In vivo migration of transplanted myoblasts requires matrix metalloproteinase activity

Regulatory factors and cell populations involved in skeletal muscle regeneration

Skeletal muscle regeneration is a complex process, which is not yet completely understood. Satellite cells, the skeletal muscle stem cells, become activated after trauma, proliferate, and migrate to the site of injury. Depending on the severity of the myotrauma, activated satellite cells form new multinucleated myofibers or fuse to damaged myofibers. The specific microenvironment of the satellite cells, the niche, controls their behavior. The niche contains several components that maintain satellite cells quiescence until they are activated. In addition, a great diversity of stimulatory and inhibitory growth factors such as IGF-1 and TGF-beta1 regulate their activity. Donor-derived satellite cells are able to improve muscle regeneration, but their migration through the muscle tissue and across endothelial layers is limited. Less than 1% of their progeny, the myoblasts, survive the first days upon intra-muscular injection. However, a range of other multipotent muscle- and non-muscle-derived stem cells are involved in skeletal muscle regeneration. These stem cells can occupy the satellite cell niche and show great potential for the treatment of skeletal muscle injuries and diseases. The aim of this review is to discuss the niche factors, growth factors, and other stem cells, which are involved in skeletal muscle regeneration. Knowledge about the factors regulating satellite cell activity and skeletal muscle regeneration can be used to improve the treatment of muscle injuries and diseases.

Role of matrix metalloproteinases in skeletal muscle: migration, differentiation, regeneration and fibrosis

Matrix metalloproteases (MMPs) are key regulatory molecules in the formation, remodeling and degradation of extracellular matrix (ECM) components in both physiological and pathological processes in many tissues. In skeletal muscle, MMPs play an important role in the homeostasis and maintenance of myofiber functional integrity by breaking down ECM and regulating skeletal muscle cell migration, differentiation and regeneration. Skeletal muscle satellite cells, a group of quiescent stem cells located between the basement membrane and the plasmalemma of myofibers, are responsible for lifelong maintenance and repairing, which can be activated and as a result migrate underneath the basement membrane to promote regeneration at the injured site. MMPs are able to degrade ECM components, thereby facilitating satellite cell migration and differentiation. This current review will focus on the critical roles of MMPs in skeletal muscle injury and repair, which include satellite cell activation with migration and differentiation. The effect of MMPs on muscle regeneration and fibrous scar tissue formation, as well as therapeutic insights for the future will be explored.

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

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

Differentiation rather than aging of muscle stem cells abolishes their telomerase activity

A general feature of stem cells is the ability to routinely proliferate to build, maintain, and repair organ systems. Accordingly, embryonic and germline, as well as some adult stem cells, produce the telomerase enzyme at various levels of expression. Our results show that, while muscle is a largely postmitotic tissue, the muscle stem cells (satellite cells) that maintain this biological system throughout adult life do indeed display robust telomerase activity. Conversely, primary myoblasts (the immediate progeny of satellite cells) quickly and dramatically downregulate telomerase activity. This work thus suggests that satellite cells, and early transient myoblasts, may be more promising therapeutic candidates for regenerative medicine than traditionally utilized myoblast cultures. Muscle atrophy accompanies human aging, and satellite cells endogenous to aged muscle can be triggered to regenerate old tissue by exogenous molecular cues. Therefore, we also examined whether these aged muscle stem cells would produce tissue that is "young" with respect to telomere maintenance. Interestingly, this work shows that the telomerase activity in muscle stem cells is largely retained into old age wintin inbred "long" telomere mice and in wild-derived short telomere mouse strains, and that age-specific telomere shortening is undetectable in the old differentiated muscle fibers of either strain. Summarily, this work establishes that young and old muscle stem cells, but not necessarily their progeny, myoblasts, are likely to produce tissue with normal telomere maintenance when used in molecular and regenerative medicine approaches for tissue repair.

Matrix metalloproteinase-1 promotes muscle cell migration and differentiation

Injured skeletal muscle has the capacity to regenerate through a highly coordinated sequence of events that involves both myoblast migration and differentiation into myofibers. Fibrosis may impede muscle regeneration by posing as a mechanical barrier to cell migration and fusion, providing inappropriate signals for cell differentiation, and limiting vascular perfusion of the injury site, subsequently leading to incomplete functional recovery. Our previous studies demonstrated that matrix metalloproteinase-1 (MMP-1) is able to digest fibrous scar tissue and improve muscle healing after injury. The goal of this study is to investigate whether MMP-1 could further enhance muscle regeneration by improving myoblast migration and differentiation. In vitro wound healing assays, flow cytometry, reverse transcriptase-polymerase chain reaction (RT-PCR), and Western blot analyses demonstrated that MMP-1 enhances myoblast migration but is not chemoattractive. We discovered that MMP-1 also enhances myoblast differentiation, which is a critical step in the sequence of muscle regeneration. In addition, RT-PCR and Western blot analyses demonstrated the up-regulation of myogenic factors after MMP-1 treatment. In vivo, we observed that myoblast transplantation was greatly improved after MMP-1 treatment within the dystrophic skeletal muscles of MDX mice. MMP-1 may therefore be able to improve muscle function recovery after injury or disease by increasing both the number of myofibers that are generated by activated myoblasts and the size of myoblast coverage area by promoting migration, thus fostering a greater degree of engraftment.

Muscle CD31(-) CD45(-) side population cells promote muscle regeneration by stimulating proliferation and migration of myoblasts

CD31(-) CD45(-) side population (SP) cells are a minor SP subfraction that have mesenchymal stem cell-like properties in uninjured skeletal muscle but that can expand on muscle injury. To clarify the role of these SP cells in muscle regeneration, we injected green fluorescent protein (GFP)-positive myoblasts with or without CD31(-) CD45(-) SP cells into the tibialis anterior muscles of immunodeficient NOD/scid mice or dystrophin-deficient mdx mice. More GFP-positive fibers were formed after co-transplantation than after transplantation of GFP-positive myoblasts alone in both mdx and NOD/scid muscles. Moreover, grafted myoblasts were more widely distributed after co-transplantation than after transplantation of myoblasts alone. Immunohistochemistry with anti-phosphorylated histone H3 antibody revealed that CD31(-) CD45(-) SP cells stimulated cell division of co-grafted myoblasts. Genome-wide gene expression analyses showed that these SP cells specifically express a variety of extracellular matrix proteins, membrane proteins, and cytokines. We also found that they express high levels of matrix metalloproteinase-2 mRNA and gelatinase activity. Furthermore, matrix metalloproteinase-2 derived from CD31(-) CD45(-) SP cells promoted migration of myoblasts in vivo. Our results suggest that CD31(-) CD45(-) SP cells support muscle regeneration by promoting proliferation and migration of myoblasts. Future studies to further define the molecular and cellular mechanisms of muscle regeneration will aid in the development of cell therapies for muscular dystrophy.

Metastatic cancer cell

Metastasis is the result of cancer cell adaptation to a tissue microenvironment at a distance from the primary tumor. Metastatic cancer cells require properties that allow them not only to adapt to a foreign microenvironment but to subvert it in a way that is conducive to their continued proliferation and survival. Recent conceptual and technological advances have contributed to our understanding of the role of the host tissue stroma in promoting tumor cell growth and dissemination and have provided new insight into the genetic makeup of cancers with high metastatic proclivity.

Tissue inhibitor of metalloproteinase-2 (TIMP-2) regulates myogenesis and beta1 integrin expression in vitro

Myogenesis in vitro involves myoblast cell cycle arrest, migration, and fusion to form multinucleated myotubes. Extracellular matrix (ECM) integrity during these processes is maintained by the opposing actions of matrix metalloproteinase (MMP) proteases and their inhibitors, the tissue inhibitor of metalloproteinases (TIMPs). Here, we report that TIMP-2, MMP-2, and MT1-MMP are differentially expressed during mouse myoblast differentiation in vitro. A specific role for TIMP-2 in myogenesis is demonstrated by altered TIMP-2(-/-) myotube formation. When differentiated in horse serum-containing medium, TIMP-2(-/-) myotubes are larger than wild-type myotubes. In contrast, when serum-free medium is used, TIMP-2(-/-) myotubes are smaller than wild-type myotubes. Regardless of culture condition, myotube size is directly correlated with MMP activity and inversely correlated with beta1 integrin expression. Treatment with recombinant TIMP-2 rescues reduced TIMP-2(-/-) myotube size and induces increased MMP-9 activation and decreased beta1 integrin expression. Treatment with either MMP-2 or MMP-9 similarly rescues reduced myotube size, but has no effect on beta1 integrin expression. These data suggest a specific regulatory relationship between TIMP-2 and beta1 integrin during myogenesis. Elucidating the role of TIMP-2 in myogenesis in vitro may lead to new therapeutic options for the use of TIMP-2 in myopathies and muscular dystrophies in vivo.

Aging alters gene expression of growth and remodeling factors in human skeletal muscle both at rest and in response to acute resistance exercise

The purpose of this investigation was to compare expression of genes that function in inflammation and stress, cell structure and signaling, or remodeling and growth in skeletal muscle of young (32 +/- 7 yr, n = 15) and elderly (72 +/- 5 yr, n = 16) healthy subjects before and after a bout of resistance leg exercises. A real-time RT-PCR method was used to screen 100 transcripts in v. lateralis biopsies obtained before and 72 h postexercise. The screen identified 15 candidates for differential expression due to aging and/or exercise that were measured quantitatively. The median levels of four mRNAs (insulin-like growth factor-1 and its binding protein IGFBP5, ciliary neurotrophic factor, and the metallopeptidase MMP2) were significantly affected by aging and were greater (1.6- to 2.3-fold, P </= 0.05) in the young than elderly muscle at both time points. The median levels of three mRNAs were significantly (P </= 0.05) affected by exercise in the young. The metallopeptidase inhibitor TIMP1 and alpha-cardiac actin mRNAs increased 2-fold and 6.5-fold, respectively, and GDF8 (myostatin) mRNA decreased by 50%. However, elderly muscle did not display any significant changes in gene expression postexercise. Thus, aging muscle shows decreased levels at rest and an impaired response to exercise for a number of mRNAs for factors potentially involved in muscle growth and remodeling. Future studies must determine the functional importance of these gene expression changes to protein synthesis, satellite cell activity, and other processes that are directly involved in the mechanisms of muscle hypertrophy.

A synthetic mechano growth factor E Peptide enhances myogenic precursor cell transplantation success

Myogenic precursor cell (MPC) transplantation is a good strategy to introduce dystrophin expression in muscles of Duchenne muscular dystrophy (DMD) patients. Insulin-like growth factor (IGF-1) promotes MPC activities, such as survival, proliferation, migration and differentiation, which could enhance the success of their transplantation. Alternative splicing of the IGF-1 mRNA produces different muscle isoforms. The mechano growth factor (MGF) is an isoform, especially expressed after a mechanical stress. A 24 amino acids peptide corresponding to the C-terminal part of the MGF E domain (MGF-Ct24E peptide) was synthesized. This peptide had been shown to enhance the proliferation and delay the terminal differentiation of C(2)C(12) myoblasts. The present study showed that the MGF-Ct24E peptide improved human MPC transplantation by modulating their proliferation and differentiation. Indeed, intramuscular or systemic delivery of this synthetic peptide significantly promoted engraftment of human MPCs in mice. In vitro experiments demonstrated that the MGF-Ct24E peptide enhanced MPC proliferation by a different mechanism than the binding to the IGF-1 receptor. Moreover, MGF-Ct24E peptide delayed human MPC differentiation while having no outcome on survival. Those combined effects are probably responsible for the enhanced transplantation success. Thus, the MGF-Ct24E peptide is an interesting agent to increase MPC transplantation success in DMD patients.

MMP-2 functions as a negative regulator of chondrogenic cell condensation via down-regulation of the FAK-integrin beta1 interaction

Matrix metalloprotease-2 (MMP-2) has the capacity to degrade cartilage extracellular matrix molecules, the turnover of which is an essential event in chondrogenesis. Here, we investigated the functional role of MMP-2 in chondrogenesis of leg bud mesenchymal cells. Small interference RNA (siRNA)-mediated knockdown of mmp-2 promoted precartilage condensation and chondrogenesis. Treatment with bafilomycin A1, an MMP-2 activator, or GM6001, an MMP inhibitor, at the pre-condensation stage resulted in the inhibition or promotion of chondrogenesis, respectively. By comparison, treatment at the post-condensation stage had little or no effect on chondrogenesis. These results indicate that MMP-2 is involved in the regulation of cell condensation. Inhibition of MMP-2 activity by mmp-2 specific siRNA increased the protein level of fibronectin, and integrins alpha5 and beta1. The interaction between focal adhesion kinase (FAK) and integrin beta1 leading to tyrosine phosphorylation of FAK was also enhanced. Moreover, inactivation of p38MAPK down-regulated the level of MMP-2 mRNA and activity, and increased mesenchymal cell condensation in parallel with enhanced phosphorylation of FAK. Taken together, our data indicate that MMP-2 mediates the inhibitory signals of p38MAPK during mesenchymal cell condensation by functioning as a negative regulator of focal adhesion activity regulated by FAK via interactions with fibronectin through integrin beta1.

Matrix metalloproteinase-1 therapy improves muscle healing

Muscle undergoes time-dependent phases of healing after injury, which ultimately results in residual fibrosis in the injured area. The use of exogenous matrix metalloproteinases (MMPs) may improve recovery after muscle injury by promoting the digestion of existing fibrous tissue and releasing local growth factors. In the current experiment, bilateral gastrocnemius (GM) lacerations were created in severe combined immunodeficient mice. Twenty-five days after injury (peak posttraumatic fibrosis), C2C12 cells (myoblasts) transduced with the LacZ reporter gene were injected with exogenous MMP-1 into the right GMs at the site of injury; the cells were also injected along with PBS (control) at the site of injury in the left GMs. The muscle tissues were examined histologically via X-gal, hemotoxylin and eosin, and Masson's trichrome staining. The MMP-treated limbs contained more regenerating myofibers than did the control limbs (MMP 170 ± 96 fibers, control 62 ± 51 fibers; P < 0.001). Less fibrous tissue was observed within MMP-treated muscles (MMP: 24 ± 11%, control: 35 ± 15%; P < 0.01). These results suggest that the direct injection of MMP-1 into the zone of injury during fibrosis can enhance muscle regeneration by increasing the number of myofibers and decreasing the amount of fibrous tissue.

Effect of Human Skin-Derived Stem Cells on Vessel Architecture, Tumor Growth, and Tumor Invasion in Brain Tumor Animal Models

Glioblastomas represent an important cause of cancer-related mortality with poor survival. Despite many advances, the mean survival time has not significantly improved in the last decades. New experimental approaches have shown tumor regression after the grafting of neural stem cells and human mesenchymal stem cells into experimental intracranial gliomas of adult rodents. However, the cell source seems to be an important limitation for autologous transplantation in glioblastoma. In the present study, we evaluated the tumor targeting and antitumor activity of human skin-derived stem cells (hSDSCs) in human brain tumor models. The hSDSCs exhibit tumor targeting characteristics in vivo when injected into the controlateral hemisphere or into the tail vein of mice. When implanted directly into glioblastomas, hSDSCs distributed themselves extensively throughout the tumor mass, reduced tumor vessel density, and decreased angiogenic sprouts. In addition, transplanted hSDSCs differentiate into pericyte cell and release high amounts of human transforming growth factor-beta1 with low expression of vascular endothelial growth factor, which may contribute to the decreased tumor cell invasion and number of tumor vessels. In long-term experiments, the hSDSCs were also able to significantly inhibit tumor growth and to prolong animal survival. Similar behavior was seen when hSDSCs were implanted into two different tumor models, the chicken embryo experimental glioma model and the transgenic Tyrp1-Tag mice. Taken together, these data validate the use of hSDSCs for targeting human brain tumors. They may represent therapeutically effective cells for the treatment of intracranial tumors after autologous transplantation.

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

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

A new pro-migratory activity on human myogenic precursor cells for a synthetic peptide within the E domain of the mechano growth factor

Duchenne muscular dystrophy (DMD) is an inherited disease that leads to progressive muscle wasting. Myogenic precursor cell transplantation is an approach that can introduce the normal dystrophin gene in the muscle fibers of the patients. Unfortunately, these myogenic precursor cells do not migrate well in the muscle and thus many injections have to be done to enable a good graft success. Recent reports have shown that there is extensive splicing of the IGF-1 gene in muscles. The MGF isoform contains a C-terminal 24 amino acids peptide in the E domain (MGF-Ct24E) that has intrinsic properties. It can promote the proliferation while delaying the differentiation of C(2)C(12) cells. Here, we demonstrated that this synthetic peptide is a motogenic factor for human precursor myogenic cells in vitro and in vivo. Indeed, MGF-Ct24E peptide can modulate members of the fibrinolytic and metalloproteinase systems, which are implicated in the migration of myogenic cells. MGF-Ct24E peptide enhances the expression of u-PA, u-PAR and MMP-7 while reducing PAI-1 activity. Moreover, it has no effect on the gelatinases MMP-2 and -9. Those combined effects can favour cell migration. Finally, we present some results suggesting that the MGF-Ct24E peptide induces these cell responses through a mechanism that does not involve the IGF-1 receptor. Thus, this MGF-Ct24E peptide has a new pro-migratory activity on human myogenic precursor cells that may be helpful in the treatment of DMD. Those results reinforce the possibility that the IGF-1Ec isoform may produce an E domain peptide that can act as a cytokine.

Identification of secreted proteins during skeletal muscle development

The differentiation program of skeletal muscle cells is exquisitely sensitive to secreted proteins. We developed a strategy to maximize the discovery of secreted proteins, using mass spectrometry-based proteomics, from cultured muscle cells, C2C12, grown in a serum-free medium. This strategy led to the identification of 80 nonredundant proteins, of which 27 were secretory proteins that were identified with a minimum of two tryptic peptides. A number of the identified secretory proteins are involved in extracellular matrix remodeling, cellular proliferation, migration, and signaling. A putative network of proteins involving matrix metalloproteinase 2, SPARC, and cystatin C that all interact with TGFbeta signaling has been postulated to contribute toward a functional role in the myogenic differentiation program.

The simultaneous treatment of MMP-2 stimulants in retinal transplantation enhances grafted cell migration into the host retina

The success of functional retinal cell transplantation has been limited by the low efficiency of the transplanted cell integration into the host retina. Given that the extracellular matrix (ECM) is thought to inhibit entry and axonal outgrowth of grafted neural cells into the host retina, modulation of the ECMs in the host environment may overcome this limitation. Here, we demonstrate that matrix metalloprotease-2 (MMP-2) expression is associated with the high migratory potential of adult rat hippocampus-derived neural stem cells compared with retinal progenitor cells. In addition, MMP-2, as well as its reported inducers concanavalin A and 17beta-estradiol, can trigger the migration of retinal progenitor cells into explanted retinas. Inhibitors of MMP-2 suppressed these effects. Intense cell migration is not required for photoreceptor transplantation; however, the environment that allows the transplanted cells to integrate is most important. Migration of the transplanted cells is a good index of the acceptance of grafted cell of the host tissue. Strategies modulating the environment by MMP-2 stimulation may provide an advance in the development of retinal transplantation.

Exercise improves the success of myoblast transplantation in mdx mice

Transplantation of normal muscle precursor cells is a potential approach to restore dystrophin expression within dystrophin [deficient] mdx mice, a model of Duchenne Muscular Dystrophy. This study aims to evaluate whether exercise could improve graft success and hybrid fiber distribution within mdx muscle. eGFP(+) Muscle precursor cells were transplanted into tibialis anterior muscles of mdx mice using a single injection trajectory. During the following weeks, muscle fiber breaks were induced by making mdx mice swim. To evaluate fiber damage, Evans blue solution was injected intraperitoneally to mice 16h before their sacrifice. Tibialis anterior muscles were then harvested and eGFP, dystrophin and Evans blue labeling were analyzed by fluorescent microscopy. Twenty minutes of exercise (i.e., swimming) were used to induce damage in about 30% of TA muscle fibers. Graft success, evaluated as the percentage of hybrid fibers which are eGFP(+), was improved by 1.9-fold after swimming 3 times per week during 4 weeks and by 1.8-fold after daily swimming. Hybrid muscle fiber transversal and longitudinal distribution were also increased after repeated physical efforts. Exercise induced fiber breaks, which improved MPC recruitment and fusion and increased long-term graft success and also transverse and longitudinal distribution of hybrid fibers.

Mannose receptor regulates myoblast motility and muscle growth

Myoblast fusion is critical for the formation, growth, and maintenance of skeletal muscle. The initial formation of nascent myotubes requires myoblast-myoblast fusion, but further growth involves myoblast-myotube fusion. We demonstrate that the mannose receptor (MR), a type I transmembrane protein, is required for myoblast-myotube fusion. Mannose receptor (MR)-null myotubes were small in size and contained a decreased myonuclear number both in vitro and in vivo. We hypothesized that this defect may arise from a possible role of MR in cell migration. Time-lapse microscopy revealed that MR-null myoblasts migrated with decreased velocity during myotube growth and were unable to migrate in a directed manner up a chemoattractant gradient. Furthermore, collagen uptake was impaired in MR-null myoblasts, suggesting a role in extracellular matrix remodeling during cell motility. These data identify a novel function for MR during skeletal muscle growth and suggest that myoblast motility may be a key aspect of regulating myotube growth.

Skeletal myogenesis by human embryonic stem cells

We have examined the myogenic potential of human embryonic stem (hES) cells in a xeno-transplantation animal model. Here we show that precursors differentiated from hES cells can undergo myogenesis in an adult environment and give rise to a range of cell types in the myogenic lineage. This study provides direct evidences that hES cells can regenerate both muscle and satellite cells in vivo and are another promising cell type for treating muscle degenerative disorders in addition to other myogenic cell types.

Myoblast transplantation for inherited myopathies: a clinical approach

Myoblast transplantation (MT) is an experimental strategy for the potential treatment of myopathies. MT has two properties that make it potentially beneficial: genetic complementation and myogenic potential. Preclinical experiments on monkeys have shown that promising results can be obtained with MT in large muscles of primates depending on two conditions: appropriate immunosuppression and cell delivery by a method of high-density injections. Preclinical work on MT is being, or may be, addressed to: develop efficient methods of donor cell delivery applicable to clinics; control or avoid acute rejection by methods with the fewest secondary effects; understand the factors that condition the early survival of donor cells following transplantation; increase the success of each individual injection; re-engineer a functional structure in muscles that degenerates to fibrosis and fat substitution; and search for precursor cells with potential advantages over myoblasts.

Interleukin-4 improves the migration of human myogenic precursor cells in vitro and in vivo

Different molecules are available to recruit new neighboring myogenic cells to the site of regeneration. Formerly called B cell stimulatory factor-1, IL-4 can now be included in the list of motogenic factors. The present report demonstrates that human IL-4 is not required for fusion between mononucleated myoblasts but is required for myotube maturation. In identifying IL-4 as a pro-migratory agent for myogenic cells, these results provide a mechanism which partly explains IL-4 demonstrated activity during differentiation. Among the different mechanisms by which IL-4 might enhance myoblast migration processes, our results indicate that there are implications of some integrins and of three major components of the fibrinolytic system. Indeed, increases in the amount of active urokinase plasminogen activator and its receptor were observed following an IL-4 treatment, while the plasminogen activator inhibitor-1 decreased. Finally, IL-4 did not modify the amount of cell surface alpha5 integrin but increased the presence of beta3 and beta1 integrins. This integrin modulation might favor myogenic cell migration and its interaction with newly formed myotubes. Therefore, IL-4 co-injection with transplanted myoblasts might be an approach to enhance the migration of transplanted cells for the treatment of a damaged myocardium or of a Duchenne Muscular Dystrophy patient.

Regulation of TIMP-2, MT1-MMP, and MMP-2 expression during C2C12 differentiation

Matrix metalloproteinases (MMPs) are zinc-dependent proteases capable of degrading extracellular matrix components. The activity of these proteases is tightly regulated through the actions of the tissue inhibitors of metalloproteinases (TIMPs). Although the regulation of MMPs and TIMPs during physiological and pathological remodeling has been investigated in a number of systems, almost nothing is known about their role in skeletal muscle differentiation. To investigate the role of MMP-mediated proteolysis during myogenesis, the regulation of TIMP-2, MT1-MMP, and MMP-2 expression was investigated during differentiation of the mouse myoblastic C2C12 cell line. We show that this trio is upregulated coincident with myogenesis. The more diffuse spatial distribution of TIMP-2 relative to MT1-MMP and MMP-2 suggests that TIMP-2 may exert MMP-independent functions during myogenesis. Elucidating the regulation of these molecules during muscle differentiation in vitro may lead to a better understanding of their role in pathological processes in muscle tissue in vivo.

Role and function of matrix metalloproteinases in the differentiation and biological characterization of mesenchymal stem cells

Matrix metalloproteinases (MMPs), known as matrixins, are Ca- and Zn-dependent endoproteinases involved in a wide variety of developmental and disease-associated processes, proving to be crucial protagonists in many physiological and pathological mechanisms. The ability of MMPs to alter, by limited proteolysis and through the fine control of tissue inhibitors of metalloproteinases, the activity or function of numerous proteins, enzymes, and receptors suggests that they are also involved in various important cellular functions during development. In this review, we focus on the differentiation of mesenchymal stem cells (including those of the myoblastic, osteoblastic, chondroblastic, neural, and apidoblastic lineages) and the possible, if unexpected, biological significance of MMPs in its regulation. The MMP system has been implicated in several differentiation events that suggests that it mediates the proliferative and prodifferentiating effect of the matrixin proteolytic cascade. We summarize these regulatory effects of MMPs on the differentiation of mesenchymal stem cells and hypothesize on the function of MMPs in the stem cell differentiation processes.

A three-dimensional in vitro model system to study the adaptation of craniofacial skeletal muscle following mechanostimulation

The aim of this study was to investigate the in vitro response of human craniofacial muscle-derived myotubes (primitive/nascent muscle fibres), in three-dimensional constructs, to strain in vitro to mimic clinical scenarios, using expression of the mechanoresponsive gene gelatinase-A/matrix metalloproteinase-2 (MMP-2) as a marker of remodelling of muscle extracellular matrix. Three-dimensional (3D) constructs of cells derived from explants of human masseter muscle (human craniofacial muscle-derived cells; hCMDC) in collagen sponges were subjected to mechanical, uniaxial strain using the Bio-Stretch system. 3D myotube constructs were exposed to the strain regimes of rapid ramp stretch (RRS) or cyclical ramp strain (CRS) with 7.5% and 15% strain. The activity of MMP-2 was assessed by zymography of construct-conditioned medium, whilst lysates of the constructs were used to measure creatine phosphokinase (CPK) activity to confirm the presence of myotubes in the strained constructs. Scanning electron microscopy of the collagen sponges and the CPK assays confirmed the presence of myotubes. MMP-2 was expressed by all the samples and controls, but expression was found to be significantly higher in those cultures strained continuously (RRS), compared to cyclical strain (CRS), and in those strained at 15% compared to 7.5%. Thus, MMP-2 expression, and hence extracellular matrix remodelling, is up-regulated in response to strain and is dependent upon the amount and type of strain to which the muscle is subjected.

Distinct modes of collagen type I proteolysis by matrix metalloproteinase (MMP) 2 and membrane type I MMP during the migration of a tip endothelial cell: insights from a computational model

Matrix metalloproteinases (MMPs) are a family of enzymes responsible for the proteolytic processing of extracellular matrix (ECM) structural proteins under physiological and pathological conditions. During sprouting angiogenesis, the MMPs expressed by a single "tip" endothelial cell exhibit proteolytic activity that allows the cells of the sprouting vessel bud to migrate into the ECM. Membrane type I matrix metalloproteinase (MT1-MMP) and the diffusible matrix metalloproteinase MMP2, in the presence of the tissue inhibitor of metalloproteinases TIMP2, constitute a system of proteins that play an important role during the proteolysis of collagen type I matrices. Here, we have formulated a computational model to investigate the proteolytic potential of such a tip endothelial cell. The cell expresses MMP2 in its proenzyme form, pro-MMP2, as well as MT1-MMP and TIMP2. The interactions of the proteins are described by a biochemically detailed reaction network. Assuming that the rate-limiting step of the migration is the ability of the tip cell to carry out proteolysis, we have estimated cell velocities for matrices of different collagen content. The estimated velocities of a few microns per hour are in agreement with experimental data. At high collagen content, proteolysis was carried out primarily by MT1-MMP and localized to the cell leading edge, whereas at lower concentrations, MT1-MMP and MMP2 were found to act in parallel, causing proteolysis in the vicinity of the leading edge. TIMP2 is a regulator of the proteolysis localization because it can shift the activity of MT1-MMP from its enzymatic toward its activatory mode, suggesting a tight mechanosensitive regulation of the enzymes and inhibitor expression. The model described here provides a foundation for quantitative studies of angiogenesis in extracellular matrices of different compositions, both in vitro and in vivo. It also identifies critical parameters whose values are not presently available and which should be determined in future experiments.

The membrane-anchored MMP-regulator RECK is a target of myogenic regulatory factors

The membrane-anchored MMP-regulator RECK is down regulated in many solid tumors; the extent of RECK down regulation correlates with poor prognosis. Forced expression of RECK in tumor cells results in suppression of angiogenesis, invasion, and metastasis. Studies on the roles and the mechanisms of regulation of the RECK gene during normal development may therefore yield important insights into how the malignant behaviors of tumor cells arise and how they can be controlled. Our previous studies indicate that mice lacking RECK die around E10.5 with reduced tissue integrity. In the present study, we have found that in later stage wild-type embryos, RECK is abundantly expressed in skeletal muscles, especially in the areas where the myoblast differentiation factor MRF4 is expressed. Consistent with this finding, the RECK-promoter is activated by MRF4 in cultured cells. In contrast, a myoblast determination factor MyoD suppresses the RECK-promoter. Myoblastic cells lacking RECK expression give rise to myotubes at higher efficiency than the cells expressing RECK, indicating that RECK suppresses myotube formation. These findings suggest that MyoD down regulates RECK to facilitate myotube formation, whereas MRF4 up regulates RECK to promote other aspects of myogenesis that require extracellular matrix integrity.

[Cell transplantation in heart failure management]

Heart failure is becoming a major issue for public health in western countries and the effect of currently available therapies is limited. Therefore cell transplantation was developed as an alternative strategy to improve cardiac structure and function. This review describes the multiple cell types and clinical trials considered for use in this indication. Most studies have been developed in models of post-ischemic heart failure. The transplantation of fetal or neonatal cardiomyocytes has proven to be functionally successful, but ethical as well as immunological and technical reasons make their clinical use limited. Recent reports, however, suggested that adult autologous cardiomyocytes could be prepared from stem cells present in various tissues (bone marrow, vessels, adult heart itself, adipose tissue). Alternatively, endothelial progenitors originating from bone marrow or peripheral blood could promote the neoangiogenesis within the scar tissue. Hematopietic stem cells prepared from bone marrow or peripheral blood have been proposed but their differentiation ability seems limited. Finally, the transplantation of skeletal muscle cells (myoblasts) in the infarcted area improved myocardial function, in correlation with the development of skeletal muscle tissue in various animal models. The latter results paved the way for the development of a first phase I clinical trial of myoblast transplantation in patients with severe post-ischemic heart failure. It required the scale-up of human cell production according to good manufacturing procedures, started in june 2000 in Paris and was terminated in november 2001, and was followed by several others. The results were encouraging and prompted the onset of a blinded, multicentric phase II clinical trial for skeletal muscle cells transplantation. Meanwhile, phase I clinical trials also evaluate the safeness and efficacy of various cell types originating from the bone marrow or the peripheral blood. However, potential side effects related to the biological properties of the cells or the delivery procedures are being reported. High quality clinical trials supported by strong pre-clinical data will help to evaluate the role of cell therapy as a potential treatment for heart failure.

Membrane independent activation of fibroblast proMMP-2 by snake venom: novel roles for venom proteinases

ProMMP-2 activation by Bothrops asper venom was investigated in mouse gastrocnemius muscle, mammalian cell culture and a cell-free system. Zymography revealed an increment of latent and activated forms of MMP-2 in muscle homogenates 1-3 days after venom injection. To clarify if venom can induce expression and activation of MMP-2, independently of the inflammatory response, venom was added to cultured human fibroblasts, endothelial and skeletal muscle cells, which expressed proMMP-2 constitutively. Venom activated proMMP-2 without promoting its expression. Venom also activated and degraded proMMP-2 in supernatants collected from fibroblast cultures, indicating that cells are not required for this activation. Pretreatment with EDTA increased MMP-2 activation and reduced degradation. Venom serine proteinases activated proMMP-2, whereas BaP1, a P-I metalloproteinase, predominantly degraded the latent and active forms of MMP-2. Moreover, pretreatment of conditioned medium with serine proteinase inhibitors greatly reduced the venom-induced activation, suggesting that venom proteinases activate MMP-2 via a serine proteinase secreted by fibroblasts. Venom also directly activated and degraded purified proMMP-2, albeit requiring a high concentration. Thus, B. asper venom proteinases activate and degrade proMMP-2 without inducing its synthesis. Serine proteinases play a dominant role in the activation, whereas metalloproteinases predominantly degrade MMP-2. Activation of proMMP-2 by snake venom proteinases, independently of the MT1-MMP/TIMP-2 pathway, extracellular matrix degradation or apoptosis, represents a novel mechanism in human fibroblasts.

Impaired spermatogenesis and male fertility defects in CIZ/Nmp4-disrupted mice

CIZ (Cas interacting zinc finger protein), also called Nmp4 (nuclear matrix protein 4), is a nucleo-cytoplasmic shuttling transcription factor that regulates the expression of collagen and matrix metalloproteinases. CIZ/Nmp4 was originally cloned by its binding to p130(Cas), a focal adhesion protein, and was recently shown to suppress BMP2 (bone mophogenetic protein 2) signalling. To explore the physiological role of CIZ/Nmp4, we disrupted CIZ/Nmp4-gene by inserting beta-galactosidase and neomycin resistance genes into the 2nd exon of CIZ/Nmp4-gene, which is utilized by all the sequenced alternative forms. CIZ-/- mice were born and grew to adulthood. Although they tend to be smaller than wild-type mice, no pathological abnormality was observed except in the testis. Histological analysis of the testes revealed variable degrees of spermatogenic cell degeneration within the seminiferous tubules of CIZ-/- mice, resembling the histology of the 'Germinal-cell aplasia with focal spermatogenesis'. Some of the CIZ-/- male mice developed infertility. TUNEL assay on testis sections revealed an increased occurrence of apoptosis of spermatogenic cells in the testes of CIZ-/- mice. CIZ/Nmp4 was co-localized with Smad1 in the testis, suggesting that a disregulation of BMP signalling could cause these phenotypes. These results suggest that CIZ/Nmp4 plays roles in the progress and the maintenance of spermatogenesis.

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.

Hepatocyte growth factor/scatter factor stimulates migration of muscle precursors in developing mouse tongue

Hepatocyte growth factor (HGF) stimulates the migration of myogenic cells during the development of skeletal muscles. The inactivation of HGF genes or that of its receptor, c-met, in mice causes hypoplasia of skeletal muscle organs, such as the tongue. Basic fibroblast growth factor (FGF-2) also induces migration of skeletal myoblasts. A comparison of the functions of HGF and FGF-2 in myogenesis revealed the crucial effect of HGF in the development of skeletal muscles. Unlike FGF-2, HGF induced migration of myoblasts from the developing mouse tongue. The differences between the activities of HGF and FGF-2 were determined by comparing their effects on the expression of matrix metalloproteinase-9 (MMP-9) in myoblasts, C2C12 cells, cultured in collagen-coated dishes. The results showed that HGF, but not FGF-2, stimulated MMP-9 expression, and that the stimulation was mediated through the activation of phosphoinositide 3-kinase (PI3K) which was not associated with FGF-2 signal transduction. Nevertheless, both growth factors exerted almost the same effect on the reduction of myogenin expression in, and on the proliferation of, C2C12 cells, suggesting that HGF, rather than FGF-2, plays a crucial role in the generation of skeletal muscles, including the tongue. Moreover, the specific role of HGF through the PI3K signal pathway is the induction of MMP-9 expression in, and the migration of, myoblasts.

Human circulating AC133(+) stem cells restore dystrophin expression and ameliorate function in dystrophic skeletal muscle

Duchenne muscular dystrophy (DMD) is a common X-linked disease characterized by widespread muscle damage that invariably leads to paralysis and death. There is currently no therapy for this disease. Here we report that a subpopulation of circulating cells expressing AC133, a well-characterized marker of hematopoietic stem cells, also expresses early myogenic markers. Freshly isolated, circulating AC133(+) cells were induced to undergo myogenesis when cocultured with myogenic cells or exposed to Wnt-producing cells in vitro and when delivered in vivo through the arterial circulation or directly into the muscles of transgenic scid/mdx mice (which allow survival of human cells). Injected cells also localized under the basal lamina of host muscle fibers and expressed satellite cell markers such as M-cadherin and MYF5. Furthermore, functional tests of injected muscles revealed a substantial recovery of force after treatment. As these cells can be isolated from the blood, manipulated in vitro, and delivered through the circulation, they represent a possible tool for future cell therapy applications in DMD disease or other muscular dystrophies.

Mutations in two matrix metalloproteinase genes, MMP-2 and MT1-MMP, are synthetic lethal in mice

The matrix metalloproteinase (MMP) family (approximately 25 members in mammals) has been implicated in extracellular matrix remodeling associated with embryonic development, cancer formation and progression, and various other physiological and pathological events. Inactivating mutations in individual matrix metalloproteinase genes in mice described so far, however, are nonlethal at least up to the first few weeks after birth, suggesting functional redundancy among MMP family members. Here, we report that mice lacking two MMPs, MMP-2 (nonmembrane type) and MT1-MMP (membrane type), die immediately after birth with respiratory failure, abnormal blood vessels, and immature muscle fibers reminiscent of central core disease. In the absence of MMP-2 and MT1-MMP, myoblast fusion in vitro is also significantly retarded. These findings suggest functional overlap in mice between the two MMPs with distinct molecular natures.

Age-related changes in the mitotic and metabolic characteristics of muscle-derived cells

Age-related sarcopenia could partly result from cumulative repeated episodes of incomplete repair and regeneration. We hypothesized that mitotic and metabolic events associated with satellite cell activation and proliferation could be altered with aging. Muscle-derived cells (mdc) were isolated from gastrocnemius and quadriceps muscles of young (3 wk old), adult (9 mo old), and old (24 mo old) Sprague-Dawley male rats (n = 10/group). The mdc from young growing rats started to proliferate earlier compared with adult and old animals. Cell cycle duration was significantly reduced with aging from 36.5 +/- 3.2 to 28.0 +/- 2.2 h. However, the proportion of noncycling (G0 phase) and cycling (G1 + S + G2 + M phases) cultured mdc was statistically unchanged among the three age groups. Significantly lower increase in c-met and proliferating cell nuclear antigen expression were observed in cultured mdc of old rats upon serum stimulation. Major changes in the expression of citrate synthase, lactate dehydrogenase, proteasome, caspase 3, plasminogen activators (PAs), and matrix metalloproteinase 2-9 (MMP2-9) were observed upon serum stimulation, but no age-related difference was noted. However, when measured on crushed muscle extracts, PAs and MMP2-9 enzyme activities were significantly decreased with aging. Our results show that cellular and biochemical events associated with the control of mdc activation and proliferation occur with aging. These alterations may participate in the accumulation of repeated episodes of incomplete repair and regeneration throughout the life span, thus contributing to the loss of skeletal muscle mass and function with aging.

GROWTH FACTORS IMPROVE THE IN VIVO MIGRATION OF HUMAN SKELETAL MYOBLASTS BY MODULATING THEIR ENDOGENOUS PROTEOLYTIC ACTIVITY

BACKGROUND

A main technological problem related to the clinical application of myoblast transplantation is the poor migration of transplanted cells. In this study, we investigated a new physiologic approach that consists of coinjecting motogenic factors insulin growth factor (IGF)-1 or basic fibroblast growth factor (bFGF) to enhance the migration of human skeletal myoblasts. Among the different ways by which those factors can induce the cell migration processes, we investigated their capacity to enhance cell endogenous proteolytic activity that will help transplanted cells to migrate through the extracellular matrix.

METHODS

In vitro, myoblasts were coincubated with bFGF or IGF-1. Growth factors effects on cell migration were evaluated using invasion chambers, and their effects on proteolytic systems were evaluated by zymography, Western blot, and reverse transcription polymerase chain reaction. In vivo, myoblasts were coinjected with growth factors and the intramuscular migration capacity was assessed using the microtube technique.

RESULTS

In vitro, the presence of IGF-1 or bFGF significantly enhanced the expression of the gelatinase matrix metalloproteinase-9 and focalized the fibrinolytic system activity at the cell membrane. In vitro and in vivo, both bFGF and IGF-1 showed strong chemokinetic potentials and improved the migration of human myoblasts. Moreover, the implication some proteinases in the in vivo enhanced migration was confirmed using specific inhibitors (BB94 or amiloride).

CONCLUSIONS

These results suggest that IGF-1 or bFGF coinjection with human myoblasts increased their proteolytic activities and consequently their migratory capacity. This study may help to develop approaches that will reduce the number of injection sites for the treatment of Duchenne muscular dystrophy patients.

Matrix metalloproteinases and skeletal muscle: A brief review

Matrix metalloproteinases (MMPs) are a family of zinc- dependent proteolytic enzymes that function mainly in the extracellular matrix, where they contribute to the development, functioning, and pathology of a wide range of tissues. This mini-review describes the MMPs and tissue inhibitors of MMPs (TIMPs) in skeletal muscle, and considers their involvement in muscle development, ischemia, myonecrosis, angiogenesis, denervation, exercise-induced injuries, disuse atrophy, muscle repair and regeneration, and inflammatory myopathies and dystrophies. Despite the very limited information currently available on MMPs and their inhibitors in skeletal muscle, it is becoming increasingly clear that they have important physiological functions in maintenance of the integrity and homeostasis of muscle fibers and of the extracellular matrix. Understanding the roles of MMPs and TIMPs may lead to the development of new drug-related treatments for various muscle disorders based on suppression or upregulation of their expression.

Tumor necrosis factor-alpha (TNF-alpha) stimulates chemotactic response in mouse myogenic cells

Migration of transplanted myogenic cells occurs during both embryogenesis and regeneration of skeletal muscles and is important for successful myoblast transplantation, but little is known about factors that promote chemotaxis of these cells. Tumor necrosis factor-alpha (TNF-alpha) is known to induce chemotactic effect on several cell types. In this study, we investigated its influence on the in vitro and in vivo motility of C2C12 and primary myoblasts. In the in vitro test performed in the blind-well Boyden chambers, we showed that TNF-alpha (50-400 U/ml) significantly enhanced the ability of myogenic cells to migrate. The dose-response curve for this factor was bell shaped, with maximum activity in the 200 U/ml range. In the in vivo test, intramuscular administration of TNF-alpha was performed by an Alzet pump connected to a perforated polyethylene microtube inserted in the tibialis anterior (TA) of CD1 mice. In these experiments, myoblasts were injected under the muscle epimysium. The recipient mice were immunosuppressed with FK506. Our results showed that, 5 days after myoblast transplantation, cells migrated further in the muscles infused with TNF-alpha than in the muscles not exposed to TNF-alpha. TNF-alpha not only has a chemotactic activity but may also modify cell migration via its action on matrix metalloproteinase (MMP) expression. The proteolytic activities of the MMPs secreted in the muscles were thus also assessed by gelatin zymography. The results showed an increased of MMP-2 and MMP-9 transcripts in the TNF-alpha-infused muscles injected with myogenic cells. Myoblast migration during transplantation may be enhanced by overlapping gradients of several effector molecules such as TNF-alpha, interferon-gamma (INF-gamma), and interleukins, released at the site of muscle injury. We propose that TNF-alpha may promote myoblast migration directly through chemotactic activity and indirectly by enhancing MMP activity at the site of muscle injury.

Skeletal muscle stem cells

Satellite cells are myogenic stem cells responsible for the post-natal growth, repair and maintenance of skeletal muscle. This review focuses on the basic biology of the satellite cell with emphasis on its role in muscle repair and parallels between embryonic myogenesis and muscle regeneration. Recent advances have altered the long-standing view of the satellite cell as a committed myogenic stem cell derived directly from the fetal myoblast. The experimental basis for this evolving perspective will be highlighted as will the relationship between the satellite cell and other newly discovered muscle stem cell populations. Finally, advances and prospects for cell-based therapies for muscular dystrophies will be addressed.

Cell therapies for inherited myopathies

PURPOSE OF REVIEW

Cell therapies for inherited myopathies are based on the implantation of normal or genetically corrected myogenic cells into the body. This review summarizes the recent progress in this field, systematized according to the factors important for success.

RECENT FINDINGS

In the choice of donor cells, myoblasts derived from satellite cells remain the best choice. Some studies on the population of muscle-derived stem cells in mice suggested that these cells may have some advantages over myoblasts; however, no results supporting this advantage have been presented in a primate model. Recent studies on bone marrow transplantation as a systemic source of myogenic precursors for the treatment of myopathies were disappointing. Concerning donor cell delivery, intramuscular myoblast injection remains the only way that can significantly introduce exogenous myogenic cells into the muscles. A recent study in primates showed some parameters of myoblast injection that could be useful in the human. Progress was made in mice to understand the factors that could favor the migration of the donor myoblasts in the host muscles. Concerning donor cell survival, analysis of immune cell infiltration dynamics allowed a better understanding of the factors implicated in early donor cell death. Progress was made on the control of acute rejection for myoblast transplantation in primates. So far, few mouse experiments have advanced the field of tolerance induction toward myogenic cells.

SUMMARY

Myoblast transplantation (intramuscular injection of satellite cell-derived myoblasts) currently remains the only cell-based therapy that has produced promising results in the context of a preclinical model such as the nonhuman primate.

Identification of a putative pathway for the muscle homing of stem cells in a muscular dystrophy model

Attempts to repair muscle damage in Duchenne muscular dystrophy (DMD) by transplanting skeletal myoblasts directly into muscles are faced with the problem of the limited migration of these cells in the muscles. The delivery of myogenic stem cells to the sites of muscle lesions via the systemic circulation is a potential alternative approach to treat this disease. Muscle-derived stem cells (MDSCs) were obtained by a MACS(R) multisort method. Clones of MDSCs, which were Sca-1+/CD34-/L-selectin+, were found to adhere firmly to the endothelium of mdx dystrophic muscles after i.v. or i.m. injections. The subpopulation of Sca-1+/CD34- MDSCs expressing L-selectin was called homing MDSCs (HMDSCs). Treatment of HMDSCs with antibodies against L-selectin prevented adhesion to the muscle endothelium. Importantly, we found that vascular endothelium from striate muscle of young mdx mice expresses mucosal addressin cell adhesion molecule-1 (MAdCAM-1), a ligand for L-selectin. Our results showed for the first time that the expression of the adhesion molecule L-selectin is important for muscle homing of MDSCs. This discovery will aid in the improvement of a potential therapy for muscular dystrophy based on the systemic delivery of MDSCs.

Dynamics of the early immune cellular reactions after myogenic cell transplantation

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