Expression of oxidative phosphorylation genes in muscle cell cultures from patients with mitochondrial myopathies

OXPHOS gene expression and control in mitochondrial disorders

The cellular consequences of deficiencies of the mitochondrial OXPHOS system include a variety of direct and secondary changes in metabolite homeostasis, such as ROS, Ca(2+), ADP/ATP, and NAD/NADH. The adaptive responses to these changes include the transcriptional responses of nuclear and mitochondrial genes that are mediated by these metabolites, control of the mitochondria permeability transition pore, and a great variety of secondary signalling elements. Among the transcriptional responses reported over more than a decade using material harboring mtDNA mutations, deletions, or depletions, nuclear and mitochondrial DNA OXPHOS genes have mostly been up-regulated. However, it is evident from the limited data in a variety of disease models that expression responses are highly diverse and inconsistent. In this article, the mechanisms and controlling elements of these transcriptional responses are reviewed. In addition, the elements that need to be evaluated, in order to gain an improved perspective of the manner in which OXPHOS genes respond and impact on mitochondrial disease expression, are highlighted.

Metallothionein isoform 2A expression is inducible and protects against ROS-mediated cell death in rotenone-treated HeLa cells

The role of MT (metallothionein) gene expression was investigated in rotenone-treated HeLa cells to induce a deficiency of NADH:ubiquinone oxidoreductase (complex I). Complex I deficiency leads to a diversity of cellular consequences, including production of ROS (reactive oxygen species) and apoptosis. HeLa cells were titrated with rotenone, resulting in dose-dependent decrease in complex I activity and elevated ROS production at activities lower than 33%. Expression of MT2A (MT isoform 2A), but not MT1A or MT1B RNA, was significantly inducible by rotenone (up to 7-fold), t-BHP (t-butyl hydroperoxide; 5-fold) and CdCl2 (50-fold), but not ZnCl2. Myxothiazol treatment did not elevate either ROS or MT2A levels, which supports a ROS-related mechanism for rotenone-induced MT2A expression. To evaluate the role of MT2A expression, MT2A and MT1B were overexpressed in HeLa cells and treated with rotenone. Compared with control and MT1B-overexpressing cells, ROS production was significantly lower and cell viability higher in MT2A-overexpressing HeLa cells when ROS production was enhanced by treatment with t-BHP. Mitochondrial membrane potential was noticeably less reduced in both MT-overexpressing cell lines. MT2A overexpression in rotenone-treated cells also significantly reduced or delayed apoptosis induction, as measured by caspase 3/7 activity and cytosolic nucleosome enrichment. We conclude that MT2A offers significant protection against the main death-causing consequences of rotenone-induced complex I deficiency in HeLa cells. Our results are in support of the protective role against oxidative stress ascribed to MTs and provide evidence that MT2A expression may be a beneficial downstream adaptive response in complex I-deficient cells.

Large functional range of steady-state levels of nuclear and mitochondrial transcripts coding for the subunits of the human mitochondrial OXPHOS system

We have measured, by reverse transcription and real-time quantitative PCR, the steady-state levels of the mitochondrial and nuclear transcripts encoding several subunits of the human oxidative phosphorylation (OXPHOS) system, in different normal tissues (muscle, liver, trachea, and kidney) and in cultured cells (normal fibroblasts, 143B osteosarcoma cells, 143B206 rho(0) cells). Five mitochondrial transcripts and nine nuclear transcripts were assessed. The measured amounts of these OXPHOS transcripts in muscle samples corroborated data obtained by others using the serial analysis of gene expression (SAGE) method to appraise gene expression in the same type of tissue. Steady-state levels for all the transcripts were found to range over more than two orders of magnitude. Most of the time, the mitochondrial H-strand transcripts were present at higher levels than the nuclear transcripts. The mitochondrial L-strand transcript ND6 was usually present at a low level. Cultured 143B cells contained significantly reduced amounts of mitochondrial transcripts in comparison with the tissue samples. In 143B206 rho(0) cells, fully depleted of mitochondrial DNA, the levels of nuclear OXPHOS transcripts were not modified in comparison with the parental cells. This observation indicated that nuclear transcription is not coordinated with mitochondrial transcription. We also observed that in the different tissues and cells, there is a transcriptional coregulation of all the investigated nuclear genes. Nuclear OXPHOS gene expression seems to be finely regulated.

Laboratory diagnosis of metabolic myopathies

The metabolic myopathies are a heterogeneous group of disorders inherited by a variety of modes that include gene defects in both the nuclear and mitochondrial genomes. Many factors impact on the expression of the pathogenic mutations that cause these disorders including genetic background, environmental factors, and coexisting disorders. Molecular technology has greatly improved the ability to make definitive diagnoses in many of the metabolic myopathies in the last decade and particularly has demonstrated that the wide diversity in the severity of mutations contributes to understanding genotype-phenotype correlations. In some cases, molecular testing obviates the necessity to perform an invasive muscle biopsy. However, it is also clear that the diagnostic yield from molecular testing is incomplete and particularly low among the mitochondrial myopathies as a group, ranging from approximately 6% to 19% in well-classified high-risk groups. Therefore, it is often essential to combine clinical, biochemical, histopathologic, and molecular data for each patient in order to arrive at a definitive diagnosis. The approach to the laboratory diagnosis of metabolic myopathies is described emphasizing both noninvasive and invasive testing, highlighting the molecular methodologies with the benefits and disadvantages of each technology, and documenting how to determine whether patients have coexisting disorders.

Laboratory approach to mitochondrial diseases

Dysfunction in mitochondrial processes has been related to several pathologies. In these disorders, the cell suffers oxidative imbalance that is mostly due to defects in pyruvate metabolism, mitochondrial fatty acids oxidation, the citric acid cycle or electron transport by the mitochondrial respiratory chain. These metabolic alterations produce mitochondrial diseases that have been related to inherited syndromes, such as MERRF or MELAS. The main affected organs are brain, skeletal muscle, kidney, heart and liver, because of the high energetic demand and the oxidative metabolism. Moreover, the relationship between mitochondrial dysfunction and neurodegenerative processes, such as Parkinson disease or Alzheimer disease, as well as ageing, has been shown. Because mitochondrias are the target of several xenobiotics, such as aspirin, AZT or alcohol consumption, mitochondrial impairment has also been proposed as a mechanism of toxicity. Most laboratory tests that are available in the diagnosis of mitochondrial illness are assayed in tissue biopsies and are usually difficult to interpret. Recently, it has been shown that non-invasive techniques, such as nuclear magnetic resonance or the 2-keto[1-(13)C]isocaproic acid breath test, may be useful to assess mitochondrial function. This article attempts to show the laboratory approach to mitochondrial diseases, reviewing new techniques that could be of great value in the research of mitochondrial function, such as the 2-keto[1-(13)C]isocaproic breath test.

Immunolabelling of mitochondrial superoxide dismutase and of Hsp60 in muscles harbouring a respiratory chain deficiency

In mitochondrial encephalomyopathies, impairment of the electron transfer chain may lead to overproduction of reduced oxygen species because oxygen consumption is decreased. Whether heat shock proteins (Hsp) are induced or not in mitochondria against oxidative stress is questionable. Muscle ragged-red fibres are the histological hallmark of most respiratory chain deficiencies in humans. They exhibit abnormal mitochondria which accumulate mainly under their sarcolemma. Within these fibres, immunolabelling demonstrated strong expression of mitochondrial manganese-dependent superoxide dismutase and a lack of expression of mitochondrial Hsp60 within the subsarcolemmal spaces. In contrast, Hsp60 was overexpressed within the intermyofibrillar mitochondria. These findings suggest enhanced generation and dismutation of superoxide anions and that processing and integration of imported precursor proteins is impaired within the subsarcolemmal mitochondrial aggregates of ragged-red fibres, whereas protein import and assembly may still be efficient in the intermyofibrillar mitochondria of these fibres.

Quantification of OXPHOS gene transcripts during muscle cell differentiation in patients with mitochondrial myopathies

The transcript levels of nuclear and mitochondrial genes involved in oxidative phosphorylation were quantified in human myoblasts and myotubes cultured from biopsies of patients harboring either heteroplasmic point mutation or deletion of mitochondrial DNA. The transcript patterns were determined by two different methodologies, competitive reverse-transcription polymerase chain reaction and classical Northern blot analysis, both referred to the mitochondrial to nuclear DNA ratio. In myoblasts from the patients with MELAS (myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) and KSS (Kearns-Sayre) syndromes, both methodologies revealed an increase of mtDNA transcript levels. A higher level of the nuclear ATP synthase beta transcript was observed in the MELAS patient cells and could be the consequence of a feedback effect of the mitochondrial DNA mutation. Moreover, the nuclear and mitochondrial transcript accumulation is more pronounced after myoblast differentiation. Thus, the OXPHOS expression is specifically altered in patients with mitochondrial diseases. The competitive RT-PCR, a rapid and sensitive technique, could be applied to investigation of mitochondrial myopathies.