Skeletal muscle-derived progenitor cells exhibit neural competence

Muscle-derived stem cells: important players in peripheral nerve repair

INTRODUCTION

Stem cell therapy for peripheral nerve repair is a rapidly evolving field in regenerative medicine. Although most studies to date have investigated stem cells originating from bone marrow or adipose, skeletal muscle has recently been recognized as an abundant and easily accessible source of stem cells. Muscle-derived stem cells (MDSCs) are a diverse population of multipotent cells with pronounced antioxidant and regenerative capacity. Areas covered: The current literature on the various roles MDSCs serve within the micro- and macro-environment of nerve injury. Furthermore, the exciting new research that is establishing MDSC-cellular therapy as an important therapeutic modality to improve peripheral nerve regeneration. Expert opinion: MDSCs are a promising therapeutic agent for the repair of peripheral nerves; MDSCs not only undergo gliogenesis and angiogenesis, but they also orchestrate larger pro-regenerative host responses. However, the isolation, transformation, and in-vivo behavior of MDSCs require further evaluation prior to clinical application.

Muscle-derived stem cells in peripheral nerve regeneration: reality or illusion?

Owing to the complicated and time-consuming regenerative process, the repair of injured peripheral nerves depends largely on ongoing stem-cell therapy. Decades ago, researchers successfully isolated and identified muscle-derived stem cells (MDSCs) and discovered their potential for multidifferentiation. MDSCs play an important role in trauma repair associated with neuromuscular and vascular injury by simultaneously promoting tissue regrowth via direct differentiation and systematic secretion under physiological conditions. However, the isolation, culture, induction and application of MDSCs require further methodological analysis before clinical application. In this review, we comprehensively discuss the challenges associated with neural regeneration and reviewed the progress of stem cell based regenerative medicine, in an effort to realize the potential of MDSCs in nerve regeneration.

Pericytes: multitasking cells in the regeneration of injured, diseased, and aged skeletal muscle

Pericytes are perivascular cells that envelop and make intimate connections with adjacent capillary endothelial cells. Recent studies show that they may have a profound impact in skeletal muscle regeneration, innervation, vessel formation, fibrosis, fat accumulation, and ectopic bone formation throughout life. In this review, we summarize and evaluate recent advances in our understanding of pericytes' influence on adult skeletal muscle pathophysiology. We also discuss how further elucidating their biology may offer new approaches to the treatment of conditions characterized by muscle wasting.

Skeletal muscle neural progenitor cells exhibit properties of NG2-glia

Reversing brain degeneration and trauma lesions will depend on cell therapy. Our previous work identified neural precursor cells derived from the skeletal muscle of Nestin-GFP transgenic mice, but their identity, origin, and potential survival in the brain are only vaguely understood. In this work, we show that Nestin-GFP+ progenitor cells share morphological and molecular markers with NG2-glia, including NG2, PDGFRα, O4, NGF receptor (p75), glutamate receptor-1(AMPA), and A2B5 expression. Although these cells exhibit NG2, they do not express other pericyte markers, such as α-SMA or connexin-43, and do not differentiate into the muscle lineage. Patch-clamp studies displayed outward potassium currents, probably carried through Kir6.1 channels. Given their potential therapeutic application, we compared their abundance in tissues and concluded that skeletal muscle is the richest source of predifferentiated neural precursor cells. We found that these cells migrate toward the neurogenic subventricular zone displaying their typical morphology and nestin-GFP expression two weeks after brain injection. For translational purposes, we sought to identify these neural progenitor cells in wild-type species by developing a DsRed expression vector under Nestin-Intron II control. This approach revealed them in nonhuman primates and aging rodents throughout the lifespan.

Dopaminergic neuronal conversion from adult rat skeletal muscle-derived stem cells in vitro

Muscle-derived stem cells reside in the skeletal muscle tissues and are known for their multipotency to differentiate toward the mesodermal lineage. Recent studies have demonstrated their capacity of neuroectodermal differentiation, including neurons and astrocytes. In this study, we investigated the possibility of dopaminergic neuronal conversion from adult rat skeletal muscle-derived stem cells. Using a neurosphere protocol, muscle-derived stem cells form neurosphere-like cell clusters after cultivation as a suspension, displaying an obvious expression of nestin and a remarkable down-regulation of myogenic associated factors desmin, MyoD, Myf5 and myogenin. Subsequently, these neurosphere-like cell clusters were further directed to dopaminergic differentiation through two major induction steps, patterning to midbrain progenitors with sonic hedgehog and fibroblast growth factor 8, followed by the differentiation to dopaminergic neurons with neurotrophic factors (glial cell line-derived neurotrophic factor) and chemicals (ascorbic acid, forskolin). After the differentiation, these cells expressed tyrosine hydroxylase, dopamine transporter, dopamine D1 receptor and synapse-associated protein synapsin I. Several genes, Nurr1, Lmx1b, and En1, which are critically related with the development of dopaminergic neurons, were also significantly up-regulated. The present results indicate that adult skeletal muscle-derived stem cells could provide a promising cell source for autologous transplantation for neurodegenerative diseases in the future, especially the Parkinson's disease.

Skeletal myogenic potential of mouse skin-derived precursors

Cell transplantation-based therapy could be an effective way for the treatment of many diseases. Among numerous somatic stem cells isolated and purified, skin-derived precursors (SKPs) are abundant autologous cells, providing a large reservoir of cells for therapeutic transplantation. Previous studies showed that SKPs could be differentiated into neural and mesodermal progeny in vitro. In the present study, we attempted to differentiate SKPs to muscle progenitors in vitro. After treatment with a combination of growth factors, SKPs were differentiated into cells expressing markers of muscle progenitors, including Pax7. Furthermore, some of these cells expressed desmin, TnT, Mstn, and Myog, suggesting their differentiation into the muscular lineage. After single point injection, the differentiation of SKPs from green fluorescent protein positive donors into muscle precursors was also demonstrated in vivo. Additionally, donor SKPs migrated to the niche of muscle progenitors, participated in the regeneration of recipient muscles, and expressed markers of muscle progenitors, including Pax7, M-cadherin, and MyoD. After recovery of donor cells from recipient muscles at 3 weeks postinjection, some of the injected SKPs were converted to myogenic precursors, based on the expression of specific markers (Pax7 and MyoD). Some of these donor cells also expressed muscle makers (desmin, TnT, and Myog). At 20 weeks postinjection, the injected SKPs were localized to recipient muscles without decreases in their population size. In summary, these findings indicated that SKPs could develop into muscle progenitors and differentiated muscle cells in vitro and in vivo, thus providing valuable autologous cells for the treatment of muscle diseases.

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.

In vitro clonal analysis of murine pluripotent stem cells isolated from skeletal muscle and adipose stromal cells

OBJECTIVE

Possible clinical utility of pluripotent stem cells (PSCs) with multilineage differentiation capacity depends on their ability to adapt to tissue-specific differentiation conditions. Previous data from our laboratory suggest that putative PSCs exhibiting an immunophenotype of CD45(-)Sca-1+CD117(-)CD90+ can be isolated from multiple tissues. In the present study, the clonal in vitro differentiation potential of two isolates of PSCs was examined.

MATERIALS AND METHODS

Clonal analysis of the differentiation potential of skeletal muscle- (SM) and adipose stromal cell (ASC)-derived PSCs into myogenic, adipogenic, and neurogenic cells was investigated by expanding single PSCs prior to specification under three separate differentiation conditions.

RESULTS

Differentiation of SM- and ASC-derived PSCs into myotubes, adipocytes, and neuronal-like cells was evident in clonal cultures promoting differentiation along these lineages. A total of 2.0%, 1.0%, and 0.33% of SM-derived clones demonstrated unipotent, bipotent, and tripotent differentiation, respectively, into combinations of myocytes, adipocytes, and neuronal cells. As a percentage of SM-derived PSCs, tripotent clones comprised 0.016% of total muscle cells. Similar results were obtained with ASC-derived PSCs, suggesting phenotypic and functional similarities between PSCs from both tissues. Following differentiation of single PSCs into three lineages, a clear and complete commitment to tissue-specific gene expression accompanied by inactivation of lineage-unrelated genes could not be demonstrated in several SM- and ASC-derived clones.

CONCLUSIONS

These data demonstrate that phenotypically defined PSCs remain functionally heterogeneous at the single-cell level and illustrate that morphologic lineage commitment may be independent of exclusive expression and/or loss of associated lineage specific genes.

Epigenetics and the plasticity of differentiation in normal and cancer stem cells

Embryonic stem cells are characterized by their differentiation to all cell types during embryogenesis. In adult life, different tissues also have somatic stem cells, called adult stem cells, which in specific niches can undergo multipotent differentiation. The use of these adult stem cells has considerable therapeutic potential for the regeneration of damaged tissues. In both embryonic and adult stem cells, differentiation is controlled by epigenetic mechanisms, and the plasticity of differentiation in these cells is associated with transcription accessibility for genes expressed in different normal tissues. Abnormalities in genetic and/or epigenetic controls can lead to development of cancer, which is maintained by self-renewing cancer stem cells. Although the genetic abnormalities produce defects in growth and differentiation in cancer stem cells, these cells have not always lost the ability to undergo differentiation through epigenetic changes that by-pass the genomic abnormalities, thus creating the basis for differentiation therapy. Like normal stem cells, cancer stem cells can show plasticity for differentiation. This plasticity of cancer stem cells is also associated with transcription accessibility for genes that are normally expressed in different tissues, including tissues other than those from which the cancers originated. This broad transcription accessibility can also contribute to the behavior of cancer cells by overexpressing genes that promote cell viability, growth and metastasis.