Protons Trigger Mitochondrial Flashes

Intermittent proton bursts of single lactic acid bacteria

Lactic acid bacteria are a kind of probiotic microorganisms that efficiently convert carbohydrates to lactic acids, thus playing essential roles in fermentation and food industry. While conventional wisdom often suggests continuous release of protons from bacteria during acidification, here we developed a methodology to measure the dynamics of proton release at the single bacteria level, and report on the discovery of a proton burst phenomenon, i.e., the intermittent efflux of protons, of single Lactobacillus plantarum bacteria. When placing an individual bacterium in an oil-sealed microwell, efflux and accumulation of protons consequently reduced the pH in the confined extracellular medium, which was monitored with fluorescent pH indicators in a high-throughput and real-time manner. In addition to the slow and continuous proton release behavior (as expected), stochastic and intermittent proton burst events were surprisingly observed with a typical timescale of several seconds. It was attributed to the regulatory response of bacteria by activating H+-ATPase to compensate the stochastic and transient depolarizations of membrane potential. These findings not only revealed an unprecedented proton burst phenomenon in lactic acid bacteria, but also shed new lights on the intrinsic roles of H+-ATPase in membrane potential homeostasis, with implications for both fermentation industry and bacterial electrophysiology.

Exploring therapeutic potential of mitophagy modulators using Drosophila models of Parkinson’s disease

Parkinson’s disease (PD) is the second most popular age-associated neurodegenerative disorder after Alzheimer’s disease. The degeneration of dopaminergic neurons, aggregation of α-synuclein (α-syn), and locomotor defects are the main characteristic features of PD. The main cause of a familial form of PD is associated with a mutation in genes such as SNCA, PINK1, Parkin, DJ-1, LRKK2, and others. Recent advances have uncovered the different underlying mechanisms of PD but the treatment of PD is still unknown due to the unavailability of effective therapies and preventive medicines in the current scenario. The pathophysiology and genetics of PD have been strongly associated with mitochondria in disease etiology. Several studies have investigated a complex molecular mechanism governing the identification and clearance of dysfunctional mitochondria from the cell, a mitochondrial quality control mechanism called mitophagy. Reduced mitophagy and mitochondrial impairment are found in both sporadic and familial PD. Pharmacologically modulating mitophagy and accelerating the removal of defective mitochondria are of common interest in developing a therapy for PD. However, despite the extensive understanding of the mitochondrial quality control pathway and its underlying mechanism, the therapeutic potential of targeting mitophagy modulation and its role in PD remains to be explored. Thus, targeting mitophagy using chemical agents and naturally occurring phytochemicals could be an emerging therapeutic strategy in PD prevention and treatment. We discuss the current research on understanding the role of mitophagy modulators in PD using Drosophila melanogaster as a model. We further explore the contribution of Drosophila in the pathophysiology of PD, and discuss comprehensive genetic analysis in flies and pharmacological drug screening to develop potential therapeutic molecules for PD.

Imaging mitochondrial calcium dynamics in the central nervous system

Mitochondrial calcium handling is a particularly active research area in the neuroscience field, as it plays key roles in the regulation of several functions of the central nervous system, such as synaptic transmission and plasticity, astrocyte calcium signaling, neuronal activity… In the last few decades, a panel of techniques have been developed to measure mitochondrial calcium dynamics, relying mostly on photonic microscopy, and including synthetic sensors, hybrid sensors and genetically encoded calcium sensors. The goal of this review is to endow the reader with a deep knowledge of the historical and latest tools to monitor mitochondrial calcium events in the brain, as well as a comprehensive overview of the current state of the art in brain mitochondrial calcium signaling. We will discuss the main calcium probes used in the field, their mitochondrial targeting strategies, their key properties and major drawbacks. In addition, we will detail the main roles of mitochondrial calcium handling in neuronal tissues through an extended report of the recent studies using mitochondrial targeted calcium sensors in neuronal and astroglial cells, in vitro and in vivo.

Signaling events that occur when cells of Escherichia coli encounter a glass surface

Microbial cells organized on solid surfaces are the most ancient form of biological communities. Yet how single cells interact with surfaces and integrate a variety of signals to establish a sessile lifestyle is poorly understood. We developed and used sensitive biosensors to determine the kinetics of second messengers’ responses to surface attachment. This allowed us to examine cell-by-cell variability of the initial signaling events and establish that some of these events depend on flagellar motor function while others do not. Environmentally determined factors, like the energetic status of the cell, can modulate all signaling events. The complex interplay between the surface interaction inputs and external conditions can now be studied using our system.

Bacterial cells interact with solid surfaces and change their lifestyle from single free-swimming cells to sessile communal structures (biofilms). Cyclic di-guanosine monophosphate (c-di-GMP) is central to this process, yet we lack tools for direct dynamic visualization of c-di-GMP in single cells. Here, we developed a fluorescent protein–based c-di-GMP–sensing system for Escherichia coli that allowed us to visualize initial signaling events and assess the role played by the flagellar motor. The sensor was pH sensitive, and the events that appeared on a seconds’ timescale were alkaline spikes in the intracellular pH. These spikes were not apparent when signals from different cells were averaged. Instead, a signal appeared on a minutes’ timescale that proved to be due to an increase in intracellular c-di-GMP. This increase, but not the alkaline spikes, depended upon a functional flagellar motor. The kinetics and the amplitude of both the pH and c-di-GMP responses displayed cell-to-cell variability indicative of the distinct ways the cells approached and interacted with the surface. The energetic status of a cell can modulate these events. In particular, the alkaline spikes displayed an oscillatory behavior and the c-di-GMP increase was modest in the presence of glucose.

Effects of Metformin on Spontaneous Ca2+ Signals in Cultured Microglia Cells under Normoxic and Hypoxic Conditions

Microglial functioning depends on Ca²⁺ signaling. By using Ca²⁺ sensitive fluorescence dye, we studied how inhibition of mitochondrial respiration changed spontaneous Ca²⁺ signals in soma of microglial cells from 5-7-day-old rats grown under normoxic and mild-hypoxic conditions. In microglia under normoxic conditions, metformin or rotenone elevated the rate and the amplitude of Ca²⁺ signals 10-15 min after drug application. Addition of cyclosporin A, a blocker of mitochondrial permeability transition pore (mPTP), antioxidant trolox, or inositol 1,4,5-trisphosphate receptor (IP3R) blocker caffeine in the presence of rotenone reduced the elevated rate and the amplitude of the signals implying sensitivity to reactive oxygen species (ROS), and involvement of mitochondrial mPTP together with IP3R. Microglial cells exposed to mild hypoxic conditions for 24 h showed elevated rate and increased amplitude of Ca²⁺ signals. Application of metformin or rotenone but not phenformin before mild hypoxia reduced this elevated rate. Thus, metformin and rotenone had the opposing fast action in normoxia after 10-15 min and the slow action during 24 h mild-hypoxia implying activation of different signaling pathways. The slow action of metformin through inhibition of complex I could stabilize Ca²⁺ homeostasis after mild hypoxia and could be important for reduction of ischemia-induced microglial activation.

Oxidative bursts of single mitochondria mediate retrograde signaling toward the ER

The emerging role of mitochondria as signaling organelles raises the question of whether individual mitochondria can initiate heterotypic communication with neighboring organelles. Using fluorescent probes targeted to the endoplasmic-reticulum-mitochondrial interface, we demonstrate that single mitochondria generate oxidative bursts, rapid redox oscillations, confined to the nanoscale environment of the interorganellar contact sites. Using probes fused to inositol 1,4,5-trisphosphate receptors (IP₃Rs), we show that Ca²⁺ channels directly sense oxidative bursts and respond with Ca²⁺ transients adjacent to active mitochondria. Application of specific mitochondrial stressors or apoptotic stimuli dramatically increases the frequency and amplitude of the oxidative bursts by enhancing transient permeability transition pore openings. Conversely, blocking interface Ca²⁺ transport via elimination of IP₃Rs or mitochondrial calcium uniporter channels suppresses ER-mitochondrial Ca²⁺ feedback and cell death. Thus, single mitochondria initiate local retrograde signaling by miniature oxidative bursts and, upon metabolic or apoptotic stress, may also amplify signals to the rest of the cell.

BacFlash signals acid-resistance gene expression in bacteria

Intracellular pH (pHi) homeostasis is crucial for cellular functions and signal transduction across all kingdoms of life. In particular, bacterial pHi homeostasis is important for physiology, ecology, and pathogenesis. Here we report an exquisite bacterial acid-resistance (AR) mechanism in which proton leak elicits a pre-emptive AR response. A single bacterial cell undergoes quantal electrochemical excitation, termed "BacFlash", which consists of membrane depolarization, transient pHi rise, and bursting production of reactive oxygen species. BacFlash ignition is dictated by acid stress in the form of proton leak across the plasma membrane and the rate of BacFlash occurrence is reversely correlated with the pHi buffering capacity. Through genome-wide screening, we further identify the ATP synthase Fo complex subunit a as the putative proton sensor for BacFlash biogenesis. Importantly, persistent BacFlash hyperactivity activates transcription of a panel of key AR genes and predisposes the cells to survive imminent extreme acid stress. These findings demonstrate a prototypical coupling between electrochemical excitation and nucleoid gene expression in prokaryotes.

LETM1: A Single Entity With Diverse Impact on Mitochondrial Metabolism and Cellular Signaling

Nearly 2 decades since its discovery as one of the genes responsible for the Wolf-Hirschhorn Syndrome (WHS), the primary function of the leucine-zipper EF-hand containing transmembrane 1 (LETM1) protein in the inner mitochondrial membrane (IMM) or the mechanism by which it regulates mitochondrial Ca²⁺ handling is unresolved. Meanwhile, LETM1 has been associated with the regulation of fundamental cellular processes, such as development, cellular respiration and metabolism, and apoptosis. This mini-review summarizes the diversity of cellular functions impacted by LETM1 and highlights the multiple roles of LETM1 in health and disease.

Acute glucose influx-induced mitochondrial hyperpolarization inactivates myosin phosphatase as a novel mechanism of vascular smooth muscle contraction

It is well-established that long-term exposure of the vasculature to metabolic disturbances leads to abnormal vascular tone, while the physiological regulation of vascular tone upon acute metabolic challenge remains unknown. Here, we found that acute glucose challenge induced transient increases in blood pressure and vascular constriction in humans and mice. Ex vivo study in isolated thoracic aortas from mice showed that glucose-induced vascular constriction is dependent on glucose oxidation in vascular smooth muscle cells. Specifically, mitochondrial membrane potential (ΔΨm), an essential component in glucose oxidation, was increased along with glucose influx and positively regulated vascular smooth muscle tone. Mechanistically, mitochondrial hyperpolarization inhibited the activity of myosin light chain phosphatase (MLCP) in a Ca2+-independent manner through activation of Rho-associated kinase, leading to cell contraction. However, ΔΨm regulated smooth muscle tone independently of the small G protein RhoA, a major regulator of Rho-associated kinase signaling. Furthermore, myosin phosphatase target subunit 1 (MYPT1) was found to be a key molecule in mediating MLCP activity regulated by ΔΨm. ΔΨm positively phosphorylated MYPT1, and either knockdown or knockout of MYPT1 abolished the effects of glucose in stimulating smooth muscle contraction. In addition, smooth muscle-specific Mypt1 knockout mice displayed blunted response to glucose challenge in blood pressure and vascular constriction and impaired clearance rate of circulating metabolites. These results suggested that glucose influx stimulates vascular smooth muscle contraction via mitochondrial hyperpolarization-inactivated myosin phosphatase, which represents a novel mechanism underlying vascular constriction and circulating metabolite clearance.

Reduction of elevated proton leak rejuvenates mitochondria in the aged cardiomyocyte

Aging-associated diseases, including cardiac dysfunction, are increasingly common in the population. However, the mechanisms of physiologic aging in general, and cardiac aging in particular, remain poorly understood. Age-related heart impairment is lacking a clinically effective treatment. Using the model of naturally aging mice and rats, we show direct evidence of increased proton leak in the aged heart mitochondria. Moreover, our data suggested ANT1 as the most likely site of mediating increased mitochondrial proton permeability in old cardiomyocytes. Most importantly, the tetra-peptide SS-31 prevents age-related excess proton entry, decreases the mitochondrial flash activity and mitochondrial permeability transition pore opening, rejuvenates mitochondrial function by direct association with ANT1 and the mitochondrial ATP synthasome, and leads to substantial reversal of diastolic dysfunction. Our results uncover the excessive proton leak as a novel mechanism of age-related cardiac dysfunction and elucidate how SS-31 can reverse this clinically important complication of cardiac aging.

Red fluorescent CEPIA indicators for visualization of Ca2+ dynamics in mitochondria

Mitochondrial Ca2+ dynamics are involved in the regulation of multifarious cellular processes, including intracellular Ca2+ signalling, cell metabolism and cell death. Use of mitochondria-targeted genetically encoded Ca2+ indicators has revealed intercellular and subcellular heterogeneity of mitochondrial Ca2+ dynamics, which are assumed to be determined by distinct thresholds of Ca2+ increases at each subcellular mitochondrial domain. The balance between Ca2+ influx through the mitochondrial calcium uniporter and extrusion by cation exchangers across the inner mitochondrial membrane may define the threshold; however, the precise mechanisms remain to be further explored. We here report the new red fluorescent genetically encoded Ca2+ indicators, R-CEPIA3mt and R-CEPIA4mt, which are targeted to mitochondria and their Ca2+ affinities are engineered to match the intramitochondrial Ca2+ concentrations. They enable visualization of mitochondrial Ca2+ dynamics with high spatiotemporal resolution in parallel with the use of green fluorescent probes and optogenetic tools. Thus, R-CEPIA3mt and R-CEPIA4mt are expected to be a useful tool for elucidating the mechanisms of the complex mitochondrial Ca2+ dynamics and their functions.

Delineating a role for the mitochondrial permeability transition pore in diabetic kidney disease by targeting cyclophilin D

Mitochondrial stress has been widely observed in diabetic kidney disease (DKD). Cyclophilin D (CypD) is a functional component of the mitochondrial permeability transition pore (mPTP) which allows the exchange of ions and solutes between the mitochondrial matrix to induce mitochondrial swelling and activation of cell death pathways. CypD has been successfully targeted in other disease contexts to improve mitochondrial function and reduced pathology. Two approaches were used to elucidate the role of CypD and the mPTP in DKD. Firstly, mice with a deletion of the gene encoding CypD (Ppif-/-) were rendered diabetic with streptozotocin (STZ) and followed for 24 weeks. Secondly, Alisporivir, a CypD inhibitor was administered to the db/db mouse model (5 mg/kg/day oral gavage for 16 weeks). Ppif-/- mice were not protected against diabetes-induced albuminuria and had greater glomerulosclerosis than their WT diabetic littermates. Renal hyperfiltration was lower in diabetic Ppif-/- as compared with WT mice. Similarly, Alisporivir did not improve renal function nor pathology in db/db mice as assessed by no change in albuminuria, KIM-1 excretion and glomerulosclerosis. Db/db mice exhibited changes in mitochondrial function, including elevated respiratory control ratio (RCR), reduced mitochondrial H2O2 generation and increased proximal tubular mitochondrial volume, but these were unaffected by Alisporivir treatment. Taken together, these studies indicate that CypD has a complex role in DKD and direct targeting of this component of the mPTP will likely not improve renal outcomes.

A mitochondria-targeted two-photon fluorescent probe for sensing and imaging pH changes in living cells

A novel two-photon pH probe, 3-benzimidazole-7-hydroxycoumarin (BHC), was designed and synthesized based on the structures of hydroxycoumarin and benzimidazole. BHC showed good linearity in the pH ranges of 3.30-5.40 (pKa = 4.20) and 6.50-8.30 (pKa = 7.20) at a maximum emission wavelength of 480 nm. BHC in acidic and alkaline media could be distinguished by an obvious spectral shift of the maximum absorption wavelength from 390 nm to 420 nm. In addition, BHC was well localized to mitochondria and successfully applied to one-photon and two-photon imaging of pH changes in the mitochondria of HeLa cells. The findings presented herein suggest that BHC can serve as an excellent fluorescent probe for selectively sensing mitochondrial pH changes with remarkable photostability and low cytotoxicity.

Temperature dependence of mitoflash biogenesis in cardiac mitochondria

Mitochondrial flashes (mitoflashes) represent fundamental biochemical and biophysical dynamics of the organelle, involving sudden depolarization of mitochondrial membrane potential (ΔΨ_m), bursting production of reactive oxygen species (ROS), and accelerated extrusion of matrix protons. Here we investigated temperature dependence of mitoflash biogenesis as well as ΔΨ_m oscillations, a subset of which overlapping with mitoflashes, in both cardiac myocytes and isolated respiring cardiac mitochondria. Unexpectedly, we found that mitoflash biogenesis was essentially temperature-independent in intact cardiac myocytes, evidenced by the constancy of frequency as well as amplitude and rise speed over 5 °C-40 °C. Moderate temperature dependence was found in single mitochondria charged by respiratory substrates, where mitoflash frequency was decreased over 5 °C-20 °C with Q₁₀ of 0.74 for Complex I substrates and 0.83 for Complex II substrate. In contrast, ΔΨ_m oscillation frequency displayed a negative temperature dependence at 5 °C-20 °C with Q₁₀ of 0.82 in intact cells, but a positive temperature dependence at 25 °C - 40 °C with Q₁₀ of 1.62 in isolated mitochondria charged with either Complex I or Complex II substrates. Moreover, the recovery speed of individual mitoflashes exhibited mild temperature dependence (Q₁₀ = 1.14-1.22). These results suggest a temperature compensation of mitoflash frequency at both the mitochondrial and extra-organelle levels, and underscore that mitoflashes and ΔΨ_m oscillations are related but distinctly different mitochondrial functional dynamics.

Isolating a reverse-mode ATP synthase-dependent mechanism of mitoflash activation

Wei-LaPierre and Dirksen discuss new work investigating the molecular events underlying mitoflash biogenesis.

Substrate-dependent and cyclophilin D-independent regulation of mitochondrial flashes in skeletal and cardiac muscle

Mitochondrial flashes (mitoflashes) are stochastic events in the mitochondrial matrix detected by mitochondrial-targeted cpYFP (mt-cpYFP). Mitoflashes are quantal bursts of reactive oxygen species (ROS) production accompanied by modest matrix alkalinization and depolarization of the mitochondrial membrane potential. Mitoflashes are fundamental events present in a wide range of cell types. To date, the precise mechanisms for mitoflash generation and termination remain elusive. Transient opening of the mitochondrial membrane permeability transition pore (mPTP) during a mitoflash is proposed to account for the mitochondrial membrane potential depolarization. Here, we set out to compare the tissue-specific effects of cyclophilin D (CypD)-deficiency and mitochondrial substrates on mitoflash activity in skeletal and cardiac muscle. In contrast to previous reports, we found that CypD knockout did not alter the mitoflash frequency or other mitoflash properties in acutely isolated cardiac myocytes, skeletal muscle fibers, or isolated mitochondria from skeletal muscle and the heart. However, in skeletal muscle fibers, CypD deficiency resulted in a parallel increase in both activity-dependent mitochondrial Ca2+ uptake and activity-dependent mitoflash activity. Increases in both mitochondrial Ca2+ uptake and mitoflash activity following electrical stimulation were abolished by inhibition of mitochondrial Ca2+ uptake. We also found that mitoflash frequency and amplitude differ greatly between intact skeletal muscle fibers and cardiac myocytes, but that this difference is absent in isolated mitochondria. We propose that this difference may be due, in part, to differences in substrate availability in intact skeletal muscle fibers (primarily glycolytic) and cardiac myocytes (largely oxidative). Overall, we find that CypD does not contribute significantly in mitoflash biogenesis under basal conditions in skeletal and cardiac muscle, but does regulate mitoflash events during muscle activity. In addition, tissue-dependent differences in mitoflash frequency are strongly regulated by mitochondrial substrate availability.

Preparation of mitochondria to measure superoxide flashes in angiosperm flowers

Background

Mitochondria are the center of energy metabolism and the production of reactive oxygen species (ROS). ROS production results in a burst of “superoxide flashes”, which is always accompanied by depolarization of mitochondrial membrane potential. Superoxide flashes have only been studied in the model plant Arabidopsis thaliana using a complex method to isolate mitochondria. In this study, we present an efficient, easier method to isolate functional mitochondria from floral tissues to measure superoxide flashes.

Method

We used 0.5 g samples to isolate mitochondria within <1.5 h from flowers of two non-transgenic plants (Magnolia denudata and Nelumbo nucifera) to measure superoxide flashes. Superoxide flashes were visualized by the pH-insensitive indicator MitoSOX Red, while the mitochondrial membrane potential (ΔΨ m) was labelled with TMRM.

Results

Mitochondria isolated using our method showed a high respiration ratio. Our results indicate that the location of ROS and mitochondria was in a good coincidence. Increased ROS together with a higher frequency of superoxide flashes was found in mitochondria isolated from the flower pistil. Furthermore, a higher rate of depolarization of the ΔΨ m was observed in the pistil. Taken together, these results demonstrate that the frequency of superoxide flashes is closely related to depolarization of the ΔΨ m in petals and pistils of flowers.

Mitoflash biogenesis and its role in the autoregulation of mitochondrial proton electrochemical potential

Individual mitochondria undergo an intermittent, all-or-none electrochemical excitation termed “mitoflash.” Feng et al. show that mitoflash occurs following build-up of mitochondrial electrochemical potential and may serve to autoregulate mitochondrial proton electrochemical potential.

Respiring mitochondria undergo an intermittent electrical and chemical excitation called mitochondrial flash (mitoflash), which transiently uncouples mitochondrial respiration from ATP production. How a mitoflash is generated and what specific role it plays in bioenergetics remain incompletely understood. Here, we investigate mitoflash biogenesis in isolated cardiac mitochondria by varying the respiratory states and substrate supply and by dissecting the involvement of different electron transfer chain (ETC) complexes. We find that robust mitoflash activity occurs once mitochondria are electrochemically charged by state II/IV respiration (i.e., no ATP synthesis at Complex V), regardless of the substrate entry site (Complex I, Complex II, or Complex IV). Inhibiting forward electron transfer abolishes, while blocking reverse electron transfer generally augments, mitoflash production. Switching from state II/IV to state III respiration, to allow for ATP synthesis at Complex V, markedly diminishes mitoflash activity. Intriguingly, when mitochondria are electrochemically charged by the ATPase activity of Complex V, mitoflashes are generated independently of ETC activity. These findings suggest that mitoflash biogenesis is mechanistically linked to the build up of mitochondrial electrochemical potential rather than ETC activity alone, and may functionally counteract overcharging of the mitochondria and hence serve as an autoregulator of mitochondrial proton electrochemical potential.

Ultrastructural Characterization of Flashing Mitochondria

Mitochondria undergo spontaneous transient elevations in matrix pH associated with drops in mitochondrial membrane potential. These mitopHlashes require a functional respiratory chain and the profusion protein optic atrophy 1, but their mechanistic basis is unclear. To gain insight on the origin of these dynamic events, we resolved the ultrastructure of flashing mitochondria by correlative light and electron microscopy. HeLa cells expressing the matrix-targeted pH probe mitoSypHer were screened for mitopHlashes and fixed immediately after the occurrence of a flashing event. The cells were then processed for imaging by serial block face scanning electron microscopy using a focused ion beam to generate ~1,200 slices of 10 nm thickness from a 28 μm × 15 μm cellular volume. Correlation of live/fixed fluorescence and electron microscopy images allowed the unambiguous identification of flashing and nonflashing mitochondria. Three-dimensional reconstruction and surface mapping revealed that each tomogram contained two flashing mitochondria of unequal sizes, one being much larger than the average mitochondrial volume. Flashing mitochondria were 10-fold larger than silent mitochondria but with a surface to volume ratio and a cristae volume similar to nonflashing mitochondria. Flashing mitochondria were connected by tubular structures, formed more membrane contact sites, and a constriction was observed at a junction between a flashing mitochondrion and a nonflashing mitochondrion. These data indicate that flashing mitochondria are structurally preserved and bioenergetically competent but form numerous membrane contact sites and are connected by tubular structures, consistent with our earlier suggestion that mitopHlashes might be triggered by the opening of fusion pores between contiguous mitochondria.

Quantitative analysis of mitoflash excited by femtosecond laser

Mitochondrial oxidative flashes (mitoflashes) are oxidative burst events in mitochondria. It is crosslinked with numerous mitochondrial molecular processes and related with pivotal cell functions such as apoptosis and respiration. In previous research, mitoflashes were found as spontaneous occasional events. It would be observed more frequently if cells were treated with proapoptotic chemicals. We show that multiple mitoflashes can be initiated by a single femtosecond-laser stimulation that was tightly focused on a diffraction-limited spot in the mitochondrial tubular structure. The mitoflash events triggered by different photostimulations are quantified and analyzed. The width and amplitude of mitoflashes are found very sensitive to stimulation parameters including laser power, exposure duration, and total incident laser energy. This study provides a quantitative investigation on the photostimulated mitoflashes. It may thus demonstrate such optical method to be a promising technique in future mitochondrial research.

Overexpressed UCP2 regulates mitochondrial flashes and reverses lipopolysaccharide-induced cardiomyocytes injury

Background: Mitochondrial flashes (mitoflashes) are transient signals from transient bursts of reactive oxygen species (ROS) and changes in pH that occur in certain physiological or pathological conditions. Mitoflashes are closely related to metabolism, cell differentiation, stress response, diseases, and aging. Sepsis can trigger mitochondrial dysfunction in myocardial cells, which leads to ROS overproduction, while uncoupling protein 2 (UCP2) can reduce ROS production. This study aims to observe whether UCP2 overexpression can regulate the frequency of mitoflashes in cardiomyocytes during sepsis and thereby play a protective role. Methods: A cell model for sepsis-induced myocardial damage was established using lipopolysaccharide (LPS). UCP2 overexpression in cardiomyocytes was achieved by adenovirus transfection. Creatinine kinase (CK), lactate dehydrogenase (LDH), tumor necrosis factor (TNF-α), and interleukin (IL-6) activities were detected, and mitochondrial membrane potentials (MMP) were measured. The frequency of mitoflashes in cardiomyocytes was observed. Results: With LPS stimulation, mitoflashes in cardiomyocytes increased significantly, and the MMP was damaged. Additionally, significant increases in CK, LDH, TNF-α, and IL-6 expression levels were observed. UCP2 overexpression can significantly reverse myocardial cell injuries that result from LPS stimulation. Compared with the LPS group, the LPS+UCP2 overexpression group showed a decrease in mitoflash frequency, an improved MMP, and decreases in CK, LDH, TNF-α, and IL-6 expression levels. Conclusion: This study is the first to demonstrate the function of UCP2 overexpression in protecting the myocardium by regulating mitoflash frequency and reversing sepsis-induced myocardial injuries.

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.

Auto-regulation in the powerhouse

Mitochondrial flashes have a central role in ensuring that ATP levels remain constant in heart cells.

Mitochondrial flashes regulate ATP homeostasis in the heart

The maintenance of a constant ATP level (‘set-point’) is a vital homeostatic function shared by eukaryotic cells. In particular, mammalian myocardium exquisitely safeguards its ATP set-point despite 10-fold fluctuations in cardiac workload. However, the exact mechanisms underlying this regulation of ATP homeostasis remain elusive. Here we show mitochondrial flashes (mitoflashes), recently discovered dynamic activity of mitochondria, play an essential role for the auto-regulation of ATP set-point in the heart. Specifically, mitoflashes negatively regulate ATP production in isolated respiring mitochondria and, their activity waxes and wanes to counteract the ATP supply-demand imbalance caused by superfluous substrate and altered workload in cardiomyocytes. Moreover, manipulating mitoflash activity is sufficient to inversely shift the otherwise stable ATP set-point. Mechanistically, the Bcl-xL-regulated proton leakage through F₁F_o-ATP synthase appears to mediate the coupling between mitoflash production and ATP set-point regulation. These findings indicate mitoflashes appear to constitute a digital auto-regulator for ATP homeostasis in the heart.

DOI:http://dx.doi.org/10.7554/eLife.23908.001

A small molecule called ATP is often referred to as the primary “energy currency” of living cells. It is required to power tasks as diverse as the general housekeeping processes that keep all cells alive to the programmed cell death response that dismantles any cells that are no longer needed. It is also crucial that cells maintain a constant level of ATP at all times, even when the supply of and demand for ATP fluctuate. This control is particularly important in the mammalian heart where the rates of ATP production and consumption change ten-fold during intense exercise. Despite intensive research over the past decades, it was still not known how cells keep ATP levels constant.

In many cell types, including heart muscle cells, ATP is mainly produced inside compartments called mitochondria. Each heart muscle cell contains between 5,000 and 8,000 mitochondria. Recent experiments have shown that ATP production in mitochondria is interrupted by ten-second bursts called “mitochondrial flashes” (or mitoflashes for short), during which the mitochondria release chemicals called reactive oxygen species. The mitoflashes are tightly linked with energy usage, and Wang, Zhang, Wu et al. have now explored if and how mitoflashes regulate ATP levels in the heart.

Experiments on isolated mitochondria from mouse heart muscle cells showed that mitoflashes inhibit the production of ATP. When the intact heart muscle cells were given excess of the building blocks needed to produce ATP – mitoflashes occurred more often. Conversely, when the cells were forced to contract more quickly, which increased demand for ATP, the mitoflashes occurred less often. Importantly, the level of ATP inside the cells actually remained constant in the experiments. Furthermore, inhibiting mitoflashes with antioxidants increased the ATP concentration in heart muscle cells. Lastly, Wang et al. demonstrated that mitoflashes could be triggered under certain conditions.

Overall, these experiments uncovered a way in which highly active cells can maintain a constant level of ATP. Future studies are needed to understand exactly how mitoflashes are initiated and how they in turn inhibit ATP production. A better understanding of these processes might uncover molecules that could be targeted by drugs to the control of the rate of ATP production to treat heart failure.

DOI:http://dx.doi.org/10.7554/eLife.23908.002

Inflammasomes on the Crossroads of Innate Immune Recognition and Metabolic Control

Inflammasomes are protein complexes formed upon encounter of microbial or damage-associated stimuli. The main output of inflammasome assembly is activation of caspase-1, a protease involved in both pro-inflammatory and host-protective responses. Defined bacterial or viral ligands have been identified for the inflammasome-forming receptors AIM2, NLRP1, and NLRC4. The signals activating other inflammasomes, NLRP3, NLRP6, and pyrin, are less well understood. Recent studies implicated several low-molecular-weight compounds traditionally linked to metabolism, not immunity, in modulation of inflammasome signaling. Furthermore, genetic, pharmacological, or pathogen-mediated interference with energy metabolism also affects inflammasome activation. Here we review the findings on how microbial- and host-derived metabolites regulate activation of the NLRP3 and NLRP6 inflammasomes. We discuss the different models of how glycolysis and mitochondrial metabolism control the NLRP3 inflammasome. Finally, we summarize the findings on metabolic control of pyrin and point to open questions to be addressed to broaden our understanding of metabolism-inflammasome interactions.

Dendritic mitoflash as a putative signal for stabilizing long-term synaptic plasticity

Mitochondrial flashes (mitoflashes) are recently discovered excitable mitochondrial events in many cell types. Here we investigate their occurrence in the context of structural long-term potentiation (sLTP) at hippocampal synapses. At dendritic spines stimulated by electric pulses, glycine, or targeted glutamate uncaging, induction of sLTP is associated with a phasic occurrence of local, quantized mitochondrial activity in the form of one or a few mitoflashes, over a 30-min window. Low-dose nigericin or photoactivation that elicits mitoflashes stabilizes otherwise short-term spine enlargement into sLTP. Meanwhile, scavengers of reactive oxygen species suppress mitoflashes while blocking sLTP. With targeted photoactivation of mitoflashes, we further show that the stabilization of sLTP is effective within the critical 30-min time-window and a spatial extent of ~2 μm, similar to that of local diffusive reactive oxygen species. These findings indicate a potential signaling role of dendritic mitochondria in synaptic plasticity, and provide new insights into the cellular function of mitoflashes.

Mitoflashes are dynamic events in mitochondria, associated with depolarization and release of reactive oxygen species, and have been associated with several cellular functions. The authors now show that in neurons, dendritic mitoflashes are involved in structural postsynaptic changes during LTP.

Deficiency of PHB complex impairs respiratory supercomplex formation and activates mitochondrial flashes

Prohibitins (PHBs; prohibitin 1, PHB1 or PHB, and prohibitin 2, PHB2) are evolutionarily conserved and ubiquitously expressed mitochondrial proteins. PHBs form multimeric ring complexes acting as scaffolds in the inner mitochondrial membrane. Mitochondrial flashes (mitoflashes) are newly discovered mitochondrial signaling events that reflect electrical and chemical excitations of the organelle. Here, we investigate the possible roles of PHBs in the regulation of mitoflash signaling. Downregulation of PHBs increases mitoflash frequency by up to 5.4-fold due to elevated basal reactive oxygen species (ROS) production in the mitochondria. Mechanistically, PHB deficiency impairs the formation of mitochondrial respiratory supercomplexes (RSCs) without altering the abundance of individual respiratory complex subunits. These impairments induced by PHB deficiency are effectively rescued by co-expression of PHB1 and PHB2, indicating that the multimeric PHB complex acts as the functional unit. Furthermore, downregulating other RSC assembly factors, including SCAFI (also known as COX7A2L), RCF1a (HIGD1A), RCF1b (HIGD2A), UQCC3 and SLP2 (STOML2), all activate mitoflashes through elevating mitochondrial ROS production. Our findings identify the PHB complex as a new regulator of RSC formation and mitoflash signaling, and delineate a general relationship among RSC formation, basal ROS production and mitoflash biogenesis.

Mitochondrial Flashes: Elemental Signaling Events in Eukaryotic Cells

Mitochondrial flashes (mitoflashes) are recently discovered mitochondrial activity which reflects chemical and electrical excitation of the organelle. Emerging evidence indicates that mitoflashes represent highly regulated, elementary signaling events that play important roles in physiological and pathophysiological processes in eukaryotes. Furthermore, they are regulated by mitochondrial ROS, Ca²⁺, and protons, and are intertwined with mitochondrial metabolic processes. As such, targeting mitoflash activity may provide a novel means for the control of mitochondrial metabolism and signaling in health and disease. In this brief review, we summarize salient features and mechanisms of biogenesis of mitoflashes, and synthesize data on mitoflash biology in the context of metabolism, cell differentiation, stress response, disease, and ageing.

Cristae remodeling causes acidification detected by integrated graphene sensor during mitochondrial outer membrane permeabilization

The intrinsic apoptotic pathway and the resultant mitochondrial outer membrane permeabilization (MOMP) via BAK and BAX oligomerization, cytochrome c (cytc) release, and caspase activation are well studied, but their effect on cytosolic pH is poorly understood. Using isolated mitochondria, we show that MOMP results in acidification of the surrounding medium. BAK conformational changes associated with MOMP activate the OMA1 protease to cleave OPA1 resulting in remodeling of the cristae and release of the highly concentrated protons within the cristae invaginations. This was revealed by utilizing a nanomaterial graphene as an optically clear and ultrasensitive pH sensor that can measure ionic changes induced by tethered mitochondria. With this platform, we have found that activation of mitochondrial apoptosis is accompanied by a gradual drop in extra-mitochondrial pH and a decline in membrane potential, both of which can be rescued by adding exogenous cytc. These findings have importance for potential pharmacological manipulation of apoptosis, in the treatment of cancer.