Pax3 is essential for skeletal myogenesis and the expression of Six1 and Eya2

Targeting legumain-mediated cell-cell interaction sensitizes glioblastoma to immunotherapy in preclinical models

Tumor-associated macrophages (TAMs) are the most prominent immune cell population in the glioblastoma (GBM) tumor microenvironment and play critical roles in promoting tumor progression and immunosuppression. Here we identified that TAM-derived legumain (LGMN) exhibited a dual role in regulating the biology of TAMs and GBM cells. LGMN promoted macrophage infiltration in a cell-autonomous manner by activating the GSK3β/STAT3 pathway. Moreover, TAM-derived LGMN activated integrin αv/AKT/p65 signaling to drive GBM cell proliferation and survival. Targeting of LGMN-directed macrophage (inhibiting GSK3β and STAT3) and GBM cell (inhibiting integrin αv) mechanisms resulted in an antitumor effect in immunocompetent GBM mouse models that was further enhanced by combination with anti-PD-1 therapy. Our study reveals a paracrine and autocrine mechanism of TAM-derived LGMN that promotes GBM progression and immunosuppression, providing effective therapeutic targets to improve immunotherapy in GBM.

The Influence of Moringa Oleifera Leaf Powder on Body Weight MYF5 and IGF-1gene Expression in Japanese Quails

This study investigates the effects of Moringa oleifera leaf powder (MOLP) supplementation on growth performance and the expression of MYF5 and IGF-1 genes in Japanese quails. A total of 116 quails were assigned to four dietary treatments: Control, Moringa-1 (2% MOLP), Moringa-2 (4% MOLP), and Moringa-3 (6% MOLP). Body weights were recorded weekly, and gene expression levels were analysed using quantitative real-time PCR. The results showed significant improvements in body weight and feed efficiency in the Moringa-2 and Moringa-3 groups compared to the Control group (p < 0.05). Additionally, the expression levels of MYF5 and IGF-1 increased dose-dependent, with the highest expression observed in the Moringa-3 group. These findings demonstrate the potential of MOL as a natural feed additive to enhance growth performance and modulate molecular pathways related to muscle development in poultry. This study provides valuable insights for sustainable poultry production and highlights the need for further research on optimal inclusion levels and the long-term effects of MOL supplementation.

Functional dissection of metabolic trait-associated gene regulation in steady state and stimulated human skeletal muscle cells

Type 2 diabetes (T2D) is a common metabolic disorder characterized by dysregulation of glucose metabolism. Genome-wide association studies have defined hundreds of signals associated with T2D and related metabolic traits, predominantly in noncoding regions. While pancreatic islets have been a focal point given their central role in insulin production and glucose homeostasis, other metabolic tissues, including liver, adipose, and skeletal muscle, also contribute to T2D pathogenesis and risk. Here, we examined context-specific genetic regulation under basal and stimulated states. Using LHCN-M2 human skeletal muscle cells, we generated transcriptomic profiles and characterized regulatory activity of 327 metabolic trait-associated variants via a massively parallel reporter assay (MPRA). To identify condition-specific effects, we compared four different conditions: (1) undifferentiated, or (2) differentiated with basal media, (3) media supplemented with the AMP analog AICAR (to simulate exercise) or (4) media containing sodium palmitate (to induce insulin resistance). RNA-seq revealed these treatments extensively perturbed transcriptional regulation, with 498-3,686 genes showing significant differential expression between pairs of conditions. Among differentially expressed genes, we observed enrichment of relevant biological pathways including muscle differentiation (undifferentiated vs. differentiated), oxidoreductase activity (differentiated vs. AICAR), and glycogen binding (differentiated vs. palmitate). The results of our MPRA found broadly different levels of activity between all conditions. Our MPRA screen revealed a shared set of 7 variants with significant allelic activity across all conditions, along with a proportional number of variants showing condition-specific allelic bias and the total number of active oligos per condition. We found that a lead variant for serum triglyceride levels, rs490972, overlaps SP transcription factor motifs and has differential regulatory activity between conditions. Comparison of MPRA activity with paired gene expression data allowed us to predict that regulatory activity at this locus is mediated by SP1 transcription factor binding. While several of the MPRA variants have been previously characterized in other metabolic tissues, none have been studied in these stimulated states. Together, this work uncovers context-dependent transcriptomic and regulatory dynamics of T2D- and metabolic trait-associated variants in skeletal muscle cells, offering new insights into their functional roles in metabolic processes.

Lin28a maintains a subset of adult muscle stem cells in an embryonic-like state

During homeostasis and after injury, adult muscle stem cells (MuSCs) activate to mediate muscle regeneration. However, much remains unclear regarding the heterogeneous capacity of MuSCs for self-renewal and regeneration. Here, we show that Lin28a is expressed in embryonic limb bud muscle progenitors, and that a rare reserve subset of Lin28a+Pax7- skeletal MuSCs can respond to injury at adult stage by replenishing the Pax7+ MuSC pool to drive muscle regeneration. Compared with adult Pax7+ MuSCs, Lin28a+ MuSCs displayed enhanced myogenic potency in vitro and in vivo upon transplantation. The epigenome of adult Lin28a+ MuSCs showed resemblance to embryonic muscle progenitors. In addition, RNA-sequencing revealed that Lin28a+ MuSCs co-expressed higher levels of certain embryonic limb bud transcription factors, telomerase components and the p53 inhibitor Mdm4, and lower levels of myogenic differentiation markers compared to adult Pax7+ MuSCs, resulting in enhanced self-renewal and stress-response signatures. Functionally, conditional ablation and induction of Lin28a+ MuSCs in adult mice revealed that these cells are necessary and sufficient for efficient muscle regeneration. Together, our findings connect the embryonic factor Lin28a to adult stem cell self-renewal and juvenile regeneration.

2,4-di-tert-butylphenol exposure impairs osteogenic differentiation

2,4-di-tert-butylphenol (2,4-DTBP) is a synthetic antioxidant used in polyethylene crosspolymer (PEX) water distribution pipes and food-related plastics. 2,4-DTBP can leach from plastic materials and has been found in breast milk, cord blood, and placental tissue, giving rise to the concern that this compound may interfere with fetal development. The objective of this study is to assess the impacts of 2,4-DTBP on cellular differentiation. Human induced pluripotent stem (HiPS) cells were differentiated into osteoblasts or myoblasts over 40 days, and analyzed for markers of somite, dermomyotome, sclerotome, myoblast, and osteoblast development. When cultured as stem cells, 2,4-DTBP did not alter cell viability and expression of markers (NANOG, OCT4). However, upon differentiation into somite-like cells, 2,4-DTBP had reduced levels of MEOX1 and TBX6 transcripts, while NANOG and OCT4 were in turn upregulated in a dose-dependent manner. At the sclerotome-like stage, PAX9 mRNA decreased by 2-fold in the 0.5 μM and 1.0 μM 2,4-DTBP exposure groups. After 40 days of differentiation into an osteoblast-like lineage, exposure to 2,4-DTBP significantly reduced expression of the osteogenesis transcripts RUNX2 and OSX in a dose-dependent manner. Further, Alizarin Red staining of calcium deposits was decreased in the 0.5 μM and 1.0 μM treatment groups. In contrast, myogenesis was not affected by 2,4-DTBP exposure. Interestingly, KEAP1 expression was significantly increased in the sclerotomal-like cells, but decreased in the dermomytomal-like cells, which may suggest a mechanism of action. Overall, this study shows that 2,4-DTBP can delay key processes during sclerotome and osteoblast development, leading to a potential for bone developmental issues in exposed individuals.

The role of MEOX1 in non-neoplastic and neoplastic diseases

Targeted gene therapy has shown durable efficacy in non-neoplastic and neoplastic patients. Therefore, finding a suitable target has become a key area of research. Mesenchyme homeobox 1 (MEOX1) is a transcriptional factor that plays a significant role in regulation of somite development. Evidence indicates that abnormalities in MEOX1 expression and function are associated with a variety of pathologies, including non-neoplastic and neoplastic diseases. MEOX1 expression is upregulated during progression of most diseases and plays a critical role in maintenance of the cellular phenotypes such as cell differentiation, cell cycle arrest and senescence, migration, and proliferation. Therefore, MEOX1 may become an important molecular target and therapeutic target. This review will discuss the current state of knowledge on the role of MEOX1 in different diseases.

The influence of 3D curve severity on paraspinal muscle fatty infiltration in patients with adolescent idiopathic scoliosis

PURPOSE

In a sex-inclusive cohort of patients with adolescent idiopathic scoliosis (AIS): (1) assess the relationship between 3D curve severity, curve flexibility, and paraspinal muscle fatty infiltration, and (2) describe three-dimensional (3D) fatty infiltration of the paraspinal muscles.

METHODS

Fat signal fraction of the paravertebral muscles was measured in pre-operative magnetic resonance images (MRIs) of males and females with AIS at the apex, ± 1, and ± 2 levels from the apex of the curve (n = 62). In a subset of patients with biplanar erect radiographic imaging (n = 35), 3D measures of deformity (axial rotation of the apical vertebrae, thoracic kyphosis, and coronal Cobb angle) were measured.

RESULTS

Contrary to previous studies, no relationship between coronal Cobb angle and fatty infiltration was found. However, axial apical rotation and sagittal Cobb angle were found to be significant predictors of paravertebral fatty infiltration (R² = 0.196-0.222). Curve concavity, female sex, and proximity to the curve apex were found to be the strongest predictors of fatty infiltration. Greater fatty infiltration of the paravertebral muscles was found on the concave side of the curve (15-24% vs. 11-13%), with increasing fatty infiltration toward the apex of the curve. Fatty infiltration was protected on the convex side of the curve, with no differences in the amount of fatty infiltration across levels.

CONCLUSION

These findings highlight that coronal curve severity and flexibility are not the primary influencing factors for the degree of paraspinal fatty infiltration in patients with AIS. This may have implications for nonsurgical rehabilitation strategies such as bracing and physical therapy.

LEVEL OF EVIDENCE

II.

Current Issues and Technical Advances in Cultured Meat Production: A Review

As the global population grows, we need a stable protein supply to meet the demands. Although plant-derived protein sources are widely available, animal meat maintains its popularity as a high-quality and savory protein source. Recently, cultured meat, also known as in vitro meat, has been suggested as a meat analog produced through in vitro cell culture technology. Cultured meat has several advantages over conventional meat, such as environmental protection, disease prevention, and animal welfare. However, cultured meat manufacturing is an emerging technology; thus, its further and dynamic development would be pivotal. Commercialization of cultured meat to the public will take a long time but cultured meat undoubtedly will come to our table someday. Here, we discuss the social and economic aspects of cultured meat production as well as the recent technical advances in cultured meat technology.

Key Genes Regulating Skeletal Muscle Development and Growth in Farm Animals

Simple Summary

Skeletal muscle mass is an important economic trait, and muscle development and growth is a crucial factor to supply enough meat for human consumption. Thus, understanding (candidate) genes regulating skeletal muscle development is crucial for understanding molecular genetic regulation of muscle growth and can be benefit the meat industry toward the goal of increasing meat yields. During the past years, significant progress has been made for understanding these mechanisms, and thus, we decided to write a comprehensive review covering regulators and (candidate) genes crucial for muscle development and growth in farm animals. Detection of these genes and factors increases our understanding of muscle growth and development and is a great help for breeders to satisfy demands for meat production on a global scale.

Abstract

Farm-animal species play crucial roles in satisfying demands for meat on a global scale, and they are genetically being developed to enhance the efficiency of meat production. In particular, one of the important breeders’ aims is to increase skeletal muscle growth in farm animals. The enhancement of muscle development and growth is crucial to meet consumers’ demands regarding meat quality. Fetal skeletal muscle development involves myogenesis (with myoblast proliferation, differentiation, and fusion), fibrogenesis, and adipogenesis. Typically, myogenesis is regulated by a convoluted network of intrinsic and extrinsic factors monitored by myogenic regulatory factor genes in two or three phases, as well as genes that code for kinases. Marker-assisted selection relies on candidate genes related positively or negatively to muscle development and can be a strong supplement to classical selection strategies in farm animals. This comprehensive review covers important (candidate) genes that regulate muscle development and growth in farm animals (cattle, sheep, chicken, and pig). The identification of these genes is an important step toward the goal of increasing meat yields and improves meat quality.

Chronological Expression of PITX2 and SIX1 Genes and the Association between Their Polymorphisms and Chicken Meat Quality Traits

Meat quality is closely related to the development of skeletal muscle, in which PITX2 and SIX1 genes play important regulatory roles. The present study firstly provided the data of chronological expression files of PITX2 and SIX1 genes in the post-hatching pectoral muscle and analyzed the association of their polymorphisms with the meat quality traits of Wuliang Mountain Black-bone (WLMB) chickens. The results showed that both PITX2 and SIX1 genes were weakly expressed in the second and third weeks, and then increased significantly from the third week to the fourth week. Furthermore, there was a significant positive correlation between the expression levels of the two genes. Twelve and one SNPs were detected in the chicken PITX2 and SIX1 genes, respectively, of which four SNPs (g.9830C > T, g.10073C > T, g.13335G > A, g.13726A > G) of the PITX2 gene and one SNP (g.564G > A) of the SIX1 gene were significantly associated with chicken meat quality traits. For the PITX2 gene, chickens with the CT genotype of g.9830C > T showed the highest meat color L*, shear force (SF), pH, and the lowest electrical conductivity (EC), and drip loss (DL) (p < 0.05 or p < 0.01); chickens with the CC genotype of g.10073C > T had the lowest L*, pH, and the highest DL (p < 0.01). For the SIX1 gene, chickens with the GG genotype of g.564G > A had the highest (p < 0.05) SF and pH. Furthermore, pH had a significant correlation with all the other meat quality traits. The current study could contribute to the research of regulatory mechanisms of meat quality and lay the foundation for improving meat quality based on marker-assisted selection in chickens.

Life-history genomic regions explain differences in Atlantic salmon marine diet specialization

Animals employ various foraging strategies along their ontogeny to acquire energy, and with varying degree of efficiencies, to support growth, maturation and subsequent reproduction events. Individuals that can efficiently acquire energy early are more likely to mature at an earlier age, as a result of faster energy gain which can fuel maturation and reproduction. We aimed to test the hypothesis that heritable resource acquisition variation that covaries with efficiency along the ontogeny would influence maturation timing of individuals. To test this hypothesis, we utilized Atlantic salmon as a model which exhibits a simple, hence trackable, genetic control of maturation age. We then monitored the variation in diet acquisition (quantified as stomach fullness and composition) of individuals with different ages, and linked it with genomic regions (haploblocks) that were previously identified to be associated with age-at-maturity. Consistent with the hypothesis, we demonstrated that one of the life-history genomic regions tested (six6) was indeed associated with age-dependent differences in stomach fullness. Prey composition was marginally linked to six6, and suggestively (but non-significantly) to vgll3 genomic regions. We further showed Atlantic salmon switched to the so-called 'feast and famine' strategy along the ontogeny, where older age groups exhibited heavier stomach content, but that came at the expense of running on empty more often. These results suggest genetic variation underlying resource utilization may explain the genetic basis of age structure in Atlantic salmon. Given that ontogenetic diet has a genetic component and the strong spatial diversity associated with these genomic regions, we predict populations with diverse maturation age will have diverse evolutionary responses to future changes in marine food web structures.

Cardiac fibroblast diversity in health and disease

The cardiac stroma plays essential roles in health and following cardiac damage. The major player of the stroma with respect to extracellular matrix deposition, maintenance and remodeling is the poorly defined fibroblast. It has long been recognized that there is considerable variability to the fibroblast phenotype. With the advent of new, high throughput analytical methods our understanding and appreciation of this heterogeneity has grown dramatically. This review aims to explore the diversity of cardiac fibroblasts and highlights new insights into the diverse nature of these cells and their progenitors as revealed by single cell sequencing and fate mapping studies. We propose that at least in part the observed heterogeneity is related to the existence of a differentiation cascade within stromal cells. Beyond in-organ heterogeneity, we also discuss how the stromal response to damage differs between non-regenerating organs such as the heart and regenerating organs such as skeletal muscle. In exploring possible causes for these differences, we outline that although fibrogenic cells from different organs overlap in many properties, they still possess organ-specific transcriptional signatures and differentiation biases that make them functionally distinct.

PAX3-FOXO1 escapes miR-495 regulation during muscle differentiation

Pax3 plays an essential role in myogenesis. Previously, we found a tumor-signature chimeric fusion RNA, PAX3-FOXO1 also present during muscle differentiation, raising the possibility of its physiological role. Here we demonstrated that the fusion is needed transiently for muscle lineage commitment. Interestingly, the fusion ortholog was not found in seven mouse muscle differentiation/regeneration systems, nor in other stem cell differentiation systems of another three mammal species. We noticed that Pax3 is expressed at a much lower level in human stem cells, and during muscle differentiation than in other mammals. Given the fact that the fusion and the parental Pax3 share common downstream targets, we reasoned that forming the fusion may be a mechanism for human cells to escape certain microRNA regulation on Pax3. By sequence comparison, we identified 16 candidate microRNAs that may specifically target the human PAX3 3'UTR. We used a luciferase reporter assay, examined the microRNAs expression, and conducted mutagenesis on the reporters, as well as a CRISPR/Cas9 mediated editing on the endogenous allele. Finally, we identified miR-495 as a microRNA that specifically targets human PAX3. Examining several other fusion RNAs revealed that the human-specificity is not limited to PAX3-FOXO1. Based on these observations, we conclude that PAX3-FOXO1 fusion RNA is absent in mouse, or other mammals we tested, the fusion RNA is a mechanism to escape microRNA, miR-495 regulation in humans, and that it is not the only human-specific fusion RNA.

Mapping molecular landmarks of human skeletal ontogeny and pluripotent stem cell-derived articular chondrocytes

Tissue-specific gene expression defines cellular identity and function, but knowledge of early human development is limited, hampering application of cell-based therapies. Here we profiled 5 distinct cell types at a single fetal stage, as well as chondrocytes at 4 stages in vivo and 2 stages during in vitro differentiation. Network analysis delineated five tissue-specific gene modules; these modules and chromatin state analysis defined broad similarities in gene expression during cartilage specification and maturation in vitro and in vivo, including early expression and progressive silencing of muscle- and bone-specific genes. Finally, ontogenetic analysis of freshly isolated and pluripotent stem cell-derived articular chondrocytes identified that integrin alpha 4 defines 2 subsets of functionally and molecularly distinct chondrocytes characterized by their gene expression, osteochondral potential in vitro and proliferative signature in vivo. These analyses provide new insight into human musculoskeletal development and provide an essential comparative resource for disease modeling and regenerative medicine.

High Fat With High Sucrose Diet Leads to Obesity and Induces Myodegeneration

Skeletal muscle utilizes both free fatty acids (FFAs) and glucose that circulate in the blood stream. When blood glucose levels acutely increase, insulin stimulates muscle glucose uptake, oxidation, and glycogen synthesis. Under these conditions, skeletal muscle preferentially oxidizes glucose while the oxidation of fatty acids (FAs) oxidation is reciprocally decreased. In metabolic disorders associated with insulin resistance, such as diabetes and obesity, both glucose uptake, and utilization muscle are significantly reduced causing FA oxidation to provide the majority of ATP for metabolic processes and contraction. Although the causes of this metabolic inflexibility or disrupted "glucose-fatty acid cycle" are largely unknown, a diet high in fat and sugar (HFS) may be a contributing factor. This metabolic inflexibility observed in models of obesity or with HFS feeding is detrimental because high rates of FA oxidation in skeletal muscle can lead to the buildup of toxic metabolites of fat metabolism and the accumulation of pro-inflammatory cytokines, which further exacerbate the insulin resistance. Further, HFS leads to skeletal muscle atrophy with a decrease in myofibrillar proteins and phenotypically characterized by loss of muscle mass and strength. Overactivation of ubiquitin proteasome pathway, oxidative stress, myonuclear apoptosis, and mitochondrial dysfunction are some of the mechanisms involved in muscle atrophy induced by obesity or in mice fed with HFS. In this review, we will discuss how HFS diet negatively impacts the various physiological and metabolic mechanisms in skeletal muscle.

Sequencing and characterization of lncRNAs in the breast muscle of Gushi and Arbor Acres chickens

Chicken muscle quality is one of the most important factors determining the economic value of poultry, and muscle development and growth are affected by genetics, environment, and nutrition. However, little is known about the molecular regulatory mechanisms of long non-coding RNAs (lncRNAs) in chicken skeletal muscle development. Our study aimed to better understand muscle development in chickens and thereby improve meat quality. In this study, Ribo-Zero RNA-Seq was used to investigate differences in the expression profiles of muscle development related genes and associated pathways between Gushi (GS) and Arbor Acres (AA) chickens. We identified two muscle tissue specific expression lncRNAs. In addition, the target genes of these lncRNAs were significantly enriched in certain biological processes and molecular functions, as demonstrated by Gene Ontology (GO) analysis, and these target genes participate in five signaling pathway, as revealed by an analysis of the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. Taken together, these data suggest that different lncRNAs might be involved in regulating chicken muscle development and growth and provide new insight into the molecular mechanisms of lncRNAs.

Identification of rare paired box 3 variant in strabismus by whole exome sequencing

AIM

To identify the potentially pathogenic gene variants that contributes to the etiology of strabismus.

METHODS

A Chinese pedigree with strabismus was collected and the exomes of two affected individuals were sequenced using the next-generation sequencing technology. The resulting variants from exome sequencing were filtered by subsequent bioinformatics methods and the candidate mutation was verified as heterozygous in the affected proposita and her mother by sanger sequencing.

RESULTS

Whole exome sequencing and filtering identified a nonsynonymous mutation c.434G-T transition in paired box 3 (PAX3) in the two affected individuals, which were predicted to be deleterious by more than 4 bioinformatics programs. This altered amino acid residue was located in the conserved PAX domain of PAX3. This gene encodes a member of the PAX family of transcription factors, which play critical roles during fetal development. Mutations in PAX3 were associated with Waardenburg syndrome with strabismus.

CONCLUSION

Our results report that the c.434G-T mutation (p.R145L) in PAX3 may contribute to strabismus, expanding our understanding of the causally relevant genes for this disorder.

Comparative muscle transcriptome associated with carcass traits of Nellore cattle

Background

Commercial cuts yield is an important trait for beef production, which affects the final value of the products, but its direct determination is a challenging procedure to be implemented in practice. The measurement of ribeye area (REA) and backfat thickness (BFT) can be used as indirect measures of meat yield. REA and BFT are important traits studied in beef cattle due to their strong implication in technological (carcass yield) and nutritional characteristics of meat products, like the degree of muscularity and total body fat. Thus, the aim of this work was to study the Longissimus dorsi muscle transcriptome of Nellore cattle, associated with REA and BFT, to find differentially expressed (DE) genes, metabolic pathways, and biological processes that may regulate these traits.

Results

By comparing the gene expression level between groups with extreme genomic estimated breeding values (GEBV), 101 DE genes for REA and 18 for BFT (false discovery rate, FDR 10%) were identified. Functional enrichment analysis for REA identified two KEGG pathways, MAPK (Mitogen-Activated Protein Kinase) signaling pathway and endocytosis pathway, and three biological processes, response to endoplasmic reticulum stress, cellular protein modification process, and macromolecule modification. The MAPK pathway is responsible for fundamental cellular processes, such as growth, differentiation, and hypertrophy. For BFT, 18 biological processes were found to be altered and grouped into 8 clusters of semantically similar terms. The DE genes identified in the biological processes for BFT were ACHE, SRD5A1, RSAD2 and RSPO3. RSAD2 has been previously shown to be associated with lipid droplet content and lipid biosynthesis.

Conclusion

In this study, we identified genes, metabolic pathways, and biological processes, involved in differentiation, proliferation, protein turnover, hypertrophy, as well as adipogenesis and lipid biosynthesis related to REA and BFT. These results enlighten some of the molecular processes involved in muscle and fat deposition, which are economically important carcass traits for beef production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3897-x) contains supplementary material, which is available to authorized users.

Gene expression profiling discerns molecular pathways elicited by ligand signaling to enhance the specification of embryonic stem cells into skeletal muscle lineage

Regulation of lineage specification and differentiation in embryonic stem (ES) cells can be achieved through the activation of endogenous signaling, an avenue for potential application in regenerative medicine. During vertebrate development, retinoic acid (RA) plays an important role in body axis elongation and mesoderm segmentation in that graded exposure to RA provides cells with positional identity and directs commitment to specific tissue lineages. Nevertheless, bexarotene, a clinically approved rexinoid, enhances the specification and differentiation of ES cells into skeletal myocytes more effectively than RA. Thus profiling the transcriptomes of ES cells differentiated with bexarotene or RA permits the identification of different genetic targets and signaling pathways that may contribute to the difference of bexarotene and RA in efficiency of myogenesis. Interestingly, bexarotene induces the early expression of a myogenic progenitor marker, Meox1, while the expression of many RA targets is also enhanced by bexarotene. Several signaling molecules involved in the progression of myogenic specification and commitment are differentially regulated by bexarotene and RA, suggesting that early targets of rexinoid allow the coordinated regulation of molecular events which leads to efficient myogenic differentiation in ES cells.

BRG1 interacts with GLI2 and binds Mef2c gene in a hedgehog signalling dependent manner during in vitro cardiomyogenesis

Background

The Hedgehog (HH) signalling pathway regulates cardiomyogenesis in vivo and in differentiating P19 embryonal carcinoma (EC) cells, a mouse embryonic stem (mES) cell model. To further assess the transcriptional role of HH signalling during cardiomyogenesis in stem cells, we studied the effects of overexpressing GLI2, a primary transducer of the HH signalling pathway, in mES cells.

Results

Stable GLI2 overexpression resulted in an enhancement of cardiac progenitor-enriched genes, Mef2c, Nkx2-5, and Tbx5 during mES cell differentiation. In contrast, pharmacological blockade of the HH pathway in mES cells resulted in lower expression of these genes. Mass spectrometric analysis identified the chromatin remodelling factor BRG1 as a protein which co-immunoprecipitates with GLI2 in differentiating mES cells. We then determined that BRG1 is recruited to a GLI2-specific Mef2c gene element in a HH signalling-dependent manner during cardiomyogenesis in P19 EC cells, a mES cell model.

Conclusions

Thus, we propose a mechanism where HH/GLI2 regulates the expression of Mef2c by recruiting BRG1 to the Mef2c gene, most probably via chromatin remodelling, to ultimately regulate in vitro cardiomyogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-016-0127-8) contains supplementary material, which is available to authorized users.

PAX3 and PAX7 as upstream regulators of myogenesis

Like other subclasses within the PAX transcription factor family, PAX3 and PAX7 play important roles in the emergence of a number of different tissues during development. PAX3 regulates neural crest and, together with its orthologue PAX7, is also expressed in parts of the central nervous system. In this chapter we will focus on their role in skeletal muscle. Both factors are key regulators of myogenesis where Pax3 plays a major role during early skeletal muscle formation in the embryo while Pax7 predominates during post-natal growth and muscle regeneration in the adult. We review the expression and functions of these factors in the myogenic context. We also discuss mechanistic aspects of PAX3/7 function and modulation of their activity by interaction with other proteins, as well as the post-transcriptional and transcriptional regulation of their expression.

Deployment of a retinal determination gene network drives directed cell migration in the sea urchin embryo

Gene regulatory networks (GRNs) provide a systems-level orchestration of an organism's genome encoded anatomy. As biological networks are revealed, they continue to answer many questions including knowledge of how GRNs control morphogenetic movements and how GRNs evolve. The migration of the small micromeres to the coelomic pouches in the sea urchin embryo provides an exceptional model for understanding the genomic regulatory control of morphogenesis. An assay using the robust homing potential of these cells reveals a ‘coherent feed-forward’ transcriptional subcircuit composed of Pax6, Six3, Six1/2, Eya, and Dach1 that is responsible for the directed homing mechanism of these multipotent progenitors. The linkages of that circuit are strikingly similar to a circuit involved in retinal specification in Drosophila suggesting that systems-level tasks can be highly conserved even though the tasks drive unrelated processes in different animals.

DOI:http://dx.doi.org/10.7554/eLife.08827.001

Within an animal embryo, groups of cells tend to move, or migrate, between different areas before they form into tissues and organs. These cell migrations are regulated by hundreds of genes, which must be expressed at the right time and in the right place. Cells use proteins called transcription factors to regulate the expression of genes. These proteins work together in circuit board-like networks called gene regulatory networks in order to drive different aspects of development, including cell migration.

The sea urchin is a useful model organism to study how animals develop. This is because these marine animals express many of the same genes as humans, but they can be easily manipulated and studied in the laboratory. In a developing sea urchin embryo, cells called the small micromeres move towards one end of animal and get incorporated into a pocket-like structure known as the coelomic pouch. From this pouch, these cells mature and eventually contribute to the adult germ cells (the precursors to the sperm and eggs).

Martik and McClay have now analyzed how small micromeres make their way to their final location in the coelomic pouch. Micromeres were labeled with a dye that fluoresces green so that they could be tracked under a microscope. This revealed that, like other moving cells, micromeres actively change their shape as they migrate. Furthermore, when micromeres were experimentally moved to abnormal locations in the sea urchin embryo, they were still able to actively home in on the coelomic pouch no matter their starting location.

Martik and McClay then identified five transcription factors expressed in the coelomic pouch in the sea urchin that are involved in this homing activity. Reducing the expression of any of these transcription factors was enough to hinder the ability of the micromeres to find their way to the coelomic pouch. Further experiments and analysis then revealed that these five transcription factors work together in a sub-circuit, which is in turn embedded in a larger gene regulatory network.

This sub-circuit that drives cell migration is unexpectedly similar to another circuit in the fruit fly Drosophila. Intriguingly, the sub-circuit in the fly controls eye development, which is unrelated to cell homing and migration. These observations raise the possibility that this circuit has been conserved as a unit over millions of years of evolution and redeployed in new networks under completely different circumstances. The data also suggest the possibility that additional conserved sub-circuits will be identified as more systems are analyzed in detail.

DOI:http://dx.doi.org/10.7554/eLife.08827.002

Loading of PAX3 to Mitotic Chromosomes Is Mediated by Arginine Methylation and Associated with Waardenburg Syndrome

PAX3 is a transcription factor critical to gene regulation in mammalian development. Mutations in PAX3 are associated with Waardenburg syndrome (WS), but the mechanism of how mutant PAX3 proteins cause WS remains unclear. Here, we found that PAX3 loads on mitotic chromosomes using its homeodomain. PAX3 WS mutants with mutations in homeodomain lose the ability to bind mitotic chromosomes. Moreover, loading of PAX3 on mitotic chromosomes requires arginine methylation, which is regulated by methyltransferase PRMT5 and demethylase JMJD6. Mutant PAX3 proteins that lose mitotic chromosome localization block cell proliferation and normal development of zebrafish. These results reveal the molecular mechanism of PAX3s loading on mitotic chromosomes and the importance of this localization pattern in normal development. Our findings suggest that PAX3 WS mutants interfere with the normal functions of PAX3 in a dominant negative manner, which is important to the understanding of the pathogenesis of Waardenburg syndrome.

The role of Six1 in the genesis of muscle cell and skeletal muscle development

The sine oculis homeobox 1 (Six1) gene encodes an evolutionarily conserved transcription factor. In the past two decades, much research has indicated that Six1 is a powerful regulator participating in skeletal muscle development. In this review, we summarized the discovery and structural characteristics of Six1 gene, and discussed the functional roles and molecular mechanisms of Six1 in myogenesis and in the formation of skeletal muscle fibers. Finally, we proposed areas of future interest for understanding Six1 gene function.

Induction of Pax3 gene expression impedes cardiac differentiation

Cell-based therapies using pluripotent stem cells hold great promise as regenerative approaches to treat many types of diseases. Nevertheless many challenges remain and, perhaps foremost, is the issue of how to direct and enhance the specification and differentiation of a desired cell type for potential therapeutics. We have examined the molecular basis for the inverse correlation of cardiac and skeletal myogenesis in small molecule-enhanced stem cell differentiation. Our study shows that activation of premyogenic factor Pax3 coincides with inhibiting gene expression of early cardiac factor GATA4. Interestingly, the inhibitory effect of small molecules on cardiac differentiation depends on the function of Pax3, but not the mesoderm factor Meox1. Thus Pax3 is an inhibitor of cardiac differentiation in lineage specification. Our studies reveal the dual roles of Pax3 in stem cell fate determinations and provide new molecular insights into small molecule-enhanced lineage specification.

Enhancing myogenic differentiation of pluripotent stem cells with small molecule inducers

Pluripotent stem cells are able to differentiate into many types of cell lineages in response to differentiation cues. However, a pure population of lineage-specific cells is desirable for any potential clinical application. Therefore, induction of the pluripotent stem cells with lineage-specific regulatory signals, or small molecule inducers, is a prerequisite for effectively directing lineage specification for cell-based therapeutics. In this article, we provide in-depth analysis of recent research findings on small molecule inducers of the skeletal muscle lineage. We also provide perspectives on how different signaling pathways and chromatin dynamics converge to direct the differentiation of skeletal myocytes.

Testosterone enhances cardiomyogenesis in stem cells and recruits the androgen receptor to the MEF2C and HCN4 genes

Since a previous study (Goldman-Johnson et al., 2008 [4]) has shown that androgens can stimulate increased differentiation of mouse embryonic stem (mES) cells into cardiomyocytes using a genomic pathway, the aim of our study is to elucidate the molecular mechanisms regulating testosterone-enhanced cardiomyogenesis. Testosterone upregulated cardiomyogenic transcription factors, including GATA4, MEF2C, and Nkx2.5, muscle structural proteins, and the pacemaker ion channel HCN4 in a dose-dependent manner, in mES cells and P19 embryonal carcinoma cells. Knock-down of the androgen receptor (AR) or treatment with anti-androgenic compounds inhibited cardiomyogenesis, supporting the requirement of the genomic pathway. Chromatin immunoprecipitation (ChIP) studies showed that testosterone enhanced recruitment of AR to the regulatory regions of MEF2C and HCN4 genes, which was associated with increased histone acetylation. In summary, testosterone upregulated cardiomyogenic transcription factor and HCN4 expression in stem cells. Further, testosterone induced cardiomyogenesis, at least in part, by recruiting the AR receptor to the regulatory regions of the MEF2C and HCN4 genes. These results provide a detailed molecular analysis of the function of testosterone in stem cells and may offer molecular insight into the role of steroids in the heart.

Functional dissection of Pax3 in paraxial mesoderm development and myogenesis

The paired box transcription factor Pax3 is well-known as a major regulator of embryonic myogenesis. Before Pax3 expression becomes restricted to the dermomyotome, this transcription factor is also expressed in the developing somites. The role of Pax3 at this early stage is unclear, in particular because of the scarce frequency of Pax3-positive cells in the early mouse embryo. Inducible gene expression in embryonic stem cells (ESCs) represents an excellent tool to overcome this limitation, since it can provide large quantities of otherwise rare embryonic populations expressing a factor of interest. Here we used engineered mouse ESCs to perform a functional analysis of Pax3 with the aim to identify the molecular determinants involved in the early functions of this transcription factor. We find that Pax3 induction during embryoid body differentiation results in the upregulation of genes expressed in the presomitic and somitic mesoderm. Moreover, we show that paraxial mesoderm induced by transient expression of Pax3 is not irreversibly committed to myogenesis rather requires sustained Pax3 expression. Using a series of deletion mutants of Pax3, which differentially affect its transcriptional activity, we map protein domains necessary for induction of paraxial mesoderm and induction of the myogenic program. The paired, homeo-, and transcriptional activation domains were each required for both processes, however, the paired-c-terminal RED domain showed a paraxial mesoderm-specific activity that was dispensable for myogenesis. These findings demonstrate and provide mechanistic insight into an early role for Pax3 in the generation of paraxial mesoderm.

Six homeoproteins directly activate Myod expression in the gene regulatory networks that control early myogenesis

In mammals, several genetic pathways have been characterized that govern engagement of multipotent embryonic progenitors into the myogenic program through the control of the key myogenic regulatory gene Myod. Here we demonstrate the involvement of Six homeoproteins. We first targeted into a Pax3 allele a sequence encoding a negative form of Six4 that binds DNA but cannot interact with essential Eya co-factors. The resulting embryos present hypoplasic skeletal muscles and impaired Myod activation in the trunk in the absence of Myf5/Mrf4. At the axial level, we further show that Myod is still expressed in compound Six1/Six4:Pax3 but not in Six1/Six4:Myf5 triple mutant embryos, demonstrating that Six1/4 participates in the Pax3-Myod genetic pathway. Myod expression and head myogenesis is preserved in Six1/Six4:Myf5 triple mutant embryos, illustrating that upstream regulators of Myod in different embryonic territories are distinct. We show that Myod regulatory regions are directly controlled by Six proteins and that, in the absence of Six1 and Six4, Six2 can compensate.

The onset of skeletal muscle formation is controlled by complex gene regulatory networks. By manipulation of these genetic pathways in the mouse embryo, we have examined the interplay between genes encoding the transcriptional regulator Pax3; the major myogenic determination proteins Myf5, Mrf4, and Myod; as well as genes encoding homeodomain proteins Six1 and Six4. In the absence of Myf5 and Six1/4, Myod expression is compromised. We demonstrate that key regulatory elements of the Myod gene are directly targeted by Six factors, including Six2, which is unexpectedly upregulated in the absence of Six1 and Six4. This work therefore reveals new aspects of the gene regulatory networks that control myogenesis.

Overexpression of Six1 gene suppresses proliferation and enhances expression of fast-type muscle genes in C2C12 myoblasts

Sine oculis homeobox 1 (Six1) homeodomain transcription factor is implicated in the genesis of muscle fiber type diversity, but its regulatory mechanisms on the formation of muscle fiber type are still poorly understood. To elucidate the biological roles of Six1 gene in muscle fiber formation, we established C2C12 cell line overexpressing Six1 and determined the effects of forced Six1 expression on muscle-specific genes expression, cell proliferation, and cell cycles. Our results indicated that Six1 overexpression could significantly promote the expression of fast-type muscle genes Atp2a1, Srl, and Mylpf. Furthermore, Six1 overexpressing C2C12 cells displayed a relative lower proliferative potential, and cell cycle analysis showed that Six1 exerted its role in cell cycle primarily through the regulation of G1/S and G2/M phases. In conclusion, Six1 plays an essential role in modulation of the fast-twitch muscle fiber phenotype through up-regulating fast-type muscle genes expression, and it could suppress the proliferation of muscle cells.

β-catenin is essential for efficient in vitro premyogenic mesoderm formation but can be partially compensated by retinoic acid signalling

Previous studies have shown that P19 cells expressing a dominant negative β-catenin mutant (β-cat/EnR) cannot undergo myogenic differentiation in the presence or absence of muscle-inducing levels of retinoic acid (RA). While RA could upregulate premyogenic mesoderm expression, including Pax3/7 and Meox1, only Pax3/7 and Gli2 could be upregulated by RA in the presence of β-cat/EnR. However, the use of a dominant negative construct that cannot be compensated by other factors is limiting due to the possibility of negative chromatin remodelling overriding compensatory mechanisms. In this study, we set out to determine if β-catenin function is essential for myogenesis with and without RA, by creating P19 cells with reduced β-catenin transcriptional activity using an shRNA approach, termed P19[shβ-cat] cells. The loss of β-catenin resulted in a reduction of skeletal myogenesis in the absence of RA as early as premyogenic mesoderm, with the loss of Pax3/7, Eya2, Six1, Meox1, Gli2, Foxc1/2, and Sox7 transcript levels. Chromatin immunoprecipitation identified an association of β-catenin with the promoter region of the Sox7 gene. Differentiation of P19[shβ-cat] cells in the presence of RA resulted in the upregulation or lack of repression of all of the precursor genes, on day 5 and/or 9, with the exception of Foxc2. However, expression of Sox7, Gli2, the myogenic regulatory factors and terminal differentiation markers remained inhibited on day 9 and overall skeletal myogenesis was reduced. Thus, β-catenin is essential for in vitro formation of premyogenic mesoderm, leading to skeletal myogenesis. RA can at least partially compensate for the loss of β-catenin in the expression of many myogenic precursor genes, but not for myoblast gene expression or overall myogenesis.

Inferring the in vivo cellular program of developing bovine skeletal muscle from expression data

We outline an in vivo cellular program of bovine longissimus muscle development inferred from expression data from 60 days post conception to 3months postnatal. Analytic challenges included changes in cellular composition, ambiguous 'diagnostic' markers of cell type and contrasts between cattle human and mouse myogenesis. Nevertheless, the expression profiles of the myosin isoforms support slow and fast muscle fibres emanating from primary and secondary myogenesis respectively, while expression of the prenatal myosin subunits is down regulated prior to birth. Of the canonical pro-myogenic transcription factors (TF), MYF6 and MYF5 are negatively co-expressed, with MYF6 displaying higher expression in the post-natal samples and MYF5, MYOG, HES6 and PAX7 displaying higher expression in early development. A set of TFs (SIX1, EYA2 and DACH2) considered important in undifferentiated murine cells were equally abundant in differentiated bovine cells. An examination of mammalian regulators of fibre composition, muscle mass and muscle metabolism, underscored the roles of PPARGC1A, TGFβ signalling and the NHR4 Nuclear Hormone Receptors on bovine muscle development. Enriched among the most variably expressed genes from the entire data set were molecules regulating mitochondrial metabolism of carbohydrate (PDK4), fat (UCP3), protein (AGXT2L1) and high energy phosphate (CKMT2). The dramatic increase in the expression of these transcripts, which may enable the peri-natal transition to metabolic independence critical for new-born herbivores, provides surprising evidence for substantial developmental remodelling of muscle mitochondria and reflects changes in nutrient availability. Overall, despite differences in size, metabolism and physiology, the muscle structural subunit expression program appears very similar in ruminants, rodents and humans.

Hedgehog signaling regulates MyoD expression and activity

The inhibition of MyoD expression is important for obtaining muscle progenitors that can replenish the satellite cell niche during muscle repair. Progenitors could be derived from either embryonic stem cells or satellite cells. Hedgehog (Hh) signaling is important for MyoD expression during embryogenesis and adult muscle regeneration. To date, the mechanistic understanding of MyoD regulation by Hh signaling is unclear. Here, we demonstrate that the Hh effector, Gli2, regulates MyoD expression and associates with MyoD gene elements. Gain- and loss-of-function experiments in pluripotent P19 cells show that Gli2 activity is sufficient and required for efficient MyoD expression during skeletal myogenesis. Inhibition of Hh signaling reduces MyoD expression during satellite cell activation in vitro. In addition to regulating MyoD expression, Hh signaling regulates MyoD transcriptional activity, and MyoD activates Hh signaling in myogenic conversion assays. Finally, Gli2, MyoD, and MEF2C form a protein complex, which enhances MyoD activity on skeletal muscle-related promoters. We therefore link Hh signaling to the function and expression of MyoD protein during myogenesis in stem cells.

Retinoic acid enhances skeletal myogenesis in human embryonic stem cells by expanding the premyogenic progenitor population

Human embryonic stem cells (hESCs) are a potential source of material for cell therapy of muscle diseases. To date, it has proven difficult to generate skeletal muscle from hESCs in high yields and within a reasonable timeframe. Further, a hESC-derived Pax3/7-positive skeletal muscle progenitor population has not yet been described. Previous studies have shown that Pax3/7-positive progenitor cells can repopulate the satellite cell niche, indicating the importance of this population for therapy. We sought to optimize the differentiation of hESCs into skeletal muscle in order to characterize myogenesis at a molecular level and shorten the time course. We treated hESCs with retinoic acid (RA) and found an enhancement of skeletal myogenesis, and the expression of the myogenic regulatory factors (MRFs) MyoD and myogenin by day 25. Furthermore, we found that RA treatment expanded the muscle progenitor pool, which occurred as a distinct Pax3(+ve) population prior to MRF expression. Non-skeletal muscle tissue types were not significantly affected. Therefore, we have identified a differentiation pathway in hESCs that provides a skeletal muscle progenitor population which can undergo myogenesis more efficiently. We propose that RA could fit into a directed culture method for deriving skeletal muscle from hESCs.

MiR-351 transiently increases during muscle regeneration and promotes progenitor cell proliferation and survival upon differentiation

MicroRNAs (miRNAs) regulate many biological processes including muscle development. However, little is known regarding miRNA regulation of muscle regeneration. Murine tibialis anterior muscle was evaluated after cardiotoxin-induced injury and used for global miRNA expression analysis. From day 1 through day 21 following injury, 298 miRNAs were significantly changed at least at one time point, including 86 miRNAs that were altered >10-fold compared with uninjured skeletal muscle. Temporal miRNA expression patterns included inflammation-related miRNAs (miR-223 and -147) that increased immediately after injury; this pattern contrasted to that of mature muscle-specific miRNAs (miR-1, -133a, and -499) that abruptly decreased following injury followed by upregulation in later regenerative events. Another cluster of miRNAs were transiently increased in the early days of muscle regeneration including miR-351, a miRNA that was also transiently expressed during myogenic progenitor cell (MPC) differentiation in vitro. Based on computational predictions, further studies demonstrated that E2f3 was a target of miR-351 in myoblasts. Moreover, knockdown of miR-351 expression inhibited MPC proliferation and promoted apoptosis during MPC differentiation, whereas miR-351 overexpression protected MPC from apoptosis during differentiation. Collectively, these observations suggest that miR-351 is involved in both the maintenance of MPC proliferation and the transition into differentiated myotubes. Thus, a novel, time-dependent sequence of molecular events during muscle regeneration has been identified; miR-351 inhibits E2f3 expression, a key regulator of cell cycle progression and proliferation, and promotes MPC proliferation and protects early differentiating MPC from apoptosis, important events in the hostile tissue environment after acute muscle injury.

Gli2 and MEF2C activate each other's expression and function synergistically during cardiomyogenesis in vitro

The transcription factors Gli2 (glioma-associated factor 2), which is a transactivator of Sonic Hedgehog (Shh) signalling, and myocyte enhancer factor 2C (MEF2C) play important roles in the development of embryonic heart muscle and enhance cardiomyogenesis in stem cells. Although the physiological importance of Shh signalling and MEF2 factors in heart development is well known, the mechanistic understanding of their roles is unclear. Here, we demonstrate that Gli2 and MEF2C activated each other's expression while enhancing cardiomyogenesis in differentiating P19 EC cells. Furthermore, dominant-negative mutant proteins of either Gli2 or MEF2C repressed each other's expression, while impairing cardiomyogenesis in P19 EC cells. In addition, chromatin immunoprecipitation (ChIP) revealed association of Gli2 to the Mef2c gene, and of MEF2C to the Gli2 gene in differentiating P19 cells. Finally, co-immunoprecipitation studies showed that Gli2 and MEF2C proteins formed a complex, capable of synergizing on cardiomyogenesis-related promoters containing both Gli- and MEF2-binding elements. We propose a model whereby Gli2 and MEF2C bind each other's regulatory elements, activate each other's expression and form a protein complex that synergistically activates transcription, enhancing cardiac muscle development. This model links Shh signalling to MEF2C function during cardiomyogenesis and offers mechanistic insight into their in vivo functions.

AKT and PAX3-FKHR cooperation enforces myogenic differentiation blockade in alveolar rhabdomyosarcoma cell

The chimeric PAX3-FKHR transcription factor is present in a majority of alveolar rhabdomyosarcoma (ARMS), an aggressive skeletal muscle cancer of childhood. PAX3-FKHR-mediated aberrant myogenic gene expression resulting in escape from terminal differentiation program is believed to contribute in ARMS development. In skeletal muscle differentiation, activation of AKT pathway leads to myogenic gene activation and terminal differentiation. Here, we report that AKT acts, in part, by modulating PAX3-FKHR transcriptional activity via phosphorylation in the maintenance of the myogenic differentiation blockade in established mouse models of ARMS cells. We observed that low levels of AKT activity are associated with elevated levels of PAX3-FKHR transcriptional activity, and AKT hyperactivation results in PAX3-FKHR phosphorylation coupled with decreased activity once cells are under differentiation-permissible conditions. Subsequent data shows that attenuated AKT activity-associated PAX3-FKHR activity is required to suppress the function of MyoD, a key myogenic regulator of muscle differentiation. Conversely, decreased PAX3-FKHR activity results in the eradication of MyoD expression and subsequent suppression of the myogenic differentiation. Thus, AKT regulation of the PAX3- FKHR suppresses myogenic gene expression in ARMS cells, causing a failure in differentiation. Evidence is presented that provides a novel molecular link between AKT and PAX3-FKHR in maintaining myogenic differentiation blockade in ARMS.

Transcription coactivator Eya2 is a critical regulator of physiological hypertrophy

Despite its significant clinical implications, physiological hypertrophy remains poorly understood. In this study, the transcription coactivator Eya2 was shown to be up-regulated during physiological hypertrophy. Transgene- or adenovirus-mediated overexpression of Eya2 led to up-regulation of mTOR, a critical mediator of physiological hypertrophy. Luciferase reporter and chromatin immunoprecipitation assays revealed that Eya2 directly binds to and activates mTOR expression. The phosphorylation of mTOR downstream molecules was significantly enhanced in Eya2 transgenic (TG) hearts, implying that the Eya2-mediated induction of mTOR expression leads to an elevated mTOR activity. The transcription factor Six1 was also up-regulated during physiological hypertrophy and formed a complex with Eya2. Luciferase reporter and electrophoretic mobility shift assays revealed that the Eya2-Six1 complex binds to and enhances the expression of mTOR in a synergistic manner. Under pressure overload, Eya2 transgenic hearts developed hypertrophy which exhibited important molecular signatures of physiological hypertrophy, as assessed by gene expression profiling and measurements of expression levels of physiological hypertrophy-related genes by quantitative (q) RT-PCR. Examination of heart sections under electron microscopy revealed that the mitochondrial integrity remained largely intact in Eya2 transgenic mice, but not in wild-type littermates, under pressure overload. This finding was confirmed by measurements of mitochondrial DNA contents and the expression levels of mitochondrial function-related genes by qRT-PCR. These data suggest that Eya2 in a physical complex with Six1 plays a critical role in physiological hypertrophy. The cardioprotective effect of Eya2 appears to be due, at least in part, to its preservation of mitochondrial integrity upon pressure overload.

Fibroblast growth factor 2 (Fgf2) inhibits differentiation of mesenchymal stem cells by inducing Twist2 and Spry4, blocking extracellular regulated kinase activation, and altering Fgf receptor expression levels

Mesenchymal stem cells (MSCs) are known to differentiate into connective tissue lineages but intracellular signaling pathways that maintain cells in an undifferentiated state remain largely unexplored. Previously, we reported that fibroblast growth factor 2 (Fgf2) reversibly inhibited multilineage differentiation of primary mouse MSCs and now identify a unique compliment of signaling proteins that are dynamically regulated by this mitogen and whose expression levels are strongly correlated with inhibition of cell differentiation. Fgf2 selectively induced expression of Twist2 and Sprouty4 (Spry4) and repressed expression of soluble frizzled related receptor 2 (Sfrp2), runt-related transcription factor 2 (Runx2), and peroxisome proliferation activated receptor gamma (Pparg). In contrast, Wnt3a induced expression of Twist but not Twist2 or Spry4 and bone morphogenetic protein 2 (Bmp2) failed to alter expression of all three genes. Moreover, pretreatment of MSCs with Fgf2 delayed extracellular regulated kinase 1 (Erk1) and Erk2 phosphorylation and repressed bone-specific gene expression during an osteoinduction time course. Alternatively, pretreatment with Wnt3a had no effect, whereas Bmp2 pretreatment augmented Erk1/2 activation and bone-specific gene expression. Fgf2 also induced expression of Fgf receptor 1 (Fgfr1) and Fgfr4 and repressed Fgfr2 and Fgfr3 expression in MSCs, whereas Wnt3a and Bmp2 had the opposite effect. Finally, immunostaining revealed that Twist and Spry4 were coexpressed in MSCs and that Fgf2 treatment altered their subcellular distribution in a manner consistent with their mode of action. Collectively, these studies demonstrate that inhibition of mouse MSC differentiation by Fgf2 is strongly correlated with upregulation of Twist2 and Spry4 and suppression of Erk1/2 activation.

Expression profiling of FSHD-1 and FSHD-2 cells during myogenic differentiation evidences common and distinctive gene dysregulation patterns

BACKGROUND

Determine global gene dysregulation affecting 4q-linked (FSHD-1) and non 4q-linked (FSHD-2) cells during early stages of myogenic differentiation. This approach has been never applied to FSHD pathogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

By in vitro differentiation of FSHD-1 and FSHD-2 myoblasts and gene chip analysis we derived that gene expression profile is altered only in FSHD-1 myoblasts and FSHD-2 myotubes. The changes seen in FSHD-1 regarded a general defect in cell cycle progression, probably due to the upregulation of myogenic markers PAX3 and MYOD1, and a deficit of factors (SUV39H1 and HMGB2) involved in D4Z4 chromatin conformation. On the other hand, FSHD-2 mytubes were characterized by a general defect in RNA metabolism, protein synthesis and degradation and, to a lesser extent, in cell cycle. Common dysregulations regarded genes involved in response to oxidative stress and in sterol biosynthetic process. Interestingly, our results also suggest that miRNAs might be implied in both FSHD-1 and FSHD-2 gene dysregulation. Finally, in both cell differentiation systems, we did not observe a gradient of altered gene expression throughout the 4q35 chromosome.

CONCLUSIONS/SIGNIFICANCE

FSHD-1 and FSHD-2 cells showed, in different steps of myogenic differentiation, a global deregulation of gene expression rather than an alteration of expression of 4q35 specific genes. In general, FSHD-1 and FSHD-2 global gene deregulation interested common and distinctive biological processes. In this regard, defects of cell cycle progression (FSHD-1 and to a lesser extent FSHD-2), protein synthesis and degradation (FSHD-2), response to oxidative stress (FSHD-1 and FSHD-2), and cholesterol homeostasis (FSHD-1 and FSHD-2) may in general impair a correct myogenesis. Taken together our results recapitulate previously reported defects of FSHD-1, and add new insights into the gene deregulation characterizing both FSHD-1 and FSHD-2, in which miRNAs may play a role.

Contribution of retinoid X receptor signaling to the specification of skeletal muscle lineage

Pluripotent stem cells possess a tremendous potential for the treatment of many diseases because of their capacity to differentiate into a variety of cell lineages. However, they provide little promise for muscle-related diseases, mainly because of the lack of small molecule inducers to efficiently direct myogenic conversion. Retinoic acid, acting through the retinoic acid receptor (RAR) and retinoid X receptor (RXR), affects stem cell fate determination in a concentration-dependent manner, but it only has a modest efficacy on the commitment of ES cells into skeletal muscle lineage. The RXR is very important for embryonic development but is generally considered to act as a silent partner of RAR in a non-permissive mode. In this study, we have examined whether activation of the RXR by rexinoid or RXR-specific signaling play a role in the specification of stem cells into muscle lineage. Our findings demonstrate that mouse ES cells generate skeletal myocytes effectively upon treatment with rexinoid at the early stage of differentiation and that on a molecular level, rexinoid-enhanced myogenesis simulates the sequential events observed in vivo. Moreover, RXR-mediated myogenic conversion requires the function of β-catenin but not RAR. Our studies establish the feasibility of applying the RXR agonist in cell-based therapies to treat muscle-related diseases. The aptitude of mouse ES cells to generate skeletal myocytes following rexinoid induction also provides a model system to study the convergence of different signaling pathways in myogenesis.

Skeletal myosin light chain kinase regulates skeletal myogenesis by phosphorylation of MEF2C

The MEF2 factors regulate transcription during cardiac and skeletal myogenesis. MEF2 factors establish skeletal muscle commitment by amplifying and synergizing with MyoD. While phosphorylation is known to regulate MEF2 function, lineage-specific regulation is unknown. Here, we show that phosphorylation of MEF2C on T(80) by skeletal myosin light chain kinase (skMLCK) enhances skeletal and not cardiac myogenesis. A phosphorylation-deficient MEF2C mutant (MEFT80A) enhanced cardiac, but not skeletal myogenesis in P19 stem cells. Further, MEFT80A was deficient in recruitment of p300 to skeletal but not cardiac muscle promoters. In gain-of-function studies, skMLCK upregulated myogenic regulatory factor (MRF) expression, leading to enhanced skeletal myogenesis in P19 cells and more efficient myogenic conversion. In loss-of-function studies, MLCK was essential for efficient MRF expression and subsequent myogenesis in embryonic stem (ES) and P19 cells as well as for proper activation of quiescent satellite cells. Thus, skMLCK regulates MRF expression by controlling the MEF2C-dependent recruitment of histone acetyltransferases to skeletal muscle promoters. This work identifies the first kinase that regulates MyoD and Myf5 expression in ES or satellite cells.

Trenbolone enhances myogenic differentiation by enhancing β-catenin signaling in muscle-derived stem cells of cattle

Testosterone is a key hormone regulating animal growth and development, which promotes skeletal muscle growth and inhibits fat deposition; however, the underlying mechanisms remain poorly defined. Because canonical Wingless and Int/β-catenin signaling promotes myogenesis, we hypothesized that testosterone regulates myogenesis through enhancing the β-catenin signaling pathway and the expression of its targeted genes. Muscle-derived stem cells were prepared from the skeletal muscle of fetal calf at day 180 of gestation and treated with or without trenbolone (10 nM), a synthetic analog of testosterone, in a myogenic medium. Trenbolone treatment increased the protein levels of MyoD and myosin heavy chain, as well as the androgen receptor content. The myogenic effect of trenbolone was blocked by cyproterone acetate, a specific inhibitor of androgen receptor, showing that the myogenic effect of trenbolone was mediated by the androgen receptor. Immunoprecipitation showed that androgen receptor and β-catenin formed a complex, which was increased by trenbolone treatment. Trenbolone activated adenosine monophosphate-activated protein kinase, which might phosphorylate β-catenin at Ser552, stabilizing β-catenin. Indeed, both cytoplasmic and nuclear β-catenin levels were increased after trenbolone treatment. As a result, β-catenin-mediated transcriptional activity was enhanced by trenbolone treatment. In conclusion, these data provide evidence that testosterone increases cellular β-catenin content which promotes the expression of β-catenin-targeted genes and myogenesis in the muscle-derived stem cells of cattle.