Long-term changes in transmembrane voltage after electroporation are governed by the interplay between nonselective leak current and ion channel activation

Electroporation causes a temporal increase in cell membrane permeability and leads to prolonged changes in transmembrane voltage (TMV) in both excitable and non-excitable cells. However, the mechanisms of these TMV changes remain to be fully elucidated. To this end, we monitored TMV over 30 min after exposing two different cell lines to a single 100 µs electroporation pulse using the FLIPR Membrane Potential dye. In CHO-K1 cells, which express very low levels of endogenous ion channels, membrane depolarization following pulse exposure could be explained by nonselective leak current, which persists until the membrane reseals, enabling the cells to recover their resting TMV. In U-87 MG cells, which express many different ion channels, we unexpectedly observed membrane hyperpolarization following the initial depolarization phase, but only at 33 °C and not at 25 °C. We developed a theoretical model, supported by experiments with ion channel inhibitors, which indicated that hyperpolarization could largely be attributed to the activation of calcium-activated potassium channels. Ion channel activation, coupled with changes in TMV and intracellular calcium, participates in various physiological processes, including cell proliferation, differentiation, migration, and apoptosis. Therefore, our study suggests that ion channels could present a potential target for influencing the biological response after electroporation.

Ih current stabilizes excitability in rodent DRG neurons and reverses hyperexcitability in a nociceptive neuron model of inherited neuropathic pain

We show here that hyperpolarization-activated current (Ih ) unexpectedly acts to inhibit the activity of dorsal root ganglion (DRG) neurons expressing WT Nav1.7, the largest inward current and primary driver of DRG neuronal firing, and hyperexcitable DRG neurons expressing a gain-of-function Nav1.7 mutation that causes inherited erythromelalgia (IEM), a human genetic model of neuropathic pain. In this study we created a kinetic model of Ih and used it, in combination with dynamic-clamp, to study Ih function in DRG neurons. We show, for the first time, that Ih increases rheobase and reduces the firing probability in small DRG neurons, and demonstrate that the amplitude of subthreshold oscillations is reduced by Ih . Our results show that Ih , due to slow gating, is not deactivated during action potentials (APs) and has a striking damping action, which reverses from depolarizing to hyperpolarizing, close to the threshold for AP generation. Moreover, we show that Ih reverses the hyperexcitability of DRG neurons expressing a gain-of-function Nav1.7 mutation that causes IEM. In the aggregate, our results show that Ih unexpectedly has strikingly different effects in DRG neurons as compared to previously- and well-studied cardiac cells. Within DRG neurons where Nav1.7 is present, Ih reduces depolarizing sodium current inflow due to enhancement of Nav1.7 channel fast inactivation and creates additional damping action by reversal of Ih direction from depolarizing to hyperpolarizing close to the threshold for AP generation. These actions of Ih limit the firing of DRG neurons expressing WT Nav1.7 and reverse the hyperexcitability of DRG neurons expressing a gain-of-function Nav1.7 mutation that causes IEM. KEY POINTS: Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, the molecular determinants of hyperpolarization-activated current (Ih ) have been characterized as a 'pain pacemaker', and thus considered to be a potential molecular target for pain therapeutics. Dorsal root ganglion (DRG) neurons express Nav1.7, a channel that is not present in central neurons or cardiac tissue. Gain-of-function mutations (GOF) of Nav1.7 identified in inherited erythromelalgia (IEM), a human genetic model of neuropathic pain, produce DRG neuron hyperexcitability, which in turn produces severe pain. We found that Ih increases rheobase and reduces firing probability in small DRG neurons expressing WT Nav1.7, and demonstrate that the amplitude of subthreshold oscillations is reduced by Ih . We also demonstrate that Ih reverses the hyperexcitability of DRG neurons expressing a GOF Nav1.7 mutation (L858H) that causes IEM. Our results show that, in contrast to cardiac cells and CNS neurons, Ih acts to stabilize DRG neuron excitability and prevents excessive firing.

Peripheral mechanisms of arthritic pain: A proposal to leverage large animals for in vitro studies

Pain arising from musculoskeletal disorders such as arthritis is one of the leading causes of disability. Whereas the past 20-years has seen an increase in targeted therapies for rheumatoid arthritis (RA), other arthritis conditions, especially osteoarthritis, remain poorly treated. Although modulation of central pain pathways occurs in chronic arthritis, multiple lines of evidence indicate that peripherally driven pain is important in arthritic pain. To understand the peripheral mechanisms of arthritic pain, various in vitro and in vivo models have been developed, largely in rodents. Although rodent models provide numerous advantages for studying arthritis pathogenesis and treatment, the anatomy and biomechanics of rodent joints differ considerably to those of humans. By contrast, the anatomy and biomechanics of joints in larger animals, such as dogs, show greater similarity to human joints and thus studying them can provide novel insight for arthritis research. The purpose of this article is firstly to review models of arthritis and behavioral outcomes commonly used in large animals. Secondly, we review the existing in vitro models and assays used to study arthritic pain, primarily in rodents, and discuss the potential for adopting these strategies, as well as likely limitations, in large animals. We believe that exploring peripheral mechanisms of arthritic pain in vitro in large animals has the potential to reduce the veterinary burden of arthritis in commonly afflicted species like dogs, as well as to improve translatability of pain research into the clinic.

Contributions of natural products to ion channel pharmacology

Covering: Up to 2020Ion channels are a vast super-family of membrane proteins that play critical physiological roles in excitable and non-excitable cells. Their biomedical importance makes them valuable and attractive drug targets for neurological, cardiovascular, gastrointestinal and metabolic diseases, and for cancer therapy and immune modulation. Current therapeutics target only a minor subset of ion channels, leaving a large unexploited space within the ion channel field. Natural products harnessed from the almost unlimited and diverse universe of compounds within the bioenvironment have been used to modulate channels for decades. In this review we highlight the impact made by natural products on ion channel pharmacology, specifically on K⁺, Na_V and Ca_V channels, and use case studies to describe the development of ion channel-modulating drugs from natural sources for the treatment of pain, heart disease and autoimmune diseases.

Potentially Cardiotoxic Diterpenoid Alkaloids from the Roots of Aconitum carmichaelii

Aconitum carmichaelii is a traditional Chinese herbal medicine used for the treatment of pain and inflammation in the joints. However, the strong cardiotoxicity hinders its use. Although diester- and monoester-type diterpenoids, e.g., aconitine, mesaconitine, and hypacaonitine, are commonly considered as the toxic components, the toxicity of A. carmichaelii cannot be completely explained by the compounds reported. To investigate further the cardiotoxic compounds and their potential mechanism, the chemical constituents were first isolated by column chromatography and identified using mass spectrometry and NMR spectroscopy. Two new hetisine-type (1 and 2) and four new aconitine-type alkaloids (3-6) were assigned. The cardiac cytotoxicity assessed on H9c2 cells indicated that the new compound 4 as well as six known alkaloids (7 and 9-13) exhibited significant toxicities. A preliminary structure-toxicity relationship study suggested that substitution at C-8 and C-10 both have a significant influence on cardiotoxicity, and such toxicity decreased in the order OBz-8, OBu-8, and OMe-8. The presence of an OH-10 group abolished the toxicity. Finally, it was found that ion channel disorder and induction of mitochondrial-mediated cell apoptosis are the possible mechanisms of cardiotoxicity among the compounds studied.

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.

The Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: from Biophysics to Pharmacology of a Unique Family of Ion Channels

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are important members of the voltage-gated pore loop channels family. They show unique features: they open at hyperpolarizing potential, carry a mixed Na/K current, and are regulated by cyclic nucleotides. Four different isoforms have been cloned (HCN1-4) that can assemble to form homo- or heterotetramers, characterized by different biophysical properties. These proteins are widely distributed throughout the body and involved in different physiologic processes, the most important being the generation of spontaneous electrical activity in the heart and the regulation of synaptic transmission in the brain. Their role in heart rate, neuronal pacemaking, dendritic integration, learning and memory, and visual and pain perceptions has been extensively studied; these channels have been found also in some peripheral tissues, where their functions still need to be fully elucidated. Genetic defects and altered expression of HCN channels are linked to several pathologies, which makes these proteins attractive targets for translational research; at the moment only one drug (ivabradine), which specifically blocks the hyperpolarization-activated current, is clinically available. This review discusses current knowledge about HCN channels, starting from their biophysical properties, origin, and developmental features, to (patho)physiologic role in different tissues and pharmacological modulation, ending with their present and future relevance as drug targets.

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.

HCN Channels Modulators: The Need for Selectivity

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, the molecular correlate of the hyperpolarization-activated current (If/Ih), are membrane proteins which play an important role in several physiological processes and various pathological conditions. In the Sino Atrial Node (SAN) HCN4 is the target of ivabradine, a bradycardic agent that is, at the moment, the only drug which specifically blocks If. Nevertheless, several other pharmacological agents have been shown to modulate HCN channels, a property that may contribute to their therapeutic activity and/or to their side effects. HCN channels are considered potential targets for developing drugs to treat several important pathologies, but a major issue in this field is the discovery of isoform-selective compounds, owing to the wide distribution of these proteins into the central and peripheral nervous systems, heart and other peripheral tissues. This survey is focused on the compounds that have been shown, or have been designed, to interact with HCN channels and on their binding sites, with the aim to summarize current knowledge and possibly to unveil useful information to design new potent and selective modulators.

Coupling of olfactory receptor and ion channel for rapid and sensitive visualization of odorant response

In the human smell sensing system, there are about 390 kinds of olfactory receptors (ORs) which bind to various odorants with different affinities and specificities. Characterization and odorant binding pattern analysis of the ORs are essential for understanding of human olfaction and to mimic the olfactory system in various applications. Although various cell-based odorant screening systems have been developed for this purpose, many human ORs (hORs) still remain orphan because of the time-consuming and labor-intensive experimental procedures of the available screening methods. In this study, we constructed an ion channel-coupled hOR for simple odorant detection by rapidly visualizing the odorant response to overcome the limitations of conventional screening systems. The hORs were coupled to the Kir6.2 potassium channel and the fusion proteins were expressed in HEK293 cells. In this system, when an odorant binds to the hORs coupled to the ion channel, a conformational change in the OR occurs, which consequently opens the ion channel to result in ion influx into the cell. This ion influx was then visualized using a membrane potential dye. Cells expressing ion channel-coupled hORs showed high sensitivity and selectivity to their specific odorants, and the odorant-hOR binding pattern was visualized to identify the response of individual hORs to various odorants, as well as the response of various hORs to various odorants. These results indicate that the ion channel-coupled hOR system can be effectively used not only for simple and fast high-throughput odorant screening, but also to visualize the odorant-hOR response pattern.

A Real-Time Screening Assay for GIRK1/4 Channel Blockers

The cardiac acetylcholine-activated K+ channel (IK,Ach) represents a novel target for drug therapy in the treatment of atrial fibrillation (AF). This channel is a member of the G-protein-coupled inward rectifier K+ (GIRK) channel superfamily and is composed of the GIRK1/4 (Kir3.1 and Kir3.4) subunits. The goal of this study was to develop a cell-based screening assay for identifying new blockers of the GIRK1/4 channel. The mouse atrial HL-1 cell line, expressing the GIRK1/4 channel, was plated in 96-well plate format, loaded with the fluorescent membrane potential-sensitive dye bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)) and measured using a fluorescent imaging plate reader (FLIPR). Application of the muscarinic agonist carbachol to the cells caused a rapid, time-dependent decrease in the fluorescent signal, indicative of K+ efflux through the GIRK1/4 channel (carbachol vs. control solution, Z′ factor = 0.5-0.6). The GIRK1/4 channel fluorescent signal was blocked by BaCl2 and enhanced by increasing the driving force for K+ across the cell membrane. To test the utility of the assay for screening GIRK1/4 channel blockers, cells were treated with a small compound library of Na+ and K+ channel modulators. Analogues of amiloride and propafenone were identified as channel blockers at concentrations less than 1 µM. Thus, the GIRK1/4 channel assay may be used in the development of new and selective agents for treating AF.

Ion channel screening plates: design, construction, and maintenance

Ion channels have provided a diverse set of therapeutic targets across all areas of the pharmaceutical industry. Many companies are pursuing this unique class of targets for areas of unmet medical need such as neuropathic and inflammatory pains. In the past, focused library screening sets had been designed for CNS and kinase targets. Our investigations were aimed at creating a similar dynamic screening set enriched for compounds targeting ion channels to aid screening efforts of this important class of targets. The key advantages of this approach for ion channel targets would be: (1) to identify tool compounds for novel targets and assist in assay validation, (2) to serve as a focused screen for non-384-well adaptable targets, and (3) to jump start a particular program, that is, catch-up to competition for validated, well-known targets.

Screening technologies for ion channel drug discovery

For every movement, heartbeat and thought, ion channels need to open and close. It is therefore not surprising that their malfunctioning leads to serious diseases. Currently, only approximately 10% of drugs, with a market value in excess of US$10 billion, act on ion channels. The systematic exploitation of this target class has started, enabled by novel assay technologies and fundamental advances of the structural and mechanistic understanding of channel function. The latter, which was rewarded with the Nobel Prize in 2003, has opened up an avenue for rational drug design. In this review we provide an overview of the current repertoire of screening technologies that has evolved to drive ion channel-targeted drug discovery towards new medicines of the future.