Intrinsic Changes and Extrinsic Influences of Myogenic Stem Cell Function During Aging

The myogenic stem cell (satellite cell) is almost solely responsible for the remarkable regeneration of adult skeletal muscle fibers after injury. The availability and the functionality of satellite cells are the determinants of efficient muscle regeneration. During aging, the efficiency of muscle regeneration declines, suggesting that the functionality of satellite cells and their progeny may be altered. Satellite cells do not sit in isolation but rather are surrounded by, and influenced by, many extrinsic factors within the muscle tissue that can alter their functionality. These factors likely change during aging and impart both reversible and irreversible changes to the satellite cells and on their proliferating progeny. In this review, we discuss the possible mechanisms of impaired muscle regeneration with respect to the biology of satellite cells. Future studies that enhance our understanding of the interactions between stem cells and the environment in which they reside will offer promise for therapeutic applications in age-related diseases.     

miRNAs在骨骼肌损伤修复过程中的调控作用

不合理的运动方式经常会引起骨骼肌损伤,骨骼肌损伤后,成体生肌性干细胞(主要是卫星细胞)被激活,进而增殖分化,与原来肌纤维融合完成修复过程。miRNAs是一种在后转录水平调节基因表达的非编码RNAs,在骨骼肌损伤与修复过程中主要通过靶向抑制一些转录因子、转录激活途径关键酶、细胞通讯连接重要蛋白以及各种信号通路中关键蛋白的翻译而发挥调控作用。为了更好地了解miRNAs在骨骼肌损伤修复过程中的作用,本文通过对miRNAs在骨骼肌损伤与修复过程中卫星细胞不同生物学状态以及信号通路等方面的作用和研究现状做一综述。     

Stem cell aging in the skeletal muscle: The importance of communication

Adult stem cells sustain tissue homeostasis and regeneration; their functional decline is often linked to aging, which is characterized by the progressive loss of physiological functions across multiple tissues and organs. The resident stem cells in skeletal muscle, termed satellite cells, are normally quiescent but activate upon injury to reconstitute the damaged tissue. In this review, we discuss the current understanding of the molecular processes that contribute to the functional failure of satellite cells during aging. This failure is due not only to intrinsic changes but also to extrinsic factors, most of which are still undefined but originate from the muscle tissue microenvironment of the satellite cells (the niche), or from the systemic environment. We also highlight the emerging applications of the powerful single-cell sequencing technologies in the study of skeletal muscle aging, particularly in the heterogeneity of the satellite cell population and the molecular interaction of satellite cells and other cell types in the niche. An improved understanding of how satellite cells communicate with their environment, and how this communication is perturbed with aging, will be helpful for defining countermeasures against loss of muscle regenerative capacity in sarcopenia.     

The origin, molecular regulation and therapeutic potential of myogenic stem cell populations

Satellite cells, originating in the embryonic dermamyotome, reside beneath the myofibre of mature adult skeletal muscle and constitute the tissue-specific stem cell population. Recent advances following the identification of markers for these cells (including Pax7, Myf5, c-Met and CD34) (CD, cluster of differentiation; c-Met, mesenchymal epithelial transition factor) have led to a greater understanding of the role played by satellite cells in the regeneration of new skeletal muscle during growth and following injury. In response to muscle damage, satellite cells harbour the ability both to form myogenic precursors and to self-renew to repopulate the stem cell niche following myofibre damage. More recently, other stem cell populations including bone marrow stem cells, skeletal muscle side population cells and mesoangioblasts have also been shown to have myogenic potential in culture, and to be able to form skeletal muscle myofibres in vivo and engraft into the satellite cell niche. These cell types, along with satellite cells, have shown potential when used as a therapy for skeletal muscle wasting disorders where the intrinsic stem cell population is genetically unable to repair non-functioning muscle tissue. Accurate understanding of the mechanisms controlling satellite cell lineage progression and self-renewal as well as the recruitment of other stem cell types towards the myogenic lineage is crucial if we are to exploit the power of these cells in combating myopathic conditions. Here we highlight the origin, molecular regulation and therapeutic potential of all the major cell types capable of undergoing myogenic differentiation and discuss their potential therapeutic application.     

The muscle stem cell niche: regulation of satellite cells during regeneration

Satellite cells are considered to be adult skeletal muscle stem cells. Their ability to regenerate large muscle defects is highly dependent on their specific niche. When these cells are cultured in vitro, the loss of this niche leads to a loss of proliferative capacity and defective regeneration when implanted back into a muscle defect. The most important aspects of the niche will be discussed--in particular, the basement membrane, the niche's mechanical properties, its supporting cells, and the influence these features have on satellite cell activation, proliferation, and differentiation. Understanding more about the control of these satellite cell activities by the niche will facilitate their recruitment and effective deployment for regenerative medicine.     

The Origin and Fate of Muscle Satellite Cells

Satellite cells represent the primary population of stem cells resident in skeletal muscle. These adult muscle stem cells facilitate the postnatal growth, remodeling, and regeneration of skeletal muscle. Given the remarkable regenerative potential of satellite cells, there is great promise for treatment of muscle pathologies such as the muscular dystrophies with this cell population. Various protocols have been developed which allow for isolation, enrichment, and expansion of satellite cell derived muscle stem cells. However, isolated satellite cells have yet to translate into effective modalities for therapeutic intervention. Broadening our understanding of satellite cells and their niche requirements should improve our in vivo and ex vivo manipulation of these cells to expedite their use for regeneration of diseased muscle. This review explores the fates of satellite cells as determined by their molecular signatures, ontogeny, and niche dependent programming.     

A population of myogenic stem cells that survives skeletal muscle aging

Age-related decline in integrity and function of differentiated adult tissues is widely attributed to reduction in number or regenerative potential of resident stem cells. The satellite cell, resident beneath the basal lamina of skeletal muscle myofibers, is the principal myogenic stem cell. Here we have explored the capacity of satellite cells within aged mouse muscle to regenerate skeletal muscle and to self-renew using isolated myofibers in tissue culture and in vivo. Satellite cells expressing Pax7 were depleted from aged muscles, and when aged myofibers were placed in culture, satellite cell myogenic progression resulted in apoptosis and fewer total differentiated progeny. However, a minority of cultured aged satellite cells generated large clusters of progeny containing both differentiated cells and new cells of a quiescent satellite-cell-like phenotype characteristic of self-renewal. Parallel in vivo engraftment assays showed that, despite the reduction in Pax7(+) cells, the satellite cell population associated with individual aged myofibers could regenerate muscle and self-renew as effectively as the larger population of satellite cells associated with young myofibers. We conclude that a minority of satellite cells is responsible for adult muscle regeneration, and that these stem cells survive the effects of aging to retain their intrinsic potential throughout life. Thus, the effectiveness of stem-cell-mediated muscle regeneration is determined by both extrinsic environmental influences and diversity in intrinsic potential of the stem cells themselves.     

Cellular and molecular regulation of muscle regeneration

Under normal circumstances, mammalian adult skeletal muscle is a stable tissue with very little turnover of nuclei. However, upon injury, skeletal muscle has the remarkable ability to initiate a rapid and extensive repair process preventing the loss of muscle mass. Skeletal muscle repair is a highly synchronized process involving the activation of various cellular responses. The initial phase of muscle repair is characterized by necrosis of the damaged tissue and activation of an inflammatory response. This phase is rapidly followed by activation of myogenic cells to proliferate, differentiate, and fuse leading to new myofiber formation and reconstitution of a functional contractile apparatus. Activation of adult muscle satellite cells is a key element in this process. Muscle satellite cell activation resembles embryonic myogenesis in several ways including the de novo induction of the myogenic regulatory factors. Signaling factors released during the regenerating process have been identified, but their functions remain to be fully defined. In addition, recent evidence supports the possible contribution of adult stem cells in the muscle regeneration process. In particular, bone marrow-derived and muscle-derived stem cells contribute to new myofiber formation and to the satellite cell pool after injury.     

Drosophila as a model of wound healing and tissue regeneration in vertebrates

Understanding the molecular basis of wound healing and regeneration in vertebrates is one of the main challenges in biology and medicine. This understanding will lead to medical advances allowing accelerated tissue repair after wounding, rebuilding new tissues/organs and restoring homeostasis. Drosophila has emerged as a valuable model for studying these processes because the genetic networks and cytoskeletal machinery involved in epithelial movements occurring during embryonic dorsal closure, larval imaginal disc fusion/regeneration, and epithelial repair are similar to those acting during wound healing and regeneration in vertebrates. Recent studies have also focused on the use of Drosophila adult stem cells to maintain tissue homeostasis. Here, we review how Drosophila has contributed to our understanding of these processes, primarily through live-imaging and genetic tools that are impractical in mammals. Furthermore, we highlight future research areas where this insect may provide novel insights and potential therapeutic strategies for wound healing and regeneration.     

Assessment of the myogenic stem cell compartment following transplantation of Pax3/Pax7-induced embryonic stem cell-derived progenitors

An effective long-term cell therapy for skeletal muscle regeneration requires donor contribution to both muscle fibers and the muscle stem cell pool. Although satellite cells have these abilities, their therapeutic potential so far has been limited due to their scarcity in adult muscle. Myogenic progenitors obtained from Pax3-engineered mouse embryonic stem (ES) cells have the ability to generate myofibers and to improve the contractility of transplanted muscles in vivo, however, whether these cells contribute to the muscle stem cell pool and are able to self-renew in vivo are still unknown. Here, we addressed this question by investigating the ability of Pax3, which plays a critical role in embryonic muscle formation, and Pax7, which is important for maintenance of the muscle satellite cell pool, to promote the derivation of self-renewing functional myogenic progenitors from ES cells. We show that Pax7, like Pax3, can drive the expansion of an ES-derived myogenic progenitor with significant muscle regenerative potential. We further demonstrate that a fraction of transplanted cells remains mononuclear, and displays key features of skeletal muscle stem cells, including satellite cell localization, response to reinjury, and contribution to muscle regeneration in secondary transplantation assays. The ability to engraft, self-renew, and respond to injury provide foundation for the future therapeutic application of ES-derived myogenic progenitors in muscle disorders.     

CD34 promotes satellite cell motility and entry into proliferation to facilitate efficient skeletal muscle regeneration

Expression of the cell surface sialomucin CD34 is common to many adult stem cell types, including muscle satellite cells. However, no clear stem cell or regeneration-related phenotype has ever been reported in mice lacking CD34, and its function on these cells remains poorly understood. Here, we assess the functional role of CD34 on satellite cell-mediated muscle regeneration. We show that Cd34(-/-) mice, which have no obvious developmental phenotype, display a defect in muscle regeneration when challenged with either acute or chronic muscle injury. This regenerative defect is caused by impaired entry into proliferation and delayed myogenic progression. Consistent with the reported antiadhesive function of CD34, knockout satellite cells also show decreased motility along their host myofiber. Altogether, our results identify a role for CD34 in the poorly understood early steps of satellite cell activation and provide the first evidence that beyond being a stem cell marker, CD34 may play an important function in modulating stem cell activity.     

Paraxial mesodermal progenitors derived from mouse embryonic stem cells contribute to muscle regeneration via differentiation into muscle satellite cells

Pluripotent embryonic stem (ES) cells hold great potential for cell-based therapies. Although several recent studies have reported the potential of ES cell-derived progenitors for skeletal muscle regeneration, how the cells contribute to reconstitution of the damaged myofibers has remained elusive. Here, we demonstrated the process of injured muscle regeneration by the engraftment of ES cell-derived mesodermal progenitors. Mesodermal progenitor cells were induced by a conventional differentiation system and isolated by flow cytometer of platelet-derived growth factor receptor-alpha (PDGFR-alpha), a marker of paraxial mesoderm, and vascular endothelial growth factor receptor-2 (VEGFR-2), a marker of lateral mesoderm. The PDGFR-alpha(+) population that represented the paraxial mesodermal character demonstrated significant engraftment when transplanted into the injured muscle of immunodeficient mouse. Moreover, the PDGFR-alpha(+) population could differentiate into the muscle satellite cells that were the stem cells of adult muscle and characterized by the expression of Pax7 and CD34. These ES cell-derived satellite cells could form functional mature myofibers in vitro and generate myofibers fused with the damaged host myofibers in vivo. On the other hand, the PDGFR-alpha(-)VEGFR-2(+) population that showed lateral mesodermal character exhibited restricted potential to differentiate into the satellite cells in injured muscle. Our results show the potential of ES cell-derived paraxial mesodermal progenitor cells to generate functional muscle stem cells in vivo without inducing or suppressing gene manipulation. This knowledge could be used to form the foundation of the development of stem cell therapies to repair diseased and damaged muscles.     

Immunological properties of mesenchymal stem cells and clinical implications

The rapid evolution of experimental data has acknowledged the critical relevance of immune biology in stem cell research. It appears that efficient transfer of stem cells to patients requires robust analyses of the immune properties as well as the responses of the stem cells to immune mediators. This review discusses the biology of adult human mesenchymal stem cells (MSCs) in the context of immunology. MSCs are pluripotent, self-renewing cells with the potential for tissue regeneration, for example the repair of bone, cartilage, tendon, ligament, skeletal muscle, and cardiac muscle. MSCs have also been shown to transdifferentiate into cells of ectodermal origin, such as neurons. MSCs are located in perfused areas of adult bone marrow, whereas hematopoietic stem cells are located in poorly perfused areas of the same organ. MSCs show bimodal, i.e. anti-inflammatory and immune-enhancing, immune responses. MSCs also regulate immune responses such as the regulation of antibody production by B cells, alterations in T cell subtypes, and immune tolerance of allogeneic transplants. MSCs also have the potential for gene delivery. This review explores the diverse clinical potential for MSCs and discusses the limitations and advantages of their immunomodulatory properties.     

Neural stem cells: plasticity and their transdifferentiation potential

The presence of resident stem cells in adult tissues is of fundamental importance for the maintenance of their structural and functional integrity. In fact, throughout life, somatic stem cells attend to the critical function of substituting terminally differentiated cells lost to physiological turnover, injury or disease. Thence, one of the basic dogmata in tissue biology holds that the differentiation potential of an adult stem cell is restricted to the generation of the mature cell lineages found in the tissue to which the stem cell belongs. A plethora of recent evidences from many groups, including ours, is now providing evidence that adult stem cells may possess a broader differentiation repertoire than expected and that their fate potential may not be as tissue specific as once thought. The initial example of an unforeseen, trans-germ layer plasticity - that seems now to emerge as a prototypic functional trait of various somatic stem cells of different origin - has come from the reported awakening of a latent hemopoietic developmental capacity in stem cells isolated from the adult mammalian brain following their transplantation into sub-lethally irradiated mice. More recently, it has been shown that adult neural stem cells can differentiate into a wide array of bodily cells of different origin when injected into the blastocyst and into myogenic cells when transplanted into the adult regenerating skeletal muscle. Moreover, bone marrow stem cells can now give rise to skeletal muscle, hepatic and brain cells, whereas muscle precursors can generate blood cells. In this article, we review some of the basic notions regarding the functional properties of the adult neural stem cells and discuss findings in the expanding area of trans-germ layer conversion, with emphasis on the neural stem cell.     

Analysis of human muscle stem cells reveals a differentiation‐resistant progenitor cell population expressing Pax7 capable of self‐renewal

Studies using mouse models have established a critical role for resident satellite stem cells in skeletal muscle development and regeneration, but little is known about this paradigm in human muscle. Here, using human muscle stem cells, we address their lineage progression, differentiation, migration, and self‐renewal. Isolated human satellite cells expressed α7‐integrin and other definitive muscle markers, were highly motile on laminin substrates and could undergo efficient myotube differentiation and myofibrillogenesis. However, only a subpopulation of the myoblasts expressed Pax7 and displayed a variable lineage progression as measured by desmin and MyoD expression. Analysis identified a differentiation‐resistant progenitor cell population that was Pax7⁺/desmin^? and capable of self‐renewal. This study extends our understanding of the role of Pax7 in regulating human satellite stem cell differentiation and self‐renewal. Developmental Dynamics 238:138–149, 2009. © 2008 Wiley‐Liss, Inc.     

Effects of skeletal muscle regeneration on the proliferation potential of satellite cells

Skeletal muscle satellite cells are myogenic stem cells that function to repair damaged muscle fibers. Participation of satellite cells in a regeneration response following muscle injury results in a significant reduction in their cumulative proliferation potential. The magnitude of the reduction is proportional to the number of regeneration responses in which the cells participate.     

骨骼肌组织工程种子细胞来源的研究进展

构建组织工程骨骼肌的种子细胞有肌卫星细胞、成肌细胞、胚胎干细胞、间充质来源的干细胞(包括骨髓来源干细胞、羊水来源干细胞、脂肪来源干细胞等).不同来源的种子细胞在体内、外均能向肌细胞系分化,或与肌源系细胞融合,表达肌源系细胞表面抗原,参与肌纤维的修复.这些种子细胞均有望构建组织工程化骨骼肌.对目前骨骼肌组织工程种子细胞来源进行综述. Abstract： Different cells could be utilized as seed cells in the construction of tissue-engineered skelet al muscle, which include satellite stem cells, myohlasts, embryonic stem cells and mesenchymal stem cells includ-ing bone marrow-derived stem cells, amniotic fluid-derived stem cells, and adipose tissue-derived stem cells. Seed cells from different origins are capable of trans-differentiating into myogenic cells or fusing with myogenic cells, expressing the surface antigen of sarcogenic cells, and participating in the regeneration of muscle fiber, both in vivo and ex vivo. This review gives a summarization of the recent progress in the research on seed cell source in skelet al muscle tissue engineering.     

Expression of functional CXCR4 by muscle satellite cells and secretion of SDF-1 by muscle-derived fibroblasts is associated with the presence of both muscle progenitors in bone marrow and hematopoietic stem/progenitor cells in muscles

We found that the murine cell lines C2C12 and G7 derived from muscle satellite cells, which are essential for muscle regeneration, express the functional CXCR4 receptor on their surface and that the specific ligand for this receptor, alpha-chemokine stromal-derived factor 1 (SDF-1), is secreted in muscle tissue. These cell lines responded to SDF-1 stimulation by chemotaxis, phosphorylation of mitogen-activated protein kinase (MAPK) p42/44 and AKT serine-threonine kinase, and calcium flux, confirming the functionality of the CXCR4 receptor. Moreover, supernatants derived from muscle fibroblasts chemoattracted both satellite cells and human CD34(+) hematopoietic stem/progenitor cells. In a similar set of experiments, supernatants from bone marrow fibroblasts were found to chemoattract CXCR4(+) satellite cells just as they chemoattract CD34(+) cells. Moreover, preincubation of both muscle satellite cells and hematopoietic stem/progenitor CD34(+) cells before chemotaxis with T140, a specific CXCR4 inhibitor, resulted in a significantly lower chemotaxis to media conditioned by either muscle- or bone marrow-derived fibroblasts. Based on these observations, we postulate that the SDF-1-CXCR4 axis is involved in chemoattracting circulating CXCR4(+) muscle stem/progenitor and circulating CXCR4(+) hematopoietic CD34(+) cells to both muscle and bone marrow tissues. Thus, it appears that tissue-specific stem cells circulating in peripheral blood could compete for SDF-1(+) niches, and this would explain, without invoking the concept of stem cell plasticity, why hematopoietic colonies can be cultured from muscles and early muscle progenitors can be cultured from bone marrow.     

Prevention of muscle aging by myofiber-associated satellite cell transplantation

Skeletal muscle is dynamic, adapting to environmental needs, continuously maintained, and capable of extensive regeneration. These hallmarks diminish with age, resulting in a loss of muscle mass, reduced regenerative capacity, and decreased functionality. Although the mechanisms responsible for this decline are unclear, complex changes within the local and systemic environment that lead to a reduction in regenerative capacity of skeletal muscle stem cells, termed satellite cells, are believed to be responsible. We demonstrate that engraftment of myofiber-associated satellite cells, coupled with an induced muscle injury, markedly alters the environment of young adult host muscle, eliciting a near-lifelong enhancement in muscle mass, stem cell number, and force generation. The abrogation of age-related atrophy appears to arise from an increased regenerative capacity of the donor stem cells, which expand to occupy both myonuclei in myofibers and the satellite cell niche. Further, these cells have extensive self-renewal capabilities, as demonstrated by serial transplantation. These near-lifelong, physiological changes suggest an approach for the amelioration of muscle atrophy and diminished function that arise with aging through myofiber-associated satellite cell transplantation.     

Adult stem cell plasticity: will engineered tissues be rejected?

The dogma that adult tissue-specific stem cells remain committed to supporting only their own tissue has been challenged; a new hypothesis, that adult stem cells demonstrate plasticity in their repertoires, is being tested. This is important because it seems possible that haematopoietic stem cells, for example, could be exploited to generate and perhaps deliver cell-based therapies deep within existing nonhaematopoietic organs. Much of the evidence for plasticity derives from histological studies of tissues from patients or animals that have received grafts of cells or whole organs, from a donor bearing (or lacking) a definitive marker. Detection in the recipient of appropriately differentiated cells bearing the donor marker is indicative of a switch in phenotype of a stem cell or a member of a transit amplifying population or of a differentiated cell. In this review, we discuss evidence for these changes occurring but do not consider the molecular basis of cell commitment. In general, the extent of engraftment is low but may be increased if tissues are damaged. In model systems of liver regeneration, the repeated application of a selection pressure increases levels of engraftment considerably; how this occurs is unclear. Cell fusion plays a part in regeneration and remodelling of the liver, skeletal muscle and even regions of the brain. Genetic disease may be amenable to some forms of cell therapy, yet immune rejection will present challenges. Graft-vs.-host disease will continue to present problems, although this may be avoided if the cells were derived from the recipient or they were tolerized. Despite great expectations for cellular therapies, there are indications that attempts to replace missing proteins could be confounded simply by the development of specific immunity that rejects the new phenotype.