Human muscle-derived stem/progenitor cells promote functional murine peripheral nerve regeneration.

Peripheral nerve injuries and neuropathies lead to profound functional deficits. Here, we have demonstrated that muscle-derived stem/progenitor cells (MDSPCs) isolated from adult human skeletal muscle (hMDSPCs) can adopt neuronal and glial phenotypes in vitro and ameliorate a critical-sized sciatic nerve injury and its associated defects in a murine model. Transplanted hMDSPCs surrounded the axonal growth cone, while hMDSPCs infiltrating the regenerating nerve differentiated into myelinating Schwann cells. Engraftment of hMDSPCs into the area of the damaged nerve promoted axonal regeneration, which led to functional recovery as measured by sustained gait improvement. Furthermore, no adverse effects were observed in these animals up to 18 months after transplantation. Following hMDSPC therapy, gastrocnemius muscles from mice exhibited substantially less muscle atrophy, an increase in muscle mass after denervation, and reorganization of motor endplates at the postsynaptic sites compared with those from PBS-treated mice. Evaluation of nerve defects in animals transplanted with vehicle-only or myoblast-like cells did not reveal histological or functional recovery. These data demonstrate the efficacy of hMDSPC-based therapy for peripheral nerve injury and suggest that hMDSPC transplantation has potential to be translated for use in human neuropathies.

The microenvironment-specific transformation of adult stem cells models malignant triton tumors.

Here, we demonstrated the differentiation potential of murine muscle-derived stem/progenitor cells (MDSPCs) toward myogenic, neuronal, and glial lineages. MDSPCs, following transplantation into a critical-sized sciatic nerve defect in mice, showed full regeneration with complete functional recovery of the injured peripheral nerve at 6 weeks post-implantation. However, several weeks after regeneration of the sciatic nerve, neoplastic growths were observed. The resulting tumors were malignant peripheral nerve sheath tumors (MPNSTs) with rhabdomyoblastic differentiation, expressing myogenic, neurogenic, and glial markers, common markers of human malignant triton tumors (MTTs). No signs of tumorigenesis were observed 17 weeks post-implantation of MDSPCs into the gastrocnemius muscles of dystrophic/mdx mice, or 1 year following subcutaneous or intravenous injection. While MDSPCs were not oncogenic in nature, the neoplasias were composed almost entirely of donor cells. Furthermore, cells isolated from the tumors were serially transplantable, generating tumors when reimplanted into mice. However, this transformation could be abrogated by differentiation of the cells toward the neurogenic lineage prior to implantation. These results establish that MDSPCs participated in the regeneration of the injured peripheral nerve but transformed in a microenvironment- and time-dependent manner, when they likely received concomitant neurogenic and myogenic differentiation signals. This microenvironment-specific transformation provides a useful mouse model for human MTTs and potentially some insight into the origins of this disease.

Human myogenic endothelial cells exhibit chondrogenic and osteogenic potentials at the clonal level.

We have previously reported the high regenerative potential of murine muscle-derived stem cells (mMDSCs) that are capable of differentiating into multiple mesodermal cell lineages, including myogenic, endothelial, chondrocytic, and osteoblastic cells. Recently, we described a putative human counterpart of mMDSCs, the myogenic endothelial cells (MECs), in adult human skeletal muscle, which efficiently repair/regenerate the injured and dystrophic skeletal muscle as well as the ischemic heart in animal disease models. Nevertheless it remained unclear whether human MECs, at the clonal level, preserve mMDSC-like chondrogenic and osteogenic potentials and classic stem cell characteristics including high proliferation and resistance to stress. Herein, we demonstrated that MECs, sorted from fresh postnatal human skeletal muscle biopsies, can be grown clonally and exhibit robust resistance to oxidative stress with no tumorigeneity. MEC clones were capable of differentiating into chondrocytes and osteoblasts under inductive conditions in vitro and participated in cartilage and bone formation in vivo. Additionally, adipogenic and angiogenic potentials of clonal MECs (cMECs) were observed. Overall, our study showed that cMECs not only display typical properties of adult stem cells but also exhibit chondrogenic and osteogenic capacities in vitro and in vivo, suggesting their potential applications in articular cartilage and bone repair/regeneration.

Prospective identification of myogenic endothelial cells in human skeletal muscle.

We document anatomic, molecular and developmental relationships between endothelial and myogenic cells within human skeletal muscle. Cells coexpressing myogenic and endothelial cell markers (CD56, CD34, CD144) were identified by immunohistochemistry and flow cytometry. These myoendothelial cells regenerate myofibers in the injured skeletal muscle of severe combined immunodeficiency mice more effectively than CD56+ myogenic progenitors. They proliferate long term, retain a normal karyotype, are not tumorigenic and survive better under oxidative stress than CD56+ myogenic cells. Clonally derived myoendothelial cells differentiate into myogenic, osteogenic and chondrogenic cells in culture. Myoendothelial cells are amenable to biotechnological handling, including purification by flow cytometry and long-term expansion in vitro, and may have potential for the treatment of human muscle disease.

Osteogenic potential of postnatal skeletal muscle-derived stem cells is influenced by donor sex.

UNLABELLED: This study compared the osteogenic differentiation of F-MDSCs and M-MDSCs. Interestingly, M-MDSCs expressed osteogenic markers and underwent mineralization more readily than F-MDSCs; a characteristic likely caused by more osteoprogenitor cells within the M-MDSCs than the F-MDSCs and/or an accelerated osteogenic differentiation of M-MDSCs. INTRODUCTION: Although therapies involving stem cells will require both female and male cells, few studies have investigated whether sex-related differences exist in their osteogenic potential. Here, we compared the osteogenic differentiation of female and male mouse skeletal muscle-derived stem cells (F- and M-MDSCs, respectively), a potential cell source for orthopedic tissue engineering. MATERIALS AND METHODS: F- and M-MDSCs were stimulated with bone morphogenetic protein (BMP)4, followed by quantification of alkaline phosphatase (ALP) activity and expression of osteogenic genes. F- and M-MDSCs were also cultured as pellets in osteogenic medium to evaluate mineralization. Single cell-derived colonies of F- and M-MDSCs were stimulated with BMP4, stained for ALP, and scored as either Low ALP+ or High ALP+ to detect the presence of osteoprogenitor cells. F- and M-MDSCs were transduced with a BMP4 retrovirus (MDSC-BMP4 cells) and used for the pellet culture and single cell-derived colony formation assays. As well, F- and M-MDSC-BMP4 cells were implanted in the intramuscular pocket of sex-matched and sex-mismatched hosts, and bone formation was monitored radiographically. RESULTS AND CONCLUSIONS: When stimulated with BMP4, both F- and M-MDSCs underwent osteogenic differentiation, although M-MDSCs had a significantly greater ALP activity and a larger increase in the expression of osteogenic genes than F-MDSCs. In the pellet culture assay, M-MDSCs showed greater mineralization than F-MDSCs. BMP4 stimulation of single cell-derived colonies from M-MDSCs showed higher levels of ALP than those from F-MDSCs. Similar results were obtained with the MDSC-BMP4 cells. In vivo, F-MDSC-BMP4 cells displayed variability in bone area and density, whereas M-MDSC-BMP4 cells showed a more consistent and denser ectopic bone formation. More bone formation was also seen in male hosts compared with female hosts, regardless of the sex of the implanted cells. These results suggest that M-MDSCs may contain more osteoprogenitor cells than F-MDSCs, which may have implications in the development of cellular therapies for bone healing.

Malignant transformation of multipotent muscle-derived cells by concurrent differentiation signals.

Recent studies have shown that germ-line determination occurs early in development and that extracellular signaling can alter this fate. This denial of a cell's fate by counteracting its intrinsic signaling pathways through extrinsic stimulation is believed to be associated with oncogenesis. Using specific populations of multipotent skeletal muscle-derived stem cells (MDSCs), we have been able to generate tumors by subjecting cells with specific lineage predilections to concomitant differentiation signals. More specifically, when a stem cell that had a predilection toward osteogenesis was implanted into a skeletal muscle, tumors formed in 25% of implanted mice. When cells predilected to undergo myogenesis were pretreated with bone morphogenetic protein 4 (BMP4) for 4 days prior to implantation, they formed tumors in 25% of mice. These same myogenic predilected cells, when transduced to express BMP4 and implanted into either a long-bone or cranial defect, formed bone, but they formed tumors in 100% of mice when implanted into the skeletal muscle. The tumors generated in this latter study were serially transplantable as long as they retained BMP4 expression. Furthermore, when we impeded the ability of the cells to undergo myogenic differentiation using small interfering RNA to the myogenic regulator MyoD1, we stopped transformation. Based on our findings, we postulate that specific MDSC populations can undergo concomitant signal-induced transformation and that the initial stages of transformation may be due to changes in the balance between the inherent nature of the cell and extrinsic signaling pathways. This theory represents a potential link between somatic stem cells and cancer and suggests an involvement of the niche/environment in transformation.

A role for cell sex in stem cell-mediated skeletal muscle regeneration: female cells have higher muscle regeneration efficiency.

We have shown that muscle-derived stem cells (MDSCs) transplanted into dystrophic (mdx) mice efficiently regenerate skeletal muscle. However, MDSC populations exhibit heterogeneity in marker profiles and variability in regeneration abilities. We show here that cell sex is a variable that considerably influences MDSCs' regeneration abilities. We found that the female MDSCs (F-MDSCs) regenerated skeletal muscle more efficiently. Despite using additional isolation techniques and cell cloning, we could not obtain a male subfraction with a regeneration capacity similar to that of their female counterparts. Rather than being directly hormonal or caused by host immune response, this difference in MDSCs' regeneration potential may arise from innate sex-related differences in the cells' stress responses. In comparison with F-MDSCs, male MDSCs have increased differentiation after exposure to oxidative stress induced by hydrogen peroxide, which may lead to in vivo donor cell depletion, and a proliferative advantage for F-MDSCs that eventually increases muscle regeneration. These findings should persuade researchers to report cell sex, which is a largely unexplored variable, and consider the implications of relying on cells of one sex.

Preclinical studies of fibroblast growth factor receptor 3 as a therapeutic target in multiple myeloma.

Dysregulation of fibroblast growth factor receptor 3 (FGFR3) by the translocation t(4;14)(p16;q32) occurs in 15% of multiple myeloma (MM) patients and confers a growth and survival advantage to malignant plasma cells. As FGFR3 is a molecular target, we assessed the therapeutic potential of the FGFR-specific tyrosine kinase inhibitors SU5402 and SU10991 in MM. SU5402 inhibited FGFR3 phosphorylation in vitro and in murine MM tumour models. B cells dependent on FGFR3 for survival were specifically sensitive to SU5402. A panel of 11 human myeloma cell lines was studied, five bearing the t(4;14) translocation. The KMS11 human myeloma cell line, which expresses constitutively active mutant FGFR3, displayed an 85% decrease in S-phase cells, a 95% increase in G0/G1 cells, and 4.5-fold increase in apoptotic cells after 72 h treatment with 10 micromol/l SU5402. Activated extracellular signal-regulated kinases 1 and 2 and signal transducer and activator of transcription 3 were rapidly down-regulated after SU5402 treatment. In human myeloma cell lines expressing wild-type FGFR3 the stimulating effect of aFGF ligand was abrogated by SU5402 treatment. Myeloma cells lacking the t(4;14) or with the t(4;14) and a secondary RAS mutation did not respond to therapy. These findings support the development of clinical trials of early intervention with FGFR3 inhibitors in t(4;14) myeloma.

RU486-inducible retrovirus-mediated caspase-3 overexpression is cytotoxic to bcl-xL-expressing myeloma cells in vitro and in vivo.

The antiapoptotic protein bcl-x(L) is upregulated in a variety of solid tumors and in primary hematologic malignancies such as multiple myeloma. Activated caspase-3 cleaves proteins essential for cell survival, including bcl-x(L). To explore the potential of caspase-3 as a cytotoxic and immunostimulatory molecule in the treatment of malignancy, an RU486-inducible caspase-3 retrovirus was constructed, validated, and used to transduce first 3T3 and subsequently murine myeloma B9BM1 cells (creating the cell line B9BM-C3). After induction, apoptotic cell death of 3T3 and B9BM-C3 cells began by 4 h and was complete by 48 h postinduction, while nontransduced cells remained viable. Annexin V staining demonstrated 43, 76, and 98% apoptotic cell death at 12, 18, and 24 h postinduction. Activation of caspase-3 was evident in induced cells and cell death could be inhibited by the addition of a caspase-3-specific inhibitor. Overexpression of the myeloma-associated oncogene FGFR3, which upregulates bcl-x(L), delayed but did not prevent caspase-3-mediated killing. B9BM-C3 cells formed tumors after subcutaneous injection in mice. Early treatment with RU486 eradicated tumors; however, rechallenge of treated mice failed to demonstrate evidence of immunoprotection. These results indicate that therapeutic attempts to induce caspase-3 in malignant cells may prove useful in the treatment of bcl-x(L)-expressing tumors.

Overexpression of the myeloma-associated oncogene fibroblast growth factor receptor 3 confers dexamethasone resistance.

Translocations involving the immunoglobulin heavy-chain switch region and fibroblast growth factor receptor 3 (FGFR3) are identified in 10% to 15% of patients with myeloma. In previous research we overexpressed FGFR3 or the constitutively active FGFR3-TD mutant in an interleukin-6 (IL-6)-dependent murine myeloma cell line, B9. FGFR3-enhanced IL-6 responsiveness increased phosphorylation of STAT3 and up-regulated Bcl-x(L). Since Bcl-x(L) was up-regulated, we have tested FGFR3-expressing B9 cells for chemotherapy sensitivity. FGFR3 expression did not alter sensitivity to melphalan or doxorubicin. In contrast, B9 cells overexpressing FGFR3 were resistant to treatment with dexamethasone, a phenomenon successfully reversed using a Bcl-x(L) antisense oligonucleotide. These data demonstrate that the overexpression of FGFR3 in B9 cells confers resistance to dexamethasone but not to anthracyclines or alkylating agents, at least in part through the up-regulation of Bcl-x(L). This finding has potential implications for the use of chemotherapy in t(4;14)-positive myeloma.