K+-dependent cerebellar granule neuron apoptosis. Role of task leak K+ channels

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Deregulation of Astroglial TASK-1 K+ Channel Decreases the Responsiveness to Perampanel-Induced AMPA Receptor Inhibition in Chronic Epilepsy Rats

Tandem of P domains in a weak inwardly rectifying K+ channel (TWIK)-related acid sensitive K+-1 channel (TASK-1) is activated under extracellular alkaline conditions (pH 7.2–8.2), which are upregulated in astrocytes (particularly in the CA1 region) of the hippocampi of patients with temporal lobe epilepsy and chronic epilepsy rats. Perampanel (PER) is a non-competitive α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) antagonist used for the treatment of focal seizures and primary generalized tonic–clonic seizures. Since AMPAR activation leads to extracellular alkaline shifts, it is likely that the responsiveness to PER in the epileptic hippocampus may be relevant to astroglial TASK-1 regulation, which has been unreported. In the present study, we found that PER ameliorated astroglial TASK-1 upregulation in responders (whose seizure activities were responsive to PER), but not non-responders (whose seizure activities were not responsive to PER), in chronic epilepsy rats. ML365 (a selective TASK-1 inhibitor) diminished astroglial TASK-1 expression and seizure duration in non-responders to PER. ML365 co-treatment with PER decreased spontaneous seizure activities in non-responders to PER. These findings suggest that deregulation of astroglial TASK-1 upregulation may participate in the responsiveness to PER, and that this may be a potential target to improve the efficacies of PER.

Subcellular Localization of Homomeric TASK3 Channels and Its Presumed Functional Significances in Trigeminal Motoneurons

Somatic expressions of either heteromeric TASK1/3 or homomeric TASK1/1 channels have been reported in various neurons, while expression of homomeric TASK3/3 channels has been re-ported only in dendrites. However, it is not known why homomeric TASK3/3 channels are hardly seen in somata of CNS neurons. Given the absence of somatic TASK3/3 channels, it should be clarified why dendritic expression of TASK3/3 channels is inevitable and necessary and how differentially distributed TASK1/1 and TASK3/3 channels play roles in soma-to-dendritic integration. Here, we addressed these questions. We found that TASK3-transfected HEK293 cells showed decreases in cell volume after being transferred from the cultured medium to HEPES Ringer, suggesting that expressions of TASK3 channels in cell bodies cause an osmolarity problem. Using TASK1- and TASK3-transfected oocytes, we also found that cGMP application slightly suppressed TASK3 currents while it largely enhanced TASK1 currents, alleviating the difference between TASK1 and TASK3 currents at physiological pH. As larger motoneurons have extensive dendritic trees while smaller motoneurons have poor ones, cGMP could integrate Ia-EPSPs to recruit small and large motoneurons synchronously by differentially modulating TASKI and TASK3 channels which were complementary distributed in soma and dendrites of motoneurons in the dorsolateral part of the trigeminal motor nucleus.

The modulation of potassium channels by estrogens facilitates neuroprotection

Estrogens, the sex hormones, have the potential to govern multiple cellular functions, such as proliferation, apoptosis, differentiation, and homeostasis, and to exert numerous beneficial influences for the cardiovascular system, nervous system, and bones in genomic and/or non-genomic ways. Converging evidence indicates that estrogens serve a crucial role in counteracting neurodegeneration and ischemic injury; they are thereby being considered as a potent neuroprotectant for preventing neurological diseases such as Alzheimer's disease and stroke. The underlying mechanism of neuroprotective effects conferred by estrogens is thought to be complex and multifactorial, and it remains obscure. It is well established that the K+ channels broadly expressed in a variety of neural subtypes determine the essential physiological features of neuronal excitability, and dysfunction of these channels is closely associated with diverse brain deficits, such as ataxia and epilepsy. A growing body of evidence supports a neuroprotective role of K+ channels in malfunctions of nervous tissues, with the channels even being a therapeutic target in clinical trials. As multitarget steroid hormones, estrogens also regulate the activity of distinct K+ channels to generate varying biological actions, and accumulated data delineate that some aspects of estrogen-mediated neuroprotection may arise from the impact on multiple K+ channels, including Kv, BK, KATP, and K2P channels. The response of these K+ channels after acute or chronic exposure to estrogens may oppose pathological abnormality in nervous cells, which serves to extend our understanding of these phenomena.

A Direct Interaction between Cyclodextrins and TASK Channels Decreases the Leak Current in Cerebellar Granule Neurons

Simple Summary

Cyclodextrins are cyclic oligosaccharides used to deplete cholesterol from cellular membranes. The effects of methyl-β-cyclodextrin (MβCD) on cellular functions originate principally from reductions in cholesterol levels. In this study, using immunocytochemistry, heterologous expression of K2P channels, and cholesterol-depleting maneuvers, we provide evidence of expression in cultured rat cerebellar granule neurons (CGNs) of TWIK-1 (K2P1), TASK-1 (K2P3), TASK-3 (K2P9), and TRESK (K2P18) channels and their association with lipid rafts using the specific lipids raft markers. In addition, we show a direct blocking with MβCD of TASK-1 and TASK-3 channels as well as for the covalently concatenated heterodimer TASK-1/TASK-3.

Abstract

Two pore domain potassium channels (K2P) are strongly expressed in the nervous system (CNS), where they play a central role in excitability. These channels give rise to background K⁺ currents, also known as IK_SO (standing-outward potassium current). We detected the expression in primary cultured cerebellar granule neurons (CGNs) of TWIK-1 (K2P1), TASK-1 (K2P3), TASK-3 (K2P9), and TRESK (K2P18) channels by immunocytochemistry and their association with lipid rafts using the specific lipids raft markers flotillin-2 and caveolin-1. At the functional level, methyl-β-cyclodextrin (MβCD, 5 mM) reduced IK_SO currents by ~40% in CGN cells. To dissect out this effect, we heterologously expressed the human TWIK-1, TASK-1, TASK-3, and TRESK channels in HEK-293 cells. MβCD directly blocked TASK-1 and TASK-3 channels and the covalently concatenated heterodimer TASK-1/TASK-3 currents. Conversely, MβCD did not affect TWIK-1- and TRESK-mediated K+ currents. On the other hand, the cholesterol-depleting agent filipin III did not affect TASK-1/TASK-3 channels. Together, the results suggest that neuronal background K⁺ channels are associated to lipid raft environments whilst the functional activity is independent of the cholesterol membrane organization.

Alternative Targets for Modulators of Mitochondrial Potassium Channels

Mitochondrial potassium channels control potassium influx into the mitochondrial matrix and thus regulate mitochondrial membrane potential, volume, respiration, and synthesis of reactive oxygen species (ROS). It has been found that pharmacological activation of mitochondrial potassium channels during ischemia/reperfusion (I/R) injury activates cytoprotective mechanisms resulting in increased cell survival. In cancer cells, the inhibition of these channels leads to increased cell death. Therefore, mitochondrial potassium channels are intriguing targets for the development of new pharmacological strategies. In most cases, however, the substances that modulate the mitochondrial potassium channels have a few alternative targets in the cell. This may result in unexpected or unwanted effects induced by these compounds. In our review, we briefly present the various classes of mitochondrial potassium (mitoK) channels and describe the chemical compounds that modulate their activity. We also describe examples of the multidirectional activity of the activators and inhibitors of mitochondrial potassium channels.

A Systematic Review of Genomic Regions and Candidate Genes Underlying Behavioral Traits in Farmed Mammals and Their Link with Human Disorders

Simple Summary

This study is a comprehensive review of genomic regions associated with animal behavior in farmed mammals (beef and dairy cattle, pigs, and sheep) which contributes to a better understanding of the biological mechanisms influencing the target indicator trait and to gene expression studies by suggesting genes likely controlling the trait, and it will be useful in optimizing genomic predictions of breeding values incorporating biological information. Behavioral mechanisms are complex traits, genetically controlled by multiple genes spread across the whole genome. The majority of the genes identified in cattle, pigs, and sheep in association with a plethora of behavioral measurements (e.g., temperament, terrain use, milking speed, tail biting, and sucking reflex) are likely controlling stimuli reception (e.g., olfactory), internal recognition of stimuli (e.g., neuroactive ligand–receptor interaction), and body response to a stimulus (e.g., blood pressure, fatty acidy metabolism, hormone signaling, and inflammatory pathways). Six genes were commonly identified between cattle and pigs. About half of the genes for behavior identified in farmed mammals were also identified in humans for behavioral, mental, and neuronal disorders. Our findings indicate that the majority of the genes identified are likely controlling animal behavioral outcomes because their biological functions as well as potentially differing allele frequencies between two breed groups (subjectively) clustered based on their temperament characteristics.

Abstract

The main objectives of this study were to perform a systematic review of genomic regions associated with various behavioral traits in the main farmed mammals and identify key candidate genes and potential causal mutations by contrasting the frequency of polymorphisms in cattle breeds with divergent behavioral traits (based on a subjective clustering approach). A total of 687 (cattle), 1391 (pigs), and 148 (sheep) genomic regions associated with 37 (cattle), 55 (pigs), and 22 (sheep) behavioral traits were identified in the literature. In total, 383, 317, and 15 genes overlap with genomic regions identified for cattle, pigs, and sheep, respectively. Six common genes (e.g., NR3C2, PITPNM3, RERG, SPNS3, U6, and ZFAT) were found for cattle and pigs. A combined gene-set of 634 human genes was produced through identified homologous genes. A total of 313 out of 634 genes have previously been associated with behavioral, mental, and neurologic disorders (e.g., anxiety and schizophrenia) in humans. Additionally, a total of 491 candidate genes had at least one statistically significant polymorphism (p-value < 0.05). Out of those, 110 genes were defined as having polymorphic regions differing in greater than 50% of exon regions. Therefore, conserved genomic regions controlling behavior were found across farmed mammal species and humans.

Antiarrhythmic Properties of Ranolazine: Inhibition of Atrial Fibrillation Associated TASK-1 Potassium Channels

Background: Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and one of the major causes of cardiovascular morbidity and mortality. Despite good progress within the past years, safe and effective treatment of AF remains an unmet clinical need. The anti-anginal agent ranolazine has been shown to exhibit antiarrhythmic properties via mainly late I_Na and I_Kr blockade. This results in prolongation of the atrial action potential duration (APD) and effective refractory period (ERP) with lower effect on ventricular electrophysiology. Furthermore, ranolazine has been shown to be effective in the treatment of AF. TASK-1 is a two-pore domain potassium (K_2P) channel that shows nearly atrial specific expression within the human heart and has been found to be upregulated in AF, resulting in shortening the atrial APD in patients suffering from AF. We hypothesized that inhibition TASK-1 contributes to the observed electrophysiological and clinical effects of ranolazine.

Methods: We used Xenopus laevis oocytes and CHO-cells as heterologous expression systems for the study of TASK-1 inhibition by ranolazine and molecular drug docking simulations to investigate the ranolazine binding site and binding characteristics.

Results: Ranolazine acts as an inhibitor of TASK-1 potassium channels that inhibits TASK-1 currents with an IC₅₀ of 30.6 ± 3.7 µM in mammalian cells and 198.4 ± 1.1 µM in X. laevis oocytes. TASK-1 inhibition by ranolazine is not frequency dependent but shows voltage dependency with a higher inhibitory potency at more depolarized membrane potentials. Ranolazine binds within the central cavity of the TASK-1 inner pore, at the bottom of the selectivity filter.

Conclusions: In this study, we show that ranolazine inhibits TASK-1 channels. We suggest that inhibition of TASK-1 may contribute to the observed antiarrhythmic effects of Ranolazine. This puts forward ranolazine as a prototype drug for the treatment of atrial arrhythmia because of its combined efficacy on atrial electrophysiology and lower risk for ventricular side effects.

Planarian regeneration as a model of anatomical homeostasis: Recent progress in biophysical and computational approaches

Planarian behavior, physiology, and pattern control offer profound lessons for regenerative medicine, evolutionary biology, morphogenetic engineering, robotics, and unconventional computation. Despite recent advances in the molecular genetics of stem cell differentiation, this model organism's remarkable anatomical homeostasis provokes us with truly fundamental puzzles about the origin of large-scale shape and its relationship to the genome. In this review article, we first highlight several deep mysteries about planarian regeneration in the context of the current paradigm in this field. We then review recent progress in understanding of the physiological control of an endogenous, bioelectric pattern memory that guides regeneration, and how modulating this memory can permanently alter the flatworm's target morphology. Finally, we focus on computational approaches that complement reductive pathway analysis with synthetic, systems-level understanding of morphological decision-making. We analyze existing models of planarian pattern control and highlight recent successes and remaining knowledge gaps in this interdisciplinary frontier field.

HIF-α/PKM2 and PI3K-AKT pathways involved in the protection by dexmedetomidine against isoflurane or bupivacaine-induced apoptosis in hippocampal neuronal HT22 cells

The present study investigated the mechanism underlying the protective effect of dexmedetomidine (Dex) on hippocampal neuronal HT22 cell apoptosis induced by the anesthetics isoflurane and bupivacaine. The cellular morphology was observed using a phase contrast microscope. The effects of anesthetics on cell proliferation were assayed using a Cell Counting Kit-8 (CCK-8). The levels of apoptosis were examined by flow cytometry utilizing Annexin V-fluorescein isothiocyanate/propidium iodide double staining, and the protein expression levels of cleaved caspase-3, phosphorylated phosphoinositide 3′-kinase (p-PI3K), p-protein kinase B (p-AKT), hypoxia inducible factor (HIF-α), pyruvate kinase M2 (PKM2), B-cell lymphoma (Bcl-2)-associated X protein (Bax), Bcl-2 and cytochrome c were detected by western blot analysis. In vitro treatment with anesthetics was identified to decrease cell proliferation (P<0.01), the effect of which was then markedly inhibited by treatment with Dex (P<0.01) or a PI3K/AKT agonist. Exposure to anesthetics induced apoptosis in HT22 cells (75.4%), which was significantly attenuated by co-treatment with Dex (26.2%) or the PI3K/AKT agonist (28.1%). Analysis of the protein expression levels revealed that exposure to anesthetics resulted in the activation of cleaved caspase-3, Bax, cytochrome c, HIF-α and PKM2 and decreased the expression levels of Bcl-2, p-PI3K and p-AKT. However, these changes were inhibited by treatment with Dex or the PI3K/AKT agonist. Dex protected hippocampal neuronal HT22 cells from anesthetic-induced apoptosis through the promotion of the PI3K/AKT pathway and inhibition of the HIF-α/PKM2 axis.

Fighting against depression with TREK-1 blockers: Past and future. A focus on spadin

Depression is a devastating mood disorder and a leading cause of disability worldwide. Depression affects approximately one in five individuals in the world and represents heavy economic and social burdens. The neurobiological mechanisms of depression are not fully understood, but evidence highlights the role of monoamine neurotransmitter balance. Several antidepressants (ADs) are marketed to treat depression and related mood disorders. However, despite their efficacy, they remain nonspecific and unsafe because they trigger serious adverse effects. Therefore, developing new molecules for new targets in depression has become a real necessity. Eight years ago, spadin was described as a natural peptide with AD properties. This 17-amino acid peptide blocks TREK-1 channels, an original target in depression. Compared to the classical AD drugs such as fluoxetine, which requires 3-4 weeks for the AD effect to manifest, spadin acts rapidly within only 4 days of treatment. The AD properties are associated with increased neurogenesis and synaptogenesis in the brain. Despite the advantages of this fast-acting AD, the in vivo stability is weak and does not last for >7 h. The present review summarizes different strategies such as retro-inverso strategy, cyclization, and shortening the spadin sequence that has led to the development and optimization of spadin as an AD. Shortened spadin analogs present increased inhibition potency for TREK-1, an improved AD activity, and prolonged in vivo bioavailability. Finally, we also discuss about other inhibitors of TREK-1 channels with a proven efficacy in treating depression in the clinic, such as fluoxetine.

TASK-3 Downregulation Triggers Cellular Senescence and Growth Inhibition in Breast Cancer Cell Lines

TASK-3 potassium channels are believed to promote proliferation and survival of cancer cells, in part, by augmenting their resistance to both hypoxia and serum deprivation. While overexpression of TASK-3 is frequently observed in cancers, the understanding of its role and regulation during tumorigenesis remains incomplete. Here, we evaluated the effect of reducing the expression of TASK-3 in MDA-MB-231 and MCF-10F human mammary epithelial cell lines through small hairpin RNA (shRNA)-mediated knockdown. Our results show that knocking down TASK-3 in fully transformed MDA-MB-231 cells reduces proliferation, which was accompanied by an induction of cellular senescence and cell cycle arrest, with an upregulation of cyclin-dependent kinase (CDK) inhibitors p21 and p27. In non-tumorigenic MCF-10F cells, however, TASK-3 downregulation did not lead to senescence induction, although cell proliferation was impaired and an upregulation of CDK inhibitors was also evident. Our observations implicate TASK-3 as a critical factor in cell cycle progression and corroborate its potential as a therapeutic target in breast cancer treatment.

The Effect of Mechanical Ventilation on TASK-1 Expression in the Brain in a Rat Model

Background and Objective

TWIK-related acid-sensitive potassium channel 1 (TASK-1) is closely related to respiratory central control and neuronal injury. We investigated the effect of MV on TASK-1's functions and explored the mechanism using a rat model.

Methods

Male Sprague-Dawley rats were randomized to three groups: (1) high tidal volume (HVt): MV for four hours with Vt at 10 mL/kg; (2) low Vt (LVt): MV for four hours with Vt at 5 mL/kg; (3) basal (BAS): anesthetized and unventilated animals. We measured lung histology and plasma and brain levels of proteins (IL-6, TNF-α, and S-100B) and determined TASK-1 levels in rat brainstems as a marker of respiratory centre activity.

Results

The LISs (lung injury scores) were significantly higher in the HVt group. Brain inflammatory cytokines levels were different to those in serum. TASK-1 levels were significantly lower in the MV groups (P = 0.002) and the HVt group tended to have a lower level of TASK-1 than the LVt group.

Conclusion

MV causes not only lung injury, but also brain injury. MV affects the regulation of the respiratory centre, perhaps causing damage to it. Inflammation is probably not the main mechanism of ventilator-related brain injury.

Shortened Spadin Analogs Display Better TREK-1 Inhibition, In Vivo Stability and Antidepressant Activity

Depression is a devastating mental disorder that affects 20% of the population worldwide. Despite their proven efficacy, antidepressants present a delayed onset of action and serious adverse effects. Seven years ago, we described spadin (PE 12-28) as a promising endogenous peptide with antidepressant activity. Spadin specifically blocks the TREK-1 channel. Previously, we showed in vivo that, spadin activity disappeared beyond 7 h after administration. In order to improve in vivo spadin stability and bioavailability, we screened spadin analogs and derivatives. From the study of spadin blood degradation products, we designed a 7 amino-acid peptide, PE 22-28. In vitro studies on hTREK-1/HEK cells by using patch-clamp technique, showed that PE 22-28 displayed a better specificity and affinity for TREK-1 channel compared to spadin, IC₅₀ of 0.12 nM vs. 40–60 nM for spadin. In the same conditions, we also pointed out that different modifications of its N or C-terminal ends maintained or abolished TREK-1 channel activity without affecting PE 22-28 affinity. In vivo, the antidepressant properties of PE 22-28 and its derivatives were demonstrated in behavioral models of depression, such as the forced swimming test. Mice treated with spadin-analogs showed a significant reduction of the immobility time. Moreover, in the novelty suppressed feeding test after a 4-day sub-chronic treatment PE 22-28 reduced significantly the latency to eat the food pellet. PE 22-28 and its analogs were able to induce neurogenesis after only a 4-day treatment with a prominent effect of the G/A-PE 22-28. On mouse cortical neurons, PE 22-28 and its derivatives enhanced synaptogenesis measured by the increase of PSD-95 expression level. Finally, the action duration of PE 22-28 and its analogs was largely improved in comparison with that of spadin, up to 23 h instead of 7 h. Taken together, our results demonstrated that PE 22-28 and its derivatives represent other promising molecules that could be an alternative to spadin in the treatment of depression.

Kcnh2 and Kcnj8 interactively regulate skin wound healing and regeneration

Previous studies indicate that ion channels are mediators of bioelectricity promoting wound closure/regeneration in nonmammalian, lower vertebrate systems. The role of ion channels however in regeneration of wounds in mammalian systems that do not regenerate as adults is not yet defined. Using a mammalian model system that allows us to determine differentially expressed genes when skin regenerates and when skin does not regenerate after wound induction, we identified two potassium channels, kcnh2 and kcnj8, to be (1) differentially expressed between the two states and (2) highly expressed after wound induction at the nonregenerative state. We also found that kcnh2 small molecule inhibitor enhanced wound healing while kcnj8 small molecule inhibitor did not. In contrast, kcnj8 activator accelerated wound healing and even augmented the effect of kcnh2 inhibition. These results provide evidence for the first time that potassium channels may mediate skin wound healing and regeneration interactively.

Localization of TASK and TREK, Two-Pore Domain K+ Channels, in Human Cytotrophoblast Cells

OBJECTIVE

Two-pore domain K+ channels (K2P), an emerging K+ channel subfamily, contribute to setting membrane potential in both electrically excitable and nonexcitable cells and, as such, influence cellular function. The multinucleate syncytiotrophoblast of human placenta, formed from the fusion of mononucleate cytotrophoblast cells, is a transporting epithelium whose function likely depends on the activity of K+ channels. We have therefore investigated the gene expression of two members of this family, TASK and TREK, in cultured human cytotrophoblast cells, and have also investigated protein expression in cytotrophoblast cells and placenta.

METHODS

We used reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting, and immunocytochemistry to investigate the gene and protein expression of TASK and TREK isoforms in both isolated cytotrophoblast cells and term placental tissue.

RESULTS

In cytotrophoblast cells, mRNAs encoding TASK1, 2, 4, 5, and TREK1 were detected, whereas weak or no signals were observed for TASK3 and TREK2. Western blotting for TASK1 in cytotrophoblast cells gave two bands of approximately 78 and 150 kd; TREK1 gave bands of approximately 90 and 130 kd. TASK1 immunofluorescence in placenta colocalized with cytokeratin-7, a trophoblast-specific marker. TREK1 predominantly stained cells around the villous perimeter and this staining was colocalized with propidium iodide nuclear staining.

CONCLUSION

Human cytotrophoblast cells from term placenta are a site of expression for various K2P genes, two of which, namely, TASK1 and TREK1, are transcribed into protein.

The potassium channels TASK2 and TREK1 regulate functional differentiation of murine skeletal muscle cells

Two-pore domain potassium (K_2P) channels influence basic cellular parameters such as resting membrane potential, cellular excitability, or intracellular Ca²⁺-concentration [Ca²⁺]_i While the physiological importance of K_2P channels in different organ systems (e.g., heart, central nervous system, or immune system) has become increasingly clear over the last decade, their expression profile and functional role in skeletal muscle cells (SkMC) remain largely unknown. The mouse SkMC cell line C2C12, wild-type mouse muscle tissue, and primary mouse muscle cells (PMMs) were analyzed using quantitative PCR, Western blotting, and immunohistochemical stainings as well as functional analysis including patch-clamp measurements and Ca²⁺ imaging. Mouse SkMC express TWIK-related acid-sensitive K⁺ channel (TASK) 2, TWIK-related K⁺ channel (TREK) 1, TREK2, and TWIK-related arachidonic acid stimulated K⁺ channel (TRAAK). Except TASK2 all mentioned channels were upregulated in vitro during differentiation from myoblasts to myotubes. TASK2 and TREK1 were also functionally expressed and upregulated in PMMs isolated from mouse muscle tissue. Inhibition of TASK2 and TREK1 during differentiation revealed a morphological impairment of myoblast fusion accompanied by a downregulation of maturation markers. TASK2 and TREK1 blockade led to a decreased K⁺ outward current and a decrease of ACh-dependent Ca²⁺ influx in C2C12 cells as potential underlying mechanisms. K_2P-channel expression was also detected in human muscle tissue by immunohistochemistry pointing towards possible relevance for human muscle cell maturation and function. In conclusion, our findings for the first time demonstrate the functional expression of TASK2 and TREK1 in muscle cells with implications for differentiation processes warranting further investigations in physiologic and pathophysiologic scenarios.

Dasatinib induces lung vascular toxicity and predisposes to pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a life-threatening disease that can be induced by dasatinib, a dual Src and BCR-ABL tyrosine kinase inhibitor that is used to treat chronic myelogenous leukemia (CML). Today, key questions remain regarding the mechanisms involved in the long-term development of dasatinib-induced PAH. Here, we demonstrated that chronic dasatinib therapy causes pulmonary endothelial damage in humans and rodents. We found that dasatinib treatment attenuated hypoxic pulmonary vasoconstriction responses and increased susceptibility to experimental pulmonary hypertension (PH) in rats, but these effects were absent in rats treated with imatinib, another BCR-ABL tyrosine kinase inhibitor. Furthermore, dasatinib treatment induced pulmonary endothelial cell apoptosis in a dose-dependent manner, while imatinib did not. Dasatinib treatment mediated endothelial cell dysfunction via increased production of ROS that was independent of Src family kinases. Consistent with these findings, we observed elevations in markers of endothelial dysfunction and vascular damage in the serum of CML patients who were treated with dasatinib, compared with CML patients treated with imatinib. Taken together, our findings indicate that dasatinib causes pulmonary vascular damage, induction of ER stress, and mitochondrial ROS production, which leads to increased susceptibility to PH development.

The Possible Role of TASK Channels in Rank-Ordered Recruitment of Motoneurons in the Dorsolateral Part of the Trigeminal Motor Nucleus

Because a rank-ordered recruitment of motor units occurs during isometric contraction of jaw-closing muscles, jaw-closing motoneurons (MNs) may be recruited in a manner dependent on their soma sizes or input resistances (IRs). In the dorsolateral part of the trigeminal motor nucleus (dl-TMN) in rats, MNs abundantly express TWIK (two-pore domain weak inwardly rectifying K channel)-related acid-sensitive-K(+) channel (TASK)-1 and TASK3 channels, which determine the IR and resting membrane potential. Here we examined how TASK channels are involved in IR-dependent activation/recruitment of MNs in the rat dl-TMN by using multiple methods. The real-time PCR study revealed that single large MNs (>35 μm) expressed TASK1 and TASK3 mRNAs more abundantly compared with single small MNs (15-20 μm). The immunohistochemistry revealed that TASK1 and TASK3 channels were complementarily distributed in somata and dendrites of MNs, respectively. The density of TASK1 channels seemed to increase with a decrease in soma diameter while there were inverse relationships between the soma size of MNs and IR, resting membrane potential, or spike threshold. Dual whole-cell recordings obtained from smaller and larger MNs revealed that the recruitment of MNs depends on their IRs in response to repetitive stimulation of the presumed Ia afferents. 8-Bromoguanosine-cGMP decreased IRs in small MNs, while it hardly changed those in large MNs, and subsequently decreased the difference in spike-onset latency between the smaller and larger MNs, causing a synchronous activation of MNs. These results suggest that TASK channels play critical roles in rank-ordered recruitment of MNs in the dl-TMN.

Heterodimerization within the TREK channel subfamily produces a diverse family of highly regulated potassium channels

Twik-related K(+) channel 1 (TREK1), TREK2, and Twik-related arachidonic-acid stimulated K(+) channel (TRAAK) form the TREK subfamily of two-pore-domain K(+) (K2P) channels. Despite sharing up to 78% sequence homology and overlapping expression profiles in the nervous system, these channels show major differences in their regulation by physiological stimuli. For instance, TREK1 is inhibited by external acidification, whereas TREK2 is activated. Here, we investigated the ability of the members of the TREK subfamily to assemble to form functional heteromeric channels with novel properties. Using single-molecule pull-down (SiMPull) from HEK cell lysate and subunit counting in the plasma membrane of living cells, we show that TREK1, TREK2, and TRAAK readily coassemble. TREK1 and TREK2 can each heterodimerize with TRAAK, but do so less efficiently than with each other. We functionally characterized the heterodimers and found that all combinations form outwardly rectifying potassium-selective channels but with variable voltage sensitivity and pH regulation. TREK1-TREK2 heterodimers show low levels of activity at physiological external pH but, unlike their corresponding homodimers, are activated by both acidic and alkaline conditions. Modeling based on recent crystal structures, along with mutational analysis, suggests that each subunit within a TREK1-TREK2 channel is regulated independently via titratable His. Finally, TREK1/TRAAK heterodimers differ in function from TRAAK homodimers in two critical ways: they are activated by both intracellular acidification and alkalinization and are regulated by the enzyme phospholipase D2. Thus, heterodimerization provides a means for diversifying functionality through an expansion of the channel types within the K2P channels.

Potassium channels in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a devastating cardiopulmonary disorder with various origins. All forms of PAH share a common pulmonary arteriopathy characterised by vasoconstriction, remodelling of the pre-capillary pulmonary vessel wall, and in situ thrombosis. Although the pathogenesis of PAH is recognised as a complex and multifactorial process, there is growing evidence that potassium channels dysfunction in pulmonary artery smooth muscle cells is a hallmark of PAH. Besides regulating many physiological functions, reduced potassium channels expression and/or activity have significant effects on PAH establishment and progression. This review describes the molecular mechanisms and physiological consequences of potassium channel modulation. Special emphasis is placed on KCNA5 (Kv1.5) and KCNK3 (TASK1), which are considered to play a central role in determining pulmonary vascular tone and may represent attractive therapeutic targets in the treatment of PAH.

Molecular bioelectricity: how endogenous voltage potentials control cell behavior and instruct pattern regulation in vivo

In addition to biochemical gradients and transcriptional networks, cell behavior is regulated by endogenous bioelectrical cues originating in the activity of ion channels and pumps, operating in a wide variety of cell types. Instructive signals mediated by changes in resting potential control proliferation, differentiation, cell shape, and apoptosis of stem, progenitor, and somatic cells. Of importance, however, cells are regulated not only by their own V_mem but also by the V_mem of their neighbors, forming networks via electrical synapses known as gap junctions. Spatiotemporal changes in V_mem distribution among nonneural somatic tissues regulate pattern formation and serve as signals that trigger limb regeneration, induce eye formation, set polarity of whole-body anatomical axes, and orchestrate craniofacial patterning. New tools for tracking and functionally altering V_mem gradients in vivo have identified novel roles for bioelectrical signaling and revealed the molecular pathways by which V_mem changes are transduced into cascades of downstream gene expression. Because channels and gap junctions are gated posttranslationally, bioelectrical networks have their own characteristic dynamics that do not reduce to molecular profiling of channel expression (although they couple functionally to transcriptional networks). The recent data provide an exciting opportunity to crack the bioelectric code, and learn to program cellular activity at the level of organs, not only cell types. The understanding of how patterning information is encoded in bioelectrical networks, which may require concepts from computational neuroscience, will have transformative implications for embryogenesis, regeneration, cancer, and synthetic bioengineering.

Involvement of leak K+ channels in neurological disorders

TWIK-related acid-sensitive K⁺ (TASK) channels give rise to leak K⁺ currents which influence the resting membrane potential and input resistance. The wide expression of TASK1 and TASK3 channels in the central nervous system suggests that these channels are critically involved in neurological disorders. It has become apparent in the past decade that TASK channels play critical roles for the development of various neurological disorders. In this review, I describe evidence for their roles in ischemia, epilepsy, learning/memory/cognition and apoptosis.

Oxidative modulation of K+ channels in the central nervous system in neurodegenerative diseases and aging

SIGNIFICANCE

Oxidative stress and damage are well-established components of neurodegenerative diseases, contributing to neuronal death during disease progression. Here, we consider key K(+) channels as target proteins that can undergo oxidative modulation, describe what is understood about how this influences disease progression, and consider regulation of these channels by gasotransmitters as a means of cellular protection.

RECENT ADVANCES

Oxidative regulation of the delayed rectifier Kv2.1 and the Ca(2+)- and voltage-sensitive BK channel are established, but recent studies contest how their redox sensitivity contributes to altered excitability, progression of neurodegenerative diseases, and healthy aging.

CRITICAL ISSUES

Both Kv2.1 and BK channels have recently been established as target proteins for regulation by the gasotransmitters carbon monoxide and hydrogen sulfide. Establishing the molecular basis of such regulation, and exactly how this influences excitability and vulnerability to apoptotic cell death will determine whether such regulation can be exploited for therapeutic benefit.

FUTURE DIRECTIONS

Developing a more comprehensive picture of the oxidative modulation of K(+) channels (and, indeed, other ion channels) within the central nervous system in health and disease will enable us to better understand processes associated with healthy aging as well as distinct processes underlying progression of neurodegenerative diseases. Advances in the growing understanding of how gasotransmitters can regulate ion channels, including redox-sensitive K(+) channels, are a research priority for this field, and will establish their usefulness in design of future approaches for the treatment of such diseases.

The family of K2P channels: salient structural and functional properties

Potassium channels participate in many biological functions, from ion homeostasis to generation and modulation of the electrical membrane potential. They are involved in a large variety of diseases. In the human genome, 15 genes code for K(+) channels with two pore domains (K2P ). These channels form dimers of pore-forming subunits that produce background conductances finely regulated by a range of natural and chemical effectors, including signalling lipids, temperature, pressure, pH, antidepressants and volatile anaesthetics. Since the cloning of TWIK1, the prototypical member of this family, a lot of work has been carried out on their structure and biology. These studies are still in progress, but data gathered so far show that K2P channels are central players in many processes, including ion homeostasis, hormone secretion, cell development and excitability. A growing number of studies underline their implication in physiopathological mechanisms, such as vascular and pulmonary hypertension, cardiac arrhythmias, nociception, neuroprotection and depression. This review gives a synthetic view of the most noticeable features of these channels.

Differential sensitivity of TREK-1, TREK-2 and TRAAK background potassium channels to the polycationic dye ruthenium red

BACKGROUND AND PURPOSE

Pharmacological separation of the background potassium currents of closely related K2P channels is a challenging problem. We previously demonstrated that ruthenium red (RR) inhibits TASK-3 (K2 P 9.1), but not TASK-1 (K2 P 3.1) channels. RR has been extensively used to distinguish between TASK currents in native cells. In the present study, we systematically investigate the RR sensitivity of a more comprehensive set of K2 P channels.

EXPERIMENTAL APPROACH

K(+) currents were measured by two-electrode voltage clamp in Xenopus oocytes and by whole-cell patch clamp in mouse dorsal root ganglion (DRG) neurons.

KEY RESULTS

RR differentiates between two closely related members of the TREK subfamily. TREK-2 (K2 P 10.1) proved to be highly sensitive to RR (IC50 = 0.2 μM), whereas TREK-1 (K2 P 2.1) was not affected by the compound. We identified aspartate 135 (D135) as the target of the inhibitor in mouse TREK-2c. D135 lines the wall of the extracellular ion pathway (EIP), a tunnel structure through the extracellular cap characteristic for K2 P channels. TREK-1 contains isoleucine in the corresponding position. The mutation of this isoleucine (I110D) rendered TREK-1 sensitive to RR. The third member of the TREK subfamily, TRAAK (K2 P 4.1) was more potently inhibited by ruthenium violet, a contaminant in some RR preparations, than by RR. DRG neurons predominantly express TREK-2 and RR-resistant TREK-1 and TRESK (K2 P 18.1) background K(+) channels. We detected the RR-sensitive leak K(+) current component in DRG neurons.

CONCLUSIONS AND IMPLICATIONS

We propose that RR may be useful for distinguishing TREK-2 (K2P 10.1) from TREK-1 (K2P 2.1) and other RR-resistant K2 P channels in native cells.

Involvement of potassium channels in the progression of cancer to a more malignant phenotype

Potassium channels are a diverse group of pore-forming transmembrane proteins that selectively facilitate potassium flow through an electrochemical gradient. They participate in the control of the membrane potential and cell excitability in addition to different cell functions such as cell volume regulation, proliferation, cell migration, angiogenesis as well as apoptosis. Because these physiological processes are essential for the correct cell function, K+ channels have been associated with a growing number of diseases including cancer. In fact, different K+ channel families such as the voltage-gated K+ channels, the ether à-go-go K+ channels, the two pore domain K+ channels and the Ca2+-activated K+ channels have been associated to tumor biology. Potassium channels have a role in neoplastic cell-cycle progression and their expression has been found abnormal in many types of tumors and cancer cells. In addition, the expression and activity of specific K+ channels have shown a significant correlation with the tumor malignancy grade. The aim of this overview is to summarize published data on K+ channels that exhibit oncogenic properties and have been linked to a more malignant cancer phenotype. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Targeting two-pore domain K(+) channels TREK-1 and TASK-3 for the treatment of depression: a new therapeutic concept

Depression is a disease that is particularly frequent, affecting up to 20% of the population in Western countries. The origins of this pathology involve multiple genes as well as environmental and developmental factors leading to a disorder that remains difficult to treat. Several therapies for depression have been developed and these mainly target monoamine neurotransmitters. However, these treatments are not only associated with numerous adverse effects, but they are also ineffective for more than one-third of patients. Therefore, the need to develop new concepts to treat depression is crucial. Recently, studies using knockout mouse models have provided evidence for a crucial role of two members of the two-pore domain potassium channel (K2P ) family, tandem P-domain weak inward rectifying K(+) (TWIK)-related K(+) channel 1 (TREK-1) and TWIK-related acid-sensitive K(+) channel 3 (TASK-3) in the pathophysiology of depression. It is believed that TREK-1 and TASK-3 antagonists could lead to the development of new antidepressants. Herein, we describe the discovery of spadin, a natural peptide released from the maturation of the neurotensin receptor-3 (also known as sortilin), which specifically blocks the activity of the TREK-1 channel and displays particular antidepressant properties, with a rapid onset of action and the absence of adverse effects. The development of such molecules may open a new era in the field of psychiatry.

Ion channels in the regulation of apoptosis

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

Silencing the KCNK9 potassium channel (TASK-3) gene disturbs mitochondrial function, causes mitochondrial depolarization, and induces apoptosis of human melanoma cells

TASK-3 (KCNK9 or K2P9.1) channels are thought to promote proliferation and/or survival of malignantly transformed cells, most likely by increasing their hypoxia tolerance. Based on our previous results that suggested mitochondrial expression of TASK-3 channels, we hypothesized that TASK-3 channels have roles in maintaining mitochondrial activity. In the present work we studied the effect of reduced TASK-3 expression on the mitochondrial function and survival of WM35 and A2058 melanoma cells. TASK-3 knockdown cells had depolarized mitochondrial membrane potential and contained a reduced amount of mitochondrial DNA. Compared to their scrambled shRNA-transfected counterparts, they demonstrated diminished responsiveness to the application of the mitochondrial uncoupler [(3-chlorophenyl)hydrazono]malononitrile (CCCP). These observations indicate impaired mitochondrial function. Further, TASK-3 knockdown cells presented reduced viability, decreased total DNA content, altered cell morphology, and reduced surface area. In contrast to non- and scrambled shRNA-transfected melanoma cell lines, which did not present noteworthy apoptotic activity, almost 50 % of the TASK-3 knockdown cells exhibited strong Annexin-V-specific immunofluorescence signal. Sequestration of cytochrome c from the mitochondria to the cytosol, increased caspase 3 activity, and translocation of the apoptosis-inducing factor from mitochondria to cell nuclei were also demonstrated in TASK-3 knockdown cells. Interference with TASK-3 channel expression, therefore, induces caspase-dependent and -independent apoptosis of melanoma cells, most likely via causing mitochondrial depolarization. Consequently, TASK-3 channels may be legitimate targets of future melanoma therapies.

The two-pore domain potassium channel KCNK5 deteriorates outcome in ischemic neurodegeneration

Potassium channels can fulfill both beneficial and detrimental roles in neuronal damage during ischemic stroke. Earlier studies have characterized a neuroprotective role of the two-pore domain potassium channels KCNK2 (TREK1) and KCNK3 (TASK1). Protective neuronal hyperpolarization and prevention of intracellular Ca(2+) overload and glutamate excitotoxicity were suggested to be the underlying mechanisms. We here identify an unexpected role for the related KCNK5 channel in a mouse model of transient middle cerebral artery occlusion (tMCAO). KCNK5 is strongly upregulated on neurons upon cerebral ischemia, where it is most likely involved in the induction of neuronal apoptosis. Hypoxic conditions elevated neuronal expression levels of KCNK5 in acute brain slices and primary isolated neuronal cell cultures. In agreement, KCNK5 knockout mice had significantly reduced infarct volumes and improved neurologic function 24 h after 60 min of tMCAO and this protective effect was preserved at later stages of infarct development. KCNK5 deficiency resulted in a significantly reduced number of apoptotic neurons, a downregulation of pro-apoptotic and upregulation of anti-apoptotic factors. Results of adoptive transfer experiments of wild-type and Kcnk5 (-/-) immune cells into Rag1 (-/-) mice prior to tMCAO exclude a major role of KCNK5 in poststroke inflammatory reactions. In summary, KCNK5 expression is induced on neurons under ischemic conditions where it most likely exerts pro-apoptotic effects. Hence, pharmacological blockade of KCNK5 might have therapeutic potential in preventing ischemic neurodegeneration.

Differential expression of two-pore domain potassium channels in rat cerebellar granule neurons

Two pore domain potassium (K2P) channels are mostly present in the central nervous system (CNS) where they play important roles in modulating neuronal excitability. K2P channels give rise to background K(+) currents (IKSO) a key component in setting and maintaining the resting membrane potential in excitable cells. Here, we studied the expression and relative abundances of K2P channels in cerebellar granule neurons (CGNs), combining molecular biology, electrophysiology and immunologic techniques. The CGN IKSO was very sensitive to external pH, as previously reported. Quantitative determination of mRNA expression level demonstrated the existence of an accumulation pattern of transcripts in CGN that encode K2P9>K2P1>K2P3>K2P18>K2P2=K2P10>K2P4>K2P5 subunits. The presence of the major K2P subunits expressed was then confirmed by Western blot and immunofluorescence analysis, demonstrating robust expression of K2P1 (TWIK-1), K2P3 (TASK-1), K2P9 (TASK-3) and K2P18 (TRESK) channel protein. Based, on these results, it is concluded that K2P1, -3, -9 and -18 subunits represent the majority component of IKSO current in CGN.

Tandem pore domain halothane-inhibited K+ channel subunits THIK1 and THIK2 assemble and form active channels

Despite a high level of sequence homology, tandem pore domain halothane-inhibited K(+) channel 1 (THIK1) produces background K(+) currents, whereas THIK2 is silent. This lack of activity is due to a unique combination of intracellular retention and weak basal activity in the plasma membrane. Here, we designed THIK subunits containing dominant negative mutations (THIK1(DN) and THIK2(DN)). THIK2(DN) mutant inhibits THIK1 currents, whereas THIK1(DN) inhibits an activated form of THIK2 (THIK2-A155P-I158D). In situ proximity ligation assays and Förster/fluorescence resonance energy transfer (FRET) experiments support a physical association between THIK1 and THIK2. Next, we expressed covalent tandems of THIK proteins to obtain expression of pure heterodimers. Td-THIK1-THIK2 (where Td indicates tandem) produces K(+) currents of amplitude similar to Td-THIK1-THIK1 but with a noticeable difference in the current kinetics. Unlike Td-THIK2-THIK2 that is mainly detected in the endoplasmic reticulum, Td-THIK1-THIK2 distributes at the plasma membrane, indicating that THIK1 can mask the endoplasmic reticulum retention/retrieval motif of THIK2. Kinetics and unitary conductance of Td-THIK1-THIK2 are intermediate between THIK1 and THIK2. Altogether, these results show that THIK1 and THIK2 form active heteromeric channels, further expanding the known repertoire of K(+) channels.

17β-estradiol downregulated the expression of TASK-1 channels in mouse neuroblastoma N2A cells

TASK channels, an acid-sensitive subgroup of two pore domain K⁺ (K2P) channels family, were widely expressed in a variety of neural tissues, and exhibited potent functions such as the regulation of membrane potential. The steroid hormone estrogen was able to interact with K⁺ channels, including voltage-gated K⁺ (Kv) and large conductance Ca²⁺-activated (BK) K⁺ channels, in different types of cells like cardiac myocytes and neurons. However, it is unclear about the effects of estrogen on TASK channels. In the present study, the expressions of two members of acid-sensitive TASK channels, TASK-1 and TASK-2, were detected in mouse neuroblastoma N2A cells by RT-PCR. Extracellular acidification (pH 6.4) weakly but statistically significantly inhibited the outward background current by 22.9 % at a holding potential of 0 mV, which inactive voltage-gated K⁺ currents, suggesting that there existed the functional TASK channels in the membrane of N2A cells. Although these currents were not altered by the acute application of 100 nM 17β-estradiol, incubation with 10 nM 17β-estradiol for 48 h reduced the mRNA level of TASK-1 channels by 40.4 % without any effect on TASK-2 channels. The proliferation rates of N2A cells were also increased by treatment with 10 nM 17β-estradiol for 48 h. These data implied that N2A cells expressed functional TASK channels and chronic exposure to 17β-estradiol downregulated the expression of TASK-1 channels and improved cell proliferation. The effect of 17β-estradiol on TASK-1 channels might be an alternative mechanism for the neuroprotective action of 17β-estradiol.

The roles of K(+) channels in cancer

Potassium channels are transmembrane proteins that selectively facilitate the flow of potassium ions down an electrochemical gradient. These molecules have been studied in great detail in the context of cell excitability, but their roles in less cell type-specific functions, such as cell proliferation, angiogenesis or cell migration, have only recently been assessed. Moreover, the importance of these channels for tumour biology has become evident. This, coupled with the fact that they are accessible proteins and that their pharmacology is well characterized, has increased the interest in investigating potassium channels as therapeutic targets in cancer patients.

Enhancement of TWIK-related acid-sensitive potassium channel 3 (TASK3) two-pore domain potassium channel activity by tumor necrosis factor α

TASK3 two-pore domain potassium (K2P) channels are responsible for native leak K channels in many cell types which regulate cell resting membrane potential and excitability. In addition, TASK3 channels contribute to the regulation of cellular potassium homeostasis. Because TASK3 channels are important for cell viability, having putative roles in both neuronal apoptosis and oncogenesis, we sought to determine their behavior under inflammatory conditions by investigating the effect of TNFα on TASK3 channel current. TASK3 channels were expressed in tsA-201 cells, and the current through them was measured using whole cell voltage clamp recordings. We show that THP-1 human myeloid leukemia monocytes, co-cultured with hTASK3-transfected tsA-201 cells, can be activated by the specific Toll-like receptor 7/8 activator, R848, to release TNFα that subsequently enhances hTASK3 current. Both hTASK3 and mTASK3 channel activity is increased by incubation with recombinant TNFα (10 ng/ml for 2-15 h), but other K2P channels (hTASK1, hTASK2, hTREK1, and hTRESK) are unaffected. This enhancement by TNFα is not due to alterations in levels of channel expression at the membrane but rather to an alteration in channel gating. The enhancement by TNFα can be blocked by extracellular acidification but persists for mutated TASK3 (H98A) channels that are no longer acid-sensitive even in an acidic extracellular environment. TNFα action on TASK3 channels is mediated through the intracellular C terminus of the channel. Furthermore, it occurs through the ASK1 pathway and is JNK- and p38-dependent. In combination, TNFα activation and TASK3 channel activity can promote cellular apoptosis.

Voltage-gated potassium channels at the crossroads of neuronal function, ischemic tolerance, and neurodegeneration

Voltage-gated potassium (Kv) channels are widely expressed in the central and peripheral nervous system and are crucial mediators of neuronal excitability. Importantly, these channels also actively participate in cellular and molecular signaling pathways that regulate the life and death of neurons. Injury-mediated increased K(+) efflux through Kv2.1 channels promotes neuronal apoptosis, contributing to widespread neuronal loss in neurodegenerative disorders such as Alzheimer's disease and stroke. In contrast, some forms of neuronal activity can dramatically alter Kv2.1 channel phosphorylation levels and influence their localization. These changes are normally accompanied by modifications in channel voltage dependence, which may be neuroprotective within the context of ischemic injury. Kv1 and Kv7 channel dysfunction leads to neuronal hyperexcitability that critically contributes to the pathophysiology of human clinical disorders such as episodic ataxia and epilepsy. This review summarizes the neurotoxic, neuroprotective, and neuroregulatory roles of Kv channels and highlights the consequences of Kv channel dysfunction on neuronal physiology. The studies described in this review thus underscore the importance of normal Kv channel function in neurons and emphasize the therapeutic potential of targeting Kv channels in the treatment of a wide range of neurological diseases.

Novel TASK channels inhibitors derived from dihydropyrrolo[2,1-a]isoquinoline

TASK channels belong to the family of K(+) channels with 4 transmembrane segments and 2 pore domains (4TM/2P) per subunit. These channels have been related to apoptosis in cerebellar granule neurons (CGN), as well as cancer in other tissues. TASK current is regulated by hormones, neurotransmitters, anesthetics and divalent cations, which are not selective. Recently, there has been found some organic compounds that inhibit TASK current selectively. In order to find other modulators, we report here a group of five dihydropyrrolo[2,1-a]isoquinolines (DPIs), four of them with putative anticancer activity, that were evaluated on TASK-1 and TASK-3 channels. The compounds 1, 2 and 3 showed IC50 < 320 μM on TASK-1 and TASK-3, intermediate activity on TASK-1/TASK-3 heterodimer, moderate effect over hslo and TREK-1 (500 μM), and practically not inhibition on Shaker-IR, herg and IRK2.1 potassium channels, when they were expressed heterologously in Xenopus laevis oocytes. In rat CGN, 500 μM of these three compounds induced a decrement by >39% of the TASK-carried leak current. Finally, only compound 1 showed significant protection (∼36%) against apoptotic death of CGN induced by K(+) deprivation. These results suggest that DPI compounds could be potential candidates for designing new selective inhibitors of TASK channels.

Altered expression of two-pore domain potassium (K2P) channels in cancer

Potassium channels have become a focus in cancer biology as they play roles in cell behaviours associated with cancer progression, including proliferation, migration and apoptosis. Two-pore domain (K_2P) potassium channels are background channels which enable the leak of potassium ions from cells. As these channels are open at rest they have a profound effect on cellular membrane potential and subsequently the electrical activity and behaviour of cells in which they are expressed. The K_2P family of channels has 15 mammalian members and already 4 members of this family (K_2P2.1, K_2P3.1, K_2P9.1, K_2P5.1) have been implicated in cancer. Here we examine the expression of all 15 members of the K_2P family of channels in a range of cancer types. This was achieved using the online cancer microarray database, Oncomine (www.oncomine.org). Each gene was examined across 20 cancer types, comparing mRNA expression in cancer to normal tissue. This analysis revealed all but 3 K_2P family members (K_2P4.1, K_2P16.1, K_2P18.1) show altered expression in cancer. Overexpression of K_2P channels was observed in a range of cancers including breast, leukaemia and lung while more cancers (brain, colorectal, gastrointestinal, kidney, lung, melanoma, oesophageal) showed underexpression of one or more channels. K_2P1.1, K_2P3.1, K_2P12.1, were overexpressed in a range of cancers. While K_2P1.1, K_2P3.1, K_2P5.1, K_2P6.1, K_2P7.1 and K_2P10.1 showed significant underexpression across the cancer types examined. This analysis supports the view that specific K_2P channels may play a role in cancer biology. Their altered expression together with their ability to impact the function of other ion channels and their sensitivity to environmental stimuli (pO2, pH, glucose, stretch) makes understanding the role these channels play in cancer of key importance.

Acid sensitive background potassium channels K2P3.1 and K2P9.1 undergo rapid dynamin-dependent endocytosis

Acid-sensitive, two-pore domain potassium channels, K(2P)3.1 and K(2P)9.1, are implicated in cardiac and nervous tissue responses to hormones, neurotransmitters and drugs. K(2P)3.1 and K(2P)9.1 leak potassium from the cell at rest and directly impact membrane potential. Hence altering channel number on the cell surface drives changes in cellular electrical properties. The rate of K(2P)3.1 and K(2P)9.1 delivery to and recovery from the plasma membrane determines both channel number at the cell surface and potassium leak from cells. This study examines the endocytosis of K(2P)3.1 and K(2P)9.1. Plasma membrane biotinylation was used to follow the fate of internalized GFP-tagged rat K(2P)3.1 and K(2P)9.1 transiently expressed in HeLa cells. Confocal fluorescence images were analyzed using Imaris software, which revealed that both channels are endocytosed by a dynamin-dependent mechanism and over the course of 60 min, move progressively toward the nucleus. Endogenous endocytosis of human K(2P)3.1 and K(2P)9.1 was examined in the lung carcinoma cell line, A549. Endogenous channels are endocytosed over a similar time-scale to the channels expressed transiently in HeLa cells. These findings both validate the use of recombinant systems and identify an endogenous model system in which K(2P)3.1 and K(2P)9.1 trafficking can be further studied.

TASK-1 channels in oligodendrocytes: a role in ischemia mediated disruption

Oligodendrocytes are the myelinating cells of the CNS and, like neurons, are highly sensitive to ischemic damage. However, the mechanisms underlying cytotoxicity in oligodendrocytes during hypoxic/ischemic episodes are not fully understood. TASK-1 is a K(+) leak channel that mediates hypoxic depolarisation in neurons. The expression and function of TASK-1 in oligodendrocytes had not previously been addressed. In this study, we investigate the expression of TASK-1 in oligodendrocytes and its role in white matter ischemic damage. Expression of TASK-1 in oligodendrocytes was investigated in the mouse brain using immunostaining. TASK-1 channel function was identified by established pharmacological and electrophysiological strategies, using the whole-cell patch clamp technique in cell cultures of oligodendrocytes from the optic nerve, a typical white matter tract. The role of TASK-1 in hypoxia was examined in isolated intact optic nerves subjected to oxygen glucose deprivation (OGD). Oligodendrocytes are strongly immunopositive for TASK-1 throughout the brain. Patch-clamp identified functional TASK-1-like leak currents in oligodendrocytes using two recognised means of inhibiting TASK-1, decreasing extracellular pH to 6.4 and exposure to the TASK-1 selective inhibitor anandamide. Incubation of optic nerves with methanandamide, a non-hydrolysable form of anandamide, significantly protected oligodendrocytes against hypoxic disruption and death in OGD. Our data demonstrate for the first time that oligodendrocytes express functional TASK-1 channels and provide compelling evidence they contribute to oligodendrocyte damage in hypoxia. Since oligodendrocyte damage is a key factor in ischemic episodes, TASK-1 may provide a potential therapeutic target in stroke and white matter disease.