Molecular mechanisms of BK channel activation

Evolution of endosymbiosis-mediated nuclear calcium signaling in land plants

The ability of fungi to establish mycorrhizal associations with plants and enhance the acquisition of mineral nutrients stands out as a key feature of terrestrial life. Evidence indicates that arbuscular mycorrhizal (AM) association is a trait present in the common ancestor of land plants,1,2,3,4 suggesting that AM symbiosis was an important adaptation for plants in terrestrial environments.5 The activation of nuclear calcium signaling in roots is essential for AM within flowering plants.6 Given that the earliest land plants lacked roots, whether nuclear calcium signals are required for AM in non-flowering plants is unknown. To address this question, we explored the functional conservation of symbiont-induced nuclear calcium signals between the liverwort Marchantia paleacea and the legume Medicago truncatula. In M. paleacea, AM fungi penetrate the rhizoids and form arbuscules in the thalli.7 Here, we demonstrate that AM germinating spore exudate (GSE) activates nuclear calcium signals in the rhizoids of M. paleacea and that this activation is dependent on the nuclear-localized ion channel DOES NOT MAKE INFECTIONS 1 (MpaDMI1). However, unlike flowering plants, MpaDMI1-mediated calcium signaling is only required for the thalli colonization but not for the AM penetration within rhizoids. We further demonstrate that the mechanism of regulation of DMI1 has diverged between M. paleacea and M. truncatula, including a key amino acid residue essential to sustain DMI1 in an inactive state. Our study reveals functional evolution of nuclear calcium signaling between liverworts and flowering plants and opens new avenues of research into the mechanism of endosymbiosis signaling.

The BK Channel Limits the Pro-Inflammatory Activity of Macrophages

Macrophages play a crucial role in the innate immune response, serving as key effector cells in the defense against pathogens. Although the role of the large-conductance voltage and calcium-activated potassium channel, also known as the KCa1.1 or BK channel, in regulating neurotransmitter release and smooth muscle contraction is well known, its potential involvement in immune regulation remains unclear. We employed BK-knockout macrophages and noted that the absence of a BK channel promotes the polarization of macrophages towards a pro-inflammatory phenotype known as M1 macrophages. Specifically, the absence of the BK channel resulted in a significant increase in the secretion of the pro-inflammatory cytokine IL-6 and enhanced the activity of extracellular signal-regulated kinases 1 and 2 (Erk1/2 kinases), Ca2+/calmodulin-dependent protein kinase II (CaMKII), and the transcription factor ATF-1 within M1 macrophages. Additionally, the lack of the BK channel promoted the activation of the AIM2 inflammasome without affecting the activation of the NLRC4 and NLRP3 inflammasomes. To further investigate the role of the BK channel in regulating AIM2 inflammasome activation, we utilized BK channel inhibitors, such as paxilline and iberiotoxin, along with the BK channel activator NS-11021. Pharmacological inactivation of the BK channel increased, and its stimulation inhibited IL-1β production following AIM2 inflammasome activation in wild-type macrophages. Moreover, wild-type macrophages displayed increased calcium influx when activated with the AIM2 inflammasome, whereas BK-knockout macrophages did not due to the impaired extracellular calcium influx upon activation. Furthermore, under conditions of a calcium-free medium, IL-1β production following AIM2 inflammasome activation was increased in both wild-type and BK-knockout macrophages. This suggests that the BK channel is required for the influx of extracellular calcium in macrophages, thus limiting AIM2 inflammasome activation. In summary, our study reveals a regulatory role of the BK channel in macrophages under inflammatory conditions.

Dual allosteric modulation of voltage and calcium sensitivities of the Slo1-LRRC channel complex

Modulation of large conductance intracellular ligand-activated potassium (BK) channel family (Slo1-3) by auxiliary subunits allows diverse physiological functions in excitable and non-excitable cells. Cryoelectron microscopy (cryo-EM) structures of voltage-gated potassium (Kv) channel complexes have provided insights into how voltage sensitivity is modulated by auxiliary subunits. However, the modulation mechanisms of BK channels, particularly as ligand-activated ion channels, remain unknown. Slo1 is a Ca2+-activated and voltage-gated BK channel and is expressed in neurons, muscle cells, and epithelial cells. Using cryo-EM and electrophysiology, we show that the LRRC26-γ1 subunit modulates not only voltage but also Ca2+ sensitivity of Homo sapiens Slo1. LRRC26 stabilizes the active conformation of voltage-senor domains of Slo1 by an extracellularly S4-locking mechanism. Furthermore, it also stabilizes the active conformation of Ca2+-sensor domains of Slo1 intracellularly, which is functionally equivalent to intracellular Ca2+ in the activation of Slo1. Such a dual allosteric modulatory mechanism may be general in regulating the intracellular ligand-activated BK channel complexes.

Incorporating physics to overcome data scarcity in predictive modeling of protein function: A case study of BK channels

Machine learning has played transformative roles in numerous chemical and biophysical problems such as protein folding where large amount of data exists. Nonetheless, many important problems remain challenging for data-driven machine learning approaches due to the limitation of data scarcity. One approach to overcome data scarcity is to incorporate physical principles such as through molecular modeling and simulation. Here, we focus on the big potassium (BK) channels that play important roles in cardiovascular and neural systems. Many mutants of BK channel are associated with various neurological and cardiovascular diseases, but the molecular effects are unknown. The voltage gating properties of BK channels have been characterized for 473 site-specific mutations experimentally over the last three decades; yet, these functional data by themselves remain far too sparse to derive a predictive model of BK channel voltage gating. Using physics-based modeling, we quantify the energetic effects of all single mutations on both open and closed states of the channel. Together with dynamic properties derived from atomistic simulations, these physical descriptors allow the training of random forest models that could reproduce unseen experimentally measured shifts in gating voltage, ∆V1/2, with a RMSE ~ 32 mV and correlation coefficient of R ~ 0.7. Importantly, the model appears capable of uncovering nontrivial physical principles underlying the gating of the channel, including a central role of hydrophobic gating. The model was further evaluated using four novel mutations of L235 and V236 on the S5 helix, mutations of which are predicted to have opposing effects on V1/2 and suggest a key role of S5 in mediating voltage sensor-pore coupling. The measured ∆V1/2 agree quantitatively with prediction for all four mutations, with a high correlation of R = 0.92 and RMSE = 18 mV. Therefore, the model can capture nontrivial voltage gating properties in regions where few mutations are known. The success of predictive modeling of BK voltage gating demonstrates the potential of combining physics and statistical learning for overcoming data scarcity in nontrivial protein function prediction.

Insights into Leishmania donovani potassium channel family and their biological functions

Leishmania donovani is the causative organism for visceral leishmaniasis. Although this parasite was discovered over a century ago, nothing is known about role of potassium channels in L. donovani. Potassium channels are known for their crucial roles in cellular functions in other organisms. Recently the presence of a calcium-activated potassium channel in L. donovani was reported which prompted us to look for other proteins which could be potassium channels and to investigate their possible physiological roles. Twenty sequences were identified in L. donovani genome and subjected to estimation of physio-chemical properties, motif analysis, localization prediction and transmembrane domain analysis. Structural predictions were also done. The channels were majorly α-helical and predominantly localized in cell membrane and lysosomes. The signature selectivity filter of potassium channel was present in all the sequences. In addition to the conventional potassium channel activity, they were associated with gene ontology terms for mitotic cell cycle, cell death, modulation by virus of host process, cell motility etc. The entire study indicates the presence of potassium channel families in L. donovani which may have involvement in several cellular pathways. Further investigations on these putative potassium channels are needed to elucidate their roles in Leishmania.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03692-y.

Incorporating physics to overcome data scarcity in predictive modeling of protein function: a case study of BK channels

Machine learning has played transformative roles in numerous chemical and biophysical problems such as protein folding where large amount of data exists. Nonetheless, many important problems remain challenging for data-driven machine learning approaches due to the limitation of data scarcity. One approach to overcome data scarcity is to incorporate physical principles such as through molecular modeling and simulation. Here, we focus on the big potassium (BK) channels that play important roles in cardiovascular and neural systems. Many mutants of BK channel are associated with various neurological and cardiovascular diseases, but the molecular effects are unknown. The voltage gating properties of BK channels have been characterized for 473 site-specific mutations experimentally over the last three decades; yet, these functional data by themselves remain far too sparse to derive a predictive model of BK channel voltage gating. Using physics-based modeling, we quantify the energetic effects of all single mutations on both open and closed states of the channel. Together with dynamic properties derived from atomistic simulations, these physical descriptors allow the training of random forest models that could reproduce unseen experimentally measured shifts in gating voltage, ΔV 1/2 , with a RMSE ∼ 32 mV and correlation coefficient of R ∼ 0.7. Importantly, the model appears capable of uncovering nontrivial physical principles underlying the gating of the channel, including a central role of hydrophobic gating. The model was further evaluated using four novel mutations of L235 and V236 on the S5 helix, mutations of which are predicted to have opposing effects on V 1/2 and suggest a key role of S5 in mediating voltage sensor-pore coupling. The measured ΔV 1/2 agree quantitatively with prediction for all four mutations, with a high correlation of R = 0.92 and RMSE = 18 mV. Therefore, the model can capture nontrivial voltage gating properties in regions where few mutations are known. The success of predictive modeling of BK voltage gating demonstrates the potential of combining physics and statistical learning for overcoming data scarcity in nontrivial protein function prediction.

Author Summary

Deep machine learning has brought many exciting breakthroughs in chemistry, physics and biology. These models require large amount of training data and struggle when the data is scarce. The latter is true for predictive modeling of the function of complex proteins such as ion channels, where only hundreds of mutational data may be available. Using the big potassium (BK) channel as a biologically important model system, we demonstrate that a reliable predictive model of its voltage gating property could be derived from only 473 mutational data by incorporating physics-derived features, which include dynamic properties from molecular dynamics simulations and energetic quantities from Rosetta mutation calculations. We show that the final random forest model captures key trends and hotspots in mutational effects of BK voltage gating, such as the important role of pore hydrophobicity. A particularly curious prediction is that mutations of two adjacent residues on the S5 helix would always have opposite effects on the gating voltage, which was confirmed by experimental characterization of four novel mutations. The current work demonstrates the importance and effectiveness of incorporating physics in predictive modeling of protein function with scarce data.

Ca2+-Activated K+ Channels in Progenitor Cells of Musculoskeletal Tissues: A Narrative Review

Musculoskeletal disorders represent one of the main causes of disability worldwide, and their prevalence is predicted to increase in the coming decades. Stem cell therapy may be a promising option for the treatment of some of the musculoskeletal diseases. Although significant progress has been made in musculoskeletal stem cell research, osteoarthritis, the most-common musculoskeletal disorder, still lacks curative treatment. To fine-tune stem-cell-based therapy, it is necessary to focus on the underlying biological mechanisms. Ion channels and the bioelectric signals they generate control the proliferation, differentiation, and migration of musculoskeletal progenitor cells. Calcium- and voltage-activated potassium (KCa) channels are key players in cell physiology in cells of the musculoskeletal system. This review article focused on the big conductance (BK) KCa channels. The regulatory function of BK channels requires interactions with diverse sets of proteins that have different functions in tissue-resident stem cells. In this narrative review article, we discuss the main ion channels of musculoskeletal stem cells, with a focus on calcium-dependent potassium channels, especially on the large conductance BK channel. We review their expression and function in progenitor cell proliferation, differentiation, and migration and highlight gaps in current knowledge on their involvement in musculoskeletal diseases.

Flavonoids as Modulators of Potassium Channels

Potassium channels are widely distributed integral proteins responsible for the effective and selective transport of K+ ions through the biological membranes. According to the existing structural and mechanistic differences, they are divided into several groups. All of them are considered important molecular drug targets due to their physiological roles, including the regulation of membrane potential or cell signaling. One of the recent trends in molecular pharmacology is the evaluation of the therapeutic potential of natural compounds and their derivatives, which can exhibit high specificity and effectiveness. Among the pharmaceuticals of plant origin, which are potassium channel modulators, flavonoids appear as a powerful group of biologically active substances. It is caused by their well-documented anti-oxidative, anti-inflammatory, anti-mutagenic, anti-carcinogenic, and antidiabetic effects on human health. Here, we focus on presenting the current state of knowledge about the possibilities of modulation of particular types of potassium channels by different flavonoids. Additionally, the biological meaning of the flavonoid-mediated changes in the activity of K+ channels will be outlined. Finally, novel promising directions for further research in this area will be proposed.

Ion channels as a therapeutic target for renal fibrosis

Renal ion channel transport and electrolyte disturbances play an important role in the process of functional impairment and fibrosis in the kidney. It is well known that there are limited effective drugs for the treatment of renal fibrosis, and since a large number of ion channels are involved in the renal fibrosis process, understanding the mechanisms of ion channel transport and the complex network of signaling cascades between them is essential to identify potential therapeutic approaches to slow down renal fibrosis. This review summarizes the current work of ion channels in renal fibrosis. We pay close attention to the effect of cystic fibrosis transmembrane conductance regulator (CFTR), transmembrane Member 16A (TMEM16A) and other Cl− channel mediated signaling pathways and ion concentrations on fibrosis, as well as the various complex mechanisms for the action of Ca2+ handling channels including Ca2+-release-activated Ca2+ channel (CRAC), purinergic receptor, and transient receptor potential (TRP) channels. Furthermore, we also focus on the contribution of Na+ transport such as epithelial sodium channel (ENaC), Na+, K+-ATPase, Na+-H+ exchangers, and K+ channels like Ca2+-activated K+ channels, voltage-dependent K+ channel, ATP-sensitive K+ channels on renal fibrosis. Proposed potential therapeutic approaches through further dissection of these mechanisms may provide new therapeutic opportunities to reduce the burden of chronic kidney disease.

Structure of the Human BK Ion Channel in Lipid Environment

Voltage-gated and ligand-modulated ion channels play critical roles in excitable cells. To understand the interplay among voltage sensing, ligand binding, and channel opening, the structures of ion channels in various functional states and in lipid membrane environments need to be determined. Here, the random spherically constrained (RSC) single-particle cryo-EM method was employed to study human large conductance voltage- and calcium-activated potassium (hBK or hSlo1) channels reconstituted into liposomes. The hBK structure was determined at 3.5 Å resolution in the absence of Ca²⁺. Instead of the common fourfold symmetry observed in ligand-modulated ion channels, a twofold symmetry was observed in hBK in liposomes. Compared with the structure of isolated hSlo1 Ca²⁺ sensing gating rings, two opposing subunits in hBK unfurled, resulting in a wider opening towards the transmembrane region of hBK. In the pore gate domain, two opposing subunits also moved downwards relative to the two other subunits.

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

State-dependent inhibition of BK channels by the opioid agonist loperamide

Large-conductance Ca²⁺-activated K⁺ (BK) channels control a range of physiological functions, and their dysfunction is linked to human disease. We have found that the widely used drug loperamide (LOP) can inhibit activity of BK channels composed of either α-subunits (BKα channels) or α-subunits plus the auxiliary γ1-subunit (BKα/γ1 channels), and here we analyze the molecular mechanism of LOP action. LOP applied at the cytosolic side of the membrane rapidly and reversibly inhibited BK current, an effect that appeared as a decay in voltage-activated BK currents. The apparent affinity for LOP decreased with hyperpolarization in a manner consistent with LOP behaving as an inhibitor of open, activated channels. Increasing LOP concentration reduced the half-maximal activation voltage, consistent with relative stabilization of the LOP-inhibited open state. Single-channel recordings revealed that LOP did not reduce unitary BK channel current, but instead decreased BK channel open probability and mean open times. LOP elicited use-dependent inhibition, in which trains of brief depolarizing steps lead to accumulated reduction of BK current, whereas single brief depolarizing steps do not. The principal effects of LOP on BK channel gating are described by a mechanism in which LOP acts as a state-dependent pore blocker. Our results suggest that therapeutic doses of LOP may act in part by inhibiting K⁺ efflux through intestinal BK channels.

Bibliometric analysis of potassium channel research

ABSTRACT Objective: To explore the research status, hotspots, and trends in research on potassium channel. Methods: The Web of Science core collection database was used as the data source and the visual analysis software Citespace5.4 R3 was used to visualize the studies of potassium channel in the past 10 years. The national/institutional distribution, journal distribution, authors, and related research were discussed. Results 17,392 articles were obtained. The USA, Peoples R China, Germany, England, and Japan were the main countries in the field and University of California was the most important institution for the study of potassium channel. PLoS One was the most productive journal and proceedings of the national academy of sciences of the united states of america was the most frequently cited journal in potassium channel research. The author with the highest number was Colin G Nichols and the author with the highest co- cited frequency was Sanguinetti MC. The three hot spots of potassium channel research were gene expression, Ca2+ activated k+ channel and nitric oxide. The top four research frontiers of potassium channel research were bk channel,blood pressure,oxidative stress and electrophysiology. Conclusion The study provides a perspective for understanding the potassium channel research and provides valuable information for potassium channel researchers to identify potential collaborators, partner institutions, hot topics and research frontiers.

Remodeling of Arterial Tone Regulation in Postnatal Development: Focus on Smooth Muscle Cell Potassium Channels

Maturation of the cardiovascular system is associated with crucial structural and functional remodeling. Thickening of the arterial wall, maturation of the sympathetic innervation, and switching of the mechanisms of arterial contraction from calcium-independent to calcium-dependent occur during postnatal development. All these processes promote an almost doubling of blood pressure from the moment of birth to reaching adulthood. This review focuses on the developmental alterations of potassium channels functioning as key smooth muscle membrane potential determinants and, consequently, vascular tone regulators. We present evidence that the pattern of potassium channel contribution to vascular control changes from K_ir2, K_v1, K_v7 and TASK-1 channels to BK_Ca channels with maturation. The differences in the contribution of potassium channels to vasomotor tone at different stages of postnatal life should be considered in treatment strategies of cardiovascular diseases associated with potassium channel malfunction.

Piezo1 and BKCa channels in human atrial fibroblasts: Interplay and remodelling in atrial fibrillation

AIMS

Atrial Fibrillation (AF) is an arrhythmia of increasing prevalence in the aging populations of developed countries. One of the important indicators of AF is sustained atrial dilatation, highlighting the importance of mechanical overload in the pathophysiology of AF. The mechanisms by which atrial cells, including fibroblasts, sense and react to changing mechanical forces, are not fully elucidated. Here, we characterise stretch-activated ion channels (SAC) in human atrial fibroblasts and changes in SAC- presence and activity associated with AF.

METHODS AND RESULTS

Using primary cultures of human atrial fibroblasts, isolated from patients in sinus rhythm or sustained AF, we combine electrophysiological, molecular and pharmacological tools to identify SAC. Two electrophysiological SAC- signatures were detected, indicative of cation-nonselective and potassium-selective channels. Using siRNA-mediated knockdown, we identified the cation-nonselective SAC as Piezo1. Biophysical properties of the potassium-selective channel, its sensitivity to calcium, paxilline or iberiotoxin (blockers), and NS11021 (activator), indicated presence of calcium-dependent 'big potassium channels' (BK_Ca). In cells from AF patients, Piezo1 activity and mRNA expression levels were higher than in cells from sinus rhythm patients, while BK_Ca activity (but not expression) was downregulated. Both Piezo1-knockdown and removal of extracellular calcium from the patch pipette resulted in a significant reduction of BK_Ca current during stretch. No co-immunoprecipitation of Piezo1 and BK_Ca was detected.

CONCLUSIONS

Human atrial fibroblasts contain at least two types of ion channels that are activated during stretch: Piezo1 and BK_Ca. While Piezo1 is directly stretch-activated, the increase in BK_Ca activity during mechanical stimulation appears to be mainly secondary to calcium influx via SAC such as Piezo1. During sustained AF, Piezo1 is increased, while BK_Ca activity is reduced, highlighting differential regulation of both channels. Our data support the presence and interplay of Piezo1 and BK_Ca in human atrial fibroblasts in the absence of physical links between the two channel proteins.

Regulatory mechanisms of mitochondrial BKCa channels

The mitochondrial BK_Ca channel (mitoBK_Ca) is a splice variant of plasma membrane BK_Ca (Maxi-K, BK_Ca, Slo1, K_Ca1.1). While a high-resolution structure of mitoBK_Ca is not available yet, functional and structural studies of the plasma membrane BK_Ca have provided important clues on the gating of the channel by voltage and Ca²⁺, as well as the interaction with auxiliary subunits. To date, we know that the control of expression of mitoBK_Ca, targeting and voltage-sensitivity strongly depends on its association with its regulatory β1-subunit, which overall participate in the control of mitochondrial Ca²⁺-overload in cardiac myocytes. Moreover, novel regulatory mechanisms of mitoBK_Ca such as β-subunits and amyloid-β have recently been proposed. However, major basic questions including how the regulatory BK_Ca-β1-subunit reaches mitochondria and the mechanism through which amyloid-β impairs mitoBK_Ca channel function remain to be addressed.

Deeper and Deeper on the Role of BK and Kir4.1 Channels in Glioblastoma Invasiveness: A Novel Summative Mechanism?

In the last decades, increasing evidence has revealed that a large number of channel protein and ion pumps exhibit impaired expression in cancers. This dysregulation is responsible for high proliferative rates as well as migration and invasiveness, reflected in the recently coined term oncochannelopathies. In glioblastoma (GBM), the most invasive and aggressive primary brain tumor, GBM cells modify their ionic equilibrium in order to change their volume as a necessary step prior to migration. This mechanism involves increased expression of BK channels and downregulation of the normally widespread Kir4.1 channels, as noted in GBM biopsies from patients. Despite a large body of work implicating BK channels in migration in response to an artificial intracellular calcium rise, little is known about how this channel acts in GBM cells at resting membrane potential (RMP), as compared to other channels that are constitutively open, such as Kir4.1. In this review we propose that a residual fraction of functionally active Kir4.1 channels mediates a small, but continuous, efflux of potassium at the more depolarized RMP of GBM cells. In addition, coinciding with transient membrane deformation and the intracellular rise in calcium concentration, brief activity of BK channels can induce massive and rapid cytosolic water loss that reduces cell volume (cell shrinkage), a necessary step for migration within the brain parenchyma.

Potassium channels in intestinal epithelial cells and their pharmacological modulation: a systematic review

Several potassium channels (KCs) have been described throughout the gastrointestinal tract. Notwithstanding, their contribution to both physiologic and pathophysiologic conditions, as inflammatory bowel disease (IBD), remains underexplored. Therefore, we aim to systematically review, for the first time, the evidence on the characteristics and modulation of KCs in intestinal epithelial cells (IECs). PubMed, Scopus, and Web of Science were searched to identify studies focusing on KCs and their modulation in IECs. The included studies were assessed using a reporting inclusiveness checklist. From the 745 identified records, 73 met the inclusion criteria; their reporting inclusiveness was moderate-high. Some studies described the physiological role of KCs, while others explored their importance in pathological settings. Globally, in IBD animal models, apical K_Ca1.1 channels, responsible for luminal secretion, were upregulated. In human colonocytes, basolateral K_Ca3.1 channels were downregulated. The pharmacological inhibition of K_2P and K_v influenced intestinal barrier function, promoting inflammation. Evidence suggests a strong association between KCs expression and secretory mechanisms in human and animal IECs. Further research is warranted to explore the usefulness of KC pharmacological modulation as a therapeutic target.

Identification and Characterization of a Novel Large-Conductance Calcium-Activated Potassium Channel Activator, CTIBD, and Its Relaxation Effect on Urinary Bladder Smooth Muscle

The large-conductance calcium-activated potassium channel (BK_Ca channel) is expressed on various tissues and is involved in smooth muscle relaxation. The channel is highly expressed on urinary bladder smooth muscle cells and regulates the repolarization phase of the spontaneous action potentials that control muscle contraction. To discover novel chemical activators of the BK_Ca channel, we screened a chemical library containing 8364 chemical compounds using a cell-based fluorescence assay. A chemical compound containing an isoxazolyl benzene skeleton (compound 1) was identified as a potent activator of the BK_Ca channel and was structurally optimized through a structure-activity relationship study to obtain 4-(4-(4-chlorophenyl)-3-(trifluoromethyl)isoxazol-5-yl)benzene-1,3-diol (CTIBD). When CTIBD was applied to the treated extracellular side of the channel, the conductance-voltage relationship of the channel shifted toward a negative value, and the maximum conductance increased in a concentration-dependent manner. CTIBD altered the gating kinetics of the channel by dramatically slowing channel closing without effecting channel opening. The effects of CTIBD on bladder muscle relaxation and micturition function were tested in rat tissue and in vivo. CTIBD concentration-dependently reduced acetylcholine-induced contraction of urinary bladder smooth muscle strips. In an acetic acid-induced overactive bladder (OAB) model, intraperitoneal injection of 20 mg/kg CTIBD effectively restored frequent voiding contraction and lowered voiding volume without affecting other bladder function parameters. Thus, our results indicate that CTIBD and its derivatives are novel chemical activators of the bladder BK_Ca channel and potential candidates for OAB therapeutics. SIGNIFICANCE STATEMENT: The novel BK_Ca channel activator CTIBD was identified and characterized in this study. CTIBD directly activates the BK_Ca channel and relaxes urinary bladder smooth muscle of rat, so CTIBD can be a potential candidate for overactive bladder therapeutics.

The action of a BK channel opener

Rockman et al. in this issue of JGP describe how NS11021 opens BK channels, which make the compound a better tool to probe physiological roles and gating mechanisms of BK channels.

Zerumbone-Induced Analgesia Modulated via Potassium Channels and Opioid Receptors in Chronic Constriction Injury-Induced Neuropathic Pain

Zerumbone, a monocyclic sesquiterpene from the wild ginger plant Zingiber zerumbet (L.) Smith, attenuates allodynia and hyperalgesia. Currently, its mechanisms of action in neuropathic pain conditions remain unclear. This study examines the involvement of potassium channels and opioid receptors in zerumbone-induced analgesia in a chronic constriction injury (CCI) neuropathic pain mice model. Male Institute of Cancer Research (ICR) mice were subjected to CCI and behavioral responses were tested on day 14. Responses toward mechanical allodynia and thermal hyperalgesia were tested with von Frey's filament and Hargreaves' tests, respectively. Symptoms of neuropathic pain were significantly alleviated following treatment with zerumbone (10 mg/kg; intraperitoneal, i.p.). However, when the voltage-dependent K⁺ channel blocker tetraethylammonium (TEA, 4 mg/kg; i.p.), ATP-sensitive K⁺ channel blocker, glibenclamide (GLIB, 10 mg/kg; i.p.); small-conductance Ca²⁺-activated K⁺ channel inhibitor apamin (APA, 0.04 mg/kg; i.p.), or large-conductance Ca²⁺-activated K⁺ channel inhibitor charybdotoxin (CHAR, 0.02 mg/kg; i.p.) was administered prior to zerumbone (10 mg/kg; i.p.), the antiallodynic and antihyperalgesic effects of zerumbone were significantly reversed. Additionally, non-specific opioid receptors antagonist, naloxone (NAL, 10 mg/kg; i.p.), selective µ-, δ- and κ-opioid receptor antagonists; β-funaltrexamine (β-FN, 40 mg/kg; i.p.), naltrindole (20 mg/kg; s.c.), nor-binaltorphamine (10 mg/kg; s.c.) respectively attenuated the antiallodynic and antihyperalgesic effects of zerumbone. This outcome clearly demonstrates the participation of potassium channels and opioid receptors in the antineuropathic properties of zerumbone. As various clinically used neuropathic pain drugs also share this similar mechanism, this compound is, therefore, a highly potential substitute to these therapeutic options.

Clinical Importance of the Human Umbilical Artery Potassium Channels

Potassium (K⁺) channels are usually predominant in the membranes of vascular smooth muscle cells (SMCs). These channels play an important role in regulating the membrane potential and vessel contractility-a role that depends on the vascular bed. Thus, the activity of K⁺ channels represents one of the main mechanisms regulating the vascular tone in physiological and pathophysiological conditions. Briefly, the activation of K⁺ channels in SMC leads to hyperpolarization and vasorelaxation, while its inhibition induces depolarization and consequent vascular contraction. Currently, there are four different types of K⁺ channels described in SMCs: voltage-dependent K⁺ (K_V) channels, calcium-activated K⁺ (K_Ca) channels, inward rectifier K⁺ (Kir) channels, and 2-pore domain K⁺ (K_2P) channels. Due to the fundamental role of K⁺ channels in excitable cells, these channels are promising therapeutic targets in clinical practice. Therefore, this review discusses the basic properties of the various types of K⁺ channels, including structure, cellular mechanisms that regulate their activity, and new advances in the development of activators and blockers of these channels. The vascular functions of these channels will be discussed with a focus on vascular SMCs of the human umbilical artery. Then, the clinical importance of K⁺ channels in the treatment and prevention of cardiovascular diseases during pregnancy, such as gestational hypertension and preeclampsia, will be explored.

BKCa Channels as Targets for Cardioprotection

The large-conductance calcium- and voltage-activated K⁺ channel (BK_Ca) are encoded by the Kcnma1 gene. They are ubiquitously expressed in neuronal, smooth muscle, astrocytes, and neuroendocrine cells where they are known to play an important role in physiological and pathological processes. They are usually localized to the plasma membrane of the majority of the cells with an exception of adult cardiomyocytes, where BK_Ca is known to localize to mitochondria. BK_Ca channels couple calcium and voltage responses in the cell, which places them as unique targets for a rapid physiological response. The expression and activity of BK_Ca have been linked to several cardiovascular, muscular, and neurological defects, making them a key therapeutic target. Specifically in the heart muscle, pharmacological and genetic activation of BK_Ca channels protect the heart from ischemia-reperfusion injury and also facilitate cardioprotection rendered by ischemic preconditioning. The mechanism involved in cardioprotection is assigned to the modulation of mitochondrial functions, such as regulation of mitochondrial calcium, reactive oxygen species, and membrane potential. Here, we review the progress made on BK_Ca channels and cardioprotection and explore their potential roles as therapeutic targets for preventing acute myocardial infarction.

Single Molecule Fluorescence Imaging Reveals the Stoichiometry of BKγ1 Subunit in Living HEK293 Cell Expression System

Large conductance Ca²⁺-activated K⁺ (BK_Ca) channels are ubiquitously expressed in plasma membrane of both excitable and non-excitable cells and possess significant physiological functions. A tetrameric complex of α subunit (BKα) forms a functional pore of BK_Ca channel. The properties of BK_Ca channel, such as voltage-dependence, Ca²⁺ sensitivity and pharmacological responses, are extensively modulated by co-expressing accessory β subunits (BKβ), which can associate with BKα in one to one manner. Although the functional significance of newly identified γ subunits (BKγ) has been revealed, the stoichiometry between BKα and BKγ1 remains unclear. In the present study, we utilized a single molecule fluorescence imaging with a total internal reflection fluorescence (TIRF) microscope to directly count the number of green fluorescent protein (GFP)-tagged BKγ1 (BKγ1-GFP) within a single BK_Ca channel complex in HEK293 cell expression system. BKγ1-GFP significantly enhanced the BK channel activity even when the intracellular Ca²⁺ concentration was kept lower, i.e., 10 nM, than the physiological resting level. BKγ1-GFP stably formed molecular complexes with BKα-mCherry in the plasma membrane. Counting of GFP bleaching steps revealed that a BK_Ca channel can contain up to four BKγ1 per channel at the maximum. These results suggest that BKγ1 forms a BK_Ca channel complex with BKα in a 1 : 1 stoichiometry in a human cell line.

14-3-3γ, a novel regulator of the large-conductance Ca2+-activated K+ channel

14-3-3γ is a small protein regulating its target proteins through binding to phosphorylated serine/threonine residues. Sequence analysis of large-conductance Ca²⁺-activated K⁺ (BK) channels revealed a putative 14-3-3 binding site in the COOH-terminal region. Our previous data showed that 14-3-3γ is widely expressed in the mouse kidney. Therefore, we hypothesized that 14-3-3γ has a novel role in the regulation of BK channel activity and protein expression. We used electrophysiology, Western blot analysis, and coimmunoprecipitation to examine the effects of 14-3-3γ on BK channels both in vitro and in vivo. We demonstrated the interaction of 14-3-3γ with BK α-subunits (BKα) by coimmunoprecipitation. In human embryonic kidney-293 cells stably expressing BKα, overexpression of 14-3-3γ significantly decreased BK channel activity and channel open probability. 14-3-3γ inhibited both total and cell surface BKα protein expression while enhancing ERK1/2 phosphorylation in Cos-7 cells cotransfected with flag-14-3-3γ and myc-BK. Knockdown of 14-3-3γ by siRNA transfection markedly increased BKα expression. Blockade of the ERK1/2 pathway by incubation with the MEK-specific inhibitor U0126 partially abolished 14-3-3γ-mediated inhibition of BK protein expression. Similarly, pretreatment of the lysosomal inhibitor bafilomycin A1 reversed the inhibitory effects of 14-3-3γ on BK protein expression. Furthermore, overexpression of 14-3-3γ significantly increased BK protein ubiquitination in embryonic kidney-293 cells stably expressing BKα. Additionally, 3 days of dietary K⁺ challenge reduced 14-3-3γ expression and ERK1/2 phosphorylation while enhancing renal BK protein expression and K⁺ excretion. These data suggest that 14-3-3γ modulates BK channel activity and protein expression through an ERK1/2-mediated ubiquitin-lysosomal pathway.

Multifractal Properties of BK Channel Currents in Human Glioblastoma Cells

Potassium channels play an important physiological role in glioma cells. In particular, voltage- and Ca2+-activated large-conductance BK channels (gBK in gliomas) are involved in the intensive growth and extensive migrating behavior of the mentioned tumor cells; thus, they may be considered as a drug target for the therapeutic treatment of glioblastoma. To enable appropriate drug design, molecular mechanisms of gBK channel activation by diverse stimuli should be unraveled as well as the way that the specific conformational states of the channel relate to its functional properties (conducting/nonconducting). There is an open debate about the actual mechanism of BK channel gating, including the question of how the channel proteins undergo a range of conformational transitions when they flicker between nonconducting (functionally closed) and conducting (open) states. The details of channel conformational diffusion ought to have its representation in the properties of the experimental signal that describes the ion-channel activity. Nonlinear methods of analysis of experimental nonstationary series can be useful for observing the changes in the number of channel substates available from geometrical and energetic points of view at given external conditions. In this work, we analyze whether the multifractal properties of the activity of glioblastoma BK channels depend on membrane potential, and which states, conducting or nonconducting, affect the total signal to a larger extent. With this aim, we carried out patch-clamp experiments at different levels of membrane hyper- and depolarization. The obtained time series of single channel currents were analyzed using the multifractal detrended fluctuation analysis (MFDFA) method in a standard form and incorporating focus-based multifractal (FMF) formalism. Thus, we show the applicability of a modified MFDFA technique in the analysis of an experimental patch-clamp time series. The obtained results suggest that membrane potential strongly affects the conformational space of the gBK channel proteins and the considered process has nonlinear multifractal characteristics. These properties are the inherent features of the analyzed signals due to the fact that the main tendencies vanish after shuffling the data.

Pro-inflammatory Cytokines Drive Deregulation of Potassium Channel Expression in Primary Synovial Fibroblasts

The synovium secretes synovial fluid, but is also richly innervated with nociceptors and acts as a gateway between avascular joint tissues and the circulatory system. Resident fibroblast-like synoviocytes' (FLS) calcium-activated potassium channels (K _Ca) change in activity in arthritis models and this correlates with FLS activation.

Objective

To investigate this activation in an in vitro model of inflammatory arthritis; 72 h treatment with cytokines TNFα and IL1β.

Methods

FLS cells were isolated from rat synovial membranes. We analyzed global changes in FLS mRNA by RNA-sequencing, then focused on FLS ion channel genes and the corresponding FLS electrophysiological phenotype and finally modeling data with ingenuity pathway analysis (IPA) and MATLAB.

Results

IPA showed significant activation of inflammatory, osteoarthritic and calcium signaling canonical pathways by cytokines, and we identified ∼200 channel gene transcripts. The large K _Ca (BK) channel consists of the pore forming Kcnma1 together with β-subunits. Following cytokine treatment, a significant increase in Kcnma1 RNA abundance was detected by qPCR and changes in several ion channels were detected by RNA-sequencing, including a loss of BK channel β-subunit expression Kcnmb1/2 and an increase in Kcnmb3. In electrophysiological experiments, there was a decrease in over-all current density at 20 mV without change in chord conductance at this potential.

Conclusion

TNFα and IL1β treatment of FLS in vitro recapitulated several common features of inflammatory arthritis at the transcriptomic level, including increase in Kcnma1 and Kcnmb3 gene expression.

Case Report on: Very Early Afterdepolarizations in HiPSC-Cardiomyocytes-An Artifact by Big Conductance Calcium Activated Potassium Current (Ibk,Ca)

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) represent an unlimited source of human CMs that could be a standard tool in drug research. However, there is concern whether hiPSC-CMs express all cardiac ion channels at physiological level and whether they might express non-cardiac ion channels. In a control hiPSC line, we found large, “noisy” outward K⁺ currents, when we measured outward potassium currents in isolated hiPSC-CMs. Currents were sensitive to iberiotoxin, the selective blocker of big conductance Ca²⁺-activated K⁺ current (I_BK,Ca). Seven of 16 individual differentiation batches showed a strong initial repolarization in the action potentials (AP) recorded from engineered heart tissue (EHT) followed by very early afterdepolarizations, sometimes even with consecutive oscillations. Iberiotoxin stopped oscillations and normalized AP shape, but had no effect in other EHTs without oscillations or in human left ventricular tissue (LV). Expression levels of the alpha-subunit (K_Ca1.1) of the BK_Ca correlated with the presence of oscillations in hiPSC-CMs and was not detectable in LV. Taken together, individual batches of hiPSC-CMs can express sarcolemmal ion channels that are otherwise not found in the human heart, resulting in oscillating afterdepolarizations in the AP. HiPSC-CMs should be screened for expression of non-cardiac ion channels before being applied to drug research.

Effects of Single Nucleotide Polymorphisms in Human KCNMA1 on BK Current Properties

BK Ca²⁺-activated K⁺ channels are important regulators of membrane excitability. Multiple regulatory mechanisms tailor BK current properties across tissues, such as alternative splicing, posttranslational modifications, and auxiliary subunits. Another potential mechanism for modulating BK channel activity is genetic variation due to single nucleotide polymorphisms (SNPs). The gene encoding the human BK α subunit, KCNMA1, contains hundreds of SNPs. However, the variation in BK channel activity due to SNPs is not well studied. Here, we screened the effects of four SNPs (A138V, C495G, N599D, and R800W) on BK currents in HEK293T cells, selected based on predicted protein pathogenicity or disease linkage. We found that the SNPs C495G and R800W had the largest effects on BK currents, affecting the conductance–voltage relationship across multiple Ca²⁺ conditions in the context of two BK channel splice variants. In symmetrical K⁺, C495G shifted the V_1/2 to more hyperpolarized potentials (by −15 to −20 mV) and accelerated activation, indicating C495G confers some gain-of-function properties. R800W shifted the V_1/2 to more depolarized potentials (+15 to +35 mV) and slowed activation, conferring loss-of-function properties. Moreover, the C495G and R800W effects on current properties were found to persist with posttranslational modifications. In contrast, A138V and N599D had smaller and more variable effects on current properties. Neither application of alkaline phosphatase to patches, which results in increased BK channel activity attributed to channel dephosphorylation, nor bidirectional redox modulations completely abrogated SNP effects on BK currents. Lastly, in physiological K⁺, C495G increased the amplitude of action potential (AP)-evoked BK currents, while R800W had a more limited effect. However, the introduction of R800W in parallel with the epilepsy-linked mutation D434G (D434G/R800W) decreased the amplitude of AP-evoked BK currents compared with D434G alone. These results suggest that in a physiological context, C495G could increase BK activation, while the effects of the loss-of-function SNP R800W could oppose the gain-of-function effects of an epilepsy-linked mutation. Together, these results implicate naturally occurring human genetic variation as a potential modifier of BK channel activity across a variety of conditions.

KCNMA1-linked channelopathy

Bailey et al. review a new neurological channelopathy associated with KCNMA1, encoding the BK voltage- and Ca²⁺-activated K⁺ channel.

KCNMA1 encodes the pore-forming α subunit of the “Big K⁺” (BK) large conductance calcium and voltage-activated K⁺ channel. BK channels are widely distributed across tissues, including both excitable and nonexcitable cells. Expression levels are highest in brain and muscle, where BK channels are critical regulators of neuronal excitability and muscle contractility. A global deletion in mouse (KCNMA1^−/−) is viable but exhibits pathophysiology in many organ systems. Yet despite the important roles in animal models, the consequences of dysfunctional BK channels in humans are not well characterized. Here, we summarize 16 rare KCNMA1 mutations identified in 37 patients dating back to 2005, with an array of clinically defined pathological phenotypes collectively referred to as “KCNMA1-linked channelopathy.” These mutations encompass gain-of-function (GOF) and loss-of-function (LOF) alterations in BK channel activity, as well as several variants of unknown significance (VUS). Human KCNMA1 mutations are primarily associated with neurological conditions, including seizures, movement disorders, developmental delay, and intellectual disability. Due to the recent identification of additional patients, the spectrum of symptoms associated with KCNMA1 mutations has expanded but remains primarily defined by brain and muscle dysfunction. Emerging evidence suggests the functional BK channel alterations produced by different KCNMA1 alleles may associate with semi-distinct patient symptoms, such as paroxysmal nonkinesigenic dyskinesia (PNKD) with GOF and ataxia with LOF. However, due to the de novo origins for the majority of KCNMA1 mutations identified to date and the phenotypic variability exhibited by patients, additional evidence is required to establish causality in most cases. The symptomatic picture developing from patients with KCNMA1-linked channelopathy highlights the importance of better understanding the roles BK channels play in regulating cell excitability. Establishing causality between KCNMA1-linked BK channel dysfunction and specific patient symptoms may reveal new treatment approaches with the potential to increase therapeutic efficacy over current standard regimens.

MitoBKCa channel is functionally associated with its regulatory β1 subunit in cardiac mitochondria

KEY POINTS

Association of plasma membrane BK_Ca channels with BK-β subunits shapes their biophysical properties and physiological roles; however, functional modulation of the mitochondrial BK_Ca channel (mitoBK_Ca ) by BK-β subunits is not established. MitoBK_Ca -α and the regulatory BK-β1 subunit associate in mouse cardiac mitochondria. A large fraction of mitoBK_Ca display properties similar to that of plasma membrane BK_Ca when associated with BK-β1 (left-shifted voltage dependence of activation, V_1/2  = -55 mV, 12 µm matrix Ca²⁺ ). In BK-β1 knockout mice, cardiac mitoBK_Ca displayed a low P_o and a depolarized V_1/2 of activation (+47 mV at 12 µm matrix Ca²⁺ ) Co-expression of BK_Ca with the BK-β1 subunit in HeLa cells doubled the density of BK_Ca in mitochondria. The present study supports the view that the cardiac mitoBK_Ca channel is functionally modulated by the BK-β1 subunit; proper targeting and activation of mitoBK_Ca shapes mitochondrial Ca²⁺ handling.

ABSTRACT

Association of the plasma membrane BK_Ca channel with auxiliary BK-β1-4 subunits profoundly affects the regulatory mechanisms and physiological processes in which this channel participates. However, functional association of mitochondrial BK (mitoBK_Ca ) with regulatory subunits is unknown. We report that mitoBK_Ca functionally associates with its regulatory subunit BK-β1 in adult rodent cardiomyocytes. Cardiac mitoBK_Ca is a calcium- and voltage-activated channel that is sensitive to paxilline with a large conductance for K⁺ of 300 pS. Additionally, mitoBK_Ca displays a high open probability (P_o ) and voltage half-activation (V_1/2  = -55 mV, n = 7) resembling that of plasma membrane BK_Ca when associated with its regulatory BK-β1 subunit. Immunochemistry assays demonstrated an interaction between mitochondrial BK_Ca -α and its BK-β1 subunit. Mitochondria from the BK-β1 knockout (KO) mice showed sparse mitoBK_Ca currents (five patches with mitoBK_Ca activity out of 28 total patches from n = 5 different hearts), displaying a depolarized V_1/2 of activation (+47 mV in 12 µm matrix Ca²⁺ ). The reduced activity of mitoBK_Ca was accompanied by a high expression of BK_Ca transcript in the BK-β1 KO, suggesting a lower abundance of mitoBK_Ca channels in this genotype. Accordingly, BK-β1subunit increased the localization of BKDEC (i.e. the splice variant of BK_Ca that specifically targets mitochondria) into mitochondria by two-fold. Importantly, both paxilline-treated and BK-β1 KO mitochondria displayed a more rapid Ca²⁺ overload, featuring an early opening of the mitochondrial transition pore. We provide strong evidence that mitoBK_Ca associates with its regulatory BK-β1 subunit in cardiac mitochondria, ensuring proper targeting and activation of the mitoBK_Ca channel that helps to maintain mitochondrial Ca²⁺ homeostasis.

The molecular nature of the 17β-Estradiol binding site in the voltage- and Ca2+-activated K+ (BK) channel β1 subunit

The accessory β1 subunit modulates the Ca²⁺- and voltage-activated K⁺ (BK) channel gating properties mainly by increasing its apparent Ca²⁺ sensitivity. β1 plays an important role in the modulation of arterial tone and blood pressure by vascular smooth muscle cells (SMCs). 17β-estradiol (E2) increases the BK channel open probability (P_o) in SMCs, through a β1 subunit-dependent modulatory effect. Here, using molecular modeling, bioinformatics, mutagenesis, and electrophysiology, we identify a cluster of hydrophobic residues in the second transmembrane domain of the β1 subunit, including the residues W163 and F166, as the binding site for E2. We further show that the increase in P_o induced by E2 is associated with a stabilization of the voltage sensor in its active configuration and an increase in the coupling between the voltage sensor activation and pore opening. Since β1 is a key molecular player in vasoregulation, the findings reported here are of importance in the design of novel drugs able to modulate BK channels.

Mechanosensitivity of the BK Channels in Human Glioblastoma Cells: Kinetics and Dynamical Complexity

BK channels are potassium selective and exhibit large single-channel conductance. They play an important physiological role in glioma cells: they are involved in cell growth and extensive migrating behavior. Due to the fact that these processes are accompanied by changes in membrane stress, here, we examine mechanosensitive properties of BK channels from human glioblastoma cells (gBK channels). Experiments were performed by the use of patch-clamp method on excised patches under membrane suction (0-40 mmHg) at membrane hyper- and depolarization. We have also checked whether channel's activity is affected by possible changes of membrane morphology after a series of long impulses of suction. Unconventionally, we also analyzed internal structure of the experimental signal to make inferences about conformational dynamics of the channel in stressed membranes. We examined the fractal long-range memory effect (by R/S Hurst analysis), the rate of changes in information by sample entropy, or correlation dimension, and characterize its complexity over a range of scales by the use of Multiscale Entropy method. The obtained results indicate that gBK channels are mechanosensitive at membrane depolarization and hyperpolarization. Prolonged suction of membrane also influences open-closed fluctuations-it decreases channel's activity at membrane hyperpolarization and, in contrary, increases channel's activity at high voltages. Both membrane strain and its "fatigue" reduce dynamical complexity of channel gating, which suggest decrease in the number of available open conformations of channel protein in stressed membranes.

Determination of the Stoichiometry between α- and γ1 Subunits of the BK Channel Using LRET

Two families of accessory proteins, β and γ, modulate BK channel gating and pharmacology. Notably, in the absence of internal Ca²⁺, the γ1 subunit promotes a large shift of the BK conductance-voltage curve to more negative potentials. However, very little is known about how α- and γ1 subunits interact. In particular, the association stoichiometry between both subunits is unknown. Here, we propose a method to answer this question using lanthanide resonance energy transfer. The method assumes that the kinetics of lanthanide resonance energy transfer-sensitized emission of the donor double-labeled α/γ1 complex is the linear combination of the kinetics of the sensitized emission in single-labeled complexes. We used a lanthanide binding tag engineered either into the α- or the γ1 subunits to bind Tb⁺³ as the donor. The acceptor (BODIPY) was attached to the BK pore-blocker iberiotoxin. We determined that γ1 associates with the α-subunit with a maximal 1:1 stoichiometry. This method could be applied to determine the stoichiometry of association between proteins within heteromultimeric complexes.

The augmentation of BK channel activity by nitric oxide signaling in rat cerebral arteries involves co-localized regulatory elements

Large conductance, Ca²⁺-activated K⁺ (BK) channels control cerebrovascular tone; however, the regulatory processes influencing these channels remain poorly understood. Here, we investigate the cellular mechanisms underlying the enhancement of BK current in rat cerebral arteries by nitric oxide (NO) signaling. In isolated cerebral myocytes, BK current magnitude was reversibly increased by sodium nitroprusside (SNP, 100 μM) and sensitive to the BK channel inhibitor, penitrem-A (100 nM). Fostriecin (30 nM), a protein phosphatase type 2A (PP2A) inhibitor, significantly prolonged the SNP-induced augmentation of BK current and a similar effect was produced by sildenafil (30 nM), a phosphodiesterase 5 (PDE5) inhibitor. Using proximity ligation assay (PLA)-based co-immunostaining, BK channels were observed to co-localize with PP2A, PDE5, and cGMP-dependent protein kinase (cGKI) (spatial restriction < 40 nm); cGKI co-localization increased following SNP exposure. SNP (10 μM) reversibly inhibited myogenic tone in cannulated cerebral arteries, which was augmented by either fostriecin or sildenafil and inhibited by penitrem-A. Collectively, these data suggest that (1) cGKI, PDE5, and PP2A are compartmentalized with cerebrovascular BK channels and determine the extent of BK current augmentation by NO/cGMP signaling, and (2) the dynamic regulation of BK activity by co-localized signaling enzymes modulates NO-evoked dilation of cerebral resistance arteries.

Maxi-K channel (BKCa) activity veils the myogenic tone of mesenteric artery in rats

Arterioles and small arteries change their tone in response to transmural pressure changes, called myogenic tone (MT). In comparison to the branches of cerebral arteries (CA) showing prominent MT, the third branches of mesenteric arteries (MA) with similar diameters show weaker MT. Here, we aimed to analyze the electrophysiological differences responsible for the weaker MT in MA (MT_MA) than MT in CA (MT_CA). We measured ionic current using patch clamp in isolated MA smooth muscle cells (MASMCs) and CA smooth muscle cells (CASMCs) of rats. MT was analyzed using video analysis of pressurized small arteries. Quantitative‐PCR (q‐PCR) and immunofluorescence confocal microscopy were used to compare the mRNA and protein expression level of big‐conductance Ca²⁺‐activated K⁺ channel (BK_Ca) subunits (Slo1α and Sloβ1). Whole‐cell patch clamp study revealed higher density of voltage‐operated Ca²⁺ channel current (I_CaV) in the MASMCs than in CASMCs. Although voltage‐gated K⁺ channel current (I_Kv) was also higher in MASMCs, treatment with Kv inhibitor (4‐aminopyridine) did not affect MT_MA. Interestingly, BK_Ca current density and the frequency of spontaneous transient outward currents (STOCs) were consistently higher in MASMCs than in CASMCs. Inside‐out patch clamp showed that the Ca²⁺‐sensitivity of BK_Ca is higher in MASMCs than in CASMCs. Iberiotoxin, a selective BK_Ca inhibitor, augmented MT_MA by a larger extent than MT_CA. Although q‐PCR analysis did not reveal a significant difference of mRNAs for Slo1α and Sloβ1, immunofluorescence image suggested higher expression of Slo1α in MASMCs than in CASMCs. Despite the large I_CaV density, the high activities of BK_Ca including the more frequent STOCs in MASMCs veils the potentially strong MT_MA.

Proximal clustering between BK and CaV1.3 channels promotes functional coupling and BK channel activation at low voltage

Ca_V-channel dependent activation of BK channels is critical for feedback control of both calcium influx and cell excitability. Here we addressed the functional and spatial interaction between BK and Ca_V1.3 channels, unique Ca_V1 channels that activate at low voltages. We found that when BK and Ca_V1.3 channels were co-expressed in the same cell, BK channels started activating near −50 mV, ~30 mV more negative than for activation of co-expressed BK and high-voltage activated Ca_V2.2 channels. In addition, single-molecule localization microscopy revealed striking clusters of Ca_V1.3 channels surrounding clusters of BK channels and forming a multi-channel complex both in a heterologous system and in rat hippocampal and sympathetic neurons. We propose that this spatial arrangement allows tight tracking between local BK channel activation and the gating of Ca_V1.3 channels at quite negative membrane potentials, facilitating the regulation of neuronal excitability at voltages close to the threshold to fire action potentials.

DOI:http://dx.doi.org/10.7554/eLife.28029.001

EAVK segment "c" sequence confers Ca2+-dependent changes to the kinetics of full-length human Ano1

Anoctamin1 (Ano1 and TMEM16A) is a calcium-activated chloride channel specifically expressed in the interstitial cells of Cajal (ICC) of the gastrointestinal tract muscularis propria. Ano1 is necessary for normal electrical slow waves and ICC proliferation. The full-length human Ano1 sequence includes an additional exon, exon "0," at the NH₂ terminus. Ano1 with exon 0 [Ano1₍₀₎] had a lower EC₅₀ for intracellular calcium ([Ca²⁺]_i) and faster chloride current (I_Cl) kinetics. The Ano1 alternative splice variant with segment "c" encoding exon 13 expresses on the first intracellular loop four additional amino acid residues, EAVK, which alter I_Cl at low [Ca²⁺]_i Exon 13 is expressed in 75-100% of Ano1 transcripts in most human tissues but only 25% in the human stomach. Our aim was to determine the effect of EAVK deletion on Ano1₍₀₎I_Cl parameters. By voltage-clamp electrophysiology, we examined I_Cl in HEK293 cells transiently expressing Ano1₍₀₎ with or without the EAVK sequence [Ano1₍₀₎ΔEAVK]. The EC₅₀ values of activating and deactivating I_Cl for [Ca²⁺]_i were 438 ± 7 and 493 ± 9 nM for Ano1₍₀₎ but higher for Ano1₍₀₎ΔEAVK at 746 ± 47 and 761 ± 26 nM, respectively. Meanwhile, the EC₅₀ values for the ratio of instantaneous to steady-state I_Cl were not different between variants. Congruently, the time constant of activation was slower for Ano1₍₀₎ΔEAVK than Ano1₍₀₎ currents at intermediate [Ca²⁺]_i These results suggest that EAVK decreases the calcium sensitivity of Ano1₍₀₎ current activation and deactivation by slowing activation kinetics. Differential expression of EAVK in the human stomach may function as a switch to increase sensitivity to [Ca²⁺]_i via faster gating of Ano1.NEW & NOTEWORTHY Calcium-activated chloride channel anoctamin1 (Ano1) is necessary for normal slow waves in the gastrointestinal interstitial cells of Cajal. Exon 0 encodes the NH₂ terminus of full-length human Ano1 [Ano1₍₀₎], while exon 13 encodes residues EAVK on its first intracellular loop. Splice variants lack EAVK more often in the stomach than other tissues. Ano1₍₀₎ without EAVK [Ano1₍₀₎ΔEAVK] has reduced sensitivity for intracellular calcium, attributable to slower kinetics. Differential expression of EAVK may function as a calcium-sensitive switch in the human stomach.

Membrane depolarization activates BK channels through ROCK-mediated β1 subunit surface trafficking to limit vasoconstriction

Membrane depolarization of smooth muscle cells (myocytes) in the small arteries that regulate regional organ blood flow leads to vasoconstriction. Membrane depolarization also activates large-conductance calcium (Ca²⁺)-activated potassium (BK) channels, which limits Ca²⁺ channel activity that promotes vasoconstriction, thus leading to vasodilation. We showed that in human and rat arterial myocytes, membrane depolarization rapidly increased the cell surface abundance of auxiliary BK β1 subunits but not that of the pore-forming BKα channels. Membrane depolarization stimulated voltage-dependent Ca²⁺ channels, leading to Ca²⁺ influx and the activation of Rho kinase (ROCK) 1 and 2. ROCK1/2-mediated activation of Rab11A promoted the delivery of β1 subunits to the plasma membrane by Rab11A-positive recycling endosomes. These additional β1 subunits associated with BKα channels already at the plasma membrane, leading to an increase in apparent Ca²⁺ sensitivity and activation of the channels in pressurized arterial myocytes and vasodilation. Thus, membrane depolarization activates BK channels through stimulation of ROCK- and Rab11A-dependent trafficking of β1 subunits to the surface of arterial myocytes.

Interplay among distinct Ca2+ conductances drives Ca2+ sparks/spontaneous transient outward currents in rat cerebral arteries

KEY POINTS

Distinct Ca²⁺ channels work in a coordinated manner to grade Ca²⁺ spark/spontaneous transient outward currents (STOCs) in rat cerebral arteries. The relative contribution of each Ca²⁺ channel to Ca²⁺ spark/STOC production depends upon their biophysical properties and the resting membrane potential of smooth muscle. Na⁺ /Ca²⁺ exchanger, but not TRP channels, can also facilitate STOC production.

ABSTRACT

Ca²⁺ sparks are generated in a voltage-dependent manner to initiate spontaneous transient outward currents (STOCs), events that moderate arterial constriction. In this study, we defined the mechanisms by which membrane depolarization increases Ca²⁺ sparks and subsequent STOC production. Using perforated patch clamp electrophysiology and rat cerebral arterial myocytes, we monitored STOCs in the presence and absence of agents that modulate Ca²⁺ entry. Beginning with Ca_V 3.2 channel inhibition, Ni²⁺ was shown to decrease STOC frequency in cells held at hyperpolarized (-40 mV) but not depolarized (-20 mV) voltages. In contrast, nifedipine, a Ca_V 1.2 inhibitor, markedly suppressed STOC frequency at -20 mV but not -40 mV. These findings aligned with the voltage-dependent profiles of L- and T-type Ca²⁺ channels. Furthermore, computational and experimental observations illustrated that Ca²⁺ spark production is intimately tied to the activity of both conductances. Intriguingly, this study observed residual STOC production at depolarized voltages that was independent of Ca_V 1.2 and Ca_V 3.2. This residual component was insensitive to TRPV4 channel modulation and was abolished by Na⁺ /Ca²⁺ exchanger blockade. In summary, our work highlights that the voltage-dependent triggering of Ca²⁺ sparks/STOCs is not tied to a single conductance but rather reflects an interplay among multiple Ca²⁺ permeable pores with distinct electrophysiological properties. This integrated orchestration enables smooth muscle to grade Ca²⁺ spark/STOC production and thus precisely tune negative electrical feedback.

Ryanodine-induced vasoconstriction of the gerbil spiral modiolar artery depends on the Ca2+ sensitivity but not on Ca2+ sparks or BK channels

Background

In many vascular smooth muscle cells (SMCs), ryanodine receptor-mediated Ca²⁺ sparks activate large-conductance Ca²⁺-activated K⁺ (BK) channels leading to lowered SMC [Ca²⁺]_i and vasodilation. Here we investigated whether Ca²⁺ sparks regulate SMC global [Ca²⁺]_i and diameter in the spiral modiolar artery (SMA) by activating BK channels.

Methods

SMAs were isolated from adult female gerbils, loaded with the Ca²⁺-sensitive flourescent dye fluo-4 and pressurized using a concentric double-pipette system. Ca²⁺ signals and vascular diameter changes were recorded using a laser-scanning confocal imaging system. Effects of various pharmacological agents on Ca²⁺ signals and vascular diameter were analyzed.

Results

Ca²⁺ sparks and waves were observed in pressurized SMAs. Inhibition of Ca²⁺ sparks with ryanodine increased global Ca²⁺ and constricted SMA at 40 cmH₂O but inhibition of Ca²⁺ sparks with tetracaine or inhibition of BK channels with iberiotoxin at 40 cmH₂O did not produce a similar effect. The ryanodine-induced vasoconstriction observed at 40 cmH₂O was abolished at 60 cmH₂O, consistent with a greater Ca²⁺-sensitivity of constriction at 40 cmH₂O than at 60 cmH₂O. When the Ca²⁺-sensitivity of the SMA was increased by prior application of 1 nM endothelin-1, ryanodine induced a robust vasoconstriction at 60 cmH₂O.

Conclusions

The results suggest that Ca²⁺ sparks, while present, do not regulate vascular diameter in the SMA by activating BK channels and that the regulation of vascular diameter in the SMA is determined by the Ca²⁺-sensitivity of constriction.

Impact of intracellular ion channels on cancer development and progression

Cancer research is nowadays focused on the identification of possible new targets in order to try to develop new drugs for curing untreatable tumors. Ion channels have emerged as "oncogenic" proteins, since they have an aberrant expression in cancers compared to normal tissues and contribute to several hallmarks of cancer, such as metabolic re-programming, limitless proliferative potential, apoptosis-resistance, stimulation of neo-angiogenesis as well as cell migration and invasiveness. In recent years, not only the plasma membrane but also intracellular channels and transporters have arisen as oncological targets and were proposed to be associated with tumorigenesis. Therefore, the research is currently focusing on understanding the possible role of intracellular ion channels in cancer development and progression on one hand and, on the other, on developing new possible drugs able to modulate the expression and/or activity of these channels. In a few cases, the efficacy of channel-targeting drugs in reducing tumors has already been demonstrated in vivo in preclinical mouse models.

Potassium channel blockers from the venom of the Brazilian scorpion Tityus serrulatus ()

Potassium (K(+)) channels are trans-membrane proteins, which play a key role in cellular excitability and signal transduction pathways. Scorpion toxins blocking the ion-conducting pore from the external side have been invaluable probes to elucidate the structural, functional, and physio-pathological characteristics of these ion channels. This review will focus on the interaction between K(+) channels and their peptide blockers isolated from the venom of the scorpion Tityus serrulatus, which is considered as the most dangerous scorpion in Brazil, in particular in Minas-Gerais State, where many casualties are described each year. The primary mechanisms of action of these K(+) blockers will be discussed in correlation with their structure, very often non-canonical compared to those of other well known K(+) channels blockers purified from other scorpion venoms. Also, special attention will be brought to the most recent data obtained by proteomic and transcriptomic analyses on Tityus serrulatus venoms and venom glands.

Ethanol Effect on BK Channels is Modulated by Magnesium

BACKGROUND

Alcoholics have been reported to have reduced levels of magnesium in both their extracellular and intracellular compartments. Calcium-dependent potassium channels (BK) are known to be one of ethanol (EtOH)'s better known molecular targets.

METHODS

Using outside-out patches from hippocampal neuronal cultures, we examined the consequences of altered intracellular Mg(2+) on the effects that EtOH has on BK channels.

RESULTS

We find that the effect of EtOH is bimodally influenced by the Mg(2+) concentration on the cytoplasmic side. More specifically, when internal Mg(2+) concentrations are ≤200 μM, EtOH decreases BK activity, whereas it increases activity when Mg(2+) is at 1 mM. Similar results are obtained when using patches from HEK cells expressing only the α-subunit of BK. When patches are made with the actin destabilizer cytochalasin D present on the cytoplasmic side, the potentiation caused by EtOH becomes independent of the Mg(2+) concentration. Furthermore, in the presence of the actin stabilizer phalloidin, EtOH causes inhibition even at Mg(2+) concentrations of 1 mM.

CONCLUSIONS

Internal Mg(2+) can modulate the EtOH effects on BK channels only when there is an intact, internal actin interaction with the channel, as is found at synapses. We propose that the EtOH-induced decrease in cytoplasmic Mg(2+) observed in frequent/chronic drinkers would decrease EtOH's actions on synaptic (e.g., actin-bound) BK channels, producing a form of molecular tolerance.