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

Challenges and Considerations of Preclinical Development for iPSC-Based Myogenic Cell Therapy

Cell therapies derived from induced pluripotent stem cells (iPSCs) offer a promising avenue in the field of regenerative medicine due to iPSCs' expandability, immune compatibility, and pluripotent potential. An increasing number of preclinical and clinical trials have been carried out, exploring the application of iPSC-based therapies for challenging diseases, such as muscular dystrophies. The unique syncytial nature of skeletal muscle allows stem/progenitor cells to integrate, forming new myonuclei and restoring the expression of genes affected by myopathies. This characteristic makes genome-editing techniques especially attractive in these therapies. With genetic modification and iPSC lineage specification methodologies, immune-compatible healthy iPSC-derived muscle cells can be manufactured to reverse the progression of muscle diseases or facilitate tissue regeneration. Despite this exciting advancement, much of the development of iPSC-based therapies for muscle diseases and tissue regeneration is limited to academic settings, with no successful clinical translation reported. The unknown differentiation process in vivo, potential tumorigenicity, and epigenetic abnormality of transplanted cells are preventing their clinical application. In this review, we give an overview on preclinical development of iPSC-derived myogenic cell transplantation therapies including processes related to iPSC-derived myogenic cells such as differentiation, scaling-up, delivery, and cGMP compliance. And we discuss the potential challenges of each step of clinical translation. Additionally, preclinical model systems for testing myogenic cells intended for clinical applications are described.

Metabolic Changes during In Vivo Maturation of PSC-Derived Skeletal Myogenic Progenitors

In vitro-generated pluripotent stem cell (PSC)-derived Pax3-induced (iPax3) myogenic progenitors display an embryonic transcriptional signature, but upon engraftment, the profile of re-isolated iPax3 donor-derived satellite cells changes toward similarity with postnatal satellite cells, suggesting that engrafted PSC-derived myogenic cells remodel their transcriptional signature upon interaction within the adult muscle environment. Here, we show that engrafted myogenic progenitors also remodel their metabolic state. Assessment of oxygen consumption revealed that exposure to the adult muscle environment promotes overt changes in mitochondrial bioenergetics, as shown by the substantial suppression of energy requirements in re-isolated iPax3 donor-derived satellite cells compared to their in vitro-generated progenitors. Mass spectrometry-based metabolomic profiling further confirmed the relationship of engrafted iPax3 donor-derived cells to adult satellite cells. The fact that in vitro-generated myogenic progenitors remodel their bioenergetic signature upon in vivo exposure to the adult muscle environment may have important implications for therapeutic applications.

Lysine methylation signaling in skeletal muscle biology: from myogenesis to clinical insights

Lysine methylation signaling is well studied for its key roles in the regulation of transcription states through modifications on histone proteins. While histone lysine methylation has been extensively studied, recent discoveries of lysine methylation on thousands of non-histone proteins has broadened our appreciation for this small chemical modification in the regulation of protein function. In this review, we highlight the significance of histone and non-histone lysine methylation signaling in skeletal muscle biology, spanning development, maintenance, regeneration, and disease progression. Furthermore, we discuss potential future implications for its roles in skeletal muscle biology as well as clinical applications for the treatment of skeletal muscle-related diseases.

Bioprocessing Considerations towards the Manufacturing of Therapeutic Skeletal and Smooth Muscle Cells

Tissue engineering approaches within the muscle context represent a promising emerging field to address the current therapeutic challenges related with multiple pathological conditions affecting the muscle compartments, either skeletal muscle or smooth muscle, responsible for involuntary and voluntary contraction, respectively. In this review, several features and parameters involved in the bioprocessing of muscle cells are addressed. The cell isolation process is depicted, depending on the type of tissue (smooth or skeletal muscle), followed by the description of the challenges involving the use of adult donor tissue and the strategies to overcome the hurdles of reaching relevant cell numbers towards a clinical application. Specifically, the use of stem/progenitor cells is highlighted as a source for smooth and skeletal muscle cells towards the development of a cellular product able to maintain the target cell's identity and functionality. Moreover, taking into account the need for a robust and cost-effective bioprocess for cell manufacturing, the combination of muscle cells with biomaterials and the need for scale-up envisioning clinical applications are also approached.

Recent advances in cell-based and cell-free therapeutic approaches for sarcopenia

Sarcopenia is a progressive loss of muscle mass and function that is connected with increased hospital expenditures, falls, fractures, and mortality. Although muscle loss has been related to aging, injury, hormonal imbalances, and diseases such as malignancies, chronic obstructive pulmonary disease, heart failure, and kidney failure, the underlying pathogenic mechanisms of sarcopenia are unclear. Exercise-based interventions and multimodal strategies are currently being considered as potential therapeutic approaches to prevent or treat these diseases. Although drug therapy research is ongoing, no drug has yet been proven to have a substantial safety and clinical value to be the first drug therapy to be licensed for sarcopenia. To better understand the molecular alterations underlying sarcopenia and effective treatments, we review leading research and available findings from the systemic change to the muscle-specific microenvironment. Furthermore, we explore possible mechanisms of sarcopenia and provide new knowledge for the development of novel cell-free and cell-based therapeutics. This review will assist researchers in developing better therapies to improve muscle health in the elderly.

Generation of muscle progenitors from human-induced pluripotent stem cells

Background

Small molecules have a role in the differentiation of human-induced pluripotent stem cells (hiPSCs) into different cell linages. The aim of this study was to evaluate the differentiation of hiPSCs into cardiac or skeletal myogenic progenitors with a single small molecule.

Methods

hiPSCs were treated with three different small molecules such as Isoxazole-9, Danazol and Givinostat in serum-free medium for 7 days. Cell viability, qRT-PCR, western blots, and immunostaining were assessed after treatment of hiPSCs with small molecules.

Results

Higher hiPSC viability was observed in hiPSCs treated with Isoxazole-9 (25 µM), Danazol (25 µM) and Givinostat (150 nM) versus control (P < 0.05). Givinostat had dual effect by generating both skeletal and cardiac progenitor cells versus Isoxazole-9 and Danazol after 7 days. Givinostat treatment induced upregulation of skeletal myogenic genes and their protein expression levels on day 4 and further increased on day 8 (P < 0.05) versus control. Furthermore,positive stained cells for Pax3, Myf5, MyoD1, dystrophin, desmin, myogenin, and β-catenin at 1 month. Givinostat increased upregulation of cardiac gene expression levels versus control after day 4 (P < 0.05), with positive stained cells for Nkx2.5, GATA4, TnT, TnI, connexin 43 and α-sarcomeric actinin at 1 month.

Conclusions

Pretreatment of hiPSCs with Givinostat represents a viable strategy for producing both cardiac/skeletal myogenic progenitors in vitro for cell therapies against myocardial infarction and Duchenne muscular dystrophy.

Human pluripotent stem cell-derived myogenic progenitors undergo maturation to quiescent satellite cells upon engraftment

Human pluripotent stem cell (hPSC)-derived myogenic progenitor cell (MPC) transplantation is a promising therapeutic approach for a variety of degenerative muscle disorders. Here, using an MPC-specific fluorescent reporter system (PAX7::GFP), we demonstrate that hPSC-derived MPCs can contribute to the regeneration of myofibers in mice following local injury and in mice deficient of dystrophin (mdx). We also demonstrate that a subset of PAX7::GFP MPCs engraft within the basal lamina of regenerated myofibers, adopt a quiescent state, and contribute to regeneration upon reinjury and in mdx mouse models. This subset of PAX7::GFP MPCs undergo a maturation process and remodel their molecular characteristics to resemble those of late-stage fetal MPCs/adult satellite cells following in vivo engraftment. These in-vivo-matured PAX7::GFP MPCs retain a cell-autonomous ability to regenerate and can repopulate in the niche of secondary recipient mice, providing a proof of principle for future hPSC-based cell therapy for muscle disorders.

Chromatin accessibility profiling identifies evolutionary conserved loci in activated human satellite cells

Satellite cells represent the main myogenic population accounting for skeletal muscle homeostasis and regeneration. While our knowledge of the signaling pathways controlling satellite cell regenerative capability is increasing, the underlying epigenetic mechanisms are still not clear, especially in the case of human satellite cells. Here, by performing chromatin accessibility profiling (ATAC-seq) in samples isolated from human and murine muscles, we investigated the changes in the epigenetic landscape occurring during the transition from activated satellite cells to myoblasts. Our analysis identifies a compendium of putative regulatory elements defining human activated satellite cells and myoblasts, respectively. A subset of these differentially accessible loci is shared by both murine and human satellite cells, includes elements associated with known self-renewal regulators, and is enriched for motifs bound by transcription factors participating in satellite cell regulation. Integration of transcriptional and epigenetic data reveals that known regulators of metabolic gene expression, such as PPARGC1A, represent potential PAX7 targets. Through characterization of genomic networks and the underlying effectors, our data represent an important starting point for decoding and manipulating the molecular mechanisms underlying human satellite cell muscle regenerative potential.

Induction of Skeletal Muscle Progenitors and Stem Cells from human induced Pluripotent Stem Cells

Induced pluripotent stem cells (iPSCs) have the potential to differentiate into various types of cells and tissues including skeletal muscle. The approach to convert these stem cells into skeletal muscle cells offers hope for patients afflicted with skeletal muscle diseases such as Duchenne muscular dystrophy (DMD). Several methods have been reported to induce myogenic differentiation with iPSCs derived from myogenic patients. An important point for generating skeletal muscle cells from iPSCs is to understand in vivo myogenic induction in development and regeneration. Current protocols of myogenic induction utilize techniques with overexpression of myogenic transcription factors such as Myod1(MyoD), Pax3, Pax7, and others, using recombinant proteins or small molecules to induce mesodermal cells followed by myogenic progenitors, and adult muscle stem cells. This review summarizes the current approaches used for myogenic induction and highlights recent improvements.

Stem cell therapy for muscular dystrophies

Muscular dystrophies are a heterogeneous group of genetic diseases, characterized by progressive degeneration of skeletal and cardiac muscle. Despite the intense investigation of different therapeutic options, a definitive treatment has not been developed for this debilitating class of pathologies. Cell-based therapies in muscular dystrophies have been pursued experimentally for the last three decades. Several cell types with different characteristics and tissues of origin, including myogenic stem and progenitor cells, stromal cells, and pluripotent stem cells, have been investigated over the years and have recently entered in the clinical arena with mixed results. In this Review, we do a roundup of the past attempts and describe the updated status of cell-based therapies aimed at counteracting the skeletal and cardiac myopathy present in dystrophic patients. We present current challenges, summarize recent progress, and make recommendations for future research and clinical trials.

Muscle progenitor specification and myogenic differentiation are associated with changes in chromatin topology

Using Hi-C, promoter-capture Hi-C (pCHi-C), and other genome-wide approaches in skeletal muscle progenitors that inducibly express a master transcription factor, Pax7, we systematically characterize at high-resolution the spatio-temporal re-organization of compartments and promoter-anchored interactions as a consequence of myogenic commitment and differentiation. We identify key promoter-enhancer interaction motifs, namely, cliques and networks, and interactions that are dependent on Pax7 binding. Remarkably, Pax7 binds to a majority of super-enhancers, and together with a cadre of interacting transcription factors, assembles feed-forward regulatory loops. During differentiation, epigenetic memory and persistent looping are maintained at a subset of Pax7 enhancers in the absence of Pax7. We also identify and functionally validate a previously uncharacterized Pax7-bound enhancer hub that regulates the essential myosin heavy chain cluster during skeletal muscle cell differentiation. Our studies lay the groundwork for understanding the role of Pax7 in orchestrating changes in the three-dimensional chromatin conformation in muscle progenitors.

Genomic Safe Harbor Expression of PAX7 for the Generation of Engraftable Myogenic Progenitors

Inducible expression of PAX7 in differentiating pluripotent stem cells (PSCs) allows massively scalable generation of human myogenic progenitors, which upon transplantation into dystrophic muscles give rise to donor-derived myofibers and satellite cells. Therefore, PSC-derived PAX7⁺ myogenic progenitors represent an attractive therapeutic approach to promote muscle regeneration. Work to date has used lentiviral vectors (LVs) that randomly integrate inducible PAX7 transgenes. Here, we investigated whether equivalent induction of the myogenic program could be achieved by targeting the PAX7 transgene into genomic safe harbor (GSH) sites. Across multiple PSC lines, we find that this approach consistently generates expandable myogenic progenitors in vitro, although scalability of expansion is moderately reduced compared with the LV approach. Importantly, transplantation of GSH-targeted myogenic progenitors produces robust engraftment, comparable with LV counterparts. These findings provide proof of concept for the use of GSH targeting as a potential alternative approach to generate therapeutic PSC-derived myogenic progenitors for clinical applications.

Tissue Stem Cells: Architects of Their Niches

Stem cells (SCs) maintain tissue homeostasis and repair wounds. Despite marked variation in tissue architecture and regenerative demands, SCs often follow similar paradigms in communicating with their microenvironmental "niche" to transition between quiescent and regenerative states. Here we use skin epithelium and skeletal muscle-among the most highly-stressed tissues in our body-to highlight similarities and differences in niche constituents and how SCs mediate natural tissue rejuvenation and perform regenerative acts prompted by injuries. We discuss how these communication networks break down during aging and how understanding tissue SCs has led to major advances in regenerative medicine.

Chromatin Landscape During Skeletal Muscle Differentiation

Cellular commitment and differentiation involve highly coordinated mechanisms by which tissue-specific genes are activated while others are repressed. These mechanisms rely on the activity of specific transcription factors, chromatin remodeling enzymes, and higher-order chromatin organization in order to modulate transcriptional regulation on multiple cellular contexts. Tissue-specific transcription factors are key mediators of cell fate specification with the ability to reprogram cell types into different lineages. A classic example of a master transcription factor is the muscle specific factor MyoD, which belongs to the family of myogenic regulatory factors (MRFs). MRFs regulate cell fate determination and terminal differentiation of the myogenic precursors in a multistep process that eventually culminate with formation of muscle fibers. This developmental progression involves the activation and proliferation of muscle stem cells, commitment, and cell cycle exit and fusion of mononucleated myoblast to generate myotubes and myofibers. Although the epigenetics of muscle regeneration has been extensively addressed and discussed over the recent years, the influence of higher-order chromatin organization in skeletal muscle regeneration is still a field of development. In this review, we will focus on the epigenetic mechanisms modulating muscle gene expression and on the incipient work that addresses three-dimensional genome architecture and its influence in cell fate determination and differentiation to achieve skeletal myogenesis. We will visit known alterations of genome organization mediated by chromosomal fusions giving rise to novel regulatory landscapes, enhancing oncogenic activation in muscle, such as alveolar rhabdomyosarcomas (ARMS).

Therapeutic Strategies for Duchenne Muscular Dystrophy: An Update

Neuromuscular disorders encompass a heterogeneous group of conditions that impair the function of muscles, motor neurons, peripheral nerves, and neuromuscular junctions. Being the most common and most severe type of muscular dystrophy, Duchenne muscular dystrophy (DMD), is caused by mutations in the X-linked dystrophin gene. Loss of dystrophin protein leads to recurrent myofiber damage, chronic inflammation, progressive fibrosis, and dysfunction of muscle stem cells. Over the last few years, there has been considerable development of diagnosis and therapeutics for DMD, but current treatments do not cure the disease. Here, we review the current status of DMD pathogenesis and therapy, focusing on mutational spectrum, diagnosis tools, clinical trials, and therapeutic approaches including dystrophin restoration, gene therapy, and myogenic cell transplantation. Furthermore, we present the clinical potential of advanced strategies combining gene editing, cell-based therapy with tissue engineering for the treatment of muscular dystrophy.

Multilineage Differentiation for Formation of Innervated Skeletal Muscle Fibers from Healthy and Diseased Human Pluripotent Stem Cells

Induced pluripotent stem cells (iPSCs) obtained by reprogramming primary somatic cells have revolutionized the fields of cell biology and disease modeling. However, the number protocols for generating mature muscle fibers with sarcolemmal organization using iPSCs remain limited, and partly mimic the complexity of mature skeletal muscle. Methods: We used a novel combination of small molecules added in a precise sequence for the simultaneous codifferentiation of human iPSCs into skeletal muscle cells and motor neurons. Results: We show that the presence of both cell types reduces the production time for millimeter-long multinucleated muscle fibers with sarcolemmal organization. Muscle fiber contractions are visible in 19–21 days, and can be maintained over long period thanks to the production of innervated multinucleated mature skeletal muscle fibers with autonomous cell regeneration of PAX7-positive cells and extracellular matrix synthesis. The sequential addition of specific molecules recapitulates key steps of human peripheral neurogenesis and myogenesis. Furthermore, this organoid-like culture can be used for functional evaluation and drug screening. Conclusion: Our protocol, which is applicable to hiPSCs from healthy individuals, was validated in Duchenne Muscular Dystrophy, Myotonic Dystrophy, Facio-Scapulo-Humeral Dystrophy and type 2A Limb-Girdle Muscular Dystrophy, opening new paths for the exploration of muscle differentiation, disease modeling and drug discovery.

Pluripotent stem cell-derived skeletal muscle fibers preferentially express myosin heavy-chain isoforms associated with slow and oxidative muscles

BACKGROUND

Skeletal muscle function is essential for health, and it depends on the proper activity of myofibers and their innervating motor neurons. Each adult muscle is composed of different types of myofibers with distinct contractile and metabolic characteristics. The proper balance of myofiber types is disrupted in most muscle degenerative disorders, representing another factor compromising muscle function. One promising therapeutic approach for the treatment of these diseases is cell replacement based on the targeted differentiation of pluripotent stem cells (PSCs) towards the myogenic lineage. We have previously shown that transient induction of Pax3 or Pax7 in PSCs allows for the generation of skeletal myogenic progenitors endowed with myogenic regenerative potential, but whether they contribute to different fiber types remains unknown.

RESULTS

Here, we investigate the fiber type composition of mouse PSC-derived myofibers upon their transplantation into dystrophic and non-dystrophic mice. Our data reveal that PSC-derived myofibers express slow and oxidative myosin heavy-chain isoforms, along with developmental myosins, regardless of the recipient background. Furthermore, transplantation of the mononuclear cell fraction re-isolated from primary grafts into secondary recipients results in myofibers that maintain preferential expression of slow and oxidative myosin heavy-chain isoforms but no longer express developmental myosins, thus indicating postnatal composition.

CONCLUSIONS

Considering oxidative fibers are commonly spared in the context of dystrophic pathogenesis, this feature of PSC-derived myofibers could be advantageous for therapeutic applications.

Towards stem cell therapies for skeletal muscle repair

Skeletal muscle is an ideal target for cell therapy. The use of its potent stem cell population in the form of autologous intramuscular transplantation represents a tantalizing strategy to slow the progression of congenital muscle diseases (such as Duchenne Muscular Dystrophy) or regenerate injured tissue following trauma. The syncytial nature of skeletal muscle uniquely permits the engraftment of stem/progenitor cells to contribute to new myonuclei and restore the expression of genes mutated in myopathies. Historically however, the implementation of this approach has been significantly limited by the inability to expand undifferentiated muscle stem cells (MuSCs) in culture whilst maintaining transplantation potential. This is crucial, as MuSC expansion and/or genetic manipulation is likely necessary for therapeutic applications. In this article, we review recent studies that have provided a number of important breakthroughs to tackle this problem. Progress towards this goal has been achieved by exploiting biochemical, biophysical and developmental paradigms to construct innovative in vitro strategies that are guiding stem cell therapies for muscle repair towards the clinic.

Stem cell-based therapies for Duchenne muscular dystrophy

Muscular dystrophies are a group of genetic muscle disorders that cause progressive muscle weakness and degeneration. Within this group, Duchenne muscular dystrophy (DMD) is the most common and one of the most severe. DMD is an X chromosome linked disease that occurs to 1 in 3500 to 1 in 5000 boys. The cause of DMD is a mutation in the dystrophin gene, whose encoded protein provides both structural support and cell signaling capabilities. So far, there are very limited therapeutic options available and there is no cure for this disease. In this review, we discuss the existing cell therapy research, especially stem cell-based, which utilize myoblasts, satellite cells, bone marrow cells, mesoangioblasts and CD133+ cells. Finally, we focus on human pluripotent stem cells (hPSCs) which hold great potential in treating DMD. hPSCs can be used for autologous transplantation after being specified to a myogenic lineage. Over the last few years, there has been a rapid development of isolation, as well as differentiation, techniques in order to achieve effective transplantation results of myogenic cells specified from hPSCs. In this review, we summarize the current methods of hPSCs myogenic commitment/differentiation, and describe the current status of hPSC-derived myogenic cell transplantation.

Transplantation studies reveal internuclear transfer of toxic RNA in engrafted muscles of myotonic dystrophy 1 mice

Background

Stem cell transplantation represents a potential therapeutic option for muscular dystrophies (MD). However, to date, most reports have utilized mouse models for recessive types of MD. Here we performed studies to determine whether myotonic dystrophy 1 (DM1), an autosomal dominant type of MD, could benefit from cell transplantation.

Methods

We injected human pluripotent stem (PS) cell-derived myogenic progenitors into the muscles of a novel mouse model combining immunodeficiency and skeletal muscle pathology of DM1 and investigated transplanted mice for engraftment as well as for the presence of RNA foci and alternative splicing pattern.

Findings

Engraftment was clearly observed in recipient mice, but unexpectedly, we detected RNA foci in donor-derived engrafted myonuclei. These foci proved to be pathogenic as we observed MBNL1 sequestration and abnormal alternative splicing in donor-derived transcripts.

Interpretation

It has been assumed that toxic CUG repeat-containing RNA forms foci in situ in the nucleus in which it is expressed, but these data suggest that CUG repeat-containing RNA may also exit the nucleus and traffic to other nuclei in the syncytial myofiber, where it can exert pathological effects.

Fund

This project was supported by funds from the LaBonte/Shawn family and NIH grants R01 AR055299 and AR071439 (R.C.R.P.). R.M-G. was funded by CONACyT-Mexico (#394378).

Cell therapy to improve regeneration of skeletal muscle injuries

Diseases that jeopardize the musculoskeletal system and cause chronic impairment are prevalent throughout the Western world. In Germany alone, ~1.8 million patients suffer from these diseases annually, and medical expenses have been reported to reach 34.2bn Euros. Although musculoskeletal disorders are seldom fatal, they compromise quality of life and diminish functional capacity. For example, musculoskeletal disorders incur an annual loss of over 0.8 million workforce years to the German economy. Among these diseases, traumatic skeletal muscle injuries are especially problematic because they can occur owing to a variety of causes and are very challenging to treat. In contrast to chronic muscle diseases such as dystrophy, sarcopenia, or cachexia, traumatic muscle injuries inflict damage to localized muscle groups. Although minor muscle trauma heals without severe consequences, no reliable clinical strategy exists to prevent excessive fibrosis or fatty degeneration, both of which occur after severe traumatic injury and contribute to muscle degeneration and dysfunction. Of the many proposed strategies, cell-based approaches have shown the most promising results in numerous pre-clinical studies and have demonstrated success in the handful of clinical trials performed so far. A number of myogenic and non-myogenic cell types benefit muscle healing, either by directly participating in new tissue formation or by stimulating the endogenous processes of muscle repair. These cell types operate via distinct modes of action, and they demonstrate varying levels of feasibility for muscle regeneration depending, to an extent, on the muscle injury model used. While in some models the injury naturally resolves over time, other models have been developed to recapitulate the peculiarities of real-life injuries and therefore mimic the structural and functional impairment observed in humans. Existing limitations of cell therapy approaches include issues related to autologous harvesting, expansion and sorting protocols, optimal dosage, and viability after transplantation. Several clinical trials have been performed to treat skeletal muscle injuries using myogenic progenitor cells or multipotent stromal cells, with promising outcomes. Recent improvements in our understanding of cell behaviour and the mechanistic basis for their modes of action have led to a new paradigm in cell therapies where physical, chemical, and signalling cues presented through biomaterials can instruct cells and enhance their regenerative capacity. Altogether, these studies and experiences provide a positive outlook on future opportunities towards innovative cell-based solutions for treating traumatic muscle injuries-a so far unmet clinical need.

Skeletal Muscle Regenerative Engineering

Skeletal muscles have the intrinsic ability to regenerate after minor injury, but under certain circumstances such as severe trauma from accidents, chronic diseases or battlefield injuries the regeneration process is limited. Skeletal muscle regenerative engineering has emerged as a promising approach to address this clinical issue. The regenerative engineering approach involves the convergence of advanced materials science, stem cell science, physical forces, insights from developmental biology, and clinical translation. This article reviews recent studies showing the potential of the convergences of technologies involving biomaterials, stem cells and bioactive factors in concert with clinical translation, in promoting skeletal muscle regeneration. Several types of biomaterials such as electrospun nanofibers, hydrogels, patterned scaffolds, decellularized tissues, and conductive matrices are being investigated. Detailed discussions are given on how these biomaterials can interact with cells and modulate their behavior through physical, chemical and mechanical cues. In addition, the application of physical forces such as mechanical and electrical stimulation are reviewed as strategies that can further enhance muscle contractility and functionality. The review also discusses established animal models to evaluate regeneration in two clinically relevant muscle injuries; volumetric muscle loss (VML) and muscle atrophy upon rotator cuff injury. Regenerative engineering approaches using advanced biomaterials, cells, and physical forces, developmental cues along with insights from immunology, genetics and other aspects of clinical translation hold significant potential to develop promising strategies to support skeletal muscle regeneration.

Directed Differentiation of Pluripotent Stem Cells by Transcription Factors

Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) have been used as promising tools for regenerative medicine, disease modeling, and drug screening. Traditional and common strategies for pluripotent stem cell (PSC) differentiation toward disease-relevant cell types depend on sequential treatment of signaling molecules identified based on knowledge of developmental biology. However, these strategies suffer from low purity, inefficiency, and time-consuming culture conditions. A growing body of recent research has shown efficient cell fate reprogramming by forced expression of single or multiple transcription factors. Here, we review transcription factor-directed differentiation methods of PSCs toward neural, muscle, liver, and pancreatic endocrine cells. Potential applications and limitations are also discussed in order to establish future directions of this technique for therapeutic purposes.

Pluripotent stem cell-derived myogenic progenitors remodel their molecular signature upon in vivo engraftment

Optimal cell-based therapies for the treatment of muscle degenerative disorders should not only regenerate fibers but provide a quiescent satellite cell pool ensuring long-term maintenance and regeneration. Conditional expression of Pax3/Pax7 in differentiating pluripotent stem cells (PSCs) allows the generation of myogenic progenitors endowed with enhanced regenerative capacity. To identify the molecular determinants underlying their regenerative potential, we performed transcriptome analyses of these cells along with primary myogenic cells from several developmental stages. Here we show that in vitro-generated PSC-derived myogenic progenitors possess a molecular signature similar to embryonic/fetal myoblasts. However, compared with fetal myoblasts, following transplantation they show superior myofiber engraftment and ability to seed the satellite cell niche, respond to multiple reinjuries, and contribute to long-term regeneration. Upon engraftment, the transcriptome of reisolated Pax3/Pax7-induced PSC-derived myogenic progenitors changes toward a postnatal molecular signature, particularly in genes involved in extracellular matrix remodeling. These findings demonstrate that Pax3/Pax7-induced myogenic progenitors remodel their molecular signature and functionally mature upon in vivo exposure to the adult muscle environment.

In Vitro Tissue-Engineered Skeletal Muscle Models for Studying Muscle Physiology and Disease

Healthy skeletal muscle possesses the extraordinary ability to regenerate in response to small-scale injuries; however, this self-repair capacity becomes overwhelmed with aging, genetic myopathies, and large muscle loss. The failure of small animal models to accurately replicate human muscle disease, injury and to predict clinically-relevant drug responses has driven the development of high fidelity in vitro skeletal muscle models. Herein, the progress made and challenges ahead in engineering biomimetic human skeletal muscle tissues that can recapitulate muscle development, genetic diseases, regeneration, and drug response is discussed. Bioengineering approaches used to improve engineered muscle structure and function as well as the functionality of satellite cells to allow modeling muscle regeneration in vitro are also highlighted. Next, a historical overview on the generation of skeletal muscle cells and tissues from human pluripotent stem cells, and a discussion on the potential of these approaches to model and treat genetic diseases such as Duchenne muscular dystrophy, is provided. Finally, the need to integrate multiorgan microphysiological systems to generate improved drug discovery technologies with the potential to complement or supersede current preclinical animal models of muscle disease is described.

Myogenic progenitor specification from pluripotent stem cells

Pluripotent stem cells represent important tools for both basic and translational science as they enable to study mechanisms of development, model diseases in vitro and provide a potential source of tissue-specific progenitors for cell therapy. Concomitantly with the increasing knowledge of the molecular mechanisms behind activation of the skeletal myogenic program during embryonic development, novel findings in the stem cell field provided the opportunity to begin recapitulating in vitro the events occurring during specification of the myogenic lineage. In this review, we will provide a perspective of the molecular mechanisms responsible for skeletal myogenic commitment in the embryo and how this knowledge was instrumental for specifying this lineage from pluripotent stem cells. In addition, we will discuss the current limitations for properly recapitulating skeletal myogenesis in the petri dish, and we will provide insights about future applications of pluripotent stem cell-derived myogenic cells.

PAX7 Targets, CD54, Integrin α9β1, and SDC2, Allow Isolation of Human ESC/iPSC-Derived Myogenic Progenitors

Pluripotent stem (PS)-cell-derived cell types hold promise for treating degenerative diseases. However, PS cell differentiation is intrinsically heterogeneous; therefore, clinical translation requires the development of practical methods for isolating progenitors from unwanted and potentially teratogenic cells. Muscle-regenerating progenitors can be derived through transient PAX7 expression. To better understand the biology, and to discover potential markers for these cells, here we investigate PAX7 genomic targets and transcriptional changes in human cells undergoing PAX7-mediated myogenic commitment. We identify CD54, integrin α9β1, and Syndecan2 (SDC2) as surface markers on PAX7-induced myogenic progenitors. We show that these markers allow for the isolation of myogenic progenitors using both fluorescent- and CGMP-compatible magnetic-based sorting technologies and that CD54+α9β1+SDC2+ cells contribute to long-term muscle regeneration in vivo. These findings represent a critical step toward enabling the translation of PS-cell-based therapies for muscle diseases.

Augmentation of musculoskeletal regeneration: role for pluripotent stem cells

The rise in the incidence of musculoskeletal diseases is attributed to an increasing ageing population. The debilitating effects of musculoskeletal diseases, coupled with a lack of effective therapies, contribute to huge financial strains on healthcare systems. The focus of regenerative medicine has shifted to pluripotent stem cells (PSCs), namely, human embryonic stem cells and human-induced PSCs, due to the limited success of adult stem cell-based interventions. PSCs constitute a valuable cell source for musculoskeletal regeneration due to their capacity for unlimited self-renewal, ability to differentiate into all cell lineages of the three germ layers and perceived immunoprivileged characteristics. This review summarizes methods for chondrogenic, osteogenic, myogenic and adipogenic differentiation of PSCs and their potential for therapeutic applications.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

Skeletal Muscle Cell Induction from Pluripotent Stem Cells

Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) have the potential to differentiate into various types of cells including skeletal muscle cells. The approach of converting ESCs/iPSCs into skeletal muscle cells offers hope for patients afflicted with the skeletal muscle diseases such as the Duchenne muscular dystrophy (DMD). Patient-derived iPSCs are an especially ideal cell source to obtain an unlimited number of myogenic cells that escape immune rejection after engraftment. Currently, there are several approaches to induce differentiation of ESCs and iPSCs to skeletal muscle. A key to the generation of skeletal muscle cells from ESCs/iPSCs is the mimicking of embryonic mesodermal induction followed by myogenic induction. Thus, current approaches of skeletal muscle cell induction of ESCs/iPSCs utilize techniques including overexpression of myogenic transcription factors such as MyoD or Pax3, using small molecules to induce mesodermal cells followed by myogenic progenitor cells, and utilizing epigenetic myogenic memory existing in muscle cell-derived iPSCs. This review summarizes the current methods used in myogenic differentiation and highlights areas of recent improvement.

Pax7 remodels the chromatin landscape in skeletal muscle stem cells

Pluripotent stem cells (PSC) hold great promise for the treatment of human skeletal muscle diseases. However, it remains challenging to convert PSC to skeletal muscle cells, and the mechanisms by which the master regulatory transcription factor, Pax7, promotes muscle stem (satellite) cell identity are not yet understood. We have taken advantage of PSC-derived skeletal muscle precursor cells (iPax7), wherein the induced expression of Pax7 robustly initiates the muscle program and enables the in vitro generation of precursors that seed the satellite cell compartment upon transplantation. Remarkably, we found that chromatin accessibility in myogenic precursors pre-figures subsequent activation of myogenic differentiation genes. We also found that Pax7 binding is generally restricted to euchromatic regions and excluded from H3K27 tri-methylated regions in muscle cells, suggesting that recruitment of this factor is circumscribed by chromatin state. Further, we show that Pax7 binding induces dramatic, localized remodeling of chromatin characterized by the acquisition of histone marks associated with enhancer activity and induction of chromatin accessibility in both muscle precursors and lineage-committed myoblasts. Conversely, removal of Pax7 leads to rapid reversal of these features on a subset of enhancers. Interestingly, another cluster of Pax7 binding sites is associated with a durably accessible and remodeled chromatin state after removal of Pax7, and persistent enhancer accessibility is associated with subsequent, proximal binding by the muscle regulatory factors, MyoD1 and myogenin. Our studies provide new insights into the epigenetic landscape of skeletal muscle stem cells and precursors and the role of Pax7 in satellite cell specification.

Epigenetic Manipulation Facilitates the Generation of Skeletal Muscle Cells from Pluripotent Stem Cells

Human pluripotent stem cells (hPSCs) have the capacity to differentiate into essentially all cell types in the body. Such differentiation can be directed to specific cell types by appropriate cell culture conditions or overexpressing lineage-defining transcription factors (TFs). Especially, for the activation of myogenic program, early studies have shown the effectiveness of enforced expression of TFs associated with myogenic differentiation, such as PAX7 and MYOD1. However, the efficiency of direct differentiation was rather low, most likely due to chromatin features unique to hPSCs, which hinder the access of TFs to genes involved in muscle differentiation. Indeed, recent studies have demonstrated that ectopic expression of epigenetic-modifying factors such as a histone demethylase and an ATP-dependent remodeling factor significantly enhances myogenic differentiation from hPSCs. In this article, we review the recent progress for in vitro generation of skeletal muscles from hPSCs through forced epigenetic and transcriptional manipulation.

Myogenic potential of mouse embryonic stem cells lacking functional Pax7 tested in vitro by 5-azacitidine treatment and in vivo in regenerating skeletal muscle

Regeneration of skeletal muscle relies on the presence of satellite cells. Satellite cells deficiency accompanying some degenerative diseases is the reason for the search for the "replacement cells" that can be used in the muscle therapies. Due to their unique properties embryonic stem cells (ESCs), as well as myogenic cells derived from them, are considered as a promising source of therapeutic cells. Among the factors crucial for the specification of myogenic precursor cells is Pax7 that sustains proper function of satellite cells. In our previous studies we showed that ESCs lacking functional Pax7 are able to form myoblasts in vitro when differentiated within embryoid bodies and their outgrowths. In the current study we showed that ESCs lacking functional Pax7, cultured in vitro in monolayer in the medium supplemented with horse serum and 5azaC, expressed higher levels of factors associated with myogenesis, such as Pdgfra, Pax3, Myf5, and MyoD. Importantly, skeletal myosin immunolocalization confirmed that myogenic differentiation of ESCs was more effective in case of cells lacking Pax7. Our in vivo studies showed that ESCs transplanted into regenerating skeletal muscles were detectable at day 7 of regeneration and the number of Pax7-/- ESCs detected was significantly higher than of control cells. Our results support the concept that lack of functional Pax7 promotes proliferation of differentiating ESCs and for this reason more of them can turn into myogenic lineage.

Emerging new tools to study and treat muscle pathologies: genetics and molecular mechanisms underlying skeletal muscle development, regeneration, and disease

Skeletal muscle is the most abundant tissue in our body, is responsible for generating the force required for movement, and is also an important thermogenic organ. Skeletal muscle is an enigmatic tissue because while on the one hand, skeletal muscle regeneration after injury is arguably one of the best-studied stem cell-dependent regenerative processes, on the other hand, skeletal muscle is still subject to many degenerative disorders with few therapeutic options in the clinic. It is important to develop new regenerative medicine-based therapies for skeletal muscle. Future therapeutic strategies should take advantage of rapidly developing technologies enabling the differentiation of skeletal muscle from human pluripotent stem cells, along with precise genome editing, which will go hand in hand with a steady and focused approach to understanding underlying mechanisms of skeletal muscle development, regeneration, and disease. In this review, I focus on highlighting the recent advances that particularly have relied on developmental and molecular biology approaches to understanding muscle development and stem cell function. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Striated muscle function, regeneration, and repair

As the only striated muscle tissues in the body, skeletal and cardiac muscle share numerous structural and functional characteristics, while exhibiting vastly different size and regenerative potential. Healthy skeletal muscle harbors a robust regenerative response that becomes inadequate after large muscle loss or in degenerative pathologies and aging. In contrast, the mammalian heart loses its regenerative capacity shortly after birth, leaving it susceptible to permanent damage by acute injury or chronic disease. In this review, we compare and contrast the physiology and regenerative potential of native skeletal and cardiac muscles, mechanisms underlying striated muscle dysfunction, and bioengineering strategies to treat muscle disorders. We focus on different sources for cellular therapy, biomaterials to augment the endogenous regenerative response, and progress in engineering and application of mature striated muscle tissues in vitro and in vivo. Finally, we discuss the challenges and perspectives in translating muscle bioengineering strategies to clinical practice.

Derivation of Skeletal Myogenic Precursors from Human Pluripotent Stem Cells Using Conditional Expression of PAX7

Cell-based therapies are considered as one of the most promising approaches for the treatment of degenerating pathologies including muscle disorders and dystrophies. Advances in the approach of reprogramming somatic cells into induced pluripotent stem (iPS) cells allow for the possibility of using the patient's own pluripotent cells to generate specific tissues for autologous transplantation. In addition, patient-specific tissue derivatives have been shown to represent valuable material for disease modeling and drug discovery. Nevertheless, directed differentiation of pluripotent stem cells into a specific lineage is not a trivial task especially in the case of skeletal myogenesis, which is generally poorly recapitulated during the in vitro differentiation of pluripotent stem cells.Here, we describe a practical and efficient method for the derivation of skeletal myogenic precursors from differentiating human pluripotent stem cells using controlled expression of PAX7. Flow cytometry (FACS) purified myogenic precursors can be expanded exponentially and differentiated in vitro into myotubes, enabling researchers to use these cells for disease modeling as well as therapeutic purposes.

A Novel Protocol for Directed Differentiation of C9orf72-Associated Human Induced Pluripotent Stem Cells Into Contractile Skeletal Myotubes

In the present study, functional skeletal myotubes were efficiently generated from human induced pluripotent stem cells using a small molecule-based approach. Myotubes derived from patients carrying the C9orf72 repeat expansion show no change in differentiation efficiency and normal TDP-43 localization after as many as 120 days in vitro when compared to unaffected controls. The protocol described in this study for the generation of skeletal myotubes from human induced pluripotent stem cells may serve as a valuable tool in drug discovery and modeling of musculoskeletal and neuromuscular diseases.

Induced pluripotent stem cells (iPSCs) offer an unlimited resource of cells to be used for the study of underlying molecular biology of disease, therapeutic drug screening, and transplant-based regenerative medicine. However, methods for the directed differentiation of skeletal muscle for these purposes remain scarce and incomplete. Here, we present a novel, small molecule-based protocol for the generation of multinucleated skeletal myotubes using eight independent iPSC lines. Through combinatorial inhibition of phosphoinositide 3-kinase (PI3K) and glycogen synthase kinase 3β (GSK3β) with addition of bone morphogenic protein 4 (BMP4) and fibroblast growth factor 2 (FGF2), we report up to 64% conversion of iPSCs into the myogenic program by day 36 as indicated by MYOG⁺ cell populations. These cells began to exhibit spontaneous contractions as early as 34 days in vitro in the presence of a serum-free medium formulation. We used this protocol to obtain iPSC-derived muscle cells from frontotemporal dementia (FTD) patients harboring C9orf72 hexanucleotide repeat expansions (rGGGGCC), sporadic FTD, and unaffected controls. iPSCs derived from rGGGGCC carriers contained RNA foci but did not vary in differentiation efficiency when compared to unaffected controls nor display mislocalized TDP-43 after as many as 120 days in vitro. This study presents a rapid, efficient, and transgene-free method for generating multinucleated skeletal myotubes from iPSCs and a resource for further modeling the role of skeletal muscle in amyotrophic lateral sclerosis and other motor neuron diseases.

Significance

Protocols to produce skeletal myotubes for disease modeling or therapy are scarce and incomplete. The present study efficiently generates functional skeletal myotubes from human induced pluripotent stem cells using a small molecule-based approach. Using this strategy, terminal myogenic induction of up to 64% in 36 days and spontaneously contractile myotubes within 34 days were achieved. Myotubes derived from patients carrying the C9orf72 repeat expansion show no change in differentiation efficiency and normal TDP-43 localization after as many as 120 days in vitro when compared to unaffected controls. This study provides an efficient, novel protocol for the generation of skeletal myotubes from human induced pluripotent stem cells that may serve as a valuable tool in drug discovery and modeling of musculoskeletal and neuromuscular diseases.

Muscle cell identity requires Pax7-mediated lineage-specific DNA demethylation

Background

Skeletal muscle stem cells enable the formation, growth, maintenance, and regeneration of skeletal muscle throughout life. The regeneration process is compromised in several pathological conditions, and muscle progenitors derived from pluripotent stem cells have been suggested as a potential therapeutic source for tissue replacement. DNA methylation is an important epigenetic mechanism in the setting and maintenance of cellular identity, but its role in stem cell determination towards the myogenic lineage is unknown. Here we addressed the DNA methylation dynamics of the major genes orchestrating the myogenic determination and differentiation programs in embryonic stem (ES) cells, their Pax7-induced myogenic derivatives, and muscle stem cells in proliferating and differentiating conditions.

Results

Our data showed a common muscle-specific DNA demethylation signature required to acquire and maintain the muscle-cell identity. This specific-DNA demethylation is Pax7-mediated, and it is a prime event in muscle stem cells gene activation. Notably, downregulation of the demethylation-related enzyme Apobec2 in ES-derived myogenic precursors reduced myogenin-associated DNA demethylation and dramatically impaired the expression of differentiation markers and, ultimately, muscle differentiation.

Conclusions

Our results underscore DNA demethylation as a key mechanism driving myogenesis and identify specific Pax7-mediated DNA demethylation signatures to acquire and maintain the muscle-cell identity. Additionally, we provide a panel of epigenetic markers for the efficient and safe generation of ES- and induced pluripotent stem cell (iPS)-derived myogenic progenitors for therapeutic applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0250-9) contains supplementary material, which is available to authorized users.

Evaluation of adipose-derived stem cells for tissue-engineered muscle repair construct-mediated repair of a murine model of volumetric muscle loss injury

Volumetric muscle loss (VML) can occur from congenital defects, muscle wasting diseases, civilian or military injuries, and as a result of surgical removal of muscle tissue (eg, cancer), all of which can lead to irrevocable functional and cosmetic defects. Current tissue engineering strategies to repair VML often employ muscle-derived progenitor cells (MDCs) as one component. However, there are some inherent limitations in their in vitro culture expansion. Thus, this study explores the potential of adipose-derived stem cells (ADSCs) as an alternative cell source to MDCs for tissue engineering of skeletal muscle. A reproducible VML injury model in murine latissimus dorsi muscle was used to evaluate tissue-engineered muscle repair (TEMR) constructs incorporating MDCs or ADSCs. Importantly, histological analysis revealed that ADSC-seeded constructs displayed regeneration potential that was comparable to those seeded with MDCs 2 months postrepair. Furthermore, morphological analysis of retrieved constructs demonstrated signs of neotissue formation, including cell fusion, fiber formation, and scaffold remodeling. Immunohistochemistry demonstrated positive staining for both structural and functional proteins. Positive staining for vascular structures indicated the potential for long-term neotissue survival and integration with existing musculature. Qualitative observation of lentivirus-Cherry-labeled donor cells by immunohistochemistry indicates that participation of ADSCs in new hybrid myofiber formation incorporating donor cells was relatively low, compared to donor MDCs. However, ADSCs appear to participate in vascularization. In summary, I have demonstrated that TEMR constructs generated with ADSCs displayed skeletal muscle regeneration potential comparable to TEMR–MDC constructs as previously reported.

Myogenic Differentiation of Mouse Embryonic Stem Cells That Lack a Functional Pax7 Gene

The transcription factor Pax7 plays a key role during embryonic myogenesis and sustains the proper function of satellite cells, which serve as adult skeletal muscle stem cells. Overexpression of Pax7 has been shown to promote the myogenic differentiation of pluripotent stem cells. However, the effects of the absence of functional Pax7 in differentiating embryonic stem cells (ESCs) have not yet been directly tested. Herein, we studied mouse stem cells that lacked a functional Pax7 gene and characterized the differentiation of these stem cells under conditions that promoted the derivation of myoblasts in vitro. We analyzed the expression of myogenic factors, such as myogenic regulatory factors and muscle-specific microRNAs, in wild-type and mutant cells. Finally, we compared the transcriptome of both types of cells and did not find substantial differences in the expression of genes related to the regulation of myogenesis. As a result, we showed that the absence of functional Pax7 does not prevent the in vitro myogenic differentiation of ESCs.

Myogenic Progenitors from Mouse Pluripotent Stem Cells for Muscle Regeneration

Muscle homeostasis is maintained by resident stem cells which, in both pathologic and non-pathologic conditions, are able to repair or generate new muscle fibers. Although muscle stem cells have tremendous regenerative potential, their application in cell therapy protocols is prevented by several restrictions, including the limited ability to grow ex vivo. Since pluripotent stem cells have the unique potential to both self-renew and expand almost indefinitely, they have become an attractive source of progenitors for regenerative medicine studies. Our lab has demonstrated that embryonic stem cell (ES)-derived myogenic progenitors retain the ability to repair existing muscle fibers and contribute to the pool of resident stem cells. Because of their relevance in both cell therapy and disease modeling, in this chapter we describe the protocol to derive myogenic progenitors from murine ES cells followed by their intramuscular delivery in a murine muscular dystrophy model.

From pluripotency to myogenesis: a multistep process in the dish

Pluripotent stem cells (PSCs), such as embryonic stem cells or induced pluripotent stem cells are a promising source of cells for regenerative medicine as they can differentiate into all cell types building a mammalian body. However, protocols leading to efficient and safe in vitro generation of desired cell types must be perfected before PSCs can be used in cell therapies or tissue engineering. In vivo, i.e. in developing mouse embryo or teratoma, PSCs can differentiate into skeletal muscle, but in vitro their spontaneous differentiation into myogenic cells is inefficient. Numerous attempts have been undertaken to enhance this process. Many of them involved mimicking the interactions occurring during embryonic myogenesis. The key regulators of embryonic myogenesis, such as Wnts proteins, fibroblast growth factor 2, and retinoic acid, have been tested to improve the frequency of in vitro myogenic differentiation of PSCs. This review summarizes the current state of the art, comparing spontaneous and directed myogenic differentiation of PSCs as well as the protocols developed this far to facilitate this process.

Generation of Functional Neuromuscular Junctions from Human Pluripotent Stem Cell Lines

Several neuromuscular diseases involve dysfunction of neuromuscular junctions (NMJs), yet there are no patient-specific human models for electrophysiological characterization of NMJ. We seeded cells of neurally-induced embryoid body-like spheres derived from induced pluripotent stem cell (iPSC) or embryonic stem cell (ESC) lines as monolayers without basic fibroblast factor (bFGF) and observed differentiation of neuronal as well as spontaneously contracting, multinucleated skeletal myotubes. The myotubes showed striation, immunoreactivity for myosin heavy chain, actin bundles typical for myo-oriented cells, and generated spontaneous and evoked action potentials (APs). The myogenic differentiation was associated with expression of MyoD1, myogenin and type I ryanodine receptor. Neurons formed end plate like structures with strong binding of α-bungarotoxin, a marker of nicotinic acetylcholine receptors highly expressed in the postsynaptic membrane of NMJs, and expressed SMI-32, a motoneuron marker, as well as SV2, a marker for synapses. Pharmacological stimulation of cholinergic receptors resulted in strong depolarization of myotube membrane and raised Ca²⁺ concentration in sarcoplasm, while electrical stimulation evoked Ca²⁺ transients in myotubes. Stimulation of motoneurons with N-Methyl-D-aspartate resulted in reproducible APs in myotubes and end plates displayed typical mEPPs and tonic activity depolarizing myotubes of about 10 mV. We conclude that simultaneous differentiation of neurons and myotubes from patient-specific iPSCs or ESCs results also in the development of functional NMJs. Our human model of NMJ may serve as an important tool to investigate normal development, mechanisms of diseases and novel drug targets involving NMJ dysfunction and degeneration.

Potential of adipose-derived stem cells in muscular regenerative therapies

Regenerative capacity of skeletal muscles resides in satellite cells, a self-renewing population of muscle cells. Several studies are investigating epigenetic mechanisms that control myogenic proliferation and differentiation to find new approaches that could boost regeneration of endogenous myogenic progenitor populations. In recent years, a lot of effort has been applied to purify, expand and manipulate adult stem cells from muscle tissue. However, this population of endogenous myogenic progenitors in adults is limited and their access is difficult and invasive. Therefore, other sources of stem cells with potential to regenerate muscles need to be examined. An excellent candidate could be a population of adult stromal cells within fat characterized by mesenchymal properties, which have been termed adipose-derived stem cells (ASCs). These progenitor adult stem cells have been successfully differentiated in vitro to osteogenic, chondrogenic, neurogenic and myogenic lineages. Autologous ASCs are multipotent and can be harvested with low morbidity; thus, they hold promise for a range of therapeutic applications. This review will summarize the use of ASCs in muscle regenerative approaches.

Generation of skeletal muscle cells from pluripotent stem cells: advances and challenges

Human pluripotent stem cells (hPSCs) possess unlimited proliferative potential while maintaining the ability to differentiate into any cell type including skeletal muscle cells (SMCs). hPSCs are amenable to genetic editing and can be derived from patient somatic cells, and thus represent a promising option for cell therapies for the treatment of degenerative diseases such as muscular dystrophies. There are unresolved challenges however associated with the derivation and scale-up of hPSCs and generation of differentiated cells in large quantity and high purity. Reported myogenic differentiation protocols are long, require cell sorting and/or rely on ectopic expression of myogenic master regulators. More recent advances have been made with the application of small molecules to enhance the myogenic differentiation efficiency and the identification of more selective markers for the enrichment of myogenic progenitors with enhanced regenerative potential. Here we review the field of myogenic differentiation and highlight areas requiring further research.

Somitogenesis: From somite to skeletal muscle

Myogenesis is controlled by an elaborate system of extrinsic and intrinsic regulatory mechanisms in all development stages. The aim of this review is to provide an overview of the different stages of myogenesis and muscle differentiation in mammals, starting from somitogenesis and analysis of the different portions that constitute the mature somite. Particular attention was paid to regulatory genes, in addition to mesodermal stem cells, which represent the earliest elements of myogenesis. Finally, the crucial role of growth factors, molecules of vital importance in contractile regulation, hormones and their function in skeletal muscle differentiation, growth and metabolism, and the role played by central nervous system, are discussed.

Coaxing stem cells for skeletal muscle repair

Skeletal muscle has a tremendous ability to regenerate, attributed to a well-defined population of muscle stem cells called satellite cells. However, this ability to regenerate diminishes with age and can also be dramatically affected by multiple types of muscle diseases, or injury. Extrinsic and/or intrinsic defects in the regulation of satellite cells are considered to be major determinants for the diminished regenerative capacity. Maintenance and replenishment of the satellite cell pool is one focus for muscle regenerative medicine, which will be discussed. There are other sources of progenitor cells with myogenic capacity, which may also support skeletal muscle repair. However, all of these myogenic cell populations have inherent difficulties and challenges in maintaining or coaxing their derivation for therapeutic purpose. This review will highlight recent reported attributes of these cells and new bioengineering approaches to creating a supply of myogenic stem cells or implants applicable for acute and/or chronic muscle disorders.

Muscle stem cells at a glance

Muscle stem cells facilitate the long-term regenerative capacity of skeletal muscle. This self-renewing population of satellite cells has only recently been defined through genetic and transplantation experiments. Although muscle stem cells remain in a dormant quiescent state in uninjured muscle, they are poised to activate and produce committed progeny. Unlike committed myogenic progenitor cells, the self-renewal capacity gives muscle stem cells the ability to engraft as satellite cells and capitulate long-term regeneration. Similar to other adult stem cells, understanding the molecular regulation of muscle stem cells has significant implications towards the development of pharmacological or cell-based therapies for muscle disorders. This Cell Science at a Glance article and accompanying poster will review satellite cell characteristics and therapeutic potential, and provide an overview of the muscle stem cell hallmarks: quiescence, self-renewal and commitment.