunits in genetic (idiopathic) epilepsy

The γ-aminobutyric acid receptor type A (GABAA receptor) is a ligand-gated chloride channel that mediates major inhibitory functions in the central nervous system. GABAA receptors function mainly as pentamers containing α, β, and either γ or δ subunits. A number of antiepileptic drugs have agonistic effects on GABAA receptors. Hence, dysfunctions of GABAA receptors have been postulated to play important roles in the etiology of epilepsy. In fact, mutations or genetic variations of the genes encoding the α1, α6, β2, β3, γ2, or δ subunits (GABRA1, GABRA6, GABRB2, GABRB3, GABRG2, and GABRD, respectively) have been associated with human epilepsy, both with and without febrile seizures. Epilepsy resulting from mutations is commonly one of following, genetic (idiopathic) generalized epilepsy (e.g., juvenile myoclonic epilepsy), childhood absence epilepsy, genetic epilepsy with febrile seizures, or Dravet syndrome. Recently, mutations of GABRA1, GABRB2, and GABRB3 were associated with infantile spasms and Lennox-Gastaut syndrome. These mutations compromise hyperpolarization through GABAA receptors, which is believed to cause seizures. Interestingly, most of the insufficiencies are not caused by receptor gating abnormalities, but by complex mechanisms, including endoplasmic reticulum (ER)-associated degradation, nonsense-mediated mRNA decay, intracellular trafficking defects, and ER stress. Thus, GABAA receptor subunit mutations are now thought to participate in the pathomechanisms of epilepsy, and an improved understanding of these mutations should facilitate our understanding of epilepsy and the development of new therapies.

Interaction between quality control systems for ER protein folding and RNA biogenesis

The endoplasmic reticulum (ER) is the intracellular organelle responsible for the synthesis, folding and assembly of proteins destined for secretion and the endomembrane system of the cell. ER quality control (ERQC) is an intensively studied surveillance mechanism that selectively degrades misfolded proteins to ensure that only properly folded proteins exit the ER en route to the Golgi compartment. Proper protein folding is indispensable for the differentiation and function of cells that secrete high levels of protein and defects in protein folding are implicated in many pathologies, including metabolic, genetic, neurodegenerative and inflammatory diseases. Accumulation of misfolded proteins in the ER activates an adaptive set of signaling pathways, collectively known as the unfolded protein response (UPR), to resolve protein misfolding and restore ER homeostasis. Nonsense-mediated RNA decay (NMD) is an RNA surveillance system that selectively degrades nascent mRNAs containing premature termination codons (PTCs). Recently, we used a genetic screen to identify genes that interact with UPR signaling in C. elegans. These studies identified NMD-associated genes that are required for ER protein folding homeostasis. These findings link the quality control systems required for ER protein folding and RNA biogenesis, provide new insights into mechanisms of ERQC and have implications on diseases of ER dysfunction and therapeutic approaches based on NMD inhibition. Here, we discuss the biological significance of these findings and future directions for study.

Trafficking-deficient mutant GABRG2 subunit amount may modify epilepsy phenotype

OBJECTIVE

Genetic epilepsies and many other human genetic diseases display phenotypic heterogeneity, often for unknown reasons. Disease severity associated with nonsense mutations is dependent partially on mutation gene location and resulting efficiency of nonsense-mediated mRNA decay (NMD) to eliminate potentially toxic proteins. Nonsense mutations in the last exon do not activate NMD, thus producing truncated proteins. We compared the protein metabolism and the impact on channel biogenesis, function, and cellular homeostasis of truncated γ2 subunits produced by GABRG2 nonsense mutations associated with epilepsy of different severities and by a nonsense mutation in the last exon unassociated with epilepsy.

METHODS

γ-Aminobutyric acid type A receptor subunits were coexpressed in non-neuronal cells and neurons. NMD was studied using minigenes that support NMD. Protein degradation rates were determined using (35) S radiolabeling pulse chase. Channel function was determined by whole cell recordings, and subunits trafficking and cellular toxicity were determined using flow cytometry, immunoblotting, and immunohistochemistry.

RESULTS

Although all GABRG2 nonsense mutations resulted in loss of γ2 subunit surface expression, the truncated subunits had different degradation rates and stabilities, suppression of wild-type subunit biogenesis and function, amounts of conjugation with polyubiquitin, and endoplasmic reticulum stress levels.

INTERPRETATION

We compared molecular phenotypes of GABRG2 nonsense mutations. The findings suggest that despite the common loss of mutant allele function, each mutation produced different intracellular levels of trafficking-deficient subunits. The concentration-dependent suppression of wild-type channel function and cellular disturbance resulting from differences in mutant subunit metabolism may contribute to associated epilepsy severities and by implication to phenotypic heterogeneity in many inherited human diseases.

Altered cortical GABAA receptor composition, physiology, and endocytosis in a mouse model of a human genetic absence epilepsy syndrome

Patients with generalized epilepsy exhibit cerebral cortical disinhibition. Likewise, mutations in the inhibitory ligand-gated ion channels, GABAA receptors (GABAARs), cause generalized epilepsy syndromes in humans. Recently, we demonstrated that heterozygous knock-out (Hetα1KO) of the human epilepsy gene, the GABAAR α1 subunit, produced absence epilepsy in mice. Here, we determined the effects of Hetα1KO on the expression and physiology of GABAARs in the mouse cortex. We found that Hetα1KO caused modest reductions in the total and surface expression of the β2 subunit but did not alter β1 or β3 subunit expression, results consistent with a small reduction of GABAARs. Cortices partially compensated for Hetα1KO by increasing the fraction of residual α1 subunit on the cell surface and by increasing total and surface expression of α3, but not α2, subunits. Co-immunoprecipitation experiments revealed that Hetα1KO increased the fraction of α1 subunits, and decreased the fraction of α3 subunits, that associated in hybrid α1α3βγ receptors. Patch clamp electrophysiology studies showed that Hetα1KO layer VI cortical neurons exhibited reduced inhibitory postsynaptic current peak amplitudes, prolonged current rise and decay times, and altered responses to benzodiazepine agonists. Finally, application of inhibitors of dynamin-mediated endocytosis revealed that Hetα1KO reduced base-line GABAAR endocytosis, an effect that probably contributes to the observed changes in GABAAR expression. These findings demonstrate that Hetα1KO exerts two principle disinhibitory effects on cortical GABAAR-mediated inhibitory neurotransmission: 1) a modest reduction of GABAAR number and 2) a partial compensation with GABAAR isoforms that possess physiological properties different from those of the otherwise predominant α1βγ GABAARs.

Neurochemical and behavioral features in genetic absence epilepsy and in acutely induced absence seizures

The absence epilepsy typical electroencephalographic pattern of sharp spikes and slow waves (SWDs) is considered to be due to an interaction of an initiation site in the cortex and a resonant circuit in the thalamus. The hyperpolarization-activated cyclic nucleotide-gated cationic I h pacemaker channels (HCN) play an important role in the enhanced cortical excitability. The role of thalamic HCN in SWD occurrence is less clear. Absence epilepsy in the WAG/Rij strain is accompanied by deficiency of the activity of dopaminergic system, which weakens the formation of an emotional positive state, causes depression-like symptoms, and counteracts learning and memory processes. It also enhances GABAA receptor activity in the striatum, globus pallidus, and reticular thalamic nucleus, causing a rise of SWD activity in the cortico-thalamo-cortical networks. One of the reasons for the occurrence of absences is that several genes coding of GABAA receptors are mutated. The question arises: what the role of DA receptors is. Two mechanisms that cause an infringement of the function of DA receptors in this genetic absence epilepsy model are proposed.

mRNA surveillance and endoplasmic reticulum quality control processes alter biogenesis of mutant GABAA receptor subunits associated with genetic epilepsies

Previous studies from our and other groups have demonstrated that the majority of γ-aminobutyric acid (GABA)(A) receptor subunit mutations produce mutant subunits with impaired biogenesis and trafficking. These GABA(A) receptor mutations include missense, nonsense, deletion, or insertion mutations that result in a frameshift with premature translation-termination codons (PTCs) and splice-site mutations. Frameshift or splice-site mutations produce mutant proteins with PTCs, thus generating nonfunctional truncated proteins. All of these mutant GABA(A) receptor subunits are subject to cellular quality control at the messenger RNA (mRNA) or protein level. These quality-control checkpoints shape the cell's response to the presence of the mutant subunits and attempt to reduce the impact of the mutant subunit on GABA(A) receptor expression and function. The check points prevent nonfunctioning or malfunctioning GABA(A) receptor subunits from trafficking to the cell surface or to synapses, and help to ensure that the receptor channels trafficked to the membrane and synapses are indeed functional. However, if and how these quality control or check points impact the posttranslational modifications of functional GABA(A) receptor channels such as receptor phosphorylation and ubiquitination and their involvement in mediating GABAergic inhibitory synaptic strength needs to be investigated in the near future.

Impaired surface αβγ GABA(A) receptor expression in familial epilepsy due to a GABRG2 frameshift mutation

The purpose of the study was to explore the pathogenic mechanisms underlying generalized epilepsy and febrile seizures plus (GEFS+) in a family with a novel γ2 subunit gene (GABRG2) frameshift mutation. Four affected and one unaffected individuals carried a c.1329delC GABRG2 mutation resulting in a subunit [γ2S(S443delC)] with a modified and elongated carboxy-terminus that is different from that of the wildtype γ2S subunit. We expressed the wildtype γ2S subunit and the predicted mutant γ2S(S443delC) subunit cDNAs in HEK293T cells and performed immunoblotting, flow cytometry and electrophysiology studies. The mutant subunit was translated as a stable protein that was larger than the wildtype γ2S subunit and was retained in the ER and not expressed on the cell surface membrane, suggesting GABRG2 haploinsufficiency. Peak GABA-evoked currents recorded from cells cotransfected with wildtype α1 and β2 subunits and mutant γ2S subunits were significantly decreased and were comparable to α1β2 receptor currents. S443delC is the first GABR epilepsy mutation predicted to abolish the natural stop codon and produce a stop codon in the 3' UTR that leads to translation of an extended peptide. The GEFS+ phenotype observed in this family is likely caused by γ2S subunit loss-of-function and possibly to dominant-negative suppression of α1β2γ2 receptors. Many GABRG2 truncation mutations result in GEFS+, but the spectrum of phenotypic severity is wider, ranging from asymptomatic individuals to the Dravet syndrome. Mechanisms influencing the severity of the phenotype are therefore complex and difficult to correlate with its demonstrable functional effects.

Decreased viability and absence-like epilepsy in mice lacking or deficient in the GABAA receptor α1 subunit

Autosomal dominant mutations S326fs328X and A322D in the GABA(A) receptor α1 subunit are associated with human absence epilepsy and juvenile myoclonic epilepsy, respectively. Because these mutations substantially reduce α1 subunit protein expression in vitro, it was hypothesized that they produce epilepsy by causing α1 subunit haploinsufficiency. However, in a mixed background strain of mice, α1 subunit deletion does not reduce viability or cause visually apparent seizures; the effects of α1 subunit deletion on electroencephalography (EEG) waveforms were not investigated. Here, we determined the effects of α1 subunit loss on viability, EEG spike-wave discharges and seizures in congenic C57BL/6J and DBA/2J mice. Deletion of α1 subunit caused strain- and sex-dependent reductions in viability. Heterozygous mice experienced EEG discharges and absence-like seizures within both background strains, and exhibited a sex-dependent effect on the discharges and viability in the C57BL/6J strain. These findings suggest that α1 subunit haploinsufficiency can produce epilepsy and may be a major mechanism by which the S326fs328X and A322D mutations cause these epilepsy syndromes.

The delicate balance between secreted protein folding and endoplasmic reticulum-associated degradation in human physiology

Protein folding is a complex, error-prone process that often results in an irreparable protein by-product. These by-products can be recognized by cellular quality control machineries and targeted for proteasome-dependent degradation. The folding of proteins in the secretory pathway adds another layer to the protein folding "problem," as the endoplasmic reticulum maintains a unique chemical environment within the cell. In fact, a growing number of diseases are attributed to defects in secretory protein folding, and many of these by-products are targeted for a process known as endoplasmic reticulum-associated degradation (ERAD). Since its discovery, research on the mechanisms underlying the ERAD pathway has provided new insights into how ERAD contributes to human health during both normal and diseases states. Links between ERAD and disease are evidenced from the loss of protein function as a result of degradation, chronic cellular stress when ERAD fails to keep up with misfolded protein production, and the ability of some pathogens to coopt the ERAD pathway. The growing number of ERAD substrates has also illuminated the differences in the machineries used to recognize and degrade a vast array of potential clients for this pathway. Despite all that is known about ERAD, many questions remain, and new paradigms will likely emerge. Clearly, the key to successful disease treatment lies within defining the molecular details of the ERAD pathway and in understanding how this conserved pathway selects and degrades an innumerable cast of substrates.

The GABRG2 nonsense mutation, Q40X, associated with Dravet syndrome activated NMD and generated a truncated subunit that was partially rescued by aminoglycoside-induced stop codon read-through

The GABRG2 nonsense mutation, Q40X, is associated with the severe epilepsy syndrome, Dravet syndrome, and is predicted to generate a premature translation-termination codon (PTC) in the GABA(A) receptor γ2 subunit mRNA in a position that codes for the first amino acid of the mutant subunit. We determined the effects of the mutation on γ2 subunit mRNA and protein synthesis and degradation, as well as on α1β2γ2 GABA(A) receptor assembly, trafficking and surface expression in HEK cells. Using bacterial artificial chromosome (BAC) constructs, we found that γ2(Q40X) subunit mRNA was degraded by nonsense mediated mRNA decay (NMD). Undegraded mutant mRNA was translated to a truncated peptide, likely the signal peptide, which was cleaved further. We also found that mutant γ2(Q40X) subunits did not assemble into functional receptors, thus decreasing GABA-evoked current amplitudes. The GABRG2(Q40X) mutation is one of several epilepsy-associated nonsense mutations that have the potential to be rescued by reading through the PTC, thus restoring full-length protein translation. As a first approach, we investigated the use of the aminoglycoside, gentamicin, to rescue translation of intact mutant subunits by inducing mRNA read-through. In the presence of gentamicin, synthesis of full length γ2 subunits was partially restored, and surface biotinylation and whole cell recording experiments suggested that rescued γ2 subunits could corporate into functional, surface GABA(A) receptors, indicating a possible direction for future therapy.

The intronic GABRG2 mutation, IVS6+2T->G, associated with childhood absence epilepsy altered subunit mRNA intron splicing, activated nonsense-mediated decay, and produced a stable truncated γ2 subunit

The intronic GABRG2 mutation, IVS6+2T→G, was identified in an Australian family with childhood absence epilepsy and febrile seizures (Kananura et al., 2002). The GABRG2 intron 6 splice donor site was found to be mutated from GT to GG. We generated wild-type and mutant γ2 subunit bacterial artificial chromosomes (BACs) driven by a CMV promoter and expressed them in HEK293T cells and expressed wild-type and mutant γ2 subunit BACs containing the endogenous hGABRG2 promoter in transgenic mice. Wild-type and mutant GABRG2 mRNA splicing patterns were determined in both BAC-transfected HEK293T cells and transgenic mouse brain, and in both, the mutation abolished intron 6 splicing at the donor site, activated a cryptic splice site, generated partial intron 6 retention, and produced a frameshift in exon 7 that created a premature translation termination codon (PTC). The resultant mutant mRNA was either degraded partially by nonsense-mediated mRNA decay or translated to a stable, truncated subunit (the γ2-PTC subunit) containing the first six GABRG2 exons and a novel frameshifted 29 aa C-terminal tail. The γ2-PTC subunit was homologous to the mollusk AChBP (acetylcholine binding protein) but was not secreted from cells. It was retained in the ER and not expressed on the surface membrane, but it did oligomerize with α1 and β2 subunits. These results suggested that the GABRG2 mutation, IVS6+2T→G, reduced surface αβγ2 receptor levels, thus reducing GABAergic inhibition, by reducing GABRG2 transcript level and producing a stable, nonfunctional truncated subunit that had a dominant-negative effect on αβγ2 receptor assembly.

Alu elements mediate large SPG11 gene rearrangements: further spatacsin mutations

PURPOSE

Hereditary spastic paraplegias compose a group of neurodegenerative disorders with a large clinical and genetic heterogeneity. Among the autosomal recessive forms, spastic paraplegia type 11 is the most common.

METHODS

To better understand the spastic paraplegia type 11 mutation spectrum, we studied a group of 54 patients with hereditary spastic paraplegia. Mutation screening was performed by PCR amplification of SPG11 coding regions and intron boundaries, followed by sequencing. For the detection of large gene rearrangements, we performed multiplex ligation-dependent probe amplification.

RESULTS

We report 13 families with spastic paraplegia type 11 carrying either novel or previously identified mutations. We describe a complex entire SPG11 rearrangement and show that large gene rearrangements are frequent among patients with spastic paraplegia type 11. Moreover, we mapped the deletion breakpoints of three different large SPG11 deletions and provide evidence for Alu microhomology-mediated exon deletion.

CONCLUSION

Our analysis shows that the high number of repeated elements in SPG11 together with the presence of recombination hotspots and the high intrinsic instability of the 15q locus all contribute toward making this genomic region more prone to large gene rearrangements. These findings enlarge the amount of data relating repeated elements with neurodegenerative disorders and highlight their importance in human disease and genome evolution.

RNA surveillance is required for endoplasmic reticulum homeostasis

The unfolded protein response (UPR) is an intracellular stress-signaling pathway that counteracts the accumulation of misfolded proteins in the endoplasmic reticulum (ER). Because defects in ER protein folding are associated with many pathological states, including metabolic, neurologic, genetic, and inflammatory diseases, it is important to understand how the UPR maintains ER protein-folding homeostasis. All metazoans have conserved the fundamental UPR transducers IRE1, ATF6, and PERK. In Caenorhabditis elegans, the UPR is required to prevent larval lethality and intestinal degeneration. Although ire-1-null worms are viable, they are particularly sensitive to ER stress. To identify genes that are required for development of ire-1-null worms, we performed a comprehensive RNA interference screen to find 10 genes that exhibit synthetic growth and intestinal defects with the ire-1(v33) mutant but not with atf-6(tm1153) or pek-1(ok275) mutants. The expression of two of these genes, exos-3 and F48E8.6, was induced by ER stress, and their knockdown in a wild-type strain caused ER stress. Because these genes encode subunits of the exosome complex that functions in mRNA surveillance, we analyzed other gene products required for nonsense-mediated mRNA decay (NMD). Our results demonstrate that defects in smg-1, smg-4, and smg-6 in C. elegans and SMG6 in mammalian cells cause ER stress and sensitize to the lethal effects of ER stress. Although ER stress did not activate mRNA surveillance complex assembly, ER stress did induce SMG6 expression, and NMD regulators were constitutively localized to the ER. Importantly, the findings demonstrate a unique and fundamental interaction where NMD-mediated mRNA quality control is required to prevent ER stress.

Novel α1 and γ2 GABAA receptor subunit mutations in families with idiopathic generalized epilepsy

Epilepsy is a heterogeneous neurological disease affecting approximately 50 million people worldwide. Genetic factors play an important role in both the onset and severity of the condition, with mutations in several ion-channel genes being implicated, including those encoding the GABA(A) receptor. Here, we evaluated the frequency of additional mutations in the GABA(A) receptor by direct sequencing of the complete open reading frame of the GABRA1 and GABRG2 genes from a cohort of French Canadian families with idiopathic generalized epilepsy (IGE). Using this approach, we have identified three novel mutations that were absent in over 400 control chromosomes. In GABRA1, two mutations were found, with the first being a 25-bp insertion that was associated with intron retention (i.e. K353delins18X) and the second corresponding to a single point mutation that replaced the aspartate 219 residue with an asparagine (i.e. D219N). Electrophysiological analysis revealed that K353delins18X and D219N altered GABA(A) receptor function by reducing the total surface expression of mature protein and/or by curtailing neurotransmitter effectiveness. Both defects would be expected to have a detrimental effect on inhibitory control of neuronal circuits. In contrast, the single point mutation identified in the GABRG2 gene, namely P83S, was indistinguishable from the wildtype subunit in terms of surface expression and functionality. This finding was all the more intriguing as the mutation exhibited a high degree of penetrance in three generations of one French Canadian family. Further experimentation will be required to understand how this mutation contributes to the occurrence of IGE in these individuals.

Fine architecture and mutation mapping of human brain inhibitory system ligand gated ion channels by high-throughput homology modeling

The common architecture of the brain inhibitory system ligand-gated ion-channels was examined at the level of each of the subunits and in their assembled pentameric arrangements. Structural modeling of the GABAA receptor, GlyR1, and the serotonin receptor, 5HTR3A, was carried out on a multi-homolog basis employing a high-throughput homology modeling pipeline. The locations of all the known mutations of each of the subunits of the receptor subfamily were mapped upon their computed structures and structural relationships between patterns of mutations in different subunits were identified, resulting in the zoning of mutations to four specific regions of the common subunit structure. These classifications may be of value in discerning probable molecular mechanisms and functional manifestations of emerging mutations and polymorphisms, providing the foundation for a family-specific predictive algorithm that may allow researchers to focus experimental effort on the most probable molecular indicators of compromised receptor function and disease mechanism.

Slow degradation and aggregation in vitro of mutant GABAA receptor gamma2(Q351X) subunits associated with epilepsy

The GABA(A) receptor γ2 subunit nonsense mutation Q351X has been associated with the genetic epilepsy syndrome generalized epilepsy with febrile seizures plus, which includes a spectrum of seizures types from febrile seizures to Dravet syndrome. Although most genetic epilepsy syndromes are mild and remit with age, Dravet syndrome has a more severe clinical course with refractory seizures associated with developmental delay and cognitive impairment. The basis for the broad spectrum of seizure phenotypes is uncertain. We demonstrated previously that the GABA(A) receptor γ2 subunit gene Q351X mutation suppressed biogenesis of wild-type partnering α1 and β2 subunits in addition to its loss of function. Here we show that γ2S(Q351X) subunits have an additional impairment of biogenesis. Mutant γ2(Q351X) subunits were degraded more slowly than wild-type γ2 subunits and formed SDS-resistant, high-molecular-mass complexes or aggregates in multiple cell types, including neurons. The half-life of γ2S(Q351X) subunits was ∼4 h, whereas that of γ2S subunits was ∼2 h. Mutant subunits formed complexes rapidly after synthesis onset. Using multiple truncated subunits, we demonstrated that aggregate formation was a general phenomenon for truncated γ2S subunits and that their Cys-loop cysteines were involved in aggregate formation. Protein aggregation is a hallmark of neurodegenerative diseases, but the effects of the mutant γ2S(Q351X) subunit aggregates on neuronal function and survival are unclear. Additional validation of the mutant subunit aggregation in vivo and determination of the involved signaling pathways will help reveal the pathological effects of these mutant subunit aggregates in the pathogenesis of genetic epilepsy syndromes.

GABA(A) receptor alpha1 subunit mutation A322D associated with autosomal dominant juvenile myoclonic epilepsy reduces the expression and alters the composition of wild type GABA(A) receptors

A GABA(A) receptor (GABA(A)R) alpha1 subunit mutation, A322D (AD), causes an autosomal dominant form of juvenile myoclonic epilepsy (ADJME). Previous studies demonstrated that the mutation caused alpha1(AD) subunit misfolding and rapid degradation, reducing its total and surface expression substantially. Here, we determined the effects of the residual alpha1(AD) subunit expression on wild type GABA(A)R expression to determine whether the AD mutation conferred a dominant negative effect. We found that although the alpha1(AD) subunit did not substitute for wild type alpha1 subunits on the cell surface, it reduced the surface expression of alpha1beta2gamma2 and alpha3beta2gamma2 receptors by associating with the wild type subunits within the endoplasmic reticulum and preventing them from trafficking to the cell surface. The alpha1(AD) subunit reduced surface expression of alpha3beta2gamma2 receptors by a greater amount than alpha1beta2gamma2 receptors, thus altering cell surface GABA(A)R composition. When transfected into cultured cortical neurons, the alpha1(AD) subunit altered the time course of miniature inhibitory postsynaptic current kinetics and reduced miniature inhibitory postsynaptic current amplitudes. These findings demonstrated that, in addition to causing a heterozygous loss of function of alpha1(AD) subunits, this epilepsy mutation also elicited a modest dominant negative effect that likely shapes the epilepsy phenotype.

Mutations affecting GABAergic signaling in seizures and epilepsy

The causes of epilepsies and epileptic seizures are multifactorial. Genetic predisposition may contribute in certain types of epilepsies and seizures, whether idiopathic or symptomatic of genetic origin. Although these are not very common, they have offered a unique opportunity to investigate the molecular mechanisms underlying epileptogenesis and ictogenesis. Among the implicated gene mutations, a number of GABAA receptor subunit mutations have been recently identified that contribute to several idiopathic epilepsies, febrile seizures, and rarely to certain types of symptomatic epilepsies, like the severe myoclonic epilepsy of infancy. Deletion of GABAA receptor genes has also been linked to Angelman syndrome. Furthermore, mutations of proteins controlling chloride homeostasis, which indirectly defines the functional consequences of GABAA signaling, have been identified. These include the chloride channel 2 (CLCN2) and the potassium chloride cotransporter KCC3. The pathogenic role of CLCN2 mutations has not been clearly demonstrated and may represent either susceptibility genes or, in certain cases, innocuous polymorphisms. KCC3 mutations have been associated with hereditary motor and sensory polyneuropathy with corpus callosum agenesis (Andermann syndrome) that often manifests with epileptic seizures. This review summarizes the recent progress in the genetic linkages of epilepsies and seizures to the above genes and discusses potential pathogenic mechanisms that contribute to the age, sex, and conditional expression of these seizures in carriers of these mutations.

Mutations in GABAA receptor subunits associated with genetic epilepsies

Mutations in inhibitory GABAA receptor subunit genes (GABRA1, GABRB3, GABRG2 and GABRD) have been associated with genetic epilepsy syndromes including childhood absence epilepsy (CAE), juvenile myoclonic epilepsy (JME), pure febrile seizures (FS), generalized epilepsy with febrile seizures plus (GEFS+), and Dravet syndrome (DS)/severe myoclonic epilepsy in infancy (SMEI). These mutations are found in both translated and untranslated gene regions and have been shown to affect the GABAA receptors by altering receptor function and/or by impairing receptor biogenesis by multiple mechanisms including reducing subunit mRNA transcription or stability, impairing subunit folding, stability, or oligomerization and by inhibiting receptor trafficking.

Novel exon nucleotide substitution at the splice junction causes a neonatal Marfan syndrome

The fibrillin-1 gene (FBN1) mutations are associated with a broad spectrum of disorders including Marfan syndrome (MFS) and show great clinical heterogeneity. An underrepresentation for mutations leading to premature termination codon (PTC) in FBN1 exons 24-32 was found in neonatal or severe MFS but the underlying cause was unclear. This study thoroughly examined two FBN1 mutations on exons 24-32 region to illustrate the molecular mechanisms underlying these FBN1 mutations on MFS etiology. Two nucleotide substitutions, c.3208G> C, the last nucleotide of exon 26, and c.3209A>G, the first nucleotide of exon 27, affecting the same amino acid, p.D1070H and p.D1070G, respectively, gave very different phenotypes. We demonstrate that c.3208G>C generates two alternatively spliced transcripts, while c.3209A>G does not affect the splicing. We further demonstrate that the aberrantly spliced transcripts do not go through nonsense-mediated decay, but rather produce unstable, premature protein peptides that are degraded by endoplasmic reticulum associated degradation. The molecular mechanism outlined here defines a model for the pathogenesis of PTC-containing mutation within the exons 24-32 of FBN1 in MFS. Furthermore, our data suggest that PTC mutation within this region may lead to early lethality in neonatal MFS.

Making sense of nonsense GABA(A) receptor mutations associated with genetic epilepsies

Nonsense mutations that generate premature translation-termination codons (PTCs) are responsible for approximately one- third of human genetic diseases. PTCs in both voltage- and ligand-gated ion channel genes, including those for sodium, potassium, nicotinic cholinergic receptor and GABA(A) receptor channels, have been associated with genetic epilepsies but the epilepsy syndromes they cause are variable. It was recently proposed that two well-established molecular pathways, nonsense-mediated decay (NMD) and endoplasmic reticulum-associated degradation (ERAD), determine the effects of PTCs in GABA(A) receptor subunit genes associated with genetic epilepsies on the cellular fates of mutant subunit mRNAs and proteins. Activation of these different molecular mechanisms might contribute in part to different clinical phenotypes in patients with GABA(A) receptor subunit gene PTCs and thus different approaches for treatment of their genetic epilepsies might be required.

Genetic basis in epilepsies caused by malformations of cortical development and in those with structurally normal brain

Epilepsy is the most common neurological disorder affecting young people. The etiologies are multiple and most cases are sporadic. However, some rare families with Mendelian inheritance have provided evidence of genes' important role in epilepsy. Two important but apparently different groups of disorders have been extensively studied: epilepsies associated with malformations of cortical development (MCDs) and epilepsies associated with a structurally normal brain (or with minimal abnormalities only). This review is focused on clinical and molecular aspects of focal cortical dysplasia, polymicrogyria, periventricular nodular heterotopia, subcortical band heterotopia, lissencephaly and schizencephaly as examples of MCDs. Juvenile myoclonic epilepsy, childhood absence epilepsy, some familial forms of focal epilepsy and epilepsies associated with febrile seizures are discussed as examples of epileptic conditions in (apparently) structurally normal brains.