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

Skeletal muscle cell protein dysregulation highlights the pathogenesis mechanism of myopathy-associated p97/VCP R155H mutations

p97/VCP, a hexametric member of the AAA-ATPase superfamily, has been associated with a wide range of cellular protein pathways, such as proteasomal degradation, the unfolding of polyubiquitinated proteins, and autophagosome maturation. Autosomal dominant p97/VCP mutations cause a rare hereditary multisystem disorder called IBMPFD/ALS (Inclusion Body Myopathy with Paget's Disease and Frontotemporal Dementia/Amyotrophic Lateral Sclerosis), characterized by progressive weakness and subsequent atrophy of skeletal muscles, and impacting bones and brains, such as Parkinson's disease, Lewy body disease, Huntington's disease, and amyotrophic lateral ALS. Among all disease-causing mutations, Arginine 155 to Histidine (R155H/+) was reported to be the most common one, affecting over 50% of IBMPFD patients, resulting in disabling muscle weakness, which might eventually be life-threatening due to cardiac and respiratory muscle involvement. Induced pluripotent stem cells (iPSCs) offer an unlimited resource of cells to study pathology's underlying molecular mechanism, perform drug screening, and investigate regeneration. Using R155H/+ patients' fibroblasts, we generated IPS cells and corrected the mutation (Histidine to Arginine, H155R) to generate isogenic control cells before differentiating them into myotubes. The further proteomic analysis allowed us to identify differentially expressed proteins associated with the R155H mutation. Our results showed that R155H/+ cells were associated with dysregulated expression of several proteins involved in skeletal muscle function, cytoskeleton organization, cell signaling, intracellular organelles organization and function, cell junction, and cell adhesion. Our findings provide molecular evidence of dysfunctional protein expression in R155H/+ myotubes and offer new therapeutic targets for treating IBMPFD/ALS.

A Myogenic Double-Reporter Human Pluripotent Stem Cell Line Allows Prospective Isolation of Skeletal Muscle Progenitors

Myogenic differentiation of human pluripotent stem cells (hPSCs) has been done by gene overexpression or directed differentiation. However, viral integration, long-term culture, and the presence of unwanted cells are the main obstacles. By using CRISPR/Cas9n, a double-reporter human embryonic stem cell (hESC) line was generated for PAX7/MYF5, allowing prospective readout. This strategy allowed pathway screen to define efficient myogenic induction in hPSCs. Next, surface marker screen allowed identification of CD10 and CD24 for purification of myogenic progenitors and exclusion of non-myogenic cells. CD10 expression was also identified on human satellite cells and skeletal muscle progenitors. In vitro and in vivo studies using transgene and/or reporter-free hPSCs further validated myogenic potential of the cells by formation of new fibers expressing human dystrophin as well as donor-derived satellite cells in NSG-mdx^4Cv mice. This study provides biological insights for myogenic differentiation of hPSCs using a double-reporter cell resource and defines an improved myogenic differentiation and purification strategy.

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.

Abnormal nuclear aggregation and myotube degeneration in myotonic dystrophy type 1

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

Human iPSC Models to Study Orphan Diseases: Muscular Dystrophies

Purpose of Review

Muscular dystrophies (MDs) are a spectrum of muscle disorders, which are caused by a number of gene mutations. The studies of MDs are limited due to lack of appropriate models, except for Duchenne muscular dystrophy (DMD), myotonic dystrophy type 1 (DM1), facioscapulohumeral muscular dystrophy (FSHD), and certain type of limb-girdle muscular dystrophy (LGMD). Human induced pluripotent stem cell (iPSC) technologies are emerging to offer a useful model for mechanistic studies, drug discovery, and cell-based therapy to supplement in vivo animal models. This review will focus on current applications of iPSC as disease models of MDs for studies of pathogenic mechanisms and therapeutic development.

Recent Findings

Many and more human disease-specific iPSCs have been or being established, which carry the natural mutation of MDs with human genomic background. These iPSCs can be differentiated into specific cell types affected in a particular MDs such as skeletal muscle progenitor cells, skeletal muscle fibers, and cardiomyocytes. Human iPSCs are particularly useful for studies of the pathogenicity at the early stage or developmental phase of MDs. High-throughput screening using disease-specific human iPSCs has become a powerful technology in drug discovery. While MD iPSCs have been generated for cell-based replacement therapy, recent advances in genome editing technologies enabled correction of genetic mutations in these cells in culture, raising hope for in vivo genome therapy, which offers a fundamental cure for these daunting inherited MDs.

Summary

Human disease-specific iPSC models for MDs are emerging as an additional tool to current disease models for elucidating disease mechanisms and developing therapeutic intervention.

Recapitulating muscle disease phenotypes with myotonic dystrophy 1 induced pluripotent stem cells: a tool for disease modeling and drug discovery

Myotonic dystrophy 1 (DM1) is a multisystem disorder primarily affecting the central nervous system, heart and skeletal muscle. It is caused by an expansion of the CTG trinucleotide repeats in the 3' untranslated region of the DMPK gene. Although patient myoblasts have been used for studying the disease in vitro, the invasiveness as well as the low accessibility to muscle biopsies motivate the development of alternative reliable myogenic models. Here, we established two DM1 induced pluripotent stem (iPS) cell lines from patient-derived fibroblasts and, using the PAX7 conditional expression system, differentiated these into myogenic progenitors and, subsequently, terminally differentiated myotubes. Both DM1 myogenic progenitors and myotubes were found to express the intranuclear RNA foci exhibiting sequestration of MBNL1. Moreover, we found the DM1-related mis-splicing, namely BIN1 exon 11 in DM1 myotubes. We used this model to test a specific therapy, antisense oligonucleotide treatment, and found that this efficiently abolished RNA foci and rescued BIN1 mis-splicing in DM1 iPS cell-derived myotubes. Together, our results demonstrate that myotubes derived from DM1 iPS cells recapitulate the critical molecular features of DM1 and are sensitive to antisense oligonucleotide treatment, confirming that these cells can be used for in vitro disease modeling and candidate drug testing or screening.This article has an associated First Person interview with the first author of the paper.

Supplementation with IL-6 and Muscle Cell Culture Conditioned Media Enhances Myogenic Differentiation of Adipose Tissue-Derived Stem Cells through STAT3 Activation

Mature skeletal muscle cells cannot be expanded in culture systems. Therefore, it is difficult to construct an in vitro model for muscle diseases. To establish an efficient protocol for myogenic differentiation of human adipose tissue-derived stem cells (hADSCs), we investigated whether addition of IL-6 and/or myocyte-conditioned media (CM) to conventional differentiation media can shorten the differentiation period. hADSCs were differentiated to myocytes using the conventional protocol or modified with the addition of 25 pg/mL IL-6 and/or C2C12 CM (25% v/v). The expression of MyoD and myogenine mRNA was significantly higher at 5⁻6 days after differentiation using the modified protocol than with the conventional protocol. mRNA and protein expression of myosin heavy chain, a marker of myotubes, was significantly upregulated at 28 and 42 days of differentiation using the modified protocol, and the level achieved after a 4-week differentiation period was similar to that achieved at 6 weeks using the conventional protocol. The expression of p-STAT3 was significantly increased when the modified protocol was used. Similarly, addition of colivelin, a STAT3 activator, instead of IL-6 and C2C12 CM, promoted the myogenic differentiation of ADSCs. The modified protocol improved differentiation efficiency and reduced the time required for differentiation of myocytes. It might be helpful to save cost and time when preparing myocytes for cell therapies and drug discovery.

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.

Stem Cells for Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a debilitating condition wherein muscle loss overwhelms the body's normal physiological repair mechanism. VML is particularly common among military service members who have sustained war injuries. Because of the high social and medical cost associated with VML and suboptimal current surgical treatments, there is great interest in developing better VML therapies. Skeletal muscle tissue engineering (SMTE) is a promising alternative to traditional VML surgical treatments that use autogenic tissue grafts, and rather uses isolated stem cells with myogenic potential to generate de novo skeletal muscle tissues to treat VML. Satellite cells are the native precursors to skeletal muscle tissue, and are thus the most commonly studied starting source for SMTE. However, satellite cells are difficult to isolate and purify, and it is presently unknown whether they would be a practical source in clinical SMTE applications. Alternative myogenic stem cells, including adipose-derived stem cells, bone marrow-derived mesenchymal stem cells, perivascular stem cells, umbilical cord mesenchymal stem cells, induced pluripotent stem cells, and embryonic stem cells, each have myogenic potential and have been identified as possible starting sources for SMTE, although they have yet to be studied in detail for this purpose. These alternative stem cell varieties offer unique advantages and disadvantages that are worth exploring further to advance the SMTE field toward highly functional, safe, and practical VML treatments. The following review summarizes the current state of satellite cell-based SMTE, details the properties and practical advantages of alternative myogenic stem cells, and offers guidance to tissue engineers on how alternative myogenic stem cells can be incorporated into SMTE research.

Development of the excitation-contraction coupling machinery and its relation to myofibrillogenesis in human iPSC-derived skeletal myocytes

Background

Human induced pluripotent stem cells-derived myogenic progenitors develop functional and ultrastructural features typical of skeletal muscle when differentiated in culture. Besides disease-modeling, such a system can be used to clarify basic aspects of human skeletal muscle development. In the present study, we focus on the development of the excitation-contraction (E-C) coupling, a process that is essential both in muscle physiology and as a tool to differentiate between the skeletal and cardiac muscle. The occurrence and maturation of E-C coupling structures (Sarcoplasmic Reticulum-Transverse Tubule (SR-TT) junctions), key molecular components, and Ca²⁺ signaling were examined, along with myofibrillogenesis.

Methods

Pax7⁺-myogenic progenitors were differentiated in culture, and developmental changes were examined from a few days up to several weeks. Ion channels directly involved in the skeletal muscle E-C coupling (RyR1 and Cav1.1 voltage-gated Ca²⁺ channels) were labeled using indirect immunofluorescence. Ultrastructural changes of differentiating cells were visualized by transmission electron microscopy. On the functional side, depolarization-induced intracellular Ca²⁺ transients mediating E-C coupling were recorded using Fura-2 ratiometric Ca²⁺ imaging, and myocyte contraction was captured by digital photomicrography.

Results

We show that the E-C coupling machinery occurs and operates within a few days post-differentiation, as soon as the myofilaments align. However, Ca²⁺ transients become effective in triggering myocyte contraction after 1 week of differentiation, when nascent myofibrils show alternate A-I bands. At later stages, myofibrils become fully organized into adult-like sarcomeres but SR-TT junctions do not reach their triadic structure and typical A-I location. This is mirrored by the absence of cross-striated distribution pattern of both RyR1 and Cav1.1 channels.

Conclusions

The E-C coupling machinery occurs and operates within the first week of muscle cells differentiation. However, while early development of SR-TT junctions is coordinated with that of nascent myofibrils, their respective maturation is not. Formation of typical triads requires other factors/conditions, and this should be taken into account when using in-vitro models to explore skeletal muscle diseases, especially those affecting E-C coupling.

Electronic supplementary material

The online version of this article (10.1186/s13395-017-0147-5) contains supplementary material, which is available to authorized users.

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.

Generation of human muscle fibers and satellite-like cells from human pluripotent stem cells in vitro

Progress toward finding a cure for muscle diseases has been slow because of the absence of relevant cellular models and the lack of a reliable source of muscle progenitors for biomedical investigation. Here we report an optimized serum-free differentiation protocol to efficiently produce striated, millimeter-long muscle fibers together with satellite-like cells from human pluripotent stem cells (hPSCs) in vitro. By mimicking key signaling events leading to muscle formation in the embryo, in particular the dual modulation of Wnt and bone morphogenetic protein (BMP) pathway signaling, this directed differentiation protocol avoids the requirement for genetic modifications or cell sorting. Robust myogenesis can be achieved in vitro within 1 month by personnel experienced in hPSC culture. The differentiating culture can be subcultured to produce large amounts of myogenic progenitors amenable to numerous downstream applications. Beyond the study of myogenesis, this differentiation method offers an attractive platform for the development of relevant in vitro models of muscle dystrophies and drug screening strategies, as well as providing a source of cells for tissue engineering and cell therapy approaches.

Direct reprogramming of urine-derived cells with inducible MyoD for modeling human muscle disease

BACKGROUND

Cellular models of muscle disease are taking on increasing importance with the large number of genes and mutations implicated in causing myopathies and the concomitant need to test personalized therapies. Developing cell models relies on having an easily obtained source of cells, and if the cells are not derived from muscle itself, a robust reprogramming process is needed. Fibroblasts are a human cell source that works well for the generation of induced pluripotent stem cells, which can then be differentiated into cardiomyocyte lineages, and with less efficiency, skeletal muscle-like lineages. Alternatively, direct reprogramming with the transcription factor MyoD has been used to generate myotubes from cultured human fibroblasts. Although useful, fibroblasts require a skin biopsy to obtain and this can limit their access, especially from pediatric populations.

RESULTS

We now demonstrate that direct reprogramming of urine-derived cells is a highly efficient and reproducible process that can be used to establish human myogenic cells. We show that this method can be applied to urine cells derived from normal individuals as well as those with muscle diseases. Furthermore, we show that urine-derived cells can be edited using CRISPR/Cas9 technology.

CONCLUSIONS

With progress in understanding the molecular etiology of human muscle diseases, having a readily available, noninvasive source of cells from which to generate muscle-like cells is highly useful.

Differentiation of control and ALS mutant human iPSCs into functional skeletal muscle cells, a tool for the study of neuromuscolar diseases

Amyotrophic Lateral Sclerosis (ALS) is a severe and fatal neurodegenerative disease characterized by progressive loss of motoneurons, muscle atrophy and paralysis. Recent evidence suggests that ALS should be considered as a multi-systemic disease, in which several cell types contribute to motoneuron degeneration. In this view, mutations in ALS linked genes in other neural and non-neural cell types may exert non-cell autonomous effects on motoneuron survival and function. Induced Pluripotent Stem Cells (iPSCs) have been recently derived from several patients with ALS mutations and it has been shown that they can generate motoneurons in vitro, providing a valuable tool to study ALS. However, the potential of iPSCs could be further valorized by generating other cell types that may be relevant to the pathology. In this paper, by taking advantage of a novel inducible system for MyoD expression, we show that both control iPSCs and iPSCs carrying mutations in ALS genes can generate skeletal muscle cells. We provide evidence that both control and mutant iPSC-derived myotubes are functionally active. This in vitro system will be instrumental to dissect the molecular and cellular pathways impairing the complex motoneuron microenvironment in ALS.

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A novel method for inducing iPSCs differentiation into muscle is presented

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Multiple inducible lines can be easily generated by a new transposable vector

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Both control and iPSCs carrying ALS mutations can generate functional muscle fibers

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This system will be instrumental to study non-cell autonomous contributions to ALS

Pax3 and Pax7 interact reciprocally and regulate the expression of cadherin-7 through inducing neuron differentiation in the developing chicken spinal cord

Pax3 and Pax7 are closely related transcription factors that are widely expressed in the developing nervous system and somites. In the CNS, both genes are expressed in the dorsal part of the neural tube during development. Pax3 and Pax7 are involved in the sonic hedgehog (Shh) signaling pathway and are inhibited by Shh overexpression. The present study confirms in vivo that Pax3 overexpression represses the expression of Pax7, whereas Pax7 overexpression endogenously enhances and ectopically induces the expression of Pax3 in the developing chicken spinal cord. Overexpression of Pax3 and Pax7 represses the endogenous expression of cadherin-7, a member of the cadherin family of morphogenetic genes, and induces its ectopic expression. The present study also shows that overexpression of Pax3 and Pax7 changes the fate and morphology of cells in the neuroepithelial layer and induces the expression of postmitotic neuronal markers. We show that both Pax3 and Pax7 promote the differentiation of neural progenitor cells into neurons. Furthermore, the downregulation of Pax3 and Pax7 with specific shRNAs results in apoptosis in the developing spinal cord. Collectively, these results suggest that the transcription factors Pax3 and Pax7 play important roles in regulating morphogenesis and cell differentiation in the developing spinal cord.

Skeletal muscle perfusion and stem cell delivery in muscle disorders using intra-femoral artery canulation in mice

Muscular dystrophies are among major inherited muscle disorders characterized by progressive muscle damage and fibrosis with no definitive cure. Recently, gene or cell based therapies have been developed to restore the missing gene expression or replace the damaged tissues. In order to test the efficiency of these therapies in mice models of muscular dystrophies, the arterial route of delivery is very advantageous as it provides uniform muscle exposure to the therapeutic agents or cells. Although there are few reports of arterial delivery of the therapeutic agents or cells in mice, there is no in-depth description and evaluation of its efficacy in perfusion of downstream muscles. This study is aimed to develop a practical method for intra-femoral artery perfusion in mice and to evaluate perfusion efficiency using near-infrared-fluorescence (NIRF) imaging as well as histology following stem cell delivery. Our results provide a practical guide to perform this delicate method in mice. By using a sensitive fluorescent dye, different muscle groups of the hindlimb have been evaluated for proper perfusion. As the final step, we have validated the efficiency of arterial cell delivery into muscles using human iPS-derived myogenic cells in an immunodeficient mouse model for Duchenne muscular dystrophy (NSG-mdx(4cv)).