Soluble activin receptor type IIB increases forward pulling tension in the mdx mouse

ACTRIIA-Fc rebalances activin/GDF versus BMP signaling in pulmonary hypertension

Human genetics, biomarker, and animal studies implicate loss of function in bone morphogenetic protein (BMP) signaling and maladaptive transforming growth factor-β (TGFβ) signaling as drivers of pulmonary arterial hypertension (PAH). Although sharing common receptors and effectors with BMP/TGFβ, the function of activin and growth and differentiation factor (GDF) ligands in PAH are less well defined. Increased expression of GDF8, GDF11, and activin A was detected in lung lesions from humans with PAH and experimental rodent models of pulmonary hypertension (PH). ACTRIIA-Fc, a potent GDF8/11 and activin ligand trap, was used to test the roles of these ligands in animal and cellular models of PH. By blocking GDF8/11- and activin-mediated SMAD2/3 activation in vascular cells, ACTRIIA-Fc attenuated proliferation of pulmonary arterial smooth muscle cells and pulmonary microvascular endothelial cells. In several experimental models of PH, prophylactic administration of ACTRIIA-Fc markedly improved hemodynamics, right ventricular (RV) hypertrophy, RV function, and arteriolar remodeling. When administered after the establishment of hemodynamically severe PH in a vasculoproliferative model, ACTRIIA-Fc was more effective than vasodilator in attenuating PH and arteriolar remodeling. Potent antiremodeling effects of ACTRIIA-Fc were associated with inhibition of SMAD2/3 activation and downstream transcriptional activity, inhibition of proliferation, and enhancement of apoptosis in the vascular wall. ACTRIIA-Fc reveals an unexpectedly prominent role of GDF8, GDF11, and activin as drivers of pulmonary vascular disease and represents a therapeutic strategy for restoring the balance between SMAD1/5/9 and SMAD2/3 signaling in PAH.

Antimyostatin Treatment in Health and Disease: The Story of Great Expectations and Limited Success

In the past 20 years, myostatin, a negative regulator of muscle mass, has attracted attention as a potential therapeutic target in muscular dystrophies and other conditions. Preclinical studies have shown potential for increasing muscular mass and ameliorating the pathological features of dystrophic muscle by the inhibition of myostatin in various ways. However, hardly any clinical trials have proven to translate the promising results from the animal models into patient populations. We present the background for myostatin regulation, clinical and preclinical results and discuss why translation from animal models to patients is difficult. Based on this, we put the clinical relevance of future antimyostatin treatment into perspective.

Mathematical Model of Muscle Wasting in Cancer Cachexia

Cancer cachexia is a debilitating condition characterized by an extreme loss of skeletal muscle mass, which negatively impacts patients' quality of life, reduces their ability to sustain anti-cancer therapies, and increases the risk of mortality. Recent discoveries have identified the myostatin/activin A/ActRIIB pathway as critical to muscle wasting by inducing satellite cell quiescence and increasing muscle-specific ubiquitin ligases responsible for atrophy. Remarkably, pharmacological blockade of the ActRIIB pathway has been shown to reverse muscle wasting and prolong the survival time of tumor-bearing animals. To explore the implications of this signaling pathway and potential therapeutic targets in cachexia, we construct a novel mathematical model of muscle tissue subjected to tumor-derived cachectic factors. The model formulation tracks the intercellular interactions between cancer cell, satellite cell, and muscle cell populations. The model is parameterized by fitting to colon-26 mouse model data, and the analysis provides insight into tissue growth in healthy, cancerous, and post-cachexia treatment conditions. Model predictions suggest that cachexia fundamentally alters muscle tissue health, as measured by the stem cell ratio, and this is only partially recovered by anti-cachexia treatment. Our mathematical findings suggest that after blocking the myostatin/activin A pathway, partial recovery of cancer-induced muscle loss requires the activation and proliferation of the satellite cell compartment with a functional differentiation program.

Anti-Inflammatory and General Glucocorticoid Physiology in Skeletal Muscles Affected by Duchenne Muscular Dystrophy: Exploration of Steroid-Sparing Agents

In Duchenne muscular dystrophy (DMD), the activation of proinflammatory and metabolic cellular pathways in skeletal muscle cells is an inherent characteristic. Synthetic glucocorticoid intake counteracts the majority of these mechanisms. However, glucocorticoids induce burdensome secondary effects, including hypertension, arrhythmias, hyperglycemia, osteoporosis, weight gain, growth delay, skin thinning, cushingoid appearance, and tissue-specific glucocorticoid resistance. Hence, lowering the glucocorticoid dosage could be beneficial for DMD patients. A more profound insight into the major cellular pathways that are stabilized after synthetic glucocorticoid administration in DMD is needed when searching for the molecules able to achieve similar pathway stabilization. This review provides a concise overview of the major anti-inflammatory pathways, as well as the metabolic effects of glucocorticoids in the skeletal muscle affected in DMD. The known drugs able to stabilize these pathways, and which could potentially be combined with glucocorticoid therapy as steroid-sparing agents, are described. This could create new opportunities for testing in DMD animal models and/or clinical trials, possibly leading to smaller glucocorticoids dosage regimens for DMD patients.

Mouse models for muscular dystrophies: an overview

Muscular dystrophies (MDs) encompass a wide variety of inherited disorders that are characterized by loss of muscle tissue associated with a progressive reduction in muscle function. With a cure lacking for MDs, preclinical developments of therapeutic approaches depend on well-characterized animal models that recapitulate the specific pathology in patients. The mouse is the most widely and extensively used model for MDs, and it has played a key role in our understanding of the molecular mechanisms underlying MD pathogenesis. This has enabled the development of therapeutic strategies. Owing to advancements in genetic engineering, a wide variety of mouse models are available for the majority of MDs. Here, we summarize the characteristics of the most commonly used mouse models for a subset of highly studied MDs, collated into a table. Together with references to key publications describing these models, this brief but detailed overview would be useful for those interested in, or working with, mouse models of MD.

Improving translatability of preclinical studies for neuromuscular disorders: lessons from the TREAT-NMD Advisory Committee for Therapeutics (TACT)

Clinical trials for rare neuromuscular diseases imply, among other investments, a high emotional burden for the whole disease community. Translation of data from preclinical studies to justify any clinical trial must be carefully pondered in order to minimize the risk of clinical trial withdrawal or failure. A rigorous distinction between proof-of-concept and preclinical efficacy studies using animal models is key to support the rationale of a clinical trial involving patients. This Review evaluates the experience accumulated by the TREAT-NMD Advisory Committee for Therapeutics, which provides detailed constructive feedback on clinical proposals for neuromuscular diseases submitted by researchers in both academia and industry, and emphasizes that a timely critical review of preclinical efficacy data from animal models, including biomarkers for specific diseases, combined with adherence to existing guidelines and standard protocols, can significantly help to de-risk clinical programs and prevent disappointments and costly engagement.

Inhibition of the Activin Receptor Type-2B Pathway Restores Regenerative Capacity in Satellite Cell-Depleted Skeletal Muscle

Degenerative myopathies typically display a decline in satellite cells coupled with a replacement of muscle fibers by fat and fibrosis. During this pathological remodeling, satellite cells are present at lower numbers and do not display a proper regenerative function. Whether a decline in satellite cells directly contributes to disease progression or is a secondary result is unknown. In order to dissect these processes, we used a genetic model to reduce the satellite cell population by ~70–80% which leads to a nearly complete loss of regenerative potential. We observe that while no overt tissue damage is observed following satellite cell depletion, muscle fibers atrophy accompanied by changes in the stem cell niche cellular composition. Treatment of these mice with an Activin receptor type-2B (AcvR2B) pathway blocker reverses muscle fiber atrophy as expected, but also restores regenerative potential of the remaining satellite cells. These findings demonstrate that in addition to controlling fiber size, the AcvR2B pathway acts to regulate the muscle stem cell niche providing a more favorable environment for muscle regeneration.

Electrical impedance myography as a biomarker of myostatin inhibition with ActRIIB-mFc: a study in wild-type mice

Aim:

We sought to determine the sensitivity of electrical impedance myography (EIM) to myofiber hypertrophy induced by treatment with various doses of ActRIIB-mFc, an inhibitor of myostatin signaling.

Methods:

Wild-type C57BL/6 J mice (n = 40, male) were treated with three different doses of ActRIIB-mFc (i.e., RAP-031) or vehicle twice weekly for 5 weeks. End point assessments included gastrocnemius EIM, force measurements, muscle mass and myofiber size quantification.

Results:

ActRIIB-mFc increased body mass, muscle mass and myofiber size across all doses. Alterations in EIM 50 kHz phase and center frequency (fc) were also present, with trends in a dose-dependent fashion. Significant correlations between EIM parameters and myofiber/functional data were identified.

Conclusion:

EIM outcomes can serve as effective biomarkers of myostatin signaling inhibition, demonstrating a dose sensitivity and correlation to standard assessments.

We were interested in studying the sensitivity of a technique, called electrical impedance myography (EIM), to noninvasively assess the size of muscle fibers. In this technique a minute electrical current is used to probe the tissue. To do so, we gave a drug (ActRIIB-mFc) to mice that enlarges muscle fibers at three different doses. We were able to show that the EIM technique was able to detect this differential effect and functional changes induced by the drug correlated to the EIM data. This work suggests that EIM will be useful as a noninvasive marker muscle health.

Identification of ligand-selective peptidic ActRIIB-antagonists using phage display technology

ActRIIB (activin receptor type-2B) is an activin receptor subtype constitutively expressed in the whole body, playing a role in cellular proliferation, differentiation, and metabolism. For its various physiological activities, ActRIIB interacts with activin and multiple other ligands including myostatin (MSTN), growth differentiation factor 11 (GDF11), and bone morphogenetic protein 9 (BMP9). Notably, the protein-protein interaction (PPI) between ActRIIB and MSTN negatively controls muscular development. Therefore, this PPI has been targeted for effective treatment of muscle degenerative diseases such as muscular dystrophy and sarcopenia. Here, we report the identification of ligand-selective peptidic ActRIIB-antagonists by phage display technology. Our peptides bound to the extracellular domain of ActRIIB, inhibited PPIs between ActRIIB expressed on the cell surface and its ligands, and subsequently suppressed activation of Smad that serves as the downstream signal of the ActRIIB pathway. Interestingly, these peptidic antagonists displayed different ligand selectivities; the AR2mini peptide inhibited multiple ligands (activin A, MSTN, GDF11, and BMP9), AR9 inhibited MSTN and GDF11, while AR8 selectively inhibited MSTN. This is the first report of artificial peptidic ActRIIB-antagonists possessing ligand-selectivity.

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Artificial ActRIIB-antagonist peptides were discovered by phage display.

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These peptides selectively bound to the extracellular domain of ActRIIB.

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They antagonized ActRIIB expressed on the cell surface.

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They presented multiple ligand selectivities.

Muscle abnormalities in osteogenesis imperfecta

Osteogenesis imperfecta (OI) is mainly characterized by bone fragility but muscle abnormalities have been reported both in OI mouse models and in children with OI. Muscle mass is decreased in OI, even when short stature is taken into account. Dynamic muscle tests aiming at maximal eccentric force production reveal functional deficits that can not be explained by low muscle mass alone. However, it appears that diaphyseal bone mass is normally adapted to muscle force. At present the determinants of muscle mass and function in OI have not been clearly defined. Physiotherapy interventions and bisphosphonate treatment appear to have some effect on muscle function in OI. Interventions targeting muscle mass have shown encouraging results in OI animal models and are an interesting area for further research.

Agents Which Inhibit NF-κB Signaling Block Spontaneous Contractile Activity and Negatively Influence Survival of Developing Myotubes

Inhibiting the NF-κB signaling pathway provides morphological and functional benefits for the mdx mouse, a model for Duchenne muscular dystrophy characterized by chronic elevations in the nuclear expression of p65, the transactivating component of the NF-κB complex. The purpose of this study was to examine p65 expression in nondystrophic and mdx myotubes using confocal immunofluorescence, and determine whether inhibitors of the NF-κB pathway alter myotube development. Primary cultures of nondystrophic and mdx myotubes had identical levels of nuclear and cytosolic p65 expression and exhibited equivalent responses to TNF-α, thus excluding the hypothesis that the lack of dystrophin is sufficient to induce increases in NF-κB signaling. The NF-κB inhibitors pyrrolidine dithiocarbamate (PDTC) and sulfasalazine decreased spontaneous contractile activity and reduced myotube viability in a dose- and time-dependent manner. Similarly, a vivo-morpholino designed to block translation of murine p65 (m-p65tb-vivomorph1) rapidly abolished spontaneous contractile activity, reduced p65 expression measured by confocal immunofluorescence, and induced cell death in primary cultures of nondystrophic and mdx myotubes. Similar effects on p65 immunofluorescence and cell viability were observed following m-p65tb-vivomorph1 exposure to spontaneously inactive C2C12 myotubes, while exposure to a control scrambled vivo morpholino had no effect. These results indicate a direct role of the NF-κB pathway in myotube development and identify a potential therapeutic limitation to the use of NF-κB inhibitors in treating Duchenne and related muscular dystrophies. J. Cell. Physiol. 231: 788-797, 2016. © 2015 Wiley Periodicals, Inc.

Postnatal Hyperplasic Effects of ActRIIB Blockade in a Severely Dystrophic Muscle

The efficacy of two ActRIIB ligand-trapping agents (RAP-031 and RAP-435) in treating muscular dystrophy was examined by determining their morphological effects on the severely dystrophic triangularis sterni (TS) muscle of the mdx mouse, a model for Duchenne muscular dystrophy. These agents trap all endogenous ligands to the ActRIIB receptor and thereby block myostatin signaling in a highly selective manner. Short-term (1 month) and long-term (3 months) in vivo treatment of 1-month-old mdx mice increased myonuclei and fiber cross section (FCS) density but did not alter individual fiber size. Vehicle-treated mdx mice exhibited age-dependent increases in myonuclei and FCS density, and age-dependent reductions in centronucleation that were each enhanced by treatment with RAP-435. Distributions of FCS area (FCSA) in the mdx TS were 90% identical to those from untreated age-matched nondystrophic mice and were unaltered by the substantial fiber hyperplasia observed with age and RAP-435 treatment. These results were inconsistent with injury-induced fiber regeneration which produces altered FCSA distributions characterized by a distinct class of smaller regenerated fibers. Nondystrophic mice exhibited a constant postnatal density of fiber cross sections and myonuclei, and RAP-435 treatment of nondystrophic mice increased TS mean FCSA but had no effects on myonuclei or FCS density. These results demonstrating a continual postnatal proliferation and fusion of satellite cells and a response to myostatin blockade characteristic of developing prenatal muscle suggest that the lack of dystrophin directly results in unrestrained postnatal satellite cell activation that is not necessarily dependent upon prior fiber degeneration. J. Cell. Physiol. 232: 1774-1793, 2017. © 2016 Wiley Periodicals, Inc.

Myostatin inhibitor ACE-031 treatment of ambulatory boys with Duchenne muscular dystrophy: Results of a randomized, placebo-controlled clinical trial

INTRODUCTION

ACE-031 is a fusion protein of activin receptor type IIB and IgG1-Fc, which binds myostatin and related ligands. It aims to disrupt the inhibitory effect on muscle development and provide potential therapy for myopathies like Duchenne muscular dystrophy (DMD).

METHODS

ACE-031 was administered subcutaneously every 2-4 weeks to DMD boys in a randomized, double-blind, placebo-controlled, ascending-dose trial. The primary objective was safety evaluation. Secondary objectives included characterization of pharmacokinetics and pharmacodynamics.

RESULTS

ACE-031 was not associated with serious or severe adverse events. The study was stopped after the second dosing regimen due to potential safety concerns of epistaxis and telangiectasias. A trend for maintenance of the 6-minute walk test (6MWT) distance in the ACE-031 groups compared with a decline in the placebo group (not statistically significant) was noted, as was a trend for increased lean body mass and bone mineral density (BMD) and reduced fat mass.

CONCLUSION

ACE-031 use demonstrated trends for pharmacodynamic effects on lean mass, fat mass, BMD, and 6MWT. Non-muscle-related adverse events contributed to the decision to discontinue the study. Myostatin inhibition is a promising therapeutic approach for DMD. Muscle Nerve 55: 458-464, 2017.

ActRIIB blockade increases force-generating capacity and preserves energy supply in exercising mdx mouse muscle in vivo

Postnatal blockade of the activin type IIB receptor (ActRIIB) represents a promising therapeutic strategy for counteracting dystrophic muscle wasting. However, its impact on muscle function and bioenergetics remains poorly documented in physiologic conditions. We have investigated totally noninvasively the effect of 8-wk administration of either soluble ActRIIB signaling inhibitor (sActRIIB-Fc) or vehicle PBS (control) on gastrocnemius muscle force-generating capacity, energy metabolism, and anatomy in dystrophic mdx mice using magnetic resonance (MR) imaging and dynamic [³¹P]-MR spectroscopy ([³¹P]-MRS) in vivo ActRIIB inhibition increased muscle volume (+33%) without changing fiber-type distribution, and increased basal animal oxygen consumption (+22%) and energy expenditure (+23%). During an in vivo standardized fatiguing exercise, maximum and total absolute contractile forces were larger (+40 and 24%, respectively) in sActRIIB-Fc treated animals, whereas specific force-generating capacity and fatigue resistance remained unaffected. Furthermore, sActRIIB-Fc administration did not alter metabolic fluxes, ATP homeostasis, or contractile efficiency during the fatiguing bout of exercise, although it dramatically reduced the intrinsic mitochondrial capacity for producing ATP. Overall, sActRIIB-Fc treatment increased muscle mass and strength without altering the fundamental weakness characteristic of dystrophic mdx muscle. These data support the clinical interest of ActRIIB blockade for reversing dystrophic muscle wasting.-Béchir, N., Pecchi, E., Vilmen, C., Le Fur, Y., Amthor, H., Bernard, M., Bendahan, D., Giannesini, B. ActRIIB blockade increases force-generating capacity and preserves energy supply in exercising mdx mouse muscle in vivo.

Emerging drugs affecting skeletal muscle function and mitochondrial biogenesis - Potential implications for sports drug testing programs

RATIONALE

A plethora of compounds potentially leading to drug candidates that affect skeletal muscle function and, more specifically, mitochondrial biogenesis, has been under (pre)clinical investigation for rare as well as more common diseases. Some of these compounds could be the object of misuse by athletes aiming at artificial and/or illicit and drug-facilitated performance enhancement, necessitating preventive and proactive anti-doping measures.

METHODS

Early warnings and the continuous retrieval and dissemination of information are crucial for sports drug testing laboratories as well as anti-doping authorities, as they assist in preparation of efficient doping control analytical strategies for potential future threats arising from new therapeutic developments. Scientific literature represents the main source of information, which yielded the herein discussed substances and therapeutic targets, which might become relevant for doping controls in the future. Where available, mass spectrometric data are presented, supporting the development of analytical strategies and characterization of compounds possibly identified in human sports drug testing samples.

RESULTS & CONCLUSIONS

Focusing on skeletal muscle and mitochondrial biogenesis, numerous substances exhibiting agonistic or antagonistic actions on different cellular 'control centers' resulting in increased skeletal muscle mass, enhanced performance (as determined with laboratory animal models), and/or elevated amounts of mitochondria have been described. Substances of interest include agonists for REV-ERBα (e.g. SR9009, SR9011, SR10067, GSK4112), sirtuin 1 (e.g. SRT1720, SRT2104), adenosine monophosphate-activated protein kinase (AMPK, e.g. AICAR), peroxisome proliferator-activated receptor (PPAR)δ (e.g. GW1516, GW0742, L165041), and inhibitory/antagonistic agents targeting the methionine-folate cycle (MOTS-c), the general control non-derepressible 5 (GCN5) acetyl transferase (e.g. CPTH2, MB-3), myostatin (e.g. MYO-029), the myostatin receptor (bimagrumab), and myostatin receptor ligands (e.g. sotatercept, ACE-031). In addition, potentially relevant drug targets were identified, e.g. with the sarcoplasmic transmembrane peptide myoregulin and the nuclear receptor corepressor 1 (NCOR-1). The antagonism of these has shown to result in substantially enhanced physical performance in animals, necessitating the monitoring of strategies such as RNA interference regarding these substances. Most drug candidates are of lower molecular mass and comprise non-natural compositions, facts which suggest approaches for their qualitative identification in doping control samples by mass spectrometry. Electrospray ionization/collision-induced dissociation mass spectra of representatives of the aforementioned substances and selected in vitro derived phase-I metabolites support this assumption, and test methods for a subset of these have been recently established. Expanding the knowledge on analytical data will further facilitate the identification of such analytes and related compounds in confiscated material as well as sports drug testing specimens.

Inhibiting myostatin signaling prevents femoral trabecular bone loss and microarchitecture deterioration in diet-induced obese rats

Besides resulting in a dramatic increase in skeletal muscle mass, myostatin (MSTN) deficiency has a positive effect on bone formation. However, the issue about whether blocking MSTN can inhibit obesity-induced bone loss has not been previously investigated. In the present study, we have evaluated the effects of MSTN blocking on bone quality in high-fat (HF), diet-induced obese rats using a prepared polyclonal antibody for MSTN (MsAb). Twenty-four rats were randomly assigned to the Control, HF and HF + MsAb groups. Rats in the HF + MsAb group were injected once a week with purified MsAb for eight weeks. The results showed that MsAb significantly reduced body and fat weight, and increased muscle mass and strength in the HF group. MicroCT analysis demonstrated that obesity-induced bone loss and architecture deterioration were significantly mitigated by MsAb treatment, as evidenced by increased bone mineral density, bone volume over total volume, trabecular number and thickness, and decreased trabecular separation and structure model index. However, neither HF diet nor MsAb treatment had an impact on femoral biomechanical properties including maximum load, stiffness, energy absorption and elastic modulus. Moreover, MsAb significantly increased adiponectin concentrations, and decreased TNF-α and IL-6 levels in diet-induced obese rats. Taken together, blocking MSTN by MsAb improves bone quality in diet-induced obese rats through a mechanotransduction pathway from skeletal muscle, and the accompanying changes occurring in the levels of circulating adipokines and pro-inflammatory cytokines may also be involved in this process. It indicates that the administration of MSTN antagonists may be a promising therapy for treating obesity and obesity-induced bone loss.

Muscle hypertrophy induced by myostatin inhibition accelerates degeneration in dysferlinopathy

Myostatin is a secreted signaling molecule that normally acts to limit muscle growth. As a result, there is extensive effort directed at developing drugs capable of targeting myostatin to treat patients with muscle loss. One potential concern with this therapeutic approach in patients with muscle degenerative diseases like muscular dystrophy is that inducing hypertrophy may increase stress on dystrophic fibers, thereby accelerating disease progression. To investigate this possibility, we examined the effect of blocking the myostatin pathway in dysferlin-deficient (Dysf^−/−) mice, in which membrane repair is compromised, either by transgenic expression of follistatin in skeletal muscle or by systemic administration of the soluble form of the activin type IIB receptor (ACVR2B/Fc). Here, we show that myostatin inhibition by follistatin transgene expression in Dysf^−/− mice results in early improvement in histopathology but ultimately exacerbates muscle degeneration; this effect was not observed in dystrophin-deficient (mdx) mice, suggesting that accelerated degeneration induced by follistatin transgene expression is specific to mice lacking dysferlin. Dysf^−/− mice injected with ACVR2B/Fc showed significant increases in muscle mass and amelioration of fibrotic changes normally seen in 8-month-old Dysf^−/− mice. Despite these potentially beneficial effects, ACVR2B/Fc treatment caused increases in serum CK levels in some Dysf^−/− mice, indicating possible muscle damage induced by hypertrophy. These findings suggest that depending on the disease context, inducing muscle hypertrophy by myostatin blockade may have detrimental effects, which need to be weighed against the potential gains in muscle growth and decreased fibrosis.

Fibronectin-based scaffold domain proteins that bind myostatin: a patent evaluation of WO2014043344

Muscular dystrophies (MD) are commonly characterized by progressive loss of muscle mass and function. It is hypothesized that therapeutic blockade of the TGF-β ligand myostatin, a negative regulator of muscle mass, will stimulate muscle growth and restore muscle function. Although many anti-myostatin targets are currently being pursued in the clinical setting, the efficacies of the tested molecules have shown mixed results. The patent WO2014043344 describes a novel approach for myostatin inhibition using a modified fibronectin type III domain that could potentially be used to treat MD and other muscle-related pathologies.

Deficient nitric oxide signalling impairs skeletal muscle growth and performance: involvement of mitochondrial dysregulation

BACKGROUND

Nitric oxide (NO), generated in skeletal muscle mostly by the neuronal NO synthases (nNOSμ), has profound effects on both mitochondrial bioenergetics and muscle development and function. The importance of NO for muscle repair emerges from the observation that nNOS signalling is defective in many genetically diverse skeletal muscle diseases in which muscle repair is dysregulated. How the effects of NO/nNOSμ on mitochondria impact on muscle function, however, has not been investigated yet.

METHODS

In this study we have examined the relationship between the NO system, mitochondrial structure/activity and skeletal muscle phenotype/growth/functions using a mouse model in which nNOSμ is absent. Also, NO-induced effects and the NO pathway were dissected in myogenic precursor cells.

RESULTS

We show that nNOSμ deficiency in mouse skeletal muscle leads to altered mitochondrial bioenergetics and network remodelling, and increased mitochondrial unfolded protein response (UPR(mt)) and autophagy. The absence of nNOSμ is also accompanied by an altered mitochondrial homeostasis in myogenic precursor cells with a decrease in the number of myonuclei per fibre and impaired muscle development at early stages of perinatal growth. No alterations were observed, however, in the overall resting muscle structure, apart from a reduced specific muscle mass and cross sectional areas of the myofibres. Investigating the molecular mechanisms we found that nNOSμ deficiency was associated with an inhibition of the Akt-mammalian target of rapamycin pathway. Concomitantly, the Akt-FoxO3-mitochondrial E3 ubiquitin protein ligase 1 (Mul-1) axis was also dysregulated. In particular, inhibition of nNOS/NO/cyclic guanosine monophosphate (cGMP)/cGMP-dependent-protein kinases induced the transcriptional activity of FoxO3 and increased Mul-1 expression. nNOSμ deficiency was also accompanied by functional changes in muscle with reduced muscle force, decreased resistance to fatigue and increased degeneration/damage post-exercise.

CONCLUSIONS

Our results indicate that nNOSμ/NO is required to regulate key homeostatic mechanisms in skeletal muscle, namely mitochondrial bioenergetics and network remodelling, UPR(mt) and autophagy. These events are likely associated with nNOSμ-dependent impairments of muscle fibre growth resulting in a deficit of muscle performance.

Pharmacological inhibition of myostatin and changes in lean body mass and lower extremity muscle size in patients receiving androgen deprivation therapy for prostate cancer

CONTEXT

Myostatin is a negative regulator of muscle growth. Androgen deprivation (ADT) is associated with muscle loss and increased body fat, and currently available therapies have limited efficacy to treat this complication. The antimyostatin peptibody (AMG 745/Mu-S) markedly attenuated muscle loss and decreased fat accumulation in orchiectomized mice.

OBJECTIVE

The objective of the study was to evaluate the safety, pharmacokinetics, and muscle efficacy of AMG 745 in men undergoing ADT for nonmetastatic prostate cancer.

METHODS

This was a randomized, blinded, placebo-controlled, multiple-dose, phase 1 study of AMG 745 given for 28 days. The end point of percentage change from baseline in lean body mass (LBM) as assessed by dual x-ray absorptiometry was prespecified.

RESULTS

Rates of adverse events (AMG 745 vs placebo) were the following: diarrhea (13% vs 9%), fatigue (13% vs 4%), contusion (10% vs 0%), and injection site bruising (6% vs 4%). Exposure increased linearly from 0.3 mg/kg to 3 mg/kg. AMG 745 significantly increased LBM in the 3 mg/kg vs the placebo groups on day 29 by 2.2% (±0.8% SE, P = 0.008); in exploratory fat mass analysis, a decrease of -2.5% (±1.0% SE, P = 0.021) was observed. Pharmacodynamic changes in muscle and fat were maintained at follow-up, 1 month after day 29.

CONCLUSION

Four weekly s.c. doses of AMG 745 were well tolerated and were associated with increased LBM and decreased fat in the men receiving ADT for nonmetastatic prostate cancer. RESULTS support further investigation of AMG 745 in clinical settings with muscle loss and atrophy.

Blockade of ActRIIB signaling triggers muscle fatigability and metabolic myopathy

Myostatin regulates skeletal muscle size via the activin receptor IIB (ActRIIB). However, its effect on muscle energy metabolism and energy-dependent muscle function remains largely unexplored. This question needs to be solved urgently since various therapies for neuromuscular diseases based on blockade of ActRIIB signaling are being developed. Here, we show in mice, that 4-month pharmacological abrogation of ActRIIB signaling by treatment with soluble ActRIIB-Fc triggers extreme muscle fatigability. This is associated with elevated serum lactate levels and a severe metabolic myopathy in the mdx mouse, an animal model of Duchenne muscular dystrophy. Blockade of ActRIIB signaling downregulates porin, a crucial ADP/ATP shuttle between cytosol and mitochondrial matrix leading to a consecutive deficiency of oxidative phosphorylation as measured by in vivo Phosphorus Magnetic Resonance Spectroscopy ((31)P-MRS). Further, ActRIIB blockade reduces muscle capillarization, which further compounds the metabolic stress. We show that ActRIIB regulates key determinants of muscle metabolism, such as Pparβ, Pgc1α, and Pdk4 thereby optimizing different components of muscle energy metabolism. In conclusion, ActRIIB signaling endows skeletal muscle with high oxidative capacity and low fatigability. The severe metabolic side effects following ActRIIB blockade caution against deploying this strategy, at least in isolation, for treatment of neuromuscular disorders.

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.

Combined effect of AAV-U7-induced dystrophin exon skipping and soluble activin Type IIB receptor in mdx mice

Adeno-associated virus (AAV)-U7-mediated skipping of dystrophin-exon-23 restores dystrophin expression and muscle function in the mdx mouse model of Duchenne muscular dystrophy. Soluble activin receptor IIB (sActRIIB-Fc) inhibits signaling of myostatin and homologous molecules and increases muscle mass and function of wild-type and mdx mice. We hypothesized that combined treatment with AAV-U7 and sActRIIB-Fc may synergistically improve mdx muscle function. Bioactivity of sActRIIB-Fc on skeletal muscle was first demonstrated in wild-type mice. In mdx mice we show that AAV-U7-mediated dystrophin restoration improved specific muscle force and resistance to eccentric contractions when applied alone. Treatment of mdx mice with sActRIIB-Fc increased body weight, muscle mass and myofiber size, but had little effect on muscle function. Combined treatment stimulated muscle growth comparable to the effect of sActRIIB-Fc alone and dystrophin rescue was similar to AAV-U7 alone. Moreover, combined treatment improved maximal tetanic force and the resistance to eccentric contraction to similar extent as AAV-U7 alone. In conclusion, combination of dystrophin exon skipping with sActRIIB-Fc brings together benefits of each treatment; however, we failed to evidence a clear synergistic effect on mdx muscle function.

TGF-β signaling in Duchenne muscular dystrophy

The TGF-β protein family consists of secreted multifunctional cytokines that control diverse processes, such as cell growth and differentiation. Aberrant expression and downstream signaling of these growth factors have been associated with multiple diseases, including muscle wasting disorders, such as Duchenne muscular dystrophy. In this review we discuss recent advances in understanding the role of TGF-β family members during normal skeletal muscle biology/regeneration and their role in muscle pathology, with a special focus on Duchenne muscular dystrophy. In addition, we will highlight progress in the development of potential therapeutics for Duchenne muscular dystrophy based on intervention of TGF-β signaling.