The C. elegans mitochondrial K+(ATP) channel: a potential target for preconditioning

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.

A reversible mitochondrial complex I thiol switch mediates hypoxic avoidance behavior in C. elegans

C. elegans react to metabolic distress caused by mismatches in oxygen and energy status via distinct behavioral responses. At the molecular level, these responses are coordinated by under-characterized, redox-sensitive processes, thought to initiate in mitochondria. Complex I of the electron transport chain is a major site of reactive oxygen species (ROS) production and is canonically associated with oxidative damage following hypoxic exposure. Here, we use a combination of optogenetics and CRISPR/Cas9-mediated genome editing to exert spatiotemporal control over ROS production. We demonstrate a photo-locomotory remodeling of avoidance behavior by local ROS production due to the reversible oxidation of a single thiol on the complex I subunit NDUF-2.1. Reversible thiol oxidation at this site is necessary and sufficient for the behavioral response to hypoxia, does not respond to ROS produced at more distal sites, and protects against lethal hypoxic exposure. Molecular modeling suggests that oxidation at this thiol residue alters the ability for NDUF-2.1 to coordinate electron transfer to coenzyme Q by destabilizing the Q-binding pocket, causing decreased complex I activity. Overall, site-specific ROS production regulates behavioral responses and these findings provide a mechanistic target to suppress the detrimental effects of hypoxia.

FNDC-1-mediated mitophagy and ATFS-1 coordinate to protect against hypoxia-reoxygenation

Mitochondrial quality control (MQC) balances organelle adaptation and elimination, and mechanistic crosstalk between the underlying molecular processes affects subsequent stress outcomes. FUNDC1 (FUN14 domain containing 1) is a mammalian mitophagy receptor that responds to hypoxia-reoxygenation (HR) stress. Here, we provide evidence that FNDC-1 is the C. elegans ortholog of FUNDC1, and that its loss protects against injury in a worm model of HR. This protection depends upon ATFS-1, a transcription factor that is central to the mitochondrial unfolded protein response (UPRmt). Global mRNA and metabolite profiling suggest that atfs-1-dependent stress responses and metabolic remodeling occur in response to the loss of fndc-1. These data support a role for FNDC-1 in non-hypoxic MQC, and further suggest that these changes are prophylactic in relation to subsequent HR. Our results highlight functional coordination between mitochondrial adaptation and elimination that organizes stress responses and metabolic rewiring to protect against HR injury.Abbreviations: AL: autolysosome; AP: autophagosome; FUNDC1: FUN14 domain containing 1; HR: hypoxia-reperfusion; IR: ischemia-reperfusion; lof: loss of function; MQC: mitochondrial quality control; PCA: principle component analysis; PPP: pentonse phosphate pathway; proK (proteinase K);UPRmt: mitochondrial unfolded protein response; RNAi: RNA interference.

Maintenance of complex I and its supercomplexes by NDUF-11 is essential for mitochondrial structure, function and health

Mitochondrial supercomplexes form around a conserved core of monomeric complex I and dimeric complex III; wherein a subunit of the former, NDUFA11, is conspicuously situated at the interface. We identified nduf-11 (B0491.5) as encoding the Caenorhabditis elegans homologue of NDUFA11. Animals homozygous for a CRISPR-Cas9-generated knockout allele of nduf-11 arrested at the second larval (L2) development stage. Reducing (but not eliminating) expression using RNAi allowed development to adulthood, enabling characterisation of the consequences: destabilisation of complex I and its supercomplexes and perturbation of respiratory function. The loss of NADH dehydrogenase activity was compensated by enhanced complex II activity, with the potential for detrimental reactive oxygen species (ROS) production. Cryo-electron tomography highlighted aberrant morphology of cristae and widening of both cristae junctions and the intermembrane space. The requirement of NDUF-11 for balanced respiration, mitochondrial morphology and development presumably arises due to its involvement in complex I and supercomplex maintenance. This highlights the importance of respiratory complex integrity for health and the potential for its perturbation to cause mitochondrial disease. This article has an associated First Person interview with Amber Knapp-Wilson, joint first author of the paper.

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.

Lifespan-extending interventions enhance lipid-supported mitochondrial respiration in Caenorhabditis elegans

Dietary restriction and reduced reproduction have been linked to long lifespans in the vast majority of species tested. Although decreased mitochondrial mass and/or function are hallmarks of aging, little is known about the mechanisms by which these organelles contribute to physiological aging or to the effects of lifespan-extending interventions, particularly with respect to oxidative phosphorylation and energy production. Here, we employed the nematode Caenorhabditis elegans to examine the effects of inhibition of germline proliferation and dietary restriction, both of which extend the lifespan of C. elegans, on mitochondrial respiratory activity in whole animals and isolated organelles. We found that oxygen consumption rates and mitochondrial mass were reduced in wild-type (WT) C. elegans subjected to bacterial deprivation (BD) compared with animals fed ad libitum (AL). In contrast, BD decreased the rate of oxygen uptake but not mitochondrial mass in germline-less glp-1(e2144ts) mutants. Interestingly, mitochondria isolated from animals subjected to BD and/or inhibition of germline proliferation showed no differences in complex I-mediated respiratory activity compared to control mitochondria, whereas both interventions enhanced the efficiency with which mitochondria utilized lipids as respiratory substrates. Notably, the combination of BD and inhibition of germline proliferation further increased mitochondrial lipid oxidation compared to either intervention alone. We also detected a striking correlation between lifespan extension in response to BD and/or inhibition of germline proliferation and the capacity of C. elegans to generate ATP from lipids. Our results thus suggest that the ability to oxidize lipids may be determinant in enhanced longevity.

Lipase-like 5 enzyme controls mitochondrial activity in response to starvation in Caenorhabditis elegans

The C. elegans lipase-like 5 (lipl-5) gene is predicted to code for a lipase homologous to the human gastric acid lipase. Its expression was previously shown to be modulated by nutritional or immune cues, but nothing is known about its impact on the lipid landscape and ensuing functional consequences. In the present work, we used mutants lacking LIPL-5 protein and found that lipl-5 is important for normal lipidome composition as well as its remodeling in response to food deprivation. Particularly, lipids with signaling functions such as ceramides and mitochondrial lipids were affected by lipl-5 silencing. In comparison with wild type worms, animals lacking LIPL-5 were enriched in cardiolipins linked to polyunsaturated C20 fatty acids and coenzyme Q-9. Differences in mitochondrial lipid composition were accompanied by differences in mitochondrial activity as mitochondria from well-fed lipl-5 mutants were significantly more able to oxidize respiratory substrates when compared with mitochondria from well-fed wild type worms. Strikingly, starvation elicited important changes in mitochondrial activity in wild type worms, but not in lipl-5 worms. This indicates that this lipase is a determinant of mitochondrial functional remodeling in response to food withdrawal.

Single-Channel Properties of the ROMK-Pore-Forming Subunit of the Mitochondrial ATP-Sensitive Potassium Channel

An increased flux of potassium ions into the mitochondrial matrix through the ATP-sensitive potassium channel (mitoK_ATP) has been shown to provide protection against ischemia-reperfusion injury. Recently, it was proposed that the mitochondrial-targeted isoform of the renal outer medullary potassium channel (ROMK) protein creates a pore-forming subunit of mitoK_ATP in heart mitochondria. Our research focuses on the properties of mitoK_ATP from heart-derived H9c2 cells. For the first time, we detected single-channel activity and describe the pharmacology of mitoK_ATP in the H9c2 heart-derived cells. The patch-clamping of mitoplasts from wild type (WT) and cells overexpressing ROMK2 revealed the existence of a potassium channel that exhibits the same basic properties previously attributed to mitoK_ATP. ROMK2 overexpression resulted in a significant increase of mitoK_ATP activity. The conductance of both channels in symmetric 150/150 mM KCl was around 97 ± 2 pS in WT cells and 94 ± 3 pS in cells overexpressing ROMK2. The channels were inhibited by 5-hydroxydecanoic acid (a mitoK_ATP inhibitor) and by Tertiapin Q (an inhibitor of both the ROMK-type channels and mitoK_ATP). Additionally, mitoK_ATP from cells overexpressing ROMK2 were inhibited by ATP/Mg²⁺ and activated by diazoxide. We used an assay based on proteinase K to examine the topology of the channel in the inner mitochondrial membrane and found that both termini of the protein localized to the mitochondrial matrix. We conclude that the observed activity of the channel formed by the ROMK protein corresponds to the electrophysiological and pharmacological properties of mitoK_ATP.

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

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

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.

Identification of a previously undetected metabolic defect in the Complex II Caenorhabditis elegans mev-1 mutant strain using respiratory control analysis

Hypometabolism may play an important role in the pathogenesis of ageing and ageing-related diseases. The nematode Caenorhabditis elegans offers the opportunity to study "living mitochondria" in a small (~1 mm) animal replete with a highly stereotypical, yet complex, anatomy and physiology. Basal oxygen consumption rate is often employed as a proxy for energy metabolism in this context. This parameter is traditionally measured using single-chamber Clark electrodes without the addition of metabolic modulators. Recently, multi-well oxygen electrodes, facilitating addition of metabolic modulators and hence study of respiratory control during different mitochondrial respiration states, have been developed. However, only limited official protocols exist for C. elegans, and key limitations of these techniques are therefore unclear. Following modification and testing of some of the existing protocols, we used these methods to explore mitochondrial bioenergetics in live nematodes of an electron transfer chain Complex II mutant strain, mev-1, and identified a previously undetected metabolic defect. We find that mev-1 mutants cannot respond adequately to increased energy demands, suggesting that oxidative phosphorylation is more severely impaired in these animals than has previously been appreciated.

Chromophore-Assisted Light Inactivation of Mitochondrial Electron Transport Chain Complex II in Caenorhabditis elegans

Mitochondria play critical roles in meeting cellular energy demand, in cell death, and in reactive oxygen species (ROS) and stress signaling. Most Caenorhabditis elegans loss-of-function (lf) mutants in nuclear-encoded components of the respiratory chain are non-viable, emphasizing the importance of respiratory function. Chromophore-Assisted Light Inactivation (CALI) using genetically-encoded photosensitizers provides an opportunity to determine how individual respiratory chain components contribute to physiology following acute lf. As proof-of-concept, we expressed the ‘singlet oxygen generator’ miniSOG as a fusion with the SDHC subunit of respiratory complex II, encoded by mev-1 in C. elegans, using Mos1-mediated Single Copy Insertion. The resulting mev-1::miniSOG transgene complemented mev-1 mutant phenotypes in kn1 missense and tm1081(lf) deletion mutants. Complex II activity was inactivated by blue light in mitochondria from strains expressing active miniSOG fusions, but not those from inactive fusions. Moreover, light-inducible phenotypes in vivo demonstrated that complex II activity is important under conditions of high energy demand, and that specific cell types are uniquely susceptible to loss of complex II. In conclusion, miniSOG-mediated CALI is a novel genetic platform for acute inactivation of respiratory chain components. Spatio-temporally controlled ROS generation will expand our understanding of how the respiratory chain and mitochondrial ROS influence whole organism physiology.

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.

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.

Evidences for an ATP-sensitive potassium channel (KATP) in muscle and fat body mitochondria of insect

In the present study, we describe the existence of mitochondrial ATP-dependent K(+) channel (mitoKATP) in two different insect tissues, fat body and muscle of cockroach Gromphadorhina coquereliana. We found that pharmacological substances known to modulate potassium channel activity influenced mitochondrial resting respiration. In isolated mitochondria oxygen consumption increased by about 13% in the presence of potassium channel openers (KCOs) such as diazoxide and pinacidil. The opening of mitoKATP was reversed by glibenclamide (potassium channel blocker) and 1 mM ATP. Immunological studies with antibodies raised against the Kir6.1 and SUR1 subunits of the mammalian ATP-sensitive potassium channel, indicated the existence of mitoKATP in insect mitochondria. MitoKATP activation by KCOs resulted in a decrease in superoxide anion production, suggesting that protection against mitochondrial oxidative stress may be a physiological role of mitochondrial ATP-sensitive potassium channel in insects.

Physiological consequences of complex II inhibition for aging, disease, and the mKATP channel

In recent years, it has become apparent that there exist several roles for respiratory complex II beyond metabolism. These include: (i) succinate signaling, (ii) reactive oxygen species (ROS) generation, (iii) ischemic preconditioning, (iv) various disease states and aging, and (v) a role in the function of the mitochondrial ATP-sensitive K(+) (mKATP) channel. This review will address the involvement of complex II in each of these areas, with a focus on how complex II regulates or may be involved in the assembly of the mKATP. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Mitochondrial ATP-sensitive potassium channel activity and hypoxic preconditioning are independent of an inwardly rectifying potassium channel subunit in Caenorhabditis elegans

Hypoxic preconditioning (HP) is an evolutionarily-conserved mechanism that protects an organism against stress. The mitochondrial ATP-sensitive K(+) channel (mK(ATP)) plays an essential role in the protective signaling, but remains molecularly undefined. Several lines of evidence suggest that mK(ATP) may arise from an inward rectifying K(+) channel (Kir). The genetic model organism Caenorhabditis elegans exhibits HP and displays mK(ATP) activity. Here, we investigate the tissue expression profile of the three C. elegans Kir genes and demonstrate that mutant strains where the irk genes have been deleted either individually or in combination can be protected by HP and exhibit robust mK(ATP) channel activity in purified mitochondria. These data suggest that the mK(ATP) in C. elegans does not arise from a Kir derived channel.

SLO-2 is cytoprotective and contributes to mitochondrial potassium transport

Mitochondrial potassium channels are important mediators of cell protection against stress. The mitochondrial large-conductance "big" K(+) channel (mBK) mediates the evolutionarily-conserved process of anesthetic preconditioning (APC), wherein exposure to volatile anesthetics initiates protection against ischemic injury. Despite the role of the mBK in cardioprotection, the molecular identity of the channel remains unknown. We investigated the attributes of the mBK using C. elegans and mouse genetic models coupled with measurements of mitochondrial K(+) transport and APC. The canonical Ca(2+)-activated BK (or "maxi-K") channel SLO1 was dispensable for both mitochondrial K(+) transport and APC in both organisms. Instead, we found that the related but physiologically-distinct K(+) channel SLO2 was required, and that SLO2-dependent mitochondrial K(+) transport was triggered directly by volatile anesthetics. In addition, a SLO2 channel activator mimicked the protective effects of volatile anesthetics. These findings suggest that SLO2 contributes to protection from hypoxic injury by increasing the permeability of the mitochondrial inner membrane to K(+).

Physiology of potassium channels in the inner membrane of mitochondria

The inner membrane of the ATP-producing organelles of endosymbiotic origin, mitochondria, has long been considered to be poorly permeable to cations and anions, since the strict control of inner mitochondrial membrane permeability is crucial for efficient ATP synthesis. Over the past 30 years, however, it has become clear that various ion channels--along with antiporters and uniporters--are present in the mitochondrial inner membrane, although at rather low abundance. These channels are important for energy supply, and some are a decisive factor in determining whether a cell lives or dies. Their electrophysiological and pharmacological characterisations have contributed importantly to the ongoing elucidation of their pathophysiological roles. This review gives an overview of recent advances in our understanding of the functions of the mitochondrial potassium channels identified so far. Open issues concerning the possible molecular entities giving rise to the observed activities and channel protein targeting to mitochondria are also discussed.

Ischemic preconditioning: the role of mitochondria and aging

Aging represents a triple threat for myocardial infarction (MI). Not only does the incidence of MI increase with age, but the heart becomes more susceptible to MI induced damage and protective interventions such as ischemic preconditioning (IPC) become less effective. Therefore, any rational therapeutic strategy must be built around the ability to combat the detrimental effects of ischemia in aged individuals. To accomplish this, we need to develop a better understanding of how ischemic damage, protection, and aging are linked. In this regard, mitochondria have emerged as a common theme. First, mitochondria contribute to cell damage during ischemia-reperfusion (IR) and are central to cell death. Second, the protective signaling pathways activated by IPC converge on mitochondria, and the opening of mitochondrial ion channels alone is sufficient to elicit protection. Finally, mitochondria clearly influence the aging process, and specific defects in mitochondrial activity are associated with age-related functional decline. This review will summarize the effects of aging on myocardial IR injury and discuss relevant and emerging strategies to protect against MI with an emphasis on mitochondrial function.

Mitochondrial dysfunction confers resistance to multiple drugs in Caenorhabditis elegans

Mutations in mitochondrial genes and inhibitors of OX-Phos make Caenorhabditis elegans resistant to multiple drugs. The anti-oxidant NAC prevents this drug-resistance, indicating that a mechanism responsive to ROS is required. The resistance generated by inhibitors of respiration is reduced in mitochondrial mutants that lack the C. elegans ortholog of PKCε.

In a previous genetic screen for Caenorhabditis elegans mutants that survive in the presence of an antimitotic drug, hemiasterlin, we identified eight strong mutants. Two of these were found to be resistant to multiple toxins, and in one of these we identified a missense mutation in phb-2, which encodes the mitochondrial protein prohibitin 2. Here we identify two additional mutations that confer drug resistance, spg-7 and har-1, also in genes encoding mitochondrial proteins. Other mitochondrial mutants, isp-1, eat-3, and clk-1, were also found to be drug-resistant. Respiratory complex inhibitors, FCCP and oligomycin, and a producer of reactive oxygen species (ROS), paraquat, all rescued wild-type worms from hemiasterlin toxicity. Worms lacking mitochondrial superoxide dismutase (MnSOD) were modestly drug-resistant, and elimination of MnSOD in the phb-2, har-1, and spg-7 mutants enhanced resistance. The antioxidant N-acetyl-l-cysteine prevented mitochondrial inhibitors from rescuing wild-type worms from hemiasterlin and sensitized mutants to the toxin, suggesting that a mechanism sensitive to ROS is necessary to trigger drug resistance in C. elegans. Using genetics, we show that this drug resistance requires pkc-1, the C. elegans ortholog of human PKCε.

Potassium channels in the mitochondria of unicellular eukaryotes and plants

The functional characterisation of potassium channels found in the mitochondria of plants and unicellular eukaryotes is critically discussed herein, with a focus on the ATP-sensitive potassium channel and the large-conductance Ca(2+)-activated potassium channel (mitoBK(Ca) channel). The physiological functions of these channels are not completely understood. We discuss the functional connections and roles of potassium channels, uncoupling protein and alternative oxidase, three energy-dissipating systems that exist in the mitochondrial respiratory chain of plants and some unicellular eukaryotes, which include preventing the production of reactive oxygen species.

Regulated production of free radicals by the mitochondrial electron transport chain: Cardiac ischemic preconditioning

Excessive production of free radicals by mitochondria is associated with, and likely contributes to, the progression of numerous pathological conditions. Nevertheless, the production of free radicals by the mitochondria may have important biological functions under normal or stressed conditions by activating or modulating redox-sensitive cellular signaling pathways. This raises the intriguing possibility that regulated mitochondrial free radical production occurs via mechanisms that are distinct from pathologies associated with oxidative damage. Indeed, the capacity of mitochondria to produce free radicals in a limited manner may play a role in ischemic preconditioning, the phenomenon whereby short bouts of ischemia protect from subsequent prolonged ischemia and reperfusion. Ischemic preconditioning can thus serve as an important model system for defining regulatory mechanisms that allow for transient, signal-inducing, production of free radicals by mitochondria. Defining how these mechanism(s) occur will provide insight into therapeutic approaches that minimize oxidative damage without altering normal cellular redox biology. The aim of this review is to present and discuss evidence for the regulated production of superoxide by the electron transport chain within the ischemic preconditioning paradigm of redox regulation.

Evidence for an ATP-sensitive K+ channel in mitoplasts isolated from Trypanosoma cruzi and Crithidia fasciculata

Mammalian mitochondria, as well as rat, plant and Caenorhabditis elegans mitochondria, possess an ATP-sensitive K+ channel (mitoK(ATP)) that has been pharmacologically characterised. Opening of mitoK(ATP) and the subsequent K+ entry into the matrix was shown to have three effects on mitochondria physiology: (i) an increase in matrix volume (swelling), (ii) an acceleration of respiration, and (iii) an increase in reactive oxygen species (ROS) production. These effects on mitochondria bioenergetics have been shown to be part of distinct intracellular signalling pathways, to protect against cell death and to modulate gene transcription. To date, such a channel or its activity has not been described in trypanosomatids. In the present study, we show pharmacological evidence for the presence of a mitoK(ATP) in trypanosomatids. Cells were incubated in a hypotonic medium followed by mild detergent exposure to isolate mitoplasts from Trypanosoma cruzi and Crithidia fasciculata. Mitoplasts swelled when incubated in KCl medium due to respiration-driven K+ entry into the matrix. Swelling was sensitive to the presence of ATP when the mitoplast suspension was incubated in K+ -containing, but not in K+ -free, medium. The ATP inhibition of swelling was reversed by the mitoK(ATP) agonist diazoxide and the diazoxide-induced swelling was inhibited by the mitoK(ATP) blockers 5-hydroxydecanoate (5HD) or glibenclamide. Similar to mammalian and rat mitochondria, trypanosomatid mitoK(ATP) activity was modulated by the general protein kinase C (PKC) agonist phorbol 12-myristate 13-acetate (PMA) and antagonist chelerythrine. As expected, the potassium ionophore valinomycin could also reverse the ATP-inhibited state but this reversal was not sensitive to 5HD or glibenclamide. Dose response curves for ATP, diazoxide and 5HD are presented. These results provide strong evidence for the presence of an ATP-sensitive K+ in trypanosomatid mitochondria.