Measurement of electrical potential, pH, and free calcium ion concentration in mitochondria of living cells by laser scanning confocal microscopy

MitoTracker Red for isolation of zone-specific hepatocytes and characterization of hepatic sublobular metabolism

BACKGROUND

The liver lobule is divided into three zones or regions: periportal (PP or Zone 1) that is highly oxidative and active in ureagenesis, pericentral (PC or Zone 3) that is more glycolytic, and midzonal (MZ or Zone 2) with intermediate characteristics.

AIM

Our goal was to isolate and metabolically characterize hepatocytes from specific sublobular zones.

METHODS

Mice were administered rhodamine123 (Rh123) or MitoTracker Red (MTR) prior to intravital imaging, liver fixation, or hepatocyte isolation. After in vivo MTR, hepatocytes were isolated and sorted based on MTR fluorescence intensity. Alternatively, E-cadherin (Ecad) and cytochrome P450 2E1 (CYP2E1) immunolabeling was performed in fixed liver slices. Ecad and CYP2E1 gene expression in sorted hepatocytes was assessed by qPCR. Oxygen consumption rates (OCR) of sorted hepatocytes were also assessed.

RESULTS

Multiphoton microscopy showed Rh123 and MTR fluorescence distributed zonally, decreasing from PP to PC in a flow-dependent fashion. In liver cross-sections, Ecad was expressed periportally and CYP2E1 pericentrally in association with high and low MTR labeling, respectively. Based on MTR fluorescence, hepatocytes were sorted into PP, MZ, and PC populations with PP and PC hepatocytes enriched in Ecad and CYP2E1, respectively. OCR of PP hepatocytes was ∼4 times that of PC hepatocytes.

CONCLUSIONS

MTR treatment in vivo delineates sublobular hepatic zones and can be used to sort hepatocytes zonally. PP hepatocytes have substantially greater OCR compared to PC and MZ. The results also indicate a sharp midzonal demarcation between hepatocytes with PP characteristics (Ecad) and those with PC features (CYP2E1). This new method to sort hepatocytes in a zone-specific fashion holds the potential to shed light on sublobular hepatocyte metabolism and regulatory pathways in health and disease.

The Interplay between Dysregulated Ion Transport and Mitochondrial Architecture as a Dangerous Liaison in Cancer

Transport of ions and nutrients is a core mitochondrial function, without which there would be no mitochondrial metabolism and ATP production. Both ion homeostasis and mitochondrial phenotype undergo pervasive changes during cancer development, and both play key roles in driving the malignancy. However, the link between these events has been largely ignored. This review comprehensively summarizes and critically discusses the role of the reciprocal relationship between ion transport and mitochondria in crucial cellular functions, including metabolism, signaling, and cell fate decisions. We focus on Ca²⁺, H⁺, and K⁺, which play essential and highly interconnected roles in mitochondrial function and are profoundly dysregulated in cancer. We describe the transport and roles of these ions in normal mitochondria, summarize the changes occurring during cancer development, and discuss how they might impact tumorigenesis.

Quantitative analysis of mitochondrial morphology and membrane potential in living cells using high-content imaging, machine learning, and morphological binning

Understanding the processes of mitochondrial dynamics (fission, fusion, biogenesis, and mitophagy) has been hampered by the lack of automated, deterministic methods to measure mitochondrial morphology from microscopic images. A method to quantify mitochondrial morphology and function is presented here using a commercially available automated high-content wide-field fluorescent microscopy platform and R programming-language-based semi-automated data analysis to achieve high throughput morphological categorization (puncta, rod, network, and large & round) and quantification of mitochondrial membrane potential. In conjunction with cellular respirometry to measure mitochondrial respiratory capacity, this method detected that increasing concentrations of toxicants known to directly or indirectly affect mitochondria (t-butyl hydroperoxide [TBHP], rotenone, antimycin A, oligomycin, ouabain, and carbonyl cyanide-p-trifluoromethoxyphenylhydrazone [FCCP]), decreased mitochondrial networked areas in cultured 661w cells to 0.60-0.80 at concentrations that inhibited respiratory capacity to 0.20-0.70 (fold change compared to vehicle). Concomitantly, mitochondrial swelling was increased from 1.4- to 2.3-fold of vehicle as indicated by changes in large & round areas in response to TBHP, oligomycin, or ouabain. Finally, the automated identification of mitochondrial location enabled accurate quantification of mitochondrial membrane potential by measuring intramitochondrial tetramethylrhodamine methyl ester (TMRM) fluorescence intensity. Administration of FCCP depolarized and administration of oligomycin hyperpolarized mitochondria, as evidenced by changes in intramitochondrial TMRM fluorescence intensities to 0.33- or 5.25-fold of vehicle control values, respectively. In summary, this high-content imaging method accurately quantified mitochondrial morphology and membrane potential in hundreds of thousands of cells on a per-cell basis, with sufficient throughput for pharmacological or toxicological evaluation.

Regulation of the mitochondrial proton gradient by cytosolic Ca²⁺ signals

Mitochondria convert the energy stored in carbohydrate and fat into ATP molecules that power enzymatic reactions within cells, and this process influences cellular calcium signals in several ways. By providing ATP to calcium pumps at the plasma and intracellular membranes, mitochondria power the calcium gradients that drive the release of Ca²⁺ from stores and the entry of Ca²⁺ across plasma membrane channels. By taking up and subsequently releasing calcium ions, mitochondria determine the spatiotemporal profile of cellular Ca²⁺ signals and the activity of Ca²⁺-regulated proteins, including Ca²⁺ entry channels that are themselves part of the Ca²⁺ circuitry. Ca²⁺ elevations in the mitochondrial matrix, in turn, activate Ca²⁺-dependent enzymes that boost the respiratory chain, increasing the ability of mitochondria to buffer calcium ions. Mitochondria are able to encode and decode Ca²⁺ signals because the respiratory chain generates an electrochemical gradient for protons across the inner mitochondrial membrane. This proton motive force (Δp) drives the activity of the ATP synthase and has both an electrical component, the mitochondrial membrane potential (ΔΨ(m)), and a chemical component, the mitochondrial proton gradient (ΔpH(m)). ΔΨ(m) contributes about 190 mV to Δp and drives the entry of Ca²⁺ across a recently identified Ca²⁺-selective channel known as the mitochondrial Ca²⁺ uniporter. ΔpH(m) contributes ~30 mV to Δp and is usually ignored or considered a minor component of mitochondria respiratory state. However, the mitochondrial proton gradient is an essential component of the chemiosmotic theory formulated by Peter Mitchell in 1961 as ΔpH(m) sustains the entry of substrates and metabolites required for the activity of the respiratory chain and drives the activity of electroneutral ion exchangers that allow mitochondria to maintain their osmolarity and volume. In this review, we summarize the mechanisms that regulate the mitochondrial proton gradient and discuss how thermodynamic concepts derived from measurements in purified mitochondria can be reconciled with our recent findings that mitochondria have high proton permeability in situ and that ΔpH(m) decreases during mitochondrial Ca²⁺ elevations.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Effect of short-term free Fatty acids elevation on mitochondrial function in skeletal muscle of healthy individuals

CONTEXT

Mitochondrial dysfunction has been proposed as an underlying mechanism in the pathogenesis of insulin resistance and type 2 diabetes mellitus.

OBJECTIVE

To determine whether mitochondrial dysfunction plays a role in the free fatty acid (FFA)-induced impairment in insulin action in skeletal muscle of healthy subjects.

DESIGN

Eleven lean normal glucose tolerant individuals received 8 h lipid and saline infusion on separate days with a euglycemic insulin clamp during the last 2 h. Vastus lateralis muscle biopsies were performed at baseline and after 6 h lipid or saline infusion. Inner mitochondrial membrane potential (Psi(m)) and mitochondrial mass were determined ex vivo by confocal microscopy.

RESULTS

Compared with saline infusion, lipid infusion reduced whole-body glucose uptake by 22% (P < 0.05). Psi(m) decreased by 33% (P < 0.005) after lipid infusion and the decrement in Psi(m) correlated with change in plasma FFA after lipid infusion (r = 0.753; P < 0.005). Mitochondrial content and morphology did not change after lipid infusion. No significant changes in genes expression, citrate synthase activity, and total ATP content were observed after either lipid or saline infusion.

CONCLUSIONS

Short-term physiological increase in plasma FFA concentration in lean normal glucose tolerant subjects induces insulin resistance and impairs mitochondrial membrane potential but has no significant effects on mitochondrial content, gene expression, ATP content, or citrate synthase activity.

A heterogeneous "resting" pool of synaptic vesicles that is dynamically interchanged across boutons in mammalian CNS synapses

Using pHluorin-tagged synaptic vesicle proteins we have examined the partitioning of these probes into recycling and nonrecycling pools at hippocampal nerve terminals in cell culture. Our studies show that for three of the major synaptic vesicle components, vGlut-1, VAMP-2, and Synaptotagmin I, approximately 50-60% of the tagged protein appears in a recycling pool that responds readily to sustained action potential stimulation by mobilizing and fusing with the plasma membrane, while the remainder is targeted to a nonrecycling, acidic compartment. The fraction of recycling and nonrecycling (or resting) pools varied significantly across boutons within an individual axon, from 100% resting (silent) to 100% recycling. Single-bouton bleaching studies show that recycling and resting pools are dynamic and exchange between synaptic boutons. The quantitative parameters that can be extracted with the approaches outlined here should help elucidate the potential functional role of the resting vesicle pool.

TOXI-SIM-A simulation tool for the analysis of mitochondrial and plasma membrane potentials

Changes in the electrochemical gradients across biological membranes are excellent indicators of pathophysiological processes, drug action, or drug toxicity. Our previous studies have utilized the potentiometric probe tetramethylrhodamine methyl ester (TMRM) to characterize changes in mitochondrial function by monitoring alterations in the mitochondrial membrane potential (Deltapsi(m)) over time during glutamate excitotoxicity. However, fluorescently charged dyes such as TMRM respond to changes in both Deltapsi(m) and the plasma membrane (Deltapsi(p)) potentials making whole cell fluorescence data difficult to interpret. Here we have implemented a mathematical model that exploits the Nernstian behaviour of TMRM and uses automated Newton based root-finding fitting (TOXI-SIM) to model changes in TMRM fluorescence from multiple cells simultaneously, providing output on changes in Deltapsi(m) and Deltapsi(p) over time. Based on Ca(2+) responses, TOXI-SIM allows for an accurate modelling of TMRM traces for different injury paradigms (necrosis, apoptosis, tolerance). TOXI-SIM is provided as a user friendly public web service for trace analysis, with an additional online data base provided for the storage and retrieval of experimental traces (http://systemsbiology.rcsi.ie/tmrm/index.html).

Evaluating mitochondrial membrane potential in cells

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

Pregnancy-specific modulatory role of mitochondria on adenosine 5'-triphosphate-induced cytosolic [Ca2+] signaling in uterine artery endothelial cells

Vascular endothelial cells respond to extracellular ATP by inositol 1,4,5-trisphosphate-mediated Ca2+ release from the endoplasmic reticulum followed by Ca2+ influx and subsequent synthesis of vasodilators. In this study, the contribution of mitochondria in shaping the ATP-induced Ca2+ increase was examined in ovine uterine artery endothelial cells from nonpregnant and pregnant (late gestation) ewes (NP- and P-UAEC, passage 4). The mitochondrial protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) induced a rapid mitochondrial depolarization. CCCP also slowly increased cytosolic [Ca2+] ([Ca2+]c), which then gradually declined to 10-20 nM above resting level. Pretreatment with CCCP for 30 min significantly inhibited both ATP and thapsigargin-induced [Ca2+]c, with inhibition in NP-UAEC more effective than in P-UAEC. Pretreatment of mitochondrial permeability transition pore inhibitor cyclosporine A did not affect CCCP-induced mitochondrial depolarization, but delayed CCCP-induced [Ca2+]c for about 12-15 min (we termed this the "window of time"). During the cyclosporine A-delayed window of time of CCCP-induced [Ca2+]c, ATP induced a normal Ca2+ response, but after this window of time, ATP-induced [Ca2+]c was significantly inhibited. Pretreatment of oligomycin B to prevent intracellular ATP depletion by F0F1-ATPase did not reduce the inhibition of ATP-induced [Ca2+]c by CCCP. Ruthenium red, a mitochondrial Ca2+ uptake blocker, did not mimic the inhibition of Ca2+ signaling by CCCP. In conclusion, our data show that mitochondrial Ca2+ depletion after dissipation of mitochondrial membrane potential with CCCP inhibits ATP-induced [Ca2+]c, mediated at the level of Ca2+ release from the endoplasmic reticulum. Moreover, our data revealed that P-UAEC is more resistant to the inhibitory effect of CCCP on [Ca2+]c than NP-UAEC.

Regional variation in the activation threshold for 1,3-DNB-induced mitochondrial permeability transition in brainstem and cortical astrocytes

1,3-Dinitrobenzene (DNB) produces edematous, glio-vascular lesions in brainstem nuclei with high energy demands. Astrocytes in vulnerable brainstem nuclei appear to be an early and selective target of DNB and other nitroaromatic compounds, though the molecular basis of this susceptibility is poorly understood. It has been postulated that mitochondria are a principal target of DNB in sensitive cell types [Neuropathol. Appl. Neurobiol. 13 (5) (1987) 371], where redox-cycling of DNB increases levels of reactive oxygen species and disrupts cellular energy metabolism. The present study investigates the role of regional differences in activation of the mitochondrial permeability transition pore (mtPTP) by DNB in brainstem and cortical astrocytes and examines the expression of Bcl-2 proteins as potential regulators of mtPTP function. Neonatal rat astrocytes were cultured from both DNB-sensitive (brainstem) and insensitive (cortex) brain regions and evaluated for DNB-induced alterations in cell morphology and mitochondrial function. Exposure to DNB resulted in rapid changes in the morphology of brainstem astrocytes consistent with loss of ion homeostasis and initiation of necrotic cell death. These changes were not observed in cortical astrocytes at corresponding concentrations of DNB and were prevented in brainstem astrocytes by the mtPTP inhibitor, bongkrekic acid, suggesting that mitochondrial dysfunction is involved in DNB-induced morphological changes in brainstem astrocytes. Mitochondrial depolarization in brainstem astrocytes was observed at DNB concentrations as low as 10 microM, whereas no loss of mitochondrial membrane potential (DeltaPsi(mt)) occurred in cortical astrocytes at less than 100 microM DNB. DNB-induced loss of DeltaPsi(mt) followed apparent first-order kinetics, with EC(50)-values for half-maximal rates of mitochondrial depolarization of approximately 23 and approximately 290 microM in brainstem cortical astrocytes, respectively. DNB-induced mitochondrial depolarization was prevented by pretreatment with bongkrekic acid, indicating that loss of DeltaPsi(mt) was mediated by activation of the mtPTP. Inhibition of succinate dehydrogenase (SDH) activity occurred in astrocytes from both brain regions exposed to DNB and was blocked in brainstem, but not cortical, astrocytes by bongkrekic acid. Constitutive expression of Bcl-X(L) was high in cortical tissue and astrocytes, whereas Bax expression was low. However, Bax was highly expressed in brainstem tissue and astrocytes and Bcl-X(L) expression was markedly lower. The expression of Bcl-2 was similar in both brain regions. These data suggest that the selective vulnerability of brainstem astrocytes to DNB is due to a lower threshold for activation of the mtPTP that is be mediated, in part, by distinct expression patterns of Bcl-2 proteins rather than by intrinsic differences in susceptibility of the electron transport chain.

Heat stress prevents mitochondrial injury in ATP-depleted renal epithelial cells

The events that precipitate cell death and the stress proteins responsible for cytoprotection during ATP depletion remain elusive. We hypothesize that exposure to metabolic inhibitors damages mitochondria, allowing proapoptotic proteins to leak into the cytosol, and suggest that heat stress-induced hsp72 accumulation prevents mitochondrial membrane injury. To test these hypotheses, renal epithelial cells were transiently ATP depleted with sodium cyanide and 2-deoxy-D-glucose in the absence of medium dextrose. Recovery from ATP depletion was associated with the release into the cytosol of cytochrome c and apoptosis-inducing factor (AIF), proapoptotic proteins that localize to the intermitochondrial membrane space. Concomitant with mitochondrial cytochrome c leak, a seven- to eightfold increase in caspase 3 activity was observed. In controls, state III mitochondrial respiration was reduced by 30% after transient exposure to metabolic inhibitors. Prior heat stress preserved mitochondrial ATP production and significantly reduced both cytochrome c release and caspase 3 activation. Despite less cytochrome c release, prior heat stress increased binding between cytochrome c and hsp72. The present study demonstrates that mitochondrial injury accompanies exposure to metabolic inhibitors. By reducing outer mitochondrial membrane injury and by complexing with cytochrome c, hsp72 could inhibit caspase activation and subsequent apoptosis.

Life imaging of peroxynitrite in rat microglial and astroglial cells: Role of superoxide and antioxidants

Free radicals, such as superoxide and nitric oxide, are known to play a role in a number of inflammatory and degenerative brain diseases, in which resident microglia upregulate the inducible nitric oxide synthase (iNOS) and thus produce large amounts of nitric oxide. Simultaneously, microglia generate superoxide mainly via NADPH-oxidase, which reacts at a diffusion-limited rate with nitric oxide to form the powerful oxidant peroxynitrite. We used mixed astroglial/microglial cultures to study the effects of iNOS induction by lipopolysaccharide and interferon-gamma on free radical formation. Using the fluorogenic compound 2,7-dihydrodichlorofluorescein diacetate, we monitored cellular peroxynitrite formation by confocal laser microscopy. Peroxynitrite formation in continuously nitric oxide-producing microglial cells was rather limited. However, activation of the superoxide-generating enzyme NADPH-oxidase dramatically increased DCF fluorescence within a few minutes. We conclude that superoxide is the limiting factor for peroxynitrite formation. Since the formation and oxidant activity of peroxynitrite depends strongly on the availability of cellular antioxidants, we investigated the capacity of several compounds to influence peroxynitrite formation. Among the substances under investigation in this study, glutathione and the synthetic compound ebselen had a major effect on preventing peroxynitrite formation, whereas ascorbate failed to decrease peroxynitrite levels.

Effects of pH on human bone marrow stromal cells in vitro: implications for tissue engineering of bone

The objective of this study was to address the hypothesis that changes in extracellular pH alter collagen gene expression, collagen synthesis, and alkaline phosphatase activity in bone marrow stromal cells (BMSCs). Potential effects of pH on cell function are of particular importance for tissue engineering because considerable effort is being placed on engineering biodegradable polymers that may generate a local acidic microenvironment on degradation. Human and murine single-cell marrow suspensions were plated at a density of 2 x 10(4) cells/cm(2). After 7 days in culture, the pH of the culture medium was adjusted to one of six ranges: > or = 7.8, 7.5.-7.7, 7.2-7.4, 6.9-7.1, 6.6-6.8, or < or = 6.5. After 48 h of exposure to an altered pH, alkaline phosphatase activity and collagen synthesis decreased significantly with decreasing pH. This decrease was two-to threefold as pH decreased from 7.5 to 6.6. In contrast, alpha1(I) procollagen mRNA levels increased two- to threefold as pH was decreased. The trend in osteocalcin mRNA expression was opposite to that of collagen. Small shifts in extracellular pH led to significant changes in the ability of BMSCs to express markers of the osteoblast phenotype. These pH effects potentially relate to the microenvironment supplied by a tissue-engineering scaffold and suggest that degrading polymer scaffolds may influence the biologic activity of the cells in the immediate environment.

Calcium flux measurements in apoptosis

Early studies in apoptosis implicated an increase in cytosolic Ca2+ as a direct mediator of DNA fragmentation. However, efforts to delineate targets for this increase in Ca2+ have been slow in evolving. Several previous studies have implicated ER Ca2+ pool depletion in the initiation of apoptosis. Our own preliminary studies confirm that many (but not all) apoptotic stimuli empty the ER store via a mechanism that is blocked by BCL-2 expression. Furthermore, ER pool depletion is not affected by broad spectrum caspase inhibitors, indicating that it occurs via a caspase-independent mechanism. Finally, our data demonstrate that ER pool depletion occurs prior to release of cytochrome c from mitochondria. Given previous work demonstrating close coordination of ER and mitochondrial Ca2+ levels, we speculate that ER-dependent changes in mitochondrial Ca2+ serve as important signals for cytochrome c release. Alternative mechanisms include activation of caspase-12 and/or the JNK pathway, both of which can be directly stimulated by depletion of the ER Ca2+ pool. Although substantial improvements in intracellular Ca2+ imaging have emerged, compelling answers to many of the present questions related to the role of Ca2+ in apoptosis await future technical improvements. The development of organelle-specific, recombinant Ca2+ probes (targeted aequorins and cameleons) certainly should facilitate some of this work, although the target cell of interest must be amenable to molecular manipulation (transfection), which precludes straightforward analysis of primary cells. Pharmacological tools (i.e., thapsigargin and DBHQ) can provide conclusive data on ER pool status without requiring an overly sophisticated image analysis system. However, confocal microscopy allows for the effective analysis of Ca2+ pools as long as dye localization is homogeneous and properly controlled. However, current techniques should be considered semiquantitative at best and will remain so until specific organelle-targeted fluorescent dyes are developed and widely available.

Human mitochondrial topoisomerase I

Tension generated in the circular mitochondrial genome during replication and transcription points to the need for mtDNA topoisomerase activity. Here we report a 601-aa polypeptide highly homologous to nuclear topoisomerase I. The N-terminal domain of this novel topoisomerase contains a mitochondrial localization sequence and lacks a nuclear localization signal. Therefore, we refer to this polypeptide as top1mt. The pattern of top1mt expression matches the requirement for high mitochondrial activity in specific tissues. top1mt is a type IB topoisomerase that requires divalent metal (Ca(2+) or Mg(2+)) and alkaline pH for optimum activity. The TOP1mt gene is highly homologous to the nuclear TOP1 gene and consists of 14 exons. It is localized on human chromosome 8q24.3.

Are mitochondria mesoscopic?

Systems whose length scales are small enough to be near the threshold where macroscopic thermodynamic descriptions fail are called mesoscopic. This is particularly appropriate for studies of mitochondrial function since basic and important ideas such as protomotive force, and models finding common applications, such as the Gouy-Chapman theory describing the diffuse ionic charge layer near the inner membrane, depend on the macroscopic thermodynamic notion of chemical potential. This paper shows that the length scales in Gouy-Chapman theory are not consistent with the premises upon which Gouy-Chapman theory was based, a failure typical of mesoscopic systems. Furthermore, while there are good reasons for a protomotive force to be a valid description for a collection of mitochondria, it is a marginal description even for an entire individual mitochondrion for similar reasons. Mitochondria are, therefore, physically mesoscopic systems.

Quantitation and origin of the mitochondrial membrane potential in human cells lacking mitochondrial DNA

Mammalian mitochondrial DNA (mtDNA) encodes 13 polypeptide components of oxidative phosphorylation complexes. Consequently, cells that lack mtDNA (termed rho degrees cells) cannot maintain a membrane potential by proton pumping. However, most mitochondrial proteins are encoded by nuclear DNA and are still imported into mitochondria in rho degrees cells by a mechanism that requires a membrane potential. This membrane potential is thought to arise from the electrogenic exchange of ATP4- for ADP3- by the adenine nucleotide carrier. An intramitochondrial ATPase, probably an incomplete FoF1-ATP synthase lacking the two subunits encoded by mtDNA, is also essential to ensure sufficient charge flux to maintain the potential. However, there are considerable uncertainties about the magnitude of this membrane potential, the nature of the intramitochondrial ATPase and the ATP flux required to maintain the potential. Here we have investigated these factors in intact and digitonin-permeabilized mammalian rho degrees cells. The adenine nucleotide carrier and ATP were essential, but not sufficient to generate a membrane potential in rho degrees cells and an incomplete FoF1-ATP synthase was also required. The maximum value of this potential was approximately 110 mV in permeabilized cells and approximately 67 mV in intact cells. The membrane potential was eliminated by inhibitors of the adenine nucleotide carrier and by azide, an inhibitor of the incomplete FoF1-ATP synthase, but not by oligomycin. This potential is sufficient to import nuclear-encoded proteins but approximately 65 mV lower than that in 143B cells containing fully functional mitochondria. Subfractionation of rho degrees mitochondria showed that the azide-sensitive ATPase activity was membrane associated. Further analysis by blue native polyacrylamide gel electrophoresis (BN/PAGE) followed by activity staining or immunoblotting, showed that this ATPase activity was an incomplete FoF1-ATPase loosely associated with the membrane. Maintenance of this membrane potential consumed about 13% of the ATP produced by glycolysis. This work has clarified the role of the adenine nucleotide carrier and an incomplete FoF1-ATP synthase in maintaining the mitochondrial membrane potential in rho degrees cells.

Mitochondrial regulation of calcium in the avian cochlear nucleus

The role of mitochondria and the endoplasmic reticulum in buffering [Ca2+]i in response to imposed calcium loads in neurons of the chick cochlear nucleus, nucleus magnocellularis (NM), was examined. Intracellular calcium concentrations were measured using fluorometric videomicroscopy. After depolarization with 125 mM KCl, NM neurons demonstrate an increase in [Ca2+]i that returns to near-basal levels within 6 min. Addition of the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP) dissipated the mitochondrial membrane potential, as evidenced by increased fluorescence when cells were loaded with rhodamine-123. Two micromolar CCCP had minimal effect on baseline [Ca2+]i. However, 2 or 10 microM CCCP interfered with the ability of NM cells to buffer [Ca2+]i in response to KCl depolarization without significantly affecting peak [Ca2+]i. Oligomycin also interfered with postdepolarization regulation of [Ca2+]i, but blocked late (7-8 min postdepolarization) increases in [Ca2+]i caused by CCCP. Thapsigargin had no effect on baseline, peak, or postdepolarization [Ca2+]i in NM cells. These results suggest that normal mitochondrial membrane potential and ATP synthesis play an important role in buffering [Ca2+]i in response to imposed calcium loads in NM neurons. Furthermore, the endoplasmic reticulum does not appear to play a significant role in either of these processes. Thus increases in mitochondrial number and function noted in NM cells after deafferentation may represent an adaptive response to an increased cytosolic calcium load.

Characterization of human and pig kidney long-chain-acyl-CoA dehydrogenases and their role in beta-oxidation

Long-chain-acyl-CoA dehydrogenase (LCADH) has been produced by recombinant techniques from the human cDNA and purified after expression in Escherichia coli. Pig kidney LCADH was purified using an optimized method which also produces apparently pure short-chain-acyl-CoA dehydrogenase (SCADH) and medium-chain-acyl-CoA dehydrogenase (MCADH) in good yields. LCADH from both sources has a maximal turnover rate (Vmax of 650-700 min(-1) at pH 7.6) with the best substrates, which is approximately fivefold higher than reported previously. The human enzyme has an approximately fivefold higher Km compared with the pig kidney enzyme with substrates of chain length from C10 to C18 and a significantly different dependence of Vmax on the chain length. Pig kidney LCADH has a similar Vmax/Km with C10 to C14 substrates as MCADH does with C6 to C10 substrates. Recombinant human LCADH, however, is significantly less efficient (approximately fourfold with C12) than purified pig kidney enzyme. We conclude that human LCADH is either quantitatively less important in beta-oxidation than in the pig, or that post-translational modifications, not present in the recombinant human enzyme, are required to optimize human LCADH activity. Our results demonstrate that LCADH is as important as the other acyl-CoA dehydrogenases in fatty acid oxidation at physiological, mitochondrial pH with optimal substrates of chain length C10-C14. The extent of the LCADH-flavin cofactor reduction observed with most substrates and the rate of the subsequent reoxidation with oxygen are markedly different from those found with human medium chain acyl-CoA dehydrogenase. Both LCADH are inactivated by the substrate analogue 2-octynoyl-CoA, possibly via covalent modification of Glu261, the active-site residue involved in deprotonation of the substrate (alpha)C-H.

Histamine induces oscillations of mitochondrial free Ca2+ concentration in single cultured rat brain astrocytes

1. The free Ca2+ concentration of mitochondria ([Ca2+]m) in cultured rat brain astrocytes was measured with a fluorescent Ca2+ indicator, rhod-2, and laser confocal microscopy. 2. Confocal images revealed a rhod-2 distribution that matched mitochondrial localization. 3. Using a Ca2+ ionophore, ionomycin, to clamp the [Ca2+]m from 0 to 100 microM in order to obtain the minimal and maximal fluorescence of rhod-2 in situ, a 3.5 +/- 0.4-fold increase in fluorescence intensity was observed, suggesting that the fluorescence of intramitochondrial rhod-2 was responding in a Ca(2+)-sensitive manner, thereby allowing measurements of [Ca2+]m in single astrocytes. 4. Exposure of fura-2-loaded astrocytes to 100 microM histamine produced a rapid and transient increase in cytosolic Ca2+ concentration ([Ca2+]c) that lasted for several tens of seconds. The spike in [Ca2+]c was frequently followed by variable numbers of repetitive oscillations of Ca2+, which appeared to dampen in amplitude with time. 5. This pattern of histamine-induced [Ca2+]c oscillations was also observed in rhod-2-loaded cells suggesting that [Ca2+]m fluctuated with a similar frequency. 6. The oscillations of [Ca2+]m, but not of [Ca2+]c, were abolished by a proton ionophore, carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), and by Ruthenium Red, a mitochondrial Ca(2+)-uniporter inhibitor. 7. These results suggest that the mitochondrial Ca2+ transport systems in cultured rat brain astrocytes are able to relay receptor-mediated [Ca2+]m oscillations into mitochondria.