In vivo functions of Drp1: lessons learned from yeast genetics and mouse knockouts

Yeast Dnm1G178R causes altered organelle dynamics and sheds light on the human DRP1G149R disease mechanism

Mitochondrial morphology is a result of regulated opposite events called fission and fusion and requires the GTPase, dynamin-related protein 1 (DRP1/Dnm1), or its homologs. A recent clinical report identified a heterozygous missense mutation in the human DRP1 that replaces Glycine (G) 149 with Arginine (R) and results in debilitating conditions in the patient. In this study, we mimicked this mutation in yeast Dnm1 (G178R) and investigated the impact of the pathogenic mutation on the protein's function. We provide evidence that the substitution of G with R in the G3 motif of the GTPase domain, renders the protein non-functional and in a dominant-negative way. The mutation hampers the distribution, localization, and function of the protein. Cells expressing the mutant variant exhibit a block in mitochondrial fission and altered peroxisome morphology and number.

OrthoID: profiling dynamic proteomes through time and space using mutually orthogonal chemical tools

Identifying proteins at organelle contact sites, such as mitochondria-associated endoplasmic reticulum membranes (MAM), is essential for understanding vital cellular processes, yet challenging due to their dynamic nature. Here we report "OrthoID", a proteomic method utilizing engineered enzymes, TurboID and APEX2, for the biotinylation (Bt) and adamantylation (Ad) of proteins close to the mitochondria and endoplasmic reticulum (ER), respectively, in conjunction with high-affinity binding pairs, streptavidin-biotin (SA-Bt) and cucurbit[7]uril-adamantane (CB[7]-Ad), for selective orthogonal enrichment of Bt- and Ad-labeled proteins. This approach effectively identifies protein candidates associated with the ER-mitochondria contact, including LRC59, whose roles at the contact site were-to the best of our knowledge-previously unknown, and tracks multiple protein sets undergoing structural and locational changes at MAM during mitophagy. These findings demonstrate that OrthoID could be a powerful proteomics tool for the identification and analysis of spatiotemporal proteins at organelle contact sites and revealing their dynamic behaviors in vital cellular processes.

DRP1: At the Crossroads of Dysregulated Mitochondrial Dynamics and Altered Cell Signaling in Cancer Cells

In the past decade, compelling evidence has accumulated that highlights the role of various subcellular structures in human disease conditions. Dysregulation of these structures greatly impacts cellular function and, thereby, disease conditions. One such organelle extensively studied for its role in several human diseases, especially cancer, is the mitochondrion. DRP1 is a GTPase that is considered the master regulator of mitochondrial fission and thereby also affects the proper functioning of the organelle. Altered signaling pathways are a distinguished characteristic of cancer cells. In this review, we aim to summarize our current understanding of the interesting crosstalk between the mitochondrial structure-function maintained by DRP1 and the signaling pathways that are affected in cancer cells. We highlight the structural aspects of DRP1, its regulation by various modifications, and the association of the protein with various cellular pathways altered in cancer. A better understanding of this association may help in identifying potential pharmacological targets for novel therapies in cancer.

Mitochondrial quality control in abdominal aortic aneurysm: From molecular mechanisms to therapeutic strategies

Mitochondria are the energy supply sites of cells and are crucial for eukaryotic life. Mitochondrial dysfunction is involved in the pathogenesis of abdominal aortic aneurysm (AAA). Multiple mitochondrial quality control (MQC) mechanisms, including mitochondrial DNA repair, biogenesis, antioxidant defense, dynamics, and autophagy, play vital roles in maintaining mitochondrial homeostasis under physiological and pathological conditions. Abnormalities in these mechanisms may induce mitochondrial damage and dysfunction leading to cell death and tissue remodeling. Recently, many clues suggest that dysregulation of MQC is closely related to the pathogenesis of AAA. Therefore, specific interventions targeting MQC mechanisms to maintain and restore mitochondrial function have become promising therapeutic methods for the prevention and treatment of AAA.

Role of Tumor Necrosis Factor Receptor 1-Reactive Oxygen Species-Caspase 11 Pathway in Neuropathic Pain Mediated by HIV gp120 With Morphine in Rats

BACKGROUND

Recent clinical research suggests that repeated use of opioid pain medications can increase neuropathic pain in people living with human immunodeficiency virus (HIV; PLWH). Therefore, it is significant to elucidate the exact mechanisms of HIV-related chronic pain. HIV infection and chronic morphine induce proinflammatory factors, such as tumor necrosis factor (TNF)α acting through tumor necrosis factor receptor I (TNFRI). HIV coat proteins and/or chronic morphine increase mitochondrial superoxide in the spinal cord dorsal horn (SCDH). Recently, emerging cytoplasmic caspase-11 is defined as a noncanonical inflammasome and can be activated by reactive oxygen species (ROS). Here, we tested our hypothesis that HIV coat glycoprotein gp120 with chronic morphine activates a TNFRI-mtROS-caspase-11 pathway in rats, which increases neuroinflammation and neuropathic pain.

METHODS

Neuropathic pain was induced by repeated administration of recombinant gp120 with morphine (gp120/M) in rats. Mechanical allodynia was assessed using von Frey filaments, and thermal latency using hotplate test. Protein expression of spinal TNFRI and cleaved caspase-11 was examined using western blots. The image of spinal mitochondrial superoxide was examined using MitoSox Red (mitochondrial superoxide indicator) image assay. Immunohistochemistry was used to examine the location of TNFRI and caspase-11 in the SCDH. Intrathecal administration of antisense oligodeoxynucleotide (AS-ODN) against TNFRI, caspase-11 siRNA, or a scavenger of mitochondrial superoxide was given for antinociceptive effects. Statistical tests were done using analysis of variance (1- or 2-way), or 2-tailed t test.

RESULTS

Intrathecal gp120/M induced mechanical allodynia and thermal hyperalgesia lasting for 3 weeks ( P < .001). Gp120/M increased the expression of spinal TNFRI, mitochondrial superoxide, and cleaved caspase-11. Immunohistochemistry showed that TNFRI and caspase-11 were mainly expressed in the neurons of the SCDH. Intrathecal administration of antisense oligonucleotides against TNFRI, Mito-Tempol (a scavenger of mitochondrial superoxide), or caspase-11 siRNA reduced mechanical allodynia and thermal hyperalgesia in the gp120/M neuropathic pain model. Spinal knockdown of TNFRI reduced MitoSox profile cell number in the SCDH; intrathecal Mito-T decreased spinal caspase-11 expression in gp120/M rats. In the cultured B35 neurons treated with TNFα, pretreatment with Mito-Tempol reduced active caspase-11 in the neurons.

CONCLUSIONS

These results suggest that spinal TNFRI-mtROS-caspase 11 signal pathway plays a critical role in the HIV-associated neuropathic pain state, providing a novel approach to treating chronic pain in PLWH with opioids.

Mitochondrial dynamics involves molecular and mechanical events in motility, fusion and fission

Mitochondria are cell organelles that play pivotal roles in maintaining cell survival, cellular metabolic homeostasis, and cell death. Mitochondria are highly dynamic entities which undergo fusion and fission, and have been shown to be very motile in vivo in neurons and in vitro in multiple cell lines. Fusion and fission are essential for maintaining mitochondrial homeostasis through control of morphology, content exchange, inheritance of mitochondria, maintenance of mitochondrial DNA, and removal of damaged mitochondria by autophagy. Mitochondrial motility occurs through mechanical and molecular mechanisms which translocate mitochondria to sites of high energy demand. Motility also plays an important role in intracellular signaling. Here, we review key features that mediate mitochondrial dynamics and explore methods to advance the study of mitochondrial motility as well as mitochondrial dynamics-related diseases and mitochondrial-targeted therapeutics.

Fission Impossible (?)-New Insights into Disorders of Peroxisome Dynamics

Peroxisomes are highly dynamic and responsive organelles, which can adjust their morphology, number, intracellular position, and metabolic functions according to cellular needs. Peroxisome multiplication in mammalian cells involves the concerted action of the membrane-shaping protein PEX11β and division proteins, such as the membrane adaptors FIS1 and MFF, which recruit the fission GTPase DRP1 to the peroxisomal membrane. The latter proteins are also involved in mitochondrial division. Patients with loss of DRP1, MFF or PEX11β function have been identified, showing abnormalities in peroxisomal (and, for the shared proteins, mitochondrial) dynamics as well as developmental and neurological defects, whereas the metabolic functions of the organelles are often unaffected. Here, we provide a timely update on peroxisomal membrane dynamics with a particular focus on peroxisome formation by membrane growth and division. We address the function of PEX11β in these processes, as well as the role of peroxisome–ER contacts in lipid transfer for peroxisomal membrane expansion. Furthermore, we summarize the clinical phenotypes and pathophysiology of patients with defects in the key division proteins DRP1, MFF, and PEX11β as well as in the peroxisome–ER tether ACBD5. Potential therapeutic strategies for these rare disorders with limited treatment options are discussed.

Mitochondrial dynamics and its impact on human health and diseases: inside the DRP1 blackbox

Mitochondria are essential organelles that play a significant role in various cellular processes apart from providing energy in eukaryotic cells. An intricate link between mitochondrial structure and function is now unequivocally accepted. Several molecular players have been identified, which are important in maintaining the structure of the organelle. Dynamin-related protein 1 (DRP1) is one such conserved protein that is a vital regulator of mitochondrial dynamics. Multidisciplinary studies have helped elucidate the structure of the protein and its mechanism of action in great detail. Mutations in various domains of the protein have been identified that are associated with debilitating conditions in patients. The involvement of the protein in disease conditions such as neurodegeneration, cancer, and cardiovascular disorders is also gaining attention. The purpose of this review is to highlight recent findings on the role of DRP1 in human disease conditions and address its importance as a therapeutic target.

Mitochondrial fusion and fission are required for proper mitochondrial function and cell proliferation in fission yeast

Mitochondria form a branched tubular network in many types of cells, depending on a balance between mitochondrial fusion and fission. How mitochondrial fusion and fission are involved in regulating mitochondrial function and cell proliferation is not well understood. Here, we dissected the roles of mitochondrial fusion and fission in mitochondrial function and cell proliferation in fission yeast. We examined mitochondrial membrane potential by staining cells with DiOC₆ and assessed mitochondrial respiration by directly measuring oxygen consumption of cells with a dissolved oxygen respirometer. We found that defects in mitochondrial fission or fusion reduce mitochondrial membrane potential and compromise mitochondrial respiration while the absence of both mitochondrial fusion and fission restores wild type-like respiration, normal membrane potential, and tubular networks of mitochondria. Moreover, we found that the absence of either mitochondrial fission or fusion prolongs the cell cycle and that the absence of both mitochondrial fusion and fission significantly delays cell cycle progression after nitrogen replenishment. The prolonged/delayed cell cycle is likely due to the deregulation of Cdc2 activation. Hence, our work not only establishes an intimate link between mitochondrial morphology and function but also underscores the importance of mitochondrial dynamics in regulating the cell cycle.

Mimicking human Drp1 disease-causing mutations in yeast Dnm1 reveals altered mitochondrial dynamics

The dynamin-related protein 1 (Drp1) and its homologs in various eukaryotes are essential to maintain mitochondrial morphology and regulate mitochondrial division. Several mutations in different domains of Drp1 have been reported, which result in debilitating conditions. Four such disease-causing mutations of the middle domain of Drp1 were mimicked in the yeast dynamin-related GTPase (Dnm1) and were characterized in this study. Mitochondrial morphology and protein function were observed to be altered to a variable extent in cells expressing the mutated variants of Dnm1. Several aspects related to the protein such as punctate formation, localization to mitochondria, dynamic behavior and structure were analyzed by microscopy, biochemical studies and molecular dynamics simulations. Significant effects on the protein structure and function were observed in cells expressing A430D and G397D mutations. Overall, our data provide insight into the molecular and cellular alterations resulting from middle domain mutations in Dnm1.

Axotomy Induces Drp1-Dependent Fragmentation of Axonal Mitochondria

It is well established that CNS axons fail to regenerate, undergo retrograde dieback, and form dystrophic growth cones due to both intrinsic and extrinsic factors. We sought to investigate the role of axonal mitochondria in the axonal response to injury. A viral vector (AAV) containing a mitochondrially targeted fluorescent protein (mitoDsRed) as well as fluorescently tagged LC3 (GFP-LC3), an autophagosomal marker, was injected into the primary motor cortex, to label the corticospinal tract (CST), of adult rats. The axons of the CST were then injured by dorsal column lesion at C4-C5. We found that mitochondria in injured CST axons near the injury site are fragmented and fragmentation of mitochondria persists for 2 weeks before returning to pre-injury lengths. Fragmented mitochondria have consistently been shown to be dysfunctional and detrimental to cellular health. Inhibition of Drp1, the GTPase responsible for mitochondrial fission, using a specific pharmacological inhibitor (mDivi-1) blocked fragmentation. Additionally, it was determined that there is increased mitophagy in CST axons following Spinal cord injury (SCI) based on increased colocalization of mitochondria and LC3. In vitro models revealed that mitochondrial divalent ion uptake is necessary for injury-induced mitochondrial fission, as inhibiting the mitochondrial calcium uniporter (MCU) using RU360 prevented injury-induced fission. This phenomenon was also observed in vivo. These studies indicate that following the injury, both in vivo and in vitro, axonal mitochondria undergo increased fission, which may contribute to the lack of regeneration seen in CNS neurons.

Mitochondrial Dynamics: A Key Role in Neurodegeneration and a Potential Target for Neurodegenerative Disease

In neurodegenerative diseases, neurodegeneration has been related to several mitochondrial dynamics imbalances such as excessive fragmentation of mitochondria, impaired mitophagy, and blocked mitochondria mitochondrial transport in axons. Mitochondria are dynamic organelles, and essential for energy conversion, neuron survival, and cell death. As mitochondrial dynamics have a significant influence on homeostasis, in this review, we mainly discuss the role of mitochondrial dynamics in several neurodegenerative diseases. There is evidence that several mitochondrial dynamics-associated proteins, as well as related pathways, have roles in the pathological process of neurodegenerative diseases with an impact on mitochondrial functions and metabolism. However, specific pathological mechanisms need to be better understood in order to propose new therapeutic strategies targeting mitochondrial dynamics that have shown promise in recent studies.

Targeting mitochondrial fission as a potential therapeutic for abdominal aortic aneurysm

AIMS

Angiotensin II (AngII) is a potential contributor to the development of abdominal aortic aneurysm (AAA). In aortic vascular smooth muscle cells (VSMCs), exposure to AngII induces mitochondrial fission via dynamin-related protein 1 (Drp1). However, pathophysiological relevance of mitochondrial morphology in AngII-associated AAA remains unexplored. Here, we tested the hypothesis that mitochondrial fission is involved in the development of AAA.

METHODS AND RESULTS

Immunohistochemistry was performed on human AAA samples and revealed enhanced expression of Drp1. In C57BL6 mice treated with AngII plus β-aminopropionitrile, AAA tissue also showed an increase in Drp1 expression. A mitochondrial fission inhibitor, mdivi1, attenuated AAA size, associated aortic pathology, Drp1 protein induction, and mitochondrial fission but not hypertension in these mice. Moreover, western-blot analysis showed that induction of matrix metalloproteinase-2, which precedes the development of AAA, was blocked by mdivi1. Mdivi1 also reduced the development of AAA in apolipoprotein E-deficient mice infused with AngII. As with mdivi1, Drp1+/- mice treated with AngII plus β-aminopropionitrile showed a decrease in AAA compared to control Drp1+/+ mice. In abdominal aortic VSMCs, AngII induced phosphorylation of Drp1 and mitochondrial fission, the latter of which was attenuated with Drp1 silencing as well as mdivi1. AngII also induced vascular cell adhesion molecule-1 expression and enhanced leucocyte adhesion and mitochondrial oxygen consumption in smooth muscle cells, which were attenuated with mdivi1.

CONCLUSION

These data indicate that Drp1 and mitochondrial fission play salient roles in AAA development, which likely involves mitochondrial dysfunction and inflammatory activation of VSMCs.

Factors Influencing Mitochondrial Function as a Key Mediator of Glucose-Induced Insulin Release: Highlighting Nicotinamide Nucleotide Transhydrogenase

Pancreatic β-cells recognize blood glucose changes and release insulin that is a peptide hormone responsible for stable glycemia. Diabetes, a chronic disorder of insulin insufficiency, leads to disturbed glucose homeostasis and multi-organ problems. Glucose and insulin are key markers in the follow-up and control of this disease. Mitochondrial metabolism of pancreatic beta cells is a crucial part of glucose-stimulated cascade of insulin secretion. Effective factors on β-cells mitochondrial function in production of compounds such as tricarboxylic acid intermediates, glutamate, nicotinamide adenine dinucleotide phosphate, and reactive oxygen species can have great effects on the secretion of insulin under diabetes. This review enhances our knowledge of factors influencing mitochondrial function as a key mediator of glucose-induced insulin release that accordingly will be helpful to further our understanding of the mechanisms implicated in the progressive beta cell failure that results in diabetes.

Neuropathic Pain: the Dysfunction of Drp1, Mitochondria, and ROS Homeostasis

Neuropathic pain affects the physical and mental health status of patients. Due to its complex pathogenesis and the adverse reactions to medicines, its treatment remains challenging. Among all the etiologies, increasing evidence has pointed to mitochondrial dysfunction. Dynamin-related protein 1 (Drp1)-mediated mitochondrial fragmentation leads to excess ROS generation, which is implicated in the pathogenesis of neuropathic pain. However, the exact mechanism remains unclear. Studies aiming to clarify the possible pathway and relationship between Drp1, mitochondria, ROS, and neuropathic pain may identify a good treatment for neuropathic pain in the clinic. As shown in this review, dysfunction of Drp1 and ROS homeostasis plays essential roles in neuropathic pain. We summarized a Drp1-mitochondrial fission-ROS cycle that potentially functions in neuropathic pain and is regulated by posttranslational modifications and Ca²⁺. Additionally, we further enumerated six Drp1 inhibitors, including Mdivi-1, P110, Drp1 antisense oligodeoxynucleotides, hyperbaric oxygen, melatonin, and β-hydroxybutyrate, as potential treatments, with the aim of providing guidance for novel molecules to be used in the clinic.

Mitochondrial Fission Mediates Endothelial Inflammation

Endothelial inflammation and mitochondrial dysfunction have been implicated in cardiovascular diseases, yet, a unifying mechanism tying them together remains limited. Mitochondrial dysfunction is frequently associated with mitochondrial fission/fragmentation mediated by the GTPase Drp1 (dynamin-related protein 1). Nuclear factor (NF)-κB, a master regulator of inflammation, is implicated in endothelial dysfunction and resultant complications. Here, we explore a causal relationship between mitochondrial fission and NF-κB activation in endothelial inflammatory responses. In cultured endothelial cells, TNF-α (tumor necrosis factor-α) or lipopolysaccharide induces mitochondrial fragmentation. Inhibition of Drp1 activity or expression suppresses mitochondrial fission, NF-κB activation, vascular cell adhesion molecule-1 induction, and leukocyte adhesion induced by these proinflammatory factors. Moreover, attenuations of inflammatory leukocyte adhesion were observed in Drp1 heterodeficient mice as well as endothelial Drp1 silenced mice. Intriguingly, inhibition of the canonical NF-κB signaling suppresses endothelial mitochondrial fission. Mechanistically, NF-κB p65/RelA seems to mediate inflammatory mitochondrial fission in endothelial cells. In addition, the classical anti-inflammatory drug, salicylate, seems to maintain mitochondrial fission/fusion balance against TNF-α via inhibition of NF-κB. In conclusion, our results suggest a previously unknown mechanism whereby the canonical NF-κB cascade and a mitochondrial fission pathway interdependently regulate endothelial inflammation.

Causal roles of mitochondrial dynamics in longevity and healthy aging

Mitochondria are organized in the cell in the form of a dynamic, interconnected network. Mitochondrial dynamics, regulated by mitochondrial fission, fusion, and trafficking, ensure restructuring of this complex reticulum in response to nutrient availability, molecular signals, and cellular stress. Aberrant mitochondrial structures have long been observed in aging and age-related diseases indicating that mitochondrial dynamics are compromised as cells age. However, the specific mechanisms by which aging affects mitochondrial dynamics and whether these changes are causally or casually associated with cellular and organismal aging is not clear. Here, we review recent studies that show specifically how mitochondrial fission, fusion, and trafficking are altered with age. We discuss factors that change with age to directly or indirectly influence mitochondrial dynamics while examining causal roles for altered mitochondrial dynamics in healthy aging and underlying functional outputs that might affect longevity. Lastly, we propose that altered mitochondrial dynamics might not just be a passive consequence of aging but might constitute an adaptive mechanism to mitigate age-dependent cellular impairments and might be targeted to increase longevity and promote healthy aging.

Pyridine nucleotide-disulphide oxidoreductase domain 2 (PYROXD2): Role in mitochondrial function

Pyridine Nucleotide-Disulphide Oxidoreductase Domain 2 (PYROXD2), a Hepatitis B virus X protein (HBx)-interacting protein, is significantly down-regulated in hepatocellular carcinoma (HCC), however its exact biological function remains unclear. The aim of this study is to investigate the subcellular localization and biological function of PYROXD2 in hepatic cells. The results showed that PYROXD2 was imported to the mitochondrial inner membrane/matrix by Tom40 and Tim23, but not Mia40. PYROXD2 151-230aa might be the mitochondrial targeting sequence. PYROXD2 interacted with complex IV subunit COX5B. Knockout of PYROXD2 decreased MMP, intracellular ROS, complex IV activity, cell proliferation, ATP content and mtDNA copy number, but increased mtROS levels and the number of immature mitochondria. In summary, our data illustrated that PYROXD2 localizes to the mitochondrial inner membrane/matrix, and it plays important roles in regulating mitochondrial function.

Dynamin-like protein 1 cleavage by calpain in Alzheimer's disease

Abnormal mitochondrial dynamics contributes to mitochondrial dysfunction in Alzheimer's disease (AD), yet the underlying mechanism remains elusive. In the current study, we reported that DLP1, the key mitochondrial fission GTPase, is a substrate of calpain which produced specific N-terminal DLP1 cleavage fragments. In addition, various AD-related insults such as exposure to glutamate, soluble amyloid-β oligomers, or reagents inducing tau hyperphosphorylation (i.e., okadaic acid) led to calpain-dependent cleavage of DLP1 in primary cortical neurons. DLP1 cleavage fragments were found in cortical neurons of CRND8 APP transgenic mice which can be inhibited by calpeptin, a potent small molecule inhibitor of calpain. Importantly, these N-terminal DLP1 fragments were also present in the human brains, and the levels of both full-length and N-terminal fragments of DLP1 and the full-length and calpain-specific cleavage product of spectrin were significantly reduced in AD brains along with significantly increased calpain. These results suggest that calpain-dependent cleavage is at least one of the posttranscriptional mechanisms that contribute to the dysregulation of mitochondrial dynamics in AD.

Proteolytic regulation of mitochondrial dynamics

Spatiotemporal changes in the abundance, shape, and cellular localization of the mitochondrial network, also known as mitochondrial dynamics, are now widely recognized to play a key role in mitochondrial and cellular physiology as well as disease states. This process involves coordinated remodeling of the outer and inner mitochondrial membranes by conserved dynamin-like guanosine triphosphatases and their partner molecules in response to various physiological and stress stimuli. Although the core machineries that mediate fusion and partitioning of the mitochondrial network have been extensively characterized, many aspects of their function and regulation are incompletely understood and only beginning to emerge. In the present review we briefly summarize current knowledge about how the key mitochondrial dynamics-mediating factors are regulated via selective proteolysis by mitochondrial and cellular proteolytic machineries.

Dynamin-related protein 1: A critical protein in the pathogenesis of neural system dysfunctions and neurodegenerative diseases

Mitochondria play a key role in the maintenance of neuronal function by continuously providing energy. Here, we will give a detailed review about the recent developments in regards to dynamin-related protein 1 (Drp1) induced unbalanced mitochondrial dynamics, excessive mitochondrial division, and neuronal injury in neural system dysfunctions and neurodegenerative diseases, including the Drp1 knockout induced mice embryonic death, the dysfunction of the Drp1-dependent mitochondrial division induced neuronal cell apoptosis and impaired neuronal axonal transportation, the abnormal interaction between Drp1 and amyloid β (Aβ) in Alzheimer's disease (AD), the mutant Huntingtin (Htt) in Huntington's disease (HD), and the Drp1-associated pathogenesis of other neurodegenerative diseases such as Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). Drp1 is required for mitochondrial division determining the size, shape, distribution, and remodeling as well as maintaining of mitochondrial integrity in mammalian cells. In addition, increasing reports indicate that the Drp1 is involved in some cellular events of neuronal cells causing some neural system dysfunctions and neurodegenerative diseases, including impaired mitochondrial dynamics, apoptosis, and several posttranslational modification induced increased mitochondrial divisions. Recent studies also revealed that the Drp1 can interact with Aβ, phosphorylated τ, and mutant Htt affecting the mitochondrial shape, size, distribution, axonal transportation, and energy production in the AD and HD neuronal cells. These changes can affect the health of mitochondria and the function of synapses causing neuronal injury and eventually leading to the dysfunction of memory, cognitive impairment, resting tremor, posture instability, involuntary movements, and progressive muscle atrophy and paralysis in patients.

ROS as Regulators of Mitochondrial Dynamics in Neurons

Mitochondrial dynamics is a complex process, which involves the fission and fusion of mitochondrial outer and inner membranes. These processes organize the mitochondrial size and morphology, as well as their localization throughout the cells. In the last two decades, it has become a spotlight due to their importance in the pathophysiological processes, particularly in neurological diseases. It is known that Drp1, mitofusin 1 and 2, and Opa1 constitute the core of proteins that coordinate this intricate and dynamic process. Likewise, changes in the levels of reactive oxygen species (ROS) lead to modifications in the expression and/or activity of the proteins implicated in the mitochondrial dynamics, suggesting an involvement of these molecules in the process. In this review, we discuss the role of ROS in the regulation of fusion/fission in the nervous system, as well as the involvement of mitochondrial dynamics proteins in neurodegenerative diseases.

Endoplasmic Reticulum-Mitochondrial Contactology: Structure and Signaling Functions

Interorganellar contacts are increasingly recognized as central to the control of cellular behavior. These contacts, which typically involve a small fraction of the endomembrane surface, are local communication hubs that resemble synapses. We propose the term contactology to denote the analysis of interorganellar contacts. Endoplasmic reticulum (ER) contacts with mitochondria were recognized several decades ago; major roles in ion and lipid transfer, signaling, and membrane dynamics have been established, while others continue to emerge. The functional diversity of ER-mitochondrial (ER-mito) contacts is mirrored in their structural heterogeneity, with subspecialization likely supported by multiple, different linker-forming protein structures. The nanoscale size of the contacts has made studying their structure, function, and dynamics difficult. This review focuses on the structure of the ER-mito contacts, methods for studying them, and the roles of contacts in Ca²⁺ and reactive oxygen species (ROS) signaling.

Downregulation of Drp1, a fission regulator, is associated with human lung and colon cancers

Dynamin-related protein 1 (Drp1), a dynamin-related GTPase, is a key regulator of mitochondrial fission. Although recent studies have shown that Drp1 plays important roles in various important cellular processes, such as maintaining proper mitochondrial function, apoptosis and necrosis, the potential involvement of Drp1 in cancer development has not been fully addressed. To explore the role of Drp1 in cancer, we examined Drp1 levels in various human cancer tissues. Tissue array analysis showed that the level of Drp1 was decreased significantly in malignant colon and lung cancer tissues, whereas no change in Drp1 was observed in breast and prostate tumors. Pairwise comparisons of cancer tissue and adjacent normal tissue from colon and lung cancer patients further confirmed decreases in Drp1 expression of 75% in colon cancer patients and 78% in lung cancer patients. Moreover, Drp1 levels were decreased further with advanced grade in both colon and lung cancers, suggesting that loss of Drp1 is associated with the progression of human lung and colon cancer. Consistent with this observation, knockdown of Drp1 increased cellular migration activity in human lung cancer cells and tumor formation in a xenograft tumor model. Taken together, these results suggest that the loss of Drp1 expression could contribute to the development of human lung and colon cancers.

Molecular Motor Dnm1 Synergistically Induces Membrane Curvature To Facilitate Mitochondrial Fission

Dnm1 and Fis1 are prototypical proteins that regulate yeast mitochondrial morphology by controlling fission, the dysregulation of which can result in developmental disorders and neurodegenerative diseases in humans. Loss of Dnm1 blocks the formation of fission complexes and leads to elongated mitochondria in the form of interconnected networks, while overproduction of Dnm1 results in excessive mitochondrial fragmentation. In the current model, Dnm1 is essentially a GTP hydrolysis-driven molecular motor that self-assembles into ring-like oligomeric structures that encircle and pinch the outer mitochondrial membrane at sites of fission. In this work, we use machine learning and synchrotron small-angle X-ray scattering (SAXS) to investigate whether the motor Dnm1 can synergistically facilitate mitochondrial fission by membrane remodeling. A support vector machine (SVM)-based classifier trained to detect sequences with membrane-restructuring activity identifies a helical Dnm1 domain capable of generating negative Gaussian curvature (NGC), the type of saddle-shaped local surface curvature found on scission necks during fission events. Furthermore, this domain is highly conserved in Dnm1 homologues with fission activity. Synchrotron SAXS measurements reveal that Dnm1 restructures membranes into phases rich in NGC, and is capable of inducing a fission neck with a diameter of 12.6 nm. Through in silico mutational analysis, we find that the helical Dnm1 domain is locally optimized for membrane curvature generation, and phylogenetic analysis suggests that dynamin superfamily proteins that are close relatives of human dynamin Dyn1 have evolved the capacity to restructure membranes via the induction of curvature mitochondrial fission. In addition, we observe that Fis1, an adaptor protein, is able to inhibit the pro-fission membrane activity of Dnm1, which points to the antagonistic roles of the two proteins in the regulation of mitochondrial fission.

Zdhhc13-dependent Drp1 S-palmitoylation impacts brain bioenergetics, anxiety, coordination and motor skills

Protein S-palmitoylation is a reversible post-translational modification mediated by palmitoyl acyltransferase enzymes, a group of Zn²⁺-finger DHHC-domain-containing proteins (ZDHHC). Here, for the first time, we show that Zdhhc13 plays a key role in anxiety-related behaviors and motor function, as well as brain bioenergetics, in a mouse model (luc) carrying a spontaneous Zdhhc13 recessive mutation. At 3 m of age, mutant mice displayed increased sensorimotor gating, anxiety, hypoactivity, and decreased motor coordination, compared to littermate controls. Loss of Zdhhc13 in cortex and cerebellum from 3- and 24 m old hetero- and homozygous male mutant mice resulted in lower levels of Drp1 S-palmitoylation accompanied by altered mitochondrial dynamics, increased glycolysis, glutaminolysis and lactic acidosis, and neurotransmitter imbalances. Employing in vivo and in vitro models, we identified that Zdhhc13-dependent Drp1 S-palmitoylation, which acting alone or in concert, enables the normal occurrence of the fission-fusion process. In vitro and in vivo direct Zdhhc13-Drp1 protein interaction was observed, confirming Drp1 as a substrate of Zdhhc13. Abnormal fission-fusion processes result in disrupted mitochondria morphology and distribution affecting not only mitochondrial ATP output but neurotransmission and integrity of synaptic structures in the brain, setting the basis for the behavioral abnormalities described in the Zdhhc13-deficient mice.

Polymorphic regulation of mitochondrial fission and fusion modifies phenotypes of microglia in neuroinflammation

Microglia are the resident macrophages of the central nervous system and play complex roles in the milieu of diseases including the primary diseases of myelin. Although mitochondria are critical for cellular functions and survival in the nervous system, alterations in and the roles of mitochondrial dynamics and associated signaling in microglia are still poorly understood. In the present study, by combining immunohistochemistry and 3D ultrastructural analyses, we show that mitochondrial fission/fusion in reactive microglia is differentially regulated from that in monocyte-derived macrophages and the ramified microglia of normal white matter in myelin disease models. Mouse cerebral microglia in vitro demonstrated that stimulation of TLR4 with lipopolysaccharide, widely used to examine microglial reactions, caused the activation of the mitochondrial fission protein, dynamin-related protein 1 (Drp1) and enhanced production of reactive oxygen species (ROS). The increase in the ROS level activated 5' adenosine monophosphate-activated protein kinase (AMPK), and facilitated elongation of mitochondria along the microtubule tracks. These results suggest that the polymorphic regulation of mitochondrial fission and fusion in reactive microglia is mediated by distinct signaling under inflammatory conditions, and modulates microglial phenotypes through the production of ROS.

Parkin suppresses Drp1-independent mitochondrial division

The cycle of mitochondrial division and fusion disconnect and reconnect individual mitochondria in cells to remodel this energy-producing organelle. Although dynamin-related protein 1 (Drp1) plays a major role in mitochondrial division in cells, a reduced level of mitochondrial division still persists even in the absence of Drp1. It is unknown how much Drp1-mediated mitochondrial division accounts for the connectivity of mitochondria. The role of a Parkinson's disease-associated protein-parkin, which biochemically and genetically interacts with Drp1-in mitochondrial connectivity also remains poorly understood. Here, we quantified the number and connectivity of mitochondria using mitochondria-targeted photoactivatable GFP in cells. We show that the loss of Drp1 increases the connectivity of mitochondria by 15-fold in mouse embryonic fibroblasts (MEFs). While a single loss of parkin does not affect the connectivity of mitochondria, the connectivity of mitochondria significantly decreased compared with a single loss of Drp1 when parkin was lost in the absence of Drp1. Furthermore, the loss of parkin decreased the frequency of depolarization of the mitochondrial inner membrane that is caused by increased mitochondrial connectivity in Drp1-knockout MEFs. Therefore, our data suggest that parkin negatively regulates Drp1-indendent mitochondrial division.

Drp1 guarding of the mitochondrial network is important for glucose-stimulated insulin secretion in pancreatic beta cells

Mitochondria form a tubular network in mammalian cells, and the mitochondrial life cycle is determined by fission, fusion and autophagy. Dynamin-related protein 1 (Drp1) has a pivotal role in these processes because it alone is able to constrict mitochondria. However, the regulation and function of Drp1 have been shown to vary between cell types. Mitochondrial morphology affects mitochondrial metabolism and function. In pancreatic beta cells mitochondrial metabolism is a key component of the glucose-induced cascade of insulin secretion. The goal of the present study was to investigate the action of Drp1 in pancreatic beta cells. For this purpose Drp1 was down-regulated by means of shDrp1 in insulin-secreting INS1 cells and mouse pancreatic islets. In INS1 cells reduced Drp1 expression resulted in diminished expression of proteins regulating mitochondrial fusion, namely mitofusin 1 and 2, and optic atrophy protein 1. Diminished mitochondrial dynamics can therefore be assumed. After down-regulation of Drp1 in INS1 cells and spread mouse islets the initially homogenous mitochondrial network characterised by a moderate level of interconnections shifted towards high heterogeneity with elongated, clustered and looped mitochondria. These morphological changes were found to correlate directly with functional alterations. Mitochondrial membrane potential and ATP generation were significantly reduced in INS1 cells after Drp1down-regulation. Finally, a significant loss of glucose-stimulated insulin secretion was demonstrated in INS1 cells and mouse pancreatic islets. In conclusion, Drp1 expression is important in pancreatic beta cells to maintain the regulation of insulin secretion.

Mitochondria and Antiviral Immunity

Mitochondria are unique dynamic organelles that evolved from free-living bacteria into endosymbionts of mammalian hosts (Sagan 1967; Hatefi 1985). They have a distinct ~16.6 kb closed circular DNA genome coding for 13 polypeptides (Taanman 1999). In addition, a majority of the ~1500 mitochondrial proteins are encoded in the nucleus and transported to the mitochondria (Bonawitz et al. 2006). Mitochondria have two membranes: an outer smooth membrane and a highly folded inner membrane called cristae, which encompasses the matrix that houses the enzymes of the tricarboxylic acid (TCA) cycle and lipid metabolism. The inner mitochondrial membrane houses the protein complexes comprising the electron transport chain (ETC) (Hatefi 1985).

Inhibition of Mitochondrial Fission Protein Reduced Mechanical Allodynia and Suppressed Spinal Mitochondrial Superoxide Induced by Perineural Human Immunodeficiency Virus gp120 in Rats

BACKGROUND

Mitochondria play an important role in many cellular and physiologic functions. Mitochondria are dynamic organelles, and their fusion and fission regulate cellular signaling, development, and mitochondrial homeostasis. The most common complaint of human immunodeficiency virus (HIV)-sensory neuropathy is pain on the soles in patients with HIV, but the exact molecular mechanisms of HIV neuropathic pain are not clear. In the present study, we investigated the role of mitochondrial dynamin-related protein 1 (Drp1, a GTPase that mediates mitochondrial fission) in the perineural HIV coat glycoprotein gp120-induced neuropathic pain state.

METHODS

Neuropathic pain was induced by the application of recombinant HIV-1 envelope protein gp120 into the sciatic nerve. Mechanical threshold was tested using von Frey filaments. The mechanical threshold response was assessed over time using the area under curves. Intrathecal administration of antisense oligodeoxynucleotide (ODN) against Drp1, mitochondrial division inhibitor-1 (mdivi-1), or phenyl-N-tert-butylnitrone (a reactive oxygen species scavenger) was given. The expression of spinal Drp1 was examined using western blots. The expression of mitochondrial superoxide in the spinal dorsal horn was examined using MitoSox imaging.

RESULTS

Intrathecal administration of either antisense ODN against Drp1 or mdivi-1 decreased mechanical allodynia (a sensation of pain evoked by nonpainful stimuli) in the gp120 model. Intrathecal ODN or mdivi-1 did not change basic mechanical threshold in sham surgery rats. Intrathecal Drp1 antisense ODN decreased the spinal expression of increased Drp1 protein induced by peripheral gp120 application. Intrathecal phenyl-N-tert-butylnitrone reduced mechanical allodynia. Furthermore, both intrathecal Drp1 antisense ODN and mdivi-1 reversed the upregulation of mitochondrial superoxide in the spinal dorsal horn in the gp120 neuropathic pain state.

CONCLUSIONS

These data suggest that mitochondrial division plays a substantial role in the HIV gp120-related neuropathic pain state through mitochondrial reactive oxygen species and provides evidence for a novel approach to treating chronic pain in patients with HIV.

MIRO GTPases in Mitochondrial Transport, Homeostasis and Pathology

The evolutionarily-conserved mitochondrial Rho (MIRO) small GTPase is a Ras superfamily member with three unique features. It has two GTPase domains instead of the one found in other small GTPases, and it also has two EF hand calcium binding domains, which allow Ca²⁺-dependent modulation of its activity and functions. Importantly, it is specifically associated with the mitochondria and via a hydrophobic transmembrane domain, rather than a lipid-based anchor more commonly found in other small GTPases. At the mitochondria, MIRO regulates mitochondrial homeostasis and turnover. In metazoans, MIRO regulates mitochondrial transport and organization at cellular extensions, such as axons, and, in some cases, intercellular transport of the organelle through tunneling nanotubes. Recent findings have revealed a myriad of molecules that are associated with MIRO, particularly the kinesin adaptor Milton/TRAK, mitofusin, PINK1 and Parkin, as well as the endoplasmic reticulum-mitochondria encounter structure (ERMES) complex. The mechanistic aspects of the roles of MIRO and its interactors in mitochondrial homeostasis and transport are gradually being revealed. On the other hand, MIRO is also increasingly associated with neurodegenerative diseases that have roots in mitochondrial dysfunction. In this review, I discuss what is currently known about the cellular physiology and pathophysiology of MIRO functions.

Mitochondrial fusion/fission dynamics in neurodegeneration and neuronal plasticity

Mitochondria are dynamic organelles that continually move, fuse and divide. The dynamic balance of fusion and fission of mitochondria determines their morphology and allows their immediate adaptation to energetic needs, keeps mitochondria in good health by restoring or removing damaged organelles or precipitates cells in apoptosis in cases of severe defects. Mitochondrial fusion and fission are essential in mammals and their disturbances are associated with several diseases. However, while mitochondrial fusion/fission dynamics, and the proteins that control these processes, are ubiquitous, associated diseases are primarily neurological disorders. Accordingly, inactivation of the main actors of mitochondrial fusion/fission dynamics is associated with defects in neuronal development, plasticity and functioning, both ex vivo and in vivo. Here, we present the central actors of mitochondrial fusion and fission and review the role of mitochondrial dynamics in neuronal physiology and pathophysiology. Particular emphasis is placed on the three main actors of these processes i.e. DRP1,MFN1-2, and OPA1 as well as on GDAP1, a protein of the mitochondrial outer membrane preferentially expressed in neurons. This article is part of a Special Issue entitled: Mitochondria & Brain.

Proliferation and fission of peroxisomes - An update

In mammals, peroxisomes perform crucial functions in cellular metabolism, signalling and viral defense which are essential to the health and viability of the organism. In order to achieve this functional versatility peroxisomes dynamically respond to molecular cues triggered by changes in the cellular environment. Such changes elicit a corresponding response in peroxisomes, which manifests itself as a change in peroxisome number, altered enzyme levels and adaptations to the peroxisomal structure. In mammals the generation of new peroxisomes is a complex process which has clear analogies to mitochondria, with both sharing the same division machinery and undergoing a similar division process. How the regulation of this division process is integrated into the cell's response to different stimuli, the signalling pathways and factors involved, remains somewhat unclear. Here, we discuss the mechanism of peroxisomal fission, the contributions of the various division factors and examine the potential impact of post-translational modifications, such as phosphorylation, on the proliferation process. We also summarize the signalling process and highlight the most recent data linking signalling pathways with peroxisome proliferation.

Metabolic dysfunction and altered mitochondrial dynamics in the utrophin-dystrophin deficient mouse model of duchenne muscular dystrophy

The utrophin-dystrophin deficient (DKO) mouse model has been widely used to understand the progression of Duchenne muscular dystrophy (DMD). However, it is unclear as to what extent muscle pathology affects metabolism. Therefore, the present study was focused on understanding energy expenditure in the whole animal and in isolated extensor digitorum longus (EDL) muscle and to determine changes in metabolic enzymes. Our results show that the 8 week-old DKO mice consume higher oxygen relative to activity levels. Interestingly the EDL muscle from DKO mouse consumes higher oxygen per unit integral force, generates less force and performs better in the presence of pyruvate thus mimicking a slow twitch muscle. We also found that the expression of hexokinase 1 and pyruvate kinase M2 was upregulated several fold suggesting increased glycolytic flux. Additionally, there is a dramatic increase in dynamin-related protein 1 (Drp 1) and mitofusin 2 protein levels suggesting increased mitochondrial fission and fusion, a feature associated with increased energy demand and altered mitochondrial dynamics. Collectively our studies point out that the dystrophic disease has caused significant changes in muscle metabolism. To meet the increased energetic demand, upregulation of metabolic enzymes and regulators of mitochondrial fusion and fission is observed in the dystrophic muscle. A better understanding of the metabolic demands and the accompanied alterations in the dystrophic muscle can help us design improved intervention therapies along with existing drug treatments for the DMD patients.

Mitochondrial division and fusion in metabolism

Mitochondria govern many metabolic processes. In addition, mitochondria sense the status of metabolism and change their functions to regulate energy production, cell death, and thermogenesis. Recent studies have revealed that mitochondrial structural remodeling through division and fusion is critical to the organelle's function. It has also become clear that abnormalities in mitochondrial division and fusion are linked to the pathophysiology of metabolic diseases such as diabetes and obesity. Here, we discuss the current understanding of the mechanisms of mitochondrial dynamics and their role in cellular and organismal metabolism.

Mitochondrial Diseases Part II: Mouse models of OXPHOS deficiencies caused by defects in regulatory factors and other components required for mitochondrial function

Mitochondrial disorders are defined as defects that affect the oxidative phosphorylation system (OXPHOS). They are characterized by a heterogeneous array of clinical presentations due in part to a wide variety of factors required for proper function of the components of the OXPHOS system. There is no cure for these disorders owing to our poor knowledge of the pathogenic mechanisms of disease. To understand the mechanisms of human disease numerous mouse models have been developed in recent years. Here we summarize the features of several mouse models of mitochondrial diseases directly related to those factors affecting mtDNA maintenance, replication, transcription, translation as well as other proteins that are involved in mitochondrial dynamics and quality control which affect mitochondrial OXPHOS function without being intrinsic components of the system. We discuss how these models have contributed to our understanding of mitochondrial diseases and their pathogenic mechanisms.

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

Mitochondrial dynamics and mitophagy have been linked to cardiovascular and neurodegenerative diseases. Here, we demonstrate that the mitochondrial division dynamin Drp1 and the Parkinson's disease-associated E3 ubiquitin ligase parkin synergistically maintain the integrity of mitochondrial structure and function in mouse heart and brain. Mice lacking cardiac Drp1 exhibited lethal heart defects. In Drp1KO cardiomyocytes, mitochondria increased their connectivity, accumulated ubiquitinated proteins, and decreased their respiration. In contrast to the current views of the role of parkin in ubiquitination of mitochondrial proteins, mitochondrial ubiquitination was independent of parkin in Drp1KO hearts, and simultaneous loss of Drp1 and parkin worsened cardiac defects. Drp1 and parkin also play synergistic roles in neuronal mitochondrial homeostasis and survival. Mitochondrial degradation was further decreased by combination of Drp1 and parkin deficiency, compared with their single loss. Thus, the physiological importance of parkin in mitochondrial homeostasis is revealed in the absence of mitochondrial division in mammals.

Interaction with the effector dynamin-related protein 1 (Drp1) is an ancient function of Rab32 subfamily proteins

The mitochondria-associated membrane (MAM) is an endoplasmic reticulum (ER) domain that forms contacts with mitochondria and accommodates Ca²⁺ transfer between the two organelles. The GTPase Rab32 regulates this function of the MAM via determining the localization of the Ca²⁺ regulatory transmembrane protein calnexin to the MAM. Another function of the MAM is the regulation of mitochondrial dynamics mediated by GTPases such as dynamin-related protein 1 (Drp1). Consistent with the importance of the MAM for mitochondrial dynamics and the role of Rab32 in MAM enrichment, the inactivation of Rab32 leads to mitochondrial collapse around the nucleus. However, Rab32 and related Rabs also perform intracellular functions at locations other than the MAM including melanosomal trafficking, autophagosome formation and maturation, and retrograde trafficking to the trans-Golgi network (TGN). This plethora of functions raises questions concerning the original cellular role of Rab32 in the last common ancestor of animals and its possible role in the last eukaryotic common ancestor (LECA). Our results now shed light on this conundrum and identify a role in Drp1-mediated mitochondrial dynamics as one common denominator of this group of Rabs, which includes the paralogues Rab32A and Rab32B, as well as the more recently derived Rab29 and Rab38 proteins. Moreover, we provide evidence that this mitochondrial function is dictated by the extent of ER-association of Rab32 family proteins.

Drp1 is dispensable for mitochondria biogenesis in induction to pluripotency but required for differentiation of embryonic stem cells

Mature mitochondria with high oxidative phosphorylation undergo fission and fusion and morphogenesis to become immature mitochondria during induced pluripotent stem (iPS) induction from somatic cells. Dynamin-related protein 1 (Drp1) is involved in mitochondria fission and biogenesis in somatic cells. We tested the role of Drp1 in the induction and maintenance of pluripotency. We show that Drp1 band shift occurs in embryonic stem cells (ESCs) and iPS cells (iPSCs) induced from fibroblasts, in association with mitochondrial morphogenesis. However, knockdown of Drp1 by shRNA does not abrogate mitochondria morphogenesis and induction of iPSCs from fibroblasts. Also, knockdown of Drp1 affects neither mitochondria fission and function as shown by normal mitochondrial membrane potential, nor proliferation and pluripotency of ESCs. Nonetheless, Drp1 knockdown negatively influences terminal differentiation of ESCs, particularly in the lineage of neurogenesis in vitro and in vivo, coincident with delayed reduction of Oct4 and Nanog during mid-differentiation. Our data suggest that Drp1 is not critical for mitochondria biogenesis in stem cell proliferation but it is required for neurogenesis likely by downregulation of pluripotency-associated genes Nanog and Oct4. ESC differentiation model could be used to model role of Drp1 in neuron development and diseases.

Biosynthesis and roles of phospholipids in mitochondrial fusion, division and mitophagy

Mitochondria move, fuse and divide in cells. The dynamic behavior of mitochondria is central to the control of their structure and function. Three conserved mitochondrial dynamin-related GTPases (i.e., mitofusin, Opa1 and Drp1 in mammals and Fzo1, Mgm1 and Dnm1 in yeast) mediate mitochondrial fusion and division. In addition to dynamins, recent studies demonstrated that phospholipids in mitochondria also play key roles in mitochondrial dynamics by interacting with dynamin GTPases and by directly changing the biophysical properties of the mitochondrial membranes. Changes in phospholipid composition also promote mitophagy, which is a selective mitochondrial degradation process that is mechanistically coupled to mitochondrial division. In this review, we will discuss the biogenesis and function of mitochondrial phospholipids.

Cyclin C: an inducer of mitochondrial division hidden in the nucleus

In response to cellular stress, mitochondria remodel their structure by organelle division and fusion. In this issue of Developmental Cell, Cooper et al. (2014) report that a nuclear protein, cyclin C, is recruited from nuclei to mitochondria upon oxidative stress and promotes mitochondrial division and apoptosis of the cell.