Cytoskeletal structure of myoblasts with the mitochondrial DNA 3243A-->G mutation and of osteosarcoma cells with respiratory chain deficiency

Fluorescence imaging of mitochondria in cultured skin fibroblasts: a useful method for the detection of oxidative phosphorylation defects

BACKGROUND

Protons are pumped from the mitochondrial matrix via oxidative phosphorylation (OXPHOS) into the intermembrane space, creating an electric membrane potential (ΔΨ) that is used for adenosine triphosphate (ATP) production. Defects in one or more of the OXPHOS complexes are associated with a variety of clinical symptoms, often making it difficult to pinpoint the causal mutation.

METHODS

In this article, a microscopic method for the quantitative evaluation of ΔΨ in cultured skin fibroblasts is described. The method using 5,5',6,6'-tetraethylbenzimidazolyl-carbocyanine iodide (JC-1) fluorescence staining was tested in a selection of OXPHOS-deficient cell lines.

RESULTS

A significant reduction of ΔΨ was found in the cell lines of patients with either an isolated defect in complex I, II, or IV or a combined defect (complex I + complex IV). ΔΨ was not reduced in the fibroblasts of two patients with severe complex V deficiency. Addition of the complex I inhibitor rotenone induced a significant reduction of ΔΨ and perinuclear relocalization of the mitochondria. In cells with a heteroplasmic mitochondrial DNA (mtDNA) defect, a more heterogeneous reduction of ΔΨ was detected.

CONCLUSION

Our data show that imaging of ΔΨ in cultured skin fibroblasts is a useful method for the evaluation of OXPHOS functioning in cultured cell lines.

Coexistence of mitochondrial and nuclear DNA mutations in a woman with mitochondrial encephalomyopathy and double cortex

We describe a 16-year-old girl with mental retardation, myoclonic epilepsy, ataxia, mitochondrial myopathy, sensorineural hearing loss, lactic acidosis, and MRI evidence of diffuse subcortical laminar heterotopia and agyria/pachygyria. Restriction fragment length polymorphism (RFLP) and DNA sequence analyses revealed two pathogenic mutations: a heteroplasmic m.3243A>G in muscle and blood, and a new heterozygous insertion at nt697 in the doublecortin gene (DCX), resulting in a frameshift after amino acid residue 232, with a premature stop codon at amino acid residue 244. This is yet another example of genetic "double trouble" resulting in a complex phenotype.

Proteome and cytoskeleton responses in osteosarcoma cells with reduced OXPHOS activity

We have recently shown disorganization of the vimentin network in cultured cells deficient in oxidative phosphorylation (OXPHOS). We describe here the cellular responses to OXPHOS deficiency in osteosarcoma cells upon complex I (CI) and complex IV (CIV) inhibition, and upon the lack of mitochondrial DNA (rho0 cells). We examined the cytoskeletal organization and the distribution of mitochondria and analysed total proteome by 2-DE and vimentin expression by ELISA. Upon CIV inhibition and in rho0 cells, the vimentin network had collapsed around the nucleus and formed thick bundles. The mitochondria formed a perinuclear crescent upon CIV inhibition, whereas they accumulated around the nucleus in the rho0 cells, where the amount of vimentin was increased. Analysis of the total proteome revealed that a lack of mitochondrial DNA or inhibition of CI or CIV led to changes in the expression of cytoskeletal and cytoskeleton-associated proteins and proteins involved in apoptosis, OXPHOS, glycolysis, the tricarboxylic acid cycle, and oxidative stress responses. Our findings suggest that a deficiency in the energy converting system and oxidative stress can lead to cytoskeletal changes.

Cytoskeletal structure in cells harboring two mutations: R133C in NOTCH3 and 5650G>A in mitochondrial DNA

We have previously described a patient with cerebral autosomal-dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL) caused by R133C mutation in NOTCH3 and with a concomitant myopathy caused by a G to A point mutation at base pair 5650 (5650G>A) in the gene encoding tRNA(Ala) in mitochondrial DNA (mtDNA). In the present study, we have examined the morphology of the cytoskeletal components in fibroblasts and myoblasts of this patient. Immunolabeling revealed that tubulin network was sparse and formed asters in these cells, whereas no changes were found in actin and vimentin networks in comparison to the control cell lines. Furthermore, mitochondria were less abundant and the branches of the mitochondrial network were reduced in number. Muscle histochemical analysis showed ragged red fibres (RRFs) and cytochrome c oxidase (COX)-negative fibres. The mean proportion of mtDNA with 5650G>A was lower in histologically normal muscle fibres than in the COX-negative fibres and in the RRFs. These findings suggest that 5650G>A is a pathogenic mtDNA mutation. However, the changes in tubulin network and mitochondrial distribution in patient fibroblasts and myoblasts cannot solely be explained by this mutation.

Upregulation of Matrix Metalloproteinase 1 and Disruption of Mitochondrial Network in Skin Fibroblasts of Patients with MERRF Syndrome

By using cDNA microarray and RT-PCR techniques, we investigated the genome-wide alteration of gene expression in skin fibroblasts from patients with myoclonic epilepsy and ragged-red fibers (MERRF) syndrome. By screening for the genes with altered levels of expression, we first discovered that matrix metalloproteinase 1 (MMP1) was highly induced in the primary culture of skin fibroblasts of a female patient in a four-generation family with MERRF syndrome. This phenomenon was confirmed in skin fibroblasts from three other MERRF patients harboring about 85% of mtDNA with A8344G mutation. A further study revealed that the expression of MMP1 could be further induced by treatment of the skin fibroblasts with 200 microM hydrogen peroxide (H2O2) and inhibited by 1 mM N-acetylcysteine. Moreover, the intracellular level of H2O2 in skin fibroblasts of the female MERRF patient was higher than those of the asymptomatic family members and age-matched healthy controls. These findings imply that the increase in the expression of MMP1 may represent one of the responses to the increased oxidative stress in the skin fibroblasts of MERRF patients. We suggest that in affected tissues the oxidative stress-elicited overexpression of MMP1, and probably other matrix metalloproteinases involved in cytoskeleton remodeling, may play an important role in the pathogenesis and progression of mitochondrial encephalomyopathies such as MERRF syndrome.