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The key role of mitochondria in the apoptotic process is well understood, but not many data are available regarding the specific role of mitochondrial DNA mutations in determining cell fate. We investigated whether two mitochondrial DNA mutations (L217R and L156R) associated with maternally-inherited Leigh syndrome may play a specific role in triggering the apoptotic cascade. Considering that different nuclear genetic factors may influence the expression of mtDNA mutations, we used a 143BTK(-) osteosarcoma cell line deprived from its own mtDNA in order to insert mutated mtDNAs. Analysis of mitochondrial features in these cybrids indicated that both mitochondrial DNA mutations produced evidence of biochemical, functional and ultrastructural modifications of mitochondria, and that these modifications were associated with an increased apoptotic proneness. Cybrids were highly susceptible to two different apoptotic stimuli, tumour necrosis factor-alpha and Staurosporin. The mechanism involved was the mitochondrial 'intrinsic' pathway, i.e. the caspase 9-driven cascade. More importantly, our results also indicated that the polarization state of the mitochondrial membrane, i.e. a constitutive hyperpolarization detected in cybrid clones, played a specific role. Interestingly, the different effects of the two mutations in terms of susceptibility to apoptosis probably reflect the deeper bioenergetic defect associated with the L217R mutation. This work provides the first evidence that hyperpolarization of mitochondria may be a 'risk factor' for cells with a deep ATPase dysfunction, such as cells from patients with maternally-inherited Leigh syndrome.

NG2 is the rat homologue of the human melanoma chondroitin sulfate proteoglycan (MCSP) preferentially expressed in dividing progenitor cells of the glial and mesenchymal lineage but downregulated after differentiation. It has recently been demonstrated that MCSP/NG2 expression is not restricted to mitotic or malignant cells. We show that MCSP/NG2 expression is detectable in the sarcolemma, and in the neuromuscular junction of human postnatal skeletal muscle, and it gradually reduces with advancing age. In human and murine myogenic cell lines, we found no clear differences in MCSP/NG2 expression between myoblasts and myotubes. Reduced levels of the core protein were found in merosin-negative congenital muscular dystrophy (MDC1A). Duchenne muscular dystrophy patients muscles exhibited an overexpression of the MCSP/NG2 core protein. In γ-sarcoglycanopathy and calpainopathy, MCSP/NG2 upregulation was restricted to regenerating myofibers. We demonstrate that MCSP/NG2 is expressed in differentiated myofibers, and appears to have a role in the pathogenesis of MDC1A and severe dystrophinopathies.

NG2 is the rat homologue of the human melanoma chondroitin sulfate proteoglycan (MCSP) preferentially expressed in dividing progenitor cells of the glial and mesenchymal lineage but downregulated after differentiation. It has recently been demonstrated that MCSP/NG2 expression is not restricted to mitotic or malignant cells. We show that MCSP/NG2 expression is detectable in the sarcolemma, and in the neuromuscular junction of human postnatal skeletal muscle, and it gradually reduces with advancing age. In human and murine myogenic cell lines, we found no clear differences in MCSP/NG2 expression between myoblasts and myotubes. Reduced levels of the core protein were found in merosin-negative congenital muscular dystrophy (MDC1A). Duchenne muscular dystrophy patients muscles exhibited an overexpression of the MCSP/NG2 core protein. In gamma-sarcoglycanopathy and calpainopathy, MCSP/NG2 upregulation was restricted to regenerating myofibers. We demonstrate that MCSP/NG2 is expressed in differentiated myofibers, and appears to have a role in the pathogenesis of MDC1A and severe dystrophinopathies.

A multidisciplinary strategy was used to identify the molecular defect in a family with Leigh syndrome (LS). The propositus presented severe developmental delay, an ataxic-spastic gait and seizures. She died at 3.5 y of age from cardiorespiratory arrest. Postmortem examination disclosed pathological features typical of LS. A 12-y-old sister is affected with the same disease. Respiratory chain enzyme complex activities in skeletal muscle biopsy were normal. Adenosine triphosphate (ATP) synthesis during oxidative phosphorylation in skin fibroblasts mitochondria showed a severely hampered ATP production. Mitochondrial DNA sequencing revealed a new mutation in the ATPase 6 gene (T9176G). Site-directed mutagenesis in Escherichia coli strains was used to measure H+ pumping and ATP synthesis. Results were comparable to findings obtained in human cells. These data corroborate the use of E. coli strains as a feasible "animal" model for functional studies in pathogenic mutations of the ATPase 6 gene.