Evidence for a resident subset of cells with SP phenotype in the C2C12 myogenic line: a tool to explore muscle stem cell biology

Deep sequencing reveals two Jurkat subpopulations with distinct miRNA profiles during camptothecin-induced apoptosis

MicroRNAs (miRNAs) are small noncoding RNAs of about 19-25 nt that regulate gene expression posttranscriptionally under various cellular conditions, including apoptosis. The miRNAs involved in modulation of apoptotic events in T cells are partially known. However, heterogeneity associated with cell lines makes it difficult to interpret gene expression signatures, especially in cancer-related cell lines. Treatment of the Jurkat T-cell leukemia cell line with the universal apoptotic drug, camptothecin, resulted in identification of two Jurkat subpopulations: one that is sensitive to camptothecin and another that is rather intrinsically resistant. We sorted apoptotic Jurkat cells from nonapoptotic ones prior to profiling miRNAs through deep sequencing. Our data showed that a total of 184 miRNAs were dysregulated. Interestingly, the apoptotic and nonapoptotic subpopulations exhibited distinct miRNA expression profiles. In particular, 6 miRNAs were inversely expressed in these two subpopulations. The pyrosequencing results were validated by real-time qPCR. Altogether, these results suggest that miRNAs modulate apoptotic events in T cells and that cellular heterogeneity requires careful interpretation of miRNA expression profiles obtained from drug-treated cell lines.

Myostatin promotes tenogenic differentiation of C2C12 myoblast cells through Smad3

Myostatin, a member of the transforming growth factor-β (TGF-β) superfamily, is expressed in developing and adult skeletal muscle and negatively regulates skeletal muscle growth. Recently, myostatin has been found to be expressed in tendons and increases tendon fibroblast proliferation and the expression of tenocyte markers. C2C12 is a mouse myoblast cell line, which has the ability to transdifferentiate into osteoblast and adipocyte lineages. We hypothesized that myostatin is capable of inducing tenogenic differentiation of C2C12 cells. We found that the expression of scleraxis, a tendon progenitor cell marker, is much higher in C2C12 than in the multipotent mouse mesenchymal fibroblast cell line C3H10T1/2. In comparison with other growth factors, myostatin significantly up-regulated the expression of the tenogenic marker in C2C12 cells under serum-free culture conditions. Immunohistochemistry showed that myostatin inhibited myotube formation and promoted the formation of spindle-shaped cells expressing tenomodulin. We examined signaling pathways essential for tenogenic differentiation to clarify the mechanism of myostatin-induced differentiation of C2C12 into tenocytes. The expression of tenomodulin was significantly suppressed by treatment with the ALK inhibitor SB341542, in contrast to p38MAPK (SB203580) and MEK1 (PD98059) inhibitors. RNAi silencing of Smad3 significantly suppressed myostatin-induced tenomodulin expression. These results indicate that myostatin has a potential role in the induction of tenogenic differentiation of C2C12 cells, which have tendon progenitor cell characteristics, through activation of Smad3-mediated signaling.

Noninvasive PET Imaging and Tracking of Engineered Human Muscle Precursor Cells for Skeletal Muscle Tissue Engineering

Transplantation of human muscle precursor cells (hMPCs) is envisioned for the treatment of various muscle diseases. However, a feasible noninvasive tool to monitor cell survival, migration, and integration into the host tissue is still missing.

METHODS

In this study, we designed an adenoviral delivery system to genetically modify hMPCs to express a signaling-deficient form of human dopamine D2 receptor (hD2R). The gene expression levels of the receptor were evaluated by reverse transcriptase polymerase chain reaction, and infection efficiency was evaluated by fluorescent microscopy. The viability, proliferation, and differentiation capacity of the transduced cells, as well as their myogenic phenotype, were determined by flow cytometry analysis and fluorescent microscopy. (18)F-fallypride and (18)F-fluoromisonidazole, two well-established PET radioligands, were assessed for their potential to image engineered hMPCs in a mouse model and their uptakes were evaluated at different time points after cell inoculation in vivo. Biodistribution studies, autoradiography, and PET experiments were performed to determine the extent of signal specificity. To address feasibility for tracking hMPCs in an in vivo model, the safety of the adenoviral gene delivery was evaluated. Finally, the harvested tissues were histologically examined to determine whether survival of the transplanted cells was sustained at different time points.

RESULTS

Adenoviral gene delivery was shown to be safe, with no detrimental effects on the primary human cells. The viability, proliferation, and differentiation capacity of the transduced cells were confirmed, and flow cytometry analysis and fluorescent microscopy showed that their myogenic phenotype was sustained. (18)F-fallypride and (18)F-fluoromisonidazole were successfully synthesized. Specific binding of (18)F-fallypride to hD2R hMPCs was demonstrated in vitro and in vivo. Furthermore, the (18)F-fluoromisonidazole signal was high at the early stages. Finally, sustained survival of the transplanted cells at different time points was confirmed histologically, with formation of muscle tissue at the site of injection.

CONCLUSION

Our proposed use of a signaling-deficient hD2R as a potent reporter for in vivo hMPC PET tracking by (18)F-fallypride is a significant step toward potential noninvasive tracking of hD2R hMPCs and bioengineered muscle tissues in the clinic.

Verapamil inhibits tumor progression of chemotherapy-resistant pancreatic cancer side population cells

Tumor side population (SP) cells display stem-like properties that can be modulated by treatment with the calcium channel blocker verapamil. Verapamil can enhance the cytotoxic effects of chemotherapeutic drugs and multi-drug resistance by targeting the transport function of the P-glycoprotein (P-gp). This study focused on the therapeutic potential of verapamil on stem-like SP tumor cells, and further investigated its chemosensitizing effects using L3.6pl and AsPC-1 pancreatic carcinoma models. As compared to parental L3.6pl cells (0.9±0.22%), L3.6pl gemcitabine-resistant cells (L3.6pl_Gres) showed a significantly higher percentage of SP cells (5.38±0.99%) as detected by Hoechst 33342/FACS assays. The L3.6pl_Gres SP cells showed stable gemcitabine resistance, enhanced colony formation ability and increased tumorigenicity. Verapamil effectively inhibited L3.6pl_Gres and AsPC-1 SP cell proliferation in vitro. A pro-apoptotic effect of verapamil was observed in L3.6pl cells, but not in L3.6pl_Gres cells, which was linked to their differential expression of P-gp and equilibrative nucleoside transporter-1 (ENT-1). In an orthotopic pancreatic cancer mouse model, both low and high dose verapamil was shown to substantially reduce L3.6pl_Gres-SP cell tumor growth and metastasis, enhance tumor apoptosis, and reduce microvascular density.

Cancer Stem and Progenitor-Like Cells as Pharmacological Targets in Breast Cancer Treatment

The present review is focused on the current role of neoplastic stem and progenitor-like cells as primary targets in the pharmacotherapy of cancer as well as in the development of new anticancer drugs. We begin by summarizing the main characteristics of these tumor-initiating cells and key concepts that support their participation in therapeutic failure. In particular, we discuss the differences between the major carcinogenesis models (ie, clonal evolution vs cancer stem cell (CSC) model) with emphasis on breast cancer (given its importance to the study of CSCs) and their implications for the development of new treatment strategies. In addition, we describe the main ways to target these cells, including the main signaling pathways that are more activated or altered in CSCs. Finally, we provide a comprehensive compilation of the most recently tested drugs.

Rapid depletion of muscle progenitor cells in dystrophic mdx/utrophin-/- mice

Duchenne muscular dystrophy (DMD) patients lack dystrophin from birth; however, muscle weakness becomes apparent only at 3-5 years of age, which happens to coincide with the depletion of the muscle progenitor cell (MPC) pools. Indeed, MPCs isolated from older DMD patients demonstrate impairments in myogenic potential. To determine whether the progression of muscular dystrophy is a consequence of the decline in functional MPCs, we investigated two animal models of DMD: (i) dystrophin-deficient mdx mice, the most commonly utilized model of DMD, which has a relatively mild dystrophic phenotype and (ii) dystrophin/utrophin double knock-out (dKO) mice, which display a similar histopathologic phenotype to DMD patients. In contrast to age-matched mdx mice, we observed that both the number and regeneration potential of dKO MPCs rapidly declines during disease progression. This occurred in MPCs at both early and late stages of myogenic commitment. In fact, early MPCs isolated from 6-week-old dKO mice have reductions in proliferation, resistance to oxidative stress and multilineage differentiation capacities compared with age-matched mdx MPCs. This effect may potentially be mediated by fibroblast growth factor overexpression and/or a reduction in telomerase activity. Our results demonstrate that the rapid disease progression in the dKO model is associated, at least in part, with MPC depletion. Therefore, alleviating MPC depletion could represent an approach to delay the onset of the histopathologies associated with DMD patients.

Defining skeletal muscle resident progenitors and their cell fate potentials

The satellite cell is the major tissue-resident stem cell underlying muscle regeneration; however, multiple non-satellite myogenic progenitors as well as non-myogenic populations that support the muscle regenerative process have been identified. PW1 is expressed in satellite cells as well as in a subset of interstitial cells with myogenic potential termed PICs (PW1+ interstitial cells). Microarray profiling revealed that PICs express a broad range of genes common to mesenchymal stem cells, whereas satellite cells express genes consistent with a committed myogenic progenitor. Isolated PICs from both young and adult muscles can differentiate into smooth and skeletal muscle and fat whereas satellite cells are restricted to a skeletal muscle fate. We demonstrate that the adipogenic potential of PICs corresponds to a subpopulation that expresses platelet derived growth factor receptor alpha (PDGFRα) and overlaps with the recently described interstitial adipogenic progenitors. By contrast, PICs with myogenic potential do not express PDGFRα. Moreover, we observe a discrete and transient population of juvenile PICs based upon SCA1 expression that disappears by 3 weeks of postnatal development coincident with a switch in the cellular and genetic mechanisms underlying postnatal muscle growth.

Sorting, identification and enrichment of side population cells in THP-1 acute monocytic leukemia cells

The objective of the present study was to examine and determine whether the human acute monocytic leukemia cell line THP-1 contains side population (SP) cells, and, if so, to increase the proportion of SP cells using arabinosylcytosine (Ara-C). Fluorescent microscopy and flow cytometry were employed to detect the percentage of SP cells in THP-1 cells. Then, SP and non-SP (NSP) cell subpopulations were collected and identified. THP-1 cells were incubated with different concentrations of Ara-C for 24 h and the proportion of SP cells was detected. Our results demonstrated that the percentage of SP cells was 1.81 ± 0.99% in THP-1 cells. A majority of the SP cells remained in the G₀/G₁ phase, and the expression of CD34⁺ and CD34⁺CD38⁻ and the proliferation ability of the SP cells were higher compared to NSP cells (P<0.05). The mRNA expression of multidrug resistance genes (ABCG2 and ABCB1), apoptosis regulation genes (Bcl-2) and the Bcl-2/Bax value of SP cells were higher than those of NSP cells. SP cells have been shown to be more tumorigenic than NSP cells. Following co-culture with Ara-C, the proportion of SP cells increased significantly and subsequently the Ara-C concentration increased. These findings suggest that the THP-1 cell line contains SP cells and that SP cells possess certain intrinsic stem cell properties and may contain a larger proportion of leukemia stem cells (LSCs). The concentrations of SP cells can be increased with Ara-C by co-culture, and this technique is a useful and important application for the study of LSCs.

Muscle Precursor Cells for the Restoration of Irreversibly Damaged Sphincter Function

Multiple modalities, including injectable bulking agents and surgery, have been used to treat stress urinary incontinence. However, none of these methods is able to fully restore normal striated sphincter muscle function. In this study, we explored the possibility of achieving functional recovery of the urinary sphincter muscle using autologous muscle precursor cells (MPCs) as an injectable, cell-based therapy. A canine model of striated urinary sphincter insufficiency was created by microsurgically removing part of the sphincter muscle in 24 dogs. Autologous MPCs were obtained, expanded in culture, and injected into the damaged sphincter muscles of 12 animals. The animals were followed for up to 6 months after injection, and urodynamic studies, functional organ bath studies, ultrastructural and histological examinations were performed. Animals receiving MPC injections demonstrated sphincter pressures of approximately 80% of normal values, while the pressures in the control animals without cells dropped and remained at 20% of normal values. Histological analysis indicated that the implanted cells survived and formed tissue, including new innervated muscle fibers, within the injected region of the sphincter. These results indicate that autologous muscle precursor cells may be able to restore otherwise irreversibly damaged urinary sphincter function clinically.

Cancer stem cells and side population cells in breast cancer and metastasis

In breast cancer it is never the primary tumour that is fatal; instead it is the development of metastatic disease which is the major cause of cancer related mortality. There is accumulating evidence that suggests that Cancer Stem Cells (CSC) may play a role in breast cancer development and progression. Breast cancer stem cell populations, including side population cells (SP), have been shown to be primitive stem cell-like populations, being long-lived, self-renewing and highly proliferative. SP cells are identified using dual wavelength flow cytometry combined with Hoechst 33342 dye efflux, this ability is due to expression of one or more members of the ABC transporter family. They have increased resistance to chemotherapeutic agents and apoptotic stimuli and have increased migratory potential above that of the bulk tumour cells making them strong candidates for the metastatic spread of breast cancer. Treatment of nearly all cancers usually involves one first-line agent known to be a substrate of an ABC transporter thereby increasing the risk of developing drug resistant tumours. At present there is no marker available to identify SP cells using immunohistochemistry on breast cancer patient samples. If SP cells do play a role in breast cancer progression/Metastatic Breast Cancer (MBC), combining chemotherapy with ABC inhibitors may be able to destroy both the cells making up the bulk tumour and the cancer stem cell population thus preventing the risk of drug resistant disease, recurrence or metastasis.

Expression of a Hoechst 33342 efflux phenomenon and common characteristics of pluripotent stem cells in a side population of amniotic fluid cells

OBJECTIVE

The aim of this study was to verify the existence of a side population (SP) of cells in second-trimester amniotic fluid.

MATERIALS AND METHODS

Amniotic fluid samples (n = 35) were obtained, and the number and size of viable amniotic fluid cells (AFCs) were analyzed. Small AFCs (SAFCs) and large AFCs (LAFCs) were isolated using a sterile 10-microm pore size strainer. Hoechst 33342 dye exclusion assay, flow cytometry analysis, reverse transcriptase polymerase chain reaction and immunocytochemistry were used to analyze the characteristics of SAFCs and LAFCs.

RESULTS

The mean concentration of viable AFCs from 16 to 21 weeks of gestation was 0.3 x 10(5), 0.8 x 10(5), 1.1 x 10(5), 1.3 x 10(5), 1.0 x 10(5) and 1.0 x 10(5) cells/mL respectively. The mean percentage of SAFCs from 16 to 21 weeks of gestation was 27.3%, 40.5%, 49.7%, 60.2%, 41.0% and 58.2%, respectively. The Hoechst 33342 efflux phenomenon was obvious among SAFCs but was rare in the LAFC population. Flow cytometry analyses showed that cell surface antigen expression on LAFCs and SAFCs were positive for CD29, CD44, CD73, CD90, CD166 and HLA-I, but negative for CD31, CD34, CD45, CD117 and HLA-II. Importantly, Nanog, Oct-4, ABCG2 and SOX2 expression in cells was easily detectable among the SAFC population. Expression of Nanog and ABCG2 was not observed among LAFCs.

CONCLUSION

Amniotic fluid contains a SP that was found mostly among the SAFCs. Enriched SP cells isolated by the efflux of Hoechst 33342 could be a novel and promising source of pluripotent-like amniotic derived stem cells for cellular therapy in the near future.

Cancer stem-like cells can be isolated with drug selection in human ovarian cancer cell line SKOV3

One emerging model for the development of drug-resistant tumors utilizes a pool of self-renewing malignant progenitors known as cancer stem cells (CSCs) or cancer-initiating cells (CICs). The purpose of this study was to propagate such CICs from the ovarian cancer cell line SKOV3. The SKOV3 sphere cells were selected using 40.0 micromol/l cisplatin and 10.0 micromol/l paclitaxel in serum-free culture system supplemented with epidermal growth factor, basic fibroblast growth factor, leukemia inhibitory factor, and insulin or standard serum-containing system. These cells formed non-adherent spheres under drug selection (cisplatin and paclitaxel) and serum-free culture system. The selected sphere cells are more resistant to cisplatin, paclitaxel, adriamycin, and methotrexate. Importantly, the sphere cells have the properties of self-renewal, with high expression of the stem cell genes Nanog, Oct4, sox2, nestin, ABCG2, CD133, and the stem cell factor receptor CD117 (c-kit). Consistently, flow cytometric analysis revealed that the sphere cells have a much higher percentage of CD133(+)/CD117(+)-positive cells (71%) than differentiated cells (33%). Moreover, the SKOV3 sphere cells are more tumorigenic. Furthermore, cDNA microarray and subsequent ontological analyses revealed that a large proportion of the classified genes were related to angiogenesis, extracellular matrix, integrin-mediated signaling pathway, cell adhesion, and cell proliferation. The subpopulation isolation from the SKOV3 cell line under this culture system offers a suitable in vitro model for studying ovarian CSCs in terms of their survival, self-renewal, and chemoresistance, and for developing therapeutic drugs that specifically interfere with ovarian CSCs.

Hypoxia induces adipogenic differentitation of myoblastic cell lines

Muscle atrophy usually accompanies fat accumulation in the muscle. In such atrophic conditions as back muscles of kyphotic spine and the rotator cuff muscles with torn tendons, blood flow might be diminished. It is known that hypoxia causes trans-differentiation of mesenchymal stem cells derived from bone marrow into adipocytes. However, it has not been elucidated yet if hypoxia turned myoblasts into adipocytes. We investigated adipogenesis in C2C12 and G8 murine myogenic cell line treated by hypoxia. Cells were also treated with the cocktail of insulin, dexamethasone and IBMX (MDI), which has been known to inhibit Wnt signaling and promote adipogenesis. Adipogenic differentiation was seen in both hypoxia and MDI. Adipogenic marker gene expression was assessed in C2C12. CCAAT/enhancer-binding protein (C/EBP) beta, alpha and peroxisome proliferator activating receptor (PPAR) gamma were increased by both hypoxia and MDI. The expression profile of Wnt10b was different between hypoxia and MDI. The mechanism for adipogenesis of myoblasts in hypoxia might be regulated by different mechanism than the modification of Wnt signaling.

Gene expression profiling in equine muscle tissues using mouse cDNA microarrays

REASONS FOR PERFORMING STUDY

Progress could be achieved by using microarrays to understand metabolic adaptations and disorders in equine muscle in response to exercise.

OBJECTIVES

To test the feasibility of using mouse cDNA microarrays to analyse gene expression profile in normal equine muscles.

METHODS

Muscular biopsies of dorsal gluteus medius and longissimus lumborum were done in 4 healthy Standardbreds. Total RNA was extracted from the muscle samples. The concentration and quality of RNA were measured before and after amplification. Gene expression profiles were measured using mouse cDNA microarrays including 15,264 unique genes representing about 11,000 documented genes. Three hybridisation tests were performed to check interspecificity, reproducibility and to compare gene expression in these muscles. For each test, a dye-swap hybridisation with Cy3 and Cy5 fluoromarkers were done and the gene list filtered according the signal level.

RESULTS

According to the specificity test, the mouse cDNA microarrays were correctly hybridised by equine muscle cDNA. All positive control genes (GAPDH, HPRT and beta-Actin) and no negative control gene (yeast, plant) hybridised. The reproducibility test demonstrated a good linearity between the duplicate hybridisations: 99.99% of the significant expressed genes have an expression ratio between 1.4 and 1/1.4 = 0.71. These limits can be considered as the thresholds to qualify as up-regulated (ratio >1.4) or downregulated (ratio <0.71). In the muscle comparison test between gluteus medius vs. longissimus lumborum, 63 genes were found up-regulated and 8 genes down-regulated. The range of gene expression ratios in the gluteus medius was 0.61-8.31 x the longissimus lumborum. This list of modulated genes was classified by functions using a gene ontology data basis.

CONCLUSION

Mouse microarrays could be used to hybridise equine RNA extracted from muscle tissues. For many genes there are large sequence identities that allowed interspecific cDNA hybridisation. The sensitivity of the method allowed quantification of up- and down-regulated genes after applying appropriate filters.

POTENTIAL RELEVANCE

Expression profiling could be used to explore the muscle metabolism changes related to exercise, training, pathology and illegal medication in horses.

Down-regulation of MyoD by calpain 3 promotes generation of reserve cells in C2C12 myoblasts

Calpain 3 is a calcium-dependent cysteine protease that is primarily expressed in skeletal muscle and is implicated in limb girdle muscular dystrophy type 2A. To date, its best characterized function is located within the sarcomere, but this protease is found in other cellular compartments, which suggests that it exerts multiple roles. Here, we present evidence that calpain 3 is involved in the myogenic differentiation process. In the course of in vitro culture of myoblasts to fully differentiated myotubes, a population of quiescent undifferentiated "reserve cells" are maintained. These reserve cells are closely related to satellite cells responsible for adult muscle regeneration. In the present work, we observe that reserve cells express higher levels of endogenous Capn3 mRNA than proliferating myoblasts. We show that calpain 3 participates in the establishment of the pool of reserve cells by decreasing the transcriptional activity of the key myogenic regulator MyoD via proteolysis independently of the ubiquitin-proteasome degradation pathway. Our results identify calpain 3 as a potential new player in the muscular regeneration process by promoting renewal of the satellite cell compartment.

Regulating fibrinolysis to engineer skeletal muscle from the C2C12 cell line

Muscles engineered from transformed cells would be a powerful model for the study of muscle physiology by allowing long-term in vitro studies of muscle adaptation. However, previously described methods either take >5 weeks to produce a tissue or use collagen as a scaffold, which decreases the specific force of the muscle, making it hard to measure the function of the constructs. The aim of this study was to rapidly engineer muscle using the C2C12 cell line in fibrin, which has a stiffness similar to muscle tissue, allowing accurate functional testing. Both the protease inhibitor aprotinin and the natural cross-linker genipin increased the length of time that muscle could be cultured, with genipin increasing the time in culture to 10 weeks. The function of the tissues was significantly affected by the batch of serum (64-78%) or thrombin (41%), the differentiation medium (78%), and the seeding protocol (38%), but was unaffected by initial cell number. Strikingly, different C2C12 clones produced up to a 3.6-fold variation in force production. Under optimal conditions, the tissues form in 10.4+/-0.3 days and remain fully functional for 5 weeks over which time they continue to mature. The optimized model described here provides rapid, reliable, and functional tissues that will be useful in the study of skeletal muscle physiology.

Emerging new paradigms for ABCG transporters

Every cell is separated from its external environment by a lipid membrane. Survival depends on the regulated and selective transport of nutrients, waste products and regulatory molecules across these membranes, a process that is often mediated by integral membrane proteins. The largest and most diverse of these membrane transport systems is the ATP binding cassette (ABC) family of membrane transport proteins. The ABC family is a large evolutionary conserved family of transmembrane proteins (>250 members) present in all phyla, from bacteria to Homo sapiens, which require energy in the form of ATP hydrolysis to transport substrates against concentration gradients. In prokaryotes the majority of ABC transporters are involved in the transport of nutrients and other macromolecules into the cell. In eukaryotes, with the exception of the cystic fibrosis transmembrane conductance regulator (CFTR/ABCC7), ABC transporters mobilize substrates from the cytoplasm out of the cell or into specific intracellular organelles. This review focuses on the members of the ABCG subfamily of transporters, which are conserved through evolution in multiple taxa. As discussed below, these proteins participate in multiple cellular homeostatic processes, and functional mutations in some of them have clinical relevance in humans.

ABCG2-mediated DyeCycle Violet efflux defined side population in benign and malignant prostate

The efflux of Hoechst 33342 by ATP-binding cassette protein G2 (ABCG2) membrane pump allows reproducible identification of a subpopulation of cells by flow cytometric analysis termed the "side population" (SP). The SP identified by constitutive Hoechst efflux contains the stem/progenitor cell population from bone marrow and many solid organs, including prostate. DyeCycle Violet (DCV) is a cell membrane permeable, fluorescent vital dye that intercalates into DNA and is a substrate for ABCG2-mediated efflux. Therefore, DCV was evaluated in this study as a tool for identification of the SP from prostate cancer cell lines and from freshly harvested human prostate tissue. SPs that demonstrated ABCG2-mediated efflux of DCV were identified in the human prostate cancer cell lines CWR-R1, DU-145 and RWPE-1, but not in the BPH-1, LAPC-4 or PC-3 cell lines. Additionally, a SP was identified in enzymatically disaggregated prostate tumors from Transgenic Adenocarcinoma of Mouse Prostate (TRAMP), human benign prostate tissue and human prostate cancer tissue. The causal role of ABCG2-mediated efflux of DCV in the identification of the SP was confirmed by loss of the SP by incubation with the specific inhibitor of ABCG2, Fumitremorgin C. Expression of ABCG2 in the SP cells was confirmed by qRT-PCR and immunofluorescence analysis. Consequently, DCV represents an important new tool for isolation of viable candidate stem cells/cancer stem cells as a SP from cultured prostate cell lines, and prostate tissue specimens, without the requirement for instrumentation with ultra-violet excitation capability and minimizing the risk of damage to DNA in the sorted population.

Real-time monitoring of cell transplantation in mouse dystrophic muscles by a secreted alkaline phosphatase reporter gene

Transplantation of muscle precursor cells (MPCs) is a promising approach for the treatment of muscular dystrophies. However, preclinical and clinical results have shown that the technology is not yet efficient enough for most therapeutic applications. Among the problems that remain unsolved are low cellular survival, poor proliferation and lack of migration of the transplanted cells. One major technical hurdle for the optimization of transplantation protocols is how to follow precisely the fate of the cells after transplantation. In this study, we examined the use of a secreted form of the mouse alkaline phosphatase (mSeAP) enzyme as the reporter system transduced into MPCs using a retroviral vector. We show that circulating mSeAP could be detected in the serum of the transplanted mice at different time points after MPC transplantation. We also found that the level of circulating mSeAP is highly correlated with the number of transplanted cells and that mSeAP is an excellent histological marker. Further, studying the levels of circulating mSeAP compared with the number of muscle fibers positive to mSeAP and to dystrophin, enabled detailed analyses of bottleneck steps for successful transplantation. Taken together, our results show that mSeAP is an excellent quantitative 'real-time' reporter gene for cell therapy preclinical studies.

Side population in oral squamous cell carcinoma possesses tumor stem cell phenotypes

To characterize the biological features of side population (SP) in oral squamous cell carcinoma (OCC), SP and non-SP were sorted and compared. The SP cells were more clonogenic and in nude mice, only 10,000 SP cells were needed for tumor development compared to 1,000,000 non-SP cells. The SP cells expressed higher levels of ABCG2, ABCB1, CD44, Oct-4, Bmi-1, NSPc1 and CK19. The SP cells generated SP and non-SP populations, whereas the non-SP cells generated only non-SP. These findings provide the first evidence that SP in OCC possesses tumor stem cell phenotypes and may play an important role in OCC tumorigenesis.

Characterization of a side population of astrocytoma cells in response to temozolomide

OBJECT

Cancer progenitor-like cells isolated by Hoechst 33342 dye efflux (termed the "side population" [SP]) have been studied in a variety of cancers, including malignant brain tumors. In this study, the authors investigate the nature of the SP phenotype in 2 glioma cell lines, U87MG and T98G, and their response to temozolomide. The roles of several adenosine triphosphate-binding cassette (ABC) multidrug transporters expressed by SP cells, in particular ABCG2, are also examined.

METHODS

Using fluorescence-activated cell sorting, the cells were separated into SP and non-SP fractions and analyzed for progenitor cell-like properties with immunofluorescence staining, quantitative real-time polymerase chain reaction, and their ability to reform glioma mass in an immune-compromised mouse. The response of the SP cells to temozolomide was investigated at the cellular and molecular levels. Small interfering RNA knockdown was used to examine the specific role of the ABCG2 transporter, and the cells' tumorigenic potential was measured using the soft agar clonogenic assay.

RESULTS

Side population cells are characterized by the presence of progenitor cell-like properties: increased expression of nestin, musashi-1, and ABCG2 were observed. In addition, only SP cells were able to reconstitute cellular heterogeneity; these cells were also more invasive than the non-SP cells, and possessed tumorigenic capacity. Temozolomide treatment increased the number of SP cells, and this corresponded to more progenitor-like cells, concurrent with elevated expression of several ABC transporters.

CONCLUSIONS

Knockdown of ABCG2 transporters did not abrogate the SP cell response to temozolomide. Upregulation of several other ABC drug transporter genes is proposed to account for this chemoresistance.

Optimization of human skeletal muscle precursor cell culture and myofiber formation in vitro

Muscle bioengineering is proposed as a treatment option for various conditions requiring restoration of muscle function. In order to allow for rapid clinical translation culture conditions have to be optimized for human application. The optimal isolation and culture technique should be able to support cell growth and differentiation using defined media only. Therefore, we have evaluated alternative culture conditions to determine the optimal growth condition for the engineering of human skeletal muscle. In this research, we present protocols for consistent isolation and growth of human muscle precursor cells (MPCs). MPCs were grown from human biopsies and expanded in culture using defined media and collagen coated dishes only. The best results were achieved using a one-step pre-plating and by supplementing the growth medium with insulin, dexamethasone, human basic fibroblast growth factor (hFGF) and human epithelial growth factor (hEGF). Detailed cell characterization using fluorescence-activated cell-sorting analysis and morphological analysis at different passages were performed. Further, the applicability of these cells for tissue engineering purposes was assessed by measuring expansion potential, formation of myofibers and fused myotubes. We have established a culture technique for human MPCs that allows for reliable cell growth and expansion using collagen coated dishes and defined media only. Cell characterization demonstrated a muscle phenotype and the ability to form myofibers in vitro.

IFATS collection: Stem cell antigen-1-positive ear mesenchymal stem cells display enhanced adipogenic potential

Hyperplasia is a major contributor to the increase in adipose tissue mass that is characteristic of obesity. However, the identity and characteristics of cells that can be committed into adipocyte lineage remain unclear. Stem cell antigen 1 (Sca-1) has been used recently as a candidate marker in the search for tissue-resident stem cells. In our quest for biomarkers of cells that can become adipocytes, we analyzed ear mesenchymal stem cells (EMSC), which can differentiate into adipocytes, osteocytes, chondrocytes, and myocytes. Our previous studies have demonstrated that EMSC abundantly expressed Sca-1. In the present study, we have analyzed the expression of adipogenic transcription factors and adipocyte-specific genes in Sca-1-enriched and Sca-1-depleted EMSC fractions. Sca-1-enriched EMSC accumulated more lipid droplets during adipogenic differentiation than Sca-1-depleted. Similarly, EMSC isolated from Sca-1(-/-) mice displayed reduced lipid accumulation relative to EMSC from wild-type controls (p < .01). Comparative analysis of the adipogenic differentiation process between Sca-1-enriched and Sca-1-depleted populations of EMSC revealed substantial differences in the gene expression. Preadipocyte factor 1, CCAAT enhancer-binding protein (C/EBP) beta, C/EBPalpha, peroxisome proliferator-activated receptor gamma2, lipoprotein lipase, and adipocyte fatty acid binding protein were expressed at significantly higher levels in the Sca-1-enriched EMSC fraction. However, the most striking observation was that leptin was detected only in the conditioned medium of Sca-1-enriched EMSC. In addition, we performed loss-of-function (Sca-1 morpholino oligonucleotide) experiments. The data presented here suggest that Sca-1 is a biomarker for EMSC with the potential to become functionally active adipocytes. Disclosure of potential conflicts of interest is found at the end of this article.

Expression of mdr1 is required for efficient long term regeneration of dystrophic muscle

The mouse mdr1a and mdr1b genes are expressed in skeletal muscle, though their precise role in muscle is unknown. Dystrophic muscle is characterized by repeated cycles of degeneration and regeneration. To explore the role of the mdr1 genes during muscle regeneration, we have created a triple knockout mouse lacking the mdr1a, mdr1b, and the dystrophin genes. The resulting ReX mice developed normally and were fertile. However, as adults, ReX had a higher proportion of degenerating muscle fibers and greater long-term loss of muscle mass than mdx. ReX muscles were also characterized by a reduced proportion of muscle side population (mSP) cells, of myogenic cells, and a reduced capacity for muscle regeneration. We found too that mSP cells derived from dystrophic muscle are more myogenic than those from normal muscle. Thus, in dystrophic muscle, the mdr1 gene plays an important role in the preservation of the mSP and of the myogenic regenerative potential. Moreover, our results suggest a hitherto unappreciated role of mdr1 in precursor cells of regenerating tissue; they therefore provide an important clue to the physiological significance of mdr1 expression in stem cells.

Histological and culture studies with respect to ABCG2 expression support the existence of a cancer cell hierarchy in human hepatocellular carcinoma

In this study, we examined the possible involvement of progenitor cells in the carcinogenesis of human hepatocellular carcinoma (HCC) using tissue specimens and cell lines. We used ATP-binding cassette transporter ABCG2 as a progenitor cell marker. Immunohistochemically, ABCG2(+) hepatocytes were observed in the periportal areas of the dysplastic nodule, and ABCG2(+) cancer cells were also scattered or focally clustered in HCC. We sorted the cultured HCC cells (HuH7 and PLC5) into ABCG2(+) and ABCG2(-) subpopulations and then subcultured them for 4 weeks. ABCG2(+) cells could generate ABCG2(+) and ABCG2(-) progenies during subculture, whereas ABCG2(-) cells bore only ABCG2(-) cells, suggesting that a cancer cell hierarchy with reference to ABCG2 exists in HCC cells and that ABCG2(+) cells reside at the higher rank in that hierarchy. Interestingly, other progenitor cell markers including cytokeratin 19 and alpha-fetoprotein were mainly expressed in ABCG2(+) subpopulations. Conversely, albumin expression was more intense in ABCG2(-) cells. In addition, the expression patterns of transcription factors (GATA6, CCAAT/enhancer-binding protein alpha, and CCAAT/enhancer-binding protein beta) in ABCG2(+) and ABCG2(-) cells resembled those during normal liver development. In conclusion, this study suggests that cancer cells with ABCG2 expression might play a central role in hepatocarcinogenesis and the maintenance of the cancer cell hierarchy of human HCC.

Reversine increases the plasticity of lineage-committed mammalian cells

Previously, a small molecule, reversine, was identified that reverses lineage-committed murine myoblasts to a more primitive multipotent state. Here, we show that reversine can increase the plasticity of C2C12 myoblasts at the single-cell level and that reversine-treated cells gain the ability to differentiate into osteoblasts and adipocytes under lineage-specific inducing conditions. Moreover, reversine is active in multiple cell types, including 3T3E1 osteoblasts and human primary skeletal myoblasts. Biochemical and cellular experiments suggest that reversine functions as a dual inhibitor of nonmuscle myosin II heavy chain and MEK1, and that both activities are required for reversine's effect. Inhibition of MEK1 and nonmuscle myosin II heavy chain results in altered cell cycle and changes in histone acetylation status, but other factors also may contribute to the activity of reversine, including activation of the PI3K signaling pathway.

Effects of Hoechst 33342 on C2C12 and PC12 cell differentiation

Accumulative evidence demonstrates that normal as well as cancer stem cells can be identified as a side population following Hoechst 33342 staining and flow cytometric analysis. This popular method is based on the ability of stem cells to efflux this fluorescent vital dye. We demonstrate that Hoechst 33342 can affect cell differentiation, suggesting potential complications in the interpretation of data.

The IGF-I splice variant MGF increases progenitor cells in ALS, dystrophic, and normal muscle

The effects of muscle splice variants of insulin-like growth factor I (IGF-I) on proliferation and differentiation were studied in human primary muscle cell cultures from healthy subjects as well as from muscular dystrophy and ALS patients. Although the initial numbers of mononucleated progenitor cells expressing desmin were lower in diseased muscle, the E domain peptide of IGF-IEc (MGF) significantly increased the numbers of progenitor cells in healthy and diseased muscle. IGF-I significantly enhances myogenic differentiation whereas MGF E peptide blocks this pathway, resulting in an increased progenitor (stem) cell pool and thus potentially facilitating repair and maintenance of this postmitotic tissue.

Gene expression profiling in blood cells of endurance horses completing competition or disqualified due to metabolic disorder

REASONS FOR PERFORMING STUDY

Genomics using cDNA microarrays could provide useful information about physiological adaptations and metabolic disorders in endurance horses.

OBJECTIVES

In order to show that genes are modulated in leucocytes in relationship with performance and clinical status of the horses, gene expression in leucocytes, haematological and biochemical parameters were compared between successful and disqualified endurance horses.

METHODS

Blood samples were collected at rest (TO) and just after a 140-160 km endurance race (T1) in 2 groups of horses: 10 continuing successful (S) and 10 disqualified horses stopped at a vet-gate for metabolic disorders (D). Total RNA was extracted from the blood cells (leucocytes), checked for purity, amplified and hybridised using mouse cDNA microarrays including 15,264 unique genes. Differential gene expressions were studied by hybridisation of each sample T1 vs. a control sample collected at TO (pool of 20 sound horses).

RESULTS

Some significant differences were observed in the haematology and biochemistry of the 2 groups (S vs. D). In Group D, rhadomyolysis was confirmed with CK 13,124 u/l and AST 1242 u/l. The list of 726 (including 603 annotated genes) significant genes was filtered according to a high P-value cut-off (P<0.00001). Among them, 130 were upregulated (expression ratio>1.5) and 288 were down-regulated (<1/1.5). Analysis of variance revealed 62 genes differentially expressed (P<0.05) in Groups D and S. The expression levels of 28 and 50 genes were significantly correlated (r>0.75) with CK and AST level in Group D, respectively. The gene ontology classification showed that more genes were up-regulated in S than in the D. More genes were down-regulated in the disqualified horses.

CONCLUSIONS

Long exercise induced many significant gene modulations in leucocytes. Some genes were expressed in relationship with the clinical phenotype observed in Group D: rhabdomyolysis and haemolysis.

POTENTIAL RELEVANCE

Some of these genes could be candidates to explain poor performance or pathologies. Further association studies with a greater number of genes should be conducted.

The origin of phenotypic heterogeneity in a clonal cell population in vitro

Background

The spontaneous emergence of phenotypic heterogeneity in clonal populations of mammalian cells in vitro is a rule rather than an exception. We consider two simple, mutually non-exclusive models that explain the generation of diverse cell types in a homogeneous population. In the first model, the phenotypic switch is the consequence of extrinsic factors. Initially identical cells may become different because they encounter different local environments that induce adaptive responses. According to the second model, the phenotypic switch is intrinsic to the cells that may occur even in homogeneous environments.

Principal Findings

We have investigated the “extrinsic” and the “intrinsic” mechanisms using computer simulations and experimentation. First, we simulated in silico the emergence of two cell types in a clonal cell population using a multiagent model. Both mechanisms produced stable phenotypic heterogeneity, but the distribution of the cell types was different. The “intrinsic” model predicted an even distribution of the rare phenotype cells, while in the “extrinsic” model these cells formed small clusters. The key predictions of the two models were confronted with the results obtained experimentally using a myogenic cell line.

Conclusions

The observations emphasize the importance of the “ecological” context and suggest that, consistently with the “extrinsic” model, local stochastic interactions between phenotypically identical cells play a key role in the initiation of phenotypic switch. Nevertheless, the “intrinsic” model also shows some other aspects of reality: The phenotypic switch is not triggered exclusively by the local environmental variations, but also depends to some extent on the phenotypic intrinsic robustness of the cells.

Molecular and cellular characterization of ABCG2 in the prostate

Background

Identification and characterization of the prostate stem cell is important for understanding normal prostate development and carcinogenesis. The flow cytometry-based side population (SP) technique has been developed to isolate putative adult stem cells in several human tissue types including the prostate. This phenotype is mainly mediated by the ATP-binding cassette membrane transporter ABCG2.

Methods

Immunolocalization of ABCG2 was performed on normal prostate tissue obtained from radical prostatectomies. Normal human prostate SP cells and ABCG2⁺ cells were isolated and gene expression was determined with DNA array analysis and RT-PCR. Endothelial cells were removed by pre-sorting with CD31.

Results

ABCG2 positive cells were localized to the prostate basal epithelium and endothelium. ABCG2⁺ cells in the basal epithelium constituted less than 1% of the total basal cell population. SP cells constituted 0.5–3% of the total epithelial fraction. The SP transcriptome was essentially the same as ABCG2⁺ and both populations expressed genes indicative of a stem cell phenotype, however, the cells also expressed many genes in common with endothelial cells.

Conclusion

These results provide gene expression profiles for the prostate SP and ABCG2⁺ cells that will be critical for studying normal development and carcinogenesis, in particular as related to the cancer stem cell concept.

Fluctuation of the SP/non-SP phenotype in the C6 glioma cell line

Using the C6 glioma cell as a paradigm, we found that (i) the clonogenicity of C6 cells is several orders of magnitude higher than the percentage of SP cells; (ii) non-SP cells are able to generate SP cells, and conversely SP cells generate non-SP cells; (iii) non-SP sorted cells behave as tumorigenic cells. Hence, in C6 cells cultured in serum-containing medium, SP cells can be generated from non-SP cells. This dynamic equilibrium explains in C6 cells the maintenance of the SP phenotype with cell passaging and demonstrates the existence of tumorigenic non-SP cells.

Progenitor cell isolation from muscle-derived cells based on adhesion properties

Adult skeletal muscle possesses remarkable regenerative capacity that has conventionally been attributed to the satellite cells. These precursor cells were thought to contain distinct populations with varying myogenic potential. Recently, the identification of multipotent stem cells capable of new myofiber formation has expanded the general view on the muscle regenerative process. Here we examined the characteristics of turkey skeletal muscle-derived cell (MDC) populations that were separated according to their adhesion abilities. We sought to determine whether these abilities could be a potential tool for separating cells with different myogenic commitment. Using the preplate technique, we showed that MDCs display a wide range of adhesion ability, allowing us to isolate a marginal fraction with initial adhesion defect. Methodological investigations revealed that this defect represents an intrinsic and well-established biological feature for these cells. In vitro behavioral and morphological analyses showed that late adherent cells (LACs) share several primitive cell characteristics. Phenotypic assessment indicated that LACs contain early stage myogenic cells and immature progenitors of satellite cells, whereas early adherent cells consist mainly of fully committed precursors. Overall, our findings demonstrate for the first time in an avian model that differential MDC adhesion properties could be used to efficiently purify cells with varying myogenic commitment, including immature progenitor cells. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Combination of quantification and observation methods for study of “Side Population” cells in their “in vitro” microenvironment

BACKGROUND

Qualitative and quantitative analyses of the rare phenotypic variants in in vitro culture systems is necessary for the understanding of cell differentiation in cell culture of primary cells or cell lines. Slide-based cytometry combines image acquisition and data treatment, and associates the power of flow cytometry (FCM) and the resolution of the microscopic studies making it suitable for the analysis of cells with rare phenotype. In this paper we develop a method that applies these principles to a particularly hot problem in cell biology, the study of stem cell like cells in cultures of primary cells, cancer cells, and various cell lines.

METHODS

The adherent cells were labeled by the fluorescent dye Hoechst 33342. The images of cell populations were collected by a two-photon microscope and processed by a software developed by us. The software allows the automated segmentation of the nuclei in a very dense cell environment, the measurement of the fluorescence intensity of each nucleus and the recording of their position in the plate. The cells with a given fluorescence intensity can then be located easily on the recorded image of the culture plate for further analysis.

RESULTS

The potential of our method is illustrated by the identification and localization of SP cells in the cultures of the C2C12 cell line. Although these cells represent only about 1% of the total population as calculated by flow cytometry, they can be identified in the culture plate with high precision by microscopy.

CONCLUSION

Cells with the rare stem-cell like phenotype can be efficiently identified in the undisturbed cultures. Since the fluorescence intensity of rare events and the position of thousands of surrounding cells are recorded at the same time, the method associates the advantage of the FCM analysis and the microscopic observation.

Molecular profile of mouse stromal mesenchymal stem cells

We determined a transcriptional profile specific for clonal stromal mesenchymal stem cells from adult and fetal hematopoietic sites. To identify mesenchymal stem cell-like stromal cell lines, we evaluated the adipocytic, osteoblastic, chondrocytic, and vascular smooth muscle differentiation potential and also the hematopoietic supportive (stromal) capacity of six mouse stromal cell lines from adult bone marrow and day 14.5 fetal liver. We found that two lines were quadripotent and also supported hematopoiesis, BMC9 from bone marrow and AFT024 from fetal liver. We then ascertained the set of genes differentially expressed in the intersection set of AFT024 and BMC9 compared with those expressed in the union set of two negative control lines, 2018 and BFC012 (both from fetal liver); 346 genes were upregulated and 299 downregulated. Using Ingenuity software, we found two major gene networks with highly significant scores. One network contained downregulated genes that are known to be implicated in osteoblastic differentiation, proliferation, or transformation. The other network contained upregulated genes that belonged to two categories, cytoskeletal genes and genes implicated in the transcriptional machinery. The data extend the concept of stromal mesenchymal stem cells to clonal cell populations derived not only from bone marrow but also from fetal liver. The gene networks described should discriminate this cell type from other types of stem cells and help define the stem cell state.

SP analysis may be used to identify cancer stem cell populations

Side populations (SP), as defined by Hoechst exclusion in flow cytometry, have been described a few years ago. While they represent only a small fraction of the whole cell population, their properties confer an important place in several investigations. SP cells express high levels of various members of ABC transporters family, such as MDR1 and BCRP, which are responsible for drug resistance. Targeting SP could improve cancer therapy by blocking these transporters. In addition, SP appear to be enriched in stem cells, cells that play a pivotal role in normal development and cancer biology. Thus, they could provide a useful tool and a readily accessible source for stem cell studies in both the normal and cancerous settings. However, these cells are poorly defined and pose challenges in their identification and isolation, particularly since they are few in number. Thus, better characterization of SP will advance our understanding of stem cells and will provide us an accessible target for drug resistance in cancer therapy.

Non-side-population hematopoietic stem cells in mouse bone marrow

Most hematopoietic stem cells (HSCs) are assumed to reside in the so-called side population (SP) in adult mouse bone marrow (BM). We report the coexistence of non-SP HSCs that do not significantly differ from SP HSCs in numbers, capacities, and cell-cycle states. When stained with Hoechst 33342 dye, the CD34(-/low) c-Kit(+)Sca-1(+)lineage marker(-) (CD34(-)KSL) cell population, highly enriched in mouse HSCs, was almost equally divided into the SP and the main population (MP) that represents non-SP cells. Competitive repopulation assays with single or 30 SP- or MP-CD34(-)KSL cells found similar degrees of repopulating activity and frequencies of repopulating cells for these populations. Secondary transplantation detected self-renewal capacity in both populations. SP analysis of BM cells from primary recipient mice suggested that the SP and MP phenotypes are interconvertible. Cell-cycle analyses revealed that CD34(-)KSL cells were in a quiescent state and showed uniform cell-cycle kinetics, regardless of whether they were in the SP or MP. Bcrp-1 expression was similarly detected in SP- and MP-CD34(-)KSL cells, suggesting that the SP phenotype is regulated not only by Bcrp-1, but also by other factors. The SP phenotype does not specify all HSCs; its identity with stem cell function thus is unlikely.

Functional properties of muscle-derived cells related to morphological characteristics

Satellite cells represent a specific lineage of myogenic progenitors that allow skeletal muscle postnatal growth and repair. They have been described as being heterogeneous in nature, a characteristic associated with functional disparities. Here, we aimed at determining whether the morphometric characteristics of freshly extracted turkey muscle-derived cells (MDC) could represent a distinctive criterion between them and could also be associated with their behavioural features. Morphometric analysis showed that MDC displayed wide cell size diversity, from 4 to 10 mum. Lineage marker analysis was performed on MDC sorted by their size using counterflow centrifugal elutriation and showed that the cell size was associated with the specific expression of myogenic markers, revealing different commitment levels. In vitro, the smallest MDC exhibited limited myogenic activity while larger MDC displayed a myogenic potential that increased with their size. Ultrastructural analysis revealed that the smallest MDC shared quiescent cell features, whereas the other cells displayed metabolic activity that also increased as a function of their size. Collectively, our results demonstrate that the size of freshly extracted MDC is indicative of their respective progression towards myogenic differentiation lineage. This criterion could be useful for the early separation of more or less committed cells in the myogenic programme.

Comparison of gene expression pattern in SP cell populations from four tissues to define common "stemness functions"

The goal of our study was to identify a subset of genes commonly expressed in Side Populations (SP), isolated by Hoechst staining followed by flow cytometry, from adult mouse bone marrow, male adult germinal cells, muscle primary culture, and mesenchymal cells. These SP cells have been proposed to be a "stem-like" population and are used here as a "model" that may reveal mechanisms which would be relevant for a better understanding of stem cell properties. Transcriptional profiles for SP and the more differentiated non-SP cells isolated from the four tissues were compared by hybridization on microarray using a common external reference. Among the 503 genes differentially expressed, which discriminate SP and non-SP cells in all the tissues, the genes upregulated in SP cells are implicated in the quiescent status of the cells, the maintenance of their pluripotency and the capacity to undergo asymmetric division. These genes may be responsible for the decision for self-renewal of these cells, whereas the repression of lineage-affiliated genes in SP cells could be responsible for their undifferentiated state. These genes, acting in concert, may be the key players that mediate the mechanisms that control stem cell functions, and our results suggest that we have identified common "stemness functions" of these "stem-like" cells.

Inhibition of Notch signaling induces myotube hypertrophy by recruiting a subpopulation of reserve cells

During muscle differentiation, a population of quiescent undifferentiated myoblasts (reserve cells) emerges among mature muscle cells. However, the molecular mechanisms underlying such cell segregation and the characterization of this subpopulation of myoblasts remain to be determined. Notch is known to control the behavior and fate of murine muscle stem cells. In this study, we examined the role of Notch in myoblast segregation. We showed that inhibition of Notch activity by either overexpressing Numb or by using a pharmacological gamma-secretase inhibitor (DAPT) enhanced differentiation of murine and human myoblasts. This effect was not restricted to in vitro culture systems since DAPT-treated zebrafish embryos also showed increased differentiation. Using C2.7 myoblasts as a model, we showed that inhibition of Notch induced myotube hypertrophy by recruiting reserve cells that do not normally fuse. We further showed that endogenous Notch-signaling components were differentially expressed and activated in reserve cells with respect to Notch 1 and CD34 expression. We identified CD34 negative reserve cells as the subpopulation of myoblasts recruited to fuse into myotubes during differentiation in response to Notch inhibition. Therefore, we showed here that the activation of Notch 1 is important to maintain a subpopulation of CD34 negative reserve cells in an undifferentiated state.

Global transcriptional characterization of SP and MP cells from the myogenic C2C12 cell line: effect of FGF6

With the use of Hoechst staining techniques, we have previously shown that the C2C12 myogenic cell line contains a side population (SP) that is largely increased in the presence of fibroblast growth factor 6 (FGF6). Here, we compared transcriptional profiles from SP and main population (MP) cells from either C2C12 or FGF6-expressing C2C12. Expression profiles of SPs show that these cells are less differentiated than MPs and display some similarities to stem cells. Moreover, principal component analysis made it possible to distinguish specific contributions of either FGF6 or differentiation effects on gene expression profiles. This demonstrated that FGF6-expanded SPs were similar to parental C2C12-derived SPs. Conversely, FGF6-treated MPs differed from parental MPs and were more related to SP cells. These results show that FGF6 pushed committed myogenic cells toward a more immature phenotype resulting in the accumulation of cells with a SP phenotype. We propose that FGF6 conditioning could provide a way to expand the pool of immature cells by myoblast dedifferentiation.

Side population is enriched in tumorigenic, stem-like cancer cells, whereas ABCG2+ and ABCG2- cancer cells are similarly tumorigenic

Recently, several human cancers including leukemia and breast and brain tumors were found to contain stem-like cancer cells called cancer stem cells (CSC). Most of these CSCs were identified using markers that identify putative normal stem cells. In some cases, stem-like cancer cells were identified using the flow cytometry-based side population technique. In this study, we first show that approximately 30% of cultured human cancer cells and xenograft tumors examined ( approximately 30 in total) possess a detectable side population. Purified side population cells from two cell lines (U373 glioma and MCF7 breast cancer) and a xenograft prostate tumor (LAPC-9) are more tumorigenic than the corresponding non-side population cells. These side population cells also possess some intrinsic stem cell properties as they generate non-side population cells in vivo, can be further transplanted, and preferentially express some "stemness" genes, including Notch-1 and beta-catenin. Because the side population phenotype is mainly mediated by ABCG2, an ATP-binding cassette half-transporter associated with multidrug resistance, we subsequently studied ABCG2+ and ABCG2- cancer cells with respect to their tumorigenicity in vivo. Although side population cells show increased ABCG2 mRNA expression relative to the non-side population cells and all cancer cells and xenograft tumors examined express ABCG2 in a small fraction (0.5-3%) of the cells, highly purified ABCG2+ cancer cells, surprisingly, have very similar tumorigenicity to the ABCG2- cancer cells. Mechanistic studies indicate that ABCG2 expression is associated with proliferation and ABCG2+ cancer cells can generate ABCG2- cells. However, ABCG2- cancer cells can also generate ABCG2+ cells. Furthermore, the ABCG2- cancer cells form more and larger clones in the long-term clonal analyses and the ABCG2- population preferentially expresses several "stemness" genes. Taken together, our results suggest that (a) the side population is enriched with tumorigenic stem-like cancer cells, (b) ABCG2 expression identifies mainly fast-cycling tumor progenitors, and (c) the ABCG2- population contains primitive stem-like cancer cells.

Adult stem cells and cancer stem cells: tie in or tear apart?

Stem cell research is one of the new frontiers of medical science. Because of the unique self-renewable ability and powerful potential to differentiate, stem cells can be viewed as the mother of all cells in the body and have been investigated as a possible tool for reversing the degeneration and damage on organs. Recently, successful isolating cancerous stem cells from leukemia, breast and brain cancers provide a new target for eliminate cancer; however, it hints an increasing caution in using adult stem cells for organ repair. Cancerous stem cells share the same properties of self-renewal and differentiation with normal stem cells, with the addition of similar phenotype of adult stem cells isolated from the same tissue. Some believe that cancerous stem cells are derived from mutation of the normal stem cells, whereas others suspect it to be from different origins. Further investigation of the intrinsic factor underlying the behavior of adult stem cells and cancerous stem cells will shed light on both the fields of tissue engineering and cancer therapy. In this review, recent progresses in the studies of adult stem cells and cancerous stem cells are summarized to facilitate a better understanding and elicit much attention in this field.

Failure of Differentiation into Mature Myotubes by Muscle Precursor Cells with the Side-Population Phenotype after Injection into Irreversibly Damaged Striated Urethral Sphincter

We previously showed that the injection of a heterogeneous population of muscle precursor cells (MPCs) into striated urethral sphincter irreversibly damaged by electrocoagulation results in the formation of functional myotubes. To gain further insights into the role played by the different types of MPCs, we elected to sort the MPCs by Hoechst 33342 staining/fluorescence-activated cell-sorter analysis before injection. We found that the side population (SP) cells (muscle stem cells) injected in isolation survived, whereas the main population (MP) cells did not. However, the SP cells failed to differentiate into mature myotubes, as observed previously with unfractionated MPCs containing both SP and MP cells. This result suggests that interactions between SP and MP cells are required for the formation of myotubes in a nonregenerating muscle. In the setting of sphincter insufficiency, injection of MPCs at different stages of maturation may be a better option than purified muscle stem cells.

Sca-1 negatively regulates proliferation and differentiation of muscle cells

Satellite cells are tissue-specific stem cells critical for skeletal muscle growth and regeneration. Upon exposure to appropriate stimuli, satellite cells produce progeny myoblasts. Heterogeneity within a population of myoblasts ensures that a subset of myoblasts readily differentiate to form myotubes, whereas other myoblasts remain undifferentiated and thus available for future muscle growth. The mechanisms that contribute to this heterogeneity in myoblasts are largely unknown. We show that satellite cells are Sca-1(neg) but give rise to myoblasts that are heterogeneous for sca-1 expression. The majority of myoblasts are sca-1(neg), rapidly divide, and are capable of undergoing myogenic differentiation to form myotubes. In contrast, a minority population is sca-1(pos), divides slower, and does not readily form myotubes. Sca-1 expression is not static but rather dynamically modulated by the microenvironment. Gain-of-function and loss-of-function experiments demonstrate that sca-1 has a functional role in regulating proliferation and differentiation of myoblasts. Myofiber size of sca-1 null muscles is altered in an age-dependent manner, with increased size observed in younger mice and decreased size in older mice. These studies reveal a novel system that reversibly modulates the myogenic behavior of myoblasts. These studies provide evidence that, rather than being a fixed property, myoblast heterogeneity can be modulated by the microenvironment.

Dedifferentiation of adult human myoblasts induced by ciliary neurotrophic factor in vitro

Ciliary neurotrophic factor (CNTF) is primarily known for its important cellular effects within the nervous system. However, recent studies indicate that its receptor can be highly expressed in denervated skeletal muscle. Here, we investigated the direct effect of CNTF on skeletal myoblasts of adult human. Surprisingly, we found that CNTF induced the myogenic lineage-committed myoblasts at a clonal level to dedifferentiate into multipotent progenitor cells--they not only could proliferate for over 20 passages with the expression absence of myogenic specific factors Myf5 and MyoD, but they were also capable of differentiating into new phenotypes, mainly neurons, glial cells, smooth muscle cells, and adipocytes. These "progenitor cells" retained their myogenic memory and were capable of redifferentiating into myotubes. Furthermore, CNTF could activate the p44/p42 MAPK and down-regulate the expression of myogenic regulatory factors (MRFs). Finally, PD98059, a specific inhibitor of p44/p42 MAPK pathway, was able to abolish the effects of CNTF on both myoblast fate and MRF expression. Our results demonstrate the myogenic lineage-committed human myoblasts can dedifferentiate at a clonal level and CNTF is a novel regulator of skeletal myoblast dedifferentiation via p44/p42 MAPK pathway.

Side Population cells isolated from different tissues share transcriptome signatures and express tissue-specific markers

Side Population (SP) cells, isolated from murine adult bone marrow (BM) based on the exclusion of the DNA dye Hoechst 33342, exhibit potent hematopoietic stem cell (HSC) activity when compared to Main Population (MP) cells. Furthermore, SP cells derived from murine skeletal muscle exhibit both hematopoietic and myogenic potential in vivo. The multipotential capacity of SP cells isolated from variable tissues is supported by an increasing number of studies. To investigate whether the SP phenotype is associated with a unique transcriptional profile, we characterized gene expression of SP cells isolated from two biologically distinct tissues, bone marrow and muscle. Comparison of SP cells with differentiated MP cells within a tissue revealed that SP cells are in an active transcriptional and translational status and underexpress genes reflecting tissue-specific functions. Direct comparison of gene expression of SP cells isolated from different tissues identified genes common to SP cells as well as genes specific to SP cells within a particular tissue and further define a muscle and bone marrow environment. This study reports gene expression of muscle SP cells, common features and differences between SP cells isolated from muscle and bone marrow, and further identifies common signaling pathways that might regulate SP cell functions.

A proposal for the physiological significance of mdr1 and Bcrp1/Abcg2 gene expression in normal tissue regeneration and after cancer therapy

Cellular multi-drug resistance (MDR), which often develops in cancer cells of patients subjected to anti-cancer treatment, remains a significant barrier to successful cancer therapy. One of the principal causes of cellular MDR development is an increased expression of ABC-transporter genes such as mdr1 and Bcrp1/Abcg2. Despite many years of intensive research, the natural biological role of mdr1 in the context of cancer has remained elusive. Some hints about this role came, however, from an observation that P-gp, the mdr1 encoded protein, is expressed widely in stem cells and from the discovery that P-gp possesses an anti-apoptotic activity independently of exogenous drug application. Here, we discuss our own and other groups' recently published works and propose an integrated view of mdr1 and Bcrp1/Abcg2 activity during tissue regeneration in normal tissues as part of a stress-induced regeneration genetic program and in cancerous tissues in response to cancer therapy.