Mitochondria form a highly dynamic tubular network, the morphology of which is regulated by frequent fission and fusion events. However, the role of mitochondrial fission in homeostasis of the organelle is still unknown. Here we report that preventing mitochondrial fission, by down-regulating expression of Drp1 in mammalian cells leads to a loss of mitochondrial DNA and a decrease of mitochondrial respiration coupled to an increase in the levels of cellular reactive oxygen species (ROS). At the cellular level, mitochondrial dysfunction resulting from the lack of fission leads to a drop in the levels of cellular ATP, an inhibition of cell proliferation and an increase in autophagy. In conclusion, we propose that mitochondrial fission is required for preservation of mitochondrial function and thereby for maintenance of cellular homeostasis.

Apoptosis, induced by a number of death stimuli, is associated with a fragmentation of the mitochondrial network. These morphological changes in mitochondria have been shown to require proteins, such as Drp1 or hFis1, which are involved in regulating the fission of mitochondria. However, the precise role of mitochondrial fission during apoptosis remains elusive. Here we report that inhibiting the fission machinery in Bax/Bak-mediated apoptosis, by down-regulating of Drp1 or hFis1, prevents the fragmentation of the mitochondrial network and partially inhibits the release of cytochrome c from the mitochondria but fails to block the efflux of Smac/DIABLO. In addition, preventing mitochondrial fragmentation does not inhibit cell death induced by Bax/Bak-dependent death stimuli, in contrast to the effects of Bcl-xL or caspase inhibition. Therefore, the fission of mitochondria is a dispensable event in Bax/Bak-dependent apoptosis.Mitochondria play a critical role in the regulation of programmed cell death by sequestering apoptogenic proteins such as cytochrome c, Smac/DIABLO, HtrA2/Omi, endonuclease G, and AIF (13,18,29,80). The release of such factors during apoptosis is regulated by a subclass of Bcl-2 proteins (12,14,63,78), including Bax and Bak. These proteins seem to be in an inactive state in healthy cells, with Bax predominantly found in the cytosol. However, during apoptosis induced by various death stimuli, including DNA damage or trophic factor deprivation, they are activated by a process requiring BH3-only Bcl-2 family members. It is thought that BH3-only proteins either bind and sequester Bcl-2 antiapoptotic proteins (this is the case for Bad and Puma) or bind to and directly activate proapoptotic proteins (tBid for example) (9,11,33,45,65,74). This results in the inactivation of Bcl-2 antiapoptotic proteins and in the oligomerization of Bax and Bak in the mitochondrial outer membrane (MOM) with a concomitant release of apoptogenic factors from the mitochondria (17,21,31,43).How permeabilization of the MOM occurs during apoptosis remains a matter of debate and has been extensively studied (for reviews, see references 4, 49, and 85). Recently, a new model has emerged based on the discovery that mitochondria fragment during cell death (32, 46-48, 61, 86). According to this model, the fission of mitochondria would be necessary for permeabilization of the MOM (3, 57, 83).Nevertheless, it is still not clear whether the fragmentation of mitochondria precedes or follows the release of apoptogenic factors (1,22,26).Mitochondrial fission and fusion are normal and frequent events in healthy cells. The protein machinery that underlies mitochondrial fission has been well characterized and extensively reviewed (53,62). In mammalian cells, at least three proteins, Drp1, hFis1, and MTP18 (75,76), are required for this process. The dynamin-related protein Drp1 is a large cytosolic GTPase that translocates to the mitochondria, where it couples GTP hydrolysis with scission of the mitochondrial tubule (59,67,68). Its recept...

Previous genomic analyses of the filamentous nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120 have identified three ferric uptake regulator (Fur) homologs with low sequence identities and probably different functions in the cell. FurA is a constitutive protein that shares the highest homology with Fur from heterotrophic bacteria and appears to be essential for in vitro growth. In this study, we have analysed the effects of FurA overexpression on the Anabaena sp. phenotype and investigated which of the observed alterations were directly operated by FurA. Overexpression of the regulator led to changes in cellular morphology, resulting in shorter filaments with rounded cells of different sizes. The furA-overexpressing strain showed a slower photoautotrophic growth and a marked decrease in the oxygen evolution rate. Overexpression of the regulator also decreased both catalase and superoxide dismutase activities, but did not lead to an increase in the levels of intracellular reactive oxygen species. By combining phenotypic studies, reverse transcription-PCR analyses and electrophoretic mobility shift assays, we identified three novel direct targets of FurA, including genes encoding a siderophore outer membrane transporter (schT), bacterial actins (mreBCD) and the PSII reaction center protein D1 (psbA). The affinity of FurA for these novel targets was markedly affected by the absence of divalent metal ions, confirming previous evidence of a critical role for the metal co-repressor in the function of the regulator in vivo. The results unravel new cellular processes modulated by FurA, supporting its role as a global transcriptional regulator in Anabaena sp. PCC 7120.

Until now, descriptions of intracellular biomineralization of amorphous inclusions involving alkaline-earth metal (AEM) carbonates other than calcium have been confined exclusively to cyanobacteria (Couradeau et al., 2012). Here, we report the first evidence of the presence of intracellular amorphous granules of AEM carbonates (calcium, strontium, and barium) in unicellular eukaryotes. These inclusions, which we have named micropearls, show concentric and oscillatory zoning on a nanometric scale. They are widespread in certain eukaryote phytoplankters of Lake Geneva (Switzerland) and represent a previously unknown type of non-skeletal biomineralization, revealing an unexpected pathway in the geochemical cycle of AEMs. We have identified Tetraselmis cf. cordiformis (Chlorophyta, Prasinophyceae) as being responsible for the formation of one micropearl type containing strontium ([Ca,Sr]CO ), which we also found in a cultured strain of Tetraselmis cordiformis. A different flagellated eukaryotic cell forms barium-rich micropearls [(Ca,Ba)CO ]. The strontium and barium concentrations of both micropearl types are extremely high compared with the undersaturated water of Lake Geneva (the Ba/Ca ratio of the micropearls is up to 800,000 times higher than in the water). This can only be explained by a high biological pre-concentration of these elements. The particular characteristics of the micropearls, along with the presence of organic sulfur-containing compounds-associated with and surrounding the micropearls-strongly suggest the existence of a yet-unreported intracellular biomineralization pathway in eukaryotic micro-organisms.

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