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Background: Mitochondria can sense signals linked to variations in energy demand to regulate nuclear gene expression. This retrograde signaling pathway is presumed to be involved in the regulation of myoblast proliferation and differentiation. Rhabdomyosarcoma cells are characterized by their failure to both irreversibly exit the cell cycle and complete myogenic differentiation. However, it is currently unknown whether mitochondria are involved in the failure of rhabdomyosarcoma cells to differentiate. Methodology/Principal Findings: Mitochondrial biogenesis and metabolism were studied in rat L6E9 myoblasts and R1H rhabdomyosacoma cells during the cell cycle and after 36 hours of differentiation. Using a combination of flow cytometry, polarographic and molecular analyses, we evidenced a marked decrease in the cardiolipin content of R1H cells cultured in growth and differentiation media, together with a significant increase in the content of mitochondrial biogenesis factors and mitochondrial respiratory chain proteins. Altogether, these data indicate that the mitochondrial inner membrane composition and the overall process of mitochondrial biogenesis are markedly altered in R1H cells. Importantly, the dysregulation of protein-to-cardiolipin ratio was associated with major deficiencies in both basal and maximal mitochondrial respiration rates. This deficiency in mitochondrial respiration probably contributes to the inability of R1H cells to decrease mitochondrial H2O2 level at the onset of differentiation. Conclusion/Significance: A defect in the regulation of mitochondrial biogenesis and mitochondrial metabolism may thus be an epigenetic mechanism that may contribute to the tumoral behavior of R1H cells. Our data underline the importance of mitochondria in the regulation of myogenic differentiation. [ABSTRACT FROM AUTHOR]

Mitochondria can sense signals linked to changes in energy demand roexpression. This retrograde signaling pathway is presumed to be affect nuclear gene involved in the regulation of myoblast proliferation and differentiation. We have investigated the regulation of mitochondrial biogenesis and production of putative retrograde signaling agents [hydrogen peroxide (H2O2) and Ca2+] during the cell cycle and the onset of differentiation in L6E9 muscle cells. The biosyinthesis of cardiolipin and mitochondrial proteins was mainly achieved in S phase, whereas the expression of mitochondrial biogenesis factors [peroxisome proliferator-activated receptor (PPAR)-α, PPAR-δ, and neuronal nitric - oxide synthase I]was regularly increased from G1 to G2M phase. In agreement with the increase in mitochondrial membrane potential, mitochondria in S and G2M phases have a significantly higher H2O2 level when compared with H2O2 phase. By contrast, the onset of differentiation was characterized by a marked reduction in mitochondrial protein expression and mitochondrial H2O2 level. The capacity of mitochondria to release Ca2+ in response to a metabolic challenge was significantly decreased at the onset of differentiation. Finally, an increase in calmodulin expression in S and G2M phases and a transitory increase in phosphorylated nuclear factor of activated T cells (NFAT) c3 in S phase was observed. NFATc3 phosphorylation was markedly decreased at the onset of differentiation. Our data point to functional links between the control of mitochondrial biogenesis and the regulation of the level of retrograde signaling agents during the cell cycle and the onset of differentiation ir L6E9 muscle cells. [ABSTRACT FROM AUTHOR]

Major modifications in energy homeostasis occur in skeletal muscle during exercise. Emerging evidence suggests that changes in energy homeostasis take part in the regulation of gene expression and contribute to muscle plasticity. A number of energy-sensing molecules have been shown to sense variations in energy homeostasis and trigger regulation of gene expression. The AMP-activated protein kinase, hypoxia-inducible factor 1, peroxisome proliferator-activated receptors, and Sirt1 proteins all contribute to altering skeletal muscle gene expression by sensing changes in the concentrations of AMP, molecular oxygen, intracellular free fatty acids, and NAD+, respectively. These molecules may therefore sense information relating to the intensity, duration, and frequency of muscle exercise. Mitochondria also contribute to the overall response, both by modulating the response of energy-sensing molecules and by generating their own signals. This review seeks to examine our current understanding of the roles that energy-sensing molecules and mitochondria can play in the regulation of gene expression in skeletal muscle. [ABSTRACT FROM AUTHOR]

Presents a study which hypothesized that mitochondrial biogenesis would be stimulated during skeletal muscle regeneration. Protein extraction for immunoblotting and enzyme assays; Details of mitochondrial isolation and respiration; Histochemical analyses of necrosis and regeneration.

Presents a study on adaptations in the mitochondrial protein import system. Process involved in mitochondrial biogenesis; Methodology; Results.

The aim of the present study was to determine whether mitochondrial activity could regulate myoblast proliferation. We demonstrate that an increase in mitochondrial activity of L6E9 myoblasts can be easily obtained by simply raising extracellular pyruvate concentration in the culture dish. Under this condition, L6E9 myoblasts underwent a rapid growth arrest in G1 + S phases concomitant to a marked cellular hypertrophy. No sign of myoblast fusion was evident. This was accompanied by the down-regulation of proliferating cell nuclear antigen expression and an increase in p21 expression. Mitochondrial biogenesis was also stimulated, as indicated by a twofold increase in mitochondrial content. These cells exhibited a large increase in the production of reactive oxygen species that could contribute to the observed phenotypic alterations. However, exposure of pyruvate-treated cells to antioxidants did not reverse growth arrest. Similarly, exposure of control cells to oxidants did not induce growth arrest. Our observations suggest that mitochondrial activity appears to play a central role in regulating myoblast proliferation. They also argue strongly in favor of a retrograde communication establishing a mitochondrial control of nuclear gene expression that could be modulated by mitochondrial activity. [Copyright &y& Elsevier]

Skeletal muscle mitochondria of the African pygmy mouse Mus mattheyi exhibit markedly reduced oxygen consumption and ATP synthesis rates but a higher mitochondrial efficiency than what would be expected from allometric trends. In the present study, we assessed whether such reduction of mitochondrial activity in M. mattheyi can limit the oxidative stress associated with an increased generation of mitochondrial reactive oxygen species. We conducted a comparative study of mitochondrial oxygen consumption, H 2 O 2 release, and electron leak (%H 2 O 2 /O) in skeletal muscle mitochondria isolated from the extremely small African pygmy mouse (M. mattheyi , ~5 g) and Mus musculus , which is a larger Mus species (~25 g). Mitochondria were energized with pyruvate, malate, and succinate, after which fluxes were measured at different steady-state rates of oxidative phosphorylation. Overall, M. mattheyi exhibited lower oxidative activity and higher electron leak than M. musculus , while the H 2 O 2 release did not differ significantly between these two Mus species. We further found that the high coupling efficiency of skeletal muscle mitochondria from M. mattheyi was associated with high electron leak. Nevertheless, data also show that, despite the higher electron leak, the lower mitochondrial respiratory capacity of M. mattheyi limits the cost of a net increase in H 2 O 2 release, which is lower than that expected for a mammals of this size. [Display omitted] • A high mitochondrial efficiency in mammals is correlated with a high electron leak. • However, the low oxidative capacity of Mus mattheyi limits the net release of reactive oxygen species. • This characteristic may contribute to its relative high longevity given its body mass. [ABSTRACT FROM AUTHOR]