Sonic hedgehog enhances somite cell viability and formation of primary slow muscle fibers in avian segmented mesoderm

Teratogenesis in the chick embryo following post-gastrulation exposure to Y-27632 -effect of Y-27632 on embryonic development

The pyridine derivative Y-27632 inhibits Rho-associated coiled-coil-containing protein kinase (ROCK) signaling, which is involved in numerous developmental processes during embryogenesis, primarily by controlling actin-cytoskeleton assembly and cell contractility. Somite formation requires rearrangement of the cytoskeleton and assists in major morphological mechanisms, including ventral body wall formation. Administration of Y-27632 impairs cytoskeletal arrangements in post-gastrulation chick embryos leading to ventral body wall defects (VBWD) at later stages of development. The aim of this study was to investigate the effect of Y-27632 on somite development in post-gastrulation chick embryos during early embryogenesis. After 60 h incubation, embryos in shell-less culture were treated with Y-27632 or vehicle for controls. Following administration, abnormality rates were assessed. In treatment groups, embryos showed a kinked longitudinal body axis. Western blot confirmed impaired ROCK downstream signaling by decreased expression of phosphorylated cofilin-2. Histology, Lysotracker studies and RT-PCR demonstrated increased cell death in somites, the neural tube and the ectoderm. RT-PCR and Western blot of factors known to be involved during somitogenesis revealed reduced expression in the treatment group compared to controls. We hypothesize that administration of Y-27632 disrupts somite development causing axial kinking and embryo malformation, which may lead to VBWD.

Downregulation of Caspase-2 Expression in Somitic Cells following Coculture with Chicken Notochord

Somites are spherical aggregations of mesodermal cells located on either sides of neural tube and are differentiated into sclerotome and dermomyotome. Notochord as an axial mesoderm has a major role in somitic cell survival and differentiation in vivo. Despite secreting the survival factors, how to notochord inhibits somitic cells apoptosis remains to be elusive. So, this study was aimed to investigate downregulation of caspase-2 expression in somitic cells upon coculturing with notochord. By using alginate system to encapsulate the isolated notochord in Somite + Notochord group, the embryonic somites were cocultured with the notochord on different days. Concurrently in somite group, the somites were cultured alone. Survival assay with MTT showed that the rate of viability in somitic cells cocultured with notochord increased from 59% on day 2 to 89.7% on day 6 but decreased to 38.5% on day 10 after coculturing. Reverse transcriptase-polymerase chain reaction and spectrophotometry analysis also confirmed these findings and showed low caspase-2 and high Bcl-2 expressions and low caspase-2 enzyme activity in somitic cells cocultured with notochord, respectively. These results clearly show that the notochord enhances survival of somitic cells in vitro through downregulating of caspase-2 expression along with triggering differentiation of somitic cells to Pax-1 expressing mesenchymal cells.

Autonomous and nonautonomous roles of Hedgehog signaling in regulating limb muscle formation

Muscle progenitor cells migrate from the lateral somites into the developing vertebrate limb, where they undergo patterning and differentiation in response to local signals. Sonic hedgehog (Shh) is a secreted molecule made in the posterior limb bud that affects patterning and development of multiple tissues, including skeletal muscles. However, the cell-autonomous and non-cell-autonomous functions of Shh during limb muscle formation have remained unclear. We found that Shh affects the pattern of limb musculature non-cell-autonomously, acting through adjacent nonmuscle mesenchyme. However, Shh plays a cell-autonomous role in maintaining cell survival in the dermomyotome and initiating early activation of the myogenic program in the ventral limb. At later stages, Shh promotes slow muscle differentiation cell-autonomously. In addition, Shh signaling is required cell-autonomously to regulate directional muscle cell migration in the distal limb. We identify neuroepithelial cell transforming gene 1 (Net1) as a downstream target and effector of Shh signaling in that context.

Sox6 is required for normal fiber type differentiation of fetal skeletal muscle in mice

Sox6, a member of the Sox family of transcription factors, is highly expressed in skeletal muscle. Despite its abundant expression, the role of Sox6 in muscle development is not well understood. We hypothesize that, in fetal muscle, Sox6 functions as a repressor of slow fiber type-specific genes. In the wild-type mouse, differentiation of fast and slow fibers becomes apparent during late fetal stages (after approximately embryonic day 16). However, in the Sox6 null-p(100H) mutant mouse, all fetal muscle fibers maintain slow fiber characteristics, as evidenced by expression of the slow myosin heavy chain MyHC-beta. Knockdown of Sox6 expression in wild-type myotubes results in a significant increase in MyHC-beta expression, supporting our hypothesis. Analysis of the MyHC-beta promoter revealed a Sox consensus sequence that likely functions as a negative cis-regulatory element. Together, our results suggest that Sox6 plays a critical role in the fiber type differentiation of fetal skeletal muscle.

Hedgehog acts directly on the zebrafish dermomyotome to promote myogenic differentiation

Vertebrate myogenesis is regulated by signaling proteins secreted from surrounding tissues. One of the most important, Sonic hedgehog, has been proposed to regulate myogenic precursor cell survival, proliferation, and differentiation in a variety of vertebrates. In zebrafish, Hedgehog signaling is both necessary and sufficient for the development of embryonic slow muscle fibers-the earliest differentiating muscle fibers. Here we investigated the function of Hedgehog signaling in another zebrafish myogenic lineage, a dermomyotomal population of cells defined by somitic pax3/7 expression. We found that Hedgehog negatively regulates the number of myogenic precursors expressing pax3/7. Hh also positively regulates the growth of embryonic fast muscle. Unlike Hedgehog's function in regulating the elongation of fast muscle fibers, this regulation is not mediated by embryonic slow muscle fibers. Instead, it is a direct Hedgehog response, cell autonomous to myogenic precursors. The regulation of myogenic precursors and their differentiation into fast fibers have a different critical time period for Hh signaling, and different requirements for specific gli gene family members of Hh activated transcription factors from the earlier promotion of embryonic slow muscle fiber differentiation. We propose that Hedgehog signaling acts at multiple times on different lineages, through different downstream pathways, to promote myogenic differentiation.

Sonic hedgehog is produced by follicular dendritic cells and protects germinal center B cells from apoptosis

The Hedgehog (Hh) signaling pathway is involved in the development of many tissues during embryogenesis, but has also been described to function in adult self-renewing tissues. In the immune system, Sonic Hedgehog (Shh) regulates intrathymic T cell development and modulates the effector functions of peripheral CD4(+) T cells. In this study we investigate whether Shh signaling is involved in peripheral B cell differentiation in mice. Shh is produced by follicular dendritic cells, mainly in germinal centers (GCs), and GC B cells express both components of the Hh receptor, Patched and Smoothened. Blockade of the Hh signaling pathway reduces the survival, and consequently the proliferation and Ab secretion, of GC B cells. Furthermore, Shh rescues GC B cells from apoptosis induced by Fas ligation. Taken together, our data suggest that Shh is one of the survival signals provided by follicular dendritic cells to prevent apoptosis in GC B cells.

Sonic hedgehog regulates early human thymocyte differentiation by counteracting the IL-7-induced development of CD34+ precursor cells

The Hedgehog (Hh) family of signaling molecules normally functions in the development of numerous tissues by regulating cellular differentiation and proliferation. Recent results have demonstrated that the different components of the Hh signaling pathway are expressed in the human thymus. In this study, we investigate the potential role of Sonic hedgehog (Shh) in human intrathymic T cell maturation. Results show that the expression of the two components of the Hh receptor, Patched and Smoothened, is mostly restricted to CD34+ precursor cells that are committing to the T cell lineage. Shh significantly increased the viability of CD34+ T cell precursors modulating bcl-2 and bax protein expression, and also inhibited their proliferation. The treatment of chimeric human-mouse fetal thymus organ cultures with Shh resulted in an arrested thymocyte differentiation and an accumulation of CD34+ progenitor cells. This effect was mainly attributed to the ability of Shh to counteract the IL-7-induced proliferation and differentiation of CD34+ cells. Shh down-regulated in the precursor cell population the expression of IL-7R as well as stromal-derived factor-1 chemokine receptor, CXCR4, and inhibited IL-7-dependent STAT5 phosphorylation. Therefore, Shh may function as a maintenance factor for intrathymic CD34+ precursor cells.

Hedgehog signaling is required for commitment but not initial induction of slow muscle precursors

In zebrafish, skeletal muscle precursors can adopt at least three distinct fates: fast, non-pioneer slow, or pioneer slow muscle fibers. Slow muscle fibers develop from adaxial cells and depend on Hedgehog signaling. We analyzed when precursors become committed to their fates and the step(s) along their differentiation pathway affected by Hedgehog. Unexpectedly, we find that embryos deficient in Hedgehog signaling still contain postmitotic adaxial cells that differentiate into fast muscle fibers instead of slow. We show that by the onset of gastrulation, slow and fast muscle precursors are already spatially segregated but uncommitted to their fates until much later, in the segmental plate when slow precursors become independent of Hedgehog. In contrast, pioneer and non-pioneer slow muscle precursors share a common lineage from the onset of gastrulation. Our results demonstrate that slow muscle precursors form independently of Hedgehog signaling and further provide direct evidence for a multipotent muscle precursor population whose commitment to the slow fate depends on Hedgehog at a late stage of development when postmitotic adaxial cells differentiate into slow muscle fibers.

Hedgehog regulation of superficial slow muscle fibres inXenopusand the evolution of tetrapod trunk myogenesis

In tetrapod phylogeny, the dramatic modifications of the trunk have received less attention than the more obvious evolution of limbs. In somites,several waves of muscle precursors are induced by signals from nearby tissues. In both amniotes and fish, the earliest myogenesis requires secreted signals from the ventral midline carried by Hedgehog (Hh) proteins. To determine if this similarity represents evolutionary homology, we have examined myogenesis in Xenopus laevis, the major species from which insight into vertebrate mesoderm patterning has been derived. Xenopus embryos form two distinct kinds of muscle cells analogous to the superficial slow and medial fast muscle fibres of zebrafish. As in zebrafish, Hh signalling is required for XMyf5 expression and generation of a first wave of early superficial slow muscle fibres in tail somites. Thus, Hh-dependent adaxial myogenesis is the likely ancestral condition of teleosts, amphibia and amniotes. Our evidence suggests that midline-derived cells migrate to the lateral somite surface and generate superficial slow muscle. This cell re-orientation contributes to the apparent rotation of Xenopussomites. Xenopus myogenesis in the trunk differs from that in the tail. In the trunk, the first wave of superficial slow fibres is missing,suggesting that significant adaptation of the ancestral myogenic programme occurred during tetrapod trunk evolution. Although notochord is required for early medial XMyf5 expression, Hh signalling fails to drive these cells to slow myogenesis. Later, both trunk and tail somites develop a second wave of Hh-independent slow fibres. These fibres probably derive from an outer cell layer expressing the myogenic determination genes XMyf5, XMyoD and Pax3 in a pattern reminiscent of amniote dermomyotome. Thus, Xenopus somites have characteristics in common with both fish and amniotes that shed light on the evolution of somite differentiation. We propose a model for the evolutionary adaptation of myogenesis in the transition from fish to tetrapod trunk.

Hedgehog can drive terminal differentiation of amniote slow skeletal muscle

BACKGROUND

Secreted Hedgehog (Hh) signalling molecules have profound influences on many developing and regenerating tissues. Yet in most vertebrate tissues it is unclear which Hh-responses are the direct result of Hh action on a particular cell type because Hhs frequently elicit secondary signals. In developing skeletal muscle, Hhs promote slow myogenesis in zebrafish and are involved in specification of medial muscle cells in amniote somites. However, the extent to which non-myogenic cells, myoblasts or differentiating myocytes are direct or indirect targets of Hh signalling is not known.

RESULTS

We show that Sonic hedgehog (Shh) can act directly on cultured C2 myoblasts, driving Gli1 expression, myogenin up-regulation and terminal differentiation, even in the presence of growth factors that normally prevent differentiation. Distinct myoblasts respond differently to Shh: in some slow myosin expression is increased, whereas in others Shh simply enhances terminal differentiation. Exposure of chick wing bud cells to Shh in culture increases numbers of both muscle and non-muscle cells, yet simultaneously enhances differentiation of myoblasts. The small proportion of differentiated muscle cells expressing definitive slow myosin can be doubled by Shh. Shh over-expression in chick limb bud reduces muscle mass at early developmental stages while inducing ectopic slow muscle fibre formation. Abundant later-differentiating fibres, however, do not express extra slow myosin. Conversely, Hh loss of function in the limb bud, caused by implanting hybridoma cells expressing a functionally blocking anti-Hh antibody, reduces early slow muscle formation and differentiation, but does not prevent later slow myogenesis. Analysis of Hh knockout mice indicates that Shh promotes early somitic slow myogenesis.

CONCLUSIONS

Taken together, the data show that Hh can have direct pro-differentiative effects on myoblasts and that early-developing muscle requires Hh for normal differentiation and slow myosin expression. We propose a simple model of how direct and indirect effects of Hh regulate early limb myogenesis.

Regulation of myosin expression during myotome formation

The first skeletal muscle fibers to form in vertebrate embryos appear in the somitic myotome. PCR analysis and in situ hybridization with isoform-specific probes reveal differences in the temporal appearance and spatial distribution of fast and slow myosin heavy chain mRNA transcripts within myotomal fibers. Embryonic fast myosin heavy chain was the first isoform expressed, followed rapidly by slow myosin heavy chains 1 and 3, with slow myosin heavy chain 2 appearing several hours later. Neonatal fast myosin heavy chain is not expressed in myotomal fibers. Although transcripts of embryonic fast myosin heavy chain were always distributed throughout the length of myotomal fibers, the mRNA for each slow myosin heavy chain isoform was initially restricted to the centrally located myotomal fiber nuclei. As development proceeded, slow myosin heavy chain transcripts spread throughout the length of myotomal fibers in order of their appearance. Explants of segments from embryos containing neural tube, notochord and somites 7-10, when incubated overnight, become innervated by motor neurons from the neural tube and express all four myosin heavy chain genes. Removal of the neural tube and/or notochord from explants prior to incubation or addition of d-tubocurare to intact explants prevented expression of slow myosin chain 2 but expression of genes encoding the other myosin heavy chain isoforms was unaffected. Thus, expression of slow myosin heavy chain 2 is dependent on functional innervation, whereas expression of embryonic fast and slow myosin heavy chain 1 and 3 are innervation independent. Implantation of sonic-hedgehog-soaked beads in vivo increased the accumulation of both fast and slow myosin heavy chain transcripts, as well as overall myotome size and individual fiber size, but had no effect on myotomal fiber phenotype. Transcripts encoding embryonic fast myosin heavy chain first appear ventrolaterally in the myotome, whereas slow myosin heavy chain transcripts first appear in fibers positioned midway between the ventrolateral and dorsomedial lips of the myotome. Therefore, models of epaxial myotome formation must account for the positioning of the oldest fibers in the more ventral-lateral region of the myotome and the youngest fibers in the dorsomedial region.

Wnt signalling regulates myogenic differentiation in the developing avian wing

The limb musculature arises by delamination of premyogenic cells from the lateral dermomyotome. Initially the cells express Pax3 but, upon entering the limb bud, they switch on the expression of MyoD and Myf5 and undergo terminal differentiation into slow or fast fibres, which have distinct contractile properties that determine how a muscle will function. In the chick, the premyogenic cells express the Wnt antagonist Sfrp2, which is downregulated as the cells differentiate, suggesting that Wnts might regulate myogenic differentiation. Here, we have investigated the role of Wnt signalling during myogenic differentiation in the developing chick wing bud by gain- and loss-of-function studies in vitro and in vivo. We show that Wnt signalling changes the number of fast and/or slow fibres. For example, in vivo, Wnt11 decreases and increases the number of slow and fast fibres, respectively, whereas overexpression of Wnt5a or a dominant-negative Wnt11 protein have the opposite effect. The latter shows that endogenous Wnt11 signalling determines the number of fast and slow myocytes. The distinct effects of Wnt5a and Wnt11 are consistent with their different expression patterns, which correlate with the ultimate distribution of slow and fast fibres in the wing. Overexpression of activated calmodulin kinase II mimics the effect of Wnt5a, suggesting that it uses this pathway. Finally, we show that overexpression of the Wnt antagonist Sfrp2 and DeltaLef1 reduces the number of myocytes. In Sfrp2-infected limbs, the number of Pax3 expressing cells was increased, suggesting that Sfrp2 blocks myogenic differentiation. Therefore, Wnt signalling modulates both the number of terminally differentiated myogenic cells and the intricate slow/fast patterning of the limb musculature.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

Myogenic regulatory factors and the specification of muscle progenitors in vertebrate embryos

Embryological and genetic studies of mouse, bird, zebrafish, and frog embryos are providing new insights into the regulatory functions of the myogenic regulatory factors, MyoD, Myf5, Myogenin, and MRF4, and the transcriptional and signaling mechanisms that control their expression during the specification and differentiation of muscle progenitors. Myf5 and MyoD genes have genetically redundant, but developmentally distinct regulatory functions in the specification and the differentiation of somite and head muscle progenitor lineages. Myogenin and MRF4 have later functions in muscle differentiation, and Pax and Hox genes coordinate the migration and specification of somite progenitors at sites of hypaxial and limb muscle formation in the embryo body. Transcription enhancers that control Myf5 and MyoD activation in muscle progenitors and maintain their expression during muscle differentiation have been identified by transgenic analysis. In epaxial, hypaxial, limb, and head muscle progenitors, Myf5 is controlled by lineage-specific transcription enhancers, providing evidence that multiple mechanisms control progenitor specification at different sites of myogenesis in the embryo. Developmental signaling ligands and their signal transduction effectors function both interactively and independently to control Myf5 and MyoD activation in muscle progenitor lineages, likely through direct regulation of their transcription enhancers. Future investigations of the signaling and transcriptional mechanisms that control Myf5 and MyoD in the muscle progenitor lineages of different vertebrate embryos can be expected to provide a detailed understanding of the developmental and evolutionary mechanisms for anatomical muscles formation in vertebrates. This knowledge will be a foundation for development of stem cell therapies to repair diseased and damaged muscles.

Slow fiber cluster pattern in pig longissimus thoracis muscle: implications for myogenesis

Recent evidence implicates fiber type proportions as playing a role in meat eating quality, and in pigs it has been suggested that the slow oxidative fibers contribute to both juiciness and tenderness. The fiber distribution in pigs is different from that found in most other species, in which the various types of skeletal muscle fiber are distributed in a "checkerboard" pattern, because in pigs the slow oxidative fibers have a clustered distribution. The initial processes leading to fiber clustering are likely to occur during myogenesis, but the precise mechanistic aetiology of this patterning and whether the slow oxidative fiber clusters occur in a random or ordered fashion is unknown. In the present study longissimus thoracis muscle from Large White crossbred pigs was sampled at the 10th rib, 48 h postmortem. Transverse cryo-sections were cut and histochemically stained to allow the identification of the main muscle fiber types: slow oxidative, fast glycolytic, and fast oxidative glycolytic. Images of the sections were captured and analyzed using point processes and Voronoi Tesselations to examine the randomness and spatial distribution of the clusters of slow oxidative fibers found in pig longissimus thoracis muscle. The results showed that an assumption of complete spatial randomness can be rejected and that a mathematical model incorporating a minimum distance of 1.7 to 2.0 microm between cluster centers produced fiber patterns similar to those observed in the original transverse sections of the muscle. In addition, if it assumed that the central fiber in each cluster is derived from primary myoblast progenitors, these results suggest that there may be some degree of repulsion between the primary fibers during the initial stages of cluster formation. The mechanistic basis of such repulsion is not clear, but it is speculated that secreted factors, such as sonic hedgehog or myostatin may play a role.

Skeletal muscle fibre type specification during embryonic development

In the last 10 years an increasing number of studies have provided an insight in the signalling mechanisms underlying myogenesis and fibre type specification during embryonic development: this paper aims to review the most relevant findings. In vertebrates a central role in muscle differentiation is played by the MyoD family, a group of transcription factors which activate transcription of muscle specific genes. In turn MyoD family is expressed in response to inductive signals coming from tissues adjacent to somites, in the first place the notochord and the neural tube. Hedgehog and Wnt are among these inductive signals and they find in the future myoblasts a response pathway which includes Ptc, Smu and Gli. The signalling mechanisms have been analysed in model organisms: mouse, chick. zebrafish and Drosophila. For some factors the orthologs in different species have been found to accomplish similar function, but for some other factors important differences are present: for example in Drosophila twist codes for a transcription factor which promotes myogenesis, whereas its ortholog in mouse tends to prevent or inhibit myogenesis. Conversely, nautilus which is the orholog of MyoD in Drosophila does not have a general function in muscle differentiation, but is required for the differentiation of a limited group of muscle fibres.

Characterization of general transcription factor 3, a transcription factor involved in slow muscle-specific gene expression

General transcription factor 3 (GTF3) binds specifically to the bicoid-like motif of the troponin I(slow) upstream enhancer. This motif is part of a sequence that restricts enhancer activity to slow muscle fibers. GTF3 contains multiple helix-loop-helix domains and an amino-terminal leucine zipper motif. Here we show that helix-loop-helix domain 4 is necessary and sufficient for binding the bicoid-like motif. Moreover, the affinity of this interaction is enhanced upon removal of amino-terminal sequences including domains 1 and 2, suggesting that an unmasking of the DNA binding surface may be a precondition for GTF3 to bind DNA in vivo. We have also investigated the interactions of six GTF3 splice variants of the mouse, three of which were identified in this study, with the troponin enhancer. The gamma-isoform lacking exon 23, and exons 26-28 that encode domain 6, interacted most avidly with the bicoid-like motif; the alpha- and beta- isoforms that include these exons fail to bind in gel retardation assays. We also show that GTF3 polypeptides associate with each other via the leucine zipper. We speculate that cells can generate a large number of GTF3 proteins with distinct DNA binding properties by alternative splicing and combinatorial association of GTF3 polypeptides.

Ventral axial organs regulate expression of myotomal Fgf-8 that influences rib development

Fgf-8 encodes a secreted signaling molecule mediating key roles in embryonic patterning. This study analyzes the expression pattern, regulation, and function of this growth factor in the paraxial mesoderm of the avian embryo. In the mature somite, expression of Fgf-8 is restricted to a subpopulation of myotome cells, comprising most, but not all, epaxial and hypaxial muscle precursors. Following ablation of the notochord and floor plate, Fgf-8 expression is not activated in the somites, in either the epaxial or the hypaxial domain, while ablation of the dorsal neural tube does not affect Fgf-8 expression in paraxial mesoderm. Contrary to the view that hypaxial muscle precursors are independent of regulatory influences from axial structures, these findings provide the first evidence for a regulatory influence of ventral, but not dorsal axial structures on the hypaxial muscle domain. Sonic hedgehog can substitute for the ventral neural tube and notochord in the initiation of Fgf-8 expression in the myotome. It is also shown that Fgf-8 protein leads to an increase in sclerotomal cell proliferation and enhances rib cartilage development in mature somites, whereas inhibition of Fgf signaling by SU 5402 causes deletions in developing ribs. These observations demonstrate: (1) a regulatory influence of the ventral axial organs on the hypaxial muscle compartment; (2) regulation of epaxial and hypaxial expression of Fgf-8 by Sonic hedgehog; and (3) independent regulation of Fgf-8 and MyoD in the hypaxial myotome by ventral axial organs. It is postulated that the notochord and ventral neural tube influence hypaxial expression of Fgf-8 in the myotome and that, in turn, Fgf-8 has a functional role in rib formation.

Slow myosins in muscle development

Myogenesis has been a system central to investigations on mechanisms of diversification within groups of differentiating cells. Diversity among cell types has been well described in striated muscle tissue at the protein and enzymatic-function levels for decades, but it is only in recent years that some understanding of the molecular mechanisms responsible for this diversity has begun to emerge. Study of the expression of the slow isoforms of the myosin heavy chain has contributed to our understanding of how cell diversity arises within skeletal and cardiac muscle. Slow MyHc isoforms are developmentally responsive to a number of cues provided by the nervous systems, the endocrine system and, later in development, to functional demands on these developing tissues. Perhaps most informative have been studies on the mechanism for regulation of slow MyHc expression in mammals and birds where studies on the calcineurin-NF-AT pathways and nuclear hormone action have been shown to control MyHC gene expression in skeletal muscle and in the developing heart. The mechanisms involved in cell diversification in myogenesis are undoubtedly more varied and complex than those currently offered to explain cell diversification, but these recent studies have broadened our understanding of the interplay between the nervous system, the endocrine system and cell lineages in controlling cell diversification. Greater focus on the first fibers and cardiomyocytes to form in the embryo are likely to bring additional insights into the mechanism crucial for establishing the patterns of diversity required for successful formation of embryonic tissues.

Regulation of myogenic differentiation in the developing limb bud

The limb myogenic precursors arise by delamination from the lateral dermomyotome in response to signals from the lateral plate mesoderm. They subsequently migrate into the developing limb bud where they switch on the expression of the myogenic regulatory factors, MyoD and Myf5, and coalese to form the dorsal and ventral muscle masses. The myogenic cells subsequently undergo terminal differentiation into slow or fast fibres which have distinct contractile properties determining how a muscle will function. In general, fast fibres contract rapidly with high force and are characterized by the expression of fast myosin heavy chains (MyHC). These fibres are needed for movement. In contrast, slow fibres express slow MyHC, contract slowly and are required for maintenance of posture. This review focuses on the molecular signals that control limb myogenic development from the initial delamination and migration of the premyogenic cells to the ultimate formation of the complex muscle pattern and differentiation of slow and fast fibres.

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Persistent myogenic capacity of the dermomyotome dorsomedial lip and restriction of myogenic competence

The dorsomedial lip (DML) of the somite dermomyotome is the source of cells for the early growth and morphogenesis of the epaxial primary myotome and the overlying dermomyotome epithelium. We have used quail-chick transplantation to investigate the mechanistic basis for DML activity. The ablated DML of chick wing-level somites was replaced with tissue fragments from various mesoderm regions of quail embryos and their capacity to form myotomal tissue assessed by confocal microscopy. Transplanted fragments from the epithelial sheet region of the dermomyotome exhibited full DML growth and morphogenetic capacity. Ventral somite fragments (sclerotome), head paraxial mesoderm or non-paraxial (lateral plate) mesoderm tested in this assay were each able to expand mitotically in concert with the surrounding paraxial mesoderm, although no myogenic potential was evident. When ablated DMLs were replaced with fragments of the dermomyotome ventrolateral lip of wing-level somites or pre-somitic mesoderm (segmental plate), myotome development was evident but was delayed or otherwise limited in some cases. Timed DML ablation-replacement experiments demonstrate that DML activity is progressive throughout the embryonic period (to at least E7) and its continued presence is necessary for the complete patterning of each myotome segment. The results of serial transplantation and BrdU pulse-chase experiments are most consistent with the conclusion that the DML consists of a self-renewing population of progenitor cells that are the primary source of cells driving the growth and morphogenesis of the myotome and dermomyotome in the epaxial domain of the body.

Embryonic and fetal rat myoblasts form different muscle fiber types in an ectopic in vivo environment

Limb muscle development is characterized by the migration of muscle precursor cells from the somite followed by myoblast differentiation and the maturation of myotubes into distinct muscle fiber types. Previous in vitro experiments have suggested that rat limb myoblasts are composed of at least two distinct myoblast subpopulations that appear in the developing hindlimb at different developmental stages. These embryonic and fetal myoblast subpopulations are believed to generate primary and secondary myotubes, respectively. To test this hypothesis, cells obtained from embryonic day 14 (ED 14) and ED 20 rat hindlimbs were analyzed for myosin heavy chain expression after long-term differentiation in adult rat brains. Fetal myoblasts from ED 20 hindlimbs produced muscle fibers with a phenotype similar to that seen in tissue culture--predominantly fast myosin with a small proportion also coexpressing slow myosin. However, injection sites populated by embryonic myoblasts from ED 14 hindlimbs produced a different phenotype from that previously reported in culture, with fibers expressing an entire array of myosin isoforms. In addition, a subpopulation of fibers expressing exclusively slow myosin was found only in the embryonic injection sites. Our results support the existence of at least three myogenic subpopulations in early rat limb buds with only one exhibiting the capability to differentiate in vitro. These findings are consistent with a model of muscle fiber type development in which the fiber type potential of myoblast populations is established before differentiation into myotubes. This process establishes myogenic subpopulations that have restricted adaptive ranges regulated by both intrinsic and extrinsic factors.

Desert Hedgehog/Patched 1 signaling specifies fetal Leydig cell fate in testis organogenesis

Establishment of the steroid-producing Leydig cell lineage is an event downstream of Sry that is critical for masculinization of mammalian embryos. Neither the origin of fetal Leydig cell precursors nor the signaling pathway that specifies the Leydig cell lineage is known. Based on the sex-specific expression patterns of Desert Hedgehog (Dhh) and its receptor Patched 1 (Ptch1) in XY gonads, we investigated the potential role of DHH/PTCH1 signaling in the origin and specification of fetal Leydig cells. Analysis of Dhh(-/-) XY gonads revealed that differentiation of fetal Leydig cells was severely defective. Defects in Leydig cell differentiation in Dhh(-/-) XY gonads did not result from failure of cell migration from the mesonephros, thought to be a possible source of Leydig cell precursors. Nor did DHH/PTCH1 signaling appear to be involved in the proliferation or survival of fetal Leydig precursors in the interstitium of the XY gonad. Instead, our results suggest that DHH/PTCH1 signaling triggers Leydig cell differentiation by up-regulating Steroidogenic Factor 1 and P450 Side Chain Cleavage enzyme expression in Ptch1-expressing precursor cells located outside testis cords.

Cooperative E-box regulation of human GLI1 by TWIST and USF

Sonic hedgehog signaling plays a critical role in vertebrate patterning, and signaling defects are associated with severe birth defects and cancer in man. GLI1 encodes a critical transcription activator in this pathway. GLI1 is expressed in human basal cell carcinomas and sarcomas. Despite the significance of the GLI1 gene in human disease, few immediate upstream regulators of GLI1 expression are known. We previously demonstrated that a 5' region, including 5' flanking sequence, an untranslated exon, and 425 bp of the first intron, regulates the human GLI1 gene. Here we show that inactivating mutations in E-box, GC box, AP-2, GATA, GSG, PuF, and Zeste sites identified three critical regulatory elements, including a GC box that binds Sp1 and two intronic E-boxes that bind USF proteins or Twist. Expression of Twist but not a frame shift mutation of Twist activates the wild-type human GLI1 regulatory sequences but not with inactivating mutations of the E-boxes. Twist activates GLI1 reporter expression through E-box +482 but requires binding of USF proteins to E-box +157. Twist mutations cause human birth defects and Twist is overexpressed in many rhabdomyosarcomas, suggesting that one of Twist's primary roles is the regulation of GLI1.

Sonic hedgehog inhibits the terminal differentiation of limb myoblasts committed to the slow muscle lineage

The proliferation, differentiation, and fusion of a small number of myogenic precursor cells must be precisely regulated during development to ensure the proper size, organization, and function of the limb musculature. We have examined the role of Sonic hedgehog (Shh) in these processes by both augmentation and inhibition of the Shh-mediated signaling pathway. Our data show that Shh regulates muscle development by repressing the terminal differentiation of early myogenic precursor cells and does not function as a myoblast mitogen. Shh function in hypaxial muscle appears to be spatially restricted to the early myoblast population within the ventral muscles of the posterior region of the limb. Furthermore, Shh appears to act as a permissive, rather than an inductive, signal for slow MyHC expression in myoblasts. Our data thus provide the foundation for a new hypothesis for Shh function in hypaxial skeletal muscle development.

Patterns of gene expression during Drosophila mesoderm development

The transcription factor Twist initiates Drosophila mesoderm development, resulting in the formation of heart, somatic muscle, and other cell types. Using a Drosophila embryo sorter, we isolated enough homozygous twist mutant embryos to perform DNA microarray experiments. Transcription profiles of twist loss-of-function embryos, embryos with ubiquitous twist expression, and wild-type embryos were compared at different developmental stages. The results implicate hundreds of genes, many with vertebrate homologs, in stage-specific processes in mesoderm development. One such gene, gleeful, related to the vertebrate Gli genes, is essential for somatic muscle development and sufficient to cause neural cells to express a muscle marker.

Hedgehog signalling is required for maintenance of myf5 and myoD expression and timely terminal differentiation in zebrafish adaxial myogenesis

Hedgehog proteins have been implicated in the control of myogenesis in the medial vertebrate somite. In the mouse, normal epaxial expression of the myogenic transcription factor gene myf5 is dependent on Sonic hedgehog. Here we examine in zebrafish the interaction between Hedgehog signals, the expression of myoD family genes, including the newly cloned zebrafish myf5, and slow myogenesis. We show that Sonic hedgehog is necessary for normal expression of both myf5 and myoD in adaxial slow muscle precursors, but not in lateral paraxial mesoderm. Expression of both genes is initiated normally in rostral presomitic mesoderm in sonic you mutants, which lack all Sonic hedgehog. Similar initiation continues during tailbud outgrowth when the cells forming caudal somites are generated. However, adaxial cells in sonic you embryos are delayed in terminal differentiation and caudal adaxial cells fail to maintain myogenic regulatory factor expression. Despite these defects, other signals are able to maintain, or reinitiate, some slow muscle development in sonic you mutants. In the cyclops mutant, the absence of floorplate-derived Tiggywinkle hedgehog and Sonic hedgehog has no discernible effect on slow adaxial myogenesis. Similarly, the absence of notochord-derived Sonic hedgehog and Echidna hedgehog in mutants lacking notochord delays, but does not prevent, adaxial slow muscle development. In contrast, removal of both Sonic hedgehog and a floorplate signal, probably Tiggywinkle hedgehog, from the embryonic midline in cyclops;sonic you double mutants essentially abolishes slow myogenesis. We conclude that several midline signals, likely to be various Hedgehogs, collaborate to maintain adaxial slow myogenesis in the zebrafish embryo. Moreover, the data demonstrate that, in the absence of this required Hedgehog signalling, expression of myf5 and myoD is insufficient to commit cells to adaxial myogenesis.

Mechanisms for the development of esophageal atresia

BACKGROUND

There is no universally accepted theory to explain esophageal embryology and the abnormal development that produces esophageal atresia.

METHODS

The impact of Adriamycin administration on the pathogenesis of esophageal atresia was studied in the rat model of VATER association, from embryonic day (ED) 10 to ED 13.

RESULTS

Tissues in the ED10 Adriamycin-exposed embryos displayed less cell proliferation as shown by the reduced population of MIB-5-labelled cells. Cell apoptosis that is characteristic of the normal ED 12 lateral epithelial folds of the foregut (the prospective site of tracheoesophageal septation) was absent in the foregut of the Adriamycin-exposed embryo. Histologic examination of the ED 11-exposed embryo showed the presence of abnormal notochord that was stretched, split, or tethered to the foregut. This contrasts with the normal embryo in which the notochord was localized in close vicinity of the ventral part of the neural tube and separated from the foregut by ample amount of mesenchyme. The abnormal localization of the notochord was accompanied by the lack of down-regulation of the sonic hedgehog (Shh) activity in the prospective site of future tracheoesophageal separation in the exposed ED 12 embryo.

CONCLUSION

The authors proposed that the ectopic location of the notochord leads to the disruption in Shh signalling that may underpin the development of esophageal atresia.

Patterning of fast and slow fibers within embryonic muscles is established independently of signals from the surrounding mesenchyme

During embryonic development, and before functional innervation, a highly stereotypic pattern of slow- and fast-contracting primary muscle fibers is established within individual muscles of the limbs, from distinct populations of myoblasts. A difference between the fiber-type pattern found within chicken and quail pectoral muscles was exploited to investigate the contributions of somite-derived myogenic precursors and lateral plate-derived mesenchymal stroma to the establishment of muscle fiber-type patterns. Chimeric chicken/quail embryos were constructed by reciprocal transplantation of somites or lateral plate mesoderm at stages prior to muscle formation. Muscle fibers derived from quail myogenic precursors that had migrated into chicken stroma showed a quail pattern of mixed fast- and slow-contracting muscle fibers. Conversely, chicken myogenic precursors that had migrated into quail stroma showed a chicken pattern of nearly exclusive fast muscle fiber formation. These results demonstrate in vivo an intrinsic commitment to fiber-type on the part of the myoblast, independent of extrinsic signals it receives from the mesenchymal stroma in which it differentiates.

The u-boot mutation identifies a Hedgehog-regulated myogenic switch for fiber-type diversification in the zebrafish embryo

Developmental programs that govern the embryonic diversification of distinct kinds of muscles in vertebrates remain obscure. For instance, the most widely recognized attribute of early diversity among skeletal myoblasts is their ability to differentiate exclusively into fibers with slow or fast contractile properties. However, we know little about the developmental basis and genetic regulation of this seminal event in vertebrate myogenesis. Here we show that in the zebrafish, the u-boot gene acts as a myogenic switch that regulates the choice of myoblasts to adopt slow versus fast fiber developmental pathways. In u-boot mutant embryos, slow muscle precursors abort their developmental program, failing to activate expression of the homeobox gene prox1 and transfating into muscle cells with fast fiber properties. Using oligonucleotide-mediated translational inhibition, we have investigated the role of prox1 in this program. We find that it functions in the terminal step of the u-boot controlled slow fiber developmental pathway in the regulation of slow myofibril assembly. Our findings provide new insight into the genetic control of slow versus fast fiber specification and differentiation and indicate that dedicated developmental pathways exist in vertebrates for the elaboration of distinct elements of embryonic muscle pattern.

Regulatory elements governing transcription in specialized myofiber subtypes

Skeletal myofibers of vertebrates acquire specialized metabolic and physiological properties as a consequence of developmental cues in the embryo and different patterns of contractile activity in the adult. The myoglobin gene is regulated stringently in muscle fibers, such that high myoglobin expression is observed in mitochondria-rich, oxidative myofibers (Types I and IIa) compared with glycolytic fibers (Type IIb). Using germ-line transgenesis and somatic cell gene transfer methods, we defined discrete regions of the murine and human genes encoding myoglobin that are sufficient to confer muscle- and fiber type-specific expression to reporter genes. Mutational analysis confirms the importance of A/T-rich, MEF2-binding motifs in myoglobin gene regulation, as suggested by previous studies using different experimental approaches. In addition, we demonstrated a previously unsuspected role for an intragenic E-box motif as a negative regulatory element contributing to the tightly regulated variation in myoglobin gene expression among particular myofiber subtypes.

Gli2 mediation of hedgehog signals in slow muscle induction in zebrafish

Zebrafish skeletal muscles are composed of two major types of muscle fibers, broadly classified as fast or slow fibers. Recent studies have demonstrated that members of the Hedgehog (Hh) family induce the formation of slow muscle fibers. Hedgehog signals are secreted proteins that function through the transcription factor Glis. We report here the characterization of a zebrafish Gli2 expression in slow and fast muscle cells and the study of the roles of Hedgehogs and Gli2 in zebrafish muscle development using two mutant strains; sonic-you (syu) and you-too (yot), respective for sonic hedgehog (shh) and Gli2 mutation. We have demonstrated that Shh and Gli2 mutation causes similar defects in slow muscle formation. There is, however, a difference in the degree of defect between these two mutants. In yot mutant embryos, development of slow muscles was completely blocked, whereas in syu mutant embryos, a small number of slow muscle cells could still form, suggesting that other Hhs were also involved in slow muscle induction. Induction of slow muscles by other Hhs appeared to require Gli2, because ectopic expression of Echidna hedgehog (Ehh) and Tiggy-winkle hedgehog (Twhh) failed to induce slow muscles in yot mutant embryos. Together, these data suggest that further Hhs, other than Shh, are also involved in the induction and differentiation of slow muscle cells and that Gli2 is required by Shh, Twhh, and Ehh, thus playing a key role in the induction and differentiation of slow muscle cells.

Compartmentalization of the somite and myogenesis in chick embryos are influenced by wnt expression

Muscles of the body and bones of the axial skeleton derive from specialized regions of somites. Somite development is influenced by adjacent structures. In particular, the dorsal neural tube and the overlying ectoderm have been shown to be necessary for the induction of myogenic precursor cells in the dermomyotome. Members of the Wnt family of signaling molecules, which are expressed in the dorsal neural tube and the ectoderm, are postulated to be responsible for this process. It is shown here that ectopically implanted Wnt-1-, -3a-, and -4-expressing cells alter the process of somite compartmentalization in vivo. An enlarged dorsal compartment results from the implantation of Wnt-expressing cells ventrally between the neural tube/notochord and epithelial somites, at the expense of the ventral compartment, the sclerotome. Thus, ectopic Wnt expression is able to override the influence of ventralizing signals arising from notochord and floor plate. This shift of the border between the two compartments was identified by an increase in the domain of Pax-3 expression and a complete loss of Pax-1 expression in somites close to the ectopic Wnt signal. The expanded expression of MyoD and desmin provides evidence that it is the myotome which increases as a result of Wnt signaling. Paraxis expression is also drastically amplified after implantation of Wnt-expressing cells indicating that Wnts are involved in the formation and maintenance of somite epithelium and suggesting that Paraxis is activated through Wnt signaling pathways. Taken together these results suggest that ectopic Wnts disturb the normal balance of signaling molecules within the somite, resulting in an enhanced recruitment of somitic cells into the myogenic lineage.

Molecular and cellular biology of avian somite development

Much of our understanding of early vertebrate embryogenesis derives from experimental work done with the chick embryo. Studies of the avian somite have played a key role in elucidating the developmental history of this important structure, the source of most muscle and bone in the organism. Here we review the development of the avian somite including morphological and molecular data on the origin of paraxial mesoderm, maturation of the segmental plate, specification and formation of somite compartments, and somite cell differentiation into cartilage and skeletal muscle.

IGFs, insulin, Shh, bFGF, and TGF-beta1 interact synergistically to promote somite myogenesis in vitro

Studies from our group and others have shown that in vitro somite myogenesis is regulated by neural tube and notochord factors including Wnt, Sonic hedgehog (Shh), and basic fibroblast growth factor (bFGF) together with transforming growth factor-beta1 (TGF-beta1). In this study we report that insulin and insulin-like growth factors I and II (IGF-I and -II) also promote myogenesis in explant cultures containing single somites or somite-sized pieces of segmental plate mesoderm from 2-day (stage 10-14) chicken embryos and that the combination of insulin/IGFs with bFGF plus TGF-beta1 promotes even higher levels of myogenesis. We also found that Shh promotes myogenesis in this in vitro system and that Shh interacts synergistically with insulin/IGFs to promote high levels of myogenesis. RT-PCR analysis detected insulin, IGF-II, insulin receptor, and IGF receptor mRNAs in both the neural tube and the somites, whereas IGF-I transcripts were detected in entire embryos but not in the neural tube or somites. Treatment of somite-neural tube cocultures with anti-insulin, anti-IGF-II, anti-insulin receptor, or anti-IGF receptor blocking antibodies caused a significant decrease in myogenesis. These results are consistent with the hypothesis that systemic IGF-I as well as insulin and IGF-II secreted by the neural tube act as additional early myogenic signals during embryogenesis. Further studies indicate that insulin, IGFs, bFGF, and Shh also stimulate somite cell proliferation and influence apoptosis.

SF/HGF is a mediator between limb patterning and muscle development

Scatter factor/hepatocyte growth factor (SF/HGF) is known to be involved in the detachment of myogenic precursor cells from the lateral dermomyotomes and their subsequent migration into the newly formed limb buds. As yet, however, nothing has been known about the role of the persistent expression of SF/HGF in the limb bud mesenchyme during later stages of limb bud development. To test for a potential role of SF/HGF in early limb muscle patterning, we examined the regulation of SF/HGF expression in the limb bud as well as the influence of SF/HGF on direction control of myogenic precursor cells in limb bud mesenchyme. We demonstrate that SF/HGF expression is controlled by signals involved in limb bud patterning. In the absence of an apical ectodermal ridge (AER), no expression of SF/HGF in the limb bud is observed. However, FGF-2 application can rescue SF/HGF expression. Excision of the zone of polarizing activity (ZPA) results in ectopic and enhanced SF/HGF expression in the posterior limb bud mesenchyme. We could identify BMP-2 as a potential inhibitor of SF/HGF expression in the posterior limb bud mesenchyme. We further demonstrate that ZPA excision results in a shift of Pax-3-positive cells towards the posterior limb bud mesenchyme, indicating a role of the ZPA in positioning of the premuscle masses. Moreover, we present evidence that, in the limb bud mesenchyme, SF/HGF increases the motility of myogenic precursor cells and has a role in maintaining their undifferentiated state during migration. We present a model for a crucial role of SF/HGF during migration and early patterning of muscle precursor cells in the vertebrate limb.