Wnt protein-mediated satellite cell conversion in adult and aged mice following voluntary wheel running

Crosstalk among canonical Wnt and Hippo pathway members in skeletal muscle and at the neuromuscular junction

Skeletal muscles are essential for locomotion, posture, and metabolic regulation. To understand physiological processes, exercise adaptation, and muscle-related disorders, it is critical to understand the molecular pathways that underlie skeletal muscle function. The process of muscle contraction, orchestrated by a complex interplay of molecular events, is at the core of skeletal muscle function. Muscle contraction is initiated by an action potential and neuromuscular transmission requiring a neuromuscular junction. Within muscle fibers, calcium ions play a critical role in mediating the interaction between actin and myosin filaments that generate force. Regulation of calcium release from the sarcoplasmic reticulum plays a key role in excitation-contraction coupling. The development and growth of skeletal muscle are regulated by a network of molecular pathways collectively known as myogenesis. Myogenic regulators coordinate the differentiation of myoblasts into mature muscle fibers. Signaling pathways regulate muscle protein synthesis and hypertrophy in response to mechanical stimuli and nutrient availability. Several muscle-related diseases, including congenital myasthenic disorders, sarcopenia, muscular dystrophies, and metabolic myopathies, are underpinned by dysregulated molecular pathways in skeletal muscle. Therapeutic interventions aimed at preserving muscle mass and function, enhancing regeneration, and improving metabolic health hold promise by targeting specific molecular pathways. Other molecular signaling pathways in skeletal muscle include the canonical Wnt signaling pathway, a critical regulator of myogenesis, muscle regeneration, and metabolic function, and the Hippo signaling pathway. In recent years, more details have been uncovered about the role of these two pathways during myogenesis and in developing and adult skeletal muscle fibers, and at the neuromuscular junction. In fact, research in the last few years now suggests that these two signaling pathways are interconnected and that they jointly control physiological and pathophysiological processes in muscle fibers. In this review, we will summarize and discuss the data on these two pathways, focusing on their concerted action next to their contribution to skeletal muscle biology. However, an in-depth discussion of the non-canonical Wnt pathway, the fibro/adipogenic precursors, or the mechanosensory aspects of these pathways is not the focus of this review.

Aging: A struggle for beneficial to overcome negative factors made by muscle and bone

Musculoskeletal health is strongly influenced by regulatory interactions of bone and muscle. Recent discoveries have identified a number of key mechanisms through which soluble factors released during exercise by bone exert positive effects on muscle and by muscle on bone. Although exercise can delay the negative effects of aging, these beneficial effects are diminished with aging. The limited response of aged muscle and bone tissue to exercise are accompanied by a failure in bone and muscle communication. Here, we propose that exercise induced beneficial factors must battle changes in circulating endocrine and inflammatory factors that occur with aging. Furthermore, sedentary behavior results in the release of negative factors impacting the ability of bone and muscle to respond to physical activity especially with aging. In this review we report on exercise responsive factors and evidence of modification occurring with aging.

Myogenic differentiation markers in muscle tissue after aerobic training

Aerobic training induces a myriad of adaptations in muscle tissue, encompassing alterations in muscle fiber type composition, hypertrophy, and metabolic capacity. Understanding the potential role of myogenic differentiation markers (MDFs), such as Pax7, MyoD, Myogenin, and myosin heavy chain (MHC) isoforms, in mediating these adaptations is of paramount importance. The review delves into the intricate molecular mechanisms underlying the regulation of MDFs following aerobic training, elucidating the role of key signaling pathways including the MAPK/ERK, PI3K/Akt, and AMPK pathways, among others. These pathways play pivotal roles in orchestrating the expression and activity of MDFs, ultimately influencing muscle adaptation and regeneration. The comprehension of MDFs in the context of aerobic training is far-reaching, offering the potential for targeted interventions to optimize muscle adaptation and regeneration. This review identifies the need for further research to unveil the precise molecular mechanisms of the activation and interaction of myogenic differentiation markers with other signaling pathways, as well as to explore their potential as therapeutic targets for muscle-related conditions. This review article also provides a thorough analysis of MDFs in muscle tissue after aerobic training, highlighting their potential clinical implications and outlining future research directions in this area.

Exercise promotes skeletal muscle growth in adolescents via modulating Mettl3-mediated m6A methylation of MyoD in muscle satellite cells

BACKGROUND

Exercise exerts positive impacts on skeletal muscle health and homeostasis. Emerging evidence suggests that m6A methylation is involved in various physiological processes. However, the impact of exercise on adolescent skeletal muscle growth and the underlying epigenetic mechanisms remain poorly understood.

METHODS

The lower-limb skeletal muscles were harvested from exercise and control groups to compare the skeletal muscle growth in adolescents. mRNA sequencing was conducted to explore the mechanisms underlying enhanced skeletal muscle growth following exercise. The effects and mechanisms of Mettl3-mediated m6A methylation on adolescent skeletal muscle growth were investigated using muscle satellite cell (MuSC)-specific Mettl3 knockout (KO) mice. The potential function of MyoD for skeletal muscle growth in adolescents was explored by phenotypes after overexpression and evaluation of in vivo myogenesis. Additionally, the effects of the methyl donor betaine on adolescent skeletal muscle growth were investigated in vitro and in vivo.

RESULTS

Exercise could promote skeletal muscle growth in adolescents. Sequencing data analysis and confirmation assays uncovered that exercise significantly increased Mettl3-mediated m6A methylation and elevated the expression levels of activation marker MyoD in MuSCs. Establishment of MuSC-specific Mettl3 KO mice further demonstrated that Mettl3-mediated m6A methylation in MyoD contributed to skeletal muscle growth during adolescence. Mettl3-mediated m6A methylation regulated MyoD mRNA stability at the posttranscriptional level in MuSCs, with a functional site at 234 bp A. Increased expression of MyoD could contribute to myogenesis of adolescent MuSCs. Furthermore, the methyl donor betaine could enhance MyoD expression, contributing to MuSCs activation and skeletal muscle growth in adolescents by boosting m6A methylation levels.

CONCLUSIONS

Exercise promoted skeletal muscle growth in adolescents through facilitating MyoD mRNA stability of MuSCs in a Mettl3-mediated m6A-dependent manner. The methyl donor betaine could be a potential alternative to exercise for promoting adolescent skeletal muscle growth by directly augmenting the global levels of m6A methylation. These findings may provide a theoretical foundation for encouraging daily fitness exercise and ensuring healthy growth in adolescents.

Exercise and nutrition benefit skeletal muscle: From influence factor and intervention strategy to molecular mechanism

Sarcopenia is a progressive systemic skeletal muscle disease induced by various physiological and pathological factors, including aging, malnutrition, denervation, and cardiovascular diseases, manifesting as the decline of skeletal muscle mass and function. Both exercise and nutrition produce beneficial effects on skeletal muscle growth and are viewed as feasible strategies to prevent sarcopenia. Mechanisms involve regulating blood flow, oxidative stress, inflammation, apoptosis, protein synthesis and degradation, and satellite cell activation through exerkines and gut microbiomes. In this review, we summarized and discussed the latest progress and future development of the above mechanisms for providing a theoretical basis and ideas for the prevention and treatment of sarcopenia.

Impaired skeletal muscle regeneration in diabetes: From cellular and molecular mechanisms to novel treatments

Diabetes represents a major public health concern with a considerable impact on human life and healthcare expenditures. It is now well established that diabetes is characterized by a severe skeletal muscle pathology that limits functional capacity and quality of life. Increasing evidence indicates that diabetes is also one of the most prevalent disorders characterized by impaired skeletal muscle regeneration, yet underlying mechanisms and therapeutic treatments remain poorly established. In this review, we describe the cellular and molecular alterations currently known to occur during skeletal muscle regeneration in people with diabetes and animal models of diabetes, including its associated comorbidities, e.g., obesity, hyperinsulinemia, and insulin resistance. We describe the role of myogenic and non-myogenic cell types on muscle regeneration in conditions with or without diabetes. Therapies for skeletal muscle regeneration and gaps in our knowledge are also discussed, while proposing future directions for the field.

Aerobic exercise training engages the canonical wnt pathway to improve pulmonary function and inflammation in COPD

BACKGROUND

We studied whether the exercise improves cigarette smoke (CS) induced chronic obstructive pulmonary disease (COPD) in mice through inhibition of inflammation mediated by Wnt/β-catenin-peroxisome proliferator-activated receptor (PPAR) γ signaling.

METHODS

Firstly, we observed the effect of exercise on pulmonary inflammation, lung function, and Wnt/β-catenin-PPARγ. A total of 30 male C57BL/6J mice were divided into the control group (CG), smoke group (SG), low-intensity exercise group (LEG), moderate-intensity exercise group (MEG), and high-intensity exercise group (HEG). All the groups, except for CG, underwent whole-body progressive exposure to CS for 25 weeks. Then, we assessed the maximal exercise capacity of mice from the LEG, MEG, and HEG, and performed an 8-week treadmill exercise intervention. Then, we used LiCl (Wnt/β-catenin agonist) and XAV939 (Wnt/β-catenin antagonist) to investigate whether Wnt/β-catenin-PPARγ pathway played a role in the improvement of COPD via exercise. Male C57BL/6J mice were randomly divided into six groups (n = 6 per group): CG, SG, LiCl group, LiCl and exercise group, XAV939 group, and XAV939 and exercise group. Mice except those in the CG were exposed to CS, and those in the exercise groups were subjected to moderate-intensity exercise training. All the mice were subjected to lung function test, lung histological assessment, and analysis of inflammatory markers in the bronchoalveolar lavage fluid, as well as detection of Wnt1, β-catenin and PPARγ proteins in the lung tissue.

RESULTS

Exercise of various intensities alleviated lung structural changes, pulmonary function and inflammation in COPD, with moderate-intensity exercise exhibiting significant and comprehensive effects on the alleviation of pulmonary inflammation and improvement of lung function. Low-, moderate-, and high-intensity exercise decreased β-catenin levels and increased those of PPARγ significantly, and only moderate-intensity exercise reduced the level of Wnt1 protein. Moderate-intensity exercise relieved the inflammation aggravated by Wnt agonist. Wnt antagonist combined with moderate-intensity exercise increased the levels of PPARγ, which may explain the highest improvement of pulmonary function observed in this group.

CONCLUSIONS

Exercise effectively decreases COPD pulmonary inflammation and improves pulmonary function. The beneficial role of exercise may be exerted through Wnt/β-catenin-PPARγ pathway.

Effects of sclerostin injection on soleus and extensor digitorum longus muscle tissue in male mice

Sclerostin, a potent inhibitor of the Wnt signaling pathway, plays a critical role in bone homeostasis. Evidence suggests that sclerostin may also be involved in crosstalk between other tissues, including muscle. This pilot study attempted to examine the effects of sclerostin on soleus and extensor digitorum longus (EDL) muscle tissue from male mice that were given continuous recombinant sclerostin injections for 4 weeks. A total of 48 10-week-old male C57BL/6J mice were assigned to be sedentary or perform 1 h treadmill running per day for 4 weeks and administered subcutaneous injections of either saline or recombinant sclerostin 5 days/week. Sclerostin injection led to a reduction in the soleus myosin heavy chain (MHC) I, MHC I/IIA, MHC IIA/X, and MHC IIB cross-sectional area (p < 0.05) with no exercise effects on these reductions. In contrast, there were no effects of sclerostin injections or exercise on the fast-twitch EDL muscle in terms of size, MHC protein, or markers of Wnt signaling. These findings provide preliminary evidence of sclerostin's endocrine role in muscle via decreases in myofiber cross-sectional area, which seems to be independent of fiber type but muscle type-specific. More studies, however, are needed to confirm these preliminary results.

The reciprocity of skeletal muscle and bone: an evolving view from mechanical coupling, secretory crosstalk to stem cell exchange

Introduction: Muscle and bone constitute the two main parts of the musculoskeletal system and generate an intricately coordinated motion system. The crosstalk between muscle and bone has been under investigation, leading to revolutionary perspectives in recent years. Method and results: In this review, the evolving concept of muscle-bone interaction from mechanical coupling, secretory crosstalk to stem cell exchange was explained in sequence. The theory of mechanical coupling stems from the observation that the development and maintenance of bone mass are largely dependent on muscle-derived mechanical loads, which was later proved by Wolff's law, Utah paradigm and Mechanostat hypothesis. Then bone and muscle are gradually recognized as endocrine organs, which can secrete various cytokines to modulate the tissue homeostasis and remodeling to each other. The latest view presented muscle-bone interaction in a more direct way: the resident mesenchymal stromal cell in the skeletal muscle, i.e., fibro-adipogenic progenitors (FAPs), could migrate to the bone injury site and contribute to bone regeneration. Emerging evidence even reveals the ectopic source of FAPs from tissue outside the musculoskeletal system, highlighting its dynamic property. Conclusion: FAPs have been established as the critical cell connecting muscle and bone, which provides a new modality to study inter-tissue communication. A comprehensive and integrated perspective of muscle and bone will facilitate in-depth research in the musculoskeletal system and promote novel therapeutic avenues in treating musculoskeletal disorders.

Molecular and Structural Alterations of Skeletal Muscle Tissue Nuclei during Aging

Aging is accompanied by a progressive loss of skeletal muscle mass and strength. The mechanisms underlying this phenomenon are certainly multifactorial and still remain to be fully elucidated. Changes in the cell nucleus structure and function have been considered among the possible contributing causes. This review offers an overview of the current knowledge on skeletal muscle nuclei in aging, focusing on the impairment of nuclear pathways potentially involved in age-related muscle decline. In skeletal muscle two types of cells are present: fiber cells, constituting the contractile muscle mass and containing hundreds of myonuclei, and the satellite cells, i.e., the myogenic mononuclear stem cells occurring at the periphery of the fibers and responsible for muscle growth and repair. Research conducted on different experimental models and with different methodological approaches demonstrated that both the myonuclei and satellite cell nuclei of aged skeletal muscles undergo several structural and molecular alterations, affecting chromatin organization, gene expression, and transcriptional and post-transcriptional activities. These alterations play a key role in the impairment of muscle fiber homeostasis and regeneration, thus contributing to the age-related decrease in skeletal muscle mass and function.

Klotho regulates the myogenic response of muscle to mechanical loading and exercise

NEW FINDINGS

What is the central question of this study? Does the hormone Klotho affect the myogenic response of muscle cells to mechanical loading or exercise? What is the main finding and its importance? Klotho prevents direct, mechanical activation of genes that regulate muscle differentiation, including genes that encode the myogenic regulatory factor myogenin and proteins in the canonical Wnt signalling pathway. Similarly, elevated levels of klotho expression in vivo prevent the exercise-induced increase in myogenin-expressing cells and reduce exercise-induced activation of the Wnt pathway. These findings demonstrate a new mechanism through which the responses of muscle to the mechanical environment are regulated.

ABSTRACT

Muscle growth is influenced by changes in the mechanical environment that affect the expression of genes that regulate myogenesis. We tested whether the hormone Klotho could influence the response of muscle to mechanical loading. Applying mechanical loads to myoblasts in vitro increased RNA encoding transcription factors that are expressed in activated myoblasts (Myod) and in myogenic cells that have initiated terminal differentiation (Myog). However, application of Klotho to myoblasts prevented the loading-induced activation of Myog without affecting loading-induced activation of Myod. This indicates that elevated Klotho inhibits mechanically-induced differentiation of myogenic cells. Elevated Klotho also reduced the transcription of genes encoding proteins involved in the canonical Wnt pathway or their target genes (Wnt9a, Wnt10a, Ccnd1). Because the canonical Wnt pathway promotes differentiation of myogenic cells, these findings indicate that Klotho inhibits the differentiation of myogenic cells experiencing mechanical loading. We then tested whether these effects of Klotho occurred in muscles of mice experiencing high-intensity interval training (HIIT) by comparing wild-type mice and klotho transgenic mice. The expression of a klotho transgene combined with HIIT synergized to tremendously elevate numbers of Pax7+ satellite cells and activated MyoD+ cells. However, transgene expression prevented the increase in myogenin+ cells caused by HIIT in wild-type mice. Furthermore, transgene expression diminished the HIIT-induced activation of the canonical Wnt pathway in Pax7+ satellite cells. Collectively, these findings show that Klotho inhibits loading- or exercise-induced activation of muscle differentiation and indicate a new mechanism through which the responses of muscle to the mechanical environment are regulated.

PAX7, a Key for Myogenesis Modulation in Muscular Dystrophies through Multiple Signaling Pathways: A Systematic Review

Muscular dystrophy is a heterogenous group of hereditary muscle disorders caused by mutations in the genes responsible for muscle development, and is generally defined by a disastrous progression of muscle wasting and massive loss in muscle regeneration. Pax7 is closely associated with myogenesis, which is governed by various signaling pathways throughout a lifetime and is frequently used as an indicator in muscle research. In this review, an extensive literature search adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines was performed to identify research that examined signaling pathways in living models, while quantifying Pax7 expression in myogenesis. A total of 247 articles were retrieved from the Web of Science (WoS), PubMed and Scopus databases and were thoroughly examined and evaluated, resulting in 19 articles which met the inclusion criteria. Admittedly, we were only able to discuss the quantification of Pax7 carried out in research affecting various type of genes and signaling pathways, rather than the expression of Pax7 itself, due to the massive differences in approach, factor molecules and signaling pathways analyzed across the research. However, we highlighted the thorough evidence for the alteration of the muscle stem cell precursor Pax7 in multiple signaling pathways described in different living models, with an emphasis on the novel approach that could be taken in manipulating Pax7 expression itself in dystrophic muscle, towards the discovery of an effective treatment for muscular dystrophy. Therefore, we believe that this could be applied to the potential gap in muscle research that could be filled by tuning the well-established marker expression to improve dystrophic muscle.

Sarcopenia: Molecular regulatory network for loss of muscle mass and function

Skeletal muscle is the foundation of human function and plays a key role in producing exercise, bone protection, and energy metabolism. Sarcopenia is a systemic disease, which is characterized by degenerative changes in skeletal muscle mass, strength, and function. Therefore, sarcopenia often causes weakness, prolonged hospitalization, falls and other adverse consequences that reduce the quality of life, and even lead to death. In recent years, sarcopenia has become the focus of in-depth research. Researchers have suggested some molecular mechanisms for sarcopenia according to different muscle physiology. These mechanisms cover neuromuscular junction lesion, imbalance of protein synthesis and breakdown, satellite cells dysfunction, etc. We summarize the latest research progress on the molecular mechanism of sarcopenia in this review in order to provide new ideas for future researchers to find valuable therapeutic targets and develop relevant prevention strategies.

Murine Models of Tenotomy-Induced Mechanical Overloading and Tail-Suspension-Induced Mechanical Unloading

Skeletal muscle is a highly plastic tissue that can alter its mass and strength in response to mechanical stimulation, such as overloading and unloading, which lead to muscle hypertrophy and atrophy, respectively. Mechanical loading in the muscle influences muscle stem cell dynamics, including activation, proliferation, and differentiation. Although experimental models of mechanical overloading and unloading have been widely used for the investigation of the molecular mechanisms regulating muscle plasticity and stem cell function, few studies have described the methods in detail. Here, we describe the appropriate procedures for tenotomy-induced mechanical overloading and tail-suspension-induced mechanical unloading, which are the most common and simple methods to induce muscle hypertrophy and atrophy in mouse models.

Genome-wide analysis of a cellular exercise model based on electrical pulse stimulation

Skeletal muscle communicates with other organs via myokines, which are secreted by muscle during exercise and exert various effects. Despite much investigation of the exercise, the underlying molecular mechanisms are still not fully understood. Here, we applied an in vitro exercise model in which cultured C2C12 myotubes were subjected to electrical pulse stimulation (EPS), which mimics contracting muscle. Based on the significantly up- and down-regulated genes in EPS, we constructed an in silico model to predict exercise responses at the transcriptional level. The in silico model revealed similarities in the transcriptomes of the EPS and exercised animals. Comparative analysis of the EPS data and exercised mouse muscle identified putative biomarkers in exercise signaling pathways and enabled to discover novel exercise-induced myokines. Biochemical analysis of selected exercise signature genes in muscle from exercised mice showed that EPS mimics in vivo exercise, at least in part, at the transcriptional level. Consequently, we provide a novel myokine, Amphiregulin (AREG), up-regulated both in vitro and in vivo, that would be a potential target for exercise mimetics.

Aerobic exercise and resistance exercise alleviate skeletal muscle atrophy through IGF-1/IGF-1R-PI3K/Akt pathway in mice with myocardial infarction

OBJECTIVES

Myocardial infarction (MI)-induced heart failure (HF) is commonly accompanied with profound effects on skeletal muscle. With the process of MI-induced HF, perturbations in skeletal muscle contribute to muscle atrophy. Exercise is viewed as a feasible strategy to prevent muscle atrophy. The aims of this study were to investigate whether exercise could alleviate MI-induced skeletal muscle atrophy via insulin-like growth factor 1 (IGF-1) pathway in mice.

MATERIALS AND METHODS

Male C57/BL6 mice were used to establish the MI model and divided into three groups: sedentary MI group, MI with aerobic exercise group and MI with resistance exercise group, sham-operated group was used as control. Exercise-trained animals were subjected to four-weeks of aerobic exercise (AE) or resistance exercise (RE). Cardiac function, muscle weight, myofiber size, levels of IGF-1 signaling and proteins related to myogenesis, protein synthesis and degradation and cell apoptosis in gastrocnemius muscle were detected. And H₂O₂-treated C2C12 cells were intervened with recombinant human IGF-1, IGF-1R inhibitor NVP-AEW541 and PI3K inhibitor LY294002 to explore the mechanism. Results：Exercises up-regulated the IGF-1/IGF-1R-phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling, increased the expressions of Pax7, myogenic regulatory factors (MRFs) and protein synthesis, reduced protein degradation and cell apoptosis in MI-mice. In vitro, IGF-1 up-regulated the levels of Pax7 and MRFs, mTOR and P70S6K, reduced MuRF1, MAFbx and inhibited cell apoptosis via IGF-1R-PI3K/Akt pathway.

CONCLUSION

AE and RE, safely and effectively, alleviate skeletal muscle atrophy by regulating the levels of myogenesis, protein degradation and cells apoptosis in mice with MI via activating IGF-1/IGF-1R-PI3K/Akt pathway.

Exogenous parathyroid hormone attenuates ovariectomy-induced skeletal muscle weakness in vivo

Osteoporosis commonly affects the elderly and is associated with significant morbidity and mortality. Loss of bone mineral density induces muscle atrophy and increases fracture risk. However, muscle lipid content and droplet size are increased by aging and mobility impairments, inversely correlated with muscle function, and a cause of reduced motor function. Teriparatide, the synthetic form of human parathyroid hormone (PTH) 1-34, has been widely used to treat osteoporosis. Although PTH positively affects muscle differentiation in vitro, the precise function and mechanisms of muscle mass and power preservation are still poorly understood, especially in vivo. In this study, we investigated the effect of PTH on skeletal muscle atrophy and dysfunction using an ovariectomized murine model. Eight-week-old female C57BL/6J mice were ovariectomized or sham-operated. Within each surgical group, the mice were divided into PTH injection or control subgroups. Motor function was evaluated based on grip strength, treadmill running, and lactic acid concentration. PTH receptor was expressed in skeletal muscle cells and myoblasts. PTH inhibited ovariectomy-induced bone loss but not uterine atrophy or increased body weight; PTH not only abolished ovariectomy-induced reduction in grip strength and maximum running speed, but also significantly reduced the ovariectomy-induced increase in lactic acid concentration (compared with that observed in the vehicle control). PTH also abrogated the ovariectomy-induced reduction in the oxidative capacity of muscle fibers, their cross-sectional area, and intramyocellular lipid content, and induced cell proliferation, cell migration, and muscle differentiation, while reducing lipid secretion by C2C12 myoblasts via the Wnt/β-catenin pathway. PTH significantly ameliorated muscle weakness and attenuated exercise-induced lactate levels in ovariectomized mice. Our in vitro study demonstrated that PTH/Wnt signaling regulated the proliferation, migration, and differentiation of myoblasts and also reduced lipid secretion in myoblasts. Thus, PTH could regulate several aspects of muscle function and physiology, and may represent a novel therapeutic strategy for patients with osteoporosis.

Meta-Analyses of Multiple Gene Expression Profiles to Screen Hub Genes Related to Osteoarthritis

BACKGROUND

This study aimed to screen and validate the crucial genes involved in osteoarthritis (OA) and explore its potential molecular mechanisms.

METHODS

Four expression profile datasets related to OA were downloaded from the Gene Expression Omnibus (GEO). The differentially expressed genes (DEGs) from 4 microarray patterns were identified by the meta-analysis method. The weighted gene co-expression network analysis (WGCNA) method was used to investigate stable modules most related to OA. In addition, a protein-protein interaction (PPI) network was built to explore hub genes in OA. Moreover, OA-related genes and pathways were retrieved from Comparative Toxicogenomics Database (CTD).

RESULTS

A total of 1,136 DEGs were identified from 4 datasets. Based on these DEGs, WGCNA further explored 370 genes included in the 3 OA-related stable modules. A total of 10 hub genes were identified in the PPI network, including AKT1, CDC42, HLA-DQA2, TUBB, TWISTNB, GSK3B, FZD2, KLC1, GUSB, and RHOG. Besides, 5 pathways including "Lysosome," "Pathways in cancer," "Wnt signaling pathway," "ECM-receptor interaction" and "Focal adhesion" in CTD and enrichment analysis and 5 OA-related hub genes (including GSK3B, CDC42, AKT1, FZD2, and GUSB) were identified.

CONCLUSION

In this study, the meta-analysis was used to screen the central genes associated with OA in a variety of gene expression profiles. Three OA-related modules (green, turquoise, and yellow) containing 370 genes were identified through WGCNA. It was discovered through the gene-pathway network that GSK3B, CDC42, AKT1, FZD2, and GUSB may be key genes related to the progress of OA and may become promising therapeutic targets.

Arrhythmogenic right ventricular cardiomyopathy and sports activity: from molecular pathways in diseased hearts to new insights into the athletic heart mimicry

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disease associated with a high risk of sudden cardiac death. Among other factors, physical exercise has been clearly identified as a strong determinant of phenotypic expression of the disease, arrhythmia risk, and disease progression. Because of this, current guidelines advise that individuals with ARVC should not participate in competitive or frequent high-intensity endurance exercise. Exercise-induced electrical and morphological para-physiological remodelling (the so-called 'athlete's heart') may mimic several of the classic features of ARVC. Therefore, the current International Task Force Criteria for disease diagnosis may not perform as well in athletes. Clear adjudication between the two conditions is often a real challenge, with false positives, that may lead to unnecessary treatments, and false negatives, which may leave patients unprotected, both of which are equally inacceptable. This review aims to summarize the molecular interactions caused by physical activity in inducing cardiac structural alterations, and the impact of sports on arrhythmia occurrence and other clinical consequences in patients with ARVC, and help the physicians in setting the two conditions apart.

MicroRNA-145, Wnt3a, and Dab2 Genes Expression Changes of the Cardiomyocytes in Hypercholesterolemic Rats Exposed to the Aerobic Training

The current study aimed to investigate the effect of a 12-week endurance training (ET) on microRNA-145 (miR-145) changes and Wnt3a and Dab2 cardiomyocytes genes expression of hypercholesterolemic Wistar male rats. Thirty-two male Wistar rats (191.2±19 g, 6-8 weeks age) were randomly assigned into the aerobic exercise-normal nutrition (ANN; n=8), hypercholesterolemic (HCL; n=8), aerobic exercise- hypercholesterolemic (ACL; n=8), and normal nutrition (NN; n=8). Hypercholesterolemia was created by adding 1% cholesterol to the food of the HCL and ACL rats. ET was done five sessions per week on nonconsecutive days for 12 weeks. Twenty-four hours after the last training session, the rats were killed, and the cardiomyocytes were removed. The expression of miR-145, Wnt3a, and Dab2 genes in cardiomyocytes was assessed by real time PCR method. The expression of miR-145 significantly increased in the ANN group in comparison with other groups (P = 0.001). Also, Dab2 gene expression significantly decreased in the ANN group in comparison with ACL (P = 0.001) and HCL (P = 0.001) groups. The results also showed that the Wnt3a in the ANN group was significantly different from NN (p=0.001), ACL, and HCL (p=0.001) groups. It can be concluded that aerobic training and cholesterol-rich foods play an essential regulatory role in the expression of miR-145, Dab2, and Wnt3a genes. However, cholesterol-rich foods appear to play a more significant regulatory role than aerobic exercise training.

Aging of the immune system and impaired muscle regeneration: A failure of immunomodulation of adult myogenesis

Skeletal muscle regeneration that follows acute injury is strongly influenced by interactions with immune cells that invade and proliferate in the damaged tissue. Discoveries over the past 20 years have identified many of the key mechanisms through which myeloid cells, especially macrophages, regulate muscle regeneration. In addition, lymphoid cells that include CD8+ T-cells and regulatory T-cells also significantly affect the course of muscle regeneration. During aging, the regenerative capacity of skeletal muscle declines, which can contribute to progressive loss of muscle mass and function. Those age-related reductions in muscle regeneration are accompanied by systemic, age-related changes in the immune system, that affect many of the myeloid and lymphoid cell populations that can influence muscle regeneration. In this review, we present recent discoveries that indicate that aging of the immune system contributes to the diminished regenerative capacity of aging muscle. Intrinsic, age-related changes in immune cells modify their expression of factors that affect the function of a population of muscle stem cells, called satellite cells, that are necessary for normal muscle regeneration. For example, age-related reductions in the expression of growth differentiation factor-3 (GDF3) or CXCL10 by macrophages negatively affect adult myogenesis, by disrupting regulatory interactions between macrophages and satellite cells. Those changes contribute to a reduction in the numbers and myogenic capacity of satellite cells in old muscle, which reduces their ability to restore damaged muscle. In addition, aging produces changes in the expression of molecules that regulate the inflammatory response to injured muscle, which also contributes to age-related defects in muscle regeneration. For example, age-related increases in the production of osteopontin by macrophages disrupts the normal inflammatory response to muscle injury, resulting in regenerative defects. These nascent findings represent the beginning of a newly-developing field of investigation into mechanisms through which aging of the immune system affects muscle regeneration.

Regenerating Urethral Striated Muscle by CRISPRi/dCas9-KRAB-Mediated Myostatin Silencing for Obesity-Associated Stress Urinary Incontinence

Overweight females are prone to obesity-associated stress urinary incontinence (OA-SUI), and there are no definitive medical therapies for this common urologic condition. This study was designed to test the hypothesis that regenerative therapy to restore urethral striated muscle (stM) and pelvic floor muscles might represent a valuable therapeutic approach. For the in vitro experiment, single-guide RNAs targeting myostatin (MSTN) were used for CRISPRi/dCas9-Kruppel associated box (KRAB)-mediated gene silencing. For the in vivo experiment, a total of 14 female lean ZUC-Leprfa 186 and 14 fatty ZUC-Leprfa 185 rats were used as control and CRISPRi-MSTN treated groups, respectively. The results indicated that lentivirus-mediated expression of MSTN CRISPRi/dCas9-KRAB caused sustained downregulation of MSTN in rat L6 myoblast cells and significantly enhanced myogenesis in vitro. In vivo, the urethral sphincter injection of lentiviral-MSTN sgRNA and lentiviral-dCas9-KRAB significantly increased the leak point pressure, the thickness of the stM layer, the ratio of stM to smooth muscle, and the number of neuromuscular junctions. Downregulation of MSTN with CRISPRi/dCas9-KRAB-mediated gene silencing significantly enhanced myogenesis in vitro and in vivo. It also improved urethral continence in the OA-SUI rat model.

Radiation-Induced Damage to Prepubertal Pax7+ Skeletal Muscle Stem Cells Drives Lifelong Deficits in Myofiber Size and Nuclear Number

During prepubertal development, muscle stem cells (satellite cells, SCs) actively contribute to myofiber growth. Because some SCs are active during this time, they may be particularly susceptible to damage. Using a Small Animal Radiation Research Platform (SARRP), we investigated the effects of local fractionated radiation treatment on prepubertal SCs. Immediately after this regimen, there was a reduction in SC number. Although surviving SCs had deficiencies in function, some myogenic potential remained. Indeed, some muscle regenerative capacity persisted immediately after irradiation. Lastly, we assessed the long-term consequences of radiation-induced SC loss during prepuberty. We observed a reduction of myofiber size and corresponding loss of nuclei in both fast- and slow-contracting muscles 14 months post-irradiation. Notably, prepubertal SC depletion mimicked these lifelong deficits. This work highlights the susceptibility of prepubertal SCs to radiation exposure. We also reveal the importance of prepubertal SC contribution to the lifelong maintenance of skeletal muscle.

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Increased sensitivity of satellite cells to irradiation during prepubertal growth

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Prepubertal irradiation leads to lifelong deficits in skeletal muscle regenerative capacity

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Lifelong reduction in myofiber size and nuclear number is a consequence of prepubertal irradiation

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Satellite cell ablation mimics the lifelong effects of prepubertal irradiation on myofiber size and nuclear number

Epigenetic mechanisms in Tendon Ageing

INTRODUCTION

Tendon is a composite material with a well-ordered hierarchical structure exhibiting viscoelastic properties designed to transfer force. It is recognized that the incidence of tendon injury increases with age, suggesting a deterioration in homeostatic mechanisms or reparative processes. This review summarizes epigenetic mechanisms identified in ageing healthy tendon.

SOURCES OF DATA

We searched multiple databases to produce a systematic review on the role of epigenetic mechanisms in tendon ageing.

AREAS OF AGREEMENT

Epigenetic mechanisms are important in predisposing ageing tendon to injury.

AREAS OF CONTROVERSY

The relative importance of epigenetic mechanisms are unknown in terms of promoting healthy ageing. It is also unknown whether these changes represent protective mechanisms to function or predispose to pathology.

GROWING POINT

Epigenetic markers in ageing tendon, which are under-researched including genome-wide chromatin accessibility, should be investigated.

AREAS TIMELY FOR DEVELOPING RESEARCH

Metanalysis through integration of multiple datasets and platforms will enable a holistic understanding of the epigenome in ageing and its relevance to disease.

Exercise rejuvenates quiescent skeletal muscle stem cells in old mice through restoration of Cyclin D1

Aging impairs tissue repair. This is pronounced in skeletal muscle, whose regeneration by muscle stem cells (MuSCs) is robust in young adult animals but inefficient in older organisms. Despite this functional decline, old MuSCs are amenable to rejuvenation through strategies that improve the systemic milieu, such as heterochronic parabiosis. One such strategy, exercise, has long been appreciated for its benefits on healthspan, but its effects on aged stem cell function in the context of tissue regeneration are incompletely understood. Here we show that exercise in the form of voluntary wheel running accelerates muscle repair in old animals and improves old MuSC function. Through transcriptional profiling and genetic studies, we discovered that the restoration of old MuSC activation ability hinges on restoration of Cyclin D1, whose expression declines with age in MuSCs. Pharmacologic studies revealed that Cyclin D1 maintains MuSC activation capacity by repressing TGFβ signaling. Taken together, these studies demonstrate that voluntary exercise is a practicable intervention for old MuSC rejuvenation. Furthermore, this work highlights the distinct role of Cyclin D1 in stem cell quiescence.

Epithelial cell senescence induces pulmonary fibrosis through Nanog-mediated fibroblast activation

Idiopathic pulmonary fibrosis (IPF) is a chronic and progressive lung disease tightly correlated with aging. The pathological features of IPF include epithelial cell senescence and abundant foci of highly activated pulmonary fibroblasts. However, the underlying mechanism between epithelial cell senescence and pulmonary fibroblast activation remain to be elucidated. In our study, we demonstrated that Nanog, as a pluripotency gene, played an essential role in the activation of pulmonary fibroblasts. In the progression of IPF, senescent epithelial cells could contribute to the activation of pulmonary fibroblasts via increasing the expression of senescence-associated secretory phenotype (SASP). In addition, we found activated pulmonary fibroblasts exhibited aberrant activation of Wnt/β-catenin signalling and elevated expression of Nanog. Further study revealed that the activation of Wnt/β-catenin signalling was responsible for senescent epithelial cell-induced Nanog phenotype in pulmonary fibroblasts. β-catenin was observed to bind to the promoter of Nanog during the activation of pulmonary fibroblasts. Targeted inhibition of epithelial cell senescence or Nanog could effectively suppress the activation of pulmonary fibroblasts and impair the development of pulmonary fibrosis, indicating a potential for the exploration of novel anti-fibrotic strategies.

Comparative molecular analysis of endurance exercise in vivo with electrically stimulated in vitro myotube contraction

Exercise has positive effects on health and improves a variety of disease conditions. An in vitro model of exercise has been developed to better understand its molecular mechanisms. While various conditions have been used to mimic in vivo exercise, no specific conditions have matched a specific type of in vivo exercise. Here, we screened various electrical pulse stimulation (EPS) conditions and compared the molecular events under each condition in myotube culture with that obtained under voluntary wheel running (VWR), a mild endurance exercise, in mice. Both EPS and VWR upregulated the mRNA levels of genes involved in the slow-type twitch ( Myh7 and Myh2) and myogenesis ( Myod and Myog) and increased the protein expression of peroxisome proliferator-activated receptor-γ coactivator-1α, which is involved in mitochondrial biogenesis. These changes were accompanied by activation of p38 and AMPK. However, neither condition induced the expression of muscle-specific E3 ligases such as MAFbx and MuRF1. Both EPS and VWR consistently induced antioxidant genes such as Sod3 and Gpx4 but did not cause similar changes in the expression levels of the calcium channel/pump-related genes Ryr and Serca. Furthermore, both EPS and VWR reduced glycogen levels but not lactate levels as assessed in post-EPS culture medium and post-VWR serum, respectively. Thus we identified an in vitro EPS condition that effectively mimics VWR in mice, which can facilitate further studies of the detailed molecular mechanisms of endurance exercise in the absence of interference from multiple tissues and organs.

NEW & NOTEWORTHY This study establishes an optimal condition for electrical pulse stimulation (EPS) in myotubes that shows a similar molecular signature as voluntary wheel running. The specific EPS condition 1) upregulates the mRNA of slow-twitch muscle components and myogenic transcription factors, 2) induces antioxidant genes without any muscle damage, and 3) promotes peroxisome proliferator-activated receptor-γ coactivator-1α and its upstream regulators involved in mitochondrial biogenesis.

Long Noncoding Ribonucleic Acid MSTRG.59589 Promotes Porcine Skeletal Muscle Satellite Cells Differentiation by Enhancing the Function of PALLD

Skeletal muscle satellite cells are a class of undifferentiated mononuclear myogenic stem cells distributed between the myofibroblast and membrane basement. Since their development determines the development of skeletal muscles, knowledge of their proliferation, differentiation, and fate is vital for understanding skeletal muscle development. Increasing evidence have shown that long noncoding RNA (lncRNA) plays an important role in regulating the development process of satellite cells. Based on the results of our previous studies, we screened lncRNA MSTRG.59589, which is highly expressed in skeletal muscle tissue. In the present study, knockdown of MSTRG.59589 significantly inhibited satellite cell differentiation at various time points, whereas overexpression of MSTRG.59589 demonstrated opposite effects. An MSTRG.59589 knockdown cell model was constructed for transcriptome sequencing, and RNA sequencing analysis screened out a large number of differentially expressed genes. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses of these differentially expressed genes revealed that they are mainly enriched in actin cytoskeleton, muscle contraction, and other pathways related to muscle development. Mechanistic analyses showed that MSTRG.59589 could promote the differentiation process of skeletal muscle satellite cells by positively regulating the expression level of the target gene PALLD. This experiment lays a theoretical foundation for deeper studies on the mechanism of MSTRG.59589 in the differentiation of porcine skeletal muscle satellite cells.

Microenergy acoustic pulses induced myogenesis of urethral striated muscle stem/progenitor cells

Background

Stress urinary incontinence (SUI) is a common disorder with high prevalence in women across their life span, but there are no non-surgical curative options for the condition. Stem cell-based therapy, especially endogenous stem cell therapy may be a potential treatment method for SUI. The aims of this study are to identify, isolate, and assay the function of urethral striated muscle derived stem/progenitor cells (uMDSCs) and to assess uMDSC response to microenergy acoustic pulses (MAP).

Methods

Urethral striated muscle was identified utilizing 3D imaging of solvent organs (3DISCO) and immunofluorescence (IF). uMDSCs were isolated and purified from Zucker Lean (ZL) (ZUC-LEAN) (ZUC-Leprfa 186) rats, with magnetic-activated cell sorting (MACS) and pre-plating methods. The stemness and differentiation potential of the uMDSCs were measured by cell proliferation, EdU, flow cytometry, IF, and Western blot.

Results

Comparison of the cell proliferation assays between MACS and pre-plating reveals the advantage of MACS over pre-plating. In addition, the study reveals that uMDSCs form myotubes when treated with MAP.

Conclusions

The uMDSCs within female rat urethral striated muscle could be a therapeutic target of MAP in managing SUI.

Podocan affects C2C12 myogenic differentiation by enhancing Wnt/β-catenin signaling

Podocan, a small leucine-rich repeat protein, is a negative regulator of cell proliferation. In this study, we demonstrated that podocan is involved in the differentiation of C2C12 murine myoblasts. Podocan expression increased with the progression of C2C12 differentiation. As expect, siRNA-mediated podocan knockdown inhibited C2C12 differentiation, as indicated by inhibition of MYOG, MYH2, and desmin expression, as well as reductions in the differentiation and fusion indices. Overexpression of podocan using dCas9 technology promoted C2C12 cell differentiation. In addition, supplementation of culture medium with podocan influenced C2C12 differentiation. Podocan knockdown reduced Wnt/β-catenin signaling, characterized by a reduction in the nuclear translocation of β-catenin, whereas podocan overexpression had the opposite effect. Furthermore, treatment with XAV939, an inhibitor of Wnt/β-catenin, reduced the podocan-mediated promotion of C2C12 differentiation. Induction of muscle injury in mice by bupivacaine administration suggested that podocan may play a role in muscle regeneration. In summary, our results suggest that podocan is required for normal C2C12 differentiation and that its role in myogenesis is mediated by the Wnt/β-catenin pathway.

Use it or lose it to age: A review of bone and muscle communication

Until recently, it was assumed that the only interaction between muscle and bone is mechanical, that the muscle acts as a pulley and the bone as a lever to move the organism. A relatively new concept is that muscle, especially contracted muscle, acts as a secretory organ, regulating metabolism. An even newer concept is that bone, especially the osteocytes in bone, act as endocrine cells targeting other organs such as kidney and more recently, muscle. These two new concepts logically led to the third concept: that muscle and bone communicate via soluble factors. Crosstalk occurs through muscle factors such as myostatin, irisin, and a muscle metabolite, β-aminoisobutyric acid, BAIBA, and through bone factors such as osteocalcin, transforming growth factor beta, TGFβ, Prostaglandin E2, PGE₂ and Wnts. Some of these factors have positive and some negative effects on the opposing tissue. One feature both bone and muscle have in common is that their tissues are mechanically loaded and many of their secreted factors are regulated by load. This mechanical loading, also known as exercise, has beneficial effects on many systems leading to the hypothesis that muscle and bone factors can be responsible for the beneficial effects of exercise. Many of the characteristics of aging and diseases associated with aging such as sarcopenia and osteoporosis and neurological conditions such as Alzheimer's disease and dementia, are delayed by exercise. This beneficial effect has been ascribed to increased blood flow increasing oxygen and nutrients, but could also be due to the secretome of the musculoskeletal system as outlined in this review.

Role of MicroRNA-141 in the Aging Musculoskeletal System: A Current Overview

MicroRNA's are small non-coding RNAs that regulate the expression of genes by targeting the 3' UTR's of mRNA. Studies reveal that miRNAs play a pivotal role in normal musculoskeletal function such as mesenchymal stem cell differentiation, survivability and apoptosis, osteogenesis, and chondrogenesis. Changes in normal miRNA expression have been linked to a number of pathological disease processes. Additionally, with aging, it is noted that there is dysregulation in the normal function of stem cell differentiation, bone formation/degradation, chondrocyte function, and muscle degeneration. Due to the change in expression of miRNA in degenerative musculoskeletal pathology, it is believed that these molecules may be at least partially responsible for cellular dysfunction. A number of miRNAs have already been identified to play a role in osteoarthritis, osteoporosis and sarcopenia. One miRNA that has become of interest recently is miRNA 141. The purpose of this article is to review the current literature available on miRNA 141 and how it could play a role in osteoporosis, osteoarthritis and musculoskeletal pathology overall.

Regulation of Wnt signaling by physical exercise in the cell biological processes of the locomotor system

In the past decade, researches on Wnt signaling in cell biology have made remarkable progress regarding our understanding of embryonic development, bone formation, muscle injury and repair, neurogenesis, and tumorigenesis. The study also showed that physical activity can reverse age-dependent decline in skeletal muscle, preventing osteoporosis, regenerative neurogenesis, hippocampal function, cognitive ability, and neuromuscular junction formation, and the age-dependent recession is highly correlated with Wnt signaling pathways. However, how the biological processes in cell and physical activity during/following exercise affect the Wnt signaling path of the locomotor system is largely unknown. In this study, we first briefly introduce the important features of the cellular biological processes of exercise in the locomotor system. Then, we discuss Wnt signaling and review the very few studies that have examined Wnt signaling pathways in cellular biological processes of the locomotor system during physical exercise.

Autophagy as a Therapeutic Target to Enhance Aged Muscle Regeneration

Skeletal muscle has remarkable regenerative capacity, relying on precise coordination between resident muscle stem cells (satellite cells) and the immune system. The age-related decline in skeletal muscle regenerative capacity contributes to the onset of sarcopenia, prolonged hospitalization, and loss of autonomy. Although several age-sensitive pathways have been identified, further investigation is needed to define targets of cellular dysfunction. Autophagy, a process of cellular catabolism, is emerging as a key regulator of muscle regeneration affecting stem cell, immune cell, and myofiber function. Muscle stem cell senescence is associated with a suppression of autophagy during key phases of the regenerative program. Macrophages, a key immune cell involved in muscle repair, also rely on autophagy to aid in tissue repair. This review will focus on the role of autophagy in various aspects of the regenerative program, including adult skeletal muscle stem cells, monocytes/macrophages, and corresponding age-associated dysfunction. Furthermore, we will highlight rejuvenation strategies that alter autophagy to improve muscle regenerative function.

Influence of hypoxic stimulation on angiogenesis and satellite cells in mouse skeletal muscle

We clarified in our previous study that hypoxic training promotes angiogenesis in skeletal muscle, but the mechanism of angiogenesis in skeletal muscle remains unknown. In this study, we investigated the influence of differences in hypoxia exposure on angiogenesis in skeletal muscles at differing ages and metabolic characteristics at which the production of reactive oxygen species and nitric oxide may differ. Ten-week-old (young) and 20-month-old (old) mice were separated into control (N), continuous hypoxia (H), and intermittent hypoxia (IH) groups. The H group was exposed to 16% O₂ hypoxia for 5 days and the IH group was exposed to 16% O₂ hypoxia at one-hour intervals during the light period for 5 days. After completion of hypoxia exposure, the soleus and gastrocnemius muscles were immediately excised, and mRNA expression of angiogenesis- and satellite cell-related genes was investigated using real-time RT-PCR. In addition, muscle fiber type composition, muscle fiber area, number of satellite cells, and capillary density were measured immunohistochemically. In the young soleus muscle, the muscle fiber area was decreased in the H group, and mRNA expression of satellite cell activation-related MyoD, MHCe, and BDNF was significantly increased. On the other hand, in the old soleus muscle, nNOS and VEGF-A mRNA expression, and the capillary density were significantly increased in the H group. In the superficial portion of the gastrocnemius, mRNA expression of FGF2, an angiogenic factor secreted by satellite cells, was significantly increased in the young IH group. In addition, a positive correlation between VEGF-A mRNA expression and nNOS mRNA expression in the soleus muscle and eNOS mRNA expression in the superficial portion of the gastrocnemius was noted. These data demonstrated that age, hypoxia exposure method and muscle metabolic characteristics are related, which results in significant differences in angiogenesis.

Cross talk between androgen and Wnt signaling potentially contributes to age-related skeletal muscle atrophy in rats

We sought to determine whether age-related gastrocnemius muscle mass loss was associated with parallel decrements in androgen receptor (AR) or select Wnt signaling markers. To test this hypothesis, serum-free and total testosterone (TEST) and gastrocnemius AR and Wnt signaling markers were analyzed in male Fischer 344 rats that were 3, 6, 12, 18, and 24 mo (mo) old ( n = 9 per group). Free and total TEST was greatest in 6 mo rats, and AR protein and Wnt5 protein levels linearly declined with aging. There were associations between Wnt5 protein levels and relative gastrocnemius mass ( r = 0.395, P = 0.007) as well as AR and Wnt5 protein levels (r = 0.670, P < 0.001). We next tested the hypothesis that Wnt5 affects muscle fiber size by treating C2C12-derived myotubes with lower (75 ng/ml) and higher (150 ng/ml) concentrations of recombinant Wnt5a protein. Both treatments increased myotube size ( P < 0.05) suggesting this ligand may affect muscle fiber size in vivo. We next tested if Wnt5a protein levels were androgen-modulated by examining 10-mo-old male Fischer 344 rats ( n = 10-11 per group) that were orchiectomized and treated with testosterone-enanthate (TEST-E); trenbolone enanthate (TREN), a nonaromatizable synthetic testosterone analogue; or a vehicle (ORX only) for 4 wk. Interestingly, TEST-E and TREN treatments increased Wnt5a protein in the androgen-sensitive levator ani/bulbocavernosus muscle compared with ORX only ( P < 0.05). To summarize, aromatizable and nonaromatizable androgens increase Wnt5a protein expression in skeletal muscle, age-related decrements in muscle AR may contribute Wnt5a protein decrements, and our in vitro data imply this mechanism may contribute to age-related muscle loss. NEW & NOTEWORTHY Results from this study demonstrate androgen and Wnt5 protein expression decrease with aging, and this may be a mechanism involved with age-related muscle loss.

Overexpression of the double homeodomain protein DUX4c interferes with myofibrillogenesis and induces clustering of myonuclei

BACKGROUND

Facioscapulohumeral muscular dystrophy (FSHD) is associated with DNA hypomethylation at the 4q35 D4Z4 repeat array. Both the causal gene DUX4 and its homolog DUX4c are induced. DUX4c is immunodetected in every myonucleus of proliferative cells, while DUX4 is present in only 1/1000 of myonuclei where it initiates a gene deregulation cascade. FSHD primary myoblasts differentiate into either atrophic or disorganized myotubes. DUX4 expression induces atrophic myotubes and associated FSHD markers. Although DUX4 silencing normalizes the FSHD atrophic myotube phenotype, this is not the case for the disorganized phenotype. DUX4c overexpression increases the proliferation rate of human TE671 rhabdomyosarcoma cells and inhibits their differentiation, suggesting a normal role during muscle differentiation.

METHODS

By gain- and loss-of-function experiments in primary human muscle cells, we studied the DUX4c impact on proliferation, differentiation, myotube morphology, and FSHD markers.

RESULTS

In primary myoblasts, DUX4c overexpression increased the staining intensity of KI67 (a proliferation marker) in adjacent cells and delayed differentiation. In differentiating cells, DUX4c overexpression led to the expression of some FSHD markers including β-catenin and to the formation of disorganized myotubes presenting large clusters of nuclei and cytoskeletal defects. These were more severe when DUX4c was expressed before the cytoskeleton reorganized and myofibrils assembled. In addition, endogenous DUX4c was detected at a higher level in FSHD myotubes presenting abnormal clusters of nuclei and cytoskeletal disorganization. We found that the disorganized FSHD myotube phenotype could be rescued by silencing of DUX4c, not DUX4.

CONCLUSION

Excess DUX4c could disturb cytoskeletal organization and nuclear distribution in FSHD myotubes. We suggest that DUX4c up-regulation could contribute to DUX4 toxicity in the muscle fibers by favoring the clustering of myonuclei and therefore facilitating DUX4 diffusion among them. Defining DUX4c functions in the healthy skeletal muscle should help to design new targeted FSHD therapy by DUX4 or DUX4c inhibition without suppressing DUX4c normal function.

Recovery from impaired muscle growth arises from prolonged postnatal accretion of myonuclei in Atrx mutant mice

Reduced muscle mass due to pathological development can occur through several mechanisms, including the loss or reduced proliferation of muscle stem cells. Muscle-specific ablation of the α-thalassemia mental retardation syndrome mutant protein, Atrx, in transgenic mice results in animals with a severely reduced muscle mass at three weeks of age; yet this muscle mass reduction resolves by adult age. Here, we explore the cellular mechanism underlying this effect. Analysis of Atrx mutant mice included testing for grip strength and rotorod performance. Muscle fiber length, fiber volume and numbers of myofiber-associated nuclei were determined from individual EDL or soleus myofibers isolated at three, five, or eight weeks. Myofibers from three week old Atrx mutant mice are smaller with fewer myofiber-associated nuclei and reduced volume compared to control animals, despite similar fiber numbers. Nonetheless, the grip strength of Atrx mutant mice was comparable to control mice when adjusted for body weight. Myofiber volume remained smaller at five weeks, becoming comparable to controls by 8 weeks of age. Concomitantly, increased numbers of myofiber-associated nuclei and Ki67⁺ myoblasts indicated that the recovery of muscle mass likely arises from the prolonged accretion of new myonuclei. This suggests that under disease conditions the muscle satellite stem cell niche can remain in a prolonged active state, allowing for the addition of a minimum number of myonuclei required to achieve a normal muscle size.

Diabetes-Induced Dysfunction of Mitochondria and Stem Cells in Skeletal Muscle and the Nervous System

Diabetes mellitus is one of the most common metabolic diseases spread all over the world, which results in hyperglycemia caused by the breakdown of insulin secretion or insulin action or both. Diabetes has been reported to disrupt the functions and dynamics of mitochondria, which play a fundamental role in regulating metabolic pathways and are crucial to maintain appropriate energy balance. Similar to mitochondria, the functions and the abilities of stem cells are attenuated under diabetic condition in several tissues. In recent years, several studies have suggested that the regulation of mitochondria functions and dynamics is critical for the precise differentiation of stem cells. Importantly, physical exercise is very useful for preventing the diabetic alteration by improving the functions of both mitochondria and stem cells. In the present review, we provide an overview of the diabetic alterations of mitochondria and stem cells and the preventive effects of physical exercise on diabetes, focused on skeletal muscle and the nervous system. We propose physical exercise as a countermeasure for the dysfunction of mitochondria and stem cells in several target tissues under diabetes complication and to improve the physiological function of patients with diabetes, resulting in their quality of life being maintained.

Low-intensity extracorporeal shock wave therapy promotes myogenesis through PERK/ATF4 pathway

AIM

Stress urinary incontinence (SUI) is a significant health problem for women. Treatments employing muscle derived stem cells (MDSCs) may be a promising approach to this prevalent, bothersome condition, but these treatments are invasive and require collection of cells from one site for injection into another. It is also unknown whether or not these cells establish themselves and function as muscle cells in the target tissues. Alternatively, low-intensity extracorporeal shock wave therapy (Li-ESWT) is non-invasive and has shown positive outcomes in the treatment of multiple musculoskeletal disorders, but the biological effects responsible for clinical success are not yet well understood. The aim of this study is to explore the possibility of employing Li-ESWT for activation of MDSCs in situ and to further elucidate the underlying biological effects and mechanisms of action in urethral muscle.

METHODS

Urethral muscle derived stem cells (uMDSCs) were harvest from Zucker Lean (ZUC-LEAN) (ZUC-Leprfa 186) rats and characterized with flow cytometry. Li-ESWT (0.02 mJ/mm² , 3 Hz, 200 pulses) and GSK2656157, an inhibitor of PERK pathway, were applied to L6 rat myoblast cells. To assess for myotube formation, we used immunofluorescence staining and western blot analysis in uMDSCs and L6 cells.

RESULTS

The results indicate that uMDSCs could form myotubes. Myotube formation was significantly increased by the Li-ESWT as was the expression of muscle heavy chain (MHC) and myogenic factor 5 (Myf5) in L6 cells in vitro. Li-ESWT activated protein kinase RNA-like ER kinase (PERK) pathway by increasing the phosphorylation levels of PERK and eukaryotic initiation factor 2a (eIF2α) and by increasing activating transcription factor 4 (ATF4). In addition, GSK2656157, an inhibitor of PERK, effectively inhibited the myotube formation in L6 rat myoblast cells. Furthermore, GSK2656157 also attenuated myotube formation induced by Li-ESWT.

CONCLUSION

In conclusion, this experiment reveals that rat uMDSCs can be isolated successfully and can form myotubes in vitro. PERK/ATF4 pathway was involved in myotube formation, and L6 rat myoblast cells were activated by Li-ESWT to form myotubes. These findings suggest that PERK/ATF4 pathway is activated by Li-ESWT. This study elucidates one of the biochemical pathways responsible for the clinical improvements seen after Li-ESWT. It is possible that this information will help to establish Li-ESWT as an acceptable treatment modality and may help to further refine the use of Li-ESWT in the clinical practice of medicine.

β-Catenin Accumulation Is Associated With Increased Expression of Nanog Protein and Predicts Maintenance of MSC Self-Renewal

Human mesenchymal stem cells (hMSCs) are self-renewing cells with the ability to differentiate into organized, functional network of cells. Recent studies have revealed that activation of the Wnt/β-catenin pathway by a glycogen synthase kinase (GSK)-3-specific pharmacological inhibitor, Bio, results in the maintenance of self-renewal in both mouse and human ES cells. The molecular mechanism behind the maintenance of hMSCs by these factors, however, is not fully understood. We found that rEGF enhances the level of β-catenin, a component of the Wnt/β-catenin signaling pathway. Furthermore, it was found that β-catenin upregulates Nanog. EGF activates the β-catenin pathway via the Ras protein and also increased the Nanog protein and gene expression levels 2 h after rEGF treatment. These results suggest that adding EGF can enhance β-catenin and Nanog expression in MSCs and facilitate EGF-mediated maintenance of MSC self-renewal. EGF was shown to augment MSC proliferation while preserving early progenitors within MSC population and thus did not induce differentiation. Thus, EGF not only can be used to expand MSC in vitro but also be utilized to autologous transplantation of MSCs in vivo.

Satellite cell activation induced by aerobic muscle adaptation in response to endurance exercise in humans and rodents

Although the requirement of satellite cells activation and expansion following injury, mechanical load or growth stimulus provoked by resistance exercise has been well established, their function in response to aerobic exercise adaptation remains unclear. A clear relationship between satellite cell expansion in fiber-type specific myosin heavy chain and aerobic performance has been related, independent of myonuclear accretion or muscle growth. However, the trigger for this activation process is not fully understood yet and it seems to be a multi-faceted and well-orchestrated process. Emerging in vitro studies suggest a role for metabolic pathways and oxygen availability for satellite cell activation, modulating the self-renewal potential and cell fate control. The goal of this review is to describe and discuss the current knowledge about the satellite cell activation and expansion in response to aerobic exercise adaptation in human and rodent models. Additionally, findings about the in vitro metabolic control, which seems be involved in the satellite cell activation and cell fate control, are presented and discussed.

Nucleoprotein supplementation enhances the recovery of rat soleus mass with reloading after hindlimb unloading-induced atrophy via myonuclei accretion and increased protein synthesis

Hindlimb unloading results in muscle atrophy and a period of reloading has been shown to partially recover the lost muscle mass. Two of the mechanisms involved in this recovery of muscle mass are the activation of protein synthesis pathways and an increase in myonuclei number. The additional myonuclei are provided by satellite cells that are activated by the mechanical stress associated with the reloading of the muscles and eventually incorporated into the muscle fibers. Amino acid supplementation with exercise also can increase skeletal muscle mass through enhancement of protein synthesis and nucleotide supplements can promote cell cycle activity. Therefore, we hypothesized that nucleoprotein supplementation, a combination of amino acids and nucleotides, would enhance the recovery of muscle mass to a greater extent than reloading alone after a period of unloading. Adult rats were assigned to 4 groups: control, hindlimb unloaded (HU; 14 days), reloaded (5 days) after hindlimb unloading (HUR), and reloaded after hindlimb unloading with nucleoprotein supplementation (HUR + NP). Compared with the HUR group, the HUR + NP group had larger soleus muscles and fiber cross-sectional areas, higher levels of phosphorylated rpS6, and higher numbers of myonuclei and myogenin-positive cells. These results suggest that nucleoprotein supplementation has a synergistic effect with reloading in recovering skeletal muscle properties after a period of unloading via rpS6 activation and satellite cell differentiation and incorporation into the muscle fibers. Therefore, this supplement may be an effective therapeutic regimen to include in rehabilitative strategies for a variety of muscle wasting conditions such as aging, cancer cachexia, muscular dystrophy, bed rest, and cast immobilization.

Regulation of Muscle Stem Cell Functions: A Focus on the p38 MAPK Signaling Pathway

Formation of skeletal muscle fibers (myogenesis) during development and after tissue injury in the adult constitutes an excellent paradigm to investigate the mechanisms whereby environmental cues control gene expression programs in muscle stem cells (satellite cells) by acting on transcriptional and epigenetic effectors. Here we will review the molecular mechanisms implicated in the transition of satellite cells throughout the distinct myogenic stages (i.e., activation from quiescence, proliferation, differentiation, and self-renewal). We will also discuss recent findings on the causes underlying satellite cell functional decline with aging. In particular, our review will focus on the epigenetic changes underlying fate decisions and on how the p38 MAPK signaling pathway integrates the environmental signals at the chromatin to build up satellite cell adaptive responses during the process of muscle regeneration, and how these responses are altered in aging. A better comprehension of the signaling pathways connecting external and intrinsic factors will illuminate the path for improving muscle regeneration in the aged.

Does skeletal muscle have an 'epi'-memory? The role of epigenetics in nutritional programming, metabolic disease, aging and exercise

Skeletal muscle mass, quality and adaptability are fundamental in promoting muscle performance, maintaining metabolic function and supporting longevity and healthspan. Skeletal muscle is programmable and can ‘remember’ early‐life metabolic stimuli affecting its function in adult life. In this review, the authors pose the question as to whether skeletal muscle has an ‘epi’‐memory? Following an initial encounter with an environmental stimulus, we discuss the underlying molecular and epigenetic mechanisms enabling skeletal muscle to adapt, should it re‐encounter the stimulus in later life. We also define skeletal muscle memory and outline the scientific literature contributing to this field. Furthermore, we review the evidence for early‐life nutrient stress and low birth weight in animals and human cohort studies, respectively, and discuss the underlying molecular mechanisms culminating in skeletal muscle dysfunction, metabolic disease and loss of skeletal muscle mass across the lifespan. We also summarize and discuss studies that isolate muscle stem cells from different environmental niches in vivo (physically active, diabetic, cachectic, aged) and how they reportedly remember this environment once isolated in vitro. Finally, we will outline the molecular and epigenetic mechanisms underlying skeletal muscle memory and review the epigenetic regulation of exercise‐induced skeletal muscle adaptation, highlighting exercise interventions as suitable models to investigate skeletal muscle memory in humans. We believe that understanding the ‘epi’‐memory of skeletal muscle will enable the next generation of targeted therapies to promote muscle growth and reduce muscle loss to enable healthy aging.

Treadmill running induces satellite cell activation in diabetic mice

Skeletal muscle-derived stem cells, termed as satellite cells, play essential roles in regeneration after muscle injury in adult skeletal muscle. Diabetes mellitus (DM), one of the most common metabolic diseases, causes impairments of satellite cell function. However, the studies of the countermeasures for the DM-induced dysfunction of satellite cells have been poor. Here, we investigated the effects of chronic running exercise on satellite cell activation in diabetic mice focused on the molecular mechanism including Notch and Wnt signaling, which are contribute to the fate determination of satellite cells. Male C57BL/6 mice 4 weeks of age were injected with streptozotocin and were randomly divided into runner group and control group. Runner group mice were performed treadmill running for 4 weeks. DM attenuated satellite cell activation and the expressions of the components of Notch and Wnt signaling. However, chronic running resulted in activation of satellite cells in diabetic mice and salvaged the inactivity of Wnt signaling but not Notch signaling. Our results suggest that chronic running induces satellite cell activation via upregulation of Wnt signaling in diabetic as well as normal mice.

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Diabetes attenuates satellite cell activation.

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Diabetes downregulates the activities of Notch and Wnt signaling.

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Treadmill running activates satellite cells in diabetic mice.

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Treadmill running can salvage diabetes-induced downregulation of Wnt signaling.

Diabetes Impairs Wnt3 Protein-induced Neurogenesis in Olfactory Bulbs via Glutamate Transporter 1 Inhibition

Diabetes is associated with impaired cognitive function. Streptozotocin (STZ)-induced diabetic rats exhibit a loss of neurogenesis and deficits in behavioral tasks involving spatial learning and memory; thus, impaired adult hippocampal neurogenesis may contribute to diabetes-associated cognitive deficits. Recent studies have demonstrated that adult neurogenesis generally occurs in the dentate gyrus of the hippocampus, the subventricular zone, and the olfactory bulbs (OB) and is defective in patients with diabetes. We hypothesized that OB neurogenesis and associated behaviors would be affected in diabetes. In this study, we show that inhibition of Wnt3-induced neurogenesis in the OB causes several behavioral deficits in STZ-induced diabetic rats, including impaired odor discrimination, cognitive dysfunction, and increased anxiety. Notably, the sodium- and chloride-dependent GABA transporters and excitatory amino acid transporters that localize to GABAergic and glutamatergic terminals decreased in the OB of diabetic rats. Moreover, GAT1 inhibitor administration also hindered Wnt3-induced neurogenesis in vitro Collectively, these data suggest that STZ-induced diabetes adversely affects OB neurogenesis via GABA and glutamate transporter systems, leading to functional impairments in olfactory performance.