Muscle regeneration by reconstitution with bone marrow or fetal liver cells from green fluorescent protein-gene transgenic mice

Of microenvironments and mammary stem cells

In most adult tissues there reside pools of stem and progenitor cells inside specialized microenvironments referred to as niches. The niche protects the stem cells from inappropriate expansion and directs their critical functions. Thus guided, stem cells are able to maintain tissue homeostasis throughout the ebb and flow of metabolic and physical demands encountered over a lifetime. Indeed, a pool of stem cells maintains mammary gland structure throughout development, and responds to the physiological demands associated with pregnancy. This review discusses how stem cells were identified in both human and mouse mammary glands; each requiring different techniques that were determined by differing biological needs and ethical constraints. These studies together create a robust portrait of mammary gland biology and identify the location of the stem cell niche, elucidate a developmental hierarchy, and suggest how the niche might be manipulated for therapeutic benefit.

The skeletal muscle satellite cell: the stem cell that came in from the cold

The muscle satellite cell was first described and actually named on the basis of its anatomic location under the basement membrane surrounding each myofiber. For many years following its discovery, electron microscopy provided the only definitive method of identification. More recently, several molecular markers have been described that can be used to detect satellite cells, making them more accessible for study at the light microscope level. Satellite cells supply myonuclei to growing myofibers before becoming mitotically quiescent in muscle as it matures. They are then activated from this quiescent state to fulfill their roles in routine maintenance, hypertrophy, and repair of adult muscle. Because muscle is able to efficiently regenerate after repeated bouts of damage, systems must be in place to maintain a viable satellite cell pool, and it was proposed over 30 years ago that self-renewal was the primary mechanism. Self-renewal entails either a stochastic event or an asymmetrical cell division, where one daughter cell is committed to differentiation whereas the second continues to proliferate or becomes quiescent. This classic model of satellite cell self-renewal and the importance of satellite cells in muscle maintenance and repair have been challenged during the past few years as bone marrow-derived cells and various intramuscular populations were shown to be able to contribute myonuclei and occupy the satellite cell niche. This is a fast-moving and dynamic field, however, and in this review we discuss the evidence that we think puts this enigmatic cell firmly back at the center of adult myogenesis.

Myogenic potential of adipose-tissue-derived cells

Adipose-tissue-derived mesenchymal stem cells can be directed towards a myogenic phenotype in vitro by the addition of specific inductive media. However, the ability of these or other adipose-tissue-associated cells to respond to ;natural' myogenic cues such as a myogenic environment has never been investigated in detail. Here, we provide evidence that a restricted subpopulation of freshly harvested adipose-tissue-derived cells possesses an intrinsic myogenic potential and can spontaneously differentiate into skeletal muscle. Conversion of adipose-tissue-derived cells to a myogenic phenotype is enhanced by co-culture with primary myoblasts in the absence of cell contact and is maximal when the two cell types are co-cultured in the same plate. Conversely, in vitro expanded adipose-tissue-derived mesenchymal stem cells require direct contact with muscle cells to generate skeletal myotubes. Finally, we show that uncultured adipose-tissue-associated cells have a high regenerative capacity in vivo since they can be incorporated into muscle fibers following ischemia and can restore significantly dystrophin expression in mdx mice.

Bone marrow transplantation improves outcome in a mouse model of congenital muscular dystrophy

We examined whether pathogenesis in dystrophin-deficient (mdx) mice and laminin-alpha2-deficient (dy) mice is ameliorated by bone marrow transplantation (BMT). Green fluorescent protein (GFP) mice were used as donors. In mdx mice, BMT failed to produce any significant differences in muscle pathology, although some GFP-positive fibers with restored dystrophin expression were observed. In contrast, in the dy mice, BMT led to a significant increase in lifespan and an increase in growth rate, muscle strength, and respiratory function. We conclude that BMT improved outcome in dy mice but not mdx mice.

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.

Pax7 and myogenic progression in skeletal muscle satellite cells

Skeletal muscle growth and regeneration are attributed to satellite cells - muscle stem cells resident beneath the basal lamina that surrounds each myofibre. Quiescent satellite cells express the transcription factor Pax7 and when activated, coexpress Pax7 with MyoD. Most then proliferate, downregulate Pax7 and differentiate. By contrast, others maintain Pax7 but lose MyoD and return to a state resembling quiescence. Here we show that Pax7 is able to drive transcription in quiescent and activated satellite cells, and continues to do so in those cells that subsequently cease proliferation and withdraw from immediate differentiation. We found that constitutive expression of Pax7 in satellite-cell-derived myoblasts did not affect MyoD expression or proliferation. Although maintained expression of Pax7 delayed the onset of myogenin expression it did not prevent, and was compatible with, myogenic differentiation. Constitutive Pax7 expression in a Pax7-null C2C12 subclone increased the proportion of cells expressing MyoD, showing that Pax7 can act genetically upstream of MyoD. However these Pax7-null cells were unable to differentiate into normal myotubes in the presence of Pax7. Therefore Pax7 may be involved in maintaining proliferation and preventing precocious differentiation, but does not promote quiescence.

The epigenetic network regulating muscle development and regeneration

This review focuses on our current knowledge of the epigenetic changes regulating gene expression at the chromatin and DNA level, independently on the primary DNA sequence, to reprogram the nuclei of muscle precursors during developmental myogenesis and muscle regeneration. These epigenetic marks provide the blueprint by which the extra-cellular cues are interpreted at the nuclear level by the transcription machinery to select the repertoire of tissue-specific genes to be expressed. The reversibility of some of these changes necessarily reflects the dynamic nature of skeletal myogenesis, which entails the progression through two antagonistic processes--proliferation and differentiation. Other epigenetic modifications are instead associated to events conventionally considered as irreversible--e.g. maintenance of lineage commitment and terminal differentiation. However, recent results support the possibility that these events can be reversed, at least upon certain experimental conditions, thereby revealing a dynamic nature of many of the epigenetic modifications underlying skeletal myogenesis. The elucidation of the epigenetic network that regulates transcription during developmental myogenesis and muscle regeneration might provide the information instrumental to devise pharmacological interventions toward selective manipulation of gene expression to promote regeneration of skeletal muscles and possibly other tissue.

Functional heterogeneity of side population cells in skeletal muscle

Skeletal muscle regeneration has been exclusively attributed to myogenic precursors, satellite cells. A stem cell-rich fraction referred to as side population (SP) cells also resides in skeletal muscle, but its roles in muscle regeneration remain unclear. We found that muscle SP cells could be subdivided into three sub-fractions using CD31 and CD45 markers. The majority of SP cells in normal non-regenerating muscle expressed CD31 and had endothelial characteristics. However, CD31(-)CD45(-) SP cells, which are a minor subpopulation in normal muscle, actively proliferated upon muscle injury and expressed not only several regulatory genes for muscle regeneration but also some mesenchymal lineage markers. CD31(-)CD45(-) SP cells showed the greatest myogenic potential among three SP sub-fractions, but indeed revealed mesenchymal potentials in vitro. These SP cells preferentially differentiated into myofibers after intramuscular transplantation in vivo. Our results revealed the heterogeneity of muscle SP cells and suggest that CD31(-)CD45(-) SP cells participate in muscle regeneration.

The Continuum of Stem Cell Transdifferentiation: Possibility of Hematopoietic Stem Cell Plasticity with Concurrent CD45 Expression

Recent years have seen a surge of scientific research examining adult stem cell plasticity. For example, the hematopoietic stem cell has been shown to give rise to skin, respiratory epithelium, intestinal epithelium, renal epithelium, liver parenchyma, pancreas, skeletal muscle, vascular endothelium, myocardium, and central nervous system (CNS) neurons. The potential for such stem cell plasticity seems to be enhanced by stressors such as injury and neoplasia. Interestingly, recent studies have demonstrated that hematopoietic stem cells may be able to adopt certain nonhematopoietic phenotypes, such as endothelial, neural, or skeletal muscle phenotypes, without entirely losing their initial hematopoietic identity. We propose that transdifferentiation can, in certain conditions, be a partial rather than a complete event, and we encourage further investigation into the phenomenon of a stem cell simultaneously expressing phenotypic features of two distinct cell fates.

Sphingomyelin Levels in the Plasma Membrane Correlate with the Activation State of Muscle Satellite Cells

Satellite cells are responsible for postnatal growth, hypertrophy, and regeneration of skeletal muscle. They are normally quiescent, and must be activated to fulfill these functions, yet little is known of how this is regulated. As a first step in determining the role of lipids in this process, we examined the dynamics of sphingomyelin in the plasma membrane. Sphingomyelin contributes to caveolae/lipid rafts, which act to concentrate signaling molecules, and is also a precursor of several bioactive lipids. Proliferating or differentiated C2C12 muscle cells did not bind lysenin, a sphingomyelin-specific binding protein, but noncycling reserve cells did. Quiescent satellite cells also bound lysenin, revealing high levels of sphingomyelin in their plasma membranes. On activation, however, the levels of sphingomyelin drop, so that lysenin did not label proliferating satellite cells. Although most satellite cell progeny differentiate, others stop cycling, maintain Pax7, downregulate MyoD, and escape immediate differentiation. Importantly, many of these Pax7-positive/MyoD-negative cells also regained lysenin binding on their surface, showing that the levels of sphingomyelin had again increased. Our observations show that quiescent satellite cells are characterized by high levels of sphingomyelin in their plasma membranes and that lysenin provides a novel marker of myogenic quiescence.

An approach for producing transgenic cloned cows by nuclear transfer of cells transfected with human alpha 1-antitrypsin gene

In an attempt to produce transgenic cloned cows secreting alpha 1-antitrypsin (alpha1-AT) protein into milk, bovine cumulus cells were transfected with a plasmid containing an alpha1-AT gene and green fluorescent protein (GFP) reporter gene using Fugene 6 as a lipid carrier. The GFP-expressing cells were selected and transferred into enucleated bovine oocytes. Couplets were fused, chemically activated and cultured. Developmental competence was monitored and the number of inner cell mass (ICM) and trophectoderm (TE) cells in blastocysts were counted after differential staining. The percentage of blastocysts was lower (P < 0.05) in transgenic cloned embryos compared to non-transgenic cloned embryos (23% versus 35%). No difference in the numbers of ICM and TE cells between the two groups of embryos was observed. One or two GFP-expressing blastocysts were transferred into the uterus of each recipient cow. Out of 49 recipient cows, three pregnancies were detected by non-return estrus and rectal palpation. However, the pregnancies failed to maintain to term; two fetuses were aborted at Day 60 and 150, respectively, and one fetus at Day 240. The genomic DNA from the aborted fetus was amplified by polymerase chain reaction (PCR) to investigate integration of the transgene in the fetus. The expected PCR product was sequenced and was identical to the sequence of alpha1-AT transgene. In conclusion, the present study demonstrated that developmental competence of cloned embryos derived from transgenic donor cells was lower than embryos derived from non-transfected donor cells. Although we failed to obtain a viable transgenic cloned calf, integration of alpha1-AT gene into the fetus presents the possibility of producing transgenic cloned cows by somatic cell nuclear transfer.

Cellular and molecular signatures of muscle regeneration: current concepts and controversies in adult myogenesis

Adult skeletal muscle generates force in a controlled and directed manner through the contraction of highly specialized, postmitotic, multinucleated myofibers. Life-long muscle function relies on maintenance and regeneration of myofibers through a highly regulated process beginning with activation of normally quiescent muscle precursor cells and proceeding with formation of proliferating progenitors that fuse to generate differentiated myofibers. In this review, we describe the historical basis and current evidence for the identification of satellite cells as adult muscle stem cells, critically evaluate contributions of other cells to adult myogenesis, and summarize existing data regarding the origins, genetic markers, and molecular regulation of satellite cells in normal, diseased, and aged muscle.

IGF-I increases bone marrow contribution to adult skeletal muscle and enhances the fusion of myelomonocytic precursors

Muscle damage has been shown to enhance the contribution of bone marrow-derived cells (BMDCs) to regenerating skeletal muscle. One responsible cell type involved in this process is a hematopoietic stem cell derivative, the myelomonocytic precursor (MMC). However, the molecular components responsible for this injury-related response remain largely unknown. In this paper, we show that delivery of insulin-like growth factor I (IGF-I) to adult skeletal muscle by three different methods-plasmid electroporation, injection of genetically engineered myoblasts, and recombinant protein injection-increases the integration of BMDCs up to fourfold. To investigate the underlying mechanism, we developed an in vitro fusion assay in which co-cultures of MMCs and myotubes were exposed to IGF-I. The number of fusion events was substantially augmented by IGF-I, independent of its effect on cell survival. These results provide novel evidence that a single factor, IGF-I, is sufficient to enhance the fusion of bone marrow derivatives with adult skeletal muscle.

Two different roles of purified CD45+c-Kit+Sca-1+Lin- cells after transplantation in muscles

Recent studies have indicated that bone marrow cells can regenerate damaged muscles and that they can adopt phenotypes of other cells by cell fusion. Our direct visualization system gave evidence of massive muscle regeneration by green fluorescent protein (GFP)-labeled CD45+c-Kit+Sca-1+Lin- cells (KSL cells), and we investigated the role of KSL cells in muscle regeneration after transplantation with or without lethal irradiation. In the early phase, GFP signals were clearly observed in all the muscles of only irradiated mice. Transverse cryostat sections showed GFP+myosin+ muscle fibers, along with numerous GFP+ hematopoietic cells in damaged muscle. These phenomena were temporary, and GFP signals had dramatically reduced 30 days after transplantation. After 6 months, GFP+ fibers could hardly be detected, but GFP+c-Met+ mononuclear cells were located beneath the basal lamina where satellite cells usually exist in both conditioned mice. Immunostaining of isolated single fibers revealed GFP+PAX7+, GFP+MyoD+, and GFP+Myf5+ satellite-like cells on the fibers. Single-fiber cultures from these mice showed proliferation of GFP+ fibers. These results indicate two different roles of KSL cells: one leading to regeneration of damaged muscles in the early phase and the other to conversion into satellite cells in the late phase.

The vast majority of bone-marrow-derived cells integrated into mdx muscle fibers are silent despite long-term engraftment

Bone-marrow-derived cells can contribute nuclei to skeletal muscle fibers. However, serial sectioning of muscle in mdx mice implanted with GFP-labeled bone marrow reveals that only 20% of the donor nuclei chronically incorporated in muscle fibers show dystrophin (or GFP) expression, which is still higher than the expected frequency of "revertant" fibers, but there is no overall increase above controls over time. Obviously, the vast majority of incorporated nuclei either never or only temporarily turn on myogenic genes; also, incorporated nuclei eventually loose the activation of the beta-actin::GFP transgene. Consequently, we attempted to enhance the expression of dystrophin. In vivo application of the chromatin-modifying agents 5-azadeoxycytidine and phenylbutyrate as well as local damage by cardiotoxin injections caused a small increase in dystrophin-positive fibers without abolishing the appearance of "silent" nuclei. The results thus confirm that endogenous repair processes and epigenetic modifications on a small-scale lead to dystrophin expression from donor nuclei. Both effects, however, remain below functionally significant levels.

Circulating myogenic progenitors and muscle repair

The capability of bone marrow derived cells to contribute to numerous peripheral tissues may hold tremendous promise for the field of regenerative medicine. In the context of skeletal muscle disease in particular, the ability of these cells to reach sites of damage through the circulation would overcome some key limitations of current cell therapy approaches. In muscle however, this non-classical repair process takes place at an exceedingly low frequency and fails to yield any measurable functional improvement. Recent advances regarding the cell types or mechanisms involved in this phenomenon may now provide direction for strategies aimed at increasing its efficiency to therapeutic levels.

Bone marrow stromal cells generate muscle cells and repair muscle degeneration

Bone marrow stromal cells (MSCs) have great potential as therapeutic agents. We report a method for inducing skeletal muscle lineage cells from human and rat general adherent MSCs with an efficiency of 89%. Induced cells differentiated into muscle fibers upon transplantation into degenerated muscles of rats and mdx-nude mice. The induced population contained Pax7-positive cells that contributed to subsequent regeneration of muscle upon repetitive damage without additional transplantation of cells. These MSCs represent a more ready supply of myogenic cells than do the rare myogenic stem cells normally found in muscle and bone marrow.

In vivo fusion of circulating fluorescent cells with dystrophin-deficient myofibers results in extensive sarcoplasmic fluorescence expression but limited dystrophin sarcolemmal expression

To investigate the therapeutic potential of bone marrow transplantation in Duchenne muscular dystrophy, green fluorescent protein-positive (GFP+) bone marrow cells were transplanted into irradiated wild-type and dystrophin-deficient mdx mice. Tibialis anterior muscles showed fivefold to sixfold more GFP+ mononucleated cells and threefold to fourfold more GFP+ myofibers in mdx than in wild-type mice. In contrast, dystrophin expression in mdx mice remained within the level of nontransplanted mdx mice, and co-expression with GFP was rare. Longitudinal sections of 5000 myofibers showed 160 GFP+ fibers, including 9 that co-expressed dystrophin. GFP was always visualized as full-length sarcoplasmic fluorescence that exceeded the span of sample length (up to 1500 microm), whereas dystrophin expression was restricted to 11 to 28% of this length. Dystrophin expression span was much shorter in GFP+ fibers (116 +/- 46 microm) than in revertant fibers (654 +/- 409 microm). These data suggest that soluble GFP diffuses far from the fusion site with a pre-existing dystrophin(-) myofiber whereas dystrophin remains mainly expressed close to the site of fusion. Because restoration of dystrophin in whole muscle fiber length is required to expect functional improvement and clinical benefits for Duchenne muscular dystrophy, future applications of cell therapies to neuromuscular disorders could be more appropriately envisaged for replacement of defective soluble sarcoplasmic proteins.

Bone marrow contribution to skeletal muscle: a physiological response to stress

Adult bone marrow-derived stem cells (BMDC) have been shown to contribute to numerous tissues after transplantation into a new host. However, whether the participation of these cells is part of the normal response to injury remains a matter of debate. Using parabiotically joined pairs of genetically labeled and wildtype mice, we show here that irradiation-induced damage of the target tissue, injection of bone marrow into the circulation, and immunological perturbation that are consequences of bone marrow transplantation are not necessary for bone marrow contribution to myofibers. Moreover, severe toxin-induced damage is not a prerequisite, as BMDC contribution to muscle is enhanced in response to increased muscle activity resulting from muscle overloading or forced exercise. Indeed, these two forms of muscle stress result in much more rapid contribution (within 1 month) than voluntary running (6 months). These results indicate that BMDC contribute to myofibers in response to physiologic stresses encountered by healthy organisms throughout life.

Production of transgenic cloned piglets from genetically transformed fetal fibroblasts selected by green fluorescent protein

This study was performed to develop a system for porcine somatic cell nuclear transfer (SCNT) and to produce human erythropoietin (hEPO)-transgenic cloned piglets. Porcine fetal fibroblasts were transfected with an expression plasmid (phEPO-GFP). In Experiment 1, the effect of transfection of phEPO-GFP transgene on development of porcine SCNT embryos was investigated. Three fetal fibroblast cell lines (two male and one female) with or without transfected with phEPO-GFP trasngene were used as donor cells for SCNT. Lower fusion rates were observed in two lines of transfected cells as compared to those of the control cells. In Experiment 2, the effect was examined of elevated Ca2+ concentration in the fusion/activation medium on development of transfected SCNT embryos. The rates of fusion and blastocyst formation were significantly increased by supplementing 1.0 mM of CaCl2 (versus 0.1 mM) into the fusion/activation medium. In Experiment 3, the effect was studied of a chemical treatment (cytochalasin B) after electric fusion/activation (F/A) on porcine transgenic SCNT embryo development. The electric F/A + cytochalasin B treatment increased total cell number in blastocysts as compared to that of electric F/A treatment alone. In Experiment 4, transgenic cloned embryos were transferred to surrogate mothers and a total of six cloned piglets were born. Transgenic cloned piglets were confirmed by polymerase chain reaction and Southern blot analysis. From a single surrogate mother, female and male transgenic cloned piglets were produced by transferring pooled SCNT embryos derived from female and male transfected donor cells. In conclusion, a system for porcine SCNT was developed and led to the successful production of hEPO transgenic cloned piglets.

Critical variables in the conversion of marrow cells to skeletal muscle

We have studied conversion of marrow cells to skeletal muscle in cardiotoxin-injured anterior tibialis muscle in a green fluorescent protein (GFP) to C57BL/6 transplantation model and ascertained that total body irradiation (TBI) with establishment of chimerism is a critical factor. Local irradiation has little effect in lower doses and was detrimental at higher doses. Whole body (1000 cGy) with shielding of the leg or a combination of 500 cGy TBI and 500 cGy local radiations was found to give the best results. In non-obese diabetic-severe combined immunodeficient (NOD-SCID) recipients, we were able to show that conversion could occur without radiation, albeit at relatively lower levels. Within 3 days of cardiotoxin injury, GFP-positive mononuclear cells were seen in the muscle, and within 2 weeks GFP-positive muscle fibers were identified. Conversion rates were increased by increasing donor-cell dose. Timing of the cardiotoxin injury relative to the transplantation was critical. These studies show that variables in transplantation and injury are critical features of marrow-to-muscle conversions. Irradiation primarily effects conversion by promoting chimerism. These data may explain the differences in the literature for the frequency of marrow-to-skeletal muscle conversion and can set a platform for future models and perhaps clinical protocols.

Muscle injuries: biology and treatment

Muscle injuries are one of the most common traumas occurring in sports. Despite their clinical importance, few clinical studies exist on the treatment of these traumas. Thus, the current treatment principles of muscle injuries have either been derived from experimental studies or been tested only empirically. Although nonoperative treatment results in good functional outcomes in the majority of athletes with muscle injuries, the consequences of failed treatment can be very dramatic, possibly postponing an athlete's return to sports for weeks or even months. Moreover, the recognition of some basic principles of skeletal muscle regeneration and healing processes can considerably help in both avoiding the imminent dangers and accelerating the return to competition. Accordingly, in this review, the authors have summarized the prevailing understanding on the biology of muscle regeneration. Furthermore, they have reviewed the existing data on the different treatment modalities (such as medication, therapeutic ultrasound, physical therapy) thought to influence the healing of injured skeletal muscle. In the end, they extend these findings to clinical practice in an attempt to propose an evidence-based approach for the diagnosis and optimal treatment of skeletal muscle injuries.

Neuronal differentiation of bone marrow-derived stromal stem cells involves suppression of discordant phenotypes through gene silencing

Tissue engineering involves the construction of transplantable tissues in which bone marrow aspirates may serve as an accessible source of autogenous multipotential mesenchymal stem cells. Increasing reports indicate that the lineage restriction of adult mesenchymal stem cells may be less established than previously believed, and stem cell-based therapeutics await the establishment of an efficient protocol capable of achieving a prescribed phenotype differentiation. We have investigated how adult mouse bone marrow-derived stromal cells (BMSCs) are guided to neurogenic and osteogenic phenotypes. Naïve BMSCs were found surprisingly active in expression of a wide range of mRNAs and proteins, including those normally reported in terminally differentiated neuronal cells and osteoblasts. The naïve BMSCs were found to exhibit voltage-dependent membrane currents similar to the neuronally guided BMSCs, although with smaller amplitudes. Once BMSCs were exposed to the osteogenic culture condition, the neuronal characteristics quickly disappeared. Our data suggest that the loss of discordant phenotypes during BMSC differentiation cannot be explained by the selection and elimination of unfit cells from the whole BMSC population. The percent ratio of live to dead BMSCs examined did not change during the first 8-10 days in either neurogenic or osteogenic differentiation media, and cell detachment was estimated at <1%. However, during this period, bone-associated extracellular matrix genes were selectively down-regulated in neuronally guided BMSCs. These data indicate that the suppression of discordant phenotypes of differentiating adult stem cells is achieved, at least in part, by silencing of superfluous gene clusters.

Mesenchymal progenitor cells localize within hematopoietic sites throughout ontogeny

Mesenchymal stem cells (MSCs) have great clinical potential for the replacement and regeneration of diseased or damaged tissue. They are especially important in the production of the hematopoietic microenvironment, which regulates the maintenance and differentiation of hematopoietic stem cells (HSCs). In the adult, MSCs and their differentiating progeny are found predominantly in the bone marrow (BM). However, it is as yet unknown in which embryonic tissues MSCs reside and whether there is a localized association of these cells within hematopoietic sites during development. To investigate the embryonic origins of these cells, we performed anatomical mapping and frequency analysis of mesenchymal progenitors at several stages of mouse ontogeny. We report here the presence of mesenchymal progenitors, with the potential to differentiate into cells of the osteogenic, adipogenic and chondrogenic lineages, in most of the sites harboring hematopoietic cells. They first appear in the aorta-gonad-mesonephros (AGM) region at the time of HSC emergence. However, at this developmental stage, their presence is independent of HSC activity. They increase numerically during development to a plateau level found in adult BM. Additionally, mesenchymal progenitors are found in the embryonic circulation. Taken together, these data show a co-localization of mesenchymal progenitor/stem cells to the major hematopoietic territories, suggesting that, as development proceeds, mesenchymal progenitors expand within these potent hematopoietic sites.

Experimental transplantation study for possible transformation of bone marrow cells in the mouse placenta

The aim of the present study is to establish a mouse model of the transplantation of bone marrow cells into the placenta in mid-gestation. The mononuclear fraction of bone marrow cells was isolated by Ficoll gradient centrifugation from the femur bones of C57BL/6 green fluorescent protein (GFP) gene transgenic (Tg) mice. After intraperitoneal injection of pentobarbital sodium, the abdominal cavities of pregnant non-Tg (C57BL/6 or ICR) mice were opened at 9.5 days postcoitum (dpc). The mononuclear fraction of bone marrow cells from Tg mice (3-5 x 10(5)cells/3 microl) was directly injected into the placental portion of the pregnant uterus, at a depth of approximately 3 mm, using a 31-gauge injector. The placenta was sampled at 14.5 dpc. Confocal laser scanning microscopic analysis of the serial sections of the sampled placenta (150-250 sections/placenta) was carried out to detect GFP-positive cells and to assess immunostaining for cytokeratin, CD34, p57(Kip2) and prolactin. Most pregnant mice survived until sampling of the placenta at 14.5-18.5 dpc (88.9% for C57BL6 and 100% for ICR). The survival rate of fetuses from mice in which the placenta was transplanted with GFP-positive bone marrow cells was approximately 50%. A small population (0.154%) of injected bone marrow cells was retained in the placental tissue. Immunohistochemically, cytokeratin, CD34 and p57(Kip2) were positively stained in 0.062%, 4.5% and 2.1% of GFP-positive cells, respectively, while prolactin was not positive in any of the cells examined. GFP-positive bone marrow cells were successfully transplanted to the murine placenta. Future investigations of the specific antigens in bone marrow cells retained in the placenta may enable a better understanding of the local regulation of placental development.

Skeletal muscle fiber-specific green autofluorescence: potential for stem cell engraftment artifacts

Adult stem cell research has lately been plagued by controversy regarding the possibility that some adult stem cells can engraft into nonautochthonous tissues. While most reports have observed some level of engraftment, the prevalence has varied in some cases by two orders of magnitude, suggesting that major technical variations may underlie these differences, possibly outweighing the biological basis of the observations. Here we describe bright green autofluorescence in a specific subset of skeletal muscle fibers that strongly resembles emission from green fluorescent protein (GFP). Moreover, we show that oxidative muscle fibers exhibit this autofluorescence, likely due to flavin, associated with NADH dehydrogenase. Finally, we demonstrate that confocal microscopy, in conjunction with spectral scanning, can be used to distinguish between GFP and autofluorescence. We suggest this autofluorescence artifact may account for some of the discrepancies in this field, particularly those describing skeletal muscle engraftment.

Isolation of Adult Mouse Myogenic Progenitors

Skeletal muscle regeneration in adults is thought to occur through the action of myogenic satellite cells located in close association with mature muscle fibers; however, these precursor cells have not been prospectively isolated, and recent studies have suggested that additional muscle progenitors, including cells of bone marrow or hematopoietic origin, may exist. To clarify the origin(s) of adult myogenic cells, we used phenotypic, morphological, and functional criteria to identify and prospectively isolate a subset of myofiber-associated cells capable at the single cell level of generating myogenic colonies at high frequency. Importantly, although muscle-engrafted cells from marrow and/or circulation localized to the same anatomic compartment as myogenic satellite cells and expressed some though not all satellite cell markers, they displayed no intrinsic myogenicity. Together, these studies describe the clonal isolation of functional adult myogenic progenitors and demonstrate that these cells do not arise from hematopoietic or other bone marrow or circulating precursors.

Mac-1(low) early myeloid cells in the bone marrow-derived SP fraction migrate into injured skeletal muscle and participate in muscle regeneration

Recent studies have shown that bone marrow (BM) cells, including the BM side population (BM-SP) cells that enrich hematopoietic stem cells (HSCs), are incorporated into skeletal muscle during regeneration, but it is not clear how and what kinds of BM cells contribute to muscle fiber regeneration. We found that a large number of SP cells migrated from BM to muscles following injury in BM-transplanted mice. These BM-derived SP cells in regenerating muscles expressed different surface markers from those of HSCs and could not reconstitute the mouse blood system. BM-derived SP/Mac-1(low) cells increased in number in regenerating muscles following injury. Importantly, our co-culture studies with activated satellite cells revealed that this fraction carried significant potential for myogenic differentiation. By contrast, mature inflammatory (Mac-1(high)) cells showed negligible myogenic activities. Further, these BM-derived SP/Mac-1(low) cells gave rise to mononucleate myocytes, indicating that their myogenesis was not caused by stochastic fusion with host myogenic cells, although they required cell-to-cell contact with myogenic cells for muscle differentiation. Taken together, our data suggest that neither HSCs nor mature inflammatory cells, but Mac-1(low) early myeloid cells in the BM-derived SP fraction, play an important role in regenerating skeletal muscles.

Hematopoietic contribution to skeletal muscle regeneration by myelomonocytic precursors

Adult bone marrow-derived cells can participate in muscle regeneration after bone marrow transplantation. In recent studies a single hematopoietic stem cell (HSC) was shown to give rise to cells that not only reconstituted all of the lineages of the blood, but also contributed to mature muscle fibers. However, the relevant HSC derivative with this potential has not yet been definitively identified. Here we use fluorescence-activated cell sorter-based protocols to test distinct hematopoietic fractions and show that only fractions containing c-kit(+) immature myelomonocytic precursors are capable of contributing to muscle fibers after i.m. injection. Although these cells belong to the myeloid lineage, they do not include mature CD11b(+) myelomonocytic cells, such as macrophages. Of the four sources of mature macrophages tested that were derived either from monocytic culture, bone marrow, peripheral blood after granulocyte colony-stimulating factor mobilization, or injured muscle, none contributed to muscle. In addition, after transplantation of bone marrow isolated from CD11b-Cre-transgenic mice into the Cre-reporter strain (Z/EG), no GFP myofibers were detected, demonstrating that macrophages expressing CD11b do not fuse with myofibers. Irrespective of the underlying mechanisms, these data suggest that the HSC derivatives that integrate into regenerating muscle fibers exist in the pool of hematopoietic cells known as myelomonocytic progenitors.

Nuclear reprogramming: A key to stem cell function in regenerative medicine

The goal of regenerative medicine is to restore form and function to damaged tissues. One potential therapeutic approach involves the use of autologous cells derived from the bone marrow (bone marrow-derived cells, BMDCs). Advances in nuclear transplantation, experimental heterokaryon formation and the observed plasticity of gene expression and phenotype reported in multiple phyla provide evidence for nuclear plasticity. Recent observations have extended these findings to show that endogenous cells within the bone marrow have the capacity to incorporate into defective tissues and be reprogrammed. Irrespective of the mechanism, the potential for new gene expression patterns by BMDCs in recipient tissues holds promise for developing cellular therapies for both proliferative and post-mitotic tissues.

Purification and cell-surface marker characterization of quiescent satellite cells from murine skeletal muscle by a novel monoclonal antibody

A novel monoclonal antibody, SM/C-2.6, specific for mouse muscle satellite cells was established. SM/C-2.6 detects mononucleated cells beneath the basal lamina of skeletal muscle, and the cells co-express M-cadherin. Single fiber analyses revealed that M-cadherin+ mononucleated cells attaching to muscle fibers are stained with SM/C-2.6. SM/C-2.6+ cells, which were freshly purified by FACS from mouse skeletal muscle, became MyoD+ in vitro in proliferating medium, and the cells differentiated into desmin+ and nuclear-MyoD+ myofibers in vitro when placed under differentiation conditions. When the sorted cells were injected into mdx mouse muscles, donor cells differentiated into muscle fibers. Flow cytometric analyses of SM/C-2.6+ cells showed that the quiescent satellite cells were c-kit-, Sca-1-, CD34+, and CD45-. More, SM/C-2.6+ cells were barely included in the side population but in the main population of cells in Hoechst dye efflux assay. These results suggest that SM/C-2.6 identifies and enriches quiescent satellite cells from adult mouse muscle, and that the antibody will be useful as a powerful tool for the characterization of cellular and molecular mechanisms of satellite cell activation and proliferation.

Muscle satellite cells adopt divergent fates: a mechanism for self-renewal?

Growth, repair, and regeneration of adult skeletal muscle depends on the persistence of satellite cells: muscle stem cells resident beneath the basal lamina that surrounds each myofiber. However, how the satellite cell compartment is maintained is unclear. Here, we use cultured myofibers to model muscle regeneration and show that satellite cells adopt divergent fates. Quiescent satellite cells are synchronously activated to coexpress the transcription factors Pax7 and MyoD. Most then proliferate, down-regulate Pax7, and differentiate. In contrast, other proliferating cells maintain Pax7 but lose MyoD and withdraw from immediate differentiation. These cells are typically located in clusters, together with Pax7−ve progeny destined for differentiation. Some of the Pax7+ve/MyoD−ve cells then leave the cell cycle, thus regaining the quiescent satellite cell phenotype. Significantly, noncycling cells contained within a cluster can be stimulated to proliferate again. These observations suggest that satellite cells either differentiate or switch from terminal myogenesis to maintain the satellite cell pool.

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

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

Robust conversion of marrow cells to skeletal muscle with formation of marrow-derived muscle cell colonies: a multifactorial process

OBJECTIVE

Murine marrow cells are capable of repopulating skeletal muscle fibers. A point of concern has been the "robustness" of such conversions. We have investigated the impact of type of cell delivery, muscle injury, nature of delivered cell, and stem cell mobilizations on marrow-to-muscle conversion.

METHODS

We transplanted green fluorescence protein (GFP)-transgenic marrow into irradiated C57BL/6 mice and then injured anterior tibialis muscle by cardiotoxin. One month after injury, sections were analyzed by standard and deconvolutional microscopy for expression of muscle and hematopoietic markers.

RESULTS

Irradiation was essential to conversion, although whether by injury or induction of chimerism is not clear. Cardiotoxin- and, to a lesser extent, PBS-injected muscles showed significant number of GFP(+) muscle fibers, while uninjected muscles showed only rare GFP(+) cells. Marrow conversion to muscle was increased by two cycles of G-CSF mobilization and to a lesser extent by G-CSF and steel or GM-CSF. Transplantation of female GFP to male C57BL/6 and GFP to ROSA26 mice showed fusion of donor cells to recipient muscle. High numbers of donor-derived muscle colonies and up to 12% GFP(+) muscle cells were seen after mobilization or direct injection. These levels of donor muscle chimerism approach levels that could be clinically significant in developing strategies for the treatment of muscular dystrophies.

CONCLUSION

In summary, the conversion of marrow to skeletal muscle cells is based on cell fusion and is critically dependent on injury. This conversion is also numerically significant and increases with mobilization.

Adult bone marrow-derived stem cells in muscle connective tissue and satellite cell niches

Skeletal muscle includes satellite cells, which reside beneath the muscle fiber basal lamina and mainly represent committed myogenic precursor cells, and multipotent stem cells of unknown origin that are present in muscle connective tissue, express the stem cell markers Sca-1 and CD34, and can differentiate into different cell types. We tracked bone marrow (BM)-derived stem cells in both muscle connective tissue and satellite cell niches of irradiated mice transplanted with green fluorescent protein (GFP)-expressing BM cells. An increasing number of GFP+ mononucleated cells, located both inside and outside of the muscle fiber basal lamina, were observed 1, 3, and 6 months after transplantation. Sublaminal cells expressed unambiguous satellite cell markers (M-cadherin, Pax7, NCAM) and fused into scattered GFP+ muscle fibers. In muscle connective tissue there were GFP+ cells located close to blood vessels that expressed the ScaI or CD34 stem-cell antigens. The rate of settlement of extra- and intralaminal compartments by BM-derived cells was compatible with the view that extralaminal cells constitute a reservoir of satellite cells. We conclude that both muscle satellite cells and stem cell marker-expressing cells located in muscle connective tissue can derive from BM in adulthood.

Myogenic fusion of human bone marrow stromal cells, but not hematopoietic cells

Following marrow transplantation in both patients and animals, cells containing donor nuclei have been detected in a variety of nonhematopoietic tissue. Whether this phenomenon represents transdifferentiation of marrow-derived cells, infiltration of blood cells, or cell fusion is still controversial. In muscle, where cell fusion occurs during normal myogenesis, fusion of marrow-derived cells with resident myotubes is a likely explanation. We tested 8 subpopulations of human bone marrow for their ability to fuse with mouse C2C12 myoblast cells. Relatively high fusion efficiency was observed with marrow stromal cells whereas hematopoietic cells, including populations enriched for stem cells, progenitor cells, and monocytes were refractory to fusion. Mouse myotubes containing human nuclei also contained transcripts for human muscle-specific genes. Injection in vivo of human stromal cells expressing green fluorescent protein (GFP) into the regenerating tibialis anterior muscle of nonobese diabetic-severe combined immunodeficient (NOD/SCID) beta 2m(-/-) mice resulted in regenerating mouse muscle fibers expressing GFP. These data suggest that marrow-derived cells contribute to myogenesis through fusion and that stromal cells are better fusion partners than hematopoietic cells.

Plasticity of Adult Stem Cells

Recent years have seen much excitement over the possibility that adult mammalian stem cells may be capable of differentiating across tissue lineage boundaries, and as such may represent novel, accessible, and very versatile effectors of therapeutic tissue regeneration. Yet studies proposing such "plasticity" of adult somatic stem cells remain controversial, and in general, existing evidence suggests that in vivo such unexpected transformations are exceedingly rare and in some cases can be accounted for by equally unexpected alternative explanations.

Single hematopoietic stem cells generate skeletal muscle through myeloid intermediates

Recent studies have shown that cells from the bone marrow can give rise to differentiated skeletal muscle fibers. However, the mechanisms and identities of the cell types involved have remained unknown, and the validity of the observation has been questioned. Here, we use transplantation of single CD45+ hematopoietic stem cells (HSCs) to demonstrate that the entire circulating myogenic activity in bone marrow is derived from HSCs and their hematopoietic progeny. We also show that ongoing muscle regeneration and inflammatory cell infiltration are required for HSC-derived contribution, which does not occur through a myogenic stem cell intermediate. Using a lineage tracing strategy, we show that myofibers are derived from mature myeloid cells in response to injury. Our results indicate that circulating myeloid cells, in response to inflammatory cues, migrate to regenerating skeletal muscle and stochastically incorporate into mature myofibers.

Participation of bone marrow-derived cells in long-term repair processes after experimental stroke

Bone marrow-derived cells participate in remodeling processes of many ischemia-associated diseases, which has raised hopes for the use of bone marrow as a source for cell-based therapeutic approaches. To study the participation of bone marrow-derived cells in a stroke model, bone marrow from C57BL/6-TgN(ACTbEGFP)1Osb mice that express green fluorescent protein (GFP) in all cells was transplanted into C57BL/6J mice. The recipient mice underwent permanent occlusion of the middle cerebral artery, and bone marrow-derived cells were tracked by fluorescence. The authors investigated the involvement of bone marrow-derived cells in repair processes 6 weeks and 6 months after infarction. Six weeks after occlusion of the artery, more than 90% of the GFP-positive cells in the infarct border zone were microglial cells. Very few GFP-positive cells expressed endothelial markers in the infarct/infarct border zone, and no bone marrow-derived cells transdifferentiated into astrocytes, neurons, or oligodendroglial cells at all time points investigated. The results indicate the need for additional experimental studies to determine whether therapeutic application of nonselected bone marrow will replenish brain cells beyond an increase in microglial engraftment.

Stem cells in adult skeletal muscle

Muscle satellite cells are a self-renewing pool of stem cells that give rise to daughter myogenic precursor cells in adult skeletal muscle, where they function in postnatal tissue growth and regeneration. Adult skeletal muscle also contains a novel stem cell population purified as a side population (SP), which actively excludes Hoechst 33342 dye. Muscle SP cells that express the hematopoietic stem cell marker Sca-1 possess the ability to differentiate into hematopoietic cells, skeletal muscle, and satellite cells following transplantation. The muscle SP fraction also contains cells expressing the hematopoietic marker CD45 that are capable of differentiation into hematopoietic cells and muscle cells. Thus, these novel muscle stem cells appear to have characteristics similar to those of hematopoietic stem cells, and can participate in muscle regeneration. This review outlines recent findings regarding different stem cell populations in skeletal muscle, and discusses their involvement in muscle regeneration.

Biological progression from adult bone marrow to mononucleate muscle stem cell to multinucleate muscle fiber in response to injury

Adult bone marrow-derived cells (BMDC) are shown to contribute to muscle tissue in a step-wise biological progression. Following irradiation-induced damage, transplanted GFP-labeled BMDC become satellite cells: membrane-ensheathed mononucleate muscle stem cells. Following a subsequent exercise-induced damage, GFP-labeled multinucleate myofibers are detected. Isolated GFP-labeled satellite cells are heritably myogenic. They express three characteristic muscle markers, are karyotypically diploid, and form clones that can fuse into multinucleate cells in culture or into myofibers after injection into mouse muscles. These results suggest that two temporally distinct injury-related signals first induce BMDC to occupy the muscle stem cell niche and then to help regenerate mature muscle fibers. The stress-induced progression of BMDC to muscle satellite cell to muscle fiber results in a contribution to as many as 3.5% of muscle fibers and is due to developmental plasticity in response to environmental cues.

Myogenic specification of side population cells in skeletal muscle

Skeletal muscle contains myogenic progenitors called satellite cells and muscle-derived stem cells that have been suggested to be pluripotent. We further investigated the differentiation potential of muscle-derived stem cells and satellite cells to elucidate relationships between these two populations of cells. FACS(R) analysis of muscle side population (SP) cells, a fraction of muscle-derived stem cells, revealed expression of hematopoietic stem cell marker Sca-1 but did not reveal expression of any satellite cell markers. Muscle SP cells were greatly enriched for cells competent to form hematopoietic colonies. Moreover, muscle SP cells with hematopoietic potential were CD45 positive. However, muscle SP cells did not differentiate into myocytes in vitro. By contrast, satellite cells gave rise to myocytes but did not express Sca-1 or CD45 and never formed hematopoietic colonies. Importantly, muscle SP cells exhibited the potential to give rise to both myocytes and satellite cells after intramuscular transplantation. In addition, muscle SP cells underwent myogenic specification after co-culture with myoblasts. Co-culture with myoblasts or forced expression of MyoD also induced muscle differentiation of muscle SP cells prepared from mice lacking Pax7 gene, an essential gene for satellite cell development. Therefore, these data document that satellite cells and muscle-derived stem cells represent distinct populations and demonstrate that muscle-derived stem cells have the potential to give rise to myogenic cells via a myocyte-mediated inductive interaction.

Bone marrow-derived CD45+ and CD45- cells reside in skeletal muscle

In this report we examined the kinetics of accumulation in muscle tissue of donor-derived CD45+ and CD45- cells from one to several weeks post-bone marrow transplantation. Only after hemopoiesis is fully reconstituted are CD45+ cells recovered from muscle tissue in proportions similar to ones we reported earlier in non-transplanted mice. Furthermore, using marked donor bone marrow cells, we document that in addition to CD45+ cells, donor-derived CD45- cells with certain phenotypic characteristics progressively accumulate in muscle post-transplantation. The turnover rate and physiologic roles of these populations in muscle are unclear and require further study.