Widespread expression of MyoD genes in Xenopus embryos is amplified in presumptive muscle as a delayed response to mesoderm induction

Nodal and churchill1 position the expression of a notch ligand during Xenopus germ layer segregation

Churchill and Nodal signaling, which participate in vertebrates’ germ layer induction, position a domain of Delta/Notch activity, which refines germ layer boundaries during frog gastrulation.

In vertebrates, Nodal signaling plays a major role in endomesoderm induction, but germ layer delimitation is poorly understood. In avian embryos, the neural/mesoderm boundary is controlled by the transcription factor CHURCHILL1, presumably through the repressor ZEB2, but there is scarce knowledge about its role in other vertebrates. During amphibian gastrulation, Delta/Notch signaling refines germ layer boundaries in the marginal zone, but it is unknown the place this pathway occupies in the network comprising Churchill1 and Nodal. Here, we show that Xenopus churchill1 is expressed in the presumptive neuroectoderm at mid-blastula transition and during gastrulation, upregulates zeb2, prevents dll1 expression in the neuroectoderm, and favors neuroectoderm over endomesoderm development. Nodal signaling prevents dll1 expression in the endoderm but induces it in the presumptive mesoderm, from where it activates Notch1 and its target gene hes4 in the non-involuting marginal zone. We propose a model where Nodal and Churchill1 position Dll1/Notch1/Hes4 domains in the marginal zone, ensuring the delimitation between mesoderm and neuroectoderm.

Making muscle: Morphogenetic movements and molecular mechanisms of myogenesis in Xenopus laevis

Xenopus laevis offers unprecedented access to the intricacies of muscle development. The large, robust embryos make it ideal for manipulations at both the tissue and molecular level. In particular, this model system can be used to fate map early muscle progenitors, visualize cell behaviors associated with somitogenesis, and examine the role of signaling pathways that underlie induction, specification, and differentiation of muscle. Several characteristics that are unique to X. laevis include myogenic waves with distinct gene expression profiles and the late formation of dermomyotome and sclerotome. Furthermore, myogenesis in the metamorphosing frog is biphasic, facilitating regeneration studies. In this review, we describe the morphogenetic movements that shape the somites and discuss signaling and transcriptional regulation during muscle development and regeneration. With recent advances in gene editing tools, X. laevis remains a premier model organism for dissecting the complex mechanisms underlying the specification, cell behaviors, and formation of the musculature system.

Agricultural applications for transgenic livestock

Transgenic animals are produced by introducing 'foreign' DNA into the genetic material of pre-implantation embryos. This DNA is present in all tissues of the resulting individual. This technique is of great importance to many aspects of biomedical science, including gene regulation, the immune system, cancer research, developmental biology, biomedicine, manufacturing and agriculture. The production of transgenic animals is one of several new and developing technologies that will have a profound impact on the genetic improvement of livestock. The rate at which these technologies are incorporated into production schemes will determine the speed at which we will be able to achieve our goal of more efficiently producing livestock that meets consumer and market demand.

FGF8 spliceforms mediate early mesoderm and posterior neural tissue formation in Xenopus

The relative contributions of different FGF ligands and spliceforms to mesodermal and neural patterning in Xenopus have not been determined, and alternative splicing, though common, is a relatively unexplored area in development. We present evidence that FGF8 performs a dual role in X. laevis and X. tropicalis early development. There are two FGF8 spliceforms, FGF8a and FGF8b, which have very different activities. FGF8b is a potent mesoderm inducer, while FGF8a has little effect on the development of mesoderm. When mammalian FGF8 spliceforms are analyzed in X. laevis, the contrast in activity is conserved. Using a loss-of-function approach, we demonstrate that FGF8 is necessary for proper gastrulation and formation of mesoderm and that FGF8b is the predominant FGF8 spliceform involved in early mesoderm development in Xenopus. Furthermore, FGF8 signaling is necessary for proper posterior neural formation; loss of either FGF8a or a reduction in both FGF8a and FGF8b causes a reduction in the hindbrain and spinal cord domains.

Transgenic animals in biomedicine and agriculture: outlook for the future

Transgenic animals are produced by introduction of 'foreign' deoxyribonucleic acid (DNA) into preimplantation embryos. The foreign DNA is inserted into the genetic material and may be expressed in tissues of the resulting individual. This technique is of great importance to many aspects of biomedical science including gene regulation, the immune system, cancer research, developmental biology, biomedicine, manufacturing and agriculture. The production of transgenic animals is one of a number of new and developing technologies that will have a profound impact on the genetic improvement of livestock. The rate at which these technologies are incorporated into production schemes will determine the speed at which we will be able to achieve our goal of more efficiently producing livestock, which meets consumer and market demand.

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.

Xenopus muscle development: from primary to secondary myogenesis

Xenopus myogenesis is characterized by specific features, different from those of mammalian and avian systems both at the cellular level and in gene expression patterns. During early embryogenesis, after the initial molecular signals inducing mesoderm, the myogenic determination factors XMyoD and XMyf-5 are activated in presomitic mesoderm in response to mesoderm-inducing factors. After these first inductions of the myogenic program, forming muscles in Xenopus can have different destinies, some of these resulting in cell death before adulthood. In particular, it is quite characteristic of this species that, during metamorphosis, the primary myotomal myofibers completely die and are progressively replaced by secondary "adult" multinucleated myofibers. This feature offers the unique opportunity to totally separate the molecular analysis of these two distinct types of myogenesis. The aim of this review is to summarize our knowledge on the cellular and molecular events as well as the epigenetic regulations involved in the construction of Xenopus muscles during development.

A single cdk inhibitor, p27Xic1, functions beyond cell cycle regulation to promote muscle differentiation in Xenopus

The molecular basis of the antagonism between cellular proliferation and differentiation is poorly understood. We have investigated the role of the cyclin-dependent kinase inhibitor p27(Xic1) in the co-ordination of cell cycle exit and differentiation during early myogenesis in vivo using Xenopus embryos. In this report, we demonstrate that p27(Xic1) is highly expressed in the developing myotome, that ablation of p27(Xic1) protein prevents muscle differentiation and that p27(Xic1) synergizes with the transcription factor MyoD to promote muscle differentiation. Furthermore, the ability of p27(Xic1) to promote myogenesis resides in an N-terminal domain and is separable from its cell cycle regulation function. This data demonstrates that a single cyclin-dependent kinase inhibitor, p27(Xic1), controls in vivo muscle differentiation in Xenopus and that regulation of this process by p27(Xic1) requires activities beyond cell cycle inhibition.

Zygotic Wnt/beta-catenin signaling preferentially regulates the expression of Myf5 gene in the mesoderm of Xenopus

Zygotic Wnt signaling has been shown to be involved in dorsoventral mesodermal patterning in Xenopus embryos, but how it regulates different myogenic gene expression in the lateral mesodermal domains is not clear. Here, we use transient exposure of embryos or explants to lithium, which mimics Wnt/beta-catenin signaling, as a tool to regulate the activation of this pathway at different times and places during early development. We show that activation of Wnt/beta-catenin signaling at the early gastrula stage rapidly induces ectopic expression of XMyf5 in both the dorsal and ventral mesoderm. In situ hybridization analysis reveals that the induction of ectopic XMyf5 expression in the dorsal mesoderm occurs within 45 min and is not blocked by the protein synthesis inhibitor cycloheximide. By contrast, the induction of XMyoD is observed after 2 h of lithium treatment and the normal expression pattern of XMyoD is blocked by cycloheximide. Analysis by RT-PCR of ectodermal explants isolated soon after midblastula transition indicates that lithium also specifically induces XMyf5 expression, which takes place 30 min following lithium treatment and is not blocked by cycloheximide, arguing strongly for an immediate-early response. In the early gastrula, inhibition of Wnt/beta-catenin signaling blocks the expression of XMyf5 and XMyoD, but not of Xbra. We further show that zygotic Wnt/beta-catenin signaling interacts specifically with bFGF and eFGF to promote XMyf5 expression in ectodermal cells. These results suggest that Wnt/beta-catenin pathway is required for regulating myogenic gene expression in the presumptive mesoderm. In particular, it may directly activate the expression of the XMyf5 gene in the muscle precursor cells.

eFGF is required for activation of XmyoD expression in the myogenic cell lineage of Xenopus laevis

This paper addresses the molecular mechanisms that regulate the transcriptional activation of the myogenic regulatory factor XmyoD in the skeletal muscle lineage of Xenopus laevis. Using antisense morpholino oligonucleotide-mediated inhibition, we show that the signalling molecule embryonic fibroblast growth factor (eFGF), which is the amphibian homologue of FGF4, is necessary for the initial activation of XmyoD transcription in myogenic cells. We demonstrate that eFGF can activate the expression of XmyoD in the absence of protein synthesis, indicating that this regulation is direct. Our data suggest that regulation of XmyoD expression may involve a labile transcriptional repressor. In addition, we show that eFGF is itself an immediate early response to activin, a molecule that mimics the endogenous mesoderm-inducing signal. We propose a model for the regulation of XmyoD within the early mesoderm, and discuss the relevance that these findings have for the understanding of myogenic specification in higher vertebrates.

MAP kinase converts MyoD into an instructive muscle differentiation factor in Xenopus

In amphibian development, muscle is specified in the dorsal lateral marginal zone (DLMZ) of the gastrula embryo. Two critical events specify the formation of skeletal muscle: the expression of the myogenic transcription factor, XMyoD, and the secretion of bone morphogenetic protein (BMP) antagonists by the adjacent Spemann organizer. Inhibition of BMP signaling during early gastrula stages converts XMyoD protein into an instructive differentiation factor in the DLMZ. Yet, the intracellular signaling factors connecting BMP antagonism and activation of XMyoD remain unknown. Our data show that BMP antagonism induces the activity of mitogen-activated protein kinase (MAPK), and that the activity of MAPK is necessary for muscle-specific differentiation. Treatment of gastrula-stage DLMZ explants with MAPK pathway inhibitors ventralized mesoderm and prevented muscle differentiation. Expression of XMyoD in ventral mesoderm weakly induced muscle formation; however, the coexpression of a constitutively active MEK1 with XMyoD efficiently induced muscle differentiation. Activation of the MAPK pathway did not induce the transcription of XMyoD, but increased its protein levels and transcriptional activity. Thus, MAPK activation is subsequent to BMP antagonism, and participates in the dorsalization of mesoderm by converting the XMyoD protein into a potent differentiation factor.

Transgenic technology and applications in swine

The introduction of foreign DNA into the genome of livestock and its stable integration into the germ line has been a major technical advance in agriculture. Production of transgenic livestock provides a method to rapidly introduce "new" genes into cattle, swine, sheep and goats without crossbreeding. It is a more extreme methodology, but in essence, not really different from crossbreeding or genetic selection in its result. Several recent developments will profoundly impact the use of transgenic technology in livestock production. These developments are: 1) the ability to isolate and maintain in vitro embryonic stem (ES) cells from preimplantation embryos, embryonic germ (EG) and somatic cells from fetuses; and somatic cells from adults, and 2) the ability to use these embryonic and somatic cells as nuclei donors in nuclear transfer or "cloning" strategies. Cell based (ES, EG, and somatic cells) strategies have several distinct advantages for use in the production of transgenic livestock that cannot be attained using pronuclear injection of DNA. There are many potential applications of transgenic methodology to develop new and improved strains of livestock. Practical applications of transgenesis in livestock production include enhanced prolificacy and reproductive performance, increased feed utilization and growth rate, improved carcass composition, improved milk production and/or composition and increased disease resistance. Development of transgenic farm animals will allow more flexibility in direct genetic manipulation of livestock.

Temporal restriction of MyoD induction and autocatalysis during Xenopus mesoderm formation

In Xenopus, the activation of the myogenic determination factors MyoD and Myf-5 in the muscle-forming region of the embryo occurs in response to mesoderm-inducing factors (MIFs). Different members of the FGF, TGF-beta, and Wnt protein families have been implicated in this process, but how MIFs induce the myogenic regulators is not known. For MyoD, the induction process may serve to locally stabilize a transient burst of ubiquitous transcription at the midblastula transition, possibly by triggering MyoD's autocatalytic loop. Here we have sought to distinguish separate activating functions during MyoD induction by analyzing when MyoD responds to different MIF signaling or to MyoD autoactivation. We show that MyoD induction depends on the developmental age of the induced cells, rather than on the type or time point of inducer application. At the permissive time, de novo MyoD induction by Activin requires less than 90 min, arguing for an immediate response, rather than a series of inductive events. MyoD autoactivation is direct, but subject to the same temporal restriction as MyoD induction by MIF signaling. Further evidence implicating MyoD autocatalysis as an essential component of the induction process comes from the observation that both autocatalysis and induction of MyoD are selectively repressed by a dominant-negative MyoD mutant. In summary, our observations let us conclude that MyoD's expression domain in the embryo results from an interplay of timed changes in cellular competence, pleiotropic signaling pathways, and autocatalysis.

Ion channels and early development of neural cells

In this review we underscore the merits of using voltage-dependent ion channels as markers for neuronal differentiation from the early stages of uncommitted embryonic blastomeres. Furthermore, a fairly large part of the review is devoted to the descriptions of the establishment of a simple model system for neural induction derived from the cleavage-arrested eight-cell ascidian embryo by pairing a single ectodermal with a single vegetal blastomere as a competent and an inducer cell, respectively. The descriptions are focused particularly on the early developmental processes of various ion channels in neuronal and other excitable membranes observed in this extraordinarily simple system, and we compare these results with those in other significant and definable systems for neural differentiation. It is stressed that this simple system, for which most of the electronic and optical methods and various injection experiments are applicable, may be useful for future molecular physiological studies on the intracellular process of differentiation of the early embryonic cells. We have also highlighted the importance of suppressive mechanisms for cellular differentiation from the experimental results, such as epidermal commitment of the cleavage-arrested one-cell Halocynthia embryos or suppression of epidermal-specific transcription of inward rectifier channels by neural induction signals. It was suggested that reciprocal suppressive mechanisms at the transcriptional level may be one of the key processes for cellular differentiation, by which exclusivity of cell types is maintained.

I-mf, a novel myogenic repressor, interacts with members of the MyoD family

During embryogenesis, cells from the ventral and dorsal parts of the somites give rise to sclerotome and dermomyotome, respectively. Dermomyotome contains skeletal muscle precursors that are determined by the MyoD family of myogenic factors. We have isolated a novel myogenic repressor, I-mf (Inhibitor of MyoD family), which is highly expressed in the sclerotome. In contrast, MyoD family members are concentrated in the dermomyotome. We demonstrate that I-mf inhibits the transactivation activity of the MyoD family and represses myogenesis. I-mf associates with MyoD family members and retains them in the cytoplasm by masking their nuclear localization signals. I-mf can also interfere with the DNA binding activity of MyoD family members. We postulate that I-mf plays an important role in the patterning of the somite early in development.

FGF is a prospective competence factor for early activin-type signals in Xenopus mesoderm induction

Normal pattern formation during embryonic development requires the regulation of cellular competence to respond to inductive signals. In the Xenopus blastula, vegetal cells release mesoderm-inducing factors but themselves become endoderm, suggesting that vegetal cells may be prevented from expressing mesodermal genes in response to the signals that they secrete. We show here that addition of low levels of basic fibroblast growth factor (bFGF) induces the ectopic expression of the mesodermal markers Xbra, MyoD and muscle actin in vegetal explants, even though vegetal cells express low levels of the FGF receptor. Activin, a potent mesoderm-inducing agent in explanted ectoderm (animal explants), does not induce ectopic expression of these markers in vegetal explants. However, activin-type signaling is present in vegetal cells, since the vegetal expression of Mix.1 and goosecoid is inhibited by the truncated activin receptor. These results, together with the observation that FGF is required for mesoderm induction by activin, support our proposal that a maternal FGF acts at the equator as a competence factor, permitting equatorial cells to express mesoderm in response to an activin-type signal. The overlap of FGF and activin-type signaling is proposed to restrict mesoderm to the equatorial region.

Cellular aggregation enhances MyoD-directed skeletal myogenesis in embryonal carcinoma cells

When introduced into P19 embryonal carcinoma cells, recombinant genes encoding MyoD converted only a small percentage (< 3%) of the transfected cells into skeletal muscle. We isolated stably transfected cells that expressed the MyoD transcript. These P19[MyoD] cells continued to express markers characteristic of undifferentiated stem cells but also expressed myf-5 and the myotonic dystrophy kinase, transcripts normally present in myoblasts but absent from P19 cells. Aggregation of P19[MyoD] cells induced the expression of myogenin, desmin, and the retinoblastoma protein and resulted in the rapid and abundant development of skeletal muscle. Both the embryonic and the slow isoforms of myosin heavy chain were present in this muscle, indicating that it resembled skeletal muscle formed from primary myoblasts. Since aggregation of P19 cells normally results in inefficient differentiation and the development of only low levels of cardiac muscle but no skeletal muscle, we conclude that MyoD imposes the skeletal muscle program on P19 cells and that the differentiation of these cells requires inductive events provided by cell aggregation.

Cellular aggregation enhances MyoD-directed skeletal myogenesis in embryonal carcinoma cells

When introduced into P19 embryonal carcinoma cells, recombinant genes encoding MyoD converted only a small percentage (< 3%) of the transfected cells into skeletal muscle. We isolated stably transfected cells that expressed the MyoD transcript. These P19[MyoD] cells continued to express markers characteristic of undifferentiated stem cells but also expressed myf-5 and the myotonic dystrophy kinase, transcripts normally present in myoblasts but absent from P19 cells. Aggregation of P19[MyoD] cells induced the expression of myogenin, desmin, and the retinoblastoma protein and resulted in the rapid and abundant development of skeletal muscle. Both the embryonic and the slow isoforms of myosin heavy chain were present in this muscle, indicating that it resembled skeletal muscle formed from primary myoblasts. Since aggregation of P19 cells normally results in inefficient differentiation and the development of only low levels of cardiac muscle but no skeletal muscle, we conclude that MyoD imposes the skeletal muscle program on P19 cells and that the differentiation of these cells requires inductive events provided by cell aggregation.

Control of somitic expression of tenascin in Xenopus embryos by myogenic factors and Brachyury

Tenascin is a large glycoprotein which is expressed in a restricted pattern in the extracellular matrix (ECM) of vertebrate embryos. Tenascin interferes with cell-fibronectin interactions in vitro, and may play a role in the control of cell migration and differentiation during development. In Xenopus, tenascin immunoreactivity is first detected at the early tailbud stage in the ECM of the most anterior somite. Thereafter, it is distributed dorsally along neural crest cell migration pathways. In this paper, we report that tenascin mRNA is most abundant in dorsal mesoderm at the neurula stage and in somites at the early tailbud stage, indicating that the initial accumulation of tenascin in the ECM is due to secretion from paraxial mesoderm. To understand how tenascin expression in somitic mesoderm is controlled, we have expressed Xbra and the myogenic factors XMyoD and XMyf5 in blastula animal cap tissue. The tenascin gene is activated by all three transcription factors. Interestingly, expression of tenascin mRNA, and accumulation of the protein in the ECM, can occur without formation of muscle. Our results suggest that tenascin regionalization in early Xenopus embryos depends on tenascin RNA expression by somitic mesoderm, where it is likely to be activated by myogenic factors.

The RSRF/MEF2 protein SL1 regulates cardiac muscle-specific transcription of a myosin light-chain gene in Xenopus embryos

We have examined the role of two RSRF/MEF2 proteins in the onset of skeletal and cardiac muscle differentiation in early Xenopus embryos. In normal development, zygotic expression of SL1 (MEF2D) precedes that of SL2 (MEF2A) by several hours, but neither gene is expressed prior to the accumulation of MyoD and Myf5 transcripts in the somitic mesoderm. Ectopic expression of the myogenic factors in explants of presumptive ectoderm induces expression of both SL1 and SL2, whereas in reciprocal experiments, neither RSRF protein activates the endogenous myoD or Myf5 genes. We conclude that SL1 and SL2 lie downstream of these myogenic factors in the skeletal myogenic pathway. SL1 is distinguished from SL2 in being expressed in the presumptive heart region of the early tailbud embryo, prior to detection of any markers for cardiac muscle differentiation. Furthermore, ectopic SL1 induces the expression of an endogenous cardiac muscle-specific myosin light-chain (XMLC2) gene in cultured blastula animal pole explants, whereas SL2 has no comparable effect. These results demonstrate that in addition to a possible role in skeletal myogenesis, SL1 also acts in vivo as a regulator of cardiac muscle-specific transcription.

The MyoD binding site is dispensable for cardiac actin gene expression in the somites of later stage Xenopus embryos

We have studied the transcription directed by the promoter of a Xenopus borealis cardiac actin gene fused to a globin reporter sequence after injection into X. laevis embryos. This promoter is known to be effective specifically in somites of neurula stage embryos, and work on a very similar X. laevis gene has indicated the importance of an upstream MyoD binding site (E box) in this process. We show that, although in the absence of the MyoD site there is a small amount of tissue-specific expression in neurulae, transcription is abundant in the tailbud embryo a few hours later. Thus, although the E box is initially essential, other sequences must adopt the same role soon afterwards.

Induction of the Xenopus organizer: expression and regulation of Xnot, a novel FGF and activin-regulated homeo box gene

We have searched for homeo box-containing genes expressed during gastrulation in Xenopus embryos with the expectation that analysis of the spatial and temporal expression of these genes will lead to greater understanding of the regionalization of the mesoderm. We describe the cloning and expression of Xnot, a novel homeo box-containing gene expressed primarily in the gastrula organizing region. We have studied the regulation of Xnot by signaling molecules involved in mesoderm induction and regionalization. Surprisingly, we found that FGF signaling is required for expression of Xnot in the gastrula organizing region, clearly implicating FGF in the induction of dorsal mesoderm. Furthermore, we found that Xnot is initially expressed throughout the embryo and that progressive translation of an unknown protein restricts expression of Xnot to the organizing region. Our results provide experimental evidence supporting the proposed division of Spemann's organizer into independently regulated organizing centers.