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.     

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.     

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.     

Mitochondrial Fission: Regulation and ER Connection

Fission and fusion of mitochondrial tubules are the main processes determining mitochondrial shape and size in cells. As more evidence is found for the involvement of mitochondrial morphology in human pathology, it is important to elucidate the mechanisms of mitochondrial fission and fusion. Mitochondrial morphology is highly sensitive to changing environmental conditions, indicating the involvement of cellular signaling pathways. In addition, the well-established structural connection between the endoplasmic reticulum (ER) and mitochondria has recently been found to play a role in mitochondrial fission. This minireview describes the latest advancements in understanding the regulatory mechanisms controlling mitochondrial morphology, as well as the ER-mediated structural maintenance of mitochondria, with a specific emphasis on mitochondrial fission.     

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.     

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.     

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.     

线粒体形态学改变与细胞凋亡

近年来,对于线粒体形态学以及其在凋亡过程中的改变和作用的研究打破了传统的观点。正常情况下,线粒体在细胞内相互连接成管网状结构,并发生着频繁的融合与分裂。融合和分裂由一系列蛋白质介导,二者之间的动态平衡维持着线粒体的形态和功能。在细胞凋亡的早期,线粒体融合和分裂失平衡,导致线粒体管网状结构碎裂和嵴的重构,这些改变对线粒体随后的变化以及凋亡的发生具有重要的意义。融合和分裂的蛋白质不仅调控线粒体形态和细胞凋亡过程,也和某些凋亡相关疾病有关。此外,促凋亡的Bcl-2蛋白可能通过改变线粒体的构形来调控凋亡过程。     

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。     

Shaping mitochondria: The complex posttranslational regulation of the mitochondrial fission protein DRP1

Mitochondria are essential and dynamic cellular organelles differing in size, subcellular distribution, and internal structure. These aspects of mitochondrial morphology are intimately controlled by a growing number of mitochondrial morphology shaping proteins. The past decade has revealed remarkable and often unexpected new insights into the molecular regulation and physiological impact of mitochondrial morphology maintenance. Obviously, proper mitochondrial dynamics, resulting from a tightly regulated equilibrium between opposing mitochondrial fusion and fission activities, is a prerequisite for normal organelle function. Consequently, a disturbance of these activities results in mitochondrial dysfunction and, thus, can lay the foundation for human disorders. Here we specifically focus on recent advances in our understanding of the regulation, activity, and function of dynamin-related protein 1, the main factor for controlled mitochondrial fission.     

Mitochondrial dynamics: Shaping and remodeling an organelle network

Mitochondria form networks that continually remodel and adapt to carry out their cellular function. The mitochondrial network is remodeled through changes in mitochondrial morphology, number, and distribution within the cell. Mitochondrial dynamics depend directly on fission, fusion, shape transition, and transport or tethering along the cytoskeleton. Over the past several years, many of the mechanisms underlying these processes have been uncovered. It has become clear that each process is precisely and contextually regulated within the cell. Here, we discuss the mechanisms regulating each aspect of mitochondrial dynamics, which together shape the network as a whole.     

Bioenergetic role of mitochondrial fusion and fission

Mitochondria are highly dynamic organelles. Frequent cycles of fusion and fission adapt the morphology of the mitochondrial compartment to the metabolic needs of the cell. Mitochondrial fusion is particularly important in respiratory active cells. It allows the spreading of metabolites, enzymes, and mitochondrial gene products throughout the entire mitochondrial compartment. This serves to optimize mitochondrial function and counteracts the accumulation of mitochondrial mutations during aging. Fragmented mitochondria are frequently found in resting cells, and mitochondrial fission plays an important role in the removal of damaged organelles by autophagy. Thus, mitochondrial fusion and fission both contribute to maintenance of mitochondrial function and optimize bioenergetic capacity. Multiple signalling pathways regulate the machinery of mitochondrial dynamics to adapt the shape of the mitochondrial compartment to the metabolic conditions of the cell. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).     

Mitochondrial dynamics and neuronal fate in Parkinson's disease

Along with the impairment of mitochondrial respiration both mitochondrial fission/fusion and mitophagy have been shown to be altered in Parkinson's disease (PD). In both genetic and toxin-induced models of PD an imbalance in mitochondrial morphology is evident, as its correction through modulation of the fission/fusion proteins has been shown to be protective. From the study of the PD-associated genes, namely PINK1 and Parkin, compromised mitochondrial clearance through mitophagy has been associated with the disease etiopathogenesis. Here we propose that an interplay between defective mitochondrial morphology and clearance arises as a crucial player in sentencing neuronal fate in PD.     

Mitochondrial dynamics and disease, OPA1

The mitochondria are dynamic organelles that constantly fuse and divide. An equilibrium between fusion and fission controls the morphology of the mitochondria, which appear as dots or elongated tubules depending the prevailing force. Characterization of the components of the fission and fusion machineries has progressed considerably, and the emerging question now is what role mitochondrial dynamics play in mitochondrial and cellular functions. Its importance has been highlighted by the discovery that two human diseases are caused by mutations in the two mitochondrial pro-fusion genes, MFN2 and OPA1. This review will focus on data concerning the function of OPA1, mutations in which cause optic atrophy, with respect to the underlying pathophysiological processes.     

线粒体分裂、融合与细胞凋亡

线粒体是高度动态变化的细胞器,其在细胞内不断分裂、融合并形成网状结构。线粒体的分裂和融合是由多种蛋白质精确调控完成的。Drp1／Dnm1p,Fis1／Fis1p,Caf4p和Mdv1p参与线粒体分裂的调控;Mfn1／2／Fzo1p控制线粒体外膜的融合,而Mgm1p／OPA1则参与线粒体内膜的融合。在细胞凋亡过程中线粒体片段化,网状结构被破坏,线粒体嵴发生重构,抑制这一过程可以部分抑制细胞色素c的释放和细胞凋亡。线粒体形态对于细胞维持正常生理代谢和机体发育起着重要的作用,一旦出现障碍会导致严重的疾病。     

Assessing mitochondrial morphology and dynamics using fluorescence wide-field microscopy and 3D image processing

Mitochondrial morphology and length change during fission and fusion and mitochondrial movement varies dependent upon the cell type and the physiological conditions. Here, we describe fundamental wide-field fluorescence microcopy and 3D imaging techniques to assess mitochondrial shape, number and length in various cell types including cancer cell lines, motor neurons and astrocytes. Furthermore, we illustrate how to assess mitochondrial fission and fusion events by 3D time-lapse imaging and to calculate mitochondrial length and numbers as a function of time. These imaging methods provide useful tools to investigate mitochondrial dynamics in health, aging and disease.     

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。     

A New Link to Mitochondrial Impairment in Tauopathies

Tauopathies like the "frontotemporal dementia with Parkinsonism linked to chromosome 17" (FTDP-17) are characterized by an aberrant accumulation of intracellular neurofibrillary tangles composed of hyperphosphorylated tau. For FTDP-17, a pathogenic tau mutation P301L was identified. Impaired mitochondrial function including disturbed dynamics such as fission and fusion are most likely major pathomechanisms of most neurodegenerative diseases. However, very little is known if tau itself affects mitochondrial function and dynamics. We addressed this question using SY5Y cells stably overexpressing wild-type (wt) and P301L mutant tau. P301L overexpression resulted in a substantial complex I deficit accompanied by decreased ATP levels and increased susceptibility to oxidative stress. This was paralleled by pronounced changes in mitochondrial morphology, decreased fusion and fission rates accompanied by reduced expression of several fission and fusion factors like OPA-1 or DRP-1. In contrast, overexpression of wt tau exhibits protective effects on mitochondrial function and dynamics including enhanced complex I activity. Our findings clearly link tau bidirectional to mitochondrial function and dynamics, identifying a novel aspect of the physiological role of tau and the pathomechanism of tauopathies.     

Transient Contraction of Mitochondria Induces Depolarization through the Inner Membrane Dynamin OPA1 Protein

Dynamin-related membrane remodeling proteins regulate mitochondrial morphology by mediating fission and fusion. Although mitochondrial morphology is considered an important factor in maintaining mitochondrial function, a direct mechanistic link between mitochondrial morphology and function has not been defined. We report here a previously unrecognized cellular process of transient contraction of the mitochondrial matrix. Importantly, we found that this transient morphological contraction of mitochondria is accompanied by a reversible loss or decrease of inner membrane potential. Fission deficiency greatly amplified this phenomenon, which functionally exhibited an increase of inner membrane proton leak. We found that electron transport activity is necessary for the morphological contraction of mitochondria. Furthermore, we discovered that silencing the inner membrane-associated dynamin optic atrophy 1 (OPA1) in fission deficiency prevented mitochondrial depolarization and decreased proton leak without blocking mitochondrial contraction, indicating that OPA1 is a factor in coupling matrix contraction to mitochondrial depolarization. Our findings show that transient matrix contraction is a novel cellular mechanism regulating mitochondrial activity through the function of the inner membrane dynamin OPA1.     

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.