TRAP1 Controls Mitochondrial Fusion/Fission Balance through Drp1 and Mff Expression

Mitochondria are dynamic organelles that change in response to extracellular stimuli. These changes are essential for normal mitochondrial/cellular function and are controlled by a tight balance between two antagonistic pathways that promote fusion and fission. Although some molecules have been identified to mediate the mitochondrial fusion and fission process, the underlying mechanisms remain unclear. Tumor necrosis factor receptor-associated protein 1 (TRAP1) is a mitochondrial molecule that regulates a variety of mitochondrial functions. Here, we examined the role of TRAP1 in the regulation of morphology. Stable TRAP1 knockdown cells showed abnormal mitochondrial morphology, and we observed significant decreases in dynamin-related protein 1 (Drp1) and mitochondrial fission factor (Mff), mitochondrial fission proteins. Similar results were obtained by transient knockdown of TRAP1 in two different cell lines, SH-SY5Y neuroblastoma cells and KNS-42 glioma cells. However, TRAP1 knockdown did not affect expression levels of fusion proteins. The reduction in Drp1 and Mff protein levels was rescued following treatment with the proteasome inhibitor MG132. These results suggest that TRAP1 regulates the expression of fission proteins and controls mitochondrial fusion/fission, which affects mitochondrial/cellular function.     

Dynamics of mitochondrial morphology in healthy cells and during apoptosis

Mitochondria exist as dynamic networks that often change shape and subcellular distribution. The number and morphology of mitochondria within a cell are controlled by precisely regulated rates of organelle fusion and fission. Recent reports have described dramatic alterations in mitochondrial morphology during the early stages of apoptotic cell death, a fragmentation of the network and the remodeling of the cristae. Surprisingly, proteins discovered to control mitochondrial morphology appear to also participate in apoptosis and proteins associated with the regulation of apoptosis have been shown to affect mitochondrial ultrastructure. In this review the recent progress in understanding the mechanisms governing mitochondrial morphology and the latest advances connecting the regulation of mitochondrial morphology with programmed cell death are discussed.     

The role of abnormal mitochondrial dynamics in the pathogenesis of Alzheimer's disease

Mitochondria play critical roles in neuronal function and almost all aspects of mitochondrial function are altered in Alzheimer neurons. Emerging evidence shows that mitochondria are dynamic organelles that undergo continuous fission and fusion, the balance of which not only controls mitochondrial morphology and number, but also regulates mitochondrial function and distribution. In this review, after a brief overview of the basic mechanisms involved in the regulation of mitochondrial fission and fusion and how mitochondrial dynamics affects mitochondrial function, we will discuss in detail our and others' recent work demonstrating abnormal mitochondrial morphology and distribution in Alzheimer's disease (AD) models and how these abnormalities may contribute to mitochondrial and synaptic dysfunction in AD. We propose that abnormal mitochondrial dynamics plays a key role in causing the dysfunction of mitochondria that ultimately damage AD neurons.     

Mitochondrial dynamics during apoptosis

Mitochondria exist as dynamic networks that often change shape and subcellular distribution. The morphology of mitochondria within a cell is controlled by precisely regulated rates of organelle fusion and fission. Several reports have described dramatic alterations in mitochondrial morphology during the early stages of apoptosis: a fragmentation of the network and the cristae remodeling. However, whether this mitochondrial fragmentation is a required step for apoptosis is highly debated. In this review the recent progress in understanding the mechanisms governing mitochondrial morphology during apoptosis and the latest advances connecting the regulation of mitochondrial morphology with apoptosis are discussed.     

ATP5B regulates mitochondrial fission and fusion in mammalian cells

Mitochondria are essential organelles that produce ATP and regulate cell growth, proliferation, and cell death. To maintain homeostasis, fusion and fission of mitochondria must be strictly regulated. Even though oligomerization of ATP synthase could affect the mitochondrial morphology, the exact mechanism is not clear. We confirmed that structure and function of ATP5B, which is a major component of the catalytic center of ATP synthase complexes, are closely connected to the mitochondrial morphology. ATP5B itself can enhance elongation of mitochondria. Moreover, mutations of the threonine residue at β-barrel domain, and the serine residue at nucleotide-binding domain of ATP5B, produce the opposite effect on the fission and fusion of mitochondrial networks. Here, we demonstrate that ATP5B is clearly involved in the mechanism of regulation for mitochondrial fusion and fission in mammalian cells.     

线粒体融合/分裂对线粒体的质量控制作用

线粒体是一种结构和功能复杂而敏感的细胞器,拥有独立于细胞核的基因组,在细胞的不同时相,生理过程和环境条件下,线粒体的形态,数量和质量,具有高度的可塑性。线粒体是细胞和生物体内最主要的能量供应场所,几乎存在于所有种类的细胞中,是一种动态变化的细胞器。正常情况下,线粒体的数量、形态以及功能维持相对稳定的状态,称之为线粒体稳态。当上述状态发生紊乱时,细胞乃至生物体形态、功能也将受到影响甚至死亡。线粒体质量控制是在细胞中维持正常状态的关键机制,决定着线粒体的命运。近年,随着线粒体研究的深入和具体,逐渐发现融合/分裂在其形态、数量、遗传物质等质量控制相关的方面挥了重要作用。本文通过探讨融合/分裂对线粒体质量控制的作用机制,总结和讨论相关前沿研究,为后期研究提供一定的理论依据。     

Sharpening the scissors

Over the past 5 yr. research in mitochondrial morphology has advanced rapidly, mainly as a result of the identification of protein factors involved in mitochondrial fission and fusion. The pathological relevance of these processes becomes clear as apoptotic cell death evidently involves mitochondrial fission and fusion machinery. Although the mechanisms by which cells maintain mitochondrial morphology are now beginning to be understood, interrelation between mitochondrial function and morphology is still not clear. This review describes the recent progress made in mitochondrial fission studies and ventures to seek an intricate link between morphology and function of mitochondria.     

Sharpening the scissors: mitochondrial fission with aid

Over the past 5 yr, research in mitochondrial morphology has advanced rapidly, mainly as a result of the identification of protein factors involved in mitochondrial fission and fusion. The pathological relevance of these processes becomes clear as apoptotic cell death evidently involves mitochondrial fission and fusion machinery. Although the mechanisms by which cells maintain mitochondrial morphology are now beginning to be understood, interrelation between mitochondrial function and morphology is still not clear. This review describes the recent progress made in mitochondrial fission studies and ventures to seek an intricate link between morphology and function of mitochondria.     

Get the balance right: mitofusins roles in health and disease

Mitochondria are highly dynamic organelles exhibiting an elaborate morphology and fine structure. Fusion and fission processes contribute to the maintenance and dynamics of mitochondrial morphology. The Mitofusins, a class of evolutionary conserved GTPases of the mitochondrial outer membrane, are essential for the controlled fusion of mitochondrial membranes. Genetic and biochemical data propose a model in which functional domains, such as the GTPase domain and the C-terminally located coiled coil structure, act in an orchestrated manner to coordinate the tethering and mitochondrial outer membrane fusion. In addition, recent reports shed new light on the physiological importance of Mitofusin function suggesting a role in mitochondrial metabolism, apoptosis as well as cellular signalling. Mutations identified in the human Mfn2 gene from patients with the peripheral neuropathy Charcot-Marie-Tooth Type 2A invoke a direct correlation between mitochondrial morphology and function.     

Bcl-xL increases mitochondrial fission,fusion, and biomass in neurons

Mitochondrial fission and fusion are linked to synaptic activity in healthy neurons and are implicated in the regulation of apoptotic cell death in many cell types. We developed fluorescence microscopy and computational strategies to directly measure mitochondrial fission and fusion frequencies and their effects on mitochondrial morphology in cultured neurons. We found that the rate of fission exceeds the rate of fusion in healthy neuronal processes, and, therefore, the fission/fusion ratio alone is insufficient to explain mitochondrial morphology at steady state. This imbalance between fission and fusion is compensated by growth of mitochondrial organelles. Bcl-x_L increases the rates of both fusion and fission, but more important for explaining the longer organelle morphology induced by Bcl-x_L is its ability to increase mitochondrial biomass. Deficits in these Bcl-x_L–dependent mechanisms may be critical in neuronal dysfunction during the earliest phases of neurodegeneration, long before commitment to cell death.     

Human MIEF1 recruits Drp1 to mitochondrial outer membranes and promotes mitochondrial fusion rather than fission

Mitochondrial morphology is controlled by two opposing processes: fusion and fission. Drp1 (dynamin-related protein 1) and hFis1 are two key players of mitochondrial fission, but how Drp1 is recruited to mitochondria and how Drp1-mediated mitochondrial fission is regulated in mammals is poorly understood. Here, we identify the vertebrate-specific protein MIEF1 (mitochondrial elongation factor 1; independently identified as MiD51), which is anchored to the outer mitochondrial membrane. Elevated MIEF1 levels induce extensive mitochondrial fusion, whereas depletion of MIEF1 causes mitochondrial fragmentation. MIEF1 interacts with and recruits Drp1 to mitochondria in a manner independent of hFis1, Mff (mitochondrial fission factor) and Mfn2 (mitofusin 2), but inhibits Drp1 activity, thus executing a negative effect on mitochondrial fission. MIEF1 also interacts with hFis1 and elevated hFis1 levels partially reverse the MIEF1-induced fusion phenotype. In addition to inhibiting Drp1, MIEF1 also actively promotes fusion, but in a manner distinct from mitofusins. In conclusion, our findings uncover a novel mechanism which controls the mitochondrial fusion-fission machinery in vertebrates. As MIEF1 is vertebrate-specific, these data also reveal important differences between yeast and vertebrates in the regulation of mitochondrial dynamics.     

哺乳动物细胞线粒体融合-分裂与钙离子信号的关系

线粒体是一种高度动态的细胞器,通过融合和分裂两个相反的过程来维持正常的形态结构。在哺乳动物中,多种因素影响线粒体的融合-分裂的平衡,但现已明确,线粒体融合的主要调节因子为Mfn1/2、OPA1,介导线粒体分裂的主要调节因子为Drp1、Fis1。新近研究发现,线粒体融合-分裂平衡的紊乱将导致线粒体结构和在细胞内分布的异常,进而影响细胞和线粒体对钙离子信号的反应;同时,钙离子也可通过多种机制影响线粒体的形态结构与分布。     

Instability of the mitofusin Fzo1 regulates mitochondrial morphology during the mating response of the yeast Saccharomyces cerevisiae

Mitochondria form a highly dynamic network that is shaped by continuous fission and fusion of these organelles. In the yeast Saccharomyces cerevisiae two machineries are involved in this process, one of which includes the mitochondrial fusion promoting GTPase Fzo1. Although a role for the F-box protein Mdm30 in regulating the stability of Fzo1 has been proposed, the molecular basis for the regulation of the fission to fusion ratio of mitochondria remains unknown. To discern the mechanism of the regulation of mitochondrial morphology, we arrested cells at different stages of the cell cycle and examined mitochondrial morphology as well as the stability of mitochondrial fission and fusion proteins. In response to a G1 arrest evoked by the mating pheromone alpha factor the mitochondrial network fragmented into small pieces, which was accompanied by dramatic down-regulation of Fzo1. Mating pheromone also triggered the degradation of Fzo1 produced under the control of a constitutive promoter, and Fzo1 was stabilized upon proteasome inhibition, indicating a role for the proteasome system in the degradation of Fzo1. However, deletion of MDM30 did not stabilize Fzo1 after mating pheromone treatment, showing a different mechanism from the previously reported process of steady state Fzo1 regulation. We show an example for a regulated change of the mitochondrial fission to fusion ratio during the life cycle of budding yeast. Proteasomal degradation of Fzo1 in response to the mating pheromone is proposed to mediate the remodeling of the mitochondrial network during the process of mating.     

Unopposed mitochondrial fission leads to severe lifespan shortening

Mitochondrial morphology is controlled by the opposing processes of fusion and fission. Previously, in baker’s yeast it was shown that reduced mitochondrial fission leads to a network-like morphology, decreased sensitivity for the induction of apoptosis and a remarkable extension of both replicative and chronological lifespan. However, the effects of reduced mitochondrial fusion on aging are so far unknown and complicated by the fact that deletion of genes encoding components of mitochondrial fusion are often lethal to higher organisms. This is also true for the mammalian OPA1 protein, which is a key regulator of mitochondrial inner membrane fusion. Baker’s yeast contains an OPA1 ortholog, Mgm1p. Deletion of Mgm1 is possible in yeast due to the fact that mitochondrial function is not essential for growth on glucose-containing media. In this study, we report that absence of mitochondrial fusion in the Δmgm1 mutant leads to a striking reduction of both replicative and chronological lifespan. Concomitantly, sensitivity to apoptosis elicitation via the reactive oxygen species hydrogen peroxide is substantially increased. These results demonstrate that the unopposed mitochondrial fission as displayed by the Δmgm1 mutant strongly affects organismal aging. Moreover, our results bear important clues for translational research to intervene into age-related degenerative processes also in multicellular organisms including humans.     

Mitochondrial Morphological Features Are Associated with Fission and Fusion Events

Mitochondria are dynamic organelles that undergo constant remodeling through the regulation of two opposing processes, mitochondrial fission and fusion. Although several key regulators and physiological stimuli have been identified to control mitochondrial fission and fusion, the role of mitochondrial morphology in the two processes remains to be determined. To address this knowledge gap, we investigated whether morphological features extracted from time-lapse live-cell images of mitochondria could be used to predict mitochondrial fate. That is, we asked if we could predict whether a mitochondrion is likely to participate in a fission or fusion event based on its current shape and local environment. Using live-cell microscopy, image analysis software, and supervised machine learning, we characterized mitochondrial dynamics with single-organelle resolution to identify features of mitochondria that are predictive of fission and fusion events. A random forest (RF) model was trained to correctly classify mitochondria poised for either fission or fusion based on a series of morphological and positional features for each organelle. Of the features we evaluated, mitochondrial perimeter positively correlated with mitochondria about to undergo a fission event. Similarly mitochondrial solidity (compact shape) positively correlated with mitochondria about to undergo a fusion event. Our results indicate that fission and fusion are positively correlated with mitochondrial morphological features; and therefore, mitochondrial fission and fusion may be influenced by the mechanical properties of mitochondrial membranes.     

Regulation of Mitochondrial Morphological Dynamics During Apoptosis by Bcl-2 family proteins: A Key in Bak?

Early during apoptosis, the mitochondrial network collapses into short punctate fragments. The seemingly morphological change, called mitochondrial fragmentation, contributes to mitochondrial injury. Mitochondrial morphology is dictated by two opposing processes, fission and fusion. It is unclear how the fission-fusion balance is tilted during apoptosis, resulting in mitochondrial fragmentation. Emerging evidence has now suggested a regulation of mitochondrial morphological dynamics by Bcl-2 family proteins. In this regulation, Bak appears to be a key. Through interaction with mitofusins, Bak may block mitochondrial fusion to induce fragmentation. By this function, Bak may collaborate with Bax to permeabilize mitochondrial outer membrane, leading to the release of apoptogenic factors.     

Inner-membrane proteins PMI/TMEM11 regulate mitochondrial morphogenesis independently of the DRP1/MFN fission/fusion pathways

Mitochondria are highly dynamic organelles that can change in number and morphology during cell cycle, development or in response to extracellular stimuli. These morphological dynamics are controlled by a tight balance between two antagonistic pathways that promote fusion and fission. Genetic approaches have identified a cohort of conserved proteins that form the core of mitochondrial remodelling machineries. Mitofusins (MFNs) and OPA1 proteins are dynamin-related GTPases that are required for outer- and inner-mitochondrial membrane fusion respectively whereas dynamin-related protein 1 (DRP1) is the master regulator of mitochondrial fission. We demonstrate here that the Drosophila PMI gene and its human orthologue TMEM11 encode mitochondrial inner-membrane proteins that regulate mitochondrial morphogenesis. PMI-mutant cells contain a highly condensed mitochondrial network, suggesting that PMI has either a pro-fission or an anti-fusion function. Surprisingly, however, epistatic experiments indicate that PMI shapes the mitochondria through a mechanism that is independent of drp1 and mfn. This shows that mitochondrial networks can be shaped in higher eukaryotes by at least two separate pathways: one PMI-dependent and one DRP1/MFN-dependent.     

Proteins That Fuse and Fragment Mitochondria in Apoptosis: Con-Fissing a Deadly Con-Fusion?

During apoptosis, mitochondria undergo multiple changes that culminate in the release of cytochrome c and other proapoptotic cofactors. Recently, a role for previously overlooked morphological changes, fission of the mitochondrial reticulum and remodeling of mitochondrial cristae, has been suggested in mammalian cells and in developmental apoptosis of C. elegans. Mitochondrial morphology is determined by fusion and fission processes, controlled by a growing set of “mitochondria-shaping” proteins, whose levels and function appear to regulate the mitochondrial pathways of cell death. Expression of pro-fusion proteins, as well as of inhibition of pro-fission molecules reduces apoptosis, suggesting a linear relationship between fragmentation and death. Mechanisms by which mitochondrial fragmentation promotes apoptosis and interactions between fragmentation and remodeling of the inner membrane are largely unclear. A tempting, unifying hypothesis suggests that fission is coupled to cristae remodeling to maximize cytochrome c release.     

Mitochondrial morphology and activity regulate furrow ingression and contractile ring dynamics in Drosophila cellularization

Mitochondria are maternally inherited in many organisms. Mitochondrial morphology and activity regulation is essential for cell survival, differentiation, and migration. An analysis of mitochondrial dynamics and function in morphogenetic events in early metazoan embryogenesis has not been carried out. In our study we find a crucial role of mitochondrial morphology regulation in cell formation in Drosophila embryogenesis. We find that mitochondria are small and fragmented and translocate apically on microtubules and distribute progressively along the cell length during cellularization. Embryos mutant for the mitochondrial fission protein, Drp1 (dynamin-related protein 1), die in embryogenesis and show an accumulation of clustered mitochondria on the basal side in cellularization. Additionally, Drp1 mutant embryos contain lower levels of reactive oxygen species (ROS). ROS depletion was previously shown to decrease myosin II activity. Drp1 loss also leads to myosin II depletion at the membrane furrow, thereby resulting in decreased cell height and larger contractile ring area in cellularization similar to that in myosin II mutants. The mitochondrial morphology and cellularization defects in Drp1 mutants are suppressed by reducing mitochondrial fusion and increasing cytoplasmic ROS in superoxide dismutase mutants. Our data show a key role for mitochondrial morphology and activity in supporting the morphogenetic events that drive cellularization in Drosophila embryos.     

Mitochondrial morphology and dynamics in <Emphasis Type="Italic">Triticum aestivum</Emphasis> roots in response to rotenone and antimycin A

Mitochondria are dynamic organelles, capable of fusion and fission as a part of cellular responses to various signals, such as the shifts in the redox status of a cell. The mitochondrial electron transport chain (ETC.) is involved in the generation of reactive oxygen species (ROS), with complexes I and III contributing the most to this process. Disruptions of ETC. can lead to increased ROS generation. Here, we demonstrate the appearance of giant mitochondria in wheat roots in response to simultaneous application of the respiratory inhibitors rotenone (complex I of mitochondrial ETC.) and antimycin A (complex III of mitochondrial ETC.). The existence of such megamitochondria was temporary, and following longer treatment with inhibitors mitochondria resumed their conventional size and oval shape. Changes in mitochondrial morphology were accompanied with a decrease in mitochondrial potential and an unexpected increase in oxygen consumption. Changes in mitochondrial morphology and activity may result from the fusion and fission of mitochondria induced by the disruption of mitochondrial ETC. Results from experiments with the inhibitor of mitochondrial fission Mdivi-1 suggest that the retarded fission may facilitate plant mitochondria to appear in a fused shape. The processes of mitochondrial fusion and fission are involved in the regulation of the efficacy of the functions of the respiratory chain complexes and ROS metabolism during stresses. The changes in morphology of mitochondria, along with the changes in their functional activity, can be a part of the strategy of the plant adaptation to stresses.