Pharmacokinetic and pharmacodynamic considerations for treating sarcoptic mange with cross-relevance to Australian wildlife

Sarcoptes scabiei is the microscopic burrowing mite responsible for sarcoptic mange, which is reported in approximately 150 mammalian species. In Australia, sarcoptic mange affects a number of native and introduced wildlife species, is particularly severe in bare-nosed wombats (Vombatus ursinus) and an emerging issue in koala and quenda. There are a variety of acaricides available for the treatment of sarcoptic mange which are generally effective in eliminating mites from humans and animals in captivity. In wild populations, effective treatment is challenging, and concerns exist regarding safety, efficacy and the potential emergence of acaricide resistance. There are risks where acaricides are used intensively or inadequately, which could adversely affect treatment success rates as well as animal welfare. While reviews on epidemiology, treatment strategies, and pathogenesis of sarcoptic mange in wildlife are available, there is currently no review evaluating the use of specific acaricides in the context of their pharmacokinetic and pharmacodynamic properties, and subsequent likelihood of emerging drug resistance, particularly for Australian wildlife. This review critically evaluates acaricides that have been utilised to treat sarcoptic mange in wildlife, including dosage forms and routes, pharmacokinetics, mode of action and efficacy. We also highlight the reports of resistance of S. scabiei to acaricides, including clinical and in vitro observations.

LiGIoNs: A computational method for the detection and classification of ligand-gated ion channels

Ligand-Gated Ion Channels (LGICs) is one of the largest groups of transmembrane proteins. Due to their major role in synaptic transmission, both in the nervous system and the somatic neuromuscular junction, LGICs present attractive therapeutic targets. During the last few years, several computational methods for the detection of LGICs have been developed. These methods are based on machine learning approaches utilizing features extracted solely from the amino acid composition. Here we report the development of LiGIoNs, a profile Hidden Markov Model (pHMM) method for the prediction and ligand-based classification of LGICs. The method consists of a library of 10 pHMMs, one per LGIC subfamily, built from the alignment of representative LGIC sequences. In addition, 14 Pfam pHMMs are used to further annotate and classify unknown protein sequences into one of the 10 LGIC subfamilies. Evaluation of the method showed that it outperforms existing methods in the detection of LGICs. On top of that, LiGIoNs is the only currently available method that classifies LGICs into subfamilies. The method is available online at http://bioinformatics.biol.uoa.gr/ligions/.

Vanilloid agonist-mediated activation of TRPV1 channels requires coordinated movement of the S1-S4 bundle rather than a quiescent state

Transient receptor potential vanilloid1 (TRPV1) channel plays an important role in a wide range of physiological and pathological processes, and a comprehensive understanding of TRPV1 gating will create opportunities for therapeutic intervention. Recent incredible advances in cryo-electron microscopy (cryo-EM) have yielded high-resolution structures of all TRPV subtypes (TRPV1-6) and all of them share highly conserved six transmembrane (TM) domains (S1-S6). As revealed by the open structures of TRPV1 in the presence of a bound vanilloid agonist (capsaicin or resiniferatoxin), TM helicesS1 to S4 form a bundle that remains quiescent during channel activation, highlighting differences in the gating mechanism of TRPV1 and voltage-gated ion channels. Here, however, we argue that the structural dynamics rather than quiescence of S1-S4 domains is necessary for capsaicin-mediated activation of TRPV1. Using fluorescent unnatural amino acid (flUAA) incorporation and voltage-clamp fluorometry (VCF) analysis, we directly observed allostery of the S1-S4 bundle upon capsaicin binding. Covalent occupation of VCF-identified sites, single-channel recording, cell apoptosis analysis, and exploration of the role of PSFL828, a novel non-vanilloid agonist we identified, have collectively confirmed the essential role of this coordinated S1-S4 motility in capsaicin-mediated activation of TRPV1. This study concludes that, in contrast to cryo-EM structural studies, vanilloid agonists are also required for S1-S4 movement during TRPV1 activation. Redefining the gating process of vanilloid agonists and the discovery of new non-vanilloid agonists will allow the evaluation of new strategies aimed at the development of TRPV1 modulators.

Glycine agonism in ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate the majority of excitatory neurotransmission in the vertebrate CNS. Classified as AMPA, kainate, delta and NMDA receptors, iGluRs are central drivers of synaptic plasticity widely considered as a major cellular substrate of learning and memory. Surprisingly however, five out of the eighteen vertebrate iGluR subunits do not bind glutamate but glycine, a neurotransmitter known to mediate inhibitory neurotransmission through its action on pentameric glycine receptors (GlyRs). This is the case of GluN1, GluN3A, GluN3B, GluD1 and GluD2 subunits, all also binding the D amino acid d-serine endogenously present in many brain regions. Glycine and d-serine action and affinities broadly differ between glycinergic iGluR subtypes. On 'conventional' GluN1/GluN2 NMDA receptors, glycine (or d-serine) acts in concert with glutamate as a mandatory co-agonist to set the level of receptor activity. It also regulates the receptor's trafficking and expression independently of glutamate. On 'unconventional' GluN1/GluN3 NMDARs, glycine acts as the sole agonist directly triggering opening of excitatory glycinergic channels recently shown to be physiologically relevant. On GluD receptors, d-serine on its own mediates non-ionotropic signaling involved in excitatory and inhibitory synaptogenesis, further reinforcing the concept of glutamate-insensitive iGluRs. Here we present an overview of our current knowledge on glycine and d-serine agonism in iGluRs emphasizing aspects related to molecular mechanisms, cellular function and pharmacological profile. The growing appreciation of the critical influence of glycine and d-serine on iGluR biology reshapes our understanding of iGluR signaling diversity and complexity, with important implications in neuropharmacology.

NACHO and 14-3-3 promote expression of distinct subunit stoichiometries of the α4β2 acetylcholine receptor

Nicotinic acetylcholine receptors (nAChRs) belong to the superfamily of pentameric ligand-gated ion channels, and in neuronal tissues, are assembled from various types of α- and β-subunits. Furthermore, the subunits α4 and β2 assemble in two predominant stoichiometric forms, (α4)2(β2)3 and (α4)3(β2)2, forming receptors with dramatically different sensitivity to agonists and allosteric modulators. However, mechanisms by which the two stoichiometric forms are regulated are not known. Here, using heterologous expression in mammalian cells, single-channel patch-clamp electrophysiology, and calcium imaging, we show that the ER-resident protein NACHO selectively promotes the expression of the (α4)2(β2)3 stoichiometry, whereas the cytosolic molecular chaperone 14-3-3η selectively promotes the expression of the (α4)3(β2)2 stoichiometry. Thus, NACHO and 14-3-3η are potential physiological regulators of subunit stoichiometry, and are potential drug targets for re-balancing the stoichiometry in pathological conditions involving α4β2 nAChRs such as nicotine dependence and epilepsy.

Concatemers to re-investigate the role of α5 in α4β2 nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are pentameric ion channels expressed in the central nervous systems. nAChRs containing the α4, β2 and α5 subunits are specifically involved in addictive processes, but their functional architecture is poorly understood due to the intricacy of assembly of these subunits. Here we constrained the subunit assembly by designing fully concatenated human α4β2 and α4β2α5 receptors and characterized their properties by two-electrodes voltage-clamp electrophysiology in Xenopus oocytes. We found that α5-containing nAChRs are irreversibly blocked by methanethiosulfonate (MTS) reagents through a covalent reaction with a cysteine present only in α5. MTS-block experiments establish that the concatemers are expressed in intact form at the oocyte surface, but that reconstitution of nAChRs from loose subunits show inefficient and highly variable assembly of α5 with α4 and β2. Mutational analysis shows that the concatemers assemble both in clockwise and anticlockwise orientations, and that α5 does not contribute to ACh binding from its principal (+) site. Reinvestigation of suspected α5-ligands such as galantamine show no specific effect on α5-containing concatemers. Analysis of the α5-D398N mutation that is linked to smoking and lung cancer shows no significant effect on the electrophysiological function, suggesting that its effect might arise from alteration of other cellular processes. The concatemeric strategy provides a well-characterized platform for mechanistic analysis and screening of human α5-specific ligands.

Implication of Neuronal Versus Microglial P2X4 Receptors in Central Nervous System Disorders

The P2X4 receptor (P2X4) is an ATP-gated cation channel that is highly permeable to Ca²⁺ and widely expressed in neuronal and glial cell types throughout the central nervous system (CNS). A growing body of evidence indicates that P2X4 plays key roles in numerous central disorders. P2X4 trafficking is highly regulated and consequently in normal situations, P2X4 is present on the plasma membrane at low density and found mostly within intracellular endosomal/lysosomal compartments. An increase in the de novo expression and/or surface density of P2X4 has been observed in microglia and/or neurons during pathological states. This review aims to summarize knowledge on P2X4 functions in CNS disorders and provide some insights into the relative contributions of neuronal and glial P2X4 in pathological contexts. However, determination of the cell-specific functions of P2X4 along with its intracellular and cell surface roles remain to be elucidated before its potential as a therapeutic target in multiple disorders can be defined.

Automated Planar Patch-Clamp Recording of P2X Receptors

P2X receptors are a structurally and functionally distinctive family of ligand-gated ion channels that play important roles in mediating extracellular adenosine 5'-triphosphate (ATP) signaling in diverse physiological and pathophysiological processes. For several decades, the "manual" patch-clamp technique was regarded as the gold standard assay for investigating ion channel properties. More recently, breakthroughs in the development of automated patch-clamp technologies are enabling the study of ion channels, with much greater throughput capacities. These automated platforms, of which there are many, generate consistent, reliable, high-fidelity data. This chapter demonstrates the versatility of one of these technologies for ligand-gated ion channels, with a particular emphasis on protocols that address some of the issues of receptor desensitization that are commonly associated with P2X receptor-mediated currents.

Serotonin discovery and stepwise disclosure of 5-HT receptor complexity over four decades. Part II. Some contributions of Manfred Göthert

About 40% of the papers within the scientific oeuvre of Manfred Göthert (1939-2019) were dedicated to serotonin (5-hydroxytryptamine, 5-HT). He was not only the witness of the gradual definition of the fourteen 5-HT receptor subtypes but also was involved directly by identifying 5-HT_1B, 5-HT_1D and 5-HT₃ receptors. Moreover, he identified presynaptic 5-HT receptors on central and/or peripheral serotoninergic, noradrenergic and/or cholinergic neurones. Two inhibitory (5-HT_1B, 5-HT_1D) and two facilitatory (5-HT₃, 5-HT₄) receptors were found, the 5-HT_1B receptor representing a possible target for antidepressant drugs. Ten years earlier than electrophysiologists, he identified ligand-gated receptors like the 5-HT₃ and the nicotinic acetylcholine (nACh) receptor as targets of halothane. Simultaneously with, but independent of, other authors he found that ethanol allosterically inhibits N-methyl-D-aspartate (NMDA) receptors, which are affected at an even lower concentration than 5-HT₃ and nACh receptors. The latter two receptors were shown to be subject to allosteric inhibition also by cannabinoids via a mechanism unrelated to cannabinoid CB₁ or CB₂ receptors; cannabinoid inhibition of 5-HT₃ receptors may represent a new target for the treatment of neuropathic pain.

KCa and KV channels modulate the venoarteriolar reflex in non-glabrous human skin with no roles of KATP channels, NOS, and COX

The venoarteriolar reflex is a local mechanism that induces vasoconstriction during venous congestion in various tissues, including skin. This response is thought to play a critical role in minimizing capillary damage or edema resulting from overperfusion, though factors that modulate this response remain largely unknown. Here, we hypothesized that nitric oxide synthase (NOS), cyclooxygenase (COX), and Ca²⁺-activated, ATP-sensitive, and voltage-gated K⁺ channels (K_Ca, K_ATP, and K_V channels, respectively) modulate the venoarteriolar reflex in human skin. Cutaneous blood flow (laser-Doppler flowmetry) was monitored during a 3-min pre-occlusion baseline and following a 3-min venous occlusion of 45 mmHg, the latter maneuver was used to induce the venoarteriolar reflex. The venoarteriolar reflex was assessed at the following forearm skin sites: Experiment 1 (n = 11): 1) lactated Ringer solution (Control), 2) 10 mM N^ω-nitro-_L-arginine (NOS inhibitor), 3) 10 mM ketorolac (COX inhibitor), and 4) combined NOS + COX inhibition; Experiment 2 (n = 15): 1) lactated Ringer solution (Control), 2) 50 mM tetraethylammonium (K_Ca channel blocker), 3) 5 mM glybenclamide (K_ATP channel blocker), and 4) 10 mM 4-aminopyridine (K_V channel blocker). Separate and combined NOS and COX inhibition as well as K_ATP channel blocker had no effect on venoarteriolar reflex. Conversely, venoarteriolar reflex was attenuated by K_Ca channel blockade (36-38%) and augmented by K_V channel blockade (38-55%). We showed that K_Ca and K_V channels modulate the venoarteriolar reflex with minimum roles of NOS, COX, and K_ATP channels in human non-glabrous forearm skin in vivo. Thus, cutaneous venoarteriolar reflex changes could reflect altered K⁺ channel function.

A common mechanism allows selective targeting of GluN2B subunit-containing N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs), especially GluN2B-containing NMDARs, are associated with neurodegenerative diseases like Parkinson, Alzheimer and Huntington based on their high Ca2+ conductivity. Overactivation leads to high intracellular Ca2+ concentrations and cell death rendering GluN2B-selective inhibitors as promising drug candidates. Ifenprodil represents the first highly potent prototypical, subtype-selective inhibitor of GluN2B-containing NMDARs. However, activity of ifenprodil on serotonergic, adrenergic and sigma receptors limits its therapeutic use. Structural reorganization of the ifenprodil scaffold to obtain 3-benzazepines retained inhibitory GluN2B activity but decreased the affinity at the mentioned non-NMDARs. While scaffold optimization improves the selectivity, the molecular inhibitory mechanism of these compounds is still not known. Here, we show a common inhibitory mechanism of ifenprodil and the related 3-benzazepines by mutational modifications of the receptor binding site, chemical modifications of the 3-benzazepine scaffold and subsequent in silico simulation of the inhibitory mechanism.

Potentiation of a neuronal nicotinic receptor via pseudo-agonist site

Neuronal nicotinic receptors containing α4 and β2 subunits assemble in two pentameric stoichiometries, (α4)3(β2)2 and (α4)2(β2)3, each with distinct pharmacological signatures; (α4)3(β2)2 receptors are strongly potentiated by the drug NS9283, whereas (α4)2(β2)3 receptors are unaffected. Despite this stoichiometry-selective pharmacology, the molecular identity of the target for NS9283 remains elusive. Here, studying (α4)3(β2)2 receptors, we show that mutations at either the principal face of the β2 subunit or the complementary face of the α4 subunit prevent NS9283 potentiation of ACh-elicited single-channel currents, suggesting the drug targets the β2-α4 pseudo-agonist sites, the α4-α4 agonist site, or both sites. To distinguish among these possibilities, we generated concatemeric receptors with mutations at specified subunit interfaces, and monitored the ability of NS9283 to potentiate ACh-elicited single-channel currents. We find that a mutation at the principal face of the β2 subunit at either β2-α4 pseudo-agonist site suppresses potentiation, whereas mutation at the complementary face of the α4 subunit at the α4-α4 agonist site allows a significant potentiation. Thus, monitoring potentiation of single concatemeric receptor channels reveals that the β2-α4 pseudo-agonist sites are required for stoichiometry-selective drug action. Together with the recently determined structure of the (α4)3(β2)2 receptor, the findings have implications for structure-guided drug design.

Physical Accuracy Leads to Biological Relevance: Best Practices For Simulating Ligand-Gated Ion Channels Interacting With General Anesthetics

Efforts to detect binding modes of general anesthetics (GAs) for pentameric ligand-gated ion channels (pLGICs) are often complicated by a large number of indicated sites, as well as the challenges of ranking sites by affinity and determining which sites are occupied at clinical concentrations. Physics-based computational methods offer a powerful route for determining affinities of ligands to isolated binding sites, but preserving accuracy is essential. This chapter describes a step-by-step approach to multiple methods for identifying candidate sites and quantifying binding affinities and also discusses limitations and common pitfalls.

Auxiliary subunits of AMPA receptors: The discovery of a forebrain-selective antagonist, LY3130481/CERC-611

Drugs originate from the discovery of compounds, natural or synthetic, that bind to proteins (receptors, enzymes, transporters, etc.), the interaction of which modulates biological cascades that have potential therapeutic benefit. Rational strategies for identifying novel drug therapies are typically based on knowledge of the structure of the target proteins and the design of new chemical entities that modulate these proteins in a beneficial manner. The present review discusses a novel approach to drug discovery based on the identification and characterization of auxiliary proteins, the transmembrane AMPA receptor regulatory proteins (TARPs) that are associated with AMPA receptors. Utilizing these auxiliary proteins in compound screening led to the discovery of the TARP-dependent-AMPA forebrain selective receptor antagonist (TDAA), LY3130481/CERC-611 that is currently in clinical development for epilepsy.

Modulation of Recombinant Human α1 Glycine Receptors by Mono- and Disaccharides: A Kinetic Study

Glycine receptors (GlyRs) mediate fast synaptic inhibition in spinal cord, brainstem, and higher brain centers. Recently, glucose was identified as a positive modulator of GlyR-mediated currents. Here, we investigated extent and kinetics of the positive modulation of recombinant human α1 glycine receptors by different mono- and disaccharides and sorbitol using patch-clamp recording techniques. Glucose and fructose augmented glycine-mediated whole-cell currents with an EC50 of 6-7 mM. At concentrations > 10 mM, the maximum of current enhancement was reached within ∼30 min. Kinetics of GlyR modulation resemble those of protein glycation. On-rates were <0.5 h for saturating concentrations of monosaccharides and ∼1.5 h for disaccharides. Off-rates were considerably slower (>24 h). Galactose, the C4-epimer of glucose, and the sugar alcohol sorbitol had no effect on GlyR currents. Recent cryoelectron microscopy structures were used to identify a potential binding site for saccharides near the ivermectin binding pocket with lysine 143 as possible attachment site. The GlyR mutant α1(K143A) was not potentiated by glucose, suggesting an involvement of this residue in glycine receptor modulation by saccharides.

Characterization of ATPase Activity of P2RX2 Cation Channel

P2X purinergic receptors are plasma membrane ATP-dependent cation channels that are broadly distributed in the mammalian tissues. P2RX2 is a modulator of auditory sensory hair cell mechanotransduction and plays an important role in hair cell tolerance to noise. In this study, we demonstrate for the first time in vitro and in cochlear neuroepithelium, that P2RX2 possesses the ATPase activity. We observed that the P2RX2 V60L human deafness mutation alters its ability to bind ATP, while the G353R has no effect on ATP binding or hydrolysis. A non-hydrolysable ATP assay using HEK293 cells suggests that ATP hydrolysis plays a significant role in the opening and gating of the P2RX2 ion channel. Moreover, the results of structural modeling of the molecule was in agreement with our experimental observations. These novel findings suggest the intrinsic ATPase activity of P2RX2 and provide molecular insights into the channel opening.

Functional Site-Directed Fluorometry

Initially developed in the mid-1990s to examine the conformational changes of the canonical Shaker voltage-gated potassium channel, functional site-directed fluorometry has since been expanded to numerous other voltage-gated and ligand-gated ion channels as well as transporters, pumps, and other integral membrane proteins. The power of functional site-directed fluorometry, also known as voltage-clamp fluorometry, lies in its ability to provide information on the conformational changes in a protein in response to changes in its environment with high temporal resolution while simultaneously monitoring the function of that protein. Over time, applications of site-directed fluorometry have expanded to examine the interactions of ion channels with modulators ranging from membrane potential to ligands to accessory protein subunits to lipids. In the future, the range of questions answerable by functional site-directed fluorometry and its interpretive power should continue to improve, making it an even more powerful technique for dissecting the conformational dynamics of ion channels and other membrane proteins.

Optogenetic Control of Mammalian Ion Channels with Chemical Photoswitches

In neurons, ligand-gated ion channels decode the chemical signal of neurotransmitters into an electric response, resulting in a transient excitation or inhibition. Neurotransmitters act on multiple receptor types and subtypes, with spatially and temporally precise patterns. Hence, understanding the neural function of a given receptor requires methods for its targeted, rapid activation/inactivation in defined brain regions. To address this, we have developed a versatile optochemical genetic strategy, which allows the reversible control of defined receptor subtypes in designated cell types, with millisecond and micrometer precision. In this chapter, we describe the engineering of light-activated and -inhibited neuronal nicotinic acetylcholine receptors, as well as their characterization and use in cultured cells.

Calcium Signalling through Ligand-Gated Ion Channels such as P2X1 Receptors in the Platelet and other Non-Excitable Cells

Ligand-gated ion channels on the cell surface are directly activated by the binding of an agonist to their extracellular domain and often referred to as ionotropic receptors. P2X receptors are ligand-gated non-selective cation channels with significant permeability to Ca(2+) whose principal physiological agonist is ATP. This chapter focuses on the mechanisms by which P2X1 receptors, a ubiquitously expressed member of the family of ATP-gated channels, can contribute to cellular responses in non-excitable cells. Much of the detailed information on the contribution of P2X1 to Ca(2+) signalling and downstream functional events has been derived from the platelet. The underlying primary P2X1-generated signalling event in non-excitable cells is principally due to Ca(2+) influx, although Na(+) entry will also occur along with membrane depolarization. P2X1 receptor stimulation can lead to additional Ca(2+) mobilization via a range of routes such as amplification of G-protein-coupled receptor-dependent Ca(2+) responses. This chapter also considers the mechanism by which cells generate extracellular ATP for autocrine or paracrine activation of P2X1 receptors. For example cytosolic ATP efflux can result from opening of pannexin anion-permeable channels or following damage to the cell membrane. Alternatively, ATP stored in specialised secretory vesicles can undergo quantal release via the process of exocytosis. Examples of physiological or pathophysiological roles of P2X1-dependent signalling in non-excitable cells are also discussed, such as thrombosis and immune responses.

Design and validation of a homogeneous time-resolved fluorescence cell-based assay targeting the ligand-gated ion channel 5-HT3A

Ligand-gated ion channels (LGICs) are considered as attractive protein targets in the search for new therapeutic agents. Nowadays, this strategy involves the capability to screen large chemical libraries. We present a new Tag-lite ligand binding assay targeting LGICs on living cells. This technology combines the use of suicide enzyme tags fused to channels of interest with homogeneous time-resolved fluorescence (HTRF) as the detection readout. Using the 5-HT3 receptor as system model, we showed that the pharmacology of the HALO-5HT3 receptor was identical to that of the native receptor. After validation of the assay by using 5-HT3 agonists and antagonists of reference, a pilot screen enabled us to identify azelastine, a well-known histamine H1 antagonist, as a potent 5-HT3 antagonist. This interesting result was confirmed with electrophysiological experiments. The method described here is easy to implement and could be applicable for other LGICs, opening new ways for the screening of chemical libraries.

Effects of monoterpenes on ion channels of excitable cells

Monoterpenes are a structurally diverse group of phytochemicals and a major constituent of plant-derived 'essential oils'. Monoterpenes such as menthol, carvacrol, and eugenol have been utilized for therapeutical purposes and food additives for centuries and have been reported to have anti-inflammatory, antioxidant and analgesic actions. In recent years there has been increasing interest in understanding the pharmacological actions of these molecules. There is evidence indicating that monoterpenes can modulate the functional properties of several types of voltage and ligand-gated ion channels, suggesting that some of their pharmacological actions may be mediated by modulations of ion channel function. In this report, we review the literature concerning the interaction of monoterpenes with various ion channels.

Role of principal ionotropic and metabotropic receptors in visceral pain

Visceral pain is the most common form of pain caused by varied diseases and a major reason for patients to seek medical consultation. It also leads to a significant economic burden due to workdays lost and reduced productivity. Further, long-term use of non-specific medications is also associated with side effects affecting the quality of life. Despite years of extensive research and the availability of several therapeutic options, management of patients with chronic visceral pain is often inadequate, resulting in frustration for both patients and physicians. This is, most likely, because the mechanisms associated with chronic visceral pain are different from those of acute pain. Accumulating evidence from years of research implicates several receptors and ion channels in the induction and maintenance of central and peripheral sensitization during chronic pain states. Understanding the specific role of these receptors will facilitate to capitalize on their unique properties to augment the therapeutic efficacy while at the same time minimizing unwanted side effects. The aim of this review is to provide a concise review of the recent literature that reports on the role of principal ionotropic receptors and metabotropic receptors in the modulation visceral pain. We also include an overview of the possibility of these receptors as potential new targets for the treatment of chronic visceral pain conditions.

Allosteric ligands and their binding sites define γ-aminobutyric acid (GABA) type A receptor subtypes

GABAA receptors (GABA(A)Rs) mediate rapid inhibitory transmission in the brain. GABA(A)Rs are ligand-gated chloride ion channel proteins and exist in about a dozen or more heteropentameric subtypes exhibiting variable age and brain regional localization and thus participation in differing brain functions and diseases. GABA(A)Rs are also subject to modulation by several chemotypes of allosteric ligands that help define structure and function, including subtype definition. The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABA(A)Rs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Two classes of pharmacologically important allosteric modulatory ligand binding sites reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site and the high-affinity, relevant to intoxication, ethanol site. The benzodiazepine site is specific for certain GABA(A)R subtypes, mainly synaptic, while the ethanol site is found at a modified benzodiazepine site on different, extrasynaptic, subtypes. In the transmembrane domain are allosteric modulatory ligand sites for diverse chemotypes of general anesthetics: the volatile and intravenous agents, barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids. The last are endogenous positive allosteric modulators. X-ray crystal structures of prokaryotic and invertebrate pentameric ligand-gated ion channels, and the mammalian GABA(A)R protein, allow homology modeling of GABA(A)R subtypes with the various ligand sites located to suggest the structure and function of these proteins and their pharmacological modulation.

The nicotinic acetylcholine receptor and its prokaryotic homologues: Structure, conformational transitions & allosteric modulation

Pentameric ligand-gated ion channels (pLGICs) play a central role in intercellular communications in the nervous system by converting the binding of a chemical messenger - a neurotransmitter - into an ion flux through the postsynaptic membrane. Here, we present an overview of the most recent advances on the signal transduction mechanism boosted by X-ray crystallography of both prokaryotic and eukaryotic homologues of the nicotinic acetylcholine receptor (nAChR) in conjunction with time-resolved analyses based on single-channel electrophysiology and Molecular Dynamics simulations. The available data consistently point to a global mechanism of gating that involves a large reorganization of the receptor mediated by two distinct quaternary transitions: a global twisting and a radial expansion/contraction of the extracellular domain. These transitions profoundly modify the organization of the interface between subunits, which host several sites for orthosteric and allosteric modulatory ligands. The same mechanism may thus mediate both positive and negative allosteric modulations of pLGICs ligand binding at topographically distinct sites. The emerging picture of signal transduction is expected to pave the way to new pharmacological strategies for the development of allosteric modulators of nAChR and pLGICs in general. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Multiple protein stationary phases: a review

Cellular membrane affinity chromatography stationary phases have been extensively used to characterize immobilized proteins and provide a direct measurement of multiple binding sites, including orthosteric and allosteric sites. This review will address the utilization of immobilized cellular and tissue fragments to characterize multiple transmembrane proteins co-immobilized onto a stationary phase. This approach will be illustrated by demonstrating that multiple transmembrane proteins were immobilized from cell lines and tissue fragments. In addition, the immobilization of individual compartments/organelles within a cell will be discussed and the changes in the proteins binding/kinetics based on their location.

Constrained geometric simulation of the nicotinic acetylcholine receptor

Constrained geometric simulations have been performed for the recently published closed-channel state of the nicotinic acetylcholine receptor. These simulations support the theory that correlated motion in the flexible β-sheet structure of the extracellular domain helps to communicate a "conformational wave", spreading from the acetylcholine binding pocket. Furthermore, we have identified key residues that act at the interface between subunits and between domains that could potentially facilitate rapid communication between the binding site and the transmembrane gate.

Crystallographic studies of pharmacological sites in pentameric ligand-gated ion channels

BACKGROUND

Pentameric ligand-gated ion channels (pLGICs) mediate fast chemical transmission of nerve signals in the central and peripheral nervous system. On the functional side, these molecules respond to the binding of a neurotransmitter (glycine, GABA, acetylcholine or 5HT3) in the extracellular domain (ECD) by opening their ionotropic pore in the transmembrane domain (TMD). The response to the neurotransmitter binding can be modulated by several chemical compounds acting at topographically distinct sites, as documented by a large body of literature. Notably, these receptors are the target of several classes of world-wide prescribed drugs, including general anesthetics, smoking cessation aids, anxiolytics, anticonvulsants, muscle relaxants, hypnotics and anti-emetics. On the structural side recent progress has been made on the crystallization of pLGICs in its different allosteric states, especially pLGICs of bacterial origin. Therefore, structure-function relationships can now be discussed at the atomic level for pLGICs.

SCOPE OF REVIEW

This review focuses on the crystallographic structure of complexes of pLGICs with a number of ligands of pharmacological interest. First, we review structural data on two key functional aspects of these receptors: the agonist-induced activation and ion transport itself. The molecular understanding of both these functional aspects is important, as they are those that most pharmacological compounds target. Next, we describe modulation sites that have recently been documented by X-ray crystallography. Finally, we propose a simple geometric classification of all these pharmacological sites in pLGICs, based on icosahedrons.

MAJOR CONCLUSIONS

This review illustrates the wealth of structural insight gained by comparing all available structures of members of the pLGIC family to rationalize the pharmacology of structurally diverse drugs acting at topographically distinct sites. It will be highlighted how sites that had been described earlier using biochemical techniques can be rationalized using structural data. Surprisingly, the use of icosahedral symmetry allows to link together several modulation sites, in a way that was totally unanticipated.

GENERAL SIGNIFICANCE

Overall, understanding the interplay between the different modulation sites at the structural level should help the design of future drugs targeting pLGICs. This article is part of a Special Issue entitled structural biochemistry and biophysics of membrane proteins.

Moxidectin and the avermectins: Consanguinity but not identity

The avermectins and milbemycins contain a common macrocyclic lactone (ML) ring, but are fermentation products of different organisms. The principal structural difference is that avermectins have sugar groups at C13 of the macrocyclic ring, whereas the milbemycins are protonated at C13. Moxidectin (MOX), belonging to the milbemycin family, has other differences, including a methoxime at C23. The avermectins and MOX have broad-spectrum activity against nematodes and arthropods. They have similar but not identical, spectral ranges of activity and some avermectins and MOX have diverse formulations for great user flexibility. The longer half-life of MOX and its safety profile, allow MOX to be used in long-acting formulations. Some important differences between MOX and avermectins in interaction with various invertebrate ligand-gated ion channels are known and could be the basis of different efficacy and safety profiles. Modelling of IVM interaction with glutamate-gated ion channels suggest different interactions will occur with MOX. Similarly, profound differences between MOX and the avermectins are seen in interactions with ABC transporters in mammals and nematodes. These differences are important for pharmacokinetics, toxicity in animals with defective transporter expression, and probable mechanisms of resistance. Resistance to the avermectins has become widespread in parasites of some hosts and MOX resistance also exists and is increasing. There is some degree of cross-resistance between the avermectins and MOX, but avermectin resistance and MOX resistance are not identical. In many cases when resistance to avermectins is noticed, MOX produces a higher efficacy and quite often is fully effective at recommended dose rates. These similarities and differences should be appreciated for optimal decisions about parasite control, delaying, managing or reversing resistances, and also for appropriate anthelmintic combination.