Mitochondrial chloride channels - What are they for?

Ion channel-mediated mitochondrial volume regulation and its relationship with mitochondrial dynamics

The mitochondrion, one of the important cellular organelles, has the major function of generating adenosine triphosphate and plays an important role in maintaining cellular homeostasis, governing signal transduction, regulating membrane potential, controlling programmed cell death and modulating cell proliferation. The dynamic balance of mitochondrial volume is an important factor required for maintaining the structural integrity of the organelle and exerting corresponding functions. Changes in the mitochondrial volume are closely reflected in a series of biological functions and pathological changes. The mitochondrial volume is controlled by the osmotic balance between the cytoplasm and the mitochondrial matrix. Thus, any disruption in the influx of the main ion, potassium, into the cells can disturb the osmotic balance between the cytoplasm and the matrix, leading to water movement between these compartments and subsequent alterations in mitochondrial volume. Recent studies have shown that mitochondrial volume homeostasis is closely implicated in a variety of diseases. In this review, we provide an overview of the main influencing factors and research progress in the field of mitochondrial volume homeostasis.

Nuclear Chloride Ion-Selective Fluorescent Probe and Its Biological Applications

Owing to the biological significance of Cl- in cells, several chemical fluorescent probes and biosensors have been constructed to monitor this anion in the cytosol and subcellular organelles. However, a fluorescent probe for the selective detection of nuclear Cl- has not been described thus far. In the current study, we developed the first nuclear Cl--selective biosensor, Cl-YFP-NLS, whose fluorescence was effectively quenched by this anion, and demonstrated that it is an efficient and powerful tool for determining the levels of nuclear Cl-. The results of cell studies using Cl-YFP-NLS as the probe suggested that the level of Cl- in the nucleus is lower than that in the cytosol. In addition, Cl-YFP-NLS along with lysosomal (Lyso-MQAE) and mitochondrial Cl--selective fluorescent probes (Mito-MQAE) were utilized to determine the effects of various substances on the levels of Cl- in subcellular organelles. The results showed that lysosomotropic agents decrease the lysosomal Cl- concentration and increase the levels of mitochondrial and nuclear Cl-. Also, observations suggested that substances capable of inducing mitochondrial outer membrane permeabilization without inducing lysosomal membrane permeabilization increase mitochondrial and nuclear Cl- concentrations but they do not affect the level of lysosomal Cl-. Moreover, a substance directly disrupting nuclear pore complexes increased the level of nuclear Cl- and did not change the levels of lysosomal and mitochondrial Cl-. Finally, nucleus-affecting substances that cause deoxyribonucleic acid damage and activate p53 and Bax increased the levels of mitochondrial and nuclear Cl- without influencing the level of lysosomal Cl-.

Intracellular Chloride Channels Regulate Endothelial Metabolic Reprogramming in Pulmonary Arterial Hypertension

Mitochondrial fission and a metabolic switch from oxidative phosphorylation to glycolysis are key features of vascular pathology in pulmonary arterial hypertension (PAH) and are associated with exuberant endothelial proliferation and apoptosis. The underlying mechanisms are poorly understood. We describe the contribution of two intracellular chloride channel proteins, CLIC1 and CLIC4, both highly expressed in PAH and cancer, to mitochondrial dysfunction and energy metabolism in PAH endothelium. Pathological overexpression of CLIC proteins induces mitochondrial fragmentation, inhibits mitochondrial cristae formation, and induces metabolic shift toward glycolysis in human pulmonary artery endothelial cells, consistent with changes observed in patient-derived cells. Interactions of CLIC proteins with structural components of the inner mitochondrial membrane offer mechanistic insights. Endothelial CLIC4 excision and mitofusin 2 supplementation have protective effects in human PAH cells and preclinical PAH. This study is the first to demonstrate the key role of endothelial intracellular chloride channels in the regulation of mitochondrial structure, biogenesis, and metabolic reprogramming in expression of the PAH phenotype.

Cannabidiol Impairs Brain Mitochondrial Metabolism and Neuronal Integrity

Background: The mechanisms underlying the clinical effects of CBD remain poorly understood. Given the increasing evidence for CBD's effects on mitochondria, we sought to examine in more detail whether CBD impacts mitochondrial function and neuronal integrity. Methods: We utilized BE(2)-M17 neuroblastoma cells or acutely isolated brain mitochondria from rodents using a Seahorse extracellular flux analyzer and a fluorescent spectrofluorophotometer assay. Mitochondrial ion channel activity and hippocampal long-term potentiation were measured using standard cellular electrophysiological methods. Spatial learning/memory function was evaluated using the Morris water maze task. Plasma concentrations of CBD were assessed with liquid chromatography-mass spectrometry, and cellular viability was evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction neuronal injury assay. Results: At low micromolar concentrations, CBD reduced mitochondrial respiration, the threshold for mitochondrial permeability transition, and calcium uptake, blocked a novel mitochondrial chloride channel, and reduced the viability of hippocampal cells. These effects were paralleled by in vitro and in vivo learning/memory deficits. We further found that these effects were independent of cannabinoid receptor 1 and mitochondrial G-protein-coupled receptor 55. Conclusion: Our results provide evidence for concentration- and dose-dependent toxicological effects of CBD, findings that may bear potential relevance to clinical populations.

Label-free metabolic imaging of non-alcoholic-fatty-liver-disease (NAFLD) liver by volumetric dynamic optical coherence tomography

Label-free metabolic imaging of non-alcoholic fatty liver disease (NAFLD) mouse liver is demonstrated ex vivo by dynamic optical coherence tomography (OCT). The NAFLD mouse is a methionine choline-deficient (MCD)-diet model, and two mice fed the MCD diet for 1 and 2 weeks are involved in addition to a normal-diet mouse. The dynamic OCT is based on repeating raster scan and logarithmic intensity variance (LIV) analysis that enables volumetric metabolic imaging with a standard-speed (50,000 A-lines/s) OCT system. Metabolic domains associated with lipid droplet accumulation and inflammation are clearly visualized three-dimensionally. Particularly, the normal-diet liver exhibits highly metabolic vessel-like structures of peri-vascular hepatic zones. The 1-week MCD-diet liver shows ring-shaped highly metabolic structures formed with lipid droplets. The 2-week MCD-diet liver exhibits fragmented vessel-like structures associated with inflammation. These results imply that volumetric LIV imaging is useful for visualizing and assessing NAFLD abnormalities.

Determinants, and implications, of the shape and size of thylakoids and cristae

Thylakoids are flattened sacs isolated from other membranes; cristae are attached to the rest of the inner mitochondrial membrane by the crista junction, but the crista lumen is separated from the intermembrane space. The shape of thylakoids and cristae involves membranes with small (5-30 nm) radii of curvature. While the mechanism of curvature is not entirely clear, it seems to be largely a function of Curt proteins in thylakoids and Mitochondrial Organising Site and Crista Organising Centre proteins and oligomeric F_OF₁ ATP synthase in cristae. A subordinate, or minimal, role is attributable to lipids with areas of their head group area greater (convex leaflet) or smaller (concave leaflet) than the area of the lipid tail; examples of the latter group are monogalactosyldiglyceride in thylakoids and cardiolipin in cristae. The volume per unit area on the lumen side of the membrane is less than that of the chloroplast stroma or cyanobacterial cytosol for thylakoids, and mitochondrial matrix for cristae. A low volume per unit area of thylakoids and cristae means a small lumen width that is the average of wider spaces between lipid parts of the membranes and the narrower gaps dominated by extra-membrane components of transmembrane proteins. These structural constraints have important implications for the movement of the electron carriers plastocyanin and cytochrome c₆ (thylakoids) and cytochrome c (cristae) and hence the separation of the membrane-associated electron donors to, and electron acceptors from, these water-soluble electron carriers. The donor/acceptor pairs, are the cytochrome fb₆Fe_nh complex and P₇₀₀⁺ in thylakoids, and Complex III and Complex IV of cristae. The other energy flux parallel to the membranes is that of the proton motive force generated by redox-powered H⁺ pumps into the lumen to the proton motive force use in ATP synthesis by H⁺ flux from the lumen through the ATP synthase. For both the electron transport and proton motive force movement, concentration differences of reduced and oxidised electron carriers and protonated and deprotonated pH buffers are involved. The need for diffusion along a congested route of these energy transfer agents may limit the separation of sources and sinks parallel to the membranes of thylakoids and cristae.

Mitochondrial Cl--Selective Fluorescent Probe for Biological Applications

Herein we describe the development of the first mitochondrial Cl^--selective fluorescent probe, Mito-MQAE, and its applications in biological systems. Fluorescence of Mito-MQAE is insensitive to pH over the physiological pH range and is quenched by Cl^- with a Stern-Volmer quenching constant of 201 M^-1 at pH 7.0. The results of cell studies using Mito-MQAE show that substances with the ability to disrupt mitochondrial membranes cause increases in the mitochondrial Cl^- concentration.

Potential role of CFTR in bisphenol A-induced malignant transformation of prostate cells via mitochondrial apoptosis

Bisphenol A (BPA) is an environmental endocrine disruptor and a risk factor for prostate cancer. The cystic fibrosis transmembrane conductance regulator (CFTR) is proposed to be a prostate cancer suppressor in some recent researches. However, the potential role and mechanism of CFTR in BPA-induced prostate cancer cells has not been well identified. In this study, BPA decreased the viability of human normal prostate RWPE-1 cells detected with a CCK-8 kit. The capacity of the cell line on soft agar colony formation, wound healing, and transwell invasion indicated malignant transformation induced by BPA. Western blot analysis demonstrated that the levels of CFTR and Bcl-2 decreased, whereas Bax level increased, and ELISA detection showed a decreased ATP level in BPA-exposed cells. Cell apoptosis was analyzed with Annexin V-FITC Detection Kit by flow cytometry. However, no significant difference was observed in cell viability and apoptosis rates compared to normal RWPE-1 cells. Our research revealed a potential role of CFTR in BPA-induced malignant transformation via mitochondrial apoptosis of normal prostate cells.

What biologists want from their chloride reporters – a conversation between chemists and biologists

Impaired chloride transport affects diverse processes ranging from neuron excitability to water secretion, which underlie epilepsy and cystic fibrosis, respectively. The ability to image chloride fluxes with fluorescent probes has been essential for the investigation of the roles of chloride channels and transporters in health and disease. Therefore, developing effective fluorescent chloride reporters is critical to characterizing chloride transporters and discovering new ones. However, each chloride channel or transporter has a unique functional context that demands a suite of chloride probes with appropriate sensing characteristics. This Review seeks to juxtapose the biology of chloride transport with the chemistries underlying chloride sensors by exploring the various biological roles of chloride and highlighting the insights delivered by studies using chloride reporters. We then delineate the evolution of small-molecule sensors and genetically encoded chloride reporters. Finally, we analyze discussions with chloride biologists to identify the advantages and limitations of sensors in each biological context, as well as to recognize the key design challenges that must be overcome for developing the next generation of chloride sensors.

Tracking intra- and inter-organelle signaling of mitochondria

Mitochondria are as highly specialized organelles and masters of the cellular energy metabolism in a constant and dynamic interplay with their cellular environment, providing adenosine triphosphate, buffering Ca²⁺ and fundamentally contributing to various signaling pathways. Hence, such broad field of action within eukaryotic cells requires a high level of structural and functional adaptation. Therefore, mitochondria are constantly moving and undergoing fusion and fission processes, changing their shape and their interaction with other organelles. Moreover, mitochondrial activity gets fine-tuned by intra- and interorganelle H⁺ , K⁺ , Na⁺ , and Ca²⁺ signaling. In this review, we provide an up-to-date overview on mitochondrial strategies to adapt and respond to, as well as affect, their cellular environment. We also present cutting-edge technologies used to track and investigate subcellular signaling, essential to the understanding of various physiological and pathophysiological processes.

A novel role and mechanism of cystic fibrosis transmembrane conductance regulator in bisphenol A-induced prostate cancer

Bisphenol A (BPA) is a well known environmental endocrine disruptor that may cause human prostate cancer through disturbing cell mitosis, proliferation, and apoptosis. As one of the most important anion channels in organisms, cystic fibrosis transmembrane conductance regulator (CFTR) is proposed as a tumor suppressor in carcinogenesis and development of prostate cancer in recent studies. Whether CFTR plays a role in BPA-induced prostate cancer needs to be further identified. In this study, two prostate cancer cell lines PC-3 and LNCaP were exposed to BPA for detecting the cytotoxic reactions by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and enzyme-linked immunosorbent assays. After the treatment with BPA for 24 hours, the cell viability was decreased significantly with increased cell apoptosis in the two cell lines. Moreover, both PC-3 and LNCaP cells had a reduced expression level of cAMP, CFTR, and adenosine triphosphate upon BPA treatment. In addition, AMPKα kinase was found upregulated to promote cell apoptosis through increasing Bax expression and decreasing Bcl-2 expression. Our study suggests a role and mechanism of CFTR in BPA-induced prostate cancer via cell apoptosis for the first time.

Calcium phosphate buffer formed in the mitochondrial matrix during preconditioning supports ΔpH formation and ischemic ATP production and prolongs cell survival -A hypothesis

Ischemic preconditioning makes cells less sensitive to oxygen deprivation. A similar effect can be achieved by increasing the calcium concentration and applying potassium channel openers. A hypothetical mechanism of preconditioning is presented. In the mitochondrial matrix, there is a calcium hydroxide buffer consisting of a few insoluble calcium phosphate minerals. During ischemia, calcium ions stored in the matrix buffer start to leak out, forming an electric potential difference, while hydroxyl ions remain in the matrix, maintaining its pH and the matrix volume. Preconditioning factors increase the matrix buffer capacity. Production of ATP during ischemia might be the relic of a pre-endosymbiotic past.

Activation of Anoctamin-1 Limits Pulmonary Endothelial Cell Proliferation via p38-Mitogen-activated Protein Kinase-Dependent Apoptosis

Hyperproliferative endothelial cells (ECs) play an important role in the pathogenesis of pulmonary arterial hypertension (PAH). Anoctamin (Ano)-1, a calcium-activated chloride channel, can regulate cell proliferation and cell cycle in multiple cell types. However, the expression and function of Ano1 in the pulmonary endothelium is unknown. We examined whether Ano1 was expressed in pulmonary ECs and if altering Ano1 activity would affect EC survival. Expression and localization of Ano1 in rat lung microvascular ECs (RLMVECs) was assessed using immunoblot, immunofluorescence, and subcellular fractionation. Cell counts, flow cytometry, and caspase-3 activity were used to assess changes in cell number and apoptosis in response to the small molecule Ano1 activator, Eact. Changes in mitochondrial membrane potential and mitochondrial reactive oxygen species (mtROS) were assessed using 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolylcarbocyanine, iodide (mitochondrial membrane potential dye) and mitochondrial ROS dye, respectively. Ano1 is expressed in RLMVECs and is enriched in the mitochondria. Activation of Ano1 with Eact reduced RLMVEC counts through increased apoptosis. Ano1 knockdown blocked the effects of Eact. Ano1 activation increased mtROS, reduced mitochondrial membrane potential, increased p38 phosphorylation, and induced release of apoptosis-inducing factor. mtROS inhibition attenuated Eact-mediated p38 phosphorylation. Pulmonary artery ECs isolated from patients with idiopathic PAH (IPAH) had higher expression of Ano1 and increased cell counts compared with control subjects. Eact treatment reduced cell counts in IPAH cells, which was associated with increased apoptosis. In summary, Ano1 is expressed in lung EC mitochondria. Activation of Ano1 promotes apoptosis of pulmonary ECs and human IPAH-pulmonary artery ECs, likely via increased mtROS and p38 phosphorylation, leading to apoptosis.

The effect of respiration buffer composition on mitochondrial metabolism and function

Functional studies on isolated mitochondria critically rely on the right choice of respiration buffer. Differences in buffer composition can lead to dramatically different respiration rates leading to difficulties in comparing prior studies. The ideal buffer facilities high ADP-stimulated respiratory rates and minimizes substrate transport effects so that the ability to distinguish between various treatments and conditions is maximal. In this study, we analyzed a variety of respiration buffers and substrate combinations to determine the optimal conditions to support mitochondrial function through ADP-stimulated respiration and uncoupled respiration using FCCP. The buffers consisted of a standard KCl based buffer (B1) and three modified buffers with chloride replaced by the K-lactobionate, sucrose, and the antioxidant taurine (B2) or K-gluconate (B3). The fourth buffer (B4) was identical to B2 except that K-lactobionate was replaced with K-gluconate. The substrate combinations consisted of metabolites that utilize different pathways of mitochondrial metabolism. To test mitochondrial function, we used isolated cardiac guinea pig mitochondria and measured oxygen consumption for three respiratory states using an Oroboros Oxygraph-2k. These states were the leak state (energized mitochondria in the absence of adenylates), ADP-stimulated state (energized mitochondria in the presence of saturating ADP concentrations), and uncoupled state (energized mitochondria in the presence of FCCP). On average across all substrate combinations, buffers B2, B3, and B4 had an increase of 16%, 26%, and 35% for the leak state, ADP-simulated state, and uncoupled state, respectively, relative to rates using B1. The common feature distinguishing these buffers from B1 is the notable lack of high chloride concentrations. Based on the respiratory rate metrics obtained with the substrate combinations, we conclude that the adenine nucleotide translocase, the dicarboxylate carrier, and the alpha-ketoglutarate exchanger are partially inhibited by chloride. Therefore, when the goal is to maximize ADP-stimulated respiration, buffers containing K-lactobionate or K-gluconate are superior choices compared to the standard KCl-based buffers.

Subconductance states of mitochondrial chloride channels: implication for functionally-coupled tetramers

Recently, it has been discovered that isoforms of intracellular chloride channels (CLIC) are present in cardiac mitochondria. By reconstituting rat cardiac mitochondrial chloride channels into bilayer lipid membranes, we detected three equally separated subconductance states with conductance increment of 45 pS and < 2% occupancy. The observed rare events of channel decomposition into substates, accompanied by disrupted gating, provide an insight into channel quaternary structure. Our findings suggest that the observed channels work as four functionally coupled subunits with synchronized gating. We discuss the putative connection of channel activity from native mitochondria with the recombinant CLIC channels. However, conclusive evidence is needed to prove this connection.

Characterization of injury in isolated rat proximal tubules during cold incubation and rewarming

Organ shortage leads to an increased utilization of marginal organs which are particularly sensitive to storage-associated damage. Cold incubation and rewarming-induced injury is iron-dependent in many cell types. In addition, a chloride-dependent component of injury has been described. This work examines the injury induced by cold incubation and rewarming in isolated rat renal proximal tubules. The tissue storage solution TiProtec^® and a chloride-poor modification, each with and without iron chelators, were used for cold incubation. Incubation was performed 4°C for up to 168 h, followed by rewarming in an extracellular buffer (3 h at 37°C). After 48, 120 and 168 h of cold incubation LDH release was lower in solutions containing iron chelators. After rewarming, injury increased especially after cold incubation in chelator-free solutions. Without addition of iron chelators LDH release showed a tendency to be higher in chloride-poor solutions. Following rewarming after 48 h of cold incubation lipid peroxidation was significantly decreased and metabolic activity was tendentially better in tubules incubated with iron chelators. Morphological alterations included mitochondrial swelling and fragmentation being partially reversible during rewarming. ATP content was better preserved in chloride-rich solutions. During rewarming, there was a further decline of ATP content in the so far best conditions and minor alterations under the other conditions, while oxygen consumption was not significantly different compared to non-stored control tubules. Results show an iron-dependent component of preservation injury during cold incubation and rewarming in rat proximal renal tubules and reveal a benefit of chloride for the maintenance of tubular energy state during cold incubation.

Stage-Specific Changes in the Water, Na+, Cl- and K+ Contents of Organelles during Apoptosis, Demonstrated by a Targeted Cryo Correlative Analytical Approach

Many studies have demonstrated changes in the levels of several ions during apoptosis, but a few recent studies have reported conflicting results concerning the changes in water content in apoptotic cells. We used a correlative light and cryo-scanning transmission electron microscopy method to quantify water and ion/element contents simultaneously at a nanoscale resolution in the various compartments of cells, from the onset to the end of apoptosis. We used stably transfected HeLa cells producing H2B-GFP to identify the stages of apoptosis in cells and for a targeted elemental analysis within condensed chromatin, nucleoplasm, mitochondria and the cytosol. We found that the compartments of apoptotic cells contained, on average, 10% more water than control cells. During mitochondrial outer membrane permeabilization, we observed a strong increase in the Na⁺ and Cl^- contents of the mitochondria and a strong decrease in mitochondrial K⁺ content. During the first step in apoptotic volume decrease (AVD), Na⁺ and Cl^- levels decreased in all cell compartments, but remained higher than those in control cells. Conversely, during the second step of AVD, Na⁺ and Cl^- levels increased considerably in the nucleus and mitochondria. During these two steps of AVD, K⁺ content decreased steadily in all cell compartments. We also determined in vivo ion status during caspase-3 activity and chromatin condensation. Finally, we found that actinomycin D-tolerant cells had water and K⁺ contents similar to those of cells entering apoptosis but lower Na⁺ and Cl^- contents than both cells entering apoptosis and control cells.

Ion Channels in Native Chloroplast Membranes: Challenges and Potential for Direct Patch-Clamp Studies

Photosynthesis without any doubt depends on the activity of the chloroplast ion channels. The thylakoid ion channels participate in the fine partitioning of the light-generated proton-motive force (p.m.f.). By regulating, therefore, luminal pH, they affect the linear electron flow and non-photochemical quenching. Stromal ion homeostasis and signaling, on the other hand, depend on the activity of both thylakoid and envelope ion channels. Experimentally, intact chloroplasts and swollen thylakoids were proven to be suitable for direct measurements of the ion channels activity via conventional patch-clamp technique; yet, such studies became infrequent, although their potential is far from being exhausted. In this paper we wish to summarize existing challenges for direct patch-clamping of native chloroplast membranes as well as present available results on the activity of thylakoid Cl⁻ (ClC?) and divalent cation-permeable channels, along with their tentative roles in the p.m.f. partitioning, volume regulation, and stromal Ca²⁺ and Mg²⁺ dynamics. Patch-clamping of the intact envelope revealed both large-conductance porin-like channels, likely located in the outer envelope membrane and smaller conductance channels, more compatible with the inner envelope location. Possible equivalent model for the sandwich-like arrangement of the two envelope membranes within the patch electrode will be discussed, along with peculiar properties of the fast-activated cation channel in the context of the stromal pH control.

Nitric oxide releases Cl(-) from acidic organelles in retinal amacrine cells

Determining the factors regulating cytosolic Cl(-) in neurons is fundamental to our understanding of the function of GABA- and glycinergic synapses. This is because the Cl(-) distribution across the postsynaptic plasma membrane determines the sign and strength of postsynaptic voltage responses. We have previously demonstrated that nitric oxide (NO) releases Cl(-) into the cytosol from an internal compartment in both retinal amacrine cells and hippocampal neurons. Furthermore, we have shown that the increase in cytosolic Cl(-) is dependent upon a decrease in cytosolic pH. Here, our goals were to confirm the compartmental nature of the internal Cl(-) store and to test the hypothesis that Cl(-) is being released from acidic organelles (AO) such as the Golgi, endosomes or lysosomes. To achieve this, we made whole cell voltage clamp recordings from cultured chick retinal amacrine cells and used GABA-gated currents to track changes in cytosolic Cl(-). Our results demonstrate that intact internal proton gradients are required for the NO-dependent release of internal Cl(-). Furthermore, we demonstrate that increasing the pH of AO leads to release of Cl(-) into the cytosol. Intriguingly, the elevation of organellar pH results in a reversal in the effects of NO. These results demonstrate that cytosolic Cl(-) is closely linked to the regulation and maintenance of organellar pH and provide evidence that acidic compartments are the target of NO.

Mitochondrial Ion Channels in Cancer Transformation

Cancer transformation involves reprograming of mitochondrial function to avert cell death mechanisms, monopolize energy metabolism, accelerate mitotic proliferation, and promote metastasis. Mitochondrial ion channels have emerged as promising therapeutic targets because of their connection to metabolic and apoptotic functions. This mini review discusses how mitochondrial channels may be associated with cancer transformation and expands on the possible involvement of mitochondrial protein import complexes in pathophysiological process.

Role of positively charged residues of the second transmembrane domain in the ion transport activity and conformation of human uncoupling protein-2

Residing at the inner mitochondrial membrane, uncoupling protein-2 (UCP2) mediates proton transport from the intermembrane space (IMS) to the mitochondrial matrix and consequently reduces the rate of ATP synthesis in the mitochondria. The ubiquitous expression of UCP2 in humans can be attributed to the protein's multiple physiological roles in tissues, including its involvement in protective mechanisms against oxidative stress, as well as glucose and lipid metabolisms. Currently, the structural properties and ion transport mechanism of UCP2 and other UCP homologues remain poorly understood. UCP2-mediated proton transport is activated by fatty acids and inhibited by di- and triphosphate purine nucleotides. UCP2 also transports chloride and some other small anions. Identification of key amino acid residues of UCP2 in its ion transport pathway can shed light on the protein's ion transport function. On the basis of our previous studies, the second transmembrane helix segment (TM2) of UCP2 exhibited chloride channel activity. In addition, it was suggested that the positively charged residues on TM2 domains of UCPs 1 and 2 were important for their chloride transport activity. On this basis, to further understand the role of these positively charged residues on the ion transport activity of UCP2, we recombinantly expressed four TM2 mutants: R76Q, R88Q, R96Q, and K104Q. The wild type UCP2 and its mutants were purified and reconstituted into liposomes, and their conformation and ion (proton and chloride) transport activity were studied. TM2 Arg residues at the matrix interface of UCP2 proved to be crucial for the protein's anion transport function, and their absence resulted in highly diminished Cl(-) transport rates. On the other hand, the two other positively charged residues of TM2, located at the UCP2-IMS interface, could participate in the salt-bridge formation in the protein and promote the interhelical tight packing in the UCP2. Absence of these residues did not influence Cl(-) transport rates, but disturbed the dense packing in UCP2 and resulted in higher UCP2-mediated proton transport rates in the presence of long chain fatty acids. Overall, the outcome of this study provides a deeper and more detailed molecular image of UCP2's ion transport mechanism.

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.

Mitochondrial chloride channels: electrophysiological characterization and pH induction of channel pore dilation

Physiological and pathological functions of mitochondria are highly dependent on the properties and regulation of mitochondrial ion channels. There is still no clear understanding of the molecular identity, regulation, and properties of anion mitochondrial channels. The inner membrane anion channel (IMAC) was assumed to be equivalent to mitochondrial centum picosiemens (mCS). However, the different properties of IMAC and mCS channels challenges this opinion. In our study, we characterized the single-channel anion selectivity and pH regulation of chloride channels from purified cardiac mitochondria. We observed that channel conductance decreased in the order: Cl⁻ > Br⁻ > I⁻ > chlorate ≈ formate > acetate, and that gluconate did not permeate under control conditions. The selectivity sequence was Br⁻ ≥ chlorate ≥ I⁻ ≥ Cl⁻ ≥ formate ≈ acetate. Measurement of the concentration dependence of chloride conductance revealed altered channel gating kinetics, which was demonstrated by prolonged mean open time value with increasing chloride concentration. The observed mitochondrial chloride channels were in many respects similar to those of mCS, but not those of IMAC. Surprisingly, we observed that acidic pH increased channel conductance and that an increase of pH from 7.4 to 8.5 reduced it. The gluconate current appeared and gradually increased when pH decreased from pH 7.0 to 5.6. Our results indicate that pH regulates the channel pore diameter in such a way that dilation increases with more acidic pH. We assume this newly observed pH-dependent anion channel property may be involved in pH regulation of anion distribution in different mitochondrial compartments.

Characterization of distinct single-channel properties of Ca²⁺ inward currents in mitochondria

Previous studies have demonstrated several molecularly distinct players involved in mitochondrial Ca²⁺ uptake. In the present study, electrophysiological recordings on mitoplasts that were isolated from HeLa cells were performed in order to biophysically and pharmacologically characterize Ca²⁺ currents across the inner mitochondrial membrane. In mitoplast-attached configuration with 105 mM Ca²⁺ as a charge carrier, three distinct channel conductances of 11, 23, and 80 pS were observed. All types of mitochondrial currents were voltage-dependent and essentially depended on the presence of Ca²⁺ in the pipette. The 23 pS channel exhibited burst kinetics. Though all channels were sensitive to ruthenium red, their sensitivity was different. The 11 and 23 pS channels exhibited a lower sensitivity to ruthenium red than the 80 pS channel. The activities of all channels persisted in the presence of cylosporin A, CGP 37187, various K⁺-channel inhibitors, and Cl⁻ channel blockers disodium 4,4′-diisothiocyanatostilbene-2,2′-disulfonate and niflumic acid. Collectively, our data identified multiple conductances of Ca²⁺ currents in mitoplasts isolated from HeLa cells, thus challenging the dogma of only one unique mitochondrial Ca²⁺ uniporter.

Modulation of the hepatocyte rough endoplasmic reticulum single chloride channel by nucleotide-Mg2+ interaction

The effect of nucleotides on single chloride channels derived from rat hepatocyte rough endoplasmic reticulum vesicles incorporated into bilayer lipid membrane was investigated. The single chloride channel currents were measured in 200/50 mmol/l KCl cis/trans solutions. Adding 2.5 mM adenosine triphosphate (ATP) and adenosine diphosphate (ADP) did not influence channel activity. However, MgATP addition inhibited the chloride channels by decreasing the channel open probability (Po) and current amplitude, whereas mixture of Mg(2+) and ADP activated the chloride channel by increasing the Po and unitary current amplitude. According to the results, there is a novel regulation mechanism for rough endoplasmic reticulum (RER) Cl(-) channel activity by intracellular MgATP and mixture of Mg(2+) and ADP that would result in significant inhibition by MgATP and activation by mixture of Mg(2+) and ADP. These modulatory effects of nucleotide-Mg(2+) complexes on chloride channels may be dependent on their chemical structure configuration. It seems that Mg-nucleotide-ion channel interactions are involved to produce a regulatory response for RER chloride channels.

ApoptoProteomics, an integrated database for analysis of proteomics data obtained from apoptotic cells

Apoptosis is the most commonly described form of programmed cell death, and dysfunction is implicated in a large number of human diseases. Many quantitative proteome analyses of apoptosis have been performed to gain insight in proteins involved in the process. This resulted in large and complex data sets that are difficult to evaluate. Therefore, we developed the ApoptoProteomics database for storage, browsing, and analysis of the outcome of large scale proteome analyses of apoptosis derived from human, mouse, and rat. The proteomics data of 52 publications were integrated and unified with protein annotations from UniProt-KB, the caspase substrate database homepage (CASBAH), and gene ontology. Currently, more than 2300 records of more than 1500 unique proteins were included, covering a large proportion of the core signaling pathways of apoptosis. Analysis of the data set revealed a high level of agreement between the reported changes in directionality reported in proteomics studies and expected apoptosis-related function and may disclose proteins without a current recognized involvement in apoptosis based on gene ontology. Comparison between induction of apoptosis by the intrinsic and the extrinsic apoptotic signaling pathway revealed slight differences. Furthermore, proteomics has significantly contributed to the field of apoptosis in identifying hundreds of caspase substrates. The database is available at http://apoptoproteomics.uio.no.

Modulation of intracellular chloride channels by ATP and Mg2+

We report the effects of ATP and Mg2+ on the activity of intracellular chloride channels. Mitochondrial and lysosomal membrane vesicles isolated from rat hearts were incorporated into bilayer lipid membranes, and single chloride channel currents were measured. The observed chloride channels (n=112) possessed a wide variation in single channel parameters and sensitivities to ATP. ATP (0.5-2 mmol/l) modulated and/or inhibited the chloride channel activities (n=38/112) in a concentration-dependent manner. The inhibition effect was irreversible (n=5/93) or reversible (n=15/93). The non-hydrolysable ATP analogue AMP-PNP had a similar inhibition effect as ATP, indicating that phosphorylation did not play a role in the ATP inhibition effect. ATP modulated the gating properties of the channels (n=6/93), decreased the channels' open dwell times and increased the gating transition rates. ATP (0.5-2 mmol/l) without the presence of Mg2+ decreased the chloride channel current (n=12/14), whereas Mg2+ significantly reversed the effect (n=4/4). We suggest that ATP-intracellular chloride channel interactions and Mg2+ modulation of these interactions may regulate different physiological and pathological processes.

Targeting of Helicobacter pylori VacA to mitochondria

One of the major virulence factors of Helicobacter pylori is the vacuolating toxin vaca. It has been known for a long time that the toxin enters host cells by endocytosis. On the other hand there is ample evidence that vaca is able to trigger apoptosis and this effect has been attributed in part to interactions with mitochondria. However, for 10 years it was difficult to reconcile the obvious accumulation of vaca in endosomes with mitochondrial targeting. The accessibility of the mitochondria to the toxin was enigmatic. In our new study, we investigated the activities of p34, the toxic subunit of vaca, in more detail. We found that the p34 N-terminus carries a unique targeting sequence for import into mitochondria and for insertion into the mitochondrial inner membrane. By forming an anion channel in this membrane, the toxin has the ability to interfere directly with mitochondrial functions. Taking into account additional results from independent studies, we discuss the implications of our findings with respect to intracellular traffic, the remarkable possibility of a direct transfer of VacA from endosomes to mitochondria and vaca-dependent cell death.