Cyclophilin D regulates mitochondrial flashes and metabolism in cardiac myocytes

An overview of ATP synthase, inhibitors, and their toxicity

Mitochondrial complex V (ATP synthase) is a remarkable molecular motor crucial in generating ATP and sustaining mitochondrial function. Its importance in cellular metabolism cannot be overstated, as malfunction of ATP synthase has been linked to various pathological conditions. Both natural and synthetic ATP synthase inhibitors have been extensively studied, revealing their inhibitory sites and modes of action. These findings have opened exciting avenues for developing new therapeutics and discovering new pesticides and herbicides to safeguard global food supplies. However, it is essential to remember that these compounds can also adversely affect human and animal health, impacting vital organs such as the nervous system, heart, and kidneys. This review aims to provide a comprehensive overview of mitochondrial ATP synthase, its structural and functional features, and the most common inhibitors and their potential toxicities.

Succinate dehydrogenase is essential for epigenetic and metabolic homeostasis in hearts

A hallmark of heart failure is a metabolic switch away from fatty acids β-oxidation (FAO) to glycolysis. Here, we show that succinate dehydrogenase (SDH) is required for maintenance of myocardial homeostasis of FAO/glycolysis. Mice with cardiomyocyte-restricted deletion of subunit b or c of SDH developed a dilated cardiomyopathy and heart failure. Hypertrophied hearts displayed a decrease in FAO, while glucose uptake and glycolysis were augmented, which was reversed by enforcing FAO fuels via a high-fat diet, which also improved heart failure of mutant mice. SDH-deficient hearts exhibited an increase in genome-wide DNA methylation associated with accumulation of succinate, a metabolite known to inhibit DNA demethylases, resulting in changes of myocardial transcriptomic landscape. Succinate induced DNA hypermethylation and depressed the expression of FAO genes in myocardium, leading to imbalanced FAO/glycolysis. Inhibition of succinate by α-ketoglutarate restored transcriptional profiles and metabolic disorders in SDH-deficient cardiomyocytes. Thus, our findings reveal the essential role for SDH in metabolic remodeling of failing hearts, and highlight the potential of therapeutic strategies to prevent cardiac dysfunction in the setting of SDH deficiency.

Small-Molecule Cyclophilin Inhibitors Potently Reduce Platelet Procoagulant Activity

Procoagulant platelets are associated with an increased risk for thrombosis. Procoagulant platelet formation is mediated via Cyclophilin D (CypD) mediated opening of the mitochondrial permeability transition pore. Inhibiting CypD activity could therefore be an interesting approach to limiting thrombosis. In this study, we investigated the potential of two novel, non-immunosuppressive, non-peptidic small-molecule cyclophilin inhibitors (SMCypIs) to limit thrombosis in vitro, in comparison with the cyclophilin inhibitor and immunosuppressant Cyclosporin A (CsA). Both cyclophilin inhibitors significantly decreased procoagulant platelet formation upon dual-agonist stimulation, shown by a decreased phosphatidylserine (PS) exposure, as well as a reduction in the loss of mitochondrial membrane potential. Furthermore, the SMCypIs potently reduced procoagulant platelet-dependent clotting time, as well as fibrin formation under flow, comparable to CsA. No effect was observed on agonist-induced platelet activation measured by P-selectin expression, as well as CypA-mediated integrin α_IIbβ₃ activation. Importantly, whereas CsA increased Adenosine 5'-diphosphate (ADP)-induced platelet aggregation, this was unaffected in the presence of the SMCypIs. We here demonstrate specific cyclophilin inhibition does not affect normal platelet function, while a clear reduction in procoagulant platelets is observed. Reducing platelet procoagulant activity by inhibiting cyclophilins with SMCypIs forms a promising strategy to limit thrombosis.

Small-molecule inhibitors of cyclophilin D as potential therapeutics in mitochondria-related diseases

Cyclophilin D (CypD) is a key regulator of mitochondrial permeability transition pore (mPTP) opening. This pathophysiological phenomenon is associated with the development of several human diseases, including ischemia-reperfusion injury and neurodegeneration. Blocking mPTP opening through CypD inhibition could be a novel and promising therapeutic approach for these conditions. While numerous CypD inhibitors have been discovered to date, none have been introduced into clinical practice, mostly owing to their high toxicity, unfavorable pharmacokinetics, and low selectivity for CypD over other cyclophilins. This review summarizes current knowledge of CypD inhibitors, with a particular focus on small-molecule compounds with regard to their in vitro activity, their selectivity for CypD, and their binding mode within the enzyme's active site. Finally, approaches for improving the molecular design of CypD inhibitors are discussed.

Functional isolation, culture and cryopreservation of adult human primary cardiomyocytes

Cardiovascular diseases are the most common cause of death globally. Accurately modeling cardiac homeostasis, dysfunction, and drug response lies at the heart of cardiac research. Adult human primary cardiomyocytes (hPCMs) are a promising cellular model, but unstable isolation efficiency and quality, rapid cell death in culture, and unknown response to cryopreservation prevent them from becoming a reliable and flexible in vitro cardiac model. Combing the use of a reversible inhibitor of myosin II ATPase, (-)-blebbistatin (Bleb), and multiple optimization steps of the isolation procedure, we achieved a 2.74-fold increase in cell viability over traditional methods, accompanied by better cellular morphology, minimally perturbed gene expression, intact electrophysiology, and normal neurohormonal signaling. Further optimization of culture conditions established a method that was capable of maintaining optimal cell viability, morphology, and mitochondrial respiration for at least 7 days. Most importantly, we successfully cryopreserved hPCMs, which were structurally, molecularly, and functionally intact after undergoing the freeze-thaw cycle. hPCMs demonstrated greater sensitivity towards a set of cardiotoxic drugs, compared to human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Further dissection of cardiomyocyte drug response at both the population and single-cell transcriptomic level revealed that hPCM responses were more pronouncedly enriched in cardiac function, whereas hiPSC-CMs responses reflected cardiac development. Together, we established a full set of methodologies for the efficient isolation and prolonged maintenance of functional primary adult human cardiomyocytes in vitro, unlocking their potential as a cellular model for cardiovascular research, drug discovery, and safety pharmacology.

Efficient Correction of a Hypertrophic Cardiomyopathy Mutation by ABEmax-NG

[Figure: see text].

Regulating quantal size of neurotransmitter release through a GPCR voltage sensor

Current models emphasize that membrane voltage (Vm) depolarization-induced Ca²⁺ influx triggers the fusion of vesicles to the plasma membrane. In sympathetic adrenal chromaffin cells, activation of a variety of G protein coupled receptors (GPCRs) can inhibit quantal size (QS) through the direct interaction of G protein Giβγ subunits with exocytosis fusion proteins. Here we report that, independently from Ca²⁺, Vm (action potential) per se regulates the amount of catecholamine released from each vesicle, the QS. The Vm regulation of QS was through ATP-activated GPCR-P2Y₁₂ receptors. D76 and D127 in P2Y₁₂ were the voltage-sensing sites. Finally, we revealed the relevance of the Vm dependence of QS for tuning autoinhibition and target cell functions. Together, membrane voltage per se increases the quantal size of dense-core vesicle release of catecholamine via Vm → P2Y₁₂(D76/D127) → Giβγ → QS → myocyte contractility, offering a universal Vm-GPCR signaling pathway for its functions in the nervous system and other systems containing GPCRs.

Prohibitin 2 deficiency impairs cardiac fatty acid oxidation and causes heart failure

Fatty acids are the most major substrate source for adult cardiac energy generation. Prohibitin 2 (PHB2), a highly conserved protein located in mitochondrial inner membrane, plays key roles in cellular energy metabolic homeostasis. However, its functions in regulating cardiac fatty acid metabolism have remained largely unknown. Our study demonstrates that cardiac-specific knockout of Phb2 leads to accumulation of lipid droplets and causes heart failure. Mechanistically, ablation of PHB2 impairs cardiac fatty acid oxidation (FAO) through downregulating carnitine palmitoyltransferase1b (CPT1b), a rate-limiting enzyme of cardiac mitochondrial FAO. Moreover, overexpression of CPT1b alleviates impaired FAO in PHB2-deficient cardiomyocytes. Thus, our study provides direct evidence for the link between PHB2 and cardiac fatty acid metabolism. Our study points out that PHB2 is a potential FAO regulator in cardiac mitochondrial inner membrane, as well as the connection between PHB2 and CPT1b and their relationships to cardiac pathology especially to cardiac fatty acid metabolic disorder.

Proximal tubule cyclophilin D regulates fatty acid oxidation in cisplatin-induced acute kidney injury

Regardless of the etiology, acute kidney injury involves aspects of mitochondrial dysfunction and ATP depletion. Fatty acid oxidation is the preferred energy source of the kidney and is inhibited during acute kidney injury. A pivotal role for the mitochondrial matrix protein, cyclophilin D in regulating overall cell metabolism is being unraveled. We hypothesize that mitochondrial interaction of proximal tubule cyclophilin D and the transcription factor PPARα modulate fatty acid beta-oxidation in cisplatin-induced acute kidney injury. Cisplatin injury resulted in histological and functional damage in the kidney with downregulation of fatty acid oxidation genes and increase of intrarenal lipid accumulation. However, proximal tubule-specific deletion of cyclophilin D protected the kidneys from the aforementioned effects. Mitochondrial translocation of PPARα, its binding to cyclophilin D, and sequestration led to inhibition of its nuclear translocation and transcription of PPARα-regulated fatty acid oxidation genes during cisplatin-induced acute kidney injury. Genetic or pharmacological inhibition of cyclophilin D preserved nuclear expression and transcriptional activity of PPARα and prevented the impairment of fatty acid oxidation and intracellular lipid accumulation. Docking analysis identified potential binding sites between PPARα and cyclophilin D. Thus, our results indicate that proximal tubule cyclophilin D elicits impaired mitochondrial fatty acid oxidation via mitochondrial interaction between cyclophilin D and PPARα. Hence, targeting their interaction may be a potential therapeutic strategy to prevent energy depletion, lipotoxicity and cell death in cisplatin-induced acute kidney injury.

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.

The Role of Mitochondria in the Mechanisms of Cardiac Ischemia-Reperfusion Injury

Mitochondria play a critical role in maintaining cellular function by ATP production. They are also a source of reactive oxygen species (ROS) and proapoptotic factors. The role of mitochondria has been established in many aspects of cell physiology/pathophysiology, including cell signaling. Mitochondria may deteriorate under various pathological conditions, including ischemia-reperfusion (IR) injury. Mitochondrial injury can be one of the main causes for cardiac and other tissue injuries by energy stress and overproduction of toxic reactive oxygen species, leading to oxidative stress, elevated calcium and apoptotic and necrotic cell death. However, the interplay among these processes in normal and pathological conditions is still poorly understood. Mitochondria play a critical role in cardiac IR injury, where they are directly involved in several pathophysiological mechanisms. We also discuss the role of mitochondria in the context of mitochondrial dynamics, specializations and heterogeneity. Also, we wanted to stress the existence of morphologically and functionally different mitochondrial subpopulations in the heart that may have different sensitivities to diseases and IR injury. Therefore, various cardioprotective interventions that modulate mitochondrial stability, dynamics and turnover, including various pharmacologic agents, specific mitochondrial antioxidants and uncouplers, and ischemic preconditioning can be considered as the main strategies to protect mitochondrial and cardiovascular function and thus enhance longevity.

Cyclophilin D participates in the inhibitory effect of high-fat diet on the expression of steroidogenic acute regulatory protein

OBJECTIVE

The high-fat diet (HFD)-induced obesity is responsible for the testosterone deficiency (TD). However, the mechanism remains unknown. Mitochondrial homeostasis is proved to be important for maintaining the function of steroidogenic acute regulatory protein (StAR), the first rate-limiting enzyme in testosterone synthesis. As the key regulator of mitochondrial membrane permeability, cyclophilin D (CypD) plays a crucial role in maintaining mitochondrial function. In this study, we sought to elucidate the role of CypD in the expression of StAR affected by HFD.

METHODS

To analyse the influence of CypD on StAR in vivo and in vitro, mouse models of HFD, CypD overexpression and CypD knockout (Ppif-/- ) as well as Leydig cells treated with palmitic acid (PA) and CypD overexpression plasmids were examined with an array of metabolic, mitochondrial function and molecular assays.

RESULTS

Compared with the normal diet mice, consistent with reduced testosterone in testes, the expressions of StAR in both mRNA and protein levels in HFD mice were down-regulated, while expressions of CypD were up-regulated. High-fat intake impaired mitochondrial function with the decrease in StAR in Leydig cells. Overexpression of CypD inhibited StAR expressions in vivo and in vitro. Compared with C57BL/6 mice with HFD, expressions of StAR were improved in Ppif-/- mice with HFD.

CONCLUSIONS

Mitochondrial CypD involved in the inhibitory effect of HFD on StAR expression in testes.

NDUFAB1 confers cardio-protection by enhancing mitochondrial bioenergetics through coordination of respiratory complex and supercomplex assembly

The impairment of mitochondrial bioenergetics, often coupled with exaggerated reactive oxygen species (ROS) production, is a fundamental disease mechanism in organs with a high demand for energy, including the heart. Building a more robust and safer cellular powerhouse holds the promise for protecting these organs in stressful conditions. Here, we demonstrate that NADH:ubiquinone oxidoreductase subunit AB1 (NDUFAB1), also known as mitochondrial acyl carrier protein, acts as a powerful cardio-protector by conferring greater capacity and efficiency of mitochondrial energy metabolism. In particular, NDUFAB1 not only serves as a complex I subunit, but also coordinates the assembly of respiratory complexes I, II, and III, and supercomplexes, through regulating iron-sulfur biosynthesis and complex I subunit stability. Cardiac-specific deletion of Ndufab1 in mice caused defective bioenergetics and elevated ROS levels, leading to progressive dilated cardiomyopathy and eventual heart failure and sudden death. Overexpression of Ndufab1 effectively enhanced mitochondrial bioenergetics while limiting ROS production and protected the heart against ischemia-reperfusion injury. Together, our findings identify that NDUFAB1 is a crucial regulator of mitochondrial energy and ROS metabolism through coordinating the assembly of respiratory complexes and supercomplexes, and thus provide a potential therapeutic target for the prevention and treatment of heart failure.

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.

Why don't mice lacking the mitochondrial Ca2+ uniporter experience an energy crisis?

Current dogma holds that the heart balances energy demand and supply effectively and sustainably by sequestering enough Ca²⁺ into mitochondria during heartbeats to stimulate metabolic enzymes in the tricarboxylic acid (TCA) cycle and electron transport chain (ETC). This process is called excitation-contraction-bioenergetics (ECB) coupling. Recent breakthroughs in identifying the mitochondrial Ca²⁺ uniporter (MCU) and its associated proteins have opened up new windows for interrogating the molecular mechanisms of mitochondrial Ca²⁺ homeostasis regulation and its role in ECB coupling. Despite remarkable progress made in the past 7 years, it has been surprising, almost disappointing, that germline MCU deficiency in mice with certain genetic background yields viable pups, and knockout of the MCU in adult heart does not cause lethality. Moreover, MCU deficiency results in few adverse phenotypes, normal performance, and preserved bioenergetics in the heart at baseline. In this review, we briefly assess the existing literature on mitochondrial Ca²⁺ homeostasis regulation and then we consider possible explanations for why MCU-deficient mice are spared from energy crises under physiological conditions. We propose that MCU and/or mitochondrial Ca²⁺ may have limited ability to set ECB coupling, that other mitochondrial Ca²⁺ handling mechanisms may play a role, and that extra-mitochondrial Ca²⁺ may regulate ECB coupling. Since the heart needs to regenerate a significant amount of ATP to assure the perpetuation of heartbeats, multiple mechanisms are likely to work in concert to match energy supply with demand.

Divergent Effects of Cyclophilin-D Inhibition on the Female Rat Heart: Acute Versus Chronic Post-Myocardial Infarction

BACKGROUND/AIMS

The mitochondrial permeability transition pore opening plays a critical role in the pathogenesis of myocardial infarction. Inhibition of cyclophilin-D (CyP-D), a key regulator of the mitochondrial permeability transition pore, has been shown to exert cardioprotective effects against ischemia-reperfusion injury on various animal models, mostly in males. However, failure of recent clinical trials requires a detailed elucidation of the cardioprotective efficacy of CyP-D inhibition. The aim of this study was to examine whether cardioprotective effects of sanglifehrin A, a potent inhibitor of CyP-D, on post-infarcted hearts depends on reperfusion.

METHODS

Acute or chronic myocardial infarction was induced by coronary artery ligation with/without subsequent reperfusion for 2 and 28 days in female Sprague-Dawley rats. Cardiac function was estimated by echocardiography. Oxygen consumption rates, ROS production, permeability transition pore opening, protein carbonylation and respiratory supercomplexes were analyzed in isolated cardiac mitochondria.

RESULTS

Sanglifehrin A significantly improved cardiac function of reperfused hearts at 2 days but failed to protect after 28 days. No protection was observed in non-reperfused post-infarcted hearts. The respiratory control index of mitochondria was significantly reduced in reperfused infarcted hearts at 2-days with no effect at 28-days post-infarction on reperfused and non-reperfused hearts. Likewise, only a minor increase in reactive oxygen species production was observed at 2-days in non-reperfused post-infarcted hearts.

CONCLUSION

This study demonstrates that CyP-D inhibition exerts cardioprotective effects in reperfused but not in non-reperfused infarcted hearts of female rats, and the effects are observed only during acute post-infarction injury.

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.

Different approaches to modeling analysis of mitochondrial swelling

Mitochondria are critical players involved in both cell life and death through multiple pathways. Structural integrity, metabolism and function of mitochondria are regulated by matrix volume due to physiological changes of ion homeostasis in cellular cytoplasm and mitochondria. Ca²⁺ and K⁺ presumably play a critical role in physiological and pathological swelling of mitochondria when increased uptake (influx)/decreased release (efflux) of these ions enhances osmotic pressure accompanied by high water accumulation in the matrix. Changes in the matrix volume in the physiological range have a stimulatory effect on electron transfer chain and oxidative phosphorylation to satisfy metabolic requirements of the cell. However, excessive matrix swelling associated with the sustained opening of mitochondrial permeability transition pores (PTP) and other PTP-independent mechanisms compromises mitochondrial function and integrity leading to cell death. The mechanisms of transition from reversible (physiological) to irreversible (pathological) swelling of mitochondria remain unknown. Mitochondrial swelling is involved in the pathogenesis of many human diseases such as neurodegenerative and cardiovascular diseases. Therefore, modeling analysis of the swelling process is important for understanding the mechanisms of cell dysfunction. This review attempts to describe the role of mitochondrial swelling in cell life and death and the main mechanisms involved in the maintenance of ion homeostasis and swelling. The review also summarizes and discusses different kinetic models and approaches that can be useful for the development of new models for better simulation and prediction of in vivo mitochondrial swelling.

Mitochondrial permeability transition in cardiac ischemia-reperfusion: whether cyclophilin D is a viable target for cardioprotection?

Growing number of studies provide strong evidence that the mitochondrial permeability transition pore (PTP), a non-selective channel in the inner mitochondrial membrane, is involved in the pathogenesis of cardiac ischemia-reperfusion and can be targeted to attenuate reperfusion-induced damage to the myocardium. The molecular identity of the PTP remains unknown and cyclophilin D is the only protein commonly accepted as a major regulator of the PTP opening. Therefore, cyclophilin D is an attractive target for pharmacological or genetic therapies to reduce ischemia-reperfusion injury in various animal models and humans. Most animal studies demonstrated cardioprotective effects of PTP inhibition; however, a recent large clinical trial conducted by international groups demonstrated that cyclosporine A, a cyclophilin D inhibitor, failed to protect the heart in patients with myocardial infarction. These studies, among others, raise the question of whether cyclophilin D, which plays an important physiological role in the regulation of cell metabolism and mitochondrial bioenergetics, is a viable target for cardioprotection. This review discusses previous studies to provide comprehensive information on the physiological role of cyclophilin D as well as PTP opening in the cell that can be taken into consideration for the development of new PTP inhibitors.

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

A novel fission-independent role of dynamin-related protein 1 in cardiac mitochondrial respiration

AIMS

Mitochondria in adult cardiomyocytes exhibit static morphology and infrequent dynamic changes, despite the high abundance of fission and fusion regulatory proteins in the heart. Previous reports have indicated that fusion proteins may bear functions beyond morphology regulation. Here, we investigated the role of fission protein, dynamin-related protein 1 (DRP1), on mitochondrial respiration regulation in adult cardiomyocytes.

METHODS AND RESULTS

By using genetic or pharmacological approaches, we manipulated the activity or protein level of fission and fusion proteins and found they mildly influenced mitochondrial morphology in adult rodent cardiomyocytes, which is in contrast to their significant effect in H9C2 cardiac myoblasts. Intriguingly, inhibiting endogenous DRP1 by dominant-negative DRP1 mutation (K38A), shRNA, or Mdivi-1 suppressed maximal respiration and respiratory control ratio in isolated mitochondria from adult mouse heart or in adult cardiomyocytes from rat. Meanwhile, basal respiration was increased due to increased proton leak. Facilitating mitofusin-mediated fusion by S3 compound, however, failed to inhibit mitochondrial respiration in adult cardiomyocytes. Mechanistically, DRP1 inhibition did not affect the maximal activity of individual respiratory chain complexes or the assembly of supercomplexes. Knocking out cyclophilin D, a regulator of mitochondrial permeability transition pore (mPTP), abolished the effect of DRP1 inhibition on respiration. Finally, DRP1 inhibition decreased transient mPTP-mediated mitochondrial flashes, delayed laser-induced mPTP opening and suppressed mitochondrial reactive oxygen species (ROS).

CONCLUSION

These results uncover a novel non-canonical function of the fission protein, DRP1 in maintaining or positively stimulating mitochondrial respiration, bioenergetics and ROS signalling in adult cardiomyocyte, which is likely independent of morphological changes.

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.

The Mitochondrial Permeability Transition Pore Regulator Cyclophilin D Exhibits Tissue-Specific Control of Metabolic Homeostasis

The mitochondrial permeability transition pore (mPTP) is a key regulator of mitochondrial function that has been implicated in the pathogenesis of metabolic disease. Cyclophilin D (CypD) is a critical regulator that directly binds to mPTP constituents to facilitate the pore opening. We previously found that global CypD knockout mice (KO) are protected from diet-induced glucose intolerance; however, the tissue-specific function of CypD and mPTP, particularly in the control of glucose homeostasis, has not been ascertained. To this end, we performed calcium retention capacity (CRC) assay to compare the importance of CypD in the liver versus skeletal muscle. We found that liver mitochondria are more dependent on CypD for mPTP opening than skeletal muscle mitochondria. To ascertain the tissue-specific role of CypD in metabolic homeostasis, we generated liver-specific and muscle-specific CypD knockout mice (LKO and MKO, respectively) and fed them either a chow diet or 45% high-fat diet (HFD) for 14 weeks. MKO mice displayed similar body weight gain and glucose intolerance compared with wild type littermates (WT), whereas LKO mice developed greater visceral obesity, glucose intolerance and pyruvate intolerance compared with WT mice. These findings demonstrate that loss of muscle CypD is not sufficient to alter whole body glucose metabolism, while the loss of liver CypD exacerbates obesity and whole-body metabolic dysfunction in mice fed HFD.

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 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.