Muscle-specific calpastatin overexpression prevents diaphragm weakness in cecal ligation puncture-induced sepsis

Mitoquinone mesylate (MitoQ) prevents sepsis-induced diaphragm dysfunction

Sepsis-induced diaphragm dysfunction is a major contributor to respiratory failure in mechanically ventilated patients. There are no pharmacological treatments for this syndrome, but studies suggest that diaphragm weakness is linked to mitochondrial free radical generation. We hypothesized that administration of mitoquinone mesylate (MitoQ), a mitochondrially targeted free radical scavenger, would prevent sepsis-induced diaphragm dysfunction. We compared diaphragm function in 4 groups of male mice: 1) sham-operated controls treated with saline (0.3 mL ip), 2) sham-operated treated with MitoQ (3.5 mg/kg/day given intraperitoneally in saline), 3) cecal ligation puncture (CLP) mice treated with saline, and 4) CLP mice treated with MitoQ. Forty-eight hours after surgery, we assessed diaphragm force generation, myosin heavy chain content, state 3 mitochondrial oxygen consumption (OCR), and aconitase activity. We also determined effects of MitoQ in female mice with CLP sepsis and in mice with endotoxin-induced sepsis. CLP decreased diaphragm specific force generation and MitoQ prevented these decrements (e.g. maximal force averaged 30.2 ± 1.3, 28.0 ± 1.3, 12.8 ± 1.9, and 30.0 ± 1.0 N/cm² for sham, sham + MitoQ, CLP, and CLP + MitoQ groups, respectively, P < 0.001). CLP also reduced diaphragm mitochondrial OCR and aconitase activity; MitoQ blocked both effects. Similar responses were observed in female mice and in endotoxin-induced sepsis. Moreover, delayed MitoQ treatment (by 6 h) was as effective as immediate treatment. These data indicate that MitoQ prevents sepsis-induced diaphragm dysfunction, preserving force generation. MitoQ may be a useful therapeutic agent to preserve diaphragm function in critically ill patients with sepsis.NEW & NOTEWORTHY This is the first study to show that mitoquinone mesylate (MitoQ), a mitochondrially targeted antioxidant, treats sepsis-induced skeletal muscle dysfunction. This biopharmaceutical agent is without known side effects and is currently being used by healthy individuals and in clinical trials in patients with various diseases. When taken together, our results suggest that MitoQ has the potential to be immediately translated into treatment for sepsis-induced skeletal muscle dysfunction.

Skeletal muscle-specific calpastatin overexpression mitigates muscle weakness in aging and extends life span

Calpain activation has been postulated as a potential contributor to the loss of muscle mass and function associated with both aging and disease, but limitations of previous experimental approaches have failed to completely examine this issue. We hypothesized that mice overexpressing calpastatin (CalpOX), an endogenous inhibitor of calpain, solely in skeletal muscle would show an amelioration of the aging muscle phenotype. We assessed four groups of mice (age in months): 1) young wild type (WT; 5.71 ± 0.43), 2) young CalpOX (5.6 ± 0.5), 3) old WT (25.81 ± 0.56), and 4) old CalpOX (25.91 ± 0.60) for diaphragm and limb muscle (extensor digitorum longus, EDL) force frequency relations. Aging significantly reduced diaphragm and EDL peak force in old WT mice, and decreased the force-time integral during a fatiguing protocol by 48% and 23% in aged WT diaphragm and EDL, respectively. In contrast, we found that CalpOX mice had significantly increased diaphragm and EDL peak force in old mice, similar to that observed in young mice. The impact of aging on the force-time integral during a fatiguing protocol was abolished in the diaphragm and EDL of old CalpOX animals. Surprisingly, we found that CalpOX had a significant impact on longevity, increasing median survival from 20.55 mo in WT mice to 24 mo in CalpOX mice (P = 0.0006).NEW & NOTEWORTHY This is the first study to investigate the role of calpastatin overexpression on skeletal muscle specific force in aging rodents. Muscle-specific overexpression of calpastatin, the endogenous calpain inhibitor, prevented aging-induced reductions in both EDL and diaphragm specific force and, remarkably, increased life span. These data suggest that diaphragm dysfunction in aging may be a major factor in determining longevity. Targeting the calpain/calpastatin pathway may elucidate novel therapies to combat skeletal muscle weakness in aging.

TGF-β Pathway Inhibition Protects the Diaphragm From Sepsis-Induced Wasting and Weakness in Rat

Sepsis is a frequent complication in patients in intensive care units (ICU). Diaphragm weakness, one of the most common symptoms observed, can lead to weaning problems during mechanical ventilation. Over the last couple of years, members of the transforming growth factor (TGF) β family, such as myostatin, activin A, and TGF-β1, have been reported to strongly trigger the activation of protein breakdown involved in muscle wasting. The aim of this study was to investigate the effect of TGF-β inhibitor LY364947 on the diaphragm during chronic sepsis.Rats were separated into four groups exposed to different experimental conditions: Control group, Septic group, Septic group with inhibitor from day 0 (LY D0), and Septic group with inhibitor from day 1 (LY D1). Sepsis was induced in rats by cecal ligation and puncture, and carried out for 7 days.Chronic sepsis was responsible for a decrease in body weight, food intake and diaphragm's mass. The inhibitor was able to abolish diaphragm wasting only in the LY D1 group. Similarly, LY364947 had a beneficial effect on the diaphragm contraction only for the LY D1 group. SMAD3 was over-expressed and phosphorylated within rats in the Septic group; however, this effect was reversed by LY364947. Calpain-1 and -2 as well as MAFbx were over-expressed within individuals in the Septic group. Yet, calpain-1 and MAFbx expressions were decreased by LY364947.With this work, we demonstrate for the first time that the inhibition of TGF-β pathway during chronic sepsis protects the diaphragm from wasting and weakness as early as one day post infection. This could lead to more efficient treatment and care for septic patients in ICU.

Emerging Strategies Targeting Catabolic Muscle Stress Relief

Skeletal muscle wasting represents a common trait in many conditions, including aging, cancer, heart failure, immobilization, and critical illness. Loss of muscle mass leads to impaired functional mobility and severely impedes the quality of life. At present, exercise training remains the only proven treatment for muscle atrophy, yet many patients are too ill, frail, bedridden, or neurologically impaired to perform physical exertion. The development of novel therapeutic strategies that can be applied to an in vivo context and attenuate secondary myopathies represents an unmet medical need. This review discusses recent progress in understanding the molecular pathways involved in regulating skeletal muscle wasting with a focus on pro-catabolic factors, in particular, the ubiquitin-proteasome system and its activating muscle-specific E3 ligase RING-finger protein 1 (MuRF1). Mechanistic progress has provided the opportunity to design experimental therapeutic concepts that may affect the ubiquitin-proteasome system and prevent subsequent muscle wasting, with novel advances made in regards to nutritional supplements, nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) inhibitors, myostatin antibodies, β₂ adrenergic agonists, and small-molecules interfering with MuRF1, which all emerge as a novel in vivo treatment strategies for muscle wasting.

MitoTEMPOL, a mitochondrial targeted antioxidant, prevents sepsis-induced diaphragm dysfunction

Clinical studies indicate that sepsis-induced diaphragm dysfunction is a major contributor to respiratory failure in mechanically ventilated patients. Currently there is no drug to treat this form of diaphragm weakness. Sepsis-induced muscle dysfunction is thought to be triggered by excessive mitochondrial free radical generation; we therefore hypothesized that therapies that target mitochondrial free radical production may prevent sepsis-induced diaphragm weakness. The present study determined whether MitoTEMPOL, a mitochondrially targeted free radical scavenger, could reduce sepsis-induced diaphragm dysfunction. Using an animal model of sepsis, we compared four groups of mice: 1) sham-operated controls, 2) animals with sepsis induced by cecal ligation puncture (CLP), 3) sham controls given MitoTEMPOL (10 mg·kg^-1·day^-1 ip), and 4) CLP animals given MitoTEMPOL. At 48 h after surgery, we measured diaphragm force generation, mitochondrial function, proteolytic enzyme activities, and myosin heavy chain (MHC) content. We also examined the effects of delayed administration of MitoTEMPOL (by 6 h) on CLP-induced diaphragm weakness. The effects of MitoTEMPOL on cytokine-mediated alterations on muscle cell superoxide generation and cell size in vitro were also assessed. Sepsis markedly reduced diaphragm force generation. Both immediate and delayed MitoTEMPOL administration prevented sepsis-induced diaphragm weakness. MitoTEMPOL reversed sepsis-mediated reductions in mitochondrial function, activation of proteolytic pathways, and decreases in MHC content. Cytokines increased muscle cell superoxide generation and decreased cell size, effects that were ablated by MitoTEMPOL. MitoTEMPOL and other compounds that target mitochondrial free radical generation may be useful therapies for sepsis-induced diaphragm weakness.

SS31, a mitochondrially targeted antioxidant, prevents sepsis-induced reductions in diaphragm strength and endurance

Sepsis-induced diaphragm dysfunction contributes to respiratory failure and mortality in critical illness. There are no treatments for this form of diaphragm weakness. Studies show that sepsis-induced muscle dysfunction is triggered by enhanced mitochondrial free radical generation. We tested the hypothesis that SS31, a mitochondrially targeted antioxidant, would attenuate sepsis-induced diaphragm dysfunction. Four groups of mice were studied: 1) sham-operated controls, 2) sham-operated+SS31 (10 mg·kg-1·day-1), 3) cecal ligation puncture (CLP), and 4) CLP+SS31. Forty-eight hours postoperatively, diaphragm strips with attached phrenic nerves were isolated, and the following were assessed: muscle-field-stimulated force-frequency curves, nerve-stimulated force-frequency curves, and muscle fatigue. We also measured calpain activity, 20S proteasomal activity, myosin heavy chain (MHC) levels, mitochondrial function, and aconitase activity, an index of mitochondrial superoxide generation. Sepsis markedly reduced diaphragm force generation; SS31 prevented these decrements. Diaphragm-specific force generation averaged 30.2 ± 1.4, 9.4 ± 1.8, 25.5 ± 2.3, and 27.9 ± 0.6 N/cm2 for sham, CLP, sham+SS31, and CLP+SS31 groups (P < 0.001). Similarly, with phrenic nerve stimulation, CLP depressed diaphragm force generation, effects prevented by SS31. During endurance trials, force was significantly reduced with CLP, and SS31 prevented these reductions (P < 0.001). Sepsis also increased diaphragm calpain activity, increased 20S proteasomal activity, decreased MHC levels, reduced mitochondrial function (state 3 rates and ATP generation), and reduced aconitase activity; SS31 prevented each of these sepsis-induced alterations (P ≤ 0.017 for all indices). SS31 prevents sepsis-induced diaphragm dysfunction, preserving force generation, endurance, and mitochondrial function. Compounds with similar mechanisms of action may be useful therapeutically to preserve diaphragm function in patients who are septic and critically ill.NEW & NOTEWORTHY Sepsis-induced diaphragm dysfunction is a major contributor to mortality and morbidity in patients with critical illness in intensive care units. Currently, there is no proven pharmacological treatment for this problem. This study provides the novel finding that administration of SS31, a mitochondrially targeted antioxidant, preserves diaphragm myosin heavy chain content and mitochondrial function, thereby preventing diaphragm weakness and fatigue in sepsis.

Taurine administration ablates sepsis induced diaphragm weakness

Infection induced diaphragm weakness is a major contributor to death and prolonged mechanical ventilation in critically ill patients. Infection induced muscle dysfunction is associated with activation of muscle proteolytic enzymes, and taurine is known to suppress proteolysis. We therefore postulated that taurine administration may prevent infection induced diaphragm dysfunction. The purpose of this study was to test this hypothesis using a clinically relevant animal model of infection, i.e. cecal ligation puncture induced sepsis (CLP). Studies were performed on (n = 5-7 mice/group): (a) sham operated controls, (b) animals with sepsis induced by CLP, (c) sham operated animals given taurine (75 mg/kg/d, intraperitoneally), and (d) CLP animals given taurine. At intervals after surgery animals were euthanized, diaphragm force generation measured in vitro, and diaphragm calpain, caspase and proteasomal activity determined. CLP elicited a large reduction in diaphragm specific force generation at 24 h (1-150 Hz, p < 0.001) and taurine significantly attenuated CLP induced diaphragm weakness at all stimulation frequencies (p < 0.001). CLP induced significant increases in diaphragm calpain, caspase and proteasomal activity; taurine administration prevented increases in the activity of all three pathways. In additional time course experiments, diaphragm force generation remained at control levels over 72 h in CLP animals treated with daily taurine administration, while CLP animals demonstrated severe, sustained reductions in diaphragm strength (p < 0.01 for all time points). Our results indicate that taurine administration prevents infection induced diaphragm weakness and reduces activation of three major proteolytic pathways. Because this agent is has been shown to be safe, non-toxic when administered to humans, taurine may have a role in treating infection induced diaphragm weakness. Future clinical studies will be needed to assess this possibility.

Diaphragm Weakness in the Critically Ill: Basic Mechanisms Reveal Therapeutic Opportunities

The diaphragm is the primary muscle of inspiration. Its capacity to respond to the load imposed by pulmonary disease is a major determining factor both in the onset of ventilatory failure and in the ability to successfully separate patients from ventilator support. It has recently been established that a very large proportion of critically ill patients exhibit major weakness of the diaphragm, which is associated with poor clinical outcomes. The two greatest risk factors for the development of diaphragm weakness in critical illness are the use of mechanical ventilation and the presence of sepsis. Loss of force production by the diaphragm under these conditions is caused by a combination of defective contractility and reduced diaphragm muscle mass. Importantly, many of the same molecular mechanisms are implicated in the diaphragm dysfunction associated with both mechanical ventilation and sepsis. This review outlines the primary cellular mechanisms identified thus far at the nexus of diaphragm dysfunction associated with mechanical ventilation and/or sepsis, and explores the potential for treatment or prevention of diaphragm weakness in critically ill patients through therapeutic manipulation of these final common pathway targets.

Diaphragm Dysfunction in Critical Illness

The diaphragm is the major muscle of inspiration, and its function is critical for optimal respiration. Diaphragmatic failure has long been recognized as a major contributor to death in a variety of systemic neuromuscular disorders. More recently, it is increasingly apparent that diaphragm dysfunction is present in a high percentage of critically ill patients and is associated with increased morbidity and mortality. In these patients, diaphragm weakness is thought to develop from disuse secondary to ventilator-induced diaphragm inactivity and as a consequence of the effects of systemic inflammation, including sepsis. This form of critical illness-acquired diaphragm dysfunction impairs the ability of the respiratory pump to compensate for an increased respiratory workload due to lung injury and fluid overload, leading to sustained respiratory failure and death. This review examines the presentation, causes, consequences, diagnosis, and treatment of disorders that result in acquired diaphragm dysfunction during critical illness.

Dual roles of calpain in facilitating Coxsackievirus B3 replication and prompting inflammation in acute myocarditis

Background

Viral myocarditis (VMC) treatment has long been lacking of effective methods. Our former studies indicated roles of calpain in VMC pathogenesis. This study aimed at verifying the potential of calpain in Coxsackievirus B3 (CVB3)-induced myocarditis treatment.

Methods

A transgenic mouse overexpressing the endogenous calpain inhibitor, calpastatin, was introduced in the study. VMC mouse model was established via intraperitoneal injection of CVB3 in transgenic and wild mouse respectively. Myocardial injury was assayed histologically (HE staining and pathology grading) and serologically (myocardial damage markers of CK-MB and cTnI). CVB3 replication was observed in vivo and in vitro via the capsid protein VP1 detection or virus titration. Inflammation/fibrotic factors of MPO, perforin, IFNγ, IL17, Smad3 and MMP2 were evaluated using western blot or immunohistology stain. Role of calpain in regulating fibroblast migration was studied in scratch assays.

Results

Calpastatin overexpression ameliorated myocardial injury induced by CVB3 infection significantly in transgenic mouse indicated by reduced peripheral CK-MB and cTnI levels and improved histology injury. Comparing with CVB3-infected wild type mouse, the transgenic mouse heart tissue carried lower virus load. The inflammation factors of MPO, perforin, IFNγ and IL17 were down-regulated accompanied with fibrotic agents of Smad3 and MMP2 inhibition. And calpain participated in the migration of fibroblasts in vitro, which further proves its role in regulating fibrosis.

Conclusion

Calpain plays dual roles of facilitating CVB3 replication and inflammation promotion. Calpain inhibition in CVB3-induced myocarditis showed significant treatment effect. Calpain might be a novel target for VMC treatment in clinical practices.

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Calpain is involved in virus replication in myocarditis.

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Calpain mediates inflammation infiltration in myocarditis.

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Calpain might be a candidate for viral myocarditis treatment.

Calcium-dependent phospholipase A2 modulates infection-induced diaphragm dysfunction

Calpain activation contributes to the development of infection-induced diaphragm weakness, but the mechanisms by which infections activate calpain are poorly understood. We postulated that skeletal muscle calcium-dependent phospholipase A2 (cPLA2) is activated by cytokines and has downstream effects that induce calpain activation and muscle weakness. We determined whether cPLA2 activation mediates cytokine-induced calpain activation in isolated skeletal muscle (C2C12) cells and infection-induced diaphragm weakness in mice. C2C12 cells were treated with the following: 1) vehicle; 2) cytomix (TNF-α 20 ng/ml, IL-1β 50 U/ml, IFN-γ 100 U/ml, LPS 10 μg/ml); 3) cytomix + AACOCF3, a cPLA2 inhibitor (10 μM); or 4) AACOCF3 alone. At 24 h, we assessed cell cPLA2 activity, mitochondrial superoxide generation, calpain activity, and calpastatin activity. We also determined if SS31 (10 μg/ml), a mitochondrial superoxide scavenger, reduced cytomix-mediated calpain activation. Finally, we determined if CDIBA (10 μM), a cPLA2 inhibitor, reduced diaphragm dysfunction due to cecal ligation puncture in mice. Cytomix increased C2C12 cell cPLA2 activity (P < 0.001) and superoxide generation; AACOCF3 and SS31 blocked increases in superoxide generation (P < 0.001). Cytomix also activated calpain (P < 0.001) and inactivated calpastatin (P < 0.01); both AACOCF3 and SS31 prevented these changes. Cecal ligation puncture reduced diaphragm force in mice, and CDIBA prevented this reduction (P < 0.001). cPLA2 modulates cytokine-induced calpain activation in cells and infection-induced diaphragm weakness in animals. We speculate that therapies that inhibit cPLA2 may prevent diaphragm weakness in infected, critically ill patients.

Glucocorticoids and Skeletal Muscle

Glucocorticoids are known to regulate protein metabolism in skeletal muscle, producing a catabolic effect that is opposite that of insulin. In many catabolic diseases, such as sepsis, starvation, and cancer cachexia, endogenous glucocorticoids are elevated contributing to the loss of muscle mass and function. Further, exogenous glucocorticoids are often given acutely and chronically to treat inflammatory conditions such as asthma, chronic obstructive pulmonary disease, and rheumatoid arthritis, resulting in muscle atrophy. This chapter will detail the nature of glucocorticoid-induced muscle atrophy and discuss the mechanisms thought to be responsible for the catabolic effects of glucocorticoids on muscle.

A conceptual framework: the early and late phases of skeletal muscle dysfunction in the acute respiratory distress syndrome

Patients with acute respiratory distress syndrome (ARDS) often develop severe diaphragmatic and limb skeletal muscle dysfunction. Impaired muscle function in ARDS is associated with increased mortality, increased duration of mechanical ventilation, and functional disability in survivors. In this review, we propose that muscle dysfunction in ARDS can be categorized into an early and a late phase. These early and late phases are based on the timing in relationship to lung injury and the underlying mechanisms. The early phase occurs temporally with the onset of lung injury, is driven by inflammation and disuse, and is marked predominantly by muscle atrophy from increased protein degradation. The ubiquitin-proteasome, autophagy, and calpain-caspase pathways have all been implicated in early-phase muscle dysfunction. Late-phase muscle weakness persists in many patients despite resolution of lung injury and cessation of ongoing acute inflammation-driven muscle atrophy. The clinical characteristics and mechanisms underlying late-phase muscle dysfunction do not involve the massive protein degradation and atrophy of the early phase and may reflect a failure of the musculoskeletal system to regain homeostatic balance. Owing to these underlying mechanistic differences, therapeutic interventions for treating muscle dysfunction in ARDS may differ during the early and late phases. Here, we review clinical and translational investigations of muscle dysfunction in ARDS, placing them in the conceptual framework of the early and late phases. We hypothesize that this conceptual model will aid in the design of future mechanistic and clinical investigations of the skeletal muscle system in ARDS and other critical illnesses.

Neutral sphingomyelinase 2 is required for cytokine-induced skeletal muscle calpain activation

Calpain contributes to infection-induced diaphragm dysfunction but the upstream mechanism(s) responsible for calpain activation are poorly understood. It is known, however, that cytokines activate neutral sphingomyelinase (nSMase) and nSMase has downstream effects with the potential to increase calpain activity. We tested the hypothesis that infection-induced skeletal muscle calpain activation is a consequence of nSMase activation. We administered cytomix (20 ng/ml TNF-α, 50 U/ml IL-1β, 100 U/ml IFN-γ, 10 μg/ml LPS) to C2C12 muscle cells to simulate the effects of infection in vitro and studied mice undergoing cecal ligation puncture (CLP) as an in vivo model of infection. In cell studies, we assessed sphingomyelinase activity, subcellular calcium levels, and calpain activity and determined the effects of inhibiting sphingomyelinase using chemical (GW4869) and genetic (siRNA to nSMase2 and nSMase3) techniques. We assessed diaphragm force and calpain activity and utilized GW4869 to inhibit sphingomyelinase in mice. Cytomix increased cytosolic and mitochondrial calcium levels in C2C12 cells (P < 0.001); addition of GW4869 blocked these increases (P < 0.001). Cytomix also activated calpain, increasing calpain activity (P < 0.02), and the calpain-mediated cleavage of procaspase 12 (P < 0.001). Procaspase 12 cleavage was attenuated by either GW4869 (P < 0.001), BAPTA-AM (P < 0.001), or siRNA to nSMase2 (P < 0.001) but was unaffected by siRNA to nSMase3. GW4869 prevented CLP-induced diaphragm calpain activation and diaphragm weakness in mice. These data suggest that nSMase2 activation is required for the development of infection-induced diaphragm calpain activation and muscle weakness. As a consequence, therapies that inhibit nSMase2 in patients may prevent infection-induced skeletal muscle dysfunction.