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PubMed2003

Fgf3 and Fgf10 are required for mouse otic placode induction.

The inner ear, which contains the sensory organs specialised for audition and balance, develops from an ectodermal placode adjacent to the developing hindbrain. Tissue grafting and recombination experiments suggest that placodal development is directed by signals arising from the underlying mesoderm and adjacent neurectoderm. In mice, Fgf3 is expressed in the neurectoderm prior to and concomitant with placode induction and otic vesicle formation, but its absence affects only the later stages of otic vesicle morphogenesis. We show here that mouse Fgf10 is expressed in the mesenchyme underlying the prospective otic placode. Embryos lacking both Fgf3 and Fgf10 fail to form otic vesicles and have aberrant patterns of otic marker gene expression, suggesting that FGF signals are required for otic placode induction and that these signals emanate from both the hindbrain and mesenchyme. These signals are likely to act directly on the ectoderm, as double mutant embryos showed normal patterns of gene expression in the hindbrain. Cell proliferation and survival were not markedly affected in double mutant embryos, suggesting that the major role of FGF signals in otic induction is to establish normal patterns of gene expression in the prospective placode. Finally, examination of embryos carrying three out of the four mutant Fgf alleles revealed intermediate phenotypes, suggesting a quantitative requirement for FGF signalling in otic vesicle formation.

Animals↗

PubMed1995

An inductive role for the endoderm in Xenopus cardiogenesis.

Heart induction in Xenopus has been thought to be dependent primarily on the interaction of the heart primordia with the Spemann organizer. We demonstrate, however, that signals derived from the deep dorsoanterior endoderm during early gastrulation are also essential for heart formation. The presence of deep endoderm dramatically enhances heart formation in explants of heart primordia, both in the presence and absence of organizer. Likewise, extirpation of the entire endoderm can decrease the frequency of heart formation in embryos that retain organizer activity. Finally, we show that the combined presence of both endoderm and organizer is necessary and sufficient to induce heart in ventral mesoderm explants that would not otherwise form heart tissue. Xenopus heart induction, therefore, may be a multistep process requiring separate dorsalization and cardiogenic signalling events. This is the first demonstration of a heart-inducing role for the endoderm in Xenopus, indicating that the mechanism of heart formation may be similar in most vertebrates.

Animals↗

PubMed1998

Early posterior neural tissue is induced by FGF in the chick embryo.

Signals that induce neural cell fate in amniote embryos emanate from a unique cell population found at the anterior end of the primitive streak. Cells in this region express a number of fibroblast growth factors (FGFs), a group of secreted proteins implicated in the induction and patterning of neural tissue in the amphibian embryo. Here we exploit the large size and accessibility of the early chick embryo to analyse the function of FGF signalling specifically during neural induction. Our results demonstrate that extraembryonic epiblast cells previously shown to be responsive to endogenous neural-inducing signals express early posterior neural genes in response to local, physiological levels of FGF signal. This neural tissue does not express anterior neural markers or undergo neuronal differentiation and forms in the absence of axial mesoderm. Prospective mesodermal tissue is, however, induced and we present evidence for both the direct and indirect action of FGFs on prospective posterior neural tissue. These findings suggest that FGF signalling underlies a specific aspect of neural induction, the initiation of the programme that leads to the generation of the posterior central nervous system.

Animals↗

PubMed2006

sli-3 negatively regulates the LET-23/epidermal growth factor receptor-mediated vulval induction pathway in Caenorhabditis elegans.

The LIN-3-LET-23-mediated inductive signaling pathway plays a major role during vulval development in C. elegans. Studies on the components of this pathway have revealed positive as well as negative regulators that function to modulate the strength and specificity of the signal transduction cascade. We have carried out genetic screens to identify new regulators of this pathway by screening for suppressors of lin-3 vulvaless phenotype. The screens recovered three loci including alleles of gap-1 and a new gene represented by sli-3. Our genetic epistasis experiments suggest that sli-3 functions either downstream or in parallel to nuclear factors lin-1 and sur-2. sli-3 synergistically interacts with the previously identified negative regulators of the let-23 signaling pathway and causes excessive cell proliferation. However, in the absence of any other mutation sli-3 mutant animals display wild-type vulval induction and morphology. We propose that sli-3 functions as a negative regulator of vulval induction and defines a branch of the inductive signaling pathway. We provide evidence that sli-3 interacts with the EGF signaling pathway components during vulval induction but not during viability and ovulation processes. Thus, sli-3 helps define specificity of the EGF signaling to induce the vulva.

Alleles↗

PubMed1987

Induction of differentiation of murine embryonal carcinoma cells by ouabain.

Murine embryonal carcinoma cells (EC) can be induced to differentiate by a variety of chemical agents, including retinoid acid (RA) and dimethyl acetamide (DMA). However, it is not known how these agents exert their effects. In this study we demonstrate that murine EC cells can also be induced to differentiate by ouabain at concentrations which inhibit Na+, K+-ATPase activity as measured by inhibition of 86Rb+ uptake. Since the pharmacologic action of ouabain is thought to be specific, we investigated the role of Na+, K+-ATPase inhibition and specific metabolic consequences of this inhibition in the induction of EC differentiation, and explored whether this might be a common mode of action for a variety of structurally diverse inducers. Although the Na+, K+-ATPase maintains ion gradients in cells, our studies failed to demonstrate a consistent role for alterations of ion flux or ion concentration on the differentiation process. Ouabain inhibited cell growth, but a direct correlation between the degree of growth inhibition and the extent of differentiation could not be demonstrated. There was also no evidence that RA or DMA induces differentiation by inhibiting the Na+, K+-ATPase. The mechanism of ouabain induction may be mediated by some alternative consequence of Na+, K+-ATPase inhibition, but it appears to be specific for that inducer and cannot be generalized to that of other inducers of EC differentiation.

Animals↗

PubMed1999

Spatial and temporal properties of ventral blood island induction in Xenopus laevis.

Questions of dorsoventral axis determination and patterning in Xenopus seek to uncover the mechanisms by which particular mesodermal fates, for example somite, are specified in the dorsal pole of the axis while other mesoderm fates, for example, ventral blood island (VBI), are specified at the ventral pole. We report here that the genes Xvent-1, Xvent-2, and Xwnt-8 do not appear to be in the pathway of VBI induction, contrary to previous reports. Results from the selective inhibition of bone morphogenetic protein (BMP) activity, a key regulator of VBI induction, by ectopic Noggin, Chordin, or dominant negative BMP ligands and receptors suggest an alternative route of VBI induction. Injection of noggin or chordin RNA into animal pole blastomeres effectively inhibited VBI development, while marginal zone injection had no effect. Cell autonomous inhibition of BMP activity in epidermis with dominant negative ligand dramatically reduced the amount of (&agr;)T3 globin expression. These results indicate that signaling activity from the Spemann Organizer alone may not be sufficient for dorsoventral patterning in the marginal zone and that an inductive interaction between presumptive VBIs and ectoderm late in gastrulation may be crucial. In agreement with these observations, other results show that in explanted blastula-stage marginal zones a distinct pattern develops with a restricted VBI-forming region at the vegetal pole that is independent of the patterning activity of the Spemann Organizer.

Actins↗

PubMed1995

Inductive signals. Revolving vertebrates.

An old idea about the relationship between arthropod and vertebrate body plans has been given new life by studies of the signalling genes controlling dorsal and ventral development in Drosophila and Xenopus.

Amino Acid Sequence↗

PubMed1997

Neural induction and antero-posterior patterning in the amphibian embryo: past, present and future.

Neural induction and patterning in competent ectoderm occurs during gastrula and early neurula stages in response to signals from dorsal mesoderm. The earliest views of antero-posterior (A-P) patterning were modified beginning in the 1930s, as complexities concerning the timing of the pattern-forming process and potential sources of the patterning signals were revealed. In the 1950s and 1960s several different models for A-P patterning were proposed, all of which, however, bear a number of similarities, including a two-component system for generating A-P axial information in the embryo. Early attempts to identify neural-inducing molecules were largely unsuccessful due to technical limitations in biochemical analyses and concerns about assaying neural responses. The advent of modern molecular genetic technology has permitted more precise tests of a number of classic observations about the timing of A-P patterning and the sources of patterning signals. While some early observations have been confirmed, a number of new concepts have emerged in recent years, particularly concerning the source of patterning signals in the embryo. Striking progress has been made in identifying putative neural-inducing molecules, and recent experiments have begun to suggest how these might contribute to A-P patterning. While the successes in recent years have been revealing, many of the classic issues concerning neural induction and patterning remain essentially as they were when first defined many decades ago. The power of modern molecular genetics, however, should permit many of these issues to be significantly clarified in the decades to come.

Amphibians↗

PubMed1991

Induction in the developing compound eye of Drosophila: multiple mechanisms restrict R7 induction to a single retinal precursor cell.

The development of the Drosophila R7 photoreceptor cell is determined by a specific inductive interaction between the R8 photoreceptor cell and a single neighboring precursor cell. This process is mediated by bride of sevenless (boss), a cell-surface bound ligand, and the sevenless (sev) tyrosine kinase receptor. The boss ligand is expressed specifically on the surface of the R8 cell, whereas the sev receptor is expressed on 5 cells contacting the developing R8 cell and other cells not in contact with R8. By altering the spatial and temporal expression of boss, we demonstrate that sev-expressing cells that do not contact R8 can assume an R7 cell fate. By contrast, the sev-expressing precursor cells to the R1-R6 photoreceptor cells that do contact R8 are nonresponsive to the inductive cue. Using the rough and Nspl mutations, we demonstrate that an early commitment to an R1-R6 cell fate blocks the pathway of sev activation in these cells.

Animals↗

PubMed2005

Correlation of a chicken stage 4 neural plate fate map with early gene expression patterns.

A number of gene markers are currently claimed to allow positive or negative visualization of the early chick neural plate at stages 3d/4, when its fate becomes determined. Some markers labeled by various authors as either "neural" or "non-neural" indeed show ectodermal expression patterns roughly correlative with widespread yet vague ideas on the shape and size of the early neural plate, based on previous fate maps. However, for technical reasons, it is not clear how precisely these expression patterns correlate with any experimentally determined fate boundaries. An eventual mismatch between fate and marker interpretation might bear importantly on ideas about gene functions and causal hypotheses in issues such as the establishment of the neural/non-neural border or the earliest mechanisms of neural regionalization. In this review, we correlated a set of epiblastic and mesendodermal gene expression patterns with the novel neuroectoderm proportions suggested by our recent fate map of the chick neural plate at stages HH 3d/4 [P. Fernández-Garre, L. Rodriguez-Gallardo, V. Gallego-Diaz, I.S. Alvarez, L. Puelles, Fate map of the chicken neural plate at stage 4, Development 129 (2002) 2807-2822.]. This analysis suggests the existence of various nested subregions of the epiblast with boundaries codefined by given sets of gene patterns. No gene expression studied reproduces exactly or even approximately the entire neural plate shape, leading to a combinatorial hypothesis on its specification. This kind of analysis (fate and molecular maps), jointly with competence maps, provides the basis for understanding gene functions and the mechanisms of neural induction, specification and regionalization. Several gene patterns observed are consistent with precocious incipient regionalization of the neural plate along the dorsoventral and anteroposterior axes.

Animals↗

PubMed1995

Limb-patterning activity and restricted posterior localization of the amino-terminal product of Sonic hedgehog cleavage.

BACKGROUND: Sonic hedgehog (Shh), a vertebrate homolog of the Drosophila segment polarity gene hedgehog (hh), has been implicated in patterning of the developing chick limb. Such a role is suggested by the restricted expression of Shh along the posterior limb bud margin, and by the observation that heterologous cells expressing Shh have limb-polarizing activity resembling that of cells from the polarizing region of the posterior limb bud margin. It has not been demonstrated, however, that the Sonic hedgehog protein (SHH) alone is sufficient for limb patterning. SHH has been shown to undergo autoproteolytic cleavage in vitro, yielding two smaller products. It is of interest, therefore, to determine whether processing of SHH occurs in the developing limb and how such processing influences the function of SHH. RESULTS: We demonstrate that SHH is proteolytically processed in developing chick limbs. Grafts of cells expressing SHH protein variants that correspond to individual cleavage products demonstrate that the ability to induce patterned gene expression and to impose morphological pattern upon the limb bud is limited to the amino-terminal product (SHH-N) of SHH proteolytic cleavage. We also demonstrate that bacterially synthesized and purified SHH-N, released from implanted beads, is sufficient for limb-patterning activity. Finally, we show that the endogenous amino-terminal cleavage product is tightly localized to the posterior margin of the limb bud. CONCLUSIONS: Our data show that, of the two cleavage products resulting from SHH autoproteolysis, SHH-N expressed in grafted heterologous cells or supplied in purified form is sufficient to impose pattern upon the developing limb. Moreover, the restricted localization of the endogenous amino-terminal SHH cleavage product to the posterior border of the chick limb bud makes it unlikely that its patterning activity results from it being distributed in a broad gradient across the antero-posterior axis. More consistent with the observed localization is a model in which the amino-terminal SHH cleavage product exerts its patterning effects by local induction in or near the polarizing region, initiating a cascade of gene expression that ultimately extends across the developing limb.

Animals↗

PubMed2000

Involvement of BMP-4/msx-1 and FGF pathways in neural induction in the Xenopus embryo.

The msx homeodomain protein is a downstream transcription factor of the bone morphogenetic protein (BMP)-4 signal and a key regulator for neural tissue differentiation. Xmsx-1 antagonizes the dorsal expression of noggin and cerberus, as revealed by in situ hybridization and reverse transcription-polymerase chain reaction assays. In animal cap explants, Xmsx-1 and BMP-4 inhibit the neural tissue differentiation induced by noggin or cerberus. A loss-of-function study using the Xmsx-1/VP-16 fusion construct indicated that neural tissue formation was directly induced by the injection of fusion ribonucleic acid, although the expression of neural cell adhesion molecule (N-CAM) in the cap was less than that in the cap injected with tBR or noggin. In contrast to the single cap assay, unexpectedly, both BMP-4 and Xmsx-1 failed to inhibit neurulation in the ectodermal explants to which the organizer mesoderm was attached. The results of cell-lineage tracing experiments indicated that the neural cells were differentiated from the animal pole tissue where the excess RNA of either BMP-4 or Xmsx-1 was injected, whereas notochord was differentiated from the organizer mesoderm. Neural tissue differentiated from BMP-4-injected ectodermal cells strongly expressed posterior neural markers, such as hoxB9 and krox20, suggesting that the posterior neural cells differentiated regardless of the existence of the BMP signal. The introduction of a dominant-negative form of the fibroblast growth factor (FGF) receptor (XFD) into the ectodermal cells drastically reduced the expression of pan and posterior neural markers (N-CAM and hoxB-9) if co-injected with BMP-4 RNA, although XFD alone at the same dose did not shut down the expression of N-CAM in the combination explants. Therefore, it is proposed that an FGF-related molecule was involved in the direct induction of posterior neural tissue in the inducing signals from the organizer mesoderm in vivo.

Animals↗

PubMed1995

lin-25, a gene required for vulval induction in Caenorhabditis elegans.

During vulval development in the Caenorhabditis elegans hermaphrodite, the fates of six vulval precursor cells (VPCs) are influenced by distinct cell signaling events. In one event, a somatic gonadal cell, the anchor cell, induces the three nearest VPCs to adopt vulval cell fates. In another event, lateral signaling between adjacent VPCs specifies one of two different vulval fates, 1 degrees and 2 degrees. Induction of vulval fates by the anchor cell is mediated by a signal transduction pathway involving let-60 Ras, lin-45 Raf, and mpk-1/sur-1 MAP kinase, whereas lateral signaling is mediated by lin-12. We have shown that the mutant phenotype of lin-25, a gene required for VPC fate specification, results from a defect in vulval induction. Genetic epistasis experiments indicate that lin-25 is required in the inductive signaling pathway downstream of let-60 Ras and the Raf/MAP kinase cascade. A decrease in induction also appears to decrease lateral signaling. We have cloned and sequenced the lin-25 gene and shown that it encodes a novel protein that may be a target of the mpk-1/sur-1 MAPK.

Amino Acid Sequence↗

PubMed1997

The genetic control of early tooth development.

Most vertebrate organs begin their initial formation by a common, developmentally conserved pattern of inductive tissue interactions between two tissues. The developing tooth germ is a prototype for such inductive tissue interactions and provides a powerful experimental system for elucidation of the genetic pathways involved in organogenesis. Members of the Msx homeobox gene family are expressed at sites of epithelial-mesenchymal interaction during embryogenesis, including the tooth. The important role that Msx genes play in tooth development is exemplified by mice lacking Msx gene function. Msxl-deficient mice exhibit an arrest in tooth development at the bud stage, while Msx2-deficient mice exhibit late defects in tooth development. The co-expression of Msx, Bmp, Lefl, and Activin beta A genes and the coincidence of tooth phenotypes in the various knockout mice suggest that these genes reside within a common genetic pathway. Results summarized here indicate that Msxl is required for the transmission of Bmp4 expression from dental epithelium to mesenchyme and also for Lefl expression. In addition, we consider the role of other signaling molecules in the epithelial-mesenchymal interactions leading to tooth formation, the role that transcription factors such as Msx play in the propagation of inductive signals, and the role of extracellular matrix. Last, as a unifying mechanism to explain the disparate tooth phenotypes in Msxl- and Msx2-deficient mice, we propose that later steps in tooth morphogenesis molecularly resemble those in early tooth development.

Ameloblasts↗

PubMed2003

Concentration dependence of inductive activity in the mixture of lens epithelium proteins.

As shown elsewhere, the mixture of proteins secreted by lens epithelium cells in the process of microcultivation can selectively induce eye and forebrain tissues in the early gastrula ectoderm (Zemchikhina et al., 2000, 2003). In the present work, the dependence of inductive activity of this protein mixture on its concentration in culture solution has been studied. The test-system was the early gastrula ectoderm of Xenopus laevis frogs. The results of the experiments revealed no direct dependence of the spectrum of induced tissues on the concentration of the protein mixture. At a concentration of 0.5 mg/ml, brain appeared being accompanied by retina, pigmented epithelium, and lentoids, while at 0.031 mg/ml a perfect lens developed along with brain, retina and pigmented epithelium. At 0.125 mg/ml not only brain with accompanying structures but also muscle fibers were equally differentiated. These data suggest a new approach to the problem of dependence of the character of induction on the concentration of inducing factors, and they enable us to suppose that this dependence is not realized as a simple concentration dependence but may de determined by some adaptive, yet not elucidation processes.

Animals↗

PubMed1986

Proximal-distal sequence of development of the skeletal tissues in the penis of rat and the inductive effect of epithelium.

The penis of adult rats comprises the corpus cavernosum penis and the proximal and distal segments of os penis which are situated proximal-distally in this order. Androgens are necessary for the phenotypic differentiation of these tissues. In the present study, genital tubercular mesenchyme of foetal rats was recombined with or without homologous or heterologous epithelia and transplanted beneath the kidney capsule of syngeneic adult male rats, and the development of the corpus cavernosum penis and the os penis in the transplants was examined. Only in the presence of epithelia can the genital tubercular mesenchyme acquire the capacity for differentiation into the corpus cavernosum penis, the proximal segment of os penis, and the distal segment of os penis in this chronological order. The inductive effect of epithelium is a permissive step in the differentiation of the os penis of rat. The epithelium seems to be necessary for the process of rudiment formation of the os penis and the corpus cavernosum penis.

Animals↗

PubMed1990

Heart induction in wild-type and cardiac mutant axolotls (Ambystoma mexicanum).

We have re-examined some of the factors affecting the induction of heart-forming mesoderm in the axolotl. The formation of functional, rhythmically contracting myocardial tissue was used as an assay. We have found that heart-forming mesoderm is fully induced and capable of completing its developmental repertoire by the end of neurulation. As has been previously reported, pharyngeal endoderm appears to be the major inductor of heart mesoderm. Unlike previous workers, we have found that the inducing activity appears to be highly localized in the mid-ventral pharyngeal endoderm. The endoderm retains its inductive properties, and the mesoderm retains at least some capacity to respond, long after the heart-forming mesoderm is apparently fully induced. We have also found that RNA extracts from pharyngeal endoderm, which are capable of causing cardiac-lethal (c/c) mutant axolotl hearts to begin beating, are not capable of inducing early wild-type heart-forming mesoderm. Based on these results, we speculate that induction of heart-forming mesoderm is a two-step process. The first signal, occurring during neurulation, directs the mesoderm to begin differentiating into cardiomyocytes, and the second, beginning in mid- to late neurulation and continuing until just prior to the onset of heartbeat, causes myofibrillogenesis and the initiation of rhythmic contractions. The latter signal, which is lacking in c/c mutant embryos, appears to be necessary to override an inhibition present in the embryonic milieu.

Ambystoma↗

PubMed2005

Wnts and the neural crest.

The neural crest is a multipotent tissue that originates between the neural epithelium and non-neural ectoderm, which can develop into numerous cell types, including neurons, glia, pigment cells, smooth muscle, cartilage and bone. Work in a variety of animal models has shown that a number of signalling factors are necessary for the induction, delamination and differentiation of neural crest cells. However one family of proteins, the Wnts, shows an overriding influence on this tissue. Here we review recent studies that pinpoint specific roles that Wnts play in the development of the neural crest.

Animals↗