The molecular pathology of respiratory-chain dysfunction in human mitochondrial myopathies

Altered skeletal muscle (mitochondrial) properties in patients with mitochondrial DNA single deletion myopathy

Background

Mitochondrial myopathy severely affects skeletal muscle structure and function resulting in defective oxidative phosphorylation. However, the major pathomechanisms and therewith effective treatment approaches remain elusive. Therefore, the aim of the present study was to investigate disease-related impairments in skeletal muscle properties in patients with mitochondrial myopathy. Accordingly, skeletal muscle biopsies were obtained from six patients with moleculargenetically diagnosed mitochondrial myopathy (one male and five females, 53 ± 9 years) and eight age- and gender-matched healthy controls (two males and six females, 58 ± 14 years) to determine mitochondrial respiratory capacity of complex I-V, mitochondrial volume density and fiber type distribution.

Results

Mitochondrial volume density (4.0 ± 0.5 vs. 5.1 ± 0.8 %) as well as respiratory capacity of complex I-V were lower (P < 0.05) in mitochondrial myopathy and associated with a higher (P < 0.001) proportion of type II fibers (65.2 ± 3.6 vs. 44.3 ± 5.9 %). Additionally, mitochondrial volume density and maximal oxidative phosphorylation capacity correlated positively (P < 0.05) to peak oxygen uptake.

Conclusion

Mitochondrial myopathy leads to impaired mitochondrial quantity and quality and a shift towards a more glycolytic skeletal muscle phenotype.

Localization of PTP-1B, SHP-2, and Src exclusively in rat brain mitochondria and functional consequences

An immunodetection study of protein tyrosine phosphatase 1B (PTP-1B), SHP-2, and Src in isolated mitochondria from different rat tissues (brain, muscle, heart, liver, and kidney) revealed their exclusive localization in the brain. Given this result, we sought whether mitochondria respond to ATP and to the general tyrosine phosphatase inhibitor orthovanadate and found little or no change in the tyrosine phosphorylation profile of mitochondria from muscle, heart, liver, and kidney. In contrast, ATP induced an enhancement in the tyrosine-phosphorylated protein profile of brain mitochondria, which was further greatly enhanced with orthovanadate and which disappeared when Src was inhibited with two inhibitors: PP2 and PP1. Importantly, we found that in brain mitochondria, ATP addition induced Src autophosphorylation at Tyr-416 in its catalytic site, leading to its activation, whereas the regulatory Tyr-527 site remained unphosphorylated. Functional implications were addressed by measurements of the enzymatic activity of each of the oxidative phosphorylation complexes in brain mitochondria in the presence of ATP. We found an increase in complex I, III, and IV activity and a decrease in complex V activity, partially reversed by Src inhibition, demonstrating that the complexes are Src substrates. These results complemented and reinforced our initial study showing that respiration of brain mitochondria was partially dependent on tyrosine phosphorylation. Therefore, the present data suggest a possible control point in the regulation of respiration by tyrosine phosphorylation of the complexes mediated by Src auto-activation.

Differentiating idiopathic inflammatory myopathies from metabolic myopathies

The metabolic myopathies are a heterogeneous group of diseases, including glycogenoses, disorders of lipid metabolism, and mitochondrial myopathies, that result primarily from inborn errors of metabolism. Most of these metabolic defects cause medical conditions that manifest early in life. Nevertheless, clinical presentations during the teenage years and adulthood are increasingly being recognized. Many of the clinical manifestations of these diseases are difficult to differentiate from those observed in the idiopathic inflammatory myopathies, especially polymyositis. A directed evaluation using the clinical, laboratory, and genetic approaches summarized in this article, however, should allow for the differentiation of most metabolic myopathies from polymyositis and other forms of idiopathic inflammatory myopathy. The diagnosis of a metabolic myopathy should be considered in patients who appear to have polymyositis but lack the characteristic changes of inflammation found on EMG, MRI, or muscle histology, or in such patients who are refractory to immunosuppressive therapy. The forearm ischemic exercise test is especially useful to screen for some inborn errors of glycogen metabolism or glycolysis and for myoadenylate deaminase deficiency. Thorough analysis of muscle tissue, including histology, histochemistry, biochemistry, and occasionally electron microscopy, is often necessary to make the diagnosis of a metabolic myopathy. Advances in molecular biology methods and knowledge of the precise genetic defects associated with these metabolic defects are dramatically increasing our capacity to diagnose patients with a widening range of myopathies. It is expected that, with further understanding of the mechanisms of the metabolic and idiopathic inflammatory myopathies, the differentiation of these disorders into their pathogenetic components, and the capacity to diagnose them will continue to improve. These are essential factors in improving genetic counseling and eventually the therapy of these serious, and currently incurable, disorders.

Mitochondrial dysfunction and neuromuscular disease

Mitochondrial diseases are a heterogeneous group of disorders with widely varying clinical features, due to defects in mitochondrial function. Involvement of both muscle and nerve is common in mitochondrial disease. In some cases, this involvement is subclinical or a minor part of a multisystem disorder, but myopathy and neuropathy are a major, often presenting, feature of a number of mitochondrial syndromes. In addition, mitochondrial dysfunction may play a role in a number of classic neuromuscular diseases. This article reviews the role of mitochondrial dysfunction in neuromuscular disease and discusses a rational approach to diagnosis and treatment of patients presenting with a neuromuscular syndrome due to mitochondrial disease.

Effects of nitric oxide and peroxynitrite on the cytochrome oxidase K(m) for oxygen: implications for mitochondrial pathology

This review summarises current knowledge about the effect of oxygen on cytochrome oxidase activity in vitro and in vivo. Cytochrome oxidase normally operates above its K(m) for oxygen in vivo. However, decreases in the intracellular oxygen concentration (hypoxia) under physiological extremes, or during pathophysiology, can cause mitochondrial respiration to become oxygen limited. Inhibitors that raise the enzyme's K(m) will induce oxygen limitation under apparently normoxic conditions. It is known that the concentrations of nitric oxide and peroxynitrite are raised in a number of pathophysiological conditions. These compounds are capable of reversibly and irreversibly raising the cytochrome oxidase K(m) for oxygen. Therefore, measurements of cell and mitochondrial respiration in vitro that fail to systematically vary oxygen through the range of physiological concentrations are likely to underestimate the effects of nitric oxide and peroxynitrite in vivo.

Contributions of mitochondria to animal physiology: from homeostatic sensor to calcium signalling and cell death

Over recent years, it has become clear that mitochondria play a central role in many key aspects of animal physiology and pathophysiology. Their central and ubiquitous task is clearly the production of ATP. Nevertheless, they also play subtle roles in glucose homeostasis, acting as the sensor for substrate supply in the transduction pathway that promotes insulin secretion by the pancreatic -cell and that modulates the excitability of the hypothalamic glucose-sensitive neurons involved in appetite control. Mitochondria may also act as sensors of availability of oxygen, the other major mitochondrial substrate, in the regulation of respiration. Mitochondria take up calcium, and the high opacity mitochondrial calcium uptake pathway provides a mechanism that couples energy demand to increased ATP production through the calcium-dependent upregulation of mitochondrial enzyme activity. Mitochondrial calcium accumulation may also have a substantial impact on the spatiotemporal dynamics of cellular calcium signals, with subtle differences of detail in different cell types. Recent work has also revealed the centrality of mitochondrial dysfunction as an irreversible step in the pathway to both necrotic and apoptotic cell death. This review looks at recent developments in these rapidly evolving areas of cell physiology in an attempt to draw together disparate areas of research into a common theme.

A 31P-magnetic resonance spectroscopy and biochemical study of the mo(vbr) mouse: potential model for the mitochondrial encephalomyopathies

31P-magnetic resonance spectroscopy (31P-MRS) provides new biochemical information on mitochondrial disorders affecting brain and muscle. To elucidate the mechanisms of mitochondrial abnormalities, however, animal models are needed. We assessed the mo(vbr) (mottled viable brindled) mouse for its value in studying (1) energetics of a mitochondrial disorder and (2) 31P-MRS changes associated with mitochondrial abnormalities in vivo. The maximal activity of succinate-cytochrome c reductase was significantly reduced in mo(vbr) muscle compared to controls, whereas cytochrome oxidase activity was only reduced in mo(vbr) brain. 31P-MRS of mo(vbr) brain showed an increased pH, but no changes in any metabolite ratios. The phosphocreatine (PCr) recovery rate after exercise was reduced in muscles from mo(vbr) mice, indicating impairment of oxidative metabolism. We conclude that mo(vbr) brain and muscle tissue have biochemical abnormalities consistent with mitochondrial impairment. The PCr recovery rate, measured by 31P-MRS, was sensitive to the muscle abnormality. This strain is best described as having chronic mitochondrial dysfunction.

Mitochondrial DNA (mtDNA) diseases: correlation of genotype to phenotype

This study examines the relationship of genotype to phenotype in 14 unselected patients who were found to harbour the A3243G transition in the mitochondrial transfer RNALeu(UUR) gene commonly associated with the syndrome of mitochondrial encephalopathy, lactic acidosis and strokes (MELAS). Only 6 of the 14 cases (43%) had seizures and recurrent strokes, the core clinical features of the MELAS phenotype. Of the remaining cases, four had an encephalomyopathy with deafness, ataxia and dementia, two had syndromes with progressive external ophthalmoplegia and two had limb weakness alone. Even within the MELAS subgroup, the majority of patients had one or more clinical manifestations considered to be atypical of the MELAS syndrome. They included developmental delay, ophthalmoparesis, pigmentary retinopathy and intestinal pseudo-obstruction. The proportion of mutant mitochondrial DNA (mtDNA) in muscle was generally higher in patients with recurrent strokes than in those without strokes, the highest levels being observed in MELAS cases with early onset disease. Studies of isolated muscle mitochondria identified a range of respiratory chain abnormalities mostly involving Complex I; immunoblots of Complex I in 3 of 10 cases showed selective loss of specific subunits encoded by nuclear genes. In the group as a whole, however, no clear correlations were observed between the severity or extent of the respiratory chain abnormality and clinical phenotype or the proportion of mutant mtDNA in biopsied skeletal muscle. These discrepancies suggest that, in patients harbouring the common MELAS3243 mutation, differences in heteroplasmy and the proportions of mutant mtDNA may not be the sole determinants of disease expression and that additional genetic mechanisms are involved in defining the range of clinical and biochemical phenotypes associated with this aberrant mitochondrial genome.

Bioenergetics of skeletal muscle in mitochondrial myopathy

31Phosphorus nuclear magnetic resonance spectroscopy was used to examine skeletal muscle in 29 patients with mitochondrial myopathy, 9 male and 20 female. Gastrocnemius was investigated in 15 patients and 30 normal subjects and finger flexor muscle (flexor digitorum superficialis, fds) in 24 patients and 35 normal controls. Both muscles were studied in 10 of the patients. Results were abnormal (outside the full range of normal values) in all but 2 patients. In 86% of patients (25/29) abnormalities were detected in resting muscle. In most cases there was a low phosphocreatine/ATP ratio, high calculated free [ADP] and low phosphorylation potential. At rest, abnormality was detected with equal ease in fds and gastrocnemius. Exercise and recovery increased the sensitivity of MRS in detecting abnormal metabolism. Finger flexion was better tolerated by patients than plantar flexion and gave bigger changes in metabolite concentrations and intracellular pH. Thus, results from fds were more easily differentiated from normal. Exercise duration was significantly shorter than in controls while phosphocreatine depletion was more rapid than normal, consistent with a shortfall in mitochondrial ATP synthesis. Nearly all patients (25/27, 93%) showed abnormalities during recovery from exercise. [ADP] was high during exercise and its recovery was delayed, providing increased drive for oxidative phosphorylation. Phosphocreatine resynthesis during recovery (which reflects oxidative ATP synthesis) was slow both in absolute terms and in relation to [ADP]. Recovery of intracellular pH after exercise was significantly more rapid than normal, consistent with an upregulation of proton efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial creatine kinase: a major constituent of pathological inclusions seen in mitochondrial myopathies

Overaccumulation of abnormally organized mitochondria in so-called "ragged-red" skeletal muscle fibers is a morphological hallmark of mitochondrial myopathies, in particular of mitochondrial encephalomyopathies. Characteristic for the abnormal mitochondria is the occurrence of highly ordered crystalline inclusions. Immuno-electron microscopy revealed that these inclusions react heavily with specific antibodies against mitochondrial creatine kinase (Mi-CK). Image processing of selected crystalline inclusions, sectioned along the crystallographic b, c planes, resulted in an averaged picture displaying an arrangement of regular, square-shaped particles with a central cavity. The overall appearance, dimensions, and symmetry of these building blocks are very reminiscent of single isolated Mi-CK octamers. Taking these findings together, it is concluded that Mi-CK octamers indeed represent the major, if not the only, component of these mitochondrial inclusions.

Mitochondrial DNA rearrangements with onset as chronic diarrhea with villous atrophy

We report two unrelated children with onset of chronic diarrhea and villous atrophy in the first years of life. Elevated plasma lactate concentrations and lactate/pyruvate and ketone body molar ratios suggested a genetic defect of oxidative phosphorylation. Analysis of the mitochondrial respiratory chain showed a complex III deficiency in muscle of both patients. Southern blot analysis provided evidence of heteroplasmic mitochondrial DNA rearrangements that involve deletion and deletion-duplication. Directly repeated sequences (10 and 11 base pairs, respectively) were present in the wild type of mitochondrial genome at the boundaries of the deletion. Neither parent of either patient had rearranged molecules in their circulating lymphocytes. It appears that a mitochondrial disorder can have chronic diarrhea and villous atrophy as the initial clinical feature. On the basis of these observations, we suggest that genetic defects of mitochondrial energy supply be considered in elucidating the origin of unexplained chronic diarrheas, especially when other, unrelated symptoms occur in the course of the disease.

Control of phosphocreatine resynthesis during recovery from exercise in human skeletal muscle

Information about the control of mitochondrial function in skeletal muscle in vivo can be obtained from the relationship between the rate of mitochondrial oxidation and the intracellular concentrations of phosphorus metabolites, although the analysis is complicated by the constraints imposed by the creatine kinase equilibrium. The rate of phosphocreatine (PCr) recovery after exercise measured by 31P MRS is an estimate of net oxidative ATP synthesis. Analysing such data from normal and abnormal human muscle, we show that the approximately exponential recovery kinetics of ADP and PCr imply that the rate of PCr resynthesis has a hyperbolic dependence on [ADP] but remains approximately linear with respect to the concentration of orthophosphate (Pi) and therefore also [PCr] and [creatine]. Both kinds of relationship are consistent with experimental data from exercising animal muscle and also with data from isolated mitochondria which suggest kinetic control of mitochondrial ATP synthesis of [ADP]. These relationships are altered in proven mitochondrial disease. This analysis offers a way to quantify mitochondrial function and its abnormalities in vivo.

Different in situ hybridization patterns of mitochondrial DNA in cytochrome c oxidase-deficient extraocular muscle fibres in the elderly

Previous studies have revealed an increase of cytochrome c oxidase-deficient fibres/cells in the skeletal and heart muscle of humans during ageing. The enzyme defect is due to a lack of both mitochondrial and nuclear coded enzyme subunits. In the present investigation in situ hybridization of mitochondrial DNA (mtDNA) has been performed on extraocular muscles of humans over 70 years of age to show whether mutated mtDNA with the so called common deletion of 4,977 basepairs at position 8,482-13,460 of mtDNA accumulates in the cytochrome c oxidase-deficient fibres. The cytochrome c oxidase-deficient fibres revealed different hybridization patterns: a normal hybridization signal with three different mtDNA probes, a reduced or lacking signal with all three probes indicating depletion of mtDNA and a selective hybridization defect with the probe recognizing the "common deletion" region of mtDNA as evidence of mtDNA deletion. The results suggest that during ageing defects of cytochrome c oxidase are associated with different molecular alterations of mtDNA. Deletion and depletion of mtDNA are not the only nor probably the leading mechanisms responsible for the loss of respiratory chain capacity during ageing. The normal hybridization signal in most of the cytochrome c oxidase-deficient fibres and the loss of mitochondrial and nuclear protein subunits indicate the involvement of other, especially nuclear factors.

Random cytochrome-C-oxidase deficiency of oxyphil cell nodules in the parathyroid gland. A mitochondrial cytopathy related to cell ageing?

Cytochrome-c-oxidase (complex IV) was histochemically studied in oncocytic adenoma (n = 10) and carcinoma of the thyroid gland (n = 3), cystadenolymphomas and oncocytic adenomas of the major salivary glands (n = 9), oncocytic neoplasia of the kidney (n = 1) and in 21 parathyroid glands with primary hyperparathyroidism and adenomatous proliferation (n = 17) and secondary hyperparathyroidism with hyperplasia (n = 4). Only in the parathyroids defects of cytochrome-c-oxidase were found being expressed in all 4 glands with hyperplasia (14 defects) and in 5 of the 17 adenomas (11 defects). All defects were confined to foci with oxyphil cell differentiation, the defect areas varying from 0.09 to 21.10 sq mm in hyperplastic glands and from 0.11 to 13.88 sq mm in adenomas, the size of the oxyphil foci varying from 0.12 sq mm-105.38 sq mm. However, not every oxyphil nodule of a gland was devoid of cytochrome-c-oxidase activity. Of 6 predominantly oxyphil adenomas, 4 showed no defects. No defects were observed either in 2 adenomas without oxyphil cells. Further enzymes of the respiratory chain, succinate dehydrogenase (complex II) and ATP synthetase, (complex V) were devoid of defects. In parathyroids with hyperplasia and oxyphil areas, defects of cytochrome-c-oxidase occurred significantly more often and tended to be larger than in adenomas, statistical analysis revealing a significant correlation between the occurrence of defects and the number of oxyphil foci but not with the total oxyphil area.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatal infantile mitochondrial cardiomyopathy and myopathy with heterogeneous tissue expression of combined respiratory chain deficiencies

A 5-month-old boy died of progressive heart failure that started at the age of 3 months. Autopsy revealed a mitochondrial cardiomyopathy and a mitochondrial myopathy of the limb muscle and diaphragm. Cytochemically random defects of cytochrome c oxidase were visualized by light and electron microscopy in the diaphragm and especially the heart muscle, the limb muscle showing a diffuse attenuation whereas the liver and kidneys reacted normally. The activities of NADH-dehydrogenase (complex I) and cytochrome c oxidase (complex IV) were severely diminished (20% residual activity of controls) in the skeletal and heart muscle. In the heart, succinate cytochrome c reductase (complex II/III) was additionally decreased to the same degree. Loss of cytochrome c oxidase activity was based on a reduction of both mitochondrial and nuclear derived subunits in the heart and diaphragm as revealed by immunohistochemical analysis, whereas the limb muscle showed a normal immunoreactive protein content. The results illustrate heterogeneous tissue expression of respiratory chain enzyme defects and demonstrate that a cardiomyopathy may be the leading presentation of a mitochondrial disorder in early infancy.