Correlations of receptor desensitization of gain-of-function GABRB3 variants with clinical severity.

Genetic variants associated with developmental and epileptic encephalopathies have been identified in the GABRB3 gene that encodes the ß3 subunit of GABAA receptors. Typically, variants alter receptor sensitivity to GABA resulting in either gain- or loss-of-function, which correlates with patient phenotypes. However, it is unclear how another important receptor property, desensitization, contributes to the greater clinical severity of gain-of-function variants. Desensitization properties of 20 gain-of-function GABRB3 variant receptors were evaluated using two-electrode voltage-clamp electrophysiology. The parameters measured included current decay rates and steady-state currents. Selected variants with increased or reduced desensitization were also evaluated using whole-cell electrophysiology in transfected mammalian cell lines. Of the 20 gain-of-function variants assessed, 13 were found to alter receptor desensitization properties. Seven variants reduced desensitization at equilibrium, which acts to worsen gain-of-function traits. Six variants accelerated current decay kinetics, which limits gain-of-function traits. All affected patients displayed severe clinical phenotypes with intellectual disability and difficult-to-treat epilepsy. Nevertheless, variants that reduced desensitization at equilibrium were associated with more severe clinical outcomes. This included younger age of first seizure onset (median 0.5 months), movement disorders (dystonia and dyskinesia), epilepsy of infancy with migrating focal seizures (EIMFS) and risk of early mortality. Variants that accelerated current decay kinetics were associated with slightly milder phenotypes with later seizure onset (median 4 months), unclassifiable developmental and epileptic encephalopathies or Lennox-Gastaut syndrome and no movement disorders. Our study reveals that gain-of-function GABRB3 variants can increase or decrease receptor desensitization properties and that there is a correlation with the degree of disease severity. Variants that reduced the desensitization at equilibrium were clustered in the transmembrane regions that constitute the channel pore and correlated with greater disease severity, while variants that accelerated current decay were clustered in the coupling loops responsible for receptor activation and correlated with lesser severity.

GABAA receptors in epilepsy: Elucidating phenotypic divergence through functional analysis of genetic variants.

Normal brain function requires a tightly regulated balance between excitatory and inhibitory neurotransmissions. γ-Aminobutyric acid type A (GABAA ) receptors represent the major class of inhibitory ion channels in the mammalian brain. Dysregulation of these receptors and/or their associated pathways is strongly implicated in the pathophysiology of epilepsy. To date, hundreds of different GABAA receptor subunit variants have been associated with epilepsy, making them a prominent cause of genetically linked epilepsy. While identifying these genetic variants is crucial for accurate diagnosis and effective genetic counselling, it does not necessarily lead to improved personalised treatment options. This is because the identification of a variant does not reveal how the function of GABAA receptors is affected. Genetic variants in GABAA receptor subunits can cause complex changes to receptor properties resulting in various degrees of gain-of-function, loss-of-function or a combination of both. Understanding how variants affect the function of GABAA receptors therefore represents an important first step in the ongoing development of precision therapies. Furthermore, it is important to ensure that functional data are produced using methodologies that allow genetic variants to be classified using clinical guidelines such as those developed by the American College of Medical Genetics and Genomics. This article will review the current knowledge in the field and provide recommendations for future functional analysis of genetic GABAA receptor variants.

Functional genomics of epilepsy-associated mutations in the GABAA receptor subunits reveal that one mutation impairs function and two are catastrophic.

A number of epilepsy-causing mutations have recently been identified in the genes of the α1, ß3, and γ2 subunits comprising the γ-aminobutyric acid type A (GABAA) receptor. These mutations are typically dominant, and in certain cases, such as the α1 and ß3 subunits, they may lead to a mix of receptors at the cell surface that contain no mutant subunits, a single mutated subunit, or two mutated subunits. To determine the effects of mutations in a single subunit or in two subunits on receptor activation, we created a concatenated protein assembly that links all five subunits of the α1ß3γ2 receptor and expresses them in the correct orientation. We created nine separate receptor variants with a single-mutant subunit and four receptors containing two subunits of the γ2R323Q, ß3D120N, ß3T157M, ß3Y302C, and ß3S254F epilepsy-causing mutations. We found that the singly mutated γ2R323Q subunit impairs GABA activation of the receptor by reducing GABA potency. A single ß3D120N, ß3T157M, or ß3Y302C mutation also substantially impaired receptor activation, and two copies of these mutants within a receptor were catastrophic. Of note, an effect of the ß3S254F mutation on GABA potency depended on the location of this mutant subunit within the receptor, possibly because of the membrane environment surrounding the transmembrane region of the receptor. Our results highlight that precise functional genomic analyses of GABAA receptor mutations using concatenated constructs can identify receptors with an intermediate phenotype that contribute to epileptic phenotypes and that are potential drug targets for precision medicine approaches.

Coadministered cannabidiol and clobazam: Preclinical evidence for both pharmacodynamic and pharmacokinetic interactions.

OBJECTIVE: Cannabidiol (CBD) has been approved by the US Food and Drug Administration (FDA) to treat intractable childhood epilepsies, such as Dravet syndrome and Lennox-Gastaut syndrome. However, the intrinsic anticonvulsant activity of CBD has been questioned due to a pharmacokinetic interaction between CBD and a first-line medication, clobazam. This recognized interaction has led to speculation that the anticonvulsant efficacy of CBD may simply reflect CBD augmenting clobazam exposure. The present study aimed to address the nature of the interaction between CBD and clobazam. METHODS: We examined whether CBD inhibits human CYP3A4 and CYP2C19 mediated metabolism of clobazam and N-desmethylclobazam (N-CLB), respectively, and performed studies assessing the effects of CBD on brain and plasma pharmacokinetics of clobazam in mice. We then used the Scn1a+/- mouse model of Dravet syndrome to examine how CBD and clobazam interact. We compared anticonvulsant effects of CBD-clobazam combination therapy to monotherapy against thermally-induced seizures, spontaneous seizures and mortality in Scn1a+/- mice. In addition, we used Xenopus oocytes expressing γ-aminobutyric acid (GABA)A receptors to investigate the activity of GABAA receptors when treated with CBD and clobazam together. RESULTS: CBD potently inhibited CYP3A4 mediated metabolism of clobazam and CYP2C19 mediated metabolism of N-CLB. Combination CBD-clobazam treatment resulted in greater anticonvulsant efficacy in Scn1a+/- mice, but only when an anticonvulsant dose of CBD was used. It is important to note that a sub-anticonvulsant dose of CBD did not promote greater anticonvulsant effects despite increasing plasma clobazam concentrations. In addition, we delineated a novel pharmacodynamic mechanism where CBD and clobazam together enhanced inhibitory GABAA receptor activation. SIGNIFICANCE: Our study highlights the involvement of both pharmacodynamic and pharmacokinetic interactions between CBD and clobazam that may contribute to its efficacy in Dravet syndrome.

Revisiting autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) mutations in the nicotinic acetylcholine receptor reveal an increase in efficacy regardless of stochiometry.

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is a genetic form of epilepsy that is caused by mutations in several genes, including genes encoding for the α4 and ß2 subunits of the nicotinic acetylcholine (nACh) receptor. Pentameric α4ß2 nACh receptors are the most abundant nicotinic receptor in the mammalian brain and form two stoichiometries, the (α4)3(ß2)2 and (α4)2(ß2)3 receptors that differ in their physiological and pharmacological properties. The purpose of this study was to investigate how ADNFLE mutations ß2V287M, ß2V287L or α4T293I manifest themselves in different receptor stoichiometries. We expressed wild-type and mutant receptors in Xenopus oocytes and measured the response to ACh and other agonists at both receptor stoichiometries. For all three mutations, the efficacy of ACh at (α4)2(ß2)3 receptors was increased. At (α4)3(ß2)2 receptors, the efficacy of activation was increased both when two molecules of agonist, either ACh or the site-selective agonist sazetidine-A, were bound at the α4-ß2 interfaces, and when a third ACh molecule was bound at the α4-α4 site. Regardless of stoichiometry, the mutations increased the current elicited by low concentrations of ACh. Further, the smoking cessation agents, nicotine, varenicline and cytisine increased activation of mutant (α4)3(ß2)2 receptors, while only nicotine increased activation of mutant (α4)2(ß2)3 receptors. Chronic exposure of all agonists reduced ACh-activation levels at low and high ACh concentrations. From this, we concluded that mutations that cause ADNFLE manifest themselves in a change in efficacy regardless of the stoichiometry of the receptor.

Oxytocin prevents ethanol actions at δ subunit-containing GABAA receptors and attenuates ethanol-induced motor impairment in rats.

Even moderate doses of alcohol cause considerable impairment of motor coordination, an effect that substantially involves potentiation of GABAergic activity at δ subunit-containing GABA(A) receptors (δ-GABA(A)Rs). Here, we demonstrate that oxytocin selectively attenuates ethanol-induced motor impairment and ethanol-induced increases in GABAergic activity at δ-GABA(A)Rs and that this effect does not involve the oxytocin receptor. Specifically, oxytocin (1 µg i.c.v.) given before ethanol (1.5 g/kg i.p.) attenuated the sedation and ataxia induced by ethanol in the open-field locomotor test, wire-hanging test, and righting-reflex test in male rats. Using two-electrode voltage-clamp electrophysiology in Xenopus oocytes, oxytocin was found to completely block ethanol-enhanced activity at α4ß1δ and α4ß3δ recombinant GABA(A)Rs. Conversely, ethanol had no effect when applied to α4ß1 or α4ß3 cells, demonstrating the critical presence of the δ subunit in this effect. Oxytocin had no effect on the motor impairment or in vitro effects induced by the δ-selective GABA(A)R agonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol, which binds at a different site on δ-GABA(A)Rs than ethanol. Vasopressin, which is a nonapeptide with substantial structural similarity to oxytocin, did not alter ethanol effects at δ-GABA(A)Rs. This pattern of results confirms the specificity of the interaction between oxytocin and ethanol at δ-GABA(A)Rs. Finally, our in vitro constructs did not express any oxytocin receptors, meaning that the observed interactions occur directly at δ-GABA(A)Rs. The profound and direct interaction observed between oxytocin and ethanol at the behavioral and cellular level may have relevance for the development of novel therapeutics for alcohol intoxication and dependence.

A pharmacological assessment of agonists and modulators at α4ß2γ2 and α4ß2δ GABAA receptors: The challenge in comparing apples with oranges.

Extrasynaptically located γ-aminobutyric acid (GABA) receptors type A are often characterized by the presence of a δ subunit in the receptor complex. δ-Containing receptors respond to low ambient concentrations of GABA, or respond to spillover of GABA from the synapse, and give rise to tonic inhibitory currents. In certain brain regions, e.g. thalamocortical neurons, tonic inhibition is estimated to represent the majority of total GABA-mediated inhibition, which has raised substantial interest in extrasynaptic receptors as potential drug targets. Thalamocortical neurons typically express α4ß2/3δ receptors, however, these have proven difficult to study in recombinant in vitro expression systems due to the inherently low current levels elicited in response to GABA. In this study, we sought to characterize a range of agonists and positive allosteric modulators at α4ß2δ and α4ß2γ2 receptors. All tested agonists (GABA, THIP, muscimol, and taurine) displayed between 8 and 22 fold increase in potency at the α4ß2δ receptor. In contrast, modulatory potencies of steroids (allopregnanolone, THDOC and alfaxalone), anesthetics (etomidate, pentobarbital) and Delta-Selective agents 1 and 2 (DS1 and DS2) were similar at α4ß2δ and α4ß2γ2 receptors. When evaluating modulatory efficacies, the neurosteroids and anesthetics displayed highest efficacy at α4ß2γ2 receptors whereas DS1 and in particular DS2 had highest efficacy at α4ß2δ receptors. Overall, several key messages emerged: (i) none of the tested compounds displayed significant selectivity and a great need for identifying new δ-selective compounds remains; (ii) α4ß2δ and α4ß2γ2 receptors have such divergent intrinsic activation properties that valid comparisons of modulator efficacies are at best challenging.

α4ßδ GABA(A) receptors are high-affinity targets for γ-hydroxybutyric acid (GHB).

γ-Hydroxybutyric acid (GHB) binding to brain-specific high-affinity sites is well-established and proposed to explain both physiological and pharmacological actions. However, the mechanistic links between these lines of data are unknown. To identify molecular targets for specific GHB high-affinity binding, we undertook photolinking studies combined with proteomic analyses and identified several GABA(A) receptor subunits as possible candidates. A subsequent functional screening of various recombinant GABA(A) receptors in Xenopus laevis oocytes using the two-electrode voltage clamp technique showed GHB to be a partial agonist at αßδ- but not αßγ-receptors, proving that the δ-subunit is essential for potency and efficacy. GHB showed preference for α4 over α(1,2,6)-subunits and preferably activated α4ß1δ (EC(50) = 140 nM) over α4ß(2/3)δ (EC(50) = 8.41/1.03 mM). Introduction of a mutation, α4F71L, in α4ß1(δ)-receptors completely abolished GHB but not GABA function, indicating nonidentical binding sites. Radioligand binding studies using the specific GHB radioligand [(3)H](E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid showed a 39% reduction (P = 0.0056) in the number of binding sites in α4 KO brain tissue compared with WT controls, corroborating the direct involvement of the α4-subunit in high-affinity GHB binding. Our data link specific GHB forebrain binding sites with α4-containing GABA(A) receptors and postulate a role for extrasynaptic α4δ-containing GABA(A) receptors in GHB pharmacology and physiology. This finding will aid in elucidating the molecular mechanisms behind the proposed function of GHB as a neurotransmitter and its unique therapeutic effects in narcolepsy and alcoholism.

Covalent trapping of methyllycaconitine at the α4-α4 interface of the α4ß2 nicotinic acetylcholine receptor: antagonist binding site and mode of receptor inhibition revealed.

The α4ß2 nicotinic acetylcholine receptors (nAChRs) are widely expressed in the brain and are implicated in a variety of physiological processes. There are two stoichiometries of the α4ß2 nAChR, (α4)2(ß2)3 and (α4)3(ß2)2, with different sensitivities to acetylcholine (ACh), but their pharmacological profiles are not fully understood. Methyllycaconitine (MLA) is known to be an antagonist of nAChRs. Using the two-electrode voltage clamp technique and α4ß2 nAChRs in the Xenopus oocyte expression system, we demonstrate that inhibition by MLA occurs via two different mechanisms; that is, a direct competitive antagonism and an apparently insurmountable mechanism that only occurs after preincubation with MLA. We hypothesized an additional MLA binding site in the α4-α4 interface that is unique to this stoichiometry. To prove this, we covalently trapped a cysteine-reactive MLA analog at an α4ß2 receptor containing an α4(D204C) mutation predicted by homology modeling to be within reach of the reactive probe. We demonstrate that covalent trapping results in irreversible reduction of ACh-elicited currents in the (α4)3(ß2)2 stoichiometry, indicating that MLA binds to the α4-α4 interface of the (α4)3(ß2)2 and providing direct evidence of ligand binding to the α4-α4 interface. Consistent with other studies, we propose that the α4-α4 interface is a structural target for potential therapeutics that modulate (α4)3(ß2)2 nAChRs.

Potency of GABA at human recombinant GABA(A) receptors expressed in Xenopus oocytes: a mini review.

GABAA receptors are members of the ligand-gated ion channel superfamily that mediate inhibitory neurotransmission in the central nervous system. They are thought to be composed of 2 alpha (α), 2 beta (ß) subunits and one other such as a gamma (γ) or delta (δ) subunit. The potency of GABA is influenced by the subunit composition. However, there are no reported systematic studies that evaluate GABA potency on a comprehensive number of subunit combinations expressed in Xenopus oocytes, despite the wide use of this heterologous expression system in structure-function studies and drug discovery. Thus, the aim of this study was to conduct a systematic characterization of the potency of GABA at 43 human recombinant GABA(A) receptor combinations expressed in Xenopus oocytes using the two-electrode voltage clamp technique. The results show that the α-subunits and to a lesser extent, the ß-subunits influence GABA potency. Of the binary and ternary combinations with and without the γ2L subunit, the α6/γ2L-containing receptors were the most sensitive to GABA, while the ß2- or ß3-subunit conferred higher sensitivity to GABA than receptors containing the ß1-subunit with the exception of the α2ß1γ2L and α6ß1γ2L subtypes. Of the δ-subunit containing GABA(A) receptors, α4/δ-containing GABA(A) receptors displayed highest GABA sensitivity, with mid-nanomolar concentrations activating α4ß1δ and α4ß3δ receptors. At α4ß2δ, GABA had low micromolar activity.

Gain-of-function and loss-of-function GABRB3 variants lead to distinct clinical phenotypes in patients with developmental and epileptic encephalopathies.

Many patients with developmental and epileptic encephalopathies present with variants in genes coding for GABAA receptors. These variants are presumed to cause loss-of-function receptors leading to reduced neuronal GABAergic activity. Yet, patients with GABAA receptor variants have diverse clinical phenotypes and many are refractory to treatment despite the availability of drugs that enhance GABAergic activity. Here we show that 44 pathogenic GABRB3 missense variants segregate into gain-of-function and loss-of-function groups and respective patients display distinct clinical phenotypes. The gain-of-function cohort (n = 27 patients) presented with a younger age of seizure onset, higher risk of severe intellectual disability, focal seizures at onset, hypotonia, and lower likelihood of seizure freedom in response to treatment. Febrile seizures at onset are exclusive to the loss-of-function cohort (n = 47 patients). Overall, patients with GABRB3 variants that increase GABAergic activity have more severe developmental and epileptic encephalopathies. This paradoxical finding challenges our current understanding of the GABAergic system in epilepsy and how patients should be treated.

The anticonvulsant zonisamide positively modulates recombinant and native glycine receptors at clinically relevant concentrations.

GABAA and glycine receptors mediate fast synaptic inhibitory neurotransmission. Despite studies showing that activation of cerebral glycine receptors could be a potential strategy in the treatment of epilepsy, few studies have assessed the effects of existing anticonvulsant therapies on recombinant or native glycine receptors. We, therefore, evaluated the actions of a series of anticonvulsants at recombinant human homo-oligomeric glycine receptor α1, α2 and α3 subtypes expressed in Xenopus oocytes using two-electrode voltage-clamp methods, and then assessed the most effective drug at native glycine receptors from entorhinal cortex neurons using whole-cell voltage-clamp recordings. Ganaxolone, tiagabine and zonisamide positively modulated glycine induced currents at recombinant homomeric glycine receptors. Of these, zonisamide was the most efficacious and exhibited an EC50 value ranging between 450 and 560 µM at α1, α2 and α3 subtypes. These values were not significantly different indicating a non-selective modulation of glycine receptors. Using a therapeutic concentration of zonisamide (100 µM), the potency of glycine was significantly shifted from 106 to 56 µM at α1, 185 to 112 µM at α2, and 245 to 91 µM at α3 receptors. Furthermore, zonisamide (100 µM) potentiated exogenous homomeric and heteromeric glycine mediated currents from layer II pyramidal cells of the lateral or medial entorhinal cortex. As therapeutic concentrations of zonisamide positively modulate recombinant and native glycine receptors, we propose that the anticonvulsant effects of zonisamide may, at least in part, be mediated via this action.

Cannabigerolic acid, a major biosynthetic precursor molecule in cannabis, exhibits divergent effects on seizures in mouse models of epilepsy.

BACKGROUND AND PURPOSE: Cannabis has been used to treat epilepsy for millennia, with such use validated by regulatory approval of cannabidiol (CBD) for Dravet syndrome. Unregulated artisanal cannabis-based products used to treat children with intractable epilepsies often contain relatively low doses of CBD but are enriched in other phytocannabinoids. This raises the possibility that other cannabis constituents might have anticonvulsant properties. EXPERIMENTAL APPROACH: We used the Scn1a+/- mouse model of Dravet syndrome to investigate the cannabis plant for phytocannabinoids with anticonvulsant effects against hyperthermia-induced seizures. The most promising, cannabigerolic acid (CBGA), was further examined against spontaneous seizures and survival in Scn1a+/- mice and in electroshock seizure models. Pharmacological effects of CBGA were surveyed across multiple drug targets. KEY RESULTS: The initial screen identified three phytocannabinoids with novel anticonvulsant properties: CBGA, cannabidivarinic acid (CBDVA) and cannabigerovarinic acid (CBGVA). CBGA was most potent and potentiated the anticonvulsant effects of clobazam against hyperthermia-induced and spontaneous seizures, and was anticonvulsant in the MES threshold test. However, CBGA was proconvulsant in the 6-Hz threshold test and a high dose increased spontaneous seizure frequency in Scn1a+/- mice. CBGA was found to interact with numerous epilepsy-relevant targets including GPR55, TRPV1 channels and GABAA receptors. CONCLUSION AND IMPLICATIONS: These results suggest that CBGA, CBDVA and CBGVA may contribute to the effects of cannabis-based products in childhood epilepsy. Although these phytocannabinoids have anticonvulsant potential and could be lead compounds for drug development programmes, several liabilities would need to be overcome before CBD is superseded by another in this class.

Conformational changes in extracellular loop 2 associated with signal transduction in the glycine receptor.

Ligand-gated ion channels efficiently couple neurotransmitter binding to the opening of an intrinsic ion channel to generate the post-synaptic potentials that are characteristic of fast synaptic transmission. In the Cys-loop family of ligand-gated ion channels, the ligand-binding site is approximately 60 Å above the channel gate. Structural modelling of related proteins and mutagenesis studies led to the hypothesis that loops 2 and 7 of the extracellular domain may couple ligand binding to receptor activation. Mutating loop 2 residues of the glycine receptor to cysteine reveals an alternating pattern of effect upon receptor function. Mutations A52C, T54C and M56C produced a threefold right-shift in EC(50) . In contrast, a 30-fold right-shift was seen for mutations E53C, T55C and D57C. Loop 2 conformational changes associated with ligand binding were assessed by measuring the rate of covalent modification of substituted cysteines by charged methane thiosulfonate reagents. We show for the first time state-dependent differences in the rate of reaction. A52C and T54C are more accessible in the resting state and M56C is more accessible in the activated state. These results demonstrate that loop 2 does undergo a conformational change as part of the mechanism that couples ligand binding to channel opening.

Ligand-gated ion channels in genetic disorders and the question of efficacy.

Whole-genome sequencing has unearthed a substantial number of individual variants in ion channels associated with genetic disorders. Ligand-gated ion channels, including glycine, γ-aminobutyric acid type A and nicotinic acetylcholine receptors, have long been known to harbour genetic variants associated with hyperekplexia and different forms of epilepsy. In some of these cases, missense variants enhance or impair the intrinsic ability of the receptor to convert ligand binding to channel opening, or the efficacy of receptor activation. We review the current understanding of how ligand-gated ion channels are activated and the properties that define the efficacy of an agonist, and how these properties can be altered by disease-causing variants. Additionally, we consider the mechanisms defining drug modulation of receptors and consider how this may differ in genetic variants. This fundamental knowledge is likely to be essential in understanding how effective treatments will be for patients with genetic variants in ligand-gated ion channels.

Venom-derived modulators of epilepsy-related ion channels.

Epilepsy is characterised by spontaneous recurrent seizures that are caused by an imbalance between neuronal excitability and inhibition. Since ion channels play fundamental roles in the generation and propagation of action potentials as well as neurotransmitter release at a subset of excitatory and inhibitory synapses, their dysfunction has been linked to a wide variety of epilepsies. Indeed, these unique proteins are the major biological targets for antiepileptic drugs. Selective targeting of a specific ion channel subtype remains challenging for small molecules, due to the high level of homology among members of the same channel family. As a consequence, there is a growing trend to target ion channels with biologics. Venoms are the best known natural source of ion channel modulators, and venom peptides are increasingly recognised as potential therapeutics due to their high selectivity and potency gained through millions of years of evolutionary selection pressure. Here we describe the major ion channel families involved in the pathogenesis of various types of epilepsy, including voltage-gated Na+, K+, Ca2+ channels, Cys-loop receptors, ionotropic glutamate receptors and P2X receptors, and currently available venom-derived peptides that target these channel proteins. Although only a small number of venom peptides have successfully progressed to the clinic, there is reason to be optimistic about their development as antiepileptic drugs, notwithstanding the challenges associated with development of any class of peptide drug.

AE Succinimide, an Analogue of Methyllycaconitine, When Bound Generates a Nonconducting Conformation of the α4ß2 Nicotinic Acetylcholine Receptor.

Nicotinic acetylcholine (nACh) receptors are pentameric ligand-gated ion channels that mediate fast synaptic transmission. The α4ß2 nACh receptor is highly expressed in the brain and exists in two functional stoichiometries: the (α4)2(ß2)3 and (α4)3(ß2)2 that differ by an ACh-binding site at the α4-α4 interface of (α4)3(ß2)2 receptors. Methyllycaconitine (MLA) is an nACh receptor antagonist, and while potent at both α7 and α4ß2 nACh receptors, it has a higher selectivity for the α7 nACh receptor. The anthranilate-succinimide ester side-chain is important for its activity and selectivity. Here we identify a simplified MLA analogue that contains only the A and E ring skeleton of MLA, AE succinimide, that binds close to the channel lumen to display insurmountable inhibition at α4ß2 nACh receptors. Although inhibition by AE succinimide was found to be voltage-dependent indicating a possible pore channel blocker, substituted-cysteine accessibility experiments indicated it did not bind between 2'-16' region of the channel pore. Instead, we found that upon binding and in the presence of ACh, there is a conformational change to the channel membrane that was identified when the compound was assessed against (α4 V13'C)ß2 nACh receptors. It was found that in the 3:2 stoichiometry the two adjacent α4 subunits containing 13' cysteine mutations formed a disulfide bond and occluded ion conductance. This was reversed by treatment with the reducing agent, dithiothreitol. Thus, AE succinimide has a different mechanism of inhibition to both MLA and other AE analogues, such as AE bicyclic alcohol, in that upon binding to an as yet unidentified site, AE succinimide in the presence of ACh induces a conformational change to the channel that generates a ligand-bound closed state.

Gain-of-function GABRB3 variants identified in vigabatrin-hypersensitive epileptic encephalopathies.

Variants in the GABRB3 gene encoding the ß3-subunit of the γ-aminobutyric acid type A ( receptor are associated with various developmental and epileptic encephalopathies. Typically, these variants cause a loss-of-function molecular phenotype whereby γ-aminobutyric acid has reduced inhibitory effectiveness leading to seizures. Drugs that potentiate inhibitory GABAergic activity, such as nitrazepam, phenobarbital or vigabatrin, are expected to compensate for this and thereby reduce seizure frequency. However, vigabatrin, a drug that inhibits γ-aminobutyric acid transaminase to increase tonic γ-aminobutyric acid currents, has mixed success in treating seizures in patients with GABRB3 variants: some patients experience seizure cessation, but there is hypersensitivity in some patients associated with hypotonia, sedation and respiratory suppression. A GABRB3 variant that responds well to vigabatrin involves a truncation variant (p.Arg194*) resulting in a clear loss-of-function. We hypothesized that patients with a hypersensitive response to vigabatrin may exhibit a different γ-aminobutyric acid A receptor phenotype. To test this hypothesis, we evaluated the phenotype of de novo variants in GABRB3 (p.Glu77Lys and p.Thr287Ile) associated with patients who are clinically hypersensitive to vigabatrin. We introduced the GABRB3 p.Glu77Lys and p.Thr287Ile variants into a concatenated synaptic and extrasynaptic γ-aminobutyric acid A receptor construct, to resemble the γ-aminobutyric acid A receptor expression by a patient heterozygous for the GABRB3 variant. The mRNA of these constructs was injected into Xenopus oocytes and activation properties of each receptor measured by two-electrode voltage clamp electrophysiology. Results showed an atypical gain-of-function molecular phenotype in the GABRB3 p.Glu77Lys and p.Thr287Ile variants characterized by increased potency of γ-aminobutyric acid A without change to the estimated maximum open channel probability, deactivation kinetics or absolute currents. Modelling of the activation properties of the receptors indicated that either variant caused increased chloride flux in response to low concentrations of γ-aminobutyric acid that mediate tonic currents. We therefore propose that the hypersensitivity reaction to vigabatrin is a result of GABRB3 variants that exacerbate GABAergic tonic currents and caution is required when prescribing vigabatrin. In contrast, drug strategies increasing tonic currents in loss-of-function variants are likely to be a safe and effective therapy. This study demonstrates that functional genomics can explain beneficial and adverse anti-epileptic drug effects, and propose that vigabatrin should be considered in patients with clear loss-of-function GABRB3 variants.