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.

Identification and Evaluation of Traditional Chinese Medicine Natural Compounds as Potential Myostatin Inhibitors: An In Silico Approach.

Myostatin (MSTN), a negative regulator of muscle mass, is reported to be increased in conditions linked with muscle atrophy, sarcopenia, and other muscle-related diseases. Most pharmacologic approaches that treat muscle disorders are ineffective, emphasizing the emergence of MSTN inhibition. In this study, we used computational screening to uncover natural small bioactive inhibitors from the Traditional Chinese Medicine database (~38,000 compounds) for the MSTN protein. Potential ligands were screened, based on binding affinity (150), physicochemical (53) and ADMET properties (17). We found two hits (ZINC85592908 and ZINC85511481) with high binding affinity and specificity, and their binding patterns with MSTN protein. In addition, molecular dynamic simulations were run on each complex to better understand the interaction mechanism of MSTN with the control (curcumin) and the hit compounds (ZINC85592908 and ZINC85511481). We determined that the hits bind to the active pocket site (Helix region) and trigger conformational changes in the MSTN protein. Since the stability of the ZINC85592908 compound was greater than the MSTN control, we believe that ZINC85592908 has therapeutic potential against the MSTN protein and may hinder downstream singling by inhibiting the MSTN protein and increasing myogenesis in the skeletal muscle tissues.

An Antisense Oligonucleotide against a Splicing Enhancer Sequence within Exon 1 of the MSTN Gene Inhibits Pre-mRNA Maturation to Act as a Novel Myostatin Inhibitor.

Antisense oligonucleotides (ASOs) are agents that modulate gene function. ASO-mediated out-of-frame exon skipping has been employed to suppress gene function. Myostatin, encoded by the MSTN gene, is a potent negative regulator of skeletal muscle growth. ASOs that induce skipping of out-of-frame exon 2 of the MSTN gene have been studied for their use in increasing muscle mass. However, no ASOs are currently available for clinical use. We hypothesized that ASOs against the splicing enhancer sequence within exon 1 of the MSTN gene would inhibit maturation of pre-mRNA, thereby suppressing gene function. To explore this hypothesis, ASOs against sequences of exon 1 of the MSTN gene were screened for their ability to reduce mature MSTN mRNA levels. One screened ASO, named KMM001, decreased MSTN mRNA levels in a dose-dependent manner and reciprocally increased MSTN pre-mRNA levels. Accordingly, KMM001 decreased myostatin protein levels. KMM001 inhibited SMAD-mediated myostatin signaling in rhabdomyosarcoma cells. Remarkably, it did not decrease GDF11 mRNA levels, indicating myostatin-specific inhibition. As expected, KMM001 enhanced the proliferation of human myoblasts. We conclude that KMM001 is a novel myostatin inhibitor that inhibits pre-mRNA maturation. KMM001 has great promise for clinical applications and should be examined for its ability to treat various muscle-wasting conditions.

Bone morphogenetic protein 2 inhibits growth differentiation factor 8-induced cell signaling via upregulation of gremlin2 expression in human granulosa-lutein cells.

BACKGROUND: Bone morphogenetic protein 2 (BMP2), growth differentiation factor 8 (GDF8) and their functional receptors are expressed in human ovarian follicles, and these two intrafollicular factors play essential roles in regulating follicle development and luteal function. As BMP antagonists, gremlin1 (GREM1) and gremlin2 (GREM2) suppress BMP signaling through blockage of ligand-receptor binding. However, whether BMP2 regulates the expression of GREM1 and GREM2 in follicular development remains to be determined. METHODS: In the present study, we investigated the effect of BMP2 on the expression of GREM1 and GREM2 and the underlying mechanisms in human granulosa-lutein (hGL) cells. An established immortalized human granulosa cell line (SVOG) and primary hGL cells were used as study models. The expression of GREM1 and GREM2 were examined following cell incubation with BMP2 at different concentrations and time courses. The TGF-ß type I inhibitors (dorsomorphin, DMH-1 and SB431542) and small interfering RNAs targeting ALK2, ALK3, SMAD2/3, SMAD1/5/8 and SMAD4 were used to investigate the involvement of the SMAD-dependent pathway. RESULTS: Our results showed that BMP2 significantly increased the expression of GREM2 (but not GREM1) in a dose- and time-dependent manner. Using a dual inhibition approach combining kinase inhibitors and siRNA-mediated knockdown, we found that the BMP2-induced upregulation of GREM2 expression was mediated by the ALK2/3-SMAD1/5-SMAD4 signaling pathway. Moreover, we demonstrated that BMP2 pretreatment significantly attenuated the GDF8-induced phosphorylation of SMAD2 and SMAD3, and this suppressive effect was reversed by knocking down GREM2 expression. CONCLUSIONS: Our findings provide new insight into the molecular mechanisms by which BMP2 modulates the cellular activity induced by GDF8 through the upregulated expression of their antagonist (GREM2).

Selective Myostatin Inhibition Spares Sublesional Muscle Mass and Myopenia-Related Dysfunction after Severe Spinal Cord Contusion in Mice.

Clinically relevant myopenia accompanies spinal cord injury (SCI), and compromises function, metabolism, body composition, and health. Myostatin, a transforming growth factor (TGF)ß family member, is a key negative regulator of skeletal muscle mass. We investigated inhibition of myostatin signaling using systemic delivery of a highly selective monoclonal antibody - muSRK-015P (40 mg/kg) - that blocks release of active growth factor from the latent form of myostatin. Adult female mice (C57BL/6) were subjected to a severe SCI (65 kdyn) at T9 and were then immediately and 1 week later administered test articles: muSRK-015P (40 mg/kg) or control (vehicle or IgG). A sham control group (laminectomy only) was included. At euthanasia, (2 weeks post-SCI) muSRK-015P preserved whole body lean mass and sublesional gastrocnemius and soleus mass. muSRK-015P-treated mice with SCI also had significantly attenuated myofiber atrophy, lipid infiltration, and loss of slow-oxidative phenotype in soleus muscle. These outcomes were accompanied by significantly improved sublesional motor function and muscle force production at 1 and 2 weeks post-SCI. At 2 weeks post-SCI, lean mass was significantly decreased in SCI-IgG mice, but was not different in SCI-muSRK-015P mice than in sham controls. Total energy expenditure (kCal/day) at 2 weeks post-SCI was lower in SCI-immunoglobulin (Ig)G mice, but not different in SCI-muSRK-015P mice than in sham controls. We conclude that in a randomized, blinded, and controlled study in mice, myostatin inhibition using muSRK-015P had broad effects on physical, metabolic, and functional outcomes when compared with IgG control treated SCI animals. These findings may identify a useful, targeted therapeutic strategy for treating post-SCI myopenia and related sequelae in humans.

Computational Identification of Dithymoquinone as a Potential Inhibitor of Myostatin and Regulator of Muscle Mass.

The skeletal muscle (SM) is the largest organ in the body and has tremendous regenerative power due to its myogenic stem cell population. Myostatin (MSTN), a protein produced by SM, is released into the bloodstream and is responsible for age-related reduced muscle fiber development. The objective of this study was to identify the natural compounds that inhibit MSTN with therapeutic potential for the management of age-related disorders, specifically muscle atrophy and sarcopenia. Sequential screening of 2000 natural compounds was performed, and dithymoquinone (DTQ) was found to inhibit MSTN with a binding free energy of -7.40 kcal/mol. Furthermore, the docking results showed that DTQ reduced the binding interaction between MSTN and its receptor, activin receptor type-2B (ActR2B). The global energy of MSTN-ActR2B was found to be reduced from -47.75 to -40.45 by DTQ. The stability of the DTQ-MSTN complex was subjected to a molecular dynamics analysis for up to 100 ns to check the stability of the complex using RMSD, RMSF, Rg, SASA, and H-bond number. The complex was found to be stable after 10 ns to the end of the simulation. These results suggest that DTQ blocks MSTN signaling through ActR2B and that it has potential use as a muscle growth-promoting agent during the aging process.

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.

Interaction of Fibromodulin and Myostatin to Regulate Skeletal Muscle Aging: An Opposite Regulation in Muscle Aging, Diabetes, and Intracellular Lipid Accumulation.

The objective of this study was to investigate fibromodulin (FMOD) and myostatin (MSTN) gene expressions during skeletal muscle aging and to understand their involvements in this process. The expressions of genes related to muscle aging (Atrogin 1 and Glb1), diabetes (RAGE and CD163), and lipid accumulation (CD36 and PPARγ) and those of FMOD and MSTN were examined in CTX-injected, aged, MSTN-/-, and high-fat diet (HFD) mice and in C2C12 myoblasts treated with ceramide or grown under adipogenic conditions. Results from CTX-injected mice and gene knockdown experiments in C2C12 cells suggested the involvement of FMOD during muscle regeneration and myoblast proliferation and differentiation. Downregulation of the FMOD gene in MSTN-/- mice, and MSTN upregulation and FMOD downregulation in FMOD and MSTN knockdown C2C12 cells, respectively, during their differentiation, suggested FMOD negatively regulates MSTN gene expression, and MSTN positively regulates FMOD gene expression. The results of our in vivo and in vitro experiments indicate FMOD inhibits muscle aging by negatively regulating MSTN gene expression or by suppressing the action of MSTN protein, and that MSTN promotes muscle aging by positively regulating the expressions of Atrogin1, CD36, and PPARγ genes in muscle.

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-ß 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 ßN-arachinoyl-5-hydroxytryptamide (C20-5HT) and ßN-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.

Strategic structure-activity relationship study on a follistatin-derived myostatin inhibitory peptide.

Myostatin, a negative regulator of muscle mass is a promising target for the treatment of muscle atrophic diseases. The novel myostatin inhibitory peptide, DF-3 is derived from the N-terminal α-helical domain of follistatin, which is an endogenous inhibitor of myostatin and other TGF-ß family members. It has been suggested that the optimization of hydrophobic residues is important to enhance the myostatin inhibition. This study describes a structure-activity relationship study focused on hydrophobic residues of DF-3 and designed to obtain a more potent peptide. A methionine residue in DF-3, which is susceptible to oxidation, was successfully converted to homophenylalanine in DF-100, and a new derivative DF-100, with four amino acid substitutions in DF-3 shows twice the potent inhibitory ability as DF-3. This report provides a new platform of a 14-mer peptide muscle enhancer.

Myostatin Inhibition-Induced Increase in Muscle Mass and Strength Was Amplified by Resistance Exercise Training, and Dietary Essential Amino Acids Improved Muscle Quality in Mice.

It has been frequently reported that myostatin inhibition increases muscle mass, but decreases muscle quality (i.e., strength/muscle mass). Resistance exercise training (RT) and essential amino acids (EAAs) are potent anabolic stimuli that synergistically increase muscle mass through changes in muscle protein turnover. In addition, EAAs are known to stimulate mitochondrial biogenesis. We have investigated if RT amplifies the anabolic potential of myostatin inhibition while EAAs enhance muscle quality through stimulations of mitochondrial biogenesis and/or muscle protein turnover. Mice were assigned into ACV (myostatin inhibitor), ACV+EAA, ACV+RT, ACV+EAA +RT, or control (CON) over 4 weeks. RT, but not EAA, increased muscle mass above ACV. Despite differences in muscle mass gain, myofibrillar protein synthesis was stimulated similarly in all vs. CON, suggesting a role for changes in protein breakdown in muscle mass gains. There were increases in MyoD expression but decreases in Atrogin-1/MAFbx expression in ACV+EAA, ACV+RT, and ACV+EAA+RT vs. CON. EAA increased muscle quality (e.g., grip strength and maximal carrying load) without corresponding changes in markers of mitochondrial biogenesis and neuromuscular junction stability. In conclusion, RT amplifies muscle mass and strength through changes in muscle protein turnover in conjunction with changes in implicated signaling, while EAAs enhance muscle quality through unknown mechanisms.

Proposal for the binding mode of the 23-mer inhibitory peptide to myostatin.

Inhibition of myostatin is a promising strategy for the treatment of amyotrophic disorders. Previously, we identified a minimum 23-mer peptide spanning positions 21-43 of a mouse myostatin precursor-derived prodomain and identified the nine key residues for effective myostatin inhibition through Ala scanning. We also reported the 23-mer peptides that show the propensity to form an α-helical structure around positions 32-36. Here, based on these findings, we conducted a docking simulation of a peptide-myostatin interaction. The results showed that by α-helix restraint docking of the 30-41 main chain, we obtained a proposed binding mode in which all nine of the key residues interact with myostatin. By analyzing the binding mode of four proposed docking models, we identified six of the myostatin residues that play an important role in the interaction with the peptide. This result provides a valuable insight into the relationship between myostatin and peptide interaction sites and may help in the design of future inhibitors.

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.

Myostatin inhibitor YK11 as a preventative health supplement for bacterial sepsis.

Muscle wasting caused by catabolic reactions in skeletal muscle is commonly observed in patients with sepsis. Myostatin, a negative regulator of muscle mass, has been reported to be upregulated in diseases associated with muscle atrophy. However, the behavior of myostatin during sepsis is not well understood. Herein, we sought to investigate the expression and regulation of myostatin in skeletal muscle in mice inoculated with gram-negative bacteria. Interestingly, the protein level of myostatin was found to increase in the muscle of septic mice simultaneously with an increase in the levels of follistatin, NF-κΒ, myogenin, MyoD, p- FOXO3a, and p-Smad2. Furthermore, the inhibition of myostatin by YK11 repressed the levels of pro-inflammatory cytokines and organ damage markers in the bloodstream and in the major organs of mice, which originally increased in sepsis; thus, myostatin inhibition by YK11 decreased the mortality rate due to sepsis. The results of this study suggest that YK11 may help revert muscle wasting during sepsis and subdue the inflammatory environment, thereby highlighting its potential as a preventive agent for sepsis-related muscle wasting.

Development of antisense-mediated myostatin knockdown for the treatment of insulin resistance.

Myostatin is a negative regulator of muscle mass and its inhibition represents a promising strategy for the treatment of muscle disorders and type 2 diabetes. However, there is currently no clinically effective myostatin inhibitor, and therefore novel methods are required. We evaluated the use of antisense phosphorodiamidate morpholino oligomers (PMO) to reduce myostatin expression in skeletal muscle and measured their effects on muscle mass and glucose uptake. C57/Bl6 mice received intramuscular or intravenous injections of anti-myostatin PMOs. Repeated intramuscular administration lead to a reduction in myostatin transcript levels (~ 20-40%), and an increase in muscle mass in chow and high-fat diet (HFD)-fed mice, but insulin-stimulated glucose uptake was reduced in PMO-treated muscles of HFD-fed mice. Five weekly intravenous administrations of 100 nmol PMO did not reduce myostatin expression, and therefore had no significant physiological effects. Unexpectedly, exon skipping levels were higher after intramuscular administration of PMO in HFD- than chow-fed mice. These results suggest that a modest PMO-induced reduction in myostatin transcript levels is sufficient to induce an increase in muscle mass, but that a greater degree of inhibition may be required to improve muscle glucose uptake.

Transcriptomic Analysis of MSTN Knockout in the Early Differentiation of Chicken Fetal Myoblasts.

In mammals, Myostatin (MSTN) is a known negative regulator of muscle growth and development, but its role in birds is poorly understood. To investigate the molecular mechanism of MSTN on muscle growth and development in chickens, we knocked out MSTN in chicken fetal myoblasts (CFMs) and sequenced the mRNA transcriptomes. The amplicon sequencing results show that the editing efficiency of the cells was 76%. The transcriptomic results showed that 296 differentially expressed genes were generated after down-regulation of MSTN, including angiotensin I converting enzyme (ACE), extracellular fatty acid-binding protein (EXFABP) and troponin T1, slow skeletal type (TNNT1). These genes are closely associated with myoblast differentiation, muscle growth and energy metabolism. Subsequent enrichment analysis showed that DEGs of CFMs were related to MAPK, Pl3K/Akt, and STAT3 signaling pathways. The MAPK and Pl3K/Akt signaling pathways are two of the three known signaling pathways involved in the biological effects of MSTN in mammals, and the STAT3 pathway is also significantly enriched in MSTN knock out chicken leg muscles. The results of this study will help to understand the possible molecular mechanism of MSTN regulating the early differentiation of CFMs and lay a foundation for further research on the molecular mechanism of MSTN involvement in muscle growth and development.

Docosanoic acid conjugation to siRNA enables functional and safe delivery to skeletal and cardiac muscles.

Oligonucleotide therapeutics hold promise for the treatment of muscle- and heart-related diseases. However, oligonucleotide delivery across the continuous endothelium of muscle tissue is challenging. Here, we demonstrate that docosanoic acid (DCA) conjugation of small interfering RNAs (siRNAs) enables efficient (~5% of injected dose), sustainable (>1 month), and non-toxic (no cytokine induction at 100 mg/kg) gene silencing in both skeletal and cardiac muscles after systemic injection. When designed to target myostatin (muscle growth regulation gene), siRNAs induced ~55% silencing in various muscle tissues and 80% silencing in heart, translating into a ~50% increase in muscle volume within 1 week. Our study identifies compounds for RNAi-based modulation of gene expression in skeletal and cardiac muscles, paving the way for both functional genomics studies and therapeutic gene modulation in muscle and heart.

The Failed Clinical Story of Myostatin Inhibitors against Duchenne Muscular Dystrophy: Exploring the Biology behind the Battle.

Myostatin inhibition therapy has held much promise for the treatment of muscle wasting disorders. This is particularly true for the fatal myopathy, Duchenne Muscular Dystrophy (DMD). Following on from promising pre-clinical data in dystrophin-deficient mice and dogs, several clinical trials were initiated in DMD patients using different modality myostatin inhibition therapies. All failed to show modification of disease course as dictated by the primary and secondary outcome measures selected: the myostatin inhibition story, thus far, is a failed clinical story. These trials have recently been extensively reviewed and reasons why pre-clinical data collected in animal models have failed to translate into clinical benefit to patients have been purported. However, the biological mechanisms underlying translational failure need to be examined to ensure future myostatin inhibitor development endeavors do not meet with the same fate. Here, we explore the biology which could explain the failed translation of myostatin inhibitors in the treatment of DMD.

Generation of myostatin edited horse embryos using CRISPR/Cas9 technology and somatic cell nuclear transfer.

The application of new technologies for gene editing in horses may allow the generation of improved sportive individuals. Here, we aimed to knock out the myostatin gene (MSTN), a negative regulator of muscle mass development, using CRISPR/Cas9 and to generate edited embryos for the first time in horses. We nucleofected horse fetal fibroblasts with 1, 2 or 5 µg of 2 different gRNA/Cas9 plasmids targeting the first exon of MSTN. We observed that increasing plasmid concentrations improved mutation efficiency. The average efficiency was 63.6% for gRNA1 (14/22 edited clonal cell lines) and 96.2% for gRNA2 (25/26 edited clonal cell lines). Three clonal cell lines were chosen for embryo generation by somatic cell nuclear transfer: one with a monoallelic edition, one with biallelic heterozygous editions and one with a biallelic homozygous edition, which rendered edited blastocysts in each case. Both MSTN editions and off-targets were analyzed in the embryos. In conclusion, CRISPR/Cas9 proved an efficient method to edit the horse genome in a dose dependent manner with high specificity. Adapting this technology sport advantageous alleles could be generated, and a precision breeding program could be developed.

The elusive promise of myostatin inhibition for muscular dystrophy.

PURPOSE OF REVIEW: Recent terminations of clinical trials of myostatin inhibitors in muscular dystrophy have raised questions about the predictiveness of mouse models for this therapeutic strategy. RECENT FINDINGS: A variety of myostatin inhibitors have been developed for preclinical and clinical studies. These inhibitors have ameliorated the phenotype of many but not all mouse models of muscular dystrophy. However, randomized double-blinded placebo controlled trials in both pediatric and adult muscular dystrophies have, as of yet, not demonstrated functional improvement. SUMMARY: The present article will review the preclinical promise of myostatin inhibitors, the clinical trial experience to date of these inhibitors in muscular dystrophy, and the potential reasons for the lack of observed translation.