Gene transfer into skeletal muscles by isogenic myoblasts

Spin infection enables efficient gene delivery to muscle stem cells

Viral vector-mediated foreign gene expression in cultured cells has been extensively used in stem cell studies to explore gene function. However, it is difficult to obtain high-quality stem cells and primary cells after viral vector infection. Here, we describe a new protocol for high-efficiency retroviral infection of primary muscle stem cell (satellite cell) cultures. We compared multiple commercially available transfection reagents to determine which was optimal for retroviral infections of primary myoblasts. Centrifugation force was also tested, and a spin infection protocol with centrifugation at 2800 × g for 90 min had the highest infection efficiency for primary myoblasts. We confirmed that infected muscle stem cells maintain cell proliferation and the capacity for in vitro and in vivo myogenic differentiation. Our new, efficient retroviral infection protocol for muscle stem cells can be applied to molecular biology experiments as well as translational studies.

Complement Deposition and Cell Death after Myoblast Transplantation

One of the problems limiting myoblast transplantation (MT) is the early death of the transplanted cells. Because complement can be fixed by myoblasts in vitro, and because it has the capacity to induce cell lysis, its possible role in the early death of transplanted myoblasts was investigated. CD1 mice and Macaca mulata monkeys were used as recipients for MT. In some mice, C3 was depleted before MT using Cobra Venom Factor. Mice were sacrificed during the first hour and up to 3 days after MT. Monkeys were biopsied 1 to 4 h after MT. Myoblast necrosis was assessed by the presence of intracellular calcium. Complement deposition was demonstrated by immunohistochemistry with anti-C3 and anti-C5b-9 neoantigen antibodies. In mice, C3 deposition was observed in damaged muscle fibers and in regions containing necrosed myoblasts. Complement depletion did not diminish the proportion of necrosed cells. In monkeys, only a small percentage of transplanted myoblasts showed C3 or C5b-9 deposition, mostly intracellular. Complement activation seems not to be implicated in directly damaging the transplanted cells, but seems secondary to cellular death. Taking into account its chemotactic functions, complement could be implicated in the migration of neutrophils and macrophages into the clusters of transplanted cells. © 1998 Elsevier Science Inc.

Muscular Tissue Engineering: Capillary-Incorporated Hybrid Muscular Tissues in Vivo Tissue Culture

Requirements for a functional hybrid muscular tissue are 1) a high density of multinucleated cells, 2) a high degree of cellular orientation, and 3) the presence of a capillary network in the hybrid tissue. Rod-shaped hybrid muscular tissues composed of C2C12 cells (skeletal muscle myoblast cell line) and type I collagen, which were prepared using the centrifugal cell-packing method reported in our previous article, were implanted into nude mice. The grafts, comprised three hybrid tissues (each dimension, diameter, approximately 0.3 mm, length, approximately 1 mm, respectively), were inserted into the subcutaneous spaces on the backs of nude mice. All nude mice that survived the implantation were sacrificed at 1, 2, and 4 wk after the implantation. The grafts were easily distinguishable from the subcutaneous tissues of host mice with implantation time. The grafts increased in size with time after implantation, and capillary networks were formed in the vicinities and on the surfaces of the grafts. One week after implantation, many capillaries formed in the vicinities of the grafts. In the central portion of the graft, few capillaries and necrotic cells were observed. Mononucleated myoblasts were densely distributed and a low number of multinucleated myotubes were scattered. Two weeks after implantation, the formation of a capillary network was induced, resulting in the surfaces of the grafts being covered by capillaries. Numerous elongated multinucleated myotubes and mononucleated myoblasts were densely distributed and numerous capillaries were observed throughout the grafts. Four weeks after implantation a dense capillary network was formed in the vicinities and on the surfaces of the grafts. In the peripheral portion of the graft, multinucleated myotubes in the vicinities of the rich capillaries were observed. Thus, hybrid muscular tissues in vitro preconstructed was remodeled in vivo, which resulted in facilitating the incorporation of capillary networks into the tissues. © 1998 Elsevier Science Inc.

Development of Approaches to Improve the Healing following Muscle Contusion

Muscle injuries are a challenging problem in traumatology, and the most frequent occurrence in sports medicine. Muscle contusions are among the most common muscle injuries. Although this injury is capable of healing, an incomplete functional recovery often occurs, depending on the severity of the blunt trauma. We have developed an animal model of muscle contusion in mice (high energy blunt trauma) and characterized the muscle's ability to heal following this injury using histology and immunohistochemistry to determine the level of muscle regeneration and the development of scar tissue. We have observed a massive muscle regeneration occurring in the first 2 wk postinjury that is subsequently followed by the development of muscle fibrosis. Based on these observations, we propose that the enhancement of muscle growth and regeneration, as well as the prevention of fibrotic development, could be used as approach(es) to improve the healing of muscle injuries. In fact, we have identified three growth factors (bFGF, IGF-1, and NGF) capable of enhancing myoblast proliferation and differentiation in vitro and improving the healing of the injured muscle in vivo. Furthermore, the ability of adenovirus to mediate direct and ex vivo gene transfer of β-galactosidase into the injured site opens possibilities of delivering an efficient and persistent expression of these growth factors in the injured muscle. These studies should help in the development of strategies to promote efficient muscle healing with complete functional recovery following muscle contusion. © 1998 Elsevier Science Inc.

A Comparison between Real-Time Quantitative PCR and DNA Hybridization for Quantitation of Male DNA following Myoblast Transplantation

The transplantation of muscle precursor cells (myoblasts) is a potential therapy for Duchenne muscular dystrophy. A commonly used method to detect cell survival is quantitation of the Y chromosome following transplantation of male donor cells into female hosts. This article presents a direct comparison between real-time quantitative PCR (Q-PCR) and the DNA hybridization (slot-blot) technique for quantitation of Y chromosome DNA. Q-PCR has a significantly greater linear quantitation range and is up to 40-fold more sensitive at low concentrations of male DNA, detecting as little as 1 ng of male DNA in each female tibialis anterior (TA) muscle. At high male DNA concentrations, accurate quantitation by Q-PCR is 2.5 times higher than the maximum possible with slot-blot. In conclusion, Q-PCR has a higher dynamic range and is more efficient than slot-blot analysis for the detection of donor cell engraftment in a transsexual transplantation model.

Control of Inflammatory Damage by Anti-Lfa-1: Increase Success of Myoblast Transplantation

Myoblast transplantation is a potential treatment for Duchenne Muscular Dystrophy. This article confirms by experiments in mice that one problem that has limited the success of clinical trials of this procedure is a rapid (within 3 days) inflammatory reaction which kills most of the injected myoblasts. The death of the transplanted myoblasts can be prevented by treating the host with a mAb against LFA-1. This led to a 27-fold increase in the number of muscle fibers expressing a reporter gene present in the donor myoblasts when the host is also adequately immunosuppressed with FK506. Therefore, both the nonspecific inflammatory reaction and the specific immune response should be adequately controlled following myoblast transplantation.

A Potential Alternative Strategy for Myoblast Transfer Therapy: The use of Sliced Muscle Grafts

Excellent long-term survival (up to 1 yr) of donor skeletal muscle cells was demonstrated using a mouse Y-chromosome specific probe, following the transplantation of grafts of whole muscles from male “normal” C57B1/10Sn mice into dystrophic muscles of female host mice. After the transplantation of equivalent sliced muscle grafts there was extensive movement of the male donor cells and fusion with host myofibres. This contrasts with the extremely poor survival of isolated myoblasts after injection into the same mouse model for Duchenne muscular dystrophy. The use of sliced muscle grafts may therefore represent a potential alternative approach to myoblast transfer therapy.

Cell Therapy in Myology: Dynamics of Muscle Precursor Cell Death after Intramuscular Administration in Non-human Primates

Cell therapy could be useful for the treatment of myopathies. A problem observed in mice, with different results and interpretations, is a significant death among the transplanted cells. We analyzed this problem in non-human primates, the animal model more similar to humans. Autologous or allogeneic myoblasts (with or without a reporter gene) were proliferated in vitro, labeled with [¹⁴C]thymidine, and intramuscularly injected in macaques. Some monkeys were immunosuppressed for long-term follow-up. Cell-grafted regions were biopsied at different intervals and analyzed by radiolabel quantification and histology. Most radiolabel was lost during the first week after injection, regardless of whether the cells were allogeneic or autologous, the culture conditions, and the use or not of immunosuppression. There was no significant difference between 1 hr and 1 day post-transplantation, a significant decrease between days 1 and 3 (45% to 83%), a significant decrease between days 3 and 7 (80% to 92%), and no significant differences between 7 days and 3 weeks. Our results confirmed in non-human primates a progressive and significant death of the grafted myoblasts during the first week after administration, relatively similar to some observations in mice but with different kinetics.

Induction of CCAAT/Enhancer-Binding Protein β Expression With the Phosphodiesterase Inhibitor Isobutylmethylxanthine Improves Myoblast Engraftment Into Dystrophic Muscle

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is the most common muscular dystrophy. Characterized by rounds of muscle degeneration and regeneration, DMD features progressive muscle wasting and is fatal. One approach for treatment is transplantation of muscle progenitor cells to repair and restore dystrophin expression to damaged muscle. However, the success of this approach has been limited by difficulties in isolating large numbers of myogenic progenitors with strong regenerative potential, poor engraftment, poor survival of donor cells, and limited migration in the diseased muscle. We demonstrate that induction of the transcription factor CCAAT/enhancer-binding protein β (C/EBPβ) using the cyclic adenosine monophosphate phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) results in enhanced myoblast expansion in culture and increased satellite cell marker expression. When equal numbers of IBMX-treated cells were transplanted into dystrophic muscle, they contributed to muscle repair more efficiently than did vehicle-treated cells and engrafted into the satellite cell niche in higher numbers, demonstrating improved cell migration from the site of injury and enhanced survival after transplantation. Thus, pharmacologic stimulation of C/EBPβ expression reprograms myoblasts to a more stem cell-like state, promotes expansion in culture, and improves engraftment such that better transplantation outcomes are achieved.

SIGNIFICANCE

Duchenne muscular dystrophy is a genetic disorder for which no cure exists. One therapeutic approach is transplantation of myogenic progenitors to restore dystrophin to damaged muscle, but this approach is limited by poor engraftment of cultured myoblasts. Transient upregulation of CCAAT/enhancer-binding protein β in primary myoblasts using the phosphodiesterase isobutylmethylxanthine (IBMX) increases satellite cell marker expression in cultured myoblasts, improves their migration, and increases their survival after transplantation. When transplanted into C57BL/10ScSn-mdx/J mice , IBMX-treated myoblasts restored dystrophin expression and were able to occupy the satellite cell niche more efficiently than controls. A myoblast culture approach that reprograms myoblasts to a more primitive state, resulting in improved transplantation outcomes and reinvigorating research into myoblast transplantation as a viable therapeutic approach, is described.

Increased Understanding of Stem Cell Behavior in Neurodegenerative and Neuromuscular Disorders by Use of Noninvasive Cell Imaging

Numerous neurodegenerative and neuromuscular disorders are associated with cell-specific depletion in the human body. This imbalance in tissue homeostasis is in healthy individuals repaired by the presence of endogenous stem cells that can replace the lost cell type. However, in most disorders, a genetic origin or limited presence or exhaustion of stem cells impairs correct cell replacement. During the last 30 years, methods to readily isolate and expand stem cells have been developed and this resulted in a major change in the regenerative medicine field as it generates sufficient amount of cells for human transplantation applications. Furthermore, stem cells have been shown to release cytokines with beneficial effects for several diseases. At present however, clinical stem cell transplantations studies are struggling to demonstrate clinical efficacy despite promising preclinical results. Therefore, to allow stem cell therapy to achieve its full potential, more insight in their in vivo behavior has to be achieved. Different methods to noninvasively monitor these cells have been developed and are discussed. In some cases, stem cell monitoring even reached the clinical setting. We anticipate that by further exploring these imaging possibilities and unraveling their in vivo behavior further improvement in stem cell transplantations will be achieved.

Strategies for functional bioscaffold-based skeletal muscle reconstruction

Tissue engineering and regenerative medicine-based strategies for the reconstruction of functional skeletal muscle tissue have included cellular and acellular approaches. The use of acellular biologic scaffold material as a treatment for volumetric muscle loss (VML) in five patients has recently been reported with a generally favorable outcome. Further studies are necessary for a better understanding of the mechanism(s) behind acellular bioscaffold-mediated skeletal muscle repair, and for combination cell-based/bioscaffold based approaches. The present overview highlights the current thinking on bioscaffold-based remodeling including the associated mechanisms and the future of scaffold-based skeletal muscle reconstruction.

Strategies for skeletal muscle tissue engineering: seed vs. soil

The most commonly used tissue engineering approach includes the ex vivo combination of site-appropriate cell(s) and scaffold material(s) to create three-dimensional constructs for tissue replacement or reconstruction. These three-dimensional combinations are typically subjected to a period of culture and conditioning (i.e., self-assembly and maturation) to promote the development of ex vivo constructs which closely mimic native target tissue. This cell-based approach is challenged by the host response to the engineered tissue construct following surgical implantation. As an alternative to the cell-based approach, acellular biologic scaffolds attract endogenous cells and remodel into partially functional mimics of native tissue upon implantation. The present review examines cell-types (i.e., seed), scaffold materials (i.e., soil), and challenges associated with functional tissue engineering. Skeletal muscle is used as the target tissue prototype but the discussed principles will largely apply to most body systems.

Combinations of Kinase Inhibitors Protecting Myoblasts against Hypoxia

Cell-based therapies to treat skeletal muscle disease are limited by the poor survival of donor myoblasts, due in part to acute hypoxic stress. After confirming that the microenvironment of transplanted myoblasts is hypoxic, we screened a kinase inhibitor library in vitro and identified five kinase inhibitors that protected myoblasts from cell death or growth arrest in hypoxic conditions. A systematic, combinatorial study of these compounds further improved myoblast viability, showing both synergistic and additive effects. Pathway and target analysis revealed CDK5, CDK2, CDC2, WEE1, and GSK3β as the main target kinases. In particular, CDK5 was the center of the target kinase network. Using our recently developed statistical method based on elastic net regression we computationally validated the key role of CDK5 in cell protection against hypoxia. This method provided a list of potential kinase targets with a quantitative measure of their optimal amount of relative inhibition. A modified version of the method was also able to predict the effect of combinations using single-drug response data. This work is the first step towards a broadly applicable system-level strategy for the pharmacology of hypoxic damage.

Satellite cells: regenerative mechanisms and applicability in muscular dystrophy

The satellite cells are long regarded as heterogeneous cell population, which is intimately linked to the processes of muscular recovery. The heterogeneous cell population may be classified by specific markers. In spite of the significant amount of variation amongst the satellite cell populations, it seems that their activity is tightly bound to the paired box 7 transcription factor expression, which is, therefore, used as a canonical marker for these cells. Muscular dystrophic diseases, such as Duchenne muscular dystrophy, elicit severe tissue injuries leading those patients to display a very specific pattern of muscular recovery abnormalities. There have been works on the application of precursors cells as a therapeutic alternative for Duchenne muscular dystrophy and initial attempts have proven the cells inefficient; however later endeavours have proposed solutions for the experiments improving significantly the results. The presence of a range of satellite cells populations indicates the existence of specific cells with enhanced capability of muscular recovery in afflicted muscles.

Rethinking regenerative medicine: a macrophage-centered approach

Regenerative medicine, a multi-disciplinary approach that seeks to restore form and function to damaged or diseased tissues and organs, has evolved significantly during the past decade. By adapting and integrating fundamental knowledge from cell biology, polymer science, and engineering, coupled with an increasing understanding of the mechanisms which underlie the pathogenesis of specific diseases, regenerative medicine has the potential for innovative and transformative therapies for heretofore unmet medical needs. However, the translation of novel technologies from the benchtop to animal models and clinical settings is non-trivial and requires an understanding of the mechanisms by which the host will respond to these novel therapeutic approaches. The role of the innate immune system, especially the role of macrophages, in the host response to regenerative medicine based strategies has recently received considerable attention. Macrophage phenotype and function have been suggested as critical and determinant factors in downstream outcomes. The constructive and regulatory, and in fact essential, role of macrophages in positive outcomes represents a significant departure from the classical paradigms of host-biomaterial interactions, which typically consider activation of the host immune system as a detrimental event. It appears desirable that emerging regenerative medicine approaches should not only accommodate but also promote the involvement of the immune system to facilitate positive outcomes. Herein, we describe the current understanding of macrophage phenotype as it pertains to regenerative medicine and suggest that improvement of our understanding of context-dependent macrophage polarization will lead to concurrent improvement in outcomes.

Administration of a soluble activin type IIB receptor promotes the transplantation of human myoblasts in dystrophic mice

Duchenne muscular dystrophy (DMD) is a recessive disease caused by a dystrophin gene mutation. Myoblast transplantation permits the introduction of the dystrophin gene into dystrophic muscle fibers. However, this strategy has so far produced limited results. Modulation of transforming growth factor-β (TGF-β) superfamily signaling promotes skeletal muscle differentiation and growth and myogenic regeneration. We investigated the possibility that the combination of TGF-β superfamily signaling inhibition with myoblast transplantation might be an effective therapeutic approach in dystrophin-deficient patients. In vitro, blocking myostatin and other ligands with a soluble form of the extracellular domain of the activin IIB receptor (ActRIIB/Fc) upregulated the expression of myogenic differentiation factors and increased human myoblast fusion. In vivo, systemic inhibition of activin IIB receptor signaling by delivery of ActRIIB/Fc increased the success of the myoblast transplantation. This effect was further increased by forcing the mice to swim weekly to induce cycles of muscle degeneration and regeneration. Treatment of dystrophic mice with ActRIIB/Fc led to increased body weight, increased skeletal muscle mass, and improved myoblast transplantation. Thus, ActRIIB/Fc represents an effective therapeutic strategy for muscular dystrophies, and its effects are enhanced when combined with muscle exercise.

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.

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.

Human skeletal muscle cells with a slow adhesion rate after isolation and an enhanced stress resistance improve function of ischemic hearts

Identification of cells that are endowed with maximum potency could be critical for the clinical success of cell-based therapies. We investigated whether cells with an enhanced efficacy for cardiac cell therapy could be enriched from adult human skeletal muscle on the basis of their adhesion properties to tissue culture flasks following tissue dissociation. Cells that adhered slowly displayed greater myogenic purity and more readily differentiated into myotubes in vitro than rapidly adhering cells (RACs). The slowly adhering cell (SAC) population also survived better than the RAC population in kinetic in vitro assays that simulate conditions of oxidative and inflammatory stress. When evaluated for the treatment of a myocardial infarction (MI), intramyocardial injection of the SACs more effectively improved echocardiographic indexes of left ventricular (LV) remodeling and contractility than the transplantation of the RACs. Immunohistological analysis revealed that hearts injected with SACs displayed a reduction in myocardial fibrosis and an increase in infarct vascularization, donor cell proliferation, and endogenous cardiomyocyte survival and proliferation in comparison with the RAC-treated hearts. In conclusion, these results suggest that adult human skeletal muscle-derived cells are inherently heterogeneous with regard to their efficacy for enhancing cardiac function after cardiac implantation, with SACs outperforming RACs.

Human muscle-derived cell populations isolated by differential adhesion rates: phenotype and contribution to skeletal muscle regeneration in Mdx/SCID mice

Muscle-derived stem cells (MDSCs) isolated from murine skeletal tissue by the preplate method have displayed the capability to commit to the myogenic lineage and regenerate more efficiently than myoblasts in skeletal and cardiac muscle in murine Duchenne Muscular Dystrophy mice (mdx). However, until now, these studies have not been translated to human muscle cells. Here, we describe the isolation, by a preplate technique, of candidate human MDSCs, which exhibit myogenic and regenerative characteristics similar to their murine counterparts. Using the preplate isolation method, we compared cells that adhere faster to the flasks, preplate 2 (PP2), and cells that adhere slower, preplate 6 (PP6). The human PP6 cells express several markers of mesenchymal stem cells and are distinct from human PP2 (a myoblast-like population) based on their expression of CD146 and myogenic markers desmin and CD56. After transplantation to the gastrocnemius muscle of mdx/SCID mice, we observe significantly higher levels of PP6 cells participating in muscle regeneration as compared with the transplantation of PP2 cells. This study supports some previous findings related to mouse preplate cells, and also identifies some differences between mouse and human muscle preplate cells.

Terminal differentiation is not a major determinant for the success of stem cell therapy - cross-talk between muscle-derived stem cells and host cells

We have found that when muscle-derived stem cells (MDSCs) are implanted into a variety of tissues only a small fraction of the donor cells can be found within the regenerated tissues and the vast majority of cells are host derived. This observation has also been documented by other investigators using a variety of different stem cell types. It is speculated that the transplanted stem cells release factors that modulate repair indirectly by mobilizing the host's cells and attracting them to the injury site in a paracrine manner. This process is loosely called a 'paracrine mechanism', but its effects are not necessarily restricted to the injury site. In support of this speculation, it has been reported that increasing angiogenesis leads to an improvement of cardiac function, while inhibiting angiogenesis reduces the regeneration capacity of the stem cells in the injured vascularized tissues. This observation supports the finding that most of the cells that contribute to the repair process are indeed chemo-attracted to the injury site, potentially through host neo-angiogenesis. Since it has recently been observed that cells residing within the walls of blood vessels (endothelial cells and pericytes) appear to represent an origin for post-natal stem cells, it is tempting to hypothesize that the promotion of tissue repair, via neo-angiogenesis, involves these blood vessel-derived stem cells. For non-vascularized tissues, such as articular cartilage, the regenerative property of the injected stem cells still promotes a paracrine, or bystander, effect, which involves the resident cells found within the injured microenvironment, albeit not through the promotion of angiogenesis. In this paper, we review the current knowledge of post-natal stem cell therapy and demonstrate the influence that implanted stem cells have on the tissue regeneration and repair process. We argue that the terminal differentiation capacity of implanted stem cells is not the major determinant of the cells regenerative potential and that the paracrine effect imparted by the transplanted cells plays a greater role in the regeneration process.

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.

A human-specific deletion in mouse Cmah increases disease severity in the mdx model of Duchenne muscular dystrophy

During the evolution of humans, an inactivating deletion was introduced in the CMAH (cytidine monophosphate-sialic acid hydroxylase) gene, which eliminated biosynthesis of the common mammalian sialic acid N-glycolylneuraminic acid from all human cells. We found that this human-specific change in sialylation capacity contributes to the marked discrepancy in phenotype between the mdx mouse model for Duchenne muscular dystrophy (DMD) and the human disease. When compared to human patients with DMD, mdx mice show reduced severity or slower development of clinically relevant disease phenotypes, despite lacking dystrophin protein in almost all muscle cells. This is especially true for the loss of ambulation, cardiac and respiratory muscle weakness, and decreased life span, all of which are major phenotypes contributing to DMD morbidity and mortality. These phenotypes occur at an earlier age or to a greater degree in mdx mice that also carry a human-like mutation in the mouse Cmah gene, possibly as a result of reduced strength and expression of the dystrophin-associated glycoprotein complex and increased activation of complement. Cmah-deficient mdx mice are a small-animal model for DMD that better approximates the human glycome and its contributions to muscular dystrophy.

3D timelapse analysis of muscle satellite cell motility

Skeletal muscle repair and regeneration requires the activity of satellite cells, a population of myogenic stem cells scattered throughout the tissue and activated to proliferate and differentiate in response to myotrauma or disease. While it seems likely that satellite cells would need to navigate local muscle tissue to reach damaged areas, relatively little data on such motility exist, and most studies have been with immortalized cell lines. We find that primary satellite cells are significantly more motile than myoblast cell lines, and that adhesion to laminin promotes primary cell motility more than fourfold over other substrates. Using timelapse videomicroscopy to assess satellite cell motility on single living myofibers, we have identified a requirement for the laminin-binding integrin α7β1 in satellite cell motility, as well as a role for hepatocyte growth factor in promoting directional persistence. The extensive migratory behavior of satellite cells resident on muscle fibers suggests caution when determining, based on fixed specimens, whether adjacent cells are daughters from the same mother cell. We also observed more persistent long-term contact between individual satellite cells than has been previously supposed, potential cell-cell attractive and repulsive interactions, and migration between host myofibers. Based on such activity, we assayed for expression of “pathfinding” cues, and found that satellite cells express multiple guidance ligands and receptors. Together, these data suggest that satellite cell migration in vivo may be more extensive than currently thought, and could be regulated by combinations of signals, including adhesive haptotaxis, soluble factors, and guidance cues. Stem Cells2009;27:2527–2538

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.

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.

Effect of VEGF on the regenerative capacity of muscle stem cells in dystrophic skeletal muscle

We have isolated a population of muscle-derived stem cells (MDSCs) that, when compared with myoblasts, display an improved regeneration capacity, exhibit better cell survival, and improve myogenesis and angiogenesis. In addition, we and others have observed that the origin of the MDSCs may reside within the blood vessel walls (endothelial cells and pericytes). Here, we investigated the role of vascular endothelial growth factor (VEGF)-mediated angiogenesis in MDSC transplantation-based skeletal muscle regeneration in mdx mice (an animal model of muscular dystrophy). We studied MDSC and MDSC transduced to overexpress VEGF; no differences were observed in vitro in terms of phenotype or myogenic differentiation. However, after in vivo transplantation, we observe an increase in angiogenesis and endogenous muscle regeneration as well as a reduction in muscle fibrosis in muscles transplanted with VEGF-expressing cells when compared to control cells. In contrast, we observe a significant decrease in vascularization and an increase in fibrosis in the muscles transplanted with MDSCs expressing soluble forms-like tyrosine kinase 1 (sFlt1) (VEGF-specific antagonist) when compared to control MDSCs. Our results indicate that VEGF-expressing cells do not increase the number of dystrophin-positive fibers in the injected mdx muscle, when compared to the control MDSCs. Together the results suggest that the transplantation of VEGF-expressing MDSCs improved skeletal muscle repair through modulation of angiogenesis, regeneration and fibrosis in the injected mdx skeletal muscle.

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.

Myoblast Xenotransplantation as a Tool to Evaluate the Appropriateness of Nanoparticular versus Cellular Trackers

Myoblast transplantation is being considered as a potential strategy to improve muscle function in myopathies; hence, it is important to identify the transplanted cells and to have available efficient reagents to track these cells. We first validated a human to mouse xenotransplantation model warranting the complete and rapid rejection of the cells. We then used this model to assess the appropriateness of a nanoparticle reagent to track the transplanted cells. Human myoblasts were loaded with ferrite nanoparticles and injected into the tibialis muscle of immunocompetent mice. Upon collection and histological analysis of muscle sections at different time points, we observed the total disappearance of the human cells within 6 days while ferrite particles remained detectable and colocalized with mouse infiltrating and neighboring cells at the injection site. These results suggest that the use of exogenous markers such as ferrite nanoparticles may lead to false-positive results and misinterpretation of cell fate.

Successful myoblast transplantation in rat tongue reconstruction

BACKGROUND

Controversy exists regarding the success of myoblast transplantation. The purpose of this study was to determine the survival of transplanted myoblasts in a rat tongue reconstruction model by using fluorescently labeled myoblasts and surgical stains to mark the location of the pocket into which transplanted cells were delivered. We evaluated tongue histology after myoblast transplantation under the hypothesis that myoblast transplantation will promote muscle regeneration and result in minimal scar tissue formation.

METHODS

Sterile solutions of 1:10 India ink, 1% methylene blue, and 1% crystal violet were applied to the inner lining of a left-sided mucosa-sparing hemiglossectomy pocket. After air-drying, the hemiglossectomy defect was filled with collagen gel and closed. The tongues were evaluated histologically at 6 weeks. Next, myoblasts were cultured and labeled with three commercially available fluorescent dyes, 5-chloromethyl-fluorescein diacetate (CMFDA), chloromethylbenzamido (CM-DiI), and fluorescently labeled microspheres (FLMs), to determine which would optimally label myoblasts in a tongue reconstruction model. Next, Lewis rats underwent left hemiglossectomy, and the created pockets were coated with 1:10 India ink. Control animals received collagen gel alone, whereas experimental animals received labeled myoblast/collagen constructs into the tongue defect. Tongues were harvested at intervals to determine the presence of labeled fluorescent cells, the relative numbers of viable myoblasts, and the degree of scarring.

RESULTS

India ink coating of the hemiglossectomy pocket caused minimal inflammation and lasted longer than the other tested dyes. CMFDA and FLMs both successfully label myoblasts for transplantation. In vivo, donor cells were observed in all specimens at week 0 with increasing numbers of cells and muscle formation, determined by desmin immunofluorescence, after 6 weeks. There was less scar tissue contracture in the experimental group and a significant increase in the amount of desmin-stained muscle in the surgical defect.

CONCLUSIONS

India ink is an appropriate vehicle for intra-operative marking of a hemiglossectomy cavity. The introduction of myoblast/collagen constructs into the rat hemiglossectomy defect increases the amount of regenerated muscle, results in less scar contracture, and may increase meaningful tongue function.

Stem and progenitor cells in skeletal muscle development, maintenance, and therapy

Satellite cells are dormant progenitors located at the periphery of skeletal myofibers that can be triggered to proliferate for both self-renewal and differentiation into myogenic cells. In addition to anatomic location, satellite cells are typified by markers such as M-cadherin, Pax7, Myf5, and neural cell adhesion molecule-1. The Pax3 and Pax7 transcription factors play essential roles in the early specification, migration, and myogenic differentiation of satellite cells. In addition to muscle-committed satellite cells, multi-lineage stem cells encountered in embryonic, as well as adult, tissues exhibit myogenic potential in experimental conditions. These multi-lineage stem cells include side-population cells, muscle-derived stem cells (MDSCs), and mesoangioblasts. Although the ontogenic derivation, identity, and localization of these non-conventional myogenic cells remain elusive, recent results suggest their ultimate origin in blood vessel walls. Indeed, purified pericytes and endothelium-related cells demonstrate high myogenic potential in culture and in vivo. Allogeneic myoblasts transplanted into Duchenne muscular dystrophy (DMD) patients have been, in early trials, largely inefficient owing to immune rejection, rapid death, and limited intramuscular migration--all obstacles that are now being alleviated, at least in part, by more efficient immunosuppression and escalated cell doses. As an alternative to myoblast transplantation, stem cells such as mesoangioblasts and CD133+ progenitors administered through blood circulation have recently shown great potential to regenerate dystrophic muscle.

Age dependence of the human skeletal muscle stem cell in forming muscle tissue

Human skeletal muscle stem cells from healthy donors aged 2-82 years (n = 13) and from three children suffering from Duchenne Muscular Dystrophy (DMD) were implanted into soleus muscles of immunoincompetent mice and were also expanded in vitro until senescence. Growth of implanted cells was quantified by structural features and by the amount of human DNA present in a muscle. Proliferative capacity in vitro and in vivo was inversely related to age of the donor. In vitro, a decline of about two mean population doublings (MPDs) per 10 years of donor's age was observed. Muscle stem cells from DMD children were prematurely aged. In general, cell preparations with low or decreasing content in desmin-positive cells produced more MPDs than age-matched high-desmin preparations and upon implantation more human DNA and more nonmyogenic than myogenic tissue. Thus, a "Desmin Factor" was derived which predicts "quality" of the human muscle tissue growing in vivo. This factor may serve as a prognostic tool.

The satellite cell as a companion in skeletal muscle plasticity: currency, conveyance, clue, connector and colander

Satellite cells are companions to voluntary muscle fibres, and are named for their intimate positional or ;satellite' relationship, as if revolving around fibres, like a satellite moon around the earth. Studies on the nature of at least some satellite cells, including their capabilities for self-renewal and for giving rise to multiple lineages in a stem cell-like function, are exploring the molecular basis of phenotypes described by markers of specialized function and gene expression in normal development, neuromuscular disease and aging. In adult skeletal muscle, the self-renewing capacity of satellite cells contributes to muscle growth, adaptation and regeneration. Muscle remodeling, such as demonstrated by changes in myofibre cross-sectional area and length, nerve and tendon junctions, and fibre-type distribution, occur in the absence of injury and provide broad functional and structural diversity among skeletal muscles. Those contributions to plasticity involve the satellite cell in at least five distinct roles, here described using metaphors for behaviour or the investigator's perspective. Satellite cells are the 'currency' of muscle; have a 'conveyance' role in adaptation by domains of cytoplasm along a myofibre; serve researchers, through a marker role, as 'clues' to various activities of muscle; are 'connectors' that physically, and through signalling and cell-fibre communications, bridge myofibres to the intra- and extra-muscular environment; and are equipped as metabolic and genetic filters or 'colanders' that can rectify or modulate particular signals. While all these roles are still under exploration, each contributes to the plasticity of skeletal muscle and thence to the overall biology and function of an organism. The use of metaphor for describing these roles helps to clarify and scrutinize the definitions that form the basis of our understanding of satellite cell biology: the metaphors provide the construct for various approaches to detect or test the nature of satellite cell functions in skeletal muscle plasticity.

Harnessing the potential of myogenic satellite cells

Adult skeletal muscle has remarkable regenerative potential, which is mainly attributable to a small population of undifferentiated skeletal muscle precursors called satellite cells. These cells reside underneath the basal lamina of skeletal myofibers and can be activated to proliferate, differentiate and fuse to form new muscle tissue. Satellite cells have long been considered promising mediators of therapeutic muscle regeneration. However, in practice, the regenerative function of such cells, which in many cases have been derived or expanded by ex vivo cultures, can be surprisingly low. A recent study from Montarras and colleagues has provided new insights into the requirements for efficient muscle engraftment from purified muscle satellite cells, suggesting possible strategies to enhance their therapeutic potential.

Differential Myocardial Infarct Repair with Muscle Stem Cells Compared to Myoblasts

Myoblast transplantation for cardiac repair has generated beneficial results in both animals and humans; however, poor viability and poor engraftment of myoblasts after implantation in vivo limit their regeneration capacity. We and others have identified and isolated a subpopulation of skeletal muscle-derived stem cells (MDSCs) that regenerate skeletal muscle more effectively than myoblasts. Here we report that in comparison with a myoblast population, MDSCs implanted into infarcted hearts displayed greater and more persistent engraftment, induced more neoangiogenesis through graft expression of vascular endothelial growth factor, prevented cardiac remodeling, and elicited significant improvements in cardiac function. MDSCs also exhibited a greater ability to resist oxidative stress-induced apoptosis compared to myoblasts, which may partially explain the improved engraftment of MDSCs. These findings indicate that MDSCs constitute an alternative to other myogenic cells for use in cardiac repair applications.

Improved success of myoblast transplantation in mdx mice by blocking the myostatin signal

BACKGROUND

: Duchenne muscular dystrophy (DMD) is caused by a dystrophin gene mutation. Transplantation of normal myoblasts results in long-term restoration of dystrophin. However, the success of this approach is compromised by the limited time of regeneration following muscle damage. Myostatin is known to be responsible for limiting skeletal muscle regeneration. Our purpose is to verify whether blocking the myostatin signal in mdx host mice or in normal myoblasts transplanted in mdx host mice would increase the extent of muscle repair and thus allow the formation of more dystrophin-positive fibers.

METHODS

: Transgenic mdx mice carrying a dominant negative form of myostatin receptor (dnActRIIB) were used to test the fiber resistance to damage and to act as a host for normal myoblast transplantation. Myoblasts obtained from nondystrophic transgenic mice carrying the dominant negative myostatin receptor were also transplanted in nontransgenic mdx mice.

RESULTS

: Transgenic mdx mice carrying the dnActRIIB gene have bigger muscles than mdx mice with the normal gene of ActRIIB. Their fiber resistance to exercise-induced damage was also greatly improved. Moreover, the success of normal myoblast transplantation was significantly enhanced in mdx/dnActRIIB mice. Finally, nondystrophic dnActRIIB myoblasts formed more abundant and bigger dystrophin positive fibers when transplanted in mdx mice.

CONCLUSIONS

: Blocking the myostatin signal in mdx mice allowed the size of muscle fibers to increase, the fiber resistance to damage induced by exercise to increase, and the success of normal myoblast transplantation to improve. The transplantation in mdx mice of dnActRIIB myoblasts formed more abundant and larger dystrophin positive fibers.

Gene therapy for arthritis

In the two years since arthritis gene therapy was last reviewed in this journal, there has been rapid progress on several fronts. Although vector development remains a slow process and long-term gene expression is not easily obtained, very encouraging preclinical data in animal models of arthritis are now emerging. Collectively, these demonstrate the principle that transfer of cytokine antagonist genes to joints has a marked anti-arthritic effect. Other options under active investigation are the transfer of cytotoxic genes to effect a surgical synovectomy, and the transfer of oligonucleotides that antagonise the actions of transcription factors. Two human clinical trials of gene therapy for rheumatoid arthritis have been initiated. There are now preliminary data suggesting that gene therapy may also be helpful in osteoarthritis, as well as in the repair of cartilage, meniscus, ligaments, tendons and bones.

Allogeneic Bone Marrow-Derived Mesenchymal Stromal Cells Promote the Regeneration of Injured Skeletal Muscle without Differentiation into Myofibers

Half-stratum laceration was performed on the tibialis anterior muscle of Sprague-Dawley (SD) rats as a skeletal muscle injury model. Bone marrow-derived mesenchymal stromal cells (BMMSCs), which were derived from enhanced green fluorescent protein (GFP) transgenic SD rats, were transplanted into the injured site. Tensile strength produced by nerve stimulation was measured for functional evaluation before sacrifice. Specimens of the tibialis anterior muscles were stained with hematoxylin and eosin, and immunohistochemically stained for histological evaluation. Our results showed that transplanted BMMSCs promoted maturation of myofibers histologically and made the injured muscle acquire almost normal muscle power functionally by 1 month after transplantation. However, the results of immunohistochemical staining could not prove that transplanted BMMSCs differentiated into or fused to skeletal myofibers, although it showed that transplanted BMMSCs seemed to differentiate into muscle precursor cells. Therefore, our results indicated that BMMSCs contributed to the regeneration of skeletal muscle by mechanisms other than fusion to myofibers after differentiation.