Muscle stem cells and regenerative myogenesis

Decline of regenerative potential of old muscle stem cells: contribution to muscle aging

Muscle stem cells (MuSCs) are required for life-long muscle regeneration. In general, aging has been linked to a decline in the numbers and the regenerative potential of MuSCs. Muscle regeneration depends on the proper functioning of MuSCs, which is itself dependent on intricate interactions with its niche components. Aging is associated with both cell-intrinsic and niche-mediated changes, which can be the result of transcriptional, posttranscriptional, or posttranslational alterations in MuSCs or in the components of their niche. The interplay between cell intrinsic alterations in MuSCs and changes in the stem cell niche environment during aging and its impact on the number and the function of MuSCs is an important emerging area of research. In this review, we discuss whether the decline in the regenerative potential of MuSCs with age is the cause or the consequence of aging skeletal muscle. Understanding the effect of aging on MuSCs and the individual components of their niche is critical to develop effective therapeutic approaches to diminish or reverse the age-related defects in muscle regeneration.

Ageing Skeletal Muscle: The Ubiquitous Muscle Stem Cell

In 1999, in a review by Beardsley, the potential of adult stem cells, in repair and regeneration was heralded (Beardsley Sci Am 281:30-31, 1999). Since then, the field of regenerative medicine has grown exponentially, with the capability of restoring or regenerating the function of damaged, diseased or aged human tissues being an underpinning motivation. If successful, stem cell therapies offer the potential to treat, for example degenerative diseases. In the subsequent 20 years, extensive progress has been made in the arena of adult stem cells (for a recent review see (Zakrzewski et al. Stem Cell Res Ther 10:68, 2019)). Prior to the growth of the adult stem cell research arena, much focus had been placed on the potential of embryonic stem cells (ESCs). The first research revealing the potential of these cells was published in 1981, when scientists reported the ability of cultured stem cells from murine embryos, to not only self-renew, but to also become all cells of the three germ layers of the developing embryo (Evans and Kaufman Nature 292:154-156, 1981), (Martin Proc Natl Acad Sci U S A 78:7634-7638, 1981). It took almost 20 years, following these discoveries, for this technology to translate to human ESCs, using donated human embryos. In 1998, Thomson et al. reported the creation of the first human embryonic cell line (Thomson et al. Science 282:1145-1147, 1998). However, research utilising human ESCs was hampered by ethical and religious constraints and indeed in 2001 George W. Bush restricted US research funding to human ESCs, which had already been banked. The contentious nature of this arena perhaps facilitated the use of and the research potential for adult stem cells. It is beyond the scope of this review to focus on ESCs, although their potential for enhancing our understanding of human development is huge (for a recent review see (Cyranoski Nature 555:428-430, 2018)). Rather, although ESCs and their epigenetic regulation will be introduced for background understanding, the focus will be on stem cells more generally, the role of epigenetics in stem cell fate, skeletal muscle, skeletal muscle stem cells, the impact of ageing on muscle wasting and the mechanisms underpinning loss, with a focus on epigenetic adaptation.

PPARs and Microbiota in Skeletal Muscle Health and Wasting

Skeletal muscle is a major metabolic organ that uses mostly glucose and lipids for energy production and has the capacity to remodel itself in response to exercise and fasting. Skeletal muscle wasting occurs in many diseases and during aging. Muscle wasting is often accompanied by chronic low-grade inflammation associated to inter- and intra-muscular fat deposition. During aging, muscle wasting is advanced due to increased movement disorders, as a result of restricted physical exercise, frailty, and the pain associated with arthritis. Muscle atrophy is characterized by increased protein degradation, where the ubiquitin-proteasomal and autophagy-lysosomal pathways, atrogenes, and growth factor signaling all play an important role. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor family of transcription factors, which are activated by fatty acids and their derivatives. PPARs regulate genes that are involved in development, metabolism, inflammation, and many cellular processes in different organs. PPARs are also expressed in muscle and exert pleiotropic specialized responses upon activation by their ligands. There are three PPAR isotypes, viz., PPARα, -β/δ, and -γ. The expression of PPARα is high in tissues with effective fatty acid catabolism, including skeletal muscle. PPARβ/δ is expressed more ubiquitously and is the predominant isotype in skeletal muscle. It is involved in energy metabolism, mitochondrial biogenesis, and fiber-type switching. The expression of PPARγ is high in adipocytes, but it is also implicated in lipid deposition in muscle and other organs. Collectively, all three PPAR isotypes have a major impact on muscle homeostasis either directly or indirectly. Furthermore, reciprocal interactions have been found between PPARs and the gut microbiota along the gut–muscle axis in both health and disease. Herein, we review functions of PPARs in skeletal muscle and their interaction with the gut microbiota in the context of muscle wasting.

Extracellular Matrix-Derived Hydrogels as Biomaterial for Different Skeletal Muscle Tissue Replacements

Recently, skeletal muscle represents a complex and challenging tissue to be generated in vitro for tissue engineering purposes. Several attempts have been pursued to develop hydrogels with different formulations resembling in vitro the characteristics of skeletal muscle tissue in vivo. This review article describes how different types of cell-laden hydrogels recapitulate the multiple interactions occurring between extracellular matrix (ECM) and muscle cells. A special attention is focused on the biochemical cues that affect myocytes morphology, adhesion, proliferation, and phenotype maintenance, underlining the importance of topographical cues exerted on the hydrogels to guide cellular orientation and facilitate myogenic differentiation and maturation. Moreover, we highlight the crucial role of 3D printing and bioreactors as useful platforms to finely control spatial deposition of cells into ECM based hydrogels and provide the skeletal muscle native-like tissue microenvironment, respectively.

Abnormal nuclear aggregation and myotube degeneration in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is caused by CTG nucleotide repeat expansions in the 3'-untranslated region (3'-UTR) of the dystrophia myotonica protein kinase (DMPK) gene. The expanded CTG repeats encode toxic CUG RNAs that cause disease, largely through RNA gain-of-function. DM1 is a fatal disease characterized by progressive muscle wasting, which has no cure. Regenerative medicine has emerged as a promising therapeutic modality for DM1, especially with the advancement of induced pluripotent stem (iPS) cell technology and therapeutic genome editing. However, there is an unmet need to identify in vitro outcome measures to demonstrate the therapeutic effects prior to in vivo clinical trials. In this study, we examined the muscle regeneration (myotube formation) in normal and DM1 myoblasts in vitro to establish outcome measures for therapeutic monitoring. We found normal proliferation of DM1 myoblasts, but abnormal nuclear aggregation during the early stage myotube formation, as well as myotube degeneration during the late stage of myotube formation. We concluded that early abnormal nuclear aggregation and late myotube degeneration offer easy and sensitive outcome measures to monitor therapeutic effects in vitro.

Methods of Isolation and Analysis of TREG Immune Infiltrates from Injured and Dystrophic Skeletal Muscle

The immune infiltrate present in acutely injured or dystrophic skeletal muscle has been shown to play an important role in the process of muscle regeneration. Our work has described, for the first time, muscle regulatory T cells (Tregs), a unique population in phenotype and function capable of promoting skeletal muscle repair. Herein, we describe the methods we have optimized to study muscle Tregs, including their isolation from injured muscle, immuno-labeling for analysis/separation by flow cytometry, and measurement of their proliferation status.

Clonogenic, myogenic progenitors expressing MCAM/CD146 are incorporated as adventitial reticular cells in the microvascular compartment of human post-natal skeletal muscle

Recent observation identifies subendothelial (mural) cells expressing MCAM, a specific system of clonogenic, self-renewing, osteoprogenitors (a.k.a, "mesenchymal stem cells") in the microvascular compartment of post-natal human bone marrow (BM). In this study, we used MCAM/CD146, as a marker to localize, isolate and assay subendothelial clonogenic cells from the microvasculature of postnatal human skeletal muscle. We show here that these cells share with their BM counterpart, anatomic position (subendothelial/adventitial) and ex vivo clonogenicity (CFU-Fs). When assayed under the stringent conditions, these cells display a high spontaneous myogenic potential (independent of co-culture with myoblasts or of in vivo fusion with local myoblasts), which is otherwise only attained in cultures of satellite cells. These muscle-derived mural cells activated a myogenic program in culture. Cultured CD146+ cells expressed the myogenic factors (Pax7, Pax3 and Myf5), NCAM/CD56, desmin as well as proteins characteristic of more advanced myogenic differentiation, such as myosin heavy chain. In vivo, these cells spontaneously generate myotubes and myofibrils. These data identify the anatomy and phenotype of a novel class of committed myogenic progenitor in human post-natal skeletal muscle of subendothelial cells associated with the abluminal surface of microvascular compartment distinct from satellite cells.

Cell death, clearance and immunity in the skeletal muscle

The skeletal muscle is an immunologically unique tissue. Leukocytes, virtually absent in physiological conditions, are quickly recruited into the tissue upon injury and persist during regeneration. Apoptosis, necrosis and autophagy coexist in the injured/regenerating muscles, including those of patients with neuromuscular disorders, such as inflammatory myopathies, dystrophies, metabolic and mitochondrial myopathies and drug-induced myopathies. Macrophages are able to alter their function in response to microenvironment conditions and as a consequence coordinate changes within the tissue from the early injury throughout regeneration and eventual healing, and regulate the activation and the function of stem cells. Early after injury, classically activated macrophages ('M1') dominate the picture. Alternatively activated M2 macrophages predominate during resolution phases and regulate the termination of the inflammatory responses. The dynamic M1/M2 transition is increasingly felt to be the key to the homeostasis of the muscle. Recognition and clearance of debris originating from damaged myofibers and from dying stem/progenitor cells, stromal cells and leukocytes are fundamental actions of macrophages. Clearance of apoptotic cells and M1/M2 transition are causally connected and represent limiting steps for muscle healing. The accumulation of apoptotic cells, which reflects their defective clearance, has been demonstrated in various tissues to prompt autoimmunity against intracellular autoantigens. In the muscle, in the presence of type I interferon, apoptotic myoblasts indeed cause the production of autoantibodies, lymphocyte infiltration and continuous cycles of muscle injury and regeneration, mimicking human inflammatory myopathies. The clearance of apoptotic cells thus modulates the homeostatic response of the skeletal muscle to injury. Conversely, defects in the process may have deleterious local effects, guiding maladaptive tissue remodeling with collagen and fat accumulation and promoting autoimmunity itself. There is strong promise for novel treatments based on new knowledge of cell death, clearance and immunity in the muscle.

MicroRNA-431 accelerates muscle regeneration and ameliorates muscular dystrophy by targeting Pax7 in mice

Skeletal muscle stem cells, called satellite cells, are a quiescent heterogeneous population. Their heterogeneity is influenced by Pax7, a well-defined transcriptional regulator of satellite cell functions that defines two subpopulations: Pax7(Hi) and Pax7(Lo). However, the mechanisms by which these subpopulations are established and maintained during myogenesis are not completely understood. Here we show that miR-431, which is predominantly expressed in the skeletal muscle, mediates satellite cell heterogeneity by fine-tuning Pax7 levels during muscle development and regeneration. In miR-431 transgenic mice, the Pax7(Lo) subpopulation is enriched, enhances myogenic differentiation and accelerates muscle regeneration. Notably, miR-431 attenuates the muscular dystrophic phenotype in mdx mice and may be a potential therapeutic target in muscular diseases. miR-431 transgenic mice are a unique genetic model for investigating the cellular features and biological functions of Pax7(Lo) satellite cells during muscle development and regeneration.

Required role of apoptotic myogenic precursors and toll-like receptor stimulation for the establishment of autoimmune myositis in experimental murine models

OBJECTIVE

Muscle regeneration is a hallmark of the idiopathic inflammatory myopathies (IIMs), a group of autoimmune disorders that are characterized by leukocyte infiltration and dysfunction of the skeletal muscle. Despite detailed studies describing the clinical and histopathologic features of IIMs, the immunopathogenesis of these disorders remains undefined. The aim of this study was to investigate the immunopathologic processes of autoimmune myositis in experimental murine models.

METHODS

Expression of the autoantigen histidyl-transfer RNA synthetase (HisRS) was analyzed in mice with acutely injured or dystrophic muscles, in inflammatory leukocytes, and in purified satellite cells. Anti-HisRS antibodies and myositis induction were assessed in mice after muscle injury and immunization with apoptotic satellite cells or C2C12 myoblasts, in the presence or absence of the Toll-like receptor 7 (TLR-7) agonist R848.

RESULTS

Muscle necrosis, leukocyte infiltration, and myofiber regeneration induced by toxic agents (cardiotoxin or glycerol) or promoted by genetic disruption of the α-sarcoglycan/dystrophin complex in mice were uniformly associated with up-regulated expression of HisRS. Although regenerating myofibers and purified satellite cells are known to show increased expression of HisRS in these settings, anti-HisRS antibodies were not detectable. However, intramuscular immunization with ultraviolet B-irradiated, HisRS-expressing apoptotic myoblasts in the presence of R848 triggered the production of anti-HisRS IgG antibodies as well as persistent lymphocyte infiltration and prolonged/delayed muscle regeneration. Conversely, intramuscular administration of R848 alone or in combination with living or postapoptotic/necrotic myoblasts failed to generate this myositis phenotype.

CONCLUSION

In the presence of TLR/adjuvant signals and underlying muscle injury, apoptotic myogenic precursors expressing high levels of autoantigen can provoke autoantibody formation and lymphocytic infiltration of muscle tissue, effectively replicating the features of IIM.

Exercise as a therapeutic strategy for primary mitochondrial cytopathies

Patients with mitochondrial cytopathies often experience exercise intolerance and may have fixed muscle weakness, leading to impaired functional capacity and lower quality of life. Endurance exercise training increases Vo 2 max, respiratory chain enzyme activity, and improves quality of life. Resistance exercise training increases muscle strength and may lower mutational burden in patients with mitochondrial DNA deletions. Both modes of exercise appear to be well tolerated. Patients with mitochondrial cytopathy should consider alternating both types of exercise to derive the benefits from each (endurance = greater aerobic fitness; resistance = greater strength). Patients should start an exercise program at a low intensity and duration, gradually increasing duration and intensity. They should "listen to their body" and not exercise on days they have fever, superimposed illness, muscle pain, or cramps, and/or if they have fasted for more than 12 hours. Children often respond best to play-based exercise and tend to enjoy intermittent activity.

An alternate protocol for establishment of primary caprine fetal myoblast cell culture: an in vitro model for muscle growth study

Cultured myoblasts have been used extensively as an in vitro model in understanding the underlying mechanisms of myogenesis. Various protocols for establishing a pure myoblast culture have been reported which involve the use of special procedures like flow cytometry and density gradient centrifugation. In goat, only a few protocols for establishing a myogenic cell culture are available and these protocols use adult muscle tissues which often does not yield sufficient numbers of precursor cells with adequate proliferative capacity. Considering the disadvantages of adult myoblasts, we are proposing an alternate protocol using caprine fetus which does not require any special procedures. In the present study, more than 90-95% fetal-derived cell populations had the typical spindle to polyhedral shape of myoblast cell and stained positive for desmin, hence confirming their myogenic origin. These cells attained the maximum confluency as early as 3-4 d against 3 wk by adult myoblasts indicating a better growth potential. Further, quantitative real-time PCR analysis revealed a higher expression (p < 0.01) of myogenic regulatory factors (i.e., myogenic determination factor 1, myogenic factor 5, and myogenin) and myostatin (MSTN) in the fetal as compared to the adult myoblasts. Consequently, higher proliferation and differentiation ability along with higher abundance of myogenic markers and MSTN make the fetal myoblasts a better in vitro model.

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 alpha7-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.

Asymmetric self-renewal and commitment of satellite stem cells in muscle

Satellite cells play a central role in mediating the growth and regeneration of skeletal muscle. However, whether satellite cells are stem cells, committed progenitors, or dedifferentiated myoblasts has remained unclear. Using Myf5-Cre and ROSA26-YFP Cre-reporter alleles, we observed that in vivo 10% of sublaminar Pax7-expressing satellite cells have never expressed Myf5. Moreover, we found that Pax7(+)/Myf5(-) satellite cells gave rise to Pax7(+)/Myf5(+) satellite cells through apical-basal oriented divisions that asymmetrically generated a basal Pax7(+)/Myf5(-) and an apical Pax7(+)/Myf5(+) cells. Prospective isolation and transplantation into muscle revealed that whereas Pax7(+)/Myf5(+) cells exhibited precocious differentiation, Pax7(+)/Myf5(-) cells extensively contributed to the satellite cell reservoir throughout the injected muscle. Therefore, we conclude that satellite cells are a heterogeneous population composed of stem cells and committed progenitors. These results provide critical insights into satellite cell biology and open new avenues for therapeutic treatment of neuromuscular diseases.

Preparation of cultured myofibers from larval salamander limbs for cellular plasticity studies

Identification of the mechanisms underlying cellular plasticity in salamander cells is important because these may give pointers to the restricted regenerative ability of mammals. The myofibers from salamanders are remarkable for their ability to undergo cellularization and cell-cycle re-entry during regeneration. Here, we describe a detailed method for the isolation and culture of larval salamander myofibers in numbers suitable for cellular plasticity studies. The basic protocol for isolation and purification of cells can be completed in 4 h.

Stem and progenitor cells in skeletal muscle development, maintenance, and therapy

Satellite cells are dormant progenitors located at the periphery of skeletal myofibers that can be triggered to proliferate for both self-renewal and differentiation into myogenic cells. In addition to anatomic location, satellite cells are typified by markers such as M-cadherin, Pax7, Myf5, and neural cell adhesion molecule-1. The Pax3 and Pax7 transcription factors play essential roles in the early specification, migration, and myogenic differentiation of satellite cells. In addition to muscle-committed satellite cells, multi-lineage stem cells encountered in embryonic, as well as adult, tissues exhibit myogenic potential in experimental conditions. These multi-lineage stem cells include side-population cells, muscle-derived stem cells (MDSCs), and mesoangioblasts. Although the ontogenic derivation, identity, and localization of these non-conventional myogenic cells remain elusive, recent results suggest their ultimate origin in blood vessel walls. Indeed, purified pericytes and endothelium-related cells demonstrate high myogenic potential in culture and in vivo. Allogeneic myoblasts transplanted into Duchenne muscular dystrophy (DMD) patients have been, in early trials, largely inefficient owing to immune rejection, rapid death, and limited intramuscular migration--all obstacles that are now being alleviated, at least in part, by more efficient immunosuppression and escalated cell doses. As an alternative to myoblast transplantation, stem cells such as mesoangioblasts and CD133+ progenitors administered through blood circulation have recently shown great potential to regenerate dystrophic muscle.

Skeletal myosphere-derived progenitor cell transplantation promotes neovascularization in delta-sarcoglycan knockdown cardiomyopathy

Bone marrow cells have been shown to contribute to neovascularization in ischemic hearts, whereas their impaired maturation to restore the delta-sarcoglycan (delta-SG) expression responsible for focal myocardial degeneration limits their utility to treat the pathogenesis of cardiomyopathy. Here, we report the isolation of multipotent progenitor cells from adult skeletal muscle, based on their ability to generate floating-myospheres. Myosphere-derived progenitor cells (MDPCs) are distinguishable from myogenic C2C12 cells and differentiate into vascular smooth muscle cells and mesenchymal progeny. The mutation in the delta-SG has been shown to develop vascular spasm to affect sarcolemma structure causing cardiomyopathy. We originally generated delta-SD knockdown (KD) mice and transplanted MDPCs into the hearts. MDPCs enhanced neoangiogenesis and restored delta-SG expression in impaired vasculatures through trans-differentiation, leading to improvement of cardiac function associated with paracrine effectors secretion. We propose that MDPCs may be the promising progenitor cells in skeletal muscle to treat delta-sarcoglycan complex mutant cardiomyopathy.

New fundamental resistance exercise determinants of molecular and cellular muscle adaptations

Abstract Physical activity relies on muscular force. In adult skeletal muscle, force results from the contraction of postmitotic, multinucleated myofibres of different contractile and metabolic properties. Myofibres can adapt to (patho-)physiological conditions of altered functional demand by radial growth, longitudinal growth, and regulation of fibre type functional gene modules. The adaptation's specificity depends on the distinct molecular and cellular events triggered by unique combinations of conditional cues. In order to derive effective and tailored exercise prescriptions, it must be determined (1) which mechano-biological condition leads to what molecular/cellular response, and (2) how this molecular/cellular response relates to the structural, contractile, and metabolic adaptation. It follows that a thorough mechano-biological description of the loading condition is imperative. Unfortunately, the definition of (resistance) exercise conditions in the past and present literature is insufficient. It is classically limited to load magnitude, number of repetitions and sets, rest in-between sets, number of interventions/week, and training period. In this review, we show why the current description is insufficient, and identify new determinants of quantitative and/or qualitative effects on skeletal muscle with respect to resistance exercise in healthy, adult humans. These new mandatory determinants comprise the fractional and temporal distribution of the contraction modes per repetition, duration of one repetition, rest in-between repetitions, time under tension, muscular failure, range of motion, recovery time, and anatomical definition. We strongly recommend to standardise the design and description of all future resistance exercise investigations by using the herein proposed set of 13 mechano-biological determinants (classical and new ones).

Mobilisation of mesenchymal cells into blood in response to skeletal muscle injury

Mesenchymal cells recruited to damaged tissues must circulate through the bloodstream. The absolute numbers of circulating mesenchymal stem cells (cMSCs) in two different models of acute and chronic skeletal muscle injury were determined. cMSCs were present in significantly higher numbers in both models than in healthy controls. These results support the hypothesis that MSCs are mobilised into the bloodstream after skeletal muscle tissue damage. These two models (acute and chronic) would be of value in the search for molecular mediators of mobilisation of MSCs into the circulation.