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.     

Mitochondrial changes in flight muscles of normal and flightless Drosophila melanogaster with age

Fine structural changes in mitochondrial morphology pertaining to size, number and growth were examined in flight muscles of normal and experimentally dewinged male Drosphila melanogaster ranging up to 26 days of age. In the normal winged flies, the number of mitochondria decreases during the first week of adult life whereas the size of individual mitochondrial profile increases significantly. Changes in mitochondrial size and number are due to the fusion of mitochondria. Fused mitochondria are extremely large in size and irregular in shape. In 26-day old normal flies, the number of mitochondria increases while the mitochondrial size is reduced indicating mitochondrial division. In comparison to the normal flies, dewinged flies exhibit a similar degree of mitochondrial fusion and growth during the first week of life. However, the extent of mitochondrial fission in 26-day old dewinged flies is greater than in the normal flies of this age. Structural mechanisms of mitochondrial fusion and fission are described. The objective of this study was to examine the relative effects of age and flight activity on the mitochondria.     

Mitochondrial network of skeletal muscle fiber

Highly specialized muscle fibers require a unique spatial organization of the mitochondrial network. Mitochondrial morphology is largely determined by the fusion and fission of these organelles. This review summarizes the current concepts of mitochondrial dynamics mechanisms and structural features of the mitochondrial network in striated muscle tissue. The role of mitochondria and their dynamics in muscle fiber physiology are also discussed.     

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 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.     

冷冻电子断层扫描研究原代培养神经元中的线粒体膜动态变化（英文）

<正>Dear Editor,Mitochondria acts as a cellular organelle that produces ATP and buffers Ca²⁺, and plays an important role in neuronal growth, survival and function^[1]. Loss of mitochondria will make the ATP supply insufficient, resulting in synaptic transmission dysfunction^[2]. Further, presynaptic mitochondrial dysfunctions are often associated with severe neurological diseases^[3].     

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.     

Bax/Bak-dependent release of DDP/TIMM8a promotes Drp1-mediated mitochondrial fission and mitoptosis during programmed cell death

Mitochondrial morphology within cells is controlled by precisely regulated rates of fusion and fission . During programmed cell death (PCD), mitochondria undergo extensive fragmentation and ultimately caspase-independent elimination through a process known as mitoptosis . Though this increased fragmentation is due to increased fission through the recruitment of the dynamin-like GTPase Drp1 to mitochondria , as well as to a block in mitochondrial fusion , cellular mechanisms underlying these processes remain unclear. Here, we describe a mechanism for the increased mitochondrial Drp1 levels and subsequent stimulation of mitochondrial fission seen during PCD. We observed Bax/Bak-mediated release of DDP/TIMM8a, a mitochondrial intermembrane space (IMS) protein , into the cytoplasm, where it binds to and promotes the mitochondrial redistribution of Drp1, a mediator of mitochondrial fission. Using both loss- and gain-of-function assays, we also demonstrate that the Drp1- and DDP/TIMM8a-dependent mitochondrial fragmentation observed during PCD is an important step in mitoptosis, which in turn is involved in caspase-independent cell death. Thus, following Bax/Bak-mediated mitochondrial outer membrane permeabilization (MOMP), IMS proteins released comprise not only apoptogenic factors such as cytochrome c involved in caspase activation but also DDP/TIMM8a, which activates Drp1-mediated fission to promote mitochondrial fragmentation and subsequently elimination during PCD.     

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.     

Thapsigargin induces biphasic fragmentation of mitochondria through calcium-mediated mitochondrial fission and apoptosis

Mitochondrial fission and fusion are the main components mediating the dynamic change of mitochondrial morphology observed in living cells. While many protein factors directly participating in mitochondrial dynamics have been identified, upstream signals that regulate mitochondrial morphology are not well understood. In this study, we tested the role of intracellular Ca(2+) in regulating mitochondrial morphology. We found that treating cells with the ER Ca(2+)-ATPase inhibitor thapsigargin (TG) induced two phases of mitochondrial fragmentation. The initial fragmentation of mitochondria occurs rapidly within minutes dependent on an increase in intracellular Ca(2+) levels, and Ca(2+) influx into mitochondria is necessary for inducing mitochondrial fragmentation. The initial mitochondrial fragmentation is a transient event, as tubular mitochondrial morphology was restored as the Ca(2+) level decreased. We were able to block the TG-induced mitochondrial fragmentation by inhibiting mitochondrial fission proteins DLP1/Drp1 or hFis1, suggesting that increased mitochondrial Ca(2+) acts upstream to activate the cellular mitochondrial fission machinery. We also found that prolonged incubation with TG induced the second phase of mitochondrial fragmentation, which was non-reversible and led to cell death as reported previously. These results suggest that Ca(2+) is involved in controlling mitochondrial morphology via intra-mitochondrial Ca(2+) signaling as well as the apoptotic process.     

Mitochondrial fusion, fission and autophagy as a quality control axis: the bioenergetic view

The mitochondrial life cycle consists of frequent fusion and fission events. Ample experimental and clinical data demonstrate that inhibition of either fusion or fission results in deterioration of mitochondrial bioenergetics. While fusion may benefit mitochondrial function by allowing the spreading of metabolites, protein and DNA throughout the network, the functional benefit of fission is not as intuitive. Remarkably, studies that track individual mitochondria through fusion and fission found that the two events are paired and that fusion triggers fission. On average each mitochondrion would go though ~5 fusion:fission cycles every hour. Measurement of Deltapsi(m) during single fusion and fission events demonstrates that fission may yield uneven daughter mitochondria where the depolarized daughter is less likely to become involved in a subsequent fusion and is more likely to be targeted by autophagy. Based on these observations we propose a mechanism by which the integration of mitochondrial fusion, fission and autophagy forms a quality maintenance mechanism. According to this hypothesis pairs of fusion and fission allow for the reorganization and sequestration of damaged mitochondrial components into daughter mitochondria that are segregated from the networking pool and then becoming eliminated by autophagy.     

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.     

Mitochondrial clustering induced by overexpression of the mitochondrial fusion protein Mfn2 causes mitochondrial dysfunction and cell death

Mitochondria change their shapes dynamically mainly through fission and fusion. Dynamin-related GTPases have been shown to mediate remodeling of mitochondrial membranes during these processes. One of these GTPases, mitofusin, is anchored at the outer mitochondrial membrane and mediates fusion of the outer membrane. We found that overexpression of a mitofusin isoform, Mfn2, drastically changes mitochondrial morphology, forming mitochondrial clusters. High-resolution microscopic examination indicated that the mitochondrial clusters consisted of small fragmented mitochondria. Inhibiting mitochondrial fission prevented the cluster formation, supporting the notion that mitochondrial clusters are formed by fission-mediated mitochondrial fragmentation and aggregation. Mitochondrial clusters displayed a decreased inner membrane potential and mitochondrial function, suggesting a functional compromise of small fragmented mitochondria produced by Mfn2 overexpression; however, mitochondrial clusters still retained mitochondrial DNA. We found that cells containing clustered mitochondria lost cytochrome c from mitochondria and underwent caspase-mediated apoptosis. These results demonstrate that mitochondrial deformation impairs mitochondrial function, leading to apoptotic cell death and suggest the presence of an intricate form-function relationship in mitochondria.     

Mitochondrial fusion, fission and autophagy as a quality control axis: The bioenergetic view

The mitochondrial life cycle consists of frequent fusion and fission events. Ample experimental and clinical data demonstrate that inhibition of either fusion or fission results in deterioration of mitochondrial bioenergetics. While fusion may benefit mitochondrial function by allowing the spreading of metabolites, protein and DNA throughout the network, the functional benefit of fission is not as intuitive. Remarkably, studies that track individual mitochondria through fusion and fission found that the two events are paired and that fusion triggers fission. On average each mitochondrion would go though ~ 5 fusion:fission cycles every hour. Measurement of Δψ_m during single fusion and fission events demonstrates that fission may yield uneven daughter mitochondria where the depolarized daughter is less likely to become involved in a subsequent fusion and is more likely to be targeted by autophagy. Based on these observations we propose a mechanism by which the integration of mitochondrial fusion, fission and autophagy forms a quality maintenance mechanism. According to this hypothesis pairs of fusion and fission allow for the reorganization and sequestration of damaged mitochondrial components into daughter mitochondria that are segregated from the networking pool and then becoming eliminated by autophagy.     

hFis1, a novel component of the mammalian mitochondrial fission machinery

The balance between the fission and fusion mechanisms regulate the morphology of mitochondria. In this study we have identified a mammalian protein that we call hFis1, which is the orthologue of the yeast Fis1p known to participate in yeast mitochondrial division. hFis1, when overexpressed in various cell types, localized to the outer mitochondrial membrane and induced mitochondrial fission. This event was inhibited by a dominant negative mutant of Drp1 (Drp1(K38A)), a major component of the fission apparatus. Fragmentation of the mitochondrial network by hFis1 was followed by the release of cytochrome c and ultimately apoptosis. Bcl-xL was able to block cytochrome c release and apoptosis but failed to prevent mitochondrial fragmentation. Our studies show that hFis1 is part of the mammalian fission machinery and suggest that regulation of the fission processes might be involved in apoptotic mechanisms.     

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。     

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.     

Divide and multiply: organelle partitioning in yeast

The mechanisms ensuring accurate partitioning of yeast vacuoles and mitochondria are distinct, yet they share common elements. Both organelles move along actin filaments, and both organelles require fusion and fission to maintain normal morphology. Recent studies have revealed that while vacuolar inheritance requires a processive myosin motor, mitochondrial inheritance requires controlled actin polymerization. Distinct sets of proteins required for the fusion and fission of each organelle have also been identified.     

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.