Mdv1p is a WD repeat protein that interacts with the dynamin-related GTPase, Dnm1p, to trigger mitochondrial division

The molecular mechanism and cellular functions of mitochondrial division

Mitochondria are highly dynamic organelles that continuously divide and fuse. These dynamic processes regulate the size, shape, and distribution of the mitochondrial network. In addition, mitochondrial division and fusion play critical roles in cell physiology. This review will focus on the dynamic process of mitochondrial division, which is highly conserved from yeast to humans. We will discuss what is known about how the essential components of the division machinery function to mediate mitochondrial division and then focus on proteins that have been implicated in division but whose functions remain unclear. We will then briefly discuss the cellular functions of mitochondrial division and the problems that arise when division is disrupted.

Mitochondrial remodeling in differentiating neuroblasts

Mitochondria are able to change their shape through fission and fusion events, leading to a continuous remodeling of the mitochondrial network. Whereas the mitochondrial fission has been extensively studied and primarily related to the onset and progression of apoptosis, the physiological function of the opposite process of fusion is far less understood. With this study we analyzed the process of mitochondrial fusion in immortalized hippocampal neuroblasts searching for a relationship with specific changes in cellular physiology. The mitochondrial dynamics was examined in every stage of the cell cycle and a link was found between the enhancement of the mitochondrial transmembrane potential DeltaPsi(m), the widespread mitochondrial fusion and the process of neurite outgrowth. An identical mitochondrial reorganization also appeared in response to the pro-differentiating agent retinoic acid. The single-cell analysis in time-lapse of the mitochondrial response to RA evidenced a free calcium raise in the mitochondrial matrix coupled with the DeltaPsi(m) increase and it confirmed the close coordination between these two events and the fusion of mitochondria. The modulation of oxidative phosphorylation by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) or pyruvate, underscored the importance of DeltaPsi(m) changes both in shaping the mitochondrial network and in regulating the rate of neurite outgrowth. We also report that the mitochondrial fusion observed during neurite outgrowth is not a consequence of the microtubule reorganization, since pharmacological treatments capable of blocking the microtubule dynamics failed to inhibit the mitochondrial remodeling in response to RA.

Mitochondrial dynamics and apoptosis

In healthy cells, mitochondria continually divide and fuse to form a dynamic interconnecting network. The molecular machinery that mediates this organelle fission and fusion is necessary to maintain mitochondrial integrity, perhaps by facilitating DNA or protein quality control. This network disintegrates during apoptosis at the time of cytochrome c release and prior to caspase activation, yielding more numerous and smaller mitochondria. Recent work shows that proteins involved in mitochondrial fission and fusion also actively participate in apoptosis induction. This review will cover the recent advances and presents competing models on how the mitochondrial fission and fusion machinery may intersect apoptosis pathways.

Positioning mitochondrial plasticity within cellular signaling cascades

Mitochondria evolved from alpha-proteobacteria captured within a host between two and three billion years ago. This origin resulted in the formation of a double-layered organelle resulting in four distinct sub-compartments: the outer membrane, the intermembrane space, the inner membrane and the matrix. The inner membrane is organized in cristae, harboring the respiratory chain and ATP synthase complexes responsible of the oxidative phosphorylation, the main energy-generating system of the cell. It is generally considered that the ultrastructure of the inner membrane provides a large variety of morphologies that facilitate metabolic output. This classical view of mitochondria as bean-shaped organelles was static until in the last decade when new imaging studies and genetic screens provided a more accurate description of a dynamic mitochondrial reticulum that fuse and divide continuously. Since then significant findings have been made in the study of machineries responsible for fusion, fission and motility, however the mechanisms and signals that regulate mitochondrial dynamics are only beginning to emerge. A growing body of evidence indicates that metabolic and cellular signals influence mitochondrial dynamics, leading to a new understanding of how changes in mitochondrial shape can have a profound impact on the functional output of the organelle. The mechanisms that regulate mitochondrial morphology are incompletely understood, but evidence to date suggests that the morphology machinery is modulated through the use of post-translational modifications, including nucleotide-binding proteins, phosphorylation, ubiquitination, SUMOylation, and changes in the lipid environment. This review focuses on the molecular switches that control mitochondrial dynamics and the integration of mitochondrial morphology within cellular signaling cascades.

Regulation of mitochondrial dynamics--characterization of fusion and fission genes in the ascomycete Podospora anserina

The filamentous ascomycete Podospora anserina is a model system for studying aging, a complex process that is regulated by multiple factors. Among these, mitochondria were shown to be of crucial importance. Recently, it was shown that the morphology of these organelles, which is dependent on dynamic fusion and fission processes, has profound effects on P. anserina aging. To further analyze this phenomenon, we characterized molecular components of the machinery regulating the dynamic behavior of mitochondria by utilizing transgenic strains in which fission genes (PaDnm1, PaFis1 and PaMdv1) and a fusion gene (PaFzo1) are overexpressed. While overexpression of PaFis1 has no phenotypic effects in the genetic background of the wild type, it surprisingly promotes mitochondrial fusion and decreases the life span in a mutant overexpressing PaDnm1. Remarkably, when grown on synthetic medium, overexpression of PaDnm1 leads to a decreased life span compared to the wild type. Increased expression of PaMdv1 results in the formation of ring-shaped mitochondria, a morphology of these organelles that has not been previously observed in P. anserina. Transformants with elevated PaFzo1 transcript levels show no altered life span, although the age-dependent fragmentation of mitochondria is impaired.

Arabidopsis ELONGATED MITOCHONDRIA1 is required for localization of DYNAMIN-RELATED PROTEIN3A to mitochondrial fission sites

Mitochondrial fission is achieved partially by the activity of self-assembling dynamin-related proteins (DRPs) in diverse organisms. Mitochondrial fission in Arabidopsis thaliana is mediated by DRP3A and DRP3B, but the other genes and molecular mechanisms involved have yet to be elucidated. To identify these genes, we screened and analyzed Arabidopsis mutants with longer and fewer mitochondria than those of the wild type. ELM1 was found to be responsible for the phenotype of elongated mitochondria. This phenotype was also observed in drp3a plants. EST and genomic sequences similar to ELM1 were found in seed plants but not in other eukaryotes. ELM1:green fluorescent protein (GFP) was found to surround mitochondria, and ELM1 interacts with both DPR3A and DRP3B. In the elm1 mutant, DRP3A:GFP was observed in the cytosol, whereas in wild-type Arabidopsis, DRP3A:GFP localized to the ends and constricted sites of mitochondria. These results collectively suggest that mitochondrial fission in Arabidopsis is mediated by the plant-specific factor ELM1, which is required for the relocalization of DRP3A (and possibly also DRP3B) from the cytosol to mitochondrial fission sites.

Dnm1p-dependent peroxisome fission requires Caf4p, Mdv1p and Fis1p

Yeast peroxisomes multiply by fission. Fission requires two dynamin-related proteins, Dnm1p and Vps1p. Using an in vivo fission assay, we show that Dnm1p-dependent peroxisome fission requires Fis1p, Caf4p and Mdv1p. Fluorescence microscopy of cells expressing GFP-tagged Caf4p and Mdv1p revealed that their association with peroxisomes relies on Fis1p. Vps1p-dependent peroxisome fission occurs independently of these factors. Vps1p contributes most to fission of peroxisomes when cells are grown on glucose. Overexpression of Dnm1p suppresses the fission defect as long as Fis1p and either Mdv1p or Caf4p are present. Conversely, overexpression of Dnm1p does not restore the vacuolar fusion defect of vps1 cells and Vps1p overexpression does not restore the mitochondrial fission defect of dnm1 cells. These data show that Vps1p and Dnm1p are part of independent fission machineries. Because the contribution of Dnm1p to peroxisome fission appears to be more pronounced in cells that proliferate peroxisomes in response to mitochondrial dysfunction, Dnm1p might be part of the mechanism that coordinates mitochondrial and peroxisomal biogenesis.

Mitochondrial death pathways in yeast and mammalian cells

In mammals, mitochondria are important mediators of programmed cell death, and this process is often regulated by Bcl-2 family proteins. However, a role for mitochondria-mediated cell death in non-mammalian species is more controversial. New evidence from a variety of sources suggests that mammalian mitochondrial fission/division proteins also have the capacity to promote programmed cell death, which may involve interactions with Bcl-2 family proteins. Homologues of these fission factors and several additional mammalian cell death regulators are conserved in flies, worms and yeast, and have been suggested to regulate programmed cell death in these species as well. However, the molecular mechanisms by which these phylogenetically conserved proteins contribute to cell death are not known for any species. Some have taken the conserved pro-death activity of mitochondrial fission factors to mean that mitochondrial fission per se, or failed attempts to undergo fission, are directly involved in cell death. Other evidence suggests that the fission function and the cell death function of these factors are separable. Here we consider the evidence for these arguments and their implications regarding the origins of programmed cell death.

The novel tail-anchored membrane protein Mff controls mitochondrial and peroxisomal fission in mammalian cells

Few components of the mitochondrial fission machinery are known, even though mitochondrial fission is a complex process of vital importance for cell growth and survival. Here, we describe a novel protein that controls mitochondrial fission. This protein was identified in a small interfering RNA (siRNA) screen using Drosophila cells. The human homologue of this protein was named Mitochondrial fission factor (Mff). Mitochondria of cells transfected with Mff siRNA form a closed network similar to the mitochondrial networks formed when cells are transfected with siRNA for two established fission proteins, Drp1 and Fis1. Like Drp1 and Fis1 siRNA, Mff siRNA also inhibits fission induced by loss of mitochondrial membrane potential, it delays cytochrome c release from mitochondria and further progression of apoptosis, and it inhibits peroxisomal fission. Mff and Fis1 are both tail anchored in the mitochondrial outer membrane, but other parts of these proteins are very different and they exist in separate 200-kDa complexes, suggesting that they play different roles in the fission process. We conclude that Mff is a novel component of a conserved membrane fission pathway used for constitutive and induced fission of mitochondria and peroxisomes.

The C. elegans Opa1 homologue EAT-3 is essential for resistance to free radicals

The C. elegans eat-3 gene encodes a mitochondrial dynamin family member homologous to Opa1 in humans and Mgm1 in yeast. We find that mutations in the C. elegans eat-3 locus cause mitochondria to fragment in agreement with the mutant phenotypes observed in yeast and mammalian cells. Electron microscopy shows that the matrices of fragmented mitochondria in eat-3 mutants are divided by inner membrane septae, suggestive of a specific defect in fusion of the mitochondrial inner membrane. In addition, we find that C. elegans eat-3 mutant animals are smaller, grow slower, and have smaller broodsizes than C. elegans mutants with defects in other mitochondrial fission and fusion proteins. Although mammalian Opa1 is antiapoptotic, mutations in the canonical C. elegans cell death genes ced-3 and ced-4 do not suppress the slow growth and small broodsize phenotypes of eat-3 mutants. Instead, the phenotypes of eat-3 mutants are consistent with defects in oxidative phosphorylation. Moreover, eat-3 mutants are hypersensitive to paraquat, which promotes damage by free radicals, and they are sensitive to loss of the mitochondrial superoxide dismutase sod-2. We conclude that free radicals contribute to the pathology of C. elegans eat-3 mutants.

Chloroplast division

Chloroplasts are descendants of cyanobacteria and divide by binary fission. Several components of the division apparatus have been identified in the past several years and we are beginning to appreciate the plastid division process at a mechanistic level. In this review, we attempt to summarize the most recent developments in the field and assemble these observations into a working model of plastid division in plants.

Vesicle, mitochondrial, and plastid division machineries with emphasis on dynamin and electron-dense rings

The original eukaryotic cells contained at least one set of double-membrane-bounded organelles (cell nucleus and mitochondria) and single-membrane-bounded organelles [endoplasmic reticulum, Golgi apparatus, lysosomes (vacuoles), and microbodies (peroxisomes)]. An increase in the number of organelles accompanied the evolution of these cells into Amoebozoa and Opisthokonta. Furthermore, the basic cells, containing mitochondria, engulfed photosynthetic Cyanobacteria, which were converted to plastids, and the cells thereby evolved into cells characteristic of the Bikonta. How did basic single- and double-membrane-bounded organelles originate from bacteria-like cells during early eukaryotic evolution? To answer this question, the important roles of the GTPase dynamin- and electron-dense rings in the promotion of diverse cellular activities in eukaryotes, including endocytosis, vesicular transport, mitochondrial division, and plastid division, must be considered. In this review, vesicle division, mitochondrial division, and plastid division machineries, including the dynamin- and electron-dense rings, and their roles in the origin and biogenesis of organelles in eukaryote cells are summarized.

Dynamin-dependent biogenesis, cell cycle regulation and mitochondrial association of peroxisomes in fission yeast

Peroxisomes were visualized for the first time in living fission yeast cells. In small, newly divided cells, the number of peroxisomes was low but increased in parallel with the increase in cell length/volume that accompanies cell cycle progression. In cells grown in oleic acid, both the size and the number of peroxisomes increased. The peroxisomal inventory of cells lacking the dynamin-related proteins Dnm1 or Vps1 was similar to that in wild type. By contrast, cells of the double mutant dnm1Delta vps1Delta contained either no peroxisomes at all or a small number of morphologically aberrant organelles. Peroxisomes exhibited either local Brownian movement or longer-range linear displacements, which continued in the absence of either microtubules or actin filaments. On the contrary, directed peroxisome motility appeared to occur in association with mitochondria and may be an indirect function of intrinsic mitochondrial dynamics. We conclude that peroxisomes are present in fission yeast and that Dnm1 and Vps1 act redundantly in peroxisome biogenesis, which is under cell cycle control. Peroxisome movement is independent of the cytoskeleton but is coupled to mitochondrial dynamics.

Alternative function for the mitochondrial SAM complex in biogenesis of alpha-helical TOM proteins

The mitochondrial outer membrane contains two preprotein translocases: the general translocase of outer membrane (TOM) and the β-barrel–specific sorting and assembly machinery (SAM). TOM functions as the central entry gate for nuclear-encoded proteins. The channel-forming Tom40 is a β-barrel protein, whereas all Tom receptors and small Tom proteins are membrane anchored by a transmembrane α-helical segment in their N- or C-terminal portion. Synthesis of Tom precursors takes place in the cytosol, and their import occurs via preexisting TOM complexes. The precursor of Tom40 is then transferred to SAM for membrane insertion and assembly. Unexpectedly, we find that the biogenesis of α-helical Tom proteins with a membrane anchor in the C-terminal portion is SAM dependent. Each SAM protein is necessary for efficient membrane integration of the receptor Tom22, whereas assembly of the small Tom proteins depends on Sam37. Thus, the substrate specificity of SAM is not restricted to β-barrel proteins but also includes the majority of α-helical Tom proteins.

Peroxisome proliferation in Hansenula polymorpha requires Dnm1p which mediates fission but not de novo formation

We show that the dynamin-like proteins Dnm1p and Vps1p are not required for re-introduction of peroxisomes in Hansenula polymorpha pex3 cells upon complementation with PEX3-GFP. Instead, Dnm1p, but not Vps1p, plays a crucial role in organelle proliferation via fission. In H. polymorpha DNM1 deletion cells (dnm1) a single peroxisome is present that forms long extensions, which protrude into developing buds and divide during cytokinesis. Budding pex11.dnm1 double deletion cells lack these peroxisomal extensions, suggesting that the peroxisomal membrane protein Pex11p is required for their formation. Life cell imaging revealed that fluorescent Dnm1p-GFP spots fluctuate between peroxisomes and mitochondria. On the other hand Pex11p is present over the entire organelle surface, but concentrates during fission at the basis of the organelle extension in dnm1 cells. Our data indicate that peroxisome fission is the major pathway for peroxisome multiplication in H. polymorpha.

Structural basis for recruitment of mitochondrial fission complexes by Fis1

Mitochondrial fission controls mitochondrial shape and physiology, including mitochondrial remodeling in apoptosis. During assembly of the yeast mitochondrial fission complex, the outer membrane protein Fis1 recruits the dynamin-related GTPase Dnm1 to mitochondria. Fis1 contains a tetratricopeptide repeat (TPR) domain and interacts with Dnm1 via the molecular adaptors Mdv1 and Caf4. By using crystallographic analysis of adaptor-Fis1 complexes, we show that these adaptors use two helices to bind to both the concave and convex surfaces of the Fis1 TPR domain. Fis1 therefore contains two interaction interfaces, a binding mode that, to our knowledge, has not been observed previously for TPR domains. Genetic and biochemical studies indicate that both binding interfaces are important for binding of Mdv1 and Caf4 to Fis1 and for mitochondrial fission activity in vivo. Our results reveal how Fis1 recruits the mitochondrial fission complex and will facilitate efforts to manipulate mitochondrial fission.

Functions and dysfunctions of mitochondrial dynamics

Recent findings have sparked renewed appreciation for the remarkably dynamic nature of mitochondria. These organelles constantly fuse and divide, and are actively transported to specific subcellular locations. These dynamic processes are essential for mammalian development, and defects lead to neurodegenerative disease. But what are the molecular mechanisms that control mitochondrial dynamics, and why are they important for mitochondrial function? We review these issues and explore how defects in mitochondrial dynamics might cause neuronal disease.

Regulation of mitochondrial fusion and division

In many organisms, ranging from yeast to humans, mitochondria fuse and divide to change their morphology in response to a multitude of signals. During the past decade, work using yeast and mammalian cells has identified much of the machinery required for fusion and division, including the dynamin-related GTPases--mitofusins (Fzo1p in yeast) and OPA1 (Mgm1p in yeast) for fusion and Drp1 (Dnm1p) for division. However, the mechanisms by which cells regulate these dynamic processes have remained largely unknown. Recent studies have uncovered regulatory mechanisms that control the activity, assembly, distribution and stability of the key components for mitochondrial fusion and division. In this review, we discuss how mitochondrial dynamics are controlled and how these events are coordinated with cell growth, mitosis, apoptosis and human diseases.

Direct binding of the dynamin-like GTPase, Dnm1, to mitochondrial dynamics protein Fis1 is negatively regulated by the Fis1 N-terminal arm

Recruitment of a dynamin-like GTPase (Drp1/Dlp1/Dnm1) to membranes requires the mitochondrial dynamics protein Fis1. Mdv1 has been proposed to act as an adaptor between Fis1 and Dnm1 in Saccharomyces cerevisiae. We show that S. cerevisiae Fis1 binds directly to Dnm1 and to Mdv1. Two Fis1 regions have been previously implicated in Mdv1 recruitment: an N-terminal "arm" and a concave surface formed by evolutionarily conserved residues in the tetratricopeptide repeat domain. Perturbing either Fis1 region does not affect Mdv1 binding, but both regions influence Dnm1 binding. Fis1 lacking its N-terminal arm binds tightly to Dnm1, and binding is abolished by mutations to the Fis1 concave surface. The Fis1-Dnm1 interaction decreases more than 100-fold in the presence of the Fis1 arm, suggesting that the arm acts in an autoinhibitory manner to restrict access to the Dnm1 binding site on Fis1. Our data indicate that the concave surface of the Fis1 tetratricopeptide repeat-like domain is evolutionarily conserved to bind the dynamin-like GTPase Dnm1 and not Mdv1 as previously predicted.

The machines that divide and fuse mitochondria

Mitochondria are derived from eubacteria; however, in most eukaryotes, novel mechanisms for the propagation of this organelle and its genome have evolved. This review focuses on what is currently known about the novel molecular machines that divide and fuse mitochondria.

The mitochondrial E3 ubiquitin ligase MARCH5 is required for Drp1 dependent mitochondrial division

We identify a mitochondrial E3 ubiquitin ligase, MARCH5, as a critical regulator of mitochondrial fission. MARCH5 RING mutants and MARCH5 RNA interference induce an abnormal elongation and interconnection of mitochondria indicative of an inhibition of mitochondrial division. The aberrant mitochondrial phenotypes in MARCH5 RING mutant–expressing cells are reversed by ectopic expression of Drp1, but not another mitochondrial fission protein Fis1. Moreover, as indicated by abnormal clustering and mitochondrial accumulation of Drp1, as well as decreased cellular mobility of YFP-Drp1 in cells expressing MARCH5 RING mutants, MARCH5 activity regulates the subcellular trafficking of Drp1, likely by impacting the correct assembly at scission sites or the disassembly step of fission complexes. Loss of this activity may account for the observed mitochondrial division defects. Finally, MARCH5 RING mutants and endogenous Drp1, but not wild-type MARCH5 or Fis1, co-assemble into abnormally enlarged clusters in a Drp1 GTPase-dependent manner, suggesting molecular interactions among these proteins. Collectively, our data suggest a model in which mitochondrial division is regulated by a MARCH5 ubiquitin-dependent switch.

Ethanol-induced death in yeast exhibits features of apoptosis mediated by mitochondrial fission pathway

Cell death in yeast (Saccharomyces cerevisiae) involves several apoptotic processes. Here, we report the first evidence of the following processes, which are also characteristic of apoptosis, in ethanol-induced cell death in yeast: chromatin condensation and fragmentation, DNA cleavage, and a requirement for de novo protein synthesis. Mitochondrial fission protein, Fis1, appears to mediate ethanol-induced apoptosis and ethanol-induced mitochondrial fragmentation. However, mitochondrial fragmentation in response to elevated ethanol levels was not correlated with cell death. Further, in the presence of ethanol, generation of reactive oxygen species was elevated in mutant fis1Delta cells. Our characterization of ethanol-induced cell death in yeast as being Fis1-mediated apoptosis is likely to pave the way to overcoming limitations in large-scale fermentation processes, such as those employed in the production of alcoholic beverages and ethanol-based biofuels.

Bax/Bak promote sumoylation of DRP1 and its stable association with mitochondria during apoptotic cell death

Dynamin-related protein 1 (DRP1) plays an important role in mitochondrial fission at steady state and during apoptosis. Using fluorescence recovery after photobleaching, we demonstrate that in healthy cells, yellow fluorescent protein (YFP)–DRP1 recycles between the cytoplasm and mitochondria with a half-time of 50 s. Strikingly, during apoptotic cell death, YFP-DRP1 undergoes a transition from rapid recycling to stable membrane association. The rapid cycling phase that characterizes the early stages of apoptosis is independent of Bax/Bak. However, after Bax recruitment to the mitochondrial membranes but before the loss of mitochondrial membrane potential, YFP-DRP1 becomes locked on the membrane, resulting in undetectable fluorescence recovery. This second phase in DRP1 cycling is dependent on the presence of Bax/Bak but independent of hFis1 and mitochondrial fragmentation. Coincident with Bax activation, we detect a Bax/Bak-dependent stimulation of small ubiquitin-like modifier-1 conjugation to DRP1, a modification that correlates with the stable association of DRP1 with mitochondrial membranes. Altogether, these data demonstrate that the apoptotic machinery regulates the biochemical properties of DRP1 during cell death.

Yeast mitochondrial division and distribution require the cortical num1 protein

Yeast mitochondrial division requires the dynamin-related Dnm1 protein. By isolating high-copy suppressors of a dominant-negative Dnm1p mutant, we uncovered an unexpected role in mitochondrial division and inheritance for Num1p, a protein previously shown to facilitate nuclear migration. num1 mutants contain an interconnected network of mitochondrial tubules, remarkably similar to cells lacking Dnm1p, and time-lapse microscopy confirms that mitochondrial fission is greatly reduced in num1Delta cells. We also find that Num1p assembles into punctate structures, which often colocalize with mitochondrial-bound Dnm1p particles. Suggesting a role for both Num1p and Dnm1p in mitochondrial inheritance, we find that num1 dnm1 double mutants accumulate mitochondria in daughter buds and that mother cells are frequently devoid of all mitochondria. Thus, our studies have revealed an additional role for Dnm1p in mitochondrial transmission through its interaction with Num1p, thereby providing a link between mitochondrial division and inheritance.

The morphology proteins Mdm12/Mmm1 function in the major beta-barrel assembly pathway of mitochondria

The beta-barrel proteins of mitochondria are synthesized on cytosolic ribosomes. The proteins are imported by the translocase of the outer membrane (TOM) and the sorting and assembly machinery (SAM). It has been assumed that the SAM(core) complex with the subunits Sam35, Sam37 and Sam50 represents the last import stage common to all beta-barrel proteins, followed by splitting in a Tom40-specific route and a route for other beta-barrel proteins. We have identified new components of the beta-barrel assembly machinery and show that the major beta-barrel pathway extends beyond SAM(core). Mdm12/Mmm1 function after SAM(core) yet before splitting of the major pathway. Mdm12/Mmm1 have been known for their role in maintenance of mitochondrial morphology but we reveal assembly of beta-barrel proteins as their primary function. Moreover, Mdm10, which functions in the Tom40-specific route, can associate with SAM(core) as well as Mdm12/Mmm1 to form distinct assembly complexes, indicating a dynamic exchange between the machineries governing mitochondrial beta-barrel assembly. We conclude that assembly of mitochondrial beta-barrel proteins represents a major function of the morphology proteins Mdm12/Mmm1.

Fis1, DLP1, and Pex11p coordinately regulate peroxisome morphogenesis

Dynamin-like protein 1 (DLP1) and Pex11pbeta function in morphogenesis of peroxisomes. In the present work, we investigated whether Fis1 is involved in fission of peroxisomes. Endogenous Fis1 was morphologically detected in peroxisomes as well as mitochondria in wild-type CHO-K1 and DLP1-defective ZP121 cells. Subcellular fractionation studies also revealed the presence of Fis1 in peroxisomes. Peroxisomal Fis1 showed the same topology, i.e., C-tail anchored membrane protein, as the mitochondrial one. Furthermore, ectopic expression of FIS1 induced peroxisome proliferation in CHO-K1 cells, while the interference of FIS1 RNA resulted in tubulation of peroxisomes, hence reducing the number of peroxisomes. Fis1 interacted with Pex11pbeta, by direct binding apparently involving the C-terminal region of Pex11pbeta in the interaction. Pex11pbeta also interacted with each other, whereas the binding of Pex11pbeta to DLP1 was not detectable. Moreover, ternary complexes comprising Fis1, Pex11pbeta, and DLP1 were detected by chemical cross-linking. We also showed that the highly conserved N-terminal domain of Pex11pbeta was required for the homo-oligomerization of Pex11pbeta and indispensable for the peroxisome-proliferating activity. Taken together, these findings indicate that Fis1 plays important roles in peroxisome division and maintenance of peroxisome morphology in mammalian cells, possibly in a concerted manner with Pex11pbeta and DLP1.

WD40 protein Mda1 is purified with Dnm1 and forms a dividing ring for mitochondria before Dnm1 in Cyanidioschyzon merolae

Mitochondria are not produced de novo but are maintained by division. Mitochondrial division is a coordinated process of positioning and constriction of the division site and fission of double membranes, in which dynamin-related protein is believed to mediate outer membrane fission. Part of the mitochondrial division machinery was purified from M phase-arrested Cyanidioschyzon merolae cells through biochemical fractionation. The dynamin-related protein Dnm1 was one of the two major proteins in the purified fraction and was accompanied by a newly identified protein CMR185C, named Mda1. Mda1 contained a predictable coiled-coil region and WD40 repeats, similarly to Mdv1 and Caf4 in yeasts. Immunofluorescence and immunoelectron microscopy showed that Mda1 localizes as a medial belt or ring on the mitochondrial outer surface throughout the division. The ring formation of Mda1 followed the plane of the ring of FtsZ, a protein that resides in the matrix. Dnm1 consistently colocalized with Mda1 only in the late stages of division. Mda1 protein was expressed through S to M phases and was phosphorylated specifically in M phase when Mda1 transformed from belt into foci and became colocalizing with Dnm1. Dephosphorylation of Mda1 in vitro increased its sedimentation coefficient, suggesting conformational changes of the macromolecule. Disassembly of the purified mitochondrial division machinery was performed by adding GTP to independently release Dnm1, suggesting that Mda1 forms a stable homo-oligomer by itself as a core structure of the mitochondrial division machinery.

Human Misato regulates mitochondrial distribution and morphology

Misato of Drosophila melanogaster and Saccharomyces cerevisiae DML1 are conserved proteins having a homologous region with a part of the GTPase family that includes eukaryotic tubulin and prokaryotic FtsZ. We characterized human Misato sharing homology with Misato of D. melanogaster and S. cerevisiae DML1. Tissue distribution of Misato exhibited ubiquitous distribution. Subcellular localization of the protein studied using anti-Misato antibody suggested that it is localized to the mitochondria. Further experiments of fractionating mitochondria revealed that Misato was localized to the outer membrane. The transfection of Misato siRNA led to growth deficiencies compared with control siRNA transfected HeLa cells, and the Misato-depleted HeLa cells showed apoptotic nuclear fragmentation resulting in cell death. After silencing of Misato, the filamentous mitochondrial network disappeared and fragmented mitochondria were observed, indicating human Misato has a role in mitochondrial fusion. To examine the effects of overexpression, COS-7 cells were transfected with cDNA encoding EGFP-Misato. Its overexpression resulted in the formation of perinuclear aggregations of mitochondria in these cells. The Misato-overexpressing cells showed low viability and had no nuclei or a small and structurally unusual ones. These results indicated that human Misato has a role(s) in mitochondrial distribution and morphology and that its unregulated expression leads to cell death.

Mitochondrial morphology and distribution in mammalian cells

It is now appreciated that mitochondria form tubular networks that adapt to the requirements of the cell by undergoing changes in their shape through fission and fusion. Proper mitochondrial distribution also appears to be required for ATP delivery and calcium regulation, and, in some cases, for cell development. While we now realise the great importance of mitochondria for the cell, we are only beginning to work out how these organelles undergo the drastic morphological changes that are essential for cellular function. Of the few known components involved in shaping mitochondria, some have been found to be essential to life and their gene mutations are linked to neurological disorders, while others appear to be recruited in the activation of cell death pathways. Here we review our current understanding of the functions of the main players involved in mitochondrial fission, fusion and distribution in mammalian cells.

Molecular machinery of mitochondrial dynamics in yeast

Mitochondria are amazingly dynamic organelles. They continuously move along cytoskeletal tracks and frequently fuse and divide. These processes are important for maintenance of mitochondrial functions, for inheritance of the organelles upon cell division, for cellular differentiation and for apoptosis. As the machinery of mitochondrial behavior has been highly conserved during evolution, it can be studied in simple model organisms, such as yeast. During the past decade, several key components of mitochondrial dynamics have been identified and functionally characterized in Saccharomyces cerevisiae. These include the mitochondrial fusion and fission machineries and proteins required for maintenance of tubular shape and mitochondrial motility. Taken together, these findings reveal a comprehensive picture that shows the cellular processes and molecular components required for mitochondrial inheritance and morphogenesis in a simple eukaryotic cell.

Mitochondrial fusion and fission in mammals

Eukaryotic cells maintain the overall shape of their mitochondria by balancing the opposing processes of mitochondrial fusion and fission. Unbalanced fission leads to mitochondrial fragmentation, and unbalanced fusion leads to mitochondrial elongation. Moreover, these processes control not only the shape but also the function of mitochondria. Mitochondrial dynamics allows mitochondria to interact with each other; without such dynamics, the mitochondrial population consists of autonomous organelles that have impaired function. Key components of the mitochondrial fusion and fission machinery have been identified, allowing initial dissection of their mechanisms of action. These components play important roles in mitochondrial function and development as well as programmed cell death. Disruption of the fusion machinery leads to neurodegenerative disease.

Mitochondrial trafficking and morphology in healthy and injured neurons

Mitochondria are the primary generators of ATP and are important regulators of intracellular calcium homeostasis. These organelles are dynamically transported along lengthy neuronal processes, presumably for appropriate distribution to cellular regions of high metabolic demand and elevated intracellular calcium, such as synapses. The removal of damaged mitochondria that produce harmful reactive oxygen species and promote apoptosis is also thought to be mediated by transport of mitochondria to autophagosomes. Mitochondrial trafficking is therefore important for maintaining neuronal and mitochondrial health while cessation of movement may lead to neuronal and mitochondrial dysfunction. Mitochondrial morphology is also dynamic and is remodeled during neuronal injury and disease. Recent studies reveal different manifestations and mechanisms of impaired mitochondrial movement and altered morphology in injured neurons. These are likely to cause varied courses toward neuronal degeneration and death. The goal of this review is to provide an appreciation of the full range of mitochondrial function, morphology and trafficking, and the critical role these parameters play in neuronal physiology and pathophysiology.

PDV1 and PDV2 mediate recruitment of the dynamin-related protein ARC5 to the plastid division site

During plastid division, the dynamin-related protein ACCUMULATION AND REPLICATION OF CHLOROPLASTS5 (ARC5) is recruited from the cytosol to the surface of the outer chloroplast envelope membrane. In Arabidopsis thaliana arc5 mutants, chloroplasts arrest during division site constriction. Analysis of mutants similar to arc5 along with map-based cloning identified PLASTID DIVISION1 (PDV1), an integral outer envelope membrane protein, and its homolog PDV2 as components of the plastid division machinery. Similar to ARC5, PDV1 localized to a discontinuous ring at the division site in wild-type plants. The midplastid PDV1 ring formed in arc5 mutants and the ARC5 ring formed in pdv1 and pdv2 mutants, but not in pdv1 pdv2. Stromal FtsZ ring assembly occurred in pdv1, pdv2, and pdv1 pdv2, as it does in arc5. Topological analysis showed that the large N-terminal region of PDV1 upstream of the transmembrane helix bearing a putative coiled-coil domain is exposed to the cytosol. Mutation of the conserved PDV1 C-terminal Gly residue did not block PDV1 insertion into the outer envelope membrane but did abolish its localization to the division site. Our results indicate that plastid division involves the stepwise localization of FtsZ, PDV1, and ARC5 at the division site and that PDV1 and PDV2 together mediate the recruitment of ARC5 to the midplastid constriction at a late stage of division.

(De)constructing mitochondria: what for?

Mitochondria are dynamic organelles, essential for cell life and death. The morphology of this organelle is determined by fusion and fission, controlled by a growing set of "mitochondria-shaping" proteins, which influence crucial signalling cascades, including apoptosis.

Mitochondria and cancer: is there a morphological connection?

Mitochondria are key players in several cellular functions including growth, division, energy metabolism, and apoptosis. The mitochondrial network undergoes constant remodelling and these morphological changes are of direct relevance for the role of this organelle in cell physiology. Mitochondrial dysfunction contributes to a number of human disorders and may aid cancer progression. Here, we summarize the recent contributions made in the field of mitochondrial dynamics and discuss their impact on our understanding of cell function and tumorigenesis.

Mitochondrial factors with dual roles in death and survival

At least in mammals, we have some understanding of how caspases facilitate mitochondria-mediated cell death, but the biochemical mechanisms by which other factors promote or inhibit programmed cell death are not understood. Moreover, most of these factors are only studied after treating cells with a death stimulus. A growing body of new evidence suggests that cell death regulators also have 'day jobs' in healthy cells. Even caspases, mitochondrial fission proteins and pro-death Bcl-2 family proteins appear to have normal cellular functions that promote cell survival. Here, we review some of the supporting evidence and stretch beyond the evidence to seek an understanding of the remaining questions.

Fis1p and Caf4p, but not Mdv1p, determine the polar localization of Dnm1p clusters on the mitochondrial surface

The mitochondrial division machinery consists of the large dynamin-related protein Dnm1p (Drp1/Dlp1 in humans), and Fis1p, Mdv1p and Caf4p. Proper assembly of Dnm1p complexes on the mitochondrial surface is crucial for balanced fission and fusion events. Using quantitative confocal microscopy, we show that Caf4p is important for the recruitment of Dnm1p to the mitochondria. The mitochondrial Dnm1p assemblies can be divided into at least two morphologically distinguishable fractions. A small subset of these assemblies appear to be present as Dnm1p-spirals (or rings) that encircle tubule constrictions, with seldom more than seven turns. A larger fraction of the Dnm1p assemblies is primarily present at one side of the mitochondrial tubules. We show that a majority of these mitochondria-associated Dnm1p clusters point towards the cell cortex. This polarized orientation is abolished in fis1Delta and caf4Delta yeast cells, but is maintained in mdv1Delta cells and after disruption of the actin cytoskeleton. This study suggests that Caf4p plays a key role in determining the polarized localization of those Dnm1p clusters that are not immediately involved in the mitochondrial fission process.

Differences in mitochondrial movement and morphology in young and mature primary cortical neurons in culture

Mitochondria have many roles critical to the function of neurons including the generation of ATP and regulation of intracellular Ca2+. Mitochondrial movement is highly dynamic in neurons and is thought to direct mitochondria to specific cellular regions of increased need and to transport damaged or old mitochondria to autophagosomes. Morphology also varies between individual mitochondria and is modulated by fusion and fission proteins such as mitofusin-1 and dynamin-related protein-1, respectively. Although mitochondrial movement and morphology are thought to be modulated to best meet cellular demands, few regulatory signals have been identified. In this study, we examined how the different cellular environments of synaptically immature and mature rat cortical neurons affect mitochondrial movement, morphology, distribution and function. In younger cells, mitochondria were more mobile, were shorter, occupied a smaller percentage of neuronal processes, and expressed greater mitofusin-1 and lower dynamin-related protein-1 protein levels compared with older cells. However, the number of mitochondria per mum of neuronal process, mitochondrial membrane potential and the amount of basally sequestered mitochondrial Ca2+ were similar. Our results suggest that while mitochondria in young neurons are functionally similar to mature neurons, their enhanced motility may permit faster energy dispersal for cellular demands, such as synaptogenesis. As cells mature, mitochondria in the processes may then elongate and reduce their motility for long-term support of synaptic structures.

The novel F-box protein Mfb1p regulates mitochondrial connectivity and exhibits asymmetric localization in yeast

Although it is clear that mitochondrial morphogenesis is a complex process involving multiple proteins in eukaryotic cells, little is known about regulatory molecules that modulate mitochondrial network formation. Here, we report the identification of a new yeast mitochondrial morphology gene called MFB1 (YDR219C). MFB1 encodes an F-box protein family member, many of which function in Skp1-Cdc53/Cullin-F-box protein (SCF) ubiquitin ligase complexes. F-box proteins also act in non-SCF complexes whose functions are not well understood. Although cells lacking Mfb1p contain abnormally short mitochondrial tubules, Mfb1p is not essential for known pathways that determine mitochondrial morphology and dynamics. Mfb1p is peripherally associated with the mitochondrial surface. Coimmunoprecipitation assays reveal that Mfb1p interacts with Skp1p in an F-box-dependent manner. However, Mfb1p does not coimmunoprecipitate with Cdc53p. The F-box motif is not essential for Mfb1p-mediated mitochondrial network formation. These observations suggest that Mfb1p acts in a complex lacking Cdc53p required for mitochondrial morphogenesis. During budding, Mfb1p asymmetrically localizes to mother cell mitochondria. By contrast, Skp1p accumulates in the daughter cell cytoplasm. Mfb1p mother cell-specific asymmetry depends on the F-box motif, suggesting that Skp1p down-regulates Mfb1p mitochondrial association in buds. We propose that Mfb1p operates in a novel pathway regulating mitochondrial tubular connectivity.

Dimeric Dnm1-G385D Interacts with Mdv1 on Mitochondria and Can Be Stimulated to Assemble into Fission Complexes Containing Mdv1 and Fis1

Interactions between yeast Dnm1p, Mdv1p, and Fis1p are required to form fission complexes that catalyze division of the mitochondrial compartment. During the formation of mitochondrial fission complexes, the Dnm1p GTPase self-assembles into large multimeric complexes on the outer mitochondrial membrane that are visualized as punctate structures by fluorescent labeling. Although it is clear that Fis1p.Mdv1p complexes on mitochondria are required for the initial recruitment of Dnm1p, it is not clear whether Dnm1p puncta assemble before or after this recruitment step. Here we show that the minimum oligomeric form of cytoplasmic Dnm1p is a dimer. The middle domain mutant protein Dnm1G385Dp forms dimers in vivo but fails to assemble into punctate structures. However, this dimeric mutant stably interacts with Mdv1p on the outer mitochondrial membrane, demonstrating that assembly of stable Dnm1p multimers is not required for Dnm1p-Mdv1p association or for mitochondrial recruitment of Dnm1p. Dnm1G385Dp is reported to be a terminal dimer in vitro. We describe conditions that allow assembly of Dnm1G385Dp into functional fission complexes on mitochondria in vivo. Using these conditions, we demonstrate that multimerization of Dnm1p is required to promote reorganization of Mdv1p from a uniform mitochondrial localization into punctate fission complexes. Our studies also reveal that Fis1p is present in these assembled fission complexes. Based on our results, we propose that Dnm1p dimers are initially recruited to the membrane via interaction with Mdv1p.Fis1p complexes. These dimers then assemble into multimers that subsequently promote the reorganization of Mdv1p into punctate fission complexes.

The dynamin-related mouse mitochondrial GTPase OPA1 alters the structure of the mitochondrial inner membrane when exogenously introduced into COS-7 cells

Mutations in the dynamin family GTPase OPA1 are reportedly the cause of autosomal dominant optic atrophy, the most frequently occurring form of hereditary optic neuropathy. But although the involvement of structural abnormalities of the enzyme in this neurodegenerative disease is clear, little is known about the cell biological and biochemical functions of OPA1. Therefore, to better understand the pathogenesis of autosomal dominant optic atrophy, we precisely analyzed the effects of exogenously introducing mouse OPA1 (mOPA1) on mitochondrial morphology in COS-7 cells. We found that exogenously introducing wild type mOPA1 caused the mitochondria to become fragmented, and moreover caused the intermembrane space to accumulate on one side of the ring-shaped mitochondrial fragments. Immunoelectron microscopic observation of the mOPA1 transfectants confirmed that the structure of the mitochondrial inner membrane had changed dramatically, accumulating on one side of the mitochondrial structures. When cells were transfected with mOPA1 containing a loss of function mutation (K301A) within the G1 GTP-binding domain, mitochondrial fragmentation still occurred. The markers for intermembrane space and matrix showed the similar morphology, which was distinctly different from the finding obtained with wild type mOPA1 transfectants. Notably, we also observed that the effect of two OPA1 missense mutations (E270K and D273A) associated with autosomal dominant optic atrophy elicit effects similar to those seen with the dominant negative K301A mutant.

Molecular mechanism of mitochondrial membrane fusion

Mitochondrial fusion requires coordinated fusion of the outer and inner membranes. This process leads to exchange of contents, controls the shape of mitochondria, and is important for mitochondrial function. Two types of mitochondrial GTPases are essential for mitochondrial fusion. On the outer membrane, the fuzzy onions/mitofusin proteins form complexes in trans that mediate homotypic physical interactions between adjacent mitochondria and are likely directly involved in outer membrane fusion. Associated with the inner membrane, the OPA1 dynamin-family GTPase maintains membrane structure and is a good candidate for mediating inner membrane fusion. In yeast, Ugo1p binds to both of these GTPases to form a fusion complex, although a related protein has yet to be found in mammals. An understanding of the molecular mechanism of fusion may have implications for Charcot-Marie-Tooth subtype 2A and autosomal dominant optic atrophy, neurodegenerative diseases caused by mutations in Mfn2 and OPA1.

Dysregulation of mitochondrial fusion and fission: an emerging concept in neurodegeneration

Mitochondrial dysfunction is increasingly being recognized as an important factor contributing to the pathogenesis of neurodegenerative disorders. However, at present, the molecular basis underlying the decline in mitochondrial function is not really understood, but recent experimental evidence has shed some light on the pivotal role of mitochondrial morphology control in this process. In particular, dysregulated mitochondrial fusion and fission events can now be regarded as playing important pathogenic roles in neurodegeneration. In healthy cells, mitochondrial morphology is maintained through a dynamic balance between fusion and fission processes, and this regulated balance seems to be required for maintaining normal mitochondrial and cellular function. Moreover, during programmed cell death, activation of mitochondrial fission occurs, leading to mitochondrial fragmentation (Karbowski et al. in J Cell Biol 164:493-499, 2004). Consequently, inhibition of mitochondrial fission results in a significantly reduced cellular susceptibility toward apoptosis. The clinical relevance of maintaining a finely tuned balance between mitochondrial fusion and fission processes is underscored by the fact that the pathogenesis of certain hereditary neurodegenerative disorders such as autosomal dominant optic atrophy (ADOA) and Charcot-Marie-Tooth neuropathy type 2A (CMT2A) can now be linked to mutations in genes encoding mediators of mitochondrial fusion. In this article, I will summarize important aspects of what is currently known about the molecular machinery regulating mitochondrial fission and fusion in mammalian cells. Special emphasis will be given to the consequences of dysregulated mitochondrial morphology with regard to the pathogenesis of neurodegenerative disorders. A detailed understanding of the mitochondrial fission and fusion machinery will be a prerequisite for the development of therapeutic approaches to inhibit the neuronal cell death underlying certain neurodegenerative disorders.

Mitochondrial morphology and dynamics in yeast and multicellular eukaryotes

Mitochondria form dynamic tubular networks that continually change their shape and move throughout the cell. In eukaryotes, these organellar gymnastics are controlled by numerous pathways that preserve proper mitochondrial morphology and function. The best understood of these are the fusion and fission pathways, which rely on conserved GTPases and their binding partners to regulate organelle connectivity and copy number in healthy cells and during apoptosis. In budding yeast, mitochondrial shape is also maintained by proteins acting in the tubulation pathway. Novel proteins and pathways that control mitochondrial dynamics continue to be discovered, indicating that the mechanisms governing this organelle's behavior are more sophisticated than previously appreciated. Here we review recent advances in the field of mitochondrial dynamics and highlight the importance of these pathways to human health.

Plant mitochondrial dynamics

Higher plant mitochondria are dynamic, pleomorphic organelles. The higher plant chondriome (all mitochondria in a cell collectively) is typically composed of numerous, physically discrete, mitochondria. However, frequent inter-mitochondrial fusion, enabling the mixing and recombination of mtDNA, ensures that the higher plant chondriome functions, at least genetically, as a discontinuous whole. Nothing is known about the genes controlling mitochondrial fusion in plants; there are no plant homologues of most of the genes known to be involved in fusion in other organisms. In contrast, the mitochondrial fission apparatus is generally conserved. Higher plant mitochondria use dynamin-like and Fis-type proteins for division; like yeast and animals, higher plants have lost the mitochondrial-specific form of the prokaryote-derived protein, FtsZ. In addition to being providers of energy for life, mitochondria provide a trigger for death. The role of mitochondrial dynamics in the initiation and promulgation of cell death is conserved in higher plants although there are specific differences in the genes and mechanisms involved relative to other higher eukaryotes.

BIGYIN, an orthologue of human and yeast FIS1 genes functions in the control of mitochondrial size and number in Arabidopsis thaliana

Reverse-genetics was used to evaluate the role of an Arabidopsis homologue of the human and yeast FIS1 genes, which are both involved in mitochondrial fission. Two independent T-DNA insertion mutants of gene At3g57090 were identified and genetically transformed to express mitochondria-targeted GFP to enable visualization of mitochondria in vivo. Plants homozygous for either of the recessive T-DNA mutant alleles, termed bigyin1-1 (bgy1-1) and bigyin1-2 (bgy1-2), displayed an abnormal mitochondrial morphology. Disruption of BIGYIN leads to a reduced number of mitochondria per cell, coupled to a large increase in the size of individual mitochondria, relative to wild-type. It is concluded that BIGYIN is an Arabidopsis FIS orthologue and is part of the Arabidopsis mitochondrial division apparatus.

Cell biology of mitochondrial dynamics

Mitochondria are the product of an ancient endosymbiotic event between an alpha-proteobacterium and an archael host. An early barrier to overcome in this relationship was the control of the bacterium's proliferation within the host. Undoubtedly, the bacterium (or protomitochondrion) would have used its own cell division apparatus to divide at first and, today a remnant of this system remains in some "ancient" and diverse eukaryotes such as algae and amoebae, the most conserved and widespread of all bacterial division proteins, FtsZ. In many of the eukaryotes that still use FtsZ to constrict the mitochondria from the inside, the mitochondria still resemble bacteria in shape and size. Eukaryotes, however, have a mitochondrial morphology that is often highly fluid, and in their tubular networks of mitochondria, division is clearly complemented by mitochondrial fusion. FtsZ is no longer used by these complex eukaryotes, and may have been replaced by other proteins better suited to sustaining complex mitochondrial networks. Although proteins that divide mitochondria from the inside are just beginning to be characterized in higher eukaryotes, many division proteins are known to act on the outside of the organelle. The most widespread of these are the dynamin-like proteins, which appear to have been recruited very early in the evolution of mitochondria. The essential nature of mitochondria dictates that their loss is intolerable to human cells, and that mutations disrupting mitochondrial division are more likely to be fatal than result in disease. To date, only one disease (Charcot-Marie-Tooth disease 2A) has been mapped to a gene that is required for mitochondrial division, whereas two other diseases can be attributed to mutations in mitochondrial fusion genes. Apart from playing a role in regulating the morphology, which might be important for efficient ATP production, research has indicated that the mitochondrial division and fusion proteins can also be important during apoptosis; mitochondrial fragmentation is an early triggering (and under many stimuli, essential) step in the pathway to cell suicide.