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PubMed1992

MyoD protein expression in Xenopus embryos closely follows a mesoderm induction-dependent amplification of MyoD transcription and is synchronous across the future somite axis.

The MyoD-related genes code for key regulators of skeletal muscle commitment and differentiation. In this study, expression of MyoD protein has been examined during Xenopus development. Protein is first detected in presumptive mesoderm at early gastrulation, directly following a dramatic increase in MyoD transcription that occurs in response to mesoderm induction. The pattern of expression resembles the muscle fate map at this time. Protein accumulates synchronously along the future somite axis, with no evidence of a spatial regulation which would explain the anterior/posterior wave of myogenic differentiation that follows MyoD expression. During gastrulation, the highest levels of MyoD are in cells next to the developing notochord, suggesting a role for the notochord in induction or maintenance of MyoD expression. After muscle differentiation, MyoD protein is degraded with a half-life of several hours, leading to very low expression in mature somites. These studies support a role for MyoD in induction of muscle mesoderm, but also point to the multi-layered regulation of these events.

Animals↗
PubMed2005

Eya 1 acts as a critical regulator for specifying the metanephric mesenchyme.

Although it is well established that the Gdnf-Ret signal transduction pathway initiates metanephric induction, no single regulator has yet been identified to specify the metanephric mesenchyme or blastema within the intermediate mesoderm, the earliest step of metanephric kidney development and the molecular mechanisms controlling Gdnf expression are essentially unknown. Previous studies have shown that a loss of Eya 1 function leads to renal agenesis that is a likely result of failure of metanephric induction. The studies presented here demonstrate that Eya 1 specifies the metanephric blastema within the intermediate mesoderm at the caudal end of the nephrogenic cord. In contrast to its specific roles in metanephric development, Eya 1 appears dispensable for the formation of nephric duct and mesonephric tubules. Using a combination of null and hypomorphic Eya 1 mutants, we now demonstrated that approximately 20% of normal Eya 1 protein level is sufficient for establishing the metanephric blastema and inducing the ureteric bud formation but not for its normal branching. Using Eya 1, Gdnf, Six 1 and Pax 2 mutant mice, we show that Eya 1 probably functions at the top of the genetic hierarchy controlling kidney organogenesis and it acts in combination with Six 1 and Pax 2 to regulate Gdnf expression during UB outgrowth and branching. These findings uncover an essential function for Eya 1 as a critical determination factor in acquiring metanephric fate within the intermediate mesoderm and as a key regulator of Gdnf expression during ureteric induction and branching morphogenesis.

Alleles↗
PubMed1999

Neural induction. A bird's eye view.

Since the discovery of the phenomenon of neural induction by Spemann and Mangold in 1924, considerable effort has been invested in identifying the signals produced by the organizer that are responsible for diverting the fate of cells from epidermal to neural. Substantial progress has been made only recently by the finding in amphibians that BMP4 is a neural inhibitor and epidermal inducer, and that endogenous antagonists of BMPs are secreted by the organizer. However, recent results in the chick point to the existence of other, upstream events required before BMP inhibition stabilizes neural fates. Here we take a critical view of the evidence for and against the view that BMP inhibition is a sufficient trigger for neural induction in different vertebrates.

Animals↗
PubMed2004

SLIT2-mediated ROBO2 signaling restricts kidney induction to a single site.

Kidney development occurs in a stereotypic position along the body axis. It begins when a single ureteric bud emerges from the nephric duct in response to GDNF secreted by the adjacent nephrogenic mesenchyme. Posterior restriction of Gdnf expression is considered critical for correct positioning of ureteric bud development. Here we show that mouse mutants lacking either SLIT2 or its receptor ROBO2, molecules known primarily for their function in axon guidance and cell migration, develop supernumerary ureteric buds that remain inappropriately connected to the nephric duct, and that the SLIT2/ROBO2 signal is transduced in the nephrogenic mesenchyme. Furthermore, we show that Gdnf expression is inappropriately maintained in anterior nephrogenic mesenchyme in these mutants. Thus our data identify an intercellular signaling system that restricts, directly or indirectly, the extent of the Gdnf expression domain, thereby precisely positioning the site of kidney induction.

Animals↗
PubMed1998

FGF-mediated mesoderm induction involves the Src-family kinase Laloo.

During embryogenesis, inductive interactions underlie the development of much of the body plan. In Xenopus laevis, factors secreted from the vegetal pole induce mesoderm in the adjacent marginal zone; members of both the transforming growth factor-beta (TGF-beta) and fibroblast growth factor (FGF) ligand families seem to have critical roles in this process. Here we report the identification and characterization of laloo, a novel participant in the signal transduction cascade linking extracellular, mesoderm-inducing signals to the nucleus, where alteration of cell fate is driven by changes in gene expression. Overexpression of laloo, a member of the Src-related gene family, in Xenopus embryos gives rise to ectopic posterior structures that frequently contain axial tissue. Laloo induces mesoderm in Xenopus ectodermal explants; this induction is blocked by reagents that disrupt the FGF signalling pathway. Conversely, expression of a dominant-inhibitory Laloo mutant blocks mesoderm induction by FGF and causes severe posterior truncations in vivo. This work provides the first evidence that a Src-related kinase is involved in vertebrate mesoderm induction.

Amino Acid Sequence↗
PubMed1998

Gli/Zic factors pattern the neural plate by defining domains of cell differentiation.

Three cell types differentiate in the early frog neural plate: neural crest at the lateral edges, floorplate at the midline and primary neurons in three bilateral stripes. Floorplate cells and ventral neurons are induced by Sonic hedgehog (Shh) and neural crest and dorsal neurons are induced by epidermal factors such as bone morphogenetic proteins (BMPs). Neurogenesis in a subset of cells within the stripes involves lateral inhibition. However, the process by which pools of precursors are defined in stereotypic domains in response to inductive signals is unknown. Here we show that frog Zic2 encodes a zinc-finger transcription factor of the Gli superfamily which is expressed in stripes that alternate with those in which primary neurons differentiate and overlap the domains of floorplate and neural crest progenitors. Zic2 inhibits neurogenesis and induces neural crest differentiation. Conversely, Gli proteins are widely expressed, induce neurogenesis and inhibit neural crest differentiation. Zic2 is therefore a vertebrate pre-pattern gene, encoding anti-neurogenic and crest-inducing functions that counteract the neurogenic but not the floorplate-inducing activity of Gli proteins. We propose that the combined function of Gli/Zic genes responds to inductive signals and induces patterned neural cell differentiation.

Animals↗
PubMed1998

Expression of the mouse cerberus-related gene, Cerr1, suggests a role in anterior neural induction and somitogenesis.

The Xenopus cerberus gene encodes a secreted factor that is expressed in the anterior endomesoderm of gastrula stage embryos and can induce the formation of ectopic heads when its mRNA is injected into Xenopus embryos [Bouwmeester, T., Kim, S., Lu, B. & De Robertis, E. M. (1996) Nature (London) 382, 595-601]. Here we describe the existence of a cerberus-related gene, Cerr1, in the mouse. Cerr1 encodes a putative secreted protein that is 48% identical to cerberus over a 110-amino acid region. Analysis of a mouse interspecific backcross panel demonstrated that Cerr1 mapped to the central portion of mouse chromosome 4. In early gastrula stage mouse embryos, Cerr1 is expressed in the anterior visceral endoderm and in the anterior definitive endoderm. In somite stage embryos, Cerr1 expression is restricted to the most recently formed somites and in the anterior presomitic mesoderm. Germ layer explant recombination assays demonstrated that Cerr1-expressing somitic-presomitic mesoderm, but not older Cerr1-nonexpressing somitic mesoderm, was able to mimic the anterior neuralizing ability of anterior mesendoderm and maintain Otx2 expression in competent ectoderm. In most Lim1-/- headless embryos, Cerr1 expression in the anterior endoderm was weak or absent. These results suggest that Cerr1 may play a role in anterior neural induction and somite formation during mouse development.

Amino Acid Sequence↗
PubMed1998

The relationships between notochord and floor plate in vertebrate development revisited.

By using the quail-chicken chimera system, we have previously shown that during development of the spinal cord, floor plate cells are inserted between neural progenitors giving rise to the alar plates. These cells are derived from the regressing Hensen's node or cordoneural hinge (HN-CNH). This common population of HN-CNH cells gives rise to three types of midline descendants: notochord, floor plate, and dorsal endoderm. Here we find that HNF3beta, an important gene in the development of the midline structures, is continuously expressed in the HN-CNH cells and their derivatives, floor plate, notochord, and dorsal endoderm. Experiments in which the notochord was removed in the posterior region of either normal chicken or of quail-chicken chimeras in which a quail HN had been grafted showed that the floor plate develops in a cell-autonomous manner in the absence of notochord. Absence of floor plate observed at the posterior level of the excision results from removal of HN-CNH material, including the future floor plate, and not from the lack of an inductive signal of notochord origin.

Animals↗
PubMed1995

Myogenesis in paraxial mesoderm: preferential induction by dorsal neural tube and by cells expressing Wnt-1.

Previous studies have demonstrated that the neural tube/notochord complex is required for skeletal muscle development within somites. In order to explore the localization of myogenic inducing signals within the neural tube, dorsal or ventral neural tube halves were cultured in contact with single somites or pieces of segmental plate mesoderm. Somites and segmental plates cultured with the dorsal half of the neural tube exhibited 70% and 85% myogenic response rates, as determined by immunostaining for myosin heavy chain. This response was slightly lower than the 100% response to whole neural tube/notochord, but was much greater than the 30% and 10% myogenic response to ventral neural tube with and without notochord. These results demonstrate that the dorsal neural tube emits a potent myogenic inducing signal which accounts for most of the inductive activity of whole neural tube/notochord. However, a role for ventral neural tube/notochord in somite myogenic induction was clearly evident from the larger number of myogenic cells induced when both dorsal neural tube and ventral neural tube/notochord were present. To address the role of a specific dorsal neural tube factor in somite myogenic induction, we tested the ability of Wnt-1-expressing fibroblasts to promote paraxial mesoderm myogenesis in vitro. We found that cells expressing Wnt-1 induced a small number of somite and segmental plate cells to undergo myogenesis. This finding is consistent with the localized dorsal neural tube inductive activity described above, but since the ventral neural tube/notochord also possesses myogenic inductive capacity yet does not express Wnt-1, additional inductive factors are likely involved.

Animals↗
PubMed1999

[Guidance of commissural axons in the neural tube--related to the induction and differentiation of ventral neurons].

The floor plate cells, located at the ventral midline of the neural tube, secrete diffusible factors such as sonic hedgehog (shh) and netrin-1. It is known that shh induces motor neurons at the basal plate, and that netrin-1 contributes to axon pathfinding of spinal commissural neurons located at the dorsal spinal cord. This review focuses on and discusses the roles of signaling molecules expressed at the floor plate, the rostral diencephalic ventral midline cells, and their underlying tissues; the notochord and the prechordal mesoderm in the induction and differentiation of ventral neurons, and in the guidance of commissural axons.

Animals↗
PubMed1991

Gene activation in the amphibian mesoderm.

Cell potency is progressively restricted in amphibian development by a series of cellular interactions called inductions. The mesoderm is believed to develop in response to the earliest known induction, in which vegetal cells of the blastula divert overlying animal hemisphere cells away from epidermal and towards mesodermal fates. We describe two early markers of mesodermal differentiation in Xenopus laevis, both mRNAs that encode DNA-binding proteins of the helix-loop-helix family. One is a frog homologue of MyoD, a gene that in transfection experiments can convert cultured fibroblasts into myoblasts. Xenopus MyoD (XMyoD) is expressed in the early myotomes, from which the axial musculature develops. The accumulation of XMyoD RNA precedes that of transcripts from the cardiac actin gene, until now the earliest known marker of the muscle lineage, this result indicating that XMyoD could play a role in initiating muscle differentiation in normal development. We show by microinjection of synthetic RNA that XMyoD can indeed activate muscle-specific gene expression in animal cap cells, which would normally form only ectoderm. However, the XMyoD-injected animal caps did not produce differentiated muscle, suggesting that additional specific factors are required for full myogenesis. The other mRNA is a relative of the twist gene of Drosophila, which is required for mesodermal differentiation in flies. This gene (Xtwi) is expressed widely in the early frog mesoderm, but not, however, in the myotomes, where XMyoD is expressed. Later, the Xtwi gene is activated, in response to a second induction, in the developing neural crest.

Animals↗
PubMed2001

Dickkopf1 and the Spemann-Mangold head organizer.

Work in amphibians indicates that inhibition of Wnt and BMP signals is essential for head development and that head induction by the Spemann-Mangold organizer may be mediated by secreted Wnt antagonists. Wnts are potent posteriorizing factors and antagonize the Spemann-Mangold organizer. Dickkopf1 (dkk1) encodes a secreted effector expressed in head organizing centers of Xenopus, mouse and zebrafish. It acts as a Wnt inhibitor and is able together with BMP inhibitors to induce the formation of ectopic embryonic heads in Xenopus. It anteriorizes both mesendoderm and neuroectoderm, promoting prechordal plate and forebrain fates. Injection of inhibitory antibodies leads to microcephaly and cyclopia. Dkk1 thus is an essential mediator of the vertebrate head organizer.

Animals↗
PubMed1998

Specification of the hindbrain fate in the zebrafish.

We determine the timing of neural commitment by hindbrain tissue in the zebrafish using microsurgical transplantation. When transplanted at shield stage to the ventral side of the embryo, presumptive hindbrain cells are not committed, as they can adapt to their environment and give rise to epidermis. In contrast, when transplanted at 80% epiboly, hindbrain cells retain their neural fate and express neural-specific antigens. Moreover, they are able to maintain regional fate, as is evident by the expression of the hindbrain-specific marker, Krox20. In addition, we observe that committed hindbrain tissues are able to induce presumptive ventral epidermis to form neural crest derivatives, otic vesicles, and neural tissues. We propose that hindbrain progenitors have acquired regional identity as a group at 80% epiboly even before making vertical contact with axial mesoderm. These results suggest that planar induction may constitute a significant component in the zebrafish neural patterning pathway.

Animals↗
PubMed1997

An essay on the similarities and differences between inductive interactions in anuran and urodele embryos.

As a first step towards providing a conceptual approach to understanding similarities and differences in the mechanisms which guide inductive interactions among related organisms (e.g. various amphibia), a set of five principles is offered here. These principles were formulated by analyzing literature examples of classical embryological phenomena and by performing experiments with activin, a peptide growth factor which is currently suspected to play for a role in mesoderm induction. Mechanisms which account, at least in part, for the observed differences between anuran and urodele inductive processes can be derived from these principles.

Animals↗
PubMed1994

Neural induction in Xenopus.

Although induction of neural tissue in vertebrates has been recognized since the experiments of Spemann and Mangold seventy years ago, only recently has the phenomenon been put onto a molecular footing. Three molecules, noggin, follistatin and fibroblast growth factor, have been shown to have neuralizing activity in various assays. These assays for neural induction and the molecular mechanisms for the action of neural inducers are reviewed here.

Activins↗
PubMed2004

The two steps of vulval induction in Oscheius tipulae CEW1 recruit common regulators including a MEK kinase.

The cell interactions that specify the spatial pattern of vulval precursor cell (VPC) fates differ between the nematodes Oscheius tipulae CEW1 and Caenorhabditis elegans. In the former, the centered pattern of fates is obtained by two successive inductions from the gonadal anchor cell, whereas in the latter, a single inductive step by the anchor cell (EGF-Ras-MAP kinase pathway) can act as a morphogen and is reinforced by lateral signaling between the vulval precursors (Notch pathway). We performed a genetic screen for vulva mutants in O. tipulae CEW1. Here we present the mutants that specifically affect the vulval induction mechanisms. Phenotypic and epistatic analyses of these mutants show that both vulval induction steps share common components, one of which appears to be MEK kinase(s). Moreover, the inductive pathway (including MEK kinase) influences the competence of the vulval precursor cells and more strikingly their division pattern as well, irrespective of their vulval fate. Finally, a comparison of vulval mutant phenotypes obtained in C. elegans and O. tipulae CEW1 highlights the evolution of vulval induction mechanisms between the two species.

Animals↗
PubMed2003

Bcl-2 mediates induction of neural differentiation.

Bcl-2 is an antiapoptotic protein expressed in a wide variety of cell types. We have found that overexpression of bcl-2 in PC12 neural crest tumor cells leads to increased expression of neural differentiation-associated genes and decreased expression of proliferation-related genes. Culture growth rate decreases as well. Overexpression of bcl-2 also leads to increased expression of TrkA and increased phosphorylation of signal transductants in, albeit not specific for, the TrkA-MEK-ERK pathway. Blocking of NGF-mediated signaling through TrkA prevents Bcl-2-associated expression changes in differentiation-associated genes, raising the possibility that Bcl-2 mediates induction of neural differentiation through TrkA/NGF signaling.

Animals↗