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Mitochondrial fusion and fission processes play a role in a variety of cell functions, including energy metabolism, cell differentiation and programmed cell death. Still, it is not clear how these processes contribute to the cell functions. Here, we investigated the role of mitochondrial remodelling on lipid metabolism in adipocytes. In 3T3-L1 pre-adipocytes, the morphology of mitochondria is organized as a continuous reticulum. Upon differentiation of adipocytes manifested by cellular triacylglycerol (TG) accumulation, mitochondrial morphology altered from filamentous to fragmented and/or punctate structures. When the mitochondrial fusion was induced in adipocytes by silencing of mitochondrial fission proteins including Fis1 and Drp1, the cellular TG content was decreased. In contrast, the silencing of mitochondrial fusion proteins including mitofusin 2 and Opa1 increased the cellular TG content followed by fragmentation of mitochondria. It also appears that polyphenolic phytochemicals, negative regulators of lipid accumulation, have mitochondrial fusion activity and that there is a good correlation between mitochondrial fusion activity and the cellular TG accumulation-reducing activity of the phytochemicals. These results suggest that cellular TG accumulation is regulated, at least in part, via mitochondrial fusion and fission processes.

Mitochondria are highly dynamic organelles in eukaryotic cells. Although the role of mitochondria in metabolism, ATP production and apoptosis is more widely recognized, alterations in mitochondrial morphology and abundance are also important for cellular functions. Here we investigated mitochondrial dynamics in synchronized HeLa cells in which the major stages of the cell cycle of the observed cells were resolved by staining phosphorylate histones H1 and H3, and showed that mitochondria exist as filamentous network structures throughout the cell cycle progression, changing their morphology, distribution, and abundance. The current results suggest that mitochondrial condensation occurred at prophase is required for the proper progression of mitochondrial division.

We found that a mouse homolog of human DNA polymerase delta interacting protein 38, referred to as Mitogenin I in this paper, and mitochondrial single-stranded DNA-binding protein (mtSSB), identified as upregulated genes in the heart of mice with juvenile visceral steatosis, play a role in the regulation of mitochondrial morphology. We demonstrated that overexpression of Mitogenin I or mtSSB increased elongated or fragmented mitochondria in mouse C2C12 myoblast cells, respectively. On the other hand, the silencing of Mitogenin I or mtSSB by RNA interference led to an increase in fragmented or elongated mitochondria in the cells, respectively, suggesting that Mitogenin I and mtSSB are involved in the processes of mitochondrial fusion and fission, respectively. In addition, we showed that the silencing of Mitogenin I resulted in an increase in the number of trypan blue-positive cells and the silencing of mtSSB resulted in an enhancement of the sensitivity of the cells to apoptotic stimulation by etoposide. The present results demonstrated that these proteins play a role in cell survival.

Subunit c is normally present as an inner mitochondrial membrane component of the F0 section of the ATP synthase complex, but in the late infantile form of neuronal ceroid lipofuscinosis (NCL) it was also found in lysosomes in high concentrations. To explore the mechanism of storage of subunit c, the rates of degradation and synthesis of subunit c were measured in fibroblast cell types from controls and patients with the late infantile form of NCL. The radiolabel from subunit c decreased with time in control cells, whereas no apparent loss of radioactivity of subunit c was found in patients' cells. There were no significant differences between control cells and cells with disease in the degradation of cytochrome oxidase subunit IV, an inner membrane protein of mitochondria. A combination of pulse-chase and subcellular fractionation analysis showed that a delay of intramitochondrial loss from prelabeled subunit c was seen in all diseased cells tested. Lysosomal appearance of labeled subunit c could be detected after chase for more than 1 week and its radioactivities were variable among diseased cell types. The biosynthetic rate of subunit c was almost the same in both control and patient cells. Northern blotting analyses showed that mRNAs for P1 and P2 genes had no significant difference in lengths and amounts between control and patient cells. Results suggest a specific failure in the degradation of subunit c after its normal inclusion in mitochondria and its consequent accumulation in lysosomes. This is the first direct evidence to show a delay of subunit c degradation in the cells from the late infantile form of NCL.

Two cDNAs encoding different signal peptides (61 and 66 amino acid residues) and the same mature protein have been cloned from a human cDNA library with a rat subunit c cDNA. The amino acid sequence of the mature human subunit c of mitochondrial H(+)-ATP synthase was completely identical with those of the mature bovine, sheep and rat subunit c.

Four cDNAs for the import precursors of oligomycin sensitivity conferring protein (OSCP), ATPase inhibitor protein (IF1) and subunit cs (encoded by P1 and P2 genes) of rat mitochondrial H(+)-ATP synthase have been cloned from a rat cDNA library. The import precursors and the mature polypeptides of rat OSCP, IF1, subunit c (P1) and subunit c (P2) consisted of 23/190, 25/82, 61/75 and 66/75 amino acids, respectively.

The nucleotide sequence of the import precursor of subunit b of human H(+)-ATP synthase has been determined from a recombinant cDNA clone isolated by screening a human kidney cDNA library with a cDNA for rat subunit b as a probe. The sequence was composed of 1,134 nucleotides including a coding region for the import precursor of subunit b and noncoding regions on the 5'- and 3'-sides. The import precursor of subunit b and its mature polypeptide deduced from the open reading frame were found to consist of 256 and 214 amino acid residues with molecular weights of 28,893 and 24,610, respectively. The presequence of 42 amino acids could be the import signal peptide for directing the protein into the mitochondrial matrix.