Lysosomal ion channels involved in cellular entry and uncoating of enveloped viruses: Implications for therapeutic strategies against SARS-CoV-2.

Ion channels are necessary for correct lysosomal function including degradation of cargoes originating from endocytosis. Almost all enveloped viruses, including coronaviruses (CoVs), enter host cells via endocytosis, and do not escape endosomal compartments into the cytoplasm (via fusion with the endolysosomal membrane) unless the virus-encoded envelope proteins are cleaved by lysosomal proteases. With the ongoing outbreak of severe acute respiratory syndrome (SARS)-CoV-2, endolysosomal two-pore channels represent an exciting and emerging target for antiviral therapies. This review focuses on the latest knowledge of the effects of lysosomal ion channels on the cellular entry and uncoating of enveloped viruses, which may aid in development of novel therapies against emerging infectious diseases such as SARS-CoV-2.

Mechano-gated channels in C. elegans.

Mechanosensation such as touch, hearing and proprioception, is functionally regulated by mechano-gated ion channels through the process of transduction. Mechano-gated channels are a subtype of gated ion channels engaged in converting mechanical stimuli to chemical or electrical signals thereby modulating sensation. To date, a few families of mechano-gated channels (DEG/ENaC, TRPN, K2P, TMC and Piezo) have been identified in eukaryotes. Using a tractable genetic model organism Caenorhabditis elegans, the molecular mechanism of mechanosensation have been the focus of much research to comprehend the process of mechanotransduction. Comprising of almost all metazoans classes of ion channels, transporters and receptors, C. elegans is a powerful genetic model to explore mechanosensitive behaviors such as touch sensation and proprioception. The nematode relies primarily on its sensory abilities to survive in its natural environment. Genetic screening, calcium imaging and electrophysiological analysis have established that ENaC proteins and TRPN channel (TRP-4 protein) can characterize mechano-gated channels in C. elegans. A recent study reported that TMCs are likely the pore-forming subunit of a mechano-gated channel in C. elegans. Nevertheless, it still remains unclear whether Piezo as well as other candidate proteins can form mechano-gated channels in C. elegans.

DeepIon: Deep learning approach for classifying ion transporters and ion channels from membrane proteins.

The movement of ions across the cell membrane is an essential for many biological processes. This study is focused on ion channels and ion transporters (pumps) as types of border guards control the incessant traffic of ions across cell membranes. Ion channels and ion transporters function to regulate membrane potential and electrical signaling and play important roles in cell proliferation, migration, apoptosis, and differentiation. In their behaviors, it is found that ion channels differ significantly from ion transporters. Therefore, a method for automatically classifying ion transporters and ion channels from membrane proteins is proposed by training deep neural networks and using the position-specific scoring matrix profile as an input. The key of novelty is the three-stage approach, in which five techniques for data normalization are used; next three imbalanced data techniques are applied to the minority classes and then, six classifiers are compared with the proposed method. © 2019 Wiley Periodicals, Inc.

Prediction of Ion Channels and their Types from Protein Sequences: Comprehensive Review and Comparative Assessment.

BACKGROUND: Ion channels are a large and growing protein family. Many of them are associated with diseases, and consequently, they are targets for over 700 drugs. Discovery of new ion channels is facilitated with computational methods that predict ion channels and their types from protein sequences. However, these methods were never comprehensively compared and evaluated. OBJECTIVE: We offer first-of-its-kind comprehensive survey of the sequence-based predictors of ion channels. We describe eight predictors that include five methods that predict ion channels, their types, and four classes of the voltage-gated channels. We also develop and use a new benchmark dataset to perform comparative empirical analysis of the three currently available predictors. RESULTS: While several methods that rely on different designs were published, only a few of them are currently available and offer a broad scope of predictions. Support and availability after publication should be required when new methods are considered for publication. Empirical analysis shows strong performance for the prediction of ion channels and modest performance for the prediction of ion channel types and voltage-gated channel classes. We identify a substantial weakness of current methods that cannot accurately predict ion channels that are categorized into multiple classes/types. CONCLUSION: Several predictors of ion channels are available to the end users. They offer practical levels of predictive quality. Methods that rely on a larger and more diverse set of predictive inputs (such as PSIONplus) are more accurate. New tools that address multi-label prediction of ion channels should be developed.

Hv 1 Proton Channels in Dinoflagellates: Not Just for Bioluminescence?

Bioluminescence in dinoflagellates is controlled by HV 1 proton channels. Database searches of dinoflagellate transcriptomes and genomes yielded hits with sequence features diagnostic of all confirmed HV 1, and show that HV 1 is widely distributed in the dinoflagellate phylogeny including the basal species Oxyrrhis marina. Multiple sequence alignments followed by phylogenetic analysis revealed three major subfamilies of HV 1 that do not correlate with presence of theca, autotrophy, geographic location, or bioluminescence. These data suggest that most dinoflagellates express a HV 1 which has a function separate from bioluminescence. Sequence evidence also suggests that dinoflagellates can contain more than one HV 1 gene.

An evolutionarily conserved gene family encodes proton-selective ion channels.

Ion channels form the basis for cellular electrical signaling. Despite the scores of genetically identified ion channels selective for other monatomic ions, only one type of proton-selective ion channel has been found in eukaryotic cells. By comparative transcriptome analysis of mouse taste receptor cells, we identified Otopetrin1 (OTOP1), a protein required for development of gravity-sensing otoconia in the vestibular system, as forming a proton-selective ion channel. We found that murine OTOP1 is enriched in acid-detecting taste receptor cells and is required for their zinc-sensitive proton conductance. Two related murine genes, Otop2 and Otop3, and a Drosophila ortholog also encode proton channels. Evolutionary conservation of the gene family and its widespread tissue distribution suggest a broad role for proton channels in physiology and pathophysiology.

Ion channels in the regulation of autophagy.

Autophagy is a cellular process in which the cell degrades and recycles its own constituents. Given the crucial role of autophagy in physiology, deregulation of autophagic machinery is associated with various diseases. Hence, a thorough understanding of autophagy regulatory mechanisms is crucially important for the elaboration of efficient treatments for different diseases. Recently, ion channels, mediating ion fluxes across cellular membranes, have emerged as important regulators of both basal and induced autophagy. However, the mechanisms by which specific ion channels regulate autophagy are still poorly understood, thus underscoring the need for further research in this field. Here we discuss the involvement of major types of ion channels in autophagy regulation.

Mapping the function of neuronal ion channels in model and experiment.

Ion channel models are the building blocks of computational neuron models. Their biological fidelity is therefore crucial for the interpretation of simulations. However, the number of published models, and the lack of standardization, make the comparison of ion channel models with one another and with experimental data difficult. Here, we present a framework for the automated large-scale classification of ion channel models. Using annotated metadata and responses to a set of voltage-clamp protocols, we assigned 2378 models of voltage- and calcium-gated ion channels coded in NEURON to 211 clusters. The IonChannelGenealogy (ICGenealogy) web interface provides an interactive resource for the categorization of new and existing models and experimental recordings. It enables quantitative comparisons of simulated and/or measured ion channel kinetics, and facilitates field-wide standardization of experimentally-constrained modeling.

Scorpion toxin peptide action at the ion channel subunit level.

This review categorizes functionally validated actions of defined scorpion toxin (SCTX) neuropeptides across ion channel subclasses, highlighting key trends in this rapidly evolving field. Scorpion envenomation is a common event in many tropical and subtropical countries, with neuropharmacological actions, particularly autonomic nervous system modulation, causing significant mortality. The primary active agents within scorpion venoms are a diverse group of small neuropeptides that elicit specific potent actions across a wide range of ion channel classes. The identification and functional characterisation of these SCTX peptides has tremendous potential for development of novel pharmaceuticals that advance knowledge of ion channels and establish lead compounds for treatment of excitable tissue disorders. This review delineates the unique specificities of 320 individual SCTX peptides that collectively act on 41 ion channel subclasses. Thus the SCTX research field has significant translational implications for pathophysiology spanning neurotransmission, neurohumoral signalling, sensori-motor systems and excitation-contraction coupling. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Voltage-Gated Sodium Channels: Evolutionary History and Distinctive Sequence Features.

Voltage-gated sodium channels (Nav) are responsible for the rising phase of the action potential. Their role in electrical signal transmission is so relevant that their emergence is believed to be one of the crucial factors enabling development of nervous system. The presence of voltage-gated sodium-selective channels in bacteria (BacNav) has raised questions concerning the evolutionary history of the ones in animals. Here we review some of the milestones in the field of Nav phylogenetic analysis and discuss some of the most important sequence features that distinguish these channels from voltage-gated potassium channels and transient receptor potential channels.

Pipeline for the identification and classification of ion channels in parasitic flatworms.

BACKGROUND: Ion channels are well characterised in model organisms, principally because of the availability of functional genomic tools and datasets for these species. This contrasts the situation, for example, for parasites of humans and animals, whose genomic and biological uniqueness means that many genes and their products cannot be annotated. As ion channels are recognised as important drug targets in mammals, the accurate identification and classification of parasite channels could provide major prospects for defining unique targets for designing novel and specific anti-parasite therapies. Here, we established a reliable bioinformatic pipeline for the identification and classification of ion channels encoded in the genome of the cancer-causing liver fluke Opisthorchis viverrini, and extended its application to related flatworms affecting humans. METHODS: We built an ion channel identification + classification pipeline (called MuSICC), employing an optimised support vector machine (SVM) model and using the Kyoto Encyclopaedia of Genes and Genomes (KEGG) classification system. Ion channel proteins were first identified and grouped according to amino acid sequence similarity to classified ion channels and the presence and number of ion channel-like conserved and transmembrane domains. Predicted ion channels were then classified to sub-family using a SVM model, trained using ion channel features. RESULTS: Following an evaluation of this pipeline (MuSICC), which demonstrated a classification sensitivity of 95.2 % and accuracy of 70.5 % for known ion channels, we applied it to effectively identify and classify ion channels in selected parasitic flatworms. CONCLUSIONS: MuSICC provides a practical and effective tool for the identification and classification of ion channels of parasitic flatworms, and should be applicable to a broad range of organisms that are evolutionarily distant from taxa whose ion channels are functionally characterised.

Phosphoinositide signaling in somatosensory neurons.

Somatosensory neurons of the dorsal root ganglia (DRG) and trigeminal ganglia (TG) are responsible for detecting thermal and tactile stimuli. They are also the primary neurons mediating pain and itch. A large number of cell surface receptors in these neurons couple to phospholipase C (PLC) enzymes leading to the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and the generation of downstream signaling molecules. These neurons also express many different ion channels, several of which are regulated by phosphoinositides. This review will summarize the knowledge on phosphoinositide signaling in DRG neurons, with special focus on effects on sensory and other ion channels.

Distinct pharmacological and molecular properties of the acid-sensitive outwardly rectifying (ASOR) anion channel from those of the volume-sensitive outwardly rectifying (VSOR) anion channel.

Expressed by many cell types, acid-sensitive outwardly rectifying (ASOR) anion channels are known to be activated by extracellular acidification and involved in acidotoxic necrotic cell death. In contrast, ubiquitously expressed volume-sensitive outwardly rectifying (VSOR) anion channels are activated by osmotic cell swelling and involved in cell volume regulation and apoptotic cell death. Distinct inhibitors to distinguish ASOR from VSOR anion channels have not been identified. Although leucine-rich repeats containing 8A (LRRC8A) was recently found to be an essential component of VSOR anion channels, the possibility of an LRRC8 family member serving as a component of ASOR anion channels has not been examined. In this study, we explored the effects of 12 known VSOR channel inhibitors and small interfering RNA (siRNA)-mediated knockdown of LRRC8 family members on ASOR and VSOR currents in HeLa cells. Among these inhibitors, eight putative VSOR blockers, including 4-(2-butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxobutyric acid (DCPIB) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), were totally ineffective at blocking ASOR channel activity, whereas suramin, R-(+)-[(2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy] acetic acid (DIOA), arachidonic acid, and niflumic acid were found to be effective ASOR anion channel antagonists. In addition, gene-silencing studies showed that no LRRC8 family members are essentially involved in ASOR anion channel activity, whereas LRRC8A is involved in VSOR anion channel activity in HeLa cells.

Classifying neuronal subclasses of the cerebellum through constellation pharmacology.

A pressing need in neurobiology is the comprehensive identification and characterization of neuronal subclasses within the mammalian nervous system. To this end, we used constellation pharmacology as a method to interrogate the neuronal and glial subclasses of the mouse cerebellum individually and simultaneously. We then evaluated the data obtained from constellation-pharmacology experiments by cluster analysis to classify cells into neuronal and glial subclasses, based on their functional expression of glutamate, acetylcholine, and GABA receptors, among other ion channels. Conantokin peptides were used to identify N-methyl-d-aspartate (NMDA) receptor subtypes, which revealed that neurons of the young mouse cerebellum expressed NR2A and NR2B NMDA receptor subunits. Additional pharmacological tools disclosed differential expression of α-amino-3-hydroxy-5-methyl-4-isoxazloepropionic, nicotinic acetylcholine, and muscarinic acetylcholine receptors in different neuronal and glial subclasses. Certain cell subclasses correlated with known attributes of granule cells, and we combined constellation pharmacology with genetically labeled neurons to identify and characterize Purkinje cells. This study illustrates the utility of applying constellation pharmacology to classify neuronal and glial subclasses in specific anatomical regions of the brain.

Ion channels in the regulation of apoptosis.

Apoptosis, a type of genetically controlled cell death, is a fundamental cellular mechanism utilized by multicellular organisms for disposal of cells that are no longer needed or potentially detrimental. Given the crucial role of apoptosis in physiology, deregulation of apoptotic machinery is associated with various diseases as well as abnormalities in development. Acquired resistance to apoptosis represents the common feature of most and perhaps all types of cancer. Therefore, repairing and reactivating apoptosis represents a promising strategy to fight cancer. Accumulated evidence identifies ion channels as essential regulators of apoptosis. However, the contribution of specific ion channels to apoptosis varies greatly depending on cell type, ion channel type and intracellular localization, pathology as well as intracellular signaling pathways involved. Here we discuss the involvement of major types of ion channels in apoptosis regulation. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

[Channels: a new way to revisit pathology]. / La pathologie revisitée par les canaux.

Many "essential" diseases that manifest themselves in the form of crises or fits (epilepsies, episodic ataxia, periodic paralyses, myotonia, heart rhythm disorders, etc.) are due to ionic channel dysfunction and are thus referred to as "channelopathies". Some of these disorders are congenital, due to mutations of genes encoding channel subunits, while others result from toxic, immune or hormonal disturbances affecting channelfunction. Channelopathies take on a wide variety of clinical forms, depending on the type of channel (sodium, potassium, calcium, chloride...) and the type of dysfunction (loss or gain of function). Some apparently unrelated diseases affecting distinct organs are due to a similar dysfunction of the same channel, revealing unsuspected relationships between organs and between medical specialties. In addition, a given syndrome can be caused by distinct channel dysfunctions. This provides new opportunities for diferential diagnosis and specific correction of the causal defects, although some treatments find applications across multiple medical specialties.

Bioinformatics approaches for functional annotation of membrane proteins.

Membrane proteins perform diverse functions in living organisms such as transporters, receptors and channels. The functions of membrane proteins have been investigated with several computational approaches, such as developing databases, analyzing the structure-function relationship and establishing algorithms to discriminate different type of membrane proteins. However, compilation of bioinformatics resources for the functions of membrane proteins is not well documented compared with their structural aspects. In this comprehensive review, we elaborately focus on three aspects of membrane protein functions: (i) databases for different types of membrane proteins based on their functions including transporters, receptors and ion channels, annotated functional data for genomes, as well as functionally important amino acid residues in membrane proteins obtained from experimental data, (ii) analysis of membrane protein functions based on their structures, motifs, amino acid properties and other features and (iii) algorithms for discriminating different types of membrane proteins and annotating them in genomic sequences. In addition, we provide a list of online resources for the databases and web servers for functional annotation of membrane proteins.

A plant homolog of animal glutamate receptors is an ion channel gated by multiple hydrophobic amino acids.

Ionotropic glutamate receptors (iGluRs) are ligand-gated cation channels that mediate neurotransmission in animal nervous systems. Homologous proteins in plants have been implicated in root development, ion transport, and several metabolic and signaling pathways. AtGLR3.4, a plant iGluR homolog from Arabidopsis thaliana, has ion channel activity and is gated by asparagine, serine, and glycine. Using heterologous expression in Xenopus oocytes, we found that another Arabidopsis iGluR homolog, AtGLR1.4, functioned as a ligand-gated, nonselective, Ca(2+)-permeable cation channel that responded to an even broader range of amino acids, none of which are agonists of animal iGluRs. Seven of the 20 standard amino acids--mainly hydrophobic ones--acted as agonists, with methionine being most effective and most potent. Nine amino acids were antagonists, and four, including glutamate and glycine, had no effect on channel activity. We constructed a model of this previously uncharacterized ligand specificity and used knockout mutants to show that AtGLR1.4 accounts for methionine-induced membrane depolarization in Arabidopsis leaves.

Ion channels regulating mast cell biology.

Mast cells play a central role in the pathophysiology of asthma and related allergic conditions. Mast cell activation leads to the degranulation of preformed mediators such as histamine and the secretion of newly synthesised proinflammatory mediators such as leukotrienes and cytokines. Excess release of these mediators contributes to allergic disease states. An influx of extracellular Ca2+ is essential for mast cell mediator release. From the Ca2+ channels that mediate this influx, to the K+ , Cl- and transient receptor potential channels that set the cell membrane potential and regulate Ca2+ influx, ion channels play a critical role in mast cell biology. In this review we provide an overview of our current knowledge of ion channel expression and function in mast cells with an emphasis on how channels interact to regulate Ca2+ signalling.