Accumulation of depolarized mitochondria within b-cells has been associated with oxidative damage and development of diabetes. To determine the source and fate of depolarized mitochondria, individual mitochondria were photolabeled and tracked through fusion and fission. Mitochondria were found to go through frequent cycles of fusion and fission in a 'kiss and run' pattern. Fission events often generated uneven daughter units: one daughter exhibited increased membrane potential (Dw m ) and a high probability of subsequent fusion, while the other had decreased membrane potential and a reduced probability for a fusion event. Together, this pattern generated a subpopulation of nonfusing mitochondria that were found to have reduced Dw m and decreased levels of the fusion protein OPA1. Inhibition of the fission machinery through DRP1 K38A or FIS1 RNAi decreased mitochondrial autophagy and resulted in the accumulation of oxidized mitochondrial proteins, reduced respiration and impaired insulin secretion. Pulse chase and arrest of autophagy at the pre-proteolysis stage reveal that before autophagy mitochondria lose Dw m and OPA1, and that overexpression of OPA1 decreases mitochondrial autophagy. Together, these findings suggest that fission followed by selective fusion segregates dysfunctional mitochondria and permits their removal by autophagy.

Assembly of mitochondria into networks supports fuel metabolism and calcium transport and is involved in the cellular response to apoptotic stimuli. A mitochondrial network is defined as a continuous matrix lumen whose boundaries limit molecular diffusion. Observation of individual networks has proven challenging in live cells that possess dense populations of mitochondria. Investigation into the electrical and morphological properties of mitochondrial networks has therefore not yielded consistent conclusions. In this study we used matrix-targeted, photoactivatable green fluorescent protein to tag single mitochondrial networks. This approach, coupled with real-time monitoring of mitochondrial membrane potential, permitted the examination of matrix lumen continuity and fusion and fission events over time. We found that adjacent and intertwined mitochondrial structures often represent a collection of distinct networks. We additionally found that all areas of a single network are invariably equipotential, suggesting that a heterogeneous pattern of membrane potential within a cell's mitochondria represents differences between discrete networks. Interestingly, fission events frequently occurred without any gross morphological changes and particularly without fragmentation. These events, which are invisible under standard confocal microscopy, redefine the mitochondrial network boundaries and result in electrically disconnected daughter units.

The mitochondrial transporter ATP binding cassette mitochondrial erythroid (ABC-me/ABCB10) is highly induced during erythroid differentiation by GATA-1 and its overexpression increases hemoglobin production rates in vitro. However, the role of ABC-me in erythropoiesis in vivo is unknown. Here we report for the first time that erythrocyte development in mice requires ABC-me. ABC-meÀ/À mice die at day 12.5 of gestation, showing nearly complete eradication of primitive erythropoiesis and lack of hemoglobinized cells at day 10.5. ABC-meÀ/À erythroid cells fail to differentiate because they exhibit a marked increase in apoptosis, both in vivo and ex vivo. Erythroid precursors are particularly sensitive to oxidative stress and ABC-me in the heart and its yeast ortholog multidrug resistance-like 1 have been shown to protect against oxidative stress. Thus, we hypothesized that increased apoptosis in ABC-meÀ/À erythroid precursors was caused by oxidative stress. Within this context, ABC-me deletion causes an increase in mitochondrial superoxide production and protein carbonylation in erythroid precursors. Furthermore, treatment of ABC-meÀ/À erythroid progenitors with the mitochondrial antioxidant MnTBAP (superoxide dismutase 2 mimetic) supports survival, ex vivo differentiation and increased hemoglobin production. Altogether, our findings demonstrate that ABC-me is essential for erythropoiesis in vivo. Cell Death and Differentiation (2012) 19, 1117-1126 doi:10.1038/cdd.2011; published online 13 January 2012A central event during erythroid development is the induction of the components responsible for heme biosynthesis. Although heme is also produced in non-erythroid cells, its biosynthesis is specifically regulated during erythroid differentiation. 1-3 Thus, mutations in some specific genes involved in heme production have been shown to cause several human blood disorders, such as sideroblastic anemias. 4 This type of anemia is characterized by abnormal erythroid differentiation with a reduction in the total number of erythrocytes. 4 In addition, there are idiopathic anemias (either genetic or druginduced) in which the primary molecular defect is unknown. Recently, a novel bioinformatic approach identified new genes involved in heme biosynthesis that could be potential therapeutic targets for these disorders. 5 Furthermore, this large-scale computational screen identified again the ATP Binding Cassette mitochondrial erythroid transporter (ABCme/ABCB10) as an important protein for heme biosynthesis. 5 We previously identified ABC-me as a downstream target of an essential transcription factor for terminal erythroid differentiation, namely GATA-1. 6 ABC-me is a homodimeric transporter located in the inner mitochondrial membrane, which shows the highest expression levels in erythroid tissues. 6,7 Importantly, during early stages of mouse development (day 10 post coitus, pc), ABC-me expression is detected exclusively at the primitive sites of hematopoiesis, namely the yolk sac blood islands. 6 Furthermore, at day 9.5-10.5 pc,...

ATP binding cassette (ABC) transporters are a diverse superfamily of energy-dependent membrane translocases. Although responsible for the majority of transmembrane transport in bacteria, they are relatively uncommon in eukaryotic mitochondria. Organellar trafficking and import, in addition to quaternary structure assembly, of mitochondrial ABC transporters is poorly understood and may offer explanations for the paucity of their diversity. Here we examine these processes in ABCB10 (ABC-me), a mitochondrial inner membrane erythroid transporter involved in heme biosynthesis. We report that ABCB10 possesses an unusually long 105-amino acid mitochondrial targeting presequence (mTP). The central subdomain of the mTP (amino acids (aa) 36 -70) is sufficient for mitochondrial import of enhanced green fluorescent protein. The N-terminal subdomain (aa 1-35) of the mTP, although not necessary for the trafficking of ABCB10 to mitochondria, participates in the proper import of the molecule into the inner membrane. We performed a series of amino acid mutations aimed at changing specific properties of the mTP. The mTP requires neither arginine residues nor predictable ␣-helices for efficient mitochondrial targeting. Disruption of its hydrophobic character by the mutation L46Q/ I47Q, however, greatly diminishes its efficacy. This mutation can be rescued by cryptic downstream (aa 106 -715) mitochondrial targeting signals, highlighting the redundancy of this protein's targeting qualities. Mass spectrometry analysis of chemically cross-linked, immunoprecipitated ABCB10 indicates that ABCB10 embedded in the mitochondrial inner membrane homodimerizes and homo-oligomerizes. A deletion mutant of ABCB10 that lacks its mTP efficiently targets to the endoplasmic reticulum. Quaternary structure assembly of ABCB10 in the ER appears to be similar to that in the mitochondria.ABC 1 transporters comprise a large and diverse family of membrane translocases (1). Their function ranges from peptide transport to phospholipid flipping to anion channel formation. Members of the ABC transporter superfamily have been implicated in numerous human diseases (including cystic fibrosis (CFTR/ABCC7), adrenoleukodystrophy (ALDP/ABCD1), Zellweger's syndrome (PMP70/ABCD3), progressive familial intrahepatic cholestasis (SPGP/ABCB11), and Stargardt macular dystrophy (ABCR/ABCA4)) (2). The basic structure of ABC transporters is relatively well conserved and includes a hydrophilic ATP binding cassette and a hydrophobic membranespanning domain (3). Whereas the large members of the family contain two of each domain, the "half-transporters" contain one of each and are predicted to dimerize (3-6). Mammalian ABC transporters are found predominantly in the plasma membrane but are also known to play essential roles in a number of organelles, including the endoplasmic reticulum, peroxisome, and the mitochondrion (2). Mammalian mitochondrial ABC transporters have recently gained attention for their role in heme biosynthesis and iron sulfur cluster synthesis (7-12). According...