The fate of myoblasts following transplantation into mature muscle

Skeletal Muscle-Derived Human Mesenchymal Stem Cells: Influence of Different Culture Conditions on Proliferative and Myogenic Capabilities

Skeletal muscle tissue is characterized by restrained self-regenerative capabilities, being ineffective in relation to trauma extension both in time span (e.g., chronic diseases) and in size (e.g., large trauma). For these reasons, tissue engineering and/or cellular therapies represent a valuable solution in the cases where the physiological healing process failed. Satellite cells, the putative skeletal muscle stem cells, have been the first solution explored to remedy the insufficient self-regeneration capacity. Nevertheless, some limitation related to donor age, muscle condition, expansion hitch, and myogenic potentiality maintenance have limited their use as therapeutic tool. To overcome this hindrance, different stem cells population with myogenic capabilities have been investigated to evaluate their real potentiality for therapeutic approaches, but, as of today, the perfect cell candidate has not been identified yet. In this work, we analyze the characteristics of skeletal muscle-derived human Mesenchymal Stem Cells (hMSCs), showing the maintenance/increment of myogenic activity upon differential culture conditions. In particular, we investigate the influence of a commercial enriched growth medium (Cyto-Grow), and of a medium enriched with either human-derived serum (H.S.) or human Platelet-rich Plasma (PrP), in order to set up a culture protocol useful for employing this cell population in clinical therapeutic strategies. The presented results reveal that both the enriched medium (Cyto-Grow) and the human-derived supplements (H.S. and PrP) have remarkable effects on hMSCs proliferation and myogenic differentiation compared to standard condition, uncovering the real possibility to exploit these human derivatives to ameliorate stem cells yield and efficacy.

Ferumoxytol Can Be Used for Quantitative Magnetic Particle Imaging of Transplanted Stem Cells

PURPOSE

To evaluate, if clinically translatable ferumoxytol nanoparticles can be used for in vivo detection and quantification of stem cell transplants with magnetic particle imaging (MPI).

PROCEDURES

Mesenchymal stem cells (MSCs) were labeled with ferumoxytol or ferucarbotran and underwent MPI, magnetic resonance imaging (MRI), Prussian blue staining, and inductively coupled plasma (ICP) spectrometry. Unlabeled, ferumoxytol, and ferucarbotran-labeled MSCs were implanted in calvarial defects of eight mice and underwent MPI, MRI, and histopathology. The iron concentration calculated according to the MPI signal intensity and T2 relaxation times of the three different groups were compared using an analysis of variance (ANOVA) with Bonferroni correction, and a p < 0.05.

RESULTS

Compared to unlabeled controls, ferumoxytol- and ferucarbotran-labeled MSC showed significantly increased iron content, MPI signal and MRI signal. The ferumoxytol MPI signal was approximately 4× weaker compared to ferucarbotran at equimolar concentrations (p = 0.0003) and approximately 1.5× weaker for labeled cells when using optimized labeling protocols (p = 0.002). In vivo, the MPI signal of ferumoxytol-labeled MSC decreased significantly between day 1 and day 14 (p = 0.0124). This was confirmed by histopathology where we observed a decrease in Prussian blue stain of MSCs at the transplant site. The MRI signal of the same transplants did not change significantly during this observation period (p = 0.93).

CONCLUSION

Ferumoxytol nanoparticles can be used for in vivo detection of stem cell transplants with MPI and provide quantitative information not attainable with MRI.

Skeletal muscle as an experimental model of choice to study tissue aging and rejuvenation

Skeletal muscle is among the most age-sensitive tissues in mammal organisms. Significant changes in its resident stem cells (i.e., satellite cells, SCs), differentiated cells (i.e., myofibers), and extracellular matrix cause a decline in tissue homeostasis, function, and regenerative capacity. Based on the conservation of aging across tissues and taking advantage of the relatively well-characterization of the myofibers and associated SCs, skeletal muscle emerged as an experimental system to study the decline in function and maintenance of old tissues and to explore rejuvenation strategies. In this review, we summarize the approaches for understanding the aging process and for assaying the success of rejuvenation that use skeletal muscle as the experimental system of choice. We further discuss (and exemplify with studies of skeletal muscle) how conflicting results might be due to variations in the techniques of stem cell isolation, differences in the assays of functional rejuvenation, or deciding on the numbers of replicates and experimental cohorts.

Adipose Tissue-Derived Stromal Cells in Matrigel Impacts the Regeneration of Severely Damaged Skeletal Muscles

In case of large injuries of skeletal muscles the pool of endogenous stem cells, i.e., satellite cells, might be not sufficient to secure proper regeneration. Such failure in reconstruction is often associated with loss of muscle mass and excessive formation of connective tissue. Therapies aiming to improve skeletal muscle regeneration and prevent fibrosis may rely on the transplantation of different types of stem cell. Among such cells are adipose tissue-derived stromal cells (ADSCs) which are relatively easy to isolate, culture, and manipulate. Our study aimed to verify applicability of ADSCs in the therapies of severely injured skeletal muscles. We tested whether 3D structures obtained from Matrigel populated with ADSCs and transplanted to regenerating mouse gastrocnemius muscles could improve the regeneration. In addition, ADSCs used in this study were pretreated with myoblasts-conditioned medium or anti-TGFβ antibody, i.e., the factors modifying their ability to proliferate, migrate, or differentiate. Analyses performed one week after injury allowed us to show the impact of 3D cultured control and pretreated ADSCs at muscle mass and structure, as well as fibrosis development immune response of the injured muscle.

Induction of muscle stem cell quiescence by the secreted niche factor Oncostatin M

The balance between stem cell quiescence and proliferation in skeletal muscle is tightly controlled, but perturbed in a variety of disease states. Despite progress in identifying activators of stem cell proliferation, the niche factor(s) responsible for quiescence induction remain unclear. Here we report an in vivo imaging-based screen which identifies Oncostatin M (OSM), a member of the interleukin-6 family of cytokines, as a potent inducer of muscle stem cell (MuSC, satellite cell) quiescence. OSM is produced by muscle fibers, induces reversible MuSC cell cycle exit, and maintains stem cell regenerative capacity as judged by serial transplantation. Conditional OSM receptor deletion in satellite cells leads to stem cell depletion and impaired regeneration following injury. These results identify Oncostatin M as a secreted niche factor responsible for quiescence induction, and for the first time establish a direct connection between induction of quiescence, stemness, and transplantation potential in solid organ stem cells.

The factors that mediate quiescence of muscle stem cells are unknown. The authors show that Oncostatin M is produced by skeletal muscle, suppresses stem cell proliferation, and that its deletion in muscle results in stem cell depletion and impaired muscle regeneration following injury in mice.

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.

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.

Xenogeneic Cell Therapy: Current Progress and Future Developments in Porcine Cell Transplantation

The multitude of distinct cell types present in mature and developing tissues display unique physiologic characteristics. Cellular therapy is a novel technology with the promise of utilizing this diversity to treat a wide range of human degenerative diseases. Intractable diseases, disorders, and injuries are characterized by cell death or aberrant cellular function. Cell transplantation can replace diseased or lost tissue to provide restorative therapy for these conditions. The limited use of cell transplants as a basis for current therapy can, in part, be attributed to the lack of available human cells suitable for transplantation. This has prevented further realization of the promise of cell transplantation as a platform technology. Accordingly, cell-based therapies such as blood transfusions, for which the cells are readily available, are a standard part of current medical practice. Despite numerous attempts to expand primary human cells in tissue culture, current technological limitations of this approach in regard to proliferative capacity and maintenance of the differentiated phenotype has prevented their use for transplantation. Further, use of human stem cells for the derivation of specific cell types for transplantation is an area of future application with great potential, but hurdles remain in regard to deriving and sufficiently expanding these multi-potential cells. Thus, it appears that primary cells are at present a superior source for transplantation. This review focuses on pigs as a source of a variety of primary cells to advance cell therapy to the clinic and implement achievement of its full potential. We outline the advantages and disadvantages of xenogeneic cell therapy while underscoring the utility of transplantable porcine cells for the treatment of human disease. © 1998 Elsevier Science Inc.

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

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

Cellular Therapies for Muscular Dystrophies: Frustrations and Clinical Successes

Cell-based therapy for muscular dystrophies was initiated in humans after promising results obtained in murine models. Early trials failed to show substantial clinical benefit, sending researchers back to the bench, which led to the discovery of many hurdles as well as many new venues to optimize this therapeutic strategy. In this review we summarize progress in preclinical cell therapy approaches, with a special emphasis on human cells potentially attractive for human clinical trials. Future perspectives for cell therapy in skeletal muscle are discussed, including the perspective of combined therapeutic approaches.

Invited review: Stem cells and muscle diseases: advances in cell therapy strategies

Despite considerable progress to increase our understanding of muscle genetics, pathophysiology, molecular and cellular partners involved in muscular dystrophies and muscle ageing, there is still a crucial need for effective treatments to counteract muscle degeneration and muscle wasting in such conditions. This review focuses on cell-based therapy for muscle diseases. We give an overview of the different parameters that have to be taken into account in such a therapeutic strategy, including the influence of muscle ageing, cell proliferation and migration capacities, as well as the translation of preclinical results in rodent into human clinical approaches. We describe recent advances in different types of human myogenic stem cells, with a particular emphasis on myoblasts but also on other candidate cells described so far [CD133+ cells, aldehyde dehydrogenase-positive cells (ALDH+), muscle-derived stem cells (MuStem), embryonic stem cells (ES) and induced pluripotent stem cells (iPS)]. Finally, we provide an update of ongoing clinical trials using cell therapy strategies.

Implanted scaffold-free prevascularized constructs promote tissue repair

To evaluate the anastomotic potential of prevascular tissue constructs generated from scaffold-free self-assembly of human endothelial and fibroblast cells, tissue constructs were implanted into athymic mice and immune-competent rats. Analysis of xenografts placed into hind limb muscle defects showed vascular anastomotic activity by 3 days after implantation and persisting for 2 weeks. Integration of the implanted prevascular tissue constructs with the host circulatory system was evident from presence of red blood cells in the implant as early as 3 days after implantation. Additionally, analysis of 3-day xenografts in the rat model showed activation of skeletal muscle satellite cells based on Pax-7 and MyoD expressions. We conclude that prevascular tissue constructs generated from scaffold-free self-assembly of human endothelial and fibroblast cells are a promising tool to provide both vascular supply and satellite cell activation toward the resolution of skeletal muscle injury.

Ex vivo gene editing of the dystrophin gene in muscle stem cells mediated by peptide nucleic acid single stranded oligodeoxynucleotides induces stable expression of dystrophin in a mouse model for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal disease caused by mutations in the dystrophin gene, which result in the complete absence of dystrophin protein throughout the body. Gene correction strategies hold promise to treating DMD. Our laboratory has previously demonstrated the ability of peptide nucleic acid single-stranded oligodeoxynucleotides (PNA-ssODNs) to permanently correct single-point mutations at the genomic level. In this study, we show that PNA-ssODNs can target and correct muscle satellite cells (SCs), a population of stem cells capable of self-renewing and differentiating into muscle fibers. When transplanted into skeletal muscles, SCs transfected with correcting PNA-ssODNs were able to engraft and to restore dystrophin expression. The number of dystrophin-positive fibers was shown to significantly increase over time. Expression was confirmed to be the result of the activation of a subpopulation of SCs that had undergone repair as demonstrated by immunofluorescence analyses of engrafted muscles using antibodies specific to full-length dystrophin transcripts and by genomic DNA analysis of dystrophin-positive fibers. Furthermore, the increase in dystrophin expression detected over time resulted in a significant improvement in muscle morphology. The ability of transplanted cells to return into quiescence and to activate upon demand was confirmed in all engrafted muscles following injury. These results demonstrate the feasibility of using gene editing strategies to target and correct SCs and further establish the therapeutic potential of this approach to permanently restore dystrophin expression into muscle of DMD patients.

Restricted Myogenic Potential of Mesenchymal Stromal Cells Isolated from Umbilical Cord

Nonhematopoietic cord blood cells and mesenchymal cells of umbilical cord Wharton's jelly have been shown to be able to differentiate into various cell types. Thus, as they are readily available and do not raise any ethical issues, these cells are considered to be a potential source of material that can be used in regenerative medicine. In our previous study, we tested the potential of whole mononucleated fraction of human umbilical cord blood cells and showed that they are able to participate in the regeneration of injured mouse skeletal muscle. In the current study, we focused at the umbilical cord mesenchymal stromal cells isolated from Wharton's jelly. We documented that limited fraction of these cells express markers of pluripotent and myogenic cells. Moreover, they are able to undergo myogenic differentiation in vitro, as proved by coculture with C2C12 myoblasts. They also colonize injured skeletal muscle and, with low frequency, participate in the formation of new muscle fibers. Pretreatment of Wharton's jelly mesenchymal stromal cells with SDF-1 has no impact on their incorporation into regenerating muscle fibers but significantly increased muscle mass. As a result, transplantation of mesenchymal stromal cells enhances the skeletal muscle regeneration.

Polystyrene-coated micropallets for culture and separation of primary muscle cells

Despite identification of a large number of adult stem cell types, current primary cell isolation and identification techniques yield heterogeneous samples, making detailed biological studies challenging. To identify subsets of isolated cells, technologies capable of simultaneous cell culture and cloning are necessary. Micropallet arrays, a new cloning platform for adherent cell types, hold great potential. However, the microstructures composing these arrays are fabricated from an epoxy photoresist 1002F, a growth surface unsuitable for many cell types. Optimization of the microstructures' surface properties was conducted for the culture of satellite cells, primary muscle cells for which improved cell isolation techniques are desired. A variety of surface materials were screened for satellite cell adhesion and proliferation and compared to their optimal substrate, gelatin-coated Petri dishes. A 1-μm thick, polystyrene copolymer was applied to the microstructures by contact printing. A negatively charged copolymer of 5% acrylic acid in 95% styrene was found to be equivalent to the control Petri dishes for cell adhesion and proliferation. Cells cultured on control dishes and optimal copolymer-coated surfaces maintained an undifferentiated state and showed similar mRNA expression for two genes indicative of cell differentiation during a standard differentiation protocol. Experiments using additional contact-printed layers of extracellular matrix proteins collagen and gelatin showed no further improvements. This micropallet coating strategy is readily adaptable to optimize the array surface for other types of primary cells.

Advances in musculoskeletal tissue engineering: moving towards therapy

Skeletal muscle can self-repair, but is unable to restore significant tissue loss, as consequence of trauma, congenital defects, tumor ablation or denervation. Intramuscular injection of autologous or allogenic derived myogenic cells (namely satellite cells and myoblasts) did not lead to efficient regeneration because of poor cell retention and survival, as well as immunorejection. In the last decade, tissue engineering looked at overcoming these problems by investigating alternative treatment options, i.e., the suspension of myogenic precursors in temporary matrix, formed by biodegradable and biocompatible materials. This approach allows to engineer custom architectured preconditioned implants, and locally deliver paracrine factors.This article reviews current and potential strategies for the repair of damaged muscle and suggests some innovative approaches for the translation to the clinical setting.

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.

Angiomyogenesis for myocardial repair

The conventional therapeutic modalities for myocardial infarction have limited success in preventing the progression of left ventricular remodeling and congestive heart failure. The heart cell therapy and therapeutic angiogenesis are two promising strategies for the treatment of ischemic heart disease. After extensive assessment of safety and effectiveness in vitro and in experimental animal studies, both of these approaches have accomplished the stage of clinical utility, albeit with limited success due to the inherent limitations and problems of each approach. Neomyogenesis without restoration of regional blood flow may be less meaningful. A combined stem-cell and gene-therapy approach of angiomyogenesis is expected to yield better results as compared with either of the approaches as a monotherapy. The combined therapy approach will help to restore the mechanical contractile function of the weakened myocardium and alleviate ischemic condition by restoration of regional blood flow. In providing an overview of both stem cell therapy and gene therapy, this article is an in-depth and critical appreciation of combined cell and gene therapy approach for myocardial repair.

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.

Alignment of skeletal muscle myoblasts and myotubes using linear micropatterned surfaces ground with abrasives

Alignment of cells plays a significant key role in skeletal muscle tissue engineering because skeletal muscle tissue in vivo has a highly organized structure consisting of long parallel multinucleated myotubes formed through differentiation and fusion of myoblasts. In the present study, we developed an easy, simple, and low-cost method for aligning skeletal muscle cells by using surfaces with linear microscale features fabricated by grinding. Iron blocks were ground in one direction with three kinds of abrasives (9 microm diamond suspension, #400 sandpaper, and #150 sandpaper) and then used as molds to make micropatterned polydimethylsiloxane (PDMS) substrates (type I, type II, and type III). Observation of the surface topography revealed that the PDMS substrates exhibited different degree of mean roughness (Ra), 0.03 microm for type I, 0.16 microm for type II, and 0.56 microm for type III, respectively. Murine skeletal muscle cell line C2C12 myoblasts were cultured and differentiated on the patterned PDMS substrates, and it was examined whether the alignment of C2C12 myoblasts and myotubes was possible. Although the cell growth and differentiation on the three types of patterned substrates were similar to those on the flat PDMS substrate as a control, the alignment of both C2C12 myoblasts and myotubes was obviously observed on types II and III, but not on type I or the control substrate. These results indicate that surfaces ground with abrasives will be useful for fabricating aligned skeletal muscle tissues.

Self-renewal and expansion of single transplanted muscle stem cells

Adult muscle satellite cells have a principal role in postnatal skeletal muscle growth and regeneration. Satellite cells reside as quiescent cells underneath the basal lamina that surrounds muscle fibres and respond to damage by giving rise to transient amplifying cells (progenitors) and myoblasts that fuse with myofibres. Recent experiments showed that, in contrast to cultured myoblasts, satellite cells freshly isolated or satellite cells derived from the transplantation of one intact myofibre contribute robustly to muscle repair. However, because satellite cells are known to be heterogeneous, clonal analysis is required to demonstrate stem cell function. Here we show that when a single luciferase-expressing muscle stem cell is transplanted into the muscle of mice it is capable of extensive proliferation, contributes to muscle fibres, and Pax7(+)luciferase(+) mononucleated cells can be readily re-isolated, providing evidence of muscle stem cell self-renewal. In addition, we show using in vivo bioluminescence imaging that the dynamics of muscle stem cell behaviour during muscle repair can be followed in a manner not possible using traditional retrospective histological analyses. By imaging luciferase activity, real-time quantitative and kinetic analyses show that donor-derived muscle stem cells proliferate and engraft rapidly after injection until homeostasis is reached. On injury, donor-derived mononucleated cells generate massive waves of cell proliferation. Together, these results show that the progeny of a single luciferase-expressing muscle stem cell can both self-renew and differentiate after transplantation in mice, providing new evidence at the clonal level that self-renewal is an autonomous property of a single adult muscle stem cell.

Cell tracking with optical imaging

Adaptability, sensitivity, resolution and non-invasiveness are the attributes that have contributed to the longstanding use of light as an investigational tool and form the basis of optical imaging (OI). OI, which encompasses numerous techniques and methods, is rapid (<5 min), inexpensive, noninvasive, nontoxic (no radiation) and has molecular (single-cell) sensitivity, which is equal to that of conventional nuclear imaging and several orders of magnitude greater than MRI. This article provides a comprehensive overview of emerging applications of OI-based techniques for in vivo monitoring of new stem cell-based therapies. Different fluorochromes for cell labeling, labeling methods and OI-based cell-tracking techniques will be reviewed with respect to their technical principles, current applications and aims for clinical translation. Advantages and limitations of these new OI-based cell-tracking techniques will be discussed. Non-invasive mapping of cells labeled with fluorochromes or OI marker genes has the potential to evolve further within the clinical realm.

Human muscular fetal cells: a potential cell source for muscular therapies

Myoblast transfer therapy has been extensively studied for a wide range of clinical applications, such as tissue engineering for muscular loss, cardiac surgery or Duchenne Muscular Dystrophy treatment. However, this approach has been hindered by numerous limitations, including early myoblast death after injection and specific immune response after transplantation with allogenic cells. Different cell sources have been analyzed to overcome some of these limitations. The object of our study was to investigate the growth potential, characterization and integration in vivo of human primary fetal skeletal muscle cells. These data together show the potential for the creation of a cell bank to be used as a cell source for muscle cell therapy and tissue engineering. For this purpose, we developed primary muscular cell cultures from biopsies of human male thigh muscle from a 16-week-old fetus and from donors of 13 and 30 years old. We show that fetal myogenic cells can be successfully isolated and expanded in vitro from human fetal muscle biopsies, and that fetal cells have higher growth capacities when compared to young and adult cells. We confirm lineage specificity by comparing fetal muscle cells to fetal skin and bone cells in vitro by immunohistochemistry with desmin and 5.1 H11 antibodies. For the feasibility of the cell bank, we ensured that fetal muscle cells retained intrinsic characteristics after 5 years cryopreservation. Finally, human fetal muscle cells marked with PKH26 were injected in normal C57BL/6 mice and were found to be present up to 4 days. In conclusion we estimate that a human fetal skeletal muscle cell bank can be created for potential muscle cell therapy and tissue engineering.

Regulation of skeletal muscle differentiation in fibroblasts by exogenous MyoD gene in vitro and in vivo

MyoD of the myogenic regulatory factors (MRFs) family regulates the skeletal muscle differentiation program. In this study, stably transfected NIH3T3-derived cell lines were established, in which exogenous MyoD was expressed at high levels. Transcriptional activation of endogenous muscle regulatory gene and induction towards the skeletal muscle lineages were observed with phase-contrast microscopy when continuously cultured in vitro. Moreover, to determine their ability of myogenic formation in vivo, the transfected cells were implanted in nude mice subcutaneously for up to 10 weeks. The morphological characterization of inductive cells was observed using transmission electron microscope and histological staining. Myogenesis of fibroblasts incubated in the medium was activated by overexpression of MyoD, and the cells were accumulated and fused into multinucleated myotubes. Correlatively, RT-PCR and immunohistochemistry confirmed the increased expression of characteristic downstream molecule myogenin and mysion heavy chains during myogenic differentiation. Ecoptic myogenesis was found and remained stable phenotype when the transfected cells were seeded in vivo. Our results suggest that MyoD can be considered to be a determining factor of myogenic lineages, and it may play an important role in the cell therapy and cell-mediated gene therapy of the skeletal muscle.

A combinatorial role for NFAT5 in both myoblast migration and differentiation during skeletal muscle myogenesis

Skeletal muscle regeneration depends on myoblast migration, differentiation and myofiber formation. Isoforms of the nuclear factor of activated T cells (NFAT) family of transcription factors display nonredundant roles in skeletal muscle. NFAT5, a new isoform of NFAT, displays many differences from NFATc1-c4. Here, we examine the role of NFAT5 in myogenesis. NFAT5+/- mice displayed a defect in muscle regeneration with fewer myofibers formed at early times after injury. NFAT5 has a muscle-intrinsic function because inhibition of NFAT5 transcriptional activity caused both a migratory and differentiation defect in cultured myoblasts. We identified Cyr61 as a target of NFAT5 signaling in skeletal muscle cells. Addition of Cyr61 to cells expressing inhibitory forms of NFAT5 rescued the migratory phenotype. These results demonstrate a role for NFAT5 in skeletal muscle cell migration and differentiation. Furthermore, as cell-cell interactions are crucial for myoblast differentiation, these data suggest that myoblast migration and differentiation are coupled and that NFAT5 is a key regulator.

Adult stem cells for cardiac repair: a choice between skeletal myoblasts and bone marrow stem cells

The real promise of a stem cell-based approach for cardiac regeneration and repair lies in the promotion of myogenesis and angiogenesis at the site of the cell graft to achieve both structural and functional benefits. Despite all of the progress and promise in this field, many unanswered questions remain; the answers to these questions will provide the much-needed breakthrough to harness the real benefits of cell therapy for the heart in the clinical perspective. One of the major issues is the choice of donor cell type for transplantation. Multiple cell types with varying potentials have been assessed for their ability to repopulate the infarcted myocardium; however, only the adult stem cells, that is, skeletal myoblasts (SkM) and bone marrow-derived stem cells (BMC), have been translated from the laboratory bench to clinical use. Which of these two cell types will provide the best option for clinical application in heart cell therapy remains arguable. With results pouring in from the long-term follow-ups of previously conducted phase I clinical studies, and with the onset of phase II clinical trials involving larger population of patients, transplantation of stem cells as a sole therapy without an adjunct conventional revascularization procedure will provide a deeper insight into the effectiveness of this approach. The present article discusses the pros and cons of using SkM and BMC individually or in combination for cardiac repair, and critically analyzes the progress made with each cell type.

Autologous transplantation of porcine myogenic precursor cells in skeletal muscle

Myoblast transplantation is a potential therapy for severe muscle trauma, myopathies and heart infarct. Success with this therapy relies on the ability to obtain cell preparations enriched in myogenic precursor cells and on their survival after transplantation. To define myoblast transplantation strategies applicable to patients, we used a large animal model, the pig. Muscle dissociation procedures adapted to porcine tissue gave high yields of cells containing at least 80% myogenic precursor cells. Autologous transplantation of 3[H]-thymidine labeled porcine myogenic precursor cells indicated 60% survival at day 1 followed by a decay to 10% at day 5 post-injection. Nuclei of myogenic precursor cells transduced with a lentivirus encoding the nls-lacZ reporter gene were present in host myotubes 8 days post-transplantation, indicating that injected myogenic precursor cells contribute to muscle regeneration. This work suggests that pig is an adequate large animal model for exploring myogenic precursor cells transplantation strategies applicable in patients.

Abortive myogenesis in denervated skeletal muscle: differentiative properties of satellite cells, their migration, and block of terminal differentiation

Little is known about the biological properties of myogenic satellite cells during post-denervation muscle atrophy. The present study investigated the differentiative capacity of satellite cells and their involvement in the compensatory regenerative process in long-term denervated rat muscle. Electron microscopy and immunocytochemical labeling of muscle tissue 1-18 months following denervation demonstrated that despite activation of satellite cells, myogenesis in denervated muscle is abortive and does not lead to the formation of normal muscle fibers. Small sizes, poor development of the contractile system in newly formed denervated myotubes, and the absence of satellite cells on the surface indicate that their differentiation typically does not progress to terminal stages. Many immature myotubes degenerate, and others survive but are embedded in a collagen lattice near their parent fibers. Interestingly, newly formed myotubes located on the surface of parent muscle fibers beneath the basal lamina typically did not contain developed myofibrils. This suggests that the contacts of daughter and parent muscle fibers block myofibrillogenesis. Assembly of sarcomeres in most cases occurs following complete spatial separation of daughter and parent muscle fibers. Another manifestation of the involvement of myogenic precursors in abortive myogenesis is the formation of clusters of underdeveloped branching myotubes surrounded by a common basal lamina. We found that myoblasts can also fuse directly with differentiated muscle fibers. The presence of satellite cells near the openings in the basal lamina and in the interstitial space indicates that myogenic precursors can migrate through the basal lamina and form myotubes at a distance from parent fibers. Our data may explain why long-term denervated skeletal muscle has a poor capacity for regeneration and functional restoration.

Skeletal muscle tissue engineering

The reconstruction of skeletal muscle tissue either lost by traumatic injury or tumor ablation or functional damage due to myopathies is hampered by the lack of availability of functional substitution of this native tissue. Until now, only few alternatives exist to provide functional restoration of damaged muscle tissues. Loss of muscle mass and their function can surgically managed in part using a variety of muscle transplantation or transposition techniques. These techniques represent a limited degree of success in attempts to restore the normal functioning, however they are not perfect solutions. A new alternative approach to addressing difficult tissue reconstruction is to engineer new tissues. Although those tissue engineering techniques attempting regeneration of human tissues and organs have recently entered into clinical practice, the engineering of skeletal muscle tissue ist still a scientific challenge. This article reviews some of the recent findings resulting from tissue engineering science related to the attempt of creation and regeneration of functional skeletal muscle tissue.

Y chromosome detection of three-dimensional tissue-engineered skeletal muscle constructs in a syngeneic rat animal model

Surgical reconstruction of muscle tissue lost by trauma or tumor ablation is limited by the lack of availability of functional native tissue substitution. Moreover, so far most inherited or acquired muscle diseases are lacking sufficient treatment, because only few alternatives exist to provide functional restoration of lost muscle tissues. Engineering those tissues and transplantation into sites of dysfunction may be an alternative approach and may allow replacement of such damaged or failing skeletal muscle tissues. Techniques attempting reconstruction of some human tissues and organs (tissue engineering) have been introduced into clinical practice recently. One major problem that previous transplantation studies were facing is the ability of detection of transplanted cells after integration. Using the Y chromosome in situ hybridization technique in a syngeneic rat model allows transplantation of cell constructs orthotopically, without manipulation of the cells, with no rejection or immunosuppression being implied, but providing a nondilutable genetic marker to identify transplanted cells. The purpose of our study was to create functional skeletal muscle tissue in vivo using the transplantation of primary myoblasts precultivated within a three-dimensional (3D) fibrin matrix and to determine the fate of the transplanted cells using the Y chromosome detection technique. 3D myoblast cultures were established derived from male donor rats and after 7 days of cultivation we performed an orthotopic transplantation of 3D cell constructs into a created muscle defect within the gracilis muscle of syngeneic female rats. Anti-desmin immunostaining and Y chromosome in situ hybridization indicated the survival and integration of transplanted male myoblasts into the female recipient animal, thus demonstrating the feasibility of this approach in tissue engineering and the research of cell transplantation in general.

Microenvironmental VEGF concentration, not total dose, determines a threshold between normal and aberrant angiogenesis

Use of long-term constitutive expression of VEGF for therapeutic angiogenesis may be limited by the growth of abnormal blood vessels and hemangiomas. We investigated the relationship between VEGF dosage and the morphology and function of newly formed blood vessels by implanting retrovirally transduced myoblasts that constitutively express VEGF164 into muscles of adult mice. Reducing VEGF dosage by decreasing the total number of VEGF myoblasts implanted did not prevent vascular abnormalities. However, when clonal populations of myoblasts homogeneously expressing different levels of VEGF were implanted, a threshold between normal and aberrant angiogenesis was found. Clonal myoblasts that expressed low to medium levels of VEGF induced growth of stable, pericyte-coated capillaries of uniform size that were not leaky and became VEGF independent, as shown by treatment with the potent VEGF blocker VEGF-TrapR1R2. In contrast, clones that expressed high levels of VEGF induced hemangiomas. Remarkably, when different clonal populations were mixed, even a small proportion of cells with high production of VEGF was sufficient to cause hemangioma growth. These results show for the first time to our knowledge that the key determinant of whether VEGF-induced angiogenesis is normal or aberrant is the microenvironmental amount of growth factor secreted, rather than the overall dose. Long-term continuous delivery of VEGF, when maintained below a threshold microenvironmental level, can lead to normal angiogenesis without other exogenous growth factors.

Engineering of muscle tissue

The loss or failure of an organ or tissue is one of the most frequent, devastating, and costly problems in health care. Tissue engineering and regenerative medicine is an emerging interdisciplinary field that applies the principles of biology and engineering to the development of viable substitutes that restore, maintain, or improve the function of human tissues and organs. Tissue engineering science has provided critical new knowledge that will deepen our understanding of the phenotype of an important category of cell types-the muscle cells-and this knowledge may enable meaningful advances in musculoskeletal tissue engineering. There are two principle strategies for the replacement of impaired muscle tissues. One approach uses the application of isolated and differentiated cells (in vivo tissue engineering), using a transport matrix for the cell delivery; the other uses in vitro-designed and pre-fabricated tissue equivalents (in vitro tissue engineering). Future developments and the decision regarding which approach is more promising depend on the elucidation of the relationships among cell growth and differentiation, the three-dimensional environment, the architecture of the cells, and gene expression of the developmental process and the survival of the cells and integration in the host in in vivo experiments. As the techniques of tissue engineering become more sophisticated and as issues such as vascularization and innervation are addressed, the usefulness of these methods for reconstructive surgery may grow significantly.

Tissue engineered muscle implantation for tongue reconstruction: a preliminary report

OBJECTIVES/HYPOTHESIS

Because current tongue reconstructive methods introduce adynamic, variably sensate tissue into the mouth, the critical functions of the tongue in articulation and deglutition may be compromised. The objective of this work was to introduce a combination of myoblasts and scaffolding material into rat hemiglossectomy defects and to examine the extent of neomuscle formation in the reconstructed area, under the hypothesis that the presence of myoblasts leads to formation of new muscle.

STUDY DESIGN

Randomized, prospective animal study.

METHODS

Myoblasts were harvested from neonatal Lewis rats, and a growth factor enriched collagen gel was prepared. Syngeneic adult animals received either hemiglossectomy alone or reconstruction with one of four experimental reconstructive preparations: collagen gel alone, collagen gel with suspended myoblasts, the gel-cell combination in undifferentiated muscle construct form by way of tissue culture for 7 days in a preformed mold, or differentiated constructs, cultured in myoblast fusion medium. After 6 or 16 weeks, animal weight gain was recorded, animals were killed, and the tongues harvested. The tissue was examined histologically, and quality of the muscular regenerate was rated on a scale according to predefined criteria.

RESULTS

Animals in all groups gained weight appropriately. In groups receiving hemiglossectomy alone or acellular (gel only) reconstruction, there was significant scarring and lack of neomuscle formation. In groups receiving myoblast transplantation, either by way of gel suspension or in the form of undifferentiated or differentiated constructs, muscle quality was superior to controls.

CONCLUSIONS

Myoblast transplantation into hemiglossectomy defects appears to lead to new muscle formation and does not inhibit normal weight gain in animals after tongue implantation.

Resetting the problem of cell death following muscle-derived cell transplantation: detection, dynamics and mechanisms

We conducted a study in mice to reevaluate and clarify many aspects of the early survival of muscle cells following transplantation. Male mouse muscle cells (primary-cultures and T-antigen-immortalized clones) labeled with [14C]thymidine and beta-galactosidase were injected into female muscles. Each label was detected in the muscles after different time periods. TUNEL, alizarin red, and immunodetection of active caspase-3 were done in muscle sections. The donor cell labels disappeared from the muscles following donor cell death, but this was not instantaneous and even if the donor cells were killed before transplantation, the first 6 hours were not enough to clear [14C]thymidine and Y chromosome. Using the cell pellet before injection as the 100% baseline for cells injected to evaluate cell death can lead to misinterpretations: the Y-chromosome band was 5-fold stronger than that of a muscle injected with cells, irrespective of whether the cells were previously killed or not. There was no evidence of an immediate massive donor cell death. Necrosis (detected by alizarin red) and apoptosis (detected by active caspase-3) were present among the donor myoblasts following transplantation. Necrosis seemed to be the most important mechanism during the first hours. T-antigen immortalized cells died earlier and more massively than primary-cultured cells, but the surviving cells proliferated more. Indeed, they seemed to exhibit more apoptosis and they triggered a more rapid CD8+ cell infiltration. As a result of our findings, many concepts concerning the early donor cell death following myoblast transplantation must be reconsidered.

Irradiation of dystrophic host tissue prior to myoblast transfer therapy enhances initial (but not long-term) survival of donor myoblasts

There is a massive and rapid death of donor myoblasts (<20% surviving) within hours after intramuscular injection in myoblast transfer therapy (MTT), due to host immune cells, especially natural killer (NK) cells. To investigate the role of host immune cells in the dramatic death of donor myoblasts, MTT experiments were performed in irradiated host mice. Cultured normal C57BL/10ScSn male donor myoblasts were injected into muscles of female C57BL/10ScSn-Dmdmdx host mice after one of three treatments: whole body irradiation (WBI) to eliminate all circulating leukocytes, WBI and bone marrow reconstitution (BMR), or local irradiation (or protection) of one limb. Similar experiments were performed in host mice after antibody depletion of NK cells. Numbers of male donor myoblasts were quantified using a Y-chromosome-specific (male) probe following total DNA extraction of injected muscles. WBI prior to MTT resulted in dramatically enhanced survival (approximately 80%) of donor myoblasts at 1 hour after MTT, supporting a central role for host inflammatory cells in the initial death of donor myoblasts seen in untreated host mice. BMR restored the massive and rapid loss (approximately 25% surviving) of donor myoblasts at 1 hour after MTT. Local pre-irradiation also resulted in increased donor myoblast numbers (approximately 35-40%) compared with untreated controls (approximately 10%) at 3 weeks after MTT. Preirradiation of host muscle with 10 Gy did not significantly stimulate proliferation of the injected donor myoblasts. Serum protein levels of TNFalpha, IL-1beta, IL-6 and IL-12 fluctuated following irradiation treatments. These combined results strongly reinforce a major role for host immune cells in the rapid death of injected cultured donor myoblasts.

Skeletal myoblast transplant in heart failure

Despite recent advances in the prevention and treatment of ischemic heart disease (IHD), treatment of patients with heart failure secondary to myocardial infarction remains a therapeutic challenge. Heart transplantation has emerged as a viable option but is fraught with problems of supply. Mechanical assist devices are extremely expensive and dynamic cardiomyoplasty has shown only limited success in the clinical setting. Recent insights into the pathogenesis of myocardial diseases and the progress made in the field of molecular biology have resulted in the development of new strategies at molecular as well as cellular levels for cardiac muscle repair. One such strategy is to augment ventricular function by means of cellular cardiomyoplasty through intracardiac cell grafting using adult and fetal cardiomyocytes, stem cells, and autologous skeletal myoblasts.

Dynamics of the early immune cellular reactions after myogenic cell transplantation

The role of immune cells in the early donor cell death/survival following myoblast transplantation is confusing, one of the reasons being the lack of data about the immune reactions following cell transplantation. We used outbred mice as hosts for transplantation of primary cultured muscle cells and T-antigen-immortalized myoblasts. The host muscles were analyzed 1 h to 7 days after cell injection. No net loss of the donor primary cultured cell population was observed in this period. The immune cellular reaction in this case was: 1) a brief (<48 h) neutrophil invasion; 2) macrophage infiltration from days 1 to 7; 3) a specific response involving CTL and few NK cells (days 6 and 7), preceded by a low CD4+ cell infiltration starting at day 3. In contrast, donor-immortalized myoblasts completely disappeared during the 7-day follow-up. In this case, an intense infiltration of CTL and macrophages, with moderate CD4+ infiltration and lower amounts of NK cells, was observed starting at day 2. The nonspecific immune response at days 0 and 1 was similar for both types of donor cells. The present observations set a basis to interpret the role of immune cells on the early death/survival of donor cells following myoblast transplantation.

Distribution and effects of a single intramuscular injection of India ink in mice

Our goal has been to study the distribution and effects following a single intramuscular injection of a substance using India ink as a tracer. We injected 30 microl India ink in the gastrocnemius muscle group of C57Bl10 mice. Hematoxylin-Eosin, Trichrome stains and polyclonal anti-laminin, anti-collagen-IV and anti-dystrophin were used. The liquid spreads in all directions mainly following the perimysial and epimysial septae. The collagen bundles act as physical barriers preventing passage of the ink particles. In the area of the injection site, necrosis of the fibres is associated with disruption of the basement membrane. In the zones adjacent and distal to the injection site, the liquid progresses by pushing the muscle fibres apart with preservation of the basement membrane. Research based on intramuscular injection of substances should take the following into consideration: a) anatomy of the muscle group injected, b) routes of distribution of the substance, c) types of lesions produced with relation to the site of injection of the liquid, and d) size of the particles of the injected substance.

Effect of injecting primary myoblasts versus putative muscle-derived stem cells on mass and force generation in mdx mice

It is well established that the injection of normal myoblasts or of muscle-derived stem cells (MDSCs) into the muscle of dystrophin-deficient mdx mice results in the incorporation of a number of donor myoblasts into the host muscle. However, the effect of the injected exogenous cells on mdx muscle mass and functional capacity has not been evaluated. This study evaluates the mass and functional capacity of the extensor digitorum longus (EDL) muscles of adult, male mdx mice that received intramuscular injections of primary myoblasts or of MDSCs (isolated by a preplating technique; Qu, Z., Balkir, L., van Deutekom, J.C., Robbins, P.D., Pruchnic, R., and Huard, J., J. Cell Biol. 1998;142:1257-1267) derived from normal mice. Evaluations were made 9 weeks after cell transplantation. Uninjected mdx EDL muscles have a mass 50% greater than that of age-matched C57BL/10J (normal) EDL muscles. Injections of either primary myoblasts or MDSCs have no effect on the mass of mdx EDL muscles. EDL muscles of mdx mice generate 43% more absolute twitch tension and 43% less specific tetanic tension then do EDL muscles of C57BL/10J mice. However, the absolute tetanic and specific twitch tension of mdx and C57BL/10J EDL muscles are similar. Injection of either primary myoblasts or MDSCs has no effect on the absolute or specific twitch and tetanic tensions of mdx muscle. Approximately 25% of the myofibers in mdx EDL muscles that received primary myoblasts react positively with antibody to dystrophin. There is no significant difference in the number of dystrophin-positive myofibers when MDSCs are injected. Regardless of the source of donor cells, dystrophin is limited to short distances (60-900 microm) along the length of the myofibers. This may, in part, explain the failure of cellular therapy to alter the contractile properties of murine dystrophic muscle.

Low-energy laser irradiation promotes the survival and cell cycle entry of skeletal muscle satellite cells

Low energy laser irradiation (LELI) has been shown to promote skeletal muscle cell activation and proliferation in primary cultures of satellite cells as well as in myogenic cell lines. Here, we have extended these studies to isolated myofibers. These constitute the minimum viable functional unit of the skeletal muscle, thus providing a close model of in vivo regeneration of muscle tissue. We show that LELI stimulates cell cycle entry and the accumulation of satellite cells around isolated single fibers grown under serum-free conditions and that these effects act synergistically with the addition of serum. Moreover, for the first time we show that LELI promotes the survival of fibers and their adjacent cells, as well as cultured myogenic cells, under serum-free conditions that normally lead to apoptosis. In both systems, expression of the anti-apoptotic protein Bcl-2 was markedly increased, whereas expression of the pro-apoptotic protein BAX was reduced. In culture, these changes were accompanied by a reduction in the expression of p53 and the cyclin-dependent kinase inhibitor p21, reflecting the small decrease in viable cells 24 hours after irradiation. These findings implicate regulation of these factors as part of the protective role of LELI against apoptosis. Taken together, our findings are of critical importance in attempts to improve muscle regeneration following injury.

Myogenic differentiation by human processed lipoaspirate cells

The use of undifferentiated cells for cell-based tissue engineering and regeneration strategies represents a promising approach for skeletal muscle repair. For such strategies to succeed, a readily available source of myogenic precursor cells must be identified. We have previously shown that cells isolated from raw human lipoaspirates, called processed lipoaspirate cells, display multilineage mesodermal potential in vitro. Because human liposuctioned fat is available in large quantities and can be harvested with low morbidity, it may be an ideal source of stem cells for tissue-engineering applications. In this study, processed lipoaspirate cells were isolated from raw lipoaspirates harvested from eight patients who underwent cosmetic surgery. Processed lipoaspirate cells were placed in promyogenic conditions for up to 6 weeks, and the expression of the myogenic markers MyoD1 and myosin heavy chain was confirmed by using structure, histology, and reverse transcriptase-polymerase chain reaction. Histologic results were quantitated as an indicator or myogenic differentiation levels. We found that induced human processed lipoaspirate cells form multinucleated cells after 3 weeks of induction, indicative of the formation of myotubes. In addition, MyoD1 and skeletal muscle myosin heavy chain are expressed at distinct time points during differentiation with MyoD1 expression preceding expression of myosin. Finally, approximately 15 percent of human processed lipoaspirate cells can be induced toward myogenic differentiation 6 weeks after induction. In summary, our findings suggest that human processed lipoaspirate cells differentiate into myogenic cells. Furthermore, these cells may be a useful source for skeletal muscle engineering and repair.

Complement and myoblast transfer therapy: donor myoblast survival is enhanced following depletion of host complement C3 using cobra venom factor, but not in the absence of C5

Myoblast transfer therapy (MTT) is a potential cell therapy for myopathies such as Duchenne Muscular Dystrophy and involves the injection of cultured muscle precursor cells ('myoblasts') isolated from normal donor skeletal muscles into dystrophic host muscle. The failure of donor myoblast survival following MTT is widely accepted as being due to the immune response of the host. The role of complement as one possible mechanism for the initial, very rapid death of myoblasts following MTT was investigated. Donor male myoblasts were injected into the tibialis anterior (TA) muscles of female host mice that were: (i) untreated; (ii) depleted of C3 complement (24 h prior to MTT) using cobra venom factor (CVF); and/or (iii) deficient in C5 complement. Quantification of surviving male donor myoblast DNA was performed using the Y-chromosome specific (Y1) probe on slot blots for samples taken at 0 h, 1 h, 24 h, 1 week and 3 weeks after MTT. Peripheral depletion of C3 was confirmed using double immunodiffusion, and local depletion of C3 in host TA muscles was confirmed by immunostaining of muscle samples. Cobra venom factor treatment significantly increased the initial survival of donor myoblasts, but there was a marked decline in myoblast numbers after 1 h and little long-term benefit by 3 weeks. Strain specific variation in the immediate survival of donor male myoblasts following MTT in untreated C57BL/10Sn, DBA-1 and DBA-2 (C5-deficient) female hosts was observed. Cobra venom factor depletion of C3 increased initial donor male myoblast survival (approximately twofold at 0 h) in C57BL/10Sn and DBA-1 host mice and approximately threefold in DBA-2 hosts at 0 h and 1 h after MTT. The rapid and extensive number (approximately 90%) of donor male myoblasts in untreated DBA-2 mice (that lack C5) indicates that activation of the membrane attack complex (MAC) plays no role in this massive initial cell death. The observation that myoblast survival was increased in all mice treated with CVF suggests that CVF may indirectly enhance donor myoblast survival by a mechanism possibly involving activated C3 fragments.

A novel means of drug delivery: myoblast-mediated gene therapy and regulatable retroviral vectors

A potentially powerful approach to drug delivery in the treatment of disease involves the use of cells to introduce genes encoding therapeutic proteins into the body. Candidate genes for delivery include those encoding secreted factors that could have broad applications ranging from treatment of inherited single-gene deficiencies to acquired disorders of the vasculature or cancer. Myoblasts, the proliferative cell type of skeletal muscle tissues, are potent tools for stable delivery of a gene of interest into the body, as they become an integral part of the muscle into which they are injected, in close proximity to the circulation. The recent development of improved tetracycline-inducible retroviral vectors allows for fine control of recombinant gene expression levels. The combination of ex vivo gene transfer using myoblasts and regulatable retroviral vectors provides a powerful toolbox with which to develop gene therapies for a number of human diseases.

Why do cultured transplanted myoblasts die in vivo? DNA quantification shows enhanced survival of donor male myoblasts in host mice depleted of CD4+ and CD8+ cells or Nk1.1+ cells

Overcoming the massive and rapid death of injected donor myoblasts is the primary hurdle for successful myoblast transfer therapy (MTT), designed as a treatment for the lethal childhood myopathy Duchenne muscular dystrophy. The injection of male myoblasts into female host mice and quantification of surviving male DNA using the Y-chromosome-specific (Y1) probe allows the speed and extent of death of donor myoblasts to be determined. Cultured normal C57BL/10Sn male donor myoblasts were injected into untreated normal C57BL/10Sn and dystrophic mdx female host mice and analyzed by slot blots using a 32P-labeled Y1 probe. The amount of male DNA from donor myoblasts showed a remarkable decrease within minutes and by 1 h represented only about 10-18% of the 2.5 x 10(5) cells originally injected (designated 100%). This declined further over 1 week to approximately 1-4%. The host environment (normal or dystrophic) as well as the extent of passaging in tissue culture (early "P3" or late "P15-20" passage) made no difference to this result. Modulation of the host response by CD4+/CD8+ -depleting antibodies administered prior to injection of the cultured myoblasts dramatically enhanced donor myoblast survival in dystrophic mdx hosts (15-fold relative to untreated hosts after 1 week). NK1.1 depletion also dramatically enhanced donor myoblast survival in dystrophic mdx hosts (21-fold after 1 week) compared to untreated hosts. These results provide a strategic approach to enhance donor myoblast survival in clinical trials of MTT.

Systemic production of human granulocyte colony-stimulating factor in nonhuman primates by transplantation of genetically modified myoblasts

Clinical use of human granulocyte-colony stimulating factor (hG-CSF) to treat various diseases involving neutropenia has been previously shown to (1) successfully increase circulating neutrophils, (2) reduce condition-related infections, and (3) cause few side effects in patients. To alleviate the symptoms of neutropenia, the patient must receive frequent injections of recombinant hG-CSF. Permanent ways to deliver stable levels of the molecule to the patient are being investigated. Among them, the transplantation of hG-CSF-secreting cells has been proposed and performed successfully in rodents, using fibroblast cell lines and primary muscle cells. We thus investigated whether similar results could be obtained by intramuscular myoblast transplantation in a large animal model. When 1-3 x 10(8) myoblasts were injected into three Macaca mulatta, hG-CSF was detected at high levels (300-900 pg/ml), which in turn led to a four- to fivefold increase in circulating neutrophils. However, both the concentrations of hG-CSF and neutrophil levels were found to decrease over time. Nonetheless, neutrophils were found at higher levels from the fourth week until the end the experiment (up to 29 weeks) in G-CSF monkeys compared with control animals. These results show that transplantation of hG-CSF-secreting myoblasts may indeed be a therapeutic option for the treatment of neutropenic patients.

Enhancement of adult muscle regeneration by primary myoblast transplantation

Extensor digitorum longus muscles (EDL) of SCID mice were induced to undergo degeneration-regeneration subsequent to orthotopic, whole-muscle transplantation. Two days after transplantation some of these muscles received injections of primary myoblasts derived from EDL muscles of transgenic mice, which express nuclear localizing beta-galactosidase under the control of the myosin light-chain 3F promoter and enhancer. Nine weeks after transplantation, regenerated muscles that received exogenous myoblasts were compared to similarly transplanted muscles that received no further treatment and to unoperated EDL muscles in order to determine the effect of myoblast transfer on muscle regeneration. Many myofibers containing donor derived myonuclei could be identified in the regenerated muscles that had received exogenous myoblasts. The mass of the muscles subjected to transplantation only was significantly less (31% less) than that of unoperated muscles. The addition of exogenous myoblasts to the regenerating EDL resulted in a muscle mass similar to that of unoperated muscles. The absolute twitch and tetanic tensions and specific twitch and tetanic tensions of transplant-only muscles were 28%, 36%, 32%, and 41%, respectively, of those of unoperated muscles. Myoblast transfer increased the absolute twitch and tetanic tensions of the regenerated muscles by 65% and 74%, respectively, and their specific twitch and tetanic tensions were increased by 41% and 48%, respectively. These data suggest a possible role for the addition of exogenous, primary myoblasts in the treatment of traumatized and/or diseased muscles that are characterized by myofiber loss.

Immunobiology and the future of myoblast transfer therapy

Myoblast transfer therapy (MTT) is a cell-mediated gene transfer method aimed at the restoration of normal dystrophin expression in Duchenne muscular dystrophy (DMD). Initial clinical MTT trials were conducted amid much controversy, as they were based on very few animal studies. Unfortunately, the trials were of little therapeutic benefit. As a result, there has been a renaissance of interest in experimental studies in animal models. In MTT, myoblasts are obtained by muscle biopsy from normal, i.e., dystrophin-positive, donors, expanded in culture, and injected directly into the muscles of dystrophic recipients. The major requirement for successful MTT is the survival of injected donor myoblasts in the host environment. However, a vast majority of donor cells fail to survive for more than 1 h after injection, and very few last beyond the first week. This review on the immunological aspects of MTT focuses in particular on the roles of specific components of the host immune response, the effects of tissue culture on donor cells, and strategies under development to circumvent the problem of donor myoblast death after injection in vivo.

Progress in myoblast transplantation: a potential treatment of dystrophies

Myoblast transplantation (MT) consists of injecting normal or genetically modified myogenic cells into muscles, where they are expected to fuse and form mature fibers. As an experimental approach to treat severe genetic muscle diseases, MT was tested in dystrophic patients at the beginning of the 1990s. Although these early clinical trials were unsuccessful, MT has progressed through the research on animal models. Many factors that may condition the success of MT were identified in the last years. The present review updates our knowledge on MT and describes the different problems that have limited its success. Factors that were first underestimated, like the specific immune response after MT, are presently well characterized. Destruction of the hybrid fibers by activated T-lymphocytes and production of antibodies against the transplanted myoblasts take place after MT and are responsible for the graft rejection. The choice of the immunosuppression seems to be very important, and FK506 is the best agent known to allow the best results after MT. Under FK506 immunosuppression, very efficient MT were obtained both in mice and monkeys. Moreover, in dystrophic mice it was demonstrated that MT ameliorates some phenotypical characteristics of the disease. The improvement of the survival of the transplanted cells and the increase of their migration into the injected tissue are presently under investigation. Some of the present research is directed also to bypass the immunosuppression by using the patient's own cells for MT. In this sense, efforts are conducted to introduce the normal gene into the patient's myoblasts before MT and to improve the ability of these cells to proliferate in vitro. Micros. Res. Tech. 48:213-222, 2000.

Skeletal muscle tissue engineering using isolated myoblasts on synthetic biodegradable polymers: preliminary studies

Skeletal muscle is responsible for the control of voluntary movement and the maintenance of structural contours of the body. Muscle loss or deficiency is encountered in various pathological states, and attempts to correct them have been employed with limited success. The aim of the present study was to tissue engineer three-dimensional vascularized skeletal muscle using isolated myoblasts attached to synthetic biodegradable polymer for tissue replacement in the enhancement of muscle regeneration. Myoblasts derived from neonatal rats (3-5-day-old), Fisher CDF-F344, were seeded onto polyglycolic acid meshes and implanted into the omentum of syngeneic adult Fisher CDF-F344 rats. Rats were sacrificed on day 30 and day 45 after the transplantation, and the cell-polymer constructs were harvested for morphological analysis. Histological analysis of the constructs were performed by hematoxylin and eosin, and immunohistochemical staining was positive for alpha sarcomeric actin and desmin skeletal muscle marker. Viable myoblasts organized between strands of degrading polymer mesh formed the new tissue, and vascularization of the entire construct was observed. Organization of neomuscle strands surrounded by vascularized tissue composed of degrading polymer and fusing myoblasts demonstrated the ability of myoblast constructs to survive, reorganize and regenerate tissue-like structures. Since myoblast transplantation to date has been limited to the cellular level of replacement, myoblast-polyglycolic acid constructs may be useful in defining the application of tissue engineering for future skeletal muscle transplantations.