Morphological dynamics of mitochondria--a special emphasis on cardiac muscle cells

Mitochondrial CDP-diacylglycerol synthase activity is due to the peripheral protein, TAMM41 and not due to the integral membrane protein, CDP-diacylglycerol synthase 1

CDP diacylglycerol synthase (CDS) catalyses the conversion of phosphatidic acid (PA) to CDP-diacylglycerol, an essential intermediate in the synthesis of phosphatidylglycerol, cardiolipin and phosphatidylinositol (PI). CDS activity has been identified in mitochondria and endoplasmic reticulum of mammalian cells apparently encoded by two highly-related genes, CDS1 and CDS2. Cardiolipin is exclusively synthesised in mitochondria and recent studies in cardiomyocytes suggest that the peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1α and β) serve as transcriptional regulators of mitochondrial biogenesis and up-regulate the transcription of the CDS1 gene. Here we have examined whether CDS1 is responsible for the mitochondrial CDS activity. We report that differentiation of H9c2 cells with retinoic acid towards cardiomyocytes is accompanied by increased expression of mitochondrial proteins, oxygen consumption, and expression of the PA/PI binding protein, PITPNC1, and CDS1 immunoreactivity. Both CDS1 immunoreactivity and CDS activity were found in mitochondria of H9c2 cells as well as in rat heart, liver and brain mitochondria. However, the CDS1 immunoreactivity was traced to a peripheral p55 cross-reactive mitochondrial protein and the mitochondrial CDS activity was due to a peripheral mitochondrial protein, TAMM41, not an integral membrane protein as expected for CDS1. TAMM41 is the mammalian equivalent of the recently identified yeast protein, Tam41. Knockdown of TAMM41 resulted in decreased mitochondrial CDS activity, decreased cardiolipin levels and a decrease in oxygen consumption. We conclude that the CDS activity present in mitochondria is mainly due to TAMM41, which is required for normal mitochondrial function.

Relative Propensities of Cytochrome c Oxidase and Cobalt Corrins for Reaction with Cyanide and Oxygen: Implications for Amelioration of Cyanide Toxicity

In aqueous media at neutral pH, the binding of two cyanide molecules per cobinamide can be described by two formation constants, K_f1 = 1.1 (±0.6) × 10⁵ M^-1 and K_f2 = 8.5 (±0.1) × 10⁴ M^-1, or an overall cyanide binding constant of ∼1 × 10¹⁰ M^-2. In comparison, the cyanide binding constants for cobalamin and a fully oxidized form of cytochrome c oxidase, each binding a single cyanide anion, were found to be 7.9 (±0.5) × 10⁴ M^-1 and 1.6 (±0.2) × 10⁷ M^-1, respectively. An examination of the cyanide-binding properties of cobinamide at neutral pH by stopped-flow spectrophotometry revealed two kinetic phases, rapid and slow, with apparent second-order rate constants of 3.2 (±0.5) × 10³ M^-1 s^-1 and 45 (±1) M^-1 s^-1, respectively. Under the same conditions, cobalamin exhibited a single slow cyanide-binding kinetic phase with a second-order rate constant of 35 (±1) M^-1 s^-1. All three of these processes are significantly slower than the rate at which cyanide is bound by complex IV during enzyme turnover (>10⁶ M^-1 s^-1). Overall, it can be understood from these findings why cobinamide is a measurably better cyanide scavenger than cobalamin, but it is unclear how either cobalt corrin can be antidotal toward cyanide intoxication as neither compound, by itself, appears able to out-compete cytochrome c oxidase for available cyanide. Furthermore, it has also been possible to unequivocally show in head-to-head comparison assays that the enzyme does indeed have greater affinity for cyanide than both cobalamin and cobinamide. A plausible resolution of the paradox that both cobalamin and cobinamide clearly are antidotal toward cyanide intoxication, involving the endogenous auxiliary agent nitric oxide, is suggested. Additionally, the catalytic consumption of oxygen by the cobalt corrins is demonstrated and, in the case of cobinamide, the involvement of cytochrome c when present. Particularly in the case of cobinamide, these oxygen-dependent reactions could potentially lead to erroneous assessment of the ability of the cyanide scavenger to restore the activity of cyanide-inhibited cytochrome c oxidase.

Computational properties of mitochondria in T cell activation and fate

In this article, we review how mitochondrial Ca²⁺ transport (mitochondrial Ca²⁺ uptake and Na⁺/Ca²⁺ exchange) is involved in T cell biology, including activation and differentiation through shaping cellular Ca²⁺ signals. Based on recent observations, we propose that the Ca²⁺ crosstalk between mitochondria, endoplasmic reticulum and cytoplasm may form a proportional–integral–derivative (PID) controller. This PID mechanism (which is well known in engineering) could be responsible for computing cellular decisions. In addition, we point out the importance of analogue and digital signal processing in T cell life and implication of mitochondrial Ca²⁺ transport in this process.

Mitochondrial deformity confined to a single cardiomyocyte in human endomyocardial biopsy specimens: Report of 4 cases

During electron microscopic examination of 156 consecutive human endomyocardial biopsy specimens, we found marked mitochondrial deformity within a single cardiomyocyte in each of 4 specimens. The deformed mitochondria were unevenly distributed, but the deformities were confined to the one cardiomyocyte. Those affected cardiomyocytes were accompanied by nonspecific degenerative changes such as nuclear hypertrophy and/or rarefaction of the myofibrils. Mitochondria in all other cells within the specimens appeared normal. Such an abnormality has never been reported to date. Each of the four cases was diagnosed with a different ailment: post-myocarditis, dilated cardiomyopathy, amyloidosis, and tachycardia-induced heart failure. However, all four cases were accompanied by left ventricular systolic dysfunction at biopsy. The very limited mitochondrial deformation may thus reflect a type of degenerative change that accompanies heart failure. <Learning objective: A marked mitochondrial deformity must have been overlooked to date, which is confined to a single cardiomyocyte in an endomyocardial biopsy specimen. Its etiology is still unknown but may reflect a type of degenerative change that accompanies heart failure.>.

Mdivi‑1 attenuates sodium azide‑induced apoptosis in H9c2 cardiac muscle cells

The aim of the current study was to investigate the effect of mitochondrial division inhibitor 1 (Mdivi-1) in sodium azide-induced cell death in H9c2 cardiac muscle cells. Mdivi-1 is a key inhibitor of the mitochondrial division protein dynamin-related protein 1 (Drp1). Mdivi-1 was added to H9c2 cells for 3 h, after which, the cells were treated with sodium azide for 24 h. Cell viability was measured by Cell Counting kit-8 assay. DAPI staining was used to observe nuclear morphology changes by microscopy. To further investigate the role of mitochondria in sodium azide-induced cell death, mitochondrial membrane potential (ΔΨm) and the cellular ATP content were determined by JC-1 staining and ATP-dependent bioluminescence assay, respectively. Reactive oxygen species (ROS) production was also assessed by use of the specific probe 2′,7′-dichlorodihydrofluorescein diacetate. In addition, the expression of Drp1 and of the apoptosis-related proteins BCL2 apoptosis regulator (Bcl-2), and BCL2 associated X (Bax) was determined by western blotting. The present findings demonstrated that pretreatment with Mdivi-1 attenuated sodium azide-induced H9c2 cell death. Mdivi-1 pretreatment also inhibited the sodium azide-induced downregulation of Bcl-2 expression and upregulation of Bax and Drp1 expression. In addition, the mitochondrion was revealed to be the target organelle of sodium azide-induced toxicity in H9c2 cells. Mdivi-1 pretreatment moderated the dissipation of ΔΨm, preserved the cellular ATP contents and suppressed the production of ROS. The results suggested that the mechanism of sodium azide-induced cell death in H9c2 cells may involve the mitochondria-dependent apoptotic pathway. The present results indicated that Mdivi-1 may have a cardioprotective effect against sodium azide-induced apoptosis in H9c2 cardiac muscle cells.

Eicosapentaenoic acid protects cardiomyoblasts from lipotoxicity in an autophagy-dependent manner

BACKGROUND AND AIMS

The cardiovascular health benefits of eicosapentaenoic acid (EPA) have been demonstrated previously; however, the exact mechanism underlying them remains unclear. Our previous study found that lipotoxicity induced cardiomyocyte apoptosis via the inhibition of autophagy. Accordingly, in this study, we investigated whether EPA attenuated lipotoxicity-induced cardiomyocyte apoptosis through autophagy regulation. The role of EPA in mitochondrial dynamics was analyzed as well.

METHODS

To explore how EPA protected against lipotoxicity-induced myocardial injury, cardiomyoblast (H9C2) cells were left untreated or were treated with 400 μM palmitic acid (PAM) and/or 80 μM EPA for 24 h.

RESULTS

Excessive PAM treatment induced apoptosis. EPA reduced this PAM-induced apoptosis; however, EPA was unable to ameliorate the effects of PAM when autophagy was blocked by 3-methyladenine and bafilomycin A1. PAM blocked the autophagic flux, thus causing the accumulation of autophagosomes and acid vacuoles, whereas EPA restored the autophagic flux. PAM caused a decrease in polyunsaturated fatty acid (PUFA) content and an increase in saturated fatty acid content in the mitochondrial membrane, while EPA was incorporated in the mitochondrial membrane and caused a significant increase in the PUFA content. PAM also decreased the mitochondrial membrane potential, whereas EPA enhanced it. Finally, PAM elevated the expressions of autophagy-related proteins (LC3I, LC3II, p62) and mitochondrial fission protein (Drp1), whereas EPA inhibited their elevation under PAM treatment.

CONCLUSIONS

EPA reduces lipotoxicity-induced cardiomyoblast apoptosis through its effects on autophagy.

Salidroside inhibits high-glucose induced proliferation of vascular smooth muscle cells via inhibiting mitochondrial fission and oxidative stress

The mitochondria are highly dynamic organelles, carefully maintaining network homeostasis by regulating mitochondrial fusion and fission. Mitochondrial dynamics are involved in the regulation of a variety of pathophysiological processes, including cell proliferation. Oxidative stress serves an important role in the remodeling of arterial vascular tissue in diabetic patients by affecting the proliferation of vascular smooth muscle cells (VSMCs). Salidroside is the primary active component of Rhodiola rosea and has been demonstrated to be an antioxidant with cardio- and vascular-protective effects, in addition to improving glucose metabolism. Therefore, the present study aimed to examine the impact of Salidroside on VSMC proliferation, reactive oxygen species (ROS) generation and mitochondrial dynamics under high glucose conditions and the potential mechanisms involved. The current study used Salidroside and a mitochondrial division inhibitor, specifically of Drp1 (Mdivi-1) to treat VSMCs under high glucose conditions for 24 h and assessed VSMCs proliferation, the state of mitochondrial fission and fusion and the expression level of proteins related to mitochondrial dynamics including dynamin-related protein (Drp1) and mitofusin 2 (Mfn2), ROS level and nicotinamide adenine dinucleotide phosphate oxidase activity. The results of the present study indicate that Salidroside and Mdivi-1 inhibit VSMC proliferation, Drp1 expression and oxidative stress and upregulate Mfn2 expression (all P<0.05). The inhibitive effect on VSMC proliferation may be partly reversed by exogenous ROS. In addition, the inhibitive effect on VSMCs proliferation and oxidative stress may also be in part reversed by Mfn2-siRNA. Collectively, these data suggest that Salidroside inhibits VSMCs proliferation induced by high-glucose and may perform its therapeutic effect via maintaining mitochondrial dynamic homeostasis and regulating oxidative stress level, with Mfn2 as a therapeutic target.

Assessment of aflatoxin B1 myocardial toxicity in rats: mitochondrial damage and cellular apoptosis in cardiomyocytes induced by aflatoxin B1

Objective

The number of deaths from heart disease is increasing worldwide. Aflatoxin B1 (AFB1), a toxin produced by the fungi Aspergillus flavus and Aspergillus parasiticus, is frequently detected in improperly processed/stored human food products. While AFB1 hepatotoxicity and carcinogenic properties have been well addressed, its myocardial toxicity is poorly documented. This study aimed to investigate myocardial toxic activity of AFB1.

Methods

Ten rats were fed with AFB1 at a dose that did not result in acute toxic reactions for 30 days and 10 vehicle-fed rats served as controls. Transmission electron microscopy was used to assess mitochondrial damage in cardiomyocytes. The terminal deoxynucleotidyl transferase-mediated UTP nick-end labelling assay was performed to detect apoptosis of cardiomyocytes. Western blotting was performed to measure apoptotic proteins (i.e., active caspase-3, Bax, and Bcl-2) in heart tissue.

Results

AFB1 treatment resulted in mitochondrial membrane disruption and disorganization of cristae, which are indicators of mitochondrial damage. Myocardial cell apoptosis was significantly higher after AFB1 treatment (22.07% ± 3.29%) compared with controls (6.27% ± 2.78%, P < 0.05). AFB1 treatment enhanced expression of active caspase-3, Bax, and Bcl-2 in cardiac tissue.

Conclusion

Various adverse effects are exerted by AFB1 on the heart, indicating AFB1 myocardial toxicity.

High glucose concentration abrogates sevoflurane post-conditioning cardioprotection by advancing mitochondrial fission but dynamin-related protein 1 inhibitor restores these effects

AIM

Hyperglycaemia-induced cell injury is a primary cause of cardiovascular complications in patients with diabetes. In vivo studies demonstrated that sevoflurane post-conditioning (SpostC) was cardioprotective against ischaemia/reperfusion injury, which was blocked by hyperglycaemia. This study investigated whether high glucose concentration abrogated SpostC cardioprotection in vitro by advancing mitochondrial fission and whether mitochondrial division inhibitor-1 (Mdivi-1) restored SpostC cardioprotection in cultured primary neonatal rat cardiomyocytes (NCMs).

METHODS

Primary cultured NCMs in low and high glucose concentrations were subjected to hypoxia/reoxygenation (H/R) injury. SpostC was carried out by adding 2.4% sevoflurane to the cells at the beginning of reoxygenation for 15 min. Cell viability, lactate dehydrogenase (LDH) level, cell death, mitochondrial morphology, mitochondrial membrane potential and mitochondrial permeability transition pore (mPTP) opening level, as well as fission- and fusion-related proteins, were measured after H/R injury. Mdivi-1 treatment was performed 40 min before hypoxia to inhibit DRP1.

RESULTS

SpostC protected cultured cardiomyocytes by increasing cell viability and reducing the LDH level and cell death following H/R, but high glucose concentration eliminated the cardioprotective effect. High glucose concentration abrogated SpostC cardioprotection via mitochondrial fragmentation (evidenced by decreased mitochondrial interconnectivity and elongation) and facilitation of mPTP opening. Decreased mitochondrial membrane potential was investigated with increased DRP1, FIS1 and MFN2 and decreased MFN1 and OPA1 expressions. Mdivi-1 (100 μmol L^-1 ) inhibited excessive mitochondrial fission and restored the cardioprotective effect of SpostC in high glucose conditions.

CONCLUSION

SpostC-induced cardioprotection against H/R injury was impaired under high glucose concentrations, but the inhibition of excess mitochondrial fission restored these effects.

Foxo3a inhibits mitochondrial fission and protects against doxorubicin-induced cardiotoxicity by suppressing MIEF2

Doxorubicin (DOX) as a chemotherapeutic drug is widely used to treat a variety of human tumors. However, a major factor limiting its clinical use is its cardiotoxicity. The molecular components and detailed mechanisms regulating DOX-induced cardiotoxicity remain largely unidentified. Here we report that Foxo3a is downregulated in the cardiomyocyte and mouse heart in response to DOX treatment. Foxo3a attenuates DOX-induced mitochondrial fission and apoptosis in cardiomyocytes. Cardiac specific Foxo3a transgenic mice show reduced mitochondrial fission, apoptosis and cardiotoxicity upon DOX administration. Furthermore, Foxo3a directly targets mitochondrial dynamics protein of 49kDa (MIEF2) and suppresses its expression at transcriptional level. Knockdown of MIEF2 reduces DOX-induced mitochondrial fission and apoptosis in cardiomyocytes and in vivo. Also, knockdown of MIEF2 protects heart from DOX-induced cardiotoxicity. Our study identifies a novel pathway composed of Foxo3a and MIEF2 that mediates DOX cardiotoxicity. This discovery provides a promising therapeutic strategy for the treatment of cancer therapy and cardioprotection.

The Three-Herb Formula Shuang-Huang-Lian stabilizes mast cells through activation of mitochondrial calcium uniporter

Mast cells (MCs) are key effector cells of IgE-FcεRI- or MrgprX2-mediated signaling event. Shuang-Huang-Lian (SHL), a herbal formula from Chinese Pharmacopoeia, has been clinically used in type I hypersensitivity. Our previous study demonstrated that SHL exerted a non-negligible effect on MC stabilization. Herein, we sought to elucidate the molecular mechanisms of the prominent anti-allergic ability of SHL. MrgprX2- and IgE-FcεRI-mediated MC activation in vitro and in vivo models were developed by using compound 48/80 (C48/80) and shrimp tropomyosin (ST), respectively. Our data showed that SHL markedly dampened C48/80- or ST-induced MC degranulation in vitro and in vivo. Mechanistic study indicated that cytosolic Ca²⁺ (Ca²⁺_[c]) level decreased rapidly and sustainably after SHL treatment, and then returned to homeostasis when SHL was withdrawn. Moreover, SHL decreases Ca²⁺_[c] levels mainly through enhancing the mitochondrial Ca²⁺ (Ca²⁺_[m]) uptake. After genetically silencing or pharmacologic inhibiting mitochondrial calcium uniporter (MCU), the effect of SHL on the Ca²⁺_[c] level and MC degranulation was significantly weakened. Simultaneously, the activation of SHL on Ca²⁺_[m] uptake was completely lost. Collectively, by activating MCU, SHL decreases Ca²⁺_[c] level to stabilize MCs, thus exerting a remarkable anti-allergic activity, which could have considerable influences on clinical practice and research.

Mitochondrial Dynamics as a Therapeutic Target for Treating Cardiac Diseases

Mitochondria are dynamic in nature and are able to shift their morphology between elongated interconnected mitochondrial networks and a fragmented disconnected arrangement by the processes of mitochondrial fusion and fission, respectively. Changes in mitochondrial morphology are regulated by the mitochondrial fusion proteins - mitofusins 1 and 2 (Mfn1 and 2), and optic atrophy 1 (Opa1) as well as the mitochondrial fission proteins - dynamin-related peptide 1 (Drp1) and fission protein 1 (Fis1). Despite having a unique spatial arrangement, cardiac mitochondria have been implicated in a variety of disorders including ischemia-reperfusion injury (IRI), heart failure, diabetes, and pulmonary hypertension. In this chapter, we review the influence of mitochondrial dynamics in these cardiac disorders as well as their potential as therapeutic targets in tackling cardiovascular disease.

Mitochondria in Structural and Functional Cardiac Remodeling

The heart must function continuously as it is responsible for both supplying oxygen and nutrients throughout the entire body, as well as for the transport of waste products to excretory organs. When facing either a physiological or pathological increase in cardiac demand, the heart undergoes structural and functional remodeling as a means of adapting to increased workload. These adaptive responses can include changes in gene expression, protein composition, and structure of sub-cellular organelles involved in energy production and metabolism. Mitochondria are essential for cardiac function, as they supply the ATP necessary to support continuous cycles of contraction and relaxation. In addition, mitochondria carry out other important processes, including synthesis of essential cellular components, calcium buffering, and initiation of cell death signals. Not surprisingly, mitochondrial dysfunction has been linked to several cardiovascular disorders, including hypertension, cardiac hypertrophy, ischemia/reperfusion and heart failure. The present chapter will discuss how changes in mitochondrial cristae structure, fusion/fission dynamics, fatty acid oxidation, ATP production, and the generation of reactive oxygen species might impact cardiac structure and function, particularly in the context of pathological hypertrophy and fibrotic response. In addition, the mechanistic role of mitochondria in autophagy and programmed cell death of cardiomyocytes will be addressed. Here we will also review strategies to improve mitochondrial function and discuss their cardioprotective potential.

Altered Mitochondrial Anatomy of Trigeminal Ganglia Neurons in Diabetes

Neurons from sensory ganglia are exposed to oxidative attack in diabetes. Altered mitochondrial morphologies are due to impaired dynamics (fusion, fission) and to cristae remodeling. This study aimed to evaluate using transmission electron microscopy mitochondrial changes in diabetic trigeminal ganglia suggestive for ignition of apoptosis, in absence of "classical" morphological signs of apoptosis. We used samples of trigeminal ganglia (from six type 2 diabetes human donors and five streptozotocin (STZ)-induced diabetic rats). In human diabetic samples we found three main distributions of mitochondria: (a) small "dark" normal mitochondria, seemingly resulted from fission processes; (b) small "dark" damaged mitochondria, with side-vesiculations (single- and double-coated), large matrix vesicles and cytosolic leakage of reactive species, mixed with larger "light" mitochondria, swollen, and with crystolysis; (c) prevailing "light" mitochondria. In STZ-treated rats a type (c) distribution prevailed, except for nociceptive neurons where we found a different distribution: large and giant mitochondria, suggestive for impaired mitochondrial fission, mitochondrial fenestrations, matrix vesicles interconnected by lamellar cristae, and mitochondrial leakage into the cytosol. Thus, the ultrastructural pattern of mitochondria damage in diabetic samples of sensory neurons may provide clues on the initiation of intrinsic apoptosis, even if the classical morphological signs of apoptosis are not present. Further studies, combining use of biochemical and ultrastructural techniques, may allow a better quantification of the degree in which mitochondrial damage, with membrane alterations and cytosolic leaks, may be used as morphological signs suggesting the point-of-no return for apoptosis. Anat Rec, 299:1561-1570, 2016. © 2016 Wiley Periodicals, Inc.

Myocardial redox status, mitophagy and cardioprotection: a potential way to amend diabetic heart?

Diabetic cardiomyopathy (DCM) is one of the major cardiovascular complications in diabetes that increase the mortality of diabetic patients. Mechanisms underlying DCM have not been fully elucidated, hindering targeted design of effective strategies to delay or treat DCM. Mitochondrial dysfunction is recognized as the driving force for the pathogenesis of DCM; therefore, maintaining cardiac mitochondrial quality is crucial for DCM prevention. Mitophagy is the process by which cells degrade abnormal or superfluous mitochondria in order to correct mitochondrial dysfunction, improve mitochondrial quality and maintain cardiac homoeostasis. Although the roles of mitophagy in various cardiomyopathies have been suggested, it remains largely unknown how the process is regulated and whether it is altered in the diabetic heart. In this review, we summarize currently available studies that investigate mitophagy in the heart, including its pathways, features and protective roles in several situations, including DCM. Due to limited data about mitophagy in diabetic hearts, future studies are required to gain a deeper understanding of the regulatory mechanisms of mitophagy in the heart and to develop mitophagy-based strategies for protecting the heart from diabetic injury.

Inhibition of Soluble Epoxide Hydrolase Limits Mitochondrial Damage and Preserves Function Following Ischemic Injury

Aims: Myocardial ischemia can result in marked mitochondrial damage leading to cardiac dysfunction, as such identifying novel mechanisms to limit mitochondrial injury is important. This study investigated the hypothesis that inhibiting soluble epoxide hydrolase (sEH), responsible for converting epoxyeicosatrienoic acids to dihydroxyeicosatrienoic acids protects mitochondrial from injury caused by myocardial infarction.

Methods: sEH null and WT littermate mice were subjected to surgical occlusion of the left anterior descending (LAD) artery or sham operation. A parallel group of WT mice received an sEH inhibitor, trans-4-[4-(3-adamantan-1-y1-ureido)-cyclohexyloxy]-benzoic acid (tAUCB; 10 mg/L) or vehicle in the drinking water 4 days prior and 7 days post-MI. Cardiac function was assessed by echocardiography prior- and 7-days post-surgery. Heart tissues were dissected into infarct, peri-, and non-infarct regions to assess ultrastructure by electron microscopy. Complexes I, II, IV, citrate synthase, PI3K activities, and mitochondrial respiration were assessed in non-infarct regions. Isolated working hearts were used to measure the rates of glucose and palmitate oxidation.

Results: Echocardiography revealed that tAUCB treatment or sEH deficiency significantly improved systolic and diastolic function post-MI compared to controls. Reduced infarct expansion and less adverse cardiac remodeling were observed in tAUCB-treated and sEH null groups. EM data demonstrated mitochondrial ultrastructure damage occurred in infarct and peri-infarct regions but not in non-infarct regions. Inhibition of sEH resulted in significant improvements in mitochondrial respiration, ATP content, mitochondrial enzymatic activities and restored insulin sensitivity and PI3K activity.

Conclusion: Inhibition or genetic deletion of sEH protects against long-term ischemia by preserving cardiac function and maintaining mitochondrial efficiency.

Does Mitochondrial Fusion Require Transmembrane Potential?

Dissipation of transmembrane potential inhibits mitochondrial fusion and thus prevents reintegration of damaged mitochondria into the mitochondrial network. Consequently, damaged mitochondria are removed by autophagy. Does transmembrane potential directly regulate the mitochondrial fusion machinery? It was shown that inhibition of ATP-synthase induces fragmentation of mitochondria while preserving transmembrane potential. Moreover, mitochondria of the yeast Saccharomyces cerevisiae retain the ability to fuse even in the absence of transmembrane potential. Metazoan mitochondria in some cases retain ability to fuse for a short period even in a depolarized state. It also seems unlikely that transmembrane potential-based regulation of mitochondrial fusion would prevent reintegration of mitochondria with damaged ATP-synthase into the mitochondrial network. Such reintegration could lead to clonal expansion of mtDNAs harboring deleterious mutations in ATP synthase. We speculate that transmembrane potential is not directly involved in regulation of mitochondrial fusion but affects mitochondrial NTP/NDP ratio, which in turn regulates their fusion.

Dynamin-related protein inhibitor downregulates reactive oxygen species levels to indirectly suppress high glucose-induced hyperproliferation of vascular smooth muscle cells

Hyperproliferation of vascular smooth muscle cells is a pathogenic mechanism common in diabetic vascular complications and is a putatively important therapeutic target. This study investigated multiple levels of biology, including cellular and organellar changes, as well as perturbations in protein synthesis and morphology. Quantitative and qualitative analysis was utilized to assess the effect of mitochondrial dynamic changes and reactive oxygen species(ROS) levels on high-glucose-induced hyperproliferation of vascular smooth muscle cells. The data demonstrated that the mitochondrial fission inhibitor Mdivi-1 and downregulation of ROS levels both effectively inhibited the high-glucose-induced hyperproliferation of vascular smooth muscle cells. Downregulation of ROS levels played a more direct role and ROS levels were also regulated by mitochondrial dynamics. Increased ROS levels induced excessive mitochondrial fission through dynamin-related protein (Drp 1), while Mdivi-1 suppressed the sensitivity of Drp1 to ROS levels, thus inhibiting excessive mitochondrial fission under high-glucose conditions. This study is the first to propose that mitochondrial dynamic changes and ROS levels interact with each other and regulate high-glucose-induced hyperproliferation of vascular smooth muscle cells. This finding provides novel ideas in understanding the pathogenesis of diabetic vascular remodeling and intervention.

Mitochondrial biogenesis and dynamics in the developing and diseased heart

In this review, Dorn et al. describe the regulatory circuitry and downstream events involved in mitochondrial biogenesis and its coordination with mitochondrial dynamics in developing and diseased hearts.

The mitochondrion is a complex organelle that serves essential roles in energy transduction, ATP production, and a myriad of cellular signaling events. A finely tuned regulatory network orchestrates the biogenesis, maintenance, and turnover of mitochondria. The high-capacity mitochondrial system in the heart is regulated in a dynamic way to generate and consume enormous amounts of ATP in order to support the constant pumping function in the context of changing energy demands. This review describes the regulatory circuitry and downstream events involved in mitochondrial biogenesis and its coordination with mitochondrial dynamics in developing and diseased hearts.

Structure and function of the N-terminal domain of the human mitochondrial calcium uniporter

The mitochondrial calcium uniporter (MCU) is responsible for mitochondrial calcium uptake and homeostasis. It is also a target for the regulation of cellular anti-/pro-apoptosis and necrosis by several oncogenes and tumour suppressors. Herein, we report the crystal structure of the MCU N-terminal domain (NTD) at a resolution of 1.50 Å in a novel fold and the S92A MCU mutant at 2.75 Å resolution; the residue S92 is a predicted CaMKII phosphorylation site. The assembly of the mitochondrial calcium uniporter complex (uniplex) and the interaction with the MCU regulators such as the mitochondrial calcium uptake-1 and mitochondrial calcium uptake-2 proteins (MICU1 and MICU2) are not affected by the deletion of MCU NTD. However, the expression of the S92A mutant or a NTD deletion mutant failed to restore mitochondrial Ca(2+) uptake in a stable MCU knockdown HeLa cell line and exerted dominant-negative effects in the wild-type MCU-expressing cell line. These results suggest that the NTD of MCU is essential for the modulation of MCU function, although it does not affect the uniplex formation.

Quality control systems in cardiac aging

Cardiac aging is an intrinsic process that results in impaired cardiac function, along with cellular and molecular changes. These degenerative changes are intimately associated with quality control mechanisms. This review provides a general overview of the clinical and cellular changes which manifest in cardiac aging, and the quality control mechanisms involved in maintaining homeostasis and retarding aging. These mechanisms include autophagy, ubiquitin-mediated turnover, apoptosis, mitochondrial quality control and cardiac matrix homeostasis. Finally, we discuss aging interventions that have been observed to impact cardiac health outcomes. These include caloric restriction, rapamycin, resveratrol, GDF11, mitochondrial antioxidants and cardiolipin-targeted therapeutics. A greater understanding of the quality control mechanisms that promote cardiac homeostasis will help to understand the benefits of these interventions, and hopefully lead to further improved therapeutic modalities.

Mitochondrial dysfunction in cardiac aging

Cardiovascular diseases are the leading cause of death in most developed nations. While it has received the least public attention, aging is the dominant risk factor for developing cardiovascular diseases, as the prevalence of cardiovascular diseases increases dramatically with increasing age. Cardiac aging is an intrinsic process that results in impaired cardiac function, along with cellular and molecular changes. Mitochondria play a great role in these processes, as cardiac function is an energetically demanding process. In this review, we examine mitochondrial dysfunction in cardiac aging. Recent research has demonstrated that mitochondrial dysfunction can disrupt morphology, signaling pathways, and protein interactions; conversely, mitochondrial homeostasis is maintained by mechanisms that include fission/fusion, autophagy, and unfolded protein responses. Finally, we describe some of the recent findings in mitochondrial targeted treatments to help meet the challenges of mitochondrial dysfunction in aging.

Impaired dynamics and function of mitochondria caused by mtDNA toxicity leads to heart failure

Cardiac mitochondrial dysfunction has been implicated in heart failure of diverse etiologies. Generalized mitochondrial disease also leads to cardiomyopathy with various clinical manifestations. Impaired mitochondrial homeostasis may over time, such as in the aging heart, lead to cardiac dysfunction. Mitochondrial DNA (mtDNA), close to the electron transport chain and unprotected by histones, may be a primary pathogenetic site, but this is not known. Here, we test the hypothesis that cumulative damage of cardiomyocyte mtDNA leads to cardiomyopathy and heart failure. Transgenic mice with Tet-on inducible, cardiomyocyte-specific expression of a mutant uracil-DNA glycosylase 1 (mutUNG1) were generated. The mutUNG1 is known to remove thymine in addition to uracil from the mitochondrial genome, generating apyrimidinic sites, which obstruct mtDNA function. Following induction of mutUNG1 in cardiac myocytes by administering doxycycline, the mice developed hypertrophic cardiomyopathy, leading to congestive heart failure and premature death after ∼2 mo. The heart showed reduced mtDNA replication, severely diminished mtDNA transcription, and suppressed mitochondrial respiration with increased Pgc-1α, mitochondrial mass, and antioxidative defense enzymes, and finally failing mitochondrial fission/fusion dynamics and deteriorating myocardial contractility as the mechanism of heart failure. The approach provides a model with induced cardiac-restricted mtDNA damage for investigation of mtDNA-based heart disease.

Role of mitochondrial fission and fusion in cardiomyocyte contractility

BACKGROUND

Mitochondria constitute 30% of cell volume and are engaged in two dynamic processes called fission and fusion, regulated by Drp-1 (dynamin related protein) and mitofusin 2 (Mfn2). Previously, we showed that Drp-1 inhibition attenuates cardiovascular dysfunction following pressure overload in aortic banding model and myocardial infarction. As dynamic organelles, mitochondria are capable of changing their morphology in response to stress. However, whether such changes can alter their function and in turn cellular function is unknown. Further, a direct role of fission and fusion in cardiomyocyte contractility has not yet been studied. In this study, we hypothesize that disrupted fission and fusion balance by increased Drp-1 and decreased Mfn2 expression in cardiomyocytes affects their contractility through alterations in the calcium and potassium concentrations.

METHODS

To verify this, we used freshly isolated ventricular myocytes from wild type mouse and transfected them with either siRNA to Drp-1 or Mfn2. Myocyte contractility studies were performed by IonOptix using a myopacer. Intracellular calcium and potassium measurements were done using flow cytometry. Immunocytochemistry (ICC) was done to evaluate live cell mitochondria and its membrane potential. Protein expression was done by western blot and immunocytochemistry.

RESULTS

We found that silencing mitochondrial fission increased the myocyte contractility, while fusion inhibition decreased contractility with simultaneous changes in calcium and potassium. Also, we observed that increase in fission prompted decrease in Serca-2a and increase in cytochrome c leakage leading to mitophagy.

CONCLUSION

Our results suggested that regulating mitochondrial fission and fusion have direct effects on overall cardiomyocyte contractility and thus function.

Thioredoxin-2 inhibits mitochondrial reactive oxygen species generation and apoptosis stress kinase-1 activity to maintain cardiac function

BACKGROUND

Thioredoxin 2 (Trx2) is a key mitochondrial protein that regulates cellular redox and survival by suppressing mitochondrial reactive oxygen species generation and by inhibiting apoptosis stress kinase-1 (ASK1)-dependent apoptotic signaling. To date, the role of the mitochondrial Trx2 system in heart failure pathogenesis has not been investigated.

METHODS AND RESULTS

Western blot and histological analysis revealed that Trx2 protein expression levels were reduced in hearts from patients with dilated cardiomyopathy, with a concomitant increase in ASK1 phosphorylation/activity. Cardiac-specific Trx2 knockout mice develop spontaneous dilated cardiomyopathy at 1 month of age with increased heart size, reduced ventricular wall thickness, and a progressive decline in left ventricular contractile function, resulting in mortality due to heart failure by ≈4 months of age. The progressive decline in cardiac function observed in cardiac-specific Trx2 knockout mice was accompanied by the disruption of mitochondrial ultrastructure, mitochondrial membrane depolarization, increased mitochondrial reactive oxygen species generation, and reduced ATP production, correlating with increased ASK1 signaling and increased cardiomyocyte apoptosis. Chronic administration of a highly selective ASK1 inhibitor improved cardiac phenotype and reduced maladaptive left ventricular remodeling with significant reductions in oxidative stress, apoptosis, fibrosis, and cardiac failure. Cellular data from Trx2-deficient cardiomyocytes demonstrated that ASK1 inhibition reduced apoptosis and reduced mitochondrial reactive oxygen species generation.

CONCLUSIONS

Our data support an essential role for mitochondrial Trx2 in preserving cardiac function by suppressing mitochondrial reactive oxygen species production and ASK1-dependent apoptosis. Inhibition of ASK1 represents a promising therapeutic strategy for the treatment of dilated cardiomyopathy and heart failure.

Crosstalk between mitogen-activated protein kinases and mitochondria in cardiac diseases: therapeutic perspectives

Cardiovascular diseases cause more mortality and morbidity worldwide than any other diseases. Although many intracellular signaling pathways influence cardiac physiology and pathology, the mitogen-activated protein kinase (MAPK) family has garnered significant attention because of its vast implications in signaling and crosstalk with other signaling networks. The extensively studied MAPKs ERK1/2, p38, JNK, and ERK5, demonstrate unique intracellular signaling mechanisms, responding to a myriad of mitogens and stressors and influencing the signaling of cardiac development, metabolism, performance, and pathogenesis. Definitive relationships between MAPK signaling and cardiac dysfunction remain elusive, despite 30 years of extensive clinical studies and basic research of various animal/cell models, severities of stress, and types of stimuli. Still, several studies have proven the importance of MAPK crosstalk with mitochondria, powerhouses of the cell that provide over 80% of ATP for normal cardiomyocyte function and play a crucial role in cell death. Although many questions remain unanswered, there exists enough evidence to consider the possibility of targeting MAPK-mitochondria interactions in the prevention and treatment of heart disease. The goal of this review is to integrate previous studies into a discussion of MAPKs and MAPK-mitochondria signaling in cardiac diseases, such as myocardial infarction (ischemia), hypertrophy and heart failure. A comprehensive understanding of relevant molecular mechanisms, as well as challenges for studies in this area, will facilitate the development of new pharmacological agents and genetic manipulations for therapy of cardiovascular diseases.

Dysregulation of Mfn2 and Drp-1 proteins in heart failure

Therapeutic approaches for cardiac regenerative mechanisms have been explored over the past decade to target various cardiovascular diseases (CVD). Structural and functional aberrations of mitochondria have been observed in CVD. The significance of mitochondrial maturation and function in cardiomyocytes is distinguished by their attribution to embryonic stem cell differentiation into adult cardiomyocytes. An abnormal fission process has been implicated in heart failure, and treatment with mitochondrial division inhibitor 1 (Mdivi-1), a specific inhibitor of dynamin related protein-1 (Drp-1), has been shown to improve cardiac function. We recently observed that the ratio of mitofusin 2 (Mfn2; a fusion protein) and Drp-1 (a fission protein) was decreased during heart failure, suggesting increased mitophagy. Treatment with Mdivi-1 improved cardiac function by normalizing this ratio. Aberrant mitophagy and enhanced oxidative stress in the mitochondria contribute to abnormal activation of MMP-9, leading to degradation of the important gap junction protein connexin-43 (Cx-43) in the ventricular myocardium. Reduced Cx-43 levels were associated with increased fibrosis and ventricular dysfunction in heart failure. Treatment with Mdivi-1 restored MMP-9 and Cx-43 expression towards normal. In this review, we discuss mitochondrial dynamics, its relation to MMP-9 and Cx-43, and the therapeutic role of fission inhibition in heart failure.

Insulin stimulates mitochondrial fusion and function in cardiomyocytes via the Akt-mTOR-NFκB-Opa-1 signaling pathway

Insulin regulates heart metabolism through the regulation of insulin-stimulated glucose uptake. Studies have indicated that insulin can also regulate mitochondrial function. Relevant to this idea, mitochondrial function is impaired in diabetic individuals. Furthermore, the expression of Opa-1 and mitofusins, proteins of the mitochondrial fusion machinery, is dramatically altered in obese and insulin-resistant patients. Given the role of insulin in the control of cardiac energetics, the goal of this study was to investigate whether insulin affects mitochondrial dynamics in cardiomyocytes. Confocal microscopy and the mitochondrial dye MitoTracker Green were used to obtain three-dimensional images of the mitochondrial network in cardiomyocytes and L6 skeletal muscle cells in culture. Three hours of insulin treatment increased Opa-1 protein levels, promoted mitochondrial fusion, increased mitochondrial membrane potential, and elevated both intracellular ATP levels and oxygen consumption in cardiomyocytes in vitro and in vivo. Consequently, the silencing of Opa-1 or Mfn2 prevented all the metabolic effects triggered by insulin. We also provide evidence indicating that insulin increases mitochondrial function in cardiomyocytes through the Akt-mTOR-NFκB signaling pathway. These data demonstrate for the first time in our knowledge that insulin acutely regulates mitochondrial metabolism in cardiomyocytes through a mechanism that depends on increased mitochondrial fusion, Opa-1, and the Akt-mTOR-NFκB pathway.

Mitochondria and mammalian reproduction

Mitochondria are cellular organelles with crucial roles in ATP synthesis, metabolic integration, reactive oxygen species (ROS) synthesis and management, the regulation of apoptosis (namely via the intrinsic pathway), among many others. Additionally, mitochondria in different organs or cell types may have distinct properties that can decisively influence functional analysis. In terms of the importance of mitochondria in mammalian reproduction, and although there are species-specific differences, these aspects involve both energetic considerations for gametogenesis and fertilization, control of apoptosis to ensure the proper production of viable gametes, and ROS signaling, as well as other emerging aspects. Crucially, mitochondria are the starting point for steroid hormone biosynthesis, given that the conversion of cholesterol to pregnenolone (a common precursor for all steroid hormones) takes place via the activity of the cytochrome P450 side-chain cleavage enzyme (P450scc) on the inner mitochondrial membrane. Furthermore, mitochondrial activity in reproduction has to be considered in accordance with the very distinct strategies for gamete production in the male and female. These include distinct gonad morpho-physiologies, different types of steroids that are more prevalent (testosterone, estrogens, progesterone), and, importantly, the very particular timings of gametogenesis. While spermatogenesis is complete and continuous since puberty, producing a seemingly inexhaustible pool of gametes in a fixed environment; oogenesis involves the episodic production of very few gametes in an environment that changes cyclically. These aspects have always to be taken into account when considering the roles of any common element in mammalian reproduction.

Antioxidant and anti-inflammatory effects of exercise: role of redox signaling

Contraction-induced production of reactive oxygen species (ROS) has been implicated in oxidative stress to skeletal muscle for the past few decades. As research advances more evidence has revealed a more complete role of ROS under both physiological and pathological conditions. The current review postulated that moderate intensity of physical exercise has antioxidant and anti-inflammatory effects due to the operation and cross-talks of several redox-sensitive signal transduction pathways. The functional roles and mechanisms of action of the nuclear factor κB, mitogen-activated protein kinase, and peroxisome proliferator-activated receptor γ co-activator 1α are highlighted.

Overexpression of ryanodine receptor type 1 enhances mitochondrial fragmentation and Ca2+-induced ATP production in cardiac H9c2 myoblasts

Ca(+) influx to mitochondria is an important trigger for both mitochondrial dynamics and ATP generation in various cell types, including cardiac cells. Mitochondrial Ca(2+) influx is mainly mediated by the mitochondrial Ca(2+) uniporter (MCU). Growing evidence also indicates that mitochondrial Ca(2+) influx mechanisms are regulated not solely by MCU but also by multiple channels/transporters. We have previously reported that skeletal muscle-type ryanodine receptor (RyR) type 1 (RyR1), which expressed at the mitochondrial inner membrane, serves as an additional Ca(2+) uptake pathway in cardiomyocytes. However, it is still unclear which mitochondrial Ca(2+) influx mechanism is the dominant regulator of mitochondrial morphology/dynamics and energetics in cardiomyocytes. To investigate the role of mitochondrial RyR1 in the regulation of mitochondrial morphology/function in cardiac cells, RyR1 was transiently or stably overexpressed in cardiac H9c2 myoblasts. We found that overexpressed RyR1 was partially localized in mitochondria as observed using both immunoblots of mitochondrial fractionation and confocal microscopy, whereas RyR2, the main RyR isoform in the cardiac sarcoplasmic reticulum, did not show any expression at mitochondria. Interestingly, overexpression of RyR1 but not MCU or RyR2 resulted in mitochondrial fragmentation. These fragmented mitochondria showed bigger and sustained mitochondrial Ca(2+) transients compared with basal tubular mitochondria. In addition, RyR1-overexpressing cells had a higher mitochondrial ATP concentration under basal conditions and showed more ATP production in response to cytosolic Ca(2+) elevation compared with nontransfected cells as observed by a matrix-targeted ATP biosensor. These results indicate that RyR1 possesses a mitochondrial targeting/retention signal and modulates mitochondrial morphology and Ca(2+)-induced ATP production in cardiac H9c2 myoblasts.

Acute inhibition of excessive mitochondrial fission after myocardial infarction prevents long-term cardiac dysfunction

Background

Ischemia and reperfusion (IR) injury remains a major cause of morbidity and mortality and multiple molecular and cellular pathways have been implicated in this injury. We determined whether acute inhibition of excessive mitochondrial fission at the onset of reperfusion improves mitochondrial dysfunction and cardiac contractility postmyocardial infarction in rats.

Methods and Results

We used a selective inhibitor of the fission machinery, P110, which we have recently designed. P110 treatment inhibited the interaction of fission proteins Fis1/Drp1, decreased mitochondrial fission, and improved bioenergetics in three different rat models of IR, including primary cardiomyocytes, ex vivo heart model, and an in vivo myocardial infarction model. Drp1 transiently bound to the mitochondria following IR injury and P110 treatment blocked this Drp1 mitochondrial association. Compared with control treatment, P110 (1 μmol/L) decreased infarct size by 28±2% and increased adenosine triphosphate levels by 70+1% after IR relative to control IR in the ex vivo model. Intraperitoneal injection of P110 (0.5 mg/kg) at the onset of reperfusion in an in vivo model resulted in improved mitochondrial oxygen consumption by 68% when measured 3 weeks after ischemic injury, improved cardiac fractional shortening by 35%, reduced mitochondrial H₂O₂ uncoupling state by 70%, and improved overall mitochondrial functions.

Conclusions

Together, we show that excessive mitochondrial fission at reperfusion contributes to long‐term cardiac dysfunction in rats and that acute inhibition of excessive mitochondrial fission at the onset of reperfusion is sufficient to result in long‐term benefits as evidenced by inhibiting cardiac dysfunction 3 weeks after acute myocardial infarction.

The mitochondrial Na+-Ca2+ exchanger, NCLX, regulates automaticity of HL-1 cardiomyocytes

Mitochondrial Ca²⁺ is known to change dynamically, regulating mitochondrial as well as cellular functions such as energy metabolism and apoptosis. The NCLX gene encodes the mitochondrial Na⁺-Ca²⁺ exchanger (NCX_mit), a Ca²⁺ extrusion system in mitochondria. Here we report that the NCLX regulates automaticity of the HL-1 cardiomyocytes. NCLX knockdown using siRNA resulted in the marked prolongation of the cycle length of spontaneous Ca²⁺ oscillation and action potential generation. The upstrokes of action potential and Ca²⁺ transient were markedly slower, and sarcoplasmic reticulum (SR) Ca²⁺ handling were compromised in the NCLX knockdown cells. Analyses using a mathematical model of HL-1 cardiomyocytes demonstrated that blocking NCX_mit reduced the SR Ca²⁺ content to slow spontaneous SR Ca²⁺ leak, which is a trigger of automaticity. We propose that NCLX is a novel molecule to regulate automaticity of cardiomyocytes via modulating SR Ca²⁺ handling.

Mitochondrial dynamics in cardiovascular health and disease

SIGNIFICANCE

Mitochondria are dynamic organelles capable of changing their shape and distribution by undergoing either fission or fusion. Changes in mitochondrial dynamics, which is under the control of specific mitochondrial fission and fusion proteins, have been implicated in cell division, embryonic development, apoptosis, autophagy, and metabolism. Although the machinery for modulating mitochondrial dynamics is present in the cardiovascular system, its function there has only recently been investigated. In this article, we review the emerging role of mitochondrial dynamics in cardiovascular health and disease.

RECENT ADVANCES

Changes in mitochondrial dynamics have been implicated in vascular smooth cell proliferation, cardiac development and differentiation, cardiomyocyte hypertrophy, myocardial ischemia-reperfusion injury, cardioprotection, and heart failure.

CRITICAL ISSUES

Many of the experimental studies investigating mitochondrial dynamics in the cardiovascular system have been confined to cardiac cell lines, vascular cells, or neonatal cardiomyocytes, in which mitochondria are distributed throughout the cytoplasm and are free to move. However, in the adult heart where mitochondrial movements are restricted by their tightly-packed distribution along myofibrils or beneath the subsarcolemma, the relevance of mitochondrial dynamics is less obvious. The investigation of transgenic mice deficient in cardiac mitochondrial fission or fusion proteins should help elucidate the role of mitochondrial dynamics in the adult heart.

FUTURE DIRECTIONS

Investigating the role of mitochondrial dynamics in cardiovascular health and disease should result in the identification of novel therapeutic targets for treating patients with cardiovascular disease, the leading cause of death and disability globally.

From structure to function: mitochondrial morphology, motion and shaping in vascular smooth muscle

The diversity of mitochondrial arrangements, which arise from the organelle being static or moving, or fusing and dividing in a dynamically reshaping network, is only beginning to be appreciated. While significant progress has been made in understanding the proteins that reorganise mitochondria, the physiological significance of the various arrangements is poorly understood. The lack of understanding may occur partly because mitochondrial morphology is studied most often in cultured cells. The simple anatomy of cultured cells presents an attractive model for visualizing mitochondrial behaviour but contrasts with the complexity of native cells in which elaborate mitochondrial movements and morphologies may not occur. Mitochondrial changes may take place in native cells (in response to stress and proliferation), but over a slow time-course and the cellular function contributed is unclear. To determine the role mitochondrial arrangements play in cell function, a crucial first step is characterisation of the interactions among mitochondrial components. Three aspects of mitochondrial behaviour are described in this review: (1) morphology, (2) motion and (3) rapid shape changes. The proposed physiological roles to which various mitochondrial arrangements contribute and difficulties in interpreting some of the physiological conclusions are also outlined.

Mitochondrial dynamics in the adult cardiomyocytes: which roles for a highly specialized cell?

Mitochondrial dynamics is a recent topic of research in the field of cardiac physiology. The study of mechanisms involved in the morphological changes and in the mobility of mitochondria is legitimate since the adult cardiomyocytes possess numerous mitochondria which occupy at least 30% of cell volume. However, architectural constraints exist in the cardiomyocyte that limit mitochondrial movements and communication between adjacent mitochondria. Still, the proteins involved in mitochondrial fusion and fission are highly expressed in these cells and could be involved in different processes important for the cardiac function. For example, they are required for mitochondrial biogenesis to synthesize new mitochondria and for the quality-control of the organelles. They are also involved in inner membrane organization and may play a role in apoptosis. More generally, change in mitochondrial morphology can have consequences in the functioning of the respiratory chain, in the regulation of the mitochondrial permeability transition pore (MPTP), and in the interactions with other organelles. Furthermore, the proteins involved in fusion and fission of mitochondria are altered in cardiac pathologies such as ischemia/reperfusion or heart failure (HF), and appear to be valuable targets for pharmacological therapies. Thus, mitochondrial dynamics deserves particular attention in cardiac research. The present review draws up a report of our knowledge on these phenomena.

Mitochondrial membrane permeabilization and cell death during myocardial infarction: roles of calcium and reactive oxygen species

Excess generation of reactive oxygen species (ROS) and cytosolic calcium accumulation play major roles in the initiation of programmed cell death during acute myocardial infarction. Cell death may include necrosis, apoptosis and autophagy, and combinations thereof. During ischemia, calcium handling between the sarcoplasmic reticulum and myofilament is disrupted and calcium is diverted to the mitochondria causing swelling. Reperfusion, while essential for survival, reactivates energy transduction and contractility and causes the release of ROS and additional ionic imbalance. During acute ischemia-reperfusion, the principal death pathways are programmed necrosis and apoptosis through the intrinsic pathway, initiated by the opening of the mitochondrial permeability transition pore and outer mitochondrial membrane permeabilization, respectively. Despite intense investigation, the mechanisms of action and modes of regulation of mitochondrial membrane permeabilization are incompletely understood. Extrinsic apoptosis, necroptosis and autophagy may also contribute to ischemia-reperfusion injury. In this review, the roles of dysregulated calcium and ROS and the contributions of Bcl-2 proteins, as well as mitochondrial morphology in promoting mitochondrial membrane permeability change and the ensuing cell death during myocardial infarction are discussed.

Differential responses to docosahexaenoic acid in primary and immortalized cardiac cells

The importance of dietary polyunsaturated fatty acids (PUFAs) in the reduction of cardiovascular disease has been recognized for many years. Docosahexaenoic acid (22:6n3, DHA) is an n-3 PUFA known to affect numerous biological functions and provide cardioprotection; however, the exact molecular and cellular protective mechanism(s) remain unknown. In contrast, DHA also possesses many anti-tumorgenic properties including suppressing cell growth and inducing apoptosis. In the present study, we investigated the effect of DHA toward H9c2 cells (an immortalized cardiac cell line) and neonatal primary cardiomyocytes (NCM). Cells were treated with 0μM, 10μM or 100μM DHA for upto 48h. Cell viability and mitochondrial activity were assayed at different time points. DHA caused a significant time- and dose-dependent decrease in cell viability and mitochondrial activity in H9c2 cells but not NCM. In addition, DHA decreased levels of TGF-β1 but increased IL-6 release in H9c2 cells. Significant induction of apoptosis was observed only in H9c2 cells, which involved activation of caspase-8 and -3 activities with a marked release of cytochrome c from mitochondria. DHA-induced severe mitochondrial damage resulting in a fragmented and punctated morphology with corresponding loss of mitochondrial membrane potential within 3h, prior to activation of caspases and cytochrome c release at 6h in H9c2 cells. Our data indicate that DHA treatment targets mitochondria, triggering collapse of mitochondrial membrane potential, increasing cellular stress and mitochondrial fragmentation resulting in apoptosis in immortalized cardiac cells, H9c2, but not neonatal primary cardiomyocyte.

Kissing and nanotunneling mediate intermitochondrial communication in the heart

Mitochondria in many types of cells are dynamically interconnected through constant fusion and fission, allowing for exchange of mitochondrial contents and repair of damaged mitochondria. However, constrained by the myofibril lattice, the ∼6,000 mitochondria in the adult mammalian cardiomyocyte display little motility, and it is unclear how, if at all, they communicate with each other. By means of target-expressing photoactivatable green fluorescent protein (PAGFP) in the mitochondrial matrix or on the outer mitochondrial membrane, we demonstrated that the local PAGFP signal propagated over the entire population of mitochondria in cardiomyocytes on a time scale of ∼10 h. Two elemental steps of intermitochondrial communications were manifested as either a sudden PAGFP transfer between a pair of adjacent mitochondria (i.e., "kissing") or a dynamic nanotubular tunnel (i.e., "nanotunneling") between nonadjacent mitochondria. The average content transfer index (fractional exchange) was around 0.5; the rate of kissing was 1‰ s(-1) per mitochondrial pair, and that of nanotunneling was about 14 times smaller. Electron microscopy revealed extensive intimate contacts between adjacent mitochondria and elongated nanotubular protrusions, providing a structural basis for the kissing and nanotunneling, respectively. We propose that, through kissing and nanotunneling, the otherwise static mitochondria in a cardiomyocyte form one dynamically continuous network to share content and transfer signals.

Mitochondrial morphology transitions and functions: implications for retrograde signaling?

In response to cellular and environmental stresses, mitochondria undergo morphology transitions regulated by dynamic processes of membrane fusion and fission. These events of mitochondrial dynamics are central regulators of cellular activity, but the mechanisms linking mitochondrial shape to cell function remain unclear. One possibility evaluated in this review is that mitochondrial morphological transitions (from elongated to fragmented, and vice-versa) directly modify canonical aspects of the organelle's function, including susceptibility to mitochondrial permeability transition, respiratory properties of the electron transport chain, and reactive oxygen species production. Because outputs derived from mitochondrial metabolism are linked to defined cellular signaling pathways, fusion/fission morphology transitions could regulate mitochondrial function and retrograde signaling. This is hypothesized to provide a dynamic interface between the cell, its genome, and the fluctuating metabolic environment.

Glutamate dehydrogenase 1 and SIRT4 regulate glial development

Congenital hyperinsulinism/hyperammonemia (HI/HA) syndrome is caused by an activation mutation of glutamate dehydrogenase 1 (GDH1), a mitochondrial enzyme responsible for the reversible interconversion between glutamate and α-ketoglutarate. The syndrome presents clinically with hyperammonemia, significant episodic hypoglycemia, seizures, and frequent incidences of developmental and learning defects. Clinical research has implicated that although some of the developmental and neurological defects may be attributed to hypoglycemia, some characteristics cannot be ascribed to low glucose and as hyperammonemia is generally mild and asymptomatic, there exists the possibility that altered GDH1 activity within the brain leads to some clinical changes. GDH1 is allosterically regulated by many factors, and has been shown to be inhibited by the ADP-ribosyltransferase sirtuin 4 (SIRT4), a mitochondrially localized sirtuin. Here we show that SIRT4 is localized to mitochondria within the brain. SIRT4 is highly expressed in glial cells, specifically astrocytes, in the postnatal brain and in radial glia during embryogenesis. Furthermore, SIRT4 protein decreases in expression during development. We show that factors known to allosterically regulate GDH1 alter gliogenesis in CTX8 cells, a novel radial glial cell line. We find that SIRT4 and GDH1 overexpression play antagonistic roles in regulating gliogenesis and that a mutant variant of GDH1 found in HI/HA patients accelerates the development of glia from cultured radial glia cells.

Targeted gene delivery to ischemic myocardium by homing peptide-guided polymeric carrier

Myocardial ischemia needs an alternative treatment such as gene therapy for the direct protection of cardiomyocytes against necrosis or apoptosis and to prevent the development of myocardial fibrosis and cardiac dysfunction. Despite the utility of gene therapy, its therapeutic use is limited due to inadequate transfection in cardiomyocytes and difficulty in directing to ischemic myocardium. Here, we present a polymeric gene carrier that is capable of targeting ischemic myocardium, resulting in high localization within the ischemic zone of the left ventricle (LV) of an ischemia/reperfusion (I/R) rat model upon systemic administration. Cystamine bisacrylamide-diamino hexane (CD) polymer was modified with the ischemic myocardium-targeted peptide (IMTP) and D-9-arginine (9R) for dual effects of the homing to ischemic myocardium and enhanced transfection efficiency with minimized polymer use. Conjugation of IMTP and 9R to CD led to an increase in transfection under hypoxia and significantly reduced the amount of polymer required for high transfection. Finally, we confirmed targeting of IMTP-CD-9R/DNA polyplex to ischemic myocardium and enhanced gene expression in LV of the I/R rat after tail vein injection. This study provides a clue that gene therapy for the treatment of myocardial ischemia can be achieved by using homing peptide-guided gene delivery systems.