Parkin-independent mitophagy requires Drp1 and maintains the integrity of mammalian heart and brain

Regulation of neuronal bioenergetics as a therapeutic strategy in neurodegenerative diseases

Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis are a heterogeneous group of debilitating disorders with multifactorial etiologies and pathogeneses that manifest distinct molecular mechanisms and clinical manifestations with abnormal protein dynamics and impaired bioenergetics. Mitochondrial dysfunction is emerging as an important feature in the etiopathogenesis of these age-related neurodegenerative diseases. The prevalence and incidence of these diseases is on the rise with the increasing global population and average lifespan. Although many therapeutic approaches have been tested, there are currently no effective treatment routes for the prevention or cure of these diseases. We present the current status of our knowledge and understanding of the involvement of mitochondrial dysfunction in these diseases and highlight recent advances in novel therapeutic strategies targeting neuronal bioenergetics as potential approach for treating these diseases.

Mitophagy Disequilibrium, a Prominent Pathological Mechanism in Metabolic Heart Diseases

With overall food intake among the general population as high as ever, metabolic syndrome (MetS) has become a global epidemic and is responsible for many serious life-threatening diseases, especially heart failure. In multiple metabolic disorders, maintaining a dynamic balance of mitochondrial number and function is necessary to prevent the overproduction of reactive oxygen species (ROS), which has been proved to be one of the important mechanisms of cardiomyocyte injury due to the mismatching of oxygen consumption and mitochondrial population and finally to heart failure. Mitophagy is a process that eliminates damaged or redundant mitochondria. It is mediated by a series of signaling molecules, including PINK, parkin, BINP3, FUNDC1, CTSD, Drp1, Rab9 and mTOR. Meanwhile, increasing evidence also showed that the interaction between ferroptosis and mitophagy interfered with mitochondrial homeostasis. This review will focus on these essential molecules and pathways of mitophagy and cell homeostasis affected by hypoxia and other stimuli in metabolic heart diseases.

Selective autophagy of intracellular organelles: recent research advances

Macroautophagy (hereafter called autophagy) is a highly conserved physiological process that degrades over-abundant or damaged organelles, large protein aggregates and invading pathogens via the lysosomal system (the vacuole in plants and yeast). Autophagy is generally induced by stress, such as oxygen-, energy- or amino acid-deprivation, irradiation, drugs, etc. In addition to non-selective bulk degradation, autophagy also occurs in a selective manner, recycling specific organelles, such as mitochondria, peroxisomes, ribosomes, endoplasmic reticulum (ER), lysosomes, nuclei, proteasomes and lipid droplets (LDs). This capability makes selective autophagy a major process in maintaining cellular homeostasis. The dysfunction of selective autophagy is implicated in neurodegenerative diseases (NDDs), tumorigenesis, metabolic disorders, heart failure, etc. Considering the importance of selective autophagy in cell biology, we systemically review the recent advances in our understanding of this process and its regulatory mechanisms. We emphasize the 'cargo-ligand-receptor' model in selective autophagy for specific organelles or cellular components in yeast and mammals, with a focus on mitophagy and ER-phagy, which are finely described as types of selective autophagy. Additionally, we highlight unanswered questions in the field, helping readers focus on the research blind spots that need to be broken.

The Interplay between Mitochondrial Morphology and Myomitokines in Aging Sarcopenia

Sarcopenia is a chronic disease characterized by the progressive loss of skeletal muscle mass, force, and function during aging. It is an emerging public problem associated with poor quality of life, disability, frailty, and high mortality. A decline in mitochondria quality control pathways constitutes a major mechanism driving aging sarcopenia, causing abnormal organelle accumulation over a lifetime. The resulting mitochondrial dysfunction in sarcopenic muscles feedbacks systemically by releasing the myomitokines fibroblast growth factor 21 (FGF21) and growth and differentiation factor 15 (GDF15), influencing the whole-body homeostasis and dictating healthy or unhealthy aging. This review describes the principal pathways controlling mitochondrial quality, many of which are potential therapeutic targets against muscle aging, and the connection between mitochondrial dysfunction and the myomitokines FGF21 and GDF15 in the pathogenesis of aging sarcopenia.

Molecular mechanisms and clinical implications of multiple forms of mitophagy in the heart

Mitochondria, the primary ATP-producing organelles, are highly abundant in cardiomyocytes. Mitochondrial function readily deteriorates in the presence of stress and, thus, maintenance of mitochondrial quality is essential for sustaining pump function in the heart. Cardiomyocytes under stress attempt to maintain mitochondrial quality primarily through dynamic changes in their morphology, namely fission and fusion, degradation, and biogenesis. Mitophagy, a mitochondria-specific form of autophagy, is a major mechanism of degradation. The level of mitophagy is altered in stress conditions, which, in turn, significantly affects mitochondrial function, cardiomyocyte survival, and death and cardiac function. Thus, mitophagy has been emerging as a promising target for treatment of cardiac conditions. To develop specific interventions, modulating the activity of mitophagy in the heart, understanding how mitochondria are degraded in a given condition is important. Increasing lines of evidence suggest that there are multiple mechanisms by which mitochondria are degraded through mitophagy in the heart. For example, in addition to the well-established mechanism commonly utilized by general autophagy, involving Atg7 and LC3, recent evidence suggests that an alternative mechanism, independent of Atg7 and LC3, also mediates mitophagy in the heart. Here, we describe molecular mechanisms through which mitochondria are degraded in the heart and discuss their functional significance. We also discuss molecular interventions to modulate the activity of mitophagy and their potential applications for cardiac conditions.

Mitochondrial Fission and Mitophagy Coordinately Restrict High Glucose Toxicity in Cardiomyocytes

Hyperglycemia-induced mitochondrial dysfunction plays a key role in the pathogenesis of diabetic cardiomyopathy. Injured mitochondrial segments are separated by mitochondrial fission and eliminated by autophagic sequestration and subsequent degradation in the lysosome, a process termed mitophagy. However, it remains poorly understood how high glucose affects the activities of, and the relationship between, mitochondrial fission and mitophagy in cardiomyocytes. In this study, we determined the functional roles of mitochondrial fission and mitophagy in hyperglycemia-induced cardiomyocyte injury. High glucose (30 mM, HG) reduced mitochondrial connectivity and particle size and increased mitochondrial number in neonatal rat ventricular cardiomyocytes, suggesting an enhanced mitochondrial fragmentation. SiRNA knockdown of the pro-fission factor dynamin-related protein 1 (DRP1) restored mitochondrial size but did not affect HG toxicity, and Mdivi-1, a DRP1 inhibitor, even increased HG-induced cardiomyocyte injury, as shown by superoxide production, mitochondrial membrane potential and cell death. However, DRP1 overexpression triggered mitochondrial fragmentation and mitigated HG-induced cardiomyocyte injury, suggesting that the increased mitochondrial fission is beneficial, rather than detrimental, to cardiomyocytes cultured under HG conditions. This is in contrast to the prevailing hypothesis that mitochondrial fragmentation mediates or contributes to HG cardiotoxicity. Meanwhile, HG reduced mitophagy flux as determined by the difference in the levels of mitochondria-associated LC3-II or the numbers of mitophagy foci indicated by the novel dual fluorescent reporter mt-Rosella in the absence and presence of the lysosomal inhibitors. The ability of HG to induce mitochondrial fragmentation and inhibit mitophagy was reproduced in adult mouse cardiomyocytes. Overexpression of Parkin, a positive regulator of mitophagy, or treatment with CCCP, a mitochondrial uncoupler, induced mitophagy and attenuated HG-induced cardiomyocyte death, while Parkin knockdown had opposite effects, suggesting an essential role of mitophagy in cardiomyocyte survival under HG conditions. Strikingly, Parkin overexpression increased mitochondrial fragmentation, while DRP1 overexpression accelerated mitophagy flux, demonstrating a reciprocal activation loop that controls mitochondrial fission and mitophagy. Thus, strategies that promote the mutual positive interaction between mitochondrial fission and mitophagy while simultaneously maintain their levels within the physiological range would be expected to improve mitochondrial health, alleviating hyperglycemic cardiotoxicity.

Rehmapicroside ameliorates cerebral ischemia-reperfusion injury via attenuating peroxynitrite-mediated mitophagy activation

Peroxynitrite (ONOO^-)-mediated mitophagy activation represents a vital pathogenic mechanism in ischemic stroke. Our previous study suggests that ONOO^- mediates Drp1 recruitment to the damaged mitochondria for excessive mitophagy, aggravating cerebral ischemia/reperfusion injury and the ONOO^--mediated mitophagy activation could be a crucial therapeutic target for improving outcome of ischemic stroke. In the present study, we tested the neuroprotective effects of rehmapicroside, a natural compound from a medicinal plant, on inhibiting ONOO^--mediated mitophagy activation, attenuating infarct size and improving neurological functions by using the in vitro cultured PC12 cells exposed to oxygen glucose deprivation with reoxygenation (OGD/RO) condition and the in vivo rat model of middle cerebral artery occlusion (MCAO) for 2 h of transient cerebral ischemia plus 22 h of reperfusion. The major discoveries include following aspects: (1) Rehmapicroside reacted with ONOO^- directly to scavenge ONOO^-; (2) Rehmapicroside decreased O₂^- and ONOO^-, up-regulated Bcl-2 but down-regulated Bax, Caspase-3 and cleaved Caspase-3, and down-regulated PINK1, Parkin, p62 and the ratio of LC3-II to LC3-I in the OGD/RO-treated PC12 cells; (3) Rehmapicroside suppressed 3-nitrotyrosine formation, Drp1 nitration as well as NADPH oxidases and iNOS expression in the ischemia-reperfused rat brains; (4) Rehmapicroside prevented the translocations of PINK1, Parkin and Drp1 into the mitochondria for mitophagy activation in the ischemia-reperfused rat brains; (5) Rehmapicroside ameliorated infarct sizes and improved neurological deficit scores in the rats with transient MCAO cerebral ischemia. Taken together, rehmapicroside could be a potential drug candidate against cerebral ischemia-reperfusion injury, and its neuroprotective mechanisms could be attributed to inhibiting the ONOO^--mediated mitophagy activation.

Mitochondrial Safeguard: a stress response that offsets extreme fusion and protects respiratory function via flickering-induced Oma1 activation

The connectivity of mitochondria is regulated by a balance between fusion and division. Many human diseases are associated with excessive mitochondrial connectivity due to impaired Drp1, a dynamin-related GTPase that mediates division. Here, we report a mitochondrial stress response, named mitochondrial safeguard, that adjusts the balance of fusion and division in response to increased mitochondrial connectivity. In cells lacking Drp1, mitochondria undergo hyperfusion. However, hyperfusion does not completely connect mitochondria because Opa1 and mitofusin 1, two other dynamin-related GTPases that mediate fusion, become proteolytically inactivated. Pharmacological and genetic experiments show that the activity of Oma1, a metalloprotease that cleaves Opa1, is regulated by short pulses of the membrane depolarization without affecting the overall membrane potential in Drp1-knockout cells. Re-activation of Opa1 and Mitofusin 1 in Drp1-knockout cells further connects mitochondria beyond hyperfusion, termed extreme fusion, leading to bioenergetic deficits. These findings reveal an unforeseen safeguard mechanism that prevents extreme fusion of mitochondria, thereby maintaining mitochondrial function when the balance is shifted to excessive connectivity.

The fluorescence toolbox for visualizing autophagy

Autophagy is an adaptive catabolic process functioning to promote cell survival in the event of inappropriate living conditions such as nutrient shortage and to cope with diverse cytotoxic insults. It is regarded as one of the key survival mechanisms of living organisms. Cells undergo autophagy to accomplish the lysosomal digestion of intracellular materials including damaged proteins, organelles, and foreign bodies, in a bulk, non-selective or a cargo-specific manner. Studies in the past decades have shed light on the association of autophagy pathways with various diseases and also highlighted the therapeutic value of autophagy modulation. Hence, it is crucial to develop effective approaches for monitoring intracellular autophagy dynamics, as a comprehensive account of methodology establishment is far from complete. In this review, we aim to provide an overview of the major current fluorescence-based techniques utilized for visualizing, sensing or measuring autophagic activities in cells or tissues, which are categorized firstly by targets detected and further by the types of fluorescence tools. We will mainly focus on the working mechanisms of these techniques, put emphasis on the insight into their roles in biomedical science and provide perspectives on the challenges and future opportunities in this field.

Drp1 Tubulates the ER in a GTPase-Independent Manner

Mitochondria are highly dynamic organelles that continuously grow, divide, and fuse. The division of mitochondria is crucial for human health. During mitochondrial division, the mechano-guanosine triphosphatase (GTPase) dynamin-related protein (Drp1) severs mitochondria at endoplasmic reticulum (ER)-mitochondria contact sites, where peripheral ER tubules interact with mitochondria. Here, we report that Drp1 directly shapes peripheral ER tubules in human and mouse cells. This ER-shaping activity is independent of GTP hydrolysis and located in a highly conserved peptide of 18 amino acids (termed D-octadecapeptide), which is predicted to form an amphipathic α helix. Synthetic D-octadecapeptide tubulates liposomes in vitro and the ER in cells. ER tubules formed by Drp1 promote mitochondrial division by facilitating ER-mitochondria interactions. Thus, Drp1 functions as a two-in-one protein during mitochondrial division, with ER tubulation and mechano-GTPase activities.

Mitochondrial division, fusion and degradation

The mitochondrion is an essential organelle for a wide range of cellular processes, including energy production, metabolism, signal transduction and cell death. To execute these functions, mitochondria regulate their size, number, morphology and distribution in cells via mitochondrial division and fusion. In addition, mitochondrial division and fusion control the autophagic degradation of dysfunctional mitochondria to maintain a healthy population. Defects in these dynamic membrane processes are linked to many human diseases that include metabolic syndrome, myopathy and neurodegenerative disorders. In the last several years, our fundamental understanding of mitochondrial fusion, division and degradation has been significantly advanced by high resolution structural analyses, protein-lipid biochemistry, super resolution microscopy and in vivo analyses using animal models. Here, we summarize and discuss this exciting recent progress in the mechanism and function of mitochondrial division and fusion.

The connection between the dynamic remodeling of the mitochondrial network and the regulation of muscle mass

The dynamic coordination of processes controlling the quality of the mitochondrial network is crucial to maintain the function of mitochondria in skeletal muscle. Changes of mitochondrial proteolytic system, dynamics (fusion/fission), and mitophagy induce pathways that affect muscle mass and performance. When muscle mass is lost, the risk of disease onset and premature death is dramatically increased. For instance, poor quality of muscles correlates with the onset progression of several age-related disorders such as diabetes, obesity, cancer, and aging sarcopenia. To date, there are no drug therapies to reverse muscle loss, and exercise remains the best approach to improve mitochondrial health and to slow atrophy in several diseases. This review will describe the principal mechanisms that control mitochondrial quality and the pathways that link mitochondrial dysfunction to muscle mass regulation.

Glioma cells are resistant to inflammation‑induced alterations of mitochondrial dynamics

Accumulating evidence suggests that inflammation is present in solid tumors. However, it is poorly understood whether inflammation exists in glioma and how it affects the metabolic signature of glioma. By analyzing immunohistochemical data and gene expression data downloaded from bioinformatic datasets, the present study revealed an accumulation of inflammatory cells in glioma, activation of microglia, upregulation of proinflammatory factors (including IL-6, IL-8, hypoxia-inducible factor-1α, STAT3, NF-κB1 and NF-κB2), destruction of mitochondrial structure and altered expression levels of electron transfer chain complexes and metabolic enzymes. By monitoring glioma cells following proinflammatory stimulation, the current study observed a remodeling of their mitochondrial network via mitochondrial fission. More than half of the mitochondria presented ring-shaped or spherical morphologies. Transmission electron microscopic analyses revealed mitochondrial swelling with partial or total cristolysis. Furthermore, proinflammatory stimuli resulted in increased generation of reactive oxygen species, decreased mitochondrial membrane potential and reprogrammed metabolism. The defective mitochondria were not eliminated via mitophagy. However, cell viability was not affected, and apoptosis was decreased in glioma cells after proinflammatory stimuli. Overall, the present findings suggested that inflammation may be present in glioma and that glioma cells may be resistant to inflammation-induced mitochondrial dysfunction.

Mitochondrial quality control mechanisms as molecular targets in cardiac ischemiareperfusion injury

Mitochondrial damage is a critical contributor to cardiac ischemia/reperfusion (I/R) injury. Mitochondrial quality control (MQC) mechanisms, a series of adaptive responses that preserve mitochondrial structure and function, ensure cardiomyocyte survival and cardiac function after I/R injury. MQC includes mitochondrial fission, mitochondrial fusion, mitophagy and mitochondria-dependent cell death. The interplay among these responses is linked to pathological changes such as redox imbalance, calcium overload, energy metabolism disorder, signal transduction arrest, the mitochondrial unfolded protein response and endoplasmic reticulum stress. Excessive mitochondrial fission is an early marker of mitochondrial damage and cardiomyocyte death. Reduced mitochondrial fusion has been observed in stressed cardiomyocytes and correlates with mitochondrial dysfunction and cardiac depression. Mitophagy allows autophagosomes to selectively degrade poorly structured mitochondria, thus maintaining mitochondrial network fitness. Nevertheless, abnormal mitophagy is maladaptive and has been linked to cell death. Although mitochondria serve as the fuel source of the heart by continuously producing adenosine triphosphate, they also stimulate cardiomyocyte death by inducing apoptosis or necroptosis in the reperfused myocardium. Therefore, defects in MQC may determine the fate of cardiomyocytes. In this review, we summarize the regulatory mechanisms and pathological effects of MQC in myocardial I/R injury, highlighting potential targets for the clinical management of reperfusion.

Defects in Mitochondrial Biogenesis Drive Mitochondrial Alterations in PARKIN-Deficient Human Dopamine Neurons

Mutations and loss of activity in PARKIN, an E3 ubiquitin ligase, play a role in the pathogenesis of Parkinson's disease (PD). PARKIN regulates many aspects of mitochondrial quality control including mitochondrial autophagy (mitophagy) and mitochondrial biogenesis. Defects in mitophagy have been hypothesized to play a predominant role in the loss of dopamine (DA) neurons in PD. Here, we show that although there are defects in mitophagy in human DA neurons lacking PARKIN, the mitochondrial deficits are primarily due to defects in mitochondrial biogenesis that are driven by the upregulation of PARIS and the subsequent downregulation of PGC-1α. CRISPR/Cas9 knockdown of PARIS completely restores the mitochondrial biogenesis defects and mitochondrial function without affecting the deficits in mitophagy. These results highlight the importance mitochondrial biogenesis versus mitophagy in the pathogenesis of PD due to inactivation or loss of PARKIN in human DA neurons.

The Ubiquitin E3 Ligase Parkin Inhibits Innate Antiviral Immunity Through K48-Linked Polyubiquitination of RIG-I and MDA5

Innate immunity is the first-line defense against antiviral or antimicrobial infection. RIG-I and MDA5, which mediate the recognition of pathogen-derived nucleic acids, are essential for production of type I interferons (IFN). Here, we identified mitochondrion depolarization inducer carbonyl cyanide 3-chlorophenylhydrazone (CCCP) inhibited the response and antiviral activity of type I IFN during viral infection. Furthermore, we found that the PTEN-induced putative kinase 1 (PINK1) and the E3 ubiquitin-protein ligase Parkin mediated mitophagy, thus negatively regulating the activation of RIG-I and MDA5. Parkin directly interacted with and catalyzed the K48-linked polyubiquitination and subsequent degradation of RIG-I and MDA5. Thus, we demonstrate that Parkin limits RLR-triggered innate immunity activation, suggesting Parkin as a potential therapeutic target for the control of viral infection.

Mitochondrial DNA Mutation, Diseases, and Nutrient-Regulated Mitophagy

A wide spectrum of human diseases, including cancer, neurodegenerative diseases, and metabolic disorders, have been shown to be associated with mitochondrial dysfunction through multiple molecular mechanisms. Mitochondria are particularly susceptible to nutrient deficiencies, and nutritional intervention is an essential way to maintain mitochondrial homeostasis. Recent advances in genetic manipulation and next-generation sequencing reveal the crucial roles of mitochondrial DNA (mtDNA) in various pathophysiological conditions. Mitophagy, a term coined to describe autophagy that targets dysfunctional mitochondria, has emerged as an important cellular process to maintain mitochondrial homeostasis and has been shown to be regulated by various nutrients and nutritional stresses. Given the high prevalence of mtDNA mutations in humans and their impact on mitochondrial function, it is important to investigate the mechanisms that regulate mtDNA mutation. Here, we discuss mitochondrial genetics and mtDNA mutations and their implications for human diseases. We also examine the role of mitophagy as a therapeutic target, highlighting how nutrients may eliminate mtDNA mutations through mitophagy.

Fenpropathrin induces degeneration of dopaminergic neurons via disruption of the mitochondrial quality control system

The synthetic pyrethroid derivative, fenpropathrin, is a widely used insecticide. However, a variety of toxic effects in mammals have been reported. In particular, fenpropathrin induces degeneration of dopaminergic neurons and parkinsonism. However, the mechanism of fenpropathrin-induced parkinsonism has remained unknown. In the present study, we investigated the toxic effects and underlying mechanisms of fenpropathrin on perturbing the dopaminergic system both in vivo and in vitro. We found that fenpropathrin induced cellular death of dopaminergic neurons in vivo. Furthermore, fenpropathrin increased the generation of reactive oxygen species, disrupted both mitochondrial function and dynamic networks, impaired synaptic communication, and promoted mitophagy in vitro. In mice, fenpropathrin was administered into the striatum via stereotaxic (ST) injections. ST-injected mice exhibited poor locomotor function at 24 weeks after the first ST injection and the number of tyrosine hydroxylase (TH)-positive cells and level of TH protein in the substantia nigra pars compacta were significantly decreased, as compared to these parameters in vehicle-treated mice. Taken together, our results demonstrate that exposure to fenpropathrin induces a loss of dopaminergic neurons and partially mimics the pathologic features of Parkinson's disease. These findings suggest that fenpropathrin may induce neuronal degeneration via dysregulation of mitochondrial function and the mitochondrial quality control system.

Investigating mitochondrial autophagy by routine transmission electron microscopy: Seeing is believing?

Mitochondrial autophagy is affected in many diseases. In the past few years, the multiple-steps process of selective degradation of mitochondria has been dissected in details by combining outcomes from different approaches. Perhaps one of the most rigorous methods to clearly visualise mitochondria undergoing autophagic engulfment and degradation, is transmission electron microscopy (TEM). In this opinion paper, we want to give a brief summary of the mitophagic process, and by which means mitophagy can be addressed, including TEM analysis. We will report examples of autophagy and mitophagy-related TEM images, and discuss how to decipher the different steps of the mitophagic process by routine TEM. In our opinion, this technique can be used as a powerful confirmatory approach for mitochondrial autophagy and can provide details of the organelle fate throughout the course of mitophagy with no substantial sample manipulation.

Mitochondrial dynamics during spermatogenesis

Mitochondrial fusion and fission (mitochondrial dynamics) are homeostatic processes that safeguard normal cellular function. This relationship is especially strong in tissues with constitutively high energy demands, such as brain, heart and skeletal muscle. Less is known about the role of mitochondrial dynamics in developmental systems that involve changes in metabolic function. One such system is spermatogenesis. The first mitochondrial dynamics gene, Fuzzy onions (Fzo), was discovered in 1997 to mediate mitochondrial fusion during Drosophila spermatogenesis. In mammals, however, the role of mitochondrial fusion during spermatogenesis remained unknown for nearly two decades after discovery of Fzo Mammalian spermatogenesis is one of the most complex and lengthy differentiation processes in biology, transforming spermatogonial stem cells into highly specialized sperm cells over a 5-week period. This elaborate differentiation process requires several developmentally regulated mitochondrial and metabolic transitions, making it an attractive model system for studying mitochondrial dynamics in vivo We review the emerging role of mitochondrial biology, and especially its dynamics, during the development of the male germ line.

Drp-1 as Potential Therapeutic Target for Lipopolysaccharide-Induced Vascular Hyperpermeability

Mitochondria-dependent apoptotic signaling has a critical role in the pathogenesis of vascular hyperpermeability (VH). Dynamin-related protein-1- (Drp-1-) mediated mitochondrial fission plays an important role in mitochondrial homeostasis. In the present study, we studied the involvement of Drp-1 in resistance to VH induced by lipopolysaccharide (LPS). To establish the model of LPS-induced VH, LPS at 15 mg/kg was injected into rats in vivo and rat pulmonary microvascular endothelial cells were exposed to 500 ng/ml LPS in vitro. We found that depletion of Drp-1 remarkedly exacerbated the mitochondria-dependent apoptosis induced by LPS, as evidenced by reduced apoptosis, mitochondrial membrane potential (MMP) depolarization, and activation of caspase-3 and caspase-9. Increased FITC-dextran flux indicated endothelial barrier disruption. In addition, overexpression of Drp-1 prevented LPS-induced endothelial hyperpermeability and upregulated mitophagy, as evidenced by the loss of mitochondrial mass and increased PINK1 expression and mitochondrial Parkin. However, the mitophagy inhibitor, 3-Methyladenine, blocked these protective effects of Drp-1. Furthermore, inhibition of Drp-1 using mitochondrial division inhibitor 1 markedly inhibited LPS-induced mitophagy and aggravated LPS-induced VH, as shown by increased FITC-dextran extravasation. These findings implied that Drp-1 strengthens resistance to mitochondria-dependent apoptosis by regulating mitophagy, suggesting Drp-1 as a possible therapeutic target in LPS-induced VH.

The long and winding road of cardiomyocyte maturation

Knowledge about the molecular mechanisms regulating cardiomyocyte (CM) proliferation and differentiation has increased exponentially in recent years. Such insights together with the availability of more efficient protocols for generation of CMs from induced pluripotent stem cells (iPSCs) have raised expectations for new therapeutic strategies to treat congenital and non-congenital heart diseases. However, the poor regenerative potential of the postnatal heart and the incomplete maturation of iPSC-derived CMs represent important bottlenecks for such therapies in future years. CMs undergo dramatic changes at the doorstep between prenatal and postnatal life, including terminal cell cycle withdrawal, change in metabolism, and further specialization of the cellular machinery required for high-performance contraction. Here, we review recent insights into pre- and early postnatal developmental processes that regulate CM maturation, laying specific focus on genetic and metabolic pathways that control transition of CMs from the embryonic and perinatal to the fully mature adult CM state. We recapitulate the intrinsic features of CM maturation and highlight the importance of external factors, such as energy substrate availability and endocrine regulation in shaping postnatal CM development. We also address recent approaches to enhance maturation of iPSC-derived CMs in vitro, and summarize new discoveries that might provide useful tools for translational research on repair of the injured human heart.

Indoxyl sulfate induces intestinal barrier injury through IRF1-DRP1 axis-mediated mitophagy impairment

Rationale: The dysfunctional gut-kidney axis forms a vicious circle, which eventually becomes a catalyst for the progression of chronic kidney disease (CKD) and occurrence of related complications. However, the pathogenic factors of CKD-associated intestinal dysfunction and its mechanism remain elusive. Methods: We first identified the protein-bound uremic toxin indoxyl sulfate (IS) as a possible contributor to intestinal barrier injury. Transepithelial electrical resistance, permeability assay and transmission electron microscopy were carried out to evaluate the damaging effect of IS on intestinal barrier in intestinal epithelial cells, IS-injected mice and CKD mice. In vitro and in vivo experiments were performed to investigate the role of IS in intestinal barrier injury and the underlying mechanism. Finally, CKD mice treated with AST-120 (an oral adsorbent for IS) and gene knockout mice were used to verify the mechanism and to explore possible interventions for IS-induced intestinal barrier injury. Results: Transepithelial electrical resistance and the expressions of tight junction-related genes were significantly suppressed by IS in intestinal epithelial cells. In vitro experiments demonstrated that IS inhibited the expression of dynamin-related protein 1 (DRP1) and mitophagic flux, whereas DRP1 overexpression attenuated IS-induced mitophagic inhibition and intestinal epithelial cell damage. Furthermore, IS suppressed DRP1 by upregulating the expression of interferon regulatory factor 1 (IRF1), and IRF1 could directly bind to the promoter region of DRP1. Additionally, the decreased expression of DRP1 and autophagosome-encapsulated mitochondria were observed in the intestinal tissues of CKD patients. Administration of AST-120 or genetic knockout of IRF1 attenuated IS-induced DRP1 reduction, mitophagic impairment and intestinal barrier injury in mice. Conclusions: These findings suggest that reducing IS accumulation or targeting the IRF1-DRP1 axis may be a promising therapeutic strategy for alleviating CKD-associated intestinal dysfunction.

Targeting mitochondrial fusion and fission proteins for cardioprotection

New treatments are needed to protect the myocardium against the detrimental effects of acute ischaemia/reperfusion (IR) injury following an acute myocardial infarction (AMI), in order to limit myocardial infarct (MI) size, preserve cardiac function and prevent the onset of heart failure (HF). Given the critical role of mitochondria in energy production for cardiac contractile function, prevention of mitochondrial dysfunction during acute myocardial IRI may provide novel cardioprotective strategies. In this regard, the mitochondrial fusion and fissions proteins, which regulate changes in mitochondrial morphology, are known to impact on mitochondrial quality control by modulating mitochondrial biogenesis, mitophagy and the mitochondrial unfolded protein response. In this article, we review how targeting these inter-related processes may provide novel treatment targets and new therapeutic strategies for reducing MI size, preventing the onset of HF following AMI.

Mechanisms and roles of mitochondrial localisation and dynamics in neuronal function

Neurons are highly polarised, complex and incredibly energy intensive cells, and their demand for ATP during neuronal transmission is primarily met by oxidative phosphorylation by mitochondria. Thus, maintaining the health and efficient function of mitochondria is vital for neuronal integrity, viability and synaptic activity. Mitochondria do not exist in isolation, but constantly undergo cycles of fusion and fission, and are actively transported around the neuron to sites of high energy demand. Intriguingly, axonal and dendritic mitochondria exhibit different morphologies. In axons mitochondria are small and sparse whereas in dendrites they are larger and more densely packed. The transport mechanisms and mitochondrial dynamics that underlie these differences, and their functional implications, have been the focus of concerted investigation. Moreover, it is now clear that deficiencies in mitochondrial dynamics can be a primary factor in many neurodegenerative diseases. Here, we review the role that mitochondrial dynamics play in neuronal function, how these processes support synaptic transmission and how mitochondrial dysfunction is implicated in neurodegenerative disease.

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.

New perspectives on the role of Drp1 isoforms in regulating mitochondrial pathophysiology

Mitochondria are dynamic organelles constantly undergoing fusion and fission. A concerted balance between the process of mitochondrial fusion and fission is required to maintain cellular health under different physiological conditions. Mutation and dysregulation of Drp1, the major driver of mitochondrial fission, has been associated with various neurological, oncological and cardiovascular disorders. Moreover, when subjected to pathological insults, mitochondria often undergo excessive fission, generating fragmented and dysfunctional mitochondria leading to cell death. Therefore, manipulating mitochondrial fission by targeting Drp1 has been an appealing therapeutic approach for cytoprotection. However, studies have been inconsistent. Studies employing Drp1 constructs representing alternate Drp1 isoforms, have demonstrated differing impacts of these isoforms on mitochondrial fission and cell death. Furthermore, there are distinct expression patterns of Drp1 isoforms in different tissues, suggesting idiosyncratic engagement in specific cellular functions. In this review, we will discuss these inherent variations among human Drp1 isoforms and how they could affect Drp1-mediated mitochondrial fission and cell death.

Mitochondrial phosphatase PGAM5 modulates cellular senescence by regulating mitochondrial dynamics

Mitochondria undergo dynamic fusion/fission, biogenesis and mitophagy in response to stimuli or stresses. Disruption of mitochondrial homeostasis could lead to cell senescence, although the underlying mechanism remains unclear. We show that deletion of mitochondrial phosphatase PGAM5 leads to accelerated retinal pigment epithelial (RPE) senescence in vitro and in vivo. Mechanistically, PGAM5 is required for mitochondrial fission through dephosphorylating DRP1. PGAM5 deletion leads to increased mitochondrial fusion and decreased mitochondrial turnover. As results, cellular ATP and reactive oxygen species (ROS) levels are elevated, mTOR and IRF/IFN-β signaling pathways are enhanced, leading to cellular senescence. Overexpression of Drp1 K38A or S637A mutant phenocopies or rescues mTOR activation and senescence in PGAM5^-/- cells, respectively. Young but not aging Pgam5^-/- mice are resistant to sodium iodate-induced RPE cell death. Our studies establish a link between defective mitochondrial fission, cellular senescence and age-dependent oxidative stress response, which have implications in age-related diseases.

Molecular cross talk among the components of the regulatory machinery of mitochondrial structure and quality control

Mitochondrial dysfunction critically impairs cellular health and often causes or affects the progression of several diseases, including neurodegenerative diseases and cancer. Thus, cells must have several ways to monitor the condition of mitochondrial quality and maintain mitochondrial health. Accumulating evidence suggests that the molecular machinery responding to spontaneous changes in mitochondrial morphology is associated with the routine mitochondrial quality control system. In this short review, we discuss recent progress made in linking mitochondrial structural dynamics and the quality control system.

The health of mitochondria is important for cellular health, and is maintained by the same mechanisms that control their shape. Mitochondria continuously divide, fuse, elongate, and shrink, forming ever-changing networks inside cells. Damaged mitochondria produce toxic byproducts and have been implicated in neurodegenerative diseases and cancer. Although changes in mitochondrial structure are known to be related to cellular health, the detailed mechanisms are not well understood. In a review, Woong Sun and Hyo Min Cho at the Korea University College of Medicine, Seoul, detail how mitochondrial fusion, division, and recycling are controlled, what signals are used to dispose of damaged mitochondria, and how the shape-control mechanisms also regulate mitochondrial quality. This review will help us to more clearly understand the structure-function relationship of mitochondria.

A Healthy Heart and a Healthy Brain: Looking at Mitophagy

Mitochondrial dysfunction is a hallmark of aging and is a major contributor to neurodegenerative diseases and various cardiovascular disorders. Mitophagy, a specialized autophagic pathway to remove damaged mitochondria, provides a critical mechanism to maintain mitochondrial quality. This function has been implicated in a tissue's ability to appropriately respond to metabolic and to bioenergetic stress, as well as to recover from mitochondrial damage. A global decline in mitophagic flux has been postulated to be linked to pathological alterations that occur in the heart and the brain as well as a general age-dependent decline in organ function. Cellular observation suggests multiple mechanistically distinct pathways converge upon and activate mitophagy. Over the past decade, additional molecular components within mitophagy have been discovered, including several disease-associated genes that are functionally implicated in mitophagy. However, the pathophysiological role of mitophagy, and how it is regulated within normal physiology or various disease states, is less well established. Here, we will review the evidence that a decline in mitophagy contributes to impaired mitochondrial homeostasis and may be particularly detrimental to postmitotic neurons and cardiomyocytes. We will discuss mitophagy's pathological significance in both neurodegenerative diseases and cardiovascular disorders. Additionally, signaling pathways regulating mitophagy are reviewed, with emphasis placed on how these pathways might contribute to disease progression. Understanding mitophagy's role in the mechanisms of disease pathogenesis should allow for the development of more efficient strategies to battle pathological conditions associated with mitochondrial dysfunction.

Bacopa monnieri inhibits apoptosis and senescence through mitophagy in human astrocytes

Benzo[a]pyrene (B[a]P), a polycyclic aromatic hydrocarbon, is a potent neurotoxic agent that is responsible for impaired neuronal development and is associated with aging. Here, it was demonstrated that extracts of Bacopa monnieri (BM), a traditional Ayurvedic medicine, diminished the B[a]P-induced apoptosis and senescence in human astrocytes. BM was demonstrated to protect the immortalized primary fetal astrocytes (IMPHFA) from B[a]P-induced apoptosis and senescence by reducing the damaged mitochondria that produced reactive oxygen species (ROS). Furthermore, it was shown that B[a]P-triggered G2 arrest could be altered by BM, thus indicating that BM could reverse the cell cycle arrest and mediate a normal cell cycle in IMPHFA cells. In addition, the lifespan of Caenorhabditis elegans was assessed, which confirmed these effects in the presence of BM, compared to the B[a]P-treated group. Furthermore, the anti-senescence and anti-apoptotic activities of BM were observed to be mediated through the protective effect of mitophagy, and inhibition of mitophagy could not protect the astrocytes from mitochondrial ROS-induced apoptosis and senescence in BM-treated cells. Moreover, it was revealed that BM induced Parkin-dependent mitophagy to exert its cytoprotective activity in IMPHFA cells. In conclusion, the anti-senescence and anti-apoptotic effects of BM in astrocytes could combat pollution and aging-related neurological disorders.

The role of Drp1 in mitophagy and cell death in the heart

Maintenance of mitochondrial function and integrity is critical for normal cell survival, particularly in non-dividing cells with a high-energy demand such as cardiomyocytes. Well-coordinated quality control mechanisms in cardiomyocytes, involving mitochondrial biogenesis, mitochondrial dynamics-fission and fusion, and mitophagy, act to protect against mitochondrial dysfunction. Mitochondrial fission, which requires dynamin-related protein 1 (Drp1), is essential for segregation of damaged mitochondria for degradation. Alterations in this process have been linked to cardiomyocyte apoptosis and cardiomyopathy. In this review, we discuss the role of Drp1 in mitophagy and apoptosis in the context of cardiac pathology, including myocardial ischemia and heart failure.

Mitophagy in cardiovascular homeostasis

Mitochondria are essential organelles that generate energy to fuel myocardial contraction. Accumulating evidence also suggests that, in the heart, mitochondria may contribute to specific aspects of disease progression through the regulations of specific metabolic intermediates, as well as the transcriptional and epigenetic states of cells. If damaged, the mitochondria and their related pathways are hindered, which may result in or contribute to the development of a wide range of cardiovascular diseases. Therefore, the maintenance of cardiac mitochondrial function and integrity through specific mitochondrial quality control mechanisms is critical for cardiovascular health. Mitophagy is part of the overall mitochondrial quality control process, and acts as a specialized autophagic pathway that mediates the lysosomal clearance of damaged mitochondria. In response to cardiac stress and injury, the pathways associated with mitophagy are triggered resulting in the removal of damaged mitochondrial, thereby maintaining cardiac homeostasis. In addition, recent studies have demonstrated an essential role for mitophagy in both developmental and disease-related metabolic transitioning of cardiac mitochondria. Here, we discuss the physiological and the pathological roles of mitophagy in the heart, the underlying molecular mechanisms, as well as potential therapeutic strategies based on mitophagic modulation.

Mitochondrial Adaptations in the Growing Heart

The heart pumps blood throughout the whole life of an organism, without rest periods during which to replenish energy or detoxify. Hence, cardiomyocytes, the working units of the heart, have mechanisms to ensure constitutive production of energy and detoxification to preserve fitness and function for decades. Even more challenging, the heart must adapt to the varying conditions of the organism from fetal life to adulthood, old age, and pathological stress. Mitochondria are at the nexus of these processes by producing not only energy but also metabolites and oxidative byproducts that can activate alarm signals and be toxic to the cell. We review basic concepts about cardiac mitochondria with a focus on their remarkable adaptations, including elimination, throughout the mammalian lifetime.

Role and Mechanisms of Mitophagy in Liver Diseases

The mitochondrion is an organelle that plays a vital role in the regulation of hepatic cellular redox, lipid metabolism, and cell death. Mitochondrial dysfunction is associated with both acute and chronic liver diseases with emerging evidence indicating that mitophagy, a selective form of autophagy for damaged/excessive mitochondria, plays a key role in the liver’s physiology and pathophysiology. This review will focus on mitochondrial dynamics, mitophagy regulation, and their roles in various liver diseases (alcoholic liver disease, non-alcoholic fatty liver disease, drug-induced liver injury, hepatic ischemia-reperfusion injury, viral hepatitis, and cancer) with the hope that a better understanding of the molecular events and signaling pathways in mitophagy regulation will help identify promising targets for the future treatment of liver diseases.

MCL-1 Inhibition by Selective BH3 Mimetics Disrupts Mitochondrial Dynamics Causing Loss of Viability and Functionality of Human Cardiomyocytes

MCL-1 is a well-characterized inhibitor of cell death that has also been shown to be a regulator of mitochondrial dynamics in human pluripotent stem cells. We used cardiomyocytes derived from human-induced pluripotent stem cells (hiPSC-CMs) to uncover whether MCL-1 is crucial for cardiac function and survival. Inhibition of MCL-1 by BH3 mimetics resulted in the disruption of mitochondrial morphology and dynamics as well as disorganization of the actin cytoskeleton. Interfering with MCL-1 function affects the homeostatic proximity of DRP-1 and MCL-1 at the outer mitochondrial membrane, resulting in decreased functionality of hiPSC-CMs. Cardiomyocytes display abnormal cardiac performance even after caspase inhibition, supporting a nonapoptotic activity of MCL-1 in hiPSC-CMs. BH3 mimetics targeting MCL-1 are promising anti-tumor therapeutics. Progression toward using BCL-2 family inhibitors, especially targeting MCL-1, depends on understanding its canonical function not only in preventing apoptosis but also in the maintenance of mitochondrial dynamics and function.

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BH3 mimetics targeting MCL-1 disrupt the mitochondrial network of human iPSC-CMs

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The BH3-mimetic-mediated effects on mitochondrial dynamics are DRP-1-dependent

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Targeting MCL-1 affects the survival and function of human cardiomyocytes

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Human iPSC-derived cardiomyocytes can be used to reveal toxicity of MCL-1 inhibitors

Mitochondrial Quality Control in Cardiomyocytes: A Critical Role in the Progression of Cardiovascular Diseases

Mitochondria serve as an energy plant and participate in a variety of signaling pathways to regulate cellular metabolism, survival and immunity. Mitochondrial dysfunction, in particular in cardiomyocytes, is associated with the development and progression of cardiovascular disease, resulting in heart failure, cardiomyopathy, and cardiac ischemia/reperfusion injury. Therefore, mitochondrial quality control processes, including post-translational modifications of mitochondrial proteins, mitochondrial dynamics, mitophagy, and formation of mitochondrial-driven vesicles, play a critical role in maintenance of mitochondrial and even cellular homeostasis in physiological or pathological conditions. Accumulating evidence suggests that mitochondrial quality control in cardiomyocytes is able to improve cardiac function, rescue dying cardiomyocytes, and prevent the deterioration of cardiovascular disease upon external environmental stress. In this review, we discuss recent progress in understanding mitochondrial quality control in cardiomyocytes. We also evaluate potential targets to prevent or treat cardiovascular diseases, and highlight future research directions which will help uncover additional mechanisms underlying mitochondrial homeostasis in cardiomyocytes.

Mitophagy: An Emerging Role in Aging and Age-Associated Diseases

Mitochondrial dysfunction constitutes one of the hallmarks of aging and is characterized by irregular mitochondrial morphology, insufficient ATP production, accumulation of mitochondrial DNA (mtDNA) mutations, increased production of mitochondrial reactive oxygen species (ROS) and the consequent oxidative damage to nucleic acids, proteins and lipids. Mitophagy, a mitochondrial quality control mechanism enabling the degradation of damaged and superfluous mitochondria, prevents such detrimental effects and reinstates cellular homeostasis in response to stress. To date, there is increasing evidence that mitophagy is significantly impaired in several human pathologies including aging and age-related diseases such as neurodegenerative disorders, cardiovascular pathologies and cancer. Therapeutic interventions aiming at the induction of mitophagy may have the potency to ameliorate these dysfunctions. In this review, we summarize recent findings on mechanisms controlling mitophagy and its role in aging and the development of human pathologies.

Mitophagy in Cardiovascular Diseases

Cardiovascular diseases are one of the leading causes of death. Increasing evidence has shown that pharmacological or genetic targeting of mitochondria can ameliorate each stage of these pathologies, which are strongly associated with mitochondrial dysfunction. Removal of inefficient and dysfunctional mitochondria through the process of mitophagy has been reported to be essential for meeting the energetic requirements and maintaining the biochemical homeostasis of cells. This process is useful for counteracting the negative phenotypic changes that occur during cardiovascular diseases, and understanding the molecular players involved might be crucial for the development of potential therapies. Here, we summarize the current knowledge on mitophagy (and autophagy) mechanisms in the context of heart disease with an important focus on atherosclerosis, ischemic heart disease, cardiomyopathies, heart failure, hypertension, arrhythmia, congenital heart disease and peripheral vascular disease. We aim to provide a complete background on the mechanisms of action of this mitochondrial quality control process in cardiology and in cardiac surgery by also reviewing studies on the use of known compounds able to modulate mitophagy for cardioprotective purposes.

Mammalian Mitophagosome Formation: A Focus on the Early Signals and Steps

Mitophagy, a conserved intracellular process by which mitochondria are eliminated via the autophagic machinery, is a quality control mechanism which facilitates maintenance of a functional mitochondrial network and cell homeostasis, making it a key process in development and longevity. Mitophagy has been linked to multiple human disorders, especially neurodegenerative diseases where the long-lived neurons are relying on clearance of old/damaged mitochondria to survive. During the past decade, the availability of novel tools to study mitophagy both in vitro and in vivo has significantly advanced our understanding of the molecular mechanisms governing this fundamental process in normal physiology and in various disease models. We here give an overview of the known mitophagy pathways and how they are induced, with a particular emphasis on the early events governing mitophagosome formation.

Autophagy-Dependent Ferroptosis: Machinery and Regulation

Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved cellular process capable of degrading various biological molecules (e.g., protein, glycogen, lipids, DNA, and RNA) and organelles (e.g., mitochondria, endoplasmic reticulum [ER] ribosomes, lysosomes, and micronuclei) via the lysosomal pathway. Ferroptosis is a type of oxidative stress-dependent regulated cell death associated with iron accumulation and lipid peroxidation. The recently discovered role of autophagy, especially selective types of autophagy (e.g., ferritinophagy, lipophagy, clockophagy, and chaperone-mediated autophagy), in driving cells toward ferroptotic death motivated us to explore the functional interactions between metabolism, immunity, and cell death. Here, we describe types of selective autophagy and discuss the regulatory mechanisms and signaling pathways of autophagy-dependent ferroptosis. We also summarize chemical modulators that are currently available for triggering or blocking autophagy-dependent ferroptosis and that may be developed for therapeutic interventions in human diseases.

Porcine Circovirus 2 Induction of ROS Is Responsible for Mitophagy in PK-15 Cells via Activation of Drp1 Phosphorylation

Mitochondrial dynamics is essential for the maintenance of cell homeostasis. Previous studies have shown that porcine circovirus 2 (PCV2) infection decreases the mitochondrial membrane potential and causes the elevation of reactive oxygen species (ROS), which may ultimately lead to mitochondrial apoptosis. However, whether PCV2 induce mitophagy remains unknown. Here we show that PCV2-induced mitophagy in PK-15 cells via Drp1 phosphorylation and PINK1/Parkin activation. PCV2 infection enhanced the phosphorylation of Drp1 and its subsequent translocation to mitochondria. PCV2-induced Drp1 phosphorylation could be suppressed by specific CDK1 inhibitor RO-3306, suggesting CDK1 as its possible upstream molecule. PCV2 infection increased the amount of ROS, up-regulated PINK1 expression, and stimulated recruitment of Parkin to mitochondria. N-acetyl-L-cysteine (NAC) markedly decreased PCV2-induced ROS, down-regulated Drp1 phosphorylation, and lessened PINK1 expression and mitochondrial accumulation of Parkin. Inhibition of Drp1 by mitochondrial division inhibitor-1 Mdivi-1 or RNA silencing not only resulted in the reduction of ROS and PINK1, improved mitochondrial mass and mitochondrial membrane potential, and decreased mitochondrial translocation of Parkin, but also led to reduced apoptotic responses. Together, our study shows that ROS induction due to PCV2 infection is responsible for the activation of Drp1 and the subsequent mitophagic and mitochondrial apoptotic responses.

Mitochondrial fission and fusion: A dynamic role in aging and potential target for age-related disease

The mitochondria is the major hub to convert energy for cellular processes. Dysregulation of mitochondrial function is one of the classical hallmarks of aging, and mitochondrial interventions have repeatedly been shown to improve outcomes in age-related diseases. Crucial to mitochondrial regulation is the dynamic nature of their network structure. Mitochondria separate and merge using fission and fusion processes in response to changes in energy and stress status. While many mitochondrial processes are already characterized in relation to aging, specific evidence in multicellular organisms causally linking mitochondrial dynamics to the regulation of lifespan is limited. There does exist, however, a large body of evidence connecting mitochondrial dynamics to other aging-related cellular processes and implicates them in a number of human diseases. Here, we discuss the mechanisms of mitochondrial fission and fusion, the current evidence of their role in aging of multicellular organisms, and how these connect to cell cycle regulation, quality control, and transmission of energy status. Finally, we discuss the current evidence implicating these processes in age-related human pathologies, such as neurodegenerative or cardio-metabolic diseases. We suggest that deeper understanding of the regulatory mechanisms within this system and downstream implications could benefit in understanding and intervention of these conditions.

A connection in life and death: The BCL-2 family coordinates mitochondrial network dynamics and stem cell fate

The B cell CLL/lymphoma-2 (BCL-2) family of proteins control the mitochondrial pathway of apoptosis, also known as intrinsic apoptosis. Direct binding between members of the BCL-2 family regulates mitochondrial outer membrane permeabilization (MOMP) after an apoptotic insult. The ability of the cell to sense stress and translate it into a death signal has been a major theme of research for nearly three decades; however, other mechanisms by which the BCL-2 family coordinates cellular homeostasis beyond its role in initiating apoptosis are emerging. One developing area of research is understanding how the BCL-2 family of proteins regulate development using pluripotent stem cells as a model system. Understanding BCL-2 family-mediated regulation of mitochondrial homeostasis in cell death and beyond would uncover new facets of stem cell maintenance and differentiation potential.

Complex Effects of Putative DRP-1 Inhibitors on Stress Responses in Mouse Heart and Rat Cardiomyoblasts

Dynamin-related protein-1 (DRP-1)-dependent mitochondrial fission may influence cardiac tolerance to ischemic or oxidative stress, presenting a potential "cardioprotective" target. Effects of dynamin inhibitors [mitochondrial division inhibitor 1 (MDIVI-1) and dynasore] on injury, mitochondrial function, and signaling proteins were assessed in distinct models: ischemia-reperfusion (I-R) in mouse hearts and oxidative stress in rat H9c2 cardiomyoblasts. Hearts exhibited substantial cell death [approx. 40 IU lactate dehydrogenase (LDH) efflux] and dysfunction (approx. 40 mmHg diastolic pressure, approx. 40% contractile recovery) following 25 minutes' ischemia. Pretreatment with 1 μM MDIVI-1 reduced dysfunction (30 mmHg diastolic pressure, approx. 55% recovery) and delayed without reducing overall cell death, whereas 5 μM MDIVI-1 reduced overall death at the same time paradoxically exaggerating dysfunction. Postischemic expression of mitochondrial DRP-1 and phospho-activation of ERK1/2 were reduced by MDIVI-1. Conversely, 1 μM dynasore worsened cell death and reduced nonmitochondrial DRP-1. Postischemic respiratory fluxes were unaltered by MDIVI-1, although a 50% fall in complex-I flux control ratio was reversed. In H9c2 myoblasts stressed with 400 μM H2O2, treatment with 50 μM MDIVI-1 preserved metabolic (MTT assay) and mitochondrial (basal respiration) function without influencing survival. This was associated with differential signaling responses, including reduced early versus increased late phospho-activation of ERK1/2, increased phospho-activation of protein kinase B (AKT), and differential changes in determinants of autophagy [reduced microtubule-associated protein 1 light chain 3b (LC3B-II/I) vs. increased Parkinson juvenile disease protein 2 (Parkin)] and apoptosis [reduced poly-(ADP-ribose) polymerase (PARP) cleavage vs. increased BCL2-associated X (BAX)/B-cell lymphoma 2 (BCL2)]. These data show MDIVI-1 (not dynasore) confers some benefit during I-R/oxidative stress. However, despite mitochondrial and metabolic preservation, MDIVI-1 exerts mixed effects on cell death versus dysfunction, potentially reflecting differential changes in survival kinase, autophagy, and apoptosis pathways. SIGNIFICANCE STATEMENT: Inhibition of mitochondrial fission is a novel approach to still elusive cardioprotection. Assessing effects of fission inhibitors on responses to ischemic or oxidative stress in hearts and cardiomyoblasts reveals mitochondrial division inhibitor 1 (MDIVI-1) and dynasore induce complex effects and limited cardioprotection. This includes differential impacts on death and dysfunction, survival kinases, and determinants of autophagy and apoptosis. Although highlighting the interconnectedness of fission and these key processes, results suggest MDIVI-1 and dynasore may be of limited value in the quest for effective cardioprotection.

FKBP8 LIRL-dependent mitochondrial fragmentation facilitates mitophagy under stress conditions

Mitochondrial quality control maintains mitochondrial function by regulating mitochondrial dynamics and mitophagy. Despite the identification of mitochondrial quality control factors, little is known about the crucial regulators coordinating both mitochondrial fission and mitophagy. Through a cell-based functional screening assay, FK506 binding protein 8 (FKBP8) was identified to target microtubule-associated protein 1 light chain 3 (LC3) to the mitochondria and to change mitochondrial morphology. Microscopy analysis revealed that the formation of tubular and enlarged mitochondria was observed in FKBP8 knockdown HeLa cells and the cortex of Fkbp8 heterozygote-knockout mouse embryos. Under iron depletion-induced stress, FKBP8 was recruited to the site of mitochondrial division through budding and colocalized with LC3. FKBP8 was also found to be required for mitochondrial fragmentation and mitophagy under hypoxic stress. Conversely, FKBP8 overexpression induced mitochondrial fragmentation in HeLa cells, human fibroblasts and mouse embryo fibroblasts (MEFs), and this fragmentation occurred in Drp1 knockout MEF cells, FIP200 knockout HeLa cells and BNIP3/NIX double knockout HeLa cells, but not in Opa1 knockout MEFs. Interestingly, we found an LIR motif-like sequence (LIRL), as well as an LIR motif, at the N-terminus of FKBP8 and LIRL was essential for both inducing mitochondrial fragmentation and binding of FKBP8 to OPA1. Together, we suggest that FKBP8 plays an essential role in mitochondrial fragmentation through LIRL during mitophagy and this activity of FKBP8 together with LIR is required for mitophagy under stress conditions.

Mitochondrially-targeted treatment strategies

Disruption of mitochondrial function is a common feature of inherited mitochondrial diseases (mitochondriopathies) and many other infectious and non-infectious diseases including viral, bacterial and protozoan infections, inflammatory and chronic pain, neurodegeneration, diabetes, obesity and cardiovascular diseases. Mitochondria therefore become an attractive target for developing new therapies. In this review we describe critical mechanisms involved in the maintenance of mitochondrial functionality and discuss strategies used to identify and validate mitochondrial targets in different diseases. We also highlight the most recent preclinical and clinical findings using molecules targeting mitochondrial bioenergetics, morphology, number, content and detoxification systems in common pathologies.

Mitophagy in the Hippocampus Is Excessive Activated After Cardiac Arrest and Cardiopulmonary Resuscitation

This study examined the activation of mitophagy following cardiac arrest (CA) and cardiopulmonary resuscitation (CPR) and the relationship between the change with time and apoptosis.

MAIN METHODS

The male Sprague-Dawley rats were randomized into four groups: Sham group, CPR24h group, CPR48h group, CPR72h group. The rat model of cardiac arrest was established by asphyxiation. We employed western blot to analyze the levels of mitophagy related proteins of hippocampus, JC-1 to detect mitochondrial membrane potential (MMP) and flow cytometry to measure the rate of apoptosis of hippocampal neurons. Moreover, we also intuitively observed the occurrence of mitophagy through electron microscopy.

KEY FINDINGS

The results showed that the levels of TOMM20 and Tim23 protein were significantly decreased after CPR, which were more remarkable following 72 h of CPR. However, the protein levels of dynamin related protein 1 (Drp1) and cytochrome C (Cyt-c) were strongly up-regulated after CPR. Meanwhile, the hippocampal MMP decreased gradually with time after CPR. Furthermore, we more intuitively verified the activation of mitophagy through electron microscopy. In addition, the rats of apoptosis rate of hippocampus after CPR were significantly increased, which were gradually enhanced over time from 24 h until at least 72 h following CPR.

SIGNIFICANCE

with the enhancement of mitophagy, the apoptosis of hippocampal neurons was gradually enhanced, which suggested mitophagy may be excessive activated and aggravating brain damage after CA and CPR.