The 'mitoflash' probe cpYFP does not respond to superoxide

Biosensors for spatiotemporal detection of reactive oxygen species in cells and tissues

Redox biology has become a major issue in numerous areas of physiology. Reactive oxygen species (ROS) have a broad range of roles from signal transduction to growth control and cell death. To understand the nature of these roles, accurate measurement of the reactive compounds is required. An increasing number of tools for ROS detection is available; however, the specificity and sensitivity of these tools are often insufficient. Furthermore, their specificity has been rarely evaluated in complex physiological conditions. Many ROS probes are sensitive to environmental conditions in particular pH, which may interfere with ROS detection and cause misleading results. Accurate detection of ROS in physiology and pathophysiology faces additional challenges concerning the precise localization of the ROS and the timing of their production and disappearance. Certain ROS are membrane permeable, and certain ROS probes move across cells and organelles. Targetable ROS probes such as fluorescent protein-based biosensors are required for accurate localization. Here we analyze these challenges in more detail, provide indications on the strength and weakness of current tools for ROS detection, and point out developments that will provide improved ROS detection methods in the future. There is no universal method that fits all situations in physiology and cell biology. A detailed knowledge of the ROS probes is required to choose the appropriate method for a given biological problem. The knowledge of the shortcomings of these probes should also guide the development of new sensors.

The path from mitochondrial ROS to aging runs through the mitochondrial permeability transition pore

Excessive production of mitochondrial reactive oxygen species (mROS) is strongly associated with mitochondrial and cellular oxidative damage, aging, and degenerative diseases. However, mROS also induces pathways of protection of mitochondria that slow aging, inhibit cell death, and increase lifespan. Recent studies show that the activation of the mitochondrial permeability transition pore (mPTP), which is triggered by mROS and mitochondrial calcium overloading, is enhanced in aged animals and humans and in aging-related degenerative diseases. mPTP opening initiates further production and release of mROS that damage both mitochondrial and nuclear DNA, proteins, and phospholipids, and also releases matrix NAD that is hydrolyzed in the intermembrane space, thus contributing to the depletion of cellular NAD that accelerates aging. Oxidative damage to calcium transporters leads to calcium overload and more frequent opening of mPTP. Because aging enhances the opening of the mPTP and mPTP opening accelerates aging, we suggest that mPTP opening drives the progression of aging. Activation of the mPTP is regulated, directly and indirectly, not only by the mitochondrial protection pathways that are induced by mROS, but also by pro-apoptotic signals that are induced by DNA damage. We suggest that the integration of these contrasting signals by the mPTP largely determines the rate of cell aging and the initiation of cell death, and thus animal lifespan. The suggestion that the control of mPTP activation is critical for the progression of aging can explain the conflicting and confusing evidence regarding the beneficial and deleterious effects of mROS on health and lifespan.

Mitochondrial transition ROS spike (mTRS) results from coordinated activities of complex I and nicotinamide nucleotide transhydrogenase

Mitochondria exhibit suppressed ATP production, membrane potential (∆Ψ_mt) polarization and reactive oxygen species (ROS) bursts during some cellular metabolic transitions. Although mitochondrial ROS release is influenced by ∆Ψ_mt and respiratory state, the relationship between these properties remains controversial primarily because they have not been measured simultaneously. We developed a multiparametric method for probing mitochondrial function that allowed precise characterization of the temporal relationship between ROS, ∆Ψ_mt and respiration. We uncovered a previously unknown spontaneous ROS spike - termed mitochondrial transition ROS spike (mTRS) - associated with re-polarization of ∆Ψ_mt that occurs at the transition between mitochondrial energy states. Pharmacological inhibition of complex CI (CI), nicotinamide nucleotide transhydrogenase (NNT) and antioxidant system significantly decreased the ability of mitochondria to exhibit mTRS. NADH levels followed a similar trend to that of ROS during the mTRS, providing a link between CI and NNT in mTRS regulation. We show that (i) mTRS is enhanced by simultaneous activation of CI and complex II (CII); (ii) CI is the principal origin of mTRS; (iii) NNT regulates mTRS via NADH- and ∆Ψ_mt-dependent mechanisms; (iv) mTRS is not a pH spike; and (v), mTRS changes in amplitude under stress conditions and its occurrence can be a signature of mitochondrial health. Collectively, we uncovered and characterized the biophysical properties and mechanisms of mTRS, and propose it as a potential diagnostic tool for CI-related dysfunctions, and as a biomarker of mitochondrial functional integrity.

Characterization of Frex as an NADH sensor for in vivo applications in the presence of NAD+ and at various pH values

The fluorescent biosensor Frex, recently introduced as a sensitive tool to quantify the NADH concentration in living cells, was characterized by time-integrated and time-resolved fluorescence spectroscopy regarding its applicability for in vivo measurements. Based on the purified sensor protein, it is shown that the NADH dependence of Frex fluorescence can be described by a Hill function with a concentration of half-maximal sensor response of K _D  ≈ 4 µM and a Hill coefficient of n ≈ 2. Increasing concentrations of NADH have moderate effects on the fluorescence lifetime of Frex, which changes by a factor of two from about 500 ps in the absence of NADH to 1 ns under fluorescence-saturating NADH concentrations. Therefore, the observed sevenfold rise of the fluorescence intensity is primarily ascribed to amplitude changes. Notably, the dynamic range of Frex sensitivity towards NADH highly depends on the NAD⁺ concentration, while the apparent K _D for NADH is only slightly affected. We found that NAD⁺ has a strong inhibitory effect on the fluorescence response of Frex during NADH sensing, with an apparent NAD⁺ dissociation constant of K _I  ≈ 400 µM. This finding was supported by fluorescence lifetime measurements, which showed that the addition of NAD⁺ hardly affects the fluorescence lifetime, but rather reduces the number of Frex molecules that are able to bind NADH. Furthermore, the fluorescence responses of Frex to NADH and NAD⁺ depend critically on pH and temperature. Thus, for in vivo applications of Frex, temperature and pH need to be strictly controlled or considered during data acquisition and analysis. If all these constraints are properly met, Frex fluorescence intensity measurements can be employed to estimate the minimum NADH concentration present within the cell at sufficiently low NAD⁺ concentrations below 100 µM.

Taking up the cudgels for the traditional reactive oxygen and nitrogen species detection assays and their use in the cardiovascular system

Reactive oxygen and nitrogen species (RONS such as H2O2, nitric oxide) confer redox regulation of essential cellular functions (e.g. differentiation, proliferation, migration, apoptosis), initiate and catalyze adaptive stress responses. In contrast, excessive formation of RONS caused by impaired break-down by cellular antioxidant systems and/or insufficient repair of the resulting oxidative damage of biomolecules may lead to appreciable impairment of cellular function and in the worst case to cell death, organ dysfunction and severe disease phenotypes of the entire organism. Therefore, the knowledge of the severity of oxidative stress and tissue specific localization is of great biological and clinical importance. However, at this level of investigation quantitative information may be enough. For the development of specific drugs, the cellular and subcellular localization of the sources of RONS or even the nature of the reactive species may be of great importance, and accordingly, more qualitative information is required. These two different philosophies currently compete with each other and their different needs (also reflected by different detection assays) often lead to controversial discussions within the redox research community. With the present review we want to shed some light on these different philosophies and needs (based on our personal views), but also to defend some of the traditional assays for the detection of RONS that work very well in our hands and to provide some guidelines how to use and interpret the results of these assays. We will also provide an overview on the "new assays" with a brief discussion on their strengths but also weaknesses and limitations.

New tools for redox biology: From imaging to manipulation

Redox reactions play a key role in maintaining essential biological processes. Deviations in redox pathways result in the development of various pathologies at cellular and organismal levels. Until recently, studies on transformations in the intracellular redox state have been significantly hampered in living systems. The genetically encoded indicators, based on fluorescent proteins, have provided new opportunities in biomedical research. The existing indicators already enable monitoring of cellular redox parameters in different processes including embryogenesis, aging, inflammation, tissue regeneration, and pathogenesis of various diseases. In this review, we summarize information about all genetically encoded redox indicators developed to date. We provide the description of each indicator and discuss its advantages and limitations, as well as points that need to be considered when choosing an indicator for a particular experiment. One chapter is devoted to the important discoveries that have been made by using genetically encoded redox indicators.

The Reactive Sulfur Species Concept: 15 Years On

Fifteen years ago, in 2001, the concept of “Reactive Sulfur Species” or RSS was advocated as a working hypothesis. Since then various organic as well as inorganic RSS have attracted considerable interest and stimulated many new and often unexpected avenues in research and product development. During this time, it has become apparent that molecules with sulfur-containing functional groups are not just the passive “victims” of oxidative stress or simple conveyors of signals in cells, but can also be stressors in their own right, with pivotal roles in cellular function and homeostasis. Many “exotic” sulfur-based compounds, often of natural origin, have entered the fray in the context of nutrition, ageing, chemoprevention and therapy. In parallel, the field of inorganic RSS has come to the forefront of research, with short-lived yet metabolically important intermediates, such as various sulfur-nitrogen species and polysulfides (S_x²⁻), playing important roles. Between 2003 and 2005 several breath-taking discoveries emerged characterising unusual sulfur redox states in biology, and since then the truly unique role of sulfur-dependent redox systems has become apparent. Following these discoveries, over the last decade a “hunt” and, more recently, mining for such modifications has begun—and still continues—often in conjunction with new, innovative and complex labelling and analytical methods to capture the (entire) sulfur “redoxome”. A key distinction for RSS is that, unlike oxygen or nitrogen, sulfur not only forms a plethora of specific reactive species, but sulfur also targets itself, as sulfur containing molecules, i.e., peptides, proteins and enzymes, preferentially react with RSS. Not surprisingly, today this sulfur-centred redox signalling and control inside the living cell is a burning issue, which has moved on from the predominantly thiol/disulfide biochemistry of the past to a complex labyrinth of interacting signalling and control pathways which involve various sulfur oxidation states, sulfur species and reactions. RSS are omnipresent and, in some instances, are even considered as the true bearers of redox control, perhaps being more important than the Reactive Oxygen Species (ROS) or Reactive Nitrogen Species (RNS) which for decades have dominated the redox field. In other(s) words, in 2017, sulfur redox is “on the rise”, and the idea of RSS resonates throughout the Life Sciences. Still, the RSS story isn’t over yet. Many RSS are at the heart of “mistaken identities” which urgently require clarification and may even provide the foundations for further scientific revolutions in the years to come. In light of these developments, it is therefore the perfect time to revisit the original hypotheses, to select highlights in the field and to question and eventually update our concept of “Reactive Sulfur Species”.

Absence of physiological Ca2+ transients is an initial trigger for mitochondrial dysfunction in skeletal muscle following denervation

BACKGROUND

Motor neurons control muscle contraction by initiating action potentials in muscle. Denervation of muscle from motor neurons leads to muscle atrophy, which is linked to mitochondrial dysfunction. It is known that denervation promotes mitochondrial reactive oxygen species (ROS) production in muscle, whereas the initial cause of mitochondrial ROS production in denervated muscle remains elusive. Since denervation isolates muscle from motor neurons and deprives it from any electric stimulation, no action potentials are initiated, and therefore, no physiological Ca²⁺ transients are generated inside denervated muscle fibers. We tested whether loss of physiological Ca²⁺ transients is an initial cause leading to mitochondrial dysfunction in denervated skeletal muscle.

METHODS

A transgenic mouse model expressing a mitochondrial targeted biosensor (mt-cpYFP) allowed a real-time measurement of the ROS-related mitochondrial metabolic function following denervation, termed "mitoflash." Using live cell imaging, electrophysiological, pharmacological, and biochemical studies, we examined a potential molecular mechanism that initiates ROS-related mitochondrial dysfunction following denervation.

RESULTS

We found that muscle fibers showed a fourfold increase in mitoflash activity 24 h after denervation. The denervation-induced mitoflash activity was likely associated with an increased activity of mitochondrial permeability transition pore (mPTP), as the mitoflash activity was attenuated by application of cyclosporine A. Electrical stimulation rapidly reduced mitoflash activity in both sham and denervated muscle fibers. We further demonstrated that the Ca²⁺ level inside mitochondria follows the time course of the cytosolic Ca²⁺ transient and that inhibition of mitochondrial Ca²⁺ uptake by Ru360 blocks the effect of electric stimulation on mitoflash activity.

CONCLUSIONS

The loss of cytosolic Ca²⁺ transients due to denervation results in the downstream absence of mitochondrial Ca²⁺ uptake. Our studies suggest that this could be an initial trigger for enhanced mPTP-related mitochondrial ROS generation in skeletal muscle.

Synchronism in mitochondrial ROS flashes, membrane depolarization and calcium sparks in human carcinoma cells

Mitochondria are major producers of reactive oxygen species (ROS) in many cells including cancer cells. However, complex interrelationships between mitochondrial ROS (mitoROS), mitochondrial membrane potential (ΔΨm) and Ca²⁺ are not completely understood. Using human carcinoma cells, we further highlight biphasic ROS dynamics: - gradual mitoROS increase followed by mitoROS flash. Also, we demonstrate heterogeneity in rates of mitoROS generation and flash initiation time. Comparing mitochondrial and near-extra-mitochondrial signals, we show that mechanisms of mitoROS flashes in single mitochondria, linked to mitochondrial permeability transition pore opening (ΔΨm collapse) and calcium sparks, may involve flash triggering by certain levels of external ROS released from the same mitochondria. In addition, mitochondria-mitochondria interactions can produce wave propagations of mitoROS flashes and ΔΨm collapses in cancer cells similar to phenomena of ROS-induced ROS release (RIRR). Our data suggest that in cancer cells RIRR, activation of mitoROS flashes and mitochondrial depolarization may involve participation of extramitochondrial-ROS produced either by individual mitochondria and/or by neighboring mitochondria. This could represent general mechanisms in ROS-ROS signaling with suggested role in both mitochondrial and cellular physiology and signaling.

L-OPA1 regulates mitoflash biogenesis independently from membrane fusion

Mitochondrial flashes mediated by optic atrophy 1 (OPA1) fusion protein are bioenergetic responses to stochastic drops in mitochondrial membrane potential (Δψ_m) whose origin is unclear. Using structurally distinct genetically encoded pH‐sensitive probes, we confirm that flashes are matrix alkalinization transients, thereby establishing the pH nature of these events, which we renamed “mitopHlashes”. Probes located in cristae or intermembrane space as verified by electron microscopy do not report pH changes during Δψ_m drops or respiratory chain inhibition. Opa1 ablation does not alter Δψ_m fluctuations but drastically decreases the efficiency of mitopHlash/Δψ_m coupling, which is restored by re‐expressing fusion‐deficient OPA1^K301A and preserved in cells lacking the outer‐membrane fusion proteins MFN1/2 or the OPA1 proteases OMA1 and YME1L, indicating that mitochondrial membrane fusion and OPA1 proteolytic processing are dispensable. pH/Δψ_m uncoupling occurs early during staurosporine‐induced apoptosis and is mitigated by OPA1 overexpression, suggesting that OPA1 maintains mitopHlash competence during stress conditions. We propose that OPA1 stabilizes respiratory chain supercomplexes in a conformation that enables respiring mitochondria to compensate a drop in Δψ_m by an explosive matrix pH flash.

Mitochondrial Ca2+ and regulation of the permeability transition pore

The mitochondrial permeability transition pore was originally described in the 1970's as a Ca2+ activated pore and has since been attributed to the pathogenesis of many diseases. Here we evaluate how each of the current models of the pore complex fit to what is known about how Ca2+ regulates the pore, and any insight that provides into the molecular identity of the pore complex. We also discuss the central role of Ca2+ in modulating the pore's open probability by directly regulating processes, such as ATP/ADP balance through the tricarboxylic acid cycle, electron transport chain, and mitochondrial membrane potential. We review how Ca2+ influences second messengers such as reactive oxygen/nitrogen species production and polyphosphate formation. We discuss the evidence for how Ca2+ regulates post-translational modification of cyclophilin D including phosphorylation by glycogen synthase kinase 3 beta, deacetylation by sirtuins, and oxidation/ nitrosylation of key residues. Lastly we introduce a novel view into how Ca2+ activated proteolysis through calpains in the mitochondria may be a driver of sustained pore opening during pathologies such as ischemia reperfusion injury.

Traditional and novel tools to probe the mitochondrial metabolism in health and disease

Mitochondrial metabolism links energy production to other essential cellular processes such as signaling, cellular differentiation, and apoptosis. In addition to producing adenosine triphosphate (ATP) as an energy source, mitochondria are responsible for the synthesis of a myriad of important metabolites and cofactors such as tetrahydrofolate, α-ketoacids, steroids, aminolevulinic acid, biotin, lipoic acid, acetyl-CoA, iron-sulfur clusters, heme, and ubiquinone. Furthermore, mitochondria and their metabolism have been implicated in aging and several human diseases, including inherited mitochondrial disorders, cardiac dysfunction, heart failure, neurodegenerative diseases, diabetes, and cancer. Therefore, there is great interest in understanding mitochondrial metabolism and the complex relationship it has with other cellular processes. A large number of studies on mitochondrial metabolism have been conducted in the last 50 years, taking a broad range of approaches. In this review, we summarize and discuss the most commonly used tools that have been used to study different aspects of the metabolism of mitochondria: ranging from dyes that monitor changes in the mitochondrial membrane potential and pharmacological tools to study respiration or ATP synthesis, to more modern tools such as genetically encoded biosensors and trans-omic approaches enabled by recent advances in mass spectrometry, computation, and other technologies. These tools have allowed the large number of studies that have shaped our current understanding of mitochondrial metabolism. WIREs Syst Biol Med 2017, 9:e1373. doi: 10.1002/wsbm.1373 For further resources related to this article, please visit the WIREs website.

Reactive Oxygen Species and Oxidative Stress in Obesity-Recent Findings and Empirical Approaches

OBJECTIVE

High levels of reactive oxygen species (ROS) are intricately linked to obesity and associated pathologies, notably insulin resistance and type 2 diabetes. However, ROS are also thought to be important in intracellular signaling, which may paradoxically be required for insulin sensitivity. Many theories have been developed to explain this apparent paradox, which have broadened our understanding of these important small molecules. While many sites for intracellular ROS production have been described, mitochondrial generated ROS remain a major contributor in most cell types. Mitochondrial ROS generation is controlled by a number of factors described in this review. Moreover, these studies have established both a demand for novel sensitive approaches to measure ROS, as well as a need to standardize and review their suitability for different applications.

METHODS

To properly assess levels of ROS and mitochondrial ROS in the development of obesity and its complications, a growing number of tools have been developed. This paper reviews many of the common methods for the investigation of ROS in mitochondria, cell, animal, and human models.

RESULTS

Available approaches can be generally divided into those that measure ROS-induced damage (e.g., DNA, lipid, and protein damage); those that measure antioxidant levels and redox ratios; and those that use novel biosensors and probes for a more direct measure of different forms of ROS (e.g., 2',7'-di-chlorofluorescein (DCF), dihydroethidium (DHE) and its mitochondrial targeted form (MitoSOX), Amplex Red, roGFP, HyPer, mt-cpYFP, ratiometric H₂ O₂ probes, and their derivatives). Moreover, this review provides caveats and strengths for the use of these techniques in different models.

CONCLUSIONS

Advances in these techniques will undoubtedly advance the understanding of ROS in obesity and may help resolve unanswered questions in the field.

Mitochondrial flashes: From indicator characterization to in vivo imaging

Mitochondrion is an organelle critically responsible for energy production and intracellular signaling in eukaryotic cells and its dysfunction often accompanies and contributes to human disease. Superoxide is the primary reactive oxygen species (ROS) produced in mitochondria. In vivo detection of superoxide has been a challenge in biomedical research. Here we describe the methods used to characterize a circularly permuted yellow fluorescent protein (cpYFP) as a biosensor for mitochondrial superoxide and pH dynamics. In vitro characterization reveals the high selectivity of cpYFP to superoxide over other ROS species and its dual sensitivity to pH. Confocal and two-photon imaging in conjunction with transgenic expression of the biosensor cpYFP targeted to the mitochondrial matrix detects mitochondrial flash events in living cells, perfused intact hearts, and live animals. The mitochondrial flashes are discrete and stochastic single mitochondrial events triggered by transient mitochondrial permeability transition (tMPT) and composed of a bursting superoxide signal and a transient alkalization signal. The real-time monitoring of single mitochondrial flashes provides a unique tool to study the integrated dynamism of mitochondrial respiration, ROS production, pH regulation and tMPT kinetics under diverse physiological and pathophysiological conditions.

In Vivo Detection of Reactive Oxygen Species and Redox Status in Caenorhabditis elegans

SIGNIFICANCE

Due to its large families of redox-active enzymes, genetic amenability, and complete transparency, the nematode Caenorhabditis elegans has the potential to become an important model for the in vivo study of redox biology.

RECENT ADVANCES

The recent development of several genetically encoded ratiometric reactive oxygen species (ROS) and redox sensors has revolutionized the quantification and precise localization of ROS and redox signals in living organisms. Only few exploratory studies have applied these sensors in C. elegans and undoubtedly much remains to be discovered in this model. As a follow-up to our recent findings that the C. elegans somatic gonad uses superoxide and hydrogen peroxide (H2O2) signals to communicate with the germline, we here analyze the patterns of H2O2 inside the C. elegans germline.

CRITICAL ISSUES

Despite the advantages of genetically encoded ROS and redox sensors over classic chemical sensors, still several general as well as C. elegans-specific issues need to be addressed. The major concerns for the application of these sensors in C. elegans are (i) decreased vitality of some reporter strains, (ii) interference of autofluorescent compartments with the sensor signal, and (iii) the use of immobilization methods that do not influence the worm's redox physiology.

FUTURE DIRECTIONS

We propose that several of the current issues may be solved by designing reporter strains carrying single copies of codon-optimized sensors. Preferably, these sensors should have their emission wavelengths in the red region, where autofluorescence is absent. Worm analysis could be optimized using four-dimensional ratiometric fluorescence microscopy of worms immobilized in microfluidic chips. Antioxid. Redox Signal. 25, 577-592.

Mitochondrial Flashes: Dump Superoxide and Dance with Protons Now

Transient changes in the physiology of individual mitochondria have recently drawn much interest. The use of a circular permuted yellow fluorescent protein (cpYFP) to monitor mitochondrial flashes and their interpretation as superoxide bursts has added confusion, however. Reviewing mitochondrial flashes in this Forum, Wang et al. again deem cpYFP to be a specific and reversible superoxide indicator, dismissing evidence that purified cpYFP is insensitive to superoxide. This interpretation lacks reproducible evidence and conflicts with the parsimony principle. We offer a constructive, transparent pathway to reach definitive clarification of contradictory reports. Antioxid. Redox Signal. 25, 550-551.

Mitochondrial Flash: Integrative Reactive Oxygen Species and pH Signals in Cell and Organelle Biology

SIGNIFICANCE

Recent breakthroughs in mitochondrial research have advanced, reshaped, and revolutionized our view of the role of mitochondria in health and disease. These discoveries include the development of novel tools to probe mitochondrial biology, the molecular identification of mitochondrial functional proteins, and the emergence of new concepts and mechanisms in mitochondrial function regulation. The discovery of "mitochondrial flash" activity has provided unique insights not only into real-time visualization of individual mitochondrial redox and pH dynamics in live cells but has also advanced understanding of the excitability, autonomy, and integration of mitochondrial function in vivo.

RECENT ADVANCES

The mitochondrial flash is a transient and stochastic event confined within an individual mitochondrion and is observed in a wide range of organisms from plants to Caenorhabditis elegans to mammals. As flash events involve multiple transient concurrent changes within the mitochondrion (e.g., superoxide, pH, and membrane potential), a number of different mitochondrial targeted fluorescent indicators can detect flash activity. Accumulating evidence indicates that flash events reflect integrated snapshots of an intermittent mitochondrial process arising from mitochondrial respiration chain activity associated with the transient opening of the mitochondrial permeability transition pore.

CRITICAL ISSUES

We review the history of flash discovery, summarize current understanding of flash biology, highlight controversies regarding the relative roles of superoxide and pH signals during a flash event, and bring forth the integration of both signals in flash genesis.

FUTURE DIRECTIONS

Investigations using flash as a biomarker and establishing its role in cell signaling pathway will move the field forward. Antioxid. Redox Signal. 25, 534-549.

Beyond the Cuvette: Redox Indicators in Biological Experiments

Redox signaling is involved in numerous physiological and pathological processes (cell cycle, gene transcription, calcium signaling, stress response, ischemia-reperfusion injury, etc.). However, its exact role in cell biology and physiology remains poorly understood, mostly due to the technical challenges that the experimenter faces while trying to detect reactive oxygen species (ROS) or redox species with adequate specificity, spatial, and temporal accuracy. Recently, tremendous efforts have been put into the development of techniques for redox detection. This Forum focuses on ex and in vivo live-imaging of ROS and redox species using fluorescent dyes. Antioxid. Redox Signal. 25, 517-519.

Regulation of Mitoflash Biogenesis and Signaling by Mitochondrial Dynamics

Mitochondria are highly dynamic organelles undergoing constant network reorganization and exhibiting stochastic signaling events in the form of mitochondrial flashes (mitoflashes). Here we investigate whether and how mitochondrial network dynamics regulate mitoflash biogenesis and signaling. We found that mitoflash frequency was largely invariant when network fragmentized or redistributed in the absence of mitofusin (Mfn) 1, Mfn2, or Kif5b. However, Opa1 deficiency decreased spontaneous mitoflash frequency due to superimposing changes in respiratory function, whereas mitoflash response to non-metabolic stimulation was unchanged despite network fragmentation. In Drp1- or Mff-deficient cells whose mitochondria hyperfused into a single whole-cell reticulum, the frequency of mitoflashes of regular amplitude and duration was again unaltered, although brief and low-amplitude “miniflashes” emerged because of improved detection ability. As the network reorganized, however, the signal mass of mitoflash signaling was dynamically regulated in accordance with the degree of network connectivity. These findings demonstrate a novel functional role of mitochondrial network dynamics and uncover a magnitude- rather than frequency-modulatory mechanism in the regulation of mitoflash signaling. In addition, our data support a stochastic trigger model for the ignition of mitoflashes.

The effect of chronic alcohol consumption on mitochondrial calcium handling in hepatocytes

The damage to liver mitochondria is universally observed in both humans and animal models after excessive alcohol consumption. Acute alcohol treatment has been shown to stimulate calcium (Ca²⁺) release from internal stores in hepatocytes. The resultant increase in cytosolic Ca²⁺ is expected to be accumulated by neighboring mitochondria, which could potentially lead to mitochondrial Ca²⁺ overload and injury. Our data indicate that total and free mitochondrial matrix Ca²⁺ levels are, indeed, elevated in hepatocytes isolated from alcohol-fed rats compared with their pair-fed control littermates. In permeabilized hepatocytes, the rates of mitochondrial Ca²⁺ uptake were substantially increased after chronic alcohol feeding, whereas those of mitochondrial Ca²⁺ efflux were decreased. The changes in mitochondrial Ca²⁺ handling could be explained by an up-regulation of the mitochondrial Ca²⁺ uniporter and loss of a cyclosporin A-sensitive Ca²⁺ transport pathway. In intact cells, hormone-induced increases in mitochondrial Ca²⁺ declined at slower rates leading to more prolonged elevations of matrix Ca²⁺ in the alcohol-fed group compared with controls. Moreover, treatment with submaximal concentrations of Ca²⁺-mobilizing hormones markedly increased the levels of mitochondrial reactive oxygen species (ROS) in hepatocytes from alcohol-fed rats, but did not affect ROS levels in controls. The changes in mitochondrial Ca²⁺ handling are expected to buffer and attenuate cytosolic Ca²⁺ increases induced by acute alcohol exposure or hormone stimulation. However, these alterations in mitochondrial Ca²⁺ handling may also lead to Ca²⁺ overload during cytosolic Ca²⁺ increases, which may stimulate the production of mitochondrial ROS, and thus contribute to alcohol-induced liver injury.

The Plant Mitochondrial Transportome: Balancing Metabolic Demands with Energetic Constraints

In plants, mitochondrial function is associated with hundreds of metabolic reactions. To facilitate these reactions, charged substrates and cofactors move across the charge-impermeable inner mitochondrial membrane via specialized transporters and must work cooperatively with the electrochemical gradient which is essential for mitochondrial function. The regulatory framework for mitochondrial metabolite transport is expected to be more complex in plants than in mammals owing to the close metabolic association between mitochondrial, plastids, and peroxisome metabolism, as well as to the major diurnal fluctuations in plant metabolic function. We propose here how recent advances can be integrated towards defining the mitochondrial transportome in plants. We also discuss what this reveals about sustaining cooperativity between bioenergetics, metabolism, and transport in typical and challenging environments.

The Roles of Mitochondrial Reactive Oxygen Species in Cellular Signaling and Stress Response in Plants

Mitochondria produce ATP via respiratory oxidation of organic acids and transfer of electrons to O2 via the mitochondrial electron transport chain. This process produces reactive oxygen species (ROS) at various rates that can impact respiratory and cellular function, affecting a variety of signaling processes in the cell. Roles in redox signaling, retrograde signaling, plant hormone action, programmed cell death, and defense against pathogens have been attributed to ROS generated in plant mitochondria (mtROS). The shortcomings of the black box-idea of mtROS are discussed in the context of mechanistic considerations and the measurement of mtROS The overall aim of this update is to better define our current understanding of mtROS and appraise their potential influence on cellular function in plants. Furthermore, directions for future research are provided, along with suggestions to increase reliability of mtROS measurements.

Practical guide for dynamic monitoring of protein oxidation using genetically encoded ratiometric fluorescent biosensors of methionine sulfoxide

In cells, physiological and pathophysiological conditions may lead to the formation of methionine sulfoxide (MetO). This oxidative modification of methionine exists in the form of two diastereomers, R and S, and may occur in both free amino acid and proteins. MetO is reduced back to methionine by methionine sulfoxide reductases (MSRs). Methionine oxidation was thought to be a nonspecific modification affecting protein functions and methionine availability. However, recent findings suggest that cyclic methionine oxidation and reduction is a posttranslational modification that actively regulates protein function akin to redox regulation by cysteine oxidation and phosphorylation. Methionine oxidation is thus an important mechanism that could play out in various physiological contexts. However, detecting MetO generation and MSR functions remains challenging because of the lack of tools and reagents to detect and quantify this protein modification. We recently developed two genetically encoded diasterospecific fluorescent sensors, MetSOx and MetROx, to dynamically monitor MetO in living cells. Here, we provide a detailed procedure for their use in bacterial and mammalian cells using fluorimetric and fluorescent imaging approaches. This method can be adapted to dynamically monitor methionine oxidation in various cell types and under various conditions.

An electrochemical platform for localized pH control on demand

Solution pH is a powerful tool for regulating many kinds of chemical activity, but is generally treated as a static property defined by a pre-selected buffer. Introducing dynamic control of pH in space, time, and magnitude can enable richer and more efficient chemistries, but is not feasible with traditional methods of titration or buffer exchange. Recent reports have featured electrochemical strategies for modifying bulk pH in constrained volumes, but only demonstrate switching between two preset values and omit spatial control entirely. Here, we use a combination of solution-borne quinones and galvanostatic excitation to enable quantitative control of pH environments that are highly localized to an electrode surface. We demonstrate highly reproducible acidification and alkalinization with up to 0.1 pH s(-1) (±0.002 pH s(-1)) rate of change across the dynamic range of our pH sensor (pH 4.5 to 7.5) in buffered solutions. Using dynamic current control, we generate and sustain 3 distinct pH microenvironments simultaneously to within ±0.04 pH for 13 minutes in a single solution, and we leverage these microenvironments to demonstrate spatially-resolved, pH-driven control of enzymatic activity. In addition to straightforward applications of spatio-temporal pH control (e.g. efficiently studying pH-dependencies of chemical interactions), the technique opens completely new avenues for implementing complex systems through dynamic control of enzyme activation, protein binding affinity, chemical reactivity, chemical release, molecular self-assembly, and many more pH-controlled processes.

Techniques for the Analysis of Protein-Protein Interactions in Vivo

Identifying key players and their interactions is fundamental for understanding biochemical mechanisms at the molecular level. The ever-increasing number of alternative ways to detect protein-protein interactions (PPIs) speaks volumes about the creativity of scientists in hunting for the optimal technique. PPIs derived from single experiments or high-throughput screens enable the decoding of binary interactions, the building of large-scale interaction maps of single organisms, and the establishment of cross-species networks. This review provides a historical view of the development of PPI technology over the past three decades, particularly focusing on in vivo PPI techniques that are inexpensive to perform and/or easy to implement in a state-of-the-art molecular biology laboratory. Special emphasis is given to their feasibility and application for plant biology as well as recent improvements or additions to these established techniques. The biology behind each method and its advantages and disadvantages are discussed in detail, as are the design, execution, and evaluation of PPI analysis. We also aim to raise awareness about the technological considerations and the inherent flaws of these methods, which may have an impact on the biological interpretation of PPIs. Ultimately, we hope this review serves as a useful reference when choosing the most suitable PPI technique.

Challenging the dogma of mitochondrial reactive oxygen species overproduction in diabetic kidney disease

The paradigm that high glucose drives overproduction of superoxide from mitochondria as a unifying theory to explain end organ damage in diabetes complications has been tightly held for more than a decade. With the recent development of techniques and probes to measure the production of distinct reactive oxygen species (ROS) in vivo, this widely held dogma is now being challenged with the emerging view that specific ROS moieties are essential for the function of specific intracellular signaling pathways and represent normal mitochondrial function. This review will provide a balanced overview of the dual nature of ROS, detailing current evidence for ROS overproduction in diabetic kidney disease, with a focus on cell types and sources of ROS. The technical aspects of measurement of mitochondrial ROS, both in isolated mitochondria and emerging in vivo methods will be discussed. The counterargument, that mitochondrial ROS production is reduced in diabetic complications, is consistent with a growing recognition that stimulation of mitochondrial biogenesis and oxidative phosphorylation activity reduces inflammation and fibrosis. It is clear that there is an urgent need to fully characterize ROS production paying particular attention to spatiotemporal aspects and to factor in the relevance of ROS in the regulation of cellular signaling in the pathogenesis of diabetic kidney disease. With improved tools and real-time imaging capacity, a greater understanding of the complex role of ROS will be able to guide novel therapeutic regimens.

Live-cell imaging approaches for the investigation of xenobiotic-induced oxidant stress

BACKGROUND

Oxidant stress is arguably a universal feature in toxicology. Research studies on the role of oxidant stress induced by xenobiotic exposures have typically relied on the identification of damaged biomolecules using a variety of conventional biochemical and molecular techniques. However, there is increasing evidence that low-level exposure to a variety of toxicants dysregulates cellular physiology by interfering with redox-dependent processes.

SCOPE OF REVIEW

The study of events involved in redox toxicology requires methodology capable of detecting transient modifications at relatively low signal strength. This article reviews the advantages of live-cell imaging for redox toxicology studies.

MAJOR CONCLUSIONS

Toxicological studies with xenobiotics of supra-physiological reactivity require careful consideration when using fluorogenic sensors in order to avoid potential artifacts and false negatives. Fortunately, experiments conducted for the purpose of validating the use of these sensors in toxicological applications often yield unexpected insights into the mechanisms through which xenobiotic exposure induces oxidant stress.

GENERAL SIGNIFICANCE

Live-cell imaging using a new generation of small molecule and genetically encoded fluorophores with excellent sensitivity and specificity affords unprecedented spatiotemporal resolution that is optimal for redox toxicology studies. This article is part of a Special Issue entitled Air Pollution, edited by Wenjun Ding, Andrew J. Ghio and Weidong Wu.

Recent advances and long-standing problems in detecting oxidative damage and reactive oxygen species in skeletal muscle

An increasingly sophisticated array of approaches are now available for the study of the activities of reactive oxygen species and oxidative modifications in skeletal muscle, but the most up-to-date techniques are not readily available to many researchers in this field due to their requirement for sophisticated mass spectrometry, imaging or other high cost technologies. Most papers published therefore rely on a number of established approaches although the choice of approach is also clearly dependent upon the experimental model and access to skeletal muscle that is available to the investigator, how much detail is required and the overall question to be addressed. Numerous reports have described the problems associated with some of the popular approaches that are widely followed, including measurement of thiobarbituric acid substances and the sole use of fluorescence-based probes such as dichlorodihydrofluorescein. This brief review reports the areas in which methods are improving to allow valid assessments to made in this area and indicates some of the more recent developments that provide alternative ways to assess the activity of individual species and endpoints in the various experimental models that may be examined.

Subcellular ROS imaging methods: Relevance for the study of calcium signaling

Recent advances in genetically encoded fluorescent probes have dramatically increased the toolkit available for imaging the intracellular environment. Perhaps the biggest improvements have been made in sensing specific reactive oxygen species (ROS) and redox changes under physiological conditions. The new generation of probes may be targeted to a wide range of subcellular environments. By targeting such probes to compartments and organelle surfaces they may be exposed to environments, which support local signal transduction and regulation. The close apposition of the endoplasmic reticulum (ER) with mitochondria and other organelles forms such a local environment where Ca(2+) dynamics are greatly enhanced compared to the bulk cytosol. We describe here how newly developed genetically encoded redox indicators (GERIs) might be used to monitor ROS and probe their interaction with Ca(2+) at both global and local level.

Making microscopy count: quantitative light microscopy of dynamic processes in living plants

Cell theory has officially reached 350 years of age as the first use of the word 'cell' in a biological context can be traced to a description of plant material by Robert Hooke in his historic publication 'Micrographia: or some physiological definitions of minute bodies'. The 2015 Royal Microscopical Society Botanical Microscopy meeting was a celebration of the streams of investigation initiated by Hooke to understand at the subcellular scale how plant cell function and form arises. Much of the work presented, and Honorary Fellowships awarded, reflected the advanced application of bioimaging informatics to extract quantitative data from micrographs that reveal dynamic molecular processes driving cell growth and physiology. The field has progressed from collecting many pixels in multiple modes to associating these measurements with objects or features that are meaningful biologically. The additional complexity involves object identification that draws on a different type of expertise from computer science and statistics that is often impenetrable to biologists. There are many useful tools and approaches being developed, but we now need more interdisciplinary exchange to use them effectively. In this review we show how this quiet revolution has provided tools available to any personal computer user. We also discuss the oft-neglected issue of quantifying algorithm robustness and the exciting possibilities offered through the integration of physiological information generated by biosensors with object detection and tracking.

Quantitative Redox Imaging Software

SIGNIFICANCE

A wealth of fluorescent reporters and imaging systems are now available to characterize dynamic physiological processes in living cells with high spatiotemporal resolution. The most reliable probes for quantitative measurements show shifts in their excitation or emission spectrum, rather than just a change in intensity, as spectral shifts are independent of optical path length, illumination intensity, probe concentration, and photobleaching, and they can be easily determined by ratiometric measurements at two wavelengths.

RECENT ADVANCES

A number of ratiometric fluorescent reporters, such as reduction-oxidation-sensitive green fluorescent protein (roGFP), have been developed that respond to the glutathione redox potential and allow redox imaging in vivo. roGFP and its derivatives can be expressed in the cytoplasm or targeted to different organelles, giving fine control of measurements from sub-cellular compartments. Furthermore, roGFP can be imaged with probes for other physiological parameters, such as reactive oxygen species or mitochondrial membrane potential, to give multi-channel, multi-dimensional 4D (x,y,z,t) images.

CRITICAL ISSUES

Live cell imaging approaches are needed to capture transient or highly spatially localized physiological behavior from intact, living specimens, which are often not accessible by other biochemical or genetic means.

FUTURE DIRECTIONS

The next challenge is to be able to extract useful data rapidly from such large (GByte) images with due care given to the assumptions used during image processing. This article describes a suite of software programs, available for download, that provide intuitive user interfaces to conduct multi-channel ratio imaging, or alternative analysis methods such as pixel-population statistics or image segmentation and object-based ratio analysis. Antioxid. Redox Signal. 24, 752-762.

Dissecting Redox Biology Using Fluorescent Protein Sensors

SIGNIFICANCE

Fluorescent protein sensors have revitalized the field of redox biology by revolutionizing the study of redox processes in living cells and organisms.

RECENT ADVANCES

Within one decade, a set of fundamental new insights has been gained, driven by the rapid technical development of in vivo redox sensing. Redox-sensitive yellow and green fluorescent protein variants (rxYFP and roGFPs) have been the central players.

CRITICAL ISSUES

Although widely used as an established standard tool, important questions remain surrounding their meaningful use in vivo. We review the growing range of thiol redox sensor variants and their application in different cells, tissues, and organisms. We highlight five key findings where in vivo sensing has been instrumental in changing our understanding of redox biology, critically assess the interpretation of in vivo redox data, and discuss technical and biological limitations of current redox sensors and sensing approaches.

FUTURE DIRECTIONS

We explore how novel sensor variants may further add to the current momentum toward a novel mechanistic and integrated understanding of redox biology in vivo. Antioxid. Redox Signal. 24, 680-712.

Imaging calcium and redox signals using genetically encoded fluorescent indicators

Calcium and redox signals are presently established as essential regulators of many cellular processes. Nevertheless, we are still far from fully understanding the physiological and pathological importance of these universal second messengers. It is becoming increasingly apparent that many cellular functions are not regulated by global changes in the abundance of Ca(2+) ions and/or reactive oxygen and nitrogen species (ROS and RNS), but by the formation of transient local micro-domains or by signaling limited to a particular cellular compartment. Therefore, it is essential to identify and quantify Ca(2+) and redox signals in single cells with a high spatial and temporal resolution. The best tools for this purpose are the genetically encoded fluorescent indicators (GEFI). These protein sensors can be targeted into different cellular compartments, feature different colors, can be used to establish transgenic animal models, and are relatively inert to the cellular environment. Based on the chemical properties of Ca(2+) and ROS/RNS, currently more sensors exist for the detection of Ca(2+)- than for redox signals. Here, we shortly describe the most popular genetically encoded fluorescent Ca(2+) and redox indicators, discuss advantages and disadvantages based on our experience, show examples of different applications, and thus provide a brief guide that will help scientists choose the right combination of Ca(2+) and redox sensors to answer specific scientific questions.

2016: A 'Mitochondria' Odyssey

The integration of the many roles of mitochondria in cellular function and the contribution of mitochondrial dysfunction to disease are major areas of research. Within this realm, the roles of mitochondria in immune defense, epigenetics, and stem cell (SC) development have recently come into the spotlight. With new understanding, mitochondria may bring together these seemingly unrelated fields, a crucial process in treatment and prevention for various diseases. In this review we describe novel findings in these three arenas, discussing the significance of the interplay between mitochondria and the cell nucleus in response to environmental cues. While we optimistically anticipate that further research in these areas can have a profound impact on disease management, we also bring forth some of the key questions and challenges that remain.

Mitochondrial redox and pH signaling occurs in axonal and synaptic organelle clusters

Redox switches are important mediators in neoplastic, cardiovascular and neurological disorders. We recently identified spontaneous redox signals in neurons at the single mitochondrion level where transients of glutathione oxidation go along with shortening and re-elongation of the organelle. We now have developed advanced image and signal-processing methods to re-assess and extend previously obtained data. Here we analyze redox and pH signals of entire mitochondrial populations. In total, we quantified the effects of 628 redox and pH events in 1797 mitochondria from intercostal axons and neuromuscular synapses using optical sensors (mito-Grx1-roGFP2; mito-SypHer). We show that neuronal mitochondria can undergo multiple redox cycles exhibiting markedly different signal characteristics compared to single redox events. Redox and pH events occur more often in mitochondrial clusters (medium cluster size: 34.1 ± 4.8 μm(2)). Local clusters possess higher mitochondrial densities than the rest of the axon, suggesting morphological and functional inter-mitochondrial coupling. We find that cluster formation is redox sensitive and can be blocked by the antioxidant MitoQ. In a nerve crush paradigm, mitochondrial clusters form sequentially adjacent to the lesion site and oxidation spreads between mitochondria. Our methodology combines optical bioenergetics and advanced signal processing and allows quantitative assessment of entire mitochondrial populations.

Mitochondrial reactive oxygen species: Do they extend or shorten animal lifespan?

Testing the predictions of the Mitochondrial Free Radical Theory of Ageing (MFRTA) has provided a deep understanding of the role of reactive oxygen species (ROS) and mitochondria in the aging process. However those data, which support MFRTA are in the majority correlative (e.g. increasing oxidative damage with age). In contrast the majority of direct experimental data contradict MFRTA (e.g. changes in ROS levels do not alter longevity as expected). Unfortunately, in the past, ROS measurements have mainly been performed using isolated mitochondria, a method which is prone to experimental artifacts and does not reflect the complexity of the in vivo process. New technology to study different ROS (e.g. superoxide or hydrogen peroxide) in vivo is now available; these new methods combined with state-of-the-art genetic engineering technology will allow a deeper interrogation of, where, when and how free radicals affect aging and pathological processes. In fact data that combine these new approaches, indicate that boosting mitochondrial ROS in lower animals is a way to extend both healthy and maximum lifespan. In this review, I discuss the latest literature focused on the role of mitochondrial ROS in aging, and how these new discoveries are helping to better understand the role of mitochondria in health and disease. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

The biosensor toolbox for plant developmental biology

Plant development is highly interconnected with the metabolic state of tissues and cells. Current research efforts focus on the identification of the links and mechanisms that govern the interplay between metabolic and gene-regulatory networks. Genetically encoded sensors that allow detection of small molecules in vivo and at high spatio-temporal resolution promise to be the tools of choice for quantifying and visualizing the dynamics of metabolite flux in plants. We provide an overview about current approaches to measure signaling molecules, such as hormones, calcium and sugars, as well as for monitoring the metabolic state via energy equivalents and pH. Biosensors show great potential to address questions of plant development but there are also limitations where alternative approaches are needed.

Therapeutic inhibition of mitochondrial reactive oxygen species with mito-TEMPO reduces diabetic cardiomyopathy

AIMS

The mitochondria are important sources of reactive oxygen species (ROS) in the heart. Mitochondrial ROS production has been implicated in the pathogenesis of diabetic cardiomyopathy, suggesting that therapeutic strategies specifically targeting mitochondrial ROS may have benefit in this disease. We investigated the therapeutic effects of mitochondria-targeted antioxidant mito-TEMPO on diabetic cardiomyopathy.

METHODS

The mitochondria-targeted antioxidant mito-TEMPO was administrated after diabetes onset in a mouse model of streptozotocin-induced type-1 diabetes and type-2 diabetic db/db mice. Cardiac adverse changes were analyzed and myocardial function assessed. Cultured adult cardiomyocytes were stimulated with high glucose, and mitochondrial superoxide generation and cell death were measured.

RESULTS

Incubation with high glucose increased mitochondria superoxide generation in cultured cardiomyocytes, which was prevented by mito-TEMPO. Co-incubation with mito-TEMPO abrogated high glucose-induced cell death. Mitochondrial ROS generation, and intracellular oxidative stress levels were induced in both type-1 and type-2 diabetic mouse hearts. Daily injection of mito-TEMPO for 30 days inhibited mitochondrial ROS generation, prevented intracellular oxidative stress levels, decreased apoptosis and reduced myocardial hypertrophy in diabetic hearts, leading to improvement of myocardial function in both type-1 and type-2 diabetic mice. Incubation with mito-TEMPO or inhibition of Nox2-containing NADPH oxidase prevented oxidative stress levels and cell death in high glucose-stimulated cardiomyocytes. Mechanistic study revealed that the protective effects of mito-TEMPO were associated with down-regulation of ERK1/2 phosphorylation.

CONCLUSIONS

Therapeutic inhibition of mitochondrial ROS by mito-TEMPO reduced adverse cardiac changes and mitigated myocardial dysfunction in diabetic mice. Thus, mitochondria-targeted antioxidants may be an effective therapy for diabetic cardiac complications.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

Levels and location are crucial in determining the effect of ROS on lifespan

Reactive oxygen species (ROS) cause molecular damage that accumulates with age and have been proposed to be one of the primary causes of aging. However, recent work indicates that ROS have beneficial roles in an organism and that the relationship between ROS and aging is complex. We have shown that increasing ROS levels or oxidative damage does not necessarily lead to decreased lifespan. We have also shown that in some cases increasing ROS can promote longevity. Further investigation of the factors that determine the effect of ROS on lifespan demonstrate that both the levels and location of ROS are important in predicting the impact of ROS on longevity. Increasing superoxide levels in the cytoplasm results in decreased lifespan, while increasing superoxide levels in the mitochondria leads to increased lifespan. Within the mitochondria, mild elevation of superoxide levels promote longevity, while high levels of superoxide are toxic. Thus, a new paradigm is emerging in which ROS are neither good nor bad but levels and location makes it so.

ROS signaling and redox biology in endothelial cells

The purpose of this review is to provide an overview of redox mechanisms, sources and antioxidants that control signaling events in ECs. In particular, we describe which molecules are involved in redox signaling and how they influence the relationship between ECs and other vascular component with regard to angiogenesis. Recent and new tools to investigate physiological ROS signaling will be also discussed. Such findings are providing an overview of the ROS biology relevant for endothelial cells in the context of normal and pathological angiogenic conditions.

Mitochondrial flash as a novel biomarker of mitochondrial respiration in the heart

Mitochondrial respiration through electron transport chain (ETC) activity generates ATP and reactive oxygen species in eukaryotic cells. The modulation of mitochondrial respiration in vivo or under physiological conditions remains elusive largely due to the lack of appropriate approach to monitor ETC activity in a real-time manner. Here, we show that ETC-coupled mitochondrial flash is a novel biomarker for monitoring mitochondrial respiration under pathophysiological conditions in cultured adult cardiac myocyte and perfused beating heart. Through real-time confocal imaging, we follow the frequency of a transient bursting fluorescent signal, named mitochondrial flash, from individual mitochondria within intact cells expressing a mitochondrial matrix-targeted probe, mt-cpYFP (mitochondrial-circularly permuted yellow fluorescent protein). This mt-cpYFP recorded mitochondrial flash has been shown to be composed of a major superoxide signal with a minor alkalization signal within the mitochondrial matrix. Through manipulating physiological substrates for mitochondrial respiration, we find a close coupling between flash frequency and the ETC electron flow, as measured by oxygen consumption rate in cardiac myocyte. Stimulating electron flow under physiological conditions increases flash frequency. On the other hand, partially block or slowdown electron flow by inhibiting the F0F1 ATPase, which represents a pathological condition, transiently increases then decreases flash frequency. Limiting electron entrance at complex I by knocking out Ndufs4, an assembling subunit of complex I, suppresses mitochondrial flash activity. These results suggest that mitochondrial electron flow can be monitored by real-time imaging of mitochondrial flash. The mitochondrial flash frequency could be used as a novel biomarker for mitochondrial respiration under physiological and pathological conditions.

Interactions between mitochondrial reactive oxygen species and cellular glucose metabolism

Mitochondrial reactive oxygen species (ROS) production and detoxification are tightly balanced. Shifting this balance enables ROS to activate intracellular signaling and/or induce cellular damage and cell death. Increased mitochondrial ROS production is observed in a number of pathological conditions characterized by mitochondrial dysfunction. One important hallmark of these diseases is enhanced glycolytic activity and low or impaired oxidative phosphorylation. This suggests that ROS is involved in glycolysis (dys)regulation and vice versa. Here we focus on the bidirectional link between ROS and the regulation of glucose metabolism. To this end, we provide a basic introduction into mitochondrial energy metabolism, ROS generation and redox homeostasis. Next, we discuss the interactions between cellular glucose metabolism and ROS. ROS-stimulated cellular glucose uptake can stimulate both ROS production and scavenging. When glucose-stimulated ROS production, leading to further glucose uptake, is not adequately counterbalanced by (glucose-stimulated) ROS scavenging systems, a toxic cycle is triggered, ultimately leading to cell death. Here we inventoried the various cellular regulatory mechanisms and negative feedback loops that prevent this cycle from occurring. It is concluded that more insight in these processes is required to understand why they are (un)able to prevent excessive ROS production during various pathological conditions in humans.

Mitochondrial activity in gametes and transmission of viable mtDNA

BACKGROUND

The retention of a genome in mitochondria (mtDNA) has several consequences, among which the problem of ensuring a faithful transmission of its genetic information through generations despite the accumulation of oxidative damage by reactive oxygen species (ROS) predicted by the free radical theory of ageing. A division of labour between male and female germ line mitochondria was proposed: since mtDNA is maternally inherited, female gametes would prevent damages by repressing oxidative phosphorylation, thus being quiescent genetic templates. We assessed mitochondrial activity in gametes of an unusual biological system (doubly uniparental inheritance of mitochondria, DUI), in which also sperm mtDNA is transmitted to the progeny, thus having to overcome the problem of maintaining genetic information viability while producing ATP for swimming.

RESULTS

Ultrastructural analysis shows no difference in the conformation of mitochondrial cristae in male and female mature gametes, while mitochondria in immature oocytes exhibit a simpler internal structure. Our data on transcriptional activity in germ line mitochondria show variability between sexes and different developmental stages, but we do not find evidence for transcriptional quiescence of mitochondria. Our observations on mitochondrial membrane potential are consistent with mitochondria being active in both male and female gametes.

CONCLUSIONS

Our findings and the literature we discussed may be consistent with the hypothesis that template mitochondria are not functionally silenced, on the contrary their activity might be fundamental for the inheritance mechanism. We think that during gametogenesis, fertilization and embryo development, mitochondria undergo selection for different traits (e.g. replication, membrane potential), increasing the probability of the transmission of functional organelles. In these phases of life cycle, the great reduction in mtDNA copy number per organelle/cell and the stochastic segregation of mtDNA variants would greatly improve the efficiency of selection. When a higher mtDNA copy number per organelle/cell is present, selection on mtDNA deleterious mutants is less effective, due to the buffering effect of wild-type variants. In our opinion, a combination of drift and selection on germ line mtDNA population, might be responsible for the maintenance of viable mitochondrial genetic information through generations, and a mitochondrial activity would be necessary for the selective process.

Monitoring methionine sulfoxide with stereospecific mechanism-based fluorescent sensors

Methionine can be reversibly oxidized to methionine sulfoxide (MetO) under physiological and pathophysiological conditions, but its use as a redox marker suffers from the lack of tools to detect and quantify MetO within cells. In this work, we created a pair of complementary stereospecific genetically encoded mechanism-based ratiometric fluorescent sensors of MetO by inserting a circularly permuted yellow fluorescent protein between yeast methionine sulfoxide reductases and thioredoxins. The two sensors, respectively named MetSOx and MetROx for their ability to detect S and R forms of MetO, were used for targeted analysis of protein oxidation, regulation and repair as well as for monitoring MetO in bacterial and mammalian cells, analyzing compartment-specific changes in MetO and examining responses to physiological stimuli.

Astrocyte sodium signaling and neuro-metabolic coupling in the brain

At tripartite synapses, astrocytes undergo calcium signaling in response to release of neurotransmitters and this calcium signaling has been proposed to play a critical role in neuron-glia interaction. Recent work has now firmly established that, in addition, neuronal activity also evokes sodium transients in astrocytes, which can be local or global depending on the number of activated synapses and the duration of activity. Furthermore, astrocyte sodium signals can be transmitted to adjacent cells through gap junctions and following release of gliotransmitters. A main pathway for activity-related sodium influx into astrocytes is via high-affinity sodium-dependent glutamate transporters. Astrocyte sodium signals differ in many respects from the well-described glial calcium signals both in terms of their temporal as well as spatial distribution. There are no known buffering systems for sodium ions, nor is there store-mediated release of sodium. Sodium signals thus seem to represent rather direct and unbiased indicators of the site and strength of neuronal inputs. As such they have an immediate influence on the activity of sodium-dependent transporters which may even reverse in response to sodium signaling, as has been shown for GABA transporters for example. Furthermore, recovery from sodium transients through Na(+)/K(+)-ATPase requires a measurable amount of ATP, resulting in an activation of glial metabolism. In this review, we present basic principles of sodium regulation and the current state of knowledge concerning the occurrence and properties of activity-related sodium transients in astrocytes. We then discuss different aspects of the relationship between sodium changes in astrocytes and neuro-metabolic coupling, putting forward the idea that indeed sodium might serve as a new type of intracellular ion signal playing an important role in neuron-glia interaction and neuro-metabolic coupling in the healthy and diseased brain.