Satellite cells and the muscle stem cell niche.

Adult skeletal muscle in mammals is a stable tissue under normal circumstances but has remarkable ability to repair after injury. Skeletal muscle regeneration is a highly orchestrated process involving the activation of various cellular and molecular responses. As skeletal muscle stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation of satellite cells not only maintains the stem cell population but also provides numerous myogenic cells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitution of a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscle regeneration is tightly regulated through the dynamic interplay between intrinsic factors within satellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. For the last half century, the advance of molecular biology, cell biology, and genetics has greatly improved our understanding of skeletal muscle biology. Here, we review some recent advances, with focuses on functions of satellite cells and their niche during the process of skeletal muscle regeneration.

Canonical Wnt signaling induces a primitive endoderm metastable state in mouse embryonic stem cells.

Activation of the canonical Wnt signaling pathway synergizes with leukemia inhibitory factor (LIF) to maintain pluripotency of mouse embryonic stem cells (mESCs). However, in the absence of LIF, Wnt signaling is unable to maintain ESCs in the undifferentiated state. To investigate the role of canonical Wnt signaling in pluripotency and lineage specification, we expressed Wnt3a in mESCs and characterized them in growth and differentiation. We found that activated canonical Wnt signaling induced the formation of a reversible metastable primitive endoderm state in mESC. Upon subsequent differentiation, Wnt3a-stimulated mESCs gave rise to large quantities of visceral endoderm. Furthermore, we determined that the ability of canonical Wnt signaling to induce a metastable primitive endoderm state was mediated by Tbx3. Our data demonstrates a specific role for canonical Wnt signaling in promoting pluripotency while at the same time priming cells for subsequent differentiation into the primitive endoderm lineage.

FOS licenses early events in stem cell activation driving skeletal muscle regeneration.

Muscle satellite cells (SCs) are a quiescent (non-proliferative) stem cell population in uninjured skeletal muscle. Although SCs have been investigated for nearly 60 years, the molecular drivers that transform quiescent SCs into the rapidly dividing (activated) stem/progenitor cells that mediate muscle repair after injury remain largely unknown. Here we identify a prominent FBJ osteosarcoma oncogene (Fos) mRNA and protein signature in recently activated SCs that is rapidly, heterogeneously, and transiently induced by muscle damage. We further reveal a requirement for FOS to efficiently initiate key stem cell functions, including cell cycle entry, proliferative expansion, and muscle regeneration, via induction of "pro-regenerative" target genes that stimulate cell migration, division, and differentiation. Disruption of one of these Fos/AP-1 targets, NAD(+)-consuming mono-ADP-ribosyl-transferase 1 (Art1), in SCs delays cell cycle entry and impedes progenitor cell expansion and muscle regeneration. This work uncovers an early-activated FOS/ART1/mono-ADP-ribosylation (MARylation) pathway that is essential for stem cell-regenerative responses.

Retinoic acid enhances skeletal muscle progenitor formation and bypasses inhibition by bone morphogenetic protein 4 but not dominant negative beta-catenin.

BACKGROUND: Understanding stem cell differentiation is essential for the future design of cell therapies. While retinoic acid (RA) is the most potent small molecule enhancer of skeletal myogenesis in stem cells, the stage and mechanism of its function has not yet been elucidated. Further, the intersection of RA with other signalling pathways that stimulate or inhibit myogenesis (such as Wnt and BMP4, respectively) is unknown. Thus, the purpose of this study is to examine the molecular mechanisms by which RA enhances skeletal myogenesis and interacts with Wnt and BMP4 signalling during P19 or mouse embryonic stem (ES) cell differentiation. RESULTS: Treatment of P19 or mouse ES cells with low levels of RA led to an enhancement of skeletal myogenesis by upregulating the expression of the mesodermal marker, Wnt3a, the skeletal muscle progenitor factors Pax3 and Meox1, and the myogenic regulatory factors (MRFs) MyoD and myogenin. By chromatin immunoprecipitation, RA receptors (RARs) bound directly to regulatory regions in the Wnt3a, Pax3, and Meox1 genes and RA activated a beta-catenin-responsive promoter in aggregated P19 cells. In the presence of a dominant negative beta-catenin/engrailed repressor fusion protein, RA could not bypass the inhibition of skeletal myogenesis nor upregulate Meox1 or MyoD. Thus, RA functions both upstream and downstream of Wnt signalling. In contrast, it functions downstream of BMP4, as it abrogates BMP4 inhibition of myogenesis and Meox1, Pax3, and MyoD expression. Furthermore, RA downregulated BMP4 expression and upregulated the BMP4 inhibitor, Tob1. Finally, RA inhibited cardiomyogenesis but not in the presence of BMP4. CONCLUSION: RA can enhance skeletal myogenesis in stem cells at the muscle specification/progenitor stage by activating RARs bound directly to mesoderm and skeletal muscle progenitor genes, activating beta-catenin function and inhibiting bone morphogenetic protein (BMP) signalling. Thus, a signalling pathway can function at multiple levels to positively regulate a developmental program and can function by abrogating inhibitory pathways. Finally, since RA enhances skeletal muscle progenitor formation, it will be a valuable tool for designing future stem cell therapies.

Pro-myogenic small molecules revealed by a chemical screen on primary muscle stem cells.

Satellite cells are the canonical muscle stem cells that regenerate damaged skeletal muscle. Loss of function of these cells has been linked to reduced muscle repair capacity and compromised muscle health in acute muscle injury and congenital neuromuscular diseases. To identify new pathways that can prevent loss of skeletal muscle function or enhance regenerative potential, we established an imaging-based screen capable of identifying small molecules that promote the expansion of freshly isolated satellite cells. We found several classes of receptor tyrosine kinase (RTK) inhibitors that increased freshly isolated satellite cell numbers in vitro. Further exploration of one of these compounds, the RTK inhibitor CEP-701 (also known as lestaurtinib), revealed potent activity on mouse satellite cells both in vitro and in vivo. This expansion potential was not seen upon exposure of proliferating committed myoblasts or non-myogenic fibroblasts to CEP-701. When delivered subcutaneously to acutely injured animals, CEP-701 increased both the total number of satellite cells and the rate of muscle repair, as revealed by an increased cross-sectional area of regenerating fibers. Moreover, freshly isolated satellite cells expanded ex vivo in the presence of CEP-701 displayed enhanced muscle engraftment potential upon in vivo transplantation. We provide compelling evidence that certain RTKs, and in particular RET, regulate satellite cell expansion during muscle regeneration. This study demonstrates the power of small molecule screens of even rare adult stem cell populations for identifying stem cell-targeting compounds with therapeutic potential.

Gene profiling on mixed embryonic stem cell populations reveals a biphasic role for beta-catenin in osteogenic differentiation.

The differentiation of embryonic stem cells (ESCs) into osteoblasts is enhanced to 60% when exposed to vitamin D3 (VD3) but leaves a remainder of one half of the cell population unidentified. To increase differentiation outcome, the known osteoinducers retinoic acid (RA) and bone morphogenetic protein-2 (BMP-2) were evaluated. Initial studies using RA and BMP-2 during early osteogenesis in addition to VD3 increased osteogenic yield in the case of RA, but surprisingly decreased osteogenesis when BMP-2 was administered together with VD3 or RA. This paper describes a comprehensive microarray study examining the gene expression profile of differentiating osteoblasts in these mixed ESC populations. In addition to five other families of signaling molecules (insulin growth factors, prostaglandin, follistatin, TGFbeta2, and Wnt molecules), we identified an endogenous expression pattern for BMPs and RA that differed from our previous exogenous administration of these molecules. By mimicking the change in expression of the RA and BMP-2 families with exogenous supplementation at the correct time, it was then possible to increase the number of ESC-derived osteoblasts to 90%. This effect was mediated through alteration in beta-catenin (CatnB) expression levels and nuclear CatnB activity, both of which are modulated by VD3, RA, and BMP-2. Our results suggest that blockage of CatnB activity by VD3 and RA is opposed by induction of CatnB activity through BMP-2 when administered together. Hence, osteoinduction, in vitro, is an intricate process involving both temporal and quantitative changes in gene expression and CatnB activity.

The solution structure of the C-terminal domain of TonB and interaction studies with TonB box peptides.

The TonB protein transduces energy from the proton gradient across the cytoplasmic membrane of Gram-negative bacteria to TonB-dependent outer membrane receptors. It is a critically important protein in iron uptake, and deletion of this protein is known to decrease virulence of bacteria in animal models. This system has been used for Trojan horse antibiotic delivery. Here, we describe the high-resolution solution structure of Escherichia coli TonB residues 103-239 (TonB-CTD). TonB-CTD is monomeric with an unstructured N terminus (103-151) and a well structured C terminus (152-239). The structure contains a four-stranded antiparallel beta-sheet packed against two alpha-helices and an extended strand in a configuration homologous to the C-terminal domain of the TolA protein. Chemical shift perturbations to the TonB-CTD (1)H-(15)N HSCQ spectrum titrated with TonB box peptides modeled from the E.coli FhuA, FepA and BtuB proteins were all equivalent, indicating that all three peptides bind to the same region of TonB. Isothermal titration calorimetry measurements demonstrate that TonB-CTD interacts with the FhuA-derived peptide with a K(D)=36(+/-7) microM. On the basis of chemical shift data, the position of Gln160, and comparison to the TolA gp3 N1 complex crystal structure, we propose that the TonB box binds to TonB-CTD along the beta3-strand.

Large-Scale Production of Mature Neurons from Human Pluripotent Stem Cells in a Three-Dimensional Suspension Culture System.

Human pluripotent stem cells (hPSCs) offer a renewable source of cells that can be expanded indefinitely and differentiated into virtually any type of cell in the human body, including neurons. This opens up unprecedented possibilities to study neuronal cell and developmental biology and cellular pathology of the nervous system, provides a platform for the screening of chemical libraries that affect these processes, and offers a potential source of transplantable cells for regenerative approaches to neurological disease. However, defining protocols that permit a large number and high yield of neurons has proved difficult. We present differentiation protocols for the generation of distinct subtypes of neurons in a highly reproducible manner, with minimal experiment-to-experiment variation. These neurons form synapses with neighboring cells, exhibit spontaneous electrical activity, and respond appropriately to depolarization. hPSC-derived neurons exhibit a high degree of maturation and survive in culture for up to 4-5 months, even without astrocyte feeder layers.

Genome-wide RNA-Seq of Human Motor Neurons Implicates Selective ER Stress Activation in Spinal Muscular Atrophy.

Spinal muscular atrophy (SMA) is caused by mutations in the SMN1 gene. Because this gene is expressed ubiquitously, it remains poorly understood why motor neurons (MNs) are one of the most affected cell types. To address this question, we carried out RNA sequencing studies using fixed, antibody-labeled, and purified MNs produced from control and SMA patient-derived induced pluripotent stem cells (iPSCs). We found SMA-specific changes in MNs, including hyper-activation of the ER stress pathway. Functional studies demonstrated that inhibition of ER stress improves MN survival in vitro even in MNs expressing low SMN. In SMA mice, systemic delivery of an ER stress inhibitor that crosses the blood-brain barrier led to the preservation of spinal cord MNs. Therefore, our study implies that selective activation of ER stress underlies MN death in SMA. Moreover, the approach we have taken would be broadly applicable to the study of disease-prone human cells in heterogeneous cultures.

Wnt/ß-catenin controls follistatin signalling to regulate satellite cell myogenic potential.

BACKGROUND: Adult skeletal muscle regeneration is a highly orchestrated process involving the activation and proliferation of satellite cells, an adult skeletal muscle stem cell. Activated satellite cells generate a transient amplifying progenitor pool of myoblasts that commit to differentiation and fuse into multinucleated myotubes. During regeneration, canonical Wnt signalling is activated and has been implicated in regulating myogenic lineage progression and terminal differentiation. METHODS: Here, we have undertaken a gene expression analysis of committed satellite cell-derived myoblasts to examine their ability to respond to canonical Wnt/ß-catenin signalling. RESULTS: We found that activation of canonical Wnt signalling induces follistatin expression in myoblasts and promotes myoblast fusion in a follistatin-dependent manner. In growth conditions, canonical Wnt/ß-catenin signalling prime myoblasts for myogenic differentiation by stimulating myogenin and follistatin expression. We further found that myogenin binds elements in the follistatin promoter and thus acts downstream of myogenin during differentiation. Finally, ectopic activation of canonical Wnt signalling in vivo promoted premature differentiation during muscle regeneration following acute injury. CONCLUSIONS: Together, these data reveal a novel mechanism by which myogenin mediates the canonical Wnt/ß-catenin-dependent activation of follistatin and induction of the myogenic differentiation process.

Inhibition of JAK-STAT signaling stimulates adult satellite cell function.

Diminished regenerative capacity of skeletal muscle occurs during adulthood. We identified a reduction in the intrinsic capacity of mouse adult satellite cells to contribute to muscle regeneration and repopulation of the niche. Gene expression analysis identified higher expression of JAK-STAT signaling targets in 3-week [corrected] 18-month-old mice [corrected]. Knockdown of Jak2 or Stat3 significantly stimulated symmetric satellite stem cell divisions on cultured myofibers. Genetic knockdown of Jak2 or Stat3 expression in prospectively isolated satellite cells markedly enhanced their ability to repopulate the satellite cell niche after transplantation into regenerating tibialis anterior muscle. Pharmacological inhibition of Jak2 and Stat3 activity similarly stimulated symmetric expansion of satellite cells in vitro and their engraftment in vivo. Intramuscular injection of these drugs resulted in a marked enhancement of muscle repair and force generation after cardiotoxin injury. Together these results reveal age-related intrinsic properties that functionally distinguish satellite cells and suggest a promising therapeutic avenue for the treatment of muscle-wasting diseases.

Integration of expressed sequence tag data flanking predicted RNA secondary structures facilitates novel non-coding RNA discovery.

Many computational methods have been used to predict novel non-coding RNAs (ncRNAs), but none, to our knowledge, have explicitly investigated the impact of integrating existing cDNA-based Expressed Sequence Tag (EST) data that flank structural RNA predictions. To determine whether flanking EST data can assist in microRNA (miRNA) prediction, we identified genomic sites encoding putative miRNAs by combining functional RNA predictions with flanking ESTs data in a model consistent with miRNAs undergoing cleavage during maturation. In both human and mouse genomes, we observed that the inclusion of flanking ESTs adjacent to and not overlapping predicted miRNAs significantly improved the performance of various methods of miRNA prediction, including direct high-throughput sequencing of small RNA libraries. We analyzed the expression of hundreds of miRNAs predicted to be expressed during myogenic differentiation using a customized microarray and identified several known and predicted myogenic miRNA hairpins. Our results indicate that integrating ESTs flanking structural RNA predictions improves the quality of cleaved miRNA predictions and suggest that this strategy can be used to predict other non-coding RNAs undergoing cleavage during maturation.