Mitochondria-targeted antioxidants protect against mechanical ventilation-induced diaphragm weakness. Crit Care Med. 2011;39:1749-1759. [PMID: 21460706 DOI: 10.1097/ccm.0b013e3182190b62]

Regulation of lysosomes in skeletal muscle during exercise, disuse and aging

Lysosomes play a critical role as a terminal organelle in autophagy flux and in regulating protein degradation, but their function and adaptability in skeletal muscle is understudied. Lysosome functions include both housekeeping and signaling functions essential for cellular homeostasis. This review focuses on the regulation of lysosomes in skeletal muscle during exercise, disuse, and aging, with a consideration of sex differences as well as the role of lysosomes in mediating the degradation of mitochondria, termed mitophagy. Exercise enhances mitophagy during elevated mitochondrial stress and energy demand. A critical response to this deviation from homeostasis is the activation of transcription factors TFEB and TFE3, which drive the expression of lysosomal and autophagic genes. Conversely, during muscle disuse, the suppression of lysosomal activity contributes to the accumulation of defective mitochondria and other cellular debris, impairing muscle function. Aging further exacerbates these effects by diminishing lysosomal efficacy, leading to the accumulation of damaged cellular components. mTORC1, a key nutrient sensor, modulates lysosomal activity by inhibiting TFEB/TFE3 translocation to the nucleus under nutrient-rich conditions, thereby suppressing autophagy. During nutrient deprivation or exercise, AMPK activation inhibits mTORC1, facilitating TFEB/TFE3 nuclear translocation and promoting lysosomal biogenesis and autophagy. TRPML1 activation by mitochondrial ROS enhances lysosomal calcium release, which is essential for autophagy and maintaining mitochondrial quality. Overall, the intricate regulation of lysosomal functions and signaling pathways in skeletal muscle is crucial for adaptation to physiological demands, and disruptions in these processes during disuse and aging underscore the ubiquitous power of exercise-induced adaptations, and also highlight the potential for targeted therapeutic interventions to preserve muscle health.

Reactive oxygen species in age-related musculoskeletal decline: implications for nutritional intervention

Musculoskeletal disorders and age-related musculoskeletal decline are major contributors to the burden of ill health seen in older subjects. Despite this increased burden, these chronic disorders of old age receive a relatively small proportion of national research funds. Much has been learned about fundamental processes involved in ageing from basic science research and this is leading to identification of key pathways that mediate ageing which may help the search for interventions to reduce age-related musculoskeletal decline. This short review will focus on the role of reactive oxygen species in age-related skeletal muscle decline and on the implications of this work for potential nutritional interventions in sarcopenia. The key physiological role of reactive oxygen species is now known to be in mediating redox signalling in muscle and other tissues and ageing leads to disruption of such pathways. In muscle, this is reflected in an age-related attenuation of specific adaptations and responses to contractile activity that impacts the ability of skeletal muscle from ageing individuals to respond to exercise. These pathways provides potential targets for identification of logical interventions that may help maintain muscle mass and function during ageing.

Mitochondrial antioxidant SkQ1 attenuates C26 cancer-induced muscle wasting in males and improves muscle contractility in female tumor-bearing mice

Mitochondrial dysfunction is a hallmark of cancer cachexia (CC). Mitochondrial reactive oxygen species (ROS) are elevated in muscle shortly after tumor onset. Targeting mitochondrial ROS may be a viable option to prevent CC. The aim of this study was to evaluate the efficacy of a mitochondria-targeted antioxidant, SkQ1, to mitigate CC in both biological sexes. Male and female Balb/c mice were injected bilaterally with colon 26 adenocarcinoma (C26) cells (total 1 × 106 cells) or PBS (equal volume control). SkQ1 was dissolved in drinking water (∼250 nmol/kg body wt/day) and administered to mice beginning 7 days following tumor induction, whereas control groups consumed normal drinking water. In vivo muscle contractility of dorsiflexors, deuterium oxide-based protein synthesis, mitochondrial respiration and mRNA content of mitochondrial, protein turnover, and calcium channel-related markers were assessed at endpoint (25 days following tumor induction). Two-way ANOVAs, followed by Tukey's post hoc test when interactions were significant (P ≤ 0.05), were performed. SkQ1 attenuated cancer-induced atrophy, promoted protein synthesis, and abated Redd1 and Atrogin induction in gastrocnemius of C26 male mice. In female mice, SkQ1 decreased muscle mass and increased catabolic signaling in the plantaris of tumor-bearing mice, as well as reduced mitochondrial oxygen consumption, regardless of tumor. However, in females, SkQ1 enhanced muscle contractility of the dorsiflexors with concurrent induction of Ryr1, Serca1, and Serca2a in TA. In conclusion, the mitochondria-targeted antioxidant SkQ1 may attenuate CC-induced muscle loss in males, while improving muscle contractile function in tumor-bearing female mice, suggesting sexual dimorphism in the effects of this mitochondrial therapy in CC.NEW & NOTEWORTHY Herein, we assess the efficacy of the mitochondria-targeted antioxidant SkQ1 to mitigate cancer cachexia (CC) in both biological sexes. We demonstrate that SkQ1 administration attenuates muscle wasting induced by C26 tumors in male, but not female, mice. Conversely, we identify that in females, SkQ1 improves muscle contractility. These phenotypic adaptations to SkQ1 are aligned with respective responses in muscle protein synthesis, mitochondrial respiration, and mRNA content of protein turnover, as well as mitochondrial and calcium handling-related markers.

Reactive oxygen species promote endurance exercise-induced adaptations in skeletal muscles

The discovery that contracting skeletal muscle generates reactive oxygen species (ROS) was first reported over 40 years ago. The prevailing view in the 1980s was that exercise-induced ROS production promotes oxidation of proteins and lipids resulting in muscle damage. However, a paradigm shift occurred in the 1990s as growing research revealed that ROS are signaling molecules, capable of activating transcriptional activators/coactivators and promoting exercise-induced muscle adaptation. Growing evidence supports the notion that reduction-oxidation (redox) signaling pathways play an important role in the muscle remodeling that occurs in response to endurance exercise training. This review examines the specific role that redox signaling plays in this endurance exercise-induced skeletal muscle adaptation. We begin with a discussion of the primary sites of ROS production in contracting muscle fibers followed by a summary of the antioxidant enzymes involved in the regulation of ROS levels in the cell. We then discuss which redox-sensitive signaling pathways promote endurance exercise-induced muscle adaptation and debate the strength of the evidence supporting the notion that redox signaling plays an essential role in muscle adaptation to endurance exercise training. In hopes of stimulating future research, we highlight several important unanswered questions in this field.

Exercise-induced Nrf2 activation increases antioxidant defenses in skeletal muscles

Following the discovery that exercise increases the production of reactive oxygen species in contracting skeletal muscles, evidence quickly emerged that endurance exercise training increases the abundance of key antioxidant enzymes in the trained muscles. Since these early observations, knowledge about the impact that regular exercise has on skeletal muscle antioxidant capacity has increased significantly. Importantly, in recent years, our understanding of the cell signaling pathways responsible for this exercise-induced increase in antioxidant enzymes has expanded exponentially. Therefore, the goals of this review are: 1) summarize our knowledge about the influence that exercise training has on the abundance of key antioxidant enzymes in skeletal muscles; and 2) to provide a state-of-the-art review of the nuclear factor erythroid 2-related factor (Nrf2) signaling pathway that is responsible for many of the exercise-induced changes in muscle antioxidant capacity. We begin with a discussion of the sources of reactive oxygen species in contracting muscles and then examine the exercise-induced changes in the antioxidant enzymes that eliminate both superoxide radicals and hydrogen peroxide in muscle fibers. We conclude with a discussion of the advances in our understanding of the exercise-induced control of the Nrf2 signaling pathway that is responsible for the expression of numerous antioxidant proteins. In hopes of stimulating future research, we also identify gaps in our knowledge about the signaling pathways responsible for the exercise-induced increases in muscle antioxidant enzymes.

A short duration of mechanical ventilation alters redox status in the diaphragm and aggravates inflammation in septic mice

BACKGROUND

Mechanical ventilation (MV) is a life support method used to treat patients with respiratory failure. High tidal volumes during MV can cause ventilator-induced lung injury (VILI), but also affect other organs, such as the diaphragm (Dia) causing ventilator-induced diaphragmatic dysfunction (VIDD). VIDD is often associated with a complicated course on MV. Sepsis can induce inflammation and oxidative stress, contributing to the impairment of the Dia and worsening of the prognosis. This study evaluated the additive or synergistic effects of a short course of mechanical ventilation on Dia in healthy and septic adult mice.

METHODS

32 adult male C57BL/6 mice were randomly into four groups: Control (CG), non-ventilated animals instilled with saline solution (PBS1x); Lipopolysaccharide (LPS), non-ventilated animals instilled with PBS solution containing lipopolysaccharide; Mechanical Ventilation (MV) for 1 h, ventilated animals instilled with PBS solution; and Mechanical Ventilation and LPS (MV+LPS), ventilated animals instilled with PBS solution containing LPS. At the end of the experimental protocol, the animals were euthanized, then blood and diaphragm tissue samples were collected.

RESULTS

Evaluation of leukocyte/blood parameters and diaphragm muscle showed that MV, LPS and the combination of both were able to increase neutrophil count, creatine kinase, inflammatory mediators and oxidative stress in all groups compared to the control. MV and sepsis combined had additive effects on inflammation and lipid peroxidation.

CONCLUSIONS

A short course of Mechanical ventilation promotes inflammation and oxidative stress and, its combination with sepsis further increases local and systemic inflammation.

Ventilator-induced diaphragm dysfunction: phenomenology and mechanism(s) of pathogenesis

Mechanical ventilation (MV) is used to support ventilation and pulmonary gas exchange in patients during critical illness and surgery. Although MV is a life-saving intervention for patients in respiratory failure, an unintended side-effect of MV is the rapid development of diaphragmatic atrophy and contractile dysfunction. This MV-induced diaphragmatic weakness is labelled as 'ventilator-induced diaphragm dysfunction' (VIDD). VIDD is an important clinical problem because diaphragmatic weakness is a risk factor for the failure to wean patients from MV. Indeed, the inability to remove patients from ventilator support results in prolonged hospitalization and increased morbidity and mortality. The pathogenesis of VIDD has been extensively investigated, revealing that increased mitochondrial production of reactive oxygen species within diaphragm muscle fibres promotes a cascade of redox-regulated signalling events leading to both accelerated proteolysis and depressed protein synthesis. Together, these events promote the rapid development of diaphragmatic atrophy and contractile dysfunction. This review highlights the MV-induced changes in the structure/function of diaphragm muscle and discusses the cell-signalling mechanisms responsible for the pathogenesis of VIDD. This report concludes with a discussion of potential therapeutic opportunities to prevent VIDD and suggestions for future research in this exciting field.

Mitochondrial bioenergetics are not associated with myofibrillar protein synthesis rates

BACKGROUND

Mitochondria represent key organelles influencing cellular homeostasis and have been implicated in the signalling events regulating protein synthesis.

METHODS

We examined whether mitochondrial bioenergetics (oxidative phosphorylation and reactive oxygen species (H2O2) emission, ROS) measured in vitro in permeabilized muscle fibres represent regulatory factors for integrated daily muscle protein synthesis rates and skeletal muscle mass changes across the spectrum of physical activity, including free-living and bed-rest conditions: n = 19 healthy, young men (26 ± 4 years, 23.4 ± 3.3 kg/m2) and following 12 weeks of resistance-type exercise training: n = 10 healthy older men (70 ± 3 years, 25.2 ± 2.1 kg/m2). Additionally, we evaluated the direct relationship between attenuated mitochondrial ROS emission and integrated daily myofibrillar and sarcoplasmic protein synthesis rates in genetically modified mice (mitochondrial-targeted catalase, MCAT).

RESULTS

Neither oxidative phosphorylation nor H2O2 emission were associated with muscle protein synthesis rates in healthy young men under free-living conditions or following 1 week of bed rest (both P > 0.05). Greater increases in GSSG concentration were associated with greater skeletal muscle mass loss following bed rest (r = -0.49, P < 0.05). In older men, only submaximal mitochondrial oxidative phosphorylation (corrected for mitochondrial content) was positively associated with myofibrillar protein synthesis rates during exercise training (r = 0.72, P < 0.05). However, changes in oxidative phosphorylation and H2O2 emission were not associated with changes in skeletal muscle mass following training (both P > 0.05). Additionally, MCAT mice displayed no differences in myofibrillar (2.62 ± 0.22 vs. 2.75 ± 0.15%/day) and sarcoplasmic (3.68 ± 0.35 vs. 3.54 ± 0.35%/day) protein synthesis rates when compared with wild-type mice (both P > 0.05).

CONCLUSIONS

Mitochondrial oxidative phosphorylation and reactive oxygen emission do not seem to represent key factors regulating muscle protein synthesis or muscle mass regulation across the spectrum of physical activity.

Acute lung injury and post-cardiac arrest syndrome: a narrative review

BACKGROUND

Post-cardiac arrest syndrome (PCAS) presents a multifaceted challenge in clinical practice, characterized by severe neurological injury and high mortality rates despite advancements in management strategies. One of the important critical aspects of PCAS is post-arrest lung injury (PALI), which significantly contributes to poor outcomes. PALI arises from a complex interplay of pathophysiological mechanisms, including trauma from chest compressions, pulmonary ischemia-reperfusion (IR) injury, aspiration, and systemic inflammation. Despite its clinical significance, the pathophysiology of PALI remains incompletely understood, necessitating further investigation to optimize therapeutic approaches.

METHODS

This review comprehensively examines the existing literature to elucidate the epidemiology, pathophysiology, and therapeutic strategies for PALI. A comprehensive literature search was conducted to identify preclinical and clinical studies investigating PALI. Data from these studies were synthesized to provide a comprehensive overview of PALI and its management.

RESULTS

Epidemiological studies have highlighted the substantial prevalence of PALI in post-cardiac arrest patients, with up to 50% of survivors experiencing acute lung injury. Diagnostic imaging modalities, including chest X-rays, computed tomography, and lung ultrasound, play a crucial role in identifying PALI and assessing its severity. Pathophysiologically, PALI encompasses a spectrum of factors, including chest compression-related trauma, pulmonary IR injury, aspiration, and systemic inflammation, which collectively contribute to lung dysfunction and poor outcomes. Therapeutically, lung-protective ventilation strategies, such as low tidal volume ventilation and optimization of positive end-expiratory pressure, have emerged as cornerstone approaches in the management of PALI. Additionally, therapeutic hypothermia and emerging therapies targeting mitochondrial dysfunction hold promise in mitigating PALI-related morbidity and mortality.

CONCLUSION

PALI represents a significant clinical challenge in post-cardiac arrest care, necessitating prompt diagnosis and targeted interventions to improve outcomes. Mitochondrial-related therapies are among the novel therapeutic strategies for PALI. Further clinical research is warranted to optimize PALI management and enhance post-cardiac arrest care paradigms.

Biomimetic nanoparticles with enhanced rapamycin delivery for autism spectrum disorder treatment via autophagy activation and oxidative stress modulation

Rationale: Autism spectrum disorder (ASD) represents a complex neurodevelopmental condition lacking specific pharmacological interventions. Given the multifaced etiology of ASD, there exist no effective treatment for ASD. Rapamycin (RAPA) can activate autophagy by inhibiting the mTOR pathway and has exhibited promising effects in treating central nervous system disorders; however, its limited ability to cross the blood-brain barrier (BBB) has hindered its clinical efficacy, leading to substantial side effects. Methods: To address this challenge, we designed a drug delivery system utilizing red blood cell membrane (CM) vesicles modified with SS31 peptides to enhance the brain penetration of RAPA for the treatment of autism. Results: The fabricated SCM@RAPA nanoparticles, with an average diameter of 110 nm, exhibit rapid release of RAPA in a pathological environment characterized by oxidative stress. In vitro results demonstrate that SCM@RAPA effectively activate cellular autophagy, reduce intracellular ROS levels, improve mitochondrial function, thereby ameliorating neuronal damage. SS31 peptide modification significantly enhances the BBB penetration and rapid brain accumulation of SCM@RAPA. Notably, SCM@RAPA nanoparticles demonstrate the potential to ameliorate social deficits, improve cognitive function, and reverse neuronal impairments in valproic acid (VPA)-induced ASD models. Conclusions: The therapeutic potential of SCM@RAPA in managing ASD signifies a paradigm shift in autism drug treatment, holding promise for clinical interventions in diverse neurological conditions.

Myalgic encephalomyelitis/chronic fatigue syndrome from current evidence to new diagnostic perspectives through skeletal muscle and metabolic disturbances

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a demanding medical condition for patients and society. It has raised much more public awareness after the COVID-19 pandemic since ME/CFS and long-COVID patients share many clinical symptoms such as debilitating chronic fatigue. However, unlike long COVID, the etiopathology of ME/CFS remains a mystery despite several decades' research. This review moves from pathophysiology of ME/CFS through the compelling evidence and most interesting hypotheses. It focuses on the pathophysiology of skeletal muscle by proposing the hypothesis that skeletal muscle tissue offers novel opportunities for diagnosis and treatment of this syndrome and that new evidence can help resolve the long-standing debate on terminology.

The mediating role of inflammaging between mitochondrial dysfunction and sarcopenia in aging: a review

Sarcopenia, characterized by the insidious reduction of skeletal muscle mass and strength, detrimentally affects the quality of life in elderly cohorts. Present therapeutic strategies are confined to physiotherapeutic interventions, signaling a critical need for elucidation of the etiological underpinnings to facilitate the development of innovative pharmacotherapies. Recent scientific inquiries have associated mitochondrial dysfunction and inflammation with the etiology of sarcopenia. Mitochondria are integral to numerous fundamental cellular processes within muscle tissue, including but not limited to apoptosis, autophagy, signaling via reactive oxygen species, and the maintenance of protein equilibrium. Deviations in mitochondrial dynamics, coupled with compromised oxidative capabilities, autophagic processes, and protein equilibrium, result in disturbances to muscular architecture and functionality. Mitochondrial dysfunction is particularly detrimental as it diminishes oxidative phosphorylation, escalates apoptotic activity, and hinders calcium homeostasis within muscle cells. Additionally, deleterious feedback loops of deteriorated respiration, exacerbated oxidative injury, and diminished quality control mechanisms precipitate the acceleration of muscular senescence. Notably, mitochondria exhibiting deficient energetic metabolism are pivotal in precipitating the shift from normative muscle aging to a pathogenic state. This analytical review meticulously examines the complex interplay between mitochondrial dysfunction, persistent inflammation, and the pathogenesis of sarcopenia. It underscores the imperative to alleviate inflammation and amend mitochondrial anomalies within geriatric populations as a strategy to forestall and manage sarcopenia. An initial overview provides a succinct exposition of sarcopenia and its clinical repercussions. The discourse then progresses to an examination of the direct correlation between mitochondrial dysfunction and the genesis of sarcopenia. Concomitantly, it accentuates potential synergistic effects between inflammatory responses and mitochondrial insufficiencies during the aging of skeletal muscle, thereby casting light upon emergent therapeutic objectives. In culmination, this review distills the prevailing comprehension of the mitochondrial and inflammatory pathways implicated in sarcopenia and delineates extant lacunae in knowledge to orient subsequent scientific inquiry.

Research progress on the pathogenesis and treatment of ventilator-induced diaphragm dysfunction

Prolonged controlled mechanical ventilation (CMV) can cause diaphragm fiber atrophy and inspiratory muscle weakness, resulting in diaphragmatic contractile dysfunction, called ventilator-induced diaphragm dysfunction (VIDD). VIDD is associated with higher rates of in-hospital deaths, nosocomial pneumonia, difficulty weaning from ventilators, and increased costs. Currently, appropriate clinical strategies to prevent and treat VIDD are unavailable, necessitating the importance of exploring the mechanisms of VIDD and suitable treatment options to reduce the healthcare burden. Numerous animal studies have demonstrated that ventilator-induced diaphragm dysfunction is associated with oxidative stress, increased protein hydrolysis, disuse atrophy, and calcium ion disorders. Therefore, this article summarizes the molecular pathogenesis and treatment of ventilator-induced diaphragm dysfunction in recent years so that it can be better served clinically and is essential to reduce the duration of mechanical ventilation use, intensive care unit (ICU) length of stay, and the medical burden.

Aberrant mitochondrial dynamics contributes to diaphragmatic weakness induced by mechanical ventilation

In critical care patients, the ""temporary inactivity of the diaphragm caused by mechanical ventilation (MV) triggers a series of events leading to diaphragmatic dysfunction and atrophy, commonly known as ventilator-induced diaphragm dysfunction (VIDD). While mitochondrial dysfunction related to oxidative stress is recognized as a crucial factor in VIDD, the exact molecular mechanism remains poorly understood. In this study, we observe that 6 h of MV triggers aberrant mitochondrial dynamics, resulting in a reduction in mitochondrial size and interaction, associated with increased expression of dynamin-related protein 1 (DRP1). This effect can be prevented by P110, a molecule that inhibits the recruitment of DRP1 to the mitochondrial membrane. Furthermore, isolated mitochondria from the diaphragms of ventilated patients exhibited increased production of reactive oxygen species (ROS). These mitochondrial changes were associated with the rapid oxidation of type 1 ryanodine receptor (RyR1) and a decrease in the stabilizing subunit calstabin 1. Subsequently, we observed that the sarcoplasmic reticulum (SR) in the ventilated diaphragms showed increased calcium leakage and reduced contractile function. Importantly, the mitochondrial fission inhibitor P110 effectively prevented all of these alterations. Taken together, the results of our study illustrate that MV leads, in the diaphragm, to both mitochondrial fragmentation and dysfunction, linked to the up-/down-regulation of 320 proteins, as assessed through global comprehensive quantitative proteomics analysis, primarily associated with mitochondrial function. These outcomes underscore the significance of developing compounds aimed at modulating the balance between mitochondrial fission and fusion as potential interventions to mitigate VIDD in human patients.

Microcirculation and Mitochondria: The Critical Unit

Critical illness is often accompanied by a hemodynamic imbalance between macrocirculation and microcirculation, as well as mitochondrial dysfunction. Microcirculatory disorders lead to abnormalities in the supply of oxygen to tissue cells, while mitochondrial dysfunction leads to abnormal energy metabolism and impaired tissue oxygen utilization, making these conditions important pathogenic factors of critical illness. At the same time, there is a close relationship between the microcirculation and mitochondria. We introduce here the concept of a "critical unit", with two core components: microcirculation, which mainly comprises the microvascular network and endothelial cells, especially the endothelial glycocalyx; and mitochondria, which are mainly involved in energy metabolism but perform other non-negligible functions. This review also introduces several techniques and devices that can be utilized for the real-time synchronous monitoring of the microcirculation and mitochondria, and thus critical unit monitoring. Finally, we put forward the concepts and strategies of critical unit-guided treatment.

Mitochondrial dysfunction and skeletal muscle atrophy: Causes, mechanisms, and treatment strategies

Skeletal muscle, which accounts for approximately 40% of total body weight, is one of the most dynamic and plastic tissues in the human body and plays a vital role in movement, posture and force production. More than just a component of the locomotor system, skeletal muscle functions as an endocrine organ capable of producing and secreting hundreds of bioactive molecules. Therefore, maintaining healthy skeletal muscles is crucial for supporting overall body health. Various pathological conditions, such as prolonged immobilization, cachexia, aging, drug-induced toxicity, and cardiovascular diseases (CVDs), can disrupt the balance between muscle protein synthesis and degradation, leading to skeletal muscle atrophy. Mitochondrial dysfunction is a major contributing mechanism to skeletal muscle atrophy, as it plays crucial roles in various biological processes, including energy production, metabolic flexibility, maintenance of redox homeostasis, and regulation of apoptosis. In this review, we critically examine recent knowledge regarding the causes of muscle atrophy (disuse, cachexia, aging, etc.) and its contribution to CVDs. Additionally, we highlight the mitochondrial signaling pathways involvement to skeletal muscle atrophy, such as the ubiquitin-proteasome system, autophagy and mitophagy, mitochondrial fission-fusion, and mitochondrial biogenesis. Furthermore, we discuss current strategies, including exercise, mitochondria-targeted antioxidants, in vivo transfection of PGC-1α, and the potential use of mitochondrial transplantation as a possible therapeutic approach.

Mitochondrial transplantation as a possible therapeutic option for sarcopenia

With advancing age, the skeletal muscle phenotype is characterized by a progressive loss of mass, strength, and quality. This phenomenon, known as sarcopenia, has a negative impact on quality of life and increases the risk of morbidity and mortality in older adults. Accumulating evidence suggests that damaged and dysfunctional mitochondria play a critical role in the pathogenesis of sarcopenia. Lifestyle modifications, such as physical activity, exercise, and nutrition, as well as medical interventions with therapeutic agents, are effective in the management of sarcopenia and offer solutions to maintain and improve skeletal muscle health. Although a great deal of effort has been devoted to the identification of the best treatment option, these strategies are not sufficient to overcome sarcopenia. Recently, it has been reported that mitochondrial transplantation may be a possible therapeutic approach for the treatment of mitochondria-related pathological conditions such as ischemia, liver toxicity, kidney injury, cancer, and non-alcoholic fatty liver disease. Given the role of mitochondria in the function and metabolism of skeletal muscle, mitochondrial transplantation may be a possible option for the treatment of sarcopenia. In this review, we summarize the definition and characteristics of sarcopenia and molecular mechanisms associated with mitochondria that are known to contribute to sarcopenia. We also discuss mitochondrial transplantation as a possible option. Despite the progress made in the field of mitochondrial transplantation, further studies are needed to elucidate the role of mitochondrial transplantation in sarcopenia. KEY MESSAGES: Sarcopenia is the progressive loss of skeletal muscle mass, strength, and quality. Although the specific mechanisms that lead to sarcopenia are not fully understood, mitochondria have been identified as a key factor in the development of sarcopenia. Damaged and dysfunctional mitochondria initiate various cellular mediators and signaling pathways, which largely contribute to the age-related loss of skeletal muscle mass and strength. Mitochondrial transplantation has been reported to be a possible option for the treatment/prevention of several diseases. Mitochondrial transplantation may be a possible therapeutic option for improving skeletal muscle health and treating sarcopenia. Mitochondrial transplantation as a possible treatment option for sarcopenia.

The beneficial role of exercise in preventing doxorubicin-induced cardiotoxicity

Doxorubicin is a highly effective chemotherapeutic agent widely used to treat a variety of cancers. However, the clinical application of doxorubicin is limited due to its adverse effects on several tissues. One of the most serious side effects of doxorubicin is cardiotoxicity, which results in life-threatening heart damage, leading to reduced cancer treatment success and survival rate. Doxorubicin-induced cardiotoxicity results from cellular toxicity, including increased oxidative stress, apoptosis, and activated proteolytic systems. Exercise training has emerged as a non-pharmacological intervention to prevent cardiotoxicity during and after chemotherapy. Exercise training stimulates numerous physiological adaptations in the heart that promote cardioprotective effects against doxorubicin-induced cardiotoxicity. Understanding the mechanisms responsible for exercise-induced cardioprotection is important to develop therapeutic approaches for cancer patients and survivors. In this report, we review the cardiotoxic effects of doxorubicin and discuss the current understanding of exercise-induced cardioprotection in hearts from doxorubicin-treated animals.

Insights into the development of insulin resistance: Unraveling the interaction of physical inactivity, lipid metabolism and mitochondrial biology

While impairments in peripheral tissue insulin signalling have a well-characterized role in the development of insulin resistance and type 2 diabetes (T2D), the specific mechanisms that contribute to these impairments remain debatable. Nonetheless, a prominent hypothesis implicates the presence of a high-lipid environment, resulting in both reactive lipid accumulation and increased mitochondrial reactive oxygen species (ROS) production in the induction of peripheral tissue insulin resistance. While the etiology of insulin resistance in a high lipid environment is rapid and well documented, physical inactivity promotes insulin resistance in the absence of redox stress/lipid-mediated mechanisms, suggesting alternative mechanisms-of-action. One possible mechanism is a reduction in protein synthesis and the resultant decrease in key metabolic proteins, including canonical insulin signaling and mitochondrial proteins. While reductions in mitochondrial content associated with physical inactivity are not required for the induction of insulin resistance, this could predispose individuals to the detrimental effects of a high-lipid environment. Conversely, exercise-training induced mitochondrial biogenesis has been implicated in the protective effects of exercise. Given mitochondrial biology may represent a point of convergence linking impaired insulin sensitivity in both scenarios of chronic overfeeding and physical inactivity, this review aims to describe the interaction between mitochondrial biology, physical (in)activity and lipid metabolism within the context of insulin signalling.

A new triphenylphosphonium-conjugated amphipathic cationic peptide with improved cell-penetrating and ROS-targeting properties

We study for the first time whether triphenylphosphonium (TPP) moiety can improve cellular delivery and redox properties of amphipathic cationic peptides based on YRFK/YrFK cell-penetrating and cytoprotective motif. TPP moiety was found to increase reducing activity of both stereoisomeric peptides in solution and on electrode surface in association with TPP-mediated intramolecular interactions. Among TPP-conjugated peptides, newly synthesized TPP3-YrFK featured both increased antioxidant efficacy and proteolytic resistance. TPP-conjugated peptides preferably mitigated endogenic ROS in mitochondria and cytoplasm of model glioblastoma cells with increased oxidative status. This anti-ROS effect was accompanied by mild reversible decrease of reduced glutathione level in the cells with relatively weak change in glutathione redox forms ratio. Such low interference with cell redox status is in accordance with non-cytotoxic nature of the compounds. Intracellular concentrations of label-free peptides were analyzed by LC-MS/MS, which showed substantial TPP-promoted penetration of YrFK motif across cell plasma membrane. However, according to ΔΨ_m analysis, TPP moiety did not profoundly enhance peptide interaction with mitochondrial inner membrane. Our study clarifies the role of TPP moiety in cellular delivery of amphipathic cationic oligopeptides. The results suggest TPP moiety as a multi-functional modifier for the oligopeptides which is capable of improving cellular pharmacokinetics and antioxidant activity as well as targeting increased ROS levels. The results encourage further investigation of TPP3-YrFK as a peptide antioxidant with multiple benefits.

Castor Oil Plant (Ricinus communis L.) Leaves Improve Dexamethasone-Induced Muscle Atrophy via Nrf2 Activation

Skeletal muscle atrophy is characterized by reduced muscle function and size. Oxidative stress contributes to muscle atrophy but can be treated with antioxidants. This study investigated the antioxidant activity of a castor oil plant leaf (Ricinus communis L.) extract (RC) and its effects on muscle atrophy. Rutin was identified as the major compound among the thirty compounds identified in RC via LC-MS/MS and was found to inhibit dexamethasone (DEX)-induced muscle atrophy and mitochondrial oxidative stress. Rutin-rich RC showed DPPH and ABTS radical scavenging activities and efficiently reduced the DEX-induced myotube atrophy and mitochondrial oxidative damage in C2C12 cells. RC supplementation prevented the loss of muscle function and muscle mass in DEX-administered mice and ameliorated DEX-induced oxidative stress via Nrf2 signaling. Taken together, both RC and rutin ameliorated muscle atrophy and helped in maintaining redox homeostasis; hence, rutin-rich RC could be a promising functional food that is beneficial for muscle health.

Redox signaling regulates skeletal muscle remodeling in response to exercise and prolonged inactivity

Skeletal muscle fibers are malleable and undergo rapid remodeling in response to increased contractile activity (i.e., exercise) or prolonged periods of muscle inactivity (e.g., prolonged bedrest). Exploration of the cell signaling pathways regulating these skeletal muscle adaptations reveal that redox signaling pathways play a key role in the control of muscle remodeling during both exercise and prolonged muscle inactivity. In this regard, muscular exercise results in an acute increase in the production of reactive oxygen species (ROS) in the contracting fibers; however, this contraction-induced rise in ROS production rapidly declines when contractions cease. In contrast, prolonged muscle disuse results in a chronic elevation in ROS production within the inactive fibers. This difference in the temporal pattern of ROS production in muscle during exercise and muscle inactivity stimulates divergent cell-signaling pathways that activate both genomic and nongenomic mechanisms to promote muscle remodeling. This review examines the role that redox signaling plays in skeletal muscle adaptation in response to both prolonged muscle inactivity and endurance exercise training. We begin with a summary of the sites of ROS production in muscle fibers followed by a review of the cellular antioxidants that are responsible for regulation of ROS levels in the cell. We then discuss the specific redox-sensitive signaling pathways that promote skeletal muscle adaptation in response to both prolonged muscle inactivity and exercise. To stimulate future research, we close with a discussion of unanswered questions in this exciting field.

SS-31 does not prevent or reduce muscle atrophy 7 days after a 65 kdyne contusion spinal cord injury in young male mice

Spinal cord injury (SCI) leads to major reductions in function, independent living, and quality of life. Disuse and paralysis from SCI leads to rapid muscle atrophy, with chronic muscle loss likely playing a role in the development of the secondary metabolic disorders often seen in those with SCI. Muscle disuse is associated with mitochondrial dysfunction. Previous evidence has suggested targeting the mitochondria with the tetrapeptide SS-31 is beneficial for muscle health in preclinical models that lead to mitochondrial dysfunction, such as cast immobilization or burn injury. We gave young male mice a sham (n = 8) or 65 kdyne thoracic contusion SCI with (n = 9) or without (n = 9) daily administration of 5.0 mg/kg SS-31. Hindlimb muscle mass and muscle bundle respiration were measured at 7 days post-SCI and molecular targets were investigated using immunoblotting, RT-qPCR, and metabolomics. SS-31 did not preserve body mass or hindlimb muscle mass 7 days post-SCI. SS-31 had no effect on soleus or plantaris muscle bundle respiration. SCI was associated with elevated levels of protein carbonylation, led to reduced protein expression of activated DRP1 and reductions in markers of mitochondrial fusion. SS-31 administration did result in reduced total DRP1 expression, as well as greater expression of inhibited DRP1. Gene expression of proinflammatory cytokines and their receptors were largely stable across groups, although SS-31 treatment led to greater mRNA expression of IL1B, TNF, and TNFRSF12A. In summation, SS-31 was not an efficacious treatment acutely after a moderate thoracic contusion SCI in young male mice.

Aldh1a1 and Scl25a30 in diaphragmatic dysfunction

New methods to prevent ventilator-induced diaphragmatic dysfunction (VIDD) are urgently needed, and the cellular basis of VIDD is poorly understood. This study evaluated whether transvenous phrenic nerve stimulation (PNS) could prevent VIDD in rabbits undergoing mechanical ventilation (MV) and explored whether oxidative stress-related genes might be candidate molecular markers for VIDD. Twenty-four adult male New Zealand white rabbits were allocated to control, MV, and PNS groups (n = 8 in each group). Rabbits in the MV and PNS groups underwent MV for 24 h. Intermittent bilateral transvenous PNS was performed in rabbits in the PNS group. Transdiaphragmatic pressure was recorded using balloon catheters. The diameters and cross-sectional areas (CSAs) of types I and II diaphragmatic fibers were measured using immunohistochemistry (IHC) techniques. Genes associated with VIDD were identified by RNA sequencing (RNA-seq), differentially expressed gene (DEG) analysis, and weighted gene co-expression network analysis (WGCNA). Reverse transcription polymerase chain reaction (RT-PCR), Western blotting, and IHC analyses were carried out to verify the transcriptome profile. Pdi_60Hz, Pdi_80Hz, and Pdi_100Hz were significantly higher in the PNS group than in the MV group at 12 and 24 h (P < 0.05 at both time points). The diameters and CSAs of types I (slow-twitch) and II (fast-twitch) fibers were significantly larger in the PNS group than in the MV group (P < 0.05). RNA-seq, RT-PCR, Western blotting, and IHC experiments identified two candidate genes associated with VIDD: Aldh1a1 and Scl25a30. The MV group had significantly higher mRNA and protein expressions of Aldh1a1/ALDH1A1 and significantly lower mRNA and protein expressions of Scl25a30/SCL25A30 than the control or PNS groups (P < 0.05). We have identified two candidate genes involved in the prevention of VIDD by transvenous PNS. These two key genes may provide a theoretical basis for targeted therapy against VIDD.

The Role of Oxidative Stress in Skeletal Muscle Myogenesis and Muscle Disease

The contractile activity, high oxygen consumption and metabolic rate of skeletal muscle cause it to continuously produce moderate levels of oxidant species, such as reactive oxygen species (ROS) and reactive nitrogen species (RNS). Under normal physiological conditions, there is a dynamic balance between the production and elimination of ROS/RNS. However, when the oxidation products exceed the antioxidant defense capacity, the body enters a state of oxidative stress. Myogenesis is an important process to maintain muscle homeostasis and the physiological function of skeletal muscle. Accumulating evidence suggests that oxidative stress plays a key role in myogenesis and skeletal muscle physiology and pathology. In this review, we summarize the sources of reactive oxygen species in skeletal muscle and the causes of oxidative stress and analyze the key role of oxidative stress in myogenesis. Then, we discuss the relationship between oxidative stress and muscle homeostasis and physiopathology. This work systematically summarizes the role of oxidative stress in myogenesis and muscle diseases and provides targets for subsequent antioxidant therapy and repair of inflammatory damage in noninflammatory muscle diseases.

Cardiovasomobility: an integrative understanding of how disuse impacts cardiovascular and skeletal muscle health

Cardiovasomobility is a novel concept that encompasses the integration of cardiovascular and skeletal muscle function in health and disease with critical modification by physical activity, or lack thereof. Compelling evidence indicates that physical activity improves health while a sedentary, or inactive, lifestyle accelerates cardiovascular and skeletal muscle dysfunction and hastens disease progression. Identifying causative factors for vascular and skeletal muscle dysfunction, especially in humans, has proven difficult due to the limitations associated with cross-sectional investigations. Therefore, experimental models of physical inactivity and disuse, which mimic hospitalization, injury, and illness, provide important insight into the mechanisms and consequences of vascular and skeletal muscle dysfunction. This review provides an overview of the experimental models of disuse and inactivity and focuses on the integrated responses of the vasculature and skeletal muscle in response to disuse/inactivity. The time course and magnitude of dysfunction evoked by various models of disuse/inactivity are discussed in detail, and evidence in support of the critical roles of mitochondrial function and oxidative stress are presented. Lastly, strategies aimed at preserving vascular and skeletal muscle dysfunction during disuse/inactivity are reviewed. Within the context of cardiovasomobility, experimental manipulation of physical activity provides valuable insight into the mechanisms responsible for vascular and skeletal muscle dysfunction that limit mobility, degrade quality of life, and hasten the onset of disease.

Identifying the key genes and functional enrichment pathways associated with feed efficiency in cattle

Residual feed intake (RFI) is a measurement of feed efficiency, and is inversely correlated with feed efficiency. The differentially expressed genes (DEGs) associated with RFI vary substantially among studies, posing great challenges in finding the RFI-related marker genes. This study attempted to resolve this issue by integrating and comparing the multiple transcriptome sequencing data associated with RFI in the cattle liver, using differential, functional enrichment, protein-protein interaction (PPI) network, weighted co-expression network (WGCNA), and gene set enrichment analyses (GSEA) to identify the candidate genes and functional enrichment pathways that are closely associated with RFI. Four candidate genes namely SHC1, GPX4, ACADL, and IGF1 were identified and validated as the marker genes for RFI. Four functional enrichment pathways, namely the fatty acid metabolism, sugar metabolism, energy metabolism, and protein ubiquitination were also found to be closely related to RFI. This study identified several genes and signaling pathways with shared characteristics, which will provide new insights into the molecular mechanisms related to the regulation of feed efficiency, and provide basis for molecular markers related to feed efficiency in beef cattle.

Mitochondria in Focus: From Function to Therapeutic Strategies in Chronic Lung Diseases

Mitochondria are essential organelles for cell metabolism, growth, and function. Mitochondria in lung cells have important roles in regulating surfactant production, mucociliary function, mucus secretion, senescence, immunologic defense, and regeneration. Disruption in mitochondrial physiology can be the central point in several pathophysiologic pathways of chronic lung diseases such as chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, and asthma. In this review, we summarize how mitochondria morphology, dynamics, redox signaling, mitophagy, and interaction with the endoplasmic reticulum are involved in chronic lung diseases and highlight strategies focused on mitochondrial therapy (mito-therapy) that could be tested as a potential therapeutic target for lung diseases.

Shared and distinct mechanisms of skeletal muscle atrophy: A narrative review

Maintenance of skeletal muscle mass and function is an incredibly nuanced balance of anabolism and catabolism that can become distorted within different pathological conditions. In this paper we intend to discuss the distinct intracellular signaling events that regulate muscle protein atrophy for a given clinical occurrence. Aside from the common outcome of muscle deterioration, several conditions have at least one or more distinct mechanisms that creates unique intracellular environments that facilitate muscle loss. The subtle individuality to each of these given pathologies can provide both researchers and clinicians with specific targets of interest to further identify and increase the efficacy of medical treatments and interventions.

Role of autophagy in muscle disease

Beside inherited muscle diseases many catabolic conditions such as insulin resistance, malnutrition, cancer growth, aging, infections, chronic inflammatory status, inactivity, obesity are characterized by loss of muscle mass, strength and function. The decrease of muscle quality and quantity increases morbidity, mortality and has a major impact on the quality of life. One of the pathogenetic mechanisms of muscle wasting is the dysregulation of the main protein and organelles quality control system of the cell: the autophagy-lysosome. This review will focus on the role of the autophagy-lysosome system in the different conditions of muscle loss. We will also dissect the signalling pathways that are involved in excessive or defective autophagy regulation. Finally, the state of the art of autophagy modulators that have been used in preclinical or clinical studies to ameliorate muscle mass will be also described.

Systemic delivery of a mitochondria targeted antioxidant partially preserves limb muscle mass and grip strength in response to androgen deprivation

Muscle mass is important for health. Decreased testicular androgen production (hypogonadism) contributes to the loss of muscle mass, with loss of limb muscle being particularly debilitating. Androgen replacement is the only pharmacological treatment, which may not be feasible for everyone. Prior work showed that markers of reactive oxygen species and markers of mitochondrial degradation pathways were higher in the limb muscle following castration. Therefore, we tested whether an antioxidant preserved limb muscle mass in male mice subjected to a castration surgery. Subsets of castrated mice were treated with resveratrol (a general antioxidant) or MitoQ (a mitochondria targeted antioxidant). Relative to the non-castrated control mice, lean mass, limb muscle mass, and grip strength were partially preserved only in castrated mice treated with MitoQ. Independent of treatment, markers of mitochondrial degradation pathways remained elevated in all castrated mice. Therefore, a mitochondrial targeted antioxidant may partially preserve limb muscle mass in response to hypogonadism.

PINK1/Parkin-mediated mitophagy in mechanical ventilation-induced diaphragmatic dysfunction

Background:

Mechanical ventilation (MV) often leads to ventilation-induced diaphragm dysfunction (VIDD). Although the development of this disorder had been linked to oxidative stress, mitochondrial energy deficiency, autophagy activation, and apoptosis in the diaphragm, it remains unclear whether the activation of mitophagy can induce VIDD. With our research, our endeavor is to uncover whether PTEN-induced putative kinase 1 (PINK1)/Parkin-mediated mitophagy affects the MV-caused diaphragmatic dysfunction

Methods:

Sprague-Dawley rats were subjected to MV treatment for 6 h (MV-6h), 12 h (MV-12h), or 24 h (MV-24h). Post MV, the diaphragm muscle compound action potential (CMAP) and cross-sectional areas (CSAs) of the diaphragm of these rats were measured. The levels of proteins of interest were examined to assess muscle health, mitochondrial dynamics, and mitophagy in the diaphragm. The co-localization of PINK1 with the mitochondrial protein marker tom20 was examined, as well as transmission electron microscopy analysis to detect changes in diaphragm mitochondrial ultrastructure.

Results:

MV-12h and MV-24h treatments resulted in a decrease in CSA of diaphragm and CMAP amplitude. In addition, the expressions of F-box (MFAbx), muscle-specific ring finger 1 (MURF1), PINK1, and p62 were elevated in rats treated with MV for 12 h and 24 h, while mfn2 expression was reduced. Rats following MV-24h treatment displayed an increase in mitochondrial dynamic protein (Drp1) and Parkin expression and microtubule-associated protein 1 light chain 3/1 (LC3II/I) ratio. Moreover, decreased SOD and GSH activity and membrane potential were observed after MV-12h and MV-24h treatment, while H₂O₂ activity increased after MV-24h treatment. In addition, a strong co-localization between PINK1 and tom20 was identified.

Conclusion:

These results reveal that MV leads to various changes in mitochondrial dynamics and significantly increases the mitophagy levels, which subsequently cause the variation in diaphragmatic function and muscle atrophy, indicating that mitophagy could be one of the possible mechanisms by which MV induces diaphragmatic dysfunction.

The reviews of this paper are available via the supplemental material section.

Mitochondria-targeted antioxidant supplementation improves 8 km time trial performance in middle-aged trained male cyclists

BACKGROUND

Exercise increases skeletal muscle reactive oxygen species (ROS) production, which may contribute to the onset of muscular fatigue and impair athletic performance. Mitochondria-targeted antioxidants such as MitoQ, which contains a ubiquinone moiety and is targeted to mitochondria through the addition of a lipophilic triphenylphosphonium cation, are becoming popular amongst active individuals as they are designed to accumulate within mitochondria and may provide targeted protection against exercise-induced oxidative stress. However, the effect of MitoQ supplementation on cycling performance is currently unknown. Here, we investigate whether MitoQ supplementation can improve cycling performance measured as time to complete an 8 km time trial.

METHOD

In a randomized, double-blind, placebo-controlled crossover study, 19 middle-aged (age: 44 ± 4 years) recreationally trained (VO_2peak: 58.5 ± 6.2 ml·kg^- 1·min^- 1, distance cycled per week during 6 months prior to study enrollment: 158.3 ± 58.4 km) male cyclists completed 45 min cycling at 70% VO_2peak followed by an 8 km time trial after 28 days of supplementation with MitoQ (20 mg·day^- 1) and a placebo. Free F₂-isoprostanes were measured in plasma samples collected at rest, after 45 min cycling at 70% VO_2peak and after completion of the time trial. Respiratory gases and measures of rating of perceived exertion (RPE) were also collected.

RESULTS

Mean completion time for the time trial was 1.3% faster with MitoQ (12.91 ± 0.94 min) compared to placebo (13.09 ± 0.95 min, p = 0.04, 95% CI [0.05, 2.64], d = 0.2). There was no difference in RPE during the time trial between conditions (p = 0.82) despite there being a 4.4% increase in average power output during the time trial following MitoQ supplementation compared to placebo (placebo; 270 ± 51 W, MitoQ; 280 ± 53 W, p = 0.04, 95% CI [0.49, 8.22], d = 0.2). Plasma F₂-isoprostanes were lower on completion of the time trial following MitoQ supplementation (35.89 ± 13.6 pg·ml^- 1) compared to placebo (44.7 ± 16.9 pg·ml^- 1 p = 0.03).

CONCLUSION

These data suggest that MitoQ supplementation may be an effective nutritional strategy to attenuate exercise-induced increases in oxidative damage to lipids and improve cycling performance.

Energetic dysfunction in sepsis: a narrative review

Background

Growing evidence associates organ dysfunction(s) with impaired metabolism in sepsis. Recent research has increased our understanding of the role of substrate utilization and mitochondrial dysfunction in the pathophysiology of sepsis-related organ dysfunction. The purpose of this review is to present this evidence as a coherent whole and to highlight future research directions.

Main text

Sepsis is characterized by systemic and organ-specific changes in metabolism. Alterations of oxygen consumption, increased levels of circulating substrates, impaired glucose and lipid oxidation, and mitochondrial dysfunction are all associated with organ dysfunction and poor outcomes in both animal models and patients. The pathophysiological relevance of bioenergetics and metabolism in the specific examples of sepsis-related immunodeficiency, cerebral dysfunction, cardiomyopathy, acute kidney injury and diaphragmatic failure is also described.

Conclusions

Recent understandings in substrate utilization and mitochondrial dysfunction may pave the way for new diagnostic and therapeutic approaches. These findings could help physicians to identify distinct subgroups of sepsis and to develop personalized treatment strategies. Implications for their use as bioenergetic targets to identify metabolism- and mitochondria-targeted treatments need to be evaluated in future studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-021-00893-7.

Hydrogen sulfide donor protects against mechanical ventilation-induced atrophy and contractile dysfunction in the rat diaphragm

Mechanical ventilation (MV) is a clinical tool providing adequate alveolar ventilation in patients that require respiratory support. Although a life-saving intervention for critically ill patients, prolonged MV results in the rapid development of inspiratory muscle weakness due to both diaphragmatic atrophy and contractile dysfunction; collectively known as "ventilator-induced diaphragm dysfunction" (VIDD). VIDD is a severe clinical problem because diaphragmatic weakness is a risk factor for difficulties in weaning patients from MV. Currently, no standard treatment to prevent VIDD exists. Nonetheless, growing evidence reveals that hydrogen sulfide (H₂ S) possesses cytoprotective properties capable of protecting skeletal muscles against several hallmarks of VIDD, including oxidative damage, accelerated proteolysis, and mitochondrial damage. Therefore, we used an established animal model of MV to test the hypothesis that treatment with sodium sulfide (H₂ S donor) will defend against VIDD. Our results confirm that sodium sulfide was sufficient to protect the diaphragm against both MV-induced fiber atrophy and contractile dysfunction. H₂ S prevents MV-induced damage to diaphragmatic mitochondria as evidenced by protection against mitochondrial uncoupling. Moreover, treatment with sodium sulfide prevented the MV-induced activation of the proteases, calpain, and caspase-3 in the diaphragm. Taken together, these results support the hypothesis that treatment with a H₂ S donor protects the diaphragm against VIDD. These outcomes provide the first evidence that H₂ S has therapeutic potential to protect against MV-induced diaphragm weakness and to reduce difficulties in weaning patients from the ventilator. Study Highlights WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC? Mechanical ventilation (MV) results in diaphragm atrophy and contractile dysfunction, known as ventilator-induced diaphragm dysfunction (VIDD). VIDD is important because diaphragm weakness is a risk factor for problems in weaning patients from MV. Currently, no accepted treatment exists to protect against VIDD. Growing evidence reveals that hydrogen sulfide (H₂ S) donors protect skeletal muscle against ischemia-reperfusion-induced injury. Nonetheless, it is unknown if treatment with a H₂ S donor can protect against VIDD. WHAT QUESTION DID THIS STUDY ADDRESS? Can treatment with an H₂ S donor protect against VIDD? WHAT DOES THIS STUDY ADD TO OUR KNOWLEDGE? This study provides the first evidence that treatment with a H₂ S donor protects against VIDD. HOW MIGHT THIS CHANGE CLINICAL PHARMACOLOGY OR TRANSLATIONAL SCIENCE? These new findings provide the basis for further exploration of H₂ S donors as a therapy to prevent VIDD and reduce the risk of problems in weaning patients from MV.

Preferent Diaphragmatic Involvement in TK2 Deficiency: An Autopsy Case Study

Our goal was to analyze postmortem tissues of an adult patient with late-onset thymidine kinase 2 (TK2) deficiency who died of respiratory failure. Compared with control tissues, we found a low mtDNA content in the patient’s skeletal muscle, liver, kidney, small intestine, and particularly in the diaphragm, whereas heart and brain tissue showed normal mtDNA levels. mtDNA deletions were present in skeletal muscle and diaphragm. All tissues showed a low content of OXPHOS subunits, and this was especially evident in diaphragm, which also exhibited an abnormal protein profile, expression of non-muscular β-actin and loss of GAPDH and α-actin. MALDI-TOF/TOF mass spectrometry analysis demonstrated the loss of the enzyme fructose-bisphosphate aldolase, and enrichment for serum albumin in the patient’s diaphragm tissue. The TK2-deficient patient’s diaphragm showed a more profound loss of OXPHOS proteins, with lower levels of catalase, peroxiredoxin 6, cytosolic superoxide dismutase, p62 and the catalytic subunits of proteasome than diaphragms of ventilated controls. Strong overexpression of TK1 was observed in all tissues of the patient with diaphragm showing the highest levels. TK2 deficiency induces a more profound dysfunction of the diaphragm than of other tissues, which manifests as loss of OXPHOS and glycolytic proteins, sarcomeric components, antioxidants and overactivation of the TK1 salvage pathway that is not attributed to mechanical ventilation.

Angiotensin 1-7 protects against ventilator-induced diaphragm dysfunction

Mechanical ventilation (MV) is a life‐saving instrument used to provide ventilatory support for critically ill patients and patients undergoing surgery. Unfortunately, an unintended consequence of prolonged MV is the development of inspiratory weakness due to both diaphragmatic atrophy and contractile dysfunction; this syndrome is labeled ventilator‐induced diaphragm dysfunction (VIDD). VIDD is clinically important because diaphragmatic weakness is an important contributor to problems in weaning patients from MV. Investigations into the pathogenesis of VIDD reveal that oxidative stress is essential for the rapid development of VIDD as redox disturbances in diaphragm fibers promote accelerated proteolysis. Currently, no standard treatment exists to prevent VIDD and, therefore, developing a strategy to avert VIDD is vital. Guided by evidence indicating that activation of the classical axis of the renin‐angiotensin system (RAS) in diaphragm fibers promotes oxidative stress and VIDD, we hypothesized that activation of the nonclassical RAS signaling pathway via angiotensin 1‐7 (Ang1‐7) will protect against VIDD. Using an established animal model of prolonged MV, our results disclose that infusion of Ang1‐7 protects the diaphragm against MV‐induced contractile dysfunction and fiber atrophy in both fast and slow muscle fibers. Further, Ang1‐7 shielded diaphragm fibers against MV‐induced mitochondrial damage, oxidative stress, and protease activation. Collectively, these results reveal that treatment with Ang1‐7 protects against VIDD, in part, due to diminishing oxidative stress and protease activation. These important findings provide robust evidence that Ang1‐7 has the therapeutic potential to protect against VIDD by preventing MV‐induced contractile dysfunction and atrophy of both slow and fast muscle fibers.

Mitochondrial Dysfunction Is a Common Denominator Linking Skeletal Muscle Wasting Due to Disease, Aging, and Prolonged Inactivity

Skeletal muscle is the most abundant tissue in the body and is required for numerous vital functions, including breathing and locomotion. Notably, deterioration of skeletal muscle mass is also highly correlated to mortality in patients suffering from chronic diseases (e.g., cancer). Numerous conditions can promote skeletal muscle wasting, including several chronic diseases, cancer chemotherapy, aging, and prolonged inactivity. Although the mechanisms responsible for this loss of muscle mass is multifactorial, mitochondrial dysfunction is predicted to be a major contributor to muscle wasting in various conditions. This systematic review will highlight the biochemical pathways that have been shown to link mitochondrial dysfunction to skeletal muscle wasting. Importantly, we will discuss the experimental evidence that connects mitochondrial dysfunction to muscle wasting in specific diseases (i.e., cancer and sepsis), aging, cancer chemotherapy, and prolonged muscle inactivity (e.g., limb immobilization). Finally, in hopes of stimulating future research, we conclude with a discussion of important future directions for research in the field of muscle wasting.

Mechanical ventilation preserves diaphragm mitochondrial function in a rat sepsis model

BACKGROUND

To describe the effect of mechanical ventilation on diaphragm mitochondrial oxygen consumption, ATP production, reactive oxygen species (ROS) generation, and cytochrome c oxidase activity and content, and their relationship to diaphragm strength in an experimental model of sepsis.

METHODS

A cecal ligation and puncture (CLP) protocol was performed in 12 rats while 12 controls underwent sham operation. Half of the rats in each group were paralyzed and mechanically ventilated. We performed blood gas analysis and lactic acid assays 6 h after surgery. Afterwards, we measured diaphragm strength and mitochondrial oxygen consumption, ATP and ROS generation, and cytochrome c oxidase activity. We also measured malondialdehyde (MDA) content as an index of lipid peroxidation, and mRNA expression of the proinflammatory interleukin-1β (IL-1β) in diaphragms.

RESULTS

CLP rats showed severe hypotension, metabolic acidosis, and upregulation of diaphragm IL-1β mRNA expression. Compared to sham controls, spontaneously breathing CLP rats showed lower diaphragm force and increased susceptibility to fatigue, along with depressed mitochondrial oxygen consumption and ATP production and cytochrome c oxidase activity. These rats also showed increased mitochondrial ROS generation and MDA content. Mechanical ventilation markedly restored mitochondrial oxygen consumption and ATP production in CLP rats; lowered mitochondrial ROS production by the complex 3; and preserved cytochrome c oxidase activity.

CONCLUSION

In an experimental model of sepsis, early initiation of mechanical ventilation restores diaphragm mitochondrial function.

Acute liver injury following acetaminophen administration does not activate atrophic pathways in the mouse diaphragm

N-acetyl-para-amino phenol (APAP, usually named paracetamol), which is commonly used for its analgesic and antipyretic properties may lead to hepatotoxicity and acute liver damage in case of overdoses. Released cytokines and oxidative stress following acute liver damage may affect other organs' function notably the diaphragm, which is particularly sensitive to oxidative stress and circulating cytokines. We addressed this issue in a mouse model of acute liver injury induced by administration of APAP. C57BL/6J mice (each n = 8) were treated with N-acetyl-para-amino phenol (APAP) to induce acute drug caused liver injury and sacrificed 12 or 24 h afterwards. An untreated group served as controls. Key markers of inflammation, proteolysis, autophagy and oxidative stress were measured in diaphragm samples. In APAP treated animals, liver damage was proven by the enhanced serum levels of alanine aminotransferase and aspartate aminotransferase. In the diaphragm, besides a significant increase in IL 6 and lipid peroxidation, no changes were observed in key markers of the proteolytic, and autophagy signaling pathways, other inflammatory markers and fiber dimensions. The first 24 h of acute liver damage did not impair diaphragm atrophic pathways although it slightly enhanced IL-6 and lipid peroxidation. Whether longer exposure might affect the diaphragm needs to be addressed in future experiments.

A 50 kdyne contusion spinal cord injury with or without the drug SS-31 was not associated with major changes in muscle mass or gene expression 14 d after injury in young male mice

Spinal cord injury (SCI) leads to rapid muscle atrophy due to paralysis/paresis and subsequent disuse. SS-31 is a mitochondrial-targeting peptide that has shown efficacy in protecting skeletal muscle mass and function in non-SCI models of muscle wasting. We aimed to determine if SS-31 could prevent muscle loss after SCI. Male C57BL/6 mice aged 9 weeks underwent sham surgery or 50 kdyne contusion SCI and were administered daily injections of vehicle or 5 mg/kg SS-31 for 14 d. Both SCI groups had sustained losses in body mass compared to Sham animals and ~10% reductions in gastrocnemius, plantaris and tibialis anterior muscle mass after SCI with no clear effect of SS-31. Measurements of protein synthesis in the soleus and plantaris were similar among all groups. mRNA expression of atrophy-associated proinflammatory cytokines was also similar among all groups. There was elevation in MYH7 mRNA and a statistical reduction in MYH2 mRNA expression in the SCI+SS-31 animals compared to Sham animals. There was an SCI-induced reduction in mRNA expression of the E3 ligase FBXO32 (MAFbx), but no effect of SS-31. In summary, a 50 kdyne contusion SCI was able to reduce body mass but was not associated with substantial muscle atrophy or alterations in gene expression profiles associated with muscle health and function 14 d post-injury. SS-31 was not associated with protection against SCI-related changes in body or muscle mass, protein synthesis or gene expression in hindlimb muscles.

The mitochondrial redistribution of eNOS is involved in lipopolysaccharide induced inflammasome activation during acute lung injury

Acute lung injury (ALI) is a devastating clinical syndrome with no effective therapies. Inflammasome activation has been reported to play a critical role in the initiation and progression of ALI. The molecular mechanisms involved in regulating the activation of inflammasome in ALI remains unresolved, although increases in mitochondrial derived reactive oxygen species (mito-ROS) are involved. Our previous work has shown that the mitochondrial redistribution of uncoupled eNOS impairs mitochondrial bioenergetics and increases mito-ROS generation. Thus, the focus of our study was to determine if lipopolysaccharide (LPS)-mediated inflammasome activation involves the mitochondrial redistribution of uncoupled eNOS. Our data show that the increase in mito-ROS involved in LPS-mediated inflammasome activation is associated with the disruption of mitochondrial bioenergetics in human lung microvascular endothelial cells (HLMVEC) and the mitochondrial redistribution of eNOS. These effects are dependent on RhoA-ROCK signaling and are mediated via increased phosphorylation of eNOS at Threonine (T)-495. A derivative of the mitochondrial targeted Szeto-Schiller peptide (SSP) attached to the antioxidant Tiron (T-SSP), significantly attenuated LPS-mediated mito-ROS generation and inflammasome activation in HLMVEC. Further, T-SSP attenuated mitochondrial superoxide production in a mouse model of sepsis induced ALI. This in turn significantly reduced the inflammatory response and attenuated lung injury. Thus, our findings show that the mitochondrial redistribution of uncoupled eNOS is intimately involved in the activation of the inflammatory response in ALI and implicate attenuating mito-ROS as a therapeutic strategy in humans.

Ubiquitin Ligases at the Heart of Skeletal Muscle Atrophy Control

Skeletal muscle loss is a detrimental side-effect of numerous chronic diseases that dramatically increases mortality and morbidity. The alteration of protein homeostasis is generally due to increased protein breakdown while, protein synthesis may also be down-regulated. The ubiquitin proteasome system (UPS) is a master regulator of skeletal muscle that impacts muscle contractile properties and metabolism through multiple levers like signaling pathways, contractile apparatus degradation, etc. Among the different actors of the UPS, the E3 ubiquitin ligases specifically target key proteins for either degradation or activity modulation, thus controlling both pro-anabolic or pro-catabolic factors. The atrogenes MuRF1/TRIM63 and MAFbx/Atrogin-1 encode for key E3 ligases that target contractile proteins and key actors of protein synthesis respectively. However, several other E3 ligases are involved upstream in the atrophy program, from signal transduction control to modulation of energy balance. Controlling E3 ligases activity is thus a tempting approach for preserving muscle mass. While indirect modulation of E3 ligases may prove beneficial in some situations of muscle atrophy, some drugs directly inhibiting their activity have started to appear. This review summarizes the main signaling pathways involved in muscle atrophy and the E3 ligases implicated, but also the molecules potentially usable for future therapies.

Master Regulators of Muscle Atrophy: Role of Costamere Components

The loss of muscle mass and force characterizes muscle atrophy in several different conditions, which share the expression of atrogenes and the activation of their transcriptional regulators. However, attempts to antagonize muscle atrophy development in different experimental contexts by targeting contributors to the atrogene pathway showed partial effects in most cases. Other master regulators might independently contribute to muscle atrophy, as suggested by our recent evidence about the co-requirement of the muscle-specific chaperone protein melusin to inhibit unloading muscle atrophy development. Furthermore, melusin and other muscle mass regulators, such as nNOS, belong to costameres, the macromolecular complexes that connect sarcolemma to myofibrils and to the extracellular matrix, in correspondence with specific sarcomeric sites. Costameres sense a mechanical load and transduce it both as lateral force and biochemical signals. Recent evidence further broadens this classic view, by revealing the crucial participation of costameres in a sarcolemmal “signaling hub” integrating mechanical and humoral stimuli, where mechanical signals are coupled with insulin and/or insulin-like growth factor stimulation to regulate muscle mass. Therefore, this review aims to enucleate available evidence concerning the early involvement of costamere components and additional putative master regulators in the development of major types of muscle atrophy.

Disturbances in Calcium Homeostasis Promotes Skeletal Muscle Atrophy: Lessons From Ventilator-Induced Diaphragm Wasting

Mechanical ventilation (MV) is often a life-saving intervention for patients in respiratory failure. Unfortunately, a common and undesired consequence of prolonged MV is the development of diaphragmatic atrophy and contractile dysfunction. This MV-induced diaphragmatic weakness is commonly labeled “ventilator-induced diaphragm dysfunction” (VIDD). VIDD is an important clinical problem because diaphragmatic weakness is a major risk factor for the failure to wean patients from MV; this inability to remove patients from ventilator support results in prolonged hospitalization and increased morbidity and mortality. Although several processes contribute to the development of VIDD, it is clear that oxidative stress leading to the rapid activation of proteases is a primary contributor. While all major proteolytic systems likely contribute to VIDD, emerging evidence reveals that activation of the calcium-activated protease calpain plays a required role. This review highlights the signaling pathways leading to VIDD with a focus on the cellular events that promote increased cytosolic calcium levels and the subsequent activation of calpain within diaphragm muscle fibers. In particular, we discuss the emerging evidence that increased mitochondrial production of reactive oxygen species promotes oxidation of the ryanodine receptor/calcium release channel, resulting in calcium release from the sarcoplasmic reticulum, accelerated proteolysis, and VIDD. We conclude with a discussion of important and unanswered questions associated with disturbances in calcium homeostasis in diaphragm muscle fibers during prolonged MV.

Muscle Stem Cell-Derived Extracellular Vesicles Reverse Hydrogen Peroxide-Induced Mitochondrial Dysfunction in Mouse Myotubes

Muscle stem cells (MuSCs) hold great potential as a regenerative therapeutic but have met numerous challenges in treating systemic muscle diseases. Muscle stem cell-derived extracellular vesicles (MuSC-EVs) may overcome these limitations. We assessed the number and size distribution of extracellular vesicles (EVs) released by MuSCs ex vivo, determined the extent to which MuSC-EVs deliver molecular cargo to myotubes in vitro, and quantified MuSC-EV-mediated restoration of mitochondrial function following oxidative injury. MuSCs released an abundance of EVs in culture. MuSC-EVs delivered protein cargo into myotubes within 2 h of incubation. Fluorescent labeling of intracellular mitochondria showed co-localization of delivered protein and mitochondria. Oxidatively injured myotubes demonstrated a significant decline in maximal oxygen consumption rate and spare respiratory capacity relative to untreated myotubes. Remarkably, subsequent treatment with MuSC-EVs significantly improved maximal oxygen consumption rate and spare respiratory capacity relative to the myotubes that were damaged but received no subsequent treatment. Surprisingly, MuSC-EVs did not affect mitochondrial function in undamaged myotubes, suggesting the cargo delivered is able to repair but does not expand the existing mitochondrial network. These data demonstrate that MuSC-EVs rapidly deliver proteins into myotubes, a portion of which co-localizes with mitochondria, and reverses mitochondria dysfunction in oxidatively-damaged myotubes.

Calpains play an essential role in mechanical ventilation-induced diaphragmatic weakness and mitochondrial dysfunction

Mechanical ventilation (MV) is a life-saving intervention for many critically ill patients. Unfortunately, an unintended consequence of prolonged MV is the rapid development of diaphragmatic atrophy and contractile dysfunction, known as ventilator-induced diaphragm dysfunction (VIDD). Although the mechanism(s) responsible for VIDD are not fully understood, abundant evidence reveals that oxidative stress leading to the activation of the major proteolytic systems (i.e., autophagy, ubiquitin-proteasome, caspase, and calpain) plays a dominant role. Of the proteolytic systems involved in VIDD, calpain has received limited experimental attention due to the longstanding dogma that calpain plays a minor role in inactivity-induced muscle atrophy. Guided by preliminary experiments, we tested the hypothesis that activation of calpains play an essential role in MV-induced oxidative stress and the development of VIDD. This premise was rigorously tested by transgene overexpression of calpastatin, an endogenous inhibitor of calpains. Animals with/without transfection of the calpastatin gene in diaphragm muscle fibers were exposed to 12 h of MV. Results confirmed that overexpression of calpastatin barred MV-induced activation of calpain in diaphragm fibers. Importantly, deterrence of calpain activation protected the diaphragm against MV-induced oxidative stress, fiber atrophy, and contractile dysfunction. Moreover, prevention of calpain activation in the diaphragm forstalled MV-induced mitochondrial dysfunction and prevented MV-induced activation of caspase-3 along with the transcription of muscle specific E3 ligases. Collectively, these results support the hypothesis that calpain activation plays an essential role in the early development of VIDD. Further, these findings provide the first direct evidence that calpain plays an important function in inactivity-induced mitochondrial dysfunction and oxidative stress in skeletal muscle fibers.

•

Inhibiting calpains during mechanical ventilation protects the diaphragm.

•

Calpains play an important role in muscle atrophy and contractile dysfunction.

•

Calpain inhibition during mechanical ventilation prevents mitochondrial dysfunction.

•

Calpain-cleaved molecules may play important signaling roles.

•

Calpain activation cross-talks with other proteolytic systems.

Comparative Efficacy of Angiotensin II Type 1 Receptor Blockers Against Ventilator-Induced Diaphragm Dysfunction in Rats

Mechanical ventilation (MV) is a life‐saving intervention for many critically ill patients. Unfortunately, prolonged MV results in the rapid development of inspiratory muscle weakness due to diaphragmatic atrophy and contractile dysfunction (termed ventilator‐induced diaphragm dysfunction (VIDD)). Although VIDD is a major risk factor for problems in weaning patients from MV, a standard therapy to prevent VIDD does not exist. However, emerging evidence suggests that pharmacological blockade of angiotensin II type 1 receptors (AT1Rs) protects against VIDD. Nonetheless, the essential characteristics of AT1R blockers (ARBs) required to protect against VIDD remain unclear. To determine the traits of ARBs that are vital for protection against VIDD, we compared the efficacy of two clinically relevant ARBs, irbesartan and olmesartan; these ARBs differ in molecular structure and effects on AT1Rs. Specifically, olmesartan blocks both angiotensin II (AngII) binding and mechanical activation of AT1Rs, whereas irbesartan prevents only AngII binding to AT1Rs. Using a well‐established preclinical model of prolonged MV, we tested the hypothesis that compared with irbesartan, olmesartan provides greater protection against VIDD. Our results reveal that irbesartan does not protect against VIDD whereas olmesartan defends against both MV‐induced diaphragmatic atrophy and contractile dysfunction. These findings support the hypothesis that olmesartan is superior to irbesartan in protecting against VIDD and are consistent with the concept that blockade of mechanical activation of AT1Rs is a required property of ARBs to shield against VIDD. These important findings provide a foundation for future clinical trials to evaluate ARBs as a therapy to protect against VIDD.