Hindlimb unloading increases muscle content of cytosolic but not nuclear Id2 and p53 proteins in young adult and aged rats

Age-Related Dysfunction in Proteostasis and Cellular Quality Control in the Development of Sarcopenia

Sarcopenia is a debilitating skeletal muscle disease that accelerates in the last decades of life and is characterized by marked deficits in muscle strength, mass, quality, and metabolic health. The multifactorial causes of sarcopenia have proven difficult to treat and involve a complex interplay between environmental factors and intrinsic age-associated changes. It is generally accepted that sarcopenia results in a progressive loss of skeletal muscle function that exceeds the loss of mass, indicating that while loss of muscle mass is important, loss of muscle quality is the primary defect with advanced age. Furthermore, preclinical models have suggested that aged skeletal muscle exhibits defects in cellular quality control such as the degradation of damaged mitochondria. Recent evidence suggests that a dysregulation of proteostasis, an important regulator of cellular quality control, is a significant contributor to the aging-associated declines in muscle quality, function, and mass. Although skeletal muscle mammalian target of rapamycin complex 1 (mTORC1) plays a critical role in cellular control, including skeletal muscle hypertrophy, paradoxically, sustained activation of mTORC1 recapitulates several characteristics of sarcopenia. Pharmaceutical inhibition of mTORC1 as well as caloric restriction significantly improves muscle quality in aged animals, however, the mechanisms controlling cellular proteostasis are not fully known. This information is important for developing effective therapeutic strategies that mitigate or prevent sarcopenia and associated disability. This review identifies recent and historical understanding of the molecular mechanisms of proteostasis driving age-associated muscle loss and suggests potential therapeutic interventions to slow or prevent sarcopenia.

The Effects of Calcium-β-Hydroxy-β-Methylbutyrate on Aging-Associated Apoptotic Signaling and Muscle Mass and Function in Unloaded but Nonatrophied Extensor Digitorum Longus Muscles of Aged Rats

Beta-hydroxy-beta-methylbutyrate (HMB), a naturally occurring leucine metabolite, has been shown to attenuate plantar flexor muscle loss and increase myogenic stem cell activation during reloading after a period of significant muscle wasting by disuse in old rodents. However, it was less clear if HMB would alter dorsiflexor muscle response to unloading or reloading when there was no significant atrophy that was induced by unloading. In this study, we tested if calcium HMB (Ca-HMB) would improve muscle function and alter apoptotic signaling in the extensor digitorum longus (EDL) of aged animals that were unloaded but did not undergo atrophy. The EDL muscle was unloaded for 14 days by hindlimb suspension (HS) in aged (34-36 mo.) male Fisher 344 × Brown Norway rats. The rats were removed from HS and allowed normal cage ambulation for 14 days of reloading (R). Throughout the study, the rats were gavaged daily with 170 mg of Ca-HMB or water 7 days prior to HS, then throughout 14 days of HS and 14 days of recovery after removing HS. The animals' body weights were significantly reduced by ~18% after 14 days of HS and continued to decline by ~22% during R as compared to control conditions; however, despite unloading, EDL did not atrophy by HS, nor did it increase in mass after R. No changes were observed in EDL twitch contraction time, force production, fatigue resistance, fiber cross-sectional area, or markers of nuclear apoptosis (myonuclei + satellite cells) after HS or R. While HS and R increased the proapoptotic Bax protein abundance, BCL-2 abundance was also increased as was the frequency of TUNEL-positive myonuclei and satellite cells, yet muscle mass and fiber cross-sectional area did not change and Ca-HMB treatment had no effect reducing apoptotic signaling. These data indicate that (i) increased apoptotic signaling preceded muscle atrophy or occurred without significant EDL atrophy and (ii) that Ca-HMB treatment did not improve EDL signaling, muscle mass, or muscle function in aged rats, when HS and R did not impact mass or function.

Protective effects of Brazilian propolis supplementation on capillary regression in the soleus muscle of hindlimb-unloaded rats

The protective effects of Brazilian propolis on capillary regression induced by chronically neuromuscular inactivity were investigated in rat soleus muscle. Four groups of male Wistar rat were used in this study; control (CON), control plus Brazilian propolis supplementation (CON + PP), 2-week hindlimb unloading (HU), and 2-week hindlimb unloading plus Brazilian propolis supplementation (HU + PP). The rats in the CON + PP and HU + PP groups received two oral doses of 500 mg/kg Brazilian propolis daily (total daily dose 1000 mg/kg) for 2 weeks. Unloading resulted in a decrease in capillary number, luminal diameter, and capillary volume, and an increase in the expression of anti-angiogenic factors, such as p53 and TSP-1, within the soleus muscle. Brazilian propolis supplementation, however, prevented these changes in capillary structure due to unloading through the stimulation of pro-angiogenic factors and suppression of anti-angiogenic factors. These results suggest that Brazilian propolis is a potential non-drug therapeutic agent against capillary regression induced by chronic unloading.

Voluntary wheel running increases satellite cell abundance and improves recovery from disuse in gastrocnemius muscles from mice

Reloading of atrophied muscles after hindlimb suspension unloading (HSU) can induce injury and prolong recovery. Low-impact exercise, such as voluntary wheel running, has been identified as a nondamaging rehabilitation therapy in rodents, but its effects on muscle function, morphology, and satellite cell activity after HSU are unclear. This study tested the hypothesis that low-impact wheel running would increase satellite cell proliferation and improve recovery of muscle structure and function after HSU in mice. Young adult male and female C57BL/6 mice ( n = 6/group) were randomly placed into five groups. These included HSU without recovery (HSU), normal ambulatory recovery for 14 days after HSU (HSU+NoWR), and voluntary wheel running recovery for 14 days after HSU (HSU+WR). Two control groups were used: nonsuspended mouse cage controls (Control) and voluntary wheel running controls (ControlWR). Satellite cell activation was evaluated by providing mice 5-bromo-2'-deoxyuridine (BrdU) in their drinking water. As expected, HSU significantly reduced in vivo maximal force, decreased in vivo fatigability, and decreased type I and IIa myosin heavy chain (MHC) abundance in plantarflexor muscles. HSU+WR mice significantly improved plantarflexor fatigue resistance, increased type I and IIa MHC abundance, increased fiber cross-sectional area, and increased the percentage of type I and IIA muscle fibers in the gastrocnemius muscle. HSU+WR mice also had a significantly greater percentage of BrdU-positive and Pax 7-positive nuclei inside muscle fibers and a greater MyoD-to-Pax 7 protein ratio compared with HSU+NoWR mice. The mechanotransduction protein Yes-associated protein (YAP) was elevated with reloading after HSU, but HSU+WR mice had lower levels of the inactive phosphorylated YAPserine127, which may have contributed to increased satellite cell activation with reloading after HSU. These results indicate that voluntary wheel running increased YAP signaling and satellite cell activity after HSU and this was associated with improved recovery. NEW & NOTEWORTHY Although satellite cell involvement in muscle remodeling has been challenged, the data in this study suggest that voluntary wheel running increased satellite cell activity and suppressed Yes-associated protein (YAP) protein relative to no wheel running and this was associated with improved muscle recovery of force, fatigue resistance, expression of type I myosin heavy chain, and greater fiber cross-sectional area after disuse.

A systematic review of p53 regulation of oxidative stress in skeletal muscle

Background: p53 is a tumor suppressor protein involved in regulating a wide array of signaling pathways. The role of p53 in the cell is determined by the type of imposed oxidative stress, its intensity and duration. The last decade of research has unravelled a dual nature in the function of p53 in mediating the oxidative stress burden. However, this is dependent on the specific properties of the applied stress and thus requires further analysis.

Methods: A systematic review was performed following an electronic search of Pubmed, Google Scholar, and ScienceDirect databases. Articles published in the English language between January 1, 1990 and March 1, 2017 were identified and isolated based on the analysis of p53 in skeletal muscle in both animal and cell culture models.

Results: Literature was categorized according to the modality of imposed oxidative stress including exercise, diet modification, exogenous oxidizing agents, tissue manipulation, irradiation, and hypoxia. With low to moderate levels of oxidative stress, p53 is involved in activating pathways that increase time for cell repair, such as cell cycle arrest and autophagy, to enhance cell survival. However, with greater levels of stress intensity and duration, such as with irradiation, hypoxia, and oxidizing agents, the role of p53 switches to facilitate increased cellular stress levels by initiating DNA fragmentation to induce apoptosis, thereby preventing aberrant cell proliferation.

Conclusion: Current evidence confirms that p53 acts as a threshold regulator of cellular homeostasis. Therefore, within each modality, the intensity and duration are parameters of the oxidative stressor that must be analyzed to determine the role p53 plays in regulating signaling pathways to maintain cellular health and function in skeletal muscle.

Abbreviations: Acadl: acyl-CoA dehydrogenase, long chain; Acadm: acyl-CoA dehydrogenase, C-4 to C-12 straight chain; AIF: apoptosis-inducing factor; Akt: protein kinase B (PKB); AMPK: AMP-activated protein kinase; ATF-4: activating transcription factor 4; ATM: ATM serine/threonine kinase; Bax: BCL2 associated X, apoptosis regulator; Bcl-2: B cell Leukemia/Lymphoma 2 apoptosis regulator; Bhlhe40: basic helix-loop-helix family member e40; BH3: Borane; Bim: bcl-2 interacting mediator of cell death; Bok: Bcl-2 related ovarian killer; COX-IV: cytochrome c oxidase IV; cGMP: Cyclic guanosine monophosphate; c-myc: proto-oncogene protein; Cpt1b: carnitine palmitoyltransferase 1B; Dr5: death receptor 5; eNOS: endothelial nitric oxide synthase; ERK: extracellular regulated MAP kinase; Fas: Fas Cell surface death receptor; FDXR: Ferredoxin Reductase; FOXO3a: forkhead box O3; Gadd45a: growth arrest and DNA damage-inducible 45 alpha; GLS2: glutaminase 2; GLUT 1 and 4: glucose transporter 1(endothelial) and 4 (skeletal muscle); GSH: Glutathione; Hes1: hes family bHLH transcription factor 1; Hey1: hes related family bHLH transcription factor with YRPW motif 1; HIFI-α: hypoxia-inducible factor 1, α-subunit; HK2: Hexokinase 2; HSP70: Heat Shock Protein 70; H₂O₂: Hydrogen Peroxide; Id2: inhibitor of DNA-binding 2; IGF-1-BP3: Insulin-like growth factor binding protein 3; IL-1β: Interleukin 1 beta; iNOS: inducible nitric oxide synthase; IRS-1: Insulin receptor substrate 1; JNK: c-Jun N-terminal kinases; LY-83583: 6-anilino-5,8-quinolinedione; inhibitor of soluble guanylate cyclase and of cGMP production; Mdm 2/ 4: Mouse double minute 2 homolog (mouse) Mdm4 (humans); mtDNA: mitochondrial DNA; MURF1: Muscle RING-finger protein-1; MyoD: Myogenic differentiation 1; MyoG: myogenin; Nanog: Nanog homeobox; NF-kB: Nuclear factor-κB; NO: nitric oxide; NoxA: phorbol-12-myristate-13-acetate-induced protein 1 (Pmaip1); NRF-1: nuclear respiratory factor 1; Nrf2: Nuclear factor erythroid 2-related factor 2; P21: Cdkn1a cyclin-dependent kinase inhibitor 1A (P21); P38 MAPK: mitogen-activated protein kinases; p53R2: p53 inducible ribonucleotide reductase gene; P66Shc: src homology 2 domain-containing transforming protein C1; PERP: p53 apoptosis effector related to PMP-22; PGC-1α: Peroxisome proliferator-activated receptor gamma coactivator 1-alpha; PGM: phosphoglucomutase; PI3K: Phosphatidylinositol-4,5-bisphosphate 3-kinase; PKCβ: protein kinase c beta; PTEN: phosphatase and tensin homolog; PTIO: 2-phenyl-4, 4, 5, 5,-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) has been used as a nitric oxide (NO) scavenger; Puma: The p53 upregulated modulator of apoptosis; PW1: paternally expressed 3 (Peg3); RNS: Reactive nitrogen species; SIRT1: sirtuin 1; SCO2: cytochrome c oxidase assembly protein; SOD2: superoxide dismutase 2; Tfam: transcription factor A mitochondrial; TIGAR: Trp53 induced glycolysis repulatory phosphatase; TNF-a: tumor necrosis factor a; TRAF2: TNF receptor associated factor 2; TRAIL: type II transmembrane protein.

Dietary resveratrol confers apoptotic resistance to oxidative stress in myoblasts

High levels of reactive oxygen species (ROS) contribute to muscle cell death in aging and disuse. We have previously found that resveratrol can reduce oxidative stress in response to aging and hindlimb unloading in rodents in vivo, but it was not known if resveratrol would protect muscle stem cells during repair or regeneration when oxidative stress is high. To test the protective role of resveratrol on muscle stem cells directly, we treated the C2C12 mouse myoblast cell line with moderate (100 μM) or very high (1 mM) levels of H2O2 in the presence or absence of resveratrol. The p21 promoter activity declined in myoblasts in response to high ROS, and this was accompanied a greater nuclear to cytoplasmic translocation of p21 in a dose-dependent matter in myoblasts as compared to myotubes. Apoptosis, as indicated by TdT-mediated dUTP nick-end labeling, was greater in C2C12 myoblasts as compared to myotubes (P<.05) after treatment with H2O2. Caspase-9, -8 and -3 activities were elevated significantly (P<.05) in myoblasts treated with H2O2. Myoblasts were more susceptible to ROS-induced oxidative stress than myotubes. We treated C2C12 myoblasts with 50 μM of resveratrol for periods up to 48 h to determine if myoblasts could be rescued from high-ROS-induced apoptosis by resveratrol. Resveratrol reduced the apoptotic index and significantly reduced the ROS-induced caspase-9, -8 and -3 activity in myoblasts. Furthermore, Bcl-2 and the Bax/Bcl-2 ratio were partially rescued in myoblasts by resveratrol treatment. Similarly, muscle stem cells isolated from mouse skeletal muscles showed reduced Sirt1 protein abundance with H2O2 treatment, but this could be reversed by resveratrol. Reduced apoptotic susceptibility in myoblasts as compared to myotubes to ROS is regulated, at least in part, by enhanced p21 promoter activity and nuclear p21 location in myotubes. Resveratrol confers further protection against ROS by improving Sirt1 levels and increasing antioxidant production, which reduces mitochondrial associated apoptotic signaling, and cell death in myoblasts.

Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention

Identification of cost-effective interventions to maintain muscle mass, muscle strength, and physical performance during muscle wasting and aging is an important public health challenge. It requires understanding of the cellular and molecular mechanisms involved. Muscle-deconditioning processes have been deciphered by means of several experimental models, bringing together the opportunities to devise comprehensive analysis of muscle wasting. Studies have increasingly recognized the importance of fatty infiltrations or intermuscular adipose tissue for the age-mediated loss of skeletal-muscle function and emphasized that this new important factor is closely linked to inactivity. The present review aims to address three main points. We first mainly focus on available experimental models involving cell, animal, or human experiments on muscle wasting. We next point out the role of intermuscular adipose tissue in muscle wasting and aging and try to highlight new findings concerning aging and muscle-resident mesenchymal stem cells called fibro/adipogenic progenitors by linking some cellular players implicated in both FAP fate modulation and advancing age. In the last part, we review the main data on the efficiency and molecular and cellular mechanisms by which exercise, replacement hormone therapies, and β-hydroxy-β-methylbutyrate prevent muscle wasting and sarcopenia. Finally, we will discuss a potential therapeutic target of sarcopenia: glucose 6-phosphate dehydrogenase.

Therapeutic exercise attenuates neutrophilic lung injury and skeletal muscle wasting

Early mobilization of critically ill patients with the acute respiratory distress syndrome (ARDS) has emerged as a therapeutic strategy that improves patient outcomes, such as the duration of mechanical ventilation and muscle strength. Despite the apparent efficacy of early mobility programs, their use in clinical practice is limited outside of specialized centers and clinical trials. To evaluate the mechanisms underlying mobility therapy, we exercised acute lung injury (ALI) mice for 2 days after the instillation of lipopolysaccharides into their lungs. We found that a short duration of moderate intensity exercise in ALI mice attenuated muscle ring finger 1 (MuRF1)-mediated atrophy of the limb and respiratory muscles and improved limb muscle force generation. Exercise also limited the influx of neutrophils into the alveolar space through modulation of a coordinated systemic neutrophil chemokine response. Granulocyte colony-stimulating factor (G-CSF) concentrations were systemically reduced by exercise in ALI mice, and in vivo blockade of the G-CSF receptor recapitulated the lung exercise phenotype in ALI mice. Additionally, plasma G-CSF concentrations in humans with acute respiratory failure (ARF) undergoing early mobility therapy showed greater decrements over time compared to control ARF patients. Together, these data provide a mechanism whereby early mobility therapy attenuates muscle wasting and limits ongoing alveolar neutrophilia through modulation of systemic neutrophil chemokines in lung-injured mice and humans.

Green tea extract attenuates muscle loss and improves muscle function during disuse, but fails to improve muscle recovery following unloading in aged rats

In this study we tested the hypothesis that green tea extract (GTE) would improve muscle recovery after reloading following disuse. Aged (32 mo) Fischer 344 Brown Norway rats were randomly assigned to receive either 14 days of hindlimb suspension (HLS) or 14 days of HLS followed by normal ambulatory function for 14 days (recovery). Additional animals served as cage controls. The rats were given GTE (50 mg/kg body wt) or water (vehicle) by gavage 7 days before and throughout the experimental periods. Compared with vehicle treatment, GTE significantly attenuated the loss of hindlimb plantaris muscle mass (-24.8% vs. -10.7%, P < 0.05) and tetanic force (-43.7% vs. -25.9%, P <0.05) during HLS. Although GTE failed to further improve recovery of muscle function or mass compared with vehicle treatment, animals given green tea via gavage maintained the lower losses of muscle mass that were found during HLS (-25.2% vs. -16.0%, P < 0.05) and force (-45.7 vs. -34.4%, P < 0.05) after the reloading periods. In addition, compared with vehicle treatment, GTE attenuated muscle fiber cross-sectional area loss in both plantaris (-39.9% vs. -23.9%, P < 0.05) and soleus (-37.2% vs. -17.6%) muscles after HLS. This green tea-induced difference was not transient but was maintained over the reloading period for plantaris (-45.6% vs. -21.5%, P <0.05) and soleus muscle fiber cross-sectional area (-38.7% vs. -10.9%, P <0.05). GTE increased satellite cell proliferation and differentiation in plantaris and soleus muscles during recovery from HLS compared with vehicle-treated muscles and decreased oxidative stress and abundance of the Bcl-2-associated X protein (Bax), yet this did not further improve muscle recovery in reloaded muscles. These data suggest that muscle recovery following disuse in aging is complex. Although satellite cell proliferation and differentiation are critical for muscle repair to occur, green tea-induced changes in satellite cell number is by itself insufficient to improve muscle recovery following a period of atrophy in old rats.

Current understanding of sarcopenia: possible candidates modulating muscle mass

The world's elderly population is expanding rapidly, and we are now faced with the significant challenge of maintaining or improving physical activity, independence, and quality of life in the elderly. Sarcopenia, the age-related loss of skeletal muscle mass, is characterized by a deterioration of muscle quantity and quality leading to a gradual slowing of movement, a decline in strength and power, increased risk of fall-related injury, and often, frailty. Since sarcopenia is largely attributed to various molecular mediators affecting fiber size, mitochondrial homeostasis, and apoptosis, the mechanisms responsible for these deleterious changes present numerous therapeutic targets for drug discovery. Muscle loss has been linked with several proteolytic systems, including the ubuiquitin-proteasome, lysosome-autophagy, and tumor necrosis factor (TNF)-α/nuclear factor-kappaB (NF-κB) systems. Although many factors are considered to regulate age-dependent muscle loss, this gentle atrophy is not affected by factors known to enhance rapid atrophy (denervation, hindlimb suspension, etc.). In addition, defects in Akt-mammalian target of rapamycin (mTOR) and serum response factor (SRF)-dependent signaling have been found in sarcopenic muscle. Intriguingly, more recent studies indicated an apparent functional defect in autophagy- and myostatin-dependent signaling in sarcopenic muscle. In this review, we summarize the current understanding of the adaptation of many regulators in sarcopenia.

Resveratrol affects the expression of glutamate cysteine ligase in the kidneys of aged rats

The aim of the present study was to evaluate the effect that a dietary intake of resveratrol (RSV) had on the expression of glutamate cysteine ligase (GCL) in the kidneys of aged rats. Young, middle-aged and aged rats were each randomly divided into two groups. The control groups were fed a controlled diet and the experimental groups received a controlled diet supplemented with RSV. GCL activity levels in the kidneys were determined. Protein content and relative gene expression levels of the two subunits of GCL were evaluated by western blot analysis and quantitative polymerase chain reaction, respectively. GCL activity levels significantly increased in the kidneys of aged rats fed the RSV-supplemented diet. In addition, RSV markedly increased the protein content and relative mRNA expression levels of the GCL subunits in the kidneys of aged rats. These observations have important implications for the development of therapeutic agents for the kidneys that may enable the elderly population to combat oxidative stress.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscle: implications for health and disease

Mitochondria have paradoxical functions within cells. Essential providers of energy for cellular survival, they are also harbingers of cell death (apoptosis). Mitochondria exhibit remarkable dynamics, undergoing fission, fusion, and reticular expansion. Both nuclear and mitochondrial DNA (mtDNA) encode vital sets of proteins which, when incorporated into the inner mitochondrial membrane, provide electron transport capacity for ATP production, and when mutated lead to a broad spectrum of diseases. Acute exercise can activate a set of signaling cascades in skeletal muscle, leading to the activation of the gene expression pathway, from transcription, to post-translational modifications. Research has begun to unravel the important signals and their protein targets that trigger the onset of mitochondrial adaptations to exercise. Exercise training leads to an accumulation of nuclear- and mtDNA-encoded proteins that assemble into functional complexes devoted to mitochondrial respiration, reactive oxygen species (ROS) production, the import of proteins and metabolites, or apoptosis. This process of biogenesis has important consequences for metabolic health, the oxidative capacity of muscle, and whole body fitness. In contrast, the chronic muscle disuse that accompanies aging or muscle wasting diseases provokes a decline in mitochondrial content and function, which elicits excessive ROS formation and apoptotic signaling. Research continues to seek the molecular underpinnings of how regular exercise can be used to attenuate these decrements in organelle function, maintain skeletal muscle health, and improve quality of life.

Epigallocatechin-3-gallate improves plantaris muscle recovery after disuse in aged rats

Aging exacerbates muscle loss and slows the recovery of muscle mass and function after disuse. In this study we investigated the potential that epigallocatechin-3-gallate (EGCg), an abundant catechin in green tea, would reduce signaling for apoptosis and promote skeletal muscle recovery in the fast plantaris muscle and the slow soleus muscle after hindlimb suspension (HLS) in senescent animals. Fischer 344 × Brown Norway inbred rats (age 34 months) received either EGCg (50 mg/kg body weight), or water daily by gavage. One group of animals received HLS for 14 days and a second group of rats received 14 days of HLS, then the HLS was removed and they recovered from this forced disuse for 2 weeks. Animals that received EGCg over the HLS followed by 14 days of recovery, had a 14% greater plantaris muscle weight (p<0.05) as compared to the animals treated with the vehicle over this same period. Plantaris fiber area was greater after recovery in EGCg (2715.2±113.8 μm(2)) vs. vehicle treated animals (1953.0±41.9 μm(2)). In addition, activation of myogenic progenitor cells was improved with EGCg over vehicle treatment (7.5% vs. 6.2%) in the recovery animals. Compared to vehicle treatment, the apoptotic index was lower (0.24% vs. 0.52%), and the abundance of pro-apoptotic proteins Bax (-22%), and FADD (-77%) was lower in EGCg treated plantaris muscles after recovery. While EGCg did not prevent unloading-induced atrophy, it improved muscle recovery after the atrophic stimulus in fast plantaris muscles. However, this effect was muscle specific because EGCg had no major impact in reversing HLS-induced atrophy in the slow soleus muscle of old rats.

Microgravity induces pelvic bone loss through osteoclastic activity, osteocytic osteolysis, and osteoblastic cell cycle inhibition by CDKN1a/p21

Bone is a dynamically remodeled tissue that requires gravity-mediated mechanical stimulation for maintenance of mineral content and structure. Homeostasis in bone occurs through a balance in the activities and signaling of osteoclasts, osteoblasts, and osteocytes, as well as proliferation and differentiation of their stem cell progenitors. Microgravity and unloading are known to cause osteoclast-mediated bone resorption; however, we hypothesize that osteocytic osteolysis, and cell cycle arrest during osteogenesis may also contribute to bone loss in space. To test this possibility, we exposed 16-week-old female C57BL/6J mice (n = 8) to microgravity for 15-days on the STS-131 space shuttle mission. Analysis of the pelvis by µCT shows decreases in bone volume fraction (BV/TV) of 6.29%, and bone thickness of 11.91%. TRAP-positive osteoclast-covered trabecular bone surfaces also increased in microgravity by 170% (p = 0.004), indicating osteoclastic bone degeneration. High-resolution X-ray nanoCT studies revealed signs of lacunar osteolysis, including increases in cross-sectional area (+17%, p = 0.022), perimeter (+14%, p = 0.008), and canalicular diameter (+6%, p = 0.037). Expression of matrix metalloproteinases (MMP) 1, 3, and 10 in bone, as measured by RT-qPCR, was also up-regulated in microgravity (+12.94, +2.98 and +16.85 fold respectively, p<0.01), with MMP10 localized to osteocytes, and consistent with induction of osteocytic osteolysis. Furthermore, expression of CDKN1a/p21 in bone increased 3.31 fold (p<0.01), and was localized to osteoblasts, possibly inhibiting the cell cycle during tissue regeneration as well as conferring apoptosis resistance to these cells. Finally the apoptosis inducer Trp53 was down-regulated by −1.54 fold (p<0.01), possibly associated with the quiescent survival-promoting function of CDKN1a/p21. In conclusion, our findings identify the pelvic and femoral region of the mouse skeleton as an active site of rapid bone loss in microgravity, and indicate that this loss is not limited to osteoclastic degradation. Therefore, this study offers new evidence for microgravity-induced osteocytic osteolysis, and CDKN1a/p21-mediated osteogenic cell cycle arrest.

Troponin T nuclear localization and its role in aging skeletal muscle

Troponin T (TnT) is known to mediate the interaction between Tn complex and tropomyosin (Tm), which is essential for calcium-activated striated muscle contraction. This regulatory function takes place in the myoplasm, where TnT binds Tm. However, recent findings of troponin I and Tm nuclear translocation in Drosophila and mammalian cells imply other roles for the Tn-Tm complex. We hypothesized that TnT plays a nonclassical role through nuclear translocation. Immunoblotting with different antibodies targeting the NH2- or COOH-terminal region uncovered a pool of fast skeletal muscle TnT3 localized in the nuclear fraction of mouse skeletal muscle as either an intact or fragmented protein. Construction of TnT3-DsRed fusion proteins led to the further observation that TnT3 fragments are closely related to nucleolus and RNA polymerase activity, suggesting a role for TnT3 in regulating transcription. Functionally, overexpression of TnT3 fragments produced significant defects in nuclear shape and caused high levels of apoptosis. Interestingly, nuclear TnT3 and its fragments were highly regulated by aging, thus creating a possible link between the deleterious effects of TnT3 and sarcopenia. We propose that changes in nuclear TnT3 and its fragments cause the number of myonuclei to decrease with age, contributing to muscle damage and wasting.

The mTORC1 signaling repressors REDD1/2 are rapidly induced and activation of p70S6K1 by leucine is defective in skeletal muscle of an immobilized rat hindlimb

Limb immobilization, limb suspension, and bed rest cause substantial loss of skeletal muscle mass, a phenomenon termed disuse atrophy. To acquire new knowledge that will assist in the development of therapeutic strategies for minimizing disuse atrophy, the present study was undertaken with the aim of identifying molecular mechanisms that mediate control of protein synthesis and mechanistic target of rapamycin complex 1 (mTORC1) signaling. Male Sprague-Dawley rats were subjected to unilateral hindlimb immobilization for 1, 2, 3, or 7 days or served as nonimmobilized controls. Following an overnight fast, rats received either saline or L-leucine by oral gavage as a nutrient stimulus. Hindlimb skeletal muscles were extracted 30 min postgavage and analyzed for the rate of protein synthesis, mRNA expression, phosphorylation state of key proteins in the mTORC1 signaling pathway, and mTORC1 signaling repressors. In the basal state, mTORC1 signaling and protein synthesis were repressed within 24 h in the soleus of an immobilized compared with a nonimmobilized hindlimb. These responses were accompanied by a concomitant induction in expression of the mTORC1 repressors regulated in development and DNA damage responses (REDD) 1/2. The nutrient stimulus produced an elevation of similar magnitude in mTORC1 signaling in both the immobilized and nonimmobilized muscle. In contrast, phosphorylation of 70-kDa ribosomal protein S6 kinase 1 (p70S6K1) on Thr(229) and Thr(389) in response to the nutrient stimulus was severely blunted. Phosphorylation of Thr(229) by PDK1 is a prerequisite for phosphorylation of Thr(389) by mTORC1, suggesting that signaling through PDK1 is impaired in response to immobilization. In conclusion, the results show an immobilization-induced attenuation of mTORC1 signaling mediated by induction of REDD1/2 and defective p70S6K1 phosphorylation.

Muscle apoptotic response to denervation, disuse, and aging

Apoptosis is a tightly regulated biological process that plays an important role in coordinating cellular proliferation and differentiation. The pathological consequences of aberrant regulation of apoptosis have been widely demonstrated in carcinogenesis, neurodegenerative diseases, autoimmune diseases, viral infections, and acquired immunodeficiency syndrome. The study of apoptosis has been initiated in skeletal muscle biology. Consistent data have indicated the activation of apoptotic events in muscle atrophic conditions including neuromuscular diseases, muscle disuse, and sarcopenia. Although these results seem to link apoptosis to muscle atrophy, the causative role of apoptosis in this process still needs to be established. Further perspective studies are desired to reveal the precise mechanism and the exact physiologic role of apoptosis in muscle adaptation. This article aims to stimulate research into apoptosis in skeletal muscle. It reviews the apoptotic response of skeletal muscle to the atrophic conditions, namely, denervation, disuse, and aging, and discusses the proposed potential physiological links of apoptosis with muscle loss.

β-Hydroxy-β-methylbutyrate reduces myonuclear apoptosis during recovery from hind limb suspension-induced muscle fiber atrophy in aged rats

β-Hydroxy-β-methylbutyrate (HMB) is a leucine metabolite shown to reduce protein catabolism in disease states and promote skeletal muscle hypertrophy in response to loading exercise. In this study, we evaluated the efficacy of HMB to reduce muscle wasting and promote muscle recovery following disuse in aged animals. Fisher 344×Brown Norway rats, 34 mo of age, were randomly assigned to receive either Ca-HMB (340 mg/kg body wt) or the water vehicle by gavage (n = 32/group). The animals received either 14 days of hindlimb suspension (HS, n = 8/diet group) or 14 days of unloading followed by 14 days of reloading (R; n = 8/diet group). Nonsuspended control animals were compared with suspended animals after 14 days of HS (n = 8) or after R (n = 8). HMB treatment prevented the decline in maximal in vivo isometric force output after 2 wk of recovery from hindlimb unloading. The HMB-treated animals had significantly greater plantaris and soleus fiber cross-sectional area compared with the vehicle-treated animals. HMB decreased the amount of TUNEL-positive nuclei in reloaded plantaris muscles (5.1% vs. 1.6%, P < 0.05) and soleus muscles (3.9% vs. 1.8%, P < 0.05). Although HMB did not significantly alter Bcl-2 protein abundance compared with vehicle treatment, HMB decreased Bax protein abundance following R, by 40% and 14% (P < 0.05) in plantaris and soleus muscles, respectively. Cleaved caspase-3 was reduced by 12% and 9% (P < 0.05) in HMB-treated reloaded plantaris and soleus muscles, compared with vehicle-treated animals. HMB reduced cleaved caspase-9 by 14% and 30% (P < 0.05) in reloaded plantaris and soleus muscles, respectively, compared with vehicle-treated animals. Although, HMB was unable to prevent unloading-induced atrophy, it attenuated the decrease in fiber area in fast and slow muscles after HS and R. HMB's ability to protect against muscle loss may be due in part to putative inhibition of myonuclear apoptosis via regulation of mitochondrial-associated caspase signaling.

Habitual exercise increases resistance of lymphocytes to oxidant-induced DNA damage by upregulating expression of antioxidant and DNA repairing enzymes

The underlying mechanisms of adaptation from staying physically active are not completely revealed. This study examined the effects of 8 and 20 weeks of habitual voluntary exercise on the susceptibility of lymphocytes to oxidant-induced DNA damage, antioxidant enzyme activities in cardiac and skeletal muscles, and circulatory antioxidant profile. Forty young adult rats were randomly assigned to sedentary control and exercise groups for an experimental period of 8 or 20 weeks. Animals assigned to exercise groups were subjected to 24 h daily free access to an in-cage running wheel with circumference of 1.19 m. A magnetic digital counter was attached to the running wheel to record daily exercise distance run by the animals. Control rats were housed in cages without a running wheel, located next to the exercised animals. Body weight and food intake were recorded weekly. After the experimental periods of 8 and 20 weeks, blood, left ventricle, soleus and plantaris muscles were collected for analysis. No significant difference was found in plasma total antioxidant capacity between exercised and control animals in the 8 and 20 week groups according to our ferric reducing/antioxidant power (FRAP) analysis. However, modified FRAP for ascorbic acid (FRASC) analysis indicated that plasma ascorbic acid content was significantly increased by 46 and 34% in 8 and 20 week exercise groups, respectively, when compared with the corresponding control groups. Superoxide dismutase (SOD) activity was significantly elevated by 39% in erythrocytes of animals exercised for 8 weeks relative to control animals. In the 20 week exercise group, Glutathione peroxidase (GPx) activity in ventricle and plantaris was significantly upregulated by 477 and 290%, respectively, relative to control values. As demonstrated by comet assay, the oxidant-induced DNA damage was significantly reduced by 21 and 45% in lymphocytes of animals exercised for 8 and 20 weeks, respectively, when compared with the corresponding control lymphocytes. Our qRT-PCR analysis showed that the transcript expression of SOD2 was significantly elevated by 939% in lymphocytes of animals exercised for 8 weeks relative to control animals. Increased expressions of SOD2 (by 19%), catalase (25%), APEX nuclease (multifunctional DNA repair enzyme) 1 (APEX1; 46%), Protein kinase, DNA-activated, catalytic polypeptide (Prkdc; 9%) and O-6-methylguanine-DNA methyltransferase (Mgmt; 26%) were found in lymphocytes of animals exercised for 20 weeks relative to control rats. These results demonstrate that habitual exercise confers increased resistance of lymphocytes to oxidant-induced DNA damage, and this protective effect is possibly attributed to the regular exercise-induced elevated expression of antioxidant and DNA repairing enzymes.

NF-κB inhibition protects against tumor-induced cardiac atrophy in vivo

Cancer cachexia is a severe wasting syndrome characterized by the progressive loss of lean body mass and systemic inflammation. It occurs in approximately 80% of patients with advanced malignancy and is the cause of 20% to 30% of all cancer-related deaths. The mechanism by which striated muscle loss occurs is the tumor release of pro-inflammatory cytokines, such as IL-1, IL-6, and TNF-α. These cytokines interact with their cognate receptors on muscle cells to enhance NF-κB signaling, which then mediates muscle loss and significant cardiac dysfunction. Genetic inhibition of NF-κB signaling has demonstrated its predominant role in skeletal muscle loss. Therefore, we tested two novel drugs designed to specifically inhibit NF-κB by targeting the IκB kinase (IKK) complex: Compound A and NEMO binding domain (NBD) peptide. Using an established mouse model of cancer cachexia (C26 adenocarcinoma), we determined how these drugs affected the development of tumor-induced cardiac atrophy and function. Echocardiographic and histological analysis revealed that both Compound A and NBD inhibit cardiac NF-κB activity and prevent the development of tumor-induced systolic dysfunction and atrophy. This protection was independent of any effects of the tumor itself (Compound A) or tumor-secreted cytokines (NBD). This study identifies for the first time, to our knowledge, that drugs targeting the IKK complex are cardioprotective against cancer cachexia-induced cardiac atrophy and systolic dysfunction, suggesting therapies that may help reduce cardiac-associated morbidities found in patients with advanced malignancies.

Protective effect of caspase inhibition on compression-induced muscle damage

There are currently no effective therapies for treating pressure-induced deep tissue injury. This study tested the efficacy of pharmacological inhibition of caspase in preventing muscle damage following sustained moderate compression. Adult Sprague-Dawley rats were subjected to prolonged moderate compression. Static pressure of 100 mm Hg compression was applied to an area of 1.5 cm2 in the tibialis region of the right limb of the rats for 6 h each day for two consecutive days. The left uncompressed limb served as intra-animal control. Rats were randomized to receive either vehicle (DMSO) as control treatment (n =8) or 6 mg kg⁻¹ of caspase inhibitor (z-VAD-fmk; n =8) prior to the 6 h compression on the two consecutive days.Muscle tissues directly underneath the compression region of the compressed limb and the same region of control limb were harvested after the compression procedure.Histological examination and biochemical/molecular measurement of apoptosis and autophagy were performed. Caspase inhibition was effective in alleviating the compression-induced pathohistology of muscle. The increases in caspase-3 protease activity, TUNEL index, apoptotic DNA fragmentation and pro-apoptotic factors (Bax, p53 and EndoG) and the decreases in anti-apoptotic factors (XIAP and HSP70) observed in compressed muscle of DMSO-treated animals were not found in animals treated with caspase inhibitor. The mRNA content of autophagic factors (Beclin-1, Atg5 and Atg12) and the protein content of LC3, FoxO3 and phospho-FoxO3 that were down-regulated in compressed muscle of DMSO-treated animals were all maintained at their basal level in the caspase inhibitor treated animals. Our data provide evidence that caspase inhibition attenuates compression-induced muscle apoptosis and maintains the basal autophagy level. These findings demonstrate that pharmacological inhibition of caspase/apoptosis is effective in alleviating muscle damage as induced by prolonged compression.

Opposing responses of apoptosis and autophagy to moderate compression in skeletal muscle

AIM

The molecular mechanism that contributes to the pathogenesis of deep pressure ulcer remains to be elucidated. This study tested the hypotheses that: (1) apoptosis and autophagy are activated in compression-induced muscle pathology and (2) apoptotic and autophagic changes precede pathohistological changes in skeletal muscle in response to prolonged moderate compression.

METHODS

  Adult Sprague-Dawley rats were subjected to an experimental model of pressure-induced deep tissue injury. Static pressure of 100 mmHg was applied to an area of 1.5 cm(2) over the mid-tibialis region of right limb of rats for one single session of 6-h compression (1D) or two sessions of 6-h compression over two consecutive days with rats sacrificed one day (2D) or immediately after (2D-IM) the compression. The left uncompressed limb served as the intra-animal control. Muscle tissues underneath compression region were collected for analysis.

RESULTS

Our histological analysis indicated that pathohistological characteristics including rounding contour of myofibres and massive nuclei accumulation were apparently demonstrated in muscles of 2D and 2D-IM. In contrast, these pathohistological changes were generally not found in muscle following 1D. Apoptotic DNA fragmentation, terminal dUTP nick-end labelling index and caspase-3 protease activity were significantly elevated in compressed muscles of all groups. Caspase-9 enzymatic activity was found to be significantly increased in compressed muscles of 2D and 2D-IM whereas increase in caspase-8 activity was exclusively found in compressed muscle of 1D. According to our immunoblot analysis, FoxO3 was significantly reduced in compressed muscles of all groups whereas Beclin-1 was decreased only in 2D. LC3-I was significantly reduced in compressed muscles of all groups while LC3-II was decreased in 2D and 1D. No significant differences were found in the protein abundance of Akt and phospho-Akt in muscles among all groups.

CONCLUSION

These data demonstrate the opposing responses of apoptosis and autophagy to moderate compression in muscle. Moreover, our findings suggest that cellular changes in apoptosis and autophagy have already taken place in the very early stage in which apparent histopathology has yet to develop in the process of compression-induced muscle pathology.

Angio-adaptation in unloaded skeletal muscle: new insights into an early and muscle type-specific dynamic process

With a remarkable plasticity, skeletal muscle adapts to an altered functional demand. Muscle angio-adaptation can either involve the growth or the regression of capillaries as respectively observed in response to endurance training or muscle unloading. Whereas the molecular mechanisms that regulate exercise-induced muscle angiogenesis have been extensively studied, understanding how muscle unloading can in contrast lead to capillary regression has received very little attention. Here we have investigated the consequences of a 9 day time course hindlimb unloading on both capillarization and expression of angio-adaptive molecules in two different rat skeletal muscles. Both soleus and plantaris muscles were atrophied similarly. In contrast, our results have shown different angio-adaptive patterns between these two muscles. Capillary regression occurred only in the soleus, a slow-twitch and oxidative postural muscle. Conversely, the level of capillarization was preserved in the plantaris, a fast-twitch and glycolytic muscle. We have also measured the time course protein expression of key pro- and anti-angiogenic signals (VEGF-A, VEGF-B, VEGF-R2, TSP-1). Our results have revealed that the angio-adaptive response to unloading was muscle-type specific, and that an integrated balance between pro- and anti-angiogenic signals plays a determinant role in regulating this process. In conclusion, we have brought new evidence that measuring the ratio between pro- and anti-angiogenic signals in order to evaluate muscle angio-adaptation was a more accurate approach than analysing the expression of molecular factors taken individually.

Aging impairs the expression of the catalytic subunit of glutamate cysteine ligase in soleus muscle under stress

This study investigated the mechanisms responsible for the disrupted homeostasis of reduced glutathione (GSH) in aging muscles with stress (14 days of hind-limb unloading [HU]). Adult and old rats were randomized into four groups: weight bearing and 3, 7, and 14 days of HU. Soleus muscles were harvested to investigate the activity or content of enzymes involved in GSH metabolism (utilization and synthesis). The activities of glutathione S transferase, glutathione reductase, gamma-glutamyl transpeptidase, and glutamate cysteine ligase (GCL) were determined. The protein content of the two subunits of GCL, catalytic subunit (GCLC) and modifier subunit (GCLM), were evaluated. The major results, failure to maintain the accelerated GCLC production and GCL activity, are associated with the GSH depletion in aging muscles with 14 days of HU. The results suggest that the regulation of GCL, especially the catalytic subunit, with stress may be compromised in aging muscles.

Changes in IL-15 expression and death-receptor apoptotic signaling in rat gastrocnemius muscle with aging and life-long calorie restriction

TNF-alpha-mediated apoptosis is enhanced in aged rodent muscles, suggesting that this pathway may be involved in sarcopenia. Interleukin-15 (IL-15), a muscle-derived anabolic cytokine, mitigates muscle wasting and apoptosis in cachectic rats. This effect is thought to occur through inhibition of TNF-alpha-triggered apoptosis. We investigated IL-15 signaling and the TNF-alpha-mediated pathway of apoptosis in the gastrocnemius muscle of Fischer344xBrown Norway rats across the ages of 8, 18, 29 and 37 months, in relation to life-long calorie restriction (CR, 40% calorie intake reduction). Aging caused loss of muscle mass and increased apoptotic DNA fragmentation, which were mitigated by CR. Protein levels of IL-15 and mRNA abundance of IL-15 receptor a-chain decreased in senescent ad libitum (AL) fed rats, but were maintained in CR rodents. Elevations of TNF-alpha, TNF-receptor 1, cleaved caspase-8 and -3 were observed at advanced age in AL rats. These changes were prevented or mitigated by CR. Our results indicate that aging is associated with decreased IL-15 signaling in rat gastrocnemius muscle, which may contribute to sarcopenia partly through enhanced TNF-alpha-mediated apoptosis. Preservation of IL-15 signaling by CR may therefore represent a further mechanism contributing to the anti-aging effect of this dietary intervention in skeletal muscle.

Phospho-ablated Id2 is growth suppressive and pro-apoptotic in proliferating myoblasts

Inhibitor of differentiation protein-2 (Id2) is a dominant negative helix-loop-helix (HLH) protein, and a positive regulator of proliferation, in various cells. The N-terminal region of Id2 contains a consensus cdk2 phosphorylation sequence SPVR, which may be involved with the induction of apoptosis, at least in myeloid 32d.3 cells. However, the role of Id2 phosphorylation at serine 5 in skeletal muscle cells is unknown. The objective of this study was to determine if the phosphorylation of Id2 at serine 5 alters its cellular localization and its role in apoptosis in C2C12 myoblasts. Overexpression of wild type Id2 decreased MyoD protein expression, which corresponded to the increased binding of Id2 to basic HLH proteins E47 and E12. Bromodeoxyuridine incorporation was significantly decreased by the overexpression of phospho-ablated Id2 (S5A); conversely, overexpression of wild type Id2 increased cellular proliferation. The subcellular localization of Id2 and phospho-mimicking Id2 (S5D) were predominantly nuclear compared to S5A. The decreased nuclear localization of S5A corresponded to a decrease in cellular proliferation, and an increase in apoptosis. These data suggest that unphosphorylated Id2 is primarily localized in the cytosol, where it is growth suppressive and potentially pro-apoptotic. These results imply that reducing unphosphorylated Id2 may improve the pool of myoblasts available for differentiation by increasing proliferation and inhibiting apoptosis.

Necdin is expressed in cachectic skeletal muscle to protect fibers from tumor-induced wasting

Skeletal muscles of subjects with advanced cancer undergo progressive wasting, referred to as cachexia. Cachexia is an important area for medical research because strategies proposed until now have yielded little benefit. We have recently identified necdin as a key player in fetal and postnatal physiological myogenesis and in muscle regeneration. Here we show that necdin is selectively expressed in muscles of cachetic mice and prove that its expression is causally linked to a protective response of the tissue against tumor-induced wasting, inhibition of myogenic differentiation and fiber regeneration. Necdin carries out this role mainly via interference with TNFalpha signaling at various levels, including regulation of expression of TNFR1 and p53, and regulation of the activity of caspase 3 and caspase 9. These data suggest that inhibition of muscle wasting using necdin is a feasible approach to treat cachexia in neoplastic patients.

Role of p53 in mitochondrial biogenesis and apoptosis in skeletal muscle

p53 is a tumor suppressor protein that also plays a role in regulating aerobic metabolism. Since skeletal muscle is a major source of whole body aerobic respiration, it is important to delineate the effects of p53 on muscle metabolism. In p53 knockout (KO) mice, we observed diminished mitochondrial content in mixed muscle and lowered peroxisome proliferator-activated receptor-γ (PPARγ) coactivator (PGC)-1α protein levels in gastrocnemius muscle. In intermyofibrillar (IMF) mitochondria, lack of p53 was associated with reduced respiration and elevated reactive oxygen species production. Permeability transition pore kinetics remained unchanged; however, IMF mitochondrial cytochrome c release was reduced and DNA fragmentation was lowered, illustrating a resistance to mitochondrially driven apoptosis in muscle of KO mice. p53-null animals displayed similar muscle strength but greater fatigability and less locomotory endurance than wild-type (WT) animals. Surprisingly, the adaptive responses in mitochondrial content to running were similar in WT and KO mice. Thus p53 may be important, but not necessary, for exercise-induced mitochondrial biogenesis. In WT animals, acute muscle contractions induced the phosphorylation of p53 in concert with increased activation of upstream kinases AMP-activated protein kinase and p38, indicating a pathway through which p53 may initiate mitochondrial biogenesis in response to contractile activity. These data illustrate a novel role for p53 in maintaining mitochondrial biogenesis, apoptosis, and performance in skeletal muscle.

Apoptotic signaling induced by H2O2-mediated oxidative stress in differentiated C2C12 myotubes

AIMS

Apoptotic signaling proteins were evaluated in postmitotic skeletal myotubes to test the hypothesis that oxidative stress induced by H(2)O(2) activates both caspase-dependent and caspase-independent apoptotic proteins in differentiated C2C12 myotubes. We hypothesized that oxidative stress would decrease anti-apoptotic protein levels in C2C12 myotubes.

MAIN METHODS

Apoptotic regulatory factors and apoptosis-associated proteins including Bcl-2, Bax, Apaf-1, XIAP, ARC, cleaved PARP, p53, p21(Cip1/Waf1), c-Myc, HSP70, CuZnSOD, and MnSOD protein content were measured by immunoblots.

KEY FINDINGS

H(2)O(2) induced apoptosis in myotubes as shown by DNA laddering and an elevation of apoptotic DNA fragmentation. Cell death ELISA showed increase in the extent of apoptotic DNA fragmentation following treatment with H(2)O(2). Treatment with 4 mM of H(2)O(2) for 24 or 96 h caused increase in Bax (56%, 227%), cytochrome c (282%, 701%), Smac/DIABLO (155%, 260%), caspase-3 protease activity (51%, 141%), and nuclear and cytosolic p53 (719%, 1581%) levels in the myotubes. As an estimate of the mitochondrial AIF release to the cytosol, AIF protein content measured in the mitochondria-free cytosolic fraction was elevated by 65% after 96 h treatment with 4 mM of H(2)O(2). AIF measured in the nuclear protein fraction increased by 74% and 352% following treatment with 4 mM of H(2)O(2) for 24 and 96 h, respectively. Bcl-2 declined in myotubes by 61% and 69% after 24 or 96 h of treatment in 4 mM H(2)O(2), respectively.

SIGNIFICANCE

These findings indicate that both caspase-dependent and caspase-independent mechanisms are involved in coordinating the activation of apoptosis induced by H(2)O(2) in differentiated myotubes.

Aging-dependent regulation of antioxidant enzymes and redox status in chronically loaded rat dorsiflexor muscles

This study compares changes in the pro-oxidant production and buffering capacity in young and aged skeletal muscle after exposure to chronic repetitive loading (RL). The dorsiflexors from one limb of young and aged rats were loaded 3 times/week for 4.5 weeks using 80 maximal stretch-shortening contractions per session. RL increased H2O2 in tibialis anterior muscles of young and aged rats and decreased the ratio of reduced/oxidized glutathione and lipid peroxidation in aged but not young adult animals. Glutathione peroxidase (GPx) activity decreased whereas catalase activity increased with RL in muscles from young and aged rats. RL increased CuZn superoxide disumutase (SOD) and Mn SOD protein concentration and CuZn SOD activity in muscles from young but not aged animals. There were no changes in protein content for GPx-1 and catalase or messenger RNA for any of the enzymes studied. These data show that aging reduces the adaptive capacity of muscles to buffer increased pro-oxidants imposed by chronic RL.

Mitochondria in skeletal muscle: adaptable rheostats of apoptotic susceptibility

Apoptosis is an essential process that plays a critical role in both tissue development and maintenance. Apoptosis has been shown to be involved in skeletal muscle atrophy resulting from chronic muscular disuse, sarcopenia, and mitochondrial myopathies. Exercise may attenuate some of the proapoptotic adaptations that occur during these conditions. This review will focus on the factors influencing mitochondrially mediated apoptosis in skeletal muscle.

Muscle disuse: adaptation of antioxidant systems is age dependent

This study investigated the age effect on antioxidant adaptation to muscle disuse. Adult and old rats were randomized into 4 groups: weight bearing (control), 3 days of hind-limb unloading (HU), 7 days of HU, and 14 days of HU. Activities of Cu-Zn superoxide dismutase (SOD), catalase, and glutathione (GSH), as well as GSH peroxidase levels were measured in the soleus. Neither disuse nor aging changed the activity of Cu-Zn SOD. The old rats had greater GSH peroxidase activity, whereas the activity of catalase had a compensatory increase with disuse, independent of age. Reduced GSH level and total glutathione (tGSH) level had age-related change with disuse. In old rats, the GSH and tGSH levels were lower with disuse, whereas the levels remained stable with disuse in adult rats. The depletion of intracellular GSH and tGSH levels of muscles from aged animals with disuse may make aged muscles more susceptible to oxidative damage.

Nuclear apoptosis contributes to sarcopenia

Apoptosis results in DNA fragmentation and, subsequently, destruction of cells containing a single nucleus. Our hypothesis is that multinucleated cells such as muscle fibers can experience apoptotic-induced loss of single nuclei (nuclear apoptosis) without destruction of the entire fiber. The loss of nuclei likely contributes to atrophy and sarcopenia. Furthermore, increased chronic activity attenuates apoptotic signaling, which may reduce sarcopenia.

Cellular adaptations in soleus muscle during recovery after hindlimb unloading

AIM

We used a model of chronic unloading followed by reloading to examine the apoptotic responses associated with soleus muscle atrophy and subsequent recovery.

METHODS

Male Wistar rats were subjected to hindlimb unloading (HU) for 2 weeks and subsequent reloading for 0, 3, 7 and 14 days. One-half of the HU-reloaded rats were administered cyclosporine A (CsA), a calcineurin (CaN) inhibitor.

RESULTS

There was fibre atrophy (73%) and a decrease in slow type I fibre/myosin heavy chain (MyHC) composition in the soleus muscle after 2 weeks of HU. Fibre size and type I MyHC composition recovered to near the age-matched control levels by recovery day 14 in non-treated, but not in CsA-treated, rats. Myonuclear number was lower and the number of apoptotic nuclei higher in 2-week HU than control rats. These values returned to control levels after 7 and 14 days of recovery, respectively, in both HU-recovery groups. After 2 weeks of HU, the levels of heat shock proteins (Hsp) 60 and 72, mitochondrial cytochrome c oxidase subunit IV (Cox IV), and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1) proteins were lower than control. The levels of all of these proteins gradually increased to or above the control levels during cage recovery in both groups.

CONCLUSION

Our results indicate that apoptotic mechanisms are involved in the modulation of myonuclear number during chronic unloading and subsequent reloading. Furthermore, it appears that CaN is related to fibre size and phenotype adaptations, but not to apoptotic responses.

Protein-DNA array-based identification of transcription factor activities differentially regulated in skeletal muscle of normal and dystrophin-deficient mdx mice

Inactivation of dystrophin gene is the primary cause of Duchenne muscular dystrophy (DMD) in humans and mdx mice. However, the underpinning mechanisms, which govern the pathogenesis of dystrophin-deficient skeletal muscle, remain poorly understood. We have previously reported activation of mitogen-activated protein kinases (MAPK), nuclear factor-kappa B (NF-kappaB), and phosphatidyl-inositol 3-kinase/Akt (PI3K/Akt) signaling pathways in diaphragm muscle of mdx mice. In this study, using a protein-DNA array-based approach, we have investigated the activation of 345 transcription factors in diaphragm muscle of 6-week old normal and dystrophin-deficient mdx mice. Our data demonstrate increased activation of a number nuclear transcription factors including AP1, HFH-3, PPARalpha, c.myb BP, ETF, Fra-1/JUN, kBF-A, N-rasBP, lactoferrin BP, Myb(2), EBP40_45, EKLF(1), p53(2), TFEB, Myc-Max; c-Rel; E2, ISRE; NF-kB; Stat1 p84/p91, Antioxidant RE, EVI-1, Stat3, AP3, p53, Stat4, AP4, HFH-1, FAST-1, Pax-5, and Beta-RE in the diaphragm muscle of mdx mice compared to corresponding normal mice. The level of activation for p53 was highest among all the transcription factors studied. Furthermore, higher activation of p53 in diaphragm muscle of mdx mice was associated with its increased phosphorylation and nuclear translocation. Collectively, our data suggest that the primary deficiency of dystrophin leads to the aberrant activation of nuclear transcription factors which might further contribute to muscle pathogenesis in mdx mice.

Prolonged treadmill training increases HSP70 in skeletal muscle but does not affect age-related functional deficits

Skeletal muscle atrophy and weakness are major causes of frailty in the elderly. Functional deficits in muscles of old humans and rodents are associated with attenuated production of heat shock proteins (HSPs) after exercise, and transgenic overexpression of HSP70 reverses this functional decline. We hypothesized that training would increase HSP70 content of muscle in adult and old wild-type mice and that this would protect against the development of age-related functional deficits. A 10-wk treadmill training protocol at 15 m/min, for 15 min, 3 days/wk resulted in a significant increase in HSP70 content of muscles of adult mice. Muscles of old untrained mice demonstrated a significant increase in HSP70 protein content and a reduction in HSP70 mRNA content compared with adult untrained mice. Training for 12 mo starting at age 12–14 mo old or for 10 wk starting from age 24 mo old resulted in modification of HSP70 protein and mRNA content to levels of adult mice. Training did not change force generation of extensor digitorum longus muscles of old mice or improve recovery after damaging contractions. The twofold increase in HSP70 content in muscles of adult mice after training may have not been sufficient to provide protection in this instance.

Effect of denervation on mitochondrially mediated apoptosis in skeletal muscle

Chronic muscle disuse induced by denervation reduces mitochondrial content and produces muscle atrophy. To investigate the molecular mechanisms responsible for these adaptations, we assessed 1) mitochondrial biogenesis- and apoptosis-related proteins and 2) apoptotic susceptibility and cell death following denervation. Rats were subjected to 5, 7, 14, 21, or 42 days of unilateral denervation of the sciatic or peroneal nerve. Muscle mass and mitochondrial content were reduced by 40–65% after 21 and 42 days of denervation. Denervation-induced decrements in mitochondrial content occurred along with 60% and 70% reductions in transcription factor A (Tfam) and peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α, respectively. After 42 days of denervation, Bax was elevated by 115% and Bcl-2 was decreased by 89%, producing a 16-fold increase in the Bax-to-Bcl-2 ratio. Mitochondrial reactive oxygen species production was markedly elevated by 5- to 7.5-fold in subsarcolemmal mitochondria after 7, 14, and 21 days of denervation, whereas reactive oxygen species production in intermyofibrillar (IMF) mitochondria was reduced by 40–50%. Subsarcolemmal and IMF mitochondrial levels of MnSOD were also reduced by 40–50% after 14–21 days of denervation. The maximal rate of IMF mitochondrial pore opening ( Vmax) was elevated by 25–35%, and time to Vmaxwas reduced by 20–25% after 14 and 21 days, indicating increased apoptotic susceptibility. Myonuclear decay, assessed by DNA fragmentation, was elevated at 7–21 days of denervation. Our data indicate that PGC-1α and Tfam are important factors that likely contribute to the reduced mitochondrial content after chronic disuse. In addition, our results illustrate that, despite the reduced mitochondrial content, denervated muscle has greater mitochondrial apoptotic susceptibility, which coincided with elevated apoptosis, and these processes may contribute to denervation-induced muscle atrophy.

Intermittent hyperthermia enhances skeletal muscle regrowth and attenuates oxidative damage following reloading

Skeletal muscle reloading following disuse is characterized by profound oxidative damage. This study tested the hypothesis that intermittent hyperthermia during reloading attenuates oxidative damage and augments skeletal muscle regrowth following immobilization. Forty animals were randomly divided into four groups: control (Con), immobilized (Im), reloaded (RC), and reloaded and heated (RH). All groups but Con were immobilized for 7 days. Animals in the RC and RH groups were then reloaded for 7 days with (RH) or without (RC) hyperthermia (41-41.5 degrees C for 30 min on alternating days) during reloading. Heating resulted in approximately 25% elevation in heat shock protein expression (P < 0.05) and an approximately 30% greater soleus regrowth (P < 0.05) in RH compared with RC. Furthermore, oxidant damage was lower in the RH group compared with RC because nitrotyrosine and 4-hydroxy-2-nonenol were returned to near baseline when heating was combined with reloading. Reduced oxidant damage was independent of antioxidant enzymes (manganese superoxide dismutase, copper-zinc superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase). In summary, these data suggest that intermittent hyperthermia during reloading attenuates oxidative stress and improves the rate of skeletal muscle regrowth during reloading after immobilization.