Inhibitory Neurosteroids and the GABAA Receptor

Forty Years Searching for Neurosteroid Binding Sites on GABAA Receptors

Brain inhibition is a vital process for controlling and sculpting the excitability of the central nervous system in healthy individuals. This level of control is provided over several timescales and involves the neurotransmitter GABA acting at inhibitory synapses to: rapidly inhibit neurons by activating the GABAA receptor; over a slower timescale, to tonically activate extrasynaptic GABAA receptors to provide a low level of background inhibition; and finally, to activate G-protein coupled GABAB receptors to control transmitter release by inhibiting presynaptic Ca2+ channels whilst providing postsynaptic inhibition via K+ channel activation. From this plethora of roles for GABA and its receptors, the GABAA receptor isoform is of major interest due to its dynamic functional plasticity, which in part, is due to being targeted by modulatory brain neurosteroids derived from sex and stress hormones. This family of neurosteroids can, depending on their structure, potentiate, activate and also inhibit the activity of GABAA receptors to affect brain inhibition. This review tracks the methods that have been deployed in probing GABAA receptors, and charts the sterling efforts made by several groups to locate the key neurosteroid binding sites that affect these important receptors. Increasing our knowledge of these binding sites will greatly facilitate our understanding of the physiological roles of neurosteroids and will help to advance their use as novel therapeutics to combat debilitating brain diseases.

Pregnenolone sulfate analogues differentially modulate GABAA receptor closed/desensitised states

BACKGROUND AND PURPOSE

GABAA receptors are regulated by numerous classes of allosteric modulators. However, regulation of receptor macroscopic desensitisation remains largely unexplored and may offer new therapeutic opportunities. Here, we report the emerging potential for modulating desensitisation with analogues of the endogenous inhibitory neurosteroid, pregnenolone sulfate.

EXPERIMENTAL APPROACH

New pregnenolone sulfate analogues were synthesised incorporating various heterocyclic substitutions located at the C-21 position on ring D. The pharmacological profiles of these compounds were assessed using electrophysiology and recombinant GABAA receptors together with mutagenesis, molecular dynamics simulations, structural modelling and kinetic simulations.

KEY RESULTS

All seven analogues retained a negative allosteric modulatory capability whilst exhibiting diverse potencies. Interestingly, we observed differential effects on GABA current decay by compounds incorporating either a six- (compound 5) or five-membered heterocyclic ring (compound 6) on C-21, which was independent of their potencies as inhibitors. We propose that differences in molecular charges, and the targeted binding of analogues to specific states of the GABAA receptor, are the most likely cause of the distinctive functional profiles.

CONCLUSIONS AND IMPLICATIONS

Our findings reveal that heterocyclic addition to inhibitory neurosteroids not only affected their potency and macroscopic efficacy but also affected innate receptor mechanisms that underlie desensitisation. Acute modulation of macroscopic desensitisation will determine the degree and duration of GABA inhibition, which are vital for the integration of neural circuit activity. Discovery of this form of modulation could present an opportunity for next-generation GABAA receptor drug design and development.

New GABA-Targeting Therapies for the Treatment of Seizures and Epilepsy: I. Role of GABA as a Modulator of Seizure Activity and Recently Approved Medications Acting on the GABA System

γ-Aminobutyric acid (GABA) is the most prevalent inhibitory neurotransmitter in the mammalian brain and has been found to play an important role in the pathogenesis or the expression of many neurological diseases, including epilepsy. Although GABA can act on different receptor subtypes, the component of the GABA system that is most critical to modulation of seizure activity is the GABAA-receptor-chloride (Cl-) channel complex, which controls the movement of Cl- ions across the neuronal membrane. In the mature brain, binding of GABA to GABAA receptors evokes a hyperpolarising (anticonvulsant) response, which is mediated by influx of Cl- into the cell driven by its concentration gradient between extracellular and intracellular fluid. However, in the immature brain and under certain pathological conditions, GABA can exert a paradoxical depolarising (proconvulsant) effect as a result of an efflux of chloride from high intracellular to lower extracellular Cl- levels. Extensive preclinical and clinical evidence indicates that alterations in GABAergic inhibition caused by drugs, toxins, gene defects or other disease states (including seizures themselves) play a causative or contributing role in facilitating or maintaning seizure activity. Conversely, enhancement of GABAergic transmission through pharmacological modulation of the GABA system is a major mechanism by which different antiseizure medications exert their therapeutic effect. In this article, we review the pharmacology and function of the GABA system and its perturbation in seizure disorders, and highlight how improved understanding of this system offers opportunities to develop more efficacious and better tolerated antiseizure medications. We also review the available data for the two most recently approved antiseizure medications that act, at least in part, through GABAergic mechanisms, namely cenobamate and ganaxolone. Differences in the mode of drug discovery, pharmacological profile, pharmacokinetic properties, drug-drug interaction potential, and clinical efficacy and tolerability of these agents are discussed.

Pre- and postsynaptic modulation of hippocampal inhibitory synaptic transmission by pregnenolone sulphate

Neurosteroids are important endogenous modulators of GABA_A receptor-mediated neurotransmission within the CNS and play a vital role in maintaining normal healthy brain function. Research has mainly focussed on neurosteroids such as allopregnanolone and tetrahydro-deoxycorticosterone (THDOC) which are allosteric potentiators of GABA_A receptors, whilst the sulphated steroids, including pregnenolone sulphate (PS), which inhibit GABA_A receptor function, have been relatively neglected. Importantly, a full description of PS effects on inhibitory synaptic transmission, at concentrations that are expected to inhibit postsynaptic GABA_A receptors, is lacking. Here, we address this deficit by recording inhibitory postsynaptic currents (IPSCs) from rat hippocampal neurons both in culture and in acute brain slices and explore the impact of PS at micromolar concentrations. We reveal that PS inhibits postsynaptic GABA_A receptors, evident from reductions in IPSC amplitude and decay time. Concurrently, PS also causes an increase in synaptic GABA release which we discover is due to the activation of presynaptic TRPM3 receptors located close to presynaptic GABA release sites. Pharmacological blockade of TRPM3 receptors uncovers a PS-evoked reduction in IPSC frequency. This second presynaptic effect is caused by PS activation of inwardly-rectifying Kir2.3 channels on interneurons, which act to depress synaptic GABA release. Overall, we provide a comprehensive characterisation of pre- and postsynaptic modulation by PS of inhibitory synaptic transmission onto hippocampal neurons which elucidates the diverse mechanisms by which this understudied neurosteroid can modulate brain function.

Role of allopregnanolone-mediated γ-aminobutyric acid A receptor sensitivity in the pathogenesis of premenstrual dysphoric disorder: Toward precise targets for translational medicine and drug development

Premenstrual dysphoric disorder (PMDD) can be conceptualized as a disorder of suboptimal sensitivity to neuroactive steroid hormones. Its core symptoms (emotional instability, irritability, depression, and anxiety) are related to the increase of stress sensitivity due to the fluctuation of hormone level in luteal phase of the menstrual cycle. In this review, we describe the emotional regulatory effect of allopregnanolone (ALLO), and summarize the relationship between ALLO and γ-aminobutyric acid A (GABA_A) receptor subunits based on rodent experiments and clinical observations. A rapid decrease in ALLO reduces the sensitivity of GABA_A receptor, and reduces the chloride influx, hindered the inhibitory effect of GABAergic neurons on pyramidal neurons, and then increased the excitability of pyramidal neurons, resulting in PMDD-like behavior. Finally, we discuss in depth the treatment of PMDD with targeted GABA_A receptors, hoping to find a precise target for drug development and subsequent clinical application. In conclusion, PMDD pathophysiology is rooted in GABA_A receptor sensitivity changes caused by rapid changes in ALLO levels. Targeting GABA_A receptors may alleviate the occurrence of PMDD.

Biosynthesis and signalling functions of central and peripheral nervous system neurosteroids in health and disease

Neurosteroids are steroid hormones synthesised de novo in the brain and peripheral nervous tissues. In contrast to adrenal steroid hormones that act on intracellular nuclear receptors, neurosteroids directly modulate plasma membrane ion channels and regulate intracellular signalling. This review provides an overview of the work that led to the discovery of neurosteroids, our current understanding of their intracellular biosynthetic machinery, and their roles in regulating the development and function of nervous tissue. Neurosteroids mediate signalling in the brain via multiple mechanisms. Here, we describe in detail their effects on GABA (inhibitory) and NMDA (excitatory) receptors, two signalling pathways of opposing function. Furthermore, emerging evidence points to altered neurosteroid function and signalling in neurological disease. This review focuses on neurodegenerative diseases associated with altered neurosteroid metabolism, mainly Niemann-Pick type C, multiple sclerosis and Alzheimer disease. Finally, we summarise the use of natural and synthetic neurosteroids as current and emerging therapeutics alongside their potential use as disease biomarkers.

Hormonal Contraception and Depression: Updated Evidence and Implications in Clinical Practice

Hormonal contraceptives are used worldwide by more than 100 million women. Some studies have been published about the possible appearance of depressive symptoms when using hormonal contraceptives, but this link is still a matter of debate. The purpose of this review is to provide an update of the literature on this issue, and to investigate the possible explanations of this problem based on animal and human studies. The main pathway responsible for menstrual cycle-related mood changes is the γ-aminobutyric acid pathway, which is sensitive to changes in the levels of progesterone and of its metabolites, the neurosteroids. In particular, allopregnanolone is a potentiating neurosteroid with anxiolytic and anti-convulsant effects whose levels change during a normal menstrual cycle together with progesterone levels. Progestins have different effects on allopregnanolone, mainly owing to their diverse androgenicity. Moreover, they might affect brain structure and function, even though the meaning of these changes has yet to be clarified. It is important to define the groups of women in which negative mood disorders are more likely to occur. Adolescence is a critical period and this age-specific vulnerability is complex and likely bidirectional. Moreover, women with a history of mood affective disorders or premenstrual dysphoric syndrome are at a higher risk when taking contraceptives. In this review, we aim to provide clinicians with advice on how to approach these difficult situations.

Turning a Drug Target into a Drug Candidate: A New Paradigm for Neurological Drug Discovery?

The conventional paradigm for developing new treatments for disease mainly involves either the discovery of new drug targets, or finding new, improved drugs for old targets. However, an ion channel found only in invertebrates offers the potential of a completely new paradigm in which an established drug target can be re-engineered to serve as a new candidate therapeutic agent. The L-glutamate-gated chloride channels (GluCls) of invertebrates are absent from vertebrate genomes, offering the opportunity to introduce this exogenous, inhibitory, L-glutamate receptor into vertebrate neuronal circuits either as a tool with which to study neural networks, or a candidate therapy. Epileptic seizures can involve L-glutamate-induced hyper-excitation and toxicity. Variant GluCls, with their inhibitory responses to L-glutamate, when engineered into human neurons, might counter the excitotoxic effects of excess L-glutamate. In reviewing recent studies on model organisms, it appears that this approach might offer a new paradigm for the development of candidate therapeutics for epilepsy.

Neurosteroids as Selective Inhibitors of Glycine Receptor Activity: Structure-Activity Relationship Study on Endogenous Androstanes and Androstenes

The ability of androstane and androstene neurosteroids with modifications at C-17, C-5, and C-3 (compounds 1-9) to influence the functional activity of inhibitory glycine and γ-aminobutyric acid (GABA) receptors was estimated. The glycine- and GABA-induced chloride current (I_Gly and I_GABA) were measured in isolated pyramidal neurons of the rat hippocampus and isolated rat cerebellar Purkinje cells, correspondingly, using the patch-clamp technique. Our results demonstrate that all the nine neurosteroids display similar biological activity, namely, they strongly inhibited I_Gly and weakly inhibited I_GABA. The threshold concentration of neurosteroids inducing effects on I_Gly was 0.1 μM, and for effects on I_GABA was 10–50 μM. Moreover, our compounds accelerated desensitization of the I_Gly with the IC₅₀ values varying from 0.12 to 0.49 μM and decreased the peak amplitude with IC₅₀ values varying from 16 to 22 μM. Interestingly, our study revealed that only compounds 4 (epiandrosterone) and 8 (dehydroepiandrosterone) were able to cause a significant change in I_GABA in 10 μM concentration. Moreover, compounds 3 (testosterone), 5 (epitestosterone), 6 (dihydroandrostenedione), and 9 (etiocholanedione) did not modulate I_GABA up to the concentration of 50 μM. Thus, we conclude that compounds 3, 5, 6, and 9 may be identified as selective modulators of I_Gly. Our results offer new avenues of investigation in the field of drug-like selective modulators of I_Gly.

Perinatal high-fat diet alters development of GABAA receptor subunits in dorsal motor nucleus of vagus

Perinatal high-fat diet (pHFD) exposure increases the inhibition of dorsal motor nucleus of the vagus (DMV) neurons, potentially contributing to the dysregulation of gastric functions. The aim of this study was to test the hypothesis that pHFD increases the inhibition of DMV neurons by disrupting GABAA receptor subunit development. In vivo gastric recordings were made from adult anesthetized Sprague-Dawley rats fed a control or pHFD (14 or 60% kcal from fat, respectively) from embryonic day 13 (E13) to postnatal day 42 (P42), and response to brainstem microinjection of benzodiazepines was assessed. Whole cell patch clamp recordings from DMV neurons assessed the functional expression of GABAA α subunits, whereas mRNA and protein expression were measured via qPCR and Western blotting, respectively. pHFD decreased basal antrum and corpus motility, whereas brainstem microinjection of L838,417 (positive allosteric modulator of α2/3 subunit-containing GABAA receptors) produced a larger decrease in gastric tone and motility. GABAergic miniature inhibitory postsynaptic currents in pHFD DMV neurons were responsive to L838,417 throughout development, unlike control DMV neurons, which were responsive only at early postnatal timepoints. Brainstem mRNA and protein expression of the GABAA α1,2, and 3 subunits, however, did not differ between control and pHFD rats. This study suggests that pHFD exposure arrests the development of synaptic GABAA α2/3 receptor subunits on DMV neurons and that functional synaptic expression is maintained into adulthood, although cellular localization may differ. The tonic activation of slower GABAA α2/3 subunit-containing receptors implies that such developmental changes may contribute to the observed decreased gastric motility. NEW & NOTEWORTHY Vagal neurocircuits involved in the control of gastric functions, satiation, and food intake are subject to significant developmental regulation postnatally, with immature GABAA receptors expressing slower α2/3-subunits, whereas mature GABAA receptor express faster α1-subunits. After perinatal high-fat diet exposure, this developmental regulation of dorsal motor nucleus of the vagus (DMV) neurons is disrupted, increasing their tonic GABAergic inhibition, decreasing efferent output, and potentially decreasing gastric motility.

Receptor variants and the development of centrally acting medications

The progressive changes in research paradigms observed in the largest pharmaceutical companies and the burgeoning of biotechnology startups over the last 10 years have generated a need for outsourcing research facilities. In parallel, progress made in the fields of genomics, protein expression in recombinant systems, and electrophysiological recording methods have offered new possibilities for the development of contract research organizations (CROs). Successful partnering between pharmaceutical companies and CROs largely depends upon the competences and scientific quality on offer for the discovery of novel active molecules and targets. Thus, it is critical to review the knowledge in the field of neuroscience research, how genetic approaches are augmenting our knowledge, and how they can be applied in the translation from the identification of potential molecules up to the first clinical trials. Taking these together, it is apparent that CROs have an important role to play in the neuroscience of drug discovery.


Visualizing pregnenolone sulfate-like modulators of NMDA receptor function reveals intracellular and plasma-membrane localization

Positive modulators of NMDA receptors are important candidates for therapeutic development to treat psychiatric disorders including autism and schizophrenia. Sulfated neurosteroids have been studied as positive allosteric modulators of NMDA receptors for years, but we understand little about the cellular fate of these compounds, an important consideration for drug development. Here we focus on a visualizable sulfated neurosteroid analogue, KK-169. As expected of a pregnenolone sulfate analogue, the compound strongly potentiates NMDA receptor function, is an antagonist of GABA_A receptors, exhibits occlusion with pregnenolone sulfate potentiation, and requires receptor domains important for pregnenolone sulfate potentiation. KK-169 exhibits somewhat higher potency than the natural parent, pregnenolone sulfate. The analogue contains a side-chain alkyne group, which we exploited for retrospective click labeling of neurons. Although the anionic sulfate group is expected to hinder cell entry, we detected significant accumulation of KK-169 in neurons with even brief incubations. Adding a photolabile diazirine group revealed that the expected plasma membrane localization of KK-169 is likely lost during fixation. Overall, our studies reveal new facets of the structure-activity relationship of neurosteroids at NMDA receptors, and their intracellular distribution suggests that sulfated neurosteroids could have unappreciated targets in addition to plasma membrane receptors.

Neurosteroids: non-genomic pathways in neuroplasticity and involvement in neurological diseases

Neurosteroids are neuroactive brain-born steroids. They can act through non-genomic and/or through genomic pathways. Genomic pathways are largely described for steroid hormones: the binding to nuclear receptors leads to transcription regulation. Pregnenolone, Dehydroepiandrosterone, their respective sulfate esters and Allopregnanolone have no corresponding nuclear receptor identified so far whereas some of their non-genomic targets have been identified. Neuroplasticity is the capacity that neuronal networks have to change their structure and function in response to biological and/or environmental signals; it is regulated by several mechanisms, including those that involve neurosteroids. In this review, after a description of their biosynthesis, the effects of Pregnenolone, Dehydroepiandrosterone, their respective sulfate esters and Allopregnanolone on their targets will be exposed. We then shall highlight that neurosteroids, by acting on these targets, can regulate neurogenesis, structural and functional plasticity. Finally, we will discuss the therapeutic potential of neurosteroids in the pathophysiology of neurological diseases in which alterations of neuroplasticity are associated with changes in neurosteroid levels.

Neurosteroid Transport in the Brain: Role of ABC and SLC Transporters

Neurosteroids, comprising pregnane, androstane, and sulfated steroids can alter neuronal excitability through interaction with ligand-gated ion channels and other receptors and have therefore a therapeutic potential in several brain disorders. They can be formed in brain cells or are synthesized by an endocrine gland and reach the brain by penetrating the blood–brain barrier (BBB). Especially sulfated steroids such as pregnenolone sulfate (PregS) and dehydroepiandrosterone sulfate (DHEAS) depend on transporter proteins to cross membranes. In this review, we discuss the involvement of ATP-binding cassette (ABC)- and solute carrier (SLC)-type membrane proteins in the transport of these compounds at the BBB and in the choroid plexus (CP), but also in the secretion from neurons and glial cells. Among the ABC transporters, especially BCRP (ABCG2) and several MRP/ABCC subfamily members (MRP1, MRP4, MRP8) are expressed in the brain and known to efflux conjugated steroids. Furthermore, several SLC transporters have been shown to mediate cellular uptake of steroid sulfates. These include members of the OATP/SLCO subfamily, namely OATP1A2 and OATP2B1, as well as OAT3 (SLC22A3), which have been reported to be expressed at the BBB, in the CP and in part in neurons. Furthermore, a role of the organic solute transporter OSTα-OSTβ (SLC51A/B) in brain DHEAS/PregS homeostasis has been proposed. This transporter was reported to be localized especially in steroidogenic cells of the cerebellum and hippocampus. To date, the impact of transporters on neurosteroid homeostasis is still poorly understood. Further insights are desirable also with regard to the therapeutic potential of these compounds.

Probing GABAA receptors with inhibitory neurosteroids

γ-aminobutyric acid type A receptors (GABA_ARs) are important components of the central nervous system and they are functionally tasked with controlling neuronal excitability. These receptors are subject to post-translational modification and also to modulation by endogenous regulators, such as the neurosteroids. These modulators can either potentiate or inhibit GABA_AR function. Whilst the former class of neurosteroids are considered to bind to and act from the transmembrane domain of the receptor, the domains that are important for the inhibitory neurosteroids remain less clear. In this study, we systematically compare a panel of recombinant synaptic-type and extrasynaptic-type GABA_ARs expressed in heterologous cell systems for their sensitivity to inhibition by the classic inhibitory neurosteroid, pregnenolone sulphate. Generally, peak GABA current responses were inhibited less compared to steady-state currents, implicating the desensitised state in inhibition. Moreover, pregnenolone sulphate inhibition increased with GABA concentration, but showed minimal voltage dependence. There was no strong dependence of inhibition on receptor subunit composition, the exception being the ρ1 receptor, which is markedly less sensitive. By using competition experiments with pregnenolone sulphate and the GABA channel blocker picrotoxinin, discrete binding sites are proposed. Furthermore, by assessing inhibition using site-directed mutagenesis and receptor chimeras comprising α, β or γ subunits with ρ1 subunits, the receptor transmembrane domains are strongly implicated in mediating inhibition and most likely the binding location for pregnenolone sulphate in GABA_ARs.

This article is part of the “Special Issue Dedicated to Norman G. Bowery”.

•

A range of GABA_A receptor subtypes are inhibited by pregenolone sulphate.

•

Peak GABA curents are less sensitive to inhibition than steady-state currents.

•

Desensitised state of GABA_A receptors most sensitive to neurosteroid inhibition.

•

Inhibition increases with GABA concentration, but not strongly voltage-dependent.

•

Pregnenolone sulphate binding site located within subunit transmembrane domains.

Crystal structures of a GABAA-receptor chimera reveal new endogenous neurosteroid-binding sites

γ-Aminobutyric acid receptors (GABA_ARs) are vital for controlling excitability in the brain. This is emphasized by the numerous neuropsychiatric disorders that result from receptor dysfunction. A critical component of most native GABA_ARs is the α subunit. Its transmembrane domain is the target for many modulators, including endogenous brain neurosteroids that impact anxiety, stress and depression, and for therapeutic drugs, such as general anesthetics. Understanding the basis for the modulation of GABA_AR function requires high-resolution structures. Here we present the first atomic structures of a GABA_AR chimera at 2.8-Å resolution, including those bound with potentiating and inhibitory neurosteroids. These structures define new allosteric binding sites for these modulators that are associated with the α-subunit transmembrane domain. Our findings will enable the exploitation of neurosteroids for therapeutic drug design to regulate GABA_ARs in neurological disorders.

Mutational Analysis at Intersubunit Interfaces of an Anionic Glutamate Receptor Reveals a Key Interaction Important for Channel Gating by Ivermectin

The broad-spectrum anthelmintic drug ivermectin (IVM) activates and stabilizes an open-channel conformation of invertebrate chloride-selective glutamate receptors (GluClRs), thereby causing a continuous inflow of chloride ions and sustained membrane hyperpolarization. These effects suppress nervous impulses and vital physiological processes in parasitic nematodes. The GluClRs are pentamers. Homopentameric receptors assembled from the Caenorhabditis elegans (C. elegans) GluClα (GLC-1) subunit can inherently respond to IVM but not to glutamate (the neurotransmitter). In contrast, heteromeric GluClα/β (GLC-1/GLC-2) assemblies respond to both ligands, independently of each other. Glutamate and IVM bind at the interface between adjacent subunits, far away from each other; glutamate in the extracellular ligand-binding domain, and IVM in the ion-channel pore periphery. To understand the importance of putative intersubunit contacts located outside the glutamate and IVM binding sites, we introduced mutations at intersubunit interfaces, between these two binding-site types. Then, we determined the effect of these mutations on the activation of the heteromeric mutant receptors by glutamate and IVM. Amongst these mutations, we characterized an α-subunit point mutation located close to the putative IVM-binding pocket, in the extracellular end of the first transmembrane helix (M1). This mutation (αF276A) moderately reduced the sensitivity of the heteromeric GluClαF276A/βWT receptor to glutamate, and slightly decreased the receptor subunits’ cooperativity in response to glutamate. In contrast, the αF276A mutation drastically reduced the sensitivity of the receptor to IVM and significantly increased the receptor subunits’ cooperativity in response to IVM. We suggest that this mutation reduces the efficacy of channel gating, and impairs the integrity of the IVM-binding pocket, likely by disrupting important interactions between the tip of M1 and the M2-M3 loop of an adjacent subunit. We hypothesize that this physical contact between M1 and the M2-M3 loop tunes the relative orientation of the ion-channel transmembrane helices M1, M2 and M3 to optimize pore opening. Interestingly, pre-exposure of the GluClαF276A/βWT mutant receptor to subthreshold IVM concentration recovered the receptor sensitivity to glutamate. We infer that IVM likely retained its positive modulation activity by constraining the transmembrane helices in a preopen orientation sensitive to glutamate, with no need for the aforementioned disrupted interactions between M1 and the M2-M3 loop.

Trapping of ivermectin by a pentameric ligand-gated ion channel upon open-to-closed isomerization

Ivermectin (IVM) is a broad-spectrum anthelmintic drug used to treat human parasitic diseases like river blindness and lymphatic filariasis. By activating invertebrate pentameric glutamate-gated chloride channels (GluCl receptors; GluClRs), IVM induces sustained chloride influx and long-lasting membrane hyperpolarization that inhibit neural excitation in nematodes. Although IVM activates the C. elegans heteromeric GluClα/β receptor, it cannot activate a homomeric receptor composed of the C. elegans GluClβ subunits. To understand this incapability, we generated a homopentameric α7-GluClβ chimeric receptor that consists of an extracellular ligand-binding domain of an α7 nicotinic acetylcholine receptor known to be potentiated by IVM, and a chloride-selective channel domain assembled from GluClβ subunits. Application of IVM prior to acetylcholine inhibited the responses of the chimeric α7-GluClβR. Adding IVM to activated α7-GluClβRs, considerably accelerated the decline of ACh-elicited currents and stabilized the receptors in a non-conducting state. Determination of IVM association and dissociation rate constants and recovery experiments suggest that, following initial IVM binding to open α7-GluClβRs, the drug induces a conformational change and locks the ion channel in a closed state for a long duration. We further found that IVM also inhibits the activation by glutamate of a homomeric receptor assembled from the C. elegans full-length GluClβ subunits.

Structural Studies of GABAA Receptor Binding Sites: Which Experimental Structure Tells us What?

Atomic resolution structures of cys-loop receptors, including one of a γ-aminobutyric acid type A receptor (GABAA receptor) subtype, allow amazing insights into the structural features and conformational changes that these pentameric ligand-gated ion channels (pLGICs) display. Here we present a comprehensive analysis of more than 30 cys-loop receptor structures of homologous proteins that revealed several allosteric binding sites not previously described in GABAA receptors. These novel binding sites were examined in GABAA receptor homology models and assessed as putative candidate sites for allosteric ligands. Four so far undescribed putative ligand binding sites were proposed for follow up studies based on their presence in the GABAA receptor homology models. A comprehensive analysis of conserved structural features in GABAA and glycine receptors (GlyRs), the glutamate gated ion channel, the bacterial homologs Erwinia chrysanthemi (ELIC) and Gloeobacter violaceus GLIC, and the serotonin type 3 (5-HT3) receptor was performed. The conserved features were integrated into a master alignment that led to improved homology models. The large fragment of the intracellular domain that is present in the structure of the 5-HT3 receptor was utilized to generate GABAA receptor models with a corresponding intracellular domain fragment. Results of mutational and photoaffinity ligand studies in GABAA receptors were analyzed in the light of the model structures. This led to an assignment of candidate ligands to two proposed novel pockets, candidate binding sites for furosemide and neurosteroids in the trans-membrane domain were identified. The homology models can serve as hypotheses generators, and some previously controversial structural interpretations of biochemical data can be resolved in the light of the presented multi-template approach to comparative modeling. Crystal and cryo-EM microscopic structures of the closest homologs that were solved in different conformational states provided important insights into structural rearrangements of binding sites during conformational transitions. The impact of structural variation and conformational motion on the shape of the investigated binding sites was analyzed. Rules for best template and alignment choice were obtained and can generally be applied to modeling of cys-loop receptors. Overall, we provide an updated structure based view of ligand binding sites present in GABAA receptors.

Inhibition of the GABAA Receptor by Sulfated Neurosteroids: A Mechanistic Comparison Study between Pregnenolone Sulfate and Dehydroepiandrosterone Sulfate

The γ-aminobutyric acid type A receptor (GABAAR) is negatively modulated by two structurally similar neurosteroids, pregnenolone sulfate (PS) and dehydroepiandrosterone sulfate (DHEAS). This study attempted to ascertain the molecular mechanisms of inhibition of the GABA-ergic current by neurosteroids. We demonstrated that the presence of the γ subunit in GABAAR enhances the efficacy of DHEAS without altering its binding affinity. A saturating concentration of DHEAS blocked approximately 75 % of currents mediated by GABAAR, which is composed of human α1, β1, and γ2S subunits, whereas the inhibition was only 35 % in GABAAR containing only α1 and β1 subunits. The IC50 values of DHEAS with and without the γ subunit were almost identical. In contrast to DHEAS, neither the affinity nor the efficacy of PS was altered by the γ subunit. When Val256 of α1 subunit was mutated to Ser, the mutant channel became resistant to inhibition by both DHEAS and PS. PS exerted its inhibitory effect by enhancing the desensitization kinetics of GABAAR possibly through promoting the interaction between the M2-M3 linker and extracellular loop 7/loop 2. Mutant α1, containing double Cys in loop 2/loop 7 and the M2-M3 linker, formed disulfide bonds three times as much fast, when treated with saturating GABA+PS, compared with GABA alone or with GABA+DHEAS. We demonstrated that PS, but not DHEAS, mediates GABA-ergic inhibition by promoting collisions between the structural elements involved in receptor desensitization, i.e., loop 2, loop 7, and the M2-M3 linker, thus following different inhibitory mechanisms.