A human dynamin-related protein controls the distribution of mitochondria

An evidence based hypothesis on the existence of two pathways of mitochondrial crista formation

Metabolic function and architecture of mitochondria are intimately linked. More than 60 years ago, cristae were discovered as characteristic elements of mitochondria that harbor the protein complexes of oxidative phosphorylation, but how cristae are formed, remained an open question. Here we present experimental results obtained with yeast that support a novel hypothesis on the existence of two molecular pathways that lead to the generation of lamellar and tubular cristae. Formation of lamellar cristae depends on the mitochondrial fusion machinery through a pathway that is required also for homeostasis of mitochondria and mitochondrial DNA. Tubular cristae are formed via invaginations of the inner boundary membrane by a pathway independent of the fusion machinery. Dimerization of the F₁F_O-ATP synthase and the presence of the MICOS complex are necessary for both pathways. The proposed hypothesis is suggested to apply also to higher eukaryotes, since the key components are conserved in structure and function throughout evolution.

DOI:http://dx.doi.org/10.7554/eLife.18853.001

Cells contain compartments called mitochondria, which are often called the powerhouses of the cell because they provide energy that drives vital cellular processes. Mitochondria have two membranes: an outer and an inner membrane. The outer membrane separates the mitochondria from the rest of the cell. The inner membrane is elaborately folded and the folds – called cristae – create a larger space to accommodate all of the protein machinery involved in producing energy. The cristae can be shaped as flat sac-like structures called lamellar cristae or as tubes known as tubular cristae.

Mitochondria are dynamic and are constantly fusing with other mitochondria and splitting up. Even though the internal architecture of mitochondria was first revealed around 60 years ago, it is still not clear how the cristae form. Harner et al. now address this question in yeast cells by combining imaging, biochemistry and genetic approaches.

The experiments show that lamellar cristae form when two mitochondria fuse with each other. The outer membranes merge and then the inner membranes start to fuse around their edges to generate the sac-like structure of lamellar cristae. A yeast protein called Mgm1 (known as Opa1 in mammals) drives the fusion of the inner membranes, but this process only takes place when enzymes called F₁F_O-ATP synthases on the inner membrane form pairs with one another. These F₁F_O-ATP synthase pairs stabilize the cristae membranes as they curve to form the sac-like structure. Later on, the formation of a group of proteins called the MICOS complex halts the fusion process to prevent the lamellar cristae from completely separating from the rest of the inner membrane.

Harner et al. also found that tubular cristae form using a different mechanism when the inner membrane of the mitochondria grows inwards. This process also requires pairs of F₁F_O-ATP synthases and the MICOS complex, but does not involve Mgm1/Opa1. Together, these findings show that lamellar and tubular cristae in yeast form using two different mechanisms. Since the key components of these mechanisms are also found in virtually all other eukaryotes, the findings of Harner et al. are also likely to apply to many other organisms including animals.

DOI:http://dx.doi.org/10.7554/eLife.18853.002

Dysfunctional mitochondrial fission impairs cell reprogramming

We have recently shown that mitochondrial fission is induced early in reprogramming in a Drp1-dependent manner; however, the identity of the factors controlling Drp1 recruitment to mitochondria was unexplored. To investigate this, we used a panel of RNAi targeting factors involved in the regulation of mitochondrial dynamics and we observed that MiD51, Gdap1 and, to a lesser extent, Mff were found to play key roles in this process. Cells derived from Gdap1-null mice were used to further explore the role of this factor in cell reprogramming. Microarray data revealed a prominent down-regulation of cell cycle pathways in Gdap1-null cells early in reprogramming and cell cycle profiling uncovered a G2/M growth arrest in Gdap1-null cells undergoing reprogramming. High-Content analysis showed that this growth arrest was DNA damage-independent. We propose that lack of efficient mitochondrial fission impairs cell reprogramming by interfering with cell cycle progression in a DNA damage-independent manner.

ER-mitochondria contacts couple mtDNA synthesis with mitochondrial division in human cells

Mitochondrial DNA (mtDNA) encodes RNAs and proteins critical for cell function. In human cells, hundreds to thousands of mtDNA copies are replicated asynchronously, packaged into protein-DNA nucleoids, and distributed within a dynamic mitochondrial network. The mechanisms that govern how nucleoids are chosen for replication and distribution are not understood. Mitochondrial distribution depends on division, which occurs at endoplasmic reticulum (ER)-mitochondria contact sites. These sites were spatially linked to a subset of nucleoids selectively marked by mtDNA polymerase and engaged in mtDNA synthesis--events that occurred upstream of mitochondrial constriction and division machine assembly. Our data suggest that ER tubules proximal to nucleoids are necessary but not sufficient for mtDNA synthesis. Thus, ER-mitochondria contacts coordinate licensing of mtDNA synthesis with division to distribute newly replicated nucleoids to daughter mitochondria.

Quantification and Characterization of UVB-Induced Mitochondrial Fragmentation in Normal Primary Human Keratinocytes

UV irradiation is a major environmental factor causing skin dryness, aging and cancer. UVB in particular triggers cumulative DNA damage, oxidative stress and mitochondrial dysfunction. The objective of our study was to provide both qualitative and quantitative analysis of how mitochondria respond to UVB irradiation in normal human epidermal keratinocytes (NHEK) of healthy donors, with the rationale that monitoring mitochondrial shape will give an indication of cell population fitness and enable the screening of bioactive agents with UVB-protective properties. Our results show that NHEK undergo dose-dependent mitochondrial fragmentation after exposure to UVB. In order to obtain a quantitative measure of this phenomenon, we implemented a novel tool for automated quantification of mitochondrial morphology in live cells based on confocal microscopy and computational calculations of mitochondrial shape descriptors. This method was used to substantiate the effects on mitochondrial morphology of UVB irradiation and of knocking-down the mitochondrial fission-mediating GTPase Dynamin-related protein 1 (DRP1). Our data further indicate that all the major mitochondrial dynamic proteins are expressed in NHEK but that their level changes were stronger after mitochondrial uncoupler treatment than following UVB irradiation or DRP1 knock-down. Our system and procedures might be of interest for the identification of cosmetic or dermatologic UVB-protective agents.

Dynamic actin cycling through mitochondrial subpopulations locally regulates the fission-fusion balance within mitochondrial networks

Mitochondria form interconnected networks that dynamically remodel in response to cellular needs. Using live-cell imaging, we investigate the role of the actin cytoskeleton in regulating mitochondrial fission and fusion. We identify cycling of actin filaments onto and off of subsets of cellular mitochondria. The association of actin filaments with mitochondrial subpopulations is transient; actin quickly disassembles, then reassembles around a distinct subpopulation, efficiently cycling through all cellular mitochondria within 14 min. The focal assembly of actin induces local, Drp1-dependent fragmentation of the mitochondrial network. On actin disassembly, fragmented mitochondria undergo rapid fusion, leading to regional recovery of the tubular mitochondrial network. Cycling requires dynamic actin polymerization and is blocked by inhibitors of both Arp2/3 and formins. We propose that cyclic assembly of actin onto mitochondria modulates the fission/fusion balance, promotes network remodelling and content mixing, and thus may serve as an essential mechanism regulating mitochondrial network homeostasis.

Mitochondria are dynamic organelles that can undergo fission and fusion. Here the authors identify a novel pathway in which actin dynamically assembles in an Arp2/3- and formin-dependent manner around a subset of cellular mitochondria, promoting localized Drp1-dependent fission and impeding fusion.

CHCHD10 mutations promote loss of mitochondrial cristae junctions with impaired mitochondrial genome maintenance and inhibition of apoptosis

CHCHD10-related diseases include mitochondrial DNA instability disorder, frontotemporal dementia-amyotrophic lateral sclerosis (FTD-ALS) clinical spectrum, late-onset spinal motor neuropathy (SMAJ), and Charcot-Marie-Tooth disease type 2 (CMT2). Here, we show that CHCHD10 resides with mitofilin, CHCHD3 and CHCHD6 within the "mitochondrial contact site and cristae organizing system" (MICOS) complex. CHCHD10 mutations lead to MICOS complex disassembly and loss of mitochondrial cristae with a decrease in nucleoid number and nucleoid disorganization. Repair of the mitochondrial genome after oxidative stress is impaired in CHCHD10 mutant fibroblasts and this likely explains the accumulation of deleted mtDNA molecules in patient muscle. CHCHD10 mutant fibroblasts are not defective in the delivery of mitochondria to lysosomes suggesting that impaired mitophagy does not contribute to mtDNA instability. Interestingly, the expression of CHCHD10 mutant alleles inhibits apoptosis by preventing cytochrome c release.

Dynamin-related protein Drp1 is required for Bax translocation to mitochondria in response to irradiation-induced apoptosis

Translocation of the pro-apoptotic protein Bax from the cytosol to the mitochondria is a crucial step in DNA damage-mediated apoptosis, and is also found to be involved in mitochondrial fragmentation. Irradiation-induced cytochrome c release and apoptosis was associated with Bax activation, but not mitochondrial fragmentation. Both Bax and Drp1 translocated from the cytosol to the mitochondria in response to irradiation. However, Drp1 mitochondrial translocation and oligomerization did not require Bax, and failed to induce apoptosis in Bax deficient diffuse large B-cell lymphoma (DLBCL) cells. Using fluorescent microscopy and the intensity correlation analysis, we demonstrated that Bax and Drp1 were colocalized and the levels of colocalization were increased by UV irradiation. Using co-immuno-precipitation, we confirmed that Bax and Drp1 were binding partners. Irradiation induced a time-associated increase in the interaction between active Bax and Drp1. Knocking down Drp1 using siRNA blocked UV irradiation-mediated Bax mitochondrial translocation. In conclusion, our findings demonstrate for the first time, that Drp1 is required for Bax mitochondrial translocation, but Drp1-induced mitochondrial fragmentation alone is not sufficient to induce apoptosis in DLBCL cells.

T-cell-restricted intracellular antigen 1 facilitates mitochondrial fragmentation by enhancing the expression of mitochondrial fission factor

Mitochondrial morphology is dynamically regulated by the formation of small fragmented units or interconnected mitochondrial networks, and this dynamic morphological change is a pivotal process in normal mitochondrial function. In the present study, we identified a novel regulator responsible for the regulation of mitochondrial dynamics. An assay using CHANG liver cells stably expressing mitochondrial-targeted yellow fluorescent protein (mtYFP) and a group of siRNAs revealed that T-cell intracellular antigen protein-1 (TIA-1) affects mitochondrial morphology by enhancing mitochondrial fission. The function of TIA-1 in mitochondrial dynamics was investigated through various biological approaches and expression analysis in human specimen. Downregulation of TIA-1-enhanced mitochondrial elongation, whereas ectopic expression of TIA-1 resulted in mitochondria fragmentation. In addition, TIA-1 increased mitochondrial activity, including the rate of ATP synthesis and oxygen consumption. Further, we identified mitochondrial fission factor (MFF) as a direct target of TIA-1, and showed that TIA-1 promotes mitochondrial fragmentation by enhancing MFF translation. TIA-1 null cells had a decreased level of MFF and less mitochondrial Drp1, a critical factor for mitochondrial fragmentation, thereby enhancing mitochondrial elongation. Taken together, our results indicate that TIA-1 is a novel factor that facilitates mitochondrial dynamics by enhancing MFF expression and contributes to mitochondrial dysfunction.

Dengue Virus Perturbs Mitochondrial Morphodynamics to Dampen Innate Immune Responses

With no antiviral drugs or widely available vaccines, Dengue virus (DENV) constitutes a public health concern. DENV replicates at ER-derived cytoplasmic structures that include substructures called convoluted membranes (CMs); however, the purpose of these membrane alterations remains unclear. We determine that DENV nonstructural protein (NS)4B, a promising drug target with unknown function, associates with mitochondrial proteins and alters mitochondria morphology to promote infection. During infection, NS4B induces elongation of mitochondria, which physically contact CMs. This restructuring compromises the integrity of mitochondria-associated membranes, sites of ER-mitochondria interface critical for innate immune signaling. The spatio-temporal parameters of CM biogenesis and mitochondria elongation are linked to loss of activation of the fission factor Dynamin-Related Protein-1. Mitochondria elongation promotes DENV replication and alleviates RIG-I-dependent activation of interferon responses. As Zika virus infection induces similar mitochondria elongation, this perturbation may protect DENV and related viruses from innate immunity and create a favorable replicative environment.

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DENV NS4B induces mitochondria elongation during viral infection

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Elongated mitochondria and virus-induced convoluted membranes are physically linked

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NS4B inhibits activation of the mitochondrial fission factor DRP1

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Mitochondria elongation alleviates DENV-induced RIG-I-dependent innate immunity

Low-dose ionizing radiation induces mitochondrial fusion and increases expression of mitochondrial complexes I and III in hippocampal neurons

High energy ionizing radiation can cause DNA damage and cell death. During clinical radiation therapy, the radiation dose could range from 15 to 60 Gy depending on targets. While 2 Gy radiation has been shown to cause cancer cell death, studies also suggest a protective potential by low dose radiation. In this study, we examined the effect of 0.2-2 Gy radiation on hippocampal neurons. Low dose 0.2 Gy radiation treatment increased the levels of MTT. Since hippocampal neurons are post-mitotic, this result reveals a possibility that 0.2 Gy irradiation may increase mitochondrial activity to cope with stimuli. Maintaining neural plasticity is an energy-demanding process that requires high efficient mitochondrial function. We thus hypothesized that low dose radiation may regulate mitochondrial dynamics and function to ensure survival of neurons. Our results showed that five days after 0.2 Gy irradiation, no obvious changes on neuronal survival, neuronal synapses, membrane potential of mitochondria, reactive oxygen species levels, and mitochondrial DNA copy numbers. Interestingly, 0.2 Gy irradiation promoted the mitochondria fusion, resulting in part from the increased level of a mitochondrial fusion protein, Mfn2, and inhibition of Drp1 fission protein trafficking to the mitochondria. Accompanying with the increased mitochondrial fusion, the expressions of complexes I and III of the electron transport chain were also increased. These findings suggest that, hippocampal neurons undergo increased mitochondrial fusion to modulate cellular activity as an adaptive mechanism in response to low dose radiation.

DNM1L-related mitochondrial fission defect presenting as refractory epilepsy

Mitochondrial fission and fusion are dynamic processes vital to mitochondrial quality control and the maintenance of cellular respiration. In dividing mitochondria, membrane scission is accomplished by a dynamin-related GTPase, DNM1L, that oligomerizes at the site of fission and constricts in a GTP-dependent manner. There is only a single previous report of DNM1L-related clinical disease: a female neonate with encephalopathy due to defective mitochondrial and peroxisomal fission (EMPF; OMIM #614388), a lethal disorder characterized by cerebral dysgenesis, seizures, lactic acidosis, elevated very long chain fatty acids, and abnormally elongated mitochondria and peroxisomes. Here, we describe a second individual, diagnosed via whole-exome sequencing, who presented with developmental delay, refractory epilepsy, prolonged survival, and no evidence of mitochondrial or peroxisomal dysfunction on standard screening investigations in blood and urine. EEG was nonspecific, showing background slowing with frequent epileptiform activity at the frontal and central head regions. Electron microscopy of skeletal muscle showed subtle, nonspecific abnormalities of cristal organization, and confocal microscopy of patient fibroblasts showed striking hyperfusion of the mitochondrial network. A panel of further bioenergetic studies in patient fibroblasts showed no significant differences versus controls. The proband's de novo DNM1L variant, NM_012062.4:c.1085G>A; NP_036192.2:p.(Gly362Asp), falls within the middle (oligomerization) domain of DNM1L, implying a likely dominant-negative mechanism. This disorder, which presents nonspecifically and affords few diagnostic clues, can be diagnosed by means of DNM1L sequencing and/or confocal microscopy.

Unexpected mitochondrial matrix localization of Parkinson's disease-related DJ-1 mutants but not wild-type DJ-1

DJ-1 has been identified as a gene responsible for recessive familial Parkinson's disease (familial Parkinsonism), which is caused by a mutation in the PARK7 locus. Consistent with the inferred correlation between Parkinson's disease and mitochondrial impairment, mitochondrial localization of DJ-1 and its implied role in mitochondrial quality control have been reported. However, the mechanism by which DJ-1 affects mitochondrial function remains poorly defined, and the mitochondrial localization of DJ-1 is still controversial. Here, we show the mitochondrial matrix localization of various pathogenic and artificial DJ-1 mutants by multiple independent experimental approaches including cellular fractionation, proteinase K protection assays, and specific immunocytochemistry. Localization of various DJ-1 mutants to the matrix is dependent on the membrane potential and translocase activity in both the outer and the inner membranes. Nevertheless, DJ-1 possesses neither an amino-terminal alpha-helix nor a predictable matrix-targeting signal, and a post-translocation processing-derived molecular weight change is not observed. In fact, wild-type DJ-1 does not show any evidence of mitochondrial localization at all. Such a mode of matrix localization of DJ-1 is difficult to explain by conventional mechanisms and implies a unique matrix import mechanism for DJ-1 mutants.

A role for septin 2 in Drp1-mediated mitochondrial fission

Mitochondria are essential eukaryotic organelles often forming intricate networks. The overall network morphology is determined by mitochondrial fusion and fission. Among the multiple mechanisms that appear to regulate mitochondrial fission, the ER and actin have recently been shown to play an important role by mediating mitochondrial constriction and promoting the action of a key fission factor, the dynamin‐like protein Drp1. Here, we report that the cytoskeletal component septin 2 is involved in Drp1‐dependent mitochondrial fission in mammalian cells. Septin 2 localizes to a subset of mitochondrial constrictions and directly binds Drp1, as shown by immunoprecipitation of the endogenous proteins and by pulldown assays with recombinant proteins. Depletion of septin 2 reduces Drp1 recruitment to mitochondria and results in hyperfused mitochondria and delayed FCCP‐induced fission. Strikingly, septin depletion also affects mitochondrial morphology in Caenorhabditis elegans, strongly suggesting that the role of septins in mitochondrial dynamics is evolutionarily conserved.

The entangled ER-mitochondrial axis as a potential therapeutic strategy in neurodegeneration: A tangled duo unchained

Endoplasmic reticulum (ER) and mitochondrial function have both been shown to be critical events in neurodegenerative diseases. The ER mediates protein folding, maturation, sorting as well acts as calcium storage. The unfolded protein response (UPR) is a stress response of the ER that is activated by the accumulation of misfolded proteins within the ER lumen. Although the molecular mechanisms underlying ER stress-induced apoptosis are not completely understood, increasing evidence suggests that ER and mitochondria cooperate to signal cell death. Similarly, calcium-mediated mitochondrial function and dynamics not only contribute to ATP generation and calcium buffering but are also a linchpin in mediating cell fate. Mitochondria and ER form structural and functional networks (mitochondria-associated ER membranes [MAMs]) essential to maintaining cellular homeostasis and determining cell fate under various pathophysiological conditions. Regulated Ca(2+) transfer from the ER to the mitochondria is important in maintaining control of pro-survival/pro-death pathways. In this review, we summarize the latest therapeutic strategies that target these essential organelles in the context of neurodegenerative diseases.

Mitochondrial dynamics, quality control and miRNA regulation in skeletal muscle: implications for obesity and related metabolic disease

The western dietary habits and sedentary lifestyle largely contributes to the growing epidemic of obesity. Mitochondria are at the front line of cellular energy homoeostasis and are implicated in the pathophysiology of obesity and obesity-related metabolic disease. In recent years, novel aspects in the regulation of mitochondrial metabolism, such as mitochondrial dynamics, mitochondrial protein quality control and post-transcriptional regulation of genes coding for mitochondrial proteins, have emerged. In this review, we discuss the recent findings concerning the dysregulation of these processes in skeletal muscle in obesogenic conditions.

SAMM50 Affects Mitochondrial Morphology through the Association of Drp1 in Mammalian Cells

Mitochondrial fission and fusion activities are important for cell survival and function. Drp1 is a GTPase protein responsible for mitochondrial division, and SAMM50 is responsible for protein sorting and assembly. We demonstrated that SAMM50 overexpression results in Drp1-dependent mitochondrial fragmentation in HeLa cells. However, the mitochondrial fragmentation induced by SAMM50 overexpression could be reversed through co-expression with MFN2. Furthermore, SAMM50 interacts with Drp1 both in vivo and in vitro. The mitochondria in SAMM50 knockdown HeLa cells displayed a swollen phenotype, and the levels of the SAM complex and OPA1, along with the mitochondrial Drp1 levels, significantly decreased. In addition, mitochondrial inheritance was impaired in SAMM50 silenced cells. These results suggest that SAMM50 affects the Drp1-dependent mitochondrial morphology.

Mitochondrial fragmentation in excitotoxicity requires ROCK activation

Mitochondria morphology constantly changes through fission and fusion processes that regulate mitochondrial function, and it therefore plays a prominent role in cellular homeostasis. Cell death progression is associated with mitochondrial fission. Fission is mediated by the mainly cytoplasmic Drp1, which is activated by different post-translational modifications and recruited to mitochondria to perform its function. Our research and other studies have shown that in the early moments of excitotoxic insult Drp1 must be nitrosylated to mediate mitochondrial fragmentation in neurons. Nonetheless, mitochondrial fission is a multistep process in which filamentous actin assembly/disassembly and myosin-mediated mitochondrial constriction play prominent roles. Here we establish that in addition to nitric oxide production, excitotoxicity-induced mitochondrial fragmentation also requires activation of the actomyosin regulator ROCK. Although ROCK1 has been shown to phosphorylate and activate Drp1, experiments using phosphor-mutant forms of Drp1 in primary cortical neurons indicate that in excitotoxic conditions, ROCK does not act directly on Drp1 to mediate fission, but may act on the actomyosin complex. Thus, these data indicate that a wider range of signaling pathways than those that target Drp1 are amenable to be inhibited to prevent mitochondrial fragmentation as therapeutic option.

Dynamin-Related Protein 1 Deficiency Improves Mitochondrial Fitness and Protects against Progression of Diabetic Nephropathy

Mitochondrial fission has been linked to the pathogenesis of diabetic nephropathy (DN). However, how mitochondrial fission affects progression of DN in vivo is unknown. Here, we report the effect of conditional podocyte-specific deletion of dynamin-related protein 1 (Drp1), an essential component of mitochondrial fission, on the pathogenesis and progression of DN. Inducible podocyte-specific deletion of Drp1 in diabetic mice decreased albuminuria and improved mesangial matrix expansion and podocyte morphology. Ultrastructure analysis revealed a significant increase in fragmented mitochondria in the podocytes of wild-type diabetic mice but a marked improvement in mitochondrial structure in Drp1-null podocytes of diabetic mice. When isolated from diabetic mice and cultured in high glucose, Drp1-null podocytes had more elongated mitochondria and better mitochondrial fitness associated with enhanced oxygen consumption and ATP production than wild-type podocytes. Furthermore, administration of a pharmacologic inhibitor of Drp1, Mdivi1, significantly blunted mitochondrial fission and rescued key pathologic features of DN in mice. Taken together, these results provide novel correlations between mitochondrial morphology and the progression of DN and point to Drp1 as a potential therapeutic target in DN.

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

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

Multiple faces of dynamin-related protein 1 and its role in Alzheimer's disease pathogenesis

Mitochondria play a large role in neuronal function by constantly providing energy, particularly at synapses. Recent studies suggest that amyloid beta (Aβ) and phosphorylated tau interact with the mitochondrial fission protein, dynamin-related protein 1 (Drp1), causing excessive fragmentation of mitochondria and leading to abnormal mitochondrial dynamics and synaptic degeneration in Alzheimer's disease (AD) neurons. Recent research also revealed Aβ-induced and phosphorylated tau-induced changes in mitochondria, particularly affecting mitochondrial shape, size, distribution and axonal transport in AD neurons. These changes affect mitochondrial health and, in turn, could affect synaptic function and neuronal damage and ultimately leading to memory loss and cognitive impairment in patients with AD. This article highlights recent findings in the role of Drp1 in AD pathogenesis. This article also highlights Drp1 and its relationships to glycogen synthase kinase 3, cyclin-dependent kinase 5, p53, and microRNAs in AD pathogenesis.

Mitochondrial dynamics regulating chemoresistance in gynecological cancers

Chemoresistance enables cancer cells to evade apoptotic stimuli and leads to poor clinical prognosis. It arises from dysregulation of signaling factors responsible for inducing cell proliferation and death and for modulating the microenvironment. In gynecologic cancers, p53 is a pivotal determinant of cisplatin sensitivity, while BCL-2 family members are associated with taxane sensitivity. Mitochondria fusion and fission dynamics are required for many mitochondrial functions and are also involved in mitochondria-mediated apoptosis, which is closely associated with chemosensitivity. Mitochondrial dynamics are controlled by a number of intracellular proteins, including fusion (Opa1 and mitofusion 1 and 2) and fission proteins (Drp1 and Fis1), which can be proapoptotic or antiapoptotic, depending on the cell types, status, and stimuli from the microenvironment. This paper describes the role of mitochondrial dynamics in the mechanism of chemoresistance and the evidence supporting a significant contribution of a hyperfusion state to chemoresistance in gynecological cancers. Moreover, we discuss our findings showing that enforced fission induces apoptosis of cancer cells and sensitizes them to chemotherapeutic agents. Understanding the regulation of mitochondrial dynamics in chemoresistance may provide insight into new biomarkers that better predict cancer chemosensitivity and may aid the development of effective therapeutic strategies for clinical management of gynecologic cancers.

Mitochondrial fission protein Drp1 regulates mitochondrial transport and dendritic arborization in cerebellar Purkinje cells

Mitochondria dynamically change their shape by repeated fission and fusion in response to physiological and pathological conditions. Recent studies have uncovered significant roles of mitochondrial fission and fusion in neuronal functions, such as neurotransmission and spine formation. However, the contribution of mitochondrial fission to the development of dendrites remains controversial. We analyzed the function of the mitochondrial fission GTPase Drp1 in dendritic arborization in cerebellar Purkinje cells. Overexpression of a dominant-negative mutant of Drp1 in postmitotic Purkinje cells enlarged and clustered mitochondria, which failed to exit from the soma into the dendrites. The emerging dendrites lacking mitochondrial transport remained short and unstable in culture and in vivo. The dominant-negative Drp1 affected neither the basal respiratory function of mitochondria nor the survival of Purkinje cells. Enhanced ATP supply by creatine treatment, but not reduced ROS production by antioxidant treatment, restored the hypomorphic dendrites caused by inhibition of Drp1 function. Collectively, our results suggest that Drp1 is required for dendritic distribution of mitochondria and thereby regulates energy supply in growing dendritic branches in developing Purkinje cells.

Dynamin-related Protein 1 Oligomerization in Solution Impairs Functional Interactions with Membrane-anchored Mitochondrial Fission Factor

Mitochondrial fission is a crucial cellular process mediated by the mechanoenzymatic GTPase, dynamin-related protein 1 (Drp1). During mitochondrial division, Drp1 is recruited from the cytosol to the outer mitochondrial membrane by one, or several, integral membrane proteins. One such Drp1 partner protein, mitochondrial fission factor (Mff), is essential for mitochondrial division, but its mechanism of action remains unexplored. Previous studies have been limited by a weak interaction between Drp1 and Mff in vitro. Through refined in vitro reconstitution approaches and multiple independent assays, we show that removal of the regulatory variable domain (VD) in Drp1 enhances formation of a functional Drp1-Mff copolymer. This protein assembly exhibits greatly stimulated cooperative GTPase activity in solution. Moreover, when Mff was anchored to a lipid template, to mimic a more physiologic environment, significant stimulation of GTPase activity was observed with both WT and ΔVD Drp1. Contrary to recent findings, we show that premature Drp1 self-assembly in solution impairs functional interactions with membrane-anchored Mff. Instead, dimeric Drp1 species are selectively recruited by Mff to initiate assembly of a functional fission complex. Correspondingly, we also found that the coiled-coil motif in Mff is not essential for Drp1 interactions, but rather serves to augment cooperative self-assembly of Drp1 proximal to the membrane. Taken together, our findings provide a mechanism wherein the multimeric states of both Mff and Drp1 regulate their collaborative interaction.

Dynamin-related protein 1 controls the migration and neuronal differentiation of subventricular zone-derived neural progenitor cells

Mitochondria are important in many essential cellular functions, including energy production, calcium homeostasis, and apoptosis. The organelles are scattered throughout the cytoplasm, but their distribution can be altered in response to local energy demands, such as cell division and neuronal maturation. Mitochondrial distribution is closely associated with mitochondrial fission, and blocking the fission-promoting protein dynamin-related protein 1 (Drp1) activity often results in mitochondrial elongation and clustering. In this study, we observed that mitochondria were preferentially localized at the leading process of migratory adult neural stem cells (aNSCs), whereas neuronal differentiating cells transiently exhibited perinuclear condensation of mitochondria. Inhibiting Drp1 activity altered the typical migratory cell morphology into round shapes while the polarized mitochondrial distribution was maintained. With these changes, aNSCs failed to migrate, and neuronal differentiation was prevented. Because Drp1 blocking also impaired the mitochondrial membrane potential, we tested whether supplementing with L-carnitine, a compound that restores mitochondrial membrane potential and ATP synthesis, could revert the defects induced by Drp1 inhibition. Interestingly, L-carnitine fully restored the aNSC defects, including cell shrinkage, migration, and impaired neuronal differentiation. These results suggest that Drp1 is required for functionally active mitochondria, and supplementing with ATP can restore the defects induced by Drp1 suppression.

Heterogeneous nuclear ribonucleoprotein A1 post-transcriptionally regulates Drp1 expression in neuroblastoma cells

Excessive mitochondrial fission is associated with the pathogenesis of neurodegenerative diseases. Dynamin-related protein 1 (Drp1) possesses specific fission activity in the mitochondria and peroxisomes. Various post-translational modifications of Drp1 are known to modulate complex mitochondrial dynamics. However, the post-transcriptional regulation of Drp1 remains poorly understood. Here, we show that the heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) regulates Drp1 expression at the post-transcriptional level. hnRNP A1 directly interacts with Drp1 mRNA at its 3′UTR region, and enhances translation potential without affecting mRNA stability. Down-regulation of hnRNP A1 induces mitochondrial elongation by reducing Drp1 expression. Moreover, depletion of hnRNP A1 suppresses 3-NP-mediated mitochondrial fission and dysfunction. In contrast, over-expression of hnRNP A1 promotes mitochondrial fragmentation by increasing Drp1 expression. Additionally, hnRNP A1 significantly exacerbates 3-NP-induced mitochondrial dysfunction and cell death in neuroblastoma cells. Interestingly, treatment with 3-NP induces subcellular translocation of hnRNP A1 from the nucleus to the cytoplasm, which accelerates the increase in Drp1 expression in hnRNP A1 over-expressing cells. Collectively, our findings suggest that hnRNP A1 controls mitochondrial dynamics by post-transcriptional regulation of Drp1.

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hnRNP A1 increases Drp1 expression through the interaction with 3′UTR of Drp1 mRNA.

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Down-regulation of hnRNP A1 increases mitochondrial elongation by reducing drp1 expression.

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Down-regulation of hnRNPA1 inhibits 3-NP-mediated mitochondrial dysfunction.

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Over-expression of hnRNP A1 potentiates 3-NP-mediated mitochondrial dysfunction and cell death.

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Treatment of 3-NP promotes translocation of hnRNP A1 to the cytoplasm and enhances Drp1 expression.

Splitting up the powerhouse: structural insights into the mechanism of mitochondrial fission

Mitochondria are dynamic organelles whose shape is regulated by the opposing processes of fission and fusion, operating in conjunction with organelle distribution along the cytoskeleton. The importance of fission and fusion homeostasis has been highlighted by a number of disease states linked to mutations in proteins involved in regulating mitochondrial morphology, in addition to changes in mitochondrial dynamics in Alzheimer's, Huntington's and Parkinson's diseases. While a number of mitochondrial morphology proteins have been identified, how they co-ordinate to assemble the fission apparatus is not clear. In addition, while the master mediator of mitochondrial fission, dynamin-related protein 1, is conserved throughout evolution, the adaptor proteins involved in its mitochondrial recruitment are not. This review focuses on our current understanding of mitochondrial fission and the proteins that regulate this process in cell homeostasis, with a particular focus on the recent mechanistic insights based on protein structures.

Molecular mechanisms for mitochondrial adaptation to exercise training in skeletal muscle

Exercise training enhances physical performance and confers health benefits, largely through adaptations in skeletal muscle. Mitochondrial adaptation, encompassing coordinated improvements in quantity (content) and quality (structure and function), is increasingly recognized as a key factor in the beneficial outcomes of exercise training. Exercise training has long been known to promote mitochondrial biogenesis, but recent work has demonstrated that it has a profound impact on mitochondrial dynamics (fusion and fission) and clearance (mitophagy), as well. In this review, we discuss the various mechanisms through which exercise training promotes mitochondrial quantity and quality in skeletal muscle.

Death-associated Protein 3 Regulates Mitochondrial-encoded Protein Synthesis and Mitochondrial Dynamics

Mitochondrial morphologies change over time and are tightly regulated by dynamic machinery proteins such as dynamin-related protein 1 (Drp1), mitofusion 1/2, and optic atrophy 1 (OPA1). However, the detailed mechanisms of how these molecules cooperate to mediate fission and fusion remain elusive. DAP3 is a mitochondrial ribosomal protein that involves in apoptosis, but its biological function has not been well characterized. Here, we demonstrate that DAP3 specifically localizes in the mitochondrial matrix. Knockdown of DAP3 in mitochondria leads to defects in mitochondrial-encoded protein synthesis and abnormal mitochondrial dynamics. Moreover, depletion of DAP3 dramatically decreases the phosphorylation of Drp1 at Ser-637 on mitochondria, enhancing the retention time of Drp1 puncta on mitochondria during the fission process. Furthermore, autophagy is inhibited in the DAP3-depleted cells, which sensitizes cells to different types of death stimuli. Together, our results suggest that DAP3 plays important roles in mitochondrial function and dynamics, providing new insights into the mechanism of a mitochondrial ribosomal protein function in cell death.

Bax/Bak-dependent, Drp1-independent Targeting of X-linked Inhibitor of Apoptosis Protein (XIAP) into Inner Mitochondrial Compartments Counteracts Smac/DIABLO-dependent Effector Caspase Activation

Efficient apoptosis requires Bax/Bak-mediated mitochondrial outer membrane permeabilization (MOMP), which releases death-promoting proteins cytochrome c and Smac to the cytosol, which activate apoptosis and inhibit X-linked inhibitor of apoptosis protein (XIAP) suppression of executioner caspases, respectively. We recently identified that in response to Bcl-2 homology domain 3 (BH3)-only proteins and mitochondrial depolarization, XIAP can permeabilize and enter mitochondria. Consequently, XIAP E3 ligase activity recruits endolysosomes into mitochondria, resulting in Smac degradation. Here, we explored mitochondrial XIAP action within the intrinsic apoptosis signaling pathway. Mechanistically, we demonstrate that mitochondrial XIAP entry requires Bax or Bak and is antagonized by pro-survival Bcl-2 proteins. Moreover, intramitochondrial Smac degradation by XIAP occurs independently of Drp1-regulated cytochrome c release. Importantly, mitochondrial XIAP actions are activated cell-intrinsically by typical apoptosis inducers TNF and staurosporine, and XIAP overexpression reduces the lag time between the administration of an apoptotic stimuli and the onset of mitochondrial permeabilization. To elucidate the role of mitochondrial XIAP action during apoptosis, we integrated our findings within a mathematical model of intrinsic apoptosis signaling. Simulations suggest that moderate increases of XIAP, combined with mitochondrial XIAP preconditioning, would reduce MOMP signaling. To test this scenario, we pre-activated XIAP at mitochondria via mitochondrial depolarization or by artificially targeting XIAP to the intermembrane space. Both approaches resulted in suppression of TNF-mediated caspase activation. Taken together, we propose that XIAP enters mitochondria through a novel mode of mitochondrial permeabilization and through Smac degradation can compete with canonical MOMP to act as an anti-apoptotic tuning mechanism, reducing the mitochondrial contribution to the cellular apoptosis capacity.

Beta-nodavirus B2 protein induces hydrogen peroxide production, leading to Drp1-recruited mitochondrial fragmentation and cell death via mitochondrial targeting

Because the role of the viral B2 protein in the pathogenesis of nervous necrosis virus infection remains unknown, the aim of the present study was to determine the effects of B2 protein on hydrogen peroxide (H₂O₂)-mediated cell death via mitochondrial targeting. Using a B2 deletion mutant, the B2 mitochondrial targeting signal sequence (⁴¹RTFVISAHAA⁵⁰) correlated with mitochondrial free radical production and cell death in fish cells, embryonic zebrafish, and human cancer cells. After treatment of grouper fin cells (GF-1) overexpressing B2 protein with the anti-oxidant drug, N-acetylcysteine (NAC), and overexpression of the antioxidant enzymes, zfCu/Zn superoxide dismutase (SOD) and zfCatalase, decreased H₂O₂ production and cell death were observed. To investigate the correlation between B2 cytotoxicity and H₂O₂ production in vivo, B2 was injected into zebrafish embryos. Cell damage, as assessed by the acridine orange assay, gradually increased over 24 h post-fertilization, and was accompanied by marked increases in H₂O₂ production and embryonic death. Increased oxidative stress, as evidenced by the up-regulation of Mn SOD, catalase, and Nrf2, was also observed during this period. Finally, B2-induced dynamin-related protein 1 (Drp1)-mediated mitochondrial fragmentation and cell death could be reversed by NAC and inhibitors of Drp1 and Mdivi in GF-1 cells. Taken together, betanodavirus B2 induces H₂O₂ production via targeting the mitochondria, where it inhibits complex II function. H₂O₂ activates Drp1, resulting in its association with the mitochondria, mitochondrial fission and cell death in vitro and in vivo.

Mitochondrial Fragmentation Due to Inhibition of Fusion Increases Cyclin B through Mitochondrial Superoxide Radicals

During the cell cycle, mitochondria undergo regulated changes in morphology. Two particularly interesting events are first, mitochondrial hyperfusion during the G₁-S transition and second, fragmentation during entry into mitosis. The mitochondria remain fragmented between late G₂- and mitotic exit. This mitotic mitochondrial fragmentation constitutes a checkpoint in some cell types, of which little is known. We bypass the ‘mitotic mitochondrial fragmentation’ checkpoint by inducing fragmented mitochondrial morphology and then measure the effect on cell cycle progression. Using Drosophila larval hemocytes, Drosophila S2R⁺ cell and cells in the pouch region of wing imaginal disc of Drosophila larvae we show that inhibiting mitochondrial fusion, thereby increasing fragmentation, causes cellular hyperproliferation and an increase in mitotic index. However, mitochondrial fragmentation due to over-expression of the mitochondrial fission machinery does not cause these changes. Our experiments suggest that the inhibition of mitochondrial fusion increases superoxide radical content and leads to the upregulation of cyclin B that culminates in the observed changes in the cell cycle. We provide evidence for the importance of mitochondrial superoxide in this process. Our results provide an insight into the need for mitofusin-degradation during mitosis and also help in understanding the mechanism by which mitofusins may function as tumor suppressors.

Mitochondrial damage: an important mechanism of ambient PM2.5 exposure-induced acute heart injury in rats

Epidemiological studies suggested that ambient fine particulate matter (PM2.5) exposure was associated with cardiovascular disease. However, the underlying mechanism, especially the mitochondrial damage mechanism, of PM2.5-induced heart acute injury is still unclear. In this study, the alterations of mitochondrial morphology and mitochondrial fission/fusion gene expression, oxidative stress, calcium homeostasis and inflammation in hearts of rats exposed to PM2.5 with different dosages (0.375, 1.5, 6.0 and 24.0mg/kg body weight) were investigated. The results indicated that the PM2.5 exposure induced pathological changes and ultra-structural damage in hearts such as mitochondrial swell and cristae disorder. Furthermore, PM2.5 exposure significantly increased specific mitochondrial fission/fusion gene (Fis1, Mfn1, Mfn2, Drp1 and OPA1) expression in rat hearts. These changes were accompanied by decreases of activities of superoxide dismutase (SOD), Na(+)K(+)-ATPase and Ca(2+)-ATPase and increases of levels of malondialdehyde (MDA), inducible nitric oxide synthase (iNOS) and nitric oxide (NO) as well as levels of pro-inflammatory mediators including TNF-α, IL-6 and IL-1β in rat hearts. The results implicate that mitochondrial damage, oxidative stress, cellular homeostasis imbalance and inflammation are potentially important mechanisms for the PM2.5-induced heart injury, and may have relations with cardiovascular disease.

Mitofusin 2 as a driver that controls energy metabolism and insulin signaling

SIGNIFICANCE

Mitochondrial dynamics is a complex process that impacts on mitochondrial biology.

RECENT ADVANCES

Recent evidence indicates that proteins participating in mitochondrial dynamics have additional cellular roles. Mitofusin 2 (Mfn2) is a potent modulator of mitochondrial metabolism with an impact on energy metabolism in muscle, liver, and hypothalamic neurons. In addition, Mfn2 is subjected to tight regulation. Hence, factors such as proinflammatory cytokines, lipid availability, or glucocorticoids block its expression, whereas exercise and increased energy expenditure promote its upregulation.

CRITICAL ISSUES

Importantly, Mfn2 controls cell metabolism and insulin signaling by limiting reactive oxygen species production and by modulation of endoplasmic reticulum stress. In this connection, it is critical to understand precisely the molecular mechanisms involved in the global actions of Mfn2.

FUTURE DIRECTIONS

Future directions should concentrate into the analysis of those mechanisms, and to fully demonstrate that Mfn2 represents a cellular hub that senses the metabolic and hormonal milieu and drives the control of metabolic homeostasis.

The mechanoenzymatic core of dynamin-related protein 1 comprises the minimal machinery required for membrane constriction

Mitochondria are dynamic organelles that continually undergo cycles of fission and fusion. Dynamin-related protein 1 (Drp1), a large GTPase of the dynamin superfamily, is the main mediator of mitochondrial fission. Like prototypical dynamin, Drp1 is composed of a mechanochemical core consisting of the GTPase, middle, and GTPase effector domain regions. In place of the pleckstrin homology domain in dynamin, however, Drp1 contains an unstructured variable domain, whose function is not yet fully resolved. Here, using time-resolved EM and rigorous statistical analyses, we establish the ability of full-length Drp1 to constrict lipid bilayers through a GTP hydrolysis-dependent mechanism. We also show the variable domain limits premature Drp1 assembly in solution and promotes membrane curvature. Furthermore, the mechanochemical core of Drp1, absent of the variable domain, is sufficient to mediate GTP hydrolysis-dependent membrane constriction.

Dynamin-related protein 1 as a therapeutic target in cardiac arrest

Despite improvements in cardiopulmonary resuscitation (CPR) quality, defibrillation technologies, and implementation of therapeutic hypothermia, less than 10 % of out-of-hospital cardiac arrest (OHCA) victims survive to hospital discharge. New resuscitation therapies have been slow to develop, in part, because the pathophysiologic mechanisms critical for resuscitation are not understood. During cardiac arrest, systemic cessation of blood flow results in whole body ischemia. CPR and the restoration of spontaneous circulation (ROSC), both result in immediate reperfusion injury of the heart that is characterized by severe contractile dysfunction. Unlike diseases of localized ischemia/reperfusion (IR) injury (myocardial infarction and stroke), global IR injury of organs results in profound organ dysfunction with far shorter ischemic times. The two most commonly injured organs following cardiac arrest resuscitation, the heart and brain, are critically dependent on mitochondrial function. New insights into mitochondrial dynamics and the role of the mitochondrial fission protein Dynamin-related protein 1 (Drp1) in apoptosis have made targeting these mechanisms attractive for IR therapy. In animal models, inhibiting Drp1 following IR injury or cardiac arrest confers protection to both the heart and brain. In this review, the relationship of the major mitochondrial fission protein Drp1 to ischemic changes in the heart and its targeting as a new therapeutic target following cardiac arrest are discussed.

New therapeutics to modulate mitochondrial dynamics and mitophagy in cardiac diseases

The processes that control the number and shape of the mitochondria (mitochondrial dynamics) and the removal of damaged mitochondria (mitophagy) have been the subject of intense research. Recent work indicates that these processes may contribute to the pathology associated with cardiac diseases. This review describes some of the key proteins that regulate these processes and their potential as therapeutic targets for cardiac diseases.

A role for the ancient SNARE syntaxin 17 in regulating mitochondrial division

Recent evidence suggests that endoplasmic reticulum (ER) tubules mark the sites where the GTPase Drp1 promotes mitochondrial fission via a largely unknown mechanism. Here, we show that the SNARE protein syntaxin 17 (Syn17) is present on raft-like structures of ER-mitochondria contact sites and promotes mitochondrial fission by determining Drp1 localization and activity. The hairpin-like C-terminal hydrophobic domain, including Lys-254, but not the SNARE domain, is important for this regulation. Syn17 also regulates ER Ca(2+) homeostasis and interferes with Rab32-mediated regulation of mitochondrial dynamics. Starvation disrupts the Syn17-Drp1 interaction, thus favoring mitochondrial elongation during autophagy. Because we also demonstrate that Syn17 is an ancient SNARE, our findings suggest that Syn17 is one of the original key regulators for ER-mitochondria contact sites present in the last eukaryotic common ancestor. As such, Syn17 acts as a switch that responds to nutrient conditions and integrates functions for the ER and autophagosomes with mitochondrial dynamics.

Mitochondrial fission augments capsaicin-induced axonal degeneration

Capsaicin, an agonist of transient receptor potential vanilloid receptor 1, induces axonal degeneration of peripheral sensory nerves and is commonly used to treat painful sensory neuropathies. In this study, we investigated the role of mitochondrial dynamics in capsaicin-induced axonal degeneration. In capsaicin-treated rodent sensory axons, axonal swellings, decreased mitochondrial stationary site length and reduced mitochondrial transport preceded axonal degeneration. Increased axoplasmic Ca(2+) mediated the alterations in mitochondrial length and transport. While sustaining mitochondrial transport did not reduce axonal swellings in capsaicin-treated axons, preventing mitochondrial fission by overexpression of mutant dynamin-related protein 1 increased mitochondrial length, retained mitochondrial membrane potentials and reduced axonal loss upon capsaicin treatment. These results establish that mitochondrial stationary site size significantly affects axonal integrity and suggest that inhibition of Ca(2+)-dependent mitochondrial fission facilitates mitochondrial function and axonal survival following activation of axonal cationic channels.

miR-27 regulates mitochondrial networks by directly targeting the mitochondrial fission factor

Mitochondrial morphology is dynamically regulated by forming small, fragmented units or interconnected networks, and this is a pivotal process that is used to maintain mitochondrial homeostasis. Although dysregulation of mitochondrial dynamics is related to the pathogenesis of several human diseases, its molecular mechanism is not fully elucidated. In this study, we demonstrate the potential role of miR-27 in the regulation of mitochondrial dynamics. Mitochondrial fission factor (MFF) mRNA is a direct target of miR-27, whose ectopic expression decreases MFF expression through binding to its 3′-untranslated region. Expression of miR-27 results in the elongation of mitochondria as well as an increased mitochondrial membrane potential and mitochondrial ATP level. Our results suggest that miR-27 is a novel regulator affecting morphological mitochondrial changes by targeting MFF.

Acute focal brain damage alters mitochondrial dynamics and autophagy in axotomized neurons

Mitochondria are key organelles for the maintenance of life and death of the cell, and their morphology is controlled by continual and balanced fission and fusion dynamics. A balance between these events is mandatory for normal mitochondrial and neuronal function, and emerging evidence indicates that mitochondria undergo extensive fission at an early stage during programmed cell death in several neurodegenerative diseases. A pathway for selective degradation of damaged mitochondria by autophagy, known as mitophagy, has been described, and is of particular importance to sustain neuronal viability. In the present work, we analyzed the effect of autophagy stimulation on mitochondrial function and dynamics in a model of remote degeneration after focal cerebellar lesion. We provided evidence that lesion of a cerebellar hemisphere causes mitochondria depolarization in axotomized precerebellar neurons associated with PTEN-induced putative kinase 1 accumulation and Parkin translocation to mitochondria, block of mitochondrial fusion by Mfn1 degradation, increase of calcineurin activity and dynamin-related protein 1 translocation to mitochondria, and consequent mitochondrial fission. Here we suggest that the observed neuroprotective effect of rapamycin is the result of a dual role: (1) stimulation of autophagy leading to damaged mitochondria removal and (2) enhancement of mitochondria fission to allow their elimination by mitophagy. The involvement of mitochondrial dynamics and mitophagy in brain injury, especially in the context of remote degeneration after acute focal brain damage, has not yet been investigated, and these findings may offer new target for therapeutic intervention to improve functional outcomes following acute brain damage.

Dynamics of mitochondrial DNA nucleoids regulated by mitochondrial fission is essential for maintenance of homogeneously active mitochondria during neonatal heart development

Mitochondria are dynamic organelles, and their fusion and fission regulate cellular signaling, development, and mitochondrial homeostasis, including mitochondrial DNA (mtDNA) distribution. Cardiac myocytes have a specialized cytoplasmic structure where large mitochondria are aligned into tightly packed myofibril bundles; however, recent studies have revealed that mitochondrial dynamics also plays an important role in the formation and maintenance of cardiomyocytes. Here, we precisely analyzed the role of mitochondrial fission in vivo. The mitochondrial fission GTPase, Drp1, is highly expressed in the developing neonatal heart, and muscle-specific Drp1 knockout (Drp1-KO) mice showed neonatal lethality due to dilated cardiomyopathy. The Drp1 ablation in heart and primary cultured cardiomyocytes resulted in severe mtDNA nucleoid clustering and led to mosaic deficiency of mitochondrial respiration. The functional and structural alteration of mitochondria also led to immature myofibril assembly and defective cardiomyocyte hypertrophy. Thus, the dynamics of mtDNA nucleoids regulated by mitochondrial fission is required for neonatal cardiomyocyte development by promoting homogeneous distribution of active mitochondria throughout the cardiomyocytes.

Mitochondrial fission factor Drp1 maintains oocyte quality via dynamic rearrangement of multiple organelles

Mitochondria are dynamic organelles that change their morphology by active fusion and fission in response to cellular signaling and differentiation. The in vivo role of mitochondrial fission in mammals has been examined by using tissue-specific knockout (KO) mice of the mitochondria fission-regulating GTPase Drp1, as well as analyzing a human patient harboring a point mutation in Drp1, showing that Drp1 is essential for embryonic and neonatal development and neuronal function. During oocyte maturation and aging, structures of various membrane organelles including mitochondria and the endoplasmic reticulum (ER) are changed dynamically, and their organelle aggregation is related to germ cell formation and epigenetic regulation. However, the underlying molecular mechanisms of organelle dynamics during the development and aging of oocytes have not been well understood. Here, we analyzed oocyte-specific mitochondrial fission factor Drp1-deficient mice and found that mitochondrial fission is essential for follicular maturation and ovulation in an age-dependent manner. Mitochondria were highly aggregated with other organelles, such as the ER and secretory vesicles, in KO oocyte, which resulted in impaired Ca(2+) signaling, intercellular communication via secretion, and meiotic resumption. We further found that oocytes from aged mice displayed reduced Drp1-dependent mitochondrial fission and defective organelle morphogenesis, similar to Drp1 KO oocytes. On the basis of these findings, it appears that mitochondrial fission maintains the competency of oocytes via multiorganelle rearrangement.

Dynamin-related protein 1 (Drp1) promotes structural intermediates of membrane division

Drp1 is a dynamin-like GTPase that mediates mitochondrial and peroxisomal division in a process dependent on self-assembly and coupled to GTP hydrolysis. Despite the link between Drp1 malfunction and human disease, the molecular details of its membrane activity remain poorly understood. Here we reconstituted and directly visualized Drp1 activity in giant unilamellar vesicles. We quantified the effect of lipid composition and GTP on membrane binding and remodeling activity by fluorescence confocal microscopy and flow cytometry. In contrast to other dynamin relatives, Drp1 bound to both curved and flat membranes even in the absence of nucleotides. We also found that Drp1 induced membrane tubulation that was stimulated by cardiolipin. Moreover, Drp1 promoted membrane tethering dependent on the intrinsic curvature of the membrane lipids and on GTP. Interestingly, Drp1 concentrated at membrane contact surfaces and, in the presence of GTP, formed discrete clusters on the vesicles. Our findings support a role of Drp1 not only in the formation of lipid tubes but also on the stabilization of tightly apposed membranes, which are intermediate states in the process of mitochondrial fission.

Calcium-induced alteration of mitochondrial morphology and mitochondrial-endoplasmic reticulum contacts in rat brown adipocytes

Mitochondria are key organelles maintaining cellular bioenergetics and integrity, and their regulation of [Ca²⁺]_i homeostasis has been investigated in many cell types. We investigated the short-term Ca-SANDOZ^® treatment on brown adipocyte mitochondria, using imaging and molecular biology techniques. Two-month-old male Wistar rats were divided into two groups: Ca-SANDOZ^® drinking or tap water (control) drinking for three days. Alizarin Red S staining showed increased Ca²⁺ level in the brown adipocytes of treated rats, and potassium pyroantimonate staining localized electron-dense regions in the cytoplasm, mitochondria and around lipid droplets. Ca-SANDOZ^® decreased mitochondrial number, but increased their size and mitochondrial cristae volume. Transmission electron microscopy revealed numerous enlarged and fusioned-like mitochondria in the Ca-SANDOZ^® treated group compared to the control, and megamitochondria in some brown adipocytes. The Ca²⁺ diet affected mitochondrial fusion as mitofusin 1 (MFN1) and mitofusin 2 (MFN2) were increased, and mitochondrial fission as dynamin related protein 1 (DRP1) was decreased. Confocal microscopy showed a higher colocalization rate between functional mitochondria and endoplasmic reticulum (ER). The level of uncoupling protein-1 (UCP1) was elevated, which was confirmed by immunohistochemistry and Western blot analysis. These results suggest that Ca-SANDOZ^® stimulates mitochondrial fusion, increases mitochondrial-ER contacts and the thermogenic capacity of brown adipocytes.

The small GTPase Arf1 modulates mitochondrial morphology and function

The small GTPase Arf1 plays critical roles in membrane traffic by initiating the recruitment of coat proteins and by modulating the activity of lipid-modifying enzymes. Here, we report an unexpected but evolutionarily conserved role for Arf1 and the ArfGEF GBF1 at mitochondria. Loss of function of ARF-1 or GBF-1 impaired mitochondrial morphology and activity in Caenorhabditis elegans. Similarly, mitochondrial defects were observed in mammalian and yeast cells. In Saccharomyces cerevisiae, aberrant clusters of the mitofusin Fzo1 accumulated in arf1-11 mutants and were resolved by overexpression of Cdc48, an AAA-ATPase involved in ER and mitochondria-associated degradation processes. Yeast Arf1 co-fractionated with ER and mitochondrial membranes and interacted genetically with the contact site component Gem1. Furthermore, similar mitochondrial abnormalities resulted from knockdown of either GBF-1 or contact site components in worms, suggesting that the role of Arf1 in mitochondrial functioning is linked to ER-mitochondrial contacts. Thus, Arf1 is involved in mitochondrial homeostasis and dynamics, independent of its role in vesicular traffic.

Mitochondrial dynamics and the innate antiviral immune response

The innate immune system has a key role in the mammalian immune response. In the cytosol, RNA viruses are sensed by the retinoic acid-inducible gene-I-like receptors, which trigger a complex signaling cascade in which mitochondrial antiviral signaling protein plays a central role in mediating the innate host response through the induction of antiviral and inflammatory responses. Hence, the mitochondrion is now emerging as a fundamental hub for innate antiviral immunity beyond its known roles in metabolic processes and the control of programmed cell death. This review summarizes the findings related to mitochondrial antiviral signaling protein, and mitochondria and their dynamics, in the innate immune response to RNA viruses.