The TGF-β Signalling Network in Muscle Development, Adaptation and Disease

The role of TGF-β signaling in muscle atrophy, sarcopenia and cancer cachexia

Skeletal muscle, comprising a significant proportion (40 to 50 percent) of total body weight in humans, plays a critical role in maintaining normal physiological conditions. Muscle atrophy occurs when the rate of protein degradation exceeds protein synthesis. Sarcopenia refers to age-related muscle atrophy, while cachexia represents a more complex form of muscle wasting associated with various diseases such as cancer, heart failure, and AIDS. Recent research has highlighted the involvement of signaling pathways, including IGF1-Akt-mTOR, MuRF1-MAFbx, and FOXO, in regulating the delicate balance between muscle protein synthesis and breakdown. Myostatin, a member of the TGF-β superfamily, negatively regulates muscle growth and promotes muscle atrophy by activating Smad2 and Smad3. It also interacts with other signaling pathways in cachexia and sarcopenia. Inhibition of myostatin has emerged as a promising therapeutic approach for sarcopenia and cachexia. Additionally, other TGF-β family members, such as TGF-β1, activin A, and GDF11, have been implicated in the regulation of skeletal muscle mass. Furthermore, myostatin cooperates with these family members to impair muscle differentiation and contribute to muscle loss. This review provides an overview of the significance of myostatin and other TGF-β signaling pathway members in muscular dystrophy, sarcopenia, and cachexia. It also discusses potential novel therapeutic strategies targeting myostatin and TGF-β signaling for the treatment of muscle atrophy.

Endocardial HDAC3 is required for myocardial trabeculation

Failure of proper ventricular trabeculation is often associated with congenital heart disease. Support from endocardial cells, including the secretion of extracellular matrix and growth factors is critical for trabeculation. However, it is poorly understood how the secretion of extracellular matrix and growth factors is initiated and regulated by endocardial cells. We find that genetic knockout of histone deacetylase 3 in the endocardium in mice results in early embryo lethality and ventricular hypotrabeculation. Single cell RNA sequencing identifies significant downregulation of extracellular matrix components in histone deacetylase 3 knockout endocardial cells. Secretome from cultured histone deacetylase 3 knockout mouse cardiac endothelial cells lacks transforming growth factor ß3 and shows significantly reduced capacity in stimulating cultured cardiomyocyte proliferation, which is remarkably rescued by transforming growth factor ß3 supplementation. Mechanistically, we identify that histone deacetylase 3 knockout induces transforming growth factor ß3 expression through repressing microRNA-129-5p. Our findings provide insights into the pathogenesis of congenital heart disease and conceptual strategies to promote myocardial regeneration.

YTHDF2 governs muscle size through a targeted modulation of proteostasis

The regulation of proteostasis is fundamental for maintenance of muscle mass and function. Activation of the TGF-β pathway drives wasting and premature aging by favoring the proteasomal degradation of structural muscle proteins. Yet, how this critical post-translational mechanism is kept in check to preserve muscle health remains unclear. Here, we reveal the molecular link between the post-transcriptional regulation of m6A-modified mRNA and the modulation of SMAD-dependent TGF-β signaling. We show that the m6A-binding protein YTHDF2 is essential to determining postnatal muscle size. Indeed, muscle-specific genetic deletion of YTHDF2 impairs skeletal muscle growth and abrogates the response to hypertrophic stimuli. We report that YTHDF2 controls the mRNA stability of the ubiquitin ligase ASB2 with consequences on anti-growth gene program activation through SMAD3. Our study identifies a post-transcriptional to post-translational mechanism for the coordination of gene expression in muscle.

Endocardial HDAC3 is required for myocardial trabeculation

BACKGROUND

Trabeculation, a key process in early heart development, is the formation of myocardial trabecular meshwork. The failure of trabeculation often leads to embryonic lethality. Support from endocardial cells, including the secretion of extracellular matrix (ECM) and growth factors is critical for trabeculation; however, it is unknown how the secretion of ECM and growth factors is initiated and regulated by endocardial cells.

METHODS

Various cellular and mouse models in conjunction with biochemical and molecular tools were employed to study the role of histone deacetylase 3 (HDAC3) in the developing endocardium.

RESULTS

We found that genetic deletion of Hdac3 in endocardial cells in mice resulted in early embryo lethality presenting as a hypotrabeculation cardiac phenotype. Single cell RNA sequencing identified several ECM components including collagens that were significantly downregulated in Hdac3 knockout (KO) endocardial cells. When cultured with supernatant from Hdac3 KO mouse cardiac endothelial cells (MCECs), wild-type mouse embryonic cardiomyocytes showed decreased proliferation, suggesting that growth signaling from Hdac3 KO MCECs is disrupted. Subsequent transcriptomic analysis revealed that transforming growth factor β3 (TGFβ3) was significantly downregulated in Hdac3 KO MCECs and Hdac3 cardiac endothelial KO hearts. Mechanistically, we identified that microRNA (miR)-129-5p was significantly upregulated in Hdac3 KO MCECs and Hdac3 cardiac endothelial KO hearts. Overexpression of miR-129-5p repressed Tgfβ3 expression in wild-type MCECs, whereas knockdown of miR-129-5p restored Tgfβ3 expression in Hdac3 KO MCECs.

CONCLUSION

Our findings reveal a critical signaling pathway in which endocardial HDAC3 promotes trabecular myocardium growth by stimulating TGFβ signaling through repressing miR-129-5p, providing novel insights into the etiology of congenital heart disease and conceptual strategies to promote myocardial regeneration.

Optimization of Xenografting Methods for Generating Human Skeletal Muscle in Mice

Xenografts of human skeletal muscle generated in mice can be used to study muscle pathology and to test drugs designed to treat myopathies and muscular dystrophies for their efficacy and specificity in human tissue. We previously developed methods to generate mature human skeletal muscles in immunocompromised mice starting with human myogenic precursor cells (hMPCs) from healthy individuals and individuals with facioscapulohumeral muscular dystrophy (FSHD). Here, we examine a series of alternative treatments at each stage in order to optimize engraftment. We show that (i) X-irradiation at 25Gy is optimal in preventing regeneration of murine muscle while supporting robust engraftment and the formation of human fibers without significant murine contamination; (ii) hMPC lines differ in their capacity to engraft; (iii) some hMPC lines yield grafts that respond better to intermittent neuromuscular electrical stimulation (iNMES) than others; (iv) some lines engraft better in male than in female mice; (v) coinjection of hMPCs with laminin, gelatin, Matrigel, or Growdex does not improve engraftment; (vi) BaCl2 is an acceptable replacement for cardiotoxin, but other snake venom preparations and toxins, including the major component of cardiotoxin, cytotoxin 5, are not; and (vii) generating grafts in both hindlimbs followed by iNMES of each limb yields more robust grafts than housing mice in cages with running wheels. Our results suggest that replacing cardiotoxin with BaCl2 and engrafting both tibialis anterior muscles generates robust grafts of adult human muscle tissue in mice.

Muscle miRNAs are influenced by sex at baseline and in response to exercise

BACKGROUND

Sex differences in microRNA (miRNA) expression profiles have been found across multiple tissues. Skeletal muscle is one of the most sex-biased tissues of the body. MiRNAs are necessary for development and have regulatory roles in determining skeletal muscle phenotype and have important roles in the response to exercise in muscle. Yet there is limited research into the role and regulation of miRNAs in the skeletal muscle at baseline and in response to exercise, a well-known modulator of miRNA expression. The aim of this study was to investigate the effect of sex on miRNA expression in the skeletal muscle at baseline and after an acute bout of high-intensity interval exercise. A total of 758 miRNAs were measured using Taqman®miRNA arrays in the skeletal muscle of 42 healthy participants from the Gene SMART study (23 males and 19 females of comparable fitness levels and aged 18-45 years), of which 308 were detected. MiRNAs that differed by sex at baseline and whose change in expression following high-intensity interval exercise differed between the sexes were identified using mixed linear models adjusted for BMI and Wpeak. We performed in silico analyses to identify the putative gene targets of the exercise-induced, sex-specific miRNAs and overrepresentation analyses to identify enriched biological pathways. We performed functional assays by overexpressing two sex-biased miRNAs in human primary muscle cells derived from male and female donors to understand their downstream effects on the transcriptome.

RESULTS

At baseline, 148 miRNAs were differentially expressed in the skeletal muscle between the sexes. Interaction analysis identified 111 miRNAs whose response to an acute bout of high-intensity interval exercise differed between the sexes. Sex-biased miRNA gene targets were enriched for muscle-related processes including proliferation and differentiation of muscle cells and numerous metabolic pathways, suggesting that miRNAs participate in programming sex differences in skeletal muscle function. Overexpression of sex-biased miRNA-30a and miRNA-30c resulted in profound changes in gene expression profiles that were specific to the sex of the cell donor in human primary skeletal muscle cells.

CONCLUSIONS

We uncovered sex differences in the expression levels of muscle miRNAs at baseline and in response to acute high-intensity interval exercise. These miRNAs target regulatory pathways essential to skeletal muscle development and metabolism. Our findings highlight that miRNAs play an important role in programming sex differences in the skeletal muscle phenotype.

Gastric Cancer with Sarcopenia: an Area Worth Focusing On

OPINION STATEMENT

Gastric cancer (GC) is the fifth most common cancer and the third leading cause of cancer death worldwide, which seriously endangers human health. A number of studies have shown that sarcopenia occurs more frequently in patients with gastric cancer than in the general population and can significantly affect the disease status and survival of patients, which is of great significance in predicting the prognosis of gastric cancer. Patients with gastric cancer may suffer sarcopenia no matter before or after surgery, and the pathogenesis is complex. Abnormal nutrient metabolism and reduced exercise are the leading causes. In addition, surgical treatment and chemotherapy for gastric cancer might participate in the physiological and pathological mechanism of sarcopenia. Generally speaking, exercise and nutritional therapy are the main prevention and treatment methods for sarcopenia. But more prospective evidence is needed to establish reasonable interventions, and other drug treatments are in their infancy. For the diagnostic criteria of sarcopenia, the cut-off values of the skeletal muscle mass index obtained from CT images vary widely and need to be standardized and unified. We also need to explore simple predictors to facilitate sarcopenia risk assessment. More research is needed to formulate more appropriate treatments for gastric cancer patients with sarcopenia.

Plum modulates Myoglianin and regulates synaptic function in D. melanogaster

Alterations in the neuromuscular system underlie several neuromuscular diseases and play critical roles in the development of sarcopenia, the age-related loss of muscle mass and function. Mammalian Myostatin (MST) and GDF11, members of the TGF-β superfamily of growth factors, are powerful regulators of muscle size in both model organisms and humans. Myoglianin (MYO), the Drosophila homologue of MST and GDF11, is a strong inhibitor of synaptic function and structure at the neuromuscular junction in flies. Here, we identified Plum, a transmembrane cell surface protein, as a modulator of MYO function in the larval neuromuscular system. Reduction of Plum in the larval body-wall muscles abolishes the previously demonstrated positive effect of attenuated MYO signalling on both muscle size and neuromuscular junction structure and function. In addition, downregulation of Plum on its own results in decreased synaptic strength and body weight, classifying Plum as a (novel) regulator of neuromuscular function and body (muscle) size. These findings offer new insights into possible regulatory mechanisms behind ageing- and disease-related neuromuscular dysfunctions in humans and identify potential targets for therapeutic interventions.

Kinin receptors regulate skeletal muscle regeneration: differential effects for B1 and B2 receptors

OBJECTIVE AND DESIGN

After traumatic skeletal muscle injury, muscle healing is often incomplete and produces extensive fibrosis. Bradykinin (BK) reduces fibrosis in renal and cardiac damage models through the B2 receptor. The B1 receptor expression is induced by damage, and blocking of the kallikrein-kinin system seems to affect the progression of muscular dystrophy. We hypothesized that both kinin B1 and B2 receptors could play a differential role after traumatic muscle injury, and the lack of the B1 receptor could produce more cellular and molecular substrates for myogenesis and fewer substrates for fibrosis, leading to better muscle healing.

MATERIAL AND METHODS

To test this hypothesis, tibialis anterior muscles of kinin receptor knockout animals were subjected to traumatic injury. Myogenesis, angiogenesis, fibrosis, and muscle functioning were evaluated.

RESULTS

Injured B1KO mice showed a faster healing progression of the injured area with a larger amount of central nucleated fiber post-injury when compared to control mice. In addition, they exhibited higher neovasculogenic capacity, maintaining optimal tissue perfusion for the post-injury phase; had higher amounts of myogenic markers with less inflammatory infiltrate and tissue destruction. This was followed by higher amounts of SMAD7 and lower amounts of p-SMAD2/3, which resulted in less fibrosis. In contrast, B2KO and B1B2KO mice showed more severe tissue destruction and excessive fibrosis. B1KO animals had better results in post-injury functional tests compared to control animals.

CONCLUSIONS

We demonstrate that injured skeletal muscle tissues have a better repair capacity with less fibrosis in the presence of B2 receptor and absence of B1 receptor, including better performances in functional tests.

GDF8 Contributes to Liver Fibrogenesis and Concomitant Skeletal Muscle Wasting

Patients with end-stage liver disease exhibit progressive skeletal muscle atrophy, highlighting a negative crosstalk between the injured liver and muscle. Our study was to determine whether TGFβ ligands function as the mediators. Acute or chronic liver injury was induced by a single or repeated administration of carbon tetrachloride. Skeletal muscle injury and repair was induced by intramuscular injection of cardiotoxin. Activin type IIB receptor (ActRIIB) ligands and growth differentiation factor 8 (Gdf8) were neutralized with ActRIIB-Fc fusion protein and a Gdf8-specific antibody, respectively. We found that acute hepatic injury induced rapid and adverse responses in muscle, which was blunted by neutralizing ActRIIB ligands. Chronic liver injury caused muscle atrophy and repair defects, which were prevented or reversed by inactivating ActRIIB ligands. Furthermore, we found that pericentral hepatocytes produce excessive Gdf8 in injured mouse liver and cirrhotic human liver. Specific inactivation of Gdf8 prevented liver injury-induced muscle atrophy, similar to neutralization of ActRIIB ligands. Inhibition of Gdf8 also reversed muscle atrophy in a treatment paradigm following chronic liver injury. Direct injection of exogenous Gdf8 protein into muscle along with acute focal muscle injury recapitulated similar dysregulated muscle regeneration as that observed with liver injury. The results indicate that injured liver negatively communicate with the muscle largely via Gdf8. Unexpectedly, inactivation of Gdf8 simultaneously ameliorated liver fibrosis in mice following chronic liver injury. In vitro, Gdf8 induced human hepatic stellate (LX-2) cells to form a septa-like structure and stimulated expression of profibrotic factors. Our findings identified Gdf8 as a novel hepatomyokine contributing to injured liver-muscle negative crosstalk along with liver injury progression.

Roles of Skeletal Muscle in Development: A Bioinformatics and Systems Biology Overview

The ability to assess various cellular events consequent to perturbations, such as genetic mutations, disease states and therapies, has been recently revolutionized by technological advances in multiple "omics" fields. The resulting deluge of information has enabled and necessitated the development of tools required to both process and interpret the data. While of tremendous value to basic researchers, the amount and complexity of the data has made it extremely difficult to manually draw inference and identify factors key to the study objectives. The challenges of data reduction and interpretation are being met by the development of increasingly complex tools that integrate disparate knowledge bases and synthesize coherent models based on current biological understanding. This chapter presents an example of how genomics data can be integrated with biological network analyses to gain further insight into the developmental consequences of genetic perturbations. State of the art methods for conducting similar studies are discussed along with modern methods used to analyze and interpret the data.

Loss of Myostatin Alters Mitochondrial Oxidative Phosphorylation, TCA Cycle Activity, and ATP Production in Skeletal Muscle

Myostatin (MSTN) is an important negative regulator of skeletal muscle growth in animals. A lack of MSTN promotes lipolysis and glucose metabolism but inhibits oxidative phosphorylation (OXPHOS). Here, we aimed to investigate the possible mechanism of MSTN regulating the mitochondrial energy homeostasis of skeletal muscle. To this end, MSTN knockout mice were generated by the CRISPR/Cas9 technique. Expectedly, the MSTN null (Mstn^-/-) mouse has a hypermuscular phenotype. The muscle metabolism of the Mstn^-/- mice was detected by an enzyme-linked immunosorbent assay, indirect calorimetry, ChIP-qPCR, and RT-qPCR. The resting metabolic rate and body temperature of the Mstn^-/- mice were significantly reduced. The loss of MSTN not only significantly inhibited the production of ATP by OXPHOS and decreased the activity of respiratory chain complexes, but also inhibited key rate-limiting enzymes related to the TCA cycle and significantly reduced the ratio of NADH/NAD+ in the Mstn^-/- mice, which then greatly reduced the total amount of ATP. Further ChIP-qPCR results confirmed that the lack of MSTN inhibited both the TCA cycle and OXPHOS, resulting in decreased ATP production. The reason may be that Smad2/3 is not sufficiently bound to the promoter region of the rate-limiting enzymes Idh2 and Idh3a of the TCA cycle, thus affecting their transcription.

Myostatin Knockout Affects Mitochondrial Function by Inhibiting the AMPK/SIRT1/PGC1α Pathway in Skeletal Muscle

Myostatin (Mstn) is a major negative regulator of skeletal muscle mass and initiates multiple metabolic changes. The deletion of the Mstn gene in mice leads to reduced mitochondrial functions. However, the underlying regulatory mechanisms remain unclear. In this study, we used CRISPR/Cas9 to generate myostatin-knockout (Mstn-KO) mice via pronuclear microinjection. Mstn-KO mice exhibited significantly larger skeletal muscles. Meanwhile, Mstn knockout regulated the organ weights of mice. Moreover, we found that Mstn knockout reduced the basal metabolic rate, muscle adenosine triphosphate (ATP) synthesis, activities of mitochondrial respiration chain complexes, tricarboxylic acid cycle (TCA) cycle, and thermogenesis. Mechanistically, expressions of silent information regulator 1 (SIRT1) and phosphorylated adenosine monophosphate-activated protein kinase (pAMPK) were down-regulated, while peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) acetylation modification increased in the Mstn-KO mice. Skeletal muscle cells from Mstn-KO and WT were treated with AMPK activator 5-aminoimidazole-4-carboxamide riboside (AICAR), and the AMPK inhibitor Compound C, respectively. Compared with the wild-type (WT) group, Compound C treatment further down-regulated the expression or activity of pAMPK, SIRT1, citrate synthase (CS), isocitrate dehydrogenase (ICDHm), and α-ketoglutarate acid dehydrogenase (α-KGDH) in Mstn-KO mice, while Mstn knockout inhibited the AICAR activation effect. Therefore, Mstn knockout affects mitochondrial function by inhibiting the AMPK/SIRT1/PGC1α signaling pathway. The present study reveals a new mechanism for Mstn knockout in regulating energy homeostasis.

miR-455-3p Is Negatively Regulated by Myostatin in Skeletal Muscle and Promotes Myoblast Differentiation

Myostatin (MSTN) is a growth and differentiation factor that regulates proliferation and differentiation of myoblasts, which in turn controls skeletal muscle growth. It may regulate myoblast differentiation by influencing miRNA expression, and the present study aimed to clarify its precise mechanism of action. Here, we found that MSTN^-/- pigs showed an overgrowth of skeletal muscle and upregulated miR-455-3p level. Intervention of MSTN expression using siMSTN in C2C12 myoblasts also showed that siMSTN significantly increased the expression of miR-455-3p. It was found that miR-455-3p directly targeted the 3'-untranslated region of Smad2 by dual-luciferase assay. qRT-PCR, Western blotting, and immunofluorescence analyses indicated that miR-455-3p overexpression or Smad2 silencing in C2C12 myoblasts significantly promoted myoblast differentiation. Furthermore, siMSTN significantly increased the expression of GATA3. The levels of miR-455-3p were considerably reduced in C2C12 myoblasts following GATA3 knockdown. Consistently, GATA3 knockdown also reduced the enhanced miR-455-3p expression caused by siMSTN. Finally, we illustrated that GATA3 has a role in myoblast differentiation regulation. Taken together, we identified the expression profiles of miRNAs in MSTN^-/- pigs and found that miR-455-3p positively regulates myoblast differentiation. In addition, we revealed that MSTN acts through the GATA3/miR-455-3p/Smad2 cascade to regulate muscle development.

Progress in Research on Antitumor Drugs and Dynamic Changes in Skeletal Muscles

Objective: To review the research progress of reltionship between antitumor drugs and the dynamic changes of the skeletal muscles during treatment phase.

Background: Sarcopenia is a common disease in patients with tumors, and it has been agreed that patients with tumors and sarcopenia experience more serious adverse reactions and have a shorter long-term survival after antitumor therapy than patients without sarcopenia. Antitumor drugs whilst beneficial for tumor regression, interferes and synergizes with cancer-induced muscle wasting/sarcopenia, induced myodemia or intramuscular fat and the two conditions often overlap making it difficult to drive conclusions. In recent years, increasing attention has been paid to the dynamic changes in skeletal muscles during antitumor drug therapy. Dynamic changes refer not only measurement skeletal muscle quantity at baseline level, but give more emphasis on the increasing or decreasing level during or end of the whole treatment course.

Methods: We retrievaled published English-language original research articles via pubmed, those studies mainly focused on repeated measurements of skeletal muscle index using computed tomography (CT) in cancer patients who received antitumor drug treatment but not received interventions that produced muscle mass change (such as exercise and nutritional interventions).

Conclusion: This article will summarize the research progress to date. Most of antineoplastic drug cause skeletal muscle loss during the treatment course, loss of L3 skeletal muscle index is always associated with poor clinical outcomes.

Sarcopenia in Children with Solid Organ Tumors: An Instrumental Era

Sarcopenia has recently been studied in both adults and children and was found to be a prognostic marker for adverse outcome in a variety of patient groups. Our research showed that sarcopenia is a relevant marker in predicting outcome in children with solid organ tumors, such as hepatoblastoma and neuroblastoma. This was especially true in very ill, high-risk groups. Children with cancer have a higher likelihood of ongoing loss of skeletal muscle mass due to a mismatch in energy intake and expenditure. Additionally, the effects of cancer therapy, hormonal alterations, chronic inflammation, multi-organ dysfunction, and a hypermetabolic state all contribute to a loss of skeletal muscle mass. Sarcopenia seems to be able to pinpoint this waste to a high degree in a new and objective way, making it an additional tool in predicting and improving outcome in children. This article focuses on the current state of sarcopenia in children with solid organ tumors. It details the pathophysiological mechanisms behind sarcopenia, highlighting the technical features of the available methods for measuring muscle mass, strength, and function, including artificial intelligence (AI)-based techniques. It also reviews the latest research on sarcopenia in children, focusing on children with solid organ tumors.

Efficient TGF-β1 Delivery to Articular Chondrocytes In Vitro Using Agro-Based Liposomes

The low efficiency in transfecting rat- and human-derived chondrocytes have been hampering developments in the field of cartilage biology. Transforming growth factor (TGF)-β1 has shown positive effects on chondrocytes, but its applications remain limited due to its short half-life, low stability and poor penetration into cartilage. Naturally derived liposomes have been shown to be promising delivery nanosystems due to their similarities with biological membranes. Here, we used agro-based rapeseed liposomes, which contains a high level of mono- and poly-unsaturated fatty acids, to efficiently deliver encapsulated TGF-β1 to rat chondrocytes. Results showed that TGF-β1 encapsulated in nano-sized rapeseed liposomes were safe for chondrocytes and did not induce any alterations of their phenotype. Furthermore, the controlled release of TGF-β1 from liposomes produced an improved response in chondrocytes, even at low doses. Altogether, these outcomes demonstrate that agro-based nanoliposomes are promising drug carriers.

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.

Receptor-Mediated Muscle Homeostasis as a Target for Sarcopenia Therapeutics

Sarcopenia is a disease characterized by age-related decline of skeletal muscle mass and function. The molecular mechanisms of the pathophysiology of sarcopenia form a complex network due to the involvement of multiple interconnected signaling pathways. Therefore, signaling receptors are major targets in pharmacological strategies in general. To provide a rationale for pharmacological interventions for sarcopenia, we herein describe several druggable signaling receptors based on their role in skeletal muscle homeostasis and changes in their activity with aging. A brief overview is presented of the efficacy of corresponding drug candidates under clinical trials. Strategies targeting the androgen receptor, vitamin D receptor, Insulin-like growth factor-1 receptor, and ghrelin receptor primarily focus on promoting anabolic action using natural ligands or mimetics. Strategies involving activin receptors and angiotensin receptors focus on inhibiting catabolic action. This review may help to select specific targets or combinations of targets in the future.

Bone Geometry Is Altered by Follistatin-Induced Muscle Growth in Young Adult Male Mice

The development of the musculoskeletal system and its maintenance depends on the reciprocal relationship between muscle and bone. The size of skeletal muscles and the forces generated during muscle contraction are potent sources of mechanical stress on the developing skeleton, and they shape bone structure during growth. This is particularly evident in hypermuscular global myostatin (Mstn)‐null mice, where larger muscles during development increase bone mass and alter bone shape. However, whether muscle hypertrophy can similarly influence the shape of bones after the embryonic and prepubertal period is unknown. To address this issue, bone structure was assessed after inducing muscle hypertrophy in the lower hindlimbs of young‐adult C57BL/6J male mice by administering intramuscular injections of recombinant adeno‐associated viral vectors expressing follistatin (FST), a potent antagonist of Mstn. Two FST isoforms were used: the full‐length 315 amino acid isoform (FST‐315) and a truncated 288 amino acid isoform (FST‐288). In both FST‐treated cohorts, muscle hypertrophy was observed, and the anterior crest of the tibia, adjacent to the tibialis anterior muscle, was lengthened. Hypertrophy of the muscles surrounding the tibia caused the adjacent cortical shell to recede inward toward the central axis: an event driven by bone resorption adjacent to the hypertrophic muscle. The findings reveal that inducing muscle hypertrophy in mice can confer changes in bone shape in early adulthood. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Targeting the Activin Receptor Signaling to Counteract the Multi-Systemic Complications of Cancer and Its Treatments

Muscle wasting, i.e., cachexia, frequently occurs in cancer and associates with poor prognosis and increased morbidity and mortality. Anticancer treatments have also been shown to contribute to sustainment or exacerbation of cachexia, thus affecting quality of life and overall survival in cancer patients. Pre-clinical studies have shown that blocking activin receptor type 2 (ACVR2) or its ligands and their downstream signaling can preserve muscle mass in rodents bearing experimental cancers, as well as in chemotherapy-treated animals. In tumor-bearing mice, the prevention of skeletal and respiratory muscle wasting was also associated with improved survival. However, the definitive proof that improved survival directly results from muscle preservation following blockade of ACVR2 signaling is still lacking, especially considering that concurrent beneficial effects in organs other than skeletal muscle have also been described in the presence of cancer or following chemotherapy treatments paired with counteraction of ACVR2 signaling. Hence, here, we aim to provide an up-to-date literature review on the multifaceted anti-cachectic effects of ACVR2 blockade in preclinical models of cancer, as well as in combination with anticancer treatments.

TMEPAI/PMEPA1 Is a Positive Regulator of Skeletal Muscle Mass

Inhibition of myostatin- and activin-mediated SMAD2/3 signaling using ligand traps, such as soluble receptors, ligand-targeting propeptides and antibodies, or follistatin can increase skeletal muscle mass in healthy mice and ameliorate wasting in models of cancer cachexia and muscular dystrophy. However, clinical translation of these extracellular approaches targeting myostatin and activin has been hindered by the challenges of achieving efficacy without potential effects in other tissues. Toward the goal of developing tissue-specific myostatin/activin interventions, we explored the ability of transmembrane prostate androgen-induced (TMEPAI), an inhibitor of transforming growth factor-β (TGF-β1)-mediated SMAD2/3 signaling, to promote growth, and counter atrophy, in skeletal muscle. In this study, we show that TMEPAI can block activin A, activin B, myostatin and GDF-11 activity in vitro. To determine the physiological significance of TMEPAI, we employed Adeno-associated viral vector (AAV) delivery of a TMEPAI expression cassette to the muscles of healthy mice, which increased mass by as much as 30%, due to hypertrophy of muscle fibers. To demonstrate that TMEPAI mediates its effects via inhibition of the SMAD2/3 pathway, tibialis anterior (TA) muscles of mice were co-injected with AAV vectors expressing activin A and TMEPAI. In this setting, TMEPAI blocked skeletal muscle wasting driven by activin-induced phosphorylation of SMAD3. In a model of cancer cachexia associated with elevated circulating activin A, delivery of AAV:TMEPAI into TA muscles of mice bearing C26 colon tumors ameliorated the muscle atrophy normally associated with cancer progression. Collectively, the findings indicate that muscle-directed TMEPAI gene delivery can inactivate the activin/myostatin-SMAD3 pathway to positively regulate muscle mass in healthy settings and models of disease.

Satellite cells and their regulation in livestock

Satellite cells are the myogenic stem and progenitor population found in skeletal muscle. These cells typically reside in a quiescent state until called upon to support repair, regeneration, or muscle growth. The activities of satellite cells are orchestrated by systemic hormones, autocrine and paracrine growth factors, and the composition of the basal lamina of the muscle fiber. Several key intracellular signaling events are initiated in response to changes in the local environment causing exit from quiescence, proliferation, and differentiation. Signals emanating from Notch, wingless-type mouse mammary tumor virus integration site family members, and transforming growth factor-β proteins mediate the reversible exit from growth 0 phase while those initiated by members of the fibroblast growth factor and insulin-like growth factor families direct proliferation and differentiation. Many of these pathways impinge upon the myogenic regulatory factors (MRF), myogenic factor 5, myogenic differentiation factor D, myogenin and MRF4, and the lineage determinate, Paired box 7, to alter transcription and subsequent satellite cell decisions. In the recent past, insight into mouse transgenic models has led to a firm understanding of regulatory events that control satellite cell metabolism and myogenesis. Many of these niche-regulated functions offer subtle differences from their counterparts in livestock pointing to the existence of species-specific controls. The purpose of this review is to examine the mechanisms that mediate large animal satellite cell activity and their relationship to those present in rodents.

Identification and characterization of Fbxl22, a novel skeletal muscle atrophy-promoting E3 ubiquitin ligase

Muscle-specific E3 ubiquitin ligases have been identified in muscle atrophy-inducing conditions. The purpose of the current study was to explore the functional role of F-box and leucine-rich protein 22 (Fbxl22), and a newly identified splice variant (Fbxl22-193), in skeletal muscle homeostasis and neurogenic muscle atrophy. In mouse C₂C₁₂ muscle cells, promoter fragments of the Fbxl22 gene were cloned and fused with the secreted alkaline phosphatase reporter gene to assess the transcriptional regulation of Fbxl22. The tibialis anterior muscles of male C57/BL6 mice (12-16 wk old) were electroporated with expression plasmids containing the cDNA of two Fbxl22 splice variants and tissues collected after 7, 14, and 28 days. Gastrocnemius muscles of wild-type and muscle-specific RING finger 1 knockout (MuRF1 KO) mice were electroporated with an Fbxl22 RNAi or empty plasmid and denervated 3 days posttransfection, and tissues were collected 7 days postdenervation. The full-length gene and novel splice variant are transcriptionally induced early (after 3 days) during neurogenic muscle atrophy. In vivo overexpression of Fbxl22 isoforms in mouse skeletal muscle leads to evidence of myopathy/atrophy, suggesting that both are involved in the process of neurogenic muscle atrophy. Knockdown of Fbxl22 in the muscles of MuRF1 KO mice resulted in significant additive muscle sparing 7 days after denervation. Targeting two E3 ubiquitin ligases appears to have a strong additive effect on protecting muscle mass loss with denervation, and these findings have important implications in the development of therapeutic strategies to treat muscle atrophy.

Multiple Roles of Transforming Growth Factor Beta in Amyotrophic Lateral Sclerosis

Transforming growth factor beta (TGFB) is a pleiotropic cytokine known to be dysregulated in many neurodegenerative disorders and particularly in amyotrophic lateral sclerosis (ALS). This motor neuronal disease is non-cell autonomous, as it affects not only motor neurons but also the surrounding glial cells, and the target skeletal muscle fibers. Here, we analyze the multiple roles of TGFB in these cell types, and how TGFB signaling is altered in ALS tissues. Data reported support a crucial involvement of TGFB in the etiology and progression of ALS, leading us to hypothesize that an imbalance of TGFB signaling, diminished at the pre-symptomatic stage and then increased with time, could be linked to ALS progression. A reduced stimulation of the TGFB pathway at the beginning of disease blocks its neuroprotective effects and promotes glutamate excitotoxicity. At later disease stages, the persistent activation of the TGFB pathway promotes an excessive microglial activation and strengthens muscular dysfunction. The therapeutic potential of TGFB is discussed, in order to foster new approaches to treat ALS.

Nox4 - RyR1 - Nox2: Regulators of micro-domain signaling in skeletal muscle

The ability for skeletal muscle to perform optimally can be affected by the regulation of Ca²⁺ within the triadic junctional space at rest. Reactive oxygen species impact muscle performance due to changes in oxidative stress, damage and redox regulation of signaling cascades. The interplay between ROS and Ca²⁺ signaling at the triad of skeletal muscle is therefore important to understand as it can impact the performance of healthy and diseased muscle. Here, we aimed to examine how changes in Ca²⁺ and redox signaling within the junctional space micro-domain of the mouse skeletal muscle fibre alters the homeostasis of these complexes. The dystrophic mdx mouse model displays increased RyR1 Ca²⁺ leak and increased NAD(P)H Oxidase 2 ROS. These alterations make the mdx mouse an ideal model for understanding how ROS and Ca²⁺ handling impact each other. We hypothesised that elevated t-tubular Nox2 ROS increases RyR1 Ca²⁺ leak contributing to an increase in cytoplasmic Ca²⁺, which could then initiate protein degradation and impaired cellular functions such as autophagy and ER stress. We found that inhibiting Nox2 ROS did not decrease RyR1 Ca²⁺ leak observed in dystrophin-deficient skeletal muscle. Intriguingly, another NAD(P)H isoform, Nox4, is upregulated in mice unable to produce Nox2 ROS and when inhibited reduced RyR1 Ca²⁺ leak. Our findings support a model in which Nox4 ROS induces RyR1 Ca²⁺ leak and the increased junctional space [Ca²⁺] exacerbates Nox2 ROS; with the cumulative effect of disruption of downstream cellular processes that would ultimately contribute to reduced muscle or cellular performance.

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Nox2 ROS does not influence RyR1 Ca²⁺ leak in skeletal muscle.

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Lack of Nox2 ROS increases Nox4 expression.

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Nox4 ROS induces RyR1 Ca²⁺ leak via S-nitrosylation.

Anti-Aging Effects of GDF11 on Skin

Human skin is composed of three layers: the epidermis, the dermis, and the hypodermis. The epidermis has four major cell layers made up of keratinocytes in varying stages of progressive differentiation. Skin aging is a multi-factorial process that affects every phase of its biology and function. The expression profiles of inflammation-related genes analyzed in resident immune cells demonstrated that these cells have a strong ability to regenerate adult skin stem cells and to produce endogenous substances such as growth differentiation factor 11 (GDF11). GDF11 appears to be the key to progenitor proliferation and/or differentiation. The preservation of youthful phenotypes has been tied to the presence of GDF11 in different human tissues, and, in the skin, this factor inhibits inflammatory responses. The protective role of GDF11 depends on a multi-factorial process implicating various types of skin cells such as keratinocytes, fibroblasts and inflammatory cells. GDF11 should be further studied for the purpose of developing novel therapies for the treatment of skin diseases.

Exercise and insulin resistance in PCOS: muscle insulin signalling and fibrosis

OBJECTIVE

Mechanisms of insulin resistance in polycystic ovary syndrome (PCOS) remain ill defined, contributing to sub-optimal therapies. Recognising skeletal muscle plays a key role in glucose homeostasis we investigated early insulin signalling, its association with aberrant transforming growth factor β (TGFβ)-regulated tissue fibrosis. We also explored the impact of aerobic exercise on these molecular pathways.

METHODS

A secondary analysis from a cross-sectional study was undertaken in women with (n = 30) or without (n = 29) PCOS across lean and overweight BMIs. A subset of participants with (n = 8) or without (n = 8) PCOS who were overweight completed 12 weeks of aerobic exercise training. Muscle was sampled before and 30 min into a euglycaemic-hyperinsulinaemic clamp pre and post training.

RESULTS

We found reduced signalling in PCOS of mechanistic target of rapamycin (mTOR). Exercise training augmented but did not completely rescue this signalling defect in women with PCOS. Genes in the TGFβ signalling network were upregulated in skeletal muscle in the overweight women with PCOS but were unresponsive to exercise training except for genes encoding LOX, collagen 1 and 3.

CONCLUSIONS

We provide new insights into defects in early insulin signalling, tissue fibrosis, and hyperandrogenism in PCOS-specific insulin resistance in lean and overweight women. PCOS-specific insulin signalling defects were isolated to mTOR, while gene expression implicated TGFβ ligand regulating a fibrosis in the PCOS-obesity synergy in insulin resistance and altered responses to exercise. Interestingly, there was little evidence for hyperandrogenism as a mechanism for insulin resistance.

Skeletal muscle in health and disease

Skeletal muscle fibres are multinucleated cells that contain postmitotic nuclei (i.e. they are no longer able to divide) and perform muscle contraction. They are formed by fusion of muscle precursor cells, and grow into elongating myofibres by the addition of further precursor cells, called satellite cells, which are also responsible for regeneration following injury. Skeletal muscle regeneration occurs in most muscular dystrophies in response to necrosis of muscle fibres. However, the complex environment within dystrophic skeletal muscle, which includes inflammatory cells, fibroblasts and fibro-adipogenic cells, together with the genetic background of the in vivo model and the muscle being studied, complicates the interpretation of laboratory studies on muscular dystrophies. Many genes are expressed in satellite cells and in other tissues, which makes it difficult to determine the molecular cause of various types of muscular dystrophies. Here, and in the accompanying poster, we discuss our current knowledge of the cellular mechanisms that govern the growth and regeneration of skeletal muscle, and highlight the defects in satellite cell function that give rise to muscular dystrophies.

Summary: The mechanisms of skeletal muscle development, growth and regeneration are described. We discuss whether these processes are dysregulated in inherited muscle diseases and identify pathways that may represent therapeutic targets.

Exercise as an anti-inflammatory therapy for cancer cachexia: a focus on interleukin-6 regulation

Cancer cachexia is a complicated disorder of extreme, progressive skeletal muscle wasting. It is directed by metabolic alterations and systemic inflammation dysregulation. Numerous studies have demonstrated that increased systemic inflammation promotes this type of cachexia and have suggested that cytokines are implicated in the skeletal muscle loss. Exercise is firmly established as an anti-inflammatory therapy that can attenuate or even reverse the process of muscle wasting in cancer cachexia. The interleukin IL-6 is generally considered to be a key player in the development of the microenvironment of malignancy; it promotes tumor growth and metastasis by acting as a bridge between chronic inflammation and cancerous tissue and it also induces skeletal muscle atrophy and protein breakdown. Paradoxically, a beneficial role for IL-6 has also been identified recently, and that is its status as a “founding member” of the myokine class of proteins. Skeletal muscle is an important source of circulating IL-6 in people who participate in exercise training. IL-6 acts as an anti-inflammatory myokine by inhibiting TNFα and improving glucose uptake through the stimulation of AMPK signaling. This review discusses the action of IL-6 in skeletal muscle tissue dysfunction and the role of IL-6 as an “exercise factor” that modulates the immune system. This review also sheds light on the main considerations related to the treatment of muscle wasting in cancer cachexia.

Chemotherapy-Induced Sarcopenia

OPINION STATEMENT

Sarcopenia is being consistently recognized as a condition not only associated with the presence of a malignancy but also induced by the oncologic therapies. Due to its negative impact on tolerance to chemotherapy and final outcome in both medical and surgical cancer patients, sarcopenia should be always considered and prevented, and, if recognized, should be appropriately treated. A CT scan at the level of the third lumbar vertebra, using an appropriate software, is the more common and easily available way to diagnose sarcopenia. It is now acknowledged that mechanisms involved in iatrogenic sarcopenia are several and depending on the type of molecule included in the regimen of chemotherapy, different pharmacologic antidotes will be required in the future. However, progression of the disease and the associated malnutrition per se are able to progressively erode the muscle mass and since sarcopenia is the hallmark of cachexia, the therapeutic approach to chemotherapy-induced sarcopenia parallels that of cachexia. This approach mainly relies on those strategies which are able to increase the lean body mass and include the use of anabolic/anti-inflammatory agents, nutritional interventions, physical exercise and, even better, a combination of different therapies. There are some phase II studies and some small controlled randomized trials which have validated these treatments using single agents or combined multimodal approaches. While these approaches may require the cooperation of some specialists (nutritionists with a specific knowledge on pathophysiology of catabolic states, accredited exercise physiologists and physiotherapists), the oncologist too should directly enter these issues to coordinate the choice and priority of the treatments. Who better than the oncologist knows the natural history of the disease, its evolution, and the probability of tolerance and response to the oncologic therapy? Only the oncologist knows when it is essential to potentiate any effort to better achieve a control of the disease, using all the available armamentarium, and when the condition is too advanced and hence requires a more palliative than supporting care. The oncologist also knows when to expect a gastrointestinal toxicity (mucositis, nausea, vomiting, and diarrhea) and hence it is more convenient using a parenteral than an enteral nutritional intervention or, on the contrary, when patient is suitable for discharge from hospital and oral supplements should be promptly tested for compliance and then prescribed. When patients are at high risk for malnutrition or if, regardless of their nutritional status, they are candidate to aggressive and potentially toxic treatments, they should undergo a jointed evaluation by the oncologist and the nutritionist and physical therapist to assess together a combined approach. In conclusion, the treatment of both cancer- or chemotherapy-related sarcopenia represents a challenge for the modern oncologist who must be able to coordinate a new panel of specialists with the same skill necessary to decide the priority of different oncologic treatments within a complex multidisciplinary context.

A Myogenic Double-Reporter Human Pluripotent Stem Cell Line Allows Prospective Isolation of Skeletal Muscle Progenitors

Myogenic differentiation of human pluripotent stem cells (hPSCs) has been done by gene overexpression or directed differentiation. However, viral integration, long-term culture, and the presence of unwanted cells are the main obstacles. By using CRISPR/Cas9n, a double-reporter human embryonic stem cell (hESC) line was generated for PAX7/MYF5, allowing prospective readout. This strategy allowed pathway screen to define efficient myogenic induction in hPSCs. Next, surface marker screen allowed identification of CD10 and CD24 for purification of myogenic progenitors and exclusion of non-myogenic cells. CD10 expression was also identified on human satellite cells and skeletal muscle progenitors. In vitro and in vivo studies using transgene and/or reporter-free hPSCs further validated myogenic potential of the cells by formation of new fibers expressing human dystrophin as well as donor-derived satellite cells in NSG-mdx^4Cv mice. This study provides biological insights for myogenic differentiation of hPSCs using a double-reporter cell resource and defines an improved myogenic differentiation and purification strategy.

TGF-β Signaling in Cellular Senescence and Aging-Related Pathology

Aging is broadly defined as the functional decline that occurs in all body systems. The accumulation of senescent cells is considered a hallmark of aging and thought to contribute to the aging pathologies. Transforming growth factor-β (TGF-β) is a pleiotropic cytokine that regulates a myriad of cellular processes and has important roles in embryonic development, physiological tissue homeostasis, and various pathological conditions. TGF-β exerts potent growth inhibitory activities in various cell types, and multiple growth regulatory mechanisms have reportedly been linked to the phenotypes of cellular senescence and stem cell aging in previous studies. In addition, accumulated evidence has indicated a multifaceted association between TGF-β signaling and aging-associated disorders, including Alzheimer’s disease, muscle atrophy, and obesity. The findings regarding these diseases suggest that the impairment of TGF-β signaling in certain cell types and the upregulation of TGF-β ligands contribute to cell degeneration, tissue fibrosis, inflammation, decreased regeneration capacity, and metabolic malfunction. While the biological roles of TGF-β depend highly on cell types and cellular contexts, aging-associated changes are an important additional context which warrants further investigation to better understand the involvement in various diseases and develop therapeutic options. The present review summarizes the relationships between TGF-β signaling and cellular senescence, stem cell aging, and aging-related diseases.

Activin A-Induced Cachectic Wasting Is Attenuated by Systemic Delivery of Its Cognate Propeptide in Male Mice

In cancer, elevated activin levels promote cachectic wasting of muscle, irrespective of tumor progression. In excess, activins A and B use the myostatin signaling pathway in muscle, triggering a decrease in protein synthesis and an increase in protein degradation, which ultimately leads to atrophy. Recently, we demonstrated that local delivery of engineered activin and myostatin propeptides (natural inhibitors of these growth factors) could induce profound muscle hypertrophy in healthy mice. Additionally, the expression of these propeptides effectively attenuated localized muscle wasting in models of dystrophy and cancer cachexia. In this study, we examined whether a systemically administered recombinant propeptide could reverse activin A-induced cachectic wasting in mice. Chinese hamster ovary cells stably expressing activin A were transplanted into the quadriceps of nude mice and caused an 85-fold increase in circulating activin A levels within 12 days. Elevated activin A induced a rapid reduction in body mass (-16%) and lean mass (-10%). In agreement with previous findings, we demonstrated that adeno-associated virus-mediated delivery of activin propeptide to the tibialis anterior muscle blocked activin-induced wasting. In addition, despite massively elevated levels of activin A in this model, systemic delivery of the propeptide significantly reduced activin-induced changes in lean and body mass. Specifically, recombinant propeptide reversed activin-induced wasting of skeletal muscle, heart, liver, and kidneys. This is the first study to demonstrate that systemic administration of recombinant propeptide therapy effectively attenuates tumor-derived activin A insult in multiple tissues.

TAZ and myostatin involved in muscle atrophy of congenital neurogenic clubfoot

BACKGROUND

Muscular atrophy is the basic defect of neurogenic clubfoot. Muscle atrophy of clubfoot needs more scientific and reasonable imaging measurement parameters to evaluate. The Hippo pathway and myostatin pathway may be directly correlated in myogenesis. In this study, we will use congenital neurogenic clubfoot muscle atrophy model to verify in vivo. Further, the antagonistic mechanism of TAZ on myostatin was studied in the C2C12 cell differentiation model.

AIM

To identify muscle atrophy in fetal neurogenic clubfoot by ultrasound imaging and detect the expression of TAZ and myostatin in gastrocnemius muscle. To elucidate the possible mechanisms by which TAZ antagonizes myostatin-induced atrophy in an in vitro cell model.

METHODS

Muscle atrophy in eight cases of fetal unilateral clubfoot with nervous system abnormalities was identified by 2D and 3D ultrasound. Western blotting and immunostaining were performed to detect expression of myostatin and TAZ. TAZ overexpression in C2C12 myotubes and the expression of associated proteins were analyzed by western blotting.

RESULTS

The maximum cross-sectional area of the fetal clubfoot on the varus side was reduced compared to the contralateral side. Myostatin was elevated in the atrophied gastrocnemius muscle, while TAZ expression was decreased. They were negatively correlated. TAZ overexpression reversed the diameter reduction of the myotube, downregulated phosphorylated Akt, and increased the expression of forkhead box O4 induced by myostatin.

CONCLUSION

Ultrasound can detect muscle atrophy of fetal clubfoot. TAZ and myostatin are involved in the pathological process of neurogenic clubfoot muscle atrophy. TAZ antagonizes myostatin-induced myotube atrophy, potentially through regulation of the Akt/forkhead box O4 signaling pathway.

The effects of CRISPR-Cas9 knockout of the TGF-β1 gene on antler cartilage cells in vitro

Background

Deer antler is the only mammalian organ that can be completely regenerated every year. Its periodic regeneration is regulated by multiple factors, including transforming growth factor β (TGF-β). This widely distributed multi-functional growth factor can control the proliferation and differentiation of many types of cell, and it may play a crucial regulatory role in antler regeneration. This study explored the role of TGF-β1 during the rapid growth of sika deer antler.

Methods

Three CRISPR-Cas9 knockout vectors targeting the TGF-β1 gene of sika deer were constructed and packaged with a lentiviral system. The expression level of TGF-β1 protein in the knockout cell line was determined using western blot, the proliferation and migration of cartilage cells in vitro were respectively determined using EdU and the cell scratch test, and the expression levels of TGF-β pathway-related genes were determined using a PCR array.

Results

Of the three gRNAs designed, pBOBI-gRNA2 had the best knockout effect. Knockout of TGF-β1 gene inhibits the proliferation of cartilage cells and enhances their migration in vitro. TGF-β signaling pathway-related genes undergo significant changes, so we speculate that when the TGF-β pathway is blocked, the BMP signaling pathway mediated by BMP4 may play a key role.

Conclusions

TGF-β1 is a newly identified regulatory factor of rapid growth in sika deer antler.

Hemojuvelin is a novel suppressor for Duchenne muscular dystrophy and age-related muscle wasting

BACKGROUND

Muscle wasting occurs in response to various physiological and pathological conditions, including ageing and Duchenne muscular dystrophy (DMD). Transforming growth factor-β1 (TGF-β1) contributes to muscle pathogenesis in elderly people and DMD patients; inhibition of TGF-β1 signalling is a promising therapeutic strategy for muscle-wasting disorders. Hemojuvelin (HJV or Hjv as the murine homologue) is a membrane-bound protein that is highly expressed in skeletal muscle, heart, and liver. In hepatic cells, Hjv acts as a coreceptor for bone morphogenetic protein, a TGF-β subfamily member. The aim of this study was to investigate whether Hjv plays an essential role in muscle physiological and pathophysiological processes by acting as a coreceptor for TGF-β1 signalling.

METHODS

Conventional and conditional Hjv knockout mice as well as mdx and aged mice transfected with Hjv overexpression vector were used to study the role of Hjv in muscle physiology and pathophysiology. qRT-PCR, western blotting, and immunohistochemistry examinations were conducted to evaluate gene, protein, and structural changes in vivo and in vitro. Exercise endurance was determined using treadmill running test, and muscle force was detected by an isometric transducer. RNA interference, immunoprecipitation, and dual-luciferase reporter assays were utilized to explore the mechanism by which Hjv regulates TGF-β1 signalling in skeletal muscle.

RESULTS

Conventional and conditional Hjv knockout mice displayed muscle atrophy, fibrosis, reduced running endurance, and muscle force. HJV was significantly down-regulated in the muscles of DMD patients (n = 3, mean age: 11.7 ± 5.7 years) and mdx mice as well as in those of aged humans (n = 10, 20% women, mean age: 75.1 ± 9.5 years) and mice. Overexpression of Hjv rescued dystrophic and age-related muscle wasting. Unlike its function in hepatic cells, the bone morphogenetic protein downstream phosphorylated p-Smad1/5/8 signalling pathway was unchanged, but TGF-β1, TGF-β receptor II (TβRII), and p-Smad2/3 expression were increased in Hjv-deficient muscles. Mechanistically, loss of Hjv promoted activation of Smad3 signalling induced by TGF-β1, whereas Hjv overexpression inhibited TGF-β1/Smad3 signalling by directly interacting with TβRII on the muscle membrane.

CONCLUSIONS

Our findings identify an unrecognized role of HJV in skeletal muscle by regulating TGF-β1/Smad3 signalling as a coreceptor for TβRII. Unlike the TGF-β1/Smad3 pathway, HJV could be a reliable drug target as its expression is not widespread. Novel therapeutic strategies could potentially be devised to interfere only with the muscle function of HJV to treat DMD and age-related muscle wasting.

Current pharmacotherapies for sarcopenia

Introduction: Sarcopenia, the age-related loss of skeletal muscle mass and function, is a global health problem that contributes to the development of physical disability, morbidity and mortality in the ageing population. Sarcopenia is now recognised in many countries as a muscle disease with an ICD-10-CM Diagnosis Code for billing care related to this condition, despite no FDA-approved treatments being currently available. Areas covered: This review highlights the current state of knowledge regarding the biological mechanisms contributing to the age-related loss of muscle mass and function and provides a summary of existing and emerging pharmacotherapies in clinical trials for sarcopenia. Expert opinion: While understanding of the pathophysiology of sarcopenia has progressed, rigorous preclinical studies that better inform clinical trials are needed to accelerate drug discovery and identify safe and effective treatments. Few drugs have been developed specifically for sarcopenia and many have failed to meet clinically relevant outcomes related to strength and physical performance. The multifactorial complexity of sarcopenia means that tailored, personalised treatments are more likely to be required than just a single intervention.

The Mechanical Stimulation of Myotubes Counteracts the Effects of Tumor-Derived Factors Through the Modulation of the Activin/Follistatin Ratio

Activin negatively affects muscle fibers and progenitor cells in aging (sarcopenia) and in chronic diseases characterized by severe muscle wasting (cachexia). High circulating activin levels predict poor survival in cancer patients. However, the relative impact of activin in mediating muscle atrophy and hampered homeostasis is still unknown. To directly assess the involvement of activin, and its physiological inhibitor follistatin, in cancer-induced muscle atrophy, we cultured C2C12 myotubes in the absence or in the presence of a mechanical stretching stimulus and in the absence or presence of C26 tumor-derived factors (CM), so as to mimic the mechanical stimulation of exercise and cancer cachexia, respectively. We found that CM induces activin release by myotubes, further exacerbating the negative effects of tumor-derived factors. In addition, mechanical stimulation is sufficient to counteract the adverse tumor-induced effects on muscle cells, in association with an increased follistatin/activin ratio in the cell culture medium, indicating that myotubes actively release follistatin upon stretching. Recombinant follistatin counteracts tumor effects on myotubes exclusively by rescuing fusion index, suggesting that it is only partially responsible for the stretch-mediated rescue. Therefore, besides activin, other tumor-derived factors may play a significant role in mediating muscle atrophy. In addition to increasing follistatin secretion mechanical stimulation induces additional beneficial responses in myotubes. We propose that in animal models of cancer cachexia and in cancer patients purely mechanical stimuli play an important role in mediating the rescue of the muscle homeostasis reported upon exercise.

Myostatin/SMAD4 signaling-mediated regulation of miR-124-3p represses glucocorticoid receptor expression and inhibits adipocyte differentiation

The mechanism of adipocyte regulation specifically in muscle and the influence of muscle tissue on intramuscular fat deposition are unknown. Our previous studies have shown that myostatin, a myokine, is involved in inhibiting the differentiation of preadipocytes and may be a potential regulator that affects the deposition of intramuscular fat. Myostatin inhibited adipogenesis by downregulating the expression of glucocorticoid receptor (GR) in porcine preadipocytes. However, the mechanism of regulation is not yet clear. In this study, we demonstrate microRNA (miR-124-3p) mediates regulation of GR by myostatin. We found that miR-124-3p can target GR 3'-UTR and negatively regulate GR expression. We demonstrate that overexpression of miR-124-3p can reduce differentiation of 3T3-L1 cells by inhibiting GR, and vice versa. The expression of miR-124-3p was upregulated in 3T3-L1 cells treated with myostatin. Further study revealed that myostatin also promotes the expression of SMAD4 and its transfer and localization to the nucleus. The activated myostatin/SMAD4 signal promotes the expression of miR-124-3p by SMAD4 binding to the promoter region of miR-124-3p. When myostatin or SMAD4 activity is inhibited, the upregulation of miR-124-3p is also inhibited. All of these findings suggested that myostatin could inhibit adipogenic differentiation of 3T3-L1 cells by activating miR-124-3p to inhibit GR. These data may provide an explanation for how myostatin signaling affects intramuscular fat deposition in a tissue-specific manner.

Comprehensive Experimental System for a Promising Model Organism Candidate for Marine Teleosts

A comprehensive experimental system for Japanese anchovy, a promising candidate model organism for marine teleosts, was established. Through the design of a rearing/spawning facility that controls the photoperiod and water temperature, one-cell eggs were continuously obtained shortly after spawning throughout the rearing period. The stages of eggs are indispensable for microinjection experiments, and we developed an efficient and robust microinjection system for the Japanese anchovy. Embryos injected with GFP mRNA showed strong whole-body GFP fluorescence and the survival rates of injected- and non-injected embryos were not significantly different, 87.5% (28 in 32 embryos) and 90.0% (45 in 50 embryos), respectively. We verified that the Tol2 transposon system, which mediates gene transfer in vertebrates, worked efficiently in the Japanese anchovy using the transient transgenesis protocol, with GFP or DsRed as the reporter gene. Finally, we confirmed that genome-editing technologies, namely Transcription Activator-Like Effector Nucleases (TALEN) and Clustered Regulatory Interspaced Short Palindromic Repeats (CRISPR)/Cas9, were applicable to the Japanese anchovy. In practice, specific gene-disrupted fishes were generated in the F₁ generation. These results demonstrated the establishment of a basic, yet comprehensive, experimental system, which could be employed to undertake experiments using the Japanese anchovy as a model organism for marine teleost fish.

TGFβi is involved in the chondrogenic differentiation of mesenchymal stem cells and is dysregulated in osteoarthritis

OBJECTIVE

Transforming growth factor-β (TGFβ) is a major regulator of cartilage homeostasis and its deregulation has been associated with osteoarthritis (OA). Deregulation of the TGFβ pathway in mesenchymal stem cells (MSCs) has been proposed to be at the onset of OA. Using a secretome analysis, we identified a member of the TGFβ family, TGFβ-induced protein (TGFβi or βIGH3), expressed in MSCs and we investigated its function and regulation during OA.

DESIGN

Cartilage, bone, synovium, infrapatellar fat pad and bone marrow-MSCs were isolated from patients with OA or healthy subjects. Chondrogenesis of BM-MSCs was induced by TGFβ3 in micropellet culture. Expression of TGFβi was quantified by RT-qPCR, ELISA or immunohistochemistry. Role of TGFβi was investigated in gain and loss of function experiments in BM-MSCs and chondrocytes.

RESULTS

TGFβi was up-regulated in early stages of chondrogenesis and its knock-down in BM-MSCs resulted in the down-regulation of mature and hypertrophic chondrocyte markers. It likely occurred through the modulation of adhesion molecules including integrin (ITG)β1, ITGβ5 and N-cadherin. We also showed that TGFβi was upregulated in vitro in a model of OA chondrocytes, and its silencing enhanced the hypertrophic marker type X collagen. In addition, TGFβi was up-regulated in bone and cartilage from OA patients while its expression was reduced in BM-MSCs. Similar findings were observed in a murine model of OA.

CONCLUSIONS

Our results revealed a dual role of TGFβi during chondrogenesis and pointed its deregulation in OA joint tissues. Modulating TGFβi in BM-MSCs might be of interest in cartilage regenerative medicine.

The Possible Role of Complete Loss of Myostatin in Limiting Excessive Proliferation of Muscle Cells (C2C12) via Activation of MicroRNAs

Myostatin (MSTN) is a member of the TGF-β superfamily that negatively regulates skeletal muscle growth and differentiation. However, the mechanism by which complete MSTN deletion limits excessive proliferation of muscle cells remains unclear. In this study, we knocked out MSTN in mouse myoblast lines using a Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR/Cas9) system and sequenced the mRNA and miRNA transcriptomes. The results show that complete loss of MSTN upregulates seven miRNAs targeting an interaction network composed of 28 downregulated genes, including TGFB1, FOS and RB1. These genes are closely associated with tumorigenesis and cell proliferation. Our study suggests that complete loss of MSTN may limit excessive cell proliferation via activation of miRNAs. These data will contribute to the treatment of rhabdomyosarcoma (RMS).

miR-26 Induces Apoptosis and Inhibits Autophagy in Non-small Cell Lung Cancer Cells by Suppressing TGF-β1-JNK Signaling Pathway

Non-small cell lung cancer (NSCLC) is one of the causes of cancer mortality worldwide. The role of miR-26 in the development and progression of NSCLC remains largely unknown. In this study we found an abnormal expression of miR-26 in human NSCLC tissues. It was found that miR-26 mimics induced cell apoptosis and promoted caspase-3, 9 activities in human NSCLC cells. The miR-26 inhibitor enhanced the expression of the light chain 3 (LC3) protein and the autophagy related genes in NSCLC cells. Moreover, miR-26 regulated apoptosis and autophagy by inhibiting TGF-β expression in a JNK dependent manner. In addition, miR-26 mimics induced cell apoptosis, was involved in the endoplasmic reticulum stress (ERS) signaling pathway. Down-regulation of the ERS, inhibited apoptosis which was induced by miR-26 mimics in NSCLC cells. In in vivo studies, TUNEL staining revealed that the number of TUNEL positive cells of the tumor tissue in the miR-26 treatment group, were significantly increased in comparison with the control group, while the number of TUNEL positive cells in the tumor tissue were remarkably decreased in the groups treated with miR-26, combined with the TGF-β1 inhibitor or JNK inhibitor. Additionally, the immunoreactivity of TGF-β1 in the cells treated with the miR-26 inhibitor, decreased in comparison to the control group. Our results indicated that miR-26 induced apoptosis and inhibited autophagy in human NSCLC cells through the TGF-β1-JNK signaling pathway, suggesting that miR-26 could be a potential novel target for the treatment of NSCLC.

Myostatin and activin blockade by engineered follistatin results in hypertrophy and improves dystrophic pathology in mdx mouse more than myostatin blockade alone

BACKGROUND

Myostatin antagonists are being developed as therapies for Duchenne muscular dystrophy due to their strong hypertrophic effects on skeletal muscle. Engineered follistatin has the potential to combine the hypertrophy of myostatin antagonism with the anti-inflammatory and anti-fibrotic effects of activin A antagonism.

METHODS

Engineered follistatin was administered to C57BL/6 mice for 4 weeks, and muscle mass and myofiber size was measured. In the mdx model, engineered follistatin was dosed for 12 weeks in two studies comparing to an Fc fusion of the activin IIB receptor or an anti-myostatin antibody. Functional measurements of grip strength and tetanic force were combined with tissue analysis for markers of necrosis, inflammation, and fibrosis to evaluate improvement in dystrophic pathology.

RESULTS

In wild-type and mdx mice, dose-dependent increases in muscle mass and quadriceps myofiber size were observed for engineered follistatin. In mdx, increases in grip strength and tetanic force were combined with improvements in muscle markers for necrosis, inflammation, and fibrosis. Improvements in dystrophic pathology were greater for engineered follistatin than the anti-myostatin antibody.

CONCLUSIONS

Engineered follistatin generated hypertrophy and anti-fibrotic effects in the mdx model.

Hyperammonemia and proteostasis in cirrhosis

PURPOSE OF REVIEW

Skeletal muscle loss or sarcopenia is a frequent complication of cirrhosis that adversely affects clinical outcomes. As skeletal muscle is the largest store of proteins in the body, proteostasis or protein homeostasis is required for maintenance of muscle mass. This review will focus on disordered skeletal muscle proteostasis in liver disease.

RECENT FINDINGS

Increased skeletal muscle uptake of ammonia initiates responses that result in disordered proteostasis including impaired protein synthesis and increased autophagy. The cellular response to the stress of hyperammonemia (hyperammonemic stress response, HASR) involves the coordinated action of diverse signaling pathways targeting the molecular mechanisms of regulation of protein synthesis. Transcriptional upregulation of myostatin, a TGFβ superfamily member, results in impaired mTORC1 signaling. Phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α) also relates to decreased global protein synthesis rates and mTORC1 signaling. Ammonia also causes mitochondrial and bioenergetic dysfunction because of cataplerosis of α-ketoglutarate. Lowering ammonia, targeting components of HASR and regulating cellular amino acid levels can potentially restore proteostasis.

SUMMARY

Signaling via myostatin and eIF2α phosphorylation causes decreases in protein synthesis and mTORC1 activity with a parallel mitochondrial dysfunction and increased autophagy contributing to proteostasis perturbations during skeletal muscle hyperammonemia of liver disease.

Disrupted Skeletal Muscle Mitochondrial Dynamics, Mitophagy, and Biogenesis during Cancer Cachexia: A Role for Inflammation

Chronic inflammation is a hallmark of cancer cachexia in both patients and preclinical models. Cachexia is prevalent in roughly 80% of cancer patients and accounts for up to 20% of all cancer-related deaths. Proinflammatory cytokines IL-6, TNF-α, and TGF-β have been widely examined for their regulation of cancer cachexia. An established characteristic of cachectic skeletal muscle is a disrupted capacity for oxidative metabolism, which is thought to contribute to cancer patient fatigue, diminished metabolic function, and muscle mass loss. This review's primary objective is to highlight emerging evidence linking cancer-induced inflammation to the dysfunctional regulation of mitochondrial dynamics, mitophagy, and biogenesis in cachectic muscle. The potential for either muscle inactivity or exercise to alter mitochondrial dysfunction during cancer cachexia will also be discussed.

Jaeumganghwa-Tang, a traditional herbal formula, improves muscle function and attenuates muscle loss in aged mice

PURPOSE

Jaeumganghwa-Tang (JGT), a traditional herbal formula composed of 12 medicinal herbs, is used for the treatment of age-related diseases. In the present study, we investigated the effects of JGT on muscle mass and function in aged mice.

METHODS

Young (5-month-old) and old (19-month-old) male C57BL/6 mice were divided into two groups each; one group received JGT (75 mg/d) and the other group received the vehicle for 6 weeks. At the end of the experimental period, muscle strength was examined using the wire hang test, and the tibialis anterior and gastrocnemius muscles were weighed. Muscle samples were further used for histological analysis to assess muscle damage, and the expression of transforming growth factor-beta was investigated via western blotting and immunohistochemistry.

RESULTS

Our results showed that treatment of old mice with JGT improved muscle strength, increased skeletal muscle mass, alleviated muscle damage, and suppressed intramuscular expression of transforming growth factor-beta.

CONCLUSION

In conclusion, JGT has beneficial effects on age-related loss of muscle mass and function. Thus, it might serve as a potential therapeutic agent for sarcopenia.

Establishment and phenotypic analysis of an Mstn knockout rat

Myostatin (Mstn) is an inhibitor of myogenesis, regulating the number and size of skeletal myocytes. In addition to its myogenic regulatory function, Mstn plays important roles in the development of adipose tissues and in metabolism. In the present study, an Mstn knockout rat model was generated using the zinc finger nuclease (ZFN) technique in order to further investigate the function and mechanism of Mstn in metabolism. The knockout possesses a frame shift mutation resulting in an early termination codon and a truncated peptide of 109 amino acids rather than the full 376 amino acids. The absence of detectable mRNA confirmed successful knockout of Mstn. Relative to wild-type (WT) littermates, Knockout (KO) rats exhibited significantly greater body weight, body circumference, and muscle mass. However, no significant differences in grip force was observed, indicating that Mstn deletion results in greater muscle mass but not greater muscle fiber strength. Additionally, KO rats were found to possess less body fat relative to WT littermates, which is consistent with previous studies in mice and cattle. The aforementioned results indicate that Mstn knockout increases muscle mass while decreasing fat content, leading to observed increases in body weight and body circumference. The Mstn knockout rat model provides a novel means to study the role of Mstn in metabolism and Mstn-related muscle hypertrophy.