Dietary nitrate does not reduce oxygen cost of exercise or improve muscle mitochondrial function in patients with mitochondrial myopathy

Nitric oxide regulation of cellular metabolism: Adaptive tuning of cellular energy

Nitric oxide can interact with a wide range of proteins including many that are involved in metabolism. In this review we have summarized the effects of NO on glycolysis, fatty acid metabolism, the TCA cycle, and oxidative phosphorylation with reference to skeletal muscle. Low to moderate NO concentrations upregulate glucose and fatty acid oxidation, while higher NO concentrations shift cellular reliance toward a fully glycolytic phenotype. Moderate NO production directly inhibits pyruvate dehydrogenase activity, reducing glucose-derived carbon entry into the TCA cycle and subsequently increasing anaploretic reactions. NO directly inhibits aconitase activity, increasing reliance on glutamine for continued energy production. At higher or prolonged NO exposure, citrate accumulation can inhibit multiple ATP-producing pathways. Reduced TCA flux slows NADH/FADH entry into the ETC. NO can also inhibit the ETC directly, further limiting oxidative phosphorylation. Moderate NO production improves mitochondrial efficiency while improving O2 utilization increasing whole-body energy production. Long-term bioenergetic capacity may be increased because of NO-derived ROS, which participate in adaptive cellular redox signaling through AMPK, PCG1-α, HIF-1, and NF-κB. However, prolonged exposure or high concentrations of NO can result in membrane depolarization and opening of the MPT. In this way NO may serve as a biochemical rheostat matching energy supply with demand for optimal respiratory function.

Cardiopulmonary exercise testing in neuromuscular disease: a systematic review

INTRODUCTION

Cardiopulmonary exercise testing (CPET) is increasingly used to determine aerobic fitness in health and disability conditions. Patients with neuromuscular diseases (NMDs) often present with symptoms of cardiac and/or skeletal muscle dysfunction and fatigue that might impede the ability to deliver maximal cardiopulmonary effort. Although an increasing number of studies report on NMDs' physical fitness, the applicability of CPET remains largely unknown.

AREAS COVERED

This systematic review synthesized evidence about the quality and feasibility of CPET in NMDs and patient's aerobic fitness. The review followed the PRISMA guidelines (PROSPERO number CRD42020211068). Between September and October 2020 one independent reviewer searched the PubMed/MEDLINE, EMBASE, SCOPUS, and Web of Science databases. Excluding reviews and protocol description articles without baseline data, all study designs using CPET to assess adult or pediatric patients with NMDs were included. The methodological quality was assessed according to the American Thoracic Society/American College of Chest Physicians (ATS/ACCP) recommendations.

EXPERT OPINION

CPET is feasible for ambulatory patients with NMDs when their functional level and the exercise modality are taken into account. However, there is still a vast potential for standardizing and designing disease-specific CPET protocols for patients with NMDs. Moreover, future studies are urged to follow the ATS/ACCP recommendations.

Exercise Testing, Physical Training and Fatigue in Patients with Mitochondrial Myopathy Related to mtDNA Mutations

Mutations in mitochondrial DNA (mtDNA) cause disruption of the oxidative phosphorylation chain and impair energy production in cells throughout the human body. Primary mitochondrial disorders due to mtDNA mutations can present with symptoms from adult-onset mono-organ affection to death in infancy due to multi-organ involvement. The heterogeneous phenotypes that patients with a mutation of mtDNA can present with are thought, at least to some extent, to be a result of differences in mtDNA mutation load among patients and even among tissues in the individual. The most common symptom in patients with mitochondrial myopathy (MM) is exercise intolerance. Since mitochondrial function can be assessed directly in skeletal muscle, exercise studies can be used to elucidate the physiological consequences of defective mitochondria due to mtDNA mutations. Moreover, exercise tests have been developed for diagnostic purposes for mitochondrial myopathy. In this review, we present the rationale for exercise testing of patients with MM due to mutations in mtDNA, evaluate the diagnostic yield of exercise tests for MM and touch upon how exercise tests can be used as tools for follow-up to assess disease course or effects of treatment interventions.

Treatment for mitochondrial diseases

Mitochondrial diseases are predominantly caused by mutations of mitochondrial or nuclear DNA, resulting in multisystem defects. Current treatments are largely supportive, and the disorders progress relentlessly. Nutritional supplements, pharmacological agents and physical therapies have been used in different clinical trials, but the efficacy of these interventions need to be further evaluated. Several recent reviews discussed some of the interventions but ignored bias in those trials. This review was conducted to discover new studies and grade the original studies for potential bias with revised Cochrane Collaboration guidelines. We focused on seven published studies and three unpublished studies; eight of these studies showed improvement in outcome measurements. In particular, two of the interventions have been tested in studies with strict design, which we believe deserve further clinical trials with a large sample. Additionally, allotopic expression of the ND4 subunit seemed to be an effective new treatment for patients with Leber hereditary optic neuropathy.

Feeding mitochondria: Potential role of nutritional components to improve critical illness convalescence

Persistent physical impairment is frequently encountered after critical illness. Recent data point towards mitochondrial dysfunction as an important determinant of this phenomenon. This narrative review provides a comprehensive overview of the present knowledge of mitochondrial function during and after critical illness and the role and potential therapeutic applications of specific micronutrients to restore mitochondrial function. Increased lactate levels and decreased mitochondrial ATP-production are common findings during critical illness and considered to be associated with decreased activity of muscle mitochondrial complexes in the electron transfer system. Adequate nutrient levels are essential for mitochondrial function as several specific micronutrients play crucial roles in energy metabolism and ATP-production. We have addressed the role of B vitamins, ascorbic acid, α-tocopherol, selenium, zinc, coenzyme Q10, caffeine, melatonin, carnitine, nitrate, lipoic acid and taurine in mitochondrial function. B vitamins and lipoic acid are essential in the tricarboxylic acid cycle, while selenium, α-tocopherol, Coenzyme Q10, caffeine, and melatonin are suggested to boost the electron transfer system function. Carnitine is essential for fatty acid beta-oxidation. Selenium is involved in mitochondrial biogenesis. Notwithstanding the documented importance of several nutritional components for optimal mitochondrial function, at present, there are no studies providing directions for optimal requirements during or after critical illness although deficiencies of these specific micronutrients involved in mitochondrial metabolism are common. Considering the interplay between these specific micronutrients, future research should pay more attention to their combined supply to provide guidance for use in clinical practise. REVISION NUMBER: YCLNU-D-17-01092R2.