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PubMed2001

Tenascin-C in developing mouse teeth: expression of splice variants and stimulation by TGFbeta and FGF.

Tenascin-C is a protein of the extracellular matrix which has been suggested to regulate organogenesis. We have analysed the expression of tenascin-C mRNA during mouse tooth development. We show that it is transiently expressed during epithelial budding in the condensed dental mesenchyme, and that it reappears later in the dental papilla mesenchyme where it persists in the dental pulp but is downregulated in odontoblasts. Probes corresponding to the domains A4, B, and D of the differentially spliced and domain 7 of the constant region of the FNIII-like domain show similar patterns of hybridization. Dental epithelium has been shown to induce tenascin-C in early dental mesenchyme, and we show that growth factors in the transforming growth factor beta (TGFbeta) and fibroblast growth factor (FGF) families can mimic this effect. FGF-4, -8 and TGFbeta-1 proteins were applied locally by beads on dissected dental mesenchyme, and tenascin-C expression was analysed after 24 h culture by reverse transcriptase-polymerase chain reaction (RT-PCR) in situ hybridization, and immunohistochemistry. FGF-4 and TGFbeta-1 stimulated tenascin-C expression in E12 dental mesenchymes. RT-PCR showed induction of several tenascin-C isoforms by both TGFbeta-1 and FGFs. We conclude that several splice forms are expressed during mouse tooth development, and that TGFbeta- and FGF-family growth factors may act as epithelial signals inducing tenascin expression in the dental mesenchyme.

Alternative Splicing↗
PubMed1990

Caenorhabditis elegans ras gene let-60 acts as a switch in the pathway of vulval induction.

The let-60 gene, an essential ras gene of the nematode Caenorhabditis elegans, acts as a switch in the inductive signalling pathway that initiates vulva formation. Recessive let-60 mutations that cause a vulvaless phenotype prevent let-60 function in response to the inductive signal. These mutations are clustered and define regions necessary either for the activation or for the action of the let-60 ras protein. Dominant let-60 mutations that cause a multivulva phenotype alter codon 13 and activate let-60 in vivo, rendering it independent of the inductive signal. The let-60 gene acts within an extensively defined genetic pathway, and other genes within this pathway seem likely to encode molecules that regulate let-60 function as well as molecules that are targets of let-60 action.

Alleles↗
PubMed1975

Repair of ultraviolet irradiation damage to a cytoplasmic component required for neural induction in the amphibian egg.

Localized ultraviolet irradiation of the amphibian egg destroys a cytoplasmic component that is required for neural induction. Destruction of that component severely diminishes the inducing capacity of the dorsal lip at gastrulation, as determined by embryological assays. Repair of the ultraviolet lesion can be achieved by replacing the dorsal lip of the irradiated embyro with a lip from an unirradiated embryo.

Animals↗
PubMed1992

LFB1 and LFB3 homeoproteins are sequentially expressed during kidney development.

LFB1 (HNF-1/HNF-1 alpha/APF) and LFB3 (vHNF-1/HNF-1 beta) are two homeoproteins involved in the transcriptional regulation of several liver-specific genes. Both genes are expressed in the polarized epithelia of a wide range of tissues, including liver, the digestive tract and kidney. We have analyzed the expression pattern of LFB1 and LFB3 in the developing rat kidney by in situ hybridization. Our results show that LFB3 transcripts can be detected in mesoderm-derived cells as soon as they are induced to differentiate into a polarized epithelium, while LFB1 transcripts appear only at a later stage when the three different segments of the nephron become apparent. LFB1 transcripts are restricted to the proximal and distal tubules, whereas LFB3 is also detected in the collecting ducts. Neither LFB1 nor LFB3 are expressed in the glomeruli or in the transition epithelia of the ureters and of the urinary bladder, none of which are involved in active transport mechanisms. The sequential activation of these two genes is also observed in transfilter organ cultures of nephrogenic mesenchyme at different stages after induction. This expression pattern suggests that LFB3 and LFB1 play a role in two critical stages of the developmentally regulated conversion of the nephric mesenchyme into a polarized epithelium: the early inductory phase (LFB3) and the postinductory phase (LFB1+LFB3).

Animals↗
PubMed1999

FAST-1 is a key maternal effector of mesoderm inducers in the early Xenopus embryo.

We have examined the role of the maternally encoded transcription factor FAST-1 in the establishment of the mesodermal transcriptional program in Xenopus embryos. FAST-1 has been shown to associate with Smad2 and Smad4, transducers of TGFbeta superfamily signals, in response to stimulation by several TGFbeta superfamily ligands. The FAST-1/Smad2/Smad4 complex binds and activates a 50 bp activin responsive element identified in the promoter of the meso-endodermal marker Mix.2. We have now used three complementary approaches to demonstrate that FAST-1 is a central regulator of mesoderm induction by ectopic TGFbeta superfamily ligands and during endogenous patterning: ectopic expression of mutationally activated FAST-1, ectopic expression of dominant inhibitory FAST-1, and injection of a blocking antibody specific for FAST-1. Expression of constitutively transcriptionally active FAST-1 fusion protein (FAST-VP16(A)) in prospective ectoderm can directly induce the same set of general and dorsal mesodermal genes, as well as some endodermal genes, as are induced by activin or Vg1. In intact embryos, this construct can induce secondary axes similar to those induced by activin or Vg1. Conversely, expression of a FAST-1-repressor fusion (FAST-En(R)) in prospective ectoderm blocks induction of mesodermal genes by activin, while expression of FAST-En(R) in intact embryos prevents general/dorsal mesodermal gene expression and axial development. Injection of a blocking antibody specific for FAST-1 prevents induction of mesodermal response genes by activin or Vg1, but not by FGF. In intact embryos, this antibody can prevent the expression of early mesodermal markers and inhibit axis formation, demonstrating that FAST-1 is a necessary component of the first steps in the specification of mesoderm.

Animals↗
PubMed1997

A Xenopus type I activin receptor mediates mesodermal but not neural specification during embryogenesis.

Activins and other ligands in the TGFbeta superfamily signal through a heteromeric complex of receptors. Disruption of signaling by a truncated type II activin receptor, XActRIIB (previously called XAR1), blocks mesoderm induction and promotes neuralization in Xenopus embryos. We report the cloning and characterization of a type I activin receptor, XALK4. Like truncated XActRIIB, a truncated mutant (tXALK4) blocks mesoderm formation both in vitro and in vivo; moreover, an active form of the receptor induces mesoderm in a ligand-independent manner. Unlike truncated XActRIIB, however, tXALK4 does not induce neural tissue. This difference is explained by the finding that tXALK4 does not block BMP4-mediated epidermal specification, while truncated XActRIIB inhibits all BMP4 responses in embryonic explants. Thus, the type I and type II activin receptors are involved in overlapping but distinct sets of embryonic signaling events.

Activin Receptors↗
PubMed1997

[Specification of cell destiny in early Caenorhabditis elegans embryo].

Embryogenesis of the nematode Caenorhabditis elegans has been described completely on a cell-by-cell basis and found to be essentially invariant. With this knowledge in hands, micromanipulated embryos and mutants have been analyzed for cell lineage defects and the distribution of specific gene products. The results challenge the classical view of cell-autonomous development in nematodes and indicate that the early embryo of C. elegans is a highly dynamic system. A network of inductive events between neighboring cells is being revealed, which is necessary to assign different developmental programs to blastomeres. In those cases where molecules involved in these cell-cell interactions have been identified, homologies to cell surface receptors, ligands and transcription factors found in other systems have become obvious.

Animals↗
PubMed1994

Wingless can bring about a mesoderm-to-ectoderm induction in Drosophila embryos.

By means of nuclear transplantations, we make mosaics in which largely wingless- embryos contain patches of wingless+ cells. In these genetic mosaics, using a standard assay for wingless function (the maintenance of engrailed expression), we uncover an induction across germ layers: Wingless made in the mesoderm can sustain engrailed expression in the ectoderm. This result makes clear that Wingless is expressed in the mesoderm until at least one hour after gastrulation and may function in this germ layer in the wild type.

Animals↗
PubMed2002

Induction and patterning of the telencephalon in Xenopus laevis.

We report an analysis of the tissue and molecular interplay involved in the early specification of the forebrain, and in particular telencephalic, regions of the Xenopus embryo. In dissection/recombination experiments, different parts of the organizer region were explanted at gastrula stage and tested for their inducing/patterning activities on either naive ectoderm or on midgastrula stage dorsal ectoderm. We show that the anterior dorsal mesendoderm of the organizer region has a weak neural inducing activity compared with the presumptive anterior notochord, but is able to pattern either neuralized stage 10.5 dorsal ectoderm or animal caps injected with BMP inhibitors to a dorsal telencephalic fate. Furthermore, we found that a subset of this tissue, the anterior dorsal endoderm, still retains this patterning activity. At least part of the dorsal telencephalic inducing activities may be reproduced by the anterior endoderm secreted molecule cerberus, but not by simple BMP inhibition, and requires the N-terminal region of cerberus that includes its Wnt-binding domain. Furthermore, we show that FGF action is both necessary and sufficient for ventral forebrain marker expression in neuralized animal caps, and possibly also required for dorsal telencephalic specification. Therefore, integration of organizer secreted molecules and of FGF, may account for patterning of the more rostral part of Xenopus CNS.

Animals↗
PubMed1995

How many signals does it take?

Although the genetics of dorsal-ventral polarity which leads to mesoderm formation in Drosophila are understood in considerable detail, subsequent molecular mechanisms involved in patterning the mesoderm primordium into individual mesodermal subtypes are poorly understood. Two papers published recently suggest strongly that an inductive signal from dorsal ectoderm is involved in subdividing the underlying mesoderm, and present evidence that one of the signalling factors is Decapentaplegic (Dpp), a member of the bone morphogenetic protein subgroup of the Transforming Growth Factor-beta (TGF-beta) super family of proteins.

Animals↗
PubMed2005

Early steps in neural crest specification.

The neural crest is a multipotent cell population that arise at the border of the neural plate and non-neural ectoderm. Studies conducted in a number of model organisms including chickens, frogs, zebrafish and mice have been instrumental in elucidating this molecular mechanisms underlying neural crest formation. Signaling molecules of the Wnt, BMP, and FGF families and their downstream effectors have been shown to mediate neural crest induction. Transcription factors including members of the Snail and SoxE gene families as well as FoxD3, c-Myc and others have been implicated in specification of the neural crest. These studies represent an important step in understanding the regulatory interactions involved in generating this complex and interesting cell type.

Animals↗
PubMed1976

Neural induction by previously induced epiblast in avian embryo in vitro.

Pieces of previously neurally induced and competent epiblast of chick and, respectively, quail primitive streak blastoderms were cultured in close contact with each other for 48 h. In several cases, both pieces differentiated into neural direction, which indicates the occurrence of a homoiogenetic induction. There was a considerable mixing of cells of different origin, especially in the undifferentiated controls. In general, the dorsoventral orientation of the previously induced epiblast was retained, but the orientation of the competent epiblast cells was more flexible and could be influenced by the neighbouring neuralised cells.

Animals↗
PubMed2001

Getting your head around Hex and Hesx1: forebrain formation in mouse.

An increasing amount of evidence suggests that in mouse there are two signalling centres required for the formation of a complete neural axis: the anterior visceral endoderm (AVE), and the node and its derivatives. Embryological and genetic studies suggest that the AVE has a head-inducing activity. In contrast, the node appears to act first as a head inducer in synergy with the AVE initiating anterior neural patterning at early stages of mouse development, and later, node derivatives are necessary for maintenance and embellishment of anterior neural character. Hex and Hesx1 are homeobox genes that are expressed in relevant tissues involved in anterior patterning. The analysis of the Hex and Hesx1 mutant mice has revealed that the lack of these genes has little or no effect on the early steps of anterior neural induction. However, both genes are required subsequently for the proper expansion of the forebrain region. We suggest that disturbance in the specification of an Fgf8 signalling centre in the anterior neural ridge may account for the anterior defects observed in these mutants.

Animals↗
PubMed1997

Induction of trunk lateral cells, the blood cell precursors, during ascidian embryogenesis.

The tadpole larvae of the ascidian Halocynthia roretzi have trunk lateral cells (TLCs) in their trunk. TLCs give rise to adult blood cells after metamorphosis. TLCs are exclusively derived from the A7.6 cell pair of 64-cell embryos. When prospective TLC blastomeres were isolated from embryos before the 16-cell stage, they failed to express TLC-specific antigen, a molecular indication of differentiation of TLCs. Isolates after the 32-cell stage, however, autonomously expressed the antigen. Results of experiments involving coisolation and recombination of blastomeres at the 16-cell stage showed that the inductive influence emanating from cells of animal hemisphere (presumptive epidermis blastomeres) is required for TLC formation. The inductive interaction takes place at the 16-cell stage, two cell cycles before the developmental fate becomes exclusively restricted to TLC formation. The inducing activity is distributed widely in animal hemisphere. By contrast, only presumptive TLC blastomeres have competence to be induced to form TLCs.

Animals↗
PubMed2003

Evolution of Brachyury proteins: identification of a novel regulatory domain conserved within Bilateria.

Orthologues of Brachyury, a subfamily of T-box transcription factors, specify distinct cell types in different metazoan phyla, suggesting that the function of these genes has changed through the course of evolution. To investigate this evolutionary process, we have compared the activities of Brachyury orthologues from all major phyla in a single cellular context, the pluripotent Xenopus laevis animal cap. In this assay, an ancestral function is revealed: most orthologues, including the Hydra protein, mimic the action of endogenous Xenopus Brachyury, in that they induce mesoderm but not endoderm. Orthologues from Drosophila and ascidians, however, display an additional derived property, represented in our assay by the induction of endoderm. Misexpression of chimeric versions of Brachyury reveals that the C-terminal half of the protein is important for the strength of the induced response but not for its specificity. In contrast, amino acids located within the T-domain and in a short N-terminal peptide are involved in restricting the activity of Brachyury proteins to induction of mesoderm and not endoderm. Possession of this N-terminal motif is correlated with early circumblastoporal expression of Brachyury orthologues. We propose that restriction of Brachyury activity by this motif plays a conserved role in the control of Bilaterian gastrulation.

Amino Acid Sequence↗
PubMed2000

Getting signals crossed in C. elegans.

The induction of an appropriate cellular response to a stimulus often depends on the intricate interplay between multiple signaling pathways. Recent work utilizing Caenorhabditis elegans has enabled the identification of points of convergence between signaling pathways and permitted the elucidation of how multiple signals work in concert to ensure a proper response.

Animals↗
PubMed1997

Identification of XLerk, an Eph family ligand regulated during mesoderm induction and neurogenesis in Xenopus laevis.

We have isolated and characterized the first Xenopus transmembrane Eph ligand, XLerk (Xenopus Ligand for Eph Receptor Tyrosine Kinases). While this ligand has 72% identity with the closest mammalian family member, Lerk-2, it is the cytoplasmic domain of this molecule that is the most conserved domain with 95% identity. XLerk exists as a maternally expressed mRNA, however, expression of transcripts and protein increase during gastrulation and again in the late swimming tadpole stage. In the adult, XLerk is expressed at low levels in most adult tissues with increased levels observed in the kidney, oocytes, ovary and testis. While low levels of XLerk expression are observed in the adult brain, in situ hybridization analysis demonstrates prominent expression in the developing olfactory system, retina, hindbrain, cranial ganglia, and somites. Furthermore, we have shown that XLerk transcripts are significantly elevated during mesoderm induction caused by activin and FGF, but not during noggin-induced neuralization. These results suggest a role for XLerk in the developing mesenchymal and nervous tissue.

Activins↗