SAHA (Vorinostat) Corrects Inhibitory Synaptic Deficits Caused by Missense Epilepsy Mutations to the GABAA Receptor γ2 Subunit

Proteostasis regulation of GABAA receptors in neuronal function and disease

The γ-aminobutyric acid type A receptors (GABAARs) are ligand-gated anion channels that mediate fast inhibitory neurotransmission in the mammalian central nervous system. GABAARs form heteropentameric assemblies comprising two α1, two β2, and one γ2 subunits as the most common subtype in mammalian brains. Proteostasis regulation of GABAARs involves subunit folding within the endoplasmic reticulum, assembling into heteropentamers, receptor trafficking to the cell surface, and degradation of terminally misfolded subunits. As GABAARs are surface proteins, their trafficking to the plasma membrane is critical for proper receptor function. Thus, variants in the genes encoding GABAARs that disrupt proteostasis result in various neurodevelopmental disorders, ranging from intellectual disability to idiopathic generalized epilepsy. This review summarizes recent progress about how the proteostasis network regulates protein folding, assembly, degradation, trafficking, and synaptic clustering of GABAARs. Additionally, emerging pharmacological approaches that restore proteostasis of pathogenic GABAAR variants are presented, providing a promising strategy to treat related neurological diseases.

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.

Various pharmacological agents in the pipeline against intractable epilepsy

Epilepsy is a noncommunicable chronic neurological disorder affecting people of all ages, with the highest prevalence in low and middle-income countries. Despite the pharmacological armamentarium, the plethora of drugs in the market, and other treatment options, 30%-35% of individuals still show resistance to the current medication, termed intractable epilepsy/drug resistance epilepsy, which contributes to 50% of the mortalities due to epilepsy. Therefore, the development of new drugs and agents is needed to manage this devastating epilepsy. We reviewed the pipeline of drugs in "ClinicalTrials. gov," which is the federal registry of clinical trials to identify drugs and other treatment options in various phases against intractable epilepsy. A total of 31 clinical trials were found regarding intractable epilepsy. Among them, 48.4% (15) are about pharmacological agents, of which 26.6% are in Phase 1, 60% are in Phase 2, and 13.3% are in Phase 3. The mechanism of action or targets of the majority of these agents are different and are more diversified than those of the approved drugs. In this article, we summarized various pharmacological agents in clinical trials, their backgrounds, targets, and mechanisms of action for the treatment of intractable epilepsy. Treatment options other than pharmacological ones, such as devices for brain stimulation, ketogenic diets, gene therapy, and others, are also summarized.

Preliminary study on the mechanism of SAHA in the treatment of refractory epilepsy induced by GABRG2(F343L) mutation

Mutations in the γ-amino butyric acid type A (GABAA) receptor γ2 subunit gene, GABRG2, have been associated with refractory epilepsy. Increasing evidence indicates that suberoylanilide hydroxamic acid (SAHA), a broad-spectrum histone acetyltransferases (HDACs) inhibitor, can inhibit seizure onset. However, the mechanisms involved remains unknown. The present study aimed to explore the anti-epileptic effect and underlying mechanisms of SAHA in the treatment of refractory epilepsy induced by GABRG2 mutation. In the zebrafish line expressing human mutant GABRG2(F343L), Tg(hGABRG2F343L), SAHA was found to reduce seizure onset, swimming activity, and neuronal activity. In both Tg(hGABRG2F343L) zebrafish and HEK293T cells transfected with GABAA receptor subunits, SAHA could improve the pan-acetylation level and reduce the expression of HDAC1/10. The decreased expressions of GABAA receptor subunits could be rescued by SAHA treatment both in vivo and in vitro, which might be the result of increased gene transcription and protein trafficking. The up-regulated acetylation of histone H3 and H4 as well as Bip expression might be involved in the process. Taken together, our data proved that both histone and non-histone acetylation might contribute to the anti-epileptic effect of SAHA in refractory epilepsy caused by GABRG2(F343L) mutation, demonstrating SAHA as a promising therapeutic agent for refractory epilepsy.

GABRG2 mutations in genetic epilepsy with febrile seizures plus: structure, roles, and molecular genetics

Genetic epilepsy with febrile seizures plus (GEFS+) is a genetic epilepsy syndrome characterized by a marked hereditary tendency inherited as an autosomal dominant trait. Patients with GEFS+ may develop typical febrile seizures (FS), while generalized tonic-clonic seizures (GTCSs) with fever commonly occur between 3 months and 6 years of age, which is generally followed by febrile seizure plus (FS+), with or without absence seizures, focal seizures, or GTCSs. GEFS+ exhibits significant genetic heterogeneity, with polymerase chain reaction, exon sequencing, and single nucleotide polymorphism analyses all showing that the occurrence of GEFS+ is mainly related to mutations in the gamma-aminobutyric acid type A receptor gamma 2 subunit (GABRG2) gene. The most common mutations in GABRG2 are separated in large autosomal dominant families, but their pathogenesis remains unclear. The predominant types of GABRG2 mutations include missense (c.983A → T, c.245G → A, p.Met199Val), nonsense (R136*, Q390*, W429*), frameshift (c.1329delC, p.Val462fs*33, p.Pro59fs*12), point (P83S), and splice site (IVS6+2T → G) mutations. All of these mutations types can reduce the function of ion channels on the cell membrane; however, the degree and mechanism underlying these dysfunctions are different and could be linked to the main mechanism of epilepsy. The γ2 subunit plays a special role in receptor trafficking and is closely related to its structural specificity. This review focused on investigating the relationship between GEFS+ and GABRG2 mutation types in recent years, discussing novel aspects deemed to be great significance for clinically accurate diagnosis, anti-epileptic treatment strategies, and new drug development.

Anti-seizure mechanisms of midazolam and valproate at the β2(L51M) variant of the GABAA receptor

Genetic sequencing is identifying an expanding number of variants of GABAA receptors associated with human epilepsies. We identified a new de novo variant of the β2 subunit (β2L51M) of the inhibitory GABAA receptor associated with seizures. Our analysis determined the pathogenicity of the variant and the effects of anti-seizure medications. Our data demonstrates that the variant reduced cell surface trafficking and peak GABA-gated currents. Synaptic currents mediated by variant-containing receptors decayed faster than wild-type and single receptor currents showed that the variant shortened the duration of receptor activity by decreasing receptor open times. We tested the effects of the anti-seizure medications, midazolam, carbamazepine and valproate and found that all three enhance variant receptor surface expression. Additionally, midazolam restored receptor function by increasing single receptor active periods and synaptic current decay times towards wild-type levels. By contrast, valproate increased synaptic peak currents, event frequency and promoted synaptic bursting. Our study identifies a new disease-causing variant to the GABAA receptor, profiles its pathogenic effects and demonstrates how anti-seizure drugs correct its functional deficits.

Disruption of the Ubiquitin-Proteasome System and Elevated Endoplasmic Reticulum Stress in Epilepsy

The epilepsies are a broad group of conditions characterized by repeated seizures, and together are one of the most common neurological disorders. Additionally, epilepsy is comorbid with many neurological disorders, including lysosomal storage diseases, syndromic intellectual disability, and autism spectrum disorder. Despite the prevalence, treatments are still unsatisfactory: approximately 30% of epileptic patients do not adequately respond to existing therapeutics, which primarily target ion channels. Therefore, new therapeutic approaches are needed. Disturbed proteostasis is an emerging mechanism in epilepsy, with profound effects on neuronal health and function. Proteostasis, the dynamic balance of protein synthesis and degradation, can be directly disrupted by epilepsy-associated mutations in various components of the ubiquitin-proteasome system (UPS), or impairments can be secondary to seizure activity or misfolded proteins. Endoplasmic reticulum (ER) stress can arise from failed proteostasis and result in neuronal death. In light of this, several treatment modalities that modify components of proteostasis have shown promise in the management of neurological disorders. These include chemical chaperones to assist proper folding of proteins, inhibitors of overly active protein degradation, and enhancers of endogenous proteolytic pathways, such as the UPS. This review summarizes recent work on the pathomechanisms of abnormal protein folding and degradation in epilepsy, as well as treatment developments targeting this area.

Repurposing Vorinostat for the Treatment of Disorders Affecting Brain

Based on the findings in recent years, we summarize the therapeutic potential of vorinostat (VOR), the first approved histone deacetylase (HDAC) inhibitor, in disorders of brain, and strategies to improve drug efficacy and reduce side effects. Scientific evidences provide a strong case for the therapeutic utility of VOR in various disorders affecting brain, including stroke, Alzheimer's disease, frontotemporal dementia, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, spinal muscular atrophy, X-linked adrenoleukodystrophy, epilepsy, Niemann-Pick type C disease, and neuropsychiatric disorders. Further elucidation of the neuroprotective and neurorestorative properties of VOR using proper clinical study designs could provide momentum towards its clinical application. To improve the therapeutic prospect, concerns on systemic toxicity and off-target actions need to be addressed along with the improvement in formulation and delivery aspects, especially with respect to solubility, permeability, and pharmacokinetic properties. Newer approaches in this regard include poly(ethylene glycol)-b-poly(DL-lactic acid) micelles, VOR-pluronic F127 micelles, encapsulation of iron complexes of VOR into PEGylated liposomes, human serum albumin bound VOR nanomedicine, magnetically guided layer-by-layer assembled nanocarriers, as well as convection-enhanced delivery. Even though targeting specific class or isoform of HDAC is projected as advantageous over pan-HDAC inhibitor like VOR, in terms of adverse effects and efficacy, till clinical validation, the idea is debated. As the VOR treatment-related adverse changes are mostly found reversible, further optimization of the therapeutic strategies with respect to dose, dosage regimen, and formulations of VOR could propel its clinical prospects.

Effects of GABAA Receptor α3 Subunit Epilepsy Mutations on Inhibitory Synaptic Signaling

Missense mutations T166M, Q242L, T336M, and Y474C in the GABA_A receptor (GABA_AR) α3 subunit gene are associated with epileptic seizures, dysmorphic features, intellectual disability, and developmental delay. When incorporated into GABA_ARs expressed in oocytes, all mutations are known to reduce GABA-evoked whole-cell currents. However, their impact on the properties of inhibitory synaptic currents (IPSCs) is unknown, largely because it is difficult to establish, much less control, the stoichiometry of GABA_AR expressed in native neuronal synapses. To circumvent this problem, we employed a HEK293 cell-neuron co-culture expression system that permits the recording of IPSCs mediated by a pure population of GABA_ARs with a defined stoichiometry. We first demonstrated that IPSCs mediated by α3-containing GABA_ARs (α3β3γ2) decay significantly slower than those mediated by α1-containing isoforms (α1β2γ2 or α1β3γ2). GABA_AR α3 mutations did not affect IPSC peak amplitudes or 10-90% rise times, but three of the mutations affected IPSC decay. T336M significantly accelerated the IPSC decay rate whereas T166M and Y474C had the opposite effect. The acceleration of IPSC decay kinetics caused by the T366M mutation was returned to wild-type-like values by the anti-epileptic medication, midazolam. Quantification experiments in HEK293 cells revealed a significant reduction in cell-surface expression for all mutants, in agreement with previous oocyte data. Taken together, our results show that impaired surface expression and altered IPSC decay rates could both be significant factors underlying the pathologies associated with these mutations.

The GABRG2 F343L allele causes spontaneous seizures in a novel transgenic zebrafish model that can be treated with suberanilohydroxamic acid (SAHA)

Background

Mutations in the γ-aminobutyric acid type A (GABA_A) receptor γ2 subunit gene, GABRG2, have been associated frequently with epilepsy syndromes with varying severities. Recently, a de novo GABRG2 mutation, c.T1027C, p.F343L, was identified in a patient with an early onset epileptic encephalopathy (EOEE). In vitro, we demonstrated that GABA_A receptors containing the mutant γ2(F343L) subunit have impaired trafficking to the cell surface. Here, we aim to validate an in vivo zebrafish model of EOEE associated with the GABRG2 mutation T1027C.

Methods

We generated a novel transgenic zebrafish (AB strain) that overexpressed mutant human γ2(F343L) subunits and provided an initial characterization of the transgenic Tg(hGABRG2^F343L) zebrafish.

Results

Real-time quantitative PCR and in situ hybridization identified a significant up-regulation of c-fos in the mutant transgenic zebrafish, which has a well-established role in epileptogenesis. In the larval stage 5 days postfertilization (dpf), freely swimming Tg(hGABRG2^F343L) zebrafish displayed spontaneous seizure-like behaviors consisting of whole-body shaking and hyperactivity during automated locomotion video tracking, and seizures can be induced by light stimulation. Using RNA sequencing, we investigated transcriptomic changes due to the presence of mutant γ2L(F343L) subunits and have found 524 genes that are differentially expressed, including up-regulation of 33 genes associated with protein processing. More specifically, protein network analysis indicated histone deacetylases (HDACs) as potential therapeutic targets, and suberanilohydroxamic acid (SAHA), a broad HDACs inhibitor, alleviated seizure-like phenotypes in mutant zebrafish larvae.

Conclusions

Overall, our Tg(hGABRG2^F343L) overexpression zebrafish model provides the first example of a human epilepsy-associated GABRG2 mutation resulting in spontaneous seizures in zebrafish. Moreover, HDAC inhibition may be worth investigating as a therapeutic strategy for genetic epilepsies caused by missense mutations in GABRG2 and possibly in other central nervous system genes that impair surface trafficking.

Proteostasis Regulators Restore Function of Epilepsy-Associated GABAA Receptors

Proteostasis deficiency in mutated ion channels leads to a variety of ion channel diseases that are caused by excessive endoplasmic reticulum-associated degradation (ERAD) and inefficient membrane trafficking. We investigated proteostasis maintenance of γ-aminobutyric acid type A (GABA_A) receptors, the primary mediators of neuronal inhibition in the mammalian central nervous system. We screened a structurally diverse, Food and Drug Administration-approved drug library and identified dinoprost (DNP) and dihydroergocristine (DHEC) as highly efficacious enhancers of surface expression of four epilepsy-causing trafficking-deficient mutant receptors. Furthermore, DNP and DHEC restore whole-cell and synaptic currents by incorporating mutated subunits into functional receptors. Mechanistic studies revealed that both drugs reduce subunit degradation by attenuating the Grp94/Hrd1/Sel1L/VCP-mediated ERAD pathway and enhance the subunit folding by promoting subunit interactions with major GABA_A receptors-interacting chaperones, BiP and calnexin. In summary, we report that DNP and DHEC remodel the endoplasmic reticulum proteostasis network to restore the functional surface expression of mutant GABA_A receptors.