Human myoblast transplantation in immunodeficient and immunosuppressed mice: evidence of rejection

Optimization of Xenografting Methods for Generating Human Skeletal Muscle in Mice

Xenografts of human skeletal muscle generated in mice can be used to study muscle pathology and to test drugs designed to treat myopathies and muscular dystrophies for their efficacy and specificity in human tissue. We previously developed methods to generate mature human skeletal muscles in immunocompromised mice starting with human myogenic precursor cells (hMPCs) from healthy individuals and individuals with facioscapulohumeral muscular dystrophy (FSHD). Here, we examine a series of alternative treatments at each stage in order to optimize engraftment. We show that (i) X-irradiation at 25Gy is optimal in preventing regeneration of murine muscle while supporting robust engraftment and the formation of human fibers without significant murine contamination; (ii) hMPC lines differ in their capacity to engraft; (iii) some hMPC lines yield grafts that respond better to intermittent neuromuscular electrical stimulation (iNMES) than others; (iv) some lines engraft better in male than in female mice; (v) coinjection of hMPCs with laminin, gelatin, Matrigel, or Growdex does not improve engraftment; (vi) BaCl2 is an acceptable replacement for cardiotoxin, but other snake venom preparations and toxins, including the major component of cardiotoxin, cytotoxin 5, are not; and (vii) generating grafts in both hindlimbs followed by iNMES of each limb yields more robust grafts than housing mice in cages with running wheels. Our results suggest that replacing cardiotoxin with BaCl2 and engrafting both tibialis anterior muscles generates robust grafts of adult human muscle tissue in mice.

A Comment on "Muscle Xenografts Reproduce Key Molecular Features of Facioscapulohumeral Muscular Dystrophy": What Is New and What Has Already been Done and Reported but Was Not Quoted?

A study was recently published that sought to develop an in vivo model of facioscapulohumeral muscular dystrophy by transplanting muscle precursor cells from a patient into immunodeficient mice. The study largely applied the methodology used by our team in a study published more than two decades ago with a similar objective, albeit for another muscular dystrophy. However, our study is not cited, leaving the wrong idea that the concept, methodology, and part of the results are original to this recent study. Although the recent study is of interest, the omission of our publication, as well as other relevant references, deprives it of an adequate scientific context. We, therefore, want to point out the importance of a careful bibliographic search in any scientific work.

Skeletal muscle-derived cell implantation for the treatment of sphincter-related faecal incontinence

Background

In an earlier pilot study with 10 women, we investigated a new approach for therapy of faecal incontinence (FI) due to obstetric trauma, involving ultrasound-guided injection of autologous skeletal muscle-derived cells (SMDC) into the external anal sphincter (EAS), and observed significant improvement. In the current study, we tested this therapeutic approach in an extended patient group: male and female patients suffering from FI due to EAS damage and/or atrophy. Furthermore, feasibility of lower cell counts and cryo-preserved SMDC was assessed.

Methods

In this single-centre, explorative, baseline-controlled clinical trial, each patient (n = 39; mean age 60.6 ± 13.81 years) received 79.4 ± 22.5 × 10⁶ cryo-preserved autologous SMDC. Changes in FI parameters, Fecal Incontinence Quality of Life (FIQL), anorectal manometry and safety from baseline to 1, 6 and 12 months post implantation were evaluated.

Results

SMDC used in this trial contained a high percentage of myogenic-expressing (CD56⁺) and muscle stem cell marker-expressing (Pax7⁺, Myf5⁺) cells. Intervention was well tolerated without any serious adverse events. After 12 months, the number of weekly incontinence episodes (WIE, primary variable), FIQL and patient condition had improved significantly. In 80.6% of males and 78.4% of females, the WIE frequency decreased by at least 50%; Wexner scores and severity of FI complaints decreased significantly, independent of gender and cause of FI.

Conclusions

Injection of SMDCs into the EAS effectively improved sphincter-related FI due to EAS damage and/or atrophy in males and females. When confirmed in a larger, placebo-controlled trial, this minimal invasive procedure has the potential to become first-line therapy for FI.

Trial registration

EU Clinical Trials Register, EudraCT 2010-023826-19 (Date of registration: 08.11.2010).

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.

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.

Human myogenic reserve cells are quiescent stem cells that contribute to muscle regeneration after intramuscular transplantation in immunodeficient mice

Satellite cells, localized within muscles in vivo, are Pax7⁺ muscle stem cells supporting skeletal muscle growth and regeneration. Unfortunately, their amplification in vitro, required for their therapeutic use, is associated with reduced regenerative potential. In the present study, we investigated if human myogenic reserve cells (MRC) obtained in vitro, represented a reliable cell source for muscle repair. For this purpose, primary human myoblasts were freshly isolated and expanded. After 2 days of differentiation, 62 ± 2.9% of the nuclei were localized in myotubes and 38 ± 2.9% in the mononucleated non-fusing MRC. Eighty percent of freshly isolated human MRC expressed a phenotype similar to human quiescent satellite cells (CD56⁺/Pax7⁺/MyoD⁻/Ki67⁻ cells). Fourteen days and 21 days after cell transplantation in immunodeficient mice, live human cells were significantly more numerous and the percentage of Pax7⁺/human lamin A/C⁺ cells was 2 fold higher in muscles of animals injected with MRC compared to those injected with human myoblasts, despite that percentage of spectrin⁺ and lamin A/C⁺ human fibers in both groups MRC were similar. Taken together, these data provide evidence that MRC generated in vitro represent a promising source of cells for improving regeneration of injured skeletal muscles.

In situ regeneration of skeletal muscle tissue through host cell recruitment

Standard reconstructive procedures for restoring normal function after skeletal muscle defects involve the use of existing host tissues such as muscular flaps. In many instances, this approach is not feasible and delays the rehabilitation process and restoration of tissue function. Currently, cell-based tissue engineering strategies have been used for reconstruction; however, donor tissue biopsy and ex vivo cell manipulation are required prior to implantation. The present study aimed to overcome these limitations by demonstrating mobilization of muscle cells into a target-specific site for in situ muscle regeneration. First, we investigated whether host muscle cells could be mobilized into an implanted scaffold. Poly(l-lactic acid) (PLLA) scaffolds were implanted in the tibialis anterior (TA) muscle of rats, and the retrieved scaffolds were characterized by examining host cell infiltration in the scaffolds. The host cell infiltrates, including Pax7+ cells, gradually increased with time. Second, we demonstrated that host muscle cells could be enriched by a myogenic factor released from the scaffolds. Gelatin-based scaffolds containing a myogenic factor were implanted in the TA muscle of rats, and the Pax7+ cell infiltration and newly formed muscle fibers were examined. By the second week after implantation, the Pax7+ cell infiltrates and muscle formation were significantly accelerated within the scaffolds containing insulin-like growth factor 1 (IGF-1). Our data suggest an ability of host stem cells to be recruited into the scaffolds with the capability of differentiating to muscle cells. In addition, the myogenic factor effectively promoted host cell recruitment, which resulted in accelerating muscle regeneration in situ.

Age-related decreases of serum-response factor levels in human mesenchymal stem cells are involved in skeletal muscle differentiation and engraftment capacity

Skeletal muscle (SkM) comprise ∼40% of human body weight. Injury or damage to this important tissue can result in physical disability, and in severe cases is difficult for its endogenous stem cell-the satellite cell-to reverse effectively. Mesenchymal stem cells (MSC) are postnatal progenitor/stem cells that possess multilineage mesodermal differentiation capacity, including toward SkM. Adult bone marrow (BM) is the best-studied source of MSCs; however, aging also decreases BMMSC numbers and can adversely affect differentiation capacity. Therefore, we asked whether human sources of developmentally early stage mesenchymal stem cells (hDE-MSCs) isolated from embryonic stem cells, fetal bone, and term placenta could be cellular sources for SkM repair. Under standard muscle-inducing conditions, hDE-MPCs differentiate toward a SkM lineage rather than cardiomyocytic or smooth muscle lineages, as evidenced by increased expression of SkM-associated markers and in vitro myotube formation. In vivo transplantation revealed that SkM-differentiated hDE-MSCs can efficiently incorporate into host SkM tissue in a mouse model of SkM injury. In contrast, adult BMMSCs do not express SkM-associated genes after in vitro SkM differentiation nor engraft in vivo. Further investigation of possible factors responsible for this difference in SkM differentiation potential revealed that, compared with adult BMMSCs, hDE-MSCs expressed higher levels of serum response factor (SRF), a transcription factor critical for SkM lineage commitment. Moreover, knockdown of SRF in hDE-MSCs resulted in decreased expression of SkM-related genes after in vitro differentiation and decreased in vivo engraftment. Our results implicate SRF as a key factor in age-related SkM differentiation capacity of MSCs, and demonstrate that hDE-MSCs are possible candidates for SkM repair.

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

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

Cellular Transplantation Alters the Disease Progression in Becker's Muscular Dystrophy

Becker's Muscular Dystrophy (BMD) is a dystrophinopathy manifested as progressive muscle degeneration. Autologous Bone Marrow Mononuclear Cells (BMMNCs) have shown some myogenic potential. The paracrine effects of the BMMNCs reduce the inflammation and are thought to reduce muscle degeneration. We treated a 39 year old dental surgeon suffering from BMD. Muscle strength was reduced when measured using modified Medical Research Council's Manual Muscle Testing (mMRC-MMT). Static sitting balance was poor. He was wheelchair dependent for ambulation and moderately independent in Activities of Daily Living (ADL). Functional Independence Measure (FIM) score was 93. Musculoskeletal Magnetic Resonance Imaging (MRI-MSK) showed moderate fatty infiltration in the muscles. Three cellular transplantations were carried out. Clinical assessment and the investigations were repeated. Progressive increase in the muscle strength was noted. Ambulation was independent using push-knee splints and minimal assistance when weary. Static and dynamic balance in sitting and standing improved. FIM score increased from 93 to 105. There was no increase in the degree of fatty infiltration, as seen on the MRI-MSK. The case study provides evidence for the putative benefits of cellular therapy in altering the disease progression in BMD. It also suggests augmented clinical benefits of combination of cellular therapy and rehabilitation.

Myoblasts and embryonic stem cells differentially engraft in a mouse model of genetic dilated cardiomyopathy

The functional and architectural benefits of embryonic stem cells (ESC) and myoblasts (Mb) transplantations into infarcted myocardium have been investigated extensively. Whereas ESC repopulated fibrotic areas and contributed to myocardial regeneration, Mb exerted their effects through paracrine secretions and scar remodeling. This therapeutic perspective, however, has been less explored in the setting of nonischemic dilated cardiomyopathies (DCMs). Our aim was to compare the integration and functional efficacy of ESC committed to cardiac fate by bone morphogenic protein 2 (BMP-2) pretreatment and Mb used as gold standard following their transplantation into the myocardium of a mouse model of laminopathy exhibiting a progressive and lethal DCM. After 4 and 8 weeks of transplantation, stabilization was observed in Mb-transplanted mice (P = 0.008) but not in groups of ESC-transplanted or medium-injected animals, where the left ventricular fractional shortening (LVFS) decreased by 32 ± 8% and 41 ± 8% respectively. Engrafted differentiated cells were consistently detected in myocardia of mice receiving Mb, whereas few or no cells were detected in the hearts of mice receiving ESC, except in two cases where teratomas were formed. These data suggest that committed ESC fail to integrate in DCM where scar tissue is absent to provide the appropriate niche, whereas the functional benefits of Mb transplantation might extend to nonischemic cardiomyopathy.

Biological approaches to improve skeletal muscle healing after injury and disease

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

Long-term engraftment of multipotent mesenchymal stromal cells that differentiate to form myogenic cells in dogs with Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an incurable genetic disease with early mortality. Multipotent mesenchymal stromal cells (MSCs) are of interest because of their ability to differentiate to form myogenic cells in situ. In the present study, methods were developed to expand cultures of MSCs and to promote the myogenic differentiation of these cells, which were then used in a new approach for the treatment of DMD. MSC cultures enriched in CD271(+) cells grew better than CD271-depleted cultures. The transduction of CD271(+) MSCs with MyoD caused myogenic differentiation in vitro and the formation of myotubes expressing late myogenic markers. CD271(+) MSCs in the myogenic cell lineage transplanted into dog leukocyte antigen (DLA)-identical dogs formed clusters of muscle-like tissue. Intra-arterial injection of the CD271(+) MSCs resulted in engraftment at the site of the cardiotoxin (CTX)-injured muscle. Dogs affected by X-linked muscular dystrophy in Japan (CXMD(J)) treated with an intramuscular injection of CD271(+) MSCs similarly developed muscle-like tissue within 8-12 weeks in the absence of immunosuppression. In the newly formed tissues, developmental myosin heavy chain (dMyHC) and dystrophin were upregulated. These findings demonstrate that a cell transplantation strategy using CD271(+) MSCs may offer a promising treatment approach for patients with DMD.

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

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

NUP98-HOXA10hd-expanded hematopoietic stem cells efficiently reconstitute bone marrow of mismatched recipients and induce tolerance

Gene therapy as well as methods capable of returning cells to a pluripotent state (iPS) have enabled the correction of genetic deficiencies in syngenic adult progenitors, reducing the need for immunosuppression in cell therapy approaches. However, in diseases involving mutations that lead to the complete lack of a protein, such as Duchenne muscular dystrophy, the main immunogens leading to rejection of transplanted cells are the therapeutic proteins themselves. In these cases even iPS cells would not circumvent the need for immunosuppression, and alternative strategies must be developed. One such potential strategy seeks to induce immune tolerance using hematopoietic stem cells originated from the same donor or iPS line from which the therapeutic progenitors are derived. However, donor hematopoietic stem cells (HSCs) are available in limiting numbers and embryonic stem (ES) cell-derived HSCs engraft poorly in adults. While these limitations have been circumvented by ectopic expression of HOXB4, overexpression of this protein is associated with inefficient lymphoid reconstitution. Here we show that adult HSCs expanded with a NUP98- HOXA10hd fusion protein sustain long-term engraftment in immunologically mismatched recipients and generate normal numbers of lymphoid cells. In addition, NUP98-HOXA10hd-expanded cells induce functional immune tolerance to a subsequent transplant of myogenic progenitors immunologically matched with the transplanted HSCs.

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.

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.

Antioxidant levels represent a major determinant in the regenerative capacity of muscle stem cells

Stem cells are classically defined by their multipotent, long-term proliferation, and self-renewal capabilities. Here, we show that increased antioxidant capacity represents an additional functional characteristic of muscle-derived stem cells (MDSCs). Seeking to understand the superior regenerative capacity of MDSCs compared with myoblasts in cardiac and skeletal muscle transplantation, our group hypothesized that survival of the oxidative and inflammatory stress inherent to transplantation may play an important role. Evidence of increased enzymatic and nonenzymatic antioxidant capacity of MDSCs were observed in terms of higher levels of superoxide dismutase and glutathione, which appears to confer a differentiation and survival advantage. Further when glutathione levels of the MDSCs are lowered to that of myoblasts, the transplantation advantage of MDSCs over myoblasts is lost when transplanted into both skeletal and cardiac muscles. These findings elucidate an important cause for the superior regenerative capacity of MDSCs, and provide functional evidence for the emerging role of antioxidant capacity as a critical property for MDSC survival post-transplantation.

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.

Evaluation of Sca-1 and c-Kit As Selective Markers for Muscle Remodelling by Nonhemopoietic Bone Marrow Cells

Bone marrow (BM)-derived cells (BMCs) have demonstrated a myogenic tissue remodeling capacity. However, because the myoremodeling is limited to approximately 1%-3% of recipient muscle fibers in vivo, there is disagreement regarding the clinical relevance of BM for therapeutic application in myodegenerative conditions. This study sought to determine whether rare selectable cell surface markers (in particular, c-Kit) could be used to identify a BMC population with enhanced myoremodeling capacity. Dystrophic mdx muscle remodeling has been achieved using BMCs sorted by expression of stem cell antigen-1 (Sca-1). The inference that Sca-1 is also a selectable marker associated with myoremodeling capacity by muscle-derived cells prompted this study of relative myoremodeling contributions from BMCs (compared with muscle cells) on the basis of expression or absence of Sca-1. We show that myoremodeling activity does not differ in cells sorted solely on the basis of Sca-1 from either muscle or BM. In addition, further fractionation of BM to a more mesenchymal-like cell population with lineage markers and CD45 subsequently revealed a stronger selectability of myoremodeling capacity with c-Kit/Sca-1 (p < .005) than with Sca-1 alone. These results suggest that c-Kit may provide a useful selectable marker that facilitates selection of cells with an augmented myoremodeling capacity derived from BM and possibly from other nonmuscle tissues. In turn, this may provide a new methodology for rapid isolation of myoremodeling capacities from muscle and nonmuscle tissues. Disclosure of potential conflicts of interest is found at the end of this article.

Widespread distribution and muscle differentiation of human fetal mesenchymal stem cells after intrauterine transplantation in dystrophic mdx mouse

Duchenne muscular dystrophy (DMD) is a common X-linked disease resulting from the absence of dystrophin in muscle. Affected boys suffer from incurable progressive muscle weakness, leading to premature death. Stem cell transplantation may be curative, but is hampered by the need for systemic delivery and immune rejection. To address these barriers to stem cell therapy in DMD, we investigated a fetal-to-fetal transplantation strategy. We investigated intramuscular, intravascular, and intraperitoneal delivery of human fetal mesenchymal stem cells (hfMSCs) into embryonic day (E) 14-16 MF1 mice to determine the most appropriate route for systemic delivery. Intramuscular injections resulted in local engraftment, whereas both intraperitoneal and intravascular delivery led to systemic spread. However, intravascular delivery led to unexpected demise of transplanted mice. Transplantation of hfMSCs into E14-16 mdx mice resulted in widespread long-term engraftment (19 weeks) in multiple organs, with a predilection for muscle compared with nonmuscle tissues (0.71% vs. 0.15%, p < .01), and evidence of myogenic differentiation of hfMSCs in skeletal and myocardial muscle. This is the first report of intrauterine transplantation of ontologically relevant hfMSCs into fully immunocompetent dystrophic fetal mice, with systemic spread across endothelial barriers leading to widespread long-term engraftment in multiple organ compartments. Although the low-level of chimerism achieved is not curative for DMD, this approach may be useful in other severe mesenchymal or enzyme deficiency syndromes, where low-level protein expression may ameliorate disease pathology.

Human muscle precursor cell regeneration in the mouse host is enhanced by growth factors

The aim of this study was to optimize human muscle formation in vivo from implanted human muscle precursor cells. We transplanted donor muscle precursor cells (MPCs) prepared from postnatal or fetal human muscle into immunodeficient host mice and showed that irradiation of host muscle significantly enhanced muscle formation by donor cells. The amount of donor muscle formed in cryodamaged host muscle was increased by exposure of donor cells to growth factors before their implantation into injured host muscle. Insulin-like growth factor type I (IGF-I) significantly increased the amount of muscle formed by postnatal human muscle cells, but not by fetal human MPCs. However, treatment of fetal muscle cells with IGF-I, in combination with basic fibroblast growth factor and plasmin, significantly increased the amount of donor muscle formed. In vivo, human MPCs formed mosaic human-mouse muscle fibers, in which each human myonucleus was associated with a zone of human sarcolemmal protein spectrin.

Transplantation of a multipotent cell population from human adipose tissue induces dystrophin expression in the immunocompetent mdx mouse

Here, we report the isolation of a human multipotent adipose-derived stem (hMADS) cell population from adipose tissue of young donors. hMADS cells display normal karyotype; have active telomerase; proliferate >200 population doublings; and differentiate into adipocytes, osteoblasts, and myoblasts. Flow cytometry analysis indicates that hMADS cells are CD44+, CD49b+, CD105+, CD90+, CD13+, Stro-1(-), CD34-, CD15-, CD117-, Flk-1(-), gly-A(-), CD133-, HLA-DR(-), and HLA-I(low). Transplantation of hMADS cells into the mdx mouse, an animal model of Duchenne muscular dystrophy, results in substantial expression of human dystrophin in the injected tibialis anterior and the adjacent gastrocnemius muscle. Long-term engraftment of hMADS cells takes place in nonimmunocompromised animals. Based on the small amounts of an easily available tissue source, their strong capacity for expansion ex vivo, their multipotent differentiation, and their immune-privileged behavior, our results suggest that hMADS cells will be an important tool for muscle cell-mediated therapy.

Initial failure in myoblast transplantation therapy has led the way toward the isolation of muscle stem cells: potential for tissue regeneration

Myoblast transfer therapy can restore dystrophin expressing myofibers in mdx mice and patients with Duchenne muscular dystrophy (DMD). However, the effectiveness of this technique is hindered by numerous limitations, including minimal distribution of cells after injection, immune rejection, and poor cell survival. Initial studies revealed that only a small population of cells was responsible for muscle regeneration. Compared with myoblast transplantation, the injection of a population of myogenic cells purified with the pre-plate technique results in a superior regeneration of dystrophin-expressing myofibers. These postnatal muscle-derived stem cells (MDSC) undergo self-renewal, display long-term proliferation, and differentiate into multiple lineages. This review examines the initial obstacles encountered in myoblast transplantation, the regenerative properties of MDSC, and the potential use of these stem cells not only for DMD therapy but also for multiple applications, including bone repair and blood reconstitution.

Innate inflammatory cells are not responsible for early death of donor myoblasts after myoblast transfer therapy

BACKGROUND

Myoblast transfer therapy (MTT) is a cell-based gene therapy representing a potential treatment for Duchenne muscular dystrophy. The rapid disappearance of donor myoblasts from transplanted muscles after MTT is one of the most controversial and significant obstacles facing research in this area. Dystrophin-deficient muscles show constitutively high levels of inflammation, thus necessitating an examination of whether inflammatory cells, specifically natural killer (NK) cells, neutrophils, and macrophages, within dystrophic muscle are responsible for poor graft survival.

METHODS

Female mdx mice were treated with RB6-8C5 monoclonal antibody, PK136 monoclonal antibody, or clodronate liposomes to systemically deplete neutrophils, NK cells, and macrophages, respectively. After each depletion regimen, the mice and age-matched controls received 5.0 x 10 male myoblasts injected longitudinally into each tibialis anterior muscle. Donor myoblast survival was assessed by Y-chromosome specific quantitative real-time polymerase chain reaction analysis. RESULTS.: The systemic depletion of host neutrophils and NK cells resulted in a transient improvement in donor myoblast survival at 72 hr and 7 days post-MTT, respectively. Systemic depletion of macrophages had no significant beneficial effect on myoblast survival. Overall, the number of detectable male donor myoblasts was similar at time 0 and 1 hr post-MTT; however, there was significant loss by 24 hr (approximately 50%-70%) followed by a continual decline in donor cell numbers.

CONCLUSIONS

Neutrophils and macrophages do not seem to play a major role in the rapid death of donor myoblasts after transplantation into dystrophic muscle. NK cells similarly seem to have no significant effect, contrary to earlier findings reported by our group.

Novel Duchenne muscular dystrophy treatment through myoblast transplantation tolerance with anti-CD45RB, anti-CD154 and mixed chimerism

Duchenne muscular dystrophy (DMD) is a fatal disease caused by a defect in the skeletal muscle protein, dystrophin. One potential therapy for DMD involves transplantation of myoblasts from normal individuals. Unfortunately, myoblast allografts are particularly immunogenic and transplant tolerance in dystrophic (mdx/mdx) mice has not yet been achieved despite using strategies successful in other allograft models. Here, we attempted to induce 'central tolerance' using either haplo- or fully allogeneic bone marrow after conditioning with low-dose (3 Gy) whole body irradiation and anti-CD154 or anti-CD45RB mAbs. With one exception, these mice lacked persistent chimerism, long-term survival of myoblast allografts, or tolerance. In contrast, the addition of anti-CD45RB to anti-CD154 uniformly resulted in long-lived high-level mixed chimerism, long-term (>100 days) engraftment of allogeneic myoblasts and deletion of donor-reactive cells. Moreover, all recipients exhibited tolerance to second myoblast allografts or donor-specific tolerance to skin transplants performed >80 days after the initial graft. Thus, we now report that anti-CD45RB synergizes with anti-CD40L to promote stable mixed chimerism and robust tolerance to myoblast allografts for the first time. This novel protocol may be applicable to future clinical trials in myoblast transplantation for treatment of DMD and for transplantation of other immunogenic allografts.

Muscle-derived stem cells: potential for muscle regeneration

Duchenne muscular dystrophy (DMD) is a devastating X-linked muscle disease characterized by progressive muscle weakness caused by the lack of dystrophin expression at the sarcolemma of muscle fibers. Although various approaches to delivering dystrophin in dystrophic muscle have been investigated extensively (e.g., cell and gene therapy), there is still no treatment that alleviates the muscle weakness in this common inherited muscle disease. The transplantation of myoblasts can enable transient delivery of dystrophin and improve the strength of injected dystrophic muscle, but this approach has various limitations, including immune rejection, poor cellular survival rates, and the limited spread of the injected cells. The isolation of muscle cells that can overcome these limitations would enhance the success of myoblast transplantation significantly. The efficiency of cell transplantation might be improved through the use of stem cells, which display unique features, including (1) self-renewal with production of progeny, (2) appearance early in development and persistence throughout life, and (3) long-term proliferation and multipotency. For these reasons, the development of muscle stem cells for use in transplantation or gene transfer (ex vivo approach) as treatment for patients with muscle disorders has become more attractive in the past few years. In this paper, we review the current knowledge regarding the isolation and characterization of stem cells isolated from skeletal muscle by highlighting their biological features and their relationship to satellite cells as well as other populations of stem cells derived from other tissues. We also describe the remarkable ability of stem cells to regenerate skeletal muscle and their potential use to alleviate the muscle weakness associated with DMD.

Dystrophin Delivery in Dystrophin-Deficient DMDmdxSkeletal Muscle by Isogenic Muscle-Derived Stem Cell Transplantation

Duchenne's muscular dystrophy (DMD) is a lethal muscle disease caused by a lack of dystrophin expression at the sarcolemma of muscle fibers. We investigated retroviral vector delivery of dystrophin in dystrophin-deficient DMD(mdx) (hereafter referred to as mdx) mice via an ex vivo approach using mdx muscle-derived stem cells (MDSCs). We generated a retrovirus carrying a functional human mini-dystrophin (RetroDys3999) and used it to stably transduce mdx MDSCs obtained by the preplate technique (MD3999). These MD3999 cells expressed dystrophin and continued to express stem cell markers, including CD34 and Sca-1. MD3999 cells injected into mdx mouse skeletal muscle were able to deliver dystrophin. Though a relatively low number of dystrophin-positive myofibers was generated within the gastrocnemius muscle, these fibers persisted for up to 24 weeks postinjection. The injection of cells from additional MDSC/Dys3999 clones into mdx skeletal muscle resulted in varying numbers of dystrophin-positive myofibers, suggesting a differential regenerating capacity among the clones. At 2 and 4 weeks postinjection, the infiltration of CD4- and CD8-positive lymphocytes and a variety of cytokines was detected within the injected site. These data suggest that the transplantation of retrovirally transduced mdx MDSCs can enable persistent dystrophin restoration in mdx skeletal muscle; however, the differential regenerating capacity observed among the MDSC/Dys3999 clones and the postinjection immune response are potential challenges facing this technology.

Extended amplification in vitro and replicative senescence: key factors implicated in the success of human myoblast transplantation

The limited success of human myoblast transplantation has been related to immune rejection, poor survival, and limited spread of injected myoblasts after transplantation. An important issue that has received little attention, but is nevertheless of fundamental importance in myoblast transplantation protocols, is the proliferative capacity of human satellite cells. Previous studies from our laboratory have demonstrated that the maximum number of divisions that a population of satellite cells can make decreases with age during the first two decades of life then stabilizes in adulthood. These observations indicate that when satellite cells are used as vectors in myoblast transplantation protocols it is important to consider donor age and the number of divisions that the cells have made prior to transplantation as limiting factors in obtaining an optimal number of donor derived muscle fibers. In this study, myoblasts derived from donors of different ages (newborn, 17 years old, and 71 years old) were isolated and amplified in culture. Their potential to participate in in vivo muscle regeneration in RAG2(-/-)/gamma(c)/C5 triple immunodeficient hosts after implantation was evaluated at 4 and 8 weeks postimplantation. Our results demonstrate that prolonged amplification in culture and the approach to replicative senescence are both important factors that may condition the success of myoblast transplantation protocols.

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.

Duchenne muscular dystrophy: current knowledge, treatment, and future prospects

The cloning of the dystrophin gene has led to major advances in the understanding of the molecular genetic basis of Duchenne, Becker, and other muscular dystrophies associated with mutations in genes encoding members of the dystrophin-associated glycoprotein complex. The recent introduction of pharmaceutical agents such as prednisone has shown great promise in delaying the progression of Duchenne muscular dystrophy but there remains a need to develop more long-term therapeutic interventions. Knowledge of the nature of the dystrophin gene and the glycoprotein complex has led many researchers to think that somatic gene replacement represents the most promising approach to treatment. The potential use of this strategy has been shown in the mdx mouse model of Duchenne muscular dystrophy, where germ line gene transfer of either a full-length or a smaller Becker-type dystrophin minigene prevents necrosis and restores normal muscle function.

Intracellular delivery of a Tat-eGFP fusion protein into muscle cells

The Tat protein from HIV-1, when fused with heterologous proteins or peptides, can traverse biological membranes in a process called "protein transduction," delivering its cargo into cells. A Tat-eGFP fusion protein was purified from bacteria to study the transduction kinetics of Tat fusion proteins into cultured myoblasts and in the muscle tissue. Correctly folded Tat-eGFP reaches a maximum intracellular level in nearly 30 min, while its endogenous fluorescence is first detected only after 14 h. The nuclear localization signal from the basic domain of Tat was not sufficient to confer nuclear localization to Tat-eGFP, suggesting that the nuclear import pathway used by the exogenously added Tat-eGFP might be sensitive to the folding state of eGFP. In mice, the direct delivery to the muscle tissue using subcutaneous injections or the intra-arterial pathway led to few positive fibers in the muscle periphery or surrounding the blood vessels. Muscles injected with Tat-eGFP showed intense labeling of the extracellular matrix (ECM), suggesting that, although Tat fusion proteins can transduce muscle fibers, their binding by components of the ECM surrounding myofibers could interfere with the intracellular transduction process.

Clonal isolation of muscle-derived cells capable of enhancing muscle regeneration and bone healing

Several recent studies suggest the isolation of stem cells in skeletal muscle, but the functional properties of these muscle-derived stem cells is still unclear. In the present study, we report the purification of muscle-derived stem cells from the mdx mouse, an animal model for Duchenne muscular dystrophy. We show that enrichment of desmin(+) cells using the preplate technique from mouse primary muscle cell culture also enriches a cell population expressing CD34 and Bcl-2. The CD34(+) cells and Bcl-2(+) cells were found to reside within the basal lamina, where satellite cells are normally found. Clonal isolation and characterization from this CD34(+)Bcl-2(+) enriched population yielded a putative muscle-derived stem cell, mc13, that is capable of differentiating into both myogenic and osteogenic lineage in vitro and in vivo. The mc13 cells are c-kit and CD45 negative and express: desmin, c-met and MNF, three markers expressed in early myogenic progenitors; Flk-1, a mouse homologue of KDR recently identified in humans as a key marker in hematopoietic cells with stem cell-like characteristics; and Sca-1, a marker for both skeletal muscle and hematopoietic stem cells. Intramuscular, and more importantly, intravenous injection of mc13 cells result in muscle regeneration and partial restoration of dystrophin in mdx mice. Transplantation of mc13 cells engineered to secrete osteogenic protein differentiate in osteogenic lineage and accelerate healing of a skull defect in SCID mice. Taken together, these results suggest the isolation of a population of muscle-derived stem cells capable of improving both muscle regeneration and bone healing.

Thymic myoid cells as a source of cells for myoblast transfer

Transplantation of disaggregated myoblasts from normal donor to the muscles of a diseased host, or reimplantation of genetically modified host myoblasts, has been suggested as a possible route to therapy for inherited myopathies such as Duchenne muscular dystrophy, or to supply missing proteins that are required systemically in diseases such as hemophilia. With two exceptions, studies of myoblast transfer in the mouse have involved transplantation of donor myoblasts isolated from adult or neonatal skeletal muscle satellite cells. In this study we present evidence that thymic myoid cells are capable of participating in the regeneration of postnatal skeletal muscle, resulting in the expression of donor-derived proteins such as dystrophin and retrovirally encoded proteins such as beta-galactosidase within host muscles. This leads us to conclude that thymic myoid cells may provide an alternative to myoblasts derived from skeletal muscle as a source of myogenic cells for myoblast transfer.

Preliminary results of myoblast injection into the urethra and bladder wall: a possible method for the treatment of stress urinary incontinence and impaired detrusor contractility

The purpose of this study is to explore the feasibility of myoblasts, the precursors of muscle fibers, injected periurethrally as a potential treatment of stress urinary incontinence. We also studied myoblast injection into the bladder wall to potentially improve detrusor contractility. A myoblast cell line was transduced with adenovirus carrying the expression of the beta-galactosidase reporter gene while in culture. The cells were incubated with fluorescent latex microspheres (FLMs) to follow the outcome of the injected cells. The tissue was harvested 3-4 days after injection; sectioned, fixed, assayed for beta-galactosidase expression, and counterstained with H+E. Photographs of the slides were taken under light and fluorescence microscopy. We have noted a large number of cells expressing beta-galactosidase and containing FLMs in the urethral and bladder walls under fluorescent microscopy (8 animals). Many regenerative myofibers expressing beta-galactosidase were also seen in the urethral and bladder walls. The fusion of injected myoblasts to form myotubes was seen in both the urethral and bladder walls. The introduction of myoblasts into the urethral and bladder wall is feasible and results in formation of myotubes and myofibers in the smooth muscle layers of the lower urinary tract. We hypothesize that myoblast injections can be used as a non-allergenic agent to enhance urethral closure and bladder function.

Exposure to tissue culture conditions can adversely affect myoblast behavior in vivo in whole muscle grafts: implications for myoblast transfer therapy

The effects of tissue culture conditions on the viability of myoblasts in whole muscles transplanted in vivo were investigated. Whole male (SJL/J) donor muscles were exposed to various tissue culture reagents and proteolytic enzymes, and allografted into female (SJL/J) host mice. Desmin immunohistochemistry was used to assess the numbers of myogenic cells (as an index of myoblast viability and the extent of regeneration) in tissue sections of whole-muscle grafts sampled on days 7 and 14. DNA quantitation with a Y-chromosome-specific probe was used to determine the total Y-1 sequence DNA (as an index of myoblast survival and proliferation) in whole-muscle grafts sampled on days 1, 3, and 7. In grafts exposed to serum-free medium, there was a delay in myoblast fusion at 7 days that was recovered by 14 days, but exposure to serum (10% or 20%) had a prolonged adverse effect on myotube formation at 14 days. DNA quantitation demonstrated that either serum-free culture medium or 10% serum enhanced the number of male cells within whole-muscle grafts at 7 days. Proteolytic digestion (even for 5 min) of whole muscles prior to grafting was extremely detrimental to myoblast survival and viability at 7 and 14 days. The unexpected finding of adverse effects of tissue culture conditions on the regeneration of whole-muscle grafts in vivo appears to parallel the major problem of the rapid death of isolated cultured donor myoblasts after injection in myoblast transfer therapy. The use of whole-muscle grafts provides an alternative and sensitive model to analyze the crucial effects of various tissue culture components on the subsequent survival and proliferation of myogenic cells in vivo.

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.

Enhanced migration and fusion of donor myoblasts in dystrophic and normal host muscle

Sliced male C57Bl/10Sn (H2-b) donor muscles were grafted into the female histocompatible muscles of untreated, FK506-treated, and T-cell depleted (with or without thymic tolerization) dystrophic (mdx; H2-b) and normal (C57Bl/10Sn; H2-b) hosts, and also into histoincompatible normal (Balb/c; H2-d) hosts. The fate of male donor nuclei was monitored on tissue sections by in situ hybridization with a Y-chromosome specific probe. The results demonstrate that the dystrophic environment is more conducive than normal muscle to donor myoblast migration, with the distance moved being threefold greater at 12 weeks in dystrophic hosts. T-cell depletion was significantly more effective than FK506 treatment at enhancing donor myoblast emigration in both histocompatible and histoincompatible hosts at 3 weeks. Furthermore, the effects of T-cell depletion were sustained in histoincompatible hosts at 12 weeks. These data endorse the use of host T-cell depletion as a promising long-term strategy to improve myoblast transfer therapy (MTT) in the clinical situation.

Matching host muscle and donor myoblasts for myosin heavy chain improves myoblast transfer therapy

Intensive efforts have been made to develop an effective therapy for Duchenne muscular dystrophy (DMD). Although myoblast transplantation has been found capable of transiently delivering dystrophin and improving the strength of the injected dystrophic muscle, this approach has been hindered by the immune rejection problems as well as the poor survival and limited spread of the injected cells. In the present study, we have investigated whether the careful selection of donor myoblasts and host muscle for the myosin heavy chain expression (MyHCs) plays a role in the success of myoblast transfer. Highly purified normal myoblasts derived from the m. soleus and m. gastrocnemius white of normal mice were transplanted into the m. soleus (containing 70% of type I fibers) and gastrocnemius white (100% of type II fibers) of dystrophin deficient mdx mice. At several time-points after injection (10, 20 and 30 days), the number of dystrophin-positive fibers was monitored and compared among the different groups. A significantly higher number and better persistence of dystrophin-positive myofibers were observed when the injected muscle and donor myoblasts expressed a similar MyHC in comparison with myoblast transfer between host muscle and donor myoblasts that were not matched for MyHC. These results suggest that careful matching between the injected myoblasts and injected muscle for the MyHC expression can improve the efficiency of myoblast-mediated gene transfer to skeletal muscle. Gene Therapy (2000) 7, 428-437.

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.

Use of growth factors to improve muscle healing after strain injury

Muscle injuries represent a large number of professional and recreational sports injuries. Muscle strains habitually occur after an eccentric contraction, which often leads to an injury located in the myotendinous junction. Treatment varies widely, depending on the severity of the trauma, but has remained limited mostly to rest, ice, compression, elevation, antiinflammatory drugs, and mobilization. The authors' research group aims to develop new biologic approaches to improve muscle healing after injuries, including muscle strains. To achieve this goal, the authors investigated several parameters that will lead to the development of new strategies to enhance muscle healing. The authors first evaluated natural muscle healing after strain injuries and showed that muscle regeneration occurs in the early phase of healing but becomes impaired with time by the development of tissue fibrosis. Several growth factors capable of improving muscle regeneration were investigated; basic fibroblast growth factor, insulin-like growth factor, and nerve growth factors were identified as substances capable of enhancing muscle regeneration and improving muscle force in the strained injured muscle. The current study should aid in the development of strategies to promote efficient muscle healing and complete recovery after strain injury.

Increased myogenic potential and fusion of matrilysin-expressing myoblasts transplanted in mice

The success of myoblast transplantation in clinical trials has been limited in part by the low dispersion of grafted cells outside the injection site. Our research group previously reported that the culture of myoblasts with concanavalin A, a stimulator of metalloproteinase production, increased their migration. Several lines of evidence also suggested that muscle cell fusion involves metalloproteinase-sensitive mechanisms. To determine whether the increased expression of metalloproteinases had an influence on myoblast fusion and dispersion through the muscle following transplantation, we generated a myoblast cell line expressing human matrilysin (MMP-7). The MMP-7-expressing myoblasts were obtained by the stable transfection of a matrilysin expression vector in a TnILacZ immortomouse myoblast clone. Matrilysin-expressing myoblasts showed a highly increased in vitro fusion index, forming seven times (p < 0.001) more myotubes than the control cell line and three times (p < 0.001) more myotubes than the Immortomyoblast parental clone. Single-site transplantation of matrilysin-expressing myoblasts generated more fibers (p < 0.001), over a greater surface (p < 0.001) than the control cell line. The cotransplantation of matrilysin-expressing myoblasts and of normal human myoblasts in SCID mice increased the number of human dystrophin-positive fibers and myotubes by sixfold. Although no significant increased migration of myoblasts outside the injection sites was observed, our results show that the metalloproteinase activity can improve the myogenic potential of myoblasts in vitro and the fusion of myoblasts with host fibers in vivo. MMP-7 expression may be useful in increasing myoblast transplantation success.

Suturing versus immobilization of a muscle laceration. A morphological and functional study in a mouse model

Muscle laceration remains a difficult problem for orthopaedic surgeons. Despite many studies related to the muscle's ability to regenerate after muscle degeneration, very few reports are available regarding structural and functional recovery after skeletal muscle laceration. We developed an animal model of muscle laceration in mice, where the gastrocnemius muscles were reproducibly transected. We compared the effect of a surgical repair versus a short period of immobilization (5 days) on the muscle healing. The natural course of muscle recovery was monitored at several points after injury using histologic, immunohistochemical, and functional testing. In the injured muscle, we observed a high number of regenerating myofibers and development of fibrotic scar tissue. Suturing the lacerated muscle immediately after injury promoted better healing of the injured muscle and prevented the development of deep scar tissue in the lacerated muscle; conversely, immobilization resulted in slower muscle regeneration and the development of a large area of scar tissue. Tetanus strength 1 month after injury was 81% of control muscles for the sutured muscles, 35% for the lacerated muscles with no treatment, and 18% for the immobilized muscles. Based on this study, suturing a muscle laceration with a modified Kessler stitch results in the best morphologic and functional healing.