Endurance exercise is protective for mice with mitochondrial myopathy

Metabolic myopathies

We consider recent developments in disorders affecting three areas of metabolism: glycogen, fatty acids, and the mitochondrial respiratory chain. Among the glycogenoses, new attention has been directed to defects of glycogen synthesis resulting in absence rather than excess of muscle glycogen ("aglycogenosis"). These include defects of glycogen synthetase and defects of glycogenin, the primer of glycogen synthesis. Considerable progress also has been made in our understanding of alterations of glycogen metabolism that result in polyglucosan storage. Among the disorders of lipid metabolism, mutations in the genes encoding two triglyceride lipases acting hand in hand cause severe generalized lipid storage myopathy, one associated with ichthyosis (Chanarin-Dorfman syndrome), the other dominated by juvenile-onset weakness. For the mitochondrial myopathies, we discuss the importance of homoplasmic mitochondrial DNA mutations and review the rapid progress made in our understanding of the coenzyme Q(10) deficiencies, which are often treatable.

Mitochondrial genetic diseases

PURPOSE OF REVIEW

Mitochondrial diseases are individually uncommon, but collectively pose a significant burden on human health. Primary mitochondrial disease is caused by defects in the mitochondrial DNA-encoded genes or in nuclear genes whose products are imported into the mitochondrion. Great strides have been made in determining the cause of mitochondrial disorders, but the clinical ability to diagnose these conditions lags behind because of phenotypic overlap between distinct genetic entities and the complexity and invasiveness of standard diagnostic testing. In this review, we evaluate new findings in mitochondrial genetics, recent developments in mitochondrial disease diagnostic testing, and emerging ideas for mitochondrial disease therapies.

RECENT FINDINGS

Clinical cohort studies have revealed important themes in patient care relative to manifestations of mitochondrial disease. Significant strides have also been made toward creating embryos free from the risk of maternally inherited mitochondrial DNA-based disease. Several new genetic causes of both nuclear and mitochondrial DNA-based diseases have been identified in the past year. In addition, novel insights have emerged from basic studies of mitochondrial biology that hold promise for the development of targeted mitochondrial disease therapies.

SUMMARY

Research on mitochondrial biology and disease continues to improve the clinical capacity to diagnose the heterogeneous group of mitochondrial diseases that afflict the pediatric population. This research also provides a framework for future approaches to devise effective mitochondrial disease therapies.

A neurological perspective on mitochondrial disease

Disruption of the most fundamental cellular energy process, the mitochondrial respiratory chain, results in a diverse and variable group of multisystem disorders known collectively as mitochondrial disease. The frequent involvement of the brain, nerves, and muscles, often in the same patient, places neurologists at the forefront of the interesting and challenging process of diagnosing and caring for these patients. Mitochondrial diseases are among the most frequently inherited neurological disorders, and can be caused by mutations in mitochondrial or nuclear DNA. Substantial progress has been made over the past decade in understanding the genetic basis of these disorders, with important implications for the general neurologist in terms of the diagnosis, investigation, and multidisciplinary management of these patients.

The microRNA miR-696 regulates PGC-1{alpha} in mouse skeletal muscle in response to physical activity

MicroRNAs (miRNAs) are small noncoding RNAs involved in posttranscriptional gene regulation that have been shown to be involved in growth, development, function, and stress responses of various organs. The purpose of this study was to identify the miRNA response to physical activity, which was related to functions such as nutrient metabolism, although the miRNAs involved are currently unknown. C57BL/6 mice were divided into exercise and control groups. The exercise group performed running exercise, with a gradual increase of the load over 4 wk. On the other hand, to examine the effect of muscle inactivity, the unilateral hindlimbs of other mice were fixed in a cast for 5 days. Microarray analysis for miRNA in gastrocnemius revealed that miR-696 was markedly affected by both exercise and immobilization, showing opposite responses to these two interventions. Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), which was increased by exercise and decreased by immobilization in the protein level, was predicted as a target regulated by miR-696. In cultured myocytes, intracellular miR-696 variation led to negative regulation of PGC-1alpha protein along with the expression of mRNAs for downstream genes. In addition, we found decreases in the biogenesis of mitochondria and fatty acid oxidation in miR-696-overexpressing myocytes compared with normal control myocytes. These observations demonstrate that miR-696 is a physical activity-dependent miRNA involved in the translational regulation of PGC-1alpha and skeletal muscle metabolism in mice.

Increased muscle PGC-1alpha expression protects from sarcopenia and metabolic disease during aging

Aging is a major risk factor for metabolic disease and loss of skeletal muscle mass and strength, a condition known as sarcopenia. Both conditions present a major health burden to the elderly population. Here, we analyzed the effect of mildly increased PGC-1alpha expression in skeletal muscle during aging. We found that transgenic MCK-PGC-1alpha animals had preserved mitochondrial function, neuromuscular junctions, and muscle integrity during aging. Increased PGC-1alpha levels in skeletal muscle prevented muscle wasting by reducing apoptosis, autophagy, and proteasome degradation. The preservation of muscle integrity and function in MCK-PGC-1alpha animals resulted in significantly improved whole-body health; both the loss of bone mineral density and the increase of systemic chronic inflammation, observed during normal aging, were prevented. Importantly, MCK-PGC-1alpha animals also showed improved metabolic responses as evident by increased insulin sensitivity and insulin signaling in aged mice. Our results highlight the importance of intact muscle function and metabolism for whole-body homeostasis and indicate that modulation of PGC-1alpha levels in skeletal muscle presents an avenue for the prevention and treatment of a group of age-related disorders.

Mitochondrial DNA mutations and human disease

Mitochondrial disorders are a group of clinically heterogeneous diseases, commonly defined by a lack of cellular energy due to oxidative phosphorylation (OXPHOS) defects. Since the identification of the first human pathological mitochondrial DNA (mtDNA) mutations in 1988, significant efforts have been spent in cataloguing the vast array of causative genetic defects of these disorders. Currently, more than 250 pathogenic mtDNA mutations have been identified. An ever-increasing number of nuclear DNA mutations are also being reported as the majority of proteins involved in mitochondrial metabolism and maintenance are nuclear-encoded. Understanding the phenotypic diversity and elucidating the molecular mechanisms at the basis of these diseases has however proved challenging. Progress has been hampered by the peculiar features of mitochondrial genetics, an inability to manipulate the mitochondrial genome, and difficulties in obtaining suitable models of disease. In this review, we will first outline the unique features of mitochondrial genetics before detailing the diseases and their genetic causes, focusing specifically on primary mtDNA genetic defects. The functional consequences of mtDNA mutations that have been characterised to date will also be discussed, along with current and potential future diagnostic and therapeutic advances.

Cytochrome c oxidase deficiency: patients and animal models

Cytochrome c oxidase (COX) deficiencies are one of the most common defects of the respiratory chain found in mitochondrial diseases. COX is a multimeric inner mitochondrial membrane enzyme formed by subunits encoded by both the nuclear and the mitochondrial genome. COX biosynthesis requires numerous assembly factors that do not form part of the final complex but participate in prosthetic group synthesis and metal delivery in addition to membrane insertion and maturation of COX subunits. Human diseases associated with COX deficiency including encephalomyopathies, Leigh syndrome, hypertrophic cardiomyopathies, and fatal lactic acidosis are caused by mutations in COX subunits or assembly factors. In the last decade, numerous animal models have been created to understand the pathophysiology of COX deficiencies and the function of assembly factors. These animal models, ranging from invertebrates to mammals, in most cases mimic the pathological features of the human diseases.