Falk R. Wiedemann

Permeabilized cell and skinned fiber techniques in studies of mitochondrial function in vivo

In this chapter we describe in details the permeabilized cell and skinned fiber techniques and their applications for studies of mitochondrial function in vivo. The experience of more than 10 years of research in four countries is summarized. The use of saponin in very low concentration (50–100 μg/ml) for permeabilisation of the sarcolemma leaves all intracellular structures, including mitochondria, completely intact. The intactness of mitochondrial function in these skinned muscle fibers is demonstrated in this work by multiple methods, such as NADH and flavoprotein fluorescence studies, fluorescence imaging, confocal immunofluorescence microscopy and respiratory analysis. Permeabilized cell and skinned fiber techniques have several very significant advantages for studies of mitochondrial function, in comparison with the traditional methods of use of isolated mitochondria: (1) very small tissue samples are required; (2) all cellular population of mitochondria can be investigated; (3) most important, however, is that mitochondria are studied in their natural surrounding. The results of research by using this method show the existence of several new phenomenon — tissue dependence of the mechanism of regulation of mitochondrial respiration, and activation of respiration by selective proteolysis. These phenomena are explained by interaction of mitochondria with other cellular structures in vivo. The details of experimental studies with use of these techniques and problems of kinetic analysis of the results are discussed. Examples of large-scale clinical application of these methods are given. (Mol Cell Biochem 184: 81–100, 1998)

Mitochondrial DNA and respiratory chain function in spinal cords of ALS patients

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by selective motor neuron death. In order to address the question of a putative role of mitochondrial dysfunction in the pathogenesis of ALS, we studied the mitochondrial DNA (mtDNA) and mitochondrial respiratory chain enzyme activities in spinal cords of ALS patients and in control subjects without neuropathologic abnormalities. Using a ‘double PCR and digestion’ technique to estimate the levels of randomly distributed point mutations in two small regions of the mtDNA, we found significantly higher levels of mutant mtDNA in the spinal cord of ALS patients compared to controls. No large‐scale rearrangements were found, but the amount of mtDNA, measured by Southern blot, was significantly lower in the ALS samples. This reduction correlated well with a decrease of citrate synthase (CS) activity, a mitochondrial marker, as were the activities of respiratory chain complexes I + III, II + III, and IV, suggesting a loss of mitochondria in ALS spinal cords.

Impairment of mitochondrial function in skeletal muscle of patients with amyotrophic lateral sclerosis

In skeletal muscle homogenates of 14 patients with sporadic amyotrophic lateral sclerosis, an approximately twofold lower specific activity of NADH:CoQ oxidoreductase in comparison to an age matched control group (n=28) was detected. This finding was confirmed by a detailed analysis of mitochondrial oxidative phosphorylation in skeletal muscle using saponin-permeabilized muscle fibers. (i) A significantly lowered maximal glutamate+malate and pyruvate+malate supported respiration of saponin-permeabilized fibers was detected in the patients group. (ii) Titrations with the specific inhibitor of NADH:CoQ oxidoreductase amytal revealed a higher sensitivity of respiration to this inhibitor indicating an elevated flux control coefficient of this enzyme. (iii) Applying functional imaging of mitochondria using ratios of NAD(P)H and flavoprotein autofluorescence images of saponin-permeabilized fibers we detected the presence of partially respiratory chain inhibited mitochondria on the single fiber level. A secondary defect of mitochondrial function due to the neurogenic changes in muscle seems to be unlikely since no mitochondrial abnormalities were detectable in biopsies of patients with spinal muscular atrophy. These results support the viewpoint that an impairment of mitochondria may be of pathophysiological significance in the etiology of amyotrophic lateral sclerosis.

Impaired mitochondrial oxidative phosphorylation in skeletal muscle of the dystrophin-deficient mdx mouse

The mdx mouse, an animal model of the Duchenne muscular dystrophy, was used for the investigation of changes in mitochondrial function associated with dystrophin deficiency. Enzymatic analysis of skeletal muscle showed an approximately 50% decrease in the activity of all respiratory chain-linked enzymes in musculus quadriceps of adult mdx mice as compared with controls, while in cardiac muscle no difference was observed. The activities of cytosolic and mitochondrial matrix enzymes were not significantly different from the control values in both cardiac and skeletal muscles. In saponin-permeabilized skeletal muscle fibers of mdx mice the maximal rates of mitochondrial respiration were about two times lower than those of controls. These changes were also demonstrated on the level of isolated mitochondria. Mdx muscle mitochondria had only 60% of maximal respiration activities of control mice skeletal muscle mitochondria and contained only about 60% of hemoproteins of mitochondrial inner membrane. Similar findings were observed in a skeletal muscle biopsy of a Duchenne muscular dystrophy patient. These data strongly suggest that a specific decrease in the amount of all mitochondrial inner membrane enzymes, most probably as result of Ca2+ overload of muscle fibers, is the reason for the bioenergetic deficits in dystrophin-deficient skeletal muscle.

Hypoxia increases activity of the BK-channel in the inner mitochondrial membrane and reduces activity of the permeability transition pore.

Hypoxia can cause severe damage to cells by initiating signaling cascades that lead to cell death. A cellular oxygen sensor, other than the respiratory chain, might exist in sensitive components of these signaling cascades. Recently, we found evidence that mitochondrial ion channels are sensitive to low levels of oxygen. We therefore studied the effects of hypoxia on the mitochondrial BK-channel (mtBK), on the mitochondrial permeability transition pore (PTP), and on their possible interaction. Using single-channel patch-clamp techniques we found that hypoxia inhibited the PTP but substantially increased the mtBK activity of mitoplasts from rat liver and astrocytes. Experiments measuring the mitochondrial membrane potential of intact rat brain mitochondria (using the fluorescence dye safranine O) during hypoxia exhibited an increased Ca2+-retention capacity implying an impaired opening of the PTP. We also found a reduced Ca2+-retention capacity with 100 nM iberiotoxin, a selective inhibitor of BK-channels. We therefore conclude that there is interaction between the mtBK and the PTP in a way that an open mtBK keeps the PTP closed. Thus, the response of mitochondrial ion channels to hypoxia could be interpreted as anti-apoptotic.

Effect of coenzyme Q10 on the mitochondrial function of skin fibroblasts from Parkinson patients.

Several lines of evidence suggest an impairment of mitochondrial function in the brain of patients with Parkinsons disease (PD). However, the presence of a detectable mitochondrial defect in extracerebral tissue of these patients remains a matter of dispute. Therefore, we investigated mitochondrial function in fibroblasts of 18 PD patients applying biochemical micromethods. Putative beneficial effects of coenzyme Q(10) (CoQ(10)), a potent antioxidant, on the mitochondrial function of skin fibroblast cultures were evaluated. Applying inhibitor titrations of the mitochondrial respiration to calculate flux control coefficients of respiratory chain complexes I and IV, we observed deficiencies of both complexes in cultivated skin fibroblasts of PD patients. Cultivation of fibroblasts in the presence of 5 microM CoQ(10) restored the activity of impaired respiratory chain complexes in the fibroblast cultures of 9 out of 18 PD patients. Our data support the presence of a generalised mitochondrial defect in at least a subgroup of patients with PD that can be partially ameliorated in vitro by coenzyme Q(10) treatment.

Re-evaluation of the dysfunction of mitochondrial respiratory chain in skeletal muscle of patients with Parkinson’s disease

Summary.The origin and tissue distribution of the mitochondrial dysfunction in Parkinson’s disease (PD) remains still a matter of controversy. To re-evaluate a probably free radical-born, generalized mitochondrial impairment in PD, we applied optimized enzymatic assays, high resolution oxygraphic measurements of permeabilized muscle fibers, and application of metabolic control analysis to skeletal muscle samples of 19 PD patients and 36 age-matched controls. We detected decreased activities of respiratory chain complexes I and IV being accompanied by increased flux control coefficients of complexes I and IV on oxygen consumption of muscle fibers. We further investigated if randomly distributed point mutations in two discrete regions of the mitochondrial DNA are increased in PD muscle, and if they could contribute to the mitochondrial impairment. Our data confirm the previously debated presence of a mild mitochondrial defect in skeletal muscle of patients with PD which is accompanied with an about 1.5 to 2-fold increase of point mutated mtDNA.

Oxygen dependence of flux control of cytochrome c oxidase – implications for mitochondrial diseases

The oxygen dependence of cytochrome c oxidase control on succinate oxidation was investigated in saponin‐permeabilized muscle fibers and isolated mitochondria from mouse quadriceps muscle applying metabolic control analysis. For this cyanide titrations of the oxygen consumption in the presence of succinate+rotenone were performed at different oxygen concentrations in the medium. While with isolated mitochondria high flux control coefficients were detected only at oxygen concentrations close to the K M value of cytochrome c oxidase, with saponin‐permeabilized fibers a significant increase of cytochrome c oxidase flux control was already observed below 130 μM oxygen. The result is in line with the high oxygen sensitivity of maximal respiration of saponin‐permeabilized muscle fibers (P50=33 μM) caused most probably by oxygen diffusion gradients through the fiber lattice. The oxygen dependence of cytochrome c oxidase flux control in muscle fibers can explain the pathological phenotype of mild cytochrome c oxidase deficiencies in mitochondrial myopathies.

Single-cell analysis of mtDNA deletion levels in sporadic amyotrophic lateral sclerosis.

One possible cause for the neuronal loss in sporadic amyotrophic lateral sclerosis (S-ALS) is an increase of free radicals, which may produce oxidative damage to susceptible biomolecules, which, in turn, can damage the mitochondrial DNA (mtDNA). Following laser microdissection of single motor neurons from paraffin-embedded autopsy tissue, we analyzed the presence of a common mtDNA deletion, the 5 kb common deletion (CD). Spinal cord neurons showed slightly higher CD detection rate in patients than controls (94% vs 75%). No significant differences were found between patients and controls for neurons derived from other motor or non-motor regions. A PCR assay of serial DNA dilutions (10-fold) showed no CD level differences between motor neurons in S-ALS and controls. These data suggest that neuronal death in S-ALS is not associated with significant accumulation of mtDNA deletions.

Detection of Respiratory Chain Defects in Cultivated Skin Fibroblasts and Skeletal Muscle of Patients with Parkinson's Disease

Parkinson’s disease (PD) is associated with a degeneration of dopaminergic neurons. The cause of this neuronal death is still unknown. Since 1989,1,2 the hypothesis of mitochondrial respiratory chain dysfunction as a possible basis for the neuronal death has been intensively debated. It is expected that the proposed mitochondrial defect as a possible pathogenic factor for PD should include mitochondria in other cell populations than neurons. Reported data obtained from different extracerebral tissues are controversial.3,4 Therefore, the direct proof of a suspected generalized mitochondrial dysfunction in PD is still missing. However, in muscle homogenates of patients with mitochondrial myopathies the mitochondrial defect is not always detectable with conventional enzyme assays for the determination of respiratory chain enzyme complexes I + III (NADH:cytochrome c reductase), II + III (succinate:cytochrome c reductase), and IV (cytochrome c oxidase).5 Therefore, we assessed the applicability of different in situ methods for the detection of putative mitochondrial respiratory chain defects in extracerebral tissues of PD patients. The functional behavior of mitochondria in digitonin-permeabilized skin fibroblasts and in saponinpermeabilized skeletal muscle fibers of PD patients was studied applying respiration experiments with different substrates using metabolic control analysis. Furthermore, we determined the redox state of the mitochondrial NAD system measuring the laser-excited autofluorescence of NAD(P)H and fluorescent flavoproteins in permeabilized skeletal muscle fibers.6 Fifteen patients with PD (4 female and 11 male, age range 37 to 78 years, mean 58 years) had a skeletal muscle and skin biopsy. The patients had had symptomatic PD for 2 to 13 years. The median UPDRS was 20 (range 4 to 56). All patients had the akinetic-rigid type of the disease. Skeletal muscle samples from diagnostic biopsies of 32 patients with discrete myopathic EMG abnormalities but no biopsy evidence for a manifest myopathy (17 male, 15 female, age range 29 to 72 years) were used as control, and all patients gave written informed consent. The study was approved by the Ethics Committee of Magdeburg University. Bundles of fibers from fresh M. vastus lateralis were used for preparation of saponin-permeabilized fibers.