Visualization of cyclosporin A and Ca2+-sensitive cyclical mitochondrial depolarizations in cell culture

Energy metabolism disturbance in migraine: From a mitochondrial point of view

Migraine is a serious central nervous system disease with a high incidence rate. Its pathogenesis is very complex, which brings great difficulties for clinical treatment. Recently, many studies have revealed that mitochondrial dysfunction may play a key role in migraine, which affects the hyperosmotic of Ca²⁺, the excessive production of free radicals, the decrease of mitochondrial membrane potential, the imbalance of mPTP opening and closing, and the decrease of oxidative phosphorylation level, which leads to neuronal energy exhaustion and apoptosis, and finally lessens the pain threshold and migraine attack. This article mainly introduces cortical spreading depression, a pathogenesis of migraine, and then damages the related function of mitochondria, which leads to migraine. Oxidative phosphorylation and the tricarboxylic acid cycle are the main ways to provide energy for the body. 95 percent of the energy needed for cell survival is provided by the mitochondrial respiratory chain. At the same time, hypoxia can lead to cell death and migraine. The pathological opening of the mitochondrial permeability transition pore can promote the interaction between pro-apoptotic protein and mitochondrial, destroy the structure of mPTP, and further lead to cell death. The increase of mPTP permeability can promote the accumulation of reactive oxygen species, which leads to a series of changes in the expression of proteins related to energy metabolism. Both Nitric oxide and Calcitonin gene-related peptide are closely related to the attack of migraine. Recent studies have shown that changes in their contents can also affect the energy metabolism of the body, so this paper reviews the above mechanisms and discusses the mechanism of brain energy metabolism of migraine, to provide new strategies for the prevention and treatment of migraine and promote the development of individualized and accurate treatment of migraine.

Acute Nitrite Exposure Induces Dysfunction and Oxidative Damage in Grass Carp Isolated Hemocytes

To evaluate the effects of nitrite on the oxidative damage of blood cells of Grass Carp Ctenopharyngodon idella, the isolated hemocytes were exposed to nitrite (0, 1, 10, or 100 mg/L) for up to 24 h. Hemoglobin (Hb) and methemoglobin (MetHb) concentrations, reactive oxygen species (ROS) and malondialdehyde (MDA) levels, mitochondrial membrane potential (∆Ψm), and antioxidant enzyme activity were assayed to assess hematological parameters and the antioxidant defense mechanism. Results showed a remarkable decrease in Hb concentration with increasing nitrite concentration after a 24-h exposure, while the MetHb concentration increased significantly in nitrite exposure groups. The levels of ROS, ∆Ψm, and MDA increased to varying degrees with increases in nitrite exposure concentration and time. The total antioxidant capacity, catalase (CAT) activity, glutathione peroxidase (GPx) activity, and glutathione content showed a trend of rising initially and then decreasing with prolonged exposure time. Superoxide dismutase (SOD) activity was higher in the 1-mg/L nitrite exposure group and lower in the 100-mg/L group than in the control. The relative messenger RNA expression ratios of cat, sod1, and gpx were up-regulated significantly in the 1- and 10-mg/L groups and then declined in the 100-mg/L group. Therefore, it can be concluded that nitrite exposure activates the antioxidant defense mechanism of Grass Carp hemocytes and that the balance of oxidant-antioxidant homeostasis will be undermined by higher nitrite doses or longer exposure periods.

Characterization of the respiration-induced yeast mitochondrial permeability transition pore

When isolated mitochondria from the yeast Saccharomyces cerevisiae oxidize respiratory substrates in the absence of phosphate and ADP, the yeast mitochondrial unselective channel, also called the yeast permeability transition pore (yPTP), opens in the inner membrane, dissipating the electrochemical gradient. ATP also induces yPTP opening. yPTP opening allows mannitol transport into isolated mitochondria of laboratory yeast strains, but mannitol is not readily permeable through the yPTP in an industrial yeast strain, Yeast Foam. The presence of oligomycin, an inhibitor of ATP synthase, allowed for respiration-induced mannitol permeability in mitochondria from this strain. Potassium (K+) had varied effects on the respiration-induced yPTP, depending on the concentration of the respiratory substrate added. At low respiratory substrate concentrations K+ inhibited respiration-induced yPTP opening, while at high substrate concentrations this effect diminished. However, at the high respiratory substrate concentrations, the presence of K+ partially prevented phosphate inhibition of yPTP opening. Phosphate was found to inhibit respiration-induced yPTP opening by binding a site on the matrix space side of the inner membrane in addition to its known inhibitory effect of donating protons to the matrix space to prevent the pH change necessary for yPTP opening. The respiration-induced yPTP was also inhibited by NAD, Mg2+, NH4 + or the oxyanion vanadate polymerized to decavanadate. The results demonstrate similar effectors of the respiration-induced yPTP as those previously described for the ATP-induced yPTP and reconcile previous strain-dependent differences in yPTP solute selectivity.

Oxidative stress defines the neuroprotective or neurotoxic properties of androgens in immortalized female rat dopaminergic neuronal cells

Males have a higher risk for developing Parkinson's disease and parkinsonism after ischemic stroke than females. Although estrogens have been shown to play a neuroprotective role in Parkinson's disease, there is little information on androgens' actions on dopamine neurons. In this study, we examined the effects of androgens under conditions of oxidative stress to determine whether androgens play a neuroprotective or neurotoxic role in dopamine neuronal function. Mitochondrial function, cell viability, intracellular calcium levels, and mitochondrial calcium influx were examined in response to androgens under both nonoxidative and oxidative stress conditions. Briefly, N27 dopaminergic cells were exposed to the oxidative stressor, hydrogen peroxide, and physiologically relevant levels of testosterone or dihydrotestosterone, applied either before or after oxidative stress exposure. Androgens, alone, increased mitochondrial function via a calcium-dependent mechanism. Androgen pretreatment protected cells from oxidative stress-induced cell death. However, treatment with androgens after the oxidative insult increased cell death, and these effects were, in part, mediated by calcium influx into the mitochondria. Interestingly, the negative effects of androgens were not blocked by either androgen or estrogen receptor antagonists. Instead, a putative membrane-associated androgen receptor was implicated. Overall, our results indicate that androgens are neuroprotective when oxidative stress levels are minimal, but when oxidative stress levels are elevated, androgens exacerbate oxidative stress damage.

Modeling the neuroprotective role of enhanced astrocyte mitochondrial metabolism during stroke

A mathematical model that integrates the dynamics of cell membrane potential, ion homeostasis, cell volume, mitochondrial ATP production, mitochondrial and endoplasmic reticulum Ca(2+) handling, IP3 production, and GTP-binding protein-coupled receptor signaling was developed. Simulations with this model support recent experimental data showing a protective effect of stimulating an astrocytic GTP-binding protein-coupled receptor (P2Y1Rs) following cerebral ischemic stroke. The model was analyzed to better understand the mathematical behavior of the equations and to provide insights into the underlying biological data. This approach yielded explicit formulas determining how changes in IP3-mediated Ca(2+) release, under varying conditions of oxygen and the energy substrate pyruvate, affected mitochondrial ATP production, and was utilized to predict rate-limiting variables in P2Y1R-enhanced astrocyte protection after cerebral ischemic stroke.

Effect of neuroprotective therapies (hypothermia and cyclosporine a) on dopamine-induced apoptosis in human neuronal SH-SY5Y cells

INTRODUCTION

This study aimed to evaluate the effect of hypothermia and CyA on neuronal survival after induced injury in a neuronal model.

METHODS

Human neuroblastoma SH-SY5Y cells were seeded and allowed to grow. To determine whether lower temperatures protect from dopamine-induced apoptosis, cells were treated with dopamine at 100 µM, at 300 µM or without dopamine and incubated at 32 °C or 37 °C for 24 hours. To assess the effect of CyA, cells were pre-incubated with CyA at 37 °C and after dopamine was added.

RESULTS

After 24 hours of incubation at 37 °C, 100 µM and 300 µM dopamine induced 42% (SD = 21) and 58% (SD = 7.9) apoptotic SH-SY5 cells, respectively. In cultures at 32 °C dopamine-induced apoptosis could be reversed by hypothermia [7% (SD = 1.4) and 3.45% (SD = 1.1) for 100 µM and 300 µM, respectively], similar to levels obtained in non-treated cells [2.4% (SD = 1.5)]. Cyclosporine A treatment did not render the expected result, since CyA-pre-treated cells and SH-SY5Y cells showed higher levels of apoptosis than those observed with dopamine alone

CONCLUSIONS

Hypothermia has a marked protective effect against apoptotic cell death induced by dopamine in a human neuroblastic cell line. The neuroprotective effect of CyA described with other apoptotic cell death stimuli was not demonstrated with our experimental conditions.

The low conductance mitochondrial permeability transition pore confers excitability and CICR wave propagation in a computational model

Mitochondria have long been known to sequester cytosolic Ca(2+) and even to shape intracellular patterns of endoplasmic reticulum-based Ca(2+) signaling. Evidence suggests that the mitochondrial network is an excitable medium which can demonstrate independent Ca(2+) induced Ca(2+) release via the mitochondrial permeability transition. The role of this excitability remains unclear, but mitochondrial Ca(2+) handling appears to be a crucial element in diverse diseases as diabetes, neurodegeneration and cardiac dysfunction that also have bioenergetic components. In this paper, we extend the modular Magnus-Keizer computational model for respiration-driven Ca(2+) handling to include a permeability transition based on a channel-like pore mechanism. We demonstrate both excitability and Ca(2+) wave propagation accompanied by depolarizations qualitatively similar to those reported in cell and isolated mitochondria preparations. These waves depend on the energy state of the mitochondria, as well as other elements of mitochondrial physiology. Our results support the concept that mitochondria can transmit state dependent signals about their function across the mitochondrial network. Our model provides the tools for predictions about the internal physiology that leads to this qualitatively different Ca(2+) excitability seen in mitochondria.

Cyclosporin A-induced oxidative stress is not the consequence of an increase in mitochondrial membrane potential

Cyclosporin A induces closure of the mitochondrial permeability transition pore. We aimed to investigate whether this closure results in concomitant increases in mitochondrial membrane potential (DeltaPsim) and the production of reactive oxygen species. Fluorescent probes were used to assess DeltaPsim (JC-1, 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-benzimidazolyl-carbocyanine iodide), reactive oxygen species [DCF, 5- (and 6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate, acetyl ester] and [Ca2+][Fluo-3, glycine N-[4-[6-[(acetyloxy)methoxy]-2,7-dichloro-3-oxo-3H-xanthen-9-yl]-2-[2-[2-[bis[2-[(acetyloxy)methoxy]-2-oxyethyl]amino]-5-methylphenoxy]ethoxy]phenyl]-N-[2-[(acetyloxy)methoxy]-2-oxyethyl]-(acetyloxy)methyl ester] in human kidney cells (HK-2 cells) and in a line of human small cell carcinoma cells (GLC4 cells), because these do not express cyclosporin A-sensitive P-glycoprotein. We used transfected GLC4 cells expressing P-glycoprotein as control for GLC4 cells. NIM811 (N-methyl-4-isoleucine-cyclosporin) and PSC833 (SDZ-PSC833) were applied as selective mitochondrial permeability transition pore and P-glycoprotein blockers, respectively. To study the effect of cyclosporin A on mitochondrial function, we isolated mitochondria from fresh pig livers. Cyclosporin A and PSC833 induced a more than two-fold increase in JC-1 fluorescence in HK-2 cells, whereas NIM811 had no effect. None of the three substances induced a significant increase in JC-1 fluorescence in GLC4 cells. Despite this, cyclosporin A, NIM811 and PSC833 induced a 1.5-fold increase in DCF fluorescence (P<0.05) and a two-fold increase in Fluo-3 fluorescence (P<0.05). Studies in isolated mitochondria showed that blockage of mitochondrial permeability transition pores by cyclosporin A affected neither DeltaPsim, ATP synthesis, nor respiration rate. The mitochondrial permeability transition pore blockers cyclosporin A and NIM811, but also the non-mitochondrial permeability transition pore blocker PSC833, induced comparable degrees of reactive oxygen species production and cytosolic [Ca2+]. Neither mitochondria, effects on P-glycoprotein nor inhibition of calcineurin therefore play a role in cyclosporin A-induced oxidative stress and disturbed Ca2+ homeostasis.

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.

Cyclical mitochondrial deltapsiM fluctuations linked to electron transport, F0F1 ATP-synthase and mitochondrial Na+/Ca+2 exchange are reduced in Alzheimer's disease cybrids

Reduced complex IV, increased oxidative stress and beta amyloid peptide secretion in Alzheimer's disease (AD) can be replicated in cybrid models. We characterized cyclical mitochondrial deltapsiM fluctuations ('flickering') in neuroblastoma cells and AD/CTL cybrids. Flickering was blocked by ATP-synthase inhibition, was not observed in rho0 cells and was not blocked by antioxidant treatment. Flickering was not affected by the Ca(+2) uniporter antagonist Ru360 but was eliminated by BAPTA or CGP37137 blockade of the mitochondrial Na(+)/Ca(+2) exchanger. AD cybrid mitochondria showed reduced flickering. Flickering seems to represent coupling of deltapsiM to F0F1 ATP-synthase; reduction of flickering in AD cybrids suggests dysfunction of this coupling.

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

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

Mitochondrial Ca2+ dynamics reveals limited intramitochondrial Ca2+ diffusion

To reveal heterogeneity of mitochondrial function on the single-mitochondrion level we have studied the spatiotemporal dynamics of the mitochondrial Ca2+ signaling and the mitochondrial membrane potential using wide-field fluorescence imaging and digital image processing techniques. Here we demonstrate first-time discrete sites--intramitochondrial hotspots--of Ca2+ uptake after Ca2+ release from intracellular stores, and spreading of Ca2+ rise within the mitochondria. The phenomenon was characterized by comparison of observations in intact cells stimulated by ATP and in plasma membrane permeabilized or in ionophore-treated cells exposed to elevated buffer [Ca2+]. The findings indicate that Ca2+ diffuses laterally within the mitochondria, and that the diffusion is limited for shorter segments of the mitochondrial network. These observations were supported by mathematical simulation of buffered diffusion. The mitochondrial membrane potential was investigated using the potentiometric dye TMRM. Irradiation-induced fluctuations (flickering) of TMRM fluorescence showed synchronicity over large regions of the mitochondrial network, indicating that certain parts of this network form electrical syncytia. The spatial extension of these syncytia was decreased by 2-aminoethoxydiphenyl borate (2-APB) or by propranolol (blockers of nonclassical mitochondrial permeabilities). Our data suggest that mitochondria form syncytia of electrical conductance whereas the passage of Ca2+ is restricted to the individual organelle.

Quantitative analysis of spontaneous mitochondrial depolarizations

Spontaneous transient depolarizations in mitochondrial membrane potential (DeltaPsi(m)), mitochondrial flickers, have been observed in isolated mitochondria and intact cells using the fluorescent probe, tetramethylrhodamine ethyl ester (TMRE). In theory, the ratio of [TMRE] in cytosol and mitochondrion allows DeltaPsi(m) to be calculated with the Nernst equation, but this has proven difficult in practice due to fluorescence quenching and binding of dye to mitochondrial membranes. We developed a new method to determine the amplitude of flickers in terms of millivolts of depolarization. TMRE fluorescence was monitored using high-speed, high-sensitivity three-dimensional imaging to track individual mitochondria in freshly dissociated smooth muscle cells. Resting mitochondrial fluorescence, an exponential function of resting DeltaPsi(m), varied among mitochondria and was approximately normally distributed. Spontaneous changes in mitochondrial fluorescence, indicating depolarizations and repolarizations in DeltaPsi(m), were observed. The depolarizations were reversible and did not result in permanent depolarization of the mitochondria. The magnitude of the flickers ranged from <10 mV to >100 mV with a mean of 17.6 +/- 1.0 mV (n = 360) and a distribution skewed to smaller values. Nearly all mitochondria flickered, and they did so independently of one another, indicating that mitochondria function as independent units in the myocytes employed here.

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.

Spontaneous mitochondrial depolarizations are independent of SR Ca2+ release

The mitochondrial membrane potential (DeltaPsi(m)) underlies many mitochondrial functions, including Ca(2+) influx into the mitochondria, which allows them to serve as buffers of intracellular Ca(2+). Spontaneous depolarizations of DeltaPsi(m), flickers, have been observed in isolated mitochondria and intact cells using the fluorescent cationic lipophile tetramethylrhodamine ethyl ester (TMRE), which distributes across the inner mitochondrial membrane in accordance with the Nernst equation. Flickers in cardiomyocytes have been attributed to uptake of Ca(2+) released from the sarcoplasmic reticulum (SR) via ryanodine receptors in focal transients called Ca(2+) sparks. We have shown previously that an increase in global Ca(2+) in smooth muscle cells causes an increase in mitochondrial Ca(2+) and depolarization of DeltaPsi(m). Here we sought to determine whether flickers in smooth muscle cells are caused by uptake of Ca(2+) released focally in Ca(2+) sparks. High-speed three-dimensional imaging was used to monitor DeltaPsi(m) in freshly dissociated myocytes from toad stomach that were simultaneously voltage clamped at 0 mV to ensure the cytosolic TMRE concentration was constant and equal to the low level in the bath (2.5 nM). This approach allows quantitative analysis of flickers as we have previously demonstrated. Depletion of SR Ca(2+) not only failed to eliminate flickers but rather increased their magnitude and frequency somewhat. Flickers were not altered in magnitude or frequency by ryanodine or xestospongin C, inhibitors of intracellular Ca(2+) release, or by cyclosporin A, an inhibitor of the permeability transition pore. Focal Ca(2+) release from the SR does not cause flickers in the cells employed here.

Old players in a new role: mitochondria-associated membranes, VDAC, and ryanodine receptors as contributors to calcium signal propagation from endoplasmic reticulum to the mitochondria

In many cell types, IP(3) and ryanodine receptor (IP(3)R/RyR)-mediated Ca(2+) mobilization from the sarcoendoplasmic reticulum (ER/SR) results in an elevation of mitochondrial matrix [Ca(2+)]. Although delivery of the released Ca(2+) to the mitochondria has been established as a fundamental signaling process, the molecular mechanism underlying mitochondrial Ca(2+) uptake remains a challenge for future studies. The Ca(2+) uptake can be divided into the following three steps: (1) Ca(2+) movement from the IP(3)R/RyR to the outer mitochondrial membrane (OMM); (2) Ca(2+) transport through the OMM; and (3) Ca(2+) transport through the inner mitochondrial membrane (IMM). Evidence has been presented that Ca(2+) delivery to the OMM is facilitated by a local coupling between closely apposed regions of the ER/SR and mitochondria. Recent studies of the dynamic changes in mitochondrial morphology and visualization of the subcellular pattern of the calcium signal provide important clues to the organization of the ER/SR-mitochondrial interface. Interestingly, key steps of phospholipid synthesis and transfer to the mitochondria have also been confined to subdomains of the ER tightly associated with the mitochondria, referred as mitochondria-associated membranes (MAMs). Through the OMM, the voltage-dependent anion channels (VDAC, porin) have been thought to permit free passage of ions and other small molecules. However, recent studies suggest that the VDAC may represent a regulated step in Ca(2+) transport from IP(3)R/RyR to the IMM. A novel proposal regarding the IMM Ca(2+) uptake site is a mitochondrial RyR that would mediate rapid Ca(2+) uptake by mitochondria in excitable cells. An overview of the progress in these directions is described in the present paper.

Spontaneous changes in mitochondrial membrane potential in cultured neurons

Using the mitochondrial membrane potential (DeltaPsi(m))-sensitive fluorescent dyes 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide (JC-1) and tetramethylrhodamine methyl ester (TMRM), we have observed spontaneous changes in the DeltaPsi(m) of cultured forebrain neurons. These fluctuations in DeltaPsi(m) appear to represent partial, transient depolarizations of individual mitochondria. The frequency of these DeltaPsi(m) fluctuations can be significantly lowered by exposure to a photo-induced oxidant burden, an ATP synthase inhibitor, or a glutamate-induced sodium load, without changing overall JC-1 fluorescence intensity. These spontaneous fluctuations in JC-1 signal were not inhibited by altering plasma membrane activity with tetrodotoxin or MK-801 or by blocking the mitochondrial permeability transition pore (PTP) with cyclosporin A. Neurons loaded with TMRM showed similar, low-amplitude, spontaneous fluctuations in DeltaPsi(m). We hypothesize that these DeltaPsi(m) fluctuations are dependent on the proper functioning of the mitochondria and reflect mitochondria alternating between the active and inactive states of oxidative phosphorylation.

Mitochondrial modulation of intracellular Ca(2+) signaling

IP3-mediated Ca(2+) release plays a fundamental role in many cell signaling processes and has been the subject of numerous modeling studies. Only recently has the important role that mitochondria play in the dynamics of intracellular Ca(2+) signaling begun to be considered in experimental work and in computational models. Mitochondria sequester large amounts of Ca(2+) and thus have a modulatory effect on intracellular Ca(2+) signaling, and mitochondrial uptake of Ca(2+), in turn, has a regulatory effect on mitochondrial function. Here we integrate a well-established model of IP3-mediated Ca(2+) signaling with a detailed model of mitochondrial Ca(2+) handling and metabolic function. The incorporation of mitochondria results in oscillations in a bistable formulation of the IP3 model, and increasing metabolic substrate decreases the frequency of these oscillations consistent with the literature. Ca(2+) spikes from the cytosol are communicated into mitochondria and are shown to induce realistic metabolic changes. The model has been formulated using a modular approach that is easy to modify and should serve as a useful basis for the investigation of questions regarding the interaction of these two systems.

Cyclosporin A inhibits inositol 1,4,5-trisphosphate-dependent Ca2+ signals by enhancing Ca2+ uptake into the endoplasmic reticulum and mitochondria

Cytosolic Ca(2+) ([Ca(2+)](i)) oscillations may be generated by the inositol 1,4,5-trisphosphate receptor (IP(3)R) driven through cycles of activation/inactivation by local Ca(2+) feedback. Consequently, modulation of the local Ca(2+) gradients influences IP(3)R excitability as well as the duration and amplitude of the [Ca(2+)](i) oscillations. In the present work, we demonstrate that the immunosuppressant cyclosporin A (CSA) reduces the frequency of IP(3)-dependent [Ca(2+)](i) oscillations in intact hepatocytes, apparently by altering the local Ca(2+) gradients. Permeabilized cell experiments demonstrated that CSA lowers the apparent IP(3) sensitivity for Ca(2+) release from intracellular stores. These effects on IP(3)-dependent [Ca(2+)](i) signals could not be attributed to changes in calcineurin activity, altered ryanodine receptor function, or impaired Ca(2+) fluxes across the plasma membrane. However, CSA enhanced the removal of cytosolic Ca(2+) by sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), lowering basal and inter-spike [Ca(2+)](i). In addition, CSA stimulated a stable rise in the mitochondrial membrane potential (DeltaPsi(m)), presumably by inhibiting the mitochondrial permeability transition pore, and this was associated with increased Ca(2+) uptake and retention by the mitochondria during a rise in [Ca(2+)](i). We suggest that CSA suppresses local Ca(2+) feedback by enhancing mitochondrial and endoplasmic reticulum Ca(2+) uptake, these actions of CSA underlie the lower IP(3) sensitivity found in permeabilized cells and the impaired IP(3)-dependent [Ca(2+)](i) signals in intact cells. Thus, CSA binding proteins (cyclophilins) appear to fine tune agonist-induced [Ca(2+)](i) signals, which, in turn, may adjust the output of downstream Ca(2+)-sensitive pathways.

Apoptotic cell death of cultured salamander photoreceptors induced by cccp: CsA-insensitive mitochondrial permeability transition

Photoreceptor degeneration is mediated by apoptosis in several animal models, although the underlying mechanisms are yet to be elucidated. We present here an apoptotic model based on a primary cell culture of tiger salamander photoreceptors, in which treatment with carbonyl cyanide m-chlorophenylhydrazone (cccp), a protonophore, induced apoptosis. Cells exposed to cccp showed condensed nuclei and displayed positive TdT-dUTP terminal nick-end labeling (TUNEL). In addition, 10–100 microM cccp rapidly induced a reduction of Delta psi(m) and &gt; or = 30 microM cccp induced a significant leakage of calcein from mitochondria to cytosol and nucleus, indicating a change in mitochondrial inner membrane permeability. Cyclosporin A (CsA), a transition pore blocker, did not prevent the cccp-induced MPT or the cccp-evoked apoptotic cell death, suggesting that cccp-induced apoptotic process was mediated by a CsA-insensitive pathway. This cell model provides an in vitro tool for studying mechanisms of photoreceptor apoptosis in isolated photoreceptors and may provide clues to the etiology of retinal degeneration.

Maintaining Mitochondrial Membrane Impermeability

Apoptosis may contribute to retinal ganglion cell loss in glaucoma and glaucoma models. Recent research has suggested that mitochondrially dependent apoptosis signaling may contribute to apoptosis in a rat model of glaucoma involving chronic increases in intraocular pressure. In some forms of apoptosis, mitochondrially dependent signaling involves increases in mitochondrial membrane permeability and the mitochondrial release of factors that signal for cell degradation. Opening of a multi-protein, mitochondrial megapore is one factor that contributes to the increased permeability and some anti-apoptotic proteins, particularly BCL-2 and BCL-X(L), bind at the megapore and facilitate megapore closure and reduce increases in mitochondrial membrane permeability. Phosphorylated protein kinase B (Akt) serves as an integrator for cellular survival signals and facilitates the megapore actions of BCL-2 and BCL-X(L), which could protect retinal ganglion cells against insults that induce apoptosis. Several anti-apoptotic agents are being evaluated for use in glaucoma, including brimonidine and propargylamines, which oppose mitochondrially dependent apoptosis through pathways involving phosphorylated Akt.

Chronic reduction in complex I function alters calcium signaling in SH-SY5Y neuroblastoma cells

Sporadic, non-familial Parkinson's disease is characterized by a 15-30% reduction in complex I activity of the electron transport chain. A pharmacological model of reduced complex I activity was created by prolonged treatment of SH-SY5Y cells with low doses (5-20 nM) of rotenone, a selective inhibitor of complex I. Short-term (less than 2 week) exposure to rotenone did not influence calcium signaling, production of reactive oxygen species, or mitochondrial morphology. However, following 2 weeks of rotenone exposure, SH-SY5Y cells showed unusual calcium dynamics, specifically multiple calcium responses to carbachol, a muscarinic agonist. These secondary calcium responses were not seen in control SH-SY5Y cells and were dependent upon calcium influx. Mitochondrial membrane potential was also reduced in low dose rotenone-treated cells. These results demonstrate that a chronic, partial reduction in complex I activity, such as that seen in Parkinson's disease, can alter cell signaling events and perhaps increase the susceptibility of cells to calcium overload and subsequent cell death.

Mitochondrial filaments and clusters as intracellular power-transmitting cables

Mitochondria exist in two interconverting forms; as small isolated particles, and as extended filaments, networks or clusters connected with intermitochondrial junctions. Extended mitochondria can represent electrically united systems, which can facilitate energy delivery from the cell periphery to the cell core and organize antioxidant defence of the cell interior when O2 is consumed by mitochondrial clusters near the the outer cell membrane, and protonic potential is transmitted to the cell core mitochondria to form ATP. As to small mitochondria, they might represent a transportable form of these organelles.

The machinery of local Ca2+ signalling between sarco-endoplasmic reticulum and mitochondria

Growing evidence suggests that propagation of cytosolic [Ca2+] ([Ca2+]c) spikes and oscillations to the mitochondria is important for the control of fundamental cellular functions. Delivery of [Ca2+]c spikes to the mitochondria may utilize activation of the mitochondrial Ca2+ uptake sites by the large local [Ca2+]c rise occurring in the vicinity of activated sarco-endoplasmic reticulum (SR/ER) Ca2+ release channels. Although direct measurement of the local [Ca2+]c sensed by the mitochondria has been difficult, recent studies shed some light onto the molecular mechanism of local Ca2+ communication between SR/ER and mitochondria. Subdomains of the SR/ER are in close contact with mitochondria and display a concentration of Ca2+ release sites, providing the conditions for an effective delivery of released Ca2+ to the mitochondrial targets. Furthermore, many functional properties of the signalling between SR/ER Ca2+ release sites and mitochondrial Ca2+ uptake sites, including transient microdomains of high [Ca2+], saturation of mitochondrial Ca2+ uptake sites by released Ca2+, connection of multiple release sites to each uptake site and quantal transmission, are analogous to the features of the coupling between neurotransmitter release sites and postsynaptic receptors in synaptic transmission. As such, Ca2+ signal transmission between SR/ER and mitochondria may utilize discrete communication sites and a closely related functional architecture to that used for synaptic signal propagation between cells.

Dynamics of the mitochondrial reticulum in live cells using Fourier imaging correlation spectroscopy and digital video microscopy

We report detailed studies of the dynamics of the mitochondrial reticulum in live cells using two independent experimental techniques: Fourier imaging correlation spectroscopy and digital video fluorescence microscopy. When both methods are used to study the same system, it is possible to directly compare measurements of preaveraged statistical dynamical quantities with their microscopic counterparts. This approach allows the underlying mechanism of the observed rates to be determined. Our results indicate that the dynamics of the reticulum structure is composed of two independent contributions, each important on very different time and length scales. During short time intervals (1-15 sec), local regions of the reticulum primarily undergo constrained thermally activated motion. During long time intervals (>15 sec), local regions of the reticulum undergo long-range "jump" motions that are associated with the action of cytoskeletal filaments. Although the frequency of the jumps depend on the physiological state of the cells, the average jump distance ( approximately 0.8 microm) is unaffected by metabolic activity. During short time intervals, the dynamics appear to be spatially heterogeneous, whereas the cumulative effect of the infrequent jumps leads to the appearance of diffusive motion in the limit of long time intervals.

Elevation of resting mitochondrial membrane potential of neural cells by cyclosporin A, BAPTA-AM, and bcl-2

This study tested the hypothesis that the activity of the mitochondrial membrane permeability transition pore (PTP) affects the resting mitochondrial membrane potential (DeltaPsi) of normal, healthy cells and that the anti-apoptotic gene product Bcl-2 inhibits the basal activity of the PTP. DeltaPsi was measured by both fluorometric and nonfluorometric methods with SY5Y human neuroblastoma cells and with GT1-7 hypothalamic cells and PC12 pheochromocytoma cells in the absence and presence of Bcl-2 gene overexpression. The resting DeltaPsi of Bcl-2 nonexpressing PC12 and wild-type SY5Y cells was increased significantly by the presence of the PTP inhibitor cyclosporin A (CsA) or by intracellular Ca(2+) chelation through exposure to the acetoxymethyl ester of 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). The DeltaPsi of Bcl-2-overexpressing PC12 cells was larger than that of Bcl-2-negative cells and not significantly increased by CsA or by Ca(2+) chelation. CsA did not present a significant effect on the DeltaPsi monitored in unstressed GT1-7 cells but did inhibit the decrease in DeltaPsi elicited by the addition of t-butyl hydroperoxide, an oxidative inducer of the mitochondrial permeability transition. These results support the hypothesis that an endogenous PTP activity can contribute to lowering the basal DeltaPsi of some cells and that Bcl-2 can regulate the endogenous activity of the mitochondrial PTP.

Nitric oxide mediates cyclosporine-induced apoptosis in cultured renal cells

BACKGROUND

The clinical use of cyclosporine (CsA) is limited by its nephrotoxicity. Apoptosis, perhaps instigated by increased nitric oxide synthase (NOS) activity, may play a role in such toxicity.

METHODS

Human mesangial cells, human tubular cells, human umbilical vein endothelial cells, or murine endothelial cells were cultured with CsA at final concentrations of 0 to 1000 ng/mL for 4 to 24 hours. As inhibitors of apoptosis, 0.01 mol/L L-nitromethylarginine (L-NAME) or 1 microg/mL cycloheximide (CHX) was added, whereas 0.01 mol/L sodium nitroprusside (as a nitric oxide donor) was used as a positive control. Apoptosis was assessed by using TUNEL method and by DNA fragmentation by electrophoresis. In addition, NOS enzymatic activity, Northern blots for inducible NOS (iNOS) mRNA, and immunohistochemically demonstrable iNOS protein were evaluated.

RESULTS

Within 12 to 24 hours, CsA significantly increased the fraction (8 to 35%) of apoptotic cells in each cell line, according to the dose. Fragmentation of DNA confirmed apoptosis. L-NAME and CHX inhibited the phenomenon, whereas sodium nitroprusside enhanced it. Each cell line significantly increased NOS activity in response to CsA, an effect blunted by L-NAME and CHX. Neither inhibitor modified the increased iNOS mRNA expression elicited by CsA. Positive staining for both iNOS and p53 proteins was observed in all cell lines incubated with CsA that were inhibited by CHX; L-NAME inhibited only p53 staining.

CONCLUSIONS

CsA induces apoptosis in various renal cell lines, and this effect is mediated by the induction of iNOS via p53. These effects may contribute to the acellular fibrosis characteristic of late CsA nephrotoxicity.

Lack of oxidative phosphorylation and low mitochondrial membrane potential decrease susceptibility to apoptosis and do not modulate the protective effect of Bcl-x(L) in osteosarcoma cells

We explored the role of low mitochondrial membrane potential (DeltaPsim) and the lack of oxidative phosphorylation in apoptosis by assessing the susceptibility of osteosarcoma cell lines with and without mitochondrial DNA to staurosporine-induced death. Our cells without mitochondrial DNA had low DeltaPsim and no functional oxidative phosphorylation. Contrary to our expectation, these cells were more resistant to staurosporine-induced death than were the parental cells. This reduced susceptibility was associated with decreased activation of caspase 3 but not with the mitochondrial permeability transition pore or cytochrome c release from the mitochondria. Apoptosis in both cell lines was associated with an increase in DeltaPsim. Bcl-x(L) could protect both cell types against caspase 3 activation and apoptosis by a mechanism that does not appear to be mediated by mitochondrial function or modulation of DeltaPsim. Nevertheless, we found that Bcl-x(L) expression can stimulate cell respiration in cells with mitochondrial DNA. Our results showed that the lack of functional oxidative phosphorylation and/or low mitochondrial membrane potential are associated with an antiapoptotic effect, possibly contributing to the development of some types of cancer. It also reinforces a model in which Bcl-x(L) can exert an antiapoptotic effect by stimulating oxidative phosphorylation and/or inhibiting caspase activation.

Regulation of energy metabolism in human cells in aging and diabetes: FoF(1), mtDNA, UCP, and ROS

Recent advances in bioenergetics consist of discoveries related to rotational coupling in ATP synthase (FoF(1)), uncoupling proteins (UCP), reactive oxygen species (ROS) and mitochondrial DNA (mtDNA). As shown in cloned sheep, mammalian genomes are composed of both nuclear DNA (nDNA) and maternal mtDNA. Oxidative phosphorylation (oxphos) varies greatly depending on cellular activities, and is regulated by both gene expression and the electrochemical potential difference of H(+) (Delta muH(+)). The expression of both mtDNA (by mtTFA) and nDNA for oxphos and UCP (by NRFs, etc.) is coordinated by a factor called PGC-1. The Delta muH(+) rotates an axis in FoF(1) that is regulated by inhibitors and ATP-sensitive K(+)-channels. We cultured human rho(o) cells (cells without mtDNA) in synthetic media and elucidated relationships among mtDNA, nDNA, Delta muH(+), UCPs, ROS, and apoptosis. These cells lack oxphos-dependent ROS formation and survive under conditions of high O(2). Cells cultured in the absence of ROS scavengers have proliferated for 40 years. UCPs lower Delta muH(+) and prevent ROS formation and resulting apoptosis. These results were applied to diabetology and gerontology. The pancreatic rho(o) cells did not secrete insulin, and mtDNA mutations caused diabetes, owing to the deficient Delta muH(+). Insulin resistance was closely related to UCPs and other energy regulators. The resulting high-glucose environment caused glycation of proteins and ROS-mediated apoptosis in vascular cells involved in diabetic complications. Telomeres, oxphos, and ROS are determinants in cellular aging. Cell division and ROS shortened telomeres and accelerated aging. In aged cells, Delta muH(+) was reduced by the slow respiration, and this change induced apoptosis. Cybrids made from aged cytoplasts and rho(o) cells showed that both decreased expression of nDNA, and somatic mutations of mtDNA are involved in the slowing of respiration in aged cells.

Permeability transition pore regulates both mitochondrial membrane potential and agonist-evoked Ca2+ signals in oligodendrocyte progenitors

In this study, we investigated the importance of mitochondrial permeability transition pore (PTP) in agonist-evoked cytosolic Ca2+ ([Ca2+]c) signals in oligodendrocyte progenitor cells (OP cells). We measured transmembrane potential across the mitochondrial inner membrane (delta psi m) and [Ca2+]c in the immediate vicinity simultaneously using tetramethylrhodamine ethyl ester (TMRE) and calcium green respectively. Stimulation of OP cells with methacholine evoked robust [Ca2+]c signals in approximately 80% of cells which were either oscillatory or showed a peak followed by a plateau. Elevations in [Ca2+]c induced by supramaximal concentrations of the agonist (> 200 microM) were accompanied by changes in delta psi m in 33-42% of the total mitochondria investigated. The mitochondria that responded either depolarized (26-29%), hyperpolarized (7-13%) or showed no change (58-67%). Thus, of the responsive mitochondria, most (70%) depolarized during agonist-evoked [Ca2+]c signals. Blockade of PTP with cyclosporin A (CSA) reduced the number of mitochondria that depolarized with a corresponding increase in the number that hyperpolarized. In addition, CSA or its analogue methyl valine-4- CSA (MeVal-CSA), reduced the frequency of agonist-evoked global [Ca2+]c oscillations. In resting cells, CSA (63%) and MeVal-CSA (77%) hyperpolarized a majority of the mitochondria suggesting that PTP is constitutively active and may show flickering openings. Such hyperpolarizations were not mimicked by either cyclosporine H or verapamil and were inhibited by Ru360, which blocks the mitochondrial uniporter. This observation suggested that in resting cells, Ca2+ ions might redistribute between cytosol and mitochondrial matrix through the uniporter and the PTP. Taken together, these data suggest that PTP may play an important role in regulating delta psi m and local [Ca2+]c signals during agonist stimulation in OP cells.

Reduced mitochondrial membrane potential and altered responsiveness of a mitochondrial membrane megachannel in p53-induced senescence

There is accumulating evidence that mitochondrial membrane potential (DeltaPsi(M)) is reduced in aged cells. In addition, a decrease of DeltaPsi(M) has been shown to be an early event in many forms of apoptosis. Here we use a mitochondrial potentiometric dye with in situ laser scanning confocal microscopic (LSCM) imaging to demonstrate that DeltaPsi(M) is dramatically decreased in both the p53-overexpressing, senescent EJ tumor cells and in pre-apoptotic PC12 cells compared to controls. Treatment with cyclosporin A (CSA), which facilitates closure of the mitochondrial permeability transition pore (PTP), was able to reverse the decrease in DeltaPsi(M) in pre-apoptotic PC12 cells but not in the senescent EJ-p53 cells. The capacity to prevent dissipation of DeltaPsi(M) in response to agents that facilitate PTP closure may differentiate cells entering apoptosis from those participating in senescence. Therefore, regulation of the closure of the mitochondrial PTP in the presence of decreased DeltaPsi(M) may be a decisional checkpoint in distinguishing between growth arrest pathways.

Mitochondrial physiology and pathology; concepts of programmed death of organelles, cells and organisms

The review summarizes the present state of our knowledge concerning alternative functions of mitochondria, namely energy conservation in forms of protonic potential and ATP, thermoregulatory energy dissipation as heat, production of useful substances, decomposition of harmful substances, control of cellular processes. The recent progress in understanding of some mitochondrion-linked pathologies is described. The role of reactive oxygen species in these processes is stressed. Possible mechanisms of programmed death of mitochondrion (mitoptosis), cell (apoptosis) and organism (phenoptosis) are considered. A concept is put forward assuming that mitoptosis is involved in some types of apoptosis whereas apoptosis can be a part of a phenoptotic cascade. It is hypothesized that septic shock, as well as the stress-induced brain and heart ischemic diseases and cancer, exemplify mechanisms of phenoptosis purifying population, community of organisms or kin from dangerous or useless individuals.