Defining a role for non-satellite stem cells in the regulation of muscle repair following exercise

Sex-based comparisons of muscle cellular adaptations after 10 weeks of progressive resistance training in middle-aged adults

Resistance training combined with adequate protein intake supports skeletal muscle strength and hypertrophy. These adaptations are supported by the action of muscle stem cells (MuSCs), which are regulated, in part, by fibro-adipogenic progenitors (FAPs) and circulating factors delivered through capillaries. It is unclear if middle-aged males and females have similar adaptations to resistance training at the cellular level. To address this gap, 27 (13 males, 14 females) middle-aged (40-64 yr) adults participated in 10 wk of whole body resistance training with dietary counseling. Muscle biopsies were collected from the vastus lateralis pre- and posttraining. Type II fiber cross-sectional area increased similarly with training in both sexes (P = 0.014). MuSC content was not altered with training; however, training increased PDGFRα⁺/CD90⁺ FAP content (P < 0.0001) and reduced PDGFRα⁺/CD90^- FAP content (P = 0.044), independent of sex. The number of CD31⁺ capillaries per fiber also increased similarly in both sexes (P < 0.05). These results suggest that muscle fiber hypertrophy, stem/progenitor cell, and capillary adaptations are similar between middle-aged males and females in response to whole body resistance training.NEW & NOTEWORTHY We demonstrate that resistance training-induced increases in fiber hypertrophy, FAP content, and capillarization are similar between middle-aged males and females.

Circulating Endothelial Progenitor and Mesenchymal Stromal Cells as Biomarkers for Monitoring Disease Status and Responses to Exercise

Noncommunicable chronic diseases, such as obesity, cardiovascular disease (CVD), and type 2 diabetes (T2D), pose significant health challenges globally. Important advances have been made in the understanding of the pathophysiologal mechanisms and treatment of noncommunicable diseases in recent years. Lack of physical activity is a primary contributor to many noncommunicable diseases including metabolic syndrome, T2D, CVD, and obesity. Certain diabetes medications and non-pharmaceutical interventions, such as physical activity and exercise, are shown to be effective in decreasing the CVD risks associated with heart disease, stroke, obesity, prediabetes, and T2D. The ability to measure and analyze circulating adult stem cells (ASCs) has gained particular interest due to their potential to identify at-risk individuals and implications in various therapeutics. Therefore, the purpose of this narrative review is to (1) provide an overview of ASCs; specifically endothelial progenitor cells (EPCs) and mesenchymal stromal cells (MSCs), (2) describe the responses of these cells to acute and chronic exercise, and (3) highlight the potential effect of exercise on EPCs and MSCs in aging and disease. EPCs are circulating cells, abundantly available in peripheral blood, bone marrow, and umbilical cord, and are defined by cell surface markers such as CD34+. EPCs are expected to play an important role in angiogenesis and neovascularization and have been implicated in the treatment of CVD. MSCs are essential for maintaining tissue and organ homeostasis. MSCs are defined as multipotent heterogeneous cells that can proliferate in vitro as plastic-adherent cells, have fibroblast-like morphology, form colonies in vitro, and can differentiate into ostyeoblasts, adipocytes, chondroblasts, and myoblasts. In the presence of aging and disease, EPCs and MSCs decrease in quantity and functional capacity. Importantly, exercise facilitates EPC differentiation and production from bone marrow and also helps to promote migration and homing to the hypoxic and damaged tissue which in turn improve angiogenesis and vasculogenesis. Similarly, exercise stimulates increases in proliferation and migratory activity of MSCs. Despite the reported benefits of exercise on EPC and MSC number and function, little is known regarding the optimal exercise prescription for aging and clinical populations. Moreover, the interactions between medications and exercise on EPCs and MSCs is currently unclear. Use of ASCs as a biomarker have the potential to revolutionize the management of patients with a variety of metabolic and obesity related disorders and also pro-inflammatory diseases. Further investigation of clinical entities are urgently needed to understand the implications of interventions such as exercise, diet, and various medications on EPC and MSC quantity and function in aging and clinical populations.

Transplantation to study satellite cell heterogeneity in skeletal muscle

Skeletal muscle has a remarkable capacity to regenerate throughout life, which is mediated by its resident muscle stem cells, also called satellite cells. Satellite cells, located periphery to the muscle fibers and underneath the basal lamina, are an indispensable cellular source for muscle regeneration. Satellite cell transplantation into regenerating muscle contributes robustly to muscle repair, thereby indicating that satellite cells indeed function as adult muscle stem cells. Moreover, satellite cells are a heterogenous population in adult tissue, with subpopulations that can be distinguished based on gene expression, cell-cycle progression, ability to self-renew, and bi-potential ability. Transplantation assays provide a powerful tool to better understand satellite cell function in vivo enabling the separation of functionally distinct satellite cell subpopulations. In this review, we focus on transplantation strategies to explore satellite cells’ functional heterogeneity, approaches targeting the recipient tissue to improve transplantation efficiency, and common strategies to monitor the behaviour of the transplanted cells. Lastly, we discuss some recent approaches to overcome challenges to enhance the transplantation potential of muscle stem cells.

Temporal disruption of neuromuscular communication and muscle atrophy following noninvasive ACL injury in rats

Many patients with anterior cruciate ligament (ACL) injuries have persistent quadriceps muscle atrophy, even after considerable time in rehabilitation. Understanding the factors that regulate muscle mass, and the time course of atrophic events, is important for identifying therapeutic interventions. With a noninvasive animal model of ACL injury, a longitudinal study was performed to elucidate key parameters underlying quadriceps muscle atrophy. Male Long-Evans rats were euthanized at 6, 12, 24, or 48 h or 1, 2, or 4 wk after ACL injury that was induced via tibial compression overload; controls were not injured. Vastus lateralis muscle size was determined by wet weight and fiber cross-sectional area (CSA). Evidence of disrupted neuromuscular communication was assessed via the expression of neural cell adhesion molecule (NCAM) and genes associated with denervation and neuromuscular junction instability. Abundance of muscle RING-finger protein-1 (MuRF-1), muscle atrophy F-box (MAFbx), and 45 s pre-rRNA along with 20S proteasome activity were determined to investigate mechanisms related to muscle atrophy. Finally, muscle damage-related parameters were assessed by measuring IgG permeability, centronucleation, CD68 mRNA, and satellite cell abundance. When compared with controls, we observed a greater percentage of NCAM-positive fibers at 6 h postinjury, followed by higher MAFbx abundance 48 h postinjury, and higher 20S proteasome activity at 1 wk postinjury. A loss of muscle wet weight, smaller fiber CSA, and the elevated expression of run-related transcription factor 1 (Runx1) were also observed at the 1 wk postinjury timepoint relative to controls. There also were no differences observed in any damage markers. These results indicate that alterations in neuromuscular communication precede the upregulation of atrophic factors that regulate quadriceps muscle mass early after noninvasive ACL injury.NEW & NOTEWORTHY A novel preclinical model of ACL injury was used to establish that acute disruptions in neuromuscular communication precede atrophic events. These data help to establish the time course of muscle atrophy after ACL injury, suggesting that clinical care may benefit from the application of acute neurogenic interventions and early gait reloading strategies.

Co-delivery of fibrin-laminin hydrogel with mesenchymal stem cell spheroids supports skeletal muscle regeneration following trauma

Volumetric muscle loss (VML) is traumatic or surgical loss of skeletal muscle with resultant functional impairment. Skeletal muscle's innate capacity for regeneration is lost with VML due to a critical loss of stem cells, extracellular matrix, and neuromuscular junctions. Consequences of VML include permanent disability or delayed amputations of the affected limb. Currently, a successful clinical therapy has not been identified. Mesenchymal stem cells (MSCs) possess regenerative and immunomodulatory properties and their three-dimensional aggregation can further enhance therapeutic efficacy. In this study, MSC aggregation into spheroids was optimized in vitro based on cellular viability, spheroid size, and trophic factor secretion. The regenerative potential of the optimized MSC spheroid therapy was then investigated in a murine model of VML injury. Experimental groups included an untreated VML injury control, intramuscular injection of MSC spheroids, and MSC spheroids encapsulated in a fibrin-laminin hydrogel. Compared to the untreated VML group, the spheroid encapsulating hydrogel group enhanced myogenic marker (i.e., MyoD and myogenin) protein expression, improved muscle mass, increased presence of centrally nucleated myofibers as well as small fibers (<500 μm² ), modulated pro- and anti-inflammatory macrophage marker expression (i.e., iNOS and Arginase), and increased the presence of CD146⁺ pericytes and CD31⁺ endothelial cells in the VML injured muscles. Future studies will evaluate the extent of functional recovery with the spheroid encapsulating hydrogel therapy.

Exercise enhances skeletal muscle regeneration by promoting senescence in fibro-adipogenic progenitors

Idiopathic inflammatory myopathies cause progressive muscle weakness and degeneration. Since high-dose glucocorticoids might not lead to full recovery of muscle function, physical exercise is also an important intervention, but some exercises exacerbate chronic inflammation and muscle fibrosis. It is unknown how physical exercise can have both beneficial and detrimental effects in chronic myopathy. Here we show that senescence of fibro-adipogenic progenitors (FAPs) in response to exercise-induced muscle damage is needed to establish a state of regenerative inflammation that induces muscle regeneration. In chronic inflammatory myopathy model mice, exercise does not promote FAP senescence or resistance against tumor necrosis factor–mediated apoptosis. Pro-senescent intervention combining exercise and pharmacological AMPK activation reverses FAP apoptosis resistance and improves muscle function and regeneration. Our results demonstrate that the absence of FAP senescence after exercise leads to muscle degeneration with FAP accumulation. FAP-targeted pro-senescent interventions with exercise and pharmacological AMPK activation may constitute a therapeutic strategy for chronic inflammatory myopathy.

Some exercises exacerbate chronic inflammation and muscle fibrosis in chronic myopathy. Here, the authors show that senescence of fibro-adipogenic progenitors (FAPs) in response to exercise induces muscle regeneration, and impaired FAP senescence worsens inflammation and fibrosis in chronic myopathy in mice.

Role of Metabolic Stress and Exercise in Regulating Fibro/Adipogenic Progenitors

Obesity is a major public health concern and is associated with decreased muscle quality (i.e., strength, metabolism). Muscle from obese adults is characterized by increases in fatty, fibrotic tissue that decreases the force producing capacity of muscle and impairs glucose disposal. Fibro/adipogenic progenitors (FAPs) are muscle resident, multipotent stromal cells that are responsible for muscle fibro/fatty tissue accumulation. Additionally, they are indirectly involved in muscle adaptation through their promotion of myogenic (muscle-forming) satellite cell proliferation and differentiation. In conditions similar to obesity that are characterized by chronic muscle degeneration, FAP dysfunction has been shown to be responsible for increased fibro/fatty tissue accumulation in skeletal muscle, and impaired satellite cell function. The role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle is just beginning to be unraveled. Thus, the present review aims to summarize the recent literature on the role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle, and the mechanisms responsible for these effects. Furthermore, we will review the role of physical activity in reversing or ameliorating the detrimental effects of obesity on FAP function.

Insulin-dependent Non-canonical Activation of Notch in Drosophila: A Story of Notch-Induced Muscle Stem Cell Proliferation

Notch plays multiple roles both in development and in adult tissue homeostasis. Notch was first identified in Drosophila in which it has then been extensively studied. Among the flag-ship Notch functions we could mention its capacity to keep precursor and stem cells in a nondifferentiated state but also its ability to activate cell proliferation that in some contexts could led to cancer. In general, both these functions involve, canonical, ligand-dependent Notch activation. However, a ligand-independent Notch activation has also been described in a few cellular contexts. Here, we focus on one of such contexts, Drosophila muscle stem cells, called AMPs, and discuss how insulin-dependent noncanonical activation of Notch pushes quiescent AMPs to proliferation.

The impact of skeletal muscle contraction on CD146+Lin- pericytes

Unaccustomed resistance exercise can initiate skeletal muscle remodeling and adaptive mechanisms that can confer protection from damage and enhanced strength with subsequent stimulation. The myofiber may provide the primary origin for adaptation, yet multiple mononuclear cell types within the surrounding connective tissue may also contribute. The purpose of this study was to evaluate the acute response of muscle-resident interstitial cells to contraction initiated by electrical stimulation (e-stim) and subsequently determine the contribution of pericytes to remodeling as a result of training. Mice were subjected to bilateral e-stim or sham treatment. Following a single session of e-stim, NG2+CD45-CD31- (NG2+Lin-) pericyte, CD146+Lin- pericyte, and PDGFRα+ fibroadipogenic progenitor cell quantity and function were evaluated via multiplex flow cytometry and targeted quantitative PCR. Relative quantity was not significantly altered 24 h postcontraction, yet unique gene signatures were observed for each cell population at 3 h postcontraction. CD146+Lin- pericytes appeared to be most responsive to contraction, and upregulation of genes related to immunomodulation and extracellular matrix remodeling was observed via RNA sequencing. Intramuscular injection of CD146+Lin- pericytes did not significantly increase myofiber size yet enhanced ECM remodeling and angiogenesis in response to repeated bouts of e-stim for 4 wk. The results from this study provide the first evidence that CD146+Lin- pericytes are responsive to skeletal muscle contraction and may contribute to the beneficial outcomes associated with exercise.

Massage increases satellite cell number independent of the age-associated alterations in sarcolemma permeability

Massage is a widely accepted manual therapy used to modulate the inflammatory response of muscle and restore function, but prolonged compression of muscle potentially causes overt injury and damage to muscle fibers. Therefore, a balance exists between the positive effects of massage and the induction of mechanical damage and injury. In addition, skeletal muscle of aged individuals displays increased stiffness, and therefore, the response to massage is likely different compared with young. We hypothesized that the aged skeletal muscle exhibits increased sarcolemmal permeability when subjected to massage compared with young skeletal muscle. Male Brown Norway/F344 rats, 10 and 30 months of age, were each divided into control, non-massaged (n = 8) and massaged (n = 8) groups. The right gastrocnemius muscle received one bout of cyclic compressive loading for 30 min at 4.5 N as a massage-mimetic. Muscles were dissected and frozen 24 h after massage. Alterations in sarcolemma permeability were quantified by measuring the level of intracellular IgG within the muscle fibers. Immunohistochemistry was performed to determine IgG inside fibers and Pax7+ cell number as an indicator of stem cell abundance. Average IgG intensity was not different between control and massaged animals at either age. However, a significant shift to the right of the density histogram indicated that massaged animals had more fibers with higher IgG intensity than control at 10 months. In addition, Pax7+ cell number was significantly elevated in massaged muscles compared with control at both ages. One bout of massage did not induce overt muscle injury, but facilitated membrane permeability, which was associated with an increase in satellite cell number. Data suggest that the load applied here, which was previously shown to induce immunomodulatory changes, does not induce overt muscle injury in young and old muscles but may result in muscle remodeling. Funded by NIH grant AG042699 and AT009268.

Decellularized extracellular matrix gelloids support mesenchymal stem cell growth and function in vitro

Volumetric muscle loss (VML) injuries are irrecoverable due to a significant loss of regenerative elements, persistent inflammation, extensive fibrosis, and functional impairment. When used in isolation, previous stem cell and biomaterial-based therapies have failed to regenerate skeletal muscle at clinically relevant levels. The extracellular matrix (ECM) microenvironment is crucial for the viability, stemness, and differentiation of stem cells. Decellularized-ECM (D-ECM) scaffolds are at the forefront of ongoing research to develop a viable therapy for VML. Due to the retention of key ECM components, D-ECM scaffolds provide an excellent substrate for the adhesion and migration of several cell types. Mesenchymal stem cells (MSCs) possess regenerative and immunomodulatory properties and are currently under investigation in clinical trials for a wide range of medical conditions. However, a major limitation to the use of MSCs in clinical applications is their poor viability at the site of transplantation. In this study, we have fabricated spherical scaffolds composed of gelatin and skeletal muscle D-ECM for the adhesion and delivery of MSCs to the site of VML injury. These spherical scaffolds termed "gelloids" supported MSC survival, expansion, trophic factor secretion, immunomodulation, and myogenic protein expression in vitro. Future studies would determine the therapeutic efficacy of this approach in a murine model of VML injury.

Biomaterial and stem cell-based strategies for skeletal muscle regeneration

Adult skeletal muscle can regenerate effectively after mild physical or chemical insult. Muscle trauma or disease can overwhelm this innate capacity for regeneration and result in heightened inflammation and fibrotic tissue deposition resulting in loss of structure and function. Recent studies have focused on biomaterial and stem cell-based therapies to promote skeletal muscle regeneration following injury and disease. Many stem cell populations besides satellite cells are implicated in muscle regeneration. These stem cells include but are not limited to mesenchymal stem cells, adipose-derived stem cells, hematopoietic stem cells, pericytes, fibroadipogenic progenitors, side population cells, and CD133⁺ stem cells. However, several challenges associated with their isolation, availability, delivery, survival, engraftment, and differentiation have been reported in recent studies. While acellular scaffolds offer a relatively safe and potentially off-the-shelf solution to cell-based therapies, they are often unable to stimulate host cell migration and activity to a level that would result in clinically meaningful regeneration of traumatized muscle. Combining stem cells and biomaterials may offer a viable therapeutic strategy that may overcome the limitations associated with these therapies when they are used in isolation. In this article, we review the stem cell populations that can stimulate muscle regeneration in vitro and in vivo. We also discuss the regenerative potential of combination therapies that utilize both stem cell and biomaterials for the treatment of skeletal muscle injury and disease. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1246-1262, 2019.

Diet-Induced Obesity Affects Muscle Regeneration After Murine Blunt Muscle Trauma-A Broad Spectrum Analysis

Injury to skeletal muscle affects millions of people worldwide. The underlying regenerative process however, is a very complex mechanism, time-wise highly coordinated, and subdivided in an initial inflammatory, a regenerative and a remodeling phase. Muscle regeneration can be impaired by several factors, among them diet-induced obesity (DIO). In order to evaluate if obesity negatively affects healing processes after trauma, we utilized a blunt injury approach to damage the extensor iliotibialis anticus muscle on the left hind limb of obese and normal weight C57BL/6J without showing any significant differences in force input between normal weight and obese mice. Magnetic resonance imaging (MRI) of the injury and regeneration process revealed edema formation and hemorrhage exudate in muscle tissue of normal weight and obese mice. In addition, morphological analysis of physiological changes revealed tissue necrosis, immune cell infiltration, extracellular matrix (ECM) remodeling, and fibrosis formation in the damaged muscle tissue. Regeneration was delayed in muscles of obese mice, with a higher incidence of fibrosis formation due to hampered expression levels of genes involved in ECM organization. Furthermore, a detailed molecular fingerprint in different stages of muscle regeneration underlined a delay or even lack of a regenerative response to injury in obese mice. A time-lapse heatmap determined 81 differentially expressed genes (DEG) with at least three hits in our model at all-time points, suggesting key candidates with a high impact on muscle regeneration. Pathway analysis of the DEG revealed five pathways with a high confidence level: myeloid leukocyte migration, regulation of tumor necrosis factor production, CD4-positive, alpha-beta T cell differentiation, ECM organization, and toll-like receptor (TLR) signaling. Moreover, changes in complement-, Wnt-, and satellite cell-related genes were found to be impaired in obese animals after trauma. Furthermore, histological satellite cell evaluation showed lower satellite cell numbers in the obese model upon injury. Ankrd1, C3ar1, Ccl8, Mpeg1, and Myog expression levels were also verified by qPCR. In summary, increased fibrosis formation, the reduction of Pax7⁺ satellite cells as well as specific changes in gene expression and signaling pathways could explain the delay of tissue regeneration in obese mice post trauma.

Muscle Stem/Progenitor Cells and Mesenchymal Stem Cells of Bone Marrow Origin for Skeletal Muscle Regeneration in Muscular Dystrophies

Muscular dystrophies represent a group of diseases which may develop in several forms, and severity of the disease is usually associated with gene mutations. In skeletal muscle regeneration and in muscular dystrophies, both innate and adaptive immune responses are involved. The regenerative potential of mesenchymal stem/stromal cells (MSCs) of bone marrow origin was confirmed by the ability to differentiate into diverse tissues and by their immunomodulatory and anti-inflammatory properties by secretion of a variety of growth factors and anti-inflammatory cytokines. Skeletal muscle comprises different types of stem/progenitor cells such as satellite cells and non-satellite stem cells including MSCs, interstitial stem cells positive for stress mediator PW1 expression and negative for PAX7 called PICs (PW1⁺/PAX7⁻ interstitial cells), fibro/adipogenic progenitors/mesenchymal stem cells, muscle side population cells and muscle resident pericytes, and all of them actively participate in the muscle regeneration process. In this review, we present biological properties of MSCs of bone marrow origin and a heterogeneous population of muscle-resident stem/progenitor cells, their interaction with the inflammatory environment of dystrophic muscle and potential implications for cellular therapies for muscle regeneration. Subsequently, we propose—based on current research results, conclusions, and our own experience—hypothetical mechanisms for modulation of the complete muscle regeneration process to treat muscular dystrophies.

Sports Injuries: Diagnosis, Prevention, Stem Cell Therapy, and Medical Sport Strategy

Sports injuries diagnosis, prevention, and treatment are the most important issues of sports medicine. Fortunately, sports injuries are often treated effectively, and people with damage recover and return to the sport in a satisfactory condition. Meanwhile, many sports injuries and complications can be prevented. In general, sports injuries include acute or chronic injuries. Given increasing in popularity, sports medicine doctors use stem cells to treat a wide variety of sports injuries, including damage to tendons, ligaments, muscles, and cartilage. Stem cell therapy to an injured area could be done through direct surgical application, stem-cell-bearing sutures, and injection. Stem cell therapy holds potential for repair and functional plasticity following sports injuries compared to traditional methods; however, the mechanism of stem cell therapy for sports injuries remains largely unknown. Medical imaging technologies provide the hope to ample the knowledge concerning basic stem cell biology in real time when transplanted into sport-induced damaged organs. Using stem cell treatment might restore continuity and regeneration and promote growth back the organ targets. Besides, using a noninvasive medical imaging method would have the long-time monitoring advantage to the stem cells transplanting individual. The multimodality imaging technique allows for studying acute pathological events following sports injuries; therefore, the use of imaging techniques in medicine permits the straight examination of dynamic regenerative events of specific stem cells following a sports injury in people.

Rehabilitation Considerations in Regenerative Medicine

Rehabilitation and regenerative medicine therapies has shown improved outcomes for tissue regeneration. Regenerative rehabilitation guides protocols regarding when to start therapy, types of stimuli administered, and graded exercise programs, taking into account biological factors and technologies designed to optimize healing potential. Although there are currently no evidence-based guidelines for rehabilitation, fundamental physical therapy principles likely apply. Immobilization tends to have deleterious effects on musculoskeletal tissues; mechanical loading promotes tissue healing and regeneration. Common physical therapy interventions may provide beneficial effects after the application of regenerative therapies. Research is needed to determine optimal rehabilitation protocols to enhance tissue healing and regeneration.

Influence of exercise and aging on extracellular matrix composition in the skeletal muscle stem cell niche

Skeletal muscle is endowed with a remarkable capacity for regeneration, primarily due to the reserve pool of muscle resident satellite cells. The satellite cell is the physiologically quiescent muscle stem cell that resides beneath the basal lamina and adjacent to the sarcolemma. The anatomic location of satellite cells is in close proximity to vasculature where they interact with other muscle resident stem/stromal cells (e.g., mesenchymal stem cells and pericytes) through paracrine mechanisms. This mini-review describes the components of the muscle stem cell niche, as well as the influence of exercise and aging on the muscle stem cell niche. Although exercise promotes ECM reorganization and stem cell accumulation, aging is associated with dense ECM deposition and loss of stem cell function resulting in reduced regenerative capacity and strength. An improved understanding of the niche elements will be valuable to inform the development of therapeutic interventions aimed at improving skeletal muscle regeneration and adaptation over the life span.

High Fat Diet-Induced Skeletal Muscle Wasting Is Decreased by Mesenchymal Stem Cells Administration: Implications on Oxidative Stress, Ubiquitin Proteasome Pathway Activation, and Myonuclear Apoptosis

Obesity can lead to skeletal muscle atrophy, a pathological condition characterized by the loss of strength and muscle mass. A feature of muscle atrophy is a decrease of myofibrillar proteins as a result of ubiquitin proteasome pathway overactivation, as evidenced by increased expression of the muscle-specific ubiquitin ligases atrogin-1 and MuRF-1. Additionally, other mechanisms are related to muscle wasting, including oxidative stress, myonuclear apoptosis, and autophagy. Stem cells are an emerging therapy in the treatment of chronic diseases such as high fat diet-induced obesity. Mesenchymal stem cells (MSCs) are a population of self-renewable and undifferentiated cells present in the bone marrow and other mesenchymal tissues of adult individuals. The present study is the first to analyze the effects of systemic MSC administration on high fat diet-induced skeletal muscle atrophy in the tibialis anterior of mice. Treatment with MSCs reduced losses of muscle strength and mass, decreases of fiber diameter and myosin heavy chain protein levels, and fiber type transitions. Underlying these antiatrophic effects, MSC administration also decreased ubiquitin proteasome pathway activation, oxidative stress, and myonuclear apoptosis. These results are the first to indicate that systemically administered MSCs could prevent muscle wasting associated with high fat diet-induced obesity and diabetes.

Transcriptomic Analysis of Adult Renal Derived Mesenchymal Stem-Like Cells

Mesenchymal stem cells (MSC) from bone marrow or adult tissues are widely studied to evaluate their potential for tissue repair. Differences in tissue of origin, donor variation, or in vitro handling exist and it is still unclear how they affect cell function and regenerative potential. Large-scale gene expression analysis of these cells not only allows researchers to compare and contrast the differences between each MSC subset but also allows for the development of better analytical tools for their characterization and utilization. Here, we describe a protocol for transcriptomics analysis of MSC-like cells derived from adult kidneys.

Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention

Identification of cost-effective interventions to maintain muscle mass, muscle strength, and physical performance during muscle wasting and aging is an important public health challenge. It requires understanding of the cellular and molecular mechanisms involved. Muscle-deconditioning processes have been deciphered by means of several experimental models, bringing together the opportunities to devise comprehensive analysis of muscle wasting. Studies have increasingly recognized the importance of fatty infiltrations or intermuscular adipose tissue for the age-mediated loss of skeletal-muscle function and emphasized that this new important factor is closely linked to inactivity. The present review aims to address three main points. We first mainly focus on available experimental models involving cell, animal, or human experiments on muscle wasting. We next point out the role of intermuscular adipose tissue in muscle wasting and aging and try to highlight new findings concerning aging and muscle-resident mesenchymal stem cells called fibro/adipogenic progenitors by linking some cellular players implicated in both FAP fate modulation and advancing age. In the last part, we review the main data on the efficiency and molecular and cellular mechanisms by which exercise, replacement hormone therapies, and β-hydroxy-β-methylbutyrate prevent muscle wasting and sarcopenia. Finally, we will discuss a potential therapeutic target of sarcopenia: glucose 6-phosphate dehydrogenase.

Pathways Implicated in Tadalafil Amelioration of Duchenne Muscular Dystrophy

Numerous therapeutic approaches for Duchenne and Becker Muscular Dystrophy (DMD and BMD), the most common X-linked muscle degenerative disease, have been proposed. So far, the only one showing a clear beneficial effect is the use of corticosteroids. Recent evidence indicates an improvement of dystrophic cardiac and skeletal muscles in the presence of sustained cGMP levels secondary to a blocking of their degradation by phosphodiesterase five (PDE5). Due to these data, we performed a study to investigate the effect of the specific PDE5 inhibitor, tadalafil, on dystrophic skeletal muscle function. Chronic pharmacological treatment with tadalafil has been carried out in mdx mice. Behavioral and physiological tests, as well as histological and biochemical analyses, confirmed the efficacy of the therapy. We then performed a microarray-based genomic analysis to assess the pattern of gene expression in muscle samples obtained from the different cohorts of animals treated with tadalafil. This scrutiny allowed us to identify several classes of modulated genes. Our results show that PDE5 inhibition can ameliorate dystrophy by acting at different levels. Tadalafil can lead to (1) increased lipid metabolism; (2) a switch towards slow oxidative fibers driven by the up-regulation of PGC-1α; (3) an increased protein synthesis efficiency; (4) a better actin network organization at Z-disk.

Mesenchymal Stromal Cells and Tissue-Specific Progenitor Cells: Their Role in Tissue Homeostasis

Multipotent mesenchymal stromal/stem cells (MSCs) reside in many human organs and comprise heterogeneous population of cells with self-renewal ability. These cells can be isolated from different tissues, and their morphology, immunophenotype, and differentiation potential are dependent on their tissue of origin. Each organ contains specific population of stromal cells which maintain regeneration process of the tissue where they reside, but some of them have much more wide plasticity and differentiate into multiple cells lineage. MSCs isolated from adult human tissues are ideal candidates for tissue regeneration and tissue engineering. However, MSCs do not only contribute to structurally tissue repair but also MSC possess strong immunomodulatory and anti-inflammatory properties and may influence in tissue repair by modulation of local environment. This paper is presenting an overview of the current knowledge of biology of tissue-resident mesenchymal stromal and progenitor cells (originated from bone marrow, liver, skeletal muscle, skin, heart, and lung) associated with tissue regeneration and tissue homeostasis.

Impact of tissue-specific stem cells on lineage-specific differentiation: a focus on the musculoskeletal system

Tissue-specific stem cells are found throughout the body and, with proper intervention and environmental cues, these stem cells exercise their capabilities for differentiation into several lineages to form cartilage, bone, muscle, and adipose tissue in vitro and in vivo. Interestingly, it has been widely demonstrated that they do not differentiate with the same efficacy during lineage-specific differentiation studies, as the tissue-specific stem cells are generally more effective when differentiating toward the tissues from which they were derived. This review focuses on four mesodermal lineages for tissue-specific stem cell differentiation: adipogenesis, chondrogenesis, myogenesis, and osteogenesis. It is intended to give insight into current multilineage differentiation and comparative research, highlight and contrast known trends regarding differentiation, and introduce supporting evidence which demonstrates particular tissue-specific stem cells' superiority in lineage-specific differentiation, along with their resident tissue origins and natural roles. In addition, some epigenetic and transcriptomic differences between stem cells which may explain the observed trends are discussed.

Muscle Regeneration with Intermuscular Adipose Tissue (IMAT) Accumulation Is Modulated by Mechanical Constraints

Sports trauma are able to induce muscle injury with fibrosis and accumulation of intermuscular adipose tissue (IMAT), which affect muscle function. This study was designed to investigate whether hypoactivity would influence IMAT accumulation in regenerating mouse skeletal muscle using the glycerol model of muscle regeneration. The animals were immediately hindlimb unloaded for 21 days after glycerol injection into the tibialis anterior (TA) muscle. Muscle fiber and adipocyte cross-sectional area (CSA) and IMAT accumulation were determined by histomorphometric analysis. Adipogenesis during regenerative processes was examined using RT-qPCR and Western blot quantification. Twenty-one days of hindlimb unloading resulted in decreases of 38% and 50.6% in the muscle weight/body weight ratio and CSA, respectively, in soleus muscle. Glycerol injection into TA induced IMAT accumulation, reaching 3% of control normal-loading muscle area. This IMAT accumulation was largely inhibited in unloading conditions (0.09%) and concomitant with a marked reduction in perilipin and FABP4 protein content, two key markers of mature adipocytes. Induction of PPARγ and C/EBPα mRNA, two markers of adipogenesis, was also decreased. Furthermore, the protein expression of PDGFRα, a cell surface marker of fibro/adipogenic progenitors, was much lower in regenerating TA from the unloaded group. Exposure of regenerating muscle to hypoactivity severely reduces IMAT development and accumulation. These results provide new insight into the mechanisms regulating IMAT development in skeletal muscle and highlight the importance of taking into account the level of mechanical constraint imposed on skeletal muscle during the regeneration processes.

17β-Estradiol and testosterone in sarcopenia: Role of satellite cells

The loss of muscle mass and strength with aging, referred to as sarcopenia, is a prevalent condition among the elderly. Although the molecular mechanisms underlying sarcopenia are unclear, evidence suggests that an age-related acceleration of myocyte loss via apoptosis might be responsible for muscle perfomance decline. Interestingly, sarcopenia has been associated to a deficit of sex hormones which decrease upon aging. The skeletal muscle ability to repair and regenerate itself would not be possible without satellite cells, a subpopulation of cells that remain quiescent throughout life. They are activated in response to stress, enabling them to guide skeletal muscle regeneration. Thus, these cells could be a key factor to overcome sarcopenia. Of importance, satellite cells are 17β-estradiol (E2) and testosterone (T) targets. In this review, we summarize potential mechanisms through which these hormones regulate satellite cells activation during skeletal muscle regeneration in the elderly. The advance in its understanding will help to the development of potential therapeutic agents to alleviate and treat sarcopenia and other related myophaties.

Pericyte response to contraction mode-specific resistance exercise training in human skeletal muscle

Skeletal muscle satellite cells (SCs) are important for muscle repair and hypertrophy in response mechanical stimuli. Neuron-glial antigen 2-positive (NG2(+)) and alkaline phosphatase-positive (ALP(+)) pericytes may provide an alternative source of myogenic progenitors and/or secrete paracrine factors to induce Pax7(+) SC proliferation and differentiation. The purpose of this study was to investigate NG2(+) and ALP(+) cell quantity, as well as SC content and activation, in human skeletal muscle following prolonged concentric (Conc) or eccentric (Ecc) resistance training. Male subjects engaged in unilateral resistance training utilizing isolated Ecc or Conc contractions. After 12 wk, muscle biopsies were analyzed for NG2(+) and ALP(+) pericytes, total Pax7(+) SCs, activated SCs (Pax7(+)MyoD(+)), and differentiating myogenic cells (Pax7(-) MyoD(+)). NG2(+) cells localized to CD31(+) vessels and the majority coexpressed ALP. NG2(+) pericyte quantity decreased following both Conc and Ecc training (P < 0.05). ALP(+) pericyte quantity declined following Conc (P < 0.05) but not Ecc training. Conversely, total Pax7(+) SC content was elevated following Conc only (P < 0.001), while Pax7(+)MyoD(+) SC content was increased following Conc and Ecc (P < 0.001). Follow up analyses demonstrated that CD90(+) and platelet-derived growth factor receptor-α (PDGFRα)(+) mononuclear cell proliferation was also increased in response to both Conc and Ecc training (P < 0.01). In summary, resistance training results in a decline in pericyte quantity and an increase in mesenchymal progenitor cell proliferation, and these events likely influence SC pool expansion and increased activation observed posttraining.

Exercise and Stem Cells

Stem cells are traditionally studied in the context of embryonic development, yet studies confirm that a fraction remains in the adult organism for the purpose of daily remodeling and rejuvenation of multiple tissues following injury. Adult stem cells (ASCs) are found in close proximity to vessels and respond to tissue-specific cues in the microenvironment that dictate their fate and function. Exercise can dramatically alter strain sensing, extracellular matrix composition, and inflammation, and such changes in the niche likely alter ASC quantity and function postexercise. The field of stem cell biology is still in its infancy and identification and terminology of ASCs continues to evolve; thus, current information regarding exercise and stem cells is lacking. This chapter summarizes the literature that reports on the ASC response to acute exercise and exercise training, with particular emphasis on hematopoietic stem cells, endothelial progenitor cells, and mesenchymal stem cells.

The acute response of pericytes to muscle-damaging eccentric contraction and protein supplementation in human skeletal muscle

Skeletal muscle pericytes increase in quantity following eccentric exercise (ECC) and contribute to myofiber repair and adaptation in mice. The purpose of the present investigation was to examine pericyte quantity in response to muscle-damaging ECC and protein supplementation in human skeletal muscle. Male subjects were divided into protein supplement (WHY; n = 12) or isocaloric placebo (CHO; n = 12) groups and completed ECC using an isokinetic dynamometer. Supplements were consumed 3 times/day throughout the experimental time course. Biopsies were collected prior to (PRE) and 3, 24, 48, and 168 h following ECC. Reflective of the damaging protocol, integrin subunits, including α7, β1A, and β1D, increased (3.8-fold, 3.6-fold and 3.9-fold, respectively, P < 0.01) 24 h post-ECC with no difference between supplements. Pericyte quantity did not change post-ECC. WHY resulted in a small, but significant, decrease in ALP(+) pericytes when expressed as a percentage of myonuclei (CHO 6.8 ± 0.3% vs. WHY 5.8 ± 0.3%, P < 0.05) or per myofiber (CHO 0.119 ± 0.01 vs. WHY 0.098 ± 0.01, P < 0.05). The quantity of myonuclei expressing serum response factor and the number of pericytes expressing serum response factor, did not differ as a function of time post-ECC or supplement. These data demonstrate that acute muscle-damaging ECC increases α7β1 integrin content in human muscle, yet pericyte quantity is largely unaltered. Future studies should focus on the capacity for ECC to influence pericyte function, specifically paracrine factor release as a mechanism toward pericyte contribution to repair and adaptation postexercise.

Return to sport after muscle injury

Skeletal muscle injuries are among the most common sports-related injuries that result in time lost from practice and competition. The cellular response to muscle injury can often result in changes made to the muscle fibers as well as the surrounding extracellular matrix during repair. This can negatively affect the force and range of the injured muscle even after the patient's return to play. Diagnosis of skeletal muscle injury involves both history and physical examinations; imaging modalities including ultrasound and magnetic resonance imaging (MRI) can also be used to assess the extent of injury. Current research is investigating potential methods, including clinical factors and MRI, by which to predict a patient's return to sports. Overall, function of acutely injured muscles seems to improve with time. Current treatment methods for skeletal muscle injuries include injections of steroids, anesthetics, and platelet-rich plasma (PRP). Other proposed methods involve inhibitors of key players in fibrotic pathways, such as transforming growth factor (TGF)-ß and angiotensin II, as well as muscle-derived stem cells.

Pericyte NF-κB activation enhances endothelial cell proliferation and proangiogenic cytokine secretion in vitro

Pericytes are skeletal muscle resident, multipotent stem cells that are localized to the microvasculature. In vivo, studies have shown that they respond to damage through activation of nuclear-factor kappa-B (NF-κB), but the downstream effects of NF-κB activation on endothelial cell proliferation and cell-cell signaling during repair remain unknown. The purpose of this study was to examine pericyte NF-κB activation in a model of skeletal muscle damage; and use genetic manipulation to study the effects of changes in pericyte NF-κB activation on endothelial cell proliferation and cytokine secretion. We utilized scratch injury to C2C12 cells in coculture with human primary pericytes to assess NF-κB activation and monocyte chemoattractant protein-1 (MCP-1) secretion from pericytes and C2C12 cells. We also cocultured endothelial cells with pericytes that expressed genetically altered NF-κB activation levels, and then quantified endothelial cell proliferation and screened the conditioned media for secreted cytokines. Pericytes trended toward greater NF-κB activation in injured compared to control cocultures (P = 0.085) and in comparison to C2C12 cells (P = 0.079). Second, increased NF-κB activation in pericytes enhanced the proliferation of cocultured endothelial cells (1.3-fold, P = 0.002). Finally, we identified inflammatory signaling molecules, including MCP-1 and interleukin 8 (IL-8) that may mediate the crosstalk between pericytes and endothelial cells. The results of this study show that pericyte NF-κB activation may be an important mechanism in skeletal muscle repair with implications for the development of therapies for musculoskeletal and vascular diseases, including peripheral artery disease.

Transcriptome analysis of cattle muscle identifies potential markers for skeletal muscle growth rate and major cell types

BACKGROUND

This study aimed to identify markers for muscle growth rate and the different cellular contributors to cattle muscle and to link the muscle growth rate markers to specific cell types.

RESULTS

The expression of two groups of genes in the longissimus muscle (LM) of 48 Brahman steers of similar age, significantly enriched for "cell cycle" and "ECM (extracellular matrix) organization" Gene Ontology (GO) terms was correlated with average daily gain/kg liveweight (ADG/kg) of the animals. However, expression of the same genes was only partly related to growth rate across a time course of postnatal LM development in two cattle genotypes, Piedmontese x Hereford (high muscling) and Wagyu x Hereford (high marbling). The deposition of intramuscular fat (IMF) altered the relationship between the expression of these genes and growth rate. K-means clustering across the development time course with a large set of genes (5,596) with similar expression profiles to the ECM genes was undertaken. The locations in the clusters of published markers of different cell types in muscle were identified and used to link clusters of genes to the cell type most likely to be expressing them. Overall correspondence between published cell type expression of markers and predicted major cell types of expression in cattle LM was high. However, some exceptions were identified: expression of SOX8 previously attributed to muscle satellite cells was correlated with angiogenesis. Analysis of the clusters and cell types suggested that the "cell cycle" and "ECM" signals were from the fibro/adipogenic lineage. Significant contributions to these signals from the muscle satellite cells, angiogenic cells and adipocytes themselves were not as strongly supported. Based on the clusters and cell type markers, sets of five genes predicted to be representative of fibro/adipogenic precursors (FAPs) and endothelial cells, and/or ECM remodelling and angiogenesis were identified.

CONCLUSIONS

Gene sets and gene markers for the analysis of many of the major processes/cell populations contributing to muscle composition and growth have been proposed, enabling a consistent interpretation of gene expression datasets from cattle LM. The same gene sets are likely to be applicable in other cattle muscles and in other species.

Myospheres are composed of two cell types: one that is myogenic and a second that is mesenchymal

Previously, in an attempt to isolate stem cells that would be capable of regenerating injured skeletal muscle, we cultured cells derived from muscle, non-adherently, in serum-free media. As a result of the culture conditions used, these cells formed spheres, and thus were referred to as myospheres. It was found that myosphere-derived cells expressed Sca-1, a marker that is not typically associated with myogenic cells, and as a result has generated some questions as to the origin of these cells. The goal of this study was to clearly determine the origin of myosphere-derived cells, and in particular to answer the question of whether myospheres contain myogenic cells. To determine if myospheres were composed of myogenic cells without altering the structure of myospheres or the culture conditions used to maintain myospheres, I isolated these cells from yellow fluorescent protein (YFP)-Myf5, YFP-MyoD, and ZsGreen-Pax7 lineage-tracing mice and monitored their growth over time. I found that myospheres do contain myogenic cells, but that these cells are gradually lost over time (within 2 months). Additionally, the use of the lineage-tracing mice gave an interesting perspective into the composition of myospheres. I found that myospheres were composed of two distinct cell types, one that is myogenic (α7 integrin+) and contains cells expressing Myf5, MyoD, and Pax7, and a second that is non-myogenic (α7 integrin-) expressing platelet-derived growth factor receptor alpha (PDGFRα) and Sca-1, both of which have been associated with fibro/adipocyte mesenchymal cells.

Pericytes: multitasking cells in the regeneration of injured, diseased, and aged skeletal muscle

Pericytes are perivascular cells that envelop and make intimate connections with adjacent capillary endothelial cells. Recent studies show that they may have a profound impact in skeletal muscle regeneration, innervation, vessel formation, fibrosis, fat accumulation, and ectopic bone formation throughout life. In this review, we summarize and evaluate recent advances in our understanding of pericytes' influence on adult skeletal muscle pathophysiology. We also discuss how further elucidating their biology may offer new approaches to the treatment of conditions characterized by muscle wasting.

Influencing the secretion of myogenic factors from mesenchymal stem cells

Mounting evidence indicates that the regenerative effect of mesenchymal stem cells in skeletal muscle is related to the secretion of factors that stimulate resident myogenic cells. However, the environmental cues that affect the secreted factors of mesenchymal stem cells are not well understood. A recent publication demonstrated that secretion of factors is dependent on cell substrate, with mesenchymal stem cells grown on laminin providing more pro-myogenic factors than those grown on collagen, and that cellular strain may also play a role. Conditioned media from mesenchymal stem cells grown on laminin and subjected to strain provided the quickest and largest stimulation to myogenic cell proliferation. The influence of cell substrate and mechanical perturbation on mesenchymal stem cells therefore appears key to secretion of factors that support myogenesis.

Substrate and strain alter the muscle-derived mesenchymal stem cell secretome to promote myogenesis

INTRODUCTION

Mesenchymal stem cells (MSCs) reside in a variety of tissues and provide a stromal role in regulating progenitor cell function. Current studies focus on identifying the specific factors in the niche that can alter the MSC secretome, ultimately determining the effectiveness and timing of tissue repair. The purpose of the present study was to evaluate the extent to which substrate and mechanical strain simultaneously regulate MSC quantity, gene expression, and secretome.

METHODS

MSCs (Sca-1+CD45-) isolated from murine skeletal muscle (muscle-derived MSCs, or mMSCs) via fluorescence-activated cell sorting were seeded onto laminin (LAM)- or collagen type 1 (COL)-coated membranes and exposed to a single bout of mechanical strain (10%, 1 Hz, 5 hours).

RESULTS

mMSC proliferation was not directly affected by substrate or strain; however, gene expression of growth and inflammatory factors and extracellular matrix (ECM) proteins was downregulated in mMSCs grown on COL in a manner independent of strain. Focal adhesion kinase (FAK) may be involved in substrate regulation of mMSC secretome as FAK phosphorylation was significantly elevated 24 hours post-strain in mMSCs plated on LAM but not COL (P <0.05). Conditioned media (CM) from mMSCs exposed to both LAM and strain increased myoblast quantity 5.6-fold 24 hours post-treatment compared with myoblasts treated with serum-free media (P <0.05). This response was delayed in myoblasts treated with CM from mMSCs grown on COL.

CONCLUSIONS

Here, we demonstrate that exposure to COL, the primary ECM component associated with tissue fibrosis, downregulates genes associated with growth and inflammation in mMSCs and delays the ability for mMSCs to stimulate myoblast proliferation.

Are we closer to having drugs to treat muscle wasting disease?

The two most common muscle wasting diseases in adults are sarcopenia and cachexia. Despite differences in their pathophysiology, it is believed that both conditions are likely to respond to drugs that increase muscle mass and muscle strength. The current gold standard in this regard is exercise training. This article provides an overview of candidate drugs to treat muscle wasting disease that are available or in development. Drugs highlighted here include ghrelin agonists, selective androgen receptor molecules, megestrol acetate, activin receptor antagonists, espindolol, and fast skeletal muscle troponin inhibitors.

Skeletal muscle wasting with disuse atrophy is multi-dimensional: the response and interaction of myonuclei, satellite cells and signaling pathways

Maintenance of skeletal muscle is essential for health and survival. There are marked losses of skeletal muscle mass as well as strength and physiological function under conditions of low mechanical load, such as space flight, as well as ground based models such as bed rest, immobilization, disuse, and various animal models. Disuse atrophy is caused by mechanical unloading of muscle and this leads to reduced muscle mass without fiber attrition. Skeletal muscle stem cells (satellite cells) and myonuclei are integrally involved in skeletal muscle responses to environmental changes that induce atrophy. Myonuclear domain size is influenced differently in fast and slow twitch muscle, but also by different models of muscle wasting, a factor that is not yet understood. Although the myonuclear domain is 3-dimensional this is rarely considered. Apoptosis as a mechanism for myonuclear loss with atrophy is controversial, whereas cell death of satellite cells has not been considered. Molecular signals such as myostatin/SMAD pathway, MAFbx, and MuRF1 E3 ligases of the ubiquitin proteasome pathway and IGF1-AKT-mTOR pathway are 3 distinctly different contributors to skeletal muscle protein adaptation to disuse. Molecular signaling pathways activated in muscle fibers by disuse are rarely considered within satellite cells themselves despite similar exposure to unloading or low mechanical load. These molecular pathways interact with each other during atrophy and also when various interventions are applied that could alleviate atrophy. Re-applying mechanical load is an obvious method to restore muscle mass, however how nutrient supplementation (e.g., amino acids) may further enhance recovery (or reduce atrophy despite unloading or ageing) is currently of great interest. Satellite cells are particularly responsive to myostatin and to growth factors. Recently, the hibernating squirrel has been identified as an innovative model to study resistance to atrophy.