Signals and myogenic regulatory factors restrict pax3 and pax7 expression to dermomyotome-like tissue in zebrafish

Restricted expression of cdc25a in the tailbud is essential for formation of the zebrafish posterior body

The vertebrate body forms from a multipotent stem cell-like progenitor population that contributes newly differentiated cells to the posterior end of the embryo. Here, in vivo analyses show that proliferation is compartmentalized at the posterior end of the zebrafish embryo via regulated expression of mitotic factor Cdc25a. Furthermore, compartmentalization of proliferation during embryogenesis is critical to both body extension and muscle cell fate. This study reveals an unexpected link between precise regulation of the cell cycle and differentiation from multipotency in the vertebrate embryo.

The vertebrate body forms from a multipotent stem cell-like progenitor population that progressively contributes newly differentiated cells to the most posterior end of the embryo. How the progenitor population balances proliferation and other cellular functions is unknown due to the difficulty of analyzing cell division in vivo. Here, we show that proliferation is compartmentalized at the posterior end of the embryo during early zebrafish development by the regulated expression of cdc25a, a key controller of mitotic entry. Through the use of a transgenic line that misexpresses cdc25a, we show that this compartmentalization is critical for the formation of the posterior body. Upon misexpression of cdc25a, several essential T-box transcription factors are abnormally expressed, including Spadetail/Tbx16, which specifically prevents the normal onset of myoD transcription, leading to aberrant muscle formation. Our results demonstrate that compartmentalization of proliferation during early embryogenesis is critical for both extension of the vertebrate body and differentiation of the multipotent posterior progenitor cells to the muscle cell fate.

Distinct regulatory mechanisms act to establish and maintain Pax3 expression in the developing neural tube

Pattern formation in developing tissues is driven by the interaction of extrinsic signals with intrinsic transcriptional networks that together establish spatially and temporally restricted profiles of gene expression. How this process is orchestrated at the molecular level by genomic cis-regulatory modules is one of the central questions in developmental biology. Here we have addressed this by analysing the regulation of Pax3 expression in the context of the developing spinal cord. Pax3 is induced early during neural development in progenitors of the dorsal spinal cord and is maintained as pattern is subsequently elaborated, resulting in the segregation of the tissue into dorsal and ventral subdivisions. We used a combination of comparative genomics and transgenic assays to define and dissect several functional cis-regulatory modules associated with the Pax3 locus. We provide evidence that the coordinated activity of two modules establishes and refines Pax3 expression during neural tube development. Mutational analyses of the initiating element revealed that in addition to Wnt signaling, Nkx family homeodomain repressors restrict Pax3 transcription to the presumptive dorsal neural tube. Subsequently, a second module mediates direct positive autoregulation and feedback to maintain Pax3 expression. Together, these data indicate a mechanism by which transient external signals are converted into a sustained expression domain by the activities of distinct regulatory elements. This transcriptional logic differs from the cross-repression that is responsible for the spatiotemporal patterns of gene expression in the ventral neural tube, suggesting that a variety of circuits are deployed within the neural tube regulatory network to establish and elaborate pattern formation.

The complex organization of tissues is established precisely and reproducibly during development. In the vertebrate neural tube, as in many other tissues, the interplay between extrinsic morphogens and intrinsic transcription factors produces spatial patterns of gene expression that delineate precursors for specific cell types. One such transcription factor, Pax3, defines the precursors of all sensory neuron subtypes and distinguishes them from precursors fated to give rise to the motor circuits. To gain insight into the molecular mechanisms by which the spinal cord is segregated into these two functional domains, we analysed the genomic regulatory sequences responsible for controlling Pax3 activity. We identified two regions of the genome, the coordinated activity of which establishes and refines Pax3 activity. We showed that the combination of activating signals from secreted Wnt factors together with Nkx family homeodomain repressors restrict Pax3 activity to the presumptive sensory region of the neural tissue. Subsequently, Pax3 acts to directly potentiate its own transcription and this autoregulation sustains Pax3 expression at later developmental stages. Together, our study reveals the way in which intrinsic and extrinsic signals are integrated by cells and converted into a sustained pattern of gene activity in the developing nervous system.

Sarcopenia and piscines: the case for indeterminate-growing fish as unique genetic model organisms in aging and longevity research

Sarcopenia and dynapenia pose significant problems for the aged, especially as life expectancy rises in developed countries. Current therapies are marginally efficacious at best, and barriers to breakthroughs in treatment may result from currently employed model organisms. Here, we argue that the use of indeterminate-growing teleost fish in skeletal muscle aging research may lead to therapeutic advancements not possible with current mammalian models. Evidence from a comparative approach utilizing the subfamily Danioninae suggests that the indeterminate growth paradigm of many teleosts arises from adult muscle stem cells with greater proliferative capacity, even in spite of smaller progenitor populations. We hypothesize that paired-box transcription factors, Pax3/7, are involved with this enhanced self-renewal and that prolonged expression of these factors may allow some fish species to escape, or at least forestall, sarcopenia/dynapenia. Future research efforts should focus on the experimental validation of these genes as key factors in indeterminate growth, both in the context of muscle stem cell proliferation and in prevention of skeletal muscle senescence.

Body wall development in lamprey and a new perspective on the origin of vertebrate paired fins

Classical hypotheses regarding the evolutionary origin of paired appendages propose transformation of precursor structures (gill arches and lateral fin folds) into paired fins. During development, gnathostome paired appendages form as outgrowths of body wall somatopleure, a tissue composed of somatic lateral plate mesoderm (LPM) and overlying ectoderm. In amniotes, LPM contributes connective tissue to abaxial musculature and forms ventrolateral dermis of the interlimb body wall. The phylogenetic distribution of this character is uncertain because lineage analyses of LPM have not been generated in anamniotes. We focus on the evolutionary history of the somatopleure to gain insight into the tissue context in which paired fins first appeared. Lampreys diverged from other vertebrates before the acquisition of paired fins and provide a model for investigating the preappendicular condition. We present vital dye fate maps that suggest the somatopleure is eliminated in lamprey as the LPM is separated from the ectoderm and sequestered to the coelomic linings during myotome extension. We also examine the distribution of postcranial mesoderm in catshark and axolotl. In contrast to lamprey, our findings support an LPM contribution to the trunk body wall of these taxa, which is similar to published data for amniotes. Collectively, these data lead us to hypothesize that a persistent somatopleure in the lateral body wall is a gnathostome synapomorphy, and the redistribution of LPM was a key step in generating the novel developmental module that ultimately produced paired fins. These embryological criteria can refocus arguments on paired fin origins and generate hypotheses testable by comparative studies on the source, sequence, and extent of genetic redeployment.

Control of muscle fibre-type diversity during embryonic development: the zebrafish paradigm

Vertebrate skeletal muscle is composed of distinct types of fibre that are functionally adapted through differences in their physiological and metabolic properties. An understanding of the molecular basis of fibre-type specification is of relevance to human health and fitness. The zebrafish provides an attractive model for investigating fibre type specification; not only are their rapidly developing embryos optically transparent, but in contrast to amniotes, the embryonic myotome shows a discrete temporal and spatial separation of fibre type ontogeny that simplifies its analysis. Here we review the current state of understanding of muscle fibre type specification and differentiation during embryonic development of the zebrafish, with a particular focus on the roles of the Prdm1a and Sox6 transcription factors, and consider the relevance of these findings to higher vertebrate muscle biology.

Oesophageal and sternohyal muscle fibres are novel Pax3-dependent migratory somite derivatives essential for ingestion

Striated muscles that enable mouth opening and swallowing during feeding are essential for efficient energy acquisition, and are likely to have played a fundamental role in the success of early jawed vertebrates. The developmental origins and genetic requirements of these muscles are uncertain. Here, we determine by indelible lineage tracing in mouse that fibres of sternohyoid muscle (SHM), which is essential for mouth opening during feeding, and oesophageal striated muscle (OSM), which is crucial for voluntary swallowing, arise from Pax3-expressing somite cells. In vivo Kaede lineage tracing in zebrafish reveals the migratory route of cells from the anteriormost somites to OSM and SHM destinations. Expression of pax3b, a zebrafish duplicate of Pax3, is restricted to the hypaxial region of anterior somites that generate migratory muscle precursors (MMPs), suggesting that Pax3b plays a role in generating OSM and SHM. Indeed, loss of pax3b function led to defective MMP migration and OSM formation, disorganised SHM differentiation, and inefficient ingestion and swallowing of microspheres. Together, our data demonstrate Pax3-expressing somite cells as a source of OSM and SHM fibres, and highlight a conserved role of Pax3 genes in the genesis of these feeding muscles of vertebrates.

The transition from differentiation to growth during dermomyotome-derived myogenesis depends on temporally restricted hedgehog signaling

The development of a functional tissue requires coordination of the amplification of progenitors and their differentiation into specific cell types. The molecular basis for this coordination during myotome ontogeny is not well understood. Dermomytome progenitors that colonize the myotome first acquire myocyte identity and subsequently proliferate as Pax7-expressing progenitors before undergoing terminal differentiation. We show that the dynamics of sonic hedgehog (Shh) signaling is crucial for this transition in both avian and mouse embryos. Initially, Shh ligand emanating from notochord/floor plate reaches the dermomyotome, where it both maintains the proliferation of dermomyotome cells and promotes myogenic differentiation of progenitors that colonized the myotome. Interfering with Shh signaling at this stage produces small myotomes and accumulation of Pax7-expressing progenitors. An in vivo reporter of Shh activity combined with mouse genetics revealed the existence of both activator and repressor Shh activities operating on distinct subsets of cells during the epaxial myotomal maturation. In contrast to observations in mice, in avians Shh promotes the differentiation of both epaxial and hypaxial myotome domains. Subsequently, myogenic progenitors become refractory to Shh; this is likely to occur at the level of, or upstream of, smoothened signaling. The end of responsiveness to Shh coincides with, and is thus likely to enable, the transition into the growth phase of the myotome.

A myogenic precursor cell that could contribute to regeneration in zebrafish and its similarity to the satellite cell

The cellular basis for mammalian muscle regeneration has been an area of intense investigation over recent decades. The consensus is that a specialized self-renewing stem cell, termed the satellite cell, plays a major role during the process of regeneration in amniotes. How broadly this mechanism is deployed within the vertebrate phylogeny remains an open question. A lack of information on the role of cells analogous to the satellite cell in other vertebrate systems is even more unexpected given the fact that satellite cells were first designated in frogs. An intriguing aspect of this debate is that a number of amphibia and many fish species exhibit epimorphic regenerative processes in specific tissues, whereby regeneration occurs by the dedifferentiation of the damaged tissue, without deploying specialized stem cell populations analogous to satellite cells. Hence, it is feasible that a cellular process completely distinct from that deployed during mammalian muscle regeneration could operate in species capable of epimorphic regeneration. In this minireview, we examine the evidence for the broad phylogenetic distribution of satellite cells. We conclude that, in the vertebrates examined so far, epimorphosis does not appear to be deployed during muscle regeneration, and that analogous cells expressing similar marker genes to satellite cells appear to be deployed during the regenerative process. However, the functional definition of these cells as self-renewing muscle stem cells remains a final hurdle to the definition of the satellite cell as a generic vertebrate cell type.

Sulf1 modulates BMP signaling and is required for somite morphogenesis and development of the horizontal myoseptum

Heparan sulfate proteoglycans (HSPGs) are glycosylated extracellular or membrane-associated proteins. Their unbranched heparan sulfate (HS) disaccharide chains interact with many growth factors and receptors, modifying their activity or diffusion. The pattern of HS sulfation can be altered by the enzymes Sulf1 and Sulf2, secreted extracellular 6-O endosulfatases, which remove specific sulfate groups from HS. Modification by Sulf enzymes changes the binding affinity of HS for protein such as ligands and receptors, affecting growth factor gradients and activities. The precise expression of these sulfatases are thought to be necessary for normal development. We have examined the role of the sulf1 gene in trunk development of zebrafish embryos. sulf1 is expressed in the developing trunk musculature and as well as in midline structures such as the notochord, floorplate and hypochord. Knockdown of sulf1 with antisense morpholinos results in poor differentiation of the somitic trunk muscle, loss of the horizontal myoseptum, lack of pigmentation along the mediolateral stripe, and improper migration of the lateral line primordium. sulf1 knockdown results in a decrease in the number of Pax7-expressing dermomyotome cells, particularly along the midline where the horizontal myoseptum develops. It also leads to decreased sdf1/cxcl12 expression along the mediolateral trunk musculature. Both the Pax7 and cxcl12 expression can be restored by inhibition pharmacological inhibition of BMP signaling, which also restores formation of the myoseptum, fast muscle development, and pigmentation patterning. Lateral line migration and neuromast deposition depend on sdf1/cxcl12 and FGF signaling respectively, both of which are disrupted in sulf1 morphants. Pharmacological activation of FGF signaling can rescue the spacing of neuromast deposition in these fish. Together this data indicate that sulf1 plays a crucial role in modulating both BMP and FGF signaling along the developing myoseptum to coordinate the morphogenesis of trunk musculature, associated pigment cells, and lateral line neuromasts.

Satellite cells and the muscle stem cell niche

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

Six1 regulates proliferation of Pax7-positive muscle progenitors in zebrafish

In the embryonic zebrafish, skeletal muscle fibres are formed from muscle progenitors in the paraxial mesoderm. The embryonic myotome is mostly constituted of fast-twitch-specific fibres, which are formed from a fast-specific progenitor cell pool. The most lateral fraction of the fast domain in the myotome of zebrafish embryos derives from the Pax7-positive dermomyotome-like cells. In this study, we show that two genes, belonging to the sine oculus class 1 (six1) genes (six1a and six1b), are both essential for the regulation of Pax7(+) cell proliferation and, consequently, in their differentiation during the establishment of the zebrafish dermomyotome. In both six1a and six1b morphant embryos, Pax7(+) cells are initially formed but fail to proliferate, as detected by reduced levels of the proliferation marker phosphohistone3 and reduced brdU incorporation. In congruence, overexpression of six1a or six1b leads to increased Pax7(+) cell number and reduced or alternatively delayed fibre cell differentiation. Bone morphogenetic protein signalling has previously been suggested to inhibit differentiation of Pax7(+) cells in the dermomyotome. Here we show that the remaining Pax7(+) cells in six1a and six1b morphant embryos also have significantly reduced pSmad1/5/8 levels and propose that this leads to a reduced proliferative activity, which may result in a premature differentiation of Pax7(+) cells in the zebrafish dermomyotome. In summary, we show a mechanism for Six1a and Six1b in establishing the Pax7(+) cell derived part of the fast muscle and suggest new important roles for Six1 in the regulation of the Pax7(+) muscle cell population through pSmad1/5/8 signalling.

Morphogenesis and cell fate determination within the adaxial cell equivalence group of the zebrafish myotome

One of the central questions of developmental biology is how cells of equivalent potential—an equivalence group—come to adopt specific cellular fates. In this study we have used a combination of live imaging, single cell lineage analyses, and perturbation of specific signaling pathways to dissect the specification of the adaxial cells of the zebrafish embryo. We show that the adaxial cells are myogenic precursors that form a cell fate equivalence group of approximately 20 cells that consequently give rise to two distinct sub-types of muscle fibers: the superficial slow muscle fibers (SSFs) and muscle pioneer cells (MPs), distinguished by specific gene expression and cell behaviors. Using a combination of live imaging, retrospective and indicative fate mapping, and genetic studies, we show that MP and SSF precursors segregate at the beginning of segmentation and that they arise from distinct regions along the anterior-posterior (AP) and dorsal-ventral (DV) axes of the adaxial cell compartment. FGF signaling restricts MP cell fate in the anterior-most adaxial cells in each somite, while BMP signaling restricts this fate to the middle of the DV axis. Thus our results reveal that the synergistic actions of HH, FGF, and BMP signaling independently create a three-dimensional (3D) signaling milieu that coordinates cell fate within the adaxial cell equivalence group.

How specific genes and signals act on initially identical cells to generate the different tissues of the body remains one of the central questions of developmental genetics. Zebrafish are a useful model system to tackle this question as the optically clear embryo allows direct imaging of forming tissues, tracking individual cells in a myriad of different genetic contexts. The zebrafish myotome, the compartment of the embryo that gives rise to skeletal muscle, is subdivided into a number of specific cell types—one of which, the adaxial cells, gives rise exclusively to muscle of the “slow twitch” class. The adaxial cells give rise to two types of slow muscle cell types, muscle pioneer cells and non-muscle pioneer slow cells, distinguished by gene expression and different cellular behaviours. In this study we use lineage tracing live imaging and the manipulation of distinct genetic pathways to demonstrate that the adaxial cells form a cell fate “equivalence group” that is specified using separate signaling pathways that operating in distinct dimensions.

Fss/Tbx6 is required for central dermomyotome cell fate in zebrafish

The dermomyotome is a pool of progenitor cells on the surface of the myotome. In zebrafish, dermomyotome precursors (anterior border cells, ABCs) can be first identified in the anterior portion of recently formed somites. They must be prevented from undergoing terminal differentiation during segmentation, even while mesodermal cells around them respond to signaling cues and differentiate.

T-box containing transcription factors regulate many aspects of mesoderm fate including segmentation and somite patterning. The fused somites (fss) gene is the zebrafish ortholog of tbx6. We demonstrate that in addition to its requirement for segmentation, fss/tbx6 is also required for the specification of ABCs and subsequently the central dermomyotome. The absence of Tbx6-dependent central dermomyotome cells in fss/tbx6 mutants is spatially coincident with a patterning defect in the myotome.

Using transgenic fish with a heat-shock inducible tbx6 gene in the fss/tbx6 mutant background, we further demonstrate that ubiquitous fss/tbx6 expression has spatially distinct effects on recovery of the dermomyotome and segment boundaries, suggesting that the mechanism of Fss/Tbx6 action is distinct with respect to dermomyotome development and segmentation.

We propose that Fss/Tbx6 is required for preventing myogenic differentiation of central dermomyotome precursors before and after segmentation and that central dermomyotome cells represent a genetically and functionally distinct subpopulation within the zebrafish dermomyotome.

Do muscle founder cells exist in vertebrates?

Skeletal muscle is formed by the iterative fusion of precursor cells (myocytes) into long multinuclear fibres. Extensive studies of fusion in Drosophila embryos have lead to a paradigm in which myoblasts are divided into two distinct subtypes - founder and fusion-competent myoblasts (FCMs) - that can fuse to each other, but not among themselves. Only founder cells can direct the formation of muscle fibres, while FCMs act as a cellular substrate. Recent studies in zebrafish and mice have demonstrated conservation of the molecules originally identified in Drosophila, but an important question remains: is vertebrate fusion regulated by specifying myocyte subtypes? Stated simply: do vertebrate founder cells exist? In light of recent findings, we argue that a different regulatory mechanism has evolved in vertebrates.

Muscle development and differentiation in the urodele Ambystoma mexicanum

Muscle differentiation has been widely described in zebrafish and Xenopus, but nothing is known about this process in amphibian urodeles. Both anatomical features and locomotor activity in urodeles are known to show intermediate features between fish and anurans. Therefore, a better understanding of myogenesis in urodeles could be useful to clarify the evolutionary changes that led to the formation of skeletal muscle in the trunk of land vertebrates. We report here a detailed morphological and molecular investigation on several embryonic stages of Ambystoma mexicanum and show that the first differentiating muscle fibers are the slow ones, originating from a myoblast population initially localized close to the notochord that forms a superficial layer on the somitic surface afterwards. Subsequently, fast fibers differentiation ensues. We also identified and cloned A. mexicanum Myf5 as a muscle-specific transcriptional factor likely involved in urodele muscle differentiation.

Xirp proteins mark injured skeletal muscle in zebrafish

Myocellular regeneration in vertebrates involves the proliferation of activated progenitor or dedifferentiated myogenic cells that have the potential to replenish lost tissue. In comparison little is known about cellular repair mechanisms within myocellular tissue in response to small injuries caused by biomechanical or cellular stress. Using a microarray analysis for genes upregulated upon myocellular injury, we identified zebrafish Xin-actin-binding repeat-containing protein1 (Xirp1) as a marker for wounded skeletal muscle cells. By combining laser-induced micro-injury with proliferation analyses, we found that Xirp1 and Xirp2a localize to nascent myofibrils within wounded skeletal muscle cells and that the repair of injuries does not involve cell proliferation or Pax7(+) cells. Through the use of Xirp1 and Xirp2a as markers, myocellular injury can now be detected, even though functional studies indicate that these proteins are not essential in this process. Previous work in chicken has implicated Xirps in cardiac looping morphogenesis. However, we found that zebrafish cardiac morphogenesis is normal in the absence of Xirp expression, and animals deficient for cardiac Xirp expression are adult viable. Although the functional involvement of Xirps in developmental and repair processes currently remains enigmatic, our findings demonstrate that skeletal muscle harbours a rapid, cell-proliferation-independent response to injury which has now become accessible to detailed molecular and cellular characterizations.

Building muscle: molecular regulation of myogenesis

The genesis of skeletal muscle during embryonic development and postnatal life serves as a paradigm for stem and progenitor cell maintenance, lineage specification, and terminal differentiation. An elaborate interplay of extrinsic and intrinsic regulatory mechanisms controls myogenesis at all stages of development. Many aspects of adult myogenesis resemble or reiterate embryonic morphogenetic episodes, and related signaling mechanisms control the genetic networks that determine cell fate during these processes. An integrative view of all aspects of myogenesis is imperative for a comprehensive understanding of muscle formation. This article provides a holistic overview of the different stages and modes of myogenesis with an emphasis on the underlying signals, molecular switches, and genetic networks.

Immunocytochemistry to study myogenesis in zebrafish

During myogenesis, cells gradually transition from mesodermal precursors to myoblasts, myocytes, and then to muscle fibers. The molecular characterization of this process requires the ability to identify each of these cell types and the factors that regulate the transitions between them. The most versatile technique for assaying cell identities in situ is immunocytochemistry, because multiple independent molecular markers of differentiation can be assayed simultaneously. The zebrafish has developed into a popular model for the study of myogenesis, and immunocytochemical techniques have been critical. We have adapted existing protocols to optimize immunocytochemistry in zebrafish, and have tested many antibodies developed against mouse, chick, and frog muscle antigens for their cross-reactivity in zebrafish. Here, we present protocols for whole mount immunocytochemistry on both formaldehyde and Carnoy's fixed embryos as well as on sectioned zebrafish tissue. We include a table of antibodies useful for experiments on the molecular biology of myogenesis in zebrafish.

Vertebrate-like regeneration in the invertebrate chordate amphioxus

An important question in biology is why some animals are able to regenerate, whereas others are not. The basal chordate amphioxus is uniquely positioned to address the evolution of regeneration. We report here the high regeneration potential of the European amphioxus Branchiostoma lanceolatum. Adults regenerate both anterior and posterior structures, including neural tube, notochord, fin, and muscle. Development of a classifier based on tail regeneration profiles predicts the assignment of young and old adults to their own class with >94% accuracy. The process involves loss of differentiated characteristics, formation of an msx-expressing blastema, and neurogenesis. Moreover, regeneration is linked to the activation of satellite-like Pax3/7 progenitor cells, the extent of which declines with size and age. Our results provide a framework for understanding the evolution and diversity of regeneration mechanisms in vertebrates.

Alkali-like myosin light chain-1 (myl1) is an early marker for differentiating fast muscle cells in zebrafish

During myogenesis, muscle precursors become divided into either fast- or slow-twitch fibres, which in the zebrafish occupy distinct domains in the embryo. Genes encoding sarcomeric proteins specific for fast or slow fibres are frequently used as lineage markers. In an attempt to identify and evaluate early definitive markers for cells in the fast-twitch pathway, we analysed genes encoding proteins contributing to the fast sarcomeric structures. The previously uncharacterized zebrafish alkali-like myosin light chain gene (myl1) was found to be expressed exclusively in cells in the fast-twitch pathway initiated at an early stage of fast fibre differentiation. Myl1 was expressed earlier, and in a more fibre type restricted manner, than any of the previously described and frequently used fast myosin light and heavy chain and troponin muscle markers mylz2, mylz3, tnni2, tnnt3a, fMyHC1.3. In summary, this study introduces a novel marker for early differentiating fast muscle cells.

Analysis of Pax7 expressing myogenic cells in zebrafish muscle development, injury, and models of disease

The transcription factor Pax7 is a marker and regulator of muscle progenitors and satellite cells that contribute to the embryonic development and postembryonic growth of skeletal muscle in vertebrates, as well as to its repair and regeneration. Here, we identify Pax7(+ve) myogenic cells in the zebrafish and characterize their behavior in postembryonic stages. Mononucleate Pax7(+ve) cells can first be found associated with myofibers at 72 hours post fertilization (hpf). To follow the behavior of muscle progenitor cells in vivo, we generated transgenic lines expressing fluorescent proteins under the control of the pax7a or pax3a promoters. We established an injury model using cardiotoxin injection and monitored cell proliferation and myogenic regulatory factor expression in myogenic precursors cells and muscle fibers after injury using proliferation markers and the transgenic lines. We also analyzed Pax7(+ve) cells in animals with dystrophic phenotypes and found an increased number compared with wild-type.

Defective cranial skeletal development, larval lethality and haploinsufficiency in Myod mutant zebrafish

Myogenic regulatory factors of the myod family (MRFs) are transcription factors essential for mammalian skeletal myogenesis. Here we show that a mutation in the zebrafish myod gene delays and reduces early somitic and pectoral fin myogenesis, reduces miR-206 expression, and leads to a persistent reduction in somite size until at least the independent feeding stage. A mutation in myog, encoding a second MRF, has little obvious phenotype at early stages, but exacerbates the loss of somitic muscle caused by lack of Myod. Mutation of both myod and myf5 ablates all skeletal muscle. Haploinsufficiency of myod leads to reduced embryonic somite muscle bulk. Lack of Myod causes a severe reduction in cranial musculature, ablating most muscles including the protractor pectoralis, a putative cucullaris homologue. This phenotype is accompanied by a severe dysmorphology of the cartilaginous skeleton and failure of maturation of several cranial bones, including the opercle. As myod expression is restricted to myogenic cells, the data show that myogenesis is essential for proper skeletogenesis in the head.

Isolation and transcriptome analysis of adult zebrafish cells enriched for skeletal muscle progenitors

INTRODUCTION

Over the past 10 years, the use of zebrafish for scientific research in the area of muscle development has increased dramatically. Although several protocols exist for the isolation of adult myoblast progenitors from larger fish, no standardized protocol exists for the isolation of myogenic progenitors from adult zebrafish muscle.

METHODS

Using a variant of a mammalian myoblast isolation protocol, zebrafish muscle progenitors have been isolated from the total dorsal myotome. These zebrafish myoblast progenitors can be cultured for several passages and then differentiated into multinucleated, mature myotubes.

RESULTS

Transcriptome analysis of these cells during myogenic differentiation revealed a strong downregulation of pluripotency genes, while, conversely, showing an upregulation of myogenic signaling and structural genes.

CONCLUSIONS

Together these studies provide a simple, yet detailed method for the isolation and culture of myogenic progenitors from adult zebrafish, while further promoting their therapeutic potential for the study of muscle disease and drug screening.

Origin of muscle satellite cells in the Xenopus embryo

We have studied the origin of muscle satellite cells in embryos of Xenopus laevis. Fate mapping at the open neural plate stage was carried out using orthotopic grafts from transgenic embryos expressing GFP. This shows that most satellite cells originate from the dorsolateral plate rather than from the paraxial mesoderm. Specification studies were made by isolation of explants from the paraxial and dorsolateral regions of neurulae and these also indicated that the satellite cell progenitors arise from the dorsolateral plate. Muscle satellite cells express Pax7, but overexpression of Pax7 in blastomeres of whole embryos that populate the myogenic areas does not induce the formation of additional satellite cells. Moreover, a dominant-negative construct, Pax7EnR, does not reduce satellite cell formation. Neither Pax7 nor other myogenic transcription factor genes will induce satellite cell formation in animal caps treated with FGF. However, BMP RNA or protein will do so, both for FGF-treated animal caps and for paraxial neurula explants. Conversely, the induction of Noggin in dorsolateral explants from HGEM-Noggin transgenic neurulae will block formation of satellite cells, showing that BMP signaling is required in vivo for satellite cell formation. We conclude that satellite cell progenitors are initially specified in the dorsal part of the lateral plate mesoderm and later become incorporated into the myotomes. The initial specification occurs at the neurula stage and depends on the ventral-to-dorsal BMP gradient in the early embryo.

Sonic hedgehog (Shh)-Gli signaling controls neural progenitor cell division in the developing tectum in zebrafish

Despite considerable progress, the mechanisms that control neural progenitor differentiation and behavior, as well as their functional integration into adult neural circuitry, are far from being understood. Given the complexity of the mammalian brain, non-mammalian models provide an excellent model to study neurogenesis, including both the cellular composition of the neurogenic microenvironment, and the factors required for precursor growth and maintenance. In particular, we chose to address the question of the control of progenitor proliferation by Sonic hedgehog (Shh) using the zebrafish dorsal mesencephalon, known as the optic tectum (OT), as a model system. Here we show that either inhibiting pharmacologically or eliminating hedgehog (Hh) signaling by using mutants that lack essential components of the Hh pathway reduces neural progenitor cell proliferation affecting neurogenesis in the OT. On the contrary, pharmacological gain-of-function experiments result in significant increase in proliferation. Importantly, Shh-dependent function controls neural progenitor cell behavior as sox2-positive cell populations were lost in the OT in the absence of Hh signaling, as evidenced in slow-muscle-omitted (smu) mutants and with timed cyclopamine inhibition. Expressions of essential components of the Hh pathway reveal for the first time a late dorsal expression in the embryonic OT. Our observations argue strongly for a role of Shh in neural progenitor biology in the OT and provide comparative data to our current understanding of progenitor/stem cell mechanisms that place Shh as a key niche factor in the dorsal brain.

Cdkn1c drives muscle differentiation through a positive feedback loop with Myod

Differentiation often requires conversion of analogue signals to a stable binary output through positive feedback. Hedgehog (Hh) signalling promotes myogenesis in the vertebrate somite, in part by raising the activity of muscle regulatory factors (MRFs) of the Myod family above a threshold. Hh is known to enhance MRF expression. Here we show that Hh is also essential at a second step that increases Myod protein activity, permitting it to promote Myogenin expression. Hh acts by inducing expression of cdkn1c (p57(Kip2)) in slow muscle precursor cells, but neither Hh nor Cdkn1c is required for their cell cycle exit. Cdkn1c co-operates with Myod to drive differentiation of several early zebrafish muscle fibre types. Myod in turn up-regulates cdkn1c, thereby providing a positive feedback loop that switches myogenic cells to terminal differentiation.

BMP regulation of myogenesis in zebrafish

In amniotes, BMP signaling from lateral plate and dorsal neural tube inhibits differentiation of muscle precursors in the dermomyotome. Here, we show that BMPs are expressed adjacent to the dermomyotome during and after segmentation in zebrafish. In addition, downstream BMP pathway members are expressed within the somite during dermomyotome development. We also show that zebrafish dermomyotome is responsive to BMP throughout its development. Ectopic overexpression of Bmp2b increases expression of the muscle precursor marker pax3, and changes the time course of myoD expression. At later stages, overexpression increases the number of Pax7+ myogenic precursors, and delays muscle differentiation, as indicated by decreased numbers of MEF2+ nuclei, decreased number of multi-nucleated muscle fibers, and an increased myotome angle. In addition, we show that while BMP overexpression is sufficient to delay myogenic differentiation, inhibition of BMP does not detectably affect this process, suggesting that other factors redundantly inhibit myogenic differentiation.

[A dermomyotome in fish?]

The dermomyotome is a transient epithelial sheet that forms from the dorsal aspect of the somite. The dermomyotome gives rise to a variety of tissues, most importantly myotomal muscle and dermis. Despite the central importance of the dermomyotome in the development of amniotes, the question of its existence in lower vertebrates has been lastingly eluded. The combination of single-cell lineage tracing and gene expression analysis has recently led to the identification in fish of a somitic sub-domain that exhibits structural and functional features of the amniote dermomyotome.

Analysis of early human neural crest development

The outstanding migration and differentiation capacities of neural crest cells (NCCs) have fascinated scientists since Wilhelm His described this cell population in 1868. Today, after intense research using vertebrate model organisms, we have gained considerable knowledge regarding the origin, migration and differentiation of NCCs. However, our understanding of NCC development in human embryos remains largely uncharacterized, despite the role the neural crest plays in several human pathologies. Here, we report for the first time the expression of a battery of molecular markers before, during, or following NCC migration in human embryos from Carnegie Stages (CS) 12 to 18. Our work demonstrates the expression of Sox9, Sox10 and Pax3 transcription factors in premigratory NCCs, while actively migrating NCCs display the additional transcription factors Pax7 and AP-2alpha. Importantly, while HNK-1 labels few migrating NCCs, p75(NTR) labels a large proportion of this population. However, the broad expression of p75(NTR) - and other markers - beyond the neural crest stresses the need for the identification of additional markers to improve our capacity to investigate human NCC development, and to enable the generation of better diagnostic and therapeutic tools.

Identification and characterization of alternative promoters of zebrafish Rtn-4/Nogo genes in cultured cells and zebrafish embryos

In mammals, the Nogo family consists of Nogo-A, Nogo-B and Nogo-C. However, there are three Rtn-4/Nogo-related transcripts were identified in zebrafish. In addition to the common C-terminal region, the N-terminal regions of Rtn4-n/Nogo-C1, Rtn4-m/Nogo-C2 and Rtn4-l/Nogo-B, respectively, contain 9, 25 and 132 amino acid residues. In this study, we isolated the 5'-upstream region of each gene from a BAC clone and demonstrated that the putative promoter regions, P1-P3, are functional in cultured cells and zebrafish embryos. A transgenic zebrafish Tg(Nogo-B:GFP) line was generated using P1 promoter region to drive green fluorescent protein (GFP) expression through Tol2-mediated transgenesis. This line recapitulates the endogenous expression pattern of Rtn4-l/Nogo-B mRNA in the brain, brachial arches, eyes, muscle, liver and intestines. In contrast, GFP expressions by P2 and P3 promoters were localized to skeletal muscles of zebrafish embryos. Several GATA and E-box motifs are found in these promoter regions. Using morpholino knockdown experiments, GATA4 and GATA6 were involved in the control of P1 promoter activity in the liver and intestine, while Myf5 and MyoD for the control of P1 and P3 promoter activities in muscles. These data demonstrate that zebrafish Rtn4/Nogo transcripts might be generated by coupling mechanisms of alternative first exons and alternative promoter usage.

The transcriptional repressor REST/NRSF modulates hedgehog signaling

The spatial and temporal control of gene expression is key to generation of specific cellular fates during development. Studies of the transcriptional repressor REST/NRSF (RE1 Silencing Transcription Factor or Neural Restrictive Silencing Factor) have provided important insight into the role that epigenetic modifications play in differential gene expression. However, the precise function of REST during embryonic development is not well understood. We have discovered a novel interaction between zebrafish Rest and the Hedgehog (Hh) signaling pathway. We observed that Rest knockdown enhances or represses Hh signaling in a context-dependant manner. In wild-type embryos and embryos with elevated Hh signaling, Rest knockdown augments transcription of Hh target genes. Conversely, in contexts where Hh signaling is diminished, Rest knockdown has the opposite effect and Hh target gene expression is further attenuated. Epistatic analysis revealed that Rest interacts with the Hh pathway at a step downstream of Smo. Furthermore, we present evidence implicating the bifunctional, Hh signaling component Gli2a as key to the Rest modulation of the Hh response. The role of Rest as a regulator of Hh signaling has broad implications for many developmental contexts where REST and Hh signaling act.

Myoblast fusion: when it takes more to make one

Cell-cell fusion is a crucial and highly regulated event in the genesis of both form and function of many tissues. One particular type of cell fusion, myoblast fusion, is a key cellular process that shapes the formation and repair of muscle. Despite its importance for human health, the mechanisms underlying this process are still not well understood. The purpose of this review is to highlight the recent literature pertaining to myoblast fusion and to focus on a comparison of these studies across several model systems, particularly the fly, zebrafish and mouse. Advances in technical analysis and imaging have allowed identification of new fusion genes and propelled further characterization of previously identified genes in each of these systems. Among the cellular steps identified as critical for myoblast fusion are migration, recognition, adhesion, membrane alignment and membrane pore formation and resolution. Importantly, striking new evidence indicates that orthologous genes govern several of these steps across these species. Taken together, comparisons across three model systems are illuminating a once elusive process, providing exciting new insights and a useful framework of genes and mechanisms.

Postembryonic fast muscle growth of teleost fish depends upon a nonuniformly distributed population of mitotically active Pax7+ precursor cells

Muscle development in teleost embryos has been shown to depend on myogenic cell recruitment from the dermomyotome (DM). However, little is known as to the cellular mechanisms that account for myotome growth after the dissociation of the DM. Here we combine immunolabeling for cell-specific markers with quantitative analysis to determine the sources and patterns of activation of myogenic cells in pearlfish larvae. Results demonstrate that appearance of mitotically active myogenic precursors inside the myotome coincides with the dissociation of the DM. Such cells are preferentially aggregated within the posterior lateral fast muscle. We therefore propose a growth model in which a pool of proliferative DM-derived precursors transferred to the posterior lateral fast muscle functions as an important source of myogenic cell spread to carry forward stratified fast muscle hyperplasia. This indicates that postembryonic teleost muscle growth includes a cellular mechanism that has no direct equivalent in the amniotes.

Pax-3 and Pax-7 label muscle progenitor cells during myotomal myogenesis in Coregonus lavaretus (Teleostei: Coregonidae)

In Coregonus lavaretus, prior the mesoderm segmentation, in cells adjacent to the notochord called adaxial cells MyoD and slow myosin heavy chain (MyHC-slow) proteins were observed. After somite formation, adaxial cells migrate towards the lateral part of the myotome and form a layer of red muscles. Deeper cells differentiate into white muscle fibres. In situ hybridization using Pax-3 molecular probe revealed, that after somitogenesis, Pax-3 is expressed in a layer of cells superficial to the myotome resembling the "external cells" (found in many teleosts species) or dermomyotome described in Amniota. During later developmental stages Pax-3 gene is expressed in cells in intermyotomal space and then in myoblasts between myotubes. In these cells Pax-7 protein was also observed. Pax-3/7 positive cells which have migrated into the myotomes differentiate into satellite cells/secondary myoblasts and participate in hypertrophic and hyperplastic growth of muscles.

Distinct and dynamic myogenic populations in the vertebrate embryo

Myogenic cells in the body of vertebrates derive from the dorsal somite, the dermomyotome, where multipotent cells are present. Regulation of cell fate choice is discussed, as is that of progenitor cell self-renewal once cells have entered the myogenic programme. Ongoing research on the formation of the first skeletal muscle, the myotome, is presented with emphasis on mechanisms controlling the early segregation of slow and fast muscle lineages that characterizes this process in the zebrafish embryo. Further insights into myogenic populations that contribute to trunk and limb development at different stages are summarized and the distinct regulatory networks that underlie the formation of head muscles are discussed.

Regulation of slow and fast muscle myofibrillogenesis by Wnt/beta-catenin and myostatin signaling

Deviation from proper muscle development or homeostasis results in various myopathic conditions. Employing genetic as well as chemical intervention, we provide evidence that a tight regulation of Wnt/β-catenin signaling is essential for muscle fiber growth and maintenance. In zebrafish embryos, gain-of-Wnt/β-catenin function results in unscheduled muscle progenitor proliferation, leading to slow and fast muscle hypertrophy accompanied by fast muscle degeneration. The effects of Wnt/β-catenin signaling on fast muscle hypertrophy were rescued by misexpression of Myostatin or p21^CIP/WAF, establishing an in vivo regulation of myofibrillogenesis by Wnt/β-catenin signaling and Myostatin. Epistatic analyses suggest a possible genetic interaction between Wnt/β-catenin and Myostatin in regulation of slow and fast twitch muscle myofibrillogenesis.

Differential requirements for myogenic regulatory factors distinguish medial and lateral somitic, cranial and fin muscle fibre populations

Myogenic regulatory factors of the Myod family (MRFs) are transcription factors essential for mammalian skeletal myogenesis. However, the roles of each gene in myogenesis remain unclear, owing partly to genetic linkage at the Myf5/Mrf4 locus and to rapid morphogenetic movements in the amniote somite. In mice, Myf5 is essential for the earliest epaxial myogenesis, whereas Myod is required for timely differentiation of hypaxially derived muscle. A second major subdivision of the somite is between primaxial muscle of the somite proper and abaxial somite-derived migratory muscle precursors. Here, we use a combination of mutant and morphant analysis to ablate the function of each of the four conserved MRF genes in zebrafish, an organism that has retained a more ancestral bodyplan. We show that a fundamental distinction in somite myogenesis is into medial versus lateral compartments, which correspond to neither epaxial/hypaxial nor primaxial/abaxial subdivisions. In the medial compartment, Myf5 and/or Myod drive adaxial slow fibre and medial fast fibre differentiation. Myod-driven Myogenin activity alone is sufficient for lateral fast somitic and pectoral fin fibre formation from the lateral compartment, as well as for cranial myogenesis. Myogenin activity is a significant contributor to fast fibre differentiation. Mrf4 does not contribute to early myogenesis in zebrafish. We suggest that the differential use of duplicated MRF paralogues in this novel two-component myogenic system facilitated the diversification of vertebrates.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

Induced early expression of mrf4 but not myog rescues myogenesis in the myod/myf5 double-morphant zebrafish embryo

Muscle regulatory factors activate myogenesis in all vertebrates, but their role has been studied in great detail only in the mouse embryo, where all but myogenin – Myod, Myf5 and Mrf4 – are sufficient to activate (albeit not completely) skeletal myogenesis. In the zebrafish embryo, myod and myf5 are required for induction of myogenesis because their simultaneous ablation prevents muscle development. Here we show that mrf4 but not myog can fully rescue myogenesis in the myod/myf5 double morphant via a selective and robust activation of myod, in keeping with its chromatin-remodelling function in vitro. Rescue does not happen spontaneously, because the gene, unlike that in the mouse embryo, is expressed only at the onset of muscle differentiation, Moreover, because of the transient nature of morpholino inhibition, we were able to investigate how myogenesis occurs in the absence of a myotome. We report that in the complete absence of a myotome, subsequent myogenesis is abolished, whereas myogenesis does proceed, albeit abnormally, when the morpholino inhibition was not complete. Therefore our data also show that the early myotome is essential for subsequent skeletal muscle differentiation and patterning in the zebrafish.

New insights into skeletal muscle development and growth in teleost fishes

Recent research has significantly broadened our understanding of how the teleost somite is patterned to achieve embryonic and postembryonic myogenesis. Medial (adaxial) cells and posterior cells of the early epithelial somite generate embryonic superficial slow and deep fast muscle fibers, respectively, whereas anterior somitic cells move laterally to form an external cell layer of undifferentiated Pax7-positive myogenic precursors surrounding the embryonic myotome. In late embryo and in larvae, some of the cells contained in the external cell layer incorporate into the myotome and differentiate into new muscle fibers, thus contributing to medio-lateral expansion of the myotome. This supports the suggestion that the teleost external cell layer is homologous to the amniote dermomyotome. Some of the signalling molecules that promote lateral movement or regulate the myogenic differentiation of external cell precursors have been identified and include stromal cell-derived factor 1 (Sdf1), hedgehog proteins, and fibroblast growth factor 8 (Fgf8). Recent studies have shed light on gene activations that underlie the differentiation and maturation of slow and fast muscle fibers, pointing out that both adaxially derived embryonic slow fibers and slow fibers formed during the myotome expansion of larvae initially and transiently bear features of the fast fiber phenotype.

Tailbud-derived Bmp4 drives proliferation and inhibits maturation of zebrafish chordamesoderm

In zebrafish, BMP signaling establishes cell identity along the dorsoventral (DV) axis during gastrulation. Owing to the early requirements of BMP activity in DV patterning, it has been difficult to assign later roles in cell fate specification to specific BMP ligands. In this study, we have taken advantage of two follistatin-like genes (fstl1 and fstl2), as well as a transgenic zebrafish line carrying an inducible truncated form of the BMP-type 1 receptor to study the role of Bmp4 outside of the context of DV specification. Characterization of fstl1/2 suggests that they exert a redundant role as BMP antagonists during late gastrulation, regulating BMP activity in axial mesoderm. Maintenance of appropriate levels of BMP signaling is crucial for the proper development of chordamesoderm, a subset of axial mesoderm that gives rise to the notochord, but not prechordal mesoderm, which gives rise to the prechordal plate. Bmp4 activity in particular is required during a crucial window beginning at late gastrulation and lasting through early somitogenesis to promote chordamesoderm proliferation. In the absence of Bmp4, the notochord precursor pool is depleted, and the notochord differentiates prematurely. Our results illustrate a role for Bmp4 in the proliferation and timely differentiation of axial tissue after DV axis specification.

Time-lapse analysis and mathematical characterization elucidate novel mechanisms underlying muscle morphogenesis

Skeletal muscle morphogenesis transforms short muscle precursor cells into long, multinucleate myotubes that anchor to tendons via the myotendinous junction (MTJ). In vertebrates, a great deal is known about muscle specification as well as how somitic cells, as a cohort, generate the early myotome. However, the cellular mechanisms that generate long muscle fibers from short cells and the molecular factors that limit elongation are unknown. We show that zebrafish fast muscle fiber morphogenesis consists of three discrete phases: short precursor cells, intercalation/elongation, and boundary capture/myotube formation. In the first phase, cells exhibit randomly directed protrusive activity. The second phase, intercalation/elongation, proceeds via a two-step process: protrusion extension and filling. This repetition of protrusion extension and filling continues until both the anterior and posterior ends of the muscle fiber reach the MTJ. Finally, both ends of the muscle fiber anchor to the MTJ (boundary capture) and undergo further morphogenetic changes as they adopt the stereotypical, cylindrical shape of myotubes. We find that the basement membrane protein laminin is required for efficient elongation, proper fiber orientation, and boundary capture. These early muscle defects in the absence of either lamininβ1 or lamininγ1 contrast with later dystrophic phenotypes in lamininα2 mutant embryos, indicating discrete roles for different laminin chains during early muscle development. Surprisingly, genetic mosaic analysis suggests that boundary capture is a cell-autonomous phenomenon. Taken together, our results define three phases of muscle fiber morphogenesis and show that the critical second phase of elongation proceeds by a repetitive process of protrusion extension and protrusion filling. Furthermore, we show that laminin is a novel and critical molecular cue mediating fiber orientation and limiting muscle cell length.

Despite the importance of muscle fiber development and tendon attachment, this process is incompletely understood in vertebrates. One critical step is muscle fiber elongation; muscle precursor cells are short and subsequent elongation/fusion generates long, multinucleate muscle fibers. Using a vertebrate model organism, the zebrafish, we find that single round myoblasts elongate to span the entire width of the myotome prior to fusion. Using rigorous and objective mathematical characterization techniques, we can further divide muscle development into three stages: short precursor cells, intercalation/elongation, and boundary capture/myotube formation. The second phase, elongation, occurs via a two-step mechanism of protrusion extension and filling. Myotube formation involves boundary capture, where the ends of muscle fibers anchor themselves to the myotome boundary and stop elongating. We show that the protein laminin is required for boundary capture, normal fiber length, and proper fiber orientation. Genetic mosaic experiments in laminin-deficient embryos reveal that boundary capture is a cell autonomous phenomenon. Wild-type (normal) cells capture the boundary appropriately and stop elongating in laminin-deficient embryos. Although adhesion to laminin has been implicated in muscular dystrophies where the attachment between muscle cells and tendons fails, no early developmental requirements for laminin in fast muscle morphogenesis have been shown until now.

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. The activity of this regulatory element is also partially dependent on E-boxes, targets of the myogenic regulatory factors, which are expressed as progenitor cells enter the myogenic program. Other FGF signaling components, notably Sprouty1, are also regulated by Pax3. In vivo manipulation of Sprouty expression reveals that FGF signaling affects the balance between Pax-positive progenitor cells and committed myoblasts. These results provide new insight into the Pax-initiated regulatory network that modulates stem cell maintenance versus tissue differentiation.

A gradient of Shh establishes mutually repressing somitic cell fates induced by Nkx3.2 and Pax3

Wnt and Sonic Hedgehog (Shh) signals are known to pattern the somite into dermomyotomal, myotomal and sclerotomal cell fates. By employing explants of presomitic mesoderm cultured with constant levels of Wnt3a conditioned medium and increasing levels of Shh, we found that differing levels of Shh signaling elicit differing responses from somitic cells: the lowest level of Shh signaling allows dermomyotomal gene expression, intermediate levels induce loss of dermomyotomal markers and activation of myogenic differentiation, and higher levels induce loss of myotomal markers and activation of sclerotomal gene expression. In addition, we have found that in the presence of high levels of Wnt signaling, instead of inducing sclerotomal markers, Shh signals act to maintain the expression of dermomyotomal and myotomal markers. One of the sclerotomal genes induced by high levels of Shh signaling is Nkx3.2. Forced expression of Nkx3.2 blocks somitic expression of the dermomyotomal marker Pax3 both in vitro and in vivo. Conversely, forced expression of Pax3 in somites can block Shh-mediated induction of sclerotomal gene expression and chondrocyte differentiation in vitro. Thus we propose that varying levels of Shh signaling act in a morphogen-like manner to elicit differing responses from somitic cells, and that Pax3 and Nkx3.2 set up mutually repressing cell fates that promote either dermomyotome/myotome or sclerotome differentiation, respectively.

Temperature until the 'eyed stage' of embryogenesis programmes the growth trajectory and muscle phenotype of adult Atlantic salmon

We investigated how adult growth in Atlantic salmon (Salmo salar L.) was affected by changing embryonic temperature from fertilization until the completion of eye pigmentation. Fertilized eggs from several hundred families were divided between four temperature treatments (2, 5, 8 or 10 degrees C) and subsequently reared in identical conditions in replicated tanks. Fish exposed to 2 and 5 degrees C treatments were significantly smaller at smoltification than groups at higher temperatures, but showed substantial compensatory catch-up growth. Remarkably, temperature during this short window of embryogenesis dictated adult myogenic phenotype three years later with significant treatment effects on the muscle fibre final number (FFN), maximum diameter, nuclear density and size distribution. FFN was highest for the 5 degrees C treatment and was reduced at higher and lower treatment temperatures. Our results require direct temperature effects on embryonic tissues, such as the stem cell-containing external cell layer, in order to produce persistent effects on juvenile and adult growth.