The role of myostatin in muscle wasting: an overview

Treatment of sporadic inclusion body myositis with bimagrumab

OBJECTIVE

To study activin signaling and its blockade in sporadic inclusion body myositis (sIBM) through translational studies and a randomized controlled trial.

METHODS

We measured transforming growth factor β signaling by SMAD2/3 phosphorylation in muscle biopsies of 50 patients with neuromuscular disease (17 with sIBM). We tested inhibition of activin receptors IIA and IIB (ActRII) in 14 patients with sIBM using one dose of bimagrumab (n = 11) or placebo (n = 3). The primary outcome was the change in right thigh muscle volume by MRI at 8 weeks. Lean body mass, strength, and function were secondary outcomes. Twelve of the patients (10 bimagrumab, 2 placebo) participated in a subsequent 16-week observation phase.

RESULTS

Muscle SMAD2/3 phosphorylation was higher in sIBM than in other muscle diseases studied (p = 0.003). Eight weeks after dosing, the bimagrumab-treated patients increased thigh muscle volume (right leg +6.5% compared with placebo, p = 0.024; left leg +7.6%, p = 0.009) and lean body mass (+5.7% compared with placebo, p = 0.014). Subsequently, bimagrumab-treated patients had improved 6-minute walking distance, which peaked at 16 weeks (+14.6%, p = 0.008) compared with placebo. There were no serious adverse events; the main adverse events with bimagrumab were mild acne and transient involuntary muscle contractions.

CONCLUSIONS

Transforming growth factor β superfamily signaling, at least through ActRII, is implicated in the pathophysiology of sIBM. Inhibition of ActRII increased muscle mass and function in this pilot trial, offering a potential novel treatment of sIBM.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that for patients with inclusion body myositis, bimagrumab increases thigh muscle volume at 8 weeks.

Molecular mechanisms and treatment targets of muscle wasting and cachexia in heart failure: an overview

PURPOSE OF REVIEW

This article aims to describe molecular pathways involved in the development of muscle wasting and cachexia, diagnostic possibilities, and potential treatments that have seen clinical testing in recent heart failure trials. An understanding of the specific changes that cause an anabolic-catabolic imbalance is an essential first step in the development of pharmaceutical intervention strategies aimed at blocking muscle wasting.

RECENT FINDINGS

Skeletal muscle mass and muscle strength are the most important determinants of exercise capacity in patients with heart failure. In contrast to cachexia, muscle wasting is not usually associated with weight loss, implying the need for sophisticated assessment methods to correctly diagnose muscle wasting, for example the use of computed tomography, magnetic resonance imaging, or dual energy X-ray absorptiometry. Simpler techniques such as handgrip strength, exercise testing, or even a biomarker may help in determining patients with a high pre-test probability of muscle wasting.

SUMMARY

Despite intensive research efforts in the field of muscle wasting during the last couple of decades, no effective treatment of muscle wasting currently exists other than exercise training. This situation remains true even though study of the molecular pathways involved in muscle wasting suggests many therapeutic targets. Easily applicable diagnostic tools may help to identify patients at risk of developing muscle wasting.

Sarcopenia in Asia: consensus report of the Asian Working Group for Sarcopenia

Sarcopenia, a newly recognized geriatric syndrome, is characterized by age-related decline of skeletal muscle plus low muscle strength and/or physical performance. Previous studies have confirmed the association of sarcopenia and adverse health outcomes, such as falls, disability, hospital admission, long term care placement, poorer quality of life, and mortality, which denotes the importance of sarcopenia in the health care for older people. Despite the clinical significance of sarcopenia, the operational definition of sarcopenia and standardized intervention programs are still lacking. It is generally agreed by the different working groups for sarcopenia in the world that sarcopenia should be defined through a combined approach of muscle mass and muscle quality, however, selecting appropriate diagnostic cutoff values for all the measurements in Asian populations is challenging. Asia is a rapidly aging region with a huge population, so the impact of sarcopenia to this region is estimated to be huge as well. Asian Working Group for Sarcopenia (AWGS) aimed to promote sarcopenia research in Asia, and we collected the best available evidences of sarcopenia researches from Asian countries to establish the consensus for sarcopenia diagnosis. AWGS has agreed with the previous reports that sarcopenia should be described as low muscle mass plus low muscle strength and/or low physical performance, and we also recommend outcome indicators for further researches, as well as the conditions that sarcopenia should be assessed. In addition to sarcopenia screening for community-dwelling older people, AWGS recommends sarcopenia assessment in certain clinical conditions and healthcare settings to facilitate implementing sarcopenia in clinical practice. Moreover, we also recommend cutoff values for muscle mass measurements (7.0 kg/m(2) for men and 5.4 kg/m(2) for women by using dual X-ray absorptiometry, and 7.0 kg/m(2) for men and 5.7 kg/m(2) for women by using bioimpedance analysis), handgrip strength (<26 kg for men and <18 kg for women), and usual gait speed (<0.8 m/s). However, a number of challenges remained to be solved in the future. Asia is made up of a great number of ethnicities. The majority of currently available studies have been published from eastern Asia, therefore, more studies of sarcopenia in south, southeastern, and western Asia should be promoted. On the other hand, most Asian studies have been conducted in a cross-sectional design and few longitudinal studies have not necessarily collected the commonly used outcome indicators as other reports from Western countries. Nevertheless, the AWGS consensus report is believed to promote more Asian sarcopenia research, and most important of all, to focus on sarcopenia intervention studies and the implementation of sarcopenia in clinical practice to improve health care outcomes of older people in the communities and the healthcare settings in Asia.

Muscle wasting: an overview of recent developments in basic research

The syndrome of cachexia, i.e., involuntary weight loss in patients with underlying diseases, sarcopenia, i.e., loss of muscle mass due to aging, and general muscle atrophy from disuse and/or prolonged bed rest have received more attention over the last decades. All lead to a higher morbidity and mortality in patients, and therefore, they represent a major socio-economic burden for the society today. This mini-review looks at recent developments in basic research that are relevant to the loss of skeletal muscle. It aims to cover the most significant publication of last 3 years on the causes and effects of muscle wasting, new targets for therapy development, and potential biomarkers for assessing skeletal muscle mass. The targets include the following: (1) E-3 ligases TRIM32, SOCS1, and SOCS3 by involving the elongin BC ubiquitin-ligase, Cbl-b, culling 7, Fbxo40, MG53 (TRIM72), and the mitochondrial Mul1; (2) the kinase MST1; and (3) the G-protein Gαi2. D(3)-creatine has the potential to be used as a novel biomarker that allows to monitor actual change in skeletal muscle mass over time. In conclusion, significant development efforts are being made by academic groups as well as numerous pharmaceutical companies to identify new target and biomarker muscles, as muscle wasting represents a great medical need, but no therapies have been approved in the last decades.

Muscle wasting: an overview of recent developments in basic research

The syndrome of cachexia, i.e. involuntary weight loss in patients with underlying diseases, sarcopenia, i.e. loss of muscle mass due to ageing, and general muscle atrophy from disuse and/or prolonged bed rest have received more attention over the last decades. All lead to a higher morbidity and mortality in patients and therefore, they represent a major socio-economic burden for the society today. This mini-review looks at recent developments in basic research that are relevant to the loss of skeletal muscle. It aims to cover the most significant publication of last three years on the causes and effects of muscle wasting, new targets for therapy development and potential biomarkers for assessing skeletal muscle mass. The targets include 1) E-3 ligases: TRIM32, SOCS1 and SOCS3 by involving the elongin BC ubiquitin-ligase, Cbl-b, culling 7, Fbxo40, MG53 (TRIM72) and the mitochondrial Mul1, 2) the kinase MST1 and 3) the G-protein Gαi2. D(3)-creatine has the potential to be used as a novel biomarker that allows to monitor actual change in skeletal muscle mass over time. In conclusion, significant development efforts are being made by academic groups as well as numerous pharmaceutical companies to identify new targets and biomarkers muscle, as muscle wasting represents a great medical need, but no therapies have been approved in the last decades.

L-leucine, beta-hydroxy-beta-methylbutyric acid (HMB) and creatine monohydrate prevent myostatin-induced Akirin-1/Mighty mRNA down-regulation and myotube atrophy

Background

The purpose of this study was to examine if L-leucine (Leu), β-hydroxy-β-methylbutyrate (HMB), or creatine monohydrate (Crea) prevented potential atrophic effects of myostatin (MSTN) on differentiated C2C12 myotubes.

Methods

After four days of differentiation, myotubes were treated with MSTN (10 ng/ml) for two additional days and four treatment groups were studied: 1) 3x per day 10 mM Leu, 2) 3x per day 10 mM HMB, 3) 3x per day 10 mM Crea, 4) DM only. Myotubes treated with DM without MSTN were analyzed as the control condition (DM/CTL). Following treatment, cells were analyzed for total protein, DNA content, RNA content, muscle protein synthesis (MPS, SUnSET method), and fiber diameter. Separate batch treatments were analyzed for mRNA expression patterns of myostatin-related genes (Akirin-1/Mighty, Notch-1, Ski, MyoD) as well as atrogenes (MuRF-1, and MAFbx/Atrogin-1).

Results

MSTN decreased fiber diameter approximately 30% compared to DM/CTL myotubes (p < 0.001). Leu, HMB and Crea prevented MSTN-induced atrophy. MSTN did not decrease MPS levels compared to DM/CTL myotubes, but MSTN treatment decreased the mRNA expression of Akirin-1/Mighty by 27% (p < 0.001) and MyoD by 26% (p < 0.01) compared to DM/CTL myotubes. shRNA experiments confirmed that Mighty mRNA knockdown reduced myotube size, linking MSTN treatment to atrophy independent of MPS. Remarkably, MSTN + Leu and MSTN + HMB myotubes had similar Akirin-1/Mighty and MyoD mRNA levels compared to DM/CTL myotubes. Furthermore, MSTN + Crea myotubes exhibited a 36% (p < 0.05) and 86% (p < 0.001) increase in Akirin-1/Mighty mRNA compared to DM/CTL and MSTN-only treated myotubes, respectively.

Conclusions

Leu, HMB and Crea may reduce MSTN-induced muscle fiber atrophy by influencing Akirin-1/Mighty mRNA expression patterns. Future studies are needed to examine if Leu, HMB and Crea independently or synergistically affect Akirin-1/Mighty expression, and how Akirin-1/Mighty expression mechanistically relates to skeletal muscle hypertrophy in vivo.

Gut barrier dysfunction and microbial translocation in cancer cachexia: a new therapeutic target

Purpose of Review

Cachexia is a complex metabolic syndrome characterized by skeletal muscle and adipose tissue loss and is frequently associated with emaciation, anorexia, systemic inflammation, and metabolic dysfunction. Lack of a clear understanding of the cause of cancer cachexia has impeded progress in identifying effective therapeutic agents. This review summarizes recent publications on the role of gut barrier function, intestinal microbiota, and inflammation in the etiology of cancer cachexia and new therapeutic interventions that may benefit treatment strategies.

Recent Findings

Significant advances have been made in understanding the composition and metabolic capabilities of the intestinal microbiota and its impact on gut barrier function with implications for certain inflammatory-based diseases. Recent studies reported associations between intestinal permeability and endotoxemia with development of cancer cachexia and other metabolic disorders. Improvements in intestinal function and weight gain along with decreased inflammation have been reported for potential therapeutic agents such as eicosapentaenoic acid, immunoglobulin isolates, and probiotics.

Summary

Continued progress in the scientific understanding of the complex interplay between the intestinal microbiota, gut barrier function, and host inflammatory responses will uncover new therapeutic targets to help avoid the serious metabolic alterations associated with cachexia.

The anabolic catabolic transforming agent (ACTA) espindolol increases muscle mass and decreases fat mass in old rats

BACKGROUND

Sarcopenia, the age-related, progressive loss of skeletal muscle mass, strength, and function, is a considerable socioeconomic burden by increasing risks of falls, fractures, and frailty. Moreover, sarcopenic patients are often obese and therapeutic options are very limited.

METHODS

Here, we assessed the efficacy of espindolol on muscle mass in 19-month-old male Wistar Han rats (weight, 555 ± 18 g), including safety issues. Rats were randomized to treatment with 3 mg/kg/day espindolol (n = 8) or placebo (n = 14) for 31 days.

RESULTS

Placebo-treated rats progressively lost body weight (-15.5 ± 7.2 g), lean mass (-1.5 ± 4.2 g), and fat mass (-15.6 ± 2.7 g), while espindolol treatment increased body weight (+8.0 ± 6.1 g, p < 0.05), particularly lean mass (+43.4 ± 3.5 g, p < 0.001), and reduced fat mass further (-38.6 ± 3.4 g, p < 0.001). Anabolic/catabolic signaling was assessed in gastrocnemius muscle. Espindolol decreased proteasome and caspase-3 proteolytic activities by approximately 50 % (all p < 0.05). Western blotting showed a reduced expression of key catabolic regulators, including NFκB, MuRF1, and LC-3 (all p < 0.01). The 50- and 26-kDa forms of myostatin were downregulated fivefold and 20-fold, respectively (both p < 0.001). Moreover, 4E-BP-1 was reduced fivefold (p < 0.01), while phospho-PI3K was upregulated fivefold (p < 0.001), although Akt expression and phosphorylation were lower compared to placebo (all p < 0.05). No regulation of p38 and expression of ERK1/2 were observed, while phosphorylation of p38 was reduced (-54 %, p < 0.001) and ERK1/2 was increased (115 and 83 %, respectively, both p < 0.01). Espindolol did not affect cardiac function (echocardiography) or clinical plasma parameters.

CONCLUSION

Espindolol reversed the effects of aging/sarcopenia, particularly loss of muscle mass and increased fat mass. Thus, espindolol is an attractive candidate drug for the treatment of sarcopenia patients.

Are we closer to having drugs to treat muscle wasting disease?

The two most common muscle wasting diseases in adults are sarcopenia and cachexia. Despite differences in their pathophysiology, it is believed that both conditions are likely to respond to drugs that increase muscle mass and muscle strength. The current gold standard in this regard is exercise training. This article provides an overview of candidate drugs to treat muscle wasting disease that are available or in development. Drugs highlighted here include ghrelin agonists, selective androgen receptor molecules, megestrol acetate, activin receptor antagonists, espindolol, and fast skeletal muscle troponin inhibitors.

Novel investigational biologics for the treatment of cancer cachexia

INTRODUCTION

Cancer cachexia is a complex multifactorial syndrome characterized by ongoing, irreversible skeletal muscle loss, leading to progressive functional impairment. Several investigational biologics targeting key inflammatory pathways and/or the myostatin/activin type II receptor pathway are in development.

AREAS COVERED

Novel therapies include ALD518, MABp1, IP-1510, OHR/AVR118, bimagrumab and REGN1033 and are discussed. For each investigational therapy, the mechanism of action, preclinical data, cachexia definition, indication and clinical data are discussed.

EXPERT OPINION

A critical look of the study protocols and two key challenges limiting the successful evaluation of these agents include: i) lack of a clinically meaningful cachexia definition; and ii) identification and treatment of cachexia in late stage. We describe our observations and clinical experience in an effort to redirect and promote successful strategies to evaluate these novel investigational biologics.

New roles for Smad signaling and phosphatidic acid in the regulation of skeletal muscle mass

Skeletal muscle is essential for normal bodily function and the loss of skeletal muscle (i.e. muscle atrophy/wasting) can have a major impact on mobility, whole-body metabolism, disease resistance, and quality of life. Thus, there is a clear need for the development of therapies that can prevent the loss, or increase, of skeletal muscle mass. However, in order to develop such therapies, we will first have to develop a thorough understanding of the molecular mechanisms that regulate muscle mass. Fortunately, our knowledge is rapidly advancing, and in this review, we will summarize recent studies that have expanded our understanding of the roles that Smad signaling and the synthesis of phosphatidic acid play in the regulation of skeletal muscle mass.

Protein kinase C theta (PKCθ) modulates the ClC-1 chloride channel activity and skeletal muscle phenotype: a biophysical and gene expression study in mouse models lacking the PKCθ

In skeletal muscle, the resting chloride conductance (gCl), due to the ClC-1 chloride channel, controls the sarcolemma electrical stability. Indeed, loss-of-function mutations in ClC-1 gene are responsible of myotonia congenita. The ClC-1 channel can be phosphorylated and inactivated by protein kinases C (PKC), but the relative contribution of each PKC isoforms is unknown. Here, we investigated on the role of PKCθ in the regulation of ClC-1 channel expression and activity in fast- and slow-twitch muscles of mouse models lacking PKCθ. Electrophysiological studies showed an increase of gCl in the PKCθ-null mice with respect to wild type. Muscle excitability was reduced accordingly. However, the expression of the ClC-1 channel, evaluated by qRT-PCR, was not modified in PKCθ-null muscles suggesting that PKCθ affects the ClC-1 activity. Pharmacological studies demonstrated that although PKCθ appreciably modulates gCl, other isoforms are still active and concur to this role. The modification of gCl in PKCθ-null muscles has caused adaptation of the expression of phenotype-specific genes, such as calcineurin and myocyte enhancer factor-2, supporting the role of PKCθ also in the settings of muscle phenotype. Importantly, the lack of PKCθ has prevented the aging-related reduction of gCl, suggesting that its modulation may represent a new strategy to contrast the aging process.

Gastric cancer does not affect the expression of atrophy-related genes in human skeletal muscle

INTRODUCTION

We evaluated the gene expression levels of atrogin-1, MuRF1, myostatin, follistatin, activin A, and inhibin alpha in skeletal muscle samples of patients with gastric cancer and controls.

METHODS

We studied 38 cancer patients and 12 controls who underwent surgery for gastric adenocarcinoma and benign abdominal diseases, respectively. A biopsy specimen was obtained from the rectus abdominis muscle from all participants. The relative gene expression of atrogin-1, MuRF1, myostatin, follistatin, activin A, and inhibin alpha was determined by quantitative real-time polymerase chain reaction analysis.

RESULTS

Atrogin-1 and MuRF1 mRNA expression was similar between cancer patients and controls and was unaffected by the disease stage or the severity of body weight loss. Transcript levels of myostatin and follistatin did not differ between cases and controls and were similar across disease stages and categories of weight loss. Finally, no differences were detected in activin A and inhibin alpha gene expression between cancer patients and controls.

CONCLUSIONS

In skeletal muscle, the gene expression of atrogin-1, MuRF1, myostatin, follistatin, activin A, and inhibin alpha is not affected by the presence of cancer. The expression of atrophy-related genes is unaffected by the disease stage and the degree of weight loss.

Hypermuscular mice with mutation in the myostatin gene display altered calcium signalling

Myostatin, a member of the transforming growth factor β family, is a potent negative regulator of skeletal muscle growth, as myostatin-deficient mice show a great increase in muscle mass. Yet the physical performance of these animals is reduced. As an explanation for this, alterations in the steps in excitation-contraction coupling were hypothesized and tested for in mice with the 12 bp deletion in the propeptide region of the myostatin precursor (Mstn(Cmpt-dl1Abc) or Cmpt). In voluntary wheel running, control C57BL/6 mice performed better than the mutant animals in both maximal speed and total distance covered. Despite the previously described lower specific force of Cmpt animals, the pCa-force relationship, determined on chemically permeabilized fibre segments, did not show any significant difference between the two mouse strains. While resting intracellular Ca(2+) concentration ([Ca(2+)]i) measured on single intact flexor digitorum brevis (FDB) muscle fibres using Fura-2 AM was similar to control (72.0 ± 1.7 vs. 78.1 ± 2.9 nM, n = 38 and 45), the amplitude of KCl-evoked calcium transients was smaller (360 ± 49 vs. 222 ± 45 nM, n = 22) in the mutant strain. Similar results were obtained using tetanic stimulation and Rhod-2 AM, which gave calcium transients that were smaller (2.42 ± 0.11 vs. 2.06 ± 0.10 ΔF/F0, n = 14 and 13, respectively) on Cmpt mice. Sarcoplasmic reticulum (SR) calcium release flux calculated from these transients showed a reduced peak (23.7 ± 3.0 vs. 15.8 ± 2.1 mM s(-1)) and steady level (5.7 ± 0.7 vs. 3.7 ± 0.5 mM s(-1)) with no change in the peak-to-steady ratio. The amplitude and spatial spread of calcium release events detected on permeabilized FDB fibres were also significantly smaller in mutant mice. These results suggest that reduced SR calcium release underlies the reduced muscle force in Cmpt animals.

Myostatin inhibitors as therapies for muscle wasting associated with cancer and other disorders

PURPOSE OF REVIEW

This review summarizes recent progress in the development of myostatin inhibitors for the treatment of muscle wasting disorders. It also focuses on findings in myostatin biology that may have implications for the development of antimyostatin therapies.

RECENT FINDINGS

There has been progress in evaluating antimyostatin therapies in animal models of muscle wasting disorders. Some programs have progressed into clinical development with initial results showing positive impact on muscle volume.In normal mice myostatin deficiency results in enlarged muscles with increased total force but decreased specific force (total force/total mass). An increase in myofibrillar protein synthesis without concomitant satellite cell proliferation and fusion leads to muscle hypertrophy with unchanged myonuclear number. A specific force reduction is not observed when atrophied muscle, the predominant therapeutic target of myostatin inhibitor therapy, is made myostatindeficient.Myostatin has been shown to be expressed by a number of tumor cell lines in mice and man.

SUMMARY

Myostatin inhibition remains a promising therapeutic strategy for a range of muscle wasting disorders.

Effects of peritoneal dialysis on protein metabolism

Protein-energy wasting is relatively common in renal patients treated with haemodialysis or peritoneal dialysis (PD) and is associated with worse outcome. In this article, we review the current state of our knowledge regarding the effects of PD on protein metabolism and the possible interactions between PD-induced changes in protein turnover and the uraemia-induced alterations in protein metabolism. Available evidence shows that PD induces a new state in muscle protein dynamics, which is characterized by decreased turnover rates and a reduced efficiency of protein turnover, a condition which may be harmful in stress conditions, when nutrient intake is diminished or during superimposed catabolic illnesses. There is a need to develop more effective treatments to enhance the nutritional status of PD patients. New approaches include the use of amino acid/keto acids-containing supplements combined with physical exercise, incremental doses of intraperitoneal amino acids, vitamin D and myostatin antagonism for malnourished patients refractory to standard nutritional therapy.

Hyperammonemia in cirrhosis induces transcriptional regulation of myostatin by an NF-κB-mediated mechanism

Loss of muscle mass, or sarcopenia, is nearly universal in cirrhosis and adversely affects patient outcome. The underlying cross-talk between the liver and skeletal muscle mediating sarcopenia is not well understood. Hyperammonemia is a consistent abnormality in cirrhosis due to impaired hepatic detoxification to urea. We observed elevated levels of ammonia in both plasma samples and skeletal muscle biopsies from cirrhotic patients compared with healthy controls. Furthermore, skeletal muscle from cirrhotics had increased expression of myostatin, a known inhibitor of skeletal muscle accretion and growth. In vivo studies in mice showed that hyperammonemia reduced muscle mass and strength and increased myostatin expression in wild-type compared with postdevelopmental myostatin knockout mice. We postulated that hyperammonemia is an underlying link between hepatic dysfunction in cirrhosis and skeletal muscle loss. Therefore, murine C2C12 myotubes were treated with ammonium acetate resulting in intracellular concentrations similar to those in cirrhotic muscle. In this system, we demonstrate that hyperammonemia stimulated myostatin expression in a NF-κB-dependent manner. This finding was also observed in primary murine muscle cell cultures. Hyperammonemia triggered activation of IκB kinase, NF-κB nuclear translocation, binding of the NF-κB p65 subunit to specific sites within the myostatin promoter, and stimulation of myostatin gene transcription. Pharmacologic inhibition or gene silencing of NF-κB abolished myostatin up-regulation under conditions of hyperammonemia. Our work provides unique insights into hyperammonemia-induced myostatin expression and suggests a mechanism by which sarcopenia develops in cirrhotic patients.

Alterations in muscle mass and contractile phenotype in response to unloading models: role of transcriptional/pretranslational mechanisms

Skeletal muscle is the largest organ system in mammalian organisms providing postural control and movement patterns of varying intensity. Through evolution, skeletal muscle fibers have evolved into three phenotype clusters defined as a motor unit which consists of all muscle fibers innervated by a single motoneuron linking varying numbers of fibers of similar phenotype. This fundamental organization of the motor unit reflects the fact that there is a remarkable interdependence of gene regulation between the motoneurons and the muscle mainly via activity-dependent mechanisms. These fiber types can be classified via the primary type of myosin heavy chain (MHC) gene expressed in the motor unit. Four MHC gene encoded proteins have been identified in striated muscle: slow type I MHC and three fast MHC types, IIa, IIx, and IIb. These MHCs dictate the intrinsic contraction speed of the myofiber with the type I generating the slowest and IIb the fastest contractile speed. Over the last ~35 years, a large body of knowledge suggests that altered loading state cause both fiber atrophy/wasting and a slow to fast shift in the contractile phenotype in the target muscle(s). Hence, this review will examine findings from three different animal models of unloading: (1) space flight (SF), i.e., microgravity; (2) hindlimb suspension (HS), a procedure that chronically eliminates weight bearing of the lower limbs; and (3) spinal cord isolation (SI), a surgical procedure that eliminates neural activation of the motoneurons and associated muscles while maintaining neurotrophic motoneuron-muscle connectivity. The collective findings demonstrate: (1) all three models show a similar pattern of fiber atrophy with differences mainly in the magnitude and kinetics of alteration; (2) transcriptional/pretranslational processes play a major role in both the atrophy process and phenotype shifts; and (3) signaling pathways impacting these alterations appear to be similar in each of the models investigated.

Elevated hydrogen peroxide and decreased catalase and glutathione peroxidase protection are associated with aging sarcopenia

Background

Sarcopenia is the progressive loss of skeletal muscle that contributes to the decline in physical function during aging. A higher level of oxidative stress has been implicated in aging sarcopenia. The current study aims to determine if the higher level of oxidative stress is a result of increased superoxide (O₂‾) production by the NADPH oxidase (NOX) enzyme or decrease in endogenous antioxidant enzyme protection.

Methods

Female Balb/c mice were assigned to 4 age groups; 6, 12, 18 and 24 months. Body weight and animal survival rates were recorded over the course of the study. Skeletal muscle tissues were collected and used to measure NOX subunit mRNA, O₂‾ levels and antioxidant enzymes.

Results

Key subunit components of NOX expression were elevated in skeletal muscle at 18 months, when sarcopenia was first evident. Increased superoxide dismutase 1 (SOD1) activity suggests an increase in O₂‾ dismutation and this was further supported by elevated levels of hydrogen peroxide (H₂O₂) and decline in catalase and glutathione peroxidase (GPx) antioxidant protection in skeletal muscle at this time. NOX expression was also higher in skeletal muscle at 24 months, however this was coupled with elevated levels of O₂‾ and a decline in SOD1 activity, compared to 6 and 12 months but was not associated with further loss of muscle mass.

Conclusions

While the source of ROS in sarcopenic muscle remains unknown, this study provides evidence that the NOX enzyme could be involved in ROS production by regulating superoxide in ageing muscles. This study also suggests that H₂O₂ is the key ROS in the onset of sarcopenia and that the decline in antioxidant protection by catalase and GPx is indicative of antioxidant dysfunction and may therefore be a major contributing factor in the development or onset of sarcopenia. Furthermore, the changes in ROS and antioxidant activity after sarcopenia was first evident gives some evidence for a compensatory mechanism, in response to insult, in order to maintain muscle integrity.

Smad3 induces atrogin-1, inhibits mTOR and protein synthesis, and promotes muscle atrophy in vivo

Myostatin, a member of the TGF superfamily, is sufficient to induce skeletal muscle atrophy. Myostatin-induced atrophy is associated with increases in E3-ligase atrogin-1 expression and protein degradation and decreases in Akt/mechanistic target of rapamycin (mTOR) signaling and protein synthesis. Myostatin signaling activates the transcription factor Smad3 (Small Mothers Against Decapentaplegic), which has been shown to be necessary for myostatin-induced atrogin-1 expression and atrophy; however, it is not known whether Smad3 is sufficient to induce these events or whether Smad3 simply plays a permissive role. Thus, the aim of this study was to address these questions with an in vivo model. To accomplish this goal, in vivo transfection of plasmid DNA was used to create transient transgenic mouse skeletal muscles, and our results show for the first time that Smad3 expression is sufficient to stimulate atrogin-1 promoter activity, inhibit Akt/mTOR signaling and protein synthesis, and induce muscle fiber atrophy. Moreover, we propose that Akt/mTOR signaling is inhibited by a Smad3-induced decrease in microRNA-29 (miR-29) expression and a subsequent increase in the translation of phosphatase and tensin homolog (PTEN) mRNA. Smad3 is also sufficient to inhibit peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) promoter activity and to increase FoxO (Forkhead Box Protein, Subclass O)-mediated signaling and the promoter activity of plasminogen activator inhibitor 1 (PAI-1). Combined, this study provides the first evidence that Smad3 is sufficient to regulate many of the events associated with myostatin-induced atrophy and therefore suggests that Smad3 signaling may be a viable target for therapies aimed at preventing myostatin-induced muscle atrophy.

Biomolecular basis of the role of diabetes mellitus in osteoporosis and bone fractures

Osteoporosis has become a serious health problem throughout the world which is associated with an increased risk of bone fractures and mortality among the people of middle to old ages. Diabetes is also a major health problem among the people of all age ranges and the sufferers due to this abnormality increasing day by day. The aim of this review is to summarize the possible mechanisms through which diabetes may induce osteoporosis. Diabetes mellitus generally exerts its effect on different parts of the body including bone cells specially the osteoblast and osteoclast, muscles, retina of the eyes, adipose tissue, endocrine system specially parathyroid hormone (PTH) and estrogen, cytokines, nervous system and digestive system. Diabetes negatively regulates osteoblast differentiation and function while positively regulates osteoclast differentiation and function through the regulation of different intermediate factors and thereby decreases bone formation while increases bone resorption. Some factors such as diabetic neuropathy, reactive oxygen species, Vitamin D, PTH have their effects on muscle cells. Diabetes decreases the muscle strength through regulating these factors in various ways and ultimately increases the risk of fall that may cause bone fractures.

Cancer cachexia pathophysiology and translational aspect of herbal medicine

About half of all cancer patients show a syndrome of cachexia, characterized by anorexia and loss of adipose tissue and skeletal muscle mass. Numerous cytokines have been postulated to play a role in the etiology of cancer cachexia. Cytokines can elicit effects that mimic leptin signaling and suppress orexigenic ghrelin and neuropeptide Y signaling, inducing sustained anorexia and cachexia not accompanied by the usual compensatory response. Furthermore, cytokines have been implicated in the induction of cancer-related muscle wasting. In particular, tumor necrosis factor-alpha, interleukin-1, interleukin-6 and interferon-gamma have been implicated in the induction of cancer-related muscle wasting. Cytokine-induced skeletal muscle wasting is probably a multifactorial process, which involves a depression in protein synthesis, an increase in protein degradation or a combination of both. Cancer patients suffer from the reduction in physical function, tolerance to anti-cancer therapy and survival, while many effective chemotherapeutic agents for cancer are burdened by toxicities that can reduce patient's quality of life or hinder their effective use. Herbal medicines have been widely used to help improve such conditions. Recent studies have shown that herbal medicines such as rikkunshito enhance ghrelin signaling and consequently improve nausea, appetite loss and cachexia associated with cancer or cancer chemotherapy, which worsens the quality of life and life expectancy of the patients. The multicomponent herbal medicines capable of targeting multiple sites could be useful for future drug discovery. Mechanistic studies and identification of active compounds could lead to new discoveries in biological and biomedical sciences.

Simultaneous surface display and secretion of proteins from mammalian cells facilitate efficient in vitro selection and maturation of antibodies

A mammalian expression system has been developed that permits simultaneous cell surface display and secretion of the same protein through alternate splicing of pre-mRNA. This enables a flexible system for in vitro protein evolution in mammalian cells where the displayed protein phenotype remains linked to genotype, but with the advantage of soluble protein also being produced without the requirement for any further recloning to allow a wide range of assays, including biophysical and cell-based functional assays, to be used during the selection process. This system has been used for the simultaneous surface presentation and secretion of IgG during antibody discovery and maturation. Presentation and secretion of monomeric Fab can also be achieved to minimize avidity effects. Manipulation of the splice donor site sequence enables control of the relative amounts of cell surface and secreted antibody. Multi-domain proteins may be presented and secreted in different formats to enable flexibility in experimental design, and secreted proteins may be produced with epitope tags to facilitate high-throughput testing. This system is particularly useful in the context of in situ mutagenesis, as in the case of in vitro somatic hypermutation.

Mechanism and novel therapeutic approaches to wasting in chronic disease

Cachexia is a multifactorial syndrome defined by continuous loss of skeletal muscle mass - with or without loss of fat mass - which cannot be fully reversed by conventional nutritional support and which may lead to progressive functional impairment and increased death risk. Its pathophysiology is characterized by negative protein and energy balance driven by a variable combination of reduced food intake and abnormal metabolism. Muscle wasting is encountered in virtually all chronic disease states in particular during advanced stages of the respective illness. Several pre-clinical and clinical studies are ongoing to ameliorate this clinical problem. The mechanisms of muscle wasting and cachexia in chronic diseases such as cancer, chronic heart failure, chronic obstructive pulmonary disease and chronic kidney disease are described. We discuss therapeutic targets and such potential modulators as appetite stimulants, selective androgen receptor modulators, amino acids and naturally occurring peptide hormones.

Cancer cachexia--pathophysiology and management

About half of all cancer patients show a syndrome of cachexia, characterized by anorexia and loss of adipose tissue and skeletal muscle mass. Cachexia can have a profound impact on quality of life, symptom burden, and a patient's sense of dignity. It is a very serious complication, as weight loss during cancer treatment is associated with more chemotherapy-related side effects, fewer completed cycles of chemotherapy, and decreased survival rates. Numerous cytokines have been postulated to play a role in the etiology of cancer cachexia. Cytokines can elicit effects that mimic leptin signaling and suppress orexigenic ghrelin and neuropeptide Y (NPY) signaling, inducing sustained anorexia and cachexia not accompanied by the usual compensatory response. Furthermore, cytokines have been implicated in the induction of cancer-related muscle wasting. Cytokine-induced skeletal muscle wasting is probably a multifactorial process, which involves a protein synthesis inhibition, an increase in protein degradation, or a combination of both. The best treatment of the cachectic syndrome is a multifactorial approach. Many drugs including appetite stimulants, thalidomide, cytokine inhibitors, steroids, nonsteroidal anti-inflammatory drugs, branched-chain amino acids, eicosapentaenoic acid, and antiserotoninergic drugs have been proposed and used in clinical trials, while others are still under investigation using experimental animals. There is a growing awareness of the positive impact of supportive care measures and development of promising novel pharmaceutical agents for cachexia. While there has been great progress in understanding the underlying biological mechanisms of cachexia, health care providers must also recognize the psychosocial and biomedical impact cachexia can have.

Mechanisms for fiber-type specificity of skeletal muscle atrophy

PURPOSE OF REVIEW

There are a variety of pathophysiologic conditions that are known to induce skeletal muscle atrophy. However, muscle wasting can occur through multiple distinct signaling pathways with differential sensitivity between selective skeletal muscle fiber subtypes. This review summarizes some of the underlying molecular mechanisms responsible for fiber-specific muscle mass regulation.

RECENT FINDINGS

Peroxisome proliferator-activated receptor gamma coactivator 1-alpha protects slow-twitch oxidative fibers from denervation/immobilization (disuse)-induced muscle atrophies. Nutrient-related muscle atrophies, such as those induced by cancer cachexia, sepsis, chronic heart failure, or diabetes, are largely restricted to fast-twitch glycolytic fibers, of which the underlying mechanism is usually related to abnormality of protein degradation, including proteasomal and lysosomal pathways. In contrast, nuclear factor kappaB activation apparently serves a dual function by inducing both fast-twitch fiber atrophy and slow-twitch fiber degeneration.

SUMMARY

Fast-twitch glycolytic fibers are more vulnerable than slow-twitch oxidative fibers under a variety of atrophic conditions related to signaling transduction of Forkhead box O family, autophagy inhibition, transforming growth factor beta family, and nuclear factor-kappaB. The resistance of oxidative fibers may result from the protection of peroxisome proliferator-activated receptor gamma coactivator 1-alpha.

Highlights of mechanistic and therapeutic cachexia and sarcopenia research 2010 to 2012 and their relevance for cardiology

Sarcopenia and cachexia are significant medical problems with a high disease-related burden in cardiovascular illness. Muscle wasting and weight loss are very frequent particularly in chronic heart failure and they relate to poor prognosis. Although clinically largely underestimated, the fields of cachexia and sarcopenia are of great relevance to cardiologists. In cachexia and sarcopenia a significant number of research publications related to basic science questions of muscle wasting and lipolysis were published between 2010 and 2012. Recently, the two processes of muscle wasting and lipolysis were found to be closely linked. Treatment research in pre-clinical models involves studies on a number of different therapeutic entities, including ghrelin, selective androgen receptor modulators (SARMs), as well as drugs targeting myostatin or melanocortin-4. In the human setting, studies using enobosarm (a SARM) and anamorelin (ghrelin) are in phase III. The last 3 years have seen significant efforts to define the field using consensus statements. In the future, these definitions should also be considered for guidelines and treatment trials in cardiovascular medicine. The current review aims to summarize important information and development in the fields of muscle wasting, sarcopenia and cachexia, focusing on findings in cardiovascular research, in order for cardiologists to have a better understanding of the progress in this still insufficiently known field.

Myostatin acts as an autocrine/paracrine negative regulator in myoblast differentiation from human induced pluripotent stem cells

Myostatin, also known as growth differentiation factor (GDF-8), regulates proliferation of muscle satellite cells, and suppresses differentiation of myoblasts into myotubes via down-regulation of key myogenic differentiation factors including MyoD. Recent advances in stem cell biology have enabled generation of myoblasts from pluripotent stem cells, but it remains to be clarified whether myostatin is also involved in regulation of artificial differentiation of myoblasts from pluripotent stem cells. Here we show that the human induced pluripotent stem (iPS) cell-derived cells that were induced to differentiate into myoblasts expressed myostatin and its receptor during the differentiation. An addition of recombinant human myostatin (rhMyostatin) suppressed induction of MyoD and Myo5a, resulting in significant suppression of myoblast differentiation. The rhMyostatin treatment also inhibited proliferation of the cells at a later phase of differentiation. RNAi-mediated silencing of myostatin promoted differentiation of human iPS-derived embryoid body (EB) cells into myoblasts. These results strongly suggest that myostatin plays an important role in regulation of myoblast differentiation from iPS cells of human origin. The present findings also have significant implications for potential regenerative medicine for muscular diseases.

Skeletal muscle protein metabolism in human heart failure

PURPOSE OF REVIEW

This review considers evidence that the clinical condition of heart failure alters skeletal muscle protein synthesis and/or breakdown to promote skeletal muscle wasting and functional decrements that ultimately contribute to the symptomology of the disease.

RECENT FINDINGS

Advanced HF is frequently accompanied by muscle atrophy and a cachectic phenotype. Protein metabolic derangements that promote this phenotype are understudied and poorly understood. Instead, most investigations have evaluated regulatory hormones/signaling pathways thought to be reflective of protein synthesis and breakdown. Several of these recent studies have provided exciting data suggesting that the dysfunctional myocardium releases catabolic agents that could promote the skeletal muscle myopathic phenotype either directly or through modulation of other regulatory systems (e.g., energy balance).

SUMMARY

Although our understanding of skeletal muscle atrophy and dysfunction in heart failure is limited, recent studies have provided clues about the nature and timing of protein metabolic dysfunction. More specifically, skeletal muscle protein metabolic derangements likely evolve during periods of disease-related stress (i.e., acute disease exacerbation and hospitalization) and potentially derive in part, from signals promoted in the damaged/dysfunctional myocardium. Despite these compelling studies, there is a surprising lack of data regarding the nature or timing of specific protein metabolic defects in heart failure.

Consilience in sarcopenia of cirrhosis

Cirrhosis is the consequence of progression of many forms of necro-inflammatory disorders of the liver with hepatic fibrosis, hepatocellular dysfunction, and vascular remodeling. Reversing the primary hepatic disorder, liver transplantation, and controlling the complications are the major management goals. Since the former options are not available to the majority of cirrhotics, treating complications remains the mainstay of therapy. Sarcopenia and/or cachexia is the most common complication and adversely affects survival, quality of life, development of other complications of cirrhosis, and outcome after liver transplantation. With the increase in number of cirrhotic patients with hepatitis C and nonalcoholic fatty liver disease, the number of patients waiting for a liver transplantation is likely to continue to increase above the currently estimated 72.3/100,000 population. One of the critical clinical questions is to determine if we can treat sarcopenia of cirrhosis without transplantation. No effective therapies exist to treat sarcopenia because the mechanism(s) of sarcopenia in cirrhosis is as yet unknown. The reasons for this include the predominantly descriptive studies to date and the advances in our understanding of skeletal muscle biology and molecular regulation of atrophy and hypertrophy not being translated into the clinical practice of hepatology. Satellite cell biology, muscle autophagy and apoptosis, and molecular signaling abnormalities in the skeletal muscle of cirrhotics are also not known. Aging of the cirrhotic and transplanted population, use of mTOR inhibitors, and the lack of definitive outcome measures to define sarcopenia and cachexia in this population add to the difficulty in increasing our understanding of hepatic sarcopenia/cachexia and developing treatment options. Recent data on the role of myostatin, AMP kinase, impaired mTOR signaling resulting in anabolic resistance in animal models, and the rapidly developing field of nutriceuticals as signaling molecules need to be evaluated in human cirrhotics. Finally, the benefits of exercise reported in other disease states with sarcopenia may not be safe in cirrhotics due to the risk of gastrointestinal variceal bleeding due to an increase in portal pressure. This article focuses on the problems facing both muscle biologists and hepatologists in developing a comprehensive approach to sarcopenia in cirrhosis.

Genetic variation in human muscle strength--opportunities for therapeutic interventions?

Inter-individual variation in muscle mass and muscular fitness is broad; being at the upper tail of the distribution not only contributes to improve elite sport performance, but is also associated with longer independent living and higher quality-of-life in the aging population. Heritability estimates of muscle phenotypes are substantial and warrant the search for genetic components underlying this individual variability. The 'kinesiogenomics' field is young, but genetic associations with muscle strength-related phenotypes have been reported already for more than 40 candidate genes, and genome-wide scans revealed several additional regions of interest in the genome. Although genetic findings may reveal attractive targets for novel muscle atrophy therapy, the benefit of exercise as a major stimulus for natural muscle mass enhancement or maintenance cannot be underestimated.