Redox regulation of diaphragm proteolysis during mechanical ventilation

Small-Molecule Inhibition of MuRF1 Prevents Early Disuse-Induced Diaphragmatic Dysfunction and Atrophy

In clinical conditions such as diaphragm paralysis or mechanical ventilation, disuse-induced diaphragmatic dysfunction (DIDD) is a condition that poses a threat to life. MuRF1 is a key E3-ligase involved in regulating skeletal muscle mass, function, and metabolism, which contributes to the onset of DIDD. We investigated if the small-molecule mediated inhibition of MuRF1 activity (MyoMed-205) protects against early DIDD after 12 h of unilateral diaphragm denervation. Wistar rats were used in this study to determine the compound's acute toxicity and optimal dosage. For potential DIDD treatment efficacy, diaphragm contractile function and fiber cross-sectional area (CSA) were evaluated. Western blotting investigated potential mechanisms underlying MyoMed-205's effects in early DIDD. Our results indicate 50 mg/kg bw MyoMed-205 as a suitable dosage to prevent early diaphragmatic contractile dysfunction and atrophy following 12 h of denervation without detectable signs of acute toxicity. Mechanistically, treatment did not affect disuse-induced oxidative stress (4-HNE) increase, whereas phosphorylation of (ser632) HDAC4 was normalized. MyoMed-205 also mitigated FoxO1 activation, inhibited MuRF2, and increased phospho (ser473) Akt protein levels. These findings may suggest that MuRF1 activity significantly contributes to early DIDD pathophysiology. Novel strategies targeting MuRF1 (e.g., MyoMed-205) have potential therapeutic applications for treating early DIDD.

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.

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.

Differential protein metabolism and regeneration in hypertrophic diaphragm and atrophic gastrocnemius muscles in hibernating Daurian ground squirrels

NEW FINDINGS

What is the central question of this study? The aim was to investigate whether diaphragm hypertrophy and gastrocnemius atrophy during hibernation of Daurian ground squirrels involve differential regulation of protein metabolism and regeneration. What is the main finding and its importance? We clarified the differences in protein metabolism and muscle regenerative potential in the diaphragm and gastrocnemius of hibernating ground squirrels, reflecting the different adaptability of muscles.

ABSTRACT

Are differences in the regulation of protein metabolism and regeneration involved in the different phenotypic adaptation mechanisms of muscle hypertrophy and atrophy in hibernators? Two fast-type muscles (diaphragm and gastrocnemius) in summer active and hibernating Daurian ground squirrels were selected to detect changes in cross-sectional area (CSA) and protein expression indicative of protein synthesis metabolism (protein expression of P-Akt, P-mTORC1, P-S6K1 and P-4E-BP1), protein degradation metabolism (MuRF1, atrogin-1, calpain-1, calpain-2, calpastatin, desmin, troponin T, Beclin1 and LC3-II) and muscle regeneration (MyoD, myogenin and myostatin). In the hibernation group compared with the summer active group, the CSA of the diaphragm muscle increased significantly by 26.1%, whereas the CSA of the gastrocnemius muscle decreased significantly by 20.4%. Our study also indicated that increased protein synthesis, decreased protein degradation and increased muscle regenerative potential contributed to diaphragm muscle hypertrophy, whereas decreased protein synthesis, increased protein degradation and decreased muscle regenerative potential contributed to gastrocnemius muscle atrophy. In conclusion, the differences in muscle regeneration and regulatory pattern of protein metabolism might contribute to the different adaptive changes observed in the diaphragm and gastrocnemius muscles of ground squirrels.

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.

SARS-CoV-2/Renin-Angiotensin System: Deciphering the Clues for a Couple with Potentially Harmful Effects on Skeletal Muscle

Severe acute respiratory syndrome coronavirus (SARS-CoV-2) has produced significant health emergencies worldwide, resulting in the declaration by the World Health Organization of the coronavirus disease 2019 (COVID-19) pandemic. Acute respiratory syndrome seems to be the most common manifestation of COVID-19. A high proportion of patients require intensive care unit admission and mechanical ventilation (MV) to survive. It has been well established that angiotensin-converting enzyme type 2 (ACE2) is the primary cellular receptor for SARS-CoV-2. ACE2 belongs to the renin–angiotensin system (RAS), composed of several peptides, such as angiotensin II (Ang II) and angiotensin (1-7) (Ang-(1-7)). Both peptides regulate muscle mass and function. It has been described that SARS-CoV-2 infection, by direct and indirect mechanisms, affects a broad range of organ systems. In the skeletal muscle, through unbalanced RAS activity, SARS-CoV-2 could induce severe consequences such as loss of muscle mass, strength, and physical function, which will delay and interfere with the recovery process of patients with COVID-19. This article discusses the relationship between RAS, SARS-CoV-2, skeletal muscle, and the potentially harmful consequences for skeletal muscle in patients currently infected with and recovering from COVID-19.

Redox Control of Proteolysis During Inactivity-Induced Skeletal Muscle Atrophy

Significance: Skeletal muscles play essential roles in key body functions including breathing, locomotion, and glucose homeostasis; therefore, maintaining healthy skeletal muscles is important. Prolonged periods of muscle inactivity (e.g., bed rest, mechanical ventilation, or limb immobilization) result in skeletal muscle atrophy and weakness. Recent Advances: Disuse skeletal muscle atrophy occurs due to both accelerated proteolysis and decreased protein synthesis with proteolysis playing a leading role in some types of inactivity-induced atrophy. Although all major proteolytic systems are involved in inactivity-induced proteolysis in skeletal muscles, growing evidence indicates that both calpain and autophagy play an important role. Regulation of proteolysis in skeletal muscle is under complex control, but it is established that activation of both calpain and autophagy is directly linked to oxidative stress. Critical Issues: In this review, we highlight the experimental evidence that supports a cause and effect link between reactive oxygen species (ROS) and activation of both calpain and autophagy in skeletal muscle fibers during prolonged inactivity. We also review the sources of oxidant production in muscle fibers during inactivity-induced atrophy, and provide a detailed discussion on how ROS activates both calpain and autophagy during disuse muscle wasting. Future Directions: Future studies are required to delineate the specific mechanisms by which ROS activates both calpain and autophagy in skeletal muscles during prolonged periods of contractile inactivity. This knowledge is essential to develop the most effective strategies to protect against disuse muscle atrophy. Antioxid. Redox Signal. 33, 559-569.

Effect of Long-Term Polytrauma on Ventilator-Induced Diaphragmatic Dysfunction in a Piglet Model

INTRODUCTION

Mechanical ventilation is known to activate oxidative stress and proteolytic pathways in the diaphragm. Trauma by inducing inflammation and activating proteolytic pathways may potentiate the effects of mechanical ventilation on the diaphragm. In a blunt chest trauma with concomitant injuries we tested the hypothesis that trauma via inflammation further activates the proteolytic pathways and worsens atrophy in the diaphragm.

MATERIAL AND METHODS

Piglets were separated into two groups and underwent 72 h of mechanical ventilation. One group received a polytrauma (PT) by unilateral femur fracture, blunt chest trauma with lung contusion, laparotomy with standardized liver incision, and a predefined hemorrhagic shock. The second mechanically ventilated group (MV) did not receive any trauma. A non-ventilated group (Con) served as control.Diaphragmatic fiber dimensions, Western Blot analyses of proteolytic pathways, and lipid peroxidation and messenger ribonucleic acid (mRNA) levels of cytokines and nuclear factor kappa b subunit p65 were measured.

RESULTS

Active Caspase-3 was significantly increased in MV (P = 0.019), and in PT (P = 0.02) compared with Con. Nuclear factor kappa b subunit p65, was upregulated in PT (P = 0.010) compared with Con. IL-6 mRNA increased significantly in PT compared with Con (P = 0.0024) but did not differ between Con and MV.

CONCLUSION

Trauma and mechanical ventilation induced proteolysis and atrophy in the diaphragm, but only polytrauma induced an inflammatory response in the diaphragm. The additional traumatic inflammatory stimulus did not increase the levels of the prementioned variables. These data underline that inflammation is not a major contributor to ventilator-induced diaphragmatic dysfunction.

TRIAL REGISTRY NUMBER

AZ 84-02.04.2014.A265 (Landesamt für Natur-, Umwelt- und Verbraucherschutz, LANUV NRW, Germany).

Skeletal muscle atrogenes: From rodent models to human pathologies

Skeletal muscle atrophy is a common side effect of most human diseases. Muscle loss is not only detrimental for the quality of life but it also dramatically impairs physiological processes of the organism and decreases the efficiency of medical treatments. While hypothesized for years, the existence of an atrophying programme common to all pathologies is still incompletely solved despite the discovery of several actors and key regulators of muscle atrophy. More than a decade ago, the discovery of a set of genes, whose expression at the mRNA levels were similarly altered in different catabolic situations, opened the way of a new concept: the presence of atrogenes, i.e. atrophy-related genes. Importantly, the atrogenes are referred as such on the basis of their mRNA content in atrophying muscles, the regulation at the protein level being sometimes more complicate to elucidate. It should be noticed that the atrogenes are markers of atrophy and that their implication as active inducers of atrophy is still an open question for most of them. While the atrogene family has grown over the years, it has mostly been incremented based on data coming from rodent models. Whether the rodent atrogenes are valid for humans still remain to be established. An "atrogene" was originally defined as a gene systematically up- or down-regulated in several catabolic situations. Even if recent works often restrict this notion to the up-regulation of a limited number of proteolytic enzymes, it is important to keep in mind the big picture view. In this review, we provide an update of the validated and potential rodent atrogenes and the metabolic pathways they belong, and based on recent work, their relevance in human physio-pathological situations. We also propose a more precise definition of the atrogenes that integrates rapid recovery when catabolic stimuli are stopped or replaced by anabolic ones.

Mitochondrial dysfunction induces muscle atrophy during prolonged inactivity: A review of the causes and effects

Prolonged skeletal muscle inactivity (e.g. limb immobilization, bed rest, mechanical ventilation, spinal cord injury, etc.) results in muscle atrophy that manifests into a decreased quality of life and in select patient populations, a higher risk of morbidity and mortality. Thus, understanding the processes that contribute to muscle atrophy during prolonged periods of muscle disuse is an important area of research. In this regard, mitochondrial dysfunction has been directly linked to the muscle wasting that occurs during extended periods of skeletal muscle inactivity. While the concept that mitochondrial dysfunction contributes to disuse muscle atrophy has been contemplated for nearly 50 years, the mechanisms connecting mitochondrial signaling events to skeletal muscle atrophy remained largely unexplained until recently. Indeed, emerging evidence reveals that mitochondrial dysfunction and the associated mitochondrial signaling events are a requirement for several forms of inactivity-induced skeletal muscle atrophy. Specifically, inactivity-induced alterations in skeletal muscle mitochondria phenotype and increased ROS emission, impaired Ca²⁺ handling, and release of mitochondria-specific proteolytic activators are established occurrences that promote fiber atrophy during prolonged periods of muscle inactivity. This review highlights the evidence that directly connects mitochondrial dysfunction and aberrant mitochondrial signaling with skeletal muscle atrophy and discusses the mechanisms linking these interconnected phenomena.

Endurance exercise protects skeletal muscle against both doxorubicin-induced and inactivity-induced muscle wasting

Repeated bouts of endurance exercise promotes numerous biochemical adaptations in skeletal muscle fibers resulting in a muscle phenotype that is protected against a variety of homeostatic challenges; these exercise-induced changes in muscle phenotype are often referred to as "exercise preconditioning." Importantly, exercise preconditioning provides protection against several threats to skeletal muscle health including cancer chemotherapy (e.g., doxorubicin) and prolonged muscle inactivity. This review summarizes our current understanding of the mechanisms responsible for exercise-induced protection of skeletal muscle fibers against both doxorubicin-induced muscle wasting and a unique form of inactivity-induced muscle atrophy (i.e., ventilator-induced diaphragm atrophy). Specifically, the first section of this article will highlight the potential mechanisms responsible for exercise-induced protection of skeletal muscle fibers against doxorubicin-induced fiber atrophy. The second segment will discuss the biochemical changes that are responsible for endurance exercise-mediated protection of diaphragm muscle against ventilator-induced diaphragm wasting. In each section, we highlight gaps in our knowledge in hopes of stimulating future research in this evolving field of investigation.

Diaphragm plasticity in aging and disease: therapies for muscle weakness go from strength to strength

The diaphragm is the main inspiratory muscle and is required to be highly active throughout the life span. The diaphragm muscle must be able to produce and sustain various behaviors that range from ventilatory to nonventilatory such as those required for airway maintenance and clearance. Throughout the life span various circumstances and conditions may affect the ability of the diaphragm muscle to generate requisite forces, and in turn the diaphragm muscle may undergo significant weakness and dysfunction. For example, hypoxic stress, critical illness, cancer cachexia, chronic obstructive pulmonary disorder, and age-related sarcopenia all represent conditions in which significant diaphragm muscle dysfunction exits. This perspective review article presents several interesting topics involving diaphragm plasticity in aging and disease that were presented at the International Union of Physiological Sciences Conference in 2017. This review seeks to maximize the broad and collective research impact on diaphragm muscle dysfunction in the search for transformative treatment approaches to improve the diaphragm muscle health during aging and disease.

Diaphragm Muscle Weakness Following Acute Sustained Hypoxic Stress in the Mouse Is Prevented by Pretreatment with N-Acetyl Cysteine

Oxygen deficit (hypoxia) is a major feature of cardiorespiratory diseases characterized by diaphragm dysfunction, yet the putative role of hypoxic stress as a driver of diaphragm dysfunction is understudied. We explored the cellular and functional consequences of sustained hypoxic stress in a mouse model. Adult male mice were exposed to 8 hours of normoxia, or hypoxia (FiO₂ = 0.10) with or without antioxidant pretreatment (N-acetyl cysteine, 200 mg/kg i.p.). Ventilation and metabolism were measured. Diaphragm muscle contractile function, myofibre size and distribution, gene expression, protein signalling cascades, and oxidative stress (TBARS) were determined. Hypoxia caused pronounced diaphragm muscle weakness, unrelated to increased respiratory muscle work. Hypoxia increased diaphragm HIF-1α protein content and activated MAPK, mTOR, Akt, and FoxO3a signalling pathways, largely favouring protein synthesis. Hypoxia increased diaphragm lipid peroxidation, indicative of oxidative stress. FoxO3 and MuRF-1 gene expression were increased. Diaphragm 20S proteasome activity and muscle fibre size and distribution were unaffected by acute hypoxia. Pretreatment with N-acetyl cysteine substantially enhanced cell survival signalling, prevented hypoxia-induced diaphragm oxidative stress, and prevented hypoxia-induced diaphragm dysfunction. Hypoxia is a potent driver of diaphragm weakness, causing myofibre dysfunction without attendant atrophy. N-acetyl cysteine protects the hypoxic diaphragm and may have application as a potential adjunctive therapy.

Nutritional Support to Counteract Muscle Atrophy

Malnutrition is an important factor contributing to muscle atrophy. Both underfeeding and obesity have negative consequences for the preservation of muscle mass and function. In addition, adequate nutrition on an exercise background is an efficacious strategy to counteract the severity of muscle loss associated with numerous clinical muscle wasting conditions. As such, significant research efforts have been dedicated to identifying optimal calorie control and the requirements of particular macro- and micronutrients in attenuating muscle atrophy. This chapter will explore current nutrition strategies with robust evidence to counteract muscle atrophy with a particular focus on protein, as well presenting evidence for other promising emergent strategies.

Diaphragm Atrophy and Weakness in the Absence of Mitochondrial Dysfunction in the Critically Ill

RATIONALE

The clinical significance of diaphragm weakness in critically ill patients is evident: it prolongs ventilator dependency and increases morbidity, duration of hospital stay, and health care costs. The mechanisms underlying diaphragm weakness are unknown, but might include mitochondrial dysfunction and oxidative stress.

OBJECTIVES

We hypothesized that weakness of diaphragm muscle fibers in critically ill patients is accompanied by impaired mitochondrial function and structure, and by increased markers of oxidative stress.

METHODS

To test these hypotheses, we studied contractile force, mitochondrial function, and mitochondrial structure in diaphragm muscle fibers. Fibers were isolated from diaphragm biopsies of 36 mechanically ventilated critically ill patients and compared with those isolated from biopsies of 27 patients with suspected early-stage lung malignancy (control subjects).

MEASUREMENTS AND MAIN RESULTS

Diaphragm muscle fibers from critically ill patients displayed significant atrophy and contractile weakness, but lacked impaired mitochondrial respiration and increased levels of oxidative stress markers. Mitochondrial energy status and morphology were not altered, despite a lower content of fusion proteins.

CONCLUSIONS

Critically ill patients have manifest diaphragm muscle fiber atrophy and weakness in the absence of mitochondrial dysfunction and oxidative stress. Thus, mitochondrial dysfunction and oxidative stress do not play a causative role in the development of atrophy and contractile weakness of the diaphragm in critically ill patients.

Anisotropic mechanosensitive pathways in the diaphragm and their implications in muscular dystrophies

The diaphragm is the "respiratory pump;" the muscle that generates pressure to allow ventilation. Diaphragm muscles play a vital function and thus are subjected to continuous mechanical loading. One of its peculiarities is the ability to generate distinct mechanical and biochemical responses depending on the direction through which the mechanical forces applied to it. Contractile forces originated from its contractile components are transmitted to other structural components of its muscle fibers and the surrounding connective tissue. The anisotropic mechanical properties of the diaphragm are translated into biochemical signals that are directionally mechanosensitive by mechanisms that appear to be unique to this muscle. Here, we reviewed the current state of knowledge on the biochemical pathways regulated by mechanical signals emphasizing their anisotropic behavior in the normal diaphragm and analyzed how they are affected in muscular dystrophies.

Astaxanthin supplementation attenuates immobilization-induced skeletal muscle fibrosis via suppression of oxidative stress

Immobilization induces skeletal muscle fibrosis characterized by increasing collagen synthesis in the perimysium and endomysium. Transforming growth factor-β1 (TGF-β1) is associated with this lesion via promoting differentiation of fibroblasts into myofibroblasts. In addition, reactive oxygen species (ROS) are shown to mediate TGF-β1-induced fibrosis in tissues. These reports suggest the importance of ROS reduction for attenuating skeletal muscle fibrosis. Astaxanthin, a powerful antioxidant, has been shown to reduce ROS production in disused muscle. Therefore, we investigated the effects of astaxanthin supplementation on muscle fibrosis under immobilization. In the present study, immobilization increased the collagen fiber area, the expression levels of TGF-β1, α-smooth muscle actin, and superoxide dismutase-1 protein and ROS production. However, these changes induced by immobilization were attenuated by astaxanthin supplementation. These results indicate the effectiveness of astaxanthin supplementation on skeletal muscle fibrosis induced by ankle joint immobilization.

Exercise: Teaching myocytes new tricks

Endurance exercise training promotes numerous cellular adaptations in both cardiac myocytes and skeletal muscle fibers. For example, exercise training fosters changes in mitochondrial function due to increased mitochondrial protein expression and accelerated mitochondrial turnover. Additionally, endurance exercise training alters the abundance of numerous cytosolic and mitochondrial proteins in both cardiac and skeletal muscle myocytes, resulting in a protective phenotype in the active fibers; this exercise-induced protection of cardiac and skeletal muscle fibers is often referred to as "exercise preconditioning." As few as 3-5 consecutive days of endurance exercise training result in a preconditioned cardiac phenotype that is sheltered against ischemia-reperfusion-induced injury. Similarly, endurance exercise training results in preconditioned skeletal muscle fibers that are resistant to a variety of stresses (e.g., heat stress, exercise-induced oxidative stress, and inactivity-induced atrophy). Many studies have probed the mechanisms responsible for exercise-induced preconditioning of cardiac and skeletal muscle fibers; these studies are important, because they provide an improved understanding of the biochemical mechanisms responsible for exercise-induced preconditioning, which has the potential to lead to innovative pharmacological therapies aimed at minimizing stress-induced injury to cardiac and skeletal muscle. This review summarizes the development of exercise-induced protection of cardiac myocytes and skeletal muscle fibers and highlights the putative mechanisms responsible for exercise-induced protection in the heart and skeletal muscles.

Diaphragm Muscle Adaptation to Sustained Hypoxia: Lessons from Animal Models with Relevance to High Altitude and Chronic Respiratory Diseases

The diaphragm is the primary inspiratory pump muscle of breathing. Notwithstanding its critical role in pulmonary ventilation, the diaphragm like other striated muscles is malleable in response to physiological and pathophysiological stressors, with potential implications for the maintenance of respiratory homeostasis. This review considers hypoxic adaptation of the diaphragm muscle, with a focus on functional, structural, and metabolic remodeling relevant to conditions such as high altitude and chronic respiratory disease. On the basis of emerging data in animal models, we posit that hypoxia is a significant driver of respiratory muscle plasticity, with evidence suggestive of both compensatory and deleterious adaptations in conditions of sustained exposure to low oxygen. Cellular strategies driving diaphragm remodeling during exposure to sustained hypoxia appear to confer hypoxic tolerance at the expense of peak force-generating capacity, a key functional parameter that correlates with patient morbidity and mortality. Changes include, but are not limited to: redox-dependent activation of hypoxia-inducible factor (HIF) and MAP kinases; time-dependent carbonylation of key metabolic and functional proteins; decreased mitochondrial respiration; activation of atrophic signaling and increased proteolysis; and altered functional performance. Diaphragm muscle weakness may be a signature effect of sustained hypoxic exposure. We discuss the putative role of reactive oxygen species as mediators of both advantageous and disadvantageous adaptations of diaphragm muscle to sustained hypoxia, and the role of antioxidants in mitigating adverse effects of chronic hypoxic stress on respiratory muscle function.

Leaky ryanodine receptors contribute to diaphragmatic weakness during mechanical ventilation

Ventilator-induced diaphragmatic dysfunction (VIDD) refers to the diaphragm muscle weakness that occurs following prolonged controlled mechanical ventilation (MV). The presence of VIDD impedes recovery from respiratory failure. However, the pathophysiological mechanisms accounting for VIDD are still not fully understood. Here, we show in human subjects and a mouse model of VIDD that MV is associated with rapid remodeling of the sarcoplasmic reticulum (SR) Ca(2+) release channel/ryanodine receptor (RyR1) in the diaphragm. The RyR1 macromolecular complex was oxidized, S-nitrosylated, Ser-2844 phosphorylated, and depleted of the stabilizing subunit calstabin1, following MV. These posttranslational modifications of RyR1 were mediated by both oxidative stress mediated by MV and stimulation of adrenergic signaling resulting from the anesthesia. We demonstrate in the murine model that such abnormal resting SR Ca(2+) leak resulted in reduced contractile function and muscle fiber atrophy for longer duration of MV. Treatment with β-adrenergic antagonists or with S107, a small molecule drug that stabilizes the RyR1-calstabin1 interaction, prevented VIDD. Diaphragmatic dysfunction is common in MV patients and is a major cause of failure to wean patients from ventilator support. This study provides the first evidence to our knowledge of RyR1 alterations as a proximal mechanism underlying VIDD (i.e., loss of function, muscle atrophy) and identifies RyR1 as a potential target for therapeutic intervention.

Renin-angiotensin system in ventilator-induced diaphragmatic dysfunction: Potential protective role of Angiotensin (1-7)

Ventilator-induced diaphragmatic dysfunction is a feared complication of mechanical ventilation that adversely affects the outcome of intensive care patients. Human and animal studies demonstrate atrophy and ultrastructural alteration of diaphragmatic muscular fibers attributable to increased oxidative stress, depression of the anabolic pathway regulated by Insulin-like growing factor 1 and increased proteolysis. The renin-angiotensin system, through its main peptide Angiotensin II, plays a major role in skeletal muscle diseases, mainly increasing oxidative stress and inducing insulin resistance, atrophy and fibrosis. Conversely, its counter-regulatory peptide Angiotensin (1-7) has a protective role in these processes. Recent data on rodent models show that renin-angiotensin system is activated after mechanical ventilation and that infusion of Angiotensin II induces diaphragmatic skeletal muscle atrophy. Given: (A) common pathways shared by ventilator-induced diaphragmatic dysfunction and skeletal muscle pathology induced by renin-angiotensin system, (B) evidences of an involvement of renin-angiotensin system in diaphragm atrophy and dysfunction, we hypothesize that renin-angiotensin system plays an important role in ventilator-induced diaphragmatic dysfunction, while Angiotensin (1-7) can have a protective effect on this pathological process. The activation of renin-angiotensin system in ventilator-induced diaphragmatic dysfunction can be demonstrated by quantification of its main components in the diaphragm of ventilated humans or animals. The infusion of Angiotensin (1-7) in an established rodent model of ventilator-induced diaphragmatic dysfunction can be used to test its potential protective role, that can be further confirmed with the infusion of Angiotensin (1-7) antagonists like A-779. Verifying this hypothesis can help in understanding the processes involved in ventilator-induced diaphragmatic dysfunction pathophysiology and open new possibilities for its prevention and treatment.

Cervical spinal cord injury exacerbates ventilator-induced diaphragm dysfunction

Cervical spinal cord injury (SCI) can dramatically impair diaphragm muscle function and often necessitates mechanical ventilation (MV) to maintain adequate pulmonary gas exchange. MV is a life-saving intervention. However, prolonged MV results in atrophy and impaired function of the diaphragm. Since cervical SCI can also trigger diaphragm atrophy, it may create preconditions that exacerbate ventilator-induced diaphragm dysfunction (VIDD). Currently, no drug therapy or clinical standard of care exists to prevent or minimize diaphragm dysfunction following SCI. Therefore, we first tested the hypothesis that initiating MV acutely after cervical SCI will exacerbate VIDD and enhance proteolytic activation in the diaphragm to a greater extent than either condition alone. Rats underwent controlled MV for 12 h following acute (∼24 h) cervical spinal hemisection injury at C2 (SCI). Diaphragm tissue was then harvested for comprehensive functional and molecular analyses. Second, we determined if antioxidant therapy could mitigate MV-induced diaphragm dysfunction after cervical SCI. In these experiments, SCI rats received antioxidant (Trolox, a vitamin E analog) or saline treatment prior to initiating MV. Our results demonstrate that compared with either condition alone, the combination of SCI and MV resulted in increased diaphragm atrophy, contractile dysfunction, and expression of atrophy-related genes, including MuRF1. Importantly, administration of the antioxidant Trolox attenuated proteolytic activation, fiber atrophy, and contractile dysfunction in the diaphragms of SCI + MV animals. These findings provide evidence that cervical SCI greatly exacerbates VIDD, but antioxidant therapy with Trolox can preserve diaphragm contractile function following acute SCI.

Inhibition of forkhead boxO-specific transcription prevents mechanical ventilation-induced diaphragm dysfunction

OBJECTIVES

Mechanical ventilation is a lifesaving measure for patients with respiratory failure. However, prolonged mechanical ventilation results in diaphragm weakness, which contributes to problems in weaning from the ventilator. Therefore, identifying the signaling pathways responsible for mechanical ventilation-induced diaphragm weakness is essential to developing effective countermeasures to combat this important problem. In this regard, the forkhead boxO family of transcription factors is activated in the diaphragm during mechanical ventilation, and forkhead boxO-specific transcription can lead to enhanced proteolysis and muscle protein breakdown. Currently, the role that forkhead boxO activation plays in the development of mechanical ventilation-induced diaphragm weakness remains unknown.

DESIGN

This study tested the hypothesis that mechanical ventilation-induced increases in forkhead boxO signaling contribute to ventilator-induced diaphragm weakness.

SETTING

University research laboratory.

SUBJECTS

Young adult female Sprague-Dawley rats.

INTERVENTIONS

Cause and effect was determined by inhibiting the activation of forkhead boxO in the rat diaphragm through the use of a dominant-negative forkhead boxO adeno-associated virus vector delivered directly to the diaphragm.

MEASUREMENTS AND MAIN RESULTS

Our results demonstrate that prolonged (12 hr) mechanical ventilation results in a significant decrease in both diaphragm muscle fiber size and diaphragm-specific force production. However, mechanically ventilated animals treated with dominant-negative forkhead boxO showed a significant attenuation of both diaphragm atrophy and contractile dysfunction. In addition, inhibiting forkhead boxO transcription attenuated the mechanical ventilation-induced activation of the ubiquitin-proteasome system, the autophagy/lysosomal system, and caspase-3.

CONCLUSIONS

Forkhead boxO is necessary for the activation of key proteolytic systems essential for mechanical ventilation-induced diaphragm atrophy and contractile dysfunction. Collectively, these results suggest that targeting forkhead boxO transcription could be a key therapeutic target to combat ventilator-induced diaphragm dysfunction.

Can antioxidants protect against disuse muscle atrophy?

Long periods of skeletal muscle inactivity (e.g. prolonged bed rest or limb immobilization) results in a loss of muscle protein and fibre atrophy. This disuse-induced muscle atrophy is due to both a decrease in protein synthesis and increased protein breakdown. Although numerous factors contribute to the regulation of the rates of protein breakdown and synthesis in skeletal muscle, it has been established that prolonged muscle inactivity results in increased radical production in the inactive muscle fibres. Further, this increase in radical production plays an important role in the regulation of redox-sensitive signalling pathways that regulate both protein synthesis and proteolysis in skeletal muscle. Indeed, it was suggested over 20 years ago that antioxidant supplementation has the potential to protect skeletal muscles against inactivity-induced fibre atrophy. Since this original proposal, experimental evidence has implied that a few compounds with antioxidant properties are capable of delaying inactivity-induced muscle atrophy. The objective of this review is to discuss the role that radicals play in the regulation of inactivity-induced skeletal muscle atrophy and to provide an analysis of the recent literature indicating that specific antioxidants have the potential to defer disuse muscle atrophy.

Chronic sustained hypoxia-induced redox remodeling causes contractile dysfunction in mouse sternohyoid muscle

Chronic sustained hypoxia (CH) induces structural and functional adaptations in respiratory muscles of animal models, however the underlying molecular mechanisms are unclear. This study explores the putative role of CH-induced redox remodeling in a translational mouse model, with a focus on the sternohyoid—a representative upper airway dilator muscle involved in the control of pharyngeal airway caliber. We hypothesized that exposure to CH induces redox disturbance in mouse sternohyoid muscle in a time-dependent manner affecting metabolic capacity and contractile performance. C57Bl6/J mice were exposed to normoxia or normobaric CH (FiO₂ = 0.1) for 1, 3, or 6 weeks. A second cohort of animals was exposed to CH for 6 weeks with and without antioxidant supplementation (tempol or N-acetyl cysteine in the drinking water). Following CH exposure, we performed 2D redox proteomics with mass spectrometry, metabolic enzyme activity assays, and cell-signaling assays. Additionally, we assessed isotonic contractile and endurance properties ex vivo. Temporal changes in protein oxidation and glycolytic enzyme activities were observed. Redox modulation of sternohyoid muscle proteins key to contraction, metabolism and cellular homeostasis was identified. There was no change in redox-sensitive proteasome activity or HIF-1α content, but CH decreased phospho-JNK content independent of antioxidant supplementation. CH was detrimental to sternohyoid force- and power-generating capacity and this was prevented by chronic antioxidant supplementation. We conclude that CH causes upper airway dilator muscle dysfunction due to redox modulation of proteins key to function and homeostasis. Such changes could serve to further disrupt respiratory homeostasis in diseases characterized by CH such as chronic obstructive pulmonary disease. Antioxidants may have potential use as an adjunctive therapy in hypoxic respiratory disease.

Lung recruitment manoeuvres do not cause haemodynamic instability or oxidative stress but improve oxygenation and lung mechanics in a newborn animal model: an observational study

BACKGROUND

Lung recruitment manoeuvres in neonates during anaesthesia are not performed routinely due to concerns about causing barotrauma, haemodynamic instability and oxidative stress.

OBJECTIVE

To assess the influence of recruitment manoeuvres and positive end-expiratory pressure (PEEP) on haemodynamics, oxidative stress, oxygenation and lung mechanics.

DESIGN

A prospective experimental study.

SETTING

Experimental Unit, La Paz University Hospital, Madrid, Spain.

ANIMALS

Eight newborn piglets (<48 h) with healthy lungs under general anaesthesia.

INTERVENTIONS

The recruitment manoeuvres in pressure-controlled ventilation (PCV) were performed along with a constant driving pressure of 15 cmH2O. After the recruitment manoeuvres, PEEP was reduced in a stepwise fashion to find the maximal dynamic compliance step (maxCDyn-PEEP). Blood oxidative stress biomarkers (lipid peroxidation products, protein carbonyls, total glutathione, oxidised glutathione, reduced glutathione and activity of glutathione peroxidase) were analysed.

MAIN OUTCOME MEASURES

Haemodynamic parameters, arterial partial pressure of oxygen (paO2), tidal volume (Vt), dynamic compliance (Cdyn) and oxidative stress biomarkers were measured.

RESULTS

The recruitment manoeuvres did not induce barotrauma. Haemodynamic instability was not detected either in the maximum pressure step (overdistension step 5) or during the entire process. No substantial differences were observed in blood oxidative stress parameters analysed as compared with their baseline values (with 0 PEEP) or the values obtained 180 min after the onset of the recruitment manoeuvres (optimal PEEP). Significant maximal values were achieved in step 14 with an increase in paO2 (32.43 ± 8.48 vs. 40.39 ± 15.66 kPa; P = 0.037), Vt (47.75 ± 13.59 vs. 73.87 ± 13.56 ml; P = 0.006) and Cdyn (2.50 ± 0.64 vs. 4.75 ± 0.88 ml cmH2O; P < 0.001). Maximal dynamic compliance step (maxCdyn-PEEP) was 2 cmH2O.

CONCLUSION

Recruitment manoeuvres in PCV with a constant driving pressure are a well tolerated open-lung strategy in a healthy-lung neonatal animal model under general anaesthesia. The recruitment manoeuvres improve oxygenation parameters and lung mechanics and do not cause barotrauma, haemodynamic instability or oxidative stress.

Mechanical ventilation triggers abnormal mitochondrial dynamics and morphology in the diaphragm

The diaphragm is a unique skeletal muscle designed to be rhythmically active throughout life, such that its sustained inactivation by the medical intervention of mechanical ventilation (MV) represents an unanticipated physiological state in evolutionary terms. Within a short period after initiating MV, the diaphragm develops muscle atrophy, damage, and diminished strength, and many of these features appear to arise from mitochondrial dysfunction. Notably, in response to metabolic perturbations, mitochondria fuse, divide, and interact with neighboring organelles to remodel their shape and functional properties-a process collectively known as mitochondrial dynamics. Using a quantitative electron microscopy approach, here we show that diaphragm contractile inactivity induced by 6 h of MV in mice leads to fragmentation of intermyofibrillar (IMF) but not subsarcolemmal (SS) mitochondria. Furthermore, physical interactions between adjacent organellar membranes were less abundant in IMF mitochondria during MV. The profusion proteins Mfn2 and OPA1 were unchanged, whereas abundance and activation status of the profission protein Drp1 were increased in the diaphragm following MV. Overall, our results suggest that mitochondrial morphological abnormalities characterized by excessive fission-fragmentation represent early events during MV, which could potentially contribute to the rapid onset of mitochondrial dysfunction, maladaptive signaling, and associated contractile dysfunction of the diaphragm.

Role of E2-Ub-conjugating enzymes during skeletal muscle atrophy

The Ubiquitin Proteasome System (UPS) is a major actor of muscle wasting during various physio-pathological situations. In the past 15 years, increasing amounts of data have depicted a picture, although incomplete, of the mechanisms implicated in myofibrillar protein degradation, from the discovery of muscle-specific E3 ligases to the identification of the signaling pathways involved. The targeting specificity of the UPS relies on the capacity of the system to first recognize and then label the proteins to be degraded with a poly-ubiquitin (Ub) chain. It is fairly assumed that the recognition of the substrate is accomplished by the numerous E3 ligases present in mammalian cells. However, most E3s do not possess any catalytic activity and E2 enzymes may be more than simple Ub-providers for E3s since they are probably important actors in the ubiquitination machinery. Surprisingly, most authors have tried to characterize E3 substrates, but the exact role of E2s in muscle protein degradation is largely unknown. A very limited number of the 35 E2s described in humans have been studied in muscle protein breakdown experiments and the vast majority of studies were only descriptive. We review here the role of E2 enzymes in skeletal muscle and the difficulties linked to their study and provide future directions for the identification of muscle E2s responsible for the ubiquitination of contractile proteins.

Inhibition of the ubiquitin-proteasome pathway does not protect against ventilator-induced accelerated proteolysis or atrophy in the diaphragm

BACKGROUND

Mechanical ventilation (MV) is a life-saving intervention in patients with acute respiratory failure. However, prolonged MV results in ventilator-induced diaphragm dysfunction (VIDD), a condition characterized by both diaphragm fiber atrophy and contractile dysfunction. Previous work has shown that calpain, caspase-3, and the ubiquitin-proteasome pathway (UPP) are all activated in the diaphragm during prolonged MV. However, although it is established that both calpain and caspase-3 are important contributors to VIDD, the role that the UPP plays in the development of VIDD remains unknown. These experiments tested the hypothesis that inhibition of the UPP will protect the diaphragm against VIDD.

METHODS

The authors tested this prediction in an established animal model of MV using a highly specific UPP inhibitor, epoxomicin, to prevent MV-induced activation of the proteasome in the diaphragm (n = 8 per group).

RESULTS

The results of this study reveal that inhibition of the UPP did not prevent ventilator-induced diaphragm muscle fiber atrophy and contractile dysfunction during 12 h of MV. Also, inhibition of the UPP does not affect MV-induced increases in calpain and caspase-3 activity in the diaphragm. Finally, administration of the proteasome inhibitor did not protect against the MV-induced increases in the expression of the E3 ligases, muscle ring finger-1 (MuRF1), and atrogin-1/MaFbx.

CONCLUSION

Collectively, these results indicate that proteasome activation does not play a required role in VIDD development during the first 12 h of MV.

EUK-134 ameliorates nNOSμ translocation and skeletal muscle fiber atrophy during short-term mechanical unloading

Reduced mechanical loading during bedrest, spaceflight, and casting, causes rapid morphological changes in skeletal muscle: fiber atrophy and reduction of slow-twitch fibers. An emerging signaling event in response to unloading is the translocation of neuronal nitric oxide synthase (nNOSμ) from the sarcolemma to the cytosol. We used EUK-134, a cell-permeable mimetic of superoxide dismutase and catalase, to test the role of redox signaling in nNOSμ translocation and muscle fiber atrophy as a result of short-term (54 h) hindlimb unloading. Fischer-344 rats were divided into ambulatory control, hindlimb-unloaded (HU), and hindlimb-unloaded + EUK-134 (HU-EUK) groups. EUK-134 mitigated the unloading-induced phenotype, including muscle fiber atrophy and muscle fiber-type shift from slow to fast. nNOSμ immunolocalization at the sarcolemma of the soleus was reduced with HU, while nNOSμ protein content in the cytosol increased with unloading. Translocation of nNOS from the sarcolemma to cytosol was virtually abolished by EUK-134. EUK-134 also mitigated dephosphorylation at Thr-32 of FoxO3a during HU. Hindlimb unloading elevated oxidative stress (4-hydroxynonenal) and increased sarcolemmal localization of Nox2 subunits gp91phox (Nox2) and p47phox, effects normalized by EUK-134. Thus, our findings are consistent with the hypothesis that oxidative stress triggers nNOSμ translocation from the sarcolemma and FoxO3a dephosphorylation as an early event during mechanical unloading. Thus, redox signaling may serve as a biological switch for nNOS to initiate morphological changes in skeletal muscle fibers.

Ventilator-induced diaphragm dysfunction: cause and effect

Mechanical ventilation (MV) is used clinically to maintain gas exchange in patients that require assistance in maintaining adequate alveolar ventilation. Common indications for MV include respiratory failure, heart failure, drug overdose, and surgery. Although MV can be a life-saving intervention for patients suffering from respiratory failure, prolonged MV can promote diaphragmatic atrophy and contractile dysfunction, which is referred to as ventilator-induced diaphragm dysfunction (VIDD). This is significant because VIDD is thought to contribute to problems in weaning patients from the ventilator. Extended time on the ventilator increases health care costs and greatly increases patient morbidity and mortality. Research reveals that only 18–24 h of MV is sufficient to develop VIDD in both laboratory animals and humans. Studies using animal models reveal that MV-induced diaphragmatic atrophy occurs due to increased diaphragmatic protein breakdown and decreased protein synthesis. Recent investigations have identified calpain, caspase-3, autophagy, and the ubiquitin-proteasome system as key proteases that participate in MV-induced diaphragmatic proteolysis. The challenge for the future is to define the MV-induced signaling pathways that promote the loss of diaphragm protein and depress diaphragm contractility. Indeed, forthcoming studies that delineate the signaling mechanisms responsible for VIDD will provide the knowledge necessary for the development of a pharmacological approach that can prevent VIDD and reduce the incidence of weaning problems.

Serological muscle loss biomarkers: an overview of current concepts and future possibilities

BACKGROUND

The skeletal muscle mass is the largest organ in the healthy body, comprising 30-40 % of the body weight of an adult man. It confers protection from trauma, locomotion, ventilation, and it represents a "sink" in glucose metabolism and a reservoir of amino acids to other tissues such as the brain and blood cells. Naturally, loss of muscle has dire consequences for health as well as functionality. Muscle loss is a natural consequence of especially aging, inactivity, and their associated metabolic dysfunction, but it is strongly accelerated in critical illness such as organ failure, sepsis, or cancer. Whether this muscle loss is considered a primary or secondary condition, it is known that muscle loss is a symptom that predicts morbidity and mortality and one that is known to impact quality of life and independence. Therefore, monitoring of muscle mass is relevant in a number of pathologies as well as in clinical trials as measures of efficacy as well as safety.

METHODS AND RESULTS

Existing biomarkers of muscle mass or muscle loss have shown to be either too unreliable or too impractical in relation to the perceived clinical benefit to reach regular clinical research or use. We suggest serological neoepitope biomarkers as a possible technology to address some of these problems. Blood biomarkers of this kind have previously been shown to respond with high sensitivity and shorter time to minimum significant change than available biomarkers of muscle mass. We provide brief reviews of existing muscle mass or function biomarker technologies, muscle protein biology, and existing neoepitope biomarkers and proceed to present tentative recommendations on how to select and detect neoepitope biomarkers.

CONCLUSION

We suggest that serological peptide biomarkers whose tissue and pathology specificity are derived from post-translational modification of proteins in tissues of interest, presenting so-called neoepitopes, represents an exciting candidate technology to fill out an empty niche in biomarker technology.

Bortezomib partially protects the rat diaphragm from ventilator-induced diaphragm dysfunction

OBJECTIVE

Controlled mechanical ventilation leads to diaphragmatic contractile dysfunction and atrophy. Since proteolysis is enhanced in the diaphragm during controlled mechanical ventilation, we examined whether the administration of a proteasome inhibitor, bortezomib, would have a protective effect against ventilator-induced diaphragm dysfunction.

DESIGN

Randomized, controlled experiment.

SETTINGS

Basic science animal laboratory.

INTERVENTIONS

Anesthetized rats were submitted for 24 hrs to controlled mechanical ventilation while receiving 0.05 mg/kg bortezomib or saline. Control rats were acutely anesthetized.

MEASUREMENTS AND MAIN RESULTS

After 24 hrs, diaphragm force production was significantly lower in mechanically ventilated animals receiving an injection of saline compared to control animals (-36%, p<.001). Importantly, administration of bortezomib improved the diaphragmatic force compared to mechanically ventilated animals receiving an injection of saline (+15%, p<.01), but force did not return to control levels. Compared to control animals, diaphragm cross-sectional area of the type IIx/b fibers was significantly decreased by 28% in mechanically ventilated animals receiving an injection of saline (p<.01) and by 16% in mechanically ventilated animals receiving an injection of bortezomib (p<.05). Diaphragmatic calpain activity was significantly increased in mechanically ventilated animals receiving an injection of saline (+52%, p<.05) and in mechanically ventilated animals receiving an injection of bortezomib (+36%, p<.05). Caspase-3 activity was increased after controlled mechanical ventilation with saline by 55% (p<.05), while it remained similar to control animals in mechanically ventilated animals receiving an injection of bortezomib. Diaphragm 20S proteasome activity was slightly increased in both ventilated groups, and the amount of ubiquitinated proteins was significantly and similarly enhanced in mechanically ventilated animals receiving an injection of saline and mechanically ventilated animals receiving an injection of bortezomib.

CONCLUSIONS

These data show that the administration of bortezomib partially protects the diaphragm from controlled mechanical ventilation-induced diaphragm contractile dysfunction without preventing atrophy. The fact that calpain activity was still increased after bortezomib treatment may explain the persistence of atrophy. Part of bortezomib effects might have been due to its ability to inhibit caspase-3 in this model.

Mitochondrial signaling contributes to disuse muscle atrophy

It is well established that long durations of bed rest, limb immobilization, or reduced activity in respiratory muscles during mechanical ventilation results in skeletal muscle atrophy in humans and other animals. The idea that mitochondrial damage/dysfunction contributes to disuse muscle atrophy originated over 40 years ago. These early studies were largely descriptive and did not provide unequivocal evidence that mitochondria play a primary role in disuse muscle atrophy. However, recent experiments have provided direct evidence connecting mitochondrial dysfunction to muscle atrophy. Numerous studies have described changes in mitochondria shape, number, and function in skeletal muscles exposed to prolonged periods of inactivity. Furthermore, recent evidence indicates that increased mitochondrial ROS production plays a key signaling role in both immobilization-induced limb muscle atrophy and diaphragmatic atrophy occurring during prolonged mechanical ventilation. Moreover, new evidence reveals that, during denervation-induced muscle atrophy, increased mitochondrial fragmentation due to fission is a required signaling event that activates the AMPK-FoxO3 signaling axis, which induces the expression of atrophy genes, protein breakdown, and ultimately muscle atrophy. Collectively, these findings highlight the importance of future research to better understand the mitochondrial signaling mechanisms that contribute to disuse muscle atrophy and to develop novel therapeutic interventions for prevention of inactivity-induced skeletal muscle atrophy.

Deficiency of inducible nitric oxide synthase attenuates immobilization-induced skeletal muscle atrophy in mice

The present study examined the effects of inducible nitric oxide synthase (iNOS) deficiency on skeletal muscle atrophy in single leg-immobilized iNOS knockout (KO) and wild-type (WT) mice. The left leg was immobilized for 1 wk, and the right leg was used as the control. Muscle weight and contraction-stimulated glucose uptake were reduced by immobilization in WT mice, which was accompanied with increased iNOS expression in skeletal muscle. Deficiency of iNOS attenuated muscle weight loss and the reduction in contraction-stimulated glucose uptake by immobilization. Phosphorylation of Akt, mTOR, and p70S6K was reduced to a similar extent by immobilization in both WT and iNOS KO mice. Immobilization decreased FoxO1 phosphorylation and increased mRNA and protein levels of MuRF1 and atrogin-1 in WT mice, which were attenuated in iNOS KO mice. Aconitase and superoxide dismutase activities were reduced by immobilization in WT mice, and deficiency of iNOS normalized these enzyme activities. Increased nitrotyrosine and carbonylated protein levels by immobilization in WT mice were reversed in iNOS KO mice. Phosphorylation of ERK and p38 was increased by immobilization in WT mice, which was reduced in iNOS KO mice. Immobilization-induced muscle atrophy was also attenuated by an iNOS-specific inhibitor N6-(1-iminoethyl)-l-lysine, and this finding was accompanied by increased FoxO1 phosphorylation and reduced MuRF1 and atrogin-1 levels. These results suggest that deficiency of iNOS attenuates immobilization-induced skeletal muscle atrophy through reduced oxidative stress, and iNOS-induced oxidative stress may be required for immobilization-induced skeletal muscle atrophy.

Both high level pressure support ventilation and controlled mechanical ventilation induce diaphragm dysfunction and atrophy

OBJECTIVES

Previous workers have demonstrated that controlled mechanical ventilation results in diaphragm inactivity and elicits a rapid development of diaphragm weakness as a result of both contractile dysfunction and fiber atrophy. Limited data exist regarding the impact of pressure support ventilation, a commonly used mode of mechanical ventilation-that permits partial mechanical activity of the diaphragm-on diaphragm structure and function. We carried out the present study to test the hypothesis that high-level pressure support ventilation decreases the diaphragm pathology associated with CMV.

METHODS

Sprague-Dawley rats were randomly assigned to one of the following five groups:1) control (no mechanical ventilation); 2) 12 hrs of controlled mechanical ventilation (12CMV); 3) 18 hrs of controlled mechanical ventilation (18CMV); 4) 12 hrs of pressure support ventilation (12PSV); or 5) 18 hrs of pressure support ventilation (18PSV).

MEASUREMENTS AND MAIN RESULTS

We carried out the following measurements on diaphragm specimens: 4-hydroxynonenal-a marker of oxidative stress, active caspase-3 (casp-3), active calpain-1 (calp-1), fiber type cross-sectional area, and specific force (sp F). Compared with the control, both 12PSV and 18PSV promoted a significant decrement in diaphragmatic specific force production, but to a lesser degree than 12CMV and 18CMV. Furthermore, 12CMV, 18PSV, and 18CMV resulted in significant atrophy in all diaphragm fiber types as well as significant increases in a biomarker of oxidative stress (4-hydroxynonenal) and increased proteolytic activity (20S proteasome, calpain-1, and caspase-3). Furthermore, although no inspiratory effort occurs during controlled mechanical ventilation, it was observed that pressure support ventilation resulted in large decrement, approximately 96%, in inspiratory effort compared with spontaneously breathing animals.

CONCLUSIONS

High levels of prolonged pressure support ventilation promote diaphragmatic atrophy and contractile dysfunction. Furthermore, similar to controlled mechanical ventilation, pressure support ventilation-induced diaphragmatic atrophy and weakness are associated with both diaphragmatic oxidative stress and protease activation.

Oxidative stress and disuse muscle atrophy: cause or consequence?

PURPOSE OF REVIEW

This review will discuss the evidence both for and against the concept that reactive oxygen species (ROS) play an important role in the regulation of inactivity-induced skeletal muscle atrophy.

RECENT FINDINGS

It is well established that prolonged skeletal muscle inactivity causes muscle fiber atrophy and a decrease in muscle force production. This disuse-induced muscle atrophy is the consequence of a loss in muscle protein resulting from increased protein degradation and decreased protein synthesis. Recent studies suggest that oxidative stress can influence cell-signaling pathways that regulate both muscle protein breakdown and synthesis during prolonged periods of disuse. Specifically, it is feasible that increased ROS production in muscle fibers can promote increased proteolysis and also depress protein synthesis during periods of skeletal muscle inactivity.

SUMMARY

Although it is established that oxidants can participate in the regulation of protein turnover in cells, there remains debate as to whether oxidative stress is required for disuse skeletal muscle atrophy. Nonetheless, based on emerging evidence we conclude that increased ROS production in skeletal muscles significantly contributes to inactivity-induced muscle atrophy.

Ontogeny of proteolytic signaling and antioxidant capacity in fetal and neonatal diaphragm

Although upregulation of protein degradation pathways contributes to the development of muscle weakness in response to muscle injury and inflammation in the adult diaphragm, less is known about the preterm diaphragm. Muscle development during the antenatal and early postnatal periods normally results in net growth. However, the structural and functional immaturity of the preterm diaphragm may predispose it to injury and inflammation induced by adverse antenatal and postnatal exposures. Characterization of the ontogeny of diaphragm protein degradation pathways in early life is essential to recognize altered signaling pathways under pathologic conditions in preterm babies. We assessed the relative role of the major proteolytic pathways and antioxidant capacity during muscle maturation in ovine fetuses and lambs from 75 days to 200 days postconceptual age. Gene expression and protein content of calpain and caspase 3 exhibited a similar profile with advancing gestation, increasing from 75 days to 100 days/128 days and subsequently decreasing gradually toward the end of gestation. In contrast, ubiquitin conjugating and ligase genes did not change during gestation. All proteolytic genes examined (except Ubiquitin) were upregulated rapidly after delivery, with a similar developmental trend observed in calpain II protein content as well as calpain protease activity. In contrast, antioxidant gene expression demonstrated a steady increase from 75 days gestation to 24 hr after birth, followed by a significant reduction at 7 weeks of postnatal age (P ≤ 0.002). The proteolytic signaling and antioxidant capacity patterns reflect the adaptive process to metabolic change and muscle maturity with development.

Alterations in intrinsic mitochondrial function with aging are fiber type-specific and do not explain differential atrophy between muscles

To determine whether mitochondrial dysfunction is causally related to muscle atrophy with aging, we examined respiratory capacity, H(2) O(2) emission, and function of the mitochondrial permeability transition pore (mPTP) in permeabilized myofibers prepared from four rat muscles that span a range of fiber type and degree of age-related atrophy. Muscle atrophy with aging was greatest in fast-twitch gastrocnemius (Gas) muscle (-38%), intermediate in both the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (Sol) muscles (-21%), and non-existent in adductor longus (AL) muscle (+47%). In contrast, indices of mitochondrial dysfunction did not correspond to this differential degree of atrophy. Specifically, despite higher protein expression for oxidative phosphorylation (oxphos) system in fast Gas and EDL, state III respiratory capacity per myofiber wet weight was unchanged with aging, whereas the slow Sol showed proportional decreases in oxphos protein, citrate synthase activity, and state III respiration. Free radical leak (H(2) O(2) emission per O(2) flux) under state III respiration was higher with aging in the fast Gas, whereas state II free radical leak was higher in the slow AL. Only the fast muscles had impaired mPTP function with aging, with lower mitochondrial calcium retention capacity in EDL and shorter time to mPTP opening in Gas and EDL. Collectively, our results underscore that the age-related changes in muscle mitochondrial function depend largely upon fiber type and are unrelated to the severity of muscle atrophy, suggesting that intrinsic changes in mitochondrial function are unlikely to be causally involved in aging muscle atrophy.

Intrinsic apoptosis in mechanically ventilated human diaphragm: linkage to a novel Fos/FoxO1/Stat3-Bim axis

Mechanical ventilation (MV) is a life-saving measure in many critically ill patients. However, prolonged MV results in diaphragm dysfunction that contributes to the frequent difficulty in weaning patients from the ventilator. The molecular mechanisms underlying ventilator-induced diaphragm dysfunction (VIDD) remain poorly understood. We report here that MV induces myonuclear DNA fragmentation (3-fold increase; P<0.01) and selective activation of caspase 9 (P<0.05) and Bcl2-interacting mediator of cell death (Bim; 2- to 7-fold increase; P<0.05) in human diaphragm. MV also statistically significantly down-regulates mitochondrial gene expression and induces oxidative stress. In cultured muscle cells, we show that oxidative stress activates each of the catabolic pathways thought to underlie VIDD: apoptotic (P<0.05), proteasomal (P<0.05), and autophagic (P<0.01). Further, silencing Bim expression blocks (P<0.05) oxidative stress-induced apoptosis. Overlapping the gene expression profiles of MV human diaphragm and H₂O₂-treated muscle cells, we identify Fos, FoxO1, and Stat3 as regulators of Bim expression as well as of expression of the catabolic markers atrogin and LC3. We thus identify a novel Fos/FoxO1/Stat3-Bim intrinsic apoptotic pathway and establish the centrality of oxidative stress in the development of VIDD. This information may help in the design of specific drugs to prevent this condition.

Exercise protects against doxorubicin-induced oxidative stress and proteolysis in skeletal muscle

Doxorubicin (Dox) is a potent antitumor agent used in cancer treatment. Unfortunately, Dox is myotoxic and results in significant reductions in skeletal muscle mass and function. Complete knowledge of the mechanism(s) by which Dox induces toxicity in skeletal muscle is incomplete, but it is established that Dox-induced toxicity is associated with increased generation of reactive oxygen species and oxidative damage within muscle fibers. Since muscular exercise promotes the expression of numerous cytoprotective proteins (e.g., antioxidant enzymes, heat shock protein 72), we hypothesized that muscular exercise will attenuate Dox-induced damage in exercise-trained muscle fibers. To test this postulate, Sprague-Dawley rats were randomly assigned to the following groups: sedentary, exercise, sedentary with Dox, or exercise with Dox. Our results show increased oxidative stress and activation of cellular proteases (calpain and caspase-3) in skeletal muscle of animals treated with Dox. Importantly, our findings reveal that exercise can prevent the Dox-induced oxidative damage and protease activation in the trained muscle. This exercise-induced protection against Dox-induced toxicity may be due, at least in part, to an exercise-induced increase in muscle levels of antioxidant enzymes and heat shock protein 72. Together, these novel results demonstrate that muscular exercise is a useful countermeasure that can protect skeletal muscle against Dox treatment-induced oxidative stress and protease activation in skeletal muscles.

Beneficial effects of a Q-ter based nutritional mixture on functional performance, mitochondrial function, and oxidative stress in rats

BACKGROUND

Mitochondrial dysfunction and oxidative stress are central mechanisms underlying the aging process and the pathogenesis of many age-related diseases. Selected antioxidants and specific combinations of nutritional compounds could target many biochemical pathways that affect both oxidative stress and mitochondrial function and, thereby, preserve or enhance physical performance.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we evaluated the potential anti-aging benefits of a Q-ter based nutritional mixture (commercially known as Eufortyn) mainly containing the following compounds: terclatrated coenzyme Q(10) (Q-ter), creatine and a standardized ginseng extract. We found that Eufortyn supplementation significantly ameliorated the age-associated decreases in grip strength and gastrocnemius subsarcolemmal mitochondria Ca(2+) retention capacity when initiated in male Fischer344 x Brown Norway rats at 21 months, but not 29 months, of age. Moreover, the increases in muscle RNA oxidation and subsarcolemmal mitochondrial protein carbonyl levels, as well as the decline of total urine antioxidant power, which develop late in life, were mitigated by Eufortyn supplementation in rats at 29 months of age.

CONCLUSIONS/SIGNIFICANCE

These data imply that Eufortyn is efficacious in reducing oxidative damage, improving the age-related mitochondrial functional decline, and preserving physical performance when initiated in animals at early midlife (21 months). The efficacy varied, however, according to the age at which the supplementation was provided, as initiation in late middle age (29 months) was incapable of restoring grip strength and mitochondrial function. Therefore, the Eufortyn supplementation may be particularly beneficial when initiated prior to major biological and functional declines that appear to occur with advancing age.

Oxidative stress is required for mechanical ventilation-induced protease activation in the diaphragm

Prolonged mechanical ventilation (MV) results in diaphragmatic weakness due to fiber atrophy and contractile dysfunction. Recent work reveals that activation of the proteases calpain and caspase-3 is required for MV-induced diaphragmatic atrophy and contractile dysfunction. However, the mechanism(s) responsible for activation of these proteases remains unknown. To address this issue, we tested the hypothesis that oxidative stress is essential for the activation of calpain and caspase-3 in the diaphragm during MV. Cause-and-effect was established by prevention of MV-induced diaphragmatic oxidative stress using the antioxidant Trolox. Treatment of animals with Trolox prevented MV-induced protein oxidation and lipid peroxidation in the diaphragm. Importantly, the Trolox-mediated protection from MV-induced oxidative stress prevented the activation of calpain and caspase-3 in the diaphragm during MV. Furthermore, the avoidance of MV-induced oxidative stress not only averted the activation of these proteases but also rescued the diaphragm from MV-induced diaphragmatic myofiber atrophy and contractile dysfunction. Collectively, these findings support the prediction that oxidative stress is required for MV-induced activation of calpain and caspase-3 in the diaphragm and are consistent with the concept that antioxidant therapy can retard MV-induced diaphragmatic weakness.