Flow cytometric evaluation of defects of the mitochondrial respiratory chain

Mitochondrial T9957C Mutation in Association with NAION and Seizures but not MELAS

PURPOSE

To assess the functional significance of the mitochondrial nt-9957 mutation in a man with non-arteritic ischemic optic neuropathy (NAION). This nt-9957 mutation has been previously reported in association with mitochondrial encephalopathy, lactic acidosis, and stroke-like events (MELAS).

METHODS

The patient was examined clinically and with magnetic resonance imaging and spectroscopy. The entire coding region of the mitochondrial genome was sequenced, and mitochondrial function was assessed by flow cytometry after staining with fluorescent dihydroethidium.

RESULTS

This 76-year-old man had optic nerve disease bilaterally and seizures, but no clinical or radiological evidence of MELAS. He had no mitochondrial DNA mutation other than the 9957. Functional testing revealed a severe defect in mitochondrial complex III activity.

CONCLUSIONS

This patient had a mitochondrial functional deficit consistent with his 9957 mutation. It seems quite likely that this mutation may be responsible for optic nerve and brain injuries.

Ethanol hyperpolarizes mitochondrial membrane potential and increases mitochondrial fraction in cultured mouse myocardial cells

Cultured mouse heart-derived myocardial and non-muscle cells were exposed to ethanol, stained with cell-permeant fluorescent vital probes, JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide) and oxidation-sensitive dihydrorhodamine 123, and analyzed by flow cytometry to elucidate ethanol-induced time-wise alterations in the mitochondrial membrane potential (DeltaPsim) and the production of reactive oxygen species (ROS). Ethanol (50 and 200 mM) not only hyperpolarized DeltaPsim of both types of cells but also dose-dependently increased ROS production at 24 h, although a 200-mM dose reduced the production until 3 h. These cell pathophysiological reactions suggest the depression of mitochondrial ATPase and mitochondrial respiratory chain. However, differences between these cells appeared after a 24-h exposure to 200 mM ethanol: the increase in ROS production was approximately twice as large for myocardial cells as for non-muscle cells; and the side-scatter parameter of light scattering significantly increased for myocardial cells, but not for non-muscle cells. All these myocyte-specific alterations indicate an increase in the mitochondrial fraction in a cell. This reaction might be a countermeasure against ethanol-induced dysfunction of mitochondrial respiration that is needed to meet the energy requirements of spontaneous myocardial contractions.

Effects of nitric oxide donors on cybrids harbouring the mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) A3243G mitochondrial DNA mutation

Reactive nitrogen and oxygen species (O2*-, H2O2, NO* and ONOO-) have been strongly implicated in the pathophysiology of neurodegenerative and mitochondrial diseases. In the present study, we examined the effects of nitrosative and/or nitrative stress generated by DETA-NO {(Z)-1-[2-aminoethyl-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate}, SIN-1 (3-morpholinosydnonimine hydrochloride) and SNP (sodium nitroprusside) on U87MG glioblastoma cybrids carrying wt (wild-type) and mutant [A3243G (Ala3243-->Gly)] mtDNA (mitochondrial genome) from a patient suffering from MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes). The mutant cybrids had reduced activity of cytochrome c oxidase, significantly lower ATP level and decreased mitochondrial membrane potential. However, endogenous levels of reactive oxygen species were very similar in all cybrids regardless of whether they carried the mtDNA defects or not. Furthermore, the cybrids were insensitive to the nitrosative and/or nitrative stress produced by either DETA-NO or SIN-1 alone. Cytotoxicity, however, was observed in response to SNP treatment and a combination of SIN-1 and glucose-deprivation. The mutant cybrids were significantly more sensitive to these insults compared with the wt controls. Ultrastructural examination of dying cells revealed several characteristic features of autophagic cell death. We concluded that nitrosative and/or nitrative stress alone were insufficient to trigger cytotoxicity in these cells, but cell death was observed with a combination of metabolic and nitrative stress. The vulnerability of the cybrids to these types of injury correlated with the cellular energy status, which were compromised by the MELAS mutation.

Flow cytometry in the study of mitochondrial respiratory chain disorders

We have developed a flow cytometric assay to measure the oxidative capacity of cultured lymphoblasts as a possible screening test for patients suspected of having a defect of the mitochondrial respiratory chain. Cells were incubated overnight in serum free media, followed by incubation with dihydroethidium with and without rotenone, and then analysed using flow cytometry to measure fluorescence. Inhibition with rotenone gave an increase in fluorescence compared to uninhibited cells. The change in fluorescence was significantly lower in four of the six patient cell lines, with a correlation between the activity of complex I and change in fluorescence. This method may be applicable to cell lines with defects in other complexes of the respiratory chain.

High frequency of somatic mitochondrial DNA mutations in human thyroid carcinomas and complex I respiratory defect in thyroid cancer cell lines

Significant progress has been made to elucidate the molecular mechanisms that determine thyroid tumor development and progression. However, most investigations have mainly focused on the genetic alterations of nuclear DNA. The potential role of mitochondrial DNA (mtDNA) mutations in thyroid tumorigenesis is not well defined. In the present study, we investigated the frequency of mtDNA mutations in 24 thyroid tumor specimens (19 primary papillary thyroid carcinomas (PTC), one follicular thyroid carcinoma, and four multinodular hyperplasias) and four thyroid cancer cell lines by sequencing the entire coding regions of mitochondrial genome. Among the 19 PTC samples tested, seven (36.8%) had somatic mutations. Somatic mtDNA mutations were also detected in one of four multinodular hyperplasias examined. All the thyroid tumor cell lines carried sequence variations that change amino acid and have not been reported previously as normal sequence variants. Flow cytometry analysis of mitochondria respiratory function in the thyroid tumor cell lines revealed a severe defect in mitochondrial complex I activity. The majority of the mutations was involved in genes located in the complex I of the mitochondrial genome. The mutations were either A --> G or C --> T transitions, often resulting in a change of a moderately or highly conserved amino acid of their corresponding protein. These data suggest that mtDNA mutations may play an important role in the thyroid tumorigenesis. Given that mtDNA mutation is present in the benign multinodular hyperplasia, it might be involved in the early stage of tumor development.