TASK1 (K(2P)3.1) K(+) channel inhibition by endothelin-1 is mediated through Rho kinase-dependent phosphorylation

Lysophosphatidic Acid and Several Neurotransmitters Converge on Rho-Kinase 2 Signaling to Manage Motoneuron Excitability

Intrinsic membrane excitability (IME) sets up neuronal responsiveness to synaptic drive. Several neurotransmitters and neuromodulators, acting through G-protein-coupled receptors (GPCRs), fine-tune motoneuron (MN) IME by modulating background K⁺ channels TASK1. However, intracellular partners linking GPCRs to TASK1 modulation are not yet well-known. We hypothesized that isoform 2 of rho-kinase (ROCK2), acting as downstream GPCRs, mediates adjustment of MN IME via TASK1. Electrophysiological recordings were performed in hypoglossal MNs (HMNs) obtained from adult and neonatal rats, neonatal knockout mice for TASK1 (task1^–/–) and TASK3 (task3^–/–, the another highly expressed TASK subunit in MNs), and primary cultures of embryonic spinal cord MNs (SMNs). Small-interfering RNA (siRNA) technology was also used to knockdown either ROCK1 or ROCK2. Furthermore, ROCK activity assays were performed to evaluate the ability of various physiological GPCR ligands to stimulate ROCK. Microiontophoretically applied H1152, a ROCK inhibitor, and siRNA-induced ROCK2 knockdown both depressed AMPAergic, inspiratory-related discharge activity of adult HMNs in vivo, which mainly express the ROCK2 isoform. In brainstem slices, intracellular constitutively active ROCK2 (aROCK2) led to H1152-sensitive HMN hyper-excitability. The aROCK2 inhibited pH-sensitive and TASK1-mediated currents in SMNs. Conclusively, aROCK2 increased IME in task3^–/–, but not in task1^–/– HMNs. MN IME was also augmented by the physiological neuromodulator lysophosphatidic acid (LPA) through a mechanism entailing G_αi/o-protein stimulation, ROCK2, but not ROCK1, activity and TASK1 inhibition. Finally, two neurotransmitters, TRH, and 5-HT, which are both known to increase MN IME by TASK1 inhibition, stimulated ROCK2, and depressed background resting currents via G_αq/ROCK2 signaling. These outcomes suggest that LPA and several neurotransmitters impact MN IME via G_αi/o/G_αq-protein-coupled receptors, downstream ROCK2 activation, and subsequent inhibition of TASK1 channels.

Ion channels as convergence points in the pathology of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a fatal disease of the cardiopulmonary system that lacks curative treatments. The main pathological event in PAH is elevated vascular resistance in the pulmonary circulation, caused by abnormal vasoconstriction and vascular remodelling. Ion channels are key determinants of vascular smooth muscle tone and homeostasis, and four PAH channelopathies (KCNK3, ABCC8, KCNA5, TRPC6) have been identified so far. However, the contribution of ion channels in other forms of PAH, which account for the majority of PAH patients, has been less well characterised. Here we reason that a variety of triggers of PAH (e.g. BMPR2 mutations, hypoxia, anorectic drugs) that impact channel function may contribute to the onset of the disease. We review the molecular mechanisms by which these ‘extrinsic’ factors converge on ion channels and provoke their dysregulation to promote the development of PAH. Ion channels of the pulmonary vasculature are therefore promising therapeutic targets because of the modulation they provide to both vasomotor tone and proliferation of arterial smooth muscle cells.

Two-Pore Domain Potassium Channel in Neurological Disorders

K2P channel is the leaky potassium channel that is critical to keep up the negative resting membrane potential for legitimate electrical conductivity of the excitable tissues. Recently, many substances and medication elements are discovered that could either straightforwardly or in a roundabout way influence the 15 distinctive K⁺ ion channels including TWIK, TREK, TASK, TALK, THIK, and TRESK. Opening and shutting of these channels or any adjustment in their conduct is thought to alter the pathophysiological condition of CNS. There is no document available till now to explain in detail about the molecular mechanism of agents acting on K2P channel. Accordingly, in this review we cover the current research and mechanism of action of these channels, we have also tried to mention the detailed effect of drugs and how the channel behavior changes by focusing on recent advances regarding activation and modulation of ion channels.

Current Drug Treatment Strategies for Atrial Fibrillation and TASK-1 Inhibition as an Emerging Novel Therapy Option

Atrial fibrillation (AF) is the most common sustained arrhythmia with a prevalence of up to 4% and an upwards trend due to demographic changes. It is associated with an increase in mortality and stroke incidences. While stroke risk can be significantly reduced through anticoagulant therapy, adequate treatment of other AF related symptoms remains an unmet medical need in many cases. Two main treatment strategies are available: rate control that modulates ventricular heart rate and prevents tachymyopathy as well as rhythm control that aims to restore and sustain sinus rhythm. Rate control can be achieved through drugs or ablation of the atrioventricular node, rendering the patient pacemaker-dependent. For rhythm control electrical cardioversion and pharmacological cardioversion can be used. While electrical cardioversion requires fasting and sedation of the patient, antiarrhythmic drugs have other limitations. Most antiarrhythmic drugs carry a risk for pro-arrhythmic effects and are contraindicated in patients with structural heart diseases. Furthermore, catheter ablation of pulmonary veins can be performed with its risk of intraprocedural complications and varying success. In recent years TASK-1 has been introduced as a new target for AF therapy. Upregulation of TASK-1 in AF patients contributes to prolongation of the action potential duration. In a porcine model of AF, TASK-1 inhibition by gene therapy or pharmacological compounds induced cardioversion to sinus rhythm. The DOxapram Conversion TO Sinus rhythm (DOCTOS)-Trial will reveal whether doxapram, a potent TASK-1 inhibitor, can be used for acute cardioversion of persistent and paroxysmal AF in patients, potentially leading to a new treatment option for AF.

Interfering with lysophosphatidic acid receptor edg2/lpa1 signalling slows down disease progression in SOD1-G93A transgenic mice

AIMS

Alterations in excitability represent an early hallmark in Amyotrophic Lateral Sclerosis (ALS). Therefore, deciphering the factors that impact motor neuron (MN) excitability offers an opportunity to uncover further aetiopathogenic mechanisms, neuroprotective agents, therapeutic targets, and/or biomarkers in ALS. Here, we hypothesised that the lipokine lysophosphatidic acid (lpa) regulates MN excitability via the G-protein-coupled receptor lpa₁ . Then, modulating lpa₁ -mediated signalling might affect disease progression in the ALS SOD1-G93A mouse model.

METHODS

The influence of lpa-lpa₁ signalling on the electrical properties, Ca²⁺ dynamic and survival of MNs was tested in vitro. Expression of lpa₁ in cultured MNs and in the spinal cord of SOD1-G93A mice was analysed. ALS mice were chronically treated with a small-interfering RNA against lpa₁ (siRNA_lpa1 ) or with the lpa₁ inhibitor AM095. Motor skills, MN loss, and lifespan were evaluated.

RESULTS

AM095 reduced MN excitability. Conversely, exogenous lpa increased MN excitability by modulating task1 'leak' potassium channels downstream of lpa₁ . Lpa-lpa₁ signalling evoked an excitotoxic response in MNs via voltage-sensitive calcium channels. Cultured SOD1-G93A MNs displayed lpa₁ upregulation and heightened vulnerability to lpa. In transgenic mice, lpa₁ was upregulated mostly in spinal cord MNs before cell loss. Chronic administration of either siRNA_lpa1 or AM095 reduced lpa₁ expression at least in MNs, delayed MN death, improved motor skills, and prolonged life expectancy of ALS mice.

CONCLUSIONS

These results suggest that stressed lpa-lpa₁ signalling contributes to MN degeneration in SOD1-G93A mice. Consequently, disrupting lpa₁ slows down disease progression. This highlights LPA₁ signalling as a potential target and/or biomarker in ALS.

Implication of Potassium Channels in the Pathophysiology of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a rare and severe cardiopulmonary disease without curative treatments. PAH is a multifactorial disease that involves genetic predisposition, epigenetic factors, and environmental factors (drugs, toxins, viruses, hypoxia, and inflammation), which contribute to the initiation or development of irreversible remodeling of the pulmonary vessels. The recent identification of loss-of-function mutations in KCNK3 (KCNK3 or TASK-1) and ABCC8 (SUR1), or gain-of-function mutations in ABCC9 (SUR2), as well as polymorphisms in KCNA5 (Kv1.5), which encode two potassium (K⁺) channels and two K⁺ channel regulatory subunits, has revived the interest of ion channels in PAH. This review focuses on KCNK3, SUR1, SUR2, and Kv1.5 channels in pulmonary vasculature and discusses their pathophysiological contribution to and therapeutic potential in PAH.

Inhibition of RELM-β prevents hypoxia-induced overproliferation of human pulmonary artery smooth muscle cells by reversing PLC-mediated KCNK3 decline

AIMS

Although resistin-like molecule β (RELM-β) is involved in the pathological processes of various lung diseases, such as pulmonary inflammation, asthma and fibrosis, its potential roles in hypoxic pulmonary arterial hypertension (PAH) remain largely unknown. The study aims to investigate whether RELM-β contributes to hypoxia-induced excessive proliferation of human pulmonary artery smooth muscle cells (PASMCs) and to explore the potential mechanisms of this process.

MAIN METHODS

Human PASMCs were exposed to normoxia or hypoxia (1% O₂) for 24 h. siRNA targeting RELM-β was transfected into cells. Protein levels of KCNK3, RELM-β, pSTAT3 and STAT3 were determined by immunoblotting. The translocation of NFATc2 and expression of KCNK3 were visualized by immunofluorescence. 5-ethynyl-2'-deoxyuridine assays and cell counting kit-8 assays were performed to assess the proliferation of PASMCs.

KEY FINDINGS

(1) Chronic hypoxia significantly decreased KCNK3 protein levels while upregulating RELM-β protein levels in human PASMCs, which was accompanied by excessive proliferation of cells. (2) RELM-β could promote human PASMCs proliferation and activate the STAT3/NFAT axis by downregulating KCNK3 protein under normoxia. (3) Inhibition of RELM-β expression effectively prevented KCNK3-mediated cell proliferation under hypoxia. (4) Phospholipase C (PLC) inhibitor U-73122 could not only prevent the hypoxia/RELM-β-induced decrease in KCNK3 protein, but also inhibit the enhanced cell viability caused by hypoxia/RELM-β. (5) Both hypoxia and RELM-β could downregulate membrane KCNK3 protein levels by enhancing endocytosis.

SIGNIFICANCE

RELM-β activation is responsible for hypoxia-induced excessive proliferation of human PASMCs. Interfering with RELM-β may alleviate the progression of hypoxic PAH by upregulating PLC-dependent KCNK3 expression.

TASK-1 and TASK-3 channels modulate pressure overload-induced cardiac remodeling and dysfunction

Tandem pore domain acid-sensitive K⁺ (TASK) channels are present in cardiac tissue; however, their contribution to cardiac pathophysiology is not well understood. Here, we investigate the role of TASK-1 and TASK-3 in the pathogenesis of cardiac dysfunction using both human tissue and mouse models of genetic TASK channel loss of function. Compared with normal human cardiac tissue, TASK-1 gene expression is reduced in association with either cardiac hypertrophy alone or combined cardiac hypertrophy and heart failure. In a pressure overload cardiomyopathy model, TASK-1 global knockout (TASK-1 KO) mice have both reduced cardiac hypertrophy and preserved cardiac function compared with wild-type mice. In contrast to the TASK-1 KO mouse pressure overload response, TASK-3 global knockout (TASK-3 KO) mice develop cardiac hypertrophy and a delayed onset of cardiac dysfunction compared with wild-type mice. The cardioprotective effects observed in TASK-1 KO mice are associated with pressure overload-induced augmentation of AKT phosphorylation and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) expression, with consequent augmentation of cardiac energetics and fatty acid oxidation. The protective effects of TASK-1 loss of function are associated with an enhancement of physiologic hypertrophic signaling and preserved metabolic functions. These findings may provide a rationale for TASK-1 channel inhibition in the treatment of cardiac dysfunction.NEW & NOTEWORTHY The role of tandem pore domain acid-sensitive K⁺ (TASK) channels in cardiac function is not well understood. This study demonstrates that TASK channel gene expression is associated with the onset of human cardiac hypertrophy and heart failure. TASK-1 and TASK-3 strongly affect the development of pressure overload cardiomyopathies in genetic models of TASK-1 and TASK-3 loss of function. The effects of TASK-1 loss of function were associated with enhanced AKT phosphorylation and expression of peroxisome proliferator-activated receptor-γ coactivator-1 (PGC-1) transcription factor. These data suggest that TASK channels influence the development of cardiac hypertrophy and dysfunction in response to injury.

Sp1-regulated expression of p11 contributes to motor neuron degeneration by membrane insertion of TASK1

Disruption in membrane excitability contributes to malfunction and differential vulnerability of specific neuronal subpopulations in a number of neurological diseases. The adaptor protein p11, and background potassium channel TASK1, have overlapping distributions in the CNS. Here, we report that the transcription factor Sp1 controls p11 expression, which impacts on excitability by hampering functional expression of TASK1. In the SOD1-G93A mouse model of ALS, Sp1-p11-TASK1 dysregulation contributes to increased excitability and vulnerability of motor neurons. Interference with either Sp1 or p11 is neuroprotective, delaying neuron loss and prolonging lifespan in this model. Nitrosative stress, a potential factor in human neurodegeneration, stimulated Sp1 expression and human p11 promoter activity, at least in part, through a Sp1-binding site. Disruption of Sp1 or p11 also has neuroprotective effects in a traumatic model of motor neuron degeneration. Together our work suggests the Sp1-p11-TASK1 pathway is a potential target for treatment of degeneration of motor neurons.

The adaptor protein p11 and K+ channel TASK1 have overlapping distributions in the CNS. Here, the authors demonstrate that the transcription factor Sp1 regulates p11 levels, which in turn affects intrinsic membrane properties and can contribute to degeneration of motor neurons in disease and injury models.

Characterization and regulation of wild-type and mutant TASK-1 two pore domain potassium channels indicated in pulmonary arterial hypertension

Key points

The TASK‐1 channel gene (KCNK3) has been identified as a possible disease‐causing gene in heritable pulmonary arterial hypertension (PAH).

In the present study, we show that novel mutated TASK‐1 channels, seen in PAH patients, have a substantially reduced current compared to wild‐type TASK‐1 channels.

These mutated TASK‐1 channels are located at the plasma membrane to the same degree as wild‐type TASK‐1 channels.

ONO‐RS‐082 and alkaline pH 8.4 both activate TASK‐1 channels but do not recover current through mutant TASK‐1 channels.

We show that the guanylate cyclase activator, riociguat, a novel treatment for PAH, enhances current through TASK‐1 channels but does not recover current through mutant TASK‐1 channels.

Abstract

Pulmonary arterial hypertension (PAH) affects ∼15–50 people per million. KCNK3, the gene that encodes the two pore domain potassium channel TASK‐1 (K2P3.1), has been identified as a possible disease‐causing gene in heritable PAH. Recently, two new mutations have been identified in KCNK3 in PAH patients: G106R and L214R. The present study aimed to characterize the functional properties and regulation of wild‐type (WT) and mutated TASK‐1 channels and determine how these might contribute to PAH and its treatment. Currents through WT and mutated human TASK‐1 channels transiently expressed in tsA201 cells were measured using whole‐cell patch clamp electrophysiology. Localization of fluorescence‐tagged channels was visualized using confocal microscopy and quantified with in‐cell and on‐cell westerns. G106R or L214R mutated channels were located at the plasma membrane to the same degree as WT channels; however, their current was markedly reduced compared to WT TASK‐1 channels. Functional current through these mutated channels could not be restored using activators of WT TASK‐1 channels (pH 8.4, ONO‐RS‐082). The guanylate cyclase activator, riociguat, enhanced current through WT TASK‐1 channels; however, similar to the other activators investigated, riociguat did not have any effect on current through mutated TASK‐1 channels. Thus, novel mutations in TASK‐1 seen in PAH substantially alter the functional properties of these channels. Current through these channels could not be restored by activators of TASK‐1 channels. Riociguat enhancement of current through TASK‐1 channels could contribute to its therapeutic benefit in the treatment of PAH.

The TASK‐1 channel gene (KCNK3) has been identified as a possible disease‐causing gene in heritable pulmonary arterial hypertension (PAH).

In the present study, we show that novel mutated TASK‐1 channels, seen in PAH patients, have a substantially reduced current compared to wild‐type TASK‐1 channels.

These mutated TASK‐1 channels are located at the plasma membrane to the same degree as wild‐type TASK‐1 channels.

ONO‐RS‐082 and alkaline pH 8.4 both activate TASK‐1 channels but do not recover current through mutant TASK‐1 channels.

We show that the guanylate cyclase activator, riociguat, a novel treatment for PAH, enhances current through TASK‐1 channels but does not recover current through mutant TASK‐1 channels.

TASK-1 (KCNK3) channels in the lung: from cell biology to clinical implications

TWIK-related acid-sensitive potassium channel 1 (TASK-1 encoded by KCNK3) belongs to the family of two-pore domain potassium channels. This gene subfamily is constitutively active at physiological resting membrane potentials in excitable cells, including smooth muscle cells, and has been particularly linked to the human pulmonary circulation. TASK-1 channels are sensitive to a wide array of physiological and pharmacological mediators that affect their activity such as unsaturated fatty acids, extracellular pH, hypoxia, anaesthetics and intracellular signalling pathways. Recent studies show that modulation of TASK-1 channels, either directly or indirectly by targeting their regulatory mechanisms, has the potential to control pulmonary arterial tone in humans. Furthermore, mutations in KCNK3 have been identified as a rare cause of both familial and idiopathic pulmonary arterial hypertension. This review summarises our current state of knowledge of the functional role of TASK-1 channels in the pulmonary circulation in health and disease, with special emphasis on current advancements in the field.

Endothelin-1 Decreases Excitability of the Dorsal Root Ganglion Neurons via ETB Receptor

Endothelin-1 (ET-1) has been demonstrated to be a pro-nociceptive as well as an anti-nociceptive agent. However, underlying molecular mechanisms for these pain modulatory actions remain unclear. In the present study, we evaluated the ability of ET-1 to alter the nociceptor excitability using a patch clamp technique in acutely dissociated rat dorsal root ganglion (DRG) neurons. ET-1 produced an increase in threshold current to evoke an action potential (I _threshold) and hyperpolarization of resting membrane potential (RMP) indicating decreased excitability of DRG neurons. I _threshold increased from 0.25 ± 0.08 to 0.33 ± 0.07 nA and hyperpolarized RMP from -57.51 ± 1.70 to -67.41 ± 2.92 mV by ET-1 (100 nM). The hyperpolarizing effect of ET-1 appears to be orchestrated via modulation of membrane conductances, namely voltage-gated sodium current (I _Na) and outward transient potassium current (I _KT). ET-1, 30 and 100 nM, decreased the peak I _Na by 41.3 ± 6.8 and 74 ± 15.2%, respectively. Additionally, ET-1 (100 nM) significantly potentiated the transient component (I _KT) of the potassium currents. ET-1-induced effects were largely attenuated by BQ-788, a selective ET_BR blocker. However, a selective ET_AR blocker BQ-123 did not alter the effects of ET-1. A selective ET_BR agonist, IRL-1620, mimicked the effect of ET-1 on I _Na in a concentration-dependent manner (IC₅₀ 159.5 ± 92.6 μM). In conclusion, our results demonstrate that ET-1 hyperpolarizes nociceptors by blocking I _Na and potentiating I _KT through selective activation of ET_BR, which may represent one of the underlying mechanisms for reported anti-nociceptive effects of ET-1.

Stretch-activated two-pore-domain (K2P) potassium channels in the heart: Focus on atrial fibrillation and heart failure

Two-pore-domain potassium (K_2P) channels modulate cellular excitability. The significance of stretch-activated cardiac K_2P channels (K_2P2.1, TREK-1, KCNK2; K_2P4.1, TRAAK, KCNK4; K_2P10.1, TREK-2, KCNK10) in heart disease has not been elucidated in detail. The aim of this work was to assess expression and remodeling of mechanosensitive K_2P channels in atrial fibrillation (AF) and heart failure (HF) patients in comparison to murine models. Cardiac K_2P channel levels were quantified in atrial (A) and ventricular (V) tissue obtained from patients undergoing open heart surgery. In addition, control mice and mouse models of AF (cAMP-response element modulator (CREM)-IbΔC-X transgenic animals) or HF (cardiac dysfunction induced by transverse aortic constriction, TAC) were employed. Human and murine KCNK2 displayed highest mRNA abundance among mechanosensitive members of the K_2P channel family (V > A). Disease-associated K_2P2.1 remodeling was studied in detail. In patients with impaired left ventricular function, atrial KCNK2 (K_2P2.1) mRNA and protein expression was significantly reduced. In AF subjects, downregulation of atrial and ventricular KCNK2 (K_2P2.1) mRNA and protein levels was observed. AF-associated suppression of atrial Kcnk2 (K_2P2.1) mRNA and protein was recapitulated in CREM-transgenic mice. Ventricular Kcnk2 expression was not significantly altered in mouse models of disease. In conclusion, mechanosensitive K_2P2.1 and K_2P10.1 K⁺ channels are expressed throughout the heart. HF- and AF-associated downregulation of KCNK2 (K_2P2.1) mRNA and protein levels suggest a mechanistic contribution to cardiac arrhythmogenesis.

Therapeutic targeting of two-pore-domain potassium (K(2P)) channels in the cardiovascular system

The improvement of treatment strategies in cardiovascular medicine is an ongoing process that requires constant optimization. The ability of a therapeutic intervention to prevent cardiovascular pathology largely depends on its capacity to suppress the underlying mechanisms. Attenuation or reversal of disease-specific pathways has emerged as a promising paradigm, providing a mechanistic rationale for patient-tailored therapy. Two-pore-domain K(+) (K(2P)) channels conduct outward K(+) currents that stabilize the resting membrane potential and facilitate action potential repolarization. K(2P) expression in the cardiovascular system and polymodal K2P current regulation suggest functional significance and potential therapeutic roles of the channels. Recent work has focused primarily on K(2P)1.1 [tandem of pore domains in a weak inwardly rectifying K(+) channel (TWIK)-1], K(2P)2.1 [TWIK-related K(+) channel (TREK)-1], and K(2P)3.1 [TWIK-related acid-sensitive K(+) channel (TASK)-1] channels and their role in heart and vessels. K(2P) currents have been implicated in atrial and ventricular arrhythmogenesis and in setting the vascular tone. Furthermore, the association of genetic alterations in K(2P)3.1 channels with atrial fibrillation, cardiac conduction disorders and pulmonary arterial hypertension demonstrates the relevance of the channels in cardiovascular disease. The function, regulation and clinical significance of cardiovascular K(2P) channels are summarized in the present review, and therapeutic options are emphasized.

Potassium Channel Subfamily K Member 3 (KCNK3) Contributes to the Development of Pulmonary Arterial Hypertension

BACKGROUND

Mutations in the KCNK3 gene have been identified in some patients suffering from heritable pulmonary arterial hypertension (PAH). KCNK3 encodes an outward rectifier K(+) channel, and each identified mutation leads to a loss of function. However, the pathophysiological role of potassium channel subfamily K member 3 (KCNK3) in PAH is unclear. We hypothesized that loss of function of KCNK3 is a hallmark of idiopathic and heritable PAH and contributes to dysfunction of pulmonary artery smooth muscle cells and pulmonary artery endothelial cells, leading to pulmonary artery remodeling: consequently, restoring KCNK3 function could alleviate experimental pulmonary hypertension (PH).

METHODS AND RESULTS

We demonstrated that KCNK3 expression and function were reduced in human PAH and in monocrotaline-induced PH in rats. Using a patch-clamp technique in freshly isolated (not cultured) pulmonary artery smooth muscle cells and pulmonary artery endothelial cells, we found that KCNK3 current decreased progressively during the development of monocrotaline-induced PH and correlated with plasma-membrane depolarization. We demonstrated that KCNK3 modulated pulmonary arterial tone. Long-term inhibition of KCNK3 in rats induced distal neomuscularization and early hemodynamic signs of PH, which were related to exaggerated proliferation of pulmonary artery endothelial cells, pulmonary artery smooth muscle cell, adventitial fibroblasts, and pulmonary and systemic inflammation. Lastly, in vivo pharmacological activation of KCNK3 significantly reversed monocrotaline-induced PH in rats.

CONCLUSIONS

In PAH and experimental PH, KCNK3 expression and activity are strongly reduced in pulmonary artery smooth muscle cells and endothelial cells. KCNK3 inhibition promoted increased proliferation, vasoconstriction, and inflammation. In vivo pharmacological activation of KCNK3 alleviated monocrotaline-induced PH, thus demonstrating that loss of KCNK3 is a key event in PAH pathogenesis and thus could be therapeutically targeted.

Upregulation of K(2P)3.1 K+ Current Causes Action Potential Shortening in Patients With Chronic Atrial Fibrillation

BACKGROUND

Antiarrhythmic management of atrial fibrillation (AF) remains a major clinical challenge. Mechanism-based approaches to AF therapy are sought to increase effectiveness and to provide individualized patient care. K(2P)3.1 (TASK-1 [tandem of P domains in a weak inward-rectifying K+ channel-related acid-sensitive K+ channel-1]) 2-pore-domain K+ (K(2P)) channels have been implicated in action potential regulation in animal models. However, their role in the pathophysiology and treatment of paroxysmal and chronic patients with AF is unknown.

METHODS AND RESULTS

Right and left atrial tissue was obtained from patients with paroxysmal or chronic AF and from control subjects in sinus rhythm. Ion channel expression was analyzed by quantitative real-time polymerase chain reaction and Western blot. Membrane currents and action potentials were recorded using voltage- and current-clamp techniques. K(2P)3.1 subunits exhibited predominantly atrial expression, and atrial K(2P)3.1 transcript levels were highest among functional K(2P) channels. K(2P)3.1 mRNA and protein levels were increased in chronic AF. Enhancement of corresponding currents in the right atrium resulted in shortened action potential duration at 90% of repolarization (APD90) compared with patients in sinus rhythm. In contrast, K(2P)3.1 expression was not significantly affected in subjects with paroxysmal AF. Pharmacological K(2P)3.1 inhibition prolonged APD90 in atrial myocytes from patients with chronic AF to values observed among control subjects in sinus rhythm.

CONCLUSIONS

Enhancement of atrium-selective K(2P)3.1 currents contributes to APD shortening in patients with chronic AF, and K(2P)3.1 channel inhibition reverses AF-related APD shortening. These results highlight the potential of K(2P)3.1 as a novel drug target for mechanism-based AF therapy.

"Disease modifying nutricals" for multiple sclerosis

The association between vitamin D and multiple sclerosis has (re)-opened new interest in nutrition and natural compounds in the prevention and treatment of this neuroinflammatory disease. The dietary amount and type of fat, probiotics and biologicals, salmon proteoglycans, phytoestrogens and protease inhibitor of soy, sodium chloride and trace elements, and fat soluble vitamins including D, A and E were all considered as disease-modifying nutraceuticals. Studies in experimental autoimmune encephalomyelitis mice suggest that poly-unsaturated fatty acids and their 'inflammation-resolving' metabolites and the gut microflora may reduce auto-aggressive immune cells and reduce progression or risk of relapse, and infection with whipworm eggs may positively change the gut-brain communication. Encouraged by the recent interest in multiple sclerosis-nutrition nature's pharmacy has been searched for novel compounds with anti-inflammatory, immune-modifying and antioxidative properties, the most interesting being the scorpion toxins that inhibit specific potassium channels of T cells and antioxidative compounds including the green tea flavonoid epigallocatechin-3-gallate, curcumin and the mustard oil glycoside from e.g. broccoli and sulforaphane. They mostly also inhibit pro-inflammatory signaling through NF-κB or toll-like receptors and stabilize the blood brain barrier. Disease modifying functions may also complement analgesic and anti-spastic effects of cannabis, its constituents, and of 'endocannabinoid enhancing' drugs or nutricals like inhibitors of fatty acid amide hydrolase. Nutricals will not solve multiple sclerosis therapeutic challenges but possibly support pharmacological interventions or unearth novel structures.

Modulation of K2P 2.1 and K2P 10.1 K(+) channel sensitivity to carvedilol by alternative mRNA translation initiation

BACKGROUND AND PURPOSE

The β-receptor antagonist carvedilol blocks a range of ion channels. K2P 2.1 (TREK1) and K2P 10.1 (TREK2) channels are expressed in the heart and regulated by alternative translation initiation (ATI) of their mRNA, producing functionally distinct channel variants. The first objective was to investigate acute effects of carvedilol on human K2P 2.1 and K2P 10.1 channels. Second, we sought to study ATI-dependent modulation of K2P K(+) current sensitivity to carvedilol.

EXPERIMENTAL APPROACH

Using standard electrophysiological techniques, we recorded currents from wild-type and mutant K2P 2.1 and K2P 10.1 channels in Xenopus oocytes and HEK 293 cells.

KEY RESULTS

Carvedilol concentration-dependently inhibited K2P 2.1 channels (IC50 ,oocytes = 20.3 μM; IC50 , HEK = 1.6 μM) and this inhibition was frequency-independent. When K2P 2.1 isoforms generated by ATI were studied separately in oocytes, the IC50 value for carvedilol inhibition of full-length channels (16.5 μM) was almost 5-fold less than that for the truncated channel variant (IC50 = 79.0 μM). Similarly, the related K2P 10.1 channels were blocked by carvedilol (IC50 ,oocytes = 24.0 μM; IC50 , HEK = 7.6 μM) and subject to ATI-dependent modulation of drug sensitivity.

CONCLUSIONS AND IMPLICATIONS

Carvedilol targets K2P 2.1 and K2P 10.1 K(+) channels. This previously unrecognized mechanism supports a general role of cardiac K2P channels as antiarrhythmic drug targets. Furthermore, the work reveals that the sensitivity of the cardiac ion channels K2P 2.1 and K2P 10.1 to block was modulated by alternative mRNA translation initiation.

Vernakalant activates human cardiac K(2P)17.1 background K(+) channels

Atrial fibrillation (AF) contributes significantly to cardiovascular morbidity and mortality. The growing epidemic is associated with cardiac repolarization abnormalities and requires the development of more effective antiarrhythmic strategies. Two-pore-domain K(+) channels stabilize the resting membrane potential and repolarize action potentials. Recently discovered K2P17.1 channels are expressed in human atrium and represent potential targets for AF therapy. However, cardiac electropharmacology of K2P17.1 channels remains to be investigated. This study was designed to elucidate human K2P17.1 regulation by antiarrhythmic drugs. Two-electrode voltage clamp and whole-cell patch clamp electrophysiology was used to record K2P currents from Xenopus oocytes and Chinese hamster ovary (CHO) cells. The class III antiarrhythmic compound vernakalant activated K2P17.1 currents in oocytes an in mammalian cells (EC50,CHO=40 μM) in frequency-dependent manner. K2P17.1 channel activation by vernakalant was specific among K2P channel family members. By contrast, vernakalant reduced K2P4.1 and K2P10.1 currents, in line with K2P2.1 blockade reported earlier. K2P17.1 open rectification characteristics and current-voltage relationships were not affected by vernakalant. The class I drug flecainide did not significantly modulate K2P currents. In conclusion, vernakalant activates K2P17.1 background potassium channels. Pharmacologic K2P channel activation by cardiovascular drugs has not been reported previously and may be employed for personalized rhythm control in patients with AF-associated reduction of K(+) channel function.

Ion channels and transporters as therapeutic targets in the pulmonary circulation

Pulmonary circulation is a low pressure, low resistance, high flow system. The low resting vascular tone is maintained by the concerted action of ion channels, exchangers and pumps. Under physiological as well as pathophysiological conditions, they are targets of locally secreted or circulating vasodilators and/or vasoconstrictors, leading to changes in expression or to posttranslational modifications. Both structural changes in the pulmonary arteries and a sustained increase in pulmonary vascular tone result in pulmonary vascular remodeling contributing to morbidity and mortality in pediatric and adult patients. There is increasing evidence demonstrating the pivotal role of ion channels such as K(+) and Cl(-) or transient receptor potential channels in different cell types which are thought to play a key role in vasoconstrictive remodeling. This review focuses on ion channels, exchangers and pumps in the pulmonary circulation and summarizes their putative pathophysiological as well as therapeutic role in pulmonary vascular remodeling. A better understanding of the mechanisms of their actions may allow for the development of new options for attenuating acute and chronic pulmonary vasoconstriction and remodeling treating the devastating disease pulmonary hypertension.

Potent and selective inhibitors of the TASK-1 potassium channel through chemical optimization of a bis-amide scaffold

TASK-1 is a two-pore domain potassium channel that is important to modulating cell excitability, most notably in the context of neuronal pathways. In order to leverage TASK-1 for therapeutic benefit, its physiological role needs better characterization; however, designing selective inhibitors that avoid the closely related TASK-3 channel has been challenging. In this study, a series of bis-amide derived compounds were found to demonstrate improved TASK-1 selectivity over TASK-3 compared to reported inhibitors. Optimization of a marginally selective hit led to analog 35 which displays a TASK-1 IC50=16 nM with 62-fold selectivity over TASK-3 in an orthogonal electrophysiology assay.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

KCNK3: new gene target for pulmonary hypertension?

Recently, KCNK3 has been identified as a new predisposing gene for pulmonary arterial hypertension (PAH) by whole-exome sequencing. Mutation in KCNK3 gene is responsible for the first channelopathy identified in PAH. PAH due to KCNK3 mutations is an autosomal dominant disease with an incomplete penetrance as previously described in PAH due to BMPR2 mutations. This discovery represents an important advance for genetic counselling, allowing identification of high risk relatives for PAH and possible screening for PAH in KCNK3 mutation carriers.

Inhibition of cardiac two-pore-domain K+ (K2P) channels by the antiarrhythmic drug vernakalant--comparison with flecainide

The mixed ion channel blocker, vernakalant (RSD1235), is effective in rapid conversion of atrial fibrillation (AF) to sinus rhythm (SR). Suppression of cardiac two-pore-domain potassium (K2P) channels causes action potential prolongation and has recently been proposed as a novel antiarrhythmic strategy. The objective of this study was to investigate acute effects of vernakalant on human K2P2.1 (TREK-1) and K2P3.1 (TASK-1) channels to provide a more complete picture of its antiarrhythmic mechanism of action. The class IC antiarrhythmic drug flecainide was studied as a comparator agent. Two-electrode voltage clamp and whole-cell patch clamp electrophysiology was used to record K2P currents from Xenopus oocytes and Chinese hamster ovary (CHO) cells. Vernakalant inhibited cardiac K2P2.1 channels expressed in Xenopus oocytes and in CHO cells. The IC50 value obtained from mammalian cells (13.3 µM) was close to the range of vernakalant levels reported in patients (2-8 µM), indicating potential clinical significance of K2P2.1 blockade. Open rectification characteristics and current-voltage relationships of K2P2.1 currents were not affected by vernakalant. Vernakalant did not significantly reduce K2P3.1 currents. Finally, the class I antiarrhythmic drug flecainide had no effect on K2P2.1 or K2P3.1 channels. In conclusion, the recently developed antiarrhythmic drug vernakalant targets human K2P2.1 K(+) background channels. This previously unrecognized inhibitory property adds to the multichannel blocking profile of vernakalant and extends the mechanistic basis for its anti-fibrillatory effect.

Cardiac expression and atrial fibrillation-associated remodeling of K₂p2.1 (TREK-1) K⁺ channels in a porcine model

AIMS

Effective management of atrial fibrillation (AF) often remains an unmet need. Cardiac two-pore-domain K(+) (K2P) channels are implicated in action potential regulation, and their inhibition has been proposed as a novel antiarrhythmic strategy. K2P2.1 (TREK-1) channels are expressed in the human heart. This study was designed to identify and functionally express porcine K2P2.1 channels. In addition, we sought to analyze cardiac expression and AF-associated K2P2.1 remodeling in a clinically relevant porcine AF model.

MAIN METHODS

Three pK2P2.1 isoforms were identified and amplified. Currents were recorded using voltage clamp electrophysiology in the Xenopus oocyte expression system. K2P2.1 remodeling was studied by quantitative real time PCR and Western blot in domestic pigs during AF induced by atrial burst pacing.

KEY FINDINGS

Human and porcine K2P2.1 proteins share 99% identity. Residues involved in phosphorylation or glycosylation are conserved. Porcine K2P2.1 channels carried outwardly rectifying K(+) currents similar to their human counterparts. In pigs, K2P2.1 was expressed ubiquitously in the heart with predominance in the atrial tissue. AF was associated with time-dependent reduction of K2P2.1 protein in the RA by 70% (7 days of AF) and 80% (21 days of AF) compared to control animals in sinus rhythm. K2P2.1 expression in the left atrium, AV node, and ventricles was not affected by AF.

SIGNIFICANCE

Similarities between porcine and human K2P2.1 channels indicate that the pig may represent a valid model for mechanistic and preclinical studies. AF-related atrial K2P2.1 remodeling has potential implications for arrhythmia maintenance and antiarrhythmic therapy.

Mechanisms of zolpidem-induced long QT syndrome: acute inhibition of recombinant hERG K(+) channels and action potential prolongation in human cardiomyocytes derived from induced pluripotent stem cells

BACKGROUND AND PURPOSE

Zolpidem, a short-acting hypnotic drug prescribed to treat insomnia, has been clinically associated with acquired long QT syndrome (LQTS) and torsade de pointes (TdP) tachyarrhythmia. LQTS is primarily attributed to reduction of cardiac human ether-a-go-go-related gene (hERG)/I(Kr) currents. We hypothesized that zolpidem prolongs the cardiac action potential through inhibition of hERG K(+) channels.

EXPERIMENTAL APPROACH

Two-electrode voltage clamp and whole-cell patch clamp electrophysiology was used to record hERG currents from Xenopus oocytes and from HEK 293 cells. In addition, hERG protein trafficking was evaluated in HEK 293 cells by Western blot analysis, and action potential duration (APD) was assessed in human-induced pluripotent stem cell (hiPSC)-derived cardiomyocytes.

KEY RESULTS

Zolpidem caused acute hERG channel blockade in oocytes (IC(50) = 61.5 μM) and in HEK 293 cells (IC(50) = 65.5 μM). Mutation of residues Y652 and F656 attenuated hERG inhibition, suggesting drug binding to a receptor site inside the channel pore. Channels were blocked in open and inactivated states in a voltage- and frequency-independent manner. Zolpidem accelerated hERG channel inactivation but did not affect I-V relationships of steady-state activation and inactivation. In contrast to the majority of hERG inhibitors, hERG cell surface trafficking was not impaired by zolpidem. Finally, acute zolpidem exposure resulted in APD prolongation in hiPSC-derived cardiomyocytes.

CONCLUSIONS AND IMPLICATIONS

Zolpidem inhibits cardiac hERG K(+) channels. Despite a relatively low affinity of zolpidem to hERG channels, APD prolongation may lead to acquired LQTS and TdP in cases of reduced repolarization reserve or zolpidem overdose.

A novel channelopathy in pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension is a devastating disease with high mortality. Familial cases of pulmonary arterial hypertension are usually characterized by autosomal dominant transmission with reduced penetrance, and some familial cases have unknown genetic causes.

METHODS

We studied a family in which multiple members had pulmonary arterial hypertension without identifiable mutations in any of the genes known to be associated with the disease, including BMPR2, ALK1, ENG, SMAD9, and CAV1. Three family members were studied with whole-exome sequencing. Additional patients with familial or idiopathic pulmonary arterial hypertension were screened for the mutations in the gene that was identified on whole-exome sequencing. All variants were expressed in COS-7 cells, and channel function was studied by means of patch-clamp analysis.

RESULTS

We identified a novel heterozygous missense variant c.608 G→A (G203D) in KCNK3 (the gene encoding potassium channel subfamily K, member 3) as a disease-causing candidate gene in the family. Five additional heterozygous missense variants in KCNK3 were independently identified in 92 unrelated patients with familial pulmonary arterial hypertension and 230 patients with idiopathic pulmonary arterial hypertension. We used in silico bioinformatic tools to predict that all six novel variants would be damaging. Electrophysiological studies of the channel indicated that all these missense mutations resulted in loss of function, and the reduction in the potassium-channel current was remedied by the application of the phospholipase inhibitor ONO-RS-082.

CONCLUSIONS

Our study identified the association of a novel gene, KCNK3, with familial and idiopathic pulmonary arterial hypertension. Mutations in this gene produced reduced potassium-channel current, which was successfully remedied by pharmacologic manipulation. (Funded by the National Institutes of Health.)

Signal transduction in the development of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a unique disease. Properly speaking, it is not a disease of the lung. It can be seen more as a microvascular disease occurring mainly in the lungs and affecting the heart. At the cellular level, the PAH paradigm is characterized by inflammation, vascular tone imbalance, pulmonary arterial smooth muscle cell proliferation and resistance to apoptosis and the presence of in situ thrombosis. At a clinical level, the aforementioned abnormal vascular properties alter physically the pulmonary circulation and ventilation, which greatly influence the right ventricle function as it highly correlates with disease severity. Consequently, right heart failure remains the principal cause of death within this cohort of patients. While current treatment modestly improve patients' conditions, none of them are curative and, as of today, new therapies are lacking. However, the future holds potential new therapies that might have positive influence on the quality of life of the patient. This article will first review the clinical presentation of the disease and the different molecular pathways implicated in the pathobiology of PAH. The second part will review tomorrow's future putative therapies for PAH.

Endothelin-1 induces hypoxia inducible factor 1α expression in pulmonary artery smooth muscle cells

Endothelin-1 (ET-1) dose-dependently increased HIF1α expression in pulmonary artery smooth muscle cells (PASMCs). Inhibition of protein synthesis did not affect ET-1-induced HIF1α expression. The maximum effect of ET-1 was similar to that caused by proteasome inhibitor MG132. Further study indicates that ET-1 also dose-dependently stimulated calcineurin activation, specific calcineurin inhibitor cyclosporine A (CsA), abolished ET-1-induced HIF1α elevation, and reversed ET-1-induced RACK1 (receptor of activated protein kinase C 1) de-phosphorylation. Endothelin receptor A was found to specifically mediate the effects of ET-1. To examine whether RACK1 is particularly involved in proteasome-dependent HIF1α degradation, RACK1 was silenced by siRNA transfection. Cells lacking RACK1 exhibited significant elevation of HIF1α protein level. Taken together, our study suggests that ET-1 suppressed proteasome-dependent HIF1α degradation by calcineurin-dependent RACK1 de-phosphorylation.

PKC-dependent activation of human K(2P) 18.1 K(+) channels

BACKGROUND AND PURPOSE

Two-pore-domain K(+) channels (K(2P) ) mediate K(+) background currents that modulate the membrane potential of excitable cells. K(2P) 18.1 (TWIK-related spinal cord K(+) channel) provides hyperpolarizing background currents in neurons. Recently, a dominant-negative loss-of-function mutation in K(2P) 18.1 has been implicated in migraine, and activation of K(2P) 18.1 channels was proposed as a therapeutic strategy. Here we elucidated the molecular mechanisms underlying PKC-dependent activation of K(2P) 18.1 currents.

EXPERIMENTAL APPROACH

Human K(2P) 18.1 channels were heterologously expressed in Xenopus laevis oocytes, and currents were recorded with the two-electrode voltage clamp technique.

KEY RESULTS

Stimulation of PKC using phorbol 12-myristate-13-acetate (PMA) activated the hK(2P) 18.1 current by 3.1-fold in a concentration-dependent fashion. The inactive analogue 4α-PMA had no effect on channel activity. The specific PKC inhibitors bisindolylmaleimide I, Ro-32-0432 and chelerythrine reduced PMA-induced channel activation indicating that PKC is involved in this effect of PMA. Selective activation of conventional PKC isoforms with thymeleatoxin (100 nM) did not reproduce K(2P) 18.1 channel activation. Current activation by PMA was not affected by pretreatment with CsA (calcineurin inhibitor) or KT 5720 (PKA inhibitor), ruling out a significant contribution of calcineurin or cross-talk with PKA to the PKC-dependent hK(2P) 18.1 activation. Finally, mutation of putative PKC phosphorylation sites did not prevent PMA-induced K(2P) 18.1 channel activation.

CONCLUSIONS AND IMPLICATIONS

We demonstrated that activation of hK(2P) 18.1 (TRESK) by PMA is mediated by PKC stimulation. Hence, PKC-mediated activation of K(2P) 18.1 background currents may serve as a novel molecular target for migraine treatment.

Animal models of pulmonary hypertension: Rho kinase inhibition

Pulmonary Hypertension is a terminology encompassing a range of etiologically different pulmonary vascular diseases. The most common is that termed pulmonary arterial hypertension or PAH; a rare but often fatal disease characterized by a mean pulmonary arterial pressure of >25 mmHg. PAH is associated with a complex etiology highlighted by core characteristics of increased pulmonary vascular resistance and elevation of mean pulmonary artery pressure. When sustained, pulmonary vascular remodeling occurs and eventually patients pass away due to right heart failure. Hypoxic pulmonary vasoconstriction is an early event occurring in pulmonary hypertension due to chronic exposure to hypoxia. While the underlying mechanisms of hypoxic pulmonary vasoconstriction may be controversial, a role for RhoA/Rho kinase mediated regulation of intracellular Ca(2+) has been recently identified. Further study suggests that RhoA may have an integral role in other pathophysiological processes such as cell proliferation and migration occurring in all forms of PH. Indeed Rho proteins are known to play essential roles in actin cytoskeleton organization in all eukaryotic cells and thus Rho and Rho-GTPases are implicated in fundamental cellular processes such as cellular proliferation, migration, adhesion, apoptosis and gene expression. This review focuses on providing an overview of the role of RhoA/Rho kinase in currently available animal models of pulmonary hypertension.