Atrogin-1, MuRF1, and FoXO, as well as phosphorylated GSK-3beta and 4E-BP1 are reduced in skeletal muscle of chronic spinal cord-injured patients

Muscle fiber atrophy and regeneration coexist in collagen VI-deficient human muscle: role of calpain-3 and nuclear factor-κB signaling

Ullrich congenital muscular dystrophy (UCMD) is a common form of muscular dystrophy associated with defects in collagen VI. It is characterized by loss of individual muscle fibers and muscle mass and proliferation of connective and adipose tissues. We sought to investigate the mechanisms by which collagen VI regulates muscle cell survival, size, and regeneration and, in particular, the potential role of the ubiquitin-proteasome and calpain-proteolytic systems. We studied muscle biopsies of UCMD (n = 6), other myopathy (n = 12), and control patients (n = 10) and found reduced expression of atrogin-1, MURF1, and calpain-3 mRNAs in UCMD cases. Downregulation of calpain-3 was associated with changes in the nuclear immunolocalization of nuclear factor-κB. We also observed increased expression versus controls of regeneration markers at the protein and RNA levels. Satellite cell numbers did not differ in collagen VI-deficient muscle versus normal nonregenerating muscle, indicating that collagen VI does not play a key role in the maintenance of the satellite cell pool. Our results indicate that alterations in calpain-3 and nuclear factor-κB signaling pathways may contribute to muscle mass loss in UCMD muscle, whereas atrogin-1 and MURF1 are not likely to play a major role.

Altered autophagy gene expression and persistent atrophy suggest impaired remodeling in chronic hemiplegic human skeletal muscle

INTRODUCTION

Upper motor neuron lesions after stroke are a major cause of disability. We aimed to determine whether skeletal muscles from these patients display typical molecular signatures of inflammation, growth arrest, and atrophy.

METHODS

Muscle biopsies were analyzed for morphological, histochemical, ultrastructural, and molecular features indicative of changes in gene expression involved in muscle atrophy.

RESULTS

Chronic hemiplegia resulted in ~9.5% atrophy, fiber type shifts, and histochemical and ultrastructural signs of impaired remodeling. TNF and TWEAK expressions were unaltered, but MSTN mRNA was lower (-73%, P < 0.05) in paretic tibialis anterior vs. age-matched controls. The expression of autophagy-related genes (BCN-1, LC3, and GABARAPL1) was lower in paretic tibialis anterior (-81%, -48%, and -60%, respectively, P < 0.01) and soleus (-85%, -54%, and -60% respectively, P < 0.01) compared with old controls.

CONCLUSIONS

Persistent atrophy in chronic spastic hemiplegia may be associated with impaired remodeling partly due to altered autophagy gene expression.

Identification of atrogin-1-targeted proteins during the myostatin-induced skeletal muscle wasting

Atrogin-1, a muscle-specific E3 ligase, targets MyoD for degradation through the ubiquitin-proteasome-mediated system. Myostatin, a member of the transforming growth factor-β superfamily, potently inhibits myogenesis by lowering MyoD levels. While atrogin-1 is upregulated by myostatin, it is currently unknown whether atrogin-1 plays a role in mediating myostatin signaling to regulate myogenesis. In this report, we have confirmed that atrogin-1 increasingly interacts with MyoD upon recombinant human myostatin (hMstn) treatment. The absence of atrogin-1, however, led to elevated MyoD levels and permitted the differentiation of atrogin-1(-/-) primary myoblast cultures despite the presence of exogenous myostatin. Furthermore, inactivation of atrogin-1 rescued myoblasts from growth inhibition by hMstn. Therefore, these results highlight the central role of atrogin-1 in regulating myostatin signaling during myogenesis. Currently, there are only two known targets of atrogin-1. Thus, we next characterized the associated proteins of atrogin-1 in control and hMstn-treated C2C12 cell cultures by stably expressing tagged atrogin-1 in myoblasts and myotubes, and sequencing the coimmunoprecipitated proteome. We found that atrogin-1 putatively interacts with sarcomeric proteins, transcriptional factors, metabolic enzymes, components of translation, and spliceosome formation. In addition, we also identified that desmin and vimentin, two components of the intermediate filament in muscle, directly interacted with and were degraded by atrogin-1 in response to hMstn. In summary, the muscle wasting effects of the myostatin-atrogin-1 axis are not only limited to the degradation of MyoD and eukaryotic translation initiation factor 3 subunit f, but also encompass several proteins that are involved in a wide variety of cellular activities in the muscle.

Nandrolone normalizes determinants of muscle mass and fiber type after spinal cord injury

Spinal cord injury (SCI) results in atrophy of skeletal muscle and changes from slow oxidative to fast glycolytic fibers, which may reflect reduced levels of peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α), increased myostatin signaling, or both. In animals, testosterone reduces loss of muscle fiber cross-sectional area and activity of enzymes of energy metabolism. To identify the molecular mechanisms behind the benefits of androgens on paralyzed muscle, male rats were spinal cord transected and treated for 8 weeks with vehicle, testosterone at a physiological replacement dose, or testosterone plus nandrolone, an anabolic steroid. Treatments were initiated immediately after SCI and continued until the day animals were euthanized. In the SCI animals, gastrocnemius muscle mass was significantly increased by testosterone plus nandrolone, but not by testosterone alone. Both treatments significantly reduced nuclear content of Smad2/3 and mRNA levels of activin receptor IIB and follistatin-like 3. Testosterone alone or with nandrolone reversed SCI-induced declines in cellular and nuclear levels of PGC-1α protein and PGC-1α mRNA levels. For PGC-1α target genes, testosterone plus nandrolone partially reversed SCI-induced decreases in levels of proteins without corresponding increases in their mRNA levels. Thus, the findings demonstrate that following SCI, signaling through activin receptors and Smad2/3 is increased, and that androgens suppress activation of this signaling pathway. The findings also indicate that androgens upregulate PGC-1α in paralyzed muscle and promote its nuclear localization, but that these effects are insufficient to fully activate transcription of PGC-1α target genes. Furthermore, the transcription of these genes is not tightly coupled with their translation.

Genetic biomarkers for ALS disease in transgenic SOD1(G93A) mice

The pathophysiological mechanisms of both familial and sporadic Amyotrophic Lateral Sclerosis (ALS) are unknown, although growing evidence suggests that skeletal muscle tissue is a primary target of ALS toxicity. Skeletal muscle biopsies were performed on transgenic SOD1^G93A mice, a mouse model of ALS, to determine genetic biomarkers of disease longevity. Mice were anesthetized with isoflurane, and three biopsy samples were obtained per animal at the three main stages of the disease. Transcriptional expression levels of seventeen genes, Ankrd1, Calm1, Col19a1, Fbxo32, Gsr, Impa1, Mef2c, Mt2, Myf5, Myod1, Myog, Nnt, Nogo A, Pax7, Rrad, Sln and Snx10, were tested in each muscle biopsy sample. Total RNA was extracted using TRIzol Reagent according to the manufacturer's protocol, and variations in gene expression were assayed by real-time PCR for all of the samples. The Pearson correlation coefficient was used to determine the linear correlation between transcriptional expression levels throughout disease progression and longevity. Consistent with the results obtained from total skeletal muscle of transgenic SOD1^G93A mice and 74-day-old denervated mice, five genes (Mef2c, Gsr, Col19a1, Calm1 and Snx10) could be considered potential genetic biomarkers of longevity in transgenic SOD1^G93A mice. These results are important because they may lead to the exploration of previously unexamined tissues in the search for new disease biomarkers and even to the application of these findings in human studies.

Muscle degeneration in rotator cuff tears

Rotator cuff tears are among the most common injuries seen by orthopedic surgeons. Although small- and medium-sized tears do well after arthroscopic and open repair, large and massive tears have been shown to develop marked muscle atrophy and fatty infiltration within the rotator cuff muscles. These pathologic changes have been found to be independent predictors of failed surgical repair with poor functional outcomes. To understand the pathophysiology of rotator cuff disease, we must first develop an understanding of the changes that occur within the cuff muscles themselves. The purpose of this review is to summarize the molecular pathways behind muscular degeneration and emphasize new findings related to the clinical relevance of muscle atrophy and fatty infiltration seen with rotator cuff tears. Understanding these molecular pathways will help guide further research and treatment options that can aim to alter expression of these pathways and improve outcomes after surgical repair of massive rotator cuff tears.

Beneficial effects of endogenous and exogenous melatonin on neural reconstruction and functional recovery in an animal model of spinal cord injury

The purpose of this study was to investigate the beneficial effects of endogenous and exogenous melatonin on functional recovery in an animal model of spinal cord injury (SCI). Eight-week-old male Sprague-Dawley (SD, 250-260 g) rats were used for contusion SCI surgery. All experimental groups were maintained under one of the following conditions: 12/12-hr light/dark (L/D) or 24:0-hr constant light (LL). Melatonin (10 mg/kg) was injected subcutaneously for 4 wk, twice daily (07:00, 19:00). Locomotor recovery, inducible nitric oxide synthase (iNOS), glial fibrillary acidic protein gene expression, and muscle atrophy-related genes, including muscle atrophy F-box (MAFbx) and muscle-specific ring-finger protein 1 (MuRF1) gene expression were evaluated. Furthermore, autophagic signaling such as Beclin-1 and LC3 protein expression was examined in the spinal cord and in skeletal muscle. The melatonin treatment resulted in increased hind-limb motor function and decreased iNOS mRNA expression in the L/D condition compared with the LL condition (P < 0.05), indicating that endogenous melatonin had neuroprotective effects. Furthermore, the MAFbx, MuRF1 mRNA level, and converted LC3 II protein expression were decreased in the melatonin-treated SCI groups under the LL (P < 0.05), possibly in response to the exogenous melatonin treatment. Therefore, it seems that both endogenous and exogenous melatonin contribute to neural recovery and to the prevention of skeletal muscle atrophy, promoting functional recovery after SCI. Finally, this study supports the benefit of endogenous melatonin and use of exogenous melatonin as a therapeutic intervention for SCI.

Myostatin induces degradation of sarcomeric proteins through a Smad3 signaling mechanism during skeletal muscle wasting

Ubiquitination-mediated proteolysis is a hallmark of skeletal muscle wasting manifested in response to negative growth factors, including myostatin. Thus, the characterization of signaling mechanisms that induce the ubiquitination of intracellular and sarcomeric proteins during skeletal muscle wasting is of great importance. We have recently characterized myostatin as a potent negative regulator of myogenesis and further demonstrated that elevated levels of myostatin in circulation results in the up-regulation of the muscle-specific E3 ligases, Atrogin-1 and muscle ring finger protein 1 (MuRF1). However, the exact signaling mechanisms by which myostatin regulates the expression of Atrogin-1 and MuRF1, as well as the proteins targeted for degradation in response to excess myostatin, remain to be elucidated. In this report, we have demonstrated that myostatin signals through Smad3 (mothers against decapentaplegic homolog 3) to activate forkhead box O1 and Atrogin-1 expression, which further promotes the ubiquitination and subsequent proteasome-mediated degradation of critical sarcomeric proteins. Smad3 signaling was dispensable for myostatin-dependent overexpression of MuRF1. Although down-regulation of Atrogin-1 expression rescued approximately 80% of sarcomeric protein loss induced by myostatin, only about 20% rescue was seen when MuRF1 was silenced, implicating that Atrogin-1 is the predominant E3 ligase through which myostatin manifests skeletal muscle wasting. Furthermore, we have highlighted that Atrogin-1 not only associates with myosin heavy and light chain, but it also ubiquitinates these sarcomeric proteins. Based on presented data we propose a model whereby myostatin induces skeletal muscle wasting through targeting sarcomeric proteins via Smad3-mediated up-regulation of Atrogin-1 and forkhead box O1.

Clinical review: Thinking outside the box--an iconoclastic view of current practice

Many advances in medicine have been achieved through challenging established dogma with revolutionary thought and novel practices. Each and every specialty is reinvigorated by regular re-evaluation of processes and practices in the light of new evidence and fresh conceptualization. Challenge can galvanize fresh thinking and new approaches, yet may also reinforce and strengthen traditional paradigms if the prevailing orthodoxy is subsequently revalidated. This article is a synopsis of a roundtable meeting held in Brussels in March 2010 designed specifically to confront doctrine with reasoned scientific argument, and to propose new ideas for advancing critical care practices and outcomes.

Distinct protein degradation profiles are induced by different disuse models of skeletal muscle atrophy

Skeletal muscle atrophy can be a consequence of many diseases, environmental insults, inactivity, age, and injury. Atrophy is characterized by active degradation, removal of contractile proteins, and a reduction in muscle fiber size. Animal models have been extensively used to identify pathways that lead to atrophic conditions. We used genome-wide expression profiling analyses and quantitative PCR to identify the molecular changes that occur in two clinically relevant mouse models of muscle atrophy: hindlimb casting and Achilles tendon laceration (tenotomy). Gastrocnemius muscle samples were collected 2, 7, and 14 days after casting or injury. The total amount of muscle loss, as measured by wet weight and muscle fiber size, was equivalent between models on day 14, although tenotomy resulted in a more rapid induction of muscle atrophy. Furthermore, tenotomy resulted in the regulation of significantly more mRNA transcripts then did casting. Analysis of the regulated genes and pathways suggest that the mechanisms of atrophy are distinct between these models. The degradation following casting was ubiquitin-proteasome mediated, while degradation following tenotomy was lysosomal and matrix-metalloproteinase mediated, suggesting a possible role for autophagy. These data suggest that there are multiple mechanisms leading to muscle atrophy and that specific therapeutic agents may be necessary to combat atrophy resulting from different conditions.

Acupuncture ameliorated skeletal muscle atrophy induced by hindlimb suspension in mice

Preventing skeletal muscle atrophy is critical for maintaining quality of life, but it is often a challenging goal for the elderly and patients with severe conditions. We hypothesized that acupuncture in place of exercise training is an alternative non-pharmacological intervention that can help to prevent muscle atrophy. To elucidate the effects of acupuncture on skeletal muscle atrophy caused by hindlimb suspension (HS), we performed acupuncture on mice according to two different methods: acupuncture with electrical stimulation (EA: electroacupuncture) and without electrical stimulation (MA: manual acupuncture). A needle was retained in the gastrocnemius muscle for 30 min every day for 2 weeks in the EA and MA groups. In the EA group, 30 min of repetitive electrical stimulation (1 Hz, 1 ms pulse width, 6.5 mA intensity) was also applied. HS significantly reduced muscle mass and the cross-sectional area of the soleus muscles. This HS-induced reduction was significantly improved in the EA group, although the level of improvement remained insufficient when compared with the control group. We found that the mRNA expression levels of atrogin-1 and MuRF1, which play a principal role in muscle-specific degradation as E3 ubiquitin ligases, were significantly increased in the HS group compared to the control group. EA and MA reduced the HS-induced upregulation of atrogin-1 (p<0.01 in EA and MA) and MuRF1 (p<0.01 in EA) mRNAs. We also found that the expression levels of PI3K, Akt1, TRPV4, adenosine A1 receptor, myostatin, and SIRT1 mRNAs tended to be increased by HS. EA and MA further increased the HS-induced upregulation of Akt1 (p<0.05 in MA) and TRPV4 (p<0.05 in MA) mRNAs. We concluded that acupuncture partially prevented skeletal muscle atrophy. This effect might be due to an increase in protein synthesis and a decrease in protein degradation.

Differential expression of metabolic genes essential for glucose and lipid metabolism in skeletal muscle from spinal cord injured subjects

Skeletal muscle plays an important role in the regulation of energy homeostasis; therefore, the ability of skeletal muscle to adapt and alter metabolic gene expression in response to changes in physiological demands is critical for energy balance. Individuals with cervical spinal cord lesions are characterized by tetraplegia, impaired thermoregulation, and altered skeletal muscle morphology. We characterized skeletal muscle metabolic gene expression patterns, as well as protein content, in these individuals to assess the impact of spinal cord injury on critical determinants of skeletal muscle metabolism. Our results demonstrate that mRNA levels and protein expression of skeletal muscle genes essential for glucose storage are reduced, whereas expression of glycolytic genes is reciprocally increased in individuals with spinal cord injury. Furthermore, expression of genes essential for lipid oxidation is coordinately reduced in spinal cord injured subjects, consistent with a marked reduction of mitochondrial proteins. Thus spinal cord injury resulted in a profound and tightly coordinated change in skeletal muscle metabolic gene expression program that is associated with the aberrant metabolic features of the tissue.

Voluntary running, skeletal muscle gene expression, and signaling inversely regulated by orchidectomy and testosterone replacement

Declines in skeletal muscle size and strength, often seen with chronic wasting diseases, prolonged or high-dose glucocorticoid therapy, and the natural aging process in mammals, are usually associated with reduced physical activity and testosterone levels. However, it is not clear whether the decline in testosterone and activity are causally related. Using a mouse model, we found that removal of endogenous testosterone by orchidectomy results in an almost complete cessation in voluntary wheel running but only a small decline in muscle mass. Testosterone replacement restored running behavior and muscle mass to normal levels. Orchidectomy also suppressed the IGF-I/Akt pathway, activated the atrophy-inducing E3 ligases MuRF1 and MAFBx, and suppressed several energy metabolism pathways, and all of these effects were reversed by testosterone replacement. The study also delineated a distinct, previously unidentified set of genes that is inversely regulated by orchidectomy and testosterone treatment. These data demonstrate the necessity of testosterone for both speed and endurance of voluntary wheel running in mice and suggest a potential mechanism for declined activity in humans where androgens are deficient.

Altered mRNA expression after long-term soleus electrical stimulation training in humans with paralysis

In humans, spinal cord injury (SCI) induces deleterious changes in skeletal muscle that may be prevented or reversed by electrical stimulation muscle training. The molecular mechanisms underlying muscle stimulation training remain unknown. We studied two unique SCI subjects whose right soleus received >6 years of training (30 minutes/day, 5 days/week). Training preserved torque, fatigue index, contractile speed, and cross-sectional area in the trained leg, but not the untrained leg. Training decreased 10 mRNAs required for fast-twitch contractions and mRNA that encodes for myostatin, an autocrine/paracrine hormone that inhibits muscle growth. Conversely, training increased 69 mRNAs that mediate the slow-twitch, oxidative phenotype, including PGC-1α, a transcriptional coactivator that inhibits muscle atrophy. When we discontinued right soleus training, training-induced effects diminished slowly, with some persisting for >6 months. Training of paralyzed muscle induces localized and long-lasting changes in skeletal muscle mRNA expression that improve muscle mass and function.

Chronic heart failure reduces Akt phosphorylation in human skeletal muscle: relationship to muscle size and function

Patients with chronic heart failure (HF) frequently lose muscle mass and function during the course of the disease. A reduction in anabolic stimuli to the muscle has been put forth as a potential mechanism underlying these alterations. The present study examined the hypothesis that skeletal muscle tissue from HF patients would show reduced IGF-1 expression and phosphorylation of signaling molecules downstream of receptor activation. To isolate the unique effect of HF on these variables, we limited the confounding effects of muscle disuse and/or acute disease exacerbation by recruiting controls (n = 11) with similar physical activity levels as HF patients (n = 11) and by testing patients at least 6 mo following any bouts of disease exacerbation/hospitalization. IGF-1 expression in skeletal muscle was similar between patients and controls. Despite this, HF patients were characterized by reduced levels of phospho-Akt/Akt (S473; -43%; P < 0.05), whereas no differences were found in total Akt protein content or phospho- or total protein content of mammalian target of rapamycin (mTOR; S2448), glycogen synthase kinase-3β (GSK-3β; S9), eukaryotic translation initiation factor 4E binding protein-1 (eIF4E-BP; T37/46), p70 ribosomal S6 kinase (p70 S6K; T389), or eIF2Bε (S540). Reduced phospho-Akt/Akt levels and phospho-mTOR/mTOR were related to decreased skeletal muscle myosin protein content (r = 0.602; P < 0.02) and knee extensor isometric torque (r = 0.550; P < 0.05), respectively. Because patients and controls were similar for age, muscle mass, and physical activity, we ascribe the observed alterations in Akt phosphorylation and its relationship to myosin protein content to the unique effects of the HF syndrome.

Structure to function: muscle failure in critically ill patients

Impaired physical function and reduced physical activity are common findings in intensive care unit (ICU) survivors. More importantly, reduced muscle strength during critical illness is an independent predictor of survival. Skeletal muscle wasting as a direct consequence of critical illness has been suggested as the cause. However, data on the physiological processes regulating muscle mass, and function, in these critically ill patients are limited as this is not only a technically challenging research area, but also the heterogeneity of the patient group adds complexity to the interpretation of results. Despite this, clinical and research interest in this area is growing. This article highlights the issues involved in measurement of muscle function and mass in critically ill patients and the physiological complexities involved in studying these patients. Although the data are limited, this article reviews the animal and healthy human data providing a rational approach to the potential pathophysiological mechanisms involved in muscle mass regulation in critically ill patients, including the established muscle wasting 'risk factors' such as ageing, immobility and systemic inflammation, all of which are common findings in the general critical care population.

Electromechanical modulation of catabolic and anabolic pathways in chronically inactive, but neurally intact, muscles

The extent and mechanisms by which neural input regulates skeletal muscle mass remain largely unknown. Adult spinal cord isolated (SI) rats were implanted unilaterally with a microstimulator, whereas the contralateral limb served as SI control (SI-C). A 100-HZ, 1-s stimulus was delivered every 30 s for 5 min, followed by a 5-min rest. This was repeated six times consecutively (SI-Stim1) or with a 9-h interval after the third bout (SI-Stim2) for 30 days (1 min of daily activity). SI-Stim1 and SI-Stim2 paradigms attenuated plantaris atrophy by 20% and 38%, respectively, whereas only SI-Stim2 blunted soleus atrophy (24%) relative to SI-C. Muscle mass changes occurred independent of the IGF-1/PI3K/Akt pathway. No relationships between SI or electromechanical stimulation and expression of several atrophy markers were observed. These data suggest that regulatory mechanisms for maintaining muscle mass previously shown in acute states of atrophy differ substantially from those observed in chronic states.