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PubMed1991

Expression of the guanine nucleotide-binding protein Go correlates with the state of neural competence in the amphibian embryo.

The nucleotide-binding protein Go is a transducing molecule closely associated with neural structures in vertebrates. Because of the potential importance of molecules of this type during the first step of neurogenesis, we have investigated the kinetics of expression of Go in the amphibian (Pleurodeles waltl) embryo, focusing our attention on the stages corresponding to the acquisition of neural competence by presumptive ectoderm and to the process of neural induction. Using affinity-purified IgGs directed against the alpha subunit of Go, Go-like immunoreaction (GoLI) is first detected at the midblastula stage in some animal cap (future ectodermal) cells just before they have attained competence to be neuralized. At the early gastrula stage, GoLI is almost exclusively expressed by neural-competent tissue as a whole, with no obvious difference between the dorsal (prospective neural) and the ventral (prospective epidermal) ectoderm. The expression of GoLI is therefore related to the state of competence of the tissue rather than to its fate. At the early neurula stage, immediately following neural induction, the expression of GoLI persists essentially in that part of ectoderm that has been diverted from epidermal differentiation towards the neural pathway; in the ventral ectoderm, as neural competence is lost GoLI disappears. Furthermore, in the neurectoderm, only approximately 70% of the cells conserve GoLI, demonstrating that immediately following neural induction the population of neurectodermal cells is not homogeneous.

Animals↗
PubMed1986

Pattern formation during vulval development in C. elegans.

Previous studies have shown that the development of the vulva of the C. elegans hermaphrodite involves six multipotential hypodermal cells as well as the gonadal anchor cell, which induces vulval formation. Our further examination of the interactions among these seven cells has led to the following model. Each hypodermal precursor cell becomes determined to adopt one of its three potential fates; each of these fates is to generate a particular cell lineage. In the absence of cellular interactions each precursor cell will generate the nonvulval cell lineage; an inductive signal from the anchor cell is required for a precursor cell to generate either of the two types of vulval cell lineages. The inductive signal is spatially graded, and the potency of the signal specifies which lineage is expressed by each of the tripotential precursor cells.

Animals↗
PubMed1996

Cerberus is a head-inducing secreted factor expressed in the anterior endoderm of Spemann's organizer.

An abundant cDNA enriched in Spemann's organizer, cerberus, was isolated by differential screening. It encodes a secreted protein that is expressed in the anterior endomesoderm. Microinjection of cerberus mRNA into Xenopus embryos induces ectopic heads, and duplicated hearts and livers. The results suggest a role for a molecule expressed in the anterior endoderm in the induction of head structures in the vertebrate embryo.

Amino Acid Sequence↗
PubMed1997

Inhibition of Caenorhabditis elegans vulval induction by gap-1 and by let-23 receptor tyrosine kinase.

During induction of the Caenorhabditis elegans hermaphrodite vulva, a signal from the anchor cell activates the LET-23 epidermal growth factor receptor (EGFR)/LET-60 Ras/MPK-1 MAP kinase signaling pathway in the vulval precursor cells. We have characterized two mechanisms that limit the extent of vulval induction. First, we found that gap-1 may directly inhibit the LET-60 Ras signaling pathway. We identified the gap-1 gene in a genetic screen for inhibitors of vulval induction. gap-1 is predicted to encode a protein similar to GTPase-activating proteins that likely functions to inhibit the signaling activity of LET-60 Ras. A loss-of-function mutation in gap-1 suppresses the vulvaless phenotype of mutations in the let-60 ras signaling pathway, but a gap-1 single mutant does not exhibit excess vulval induction. Second, we found that let-23 EGFR prevents vulval induction in a cell-nonautonomous manner, in addition to its cell-autonomous role in activating the let-60 ras/mpk-1 signaling pathway. Using genetic mosaic analysis, we show that let-23 activity in the vulval precursor cell closest to the anchor cell (P6.p) prevents induction of vulval precursor cells further away from the anchor cell (P3.p, P4.p, and P8.p). This result suggests that LET-23 in proximal vulval precursor cells might bind and sequester the inductive signal LIN-3 EGF, thereby preventing diffusion of the inductive signal to distal vulval precursor cells.

Amino Acid Sequence↗
PubMed2003

The C. elegans G-protein-coupled receptor SRA-13 inhibits RAS/MAPK signalling during olfaction and vulval development.

In C. elegans, the RAS/MAPK pathway is used in different tissues to regulate various cell fate decisions. Several positive and negative regulators tightly control the activity of the RAS/MAPK pathway at different steps. We demonstrate a link between a G-protein-coupled receptor signalling pathway and the RAS/MAPK cascade. SRA-13, a member of the SRA family of chemosensory receptors, negatively regulates RAS/MAPK signalling during vulval induction and the olfaction of volatile attractants. Epistasis analysis indicates that SRA-13 inhibits the RAS/MAPK pathway at the level or upstream of MAPK. In both tissues, the vulval precursor cells and the chemosensory neurones, SRA-13 acts through the GPA-5 Galpha protein subunit, suggesting a common mechanism of crosstalk. Moreover, we find that vulval induction is repressed by food withdrawal during larval development and that SRA-13 activity is required for the suppression of vulval induction in response to food starvation. Thus, SRA-13 may serve to adapt the activity of the RAS/MAPK pathway to environmental conditions.

Amino Acid Sequence↗
PubMed1992

Synergistic principles of development: overlapping patterning systems in Xenopus mesoderm induction.

The first inductive event in Xenopus development establishes the mesoderm at the equator of the developing embryo. As part of this process, the dorsal-ventral and anterior-posterior axes of the embryo are initially established. A number of signalling molecules which may play a role in mesodermal induction and patterning have been identified in the last several years, including members of the FGF, TGF-beta and Wnt gene families. A variety of experiments, using either purified factors or injection of RNA encoding these factors, have added to the wealth of classical embryological experimental data collected over the last century. We have synthesized some recent results with the classical data to provide a framework for examining the process of mesoderm induction, and to formulate putative roles for some of the different factors. We incorporate these ideas into a working model of mesoderm induction that provides a basis for future experimental directions. Finally, we suggest that mesoderm induction may not be a discrete set of well separated events, but instead may be a process involving partially overlapping signals that produce the same pattern.

Animals↗
PubMed1996

Identified central neurons convey a mitogenic signal from a peripheral target to the CNS.

Regulation of central neurogenesis by a peripheral target has been previously demonstrated in the ventral nerve cord of the leech Hirudo medicinalis (Baptista, C. A., Gershon, T. R. and Macagno, E. R. (1990). Nature 346, 855-858) Specifically, innervation of the male genitalia by the fifth and sixth segmental ganglia (the sex ganglia) was shown to trigger the birth of several hundred central neurons (PIC neurons) in these ganglia. As reported here, removal of the target early during induction shows that PIC neurons can be independently induced in each side of a ganglion, indicating that the inductive signal is both highly localized and conveyed to each hemiganglion independently. Further, since recent observations (Becker, T., Berliner, A. J., Nitabach, M. N., Gan, W.-B. and Macagno, E. R. (1995). Development, 121, 359-369) had indicated that efferent projections are probably involved in this phenomenon, we individually ablated all possible candidates, which led to the identification of two central neurons that appear to play significant roles in conveying the inductive signal to the CNS. Ablation of a single ML neuron reduced cell proliferation in its own hemiganglion by nearly 50%, on the average. In contrast, proliferation on the opposite side of the ganglion increased by about 25%, suggesting the possibility of a compensatory response by the remaining contralateral ML neuron. Simultaneous ablation of both ML neurons in a sex ganglion caused similar reductions in cell proliferation in each hemiganglion. Deletion of a single AL neuron produced a weaker (7%) but nonetheless reproducible reduction. Ablation of the other nine central neurons that might have been involved in PIC neuron induction had no detectable effect. Both ML and AL neurons exhibit ipsilateral peripheral projections, and both arborize mostly in the hemiganglion where they reside. Thus, we conclude that peripheral regulation of central neurogenesis is mediated in the leech by inductive signals conveyed retrogradely to each hemiganglion by specific central neurons that innervate this target and the hemiganglion they affect.

Animals↗
PubMed1997

Induction of identified mesodermal cells by CNS midline progenitors in Drosophila.

The Drosophila ventral midline cells generate a discrete set of CNS lineages, required for proper patterning of the ventral ectoderm. Here we provide the first evidence that the CNS midline cells also exert inductive effects on the mesoderm. Mesodermal progenitors adjacent to the midline progenitor cells give rise to ventral somatic mucles and a pair of unique cells that come to lie dorsomedially on top of the ventral nerve cord, the so-called DM cells. Cell ablation as well as cell transplantation experiments indicate that formation of the DM cells is induced by midline progenitors in the early embryo. These results are corroborated by genetic analyses. Mutant single minded embryos lack the CNS midline as well as the DM cells. Embryos mutant for any of the spitz group genes, which primarily express defects in the midline glial cell lineages, show reduced formation of the DM cells. Conversely, directed overexpression of secreted SPITZ by some or all CNS midline cells leads to the formation of additional DM cells. Furthermore we show that DM cell development does not depend on the absolute concentration of a local inductor but appears to require a graded source of an inducing signal. Thus, the Drosophila CNS midline cells play a central inductive role in patterning the mesoderm as well as the underlying ectoderm.

Animals↗
PubMed1984

Ultrastructural identification of the hemopoietic inductive microenvironment in the human embryonic liver.

Reciprocal interaction between the hemopoietic organ stromal cells and the cells of the granulocytic, megakaryocytic and erythrocytic series in the human liver obtained from 109 embryos 28 to 49 days after ovulation and 76 fetuses from 8 to 22 weeks of gestation were investigated by light and electron microscopy. The close association of stromal cells with immature cells of the three series was confirmed under the electron microscope and a presumptive HIM (hemopoietic inductive microenvironment) was visualized. A majority of immature erythroblasts intruded into the cytoplasm of the hepatocytes, so the presumptive hemopoietic stem cell types II and IV are undoubtedly differentiated into cells of the erythroid line by contact with hepatocytes at a certain stage of maturation. Granulopoiesis developed among the reticular cells around the ductus venosus--or large arteries in hepatic parenchyma--and the cells of the granulocytic series were enclosed by thin cytoplasmic projections of mesenchymal cells. Neither erythropoiesis nor megakaryopoiesis was noted here. Therefore the compartments composed of one or more reticular cells around the ductus venosus or large arteries seem to have a capacity to regulate the differentiation of the presumptive hemopoietic stem cells type IV into cells of the granulocytic series. This differentiation of presumptive hemopoietic stem cell types II and IV into the megakaryocytic series is belived to be induced by the presence of the microenvironments that consist of foci of a few reticular cells in the hepatic parenchyma, as immature cells of megakaryocytic lineage were encircled by the cytoplasmic projection of one or more reticular cells among hepatocytes. Erythroblastic islets are concluded to be a kind of HIM, where erythroblasts loosely adhere to the central macrophages and undergo mitoses and maturation.

Erythrocytes↗
PubMed1990

Formation of ectopic neurepithelium in chick blastoderms: age-related capacities for induction and self-differentiation following transplantation of quail Hensen's nodes.

Hensen's node, regarded as the avian and mammalian homologue of Spemann's neural inducer (i.e., the amphibian dorsal blastoporal lip), has been transplanted in many previous studies to the germinal crescent of avian blastoderms to examine ectopic neural induction. All these studies have suffered from one or more major shortcomings, the most significant of which has been the lack of a reliable cell marker to determine the contributions of graft cells to ectopic embryos. In the absence of such marker, induced (i.e., derived from the host) and self-differentiated (i.e., derived from the graft) neurepithelium cannot be distinguished from one another with certainty. We have transplanted quail Hensen's nodes to chick host blastoderms and have subsequently used the quail nucleolar heterochromatin marker to identify graft cells unequivocally. We systematically varied both donor and host ages (i.e., stages 3-8 and 3-5, respectively) to examine the effects of age on ectopic neural induction and self-differentiation. Our results demonstrate that the age of the donor is more critical than that of the host over the stages examined. With advancing donor age, the frequency of host induction decreases, while the frequency of graft self-differentiation increases. Previous studies not using cell markers have concluded that the craniocaudal level of the induced neuraxis is determined by the age of the donor, that is, young donors induce cranial neuraxial levels, whereas old donors induce caudal levels. By contrast, we found that with grafts from older donors, neurepithelium was more commonly self-differentiated rather than induced and that progressively more caudal levels of the neuraxis self-differentiated with advancing donor age. Induction of caudal neuraxial levels never occurred in the absence of induced cranial levels. The frequency of neural induction was inversely correlated with the age of the donor and directly correlated with the quantity of graft endodermal cells contributed to the ectopic embryo, supporting a previous assertion that in avian embryos, the earliest and principal source of neural inducer lies within the endoderm rather than mesoderm. From our results, we propose that the role of neural induction is to produce neurepithelium of unspecified regional character, and that the formation of regional character depends on subsequent morphogenetic events.

Age Factors↗
PubMed1997

Mesoderm induction by BMP-4 and -7 heterodimers.

Bone morphogenetic proteins (BMPs) are peptide growth factors belonging to the TGF-beta superfamily. A large number of these ligands, including BMP-2, -4 and -7 is expressed during early embryogenesis in the vertebrate embryo. In this study, we demonstrate that BMP-7 has ventralizing activity both in ectodermal explants as well as in whole embryos. As it was the case for BMP-2 and BMP-4, BMP-7 is a very poor inducer when provided as a homodimer protein. Because of this weak mesoderm inducing activity, it has been suggested that mesoderm induction by BMPs might represent an artifact of overexpression. We provide evidence demonstrating that unlike the homodimers of BMP-4 or BMP-7, the purified recombinant heterodimer of Xenopus BMP-4 and BMP-7 (BMP-4/7) has a potent mesoderm inducing activity at physiological concentrations. These results provide the first evidence for an embryonic function of BMP-4/7 heterodimers in the vertebrate embryo.

Animals↗
PubMed1997

Regulation of epidermal induction by BMP2 and BMP7 signaling.

The specification of neural fate in Xenopus embryos has been shown to be under regulation by negative factors. The secreted protein bone morphogenetic protein-4 (BMP4) has been identified as one of these factors: in the early gastrula ectoderm, BMP4 can both inhibit neural fate and induce epidermis. In this study, we show that two other Xenopus BMP genes, BMP2 and BMP7, are endowed with the same types of activities. First, we show that expression of a dominant negative form of the BMP2 ligand, which blocks normal processing of the wild-type ligand, causes neuralization of Xenopus ectoderm. Second, we have isolated the Xenopus BMP2/7 receptor (XALK2) and generated a constitutively active mutant that signals in a ligand-independent manner. We show that signals from the activated BMP2/7 receptor also inhibit neuralization and induce epidermis in dissociated ectoderm cells. Consistent with both findings we show that secreted BMP2 and BMP7 ligands can also mediate neural inhibition and epidermal induction. These results suggest that both BMP2 and BMP7 may be involved independently or together with BMP4 in the inhibition of the neural fate and the onset of the epidermal induction pathway in vivo. This further supports the idea that epidermal induction is due to the effects of multiple signals from heterogeneous BMP genes.

Animals↗
PubMed1998

The ability to initiate an axis in the avian blastula is concentrated mainly at a posterior site.

Cell interactions during early vertebrate development are crucial for embryonic mesoderm induction and axis initiation. In the avian embryo two unique layers of cells, the epiblast and the hypoblast, constitute the blastoderm before the primitive streak develops (stage XIII). It was suggested that cells of the hypoblast have the ability to induce competent cells in the epiblast to form the mesoderm and to initiate the embryonic axis. Recent results suggest, however, that at stage XIII the hypoblast does not act by inducing the epiblast to form a primitive streak. Since the hypoblast at stage XIII does not induce the epiblast, experiments were designed in this work to examine whether other subregions of the avian blastula have the ability to initiate the embryonic axis. To distinguish the contribution of a particular fragment to the formation of the embryonic axis, cell-marking examinations with lysinated rhodamine dextran (LRD) were designed. The results of the experimental series discussed in this report suggest that at stage XIII it is mainly the posterior side of the marginal zone and/or of the posterior region of the epiblast layer which has the abilities to initiate the embryonic axis. However, the posteriolateral part of the marginal zone region also has such abilities, which are inhibited during normal development. LRD examinations have demonstrated that a graft of a particular posterior blastoderm region, or posteriolateral marginal zone, can initiate an ectopic streak, and is able to recruit other neighboring cells to the developing ectopic streak. No evidence was found that Koller's sickle itself can initiate an ectopic axis in the epiblast at stage XIII. It is proposed that the cells which are important to initiate the avian embryonic axis are concentrated mainly at the region of the posterior marginal zone including Koller's sickle and in the posterior region of the epiblast layer. The cells in this region, which also express the goosecoid and cVg1 genes, may have organizer properties which induce the mesoderm and determine the initiation site of gastrulation in the chick embryo.

Animals↗
PubMed1998

Induction of the zebrafish ventral brain and floorplate requires cyclops/nodal signalling.

Zebrafish cyclops (cyc) mutations cause deficiencies in the dorsal mesendoderm and ventral neural tube, leading to neural defects and cyclopia. Here we report that cyc encodes a transforming growth factor-beta (TGF-beta)-related intercellular signalling molecule that is similar to mouse nodal. cyc is expressed in dorsal mesendoderm at gastrulation and in the prechordal plate until early somitogenesis. Expression reappears transiently in the left lateral-plate mesoderm, and in an unprecedented asymmetric pattern in the left forebrain. Injection of cyc RNA non-autonomously restores sonic hedgehog-expressing cells of the ventral brain and floorplate that are absent in cyc mutants, whereas inducing activities are abolished by cyc, a mutation of a conserved cysteine in the mature ligand. Our results indicate that cyc provides an essential non-cell-autonomous signal at gastrulation, leading to induction of the floorplate and ventral brain.

Animals↗
PubMed1995

Induction of dopaminergic neuron phenotype in the midbrain by Sonic hedgehog protein.

Loss of substantia nigra dopaminergic neurons, which develop from the ventral region of the midbrain, is associated with Parkinson's disease. During embryogenesis, induction of these and other ventral neurons is influenced by interactions with the induction of mesoderm of the notochord and the floor plate, which lies at the ventral midline of the developing CNS. Sonic hedgehog encodes a secreted peptide, which is expressed in notochord and floor plate cells and can induce appropriate ventral cell types in the basal forebrain and spinal cord. Here we demonstrate that Sonic hedgehog is sufficient to induce dopaminergic and other neuronal phenotypes in chick mesencephalic explants in vitro. We find that Sonic hedgehog is a general ventralizing signal in the CNS, the specific response being determined by the receiving cells. These results suggest that Sonic hedgehog may have utility in the induction of clinically important cell types.

Animals↗
PubMed2001

Distinct regulators control the expression of the mid-hindbrain organizer signal FGF8.

Local expression of FGF8 at the mid/hindbrain boundary (MHB) governs the development of multiple neurons and support cells. Here we show that the paired-domain protein Pax2 is necessary and sufficient for the induction of FGF8 in part by regulating the expression of Pax5&8. A network of transcription and secreted factors, including En1, Otx2, Gbx2, Grg4 and Wnt1&4, that is established independently of Pax2, further refines the expression domain and level of FGF8 at the MHB through opposing effects on Pax2 activity. Our results indicate that the expression of local organizing factors is controlled by combinatorial interaction between inductive and modulatory factors.

Animals↗
PubMed1989

The specification of heart mesoderm occurs during gastrulation in Xenopus laevis.

The establishment of heart mesoderm during Xenopus development has been examined using an assay for heart differentiation in explants and explant combinations in culture. Previous studies using urodele embryos have shown that the heart mesoderm is induced by the prospective pharyngeal endoderm during neurula and postneurula stages. In this study, we find that the specification of heart mesoderm must begin well before the end of gastrulation in Xenopus embryos. Explants of prospective heart mesoderm isolated from mid- or late neurula stages were capable of heart formation in nearly 100% of cases, indicating that the specification of heart mesoderm is complete by midneurula stages. Moreover, inclusion of pharyngeal endoderm had no statistically significant effect upon either the frequency of heart formation or the timing of the initiation of heartbeat in explants of prospective heart mesoderm isolated after the end of gastrulation. When the superficial pharyngeal endoderm was removed at the beginning of gastrulation, experimental embryos formed hearts, as did explants of prospective heart mesoderm from such embryos. These results indicate that the inductive interactions responsible for the establishment of heart mesoderm occur prior to the end of gastrulation and do not require the participation of the superficial pharyngeal endoderm.

Animals↗
PubMed1996

The Xenopus homologue of hepatocyte growth factor-like protein is specifically expressed in the presumptive neural plate during gastrulation.

Using a RT-PCR approach, we were able to isolate a cDNA encoding the Xenopus homologue of hepatocyte growth factor-like protein, which we have termed accordingly Xhl. The deduced Xhl protein consists of 717 amino acids, contains four putative kringle domains and a serine protease-like domain characteristic for mammalian HGF and HGF-like protein. The mRNA of Xhl is exclusively expressed in the midline of the prospective neural plate during the period of neural induction, only. Ectopic expression of Xhl causes a 'spina bifida'-like phenotype with enlargement of neural tissue. Activation of Xhl mRNA transcription can be induced by delayed reaggregation of animal caps and appears to require vertical rather than planar signals from the organizer. These data suggest that Xhl is involved in the formation of the embryonic nervous system of Xenopus.

Amino Acid Sequence↗