External Ba2+ block of the two-pore domain potassium channel TREK-1 defines conformational transition in its selectivity filter

A novel K+ -dependent Na+ uptake mechanism during low pH exposure in adult zebrafish (Danio rerio): New tricks for old dogma

AIM

To determine whether Na⁺ uptake in adult zebrafish (Danio rerio) exposed to acidic water adheres to traditional models reliant on Na⁺ /H⁺ Exchangers (NHEs), Na⁺ channels and Na⁺ /Cl^- Cotransporters (NCCs) or if it occurs through a novel mechanism.

METHODS

Zebrafish were exposed to control (pH 8.0) or acidic (pH 4.0) water for 0-12 hours during which ²² Na⁺ uptake ( J Na in ), ammonia excretion, net acidic equivalent flux and net K⁺ flux ( J H net ) were measured. The involvement of NHEs, Na⁺ channels, NCCs, K⁺ -channels and K⁺ -dependent Na⁺ /Ca²⁺ exchangers (NCKXs) was evaluated by exposure to Cl^- -free or elevated [K⁺ ] water, or to pharmacological inhibitors. The presence of NCKXs in gill was examined using RT-PCR.

RESULTS

J Na in was strongly attenuated by acid exposure, but gradually recovered to control rates. The systematic elimination of each of the traditional models led us to consider K⁺ as a counter substrate for Na⁺ uptake during acid exposure. Indeed, elevated environmental [K⁺ ] inhibited J Na in during acid exposure in a concentration-dependent manner, with near-complete inhibition at 10 mM. Moreover, J H net loss increased approximately fourfold at 8-10 hours of acid exposure which correlated with recovered J Na in in 1:1 fashion, and both J Na in and J H net were sensitive to tetraethylammonium (TEA) during acid exposure. Zebrafish gills expressed mRNA coding for six NCKX isoforms.

CONCLUSIONS

During acid exposure, zebrafish engage a novel Na⁺ uptake mechanism that utilizes the outwardly directed K⁺ gradient as a counter-substrate for Na⁺ and is sensitive to TEA. NKCXs are promising candidates to mediate this K⁺ -dependent Na⁺ uptake, opening new research avenues about Na⁺ uptake in zebrafish and other acid-tolerant aquatic species.

Multifaceted Regulation of Potassium-Ion Channels by Graphene Quantum Dots

Graphene quantum dots (GQDs) are emerging as a versatile nanomaterial with numerous proposed biomedical applications. Despite the explosion in potential applications, the molecular interactions between GQDs and complex biomolecular systems, including potassium-ion (K+) channels, remain largely unknown. Here, we use molecular dynamics (MD) simulations and electrophysiology to study the interactions between GQDs and three representative K+ channels, which participate in a variety of physiological processes and are closely related to many disease states. Using MD simulations, we observed that GQDs adopt distinct contact poses with each of the three structurally distinct K+ channels. Our electrophysiological characterization of the effects of GQDs on channel currents revealed that GQDs interact with the extracellular voltage-sensing domain (VSD) of a Kv1.2 channel, augmenting current by left-shifting the voltage dependence of channel activation. In contrast, GQDs form a "lid" cluster over the extracellular mouth of inward rectifier Kir3.2, blocking the channel pore and decreasing the current in a concentration-dependent manner. Meanwhile, GQDs accumulate on the extracellular "cap domain" of K2P2 channels and have no apparent impact on the K+ flux through the channel. These results reveal a surprising multifaceted regulation of K+ channels by GQDs, which might help de novo design of nanomaterial-based channel probe openers/inhibitors that can be used to further discern channel function.

Next-generation inward rectifier potassium channel modulators: discovery and molecular pharmacology

Inward rectifying potassium (Kir) channels play important roles in both excitable and nonexcitable cells of various organ systems and could represent valuable new drug targets for cardiovascular, metabolic, immune, and neurological diseases. In nonexcitable epithelial cells of the kidney tubule, for example, Kir1.1 (KCNJ1) and Kir4.1 (KCNJ10) are linked to sodium reabsorption in the thick ascending limb of Henle's loop and distal convoluted tubule, respectively, and have been explored as novel-mechanism diuretic targets for managing hypertension and edema. G protein-coupled Kir channels (Kir3) channels expressed in the central nervous system are critical effectors of numerous signal transduction pathways underlying analgesia, addiction, and respiratory-depressive effects of opioids. The historical dearth of pharmacological tool compounds for exploring the therapeutic potential of Kir channels has led to a molecular target-based approach using high-throughput screen (HTS) of small-molecule libraries and medicinal chemistry to develop "next-generation" Kir channel modulators that are both potent and specific for their targets. In this article, we review recent efforts focused specifically on discovery and improvement of target-selective molecular probes. The reader is introduced to fluorescence-based thallium flux assays that have enabled much of this work and then provided with an overview of progress made toward developing modulators of Kir1.1 (VU590, VU591), Kir2.x (ML133), Kir3.X (ML297, GAT1508, GiGA1, VU059331), Kir4.1 (VU0134992), and Kir7.1 (ML418). We discuss what is known about the small molecules' molecular mechanisms of action, in vitro and in vivo pharmacology, and then close with our view of what critical work remains to be done.

FOXP1 negatively regulates intrinsic excitability in D2 striatal projection neurons by promoting inwardly rectifying and leak potassium currents

Heterozygous loss-of-function mutations in the transcription factor FOXP1 are strongly associated with autism. Dopamine receptor 2 expressing (D2) striatal projection neurons (SPNs) in heterozygous Foxp1 (Foxp1^+/-) mice have higher intrinsic excitability. To understand the mechanisms underlying this alteration, we examined SPNs with cell-type specific homozygous Foxp1 deletion to study cell-autonomous regulation by Foxp1. As in Foxp1^+/- mice, D2 SPNs had increased intrinsic excitability with homozygous Foxp1 deletion. This effect involved postnatal mechanisms. The hyperexcitability was mainly due to down-regulation of two classes of potassium currents: inwardly rectifying (K_IR) and leak (K_Leak). Single-cell RNA sequencing data from D2 SPNs with Foxp1 deletion indicated the down-regulation of transcripts of candidate ion channels that may underlie these currents: Kcnj2 and Kcnj4 for K_IR and Kcnk2 for K_Leak. This Foxp1-dependent regulation was neuron-type specific since these same currents and transcripts were either unchanged, or very little changed, in D1 SPNs with cell-specific Foxp1 deletion. Our data are consistent with a model where FOXP1 negatively regulates the excitability of D2 SPNs through K_IR and K_Leak by transcriptionally activating their corresponding transcripts. This, in turn, provides a novel example of how a transcription factor may regulate multiple genes to impact neuronal electrophysiological function that depends on the integration of multiple current types - and do this in a cell-specific fashion. Our findings provide initial clues to altered neuronal function and possible therapeutic strategies not only for FOXP1-associated autism but also for other autism forms associated with transcription factor dysfunction.

TOK channels use the two gates in classical K+ channels to achieve outward rectification

TOKs are outwardly rectifying K⁺ channels in fungi with two pore-loops and eight transmembrane spans. Here, we describe the TOKs from four pathogens that cause the majority of life-threatening fungal infections in humans. These TOKs pass large currents only in the outward direction like the canonical isolate from Saccharomyces cerevisiae (ScTOK), and distinct from other K⁺ channels. ScTOK, AfTOK1 (Aspergillus fumigatus), and H99TOK (Cryptococcus neoformans grubii) are K⁺ -selective and pass current above the K⁺ reversal potential. CaTOK (Candida albicans) and CnTOK (Cryptococcus neoformans neoformans) pass both K⁺ and Na⁺ and conduct above a reversal potential reflecting the mixed permeability of their selectivity filter. Mutations in CaTOK and ScTOK at sites homologous to those that open the internal gates in classical K⁺ channels are shown to produce inward TOK currents. A favored model for outward rectification is proposed whereby the reversal potential determines ion occupancy, and thus, conductivity, of the selectivity filter gate that is coupled to an imperfectly restrictive internal gate, permitting the filter to sample ion concentrations on both sides of the membrane.

Ion currents, action potentials, and noradrenergic responses in rat pulmonary vein and left atrial cardiomyocytes

The electrophysiological properties of pulmonary vein (PV)‐cardiomyocytes, and their responses to the sympathetic neurotransmitter, noradrenaline (NA), are thought to differ from those of the left atrium (LA) and contribute to atrial ectopy. The aim of this study was to examine rat PV cardiomyocyte electrophysiology and responses to NA in comparison with LA cells. LA and PV cardiomyocytes were isolated from adult male Wistar rat hearts, and membrane potentials and ion currents recorded at 36°C using whole‐cell patch‐clamp techniques. PV and LA cardiomyocytes did not differ in size. In control, there were no differences between the two cell‐types in zero‐current potential or action potential duration (APD) at 1 Hz, although the incidence of early afterdepolarizations (EADs) was greater in PV than LA cardiomyocytes. The L‐type Ca²⁺ current (I_CaL) was ~×1.5 smaller (p = .0029, Student's t test) and the steady‐state K⁺ current (I_Kss) was ~×1.4 larger (p = .0028, Student's t test) in PV than in LA cardiomyocytes. PV cardiomyocyte inward‐rectifier current (I_K1) was slightly smaller than LA cardiomyocyte I_K1. In LA cardiomyocytes, NA significantly prolonged APD₃₀. In PV cells, APD₃₀ responses to 1 μM NA were heterogeneous: while the mean percentage change in APD₃₀ was not different from 0 (16.5 ± 9.7%, n cells/N animals = 12/10, p = .1177, one‐sample t test), three cells showed shortening (‐18.8 ± 6.0%) whereas nine showed prolongation (28.3 ± 10.1%, p = .008, Student's t test). NA had no effect on I_K1 in either cell‐type but inhibited PV I_Kss by 41.9 ± 4.1% (n/N = 23/11 p < .0001), similar to LA cells. NA increased I_CaL in most PV cardiomyocytes (median × 2.2‐increase, p < .0001, n/N = 32/14, Wilcoxon‐signed‐rank test), although in 7/32 PV cells I_CaL was decreased following NA. PV cardiomyocytes differ from LA cells and respond heterogeneously to NA.

Spadin Selectively Antagonizes Arachidonic Acid Activation of TREK-1 Channels

TREK-1 channel activity is a critical regulator of neuronal, cardiac, and smooth muscle physiology and pathology. The antidepressant peptide, spadin, has been proposed to be a TREK-1-specific blocker. Here we sought to examine the mechanism of action underlying spadin inhibition of TREK-1 channels. Heterologous expression in Xenopus laevis oocytes and electrophysiological analysis using two-electrode voltage clamp in standard bath solutions was used to characterize the pharmacological profile of wild-type and mutant murine TREK-1 and TREK-2 channels using previously established human K_2P activators; arachidonic acid (AA), cis-4,7,10,13,16,19-docosahexaenoic acid (DHA), BL-1249, and cinnamyl-3,4-dihydroxy-α-cyanocinnamate (CDC) and inhibitors; spadin and barium (Ba²⁺). Mouse TREK-1 and TREK-2 channel currents were both significantly increased by AA, BL-1249, and CDC, similar to their human homologs. Under basal conditions, both TREK-1 and TREK-2 currents were insensitive to application of spadin, but could be blocked by Ba²⁺. Spadin did not significantly inhibit either TREK-1 or TREK-2 currents either chemically activated by AA, BL-1249, or CDC, or structurally activated via a gating mutation. However, pre-exposure to spadin significantly perturbed the subsequent activation of TREK-1 currents by AA, but not TREK-2. Furthermore, spadin was unable to prevent activation of TREK-1 by BL-1249, CDC, or the related bioactive lipid, DHA. Spadin specifically antagonizes the activation of TREK-1 channels by AA, likely via an allosteric mechanism. Lack of intrinsic activity may explain the absence of clinical side effects during antidepressant therapy.

Development of a Novel Cell-Based Assay System for High-Throughput Screening of Compounds Acting on Background Two-Pore Domain K+ Channels

Two-pore domain K⁺ (K_2P) channels are thought to be druggable targets. However, only a few agents specific for K_2P channels have been identified, presumably due to the lack of an efficient screening system. To develop a new high-throughput screening (HTS) system targeting these channels, we have established a HEK293-based "test cell" expressing a mutated Na⁺ channel (Nav1.5) with markedly slowed inactivation, as well as a K⁺ channel (Kir2.1) that sets the membrane potential quite negative, close to K⁺ equilibrium potential. We found in this system that Kir2.1 block by 100 μM Ba²⁺ application consistently elicited a large depolarization like a long-lasting action potential. This maneuver resulted in cell death, presumably due to the sustained Na⁺ influx. When either the TWIK-related acid-sensitive K⁺ (TASK)-1 or TASK-3 channel was expressed in the test cells, Ba²⁺-induced cell death was markedly weakened. Stronger activation of TASK-1 by extracellular acidification further decreased the cell death. In contrast, the presence of K_2P channel blockers enhanced cell death. IC₅₀ values for TASK-1 and/or TASK-3 blockers acquired by measurements of relative cell viability were comparable to those obtained using patch-clamp recordings. Both blockers and openers of K_2P channels can be accurately assessed with high efficiency and throughput by this novel HTS system.

Involvement of the TREK-1 channel in human alveolar cell membrane potential and its regulation by inhibitors of the chloride current

K⁺ channels of the alveolar epithelium control the driving force acting on the ionic and solvent flow through the cell membrane contributing to the maintenance of cell volume and the constitution of epithelial lining fluid. In the present work, we analyze the effect of the Cl^- channel inhibitors: (4-[(2-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-inden-5-yl)oxy] butanoic acid (DCPIB) and 9-anthracenecarboxylic acid (9-AC) on the total current in a type II pneumocytes (A549 cell line) model by patch clamp, immunocytochemical, and gene knockdown techniques. We noted that DCPIB and 9-AC promote the activation of K conductance. In fact, they significantly increase the intensity of the current and shift its reversal potential to values more negative than the control. By silencing outward rectifier channel in its anoctamin 6 portion, we excluded a direct involvement of Cl^- ions in modulation of I_K and, by means of functional tests with its specific inhibitor spadin, we identified the TREK-1 channel as the presumable target of both drugs. As the activity of TREK-1 has a key role for the correct functioning of the alveolar epithelium, the identification of DCPIB and 9-AC molecules as its activators suggests their possible use to build new pharmacological tools for the modulation of this channel.

Mutation of a single residue promotes gating of vertebrate and invertebrate two-pore domain potassium channels

Mutations that modulate the activity of ion channels are essential tools to understand the biophysical determinants that control their gating. Here, we reveal the conserved role played by a single amino acid position (TM2.6) located in the second transmembrane domain of two-pore domain potassium (K2P) channels. Mutations of TM2.6 to aspartate or asparagine increase channel activity for all vertebrate K2P channels. Using two-electrode voltage-clamp and single-channel recording techniques, we find that mutation of TM2.6 promotes channel gating via the selectivity filter gate and increases single channel open probability. Furthermore, channel gating can be progressively tuned by using different amino acid substitutions. Finally, we show that the role of TM2.6 was conserved during evolution by rationally designing gain-of-function mutations in four Caenorhabditis elegans K2P channels using CRISPR/Cas9 gene editing. This study thus describes a simple and powerful strategy to systematically manipulate the activity of an entire family of potassium channels.

Mutations that modulate the activity of ion channels are essential tools to understand the biophysical determinants that control their gating. Here authors reveal the role played by a single residue in the second transmembrane domain of vertebrate and invertebrate two-pore domain potassium channels.

Modulation of Hyperpolarization-Activated Inward Current and Thalamic Activity Modes by Different Cyclic Nucleotides

The hyperpolarization-activated inward current, I_h, plays a key role in the generation of rhythmic activities in thalamocortical (TC) relay neurons. Cyclic nucleotides, like 3′,5′-cyclic adenosine monophosphate (cAMP), facilitate voltage-dependent activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels by shifting the activation curve of I_h to more positive values and thereby terminating the rhythmic burst activity. The role of 3′,5′-cyclic guanosine monophosphate (cGMP) in modulation of I_h is not well understood. To determine the possible role of the nitric oxide (NO)-sensitive cGMP-forming guanylyl cyclase 2 (NO-GC2) in controlling the thalamic I_h, the voltage-dependency and cGMP/cAMP-sensitivity of I_h was analyzed in TC neurons of the dorsal part of the lateral geniculate nucleus (dLGN) in wild type (WT) and NO-GC2-deficit (NO-GC2^−/−) mice. Whole cell voltage clamp recordings in brain slices revealed a more hyperpolarized half maximal activation (V_1/2) of I_h in NO-GC2^−/− TC neurons compared to WT. Different concentrations of 8-Br-cAMP/8-Br-cGMP induced dose-dependent positive shifts of V_1/2 in both strains. Treatment of WT slices with lyase enzyme (adenylyl and guanylyl cyclases) inhibitors (SQ22536 and ODQ) resulted in further hyperpolarized V_1/2. Under current clamp conditions NO-GC2^−/− neurons exhibited a reduction in the I_h-dependent voltage sag and reduced action potential firing with hyperpolarizing and depolarizing current steps, respectively. Intrathalamic rhythmic bursting activity in brain slices and in a simplified mathematical model of the thalamic network was reduced in the absence of NO-GC2. In freely behaving NO-GC2^−/− mice, delta and theta band activity was enhanced during active wakefulness (AW) as well as rapid eye movement (REM) sleep in cortical local field potential (LFP) in comparison to WT. These findings indicate that cGMP facilitates I_h activation and contributes to a tonic activity in TC neurons. On the network level basal cGMP production supports fast rhythmic activity in the cortex.

TRESK background potassium channel is not gated at the helix bundle crossing near the cytoplasmic end of the pore

Two-pore domain K⁺ channels (K_2P) are responsible for background K⁺ currents and regulate the resting membrane potential and cellular excitability. Their activity is controlled by a large variety of physicochemical factors and intracellular signaling pathways. The majority of these effects converge on the intracellular C-terminus of the channels, resulting in the modification of the gating at the selectivity filter. Another gating mechanism, the activation gate at the helix bundle crossing is also well documented in other K⁺ channel families, however, it remains uncertain whether this type of gating is functional in K_2P channels. The regulation of TWIK-related spinal cord K⁺ channel (TRESK) is different from the other K_2P channels. Regulatory factors acting via the C-terminus are not known, instead channel activity is modified by the phosphorylation/dephosphorylation of the unusually long intracellular loop between the 2^nd and 3^rd transmembrane segments. These unique structural elements of the regulation lead us to examine channel gating at the bundle crossing region. Ba²⁺ was applied to the intracellular side of excised membrane patches and the characteristics of the channel block were determined. We compared the kinetics of the development of Ba²⁺ block when the channels were phosphorylated (inhibited) or dephosphorylated (activated) and also in different mutants mimicking the two functional states. Neither the phosphorylation/dephosphorylation nor the point mutations influenced the development of Ba²⁺ block, suggesting that the conformational changes of the bundle crossing region do not contribute to the phosphorylation-dependent gating of TRESK.

TREK-1 Channel Expression in Smooth Muscle as a Target for Regulating Murine Intestinal Contractility: Therapeutic Implications for Motility Disorders

Gastrointestinal (GI) motility disorders such as irritable bowel syndrome (IBS) can occur when coordinated smooth muscle contractility is disrupted. Potassium (K⁺) channels regulate GI smooth muscle tone and are key to GI tract relaxation, but their molecular and functional phenotypes are poorly described. Here we define the expression and functional roles of mechano-gated K_2P channels in mouse ileum and colon. Expression and distribution of the K_2P channel family were investigated using quantitative RT-PCR (qPCR), immunohistochemistry and confocal microscopy. The contribution of mechano-gated K_2P channels to mouse intestinal muscle tension was studied pharmacologically using organ bath. Multiple K_2P gene transcripts were detected in mouse ileum and colon whole tissue preparations. Immunohistochemistry confirmed TREK-1 expression was smooth muscle specific in both ileum and colon, whereas TREK-2 and TRAAK channels were detected in enteric neurons but not smooth muscle. In organ bath, mechano-gated K_2P channel activators (Riluzole, BL-1249, flufenamic acid, and cinnamyl 1-3,4-dihydroxy-alpha-cyanocinnamate) induced relaxation of KCl and CCh pre-contracted ileum and colon tissues and reduced the amplitude of spontaneous contractions. These data reveal the specific expression of mechano-gated K_2P channels in mouse ileum and colon tissues and highlight TREK-1, a smooth muscle specific K_2P channel in GI tract, as a potential therapeutic target for combating motility pathologies arising from hyper-contractility.

Potassium channel dysfunction underlies Purkinje neuron spiking abnormalities in spinocerebellar ataxia type 2

Alterations in Purkinje neuron firing often accompany ataxia, but the molecular basis for these changes is poorly understood. In a mouse model of spinocerebellar ataxia type 2 (SCA2), a progressive reduction in Purkinje neuron firing frequency accompanies cell atrophy. We investigated the basis for altered Purkinje neuron firing in SCA2. A reduction in the expression of large-conductance calcium-activated potassium (BK) channels and Kv3.3 voltage-gated potassium channels accompanies the inability of Purkinje neurons early in disease to maintain repetitive spiking. In association with prominent Purkinje neuron atrophy, repetitive spiking is restored, although at a greatly reduced firing frequency. In spite of a continued impairment in spike repolarization and a persistently reduced BK channel mediated afterhyperpolarization (AHP), repetitive spiking is maintained, through the increased activity of barium-sensitive potassium channels, most consistent with inwardly rectifying potassium (Kir) channels. Increased activity of Kir channels results in the generation of a novel AHP not seen in wild-type Purkinje neurons that also accounts for the reduced firing frequency late in disease. Homeostatic changes in Purkinje neuron morphology that help to preserve repetitive spiking can also therefore have deleterious consequences for spike frequency. These results suggest that the basis for spiking abnormalities in SCA2 differ depending on disease stage, and interventions targeted towards correcting potassium channel dysfunction in ataxia need to be tailored to the specific stage in the degenerative process.

Down-regulation of Inwardly Rectifying K+ Currents in Astrocytes Derived from Patients with Monge's Disease

Chronic mountain sickness (CMS) or Monge's disease is a disease in highlanders. These patients have a variety of neurologic symptoms such as migraine, mental fatigue, confusion, dizziness, loss of appetite, memory loss and neuronal degeneration. The cellular and molecular mechanisms underlying CMS neuropathology is not understood. In the previous study, we demonstrated that neurons derived from CMS patients' fibroblasts have a decreased expression and altered gating properties of voltage-gated sodium channel. In this study, we further characterize the electrophysiological properties of iPSC-derived astrocytes from CMS patients. We found that the current densities of the inwardly rectifying potassium (Kir) channels in CMS astrocytes (-5.7 ± 2.2 pA/pF at -140 mV) were significantly decreased as compared to non-CMS (-28.4 ± 3.4 pA/pF at -140 mV) and sea level subjects (-28.3 ± 5.3 pA/pF at -140 mV). We further demonstrated that the reduced Kir current densities in CMS astrocytes were caused by their decreased protein expression of Kir4.1 and Kir2.3 channels, while single channel properties (i.e., P_o, conductance) of Kir channel in CMS astrocytes were not altered. In addition, we found no significant differences of outward potassium currents between CMS and non-CMS astrocytes. As compared to non-CMS and sea level subjects, the K⁺ uptake ability in CMS astrocytes was significantly decreased. Taken together, our results suggest that down-regulation of Kir channels and the resulting decreased K⁺ uptake ability in astrocytes could be one of the major molecular mechanisms underlying the neurologic manifestations in CMS patients.

Exploring the Nanotoxicology of MoS2: A Study on the Interaction of MoS2 Nanoflakes and K+ Channels

Molybdenum disulfide (MoS₂) nanomaterial has recently found various applications in the biomedical field mainly due to its outstanding physicochemical properties. However, little is known about its interactions with biological systems at the atomic level, which intimately relates to the biocompatibility of the material. To provide insights into the effects of MoS₂ in biological entities, we investigated the interactions of MoS₂ with proteins from a functionally important membrane family, the ubiquitous potassium (K⁺) channels. Here, we study four representative K⁺ channels-KcsA, Kir3.2, the Kv1.2 paddle chimera, and K2P2-to investigate their interactions with a triangular MoS₂ nanoflake using Molecular Dynamics (MD) simulations combined with electrophysiology experiments. These particular K⁺ channels were selected based on the diversity in their structure; that is, although these K⁺ channels display similar structural motifs, they also contain significant differences related to their particular function. Our results indicate that the MoS₂ nanoflake is able to stably bind to three out of the four channels, albeit through distinct binding modes. The binding mode between each channel and MoS₂ underlies the specific deleterious influence on the channel's basic physiological function: For KcsA, MoS₂ binds on the extracellular loops, which indirectly destroys the delicate structure of the selectivity filter causing a strong leak of K⁺ ions. In the binding mode with Kir3.2, the MoS₂ nanoflake completely covers the entrance to the channel pore affecting the normal ion conduction. For the Kv1.2 chimera, the MoS₂ nanoflake prefers to bind into a crevice located at the extracellular side of the Voltage Sensor Domain (VSD). Interestingly, the crevice involves the N-terminal segment of S4, a crucial transmembrane helix which directly controls the gating process of the Kv1.2 chimera channel by electromechanical coupling the VSD to the transmembrane electric field. MoS₂ in contact with S4 from the Kv1.2 chimera, potentially influences the channel's gating process from open to closed states. In all three systems, the van der Waals contribution to the total energy dominates the binding interactions; also, hydrophobic residues contribute the most contact points, which agrees with the strong hydrophobic character of the MoS₂ nanomaterial. Electrophysiology recordings using two-electrode voltage-clamp show that currents of Kir3.2 and Kv1.2 are both blocked by the MoS₂ nanoflakes in a concentration-dependent way. While the background K⁺ channel, K2P2 (TREK-1), identified as a negative control, is not blocked by the MoS₂ nanoflakes. The large and rigid extracellular domain of K2P2 appears to protect the channel from disturbance by the nanoflakes. Intrinsic chemical properties of MoS₂, together with the specific features of the channels, such as the electrostatic character and complex surface architecture, determine the critical details of the binding events. These findings might shed light on the potential nanotoxicology of MoS₂ nanomaterials and help us to understand the underlying molecular mechanism.

T2N as a new tool for robust electrophysiological modeling demonstrated for mature and adult-born dentate granule cells

Compartmental models are the theoretical tool of choice for understanding single neuron computations. However, many models are incomplete, built ad hoc and require tuning for each novel condition rendering them of limited usability. Here, we present T2N, a powerful interface to control NEURON with Matlab and TREES toolbox, which supports generating models stable over a broad range of reconstructed and synthetic morphologies. We illustrate this for a novel, highly detailed active model of dentate granule cells (GCs) replicating a wide palette of experiments from various labs. By implementing known differences in ion channel composition and morphology, our model reproduces data from mouse or rat, mature or adult-born GCs as well as pharmacological interventions and epileptic conditions. This work sets a new benchmark for detailed compartmental modeling. T2N is suitable for creating robust models useful for large-scale networks that could lead to novel predictions. We discuss possible T2N application in degeneracy studies.

Contribution of two-pore K+ channels to cardiac ventricular action potential revealed using human iPSC-derived cardiomyocytes

Two-pore K⁺ (K_2p) channels have been described in modulating background conductance as leak channels in different physiological systems. In the heart, the expression of K_2p channels is heterogeneous with equivocation regarding their functional role. Our objective was to determine the K_2p expression profile and their physiological and pathophysiological contribution to cardiac electrophysiology. Induced pluripotent stem cells (iPSCs) generated from humans were differentiated into cardiomyocytes (iPSC-CMs). mRNA was isolated from these cells, commercial iPSC-CM (iCells), control human heart ventricular tissue (cHVT), and ischemic (iHF) and nonischemic heart failure tissues (niHF). We detected 10 K_2p channels in the heart. Comparing quantitative PCR expression of K_2p channels between human heart tissue and iPSC-CMs revealed K_2p1.1, K_2p2.1, K_2p5.1, and K_2p17.1 to be higher expressed in cHVT, whereas K_2p3.1 and K_2p13.1 were higher in iPSC-CMs. Notably, K_2p17.1 was significantly lower in niHF tissues compared with cHVT. Action potential recordings in iCells after K_2p small interfering RNA knockdown revealed prolongations in action potential depolarization at 90% repolarization for K_2p2.1, K_2p3.1, K_2p6.1, and K_2p17.1. Here, we report the expression level of 10 human K_2p channels in iPSC-CMs and how they compared with cHVT. Importantly, our functional electrophysiological data in human iPSC-CMs revealed a prominent role in cardiac ventricular repolarization for four of these channels. Finally, we also identified K_2p17.1 as significantly reduced in niHF tissues and K_2p4.1 as reduced in niHF compared with iHF. Thus, we advance the notion that K_2p channels are emerging as novel players in cardiac ventricular electrophysiology that could also be remodeled in cardiac pathology and therefore contribute to arrhythmias.NEW & NOTEWORTHY Two-pore K⁺ (K_2p) channels are traditionally regarded as merely background leak channels in myriad physiological systems. Here, we describe the expression profile of K_2p channels in human-induced pluripotent stem cell-derived cardiomyocytes and outline a salient role in cardiac repolarization and pathology for multiple K_2p channels.

Potentiation of Schaffer-Collateral CA1 Synaptic Transmission by eEF2K and p38 MAPK Mediated Mechanisms

The elongation factor 2 kinase (eEF2K), likewise known as CaMKIII, has been demonstrated to be involved in antidepressant responses of NMDA receptor antagonists. Even so, it remains open whether direct inhibition of eEF2K without altering up-stream or other signaling pathways affects hippocampal synaptic transmission and neuronal network synchrony. Inhibition of eEF2K by the selective and potent eEF2K inhibitor A-484954 induced a fast pre-synaptically mediated enhancement of synaptic transmission and synchronization of neural network activity. The eEF2K-inhibition mediated potentiation of synaptic transmission of hippocampal CA1 neurons is most notably independent of protein synthesis and does not rely on protein kinase C, protein kinase A or mitogen-activated protein kinase (MAPK)/extracellular signal-regulated protein kinase 1/2. Moreover, the strengthening of synaptic transmission in the response to the inhibition of eEF2K was strongly attenuated by the inhibition of p38 MAPK. In addition, we show the involvement of barium-sensitive and more specific the TWIK-related potassium-1 (TREK-1) channels in the eEF2K-inhibition mediated potentiation of synaptic transmission. These findings reveal a novel pathway of eEF2K mediated regulation of hippocampal synaptic transmission. Further research is required to study whether such compounds could be beneficial for the development of mood disorder treatments with a fast-acting antidepressant response.

Unambiguous observation of blocked states reveals altered, blocker-induced, cardiac ryanodine receptor gating

The flow of ions through membrane channels is precisely regulated by gates. The architecture and function of these elements have been studied extensively, shedding light on the mechanisms underlying gating. Recent investigations have focused on ion occupancy of the channel’s selectivity filter and its ability to alter gating, with most studies involving prokaryotic K⁺ channels. Some studies used large quaternary ammonium blocker molecules to examine the effects of altered ionic flux on gating. However, the absence of blocking events that are visibly distinct from closing events in K⁺ channels makes unambiguous interpretation of data from single channel recordings difficult. In this study, the large K⁺ conductance of the RyR2 channel permits direct observation of blocking events as distinct subconductance states and for the first time demonstrates the differential effects of blocker molecules on channel gating. This experimental platform provides valuable insights into mechanisms of blocker-induced modulation of ion channel gating.

Myometrial relaxation of mice via expression of two pore domain acid sensitive K(+) (TASK-2) channels

Myometrial relaxation of mouse via expression of two-pore domain acid sensitive (TASK) channels was studied. In our previous report, we suggested that two-pore domain acid-sensing K⁺ channels (TASK-2) might be one of the candidates for the regulation of uterine circular smooth muscles in mice. In this study, we tried to show the mechanisms of relaxation via TASK-2 channels in marine myometrium. Isometric contraction measurements and patch clamp technique were used to verify TASK conductance in murine myometrium. Western blot and immunehistochemical study under confocal microscopy were used to investigate molecular identity of TASK channel. In this study, we showed that TEA and 4-AP insensitive non-inactivating outward K⁺ current (NIOK) may be responsible for the quiescence of murine pregnant longitudinal myometrium. The characteristics of NIOK coincided with two-pore domain acid-sensing K⁺ channels (TASK-2). NIOK in the presence of K⁺ channel blockers was inhibited further by TASK inhibitors such as quinidine, bupivacaine, lidocaine, and extracellular acidosis. Furthermore, oxytocin and estrogen inhibited NIOK in pregnant myometrium. When compared to non-pregnant myometrium, pregnant myometrium showed stronger inhibition of NIOK by quinidine and increased immunohistochemical expression of TASK-2. Finally, TASK-2 inhibitors induced strong myometrial contraction even in the presence of L-methionine, a known inhibitor of stretch-activated channels in the longitudinal myometrium of mouse. Activation of TASK-2 channels seems to play an essential role for relaxing uterus during pregnancy and it might be one of the alternatives for preventing preterm delivery.

Apoptosis of Corneal Epithelial Cells Caused by Ultraviolet B-induced Loss of K(+) is Inhibited by Ba(2.)

UVB exposure at ambient outdoor levels triggers rapid K(+) loss and apoptosis in human corneal limbal epithelial (HCLE) cells cultured in medium containing 5.5 mM K(+), but considerably less apoptosis occurs when the medium contains the high K(+) concentration that is present in tears (25 mM). Since Ba(2+) blocks several K(+) channels, we tested whether Ba(2+)-sensitive K(+) channels are responsible for some or all of the UVB-activated K(+) loss and subsequent activation of the caspase cascade and apoptosis. Corneal epithelial cells in culture were exposed to UVB at 80 or 150 mJ/cm(2). Patch-clamp recording was used to measure UVB-induced K(+) currents. Caspase-activity and TUNEL assays were performed on HCLE cells exposed to UVB followed by incubation in the presence or absence of Ba(2+). K(+) currents were activated in HCLE cells following UVB-exposure. These currents were reversibly blocked by 5 mM Ba(2+). When HCLE cells were incubated with 5 mM Ba(2+) after exposure to UVB, activation of caspases-9, -8, and -3 and DNA fragmentation were significantly decreased. The data confirm that UVB-induced K(+) current activation and loss of intracellular K(+) leads to activation of the caspase cascade and apoptosis. Extracellular Ba(2+) inhibits UVB-induced apoptosis by preventing loss of intracellular K(+) when K(+) channels are activated. Ba(2+) therefore has effects similar to elevated extracellular K(+) in protecting HCLE cells from UVB-induced apoptosis. This supports our overall hypothesis that elevated K(+) in tears contributes to protection of the corneal epithelium from adverse effects of ambient outdoor UVB.

Intersubunit Concerted Cooperative and cis-Type Mechanisms Modulate Allosteric Gating in Two-Pore-Domain Potassium Channel TREK-2

In response to diverse stimuli, two-pore-domain potassium channel TREK-2 regulates cellular excitability, and hence plays a key role in mediating neuropathic pain, mood disorders and ischemia through. Although more and more input modalities are found to achieve their modulations via acting on the channel, the potential role of subunit interaction in these modulations remains to be explored. In the current study, the deletion (lack of proximal C-terminus, ΔpCt) or point mutation (G312A) was introduced into TREK-2 subunits to limit K⁺ conductance and used to report subunit stoichiometry. The constructs were then combined with wild type (WT) subunit to produce concatenated dimers with defined composition, and the gating kinetics of these channels to 2-Aminoethoxydiphenyl borate (2-APB) and extracellular pH (pH_o) were characterized. Our results show that combination of WT and ΔpCt/G312A subunits reserves similar gating properties to that of WT dimmers, suggesting that the WT subunit exerts dominant and positive effects on the mutated one, and thus the two subunits controls channel gating via a concerted cooperative manner. Further introduction of ΔpCt into the latter subunit of heterodimeric channel G312A-WT or G312A-G312A attenuated their sensitivity to 2-APB and pH_o alkalization, implicating that these signals were transduced by a cis-type mechanism. Together, our findings elucidate the mechanisms for how the two subunits control the pore gating of TREK-2, in which both intersubunit concerted cooperative and cis-type manners modulate the allosteric regulations induced by 2-APB and pH_o alkalization.

Allosteric coupling between proximal C-terminus and selectivity filter is facilitated by the movement of transmembrane segment 4 in TREK-2 channel

TREK-2, a member of two-pore-domain potassium channel family, regulates cellular excitability in response to diverse stimuli. However, how such stimuli control channel function remains unclear. Here, by characterizing the responses of cytosolic proximal C-terminus deletant (ΔpCt) and transmembrane segment 4 (M4)-glycine hinge mutant (G312A) to 2-Aminoethoxydiphenyl borate (2-APB), an activator of TREK-2, we show that the transduction initiated from pCt domain is allosterically coupled with the conformation of selectivity filter (SF) via the movements of M4, without depending on the original status of SF. Moreover, ΔpCt and G312A also exhibited blunted responses to extracellular alkalization, a model to induce SF conformational transition. These results suggest that the coupling between pCt domain and SF is bidirectional, and M4 movements are involved in both processes. Further mechanistic exploration reveals that the function of Phe316, a residue close to the C-terminus of M4, is associated with such communications. However, unlike TREK-2, M4-hinge of TREK-1 only controls the transmission from pCt to SF, rather than SF conformational changes triggered by pHo changes. Together, our findings uncover the unique gating properties of TREK-2, and elucidate the mechanisms for how the extracellular and intracellular stimuli harness the pore gating allosterically.

The isoforms generated by alternative translation initiation adopt similar conformation in the selectivity filter in TREK-2

TREK-2 (TWIK-related K(+) channel-2), a member of two-pore domain potassium (K2P) channel family, tunes cellular excitability via conducting leak or background currents. In TREK-2, the isoforms generated by alternative translation initiation (ATI) mechanism exhibit large divergence in unitary conductance, but similar in selectivity to K(+). Up to now, the structural basis for this similarity in ion selectivity is unknown. Here, we report that externally applied Ba(2+) inhibits the currents of TREK-2 in a concentration- and time-dependent manner. The blocking effect is blunted by elevated extracellular K(+) or mutation of S4 K(+) binding site, which suggests that the inhibitory mechanism of Ba(2+) is due to its competitive docking properties within the selectivity filter (SF). Next, we demonstrate that all the ATI isoforms exhibit analogous behaviors upon the application of Ba(2+) and alteration of extracellular pH (pHo), which acts on the outer position of the SF. These results strongly support the notion that all the ATI isoforms of TREK-2 possess resembled SF conformation in S4 site and the position defined by pHo, which implicates that neither the role of N-terminus (Nt) nor the unitary conductance is associated with SF conformation. Our findings might help to understand the detail gating mechanism of TREK-2 and K2P channels.

Metabolism regulates the spontaneous firing of substantia nigra pars reticulata neurons via KATP and nonselective cation channels

Neurons use glucose to fuel glycolysis and provide substrates for mitochondrial respiration, but neurons can also use alternative fuels that bypass glycolysis and feed directly into mitochondria. To determine whether neuronal pacemaking depends on active glucose metabolism, we switched the metabolic fuel from glucose to alternative fuels, lactate or β-hydroxybutyrate, while monitoring the spontaneous firing of GABAergic neurons in mouse substantia nigra pars reticulata (SNr) brain slices. We found that alternative fuels, in the absence of glucose, sustained SNr spontaneous firing at basal rates, but glycolysis may still be supported by glycogen in the absence of glucose. To prevent any glycogen-fueled glycolysis, we directly inhibited glycolysis using either 2-deoxyglucose or iodoacetic acid. Inhibiting glycolysis in the presence of alternative fuels lowered SNr firing to a slower sustained firing rate. Surprisingly, we found that the decrease in SNr firing was not mediated by ATP-sensitive potassium (KATP) channel activity, but if we lowered the perfusion flow rate or omitted the alternative fuel, KATP channels were activated and could silence SNr firing. The KATP-independent slowing of SNr firing that occurred with glycolytic inhibition in the presence of alternative fuels was consistent with a decrease in a nonselective cationic conductance. Although mitochondrial metabolism alone can prevent severe energy deprivation and KATP channel activation in SNr neurons, active glucose metabolism appears important for keeping open a class of ion channels that is crucial for the high spontaneous firing rate of SNr neurons.

Inhibition of a TREK-like K+ channel current by noradrenaline requires both β1- and β2-adrenoceptors in rat atrial myocytes

AIMS

Noradrenaline plays an important role in the modulation of atrial electrophysiology. However, the identity of the modulated channels, their mechanisms of modulation, and their role in the action potential remain unclear. This study aimed to investigate the noradrenergic modulation of an atrial steady-state outward current (IKss).

METHODS AND RESULTS

Rat atrial myocyte whole-cell currents were recorded at 36°C. Noradrenaline potently inhibited IKss (IC50 = 0.90 nM, 42.1 ± 4.3% at 1 µM, n = 7) and potentiated the L-type Ca(2+) current (ICaL, EC50 = 136 nM, 205 ± 40% at 1 µM, n = 6). Noradrenaline-sensitive IKss was weakly voltage-dependent, time-independent, and potentiated by the arachidonic acid analogue, 5,8,11,14-eicosatetraynoic acid (EYTA; 10 µM), or by osmotically induced membrane stretch. Noise analysis revealed a unitary conductance of 8.4 ± 0.42 pS (n = 8). The biophysical/pharmacological properties of IKss indicate a TREK-like K(+) channel. The effect of noradrenaline on IKss was abolished by combined β1-/β2-adrenoceptor antagonism (1 µM propranolol or 10 µM β1-selective atenolol and 100 nM β2-selective ICI-118,551 in combination), but not by β1- or β2-antagonist alone. The action of noradrenaline could be mimicked by β2-agonists (zinterol and fenoterol) in the presence of β1-antagonist. The action of noradrenaline on IKss, but not on ICaL, was abolished by pertussis toxin (PTX) treatment. The action of noradrenaline on ICaL was mediated by β1-adrenoceptors via a PTX-insensitive pathway. Noradrenaline prolonged APD30 by 52 ± 19% (n = 5; P < 0.05), and this effect was abolished by combined β1-/β2-antagonism, but not by atenolol alone.

CONCLUSION

Noradrenaline inhibits a rat atrial TREK-like K(+) channel current via a PTX-sensitive mechanism involving co-operativity of β1-/β2-adrenoceptors that contributes to atrial APD prolongation.

Activation of neurotensin receptor 1 facilitates neuronal excitability and spatial learning and memory in the entorhinal cortex: beneficial actions in an Alzheimer's disease model

Neurotensin (NT) is a tridecapeptide distributed in the CNS, including the entorhinal cortex (EC), a structure that is crucial for learning and memory and undergoes the earliest pathological alterations in Alzheimer's disease (AD). Whereas NT has been implicated in modulating cognition, the cellular and molecular mechanisms by which NT modifies cognitive processes and the potential therapeutic roles of NT in AD have not been determined. Here we examined the effects of NT on neuronal excitability and spatial learning in the EC, which expresses high density of NT receptors. Brief application of NT induced persistent increases in action potential firing frequency, which could last for at least 1 h. NT-induced facilitation of neuronal excitability was mediated by downregulation of TREK-2 K(+) channels and required the functions of NTS1, phospholipase C, and protein kinase C. Microinjection of NT or NTS1 agonist, PD149163, into the EC increased spatial learning as assessed by the Barnes Maze Test. Activation of NTS1 receptors also induced persistent increases in action potential firing frequency and significantly improved the memory status in APP/PS1 mice, an animal model of AD. Our study identifies a cellular substrate underlying learning and memory and suggests that NTS1 agonists may exert beneficial actions in an animal model of AD.

Dorsal raphe serotonin neurons in mice: immature hyperexcitability transitions to adult state during first three postnatal weeks suggesting sensitive period for environmental perturbation

Trauma during early life is a major risk factor for the development of anxiety disorders and suggests that the developing brain may be particularly sensitive to perturbation. Increased vulnerability most likely involves altering neural circuits involved in emotional regulation. The role of serotonin in emotional regulation is well established, but little is known about the postnatal development of the raphe where serotonin is made. Using whole-cell patch-clamp recording and immunohistochemistry, we tested whether serotonin circuitry in the dorsal and median raphe was functionally mature during the first 3 postnatal weeks in mice. Serotonin neurons at postnatal day 4 (P4) were hyperexcitable. The increased excitability was due to depolarized resting membrane potential, increased resistance, increased firing rate, lack of 5-HT1A autoreceptor response, and lack of GABA synaptic activity. Over the next 2 weeks, membrane resistance decreased and resting membrane potential hyperpolarized due in part to potassium current activation. The 5-HT1A autoreceptor-mediated inhibition did not develop until P21. The frequency of spontaneous inhibitory and excitatory events increased as neurons extended and refined their dendritic arbor. Serotonin colocalized with vGlut3 at P4 as in adulthood, suggesting enhanced release of glutamate alongside enhanced serotonin release. Because serotonin affects circuit development in other brain regions, altering the developmental trajectory of serotonin neuron excitability and release could have many downstream consequences. We conclude that serotonin neuron structure and function change substantially during the first 3 weeks of life during which external stressors could potentially alter circuit formation.

Temperature sensitivity of two-pore (K2P) potassium channels

At normal body temperature, the two-pore potassium channels TREK-1 (K2P2.1/KCNK2), TREK-2 (K2P10.1/KCNK10), and TRAAK (K2P4.1/KCNK2) regulate cellular excitability by providing voltage-independent leak of potassium. Heat dramatically potentiates K2P channel activity and further affects excitation. This review focuses on the current understanding of the physiological role of heat-activated K2P current, and discusses the molecular mechanism of temperature gating in TREK-1, TREK-2, and TRAAK.

Photoactivation of bone marrow mesenchymal stromal cells with diode laser: effects and mechanisms of action

Mesenchymal stromal cells (MSCs) are a promising cell candidate in tissue engineering and regenerative medicine. Their proliferative potential can be increased by low-level laser irradiation (LLLI), but the mechanisms involved remain to be clarified. With the aim of expanding the therapeutic application of LLLI to MSC therapy, in the present study we investigated the effects of 635 nm diode laser on mouse MSC proliferation and investigated the underlying cellular and molecular mechanisms, focusing the attention on the effects of laser irradiation on Notch-1 signal activation and membrane ion channel modulation. It was found that MSC proliferation was significantly enhanced after laser irradiation, as judged by time lapse videomicroscopy and EdU incorporation. This phenomenon was associated with the up-regulation and activation of Notch-1 pathway, and with increased membrane conductance through voltage-gated K(+) , BK and Kir, channels and T- and L-type Ca(2+) channels. We also showed that MSC proliferation was mainly dependent on Kir channel activity, on the basis that the cell growth and Notch-1 up-regulation were severely decreased by the pre-treatment with the channel inhibitor Ba(2+) (0.5 mM). Interestingly, the channel inhibition was also able to attenuate the stimulatory effects of diode laser on MSCs, thus providing novel evidence to expand our knowledge on the mechanisms of biostimulation after LLLI. In conclusions, our findings suggest that diode laser may be a valid approach for the preconditioning of MSCs in vitro prior cell transplantation.

Metabolic and thermal stimuli control K(2P)2.1 (TREK-1) through modular sensory and gating domains

The two-pore domain potassium channel K_2P2.1 (TREK-1) responds to extracellular and intracellular stimuli, including pH and temperature. This study elucidates how the intracellular sensor element relays metabolic and thermal stimuli to the extracellular C-type gating element.

K_2P2.1 (TREK-1) is a polymodal two-pore domain leak potassium channel that responds to external pH, GPCR-mediated phosphorylation signals, and temperature through the action of distinct sensors within the channel. How the various intracellular and extracellular sensory elements control channel function remains unresolved. Here, we show that the K_2P2.1 (TREK-1) intracellular C-terminal tail (Ct), a major sensory element of the channel, perceives metabolic and thermal commands and relays them to the extracellular C-type gate through transmembrane helix M4 and pore helix 1. By decoupling Ct from the pore-forming core, we further demonstrate that Ct is the primary heat-sensing element of the channel, whereas, in contrast, the pore domain lacks robust temperature sensitivity. Together, our findings outline a mechanism for signal transduction within K_2P2.1 (TREK-1) in which there is a clear crosstalk between the C-type gate and intracellular Ct domain. In addition, our findings support the general notion of the existence of modular temperature-sensing domains in temperature-sensitive ion channels. This marked distinction between gating and sensory elements suggests a general design principle that may underlie the function of a variety of temperature-sensitive channels.