Engraftment of ES-Derived Myogenic Progenitors in a Severe Mouse Model of Muscular Dystrophy

Transplantation of PSC-derived myogenic progenitors counteracts disease phenotypes in FSHD mice

Facioscapulohumeral muscular dystrophy (FSHD) is a genetically dominant progressive myopathy caused by improper silencing of the DUX4 gene, leading to fibrosis, muscle atrophy, and fatty replacement. Approaches focused on muscle regeneration through the delivery of stem cells represent an attractive therapeutic option for muscular dystrophies. To investigate the potential for cell transplantation in FSHD, we have used the doxycycline-regulated iDUX4pA-HSA mouse model in which low-level DUX4 can be induced in skeletal muscle. We find that mouse pluripotent stem cell (PSC)-derived myogenic progenitors engraft in muscle actively undergoing DUX4-mediated degeneration. Donor-derived muscle tissue displayed reduced fibrosis and importantly, engrafted muscles showed improved contractile specific force compared to non-transplanted controls. These data demonstrate the feasibility of replacement of diseased muscle with PSC-derived myogenic progenitors in a mouse model for FSHD, and highlight the potential for the clinical benefit of such a cell therapy approach.

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.

Engraftment of human induced pluripotent stem cell-derived myogenic progenitors restores dystrophin in mice with duchenne muscular dystrophy

Background

Duchenne muscular dystrophy (DMD) is a devastating genetic muscular disorder with no effective treatment that is caused by the loss of dystrophin. Human induced pluripotent stem cells (hiPSCs) offer a promising unlimited resource for cell-based therapies of muscular dystrophy. However, their clinical applications are hindered by inefficient myogenic differentiation, and moreover, the engraftment of non-transgene hiPSC-derived myogenic progenitors has not been examined in the mdx mouse model of DMD.

Methods

We investigated the muscle regenerative potential of myogenic progenitors derived from hiPSCs in mdx mice. The hiPSCs were transfected with enhanced green fluorescent protein (EGFP) vector and defined as EGFP hiPSCs. Myogenic differentiation was performed on EGFP hiPSCs with supplementary of basic fibroblast growth factor, forskolin, 6-bromoindirubin-3′-oxime as well as horse serum. EGFP hiPSCs-derived myogenic progenitors were engrafted into mdx mice via both intramuscular and intravenous injection. The restoration of dystrophin expression, the ratio of central nuclear myofibers, and the transplanted cells-derived satellite cells were accessed after intramuscular and systemic transplantation.

Results

We report that abundant myogenic progenitors can be generated from hiPSCs after treatment with these three small molecules, with consequent terminal differentiation giving rise to mature myotubes in vitro. Upon intramuscular or systemic transplantation into mdx mice, these myogenic progenitors engrafted and contributed to human-derived myofiber regeneration in host muscles, restored dystrophin expression, ameliorated pathological lesions, and seeded the satellite cell compartment in dystrophic muscles.

Conclusions

This study demonstrates the muscle regeneration potential of myogenic progenitors derived from hiPSCs using non-transgenic induction methods. Engraftment of hiPSC-derived myogenic progenitors could be a potential future therapeutic strategy to treat DMD in a clinical setting.

Efficient engraftment of pluripotent stem cell-derived myogenic progenitors in a novel immunodeficient mouse model of limb girdle muscular dystrophy 2I

BACKGROUND

Defects in α-dystroglycan (DG) glycosylation characterize a group of muscular dystrophies known as dystroglycanopathies. One of the key effectors in the α-DG glycosylation pathway is the glycosyltransferase fukutin-related protein (FKRP). Mutations in FKRP lead to a large spectrum of muscular dystrophies, including limb girdle muscular dystrophy 2I (LGMD2I). It remains unknown whether stem cell transplantation can promote muscle regeneration and ameliorate the muscle wasting phenotype associated with FKRP mutations.

RESULTS

Here we transplanted murine and human pluripotent stem cell-derived myogenic progenitors into a novel immunodeficient FKRP-mutant mouse model by intra-muscular injection. Upon both mouse and human cell transplantation, we observe the presence of donor-derived myofibers even in absence of pre-injury, and the rescue of α-DG functional glycosylation, as shown by IIH6 immunoreactivity. The presence of donor-derived cells expressing Pax7 under the basal lamina is indicative of satellite cell engraftment, and therefore, long-term repopulation potential. Functional assays performed in the mouse-to-mouse cohort revealed enhanced specific force in transplanted muscles compared to PBS-injected controls.

CONCLUSIONS

Altogether, our data demonstrate for the first time the suitability of a cell-based therapeutic approach to improve the muscle phenotype of dystrophic FKRP-mutant mice.

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.

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.

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.

Recapitulating early development of mouse musculoskeletal precursors of the paraxial mesoderm in vitro

Body skeletal muscles derive from the paraxial mesoderm, which forms in the posterior region of the embryo. Using microarrays, we characterize novel mouse presomitic mesoderm (PSM) markers and show that, unlike the abrupt transcriptome reorganization of the PSM, neural tube differentiation is accompanied by progressive transcriptome changes. The early paraxial mesoderm differentiation stages can be efficiently recapitulated in vitro using mouse and human pluripotent stem cells. While Wnt activation alone can induce posterior PSM markers, acquisition of a committed PSM fate and efficient differentiation into anterior PSM Pax3+ identity further requires BMP inhibition to prevent progenitors from drifting to a lateral plate mesoderm fate. When transplanted into injured adult muscle, these precursors generated large numbers of immature muscle fibers. Furthermore, exposing these mouse PSM-like cells to a brief FGF inhibition step followed by culture in horse serum-containing medium allows efficient recapitulation of the myogenic program to generate myotubes and associated Pax7+ cells. This protocol results in improved in vitro differentiation and maturation of mouse muscle fibers over serum-free protocols and enables the study of myogenic cell fusion and satellite cell differentiation.

Generation of skeletal myogenic progenitors from human pluripotent stem cells using non-viral delivery of minicircle DNA

Currently, the most efficient and promising approach for generating large numbers of engraftable human skeletal myogenic progenitors from pluripotent stem cells requires the conditional in vitro overexpression of PAX7 using lentiviral vectors. Because a non-integrating approach would be preferable to eliminate or minimize the risk associated with random genomic integration, here we investigate whether transient expression of PAX7 using minicircle DNA would enable the generation of functional pluripotent stem cell-derived myogenic progenitors, equivalent to those generated by lentivirus. Our results demonstrate that upon multiple transfections, the minicircle approach allows for the scalable generation of myogenic progenitors and these undergo efficient terminal differentiation in vitro. However, transplantation of minicircle-generated myogenic progenitors resulted in limited engraftment. This is probably due to less efficient delivery and more transient PAX7 expression in these cultures since PAX7 downregulation is accompanied by high level of spontaneous differentiation. Thus, although the in vitro data shows that the minicircle approach could potentially replace the use of lentivirus, improvements in the transfection/expression system will be necessary before it will be a feasible strategy for the generation of myogenic progenitors for cell replacement therapy.

Stem Cell Differentiation Toward the Myogenic Lineage for Muscle Tissue Regeneration: A Focus on Muscular Dystrophy

Skeletal muscle tissue engineering is one of the important ways for regenerating functionally defective muscles. Among the myopathies, the Duchenne muscular dystrophy (DMD) is a progressive disease due to mutations of the dystrophin gene leading to progressive myofiber degeneration with severe symptoms. Although current therapies in muscular dystrophy are still very challenging, important progress has been made in materials science and in cellular technologies with the use of stem cells. It is therefore useful to review these advances and the results obtained in a clinical point of view. This article focuses on the differentiation of stem cells into myoblasts, and their application in muscular dystrophy. After an overview of the different stem cells that can be induced to differentiate into the myogenic lineage, we introduce scaffolding materials used for muscular tissue engineering. We then described some widely used methods to differentiate different types of stem cell into myoblasts. We highlight recent insights obtained in therapies for muscular dystrophy. Finally, we conclude with a discussion on stem cell technology. We discussed in parallel the benefits brought by the evolution of the materials and by the expansion of cell sources which can differentiate into myoblasts. We also discussed on future challenges for clinical applications and how to accelerate the translation from the research to the clinic in the frame of DMD.

Pax3-induced expansion enables the genetic correction of dystrophic satellite cells

Background

Satellite cells (SCs) are indispensable for muscle regeneration and repair; however, due to low frequency in primary muscle and loss of engraftment potential after ex vivo expansion, their use in cell therapy is currently unfeasible. To date, an alternative to this limitation has been the transplantation of SC-derived myogenic progenitor cells (MPCs), although these do not hold the same attractive properties of stem cells, such as self-renewal and long-term regenerative potential.

Methods

We develop a method to expand wild-type and dystrophic fresh isolated satellite cells using transient expression of Pax3. This approach can be combined with genetic correction of dystrophic satellite cells and utilized to promote muscle regeneration when transplanted into dystrophic mice.

Results

Here, we show that SCs from wild-type and dystrophic mice can be expanded in culture through transient expression of Pax3, and these expanded activated SCs can regenerate the muscle. We test this approach in a gene therapy model by correcting dystrophic SCs from a mouse lacking dystrophin using a Sleeping Beauty transposon carrying the human μDYSTROPHIN gene. Transplantation of these expanded corrected cells into immune-deficient, dystrophin-deficient mice generated large numbers of dystrophin-expressing myofibers and improved contractile strength. Importantly, in vitro expanded SCs engrafted the SC compartment and could regenerate muscle after secondary injury.

Conclusion

These results demonstrate that Pax3 is able to promote the ex vivo expansion of SCs while maintaining their stem cell regenerative properties.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0061-7) contains supplementary material, which is available to authorized users.

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.

Sdf-1 (CXCL12) induces CD9 expression in stem cells engaged in muscle regeneration

INTRODUCTION

Understanding the mechanism of stem cell mobilization into injured skeletal muscles is a prerequisite step for the development of muscle disease therapies. Many of the currently studied stem cell types present myogenic potential; however, when introduced either into the blood stream or directly into the tissue, they are not able to efficiently engraft injured muscle. For this reason their use in therapy is still limited. Previously, we have shown that stromal-derived factor-1 (Sdf-1) caused the mobilization of endogenous (not transplanted) stem cells into injured skeletal muscle improving regeneration. Here, we demonstrate that the beneficial effect of Sdf-1 relies on the upregulation of the tetraspanin CD9 expression in stem cells.

METHODS

The expression pattern of adhesion proteins, including CD9, was analysed after Sdf-1 treatment during regeneration of rat skeletal muscles and mouse Pax7-/- skeletal muscles, that are characterized by the decreased number of satellite cells. Next, we examined the changes in CD9 level in satellite cells-derived myoblasts, bone marrow-derived mesenchymal stem cells, and embryonic stem cells after Sdf-1 treatment or silencing expression of CXCR4 and CXCR7. Finally, we examined the potential of stem cells to fuse with myoblasts after Sdf-1 treatment.

RESULTS

In vivo analyses of Pax7-/- mice strongly suggest that Sdf-1-mediates increase in CD9 levels also in mobilized stem cells. In the absence of CXCR4 receptor the effect of Sdf-1 on CD9 expression is blocked. Next, in vitro studies show that Sdf-1 increases the level of CD9 not only in satellite cell-derived myoblasts but also in bone marrow derived mesenchymal stem cells, as well as embryonic stem cells. Importantly, the Sdf-1 treated cells migrate and fuse with myoblasts more effectively.

CONCLUSIONS

We suggest that Sdf-1 binding CXCR4 receptor improves skeletal muscle regeneration by upregulating expression of CD9 and thus, impacting at stem cells mobilization to the injured muscles.

Fetal stem cells and skeletal muscle regeneration: a therapeutic approach

More than 40% of the body mass is represented by muscle tissue, which possesses the innate ability to regenerate after damage through the activation of muscle-specific stem cells, namely satellite cells. Muscle diseases, in particular chronic degenerative states of skeletal muscle such as dystrophies, lead to a perturbation of the regenerative process, which causes the premature exhaustion of satellite cell reservoir due to continuous cycles of degeneration/regeneration. Nowadays, the research is focused on different therapeutic approaches, ranging from gene and cell to pharmacological therapy, but still there is no definitive cure in particular for genetic muscle disease. Keeping this in mind, in this article, we will give special consideration to muscle diseases and the use of fetal derived stem cells as a new approach for therapy. Cells of fetal origin, from cord blood to placenta and amniotic fluid, can be easily obtained without ethical concern, expanded and differentiated in culture, and possess immune-modulatory properties. The in vivo approach in animal models can be helpful to study the mechanism underneath the operating principle of the stem cell reservoir, namely the niche, which holds great potential to understand the onset of muscle pathologies.

WNT3A promotes myogenesis of human embryonic stem cells and enhances in vivo engraftment

The ability of human embryonic stem cells (hESCs) to differentiate into skeletal muscle cells is an important criterion in using them as a cell source to ameliorate skeletal muscle impairments. However, differentiation of hESCs into skeletal muscle cells still remains a challenge, often requiring introduction of transgenes. Here, we describe the use of WNT3A protein to promote in vitro myogenic commitment of hESC-derived cells and their subsequent in vivo function. Our findings show that the presence of WNT3A in culture medium significantly promotes myogenic commitment of hESC-derived progenitors expressing a mesodermal marker, platelet-derived growth factor receptor-α (PDGFRA), as evident from the expression of myogenic markers, including DES, MYOG, MYH1, and MF20. In vivo transplantation of these committed cells into cardiotoxin-injured skeletal muscles of NOD/SCID mice reveals survival and engraftment of the donor cells. The cells contributed to the regeneration of damaged muscle fibers and the satellite cell compartment. In lieu of the limited cell source for treating skeletal muscle defects, the hESC-derived PDGFRA(+) cells exhibit significant in vitro expansion while maintaining their myogenic potential. The results described in this study provide a proof-of-principle that myogenic progenitor cells with in vivo engraftment potential can be derived from hESCs without genetic manipulation.

Stem cell transplantation for muscular dystrophy: the challenge of immune response

Treating muscle disorders poses several challenges to the rapidly evolving field of regenerative medicine. Considerable progress has been made in isolating, characterizing, and expanding myogenic stem cells and, although we are now envisaging strategies to generate very large numbers of transplantable cells (e.g., by differentiating induced pluripotent stem cells), limitations directly linked to the interaction between transplanted cells and the host will continue to hamper a successful outcome. Among these limitations, host inflammatory and immune responses challenge the critical phases after cell delivery, including engraftment, migration, and differentiation. Therefore, it is key to study the mechanisms and dynamics that impair the efficacy of cell transplants in order to develop strategies that can ultimately improve the outcome of allogeneic and autologous stem cell therapies, in particular for severe disease such as muscular dystrophies. In this review we provide an overview of the main players and issues involved in this process and discuss potential approaches that might be beneficial for future regenerative therapies of skeletal muscle.

Emerging gene editing strategies for Duchenne muscular dystrophy targeting stem cells

The progressive loss of muscle mass characteristic of many muscular dystrophies impairs the efficacy of most of the gene and molecular therapies currently being pursued for the treatment of those disorders. It is becoming increasingly evident that a therapeutic application, to be effective, needs to target not only mature myofibers, but also muscle progenitors cells or muscle stem cells able to form new muscle tissue and to restore myofibers lost as the result of the diseases or during normal homeostasis so as to guarantee effective and lost lasting effects. Correction of the genetic defect using oligodeoxynucleotides (ODNs) or engineered nucleases holds great potential for the treatment of many of the musculoskeletal disorders. The encouraging results obtained by studying in vitro systems and model organisms have set the groundwork for what is likely to become an emerging field in the area of molecular and regenerative medicine. Furthermore, the ability to isolate and expand from patients various types of muscle progenitor cells capable of committing to the myogenic lineage provides the opportunity to establish cell lines that can be used for transplantation following ex vivo manipulation and expansion. The purpose of this article is to provide a perspective on approaches aimed at correcting the genetic defect using gene editing strategies and currently under development for the treatment of Duchenne muscular dystrophy (DMD), the most sever of the neuromuscular disorders. Emphasis will be placed on describing the potential of using the patient own stem cell as source of transplantation and the challenges that gene editing technologies face in the field of regenerative biology.

Molecular and cell-based therapies for muscle degenerations: a road under construction

Despite the advances achieved in understanding the molecular biology of muscle cells in the past decades, there is still need for effective treatments of muscular degeneration caused by muscular dystrophies and for counteracting the muscle wasting caused by cachexia or sarcopenia. The corticosteroid medications currently in use for dystrophic patients merely help to control the inflammatory state and only slightly delay the progression of the disease. Unfortunately, walkers and wheel chairs are the only options for such patients to maintain independence and walking capabilities until the respiratory muscles become weak and the mechanical ventilation is needed. On the other hand, myostatin inhibition, IL-6 antagonism and synthetic ghrelin administration are examples of promising treatments in cachexia animal models. In both dystrophies and cachectic syndrome the muscular degeneration is extremely relevant and the translational therapeutic attempts to find a possible cure are well defined. In particular, molecular-based therapies are common options to be explored in order to exploit beneficial treatments for cachexia, while gene/cell therapies are mostly used in the attempt to induce a substantial improvement of the dystrophic muscular phenotype. This review focuses on the description of the use of molecular administrations and gene/stem cell therapy to treat muscular degenerations. It reviews previous trials using cell delivery protocols in mice and patients starting with the use of donor myoblasts, outlining the likely causes for their poor results and briefly focusing on satellite cell studies that raise new hope. Then it proceeds to describe recently identified stem/progenitor cells, including pluripotent stem cells and in relationship to their ability to home within a dystrophic muscle and to differentiate into skeletal muscle cells. Different known features of various stem cells are compared in this perspective, and the few available examples of their use in animal models of muscular degeneration are reported. Since non coding RNAs, including microRNAs (miRNAs), are emerging as prominent players in the regulation of stem cell fates we also provides an outline of the role of microRNAs in the control of myogenic commitment. Finally, based on our current knowledge and the rapid advance in stem cell biology, a prediction of clinical translation for cell therapy protocols combined with molecular treatments is discussed.

Stem cells for skeletal muscle regeneration: therapeutic potential and roadblocks

Conditions involving muscle wasting, such as muscular dystrophies, cachexia, and sarcopenia, would benefit from approaches that promote skeletal muscle regeneration. Stem cells are particularly attractive because they are able to differentiate into specialized cell types while retaining the ability to self-renew and, thus, provide a long-term response. This review will discuss recent advancements on different types of stem cells that have been attributed to be endowed with muscle regenerative potential. We will discuss the nature of these cells and their advantages and disadvantages in regards to therapy for muscular dystrophies.

Directed in vitro myogenesis of human embryonic stem cells and their in vivo engraftment

Development of human embryonic stem cell (hESC)-based therapy requires derivation of in vitro expandable cell populations that can readily differentiate to specified cell types and engraft upon transplantation. Here, we report that hESCs can differentiate into skeletal muscle cells without genetic manipulation. This is achieved through the isolation of cells expressing a mesodermal marker, platelet-derived growth factor receptor-α (PDGFRA), following embryoid body (EB) formation. The ESC-derived cells differentiated into myoblasts in vitro as evident by upregulation of various myogenic genes, irrespective of the presence of serum in the medium. This result is further corroborated by the presence of sarcomeric myosin and desmin, markers for terminally differentiated cells. When transplanted in vivo, these pre-myogenically committed cells were viable in tibialis anterior muscles 14 days post-implantation. These hESC-derived cells, which readily undergo myogenic differentiation in culture medium containing serum, could be a viable cell source for skeletal muscle repair and tissue engineering to ameliorate various muscle wasting diseases.

An ex vivo gene therapy approach to treat muscular dystrophy using inducible pluripotent stem cells

Duchenne muscular dystrophy is a progressive and incurable neuromuscular disease caused by genetic and biochemical defects of the dystrophin-glycoprotein complex. Here we show the regenerative potential of myogenic progenitors derived from corrected dystrophic induced pluripotent stem (iPS) cells generated from fibroblasts of mice lacking both dystrophin and utrophin. We correct the phenotype of dystrophic iPS cells using a Sleeping Beauty transposon carrying the micro-utrophin (μUTRN) gene, differentiate these cells into skeletal muscle progenitors, and transplant them back into dystrophic mice. Engrafted muscles displayed large numbers of micro-utrophin-positive myofibers, with biochemically restored dystrophin-glycoprotein complex and improved contractile strength. The transplanted cells seed the satellite cell compartment, responded properly to injury and exhibit neuromuscular synapses. We also detect muscle engraftment after systemic delivery of these corrected progenitors. These results represent an important advance toward the future treatment of muscular dystrophies using genetically corrected autologous iPS cells.