Respiratory control and oxidative phosphorylation in the dystrophic muscle

AMPK is mitochondrial medicine for neuromuscular disorders

Duchenne muscular dystrophy (DMD), myotonic dystrophy type 1 (DM1), and spinal muscular atrophy (SMA) are the most prevalent neuromuscular disorders (NMDs) in children and adults. Central to a healthy neuromuscular system are the processes that govern mitochondrial turnover and dynamics, which are regulated by AMP-activated protein kinase (AMPK). Here, we survey mitochondrial stresses that are common between, as well as unique to, DMD, DM1, and SMA, and which may serve as potential therapeutic targets to mitigate neuromuscular disease. We also highlight recent advances that leverage a mutation-agnostic strategy featuring physiological or pharmacological AMPK activation to enhance mitochondrial health in these conditions, as well as identify outstanding questions and opportunities for future pursuit.

Mitochondrial Stress Responses in Duchenne muscular dystrophy: Metabolic Dysfunction or Adaptive Reprogramming?

Mitochondrial stress may be a secondary contributor to muscle weakness in inherited muscular dystrophies. Duchenne muscular dystrophy has received the majority of attention whereby most discoveries suggest mitochondrial ATP synthesis may be reduced. However, not all studies support this finding. Furthermore, some studies have reported increased mitochondrial reactive oxygen species and propensity for permeability transition pore formation as an inducer of apoptosis, although divergent findings have also been described. A closer examination of the literature suggests the degree and direction of mitochondrial stress responses may depend on the progression of the disease, the muscle type examined, the mouse model employed with regards to pre-clinical research, the precise metabolic pathways in consideration, and in some cases the in vitro technique used to assess a given mitochondrial bioenergetic function. One intent of this review is to provide careful considerations for future experimental designs to resolve the heterogeneous nature of mitochondrial stress during the progression of Duchenne muscular dystrophy. Such considerations have implications for other muscular dystrophies as well which are addressed briefly herein. A renewed perspective of the term 'mitochondrial dysfunction' is presented whereby stress responses might be re-explored in future investigations as direct contributors to myopathy vs an adaptive 'reprogramming' intended to maintain homeostasis in the face of disease stressors themselves. In so doing, the prospective development of mitochondrial enhancement therapies can be driven by advances in perspectives as much as experimental approaches when resolving the precise relationships between mitochondrial remodelling and muscle weakness in Duchenne and, indeed, other muscular dystrophies.

Adenylosuccinic acid therapy ameliorates murine Duchenne Muscular Dystrophy

Arising from the ablation of the cytoskeletal protein dystrophin, Duchenne Muscular Dystrophy (DMD) is a debilitating and fatal skeletal muscle wasting disease underpinned by metabolic insufficiency. The inability to facilitate adequate energy production may impede calcium (Ca²⁺) buffering within, and the regenerative capacity of, dystrophic muscle. Therefore, increasing the metabogenic potential could represent an effective treatment avenue. The aim of our study was to determine the efficacy of adenylosuccinic acid (ASA), a purine nucleotide cycle metabolite, to stimulate metabolism and buffer skeletal muscle damage in the mdx mouse model of DMD. Dystrophin-positive control (C57BL/10) and dystrophin-deficient mdx mice were treated with ASA (3000 µg.mL⁻¹) in drinking water. Following the 8-week treatment period, metabolism, mitochondrial density, viability and superoxide (O₂⁻) production, as well as skeletal muscle histopathology, were assessed. ASA treatment significantly improved the histopathological features of murine DMD by reducing damage area, the number of centronucleated fibres, lipid accumulation, connective tissue infiltration and Ca²⁺ content of mdx tibialis anterior. These effects were independent of upregulated utrophin expression in the tibialis anterior. ASA treatment also increased mitochondrial viability in mdx flexor digitorum brevis fibres and concomitantly reduced O₂⁻ production, an effect that was also observed in cultured immortalised human DMD myoblasts. Our data indicates that ASA has a protective effect on mdx skeletal muscles.

Revisiting the dystrophin-ATP connection: How half a century of research still implicates mitochondrial dysfunction in Duchenne Muscular Dystrophy aetiology

Duchenne Muscular Dystrophy (DMD) is a fatal neuromuscular disease that is characterised by dystrophin-deficiency and chronic Ca(2+)-induced skeletal muscle wasting, which currently has no cure. DMD was once considered predominantly as a metabolic disease due to the myriad of metabolic insufficiencies evident in the musculature, however this aspect of the disease has been extensively ignored since the discovery of dystrophin. The collective historical and contemporary literature documenting these metabolic nuances has culminated in a series of studies that importantly demonstrate that metabolic dysfunction exists independent of dystrophin expression and a mild disease phenotype can be expressed even in the complete absence of dystrophin expression. Targeting and supporting metabolic pathways with anaplerotic and other energy-enhancing supplements has also shown therapeutic value. We explore the hypothesis that DMD is characterised by a systemic mitochondrial impairment that is central to disease aetiology rather than a secondary pathophysiological consequence of dystrophin-deficiency.

Effect of creatine monohydrate in improving cellular energetics and muscle strength in ambulatory Duchenne muscular dystrophy patients: a randomized, placebo-controlled 31P MRS study

Randomized, placebo-controlled single blinded study was carried out to evaluate the effect of oral creatine supplementation on cellular energetics, manual muscle test (MMT) score and functional status in steroid-naive, ambulatory boys suffering with Duchenne muscular dystrophy (DMD; n=33). Eighteen patients received creatine monohydrate (Cr; 5 g/day for 8 weeks), while 15 received placebo (500 mg of vitamin C). Phosphorus metabolite ratios were determined from the right calf muscle of patients using phosphorus magnetic resonance spectroscopy ((31)P MRS) both prior to (baseline) and after supplementation of Cr or placebo. In addition, metabolite ratios were determined in normal calf muscle of age and sex matched controls (n=8). Significant differences in several metabolite ratios were observed between controls and DMD patients indicating a lower energy state in these patients. Analysis using analysis of covariance adjusted for age and stature showed that the mean phosphocreatine (PCr)/inorganic phosphate (Pi) ratio in patients treated with Cr (4.7; 95% CI; 3.9-5.6) was significantly higher (P=.03) compared to the placebo group (3.3; 95% CI; 2.5-4.2). The mean percentage increase in PCr/Pi ratio was also more in patients <7 years of age compared to older patients after Cr supplementation indicating variation in therapeutic effect with the age. In the placebo group, significant reduction in PCr/Pi (P=.0009), PCr/t-ATP (P=.05) and an increase in phosphodiester (PDE)/PCr ratios was observed after supplementation. Further, in the placebo group, patients <7 years showed reduction of PCr/t-ATP and Pi/t-ATP compared to older patients (>7 years), after supplementation. These results imply that the significant difference observed in PCr/Pi ratio between the Cr and the placebo groups after supplementation may be attributed to a decrease of PCr in the placebo group and an increase in PCr in the Cr group. Changes in MMT score between the two groups was significant (P=.04); however, no change in functional scale (P=.19) was observed. Parents reported subjective improvement on Cr supplementation versus worsening in placebo (P=.02). Our results indicated that Cr was well tolerated and oral Cr significantly improved the muscle PCr/Pi ratio and preserved the muscle strength in short term. However, this study provides no evidence that creatine will prove beneficial after long-term treatment, or have any positive effect on patient lifespan.

Components of energy expenditure in the mdx mouse model of Duchenne muscular dystrophy

Previous observations showing that basal heat production rates and glucose metabolism were reduced in mdx mouse skeletal muscles incubated in vitro led us to study the components of total energy expenditure by open-circuit indirect calorimetry in the intact, free-moving mdx mouse. Our purpose was to verify if the mdx mouse exhibited whole-body alterations in energy metabolism. The results revealed that total and basal energy expenditure, as well as spontaneous activity, energetic cost of activity, and, therefore, energy expended in relation to activity were not significantly different in C57B1/10 (control) and in dystrophic (mdx) mice. In contrast, the thermic effect of food was 32% larger in mdx than in control mice and was accompanied by significant differences in post-prandial glucose and lipid oxidation. The present in vivo study could not show a direct demonstration that impaired glucose metabolism by skeletal muscles participated in this phenomenon. However, since post-prandial glucose metabolism by skeletal muscles contributes a significant part of the thermic effect of food, the present data are in line with previous studies in vitro that show that mdx mouse skeletal muscles probably suffer an impaired control of their energy metabolism.

The biochemical basis of mitochondrial diseases

Dysfunctioning of human mitochondria is found in a rapidly increasing number of patients. The mitochondrial system for energy transduction is very vulnerable to damage by genetic and environmental factors. A primary mitochondrial disease is caused by a genetic defect in a mitochondrial enzyme or translocator. More than 60 mitochondrial enzyme deficiencies have been reported. Secondary mitochondrial defects are caused by lack of compounds to enable a proper mitochondrial function or by inhibition of that function. This may result from malnutrition, circulatory or hormonal disturbances, viral infection, poisoning, or an extramitochondrial error of metabolism. Once mitochondrial ATP synthesis decreases, secondary mitochondrial lesions may be generated further, due to changes in synthesis and degradation of mitochondrial phospholipids and proteins, to mitochondrial antibody formation following massive degradation, to accumulation of toxic products as excess acyl-CoA, to the depletion of Krebs cycle intermediates, and to the increase of free radical formation and lipid peroxidation.

Activation of mitochondrial ATPase as evidence of loosely coupled oxidative phosphorylation in various skeletal muscle disorders. A histochemical fine-structural study

Combined histochemical and biochemical studies have shown, that the histochemical activity of mitochondrial Mg2+-activated ATPase closely correlates with the coupling state of oxidative phosphorylation (Meijer and Vloedman 1980). Using this histochemical method 646 unselected skeletal muscle biopsies have been investigated. Activation of the enzyme, i.e. loosely coupled mitochondria were present either focally or diffusely expressed in 28% of the biopsies irrespective of the underlying disorder. Most often it was found in mitochondrial myopathies and in progressive muscular dystrophy type Duchenne; in a lesser degree it was also present in neurogenic atrophy and in various other disorders. Ninety two percent of all cases with loose coupling showed mitochondrial proliferations. On the other hand in 20% of all cases with mitochondrial proliferations including 19 cases of diffuse mitochondrial myopathy and 3 of progressive external ophthalmoplegia no activation of the enzyme was found. The results show that loose coupling is closely but not absolutely associated with mitochondrial proliferation, it is present in mitochondrial myopathies but also in various other muscular disorders with different pathogenesis.

Follow-up study of a myopathy with loosely coupled mitochondria

This communication deals with a clinical, histological, histochemical and biochemical follow-up study of a boy with an ameliorating myopathy with loosely coupled mitochondria who was studied earlier. In the course of about 15 years the clinical condition of the patient improved greatly. In a biopsy of skeletal muscle the same pathological features were found as before, but the pathological changes were much less severe. In contrast to earlier findings, the majority of the fibres with a loosely coupled state of the mitochondria belonged to the anaerobic type II fibres.

Mitochondrial functions in chronic spinal muscular atrophy

We determined the respiration rate, respiratory control and ADP/O ratios, with different substrates in mitochondria isolated from seven patients with chronic spinal muscular atrophy and compared them with normal human muscle. In all cases studied, a severe alteration of the respiratory control with variable derangement of oxidative phosphorylation was found. Similar findings have been described in other neuromuscular disorders including the so-called "mitochondrial myopathy". We believe that this disturbance of mitochondrial function is non specific and only the hypermetabolic syndrome of Luft could be considered biochemically as a "mitochondrial myopathy", a disorder selectively involving mitochondria of skeletal muscle.

Early changes of muscle mitochondria in Duchenne dystrophy. Partition and activity of mitochondrial enzymes in fractionated muscle of unaffected boys and adults and patients

(1) Biopsies from the gastrocnemius muscle of patients with Duchenne dystrophy were partitioned into a myofibrillar plus nuclear fraction, a mitochondrial fraction and a supernatant fraction. The fractions were assayed for mitochondrial enzymes and protein, in order to obtain information about the integrity of mitochondrial structure and function. Muscles from boys and adults without neuromuscular disease were treated likewise. (2) In adults, muscle possesses a significantly higher specific activity (on protein basis) of monoamine oxidase and rotenone-insenitive NADH-cytochrome c reductase (RINCR) than in boys. In childhood, monoamine oxidase activity increases with age. At the age of 5 yr, the specific activity is 50% of the adult value. RINCR activity is constant in childhood. With adolescence it increases from 20 +/- 2 (SEM) to 35 +/- 6 mumoles cytochrome c reduced per min per g protein, and it remains at this level. Palmitoyl-CoA synthetase activity remains constant with age. (3) In Duchenne dystrophy the extractable protein content from muscle is decreased to 75%. The specific activities of the matrix enzymes propionyl-CoA carboxylase and glutamate dehydrogenase are 1.8 and 2.8 times increased, the inner membrane enzyme cytochrome c oxidase is 2.8 times increased, the inner membrane enzyme cytochrome c oxidase is 2.8 times increased. Of the outer membrane enzymes RINCR is 2.0 times increased, while palmitoyl-CoA synthetase is not changed in acitivity. In Duchenne dystrophy monoamine oxidase activity also increases with age. In part this may be due to mitochondria from adipose tissue and macrophages, which are increasingly present in older patients. The specific activities of enzymes with a predominant cytosolic localisation, creatine kinase and adenylate kinase, are increased by a factor of 1.5 and 1.7. (4) The subcellular distribution of the studied enzymes in human skeletal muscle was found to be similar as in animal studies. In mitochondrial fractions from Duchenne patients the recoveries of the following enzymes are decreased: glutamate dehydrogenase (from 25 to 9%), creatine kinase (1.1-0.66%), adenylate kinase (0.44-0.22%), hexokinase (7.1-2.7%), monoamine oxidase (36-21%), RINCR (30-17%), and palmitoyl-CoA synthetase (40-21%). The recoveries of last 3 mitochondrial outer membrane enzymes in the supernatant fractions are correspondingly increased. These results indicate an increased fragility of the mitochondrial membranes in dystrophic muscles. (5) The reported changes are clearly evident in a one-year-old patient, which indicates that the mitochondria are involved early in the disease process.

The histochemical characterization of the coupling state of skeletal muscle mitochondria

Isolated mitochondria from skeletal muscles of human and animals with neuromuscular diseases may reveal a loosely coupled state of oxidative phosphorylation, which is characterized by a normal phosphorylation in the presence of a phosphate acceptor and a maximal respiration in the absence of a phosphate acceptor. Moreover in these cases activity of mitochondrial Mg2+-stimulated ATPase is strongly increased and cannot be stimulated by the uncoupler 2,4-dinitrophenol. In this communication a histochemical technique for the demonstration of activity of mitochondrial Mg2+-stimulated ATPase to characterize the coupling state of muscle mitochondria in tissue sections, is described. This tissue-saving technique is especially suitable for the study of human skeletal muscle diseases.

Fibroblast cultures in Duchenne muscular dystrophy. Alterations in synthesis and secretion of collagen and noncollagen proteins

Primary skin fibroblast cultures were grown from forearm pinch skin biopsies obtained from 24 patients with Duchenne muscular dystrophy (DMD) and ten normal controls matched for sex and age. The first subcultures were grown for 7 days and incubated with L-(3H)-proline for 24 hours. Intracellular collagen incorporation was significantly decreased (2.2 X) and extracellular collagen incorporation significantly increased (1.8 X) in fibroblast cultures from patients with DMD by both collagenase assay and polyacrylamide gel electrophoresis. The synthesis of noncollagen proteins showed low values from the DMD fibroblast cultures. The alterations in synthesis and secretion of collagen and noncollagen proteins were characteristic only for the log phase of DMD fibroblasts.