Mechanism of neurodegeneration of neurons with mitochondrial DNA mutations

Mutations of mitochondrial DNA are associated with a wide spectrum of disorders, primarily affecting the central nervous system and muscle function. The specific consequences of mitochondrial DNA mutations for neuronal pathophysiology are not understood. In order to explore the impact of mitochondrial mutations on neuronal biochemistry and physiology, we have used fluorescence imaging techniques to examine changes in mitochondrial function in neurons differentiated from mouse embryonic stem-cell cybrids containing mitochondrial DNA polymorphic variants or mutations. Surprisingly, in neurons carrying a severe mutation in respiratory complex I (<10% residual complex I activity) the mitochondrial membrane potential was significantly increased, but collapsed in response to oligomycin, suggesting that the mitochondrial membrane potential was maintained by the F₁F_o ATPase operating in ‘reverse’ mode. In cells with a mutation in complex IV causing ∼40% residual complex IV activity, the mitochondrial membrane potential was not significantly different from controls. The rate of generation of mitochondrial reactive oxygen species, measured using hydroethidium and signals from the mitochondrially targeted hydroethidine, was increased in neurons with both the complex I and complex IV mutations. Glutathione was depleted, suggesting significant oxidative stress in neurons with a complex I deficiency, but not in those with a complex IV defect. In the neurons with complex I deficiency but not the complex IV defect, neuronal death was increased and was attenuated by reactive oxygen species scavengers. Thus, in neurons with a severe mutation of complex I, the maintenance of a high potential by F₁F_o ATPase activity combined with an impaired respiratory chain causes oxidative stress which promotes cell death.

Mitochondria mediated cell death in diabetes

Mitochondrial dysfunction plays a role in the pathogenesis of a wide range of diseases that involve disordered cellular fuel metabolism and survival/death pathways, including neurodegenerative diseases, cancer and diabetes. Cytokine, virus recognition and cellular stress pathways converging on mitochondria cause apoptotic and/or necrotic cell death of beta-cells in type-1 diabetes. Moreover, since mitochondria generate crucial metabolic signals for glucose stimulated insulin secretion (GSIS), mitochondrial dysfunction underlies both the functional derangement of GSIS and (over-nutrition) stress-induced apoptotic/necrotic beta-cell death, hallmarks of type-2 diabetes. The apparently distinct mechanisms governing beta-cell life/death decisions during the development of diabetes provide a remarkable example where remote metabolic, immune and stress signalling meet with mitochondria mediated apoptotic/necrotic death pathways to determine the fate of the beta-cell. We summarize the main findings supporting such a pivotal role of mitochondria in beta-cell death in the context of current trends in diabetes research.

Extracellular growth factors and mitogens cooperate to drive mitochondrial biogenesis

Cells generate new organelles when stimulated by extracellular factors to grow and divide; however, little is known about how growth and mitogenic signalling pathways regulate organelle biogenesis. Using mitochondria as a model organelle, we have investigated this problem in primary Schwann cells, for which distinct factors act solely as mitogens (neuregulin) or as promoters of cell growth (insulin-like growth factor 1; IGF1). We find that neuregulin and IGF1 act synergistically to increase mitochondrial biogenesis and mitochondrial DNA replication, resulting in increased mitochondrial density in these cells. Moreover, constitutive oncogenic Ras signalling results in a further increase in mitochondrial density. This synergistic effect is seen at the global transcriptional level, requires both the ERK and phosphoinositide 3-kinase (PI3K) signalling pathways and is mediated by the transcription factor ERRalpha. Interestingly, the effect is independent of Akt-TOR signalling, a major regulator of cell growth in these cells. This separation of the pathways that drive mitochondrial biogenesis and cell growth provides a mechanism for the modulation of mitochondrial density according to the metabolic requirements of the cell.

Impaired mitochondrial bioenergetics determines glutamate-induced delayed calcium deregulation in neurons

BACKGROUND

Accumulation of glutamate in ischaemic CNS is thought to amplify neuronal death during a stroke. Exposure of neurons to toxic glutamate concentrations causes an initial transient increase in [Ca(2+)](c) followed by a delayed increase commonly termed delayed [Ca(2+)](c) deregulation (DCD).

METHODS

We have used fluorescence imaging techniques to explore differences in glutamate-induced DCD in rat hippocampal neurons after different periods of time in culture (days in vitro; DIV).

RESULTS

The amplitude of both the initial [Ca(2+)](c) signal and the number of cells showing DCD in response to glutamate increased with the duration of culture. The capacity of mitochondria to accumulate calcium in permeabilised neurons decreased with time in culture, although mitochondrial membrane potential at rest did not change. The rate of ATP consumption, measured as an increase in [Mg(2+)](c) following inhibition of ATP synthesis, was lower in 'young' neurons. The sensitivity of 'young' neurons to glutamate-induced DCD approximated to that of 'old' neurons when mitochondrial function was impaired using either FCCP or oligomycin. Further, following such treatment, cells showed a DCD-like response to increased [Ca(2+)](c) induced by KCl induced depolarisation which was never otherwise seen.

GENERAL SIGNIFICANCE

Thus, changes in cellular bioenergetics dictate the onset of DCD in response to glutamate.

Mitochondrial function and redox state in mammalian embryos

Mitochondria play a central and multifaceted role in the mammalian egg and early embryo, contributing to many different aspects of early development. While the contribution of mitochondria to energy production is fundamental, other roles for mitochondria are starting to emerge. Mitochondria are central to intracellular redox metabolism as they produce reactive oxygen species (ROS, the mediators of oxidative stress) and they can generate TCA cycle intermediates and reducing equivalents that are used in antioxidant defence. A high cytosolic lactate dehydrogenase activity coupled with dynamic levels of cytosolic pyruvate is responsible for a very dynamic intracellular redox state in the oocyte and embryo. Mammalian embryos have a low glucose metabolism during the earliest stages of development, as both glycolysis and the pentose phosphate pathway are suppressed. The mitochondrial TCA cycle is therefore the major source of reducing equivalents in the cytosol so that any change in mitochondrial function in the embryo will be reflected in changes in the intracellular redox state. In the mouse, the metabolic substrates used by the oocyte and early embryo each have a different impact on the intracellular redox state. Pyruvate which oxidises the cytosolic redox state, acts as an energetic and redox substrate whereas lactate, which reduces the cytosolic redox state, acts only as a redox substrate. Mammalian early embryos are very sensitive to oxidative stress which can cause permanent developmental arrest before zygotic genome activation and apoptosis in the blastocyst. The oocyte stockpiles antioxidant defence for the early embryo to cope with exogenous and endogenous oxidant insults arising during early development. Mitochondria provide ATP for glutathione (GSH) production during oocyte maturation and also participate in the regeneration of NADPH and GSH during early development. Finally, a number of pathological conditions or environmental insults impair early development by altering mitochondrial function, illustrating the centrality of mitochondrial function in embryo development.

Mitochondria modulate the spatio-temporal properties of intra- and intercellular Ca2+ signals in cochlear supporting cells

In the cochlea, cell damage triggers intercellular Ca2+ waves that propagate through the glial-like supporting cells that surround receptor hair cells. These Ca2+ waves are thought to convey information about sensory hair cell-damage to the surrounding supporting cells within the cochlear epithelium. Mitochondria are key regulators of cytoplasmic Ca2+ concentration ([Ca2+](cyt)), and yet little is known about their role during the propagation of such intercellular Ca2+ signalling. Using neonatal rat cochlear explants and fluorescence imaging techniques, we explore how mitochondria modulate supporting cell [Ca2+](cyt) signals that are triggered by ATP or by hair cell damage. ATP application (0.1-50 microM) caused a dose dependent increase in [Ca2+](cyt) which was accompanied by an increase in mitochondrial calcium. Blocking mitochondrial Ca2+ uptake by dissipating the mitochondrial membrane potential using CCCP and oligomycin or using Ru360, an inhibitor of the mitochondrial Ca2+ uniporter, enhanced the peak amplitude and duration of ATP-induced [Ca2+](cyt) transients. In the presence of Ru360, the mean propagation velocity, amplitude and extent of spread of damage-induced intercellular Ca2+ waves was significantly increased. Thus, mitochondria function as spatial Ca2+ buffers during agonist-evoked [Ca2+](cyt) signalling in cochlear supporting cells and play a significant role in regulating the spatio-temporal properties of intercellular Ca2+ waves.

IF(1): setting the pace of the F(1)F(o)-ATP synthase

When mitochondrial function is compromised and the mitochondrial membrane potential (Deltapsi(m)) falls below a threshold, the F(1)F(o)-ATP synthase can reverse, hydrolysing ATP to pump protons out of the mitochondrial matrix. Although this activity can deplete ATP and precipitate cell death, it is limited by the mitochondrial protein IF(1), an endogenous F(1)F(o)-ATPase inhibitor. IF(1), therefore, preserves ATP at the expense of Deltapsi(m). Despite a wealth of detailed knowledge on the biochemistry of the interaction of IF(1) and the F(1)F(o)-ATPase, little is known about its physiological activity. Emerging research suggests that IF(1) has a wider ranging impact on mitochondrial structure and function than previously thought.

Multiphoton imaging reveals differences in mitochondrial function between nephron segments

Mitochondrial dysfunction may play a role in the pathogenesis of several renal diseases. Although functional roles and metabolic demands differ among tubule segments, relatively little is known about the properties of mitochondria in different parts of the nephron. Clinically, the proximal tubule seems particularly vulnerable to mitochondrial toxicity. In this study, we used multiphoton imaging of live rat kidney slices to investigate differences in mitochondrial function along the nephron. The mitochondrial membrane potential was markedly higher in distal than proximal tubules. Inhibition of respiration rapidly collapsed the membrane potential in proximal tubules, but potential was better maintained in distal tubules. Inhibition of the F1F(o)-ATPase abolished this difference, suggesting that maintenance of potential via ATPase activity is more effective in distal than proximal tubules. Immunostaining revealed that the ratio of the expression of ATPase to IF1, an endogenous inhibitor of the mitochondrial ATPase, was lower in proximal tubules than in distal tubules. Production of reactive oxygen species was higher in proximal than distal cells, but inhibition of NADPH oxidase eliminated this difference. Glutathione levels were higher in proximal tubules. Overall, mitochondria in the proximal tubules were in a more oxidized state than those in the distal tubules. In summary, there are axial differences in mitochondrial function along the nephron, which may contribute to the pattern and pathophysiology of some forms of renal injury.

IF1, the endogenous regulator of the F(1)F(o)-ATPsynthase, defines mitochondrial volume fraction in HeLa cells by regulating autophagy

The protein IF1 limits mitochondrial ATP consumption when mitochondrial respiration is impaired by inhibiting the 'reverse' activity of the F(1)F(o)-ATPsynthase. We have found that IF1 also increases F(1)F(o)-ATPsynthase activity in respiring mitochondria, promoting its dimerization and increasing the density of mitochondrial cristae. We also noted that IF1 overexpression was associated with an increase in mitochondrial volume fraction that was conversely reduced when IF1 was knocked down using small interfering RNA (siRNA). The volume change did not correlate with the level of transcription factors involved in mitochondrial biogenesis. However, autophagy was dramatically increased in the IF1siRNA treated cells (-IF1), assessed by quantifying LC3-GFP translocation to autophagosomes, whilst levels of autophagy were low in IF1 overexpressing cells (+IF1). The increase in LC3-GFP labelled autophagosomes in -IF1 cells was prevented by the superoxide dismutase mimetic, manganese (III) tetrakis (4-benzoic acid) porphyrin (MnTBAP). An increase in the basal rate of generation of reactive oxygen species (ROS) in -IF1 cells was demonstrated using the fluorescent probe dihydroethidium (DHE). Thus, IF1 appears to limit mitochondrial ROS generation, limiting autophagy which is increased by IF1 knockdown. Variations in IF1 expression level may therefore play a significant role in defining both resting rates of ROS generation and cellular mitochondrial content.

PINK1-associated Parkinson's disease is caused by neuronal vulnerability to calcium-induced cell death

Mutations in PINK1 cause autosomal recessive Parkinson's disease. PINK1 is a mitochondrial kinase of unknown function. We investigated calcium homeostasis and mitochondrial function in PINK1-deficient mammalian neurons. We demonstrate physiologically that PINK1 regulates calcium efflux from the mitochondria via the mitochondrial Na(+)/Ca(2+) exchanger. PINK1 deficiency causes mitochondrial accumulation of calcium, resulting in mitochondrial calcium overload. We show that calcium overload stimulates reactive oxygen species (ROS) production via NADPH oxidase. ROS production inhibits the glucose transporter, reducing substrate delivery and causing impaired respiration. We demonstrate that impaired respiration may be restored by provision of mitochondrial complex I and II substrates. Taken together, reduced mitochondrial calcium capacity and increased ROS lower the threshold of opening of the mitochondrial permeability transition pore (mPTP) such that physiological calcium stimuli become sufficient to induce mPTP opening in PINK1-deficient cells. Our findings propose a mechanism by which PINK1 dysfunction renders neurons vulnerable to cell death.

Deletion of the von Hippel-Lindau gene in pancreatic beta cells impairs glucose homeostasis in mice

Defective insulin secretion in response to glucose is an important component of the beta cell dysfunction seen in type 2 diabetes. As mitochondrial oxidative phosphorylation plays a key role in glucose-stimulated insulin secretion (GSIS), oxygen-sensing pathways may modulate insulin release. The von Hippel-Lindau (VHL) protein controls the degradation of hypoxia-inducible factor (HIF) to coordinate cellular and organismal responses to altered oxygenation. To determine the role of this pathway in controlling glucose-stimulated insulin release from pancreatic beta cells, we generated mice lacking Vhl in pancreatic beta cells (betaVhlKO mice) and mice lacking Vhl in the pancreas (PVhlKO mice). Both mouse strains developed glucose intolerance with impaired insulin secretion. Furthermore, deletion of Vhl in beta cells or the pancreas altered expression of genes involved in beta cell function, including those involved in glucose transport and glycolysis, and isolated betaVhlKO and PVhlKO islets displayed impaired glucose uptake and defective glucose metabolism. The abnormal glucose homeostasis was dependent on upregulation of Hif-1alpha expression, and deletion of Hif1a in Vhl-deficient beta cells restored GSIS. Consistent with this, expression of activated Hif-1alpha in a mouse beta cell line impaired GSIS. These data suggest that VHL/HIF oxygen-sensing mechanisms play a critical role in glucose homeostasis and that activation of this pathway in response to decreased islet oxygenation may contribute to beta cell dysfunction.

Expression of mutant SOD1 in astrocytes induces functional deficits in motoneuron mitochondria

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by motoneuron degeneration resulting in paralysis and eventual death. ALS is regarded as a motoneuron-specific disorder but increasing evidence indicates non-neuronal cells play a significant role in disease pathogenesis. Although the precise aetiology of ALS remains unclear, mutations in the superoxide dismutase (SOD1) gene are known to account for approximately 20% of familial ALS. We examined the influence of SOD1(G93A) expression in astrocytes on mitochondrial homeostasis in motoneurons in a primary astrocyte : motoneuron co-culture model. SOD1(G93A) expression in astrocytes induced changes in mitochondrial function of both SOD1(G93A) and wild-type motoneurons. In the presence of SOD1(G93A) astrocytes, mitochondrial redox state of both wild-type and SOD1(G93A) motoneurons was more reduced and mitochondrial membrane potential decreased. While intra-mitochondrial calcium levels [Ca(2+)](m) were elevated in SOD1(G93A) motoneurons, changes in mitochondrial function did not correlate with [Ca(2+)](m). Thus, expression of SOD1(G93A) in astrocytes directly alters mitochondrial function even in embryonic motoneurons, irrespective of genotype. These early deficits in mitochondrial function induced by surrounding astrocytes may increase the vulnerability of motoneurons to other neurotoxic mechanisms involved in ALS pathogenesis.

Renal function and mitochondrial cytopathy (MC): more questions than answers?

Our knowledge of mitochondrial biology has advanced significantly in the last 10 years. The effects of mitochondrial dysfunction or cytopathy (MC) on the heart and neuromuscular system are well known, and its involvement in the pathophysiology of several common clinical disorders such as diabetes, hyperlipidaemia and hypertension, is just beginning to emerge; however, its contribution to renal disease has received much less attention, and the available literature raises some interesting questions: Why do children with MC commonly present with a renal phenotype that is often quite different from adults? How does a mutation in mitochondrial DNA (mtDNA) lead to disease at the cellular level, and how can a single mtDNA point mutation result in such a variety of renal- and non-renal phenotypes in isolation or combined? Why are some regions of the nephron seemingly more sensitive to mitochondrial dysfunction and damage by mitochondrial toxins? Perhaps most important of all, what can be done to diagnose and treat MC, now and in the future? In this review we summarize our current understanding of the relationship between mitochondrial biology, renal physiology and clinical nephrology, in an attempt to try to answer some of these questions. Although MC is usually considered a rare defect, it is almost certainly under-diagnosed. A greater awareness and understanding of kidney involvement in MC might lead to new treatment strategies for diseases in which mitochondrial dysfunction is secondary to toxic or ischaemic injury, rather than to an underlying genetic mutation.

Regulation of mitochondrial structure and function by the F1Fo-ATPase inhibitor protein, IF1

When mitochondrial respiration is compromised, the F(1)F(o)-ATP synthase reverses and consumes ATP, serving to maintain the mitochondrial membrane potential (Delta psi(m)). This process is mitigated by IF(1). As little is known of the cell biology of IF(1), we have investigated the functional consequences of varying IF(1) expression. We report that, (1) during inhibition of respiration, IF(1) conserves ATP at the expense of Delta psi(m); (2) overexpression of IF(1) is protective against ischemic injury; (3) relative IF(1) expression level varies between tissues and cell types and dictates the response to inhibition of mitochondrial respiration; (4) the density of mitochondrial cristae is increased by IF(1) overexpression and decreased by IF(1) suppression; and (5) IF(1) overexpression increases the formation of dimeric ATP synthase complexes and increases F(1)F(o)-ATP synthase activity. Thus, IF(1) regulates mitochondrial function and structure under both physiological and pathological conditions.

PINK1 is necessary for long term survival and mitochondrial function in human dopaminergic neurons

Parkinson's disease (PD) is a common age-related neurodegenerative disease and it is critical to develop models which recapitulate the pathogenic process including the effect of the ageing process. Although the pathogenesis of sporadic PD is unknown, the identification of the mendelian genetic factor PINK1 has provided new mechanistic insights. In order to investigate the role of PINK1 in Parkinson's disease, we studied PINK1 loss of function in human and primary mouse neurons. Using RNAi, we created stable PINK1 knockdown in human dopaminergic neurons differentiated from foetal ventral mesencephalon stem cells, as well as in an immortalised human neuroblastoma cell line. We sought to validate our findings in primary neurons derived from a transgenic PINK1 knockout mouse. For the first time we demonstrate an age dependent neurodegenerative phenotype in human and mouse neurons. PINK1 deficiency leads to reduced long-term viability in human neurons, which die via the mitochondrial apoptosis pathway. Human neurons lacking PINK1 demonstrate features of marked oxidative stress with widespread mitochondrial dysfunction and abnormal mitochondrial morphology. We report that PINK1 plays a neuroprotective role in the mitochondria of mammalian neurons, especially against stress such as staurosporine. In addition we provide evidence that cellular compensatory mechanisms such as mitochondrial biogenesis and upregulation of lysosomal degradation pathways occur in PINK1 deficiency. The phenotypic effects of PINK1 loss-of-function described here in mammalian neurons provides mechanistic insight into the age-related degeneration of nigral dopaminergic neurons seen in PD.

Mitochondria: the hub of cellular Ca2+ signaling

Mitochondria couple cellular metabolic state with Ca(2+) transport processes. They therefore control not only their own intra-organelle [Ca(2+)], but they also influence the entire cellular network of cellular Ca(2+) signaling, including the endoplasmic reticulum, the plasma membrane, and the nucleus. Through the detailed study of mitochondrial roles in Ca(2+) signaling, a remarkable picture of inter-organelle communication has emerged. We here review the ways in which this system provides integrity and flexibility for the cell to cope with the countless demands throughout its life cycle and discuss briefly the mechanisms through which it can also drive cell death.

Mechanisms underlying the loss of mitochondrial membrane potential in glutamate excitotoxicity

Glutamate excitotoxicity amplifies neuronal death following stroke. We have explored the mechanisms underlying the collapse of mitochondrial potential (Deltapsi(m)) and loss of [Ca(2+)](c) homeostasis in rat hippocampal neurons in culture following toxic glutamate exposure. The collapse of Deltapsi(m) is multiphasic and Ca(2+)-dependent. Glutamate induced a decrease in NADH autofluorescence which preceded the loss of Deltapsi(m). Both the decrease in NADH signal and the loss of Deltapsi(m) were suppressed by Ru360 and both were delayed by inhibition of PARP (by 3-AB or DPQ). During this period, addition of mitochondrial substrates (methyl succinate and TMPD-ascorbate) or buffering [Ca(2+)](i) (using BAPTA-AM or EGTA-AM), rescued Deltapsi(m). These data suggest that mitochondrial Ca(2+) uptake activates PARP which in turn depletes NADH, promoting the initial collapse of Deltapsi(m). After > approximately 20 min, buffering Ca(2+) or substrate addition failed to restore Deltapsi(m). In neurons from cyclophilin D-/- (cypD-/-) mice or in cells treated with cyclosporine A, removal of Ca(2+) restored Deltapsi(m) even after 20 min of glutamate exposure, suggesting involvement of the mPTP in the irreversible depolarisation seen in WT cells. Thus, mitochondrial depolarisation represents two consecutive but distinct processes driving cell death, the first of which is reversible while the second is not.

Mitochondrial ND5 gene variation associated with encephalomyopathy and mitochondrial ATP consumption

Mitochondrial encephalomyopathy and lactic acidosis with strokelike episodes (MELAS) is a severe young onset stroke disorder without effective treatment. We have identified a MELAS patient harboring a 13528A-->G mitochondrial DNA (mtDNA) mutation in the Complex I ND5 gene. This mutation was homoplasmic in mtDNA from patient muscle and nearly homoplasmic (99.9%) in blood. Fibroblasts from the patient exhibited decreased mitochondrial membrane potential (Deltapsim) and increased lactate production, consistent with impaired mitochondrial function. Transfer of patient mtDNA to a new nuclear background using transmitochondrial cybrid fusions confirmed the pathogenicity of the 13528A-->G mutation; Complex I-linked respiration and Deltapsim were both significantly reduced in patient mtDNA cybrids compared with controls. Inhibition of the adenine nucleotide translocase or the F1F0-ATPase with bongkrekic acid or oligomycin caused a loss of potential in patient mtDNA cybrid mitochondria, indicating a requirement for glycolytically generated ATP to maintain Deltapsim. This was confirmed by inhibition of glycolysis with 2-deoxy-D-glucose, which caused depletion of ATP and mitochondrial depolarization in patient mtDNA cybrids. These data suggest that in response to impaired respiration due to the mtDNA mutation, mitochondria consume ATP to maintain Deltapsim, representing a potential pathophysiological mechanism in human mitochondrial disease.

Endothelial mitochondria: contributing to vascular function and disease

Disturbances in vascular function contribute to the development of several diseases of increasing prevalence and thereby contribute significantly to human mortality and morbidity. Atherosclerosis, diabetes, heart failure, and ischemia with attendant reperfusion injury share many of the same risk factors, among the most important being oxidative stress and alterations in the blood concentrations of compounds that influence oxidative stress, such as oxidized low-density lipoprotein. In this review, we focus on endothelial cells: cells in the frontline against these disturbances. Because ATP supplies in endothelial cells are relatively independent of mitochondrial oxidative pathways, the mitochondria of endothelial cells have been somewhat neglected. However, they are emerging as agents with diverse roles in modulating the dynamics of intracellular calcium and the generation of reactive oxygen species and nitric oxide. The mitochondria may also constitute critical "targets" of oxidative stress, because survival of endothelial cells can be compromised by opening of the mitochondrial permeability transition pore or by mitochondrial pathways of apoptosis. In addition, evidence suggests that endothelial mitochondria may play a "reconnaissance" role. For example, although the exact mechanism remains obscure, endothelial mitochondria may sense levels of oxygen in the blood and relay this information to cardiac myocytes as well as modulating the vasodilatory response mediated by endothelial nitric oxide.

Three distinct mechanisms generate oxygen free radicals in neurons and contribute to cell death during anoxia and reoxygenation

Ischemia is a major cause of brain damage, and patient management is complicated by the paradoxical injury that results from reoxygenation. We have now explored the generation of reactive oxygen species (ROS) in hippocampal and cortical neurons in culture in response to oxygen and glucose deprivation or metabolic inhibition and reoxygenation. Fluorescence microscopy was used to measure the rate of ROS generation using hydroethidine, dicarboxyfluorescein diacetate, or MitoSOX. ROS generation was correlated with changing mitochondrial potential (rhodamine 123), [Ca2+]c (fluo-4, fura-2, or Indo-1), or ATP consumption, indicated by increased [Mg2+]c. We found that three distinct mechanisms contribute to neuronal injury by generating ROS and oxidative stress, each operating at a different stage of ischemia and reperfusion. In response to hypoxia, mitochondria generate an initial burst of ROS, which is curtailed once mitochondria depolarize or prevented by previous depolarization with uncoupler. A second phase of ROS generation that followed after a delay was blocked by the xanthine oxidase (XO) inhibitor oxypurinol. This phase correlated with a rise in [Mg2+]c, suggesting XO activation by accumulating products of ATP consumption. A third phase of ROS generation appeared at reoxygenation. This was blocked by NADPH oxidase inhibitors and was absent in cells from gp91(phox-/-) knock-out mice. It was Ca2+ dependent, suggesting activation by increased [Ca2+]c during anoxia, itself partly attributable to glutamate release. Inhibition of either the NADPH oxidase or XO was significantly neuroprotective. Thus, oxidative stress contributes to cell death over and above the injury attributable to energy deprivation.

Assessing mitochondrial potential, calcium, and redox state in isolated mammalian cells using confocal microscopy

Mitochondria play a vital role in the regulation of intracellular calcium dynamics. Fluorescent dyes can be used to provide a direct measurement of the redox state, mitochondrial membrane potential, and mitochondrial calcium content. The simplicity of this approach lends itself to high-throughput assays and time-resolved analyses; however, care must be taken to avoid artifactual results. We outline general methods using confocal microscopy for analysis of the redox state, mitochondrial membrane potential, and mitochondrial calcium content in adult cardiomyocytes. We demonstrate how these parameters can be analyzed in parallel using the emission spectra "fingerprinting" method even when emission spectra overlap.

Regulation of redox metabolism in the mouse oocyte and embryo

Energy homeostasis of the oocyte is a crucial determinant of fertility. Following ovulation, the oocyte is exposed to the unique environment of the Fallopian tube, and this is reflected in a highly specialised biochemistry. The minute amounts of tissue available have made the physiological analysis of oocyte intermediary metabolism almost impossible. We have therefore used confocal imaging of mitochondrial and cytosolic redox state under a range of conditions to explore the oxidative metabolism of intermediary substrates. It has been known for some time that the early mouse embryo metabolises external pyruvate and lactate but not glucose to produce ATP. We now show at the level of single oocytes, that supplied glucose has no effect on the redox potential of the oocyte. Pyruvate is a cytosolic oxidant but a mitochondrial reductant, while lactate is a strong cytosolic reductant via the activity of lactate dehydrogenase. Unexpectedly, lactate-derived pyruvate appears to be diverted from mitochondrial oxidation. Our approach also reveals that the level of reduced glutathione (GSH) in the oocyte is maintained by glutathione reductase, which oxidises intracellular NADPH to reduce oxidised glutathione. Surprisingly, NADPH does not seem to be supplied by the pentose phosphate pathway in the unfertilised oocyte but rather by cytosolic NADP-dependent isocitrate dehydrogenase. Remarkably, we also found that the oxidant action of pyruvate impairs development, demonstrating the fundamental importance of redox state on early development.

Calcium microdomains and oxidative stress

The phenomenon of calcium microdomains is firmly established in the field of subcellular physiology. These regions of localized, transient calcium increase are exemplified by the spontaneous 'sparks' released through the ryanodine receptor in myocytes, but include subplasmalemmal microdomains, focal calcium oscillations and microdomains enclosed within organelles, such as the endoplasmic reticulum, golgi and mitochondria. Increasing evidence suggests that oxidative stress regulates both the formation and disappearance of microdomains. Calcium release channels and transporters are all modulated by redox state, while several mechanisms that generate oxidative or nitrosative stress are regulated by calcium. Here, we discuss the evidence for the regulation of calcium microdomains by redox state, and, by way of example, demonstrate that the frequency of calcium sparks in cardiomyocytes is increased in response to oxidative stress. We consider the evidence for the existence of analogous microdomains of reactive oxygen and nitrogen species and suggest that the refinement of imaging techniques for these species might lead to similar concepts. The interaction between Ca(2+) microdomains and proteins that modulate their formation results in a complex and dynamic, spatial signaling mechanism, which is likely to be broadly applicable to different cell types, adding new dimensions to the calcium signaling 'toolkit'.

FCCP is cardioprotective at concentrations that cause mitochondrial oxidation without detectable depolarisation

OBJECTIVE

The role of mitochondria and in particular of mitochondrial uncoupling in the mechanism of cardioprotection is not defined. In the accompanying paper we have shown that pretreatment of isolated rat hearts with a low concentration (100 nM) of FCCP, prior to global ischaemia, is cardioprotective, while 300 nM FCCP exacerbates injury. Here we define the mitochondrial responses to increasing concentrations of FCCP and also to explore the equivalence of the cardioprotective doses of diazoxide.

METHODS

Changes in mitochondrial respiration in response to FCCP and diazoxide were determined in isolated rat ventricular myocytes. In addition, mitochondrial state was monitored using confocal microscopy to record mitochondrial potential (TMRM) and redox state (NADH) during FCCP and diazoxide treatment. Myocytes were also voltage-clamped and whole cell currents recorded in response to 100 nM FCCP.

RESULTS

FCCP (10-1000 nM) caused significant dose-dependent increase in oxygen consumption. Diazoxide (30 microM) failed to cause any measurable change in mitochondrial function. FCCP at 100 nM caused mitochondrial oxidation, but no change in mitochondrial membrane potential or (sarc)K(ATP) channel current, while at 300 nM, FCCP caused significant mitochondrial depolarisation. Diazoxide failed to induce any mitochondrial oxidation or depolarisation.

CONCLUSIONS

Concentrations of FCCP that cause mitochondrial oxidation without depolarisation are cardioprotective. Higher FCCP concentrations dissipate mitochondrial membrane potential and exacerbate injury. This establishes the principle that mild mitochondrial uncoupling activates a protective mechanism. Diazoxide did not cause mitochondrial oxidation or mitochondrial depolarisation, suggesting it induces protection via another mechanism.