Efficacy of myoblast transplantation in nonhuman primates following simple intramuscular cell injections: toward defining strategies applicable to humans

Human MuStem cells are competent to fuse with nonhuman primate myofibers in a clinically relevant transplantation context: A proof-of-concept study

We previously reported that human muscle-derived stem cells (hMuStem cells) contribute to tissue repair after local administration into injured skeletal muscle or infarcted heart in immunodeficient rodent models. However, extrapolation of these findings to a clinical context is problematic owing to the considerable differences often seen between in vivo findings in humans versus rodents. Therefore, we investigated whether the muscle regenerative behavior of hMuStem cells is maintained in a clinically relevant transplantation context. Human MuStem cells were intramuscularly administered by high-density microinjection matrices into nonhuman primates receiving tacrolimus-based immunosuppression thereby reproducing the protocol that has so far produced the best results in clinical trials of cell therapy in myopathies. Four and 9 weeks after administration, histological analysis of cell injection sites revealed large numbers of hMuStem cell-derived nuclei in all cases. Most graft-derived nuclei were distributed in small myofiber groups in which no signs of a specific immune response were observed. Importantly, hMuStem cells contributed to simian tissue repair by fusing mainly with host myofibers, demonstrating their capacity for myofiber regeneration in this model. Together, these findings obtained in a valid preclinical model provide new insights supporting the potential of hMuStem cells in future cell therapies for muscle diseases.

Muscle Precursor Cells Enhance Functional Muscle Recovery and Show Synergistic Effects With Postinjury Treadmill Exercise in a Muscle Injury Model in Rats

BACKGROUND

Skeletal muscle injuries represent a major concern in sports medicine. Cell therapy has emerged as a promising therapeutic strategy for muscle injuries, although the preclinical data are still inconclusive and the potential clinical use of cell therapy has not yet been established.

PURPOSE

To evaluate the effects of muscle precursor cells (MPCs) on muscle healing in a small animal model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 27 rats were used in the study. MPCs were isolated from rat (n = 3) medial gastrocnemius muscles and expanded in primary culture. Skeletal muscle injury was induced in 24 rats, and the animals were assigned to 3 groups. At 36 hours after injury, animals received treatment based on a single ultrasound-guided MPC (10⁵ cells) injection (Cells group) or MPC injection in combination with 2 weeks of daily exercise training (Cells+Exercise group). Animals receiving intramuscular vehicle injection were used as controls (Vehicle group). Muscle force was determined 2 weeks after muscle injury, and muscles were collected for histological and immunofluorescence evaluation.

RESULTS

Red fluorescence-labeled MPCs were successfully transplanted in the site of the injury by ultrasound-guided injection and were localized in the injured area after 2 weeks. Transplanted MPCs participated in the formation of regenerating muscle fibers as corroborated by the co-localization of red fluorescence with developmental myosin heavy chain (dMHC)-positive myofibers by immunofluorescence analysis. A strong beneficial effect on muscle force recovery was detected in the Cells and Cells+Exercise groups (102.6% ± 4.0% and 101.5% ± 8.5% of maximum tetanus force of the injured vs healthy contralateral muscle, respectively) compared with the Vehicle group (78.2% ± 5.1%). Both Cells and Cells+Exercise treatments stimulated the growth of newly formed regenerating muscles fibers, as determined by the increase in myofiber cross-sectional area (612.3 ± 21.4 µm² and 686.0 ± 11.6 µm², respectively) compared with the Vehicle group (247.5 ± 10.7 µm²), which was accompanied by a significant reduction of intramuscular fibrosis in Cells and Cells+Exercise treated animals (24.2% ± 1.3% and 26.0% ± 1.9% of collagen type I deposition, respectively) with respect to control animals (40.9% ± 4.1% in the Vehicle group). MPC treatment induced a robust acceleration of the muscle healing process as demonstrated by the decreased number of dMHC-positive regenerating myofibers (enhanced replacement of developmental myosin isoform by mature myosin isoforms) (4.3% ± 2.6% and 4.1% ± 1.5% in the Cells and Cells+Exercise groups, respectively) compared with the Vehicle group (14.8% ± 13.9%).

CONCLUSION

Single intramuscular administration of MPCs improved histological outcome and force recovery of the injured skeletal muscle in a rat injury model that imitates sports-related muscle injuries. Cell therapy showed a synergistic effect when combined with an early active rehabilitation protocol in rats, which suggests that a combination of treatments can generate novel therapeutic strategies for the treatment of human skeletal muscle injuries.

CLINICAL RELEVANCE

Our study demonstrates the strong beneficial effect of MPC transplant and the synergistic effect when the cell therapy is combined with an early active rehabilitation protocol for muscle recovery in rats; this finding opens new avenues for the development of effective therapeutic strategies for muscle healing and clinical trials in athletes undergoing MPC transplant and rehabilitation protocols.

Myogenic progenitor cell transplantation for muscle regeneration following hindlimb ischemia and reperfusion

Background

Muscle is severely affected by ischemia/reperfusion injury (IRI). Quiescent satellite cells differentiating into myogenic progenitor cells (MPC) possess a remarkable regenerative potential. We herein established a model of local application of MPC in murine hindlimb ischemia/reperfusion to study cell engraftment and differentiation required for muscle regeneration.

Methods

A clamping model of murine (C57b/6 J) hindlimb ischemia was established to induce IRI in skeletal muscle. After 2 h (h) warm ischemic time (WIT) and reperfusion, reporter protein expressing MPC (TdTomato or Luci-GFP, 1 × 10⁶ cells) obtained from isolated satellite cells were injected intramuscularly. Surface marker expression and differentiation potential of MPC were analyzed in vitro by flow cytometry and differentiation assay. In vivo bioluminescence imaging and histopathologic evaluation of biopsies were performed to quantify cell fate, engraftment and regeneration.

Results

2h WIT induced severe IRI on muscle, and muscle fiber regeneration as per histopathology within 14 days after injury. Bioluminescence in vivo imaging demonstrated reporter protein signals of MPC in 2h WIT animals and controls over the study period (75 days). Bioluminescence signals were detected at the injection site and increased over time. TdTomato expressing MPC and myofibers were visible in host tissue on postoperative days 2 and 14, respectively, suggesting that injected MPC differentiated into muscle fibers. Higher reporter protein signals were found after 2h WIT compared to controls without ischemia, indicative for enhanced growth and/or engraftment of MPC injected into IRI-affected muscle antagonizing muscle damage caused by IRI.

Conclusion

WIT-induced IRI in muscle requests increased numbers of injected MPC to engraft and persist, suggesting a possible rational for cell therapy to antagonize IRI. Further investigations are needed to evaluate the regenerative capacity and therapeutic advantage of MPC in the setting of ischemic limb injury.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02208-w.

Human Muscle Precursor Cells Form Human-Derived Myofibers in Skeletal Muscles of Nonhuman Primates: A Potential New Preclinical Setting to Test Myogenic Cells of Human Origin for Cell Therapy of Myopathies

This study aimed to verify if human myogenic cells could participate in muscle regeneration in macaques. This experimental setting would grant researchers a model that could better evaluate the effects of cell therapies in myopathies with a better translation to human patients. Human muscle precursor cells (MPCs) were cultured in vitro and transduced with ß-galactosidase. The cells were subsequently injected into 1-cm3 muscle regions of 6 macaques immunosuppressed with tacrolimus and dexamethasone. Allogeneic ß-galactosidase+ MPCs were injected in other regions as positive controls. Some cell-grafted regions were electroporated to induce extensive muscle regeneration. MPC-grafted regions were sampled 1 month later and analyzed by histology. There were ß-galactosidase+ myofibers in both the regions grafted with human and macaque MPCs. Electroporation increased the engraftment of human MPCs in the same way as in macaque allografts. The histological analysis (hematoxylin and eosin, CD8, and CD4 immunodetection) demonstrated an absence of cellular rejection in most MPC-grafted regions, as well as minimal lymphocytic infiltration in the regions transplanted with human MPCs in the individual with the lowest tacrolimus levels. Circulating de novo anti-donor antibodies were not detected. In conclusion, we report the successful engraftment of human myogenic cells in macaques, which was possible using tacrolimus-based immunosuppression.

De Novo Circulating Antidonor's Cell Antibodies During Induced Acute Rejection of Allogeneic Myofibers in Myogenic Cell Transplantation: A Study in Nonhuman Primates

Background

Transplantation of myogenic cells has potential applications in the treatment of muscle pathologies. Excluding purely autologous cell transplantation, graft viability depends on an adequate control of acute rejection (AR). To contribute in understanding AR in this context, we analyzed whether de novo circulating antibodies against donor’s cells are detected during induced AR of graft-derived myofibers in nonhuman primates.

Methods

We allotransplanted satellite cell-derived myoblasts in macaques immunosuppressed with tacrolimus. To induce AR of graft-derived myofibers, we administered tacrolimus for 4 weeks to allow complete myofiber formation, and then we stopped tacrolimus administration. Cell-grafted sites were biopsied at tacrolimus withdrawal and then every 2 weeks and analyzed by histology until AR completion. Blood samples were taken before immunosuppression, at tacrolimus withdrawal and then every 2 weeks to detect antibodies against the donor’s cells by flow cytometry.

Results

There was an increase of antibodies against the donor’s cells related to AR in all monkeys. This increase was variable in intensity, and preceded, coincided or followed the histological evidence of AR (focal accumulations of lymphocytes) and/or the loss of myofibers of donor origin, and remained until the end of the follow-up (up to 8 weeks after tacrolimus withdrawal).

Conclusions

Flow cytometry detection of de novo circulating antibodies against the donor’s cells was consistently associated with AR. A clear increase in this antibody detection indicated current or recent AR. Smaller increases in comparison to the preimmunosuppression values were not associated with AR.

The Process of Engraftment of Myogenic Cells in Skeletal Muscles of Primates: Understanding Clinical Observations and Setting Directions in Cell Transplantation Research

We studied in macaques the evolution of the intramuscular transplantation of muscle precursor cells between the time of administration and the time at which the graft is considered stable. Satellite cell–derived myoblasts labeled with ß-galactosidase were transplanted into 1 cm³ muscle regions following cell culture and transplantation protocols similar to our last clinical trials. These regions were biopsied 1 h, 1, 3, 7 d, and 3 wk later and analyzed by histology. We observed that the cell suspension leaks from the muscle bundles during injection toward the epimysium and perimysium, where most cells accumulate after transplantation. We observed evidence of necrosis, apoptosis, and mitosis in the accumulations of grafted cells, and of potential migration to participate in myofiber regeneration in the surrounding muscle bundles. After 3 wk, the compact accumulations of grafted cells left only some graft-derived myotubes and small myofibers in the perimysium. Hybrid myofibers were abundant in the muscle fascicles at 3 wk posttransplantation, and they most likely occur by grafted myoblasts that migrated from the peripheral accumulations than by the few remaining within the fascicles immediately after injection. These observations explain the findings in clinical trials of myoblast transplantation and provide information for the future research in cell therapy in myology.

Skeletal Muscle Regenerative Potential of Human MuStem Cells following Transplantation into Injured Mice Muscle

After intra-arterial delivery in the dystrophic dog, allogeneic muscle-derived stem cells, termed MuStem cells, contribute to long-term stabilization of the clinical status and preservation of the muscle regenerative process. However, it remains unknown whether the human counterpart could be identified, considering recent demonstrations of divergent features between species for several somatic stem cells. Here, we report that MuStem cells reside in human skeletal muscle and display a long-term ability to proliferate, allowing generation of a clinically relevant amount of cells. Cultured human MuStem (hMuStem) cells do not express hematopoietic, endothelial, or myo-endothelial cell markers and reproducibly correspond to a population of early myogenic-committed progenitors with a perivascular/mesenchymal phenotypic signature, revealing a blood vessel wall origin. Importantly, they exhibit both myogenesis in vitro and skeletal muscle regeneration after intramuscular delivery into immunodeficient host mice. Together, our findings provide new insights supporting the notion that hMuStem cells could represent an interesting therapeutic candidate for dystrophic patients.

Technical advantage of recombinant collagenase for isolation of muscle stem cells

Background

Muscle satellite cells are resident skeletal muscle stem cells responsible for muscle regeneration. Isolation of satellite cells is a critical process for clinical application such as drug screening and cell transplantation. Fluorescence-activated cell sorting (FACS) enables the direct isolation of satellite cells from muscle tissue. During the process used to isolate satellite cells from skeletal muscle, enzymatic digestion is the first step. Therefore, the evaluation and standardization of enzymes is important not only for reproducibility of cellular yield and viability, but also for traceability of material used in protocols.

Methods

The comparison of muscle digestion was performed either by a mixture of recombinant collagenase G (ColG) and collagenase H (ColH) or by a conventional collagenase II. The degree of cell damage and surface antigen expression upon collagenase treatment were analyzed by FACS. To investigate whether satellite cells isolated using recombinant collagenase can regenerate injured muscle, satellite cells were cultured, transplanted into injured muscles, and analyzed by immunostaining.

Results

We show that ColG and ColH were efficient to isolate satellite cells from mouse skeletal muscle tissue. Digestion with a combination of ColG and ColH enriched satellite cells with intact surface antigens such as α7 and β1 integrins. Furthermore, satellite cells isolated using ColG and ColH dramatically proliferated and remained undifferentiated in vitro. When transplanted, satellite cells isolated using ColG and ColH enhanced the therapeutic efficacy in vivo.

Conclusions

Our results provide an efficient method of satellite cell preparation using recombinant collagenases with a high cell yield, viability of cells, and regeneration potency to fit the biological raw material criteria.

Use of Repeating Dispensers to Increase the Efficiency of the Intramuscular Myogenic Cell Injection Procedure

Intramuscular myoblast transplantation in humans and nonhuman primates requires precise repetitive cell injections very close to each other. Performed with syringes operated manually throughout large regions, this procedure takes a lot of time, becoming tiring and thus imprecise. We tested two repetitive dispensers with Hamilton syringes as cell injection devices to facilitate this procedure. Monkeys received intramuscular allotransplantations of β-galactosidase-labeled myoblasts, using either a monosyringe or a multisyringe repeating dispenser. The monosyringe repeating dispenser allowed performing cell injections faster and easier than with a manually operated syringe. The multisyringe dispenser accelerated the procedure still more, but it was not ergonomic. Biopsies of the myoblast-injected sites 1 month later showed abundant β-galactosidase-positive myofibers, with the same density and morphological pattern observed following myoblast transplantation with a syringe operated manually. We recommend the monosyringe repeating dispenser for myoblast transplantation in skeletal muscles and maybe in the heart.

In Utero Stem Cell Transplantation: Potential Therapeutic Application for Muscle Diseases

Muscular dystrophies, myopathies, and traumatic muscle injury and loss encompass a large group of conditions that currently have no cure. Myoblast transplantations have been investigated as potential cures for these conditions for decades. However, current techniques lack the ability to generate cell numbers required to produce any therapeutic benefit. In utero stem cell transplantation into embryos has been studied for many years mainly in the context of hematopoietic cells and has shown to have experimental advantages and therapeutic applications. Moreover, patient-derived cells can be used for experimental transplantation into nonhuman animal embryos via in utero injection as the immune response is absent at such early stages of development. We therefore propose in utero transplantation as a potential method to generate patient-derived humanized skeletal muscle as well as muscle stem cells in animals for therapeutic purposes as well as patient-specific drug screening.

Stem cells migration during skeletal muscle regeneration - the role of Sdf-1/Cxcr4 and Sdf-1/Cxcr7 axis

The skeletal muscle regeneration occurs due to the presence of tissue specific stem cells - satellite cells. These cells, localized between sarcolemma and basal lamina, are bound to muscle fibers and remain quiescent until their activation upon muscle injury. Due to pathological conditions, such as extensive injury or dystrophy, skeletal muscle regeneration is diminished. Among the therapies aiming to ameliorate skeletal muscle diseases are transplantations of the stem cells. In our previous studies we showed that Sdf-1 (stromal derived factor −1) increased migration of stem cells and their fusion with myoblasts in vitro. Importantly, we identified that Sdf-1 caused an increase in the expression of tetraspanin CD9 - adhesion protein involved in myoblasts fusion. In the current study we aimed to uncover the details of molecular mechanism of Sdf-1 action. We focused at the Sdf-1 receptors - Cxcr4 and Cxcr7, as well as signaling pathways induced by these molecules in primary myoblasts, as well as various stem cells - mesenchymal stem cells and embryonic stem cells, i.e. the cells of different migration and myogenic potential. We showed that Sdf-1 altered actin organization via FAK (focal adhesion kinase), Cdc42 (cell division control protein 42), and Rac-1 (Ras-Related C3 Botulinum Toxin Substrate 1). Moreover, we showed that Sdf-1 modified the transcription profile of genes encoding factors engaged in cells adhesion and migration. As the result, cells such as primary myoblasts or embryonic stem cells, became characterized by more effective migration when transplanted into regenerating muscle.

Satellite Cells and Skeletal Muscle Regeneration

Skeletal muscles are essential for vital functions such as movement, postural support, breathing, and thermogenesis. Muscle tissue is largely composed of long, postmitotic multinucleated fibers. The life-long maintenance of muscle tissue is mediated by satellite cells, lying in close proximity to the muscle fibers. Muscle satellite cells are a heterogeneous population with a small subset of muscle stem cells, termed satellite stem cells. Under homeostatic conditions all satellite cells are poised for activation by stimuli such as physical trauma or growth signals. After activation, satellite stem cells undergo symmetric divisions to expand their number or asymmetric divisions to give rise to cohorts of committed satellite cells and thus progenitors. Myogenic progenitors proliferate, and eventually differentiate through fusion with each other or to damaged fibers to reconstitute fiber integrity and function. In the recent years, research has begun to unravel the intrinsic and extrinsic mechanisms controlling satellite cell behavior. Nonetheless, an understanding of the complex cellular and molecular interactions of satellite cells with their dynamic microenvironment remains a major challenge, especially in pathological conditions. The goal of this review is to comprehensively summarize the current knowledge on satellite cell characteristics, functions, and behavior in muscle regeneration and in pathological conditions.

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.

Cell therapy in muscular dystrophies: many promises in mice and dogs, few facts in patients

INTRODUCTION

Muscular dystrophies (MDs) are genetic diseases that produce progressive loss of skeletal muscle fibers. Cell therapy (CT) is an experimental approach to treat MD. The first clinical trials of CT in MD conducted in the 1990s were based on myoblast transplantation (MT). Since they did not yield the expected results, several researchers sought to discover other cells with more advantageous properties than myoblasts whereas others sought to improve MT.

AREAS COVERED

We explain the properties that are required for a cell to be used in CT of MD. We briefly review most of the cells that were proposed for this CT, and to what extent these properties were met not only in laboratory animals but also in clinical trials.

EXPERT OPINION

Although the repertoire of cells proposed for CT of MD has been expanded since the 1990s, only myoblasts have currently demonstrated unequivocally to significantly engraft in humans. Indeed, MT for MD involves significant technical challenges that need be solved. While it would be ideal to find cells involving less technical challenges for CT of MD, there is so far no clinical evidence that this is possible and therefore the work to improve MT should continue.

Current understanding of molecular pathology and treatment of cardiomyopathy in duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a genetic muscle disorder caused by mutations in the Dmd gene resulting in the loss of the protein dystrophin. Patients do not only experience skeletal muscle degeneration, but also develop severe cardiomyopathy by their second decade, one of the main causes of death. The absence of dystrophin in the heart renders cardiomyocytes more sensitive to stretch-induced damage. Moreover, it pathologically alters intracellular calcium (Ca²⁺) concentration, neuronal nitric oxide synthase (nNOS) localization and mitochondrial function and leads to inflammation and necrosis, all contributing to the development of cardiomyopathy. Current therapies only treat symptoms and therefore the need for targeting the genetic defect is immense. Several preclinical therapies are undergoing development, including utrophin up-regulation, stop codon read-through therapy, viral gene therapy, cell-based therapy and exon skipping. Some of these therapies are undergoing clinical trials, but these have predominantly focused on skeletal muscle correction. However, improving skeletal muscle function without addressing cardiac aspects of the disease may aggravate cardiomyopathy and therefore it is essential that preclinical and clinical focus include improving heart function. This review consolidates what is known regarding molecular pathology of the DMD heart, specifically focusing on intracellular Ca²⁺, nNOS and mitochondrial dysregulation. It briefly discusses the current treatment options and then elaborates on the preclinical therapeutic approaches currently under development to restore dystrophin thereby improving pathology, with a focus on the heart.

Stem cell transplantation for treating Duchenne muscular dystrophy: A Web of Science-based literature analysis

OBJECTIVE:

To identify global research trends in stem cell transplantation for treating Duchenne muscular dystrophy using a bibliometric analysis of Web of Science.

DATA RETRIEVAL:

We performed a bibliometric analysis of studies on stem cell transplantation for treating Duchenne muscular dystrophy from 2002 to 2011 retrieved from Web of Science.

SELECTION CRITERIA:

Inclusion criteria: (a) peer-reviewed published articles on stem cell transplantation for treating Duchenne muscular dystrophy indexed in Web of Science; (b) original research articles, reviews, meeting abstracts, proceedings papers, book chapters, editorial material, and news items; and (c) publication between 2002 and 2011. Exclusion criteria: (a) articles that required manual searching or telephone access; (b) documents that were not published in the public domain; and (c) corrected papers.

MAIN OUTCOME MEASURES:

(1) Annual publication output; (2) distribution according to subject areas; (3) distribution according to journals; (4) distribution according to country; (5) distribution according to institution; (6) distribution according to institution in China; (7) distribution according to institution that cooperated with Chinese institutions; (8) top-cited articles from 2002 to 2006; (9) top-cited articles from 2007 to 2011.

RESULTS:

A total of 318 publications on stem cell transplantation for treating Duchenne muscular dystrophy were retrieved from Web of Science from 2002 to 2011, of which almost half derived from American authors and institutes. The number of publications has gradually increased over the past 10 years. Most papers appeared in journals with a focus on gene and molecular research, such as Molecular Therapy, Neuromuscular Disorders, and PLoS One. The 10 most-cited papers from 2002 to 2006 were mostly about different kinds of stem cell transplantation for muscle regeneration, while the 10 most-cited papers from 2007 to 2011 were mostly about new techniques of stem cell transplantation for treating Duchenne muscular dystrophy.

CONCLUSION:

The publications on stem cell transplantation for treating Duchenne muscular dystrophy were relatively few. It also needs more research to confirm that stem cell therapy is a reliable treatment for Duchenne muscular dystrophy.

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.

Electroporation as a method to induce myofiber regeneration and increase the engraftment of myogenic cells in skeletal muscles of primates

Engraftment of intramuscularly transplanted myogenic cells in mice can be optimized after induction of massive myofiber damage that triggers myofiber regeneration and recruitment of grafted cells; this generally involves either myotoxin injection or cryodamage. There are no effective methods to produce a similar process in the muscles of large mammals such as primates. In this study, we tested the use of intramuscular electroporation for this purpose in 11 macaques. The test sites were 1 cm of skeletal muscle. Each site was treated with 3 penetrations of a 2-needle electrode with 1 cm spacing, applying 3 pulses of 400 V/cm, for a duration of 5 milliseconds and a delay of 200 milliseconds during each penetration. Transplantation of β-galactosidase-labeled myoblasts was done in electroporated and nonelectroporated sites. Electroporation induced massive myofiber necrosis that was followed by efficient muscle regeneration. Myoblast engraftment was substantially increased in electroporated compared with nonelectroporated sites. This suggests that electroporation may be a useful tool to study muscle regeneration in primates and other large mammals and as a method for increasing the engraftment of myoblasts and other myogenic cells in intramuscular transplantation.

The mdx mouse model as a surrogate for Duchenne muscular dystrophy

Research into fundamental principles and the testing of therapeutic hypotheses for treatment of human disease is commonly performed on mouse models of human diseases. Although this is often the only practicable approach, it carries a number of caveats arising from differences between the two species. This review focuses on the example of skeletal muscle disease, in particular muscular dystrophy, to identify some of the principal classes of obstacles to translation of data from mouse to humans. Of these, the difference in scale is one of the most commonly ignored, and is of particular interest because it has quite major repercussions for evaluation of some classes of intervention and of outcome criteria, while having comparatively little bearing on others. Likewise, inter-species differences and similarities in cell and molecular biological mechanisms underlying development, growth and response to pathological processes should be considered on an individual basis. An awareness of such distinctions is crucial if we are to avoid misjudging the likely applicability to humans of results obtained on mouse models.

Intramuscular transplantation of myogenic cells in primates: importance of needle size, cell number, and injection volume

The aim of this study was to quantitatively define the main measurable technical parameters for the intramuscular transplantation of myogenic cells in primates. Myoblasts transduced with the gene coding for β-galactosidase were injected into the skeletal muscles of 15 monkeys. The following parameters were studied: needle size, number of cells per injection, and volume of cell suspension per injection. Monkeys were immunosuppressed with tacrolimus. The cell-injected sites were biopsied 1 or 2 months later. Biopsies were examined histologically to assess the myoblast engraftment and the muscle structure. The conclusions were as follows: (1) Needles should be thin enough to avoid important tissue damage and allow muscle regeneration as satisfactory as possible. Among those tested, 27G should be the choice if the length is consistent with depth of injection. (2) At least 100,000 cells should be delivered per centimeter of needle trajectory. (3) The smallest volumes of cell suspension per injection should be used. In this study, 1 µl/cm of injection trajectory was sufficient. In principle, these parameters apply to muscles in which no damage occurred other than the injections.

Myoblasts derived from normal hESCs and dystrophic hiPSCs efficiently fuse with existing muscle fibers following transplantation

Human embryonic stem cells (hESCs) and human-induced pluripotent stem cells (hiPSCs) have an endless self-renewal capacity and can theoretically differentiate into all types of lineages. They thus represent an unlimited source of cells for therapies of regenerative diseases, such as Duchenne muscular dystrophy (DMD), and for tissue repair in specific medical fields. However, at the moment, the low number of efficient specific lineage differentiation protocols compromises their use in regenerative medicine. We developed a two-step procedure to differentiate hESCs and dystrophic hiPSCs in myogenic cells. The first step was a culture in a myogenic medium and the second step an infection with an adenovirus expressing the myogenic master gene MyoD. Following infection, the cells expressed several myogenic markers and formed abundant multinucleated myotubes in vitro. When transplanted in the muscle of Rag/mdx mice, these cells participated in muscle regeneration by fusing very well with existing muscle fibers. Our findings provide an effective method that will permit to use hESCs or hiPSCs for preclinical studies in muscle repair.

Preconditioning approach in stem cell therapy for the treatment of infarcted heart

Nearly two decades of research in regenerative medicine have been focused on the development of stem cells as a therapeutic option for treatment of the ischemic heart. Given the ability of stem cells to regenerate the damaged tissue, stem-cell-based therapy is an ideal approach for cardiovascular disorders. Preclinical studies in experimental animal models and clinical trials to determine the safety and efficacy of stem cell therapy have produced encouraging results that promise angiomyogenic repair of the ischemically damaged heart. Despite these promising results, stem cell therapy is still confronted with issues ranging from uncertainty about the as-yet-undetermined "ideal" donor cell type to the nonoptimized cell delivery strategies to harness optimal clinical benefits. Moreover, these lacunae have significantly hampered the progress of the heart cell therapy approach from bench to bedside for routine clinical applications. Massive death of donor cells in the infarcted myocardium during acute phase postengraftment is one of the areas of prime concern, which immensely lowers the efficacy of the procedure. An overview of the published data relevant to stem cell therapy is provided here and the various strategies that have been adopted to develop and optimize the protocols to enhance donor stem cell survival posttransplantation are discussed, with special focus on the preconditioning approach.

Intramuscular Transplantation of Muscle Precursor Cells over-expressing MMP-9 improves Transplantation Success

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin in muscles. A therapeutic approach to restore dystrophin expression in DMD patient's muscles is the transplantation of muscle precursor cells (MPCs). However, this transplantation is limited by the low MPC capacity to migrate beyond the injection trajectory. Matrix metalloproteases (MMPs) are key regulatory molecules in the remodeling of extracellular matrix (ECM) components. MPCs over-expressing MMP-9 were tested by zymography, migration and invasion assays in vitro and by transplantation in mouse muscle. In vitro, MPCs over-expressing MMP-9 have a better invasion capacity than control MPCs. When these cells are transplanted in mouse muscles, the transplantation success is increased by more than 50% and their dispersion is higher than normal cells. MMP-9 over-expression could thus be an approach to improve cell transplantation in DMD patients by increasing the dispersion capacity of transplanted cells.

Progress in therapy for Duchenne muscular dystrophy

Duchenne muscular dystrophy is a devastating muscular dystrophy of childhood. Mutations in the dystrophin gene destroy the link between the internal muscle filaments and the extracellular matrix, resulting in severe muscle weakness and progressive muscle wasting. There is currently no cure and, whilst palliative treatment has improved, affected boys are normally confined to a wheelchair by 12 years of age and die from respiratory or cardiac complications in their twenties or thirties. Therapies currently being developed include mutation-specific treatments, DNA- and cell-based therapies, and drugs which aim to modulate cellular pathways or gene expression. This review aims to provide an overview of the different therapeutic approaches aimed at reconstructing the dystrophin-associated protein complex, including restoration of dystrophin expression and upregulation of the functional homologue, utrophin.

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.

Past, present and future of myoblast transplantation in the treatment of Duchenne muscular dystrophy

DMD is a genetic X-linked recessive disease that affects approximately one in 3500 male births. Boys with DMD have progressive and predictable muscle destruction because of the absence of Dys, a protein present under the muscle fiber membrane. Dys deficiency induces contraction-related membrane damages, activation of inflammatory-necrosis-fibrosis up to the cardiac-diaphragmatic failure and death. This review supports the therapeutic role of MT associated with immunosuppression in DMD patients, describing the history and the rationale of such approach. The authors underline the importance to evaluate a protocol of myoblast intradermal multi-injection to apply in young DMD patients

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.

Laminin-111: a potential therapeutic agent for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) still needs effective treatments, and myoblast transplantation (MT) is considered as an approach to repair damaged skeletal muscles. DMD is due to the complete loss of dystrophin from muscles. The lack of link between the contracting apparatus and the extracellular matrix leads to frequent damage to the sarcolemma triggering muscle fiber necrosis. Laminins are major proteins in the extracellular matrix. Laminin-111 is normally present in skeletal and cardiac muscles in mice and humans but only during embryonic development. In this study, we showed that intramuscular injection of laminin-111 increased muscle strength and resistance in mdx mice. We also used laminin-111 as a coadjuvant in MT, and we showed this protein decreased considerably the repetitive cycles of degeneration, inflammatory reaction, and regeneration. Moreover, MT is significantly improved. To explain the improvement, we confirmed with the same myoblast cell batch that laminin-111 improves proliferation and drastically increases migration in vitro. These results are extremely important because DMD could be treated only by the injection of a recombinant protein, a simple and safe therapy to prevent loss of muscle function. Moreover, the improvement in MT would be significant to treat the muscles of DMD patients who are already weak.

In vivo fluorescence imaging of muscle cell regeneration by transplanted EGFP-labeled myoblasts

In vivo fluorescence imaging (FLI) enables monitoring fluorescent protein (FP)-labeled cells and proteins in living organisms noninvasively. Here, we examined whether this modality could reach a sufficient sensitivity to allow evaluation of the regeneration process of enhanced green fluorescent protein (eGFP)-labeled muscle precursors (myoblasts). Using a basic FLI station, we were able to detect clear fluorescence signals generated by 40,000 labeled cells injected into a tibialis anterior (TA) muscle of mouse. We observed that the signal declined to approximately 25% on the 48 hours of cell injection followed by a recovery starting at the second day and reached a peak of approximately 45% of the original signal by the 7th day, suggesting that the survived population underwent a limited run of proliferation before differentiation. To assess whether transplanted myoblasts could form satellite cells, we injured the transplanted muscles repeatedly with cardiotoxin. We observed a recovery of fluorescence signal following a disappearance of the signal after each cardiotoxin injection. Histology results showed donor-derived cells located underneath basal membrane and expressing Pax7, confirming that the regeneration observed by imaging was indeed mediated by donor-derived satellite cells. Our results show that FLI is a powerful tool that can extend our ability to unveil complicated biological processes such as stem cell-mediated regeneration.

A First Semimanual Device for Clinical Intramuscular Repetitive Cell Injections

Intramuscular cell transplantation in humans requires so far meticulous repetitive cell injections. Performed percutaneously with syringes operated manually, the procedure is very time consuming and requires a lot of concentration to deliver the cells exactly in the required region. This becomes impractical and inaccurate for large volumes of muscle. In order to accelerate this task, to render it more precise, and to perform injections more reproducible in large volumes of muscle, we developed a specific semimanual device for intramuscular repetitive cell injections. Our prototype delivers very small quantities of cell suspension, homogeneously throughout several needles, from a container in the device. It was designed in order to deliver the cells as best as possible only in a given subcutaneous region (in our case, skeletal muscles accessible from the surface), avoiding wasting in skin and hypodermis. The device was tested in monkeys by performing intramuscular allotransplantations of β-galactosidase-labeled myoblasts. During transplantations, it was more ergonomic and considerably faster than manually operated syringes, facilitating the cell graft in whole limb muscles. Biopsies of the myoblast-injected muscles 1 month later showed abundant β-galactosidase-positive myofibers with homogeneous distribution through the biopsy sections. This is the first device specifically designed for the needs of intramuscular cell transplantation in a clinical context.

Methylguanine DNA methyltransferase-mediated drug resistance-based selective enrichment and engraftment of transplanted stem cells in skeletal muscle

Cell replacement therapy using stem cell transplantation holds much promise in the field of regenerative medicine. In the area of hematopoietic stem cell transplantation, O(6)-methylguanine-DNA methyltransferase MGMT (P140K) gene-mediated drug resistance-based in vivo enrichment strategy of donor stem cells has been shown to achieve up to 75%-100% donor cell engraftment in the host's hematopoietic stem cell compartment following repeated rounds of selection. This strategy, however, has not been applied in any other organ system. We tested the feasibility of using this MGMT (P140K)-mediated enrichment strategy for cell transplantation in skeletal muscles of mice. We demonstrate that muscle cells expressing an MGMT (P140K) drug resistance gene can be protected and selectively enriched in response to alkylating chemotherapy both in vitro and in vivo. Upon transplantation of MGMT (P140K)-expressing male CD34(+ve) donor stem cells isolated from regenerating skeletal muscle into injured female muscle treated with alkylating chemotherapy, donor cells showed enhanced engraftment in the recipient muscle 7 days following transplantation as examined by quantitative-polymerase chain reaction using Y-chromosome specific primers. Fluorescent in situ hybridization analysis using a Y-chromosome paint probe revealed donor-derived de novo muscle fiber formation in the recipient muscle 14 days following transplantation, with approximately 12.5% of total nuclei within the regenerated recipient muscle being of donor origin. Following engraftment, the chemo-protected donor CD34(+ve) cells induced substantial endogenous regeneration of the chemo-ablated host muscle that is otherwise unable to self-regenerate. We conclude that the MGMT (P140K)-mediated enrichment strategy can be successfully implemented in muscle.

Growth Factor Coinjection Improves the Migration Potential of Monkey Myogenic Precursors without Affecting Cell Transplantation Success

Duchenne muscular dystrophy (DMD) is an inherited disease and a main target of myogenic cell transplantation (MT). After the failure of the first clinical trials with DMD patients, the poor migration of transplanted cells has been suspected to be a major problem for a more effective clinical application of MT. Previous investigations suggested that the quantity and dispersion of myofibers containing donor cell nuclei might be improved by increasing the migration of the transplanted cells outside the injection sites. Because the coinjection of motogenic factors with human myoblasts enhanced their intramuscular migration following MT in SCID mice, the present study aimed to investigate whether this approach was appropriate to increase MT success in muscles of nonhuman primates. In vitro studies indicated that IGF-1 or bFGF increased components of proteolytic systems involved in myoblast migration. In vitro and in vivo experiments also demonstrated that coinjection of bFGF or IGF-1 was able to improve monkey myogenic cell migration and invasion. Sixty hours after MT in skeletal muscle tissue, the migration distances reached by monkey myoblasts increased by nearly twofold when one of the growth factors was coinjected with the cells. However, long-term observations in adult monkeys suggest that promigratory treatments are not intrinsically sufficient to improve the success of MT. Even if short-term observations reveal that grafted cells are not always trapped inside the injection site and in spite of the fact that both factors enhanced transplanted cell migration, myofibers including grafted cell nuclei were still restrained to the injection trajectory without notable difference in their amount or their dispersion. The incapacity of transplanted cells to fuse with undamaged myofibers, which are located outside the injection sites, is a priority problem to solve in order to improve transplantation success and reduce the number of injections required for the treatment of DMD patients.

Noninvasive evaluation of immunosuppressive drug efficacy on acute donor cell survival

PURPOSE

The therapeutic benefits of cell transplantation may depend on the survival of sufficient numbers of grafted cells. We evaluate four potent immunosuppressive medications aimed at preventing acute donor cell death.

PROCEDURES AND RESULTS

Embryonic rat H9c2 myoblasts were stably transduced to express firefly luciferase reporter gene (H9c2-Fluc). H9c2-Fluc cells (3x10(6)) were injected into thigh muscles of Sprague-Dawley rats (N=30) treated with cyclosporine, dexamethasone, mycophenolate mofetil, tacrolimus, or saline from day -3 to day +14. Longitudinal optical bioluminescence imaging was performed over two weeks. Fluc activity was 40.0+/-12.1% (dexamethasone), 30.5+/-12.5% (tacrolimus), and 21.5+/-3.5% (mycophenolate) vs. 12.0+/-5.0% (control) and 8.3+/-5.0% (cyclosporine) at day 4 (P<0.05). However, by day 14, cell signals had decreased drastically to <10% for all groups despite drug therapy.

CONCLUSION

This study demonstrates the ability of optical molecular imaging for tracking cell survival noninvasively and raises important questions with regard to the overall efficacy of immunosuppressives for prolonging transplanted cell survival.

Strategies to promote donor cell survival: combining preconditioning approach with stem cell transplantation

Stem cell transplantation has emerged as a potential modality in cardiovascular therapeutics due to their inherent characteristics of self-renewal, unlimited capacity for proliferation and ability to cross lineage restrictions and adopt different phenotypes. Constrained by extensive death in the unfriendly milieu of ischemic myocardium, the results of heart cell therapy in experimental animal models as well as clinical studies have been less than optimal. Several factors which play a role in early cell death after engraftment in the ischemic myocardium include: absence of survival factors in the transplanted heart, disruption of cell-cell interaction coupled with loss of survival signals from matrix attachments, insufficient vascular supply and elaboration of inflammatory cytokines resulting from ischemia and/or cell death. This article reviews various signaling pathways involved in triggering highly complex forms of cell death and provides critical appreciation of different novel anti-death strategies developed from the knowledge gained from using an ischemic preconditioning approach. The use of pharmacological preconditioning for up-regulation of pro-survival proteins and cardiogenic markers in the transplanted stem cells will be discussed.

Ischemic central necrosis in pockets of transplanted myoblasts in nonhuman primates: implications for cell-transplantation strategies

BACKGROUND

Several cell-transplantation strategies implicate the injection of cells into tissues. Avascular accumulations of implanted cells are then formed. Because the diffusion of oxygen and nutrients from the surrounding tissue throughout the implanted cell accumulations may be limited, central ischemic necrosis could develop. We analyzed this possibility after myoblast transplantation in nonhuman primates.

METHODS

Macaca monkeys were injected intramuscularly with different amounts of myoblasts per single site. These sites were sampled 1 hr later and at posttransplantation days 1, 3, 5, and 7 and analyzed by histological techniques.

RESULTS

One day posttransplantation, the largest pockets of implanted cells showed cores of massive necrosis. The width of the peripheral layer of living cells was approximately 100-200 microm. We thus analyzed the relationship between the amount of myoblasts injected per site and the volume of ischemic necrosis. Delivering 0.1 x 10(6) and 0.3 x 10(6) myoblasts did not produce ischemic necrosis; pockets of 1 x 10(6), 3 x 10(6), 10 x 10(6), and 20 x 10(6) myoblasts exhibited, respectively, a mean of 2%, 9%, 41%, and 59% of central necrosis. Intense macrophage infiltration took place in the muscle, invading the accumulations of necrotic cells and eliminating them by posttransplantation days 5 to 7.

CONCLUSIONS

The desire to create more neoformed tissue by delivering more cells per injection site is confronted with the fact that the acute survival of the implanted cells is restricted to the peripheral layer that can profit of the diffusion of oxygen and nutriments from the surrounding recipient's tissue.

Autologous transplantation of SM/C-2.6(+) satellite cells transduced with micro-dystrophin CS1 cDNA by lentiviral vector into mdx mice

Duchenne muscular dystrophy (DMD) is a lethal muscle disorder caused by mutations in the dystrophin gene. Transplantation of autologous myogenic cells genetically corrected ex vivo is a possible treatment for this disorder. In order to test the regenerative efficiency of freshly isolated satellite cells, we purified quiescent satellite cells from limb muscles of 8-12-week-old green fluorescent protein-transgenic (GFP-Tg) mice using SM/C-2.6 (a recently developed monoclonal antibody) and flow cytometry. Freshly isolated satellite cells were shown to participate in muscle regeneration more efficiently than satellite cell-derived myoblasts passaged in vitro do, when transplanted into tibialis anterior (TA) muscles of 8-12-week-old cardiotoxin-injected C57BL/6 mice and 5-week-old dystrophin-deficient mdx mice, and analyzed at 4 weeks after injection. Importantly, expansion of freshly isolated satellite cells in vitro without passaging had no detrimental effects on their regenerative capacity. Therefore we directly isolated satellite cells from 5-week-old mdx mice using SM/C-2.6 antibody and cultured them with lentiviral vectors expressing micro-dystrophin CS1. The transduced cells were injected into TA muscles of 5-week-old mdx mice. At 4 weeks after transplantation, the grafted cells efficiently contributed to regeneration of mdx dystrophic muscles and expressed micro-dystrophin at the sarcolemma. These results suggest that there is potential for lentiviral vector-mediated ex vivo gene therapy for DMD.

Induction of Anoikis following myoblast transplantation into SCID mouse muscles requires the Bit1 and FADD pathways

Seventy-five percent of the myoblasts transplanted in the mouse muscle die during the first 4 days following transplantation. The purpose of this study was to determine if anoikis plays a role in this phenomenon. Survival and proliferation of myoblasts in vitro were determined by Hoescht-PI labeling and cell counts respectively. In vivo cell survival and proliferation were quantified by injecting human male myoblasts labeled with (14)C-thymidine in SCID mouse muscles. Survival and proliferation of the transplanted myoblasts were evaluated by scintigraphy and quantitative PCR of human Y chromosomal DNA. Inclusion of the extracellular matrix protein fibronectin enhanced transplanted myoblast survival by 1.7-fold while vitronectin improved their proliferation by 1.8-fold. Reductions in FADD and Bit1 expression reduced anoikis in vitro and improved the injected myoblast survival in vivo. Ectopic expression of the anti-apoptotic protein Bcl-2 completely abolished myoblast anoikis in vitro and enhanced cell survival by 3.1-fold in vivo. Cell death following transplantation appears to me mediated in part by anoikis. Inclusion of extracellular matrix proteins enhanced both survival and proliferation. Reduced expression of the proapoptotic proteins Bit1 and FADD or overexpression of Bcl-2 improved myoblast survival.

Autologous Transplantation of Muscle Precursor Cells Modified with a Lentivirus for Muscular Dystrophy: Human Cells and Primate Models

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin. We tested the ability of lentiviral vectors to deliver a transgene into myogenic cells before their transplantation. Enhanced green fluorescent protein (eGFP) transgene was efficiently transferred into cells and eGFP-positive fibers were generated following transplantation. An eGFP-micro-dystrophin transgene under the control of a cytomegalovirus promoter was then transferred with the same viral vector but caused some toxicity to the mono-nucleated cells. We then used instead a muscle creatine kinase promoter. Dystrophin expression was observed in the muscle fibers after the transplantation of such genetically modified cells into mdx and severe combined immunodeficient mice. Micro-dystrophin expression was also observed in monkey muscles a month after allogenic or autologous transplantation of genetically modified myoblasts. Therapeutic exon skipping was induced by infecting myoblasts of a DMD patient, deleted for dystrophin exons 49 and 50, with a lentivirus expressing a U7 small nuclear RNA containing antisense sequences against exon 51. The modification led to correct exon skipping and to the expression of a quasi-dystrophin in vitro and in vivo. These results demonstrate the feasibility of lentiviral-based ex vivo gene therapy for DMD.

Comparison of intracardiac cell transplantation: autologous skeletal myoblasts versus bone marrow cells

An increasing number of patients living with cardiovascular disease (CVD) and still unacceptably high mortality created an urgent need to effectively treat and prevent disease-related events. Within the past 5 years, skeletal myoblasts (SKMBs) and bone marrow (or blood)-derived mononuclear cells (BMNCs) have demonstrated preclinical efficacy in reducing ischemia and salvaging already injured myocardium, and in preventing left ventricular (LV) remodeling, respectively. These findings have been translated into clinical trials, so far totaling over 200 patients for SKMBs and over 800 patients for BMNCs. These safety/feasibility and early phase II studies showed promising but somewhat conflicting symptomatic and functional improvements, and some safety concerns have arisen. However, the patient population, cell type, dose, time and mode of delivery, and outcome measures differed, making comparisons problematic. In addition, the mechanisms through which cells engraft and deliver their beneficial effects remain to be fully elucidated. It is now time to critically evaluate progress made and challenges encountered in order to select not only the most suitable cells for cardiac repair but also to define appropriate patient populations and outcome measures. Reiterations between bench and bedside will increase the likelihood of cell therapy success, reduce the time to development of combined of drug- and cell-based disease management algorithms, and offer these therapies to patients to achieve a greater reduction of symptoms and allow for a sustained improvement of quality of life.

First test of a "high-density injection" protocol for myogenic cell transplantation throughout large volumes of muscles in a Duchenne muscular dystrophy patient: eighteen months follow-up

A 26-years old Duchenne muscular dystrophy (DMD) patient received normal muscle-precursor cells, proliferated in vitro and implanted in a thenar eminence, biceps brachii, and in a portion of a gastrocnemius by injections placed 1mm from each other or less. Saline was injected in the contralateral gastrocnemius. The patient was immunosuppressed with tacrolimus. The protocol of cell transplantation was well tolerated and did not cause permanent sequels. Some injected sites were biopsied at 1, 14 and 18 months post-transplantation. Muscles were replaced by fat and fibrosis. In the cell-grafted site of the gastrocnemius, 27.5% of the myofiber profiles expressed donor-derived dystrophin 1 month post-transplantation and 34.5% 18 months post-transplantation. The contralateral gastrocnemius was dystrophin-negative. Myofibers were virtually absent in the biceps brachii, where only two dystrophin-positive myofibers were observed. In conclusion, a "high-density injection" protocol was feasible for intramuscular cell-transplantation in a DMD patient and long-term expression of donor-derived dystrophin was observed.

Stem cell based therapies to treat muscular dystrophy

Muscular dystrophies comprise a heterogeneous group of neuromuscular disorders, characterized by progressive muscle wasting, for which no satisfactory treatment exists. Multiple stem cell populations, both of adult or embryonic origin, display myogenic potential and have been assayed for their ability to correct the dystrophic phenotype. To date, many of these described methods have failed, underlying the need to identify the mechanisms controlling myogenic potential, homing of donor populations to the musculature, and avoidance of the immune response. Recent results focus on the fresh isolation of satellite cells and the use of multiple growth factors to promote mesangioblast migration, both of which promote muscle regeneration. Throughout this chapter, various stem cell based therapies will be introduced and evaluated based on their potential to treat muscular dystrophy in an effective and efficient manner.