Glycine Receptor Subtypes and Their Roles in Nociception and Chronic Pain

Methods for negating the impact of zinc contamination to allow characterization of positive allosteric modulators of glycine receptors

Zinc is a ubiquitous contaminant in many buffers, purified products and common labware that has previously been suggested to impact on the results of functional GlyR studies and may inadvertently cause the effectiveness of some GlyR modulators to be over-estimated. This could greatly impact the assessment of potential drug-candidates and contribute to the reduced effectiveness of compounds that reach clinical stages. This is especially true for GlyR modulators being developed for pain therapeutics due to the changes in spinal zinc concentrations that have been observed during chronic pain conditions. In this study we use two-electrode voltage clamp electrophysiology to evaluate the metal chelators tricine and Ca-EDTA, and show that tricine produces inhibitory effects at GlyRα1 that are not mediated by zinc. We also utilized the zinc insensitive W170S mutation as a tool to validate metal chelators and confirm that zinc contamination has not impacted the examination of lipid modulators previously developed by our lab. This study helps to further develop methods to negate the impact of contaminating zinc in functional studies of GlyRs which should be incorporated into future studies that seek to characterize the activity of novel modulators at GlyRs.

Case report: A case of anti-glycine receptor encephalomyelitis triggered by post-transplant or COVID-19 infection?

Even though long-term immunosuppressant drugs (ISD) are employed to inhibit immune system activity, enhancing graft functionality and patient survival in solid organ transplantation (SOT), these transplants often lead to immune complications, with post-transplant autoimmune diseases of the central nervous system (CNS) being uncommon. Here, we detail the case of a 66-year-old woman who underwent a renal transplantation 8 months prior, who was admitted with subacute onset of encephalomyelitis, accompanied by headaches, paraplegia, weakness, vomiting, and abdominal pain, with a positive COVID-19 nasopharyngeal swab test 1 month before admission. MRI scans of the brain revealed multiple lesions in the white matter of the bilateral deep frontal lobe, the left temporal lobe and insula lobe. Additionally, there were multiple short segment lesions in the spinal cord and subdural hematoma at T1, T6-T7 posterior. The serum revealed a positive result for GlyR-IgG. Following the administration of corticosteroid and intravenous immunoglobulin, there was a significant improvement in the patient's symptoms within 2 weeks, and her brain MRI showed a reduction in the lesion. Despite its rarity, we believe this to be the inaugural documentation of anti-GlyR encephalomyelitis occurring during renal transplantation. A full panel of antibodies for autoimmune encephalomyelitis is the key leading to the diagnosis.

Reducing the harms of alcohol: nutritional interventions and functional alcohol alternatives

The health risks and harm associated with regular alcohol consumption are well documented. In a recent WHO statement published in The Lancet Public Health alcohol consumption has been estimated to contribute worldwide to 3 million deaths in 2016 while also being responsible for 5·1% of the global burden of disease and injury. The total elimination of alcohol consumption, which has been long imbedded in human culture and society, is not practical and prohibition policies have proved historically ineffective. However, valuable strategies to reduce alcohol harms are already available and improved alternative approaches are currently being developed. Here, we will review and discuss recent advances on two main types of approaches, that is nutritional interventions and functional alcohol alternatives.

Gating mechanism of the human α1β GlyR by glycine

Glycine receptors (GlyRs) are members of the Cys-loop receptors that constitute a major portion of neurotransmitter receptors in the human nervous system. GlyRs are found in the spinal cord and brain mediating locomotive, sensory and cognitive functions, and are targets for pharmaceutical development. GlyRs share a general gating scheme with Cys-loop receptor family members, but the underlying mechanism is unclear. Recent resolution of heteromeric GlyRs structures in multiple functional states identified an invariable 4:1 α:β subunit stoichiometry and provided snapshots in the gating cycle, challenging previous beliefs and raising the fundamental questions of how α and β subunit functions in glycine binding and channel activation. In addition, how a single glycine-bound extracellular domain conformation leads to structurally and functionally different open and desensitized states remained enigmatic. In this study, we characterized in detail equilibrium properties as well as the transition kinetics between functional states. We show that while all allosteric sites bind cooperatively to glycine, occupation of 2 sites at the α-α interfaces is necessary and sufficient for GlyR activation. We also demonstrate differential glycine concentration dependence of desensitization rate, extent, and its recovery, which suggests separate but concerted roles of ligand-binding and ionophore reorganization. Based on these observations and available structural information, we developed a comprehensive quantitative gating model that accurately predicts both equilibrium and kinetical properties throughout glycine gating cycle. This model likely applies generally to the Cys-loop receptor family and informs on pharmaceutical endeavors in function modulation of this receptor family.

The diverse family of Cys-loop receptors in Caenorhabditis elegans: insights from electrophysiological studies

Cys-loop receptors integrate a large family of pentameric ligand-gated ion channels that mediate fast ionotropic responses in vertebrates and invertebrates. Their vital role in converting neurotransmitter recognition into an electrical impulse makes these receptors essential for a great variety of physiological processes. In vertebrates, the Cys-loop receptor family includes the cation-selective channels, nicotinic acetylcholine and 5-hydroxytryptamine type 3 receptors, and the anion-selective channels, GABAA and glycine receptors, whereas in invertebrates, the repertoire is significantly larger. The free-living nematode Caenorhabditis elegans has the largest known Cys-loop receptor family as well as unique receptors that are absent in vertebrates and constitute attractive targets for anthelmintic drugs. Given the large number and variety of Cys-loop receptor subunits and the multiple possible ways of subunit assembly, C. elegans offers a large diversity of receptors although only a limited number of them have been characterized to date. C. elegans has emerged as a powerful model for the study of the nervous system and human diseases as well as a model for antiparasitic drug discovery. This nematode has also shown promise in the pharmaceutical industry search for new therapeutic compounds. C. elegans is therefore a powerful model organism to explore the biology and pharmacology of Cys-loop receptors and their potential as targets for novel therapeutic interventions. In this review, we provide a comprehensive overview of what is known about the function of C. elegans Cys-loop receptors from an electrophysiological perspective.

Metabolic Biomarkers Differentiate a Surgical Intervertebral Disc from a Nonsurgical Intervertebral Disc

BACKGROUND

Degeneration of the intervertebral disc (IVD) is caused by disturbances in metabolic processes, which lead to structural disorders. The aim of this report is to analyze metabolic disorders in the degeneration process by comparing control discs with degenerated discs. In our research on the nucleus pulposus (NP), we used NMR spectroscopy of extracts of hydrophilic and hydrophobic compounds of the tissue.

METHODS

Nuclear magnetic resonance (NMR) spectroscopy allows the study of biochemistry and cellular metabolism in vitro. Hydrophilic and hydrophobic compounds were extracted from the NP of the intervertebral disc. In the NMR spectra, metabolites were identified and quantitatively analyzed. The results of our research indicate disturbances in the biosynthesis and metabolism of cholesterol, the biosynthesis and degradation of various fatty acid groups, ketone bodies, or lysine, and the metabolism of glycerophospholipids, purines, glycine, inositol, galactose, alanine, glutamate, and pyruvate in the biosynthesis of valine and isoleucine, leucine. All these disorders indicate pathomechanisms related to oxidative stress, energy, neurotransmission disturbances, and disturbances in the structure and functioning of cell membranes, inflammation, or chronic pain generators.

CONCLUSIONS

NMR spectroscopy allows the identification of metabolites differentiating surgical from nonsurgical discs. These data may provide guidance in in vivo MRS studies in assessing the severity of lesions of the disc.

Recent Advances in Computer-Aided Structure-Based Drug Design on Ion Channels

Ion channels play important roles in fundamental biological processes, such as electric signaling in cells, muscle contraction, hormone secretion, and regulation of the immune response. Targeting ion channels with drugs represents a treatment option for neurological and cardiovascular diseases, muscular degradation disorders, and pathologies related to disturbed pain sensation. While there are more than 300 different ion channels in the human organism, drugs have been developed only for some of them and currently available drugs lack selectivity. Computational approaches are an indispensable tool for drug discovery and can speed up, especially, the early development stages of lead identification and optimization. The number of molecular structures of ion channels has considerably increased over the last ten years, providing new opportunities for structure-based drug development. This review summarizes important knowledge about ion channel classification, structure, mechanisms, and pathology with the main focus on recent developments in the field of computer-aided, structure-based drug design on ion channels. We highlight studies that link structural data with modeling and chemoinformatic approaches for the identification and characterization of new molecules targeting ion channels. These approaches hold great potential to advance research on ion channel drugs in the future.

Molecular determinants of tetrahydrocannabinol binding to the glycine receptor

The recognition of Cannabis as a source of new compounds suitable for medical use has attracted strong interest from the scientific community in its research, and substantial progress has accumulated regarding cannabinoids' activity; however, a thorough description of their molecular mechanisms of action remains a task to complete. Highlighting their complex pharmacology, the list of cannabinoids' interactors has vastly expanded beyond the canonical cannabinoid receptors. Among those, we have focused our study on the glycine receptor (GlyR), an ion channel involved in the modulation of nervous system responses, including, to our interest, sensitivity to peripheral pain. Here, we report the use of computational methods to investigate possible binding modes between the GlyR and Δ⁹ -tetrahydrocannabinol (THC). After obtaining a first pose for the THC binding from a biased molecular docking simulation and subsequently evaluating it by molecular dynamic simulations, we found a dynamic system with an identifiable representative binding mode characterized by the specific interaction with two transmembrane residues (Phe293 and Ser296). Complementarily, we assessed the role of membrane cholesterol in this interaction and positively established its relevance for THC binding to GlyR. Lastly, the use of restrained molecular dynamics simulations allowed us to refine the description of the binding mode and of the cholesterol effect. Altogether, our findings contribute to the current knowledge about the GlyR-THC mode of binding and propose a new starting point for future research on how cannabinoids in general, and THC in particular, modulate pain perception in view of its possible clinical applications.

Positive Allosteric Modulators of Glycine Receptors and Their Potential Use in Pain Therapies

Glycine receptors are ligand-gated ion channels that mediate synaptic inhibition throughout the mammalian spinal cord, brainstem, and higher brain regions. They have recently emerged as promising targets for novel pain therapies due to their ability to produce antinociception by inhibiting nociceptive signals within the dorsal horn of the spinal cord. This has greatly enhanced the interest in developing positive allosteric modulators of glycine receptors. Several pharmaceutical companies and research facilities have attempted to identify new therapeutic leads by conducting large-scale screens of compound libraries, screening new derivatives from natural sources, or synthesizing novel compounds that mimic endogenous compounds with antinociceptive activity. Advances in structural techniques have also led to the publication of multiple high-resolution structures of the receptor, highlighting novel allosteric binding sites and providing additional information for previously identified binding sites. This has greatly enhanced our understanding of the functional properties of glycine receptors and expanded the structure activity relationships of novel pharmacophores. Despite this, glycine receptors are yet to be used as drug targets due to the difficulties in obtaining potent, selective modulators with favorable pharmacokinetic profiles that are devoid of side effects. This review presents a summary of the structural basis for how current compounds cause positive allosteric modulation of glycine receptors and discusses their therapeutic potential as analgesics. SIGNIFICANCE STATEMENT: Chronic pain is a major cause of disability, and in Western societies, this will only increase as the population ages. Despite the high level of prevalence and enormous socioeconomic burden incurred, treatment of chronic pain remains limited as it is often refractory to current analgesics, such as opioids. The National Institute for Drug Abuse has set finding effective, safe, nonaddictive strategies to manage chronic pain as their top priority. Positive allosteric modulators of glycine receptors may provide a therapeutic option.

Pain modulation in the spinal cord

The sensory inflow from the periphery that triggers innocuous and painful sensations is highly complex, capturing key elements of the nature of any stimulus, its location, intensity, and duration, and converting this to dynamic action potential firing across a wide population of afferents. While sensory afferents are highly specialized to detect these features, their input to the spinal cord also triggers active processing and modulation there which determines its output, to drive the sensory percept experienced and behavioral responses. Focus on such active spinal modulation was arguably first introduced by Melzack and Wall in their Spinal Cord Gate Control theory. This theory has had a profound influence on our understanding of pain, and especially its processing, as well as leading directly to the development of clinical interventions, and its historical importance certainly needs to be fully recognized. However, the enormous progress we are making in the understanding of the function of the somatosensory system, means that it is time to incorporate these newly discovered features into a more complex and accurate model of spinal sensory modulation.