Quadrupolar, Highly Polarized Dyes: Emission Dependence on Viscosity and Selective Mitochondria Staining

Quadrupolar A-D-A-type 1,4-dihydropyrrolo[3,2-b]pyrroles (DHPPs) bearing pyridinium and quinolinium substituents emit in the 500-600 nm region. The enhancement of electronic communication between the electron-rich heterocyclic core and electron-deficient peripheral substituents turned out to be crucial for achieving emission enhancement in viscous media. DHPP bearing two 4-pyridinium substituents has optical brightness 34,000 in glycerol and only 700 in MeOH, as evidenced by measurements of the emission intensity and fluorescence lifetimes in a series of polar solvents. Such behavior makes it an excellent candidate for viscosity probes in fluorescence microscopy, as demonstrated by the fluorescence imaging of H9C2 cardiomyocytes.

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.

Loss of the large conductance calcium-activated potassium channel causes an increase in mitochondrial reactive oxygen species in glioblastoma cells

Mitochondrial potassium (mitoK) channels play an important role in cellular physiology. These channels are expressed in healthy tissues and cancer cells. Activation of mitoK channels can protect neurons and cardiac tissue against injury induced by ischemia-reperfusion. In cancer cells, inhibition of mitoK channels leads to an increase in mitochondrial reactive oxygen species, which leads to cell death. In glioma cell activity of the mitochondrial, large conductance calcium-activated potassium (mitoBKCa) channel is regulated by the mitochondrial respiratory chain. In our project, we used CRISPR/Cas9 technology in human glioblastoma U-87 MG cells to generate knockout cell lines lacking the α-subunit of the BKCa channel encoded by the KCNMA1 gene, which also encodes cardiac mitoBKCa. Mitochondrial patch-clamp experiments showed the absence of an active mitoBKCa channel in knockout cells. Additionally, the absence of this channel resulted in increased levels of mitochondrial reactive oxygen species. However, analysis of the mitochondrial respiration rate did not show significant changes in oxygen consumption in the cell lines lacking BKCa channels compared to the wild-type U-87 MG cell line. These observations were reflected in the expression levels of selected mitochondrial genes, organization of the respiratory chain, and mitochondrial morphology, which did not show significant differences between the analyzed cell lines. In conclusion, we show that in U-87 MG cells, the pore-forming subunit of the mitoBKCa channel is encoded by the KCNMA1 gene. Additionally, the presence of this channel is important for the regulation of reactive oxygen species levels in mitochondria.

Electrochemical studies of the mitochondrial ROMK2 potassium channel activity reconstituted into the free-standing and tethered bilayer lipid membranes

The renal-outer-medullary‑potassium (ROMK2) channel modulates potassium transport in the kidney. It has been postulated that the ROMK2 is the pore-forming subunit of the mitochondrial ATP-sensitive potassium channel as a mediator of cardioprotection. In this study, cell-free synthesis of the ROMK2 was performed in presence of membrane scaffold protein (MSP1D1) nanodiscs. Activity measurements were achieved after channel reconstitution into the planar lipid bilayer and tethered bilayer lipid membranes. Both methods allowed for monitoring of channel function, verified with channel blocking and activation/re-activation experiments. The primary function of the mitochondrial potassium channels is to regulate the potential of the mitochondrial membrane, which allows them to play an important role in cytoprotection. This work focuses on obtaining the ROMK2 using a cell-free expression system, followed by the incorporation of the channel protein into the lipid bilayer and studying the influence of voltage changes and molecular modulators on channel activity. Channel activity was measured after its reconstitution into two models of lipid bilayers - BLM (Bilayer Lipid Membrane) and tBLM (Tethered Bilayer Lipid Membrane) deposited on a solid gold electrode. These two model membranes and electrochemical measurements made it possible to measure the flux of K⁺ ions in the presence of channel modulators.

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.

Pharmacological Characterization of a Recombinant Mitochondrial ROMK2 Potassium Channel Expressed in Bacteria and Reconstituted in Planar Lipid Bilayers

In the inner mitochondrial membrane, several potassium channels that play a role in cell life and death have been identified. One of these channels is the ATP-regulated potassium channel (mitoK_ATP). The ROMK2 potassium channel is a potential molecular component of the mitoK_ATP channel. The current study aimed to investigate the pharmacological modulation of the activity of the ROMK2 potassium channel expressed in Escherichia coli bacteria. ROMK2 was solubilized in polymer nanodiscs and incorporated in planar lipid bilayers. The impact of known mitoK_ATP channel modulators on the activity of the ROMK2 was characterized. We found that the ROMK2 channel was activated by the mitoK_ATP channel opener diazoxide and blocked by mitoK_ATP inhibitors such as ATP/Mg²⁺, 5-hydroxydecanoic acid, and antidiabetic sulfonylurea glibenclamide. These results indicate that the ROMK2 potassium protein may be a pore-forming subunit of mitoK_ATP and that the impact of channel modulators is not related to the presence of accessory proteins.

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.

A sensitive zinc probe operating via enhancement of excited-state intramolecular charge transfer

Novel highly sensitive fluorescent probes for zinc cations based on the diketopyrrolopyrrole scaffold were designed and synthesized. Large bathochromic shifts (≈80 nm) of fluorescence are observed when the Zn²⁺-recognition unit (di-(2-picolyl)amine) is bridged with the fluorophore possessing an additional pyridine unit able to participate in the coordination process. This effect originates from the dipolar architecture and the increasing electron-withdrawing properties of the diketopyrrolopyrrole core upon addition of the cation. The new, greenish-yellow emitting probes, which operate via modulation of intramolecular charge transfer, are very sensitive to the presence of Zn²⁺. Introduction of a morpholine unit in the diketopyrrolopyrrole structure induces a selective six-fold increase of the emission intensity upon zinc coordination. Importantly, the presence of other divalent biologically relevant metal cations has negligible effects and typically even at a 100-fold higher concentration of Mg²⁺/Zn²⁺, the effect is comparable. Computational studies rationalize the strong bathochromic shift upon Zn²⁺-complexation. Decorating the probes with the triphenylphosphonium cation and morpholine unit enables selective localization in the mitochondria and the lysosome of cardiac H9C2 cells, respectively.

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.

Current Challenges of Mitochondrial Potassium Channel Research

In this paper, the current challenges of mitochondrial potassium channels research were critically reviewed. Even though recent progress in understanding K⁺ traffic in mitochondria has been substantial, some basic issues of this process remain unresolved. Here, we focused on the critical discussion of the molecular identity of various mitochondrial potassium channels. This point helps to clarify why there are different potassium channels in specific mitochondria. We also described interactions of mitochondrial potassium channel subunits with other mitochondrial proteins. Posttranslational modifications of mitochondrial potassium channels and their import are essential but unexplored research areas. Additionally, problems with the pharmacological targeting of mitochondrial potassium channel were illustrated. Finally, the limitation of the techniques used to measure mitochondrial potassium channels was explained. We believe that recognizing these problems may be interesting for readers but will also help to progress the field of mitochondrial potassium channels.

Probing the flux of mitochondrial potassium using an azacrown-diketopyrrolopyrrole based highly sensitive probe

The diketopyrrolopyrrole bearing an aza-18-crown-6 as a binding unit as well as a PPh₃⁺ group is highly sensitive towards K⁺ and localizes selectively in mitochondria of cardiac H9C2 cells. Fast efflux/influx of mitochondrial K⁺ can be observed upon stimulation with nigericin.

Mitochondrial potassium channels: A novel calcitriol target

BACKGROUND

Calcitriol (an active metabolite of vitamin D) modulates the expression of hundreds of human genes by activation of the vitamin D nuclear receptor (VDR). However, VDR-mediated transcriptional modulation does not fully explain various phenotypic effects of calcitriol. Recently a fast non-genomic response to vitamin D has been described, and it seems that mitochondria are one of the targets of calcitriol. These non-classical calcitriol targets open up a new area of research with potential clinical applications. The goal of our study was to ascertain whether calcitriol can modulate mitochondrial function through regulation of the potassium channels present in the inner mitochondrial membrane.

METHODS

The effects of calcitriol on the potassium ion current were measured using the patch-clamp method modified for the inner mitochondrial membrane. Molecular docking experiments were conducted in the Autodock4 program. Additionally, changes in gene expression were investigated by qPCR, and transcription factor binding sites were analyzed in the CiiiDER program.

RESULTS

For the first time, our results indicate that calcitriol directly affects the activity of the mitochondrial large-conductance Ca²⁺-regulated potassium channel (mitoBK_Ca) from the human astrocytoma (U-87 MG) cell line but not the mitochondrial calcium-independent two-pore domain potassium channel (mitoTASK-3) from human keratinocytes (HaCaT). The open probability of the mitoBK_Ca channel in high calcium conditions decreased after calcitriol treatment and the opposite effect was observed in low calcium conditions. Moreover, using the AutoDock4 program we predicted the binding poses of calcitriol to the calcium-bound BK_Ca channel and identified amino acids interacting with the calcitriol molecule. Additionally, we found that calcitriol influences the expression of genes encoding potassium channels. Such a dual, genomic and non-genomic action explains the pleiotropic activity of calcitriol.

CONCLUSIONS

Calcitriol can regulate the mitochondrial large-conductance calcium-regulated potassium channel. Our data open a new chapter in the study of non-genomic responses to vitamin D with potential implications for mitochondrial bioenergetics and cytoprotective mechanisms.

Professor Lech Wojtczak (1926-2019) as remembered by his four former students

The sudden death of Professor Lech Wojtczak, the great Polish biochemist and a remarkable man, our Mentor and Friend, left us in sorrow and emptiness difficult to accept. Two years have passed already from this event and our memories seem to be even more vivid, and his absence even more felt. Hence we decided to put on paper our personal reflections on Lech Wojtczak, each of us concentrating on a slightly different aspect of this towering figure. We tried to focus on memories and comments that were not mentioned in official obituaries that followed His passing away. Therefore do not expect to find here a comprehensive text on the Founder of Polish Bioenergetics, and a famous Polish biochemist, but rather a set of subjective comments on a man who made us scientists. Our memories are presented in a chronological order. The first chapter is by Professor Jolanta Barańska, who joined the group of Lech Wojtczak in 1968, followed by a chapter by Professor Maciej J. Nałęcz, who joined Lech in 1976, then Professor Konrad S. Famulski (1978) and finally followed by a chapter by Professor Adam Szewczyk, the youngest, joining the group in 1984.

Properties of electrode-supported lipid cubic mesophase films with embedded gramicidin A: structure and ion-transport studies

The lipid cubic phase (LCP) is a nanomaterial composed of water channels surrounded by lipid bilayers. LCPs are stable at room temperature and are biocompatible. These features make the lipid cubic phases similar to biological membranes, and hence, are favorable for embedding membrane proteins. We show that the monoolein cubic phase deposited on the electrode forms a 3D lipid bilayer film convenient for electrochemical investigations of membrane proteins. In this research, we studied the effect of embedding an ionophoric peptide, gramicidin A (gA), on the structure and properties of the LCP film. The phase identity and structural parameters of the gramicidin-doped phase were characterized by small-angle X-ray scattering (SAXS). The potassium ion transport through the film were studied by electroanalytical methods: alternating current voltammetry (ACV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). Increased values for the current of the gramicidin-doped cubic phase compared to the empty cubic phase and changes of the EIS parameters confirmed that the peptide remained in the film in its active dimeric form. Our results show that the LCP can be considered a suitable 3D biomimetic film for the investigation of ion channels and other transporting membrane proteins, and for their application in electrochemical sensors.

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.

Red emissive sulfone-rhodols as mitochondrial imaging agents

The controlled hydrolysis of sulfone-rhodamines affords a series of core-modified red-emitting rhodols, the fluorescence of which is sensitive to solvent polarity with pronounced bathochromic shifts recorded in both DMSO and CH3CN combined with an up to 8-fold increase in the fluorescence quantum yield.

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.

Molecular and Functional Effects of Loss of Cytochrome c Oxidase Subunit 8A

In this work we studied molecular and functional effects of the loss of the smallest nuclear encoded subunit of cytochrome c oxidase COX8A in fibroblasts from a patient with a homozygous splice site mutation and in CRISPR/Cas9 genome-edited HEK293T cells. In both cellular model systems, between 20 to 30% of the residual enzymatic activity of cytochrome c oxidase (COX) was detectable. In immunoblots of BN-PAGE separated mitochondria from both cellular models almost no monomers and dimers of the fully assembled COX could be visualized. Interestingly, supercomplexes of COX formed with complex III and also with complexes I and III retained considerable immunoreactivity, while nearly no immunoreactivity attributable to subassemblies was found. That indicates that COX lacking subunit 8A is stabilized in supercomplexes, while monomers and dimers are rapidly degraded. With transcriptome analysis by 3'-RNA sequencing we failed to detect in our cellular models of COX8A deficiency transcriptional changes of genes involved in the mitochondrial unfolded protein response (mtUPR) and the integrated stress response (ISR). Thus, our data strongly suggest that the smallest subunit of cytochrome c oxidase COX8A is required for maintenance of the structural stability of COX monomers and dimers.

Regulation of Lipid Bilayer Ion Permeability by Antibacterial Polymethyloxazoline-Polyethyleneimine Copolymers

Amphiphilic antimicrobial polymers display activity against the outer bacterial cell membrane, triggering various physiological effects. We investigated the regulation of ion transport across the lipid bilayer to understand differences in biological activity for a series of amphiphilic polymethyloxazoline - polyethyleneimine copolymers. The results confirmed that the tested structures were able to increase the permeability of the lipid bilayer (LB) membrane or its rupture. Black lipid membrane (BLM) experiments show that the triggered conductance profile and its character is strongly correlated with the polymer structure and zeta potential. The polymer exhibiting the highest antimicrobial activity promotes ion transport by using a unique mechanism and step-like characteristics with well-defined discreet openings and closings. The molecule was incorporated into the membrane in a reproducible way, and the observed channel-like activity could be responsible for the antibacterial activity of this molecule.

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.

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.

The monomers, oligomers, and fibrils of amyloid-β inhibit the activity of mitoBKCa channels by a membrane-mediated mechanism

A causative agent of Alzheimer's disease (AD) is a short amphipathic peptide called amyloid beta (Aβ). Aβ monomers undergo structural changes leading to their oligomerization or fibrillization. The monomers as well as all aggregated forms of Aβ, i.e., oligomers, and fibrils, can bind to biological membranes, thereby modulating membrane mechanical properties. It is also known that some isoforms of the large-conductance calcium-activated potassium (BK_Ca) channel, including the mitochondrial BK_Ca (mitoBK_Ca) channel, respond to mechanical changes in the membrane. Here, using the patch-clamp technique, we investigated the impact of full-length Aβ (Aβ_1-42) and its fragment, Aβ_25-35, on the activity of mitoBK_Ca channels. We found that all forms of Aβ inhibited the activity of the mitoBK_Ca channel in a concentration-dependent manner. Since monomers, oligomers, and fibrils of Aβ exhibit different molecular characteristics and structures, we hypothesized that the inhibition was not due to direct peptide-protein interactions but rather to membrane-binding of the Aβ peptides. Our findings supported this hypothesis by showing that Aβ peptides block mitoBK_Ca channels irrespective of the side of the membrane to which they are applied. In addition, we found that the enantiomeric peptide, D-Aβ_1-42, demonstrated similar inhibitory activity towards mitoBK_Ca channels. As a result, we proposed a general model in which all Aβ forms i.e., monomers, oligomers, and amyloid fibrils, contribute to the progression of AD by exerting a modulatory effect on mechanosensitive membrane components.

[Report from "8th MITOCHONDRION: in memory of Professor Lech Wojtczak"]

Relacja z "8th MITOCHONDRION: in memory of Professor Lech Wojtczak".

Chloride channel blocker IAA-94 increases myocardial infarction by reducing calcium retention capacity of the cardiac mitochondria

Indanyloxyacetic acid-94 (IAA-94), an intracellular chloride channel blocker, is shown to ablate cardioprotection rendered by ischemic preconditioning (IPC), N (6)-2-(4-aminophenyl) ethyladenosine or the PKC activator phorbol 12-myristate 13-acetate and cyclosporin A (CsA) in both ex-vivo and in-vivo ischemia-reperfusion (IR) injury. Thus signifying the role of the IAA-94 sensitive chloride channels in mediating cardio-protection upon IR injury. Although IAA-94 sensitive chloride currents are recorded in cardiac mitoplast, there is still a lack of understanding of the mechanism by which IAA-94 increases myocardial infarction (MI) by IR injury. Mitochondria are the key arbitrators of cell life and death pathways. Both oxidative stress and calcium overload in the mitochondria, elicit pathways resulting in the opening of mitochondrial permeability transition pore (mPTP) leading to cell death. Therefore, in this study we explored the role of IAA-94 in MI and in maintaining calcium retention capacity (CRC) of cardiac mitochondria after IR. IAA-94 inhibited the CRC of the isolated cardiac mitochondria in a concentration-dependent manner as measured spectrofluorimetrically using calcium green-5 N. Interestingly, IAA-94 did not change the mitochondrial membrane potential. Further, CsA a blocker of mPTP opening could not override the effect of IAA-94. We also showed for the first time that IAA-94 perfusion after ischemic event augments MI by reducing the CRC of mitochondria. To conclude, our results demonstrate that the mechanism of IAA-94 mediated cardio-deleterious effects is via modulating the mitochondria CRC, thereby playing a role in mPTP opening. These findings highlight new pharmacological targets, which can mediate cardioprotection from IR 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.

Mitochondrial potassium channels - an overview

Mitochondria play a fundamental role in ATP synthesis within the majority of mammalian cells. Potassium channels present in the inner mitochondrial membrane are fine regulators of mitochondrial function, based on inner membrane K+ permeability. These channels are regulated by a plethora of factors and conditions in a way similar to plasma membrane potassium channels. Regulators of mitochondrial potassium channels include the membrane potential, calcium ions, free fatty acids and ATP levels within the cells. Recently, it was shown that these channels are regulated by the respiratory chain, stretching of the membrane and phosphorylation. The essential interest that has driven studies of mitochondrial potassium channels for nearly 25 years is their role in cytoprotection and in cell death. Mitochondrial potassium channels have been described in neurons, astrocytoma, cardiac and skeletal muscles, fibroblasts, keratinocytes and endothelial cells. In this overview, we summarize the current knowledge of mitochondrial potassium channels. This summary will be done with a special focus on studies performed over the last 20 years in the Laboratory of Intracellular Ion Channels at the Nencki Institute. These include studies on the electrophysiological and pharmacological properties of mitochondrial potassium channels and on their regulation by endogenous intracellular substances. Additionally, the regulation of mitochondrial potassium channels by the respiratory chain and by stretching of the inner mitochondrial membrane will be reviewed. Properties of mitochondrial potassium channels in various organisms will also be summarized.

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.

cGMP-Elevating Compounds and Ischemic Conditioning Provide Cardioprotection Against Ischemia and Reperfusion Injury via Cardiomyocyte-Specific BK Channels

BACKGROUND

The nitric oxide-sensitive guanylyl cyclase/cGMP-dependent protein kinase type I signaling pathway can afford protection against the ischemia/reperfusion injury that occurs during myocardial infarction. Reportedly, voltage and Ca²⁺-activated K⁺ channels of the BK type are stimulated by cGMP/cGMP-dependent protein kinase type I, and recent ex vivo studies implicated that increased BK activity favors the survival of the myocardium at ischemia/reperfusion. It remains unclear, however, whether the molecular events downstream of cGMP involve BK channels present in cardiomyocytes or in other cardiac cell types.

METHODS

Gene-targeted mice with a cardiomyocyte- or smooth muscle cell-specific deletion of the BK (CMBK or SMBK knockouts) were subjected to the open-chest model of myocardial infarction. Infarct sizes of the conditional mutants were compared with litter-matched controls, global BK knockout, and wild-type mice. Cardiac damage was assessed after mechanical conditioning or pharmacological stimulation of the cGMP pathway and by using direct modulators of BK. Long-term outcome was studied with respect to heart functions and cardiac fibrosis in a chronic myocardial infarction model.

RESULTS

Global BK knockouts and CMBK knockouts, in contrast with SMBK knockouts, exhibited significantly larger infarct sizes compared with their respective controls. Ablation of CMBK resulted in higher serum levels of cardiac troponin I and elevated amounts of reactive oxygen species, lower phosphorylated extracellular receptor kinase and phosphorylated AKT levels and an increase in myocardial apoptosis. Moreover, CMBK was required to allow beneficial effects of both nitric oxide-sensitive guanylyl cyclase activation and inhibition of the cGMP-degrading phosphodiesterase-5, ischemic preconditioning, and postconditioning regimens. To this end, after 4 weeks of reperfusion, fibrotic tissue increased and myocardial strain echocardiography was significantly compromised in CMBK-deficient mice.

CONCLUSIONS

Lack of CMBK channels renders the heart more susceptible to ischemia/reperfusion injury, whereas the pathological events elicited by ischemia/reperfusion do not involve BK in vascular smooth muscle cells. BK seems to permit the protective effects triggered by cinaciguat, riociguat, and different phosphodiesterase-5 inhibitors and beneficial actions of ischemic preconditioning and ischemic postconditioning by a mechanism stemming primarily from cardiomyocytes. This study establishes mitochondrial CMBK channels as a promising target for limiting acute cardiac damage and adverse long-term events that occur after myocardial infarction.

Effectors of large-conductance calcium-activated potassium channel modulate glutamate excitotoxicity in organotypic hippocampal slice cultures

Mitochondria have been suggested as a potential target for cytoprotective strategies. It has been shown that increased K+ uptake mediate by mitochondrial ATP-regulated potassium channels (mitoKATP channel) or large-conductance Ca2+-activated potassium channels (mitoBKCa channel) may provide protection in different models of cell death. Since recent findings demonstrated the presence of BKCa channels in neuronal mitochondria, the goal of the present study was to test the potential neuroprotective effects of BKCa channel modulators. Using organotypic hippocampal slice cultures exposed to glutamate, we demonstrated that preincubation of the slices with the BKCa channel opener NS1619 resulted in decreased neuronal cell death measured as reduced uptake of propidium iodide. This neuroprotective effect was reversed by preincubation with the BKCa channel inhibitors paxilline and Iberiotoxin (IbTx). Moreover, mitochondrial respiration measurements revealed that NS1619 induced an IbTx-sensitive increase in state 2 respiration of isolated brain mitochondria. In addition, electrophysiological patch-clamp studies confirmed the presence of BKCa channels in mitoplasts isolated from embryonic hippocampal cells. Taken together, our results confirm presence of BKCa channel in rat hippocampal neurons mitochondria and suggest putative role for mitoBKCa in neuroprotection.

Guide to the Pharmacology of Mitochondrial Potassium Channels

This chapter provides a critical overview of the available literature on the pharmacology of mitochondrial potassium channels. In the first part, the reader is introduced to the topic, and eight known protein contributors to the potassium permeability of the inner mitochondrial membrane are presented. The main part of this chapter describes the basic characteristics of each channel type mentioned in the introduction. However, the most important and valuable information included in this chapter concerns the pharmacology of mitochondrial potassium channels. Several available channel modulators are critically evaluated and rated by suitability for research use. The last figure of this chapter shows the results of this evaluation at a glance. Thus, this chapter can be very useful for beginners in this field. It is intended to be a time- and resource-saving guide for those searching for proper modulators of mitochondrial potassium channels.

SERCA, complex I of the respiratory chain and ATP-synthase inhibition are involved in pleiotropic effects of NS1619 on endothelial cells

A large conductance potassium (BKCa) channel opener, NS1619 (1,3-dihydro-1- [2-hydroxy-5-(trifluoromethyl) phenyl]-5-(trifluoromethyl)-2H-benzimidazole-2-one), is well known for its protective effects against ischemia-reperfusion injury; however, the exact mode of its action remains unclear. The aim of this study was to characterize the effect of NS1619 on endothelial cells. The endothelial cell line EA.hy926, guinea pig hearts and submitochondrial particles isolated from the heart were used. In the isolated guinea pig hearts, which were perfused using the Langendorff technique, NS1619 caused a dose-dependent increase in coronary flow that was inhibited by L-NAME. In EA.hy926 cells, NS1619 also caused a dose-dependent increase in the intracellular calcium ion concentration [Ca(2+)]i, as measured using the FURA-2 fluorescent probe. Moreover, NS1619 decreased the oxygen consumption rate in EA.hy926 cells, as assessed using a Clark-type oxygen electrode. However, when NS1619 was applied in the presence of oligomycin, the oxygen consumption increased. NS1619 also decreased the mitochondrial membrane potential, as measured using a JC-1 fluorescent probe in the presence and absence of oligomycin. Additionally, the application of NS1619 to submitochondrial particles inhibited ATP synthase. In summary, NS1619 has pleiotropic actions on EA.hy926 cells and acts not only as an opener of the BKCa channel in EA.hy926 cells but also as an inhibitor of the respiratory chain component, sarcoplasmic reticulum ATPase, which leads to the release of Ca(2+) from the endoplasmic reticulum. Furthermore, NS1619 has the oligomycin-like property of inhibiting mitochondrial ATP synthase.

What do we not know about mitochondrial potassium channels?

In this review, we summarize our knowledge about mitochondrial potassium channels, with a special focus on unanswered questions in this field. The following potassium channels have been well described in the inner mitochondrial membrane: ATP-regulated potassium channel, Ca(2+)-activated potassium channel, the voltage-gated Kv1.3 potassium channel, and the two-pore domain TASK-3 potassium channel. The primary functional roles of these channels include regulation of mitochondrial respiration and the alteration of membrane potential. Additionally, they modulate the mitochondrial matrix volume and the synthesis of reactive oxygen species by mitochondria. Mitochondrial potassium channels are believed to contribute to cytoprotection and cell death. In this paper, we discuss fundamental issues concerning mitochondrial potassium channels: their molecular identity, channel pharmacology and functional properties. Attention will be given to the current problems present in our understanding of the nature of mitochondrial potassium channels. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

[What we don't know about mitochondrial potassium channels?]

In the current work the authors present the most interesting, yet not fully understood issues regarding origin, function and pharmacology of the mitochondrial potassium channels. There are eight potassium channels known to contribute to the potassium permeability of the inner mitochondrial membrane: ATP-regulated channel, calcium-regulated channels of large, intermediate and small conductance, voltage-regulated Kv1.3 and Kv7.4 channels, two-pore-domain TASK-3 channel and SLO2 channel. The primary function of the mitochondrial potassium channels is regulation of the mitochondrial membrane potential. Additionally, mitochondrial potassium channels alter cellular respiration, regulation of the mitochondrial volume and ROS synthesis. However, mechanisms underlying these processes are not fully understood yet. In this work, the authors not only present available knowledge about this topic, but also put certain hypotheses that may set the direction for the future research on these proteins.

The influence of the time-of-day administration of sunitinib on the penetration through the blood-brain and blood-aqueous humour barriers in rabbits

OBJECTIVE

Sunitinib is a multiple tyrosine kinase inhibitor (TKI) that exerts anti-tumor and antiangiogenic activity. It is used for the treatment of metastatic gastrointestinal stromal tumours, renal cell carcinoma and pancreatic neuroendocrine tumours. A few studies confirm the anti-tumour activity of sunitinib in brain tumours and uveal melanoma, as well as its efficacy in the reduction of brain metastases of some primary cancers. Therefore, the penetration of sunitinib through the blood-brain barrier (BBB) and blood-aqueous humour barrier (BAB) is an issue of growing interest. The aim of the study was to investigate the influence of the time-of-day administration on the penetration of sunitinib into the cerebrospinal fluid (CSF) and aqueous humour (AH).

MATERIALS AND METHODS

The rabbits were divided into two groups: I (control group)--receiving sunitinib at 8 a.m., and II--receiving sunitinib at 8 p.m. Sunitinib was administered p.o. at a single dose of 25 mg. The concentrations of sunitinib and its active metabolite (SU12662) in the plasma, CSF, AH were measured with the validated HPLC-UV method.

RESULTS

The plasma AUC0-t for sunitinib in group I was 2051.8 ng × h/mL, whereas in group II it was 3069.3 ng × h/mL. The aqueous humour AUC0-t for sunitinib in thr groups were 43.2 and 76.3 ng × h/mL, respectively. The cerebrospinal AUC0-t for sunitinib in groups I and II were 55.5 and 66.3 ng × h/mL, respectively.

CONCLUSIONS

After the evening administration (8 p.m.) the exposure to sunitinib in the rabbits' plasma, AH and CSF was higher than after the morning administration (8 a.m.), but the degree of sunitinib penetration through the BAB and BBB was very low (< 5%) and comparable in both groups.