Fluorescence imaging of metabolic responses in single mitochondria

Effect of curcumin on malignant hepatocytes and mitochondria studied using atomic force microscopy

Mitochondria are emerging as potential targets for the cancer treatment. In this study, the effects of curcumin on the activity, migration, and mitochondrial membrane potential (MMP) of malignant hepatocytes (SMMC-7721 cells) were determined using cell viability, migration, and MMP assays. Changes in the morphology and biomechanics of SMMC-7721 cells and their mitochondria were studied using both optical microscopy and atomic force microscopy (AFM). The cell survival rate, migration and MMP depended on the concentration of curcumin. Optical microscopy studies showed that curcumin altered the cell morphology. AFM studies showed that the changes in the morphology and nanomechanics of SMMC-7721 cells and their mitochondria, were induced by curcumin. As the concentration of curcumin increased, the cell length, width, and adhesion decreased, but the height, roughness and Young's modulus increased. In contrast, the mitochondrial length, width, height and roughness increased, but the adhesion and Young's modulus decreased. There was a close relationship between mitochondria and cells in terms of function, morphology and biomechanics. This study shows the effects of curcumin on SMMC-7721 cells and their mitochondria from biology and biophysics perspectives. The findings aid in comprehensively understanding the interactions between mitochondria and malignant hepatocytes.

Reduction of alcohol-induced mitochondrial damage with ginsenoside Rg1 studied by atomic force microscopy

The quantification of mitochondrial morphology and mechanical properties is useful for the diagnosis and treatment of mitochondrial and alcoholic liver disease. In this study, the effects of ginsenoside Rg1 (G-Rg1) on the morphology and mechanical properties of mitochondria that had suffered alcohol-induced damage were investigated under near-physiological conditions. Additionally, the morphological and mechanical properties of mitochondria were quantified through atomic force microscopy. Atomic force microscopy revealed that alcohol-induced significant morphological changes in mitochondria. Compared with that of the mitochondria of normal hepatocytes, the average surface area of the damaged mitochondria was found to have increased significantly under the influence of alcohol. Furthermore, the mitochondrial area tended to be normal under the action of G-Rg1, whilst other parameters (length, width and perimeter) were significantly different from those of the mitochondria with the alcohol-induced damage. Simultaneously, alcohol significantly reduced the adhesion and elastic modulus of mitochondria, whilst the adhesion and elastic modulus of mitochondria in the G-Rg1 treatment group were closer to the values of normal mitochondria. This study overall showed that G-Rg1 could effectively alleviate the swelling and anomalous mechanical properties of mitochondria induced by alcohol.

Direct Analysis of Mitochondrial Damage Caused by Misfolded/Destabilized Proteins

Protein quality control is essential for cellular homeostasis. In this study, we examined the effect of improperly folded proteins that do not form amyloid fibrils on mitochondria, which play important roles in ATP production and cell death. First, we prepared domain 3 of the dengue envelope protein in wild type and four mutants with widely different biophysical properties in misfolded/aggregated or destabilized states. The effects of the different proteins were detected using fluorescence microscopy and Western blotting, which revealed that three of the five proteins disrupted both inner and outer membrane integrity, while the other two proteins, including the wild type, did not. Next, we examined the common characteristics of the proteins that displayed toxicity against mitochondria by measuring oligomer size, molten globule-like properties, and thermal stability. The common feature of all three toxic proteins was thermal instability. Therefore, our data strongly suggest that thermally unstable proteins generated in the cytosol can cause cellular damage by coming into direct contact with mitochondria. More importantly, we revealed that this damage is not amyloid-specific.

Effects of proton pumping on the structural rigidity of cristae in mitochondria

Mitochondria change their morphology and inner membrane structure depending on their activity. Since mitochondrial activity also depends on their structure, it is important to elucidate the interrelationship between the activity and structure of mitochondria. However, the mechanism by which mitochondrial activity affects the structure of cristae, the folded structure of the inner membrane, is not well understood. In this study, the effect of the mitochondrial activity on the cristae structure was investigated by examining the structural rigidity of cristae. Taking advantage of the fact that unfolding of cristae induces mitochondrial swelling, we investigated the relationship between mitochondrial activity and the susceptibility to swelling. The swelling of individual isolated mitochondria exposed to a hypotonic solution was observed with an optical microscope. The presence of respiratory substrates (malate and glutamate) increased the percentage of mitochondria that underwent swelling, and the further addition of rotenone or KCN (inhibitors of proton pumps) reversed the increase. In the absence of respiratory substrates, acidification of the buffer surrounding the mitochondria also increased the percentage of swollen mitochondria. These observations suggest that acidification of the outer surface of inner membranes, especially intracristal space, by proton translocation from the matrix to the intracristal space, decreases the structural rigidity of the cristae. This interpretation was verified by the observation that ADP or CCCP, which induces proton re-entry to the matrix, suppressed the mitochondrial swelling in the presence of respiratory substrates. The addition of CCCP to the cells induced a morphological change in mitochondria from an initial elongated structure to a largely curved structure at pH 7.4, but there were no morphological changes when the pH of the cytosol dropped to 6.2. These results suggest that a low pH in the intracristal space may be helpful in maintaining the elongated structure of mitochondria. The present study shows that proton pumping by the electron transfer chain is the mechanism underlying mitochondrial morphology and the flexibility of cristae structure.

Effects of Matrix pH on Spontaneous Transient Depolarization and Reactive Oxygen Species Production in Mitochondria

Reactive oxygen species (ROS) oxidize surrounding molecules and thus impair their functions. Since mitochondria are a major source of ROS, suppression of ROS overproduction in the mitochondria is important for cells. Spontaneous transient depolarization of individual mitochondria is a physiological phenomenon widely observed from plants to mammals. Mitochondrial uncoupling can reduce ROS production; therefore, it is conceivable that transient depolarization could reduce ROS production. However, transient depolarization has been observed with increased ROS production. Therefore, the exact contribution of transient depolarization to ROS production has not been elucidated. In this study, we examined how the spontaneous transient depolarization occurring in individual mitochondria affected ROS production. When the matrix pH increased after the addition of malate or exposure of the isolated mitochondria to a high-pH buffer, transient depolarization was stimulated. Similar stimulation by an increased matrix pH was also observed in the mitochondria in intact H9c2 cells. Modifying the mitochondrial membrane potential and matrix pH by adding K⁺ in the presence of valinomycin, a K⁺ ionophore, clarified that an increase in the matrix pH is a major cause of ROS generation. When we added ADP in the presence of oligomycin to suppress the transient depolarization without decreasing the matrix pH, we observed the suppression of mitochondrial respiration, increased matrix pH, and enhanced ROS production. Based on these results, we propose a model where spontaneous transient depolarization occurs during increased proton influx through proton channels opened by increased matrix pH, leading to the suppression of ROS production. This study improves our understanding of mitochondrial behavior.

Partial contribution of mitochondrial permeability transition to t-butyl hydroperoxide-induced cell death

Mitochondrial permeability transition (MPT) is thought to determine cell death under oxidative stress. However, MPT inhibitors only partially suppress oxidative stress-induced cell death. Here, we demonstrate that cells in which MPT is inhibited undergo cell death under oxidative stress. When C6 cells were exposed to 250 μM t-butyl hydroperoxide (t-BuOOH), the loss of a membrane potential-sensitive dye (tetramethylrhodamine ethyl ester, TMRE) from mitochondria was observed, indicating mitochondrial depolarization leading to cell death. The fluorescence of calcein entrapped in mitochondria prior to addition of t-BuOOH was significantly decreased to 70% after mitochondrial depolarization. Cyclosporin A suppressed the decrease in mitochondrial calcein fluorescence, but not mitochondrial depolarization. These results show that t-BuOOH induced cell death even when it did not induce MPT. Prior to MPT, lactate production and respiration were hampered. Taken together, these data indicate that the decreased turnover rate of glycolysis and mitochondrial respiration may be as vital as MPT for cell death induced under moderate oxidative stress.

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Cell death was induced in C6 cells by 250 μM t-BuOOH.

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Mitochondrial permeability transition (MPT) occurred before cell death.

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MPT was confirmed by observing calcein fluorescence in mitochondria.

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MPT inhibition did not prevent depolarization of mitochondria and cell death.

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Contribution of MPT to cell death is partial under moderate oxidative stress.

Nanohole Array-Directed Trapping of Mammalian Mitochondria Enabling Single Organelle Analysis

We present periodic nanohole arrays fabricated in free-standing metal-coated nitride films as a platform for trapping and analyzing single organelles. When a microliter-scale droplet containing mitochondria is dispensed above the nanohole array, the combination of evaporation and capillary flow directs individual mitochondria to the nanoholes. Mammalian mitochondria arrays were rapidly formed on chip using this technique without any surface modification steps, microfluidic interconnects, or external power sources. The trapped mitochondria were depolarized on chip using an ionophore with results showing that the organelle viability and behavior were preserved during the on-chip assembly process. Fluorescence signal related to mitochondrial membrane potential was obtained from single mitochondria trapped in individual nanoholes revealing statistical differences between the behavior of polarized vs depolarized mammalian mitochondria. This technique provides a fast and stable route for droplet-based directed localization of organelles-on-a-chip with minimal limitations and complexity, as well as promotes integration with other optical or electrochemical detection techniques.

Analysis of mitochondrial mechanical dynamics using a confocal fluorescence microscope with a bent optical fibre

The cells in the cardiovascular system are constantly subjected to mechanical forces created by blood flow and the beating heart. The effect of forces on cells has been extensively investigated, but their effect on cellular organelles such as mitochondria remains unclear. We examined the impact of nano-Newton forces on mitochondria using a bent optical fibre (BOF) with a flat-ended tip (diameter exceeding 2 μm) and a confocal fluorescence microscope. By indenting a single mitochondrion with the BOF tip, we found that the mitochondrial elastic modulus was proportional to the (-1/2) power of the mitochondrial radius in the 9.6-115 kPa range. We stained the mitochondria with a potential-metric dye (TMRE) and measured the changes in TMRE fluorescence intensity. We confirmed that more active mitochondria exhibit a higher frequency of repetitive transient depolarization. The same trend was observed at forces lower than 50 nN. We further showed that the depolarization frequency of mitochondria decreases under an extremely large force (nearly 100 nN). We conclude that mitochondrial function is affected by physical environmental factors, such as external forces at the nano-Newton level.

Isolation of mitochondria by gentle cell membrane disruption, and their subsequent characterization

Mitochondria play a key role in several physiological processes as in integrating signals in the cell. However, understanding of the mechanism by which mitochondria sense and respond to signals has been limited due to the lack of an appropriate model system. In this study, we developed a method to isolate and characterize mitochondria without cell homogenization. By gently pipetting cells treated with streptolysin-O, a pore-forming membrane protein, we disrupted the cell membrane and were able to isolate both elongated and spherical mitochondria. Fluorescence imaging combined with super resolution microscopy showed that both the outer and inner membranes of the elongated mitochondria isolated using the newly developed method were intact. In addition, a FRET-based ATP sensor expressed in the mitochondrial matrix demonstrated that ATP generation by FoF1-ATPase in the isolated elongated mitochondria was as high as that in intracellular mitochondria. On the other hand, some of the spherical mitochondria isolated with this method had the outer membrane that no longer encapsulated the inner membrane. In addition, all mitochondria isolated using conventional procedures involving homogenization were spherical, many of them had damaged membranes, and low levels of ATP generation. Our results suggest that elongated mitochondria isolated from cells through gentle cell membrane disruption using a pore-forming protein tend to be more similar to intracellular mitochondria, having an intact membrane system and higher activity than spherical mitochondria.

Significant correlation between refractive index and activity of mitochondria: single mitochondrion study

Measurements of refractive indices (RIs) of intracellular components can provide useful information on the structure and function of cells. The present study reports, for the first time, determination of the RI of an isolated mitochondrion in isotonic solution using retardation-modulated differential interference contrast microscopy. The value was 1.41 ± 0.01, indicating that mitochondria are densely packed with molecules having high RIs. Further, the RIs of each mitochondrion were significantly correlated with the mitochondrial membrane potential, an index of mitochondrial activity. These results will provide useful information on the structures and functions of cells based on the intracellular distribution of RIs.

Detection of swelling of single isolated mitochondrion with optical microscopy

Volume regulation under osmotic loading is one of the most fundamental functions in cells and organelles. However, the effective method to detect volume changes of a single organelle has not been developed. Here, we present a novel technique for detecting volume changes of a single isolated mitochondrion in aqueous solution based on the transmittance of the light through the mitochondrion. We found that 70% and 21% of mitochondria swelled upon addition of a hypotonic solution and Ca(2+), respectively. These results show the potential of the present technique to detect the physiological volume changes of individual small organelles such as mitochondria.

Monitoring metabolic responses of single mitochondria within poly(dimethylsiloxane) wells: study of their endogenous reduced nicotinamide adenine dinucleotide evolution

It is now demonstrated that mitochondria individually function differently because of specific energetic needs in cell compartments but also because of the genetic heterogeneity within the mitochondrial pool-network of a cell. Consequently, understanding mitochondrial functioning at the single organelle level is of high interest for biomedical research, therefore being a target for analyticians. In this context, we developed easy-to-build platforms of milli- to microwells for fluorescence microscopy of single isolated mitochondria. Poly(dimethylsiloxane) (PDMS) was determined to be an excellent material for mitochondrial deposition and observation of their NADH content. Because of NADH autofluorescence, the metabolic status of each mitochondrion was analyzed following addition of a respiratory substrate (stage 2), ethanol herein, and a respiratory inhibitor (stage 3), Antimycin A. Mean levels of mitochondrial NADH were increased by 32% and 62% under stages 2 and 3, respectively. Statistical studies of NADH value distributions evidenced different types of responses, at least three, to ethanol and Antimycin A within the mitochondrial population. In addition, we showed that mitochondrial ability to generate high levels of NADH, that is its metabolic performance, is not correlated either to the initial energetic state or to the respective size of each mitochondrion.

Extramitochondrial domain rich in carbonic anhydrase activity improves myocardial energetics

CO2 is produced abundantly by cardiac mitochondria. Thus an efficient means for its venting is required to support metabolism. Carbonic anhydrase (CA) enzymes, expressed at various sites in ventricular myocytes, may affect mitochondrial CO2 clearance by catalyzing CO2 hydration (to H(+) and HCO3(-)), thereby changing the gradient for CO2 venting. Using fluorescent dyes to measure changes in pH arising from the intracellular hydration of extracellularly supplied CO2, overall CA activity in the cytoplasm of isolated ventricular myocytes was found to be modest (2.7-fold above spontaneous kinetics). Experiments on ventricular mitochondria demonstrated negligible intramitochondrial CA activity. CA activity was also investigated in intact hearts by (13)C magnetic resonance spectroscopy from the rate of H(13)CO3(-) production from (13)CO2 released specifically from mitochondria by pyruvate dehydrogenase-mediated metabolism of hyperpolarized [1-(13)C]pyruvate. CA activity measured upon [1-(13)C]pyruvate infusion was fourfold higher than the cytoplasm-averaged value. A fluorescent CA ligand colocalized with a mitochondrial marker, indicating that mitochondria are near a CA-rich domain. Based on immunoreactivity, this domain comprises the nominally cytoplasmic CA isoform CAII and sarcoplasmic reticulum-associated CAXIV. Inhibition of extramitochondrial CA activity acidified the matrix (as determined by fluorescence measurements in permeabilized myocytes and isolated mitochondria), impaired cardiac energetics (indexed by the phosphocreatine-to-ATP ratio measured by (31)P magnetic resonance spectroscopy of perfused hearts), and reduced contractility (as measured from the pressure developed in perfused hearts). These data provide evidence for a functional domain of high CA activity around mitochondria to support CO2 venting, particularly during elevated and fluctuating respiratory activity. Aberrant distribution of CA activity therefore may reduce the heart's energetic efficiency.

Paraquat toxicity induced by voltage-dependent anion channel 1 acts as an NADH-dependent oxidoreductase

Paraquat (PQ), a herbicide used worldwide, causes fatal injury to organs upon high dose ingestion. Treatments for PQ poisoning are unreliable, and numerous deaths have been attributed inappropriate usage of the agent. It is generally speculated that a microsomal drug-metabolizing enzyme system is responsible for PQ toxicity. However, recent studies have demonstrated cytotoxicity via mitochondria, and therefore, the cytotoxic mechanism remains controversial. Here, we demonstrated that mitochondrial NADH-dependent PQ reductase containing a voltage-dependent anion channel 1 (VDAC1) is responsible for PQ cytotoxicity. When mitochondria were incubated with NADH and PQ, superoxide anion (O(2)(*)) was produced, and the mitochondria ruptured. Outer membrane extract oxidized NADH in a PQ dose-dependent manner, and oxidation was suppressed by VDAC inhibitors. Zymographic analysis revealed the presence of VDAC1 protein in the oxidoreductase, and the direct binding of PQ to VDAC1 was demonstrated using biotinylated PQ. VDAC1-overexpressing cells showed increased O(2)(*) production and cytotoxicity, both of which were suppressed in VDAC1 knockdown cells. These results indicated that a VDAC1-containing mitochondrial system is involved in PQ poisoning. These insights into the mechanism of PQ poisoning not only demonstrated novel physiological functions of VDAC protein, but they may facilitate the development of new therapeutic approaches.

A comparison of Zn2+- and Ca2+-triggered depolarization of liver mitochondria reveals no evidence of Zn2+-induced permeability transition

Intracellular Zn(2+) toxicity is associated with mitochondrial dysfunction. Zn(2+) depolarizes mitochondria in assays using isolated organelles as well as cultured cells. Some reports suggest that Zn(2+)-induced depolarization results from the opening of the mitochondrial permeability transition pore (mPTP). For a more detailed analysis of this relationship, we compared Zn(2+)-induced depolarization with the effects of Ca(2+) in single isolated rat liver mitochondria monitored with the potentiometric probe rhodamine 123. Consistent with previous work, we found that relatively low levels of Ca(2+) caused rapid, complete and irreversible loss of mitochondrial membrane potential, an effect that was diminished by classic inhibitors of mPT, including high Mg(2+), ADP and cyclosporine A. Zn(2+) also depolarized mitochondria, but only at relatively high concentrations. Furthermore Zn(2+)-induced depolarization was slower, partial and sometimes reversible, and was not affected by inhibitors of mPT. We also compared the effects of Ca(2+) and Zn(2+) in a calcein-retention assay. Consistent with the well-documented ability of Ca(2+) to induce mPT, we found that it caused rapid and substantial loss of matrix calcein. In contrast, calcein remained in Zn(2+)-treated mitochondria. Considered together, our results suggest that Ca(2+) and Zn(2+) depolarize mitochondria by considerably different mechanisms, that opening of the mPTP is not a direct consequence of Zn(2+)-induced depolarization, and that Zn(2+) is not a particularly potent mitochondrial inhibitor.

Involvement of cyclophilin D in mitochondrial permeability transition induction in intact cells

The mitochondrial permeability transition (MPT) is involved in both Ca(2+) signaling and cell death. The present study aimed to clarify the involvement of cyclophilin D, a peptidyl prolyl cis-trans isomerase (PPIase), in MPT induction in intact cells. To achieve this, we used C6 cells overexpressing wild-type or PPIase-deficient cyclophilin D, and measured the inner mitochondrial membrane permeability to calcein, a 623-Da hydrophilic fluorescent molecule, to evaluate MPT induction. In vector control cells, the percentage of MPT induction by ionomycin increased as the Ca(2+) concentration in the extracellular medium increased. This result indicates that the present method is valid for numerical evaluation of MPT induction. In C6 cells expressing the PPIase-deficient mutant, the percentage of MPT induction was significantly decreased compared with wild-type CypD-overexpressing cells or vector control cells. These results suggest that cyclophilin D is involved in MPT induction by Ca(2+) in intact cells.

Mitochondrial membrane potential in axons increases with local nerve growth factor or semaphorin signaling

Neurons concentrate mitochondria at sites in the cell that have a high demand for ATP and/or calcium buffering. To accomplish this, mitochondrial transport and docking are thought to respond to intracellular signaling pathways. However, the cell might also concentrate mitochondrial function by locally modulating mitochondrial activity. We tested this hypothesis by measuring the membrane potential of individual mitochondria throughout the axons of chick sensory neurons using the dye tetramethylrhodamine methylester (TMRM). We found no difference in the TMRM mitochondrial-to-cytoplasmic fluorescence ratio (F(m)/F(c)) among three functionally distinct regions: axonal branch points, distal axons, and the remaining axon shaft. In addition, we found no difference in F(m)/F(c) among stationary, retrogradely moving, or anterogradely moving mitochondria. However, F(m)/F(c) was significantly higher in the lamellipodia of growth cones, and among a small fraction of mitochondria throughout the axon. To identify possible signals controlling membrane potential, we used beads covalently coupled to survival and guidance cues to provide a local stimulus along the axon shaft. NGF- or semaphorin 3A-coupled beads caused a significant increase in F(m)/F(c) in the immediately adjacent region of axon, and this was diminished in the presence of the PI3 (phosphatidylinositol-3) kinase inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one] or the MAP (mitogen-activated protein) kinase inhibitor U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-amino-phenylthio]butadiene), demonstrating that signaling pathways downstream of both ligands affect the DeltaPsi(m) of mitochondria. In addition, general inhibition of receptor tyrosine kinase activity produced a profound global decrease in F(m)/F(c). Thus, two guidance molecules that exert different effects on growth cone motility both elicit local, receptor-mediated increases in membrane potential.

Evaluating mitochondrial membrane potential in cells

Permeant cationic fluorescent probes are widely employed to monitor mitochondrial transmembrane potential and its changes. The application of such potential-dependent probes in conjunction with both fluorescence microscopy and fluorescence spectroscopy allows the monitoring of mitochondrial membrane potential in individual living cells as well as in large population of cells. These approaches to the analysis of membrane potential is of extremely high value to obtain insights into both the basic energy metabolism and its dysfunction in pathologic cells. However, the use of fluorescent molecules to probe biological phenomena must follow the awareness of some principles of fluorescence emission, quenching, and quantum yield since it is a very sensitive tool, but because of this extremely high sensitivity it is also strongly affected by the environment. In addition, the instruments used to monitor fluorescence and its changes in biological systems have also to be employed with cautions due to technical limits that may affect the signals. We have therefore undertaken to review the most currently used analytical methods, providing a summary of practical tips that should precede data acquisition and subsequent analysis. Furthermore, we discuss the application and feasibility of various techniques and discuss their respective strength and weakness.

Stability of membrane potential in heart mitochondria: single mitochondrion imaging

Mitochondrial membrane potential (delta psi(m)) plays an important role in cellular activity. Although delta psi(m) of intracellular mitochondria are relatively stable, the recent experiments with isolated mitochondria demonstrate that individual mitochondria show frequent fluctuations of delta psi(m). The current study is performed to investigate the factors that stabilize delta psi(m) in cells by observing delta psi(m) of individual isolated mitochondria with fluorescence microscopy. Here, we report that (1) the transient depolarizations are also induced for mitochondria in plasma membrane permeabilized cells, (2) almost all mitochondria isolated from porcine hearts show the transient depolarizations that is enhanced with the net efflux of protons from the matrix to the intermembrane space, and (3) ATP and ADP significantly inhibit the transient depolarizations by plural mechanisms. These results suggest that the suppression of acute alkalinization of the matrix together with the presence of ATP and ADP contributes to the stabilization of delta psi(m) in cells.

Distinct characteristics of Ca(2+)-induced depolarization of isolated brain and liver mitochondria

Ca(2+)-induced mitochondrial depolarization was studied in single isolated rat brain and liver mitochondria. Digital imaging techniques and rhodamine 123 were used for mitochondrial membrane potential measurements. Low Ca(2+) concentrations (about 30--100 nM) initiated oscillations of the membrane potential followed by complete depolarization in brain mitochondria. In contrast, liver mitochondria were less sensitive to Ca(2+); 20 microm Ca(2+) was required to depolarize liver mitochondria. Ca(2+) did not initiate oscillatory depolarizations in liver mitochondria, where each individual mitochondrion depolarized abruptly and irreversibly. Adenine nucleotides dramatically reduced the oscillatory depolarization in brain mitochondria and delayed the onset of the depolarization in liver mitochondria. In both type of mitochondria, the stabilizing effect of adenine nucleotides completely abolished by an inhibition of adenine nucleotide translocator function with carboxyatractyloside, but was not sensitive to bongkrekic acid. Inhibitors of mitochondrial permeability transition cyclosporine A and bongkrekic acid also delayed Ca(2+)-depolarization. We hypothesize that the oscillatory depolarization in brain mitochondria is associated with the transient conformational change of the adenine nucleotide translocator from a specific transporter to a non-specific pore, whereas the non-oscillatory depolarization in liver mitochondria is caused by the irreversible opening of the pore.

Effects of disialoganglioside GD3 on the mitochondrial membrane potential

GD3 is an intracellular mediator of apoptotic signaling. Although GD3 is known to directly act on mitochondria, the dynamic responses of individual mitochondria to GD3 remain to be elucidated. In the current study, the membrane potential of single mitochondria is observed in the presence of GD3 or its analogues. Here, we report that (1) GD3 specifically induces gradual depolarizations of the inner membrane by a mechanism that differs from the permeability transition, and (2) the GD3-induced depolarizations are suppressed by cyclosporin A. These results suggest that GD3 depolarizes mitochondria by a mechanism distinct from but relevant to the permeability transition.

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.

Fluctuations in mitochondrial membrane potential in single isolated brain mitochondria: modulation by adenine nucleotides and Ca2+

In this study we investigated fluctuations in mitochondrial membrane potential (DeltaPsim) in single isolated brain mitochondria using fluorescence imaging. Mitochondria were attached to coverslips and perfused with K+-based buffer containing 20 microM EDTA, supplemented with malate and glutamate, and rhodamine 123 for DeltaPsim determination. DeltaPsim fluctuations were triggered by mitochondrial Ca2+ uptake since they were inhibited by both ruthenium red, a Ca2+-uniporter blocker, and by high concentrations of EGTA. A very low concentration of Ca2+ (approximately 30 nM) was required to initiate the fluctuations. Both ATP and ADP reversibly inhibited DeltaPsim fluctuations, with maximal effects occurring at 100 microM. The effect of nucleotides could not be explained by the reversed mode of mitochondrial ATP-synthase, since oligomycin was not effective and nonhydrolysable analogs of ATP and ADP did not stop the fluctuations. The effects of adenine nucleotides were abolished by blockade of the adenine nucleotide translocator with carboxyatractyloside, but were insensitive to another inhibitor, bongkrekic acid. ATP-sensitive K+-channels are not involved in the mechanism of DeltaPsim fluctuations, since the inhibitor 5-hydroxydecanoate or the activator diazoxide did not affect dynamics of DeltaPsim. We suggest DeltaPsim fluctuations in brain mitochondria are not spontaneous, but are triggered by Ca2+ and are modulated by adenine nucleotides, possibly from the matrix side of the inner mitochondrial membrane.

Spontaneous changes in mitochondrial membrane potential in single isolated brain mitochondria

In this study we measured DeltaPsim in single isolated brain mitochondria using rhodamine 123. Mitochondria were attached to coverslips and superfused with K(+)-based HEPES-buffer medium supplemented with malate and glutamate. In approximately 70% of energized mitochondria we observed large amplitude spontaneous fluctuations in DeltaPsim with a time course comparable to that observed previously in mitochondria of intact cells. The other 30% of mitochondria maintained a stable DeltaPsim. Some of the "stable" mitochondria began to fluctuate spontaneously during the recording period. However, none of the initially fluctuating mitochondria became stable. Upon the removal of substrates from the medium or application of small amounts of Ca(2+), rhodamine 123 fluorescence rapidly dropped to background values in fluctuating mitochondria, while nonfluctuating mitochondria depolarized with a delay and often began to fluctuate before complete depolarization. The changes in DeltaPsim were not connected to oxidant production since reducing illumination or the addition of antioxidants had no effect on DeltaPsim. Fluctuating mitochondria did not lose calcein, nor was there any effect of cyclosporin A on DeltaPsim, which ruled out a contribution of permeability transition. We conclude that the fluctuations in DeltaPsim reflect an intermediate, unstable state of mitochondria that may lead to or reflect mitochondrial dysfunction.

Advances in the analysis of single mitochondria

Mitochondria are both morphologically and functionally diverse, and this variety is thought to have important biological ramifications. The development of methods to probe the properties of individual mitochondria is therefore of utmost importance. Recent advances have been made using in situ microscopy techniques and methods to investigate isolated mitochondria, including flow cytometry, capillary electrophoresis, patch-clamping and optical trapping. Such techniques have been used to study metabolism, mitochondrial calcium homeostasis, mitochondrial membrane potential, apoptosis, and other properties.