Mitochondrial fusion and division play important roles in the regulation of apoptosis. Mitochondrial fusion proteins attenuate apoptosis by inhibiting release of cytochrome c from mitochondria, in part by controlling cristae structures. Mitochondrial division promotes apoptosis by an unknown mechanism. We addressed how division proteins regulate apoptosis using inhibitors of mitochondrial division identified in a chemical screen. The most efficacious inhibitor, mdivi-1 (for mitochondrial division inhibitor) attenuates mitochondrial division in yeast and mammalian cells by selectively inhibiting the mitochondrial division dynamin. In cells, mdivi-1 retards apoptosis by inhibiting mitochondrial outer membrane permeabilization. In vitro, mdivi-1 potently blocks Bid-activated Bax/Bak-dependent cytochrome c release from mitochondria. These data indicate the mitochondrial division dynamin directly regulates mitochondrial outer membrane permeabilization independent of Drp1-mediated division. Our findings raise the interesting possibility that mdivi-1 represents a class of therapeutics for stroke, myocardial infarction, and neurodegenerative diseases.

Statement MITO-MAP, a high-density genetic interaction map in budding yeast, identifies a mitochondrial inner membrane–associated complex that promotes normal mitochondrial membrane organization and morphology.

Mitochondrial outer- and inner-membrane fusion events are coupled in vivo but separable and mechanistically distinct in vitro, indicating that separate fusion machines exist in each membrane. Outer-membrane fusion requires trans interactions of the dynamin-related GTPase Fzo1, GTP hydrolysis, and an intact inner-membrane proton gradient. Inner-membrane fusion also requires GTP hydrolysis but distinctly requires an inner-membrane electrical potential. The protein machinery responsible for inner-membrane fusion is unknown. Here, we show that the conserved intermembrane-space dynamin-related GTPase Mgm1 is required to tether and fuse mitochondrial inner membranes. We observe an additional role of Mgm1 in inner-membrane dynamics, specifically in the maintenance of crista structures. We present evidence that trans Mgm1 interactions on opposing inner membranes function similarly to tether and fuse inner membranes as well as maintain crista structures and propose a model for how the mitochondrial dynamins function to facilitate fusion.

Impaired regulation of mitochondrial dynamics, which shifts the balance towards fission, is associated with neuronal death in age-related neurodegenerative diseases, such as Alzheimer's disease or Parkinson's disease. A role for mitochondrial dynamics in acute brain injury, however, has not been elucidated to date. Here, we investigated the role of dynamin-related protein 1 (Drp1), one of the key regulators of mitochondrial fission, in neuronal cell death induced by glutamate toxicity or oxygen-glucose deprivation (OGD) in vitro, and after ischemic brain damage in vivo. Drp1 siRNA and small molecule inhibitors of Drp1 prevented mitochondrial fission, loss of mitochondrial membrane potential (MMP), and cell death induced by glutamate or tBid overexpression in immortalized hippocampal HT-22 neuronal cells. Further, Drp1 inhibitors protected primary neurons against glutamate excitotoxicity and OGD, and reduced the infarct volume in a mouse model of transient focal ischemia. Our data indicate that Drp1 translocation and associated mitochondrial fission are key features preceding the loss of MMP and neuronal cell death. Thus, inhibition of Drp1 is proposed as an efficient strategy of neuroprotection against glutamate toxicity and OGD in vitro and ischemic brain damage in vivo. Mitochondria play crucial roles in energy metabolism, regulation of free radical formation and calcium storage, thereby determining essential metabolic functions and cell survival. 1 Further, mitochondria are highly dynamic organelles that undergo constant fission and fusion and these morphological changes are required for efficient ATP production, calcium buffering, regulation of signal transduction and apoptosis. 2 In neurons, mitochondrial fission is also essential for axonal transport of the organelles into areas of high metabolic demand, 3 whereas mitochondrial fusion supports substitution and regeneration of mitochondrial proteins, mtDNA repair and functional recovery. 2,4 Consistent with the critical roles of mitochondrial dynamics in neurons, defects in mitochondrial fission and fusion proteins are associated with a wide array of inherited or acquired neurodegenerative diseases such as Charcot-Marie-Tooth disease or Alzheimer's disease, respectively. 2 Fission and fusion defects may limit mitochondrial motility, decrease energy production, promote oxidative stress and lead to accumulating of mtDNA defects, thereby promoting neuronal dysfunction and cell death. 1 Recently, enhanced mitochondrial fragmentation was associated with induction of neuronal death triggered by oxidative stress. 5 These data imply that during neuronal cell death, the tubular mitochondrial network is fragmented into smaller and functionally impaired organelles. 5 It is, however, a matter of ongoing controversy, whether mitochondrial fragmentation is cause or consequence in programmed cell death.Current knowledge of the mechanisms regulating mitochondrial dynamics indicates that fission and fusion of mitochondria are under control of highly conserved dynamin-relate...

Abalance between fission and fusion events determines the morphology of mitochondria. In yeast, mitochondrial fission is regulated by the outer membrane–associated dynamin-related GTPase, Dnm1p. Mitochondrial fusion requires two integral outer membrane components, Fzo1p and Ugo1p. Interestingly, mutations in a second mitochondrial-associated dynamin-related GTPase, Mgm1p, produce similar phenotypes to fzo1 and ugo cells. Specifically, mutations in MGM1 cause mitochondrial fragmentation and a loss of mitochondrial DNA that are suppressed by abolishing DNM1-dependent fission. In contrast to fzo1 ts mutants, blocking DNM1-dependent fission restores mitochondrial fusion in mgm1 ts cells during mating. Here we show that blocking DNM1-dependent fission in Δmgm1 cells fails to restore mitochondrial fusion during mating. To examine the role of Mgm1p in mitochondrial fusion, we looked for molecular interactions with known fusion components. Immunoprecipitation experiments revealed that Mgm1p is associated with both Ugo1p and Fzo1p in mitochondria, and that Ugo1p and Fzo1p also are associated with each other. In addition, genetic analysis of specific mgm1 alleles indicates that Mgm1p's GTPase and GTPase effector domains are required for its ability to promote mitochondrial fusion and that Mgm1p self-interacts, suggesting that it functions in fusion as a self-assembling GTPase. Mgm1p's localization within mitochondria has been controversial. Using protease protection and immuno-EM, we have shown previously that Mgm1p localizes to the intermembrane space, associated with the inner membrane. To further test our conclusions, we have used a novel method using the tobacco etch virus protease and confirm that Mgm1p is present in the intermembrane space compartment in vivo. Taken together, these data suggest a model where Mgm1p functions in fusion to remodel the inner membrane and to connect the inner membrane to the outer membrane via its interactions with Ugo1p and Fzo1p, thereby helping to coordinate the behavior of the four mitochondrial membranes during fusion.