Dystrophin delivery in dystrophin-deficient DMDmdx skeletal 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.     

Spontaneous and evoked intracellular calcium transients in donor-derived myocytes following intracardiac myoblast transplantation

Skeletal myoblast transplantation is a potential treatment for congestive heart failure. To study the functional activity of both donor and host myocytes following transplantation, skeletal myoblasts expressing an enhanced green fluorescent protein (EGFP) transgene were transplanted into hearts of nontransgenic recipients, and changes in intracellular calcium concentration ([Ca2+]i) were monitored in donor and host cells. While the vast majority of donor-derived myocytes were observed to be functionally isolated from the host myocardium, a small population of donor myocytes exhibited action potential-induced calcium transients in synchrony with adjacent host cardiomyocytes. In many cases, the durations of these [Ca2+]i transients were heterogeneous compared with those in neighboring host cardiomyocytes. In other studies, EGFP-expressing donor myoblasts were transplanted into the hearts of adult transgenic recipient mice expressing a cardiomyocyte-restricted beta-gal reporter gene. A small population of myocytes was observed to express both reporter transgenes, indicating that the transplanted myoblasts fused with host cardiomyocytes at a very low frequency. These cells also expressed connexin43, a component of gap junctions. Thus engraftment of skeletal myoblasts generated spatial heterogeneity of [Ca2+]i signaling at the myocardial/skeletal muscle interface, most likely as a consequence of fusion events between donor myoblasts and host cardiomyocytes.     

Nerve growth factor improves the muscle regeneration capacity of muscle stem cells in dystrophic muscle

Researchers have attempted to use gene- and cell-based therapies to restore dystrophin and alleviate the muscle weakness that results from Duchenne muscular dystrophy (DMD). Our research group has isolated populations of muscle-derived stem cells (MDSCs) from the postnatal skeletal muscle of mice. In comparison with satellite cells, MDSCs display an improved transplantation capacity in dystrophic mdx muscle that we attribute to their ability to undergo long-term proliferation, self-renewal, and multipotent differentiation, including differentiation toward endothelial and neuronal lineages. Here we tested whether the use of nerve growth factor (NGF) improves the transplantation efficiency of MDSCs. We used two methods of in vitro NGF stimulation: retroviral transduction of MDSCs with a CL-NGF vector and direct stimulation of MDSCs with NGF protein. Neither method of NGF treatment changed the marker profile or proliferation behavior of the MDSCs, but direct stimulation with NGF protein significantly reduced the in vitro differentiation ability of the cells. NGF stimulation also significantly enhanced the engraftment efficiency of MDSCs transplanted within the dystrophic muscle of mdx mice, resulting in the regeneration of numerous dystrophin-positive muscle fibers. These findings highlight the importance of NGF as a modulatory molecule, the study of which will broaden our understanding of its biologic role in the regeneration and repair of skeletal muscle by musclederived cells.     

Effect of VEGF on the Regenerative Capacity of Muscle Stem Cells in Dystrophic Skeletal Muscle

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

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.     

Gene therapy and tissue engineering based on muscle-derived stem cells

Skeletal muscle represents a convenient source of stem cells for cell-based tissue and genetic engineering. Muscle-derived stem cells (MDSCs) exhibit both multipotentiality and self-renewal capabilities, and are considered to be distinct from the well-studied satellite cell, another type of muscle stem cell that is capable of self-renewal and myogenic lineage differentiation. The MDSC appears to have less restricted differentiation capabilities as compared with the satellite cell, and may be a precursor of the satellite cell. This review considers the evidence for the existence of MDSCs as well as their origin. We will discuss recent investigations highlighting the potential of stem cell transplantation for the treatment of skeletal, cardiac and smooth muscle injuries and disease. We will highlight challenges in bridging the gap between understanding basic stem cell biology and clinical utilization for cell therapy.     

Differential myocardial infarct repair with muscle stem cells compared to myoblasts.

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

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.     

Transplanted hematopoietic stem cells demonstrate impaired sarcoglycan expression after engraftment into cardiac and skeletal muscle

Pluripotent bone marrow-derived side population (BM-SP) stem cells have been shown to repopulate the hematopoietic system and to contribute to skeletal and cardiac muscle regeneration after transplantation. We tested BM-SP cells for their ability to regenerate heart and skeletal muscle using a model of cardiomyopathy and muscular dystrophy that lacks delta-sarcoglycan. The absence of delta-sarcoglycan produces microinfarcts in heart and skeletal muscle that should recruit regenerative stem cells. Additionally, sarcoglycan expression after transplantation should mark successful stem cell maturation into cardiac and skeletal muscle lineages. BM-SP cells from normal male mice were transplanted into female delta-sarcoglycan-null mice. We detected engraftment of donor-derived stem cells into skeletal muscle, with the majority of donor-derived cells incorporated within myofibers. In the heart, donor-derived nuclei were detected inside cardiomyocytes. Skeletal muscle myofibers containing donor-derived nuclei generally failed to express sarcoglycan, with only 2 sarcoglycan-positive fibers detected in the quadriceps muscle from all 14 mice analyzed. Moreover, all cardiomyocytes with donor-derived nuclei were sarcoglycan-negative. The absence of sarcoglycan expression in cardiomyocytes and skeletal myofibers after transplantation indicates impaired differentiation and/or maturation of bone marrow-derived stem cells. The inability of BM-SP cells to express this protein severely limits their utility for cardiac and skeletal muscle regeneration.     

Functional Overloading of Dystrophic Mice Enhances Muscle-Derived Stem Cell Contribution to Muscle Contractile Capacity

Ambrosio F, Ferrari RJ, Fitzgerald GK, Carvell G, Boninger ML, Huard J. Functional overloading of dystrophic mice enhances muscle-derived stem cell contribution to muscle contractile capacity. Arch Phys Med Rehabil

Objectives

To evaluate the effect of functional overloading on the transplantation of muscle derived stem cells (MDSCs) into dystrophic muscle and the ability of transplanted cells to increase dystrophic muscle's ability to resist overloading-induced weakness.

Design

Cross-sectional.

Setting

Laboratory.

Animals

Male mice (N=10) with a dystrophin gene mutation.

Interventions

MDSCs were intramuscularly transplanted into the extensor digitorum longus muscle (EDL). Functional overloading of the EDL was performed by surgical ablation of the EDL's synergist.

Main Outcome Measures

The total number of dystrophin-positive fibers/cross-section (as a measure of stem cell engraftment), the average number of CD31+ cells (as a measure of capillarity), and in vitro EDL contractile strength. Independent t tests were used to investigate the effect of overloading on engraftment, capillarity, and strength. Paired t tests were used to investigate the effect of MDSC engraftment on strength and capillarity.

Results

MDSC transplantation protects dystrophic muscles against overloading-induced weakness (specific twitch force: control 4.5N/cm²±2.3; MDSC treated 7.9N/cm²±1.4) (P=.02). This improved force production following overloading is concomitant with an increased regeneration by transplanted MDSCs (MDSC: 26.6±20.2 dystrophin-positive fibers/cross-section; overloading + MDSC: 170.6±130.9 dystrophin-positive fibers/cross-section [P=.03]). Overloading-induced increases in skeletal muscle capillarity is significantly correlated with increased MDSC engraftment (R²=.80, P=.01).

Conclusions

These findings suggest that the functional contribution of transplanted MDSCs may rely on activity-dependent mechanisms, possibly mediated by skeletal muscle vascularity. Rehabilitation modalities may play an important role in the development of stem cell transplantation strategies for the treatment of muscular dystrophy.     

Fusion of bone marrow-derived stem cells with striated muscle may not be sufficient to activate muscle genes

Several studies have demonstrated the existence of pluripotent bone marrow-derived stem cells capable of homing to injured cardiac and skeletal muscle; however, there has been little evidence demonstrating the induction of tissue-specific endogenous genes in donor stem cells following engraftment. A new study in this issue reports an intriguing finding that raises additional concerns relating to stem cell plasticity and stem cell therapy in an already heated and controversial field. The study demonstrates that wild-type bone marrow-derived side population stem cells are indeed readily incorporated into both skeletal and cardiac muscle when transplanted into mice that lack delta-sarcoglycan -- a model of cardiomyopathy and muscular dystrophy. However, these cells fail to express sarcoglycan and thus to repair the tissue, which suggests that this stem cell population has limited potential for cardiac and skeletal muscle regeneration.     

Long-term persistence of donor nuclei in a Duchenne muscular dystrophy patient receiving bone marrow transplantation

Duchenne muscular dystrophy (DMD) is a severe progressive muscle-wasting disorder caused by mutations in the dystrophin gene. Studies have shown that bone marrow cells transplanted into lethally irradiated mdx mice, the mouse model of DMD, can become part of skeletal muscle myofibers. Whether human marrow cells also have this ability is unknown. Here we report the analysis of muscle biopsies from a DMD patient (DMD-BMT1) who received bone marrow transplantation at age 1 year for X-linked severe combined immune deficiency and who was diagnosed with DMD at age 12 years. Analysis of muscle biopsies from DMD-BMT1 revealed the presence of donor nuclei within a small number of muscle myofibers (0.5-0.9%). The majority of the myofibers produce a truncated, in-frame isoform of dystrophin lacking exons 44 and 45 (not wild-type). The presence of bone marrow-derived donor nuclei in the muscle of this patient documents the ability of exogenous human bone marrow cells to fuse into skeletal muscle and persist up to 13 years after transplantation.     

骨髓基质干细胞向肌细胞分化及移植的治疗作用

背景：骨髓基质干细胞属多能干细胞,在适当的条件下可转分化为各种肌肉细胞,在移植后可改善心脏或骨骼肌的整体功能。如何优化诱导分化方法,改善移植途径以及移植后改善组织功能的机制成为现今研究的热点。目的：就近年来骨髓基质干细胞诱导分化为肌细胞,以及其用作细胞移植治疗心肌梗死、肌肉萎缩等肌细胞坏死性疾病的研究进行综述。方法：应用计算机以bone marrow cell,muscle cell检索词,检索Pubmed数据库（2005/2010）;以＂骨髓基质干细胞、肌细胞＂为检索词,检索中国期刊网全文数据库（2007/2010）;以＂骨髓基质干细胞、肌细胞、分化、心脏＂为检索词,检索万方数据库（2007/2010）。共收集220篇关于外周血干细胞方面的文献,中文61篇,英文159篇,排除发表时间较早、重复及类似研究,纳入21篇符合标准的文献。结果与结论：骨髓基质干细胞可以在体外扩增,但其表面特异性标记尚无定论,因此要分离得到完全纯化的骨髓基质干细胞还有一定的困难,当前只有STRO-1抗原是较为肯定的间质干细胞表面抗原,可用于鉴定分离骨髓中的间质干细胞。大量研究表明,通过体外诱导剂或体内移植可促使骨髓基质干细胞转化成各种组织细胞,包括心肌细胞和骨骼肌细胞,并且在移植后可观察到肌组织功能得到明显改善,但其机制目前尚无充分了解。此外,尽管各种诱导方法诱导骨髓基质干细胞分化的结果是肯定的,以及体内移植也表现出改善坏死肌组织功能的潜能,但目前体外诱导程度仍然较低,且体内功能改善也有局限性,因此,如何改善诱导方法,提高移植效率及功能改善有待进一步研究。     

Gene transfer of connexin43 into skeletal muscle

Cellular cardiomyoplasty using skeletal myoblasts may be beneficial for infarct repair. One drawback to skeletal muscle cells is their lack of gap junction expression after differentiation, thus preventing electrical coupling to host cardiomyocytes. We sought to overexpress the gap junction protein connexin43 (Cx43) in differentiated skeletal myotubes, using retroviral, adenoviral, and plasmid-mediated gene transfer. All strategies resulted in overexpression of Cx43 in cultured myotubes, but expression of Cx43 from constitutive viral promoters caused significant death upon differentiation. Dye transfer studies showed that surviving myotubes contained functional gap junctions, however. Retrovirally transfected myoblasts did not express Cx43 after grafting into the heart, possibly due to promoter silencing. Adenovirally transfected myoblasts expressed abundant Cx43 after forming myotubes in cardiac grafts, but grafts showed signs of injury at 1 week and had died by 2 weeks. Interestingly, transfection of already differentiated myotubes with adenoviral Cx43 was nontoxic, implying a window of vulnerability during differentiation. To test this hypothesis, Cx43 was expressed from the muscle creatine kinase (MCK) promoter, which is active only after myocyte differentiation. The MCK promoter resulted in high levels of Cx43 expression in differentiated myotubes but did not cause cell death during differentiation. MCK-Cx43-transfected myoblasts formed viable cardiac grafts and, in some cases, Cx43-expressing myotubes were in close apposition to host cardiomyocytes, possibly allowing electrical coupling. Thus, high levels of Cx43 during skeletal muscle differentiation cause cell death. When, however, expression of Cx43 is delayed until after differentiation, using the MCK promoter, myotubes are viable and express gap junction proteins after grafting in the heart. This strategy may permit electrical coupling of skeletal and cardiac muscle for cardiac repair.     

Systemic administration of micro-dystrophin restores cardiac geometry and prevents dobutamine-induced cardiac pump failure.

Duchenne muscular dystrophy (DMD) is a fatal disease of striated muscle deterioration resulting from the loss of the cytoskeletal protein dystrophin. Most patients develop significant cardiomyopathy, with heart failure being the second leading cause of death in DMD. Compared with the extensive studies on skeletal muscle defects and potential therapy in DMD, very little attention has been directed at the increasing incidence of heart failure in DMD. Here we show that a single systemic injection of recombinant adeno-associated virus (rAAV2/6) harboring micro-dystrophin leads to extensive cardiac transduction, with micro-dystrophin correctly localized at the periphery of the cardiac myocytes and functionally associated with the sarcolemmal membrane. Significantly, micro-dystrophin gene transfer corrected the baseline end-diastolic volume defect in the mdx mouse heart and prevented cardiac pump failure induced by dobutamine stress testing in vivo, although several parameters of systolic function were not corrected. These results demonstrate that systemic gene delivery of micro-dystrophin can restore ventricular distensibility and protect the mdx myocardium from pump dysfunction during adrenergic stimulation in vivo.     

应用改良差速贴壁法和有限稀释技术分离培养小鼠来源肌源干细胞

背景：研究者们通过多种方法从肌组织中分离得到肌源干细胞,并应用于各类组织工程和再生医学研究。目的：结合改良的差速贴壁法和有限稀释技术分离小鼠来源肌源干细胞,并培养其单细胞克隆和亚克隆集落。方法：以新生C57BL/6小鼠四肢作为肌组织取材对象,经三重酶消化和细胞筛过滤,运用改良的差速贴壁法分离出肌源干细胞,予细胞特异标记物以免疫组织化学染色;以有限稀释技术克隆培养的方法,获得稳定的肌源干细胞单克隆和亚克隆集落。结果与结论：差速贴壁培养过程中,肌性细胞占比逐渐增高,首次贴壁1h可以获得足够数量的细胞进行第6次贴壁培养;肌源干细胞需72h左右贴壁生长,经10d左右可以增殖为300～500细胞数量的集落,细胞形态以小圆形细胞为主,并有少量梭形细胞,肌源干细胞能够维持形态并持续增殖;应用有限稀释技术可获得肌源干细胞单克隆和亚克隆集落,肌源干细胞克隆细胞均呈现Desmin染色阳性,Sca-1染色阳性,阳性率为（92.3±4.1）%。提示应用preplate法和有限稀释技术可以分离得到小鼠来源肌源干细胞及其克隆集落。     

兔肌源性干细胞的生物学特性及表型分析

背景：骨髓间质干细胞作为种子细胞具有巨大的分化潜力和优势，但对于再生障碍性贫血或骨髓源性肿瘤等疾病的应用受限。现已发现肌源性干细胞具备与其相似的优越性，已引起相关领域研究者的注意。目的：观察兔肌源性干细胞的生物学特性，并对其进行表型分析。设计、时间和地点：细胞体外观察，于2005-08／2006-03在中山大学附属第二医院医研中心完成。材料：清洁级1．5月龄新西兰大白兔1只，增殖培养基为DMEM—LG＋体积分数0．1胎牛血清＋100gm/L血清，融合培养基为DMEM-LC＋2％胎牛血清。方法：兔麻醉后取大腿肌肉，采用差速贴壁Preplate技术分离培养肌源性干细胞，以Ⅺ胶原酶、dispase蛋白酶及胰蛋白酶分步消化，沉淀用增殖培养基重悬，接种在胶原包被的培养瓶中，该培养瓶为PP1。PP1于37℃、含体积分数为0．05的CO2培养箱中静置过夜，随后将悬液转移到另一个胶原包被的培养瓶，此为PP2。随后同法建立PP3，PP4，PP5，PP6。将PP6接种到6孔板进行融合实验，分为两组：一组使用增殖培养基培养，在汇合度超过50％后继续培养，不传代；另一组使用融合培养基进行培养，汇合度达30％时传代培养。主要观察指标：收集PP1～PP6，采用流式细胞仪、免疫细胞化学及Western Blot法鉴定细胞表型。通过不同汇合度及不同浓度血清培养，检测PP6融合情况。结果：PP6细胞〉80％为desmin^＋，〉70％为Bcl-2^＋，〉95％为CD45，提示为高浓度肌源性干细胞，随着纯化步骤的进行，α-SMA表达逐渐减弱，至高度纯化的PP6时己无α-SMA表达。PP6在高汇合度（〉50％）或低血清（仅含2％血清）培养时，极易融合成肌管或肌细胞链，骨骼肌肌球蛋白呈阳性表达。结论：肌源性干细胞具有高水平表达desmin，Bcl-2，极低水平表达CD45，不表达α-SMA的生物学特性，在高汇合度或低血清培养条件下能够多向分化。     

Blocking the Myostatin Signal With a Dominant Negative Receptor Improves the Success of Human Myoblast Transplantation in Dystrophic Mice

Duchenne muscular dystrophy (DMD) is a recessive disease caused by a dystrophin gene mutation. Myoblast transplantation permits to introduce the dystrophin gene in dystrophic muscle fibers. However, the success of this approach is reduced by the short duration of the regeneration following the transplantation, which reduces the number of hybrid fibers. Myostatin (MSTN) is a negative regulator of skeletal muscle development and responsible for limiting regeneration. It binds with high affinity to the activin type IIB receptor (ActRIIB). Our aim was to verify whether the success of the myoblast transplantation is enhanced by blocking the MSTN signal with expression of a dominant negative mutant of ActRIIB (dnActRIIB). In vitro, blocking MSTN activity with a lentivirus carrying dnActRIIB increased proliferation and fusion of human myoblasts because MSTN regulates the expression of several myogenic regulatory factors. In vivo, myoblasts infected with the dnActRIIB lentivirus were transplanted in immunodeficient dystrophic mice. Dystrophin immunostaining of tibialis anterior (TA) cross-sections of these mice 1 month post-transplantation revealed more human dystrophin-positive myofibers following the transplantation of dnActRIIB myoblasts than of control myoblasts. Thus, blocking the MSTN signal with dnActRIIB improved the success of myoblast transplantation by increasing the myoblast proliferation and fusion and changed the expression of myogenic regulatory factors.