Hysteretic Behavior in Voltage-Gated Channels

Evaluation of Small-Molecule Candidates as Modulators of M-Type K+ Currents: Impacts on Current Amplitude, Gating, and Voltage-Dependent Hysteresis

The core subunits of the KV7.2, KV7.3, and KV7.5 channels, encoded by the KCNQ2, KCNQ3, and KCNQ5 genes, are expressed across various cell types and play a key role in generating the M-type K+ current (IK(M)). This current is characterized by an activation threshold at low voltages and displays slow activation and deactivation kinetics. Variations in the amplitude and gating kinetics of IK(M) can significantly influence membrane excitability. Notably, IK(M) demonstrates distinct voltage-dependent hysteresis when subjected to prolonged isosceles-triangular ramp pulses. In this review, we explore various small-molecule modulators that can either inhibit or enhance the amplitude of IK(M), along with their perturbations on its gating kinetics and voltage-dependent hysteresis. The inhibitors of IK(M) highlighted here include bisoprolol, brivaracetam, cannabidiol, nalbuphine, phenobarbital, and remdesivir. Conversely, compounds such as flupirtine, kynurenic acid, naringenin, QO-58, and solifenacin have been shown to enhance IK(M). These modulators show potential as pharmacological or therapeutic strategies for treating certain disorders linked to gain-of-function or loss-of-function mutations in M-type K+ (KV7x or KCNQx) channels.

Effective suppression of Ih and INa caused by capsazepine, known to be a blocker of TRPV1 receptor

A synthetic inhibitor of capsaicin-induced TRPV1 channel activation is called capsazepine (CPZ). In this study, we aimed to explore the effects of CPZ on hyperpolarization-activated cationic current (Ih) and voltage-gated Na + current (INa) in pituitary tumor (GH3) cells. Through patch-clamp recordings, we found that CPZ concentration-dependently inhibited Ih amplitude and slowed its activation time course. The IC50 and KD values were 3.1 and 3.16 μM, respectively. CPZ also shifted the steady-state activation curve of Ih towards a more hyperpolarized potential. However, there was no change in the gating charge of the curve. A modified Markovian model predicted the CPZ-induced decrease in the voltage-dependent hysteresis of Ih. CPZ suppressed INa in GH3 cells, without altering its activation or inactivation time course. Additionally, exposure to CPZ reduced spontaneous firing. These findings suggest that CPZ's inhibitory effects on Ih and INa are direct and not dependent on vanilloid receptor binding. This could provide light on an unidentified ionic mechanism influencing the membrane excitability of neurons and endocrine or neuroendocrine cells in vivo.

Different fluorescent labels report distinct components of spHCN channel voltage sensor movement

We used voltage clamp fluorometry to probe the movement of the S4 helix in the voltage-sensing domain of the sea urchin HCN channel (spHCN) expressed in Xenopus oocytes. We obtained markedly different fluorescence responses with either ALEXA-488 or MTS-TAMRA covalently linked to N-terminal Cys332 of the S4 helix. With hyperpolarizing steps, ALEXA-488 fluorescence increased rapidly, consistent with it reporting the initial inward movement of S4, as previously described. In contrast, MTS-TAMRA fluorescence increased more slowly and its early phase correlated with that of channel opening. Additionally, a slow fluorescence component that tracked the development of the mode shift, or channel hysteresis, could be resolved with both labels. We quantitated this component as an increased deactivation tail current delay with concomitantly longer activation periods and found it to depend strongly on the presence of K+ ions in the pore. Using collisional quenching experiments and structural predictions, we established that ALEXA-488 was more exposed to solvent than MTS-TAMRA. We propose that components of S4 movement during channel activation can be kinetically resolved using different fluorescent probes to reveal distinct biophysical properties. Our findings underscore the need to apply caution when interpreting voltage clamp fluorometry data and demonstrate the potential utility of different labels to interrogate distinct biophysical properties of voltage-gated membrane proteins.

Investigating the influence of XAV-939, a tankyrase inhibitor, on the density and gating of erg-mediated K+ currents in mouse MA-10 Leydig tumor cells

XAV-939(XAV) is a chemical compound that inhibits the activity of tankyrase. However, the precise way in which XAV alters membrane ionic currents is not well understood. In this study,our goal was to examine the impact of XAV on the ionic currents in mouse MA-10 Leydig cells, specifically focusing on the magnitude, gating properties,and voltage-dependent hysteresis of erg-mediated K+currents(IK(erg)). In our whole-cell current recordings we observed that the addition of XAV inhibited the density of IK(erg) in a concentration-dependent manner with an IC50 of 3.1 μM. Furthermore we found that continued exposure to XAV, further addition of neither liraglutide nor insulin-like growth factor-1 counteracted XAV-mediated inhibition of IK(erg). Additionally the presence of XAV suppressed the mean current versus voltage relationship of IK(erg) across the entire voltage-clamp step analyzed. This compound shifted the steady-state activation curve of IK(erg) to a less negative potential by approximately 12 mV. The presence of XAV increased the time constant of deactivating IK(erg) in MA-10 cells. The voltage-dependent clockwise hysteresis of IK(erg) responding to prolonged upright isosceles-triangular ramp voltage became diminished by adding XAV; moreover subsequent addition of NS3623 effectively reversed XAV-induced decrease of hysteretic area of IK(erg). XAV also inhibited the proliferation of this cell line and the IC50 value of XAV-induced inhibition of cell proliferation was 2.8M. Overall the suppression of IK(erg) by XAV may serve as a significant ionic mechanism that contribute to the functional properties of MA-10 cells. However, it is important to note that this effect cannot be attributed solely to the inhibition of tankyrase.

Effect of a sensing charge mutation on the deactivation of KV7.2 channels

Potassium-selective, voltage-gated channels of the KV7 family are critical regulators of electrical excitability in many cell types. Removing the outermost putative sensing charge (R198) of the human KV7.2 shifts its activation voltage dependence toward more negative potentials. This suggests that removing a charge "at the top" of the fourth (S4) segment of the voltage-sensing domain facilitates activation. Here, we hypothesized that restoring that charge would bring back the activation to its normal voltage range. We introduced the mutation R198H in KV7.2 with the idea that titrating the introduced histidine with protons would reinstate the sensing charge. As predicted, the mutant's activation voltage dependence changed as a function of the external pH (pHEXT) while modest changes in the activation voltage dependence were observed with the wild-type (WT) channel. On the other hand, the deactivation kinetics of the R198H mutant was remarkably sensitive to pHEXT changes, readily deactivating at pHEXT 6, while becoming slower to deactivate at pHEXT 8. In contrast, the KV7.2 WT displayed modest changes in the deactivation kinetics as a function of pHEXT. This suggested that the charge of residue 198 was critical for deactivation. However, in a surprising turn, the mutant R198Q-a non-titratable mutation-also displayed a high pHEXT sensitivity activity. We thus concluded that rather than the charge at position 198, the protonation status of the channel's extracellular face modulates the open channel stabilization and that the charge of residue 198 is required for the voltage sensor to effectively deactivate the channel, overcoming the stabilizing effect of high pHEXT.

Modulation of Voltage-Gating and Hysteresis of Lysenin Channels by Cu2+ Ions

The intricate voltage regulation presented by lysenin channels reconstituted in artificial lipid membranes leads to a strong hysteresis in conductance, bistability, and memory. Prior investigations on lysenin channels indicate that the hysteresis is modulated by multivalent cations which are also capable of eliciting single-step conformational changes and transitions to stable closed or sub-conducting states. However, the influence on voltage regulation of Cu2+ ions, capable of completely closing the lysenin channels in a two-step process, was not sufficiently addressed. In this respect, we employed electrophysiology approaches to investigate the response of lysenin channels to variable voltage stimuli in the presence of small concentrations of Cu2+ ions. Our experimental results showed that the hysteretic behavior, recorded in response to variable voltage ramps, is accentuated in the presence of Cu2+ ions. Using simultaneous AC/DC stimulation, we were able to determine that Cu2+ prevents the reopening of channels previously closed by depolarizing potentials and the channels remain in the closed state even in the absence of a transmembrane voltage. In addition, we showed that Cu2+ addition reinstates the voltage gating and hysteretic behavior of lysenin channels reconstituted in neutral lipid membranes in which lysenin channels lose their voltage-regulating properties. In the presence of Cu2+ ions, lysenin not only regained the voltage gating but also behaved like a long-term molecular memory controlled by electrical potentials.

Effects of simulated hypo-gravity on lower limb kinematic and electromyographic variables during anti-gravitational treadmill walking

Introduction: This study investigated kinematic and EMG changes in gait across simulated gravitational unloading levels between 100% and 20% of normal body weight. This study sought to identify if each level of unloading elicited consistent changes-particular to that percentage of normal body weight-or if the changes seen with unloading could be influenced by the previous level(s) of unloading. Methods: 15 healthy adult participants (26.3 ± 2.5 years; 53% female) walked in an Alter-G anti-gravity treadmill unloading system (mean speed: 1.49 ± 0.37 mph) for 1 min each at 100%, 80%, 60%, 40% and 20% of normal body weight, before loading back to 100% in reverse order. Lower-body kinematic data were captured by inertial measurement units, and EMG data were collected from the rectus femoris, biceps femoris, medial gastrocnemius, and anterior tibialis. Data were compared across like levels of load using repeated measures ANOVA and statistical parametric mapping. Difference waveforms for adjacent levels were created to examine the rate of change between different unloading levels. Results: This study found hip, knee, and ankle kinematics as well as activity in the rectus femoris, and medial gastrocnemius were significantly different at the same level of unloading, having arrived from a higher, or lower level of unloading. There were no significant changes in the kinematic difference waveforms, however the waveform representing the change in EMG between 100% and 80% load was significantly different from all other levels. Discussion: This study found that body weight unloading from 100% to 20% elicited distinct responses in the medial gastrocnemius, as well as partly in the rectus femoris. Hip, knee, and ankle kinematics were also affected differentially by loading and unloading, especially at 40% of normal body weight. These findings suggest the previous level of gravitational load is an important factor to consider in determining kinematic and EMG responses to the current level during loading and unloading below standard g. Similarly, the rate of change in kinematics from 100% to 20% appears to be linear, while the rate of change in EMG was non-linear. This is of particular interest, as it suggests that kinematic and EMG measures decouple with unloading and may react to unloading uniquely.

About hysteresis in Shaker: A note on Cowgill and Chanda

This letter proposes an alternative explanation to the work published by Cowgill and Chanda on the nature of hysteresis in the voltage-gated, potassium-selective channel Shaker.

Charge-voltage curves of Shaker potassium channel are not hysteretic at steady state

Charge-voltage curves of many voltage-gated ion channels exhibit hysteresis but such curves are also a direct measure of free energy of channel gating and, hence, should be path-independent. Here, we identify conditions to measure steady-state charge-voltage curves and show that these are curves are not hysteretic. Charged residues in transmembrane segments of voltage-gated ion channels (VGICs) sense and respond to changes in the electric field. The movement of these gating charges underpins voltage-dependent activation and is also a direct metric of the net free-energy of channel activation. However, for most voltage-gated ion channels, the charge-voltage (Q-V) curves appear to be dependent on initial conditions. For instance, Q-V curves of Shaker potassium channel obtained by hyperpolarizing from 0 mV is left-shifted compared to those obtained by depolarizing from a holding potential of -80 mV. This hysteresis in Q-V curves is a common feature of channels in the VGIC superfamily and raises profound questions about channel energetics because the net free-energy of channel gating is a state function and should be path independent. Due to technical limitations, conventional gating current protocols are limited to test pulse durations of <500 ms, which raises the possibility that the dependence of Q-V on initial conditions reflects a lack of equilibration. Others have suggested that the hysteresis is fundamental thermodynamic property of voltage-gated ion channels and reflects energy dissipation due to measurements under non-equilibrium conditions inherent to rapid voltage jumps (Villalba-Galea. 2017. Channels. https://doi.org/10.1080/19336950.2016.1243190). Using an improved gating current and voltage-clamp fluorometry protocols, we show that the gating hysteresis arising from different initial conditions in Shaker potassium channel is eliminated with ultra-long (18-25 s) test pulses. Our study identifies a modified gating current recording protocol to obtain steady-state Q-V curves of a voltage-gated ion channel. Above all, these findings demonstrate that the gating hysteresis in Shaker channel is a kinetic phenomenon rather than a true thermodynamic property of the channel and the charge-voltage curve is a true measure of the net-free energy of channel gating.

Rufinamide, a Triazole-Derived Antiepileptic Drug, Stimulates Ca2+-Activated K+ Currents While Inhibiting Voltage-Gated Na+ Currents

Rufinamide (RFM) is a clinically utilized antiepileptic drug that, as a triazole derivative, has a unique structure. The extent to which this drug affects membrane ionic currents remains incompletely understood. With the aid of patch clamp technology, we investigated the effects of RFM on the amplitude, gating, and hysteresis of ionic currents from pituitary GH₃ lactotrophs. RFM increased the amplitude of Ca²⁺-activated K⁺ currents (I_K(Ca)) in pituitary GH₃ lactotrophs, and the increase was attenuated by the further addition of iberiotoxin or paxilline. The addition of RFM to the cytosolic surface of the detached patch of membrane resulted in the enhanced activity of large-conductance Ca²⁺-activated K⁺ channels (BK_Ca channels), and paxilline reversed this activity. RFM increased the strength of the hysteresis exhibited by the BK_Ca channels and induced by an inverted isosceles-triangular ramp pulse. The peak and late voltage-gated Na⁺ current (I_Na) evoked by rapid step depolarizations were differentially suppressed by RFM. The molecular docking approach suggested that RFM bound to the intracellular domain of K_Ca1.1 channels with amino acid residues, thereby functionally affecting BK_Ca channels' activity. This study is the first to present evidence that, in addition to inhibiting the I_Na, RFM effectively modifies the I_K(Ca), which suggests that it has an impact on neuronal function and excitability.

Effective Perturbations by Small-Molecule Modulators on Voltage-Dependent Hysteresis of Transmembrane Ionic Currents

The non-linear voltage-dependent hysteresis (Hys_(V)) of voltage-gated ionic currents can be robustly activated by the isosceles-triangular ramp voltage (V_ramp) through digital-to-analog conversion. Perturbations on this Hys_(V) behavior play a role in regulating membrane excitability in different excitable cells. A variety of small molecules may influence the strength of Hys_(V) in different types of ionic currents elicited by long-lasting triangular V_ramp. Pirfenidone, an anti-fibrotic drug, decreased the magnitude of I_h's Hys_(V) activated by triangular V_ramp, while dexmedetomidine, an agonist of α₂-adrenoceptors, effectively suppressed I_h as well as diminished the Hys_(V) strength of I_h. Oxaliplatin, a platinum-based anti-neoplastic drug, was noted to enhance the I_h's Hys_(V) strength, which is thought to be linked to the occurrence of neuropathic pain, while honokiol, a hydroxylated biphenyl compound, decreased I_h's Hys_(V). Cell exposure to lutein, a xanthophyll carotenoid, resulted in a reduction of I_h's Hys_(V) magnitude. Moreover, with cell exposure to UCL-2077, SM-102, isoplumbagin, or plumbagin, the Hys_(V) strength of erg-mediated K⁺ current activated by triangular V_ramp was effectively diminished, whereas the presence of either remdesivir or QO-58 respectively decreased or increased Hys_(V) magnitude of M-type K⁺ current. Zingerone, a methoxyphenol, was found to attenuate Hys_(V) (with low- and high-threshold loops) of L-type Ca²⁺ current induced by long-lasting triangular V_ramp. The Hys_(V) properties of persistent Na⁺ current (I_Na(P)) evoked by triangular V_ramp were characterized by a figure-of-eight (i.e., ∞) configuration with two distinct loops (i.e., low- and high-threshold loops). The presence of either tefluthrin, a pyrethroid insecticide, or t-butyl hydroperoxide, an oxidant, enhanced the Hys_(V) strength of I_Na(P). However, further addition of dapagliflozin can reverse their augmenting effects in the Hys_(V) magnitude of the current. Furthermore, the addition of esaxerenone, mirogabalin, or dapagliflozin was effective in inhibiting the strength of I_Na(P). Taken together, the observed perturbations by these small-molecule modulators on Hys_(V) strength in different types of ionic currents evoked during triangular V_ramp are expected to influence the functional activities (e.g., electrical behaviors) of different excitable cells in vitro or in vivo.

The Modulation of Ubiquinone, a Lipid Antioxidant, on Neuronal Voltage-Gated Sodium Current

Ubiquinone, composed of a 1,4-benzoquinone and naturally produced in the body, actively participates in the mitochondrial redox reaction and functions as an endogenous lipid antioxidant, protecting against peroxidation in the pituitary-dependent hormonal system. However, the questions of if and how ubiquinone directly affects neuronal ionic currents remain largely unsettled. We investigated its effects on ionic currents in pituitary neurons (GH3 and MMQ cells) with the aid of patch-clamp technology. Ubiquinone decreased the peak amplitude of the voltage-gated Na⁺ current (I_Na) with a slowing of the inactivation rate. Neither menadione nor superoxide dismutase modified the ubiquinone-induced I_Na inhibition. In response to an isosceles-triangular ramp pulse, the persistent I_Na (I_Na(P)) at high- and low- threshold potentials occurred concurrently with a figure-eight hysteresis loop. With ubiquinone, the I_Na(P) increased with no change in the intersection voltage, and the magnitude of the voltage-dependent hysteresis of the current was enhanced. Ubiquinone was ineffective in modifying the gating of hyperpolarization-activated cation currents. In MMQ lactotrophs, ubiquinone effectively decreased the amplitude of the I_Na and the current inactivation rate. In sum, the effects of ubiquinone demonstrated herein occur upstream of its effects on mitochondrial redox processes, involved in its modulation of sodium channels and neuronal excitability.

Characterization in Inhibitory Effectiveness of Carbamazepine in Voltage-Gated Na+ and Erg-Mediated K+ Currents in a Mouse Neural Crest-Derived (Neuro-2a) Cell Line

Carbamazepine (CBZ, Tegretol^®) is an anticonvulsant used in the treatment of epilepsy and neuropathic pain; however, several unwanted effects of this drug have been noticed. Therefore, the regulatory actions of CBZ on ionic currents in electrically excitable cells need to be reappraised, although its efficacy in suppressing voltage-gated Na⁺ current (I_Na) has been disclosed. This study was undertaken to explore the modifications produced by CBZ on ionic currents (e.g., I_Na and erg-mediated K⁺ current [I_K(erg)]) measured from Neuro-2a (N2a) cells. In these cells, we found that this drug differentially suppressed the peak (transient, I_Na(T)) and sustained (late, I_Na(L)) components of I_Na in a concentration-dependent manner with effective IC₅₀ of 56 and 18 μM, respectively. The overall current–voltage relationship of I_Na(T) with or without the addition of CBZ remained unchanged; however, the strength (i.e., ∆area) in the window component of I_Na (I_Na(W)) evoked by the short ascending ramp pulse (V_ramp) was overly lessened in the CBZ presence. Tefluthrin (Tef), a synthetic pyrethroid, known to stimulate I_Na, augmented the strength of the voltage-dependent hysteresis (Hys_(V)) of persistent I_Na (I_Na(P)) in response to the isosceles-triangular V_ramp; moreover, further application of CBZ attenuated Tef-mediated accentuation of I_Na(P)’s Hys_(V). With a two-step voltage protocol, the recovery of I_Na(T) inactivation seen in Neuro-2a cells became progressively slowed by adding CBZ; however, the cumulative inhibition of I_Na(T) evoked by pulse train stimulation was enhanced during exposure to this drug. Neuro-2a-cell exposure to CBZ (100 μM), the magnitude of erg-mediated K⁺ current measured throughout the entire voltage-clamp steps applied was mildly inhibited. The docking results regarding the interaction of CBZ and voltage-gate Na⁺ (Na_V) channel predicted the ability of CBZ to bind to some amino-acid residues in Na_V due to the existence of a hydrogen bond or hydrophobic contact. It is conceivable from the current investigations that the I_Na (I_Na(T), I_Na(L), I_Na(W), and I_Na(P)) residing in Neuro-2a cells are susceptible to being suppressed by CBZ, and that its block on I_Na(L) is larger than that on I_Na(T). Collectively, the magnitude and gating of Na_V channels produced by the CBZ presence might have an impact on its anticonvulsant and analgesic effects occurring in vivo.

Characterization of Inhibitory Capability on Hyperpolarization-Activated Cation Current Caused by Lutein (β,ε-Carotene-3,3'-Diol), a Dietary Xanthophyll Carotenoid

Lutein (β,ε-carotene-3,3'-diol), a xanthophyll carotenoid, is found in high concentrations in the macula of the human retina. It has been recognized to exert potential effectiveness in antioxidative and anti-inflammatory properties. However, whether and how its modifications on varying types of plasmalemmal ionic currents occur in electrically excitable cells remain incompletely answered. The current hypothesis is that lutein produces any direct adjustments on ionic currents (e.g., hyperpolarization-activated cation current, Ih [or funny current, If]). In the present study, GH3-cell exposure to lutein resulted in a time-, state- and concentration-dependent reduction in Ih amplitude with an IC50 value of 4.1 μM. There was a hyperpolarizing shift along the voltage axis in the steady-state activation curve of Ih in the presence of this compound, despite being void of changes in the gating charge of the curve. Under continued exposure to lutein (3 μM), further addition of oxaliplatin (10 μM) or ivabradine (3 μM) could be effective at either reversing or further decreasing lutein-induced suppression of hyperpolarization-evoked Ih, respectively. The voltage-dependent anti-clockwise hysteresis of Ih responding to long-lasting inverted isosceles-triangular ramp concentration-dependently became diminished by adding this compound. However, the addition of 10 μM lutein caused a mild but significant suppression in the amplitude of erg-mediated or A-type K+ currents. Under current-clamp potential recordings, the sag potential evoked by long-lasting hyperpolarizing current stimulus was reduced under cell exposure to lutein. Altogether, findings from the current observations enabled us to reflect that during cell exposure to lutein used at pharmacologically achievable concentrations, lutein-perturbed inhibition of Ih would be an ionic mechanism underlying its changes in membrane excitability.

The 70-year search for the voltage sensing mechanism of ion channels

This retrospective on the voltage‐sensing mechanisms and gating models of ion channels begins in 1952 with the charged gating particles postulated by Hodgkin and Huxley, viewed as charges moving across the membrane and controlling its permeability to Na⁺ and K⁺ ions. Hodgkin and Huxley postulated that their movement should generate small and fast capacitive currents, which were recorded 20 years later as gating currents. In the early 1980s, several voltage‐dependent channels were cloned and found to share a common architecture: four homologous domains or subunits, each displaying six transmembrane α‐helical segments, with the fourth segment (S4) displaying four to seven positive charges invariably separated by two non‐charged residues. This immediately suggested that this segment was serving as the voltage sensor of the channel (the molecular counterpart of the charged gating particle postulated by Hodgkin and Huxley) and led to the development of the sliding helix model. Twenty years later, the X‐ray crystallographic structures of many voltage‐dependent channels allowed investigation of their gating by molecular dynamics. Further understanding of how channels gate will benefit greatly from the acquisition of high‐resolution structures of each of their relevant functional or structural states. This will allow the application of molecular dynamics and other approaches. It will also be key to investigate the energetics of channel gating, permitting an understanding of the physical and molecular determinants of gating. The use of multiscale hierarchical approaches might finally prove to be a rewarding strategy to overcome the limits of the various single approaches to the study of channel gating.

Endogenous pannexin1 channels form functional intercellular cell-cell channels with characteristic voltage-dependent properties

Pannexin1 is a glycoprotein that has been shown to form functional plasma membrane channels and mediate many cellular signaling pathways. However, the formation and function of pannexin1-based intercellular cell–cell channels in mammalian cells and vertebrate tissue is a question of substantial debate. This work provides robust electrophysiological evidence to demonstrate that endogenously expressed human pannexin1 forms cell–cell channels and lays the groundwork for studying a potential new type of electrical synapses between many mammalian cell types that endogenously express pannexin1.

The occurrence of intercellular channels formed by pannexin1 has been challenged for more than a decade. Here, we provide an electrophysiological characterization of exogenous human pannexin1 (hPanx1) cell–cell channels expressed in HeLa cells knocked out for connexin45. The observed hPanx1 cell–cell channels show two phenotypes: O-state and S-state. The former displayed low transjunctional voltage (V_j) sensitivity and single-channel conductance of ∼175 pS, with a substate of ∼35 pS; the latter showed a peculiar dynamic asymmetry in V_j dependence and single-channel conductance identical to the substate conductance of the O-state. S-state hPanx1 cell–cell channels were also identified between TC620 cells, a human oligodendroglioma cell line that endogenously expresses hPanx1. In these cells, dye and electrical coupling increased with temperature and were strongly reduced after hPanx1 expression was knocked down by small interfering RNA or inhibited with Panx1 mimetic inhibitory peptide. Moreover, cell–cell coupling was augmented when hPanx1 levels were increased with a doxycycline-inducible expression system. Application of octanol, a connexin gap junction (GJ) channel inhibitor, was not sufficient to block electrical coupling between HeLa KO Cx45-hPanx1 or TC620 cell pairs. In silico studies suggest that several arginine residues inside the channel pore may be neutralized by hydrophobic interactions, allowing the passage of DAPI, consistent with dye coupling observed between TC620 cells. These findings demonstrate that endogenously expressed hPanx1 forms intercellular cell–cell channels and their unique properties resemble those described in innexin-based GJ channels. Since Panx1 is ubiquitously expressed, finding conditions to recognize Panx1 cell–cell channels in different cell types might require special attention.

The Evidence for Effective Inhibition of INa Produced by Mirogabalin ((1R,5S,6S)-6-(aminomethyl)-3-ethyl-bicyclo [3.2.0] hept-3-ene-6-acetic acid), a Known Blocker of CaV Channels

Mirogabalin (MGB, Tarlige^®), an inhibitor of the α₂δ-1 subunit of voltage-gated Ca²⁺ (Ca_V) channels, is used as a way to alleviate peripheral neuropathic pain and diabetic neuropathy. However, to what extent MGB modifies the magnitude, gating, and/or hysteresis of various types of plasmalemmal ionic currents remains largely unexplored. In pituitary tumor (GH₃) cells, we found that MGB was effective at suppressing the peak (transient, I_Na(T)) and sustained (late, I_Na(L)) components of the voltage-gated Na⁺ current (I_Na) in a concentration-dependent manner, with an effective IC₅₀ of 19.5 and 7.3 μM, respectively, while the K_D value calculated on the basis of minimum reaction scheme was 8.2 μM. The recovery of I_Na(T) inactivation slowed in the presence of MGB, although the overall current-voltage relation of I_Na(T) was unaltered; however, there was a leftward shift in the inactivation curve of the current. The magnitude of the window (I_Na(W)) or resurgent I_Na (I_Na(R)) evoked by the respective ascending or descending ramp pulse (V_ramp) was reduced during cell exposure to MGB. MGB-induced attenuation in I_Na(W) or I_Na(R) was reversed by the further addition of tefluthrin, a pyrethroid insecticide known to stimulate I_Na. MGB also effectively lessened the strength of voltage-dependent hysteresis of persistent I_Na in response to the isosceles triangular V_ramp. The cumulative inhibition of I_Na(T), evoked by pulse train stimulation, was enhanced in its presence. Taken together, in addition to the inhibition of Ca_V channels, the Na_V channel attenuation produced by MGB might have an impact in its analgesic effects occurring in vivo.

Evidence for Inhibitory Perturbations on the Amplitude, Gating, and Hysteresis of A-Type Potassium Current, Produced by Lacosamide, a Functionalized Amino Acid with Anticonvulsant Properties

Lacosamide (Vimpat^®, LCS) is widely known as a functionalized amino acid with promising anti-convulsant properties; however, adverse events during its use have gradually appeared. Despite its inhibitory effect on voltage-gated Na⁺ current (I_Na), the modifications on varying types of ionic currents caused by this drug remain largely unexplored. In pituitary tumor (GH₃) cells, we found that the presence of LCS concentration-dependently decreased the amplitude of A-type K⁺ current (I_K(A)) elicited in response to membrane depolarization. The I_K(A) amplitude in these cells was sensitive to attenuation by the application of 4-aminopyridine, 4-aminopyridine-3-methanol, or capsaicin but not by that of tetraethylammonium chloride. The effective IC₅₀ value required for its reduction in peak or sustained I_K(A) was calculated to be 102 or 42 µM, respectively, while the value of the dissociation constant (K_D) estimated from the slow component in I_K(A) inactivation at varying LCS concentrations was 52 µM. By use of two-step voltage protocol, the presence of this drug resulted in a rightward shift in the steady-state inactivation curve of I_K(A) as well as in a slowing in the recovery time course of the current block; however, no change in the gating charge of the inactivation curve was detected in its presence. Moreover, the LCS addition led to an attenuation in the degree of voltage-dependent hysteresis for I_K(A) elicitation by long-duration triangular ramp voltage commands. Likewise, the I_K(A) identified in mouse mHippoE-14 neurons was also sensitive to block by LCS, coincident with an elevation in the current inactivation rate. Collectively, apart from its canonical action on I_Na inhibition, LCS was effective at altering the amplitude, gating, and hysteresis of I_K(A) in excitable cells. The modulatory actions on I_K(A), caused by LCS, could interfere with the functional activities of electrically excitable cells (e.g., pituitary tumor cells or hippocampal neurons).

Human‑made electromagnetic fields: Ion forced‑oscillation and voltage‑gated ion channel dysfunction, oxidative stress and DNA damage (Review)

Exposure of animals/biological samples to human‑made electromagnetic fields (EMFs), especially in the extremely low frequency (ELF) band, and the microwave/radio frequency (RF) band which is always combined with ELF, may lead to DNA damage. DNA damage is connected with cell death, infertility and other pathologies, including cancer. ELF exposure from high‑voltage power lines and complex RF exposure from wireless communication antennas/devices are linked to increased cancer risk. Almost all human‑made RF EMFs include ELF components in the form of modulation, pulsing and random variability. Thus, in addition to polarization and coherence, the existence of ELFs is a common feature of almost all human‑made EMFs. The present study reviews the DNA damage and related effects induced by human‑made EMFs. The ion forced‑oscillation mechanism for irregular gating of voltage‑gated ion channels on cell membranes by polarized/coherent EMFs is extensively described. Dysfunction of ion channels disrupts intracellular ionic concentrations, which determine the cell's electrochemical balance and homeostasis. The present study shows how this can result in DNA damage through reactive oxygen species/free radical overproduction. Thus, a complete picture is provided of how human‑made EMF exposure may indeed lead to DNA damage and related pathologies, including cancer. Moreover, it is suggested that the non‑thermal biological effects attributed to RF EMFs are actually due to their ELF components.

Effective Accentuation of Voltage-Gated Sodium Current Caused by Apocynin (4'-Hydroxy-3'-methoxyacetophenone), a Known NADPH-Oxidase Inhibitor

Apocynin (aPO, 4'-Hydroxy-3'-methoxyacetophenone) is a cell-permeable, anti-inflammatory phenolic compound that acts as an inhibitor of NADPH-dependent oxidase (NOX). However, the mechanisms through which aPO can interact directly with plasmalemmal ionic channels to perturb the amplitude or gating of ionic currents in excitable cells remain incompletely understood. Herein, we aimed to investigate any modifications of aPO on ionic currents in pituitary GH₃ cells or murine HL-1 cardiomyocytes. In whole-cell current recordings, GH₃-cell exposure to aPO effectively stimulated the peak and late components of voltage-gated Na+ current (I_Na) with different potencies. The EC₅₀ value of aPO required for its differential increase in peak or late I_Na in GH₃ cells was estimated to be 13.2 or 2.8 μM, respectively, whereas the K_D value required for its retardation in the slow component of current inactivation was 3.4 μM. The current-voltage relation of I_Na was shifted slightly to more negative potential during cell exposure to aPO (10 μM); however, the steady-state inactivation curve of the current was shifted in a rightward direction in its presence. Recovery of peak I_Na inactivation was increased in the presence of 10 μM aPO. In continued presence of aPO, further application of rufinamide or ranolazine attenuated aPO-stimulated I_Na. In methylglyoxal- or superoxide dismutase-treated cells, the stimulatory effect of aPO on peak I_Na remained effective. By using upright isosceles-triangular ramp pulse of varying duration, the amplitude of persistent I_Na measured at low or high threshold was enhanced by the aPO presence, along with increased hysteretic strength appearing at low or high threshold. The addition of aPO (10 μM) mildly inhibited the amplitude of erg-mediated K+ current. Likewise, in HL-1 murine cardiomyocytes, the aPO presence increased the peak amplitude of I_Na as well as decreased the inactivation or deactivation rate of the current, and further addition of ranolazine or esaxerenone attenuated aPO-accentuated I_Na. Altogether, this study provides a distinctive yet unidentified finding that, despite its effectiveness in suppressing NOX activity, aPO may directly and concertedly perturb the amplitude, gating and voltage-dependent hysteresis of I_Na in electrically excitable cells. The interaction of aPO with ionic currents may, at least in part, contribute to the underlying mechanisms through which it affects neuroendocrine, endocrine or cardiac function.

Effective Perturbations of the Amplitude, Gating, and Hysteresis of IK(DR) Caused by PT-2385, an HIF-2α Inhibitor

PT-2385 is currently regarded as a potent and selective inhibitor of hypoxia-inducible factor-2α (HIF-2α), with potential antineoplastic activity. However, the membrane ion channels changed by this compound are obscure, although it is reasonable to assume that the compound might act on surface membrane before entering the cell´s interior. In this study, we intended to explore whether it and related compounds make any adjustments to the plasmalemmal ionic currents of pituitary tumor (GH₃) cells and human 13-06-MG glioma cells. Cell exposure to PT-2385 suppressed the peak or late amplitude of delayed-rectifier K⁺ current (I_K(DR)) in a time- and concentration-dependent manner, with IC₅₀ values of 8.1 or 2.2 µM, respectively, while the K_D value in PT-2385-induced shortening in the slow component of I_K(DR) inactivation was estimated to be 2.9 µM. The PT-2385-mediated block of I_K(DR) in GH₃ cells was little-affected by the further application of diazoxide, cilostazol, or sorafenib. Increasing PT-2385 concentrations shifted the steady-state inactivation curve of I_K(DR) towards a more hyperpolarized potential, with no change in the gating charge of the current, and also prolonged the time-dependent recovery of the I_K(DR) block. The hysteretic strength of I_K(DR) elicited by upright or inverted isosceles-triangular ramp voltage was decreased during exposure to PT-2385; meanwhile, the activation energy involved in the gating of I_K(DR) elicitation was noticeably raised in its presence. Alternatively, the presence of PT-2385 in human 13-06-MG glioma cells effectively decreased the amplitude of I_K(DR). Considering all of the experimental results together, the effects of PT-2385 on ionic currents demonstrated herein could be non-canonical and tend to be upstream of the inhibition of HIF-2α. This action therefore probably contributes to down-streaming mechanisms through the changes that it or other structurally resemblant compounds lead to in the perturbations of the functional activities of pituitary cells or neoplastic astrocytes, in the case that in vivo observations occur.

Synchronization of gene expression across eukaryotic communities through chemical rhythms

The synchronization is a recurring phenomenon in neuroscience, ecology, human sciences, and biology. However, controlling synchronization in complex eukaryotic consortia on extended spatial-temporal scales remains a major challenge. Here, to address this issue we construct a minimal synthetic system that directly converts chemical signals into a coherent gene expression synchronized among eukaryotic communities through rate-dependent hysteresis. Guided by chemical rhythms, isolated colonies of yeast Saccharomyces cerevisiae oscillate in near-perfect synchrony despite the absence of intercellular coupling or intrinsic oscillations. Increased speed of chemical rhythms and incorporation of feedback in the system architecture can tune synchronization and precision of the cell responses in a growing cell collectives. This synchronization mechanism remain robust under stress in the two-strain consortia composed of toxin-sensitive and toxin-producing strains. The sensitive cells can maintain the spatial-temporal synchronization for extended periods under the rhythmic toxin dosages produced by killer cells. Our study provides a simple molecular framework for generating global coordination of eukaryotic gene expression through dynamic environment.

Characterization of Direct Perturbations on Voltage-Gated Sodium Current by Esaxerenone, a Nonsteroidal Mineralocorticoid Receptor Blocker

Esaxerenone (ESAX; CS-3150, Minnebro^®) is known to be a newly non-steroidal mineralocorticoid receptor (MR) antagonist. However, its modulatory actions on different types of ionic currents in electrically excitable cells remain largely unanswered. The present investigations were undertaken to explore the possible perturbations of ESAX on the transient, late and persistent components of voltage-gated Na⁺ current (I_Na) identified from pituitary GH₃ or MMQ cells. GH₃-cell exposure to ESAX depressed the transient and late components of I_Na with varying potencies. The IC₅₀ value of ESAX required for its differential reduction in peak or late I_Na in GH₃ cells was estimated to be 13.2 or 3.2 μM, respectively. The steady-state activation curve of peak I_Na remained unchanged during exposure to ESAX; however, recovery of peak I_Na block was prolonged in the presence 3 μM ESAX. In continued presence of aldosterone (10 μM), further addition of 3 μM ESAX remained effective at inhibiting I_Na. ESAX (3 μM) potently reversed Tef-induced augmentation of I_Na. By using isosceles-triangular ramp pulse with varying durations, the amplitude of persistent I_Na measured at high or low threshold was enhanced by the presence of tefluthrin (Tef), in combination with the appearance of the figure-of-eight hysteretic loop; moreover, hysteretic strength of the current was attenuated by subsequent addition of ESAX. Likewise, in MMQ lactotrophs, the addition of ESAX also effectively decreased the peak amplitude of I_Na along with the increased current inactivation rate. Taken together, the present results provide a noticeable yet unidentified finding disclosing that, apart from its antagonistic effect on MR receptor, ESAX may directly and concertedly modify the amplitude, gating properties and hysteresis of I_Na in electrically excitable cells.

Effective Activation of BKCa Channels by QO-40 (5-(Chloromethyl)-3-(Naphthalen-1-yl)-2-(Trifluoromethyl)Pyrazolo [1,5-a]pyrimidin-7(4H)-one), Known to Be an Opener of KCNQ2/Q3 Channels

QO-40 (5-(chloromethyl)-3-(naphthalene-1-yl)-2-(trifluoromethyl) pyrazolo[1,5-a]pyrimidin-7(4H)-one) is a novel and selective activator of KCNQ2/KCNQ3 K⁺ channels. However, it remains largely unknown whether this compound can modify any other type of plasmalemmal ionic channel. The effects of QO-40 on ion channels in pituitary GH₃ lactotrophs were investigated in this study. QO-40 stimulated Ca²⁺-activated K⁺ current (I_K(Ca)) with an EC₅₀ value of 2.3 μM in these cells. QO-40-stimulated I_K(Ca) was attenuated by the further addition of GAL-021 or paxilline but not by linopirdine or TRAM-34. In inside-out mode, this compound added to the intracellular leaflet of the detached patches stimulated large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels with no change in single-channel conductance; however, there was a decrease in the slow component of the mean closed time of BK_Ca channels. The K_D value required for the QO-40-mediated decrease in the slow component at the mean closure time was 1.96 μM. This compound shifted the steady-state activation curve of BK_Ca channels to a less positive voltage and decreased the gating charge of the channel. The application of QO-40 also increased the hysteretic strength of BK_Ca channels elicited by a long-lasting isosceles-triangular ramp voltage. In HEK293T cells expressing α-hSlo, QO-40 stimulated BK_Ca channel activity. Overall, these findings demonstrate that QO-40 can interact directly with the BK_Ca channel to increase the amplitude of I_K(Ca) in GH₃ cells.

Effective Activation by Kynurenic Acid and Its Aminoalkylated Derivatives on M-Type K+ Current

Kynurenic acid (KYNA, 4-oxoquinoline-2-carboxylic acid), an intermediate of the tryptophan metabolism, has been recognized to exert different neuroactive actions; however, the need of how it or its aminoalkylated amide derivative N-(2-(dimethylamino)ethyl)-3-(morpholinomethyl)-4-oxo-1,4-dihydroquinoline-2-carboxamide (KYNA-A4) exerts any effects on ion currents in excitable cells remains largely unmet. In this study, the investigations of how KYNA and other structurally similar KYNA derivatives have any adjustments on different ionic currents in pituitary GH₃ cells and hippocampal mHippoE-14 neurons were performed by patch-clamp technique. KYNA or KYNA-A4 increased the amplitude of M-type K⁺ current (I_K(M)) and concomitantly enhanced the activation time course of the current. The EC₅₀ value required for KYNA- or KYNA-A4 -stimulated I_K(M) was yielded to be 18.1 or 6.4 μM, respectively. The presence of KYNA or KYNA-A4 shifted the relationship of normalized I_K(M)-conductance versus membrane potential to more depolarized potential with no change in the gating charge of the current. The voltage-dependent hysteretic area of I_K(M) elicited by long-lasting triangular ramp pulse was observed in GH₃ cells and that was increased during exposure to KYNA or KYNA-A4. In cell-attached current recordings, addition of KYNA raised the open probability of M-type K⁺ channels, along with increased mean open time of the channel. Cell exposure to KYNA or KYNA-A4 mildly inhibited delayed-rectifying K⁺ current; however, neither erg-mediated K⁺ current, hyperpolarization-activated cation current, nor voltage-gated Na⁺ current in GH₃ cells was changed by KYNA or KYNA-A4. Under whole-cell, current-clamp recordings, exposure to KYNA or KYNA-A4 diminished the frequency of spontaneous action potentials; moreover, their reduction in firing frequency was attenuated by linopirdine, yet not by iberiotoxin or apamin. In hippocampal mHippoE-14 neurons, the addition of KYNA also increased the I_K(M) amplitude effectively. Taken together, the actions presented herein would be one of the noticeable mechanisms through which they modulate functional activities of excitable cells occurring in vivo.