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The Division and Inheritance of Mitochondria
THE DIVISION A N D INHERITANCE OF MITOC H0NDRIA
Michael P. Yaffe I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 11. Mitochondrial Form and Behavior in the Cell . . . . . . . . . . . . . . . . . 342 111. Mitochondrial Movement and the Cytoskeleton . . . . . . . . . . . . . . . . 343 IV. Mutants of Mitochondrial Distribution and Morphology . . . . . . . . . . . . 345 V. Models of Mitochondrial Division . . . . . . . . . . . . . . . . . . . . . . . 348 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
1. INTRODUCTION The division and inheritance of mitochondria are essential elements of cell proliferation. During every cell cycle, mitochondrial mass is duplicated and the mitochondria are distributed among daughter cells. This mitochondrial division and inheritance ensures the, continuation of mitochondrial populations and provides daughter cells with adequate mitochondna to perform a myriad of metabolic reactions throughout the cell cycle. Key components of mitochondrial growth, the replication and expression of the mitochondrial genetic system and the import of nuclear-encoded proteins into mitochondria, have been the focus of intense study, Advances in Molecular and Cell Biology, Volume 17, pages 341-350. Copyright 0 1996 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0144-9
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yet little is known about the mechanisms responsible for faithful transmission of mitochondria to daughter cells.
11. MITOCHONDRIA1 FORM AND BEHAVIOR IN THE CELL The consideration of mitochondrial division and 'inheritance benefits from an understanding of the shape and distribution of mitochondria within the eukaryotic cell. Electron microscopic studies combining analysis of serial sections with three-dimensional reconstructions (Stevens, 1977; Blank and Arnold, 1981) and fluorescence microscopy employing mitochondrial-specific dyes (Chen, 1988; Bereiter-Hahn, 1976) have revealed that many types of cells contain an elaborate, reticulated, mitochondrial network. Rather than the spherical or cigar-shaped structures suggested by electron micrographs of single cellular cross-sections, mitochondria often exist in cells as branched, tubular networks with connected, snake-like domains reaching numerous cytoplasmic regions. These mitochondrial tubules may be entirely interconnected so that the cells contain, in essence, a single giant mitochondrion. Such a situation has been documented for unicellular algae (Blank and Arnold, 1981) and yeast cells under certain growth conditions (Stevens, 1977). Extensive mitochondrial networks also have been described in a variety of mammalian cells (Johnson et al., 1980; Schnedl, 1974; Bakeeva et al., 1986). Mitochondria, or the cellularmitochondnal reticulum, also displays a remarkable plasticity in form and positioning within cells. Microscopic studies of isolated plant cells and of animal cells in culture have revealed that mitochondria undergo frequent fisions and fissions throughout the cell cycle (Bereiter-Hahn, 1990). Morphological changes also can occur in response to altered nutritional status or growth state of cells (Stevens, 1977) during the differentiation of certain tissues (Munn, 1974) and in response to certain pathological conditions (Munn, 1974). In the yeast Saccharomyces cerevisiae, mitochondrial number and morphology alter with changes in carbon source in the media and with the growth phase (Stevens, 1981). During exponential growth on glucose, yeast cells often contain one giant mitochondrion in the form of an extended reticulum, comprising the bulk of the mitochondrial mass. Several smaller mitochondria can also be present. When such cells are changed to a nonfermentable carbon-source, the total mass of mitochondria increases two to fourfold and is distributed as a giant mitochondrial reticulum along with 6-10 additional mitochondria. During stationary phase, the mitochondrial mass redistributes into 20-40 discrete organelles. A number of intracellularmitochondrial movements that appear to be specifically controlled or programmed have been described in several diverse types of cells. In the fission yeast, Schizosaccharomycespombe, the mitochondria aggregate and move as two masses to either end of the cell during mitosis (Hirano et al., 1988). Changes in mitochondrial positioning have been observed in newly plated fibroblasts in which mitochondria are initially found in the cell periphery and subsequently migrate to the perinuclear region (Bereiter-Hahn et al., 1990). Per-
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inuclear aggregation of mitochondria was increased by stimulation of quiescent BALBc/3T3 cells with growth factors (Chen, 1988). As chick fibroblasts cease migration, their mitochondria redistribute from a tight perinuclear localizationto a broader distributionthroughout the cytoplasm (Couchman and Rees, 1982).During the development of Xenopus oocytes, many of the mitochondria aggregate into a prominent cytoplasmic mass, “the mitochondrial cloud,” as the cells become stage I oocytes (Heasman et al., 1984).This mitochondrial mass divides into two portions as oogenesis proceeds: a group of organelles moves toward the plasma membrane and forms a cortical layer in the vegetal hemisphere, while the majority of mitochondria remain near the nucleus until late in oogenesis when they disperse throughout the cytoplasm of the animal hemisphere (Mignotte et al., 1989). Evidence of abnormal mitochondrial movement was detected in a study of epithelial cells from a nematode mutant (Hedgecock and Thomson, 1982). Mitochondria are normally dispersed through the cytoplasm in this cell-type but were found abnormallyclustered in the centers ofthe mutant cells. This mutation did not appear to affect the segregation of mitochondria during cell division. One of the earliest events in the cell division cycle of the yeast Saccharomyces cerevisiae is the movement of mitochondria into the growing bud (Stevens, 1981). Almost as soon as a bud is apparent, a portion of a mitochondrion is found in this region of the cell. As the bud grows, it is filled with more mitochondria (and other organelles) until, at cytokinesis, the daughter cell is provisioned with a mitochondrial content slightly exceeding that ofthe mother cell (Stevens, 1977).This pattern of mitochondrial movement into the yeast bud occurs without regard to the carbon source or environmental conditions (i.e., even in cells growing anaerobically on glucose) and appears to be independent of microtubule function (Huffaker et al., 1988) and nuclear division (Thomas and Botstein, 1986). The identification of recessive mutations that block mitochondrial movement into buds (McConnell et al., 1990) suggests that mitochondrial inheritance is a specific and active process. Many of the molecular details and control of this mitochondrial movement have yet to be described.
111. MITOCHONDRIA1MOVEMENT AND THE CYTOSKE1ETON Studies with cells from a number of diverse organisms have implicated the cytoskeleton as playing a major role in the positioning and distribution of mitochondria. Mitochondria exhibit saltatory motion (Aufderheide, 1977; Adams, 1982) characteristic of transport along cytoskeletal components. Specific intracellular positions of mitochondria have been correlated with microtubulesin some cell types (Heggeness et al., 1978; Ball and Singer, 1982; Couchman and Rees, 1982). In other types of cells, mitochondrial position correlates with the distribution of intermediate filaments (David-Ferreira and David-Ferreira, 1980; Mose-Larsen et al., 1982; Chen, 1988). A study of mitochondrial distribution in living cells
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implicated both microtubules and intermediate filament networks as contributing to the positioning of mitochondria (Summerhayes et al., 1983). Studies with mammalian neuronal cells have documented transport along microtubules as a key component of the axonal transport of mitochondria and other organelles (Vale, 1987), and movement of organelles along microtubules has been described for certain non-neuronal animal cells (Vale, 1987).Video and fluorescencemicroscopy of Neurospora crassa cells have indicated that mitochondria migrate along cytoplasmic microtubules (Steinberg and Schliwa, 1993). Electron microscopy of rapidly frozen, frog neurons revealed short cross-bridges of thin filaments between mitochondria and microtubules and between mitochondria and neurofilaments (a type of intermediate filament; Hirokawa, 1982). In vitro, purified tubulin binds directly to isolated mitochondria (Bernier-Valentin and Rousset, 1982), although the physiological relevance of this interaction is unknown. Reconstitutionstudies using permeabilized cells or isolated cellular components have identified proteins involved in the movement of particles along microtubules (Vale, 1987).These investigationshave implicated two key proteins, kinesin (Vale, 1987) and cytoplasmic dynein (Lye et al., 1987; Schroer et al., 1989), as microtubule-based motors found ubiquitously in eukaryotic cells. Bound kinesin will drive the movement of vesicles or even latex beads toward the plus-end of microtubules while dynein drives movement toward the minus-end. The in vitro studies have suggested also that other, as yet unidentified, proteins are required for the similar movements of purified organelles (Schroer et al., 1988). Although the force-generating proteins, kinesin and cytoplasmic dynein, have been implicated in the positioning of organelleswithin nondividing cells and in the axonal transport of cellular particles, little is known of their role in organellar distribution during mitosis. Cytoplasmic microtubules largely disassemble prior to mitosis (Saxton et al., 1984),so the role of microtubulesin the distribution of organelles during the mitotic phase of the cell cycle is obscure. Studies with the filamentous fungus Aspergillus nidulans demonstrated that the inhibition of microtubule function with the agent benomyl or by mutations in P-tubulin blocked the movement and division of the nucleus but had no effect on the movement of mitochondria (Oakley and Reinhart, 1985). Additionally, mitochondria and other organelles migrate into the growing buds of the yeast Saccharomyces cerevisiae even in the presence of inhibitors or mutations that disrupt microtubule function (Huffaker et al., 1988; Jacobs et al., 1988). Actin microfilaments may also play a role in mitochondrial distribution.In some mammalian cells, regions with mitochondrial and organellar streaming movement correlate with areas of change in the structure of actin-like microfilament bundles (Wang and Goldman, 1978). Actin-mediated organellar transport has been characterized in characean algal cells (Kachar and Reese, 1988) where endoplasmic reticulum and other organelles contact actin filament bundles during cytoplasmic streaming. Using an in vitm reconstituted system, Adams and Pollard (1986)
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demonstratedthe functionof myosin-I in the active translocation of organelles from Acanthamoebu along actin microfilaments. In Succharomyces cerevisiae, mutations in the single actin gene cause aberrant transport of materials into a new bud (Novick and Botstein, 1985),but it is difficult to assess the specific role of actin in mitochondrialmovement in these mutants since a number of cellular processes are disrupted and buds do not form. Drubin and colleagues (1993) have described clumped mitochondria in yeast cells harboring certain mutant actin alleles, and these observations suggest a role for the actin-myosin cytoskeleton in the distribution or organizationof mitochondria. The specific binding of actin to the mitochondrial outer membrane has been described (Pardo et al., 1983), but the functional significance of this interaction remains to be determined. Mitochondrial behavior may depend on several different cytoskeletal systems. For example, microtubules may mediate the positioning and division of mitochondria during interphase, while interactions with intermediate filaments could determine mitochondrial distribution during mitosis. An alternative mechanism of mitochondrial movement (discussed by BereiterHahn, 1990) is that mitochondria move by changing shape and “creeping” through the cytoplasm. Such creeping would employ components and processes internal to the organelle and might be directed by gradients of ADP or other small molecules in the cytoplasm. This model envisions cytoskeletal components as playing a passive role by providing a structure upon which the mitochondria can maneuver.
IV. MUTANTS OF MITOCHONDRIA1 DISTRIBUTION AND MORPHOLOGY One approach to the study of mitochondrial division and inheritance has been the isolation and analysis of Succharomyces cerevisiue mutants possessing conditional defects in mitochondrial morphology and distribution (McConnell et al., 1990). These mdm mutants were isolated by screening a collection of temperaturesensitive yeast strains by fluorescence microscopy to identify cells that failed to transfer mitochondria into developing buds during incubation at 37 “C. A number of such mutant strains have been identified (McConnell et al., 1990),and many of these also display aberrant mitochondrial morphologies. Genetic analysis has revealed that the morphological, distribution, and temperature-sensitive growth phenotypes in mdm mutants result from single, recessive, nuclear mutations. The mutations define at least 20 complementationgroups. Microscopic studies employing indirect immunofluorescence and electron microscopy have confirmed the absence of mitochondria from buds that develop at the nonpermissive temperature in the mdm strains (McConnell et al., 1990). These studies also allowed an examination of the effect of the mutations on other cellular structures. Two classes of mdm mutants have been identified: those in which only the mitochondria are affected by the mutant lesions and those in which the transfer of both mitochondria and nuclei into daughter buds are defective at the nonpermis-
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sive temperature. None of the mutants have been found to affect the secretory process, the development of vacuoles or the transfer of vacuolar material into buds, the assembly and fimction of microtubules, or the cellular distribution of the actin cytoskeleton. The analysis of one mutant, mdml, has led to the identification of a new cytoskeletal component that mediates mitochondrial inheritance in yeast (McConnell and Yaffe, 1992). The mdml mutation affects transfer of both mitochondria and nuclei into daughter buds. Isolation and analysis of the wild-type MDMl gene revealed its product to be a 52 kDa protein with modest sequence similarity to intermediate filament proteins of animal cells. The protein, Mdmlp, is localized to a unique series of punctate structures distributed throughout the yeast cytoplasm. These structuresdisappear or disassemble in the mdml-1 mutant during incubation at the nonpermissive temperature (37OC), although Mdmlp protein levels remain constant in the cells. Mdmlp is an intermediate filament-formingprotein. Mdm lp expressed in E. coli and purified from bacterial inclusion bodies readily formed into filamentsof 10nm diameter in vitro (McConnell and Yaffe, 1993). Conditions of filament selfassembly were essentially those under which the animal proteins, vimentin and desmin, formed intermediate-sized filaments. Mutant Mdmlp, purified from bacteria expressing the mdml-1 gene, was temperature-sensitive for intermediate filament assembly in vitro: 10 nm filaments formed at 4°C or 23°C but failed to form at 37°C. However, filaments formed of the mutant protein at 4°C were stable when subsequently incubated at 37OC, indicating that the mdml-1 mutation prevents filament assembly.The relationshipbetween the filaments formed of Mdmlp in vitro and the punctate Mdmlp structures in the cell is unclear. Mdmlp may assemble into a different type of structure in the cellular environment or, alternatively, the punctate structures may be intersections of a number of filaments, with individual filaments being undetectable by currently available methods. For mitochondrial inheritance, the Mdmlp structures may constitute a network upon which mitochondria are transported. Asecond mdm mutant has revealed a role for unsaturated fatty acids in mitochondrial movement. The mdm2 mutation appeared to affect specificallythe inheritance of mitochondria at the nonpermissive temperature (McConnell et al., 1990). This lesion also blocked a second type of mitochondrial movement: distribution of mitochondria throughout cytoplasmic projections induced by exposure of yeast cells to mating pheromones. Mitochondria in mdm2 cells appeared to aggregate following a shift of cells to the nonpermissive temperature. Cloning and analysis of MDM2 (Stewart and Yaffe, 1991) revealed its identity with a previously identified gene, OLEl, encoding fatty acid desaturase. Consistentwith this identification, cellular levels of unsaturated fatty acids decreased following a shift of mdm2 cells to the nonpermissive temperature. Furthermore, the addition of oleic acid to culture media cured both the temperature-sensitive growth and mitochondrial distribution defects of mdmZ cells. The hnction of unsaturated fatty acids in mitochondrial
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movement might be related to some gross physical properties of the mitochondrial membranes ( e g , membrane fluidity) or could reflect a more specific requirement for unsaturated fatty acid in sustaining the activity or structure of a protein that mediates interaction of mitochondria with other cellular components. Analysis of a third mdm mutant has highlighted the relationship between mitochondrial morphology and mitochondrial inheritance. Yeast cells with the mdmlO lesion possess giant, spherical mitochondria, as well as a defect in mitochondrial inheritance (Sogo and Yaffe, 1994). The giant mitochondria contain classical structural features and are competent for respiration. MDMlO encodes a 57 kDa polypeptide that is an integral protein of the mitochondrial outer membrane. A portion of this protein is exposed to the cytoplasm and may mediate the interaction of mitochondria with cytoskeletal components. Function of the MdmlOp was further revealed by analyzing the cellular distribution and morphology of mitochondria in cells following a controlled cessation of MDMIO expression. The depletion of MdmlOp led to a progressive condensation of snake-like mitochondna into thicker structures and, eventually, into one or a few giant, mitochondrialballs. Empty daughter buds, reflecting defectivemitochondrial inheritance, also appeared as Mdml Op levels dropped. Re-expression of MDMlO in cells with giant mitochondria led to a rapid return of normal mitochondrial morphology. This return to snake-like mitochondria seemed to occur by a stretching-out and fragmentation of the giant organelles. A second approach fbrther illustrated the reversible nature of the giant mitochondrial phenotype. Haploid mdml0 cells (containing giant mitochondria)were mated to wild-type cells of the opposite mating type. The mitochondrial morphologies in the resulting diploid zygotes were analyzed by fluorescence microscopy. Initially, zygotes contained normal mitochondria on one side and a giant mitochondrion on the other side of the cell. Over the subsequent hour, the giant mitochondrion fragmented into numerous snake-like organelles until both sides of the zygote displayed wild-type mitochondrial morphologies. The giant mitochondria in the mdmlO mutant appeared to be defectivecfsr both movement and division. One model to explain how all of these defects could result from a loss of Mdm10p would be that interactions between mitochondria and cytoskeleton mediate the control of mitochondrial morphology, mitochondrial division, and mitochondrial movement. Mdm lop might serve as an anchor-point on the mitochondrial surface for attachment of the organelle to cytoskeletal networks. Another explanation for all of the phenotypes of the mdmlO mutant is that mitochondrialmovement and division require an extended, snake-like morphology, and their failure to occur in mdmlO is a secondary consequence of the dramatic morphological changes. A second, distinct mutant with altered mitochondrial morphology, mmml, was recently described by Burgess and colleagues (1994). This mutant displays phenotypes very similar to those of mdml0, and MMMl also encodes an integral protein of the mitochondrial outer membrane. A third new mutant, mdml2, possesses giant
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mitochondria and defective mitochondrial inheritance (Burger and Yaffe, 1994). These various mutants may define subunits of a protein complex localized to the surface of mitochondria.
V. MODELS OF MITOCHONDRIA1 DIVISION The continuous increase in mitochondrial mass throughout the cell cycle is accompanied by a parallel increase in mitochondrial number (Schnedl, 1974; Posakony et al., 1977). This increase in number occurs by the division of pre-existing mitochondria. Additionally,mitochondria divide in response to various physiological changes (as described previously). Mitochondrial division does not appear to be tightly coordinated with the cell division cycle. Although the amount (i.e., mass) of mitochondria is likely to be regulated by metabolic needs of the cell, factors governing the number of mitochondria are unknown. Additionally, as with mitochondrial inheritance, the mechanism of mitochondrial division has yet to be described. Electron microscopic images ofmitochondria in a variety of cells have suggested several possible models for division. One possibility is that extended mitochondrial structures are pulled apart by molecular motors attached to opposite sides of the organelle and moving in opposite directions along a cytoskeletal track. Such a mechanism might involve kinesin, dynein, or myosin-like motor proteins attached to the outer membrane and could proceed by an initial stretching of the organelle followed by the development of a division furrow. A second possible mechanism is that an encircling band of cytoskeletal proteins constricts the organelle and eventually pinches it in two. This model resembles cytokinesis and might employ an actinomyosin system. A third model is that a division septum or similar structure is assembled within the mitochondrion, facilitating a separation into two distinct organelles. This mechanism might recall the prokaryotic origins of mitochondria and rely on activitiesinternal to the organelle. Ahrther understanding of mitochondrial division may emerge from genetic studies of mutant cells defective in mitochondrial fission and biochemical analysis using systems reconstituted from purified components.
REFERENCES Adams, R. J. (1982). Organelle movement in axons depends on ATP. Nature 297,327-329. Adams. R. J., & Pollard, T.D. ( 1986). Propulsion of organellesisolated from Acanthamoeba along actin filaments by myosin-I. Nature 322,754-756. Aufderheide, K. J . (1977). Saltatory motility of uninserted trichocysts and mitochondria in Paramecium fetraurelia. Science 198,29%300. Bakeeva, L. E., Chentsov, U. S., & Skulachev, V. P. (1986). Mitochondria1 framework (reticulum mitochondriale) in rat diaphragm muscle. Biochim. Biophys. Acta 501,349-369. Ball, E. H., & Singer, S. J. (1982). Mitochondria are associated with microtubules and not with intermediate filaments in cultured fibroblasts. Proc. Natl. Acad. Sci. USA 79, 123-126.
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Bereiter-Hahn, J. ( 1976). Dimethylaminostyrylmethylpyridiniuznidine(DASPMI), a fluorescent probe for mitochondria in situ. Biochim. Biophys. Acta 423, 1-14. Bereiter-Hahn, J. (1990). Behavior of mitochondria in the living cell. Int. Rev. Cytol. 122, 1-61. Bereiter-Hahn, J., Luck, M., Miebach, T., Stelzer, K.E., & Voth, M. (1990). Spreading of trypsinized cells: Cytoskeletal dynamics and energy requirements. J. Cell Sci. 96, 171-188. Bemier-Valentin, F., & Rousset, B. (1982). Interaction of tubulin with rat liver mitochondria. J. Biol. Chem. 257,7092-7099. Blank, R., & Arnold, C.-G. (1981). Structural changes of mitochondria in Chlamydomonas reinhardii after chloramphenicol treatment. Eur. J. Cell Biol. 24,244-251. Burger, K. H., & Yaffe, M. P. (1994). Unpublished results. Burgess, S. M., Delannoy, M., & Jensen, R. E. (1 994). MMMl encodes a mitochondrial outer membrane protein essential for establishing and maintaining the structure of yeast mitochondria. J. Cell Biol. 126, 1375-1391. Chen, L. B. (1988). Mitochondria1membrane potential in livingcells. Ann. Rev. Cell Biol. 4, 155-181. Couchman, J. R., & Rees, D. A. (1982). Organelle-cytoskeleton relationships in fibroblasts: Mitochondria, Golgi apparatus, and endoplasmic reticulum in phases ofmovement and growth. Eur. J. Cell Biol. 27,47-54. David-Ferreira, K. L., & David-Ferreira, J. F. (1980).Association between intermediate-sized filaments and mitochondria in rat Leidyg cells. Int. Cell Biol. Rep. 4, 655-662. Drubin, D. G., Jones, H. D., & Werhnan, K. F. (1993). Actin structure and function: roles in mitochondrial organization and morphogenesis in budding yeast and identification of the phalloidin-binding site. Mol. Biol. Cell 4, 1277-1294. Heasman, J., Quarmby, J., & Wylie, C. C. (1984). The mitochondrial cloud ofXenopus oocytes: The source of germinal granule material. Dev. Biol. 105,45%469. Hedgecock, E. M., & Thomson, J. N. (I 982). A gene required for nuclear and mitochondrial attachment in the nematode Caenorhabditis elegans. Cell 30,321-330. Heggeness, M. H., Simon, M., & Singer, S. J. (1978). Association of mitochondria with microtubules in cultured cells. Proc. Natl. Acad. Sci. USA 75,3863-3866. Hirano, T., Hiraoka, Y., & Yanagida, M. (1988). A temperature-sensitive mutation of the Schizosacchuromyces pombe gene nuc2+ that encodes a nuclear scaffold-like protein blocks spindle elongation in mitotic anaphase. J. Cell Biol. 106, 1171-1 183. Hirokawa, N. (1982). Cross-linker system between neurofilaments, microtubules, and membranous organelles in frogaxons revealed by the quick-freeze, deep-etch method. J. Cell Biol. 94, 129-142. Huffaker, T. C., Thomas, J. H., & Botstein, D. (1988). Diverse effects of P-tubulin mutations on microtubule formation and function. J. Cell Biol. 106. 1997-2010. Jacobs, C. W., Adams, A. E. M., Szaniszlo, P. J., & Pringle, J. R. (1988). Functions of microtubules in the Succharomyces cerevisiae cell cycle. J. Cell Biol. 107, 140%1426. Johnson, L. V., Walsh, M. L., & Chen, L. B. (1980). Localization of mitochondria in living cells with rhodamine 123. Proc. Natl. Acad. Sci. USA 77,990. Kachar, B., & Reese, T. S. (1988). The mechanism of cytoplasmic streaming in Characean algal cells: Sliding of endoplasmic reticulum along actin filaments. J. Cell Biol. 106, 1545-1552. Lye, R. J., Porter, M. E., Scholey, J. M., & Mclntosh, J. R. (1987). Identification of a microtubule-based cytoplasmic motor in the nematode C. elegans. Cell 51,30%318. McConnell, S. J., Stewart, L. C., Talin, A., & Yaffe, M. P. (1990). Temperature-sensitive yeast mutants defective in mitochondrial inheritance. J. Cell Biol. 1 11,967-976. McConnell, S. J., & Yaffe, M. P. (1992). Nuclear and mitochondrial inheritance in yeast depends on novel cytoplasmic structures defined by the MDMl protein. J. Cell Biol. 118,385-395. McConnell, S. J., & Yaffe, M. P. (1993). Intermediate filament formation by a yeast protein essential for organelle inheritance. Science 260,687-689. Mignotte, F., Tourte, M., & Mounolou, J-C. (1989). Segregation of mitochondria in the cytoplasm of Xenopus vitellogenic oocytes. Biol. Cell 60,97-102.
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Mose-Larsen, P., Bravo, R., Fey, S. J., Small, J. V., & Celis, J. E. (1982). Putative association of mitochondria with a subpopulation of intermediate-sized filaments in cultured human skin fibroblasts. Cell 3 1,681-692. Munn, E. A. (1974). The Structure of Mitochondria. Academic Press, London. Novick, P., & Botstein, D. (1985). Phenotypic analysis of temperature-sensitive yeast actin mutants. Cell 40,405-416. Oakley, B. R., & Reinhart, J. E. (1985). Mitochondria and nuclei move by different mechanisms in Aspergillus nidulans. J. Cell Biol. 101,2392-2397. Pardo, J. V., Pittenger, M. F., & Craig, S. W. (1983). Subcellularsortingofisoactins:Selective association of O-actin with skeletal muscle mitochondria. Cell 32, 1093-1103. Posakony, J. W., England, J. M., & Attardi, G. (1977). Mitochondrial growth and division during the cell cycle in HeLa cells. J. Cell. Biol. 74,468-491. Saxton, W. M., Stemple, D. L., Leslie, R. J.. Salmon, E. D., Zavortink, M., & Mclntosh, J. R. (1984). Tubulin dynamics in cultured mammalian cells. J. Cell Biol. 99, 2175-2186. Schnedl, W. (1974). Changes in mitochondria during the cell cycle. Cytobiol. 8,403-41 1. Schrmr, T. A., Schnapp, B. J., Reese, T.S., & Sheetz, M. P. (1988).The role ofkinesin and other soluble factors in organelle movement along microtubules. J. Cell Biol. 107. 1785-1792. Schroer, T. A., Steuer, E. R., & Sheetz. M. P. (1989). Cytoplasmic dynein is a minus end-directed motor for membranous organelles. Cell 56.937-946. Sogo, L. F., & Yaffe, M. P. (1994). Regulation of mitochondrial morphology and inheritance by MdmIOp, a protein ofthe mitochondrial outer membrane. J. Cell Biol. 126, 1361-1373. Steinberg, G., & Schliwa, M. (1993). Organelle movements in the wild type and wall-less fz: sg; 0s-I mutants of Neurospora crassa are mediated by cytoplasmic microtubules. J. Cell Sci. 106, 555-564. Stevens, B. (1981). Mitochondrial structure. In: The Molecular Biology of the Yeast Saccharomyces cerevisiae, Vol. 1 (Strathem, J. N., Jones, E. W., & Broach, J. R., Eds.). Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, pp. 471-504. Stevens, B. J. (1977).Variation in number and volume of the mitochondria in yeast according to growth conditions. A study based on serial sectioning and computer graphics reconstruction. Biol. Cell 28,37-56. Stewart, L. C., & Yaffe, M. P. (1991). A role for unsaturated fatty acids in mitochondrial movement and inheritance. J. Cell Biol. 115, 1249-1257. Summerhayes, 1. C., Wong, D., & Chen, L. B. (1983). Effect of microtubules and intermediate filaments on mitochondrial distribution. J. Cell Sci. 61, 87-105. Thomas, J. H., & Botstein, D. (1986). A gene required for the separation of chromosomes on the spindle apparatus. Cell 44,65-76. Vale, R. D. (1987). lntracellular transport using microtubule-based motors. Ann. Rev. Cell Biol. 3, 347-378. Wang, E., & Goldman, R. D. (1978). Functions of cytoplasmic fibers in intracellular movements in BHK-21 cells. J. Cell Biol. 79, 708-726.
Michael P. Yaffe I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 11. Mitochondrial Form and Behavior in the Cell . . . . . . . . . . . . . . . . . 342 111. Mitochondrial Movement and the Cytoskeleton . . . . . . . . . . . . . . . . 343 IV. Mutants of Mitochondrial Distribution and Morphology . . . . . . . . . . . . 345 V. Models of Mitochondrial Division . . . . . . . . . . . . . . . . . . . . . . . 348 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
1. INTRODUCTION The division and inheritance of mitochondria are essential elements of cell proliferation. During every cell cycle, mitochondrial mass is duplicated and the mitochondria are distributed among daughter cells. This mitochondrial division and inheritance ensures the, continuation of mitochondrial populations and provides daughter cells with adequate mitochondna to perform a myriad of metabolic reactions throughout the cell cycle. Key components of mitochondrial growth, the replication and expression of the mitochondrial genetic system and the import of nuclear-encoded proteins into mitochondria, have been the focus of intense study, Advances in Molecular and Cell Biology, Volume 17, pages 341-350. Copyright 0 1996 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0144-9
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yet little is known about the mechanisms responsible for faithful transmission of mitochondria to daughter cells.
11. MITOCHONDRIA1 FORM AND BEHAVIOR IN THE CELL The consideration of mitochondrial division and 'inheritance benefits from an understanding of the shape and distribution of mitochondria within the eukaryotic cell. Electron microscopic studies combining analysis of serial sections with three-dimensional reconstructions (Stevens, 1977; Blank and Arnold, 1981) and fluorescence microscopy employing mitochondrial-specific dyes (Chen, 1988; Bereiter-Hahn, 1976) have revealed that many types of cells contain an elaborate, reticulated, mitochondrial network. Rather than the spherical or cigar-shaped structures suggested by electron micrographs of single cellular cross-sections, mitochondria often exist in cells as branched, tubular networks with connected, snake-like domains reaching numerous cytoplasmic regions. These mitochondrial tubules may be entirely interconnected so that the cells contain, in essence, a single giant mitochondrion. Such a situation has been documented for unicellular algae (Blank and Arnold, 1981) and yeast cells under certain growth conditions (Stevens, 1977). Extensive mitochondrial networks also have been described in a variety of mammalian cells (Johnson et al., 1980; Schnedl, 1974; Bakeeva et al., 1986). Mitochondria, or the cellularmitochondnal reticulum, also displays a remarkable plasticity in form and positioning within cells. Microscopic studies of isolated plant cells and of animal cells in culture have revealed that mitochondria undergo frequent fisions and fissions throughout the cell cycle (Bereiter-Hahn, 1990). Morphological changes also can occur in response to altered nutritional status or growth state of cells (Stevens, 1977) during the differentiation of certain tissues (Munn, 1974) and in response to certain pathological conditions (Munn, 1974). In the yeast Saccharomyces cerevisiae, mitochondrial number and morphology alter with changes in carbon source in the media and with the growth phase (Stevens, 1981). During exponential growth on glucose, yeast cells often contain one giant mitochondrion in the form of an extended reticulum, comprising the bulk of the mitochondrial mass. Several smaller mitochondria can also be present. When such cells are changed to a nonfermentable carbon-source, the total mass of mitochondria increases two to fourfold and is distributed as a giant mitochondrial reticulum along with 6-10 additional mitochondria. During stationary phase, the mitochondrial mass redistributes into 20-40 discrete organelles. A number of intracellularmitochondrial movements that appear to be specifically controlled or programmed have been described in several diverse types of cells. In the fission yeast, Schizosaccharomycespombe, the mitochondria aggregate and move as two masses to either end of the cell during mitosis (Hirano et al., 1988). Changes in mitochondrial positioning have been observed in newly plated fibroblasts in which mitochondria are initially found in the cell periphery and subsequently migrate to the perinuclear region (Bereiter-Hahn et al., 1990). Per-
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inuclear aggregation of mitochondria was increased by stimulation of quiescent BALBc/3T3 cells with growth factors (Chen, 1988). As chick fibroblasts cease migration, their mitochondria redistribute from a tight perinuclear localizationto a broader distributionthroughout the cytoplasm (Couchman and Rees, 1982).During the development of Xenopus oocytes, many of the mitochondria aggregate into a prominent cytoplasmic mass, “the mitochondrial cloud,” as the cells become stage I oocytes (Heasman et al., 1984).This mitochondrial mass divides into two portions as oogenesis proceeds: a group of organelles moves toward the plasma membrane and forms a cortical layer in the vegetal hemisphere, while the majority of mitochondria remain near the nucleus until late in oogenesis when they disperse throughout the cytoplasm of the animal hemisphere (Mignotte et al., 1989). Evidence of abnormal mitochondrial movement was detected in a study of epithelial cells from a nematode mutant (Hedgecock and Thomson, 1982). Mitochondria are normally dispersed through the cytoplasm in this cell-type but were found abnormallyclustered in the centers ofthe mutant cells. This mutation did not appear to affect the segregation of mitochondria during cell division. One of the earliest events in the cell division cycle of the yeast Saccharomyces cerevisiae is the movement of mitochondria into the growing bud (Stevens, 1981). Almost as soon as a bud is apparent, a portion of a mitochondrion is found in this region of the cell. As the bud grows, it is filled with more mitochondria (and other organelles) until, at cytokinesis, the daughter cell is provisioned with a mitochondrial content slightly exceeding that ofthe mother cell (Stevens, 1977).This pattern of mitochondrial movement into the yeast bud occurs without regard to the carbon source or environmental conditions (i.e., even in cells growing anaerobically on glucose) and appears to be independent of microtubule function (Huffaker et al., 1988) and nuclear division (Thomas and Botstein, 1986). The identification of recessive mutations that block mitochondrial movement into buds (McConnell et al., 1990) suggests that mitochondrial inheritance is a specific and active process. Many of the molecular details and control of this mitochondrial movement have yet to be described.
111. MITOCHONDRIA1MOVEMENT AND THE CYTOSKE1ETON Studies with cells from a number of diverse organisms have implicated the cytoskeleton as playing a major role in the positioning and distribution of mitochondria. Mitochondria exhibit saltatory motion (Aufderheide, 1977; Adams, 1982) characteristic of transport along cytoskeletal components. Specific intracellular positions of mitochondria have been correlated with microtubulesin some cell types (Heggeness et al., 1978; Ball and Singer, 1982; Couchman and Rees, 1982). In other types of cells, mitochondrial position correlates with the distribution of intermediate filaments (David-Ferreira and David-Ferreira, 1980; Mose-Larsen et al., 1982; Chen, 1988). A study of mitochondrial distribution in living cells
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implicated both microtubules and intermediate filament networks as contributing to the positioning of mitochondria (Summerhayes et al., 1983). Studies with mammalian neuronal cells have documented transport along microtubules as a key component of the axonal transport of mitochondria and other organelles (Vale, 1987), and movement of organelles along microtubules has been described for certain non-neuronal animal cells (Vale, 1987).Video and fluorescencemicroscopy of Neurospora crassa cells have indicated that mitochondria migrate along cytoplasmic microtubules (Steinberg and Schliwa, 1993). Electron microscopy of rapidly frozen, frog neurons revealed short cross-bridges of thin filaments between mitochondria and microtubules and between mitochondria and neurofilaments (a type of intermediate filament; Hirokawa, 1982). In vitro, purified tubulin binds directly to isolated mitochondria (Bernier-Valentin and Rousset, 1982), although the physiological relevance of this interaction is unknown. Reconstitutionstudies using permeabilized cells or isolated cellular components have identified proteins involved in the movement of particles along microtubules (Vale, 1987).These investigationshave implicated two key proteins, kinesin (Vale, 1987) and cytoplasmic dynein (Lye et al., 1987; Schroer et al., 1989), as microtubule-based motors found ubiquitously in eukaryotic cells. Bound kinesin will drive the movement of vesicles or even latex beads toward the plus-end of microtubules while dynein drives movement toward the minus-end. The in vitro studies have suggested also that other, as yet unidentified, proteins are required for the similar movements of purified organelles (Schroer et al., 1988). Although the force-generating proteins, kinesin and cytoplasmic dynein, have been implicated in the positioning of organelleswithin nondividing cells and in the axonal transport of cellular particles, little is known of their role in organellar distribution during mitosis. Cytoplasmic microtubules largely disassemble prior to mitosis (Saxton et al., 1984),so the role of microtubulesin the distribution of organelles during the mitotic phase of the cell cycle is obscure. Studies with the filamentous fungus Aspergillus nidulans demonstrated that the inhibition of microtubule function with the agent benomyl or by mutations in P-tubulin blocked the movement and division of the nucleus but had no effect on the movement of mitochondria (Oakley and Reinhart, 1985). Additionally, mitochondria and other organelles migrate into the growing buds of the yeast Saccharomyces cerevisiae even in the presence of inhibitors or mutations that disrupt microtubule function (Huffaker et al., 1988; Jacobs et al., 1988). Actin microfilaments may also play a role in mitochondrial distribution.In some mammalian cells, regions with mitochondrial and organellar streaming movement correlate with areas of change in the structure of actin-like microfilament bundles (Wang and Goldman, 1978). Actin-mediated organellar transport has been characterized in characean algal cells (Kachar and Reese, 1988) where endoplasmic reticulum and other organelles contact actin filament bundles during cytoplasmic streaming. Using an in vitm reconstituted system, Adams and Pollard (1986)
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demonstratedthe functionof myosin-I in the active translocation of organelles from Acanthamoebu along actin microfilaments. In Succharomyces cerevisiae, mutations in the single actin gene cause aberrant transport of materials into a new bud (Novick and Botstein, 1985),but it is difficult to assess the specific role of actin in mitochondrialmovement in these mutants since a number of cellular processes are disrupted and buds do not form. Drubin and colleagues (1993) have described clumped mitochondria in yeast cells harboring certain mutant actin alleles, and these observations suggest a role for the actin-myosin cytoskeleton in the distribution or organizationof mitochondria. The specific binding of actin to the mitochondrial outer membrane has been described (Pardo et al., 1983), but the functional significance of this interaction remains to be determined. Mitochondrial behavior may depend on several different cytoskeletal systems. For example, microtubules may mediate the positioning and division of mitochondria during interphase, while interactions with intermediate filaments could determine mitochondrial distribution during mitosis. An alternative mechanism of mitochondrial movement (discussed by BereiterHahn, 1990) is that mitochondria move by changing shape and “creeping” through the cytoplasm. Such creeping would employ components and processes internal to the organelle and might be directed by gradients of ADP or other small molecules in the cytoplasm. This model envisions cytoskeletal components as playing a passive role by providing a structure upon which the mitochondria can maneuver.
IV. MUTANTS OF MITOCHONDRIA1 DISTRIBUTION AND MORPHOLOGY One approach to the study of mitochondrial division and inheritance has been the isolation and analysis of Succharomyces cerevisiue mutants possessing conditional defects in mitochondrial morphology and distribution (McConnell et al., 1990). These mdm mutants were isolated by screening a collection of temperaturesensitive yeast strains by fluorescence microscopy to identify cells that failed to transfer mitochondria into developing buds during incubation at 37 “C. A number of such mutant strains have been identified (McConnell et al., 1990),and many of these also display aberrant mitochondrial morphologies. Genetic analysis has revealed that the morphological, distribution, and temperature-sensitive growth phenotypes in mdm mutants result from single, recessive, nuclear mutations. The mutations define at least 20 complementationgroups. Microscopic studies employing indirect immunofluorescence and electron microscopy have confirmed the absence of mitochondria from buds that develop at the nonpermissive temperature in the mdm strains (McConnell et al., 1990). These studies also allowed an examination of the effect of the mutations on other cellular structures. Two classes of mdm mutants have been identified: those in which only the mitochondria are affected by the mutant lesions and those in which the transfer of both mitochondria and nuclei into daughter buds are defective at the nonpermis-
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sive temperature. None of the mutants have been found to affect the secretory process, the development of vacuoles or the transfer of vacuolar material into buds, the assembly and fimction of microtubules, or the cellular distribution of the actin cytoskeleton. The analysis of one mutant, mdml, has led to the identification of a new cytoskeletal component that mediates mitochondrial inheritance in yeast (McConnell and Yaffe, 1992). The mdml mutation affects transfer of both mitochondria and nuclei into daughter buds. Isolation and analysis of the wild-type MDMl gene revealed its product to be a 52 kDa protein with modest sequence similarity to intermediate filament proteins of animal cells. The protein, Mdmlp, is localized to a unique series of punctate structures distributed throughout the yeast cytoplasm. These structuresdisappear or disassemble in the mdml-1 mutant during incubation at the nonpermissive temperature (37OC), although Mdmlp protein levels remain constant in the cells. Mdmlp is an intermediate filament-formingprotein. Mdm lp expressed in E. coli and purified from bacterial inclusion bodies readily formed into filamentsof 10nm diameter in vitro (McConnell and Yaffe, 1993). Conditions of filament selfassembly were essentially those under which the animal proteins, vimentin and desmin, formed intermediate-sized filaments. Mutant Mdmlp, purified from bacteria expressing the mdml-1 gene, was temperature-sensitive for intermediate filament assembly in vitro: 10 nm filaments formed at 4°C or 23°C but failed to form at 37°C. However, filaments formed of the mutant protein at 4°C were stable when subsequently incubated at 37OC, indicating that the mdml-1 mutation prevents filament assembly.The relationshipbetween the filaments formed of Mdmlp in vitro and the punctate Mdmlp structures in the cell is unclear. Mdmlp may assemble into a different type of structure in the cellular environment or, alternatively, the punctate structures may be intersections of a number of filaments, with individual filaments being undetectable by currently available methods. For mitochondrial inheritance, the Mdmlp structures may constitute a network upon which mitochondria are transported. Asecond mdm mutant has revealed a role for unsaturated fatty acids in mitochondrial movement. The mdm2 mutation appeared to affect specificallythe inheritance of mitochondria at the nonpermissive temperature (McConnell et al., 1990). This lesion also blocked a second type of mitochondrial movement: distribution of mitochondria throughout cytoplasmic projections induced by exposure of yeast cells to mating pheromones. Mitochondria in mdm2 cells appeared to aggregate following a shift of cells to the nonpermissive temperature. Cloning and analysis of MDM2 (Stewart and Yaffe, 1991) revealed its identity with a previously identified gene, OLEl, encoding fatty acid desaturase. Consistentwith this identification, cellular levels of unsaturated fatty acids decreased following a shift of mdm2 cells to the nonpermissive temperature. Furthermore, the addition of oleic acid to culture media cured both the temperature-sensitive growth and mitochondrial distribution defects of mdmZ cells. The hnction of unsaturated fatty acids in mitochondrial
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movement might be related to some gross physical properties of the mitochondrial membranes ( e g , membrane fluidity) or could reflect a more specific requirement for unsaturated fatty acid in sustaining the activity or structure of a protein that mediates interaction of mitochondria with other cellular components. Analysis of a third mdm mutant has highlighted the relationship between mitochondrial morphology and mitochondrial inheritance. Yeast cells with the mdmlO lesion possess giant, spherical mitochondria, as well as a defect in mitochondrial inheritance (Sogo and Yaffe, 1994). The giant mitochondria contain classical structural features and are competent for respiration. MDMlO encodes a 57 kDa polypeptide that is an integral protein of the mitochondrial outer membrane. A portion of this protein is exposed to the cytoplasm and may mediate the interaction of mitochondria with cytoskeletal components. Function of the MdmlOp was further revealed by analyzing the cellular distribution and morphology of mitochondria in cells following a controlled cessation of MDMIO expression. The depletion of MdmlOp led to a progressive condensation of snake-like mitochondna into thicker structures and, eventually, into one or a few giant, mitochondrialballs. Empty daughter buds, reflecting defectivemitochondrial inheritance, also appeared as Mdml Op levels dropped. Re-expression of MDMlO in cells with giant mitochondria led to a rapid return of normal mitochondrial morphology. This return to snake-like mitochondria seemed to occur by a stretching-out and fragmentation of the giant organelles. A second approach fbrther illustrated the reversible nature of the giant mitochondrial phenotype. Haploid mdml0 cells (containing giant mitochondria)were mated to wild-type cells of the opposite mating type. The mitochondrial morphologies in the resulting diploid zygotes were analyzed by fluorescence microscopy. Initially, zygotes contained normal mitochondria on one side and a giant mitochondrion on the other side of the cell. Over the subsequent hour, the giant mitochondrion fragmented into numerous snake-like organelles until both sides of the zygote displayed wild-type mitochondrial morphologies. The giant mitochondria in the mdmlO mutant appeared to be defectivecfsr both movement and division. One model to explain how all of these defects could result from a loss of Mdm10p would be that interactions between mitochondria and cytoskeleton mediate the control of mitochondrial morphology, mitochondrial division, and mitochondrial movement. Mdm lop might serve as an anchor-point on the mitochondrial surface for attachment of the organelle to cytoskeletal networks. Another explanation for all of the phenotypes of the mdmlO mutant is that mitochondrialmovement and division require an extended, snake-like morphology, and their failure to occur in mdmlO is a secondary consequence of the dramatic morphological changes. A second, distinct mutant with altered mitochondrial morphology, mmml, was recently described by Burgess and colleagues (1994). This mutant displays phenotypes very similar to those of mdml0, and MMMl also encodes an integral protein of the mitochondrial outer membrane. A third new mutant, mdml2, possesses giant
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mitochondria and defective mitochondrial inheritance (Burger and Yaffe, 1994). These various mutants may define subunits of a protein complex localized to the surface of mitochondria.
V. MODELS OF MITOCHONDRIA1 DIVISION The continuous increase in mitochondrial mass throughout the cell cycle is accompanied by a parallel increase in mitochondrial number (Schnedl, 1974; Posakony et al., 1977). This increase in number occurs by the division of pre-existing mitochondria. Additionally,mitochondria divide in response to various physiological changes (as described previously). Mitochondrial division does not appear to be tightly coordinated with the cell division cycle. Although the amount (i.e., mass) of mitochondria is likely to be regulated by metabolic needs of the cell, factors governing the number of mitochondria are unknown. Additionally, as with mitochondrial inheritance, the mechanism of mitochondrial division has yet to be described. Electron microscopic images ofmitochondria in a variety of cells have suggested several possible models for division. One possibility is that extended mitochondrial structures are pulled apart by molecular motors attached to opposite sides of the organelle and moving in opposite directions along a cytoskeletal track. Such a mechanism might involve kinesin, dynein, or myosin-like motor proteins attached to the outer membrane and could proceed by an initial stretching of the organelle followed by the development of a division furrow. A second possible mechanism is that an encircling band of cytoskeletal proteins constricts the organelle and eventually pinches it in two. This model resembles cytokinesis and might employ an actinomyosin system. A third model is that a division septum or similar structure is assembled within the mitochondrion, facilitating a separation into two distinct organelles. This mechanism might recall the prokaryotic origins of mitochondria and rely on activitiesinternal to the organelle. Ahrther understanding of mitochondrial division may emerge from genetic studies of mutant cells defective in mitochondrial fission and biochemical analysis using systems reconstituted from purified components.
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