Eya1 and Eya2 proteins are required for hypaxial somitic myogenesis in the mouse embryo

Building Haplotype-Resolved 3D Genome Maps of Chicken Skeletal Muscle

Haplotype-resolved 3D chromatin architecture related to allelic differences in avian skeletal muscle development has not been addressed so far, although chicken husbandry for meat consumption has been prevalent feature of cultures on every continent for more than thousands of years. Here, high-resolution Hi-C diploid maps (1.2-kb maximum resolution) are generated for skeletal muscle tissues in chicken across three developmental stages (embryonic day 15 to day 30 post-hatching). The sequence features governing spatial arrangement of chromosomes and characterize homolog pairing in the nucleus, are identified. Multi-scale characterization of chromatin reorganization between stages from myogenesis in the fetus to myofiber hypertrophy after hatching show concordant changes in transcriptional regulation by relevant signaling pathways. Further interrogation of parent-of-origin-specific chromatin conformation supported that genomic imprinting is absent in birds. This study also reveals promoter-enhancer interaction (PEI) differences between broiler and layer haplotypes in skeletal muscle development-related genes are related to genetic variation between breeds, however, only a minority of breed-specific variations likely contribute to phenotypic divergence in skeletal muscle potentially via allelic PEI rewiring. Beyond defining the haplotype-specific 3D chromatin architecture in chicken, this study provides a rich resource for investigating allelic regulatory divergence among chicken breeds.

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.

Overlapping functions of SIX homeoproteins during embryonic myogenesis

Four SIX homeoproteins display a combinatorial expression throughout embryonic developmental myogenesis and they modulate the expression of the myogenic regulatory factors. Here, we provide a deep characterization of their role in distinct mouse developmental territories. We showed, at the hypaxial level, that the Six1:Six4 double knockout (dKO) somitic precursor cells adopt a smooth muscle fate and lose their myogenic identity. At the epaxial level, we demonstrated by the analysis of Six quadruple KO (qKO) embryos, that SIX are required for fetal myogenesis, and for the maintenance of PAX7+ progenitor cells, which differentiated prematurely and are lost by the end of fetal development in qKO embryos. Finally, we showed that Six1 and Six2 are required to establish craniofacial myogenesis by controlling the expression of Myf5. We have thus described an unknown role for SIX proteins in the control of myogenesis at different embryonic levels and refined their involvement in the genetic cascades operating at the head level and in the genesis of myogenic stem cells.

The role of Eya1 and Eya2 in the taste system of mice from embryonic stage to adulthood

Members of the Eya family, which are a class of transcription factors with phosphatase activity, are widely expressed in cranial sensory organs during development. However, it is unclear whether these genes are expressed in the taste system during development and whether they play any role in specifying taste cell fate. In this study, we report that Eya1 is not expressed during embryonic tongue development but that Eya1-expressing progenitors in somites or pharyngeal endoderm give rise to tongue musculature or taste organs, respectively. In the Eya1-deficient tongues, these progenitors do not proliferate properly, resulting in a smaller tongue at birth, impaired growth of taste papillae, and disrupted expression of Six1 in the papillary epithelium. On the other hand, Eya2 is specifically expressed in endoderm-derived circumvallate and foliate papillae located on the posterior tongue during development. In adult tongues, Eya1 is predominantly expressed in IP3R3-positive taste cells in the taste buds of the circumvallate and foliate papillae, while Eya2 is persistently expressed in these papillae at higher levels in some epithelial progenitors and at lower levels in some taste cells. We found that conditional knockout of Eya1 in the third week or Eya2 knockout reduced Pou2f3+, Six1+ and IP3R3+ taste cells. Our data define for the first time the expression patterns of Eya1 and Eya2 during the development and maintenance of the mouse taste system and suggest that Eya1 and Eya2 may act together to promote lineage commitment of taste cell subtypes.

Retinal determination gene networks: from biological functions to therapeutic strategies

The retinal determinant gene network (RDGN), originally discovered as a critical determinator in Drosophila eye specification, has become an important regulatory network in tumorigenesis and progression, as well as organogenesis. This network is not only associated with malignant biological behaviors of tumors, such as proliferation, and invasion, but also regulates the development of multiple mammalian organs. Three members of this conservative network have been extensively investigated, including DACH, SIX, and EYA. Dysregulated RDGN signaling is associated with the initiation and progression of tumors. In recent years, it has been found that the members of this network can be used as prognostic markers for cancer patients. Moreover, they are considered to be potential therapeutic targets for cancer. Here, we summarize the research progress of RDGN members from biological functions to signaling transduction, especially emphasizing their effects on tumors. Additionally, we discuss the roles of RDGN members in the development of organs and tissue as well as their correlations with the pathogenesis of chronic kidney disease and coronary heart disease. By summarizing the roles of RDGN members in human diseases, we hope to promote future investigations into RDGN and provide potential therapeutic strategies for patients.

Eutherian-Specific Functions of BetaM Acquired through Atp1b4 Gene Co-Option in the Regulation of MyoD Expression

Vertebrate ATP1B4 genes represent a rare instance of orthologous gene co-option, resulting in radically different functions of the encoded BetaM proteins. In lower vertebrates, BetaM is a Na, K-ATPase β-subunit that is a component of ion pumps in the plasma membrane. In placental mammals, BetaM lost its ancestral role and, through structural alterations of the N-terminal domain, became a skeletal and cardiac muscle-specific protein of the inner nuclear membrane, highly expressed during late fetal and early postnatal development. We previously determined that BetaM directly interacts with the transcriptional co-regulator SKI-interacting protein (SKIP) and is implicated in the regulation of gene expression. This prompted us to investigate a potential role for BetaM in the regulation of muscle-specific gene expression in neonatal skeletal muscle and cultured C2C12 myoblasts. We found that BetaM can stimulate expression of the muscle regulatory factor (MRF), MyoD, independently of SKIP. BetaM binds to the distal regulatory region (DRR) of MyoD, promotes epigenetic changes associated with activation of transcription, and recruits the SWI/SNF chromatin remodeling subunit, BRG1. These results indicate that eutherian BetaM regulates muscle gene expression by promoting changes in chromatin structure. These evolutionarily acquired new functions of BetaM might be very essential and provide evolutionary advantages to placental mammals.

Copy number variations analysis in a cohort of 47 fetuses and newborns with congenital diaphragmatic hernia

OBJECTIVES

The congenital diaphragmatic hernia (CDH), characterized by malformation of the diaphragm and lung hypoplasia, is a common and severe birth defect that affects around 1 in 4000 live births. However, the etiology of most cases of CDH remains unclear. The aim of this study was to perform a retrospective analysis of copy number variations (CNVs) using a high-resolution array comparative genomic hybridization (array-CGH) in a cohort of fetuses and newborns with CDH.

METHODS

Forty seven fetuses and newborns with either isolated or syndromic CDH were analyzed by oligonucleotide-based array-CGH Agilent 180K technique.

RESULTS

A mean of 10.2 CNVs was detected by proband with a total number of 480 CNVs identified based on five categories: benign, likely benign, of uncertain signification, likely pathogenic, and pathogenic. Diagnostic performance was estimated at 19.15% (i.e., likely pathogenic and pathogenic CNVs) for both CDH types. We identified 11 potential candidate genes: COL25A1, DSEL, EYA1, FLNA, MECOM, NRXN1, RARB, SPATA13, TJP2, XIRP2, and ZFPM2.

CONCLUSION

We suggest that COL25A1, DSEL, EYA1, FLNA, MECOM, NRXN1, RARB, SPATA13, TJP2, XIRP2, and ZFPM2 genes may be related to CDH occurrence. Thus, this study provides a possibility for new methods of a positive diagnosis.

Cell fate decisions, transcription factors and signaling during early retinal development

The development of the vertebrate eyes is a complex process starting from anterior-posterior and dorso-ventral patterning of the anterior neural tube, resulting in the formation of the eye field. Symmetrical separation of the eye field at the anterior neural plate is followed by two symmetrical evaginations to generate a pair of optic vesicles. Next, reciprocal invagination of the optic vesicles with surface ectoderm-derived lens placodes generates double-layered optic cups. The inner and outer layers of the optic cups develop into the neural retina and retinal pigment epithelium (RPE), respectively. In vitro produced retinal tissues, called retinal organoids, are formed from human pluripotent stem cells, mimicking major steps of retinal differentiation in vivo. This review article summarizes recent progress in our understanding of early eye development, focusing on the formation the eye field, optic vesicles, and early optic cups. Recent single-cell transcriptomic studies are integrated with classical in vivo genetic and functional studies to uncover a range of cellular mechanisms underlying early eye development. The functions of signal transduction pathways and lineage-specific DNA-binding transcription factors are dissected to explain cell-specific regulatory mechanisms underlying cell fate determination during early eye development. The functions of homeodomain (HD) transcription factors Otx2, Pax6, Lhx2, Six3 and Six6, which are required for early eye development, are discussed in detail. Comprehensive understanding of the mechanisms of early eye development provides insight into the molecular and cellular basis of developmental ocular anomalies, such as optic cup coloboma. Lastly, modeling human development and inherited retinal diseases using stem cell-derived retinal organoids generates opportunities to discover novel therapies for retinal diseases.

Manzamine-A Alters In Vitro Calvarial Osteoblast Function

Manzamine-A is a marine-derived alkaloid which has anti-viral and anti-proliferative properties and is currently being investigated for its efficacy in the treatment of certain viruses (malaria, herpes, HIV-1) and cancers (breast, cervical, colorectal). Manzamine-A has been found to exert effects via modulation of SIX1 gene expression, a gene critical to craniofacial development via the WNT, NOTCH, and PI3K/AKT pathways. To date little work has focused on Manzamine-A and how its use may affect bone. We hypothesize that Manzamine-A, through SIX1, alters bone cell activity. Here, we assessed the effects of Manzamine-A on cells that are responsible for the generation of bone, pre-osteoblasts and osteoblasts. PCR, qrtPCR, MTS cell viability, Caspase 3/7, and functional assays were used to test the effects of Manzamine-A on these cells. Our data suggests Six1 is highly expressed in osteoblasts and their progenitors. Further, osteoblast progenitors and osteoblasts exhibit great sensitivity to Manzamine-A treatment exhibited by a significant decrease in cell viability, increase in cellular apoptosis, and decrease in alkaline phosphatase activity. In silico binding experiment showed that manzamine A potential as an inhibitor of cell proliferation and survival proteins, i.e., Iκb, JAK2, AKT, PKC, FAK, and Bcl-2. Overall, our data suggests Manzamine-A may have great effects on bone health overall and may disrupt skeletal development, homeostasis, and repair.

The interconnected relationships between middle ear bulla size, cavitation defects, and chronic otitis media revealed in a syndromic mouse model

High incidence of chronic otitis media is associated with human craniofacial syndromes, suggesting that defects in the formation of the middle ear and associated structures can have a knock-on effect on the susceptibility to middle ear inflammation. Patients with branchio-oto-renal (BOR) syndrome have several defects in the ear leading to both sensorineural and conductive hearing loss, including otitis media. 40% of BOR syndrome cases are due to Eya1 haploinsufficiency, with mouse models affecting Eya1, mimicking many of the defects found in patients. Here, we characterize the onset, consequences, and underlying causes of chronic otitis media in Eya1 heterozygous mice. Cavitation defects were evident in these mice from postnatal day (P)11 onwards, with mesenchyme around the promontory and attic regions of the middle ear space. This mesenchyme was still prominent in adult Eya1 heterozygous mice, while the wild-type littermates had fully aerated ears from P14 onwards. MicroCT analysis highlighted a significantly smaller bulla, confirming the link between bulla size defects and the ability of the mesenchyme to retract successfully. Otitis media was observed from P14, often presenting unilaterally, resulting in hyperplasia of the middle ear mucosa, expansion of secretory cells, defects in the motile cilia, and changes in basal epithelial cell markers. A high incidence of otitis media was identified in older mice but only associated with ears with retained mesenchyme. To understand the impact of the environment, the mouse line was rederived onto a super-clean environment. Cavitation defects were still evident at early stages, but these generally resolved over time, and importantly, no signs of otitis media were observed at 6 weeks. In conclusion, we show that a small bulla size is closely linked to defects in cavitation and the presence of retained mesenchyme. A delay in retraction of the mesenchyme predates the onset of otitis media, making the ears susceptible to its development. Early exposure to OM appears to exacerbate the cavitation defect, with mesenchyme evident in the middle ear throughout the animal’s life. This highlights that permanent damage to the middle ear can arise as a consequence of the early onset of OM.

Genetically Modified Mouse Models of Congenital Diaphragmatic Hernia: Opportunities and Limitations for Studying Altered Lung Development

Congenital diaphragmatic hernia (CDH) is a relatively common and life-threatening birth defect, characterized by an abnormal opening in the primordial diaphragm that interferes with normal lung development. As a result, CDH is accompanied by immature and hypoplastic lungs, being the leading cause of morbidity and mortality in patients with this condition. In recent decades, various animal models have contributed novel insights into the pathogenic mechanisms underlying CDH and associated pulmonary hypoplasia. In particular, the generation of genetically modified mouse models, which show both diaphragm and lung abnormalities, has resulted in the discovery of multiple genes and signaling pathways involved in the pathogenesis of CDH. This article aims to offer an up-to-date overview on CDH-implicated transcription factors, molecules regulating cell migration and signal transduction as well as components contributing to the formation of extracellular matrix, whilst also discussing the significance of these genetic models for studying altered lung development with regard to the human situation.

Epigenetic mechanisms of DNA methylation in the transgenerational effect of ethylhexyl salicylate on zebrafish

In this study, a 120-day whole-life cycle exposure and oviposition experiment on zebrafish with maternal and paternal mixed mating strategy was conducted to investigate the epigenetic mechanism of DNA methylation in ethylhexyl salicylate (EHS, 1, 10, 100 μg/L)-induced transgenerational effects. Results showed that EHS could induce the decrease of DNA methyltransferase 1 (DNMT1) activity and average global DNA methylation level in maternal parents and the increase of the above indexes in paternal parents, while the change of glycine N-methyltransferase activity was opposite to DNMT1. The average global DNA methylation levels were significantly increased in the offsprings of both parents exposed and father-only exposed to EHS, suggesting that EHS-induced epigenetic modifications may be stable and heritable. Hierarchical clustering analysis of promoter at different methylation sites showed that the DNA methylation pattern of offsprings were similar to that of the paternal parents, meaning that the offsprings may have inherited paternal DNA methylation pattern with eya2, pcdh2g5 and pcdh2g1 as key genes and lead to high locomotor activity in offsprings. KEGG pathway analysis showed that parental exposure to EHS may interfere with the central nervous system, insulin function system, melanogenesis system and the normal development of somatic axis of offsprings.

Comparative development of limb musculature in phylogenetically and ecologically divergent lizards

BACKGROUND

Squamate reptiles (lizards, snakes, and amphisbaenians) exhibit incredible diversity in their locomotion, behavior, morphology, and ecological breadth. Although they often are used as models of locomotor diversity, surprisingly little attention has been given to muscle development in squamate reptiles. In fact, the most detailed examination was conducted almost 80 years ago and solely focused on the proximal limb regions. Herein, we present forelimb and hindlimb muscle morphogenesis data for three lizard species with different locomotion and feeding strategies: the desert grassland whiptail lizard, the central bearded dragon, and the veiled chameleon. This study fills critical gaps in our understanding of muscle morphogenesis in squamate reptiles and presents a comparative and temporospatial analysis of muscle development.

RESULTS

Our results reveal a conserved pattern of early muscle development among lizards with different adult morphologies and ecologies. The variations that exist are concentrated in distal regions, particularly the specialized autopodia of chameleons, where differentiation of muscles associated with the digits is delayed.

CONCLUSIONS

The chameleon autopod provides an example of major evolutionary modifications to the skeleton with only minor disruption of the conserved order and pattern of limb muscle development. This robustness of muscle patterning facilitates the evolution of extreme yet functional phenotypes.

Targeting EYA2 tyrosine phosphatase activity in glioblastoma stem cells induces mitotic catastrophe

Glioblastoma ranks among the most lethal of primary brain malignancies, with glioblastoma stem cells (GSCs) at the apex of tumor cellular hierarchies. Here, to discover novel therapeutic GSC targets, we interrogated gene expression profiles from GSCs, differentiated glioblastoma cells (DGCs), and neural stem cells (NSCs), revealing EYA2 as preferentially expressed by GSCs. Targeting EYA2 impaired GSC maintenance and induced cell cycle arrest, apoptosis, and loss of self-renewal. EYA2 displayed novel localization to centrosomes in GSCs, and EYA2 tyrosine (Tyr) phosphatase activity was essential for proper mitotic spindle assembly and survival of GSCs. Inhibition of the EYA2 Tyr phosphatase activity, via genetic or pharmacological means, mimicked EYA2 loss in GSCs in vitro and extended the survival of tumor-bearing mice. Supporting the clinical relevance of these findings, EYA2 portends poor patient prognosis in glioblastoma. Collectively, our data indicate that EYA2 phosphatase function plays selective critical roles in the growth and survival of GSCs, potentially offering a high therapeutic index for EYA2 inhibitors.

Master regulators of skeletal muscle lineage development and pluripotent stem cells differentiation

In vertebrates, the skeletal muscles of the body and their associated stem cells originate from muscle progenitor cells, during development. The specification of the muscles of the trunk, head and limbs, relies on the activity of distinct genetic hierarchies. The major regulators of trunk and limb muscle specification are the paired-homeobox transcription factors PAX3 and PAX7. Distinct gene regulatory networks drive the formation of the different muscles of the head. Despite the redeployment of diverse upstream regulators of muscle progenitor differentiation, the commitment towards the myogenic fate requires the expression of the early myogenic regulatory factors MYF5, MRF4, MYOD and the late differentiation marker MYOG. The expression of these genes is activated by muscle progenitors throughout development, in several waves of myogenic differentiation, constituting the embryonic, fetal and postnatal phases of muscle growth. In order to achieve myogenic cell commitment while maintaining an undifferentiated pool of muscle progenitors, several signaling pathways regulate the switch between proliferation and differentiation of myoblasts. The identification of the gene regulatory networks operating during myogenesis is crucial for the development of in vitro protocols to differentiate pluripotent stem cells into myoblasts required for regenerative medicine.

Eya1 protein distribution during embryonic development of Xenopus laevis

Eya1 and other Eya proteins are important regulators of progenitor proliferation, cell differentiation and morphogenesis in all three germ layers. At present, most of our knowledge of Eya1 distribution is based on in situ hybridization for Eya1 mRNA. However, to begin to dissect the mechanisms underlying Eya1 functions, we need a better understanding of the spatiotemporal distribution of Eya1 proteins during embryonic development, their subcellular localization and their levels of expression in various tissues. Here we report the localization of Eya1 protein throughout embryonic development from neural plate stages to tadpole stages of Xenopus laevis using a specific antibody for Xenopus Eya1. Our study confirms the expression of Eya1 protein in cranial placodes, placodally derived sensory primordia (olfactory epithelium, otic vesicle, lateral line primordia) and cranial ganglia, as well as in somites, secondary heart field and pharyngeal endoderm. In addition, we report here a novel expression of Eya1 proteins in scattered epidermal cells in Xenopus. Our findings also reveal that, while being predominantly expressed in nuclei in most expression domains, Eya1 protein is also localized to the cytoplasm, in particular in the early preplacodal ectoderm, some placode-derived ganglia and a subset of epidermal cells. While some cytoplasmic roles of Eya1 have been previously described in other contexts, the functions of cytoplasmic Eya1 in the preplacodal ectoderm, cranial ganglia and epidermal cells remain to be investigated.

ITGB6 inhibits the proliferation of porcine skeletal muscle satellite cells

The formation of embryonic muscle fibers determines the amount of postnatal muscles and is regulated by a variety of signaling pathways and transcription factors. Previously, by using chromatin immunoprecipitation-sequencing and RNA-Seq techniques, we identified a large number of genes that are regulated by H3K27me3 in porcine embryonic skeletal muscles. Among these genes, we found that ITGB6 is regulated by H3K27me3. However, its function in muscle development is unknown. In this study, we first verified that ITGB6 was differentially regulated by H3K27me3 and that its expression levels were upregulated in porcine skeletal muscles at embryonic Days 33, 65, and 90. Then, we performed gain- or loss-of-function studies on porcine skeletal muscle satellite cells to study the role of ITGB6 in porcine skeletal muscle development. The proliferation of porcine skeletal muscle satellite cells was studied through real-time polymerase chain reaction, Cell Counting Kit-8, 5-ethynyl-2'-deoxyuridine staining, Western blot, and flow cytometry analyses. We found that the ITGB6 gene was regulated by H3K27me3 during muscle development and had an inhibitory effect on the proliferation of porcine skeletal muscle satellite cells.

The Eyes Absent proteins in development and in developmental disorders

The Eyes Absent (EYA) transactivator-phosphatase proteins are important contributors to cell-fate determination processes and to the development of multiple organs. The transcriptional regulatory activity as well as the protein tyrosine phosphatase activities of the EYA proteins can independently contribute to proliferation, differentiation, morphogenesis and tissue homeostasis in different contexts. Aberrant EYA levels or activity are associated with numerous syndromic and non-syndromic developmental disorders, as well as cancers. Commensurate with the multiplicity of biochemical activities carried out by the EYA proteins, they impact upon a range of cellular signaling pathways. Here, we provide a broad overview of the roles played by EYA proteins in development, and highlight the molecular signaling pathways known to be linked with EYA-associated organ development and developmental disorders.

M-Cadherin Is a PAX3 Target During Myotome Patterning

PAX3 belongs to the paired-homeobox family of transcription factors and plays a key role as an upstream regulator of muscle progenitor cells during embryonic development. Pax3-mutant embryos display impaired somite development, yet the consequences for myotome formation have not been characterized. The early myotome is formed by PAX3-expressing myogenic cells that delaminate from the dermomyotomal lips and migrate between the dermomyotome and sclerotome where they terminally differentiate. Here we show that in Pax3-mutant embryos, myotome formation is impaired, displays a defective basal lamina and the regionalization of the structural protein Desmin is lost. In addition, this phenotype is more severe in embryos combining Pax3-null and Pax3 dominant-negative alleles. We identify the adhesion molecule M-Cadherin as a PAX3 target gene, the expression of which is modulated in the myotome according to Pax3 gain- and loss-of-function alleles analyzed. Taken together, we identify M-Cadherin as a PAX3-target linked to the formation of the myotome.

Eya2 expression during mouse embryonic development revealed by Eya2lacZ knockin reporter and homozygous mice show mild hearing loss

BACKGROUND

Eya2 expression during mouse development has been studied by in situ hybridization and it has been shown to be involved skeletal muscle development and limb formation. Here, we generated Eya2 knockout (Eya2^- ) and a lacZ knockin reporter (Eya2^lacZ ) mice and performed a detailed expression analysis for Eya2^lacZ at different developmental stages to trace Eya2^lacZ -positive cells in Eya2-null mice. We describe that Eya2 is not only expressed in cranial sensory and dorsal root ganglia, retina and olfactory epithelium, and somites as previously reported, but also Eya2 is specifically detected in other organs during mouse development.

RESULTS

We found that Eya2 is expressed in ocular and trochlear motor neurons. In the inner ear, Eya2^lacZ is specifically expressed in differentiating hair cells in both vestibular and cochlear sensory epithelia of the inner ear and Eya2^-/- or Eya2^lacZ/lacZ mice displayed mild hearing loss. Furthermore, we detected Eya2 expression during both salivary gland and thymus development and Eya2-null mice had a smaller thymus.

CONCLUSIONS

As Eya2 is coexpressed with other members of the Eya family genes, these results together highlight that Eya2 as a potential regulator may act synergistically with other Eya genes to regulate the differentiation of the inner ear sensory hair cells and the formation of the salivary gland and thymus.

Axial skeleton anterior-posterior patterning is regulated through feedback regulation between Meis transcription factors and retinoic acid

Vertebrate axial skeletal patterning is controlled by co-linear expression of Hox genes and axial level-dependent activity of HOX protein combinations. MEIS transcription factors act as co-factors of HOX proteins and profusely bind to Hox complex DNA; however, their roles in mammalian axial patterning remain unknown. Retinoic acid (RA) is known to regulate axial skeletal element identity through the transcriptional activity of its receptors; however, whether this role is related to MEIS/HOX activity remains unknown. Here, we study the role of Meis in axial skeleton formation and its relationship to the RA pathway in mice. Meis elimination in the paraxial mesoderm produces anterior homeotic transformations and rib mis-patterning associated to alterations of the hypaxial myotome. Although Raldh2 and Meis positively regulate each other, Raldh2 elimination largely recapitulates the defects associated with Meis deficiency, and Meis overexpression rescues the axial skeletal defects in Raldh2 mutants. We propose a Meis-RA-positive feedback loop, the output of which is Meis levels, that is essential to establish anterior-posterior identities and patterning of the vertebrate axial skeleton.

Fine-tuning of the PAX-SIX-EYA-DACH network by multiple microRNAs controls embryo myogenesis

MicroRNAs (miRNAs), short non-coding RNAs, which act post-transcriptionally to regulate gene expression, are of widespread significance during development and disease, including muscle disease. Advances in sequencing technology and bioinformatics led to the identification of a large number of miRNAs in vertebrates and other species, however, for many of these miRNAs specific roles have not yet been determined. LNA in situ hybridisation has revealed expression patterns of somite-enriched miRNAs, here we focus on characterising the functions of miR-128. We show that antagomiR-mediated knockdown (KD) of miR-128 in developing chick somites has a negative impact on skeletal myogenesis. Computational analysis identified the transcription factor EYA4 as a candidate target consistent with the observation that miR-128 and EYA4 display similar expression profiles. Luciferase assays confirmed that miR-128 interacts with the EYA4 3'UTR. In vivo experiments also suggest that EYA4 is regulated by miR-128. EYA4 is a member of the PAX-SIX-EYA-DACH (PSED) network of transcription factors. Therefore, we identified additional candidate miRNA binding sites in the 3'UTR of SIX1/4, EYA1/2/3 and DACH1. Using the miRanda algorithm, we found sites for miR-128, as well as for other myogenic miRNAs, miR-1a, miR-206 and miR-133a, some of these were experimentally confirmed as functional miRNA target sites. Our results reveal that miR-128 is involved in regulating skeletal myogenesis by directly targeting EYA4 with indirect effects on other PSED members, including SIX4 and PAX3. Hence, the inhibitory effect on myogenesis observed after miR-128 knockdown was rescued by concomitant knockdown of PAX3. Moreover, we show that the PSED network of transcription factors is co-regulated by multiple muscle-enriched microRNAs.

The PAX-FOXO1s trigger fast trans-differentiation of chick embryonic neural cells into alveolar rhabdomyosarcoma with tissue invasive properties limited by S phase entry inhibition

The chromosome translocations generating PAX3-FOXO1 and PAX7-FOXO1 chimeric proteins are the primary hallmarks of the paediatric fusion-positive alveolar subtype of Rhabdomyosarcoma (FP-RMS). Despite the ability of these transcription factors to remodel chromatin landscapes and promote the expression of tumour driver genes, they only inefficiently promote malignant transformation in vivo. The reason for this is unclear. To address this, we developed an in ovo model to follow the response of spinal cord progenitors to PAX-FOXO1s. Our data demonstrate that PAX-FOXO1s, but not wild-type PAX3 or PAX7, trigger the trans-differentiation of neural cells into FP-RMS-like cells with myogenic characteristics. In parallel, PAX-FOXO1s remodel the neural pseudo-stratified epithelium into a cohesive mesenchyme capable of tissue invasion. Surprisingly, expression of PAX-FOXO1s, similar to wild-type PAX3/7, reduce the levels of CDK-CYCLIN activity and increase the fraction of cells in G1. Introduction of CYCLIN D1 or MYCN overcomes this PAX-FOXO1-mediated cell cycle inhibition and promotes tumour growth. Together, our findings reveal a mechanism that can explain the apparent limited oncogenicity of PAX-FOXO1 fusion transcription factors. They are also consistent with certain clinical reports indicative of a neural origin of FP-RMS.

The Mediator Subunit, Med23 Is Required for Embryonic Survival and Regulation of Canonical WNT Signaling During Cranial Ganglia Development

Development of the vertebrate head is a complex and dynamic process, which requires integration of all three germ layers and their derivatives. Of special importance are ectoderm-derived cells that form the cranial placodes, which then differentiate into the cranial ganglia and sensory organs. Critical to a fully functioning head, defects in cranial placode and sensory organ development can result in congenital craniofacial anomalies. In a forward genetic screen aimed at identifying novel regulators of craniofacial development, we discovered an embryonically lethal mouse mutant, snouty, which exhibits malformation of the facial prominences, cranial nerves and vasculature. The snouty mutation was mapped to a single nucleotide change in a ubiquitously expressed gene, Med23, which encodes a subunit of the global transcription co-factor complex, Mediator. Phenotypic analyses revealed that the craniofacial anomalies, particularly of the cranial ganglia, were caused by a failure in the proper specification of cranial placode neuronal precursors. Molecular analyses determined that defects in cranial placode neuronal differentiation in Med23 ^sn/sn mutants were associated with elevated WNT/β-catenin signaling, which can be partially rescued through combined Lrp6 and Wise loss-of-function. Our work therefore reveals a surprisingly tissue specific role for the ubiquitously expressed mediator complex protein Med23 in placode differentiation during cranial ganglia development. This highlights the importance of coupling general transcription to the regulation of WNT signaling during embryogenesis.

Deep whole-genome sequencing of multiple proband tissues and parental blood reveals the complex genetic etiology of congenital diaphragmatic hernias

The diaphragm is critical for respiration and separation of the thoracic and abdominal cavities, and defects in diaphragm development are the cause of congenital diaphragmatic hernias (CDH), a common and often lethal birth defect. The genetic etiology of CDH is complex. Single-nucleotide variants (SNVs), insertions/deletions (indels), and structural variants (SVs) in more than 150 genes have been associated with CDH, although few genes are recurrently mutated in multiple individuals and mutated genes are incompletely penetrant. This suggests that multiple genetic variants in combination, other not-yet-investigated classes of variants, and/or nongenetic factors contribute to CDH etiology. However, no studies have comprehensively investigated in affected individuals the contribution of all possible classes of variants throughout the genome to CDH etiology. In our study, we used a unique cohort of four individuals with isolated CDH with samples from blood, skin, and diaphragm connective tissue and parental blood and deep whole-genome sequencing to assess germline and somatic de novo and inherited SNVs, indels, and SVs. In each individual we found a different mutational landscape that included germline de novo and inherited SNVs and indels in multiple genes. We also found in two individuals a 343 bp deletion interrupting an annotated enhancer of the CDH-associated gene GATA4, and we hypothesize that this common SV (found in 1%-2% of the population) acts as a sensitizing allele for CDH. Overall, our comprehensive reconstruction of the genetic architecture of four CDH individuals demonstrates that the etiology of CDH is heterogeneous and multifactorial.

TBX3 acts as tissue-specific component of the Wnt/β-catenin transcriptional complex

BCL9 and PYGO are β-catenin cofactors that enhance the transcription of Wnt target genes. They have been proposed as therapeutic targets to diminish Wnt signaling output in intestinal malignancies. Here we find that, in colorectal cancer cells and in developing mouse forelimbs, BCL9 proteins sustain the action of β-catenin in a largely PYGO-independent manner. Our genetic analyses implied that BCL9 necessitates other interaction partners in mediating its transcriptional output. We identified the transcription factor TBX3 as a candidate tissue-specific member of the β-catenin transcriptional complex. In developing forelimbs, both TBX3 and BCL9 occupy a large number of Wnt-responsive regulatory elements, genome-wide. Moreover, mutations in Bcl9 affect the expression of TBX3 targets in vivo, and modulation of TBX3 abundance impacts on Wnt target genes transcription in a β-catenin- and TCF/LEF-dependent manner. Finally, TBX3 overexpression exacerbates the metastatic potential of Wnt-dependent human colorectal cancer cells. Our work implicates TBX3 as context-dependent component of the Wnt/β-catenin-dependent transcriptional complex.

Development of the amniote ventrolateral body wall

In vertebrates, the trunk consists of the musculoskeletal structures of the back and the ventrolateral body wall, which together enclose the internal organs of the circulatory, digestive, respiratory and urogenital systems. This review gives an overview on the development of the thoracic and abdominal wall during amniote embryogenesis. Specifically, I briefly summarize relevant historical concepts and the present knowledge on the early embryonic development of ribs, sternum, intercostal muscles and abdominal muscles with respect to anatomical bauplan, origin and specification of precursor cells, initial steps of pattern formation, and cellular and molecular regulation of morphogenesis.

Myogenesis control by SIX transcriptional complexes

SIX homeoproteins were first described in Drosophila, where they participate in the Pax-Six-Eya-Dach (PSED) network with eyeless, eyes absent and dachsund to drive synergistically eye development through genetic and biochemical interactions. The role of the PSED network and SIX proteins in muscle formation in vertebrates was subsequently identified. Evolutionary conserved interactions with EYA and DACH proteins underlie the activity of SIX transcriptional complexes (STC) both during embryogenesis and in adult myofibers. Six genes are expressed throughout muscle development, in embryonic and adult proliferating myogenic stem cells and in fetal and adult post-mitotic myofibers, where SIX proteins regulate the expression of various categories of genes. In vivo, SIX proteins control many steps of muscle development, acting through feedforward mechanisms: in the embryo for myogenic fate acquisition through the direct control of Myogenic Regulatory Factors; in adult myofibers for their contraction/relaxation and fatigability properties through the control of genes involved in metabolism, sarcomeric organization and calcium homeostasis. Furthermore, during development and in the adult, SIX homeoproteins participate in the genesis and the maintenance of myofibers diversity.

Mechanism and Functions of Identified miRNAs in Poultry Skeletal Muscle Development – A Review

Development of the skeletal muscle goes through several complex processes regulated by numerous genetic factors. Although much efforts have been made to understand the mechanisms involved in increased muscle yield, little work is done about the miRNAs and candidate genes that are involved in the skeletal muscle development in poultry. Comprehensive research of candidate genes and single nucleotide related to poultry muscle growth is yet to be experimentally unraveled. However, over a few periods, studies in miRNA have disclosed that they actively participate in muscle formation, differentiation, and determination in poultry. Specifically, miR-1, miR-133, and miR-206 influence tissue development, and they are highly expressed in the skeletal muscles. Candidate genes such as CEBPB, MUSTN1, MSTN, IGF1, FOXO3, mTOR, and NFKB1, have also been identified to express in the poultry skeletal muscles development. However, further researches, analysis, and comprehensive studies should be made on the various miRNAs and gene regulatory factors that influence the skeletal muscle development in poultry. The objective of this review is to summarize recent knowledge in miRNAs and their mode of action as well as transcription and candidate genes identified to regulate poultry skeletal muscle development.

Master control: transcriptional regulation of mammalian Myod

MYOD is a master regulator of the skeletal myogenic program. But what regulates expression of Myod? More than 20 years ago, studies established that Myod expression is largely controlled by just two enhancer regions located within a region 24 kb upstream of the transcription start site in mammals, which regulate Myod expression in the embryo, fetus and adult. Despite this apparently simple arrangement, Myod regulation is complex, with different combinations of transcription factors acting on these enhancers in different muscle progenitor cells and phases of differentiation. A range of epigenetic modifications in the Myod upstream region also play a part in activating and repressing Myod expression during development and regeneration. Here the evidence for this binding at Myod control regions is summarized, giving an overview of our current understanding of Myod expression regulation in mammals.

Pax3 cooperates with Ldb1 to direct local chromosome architecture during myogenic lineage specification

Chromatin looping allows enhancer-bound regulatory factors to influence transcription. Large domains, referred to as topologically associated domains, participate in genome organization. However, the mechanisms underlining interactions within these domains, which control gene expression, are not fully understood. Here we report that activation of embryonic myogenesis is associated with establishment of long-range chromatin interactions centered on Pax3-bound loci. Using mass spectrometry and genomic studies, we identify the ubiquitously expressed LIM-domain binding protein 1 (Ldb1) as the mediator of looping interactions at a subset of Pax3 binding sites. Ldb1 is recruited to Pax3-bound elements independently of CTCF-Cohesin, and is necessary for efficient deposition of H3K4me1 at these sites and chromatin looping. When Ldb1 is deleted in Pax3-expressing cells in vivo, specification of migratory myogenic progenitors is severely impaired. These results highlight Ldb1 requirement for Pax3 myogenic activity and demonstrate how transcription factors can promote formation of sub-topologically associated domain interactions involved in lineage specification.

Muscle precursor cell movements in zebrafish are dynamic and require Six family genes

Muscle precursors need to be correctly positioned during embryonic development for proper body movement. In zebrafish, a subset of hypaxial muscle precursors from the anterior somites undergo long-range migration, moving away from the trunk in three streams to form muscles in distal locations such as the fin. We mapped long-distance muscle precursor migrations with unprecedented resolution using live imaging. We identified conserved genes necessary for normal precursor motility (six1a, six1b, six4a, six4b and met). These genes are required for movement away from somites and later to partition two muscles within the fin bud. During normal development, the middle muscle precursor stream initially populates the fin bud, then the remainder of this stream contributes to the posterior hypaxial muscle. When we block fin bud development by impairing retinoic acid synthesis or Fgfr function, the entire stream contributes to the posterior hypaxial muscle indicating that muscle precursors are not committed to the fin during migration. Our findings demonstrate a conserved muscle precursor motility pathway, identify dynamic cell movements that generate posterior hypaxial and fin muscles, and demonstrate flexibility in muscle precursor fates.

VEGFC acts as a double-edged sword in renal cell carcinoma aggressiveness

Hypoxic zones are common features of metastatic tumors. Due to inactivation of the von Hippel-Lindau gene (VHL), renal cell carcinomas (RCC) show constitutive stabilization of the alpha subunit of the hypoxia-inducible factor (HIF). Thus, RCC represents a model of chronic hypoxia. Development of the lymphatic network is dependent on vascular endothelial growth factor C (VEGFC) and lies at the front line of metastatic spreading. Here, we addressed the role of VEGFC in RCC aggressiveness and the regulation of its expression in hypoxia. Methods: Transcriptional and post transcriptional regulation of VEGFC expression was evaluated by qPCR and with reporter genes. The involvement of HIF was evaluated using a siRNA approach. Experimental RCC were performed with immuno-competent/deficient mice using human and mouse cells knocked-out for the VEGFC gene by a CRISPR/Cas9 method. The VEGFC axis was analyzed with an online available data base (TCGA) and using an independent cohort of patients. Results: Hypoxia induced VEGFC protein expression but down-regulated VEGFC gene transcription and mRNA stability. Increased proliferation, migration, over-activation of the AKT signaling pathway and enhanced expression of mesenchymal markers characterized VEGFC-/- cells. VEGFC-/- cells did not form tumors in immuno-deficient mice but developed aggressive tumors in immuno-competent mice. These tumors showed down-regulation of markers of activated lymphocytes and M1 macrophages, and up-regulation of M2 macrophages markers and programmed death ligand 1 (PDL1). Over-expression of lymphangiogenic genes including VEGFC was linked to increased disease-free and overall survival in patients with non-metastatic tumors, whereas its over-expression correlated with decreased progression-free and overall survival of metastatic patients. Conclusion: Our study revisited the admitted dogma linking VEGFC to tumor aggressiveness. We conclude that targeting VEGFC for therapy must be considered with caution.

Eya2 is critical for the E2A‑HLF‑mediated immortalization of mouse hematopoietic stem/progenitor cells

The immunoglobulin enhancer‑binding factor/hepatic leukemia factor (E2A‑HLF) oncogenic fusion gene, generated by t(17;19)(q22;p13) translocation in childhood B‑cell acute lymphoblastic leukemia with a very poor prognosis, encodes a chimeric transcription factor in which the transactivation domains of E2A are fused to the DNA‑binding and dimerization domain of HLF. E2A‑HLF has been demonstrated to have an anti‑apoptotic effect. However, the molecular mechanism underlying E2A‑HLF‑mediated leukemogenesis remains unclear. The present study identified EYA transcriptional coactivator and phosphatase 2 (Eya2), the forced expression of which is known to immortalize mouse hematopoietic stem/progenitor cells (HSPCs), as a direct target molecule downstream of E2A‑HLF. E2A‑HLF‑immortalized mouse HSPCs expressed Eya2 at a high level in the aberrant self‑renewal program. Chromatin immunoprecipitation‑quantitative polymerase chain reaction and a reporter assay revealed that E2A‑HLF enhanced the Eya2 expression by binding to the promoter region containing the E2A‑HLF‑binding consensus sequence. Eya2 knockdown in E2A‑HLF‑immortalized cells resulted in reduced colony‑forming efficiency. These results suggest a critical role of Eya2 in E2A‑HLF‑mediated leukemogenesis.

Transcriptomic landscape of the blastema niche in regenerating adult axolotl limbs at single-cell resolution

Regeneration of complex multi-tissue structures, such as limbs, requires the coordinated effort of multiple cell types. In axolotl limb regeneration, the wound epidermis and blastema have been extensively studied via histology, grafting, and bulk-tissue RNA-sequencing. However, defining the contributions of these tissues is hindered due to limited information regarding the molecular identity of the cell types in regenerating limbs. Here we report unbiased single-cell RNA-sequencing on over 25,000 cells from axolotl limbs and identify a plethora of cellular diversity within epidermal, mesenchymal, and hematopoietic lineages in homeostatic and regenerating limbs. We identify regeneration-induced genes, develop putative trajectories for blastema cell differentiation, and propose the molecular identity of fibroblast-like blastema progenitor cells. This work will enable application of molecular techniques to assess the contribution of these populations to limb regeneration. Overall, these data allow for establishment of a putative framework for adult axolotl limb regeneration.

Limb regeneration requires a blastema with progenitor cells, immune cells, and an overlying wound epidermis, but molecular identities of these populations are unclear. Here, the authors use single-cell RNA-sequencing to identify transcriptionally distinct cell populations in adult axolotl limb blastemas.

miR-219a-5p represses migration and invasion of osteosarcoma cells via targeting EYA2

EYA2 is the developmental transcription factor and phosphatase, playing an important role in numerous species in regulating cell death and differentiation. Recent studies showed that EYA2 is dysregulated and involved in the progression of various cancers. However, the expression and role of EYA2 in osteosarcoma remains unclear. Here, we found that EYA2 expression was evidently upregulated osteosarcoma (OS) tissue and cell lines. Next, we predicted EYA2-targeting miRNAs, which was further evaluated using a dual luciferase reporter assay. We found that miR-219a-5p significantly repressed EYA2 expression via binding to the 3'-UTR of EYA2. Furthermore, overexpressed miR-219a-5p significantly repressed OS cell invasion and migration, which was reversed by overexpressed EYA2. While silenced miR-219a-5p induced OS cell invasion and migration, which was reversed by silenced EYA2. In conclusion, our study revealed that miR-219a-5p function as tumour suppressor regulates OS cell invasiveness by repressing EYA2 expression.

Establishment of stably expandable induced myogenic stem cells by four transcription factors

Life-long regeneration of healthy muscle by cell transplantation is an ideal therapy for patients with degenerative muscle diseases. Yet, obtaining muscle stem cells from patients is very limited due to their exhaustion in disease condition. Thus, development of a method to obtain healthy myogenic stem cells is required. Here, we showed that the four transcription factors, Six1, Eya1, Esrrb, and Pax3, converts fibroblasts into induced myogenic stem cells (iMSCs). The iMSCs showed effective differentiation into multinucleated myotubes and also higher proliferation capacity than muscle derived stem cells both in vitro and in vivo. The iMSCs do not lose their proliferation capacity though the passaging number is increased. We further isolated CD106-negative and α7-integrin-positive iMSCs (sort-iMSCs) showing higher myogenic differentiation capacity than iMSCs. Moreover, genome-wide transcriptomic analysis of iMSCs and sort-iMSCs, followed by network analysis, revealed the genes and signaling pathways associated with enhanced proliferation and differentiation capacity of iMSCs and sort-iMSCs, respectively. The stably expandable iMSCs provide a new source for drug screening and muscle regenerative therapy for muscle wasting disease.

Stepwise participation of HGF/MET signaling in the development of migratory muscle precursors during vertebrate evolution

BACKGROUND

The skeletal musculature of gnathostomes, which is derived from embryonic somites, consists of epaxial and hypaxial portions. Some hypaxial muscles, such as tongue and limb muscles, undergo de-epithelialization and migration during development. Delamination and migration of these myoblasts, or migratory muscle precursors (MMPs), is generally thought to be regulated by hepatocyte growth factor (HGF) and receptor tyrosine kinase (MET) signaling. However, the prevalence of this mechanism and the expression patterns of the genes involved in MMP development across different vertebrate species remain elusive.

RESULTS

We performed a comparative analysis of Hgf and Met gene expression in several vertebrates, including mouse, chicken, dogfish (Scyliorhinus torazame), and lamprey (Lethenteron camtschaticum). While both Hgf and Met were expressed during development in the mouse tongue muscle, and in limb muscle formation in the mouse and chicken, we found no clear evidence for the involvement of HGF/MET signaling in MMP development in shark or lamprey embryos.

CONCLUSIONS

Our results indicate that the expressions and functions of both Hgf and Met genes do not represent shared features of vertebrate MMPs, suggesting a stepwise participation of HGF/MET signaling in MMP development during vertebrate evolution.

Eya3 promotes breast tumor-associated immune suppression via threonine phosphatase-mediated PD-L1 upregulation

Eya proteins are critical developmental regulators that are highly expressed in embryogenesis but downregulated after development. Amplification and/or re-expression of Eyas occurs in many tumor types. In breast cancer, Eyas regulate tumor progression by acting as transcriptional cofactors and tyrosine phosphatases. Intriguingly, Eyas harbor a separate threonine (Thr) phosphatase activity, which was previously implicated in innate immunity. Here we describe what we believe to be a novel role for Eya3 in mediating triple-negative breast cancer-associated immune suppression. Eya3 loss decreases tumor growth in immune-competent mice and is associated with increased numbers of infiltrated CD8+ T cells, which, when depleted, reverse the effects of Eya3 knockdown. Mechanistically, Eya3 utilizes its Thr phosphatase activity to dephosphorylate Myc at pT58, resulting in a stabilized form. We show that Myc is required for Eya3-mediated increases in PD-L1, and that rescue of PD-L1 in Eya3-knockdown cells restores tumor progression. Finally, we demonstrate that Eya3 significantly correlates with PD-L1 in human breast tumors, and that tumors expressing high levels of Eya3 have a decreased CD8+ T cell signature. Our data uncover a role for Eya3 in mediating tumor-associated immune suppression, and suggest that its inhibition may enhance checkpoint therapies.

Myogenic regulatory factors: The orchestrators of myogenesis after 30 years of discovery

Prenatal and postnatal myogenesis share many cellular and molecular aspects. Myogenic regulatory factors are basic Helix-Loop-Helix transcription factors that indispensably regulate both processes. These factors (Myf5, MyoD, Myogenin, and MRF4) function as an orchestrating cascade, with some overlapped actions. Prenatally, myogenic regulatory factors are restrictedly expressed in somite-derived myogenic progenitor cells and their derived myoblasts. Postnatally, myogenic regulatory factors are important in regulating the myogenesis process via satellite cells. Many positive and negative regulatory mechanisms exist either between myogenic regulatory factors themselves or between myogenic regulatory factors and other proteins. Upstream factors and signals are also involved in the control of myogenic regulatory factors expression within different prenatal and postnatal myogenic cells. Here, the authors have conducted a thorough and an up-to-date review of the myogenic regulatory factors since their discovery 30 years ago. This review discusses the myogenic regulatory factors structure, mechanism of action, and roles and regulations during prenatal and postnatal myogenesis. Impact statement Myogenic regulatory factors (MRFs) are key players in the process of myogenesis. Despite a considerable amount of literature regarding these factors, their exact mechanisms of actions are still incompletely understood with several overlapped functions. Herein, we revised what has hitherto been reported in the literature regarding MRF structures, molecular pathways that regulate their activities, and their roles during pre- and post-natal myogenesis. The work submitted in this review article is considered of great importance for researchers in the field of skeletal muscle formation and regeneration, as it provides a comprehensive summary of all the biological aspects of MRFs and advances a better understanding of the cellular and molecular mechanisms regulating myogenesis. Indeed, attaining a better understanding of MRFs could be utilized in developing novel therapeutic protocols for multiple myopathies.

PITX1 promotes chondrogenesis and myogenesis in mouse hindlimbs through conserved regulatory targets

The PITX1 transcription factor is expressed during hindlimb development, where it plays a critical role in directing hindlimb growth and the specification of hindlimb morphology. While it is known that PITX1 regulates hindlimb formation, in part, through activation of the Tbx4 gene, other transcriptional targets remain to be elucidated. We have used a combination of ChIP-seq and RNA-seq to investigate enhancer regions and target genes that are directly regulated by PITX1 in embryonic mouse hindlimbs. In addition, we have analyzed PITX1 binding sites in hindlimbs of Anolis lizards to identify ancient PITX1 regulatory targets. We find that PITX1-bound regions in both mouse and Anolis hindlimbs are strongly associated with genes implicated in limb and skeletal system development. Gene expression analyses reveal a large number of misexpressed genes in the hindlimbs of Pitx1^-/- mouse embryos. By intersecting misexpressed genes with genes that have neighboring mouse PITX1 binding sites, we identified 440 candidate targets of PITX1. Of these candidates, 68 exhibit ultra-conserved PITX1 binding events that are shared between mouse and Anolis hindlimbs. Among the ancient targets of PITX1 are important regulators of cartilage and skeletal muscle development, including Sox9 and Six1. Our data suggest that PITX1 promotes chondrogenesis and myogenesis in the hindlimb by direct regulation of several key members of the cartilage and muscle transcriptional networks.

Respective contribution of the cephalic neural crest and mesoderm to SIX1-expressing head territories in the avian embryo

Background

Vertebrate head development depends on a series of interactions between many cell populations of distinct embryological origins. Cranial mesenchymal tissues have a dual embryonic source: - the neural crest (NC), which generates most of craniofacial skeleton, dermis, pericytes, fat cells, and tenocytes; and - the mesoderm, which yields muscles, blood vessel endothelia and some posterior cranial bones. The molecular players that orchestrate co-development of cephalic NC and mesodermal cells to properly construct the head of vertebrates remain poorly understood. In this regard, Six1 gene, a vertebrate homolog of Drosophila Sine Oculis, is known to be required for development of ear, nose, tongue and cranial skeleton. However, the embryonic origin and fate of Six1-expressing cells have remained unclear. In this work, we addressed these issues in the avian embryo model by using quail-chick chimeras, cephalic NC cultures and immunostaining for SIX1.

Results

Our data show that, at early NC migration stages, SIX1 is expressed by mesodermal cells but excluded from the NC cells (NCC). Then, SIX1 becomes widely expressed in NCC that colonize the pre-otic mesenchyme. In contrast, in the branchial arches (BAs), SIX1 is present only in mesodermal cells that give rise to jaw muscles. At later developmental stages, the distribution of SIX1-expressing cells in mesoderm-derived tissues is consistent with a possible role of this factor in the myogenic program of all types of head muscles, including pharyngeal, extraocular and tongue muscles. In NC derivatives, SIX1 is notably expressed in perichondrium and chondrocytes of the nasal septum and in the sclera, although other facial cartilages such as Meckel’s were negative at the stages considered. Moreover, in cephalic NC cultures, chondrocytes and myofibroblasts, not the neural and melanocytic cells express SIX1.

Conclusion

The present results point to a dynamic tissue-specific expression of SIX1 in a variety of cephalic NC- and mesoderm-derived cell types and tissues, opening the way for further analysis of Six1 function in the coordinated development of these two cellular populations during vertebrate head formation.

Electronic supplementary material

The online version of this article (10.1186/s12861-017-0155-z) contains supplementary material, which is available to authorized users.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

Gene expression profiling in Pekin duck embryonic breast muscle

Lean-type Pekin duck is a breed gained through long-term selection and great effort has been exerted to understand the mechanisms underlying increased muscle yields. However, the genes involved in Pekin duck embryonic breast muscle development have not been explored to date. In this study, we investigated gene expression profiles in Pekin Duck embryonic breast muscle at hatched day 13 (E13), E19, and E27 using RNA-seq. In total, we produced 519,312,178 raw reads resulting in 497,348,158 high-quality reads after filtering. The mapping, distribution of reads along annotated genes, and consistency across replicates demonstrates the high quality of the RNA-seq data used in this study, allowing us to continue with the downstream analysis. Significantly fewer differentially expressed genes (DEGs) were identified between E13 and E19 (203 DEGs) compared to E27 and E19 (2,797 DEGs). Many DEGs highly expressed in E19 are involved in metabolic processes and cell division. KEGG analysis showed many pathways associated with fat development were significantly enriched for DEGs highly expressed in E27. These results provide a basis for the further investigation of the mechanisms involved in Pekin duck embryonic breast muscle development.

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.

Arginine Methylation by PRMT1 Regulates Muscle Stem Cell Fate

Quiescent muscle stem cells (MSCs) become activated in response to skeletal muscle injury to initiate regeneration. Activated MSCs proliferate and differentiate to repair damaged fibers or self-renew to maintain the pool and ensure future regeneration. The balance between self-renewal, proliferation, and differentiation is a tightly regulated process controlled by a genetic cascade involving determinant transcription factors such as Pax7, Myf5, MyoD, and MyoG. Recently, there have been several reports about the role of arginine methylation as a requirement for epigenetically mediated control of muscle regeneration. Here we report that the protein arginine methyltransferase 1 (PRMT1) is expressed in MSCs and that conditional ablation of PRMT1 in MSCs using Pax7^CreERT2 causes impairment of muscle regeneration. Importantly, PRMT1-deficient MSCs have enhanced cell proliferation after injury but are unable to terminate the myogenic differentiation program, leading to regeneration failure. We identify the coactivator of Six1, Eya1, as a substrate of PRMT1. We show that PRMT1 methylates Eya1 in vitro and that loss of PRMT1 function in vivo prevents Eya1 methylation. Moreover, we observe that PRMT1-deficient MSCs have reduced expression of Eya1/Six1 target MyoD due to disruption of Eya1 recruitment at the MyoD promoter and subsequent Eya1-mediated coactivation. These findings suggest that arginine methylation by PRMT1 regulates muscle stem cell fate through the Eya1/Six1/MyoD axis.

Dissecting the pre-placodal transcriptome to reveal presumptive direct targets of Six1 and Eya1 in cranial placodes

The pre-placodal ectoderm, marked by the expression of the transcription factor Six1 and its co-activator Eya1, develops into placodes and ultimately into many cranial sensory organs and ganglia. Using RNA-Seq in Xenopus laevis we screened for presumptive direct placodal target genes of Six1 and Eya1 by overexpressing hormone-inducible constructs of Six1 and Eya1 in pre-placodal explants, and blocking protein synthesis before hormone-inducing nuclear translocation of Six1 or Eya1. Comparing the transcriptome of explants with non-induced controls, we identified hundreds of novel Six1/Eya1 target genes with potentially important roles for placode development. Loss-of-function studies confirmed that target genes encoding known transcriptional regulators of progenitor fates (e.g. Sox2, Hes8) and neuronal/sensory differentiation (e.g. Ngn1, Atoh1, Pou4f1, Gfi1) require Six1 and Eya1 for their placodal expression. Our findings provide insights into the gene regulatory network regulating placodal neurogenesis downstream of Six1 and Eya1 suggesting new avenues of research into placode development and disease.

MyoD reprogramming requires Six1 and Six4 homeoproteins: genome-wide cis-regulatory module analysis

Myogenic regulatory factors of the MyoD family have the ability to reprogram differentiated cells toward a myogenic fate. In this study, we demonstrate that Six1 or Six4 are required for the reprogramming by MyoD of mouse embryonic fibroblasts (MEFs). Using microarray experiments, we found 761 genes under the control of both Six and MyoD. Using MyoD ChIPseq data and a genome-wide search for Six1/4 MEF3 binding sites, we found significant co-localization of binding sites for MyoD and Six proteins on over a thousand mouse genomic DNA regions. The combination of both datasets yielded 82 genes which are synergistically activated by Six and MyoD, with 96 associated MyoD+MEF3 putative cis-regulatory modules (CRMs). Fourteen out of 19 of the CRMs that we tested demonstrated in Luciferase assays a synergistic action also observed for their cognate gene. We searched putative binding sites on these CRMs using available databases and de novo search of conserved motifs and demonstrated that the Six/MyoD synergistic activation takes place in a feedforward way. It involves the recruitment of these two families of transcription factors to their targets, together with partner transcription factors, encoded by genes that are themselves activated by Six and MyoD, including Mef2, Pbx-Meis and EBF.