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. -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.

A tensile ring drives tissue flows to shape the gastrulating amniote embryo.

Tissue morphogenesis is driven by local cellular deformations that are powered by contractile actomyosin networks. How localized forces are transmitted across tissues to shape them at a mesoscopic scale is still unclear. Analyzing gastrulation in entire avian embryos, we show that it is driven by the graded contraction of a large-scale supracellular actomyosin ring at the margin between the embryonic and extraembryonic territories.

PAX3 Confers Functional Heterogeneity in Skeletal Muscle Stem Cell Responses to Environmental Stress.

Muscle satellite cells (MuSCs) are the quiescent muscle stem cells required for adult skeletal muscle repair. The impact of environmental stress such as pollution on MuSC behavior remains unexplored. We evaluated the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure, a ubiquitous and highly toxic pollutant, on MuSCs by combining in vivo mouse molecular genetic models with ex vivo studies.

Timed Collinear Activation of Hox Genes during Gastrulation Controls the Avian Forelimb Position.

Limb position along the body is highly consistent within one species but very variable among vertebrates. Despite major advances in our understanding of limb patterning in three dimensions, how limbs reproducibly form along the antero-posterior axis remains largely unknown. Hox genes have long been suspected to control limb position; however, supporting evidences are mostly correlative and their role in this process is unclear.

Cell Division Drives Epithelial Cell Rearrangements during Gastrulation in Chick.

During early embryonic development, cells are organized as cohesive epithelial sheets that are continuously growing and remodeled without losing their integrity, giving rise to a wide array of tissue shapes. Here, using live imaging in chick embryo, we investigate how epithelial cells rearrange during gastrulation. We find that cell division is a major rearrangement driver that powers dramatic epithelial cell intercalation events.

Notch regulation of myogenic versus endothelial fates of cells that migrate from the somite to the limb.

Multipotent Pax3-positive (Pax3(+)) cells in the somites give rise to skeletal muscle and to cells of the vasculature. We had previously proposed that this cell-fate choice depends on the equilibrium between Pax3 and Foxc2 expression. In this study, we report that the Notch pathway promotes vascular versus skeletal muscle cell fates.

Itm2a is a Pax3 target gene, expressed at sites of skeletal muscle formation in vivo.

The paired-box homeodomain transcription factor Pax3 is a key regulator of the nervous system, neural crest and skeletal muscle development. Despite the important role of this transcription factor, very few direct target genes have been characterized. We show that Itm2a, which encodes a type 2 transmembrane protein, is a direct Pax3 target in vivo, by combining genetic approaches and in vivo chromatin immunoprecipitation assays.

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.

Lack of in vivo functional compensation between Pax family groups II and III in rodents.

Pax genes encode evolutionarily conserved transcription factors that play critical roles in embryonic development and organogenesis. Pax proteins are subdivided into four subfamilies: group I (Pax1and 9), II (Pax2, 5, and 8), III (Pax3 and 7), and IV (Pax4 and 6), based on the presence of a paired domain, an octapeptide motif and part or all of the homeodomain. Studies of the evolution of this gene family are incomplete.

A Pax3/Dmrt2/Myf5 regulatory cascade functions at the onset of myogenesis.

All skeletal muscle progenitor cells in the body derive from the dermomyotome, the dorsal epithelial domain of developing somites. These multipotent stem cells express Pax3, and this expression is maintained in the myogenic lineage where Pax3 plays an important role. Identification of Pax3 targets is therefore important for understanding the mechanisms that underlie the onset of myogenesis.

Muscle stem cell behavior is modified by microRNA-27 regulation of Pax3 expression.

Skeletal muscle stem cells are regulated by Pax3/7. During development, Pax3 is required for the maintenance of these cells in the somite and their migration to sites of myogenesis; high levels of Pax3 interfere with muscle cell differentiation, both in the embryo and in the adult. Quantitative fine-tuning of Pax3 is critical, and microRNAs provide a potential mechanism.

The regulatory mechanisms that underlie inappropriate transcription of the myogenic determination gene Myf5 in the central nervous system.

Myf5 is a key myogenic determination factor, specifically present at sites of myogenesis. Surprisingly, during mouse development, this gene is also transcribed in restricted areas of the central nervous system, although the Myf5 protein is not detectable. We have investigated the regulation of Myf5 expression in the central nervous system.

Pax3 regulation of FGF signaling affects the progression of embryonic progenitor cells into the myogenic program.

Pax3/7-dependent stem cells play an essential role in skeletal muscle development. We now show that Fgfr4 lies genetically downstream from Pax3 and is a direct target. In chromatin immunoprecipitation (ChIP)-on-chip experiments, Pax3 binds to a sequence 3' of the Fgfr4 gene that directs Pax3-dependent expression at sites of myogenesis in transgenic mouse embryos.

Myogenic progenitor cells in the mouse embryo are marked by the expression of Pax3/7 genes that regulate their survival and myogenic potential.

The transcription factors Pax3 and Pax7 are important regulators of myogenic cell fate, as demonstrated by genetic manipulations in the mouse embryo. Pax3 lies genetically upstream of MyoD and has also been shown recently to directly control Myf5 transcription in derivatives of the hypaxial somite, where it also plays an important role in ensuring cell survival. Both Pax3 and Pax7 are expressed in myogenic progenitor cells derived from the central dermomyotome that make a major contribution to skeletal muscle growth.

A novel genetic hierarchy functions during hypaxial myogenesis: Pax3 directly activates Myf5 in muscle progenitor cells in the limb.

We address the molecular control of myogenesis in progenitor cells derived from the hypaxial somite. Null mutations in Pax3, a key regulator of skeletal muscle formation, lead to cell death in this domain. We have developed a novel allele of Pax3 encoding a Pax3-engrailed fusion protein that acts as a transcriptional repressor.

Pax3 and Pax7 have distinct and overlapping functions in adult muscle progenitor cells.

The growth and repair of skeletal muscle after birth depends on satellite cells that are characterized by the expression of Pax7. We show that Pax3, the paralogue of Pax7, is also present in both quiescent and activated satellite cells in many skeletal muscles. Dominant-negative forms of both Pax3 and -7 repress MyoD, but do not interfere with the expression of the other myogenic determination factor, Myf5, which, together with Pax3/7, regulates the myogenic differentiation of these cells.

A Pax3/Pax7-dependent population of skeletal muscle progenitor cells.

During vertebrate development, successive phases of embryonic and fetal myogenesis lead to the formation and growth of skeletal muscles. Although the origin and molecular regulation of the earliest embryonic muscle cells is well understood, less is known about later stages of myogenesis. We have identified a new cell population that expresses the transcription factors Pax3 and Pax7 (paired box proteins 3 and 7) but no skeletal-muscle-specific markers.

Mrf4 determines skeletal muscle identity in Myf5:Myod double-mutant mice.

In vertebrates, skeletal muscle is a model for the acquisition of cell fate from stem cells. Two determination factors of the basic helix-loop-helix myogenic regulatory factor (MRF) family, Myf5 and Myod, are thought to direct this transition because double-mutant mice totally lack skeletal muscle fibres and myoblasts. In the absence of these factors, progenitor cells remain multipotent and can change their fate.

Divergent functions of murine Pax3 and Pax7 in limb muscle development.

Pax genes encode evolutionarily conserved transcription factors that play critical roles in development. Pax3 and Pax7 constitute one of the four Pax subfamilies. Despite partially overlapping expression domains, mouse mutations for Pax3 and Pax7 have very different consequences.

An enhancer directs differential expression of the linked Mrf4 and Myf5 myogenic regulatory genes in the mouse.

The myogenic regulatory factors, Mrf4 and Myf5, play a key role in skeletal muscle formation. An enhancer trap approach, devised to isolate positive-acting elements from a 200-kb YAC covering the mouse Mrf4-Myf5 locus in a C2 myoblast assay, yielded an enhancer, A17, which mapped at -8 kb 5' of Mrf4 and -17 kb 5' of Myf5. An E-box bound by complexes containing the USF transcription factor is critical for enhancer activity.

The transcriptional activator PAX3-FKHR rescues the defects of Pax3 mutant mice but induces a myogenic gain-of-function phenotype with ligand-independent activation of Met signaling in vivo.

Pax3 is a key transcription factor implicated in development and human disease. To dissect the role of Pax3 in myogenesis and establish whether it is a repressor or activator, we generated loss- and gain-of-function alleles by targeting an nLacZ reporter and a sequence encoding the oncogenic fusion protein PAX3-FKHR into the Pax3 locus. Rescue of the Pax3 mutant phenotypes by PAX3-FKHR suggests that Pax3 acts as a transcriptional activator during embryogenesis.

[The mouse as a model of heart morphogenesis in mammals: origin and lineage of myocytes].

In order to follow cardiac precursor cells, we have adopted a retrospective clonal approach, based on the nlaacZ genetic label. Random clones were generated and observed at different developmental stages in murine myocardium. The distribution of these clones in clusters suggest for the first time that cells fated to form myocardium proliferate in two steps.

A retrospective clonal analysis of the myocardium reveals two phases of clonal growth in the developing mouse heart.

Key molecules which regulate the formation of the heart have been identified; however, the mechanism of cardiac morphogenesis remains poorly understood at the cellular level. We have adopted a genetic approach, which permits retrospective clonal analysis of myocardial cells in the mouse embryo, based on the targeting of an nlaacZ reporter to the alpha-cardiac actin gene. A rare intragenic recombination event leads to a clone of beta-galactosidase-positive myocardial cells.

Analysis of a key regulatory region upstream of the Myf5 gene reveals multiple phases of myogenesis, orchestrated at each site by a combination of elements dispersed throughout the locus.

Myf5 is the first myogenic regulatory factor to be expressed in the mouse embryo and it determines the entry of cells into the skeletal muscle programme. A region situated between -58 kb and -48 kb from the gene directs Myf5 transcription at sites where muscles will form. We now show that this region consists of a number of distinct regulatory elements that specifically target sites of myogenesis in the somite, limbs and hypoglossal cord, and also sites of Myf5 transcription in the central nervous system.