The myoD gene family: nodal point during specification of the muscle cell lineage

Origin of vertebrate limb muscle: the role of progenitor and myoblast populations

Muscle development, growth, and regeneration take place throughout vertebrate life. In amniotes, myogenesis takes place in four successive, temporally distinct, although overlapping phases. Understanding how embryonic, fetal, neonatal, and adult muscle are formed from muscle progenitors and committed myoblasts is an area of active research. In this review we examine recent expression, genetic loss-of-function, and genetic lineage studies that have been conducted in the mouse, with a particular focus on limb myogenesis. We synthesize these studies to present a current model of how embryonic, fetal, neonatal, and adult muscle are formed in the limb.

Transcriptional networks that regulate muscle stem cell function

Muscle stem cells comprise different populations of stem and progenitor cells found in embryonic and adult tissues. A number of signaling and transcriptional networks are responsible for specification and survival of these cell populations and regulation of their behavior during growth and regeneration. Muscle progenitor cells are mostly derived from the somites of developing embryos, while satellite cells are the progenitor cells responsible for the majority of postnatal growth and adult muscle regeneration. In resting muscle, these stem cells are quiescent, but reenter the cell cycle during their activation, whereby they undergo decisions to self-renew, proliferate, or differentiate and fuse into multinucleated myofibers to repair damaged muscle. Regulation of muscle stem cell activity is under the precise control of a number of extrinsic signaling pathways and active transcriptional networks that dictate their behavior, fate, and regenerative potential. Here, we review the networks responsible for these different aspects of muscle stem cell biology and discuss prevalent parallels between mechanisms regulating the activity of embryonic muscle progenitor cells and adult satellite cells.

Networking the nucleus

The nuclei of differentiating cells exhibit several fundamental principles of self-organization. They are composed of many dynamical units connected physically and functionally to each other—a complex network—and the different parts of the system are mutually adapted and produce a characteristic end state. A unique cell-specific signature emerges over time from complex interactions among constituent elements that delineate coordinate gene expression and chromosome topology. Each element itself consists of many interacting components, all dynamical in nature. Self-organizing systems can be simplified while retaining complex information using approaches that examine the relationship between elements, such as spatial relationships and transcriptional information. These relationships can be represented using well-defined networks. We hypothesize that during the process of differentiation, networks within the cell nucleus rewire according to simple rules, from which a higher level of order emerges. Studying the interaction within and among networks provides a useful framework for investigating the complex organization and dynamic function of the nucleus.

In vitro characterization of proliferation and differentiation of trout satellite cells

Fish satellite cells have been extracted from various species, but the myogenic characteristics of these cells in culture remain largely unknown. We show here that 60%-70% of the adherent cells are myogenic based on their immunoreactivity for the myogenic regulatory factor MyoD. In DMEM containing 10% fetal calf serum (FCS), trout myoblasts display rapid expression of myogenin (18% of myogenin-positive cells at day 2) combined with rapid fusion into myotubes (50% of myogenin-positive nuclei and 30% nuclei in myosin heavy chain [MyHC]-positive cells at day 7). These kinetics of differentiation are reminiscent of the behavior of fetal myoblasts in mammals. However, not all the myogenic cells differentiate; this subpopulation of cells might correspond to the previously named "reserve" cells. More than 90% of the BrdU-positive cells are also positive for MyoD, indicating that myogenic cells proliferate in vitro. By contrast, less than 1% of myogenin-positive cells are positive for BrdU suggesting that myogenin expression occurs only in post-mitotic cells. In order to maximize either the proliferation or the differentiation of cells, we have defined new culture conditions based on the use of a proliferation medium (F10+10%FCS) and a differentiation medium (DMEM+2%FCS). Three days after switching the medium, the differentiation index (% MyHC-positive nuclei) is 40-fold higher than that in proliferation medium, whereas the proliferation index (% BrdU-positive nuclei) is three-fold lower. Stimulation of cell proliferation by insulin-like growth factor 1 (IGF1), IGF2, and FGF2 is greater in F10 medium. The characterization of these extracted muscle cells thus validates the use of this in vitro system of myogenesis in further studies of the myogenic activity of growth factors in trout.

MyoD directly up-regulates premyogenic mesoderm factors during induction of skeletal myogenesis in stem cells

Gain- and loss-of-function experiments have illustrated that the family of myogenic regulatory factors is necessary and sufficient for the formation of skeletal muscle. Furthermore, MyoD required cellular aggregation to induce myogenesis in P19 embryonal carcinoma stem cells. To determine the mechanism by which stem cells can be directed into skeletal muscle, a time course of P19 cell differentiation was examined in the presence and absence of exogenous MyoD. By quantitative PCR, the first MyoD up-regulated transcripts were the premyogenic mesoderm factors Meox1, Pax7, Six1, and Eya2 on day 4 of differentiation. Subsequently, the myoblast markers myogenin, MEF2C, and Myf5 were up-regulated, leading to skeletal myogenesis. These results were corroborated by Western blot analysis, showing up-regulation of Pax3, Six1, and MEF2C proteins, prior to myogenin protein expression. To determine at what stage a dominant-negative MyoD/EnR mutant could inhibit myogenesis, stable cell lines were created and examined. Interestingly, the premyogenic mesoderm factors, Meox1, Pax3/7, Six1, Eya2, and Foxc1, were down-regulated, and as expected, skeletal myogenesis was abolished. Finally, to identify direct targets of MyoD in this system, chromatin immunoprecipitation experiments were performed. MyoD was observed associated with regulatory regions of Meox1, Pax3/7, Six1, Eya2, and myogenin genes. Taken together, MyoD directs stem cells into the skeletal muscle lineage by binding and activating the expression of premyogenic mesoderm genes, prior to activating myoblast genes.

Making memories that last a lifetime: heritable functions of self-renewing memory CD8 T cells

Clonal expansion of virus-specific naive T cells during an acute viral infection results in the formation of memory CD8 T cells that provide the host with long-term protective immunity against the pathogen. Memory CD8 T cells display enhanced effector functions compared with their naive precursors, allowing them to respond more rapidly and effectively to antigen re-encounter. The enhanced functions of memory CD8 T cells are mediated by heritable changes in gene regulation. Expression of select transcription factors along with locus-specific epigenetic modifications are coupled to and are essential in the formation of memory-specific gene expression patterns. Here, we will review the changes in gene expression that accompany development of memory CD8 T cells and discuss chromatin modifications as a potential means for heritable propagation of these changes during homeostatic cell division of self-renewing memory CD8 T cells. Also, we will discuss therapies that manipulate heritable gene regulation as a potential mechanism to restore function to non-functional memory CD8 T cells to combat chronic viral infection.

MafA and MafB regulate genes critical to beta-cells in a unique temporal manner

OBJECTIVE

Several transcription factors are essential to pancreatic islet β-cell development, proliferation, and activity, including MafA and MafB. However, MafA and MafB are distinct from others in regard to temporal and islet cell expression pattern, with β-cells affected by MafB only during development and exclusively by MafA in the adult. Our aim was to define the functional relationship between these closely related activators to the β-cell.

RESEARCH DESIGN AND METHODS

The distribution of MafA and MafB in the β-cell population was determined immunohistochemically at various developmental and perinatal stages in mice. To identify genes regulated by MafB, microarray profiling was performed on wild-type and MafB(-/-) pancreata at embryonic day 18.5, with candidates evaluated by quantitative RT-PCR and in situ hybridization. The potential role of MafA in the expression of verified targets was next analyzed in adult islets of a pancreas-wide MafA mutant (termed MafA(ΔPanc)).

RESULTS

MafB was produced in a larger fraction of β-cells than MafA during development and found to regulate potential effectors of glucose sensing, hormone processing, vesicle formation, and insulin secretion. Notably, expression from many of these genes was compromised in MafA(ΔPanc) islets, suggesting that MafA is required to sustain expression in adults.

CONCLUSIONS

Our results provide insight into the sequential manner by which MafA and MafB regulate islet β-cell formation and maturation.

Symmetry breaking in plants: molecular mechanisms regulating asymmetric cell divisions in Arabidopsis

Asymmetric cell division generates cell types with different specialized functions or fates. This type of division is critical to the overall cellular organization and development of many multicellular organisms. In plants, regulated asymmetric cell divisions are of particular importance because cell migration does not occur. The influence of extrinsic cues on asymmetric cell division in plants is well documented. Recently, candidate intrinsic factors have been identified and links between intrinsic and extrinsic components are beginning to be elucidated. A novel mechanism in breaking symmetry was revealed that involves the movement of typically intrinsic factors between plant cells. As we learn more about the regulation of asymmetric cell divisions in plants, we can begin to reflect on the similarities and differences between the strategies used by plants and animals. Focusing on the underlying molecular mechanisms, this article describes three selected cases of symmetry-breaking events in the model plant Arabidopsis thaliana. These examples occur in early embryogenesis, stomatal development, and ground tissue formation in the root.

Lessons for cardiac regeneration and repair through development

Cell-based regenerative strategies have the potential to revolutionize the way cardiovascular injury is treated, but successful therapies will require a precise understanding of the mechanisms that dictate cell fate, survival and differentiation. Recent advances in the study of cardiac development hold promise for unlocking the keys for successful therapies. Using mouse models and embryonic stem cells, researchers are uncovering cardiac progenitor cells in both embryonic and adult contexts. Furthermore, the signaling molecules and transcriptional regulators that govern these cells and their behavior are being revealed. Here, we focus on the recent advances in these areas of cardiac developmental research and their impact on the expanding field of regenerative medicine.

Predicting embryonic patterning using mutual entropy fitness and in silico evolution

During vertebrate embryogenesis, the expression of Hox genes that define anterior-posterior identity follows general rules: temporal colinearity and posterior prevalence. A mathematical measure for the quality or fitness of the embryonic pattern produced by a gene regulatory network is derived. Using this measure and in silico evolution we derive gene interaction networks for anterior-posterior (AP) patterning under two developmental paradigms. For patterning during growth (paradigm I), which is appropriate for vertebrates and short germ-band insects, the algorithm creates gene expression patterns reminiscent of Hox gene expression. The networks operate through a timer gene, the level of which measures developmental progression (a candidate is the widely conserved posterior morphogen Caudal). The timer gene provides a simple mechanism to coordinate patterning with growth rate. The timer, when expressed as a static spatial gradient, functions as a classical morphogen (paradigm II), providing a natural way to derive the AP patterning, as seen in long germ-band insects that express their Hox genes simultaneously, from the ancestral short germ-band system. Although the biochemistry of Hox regulation in higher vertebrates is complex, the actual spatiotemporal expression phenotype is not, and simple activation and repression by Hill functions suffices in our model. In silico evolution provides a quantitative demonstration that continuous positive selection can generate complex phenotypes from simple components by incremental evolution, as Darwin proposed.

Mitotic bookmarking of genes: a novel dimension to epigenetic control

Regulatory machinery is focally organized in the interphase nucleus. The information contained in these focal nuclear microenvironments must be inherited during cell division to sustain physiologically responsive gene expression in progeny cells. Recent results suggest that focal mitotic retention of phenotypic transcription factors at promoters together with histone modifications and DNA methylation--a mechanism collectively known as gene bookmarking--is a novel parameter of inherited epigenetic control that sustains cellular identity after mitosis. The epigenetic signatures imposed by bookmarking poise genes for activation or suppression following mitosis. We discuss the implications of phenotypic transcription factor retention on mitotic chromosomes in biological control and disease.

Pax3 induces differentiation of juvenile skeletal muscle stem cells without transcriptional upregulation of canonical myogenic regulatory factors

Pax3 is an essential myogenic regulator of fetal and embryonic development, but its role in postnatal myogenesis remains a topic of debate. We show that constitutive expression of Pax3 in postnatal, juvenile mouse skeletal muscle stem cells, a subset of the heterogeneous satellite cell pool highly enriched for myogenic activity, potently induces differentiation. This differentiation-promoting activity stands in contrast to the differentiation-inhibiting effects of Pax3 in the commonly used mouse myoblast cell line C2C12. Pax3 mRNA levels in distinct muscles correlate with the rate of myogenic differentiation of their muscle stem cells. Although Pax3 controls embryonic myogenesis through regulation of the canonical myogenic regulatory factors (MRFs) Myf-5, MyoD, myogenin and Mrf4, we find that in postnatal muscle stem cells, ectopic Pax3 expression fails to induce expression of any of these factors. Unexpectedly, overexpression of neither Myf-5 nor myogenin is sufficient to induce differentiation of juvenile stem cells; and knockdown of Myf-5, rather than inhibiting differentiation, promotes it. Taken together, our results suggest that there are distinct myogenic regulatory pathways that control the embryonic development, juvenile myogenesis and adult regeneration of skeletal myofibers.

Arsenic inhibits myogenic differentiation and muscle regeneration

BACKGROUND

The incidence of low birth weights is increased in offspring of women who are exposed to high concentrations of arsenic in drinking water compared with other women. We hypothesized that effects of arsenic on birth weight may be related to effects on myogenic differentiation.

OBJECTIVE

We investigated the effects of arsenic trioxide (As2O3) on the myogenic differentiation of myoblasts in vitro and muscle regeneration in vivo.

METHODS

C2C12 myoblasts and primary mouse and human myoblasts were cultured in differentiation media with or without As2O3 (0.1-0.5 microM) for 4 days. Myogenic differentiation was assessed by myogenin and myosin heavy chain expression and multinucleated myotube formation in vitro; skeletal muscle regeneration was tested using an in vivo mouse model with experimental glycerol myopathy.

RESULTS

A submicromolar concentration of As2O3 dose-dependently inhibited myogenic differentiation without apparent effects on cell viability. As2O3 significantly and dose-dependently decreased phosphorylation of Akt and p70s6k proteins during myogenic differentiation. As2O3-induced inhibition in myotube formation and muscle-specific protein expression was reversed by transfection with the constitutively active form of Akt. Sections of soleus muscles stained with hematoxylin and eosin showed typical changes of injury and regeneration after local glycerol injection in mice. Regeneration of glycerol-injured soleus muscles, myogenin expression, and Akt phosphorylation were suppressed in muscles isolated from As2O3-treated mice compared with untreated mice.

CONCLUSION

Our results suggest that As2O3 inhibits myogenic differentiation by inhibiting Akt-regulated signaling.

Polymorphisms of MRF4 and H-FABP genes association with growth traits in Qinchuan cattle and related hybrids

PCR-RFLP was applied to analyse polymorphisms within the MRF4 and heart fatty acid-binding protein (H-FABP) gene for correlation studies with growth traits in three-month-old Qinchuan (QQ), Qinchuan × Limousin (LQ) and Qinchuan × Red Angus (AQ) cattle. The results showed that 874 bp PCR products of MRF4 digested with XbaI and 2,075 bp PCR products of H-FABP digested with HaeIII were polymorphic in the three populations. Moreover, the frequencies of allele A at MRF4 locus and allele B at H-FABP locus in the QQ, AQ, and LQ populations were 0.8358/0.8888/0.8273 and 0.8358/0.7500/0.8195 respectively. Allele A at MRF4 locus and allele B at H-FABP locus were dominant in the three populations. No statistically significant differences in growth traits were observed among the genotypes of the all three populations at H-FABP locus. However, the association of MRF4 polymorphism with growth traits was then determined in all three populations. The body weight, withers height, heart girth and height at hip cross of individuals with genotype AA were higher than those with genotype AB or BB (P < 0.05). Therefore, we suggest that the MRF4 gene may function in the control or expression of growth traits, particularly body weight, withers height, heart girth and height at hip cross.

The formation of the embryonic mouse heart: heart fields and myocardial cell lineages

During cardiogenesis in the mouse, the second heart field (SHF) is the source of the myocardium of the outflow tract and it contributes to other regions of the heart with the exception of the primitive left ventricle. This contribution corresponds with that of the second myocardial cell lineage, identified by retrospective clonal analysis. Gene regulatory networks, signaling pathways, and heterogeneity within the SHF are discussed, together with the question of regulation of myocardial progenitor cells within the first heart field. The extension of the SHF into the mesodermal core of the arches also gives rise to endothelial cells of the pharyngeal arch arteries. Knowledge about the origin and genetic regulation of cells that contribute to the heart and associated vasculature is important for the diagnosis and treatment of congenital heart malformations.

The influence of acoustic and static stimuli on development of inner ear sensory epithelia

Mechanical stimuli affect differentiation of specific cell types in several organs of mouse fetuses that develop without any skeletal musculature. To that end, we employed Myf5(-/-):MyoD(-/-) mouse embryos that completely lack skeletal musculature, and analyzed the development of sensory fields in the inner ear. Amyogenic fetuses lack skeletal muscles that move the chain of three middle ear ossicles which normally transfers sound vibrations. They also cannot tilt their head, which prevents the perception of angular acceleration. While our findings in the spiral organ of Corti are surprisingly normal, our results show that the development of cristae ampullaris, vestibular sensory fields sensitive to the angular acceleration, was the most affected. In cristae, hair cells and supporting cells were significantly smaller in the mutant embryos, but hair cells completely lacked tenascin, while supporting cells were more numerous. In maculae, supporting cells were significantly smaller but more numerous in the mutants. Here, we propose that our finding of a specific type I hair cell absence in the mutant's crista may now be employed in the identification of a profile of genes specific for the lacking cell type.

Algorithm of myogenic differentiation in higher-order organisms

Cell fate determination is governed by complex signaling molecules at appropriate concentrations that regulate the cell decision-making process. In vertebrates, however, concentration and kinetic parameters are practically unknown, and therefore the mechanism by which these molecules interact is obscure. In myogenesis, for example, multipotent cells differentiate into skeletal muscle as a result of appropriate interplay between several signaling molecules, which is not sufficiently characterized. Here we demonstrate that treatment of biochemical events with SAT (satisfiability) formalism, which has been primarily applied for solving decision-making problems, can provide a simple conceptual tool for describing the relationship between causes and effects in biological phenomena. Specifically, we applied the Łukasiewicz logic to a diffusible protein system that leads to myogenesis. The creation of an automaton that describes the myogenesis SAT problem has led to a comprehensive overview of this non-trivial phenomenon and also to a hypothesis that was subsequently verified experimentally. This example demonstrates the power of applying Łukasiewicz logic in describing and predicting any decision-making problem in general, and developmental processes in particular.

Activated protein C therapy slows ALS-like disease in mice by transcriptionally inhibiting SOD1 in motor neurons and microglia cells

Activated protein C (APC) is a signaling protease with anticoagulant activity. Here, we have used mice expressing a mutation in superoxide dismutase-1 (SOD1) that is linked to amyotrophic lateral sclerosis (ALS) to show that administration of APC or APC analogs with reduced anticoagulant activity after disease onset slows disease progression and extends survival. A proteolytically inactive form of APC with reduced anticoagulant activity provided no benefit. APC crossed the blood-spinal cord barrier in mice via endothelial protein C receptor. When administered after disease onset, APC eliminated leakage of hemoglobin-derived products across the blood-spinal cord barrier and delayed microglial activation. In microvessels, motor neurons, and microglial cells from SOD1-mutant mice and in cultured neuronal cells, APC transcriptionally downregulated SOD1. Inhibition of SOD1 synthesis in neuronal cells by APC required protease-activated receptor-1 (PAR1) and PAR3, which inhibited nuclear transport of the Sp1 transcription factor. Diminished mutant SOD1 synthesis by selective gene excision within endothelial cells did not alter disease progression, which suggests that diminished mutant SOD1 synthesis in other cells, including motor neurons and microglia, caused the APC-mediated slowing of disease. The delayed disease progression in mice after APC administration suggests that this approach may be of benefit to patients with familial, and possibly sporadic, ALS.

Vestigial-like 2 acts downstream of MyoD activation and is associated with skeletal muscle differentiation in chick myogenesis

The co-factor Vestigial-like 2 (Vgl-2), in association with the Scalloped/Tef/Tead transcription factors, has been identified as a component of the myogenic program in the C2C12 cell line. In order to understand Vgl-2 function in embryonic muscle formation, we analysed Vgl-2 expression and regulation during chick embryonic development. Vgl-2 expression was associated with all known sites of skeletal muscle formation, including those in the head, trunk and limb. Vgl-2 was expressed after the myogenic factor MyoD, regardless of the site of myogenesis. Analysis of Vgl-2 regulation by Notch signalling showed that Vgl-2 expression was down-regulated by Delta1-activated Notch, similarly to the muscle differentiation genes MyoD, Myogenin,Desmin, and Mef2c, while the expression of the muscle progenitor markers such as Myf5, Six1 and FgfR4 was not modified. Moreover, we established that the Myogenic Regulatory Factors (MRFs) associated with skeletal muscle differentiation (MyoD, Myogenin and Mrf4) were sufficient to activate Vgl-2 expression, while Myf5 was not able to do so. The Vgl-2 endogenous expression, the similar regulation of Vgl-2 and that of MyoD and Myogenin by Notch signalling, and the positive regulation of Vgl-2 by these MRFs suggest that Vgl-2 acts downstream of MyoD activation and is associated with the differentiation step in embryonic skeletal myogenesis.

An evolutionarily conserved Myostatin proximal promoter/enhancer confers basal levels of transcription and spatial specificity in vivo

Myostatin (Mstn) is a negative regulator of skeletal muscle mass, and Mstn mutations are responsible for the double muscling phenotype observed in many animal species. Moreover, Mstn is a positive regulator of adult muscle stem cell (satellite cell) quiescence, and hence, Mstn is being targeted in therapeutic approaches to muscle diseases. In order to better understand the mechanisms underlying Mstn regulation, we searched for the gene's proximal enhancer and promoter elements, using an evolutionary approach. We identified a 260-bp-long, evolutionary conserved region upstream of tetrapod Mstn and teleost mstn b genes. This region contains binding sites for TATA binding protein, Meis1, NF-Y, and for CREB family members, suggesting the involvement of cAMP in Myostatin regulation. The conserved fragment was able to drive reporter gene expression in C2C12 cells in vitro and in chicken somites in vivo; both normally express Mstn. In contrast, the reporter construct remained silent in the avian neural tube that normally does not express Mstn. This suggests that the identified element serves as a minimal promoter, harboring some spatial specificity. Finally, using bioinformatic approaches, we identified additional genes in the human genome associated with sequences similar to the Mstn proximal promoter/enhancer. Among them are genes important for myogenesis. This suggests that Mstn and these genes may form a synexpression group, regulated by a common signaling pathway.

Xenopus oocytes reactivate muscle gene transcription in transplanted somatic nuclei independently of myogenic factors

Transplantation into eggs or oocytes is an effective means of achieving the reprogramming of somatic cell nuclei. We ask here whether the provision of gene-specific transcription factors forms part of the mechanism by which a gene that is repressed in somatic cells is transcribed in oocytes. We find that M1 oocytes have an extremely strong transcription-inducing activity. They cause muscle genes of nuclei from non-muscle somatic cells, after injection into oocytes, to be transcribed to nearly the same extent as muscle genes in muscle cells. We show, surprisingly, that the myogenic factor MyoD and other known myogenic factors are not required to induce the transcription of muscle genes in a range of non-muscle somatic cell nuclei after transplantation to Xenopus oocytes. The overexpression of Id, a dominant-negative repressor of MyoD, prevents maternal MyoD from binding to its consensus sequences; nevertheless, muscle genes are activated in somatic nuclei to the same extent as without Id. We conclude that M1 oocytes can reprogram somatic nuclei in a different way to other experimental procedures: oocytes do not suppress the transcription of inappropriate genes and they activate a gene without the help of its known transcription factors. We suggest that these characteristics might be a special property of amphibian oocytes, and possibly of oocytes in general.

Pbx acts with Hand2 in early myocardial differentiation

Transcription factors of the basic helix-loop-helix (bHLH) family are critical regulators of muscle cell differentiation. For example, Myod drives skeletal muscle differentiation, and Hand2 potentiates cardiac muscle differentiation. Understanding how these bHLH factors regulate distinct transcriptional targets in a temporally and spatially controlled manner is critical for understanding their activity in cellular differentiation. We previously showed that Pbx homeodomain proteins modulate the activity of Myod to promote the differentiation of fast-twitch skeletal muscle. Here, we test the hypothesis that Pbx proteins are also necessary for cardiac muscle differentiation through interacting with Hand2. We show that Pbx proteins are required for the activation of cardiac muscle differentiation in zebrafish embryos. Loss of Pbx activity leads to delay of myocardial differentiation and subsequent defective cardiac morphogenesis, similar to reduced Hand2 activity. Genetic interaction experiments support the hypothesis that Pbx proteins modulate the activity of Hand2 in myocardial differentiation. Furthermore, we show that Pbx proteins directly bind the promoter of the myocardial differentiation gene myl7 in vitro, supporting a direct role for Pbx proteins in promoting cardiac muscle differentiation. Our findings demonstrate new roles for Pbx proteins in vertebrate cardiac development and also provide new insight into connections between the transcriptional regulation of skeletal and cardiac muscle differentiation programs.

Ghrelin inhibits early osteogenic differentiation of C3H10T1/2 cells by suppressing Runx2 expression and enhancing PPARgamma and C/EBPalpha expression

Ghrelin is a 28-residue peptide identified in the stomach as an endogenous ligand of the growth hormone secretagogue receptor that is expressed in a variety of peripheral tissues, as well as in the brain. In previous studies, ghrelin has been shown to stimulate both adipogenic differentiation from preadipocytes and osteogenic differentiation from preosteoblasts or primary osteoblasts. This study was undertaken to investigate the direct effect of ghrelin on the lineage allocation of mesenchymal stem cells (MSCs). We identified ghrelin receptor mRNA in C3H10T1/2 cells, and we found the levels of this mRNA to be attenuated during osteogenic differentiation. Treatment of cells with ghrelin resulted in both proliferation and inhibition of caspase-3 activity. In addition, ghrelin decreased serum deprivation-induced bax protein expression and release of cytochrome c from the mitochondria, whereas it increased bcl-2 protein expression. Moreover, ghrelin inhibited early osteogenic differentiation, as shown by alkaline phosphatase activity and staining, and inhibited osteoblast-specific genes expression by altering Runx2, PPARgamma, and C/EBPalpha protein expression.

Rhabdomyosarcoma in children

PURPOSE OF REVIEW

Rhabdomyosarcoma is a rare childhood cancer that affects only approximately 300 children per year in the United States. The purpose of this review is to provide the reader a greater understanding of the complex diagnosis, assessment and treatment of rhabdomyosarcoma in children.

RECENT FINDINGS

This review focuses on the new risk classification that is the foundation of all present rhabdomyosarcoma protocols developed by the Children's Oncology Group of the United States and Canada. The new risk classification of low, intermediate and high encompasses the staging and grouping categories that were previously utilized.

SUMMARY

This review also provides a complete list of diagnostic tests and imaging required to identify rhabdomyosarcoma in any body site. Rapid diagnosis and recognition of this rare disorder will facilitate long-term survival. Rhabdomyosarcoma today has an overall survival of 70%, depending on the site, and in orbital and other sites survival is as high as 90%. The treatment approaches that have led to this doubling in survival over the last 25 years are reviewed. For a practitioner, this review can be used as a reference when a child with a suspicious mass is encountered.

Progenitors of skeletal muscle satellite cells express the muscle determination gene, MyoD

Satellite cells are tissue-specific stem cells responsible for skeletal muscle growth and regeneration. Although satellite cells were identified almost 50 years ago, the identity of progenitor populations from which they derive remains controversial. We developed MyoD(iCre) knockin mice, and used Cre/lox lineage analysis to determine whether satellite cell progenitors express MyoD, a marker of myogenic commitment. Recombination status of satellite cells was determined by confocal microscopy of isolated muscle fibers and by electron microscopic observation of muscle tissue fixed immediately following isolation, using R26R-EYFP and R26R (beta-gal) reporter mice, respectively. We show that essentially all adult satellite cells associated with limb and body wall musculature, as well as the diaphragm and extraocular muscles, originate from MyoD+ progenitors. Neonatal satellite cells were Cre-recombined, but only a small minority exhibited ongoing Cre expression, indicating that most satellite cells had expressed MyoD prenatally. We also show that satellite cell development in MyoD-null mice is not due to functional compensation by MyoD non-expressing lineages. The results suggest that satellite cells are derived from committed myogenic progenitors, irrespective of the anatomical location, embryological origin, or physiological properties of associated musculature.

MyoD and E-protein heterodimers switch rhabdomyosarcoma cells from an arrested myoblast phase to a differentiated state

Rhabdomyosarcomas are characterized by expression of myogenic specification genes, such as MyoD and/or Myf5, and some muscle structural genes in a population of cells that continues to replicate. Because MyoD is sufficient to induce terminal differentiation in a variety of cell types, we have sought to determine the molecular mechanisms that prevent MyoD activity in human embryonal rhabdomyosarcoma cells. In this study, we show that a combination of inhibitory Musculin:E-protein complexes and a novel splice form of E2A compete with MyoD for the generation of active full-length E-protein:MyoD heterodimers. A forced heterodimer between MyoD and the full-length E12 robustly restores differentiation in rhabdomyosarcoma cells and broadly suppresses multiple inhibitory pathways. Our studies indicate that rhabdomyosarcomas represent an arrested progress through a normal transitional state that is regulated by the relative abundance of heterodimers between MyoD and the full-length E2A proteins. The demonstration that multiple inhibitory mechanisms can be suppressed and myogenic differentiation can be induced in the RD rhabdomyosarcomas by increasing the abundance of MyoD:E-protein heterodimers suggests a central integrating function that can be targeted to force differentiation in muscle cancer cells.

Retrodifferentiation--a mechanism for cellular regeneration?

Cellular differentiation can be characterized by the acquisition of specified properties during several steps of development whereby the original stem- or precursor-like populations can finally obtain a certain phenotype with highly specific cell functions. The continuing maturation process can be paralleled by progressively reduced proliferative capacity in various cell types functioning as postmitotic tissues. Conversely, other cell populations (e.g., distinct immune cells) may carry out their specific function upon stimulation of proliferation. While these differentiated phenotypes perform their appropriate specific duties throughout the functioning organism, nature may provide an interesting alternative within this concept of life: sometimes, differentiation steps appear to be reversible. Thus, retrograde differentiation--also termed retrodifferentiation--and accordingly rejuvenation may occur when differentiated cells lose their specific properties acquired during previous steps of maturation. Consequently, retrodifferentiation and rejuvenation could provide enormous potential for tissue repair and cell renewal; however, regulatory dysfunctions within these retrograde developments may also involve the risk of tumor promotion.

VITO-2, a new SID domain protein, is expressed in the myogenic lineage during early mouse embryonic development

MCAT elements and its cognate binding partners, the transcription enhancer factors (TEFs) play important roles in the regulation of expression of several muscle-specific genes. The biological effects of TEFs strongly depend on different co-factors, which might act as co-activators or anti-repressors to enable transcriptional activation of target genes by TEFs. Previously, we have cloned and characterized VITO-1, which acts as a skeletal muscle-specific transcriptional co-activator of TEFs. Here we describe the cloning and expression profile of a related gene, VITO-2 (also termed Vgl-3), which shares a high homology with VITO-1 in the SID domain responsible for interaction with TEFs. During early embryonic and fetal development VITO-2 is mainly expressed in the myogenic lineage with an onset of expression in the myotomes of somites VI at E9.5 slightly later than VITO-1. At later developmental stages VITO-2 is predominantly found in the nervous system. In adult mice VITO-2 was detected in different tissues, including skeletal muscle, heart, kidney, liver and brain, where it was found in cortical and cerebellar neurons as well as in Purkinje cells. The expression of VITO-2 in the mesoderm was repressed by the notch/delta pathway and activated by Myf-5 since Dll-1 mutant showed an aberrant expression of VITO-2 but not VITO-1 in the tail bud and in the caudal neural tube at E10.5 while Myf-5 mutant mice lack expression of VITO-1 and VITO-2 in somites until E10.5.

Mapping key features of transcriptional regulatory circuitry in embryonic stem cells

The process by which a single fertilized egg develops into a human being with more than 200 cell types--each with a distinct gene expression pattern controlling its cellular state--is poorly understood. Knowledge of the transcriptional regulatory circuitry that establishes and maintains gene expression programs in mammalian cells is fundamental to understanding development and should provide the foundation for improved diagnosis and treatment of disease. Although it is not yet feasible to map the entirety of this circuitry in vertebrate cells, recent work in embryonic stem (ES) cells has demonstrated that core features of the circuitry can be discovered through studies involving selected regulators. Here, we highlight the fundamental insights that have emerged from studies that examined the role of transcription factors, chromatin regulators, signaling pathways, and noncoding RNAs in the regulatory circuitry of ES cells. Maps of regulatory circuitry and the insights that have emerged from these studies have improved our understanding of global gene expression and are facilitating efforts to reprogram cells for disease therapeutics and regenerative medicine.

Genomewide analysis of PRC1 and PRC2 occupancy identifies two classes of bivalent domains

In embryonic stem (ES) cells, bivalent chromatin domains with overlapping repressive (H3 lysine 27 tri-methylation) and activating (H3 lysine 4 tri-methylation) histone modifications mark the promoters of more than 2,000 genes. To gain insight into the structure and function of bivalent domains, we mapped key histone modifications and subunits of Polycomb-repressive complexes 1 and 2 (PRC1 and PRC2) genomewide in human and mouse ES cells by chromatin immunoprecipitation, followed by ultra high-throughput sequencing. We find that bivalent domains can be segregated into two classes—the first occupied by both PRC2 and PRC1 (PRC1-positive) and the second specifically bound by PRC2 (PRC2-only). PRC1-positive bivalent domains appear functionally distinct as they more efficiently retain lysine 27 tri-methylation upon differentiation, show stringent conservation of chromatin state, and associate with an overwhelming number of developmental regulator gene promoters. We also used computational genomics to search for sequence determinants of Polycomb binding. This analysis revealed that the genomewide locations of PRC2 and PRC1 can be largely predicted from the locations, sizes, and underlying motif contents of CpG islands. We propose that large CpG islands depleted of activating motifs confer epigenetic memory by recruiting the full repertoire of Polycomb complexes in pluripotent cells.

Polycomb-group (PcG) proteins play essential roles in the epigenetic regulation of gene expression during development. PcG proteins are repressors that catalyze lysine 27 tri-methylation on histone H3. They are antagonized by trithorax-group proteins that catalyze lysine 4 tri-methylation. Recent studies of ES cells revealed a novel chromatin pattern consisting of overlapping lysine 27 and lysine 4 tri-methylation. Genomic regions with these opposing modifications were termed “bivalent domains” and proposed to silence developmental regulators while keeping them “poised” for alternate fates. However, our understanding of PcG regulation and bivalent domains remains limited. For instance, bivalent domains affect over 2,000 promoters with diverse functions, which suggests that they may function in diverse cellular processes. Moreover, the mechanisms that underlie the targeting of PcG complexes to specific genomic regions remain completely unknown. To gain insight into these issues, we used ultra high-throughput sequencing to map PcG complexes and related modifications genomewide in human and mouse ES cells. The data identify two classes of bivalent domains with distinct regulatory properties. They also reveal striking relationships between genome sequence and chromatin state that suggest a prominent role for the DNA sequence in dictating the genomewide localization of PcG complexes and, consequently, bivalent domains in ES cells.

Effect of Smad3-mediated transforming growth factor-beta1 signaling on satellite cell proliferation and differentiation in chickens

Transforming growth factor-beta1 (TGF-beta1) is a potent inhibitor of muscle cell proliferation and differentiation. The TGF-beta1 signal is carried by Smad proteins into the cell nucleus, resulting in the regulation of the expression of key myogenic regulatory factors including MyoD and myogenin during myogenesis. However, to date, the molecular mechanism of the inhibition by Smad-mediated TGF-beta1 signaling on the function of the myogenic regulatory factors has not been well understood. The present study was designed to investigate the effect of TGF-beta1 on satellite cell proliferation and differentiation by a Smad3-dependent signaling pathway. A chicken line, low score normal (LSN) with reduced muscling and upregulated TGF-beta1 expression, was used and compared with a normal chicken line. In LSN satellite cell cultures, both MyoD and myogenin expression was significantly decreased compared with the normal cells. Furthermore, in response to exogenous TGF-beta1, the normal satellite cells had a significant decrease in both MyoD and myogenin expression, which suggests that TGF-beta1 inhibited MyoD and myogenin expression, resulting in decreased satellite cell proliferation and differentiation. The expression of Smad3 and Smad7, key proteins of the Smad family, was greater in the LSN cultures than that measured in the normal culture. The addition of TGF-beta1 reduced Smad3 expression, but did not affect the expression of Smad7. The reduction of Smad3 in response to TGF-beta1 suggests that a negative regulatory feedback is likely involved in LSN satellite cell proliferation and differentiation. The overexpression of Smad3 inhibited both MyoD and myogenin expression in normal and LSN satellite cells. In contrast, the underexpression of Smad3 increased the expression of MyoD and myogenin in the LSN cells. However, in the normal cells, only myogenin expression was increased by Smad3 overexpression, but not MyoD. These data together suggest that LSN satellite cells are more responsive to a Smad3-dependent TGF-beta1 signaling pathway than normal satellite cells, and a Smad3-independent pathway is also likely involved in the regulation of satellite cell proliferation and differentiation.

Proteomic identification and functional validation of activins and bone morphogenetic protein 11 as candidate novel muscle mass regulators

Myostatin is a secreted TGF-beta family member that controls skeletal muscle growth. Humans, cattle, and dogs carrying natural loss-of-function mutations in the myostatin gene and myostatin knockout mice exhibit significant increases in skeletal muscle mass. Treatment of adult mice with antimyostatin antibodies also resulted in significant muscle mass increases. However, myostatin-knockout mice that were treated with a soluble form of the activin type II receptor (ActRII) B increased their muscle mass by an additional 15-25%, indicating that there is at least one additional ligand, in addition to myostatin, that functions to limit muscle growth. Here, both soluble ActRII and -IIB fragment-crystallizable proteins were used to affinity purify their native ligands from human and mouse sera. Using mass spectrometry-based proteomics and in vitro binding assays we have identified and confirmed that a number of TGF-beta family members, including myostatin, activins-A, -B, and -AB, bone morphogenetic proteins (BMPs) -9, -10, and -11, bind to both ActRIIs. Many of these factors, such as BMPs-11, -9, and -10 were discovered in systemic circulation for the first time, indicating that these ligands may also act in an endocrine fashion. Using a promoter-specific gene reporter assay, we demonstrated that soluble ActRIIB fragment-crystallizable proteins can inhibit the canonical signaling induced by these ligands. In addition, like myostatin, these factors were able to block the differentiation of myoblast cells into myotubes. However, in addition to myostatin, only BMP-11, and activins-A, -B, and -AB could be blocked from inhibiting the myoblast-to-myotube differentiation with both soluble ActRIIs, thus implicating them as potential novel regulators of muscle growth.

Mechanisms for reaching the differentiated state: Insights from neural crest-derived melanocytes

Black pigment cells, or melanocytes, are the major contributing cells to pigmentation in vertebrate organisms. Although the function of these cells is distinct depending on the organism, the events involved in their development are remarkably similar. Here, we review the mechanisms involved in the early development of melanocytes from neural crest, many of which are conserved in organisms as diverse as zebrafish, birds and humans. We also discuss recent studies that provide further insight into how melanocyte differentiation is achieved and maintained.

All muscle satellite cells are equal, but are some more equal than others?

Skeletal muscle is an accessible adult stem-cell model in which differentiated myofibres are maintained and repaired by a self-renewing stem-cell compartment. These resident stem cells, which are known as satellite cells, lie on the surface of the muscle fibre, between the plasmalemma and overlying basal lamina. Although they are normally mitotically quiescent in adult muscle, satellite cells can be activated when needed to generate myoblasts, which eventually differentiate to provide new myonuclei for the homeostasis, hypertrophy and repair of muscle fibres, or fuse together to form new myofibres for regeneration. Satellite cells also self-renew in order to maintain a viable stem-cell pool that is able to respond to repeated demand. The study of the control of self-renewal has led to the idea that the satellite-cell pool might be heterogeneous: that is it might contain both self-renewing satellite `stem' cells and myogenic precursors with limited replicative potential in the same anatomical location. The regulatory circuits that control satellite-cell self-renewal are beginning to be deciphered, with Pax7, and Notch and Wnt signalling being clearly implicated. This Commentary seeks to integrate these interesting new findings into the wider context of satellite-cell biology, and to highlight some of the many outstanding questions.

Morphogenetic competence of HNF4 alpha-deficient mouse hepatic cells

BACKGROUND/AIMS

To specify roles of HNF 4 alpha in mouse liver development, we have analyzed the ex vivo morphogenetic potential of HNF4 alpha-null embryonic hepatic cells.

METHODS

Using mice with floxed or deficiency alleles of HNF4 alpha, hepatic cells lacking this transcription factor were explanted into primary culture and derived into cell lines.

RESULTS

Contrary to behavior in vivo where HNF4 alpha-null liver cells fail to show normal polarity and epithelialization, e18.5 hepatic cells in primary culture from mutant embryos show restoration of apical expression of tight junction protein-1 and of transcripts for E-cadherin. Clones of control and HNF4 alpha-null cell lines were indistinguishable, even when differentiation of bile canalicular formation was induced. HNF4 alpha-null and control cell lines showed similar potential to colonize livers of the murine ALB-uPA/SCID model of liver regeneration, but null cells formed only bile ducts and not clusters of hepatocytes. Finally, analysis of mutant embryonic livers revealed a transcriptional signature consistent with a stress response, which could underlie the morphogenetic defects observed in vivo.

CONCLUSIONS

We conclude that the lack of epithelialization characteristic of the HNF4 alpha-null embryonic liver is due, at least in part, to non-cell autonomous defects, and that null cells do not suffer intrinsic defects in polarization.

DUX4c, an FSHD candidate gene, interferes with myogenic regulators and abolishes myoblast differentiation

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant neuromuscular disease. It maps to the D4Z4 repeat array at 4q35, and correlates with a repeat contraction which derepresses transcription of local genes. Which, if any, of these genes is pathogenic to muscle, and through what molecular mechanism is unknown. The present study investigates the function of one candidate gene, DUX4c, encoded by a truncated inverted D4Z4 element located 42 kb proximal to the D4Z4 repeats. Using a gain of function approach we tested DUX4c for toxicity and effects on differentiation in C2C12 myoblasts. DUX4c-expressing myoblasts appear morphologically normal but have reduced expression of myogenic regulators, including MyoD and Myf5. Consistent with this, DUX4c-expressing myoblasts are unable to differentiate into myotubes. Furthermore, overexpression of Myf5 or MyoD rescued myoblast differentiation, suggesting that reductions in expression of these regulators are the relevant DUX4c-induced physiological changes that inhibit differentiation. Our results suggest that upregulation of DUX4c can have a deleterious effect on muscle homeostasis and regeneration, and point to a possible role for DUX4c in the pathology of FSHD.