Mitochondrial morphology is crucial for tissue homeostasis, but its role in cell differentiation is unclear. We found that mitochondrial fusion was required for proper cardiomyocyte development. Ablation of mitochondrial fusion proteins Mitofusin 1 and 2 in the embryonic mouse heart, or gene-trapping of Mitofusin 2 or Optic atrophy 1 in mouse embryonic stem cells (ESCs), arrested mouse heart development and impaired differentiation of ESCs into cardiomyocytes. Gene expression profiling revealed decreased levels of transcription factors transforming growth factor-β/bone morphogenetic protein, serum response factor, GATA4, and myocyte enhancer factor 2, linked to increased Ca(2+)-dependent calcineurin activity and Notch1 signaling that impaired ESC differentiation. Orchestration of cardiomyocyte differentiation by mitochondrial morphology reveals how mitochondria, Ca(2+), and calcineurin interact to regulate Notch1 signaling.

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Caspases and the cytotoxic lymphocyte protease granzyme B (GB) induce reactive oxygen species (ROS) formation, loss of transmembrane potential and mitochondrial outer membrane permeabilization (MOMP). Whether ROS are required for GBmediated apoptosis and how GB induces ROS is unclear. Here, we found that GB induces cell death in an ROS-dependent manner, independently of caspases and MOMP. GB triggers ROS increase in target cell by directly attacking the mitochondria to cleave NDUFV1, NDUFS1 and NDUFS2 subunits of the NADH: ubiquinone oxidoreductase complex I inside mitochondria. This leads to mitocentric ROS production, loss of complex I and III activity, disorganization of the respiratory chain, impaired mitochondrial respiration and loss of the mitochondrial cristae junctions. Furthermore, we have also found that GB-induced mitocentric ROS are necessary for optimal apoptogenic factor release, rapid DNA fragmentation and lysosomal rupture. Interestingly, scavenging the ROS delays and reduces many of the features of GB-induced death. Consequently, GB-induced ROS significantly promote apoptosis. Cell Death and Differentiation (2015) 22, 862-874; doi:10.1038/cdd.2014.180; published online 31 October 2014To induce cell death, human granzyme B (GB) activates effector caspase-3 or acts directly on key caspase substrates, such as the proapoptotic BH3 only Bcl-2 family member Bid, inhibitor of caspase-activated DNase (ICAD), poly-(ADPribose) polymerase-1 (PARP-1), lamin B, nuclear mitotic apparatus protein 1 (NUMA1), catalytic subunit of the DNAdependent protein kinase (DNA-PKcs) and tubulin. [1][2][3] Consequently, caspase inhibitors have little effect on human GB-mediated cell death and DNA fragmentation. 2 GB causes reactive oxygen species (ROS) production, dissipation of the mitochondrial transmembrane potential (ΔΨm) and MOMP, which leads to the release of apoptogenic factors such as cytochrome c (Cyt c), HtrA2/Omi, endonuclease G (Endo G), Smac/Diablo and apoptosis-inducing factor, from the mitochondrial intermembrane space to the cytosol. [4][5][6][7][8][9][10][11] Interestingly, cells deficient for Bid, Bax and Bak are still sensitive to human GB-induced cell death, 5,11-13 suggesting that human GB targets the mitochondria in another way that needs to be characterized. Altogether, much attention has been focused on the importance of MOMP in the execution of GB-mediated cell death, leaving unclear whether ROS production is a bystander effect or essential to the execution of GBinduced apoptosis. The mitochondrial NADH: ubiquinone oxidoreductase complex I is a key determinant in steadystate ROS production. This 1 MDa complex, composed of 44 subunits, 14 couples the transfer of two electrons from NADH to ubiquinone with the translocation of four protons to generate the ΔΨm. The importance of ROS has been previously demonstrated for caspase-3 and granzyme A (GA) pathways through the cleavage of NDUFS1 and NDUFS3, respectively. [15][16][17][18] GA induces cell death in a Bcl-2-insensitive and caspase-and MOMP-indepen...

Generation of various kinds of trans-mitochondrial mice, mito-mice, each carrying mtDNAs with a different pathogenic mutation, is required for precise investigation of the pathogenesis of mitochondrial diseases. This study used two respiration-deficient mouse cell lines as donors of mtDNAs with possible pathogenic mutations. One cell line expressed 45-50% respiratory activity due to mouse mtDNAs with a T6589C missense mutation in the COI gene (T6589C mtDNA) and the other expressed 40% respiratory activity due to rat (Rattus norvegicus) mtDNAs in mouse cells. By cytoplasmic transfer of these mtDNAs to mouse ES cells, we isolated respiration-deficient ES cells. We obtained chimeric mice and generated their F(6) progeny carrying mouse T6589C mtDNAs by its female germ line transmission. They were respiration-deficient and thus could be used as models of mitochondrial diseases caused by point mutations in mtDNA structural genes. However, chimeric mice and mito-mice carrying rat mtDNAs were not obtained, suggesting that significant respiration defects or some deficits induced by rat mtDNAs in mouse ES cells prevented their differentiation to generate mice carrying rat mtDNAs.

The proportions of mutant and wild-type mtDNA are crucial in determining the severity of mitochondrial diseases. It has been generally considered that deletion-mutant mtDNA has replication advantages and accumulates with time.