In vitro and in vivo hematopoietic potential of human stem cells residing in muscle tissue

Characterization of distinct mesenchymal-like cell populations from human skeletal muscle in situ and in vitro

Human skeletal muscle is an essential source of various cellular progenitors with potential therapeutic perspectives. We first used extracellular markers to identify in situ the main cell types located in a satellite position or in the endomysium of the skeletal muscle. Immunohistology revealed labeling of cells by markers of mesenchymal (CD13, CD29, CD44, CD47, CD49, CD62, CD73, CD90, CD105, CD146, and CD15 in this study), myogenic (CD56), angiogenic (CD31, CD34, CD106, CD146), hematopoietic (CD10, CD15, CD34) lineages. We then analysed cell phenotypes and fates in short- and long-term cultures of dissociated muscle biopsies in a proliferation medium favouring the expansion of myogenic cells. While CD56(+) cells grew rapidly, a population of CD15(+) cells emerged, partly from CD56(+) cells, and became individualized. Both populations expressed mesenchymal markers similar to that harboured by human bone marrow-derived mesenchymal stem cells. In differentiation media, both CD56(+) and CD15(+) cells shared osteogenic and chondrogenic abilities, while CD56(+) cells presented a myogenic capacity and CD15(+) cells presented an adipogenic capacity. An important proportion of cells expressed the CD34 antigen in situ and immediately after muscle dissociation. However, CD34 antigen did not persist in culture and this initial population gave rise to adipogenic cells. These results underline the diversity of human muscle cells, and the shared or restricted commitment abilities of the main lineages under defined conditions.

Defining the heterogeneity of skeletal muscle-derived side and main population cells isolated immediately ex vivo

Myoblast transfer therapy for Duchenne muscular dystrophy (DMD) largely fails due to cell death and inability of transplanted cells to engraft in diseased muscles. One method attempting to enrich for cell subpopulations is the Hoechst 33342 dye exclusion assay, yielding a side population (SP) thought to be progenitor enriched and a main population (MP). However, in vitro and transplant studies yielded inconsistent results relative to downstream progeny. Cell surface markers expressed by skeletal muscle-derived MP and SP cells have not been fully characterized directly ex vivo. Using flow cytometry, MP and SP cells were characterized based on their expression of several well-accepted progenitor cell antigens. Both the MP and SP populations are heterogeneous and overlapping in the cells they contain. The percentages of cells in each population vary with species and specific muscle examined. MP and SP populations contain both satellite and multipotent progenitor cells, based on expression of CD34, Sca-1, Pax7, and M-cadherin. Thus, isolation using this procedure cannot be used to predict downstream differentiation outcomes, and explains the conflicting literature on these cells. Hoechst dye also results in significant mortality of sorted cells. As defined subpopulations are easily obtained using flow cytometry, sorting immediately ex vivo based on accepted myogenic precursor cell markers will yield superior results in terms of cell homogeneity for transplantation therapy.

Isolation and characterization of human muscle-derived cells

OBJECTIVES

To isolate and characterize human muscle-derived cells (MDCs) for future management applications on lower urinary tract symptoms, including stress urinary incontinence and bladder reconstitution. The development of muscle stem cells for transplantation or gene transfer in patients with muscle disorders has become more attractive and challenging recently.

METHODS

Human MDCs were isolated from the skeletal muscles of the limbs. The muscle tissues were minced, digested at 37 degrees C by 0.2% collagenase, trypsinized, filtered, and cultured in F12 medium with 15% fetal bovine serum at 37 degrees C. Human MDCs were then isolated using a modified preplate technique. After isolation, the MDCs were characterized by immunohistochemistry, flow cytometry, and indirect immunofluorescence.

RESULTS

The growth doubling time of the MDCs was approximately 24 hours. Immunohistochemistry study was performed with the stem cell markers CD34, CD117, vascular cell adhesion molecule, and vascular endothelial growth factor receptor 2, and the relative stem cell position was identified. Positive immunofluorescence outcomes were found with the stem cell markers, myoblast markers CXCR4, CD56, desmin, and a fibroblast marker AB-1. Flow cytometry analysis identified markers CD34 and CD56 in the isolated MDCs, with a percentage of 5.12% and 10.34%, respectively.

CONCLUSIONS

The isolation and characterization of human MDCs was successfully achieved. Human MDCs might have the potential to be a novel tool for the management of stress urinary incontinence and bladder reconstitution.

In vitro and in vivo evaluation of the haematopoietic potential of skeletal muscle in a non-human primate model

This study was aimed at evaluating the in vitro and in vivo haematopoietic potential in macaque skeletal muscle cells. Biopsy samples showed the presence of CD34(+) (7.6%), CD90(+) (8.4%), CD117(+), CD31(+), side population (SP) cells (7-10%) and a low number of CD45(+) cells. In clonogenic and long-term culture-initiating cell assays, no haematopoietic potential could be detected in either total mononuclear cells or SP cells. Regarding in vivo studies, two animals were transplanted with unfractionated fresh muscle cells after lethal irradiation. Both animals died early after transplant without any evidence of haematopoietic reconstitution. In two other monkeys, harvested muscle cells were frozen and secondarily marked using a green fluorescent protein (GFP)-lentiviral vector. After sublethal irradiation, both animals were transplanted with GFP-expressing muscle cells followed by a bone marrow rescue. Both animals had haematopoietic reconstitution at days 22 and 25, but no GFP-expressing haematopoietic cells could be detected by flow cytometry, either in the blood or in clonogenic cells from marrow aspirates. Using PCR assays, GFP(+) cells were detected in a single marrow sample of one animal at 41 days after transplantation. These results strongly suggest that as opposed to murine muscle, the non-human primate skeletal muscle does not harbour cells with a straightforward haematopoietic potential.

RNAi therapeutics: principles, prospects and challenges

RNA interference (RNAi) was discovered less than a decade ago and already there are human clinical trials in progress or planned. A major advantage of RNAi versus other antisense based approaches for therapeutic applications is that it utilizes cellular machinery that efficiently allows targeting of complementary transcripts, often resulting in highly potent down-regulation of gene expression. Despite the excitement about this remarkable biological process for sequence specific gene regulation, there are a number of hurdles and concerns that must be overcome prior to making RNAi a real therapeutic modality, which include off-target effects, triggering of type I interferon responses, and effective delivery in vivo. This review discusses mechanistic aspects of RNAi, the potential problem areas and solutions and therapeutic applications. It is anticipated that RNAi will be a major therapeutic modality within the next several years, and clearly warrants intense investigation to fully understand the mechanisms involved.

Muscling in on stem cells

Skeletal muscle is one of the few adult tissues that possesses the capacity for regeneration (restoration of lost functional tissue) as opposed to repair. This capacity is due to the presence of 'muscle stem cells' known as satellite cells. Detailed investigation of these cells over the past 50 years has revealed that both these and other cells within the skeletal muscle complex are capable of regenerating both muscle and other cell types as well. Here, we review this information, and suggest that skeletal muscle is an exciting reservoir of cells for regenerating skeletal muscle itself, as well as other cell types.

Identification of tissue-specific vasculogenic cells originating from murine uterus

Endometrium is a highly regenerative adult tissue that undergoes repeated degeneration and regeneration following menarche. Therefore, it is believed that endometrium contains stem and/or progenitor cells in order to compensate for the regeneration of tissue components. We report here that stem-like cells having vasculogenic potential are present in the uterus. Enzymatically extracted cells from murine uteri were characterized and fractionated into four subpopulations by flowcytometry; CD34(+)/45(-) (Ut-34), CD34(-)/45(-) (Ut-DN) and the remaining CD45(+) cell fractions (CD34(+)/45(+) and CD34(-)/45(+) cells). The Ut-34 and Ut-DN fractions were mostly negative for putative endothelial cell (EC) markers, such as CD31, Flk-1, c-kit and VE-cadherin, although the Ut-DN fraction contained 2.8% CD31(+) cells. Ut-DN cells were further divided into CD31(+) and CD31(-) fractions. Three cell populations were obtained from green fluorescence protein (GFP) transgenic mice and were transplanted into injured wild-type mouse skeletal muscle. At 4 weeks after cell transplantation, donor-derived vascular smooth muscle and ECs were observed in the injured recipient muscle. A similar trend was observed in the Ut-34 group, but differentiation into vascular smooth muscle was predominant. In contrast, the Ut-DN/31(+) cell-transplanted group showed preferential differentiation into vascular ECs, thus suggesting that they were relatively committed preexisting ECs. These characteristics were also seen in vitro, in clonal cell cultures. Interestingly, donor derived Ut-DN/31(+), Ut-DN/31(-) and Ut-34 cells could not be identified after bone marrow (BM) transplantation, thus confirming that they are not derived from BM. It therefore appeared that tissue-specific vasculogenic cells are present in the murine uterus and that they exhibit vascular formation, even in different tissue microenvironments.

A unique population of bone marrow cells migrates to skeletal muscle via hepatocyte growth factor/c-met axis

Cells expressing the CD45-associated hematopoietic marker are predominantly present in the mammalian bone marrow (BM), but have recently been shown to also reside in the skeletal muscle and potentially participate in muscle repair. Despite the consistent observations, the specific relationship and potential migration of CD45+ cells in the BM versus CD45+ cells residing in the muscle remain unclear, in addition to any understanding of the factors that may regulate the trafficking of CD45+-derived BM cells to skeletal muscle upon i.v. transplantation. Here, transplantation of BM-derived cells fully replaced the CD45+ fraction of skeletal muscle, but gave rise to progenitor cells with distinct hematopoietic lineage capacity from CD45+ cells residing in the BM. Using transwell migration assays, a subset of BM cells was shown to migrate exclusively to mature skeletal muscle cells and not BM-derived stromal cells. Unlike migration of BM cells to stroma, myofiber induced migration of BM-derived cells was not affected by stromal-derived factor-1 (SDF-1) neutralization or CXCR4-blocking antibody, but could be reduced by addition of c-met-blocking antibody and augmented by hepatocyte growth factor (HGF), the putative ligand for c-met. We suggest that the BM compartment consists of a functionally complex population of CD45+ progenitors that includes a subset of HGF/c-met responsive cells capable of migration to skeletal muscle. This previously unappreciated basis for cellular tracking now aids in defining regulatory networks that distinguish the stem cell niche of the BM versus skeletal muscle microenvironments.

Perivascular cells expressing annexin A5 define a novel mesenchymal stem cell-like population with the capacity to differentiate into multiple mesenchymal lineages

The annexin A5 gene (Anxa5) was recently found to be expressed in the developing and adult vascular system as well as the skeletal system. In this paper, the expression of an Anxa5-lacZ fusion gene was used to define the onset of expression in the vasculature and to characterize these Anxa5-lacZ-expressing vasculature-associated cells. After blastocyst implantation, Anxa5-lacZ-positive cells were first detected in extra-embryonic tissues and in angioblast progenitors forming the primary vascular plexus. Later, expression is highly restricted to perivascular cells in most blood vessels resembling pericytes or vascular smooth muscle cells. Viable Anxa5-lacZ+ perivascular cells were isolated from embryos as well as adult brain meninges by specific staining with fluorescent X-gal substrates and cell-sorting. These purified lacZ+ cells specifically express known markers of pericytes, but also markers characteristic for stem cell populations. In vitro and in vivo differentiation experiments show that this cell pool expresses early markers of chondrogenesis, is capable of forming a calcified matrix and differentiates into adipocytes. Hence, Anxa5 expression in perivascular cells from mouse defines a novel population of cells with a distinct developmental potential.

Diseased muscles that lack dystrophin or laminin-alpha2 have altered compositions and proliferation of mononuclear cell populations

Background

Multiple types of mononucleate cells reside among the multinucleate myofibers in skeletal muscles and these mononucleate cells function in muscle maintenance and repair. How neuromuscular disease might affect different types of muscle mononucleate cells had not been determined. In this study, therefore, we examined how two neuromuscular diseases, dystrophin-deficiency and laminin-α2-deficiency, altered the proliferation and composition of different subsets of muscle-derived mononucleate cells.

Methods

We used fluorescence-activated cell sorting combined with bromodeoxyuridine labeling to examine proliferation rates and compositions of mononuclear cells in diseased and healthy mouse skeletal muscle. We prepared mononucleate cells from muscles of mdx (dystrophin-deficient) or Lama2^-/- (laminin-α2-deficient) mice and compared them to cells from healthy control muscles. We enumerated subsets of resident muscle cells based on Sca-1 and CD45 expression patterns and determined the proliferation of each cell subset in vivo by BrdU incorporation.

Results

We found that the proliferation and composition of the mononucleate cells in dystrophin-deficient and laminin-α2-deficient diseased muscles are different than in healthy muscle. The mdx and Lama2^-/- muscles showed similar significant increases in CD45⁺ cells compared to healthy muscle. Changes in proliferation, however, differed between the two diseases with proliferation increased in mdx and decreased in Lama2^-/- muscles compared to healthy muscles. In particular, the most abundant Sca-1^-/CD45^- subset, which contains muscle precursor cells, had increased proliferation in mdx muscle but decreased proliferation in Lama2^-/- muscles.

Conclusion

The similar increases in CD45⁺ cells, but opposite changes in proliferation of muscle precursor cells, may underlie aspects of the distinct pathologies in the two diseases.

Potential and origin of the hematopoietic population in human skeletal muscle

While mononuclear cells isolated from murine skeletal muscle were shown to be capable of hematopoietic activity, similar hematopoietic cells (HC) recently were reported to exist in primate muscle. We investigated muscle-derived HC from young and adult human subjects. Although hematopoietic stem cells were rare in muscle, their frequency nonetheless was approximately four times greater than in peripheral blood. These cells in muscle appeared to originate from CD45(+) bone marrow cells. Our results suggested an additional function of human skeletal muscle as a reservoir of HC.

Hematopoietic capacity of adult human skeletal muscle is negligible

Using experimental mouse models, hematopoietic potential has been shown to exist within skeletal muscle. In humans, the clinical utility of using muscle-derived hematopoietic progenitors remains uncertain. Here, we evaluate the hematopoietic potential of human skeletal muscle. De novo adult muscle contained markedly reduced levels of hematopoietic colony-forming units (hCFU) and negligible responsiveness to hematopoietic ex vivo culture conditions that augment hematopoietic activity of fetal muscle. Neither fetal nor adult muscle yielded significant engraftment in transplanted immune-deficient mice. Although adult muscle possessed 1.5+/-0.9 hCFU/g, similar hematopoietic activity (2.3+/-0.17 hCFU) could also be demonstrated from as little as 3-10 microl of contaminating peripheral blood. We suggest that the clinical utility of adult skeletal muscle as an alternative source of hematopoietic cells in humans appears limited due to the low yield of blood-forming precursors and their lack of responsiveness to ex vivo expansion.

Muscle stem cells and regenerative myogenesis

A population of myogenic progenitors termed satellite cells undertakes postnatal development and repair of skeletal muscle. Studies have indicated that atypical myogenic precursors can also participate in muscle regeneration. The source of this regenerative capacity has been attributed to "adult stem cells" that represent poorly understood multipotent cell lineages, believed to reside in all adult tissue populations. Here we review the origin and location of muscle satellite cells and stem cells, as well as the mechanisms by which they may be specified. We discuss how the experimental models utilized raise important questions regarding the validity of extrapolating these findings.

Unlocking the potential of the human genome with RNA interference

The discovery of RNA interference (RNAi) may well be one of the transforming events in biology in the past decade. RNAi can result in gene silencing or even in the expulsion of sequences from the genome. Harnessed as an experimental tool, RNAi has revolutionized approaches to decoding gene function. It also has the potential to be exploited therapeutically, and clinical trials to test this possibility are already being planned.

Adult stem cells

Development of a multicellular organism is accomplished through a series of events that are preprogrammed in the genome. These events encompass cellular proliferation, lineage commitment, lineage progression, lineage expression, cellular inhibition, and regulated apoptosis. The sequential progression of cells through these events results in the formation of the differentiated cells, tissues, and organs that constitute an individual. Although most cells progress through this sequence during development, a few cells leave the developmental continuum to become reserve precursor cells. The reserve precursor cells are involved in the continual maintenance and repair of the tissues and organs throughout the life span of the individual. Until recently it was generally assumed that the precursor cells in postnatal individuals were limited to lineage-committed progenitor cells specific for various tissues. However, studies by Young, his colleagues, and others have demonstrated the presence of two categories of precursor cells that reside within the organs and tissues of postnatal animals. These two categories of precursor cells are lineage-committed (multipotent, tripotent, bipotent, and unipotent) progenitor cells and lineage-uncommitted pluripotent (epiblastic-like, ectodermal, mesodermal, and endodermal) stem cells. These reserve precursor cells provide for the continual maintenance and repair of the organism after birth.

Regeneration of skeletal muscle from transplanted immortalised myoblasts is oligoclonal

Myoblasts transplanted into muscles of recipient mice mostly die, only a minor stem cell-like subpopulation surviving and participating in muscle regeneration. To investigate this phenomenon further, we used a retrovirus expressing beta-galactosidase to provide a unique marker for satellite-cell-derived muscle precursor cells, before transplanting them into myopathic mdx nu/nu mouse muscle. We employed inverse polymerase chain reaction to identify viral integrations, to follow the fate of clones present within the injected cells. Mass-infected cultures contained many marked clones, some of which contributed disproportionately to muscle regeneration. Although no particular clones showed overall predominance, some were present in more than one injected muscle, an eventuality unlikely to arise by chance. Conversely, in grafts of muscle precursor cells that had either been labelled as sparse satellite-cell derived cultures, or had been cloned, all clones were shown to be able to survive and form muscle in vivo. Moreover, all clones contributed to further generations of new-formed muscle fibres following a series of injuries administered to injected muscles, demonstrating that some cells of each clone had been retained as stem-cell-like muscle precursors. Furthermore, retrovirally marked satellite-cell-derived clones were derived from muscles that had been injected with marked muscle precursor cells. These cells formed muscle following their transplantation into a new host mouse, confirming their stem cell properties.

Extracellular nucleotides are potent stimulators of human hematopoietic stem cells in vitro and in vivo

Although extracellular nucleotides support a wide range of biologic responses of mature blood cells, little is known about their effect on blood cell progenitor cells. In this study, we assessed whether receptors for extracellular nucleotides (P2 receptors [P2Rs]) are expressed on human hematopoietic stem cells (HSCs), and whether activation by their natural ligands, adenosine triphosphate (ATP) and uridine triphosphate (UTP), induces HSC proliferation in vitro and in vivo. Our results demonstrated that CD34(+) HSCs express functional P2XRs and P2YRs of several subtypes. Furthermore, stimulation of CD34(+) cells with extracellular nucleotides caused a fast release of Ca(2+) from intracellular stores and an increase in ion fluxes across the plasma membrane. Functionally, ATP and, to a higher extent, UTP acted as potent early acting growth factors for HSCs, in vitro, because they strongly enhanced the stimulatory activity of several cytokines on clonogenic CD34(+) and lineage-negative CD34(-) progenitors and expanded more primitive CD34(+)-derived long-term culture-initiating cells. Furthermore, xenogenic transplantation studies showed that short-term preincubation with UTP significantly expanded the number of marrow-repopulating HSCs in nonobese diabetic/severe combined immunodeficiency mice. Our data suggest that extracellular nucleotides may provide a novel and powerful tool to modulate HSC functions.

Systemic delivery of human microdystrophin to regenerating mouse dystrophic muscle by muscle progenitor cells

Cell-based therapy for Duchenne muscular dystrophy patients and mdx mice has proven to be a safe but ineffective form of treatment. Recently, a group of cells called muscle side population (SP) cells have been isolated based on their ability to efflux the DNA-binding dye Hoechst. To understand the potential of skeletal muscle SP cells to serve as precursors for muscle, SP cells from the two mice strains mdx(5cv) and C57BL/6N were isolated, transduced, and transplanted. Under coculture conditions with myogenic cells, some cells within the SP cell population can give rise to early Pax7-positive satellite cells and other later stage myogenic cells. Transduced SP cells were transplanted via the tail vein and were shown to successfully deliver enhanced GFP and human microdystrophin to the skeletal muscle of nonirradiated mdx(5cv) mice, thus demonstrating their ability to travel through the capillaries and enter into damaged muscle. These results demonstrate that i.v. delivery of genes via SP cells is possible and that these SP cells are capable of recapitulating the myogenic lineage. Because this approach shows definitive engraftment by using autologous transplantation of noninjured recipients, our data may have substantial implications for therapy of muscular dystrophy.

Evaluation of Hematopoietic Potential Generated by Transplantation of Muscle-Derived Stem Cells in Mice

Muscle tissue of adult mice has been shown to contain stem cells with hematopoietic repopulation ability in vivo. To determine the functional characteristics of stem cells giving rise to this hematopoietic activity, we have performed hematopoietic reconstitution experiments by the use of muscle versus marrow transplantation in lethally irradiated mice and followed the fate of transplanted cells by Y-chimerism using PCR and fluorescence in situ hybridization (FISH) analysis. We report here that transplantation of murine muscle generate a major hematopoietic chimerism at the level of CFU-C, CFU-S, and terminally-differentiated cells in three generations of lethally irradiated mice followed up to 1 year after transplantation. This potential is totally abolished when muscle grafts were performed by the use of muscle from previously irradiated mice. As compared to marrow transplantation, muscle transplants were able to generate similar potencies to give rise to myeloid, T, B, and natural killer (NK) cells. Interestingly, marrow stem cells that have been generated in primary and then in secondary recipients were able to contribute efficiently to myofibers in the muscle tissue of tertiary recipients. Altogether, our data demonstrate that muscle-derived stem cells present a major hematopoietic repopulating ability with evidence of self-replication in vivo. They are radiation-sensitive and similar to marrow-derived stem cells in terms of their ability to generate multilineage hematopoiesis. Finally, our data demonstrate that muscle-derived hematopoietic stem cells do not lose their ability to contribute to myofiber generation after at least two rounds of serial transplantation, suggesting a potential that is probably equivalent to that generated by marrow transplantation.

NOD/LtSz-Rag1nullPfpnull mice: a new model system with increased levels of human peripheral leukocyte and hematopoietic stem-cell engraftment

BACKGROUND

A critical need exists for effective small-animal models that accept engraftment of human hematopoietic progenitor cells and mature lymphocytes. The purpose of this study was to determine the phenotypic effects of perforin (Pfp) deficiency on nonobese diabetic (NOD)-Rag1null mice and to evaluate the ability of NOD/LtSz-Rag1nullPfpnull recipients to support engraftment with human hematolymphoid cells.

METHODS

A new genetic stock of NOD mice doubly homozygous for targeted mutations at the recombination activating gene (Rag)-1 and Pfp genes was developed. NOD/LtSz-Rag1nullPfpnull mice were studied for immunopathologic and hematologic abnormalities. The ability of these mice to support engraftment with human peripheral blood mononuclear cells (PBMC) and umbilical-cord blood hematopoietic progenitor cells was assessed.

RESULTS

NOD/LtSz-Rag1nullPfpnull mice lacked mature B cells, T cells, natural killer (NK) cell cytotoxic activity and were devoid of serum immunoglobulin (Ig) throughout a 37-week lifespan. These mice supported heightened engraftment with human PBMC as compared with NOD/LtSz-Rag1null controls as evidenced by a 4- to 5-fold increase in percentages of human lymphocytes and a 7- to 13-fold increase in percentages of CD4+ T cells in the peripheral blood and spleen. Total numbers of human CD4+ T cells were increased approximately 20-fold in the spleens of NOD/LtSz-Rag1nullPfpnull mice. These mice also showed approximately 12-fold higher levels of engraftment with human umbilical-cord blood cells compared with NOD/LtSz-Rag1null mice.

CONCLUSIONS

NOD/LtSz-Rag1nullPfpnull mice are devoid of mature B cell, T cell, and NK cell cytotoxic activity, engraft at high levels with human PBMC, and hematopoietic progenitor cells and provide a new NK cell-deficient model for human hematolymphoid cell engraftment.

Retrovirally transduced muscle-derived cells contribute to hematopoiesis at very low levels in the nonhuman primate model

Recent studies have suggested a remarkable potential of adult stem cells from a variety of organs to give rise to cells of disparate organs, but evidence of such potential at a clonal level is lacking in most if not all studies to date. To assess directly the hematopoietic potential of muscle-derived cells in a relevant large animal, we initiated retroviral-tagging studies in the rhesus macaque to allow tracking at the clonal level by integration site analysis. Four rhesus macaques underwent transplantation with transduced muscle-derived cells after lethal irradiation followed by delayed infusion of an autologous hematopoietic graft. The first animal showed no evidence of hematopoietic recovery and, despite infusion of the backup hematopoietic graft, succumbed due to complications of prolonged cytopenias. In the remaining three animals, the overall contribution of retrovirally tagged muscle-derived cells toward hematopoiesis was exceedingly low. Retroviral integration site analysis among clonally derived muscle cells and bone marrow cells in vivo in one animal suggests a common source. These results demonstrate that harvesting disparate organs for cellular therapy is currently highly inefficient at best.

Functional and kinetic characterization of granulocyte colony-stimulating factor-primed CD34− human stem cells

We assessed the functional properties and the kinetic status in vitro, and the engraftment potential in vivo of human haematopoietic stem cells according to the expression of CD34 antigen. Lin-CD34- and Lin-CD34+ cells were isolated from granulocyte colony-stimulating factor-primed peripheral blood (PB) cells of healthy donors. The CD34- cell fraction did not contain either clonogenic cells in semisolid culture or long-term culture initiating cells (LTC-IC). However, stroma-dependent liquid cultures and cytokines induced CD34 expression on a minority of stem cells, acquisition of clonogenic capacity and generation of LTC-IC. Significantly higher percentages of quiescent G0 cells and lower percentages of cycling G1 cells were found in Lin-CD34- cells when compared with Lin-CD34+ cells. Kinetic quiescence of Lin-CD34- cells was associated with a significantly higher expression of the negative regulators of the cell cycle, p27Kip1 and p21(cip1/waf1). Cytokine-mediated induction of CD34, in vitro, resulted in cycling of stem cells and downregulation of p27. There was a higher rate of human long-term engraftment in immunocompromised non-obese diabetic (NOD)/recombination activating gene 1null and NOD/severe combined immunodeficient-beta2microglobulin(null) mice injected with CD34+ cells. Thus, our study indicated that CD34 expression on human PB stem cells was associated with haematopoietic activity, cell-cycle recruitment and downregulation of p27Kip1 in vitro and higher engraftment capacity in vivo.

Wnt signaling induces the myogenic specification of resident CD45+ adult stem cells during muscle regeneration

The observation that CD45(+) stem cells injected into the circulation participate in muscle regeneration raised the question of whether CD45(+) stem cells resident in muscle play a physiological role during regeneration. We found that CD45(+) cells cultured from uninjured muscle were uniformly nonmyogenic. However, CD45(+) cells purified from regenerating muscle readily gave rise to determined myoblasts. The number of CD45(+) cells in muscle rapidly expanded following injury, and a high proportion entered the cell cycle. Investigation of candidate pathways involved in embryonic myogenesis revealed that Wnt signaling was sufficient to induce the myogenic specification of muscle-derived CD45(+) stem cells. Moreover, injection of the Wnt antagonists sFRP2/3 into regenerating muscle markedly reduced CD45(+) stem cell proliferation and myogenic specification. Our data therefore suggest that mobilization of resident CD45(+) stem cells is an important factor in regeneration after injury and highlight the Wnt pathway as a potential therapeutic target for degenerative neuromuscular disease.

Stem-cell "plasticity": befuddled by the muddle

In the past 4 years, multiple reports have suggested that stem cells derived from adult tissues can differentiate outside their tissue of origin, challenging long-accepted tenets of developmental biology. This concept of stem-cell "plasticity" has helped to galvanize research on stem cells due to the myriad therapeutic possibilities. However, there are wide discrepancies in the reported frequencies of so-called transdifferentiation events, from recent reports of negative data to reports of the contribution in some tissues and systems reaching as much as 20%. The evidence for and against stem-cell plasticity is reviewed here as well as some of the possible sources of the experimental variation.

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.