Mitofusin-2 is a major determinant of oxidative stress-mediated heart muscle cell apoptosis

Dexrazoxane Protects Cardiomyocyte from Doxorubicin-Induced Apoptosis by Modulating miR-17-5p

The usage of doxorubicin is hampered by its life-threatening cardiotoxicity in clinical practice. Dexrazoxane is the only cardioprotective medicine approved by the FDA for preventing doxorubicin-induced cardiac toxicity. Nevertheless, the mechanism of dexrazoxane is incompletely understood. The aim of our study is to investigate the possible molecular mechanism of dexrazoxane against doxorubicin-induced cardiotoxicity. We established a doxorubicin-induced mouse and cardiomyocyte injury model. Male C57BL/6J mice were randomly distributed into a control group (Con), a doxorubicin treatment group (DOX), a doxorubicin plus dexrazoxane treatment group (DOX+DEX), and a dexrazoxane treatment group (DEX). Echocardiography and histology analyses were performed to evaluate heart function and structure. DNA laddering, qRT-PCR, and Western blot were performed on DOX-treated cardiomyocytes with/without DEX treatment in vitro. Cardiomyocytes were then transfected with miR-17-5p mimics or inhibitors in order to analyze its downstream target. Our results demonstrated that dexrazoxane has a potent effect on preventing cardiac injury induced by doxorubicin in vivo and in vitro by reducing cardiomyocyte apoptosis. MicroRNA plays an important role in cardiovascular diseases. Our data revealed that dexrazoxane could upregulate the expression of miR-17-5p, which plays a cytoprotective role in response to hypoxia by regulating cell apoptosis. Furthermore, the miRNA and protein analysis revealed that miR-17-5p significantly attenuated phosphatase and tensin homolog (PTEN) expression in cardiomyocytes exposed to doxorubicin. Taken together, dexrazoxane might exert a cardioprotective effect against doxorubicin-induced cardiomyocyte apoptosis by regulating the expression of miR-17-5p/PTEN cascade.

Mitochondrial Quality Control: Role in Cardiac Models of Lethal Ischemia-Reperfusion Injury

The current standard of care for acute myocardial infarction or 'heart attack' is timely restoration of blood flow to the ischemic region of the heart. While reperfusion is essential for the salvage of ischemic myocardium, re-introduction of blood flow paradoxically kills (rather than rescues) a population of previously ischemic cardiomyocytes-a phenomenon referred to as 'lethal myocardial ischemia-reperfusion (IR) injury'. There is long-standing and exhaustive evidence that mitochondria are at the nexus of lethal IR injury. However, during the past decade, the paradigm of mitochondria as mediators of IR-induced cardiomyocyte death has been expanded to include the highly orchestrated process of mitochondrial quality control. Our aims in this review are to: (1) briefly summarize the current understanding of the pathogenesis of IR injury, and (2) incorporating landmark data from a broad spectrum of models (including immortalized cells, primary cardiomyocytes and intact hearts), provide a critical discussion of the emerging concept that mitochondrial dynamics and mitophagy (the components of mitochondrial quality control) may contribute to the pathogenesis of cardiomyocyte death in the setting of ischemia-reperfusion.

The role of mitochondrial fusion and fission in the process of cardiac oxidative stress

Mitochondria are the energy suppliers in the cell and undergo constant fusion and fission to meet metabolic demand during the cell life cycle. Well-balanced mitochondrial dynamics are extremely important and necessary for cell survival as well as for tissue homeostasis. Cardiomyocytes contain large numbers of mitochondria to satisfy the high energy demand. It has been established that deregulated processes of mitochondrial dynamics play a major role in myocardial cell death. Currently, cardiac mitochondrial cell death pathways attract great attention in the cell biology and regenerative medicine fields. Importantly, mitochondrial dynamics are tightly linked to oxidative stress-induced cardiac damage. This review summarizes molecular mechanisms of mitochondrial fusion and fission processes and their potential roles in myocardial cell death triggered by oxidative stress. Advances in understanding the effect of both normal and abnormal mitochondrial dynamics on heart protection will lead to significant improvement of therapeutic discoveries.

miR-154-5p Functions as an Important Regulator of Angiotensin II-Mediated Heart Remodeling

Chronic hypertension, valvular heart disease, and heart infarction cause cardiac remodeling and potentially lead to a series of pathological and structural changes in the left ventricular myocardium and a progressive decrease in heart function. Angiotensin II (AngII) plays a key role in the onset and development of cardiac remodeling. Many microRNAs (miRNAs), including miR-154-5p, may be involved in the development of cardiac remolding, but the underlying molecular mechanisms remain unclear. We aimed to characterize the function of miR-154-5p and reveal its mechanisms in cardiac remodeling induced by AngII. First, angiotensin II led to concurrent increases in miR-154-5p expression and cardiac remodeling in adult C57BL/6J mice. Second, overexpression of miR-154-5p to a level similar to that induced by AngII was sufficient to trigger cardiomyocyte hypertrophy and apoptosis, which is associated with profound activation of oxidative stress and inflammation. Treatment with a miR-154-5p inhibitor noticeably reversed these changes. Third, miR-154-5p directly inhibited arylsulfatase B (Arsb) expression by interacting with its 3′-UTR and promoted cardiomyocyte hypertrophy and apoptosis. Lastly, the angiotensin type 1 receptor blocker telmisartan attenuated AngII-induced cardiac hypertrophy, apoptosis, and fibrosis by blocking the increase in miR-154-5p expression. Moreover, upon miR-154-5p overexpression in isolated cardiomyocytes, the protective effect of telmisartan was partially abolished. Based on these results, increased cardiac miR-154-5p expression is both necessary and sufficient for AngII-induced cardiomyocyte hypertrophy and apoptosis, suggesting that the upregulation of miR-154-5p may be a crucial pathological factor and a potential therapeutic target for cardiac remodeling.

MiR-93 regulates vascular smooth muscle cell proliferation, and neointimal formation through targeting Mfn2

Background/Aims: Vascular smooth muscle cell (VSMC) hyperplasia plays important roles in the pathogenesis of many vascular diseases, such as atherosclerosis and restenosis. Many microRNAs (miRs) have recently been reported to regulate the proliferation and migration of VSMC. In the current study, we aimed to explore the function of miR-93 in VSMCs and its molecular mechanism. Methods: First, qRT-PCR and immunofluorescence assays were performed to determine miR-93 expression in rat VSMCs following carotid artery injury in vivo and platelet-derived growth factor-BB (PDGF-BB) stimulation in vitro. Next, the biological role of miR-93 in rat VSMC proliferation and migration was examined in vivo and vitro. EdU incorporation assay and MTT assay for measuring cell proliferation, Transwell cell invasion assay and Cell scratch wound assay for measuring cell migration. Then, the targets of miR-93 were identified. Finally, the expression levels of proteins in the Raf-ERK1/2 pathway were measured by western blot. Results: MiR-93 was upregulated in rat VSMCs following carotid artery injury in vivo. Similar results were observed in ex vivo cultured VSMCs after PDGF-BB treatment. MiR-93 inhibition suppressed neointimal formation after carotid artery injury. Moreover, our results demonstrated that a miR-93 inhibitor suppressed the PDGF-BB induced proliferation and migration of in VSMC. This inhibitor also decreased the expression levels of MMP2 and cyclin D1. Mechanistically, we discovered that mitofusin 2(Mfn2) is a direct target of miR-93. Furthermore, an analysis of the signaling events revealed that miR-93-mediated VSMC proliferation and migration occurred via the Raf-ERK1/2 pathway. Conclusions: Our findings suggest that miR-93 promotes VSMCs proliferation and migration by targeting Mfn2. MiR-93 may be a new target for treating in-stent restenosis.

Mul1 restrains Parkin-mediated mitophagy in mature neurons by maintaining ER-mitochondrial contacts

Chronic mitochondrial stress associates with major neurodegenerative diseases. Recovering stressed mitochondria constitutes a critical step of mitochondrial quality control and thus energy maintenance in early stages of neurodegeneration. Here, we reveal Mul1-Mfn2 pathway that maintains neuronal mitochondrial integrity under stress conditions. Mul1 deficiency increases Mfn2 activity that triggers the first phasic mitochondrial hyperfusion and also acts as an ER-Mito tethering antagonist. Reduced ER-Mito coupling leads to increased cytoplasmic Ca²⁺ load that activates calcineurin and induces the second phasic Drp1-dependent mitochondrial fragmentation and mitophagy. Overexpressing Mfn2, but not Mfn1, mimics Mul1-deficient phenotypes, while expressing PTPIP51, an ER-Mito anchoring protein, suppresses Parkin-mediated mitophagy. Thus, by regulating mitochondrial morphology and ER-Mito contacts, Mul1-Mfn2 pathway plays an early checkpoint role in maintaining mitochondrial integrity. Our study provides new mechanistic insights into neuronal mitochondrial maintenance under stress conditions, which is relevant to several major neurodegenerative diseases associated with mitochondrial dysfunction and altered ER-Mito interplay.

Little is known about the pathways that maintain mitochondrial structure and function under neuronal stress conditions. Here, authors demonstrate that the Mul1-Mfn2 pathway plays a checkpoint role in maintaining mitochondrial integrity and energy maintenance by ensuring ER-mitochondrial tethering and preventing mitophagy.

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.

Genetic Deletion or Pharmacological Inhibition of Soluble Epoxide Hydrolase Ameliorates Cardiac Ischemia/Reperfusion Injury by Attenuating NLRP3 Inflammasome Activation

Activation of the nucleotide-binding oligomerization domain-like receptor (NLR) family pyrin domain containing 3 (NLRP3) inflammasome cascade has a role in the pathogenesis of ischemia/reperfusion (IR) injury. There is growing evidence indicating cytochrome p450 (CYP450)-derived metabolites of n-3 and n-6 polyunsaturated fatty acids (PUFAs) possess both adverse and protective effects in the heart. CYP-derived epoxy metabolites are rapidly hydrolyzed by the soluble epoxide hydrolase (sEH). The current study hypothesized that the cardioprotective effects of inhibiting sEH involves limiting activation of the NLRP3 inflammasome. Isolated hearts from young wild-type (WT) and sEH null mice were perfused in the Langendorff mode with either vehicle or the specific sEH inhibitor t-AUCB. Improved post-ischemic functional recovery and better mitochondrial respiration were observed in both sEH null hearts or WT hearts perfused with t-AUCB. Inhibition of sEH markedly attenuated the activation of the NLRP3 inflammasome complex and limited the mitochondrial localization of the fission protein dynamin-related protein-1 (Drp-1) triggered by IR injury. Cardioprotective effects stemming from the inhibition of sEH included preserved activities of both cytosolic thioredoxin (Trx)-1 and mitochondrial Trx-2 antioxidant enzymes. Together, these data demonstrate that inhibiting sEH imparts cardioprotection against IR injury via maintaining post-ischemic mitochondrial function and attenuating a detrimental innate inflammatory response.

Expression Profile of microRNAs in Hypertrophic Cardiomyopathy and Effects of microRNA-20 in Inducing Cardiomyocyte Hypertrophy Through Regulating Gene MFN2

Myocardial hypertrophy is an important cause of heart failure and sudden death. Studies have shown that Mitofusin-2 (MFN2) is downregulated in myocardial hypertrophy, but the upstream regulation mechanism underlying its downexpression in cardiomyocytes is still unclear. This study aims to identify the expression profile of microRNAs (miRNAs) in hypertrophic cardiomyopathy (HCM) and explore the function of miRNA-20 in inducing cardiomyocyte hypertrophy through regulating MFN2. Through miRNA + mRNA microarray analysis, 1451 miRNAs were identified, 367 miRNAs expressed differently between groups. Meanwhile, a number of 24,718 mRNAs were identified, among which 5850 mRNAs were upregulated and 3005 mRNAs were downregulated in HCM group compared with the control group. Expression of hsa-miRNA-20a-5p was 2.26 times higher in the HCM group compared with the control group and 7 target gene prediction programs predicted MFN2 as a target of miRNA-20. In vitro model of hypertrophic cardiomyocytes displayed high expression level of miRNA-20, atrial natriuretic peptide (ANP) mRNA, and protein, accompanying low expression level of Mfn2 mRNA and protein, which meant miRNA-20 played a role in cardiomyocyte hypertrophy and might interact with MFN2 to function. Thereafter, overexpression of miRNA-20 led to cell hypertrophy accompanied with lowly expressed Mfn2 mRNA and protein. When transfected with miRNA-20 inhibitors, the expression of miRNA-20 and ANP gene was attenuated and MFN2 was the other way around. The cell surface area of Ang II group and mimic group was significantly larger compared with the control group, and in the inhibitor+Ang II group, the area was significantly decreased compared with the Ang II group. Dual-luciferase assays showed that miRNA-20 bound to 3' untranslated region of MFN2 and inhibited its expression. In conclusion, hypertrophic myocardium and normal myocardium have different miRNA expression profiles and the effect of miRNA-20 reducing the expression of MFN2 plays a role in promoting cardiomyocyte hypertrophy.

Mitochondrial Morphofunction in Mammalian Cells

Significance: In addition to their classical role in cellular ATP production, mitochondria are of key relevance in various (patho)physiological mechanisms including second messenger signaling, neuro-transduction, immune responses and death induction. Recent Advances: Within cells, mitochondria are motile and display temporal changes in internal and external structure ("mitochondrial dynamics"). During the last decade, substantial empirical and in silico evidence was presented demonstrating that mitochondrial dynamics impacts on mitochondrial function and vice versa. Critical Issues: However, a comprehensive and quantitative understanding of the bidirectional links between mitochondrial external shape, internal structure and function ("morphofunction") is still lacking. The latter particularly hampers our understanding of the functional properties and behavior of individual mitochondrial within single living cells. Future Directions: In this review we discuss the concept of mitochondrial morphofunction in mammalian cells, primarily using experimental evidence obtained within the last decade. The topic is introduced by briefly presenting the central role of mitochondria in cell physiology and the importance of the mitochondrial electron transport chain (ETC) therein. Next, we summarize in detail how mitochondrial (ultra)structure is controlled and discuss empirical evidence regarding the equivalence of mitochondrial (ultra)structure and function. Finally, we provide a brief summary of how mitochondrial morphofunction can be quantified at the level of single cells and mitochondria, how mitochondrial ultrastructure/volume impacts on mitochondrial bioreactions and intramitochondrial protein diffusion, and how mitochondrial morphofunction can be targeted by small molecules.

Cardioprotective effects of CYP-derived epoxy metabolites of docosahexaenoic acid involve limiting NLRP3 inflammasome activation

Impaired mitochondrial function and activation of NLRP3 inflammasome cascade has a significant role in the pathogenesis of myocardial ischemia–reperfusion (IR) injury. The current study investigated whether eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), or their corresponding CYP epoxygenase metabolites 17,18-epoxyeicosatetraenoic acid (17,18-EEQ) and 19,20-epoxydocosapentaenoic acid (19,20-EDP) protect against IR injury. Isolated mouse hearts were perfused in the Langendorff mode with vehicle, DHA, 19,20-EDP, EPA, or 17,18-EEQ and subjected to 30 min of ischemia and followed by 40 min of reperfusion. In contrast with EPA and 17,18-EEQ, DHA and 19,20-EDP exerted cardioprotection, as shown by a significant improvement in postischemic functional recovery associated with significant attenuation of NLRP3 inflammasome complex activation and preserved mitochondrial function. Hearts perfused with DHA or 19,20-EDP displayed a marked reduction in localization of mitochondrial Drp-1 and Mfn-2 as well as maintained Opa-1 levels. DHA and 19,20-EDP preserved the activities of both the cytosolic Trx-1 and mitochondrial Trx-2. DHA cardioprotective effect was attenuated by the CYP epoxygenase inhibitor N-(methysulfonyl)-2-(2-propynyloxy)-benzenehexanamide. In conclusion, our data indicate a differential cardioprotective response between DHA, EPA, and their active metabolites toward IR injury. Interestingly, 19,20-EDP provided the best protection against IR injury via maintaining mitochondrial function and thereby reducing the detrimental NLRP3 inflammasome responses.

Mitochondrial Dynamics Impairment in Dexamethasone-Treated Neuronal Cells

Dexamethasone is an approved steroid for clinical use to activate or suppress cytokines, chemokines, inflammatory enzymes and adhesion molecules. It enters the brain, by-passing the blood brain barrier, and acts through genomic mechanisms. High levels of dexamethasone are able to induce neuronal cell loss, reduce neurogenesis and cause neuronal dysfunction. The exact mechanisms of steroid, especially the dexamethasone contribute to neuronal damage remain unclear. Therefore, the present study explored the mitochondrial dynamics underlying dexamethasone-induced toxicity of human neuroblastoma SH-SY5Y cells. Neuronal cells treatment with the dexamethasone resulted in a marked decrease in cell proliferation. Dexamethasone-induced neurotoxicity also caused upregulation of mitochondrial fusion and cleaved caspase-3 proteins expression. Mitochondria fusion was found in large proportions of dexamethasone-treated cells. These results suggest that dexamethasone-induced hyperfused mitochondrial structures are associated with a caspase-dependent death process in dexamethasone-induced neurotoxicity. These findings point to the high dosage of dexamethasone as being neurotoxic through impairment of mitochondrial dynamics.

Propofol Ameliorates H9c2 Cells Apoptosis Induced by Oxygen Glucose Deprivation and Reperfusion Injury via Inhibiting High Levels of Mitochondrial Fusion and Fission

Background: The cardioprotective effect of propofol on ischemia-reperfusion injury (I/R injury) is partly due to suppressing apoptosis. Mitochondrial dynamics are also involved in apoptosis. Mitochondrial fusion and fission lead to mitochondrial morphological changes. However, whether suppressing apoptosis effect of propofol against ischemia-reperfusion injury in the heart is via regulating mitochondrial morphology remains unclear. Methods: H9c2 cells underwent oxygen glucose deprivation (OGD) followed by reperfusion to simulate cardiomyocytes ischemia/reperfusion injury. Cell viability, apoptosis ratio and intracellular reactive oxygen species (ROS) were assessed, respectively. Mitochondrial membrane dynamin family proteins, extracellular signal regulated kinase 1 and 2 (ERK1/2), phosphorylated extracellular signal regulated kinase 1 and 2 (p-ERK1/2) and proteins related to intrinsic apoptosis pathways were detected by western blotting. The mitochondrial morphology and the distribution of dynamin-related protein 1 (Drp1) were observed by using laser confocal microscopy. Results: Propofol enhanced the survival of H9c2 cells, decreased ROS levels and inhibited apoptosis during oxygen glucose deprivation/reperfusion (OGD/R) injury. Mitochondrial fission in H9c2 cells was inhibited by propofol during OGD injury. Propofol alleviated high levels of mitochondrial fusion and fission during OGD/R in H9c2 cells, by regulating mitochondrial membrane remodeling dynamin family proteins. Propofol inhibited Drp1 colocalization with mitochondria in H9c2 cells during OGD/R injury. Moreover, Drp1 phosphorylation was inhibited by propofol through decreasing ERK activation during OGD/R injury. We found that propofol ameliorated H9c2 cells apoptosis during OGD/R via inhibiting mitochondrial cytochrome c release and caspase-9, caspase-6, caspase-7 and caspase-3 activation. Conclusion: Propofol suppresses H9c2 cells apoptosis during OGD/R injury via inhibiting intrinsic apoptosis pathway, which may be partly due to reducing high levels of mitochondrial fusion and fission induced by OGD/R injury.

MTP18 overexpression contributes to tumor growth and metastasis and associates with poor survival in hepatocellular carcinoma

Background

Human MTP18 (mitochondrial protein 18 kDa) is a novel nuclear-encoded mitochondrial membrane protein that is involved in controlling mitochondrial fission. Our bioinformatic analysis of TCGA data revealed an aberrant overexpression of MTP18 in hepatocellular carcinoma (HCC). We analyzed its biological effects and prognostic significance in this malignancy.

Methods

MTP18 expression was evaluated by qRT-PCR and western blot analysis in 20 paired tumor and peritumor tissues. Clinical impact of MTP18 overexpression was assessed in 156 patients with HCC. The effects of MTP18 knockdown or overexpression on cell growth and metastasis were determined by cell proliferation, colony formation, cell cycle, apoptosis, migration, and invasion assays. Furthermore, the underlying molecular mechanisms by which MTP18 overexpression promoted HCC cell growth and metastasis were explored.

Results

MTP18 was commonly overexpressed in HCC tissues mainly due to the downregulation of miR-125b, which significantly contributed to poor prognosis of HCC patients. Functional experiments revealed that MTP18 promoted both the growth and metastasis of HCC cells by inducing the progression of cell cycle, epithelial to mesenchymal transition (EMT) and production of MMP–9, and suppressing cell apoptosis. Mechanistically, increased mitochondrial fission and subsequent ROS production was found to be involved in the promotion of growth and metastasis by MTP18 in HCC cells.

Conclusions

MTP18 plays a pivotal oncogenic role in hepatocellular carcinogenesis; its overexpression may serve as a novel prognostic factor and a therapeutic target in HCC.

Mitochondria and cardiovascular diseases-from pathophysiology to treatment

Mitochondria are the source of cellular energy production and are present in different types of cells. However, their function is especially important for the heart due to the high demands in energy which is achieved through oxidative phosphorylation. Mitochondria form large networks which regulate metabolism and the optimal function is achieved through the balance between mitochondrial fusion and mitochondrial fission. Moreover, mitochondrial function is upon quality control via the process of mitophagy which removes the damaged organelles. Mitochondrial dysfunction is associated with the development of numerous cardiac diseases such as atherosclerosis, ischemia-reperfusion (I/R) injury, hypertension, diabetes, cardiac hypertrophy and heart failure (HF), due to the uncontrolled production of reactive oxygen species (ROS). Therefore, early control of mitochondrial dysfunction is a crucial step in the therapy of cardiac diseases. A number of anti-oxidant molecules and medications have been used but the results are inconsistent among the studies. Eventually, the aim of future research is to design molecules which selectively target mitochondrial dysfunction and restore the capacity of cellular anti-oxidant enzymes.

Reactive Oxygen Species in Metabolic and Inflammatory Signaling

Reactive oxygen species (ROS) are well known for their role in mediating both physiological and pathophysiological signal transduction. Enzymes and subcellular compartments that typically produce ROS are associated with metabolic regulation, and diseases associated with metabolic dysfunction may be influenced by changes in redox balance. In this review, we summarize the current literature surrounding ROS and their role in metabolic and inflammatory regulation, focusing on ROS signal transduction and its relationship to disease progression. In particular, we examine ROS production in compartments such as the cytoplasm, mitochondria, peroxisome, and endoplasmic reticulum and discuss how ROS influence metabolic processes such as proteasome function, autophagy, and general inflammatory signaling. We also summarize and highlight the role of ROS in the regulation metabolic/inflammatory diseases including atherosclerosis, diabetes mellitus, and stroke. In order to develop therapies that target oxidative signaling, it is vital to understand the balance ROS signaling plays in both physiology and pathophysiology, and how manipulation of this balance and the identity of the ROS may influence cellular and tissue homeostasis. An increased understanding of specific sources of ROS production and an appreciation for how ROS influence cellular metabolism may help guide us in the effort to treat cardiovascular diseases.

Novel insights into the role of mitochondrial fusion and fission in cardiomyocyte apoptosis induced by ischemia/reperfusion

As the main source of energy in the body, mitochondria are highly dynamic organelles, which are constantly going through fusion and fission. The fine balance of mitochondrial fusion and fission plays an important role in maintaining the stability of cardiomyocyte homeostasis. The processes of mitochondrial fusion and fission are very complex, which is mediated by fusion and fission proteins. Disruptions in these processes through controlling fusion and fission proteins affect mitochondrial functions and cardiomyocyte survival. Ischemia/reperfusion (I/R) can regulate the expression and post-translational modifications of fusion and fission proteins thereby inducing the abnormality of mitochondrial fusion and fission and cardiomyocyte apoptosis. Furthermore, intervention with the expression and function of fusion and fission proteins influences on cardiomyocyte apoptosis under I/R conditions. In this review, we focus on the current developments in the effects of mitochondrial fusion and fission on cardiomyocyte functions, the implications for cardiomyocyte apoptosis in response to I/R, and possible mechanisms. And we review their roles as a potential therapeutic target for treating I/R-induced cardiomyocyte injury.

Mitofusin 2: from functions to disease

Mitochondria are highly dynamic organelles whose functions are essential for cell viability. Within the cell, the mitochondrial network is continuously remodeled through the balance between fusion and fission events. Moreover, it dynamically contacts other organelles, particularly the endoplasmic reticulum, with which it enterprises an important functional relationship able to modulate several cellular pathways. Being mitochondria key bioenergetics organelles, they have to be transported to all the specific high-energy demanding sites within the cell and, when damaged, they have to be efficiently removed. Among other proteins, Mitofusin 2 represents a key player in all these mitochondrial activities (fusion, trafficking, turnover, contacts with other organelles), the balance of which results in the appropriate mitochondrial shape, function, and distribution within the cell. Here we review the structural and functional properties of Mitofusin 2, highlighting its crucial role in several cell pathways, as well as in the pathogenesis of neurodegenerative diseases, metabolic disorders, cardiomyopathies, and cancer.

Signalling mechanisms underlying doxorubicin and Nox2 NADPH oxidase-induced cardiomyopathy: involvement of mitofusin-2

BACKGROUND AND PURPOSE

The anthracycline doxorubicin (DOX), although successful as a first-line cancer treatment, induces cardiotoxicity linked with increased production of myocardial ROS, with Nox2 NADPH oxidase-derived superoxide reported to play a key role. The aim of this study was to identify novel mechanisms underlying development of cardiac remodelling/dysfunction further to DOX-stimulated Nox2 activation.

EXPERIMENTAL APPROACH

Nox2-/- and wild-type (WT) littermate mice were administered DOX (12 mg·kg-1 over 3 weeks) prior to study at 4 weeks. Detailed mechanisms were investigated in murine HL-1 cardiomyocytes, employing a robust model of oxidative stress, gene silencing and pharmacological tools.

KEY RESULTS

DOX-induced cardiac dysfunction, cardiomyocyte remodelling, superoxide production and apoptosis in WT mice were attenuated in Nox2-/- mice. Transcriptional analysis of left ventricular tissue identified 152 differentially regulated genes (using adjusted P < 0.1) in DOX-treated Nox2-/- versus WT mice, and network analysis highlighted 'Cell death and survival' as the biological function most significant to the dataset. The mitochondrial membrane protein, mitofusin-2 (Mfn2), appeared as a strong candidate, with increased expression (1.5-fold), confirmed by qPCR (1.3-fold), matching clear published evidence of promotion of cardiomyocyte cell death. In HL-1 cardiomyocytes, targeted siRNA knockdown of Nox2 decreased Mfn2 protein expression, but not vice versa. While inhibition of Nox2 activity along with DOX treatment attenuated its apoptotic and cytotoxic effects, reduced apoptosis after Mfn2 silencing reflected a sustained cytotoxic response and reduced cell viability.

CONCLUSIONS AND IMPLICATIONS

DOX-induced and Nox2-mediated up-regulation of Mfn2, rather than contributing to cardiomyocyte dysfunction through apoptotic pathways, appears to promote a protective mechanism.

LINKED ARTICLES

This article is part of a themed section on New Insights into Cardiotoxicity Caused by Chemotherapeutic Agents. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.21/issuetoc.

Impairment of Mitochondrial Biogenesis and Dynamics Involved in Isoniazid-Induced Apoptosis of HepG2 Cells Was Alleviated by p38 MAPK Pathway

Isoniazid (INH), a widely used first-line antitubercular drug, has been noted to be associated with hepatotoxicity. In spite of extensive researches over many decades, the mechanism of INH-induced hepatotoxicity still remains poorly understood. Recently, mitochondrial toxicity has been emerging as a new paradigm for INH-induced hepatotoxicity. In this study, we showed that INH impaired mitochondrial biogenesis and dynamics in human hepatocarcinoma HepG2 cells. INH reduced mitochondrial membrane potential (MMP) and induced mitochondria swelling. INH also inhibited the protein expressions of three major mitochondrial biogenesis regulators, SIRT1, PGC1α and NRF1, along with increased acetylation of PGC1α. Meanwhile, INH decreased the number of mitochondria, accompanied by decreased expression of mitochondrial protein COX IV. INH caused mitochondrial fragmentation involving decreased levels of the fusion protein MFN2 as well as the fission protein DRP1. INH-reduced DRP1 expression was associated with the increase of apoptosis, suggesting the existence of pro-survival fission and its involvement in mitochondrial quality control. INH activated p38 MAPK, whereas inhibition of p38 MAPK aggravated INH-induced decreases of SIRT1, PGC1α, NRF1, COX IV and DRP1 expressions. P38 MAPK inhibition also further up-regulated the acetylation of PGC1α and exacerbated INH-induced MMP loss, mitochondrial swelling and apoptosis. Taken together, INH-activated p38 MAPK induced mitochondrial biogenesis to alleviate apoptosis through partly recovering SIRT1-PGC1α pathway activation. In the meantime, p38 MAPK activation by INH promoted protective mitochondrial fission to alleviate apoptosis by partial recovery of DRP1 expression.

The Role of Autophagy in the Heart

Autophagy is an evolutionarily conserved mechanism by which cytoplasmic elements are degraded intracellularly. Autophagy has also emerged as a major regulator of cardiac homeostasis and function. Autophagy preserves cardiac structure and function under baseline conditions and is activated during stress, limiting damage under most conditions. It reduces injury and preserves cardiac function during ischemia. It also reduces chronic ischemic remodeling and mediates the cardiac adaptation to pressure overload by restricting misfolded protein accumulation, mitochondrial dysfunction, and oxidative stress. Impairment of autophagy is involved in the development of diabetes and aging-induced cardiac abnormalities. Autophagy defects contribute to the development of cardiac proteinopathy and doxorubicin-induced cardiomyopathy. However, massive activation of autophagy may be detrimental for the heart in certain stress conditions, such as reperfusion injury. In this review, we discuss recent evidence supporting the important role of autophagy and mitophagy in the regulation of cardiac homeostasis and adaptation to stress.

Roles of mitochondrial dynamics modulators in cardiac ischaemia/reperfusion injury

The current therapeutic strategy for the management of acute myocardial infarction (AMI) is to return blood flow into the occluded coronary artery of the heart, a process defined as reperfusion. However, reperfusion itself can increase mortality rates in AMI patients because of cardiac tissue damage and dysfunction, which is termed ‘ischaemia/reperfusion (I/R) injury’. Mitochondria play an important role in myocardial I/R injury as disturbance of mitochondrial dynamics, especially excessive mitochondrial fission, is a predominant cause of cardiac dysfunction. Therefore, pharmacological intervention and therapeutic strategies which modulate the mitochondrial dynamics balance during I/R injury could exert great beneficial effects to the I/R heart. This review comprehensively summarizes and discusses the effects of mitochondrial fission inhibitors as well as mitochondrial fusion promoters on cardiac and mitochondrial function during myocardial I/R injury. The comparison of the effects of both compounds given at different time‐points during the course of I/R injury (i.e. prior to ischaemia, during ischaemia and at the reperfusion period) are also summarized and discussed. Finally, this review also details important information which may contribute to clinical practices using these drugs to improve the quality of life in AMI patients.

Increased mitochondrial fusion allows the survival of older animals in diverse C. elegans longevity pathways

Mitochondria are dynamic organelles that undergo fusion and fission events. Mitochondrial dynamics are required for mitochondrial viability and for responses to changes in bioenergetic status. Here we describe an insulin-signaling and SCFLIN-23-regulated pathway that controls mitochondrial fusion in Caenorhabditis elegans by repressing the expression of the mitochondrial proteases SPG-7 and PPGN-1. This pathway is required for mitochondrial fusion in response to physical exertion, and for the associated extension in lifespan. We show that diverse longevity pathways exhibit increased levels of elongated mitochondria. The increased mitochondrial fusion is essential for longevity in the diverse longevity pathways, as inhibiting mitochondrial fusion reduces their lifespans to wild-type levels. Our results suggest that increased mitochondrial fusion is not a major driver of longevity, but rather is essential to allow the survival of older animals beyond their normal lifespan in diverse longevity pathways.Mitochondria can undergo shape changes as a result of fusion and fission events. Here the authors describe how insulin signalling regulates mitochondrial fusion in C. elegans, and show that mitochondrial fusion is necessary, but not sufficient, for longevity of worms with mutations that increase lifespan.

Penehyclidine hydrochloride preconditioning provides cardiac protection in a rat model of myocardial ischemia/reperfusion injury via the mechanism of mitochondrial dynamics mechanism

To investigate that penehyclidine hydrochloride (PHC) plays a cardiac protection role in myocardial ischemia/reperfusion injury (IRI) through mitochondrial dynamics mechanism. Rat model of myocardial I/R injury was established by the ligation of left anterior descending coronary artery for 30min followed by 3h perfusion. Before I/R, the rats were pretreated with or without PHC. Cardiac function was measured by echocardiography. The activities/levels of myocardial enzymes, oxidants and antioxidant enzymes were detected. Evans blue/TTC double staining was performed to assess infarct size. Cell apoptosis was evaluated by TUNEL assay. Western blot and real time fluorescent quantitative PCR was performed to analyze the expression of Drp₁, Mfn₁, Mfn₂. Meanwhile, the rats were given a single injection of PHC before I/R. The effects of PHC on myocardial infarct and cardiac function were investigated after 7 days post-reperfusion. Our results showed that PHC pretreatment improved imbalance of mitochondrial dynamics induced by oxidative stressor in IRI. PHC preconditioning alleviated apoptotic rate of cell by improving the imbalance of mitochondrial dynamics in IRI. Meanwhile, we showed that PHC remarkably improved cardiac function, myocardial injury by decreasing infarct size and attenuated levels of myocardial enzyme. Additionally, PHC also exerted long-term cardiac protection in a rat model of I/R injury by decreasing infarct size and improving cardiac function. These results suggested that PHC could efficiently protect the rats against I/R-induced myocardial injury via the mechanism of mitochondrial dynamics.

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

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.

Human biallelic MFN2 mutations induce mitochondrial dysfunction, upper body adipose hyperplasia, and suppression of leptin expression

MFN2 encodes mitofusin 2, a membrane-bound mediator of mitochondrial membrane fusion and inter-organelle communication. MFN2 mutations cause axonal neuropathy, with associated lipodystrophy only occasionally noted, however homozygosity for the p.Arg707Trp mutation was recently associated with upper body adipose overgrowth. We describe similar massive adipose overgrowth with suppressed leptin expression in four further patients with biallelic MFN2 mutations and at least one p.Arg707Trp allele. Overgrown tissue was composed of normal-sized, UCP1-negative unilocular adipocytes, with mitochondrial network fragmentation, disorganised cristae, and increased autophagosomes. There was strong transcriptional evidence of mitochondrial stress signalling, increased protein synthesis, and suppression of signatures of cell death in affected tissue, whereas mitochondrial morphology and gene expression were normal in skin fibroblasts. These findings suggest that specific MFN2 mutations cause tissue-selective mitochondrial dysfunction with increased adipocyte proliferation and survival, confirm a novel form of excess adiposity with paradoxical suppression of leptin expression, and suggest potential targeted therapies.

Doxycycline protects against ROS-induced mitochondrial fragmentation and ISO-induced heart failure

In addition to their anti-bacterial action, tetracyclines also have complex biological effects, including the modification of mitochondrial protein synthesis, metabolism and gene-expression. Long-term clinical studies have been performed using tetracyclines, without significant side effects. Previous studies demonstrated that doxycycline (DOX), a major tetracyclin antibiotic, exerted a protective effect in animal models of heart failure; however, its exact molecular mechanism is still unknown. Here, we provide the first evidence that DOX reduces oxidative stress-induced mitochondrial fragmentation and depolarization in H9c2 cardiomyocytes and beneficially alters the expression of Mfn-2, OPA-1 and Drp-1 -the main regulators of mitochondrial fusion and fission-in our isoproterenol (ISO)-induced heart failure model, ultimately decreasing the severity of heart failure. In mitochondria, oxidative stress causes a shift toward fission which leads to mitochondrial fragmentation and cell death. Protecting mitochondria from oxidative stress, and the regulation of mitochondrial dynamics by drugs that shift the balance toward fusion, could be a novel therapeutic approach for heart failure. On the basis of our findings, we raise the possibility that DOX could be a novel therapeutic agent in the future treatment of heart failure.

Exogenous H2S regulates endoplasmic reticulum-mitochondria cross-talk to inhibit apoptotic pathways in STZ-induced type I diabetes

The upregulation of reactive oxygen species (ROS) is a primary cause of cardiomyocyte apoptosis in diabetes cardiomyopathy (DCM). Mitofusin-2 (Mfn-2) is a key protein that bridges the mitochondria and endoplasmic reticulum (ER). Hydrogen sulfide (H₂S)-mediated cardioprotection is related to antioxidant effects. The present study demonstrated that H₂S inhibited the interaction between the ER and mitochondrial apoptotic pathway. This study investigated cardiac function, ultrastructural changes in the ER and mitochondria, apoptotic rate using TUNEL, and the expression of ER stress-associated proteins and mitochondrial apoptotic proteins in cardiac tissues in STZ-induced type I diabetic rats treated with or without NaHS (donor of H₂S). Mitochondria of cardiac tissues were isolated, and MPTP opening and cytochrome c (cyt C) and Mfn-2 expression were also detected. Our data showed that hyperglycemia decreased the cardiac function by ultrasound cardiogram, and the administration of exogenous H₂S ameliorated these changes. We demonstrated that the expression of ER stress sensors and apoptotic rates were elevated in cardiac tissue of DCM and cultured H9C2 cells, but the expression of these proteins was reduced following exogenous H₂S treatment. The expression of mitochondrial apoptotic proteins, cyt C, and mPTP opening was decreased following treatment with exogenous H₂S. In our experiment, the expression and immunofluorescence of Mfn-2 were both decreased after transfection with Mfn-2-siRNA. Hyperglycemia stimulated ER interactions and mitochondrial apoptotic pathways, which were inhibited by exogenous H₂S treatment through the regulation of Mfn-2 expression.

Novel putative pharmacological therapies to protect the right ventricle in pulmonary hypertension: a review of current literature

Pulmonary hypertension (PH) is defined by elevated mean pulmonary artery pressure following the pathological remodelling of small pulmonary arteries. An increase in right ventricular (RV) afterload results in RV hypertrophy and RV failure. The pathophysiology of PH, and RV remodelling in particular, is not well understood, thus explaining, at least in part, why current PH therapies have a limited effect. Existing therapies mostly target the pulmonary circulation. Because the remodelled RV fails to support normal cardiac function, patients eventually succumb from RV failure. Developing novel therapies that directly target the function of the RV may therefore benefit patients with PH. In the past decade, several promising studies have investigated novel cardioprotective strategies in experimental models of PH. This review aims to comprehensively discuss and highlight these novel experimental approaches to confer, in the long-term, greater health benefit in patients with PH.

Barth syndrome cardiomyopathy

Barth syndrome (BTHS) is an inherited form of cardiomyopathy, caused by a mutation within the gene encoding the mitochondrial transacylase tafazzin. Tafazzin is involved in the biosynthesis of the unique phospholipid cardiolipin (CL), which is almost exclusively found in mitochondrial membranes. CL directly interacts with a number of essential protein complexes in the mitochondrial membranes including the respiratory chain, mitochondrial metabolite carriers, and proteins, involved in shaping mitochondrial morphology. Here we describe, how in BTHS CL deficiency causes changes in the morphology of mitochondria, structural changes in the respiratory chain, decreased respiration, and increased generation of reactive oxygen species. A large number of cellular and animal models for BTHS have been established to elucidate how mitochondrial dysfunction induces sarcomere disorganization and reduced contractility, resulting in dilated cardiomyopathy in vivo.

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.

Mitofusin-2 prevents skeletal muscle wasting in cancer cachexia

Cancer cachexia remains a leading cause of morbidity and mortality worldwide, despite extensive research and clinical trials. The prominent clinical feature of cancer cachexia is the continuous loss of skeletal muscle that cannot be fully reversed by conventional nutritional support, and that leads to progressive functional impairment. The mechanism underlying muscle loss in patients with cachexia is poorly understood. The present study analyzed 21 cancer patients with or without cachexia, and demonstrated that mitofusin-2 (Mfn2) was downregulated in the rectus abdominis of patients with cachexia, which was associated with body weight loss. In vitro cell experiments indicated that loss of Mfn2 was associated with atrophy of the C2C12 mouse myoblast cell line. Furthermore, in vivo animal experiments demonstrated that cachexia decreased gastrocnemius muscle mass and Mfn2 expression, and overexpression of Mfn2 in gastrocnemius muscle was able to partially attenuate cachexia-induced gastrocnemius muscle loss. The results of the present study suggested that Mfn2 is involved in cachexia-induced muscle loss and may serve as a potential target for therapy of cachexia.

The antioxidant edaravone prevents cardiac dysfunction by suppressing oxidative stress in type 1 diabetic rats and in high-glucose-induced injured H9c2 cardiomyoblasts

Edaravone, a radical scavenger, has been recognized as a potential protective agent for cardiovascular diseases. However, little is known about the effect of edaravone in cardiac complications associated with diabetes. Here, we have demonstrated that edaravone prevents cardiac dysfunction and apoptosis in the streptozotocin-induced type 1 diabetic rat heart. Mechanistic studies revealed that edaravone treatment improved cardiac function and restored superoxide dismutase levels. In addition, treatment of diabetic animals by edaravone increased protein expressions of sirtuin-1 (SIRT-1), peroxisome proliferator activated receptor γ coactivator α (PGC-1α), nuclear factor like-2 (NRF-2), and B cell lymphoma 2 (Bcl-2), and reduced protein expressions of Bax and Caspase-3 compared to the control group. High glucose incubation resulted in the production of reactive oxygen species (ROS) and cell death. Treatment of high-glucose-incubated H9c2 cells by edaravone reduced ROS production and cell death. In addition, the treatment of high-glucose-incubated H9c2 cells by edaravone increased the activity of antioxidative stress by increasing SIRT-1, PGC-1α, and NRF-2, and this treatment also reduced apoptosis by increasing Bcl-2 expression and reducing Bax and Caspase-3 expressions. Knockdown SIRT-1 with small interferer RNA abolished the effects of edaravone. Overall, our data demonstrated that edaravone may be an effective agent against the development of diabetic cardiomyopathy.

Diastolic dysfunction in prediabetic male rats: Role of mitochondrial oxidative stress

Although incidence and prevalence of prediabetes are increasing, little is known about its cardiac effects. Therefore, our aim was to investigate the effect of prediabetes on cardiac function and to characterize parameters and pathways associated with deteriorated cardiac performance. Long-Evans rats were fed with either control or high-fat chow for 21 wk and treated with a single low dose (20 mg/kg) of streptozotocin at week 4 High-fat and streptozotocin treatment induced prediabetes as characterized by slightly elevated fasting blood glucose, impaired glucose and insulin tolerance, increased visceral adipose tissue and plasma leptin levels, as well as sensory neuropathy. In prediabetic animals, a mild diastolic dysfunction was observed, the number of myocardial lipid droplets increased, and left ventricular mass and wall thickness were elevated; however, no molecular sign of fibrosis or cardiac hypertrophy was shown. In prediabetes, production of reactive oxygen species was elevated in subsarcolemmal mitochondria. Expression of mitofusin-2 was increased, while the phosphorylation of phospholamban and expression of Bcl-2/adenovirus E1B 19-kDa protein-interacting protein 3 (BNIP3, a marker of mitophagy) decreased. However, expression of other markers of cardiac auto- and mitophagy, mitochondrial dynamics, inflammation, heat shock proteins, Ca²⁺/calmodulin-dependent protein kinase II, mammalian target of rapamycin, or apoptotic pathways were unchanged in prediabetes. This is the first comprehensive analysis of cardiac effects of prediabetes indicating that mild diastolic dysfunction and cardiac hypertrophy are multifactorial phenomena that are associated with early changes in mitophagy, cardiac lipid accumulation, and elevated oxidative stress and that prediabetes-induced oxidative stress originates from the subsarcolemmal mitochondria.

Selective mitochondrial depletion, apoptosis resistance, and increased mitophagy in human Charcot-Marie-Tooth 2A motor neurons

Charcot-Marie-Tooth 2A (CMT2A) is an inherited peripheral neuropathy caused by mutations in MFN2, which encodes a mitochondrial membrane protein involved in mitochondrial network homeostasis. Because MFN2 is expressed ubiquitously, the reason for selective motor neuron (MN) involvement in CMT2A is unclear. To address this question, we generated MNs from induced pluripotent stem cells (iPSCs) obtained from the patients with CMT2A as an in vitro disease model. CMT2A iPSC-derived MNs (CMT2A-MNs) exhibited a global reduction in mitochondrial content and altered mitochondrial positioning without significant differences in survival and axon elongation. RNA sequencing profiles and protein studies of key components of the apoptotic executioner program (i.e. p53, BAX, caspase 8, cleaved caspase 3, and the anti-apoptotic marker Bcl2) demonstrated that CMT2A-MNs are more resistant to apoptosis than wild-type MNs. Exploring the balance between mitochondrial biogenesis and the regulation of autophagy-lysosome transcription, we observed an increased autophagic flux in CMT2A-MNs that was associated with increased expression of PINK1, PARK2, BNIP3, and a splice variant of BECN1 that was recently demonstrated to be a trigger for mitochondrial autophagic removal. Taken together, these data suggest that the striking reduction in mitochondria in MNs expressing mutant MFN2 is not the result of impaired biogenesis, but more likely the consequence of enhanced mitophagy. Thus, these pathways represent possible novel molecular therapeutic targets for the development of an effective cure for this disease.

Mitofusins, from Mitochondria to Metabolism

Mitochondrial architecture is involved in several functions crucial for cell viability, proliferation, senescence, and signaling. In particular, mitochondrial dynamics, through the balance between fusion and fission events, represents a central mechanism for bioenergetic adaptation to metabolic needs of the cell. As key regulators of mitochondrial dynamics, the fusogenic mitofusins have recently been linked to mitochondrial biogenesis and respiratory functions, impacting on cell fate and organism homeostasis. Here we review the implication of mitofusins in the regulation of mitochondrial metabolism, and their consequence on energy homeostasis at the cellular and physiological level, highlighting their crucial role in metabolic disorders, cancer, and aging.

Clinical significance of mitofusin-2 and its signaling pathways in hepatocellular carcinoma

BACKGROUND

The mitochondrial GTPase mitofusin-2 (MFN2) gene encodes a mitochondrial membrane protein that can induce apoptosis of hepatocellular carcinoma (HCC) via the mitochondrial apoptotic pathway, as validated in our previous research. However, little is known of the clinical significance of MFN2 expression and its signaling pathways in HCC.

METHODS

MFN2 mRNA expression in tumor and adjacent non-tumor tissues from 115 patients with HCC was investigated using quantitative real-time PCR. The association of the MFN2 mRNA expression level with clinical and pathological parameters was evaluated statistically, while a comparative microarray analysis was used to identify MFN2 signaling pathways in HepG2 cells.

RESULTS

MFN2 was significantly (p < 0.0001) downregulated in HCC tissues. Low MFN2 expression was significantly correlated with sex and preoperative alpha-fetoprotein (p < 0.05). Both a Kaplan-Meier survival curve and multivariate analyses showed that MFN2 was related to overall survival. A comparative gene expression microarray revealed 211 upregulated (58 %) and 153 downregulated (42 %) genes. Eighteen pathways were identified as the most significant pathways correlated with MFN2.

CONCLUSIONS

Low MFN2 expression in HCC indicated a worse overall survival. Crucial signaling molecules such as PI3K-AKT, cytokine receptor, and focal adhesion may participate in MFN2-mediated signaling pathway changes in HCC.

Increased mitochondrial fission promotes autophagy and hepatocellular carcinoma cell survival through the ROS-modulated coordinated regulation of the NFKB and TP53 pathways

Mitochondrial morphology is dynamically remodeled by fusion and fission in cells, and dysregulation of this process is closely implicated in tumorigenesis. However, the mechanism by which mitochondrial dynamics influence cancer cell survival is considerably less clear, especially in hepatocellular carcinoma (HCC). In this study, we systematically investigated the alteration of mitochondrial dynamics and its functional role in the regulation of autophagy and HCC cell survival. Furthermore, the underlying molecular mechanisms and therapeutic application were explored in depth. Mitochondrial fission was frequently upregulated in HCC tissues mainly due to an elevated expression ratio of DNM1L to MFN1, which significantly contributed to poor prognosis of HCC patients. Increased mitochondrial fission by forced expression of DNM1L or knockdown of MFN1 promoted the survival of HCC cells both in vitro and in vivo mainly by facilitating autophagy and inhibiting mitochondria-dependent apoptosis. We further demonstrated that the survival-promoting role of increased mitochondrial fission was mediated via elevated ROS production and subsequent activation of AKT, which facilitated MDM2-mediated TP53 degradation, and NFKBIA- and IKK-mediated transcriptional activity of NFKB in HCC cells. Also, a crosstalk between TP53 and NFKB pathways was involved in the regulation of mitochondrial fission-mediated cell survival. Moreover, treatment with mitochondrial division inhibitor-1 significantly suppressed tumor growth in an in vivo xenograft nude mice model. Our findings demonstrate that increased mitochondrial fission plays a critical role in regulation of HCC cell survival, which provides a strong evidence for this process as drug target in HCC treatment.

Mitochondrial fission/fusion and cardiomyopathy

Mitochondria are highly abundant in and essential to the beat-to-beat contractile performance of hearts. However, relatively few cardiac diseases have been attributed to primary mitochondrial dysfunction. The paucity of evidence for 'primary mitochondrial cardiac diseases' may be because such an entity does not exist. Alternately, the consequences of mitochondrial dysfunction on hearts may be so severe that long-term viability is severely impaired and affected individuals are therefore not included in standard genetic screens of adult heart disease subjects. Here, I review accumulating experimental evidence that impairing mitochondrial fission or fusion causes cardiomyopathy in otherwise normal mice, and consider how these data could motivate screening of perinatal cardiomyopathy subjects for damaging mutations of mitochondrial fission and fusion factors.

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.

Effect of breviscapine against hepatic ischemia reperfusion injury

BACKGROUND

Breviscapine is an active ingredient extracted from traditional Chinese medicine Erigeron breviscapus. The purpose of this study was to investigate the effect of breviscapine injection on hepatic ischemia and/or reperfusion injury.

METHODS

Forty rats were randomly divided into five groups (n = 8): Sham group, Ischemia reperfusion 1 (I/R1) + normal saline (NS) group, I/R1 + breviscapine (Bre), I/R2 + NS group, and I/R2 + Bre group. Group1 and group2 represent ischemia time for 10 min and 30 min, respectively. Breviscapine or normal saline was administered to rats (single dose of 10 mg/Kg, intravenously) 30 min before hepatic ischemia. Serum transaminases, histopathologic changes, malondialdehyde (MDA), and superoxide dismutase (SOD) in liver tissues were evaluated. The expression level of mitochondrial fusion 2 (Mfn2) was also investigated.

RESULTS

After 24-h reperfusion, based on the histopathologic analysis, compared with NS control group, the liver function was improved in breviscapine group. Liver enzymes aspartate and alanine aminotransferase levels were significantly lower in the I/R + Bre group, when compared with the I/R + NS group. Pretreatment with breviscapine reduced MDA level (P < 0.05) and increased SOD activity significantly in I/R + Bre compared with I/R + NS group. Western blot and RT-q polymerase chain reaction showed that Mfn2 was significantly downregulated in breviscapine preconditioning group as compared to normal saline control group.

CONCLUSIONS

Breviscapine preconditioning attenuates liver ischemia reperfusion injury via inhibiting liver oxidative stress reaction. The protective mechanism probably inhibits Mfn2 protein and mRNA expression.

Appoptosin interacts with mitochondrial outer-membrane fusion proteins and regulates mitochondrial morphology

Mitochondrial morphology is regulated by fusion and fission machinery. Impaired mitochondria dynamics cause various diseases, including Alzheimer's disease. Appoptosin (encoded by SLC25A38) is a mitochondrial carrier protein that is located in the mitochondrial inner membrane. Appoptosin overexpression causes overproduction of reactive oxygen species (ROS) and caspase-dependent apoptosis, whereas appoptosin downregulation abolishes β-amyloid-induced mitochondrial fragmentation and neuronal death during Alzheimer's disease. Herein, we found that overexpression of appoptosin resulted in mitochondrial fragmentation in a manner independent of its carrier function, ROS production or caspase activation. Although appoptosin did not affect levels of mitochondrial outer-membrane fusion (MFN1 and MFN2), inner-membrane fusion (OPA1) and fission [DRP1 (also known as DNM1L) and FIS1] proteins, appoptosin interacted with MFN1 and MFN2, as well as with the mitochondrial ubiquitin ligase MITOL (also known as MARCH5) but not OPA1, FIS1 or DRP1. Appoptosin overexpression impaired the interaction between MFN1 and MFN2, and mitochondrial fusion. By contrast, co-expression of MFN1, MITOL and a dominant-negative form of DRP1, DRP1(K38A), partially rescued appoptosin-induced mitochondrial fragmentation and apoptosis, whereas co-expression of FIS1 aggravated appoptosin-induced apoptosis. Together, our results demonstrate that appoptosin can interact with mitochondrial outer-membrane fusion proteins and regulates mitochondrial morphology.

Rg1 prevents myocardial hypoxia/reoxygenation injury by regulating mitochondrial dynamics imbalance via modulation of glutamate dehydrogenase and mitofusin 2

PURPOSE

Mitochondrial dysfunction is a prominent feature of ischemia heart disease but the underlying mechanism of dynamics (fusion/fission) is still unclear. Here we investigated a novel function and underlying mechanism of Rg1 on an in vitro cardiomyocyte model of hypoxia/reoxygenation (H/R).

METHODS

Cellular cytotoxicity was evaluated by MTT, mitochondrial viable staining, and cardiac marker detection. Mitochondrial function was evaluated by ATP content measurement, MMP determination, ROS, OCR and ECAR assay. Mitochondrial dynamics was investigated by Live-cell imaging with time-lapse fluorescence microscopy and morphological features were evaluated by the high-content image analysis. Mitochondrial fusion and fission-related proteins, GDH were determined by Western blot, RT-PCR and immunofluorescence.

RESULTS

Rg1 moderated GDH dysregulation and then protected against H/R-induced cellular damage and mitochondrial dysfunction in a dose-dependent manner. Rg1 significantly increased mitochondrial length, reduced the number of cells with fragmented mitochondria and up-regulated the MFN2 expression finally leading to preventing the imbalance of mitochondrial dynamics following H/R. Knock-down of MFN2 by specific siRNA completely abolished the ability of Rg1 to cell survival by H/R.

CONCLUSION

Rg1 through modulation of GDH and MFN2 maintained mitochondrial dynamics that resulted in protection against H/R-induced cardiomyocyte injury. All these results put forward a new protective mechanism of Rg1 on the therapeutic potential in cardiac I/R disorders.

Mitofusin 2 ameliorates hypoxia-induced apoptosis via mitochondrial function and signaling pathways

Mitochondrial dynamics play a critical role in mitochondrial function and signaling. Although mitochondria play a critical role in hypoxia/ischemia, the further mechanisms between mitochondrial dynamics and ischemia are still unclear. The current study aimed to determine the role of mitofusin 2, a key regulator of mitochondrial fusion, in a hypoxic model and to explore a novel strategy for cerebral ischemia via modulation of mitochondrial dynamics. To the best of our knowledge, this is the first study to investigate both mitochondrial function and molecular pathways to determine the role of mitofusin 2 in hypoxia-induced neuronal apoptosis. In vivo, C57BL/6 mice (male, 19-25g) underwent a permanent middle cerebral artery occlusion for 12 or 24h (n=6 per group). In vitro, cobalt chloride was used to mimic hypoxia in immortalized hippocampal neurons. Down- or up-regulation of Mfn2 was induced to investigate the role of Mfn2 in hypoxia, especially in mitochondrial function and signaling pathways. The findings demonstrated that decreased mitofusin 2 occurred both in vivo and in vitro hypoxic models; second, the anti-apoptotic effect of Mfn2 may work via restoration of mitochondrial function; third, the modulation of the B Cell Leukemia 2/Bcl-2 Associated X protein and extracellular signal-regulated kinase 1/2 signaling pathways highlight the role of Mfn2 in signaling pathways beyond fusion. In summary, depletion of mitofusin 2 would lead to apoptosis both in normal or hypoxic conditions; however, mitofusin 2 overexpression could attenuate hypoxia-induced apoptosis, which represents a potential novel strategy for neuroprotection against ischemic brain damage.