Blockade of activin type II receptors with a dual anti-ActRIIA/IIB antibody is critical to promote maximal skeletal muscle hypertrophy

GDF8 and activin A are the key negative regulators of muscle mass in postmenopausal females: a randomized phase I trial

Evolutionary pressures to protect against food scarcity likely resulted in highly-conserved pathways designed to minimize energy expenditure, one of which involves the minimization of muscle mass; these mechanisms may be counter-productive in a modern world suffering from obesity and sarcopenia. Growth differentiation factor 8 (GDF8)/myostatin, acting via ActRIIA/B receptors, is the best-characterized negative regulator of muscle mass, leading to therapeutic efforts to augment muscle growth by blocking GDF8 or ActRIIA/B. ActRIIA/B blockade approximately doubles the muscle increase of GDF8 blockade, and as ActRIIA/B responds to multiple other TGFβ-family members, this implies other ligands might also regulate muscle mass. Previously, we suggested that activin A (ActA) is the key second negative regulator acting via ActRIIA/B, as blockade of both GDF8 and ActA in mice/monkeys matches the muscle growth of ActRIIA/B blockade. Here, we extend these observations to humans in a two-part, randomized, placebo-controlled Phase 1 trial ( www.clinicaltrials.gov , NCT02943239) conducted at two sites in New Zealand. Eligible subjects included healthy postmenopausal females aged 45-70 years and males aged 35-60 years not intending to father children, with a body mass index of 18-32 kg/m2. Part I tested single-dose administration of anti-GDF8 alone, anti-ActA alone, several dose combinations of anti-GDF8 + anti-ActA, or placebo in healthy postmenopausal females; part II tested multiple-dose administration of anti-ActA alone or placebo in healthy postmenopausal females, combination anti-GDF8 + anti-ActA or placebo in healthy postmenopausal females, and anti-ActA alone or placebo in healthy males. The primary outcome measure was the incidence and severity of treatment-emergent adverse events through week 16 for the single-dose part of the study and through week 40 for the multiple-dose part of the study. Secondary endpoints included percent and absolute change in thigh muscle volume, percent and absolute change in total and regional body composition, pharmacokinetic profiles of the GDF8 and ActA mAbs in serum over time, changes in serum total GDF8 and total ActA levels over time, and the presence of anti-drug antibodies against the GDF8 mAb or the ActA mAb. Magnetic resonance imaging was used to quantitate changes in thigh muscle volume and dual x-ray absorptiometry was used to quantitate changes in regional body composition (total lean mass, appendicular lean body mass, android fat mass, and total fat mass). A total of 82 subjects were enrolled (48 in the single-dose part and 34 in the multiple-dose part of the study). Baseline demographic and clinical characteristics were generally balanced across the single- and multiple-dose parts of the study. Combining GDF8 and ActA blocking antibodies led to greater muscle growth than either antibody alone; increases in muscle were accompanied by reductions in fat. The observed clinical effects on muscle and fat paralleled mAb exposure in serum. The combination was generally well tolerated, and no subjects tested positive for anti-drug antibodies post-treatment. These results suggest that GDF8 and ActA are the dominant negative regulators of muscle mass in humans, and that combined blockade may be a promising therapeutic approach in muscle atrophy and obesity settings.

GDF8 and activin A blockade protects against GLP-1-induced muscle loss while enhancing fat loss in obese male mice and non-human primates

Glucagon-like peptide-1 receptor agonists act via appetite suppression and caloric restriction. These treatments can result in significant muscle loss, likely due to evolutionary mechanisms protecting against food scarcity as muscle is a major energy utilizer. One mechanism that reduces muscle mass involves activation of type II activin receptors, ActRIIA/B, which yield profound muscle growth in humans when blocked. We previously demonstrated GDF8, also known as myostatin, and activin A are the two major ActRIIA/B ligands mediating muscle minimization. Here, we report that dual blockade can also prevent muscle loss associated with glucagon-like peptide-1 receptor agonists - and even increase muscle mass - in both obese mice and non-human primates; moreover, this muscle preservation enhances fat loss and is metabolically beneficial. These data raise the possibility that supplementing glucagon-like peptide-1 receptor agonist treatment with GDF8 and activin A blockade could greatly improve the quality of weight loss during the treatment of obesity in humans.

Skeletal muscle growth to combat diabetes and obesity: the potential role of muscle-secreted factors

As the prevalence of obesity and metabolic disease continues to climb, the need for effective therapeutic interventions remains high. The growth of skeletal muscle (SkM) greatly influences systemic metabolism across the whole body, making this tissue an important therapeutic target to combat the rise of metabolic dysfunction. Transgenic rodent models of targeted SkM growth exhibit profound improvements in various remote tissues, including adipose tissue and the liver. It is currently unclear how selective stimulation of SkM growth alters the metabolism of distant tissues; however, evidence suggests that muscle-secreted factors may be involved. Here, we aim to provide basic biomedical researchers with a summary of the current knowledge regarding various muscle-secreted factors regulated by anabolic pathways and proteins in SkM, as well as their systemic metabolic effects, to implicate them in the whole-body metabolic effects of SkM growth. In this review, we also identify several knowledge gaps in this field, future directions of investigation, and implications for therapeutic interventions such as resistance exercise and pharmacology.

ActRII or BMPR ligands inhibit skeletal myoblast differentiation, and BMPs promote heterotopic ossification in skeletal muscles in mice

BACKGROUND

Prior studies suggested that canonical Activin Receptor II (ActRII) and BMP receptor (BMPR) ligands can have opposing, distinct effects on skeletal muscle depending in part on differential downstream SMAD activation. It was therefore of interest to test ActRII ligands versus BMP ligands in settings of muscle differentiation and in vivo.

METHODS AND RESULTS

In human skeletal muscle cells, both ActRII ligands and BMP ligands inhibited myogenic differentiation: ActRII ligands in a SMAD2/3-dependent manner, and BMP ligands via SMAD1/5. Surprisingly, a neutralizing ActRIIA/B antibody mitigated the negative effects of both classes of ligands, indicating that some BMPs act at least partially through the ActRII receptors in skeletal muscle. Gene expression analysis showed that both ActRII and BMP ligands repress muscle differentiation genes in human myoblasts and myotubes. In mice, hepatic BMP9 over-expression induced liver toxicity, caused multi-organ wasting, and promoted a pro-atrophy gene signature despite elevated SMAD1/5 signaling in skeletal muscle. Local overexpression of BMP7 or BMP9, achieved by intramuscular AAV delivery, induced heterotopic ossification. Elevated SMAD1/5 signaling with increased expression of BMP target genes was also observed in sarcopenic muscles of old rats.

CONCLUSIONS

The canonical ActRII ligand-SMAD2/3 and BMP ligand-SMAD1/5 axes can both block human myoblast differentiation. Our observations further demonstrate the osteoinductive function of BMP ligands while pointing to a potential relevancy of blocking the BMP-SMAD1/5 axis in the setting of therapeutic anti-ActRIIA/B inhibition.

CRISPR/Cas9-Targeted Myostatin Deletion Improves the Myogenic Differentiation Parameters for Muscle-Derived Stem Cells in Mice

Skeletal muscle plays a pivotal role in physical activity, protein storage and energy utilization. Skeletal muscle wasting due to immobilization, aging, muscular dystrophy and cancer cachexia has negative impacts on the quality of life. The deletion of myostatin, a growth and differentiation factor-8 (GDF-8) augments muscle mass through hyperplasia and hypertrophy of muscle fibers. The present study examines the impact of myostatin deletion using CRISPR/Cas9 editing on the myogenic differentiation (MD) of C2C12 muscle stem cells. A total of five myostatin loci were targeted using guided RNAs that had been previously cloned into a vector. The clones were transfected in C2C12 cells via electroporation. The cell viability and MD of myostatin-edited clones (Mstn-/-) were compared with C2C12 (Mstn+/+) using a series of assays, including MTT, sulforhodamine B, immunocytochemistry, morphometric analysis and RT-qPCR. The clones sequenced showed evidence of nucleotides deletion in Mstn-/- cells. Mstn-/- cells demonstrated a normal physiological performance and lack of cytotoxicity. Myostatin depletion promoted the myogenic commitment as evidenced by upregulated MyoD and myogenin expression. The number of MyoD-positive cells was increased in the differentiated Mstn-/- clones. The Mstn-/- editing upregulates both mTOR and MyH expression, as well as increasing the size of myotubes. The differentiation of Mstn-/- cells upregulates ActRIIb; in contrast, it downregulates decorin expression. The data provide evidence of successful CRISPR/Cas9-mediated myostatin deletion. In addition, targeting myostatin could be a beneficial therapeutic strategy to promote MD and to restore muscle loss. In conclusion, the data suggest that myostatin editing using CRISPR/Cas9 could be a potential therapeutic manipulation to improve the regenerative capacity of muscle stem cells before in vivo application.

N-glycosylation of ACTRIIB enhances protein stability leading to rapid cell proliferation and strong resistance to docetaxel in nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a malignant tumor predominantly influenced by Epstein-Barr virus infection and genetic factors. The transforming growth factor-beta (TGF-β) superfamily is implicated in various cellular processes, including tumorigenesis. This study aimed to detect the role of one TGF-β superfamily member activin receptor type IIB (ACTRIIB) in NPC. This study analyzed NPC datasets, including GSE12452, GSE102349, and GSE53819. ACTRIIB expression and N-glycosylation levels were assessed by western blot, real-time PCR, immunofluorescence, and immunohistochemistry in NPC cells and tissues. As indicated by the datasets, ACTRIIB was significantly upregulated in NPC tissues, and the up-regulation was associated with poor prognosis. This study confirmed the N-glycosylation of ACTRIIB primarily at the forty-second amino acid, an asparagine. The N-glycosylation of ACTRIIB promoted the localization of ACTRIIB to the cell membrane and prevented the degradation of the protein by lysosomes, through which ACTRIIB activated the downstream Smard1/2 to promote tumor cell proliferation and invasion. Inhibition of N-glycosylation or knockdown of ACTRIIB resulted in reduced cell proliferation and invasion and increased the cell sensitivity to docetaxel. In conclusion, N-glycosylation of ACTRIIB was a critical post-translational modification that enhanced protein stability and induced membrane localization, which facilitates the functions of ACTRIIB in cell proliferation and invasion in NPC. Inhibition of ACTRIIB N-glycosylation could potentially serve as a therapeutic strategy to improve the efficacy of chemotherapy in NPC.

The Effect of Anti-Activin Receptor Type IIA and Type IIB Antibody on Muscle, Bone and Blood in Healthy and Osteosarcopenic Mice

BACKGROUND

Anti-Activin Receptor Type IIA and Type IIB antibody (αActRIIA/IIB ab) is a recently developed drug class that targets the activin receptor signalling pathway. Inhibition of receptor ligands (activins, myostatin, growth differentiation factor 11, etc.) can lead to skeletal muscle hypertrophy, bone formation, and increased haematopoiesis. Despite the αActRIIA/IIB ab, bimagrumab, having progressed to clinical trials, two crucial questions about αActRIIA/IIB ab therapy remain: Does αActRIIA/IIB ab influence bone metabolism and bone strength similarly to its generic classmates (activin receptor-based ligand traps)? Does αActRIIA/IIB ab affect red blood cell parameters, thereby increasing the risk of thromboembolism, similar to its generic classmates? Therefore, the aim of the present study was to investigate the therapeutic potential of αActRIIA/IIB ab in a mouse model of concurrent sarcopenia and osteopenia and to investigate the effect on bone and haematopoiesis in more detail.

METHODS

In C57BL/6JRj mice, combined sarcopenia and osteopenia were induced locally by injecting botulinum toxin A into the right hindlimb, resulting in acute muscle paresis. Immediately after immobilization, mice received twice-weekly intraperitoneal injections with αActRIIA/IIB ab (10 mg/kg) for 21 days, after which they were sacrificed. Muscle mass, skeletal muscle fibre size and Smad2 expression were analysed in the rectus femoris and gastrocnemius muscles. Bone mass and bone microstructure were analysed in the trabecular bone at the distal femoral metaphysis, while the cortical bone was analysed at the femoral mid-diaphysis. In a substudy, the effect on haematopoiesis was explored 2 and 7 days after a single αActRIIA/IIB ab (30 mg/kg) injection in C57BL/6JRj mice.

RESULTS

αActRIIA/IIB ab caused a large increase in muscle mass in both healthy (+21%) and immobilized (sarcopenic and osteopenic) (+12%) mice. Furthermore, αActRIIA/IIB ab increased trabecular bone (bone volume fraction) for both healthy (+65%) and immobilized (+44%) mice. For cortical bone, αActRIIA/IIB ab caused a small, but significant, increase in bone area (+6%) for immobilized mice, but not for healthy mice. Treatment with αActRIIA/IIB ab did not change red blood cell count, haemoglobin concentration or mean cell volume after either 2 or 7 days.

CONCLUSIONS

Treatment with αActRIIA/IIB ab caused a significant increase in both skeletal muscle mass and bone parameters in both healthy and immobilized mice, suggesting a potential in the treatment of concurrent osteopenia and sarcopenia. Interestingly, the bone anabolic effect of the treatment was much more pronounced on trabecular bone than on cortical bone. There was no pronounced effect of short-term treatment on haematopoiesis.

The impact of weight loss on fat-free mass, muscle, bone and hematopoiesis health: Implications for emerging pharmacotherapies aiming at fat reduction and lean mass preservation

Similar to bariatric surgery, incretin receptor agonists have revolutionized the treatment of obesity, achieving up to 15-25 % weight loss in many patients, i.e., at a rate approaching that achieved with bariatric surgery. However, over 25 % of total weight lost from both surgery and pharmacotherapy typically comes from fat-free mass, including skeletal muscle mass, which is often overlooked and can impair metabolic health and increase the risk of subsequent sarcopenic obesity. Loss of muscle and bone as well as anemia can compromise physical function, metabolic rate, and overall health, especially in older adults. The myostatin-activin-follistatin-inhibin system, originally implicated in reproductive function and subsequently muscle regulation, appears to be crucial for muscle and bone maintenance during weight loss. Activins and myostatin promote muscle degradation, while follistatins inhibit their activity in states of negative energy balance, thereby preserving lean mass. Novel compounds in the pipeline, such as Bimagrumab, Trevogrumab, and Garetosmab-which inhibit activin and myostatin signaling-have demonstrated promise in preventing muscle loss while promoting fat loss. Either alone or combined with incretin receptor agonists, these medications may enhance fat loss while preserving or even increasing muscle and bone mass, offering a potential solution for improving body composition and metabolic health during significant weight loss. Since this dual therapeutic approach could help address the challenges of muscle and bone loss during weight loss, well-designed studies are needed to optimize these strategies and assess long-term benefits. For the time being, considerations like advanced age and prefrailty may affect the choice of suitable candidates in clinical practice for current and emerging anti-obesity medications due to the associated risk of sarcopenia.

Reduced myotube diameter induced by combined inhibition of transforming growth factor-β type I receptors Acvr1b and Tgfbr1 is associated with enhanced β1-syntrophin expression

Simultaneous inhibition of transforming growth factor-β (TGF-β) type I receptors Acvr1b and Tgfbr1 signalling has been associated with excessive skeletal muscle hypertrophy in vivo. However, it remains unclear whether the increased muscle mass in vivo is a direct result of inhibition of intracellular TGF-β signalling or whether this is an indirect effect of an altered extracellular anabolic environment. Here, we tested whether individual or simultaneous knockdown of TGF-β type I receptors in C2C12 myotubes was sufficient to induce muscle hypertrophy. The expression levels of TGF-β type I receptors Acvr1b and Tgfbr1 in myotubes were knocked down individually or in combination in the absence or presence of TGF-β1 and myostatin. Knocking down either Acvr1b or Tgfbr1 did not significantly change cell phenotype. Unexpectedly, simultaneous knockdown of both receptors reduced C2C12 myotube diameter, mRNA expression levels of Hgf, Ccn2 and Mymx with or without TGF-β1 and myostatin administration. In spite of decreased phosphorylation of Smad2/3, phosphorylation of P70S6K was reduced. In addition, the gene expression level of β1-syntrophin (Sntb1), which encodes a protein associated with the dystrophin-glycoprotein complex, was increased. Parallel experiments where Sntb1 gene expression was reduced showed an increase in myotube diameter and fusion of C2C12 myoblasts. Together, these results indicate that the knockdown of both TGF-β type I receptors reduced myotube diameter. This atrophic effect was attributed to reduced protein synthesis signalling and an increased expression of β1-syntrophin. These results have implications for our fundamental understanding of how TGF-β signalling regulates skeletal muscle size.

Advancements in Drug Delivery Systems for the Treatment of Sarcopenia: An Updated Overview

Since sarcopenia is a progressive condition that leads to decreased muscle mass and function, especially in elderly people, it is a public health problem that requires attention from researchers. This review aims to highlight drug delivery systems that have a high and efficient therapeutic potential for sarcopenia. Current as well as future research needs to consider the barriers encountered in the realization of delivery systems, such as the route of administration, the interaction of the systems with the aggressive environment of the human body, the efficient delivery and loading of the systems with therapeutic agents, and the targeted delivery of therapeutic agents into the muscle tissue without creating undesirable adverse effects. Thus, this paper sets the framework of existing drug delivery possibilities for the treatment of sarcopenia, serving as an inception point for future interdisciplinary studies.

Skeletal Muscle Injury in Chronic Kidney Disease-From Histologic Changes to Molecular Mechanisms and to Novel Therapies

Chronic kidney disease (CKD) is associated with significant reductions in lean body mass and in the mass of various tissues, including skeletal muscle, which causes fatigue and contributes to high mortality rates. In CKD, the cellular protein turnover is imbalanced, with protein degradation outweighing protein synthesis, leading to a loss of protein and cell mass, which impairs tissue function. As CKD itself, skeletal muscle wasting, or sarcopenia, can have various origins and causes, and both CKD and sarcopenia share common risk factors, such as diabetes, obesity, and age. While these pathologies together with reduced physical performance and malnutrition contribute to muscle loss, they cannot explain all features of CKD-associated sarcopenia. Metabolic acidosis, systemic inflammation, insulin resistance and the accumulation of uremic toxins have been identified as additional factors that occur in CKD and that can contribute to sarcopenia. Here, we discuss the elevation of systemic phosphate levels, also called hyperphosphatemia, and the imbalance in the endocrine regulators of phosphate metabolism as another CKD-associated pathology that can directly and indirectly harm skeletal muscle tissue. To identify causes, affected cell types, and the mechanisms of sarcopenia and thereby novel targets for therapeutic interventions, it is important to first characterize the precise pathologic changes on molecular, cellular, and histologic levels, and to do so in CKD patients as well as in animal models of CKD, which we describe here in detail. We also discuss the currently known pathomechanisms and therapeutic approaches of CKD-associated sarcopenia, as well as the effects of hyperphosphatemia and the novel drug targets it could provide to protect skeletal muscle in CKD.

Cell type-specific transforming growth factor-β (TGF-β) signaling in the regulation of salivary gland fibrosis and regeneration

Salivary gland damage and hypofunction result from various disorders, including autoimmune Sjögren's disease (SjD) and IgG4-related disease (IgG4-RD), as well as a side effect of radiotherapy for treating head and neck cancers. There are no therapeutic strategies to prevent the loss of salivary gland function in these disorders nor facilitate functional salivary gland regeneration. However, ongoing aquaporin-1 gene therapy trials to restore saliva flow show promise. To identify and develop novel therapeutic targets, we must better understand the cell-specific signaling processes involved in salivary gland regeneration. Transforming growth factor-β (TGF-β) signaling is essential to tissue fibrosis, a major endpoint in salivary gland degeneration, which develops in the salivary glands of patients with SjD, IgG4-RD, and radiation-induced damage. Though the deposition and remodeling of extracellular matrix proteins are essential to repair salivary gland damage, pathological fibrosis results in tissue hardening and chronic salivary gland dysfunction orchestrated by multiple cell types, including fibroblasts, myofibroblasts, endothelial cells, stromal cells, and lymphocytes, macrophages, and other immune cell populations. This review is focused on the role of TGF-β signaling in the development of salivary gland fibrosis and the potential for targeting TGF-β as a novel therapeutic approach to regenerate functional salivary glands. The studies presented highlight the divergent roles of TGF-β signaling in salivary gland development and dysfunction and illuminate specific cell populations in damaged or diseased salivary glands that mediate the effects of TGF-β. Overall, these studies strongly support the premise that blocking TGF-β signaling holds promise for the regeneration of functional salivary glands.

Multimodal cell atlas of the ageing human skeletal muscle

Muscle atrophy and functional decline (sarcopenia) are common manifestations of frailty and are critical contributors to morbidity and mortality in older people1. Deciphering the molecular mechanisms underlying sarcopenia has major implications for understanding human ageing2. Yet, progress has been slow, partly due to the difficulties of characterizing skeletal muscle niche heterogeneity (whereby myofibres are the most abundant) and obtaining well-characterized human samples3,4. Here we generate a single-cell/single-nucleus transcriptomic and chromatin accessibility map of human limb skeletal muscles encompassing over 387,000 cells/nuclei from individuals aged 15 to 99 years with distinct fitness and frailty levels. We describe how cell populations change during ageing, including the emergence of new populations in older people, and the cell-specific and multicellular network features (at the transcriptomic and epigenetic levels) associated with these changes. On the basis of cross-comparison with genetic data, we also identify key elements of chromatin architecture that mark susceptibility to sarcopenia. Our study provides a basis for identifying targets in the skeletal muscle that are amenable to medical, pharmacological and lifestyle interventions in late life.

Advances in sarcopenia: mechanisms, therapeutic targets, and intervention strategies

Sarcopenia is a multifactorial condition characterized by loss of muscle mass. It poses significant health risks in older adults worldwide. Both pharmacological and non-pharmacological approaches are reported to address this disease. Certain dietary patterns, such as adequate energy intake and essential amino acids, have shown positive outcomes in preserving muscle function. Various medications, including myostatin inhibitors, growth hormones, and activin type II receptor inhibitors, have been evaluated for their effectiveness in managing sarcopenia. However, it is important to consider the variable efficacy and potential side effects associated with these treatments. There are currently no drugs approved by the Food and Drug Administration for sarcopenia. The ongoing research aims to develop more effective strategies in the future. Our review of research on disease mechanisms and drug development will be a valuable contribution to future research endeavors.

Antibody blockade of activin type II receptors preserves skeletal muscle mass and enhances fat loss during GLP-1 receptor agonism

OBJECTIVE

Glucagon-like peptide 1 (GLP-1) receptor agonists reduce food intake, producing remarkable weight loss in overweight and obese individuals. While much of this weight loss is fat mass, there is also a loss of lean mass, similar to other approaches that induce calorie deficit. Targeting signaling pathways that regulate skeletal muscle hypertrophy is a promising avenue to preserve lean mass and modulate body composition. Myostatin and Activin A are TGFβ-like ligands that signal via the activin type II receptors (ActRII) to antagonize muscle growth. Pre-clinical and clinical studies demonstrate that ActRII blockade induces skeletal muscle hypertrophy and reduces fat mass. In this manuscript, we test the hypothesis that combined ActRII blockade and GLP-1 receptor agonism will preserve muscle mass, leading to improvements in skeletomuscular and metabolic function and enhanced fat loss.

METHODS

In this study, we explore the therapeutic potential of bimagrumab, a monoclonal antibody against ActRII, to modify body composition alone and during weight loss induced by GLP-1 receptor agonist semaglutide in diet-induced obese mice. Mechanistically, we define the specific role of the anabolic kinase Akt in mediating the hypertrophic muscle effects of ActRII inhibition in vivo.

RESULTS

Treatment of obese mice with bimagrumab induced a ∼10 % increase in lean mass while simultaneously decreasing fat mass. Daily treatment of obese mice with semaglutide potently decreased body weight; this included a significant decrease in both muscle and fat mass. Combination treatment with bimagrumab and semaglutide led to superior fat mass loss while simultaneously preserving lean mass despite reduced food intake. Treatment with both drugs was associated with improved metabolic outcomes, and increased lean mass was associated with improved exercise performance. Deletion of both Akt isoforms in skeletal muscle modestly reduced, but did not prevent, muscle hypertrophy driven by ActRII inhibition.

CONCLUSIONS

Collectively, these data demonstrate that blockade of ActRII signaling improves body composition and metabolic parameters during calorie deficit driven by GLP-1 receptor agonism and demonstrate the existence of Akt-independent pathways supporting muscle hypertrophy in the absence of ActRII signaling.

Unveiling the Potential of Nelumbo nucifera-Derived Liensinine to Target The Myostatin Protein and to Counteract Muscle Atrophy

Muscle atrophy refers to a decline in muscle mass and function, which has become a global concern due to the aging population. Various clinical trials have investigated the inhibitors of myostatin (MSTN). They have shown promising improvements in muscle function and quality of life. However, there are no drugs specifically targeting MSTN that have been approved for clinical use. In this study, we virtually screened liensinine (LIE), a food (Nelumbo nucifera)-derived compound, with low toxicity, from over 1.1 million compounds. We subsequently identified it as a potential candidate that targets MSTN by a cellular thermal shift assay (CETSA) and drug affinity response target stability (DARTS) assay. Further validation through cellular and in vivo studies demonstrated its promising potential in combating muscle atrophy. The mechanism of action may involve hindering the interaction between MSTN and the activin receptor type IIB (ActRIIB) and downregulating the expression of downstream proteins, including the muscle RING-finger protein-1 (MuRF-1) and muscle atrophy F-box (MAFbx)/Atrogin-1, ultimately promoting muscle regeneration. These results provide a strong foundation for future studies to explore the therapeutic potential of LIE in clinical settings.

The MuSK-BMP pathway maintains myofiber size in slow muscle through regulation of Akt-mTOR signaling

Myofiber size regulation is critical in health, disease, and aging. MuSK (muscle-specific kinase) is a BMP (bone morphogenetic protein) co-receptor that promotes and shapes BMP signaling. MuSK is expressed at all neuromuscular junctions and is also present extrasynaptically in the mouse soleus, whose predominantly oxidative fiber composition is akin to that of human muscle. To investigate the role of the MuSK-BMP pathway in vivo, we generated mice lacking the BMP-binding MuSK Ig3 domain. These ∆Ig3-MuSK mice are viable and fertile with innervation levels comparable to wild type. In 3-month-old mice, myofibers are smaller in the slow soleus, but not in the fast tibialis anterior (TA). Transcriptomic analysis revealed soleus-selective decreases in RNA metabolism and protein synthesis pathways as well as dysregulation of IGF1-Akt-mTOR pathway components. Biochemical analysis showed that Akt-mTOR signaling is reduced in soleus but not TA. We propose that the MuSK-BMP pathway acts extrasynaptically to maintain myofiber size in slow muscle by promoting protein synthetic pathways including IGF1-Akt-mTOR signaling. These results reveal a novel mechanism for regulating myofiber size in slow muscle and introduce the MuSK-BMP pathway as a target for promoting muscle growth and combatting atrophy.

Sarcopenia and Diabetes: A Detrimental Liaison of Advancing Age

Sarcopenia is an age-related clinical complaint characterized by the progressive deterioration of skeletal muscle mass and strength over time. Type 2 diabetes (T2D) is associated with faster and more relevant skeletal muscle impairment. Both conditions influence each other, leading to negative consequences on glycemic control, cardiovascular risk, general health status, risk of falls, frailty, overall quality of life, and mortality. PubMed/Medline, Scopus, Web of Science, and Google Scholar were searched for research articles, scientific reports, observational studies, clinical trials, narrative and systematic reviews, and meta-analyses to review the evidence on the pathophysiology of di-abetes-induced sarcopenia, its relevance in terms of glucose control and diabetes-related outcomes, and diagnostic and therapeutic challenges. The review comprehensively addresses key elements for the clinical definition and diagnostic criteria of sarcopenia, the pathophysiological correlation be-tween T2D, sarcopenia, and related outcomes, a critical review of the role of antihyperglycemic treatment on skeletal muscle health, and perspectives on the role of specific treatment targeting myokine signaling pathways involved in glucose control and the regulation of skeletal muscle metabolism and trophism. Prompt diagnosis and adequate management, including lifestyle inter-vention, health diet programs, micronutrient supplementation, physical exercise, and pharmaco-logical treatment, are needed to prevent or delay skeletal muscle deterioration in T2D.

Current and investigational medications for the treatment of sarcopenia

Sarcopenia, defined as the loss of muscle mass and function, is a widely prevalent and severe condition in older adults. Since 2016, it is recognized as a disease. Strength exercise training and nutritional support are the frontline treatment of sarcopenia, with no drug currently approved for this indication. However, new therapeutic options are emerging. In this review, we evidenced that only very few trials have focused on sarcopenia/sarcopenic patients. Most drug trials were performed in different clinical older populations (e.g., men with hypogonadism, post-menopausal women at risk for osteoporosis), and their efficacy were tested separately on the components of sarcopenia (muscle mass, muscle strength and physical performances). Results from trials testing the effects of Testosterone, Selective Androgen Receptor Modulators (SARMs), Estrogen, Dehydroepiandrosterone (DHEA), Insulin-like Growth Factor-1 (IGF-1), Growth Hormone (GH), GH Secretagogue (GHS), drug targeting Myostatin and Activin receptor pathway, Vitamin D, Angiotensin Converting Enzyme inhibitors (ACEi) and Angiotensin Receptor Blockers (ARBs), or β-blockers, were compiled. Although some drugs have been effective in improving muscle mass and/or strength, this was not translated into clinically relevant improvements on physical performance. Finally, some promising molecules investigated in on-going clinical trials and in pre-clinical phase were summarized, including apelin and irisin.

Ginsenoside Rg5 promotes muscle regeneration via p38MAPK and Akt/mTOR signaling

Background

Skeletal muscles play a key role in physical activity and energy metabolism. The loss of skeletal muscle mass can cause problems related to metabolism and physical activity. Studies are being conducted to prevent such diseases by increasing the mass and regeneration capacity of muscles. Ginsenoside Rg5 has been reported to exhibit a broad range of pharmacological activities. However, studies on the effects of Rg5 on muscle differentiation and growth are scarce.

Methods

To investigate the effects of Rg5 on myogenesis, C2C12 myoblasts were induced to differentiate with Rg5, followed by immunoblotting, immunostaining, and qRT-PCR for myogenic markers and promyogenic signaling (p38MAPK). Immunoprecipitation confirmed that Rg5 increased the interaction between MyoD and E2A via p38MAPK. To investigate the effects of Rg5 on prevention of muscle mass loss, C2C12 myotubes were treated with dexamethasone to induce muscle atrophy. Immunoblotting, immunostaining, and qRT-PCR were performed for myogenic markers, Akt/mTOR signaling for protein synthesis, and atrophy-related genes (Atrogin-1 and MuRF1).

Results

Rg5 promoted C2C12 myoblast differentiation through phosphorylation of p38MAPK and MyoD/E2A heterodimerization. Furthermore, Rg5 stimulated C2C12 myotube hypertrophy via phosphorylation of Akt/mTOR. Phosphorylation of Akt induces FoxO3a phosphorylation, which reduces the expression of Atrogin-1 and MuRF1.

Conclusion

This study provides an understanding of how Rg5 promotes myogenesis and hypertrophy and prevents dexamethasone-induced muscle atrophy. The study is the first, to the best of our knowledge, to show that Rg5 promotes muscle regeneration and to suggest that Rg5 can be used for therapeutic intervention of muscle weakness and atrophy, including cancer cachexia.

GDF11 and aging biology - controversies resolved and pending

Since the exogenous administration of GDF11, a TGF-ß superfamily member, was reported to have beneficial effects in some models of human disease, there have been many research studies in GDF11 biology. However, many studies have now confirmed that exogenous administration of GDF11 can improve physiology in disease models, including cardiac fibrosis, experimental stroke, and disordered metabolism. GDF11 is similar to GDF8 (also called Myostatin), differing only by 11 amino acids in their mature signaling domains. These two proteins are now known to be biochemically different both in vitro and in vivo. GDF11 is much more potent than GDF8 and induces more strongly SMAD2 phosphorylation in the myocardium compared to GDF8. GDF8 and GDF11 prodomain are only 52% identical and are cleaved by different Tolloid proteases to liberate the mature signaling domain from inhibition of the prodomain. Here, we review the state of GDF11 biology, highlighting both resolved and remaining controversies.

Mechanisms of sarcopenia in liver cirrhosis and the role of myokines

Sarcopenia is a syndrome characterized by a decline in skeletal muscle quantity and/or quality, strength and performance, leading to unfortunate events, such as injurious falls or even death. It is not identical to frailty and malnutrition, even though there is a significant overlap among these syndromes. In patients with liver cirrhosis (LC), sarcopenia is classified as secondary and has been associated with increased morbidity and mortality during the pre- and post-transplantation period. It can be a result of malnutrition, hyperammonemia, low physical activity, endocrine abnormalities, accelerated starvation, metabolic disturbances, altered gut function leading to chronic inflammation, and alcohol abuse. Myokines are peptides mainly synthesized by contracting muscle and adipose tissue cells and may play a key role in the pathophysiology of sarcopenia. More than a hundred myokines have been recognized, but only a few have been investigated. They can be classified as negative regulators, such as myostatin, tumor growth factor-β, activins, growth differentiation factor-11, and positive regulators of muscle growth including follistatin, bone morphogenic proteins, and irisin. So far, only myostatin, follistatin, irisin and decorin have been studied in LC-associated sarcopenia. In this review, we focused on the mechanisms of cirrhosis-related sarcopenia and the role of myokines that have already been studied in the literature, either as markers helping in the diagnostic evaluation of sarcopenia, or as prognostic factors of survival. Standard therapeutic options to prevent or treat sarcopenia in LC are also being reported, as well as the possible therapeutic implication of myokines.

Activation of β-catenin in mesenchymal progenitors leads to muscle mass loss

Loss of muscle mass is a common manifestation of chronic disease. We find the canonical Wnt pathway to be activated in mesenchymal progenitors (MPs) from cancer-induced cachectic mouse muscle. Next, we induce β-catenin transcriptional activity in murine MPs. As a result, we observe expansion of MPs in the absence of tissue damage, as well as rapid loss of muscle mass. Because MPs are present throughout the organism, we use spatially restricted CRE activation and show that the induction of tissue-resident MP activation is sufficient to induce muscle atrophy. We further identify increased expression of stromal NOGGIN and ACTIVIN-A as key drivers of atrophic processes in myofibers, and we verify their expression by MPs in cachectic muscle. Finally, we show that blocking ACTIVIN-A rescues the mass loss phenotype triggered by β-catenin activation in MPs, confirming its key functional role and strengthening the rationale for targeting this pathway in chronic disease.

MeCP2 regulates Gdf11, a dosage-sensitive gene critical for neurological function

Loss- and gain-of-function of MeCP2 causes Rett syndrome (RTT) and MECP2 duplication syndrome (MDS), respectively. MeCP2 binds methyl-cytosines to finely tune gene expression in the brain, but identifying genes robustly regulated by MeCP2 has been difficult. By integrating multiple transcriptomics datasets, we revealed that MeCP2 finely regulates growth differentiation factor 11 (Gdf11). Gdf11 is down-regulated in RTT mouse models and, conversely, up-regulated in MDS mouse models. Strikingly, genetically normalizing Gdf11 dosage levels improved several behavioral deficits in a mouse model of MDS. Next, we discovered that losing one copy of Gdf11 alone was sufficient to cause multiple neurobehavioral deficits in mice, most notably hyperactivity and decreased learning and memory. This decrease in learning and memory was not due to changes in proliferation or numbers of progenitor cells in the hippocampus. Lastly, loss of one copy of Gdf11 decreased survival in mice, corroborating its putative role in aging. Our data demonstrate that Gdf11 dosage is important for brain function.

Myostatin: A Skeletal Muscle Chalone

Myostatin (GDF-8) was discovered 25 years ago as a new transforming growth factor-β family member that acts as a master regulator of skeletal muscle mass. Myostatin is made by skeletal myofibers, circulates in the blood, and acts back on myofibers to limit growth. Myostatin appears to have all of the salient properties of a chalone, which is a term proposed over a half century ago to describe hypothetical circulating, tissue-specific growth inhibitors that control tissue size. The elucidation of the molecular, cellular, and physiological mechanisms underlying myostatin activity suggests that myostatin functions as a negative feedback regulator of muscle mass and raises the question as to whether this type of chalone mechanism is unique to skeletal muscle or whether it also operates in other tissues.

Upregulation of CCL5/RANTES Gene Expression in the Diaphragm of Mice with Cholestatic Liver Disease

Chronic liver diseases are a group of pathologies affecting the liver with high prevalence worldwide. Among them, cholestatic chronic liver diseases (CCLD) are characterized by alterations in liver function and increased plasma bile acids. Secondary to liver disease, under cholestasis, is developed sarcopenia, a skeletal muscle dysfunction with decreased muscle mass, strength, and physical function. CCL5/RANTES is a chemokine involved in the immune and inflammatory response. Indeed, CCL5 is a myokine because it is produced by skeletal muscle. Several studies show that bile acids induce CCL5/RANTES expression in liver cells. However, it is unknown if the expression of CCL5/RANTES is changed in the skeletal muscle of mice with cholestatic liver disease. We used a murine model of cholestasis-induced sarcopenia by intake of hepatotoxin 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC diet), in which we detected the mRNA levels for ccl5. We determined that mice fed the DDC diet presented high levels of serum bile acids and developed typical features of sarcopenia. Under these conditions, we detected the ccl5 gene expression in diaphragm muscle showing elevated mRNA levels compared to mice fed with a standard diet (chow diet). Our results collectively suggest an increased ccl5 gene expression in the diaphragm muscle concomitantly with elevated serum bile acids and the development of sarcopenia.

Pharmacokinetics and Pharmacodynamics of Bimagrumab (BYM338)

BACKGROUND

Bimagrumab is a human monoclonal antibody binding to the activin type II receptor with therapeutic potential in conditions of muscle wasting and obesity. This phase I study evaluated the pharmacokinetics (PK), pharmacodynamics (PD), and safety of various dose regimens of bimagrumab and routes of administration in healthy older adults.

METHODS

This was a randomized, double-blind, placebo-controlled, parallel-arm, multiple-dose study in older adult men and women (aged ≥ 70 years, body mass index [BMI] 18-34 kg/m²) with stable health and diet. The study comprised seven treatment groups (Cohorts 1-7). Participants received bimagrumab or placebo treatment every 4 weeks for three doses (Cohorts 1 [700 mg] and 2 [210 mg] intravenous infusion; Cohorts 3 [1500 mg] and 4 [525 mg] subcutaneous infusion), or every week for 12 doses (Cohorts 5 [300 mg], 6 [150 mg], and 7 [52.5 mg] subcutaneous bolus injection) and were followed up until week 20. Blood samples were collected for bimagrumab PK analysis. PD were assessed by dual energy X-ray absorptiometry to quantify the change from baseline in lean body mass (LBM) and fat body mass (FBM) compared with placebo. Safety was assessed throughout the study.

RESULTS

Eighty-four of 91 (92.3%) randomized participants (mean age 74.5 years; BMI 28.0 kg/m²) completed the study. Demographic characteristics were generally balanced across the groups. A target-mediated drug disposition profile was observed following both intravenous and subcutaneous administration. The absolute subcutaneous bioavailability was estimated at approximately 40%. LBM increased by 4-6% (1.5-2 kg) from baseline throughout the treatment period for intravenous and subcutaneous regimens, except for the 52.5 mg subcutaneous dose, which did not differ from placebo. Concurrently, there was a decrease in FBM (approximately 2-3 kg) for all intravenous and subcutaneous regimens. Bimagrumab was generally safe and well tolerated; adverse events were mostly mild to moderate in severity.

CONCLUSIONS

Dose levels of bimagrumab administered weekly subcutaneously resulted in PK profiles and PD effects comparable with monthly intravenous dosing, which supports the feasibility of the subcutaneous route of administration for bimagrumab for future clinical development.

Implications of pre-transplant sarcopenia and frailty in patients with non-alcoholic steatohepatitis and alcoholic liver disease

Frailty manifesting as sarcopenia is an independent risk factor for mortality in cirrhosis, and often presents in low model for end-stage liver disease (MELD) patients. Its etiology is multifactorial, but key physiologic changes culminate in altered energy utilization in the fasting state, preferentially utilizing muscle amino acids for gluconeogenesis thereby promoting sarcopenia. Hyperammonemia alters the circulating amino acid profile, diminishing pro-muscle branched-chain amino acids like leucine. The metabolic syndrome worsens sarcopenia through multi-tissue insulin resistance. Alcohol also exacerbates sarcopenia as a direct muscle toxin and inhibitor of growth signaling. Therapy is aimed at alcohol cessation, frequent high-protein meals, branched-chain amino acid supplementation, and diminished time spent fasting. Moderate exercise can improve muscle mass and muscle quality, though precise exercise regimens have not yet been explicitly determined. Studies are ongoing into the effects of myostatin antagonists and insulin sensitizers. The Liver Frailty Index can predict patients most at risk of poor outcome and should be considered in the management of all cirrhotic patients. Specialty testing like dual-energy X-ray absorptiometry (DEXA) scanning and cross-sectional estimates of muscle mass are areas of active research and may play a future role in clinical risk-stratification.

Muscle Wasting in Chronic Kidney Disease: Mechanism and Clinical Implications—A Narrative Review

Muscle wasting, known to develop in patients with chronic kidney disease (CKD), is a deleterious consequence of numerous complications associated with deteriorated renal function. Muscle wasting in CKD mainly involves dysregulated muscle protein metabolism and impaired muscle cell regeneration. In this narrative review, we discuss the cardinal role of the insulin-like growth factor 1 and myostatin signaling pathways, which have been extensively investigated using animal and human studies, as well as the emerging concepts in microRNA- and gut microbiota-mediated regulation of muscle mass and myogenesis. To ameliorate muscle loss, therapeutic strategies, including nutritional support, exercise programs, pharmacological interventions, and physical modalities, are being increasingly developed based on advances in understanding its underlying pathophysiology.

Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

Pathophysiological mechanisms leading to muscle loss in chronic kidney disease

Loss of muscle proteins is a deleterious consequence of chronic kidney disease (CKD) that causes a decrease in muscle strength and function, and can lead to a reduction in quality of life and increased risk of morbidity and mortality. The effectiveness of current treatment strategies in preventing or reversing muscle protein losses is limited. The limitations largely stem from the systemic nature of diseases such as CKD, which stimulate skeletal muscle protein degradation pathways while simultaneously activating mechanisms that impair muscle protein synthesis and repair. Stimuli that initiate muscle protein loss include metabolic acidosis, insulin and IGF1 resistance, changes in hormones, cytokines, inflammatory processes and decreased appetite. A growing body of evidence suggests that signalling molecules secreted from muscle can enter the circulation and subsequently interact with recipient organs, including the kidneys, while conversely, pathological events in the kidney can adversely influence protein metabolism in skeletal muscle, demonstrating the existence of crosstalk between kidney and muscle. Together, these signals, whether direct or indirect, induce changes in the levels of regulatory and effector proteins via alterations in mRNAs, microRNAs and chromatin epigenetic responses. Advances in our understanding of the signals and processes that mediate muscle loss in CKD and other muscle wasting conditions will support the future development of therapeutic strategies to reduce muscle loss.

Structures of activin ligand traps using natural sets of type I and type II TGFβ receptors

The 30+ unique ligands of the TGFβ family signal by forming complexes using different combinations of type I and type II receptors. Therapeutically, the extracellular domain of a single receptor fused to an Fc molecule can effectively neutralize subsets of ligands. Increased ligand specificity can be accomplished by using the extracellular domains of both the type I and type II receptor to mimic the naturally occurring signaling complex. Here, we report the structure of one "type II-type I-Fc" fusion, ActRIIB-Alk4-Fc, in complex with two TGFβ family ligands, ActA, and GDF11, providing a snapshot of this therapeutic platform. The study reveals that extensive contacts are formed by both receptors, replicating the ternary signaling complex, despite the inherent low affinity of Alk4. Our study shows that low-affinity type I interactions support altered ligand specificity and can be visualized at the molecular level using this platform.

SLIT3 promotes myogenic differentiation as a novel therapeutic factor against muscle loss

BACKGROUND

Sarcopenia and osteoporosis frequently co-occur in the elderly and have common pathophysiological determinants. Slit guidance ligand 3 (SLIT3) has been recently discovered as a novel therapeutic factor against osteoporosis, and a SLIT3 fragment containing the second leucine-rich repeat domain (LRRD2) had a therapeutic efficacy against osteoporosis. However, a role of SLIT3 in the skeletal muscle is unknown.

METHODS

Skeletal muscle mass, strength, and/or physical activity were evaluated in Slit3^-/- , ovariectomized, and aged mice, based on the measurements of muscle weight and grip strength, Kondziella's inverted hanging test, and/or wheel-running test. Skeletal muscles were also histologically evaluated by haematoxylin and eosin staining and/or immunofluorescence. The ovariectomized and aged mice were intravenously injected with recombinant SLIT3 LRRD2 for 4 weeks. C2C12 cells were used to know cellular effects of SLIT3, such as in vitro myogenesis, fusion, cell viability, and proliferation, and also used to evaluate its molecular mechanisms by immunocytochemistry, immunoprecipitation, western blotting, real-time PCR, siRNA transfection, and receptor-ligand binding ELISA.

RESULTS

Slit3-deficient mice exhibited decreased skeletal muscle mass, muscle strength, and physical activity. The relative masses of gastrocnemius and soleus were lower in the Slit3^-/- mice (0.580 ± 0.039% and 0.033 ± 0.003%, respectively) than those in the WT littermates (0.622 ± 0.043% and 0.038 ± 0.003%, respectively) (all, P < 0.05). Gastrocnemius of Slit3^-/- mice showed the reduced number of Type I and Type IIa fibres (all, P < 0.05), but not of Type IIb and Type IIx fibres. SLIT3 activated β-catenin signalling by promoting its release from M-cadherin, thereby increasing myogenin expression to stimulate myoblast differentiation. In vitro experiments involving ROBO2 expression, knockdown, and interaction with SLIT3 indicated that ROBO2 functions as a SLIT3 receptor to aid myoblast differentiation. SLIT3 LRRD2 dissociated M-cadherin-bound β-catenin and up-regulated myogenin expression to increase myoblast differentiation, in a manner similar to full-length SLIT3. Systemic treatment with SLIT3 LRRD2 increased skeletal muscle mass in both ovariectomized and aged mice (all, P < 0.05). The relative masses of gastrocnemius and soleus were higher in the treated aged mice (0.548 ± 0.045% and 0.033 ± 0.005%, respectively) than in the untreated aged mice (0.508 ± 0.016% and 0.028 ± 0.003%, respectively) (all, P < 0.05). SLIT3 LRRD2 treatment increased the hanging duration of the aged mice by approximately 1.7-fold (P < 0.05).

CONCLUSIONS

SLIT3 plays a sarcoprotective role by activating β-catenin signalling. SLIT3 LRRD2 can potentially be used as a therapeutic agent against muscle loss.

Myostatin: Basic biology to clinical application

Myostatin is a member of the transforming growth factor (TGF)-β superfamily. It is expressed by animal and human skeletal muscle cells where it limits muscle growth and promotes protein breakdown. Its effects are influenced by complex mechanisms including transcriptional and epigenetic regulation and modulation by extracellular binding proteins. Due to its actions in promoting muscle atrophy and cachexia, myostatin has been investigated as a promising therapeutic target to counteract muscle mass loss in experimental models and patients affected by different muscle-wasting conditions. Moreover, growing evidence indicates that myostatin, beyond to regulate skeletal muscle growth, may have a role in many physiologic and pathologic processes, such as obesity, insulin resistance, cardiovascular and chronic kidney disease. In this chapter, we review myostatin biology, including intracellular and extracellular regulatory pathways, and the role of myostatin in modulating physiologic processes, such as muscle growth and aging. Moreover, we discuss the most relevant experimental and clinical evidence supporting the extra-muscle effects of myostatin. Finally, we consider the main strategies developed and tested to inhibit myostatin in clinical trials and discuss the limits and future perspectives of the research on myostatin.

Therapeutic Implications of miRNAs for Muscle-Wasting Conditions

MicroRNAs (miRNAs) are small, non-coding RNA molecules that are mainly involved in translational repression by binding to specific messenger RNAs. Recently, miRNAs have emerged as biomarkers, relevant for a multitude of pathophysiological conditions, and cells can selectively sort miRNAs into extracellular vesicles for paracrine and endocrine effects. In the overall context of muscle-wasting conditions, a multitude of miRNAs has been implied as being responsible for the typical dysregulation of anabolic and catabolic pathways. In general, chronic muscle disorders are associated with the main characteristic of a substantial loss in muscle mass. Muscular dystrophies (MDs) are a group of genetic diseases that cause muscle weakness and degeneration. Typically, MDs are caused by mutations in those genes responsible for upholding the integrity of muscle structure and function. Recently, the dysregulation of miRNA levels in such pathological conditions has been reported. This revelation is imperative for both MDs and other muscle-wasting conditions, such as sarcopenia and cancer cachexia. The expression levels of miRNAs have immense potential for use as potential diagnostic, prognostic and therapeutic biomarkers. Understanding the role of miRNAs in muscle-wasting conditions may lead to the development of novel strategies for the improvement of patient management.

Allulose Attenuated Age-Associated Sarcopenia via Regulating IGF-1 and Myostatin in Aged Mice

SCOPE

Allulose is shown to increase the muscle weight in diet-induced obese mice. However, there are no studies on the effects of allulose in age-associated sarcopenia. This study aims to elucidate the mechanisms of action for allulose in age associated by analyzing the transcriptional patterns in aged mice.

METHODS AND RESULTS

The 48-week-old mice are fed with AIN-93diet containing allulose for 12 weeks. Allulose supplementation increases the muscle mass and grip strength in aged mice. Allulose increases the insulin-like growth factor 1 (IGF-1) and its downstream factor expressions which 40 are related protein synthesis, while inhibits the myostatin expression related protein degradation. In mRNA-seq analysis, allulose supplementation significantly decreases in Adiponectin, Adipsin, cell death inducing DFFA like effector (CIDEC), Haptoglobin, Neuroglobin, and stearoyl-CoA desaturase-1 (SCD1) and increases in cytokine-inducible SH2-containing protein (CISH) and ceramide synthase 1 (CerS1) that are regulate protein turn over in gastrocnemius. Also, allulose alleviates autophagy in muscle with regulated mammalian target of rapamycin (mTOR) signaling pathway and increases the anti-oxidant enzyme activity.

CONCLUSION

These findings suggest that allulose improves the age-associated sarcopenia with enhancing antioxidant properties by altering mRNA and protein expression.

ActRIIB:ALK4-Fc alleviates muscle dysfunction and comorbidities in murine models of neuromuscular disorders

Patients with neuromuscular disorders suffer from a lack of treatment options for skeletal muscle weakness and disease comorbidities. Here, we introduce as a potential therapeutic agent a heterodimeric ligand-trapping fusion protein, ActRIIB:ALK4-Fc, which comprises extracellular domains of activin-like kinase 4 (ALK4) and activin receptor type IIB (ActRIIB), a naturally occurring pair of type I and II receptors belonging to the TGF-β superfamily. By surface plasmon resonance (SPR), ActRIIB:ALK4-Fc exhibited a ligand binding profile distinctly different from that of its homodimeric variant ActRIIB-Fc, sequestering ActRIIB ligands known to inhibit muscle growth but not trapping the vascular regulatory ligand bone morphogenetic protein 9 (BMP9). ActRIIB:ALK4-Fc and ActRIIB-Fc administered to mice exerted differential effects - concordant with SPR results - on vessel outgrowth in a retinal explant assay. ActRIIB:ALK4-Fc induced a systemic increase in muscle mass and function in wild-type mice and in murine models of Duchenne muscular dystrophy (DMD), amyotrophic lateral sclerosis (ALS), and disuse atrophy. Importantly, ActRIIB:ALK4-Fc improved neuromuscular junction abnormalities in murine models of DMD and presymptomatic ALS and alleviated acute muscle fibrosis in a DMD model. Furthermore, in combination therapy ActRIIB:ALK4-Fc increased the efficacy of antisense oligonucleotide M12-PMO on dystrophin expression and skeletal muscle endurance in an aged DMD model. ActRIIB:ALK4-Fc shows promise as a therapeutic agent, alone or in combination with dystrophin rescue therapy, to alleviate muscle weakness and comorbidities of neuromuscular disorders.

Understanding of sarcopenia: from definition to therapeutic strategies

Sarcopenia refers to the gradual loss of skeletal muscle mass and function along with aging and is a social burden due to growing healthcare cost associated with a super-aging society. Therefore, researchers have established guidelines and tests to diagnose sarcopenia. Several studies have been conducted actively to reveal the cause of sarcopenia and find an economic therapy to improve the quality of life in elderly individuals. Sarcopenia is caused by multiple factors such as reduced regenerative capacity, imbalance in protein turnover, alteration of fat and fibrotic composition in muscle, increased reactive oxygen species, dysfunction of mitochondria and increased inflammation. Based on these mechanisms, nonpharmacological and pharmacological strategies have been developed to prevent and treat sarcopenia. Although several studies are currently in progress, no treatment is available yet. This review presents the definition of sarcopenia and summarizes recent understanding on the detailed mechanisms, diagnostic criteria, and strategies for prevention and treatment.

Advances in research on pharmacotherapy of sarcopenia

Sarcopenia is a comprehensive degenerative disease with the progressive loss of skeletal muscle mass with age, accompanied by the loss of muscle strength and muscle dysfunction. As a new type of senile syndrome, sarcopenia seriously threatens the health of the elderly. The first-line treatment for sarcopenia is exercise and nutritional supplements. However, pharmacotherapy will provide more reliable and sustainable interventions in geriatric medicine. Clinical trials of new drugs targeting multiple molecules are ongoing. This article focuses on the latest progress in pharmacotherapeutic approaches of sarcopenia in recent years by comprehensively reviewing the clinical outcomes of the existing and emerging pharmacotherapies as well as the molecular mechanisms underlying their therapeutic benefits and side effects.

Local versus systemic control of bone and skeletal muscle mass by components of the transforming growth factor-β signaling pathway

Skeletal muscle and bone homeostasis are regulated by members of the myostatin/GDF-11/activin branch of the transforming growth factor-β superfamily, which share many regulatory components, including inhibitory extracellular binding proteins and receptors that mediate signaling. Here, we present the results of genetic studies demonstrating a critical role for the binding protein follistatin (FST) in regulating both skeletal muscle and bone. Using an allelic series corresponding to varying expression levels of endogenous Fst, we show that FST acts in an exquisitely dose-dependent manner to regulate both muscle mass and bone density. Moreover, by employing a genetic strategy to target Fst expression only in the posterior (caudal) region of the animal, we show that the effects of Fst loss are mostly restricted to the posterior region, implying that locally produced FST plays a much more important role than circulating FST with respect to regulation of muscle and bone. Finally, we show that targeting receptors for these ligands specifically in osteoblasts leads to dramatic increases in bone mass, with trabecular bone volume fraction being increased by 12- to 13-fold and bone mineral density being increased by 8- to 9-fold in humeri, femurs, and lumbar vertebrae. These findings demonstrate that bone, like muscle, has an enormous inherent capacity for growth that is normally kept in check by this signaling system and suggest that the extent to which this regulatory mechanism may be used throughout the body to regulate tissue mass may be more significant than previously appreciated.

Cancer-Mediated Muscle Cachexia: Etiology and Clinical Management

Muscle cachexia has a major detrimental impact on cancer patients, being responsible for 30% of all cancer deaths. It is characterized by a debilitating loss in muscle mass and function, which ultimately deteriorates patients' quality of life and dampens therapeutic treatment efficacy. Muscle cachexia stems from widespread alterations in whole-body metabolism as well as immunity and neuroendocrine functions and these global defects often culminate in aberrant signaling within skeletal muscle, causing muscle protein breakdown and attendant muscle atrophy. This review summarizes recent landmark discoveries that significantly enhance our understanding of the molecular etiology of cancer-driven muscle cachexia and further discuss emerging therapeutic approaches seeking to simultaneously target those newly discovered mechanisms to efficiently curb this lethal syndrome.

Identification of Molecules from Coffee Silverskin That Suppresses Myostatin Activity and Improves Muscle Mass and Strength in Mice

Coffee has been shown to attenuate sarcopenia, the age-associated muscle atrophy. Myostatin (MSTN), a member of the TGF-β growth/differentiation factor superfamily, is a potent negative regulator of skeletal muscle mass, and MSTN-inhibition increases muscle mass or prevents muscle atrophy. This study, thus, investigated the presence of MSTN-inhibitory capacity in coffee extracts. The ethanol-extract of coffee silverskin (CSE) but not other extracts demonstrated anti-MSTN activity in a pGL3-(CAGA)₁₂-luciferase reporter gene assay. CSE also blocked Smad3 phosphorylation induced by MSTN but not by GDF11 or Activin A in Western blot analysis, demonstrating its capacity to block the binding of MSTN to its receptor. Oral administration of CSE significantly increased forelimb muscle mass and grip strength in mice. Using solvent partitioning, solid-phase chromatography, and reverse-phase HPLC, two peaks having MSTN-inhibitory capacity were purified from CSE. The two peaks were identified as ^βN-arachinoyl−5-hydroxytryptamide (C₂₀−5HT) and ^βN-behenoyl−5-hydroxytryptamide (C₂₂−5HT) using mass spectrometry and NMR analysis. In summary, the results show that CSE has the MSTN-inhibitory capacity, and C₂₀−5HT and C₂₂−5HT are active components of CSE-suppressing MSTN activity, suggesting the potential of CSE, C₂₀−5HT, and C₂₂−5HT being developed as agents to combat muscle atrophy and metabolic syndrome.

Targeting the myostatin signaling pathway to treat muscle loss and metabolic dysfunction

Since the discovery of myostatin (MSTN; also known as GDF-8) as a critical regulator of skeletal muscle mass in 1997, there has been an extensive effort directed at understanding the cellular and physiological mechanisms underlying MSTN activity, with the long-term goal of developing strategies and agents capable of blocking MSTN signaling to treat patients with muscle loss. Considerable progress has been made in elucidating key components of this regulatory system, and in parallel with this effort has been the development of numerous biologics that have been tested in clinical trials for a wide range of indications, including muscular dystrophy, sporadic inclusion body myositis, spinal muscular atrophy, cachexia, muscle loss due to aging or following falls, obesity, and type 2 diabetes. Here, I review what is known about the MSTN regulatory system and the current state of efforts to target this pathway for clinical applications.

Principles of the activin receptor signaling pathway and its inhibition

This review captures the anabolic and stimulatory effects observed with inhibition of the transforming growth factor β superfamily in muscle, blood, and bone. New medicinal substances that rectify activin, myostatin, and growth differentiation factor 11 signaling give hope to the many whose lives are affected by deterioration of these tissues. The review first covers the origin, structure, and common pathway of activins, myostatin, and growth differentiation factor 11 along with the pharmacodynamics of the new class of molecules designed to oppose the activin receptor signaling pathway. Current terminology surrounding this new class of molecules is inconsistent and does not infer functionality. Adopting inhibitors of the activin receptor signaling pathway (IASPs) as a generic term is proposed because it encapsulates the molecular mechanisms along the pathway trajectory. To conclude, a pragmatic classification of IASPs is presented that integrates functionality and side effects based on the data available from animals and humans. This provides researchers and clinicians with a tool to tailor IASPs therapy according to the need of projects or patients and with respect to side effects.

activin-2 is required for regeneration of polarity on the planarian anterior-posterior axis

Planarians are flatworms and can perform whole-body regeneration. This ability involves a mechanism to distinguish between anterior-facing wounds that require head regeneration and posterior-facing wounds that require tail regeneration. How this head-tail regeneration polarity decision is made is studied to identify principles underlying tissue-identity specification in regeneration. We report that inhibition of activin-2, which encodes an Activin-like signaling ligand, resulted in the regeneration of ectopic posterior-facing heads following amputation. During tissue turnover in uninjured planarians, positional information is constitutively expressed in muscle to maintain proper patterning. Positional information includes Wnts expressed in the posterior and Wnt antagonists expressed in the anterior. Upon amputation, several wound-induced genes promote re-establishment of positional information. The head-versus-tail regeneration decision involves preferential wound induction of the Wnt antagonist notum at anterior-facing over posterior-facing wounds. Asymmetric activation of notum represents the earliest known molecular distinction between head and tail regeneration, yet how it occurs is unknown. activin-2 RNAi animals displayed symmetric wound-induced activation of notum at anterior- and posterior-facing wounds, providing a molecular explanation for their ectopic posterior-head phenotype. activin-2 RNAi animals also displayed anterior-posterior (AP) axis splitting, with two heads appearing in anterior blastemas, and various combinations of heads and tails appearing in posterior blastemas. This was associated with ectopic nucleation of anterior poles, which are head-tip muscle cells that facilitate AP and medial-lateral (ML) pattern at posterior-facing wounds. These findings reveal a role for Activin signaling in determining the outcome of AP-axis-patterning events that are specific to regeneration.

A central problem in animal regeneration is how animals determine what body part to regenerate. Planarians are flatworms that can regenerate any missing body region, and are studied to identify mechanisms underlying regeneration. At transverse amputation planes, a poorly understood mechanism specifies regeneration of either a head or a tail. This head-versus-tail regeneration decision-making process is referred to as regeneration polarity and has been studied for over a century to identify mechanisms that specify what to regenerate. The gene notum, which encodes a Wnt antagonist, is induced within hours after injury preferentially at anterior-facing wounds, where it specifies head regeneration. We report that Activin signaling is required for regeneration polarity, and the underlying asymmetric activation of notum at anterior- over posterior-facing wounds. We propose that Activin signaling is involved in regeneration-specific responses broadly in the animal kingdom.

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.

The emerging role of activins in renal disease

PURPOSE OF REVIEW

This review highlights recent discoveries and advances that have been made in understanding the role of the TGFβ superfamily members activins, and in particular, activin A (ActA), in renal disease.

RECENT FINDINGS

A deleterious role for ActA in renal disease and its complications has begun to emerge. We summarize data supporting an important contribution of ActA to kidney fibrosis and inflammation of varying causes, as well as its role in the development of a particular bone mineral disorder seen in chronic kidney disease (CKD) called mineral bone disorder (MBD), including vascular calcification. Finally, we discuss ActA in the context of anemia associated with chronic kidney disease and review potential approaches to treatment based on ActA blockade.

SUMMARY

ActA is an important contributor to the pathogenesis of acute and chronic kidney disease of varying causes. Preclinical studies show that ActA inhibition, through various approaches, is protective in rodent models of kidney disease. The potential adverse effects of some of these approaches can be attributed to their targeting of other TGFβ family ligands. Further preclinical and clinical investigations testing the therapeutic efficacy of more selective ActA inhibition on the progression of acute and chronic kidney disease and its impact on bone-mineral disorder would more definitively establish its role in renal disease.

Safety and pharmacokinetics of bimagrumab in healthy older and obese adults with body composition changes in the older cohort

BACKGROUND

Bimagrumab prevents activity of myostatin and other negative regulators of skeletal muscle mass. This randomized double-blind, placebo-controlled study investigated safety, pharmacokinetics (PK), and pharmacodynamics of bimagrumab in healthy older and obese adults.

METHODS

A cohort of older adults (aged 70-85 years) received single intravenous infusions of bimagrumab 30 mg/kg (n = 6) or 3 mg/kg (n = 6) or placebo (n = 4) and was followed for 20 weeks. A second cohort of obese participants [body mass index (BMI) 30-45 kg/m² , aged 18-65 years] received a single intravenous infusion of bimagrumab 30 mg/kg (n = 6) or placebo (n = 2) and was followed for 12 weeks. Outcomes included the safety, tolerability, and PK of bimagrumab, in both cohorts. Measures of pharmacodynamics were performed in the older adult cohort to evaluate the effects of bimagrumab on thigh muscle volume (TMV), total lean body mass (LBM), total fat body mass, and muscle strength.

RESULTS

All 24 randomized participants completed the study. The older adults had a mean (±SD) age of 74.5 ± 3.4 years and BMI of 26.5 ± 3.5 kg/m² . The obese participants had a mean (±SD) age of 40.4 ± 11.8 years, weight of 98.0 ± 11.3 kg, and BMI of 34.3 ± 3.9 kg/m² . Adverse events in both cohorts were mostly mild. In older adults, most commonly reported adverse events were upper respiratory tract infection, rash, and diarrhoea (each 3/16, 19%). Obese participants reported muscle spasms and rash (both 5/8, 63%) most often. Non-linearity was observed in the PK concentration profiles of both cohorts due to target-mediated drug disposition. Bimagrumab 3 and 30 mg/kg increased mean (±SD) TMV (Week 4: 5.3 ± 1.8% and 6.1 ± 2.2%, vs. placebo: 0.5 ± 2.1%, both P ≤ 0.02) and LBM (Week 4: 6.0 ± 3.2%, P = 0.03 and 2.4 ± 2.2%, vs. placebo: 0.1 ± 2.4%), which were maintained longer with higher dose level, while total fat body mass (Week 4: -2.7 ± 2.9% and -1.6 ± 3.0%, vs. placebo: -2.3 ± 3.2%) decreased from baseline in older adults, with no change in muscle strength.

CONCLUSIONS

Bimagrumab was safe and well tolerated and demonstrated similar PK in older and obese adults. A single dose of bimagrumab rapidly increased TMV and LBM and decreased body adiposity in older adults. Muscle hypertrophy and fat loss were sustained with extended drug exposure.