High Throughput Techniques for Discovering New Glycine Receptor Modulators and their Binding Sites

Assaying Proliferation Characteristics of Cells Cultured Under Static Versus Periodic Conditions

Two-dimensional in vitro culture models are widely being employed for assessing a vast variety of biological questions in different scientific fields. Common in vitro culture models are typically maintained under static conditions, where the surrounding culture medium is replaced every few days-typically every 48 to 72 h-with the aim to remove metabolites and to replenish nutrients. Although this approach is sufficient for supporting cellular survival and proliferation, static culture conditions do mostly not reflect the in vivo situation where cells are continuously being perfused by extracellular fluid, and thus, create a less-physiological environment. In order to evaluate whether the proliferation characteristics of cells in 2D culture maintained under static conditions differ from cells kept in a dynamic environment, in this chapter, we provide a protocol for differential analysis of cellular growth under static versus pulsed-perfused conditions, mimicking continuous replacement of extracellular fluid in the physiological environment. The protocol involves long-term life-cell high-content time-lapse imaging of fluorescent cells at 37 °C and ambient CO₂ concentration using multi-parametric biochips applicable for microphysiological analysis of cellular vitality. We provide instructions and useful information for (i) the culturing of cells in biochips, (ii) setup of cell-laden biochips for culturing cells under static and pulsed-perfused conditions, (iii) long-term life-cell high-content time-lapse imaging of fluorescent cells in biochips, and (iv) quantification of cellular proliferation from image series generated from imaging of differentially cultured cells.

3D Printed Lab-on-a-Chip Platform for Chemical Stimulation and Parallel Analysis of Ion Channel Function

Functional imaging has been a widely established method for the assessment of ion channel function in vitro. Conventional infrastructure used for in vitro functional analysis of ion channels is typically proprietary, non-customizable, expensive, and requires a high level of skill to use and maintain. 3D desktop printing, which is employed in the rapid prototyping field, allows for quick engineering of alternatives to conventional imaging infrastructure that are customizable, low cost, and user friendly. Here, we describe an ultra-low-cost microfluidic lab-on-a-chip (LOC) device manufactured using acrylonitrile butadiene styrene (ABS) for in vitro functional imaging of ion channels that can quickly and easily be reconstructed using three-dimensional (3D) desktop printing. The device is light weight (<5 g), small (20 mm × 49 mm), and extremely low cost (<EUR 1). We simulate fluidics within the printed channels and assess the suitability of the engineered chamber to generate homogeneous mixtures during solution exchange. We demonstrate the usability of the 3D printed microfluidic device in a case study using Fluo-4-loaded human embryonal kidney-derived (HEK293) cells, recombinantly expressing the capsaicin receptor, transient receptor potential vanilloid receptor type 1 (TRPV1), as a model system. In the case study, we confirm its applicability to solution exchange for chemical stimulation and parallel functional time-lapse fluorescence microscopy-based calcium imaging. We assess the suitability of ABS for culturing HEK293 cells inside the microfluidic LOC, based on qualitative analysis of microscopic transmission light images of ABS-exposed HEK293 cells and confirm the previously reported biocompatibility of ABS. To highlight the versatility of the 3D printed microfluidic device, we provide an example for multiplication of the shown concept within a 3D printed multichannel microfluidic LOC to be used, for example, in a higher throughput format for parallelized functional analysis of ion channels. While this work focusses on Ca²⁺ imaging with TRPV1 channels, the device may also be useful for application with other ion channel types and in vitro models.

Proliferation characteristics of cells cultured under periodic versus static conditions

In vitro culture models have become an indispensable tool for assessing a vast variety of biological questions in many scientific fields. However, common in vitro cultures are maintained under static conditions, which do not reflect the in vivo situation and create a non-physiological environment. To assess whether the growth characteristics of cells cultured at pulsed-perfused versus static conditions differ, we observed the growth of differentially cultured cells in vitro by life-cell time-lapse imaging of recombinant HEK293^YFPI152L cells, stably expressing yellow fluorescent protein. Cells were grown for ~ 30 h at 37 °C and ambient CO₂ concentration in biochips mounted into a custom-designed 3D printed carrier and were imaged at a rate of ten images per hour using a fluorescence microscope with environment control infrastructure. Cells in one chip were maintained under static conditions whereas cells in another chip were recurrently perfused with fresh media. Generated image series were quantitatively analyzed using a custom-modified cell detection software. Imaging data averaged from four biological replicates per culturing condition demonstrate that cells cultured under conventional conditions exhibit an exponential growth rate. In contrast, cells cultured in periodic mode exhibited a non-exponential growth rate. Our data clearly indicate differential growth characteristics of cells cultured under periodic versus static conditions highlighting the impact of the culture conditions on the physiology of cells in vitro.

Phenotyping Cellular Viability by Functional Analysis of Ion Channels: GlyR-Targeted Screening in NT2-N Cells

Glycine receptor chloride channels (GlyRs) are attractive drug targets for therapeutic intervention and are also more and more recognized in the context of in vitro neurotoxicity and developmental neurotoxicity testing. Assaying the functional properties of GlyR can serve as an indicator of cellular viability and the integrity of the developing and mature central nervous system. Human pluripotent NTERA-2 (NT2) stem cells undergo neuronal differentiation upon stimulation with retinoic acid and express a large variety of neuronal proteins-including GlyR. YFP-I152L, a halide-sensitive variant of yellow fluorescent protein, allows high-throughput fluorescence-based functional analysis of GlyRs in NT2 cells. Here we describe a protocol for phenotyping of cellular viability by functional analysis of GlyR in neuronally differentiated NT2 (NT2-N) cells using YFP-I152L as a reporter of functional integrity of GlyRs. The protocol describes neuronal differentiation of NT2 stem cells, transient transfection of NT2-N cells with YFP-I152L as well as functional imaging and analysis of data from high-content imaging.

Structure/Function Studies of the α4 Subunit Reveal Evolutionary Loss of a GlyR Subtype Involved in Startle and Escape Responses

Inhibitory glycine receptors (GlyRs) are pentameric ligand-gated anion channels with major roles in startle disease/hyperekplexia (GlyR α1), cortical neuronal migration/autism spectrum disorder (GlyR α2), and inflammatory pain sensitization/rhythmic breathing (GlyR α3). However, the role of the GlyR α4 subunit has remained enigmatic, because the corresponding human gene (GLRA4) is thought to be a pseudogene due to an in-frame stop codon at position 390 within the fourth membrane-spanning domain (M4). Despite this, a recent genetic study has implicated GLRA4 in intellectual disability, behavioral problems and craniofacial anomalies. Analyzing data from sequenced genomes, we found that GlyR α4 subunit genes are predicted to be intact and functional in the majority of vertebrate species—with the exception of humans. Cloning of human GlyR α4 cDNAs excluded alternative splicing and RNA editing as mechanisms for restoring a full-length GlyR α4 subunit. Moreover, artificial restoration of the missing conserved arginine (R390) in the human cDNA was not sufficient to restore GlyR α4 function. Further bioinformatic and mutagenesis analysis revealed an additional damaging substitution at K59 that ablates human GlyR α4 function, which is not present in other vertebrate GlyR α4 sequences. The substitutions K59 and X390 were also present in the genome of an ancient Denisovan individual, indicating that GLRA4 has been a pseudogene for at least 30,000–50,000 years. In artificial synapses, we found that both mouse and gorilla α4β GlyRs mediate synaptic currents with unusually slow decay kinetics. Lastly, to gain insights into the biological role of GlyR α4 function, we studied the duplicated genes glra4a and glra4b in zebrafish. While glra4b expression is restricted to the retina, using a novel tol2-GAL4FF gene trap line (SAIGFF16B), we found that the zebrafish GlyR α4a subunit gene (glra4a) is strongly expressed in spinal cord and hindbrain commissural neurones. Using gene knockdown and a dominant-negative GlyR α4a^R278Q mutant, we found that GlyR α4a contributes to touch-evoked escape behaviors in zebrafish. Thus, although GlyR α4 is unlikely to be involved in human startle responses or disease states, this subtype may contribute to escape behaviors in other organisms.

Structural changes at the myrtenol backbone reverse its positive allosteric potential into inhibitory GABAA receptor modulation

GABAA receptors are ligand-gated anion channels that form pentameric arrangements of various subunits. Positive allosteric modulators of GABAA receptors have been reported as being isolated either from plants or synthesized analogs of known GABAA receptor targeting drugs. Recently, we identified monoterpenes, e.g. myrtenol as a positive allosteric modulator at α1β2 GABAA receptors. Here, along with pharmacophore-based virtual screening studies, we demonstrate that scaffold modifications of myrtenol resulted in the loss of modulatory activity. Two independent approaches, fluorescence-based compound analysis and electrophysiological recordings in whole-cell configurations were used for analysis of transfected cells. C-atoms 1 and 2 of the myrtenol backbone were identified as crucial to preserve positive allosteric potential. A modification at C-atom 2 and lack of the hydroxyl group at C-atom 1 exhibited significantly reduced GABAergic currents at α1β2, α1β2γ, α2β3, α2β3γ and α4β3δ receptors. This effect was independent of the γ2 subunit. A sub-screen with side chain length and volume differences at the C-atom 1 identified two compounds that inhibited GABAergic responses but without receptor subtype specificity. Our combined approach of pharmacophore-based virtual screening and functional readouts reveals that side chain modifications of the bridged six-membered ring structure of myrtenol are crucial for its modulatory potential at GABAA receptors.

Identification of gelsemine metabolites in rat liver S9 by high-performance liquid chromatography/quadrupole-time-of-flight mass spectrometry

RATIONALE

Gelsemine has been extensively studied because of its anti-tumor, immunomodulatory, insecticidal itching and other significant effects. However, limited information on the pharmacokinetics and metabolism of gelsemine has been reported. Therefore, the purpose of the present study was to investigate the in vitro metabolism of gelsemine in rat liver S9 by using rapid and accurate high-performance liquid chromatography/ quadrupole-time-of-flight mass spectrometry (HPLC/QqTOF-MS).

METHODS

The incubation mixture was processed with 15% trichloroacetic acid. Multiple scans of gelsemine metabolites and accurate mass measurements were automatically performed simultaneously through data-dependent acquisition in only 30 min. The structural elucidations of these metabolites were performed by comparing their changes in accurate molecular masses and product ions with those of the parent drug.

RESULTS

Five metabolites of gelsemine were identified in rat liver S9. Of these, four metabolites of gelsemine were identified for the first time. The present results showed that the metabolic pathways of gelsemine are oxidation, demethylation, and dehydrogenation in rat liver S9.

CONCLUSIONS

In this study, metabolites of gelsemine in liver S9 were identified and elucidated firstly using the HPLC/QqTOF-MS method. The proposed metabolic pathways of gelsemine in liver S9 will provide a basis for further studies of the in vivo metabolism of gelsemine in animals and humans.

Progress in automating patch clamp cellular physiology

Patch clamp electrophysiology has transformed research in the life sciences over the last few decades. Since their inception, automatic patch clamp platforms have evolved considerably, demonstrating the capability to address both voltage- and ligand-gated channels, and showing the potential to play a pivotal role in drug discovery and biomedical research. Unfortunately, the cell suspension assays to which early systems were limited cannot recreate biologically relevant cellular environments, or capture higher order aspects of synaptic physiology and network dynamics. In vivo patch clamp electrophysiology has the potential to yield more biologically complex information and be especially useful in reverse engineering the molecular and cellular mechanisms of single-cell and network neuronal computation, while capturing important aspects of human disease mechanisms and possible therapeutic strategies. Unfortunately, it is a difficult procedure with a steep learning curve, which has restricted dissemination of the technique. Luckily, in vivo patch clamp electrophysiology seems particularly amenable to robotic automation. In this review, we document the development of automated patch clamp technology, from early systems based on multi-well plates through to automated planar-array platforms, and modern robotic platforms capable of performing two-photon targeted whole-cell electrophysiological recordings in vivo.

Modulation of Glycine-Mediated Spinal Neurotransmission for the Treatment of Chronic Pain

Chronic pain constitutes a significant and expanding worldwide health crisis. Currently available analgesics poorly serve individuals suffering from chronic pain, and new therapeutic agents that are more effective, safer, and devoid of abuse liabilities are desperately needed. Among the myriad of cellular and molecular processes contributing to chronic pain, spinal disinhibition of pain signaling to higher cortical centers plays a critical role. Accumulating evidence shows that glycinergic inhibitory neurotransmission in the spinal cord dorsal horn gates nociceptive signaling, is essential in maintaining physiological pain sensitivity, and is diminished in pathological pain states. Thus, it is hypothesized that agents capable of enhancing glycinergic tone within the dorsal horn could obtund nociceptor signaling to the brain and serve as analgesics for persistent pain. This Perspective highlights the potential that pharmacotherapies capable of increasing inhibitory spinal glycinergic neurotransmission hold in providing new and transformative analgesic therapies for the treatment of chronic pain.

Using automated patch clamp electrophysiology platforms in pain-related ion channel research: insights from industry and academia

Automated patch clamp (APC) technology was first developed at the turn of the millennium. The increased throughput it afforded promised a new paradigm in ion channel recordings, offering the potential to overcome the time-consuming, low-throughput bottleneck, arising from manual patch clamp investigations. This has relevance to the fast-paced development of novel therapies for chronic pain. This review highlights the advances in technology, using select examples that have facilitated APC usage in both industry and academia. It covers both first generation and the latest developments in second-generation platforms. In addition, it also provides an overview of the pain research field and how APC platforms have furthered our understanding of ion channel research and the development of pharmacological tools and therapeutics. APC platforms have much to offer to the ion channel research community, and this review highlights areas of best practice for both academia and industry. The impact of APC platforms and the prospects of ion channel research and improved therapeutics for chronic pain will be evaluated.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Ivermectin-Activated, Cation-Permeable Glycine Receptors for the Chemogenetic Control of Neuronal Excitation

The ability to control neuronal activation is rapidly advancing our understanding of brain function and is widely viewed as having eventual therapeutic application. Although several highly effective optogenetic, optochemical genetic, and chemogenetic techniques have been developed for this purpose, new approaches may provide better solutions for addressing particular questions and would increase the number of neuronal populations that can be controlled independently. An early chemogenetic neuronal silencing method employed a glutamate receptor Cl^- channel engineered for activation by 1-3 nM ivermectin. This construct has been validated in vivo. Here, we sought to develop cation-permeable ivermectin-gated receptors that were either maximally Ca²⁺-permeable so as to induce neuro-excitotoxic cell death or minimally Ca²⁺-permeable so as to depolarize neurons with minimal excitotoxic risk. Our constructs were based on the human α1 glycine receptor Cl^- channel due to its high conductance, human origin, high ivermectin sensitivity (once mutated), and because pore mutations that render it permeable to Na⁺ alone or Na⁺ plus Ca²⁺ are well characterized. We developed a Ca²⁺-impermeable excitatory receptor by introducing the F207A/P-2'Δ/A-1'E/T13'V/A288G mutations and a Ca²⁺-permeable excitatory receptor by introducing the F207A/A-1'E/A288G mutations. The latter receptor efficiently induces cell death and strongly depolarizes neurons at nanomolar ivermectin concentrations.

Step-by-step guide to building an inexpensive 3D printed motorized positioning stage for automated high-content screening microscopy

High-content screening microscopy relies on automation infrastructure that is typically proprietary, non-customizable, costly and requires a high level of skill to use and maintain. The increasing availability of rapid prototyping technology makes it possible to quickly engineer alternatives to conventional automation infrastructure that are low-cost and user-friendly. Here, we describe a 3D printed inexpensive open source and scalable motorized positioning stage for automated high-content screening microscopy and provide detailed step-by-step instructions to re-building the device, including a comprehensive parts list, 3D design files in STEP (Standard for the Exchange of Product model data) and STL (Standard Tessellation Language) format, electronic circuits and wiring diagrams as well as software code. System assembly including 3D printing requires approx. 30h. The fully assembled device is light-weight (1.1kg), small (33×20×8cm) and extremely low-cost (approx. EUR 250). We describe positioning characteristics of the stage, including spatial resolution, accuracy and repeatability, compare imaging data generated with our device to data obtained using a commercially available microplate reader, demonstrate its suitability to high-content microscopy in 96-well high-throughput screening format and validate its applicability to automated functional Cl^-- and Ca²⁺-imaging with recombinant HEK293 cells as a model system. A time-lapse video of the stage during operation and as part of a custom assembled screening robot can be found at https://vimeo.com/158813199.

A Recombinant Human Pluripotent Stem Cell Line Stably Expressing Halide-Sensitive YFP-I152L for GABAAR and GlyR-Targeted High-Throughput Drug Screening and Toxicity Testing

GABA_ARs and GlyRs are considered attractive drug targets for therapeutic intervention and are also increasingly recognized in the context of in vitro neurotoxicity (NT) and developmental neurotoxicity (DNT) testing. However, systematic human-specific GABA_AR and GlyR-targeted drug screening and toxicity testing is hampered due to lack of appropriate in vitro models that express native GABA_ARs and GlyRs. We have established a human pluripotent stem cell line (NT2) stably expressing YFP-I152L, a halide-sensitive variant of yellow fluorescent protein (YFP), allowing for fluorescence-based functional analysis of chloride channels. Upon stimulation with retinoic acid, NT2 cells undergo neuronal differentiation and allow pharmacological and toxicological evaluation of native GABA_ARs and GlyRs at different stages of brain maturation. We applied the cell line in concentration-response experiments with the neurotransmitters GABA and glycine as well as with the drugs strychnine, picrotoxin, fipronil, lindane, bicuculline, and zinc and demonstrate that the established in vitro model is applicable to GABA_AR and GlyR-targeted pharmacological and toxicological profiling. We quantified the proportion of GABA_AR and GlyR-sensitive cells, respectively, and identified percentages of approximately 20% each within the overall populations, rendering the cells a suitable model for systematic in vitro GABA_AR and GlyR-targeted screening in the context of drug development and NT/DNT testing.

Towards in vitro DT/DNT testing: Assaying chemical susceptibility in early differentiating NT2 cells

Human pluripotent embryonal carcinoma (NT2) cells are increasingly considered as a suitable model for in vitro toxicity testing, e.g. developmental toxicity and neurotoxicity (DT/DNT) studies, as they undergo neuronal differentiation upon stimulation with retinoic acid (RA) and permit toxicity testing at different stages of maturation. NT2 cells have recently been reported to show specific changes in dielectric resistance profiles during differentiation which can be observed as early as 24h upon RA-stimulation. These observations suggest altered susceptibility to chemicals at an early stage of differentiation. However, chemical susceptibility of early differentiating NT cells has not yet been studied. To address this question, we have established a cell fitness screening assay based on the analysis of intracellular ATP levels and we applied the assay in a large-scale drug screening experiment in NT2 stem cells and early differentiating NT2 cells. Subsequent analysis of ranked fitness phenotypes revealed 19 chemicals with differential toxicity profile in early differentiating NT2 cells. To evaluate whether any of the identified drugs have previously been associated with DT/DNT, we conducted a literature search on the identified molecules and quantified the fraction of chemicals assigned to the FDA (Food and Drug Administration) pregnancy risk categories (PRC) N, A, B, C, D, and X in the hit list and the small molecule library. While the fractions of the categories N and B were decreased (0.81 and 0.35-fold), the classes C, D and X were increased (1.35, 1.47 and 3.27-fold) in the hit list compared to the chemical library. From these data as well as from the literature review, identifying large fractions of chemicals being directly (∼42%) and indirectly associated with DT/DNT (∼32%), we conclude that our method may be beneficial to systematic in vitro-based primary screening for developmental toxicants and neurotoxicants and we propose cell fitness screening in early differentiating NT2 cells as a strategy for evaluating chemical susceptibility at different stages of differentiation to reduce animal testing in the context of the 3Rs.

4-bromopropofol decreases action potential generation in spinal neurons by inducing a glycine receptor-mediated tonic conductance

BACKGROUND AND PURPOSE

Impaired function of spinal strychnine-sensitive glycine receptors gives rise to chronic pain states and movement disorders. Therefore, increased activity of glycine receptors should help to treat such disorders. Although compounds targeting glycine receptors with a high selectivity are lacking, halogenated analogues of propofol have recently been considered as potential candidates. Therefore we asked whether 4-bromopropofol attenuated the excitability of spinal neurons by promoting glycine receptor-dependent inhibition.

EXPERIMENTAL APPROACH

The actions of sub-anaesthetic concentrations of propofol and 4-bromopropofol were investigated in spinal tissue cultures prepared from mice. Drug-induced alterations in action potential firing were monitored by extracellular multi-unit recordings. The effects on GABAA and glycine receptor-mediated inhibition were quantified by whole-cell voltage-clamp recordings.

KEY RESULTS

Low concentrations of 4-bromopropofol (50 nM) reduced action potential activity of ventral horn neurons by about 30%, compared with sham-treated slices. This effect was completely abolished by strychnine (1 μM). In voltage-clamped neurons, 4-bromopropofol activated glycine receptors, generating a tonic current of 65 ± 10 pA, while GABAA - and glycine receptor-mediated synaptic transmission remained unaffected.

CONCLUSIONS AND IMPLICATIONS

The highest glycine levels in the CNS are found in the ventral horn of the spinal cord, a region mediating pain-induced motor reflexes and participating in the control of muscle tone. 4-Bromopropofol may serve as a starting point for the development of non-sedative, non-addictive, muscle relaxants and analgesics to be used to treat low back pain.

Gelsemium analgesia and the spinal glycine receptor/allopregnanolone pathway

Gelsemium, a small genus of flowering plant from the family Loganiaceae, comprises five species including the popular Gelsemium sempervirens Ait. and Gelsemium elegans Benth., which are indigenous to North America and China/East Asia, respectively. Approximately 120 alkaloids have been isolated and identified from Gelsemium, with the predominant indole alkaloids including gelsemine, koumine, gelsemicine, gelsenicine, gelsedine, sempervirine, koumidine, koumicine and humantenine. Gelsemine is the principal active alkaloid in G. sempervirens Ait., and koumine and gelsemine are the most and second-most dominant alkaloids in G. elegans Benth. Gelsemium extract and its active alkaloids serve a variety of biological functions, including neurobiological, immunosuppressive and antitumor effects, and have traditionally been used to treat pain, neuralgia, anxiety, insomnia, asthma, respiratory ailments and cancers. This review focuses on animal-based studies of Gelsemium as a pain treatment and its mechanism of action. In contrast to morphine, when administered intrathecally and systemically, koumine, gelsemine and gelsenicine have marked antinociception in inflammatory, neuropathic and bone cancer pains without inducing antinociceptive tolerance. Gelsemium and its active alkaloids may produce antinociception by activating the spinal α3 glycine/allopregnanolone pathway. The results of this review support the clinical use of Gelsemium and suggest that its active alkaloids may be developed to treat intractable and other types of pain, preferably after chemical modification. However, Gelsemium is a known toxic plant, and its toxicity limits its appropriate dosage and clinical use. To avoid or decrease the side/toxic effects of Gelsemium, an individual monomer of highly potent alkaloids must be selected, or alkaloids that exhibit greater α3 glycine receptor selectivity may be discovered or modified.

Fluorescence-based high-throughput functional profiling of ligand-gated ion channels at the level of single cells

Ion channels are involved in many physiological processes and are attractive targets for therapeutic intervention. Their functional properties vary according to their subunit composition, which in turn varies in a developmental and tissue-specific manner and as a consequence of pathophysiological events. Understanding this diversity requires functional analysis of ion channel properties in large numbers of individual cells. Functional characterisation of ligand-gated channels involves quantitating agonist and drug dose-response relationships using electrophysiological or fluorescence-based techniques. Electrophysiology is limited by low throughput and high-throughput fluorescence-based functional evaluation generally does not enable the characterization of the functional properties of each individual cell. Here we describe a fluorescence-based assay that characterizes functional channel properties at single cell resolution in high throughput mode. It is based on progressive receptor activation and iterative fluorescence imaging and delivers >100 dose-responses in a single well of a 384-well plate, using α1-3 homomeric and αβ heteromeric glycine receptor (GlyR) chloride channels as a model system. We applied this assay with transiently transfected HEK293 cells co-expressing halide-sensitive yellow fluorescent protein and different GlyR subunit combinations. Glycine EC₅₀ values of different GlyR isoforms were highly correlated with published electrophysiological data and confirm previously reported pharmacological profiles for the GlyR inhibitors, picrotoxin, strychnine and lindane. We show that inter and intra well variability is low and that clustering of functional phenotypes permits identification of drugs with subunit-specific pharmacological profiles. As this method dramatically improves the efficiency with which ion channel populations can be characterized in the context of cellular heterogeneity, it should facilitate systems-level analysis of ion channel properties in health and disease and the discovery of therapeutics to reverse pathological alterations.

Emerging approaches to probing ion channel structure and function

Ion channels, as membrane proteins, are the sensors of the cell. They act as the first line of communication with the world beyond the plasma membrane and transduce changes in the external and internal environments into unique electrical signals to shape the responses of excitable cells. Because of their importance in cellular communication, ion channels have been intensively studied at the structural and functional levels. Here, we summarize the diverse approaches, including molecular and cellular, chemical, optical, biophysical, and computational, used to probe the structural and functional rearrangements that occur during channel activation (or sensitization), inactivation (or desensitization), and various forms of modulation. The emerging insights into the structure and function of ion channels by multidisciplinary approaches allow the development of new pharmacotherapies as well as new tools useful in controlling cellular activity.

Mechanism of action of the insecticides, lindane and fipronil, on glycine receptor chloride channels

BACKGROUND AND PURPOSE

Docking studies predict that the insecticides, lindane and fipronil, block GABA(A) receptors by binding to 6' pore-lining residues. However, this has never been tested at any Cys-loop receptor. The neurotoxic effects of these insecticides are also thought to be mediated by GABA(A) receptors, although a recent morphological study suggested glycine receptors mediated fipronil toxicity in zebrafish. Here we investigated whether human α1, α1β, α2 and α3 glycine receptors were sufficiently sensitive to block by either compound as to represent possible neurotoxicity targets. We also investigated the mechanisms by which lindane and fipronil inhibit α1 glycine receptors.

EXPERIMENTAL APPROACH

Glycine receptors were recombinantly expressed in HEK293 cells and insecticide effects were studied using patch-clamp electrophysiology.

KEY RESULTS

Both compounds completely inhibited all tested glycine receptor subtypes with IC(50) values ranging from 0.2-2 µM, similar to their potencies at vertebrate GABA(A) receptors. Consistent with molecular docking predictions, both lindane and fipronil interacted with 6' threonine residues via hydrophobic interactions and hydrogen bonds. In contrast with predictions, we found no evidence for lindane interacting at the 2' level. We present evidence for fipronil binding in a non-blocking mode in the anaesthetic binding pocket, and for lindane as an excellent pharmacological tool for identifying the presence of β subunits in αβ heteromeric glycine receptors.

CONCLUSIONS AND IMPLICATIONS

This study implicates glycine receptors as novel vertebrate toxicity targets for fipronil and lindane. Furthermore, lindane interacted with pore-lining 6' threonine residues, whereas fipronil may have both pore and non-pore binding sites.

Molecular determinants of ivermectin sensitivity at the glycine receptor chloride channel

Ivermectin is an anthelmintic drug that works by activating glutamate-gated chloride channel receptors (GluClRs) in nematode parasites. GluClRs belong to the Cys-loop receptor family that also includes glycine receptor (GlyR) chloride channels. GluClRs and A288G mutant GlyRs are both activated by low nanomolar ivermectin concentrations. The crystal structure of the Caenorhabditis elegans α GluClR complexed with ivermectin has recently been published. Here, we probed ivermectin sensitivity determinants on the α1 GlyR using site-directed mutagenesis and electrophysiology. Based on a mutagenesis screen of transmembrane residues, we identified Ala288 and Pro230 as crucial sensitivity determinants. A comparison of the actions of selamectin and ivermectin suggested the benzofuran C05-OH was required for high efficacy. When taken together with docking simulations, these results supported a GlyR ivermectin binding orientation similar to that seen in the GluClR crystal structure. However, whereas the crystal structure shows that ivermectin interacts with the α GluClR via H-bonds with Leu218, Ser260, and Thr285 (α GluClR numbering), our data indicate that H-bonds with residues homologous to Ser260 and Thr285 are not important for high ivermectin sensitivity or direct agonist efficacy in A288G α1 GlyRs or three other GluClRs. Our data also suggest that van der Waals interactions between the ivermectin disaccharide and GlyR M2-M3 loop residues are unimportant for high ivermectin sensitivity. Thus, although our results corroborate the ivermectin binding orientation as revealed by the crystal structure, they demonstrate that some of the binding interactions revealed by this structure do not pertain to other highly ivermectin-sensitive Cys-loop receptors.

Recent progress in understanding the interaction between avermectins and ligand-gated ion channels: putting the pests to sleep

Avermectins and milbemycins are an important family of anthelmintics, insecticides and acaricides. Their mode of action is as positive allosteric modulators of ligand-gated chloride channels, and at higher concentrations, they gate some channels directly. Though it has long been known that the avermectins do not compete for the ligand binding site, the actual site at which they interact with their receptors has been unclear. Recent data demonstrate the importance to drug binding of amino acid residues predicted to line a water-filled pocket in the channel domain. This pocket acts as the binding site for anaesthetics and other modulators of mammalian GABA(A) and glycine receptors, suggesting similarities in the mode of action between these drugs and the avermectins/milbemycins.

Identification of novel specific allosteric modulators of the glycine receptor using phage display

The glycine receptor (GlyR) is a member of the Cys-loop superfamily of ligand-gated ion channels and the major mediator of inhibitory neurotransmission in the spinal cord and brainstem. Many allosteric modulators affect the functioning of members of this superfamily, with some such as benzodiazepines showing great specificity and others such as zinc, alcohols, and volatile anesthetics acting on multiple members. To date, no potent and efficacious allosteric modulator acting specifically at the GlyR has been identified, hindering both experimental characterization of the receptor and development of GlyR-related therapeutics. We used phage display to identify novel peptides that specifically modulate GlyR function. Peptide D12-116 markedly enhanced GlyR currents at low micromolar concentrations but had no effects on the closely related gamma-aminobutyric acid type A receptors. This approach can readily be adapted for use with other channels that currently lack specific allosteric modulators.

An improved ivermectin-activated chloride channel receptor for inhibiting electrical activity in defined neuronal populations

The ability to silence the electrical activity of defined neuronal populations in vivo is dramatically advancing our understanding of brain function. This technology may eventually be useful clinically for treating a variety of neuropathological disorders caused by excessive neuronal activity. Several neuronal silencing methods have been developed, with the bacterial light-activated halorhodopsin and the invertebrate allatostatin-activated G protein-coupled receptor proving the most successful to date. However, both techniques may be difficult to implement clinically due to their requirement for surgically implanted stimulus delivery methods and their use of nonhuman receptors. A third silencing method, an invertebrate glutamate-gated chloride channel receptor (GluClR) activated by ivermectin, solves the stimulus delivery problem as ivermectin is a safe, well tolerated drug that reaches the brain following systemic administration. However, the limitations of this method include poor functional expression, possibly due to the requirement to coexpress two different subunits in individual neurons, and the nonhuman origin of GluClR. Here, we describe the development of a modified human alpha1 glycine receptor as an improved ivermectin-gated silencing receptor. The crucial development was the identification of a mutation, A288G, which increased ivermectin sensitivity almost 100-fold, rendering it similar to that of GluClR. Glycine sensitivity was eliminated via the F207A mutation. Its large unitary conductance, homomeric expression, and human origin may render the F207A/A288G alpha1 glycine receptor an improved silencing receptor for neuroscientific and clinical purposes. As all known highly ivermectin-sensitive GluClRs contain an endogenous glycine residue at the corresponding location, this residue appears essential for exquisite ivermectin sensitivity.