Disruption of Fusion Results in Mitochondrial Heterogeneity and Dysfunction

Mitochondria undergo continual cycles of fusion and fission, and the balance of these opposing processes regulates mitochondrial morphology. Paradoxically, cells invest many resources to maintain tubular mitochondrial morphology, when reducing both fusion and fission simultaneously achieves the same end. This observation suggests a requirement for mitochondrial fusion, beyond maintenance of organelle morphology. Here, we show that cells with targeted null mutations in Mfn1 or Mfn2 retained low levels of mitochondrial fusion and escaped major cellular dysfunction. Analysis of these mutant cells showed that both homotypic and heterotypic interactions of Mfns are capable of fusion. In contrast, cells lacking both Mfn1 and Mfn2 completely lacked mitochondrial fusion and showed severe cellular defects, including poor cell growth, widespread heterogeneity of mitochondrial membrane potential, and decreased cellular respiration. Disruption of OPA1 by RNAi also blocked all mitochondrial fusion and resulted in similar cellular defects. These defects in Mfn-null or OPA1-RNAi mammalian cells were corrected upon restoration of mitochondrial fusion, unlike the irreversible defects found in fzoΔ yeast. In contrast, fragmentation of mitochondria, without severe loss of fusion, did not result in such cellular defects. Our results showed that key cellular functions decline as mitochondrial fusion is progressively abrogated. Mitochondria undergo continual cycles of fusion and fission, and the balance of these opposing processes regulates mitochondrial morphology. Paradoxically, cells invest many resources to maintain tubular mitochondrial morphology, when reducing both fusion and fission simultaneously achieves the same end. This observation suggests a requirement for mitochondrial fusion, beyond maintenance of organelle morphology. Here, we show that cells with targeted null mutations in Mfn1 or Mfn2 retained low levels of mitochondrial fusion and escaped major cellular dysfunction. Analysis of these mutant cells showed that both homotypic and heterotypic interactions of Mfns are capable of fusion. In contrast, cells lacking both Mfn1 and Mfn2 completely lacked mitochondrial fusion and showed severe cellular defects, including poor cell growth, widespread heterogeneity of mitochondrial membrane potential, and decreased cellular respiration. Disruption of OPA1 by RNAi also blocked all mitochondrial fusion and resulted in similar cellular defects. These defects in Mfn-null or OPA1-RNAi mammalian cells were corrected upon restoration of mitochondrial fusion, unlike the irreversible defects found in fzoΔ yeast. In contrast, fragmentation of mitochondria, without severe loss of fusion, did not result in such cellular defects. Our results showed that key cellular functions decline as mitochondrial fusion is progressively abrogated. Mitochondrial dynamics involves the two opposing processes of fusion and fission (1Shaw J.M. Nunnari J. Trends Cell Biol. 2002; 12: 178-184Abstract Full Text Full Text PDF PubMed Scopus (302) Google Scholar, 2Chen H. Chan D.C. Curr. Top. Dev. Biol. 2004; 59: 119-144Crossref PubMed Scopus (136) Google Scholar). In cultured cells, an equilibrium is achieved between the two processes, and thus a relatively stable but dynamic morphology is maintained. In mouse embryonic fibroblasts (MEFs), 1The abbreviations used are: MEF, mouse embryonic fibroblast; Mfn, mitofusin; Mfn-dm, mitofusin double mutant; OPA1-OE, OPA1 overexpressing; RNAi, RNA interference; ΔΨ, membrane potential; mtDNA, mitochondrial DNA; shRNA, short hairpin RNA; GFP, green fluorescent protein; PEG, polyethylene glycol. 1The abbreviations used are: MEF, mouse embryonic fibroblast; Mfn, mitofusin; Mfn-dm, mitofusin double mutant; OPA1-OE, OPA1 overexpressing; RNAi, RNA interference; ΔΨ, membrane potential; mtDNA, mitochondrial DNA; shRNA, short hairpin RNA; GFP, green fluorescent protein; PEG, polyethylene glycol. for example, a tubular network of mitochondria normally predominates. If fusion is inhibited, however, unopposed fission leads to mitochondrial fragmentation (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar). In mammalian cells, the large mitochondrial GTPases Mitofusin 1 (Mfn1), Mitofusin 2 (Mfn2), and OPA1 play important roles in mitochondrial fusion (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar, 4Olichon A. Baricault L. Gas N. Guillou E. Valette A. Belenguer P. Lenaers G. J. Biol. Chem. 2003; 278: 7743-7746Abstract Full Text Full Text PDF PubMed Scopus (856) Google Scholar, 5Griparic L. van der Wel N.N. Orozco I.J. Peters P.J. van der Bliek A.M. J. Biol. Chem. 2004; 279: 18792-18798Abstract Full Text Full Text PDF PubMed Scopus (330) Google Scholar, 6Cipolat S. Martins de Brito O. Dal Zilio B. Scorrano L. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 15927-15932Crossref PubMed Scopus (870) Google Scholar, 7Santel A. Fuller M.T. J. Cell Sci. 2001; 114: 867-874Crossref PubMed Google Scholar, 8Rojo M. Legros F. Chateau D. Lombes A. J. Cell Sci. 2002; 115: 1663-1674Crossref PubMed Google Scholar). In humans, mutations in Mfn2 cause Charcot-Marie-Tooth neuropathy type 2A (9Zuchner S. Mersiyanova I.V. Muglia M. Bissar-Tadmouri N. Rochelle J. Dadali E.L. Zappia M. Nelis E. Patitucci A. Senderek J. Parman Y. Evgrafov O. Jonghe P.D. Takahashi Y. Tsuji S. Pericak-Vance M.A. Quattrone A. Battologlu E. Polyakov A.V. Timmerman V. Schroder J.M. Vance J.M. Nat. Genet. 2004; 36: 449-451Crossref PubMed Scopus (1206) Google Scholar), and mutations in OPA1 cause dominant optic atrophy (10Alexander C. Votruba M. Pesch U.E. Thiselton D.L. Mayer S. Moore A. Rodriguez M. Kellner U. Leo-Kottler B. Auburger G. Bhattacharya S.S. Wissinger B. Nat. Genet. 2000; 26: 211-215Crossref PubMed Scopus (1037) Google Scholar, 11Delettre C. Lenaers G. Griffoin J.M. Gigarel N. Lorenzo C. Belenguer P. Pelloquin L. Grosgeorge J. Turc-Carel C. Perret E. Astarie-Dequeker C. Lasquellec L. Arnaud B. Ducommun B. Kaplan J. Hamel C.P. Nat. Genet. 2000; 26: 207-210Crossref PubMed Scopus (1130) Google Scholar). Mice lacking either of the mitofusins die as embryos, with Mfn2-null mice showing placental insufficiency (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar). To understand these diverse tissue-specific phenotypes, it will be critical to elucidate the relative importance of Mfn1, Mfn2, and OPA1 in specific cell types. In particular, Mfn1 and Mfn2 appear to play similar roles in mitochondrial fusion, but the degree of redundancy between these homologs and whether they can cooperate in mitochondrial fusion remains to be determined. On a cellular level, the best known function of mitochondrial fusion is to maintain mitochondrial morphology (12Bleazard W. McCaffery J.M. King E.J. Bale S. Mozdy A. Tieu Q. Nunnari J. Shaw J.M. Nat. Cell Biol. 1999; 1: 298-304Crossref PubMed Scopus (573) Google Scholar, 13Sesaki H. Jensen R.E. J. Cell Biol. 1999; 147: 699-706Crossref PubMed Scopus (425) Google Scholar). Given the highly ordered arrangement of mitochondrial networks in some cell types (14Bakeeva L.E. Chentsov Y.S. Skulachev V.P. Eur. J. Cell Biol. 1981; 25: 175-181PubMed Google Scholar), it is likely that mitochondrial shape indeed has important consequences for mitochondrial function. Nevertheless, there are clues that mitochondrial fusion has additional functions beyond regulation of morphology. First, the in vivo rates of mitochondrial fusion and fission far exceed those necessary to maintain tubular mitochondrial morphology. Cells with greatly reduced rates can have tubular mitochondria, as long as the fusion and fission rates are appropriately balanced (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar). Second, in Mfn1-null or Mfn2-null cells, a small subset of mitochondria lose their membrane potential (ΔΨ) (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar). We have hypothesized that mitochondrial dynamics protects mitochondria by ensuring that stochastic depletion of materials, such as metabolic substrates or mitochondrial DNA (mtDNA), is transient. Without fusion, however, these deficiencies persist, accumulate, and result in cellular dysfunction. In this study, we examined the dependence of mitochondrial fusion on Mfn1, Mfn2, and OPA1 and demonstrated that cells lacking all mitochondrial fusion show defects in key cellular functions. Fusion-defective cells have dramatically reduced growth rates, reduced respiration, and a highly heterogeneous mitochondrial population prone to loss of ΔΨ. Cell Lines and Culture—To generate Mfn-dm cells, MEFs were first derived from Mfn1–/–, Mfn2-/conditional embryos (carrying one copy of a loxP-flanked conditional allele of Mfn2), as described previously (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar). Transient transfection with a Cre-enhanced GFP expression plasmid, followed by cell sorting for Cre-GFP expression, was used to isolate Mfn-dm cells. The genotypes of all mutant cells were confirmed by PCR analysis. Mfn-dm and OPA1-RNAi lines were derived and maintained in rich medium (Dulbecco's modified Eagle's medium supplemented with 15% fetal calf serum, 1 mm pyruvate, 50 μg/ml uridine). All other cell lines were maintained in standard medium (Dulbecco's modified Eagle's medium supplemented with 10% bovine calf serum). All experiments involving retroviral infection were conducted on both freshly infected bulk cultures and multiple clonal lines. Mitochondrial ΔΨ was monitored, as described previously (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar), except that mitochondria were detected by expression Su9-GFP (enhanced GFP with the mitochondrial presequence of subunit 9 of the F0-ATPase). Z-stacks of images acquired on a Zeiss Axiovert 200 m microscope were deconvolved using the iterative algorithm in AxioVision 4.2 and presented as maximum intensity projections. For cell growth measurements, parental control cultures were split in two and either mock-infected or infected with the experimental virus. Cells were then plated onto two 6-well plates, at 6 × 104 cells/well in their respective growth media. 4 h later, the cells were counted, in triplicate, to constitute the day 0 score. At this time, the medium in the remaining 9 wells of cell was to standard The cells were then counted, in triplicate, on and RNA RNA we retroviral short hairpin from the The was as described previously 2002; PubMed Scopus Google Scholar). The GFP of G. and J. of and of J. of were with and The was then as a to the retroviral and were then as OPA1 was by for to was from these and used to as described previously (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar). The were and were conducted using the of L. and A. van der (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar), and fusion was as described previously (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar), except that cells were plated and in rich Cell fusion was to for h in the of with for h did not the fusion To membrane fusion in OPA1-RNAi cells, cells the mitochondrial membrane (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google were The was detected by the is a Mfn1 that has dominant on mitochondrial morphology. The cells were with a on a Zeiss microscope using AxioVision Zeiss was in cells using a as described previously G. G. Cell Biol. 2001; PubMed Google Scholar). In 2 × cells in of mm mm mm mm were to the and was 1 of the was was as the a of 1 2 At mm was the and the maximum was for 1 rates were in cells, as described previously S. P. C. G. A. J. Biol. Chem. 2003; 278: Full Text Full Text PDF PubMed Scopus Google Scholar). of Mitochondrial in Cells or targeted we previously that and double mutant cells have mitochondria, of in mitochondrial fusion (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar, Detmer S.A. H. McCaffery J.M. Chan D.C. 2004; PubMed Scopus Google Scholar). it remains to degree these homologs are and whether they cooperate to levels of mitochondrial fusion. To these we a of mitochondrial morphology and fusion in the mutant cell lines. MEFs of all genotypes and showed mitochondrial they be We found that all mutant cells be cells showed fragmentation in mitochondrial cells showed fragmentation but some short mitochondrial in Cells with mitochondrial in were in of whether fragmentation was images of these are in 1 for of the mutant of these the tubular network found in cells Mfn2-null cells showed all showed mitochondrial a short in to mitochondrial and In contrast, Mfn1-null MEFs and showed severe mitochondrial with of cells showing small mitochondrial Mfn1-null cells of be from Mfn2-null cells of by the of the Mfn1-null mitochondrial The of Mfn1-null cells showed a of small and short Mfn1-null cells morphology. The of mitochondria in Mfn2-null cells suggests that cells Mfn1 fusion cells all Mfn-dm cells and to and showed mitochondrial or in to Mfn1-null or Mfn2-null cells of Mfn-dm cells showed heterogeneity in the mitochondrial by a of large and small 1 the of these mutant cells and cells the same have mitochondrial on their We also the mitochondrial morphology of cells with expression of in mitochondrial dynamics A. Baricault L. Gas N. Guillou E. Valette A. Belenguer P. Lenaers G. J. Biol. Chem. 2003; 278: 7743-7746Abstract Full Text Full Text PDF PubMed Scopus (856) Google Scholar, 5Griparic L. van der Wel N.N. Orozco I.J. Peters P.J. van der Bliek A.M. J. Biol. Chem. 2004; 279: 18792-18798Abstract Full Text Full Text PDF PubMed Scopus (330) Google Scholar, 6Cipolat S. Martins de Brito O. Dal Zilio B. Scorrano L. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 15927-15932Crossref PubMed Scopus (870) Google Scholar). cells were by MEFs with short hairpin RNA This all of and OPA1-RNAi cells of OPA1 levels cells a OPA1 were also results were with and D. previously both and of OPA1 to mitochondrial fragmentation A. Baricault L. Gas N. Guillou E. Valette A. Belenguer P. Lenaers G. J. Biol. Chem. 2003; 278: 7743-7746Abstract Full Text Full Text PDF PubMed Scopus (856) Google Scholar, 5Griparic L. van der Wel N.N. Orozco I.J. Peters P.J. van der Bliek A.M. J. Biol. Chem. 2004; 279: 18792-18798Abstract Full Text Full Text PDF PubMed Scopus (330) Google Scholar, 6Cipolat S. Martins de Brito O. Dal Zilio B. Scorrano L. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 15927-15932Crossref PubMed Scopus (870) Google Scholar). we found that the mitochondrial of the two types of cells be cells small mitochondrial that were the In contrast, cells mitochondria that were highly heterogeneous in a similar to that of Mfn-dm cells. OPA1-RNAi and Mfn-dm cells some large mitochondrial in by a of small mitochondrial For both Mfn-dm Detmer S.A. H. McCaffery J.M. Chan D.C. 2004; PubMed Scopus Google and OPA1-RNAi cells this mitochondrial fragmentation was of the the tubular network of for OPA1 and in types of mitochondrial fragmentation be by in the degree of mitochondrial fusion. To this we used the cell fusion to mitochondrial fusion in mutant cell lines In cell were for mitochondrial fusion. with results Detmer S.A. H. McCaffery J.M. Chan D.C. 2004; PubMed Scopus Google Scholar), cells demonstrated fusion of all their mitochondria in the of cell fusion h Mfn-dm cells showed fusion and D. fusion and and and and and and and in a In contrast, we detected fusion in cells Mfn1 or Cell Mfn2 showed and fusion of cell fusion was found in one of cell Mfn1 showed fusion but a showed fusion These levels of fusion likely the of mitochondrial in Mfn1-null and Mfn2-null cells and in Mfn1 has a in mitochondrial fusion the mitofusins are in that a is capable of levels of fusion. mitofusins have to be on mitochondria to fusion Detmer S.A. H. McCaffery J.M. Chan D.C. 2004; PubMed Scopus Google Scholar), these results that homotypic interactions and can fusion. cell between Mfn1-null and Mfn2-null cells also fusion, with showing fusion and showing fusion This result for the first time, that heterotypic interactions can fusion at expression OPA1 RNAi that a of OPA1 levels to of results in of mitochondrial fusion by S. Martins de Brito O. Dal Zilio B. Scorrano L. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 15927-15932Crossref PubMed Scopus (870) Google Scholar). of the of this RNAi it was not to whether OPA1 is for mitochondrial fusion or the of fusion. We this with OPA1-RNAi cells, levels of OPA1 In fusion OPA1-RNAi cells were of mitochondrial fusion This fusion likely the similar defects in mitochondria in OPA1-RNAi and Mfn-dm cells. these results demonstrated that are for mitochondrial fusion. standard fusion on mitochondrial we also used an mitochondrial membrane to whether membrane fusion is in OPA1-RNAi cells. mitochondrial fusion was that both and membrane fusion In contrast, cells their mitochondria as as cells that the mitochondrial fragmentation in these cells was not to the of fusion. We also found levels of fusion at the that cells have a fusion In of mitochondrial fragmentation by OPA1 L. van der Wel N.N. Orozco I.J. Peters P.J. van der Bliek A.M. J. Biol. Chem. 2004; 279: 18792-18798Abstract Full Text Full Text PDF PubMed Scopus (330) Google Scholar, Y. J. Biol. Chem. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar), it has whether fragmentation is of reduced fusion or Our the with this we found that mitochondrial fragmentation to OPA1 was blocked when the fission was by of Cell to defects in mitochondrial morphology, we that both OPA1-RNAi and Mfn-dm cells cell growth is a of cell we this growth by cell the of 4 Mfn2-null cells of Mfn1 by to of the parental Mfn2-null control Mfn-dm cells also showed This growth was as demonstrated by the growth of such cells upon of Mfn1 and Mfn2 cells lacking Mfn2 or Mfn1 showed growth that the levels of fusion are to severe of cell function. to Mfn-dm cells, OPA1-RNAi cells also and 15% of the of mock-infected control cells cells, with but mitochondria, at the same as of Mitochondrial cell growth in cells in mitochondrial fusion to specific mitochondrial defects To mitochondrial ΔΨ, cells were with a mitochondrial on ΔΨ. This was with a that to the mitochondrial in cells with low mitochondrial ΔΨ B. W. 2000; PubMed Scopus Google Scholar). For the described results were when a mitochondrial membrane was used of with (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar), we found that both Mfn1-null and Mfn2-null cells have heterogeneity in mitochondrial Mfn1-null cells showed some mitochondria with but this was to a mitochondria Mfn2-null cells, a of cells showed heterogeneity in and this was to a In contrast, of Mfn-dm cells showed widespread heterogeneity of mitochondrial ΔΨ, with a large of the mitochondrial population to with The loss of mitochondrial ΔΨ was with restoration of ΔΨ in Mfn-null cells of OPA1 by RNAi to loss of mitochondrial ΔΨ Cells OPA1 showed ΔΨ defects with the fusion these results that the of mitochondrial ΔΨ loss with the of fusion Mitochondrial the of mutant cells, we used to the of in cells. We first the and then a to the of the a in in at MEFs a that was capable of upon of the similar was in cells In contrast, Mfn-dm cells reduced respiration, and to upon the of OPA1-RNAi cells an severe in and upon the of The severe found in OPA1-RNAi cells, as with that of Mfn-dm cells, was highly between multiple To the in cell we in cells. we used and to for and for was for all in the mutant lines important of the defects was that they were Mfn-dm cells a upon expression of Mfn1 and Mfn2 and expression of OPA1 in OPA1-RNAi cells completely and the severe in OPA1-RNAi cells. This unlike in defects in mammalian cells are not to a loss of mtDNA, and suggests that of fusion respiration. is to that cells lacking Mfn1 or Mfn2 in cell growth or bulk that low levels of mitochondrial fusion are to at some of the defects of Mfns and OPA1 in Mitochondrial is some that Mfn1 and Mfn2 have S. Martins de Brito O. Dal Zilio B. Scorrano L. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 15927-15932Crossref PubMed Scopus (870) Google Scholar, N. Y. J. Cell Sci. 2004; PubMed Scopus Google Scholar), but they have similar roles in mitochondrial membrane fusion. these two mitofusin homologs have expression it is important to whether they play roles in they are and whether they can cooperate to mitochondrial fusion. Our showed that both Mfn1 and Mfn2 play a major in levels of mitochondrial fusion. In the of either mitochondrial fragmentation results from a in the of mitochondrial fusion, as by the fusion Nevertheless, a mitofusin is to generate mitochondrial fusion that can be In mouse Mfn1 to play a Mfn2, as by the loss of mitochondrial fusion and severe fragmentation found in Mfn1-null suggests that mitofusins in to mitochondrial and fusion Detmer S.A. H. McCaffery J.M. Chan D.C. 2004; PubMed Scopus Google Scholar). Our fusion using of Mfn1-null and Mfn2-null cells, that mitochondrial fusion, in can be by in Mfn1 homotypic Mfn2 homotypic and heterotypic Our results the observation and by A. Baricault L. Gas N. Guillou E. Valette A. Belenguer P. Lenaers G. J. Biol. Chem. 2003; 278: 7743-7746Abstract Full Text Full Text PDF PubMed Scopus (856) Google Scholar, 5Griparic L. van der Wel N.N. Orozco I.J. Peters P.J. van der Bliek A.M. J. Biol. Chem. 2004; 279: 18792-18798Abstract Full Text Full Text PDF PubMed Scopus (330) Google Scholar, 6Cipolat S. Martins de Brito O. Dal Zilio B. Scorrano L. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 15927-15932Crossref PubMed Scopus (870) Google Scholar, Y. J. Biol. Chem. 2002; Full Text Full Text PDF PubMed Scopus Google that as as depletion of OPA1 mitochondrial We that OPA1 all mitochondrial fusion. In contrast, fragmentation to of OPA1 not result from a fusion but is The for this fragmentation remains to be is that of OPA1 can to an in OPA1 similar is in yeast. is the for of M. F. C. W. J. Biol. Chem. 2003; 278: Full Text Full Text PDF PubMed Scopus Google Scholar, S. M. 2003; PubMed Scopus Google Scholar). The balance of long and short of is critical for maintenance of tubular mitochondrial morphology H. Jensen R.E. 2003; PubMed Scopus Google Scholar, M. C. W. J. Cell Biol. 2004; PubMed Scopus Google Scholar). lacking the long of and have mitochondria that are H. Jensen R.E. 2003; PubMed Scopus Google Scholar). of Mitochondrial have the of mitochondrial fusion and fission in maintenance of mitochondrial morphology (3Chen H. Detmer S.A. Ewald A.J. Griffin E.E. Fraser S.E. Chan D.C. J. Cell Biol. 2003; 160: 189-200Crossref PubMed Scopus (1706) Google Scholar, W. McCaffery J.M. King E.J. Bale S. Mozdy A. Tieu Q. Nunnari J. Shaw J.M. Nat. Cell Biol. 1999; 1: 298-304Crossref PubMed Scopus (573) Google Scholar, 13Sesaki H. Jensen R.E. J. Cell Biol. 1999; 147: 699-706Crossref PubMed Scopus (425) Google Scholar). is likely on the cell the shape of mitochondria can their For example, it has hypothesized that the and of mitochondria in some cells the of or membrane potential to specific of the cell V.P. Trends Sci. 2001; 26: Full Text Full Text PDF PubMed Scopus Google Scholar). In to this in regulation of organelle that mitochondrial fusion is important for mitochondrial function. Mfn1-null and Mfn2-null MEFs highly mitochondria but mitochondrial fusion to major cellular dysfunction. In contrast, Mfn-dm cells and OPA1-RNAi cells mitochondrial fusion. These cells show widespread heterogeneity in mitochondrial ΔΨ, and reduced In measurements, these cells at rates in the of an and these rates are with cells. these cells have heterogeneity in their mitochondria, this a for the reduced of the mutant cells have or for respiration. These not appear to be to mitochondrial cells have mitochondria without loss of mitochondrial fusion and not cellular defects. of is that the cell growth and defects are not to an irreversible loss of as in of the cell function. remains a that the severe defects in Mfn-dm and OPA1-RNAi cells be to as functions of Mfns or from fusion. This is of a with this is and has to play a in membrane A. Baricault L. Gas N. Guillou E. Valette A. Belenguer P. Lenaers G. J. Biol. Chem. 2003; 278: 7743-7746Abstract Full Text Full Text PDF PubMed Scopus (856) Google Scholar). additional functions the in OPA1-RNAi cells is severe in Mfn-dm cells. we the that the defects in Mfn-dm and OPA1-RNAi cells are of the of mitochondrial fusion. are and appear to play a in mitochondrial membrane fusion Detmer S.A. H. McCaffery J.M. Chan D.C. 2004; PubMed Scopus Google Scholar). In the in Mfn-dm and OPA1-RNAi cells are with the that they from a major for will be to the loss of fusion in mammalian cells results in mitochondrial dysfunction. In some experimental mitochondrial fusion has with on mitochondrial function. Cell between parental cells mutant result in restoration of Nat. Genet. 2001; PubMed Scopus (330) Google Scholar), that of can in mice mutations are from mitochondrial a critical of mutant is A. Nat. 2001; PubMed Scopus Google Scholar). These results that mitochondrial fusion can of and of mitochondrial function. We are to G. J. J. L. and A. van der Bliek for of and We J. for on with