Transient mitochondrial permeability transition mediates excitotoxicity in glutamate-sensitive NSC34D motor neuron-like cells

Imeglimin Is Neuroprotective Against Ischemic Brain Injury in Rats-a Study Evaluating Neuroinflammation and Mitochondrial Functions

Imeglimin is a novel oral antidiabetic drug modulating mitochondrial functions. However, neuroprotective effects of this drug have not been investigated. The aim of this study was to investigate effects of imeglimin against ischemia-induced brain damage and neurological deficits and whether it acted via inhibition of mitochondrial permeability transition pore (mPTP) and suppression of microglial activation. Ischemia in rats was induced by permanent middle cerebral artery occlusion (pMCAO) for 48 h. Imeglimin (135 μg/kg/day) was injected intraperitoneally immediately after pMCAO and repeated after 24 h. Immunohistochemical staining was used to evaluate total numbers of neurons, astrocytes, and microglia as well as interleukin-10 (IL-10) producing cells in brain slices. Respiration of isolated brain mitochondria was assessed using high-resolution respirometry. Assessment of ionomycin-induced mPTP opening in intact cultured primary rat neuronal, astrocytic, and microglial cells was performed using fluorescence microscopy. Treatment with imeglimin significantly decreased infarct size, brain edema, and neurological deficits after pMCAO. Moreover, imeglimin protected against pMCAO-induced neuronal loss as well as microglial proliferation and activation, and increased the number of astrocytes and the number of cells producing anti-inflammatory cytokine IL-10 in the ischemic hemisphere. Imeglimin in vitro acutely prevented mPTP opening in cultured neurons and astrocytes but not in microglial cells; however, treatment with imeglimin did not prevent ischemia-induced mitochondrial respiratory dysfunction after pMCAO. This study demonstrates that post-stroke treatment with imeglimin exerts neuroprotective effects by reducing infarct size and neuronal loss possibly via the resolution of neuroinflammation and partly via inhibition of mPTP opening in neurons and astrocytes.

Bacopa monnieri attenuates glutamate-induced nociception and brain mitochondrial toxicity in Zebrafish

Bacopa monnieri L. (BM; Family: Scrophulariaceae), commonly known as Brahmi, is traditionally used as a nootropic agent. BM also exhibits significant analgesic activity in experimental models of pain. However, the effect of Bacopa monnieri against glutamate-induced nociception in zebrafish is yet to be explored in experimental condition. Therefore, the present study was designed to evaluate the effect of BM against glutamate-induced nociception and brain mitochondrial toxicity in adult zebrafish (Danio rerio). BM at 0.625, 1.25 and 2.5 mg/ml was administered to adult zebrafish and after half an hour glutamate was injected through i.m. route of administration. Indomethacin was used as standard drug. After behavioral analysis, the fish were euthanized and the brain was isolated and stored for further biochemical analysis. BM (1.25 and 2.5 mg/ml) and indomethacin significantly attenuated the glutamate-induced increase in number of line crossing compared to control group animals. Additionally, BM (1.25 and 2.5 mg/ml) and indomethacin significantly reduced the glutamate induced increase in cytosolic calcium level. Further, there was a substantial improvement in mitochondrial function, integrity and bioenergetics in term of respiratory control rate and ADP/O in zebrafish brain. Moreover, BM (1.25 and 2.5 mg/ml) and indomethacin significantly reduced the glutamate-induced mitochondria-dependent apoptosis in zebrafish brain. Therefore, BM could be a potential alternative drug candidate in the management of pain.

Mitoflash generated at the Qo site of mitochondrial Complex III

The previous research has shown that mitochondrial flash (mitoflash) genesis are functionally and mechanistically integrated with mitochondrial electron transport chain (ETC) energy metabolism. However, the response of mitoflash to superoxide is not entirely consistent with the response of MitoSOX Red. The generation mechanism of mitoflash is still unclear. Here, we investigated mitoflash activities, using the different combinations of ETC substrates and inhibitors, in permeabilized cardiomyocytes or hearts. We found that blocking the complete electron flow, from Complex I to IV, with any one of ETC inhibitors including rotenone (Rot), antimycin A (AntA), myxothiazol (Myxo), stigmatellin, and sodium cyanide, will lead to the abolishment of mitoflashes triggered by substrates in adult permeabilized cardiomyocytes. However, Myxo boosted mitoflashes triggered by the reverse electron of N,N,N',N'-tetramethyl-p-phenylenediamine/ascorbate. Moreover, Rot and AntA furtherly enhanced mitoflash activity rather than depressed it, suggesting that mitoflashes generated at the Complex III Qo site. Meanwhile, the inhibition of Complex III protein expression resulted in the activity of Complex III decrease, which decreased mitoflash frequency. The function defect (no change of protein level) of the Qo site of Complex III in aging hearts augmented mitoflash generation confirmed the Qo site function was critical to mitoflash genesis. Thus, our results indicate that mitoflash detected by circularly permuted yellow fluorescent protein is generated at the Qo site of Complex III.

Machine learning-based classification of mitochondrial morphology in primary neurons and brain

The mitochondrial network continually undergoes events of fission and fusion. Under physiologic conditions, the network is in equilibrium and is characterized by the presence of both elongated and punctate mitochondria. However, this balanced, homeostatic mitochondrial profile can change morphologic distribution in response to various stressors. Therefore, it is imperative to develop a method that robustly measures mitochondrial morphology with high accuracy. Here, we developed a semi-automated image analysis pipeline for the quantitation of mitochondrial morphology for both in vitro and in vivo applications. The image analysis pipeline was generated and validated utilizing images of primary cortical neurons from transgenic mice, allowing genetic ablation of key components of mitochondrial dynamics. This analysis pipeline was further extended to evaluate mitochondrial morphology in vivo through immunolabeling of brain sections as well as serial block-face scanning electron microscopy. These data demonstrate a highly specific and sensitive method that accurately classifies distinct physiological and pathological mitochondrial morphologies. Furthermore, this workflow employs the use of readily available, free open-source software designed for high throughput image processing, segmentation, and analysis that is customizable to various biological models.

Highly Efficient Conversion of Motor Neuron-Like NSC-34 Cells into Functional Motor Neurons by Prostaglandin E2

Motor neuron diseases are a group of progressive neurological disorders that degenerate motor neurons. The neuroblastoma × spinal cord hybrid cell line NSC-34 is widely used as an experimental model in studies of motor neuron diseases. However, the differentiation efficiency of NSC-34 cells to neurons is not always sufficient. We have found that prostaglandin E₂ (PGE₂) induces morphological differentiation in NSC-34 cells. The present study investigated the functional properties of PGE₂-differentiated NSC-34 cells. Retinoic acid (RA), a widely-used agent inducing cell differentiation, facilitated neuritogenesis, which peaked on day 7, whereas PGE₂-induced neuritogenesis took only 2 days to reach the same level. Whole-cell patch-clamp recordings showed that the current threshold of PGE₂-treated cell action potentials was lower than that of RA-treated cells. PGE₂ and RA increased the protein expression levels of neuronal differentiation markers, microtubule-associated protein 2c and synaptophysin, and to the same extent, motor neuron-specific markers HB9 and Islet-1. On the other hand, protein levels of choline acetyltransferase and basal release of acetylcholine in PGE₂-treated cells were higher than in RA-treated cells. These results suggest that PGE₂ is a rapid and efficient differentiation-inducing factor for the preparation of functionally mature motor neurons from NSC-34 cells.

The Molecular Mechanisms Underlying Prostaglandin D2-Induced Neuritogenesis in Motor Neuron-Like NSC-34 Cells

Prostaglandins are a group of physiologically active lipid compounds derived from arachidonic acid. Our previous study has found that prostaglandin E2 promotes neurite outgrowth in NSC-34 cells, which are a model for motor neuron development. However, the effects of other prostaglandins on neuronal differentiation are poorly understood. The present study investigated the effect of prostaglandin D2 (PGD2) on neuritogenesis in NSC-34 cells. Exposure to PGD2 resulted in increased percentages of neurite-bearing cells and neurite length. Although D-prostanoid receptor (DP) 1 and DP2 were dominantly expressed in the cells, BW245C (a DP1 agonist) and 15(R)-15-methyl PGD2 (a DP2 agonist) had no effect on neurite outgrowth. Enzyme-linked immunosorbent assay demonstrated that PGD2 was converted to 15-deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2) under cell-free conditions. Exogenously applied 15d-PGJ2 mimicked the effect of PGD2 on neurite outgrowth. GW9662, a peroxisome proliferator-activated receptor-gamma (PPARγ) antagonist, suppressed PGD2-induced neurite outgrowth. Moreover, PGD2 and 15d-PGJ2 increased the protein expression of Islet-1 (the earliest marker of developing motor neurons), and these increases were suppressed by co-treatment with GW9662. These results suggest that PGD2 induces neuritogenesis in NSC-34 cells and that PGD2-induced neurite outgrowth was mediated by the activation of PPARγ through the metabolite 15d-PGJ2.

Glial Cell AMPA Receptors in Nervous System Health, Injury and Disease

Glia form a central component of the nervous system whose varied activities sustain an environment that is optimised for healthy development and neuronal function. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA)-type glutamate receptors (AMPAR) are a central mediator of glutamatergic excitatory synaptic transmission, yet they are also expressed in a wide range of glial cells where they influence a variety of important cellular functions. AMPAR enable glial cells to sense the activity of neighbouring axons and synapses, and as such many aspects of glial cell development and function are influenced by the activity of neural circuits. However, these AMPAR also render glia sensitive to elevations of the extracellular concentration of glutamate, which are associated with a broad range of pathological conditions. Excessive activation of AMPAR under these conditions may induce excitotoxic injury in glial cells, and trigger pathophysiological responses threatening other neural cells and amplifying ongoing disease processes. The aim of this review is to gather information on AMPAR function from across the broad diversity of glial cells, identify their contribution to pathophysiological processes, and highlight new areas of research whose progress may increase our understanding of nervous system dysfunction and disease.

Neuroprotection by urate on the mutant hSOD1-related cellular and Drosophila models of amyotrophic lateral sclerosis: Implication for GSH synthesis via activating Akt/GSK3β/Nrf2/GCLC pathways

Oxidative stress has been considered as a principal mechanism of motor neuron death in amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease which could be caused by dominant mutations in an antioxidant enzyme superoxide dismutase-1 (SOD1). The aim of the present study was to investigate the potential neuroprotective effects and mechanisms of urate, an important endogenous antioxidant and a biomarker of favorable ALS progression rates, in the mutant human SOD1-related cellular and Drosophila models of ALS. Our results showed that urate treatment provided neuroprotective effects as confirmed by enhanced survival, attenuated motor impairments, reduced oxidative damage and increased antioxidant defense in hSOD1-G85R-expressing Drosophila models of ALS. In vitro studies, we demonstrated that urate protected motor neurons (NSC-34 cells) against hSOD1-G93A-induced cell damage and apoptosis by decreasing reactive oxygen specials (ROS) production and oxidative damage. Moreover, urate markedly increased the expression and activation of nuclear factor erythroid 2-related factor 2 (Nrf2), stimulated Nrf2-targeted antioxidant gene glutathione cysteine ligase catalytic subunit (GCLC) expression and glutathione (GSH) synthesis by upregulating Akt/GSK3β pathway. Furthermore, the inhibition of Akt pathway with LY294002 abolished urate-mediated elevation of GSH synthesis and neuroprotective effects both in vivo and in vitro. Overall, these results suggested that, in addition to its direct scavenging of ROS, urate markedly enhanced GSH expression by activating Akt/GSK3β/Nrf2/GCLC pathway, and thus offering neuroprotective effects on motor neurons against oxidative stress.

Pyridine nucleotides regulate the superoxide anion flash upon permeabilization of mitochondrial membranes: An MCLA-based study

The permeabilization of mitochondrial membranes via permeability transition pore opening or by the pore-forming peptide alamethicin causes a flash of superoxide anion (SA) and hydrogen peroxide production and the inhibition of matrix aconitase. It was shown using the SA probe 3,7-dihydro-2-methyl-6-(4-methoxyphenyl)imidazol[1,2-a]pyrazine-3-one (MCLA) that the substrates of NAD-dependent dehydrogenases, inhibitors of the respiratory chain, and NAD(P)H at millimolar concentrations suppressed or delayed SA flashes. In the presence of added NADH and NADPH, SA flashes were observed only after considerable oxidation of pyridine nucleotides. The production of SA was maximal at NADPH and NADH redox potentials from -315 to -295 mV and from -325 to -270 mV, respectively, depending on NAD(P)H concentration. SA generation supported by NADPH was severalfold greater than that supported by NADH. In intact mitochondria, NADPH- and NADH-dependent SA generation was negligible. Respiratory substrates at physiological or lower concentrations were incapable of suppressing the NADPH-supported SA flash. These data indicate that, in conditions close to pathophysiological, matrix NADPH oxidoreductase(s), presumably, an adrenodoxin reductase in complex with adrenodoxin, can essentially contribute to SA flashes associated with transient or irreversible permeability transition pore opening or membrane permeabilization by another mechanism.

α-Lipoic acid attenuates oxidative stress and neurotoxicity via the ERK/Akt-dependent pathway in the mutant hSOD1 related Drosophila model and the NSC34 cell line of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a degenerative disease with a progressive loss of motor neurons in the central nervous system (CNS). However, there are unsolved problems with the therapies for this disease. α-Lipoic acid (LA) is a natural, universal antioxidant capable of scavenging hydroxyl radicals as well as regenerating a series of antioxidant enzymes that has been widely used in clinical settings. This study aimed to evaluate the antioxidant and neuroprotective effects of LA in ALS cell and Drosophila models with mutant G85R and G93A hSOD1 genes. The biological effects of LA and the protein levels of several antioxidant factors were examined, as were those of phospho-Akt and phospho-ERK. Furthermore, specific inhibitors of the PI3K/Akt and MEK/ERK signaling pathways were used to analyze their effects on LA-induced antioxidant expression in vivo and in vitro. Evidences showed that the mutant hSOD1 resulted in the increased oxidative stress, abnormal antioxidant signaling and pathological behaviors in motor performance and survival compared with non-mutant hSOD1 models, treatment with LA improved motor activity and survival in transgenic flies, prevented NSC34 cells from mutant hSOD1 or H₂O₂ induced decreased antioxidant enzymes as well as increased ROS levels. In addition, LA regulated the expression levels of antioxidant proteins in a dose- and periodical time-dependent manner, which might be mediated by ERK/Akt pathway activation and independent from the mutant hSOD1 gene. Our observations suggest that LA exerts strong and positive antioxidant and neuroprotective effects through the activation of the ERK-Akt pathway in hSOD1 ALS models.

Calcium Dynamics as a Machine for Decoding Signals

Calcium (Ca²⁺) is considered one of the most-important biological cations, because it is implicated in cell physiopathology and cell fate through a finely tuned signaling system. In support of this notion, Ca²⁺ is the primary driver of cell proliferation and cell growth; however, it is also intimately linked to cell death. Functional abnormalities or mutations in proteins that mediate Ca²⁺ homeostasis usually lead to a plethora of diseases and pathogenic states, including cancer, heart failure, diabetes, and neurodegenerative disease. In this review, we examine recent discoveries in the highly localized nature of Ca²⁺-dependent signal transduction and its roles in cell fate, inflammasome activation, and synaptic transmission.

Mitochondria Initiate and Regulate Sarcopenia

We present the hypothesis that an accumulation of dysfunctional mitochondria initiates a signaling cascade leading to motor neuron and muscle fiber death and culminating in sarcopenia. Interactions between neural and muscle cells that contain dysfunctional mitochondria exacerbate sarcopenia. Preventing sarcopenia will require identifying mitochondrial sources of dysfunction that are reversible.

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.

CaMKII induces permeability transition through Drp1 phosphorylation during chronic β-AR stimulation

Mitochondrial permeability transition pore (mPTP) is involved in cardiac dysfunction during chronic β-adrenergic receptor (β-AR) stimulation. The mechanism by which chronic β-AR stimulation leads to mPTP openings is elusive. Here, we show that chronic administration of isoproterenol (ISO) persistently increases the frequency of mPTP openings followed by mitochondrial damage and cardiac dysfunction. Mechanistically, this effect is mediated by phosphorylation of mitochondrial fission protein, dynamin-related protein 1 (Drp1), by Ca²⁺/calmodulin-dependent kinase II (CaMKII) at a serine 616 (S616) site. Mutating this phosphorylation site or inhibiting Drp1 activity blocks CaMKII- or ISO-induced mPTP opening and myocyte death in vitro and rescues heart hypertrophy in vivo. In human failing hearts, Drp1 phosphorylation at S616 is increased. These results uncover a pathway downstream of chronic β-AR stimulation that links CaMKII, Drp1 and mPTP to bridge cytosolic stress signal with mitochondrial dysfunction in the heart.

β-adrenergic receptor signaling induces mitochondrial permeability transition pore (mPTP) opening. Here, Xu et al. show that this effect is mediated by phosphorylation of mitochondrial fission protein Drp1 by CamKII, which increases the frequency of transient mPTP opening.

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.

Mitochondrial Dysfunction during the Early Stages of Excitotoxic Spinal Motor Neuron Degeneration in Vivo

Glutamate excitotoxicity and mitochondrial dysfunction are involved in motor neuron degeneration process during amyotrophic lateral sclerosis (ALS). We have previously shown that microdialysis perfusion of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) in the lumbar region of the rat spinal cord produces permanent paralysis of the ipsilateral hindlimb and death of motor neurons by a Ca(2+)-dependent mechanism, in a process that starts 2-3 h after AMPA perfusion. Co-perfusion with different energy metabolic substrates, mainly pyruvate, prevented the paralysis and motor neuron degeneration induced by AMPA, suggesting that mitochondrial energetic deficiencies are involved in this excitotoxic motor neuron death. To test this, in the present work, we studied the functional and ultrastructural characteristics of mitochondria isolated from the ventral horns of lumbar spinal cords of rats, at the beginning of the AMPA-induced degeneration process, when motor neurons are still alive. Animals were divided in four groups: perfused with AMPA, AMPA + pyruvate, and pyruvate alone and Krebs-Ringer medium as controls. Mitochondria from the AMPA-treated group showed decreased oxygen consumption rates, respiratory controls, and transmembrane potentials. Additionally, activities of the respiratory chain complexes I and IV were significantly decreased. Electron microscopy showed that mitochondria from AMPA-treated rats presented swelling, disorganized cristae and disrupted membranes. Remarkably, in the animals co-perfused with AMPA and pyruvate all these abnormalities were prevented. We conclude that mitochondrial dysfunction plays a crucial role in spinal motor neuron degeneration induced by overactivation of AMPA receptors in vivo. These mechanisms could be involved in ALS motor neuron degeneration.

NSC-34 Motor Neuron-Like Cells Are Unsuitable as Experimental Model for Glutamate-Mediated Excitotoxicity

Glutamate-induced excitotoxicity is a major contributor to motor neuron degeneration in the pathogenesis of amyotrophic lateral sclerosis (ALS). The spinal cord × Neuroblastoma hybrid cell line (NSC-34) is often used as a bona fide cellular model to investigate the physiopathological mechanisms of ALS. However, the physiological response of NSC-34 to glutamate remains insufficiently described. In this study, we evaluated the relevance of differentiated NSC-34 (NSC-34D) as an in vitro model for glutamate excitotoxicity studies. NSC-34D showed morphological and physiological properties of motor neuron-like cells and expressed glutamate receptor subunits GluA1-4, GluN1 and GluN2A/D. Despite these diverse characteristics, no specific effect of glutamate was observed on cultured NSC-34D survival and morphology, in contrast to what has been described in primary culture of motor neurons (MN). Moreover, a small non sustained increase in the concentration of intracellular calcium was observed in NSC-34D after exposure to glutamate compared to primary MN. Our findings, together with the inability to obtain cultures containing only differentiated cells, suggest that the motor neuron-like NSC-34 cell line is not a suitable in vitro model to study glutamate-induced excitotoxicity. We suggest that the use of primary cultures of MN is more suitable than NSC-34 cell line to explore the pathogenesis of glutamate-mediated excitotoxicity at the cellular level in ALS and other motor neuron diseases.

The mitochondrial Ca2+ uniporter: regulation by auxiliary subunits and signal transduction pathways

Mitochondrial Ca(2+) homeostasis, the Ca(2+) influx-efflux balance, is responsible for the control of numerous cellular functions, including energy metabolism, generation of reactive oxygen species, spatiotemporal dynamics of Ca(2+) signaling, and cell growth and death. Recent discovery of the molecular identity of the mitochondrial Ca(2+) uniporter (MCU) provides new possibilities for application of genetic approaches to study the mitochondrial Ca(2+) influx mechanism in various cell types and tissues. In addition, the subsequent discovery of various auxiliary subunits associated with MCU suggests that mitochondrial Ca(2+) uptake is not solely regulated by a single protein (MCU), but likely by a macromolecular protein complex, referred to as the MCU-protein complex (mtCUC). Moreover, recent reports have shown the potential role of MCU posttranslational modifications in the regulation of mitochondrial Ca(2+) uptake through mtCUC. These observations indicate that mtCUCs form a local signaling complex at the inner mitochondrial membrane that could significantly regulate mitochondrial Ca(2+) handling, as well as numerous mitochondrial and cellular functions. In this review we discuss the current literature on mitochondrial Ca(2+) uptake mechanisms, with a particular focus on the structure and function of mtCUC, as well as its regulation by signal transduction pathways, highlighting current controversies and discrepancies.

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.