Rafting through traffic: Membrane domains in cellular logistics

The intricate and tightly regulated organization of eukaryotic cells into spatially and functionally distinct membrane-bound compartments is a defining feature of complex organisms. These compartments are defined by their lipid and protein compositions, with their limiting membrane as the functional interface to the rest of the cell. Thus, proper segregation of membrane proteins and lipids is necessary for the maintenance of organelle identity, and this segregation must be maintained despite extensive, rapid membrane exchange between compartments. Sorting processes of high efficiency and fidelity are required to avoid potentially deleterious mis-targeting and maintain cellular function. Although much molecular machinery associated with membrane traffic (i.e. membrane budding/fusion/fission) has been characterized both structurally and biochemically, the mechanistic details underlying the tightly regulated distribution of membranes between subcellular locations remain to be elucidated. This review presents evidence for the role of ordered lateral membrane domains known as lipid rafts in both biosynthetic sorting in the late secretory pathway, as well as endocytosis and recycling to/from the plasma membrane. Although such evidence is extensive and the involvement of membrane domains in sorting is definitive, specific mechanistic details for raft-dependent sorting processes remain elusive.     

Lipid signaling on the mitochondrial surface

Regulated production and elimination of the signaling lipids phosphatidic acid (PA), diacylglycerol (DAG), and phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) creates a complex and interconnected signaling network that modulates a wide variety of eukaryotic cell biological events. PA production at the plasma membrane and on trafficking membrane organelles by classical Phospholipase D (PLD) through the hydrolysis of phosphatidylcholine (PC) has been studied widely. In this chapter, we review a newly identified, non-canonical member of the PLD superfamily, MitoPLD, which localizes to the mitochondrial surface and plays a role in mitochondrial fusion via the hydrolysis of cardiolipin (CL) to generate PA. The role of PA in facilitating the mitochondrial fusion event carried out by proteins known as Mitofusins is intriguing in light of the role classic PLD-generated PA plays in facilitating SNARE-mediated fusion of secretory membrane vesicles into the plasma membrane. In addition, however, PA on the mitochondrial surface may also trigger a signaling cascade that elevates DAG, leading to downstream events that affect mitochondrial fission and energy production. PA production on the mitochondrial surface may also stimulate local production of PI4,5P₂ to facilitate mitochondrial fission and subcellular trafficking or facilitate Ca²⁺ influx.     

Mitochondrial shaping cuts

A broad range of cellular processes are regulated by proteolytic events. Proteolysis has now also been established to control mitochondrial morphology which results from the balanced action of fusion and fission. Two out of three known core components of the mitochondrial fusion machinery are under proteolytic control. The GTPase Fzo1 in the outer membrane of mitochondria is degraded along two independent proteolytic pathways. One controls mitochondrial fusion in vegetatively growing cells, the other one acts upon mating factor-induced cell cycle arrest. Fusion also depends on proteolytic processing of the GTPase Mgm1 by the rhomboid protease Pcp1 in the inner membrane of mitochondria. Functional links of AAA proteases or other proteolytic components to mitochondrial dynamics are just emerging. This review summarises the current understanding of regulatory roles of proteolytic processes for mitochondrial plasticity.     

Mitochondrial shaping cuts

A broad range of cellular processes are regulated by proteolytic events. Proteolysis has now also been established to control mitochondrial morphology which results from the balanced action of fusion and fission. Two out of three known core components of the mitochondrial fusion machinery are under proteolytic control. The GTPase Fzo1 in the outer membrane of mitochondria is degraded along two independent proteolytic pathways. One controls mitochondrial fusion in vegetatively growing cells, the other one acts upon mating factor-induced cell cycle arrest. Fusion also depends on proteolytic processing of the GTPase Mgm1 by the rhomboid protease Pcp1 in the inner membrane of mitochondria. Functional links of AAA proteases or other proteolytic components to mitochondrial dynamics are just emerging. This review summarises the current understanding of regulatory roles of proteolytic processes for mitochondrial plasticity.     

From dynamin related proteins structures and oligomers to membrane fusion mediated by mitofusins

Mitochondria assemble in a highly dynamic network where interconnected tubules evolve in length and size through regulated cycles of fission and fusion of mitochondrial membranes thereby adapting to cellular needs. Mitochondrial fusion and fission processes are mediated by specific sets of mechano-chemical large GTPases that belong to the Dynamin-Related Proteins (DRPs) super family. DRPs bind to cognate membranes and auto-oligomerize to drive lipid bilayers remodeling in a nucleotide dependent manner. Although structural characterization and mechanisms of DRPs that mediate membrane fission are well established, the capacity of DRPs to mediate membrane fusion is only emerging. In this review, we discuss the distinct structures and mechanisms of DRPs that trigger the anchoring and fusion of biological membranes with a specific focus on mitofusins that are dedicated to the fusion of mitochondrial outer membranes. In particular, we will highlight oligomeric assemblies of distinct DRPs and confront their mode of action against existing models of mitofusins assemblies with emphasis on recent biochemical, structural and computational reports. As we will see, the literature brings valuable insights into the presumed macro-assemblies mitofusins may form during anchoring and fusion of mitochondrial outer membranes.     

Lipids of mitochondria

A unique organelle for studying membrane biochemistry is the mitochondrion whose functionality depends on a coordinated supply of proteins and lipids. Mitochondria are capable of synthesizing several lipids autonomously such as phosphatidylglycerol, cardiolipin and in part phosphatidylethanolamine, phosphatidic acid and CDP-diacylglycerol. Other mitochondrial membrane lipids such as phosphatidylcholine, phosphatidylserine, phosphatidylinositol, sterols and sphingolipids have to be imported. The mitochondrial lipid composition, the biosynthesis and the import of mitochondrial lipids as well as the regulation of these processes will be main issues of this review article. Furthermore, interactions of lipids and mitochondrial proteins which are highly important for various mitochondrial processes will be discussed. Malfunction or loss of enzymes involved in mitochondrial phospholipid biosynthesis lead to dysfunction of cell respiration, affect the assembly and stability of the mitochondrial protein import machinery and cause abnormal mitochondrial morphology or even lethality. Molecular aspects of these processes as well as diseases related to defects in the formation of mitochondrial membranes will be described.     

Regulation of mitochondrial morphology by membrane potential,and DRP1-dependent division and FZO1-dependent fusion reaction in mammalian cells

Mitochondria are dynamic organelles that undergo frequent fission and fusion or branching. To analyze the mitochondrial fusion reaction, mitochondria were separately labeled with green or red fluorescent protein (GFP and RFP, respectively) in HeLa cells, and the cells were fused using hemagglutinating virus of Japan (HVJ). The resulting mixing of the fluorescent reporters was then followed using fluorescence microscopy. This system revealed that mitochondria fuse frequently in mammalian cells, and the fusion depends on the membrane potential across the inner membrane. The protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP), led to fragmentation of the mitochondria and inhibited the fusion reaction. Removal of CCCP recovered the fusion activity to reform filamentous mitochondrial networks. Analysis of the effects of GTP-binding proteins, DRP1 and two FZO1 isoforms, and the GTPase-domain mutants on the CCCP-induced mitochondrial morphologic changes revealed that DRP1 and FZO1 are involved in membrane budding and fusion, respectively. Furthermore, a HVJ-dependent cell fusion assay combined with RNA interference (RNAi) demonstrated that both FZO1 isoforms are essential and must be acting in cis for the mitochondrial fusion reaction to occur.     

Mitochondrial outer-membrane permeabilization and remodelling in apoptosis

Many human pathologies are associated with defects in mitochondria such as diabetes, neurodegenerative diseases or cancer. This tiny organelle is involved in a plethora of processes in mammalian cells, including energy production, lipid metabolism and cell death. In the so-called intrinsic apoptotic pathway, the outer mitochondrial membrane (MOM) is premeabilized by the pro-apoptotic Bcl-2 members Bax and Bak, allowing the release of apoptogenic factors such as cytochrome c from the inter-membrane space into the cytosol. At the same time, mitochondria fragment in response to Drp-1 activation suggesting that mitochondrial fission could play a role in mitochondrial outer-membrane permeabilization (MOMP). In this review, we will discuss the link that could exist between mitochondrial fission and fusion machinery, Bcl-2 family members and MOMP.     

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.     

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。     

Mitochondrial dynamics: to be in good shape to survive

Mitochondria are essential organelles of all eukaryotic cells that play a key role in several physiological processes and are involved in the pathology of many diseases. These organelles form a highly dynamic network, which results from continuous fusion and fission processes. Importance of these processes is underlined by inherited human diseases caused by mutations in two mitochondrial pro-fusion genes: Charcot-Marie-Tooth disease, caused by mutations in Mitofusin 2 gene and ADOA due to mutations in OPA1. During apoptosis, the mitochondrial network is disintegrated and the outer mitochondrial membrane permeabilized, which results in the release of several apoptogenic proteins, including cytochrome c. Although modulating mitochondrial fusion and fission machineries has been reported to influence the apoptotic response to various stimuli, it is still unclear whether fission is absolutely required for apoptosis. In this review, we present the latest progress in the field of mitochondrial dynamics with a particular emphasis on its implication in apoptosis and in diseases.     

综述: 与神经退行性疾病相关的RNA结合蛋白在线粒体损伤中的作用

线粒体是细胞内制造能量的细胞器,它还负责各种细胞信号的整合,参与协调多种复杂的细胞功能.线粒体是动态变化的,连续不断地进行分裂与融合,这是其功能维持和增殖遗传的关键.在过去20年中,参与线粒体分裂与融合的核心因子陆续被发现,它们在进化上高度保守,但是在形成分裂与融合复合物中的详细分子机制还有待于深入研究.线粒体分裂与融合的动态变化,是线粒体质量控制的重要组成部分,其动态平衡在细胞发育和稳态维持中起重要作用.线粒体动态变化失衡和功能失调,则会导致多种神经退行性疾病的发生.这些研究的发现为探索线粒体生物学及与疾病的关系开拓了令人振奋的新方向.     

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.     

Organellar vs cellular control of mitochondrial dynamics

Mitochondrial dynamics, the fusion and fission of individual mitochondrial units, is critical to the exchange of the metabolic, genetic and proteomic contents of individual mitochondria. In this regard, fusion and fission events have been shown to modulate mitochondrial bioenergetics, as well as several cellular processes including fuel sensing, ATP production, autophagy, apoptosis, and the cell cycle. Regulation of the dynamic events of fusion and fission occur at two redundant and interactive levels. Locally, the microenvironment of the individual mitochondrion can alter its ability to fuse, divide or move through the cell. Globally, nuclear-encoded processes and cellular ionic and second messenger systems can alter or activate mitochondrial proteins, regulate mitochondrial dynamics and concomitantly change the condition of the mitochondrial population. In this review we investigate the different global and local signals that control mitochondrial biology. This discussion is carried out to clarify the different signals that impact the status of the mitochondrial population.     

A new pathway for mitochondrial quality control: mitochondrial‐derived vesicles

The last decade has been marked by tremendous progress in our understanding of the cell biology of mitochondria, with the identification of molecules and mechanisms that regulate their fusion, fission, motility, and the architectural transitions within the inner membrane. More importantly, the manipulation of these machineries in tissues has provided links between mitochondrial dynamics and physiology. Indeed, just as the proteins required for fusion and fission were identified, they were quickly linked to both rare and common human diseases. This highlighted the critical importance of this emerging field to medicine, with new hopes of finding drugable targets for numerous pathologies, from neurodegenerative diseases to inflammation and cancer. In the midst of these exciting new discoveries, an unexpected new aspect of mitochondrial cell biology has been uncovered; the generation of small vesicular carriers that transport mitochondrial proteins and lipids to other intracellular organelles. These mitochondrial‐derived vesicles (MDVs) were first found to transport a mitochondrial outer membrane protein MAPL to a subpopulation of peroxisomes. However, other MDVs did not target peroxisomes and instead fused with the late endosome, or multivesicular body. The Parkinson's disease‐associated proteins Vps35, Parkin, and PINK1 are involved in the biogenesis of a subset of these MDVs, linking this novel trafficking pathway to human disease. In this review, we outline what has been learned about the mechanisms and functional importance of MDV transport and speculate on the greater impact of these pathways in cellular physiology.     

Intersection of mitochondrial fission and fusion machinery with apoptotic pathways: Role of Mcl‐1

Mitochondria actively contribute to apoptotic cell death through mechanisms including the loss of integrity of the outer mitochondrial membrane, the release of intermembrane space proteins, such as cytochrome c, in the cytosol and the caspase cascade activation. This process is the result of careful cooperation not only among members of the Bcl‐2 family but also dynamin‐related proteins. These events are often accompanied by fission of the organelle, thus linking mitochondrial dynamics to apoptosis. Emerging evidences are suggesting a fine regulation of mitochondrial morphology by Bcl‐2 family members and active participation of fission–fusion proteins in apoptosis. The debate whether in mitochondrial morphogenesis the role of Bcl‐2 family members is functionally distinct from their role in apoptosis is still open and, above all, which morphological changes are associated with cell death sensitisation. This review will cover the findings on how the mitochondrial fission and fusion machinery may intersect apoptotic pathways focusing on recent advances on the key role played by Mcl‐1.     

Mitochondrial dynamics in filamentous fungi

Mitochondria are essential organelles of eukaryotic cells. They grow continuously throughout the cell cycle and are inherited by daughter cells upon cell division. Inheritance of mitochondria and maintenance of mitochondrial distribution and morphology require active transport of the organelles along the cytoskeleton and depend on membrane fission and fusion events. Many of the molecular components and cellular mechanisms mediating these complex processes have been conserved during evolution across the borders of the fungal and animal kingdoms. During the past few decades, several constituents of the cellular machinery mediating mitochondrial behavior have been identified and functionally characterized. Here, we review the contributions of fungi, with special emphasis on the filamentous fungus Neurospora crassa, to our current understanding of mitochondrial morphogenesis and inheritance.     

Mitochondrial dynamic changes in health and genetic diseases

Mitochondria are highly specialized in function, but mitochondrial and, therefore, cellular integrity is maintained through their dynamic nature. Through the frequent processes of fusion and fission, mitochondria continuously change in shape and adjust function to meet cellular requirements. Abnormalities in fusion/fission dynamics generate cellular dysfunction that may lead to diseases. Mutations in the genes encoding mitochondrial fusion/fission proteins, such as MFN2 and OPA1, have been associated with an increasing number of genetic disorders, including Charcot-Marie-Tooth disease type 2A (CMT2A) and autosomal dominant optic atrophy. In this review, we address the mitochondrial dynamic changes in several important genetic diseases, which will bring the new insight of clinical relevance of mitochondrial genetics.     

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.     

Tying trafficking to fusion and fission at the mighty mitochondria

The mitochondrion is a unique organelle that serves as the main site of ATP generation needed for energy in the cell. However, mitochondria also play essential roles in cell death through apoptosis and necrosis, as well as a variety of crucial functions related to stress regulation, autophagy, lipid synthesis and calcium storage. There is a growing appreciation that mitochondrial function is regulated by the dynamics of its membrane fusion and fission; longer, fused mitochondria are optimal for ATP generation, whereas fission of mitochondria facilitates mitophagy and cell division. Despite the significance of mitochondrial homeostasis for such crucial cellular events, the intricate regulation of mitochondrial fusion and fission is only partially understood. Until very recently, only a single mitochondrial fission protein had been identified. Moreover, only now have researchers turned to address the upstream machinery that regulates mitochondrial fusion and fission proteins. Herein, we review the known GTPases involved in mitochondrial fusion and fission, but also highlight recent studies that address the mechanisms by which these GTPases are regulated. In particular, we draw attention to a substantial new body of literature linking endocytic regulatory proteins, such as the retromer VPS35 cargo selection complex subunit, to mitochondrial homeostasis. These recent studies suggest that relationships and cross‐regulation between endocytic and mitochondrial pathways may be more widespread than previously assumed.