A novel pan-negative-gating modulator of KCa2/3 channels, fluoro-di-benzoate, RA-2, inhibits endothelium-derived hyperpolarization-type relaxation in coronary artery and produces bradycardia in vivo

KCa 2.2 (KCNN2): A physiologically and therapeutically important potassium channel

One group of the K+ ion channels, the small-conductance Ca2+ -activated potassium channels (KCa 2.x, also known as SK channels family), is widely expressed in neurons as well as the heart, endothelial cells, etc. They are named small-conductance Ca2+ -activated potassium channels (SK channels) due to their comparatively low single-channel conductance of about ~10 pS. These channels are insensitive to changes in membrane potential and are activated solely by rises in the intracellular Ca2+ . According to the phylogenic research done on the KCa 2.x channels family, there are three channels' subtypes: KCa 2.1, KCa 2.2, and KCa 2.3, which are encoded by KCNN1, KCNN2, and KCNN3 genes, respectively. The KCa 2.x channels regulate neuronal excitability and responsiveness to synaptic input patterns. KCa 2.x channels inhibit excitatory postsynaptic potentials (EPSPs) in neuronal dendrites and contribute to the medium afterhyperpolarization (mAHP) that follows the action potential bursts. Multiple brain regions, including the hippocampus, express the KCa 2.2 channel encoded by the KCNN2 gene on chromosome 5. Of particular interest, rat cerebellar Purkinje cells express KCa 2.2 channels, which are crucial for various cellular processes during development and maturation. Patients with a loss-of-function of KCNN2 mutations typically exhibit extrapyramidal symptoms, cerebellar ataxia, motor and language developmental delays, and intellectual disabilities. Studies have revealed that autosomal dominant neurodevelopmental movement disorders resembling rodent symptoms are caused by heterozygous loss-of-function mutations, which are most likely to induce KCNN2 haploinsufficiency. The KCa 2.2 channel is a promising drug target for spinocerebellar ataxias (SCAs). SCAs exhibit the dysregulation of firing in cerebellar Purkinje cells which is one of the first signs of pathology. Thus, selective KCa 2.2 modulators are promising potential therapeutics for SCAs.

KCa2 and KCa3.1 Channels in the Airways: A New Therapeutic Target

K⁺ channels are involved in many critical functions in lung physiology. Recently, the family of Ca²⁺-activated K⁺ channels (K_Ca) has received more attention, and a massive amount of effort has been devoted to developing selective medications targeting these channels. Within the family of K_Ca channels, three small-conductance Ca²⁺-activated K⁺ (K_Ca2) channel subtypes, together with the intermediate-conductance K_Ca3.1 channel, are voltage-independent K⁺ channels, and they mediate Ca²⁺-induced membrane hyperpolarization. Many K_Ca2 channel members are involved in crucial roles in physiological and pathological systems throughout the body. In this article, different subtypes of K_Ca2 and K_Ca3.1 channels and their functions in respiratory diseases are discussed. Additionally, the pharmacology of the K_Ca2 and K_Ca3.1 channels and the link between these channels and respiratory ciliary regulations will be explained in more detail. In the future, specific modulators for small or intermediate Ca²⁺-activated K⁺ channels may offer a unique therapeutic opportunity to treat muco-obstructive lung diseases.

Channelopathy of small- and intermediate-conductance Ca2+-activated K+ channels

Small- and intermediate-conductance Ca2+-activated K+ (KCa2.x/KCa3.1 also called SK/IK) channels are gated exclusively by intracellular Ca2+. The Ca2+ binding protein calmodulin confers sub-micromolar Ca2+ sensitivity to the channel-calmodulin complex. The calmodulin C-lobe is constitutively associated with the proximal C-terminus of the channel. Interactions between calmodulin N-lobe and the channel S4-S5 linker are Ca2+-dependent, which subsequently trigger conformational changes in the channel pore and open the gate. KCNN genes encode four subtypes, including KCNN1 for KCa2.1 (SK1), KCNN2 for KCa2.2 (SK2), KCNN3 for KCa2.3 (SK3), and KCNN4 for KCa3.1 (IK). The three KCa2.x channel subtypes are expressed in the central nervous system and the heart. The KCa3.1 subtype is expressed in the erythrocytes and the lymphocytes, among other peripheral tissues. The impact of dysfunctional KCa2.x/KCa3.1 channels on human health has not been well documented. Human loss-of-function KCa2.2 mutations have been linked with neurodevelopmental disorders. Human gain-of-function mutations that increase the apparent Ca2+ sensitivity of KCa2.3 and KCa3.1 channels have been associated with Zimmermann-Laband syndrome and hereditary xerocytosis, respectively. This review article discusses the physiological significance of KCa2.x/KCa3.1 channels, the pathophysiology of the diseases linked with KCa2.x/KCa3.1 mutations, the structure-function relationship of the mutant KCa2.x/KCa3.1 channels, and potential pharmacological therapeutics for the KCa2.x/KCa3.1 channelopathy.

KCa-Related Neurological Disorders: Phenotypic Spectrum and Therapeutic Indications

Although potassium channelopathies have been linked to a wide range of neurological conditions, the underlying pathogenic mechanism is not always clear, and a systematic summary of clinical manifestation is absent. Several neurological disorders have been associated with alterations of calcium-activated potassium channels (KCa channels), such as loss- or gain-of-function mutations, post-transcriptional modification, etc. Here, we outlined the current understanding of the molecular and cellular properties of three subtypes of KCa channels, including big conductance KCa channels (BK), small conductance KCa channels (SK), and the intermediate conductance KCa channels (IK). Next, we comprehensively reviewed the loss- or gain-of-function mutations of each KCa channel and described the corresponding mutation sites in specific diseases to broaden the phenotypic-genotypic spectrum of KCa-related neurological disorders. Moreover, we reviewed the current pharmaceutical strategies targeting KCa channels in KCa-related neurological disorders to provide new directions for drug discovery in anti-seizure medication.

Inhibition of KCa3.1 Channels Suppresses Atrial Fibrillation via the Attenuation of Macrophage Pro-inflammatory Polarization in a Canine Model With Prolonged Rapid Atrial Pacing

Aims: To investigate the role of KCa3. 1 inhibition in macrophage pro-inflammatory polarization and vulnerability to atrial fibrillation (AF) in a canine model with prolonged rapid atrial pacing.

Materials and Methods: Twenty beagle dogs (weighing 8–10 kg) were randomly assigned to a sham group (n = 6), pacing group (n = 7) and pacing+TRAM-34 group (n = 7). An experimental model of AF was established by rapid pacing. TRAM-34 was administered to the Pacing+TRAM-34 group by slow intravenous injection (10 mg/kg), 3 times each day. After 7 days of pacing, the electrophysiology was measured in vivo. The levels of interleukin-1β (IL-1β), monocyte chemotactic protein-1 (MCP-1), tumor necrosis factor-α (TNF-α), CD68, c-Fos, p38, and NF-κB p65 in both atriums were measured by Western blotting, and the levels of inducible nitric oxide synthase (iNOS) and arginase1 (Arg-1) were measured by real-time PCR. Macrophage and KCa3.1 in macrophage in the atrium were quantized following double labeled immunofluorescent.

Results: Greater inducibility of AF, an extended duration of AF and lower atrial effective refractory period (AERP) were observed in the pacing group compared with those in the sham group. Both CD68-labeled macrophage and the expression of KCa3.1 in macrophage were elevated in the pacing group and inhibited by TRAM-34, led to higher iNOS expression, lower Arg-1 expression, elevated levels of IL-1β, MCP-1, and TNF-α in the atria, which could be reversed by TRAM-34 treatment (all P < 0.01). KCa3.1 channels were possibly activated via the p38/AP-1/NF-κB signaling pathway.

Conclusions: Inhibition of KCa3.1 suppresses vulnerability to AF by attenuating macrophage pro-inflammatory polarization and inflammatory cytokine secretion in a canine model with prolonged rapid atrial pacing.

Conditional KCa3.1-transgene induction in murine skin produces pruritic eczematous dermatitis with severe epidermal hyperplasia and hyperkeratosis

Ion channels have recently attracted attention as potential mediators of skin disease. Here, we explored the consequences of genetically encoded induction of the cell volume-regulating Ca2+-activated KCa3.1 channel (Kcnn4) for murine epidermal homeostasis. Doxycycline-treated mice harboring the KCa3.1+-transgene under the control of the reverse tetracycline-sensitive transactivator (rtTA) showed 800-fold channel overexpression above basal levels in the skin and solid KCa3.1-currents in keratinocytes. This overexpression resulted in epidermal spongiosis, progressive epidermal hyperplasia and hyperkeratosis, itch and ulcers. The condition was accompanied by production of the pro-proliferative and pro-inflammatory cytokines, IL-β1 (60-fold), IL-6 (33-fold), and TNFα (26-fold) in the skin. Treatment of mice with the KCa3.1-selective blocker, Senicapoc, significantly suppressed spongiosis and hyperplasia, as well as induction of IL-β1 (-88%) and IL-6 (-90%). In conclusion, KCa3.1-induction in the epidermis caused expression of pro-proliferative cytokines leading to spongiosis, hyperplasia and hyperkeratosis. This skin condition resembles pathological features of eczematous dermatitis and identifies KCa3.1 as a regulator of epidermal homeostasis and spongiosis, and as a potential therapeutic target.

Adipose‑derived stem cells overexpressing SK4 calcium‑activated potassium channel generate biological pacemakers

Recent studies have suggested that calcium-activated potassium channel (K_Ca) agonists increase the proportion of mouse embryonic stem cell-derived cardiomyocytes and promote the differentiation of pacemaker cells. In the present study, it was hypothesized that adipose-derived stem cells (ADSCs) can differentiate into pacemaker-like cells via over-expression of the SK4 gene. ADSCs were transduced with a recombinant adenovirus vector carrying the mouse SK4 gene, whereas the control group was transduced with GFP vector. ADSCs transduced with SK4 vector were implanted into the rat left ventricular free wall. Complete atrioventricular block (AVB) was established in isolated perfused rat hearts after 2 weeks. SK4 was successfully and stably expressed in ADSCs following transduction. The mRNA levels of the pluripotent markers Oct-4 and Sox-2 declined and that of the transcription factor Shox2 was upregulated following SK4 transduction. The expression of α-actinin and hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4) increased in the SK4 group. The hyperpolarizing activated pacemaker current I_f (8/20 cells) was detected in ADSCs transduced with SK4, but not in the GFP group. Furthermore, SK4 transduction induced the expression of p-ERK1/2 and p-p38 MAPK. In the ex vivo experiments, the heart rate of the SK4 group following AVB establishment was significantly higher compared with that in the GFP group. Immunofluorescence revealed that the transduced ADSCs were successfully implanted and expressed HCN4 in the SK4 group. In conclusion, SK4 induced ADSCs to differentiate into cardiomyocyte-like and pacemaker-like cells via activation of the extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase pathways. Therefore, ADSCs transduced with SK4 may be used to generate biological pacemakers in ex vivo rat hearts.

Pharmacology of Small- and Intermediate-Conductance Calcium-Activated Potassium Channels

The three small-conductance calcium-activated potassium (K_Ca2) channels and the related intermediate-conductance K_Ca3.1 channel are voltage-independent K⁺ channels that mediate calcium-induced membrane hyperpolarization. When intracellular calcium increases in the channel vicinity, it calcifies the flexible N lobe of the channel-bound calmodulin, which then swings over to the S4-S5 linker and opens the channel. K_Ca2 and K_Ca3.1 channels are highly druggable and offer multiple binding sites for venom peptides and small-molecule blockers as well as for positive- and negative-gating modulators. In this review, we briefly summarize the physiological role of K_Ca channels and then discuss the pharmacophores and the mechanism of action of the most commonly used peptidic and small-molecule K_Ca2 and K_Ca3.1 modulators. Finally, we describe the progress that has been made in advancing K_Ca3.1 blockers and K_Ca2.2 negative- and positive-gating modulators toward the clinic for neurological and cardiovascular diseases and discuss the remaining challenges.

Pharmacological activation of TRPV4 produces immediate cell damage and induction of apoptosis in human melanoma cells and HaCaT keratinocytes

Background

TRPV4 channels are calcium-permeable cation channels that are activated by several physicochemical stimuli. Accordingly, TRPV4 channels have been implicated in the regulation of osmosensing, mechanotransduction, thermosensation, and epithelial/endothelial barrier functions. Whether TRPV4 is also mechanistically implicated in melanoma cell proliferation is not clear. Here, we hypothesized that TRPV4 is expressed in human melanoma and that pharmacological activation interferes with cell proliferation.

Methodology/Principal findings

TRPV4 functions were studied in melanoma cell lines (A375, SK-MEL-28, MKTBR), immortalized non-cancer keratinocytes (HaCaT), and murine 3T3 fibroblasts by patch-clamp, qRT-PCR, intracellular calcium measurements, cell proliferation, and flow cytometric assays of apoptosis and cell cycle. The selective TRPV4-activator, GSK1016790A, elicited non-selective cation currents with TRPV4-typical current-voltage-relationship in all cell lines. GSK1016790A-induced currents were blocked by the TRPV4-blocker, HC067047. TRPV4 mRNA expression was demonstrated by qRT-PCR. In A375 cells, TRPV4 activation was frequently paralleled by co-activation of calcium/calmodulin-regulated KCa3.1 channels. Light microscopy showed that TRPV4-activation produced rapid cellular disarrangement, nuclear densification, and detachment of a large fraction of all melanoma cell lines and HaCaT cells. TRPV4-activation induced apoptosis and drastically inhibited A375 and HaCaT proliferation that could be partially prevented by HC067047.

Conclusions/Significance

Our study showed that TRPV4 channels were functionally expressed in human melanoma cell lines and in human keratinocytes. Pharmacological TRPV4 activation in human melanoma cells and keratinocytes caused severe cellular disarrangement, necrosis and apoptosis. Pharmacological targeting of TRPV4 could be an alternative or adjuvant therapeutic strategy to treat melanoma progression and other proliferative skin disorders.

SK4 K+ channels are therapeutic targets for the treatment of cardiac arrhythmias

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a stress‐provoked ventricular arrhythmia, which also manifests sinoatrial node (SAN) dysfunction. We recently showed that SK4 calcium‐activated potassium channels are important for automaticity of cardiomyocytes derived from human embryonic stem cells. Here SK4 channels were identified in human induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CMs) from healthy and CPVT2 patients bearing a mutation in calsequestrin 2 (CASQ2‐D307H) and in SAN cells from WT and CASQ2‐D307H knock‐in (KI) mice. TRAM‐34, a selective blocker of SK4 channels, prominently reduced delayed afterdepolarizations and arrhythmic Ca²⁺ transients observed following application of the β‐adrenergic agonist isoproterenol in CPVT2‐derived hiPSC‐CMs and in SAN cells from KI mice. Strikingly, in vivo ECG recording showed that intraperitoneal injection of the SK4 channel blockers, TRAM‐34 or clotrimazole, greatly reduced the arrhythmic features of CASQ2‐D307H KI and CASQ2 knockout mice at rest and following exercise. This work demonstrates the critical role of SK4 Ca²⁺‐activated K⁺ channels in adult pacemaker function, making them promising therapeutic targets for the treatment of cardiac ventricular arrhythmias such as CPVT.

Down-regulation of KCa2.3 channels causes erectile dysfunction in mice

Modulation of endothelial calcium-activated K⁺ channels has been proposed as an approach to restore arterial endothelial cell function in disease. We hypothesized that small-conductance calcium-activated K⁺ channels (K_Ca2.3 or SK3) contributes to erectile function. The research was performed in transgenic mice with overexpression (K_Ca2.3^T/T(−Dox)) or down-regulation (K_Ca2.3^T/T(+Dox)) of the K_Ca2.3 channels and wild-type C57BL/6-mice (WT). QPCR revealed that K_Ca2.3 and K_Ca1.1 channels were the most abundant in mouse corpus cavernosum. K_Ca2.3 channels were found by immunoreactivity and electron microscopy in the apical-lateral membrane of endothelial cells in the corpus cavernosum. Norepinephrine contraction was enhanced in the corpus cavernosum of K_Ca2.3^T/T(+Dox)versus K_Ca2.3^T/T(−Dox) mice, while acetylcholine relaxation was only reduced at 0.3 µM and relaxations in response to the nitric oxide donor sodium nitroprusside were unaltered. An opener of K_Ca2 channels, NS309 induced concentration-dependent relaxations of corpus cavernosum. Mean arterial pressure was lower in K_Ca2.3^T/T(−Dox) mice compared with WT and K_Ca2.3^T/T(+Dox) mice. In anesthetized mice, cavernous nerve stimulation augmented in frequency/voltage dependent manner erectile function being lower in K_Ca2.3^T/T(+Dox) mice at low frequencies. Our findings suggest that down-regulation of K_Ca2.3 channels contributes to erectile dysfunction, and that pharmacological activation of K_Ca2.3 channels may have the potential to restore erectile function.

Inhibition of Intermediate-Conductance Calcium-Activated K Channel (KCa3.1) and Fibroblast Mitogenesis by α-Linolenic Acid and Alterations of Channel Expression in the Lysosomal Storage Disorders, Fabry Disease, and Niemann Pick C

The calcium/calmodulin-gated KCa3.1 channel regulates normal and abnormal mitogenesis by controlling K⁺-efflux, cell volume, and membrane hyperpolarization-driven calcium-entry. Recent studies suggest modulation of KCa3.1 by omega-3 fatty acids as negative modulators and impaired KCa3.1 functions in the inherited lysosomal storage disorder (LSD), Fabry disease (FD). In the first part of present study, we characterize KCa3.1 in murine and human fibroblasts and test the impact of omega-3 fatty acids on fibroblast proliferation. In the second, we study whether KCa3.1 is altered in the LSDs, FD, and Niemann-Pick disease type C (NPC). Our patch-clamp and mRNA-expression studies on murine and human fibroblasts show functional expression of KCa3.1. K_Ca currents display the typical pharmacological fingerprint of KCa3.1: Ca²⁺-activation, potentiation by the positive-gating modulators, SKA-31 and SKA-121, and inhibition by TRAM-34, Senicapoc (ICA-17043), and the negative-gating modulator, 13b. Considering modulation by omega-3 fatty acids we found that α-linolenic acid (α-LA) and docosahexanenoic acid (DHA) inhibit KCa3.1 currents and strongly reduce fibroblast growth. The α-LA-rich linseed oil and γ-LA-rich borage oil at 0.5% produce channel inhibition while α-LA/γ-LA-low oils has no anti-proliferative effect. Concerning KCa3.1 in LSD, mRNA expression studies, and patch-clamp on primary fibroblasts from FD and NPC patients reveal lower KCa3.1-gene expression and membrane expression than in control fibroblasts. In conclusion, the omega-3 fatty acid, α-LA, and α-LA/γ-LA-rich plant oils, inhibit fibroblast KCa3.1 channels and mitogenesis. Reduced fibroblast KCa3.1 functions are a feature and possible biomarker of cell dysfunction in FD and NPC and supports the concept that biased lipid metabolism is capable of negatively modulating KCa3.1 expression.

Emerging roles of calcium-activated K channels and TRPV4 channels in lung oedema and pulmonary circulatory collapse

It has been suggested that the transient receptor potential cation (TRP) channel subfamily V (vanilloid) type 4 (TRPV4) and intermediate conductance calcium-activated potassium (KCa3.1) channels contribute to endothelium-dependent vasodilation. Here, we summarize very recent evidence for a synergistic interplay of TRPV4 and KCa3.1 channels in lung disease. Among the endothelial Ca²⁺ -permeable TRPs, TRPV4 is best characterized and produces arterial dilation by stimulating Ca²⁺ -dependent nitric oxide synthesis and endothelium-dependent hyperpolarization. Besides these roles, some TRP channels control endothelial/epithelial barrier functions and vascular integrity, while KCa3.1 channels provide the driving force required for Cl^- and water transport in some cells and most secretory epithelia. The three conditions, increased pulmonary venous pressure caused by left heart disease, high inflation pressure and chemically induced lung injury, may lead to activation of TRPV4 channels followed by Ca²⁺ influx leading to activation of KCa3.1 channels in endothelial cells ultimately leading to acute lung injury. We find that a deficiency in KCa3.1 channels protects against TRPV4-induced pulmonary arterial relaxation, fluid extravasation, haemorrhage, pulmonary circulatory collapse and cardiac arrest in vivo. These data identify KCa3.1 channels as crucial molecular components in downstream TRPV4 signal transduction and as a potential target for the prevention of undesired fluid extravasation, vasodilatation and pulmonary circulatory collapse.

Vascular Reactivity Profile of Novel KCa 3.1-Selective Positive-Gating Modulators in the Coronary Vascular Bed

Opening of intermediate-conductance calcium-activated potassium channels (KC a 3.1) produces membrane hyperpolarization in the vascular endothelium. Here, we studied the ability of two new KC a 3.1-selective positive-gating modulators, SKA-111 and SKA-121, to (1) evoke porcine endothelial cell KC a 3.1 membrane hyperpolarization, (2) induce endothelium-dependent and, particularly, endothelium-derived hyperpolarization (EDH)-type relaxation in porcine coronary arteries (PCA) and (3) influence coronary artery tone in isolated rat hearts. In whole-cell patch-clamp experiments on endothelial cells of PCA (PCAEC), KC a currents evoked by bradykinin (BK) were potentiated ≈7-fold by either SKA-111 or SKA-121 (both at 1 μM) and were blocked by a KC a 3.1 blocker, TRAM-34. In membrane potential measurements, SKA-111 and SKA-121 augmented bradykinin-induced hyperpolarization. Isometric tension measurements in large- and small-calibre PCA showed that SKA-111 and SKA-121 potentiated endothelium-dependent relaxation with intact NO synthesis and EDH-type relaxation to BK by ≈2-fold. Potentiation of the BK response was prevented by KC a 3.1 inhibition. In Langendorff-perfused rat hearts, SKA-111 potentiated coronary vasodilation elicited by BK. In conclusion, our data show that positive-gating modulation of KC a 3.1 channels improves BK-induced membrane hyperpolarization and endothelium-dependent relaxation in small and large PCA as well as in the coronary circulation of rats. Positive-gating modulators of KC a 3.1 could be therapeutically useful to improve coronary blood flow and counteract impaired coronary endothelial dysfunction in cardiovascular disease.

Pharmacological gating modulation of small- and intermediate-conductance Ca(2+)-activated K(+) channels (KCa2.x and KCa3.1)

This short review discusses pharmacological modulation of the opening/closing properties (gating) of small- and intermediate-conductance Ca(2+)-activated K(+) channels (KCa2 and KCa3.1) with special focus on mechanisms-of-action, selectivity, binding sites, and therapeutic potentials. Despite KCa channel gating-modulation being a relatively novel field in drug discovery, efforts in this area have already revealed a surprising plethora of pharmacological sites-of-actions and channel subtype selectivity exerted by different chemical classes. The currently published positive modulators show that such molecules are potentially useful for the treatment of various neurodegenerative disorders such as ataxia, alcohol dependence, and epilepsy as well as hypertension. The negative KCa2 modulators are very effective agents for atrial fibrillation. The prediction is that further unraveling of the molecular details of gating pharmacology will allow for the design of even more potent and subtype selective KCa modulators entering into drug development for these indications.