The mitochondrial inner membrane anion channel is inhibited by DIDS

Recent progress in ferroptosis: inducers and inhibitors

Ferroptosis is a new iron-dependent form of programmed cell death characterized by iron accumulation and lipid peroxidation. In recent years, ferroptosis has garnered enormous interest in disease treatment research communities in pursuit to reveal the mechanism and key targets of ferroptosis because ferroptosis is closely related to the pathophysiological processes of many diseases. Recent studies have shown some key targets, such as glutathione peroxidase 4 (GPX4) and System Xc^-, and several inducers and inhibitors have been developed to regulate these key targets. With the emergence of new ferroptosis targets, studies on inducers and inhibitors have made new developments. The selection and use of inducers and inhibitors are very important for related work. This paper briefly introduces important regulatory targets in the ferroptosis metabolic pathway, lists and categorizes commonly used and recently developed inducers and inhibitors, and discusses their medical application. The paper ends of with potential future research direction for ferroptosis.

Molecular logic of salt taste reception in special reference to transmembrane channel-like 4 (TMC4)

The taste is biologically of intrinsic importance. It almost momentarily perceives environmental stimuli for better survival. In the early 2000s, research into taste reception was greatly developed with discovery of the receptors. However, the mechanism of salt taste reception is not fully elucidated yet and many questions still remain. At present, next-generation sequencing and genome-editing technologies are available which would become pivotal tools to elucidate the remaining issues. Here we review current mechanisms of salt taste reception in particular and characterize the properties of transmembrane channel-like 4 as a novel salt taste-related molecule that we found using these sophisticated tools.

Mitochondrial ion channels in cardiac function

Mitochondria have been recognized as key organelles in cardiac physiology and are potential targets for clinical interventions to improve cardiac function. Mitochondrial dysfunction has been accepted as a major contributor to the development of heart failure. The main function of mitochondria is to meet the high energy demands of the heart by oxidative metabolism. Ionic homeostasis in mitochondria directly regulates oxidative metabolism, and any disruption in ionic homeostasis causes mitochondrial dysfunction and eventually contractile failure. The mitochondrial ionic homeostasis is closely coupled with inner mitochondrial membrane potential. To regulate and maintain ionic homeostasis, mitochondrial membranes are equipped with ion transporting proteins. Ion transport mechanisms involving several different ion channels and transporters are highly efficient and dynamic, thus helping to maintain the ionic homeostasis of ions as well as their salts present in the mitochondrial matrix. In recent years, several novel proteins have been identified on the mitochondrial membranes and these proteins are actively being pursued in research for roles in the organ as well as organelle physiology. In this article, the role of mitochondrial ion channels in cardiac function is reviewed. In recent times, the major focus of the mitochondrial ion channel field is to establish molecular identities as well as assigning specific functions to them. Given the diversity of mitochondrial ion channels and their unique roles in cardiac function, they present novel and viable therapeutic targets for cardiac diseases.

TMC4 is a novel chloride channel involved in high-concentration salt taste sensation

"Salty taste" sensation is evoked when sodium and chloride ions are present together in the oral cavity. The presence of an epithelial cation channel that receives Na+ has previously been reported. However, no molecular entity involving Cl- receptors has been elucidated. We report the strong expression of transmembrane channel-like 4 (TMC4) in the circumvallate and foliate papillae projected to the glossopharyngeal nerve, mediating a high-concentration of NaCl. Electrophysiological analysis using HEK293T cells revealed that TMC4 was a voltage-dependent Cl- channel and the consequent currents were completely inhibited by NPPB, an anion channel blocker. TMC4 allowed permeation of organic anions including gluconate, but their current amplitudes at positive potentials were less than that of Cl-. Tmc4-deficient mice showed significantly weaker glossopharyngeal nerve response to high-concentration of NaCl than the wild-type littermates. These results indicated that TMC4 is a novel chloride channel that responds to high-concentration of NaCl.

Electro-pharmacological profiles of two brain mitoplast anion channels: Inferences from single channel recording

We have characterized the conduction and blocking properties of two different chloride channels from brain mitochondrial inner membranes after incorporation into planar lipid bilayers. Our experiments revealed the existence of channels with a mean conductance of 158 ± 7 and 301 ± 8 pS in asymmetrical 200 mM cis/50 mM trans KCl solutions. We determined that the channels were ten times more permeable for Cl⁻ than for K⁺, calculated from the reversal potential using the Goldman-Hodgkin-Katz equation. The channels were bell-shaped voltage dependent, with maximum open probability 0.9 at ± 20 mV. Two mitochondrial chloride channels were blocked after the addition of 10 µM DIDS. In addition, 158 pS chloride channel was blocked by 300 nM NPPB, acidic pH and 2.5 mM ATP, whereas the 301 pS chloride channel was blocked by 600 µM NPPB but not by acidic pH or ATP. Gating and conducting behaviors of these channels were unaffected by Ca²⁺. These results demonstrate that the 158 pS anion channel present in brain mitochondrial inner membrane, is probably identical to IMAC and 301 pS Cl channel displays different properties than those classically described for mitochondrial anion channels.

Anion Channels of Mitochondria

Mitochondria are the "power house" of a cell continuously generating ATP to ensure its proper functioning. The constant production of ATP via oxidative phosphorylation demands a large electrochemical force that drives protons across the highly selective and low-permeable mitochondrial inner membrane. Besides the conventional role of generating ATP, mitochondria also play an active role in calcium signaling, generation of reactive oxygen species (ROS), stress responses, and regulation of cell-death pathways. Deficiencies in these functions result in several pathological disorders like aging, cancer, diabetes, neurodegenerative and cardiovascular diseases. A plethora of ion channels and transporters are present in the mitochondrial inner and outer membranes which work in concert to preserve the ionic equilibrium of a cell for the maintenance of cell integrity, in physiological as well as pathophysiological conditions. For, e.g., mitochondrial cation channels KATP and BKCa play a significant role in cardioprotection from ischemia-reperfusion injury. In addition to the cation channels, mitochondrial anion channels are equally essential, as they aid in maintaining electro-neutrality by regulating the cell volume and pH. This chapter focusses on the information on molecular identity, structure, function, and physiological relevance of mitochondrial chloride channels such as voltage dependent anion channels (VDACs), uncharacterized mitochondrial inner membrane anion channels (IMACs), chloride intracellular channels (CLIC) and the aspects of forthcoming chloride channels.

Mitochondrial translocator protein (TSPO): From physiology to cardioprotection

The mitochondrial translocator protein (TSPO) is a high affinity cholesterol binding protein which is primarily located in the outer mitochondrial membrane where it has been shown to interact with proteins implicated in mitochondrial permeability transition pore (mPTP) formation. TSPO is found in different species and is expressed at high levels in tissues that synthesize steroids but is also present in other peripheral tissues especially in the heart. TSPO has been involved in the import of cholesterol into mitochondria, a key step in steroidogenesis. This constitutes the main established function of the protein which was recently challenged by genetic studies. TSPO has also been associated directly or indirectly with a wide range of cellular functions such as apoptosis, cell proliferation, differentiation, regulation of mitochondrial function or porphyrin transport. In the heart the role of TSPO remains undefined but a growing body of evidence suggests that TSPO plays a critical role in regulating physiological cardiac function and that TSPO ligands may represent interesting drugs to protect the heart under pathological conditions. This article briefly reviews current knowledge regarding TSPO and discusses its role in the cardiovascular system under physiological and pathologic conditions. More particularly, it provides evidence that TSPO can represent an alternative strategy to develop new pharmacological agents to protect the myocardium against ischemia-reperfusion injury.

Appearance of fast astrocytic component in voltage-sensitive dye imaging of neural activity

Background

Voltage-sensitive dye (VSD) imaging and intrinsic optical signals (IOS) are widely used methods for monitoring spatiotemporal neural activity in extensive networks. In spite of that, identification of their major cellular and molecular components has not been concluded so far.

Results

We addressed these issues by imaging spatiotemporal spreading of IOS and VSD transients initiated by Schaffer collateral stimulation in rat hippocampal slices with temporal resolution comparable to standard field potential recordings using a 464-element photodiode array. By exploring the potential neuronal and astroglial molecular players in VSD and IOS generation, we identified multiple astrocytic mechanisms that significantly contribute to the VSD signal, in addition to the expected neuronal targets. Glutamate clearance through the astroglial glutamate transporter EAAT2 has been shown to be a significant player in VSD generation within a very short (<5 ms) time-scale, indicating that astrocytes do contribute to the development of spatiotemporal VSD transients previously thought to be essentially neuronal. In addition, non-specific anion channels, astroglial K⁺ clearance through K_ir4.1 channel and astroglial Na⁺/K⁺ ATPase also contribute to IOS and VSD transients.

Conclusion

VSD imaging cannot be considered as a spatially extended field potential measurement with predominantly neuronal origin, instead it also reflects a fast communication between neurons and astrocytes.

The mitochondrial translocator protein and arrhythmogenesis in ischemic heart disease

Mitochondrial dysfunction is a hallmark of multiple cardiovascular disorders, including ischemic heart disease. Although mitochondria are well recognized for their role in energy production and cell death, mechanisms by which they control excitation-contraction coupling, excitability, and arrhythmias are less clear. The translocator protein (TSPO) is an outer mitochondrial membrane protein that is expressed in multiple organ systems. The abundant expression of TSPO in macrophages has been leveraged to image the immune response of the heart to inflammatory processes. More recently, the recognition of TSPO as a regulator of energy-dissipating mitochondrial pathways has extended its utility from a diagnostic marker of inflammation to a therapeutic target influencing diverse pathophysiological processes. Here, we provide an overview of the emerging role of TSPO in ischemic heart disease. We highlight the importance of TSPO in the regenerative process of reactive oxygen species (ROS) induced ROS release through its effects on the inner membrane anion channel (IMAC) and the permeability transition pore (PTP). We discuss evidence implicating TSPO in arrhythmogenesis in the settings of acute ischemia-reperfusion injury and myocardial infarction.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

DIDS (4,4-diisothiocyanatostilbenedisulphonic acid) induces apoptotic cell death in a hippocampal neuronal cell line and is not neuroprotective against ischemic stress

DIDS is a commonly used anion channel antagonist that is putatively cytoprotective against ischemic insult. However, recent reports indicate potentially deleterious secondary effects of DIDS. To assess the impact of DIDS on cellular viability comprehensively we examined neuronal morphology and function through 24 hours treatment with ACSF ± DIDS (40 or 400 µM). Control cells were unchanged, whereas DIDS induced an apoptotic phenotype (chromatin condensation, nuclear fragmentation and cleavage of the nuclear membrane protein lamin A, expression of pro-apoptotic proteins c-Jun N-terminal kinase 3, caspase 3, and cytochrome C, Annexin V staining, RNA degradation, and oligonucleosomal DNA cleavage). These deleterious effects were mediated by DIDS in a dose- and time-dependant manner, such that higher [DIDS] induced apoptosis more rapidly while apoptosis was observed at lower [DIDS] with prolonged exposure. In an apparent paradox, despite a clear overall apoptotic phenotype, certain hallmarks of apoptosis were not present in DIDS treated cells, including mitochondrial fission and loss of plasma membrane integrity. We conclude that DIDS induces apoptosis in cultured hippocampal neurons, in spite of the fact that some common hallmarks of cell death pathways are prevented. These contradictory effects may cause false-positive results in certain assays and future evaluations of DIDS as a neuroprotective agent should incorporate multiple viability assays.

Baicalein Preconditioning Protects Cardiomyocytes from Ischemia-Reperfusion Injury via Mitochondrial Oxidant Signaling

Previous studies suggest baicalein, in addition to its antioxidant effects, protects against hypoxia/reoxygenation injury via its pro-oxidant properties. We hypothesize that a brief period of baicalein treatment prior to ischemia/reperfusion (I/R) may trigger preconditioning protection via a mitochondrial pro-oxidant mechanism. Using an established chick cardiomyocyte model of I/R, cells were preconditioned with baicalein (10 μM) for 10 min followed by 10-min wash prior to I/R. Intracellular oxidants were measured using 2′, 7′-dichlorofluorescin diacetate (DCFH/DA). Cell viability was assessed by propidium iodide and apoptosis determined by DNA fragmentation. Baicalein induced a transient but significant increase of DCF fluorescence within the 10-min preconditioning period, and led to significant reduction of cell death (38.9 ± 1.8% vs. 58.7 ± 1.2% in I/R control, n = 6, p < 0.001) and DNA fragmentation after I/R. Cotreatment with N-acetylcysteine (500 μM), mitochondrial complex III electron transport chain inhibitor myxothiazol (1 μM), mitochondrial KATP channel blocker 5-hydroxydecanoate-Na (5-HD, 500 μM) or anion channel inhibitor 4′, 4′-diisothiocyanato-stilbene-2, 2′-disulfonic acid (DIDS, 200 μM) resulted in significant abrogation of oxidant increase during induction as well as the protection conferred by baicalein preconditioning. These results suggest that baicalein preconditioning exhibits significant anti-apoptotic protection against cardiomyocyte I/R injury by mitochondrial oxidant signaling, which was in part mediated by mitochondrial KATP channel and anion channel opening.

An in vitro ischemic penumbral mimic perfusate increases NADPH oxidase-mediated superoxide production in cultured hippocampal neurons

The currently accepted scheme for reactive oxygen species production during ischemia/reperfusion injury is characterized by a deleterious mitochondria-derived burst of radical generation during reperfusion; however, recent examination of the penumbra suggests a central role for NADPH-oxidase (Nox)-mediated radical generation during the ischemic period. Therefore, we utilized a novel in vitro model of the penumbra to examine the free radical profile of ischemic murine hippocampal neurons using electron paramagnetic resonance spectroscopy, and also the role of Nox in this generation and in cell fate. We report that free radical production increased ~75% at 2 h of ischemia, and this increase was abolished by: (1) scavenging of extracellular free radicals with superoxide dismutase (SOD), (2) a general anion channel antagonist, or (3) the Nox inhibitor apocynin. Similarly, at 24 h of ischemia, [ATP] decreased >95% and vital dye uptake increased 6-fold relative to controls; whereas apocynin, the Cl(-) channel antagonist 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), or the free radical scavenger N-acetyl cysteine (NAC) each provided moderate neuroprotection, ameliorating 13-32% of [ATP]-depletion and 19-56% of vital dye uptake at 24 h. Our results support a cytotoxic role for Nox-mediated free radical production from penumbral neurons during the ischemic period.

Regulation of mitochondrial function by voltage dependent anion channels in ethanol metabolism and the Warburg effect

Voltage dependent anion channels (VDAC) are highly conserved proteins that are responsible for permeability of the mitochondrial outer membrane to hydrophilic metabolites like ATP, ADP and respiratory substrates. Although previously assumed to remain open, VDAC closure is emerging as an important mechanism for regulation of global mitochondrial metabolism in apoptotic cells and also in cells that are not dying. During hepatic ethanol oxidation to acetaldehyde, VDAC closure suppresses exchange of mitochondrial metabolites, resulting in inhibition of ureagenesis. In vivo, VDAC closure after ethanol occurs coordinately with mitochondrial uncoupling. Since acetaldehyde passes through membranes independently of channels and transporters, VDAC closure and uncoupling together foster selective and more rapid oxidative metabolism of toxic acetaldehyde to nontoxic acetate by mitochondrial aldehyde dehydrogenase. In single reconstituted VDAC, tubulin decreases VDAC conductance, and in HepG2 hepatoma cells, free tubulin negatively modulates mitochondrial membrane potential, an effect enhanced by protein kinase A. Tubulin-dependent closure of VDAC in cancer cells contributes to suppression of mitochondrial metabolism and may underlie the Warburg phenomenon of aerobic glycolysis. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Metabolic stress, reactive oxygen species, and arrhythmia

Cardiac arrhythmias can cause sudden cardiac death (SCD) and add to the current heart failure (HF) health crisis. Nevertheless, the pathological processes underlying arrhythmias are unclear. Arrhythmic conditions are associated with systemic and cardiac oxidative stress caused by reactive oxygen species (ROS). In excitable cardiac cells, ROS regulate both cellular metabolism and ion homeostasis. Increasing evidence suggests that elevated cellular ROS can cause alterations of the cardiac sodium channel (Na(v)1.5), abnormal Ca(2+) handling, changes of mitochondrial function, and gap junction remodeling, leading to arrhythmogenesis. This review summarizes our knowledge of the mechanisms by which ROS may cause arrhythmias and discusses potential therapeutic strategies to prevent arrhythmias by targeting ROS and its consequences. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Single channel characterization of the mitochondrial ryanodine receptor in heart mitoplasts

Heart mitochondria utilize multiple Ca(2+) transport mechanisms. Among them, the mitochondrial ryanodine receptor provides a fast Ca(2+) uptake pathway across the inner membrane to control "excitation and metabolism coupling." In the present study, we identified a novel ryanodine-sensitive channel in the native inner membrane of heart mitochondria and characterized its pharmacological and biophysical properties by directly patch clamping mitoplasts. Four distinct channel conductances of ∼100, ∼225, ∼700, and ∼1,000 picosiemens (pS) in symmetrical 150 mm CsCl were observed. The 225 pS cation-selective channel exhibited multiple subconductance states and was blocked by high concentrations of ryanodine and ruthenium red, known inhibitors of ryanodine receptors. Ryanodine exhibited a concentration-dependent modulation of this channel, with low concentrations stabilizing a subconductance state and high concentrations abolishing activity. The 100, 700, and 1,000 pS conductances exhibited different channel characteristics and were not inhibited by ryanodine. Taken together, these findings identified a novel 225 pS channel as the native mitochondrial ryanodine receptor channel activity in heart mitoplasts with biophysical and pharmacological properties that distinguish it from previously identified mitochondrial ion channels.

P2X(7) receptor antagonists display agonist-like effects on cell signaling proteins

BACKGROUND

The activation of various P2 receptors (P2R) by extracellular nucleotides promotes diverse cellular events, including the stimulation of cell signaling protein and increases in [Ca(2+)](i). We report that some agents that can block P2X(7)R receptors also promote diverse P2X(7)R-independent effects on cell signaling.

METHODS

We exposed native rat parotid acinar cells, salivary gland cell lines (Par-C10, HSY, HSG), and PC12 cells to suramin, DIDS (4,4'-diisothiocyano stilbene-2,2'-disulfonic acid), Cibacron Blue 3GA, Brilliant Blue G, and the P2X(7)R-selective antagonist A438079, and examined the activation/phosphorylation of ERK1/2, PKCδ, Src, CDCP1, and other signaling proteins.

RESULTS

With the exception of suramin, these agents blocked the phosphorylation of ERK1/2 by BzATP in rat parotid acinar cells; but higher concentrations of suramin blocked ATP-stimulated (45)Ca(2+) entry. Aside from A438079, these agents increased the phosphorylation of ERK1/2, Src, PKCδ, and other proteins (including Dok-1) within minutes in an agent- and cell type-specific manner in the absence of a P2X(7)R ligand. The stimulatory effect of these compounds on the tyrosine phosphorylation of CDCP1 and its Src-dependent association with PKCδ was blocked by knockdown of CDCP1, which also blocked Src and PKCδ phosphorylation.

CONCLUSIONS

Several agents used as P2X(7)R blockers promote the activation of various signaling proteins and thereby act more like receptor agonists than antagonists.

GENERAL SIGNIFICANCE

Some compounds used to block P2 receptors have complicated effects that may confound their use in blocking receptor activation and other biological processes for which they are employed, including their use as blockers of various ion transport proteins.

Cardiac mitochondria and arrhythmias

Despite a high prevalence of sudden cardiac death throughout the world, the mechanisms that lead to ventricular arrhythmias are not fully understood. Over the last 20 years, a growing body of evidence indicates that cardiac mitochondria are involved in the genesis of arrhythmia. In this review, we have attempted to describe the role that mitochondria play in altering the heart's electrical function by introducing heterogeneity into the cardiac action potential. Specifically, we have focused on how the energetic status of the mitochondrial network can alter sarcolemmal potassium fluxes through ATP-sensitive potassium channels, creating a 'metabolic sink' for depolarizing wave-fronts and introducing conditions that favour catastrophic arrhythmia. Mechanisms by which mitochondria depolarize under conditions of oxidative stress are characterized, and the contributions of several mitochondrial ion channels to mitochondrial depolarization are presented. The inner membrane anion channel in particular opens upstream of other inner membrane channels during metabolic stress, and may be an effective target to prevent the metabolic oscillations that create action potential lability. Finally, we discuss therapeutic strategies that prevent arrhythmias by preserving mitochondrial membrane potential in the face of oxidative stress, supporting the notion that treatments aimed at cardiac mitochondria have significant potential in attenuating electrical dysfunction in the heart.

Role of arachidonic acid, lipoxygenase, and mitochondrial depolarization in reperfusion arrhythmias

We have sought evidence that arachidonic acid (AA) induces mitochondrial depolarization in isolated myocytes by a lipoxygenase (LOX)-dependent mechanism and that such depolarization might contribute to arrhythmogenesis following ischemia-reperfusion injury. A method was developed for measuring mitochondrial depolarization in isolated adult rat myocytes in suspension, using tetramethylrhodamine ethyl ester. The addition of AA to myocytes resulted in mitochondrial depolarization that was inhibited by the LOX inhibitor baicalein, by the reactive oxygen species (ROS) scavenger mercaptoproprionylglycine, and by the anion channel inhibitor diisothiocyanatostilbene-disulfonic acid (DIDS). AA induced mitochondrial uncoupling and mitochondrial ATPase activity in myocytes, but both were insensitive to baicalein. We conclude that the metabolic effect of AA in myocytes puts mitochondria into an energetically compromised state where membrane potential is easily changed by the DIDS-sensitive LOX/ROS-mediated opening of an inner membrane anion channel. In an in vivo anesthetized rat model of coronary artery occlusion, baicalein was found to strongly inhibit arrhythmias induced by ischemia-reperfusion injury. Arrhythmias following ischemia-reperfusion injury have been previously associated with DIDS-sensitive ROS-mediated mitochondrial depolarization, and free fatty acids including AA were previously found to accumulate during such injury. We therefore conclude that arrhythmias following ischemia-reperfusion injury might originate from mitochondrial depolarization mediated by LOX and AA.

Mitochondrial benzodiazepine receptors mediate cardioprotection of estrogen against ischemic ventricular fibrillation

The cardioprotective effects of estrogen remain controversial in clinical practice. Previous reports have shown that cardioprotective mechanisms converge on the mitochondria, but the role of mitochondria in estrogen's actions on cardiac arrhythmias is unclear. Here, we report that stimulation or inhibition of mitochondrial benzodiazepine receptors (mBzR) affected ventricular fibrillation (VF) almost in an "all-or-none" manner in an in vitro rat heart model of ischemic VF. Low concentrations of estrogen did not provide antiarrhythmic effects; however, the combination of mBzR activator and estrogen reduced VF incidence in hearts from either gender. Such synergistic actions also enabled cardiomyocytes to resist metabolic stress-induced intracellular [Ca(2+)](i) overload. Ligand binding experiments revealed that estrogen itself did not affect mBzR activity under basal conditions but promoted its up-regulation under myocardial ischemia. Our results suggest that mBzR may be an important molecule for ischemic arrhythmia and may act as a molecular switch for estrogen's antiarrhythmic effects. This finding provides a clue for elucidating the conflicting results regarding estrogen's cardiac effects in clinical studies and also suggests potential new strategies for hormone treatment in the female population.

Ethanol exposure decreases mitochondrial outer membrane permeability in cultured rat hepatocytes

Mitochondrial metabolism depends on movement of hydrophilic metabolites through the mitochondrial outer membrane via the voltage-dependent anion channel (VDAC). Here we assessed VDAC permeability of intracellular mitochondria in cultured hepatocytes after plasma membrane permeabilization with 8 microM digitonin. Blockade of VDAC with Koenig's polyanion inhibited uncoupled and ADP-stimulated respiration of permeabilized hepatocytes by 33% and 41%, respectively. Tenfold greater digitonin (80 microM) relieved KPA-induced inhibition and also released cytochrome c, signifying mitochondrial outer membrane permeabilization. Acute ethanol exposure also decreased respiration and accessibility of mitochondrial adenylate kinase (AK) of permeabilized hepatocytes membranes by 40% and 32%, respectively. This inhibition was reversed by high digitonin. Outer membrane permeability was independently assessed by confocal microscopy from entrapment of 3 kDa tetramethylrhodamine-conjugated dextran (RhoDex) in mitochondria of mechanically permeabilized hepatocytes. Ethanol decreased RhoDex entrapment in mitochondria by 35% of that observed in control cells. Overall, these results demonstrate that acute ethanol exposure decreases mitochondrial outer membrane permeability most likely by inhibition of VDAC.

Opposite effects of two resveratrol (trans-3,5,4'-trihydroxystilbene) tetramers, vitisin A and hopeaphenol, on apoptosis of myocytes isolated from adult rat heart

It has been reported that resveratrol (trans-3,5,4'-trihydroxystilbene) from Vitis plants has various cardioprotective effects. Vitis plants also include various resveratrol tetramers. The aim of our study is to clarify the pharmacological properties of resveratrol tetramers. We isolated two resveratrol tetramers as major products of Vitis plants. One is vitisin A, a complex of two resveratrol dimers, (+)-epsilon-viniferin and ampelopsin B, and the other is hopeaphenol, composed of 2 mol ampelopsin B. Vitisin A (30-300 nM) unexpectedly dose-dependently facilitated swelling and depolarization of mitochondria and cytochrome c release from mitochondria, which are indices of cardiomyocyte apoptosis. Furthermore, vitisin A induced apoptosis in the primary culture of adult rat ventricular myocytes. On the other hand, hopeaphenol (1-10 microM) dose-dependently inhibited Ca(2+) (30 microM)-induced mitochondrial depolarization and cytochrome c release from mitochondria but had not affected mitochondrial swelling. Moreover, hopeaphenol inhibited vitisin A-induced apoptosis. In structural and functional studies, we further confirmed that vitisin B, one of the resveratrol tetramers having (+)-epsilon-viniferin unit, induces mitochondrial swelling and cytochrome c release from mitochondria like vitisin A and that vitisifuran A, one of the resveratrol tetramers having the ampelopsin B unit, inhibits Ca(2+)-induced cytochrome c release from mitochondria like hopeaphenol. These results show that resveratrol tetramers have at least two opposite effects on cardiomyocytes; the one having the (+)-epsilon-viniferin unit induces cardiomyocyte apoptosis, and the other having ampelopsin B but not (+)-epsilon-viniferin unit inhibits it.

Air pollution induces enhanced mitochondrial oxidative stress in cystic fibrosis airway epithelium

We studied the effects of airborne particulate matters (PM) on cystic fibrosis (CF) epithelium. We noted that PM enhanced human CF bronchial epithelial apoptosis, activated caspase-9 and PARP-1; and reduced mitochondrial membrane potential. Mitochondrial inhibitors (4,4-diisothiocyanatostilbene-2,2'disulfonic acid, rotenone and thenoyltrifluoroacetone) blocked PM-induced generation of reactive oxygen species and apoptosis. PM upregulated pro-apoptotic Bad, Bax, p53 and p21; and enhanced mitochondrial localization of Bax. The anti-apoptotic Bcl-2, Bcl-xl, Mcl-1 and Xiap remained unchanged; however, overexpression of Bcl-xl blocked PM-induced apoptosis. Accordingly, we provide the evidence that PM enhances oxidative stress and mitochondrial signaling mediated apoptosis via the modulation of Bcl family proteins in CF.

Hydrogen sulfide decreases adenosine triphosphate levels in aortic rings and leads to vasorelaxation via metabolic inhibition

AIMS

Hydrogen sulfide (H(2)S) at low concentrations serves as a physiological endogenous vasodilator molecule, while at higher concentrations it can trigger cytotoxic effects. The aim of our study was to elucidate the potential mechanisms responsible for the effects of H(2)S on vascular tone.

MAIN METHODS

We measured the vascular tone in vitro in precontracted rat thoracic aortic rings and we have tested the effect of different oxygen levels and a variety of inhibitors affecting known vasodilatory pathways. We have also compared the vascular effect of high concentrations of H(2)S to those of pharmacological inhibitors of oxidative phosphorylation. Furthermore, we measured adenosine triphosphate (ATP)-levels in the same vascular tissues.

KEY FINDINGS

We have found that in rat aortic rings: (1) H(2)S decreases ATP levels; (2) relaxations to H(2)S depend on the ambient oxygen concentration; (3) prostaglandins do not take part in the H(2)S induced relaxations; (4) the 3':5'-cyclic guanosine monophosphate (cGMP)-nitric oxide (NO) pathway does not have a role in the relaxations (5) the role of K(ATP) channels is limited, while Cl(-)/HCO(3)(-) channels have a role in the relaxations. (6): We have observed that high concentrations of H(2)S relax the aortic rings in a fashion similar to sodium cyanide, and both agents reduce cellular ATP levels to a comparable degree.

SIGNIFICANCE

H(2)S, a new gasotransmitter of emerging importance, leads to relaxation via Cl(-)/HCO(3)(-) channels and metabolic inhibition and the interactions of these two factors depend on the oxygen levels of the tissue.

Nicotine induces resistance to chemotherapy by modulating mitochondrial signaling in lung cancer

Continued smoking causes tumor progression and resistance to therapy in lung cancer. Carcinogens possess the ability to block apoptosis, and thus may induce development of cancers and resistance to therapy. Tobacco carcinogens have been studied widely; however, little is known about the agents that inhibit apoptosis, such as nicotine. We determine whether mitochondrial signaling mediates antiapoptotic effects of nicotine in lung cancer. A549 cells were exposed to nicotine (1 muM) followed by cisplatin (35 muM) plus etoposide (20 muM) for 24 hours. We found that nicotine prevented chemotherapy-induced apoptosis, improved cell survival, and caused modest increases in DNA synthesis. Inhibition of mitogen-activated protein kinase (MAPK) and Akt prevented the antiapoptotic effects of nicotine and decreased chemotherapy-induced apoptosis. Small interfering RNA MAPK kinase-1 blocked antiapoptotic effects of nicotine, whereas small interfering RNA MAPK kinase-2 blocked chemotherapy-induced apoptosis. Nicotine prevented chemotherapy-induced reduction in mitochondrial membrane potential and caspase-9 activation. Antiapoptotic effects of nicotine were blocked by mitochondrial anion channel inhibitor, 4,4'diisothiocyanatostilbene-2,2'disulfonic acid. Chemotherapy enhanced translocation of proapoptotic Bax to the mitochondria, whereas nicotine blocked these effects. Nicotine up-regulated Akt-mediated antiapoptotic X-linked inhibitor of apoptosis protein and phosphorylated proapoptotic Bcl2-antagonist of cell death. The A549-rho0 cells, which lack mitochondrial DNA, demonstrated partial resistance to chemotherapy-induced apoptosis, but blocked the antiapoptotic effects of nicotine. Accordingly, we provide evidence that nicotine modulates mitochondrial signaling and inhibits chemotherapy-induced apoptosis in lung cancer. The mitochondrial regulation of nicotine imposes an important mechanism that can critically impair the treatment of lung cancer, because many cancer-therapeutic agents induce apoptosis via the mitochondrial death pathway. Strategies aimed at understanding nicotine-mediated signaling may facilitate the development of improved therapies in lung cancer.

Importance of bicarbonate transport for ischaemia-induced apoptosis of coronary endothelial cells

Bicarbonate transport (BT) has been previously shown to participate in apoptosis induced by various stress factors. However, the precise role of BT in ischaemia-induced apoptosis is still unknown. To investigate this subject, rat coronary endothelial cells (EC) were exposed to simulated ischaemia (glucose free anoxia at Ph 6.4) for 2 hrs and cells undergoing apoptosis were visualized by nuclear staining or by determination of cas-pase- 3 activity. To inhibit BT, EC were either treated with the inhibitor of BT 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 300 mumol/l) or exposed to ischaemia in bicarbonate free, 4-(2-hydroxyethyl)-I-piperazi-neethanesulphonic acid (HEPES)-buffered medium. Simulated ischaemia in bicarbonate-buffered medium (Bic) increased caspase-3 activity and the number of apoptotic cell (23.7 + 1.4%versus 5.1 + 1.2% in control). Omission of bicarbonate during ischaemia further significantly increased caspase-3 activity and the number of apoptotic cells (36.7 1.7%). Similar proapoptotic effect was produced by DIDS treatment during ischaemia in Bic, whereas DIDS had no effect when applied in bicarbonate-free, HEPES-buffered medium (Hep). Inhibition of BT was without influence on cytosolic acidification during ischaemia and slightly reduced cytosolic Ca(2+) accumulation. Initial characterization of the underlying mechanism leading to apoptosis induced by BT inhibition revealed activation of the mitochondrial pathway of apoptosis, i.e., increase of cytochrome C release, depolarization of mitochondria and translocation of Bax protein to mitochondria. In contrast, no activation of death receptor-dependent pathway (caspase-8 cleavage) and endoplasmic reticulum- dependent pathway (caspase-12 cleavage) was detected. In conclusion, BT plays an important role in ischaemia-induced apoptosis of coronary EC by suppression of mitochondria-dependent apoptotic pathway.

Inhibition of anion channels derived from mitochondrial membranes of the rat heart by stilbene disulfonate--DIDS

The objective of this work was to characterize in more detail the inhibition effect of diisothiocyanatostilbene-2',2-disulfonic acid (DIDS) on anion channels isolated from the rat heart mitochondria. The channels reconstituted into a planar lipid membrane displayed limited powers of discrimination between anions and cations and the ion conductance measured under asymmetric (250/50 mM KCl, cis/trans) and symmetric (150 mM KCl) conditions was approximately 100 pS. DIDS caused a dramatic decrease in the channel activity (IC(50) = 11.7 +/- 3.1 microM) only when it was added to the cis side of a planar lipid membrane. The inhibition was accompanied by the significant prolongation of closings and the shortening of openings within the burst as well as gaps between bursts were prolonged and durations of bursts were reduced. The blockade was complete and irreversible when concentration of DIDS was increased up to 200 microM. Our data indicate that DIDS is an allosteric blocker of mitochondrial anion channels and this specific effect could be used as a tool for reliable identification of anion channels on the functional level.

Inhibitory effect of DIDS, NPPB, and phloretin on intracellular chloride channels

We studied the effects of the chloride channel blockers, 5-nitro-2-(phenylpropylamino)-benzoate (NPPB), dihydro-4,4' diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), and phloretin on H2O2-induced primary culture cardiomyocyte apoptosis and activity of intracellular chloride channels obtained from rat heart mitochondrial and lysosomal vesicles. The chloride channel blockers (100 micromol/l) inhibited the H2O2-induced cardiomyocytes apoptosis. We characterized the effect of the blockers on single channel properties of the chloride channels derived from the mitochondrial and lysosomal vesicles incorporated into a bilayer lipid membrane. The single chloride channel currents were measured in 250:50 mmol/l KCl cis/trans solutions. NPPB, DIDS, and phloretin inhibited the chloride channels by decreasing the channel open probability in a concentration-dependent manner with EC50 values of 42, 7, and 20 micromol/l, respectively. NPPB and phloretin inhibited the channel's conductance and open dwell time, indicating that they could affect the chloride selective filter, pore permeability, and gating mechanism of the chloride channels. DIDS and NPPB inhibited the channels from the other side than bongkrekic acid and carboxyatractyloside. The results may contribute to understand a possible involvement of intracellular chloride channels in apoptosis and cardioprotection.

Intracellular cholesterol changes induced by translocator protein (18 kDa) TSPO/PBR ligands

One of the main functions of the translocator protein (18 kDa) or TSPO, previously known as peripheral-type benzodiazepine receptor, is the regulation of cholesterol import into mitochondria for steroid biosynthesis. In this paper we show that TSPO ligands induce changes in the distribution of intracellular cholesterol in astrocytes and fibroblasts. NBD-cholesterol, a fluorescent analog of cholesterol, was rapidly removed from membranes and accumulated into lipid droplets. This change was followed by a block of cholesterol esterification, but not by modification of intracellular cholesterol synthesis. NBD-cholesterol droplets were in part released in the medium, and increased cholesterol efflux was observed in [(3)H]cholesterol-prelabeled cells. TSPO ligands also induced a prominent shrinkage and depolarization of mitochondria and depletion of acidic vesicles with cytoplasmic acidification. Consistent with NBD-cholesterol changes, MTT assay showed enhanced accumulation of formazan into lipid droplets and inhibition of formazan exocytosis after treatment with TSPO ligands. The effects of specific TSPO ligands PK 11195 and Ro5-4864 were reproduced by diazepam, which binds with high affinity both TSPO and central benzodiazepine receptors, but not by clonazepam, which binds exclusively to GABA receptor, and other amphiphilic substances such as DIDS and propranolol. All these effects and the parallel immunocytochemical detection of TSPO in potentially steroidogenic cells (astrocytes) and non-steroidogenic cells (fibroblasts) suggest that TSPO is involved in the regulation and trafficking of intracellular cholesterol by means of mechanisms not necessarily related to steroid biosynthesis.

Stilbene derivatives inhibit the activity of the inner mitochondrial membrane chloride channels

Ion channels selective for chloride ions are present in all biological membranes, where they regulate the cell volume or membrane potential. Various chloride channels from mitochondrial membranes have been described in recent years. The aim of our study was to characterize the effect of stilbene derivatives on single-chloride channel activity in the inner mitochondrial membrane. The measurements were performed after the reconstitution into a planar lipid bilayer of the inner mitochondrial membranes from rat skeletal muscle (SMM), rat brain (BM) and heart (HM) mitochondria. After incorporation in a symmetric 450/450 mM KCl solution (cis/trans), the chloride channels were recorded with a mean conductance of 155 ± 5 pS (rat skeletal muscle) and 120 ± 16 pS (rat brain). The conductances of the chloride channels from the rat heart mitochondria in 250/50 mM KCl (cis/trans) gradient solutions were within the 70–130 pS range. The chloride channels were inhibited by these two stilbene derivatives: 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) and 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid (SITS). The skeletal muscle mitochondrial chloride channel was blocked after the addition of 1 mM DIDS or SITS, whereas the brain mitochondrial channel was blocked by 300 μM DIDS or SITS. The chloride channel from the rat heart mitochondria was inhibited by 50–100 μM DIDS. The inhibitory effect of DIDS was irreversible. Our results confirm the presence of chloride channels sensitive to stilbene derivatives in the inner mitochondrial membrane from rat skeletal muscle, brain and heart cells.

A wave of reactive oxygen species (ROS)-induced ROS release in a sea of excitable mitochondria

Once considered simply as the main source of ATP, mitochondria are now implicated in the control of many additional aspects of cell physiology, such as calcium signaling, and pathology, as in injury incurred on ischemia and subsequent reperfusion (I/R). Mitochondrial respiration is ordinarily accompanied by low-level ROS production, but they can respond to elevated ROS concentrations by increasing their own ROS production, a phenomenon termed ROS-induced ROS release (RIRR). Two modes of RIRR have been described. In the first mode of RIRR, enhanced ROS leads to mitochondrial depolarization via activation of the MPTP, yielding a short-lived burst of ROS originating from the mitochondrial electron transport chain (ETC). The second mode of RIRR is MPTP independent but is regulated by the mitochondrial benzodiazepine receptor (mBzR). Increased ROS in the mitochondrion triggers opening of the inner mitochondrial membrane anion channel (IMAC), resulting in a brief increase in ETC-derived ROS. Both modes of RIRR have been shown to transmit localized mitochondrial perturbations throughout the cardiac cell in the form of oscillations or waves but are kinetically distinct and may involve different ROS that serve as second messengers. In this review, we discuss the mechanisms of these different modes of RIRR.

Electrochemical gradients for Na+, K+,Cl– and H+ across the apical membrane in Malpighian (renal) tubule cells ofRhodnius prolixus

Measurements of intracellular and luminal ion activities as well as membrane potential were used to calculate electrochemical gradients for Cl–, Na+, K+ and H+ across the apical membrane during fluid secretion by Malpighian tubules of Rhodnius prolixus. The results show that the contribution of Na+/H+ and/or K+/H+ exchangers to fluid secretion is feasible both in unstimulated and serotonin-stimulated tubules. Similarly, the electrochemical potential for Cl– is consistent with the passive movement of Cl– from cell to lumen through Cl– channels. The contribution of apical K+:Cl– cotransport and/or paracellular Cl– movement to net transepithelial ion transport is thermodynamically unfeasible. pH in the lumen (pH 6.08±0.1, N=6) was more acid than in the bath (pH 7.25±0.01, N=26) and serotonin stimulation produced a significant increase in lumen pH to 6.32±0.04 (N=5). Intracellular pH was 6.97±0.01 and 6.82±0.04 in unstimulated and serotonin-stimulated tubules, respectively. Lumen pH was altered whereas intracellular pH was tightly regulated during serotonin and bumetanide treatment. Furthermore, DIDS or amiloride treatment did not affect intracellular pH. However, intracellular pH shifted 0.25 pH units more acid in Na+-free saline, suggesting that a Na+-dependent pH regulatory mechanism is at play in steady state pH regulation during fluid secretion by Malpighian tubules of Rhodnius prolixus. The data are consistent with a role for a basolateral Na+/H+ exchanger in intracellular pH regulation during fluid secretion.

The mitochondrial origin of postischemic arrhythmias

Recovery of the mitochondrial inner membrane potential (DeltaPsi(m)) is a key determinant of postischemic functional recovery of the heart. Mitochondrial ROS-induced ROS release causes the collapse of DeltaPsi(m) and the destabilization of the action potential (AP) through a mechanism involving a mitochondrial inner membrane anion channel (IMAC) modulated by the mitochondrial benzodiazepine receptor (mBzR). Here, we test the hypothesis that this mechanism contributes to spatiotemporal heterogeneity of DeltaPsi(m) during ischemia-reperfusion (IR), thereby promoting abnormal electrical activation and arrhythmias in the whole heart. High-resolution optical AP mapping was performed in perfused guinea pig hearts subjected to 30 minutes of global ischemia followed by reperfusion. Typical electrophysiological responses, including progressive AP shortening followed by membrane inexcitablity in ischemia and ventricular fibrillation upon reperfusion, were observed in control hearts. These responses were reduced or eliminated by treatment with the mBzR antagonist 4'-chlorodiazepam (4'-Cl-DZP), which blocks depolarization of DeltaPsi(m). When applied throughout the IR protocol, 4'-Cl-DZP blunted AP shortening and prevented reperfusion arrhythmias. Inhibition of ventricular fibrillation was also achieved by bolus infusion of 4'-Cl-DZP just before reperfusion. Conversely, treatment with an agonist of the mBzR that promotes DeltaPsi(m) depolarization exacerbated IR-induced electrophysiological changes and failed to prevent arrhythmias. The effects of these compounds were consistent with their actions on IMAC and DeltaPsi(m). These findings directly link instability of DeltaPsi(m) to the heterogeneous electrophysiological substrate of the postischemic heart and highlight the mitochondrial membrane as a new therapeutic target for arrhythmia prevention in ischemic heart disease.

Generation of metabolic oscillations by mitoKATP and ATP synthase during simulated ischemia in ventricular myocytes

Metabolic oscillations and the concomitant periodic activations of sarcolemmal ATP-sensitive K(+) channels (sarcK(ATP)) have recently been proposed as one mechanism underlying ischemia-related arrhythmia. In this study, we investigated the role of mitochondrial ATP-sensitive K(+) channels (mitoK(ATP)) and ATP synthase in the generation of metabolic oscillations during simulated ischemia from rat ventricular myocytes using patch-clamp technique and fluorescence microscopy. We have found that the combined application of creatine kinase (CK) inhibitor, 2,4-dinitrofluorobenzene, with cyanide, electron-transport-chain inhibitor causes oscillatory activations of sarcK(ATP). The oscillatory activations of sarcK(ATP) were accompanied by large periodic depolarizations in mitochondrial membrane potential (Psi(m)). 5-Hydroxydecanoate, an inhibitor of mitoK(ATP), halted the oscillations in Psi(m) at repolarized state, whereas oligomycin, an inhibitor of ATP synthase, halted them at depolarized state. In both conditions, oscillatory activations of sarcK(ATP) were abolished. Inhibitors of adenine nucleotide translocator and permeability transition pore had no effect on the oscillations in Psi(m) and sarcK(ATP). 4,4'-diisothiocyanatostilbene-2,2'-disulfonate, an inhibitor of mitochondrial inner-membrane anion channel (IMAC), caused a full depolarization in Psi(m) and activation of sarcK(ATP), finally resulting in irreversible hypercontracture. Taken together, oscillations in Psi(m) can be explained by balance between depolarizing power of mitoK(ATP) and repolarizing power of the reverse activity of ATP synthase. ATP consumption by ATP synthase in reverse mode links periodic depolarizations in Psi(m) to oscillatory activation of sarcK(ATP). Considering that such oscillations were not induced by cyanide alone, CK system may act as an important buffer, inhibiting arrhythmia during ischemia.

Mitochondrial criticality: a new concept at the turning point of life or death

A variety of stressors can cause the collapse of mitochondrial membrane potential (DeltaPsi(m)), but the events leading up to this catastrophic cellular event are not well understood at the mechanistic level. Based on our recent studies of oscillations in mitochondrial energetics, we have coined the term "mitochondrial criticality" to describe the state in which the mitochondrial network of cardiomyocytes becomes very sensitive to small perturbations in reactive oxygen species (ROS), resulting in the scaling of local mitochondrial uncoupling and DeltaPsi(m) loss to the whole cell, and the myocardial syncytium. At the point of criticality, the dynamics of the mitochondrial network bifurcate to oscillatory behavior. These energetic changes are translated into effects on the electrical excitability of the cell, inducing dramatic changes in the morphology and the threshold for activating an action potential. Emerging evidence suggests that this mechanism, by creating spatial and temporal heterogeneity of excitability in the heart during ischemia and reperfusion, underlies the genesis of potentially lethal cardiac arrhythmias.

Lung surfactant gelation induced by epithelial cells exposed to air pollution or oxidative stress

Lung surfactant lowers surface tension and adjusts interfacial rheology to facilitate breathing. A novel instrument, the interfacial stress rheometer (ISR), uses an oscillating magnetic needle to measure the shear viscosity and elasticity of a surfactant monolayer at the air-water interface. The ISR reveals that calf lung surfactant, Infasurf, exhibits remarkable fluidity, even when exposed to air pollution residual oil fly ash (ROFA), hydrogen peroxide (H2O2), or conditioned media from resting A549 alveolar epithelial cells (AEC). However, when Infasurf is exposed to a subphase of the soluble fraction of ROFA- or H2O2-treated AEC conditioned media, there is a prominent increase in surfactant elasticity and viscosity, representing two-dimensional gelation. Surfactant gelation is decreased when ROFA-AEC are pretreated with inhibitors of cellular reactive oxygen species (ROS), or with a mitochondrial anion channel inhibitor, as well as when A549-rho0 cells that lack mitochondrial DNA and functional electron transport are investigated. These results implicate both mitochondrial and nonmitochondrial ROS generation in ROFA-AEC-induced surfactant gelation. A549 cells treated with H2O2 demonstrate a dose-dependent increase in lung surfactant gelation. The ISR is a unique and sensitive instrument to characterize surfactant gelation induced by oxidatively stressed AEC.

Coordinated behavior of mitochondria in both space and time: a reactive oxygen species-activated wave of mitochondrial depolarization

Reactive oxygen species (ROS) can trigger a transient burst of mitochondrial ROS production via ROS activation of the mitochondrial permeability transition pore (MPTP), a phenomenon termed ROS-induced ROS release (RIRR). The goal of this study was to investigate if the generation of ROS in a discrete region of a cardiomyocyte could serve to propagate RIRR-mediated mitochondrial depolarizations throughout a cell. Our experiments revealed that localized RIRR activated either RIRR-mediated fluctuations in mitochondrial membrane potential (time period: 3-10 min) or a traveling wave of depolarization of the cell's mitochondria (velocity: approximately 5 microm/min). Both phenomena appeared to be mediated by the mitochondrial permeability transition pore and eventually encompassed the majority of the mitochondrial population of both isolated rat and rabbit cardiomyocytes. Furthermore, depolarization was often reversible; the waves of depolarization were then followed by a rapid (approximately 40 microm/min) repolarization wave of the mitochondria. We show that the RIRR can function to communicate the mitochondrial permeability transition from one mitochondrion to another in the isolated adult cardiomyocyte.

Repetitive transient depolarizations of the inner mitochondrial membrane induced by proton pumping

Single mitochondria show the spontaneous fluctuations of DeltaPsim. In this study, to examine the mechanism of the fluctuations, we observed DeltaPsim in single isolated heart mitochondria using time-resolved fluorescence microscopy. Addition of malate, succinate, or ascorbate plus TMPD to mitochondria induced polarization of the inner membrane followed by repeated cycles of rapid depolarizations and immediate repolarizations. ADP significantly decreased the frequency of the rapid depolarizations, but the ADP effect was counteracted by oligomycin. On the other hand, the rapid depolarizations did not occur when mitochondria were polarized by the efflux of K(+) from the matrix. The rapid depolarizations became frequent with the increase in the substrate concentration or pH of the buffer. These results suggest that the rapid depolarizations depend on the net translocation of protons from the matrix. The frequency of the rapid depolarizations was not affected by ROS scavengers, Ca(2+), CsA, or BA. In addition, the obvious increase in the permeability of the inner membrane to calcein (MW 623) that was entrapped in the matrix was not observed upon the transient depolarization. The mechanisms of the spontaneous oscillations of DeltaPsim are discussed in relation to the matrix pH and the permeability transitions.

Mitochondrial requirement for endothelial responses to cyclic strain: implications for mechanotransduction

Mechanical strain triggers a variety of cellular responses, but the underlying mechanotransduction process has not been established. Endothelial cells (EC) respond to mechanical strain by upregulating adhesion molecule expression through a signaling process involving reactive oxygen species (ROS), but the site of their generation is unknown. Mitochondria anchor to the cytoskeleton and could function as mechanotransducers by releasing ROS during cytoskeletal strain. In human umbilical vein EC (HUVEC), ROS production increased 221 ± 17% during 6 h of cyclic strain vs. unstrained controls. Mitochondrial inhibitors diphenylene iodonium or rotenone abrogated this response, whereas inhibitors of nitric oxide (NO) synthase (l-nitroarginine), xanthine oxidase (allopurinol), or NAD(P)H oxidase (apocynin) had no effect. The antioxidants ebselen and diethyldithiocarbamate inhibited the increase in ROS, but the NO scavenger Hb had no effect. Thus strain induces ROS release from mitochondria. In other studies, HUVEC were rendered mitochondria deficient (ρ0EC) to determine the requirement for electron transport in the response to strain. Strain-induced 2′7′-dichlorofluorescein fluorescence was attenuated by >80% in ρ0EC compared with HUVEC (43 ± 7 vs. 221 ± 17%). Treatment with cytochalasin D abrogated strain-induced ROS production, indicating a requirement for the actin cytoskeleton. Cyclic strain (6 h) increased VCAM-1 expression in wild-type but not ρ0EC. Increases in NF-κB activation and VCAM-1 mRNA expression during strain were prevented by antioxidants. These findings demonstrate that mitochondria function as mechanotransducers in endothelium by increasing ROS signaling, which is required for strain-induced increase in VCAM-1 expression via NF-κB.

Mitochondrial membrane potential (DeltaPsi) and Ca(2+)-induced differentiation in HaCaT keratinocytes

We have used the human calcium- and temperature-dependent (HaCaT) keratinocyte cell line to elucidate mechanisms of switching from a proliferating to a differentiating state. When grown in low calcium medium (<0.1 mM) HaCaT cells proliferate. However, an increase in the calcium concentration of the culture medium, [Ca(2+)](0), induces growth arrest and the cells start to differentiate. Numerous studies have already shown that the increase in [Ca(2+)](0) results in acute and sustained increases in intracellular calcium concentration, [Ca(2+)](i). We find that the Ca(2+)-induced cell differentiation of HaCaT cells is also accompanied by a significant decrease in mitochondrial membrane potential, DeltaPsi. By combining patch-clamp electrophysiological recordings and microspectrofluorimetric measurements of DeltaPsi on single cells, we show that the increase in [Ca(2+)](i) led to DeltaPsi depolarization. In addition, we report that tetraethylammonium (TEA), a blocker of plasma membrane K(+) channels, which is known to inhibit cell proliferation, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), a blocker of plasma membrane Cl(-) channels, also affect DeltaPsi. Both these agents stimulate HaCaT cell differentiation. These data therefore strongly suggest a direct causal relationship between depolarization of DeltaPsi and the inhibition of proliferation and induction of differentiation in HaCaT keratinocytes.

Temperature Dependence of the Mitochondrial Inner Membrane Anion Channel

The mitochondrial inner membrane anion channel (IMAC) carries a wide variety of anions and is postulated to be involved in mitochondrial volume homeostasis in conjunction with the K+/H+ antiporter, thus allowing the respiratory chain proton pumps to drive salt efflux. How it is regulated is uncertain; however, it is inhibited by matrix Mg2+ and matrix protons. Previously determined values for the IC50 suggested that the channel would be closed under physiological conditions. In a previous study (Liu, G., Hinch, B., Davatol-Hag, H., Lu, Y., Powers, M., and Beavis, A. D. (1996) J. Biol. Chem. 271, 19717-19723), it was demonstrated that the channel is highly temperature-dependent, and that a large component of this sensitivity resulted from an effect on the pIC50 for protons. We have now investigated the effect of temperature on the inhibition by Mg2+ and have found that it too is temperature-dependent. When the temperature is raised from 20 degrees C to 45 degrees C, the IC50 increases from 22 to 350 microm at pH 7.4 and from 80 to 1.5 mm at pH 8.4, respectively. The Arrhenius plot for the IC50 is linear with a slope = -80 kJ/mol. The IC50 is also strongly pH-dependent, and at 37 degrees C increases from 90 microm at pH 7.4 to 1230 microm at pH 8.4. In view of the extremely rapid fluxes that IMAC is capable of conducting at 37 degrees C, we conclude that inhibition by matrix Mg2+ and protons is necessary to limit its activity under physiological conditions. We conclude that the primary role of Mg2+ is to ensure IMAC is poised to allow regulation by small changes in pH in the physiological range. This control is mediated by a direct effect of H+ on the activity, in addition to an indirect effect mediated by a change in the Mg2+ IC50. The question that remains is not whether IMAC can be active at physiological concentrations of Mg2+ and H+, but what other factors might increase its sensitivity to changes in mitochondrial volume.

Synchronized whole cell oscillations in mitochondrial metabolism triggered by a local release of reactive oxygen species in cardiac myocytes

Reactive oxygen species (ROS) and/or Ca2+ overload can trigger depolarization of mitochondrial inner membrane potential (DeltaPsim) and cell injury. Little is known about how loss of DeltaPsim in a small number of mitochondria might influence the overall function of the cell. Here we employ the narrow focal excitation volume of the two-photon microscope to examine the effect of local mitochondrial depolarization in guinea pig ventricular myocytes. Remarkably, a single local laser flash triggered synchronized and self-sustained oscillations in DeltaPsim, NADH, and ROS after a delay of approximately 40s, in more than 70% of the mitochondrial population. Oscillations were initiated only after a specific threshold level of mitochondrially produced ROS was exceeded, and did not involve the classical permeability transition pore or intracellular Ca2+ overload. The synchronized transitions were abolished by several respiratory inhibitors or a superoxide dismutase mimetic. Anion channel inhibitors potentiated matrix ROS accumulation in the flashed region, but blocked propagation to the rest of the myocyte, suggesting that an inner membrane, superoxide-permeable, anion channel opens in response to free radicals. The transitions in mitochondrial energetics were tightly coupled to activation of sarcolemmal KATP currents, causing oscillations in action potential duration, and thus might contribute to catastrophic arrhythmias during ischemia-reperfusion injury.

O(2) sensing in hypoxic pulmonary vasoconstriction: the mitochondrial door re-opens

The identity of the O(2) sensor underlying the hypoxic pulmonary vasoconstriction (HPV) response has been sought for more than 50 years. Recently, the mitochondria have again come into sharp focus as the cellular organelle responsible for triggering the events that culminate in pulmonary artery constriction. Studies from different laboratories propose two disparate models to explain how mitochondria react to a decrease in P(O(2)). One model proposes that hypoxia slows or inhibits mitochondrial electron transport resulting in the accumulation of reducing equivalents and a decrease in the generation of reactive oxygen species (ROS). This is proposed to activate a redox-sensitive pathway leading to pulmonary vasoconstriction. A second and opposing model suggests that hypoxia triggers a paradoxical increase in mitochondrial ROS generation. This increase would then lead to the activation of an oxidant-sensitive signaling transduction pathway leading to HPV. This article summarizes the potential involvement of mitochondria in these two very different models.

Mitochondrial ROS initiate phosphorylation of p38 MAP kinase during hypoxia in cardiomyocytes

The p38 mitogen-activated protein kinase (MAPK) is phosphorylated in response to oxidative stress. Mitochondria in cardiomyocytes increase their generation of reactive oxygen species (ROS) during hypoxia (1-5% O(2)). These ROS participate in signal transduction pathways involved in adaptive responses, including ischemic preconditioning and gene transcription. The present study therefore tested the hypothesis that hypoxia induces p38 MAPK phosphorylation by augmenting mitochondrial ROS generation. In cardiomyocytes, phosphorylation of p38 was observed in a PO(2)-dependent manner during hypoxia. This response was inhibited by rotenone, thenoyltrifluoroacetone, and myxothiazol, inhibitors of mitochondrial complexes I, II, and III, respectively. A similar inhibition was observed in the cells pretreated with anion channel inhibitor DIDS, which may block ROS release from mitochondria. During normoxia, increases in mitochondrial ROS elicited by azide (1-2 mM) or by the mitochondrial inhibitor antimycin A caused increased phosphorylation of p38. Brief treatment with exogenous H(2)O(2) during normoxia also induced phosphorylation of p38 as hypoxia, but this effect was not abolished by myxothiazol or DIDS. The antioxidant N-acetyl-cysteine abolished the p38 response to hypoxia, presumably by scavenging H(2)O(2), but the mitogen extracellular receptor kinase inhibitor PD-98059 did not inhibit p38 phosphorylation during hypoxia. Thus physiological hypoxia leads to p38 phosphorylation through a mechanism that requires electron flux in the proximal region of the mitochondrial electron transport chain, which suggests that either H(2)O(2) or superoxide participates in activating that process.

Model for hypoxic pulmonary vasoconstriction involving mitochondrial oxygen sensing

We tested whether mitochondria function as the O(2) sensor underlying hypoxic pulmonary vasoconstriction (HPV). In buffer-perfused rat lungs, rotenone, myxothiazol, and diphenyleneiodonium, which inhibit mitochondria in the proximal region of the electron transport chain (ETC), abolished HPV without attenuating the response to U46619. Cyanide and antimycin A inhibit electron transfer in the distal region of the ETC, but they did not abolish HPV. Cultured pulmonary artery (PA) myocytes contract in response to hypoxia or to U46619. The hypoxic response was abolished while the response to U46619 was maintained in mutant (rho(0)) PA myocytes lacking a mitochondrial ETC. To test whether reactive oxygen species (ROS) derived from mitochondria act as signaling agents in HPV, the antioxidants pyrrolidinedithiocarbamate and ebselen and the Cu,Zn superoxide dismutase inhibitor diethyldithiocarbamate were used. These abolished HPV without affecting contraction to U46619, suggesting that ROS act as second messengers. In cultured PA myocytes, oxidation of intracellular 2',7'-dichlorofluorescin diacetate (DCFH) dye increased under 2% O(2), indicating that myocytes increase their generation of H(2)O(2) during hypoxia. This was attenuated by myxothiazol, implicating mitochondria as the source of increased ROS during HPV. These results indicate that mitochondrial ATP is not required for HPV, that mitochondria function as O(2) sensors during hypoxia, and that ROS generated in the proximal region of the ETC act as second messengers in the response.

Reactive oxygen species generated at mitochondrial complex III stabilize hypoxia-inducible factor-1alpha during hypoxia: a mechanism of O2 sensing

During hypoxia, hypoxia-inducible factor-1alpha (HIF-1alpha) is required for induction of a variety of genes including erythropoietin and vascular endothelial growth factor. Hypoxia increases mitochondrial reactive oxygen species (ROS) generation at Complex III, which causes accumulation of HIF-1alpha protein responsible for initiating expression of a luciferase reporter construct under the control of a hypoxic response element. This response is lost in cells depleted of mitochondrial DNA (rho(0) cells). Overexpression of catalase abolishes hypoxic response element-luciferase expression during hypoxia. Exogenous H(2)O(2) stabilizes HIF-1alpha protein during normoxia and activates luciferase expression in wild-type and rho(0) cells. Isolated mitochondria increase ROS generation during hypoxia, as does the bacterium Paracoccus denitrificans. These findings reveal that mitochondria-derived ROS are both required and sufficient to initiate HIF-1alpha stabilization during hypoxia.

Redox regulation of p53 during hypoxia

The transcription factor p53 can induce growth arrest or death in cells. Tumor cells that develop mutations in p53 demonstrate a diminished apoptotic potential, which may contribute to growth and tumor metastasis. Cellular levels of p53 are stabilized during hypoxia. The present study tested the hypothesis that reactive oxygen species (ROS) released from mitochondria regulate the cytosolic redox state and are required for the stabilization of p53 protein levels in response to hypoxia. Our results indicate that hypoxia (1.5% O2) increases mitochondrial ROS generation and increases p53 protein levels in human breast carcinoma MCF-7 cells and in normal human diploid fibroblast IMR-90 cells. MCF-7 cells depleted of their mitochondrial DNA (rho(o) cells) failed to stabilize p53 protein levels during hypoxia. The antioxidant N-acetylcysteine and the Cu/Zn superoxide dismutase inhibitor diethyldithiocarbamic acid abolished the hypoxia-induced increases in ROS and p53 levels. Rotenone, an inhibitor of mitochondrial complex I, and 4,4'-diisothiocyanato-stilbene-2,2'-disulfonate, a mitochondrial anion channel inhibitor, also abolished the increase in ROS signal and p53 levels during hypoxia. The p53-dependent gene p21WAF1/CIP1 was also induced by hypoxia in both MCF-7 and IMR-90 cells without affecting the growth rate of either cell line. In contrast, both cell lines exhibited increases in p21WAF1/CIP1 expression and growth arrest after gamma irradiation. Primary chick cardiac myocytes and murine embryonic fibroblasts also showed an increase in p53 protein levels in response to hypoxia without cell death or growth arrest. These results indicate that mitochondria regulate p53 protein levels during hypoxia through a redox-dependent mechanism involving ROS. Despite p53-induction, hypoxia alone does not cause either growth arrest or cell death.