Mitochondria are morphologically and functionally heterogeneous within cells

Group IVC cytosolic phospholipase A2gamma is farnesylated and palmitoylated in mammalian cells

Cytosolic phospholipase A(2)gamma (cPLA(2)gamma) is a member of the group IV family of intracellular phospholipase A(2) enzymes, but unlike the well-studied cPLA(2)alpha, it is constitutively bound to membrane and is calcium independent. cPLA(2)gamma contains a C-terminal CaaX sequence and is radiolabeled by mevalonic acid when expressed in cPLA(2)alpha-deficient immortalized lung fibroblasts (IMLF(-/-)). The radiolabel associated with cPLA(2)gamma was identified as the farnesyl group. The protein farnesyltransferase inhibitor BMS-214662 prevented the incorporation of [(3)H]mevalonic acid into cPLA(2)gamma and partially suppressed serum-stimulated arachidonic acid release from IMLF(-/-) and undifferentiated human skeletal muscle (SkMc) cells overexpressing cPLA(2)gamma, but not from cells overexpressing cPLA(2)alpha. However, BMS-214662 did not alter the amount of cPLA(2)gamma associated with membrane. These results were consistent in COS cells expressing the C538S cPLA(2)gamma prenylation mutant. cPLA(2)gamma also contains a classic myristoylation site and several potential palmitoylation sites and was found to be acylated with oleic and palmitic acids but not myristoylated. Immunofluorescence microscopy revealed that cPLA(2)gamma is associated with mitochondria in IMLF(-/-), SkMc cells, and COS cells.

Metformin prevents high-glucose-induced endothelial cell death through a mitochondrial permeability transition-dependent process

Hyperglycemia-induced oxidative stress is detrimental for endothelial cells, contributing to the vascular complications of diabetes. The mitochondrial permeability transition pore (PTP) is an oxidative stress-sensitive channel involved in cell death; therefore, we have examined its potential role in endothelial cells exposed to oxidative stress or high glucose level. Metformin, an antihyperglycemic agent used in type 2 diabetes, was also investigated because it inhibits PTP opening in transformed cell lines. Cyclosporin A (CsA), the reference PTP inhibitor, and a therapeutic dose of metformin (100 micromol/l) led to PTP inhibition in permeabilized human microvascular endothelial cells (HMEC-1). Furthermore, exposure of intact HMEC-1 or primary endothelial cells from either human umbilical vein or bovine aorta to the oxidizing agent tert-butylhydroperoxide or to 30 mmol/l glucose triggered PTP opening, cytochrome c decompartmentalization, and cell death. CsA or metformin prevented all of these effects. The antioxidant N-acetyl-l-cysteine also prevented hyperglycemia-induced apoptosis. We conclude that 1) elevated glucose concentration leads to an oxidative stress that favors PTP opening and subsequent cell death in several endothelial cell types and 2) metformin prevents this PTP opening-related cell death. We propose that metformin improves diabetes-associated vascular disease both by lowering blood glucose and by its effect on PTP regulation.

Flow-cytometric monitoring of mitochondrial depolarisation: from fluorescence intensities to millivolts

Redistribution potentiometric dyes represent a powerful tool for monitoring membrane potential of mitochondria, especially when these dyes are used with flow cytometry. In particular, tetramethylrhodamine methyl ester proved to be suitable for the screening of mitochondrial membrane potential in cultured human skin fibroblasts from patients suffering from different defects of oxidative phosphorylation. We have developed a method that makes it possible to measure the changes in mitochondrial membrane potential, or to assess the differences between respective mitochondrial membrane potentials in investigated cells and controls in the absolute scale of millivolts. Our approach employs the fact that a logarithmic transformation of Nernst equation-controlled intensity of fluorescence from potentiometric dyes accumulated in mitochondria leads to a linear scale for mitochondrial membrane potentials.

Excitotoxic injury to mitochondria isolated from cultured neurons

Neuronal death in response to excitotoxic levels of glutamate is dependent upon mitochondrial Ca2+ accumulation and is associated with a drop in ATP levels and a loss in ionic homeostasis. Yet the mapping of temporal events in mitochondria subsequent to Ca2+ sequestration is incomplete. By isolating mitochondria from primary cultures, we discovered that glutamate treatment of cortical neurons for 10 min caused 44% inhibition of ADP-stimulated respiration, whereas the maximal rate of electron transport (uncoupler-stimulated respiration) was inhibited by approximately 10%. The Ca2+ load in mitochondria from glutamate-treated neurons was estimated to be 167 +/- 19 nmol/mg protein. The glutamate-induced Ca2+ load was less than the maximal Ca2+ uptake capacity of the mitochondria determined in vitro (363 +/- 35 nmol/mg protein). Comparatively, mitochondria isolated from cerebellar granule cells demonstrated a higher Ca2+ uptake capacity (686 +/- 71 nmol/mg protein) than the cortical mitochondria, and the glutamate-induced load of Ca2+ was a smaller percentage of the maximal Ca2+ uptake capacity. Thus, this study indicated that Ca(2+)-induced impairment of mitochondrial ATP production is an early event in the excitotoxic cascade that may contribute to decreased cellular ATP and loss of ionic homeostasis that precede commitment to neuronal death.

Within the cell: analytical techniques for subcellular analysis

This review covers recent developments in the preparation, manipulation, and analyses of subcellular environments. In particular, it highlights approaches for (1) separation and detection of individual organelles, (2) preparation of ultra-pure organelle fractions, and (3) utilization of novel labeling strategies. These approaches, based on innovative technologies such as microfluidics, immunoisolation, mass spectrometry and electrophoresis, suggest that subcellular analyses will soon become as commonplace as single cell and bulk cellular assays.

Characterization of depolarization and repolarization phases of mitochondrial membrane potential fluctuations induced by tetramethylrhodamine methyl ester photoactivation

Depolarization and repolarization phases (D and R phases, respectively) of mitochondrial potential fluctuations induced by photoactivation of the fluorescent probe tetramethylrhodamine methyl ester (TMRM) were analyzed separately and investigated using specific inhibitors and substrates. The frequency of R phases was significantly inhibited by oligomycin and aurovertin (mitochondrial ATP synthase inhibitors), rotenone (mitochondrial complex I inhibitor) and iodoacetic acid (inhibitor of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase). Succinic acid (mitochondrial complex II substrate, given in the permeable form of dimethyl ester) abolished the rotenone-induced inhibition of R phases. Taken together, these findings indicate that the activity of both respiratory chain and ATP synthase were required for the recovery of the mitochondrial potential. The frequency of D phases prevailed over that of R phases in all experimental conditions, resulting in a progressive depolarization of mitochondria accompanied by NAD(P)H oxidation and Ca2+ influx. D phases were not blocked by cyclosporin A (inhibitor of the permeability transition pore) or o-phenyl-EGTA (a Ca2+ chelator), suggesting that the permeability transition pore was not involved in mitochondrial potential fluctuations.

Activated mitofusin 2 signals mitochondrial fusion, interferes with Bax activation, and reduces susceptibility to radical induced depolarization

Mitochondrial fusion in higher eukaryotes requires at least two essential GTPases, Mitofusin 1 and Mitofusin 2 (Mfn2). We have created an activated mutant of Mfn2, which shows increased rates of nucleotide exchange and decreased rates of hydrolysis relative to wild type Mfn2. Mitochondrial fusion is stimulated dramatically within heterokaryons expressing this mutant, demonstrating that hydrolysis is not requisite for the fusion event, and supporting a role for Mfn2 as a signaling GTPase. Although steady-state mitochondrial fusion required the conserved intermembrane space tryptophan residue, this requirement was overcome within the context of the hydrolysis-deficient mutant. Furthermore, the punctate localization of Mfn2 is lost in the dominant active mutants, indicating that these sites are functionally controlled by changes in the nucleotide state of Mfn2. Upon staurosporine-stimulated cell death, activated Bax is recruited to the Mfn2-containing puncta; however, Bax activation and cytochrome c release are inhibited in the presence of the dominant active mutants of Mfn2. The dominant active form of Mfn2 also protected the mitochondria against free radical-induced permeability transition. In contrast to staurosporine-induced outer membrane permeability transition, pore opening induced through the introduction of free radicals was dependent upon the conserved intermembrane space residue. This is the first evidence that Mfn2 is a signaling GTPase regulating mitochondrial fusion and that the nucleotide-dependent activation of Mfn2 concomitantly protects the organelle from permeability transition. The data provide new insights into the critical relationship between mitochondrial membrane dynamics and programmed cell death.

Mitochondrial calcium ion and membrane potential transients follow the pattern of epileptiform discharges in hippocampal slice cultures

Emerging evidence suggests that mitochondrial dysfunction contributes to the pathophysiology of epilepsy. Recurrent mitochondrial Ca2+ ion load during seizures might act on mitochondrial membrane potential (DeltaPsim) and proton motive force. By using electrophysiology and confocal laser-scanning microscopy, we investigated the effects of epileptiform activity, as induced by low-Mg2+ ion perfusion in hippocampal slice cultures, on changes in DeltaPsim and in mitochondrial Ca2+ ion concentration ([Ca2+]m). The mitochondrial compartment was identified by monitoring DeltaPsim in the soma and dendrites of patched CA3 pyramidal cells using the mitochondria-specific voltage-sensitive dye rhodamine-123 (Rh-123). Interictal activity was accompanied by localized mitochondrial depolarization that was restricted to a few mitochondria in small dendrites. In contrast, robust Rh-123 release into the cytosol was observed during seizure-like events (SLEs), indicating simultaneous depolarization of mitochondria. This was critically dependent on Ca2+ ion uptake and extrusion, because inhibition of the mitochondrial Ca2+ ion uniporter by Ru360 and the mitochondrial Na+/Ca2+ ion exchanger by 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one but not the inhibitor of mitochondrial permeability transition pore, cyclosporin A, decreased the SLE-associated mitochondrial depolarization. The Ca2+ ion dependence of simultaneous mitochondrial depolarization suggested enhanced Ca2+ ion cycling across mitochondrial membranes during epileptiform activity. Indeed, [Ca2+]m fluctuated during interictal activity in single dendrites, and these fluctuations spread over the entire mitochondrial compartment during SLEs, as revealed using mitochondria-specific dyes (rhod-2 and rhod-ff) and spatial frequency-based image analysis. These findings strengthen the hypothesis that epileptic activity results in Ca2+ ion-dependent changes in mitochondrial function that might contribute to the neuronal injury during epilepsy.

Proteomic and functional analyses reveal a mitochondrial dysfunction in P301L tau transgenic mice

Transgenic mice overexpressing the P301L mutant human tau protein exhibit an accumulation of hyperphosphorylated tau and develop neurofibrillary tangles. The consequences of tau pathology were investigated here by proteomics followed by functional analysis. Mainly metabolism-related proteins including mitochondrial respiratory chain complex components, antioxidant enzymes, and synaptic proteins were identified as modified in the proteome pattern of P301L tau mice. Significantly, the reduction in mitochondrial complex V levels in the P301L tau mice revealed using proteomics was also confirmed as decreased in human P301L FTDP-17 (frontotemporal dementia with parkinsonism linked to chromosome 17) brains. Functional analysis demonstrated a mitochondrial dysfunction in P301L tau mice together with reduced NADH-ubiquinone oxidoreductase activity and, with age, impaired mitochondrial respiration and ATP synthesis. Mitochondrial dys-function was associated with higher levels of reactive oxygen species in aged transgenic mice. Increased tau pathology as in aged homozygous P301L tau mice revealed modified lipid peroxidation levels and the up-regulation of antioxidant enzymes in response to oxidative stress. Furthermore, P301L tau mitochondria displayed increased vulnerability toward beta-amyloid (Abeta) peptide insult, suggesting a synergistic action of tau and Abeta pathology on the mitochondria. Taken together, we conclude that tau pathology involves a mitochondrial and oxidative stress disorder possibly distinct from that caused by Abeta.

Mitochondrial function and actin regulate dynamin-related protein 1-dependent mitochondrial fission

Mitochondria display a variety of shapes, ranging from small and spherical or the classical tubular shape to extended networks. Shape transitions occur frequently and include fusion, fission, and branching. It was reported that some mitochondrial shape transitions are developmentally regulated, whereas others were linked to disease or apoptosis. However, if and how mitochondrial function controls mitochondrial shape through regulation of mitochondrial fission and fusion is unclear. Here, we show that inhibitors of electron transport, ATP synthase, or the permeability transition pore (mtPTP) induced reversible mitochondrial fission. Mitochondrial fission depended on dynamin-related protein 1 (DRP1) and F-actin: Disruption of F-actin attenuated fission and recruitment of DRP1 to mitochondria. In contrast, uncoupling of electron transport and oxidative phosphorylation caused mitochondria to adopt a distinct disk shape. This shape change was independent of the cytoskeleton and DRP1 and was most likely caused by swelling. Thus, disruption of mitochondrial function rapidly and reversibly altered mitochondrial shape either by activation of DRP1-dependent fission or by swelling, indicating a close relationship between mitochondrial fission, shape, and function. Furthermore, our results suggest that the actin cytoskeleton is involved in mitochondrial fission by facilitating mitochondrial recruitment of DRP1.

Divergent mitochondrial and endoplasmic reticulum association of DMPK splice isoforms depends on unique sequence arrangements in tail anchors

Myotonic dystrophy protein kinase (DMPK) is a Ser/Thr-type protein kinase with unknown function, originally identified as the product of the gene that is mutated by triplet repeat expansion in patients with myotonic dystrophy type 1 (DM1). Alternative splicing of DMPK transcripts results in multiple protein isoforms carrying distinct C termini. Here, we demonstrate by expressing individual DMPKs in various cell types, including C(2)C(12) and DMPK(-/-) myoblast cells, that unique sequence arrangements in these tails control the specificity of anchoring into intracellular membranes. Mouse DMPK A and C were found to associate specifically with either the endoplasmic reticulum (ER) or the mitochondrial outer membrane, whereas the corresponding human DMPK A and C proteins both localized to mitochondria. Expression of mouse and human DMPK A-but not C-isoforms in mammalian cells caused clustering of ER or mitochondria. Membrane association of DMPK isoforms was resistant to alkaline conditions, and mutagenesis analysis showed that proper anchoring was differentially dependent on basic residues flanking putative transmembrane domains, demonstrating that DMPK tails form unique tail anchors. This work identifies DMPK as the first kinase in the class of tail-anchored proteins, with a possible role in organelle distribution and dynamics.

Regulating cell survival by controlling cellular energy production: novel functions for ancient signaling pathways?

Cell survival is maintained by growth factors and critically depends on sufficient energy supply. New evidence suggests that a rise in cellular energy production is not merely a homeostatic response to increased demand but subject to regulation by extrinsic factors. The mechanisms operating in this control are largely enigmatic. Work on transformed cells identified direct targeting of glycolytic enzymes by signaling proteins as one possibility. But mitochondrial oxidative phosphorylation and biogenesis may also be subject to regulation by growth and survival factors. Both, positive and negative regulators of cell survival impinge on the processes of cellular energy production to regulate growth and survival versus death.

Mitochondria in health and disease: perspectives on a new mitochondrial biology

The integrity of mitochondrial function is fundamental to cell life. It follows that disturbances of mitochondrial function will lead to disruption of cell function, expressed as disease or even death. In this review, I consider recent developments in our knowledge of basic aspects of mitochondrial biology as an essential step in developing our understanding of the contributions of mitochondria to disease. The identification of novel mechanisms that govern mitochondrial biogenesis and replication, and the delicately poised signalling pathways that coordinate the mitochondrial and nuclear genomes are discussed. As fluorescence imaging has made the study of mitochondrial function within cells accessible, the application of that technology to the exploration of mitochondrial bioenergetics is reviewed. Mitochondrial calcium uptake plays a major role in influencing cell signalling and in the regulation of mitochondrial function, while excessive mitochondrial calcium accumulation has been extensively implicated in disease. Mitochondria are major producers of free radical species, possibly also of nitric oxide, and are also major targets of oxidative damage. Mechanisms of mitochondrial radical generation, targets of oxidative injury and the potential role of uncoupling proteins as regulators of radical generation are discussed. The role of mitochondria in apoptotic and necrotic cell death is seminal and is briefly reviewed. This background leads to a discussion of ways in which these processes combine to cause illness in the neurodegenerative diseases and in cardiac reperfusion injury. The demands of mitochondria and their complex integration into cell biology extends far beyond the provision of ATP, prompting a radical change in our perception of mitochondria and placing these organelles centre stage in many aspects of cell biology and medicine.

Mitochondrial function is a critical determinant of IL-1-induced ERK activation

Interleukin-1 (IL-1) is a potent, proinflammatory cytokine, but local environmental factors in inflamed sites or in sepsis may affect cell metabolism and energetics, including the amplitude and duration of IL-1-induced signals, thereby leading to loss of tissue homeostasis. Currently, the mechanisms by which disruption of cell energetics affects inflammatory signaling are incompletely understood. Here, we examined the impact of cell energetics and mitochondrial function on the regulation of IL-1-induced Ca2+ signals and ERK activation in human gingival fibroblasts, cells that are important targets for IL-1-induced destruction of extracellular matrix in inflamed connective tissues. In untreated cells, IL-1 induced a prolonged increase of free intracellular calcium, which was required for ERK activation. Inhibition of cellular energetics by selective depolarization of mitochondria blocked Ca2+ uptake and almost completely abolished IL-1-induced cytosolic Ca2+ signals and ERK activation. IL-1 caused rapid Ca2+ release from the endoplasmic reticulum (ER), concomitant with mitochondrial Ca2+ uptake from ER and non-ER stores. Disruption of mitochondrial energetics abrogated IL-1 induced Ca2+ release from the ER but left other vital cellular functions intact. The negative effect of mitochondrial depolarization on ER release was bypassed by BAPTA/AM, indicating that mitochondrial Ca2+ buffering is the key mechanism in regulating ER release. Thus, in gingival fibroblasts, mitochondrial Ca2+ uptake is essential not only for shaping the kinetics and duration, but also the generation of, IL-1-induced Ca2+ signals. Consequently, mitochondria regulate key downstream effectors of IL-1, including MAP kinases.

pH difference across the outer mitochondrial membrane measured with a green fluorescent protein mutant

In this study we have generated a EYFP targeted to the mitochondrial intermembrane space (MIMS-EYFP) to determine for the first time the pH within this compartment. The fragment encoding HAI-tagged EYFP was fused with the C-terminus of glycerol-phosphate dehydrogenase, an integral protein of the inner mitochondrial membrane. Human ECV304 cells transiently transfected with MIMS-EYFP showed the typical mitochondrial network, co-localized with MitoTracker Red. Following the calibration procedure, an estimation of the pH value in the intermembrane space was obtained. This value (6.88+/-0.09) was significantly lower than that determined in the cytosol after transfection with a cytosolic EYFP (7.59+/-0.01). Further, the pH of the mitochondrial matrix, determined with a EYFP targeted to this subcompartment, was 0.9 pH units higher than that in the intermembrane space. In conclusion, MIMS-EYFP represents a novel powerful tool to monitor pH changes in the mitochondrial intermembrane space of live cells.

Calcium and mitochondria: mechanisms and functions of a troubled relationship

Mitochondria promptly respond to Ca(2+)-mediated cell stimulations with a rapid accumulation of the cation into the matrix. In this article, we review (i) the basic principles of mitochondrial Ca(2+) transport, (ii) the physiological/pathological role of mitochondrial Ca(2+) uptake, (iii) the regulatory mechanisms that may operate in vivo, and (iv) the new targeted Ca(2+) probes that allowed the "rediscovery" of these organelles in calcium signalling.

Influence of a mitochondrial genetic defect on capacitative calcium entry and mitochondrial organization in the osteosarcoma cells

Effects of T8993G mutation in mitochondrial DNA (mtDNA), associated with neurogenical muscle weakness, ataxia and retinitis pigmentosa (NARP), on the cytoskeleton, mitochondrial network and calcium homeostasis in human osteosarcoma cells were investigated. In 98% NARP and rho(0) (lacking mtDNA) cells, the organization of the mitochondrial network and actin cytoskeleton was disturbed. Capacitative calcium entry (CCE) was practically independent of mitochondrial energy status in osteosarcoma cell lines. The significantly slower Ca(2+) influx rates observed in 98% NARP and rho(0), in comparison to parental cells, indicates that proper actin cytoskeletal organization is important for CCE in these cells.

Estrogen, mitochondria, and growth of cancer and non-cancer cells

In this review, we discuss estrogen actions on mitochondrial function and the possible implications on cell growth. Mitochondria are important targets of estrogen action. Therefore, an in-depth analysis of interaction between estrogen and mitochondria; and mitochondrial signaling to nucleus are pertinent to the development of new therapy strategies for the treatment of estrogen-dependent diseases related to mitochondrial disorders, including cancer.

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.

Gender-related differences in morphology and thermogenic capacity of brown adipose tissue mitochondrial subpopulations

To investigate the possible existence of a gender dimorphism in the morphology and functionality of brown adipose tissue (BAT) mitochondrial subpopulations, we obtained three mitochondrial fractions - heavy, medium and light - by differential centrifugation. Electron microscopic analysis was carried out and mitochondrial protein content, cytochrome c oxidase and ATP synthase activities, mitochondrial DNA content and UCP1 protein levels were measured in each mitochondrial fraction. Female rats showed a greater mitochondrial size than males, with a different distribution pattern of the subpopulations. These differences were accompanied by higher oxidative and thermogenic capacities and a higher protein content in female rat BAT. This tissue also showed a greater tendency to respiratory chain uncoupling, as well as a close coordination between the oxidative, phosphorylative and thermogenic processes. These differences were found in the heavy subpopulation but not in the light one. Our results demonstrate that female rat BAT shows a highly differentiated mitochondrial pool, with the heavy mitochondrial subpopulation as the main responsible for the greater thermogenic activity of this tissue. In addition, it seems that there is a differential regulation of the mitochondrial growth cycle between genders in BAT, which leads to enhanced thermogenic capacity in female rat mitochondria.

Mitochondria form a filamentous reticular network in hippocampal dendrites but are present as discrete bodies in axons: A three-dimensional ultrastructural study

The fine structure of mitochondria and smooth endoplasmic reticulum (SER) was studied via electron microscopy in dendritic and axonal neuronal segments of hippocampal areas CA1, CA3, and dentate gyrus (DG) of both ground squirrels in normothermic and hibernating conditions, and rats. Ultrathin serial sections of approximately 60 nm (up to 150 per series) were taken and three-dimensional (3D) reconstructions made of dendritic segments, up to 36 microm in length. Mitochondria were demonstrated to be present in filamentous form in every dendrite examined, in each of the hippocampal regions studied, whether in rat or ground squirrel. In addition, apparent continuity between the outer mitochondrial membrane and that of SER was observed by 3D reconstructions of very ultrathin (20 nm) serial sections prepared from dendritic segments. It is believed that SER penetrate into the heads of thin and mushroom spines but mitochondria do not enter the heads of these types of spines in dentate gyrus or CA1 of either rat or ground squirrel. However, in CA3 we have shown here that mitochondria penetrate into the base of the large thorny excrescences. Mushroom dendritic spines (but not thin spines) contained puncta adherentia, formed between pre- and postsynaptic membranes. In contrast to dendrites, the mitochondrial population of axonal processes in the same hippocampal regions were found only in the form of discrete bodies no more than 3 microm in length. The issue of the likely function of this network in dendrites and its potential role in calcium movement is discussed.

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.

Age-related decline in metabolic competence of small and medium-sized synaptic mitochondria

A computer-assisted morphometric investigation of cytochrome oxidase (COX) activity, selectively evidenced by preferential diaminobenzidine cytochemistry, has been carried out on synaptic mitochondria in the cerebellar cortex of adult and old rats. The ratio (R) of the area of the cytochemical precipitate (CPA) to the overall area of each mitochondrion (MA) was calculated. R refers to the fraction of the inner mitochondrial membrane actively involved in cellular respiration, thus its quantitative estimation constitutes a reliable index of the mitochondrial metabolic competence (MMC). In adult rats a significant negative correlation between MA and R values was found, while in old animals there was just a positive trend. Paired-quartile comparisons of R values showed a significant age-related decrease in small and medium-sized mitochondria, whereas the lowest and not significant age-related reduction was found in oversized organelles. A paired decrease in number and increase in size is reported to be a general trend for mitochondria during aging, but oversized organelles, according to their low R value, constitute a scanty, though functional, compensating reaction. Thus, the present findings support the argument that the currently reported age-related cellular metabolic decay appears to rely both on the decline in MMC of the small and medium-sized mitochondria, and on their specific reduction in number. This novel result is of biological relevance since it is largely the small and medium-sized mitochondria that are required for the provision of adequate amounts of ATP for actual cellular performance, while the significantly enlarged organelles are thought to represent an intermediate ultrastructural feature in mitochondrial genesis and/or remodelling.

Excitotoxic calcium overload in a subpopulation of mitochondria triggers delayed death in hippocampal neurons

In neurons, excitotoxic stimulation induces mitochondrial calcium overload and the release of pro-apoptotic proteins, which triggers delayed cell death. The precise mechanisms of apoptogen release, however, remain controversial. To characterize the linkage between mitochondrial calcium load and cell vulnerability, and to test the hypothesis that only a subpopulation of mitochondria damaged by calcium overload releases apoptogens, we have measured directly the concentrations of total Ca (free plus bound) in individual mitochondria and monitored in parallel structural changes and the subcellular localization of pro-apoptotic cytochrome c after NMDA overstimulation in cultured hippocampal neurons. Beyond transient elevation of cytosolic calcium and perturbation of Na+/K+ homeostasis, NMDA stimulation induced dramatic, but mainly reversible, changes in mitochondria, including strong calcium elevation, membrane potential depolarization, and variable swelling. Elevation of matrix Ca in the approximately one-third of mitochondria that were strongly swollen, as well as the absence of swelling when Ca2+ entry was abolished, indicate an essential role for Ca overload. Shortly after NMDA exposure, cytochrome c, normally localized to mitochondria, became diffusely distributed in the cytoplasm, coincident with the appearance of severely swollen mitochondria with ruptured outer membranes; under these conditions, cytochrome c was retained in intact mitochondria, implying that it was released mainly from damaged mitochondria. Consistent with the role of mitochondrial Ca overload, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone decreased Ca accumulation, prevented cytochrome c release, and was neuroprotective. These results support a mechanism in which delayed excitotoxic death involves apoptogen release from a subpopulation of calcium-overloaded mitochondria, whereas other, undamaged mitochondria maintain normal function.

Reduced mitochondrial buffering of voltage-gated calcium influx in aged rat basal forebrain neurons

Alterations of neuronal Ca(2+) homeostatic mechanisms could be responsible for many of the cognitive deficits associated with aging in mammals. Mitochondrial participation in Ca(2+) signaling is now recognized as a prominent feature in neuronal physiology. We combined voltage-clamp electrophysiology with Ca(2+)-sensitive ratiometric microfluorimetry and laser scanning confocal microscopy to investigate the participation in Ca(2+) buffering of in situ mitochondria in acutely dissociated basal forebrain neurons from young and aged F344 rats. By pharmacologically blocking mitochondrial Ca(2+) uptake, we determined that mitochondria were not involved in rapid buffering of small Ca(2+) influx through voltage-gated Ca(2+) channels (VGCCs) in the somatic compartment. For larger Ca(2+) influx, aged mitochondria showed a significant buffering deficit. Evidence obtained with the potentiometric indicator, JC-1, suggests a significantly reduced mitochondrial membrane potential in aged neurons. These results support the interpretation that there is a fundamental difference in the way young and aged neurons buffer Ca(2+), and a corresponding difference in the quality of the Ca(2+) signal experienced by young and aged neurons for different intensities of cytoplasmic Ca(2+) influx.

Mitochondrial localization as a determinant of capacitative Ca2+ entry in HeLa cells

Whether different subsets of mitochondria play distinct roles in shaping intracellular Ca2+ signals is presently unresolved. Here, we determine the role of mitochondria located beneath the plasma membrane in controlling (a) Ca2+ release from the endoplasmic reticulum (ER) and (b) capacitative Ca2+ entry. By over-expression of the dynactin subunit dynamitin, and consequent inhibition of the fission factor, dynamin-related protein (Drp-1), mitochondria were relocalised from the plasma membrane towards the nuclear periphery in HeLa cells. The impact of these changes on free calcium concentration in the cytosol ([Ca2+]c), mitochondria ([Ca2+]m) and ER ([Ca2+]ER) was then monitored with specifically-targeted aequorins. Whilst dynamitin over-expression increased the number of close contacts between the ER and mitochondria by >2.5-fold, assessed using organelle-targeted GFP variants, histamine-induced changes in organellar [Ca2+] were unaffected. By contrast, Ca2+ influx elicited significantly smaller increases in [Ca2+]c and [Ca2+]m in dynamitin-expressing than in control cells. These data suggest that the strategic localisation of a subset of mitochondria beneath the plasma membrane is required for normal Ca2+ influx, but that the transfer of Ca2+ ions between the ER and mitochondria is relatively insensitive to gross changes in the spatial relationship between these two organelles.

Functional coupling as a basic mechanism of feedback regulation of cardiac energy metabolism

In this review we analyze the concepts and the experimental data on the mechanisms of the regulation of energy metabolism in muscle cells. Muscular energetics is based on the force-length relationship, which in the whole heart is expressed as a Frank-Starling law, by which the alterations of left ventricle diastolic volume change linearly both the cardiac work and oxygen consumption. The second basic characteristics of the heart is the metabolic stability--almost constant levels of high energy phosphates, ATP and phosphocreatine, which are practically independent of the workload and the rate of oxygen consumption, in contrast to the fast-twitch skeletal muscle with no metabolic stability and rapid fatigue. Analysis of the literature shows that an increase in the rate of oxygen consumption by order of magnitude, due to Frank-Starling law, is observed without any significant changes in the intracellular calcium transients. Therefore, parallel activation of contraction and mitochondrial respiration by calcium ions may play only a minor role in regulation of respiration in the cells. The effective regulation of the respiration under the effect of Frank-Starling law and metabolic stability of the heart are explained by the mechanisms of functional coupling within supramolecular complexes in mitochondria, and at the subcellular level within the intracellular energetic units. Such a complex structural and functional organisation of heart energy metabolism can be described quantitatively by mathematical models.

OPA1 requires mitofusin 1 to promote mitochondrial fusion

The regulated equilibrium between mitochondrial fusion and fission is essential to maintain integrity of the organelle. Mechanisms of mitochondrial fusion are largely uncharacterized in mammalian cells. It is unclear whether OPA1, a dynamin-related protein of the inner membrane mutated in autosomal dominant optic atrophy, participates in fusion or fission. OPA1 promoted the formation of a branched network of elongated mitochondria, requiring the integrity of both its GTPase and C-terminal coiled-coil domain. Stable reduction of OPA1 levels by RNA interference resulted in small, fragmented, and scattered mitochondria. Levels of OPA1 did not affect mitochondrial docking, but they correlated with the extent of fusion as measured by polyethylene glycol mitochondrial fusion assays. A genetic analysis proved that OPA1 was unable to tubulate and fuse mitochondria lacking the outer membrane mitofusin 1 but not mitofusin 2. Our data show that OPA1 functionally requires mitofusin 1 to regulate mitochondrial fusion and reveal a specific functional difference between mitofusin 1 and 2.

Mitochondrial regular arrangement in muscle cells: a "crystal-like" pattern

The aim of this work was to characterize quantitatively the arrangement of mitochondria in heart and skeletal muscles. We studied confocal images of mitochondria in nonfixed cardiomyocytes and fibers from soleus and white gastrocnemius muscles of adult rats. The arrangement of intermyofibrillar mitochondria was analyzed by estimating the densities of distribution of mitochondrial centers relative to each other (probability density function). In cardiomyocytes (1,820 mitochondrial centers marked), neighboring mitochondria are aligned along a rectangle, with distance between the centers equal to 1.97 +/- 0.43 and 1.43 +/- 0.43 microm in the longitudinal and transverse directions, respectively. In soleus (1,659 mitochondrial centers marked) and white gastrocnemius (621 pairs of mitochondria marked), mitochondria are mainly organized in pairs at the I-band level. Because of this organization, there are two distances characterizing mitochondrial distribution in the longitudinal direction in these muscles. The distance between mitochondrial centers in the longitudinal direction within the same I band is 0.91 +/- 0.11 and 0.61 +/- 0.07 microm in soleus and white gastrocnemius, respectively. The distance between mitochondrial centers in different I bands is approximately 3.7 and approximately 3.3 microm in soleus and gastrocnemius, respectively. In the transverse direction, the mitochondria are packed considerably closer to each other in soleus than in white gastrocnemius, with the distance equal to 0.75 +/- 0.22 microm in soleus and 1.09 +/- 0.41 microm in gastrocnemius. Our results show that intermyofibrillar mitochondria are arranged in a highly ordered crystal-like pattern in a muscle-specific manner with relatively small deviation in the distances between neighboring mitochondria. This is consistent with the concept of the unitary nature of the organization of the muscle energy metabolism.

Amyloid beta-induced changes in nitric oxide production and mitochondrial activity lead to apoptosis

Increasing evidence suggests an important role of mitochondrial dysfunction in the pathogenesis of Alzheimer's disease. Thus, we investigated the effects of acute and chronic exposure to increasing concentrations of amyloid beta (Abeta) on mitochondrial function and nitric oxide (NO) production in vitro and in vivo. Our data demonstrate that PC12 cells and human embryonic kidney cells bearing the Swedish double mutation in the amyloid precursor protein gene (APPsw), exhibiting substantial Abeta levels, have increased NO levels and reduced ATP levels. The inhibition of intracellular Abeta production by a functional gamma-secretase inhibitor normalizes NO and ATP levels, indicating a direct involvement of Abeta in these processes. Extracellular treatment of PC12 cells with comparable Abeta concentrations only leads to weak changes, demonstrating the important role of intracellular Abeta. In 3-month-old APP transgenic (tg) mice, which exhibit no plaques but already detectable Abeta levels in the brain, reduced ATP levels can also be observed showing the in vivo relevance of our findings. Moreover, we could demonstrate that APP is present in the mitochondria of APPsw PC12 cells. This presence might be directly involved in the impairment of cytochrome c oxidase activity and depletion of ATP levels in APPsw PC12 cells. In addition, APPsw human embryonic kidney cells, which produce 20-fold increased Abeta levels compared with APPsw PC12 cells, and APP tg mice already show a significantly decreased mitochondrial membrane potential under basal conditions. We suggest a hypothetical sequence of pathogenic steps linking mutant APP expression and amyloid production with enhanced NO production and mitochondrial dysfunction finally leading to cell death.

Cytoplasmic dynein regulates the subcellular distribution of mitochondria by controlling the recruitment of the fission factor dynamin-related protein-1

While the subcellular organisation of mitochondria is likely to influence many aspects of cell physiology, its molecular control is poorly understood. Here, we have investigated the role of the retrograde motor protein complex, dynein-dynactin, in mitochondrial localisation and morphology. Disruption of dynein function, achieved in HeLa cells either by over-expressing the dynactin subunit, dynamitin (p50), or by microinjection of an anti-dynein intermediate chain antibody, resulted in (a) the redistribution of mitochondria to the nuclear periphery, and (b) the formation of long and highly branched mitochondrial structures. Suggesting that an alteration in the balance between mitochondrial fission and fusion may be involved in both of these changes, overexpression of p50 induced the translocation of the fission factor dynamin-related protein (Drp1) from mitochondrial membranes to the cytosol and microsomes. Moreover, a dominant-negative-acting form of Drp1 mimicked the effects of p50 on mitochondrial morphology, while wild-type Drp1 almost completely restored normal mitochondrial distribution in p50 over-expressing cells. Thus, the dynein/dynactin complex plays an unexpected role in the regulation of mitochondrial morphology in living cells, by controlling the recruitment of Drp1 to these organelles.

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.

The role of mitochondria in the establishment of oocyte functional competence

Mitochondria are maternally inherited, semi-autonomous organelles with their own genomes (mtDNA), largely responsible for the generation of energy in the form of cellular ATP. However, mitochondrial replication and transcription of mtDNA do not commence until well into embryonic differentiation. This means that the oocyte needs to contain sufficient stocks of functioning mitochondria to fuel the first few days of embryonic development. In this review, I examine how qualitative and quantitative aspects of mitochondria help us define the notion of functional competence.

Mitochondrial swelling and generation of reactive oxygen species induced by photoirradiation are heterogeneously distributed

Abundant evidence has been gathered to show that overproduction of reactive oxygen species (ROS) can lead to the opening of the mitochondrial permeability transition pore (MPTP) and result in apoptosis in mammalian cells. The information regarding spatial and temporal regulation of intracellular ROS formation related to the MPTP opening, however, is relatively limited. In this study, we used a fluorescent probe, dihydro-2',7'-dichloroforescin (DCF), to detect intracellular ROS levels in different compartments of the cell in a time-resolved manner. The roles of mitochondrial ROS (mROS) in the MPTP opening and mitochondrial membrane potential drop were investigated by using H(2)DCFDA coloaded with a mitochondrial marker dye MitoTracker Red, and by a mitochondrial membrane potential dye tetramethyl rhodamine ethyl ester. We applied multiphoton laser scanning microscopy to avoid autooxidation and bleaching of DCF so that long-term visualization of intracellular ROS formation could be performed. Moreover, we noted that the resting mROS levels of different mitochondria were not homogeneous. After cells had been exposed to photoirradiation, the intracellular ROS gradually increased but the heterogeneity of mROS was maintained. Later, swelling was observed in mitochondria that contained higher levels of ROS, indicating the opening of the MPTP. In cells in which all the mitochondria swelled, they were translocated to the perinuclear area, which became the site of ROS production. At this stage, mROS reached the highest level concomitantly with a complete loss of mitochondrial membrane potential, indicating full opening of the MPTP. At the end, photoirradiation resulted in apoptotic cell death. In summary, we demonstrated by multiphoton laser scanning microscopy that photoirradiation induces heterogeneous intracellular ROS formation and mitochondrial permeability transition pore opening in single intact cells. These observations imply the existence of a microdomain in the regulation of mROS formation and subsequent opening of the MPTP.

Human cytomegalovirus-induced host cell enlargement is iron dependent

A hallmark of human cytomegalovirus (HCMV) infection is the characteristic enlargement of the host cells (i.e., cytomegaly). Because iron (Fe) is required for cell growth and Fe chelators inhibit viral replication, we investigated the effects of HCMV infection on Fe homeostasis in MRC-5 fibroblasts. Using the metallosensitive fluorophore calcein and the Fe chelator salicylaldehyde isonicotinoyl hydrazone (SIH), the labile iron pool (LIP) in mock-infected cells was determined to be 1.04 +/- 0.05 microM. Twenty-four hours postinfection (hpi), the size of the LIP had nearly doubled. Because cytomegaly occurs between 24 and 96 hpi, access to this larger LIP could be expected to facilitate enlargement to approximately 375% of the initial cell size. The ability of Fe chelation by 100 microM SIH to limit enlargement to approximately 180% confirms that the LIP plays a major role in cytomegaly. The effect of SIH chelation on the mitochondrial membrane potential (DeltaPsi(M)) and morphology was studied using the mitochondrial voltage-sensitive dye JC-1. The mitochondria in mock-infected cells were heterogeneous with a broad distribution of DeltaPsi(M) and were threadlike. In contrast, the mitochondria of HCMV-infected cells had a more depolarized DeltaPsi(M) distributed over a narrow range and were grainlike in appearance. However, the HCMV-induced alteration in DeltaPsi(M) was not affected by SIH chelation. We conclude that the development of cytomegaly is inhibited by Fe chelation and may be facilitated by an HCMV-induced increase in the LIP.

The functional organization of mitochondrial genomes in human cells

Background

We analyzed the organization and function of mitochondrial DNA in a stable human cell line (ECV304, which is also known as T-24) containing mitochondria tagged with the yellow fluorescent protein.

Results

Mitochondrial DNA is organized in ~475 discrete foci containing 6–10 genomes. These foci (nucleoids) are tethered directly or indirectly through mitochondrial membranes to kinesin, marked by KIF5B, and microtubules in the surrounding cytoplasm. In living cells, foci have an apparent diffusion constant of 1.1 × 10^-3 μm²/s, and mitochondria always split next to a focus to distribute all DNA to one daughter. The kinetics of replication and transcription (monitored by immunolabelling after incorporating bromodeoxyuridine or bromouridine) reveal that each genome replicates independently of others in a focus, and that newly-made RNA remains in a focus (residence half-time ~43 min) long after it has been made. This mitochondrial RNA colocalizes with components of the cytoplasmic machinery that makes and imports nuclear-encoded proteins – that is, a ribosomal protein (S6), a nascent peptide associated protein (NAC), and the translocase in the outer membrane (Tom22).

Conclusions

The results suggest that clusters of mitochondrial genomes organize the translation machineries on both sides of the mitochondrial membranes. Then, proteins encoded by the nuclear genome and destined for the mitochondria will be made close to mitochondrial-encoded proteins so that they can be assembled efficiently into mitochondrial complexes.

Stress induction and mitochondrial localization of Oxr1 proteins in yeast and humans

Reactive oxygen species (ROS) are critical molecules produced as a consequence of aerobic respiration. It is essential for cells to control the production and activity of such molecules in order to protect the genome and regulate cellular processes such as stress response and apoptosis. Mitochondria are the major source of ROS within the cell, and as a result, numerous proteins have evolved to prevent or repair oxidative damage in this organelle. The recently discovered OXR1 gene family represents a set of conserved eukaryotic genes. Previous studies of the yeast OXR1 gene indicate that it functions to protect cells from oxidative damage. In this report, we show that human and yeast OXR1 genes are induced by heat and oxidative stress and that their proteins localize to the mitochondria and function to protect against oxidative damage. We also demonstrate that mitochondrial localization is required for Oxr1 protein to prevent oxidative damage.

Axonal mitochondrial transport and potential are correlated

Disruption of axonal transport leads to a disorganized distribution of mitochondria and other organelles and is thought to be responsible for some types of neuronal disease. The reason for bidirectional transport of mitochondria is unknown. We have developed and applied a set of statistical methods and found that axonal mitochondria are uniformly distributed. Analysis of fast axonal transport showed that the uniform distribution arose from the clustering of the stopping events of fast axonal transport in the middle of the gaps between stationary mitochondria. To test whether transport was correlated with ATP production, we added metabolic inhibitors locally by micropipette. Whereas applying CCCP (a mitochondrial uncoupler) blocked mitochondrial transport, as has been previously reported, treatment with antimycin (an inhibitor of electron transport at complex III) caused increases in retrograde mitochondrial transport. Application of 2-deoxyglucose did not decrease transport compared with the mannitol control. To determine whether mitochondrial transport was correlated with mitochondrial potential, we stained the neurons with the mitochondrial potential-sensing dye JC-1. We found that approximately 90% of mitochondria with high potential were transported towards the growth cone and approximately 80% of mitochondria with low potential were transported towards the cell body. These experiments show for the first time that a uniform mitochondrial distribution is generated by local regulation of the stopping events of fast mitochondrial transport, and that the direction of mitochondrial transport is correlated with mitochondrial potential. These results have implications for axonal clogging, autophagy, apoptosis and Alzheimer's disease.

Role of mitochondrial membrane permeabilization in apoptosis and cancer

The release of proteins from the intermembrane space of mitochondria is one of the pivotal events in the apoptotic process, which can lead to the activation of caspases and the ultimate demise of the cell. How these proteins exit the mitochondria is still a matter of intense debate. Here, we discuss the possible mechanisms behind the release of apoptogenic proteins, the ways in which cancer cells subvert these mechanisms, and the therapeutic regimens that aim to promote the timely loss of integrity of the outer mitochondrial membrane.

Ca2+ Homeostasis during Mitochondrial Fragmentation and Perinuclear Clustering Induced by hFis1

Mitochondria modulate Ca(2+) signals by taking up, buffering, and releasing Ca(2+) at key locations near Ca(2+) release or influx channels. The role of such local interactions between channels and organelles is difficult to establish in living cells because mitochondria form an interconnected network constantly remodeled by coordinated fusion and fission reactions. To study the effect of a controlled disruption of the mitochondrial network on Ca(2+) homeostasis, we took advantage of hFis1, a protein that promotes mitochondrial fission by recruiting the dynamin-related protein, Drp1. hFis1 expression in HeLa cells induced a rapid and complete fragmentation of mitochondria, which redistributed away from the plasma membrane and clustered around the nucleus. Despite the dramatic morphological alteration, hFis1-fragmented mitochondria maintained a normal transmembrane potential and pH and took up normally the Ca(2+) released from intracellular stores upon agonist stimulation, as measured with a targeted ratiometric pericam probe. In contrast, hFis1-fragmented mitochondria took up more slowly the Ca(2+) entering across plasma membrane channels, because the Ca(2+) ions reaching mitochondria propagated faster and in a more coordinated manner in interconnected than in fragmented mitochondria. In parallel cytosolic fura-2 measurements, the capacitative Ca(2+) entry (CCE) elicited by store depletion was only marginally reduced by hFis1 expression. Regardless of mitochondria shape and location, disruption of mitochondrial potential with uncouplers or oligomycin/rotenone reduced CCE by approximately 35%. These observations indicate that close contact to Ca(2+) influx channels is not required for CCE modulation and that the formation of a mitochondrial network facilitates Ca(2+) propagation within interconnected mitochondria.

Poly(ADP-ribose) polymerase-1-mediated cell death in astrocytes requires NAD+ depletion and mitochondrial permeability transition

Extensive activation of poly(ADP-ribose) polymerase-1 (PARP-1) by DNA damage is a major cause of caspase-independent cell death in ischemia and inflammation. Here we show that NAD(+) depletion and mitochondrial permeability transition (MPT) are sequential and necessary steps in PARP-1-mediated cell death. Cultured mouse astrocytes were treated with the cytotoxic concentrations of N-methyl-N'-nitro-N-nitrosoguanidine or 3-morpholinosydnonimine to induce DNA damage and PARP-1 activation. The resulting cell death was preceded by NAD(+) depletion, mitochondrial membrane depolarization, and MPT. Sub-micromolar concentrations of cyclosporin A blocked MPT and cell death, suggesting that MPT is a necessary step linking PARP-1 activation to cell death. In astrocytes, extracellular NAD(+) can raise intracellular NAD(+) concentrations. To determine whether NAD(+) depletion is necessary for PARP-1-induced MPT, NAD(+) was restored to near-normal levels after PARP-1 activation. Restoration of NAD(+) enabled the recovery of mitochondrial membrane potential and blocked both MPT and cell death. Furthermore, both cyclosporin A and NAD(+) blocked translocation of the apoptosis-inducing factor from mitochondria to nuclei, a step previously shown necessary for PARP-1-induced cell death. These results suggest that NAD(+) depletion and MPT are necessary intermediary steps linking PARP-1 activation to AIF translocation and cell death.

Functional heterogeneity of mitochondria after cardiac cold ischemia and reperfusion revealed by confocal imaging

BACKGROUND

Mitochondria play a critical role in ischemia-reperfusion injury of the heart. The purpose of the present study was to analyze the intracellular region-specific functional state of mitochondria after cold ischemia-reperfusion in a rat heart transplant model.

METHODS

Imaging of the mitochondrial functional state in situ in nonfixed myocardial fibers was performed by confocal microscopy of mitochondrial flavoprotein autofluorescence as redox state indicator; fluorescence of Rhod-2, a specific probe for mitochondrial calcium; and of tetramethylrhodamine ethyl ester fluorescence to monitor the mitochondrial membrane potential.

RESULTS

This imaging demonstrated that, in contrast to control fibers, 10-hr heart cold storage, heterotopic cardiac transplantation, and 24-hr reperfusion result in a highly heterogeneous mitochondrial functional state (mitochondrial calcium content, redox state, and inner membrane potential), thus suggesting local permeability transitions and heterogeneous mitochondrial damage.

CONCLUSIONS

Imaging of in situ mitochondria allows topologic assessment of mitochondrial defects and heterogeneity, consequently providing new insights into the mechanisms of cardiac ischemia-reperfusion injury.

[Organization and dynamics of the mitochondrial compartment]

Mitochondria are essential organelles that are involved in numerous metabolic pathways and produce the major part of intracellular ATP by oxidative phosphorylation. Their ultrastructure was solved in the 1950s by electron microscopic analysis of ultrathin sections. Based on these pioneering studies and on the endosymbiotic origin of mitochondria, cells are often assumed to contain numerous independent mitochondria with a size similar to that of bacteria. However, electron microscopy of thick sections reveals that mitochondria form elongated and branched filaments. Optical microscopy of living cells demonstrates that mitochondrial filaments continuously modify their position and morphology and that they undergo frequent fission and fusion reactions. In this review, we revise the actual knowledge on the ultrastructure, the organization and the dynamics of the mitochondrial compartment. We review recent findings showing that mitochondria exchange molecules by fusion and we present the main proteins involved in mitochondrial fusion and fission reactions. Finally, we discuss the functional and physiological relevance of mitochondrial dynamics.

Altered mitochondrial structure and motion dynamics in living cells with energy metabolism defects revealed by real time microscope imaging

Using the real time microscope (RTM), a system applying new developments in light microscopy, we documented the spatial and temporal dynamics of mitochondrial behavior in human cultured skin fibroblasts. Without the use of stains or probes, we resolved fibroblast mitochondria as dark slender filaments of approximately 0.2 m wide and up to 10 m long, as well as a few smaller ovoid forms. In the living cell, the three most common mitochondrial movements were: (1) small oscillatory movements; (2) larger movements including filament extension, retraction, and branching as well as combinations of these actions; and (3) whole transit movements of single mitochondrial filaments. Skin fibroblasts from patients with mitochondrial complex I deficiency and normal fibroblasts during incubation with rotenone, or antimycin A, contained higher proportions of mitochondria in the swollen filamentous forms, nodal filaments, and ovoid forms rather than the slender filamentous forms in normal cells. Interestingly, decreased motility was observed with the more ovoid mitochondrial forms compared to the filamentous forms. We conclude that mitochondrial morphology and dynamic motion are strongly associated with changes in mitochondrial energy metabolism. Images documenting our observations are presented both at single time points and as QuickTime videos.

Heterogeneity in the Effect of Albumin and Other Resuscitation Fluids on Intracellular Oxygen Free Radical Production

BACKGROUND

We hypothesized that by studying a suspension of a single cell type in various resuscitation fluids, we could compare their effects on parameters associated with cell death.

METHODS

Jurkat cells were suspended in resuscitation fluids. Using flow cytometry, light scatter, phosphatidylserine translocation, propidium iodide uptake, and intracellular H2O2 production were measured.

RESULTS

Buffer (pH, 7.4) and albumin added to Ringer's lactate inhibited the adverse changes, including intracellular oxygen free radical production. Oxygen free radical production was variable within the cell population and inhibited by albumin but not other colloids or crystalloids. This correlated well with the ability of albumin to enhance metabolic activity. A flavoprotein inhibitor blocked H2O2 production, suggesting that mitochondria are the source of the H2O2 and the variability.

CONCLUSION

Oxygen free radical inhibition by albumin could explain both its beneficial and its harmful effects.

Mitochondria in tumor cells studied by laser scanning confocal microscopy

We present here a confocal fluorescence microscopy study of mitochondria in sensitive and resistant carcinoma cells by using two potentiometric probes of mitochondria, rhodamine 123 (R123) and dimethylaminostyryl-methylpyridiniumiodine. We have found that active mitochondria in sensitive MCF-7 and multidrug resistant MCF-7/DX carcinoma cells are very different in localization and morphology. In sensitive cells active mitochondria are found in the perinuclear region, whereas in the multidrug resistance (MDR) subline they are confined to the cell periphery. Interestingly, the MDR revertant verapamil has been found to restore in MCF-7/DX cells the same pattern of active mitochondria seen in sensitive cells. We have also studied R123 in human lung carcinoma A549 cells, which display a low responsivity to doxorubicin, and overexpress the lung resistance-related protein. In addition to perinuclear mitochondria, peripheral mitochondria with weaker fluorescence can be seen in this cell line. Interestingly, in the two examined carcinoma lines we have been able to recognize by image analysis a common new star-lobed morphology. Our results indicate that in resistant carcinoma cells two populations of mitochondria coexist with different localization, morphology, and activity.

Subcellular heterogeneity in mitochondrial red-ox responses to KATP channel agonists in freshly isolated rabbit cardiomyocytes

We have used the technique of fluorescent microscopy imaging supplemented with the refined analysis of temporal cartography of the cell fluorescence to investigate the mechanisms of regulation of mitochondrial function and its red-ox state in cardiac cells in vivo. Autofluorescence of flavoproteins of the respiratory chain in the isolated rabbit cardiomyocytes was registered before and after application of mitochondrial KATP channel opener diazoxide (100 and 400 microM). Diazoxide addition resulted in oxidation of flavoproteins. Detailed analysis of these responses showed that they were heterogeneous over space and time. The local responses show rapid jumps. In a few cells, metabolic oscillations developed and could be recorded for tens of minutes. Under these conditions the cells appeared divided into a small number of regions in which mitochondria function synchronously. Local pattern of oxidation switches again and again from a reduced state to the same level of oxidation. All these phenomena where absent when the cells were permeabilized by saponin giving a direct access to mitochondrial KATP channel opener. Cross-correlation analysis revealed a high degree of homogeneity for cells presenting metabolic oscillations, contrarily to those displaying a smooth increase in fluorescence in response to diazoxide. The results are consistent with the view that mitochondria form independent functional units whose behaviour can be synchronised by some unknown cellular factors or metabolites.

Axonal degeneration in paraplegin-deficient mice is associated with abnormal mitochondria and impairment of axonal transport

In several neurodegenerative diseases, axonal degeneration occurs before neuronal death and contributes significantly to patients' disability. Hereditary spastic paraplegia (HSP) is a genetically heterogeneous condition characterized by selective degeneration of axons of the corticospinal tracts and fasciculus gracilis. HSP may therefore be considered an exemplary disease to study the local programs mediating axonal degeneration. We have developed a mouse model for autosomal recessive HSP due to mutations in the SPG7 gene encoding the mitochondrial ATPase paraplegin. Paraplegin-deficient mice are affected by a distal axonopathy of spinal and peripheral axons, characterized by axonal swelling and degeneration. We found that mitochondrial morphological abnormalities occurred in synaptic terminals and in distal regions of axons long before the first signs of swelling and degeneration and correlated with onset of motor impairment during a rotarod test. Axonal swellings occur through massive accumulation of organelles and neurofilaments, suggesting impairment of anterograde axonal transport. Retrograde axonal transport is delayed in symptomatic mice. We speculate that local failure of mitochondrial function may affect axonal transport and cause axonal degeneration. Our data suggest that a timely therapeutic intervention may prevent the loss of axons.