Search

Mitochondrial disorders are challenging to diagnose. Exome sequencing has greatly enhanced the diagnostic precision of these disorders although interpreting variants of uncertain significance (VUS) remains a formidable obstacle. Whether specific mitochondrial morphological changes can aid in the classification of these variants is unknown. Here, we describe two families (four patients), each with a VUS in a gene known to affect the morphology of mitochondria through a specific role in the fission-fusion balance. In the first, the missense variant in MFF, encoding a fission factor, was associated with impaired fission giving rise to a characteristically over-tubular appearance of mitochondria. In the second, the missense variant in DNAJA3, which has no listed OMIM phenotype, was associated with fragmented appearance of mitochondria consistent with its published deficiency states. In both instances, the highly specific phenotypes allowed us to upgrade the classification of the variants. Our results suggest that, in select cases, mitochondrial "dysmorphology" can be helpful in interpreting variants to reach a molecular diagnosis.

Porcine epidemic diarrhea virus (PEDV) infection causes huge economic losses to the pig industry worldwide. DNAJA3, a member of the Hsp40 family proteins, is known to play an important role in the replication of several viruses. However, it remains unknown if it interacts with PEDV. We found that DNAJA3 interacted with PEDV S1, initially with yeast two-hybrid screening and later with Co-IP, GST pull-down, and confocal imaging. Further experiments showed the functional relationship between DNAJA3 and PEDV in the infected IPEC-J2 cells. DNAJA3 overexpression significantly inhibited PEDV replication while its knockdown had the opposite effect, suggesting that it is a negative regulator of PEDV replication. In addition, DNAJA3 expression could be downregulated by PEDV infection possibly as the viral strategy to evade the suppressive role of DNAJA3. By gene silencing and overexpression, we were able to show that DNAJA3 inhibited PEDV adsorption to IPEC-J2 cells but did not affect virus invasion. In conclusion, our study provides clear evidence that DNAJA3 mediates PEDV adsorption to host cells and plays an antiviral role in IPEC-J2 cells.

Mitochondrial dysfunction may activate innate immunity, e.g. upon abnormal handling of mitochondrial DNA in TFAM mutants or in altered mitophagy. Recent reports showed that also deletion of mitochondrial matrix peptidase ClpP in mice triggers transcriptional upregulation of inflammatory factors. Here, we studied ClpP-null mouse brain at two ages and mouse embryonal fibroblasts, to identify which signaling pathways are responsible, employing mass spectrometry, subcellular fractionation, immunoblots, and reverse transcriptase polymerase chain reaction. Several mitochondrial unfolded protein response factors showed accumulation and altered migration in blue-native gels, prominently the co-chaperone DNAJA3. Its mitochondrial dysregulation increased also its extra-mitochondrial abundance in the nucleus, a relevant observation given that DNAJA3 modulates innate immunity. Similar observations were made for STAT1, a putative DNAJA3 interactor. Elevated expression was observed not only for the transcription factorsStat1/2, but also for two interferon-stimulated genes (Ifi44,Gbp3). Inflammatory responses were strongest for the RLR pattern recognition receptors (Ddx58,Ifih1,Oasl2,Trim25) and several cytosolic nucleic acid sensors (Ifit1,Ifit3,Oas1b,Ifi204,Mnda). The consistent dysregulation of these factors from an early age might influence also human Perrault syndrome, where ClpP loss-of-function leads to early infertility and deafness, with subsequent widespread neurodegeneration.

RW was supported by a scholarship from the Carl Zeiss Foundation. CK received funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), GRK 1715. E-MP was supported by a Landesgraduiertenstipendium (Friedrich Schiller University Jena). Work done in the group of OHK was done by MB and is funded by DFG, Project-ID 393547839-SFB 1361 and DFG, GRK 2291/9-1, project number 427404172. Open Access funding enabled and organized by Projekt DEAL. Overexpression of MYC is a genuine cancer driver in lymphomas and related to poor prognosis. However, therapeutic targeting of the transcription factor MYC remains challenging. Here, we show that inhibition of the histone deacetylase 6 (HDAC6) using the HDAC6 inhibitor Marbostat-100 (M-100) reduces oncogenic MYC levels and prevents lymphomagenesis in a mouse model of MYC-induced aggressive B-cell lymphoma. M-100 specifically alters protein-protein interactions by switching the acetylation state of HDAC6 substrates, such as tubulin. Tubulin facilitates nuclear import of MYC, and MYC-dependent B-cell lymphoma cells rely on continuous import of MYC due to its high turn-over. Acetylation of tubulin impairs this mechanism and enables proteasomal degradation of MYC. M-100 targets almost exclusively B-cell lymphoma cells with high levels of MYC whereas non-tumor cells are not affected. M-100 induces massive apoptosis in human and murine MYC-overexpressing B-cell lymphoma cells. We identified the heat-shock protein DNAJA3 as an interactor of tubulin in an acetylation-dependent manner and overexpression of DNAJA3 resulted in a pronounced degradation of MYC. We propose a mechanism by which DNAJA3 associates with hyperacetylated tubulin in the cytoplasm to control MYC turnover. Taken together, our data demonstrate a beneficial role of HDAC6 inhibition in MYC-dependent B-cell lymphoma.

Most mitochondrial precursor polypeptides are imported from the cytosol into the mitochondrion, where they must efficiently undergo folding. Mitochondrial precursors are imported as unfolded polypeptides. For proteins of the mitochondrial matrix and inner membrane, two separate chaperone systems, HSP60 and mitochondrial HSP70 (mtHSP70), facilitate protein folding. We show that LONP1, an AAA+ protease of the mitochondrial matrix, works with the mtHSP70 chaperone system to promote mitochondrial protein folding. Inhibition of LONP1 results in aggregation of a protein subset similar to that caused by knockdown of DNAJA3, a co-chaperone of mtHSP70. LONP1 is required for DNAJA3 and mtHSP70 solubility, and its ATPase, but not its protease activity, is required for this function. In vitro, LONP1 shows an intrinsic chaperone-like activity and collaborates with mtHSP70 to stabilize a folding intermediate of OXA1L. Our results identify LONP1 as a critical factor in the mtHSP70 folding pathway and demonstrate its proposed chaperone activity., Most mitochondrial proteins are imported from the cytosol and must fold in the mitochondria. Here, the authors show that the mitochondrial protease LONP1 plays a critical role in the mtHSP70 chaperone system independently of its protease activity.

Tid1, a mitochondrial co-chaperone protein, acts as a tumor suppressor in various cancer types. However, the role of Tid1 in hepatocellular carcinoma (HCC) remains unclear. First, we found that a low endogenous Tid1 protein level was observed in poorly differentiated HCC cell lines. Further, upregulation/downregulation of Tid1 abrogated/promoted the malignancy of human HCC cell lines, respectively. Interestingly, Tid1 negatively modulated the protein level of Nrf2. Tissue assays from 210 surgically resected HCC patients were examined by immunohistochemistry (IHC) analyses. The protein levels of Tid1 in the normal and tumor part of liver tissues were correlated with the clinical outcome of the 210 HCC cases. In multivariate analysis, we discovered that tumor size &gt, 5 cm, multiple tumors, presence of vascular invasion, low Tid1 expression in the non-tumor part, and high Nrf2 expression in the non-tumor part were significant factors associated with worse recurrence-free survival (RFS). A scoring system by integrating the five clinical and pathological factors predicts the RFS among HCC patients after surgical resection. Together, Tid1, serving as a tumor suppressor, has a prognostic role for surgically resected HCC to predict RFS.

Mitochondrial dynamics are increasingly recognized to play an important role in regulating mitochondrial function in response to diverse stimuli. Given the overlap in the physiological processes influenced by mitochondria and the physiological processes disrupted in tumor cells, we speculate that tumor cells alter mitochondrial shape to promote the tumorigenic phenotype. Here, we briefly review the evidence linking changes in mitochondrial fusion and fission to a number of key tumorigenic processes, including metabolic rewiring, inhibition of cell death, cell migration, cell proliferation and self-renewal capacity. The role of mitochondrial dynamics in tumor growth is an important emerging area of research, a better understanding of which may lead to promising new therapeutic options for the treatment of cancer.

Recent advancement in mitochondrial research has significantly extended our knowledge on the role and regulation of mitochondria in health and disease. One important breakthrough is the delineation of how mitochondrial morphological changes, termed mitochondrial dynamics, are coupled to the bioenergetics and signaling functions of mitochondria. In general, it is believed that fusion leads to an increased mitochondrial respiration efficiency and resistance to stress-induced dysfunction while fission does the contrary. This concept seems not applicable to adult cardiomyocytes. The mitochondria in adult cardiomyocytes exhibit fragmented morphology (tilted towards fission) and show less networking and movement as compared to other cell types. However, being the most energy-demanding cells, cardiomyocytes in the adult heart possess vast number of mitochondria, high level of energy flow, and abundant mitochondrial dynamics proteins. This apparent discrepancy could be explained by recently identified new functions of the mitochondrial dynamics proteins. These "non-canonical" roles of mitochondrial dynamics proteins range from controlling inter-organelle communication to regulating cell viability and survival under metabolic stresses. Here, we summarize the newly identified non-canonical roles of mitochondrial dynamics proteins. We focus on how these fission and fusion independent roles of dynamics proteins regulate mitochondrial bioenergetics. We also discuss potential molecular mechanisms, unique intracellular location, and the cardiovascular disease relevance of these non-canonical roles of the dynamics proteins. We propose that future studies are warranted to differentiate the canonical and non-canonical roles of dynamics proteins and to identify new approaches for the treatment of heart diseases. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Mitochondria are fundamental structures that fulfil important and diverse functions within cells, including cellular respiration and iron-sulfur cluster biogenesis. Mitochondrial function is reliant on the organelles proteome, which is maintained and adjusted depending on cellular requirements. The majority of mitochondrial proteins are encoded by nuclear genes and must be trafficked to, and imported into the organelle following synthesis in the cytosol. These nuclear-encoded mitochondrial precursors utilise dynamic and multimeric translocation machines to traverse the organelles membranes and be partitioned to the appropriate mitochondrial subcompartment. Yeast model systems have been instrumental in establishing the molecular basis of mitochondrial protein import machines and mechanisms, however unique players and mechanisms are apparent in higher eukaryotes. Here, we review our current knowledge on mitochondrial protein import in human cells and how dysfunction in these pathways can lead to disease.

Mitochondrial function is coupled to metabolic and survival pathways through both direct signaling cascades and dynamic changes in mitochondrial morphology. For example, a hyperfused mitochondrial reticulum is activated upon cellular stress and is protective against cell death. As part of a genome-wide small inhibitory ribonucleic acid screen, we identified the central redox regulator, Keap1, as a novel regulator of mitochondrial morphology. Here, we aimed to determine the mechanism through which redox signaling and Keap1 mediate changes in mitochondrial morphology.We found that the Nrf2 transcription factor is required for mitochondrial hyperfusion induced by knockdown of Keap1. Nrf2, which is negatively regulated by Keap1, mediates the cell's response to stress by controlling the expression of several hundred genes, including proteasome expression. We next showed that increased proteasome activity, a result of increased Nrf2 activity, is responsible for the degradation of the mitochondrial fission protein Drp1, which occurs in an ubiquitin-independent manner.Our study described a novel pathway by which Nrf2 activation, known to occur in response to increased oxidative stress, decreases mitochondrial fission and contributes to a hyperfused mitochondrial network.This study has identified the Keap1-Nrf2 nexus and modulation of proteasomal activity as novel avenues to inhibit mitochondrial fission. These findings are important, because inhibiting mitochondrial fission is a promising therapeutic approach to restore the balance between fission and fusion, which is attractive for an increasing number of disorders linked to mitochondrial dysfunction. Antioxid. Redox Signal. 27, 1447-1459.

The anti-apoptotic BCL2 family of proteins elicits a broad cell survival program mainly by promoting cell migration, invasion, and metastasis. High expression level of BCL2 family proteins is a characteristic feature of cancer cells, especially in cisplatin-resistant cancer cells. Recent studies have shown that BCL2 family proteins play a housekeeping role in modulating mitochondrial dynamics. However, it is not clear whether BCL2 family proteins are relevant to mitochondrial fission and fusion in cisplatin-resistant ovarian cancer cells. Here, we report that the BCL2/BCLXL inhibitor ABT737 induced apoptosis more potently in cisplatin-resistant SKOV3/DDP ovarian cancer cells than in cisplatin-sensitive SKOV3 ovarian cancer cells. ABT737 significantly increased levels of DRP1 in mitochondria and increased rates of mitochondrial fission, and then induced cytochrome C release from mitochondria and mitophagy in SKOV3/DDP cells. Mdivi-1, a selective inhibitor of DRP1, weakened ABT737-induced mitochondrial fission, intrinsic apoptotic pathways, and mitophagy in SKOV3/DDP cells. Taken together, these results demonstrate a novel function of ABT737 in inducing DRP1-dependent apoptotic mitochondrial fission and highlight that targeting anti-apoptotic BCL2 family proteins may be an emerging therapeutic strategy for patients with cisplatin-resistant ovarian cancer.

Mitochondria form a cellular network of organelles, or cellular compartments, that efficiently couple nutrients to energy production in the form of ATP. As cancer cells rely heavily on glycolysis, historically mitochondria and the cellular pathways in place to maintain mitochondrial activities were thought to be more relevant to diseases observed in non-dividing cells such as muscles and neurons. However, more recently it has become clear that cancers rely heavily on mitochondrial activities including lipid, nucleotide and amino acid synthesis, suppression of mitochondria-mediated apoptosis as well as oxidative phosphorylation (OXPHOS) for growth and survival. Considering the variety of conditions and stresses that cancer cell mitochondria may incur such as hypoxia, reactive oxygen species and mitochondrial genome mutagenesis, we examine potential roles for a mitochondrial-protective transcriptional response known as the mitochondrial unfolded protein response (UPRmt) in cancer cell biology.

Mitochondria-shaping proteins control the dynamic equilibrium between fusion and fission of the mitochondrial network. Their balance is strictly required to regulate various processes, including the quality of mitochondria, cell metabolism, cell death, proliferation and cell migration. Alterations in these processes are frequently encountered in cancer, during both its onset and later progression, as evidence emerge connecting alterations in mitochondrial dynamics with cancer development. In recent years, novel therapeutic approaches to fight against different human tumors aim at exploiting the immune system's ability to specifically recognize tumor antigens, thus killing malignant cells in a process named immune-surveillance. Interestingly, data are accumulating on the role that mitochondrial dynamics play also for the correct function of both the innate and the adaptive immune system. By this review, we overview how mitochondrial dynamics can affect various processes during cancer development, acting directly on tumor cells or indirectly on cells responsible for tumor aggression and defence.

Functional and structural damages to mitochondria have been critically associated with the pathogenesis of Down syndrome (DS), a human multifactorial disease caused by trisomy of chromosome 21 and associated with neurodevelopmental delay, intellectual disability and early neurodegeneration. Recently, we demonstrated in neural progenitor cells (NPCs) isolated from the hippocampus of Ts65Dn mice -a widely used model of DS - a severe impairment of mitochondrial bioenergetics and biogenesis and reduced NPC proliferation. Here we further investigated the origin of mitochondrial dysfunction in DS and explored a possible mechanistic link among alteration of mitochondrial dynamics, mitochondrial dysfunctions and defective neurogenesis in DS. We first analyzed mitochondrial network and structure by both confocal and transmission electron microscopy as well as by evaluating the levels of key proteins involved in the fission and fusion machinery. We found a fragmentation of mitochondria due to an increase in mitochondrial fission associated with an up-regulation of dynamin-related protein 1 (Drp1), and a decrease in mitochondrial fusion associated with a down-regulation of mitofusin 2 (Mnf2) and increased proteolysis of optic atrophy 1 (Opa1). Next, using the well-known neuroprotective agent mitochondrial division inhibitor 1 (Mdivi-1), we assessed whether the inhibition of mitochondrial fission might reverse alteration of mitochondrial dynamics and mitochondrial dysfunctions in DS neural progenitors cells. We demonstrate here for the first time, that Mdivi-1 restores mitochondrial network organization, mitochondrial energy production and ultimately improves proliferation and neuronal differentiation of NPCs. This research paves the way for the discovery of new therapeutic tools in managing some DS-associated clinical manifestations.

Mitochondrial fatty acid synthesis (mtFAS) is a highly conserved pathway essential for mitochondrial biogenesis. The mtFAS process is required for mitochondrial respiratory chain assembly and function, synthesis of the lipoic acid cofactor indispensable for the function of several mitochondrial enzyme complexes and essential for embryonic development in mice. Mutations in human mtFAS have been reported to lead to neurodegenerative disease. The source of malonyl-CoA for mtFAS in mammals has remained unclear. We report the identification of a conserved vertebrate mitochondrial isoform of ACC1 expressed from an ACACA transcript splicing variant. A specific knockdown (KD) of the corresponding transcript in mouse cells, or CRISPR/Cas9-mediated inactivation of the putative mitochondrial targeting sequence in human cells, leads to decreased lipoylation and mitochondrial fragmentation. Simultaneous KD of ACSF3, encoding a mitochondrial malonyl-CoA synthetase previously implicated in the mtFAS process, resulted in almost complete ablation of protein lipoylation, indicating that these enzymes have a redundant function in mtFAS. The discovery of a mitochondrial isoform of ACC1 required for lipoic acid synthesis has intriguing consequences for our understanding of mitochondrial disorders, metabolic regulation of mitochondrial biogenesis and cancer.

Mitochondria are critically involved in reactive oxygen species (ROS)-dependent lung diseases, such as lung fibrosis, asbestos, chronic airway diseases and lung cancer. Mitochondrial DNA (mtDNA) encodes mitochondrial proteins and is more sensitive to oxidants than nuclear DNA. Damage to mtDNA causes mitochondrial dysfunction, including electron transport chain impairment and mitochondrial membrane potential loss. Furthermore, damaged mtDNA also acts as a damage-associated molecular pattern (DAMP) that drives inflammatory and immune responses. In this review, crosstalk among alveolar epithelial cells, alveolar macrophages and mitochondria is examined. ROS-related transcription factors and downstream cell signaling pathways are also discussed. We conclude that targeting oxidative stress with antioxidant agents, such as thiol molecules, polyphenols and superoxide dismutase (SOD), and promoting mitochondrial biogenesis should be considered as novel strategies for treating lung diseases that currently have no effective treatment options.

The production of reactive species contributes to the age-dependent accumulation of dysfunctional mitochondria and protein aggregates, all of which are associated with neurodegeneration. A putative mediator of these effects is the lipid peroxidation product 4-hydroxynonenal (4-HNE), which has been shown to inhibit mitochondrial function, and accumulate in the postmortem brains of patients with neurodegenerative diseases. This deterioration in mitochondrial quality could be due to direct effects on mitochondrial proteins, or through perturbation of the macroautophagy/autophagy pathway, which plays an essential role in removing damaged mitochondria. Here, we use a click chemistry-based approach to demonstrate that alkyne-4-HNE can adduct to specific mitochondrial and autophagy-related proteins. Furthermore, we found that at lower concentrations (5–10 μM), 4-HNE activates autophagy, whereas at higher concentrations (15 μM), autophagic flux is inhibited, correlating with the modification of key autophagy proteins at higher concentrations of alkyne-4-HNE. Increasing concentrations of 4-HNE also cause mitochondrial dysfunction by targeting complex V (the ATP synthase) in the electron transport chain, and induce significant changes in mitochondrial fission and fusion protein levels, which results in alterations to mitochondrial network length. Finally, inhibition of autophagy initiation using 3-methyladenine (3MA) also results in a significant decrease in mitochondrial function and network length. These data show that both the mitochondria and autophagy are critical targets of 4-HNE, and that the proteins targeted by 4-HNE may change based on its concentration, persistently driving cellular dysfunction.

Hypertonia is a neurological dysfunction associated with a number of central nervous system disorders, including cerebral palsy, Parkinson’s disease, dystonia, and epilepsy. Genetic studies have identified a homozygous truncation mutation in Trak1 that causes hypertonia in mice. Moreover, elevated Trak1 protein expression is associated with several types of cancers and variants in Trak1 are linked to childhood absence epilepsy in humans. Despite the importance of Trak1 in health and disease, the mechanisms of Trak1 action remain unclear and the pathogenic effects of Trak1 mutation are unknown. Here we report that Trak1 has a crucial function in regulation of mitochondrial fusion. Depletion of Trak1 inhibits mitochondrial fusion, resulting in mitochondrial fragmentation, whereas overexpression of Trak1 elongates and enlarges mitochondria. Our analyses revealed that Trak1 interacts and colocalizes with mitofusins on the outer mitochondrial membrane and functions with mitofusins to promote mitochondrial tethering and fusion. Furthermore, Trak1 is required for stress-induced mitochondrial hyperfusion and pro-survival response. We found that hypertonia-associated mutation impairs Trak1 mitochondrial localization and its ability to facilitate mitochondrial tethering and fusion. Our findings uncover a novel function of Trak1 as a regulator of mitochondrial fusion and provide evidence linking dysregulated mitochondrial dynamics to hypertonia pathogenesis.

Pathogenic mtDNA mutations associated with alterations of respiratory complex I, mitochondrial proliferation (oncocytic-like phenotype) and increase in antioxidant response were previously reported in type I endometrial carcinoma (EC). To evaluate whether in the presence of pathogenic mtDNA mutations other mitochondrial adaptive processes were triggered by cancer cells, the expression level of proteins involved in mitochondrial dynamics, mitophagy, proteolysis and apoptosis were evaluated in type I ECs harboring pathogenic mtDNA mutations and complex I deficiency. An increase in the fission protein Drp1, in the mitophagy protein BNIP3, in the mitochondrial protease CLPP, in the antioxidant and anti-apoptotic protein ALR and in Bcl-2 as well as a decrease in the fusion protein Mfn2 were found in cancer compared to matched non malignant tissue. Moreover, the level of these proteins was measured in type I EC, in hyperplastic (the premalignant form) and in non malignant tissues to verify whether the altered expression of these proteins is a common feature of endometrial cancer and eventually of hyperplastic tissue. This analysis confirmed in type I EC samples, but not in hyperplasia, an alteration of the expression level of these proteins. These results suggest that in this cancer mitochondrial fission, antioxidant and anti-apoptotic response may be activated, as well as the discharge of damaged mitochondrial proteins as mitochondrial adaptation processes to mitochondrial dysfunction.

By generating the majority of a cell's ATP, mitochondria permit a vast range of reactions necessary for life. Mitochondria also perform other vital functions including biogenesis and assembly of iron-sulfur proteins, maintenance of calcium homeostasis, and activation of apoptosis. Accordingly, mitochondrial dysfunction has been linked with the pathology of many clinical conditions including cancer, type 2 diabetes, cardiomyopathy, and atherosclerosis. The ongoing maintenance of mitochondrial structure and function requires the import of nuclear-encoded proteins and for this reason, mitochondrial protein import is indispensible for cell viability. As mitochondria play central roles in determining if cells live or die, a comprehensive understanding of mitochondrial structure, protein import, and function is necessary for identifying novel drugs that may destroy harmful cells while rescuing or protecting normal ones to preserve tissue integrity. This review summarizes our current knowledge on mitochondrial architecture, mitochondrial protein import, and mitochondrial function. Our current comprehension of how mitochondrial functions maintain cell homeostasis and how cell death occurs as a result of mitochondrial stress are also discussed.

Mitochondrial integrity is indispensable for cardiac health. With the advent of modern imaging technologies, mitochondrial motility and dynamics within the cell are extensively studied. Terminally differentiated and well-structured cardiomyocytes depict little mitochondrial division and fusion, questioning the contribution of mitochondrial fusion proteins (Mitofusin 1/2 and Optic Atrophy 1 protein) and fission factors (Dynamin-like protein 1 and mitochondrial fission 1 protein) in cardiomyocyte homeostasis. Emerging evidences suggest that alterations in mitochondrial morphology from globular, elongated network to punctate fragmented disconnected structures are a pathological response to ensuing cardiac stress and cardiomyocyte cell death, bringing forth the following question, "what maintains this balance between fusion and fission?" The answer hinges upon the classical "junk" DNA: microRNAs, the endogenous non-coding RNAs. Because of their essential role in numerous signaling pathways, microRNAs are considered to play major roles in the pathogenesis of various diseases. Mitochondria are not exempted from microRNA-mediated regulation. This review defines the importance of mitochondrial structural integrity and the microRNA-mitochondrial dynamics tandem, an imminent dimension of the cardiac homeostasis network. Elucidating their coordinated interaction could spur RNA-based therapeutics for resuscitating functional mitochondrial population during cardiovascular disorders.

Mitochondrial dysfunction is a hallmark of many diseases. The retrograde signaling initiated by dysfunctional mitochondria can bring about global changes in gene expression that alters cell morphology and function. Typically, this is attributed to disruption of important mitochondrial functions, such as ATP production, integration of metabolism, calcium homeostasis and regulation of apoptosis. Recent studies showed that in addition to these factors, mitochondrial dynamics might play an important role in stress signaling. Normal mitochondria are highly dynamic organelles whose size, shape and network are controlled by cell physiology. Defective mitochondrial dynamics play important roles in human diseases. Mitochondrial DNA defects and defective mitochondrial function have been reported in many cancers. Recent studies show that increased mitochondrial fission is a pro-tumorigenic phenotype. In this paper, we have explored the current understanding of the role of mitochondrial dynamics in pathologies. We present new data on mitochondrial dynamics and dysfunction to illustrate a causal link between mitochondrial DNA defects, excessive fission, mitochondrial retrograde signaling and cancer progression. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

Mitochondrial dysfunction has been associated with obesity and metabolic disorders. However, whether mitochondrial perturbation in a single tissue influences mitochondrial function and metabolic status of another distal tissue remains largely unknown. We analyzed the nonautonomous role of muscular mitochondrial dysfunction in Drosophila. Surprisingly, impaired muscle mitochondrial function via complex I perturbation results in simultaneous mitochondrial dysfunction in the fat body (the fly adipose tissue) and subsequent triglyceride accumulation, the major characteristic of obesity. RNA-sequencing (RNA-seq) analysis, in the context of muscle mitochondrial dysfunction, revealed that target genes of the TGF-β signaling pathway were induced in the fat body. Strikingly, expression of the TGF-β family ligand, Activin-β (Actβ), was dramatically increased in the muscles by NF-κB/Relish (Rel) signaling in response to mitochondrial perturbation, and decreasing Actβ expression in mitochondrial-perturbed muscles rescued both the fat body mitochondrial dysfunction and obesity phenotypes. Thus, perturbation of muscle mitochondrial activity regulates mitochondrial function in the fat body nonautonomously via modulation of Activin signaling.

Objective Insulin release from pancreatic β-cells is controlled by plasma glucose levels via mitochondrial fuel metabolism. Therefore, insulin secretion is critically dependent on mitochondrial DNA (mtDNA) and the genes it encodes. Mitochondrial transcription factor B2 (TFB2M) controls transcription of mitochondrial-encoded genes. However, its precise role in mitochondrial metabolism in pancreatic β-cells and, consequently, in insulin secretion remains unknown. Methods To elucidate the role of TFB2M in mitochondrial function and insulin secretion in vitro and in vivo, mice with a β-cell specific homozygous or heterozygous knockout of Tfb2m and rat clonal insulin-producing cells in which the gene was silenced were examined with an array of metabolic and functional assays. Results There was an effect of gene dosage on Tfb2m expression and function. Loss of Tfb2m led to diabetes due to disrupted transcription of mitochondrial DNA (mtDNA) and reduced mtDNA content. The ensuing mitochondrial dysfunction activated compensatory mechanisms aiming to limit cellular dysfunction and damage of β-cells. These processes included the mitochondrial unfolded protein response, mitophagy, and autophagy. Ultimately, however, these cell-protective systems were overridden, leading to mitochondrial dysfunction and activation of mitochondrial-dependent apoptotic pathways. In this way, β-cell function and mass were reduced. Together, these perturbations resulted in impaired insulin secretion, progressive hyperglycemia, and, ultimately, development of diabetes. Conclusions Loss of Tfb2m in pancreatic β-cells results in progressive mitochondrial dysfunction. Consequently, insulin secretion in response to metabolic stimuli is impaired and β-cell mass reduced. Our findings indicate that TFB2M plays an important functional role in pancreatic β-cells. Perturbations of its actions may lead to loss of functional β-cell mass, a hallmark of T2D., Highlights • Loss of TFB2M leads to mitochondrial dysfunction and impaired insulin secretion. • There was an effect of gene dosage on Tfb2m expression and function. • TFB2M plays a key role in cellular and mitochondrial function in pancreatic β-cells.

Regulated inter-mitochondrial fusion/fission is essential for maintaining optimal mitochondrial respiration and control of apoptosis and autophagy. In mammals, mitochondrial fusion is controlled by outer membrane GTPases MFN1 and MFN2 and by inner membrane (IM) GTPase OPA1. Disordered mitochondrial fusion/fission contributes to various pathologies, and MFN2 or OPA1 mutations underlie neurodegenerative diseases. Here, we show that the WBSCR16 protein is primarily associated with the outer face of the inner mitochondrial membrane and is important for mitochondrial fusion. We provide evidence of a WBSCR16/OPA1 physical interaction in the intact cell and of a WBSCR16 function as an OPA1-specific guanine nucleotide exchange factor (GEF). Homozygosity for a Wbscr16 mutation causes early embryonic lethality, whereas neurons of mice heterozygous for the mutation have mitochondria with reduced membrane potential and increased susceptibility to fragmentation upon exposure to stress, suggesting roles for WBSCR16 deficits in neuronal pathologies.

Parkin/PINK1-mediated mitophagy is implicated in the pathogenesis of Parkinson's disease (PD). Prior to elimination of damaged mitochondria, Parkin translocates to mitochondria and induces mitochondrial clustering. While the mechanism of PINK1-dependent Parkin redistribution to mitochondria is now becoming clear, the role of mitochondrial clustering has been less well understood. In our study, we found that loss of p62 disrupted mitochondrial aggregation and specifically sensitized Parkin-expressing cells to apoptosis induced by mitochondrial depolarization. Notably, altering mitochondrial aggregation through regulating p62 or other methods was sufficient to affect such apoptosis. Moreover, disruption of mitochondrial aggregation promoted proteasome-dependent degradation of outer mitochondrial membrane (OMM) proteins. The accelerated degradation in turn facilitated cytochrome c release from mitochondria, leading to apoptosis. Together, our study demonstrates a protective role of mitochondrial clustering in mitophagy and helps in understanding how aggregation defends cells against stress.

The mitochondrial network is a major site of ATP production through the coupled integration of the electron transport chain (ETC) with oxidative phosphorylation. In melanoma arising from the V600E mutation in the kinase v-RAF murine sarcoma viral oncogene homolog B (BRAFV600E), oncogenic signaling enhances glucose-dependent metabolism while reducing mitochondrial ATP production. Likewise, when BRAFV600E is pharmacologically inhibited by targeted therapies (e.g. PLX-4032/vemurafenib), glucose metabolism is reduced, and cells increase mitochondrial ATP production to sustain survival. Therefore, collateral inhibition of oncogenic signaling and mitochondrial respiration may help enhance the therapeutic benefit of targeted therapies. Honokiol (HKL) is a well tolerated small molecule that disrupts mitochondrial function; however, its underlying mechanisms and potential utility with targeted anticancer therapies remain unknown. Using wild-type BRAF and BRAFV600E melanoma model systems, we demonstrate here that HKL administration rapidly reduces mitochondrial respiration by broadly inhibiting ETC complexes I, II, and V, resulting in decreased ATP levels. The subsequent energetic crisis induced two cellular responses involving cyclin-dependent kinases (CDKs). First, loss of CDK1-mediated phosphorylation of the mitochondrial division GTPase dynamin-related protein 1 promoted mitochondrial fusion, thus coupling mitochondrial energetic status and morphology. Second, HKL decreased CDK2 activity, leading to G1 cell cycle arrest. Importantly, although pharmacological inhibition of oncogenic MAPK signaling increased ETC activity, co-treatment with HKL ablated this response and vastly enhanced the rate of apoptosis. Collectively, these findings integrate HKL action with mitochondrial respiration and shape and substantiate a pro-survival role of mitochondrial function in melanoma cells after oncogenic MAPK inhibition.

The hormone erythropoietin (EPO) has been demonstrated to protect against chemotherapy drug doxorubicin (DOX)-induced cardiotoxicity, but the underlying mechanism remains obscure. We hypothesized that silent mating type information regulation 2 homolog 1 (SIRT1), an NAD+-dependent protein deacetylase that activates peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), plays a crucial role in regulating mitochondrial function and mediating the beneficial effect of EPO. Our study in human cardiomyocyte AC16 cells showed that DOX-induced cytotoxicity and mitochondrial dysfunction, as manifested by decreased mitochondrial DNA (mtDNA) copy number, mitochondrial membrane potential, and increased mitochondrial superoxide accumulation, can be mitigated by EPO pretreatment. EPO was found to upregulate SIRT1 activity and protein expression to reverse DOX-induced acetylation of PGC-1α and suppression of a suite of PGC-1α-activated genes involved in mitochondrial function and biogenesis, such as nuclear respiratory factor-1 (NRF1), mitochondrial transcription factor A (TFAM), citrate synthase (CS), superoxide dismutase 2 (SOD2), cytochrome c oxidase IV (COXIV), and voltage-dependent anion channel (VDAC). Silencing of SIRT1 via small RNA interference sensitized AC16 cells to DOX-induced cytotoxicity and reduction in mtDNA copy number. Although with SIRT1 silenced, EPO could reverse to some extent DOX-induced mitochondrial superoxide accumulation, loss of mitochondrial membrane potential and ATP depletion, it failed to normalize protein expression of PGC-1α and its downstream genes. Taken together, our results indicated that EPO may activate SIRT1 to enhance mitochondrial function and protect against DOX-induced cardiotoxicity.

Sirtuins connect energy generation and metabolic stress to the cellular acetylome. Currently, only the mitochondrial sirtuins (SIRT3-5) and SIRT1 have been shown to direct mitochondrial function; however, Aims: NAD-dependent protein deacetylase sirtuin-2 (SIRT2), the primary cytoplasmic sirtuin, is not yet reported to associate with mitochondria.This study revealed a novel physiological function of SIRT2: the regulation of mitochondrial function. First, the acetylation of several metabolic mitochondrial proteins was found to be altered in Sirt2-deficient mice, which was, subsequently, validated by immunoprecipitation experiments in which the acetylated mitochondrial proteins directly interacted with SIRT2. Moreover, immuno-gold electron microscopic images of mouse brains showed that SIRT2 associates with the inner mitochondrial membrane in central nervous system cells. The loss of Sirt2 increased oxidative stress, decreased adenosine triphosphate levels, and altered mitochondrial morphology at the cellular and tissue (i.e., brain) level. Furthermore, the autophagic/mitophagic processes were dysregulated in Sirt2-deficient neurons and mouse embryonic fibroblasts.For the first time it is shown that SIRT2 directs mitochondrial metabolism.Together, these findings support that SIRT2 functions as a mitochondrial sirtuin, as well as a regulator of autophagy/mitophagy to maintain mitochondrial biology, thus facilitating cell survival. Antioxid. Redox Signal. 26, 849-863.

Mitochondrial ribosomes evolved from prokaryotic ribosomes, with which they therefore share more common features than with their counterparts in the cytosol. Yet, mitochondrial ribosomes are highly diverse in structure and composition, having undergone considerable changes, including reduction of their RNA component and varying degree of acquisition of novel proteins in various phylogenetic lineages. Here, we present functional analysis of three putative mitochondrial ribosome-associated proteins (RSM22, mtYsxC and PNKD-like) in Trypanosoma brucei, originally identified by database mining. While in other systems the homologs of RSM22 are known as components of mitochondrial ribosomes, YsxC was linked with ribosomes only in bacteria. The PNKD-like protein shows similarity to a human protein, the defects of which cause PNKD (paroxysmal non-kinesigenic dyskinesia). Here we show that all three proteins are important for the growth of T. brucei. They play an important function in mitochondrial translation, as their ablation by RNAi rapidly and severely affected the de novo synthesis of mitochondrial proteins. Moreover, following the RNAi-mediated depletion of RSM22, structure of the small subunit of mitochondrial ribosome becomes severely compromised, suggesting a role of RSM22 in ribosomal assembly and/or stability.

Mitochondrial dynamics play an important role within several pathological conditions, including cancer and neurological diseases. For the purpose of identifying therapies that target aberrant regulation of the mitochondrial dynamics machinery and characterizing the regulating signaling pathways, there is a need for label-free means to detect the dynamic alterations in mitochondrial morphology. We present the use of dielectrophoresis for label-free quantification of intracellular mitochondrial modifications that alter cytoplasmic conductivity, and these changes are benchmarked against label-based image analysis of the mitochondrial network. This is validated by quantifying the mitochondrial alterations that are carried out by entirely independent means on two different cell lines: human embryonic kidney cells and mouse embryonic fibroblasts. In both cell lines, the inhibition of mitochondrial fission that leads to a mitochondrial structure of higher connectivity is shown to substantially enhance conductivity of the cell interior, as apparent from the significantly higher positive dielectrophoresis levels in the 0.5-15 MHz range. Using single-cell velocity tracking, we show ∼10-fold higher positive dielectrophoresis levels at 0.5 MHz for cells with a highly connected versus those with a highly fragmented mitochondrial structure, suggesting the feasibility for frequency-selective dielectrophoretic isolation of cells to aid the discovery process for development of therapeutics targeting the mitochondrial machinery.

CCAR2 (cell cycle and apoptosis regulator protein 2; formerly DBC1, deleted in breast cancer 1) functions in diverse cellular processes including responses to genotoxic and metabolic stresses. However, its role in the mitochondrial stress response has not been fully elucidated. To investigate how CCAR2 regulates stress response, we purified CCAR2-containing complexes. Interestingly, the results revealed that CCAR2 localized to the mitochondria, and also bound Hsp60 (heat shock protein 60), a mitochondrial chaperone. The binding of CCAR2 to Hsp60 increased following rotenone-induced mitochondrial stress. The deficiencies in CCAR2 and Hsp60 also disrupted the mitochondrial membrane potential, thereby promoting apoptosis following mitochondrial stress. In summary, the CCAR2-Hsp60 complex promoted cell survival during mitochondrial stress-induced apoptosis. These data suggest that CCAR2 is critical for maintaining mitochondrial homeostasis in response to stress.

Mitochondria have many functions, including ATP generation. The electron transport chain (ETC) and the coupled ATP synthase generate ATP by consuming oxygen. Reactive oxygen species (ROS) are also produced by ETC, and ROS damage deoxyribonucleic acids, membrane lipids and proteins. Recent analysis indicate that mitochondrial unfolded protein response (UPRmt), which enhances expression of mitochondrial chaperones and proteases to remove damaged proteins, is activated when damaged proteins accumulate in the mitochondria. In Caenorhabditis elegans, HAF-1, a putative ortholog of human ABCB10, plays an essential role in signal transduction from mitochondria to nuclei to enhance UPRmt. Therefore, it is possible that ABCB10 has a role similar to that of HAF-1. However, it has not been reported whether ABCB10 is a factor in the signal transduction pathway to enhance UPRmt. In this study, ABCB10 was depleted in HepG2 cells using small interfering RNA (siRNA), and the effect was examined. ABCB10 depletion upregulated ROS and the expression of ROS-detoxifying enzymes (SOD2, GSTA1, and GSTA2), and SESN3, a protein induced by ROS to protect the cell from oxidative stress. In addition, ABCB10 depletion significantly decreased expression of UPRmt-related mitochondrial chaperones (HSPD1 and DNAJA3), and a mitochondrial protease (LONP1). However, the putative activity of ABCB10 to export peptides from mitochondria was not lost by ABCB10 depletion. Altogether, these data suggest that ABCB10 is involved in UPRmt signaling pathway similar to that of HAF-1, although ABCB10 probably does not participate in peptide export from mitochondria.

Mitochondrial cytopathies are a heterogeneous group of systemic disorders caused by mutations in mitochondrial or nuclear genome. The review presents some data on pathogenic mutations in mitochondrial DNA leading to the imbalance in the oxidation phosphorylation processes and energy metabolism in the cells and eventually to the development of mitochondrial cytopathy. The pathways of medicated correction are examined, which are aimed at obtaining optimal energy efficiency of mitochondria with impaired functions, increase of the efficiency of energy metabolism in the tissues, as well as prevention of mitochondrial membrane damage by free radicals using antioxidants and membrane protectors. A conclusion is drawn on the inefficiency of currently used therapeutic strategies and the necessity of new approaches, which can be gene therapy of mitochondrial diseases. Some modern methods for gene defects correction, capable of restoring or removing the damaged gene, expressing full gene product, or blocking the mutant or strange genes work are analyzed. It is shown that the described approaches to the gene therapy of human mitochondrial diseases demand the introduction of foreign sequences into nuclear or mitochondrial genome of a living person, which completely excludes their practical application because of the uncertainty of the outcome. A perspective approach in solving this problem may be a creation of a system allowing the correction of defect genes without introducing synthetic nucleotides into the human genome. Phenotypic selection combined with a capacity of homologous recombination, artificially imparted to mitochondria of yeast Yarrowia lipolytica, allows for replication of intact human mitochondrial DNA in yeast mitochondria, supporting a full-size native human mitochondrial DNA in the yeast cells and eliminating pathogenic mutations by means of standard sitedirected PCR mutagenesis. After the correction in the Y. lipolytica cells, copies of mitochondrial DNA of an individual patient may be returned to him using the transfection of mesenchymal stromal cells followed by selection of transfectants grown in minimal culture media, in which the cells with higher respiratory mitochondrial activity will gain the advantage.

Heat-shock proteins (HSPs) play a crucial role in maintaining protein stability for cell survival during stress-induced insults. Overexpression of HSPs in cancer cells results in antiapoptotic activity contributing to cancer cell survival and restricting the efficacy of cytotoxic chemotherapy, which continues to play an important role in the treatment of many cancers, including triple-negative breast cancer (TNBC). First-line therapy for TNBC includes anthracycline antibiotics, which are associated with serious dose-dependent side effects and the development of resistance. We previously identified YDJ1, which encodes a heat-shock protein 40 (HSP40), as an important factor in the cellular response to anthracyclines in yeast, with mutants displaying over 100-fold increased sensitivity to doxorubicin. In humans, the DNAJA HSP40s are homologues of YDJ1. To determine the role of DNAJAs in the cellular response to cytotoxic drugs, we investigated their ability to rescue ydj1Δ mutants from exposure to chemotherapeutic agents. Our results indicate that DNAJA1 and DNAJA2 provide effective protection, while DNAJA3 and DNAJA4 did not. The level of complementation was also dependent on the agent used, with DNAJA1 and DNAJA2 rescuing the ydj1Δ strain from doxorubicin, cisplatin, and heat shock. DNAJA3 and DNAJA4 did not rescue the ydj1Δ strain and interfered with the cellular response to stress when expressed in wild type background. DNAJA1 and DNAJA2 protect the cell from proteotoxic damage caused by reactive oxygen species (ROS) and are not required for repair of DNA double-strand breaks. These data indicate that the DNAJAs play a role in the protection of cells from ROS-induced cytotoxic stress.

Experimental evidence supports an intricate association between inflammation and mitochondrial dysfunction as main contributors of neurological diseases. Inflammatory mediators produced by activated microglia and infiltrated immune cells trigger intracellular signalling cascades that can alter cellular mitochondrial metabolism. Cytokines, particularly tumor necrosis factor-alpha, impede mitochondrial oxidative phosphorylation and associated ATP production and instigate mitochondrial reactive oxygen species production. This culminates in mitochondrial membrane permeabilization, altered mitochondrial dynamics and might ultimately result in cell death. When severely injured mitochondria are not appropriately removed by mitophagy they can release their contents into the cytosol and extracellular environment and thereby amplify the inflammatory process. Here we provide a comprehensive overview on how inflammatory mediators impair mitochondrial metabolism and discuss how defective mitochondria can elicit and potentiate an inflammatory response.

Myocardial apoptosis play a vital role in pathogenesis of cardiovascular diseases. The intrinsic pathway of apoptosis (mitochondrial apoptosis pathway) and abnormal mitochondrial fission and fusion have a detrimental effect on cells under a variety of intracellular stresses including hypoxia, oxidative stress, drug toxicity or DNA damage and contributes to the development of heart failure (HF), myocardial infarction (MI), diabetic cardiomyopathy and ischaemia/reperfusion injury (I/R). MicroRNAs (miRNAs) are endogenous short non-coding RNAs, which target 3'-untranslated region of mRNA to switch off gene expression. They play crucial roles in regulating complicated cardiac signalling and transcriptional events during cardiac development as well as in diseased condition. In this review, we summarize the molecular mechanism of mitochondrial apoptosis in cardiac cells and influence of miRNAs on them. MiRNAs regulate cardiac mitochondrial apoptosis by exert their effects on mitochondrial fission and fusion, reactive oxygen species (ROS) generation and Ca2+ homeostasis, Bcl-2 family members, and other mitochondrial function proteins. This advancement in understanding mechanism of cardiac cells death provides us new therapy targets for cardiovascular diseases associated with mitochondrial dysfunctions.

Mitochondria play a key role in energy production, calcium homeostasis, cell survival, and death. Adverse stimulations including neurodegenerative diseases may result in mitochondrial dynamic imbalance, free radical production, calcium accumulation, intrinsic cell death pathway activation and eventually cell death. Therefore, preserving or promoting mitochondrial function is a potential therapeutic target for the treatment of neurodegenerative disorders. Mitochondrial biogenesis is a process by which new mitochondria are produced from existing mitochondria. This biogenesis process is regulated by Peroxisome proliferator-activated receptor-gamma (PPARγ) coactivator-1alpha (PGC-1α). Once being activated by either phosphorylation or de-acetylation, PGC-1α activates nuclear respiratory factor 1 and 2 (NRF1 and NRF2), and subsequently mitochondrial transcription factor A (Tfam). The activation of this PGC-1α - NRF -Tfam pathway leads to synthesis of mitochondrial DNA and proteins and generation of new mitochondria. © 2017 Wiley Periodicals, Inc.

Loss of proteostasis underlies ageing and neurodegeneration characterized by the accumulation of protein aggregates and mitochondrial dysfunction. Although many neurodegenerative-disease-associated proteins can be found in mitochondria, it remains unclear how mitochondrial dysfunction and protein aggregation could be related. In dividing yeast cells, protein aggregates that form under stress or during ageing are preferentially retained by the mother cell, in part through tethering to mitochondria, while the disaggregase Hsp104 helps to dissociate aggregates and thereby enables refolding or degradation of misfolded proteins. Here we show that, in yeast, cytosolic proteins prone to aggregation are imported into mitochondria for degradation. Protein aggregates that form under heat shock contain both cytosolic and mitochondrial proteins and interact with the mitochondrial import complex. Many aggregation-prone proteins enter the mitochondrial intermembrane space and matrix after heat shock, and some do so even without stress. Timely dissolution of cytosolic aggregates requires the mitochondrial import machinery and proteases. Blocking mitochondrial import but not proteasome activity causes a marked delay in the degradation of aggregated proteins. Defects in cytosolic Hsp70s leads to enhanced entry of misfolded proteins into mitochondria and elevated mitochondrial stress. We term this mitochondria-mediated proteostasis mechanism MAGIC (mitochondria as guardian in cytosol) and provide evidence that it may exist in human cells.

Mitochondrial function is central to many different processes in the cell, from oxidative phosphorylation to the synthesis of iron–sulfur clusters. Therefore, mitochondrial dysfunction underlies a diverse array of diseases, from neurodegenerative diseases to cancer. Stress can be communicated to the cytosol and nucleus from the mitochondria through many different signals, and in response the cell can effect everything from transcriptional to post-transcriptional responses to protect the mitochondrial network. How these responses are coordinated have only recently begun to be understood. In this review, we explore how the cell maintains mitochondrial function, focusing on the mitochondrial unfolded protein response (UPRmt), a transcriptional response that can activate a wide array of programs to repair and restore mitochondrial function.

Mitochondrial health is critically dependent on the ability of mitochondria to undergo changes in mitochondrial morphology, a process which is regulated by mitochondrial shaping proteins. Mitochondria undergo fission to generate fragmented discrete organelles, a process which is mediated by the mitochondrial fission proteins (Drp1, hFIS1, Mff and MiD49/51), and is required for cell division, and to remove damaged mitochondria by mitophagy. Mitochondria undergo fusion to form elongated interconnected networks, a process which is orchestrated by the mitochondrial fusion proteins (Mfn1, Mfn2 and OPA1), and which enables the replenishment of damaged mitochondrial DNA. In the adult heart, mitochondria are relatively static, are constrained in their movement, and are characteristically arranged into 3 distinct subpopulations based on their locality and function (subsarcolemmal, myofibrillar, and perinuclear). Although the mitochondria are arranged differently, emerging data supports a role for the mitochondrial shaping proteins in cardiac health and disease. Interestingly, in the adult heart, it appears that the pleiotropic effects of the mitochondrial fusion proteins, Mfn2 (endoplasmic reticulum-tethering, mitophagy) and OPA1 (cristae remodeling, regulation of apoptosis, and energy production) may play more important roles than their pro-fusion effects. In this review article, we provide an overview of the mitochondrial fusion and fission proteins in the adult heart, and highlight their roles as novel therapeutic targets for treating cardiac disease.