The fate of individual myoblasts after transplantation into muscles of DMD patients

Mouse regenerating myofibers detected as false-positive donor myofibers with anti-human spectrin

Abstract Stem cell transplantation is being tested as a potential therapy for a number of diseases. Stem cells isolated directly from tissue specimens or generated via reprogramming of differentiated cells require rigorous testing for both safety and efficacy in preclinical models. The availability of mice with immune-deficient background that carry additional mutations in specific genes facilitates testing the efficacy of cell transplantation in disease models. The muscular dystrophies are a heterogeneous group of disorders, of which Duchenne muscular dystrophy is the most severe and common type. Cell-based therapy for muscular dystrophy has been under investigation for several decades, with a wide selection of cell types being studied, including tissue-specific stem cells and reprogrammed stem cells. Several immune-deficient mouse models of muscular dystrophy have been generated, in which human cells obtained from various sources are injected to assess their preclinical potential. After transplantation, the presence of engrafted human cells is detected via immunofluorescence staining, using antibodies that recognize human, but not mouse, proteins. Here we show that one antibody specific to human spectrin, which is commonly used to evaluate the efficacy of transplanted human cells in mouse muscle, detects myofibers in muscles of NOD/Rag1(null)mdx(5cv), NOD/LtSz-scid IL2Rγ(null) mice, or mdx nude mice, irrespective of whether they were injected with human cells. These "reactive" clusters are regenerating myofibers, which are normally present in dystrophic tissue and the spectrin antibody is likely recognizing utrophin, which contains spectrin-like repeats. Therefore, caution should be used in interpreting data based on detection of single human-specific proteins, and evaluation of human stem cell engraftment should be performed using multiple human-specific labeling strategies.

Inhibition of Drp1-dependent mitochondrial division impairs myogenic differentiation

Mitochondria are dynamic organelles forming a tubular network that is continuously fusing and dividing to control their morphology and functions. Recent literature has shed new light on a potential link between the dynamic behavior of mitochondria and muscle development. In this study, we investigate the role of mitochondrial fission factor dynamin-related protein 1 (Drp1) in myogenic differentiation. We found that differentiation of C2C12 myoblasts induced by serum starvation was accompanied by a gradual increase in Drp1 protein expression (to ∼350% up to 3 days) and a fast reduction of Drp1 phosphorylation at Ser-637 (to ∼30%) resulting in translocation of Drp1 protein from the cytosol to mitochondria. During differentiation, treatment of myoblasts with mitochondrial division inhibitor (mdivi-1), a specific inhibitor of Drp1 GTPase activity, caused extensive formation of elongated mitochondria, which coincided with increased apoptosis evidenced by both enhanced caspase-3 activity and increased number of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive cells. Furthermore, the mdivi-1-treated myotubes (day 3 in differentiation media) showed a reduction in mitochondrial DNA content, mitochondrial mass, and membrane potential in a dose-dependent manner indicating defects in mitochondrial biogenesis during myogenic differentiation. Most interestingly, mdivi-1 treatment significantly suppressed myotube formation in both C2C12 cells and primary myoblasts. Likewise, stable overexpression of a dominant negative mutant Drp1 (K38A) dramatically reduced myogenic differentiation. These data suggest that Drp-1-dependent mitochondrial division is a necessary step for successful myogenic differentiation, and perturbation of mitochondrial dynamics hinders normal mitochondrial adaptations during muscle development. Therefore, in the present study, we report a novel physiological role of mitochondrial dynamics in myogenic differentiation.

A high-content, high-throughput siRNA screen identifies cyclin D2 as a potent regulator of muscle progenitor cell fusion and a target to enhance muscle regeneration

Cell-mediated regenerative approaches using muscle progenitor cells hold promises for the treatment of many forms of muscle disorders. Their applicability in the clinic, however, is hindered by the low levels of regeneration obtained after transplantation and the large number of cells required to achieve an effect. To better understand the mechanisms that regulate the temporal switch of replicating muscle progenitor cells into terminally differentiated cells and to develop new strategies that could enhance muscle regeneration, we have developed and performed a high-throughput screening (HTS) capable of identifying genes that play active roles during myogenesis. Secondary and tertiary screens were used to confirm the effects of RNAi in vitro and in vivo and to select for candidate hits that significantly increase regeneration into skeletal muscles. Downregulation of cyclin D2 (CCND2) was shown to dramatically enhance myogenic differentiation of muscle progenitor cells and to induce a robust regeneration after cell transplantation into skeletal muscles of dystrophin-deficient mice. Protein interaction network and pathway analysis revealed that CCND2 directly interacts with the cyclin-dependent kinase Cdk4 to inhibit phosphorylation of the retinoblastoma protein (pRb), thus blocking the activation of the myogenic switch during fusion. These studies identify CCND2 as a new key regulator of terminal differentiation in muscle progenitor cells and open new possibilities for the treatment of many forms of muscle disorders characterized by impaired regeneration and loss of muscle mass.

Phospho-tyrosine phosphatase inhibitor Bpv(Hopic) enhances C2C12 myoblast migration in vitro. Requirement of PI3K/AKT and MAPK/ERK pathways

Muscle progenitor cell migration is an important step in skeletal muscle myogenesis and regeneration. Migration is required for muscle precursors to reach the site of damage and for the alignment of myoblasts prior to their fusion, which ultimately contributes to muscle regeneration. Limited spreading and migration of donor myoblasts are reported problems of myoblast transfer therapy, a proposed therapeutic strategy for Duchenne Muscular Dystrophy, warranting further investigation into different approaches for improving the motility and homing of these cells. In this article, the effect of protein phospho-tyrosine phosphatase and PTEN inhibitor BpV(Hopic) on C2C12 myoblast migration and differentiation was investigated. Applying a wound healing migration model, it is reported that 1 μM BpV(Hopic) is capable of enhancing the migration of C2C12 myoblasts by approximately 40 % in the presence of myotube conditioned media, without significantly affecting their capacity to differentiate and fuse into multinucleated myotubes. Improved migration of myoblasts treated with 1 μM BpV(Hopic) was associated with activation of PI3K/AKT and MAPK/ERK pathways, while their inhibition with either LY294002 or UO126, respectively, resulted in a reduction of C2C12 migration back to control levels. These results propose that bisperoxovanadium compounds may be considered as potential tools for enhancing the migration of myoblasts, while not reducing their differentiation capacity and underpin the importance of PI3K/AKT and MAPK/ERK signalling for the process of myogenic progenitor migration.

Pluripotent stem cells and gene therapy

Human pluripotent stem cells represent an accessible cell source for novel cell-based clinical research and therapies. With the realization of induced pluripotent stem cells (iPSCs), it is possible to produce almost any desired cell type from any patient's cells. Current developments in gene modification methods have opened the possibility for creating genetically corrected human iPSCs for certain genetic diseases that could be used later in autologous transplantation. Promising preclinical studies have demonstrated correction of disease-causing mutations in a number of hematological, neuronal, and muscular disorders. This review aims to summarize these recent advances with a focus on iPSC generation techniques, as well as gene modification methods. We will then further discuss some of the main obstacles remaining to be overcome before successful application of human pluripotent stem cell-based therapy arrives in the clinic and what the future of stem cell research may look like.

Glutathione peroxidase 3, a new retinoid target gene, is crucial for human skeletal muscle precursor cell survival

Protection of satellite cells from cytotoxic damages is crucial to ensure efficient adult skeletal muscle regeneration and to improve therapeutic efficacy of cell transplantation in degenerative skeletal muscle diseases. It is therefore important to identify and characterize molecules and their target genes that control the viability of muscle stem cells. Recently, we demonstrated that high aldehyde dehydrogenase activity is associated with increased viability of human myoblasts. In addition to its detoxifying activity, aldehyde dehydrogenase can also catalyze the irreversible oxidation of vitamin A to retinoic acid; therefore, we examined whether retinoic acid is important for myoblast viability. We showed that when exposed to oxidative stress induced by hydrogen peroxide, adherent human myoblasts entered apoptosis and lost their capacity for adhesion. Pre-treatment with retinoic acid reduced the cytotoxic damage ex vivo and enhanced myoblast survival in transplantation assays. The effects of retinoic acid were maintained in dystrophic myoblasts derived from facioscapulohumeral patients. RT-qPCR analysis of antioxidant gene expression revealed glutathione peroxidase 3 (Gpx3), a gene encoding an antioxidant enzyme, as a potential retinoic acid target gene in human myoblasts. Knockdown of Gpx3 using short interfering RNA induced elevation in reactive oxygen species and cell death. The anti-cytotoxic effects of retinoic acid were impaired in GPx3-inactivated myoblasts, which indicates that GPx3 regulates the antioxidative effects of retinoic acid. Therefore, retinoid status and GPx3 levels may have important implications for the viability of human muscle stem cells.

Concise review: stem cell therapy for muscular dystrophies

Muscular dystrophy comprises a group of genetic diseases that cause progressive weakness and degeneration of skeletal muscle resulting from defective proteins critical to muscle structure and function. This leads to premature exhaustion of the muscle stem cell pool that maintains muscle integrity during normal use and exercise. Stem cell therapy holds promise as a treatment for muscular dystrophy by providing cells that can both deliver functional muscle proteins and replenish the stem cell pool. Here, we review the current state of research on myogenic stem cells and identify the important challenges that must be addressed as stem cell therapy is brought to the clinic.

Biological approaches to improve skeletal muscle healing after injury and disease

Skeletal muscle injury and repair are complex processes, including well-coordinated steps of degeneration, inflammation, regeneration, and fibrosis. We have reviewed the recent literature including studies by our group that describe how to modulate the processes of skeletal muscle repair and regeneration. Antiinflammatory drugs that target cyclooxygenase-2 were found to hamper the skeletal muscle repair process. Muscle regeneration phase can be aided by growth factors, including insulin-like growth factor-1 and nerve growth factor, but these factors are typically short-lived, and thus more effective methods of delivery are needed. Skeletal muscle damage caused by traumatic injury or genetic diseases can benefit from cell therapy; however, the majority of transplanted muscle cells (myoblasts) are unable to survive the immune response and hypoxic conditions. Our group has isolated neonatal skeletal muscle derived stem cells (MDSCs) that appear to repair muscle tissue in a more effective manner than myoblasts, most likely due to their better resistance to oxidative stress. Enhancing antioxidant levels of MDSCs led to improved regenerative potential. It is becoming increasingly clear that stem cells tissue repair by direct differentiation and paracrine effects leading to neovascularization of injured site and chemoattraction of host cells. The factors invoked in paracrine action are still under investigation. Our group has found that angiotensin II receptor blocker (losartan) significantly reduces fibrotic tissue formation and improves repair of murine injured muscle. Based on these data, we have conducted a case study on two hamstring injury patients and found that losartan treatment was well tolerated and possibly improved recovery time. We believe this medication holds great promise to optimize muscle repair in humans.

FACS-purified myoblasts producing controlled VEGF levels induce safe and stable angiogenesis in chronic hind limb ischemia

We recently developed a method to control the in vivo distribution of vascular endothelial growth factor (VEGF) by high throughput Fluorescence-Activated Cell Sorting (FACS) purification of transduced progenitors such that they homogeneously express specific VEGF levels. Here we investigated the long-term safety of this method in chronic hind limb ischemia in nude rats. Primary myoblasts were transduced to co-express rat VEGF-A₁₆₄ (rVEGF) and truncated ratCD8a, the latter serving as a FACS-quantifiable surface marker. Based on the CD8 fluorescence of a reference clonal population, which expressed the desired VEGF level, cells producing similar VEGF levels were sorted from the primary population, which contained cells with very heterogeneous VEGF levels. One week after ischemia induction, 12 × 10⁶ cells were implanted in the thigh muscles. Unsorted myoblasts caused angioma-like structures, whereas purified cells only induced normal capillaries that were stable after 3 months. Vessel density was doubled in engrafted areas, but only approximately 0.1% of muscle volume showed cell engraftment, explaining why no increase in total blood flow was observed. In conclusion, the use of FACS-purified myoblasts granted the cell-by-cell control of VEGF expression levels, which ensured long-term safety in a model of chronic ischemia. Based on these results, the total number of implanted cells required to achieve efficacy will need to be determined before a clinical application.

The skeletal muscle satellite cell: still young and fascinating at 50

The skeletal muscle satellite cell was first described and named based on its anatomic location between the myofiber plasma and basement membranes. In 1961, two independent studies by Alexander Mauro and Bernard Katz provided the first electron microscopic descriptions of satellite cells in frog and rat muscles. These cells were soon detected in other vertebrates and acquired candidacy as the source of myogenic cells needed for myofiber growth and repair throughout life. Cultures of isolated myofibers and, subsequently, transplantation of single myofibers demonstrated that satellite cells were myogenic progenitors. More recently, satellite cells were redefined as myogenic stem cells given their ability to self-renew in addition to producing differentiated progeny. Identification of distinctively expressed molecular markers, in particular Pax7, has facilitated detection of satellite cells using light microscopy. Notwithstanding the remarkable progress made since the discovery of satellite cells, researchers have looked for alternative cells with myogenic capacity that can potentially be used for whole body cell-based therapy of skeletal muscle. Yet, new studies show that inducible ablation of satellite cells in adult muscle impairs myofiber regeneration. Thus, on the 50th anniversary since its discovery, the satellite cell's indispensable role in muscle repair has been reaffirmed.

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

Controlled myogenic differentiation of mouse embryonic stem cells by Pax3 combined with purification of PDGFαR+Flk-1- paraxial mesoderm results in the efficient in vitro generation of early skeletal myogenic progenitors. Upon transplantation into dystrophin-deficient mdx mice, these progenitors promote significant regeneration that is accompanied by improvement in muscle contractility. In this study, we aimed to raise the bar and assess the therapeutic potential of these cells in a more clinically relevant model of muscular dystrophy: the dystrophin-utrophin double-knockout (dKO) mouse. Unlike mdx mice, which display a mild phenotype, dKO mice are severely ill, displaying progressive muscle wasting, impaired mobility, and premature death. Here we show that in this very severe model of DMD, transplantation of Pax3-induced ES-derived skeletal myogenic progenitors results in significant engraftment as evidenced by the presence of Dystrophin+ myofibers with restoration of β-dystroglycan and eNOS within the sarcolemma, and enhanced strengthen of treated muscles. These findings demonstrate that ES-derived myogenic cell preparations are capable of engrafting in severely dystrophic muscle, and promote significant regeneration, providing a rationale for further studies on the potential therapeutic application of these cells in muscular dystrophies.

Canine models of Duchenne muscular dystrophy and their use in therapeutic strategies

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder in which the loss of dystrophin causes progressive degeneration of skeletal and cardiac muscle. Potential therapies that carry substantial risk, such as gene- and cell-based approaches, must first be tested in animal models, notably the mdx mouse and several dystrophin-deficient breeds of dogs, including golden retriever muscular dystrophy (GRMD). Affected dogs have a more severe phenotype, in keeping with that of DMD, so may better predict disease pathogenesis and treatment efficacy. Various phenotypic tests have been developed to characterize disease progression in the GRMD model. These biomarkers range from measures of strength and joint contractures to magnetic resonance imaging. Some of these tests are routinely used in clinical veterinary practice, while others require specialized equipment and expertise. By comparing serial measurements from treated and untreated groups, one can document improvement or delayed progression of disease. Potential treatments for DMD may be broadly categorized as molecular, cellular, or pharmacologic. The GRMD model has increasingly been used to assess efficacy of a range of these therapies. A number of these studies have provided largely general proof-of-concept for the treatment under study. Others have demonstrated efficacy using the biomarkers discussed. Importantly, just as symptoms in DMD vary among patients, GRMD dogs display remarkable phenotypic variation. Though confounding statistical analysis in preclinical trials, this variation offers insight regarding the role that modifier genes play in disease pathogenesis. By correlating functional and mRNA profiling results, gene targets for therapy development can be identified.

Murine and human myogenic cells identified by elevated aldehyde dehydrogenase activity: implications for muscle regeneration and repair

BACKGROUND

Despite the initial promise of myoblast transfer therapy to restore dystrophin in Duchenne muscular dystrophy patients, clinical efficacy has been limited, primarily by poor cell survival post-transplantation. Murine muscle derived stem cells (MDSCs) isolated from slowly adhering cells (SACs) via the preplate technique, induce greater muscle regeneration than murine myoblasts, primarily due to improved post-transplantation survival, which is conferred by their increased stress resistance capacity. Aldehyde dehydrogenase (ALDH) represents a family of enzymes with important morphogenic as well as oxidative damage mitigating roles and has been found to be a marker of stem cells in both normal and malignant tissue. In this study, we hypothesized that elevated ALDH levels could identify murine and human muscle derived cell (hMDC) progenitors, endowed with enhanced stress resistance and muscle regeneration capacity.

METHODOLOGY/PRINCIPAL FINDINGS

Skeletal muscle progenitors were isolated from murine and human skeletal muscle by a modified preplate technique and unfractionated enzymatic digestion, respectively. ALDH(hi) subpopulations isolated by fluorescence activate cell sorting demonstrated increased proliferation and myogenic differentiation capacities compared to their ALDH(lo) counterparts when cultivated in oxidative and inflammatory stress media conditions. This behavior correlated with increased intracellular levels of reduced glutathione and superoxide dismutase. ALDH(hi) murine myoblasts were observed to exhibit an increased muscle regenerative potential compared to ALDH(lo) myoblasts, undergo multipotent differentiation (osteogenic and chondrogenic), and were found predominately in the SAC fraction, characteristics that are also observed in murine MDSCs. Likewise, human ALDH(hi) hMDCs demonstrated superior muscle regenerative capacity compared to ALDH(lo) hMDCs.

CONCLUSIONS

The methodology of isolating myogenic cells on the basis of elevated ALDH activity yielded cells with increased stress resistance, a behavior that conferred increased regenerative capacity of dystrophic murine skeletal muscle. This result demonstrates the critical role of stress resistance in myogenic cell therapy as well as confirms the role of ALDH as a marker for rapid isolation of murine and human myogenic progenitors for cell therapy.

Suppression of GSK-3β activation by M-cadherin protects myoblasts against mitochondria-associated apoptosis during myogenic differentiation

Apoptosis occurs concurrently with differentiation of muscle progenitor cells (MPCs) before they fuse to form myotubes. Dysregulated apoptosis in MPCs contributes to the low regeneration capability in aged muscle and decreases the survival rate of donor cells in stem cell-based therapies for muscular dystrophies. This study investigated the role of the M-cadherin/PI3K/Akt/GSK-3β signaling pathway in regulating apoptosis during differentiation of MPCs. Disruption of M-cadherin-dependent cell-cell adhesion by M-cadherin RNA interference in confluent C2C12 myoblasts sensitized the cells to mitochondria-associated intrinsic apoptosis induced by cell confluence or serum starvation. Further investigation of this pathway revealed that M-cadherin-mediated signaling suppressed GSK-3β activation by enhancing the PI3K/AKT-dependent inhibitory phosphorylation of Ser9 in GSK-3β. Overexpression of wild-type GSK-3β in confluent C2C12 myoblasts exacerbated the apoptosis, whereas chemical inhibition of GSK-3β using TDZD-8, or forced expression of constitutively active Akt (myrAkt), or a kinase-deficient GSK-3β mutant [GSK-3β(K85R)], attenuated apoptosis and rescued the impaired myogenic differentiation that is caused by M-cadherin RNA interference. These data suggest that M-cadherin-mediated signaling prevents acceleration of mitochondria-associated intrinsic apoptosis in MPCs by suppressing GSK-3β activation during myogenic differentiation.

Immune response and mitochondrial metabolism are commonly deregulated in DMD and aging skeletal muscle

Duchenne Muscular Dystrophy (DMD) is a complex process involving multiple pathways downstream of the primary genetic insult leading to fatal muscle degeneration. Aging muscle is a multifactorial neuromuscular process characterized by impaired muscle regeneration leading to progressive atrophy. We hypothesized that these chronic atrophying situations may share specific myogenic adaptative responses at transcriptional level according to tissue remodeling. Muscle biopsies from four young DMD and four AGED subjects were referred to a group of seven muscle biopsies from young subjects without any neuromuscular disorder and explored through a dedicated expression microarray. We identified 528 differentially expressed genes (out of 2,745 analyzed), of which 328 could be validated by an exhaustive meta-analysis of public microarray datasets referring to DMD and Aging in skeletal muscle. Among the 328 validated co-expressed genes, 50% had the same expression profile in both groups and corresponded to immune/fibrosis responses and mitochondrial metabolism. Generalizing these observed meta-signatures with large compendia of public datasets reinforced our results as they could be also identified in other pathological processes and in diverse physiological conditions. Focusing on the common gene signatures in these two atrophying conditions, we observed enrichment in motifs for candidate transcription factors that may coordinate either the immune/fibrosis responses (ETS1, IRF1, NF1) or the mitochondrial metabolism (ESRRA). Deregulation in their expression could be responsible, at least in part, for the same transcriptome changes initiating the chronic muscle atrophy. This study suggests that distinct pathophysiological processes may share common gene responses and pathways related to specific transcription factors.

Oxygen cycling in conjunction with stem cell transplantation induces NOS3 expression leading to attenuation of fibrosis and improved cardiac function

AIMS

Myocardial infarction (MI) is associated with irreversible loss of viable cardiomyocytes. Cell therapy is a potential option to replace the lost cardiomyocytes and restore cardiac function. However, cell therapy is faced with a number of challenges, including survival of the transplanted cells in the infarct region, which is characterized by abundant levels of oxidants and lack of a pro-survival support mechanism. The goal of the present study was to evaluate the effect of supplemental oxygenation on cell engraftment and functional recovery in a rat model.

METHODS AND RESULTS

MI was induced in rats by a 60-min occlusion of the coronary artery, followed by restoration of flow. Mesenchymal stem cells (MSCs), isolated from adult rat bone marrow, were transplanted in the MI region. Rats with transplanted MSCs were exposed to hyperbaric oxygen (HBO: 100% O(2), 2 atmospheres absolute) for 90 min, 5 days/week for 4 weeks. The experimental groups were: MI (control), Ox (MI + HBO), MSC (MI + MSC), and MSC + Ox (MI + MSC + HBO). HBO exposure (oxygenation) was started 3 days after induction of MI. MSCs were transplanted 1 week after induction of MI. Echocardiography showed a significant recovery of cardiac function in the MSC + Ox group, when compared with the MI or MSC group. Oxygenation increased the engraftment of MSCs and vascular density in the infarct region. Molecular analysis of infarct tissue showed a four-fold increase in NOS3 expression in the MSC + Ox group compared with the MI group.

CONCLUSIONS

The results showed that post-MI exposure of rats to daily cycles of hyperoxygenation (oxygen cycling) improved stem cell engraftment, cardiac function, and increased NOS3 expression.

Long term follow-up of the endovascular trans-vessel wall technique for parenchymal access in rabbit with full clinical integration

Objective

Endovascular techniques are providing options to surgical/percutaneous cell transplantation methods. Some cells, e.g. insulin producing cells, are not suitable for intra-luminal transplantation and for such cells, other options must be found. We have constructed a “nanocatheter” with a penetrating tip for vessel perforation, thereby creating a working channel for parenchymal access by endovascular technique. To finish the procedure safely, the distal tip is detached to provide a securing plug in the vessel wall defect.

Materials and Methods

We have performed interventions with full clinical integration in the superior mesenteric artery (SMA), the subclavian artery and the external carotid artery in rabbits. No hemorrhagic- or thromboembolic events occurred during the procedure. Stenosis formation and distal embolisation were analyzed by angiography and macroscopic inspection during autopsy at five, 30 and 80 days. All animals and implanted devices were also evaluated by micro-dissections and histochemical analysis.

Results

In this study we show safety data on the trans-vessel wall technique by behavioral, angiographical and histological analysis. No stenosis formation was observed at any of the follow-up time points. No animals or organs have shown any signs of distress due to the intervention. Histological examination showed no signs of hemorrhage, excellent biocompatibility with no inflammation and a very limited fibrous capsule formation around the device, comparable to titanium implants. Further, no histological changes were detected in the endothelia of the vessels subject to intervention.

Conclusions

The trans-vessel wall technique can be applied for e.g. cell transplantations, local substance administration and tissue sampling with low risk for complications during the procedure and low risk for hemorrhage, stenosis development or adverse tissue reactions with an 80 days follow-up time. The benefit should be greatest in organs that are difficult or risky to reach with surgical techniques, such as the pancreas, the CNS and the heart.

Three-dimensional porous scaffold allows long-term wild-type cell delivery in dystrophic muscle

Duchenne muscular dystrophy (DMD) is caused by the lack of dystrophin; affected muscles are characterized by continuous bouts of muscle degeneration, eventually leading to the exhaustion of the endogenous satellite cell pool. At present, only palliative treatments are available, although several gene and cell therapy-based approaches are being studied. In this study we proposed to overcome the limitations hampering intramuscular cell injection by using a biomaterial-based strategy. In particular, we used a three-dimensional (3D) collagen porous scaffold to deliver myogenic precursor cells (MPCs) in vivo in the mdx murine model of DMD. MPCs, derived from single fibres of wild-type donors, were expanded in vitro, seeded onto collagen scaffolds and implanted into the tibialis anterior muscles of normal and mdx mice. As a control, cells were delivered via direct intramuscular cell injection in the contralateral muscles. Scaffold-delivered MPCs displayed lower apoptosis and higher proliferation than injected cells; in terms of dystrophin restoration, collagen scaffolds yielded better results than direct injections. Importantly, time-course experiments indicated that the scaffolds acted as a cell reservoir, although cell migration was mostly contained within 400 µm from the scaffold-host tissue interface.

Aldehyde dehydrogenase activity promotes survival of human muscle precursor cells

Aldehyde dehydrogenases (ALDH) are a family of enzymes that efficiently detoxify aldehydic products generated by reactive oxygen species and might therefore participate in cell survival. Because ALDH activity has been used to identify normal and malignant cells with stem cell properties, we asked whether human myogenic precursor cells (myoblasts) could be identified and isolated based on their levels of ALDH activity. Human muscle explant-derived cells were incubated with ALDEFLUOR, a fluorescent substrate for ALDH, and we determined by flow cytometry the level of enzyme activity. We found that ALDH activity positively correlated with the myoblast-CD56⁺ fraction in those cells, but, we also observed heterogeneity of ALDH activity levels within CD56-purified myoblasts. Using lentiviral mediated expression of shRNA we demonstrated that ALDH activity was associated with expression of Aldh1a1 protein. Surprisingly, ALDH activity and Aldh1a1 expression levels were very low in mouse, rat, rabbit and non-human primate myoblasts. Using different approaches, from pharmacological inhibition of ALDH activity by diethylaminobenzaldehyde, an inhibitor of class I ALDH, to cell fractionation by flow cytometry using the ALDEFLUOR assay, we characterized human myoblasts expressing low or high levels of ALDH. We correlated high ALDH activity ex vivo to resistance to hydrogen peroxide (H₂O₂)-induced cytotoxic effect and in vivo to improved cell viability when human myoblasts were transplanted into host muscle of immune deficient scid mice. Therefore detection of ALDH activity, as a purification strategy, could allow non-toxic and efficient isolation of a fraction of human myoblasts resistant to cytotoxic damage.

Engineering skeletal muscle tissue--new perspectives in vitro and in vivo

Muscle tissue engineering (TE) has not yet been clinically applied because of several problems. However, the field of skeletal muscle TE has been developing tremendously and new approaches and techniques have emerged. This review will highlight recent developments in the field of nanotechnology, especially electrospun nanofibre matrices, as well as potential cell sources for muscle TE. Important developments in cardiac muscle TE and clinical studies on Duchenne muscular dystrophy (DMD) will be included to show their implications on skeletal muscle TE.

Stem cells in clinical practice: applications and warnings

Stem cells are a relevant source of information about cellular differentiation, molecular processes and tissue homeostasis, but also one of the most putative biological tools to treat degenerative diseases. This review focuses on human stem cells clinical and experimental applications. Our aim is to take a correct view of the available stem cell subtypes and their rational use in the medical area, with a specific focus on their therapeutic benefits and side effects. We have reviewed the main clinical trials dividing them basing on their clinical applications, and taking into account the ethical issue associated with the stem cell therapy.

Intramuscular cell transplantation as a potential treatment of myopathies: clinical and preclinical relevant data

INTRODUCTION

Myopathies produce deficits in skeletal muscle function and, in some cases, literally progressive loss of skeletal muscles. The transplantation of cells able to differentiate into myofibers is an experimental strategy for the potential treatment of some of these diseases.

AREAS COVERED

Among the two routes used to deliver cells to skeletal muscles, that is intramuscular and intravascular, this paper focuses on the intramuscular route due to our expertise and because it is the most used in animal experiments and the only tested so far in humans. Given the absence of recent reviews about clinical observations and the profusion based on mouse results, this review prioritizes observations made in humans and non-human primates. The review provides a vision of cell transplantation in myology centered on what can be learned from clinical trials and from preclinical studies in non-human primates and leading mouse studies.

EXPERT OPINION

Experiments on myogenic cell transplantation in mice are essential to quickly identify potential treatments, but studies showing the possibility to scale up the methods in large mammals are indispensable to determine their applicability in humans and to design clinical protocols.

Stem cell therapies to treat muscular dystrophy: progress to date

Muscular dystrophies are heritable, heterogeneous neuromuscular disorders and include Duchenne and Becker muscular dystrophies (DMD and BMD, respectively). DMD patients exhibit progressive muscle weakness and atrophy followed by exhaustion of muscular regenerative capacity, fibrosis, and eventually disruption of the muscle tissue architecture. In-frame mutations in the dystrophin gene lead to expression of a partially functional protein, resulting in the milder BMD. No effective therapies are available at present. Cell-based therapies have been attempted in an effort to promote muscle regeneration, with the hope that the host cells would repopulate the muscle and improve muscle function and pathology. Injection of adult myoblasts has led to the development of new muscle fibers, but several limitations have been identified, such as poor cell survival and limited migratory ability. As an alternative to myoblasts, stem cells were considered preferable for therapeutic applications because of their capacity for self-renewal and differentiation potential. In recent years, encouraging results have been obtained with adult stem cells to treat human diseases such as leukemia, Parkinson's disease, stroke, and muscular dystrophies. Embryonic stem cells (ESCs) can be derived from mammalian embryos in the blastocyst stage, and because they can differentiate into a wide range of specialized cells, they hold potential for use in treating almost all human diseases. Several ongoing studies focus on this possibility, evaluating differentiation of specific cell lines from human ESCs (hESCs) as well as the potential tumorigenicity of hESCs. The most important limitation with using hESCs is that it requires destruction of human blastocysts or embryos. Conversely, adult stem cells have been identified in various tissues, where they serve to maintain, generate, and replace terminally differentiated cells within their specific tissue as the need arises for cell turnover or from tissue injury. Moreover, these cells can participate in regeneration of more than just their specific tissue type. Here we describe multiple types of muscle- and fetal-derived myogenic stem cells, their characterization, and their possible use in treating muscular dystrophies such as DMD and BMD. We also emphasize that the most promising possibility for the management and therapy of DMD and BMD is a combination of different approaches, such as gene and stem cell therapy.

Myoblast transplantation: a possible surgical treatment for a severe pediatric disease

Duchenne muscular dystrophy (DMD) is a genetic X-linked recessive orphan disease that affects approximately 1 in 3 500 male births. Boys with DMD have progressive and predictable muscle destruction due to the absence of dystrophin, a protein present under the muscle fiber membrane. This absence induces contraction-related membrane damage and activation of inflammatory necrosis and fibrosis, leading to cardiac/diaphragmatic failure and death. The authors support the therapeutic role of myoblast transplantation in DMD, and describe the history and rationale for such an approach.

Regulatory factors and cell populations involved in skeletal muscle regeneration

Skeletal muscle regeneration is a complex process, which is not yet completely understood. Satellite cells, the skeletal muscle stem cells, become activated after trauma, proliferate, and migrate to the site of injury. Depending on the severity of the myotrauma, activated satellite cells form new multinucleated myofibers or fuse to damaged myofibers. The specific microenvironment of the satellite cells, the niche, controls their behavior. The niche contains several components that maintain satellite cells quiescence until they are activated. In addition, a great diversity of stimulatory and inhibitory growth factors such as IGF-1 and TGF-beta1 regulate their activity. Donor-derived satellite cells are able to improve muscle regeneration, but their migration through the muscle tissue and across endothelial layers is limited. Less than 1% of their progeny, the myoblasts, survive the first days upon intra-muscular injection. However, a range of other multipotent muscle- and non-muscle-derived stem cells are involved in skeletal muscle regeneration. These stem cells can occupy the satellite cell niche and show great potential for the treatment of skeletal muscle injuries and diseases. The aim of this review is to discuss the niche factors, growth factors, and other stem cells, which are involved in skeletal muscle regeneration. Knowledge about the factors regulating satellite cell activity and skeletal muscle regeneration can be used to improve the treatment of muscle injuries and diseases.

Defining the heterogeneity of skeletal muscle-derived side and main population cells isolated immediately ex vivo

Myoblast transfer therapy for Duchenne muscular dystrophy (DMD) largely fails due to cell death and inability of transplanted cells to engraft in diseased muscles. One method attempting to enrich for cell subpopulations is the Hoechst 33342 dye exclusion assay, yielding a side population (SP) thought to be progenitor enriched and a main population (MP). However, in vitro and transplant studies yielded inconsistent results relative to downstream progeny. Cell surface markers expressed by skeletal muscle-derived MP and SP cells have not been fully characterized directly ex vivo. Using flow cytometry, MP and SP cells were characterized based on their expression of several well-accepted progenitor cell antigens. Both the MP and SP populations are heterogeneous and overlapping in the cells they contain. The percentages of cells in each population vary with species and specific muscle examined. MP and SP populations contain both satellite and multipotent progenitor cells, based on expression of CD34, Sca-1, Pax7, and M-cadherin. Thus, isolation using this procedure cannot be used to predict downstream differentiation outcomes, and explains the conflicting literature on these cells. Hoechst dye also results in significant mortality of sorted cells. As defined subpopulations are easily obtained using flow cytometry, sorting immediately ex vivo based on accepted myogenic precursor cell markers will yield superior results in terms of cell homogeneity for transplantation therapy.

CXCR4 enhances engraftment of muscle progenitor cells

Cell-based therapy is a possible avenue for the treatment of Duchenne muscular dystrophy (DMD), an X-linked skeletal muscle-wasting disease. We have demonstrated that cultured myogenic progenitors derived from the adult skeletal muscle side population can engraft into dystrophic fibers of non-irradiated, non-chemically injured mouse models of DMD (mdx(5cv)) after intravenous and intraarterial transplantation, with engraftment rates approaching 10%. In an effort to elucidate the cell-surface markers that promote progenitor cell extravasation and engraftment after systemic transplantation, we found that expression of the chemokine receptor CXCR4, whose ligand SDF-1 is overexpressed in dystrophic muscle, enhances the extravasation of these cultured progenitor cells into skeletal muscle after intraarterial transplantation. At 1 day post-transplantation, mice that received CXCR4-positive enhanced green fluorescent protein (eGFP)-positive cultured cells derived from the skeletal muscle side population displayed significantly higher amounts of eGFP-positive mononuclear cells in quadriceps and tibialis anterior than mice that received CXCR4-negative eGFP-positive cells derived from the same cultured population. At 30 days posttransplantation, significantly higher engraftment rates of donor cells were observed in mice that received CXCR4-positive cells compared with mice transplanted with CXCR4-negative fractions. Our data suggest that CXCR4 expression by muscle progenitor cells increases their extravasation into skeletal muscle shortly after transplantation. Furthermore, this enhanced extravasation likely promotes higher donor cell engraftment rates over time.

Cell based therapy for Duchenne muscular dystrophy

Mutations in the dystrophin gene cause an X-linked genetic disorder: Duchenne muscular dystrophy (DMD). Stem cell therapy is an attractive method to treat DMD because a small number of cells are required to obtain a therapeutic effect. Here, we discussed about multiple types of myogenic stem cells and their possible use to treat DMD. The identification of a stem cell population providing efficient muscle regeneration is critical for the progression of cell therapy for DMD. We speculated that the most promising possibility for the treatment of DMD is a combination of different approaches, such as gene and stem cell therapy.

Engraftment of embryonic stem cell-derived myogenic progenitors in a dominant model of muscular dystrophy

Muscular dystrophies (MDs) consist of a genetically heterogeneous group of disorders, recessive or dominant, characterized by progressive skeletal muscle weakening. To date, no effective treatment is available. Experimental strategies pursuing muscle regeneration through the transplantation of stem cell preparations have brought hope to patients affected by this disorder. Efficacy has been demonstrated in recessive MD models through contribution of wild-type nuclei to the muscle fiber heterokaryon; however, to date, there has been no study investigating the efficacy of a cell therapy in a dominant model of MD. We have recently demonstrated that Pax3-induced embryonic stem (ES) cell-derived myogenic progenitors are able to engraft and improve muscle function in mdx mice, a recessive mouse model for Duchenne MD. To assess whether this therapeutic effect can be extended to a dominant type of muscle disorder, here we transplanted these cells into FRG1 transgenic mice, a dominant model that has been associated with facioscapulohumeral muscular dystrophy. Our results show that Pax3-induced ES-derived myogenic progenitors are capable of significant engraftment after intramuscular or systemic transplantation into Frg1 mice. Analyses of contractile parameters revealed functional improvement in treated muscles of male mice, but not females, which are less severely affected. This study is the first to use Frg1 transgenic mice to assess muscle regeneration as well as to support the use of a cell-based therapy for autosomal dominant types of MD.

Skeletal myoblast transplantation for attenuation of hyperglycaemia, hyperinsulinaemia and glucose intolerance in a mouse model of type 2 diabetes mellitus

AIMS/HYPOTHESIS

We aimed to demonstrate the feasibility and efficacy of intra-muscular transplantation of human skeletal myoblasts (hSkMs) for attenuation of hyperglycaemia and improvement of insulin sensitivity using a mouse model of type 2 diabetes mellitus.

METHODS

KK Cg-Ay/J mice, aged 12 to 14 weeks, underwent an initial intraperitoneal glucose tolerance test (GTT) and were divided into the following groups: KK control group, basal medium (M199) only; KK myoblast group, with hSkM transplantation; KK fibroblast group, with human fibroblast transplantation. Non-diabetic C57BL mice were used as an additional normal control and also had hSkM transplantation. Cells were transplanted intra-muscularly into the skeletal muscles of the mice. All animals were treated with ciclosporin for 6 weeks only. HbA(1c) and fasting GTT, as well as serum adiponectin, cholesterol, insulin and triacylglycerol were studied.

RESULTS

Immunohistochemistry studies showed extensive survival of the transplanted hSkMs in the skeletal muscles at 12 weeks, with nuclei of the hSkMs integrated into the host muscle fibres. Repeat GTT showed a significant decrease in glucose concentrations in the KK myoblast group compared with the KK control and KK fibroblast groups. The KK myoblast group also had reduced mean HbA(1c), cholesterol, insulin and triacylglycerol, and increased adiponectin compared with the KK control and KK fibroblast groups. C57BL mice showed no change in glucose homeostasis after hSkM transplant.

CONCLUSIONS/INTERPRETATION

Human skeletal myoblast transplantation attenuated hyperglycaemia and hyperinsulinaemia and improved glucose tolerance in the KK mouse. This novel approach of improving muscle insulin resistance may be a potential alternative treatment for type 2 diabetes mellitus.

The dynamics of long-term transgene expression in engrafted neural stem cells

To assess the dynamics and confounding variables that influence transgene expression in neural stem cells (NSCs), we generated distinct NSC clones from the same pool of cells, carrying the same reporter gene transcribed from the same promoter, transduced by the same retroviral vector, and transplanted similarly at the same differentiation state, at the same time and location, into the brains of newborn mouse littermates, and monitored in parallel for over a year in vivo (without immunosuppression). Therefore, the sole variables were transgene chromosomal insertion site and copy number. We then adapted and optimized a technique that tests, at the single cell level, persistence of stem cell-mediated transgene expression in vivo based on correlating the presence of the transgene in a given NSC's nucleus (by fluorescence in situ hybridization [FISH]) with the frequency of that transgene's product within the same cell (by combined immunohistochemistry [IHC]). Under the above-stated conditions, insertion site is likely the most contributory variable dictating transgene downregulation in an NSC after 3 months in vivo. We also observed that this obstacle could be effectively and safely counteracted by simple serial infections (as few as three) inserting redundant copies of the transgene into the prospective donor NSC. (The preservation of normal growth control mechanisms and an absence of tumorigenic potential can be readily screened and ensured ex vivo prior to transplantation.) The combined FISH/IHC strategy employed here for monitoring the dynamics of transgene expression at the single cell level in vivo may be used for other types of therapeutic and housekeeping genes in endogenous and exogenous stem cells of many organs and lineages.

Cooperation of Mtmr8 with PI3K regulates actin filament modeling and muscle development in zebrafish

BACKGROUND

It has been shown that mutations in at least four myotubularin family genes (MTM1, MTMR1, 2 and 13) are causative for human neuromuscular disorders. However, the pathway and regulative mechanism remain unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we reported a new role for Mtmr8 in neuromuscular development of zebrafish. Firstly, we cloned and characterized zebrafish Mtmr8, and revealed the expression pattern predominantly in the eye field and somites during early somitogenesis. Using morpholino knockdown, then, we observed that loss-of-function of Mtmr8 led to defects in somitogenesis. Subsequently, the possible underlying mechanism and signal pathway were examined. We first checked the Akt phosphorylation, and observed an increase of Akt phosphorylation in the morphant embryos. Furthermore, we studied the PH/G domain function within Mtmr8. Although the PH/G domain deletion by itself did not result in embryonic defect, addition of PI3K inhibitor LY294002 did give a defective phenotype in the PH/G deletion morphants, indicating that the PH/G domain was essential for Mtmr8's function. Moreover, we investigated the cooperation of Mtmr8 with PI3K in actin filament modeling and muscle development, and found that both Mtmr8-MO1 and Mtmr8-MO2+LY294002 led to the disorganization of the actin cytoskeleton. In addition, we revealed a possible participation of Mtmr8 in the Hedgehog pathway, and cell transplantation experiments showed that Mtmr8 worked in a non-cell autonomous manner in actin modeling.

CONCLUSION/SIGNIFICANCE

The above data indicate that a conserved functional cooperation of Mtmr8 with PI3K regulates actin filament modeling and muscle development in zebrafish, and reveal a possible participation of Mtmr8 in the Hedgehog pathway. Therefore, this work provides a new clue to study the physiological function of MTM family members.

Isolation of a slowly adhering cell fraction containing stem cells from murine skeletal muscle by the preplate technique

This protocol details a procedure, known as the modified preplate technique, which is currently used in our laboratory to isolate muscle cells on the basis of selective adhesion to collagen-coated tissue culture plates. By employing this technique to murine skeletal muscle, we have been able to isolate a rapidly adhering cell (RAC) fraction within the earlier stages of the process, whereas a slowly adhering cell (SAC) fraction containing muscle-derived stem cells is obtained from the later stages of the process. This protocol outlines the methods and materials needed to isolate RAC and SAC populations from murine skeletal muscle. The procedure involves mechanical and enzymatic digestion of skeletal muscle tissue with collagenase XI, dispase and trypsin followed by plating the resultant muscle slurry on collagen type I-coated flasks where the cells adhere at different rates. The entire preplate technique requires 5 d to obtain the final preplate SAC population. Two to three additional days are usually required before this population is properly established. We also detail additional methodologies designed to further enrich the resultant cell population by continuing the modified preplating process on the SAC population. This process is known as replating and requires further time.