A large-conductance calcium-regulated K+ channel in human dermal fibroblast mitochondria

mitoBKCa is functionally expressed in murine and human breast cancer cells and potentially contributes to metabolic reprogramming

Alterations in the function of K+ channels such as the voltage- and Ca2+-activated K+ channel of large conductance (BKCa) reportedly promote breast cancer (BC) development and progression. Underlying molecular mechanisms remain, however, elusive. Here, we provide electrophysiological evidence for a BKCa splice variant localized to the inner mitochondrial membrane of murine and human BC cells (mitoBKCa). Through a combination of genetic knockdown and knockout along with a cell permeable BKCa channel blocker, we show that mitoBKCa modulates overall cellular and mitochondrial energy production, and mediates the metabolic rewiring referred to as the 'Warburg effect', thereby promoting BC cell proliferation in the presence and absence of oxygen. Additionally, we detect mitoBKCa and BKCa transcripts in low or high abundance, respectively, in clinical BC specimens. Together, our results emphasize, that targeting mitoBKCa could represent a treatment strategy for selected BC patients in future.

Exploring the Impact of BKCa Channel Function in Cellular Membranes on Cardiac Electrical Activity

This review paper delves into the current body of evidence, offering a thorough analysis of the impact of large-conductance Ca2+-activated K+ (BKCa or BK) channels on the electrical dynamics of the heart. Alterations in the activity of BKCa channels, responsible for the generation of the overall magnitude of Ca2+-activated K+ current at the whole-cell level, occur through allosteric mechanisms. The collaborative interplay between membrane depolarization and heightened intracellular Ca2+ ion concentrations collectively contribute to the activation of BKCa channels. Although fully developed mammalian cardiac cells do not exhibit functional expression of these ion channels, evidence suggests their presence in cardiac fibroblasts that surround and potentially establish close connections with neighboring cardiac cells. When cardiac cells form close associations with fibroblasts, the high single-ion conductance of these channels, approximately ranging from 150 to 250 pS, can result in the random depolarization of the adjacent cardiac cell membranes. While cardiac fibroblasts are typically electrically non-excitable, their prevalence within heart tissue increases, particularly in the context of aging myocardial infarction or atrial fibrillation. This augmented presence of BKCa channels' conductance holds the potential to amplify the excitability of cardiac cell membranes through effective electrical coupling between fibroblasts and cardiomyocytes. In this scenario, this heightened excitability may contribute to the onset of cardiac arrhythmias. Moreover, it is worth noting that the substances influencing the activity of these BKCa channels might influence cardiac electrical activity as well. Taken together, the BKCa channel activity residing in cardiac fibroblasts may contribute to cardiac electrical function occurring in vivo.

Machine Learning Methods for the Analysis of the Patch-Clamp Signals

Patch-clamp technique provides a unique possibility to record the ion channels' activity. This method enables tracking the changes in their functional states at controlled conditions on a real-time scale. Kinetic parameters evaluated for the patch-clamp signals form the fundamentals of electrophysiological characteristics of the channel functioning. Nevertheless, the noisy series of ionic currents flowing through the channel protein(s) seem to be bountiful of information, and the standard data processing techniques likely unravel only its part. Rapid development of artificial intelligence (AI) techniques, especially machine learning (ML), gives new prospects for whole channelology. Here we consider the question of the AI applications in the patch-clamp signal analysis. It turns out that the AI methods may not only enable for automatizing of signal analysis, but also they can be used in finding inherent patterns of channel gating and allow the researchers to uncover the details of gating machinery, which had been never considered before. In this work, we outline the currently known AI methods that turned out to be utilizable and useful in the analysis of patch-clamp signals. This chapter can be considered an introductory guide to the application of AI methods in the analysis of the time series of channel currents (together with its advantages, disadvantages, and limitations), but we also propose new possible directions in this field.

Lack of activity of the mitochondrial large-conductance calcium-regulated potassium channels in senescent vascular smooth muscle cells

A limited number of studies have shown functional changes in mitochondrial ion channels in aging and senescent cells. We have identified, for the first time, mitochondrial large-conductance calcium-regulated potassium channels in human smooth muscle mitochondria. This channel, with a conductance of 273 pS, was regulated by calcium ions and membrane potential. Additionally, it was activated by the potassium channel opener NS11021 and blocked by paxilline. Importantly, we have shown that senescence of these cells induced by hydrogen peroxide treatment leads to the disappearance of potassium channel protein levels and channel activity measured by the single channel patch-clamp technique. Our data suggest that disturbances in the expression of mitochondrial large conductance calcium-regulated potassium channels may be hallmarks of cellular senescence and contribute to the misregulation of mitochondrial function in senescent cells.

Signaling Roleplay between Ion Channels during Mammalian Sperm Capacitation

In order to accomplish their primary goal, mammalian spermatozoa must undergo a series of physiological, biochemical, and functional changes crucial for the acquisition of fertilization ability. Spermatozoa are highly polarized cells, which must swiftly respond to ionic changes on their passage through the female reproductive tract, and which are necessary for male gametes to acquire their functional competence. This review summarizes the current knowledge about specific ion channels and transporters located in the mammalian sperm plasma membrane, which are intricately involved in the initiation of changes within the ionic milieu of the sperm cell, leading to variations in the sperm membrane potential, membrane depolarization and hyperpolarization, changes in sperm motility and capacitation to further lead to the acrosome reaction and sperm-egg fusion. We also discuss the functionality of selected ion channels in male reproductive health and/or disease since these may become promising targets for clinical management of infertility in the future.

Mitochondrial Ion Channels

Mitochondria are involved in multiple cellular tasks, such as ATP synthesis, metabolism, metabolite and ion transport, regulation of apoptosis, inflammation, signaling, and inheritance of mitochondrial DNA. The majority of the correct functioning of mitochondria is based on the large electrochemical proton gradient, whose component, the inner mitochondrial membrane potential, is strictly controlled by ion transport through mitochondrial membranes. Consequently, mitochondrial function is critically dependent on ion homeostasis, the disturbance of which leads to abnormal cell functions. Therefore, the discovery of mitochondrial ion channels influencing ion permeability through the membrane has defined a new dimension of the function of ion channels in different cell types, mainly linked to the important tasks that mitochondrial ion channels perform in cell life and death. This review summarizes studies on animal mitochondrial ion channels with special focus on their biophysical properties, molecular identity, and regulation. Additionally, the potential of mitochondrial ion channels as therapeutic targets for several diseases is briefly discussed.

Activation of Endothelial Large Conductance Potassium Channels Protects against TNF-α-Induced Inflammation

Elevated TNF-α levels in serum and broncho-alveolar lavage fluid of acute lung injury patients correlate with mortality rates. We hypothesized that pharmacological plasma membrane potential (Em) hyperpolarization protects against TNF-α-induced CCL-2 and IL-6 secretion from human pulmonary endothelial cells through inhibition of inflammatory Ca2+-dependent MAPK pathways. Since the role of Ca2+ influx in TNF-α-mediated inflammation remains poorly understood, we explored the role of L-type voltage-gated Ca2+ (CaV) channels in TNF-α-induced CCL-2 and IL-6 secretion from human pulmonary endothelial cells. The CaV channel blocker, Nifedipine, decreased both CCL-2 and IL-6 secretion, suggesting that a fraction of CaV channels is open at the significantly depolarized resting Em of human microvascular pulmonary endothelial cells (-6 ± 1.9 mV), as shown by whole-cell patch-clamp measurements. To further explore the role of CaV channels in cytokine secretion, we demonstrated that the beneficial effects of Nifedipine could also be achieved by Em hyperpolarization via the pharmacological activation of large conductance K+ (BK) channels with NS1619, which elicited a similar decrease in CCL-2 but not IL-6 secretion. Using functional gene enrichment analysis tools, we predicted and validated that known Ca2+-dependent kinases, JNK-1/2 and p38, are the most likely pathways to mediate the decrease in CCL-2 secretion.

Flavonoids as new regulators of mitochondrial potassium channels: contribution to cardioprotection

Objectives

Acute myocardial ischemia is one of the major causes of illness in western society. Reduced coronary blood supply leads to cell death and loss of cardiomyocyte population, resulting in serious and often irreversible consequences on myocardial function. Mitochondrial potassium (mitoK) channels have been identified as fine regulators of mitochondrial function and, consequently, in the metabolism of the whole cell, and in the mechanisms underlying the cardioprotection. Interestingly, mitoK channels represent a novel putative target for treating cardiovascular diseases, particularly myocardial infarction, and their modulators represent an interesting tool for pharmacological intervention. In this review, we took up the challenge of selecting flavonoids that show cardioprotective properties through the activation of mitoK channels.

Key findings

A brief overview of the main information on mitoK channels and their participation in the induction of cytoprotective processes was provided. Then, naringenin, quercetin, morin, theaflavin, baicalein, epigallocatechin gallate, genistein, puerarin, luteolin and proanthocyanidins demonstrated to be effective modulators of mitoK channels activity, mediating many beneficial effects.

Summary

The pathophysiological role of mitoK channels has been investigated as well as the impact of flavonoids on this target with particular attention to their potential role in the prevention of cardiovascular disorders.

Luteolin-Induced Activation of Mitochondrial BKCa Channels: Undisclosed Mechanism of Cytoprotection

Luteolin (LUT) is a well-known flavonoid that exhibits a number of beneficial properties. Among these, it shows cardioprotective effects, as confirmed by numerous studies. However, its effect on mitochondrial potassium channels, the activation of which is related to cytoprotection, as well as on heart ischemia/reperfusion (I/R) damage prevention, has not yet been investigated. The large conductance calcium-regulated potassium channel (mitoBKCa) has been identified in both the mitochondria of the vascular endothelial cells, which plays a significant role in the functioning of the cardiovascular system under oxidative stress-related conditions, and in the mitochondria of cardiomyocytes, where it is deeply involved in cardiac protection against I/R injury. Therefore, the aim of this study was to explore the role of the mitoBKCa channel in luteolin-induced cytoprotection. A number of in vitro, in vivo, ex vivo and in silico studies have confirmed that luteolin activates this channel in the mitochondria of cardiomyocytes and endothelial cells, which in turn leads to the protection of the endothelium and a significant reduction in the extent of damage resulting from myocardial infarction, where this effect was partially abolished by the mitoBKCa channel blocker paxilline. In conclusion, these results suggest that luteolin has cardioprotective effects, at least in part, through the activation of the mitoBKCa channel, shedding light on a new putative mechanism of action.

Methods of Measuring Mitochondrial Potassium Channels: A Critical Assessment

In this paper, the techniques used to study the function of mitochondrial potassium channels are critically reviewed. The majority of these techniques have been known for many years as a result of research on plasma membrane ion channels. Hence, in this review, we focus on the critical evaluation of techniques used in the studies of mitochondrial potassium channels, describing their advantages and limitations. Functional analysis of mitochondrial potassium channels in comparison to that of plasmalemmal channels presents additional experimental challenges. The reliability of functional studies of mitochondrial potassium channels is often affected by the need to isolate mitochondria and by functional properties of mitochondria such as respiration, metabolic activity, swelling capacity, or high electrical potential. Three types of techniques are critically evaluated: electrophysiological techniques, potassium flux measurements, and biochemical techniques related to potassium flux measurements. Finally, new possible approaches to the study of the function of mitochondrial potassium channels are presented. We hope that this review will assist researchers in selecting reliable methods for studying, e.g., the effects of drugs on mitochondrial potassium channel function. Additionally, this review should aid in the critical evaluation of the results reported in various articles on mitochondrial potassium channels.

Alternative Targets for Modulators of Mitochondrial Potassium Channels

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

BK in Double-Membrane Organelles: A Biophysical, Pharmacological, and Functional Survey

In the 1970s, calcium-activated potassium currents were recorded for the first time. In 10years, this Ca²⁺-activated potassium channel was identified in rat skeletal muscle, chromaffin cells and characterized in skeletal muscle membranes reconstituted in lipid bilayers. This calcium- and voltage-activated potassium channel, dubbed BK for “Big K” due to its large ionic conductance between 130 and 300 pS in symmetric K⁺. The BK channel is a tetramer where the pore-forming α subunit contains seven transmembrane segments. It has a modular architecture containing a pore domain with a highly potassium-selective filter, a voltage-sensor domain and two intracellular Ca²⁺ binding sites in the C-terminus. BK is found in the plasma membrane of different cell types, the inner mitochondrial membrane (mitoBK) and the nuclear envelope’s outer membrane (nBK). Like BK channels in the plasma membrane (pmBK), the open probability of mitoBK and nBK channels are regulated by Ca²⁺ and voltage and modulated by auxiliary subunits. BK channels share common pharmacology to toxins such as iberiotoxin, charybdotoxin, paxilline, and agonists of the benzimidazole family. However, the precise role of mitoBK and nBK remains largely unknown. To date, mitoBK has been reported to play a role in protecting the heart from ischemic injury. At the same time, pharmacology suggests that nBK has a role in regulating nuclear Ca²⁺, membrane potential and expression of eNOS. Here, we will discuss at the biophysical level the properties and differences of mitoBK and nBK compared to those of pmBK and their pharmacology and function.

Cytoprotective effects of the flavonoid quercetin by activating mitochondrial BKCa channels in endothelial cells

Mitochondrial potassium channels have been implicated in cytoprotective mechanisms. Activation of the mitochondrial large-conductance Ca²⁺-regulated potassium (mitoBK_Ca) channel is important for protecting brain tissue against stroke damage as well as heart tissue against ischemia damage. In this paper, we examine the effect of the natural flavonoid quercetin as an activator of the mitoBK_Ca channel. Quercetin has a beneficial effect on many processes in the human body and interacts with many receptors and signaling pathways. We found that quercetin acts on mitochondria as a mitoBK_Ca channel opener. The activation observed with the patch-clamp technique was potent and increased the channel open probability from approximately 0.35 to 0.95 at + 40 mV in the micromolar concentration range. Moreover, quercetin at a concentration of 10 µM protected cells by reducing damage from treatment factors (tumor necrosis factor α and cycloheximide) by 40%, enhancing cellular migration and depolarizing the mitochondrial membrane. Moreover, the presence of quercetin increased the gene expression and protein level of the mitoBK_Ca β3 regulatory subunit. The observed cytoprotective effects suggested the involvement of BK_Ca channel activation. Additionally, the newly discovered mitoBK_Ca activator quercetin elucidates a new mitochondrial pathway that is beneficial for vascular endothelial cells.

Routes for Potassium Ions across Mitochondrial Membranes: A Biophysical Point of View with Special Focus on the ATP-Sensitive K+ Channel

Potassium ions can cross both the outer and inner mitochondrial membranes by means of multiple routes. A few potassium-permeable ion channels exist in the outer membrane, while in the inner membrane, a multitude of different potassium-selective and potassium-permeable channels mediate K+ uptake into energized mitochondria. In contrast, potassium is exported from the matrix thanks to an H+/K+ exchanger whose molecular identity is still debated. Among the K+ channels of the inner mitochondrial membrane, the most widely studied is the ATP-dependent potassium channel, whose pharmacological activation protects cells against ischemic damage and neuronal injury. In this review, we briefly summarize and compare the different hypotheses regarding the molecular identity of this patho-physiologically relevant channel, taking into account the electrophysiological characteristics of the proposed components. In addition, we discuss the characteristics of the other channels sharing localization to both the plasma membrane and mitochondria.

Multidimensional Regulation of Cardiac Mitochondrial Potassium Channels

Mitochondria play a fundamental role in the energetics of cardiac cells. Moreover, mitochondria are involved in cardiac ischemia/reperfusion injury by opening the mitochondrial permeability transition pore which is the major cause of cell death. The preservation of mitochondrial function is an essential component of the cardioprotective mechanism. The involvement of mitochondrial K+ transport in this complex phenomenon seems to be well established. Several mitochondrial K+ channels in the inner mitochondrial membrane, such as ATP-sensitive, voltage-regulated, calcium-activated and Na+-activated channels, have been discovered. This obliges us to ask the following question: why is the simple potassium ion influx process carried out by several different mitochondrial potassium channels? In this review, we summarize the current knowledge of both the properties of mitochondrial potassium channels in cardiac mitochondria and the current understanding of their multidimensional functional role. We also critically summarize the pharmacological modulation of these proteins within the context of cardiac ischemia/reperfusion injury and cardioprotection.

Identification of the Large-Conductance Ca2+-Regulated Potassium Channel in Mitochondria of Human Bronchial Epithelial Cells

Mitochondria play a key role in energy metabolism within the cell. Potassium channels such as ATP-sensitive, voltage-gated or large-conductance Ca²⁺-regulated channels have been described in the inner mitochondrial membrane. Several hypotheses have been proposed to describe the important roles of mitochondrial potassium channels in cell survival and death pathways. In the current study, we identified two populations of mitochondrial large-conductance Ca²⁺-regulated potassium (mitoBK_Ca) channels in human bronchial epithelial (HBE) cells. The biophysical properties of the channels were characterized using the patch-clamp technique. We observed the activity of the channel with a mean conductance close to 285 pS in symmetric 150/150 mM KCl solution. Channel activity was increased upon application of the potassium channel opener NS11021 in the micromolar concentration range. The channel activity was completely inhibited by 1 µM paxilline and 300 nM iberiotoxin, selective inhibitors of the BK_Ca channels. Based on calcium and iberiotoxin modulation, we suggest that the C-terminus of the protein is localized to the mitochondrial matrix. Additionally, using RT-PCR, we confirmed the presence of α pore-forming (Slo1) and auxiliary β3-β4 subunits of BK_Ca channel in HBE cells. Western blot analysis of cellular fractions confirmed the mitochondrial localization of α pore-forming and predominately β3 subunits. Additionally, the regulation of oxygen consumption and membrane potential of human bronchial epithelial mitochondria in the presence of the potassium channel opener NS11021 and inhibitor paxilline were also studied. In summary, for the first time, the electrophysiological and functional properties of the mitoBK_Ca channel in a bronchial epithelial cell line were described.

Single channel properties of mitochondrial large conductance potassium channel formed by BK-VEDEC splice variant

The activation of mitochondrial large conductance calcium-activated potassium (mitoBKCa) channels increases cell survival during ischemia/reperfusion injury of cardiac cells. The basic biophysical and pharmacological properties of mitoBKCa correspond to the properties of the BKCa channels from the plasma membrane. It has been suggested that the VEDEC splice variant of the KCNMA1 gene product encoding plasma membrane BKCa is targeted toward mitochondria. However there has been no direct evidence that this protein forms a functional channel in mitochondria. In our study, we used HEK293T cells to express the VEDEC splice variant and observed channel activity in mitochondria using the mitoplast patch-clamp technique. For the first time, we found that transient expression with the VEDEC isoform resulted in channel activity with the conductance of 290 ± 3 pS. The channel was voltage-dependent and activated by calcium ions. Moreover, the activity of the channel was stimulated by the potassium channel opener NS11021 and inhibited by hemin and paxilline, which are known BKCa channel blockers. Immunofluorescence experiments confirmed the partial colocalization of the channel within the mitochondria. From these results, we conclude that the VEDEC isoform of the BKCa channel forms a functional channel in the inner mitochondrial membrane. Additionally, our data show that HEK293T cells are a promising experimental model for expression and electrophysiological studies of mitochondrial potassium channels.

Mitochondrial K+ channels and their implications for disease mechanisms

The field of mitochondrial ion channels underwent a rapid development during the last decade, thanks to the molecular identification of some of the nuclear-encoded organelle channels and to advances in strategies allowing specific pharmacological targeting of these proteins. Thereby, genetic tools and specific drugs aided definition of the relevance of several mitochondrial channels both in physiological as well as pathological conditions. Unfortunately, in the case of mitochondrial K⁺ channels, efforts of genetic manipulation provided only limited results, due to their dual localization to mitochondria and to plasma membrane in most cases. Although the impact of mitochondrial K⁺ channels on human diseases is still far from being genuinely understood, pre-clinical data strongly argue for their substantial role in the context of several pathologies, including cardiovascular and neurodegenerative diseases as well as cancer. Importantly, these channels are druggable targets, and their in-depth investigation could thus pave the way to the development of innovative small molecules with huge therapeutic potential. In the present review we summarize the available experimental evidence that mechanistically link mitochondrial potassium channels to the above pathologies and underline the possibility of exploiting them for therapy.

Application of Machine-Learning Methods to Recognize mitoBK Channels from Different Cell Types Based on the Experimental Patch-Clamp Results

(1) Background: In this work, we focus on the activity of large-conductance voltage- and Ca2+-activated potassium channels (BK) from the inner mitochondrial membrane (mitoBK). The characteristic electrophysiological features of the mitoBK channels are relatively high single-channel conductance (ca. 300 pS) and types of activating and deactivating stimuli. Nevertheless, depending on the isoformal composition of mitoBK channels in a given membrane patch and the type of auxiliary regulatory subunits (which can be co-assembled to the mitoBK channel protein) the characteristics of conformational dynamics of the channel protein can be altered. Consequently, the individual features of experimental series describing single-channel activity obtained by patch-clamp method can also vary. (2) Methods: Artificial intelligence approaches (deep learning) were used to classify the patch-clamp outputs of mitoBK activity from different cell types. (3) Results: Application of the K-nearest neighbors algorithm (KNN) and the autoencoder neural network allowed to perform the classification of the electrophysiological signals with a very good accuracy, which indicates that the conformational dynamics of the analyzed mitoBK channels from different cell types significantly differs. (4) Conclusion: We displayed the utility of machine-learning methodology in the research of ion channel gating, even in cases when the behavior of very similar microbiosystems is analyzed. A short excerpt from the patch-clamp recording can serve as a “fingerprint” used to recognize the mitoBK gating dynamics in the patches of membrane from different cell types.

Patch-Clamp Recording of the Activity of Ion Channels in the Inner Mitochondrial Membrane

Mitochondria are intracellular organelles, which play a crucial role in the generation of ATP. Mitochondria are surrounded by a double membrane, consisting of a smooth outer membrane (OMM) and a markedly folded inner mitochondrial membrane (IMM). Mitochondrion that has been stripped of its outer membrane, leaving the inner membrane intact is called mitoplast. There is a number of different transport proteins located in the inner mitochondrial membrane including ion channels that mediate fluxes of potassium, calcium, and chloride ions. These channels regulate the mitochondrial membrane potential, respiration, and production of reactive oxygen species. The stability of mitoplasts offers the possibility of measuring the activity of ion channels from IMM using the patch-clamp technique. Electrophysiological measurements of currents through ion channels in the IMM permit discovery of unique properties of these channels with the aim of new specific pharmacological therapies. In this chapter, we describe the isolation of mitochondria, preparation of mitoplast for patch-clamp recordings and single-mitoplast PCR experiments, which can be helpful in mastering the technique of recording the activity of mitochondrial ion channels.

Dynamical diversity of mitochondrial BK channels located in different cell types

Mitochondrial large-conductance voltage- and Ca²⁺-activated potassium channels (mitoBK) exhibit substantial similarities in their physiology regardless of the channel's location. Nevertheless, depending on the cell type, composition of membranes can vary, and mitoBK channels can be expressed in different splice variants as well as they can be co-assembled with different types of auxiliary β subunits. These factors can modulate their voltage- and Ca²⁺-sensitivity, and single-channel current kinetics. It is still an open question to what extent the mentioned factors can affect the complexity of the conformational dynamics of the mitoBK channel gating. In this work the dynamical diversity of mitoBK channels from different cell types was unraveled by the use of nonlinear methods of analysis: multifractal detrended fluctuation analysis (MFDFA) and multiscale entropy (MSE). These techniques were applied to the experimental series of single channel currents. It turns out that the differences in the mitoBK expression systems influence gating machinery by changing the scheme of switching between the stable channel conformations, and affecting the average number of available channel conformations (this effect is visible for mitoBK channels in glioblastoma cells). The obtained results suggest also that a pathological dynamics can be represented by signals of relatively low complexity (low MSE of the mitoBK channel gating in glioblastoma).

Mitochondrial Potassium Channels as Druggable Targets

Mitochondrial potassium channels have been described as important factors in cell pro-life and death phenomena. The activation of mitochondrial potassium channels, such as ATP-regulated or calcium-activated large conductance potassium channels, may have cytoprotective effects in cardiac or neuronal tissue. It has also been shown that inhibition of the mitochondrial Kv1.3 channel may lead to cancer cell death. Hence, in this paper, we examine the concept of the druggability of mitochondrial potassium channels. To what extent are mitochondrial potassium channels an important, novel, and promising drug target in various organs and tissues? The druggability of mitochondrial potassium channels will be discussed within the context of channel molecular identity, the specificity of potassium channel openers and inhibitors, and the unique regulatory properties of mitochondrial potassium channels. Future prospects of the druggability concept of mitochondrial potassium channels will be evaluated in this paper.

Regulation of the Mitochondrial BKCa Channel by the Citrus Flavonoid Naringenin as a Potential Means of Preventing Cell Damage

Naringenin, a flavanone obtained from citrus fruits and present in many traditional Chinese herbal medicines, has been shown to have various beneficial effects on cells both in vitro and in vivo. Although the antioxidant activity of naringenin has long been believed to be crucial for its effects on cells, mitochondrial pathways (including mitochondrial ion channels) are emerging as potential targets for the specific pharmacological action of naringenin in cardioprotective strategies. In the present study, we describe interactions between the mitochondrial large-conductance calcium-regulated potassium channel (mitoBK_Ca channel) and naringenin. Using the patch-clamp method, we showed that 10 µM naringenin activated the mitoBK_Ca channel present in endothelial cells. In the presence of 30 µM Ca²⁺, the increase in the mitoBK_Ca channel probability of opening from approximately 0.25 to 0.50 at −40 mV was observed. In addition, regulation of the mitoBK_Ca channel by naringenin was dependent on the concentration of calcium ions. To confirm our data, physiological studies on the mitochondria were performed. An increase in oxygen consumption and a decrease in membrane potential was observed after naringenin treatment. In addition, contributions of the mitoBK_Ca channel to apoptosis and necrosis were investigated. Naringenin protected cells against damage induced by tumor necrosis factor α (TNF-α) in combination with cycloheximide. In this study, we demonstrated that the flavonoid naringenin can activate the mitoBK_Ca channel present in the inner mitochondrial membrane of endothelial cells. Our studies describing the regulation of the mitoBK_Ca channel by this natural, plant-derived substance may help to elucidate flavonoid-induced cytoprotective mechanisms.

Heme is required for carbon monoxide activation of mitochondrial BKCa channel

Carbon monoxide (CO) is an endogenously synthesized gaseous mediator and is involved in the regulation of numerous physiological processes. Mitochondria, in which hemoproteins are abundant, are among the targets for CO action. Large-conductance calcium-activated (mitoBK_Ca) channels in the inner mitochondrial membrane share multiple biophysical similarities with the BK_Ca channels of the plasma membrane and could be a potential target for CO. To test this hypothesis, the activity of the mitoBK_Ca channels in human astrocytoma U-87 MG cell mitochondria was assessed with the patch-clamp technique. The effects of CO-releasing molecules (CORMs), such as CORM-2, CORM-401, and CORM-A1, were compared to the application of a CO-saturated solution to the mitoBK_Ca channels in membrane patches. The applied CORMs showed pleiotropic effects including channel inhibition, while the CO-containing solution did not significantly modulate channel activity. Interestingly, CO applied to the mitoBK_Ca channels, which were inhibited by exogenously added heme, stimulated the channel. To summarize, our findings indicate a requirement of heme binding to the mitoBK_Ca channel for channel modulation by CO and suggest that CORMs might have complex unspecific effects on mitoBK_Ca channels.

Mitochondrial bioenergetics and redox dysfunctions in hypercholesterolemia and atherosclerosis

In the first part of this review, we summarize basic mitochondrial bioenergetics concepts showing that mitochondria are critical regulators of cell life and death. Until a few decades ago, mitochondria were considered to play essential roles only in respiration, ATP formation, non-shivering thermogenesis and a variety of metabolic pathways. However, the concept presented by Peter Mitchell regarding coupling between electron flow and ATP synthesis through the intermediary of a H⁺ electrochemical potential leads to the recognition that the proton-motive force also regulates a series of relevant cell signalling processes, such as superoxide generation, redox balance and Ca²⁺ handling. Alterations in these processes lead to cell death and disease states. In the second part of this review, we discuss the role of mitochondrial dysfunctions in the specific context of hypercholesterolemia-induced atherosclerosis. We provide a literature analysis that indicates a decisive role of mitochondrial redox dysfunction in the development of atherosclerosis and discuss the underlying molecular mechanisms. Finally, we highlight the potential mitochondrial-targeted therapeutic strategies that are relevant for atherosclerosis.

Specific BK Channel Activator NS11021 Protects Rat Renal Proximal Tubular Cells from Cold Storage-Induced Mitochondrial Injury In Vitro

Kidneys from deceased donors used for transplantation are placed in cold storage (CS) solution during the search for a matched recipient. However, CS causes mitochondrial injury, which may exacerbate renal graft dysfunction. Here, we explored whether adding NS11021, an activator of the mitochondrial big-conductance calcium-activated K⁺ (mitoBK) channel, to CS solution can mitigate CS-induced mitochondrial injury. We used normal rat kidney proximal tubular epithelial (NRK) cells as an in vitro model of renal cold storage (18 h) and rewarming (2 h) (CS + RW). Western blots detected the pore-forming α subunit of the BK channel in mitochondrial fractions from NRK cells. The fluorescent K⁺-binding probe, PBFI-AM, revealed that isolated mitochondria from NRK cells exhibited mitoBK-mediated K⁺ uptake, which was impaired ~70% in NRK cells subjected to CS + RW compared to control NRK cells maintained at 37 °C. Importantly, the addition of 1 μM NS11021 to CS solution prevented CS + RW-induced impairment of mitoBK-mediated K⁺ uptake. The NS11021–treated NRK cells also exhibited less cell death and mitochondrial injury after CS + RW, including mitigated mitochondrial respiratory dysfunction, depolarization, and superoxide production. In summary, these new data show for the first time that mitoBK channels may represent a therapeutic target to prevent renal CS-induced injury.

Naringenin as an opener of mitochondrial potassium channels in dermal fibroblasts

Flavonoids belong to a large group of polyphenolic compounds that are widely present in plants. Certain flavonoids, including naringenin, have cytoprotective properties. Although the antioxidant effect has long been thought to be a crucial factor accounting for the cellular effects of flavonoids, mitochondrial channels have emerged recently as targets for cytoprotective strategies. In the present study, we characterized interactions between naringenin and the mitochondrial potassium (mitoBK_{C a} and mitoK_ATP ) channels recently described in dermal fibroblasts. With the use of the patch-clamp technique and mitoplasts isolated from primary human dermal fibroblast cells, our study shows that naringenin in micromolar concentrations leads to an increase in mitoBK_{C a} channel activity. The opening probability of the channel decreased from 0.97 in the control conditions (200 μmol/L Ca²⁺ ) to 0.06 at a low Ca²⁺ level (1 μmol/L) and increased to 0.85 after the application of 10 μmol/L naringenin. Additionally, the activity of the mitoK_ATP channel increased following the application of 10 μmol/L naringenin. To investigate the effects of naringenin on mitochondrial function, the oxygen consumption of dermal fibroblast cells was measured in potassium-containing media. The addition of naringenin significantly and dose-dependently increased the respiratory rate from 5.8 ± 0.2 to 14.0 ± 0.6 nmol O₂  × min^-1  × mg protein^-1 . Additionally, a Raman spectroscopy analysis of skin penetration indicated that the naringenin was distributed in all skin layers, including the epidermis and dermis. In this study, we demonstrated that a flavonoid, naringenin, can activate two potassium channels present in the inner mitochondrial membrane of dermal fibroblasts.

Mechanosensitivity of mitochondrial large-conductance calcium-activated potassium channels

Potassium channels have been discovered in the inner mitochondrial membrane of various cells. These channels can regulate the mitochondrial membrane potential, the matrix volume, respiration and reactive species generation. Therefore, it is believed that their activation is cytoprotective in various tissues. In our study, the single-channel activity of a large-conductance calcium-activated potassium channel (mitoBK_Ca) was measured by the patch-clamp technique on mitoplasts derived from mitochondria isolated from human glioma U-87 MG cells. Here, we show for the first time that mechanical stimulation of mitoBK_Ca channels results in an increased probability of channel opening. However, the mechanosensitivity of mitoBK_Ca channels was variable with some channels exhibiting no mechanosensitivity. We detected the expression of mechanosensitive BK_Ca-STREX exon in U-87 MG cells and hypotesize, based on previous studies demonstrating the presence of multiple BK_Ca splice variants that variable mechanosensitivity of mitoBK_Ca could be the result of the presence of diverse BK_Ca isoforms in mitochondria of U-87 MG cells. Our findings indicate the possible involvement of the mitoBK_Ca channel in mitochondria activities in which changes in membrane tension and shape play a crucial role, such as fusion/fission and cristae remodeling.

Evidence for a mitochondrial ATP-regulated potassium channel in human dermal fibroblasts

Mitochondrial ATP-regulated potassium channels are present in the inner membrane of the mitochondria of various cells. In the present study, we show for the first time mitochondrial ATP-regulated potassium channels in human dermal fibroblast cells. Using the patch-clamp technique on the inner mitochondrial membrane of fibroblasts, we detected a potassium channel with a mean conductance equal to 100 pS in symmetric 150 mM KCl. The activity of this channel was inhibited by a complex of ATP/Mg²⁺ and activated by potassium channel openers such as diazoxide or BMS 191095. Channel activity was inhibited by antidiabetic sulfonylurea glibenclamide and 5-hydroxydecanoic acid. The influence of substances modulating ATP-regulated potassium channel activity on oxygen consumption and membrane potential of isolated fibroblast mitochondria was also studied. Additionally, the potassium channel opener diazoxide lowered the amount of superoxide formed in isolated fibroblast mitochondria. Using reverse transcriptase-PCR, we found an mRNA transcript for the KCNJ1(ROMK) channel. The presence of ROMK protein was observed in the inner mitochondrial membrane fraction. Moreover, colocalization of the ROMK protein and a mitochondrial marker in the mitochondria of fibroblast cells was shown by immunofluorescence. In summary, the ATP-regulated mitochondrial potassium channel in a dermal fibroblast cell line have been identified.

The Slo(w) path to identifying the mitochondrial channels responsible for ischemic protection

Mitochondria play an important role in tissue ischemia and reperfusion (IR) injury, with energetic failure and the opening of the mitochondrial permeability transition pore being the major causes of IR-induced cell death. Thus, mitochondria are an appropriate focus for strategies to protect against IR injury. Two widely studied paradigms of IR protection, particularly in the field of cardiac IR, are ischemic preconditioning (IPC) and volatile anesthetic preconditioning (APC). While the molecular mechanisms recruited by these protective paradigms are not fully elucidated, a commonality is the involvement of mitochondrial K⁺ channel opening. In the case of IPC, research has focused on a mitochondrial ATP-sensitive K⁺ channel (mitoK_ATP), but, despite recent progress, the molecular identity of this channel remains a subject of contention. In the case of APC, early research suggested the existence of a mitochondrial large-conductance K⁺ (BK, big conductance of potassium) channel encoded by the Kcnma1 gene, although more recent work has shown that the channel that underlies APC is in fact encoded by Kcnt2 In this review, we discuss both the pharmacologic and genetic evidence for the existence and identity of mitochondrial K⁺ channels, and the role of these channels both in IR protection and in regulating normal mitochondrial function.