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PubMed1999

Analysis of chicken Wnt-13 expression demonstrates coincidence with cell division in the developing eye and is consistent with a role in induction.

We used a degenerate polymerase chain reaction (PCR) strategy to search for Wnt RNA in developing ocular tissues. We isolated a Macaca monkey Wnt-13 PCR fragment, orthologous to the human and murine Wnt-13 and Xenopus Wnt-2b, and a chick Wnt13 cDNA. Wnt-13 is a member of the Wnt-1 class of transforming Wnt molecules. In situ RNA hybridization revealed a dynamic Wnt-13 expression pattern in numerous developing tissues. Within the eye, Wnt-13 is expressed in the proliferative epithelium of the lens and both pigmented and non-pigmented layers of the ciliary margin. In vitro BrdU incorporation studies coupled with in situ hybridization showed that cWnt-13 expression domains in the lens were coincident with cell division. In addition to the eye, cWnt-13 was expressed in head ectoderm, prospective forelimb mesenchyme, lung bud, pharyngeal arches, the brain, as well as the otic vesicle. Our data are consistent with previous observations linking transforming Wnts with cell division and implicate a cascade of events involving cWnt-13 first in dorsoventral patterning and later in cell proliferation regulation associated with lens development. Dev Dyn 1999;215:215-224.

Amino Acid Sequence↗
PubMed1989

Expression of nicotinic acetylcholine receptors in aneural Xenopus embryos.

During gastrulation in vertebrate embryos, the mesoderm moves inward and under the ectoderm and these two cell layers subsequently differentiate in close proximity to each other, providing an opportunity for the exchange of inductive signals. This study examines whether the activation of muscle nicotinic acetylcholine receptor (AChR) genes and the subsequent expression of receptors in Xenopus myotomal muscle are dependent on interaction between the ectoderm and the mesoderm, or their derivatives, after the onset of gastrulation. We eliminated such interaction by inducing total exogastrulation of Xenopus embryos. During exogastrulation, the mesoderm moves away from the ectoderm, and the nervous system fails to develop. Single channel recordings from the myotomal muscle of exogastrulated embryos revealed the presence of two major classes of AChRs, which could be distinguished on the basis of channel conductance. The current amplitudes, conductances, reversal potentials, and open times of these channels closely resembled those reported for the two major classes of AChR channels normally expressed in vivo. We conclude that interaction between ectoderm and mesoderm following the onset of gastrulation is not required for the future expression of the major classes of AChRs in myotomal muscle.

Acetylcholine↗
PubMed1990

The role of the dorsal lip in the induction of heart mesoderm in Xenopus laevis.

We have examined the tissue interactions responsible for the expression of heart-forming potency during gastrulation. By comparing the specification of different regions of the marginal zone, we show that heart-forming potency is expressed only in explants containing both the dorsal lip of the blastopore and deep mesoderm between 30 degrees and 45 degrees lateral to the dorsal midline. Embryos from which both of these 30 degrees-45 degrees dorsolateral regions have been removed undergo heart formation in two thirds of cases, as long as the dorsal lip is left intact. If the dorsal lip is removed along with the 30 degrees-45 degrees regions, heart formation does not occur. These results indicate that the dorsolateral deep mesoderm must interact with the dorsal lip in order to express heart-forming potency. Transplantation of the dorsal lip into the ventral marginal zone of host embryos results in the formation of a secondary axis; in over half of cases, this secondary axis includes a heart derived from the host mesoderm. These findings suggest that the establishment of heart mesoderm is initiated by a dorsalizing signal from the dorsal lip of the blastopore.

Animals↗
PubMed1994

Floor plate and motor neuron induction by vhh-1, a vertebrate homolog of hedgehog expressed by the notochord.

The differentiation of distinct cell types in the ventral neural tube depends on local inductive signals from the notochord. We have isolated a vertebrate homolog of the Drosophila segment polarity gene hedgehog (hh) from zebrafish and rat, termed vhh-1. vhh-1 is expressed in the node, notochord, floor plate, and posterior limb bud mesenchyme. Each of these cell groups has floor plate inducing activity, suggesting that the vhh-1 gene may encode a floor plate-inducing molecule. Widespread expression of rat vhh-1 in frog embryos leads to ectopic floor plate differentiation in the neural tube. In vitro tests for the signaling functions of vhh-1 demonstrate that COS cells expressing the rat vhh-1 gene induce floor plate and motor neuron differentiation in neural plate explants. vhh-1 may, therefore, contribute to the floor plate and motor neuron inducing activities of the notochord.

Amino Acid Sequence↗
PubMed2000

Imaging patterns of calcium transients during neural induction in Xenopus laevis embryos.

Through the injection of f-aequorin (a calcium-sensitive bioluminescent reporter) into the dorsal micromeres of 8-cell stage Xenopus laevis embryos, and the use of a Photon Imaging Microscope, distinct patterns of calcium signalling were visualised during the gastrulation period. We present results to show that localised domains of elevated calcium were observed exclusively in the anterior dorsal part of the ectoderm, and that these transients increased in number and amplitude between stages 9 to 11, just prior to the onset of neural induction. During this time, however, no increase in cytosolic free calcium was observed in the ventral ectoderm, mesoderm or endoderm. The origin and role of these dorsal calcium-signalling patterns were also investigated. Calcium transients require the presence of functional L-type voltage-sensitive calcium channels. Inhibition of channel activation from stages 8 to 14 with the specific antagonist R(+)BayK 8644 led to a complete inhibition of the calcium transients during gastrulation and resulted in severe defects in the subsequent formation of the anterior nervous system. BayK treatment also led to a reduction in the expression of Zic3 and geminin in whole embryos, and of NCAM in noggin-treated animal caps. The possible role of calcium transients in regulating developmental gene expression is discussed.

Aequorin↗
PubMed1999

Expression of Pax-3 in the lateral neural plate is dependent on a Wnt-mediated signal from posterior nonaxial mesoderm.

During early patterning of the vertebrate neuraxis, the expression of the paired-domain transcription factor Pax-3 is induced in the lateral portions of the posterior neural plate via posteriorizing signals emanating from the late organizer and posterior nonaxial mesoderm. Using a dominant-negative approach, we show in explant assays that Pax-3 inductive activities from the organizer do not depend on FGF, retinoic acid, or XWnt-8, either alone or in combination, suggesting that the organizer may produce an unknown posteriorizing factor. However, Pax-3 inductive signals from posterior nonaxial mesoderm are Wnt-dependent. We show that Pax-3 expression in the lateral neural plate expands in XWnt-8-injected embryos and is blocked by dominant-negative XWnt-8. Similarly, we show that the homeodomain transcription factor Msx-1, which like Pax-3 is an early marker of the lateral neural plate, is induced by posterior nonaxial mesoderm and blocked by dominant-negative XWnt-8. Finally, we show that Rohon-Beard primary neurons, a cell type that develops within the lateral neural plate, are also blocked in vivo by dominant-negative Xwnt-8. Together these data support a model in which patterning of the lateral neural plate by Wnt-mediated signals is an early event that establishes a posteriolateral domain, marked by Pax-3 and Msx-1 expression, from which Rohon-Beard cells and neural crest will subsequently arise.

Animals↗
PubMed1998

A small population of anterior cells patterns the forebrain during zebrafish gastrulation.

During gastrulation in vertebrates, dorsal ectoderm is induced to form neural tissue that later gives rise to the brain and spinal cord. This induction depends on signals arising from a group of cells on the dorsal side of the gastrula. This group of cells constitutes the organizer. It is thought that the organizer initially induces neural tissue with anterior, or forebrain, character, and that other signals subsequently posteriorize neural tissue in the trunk. Here we show that development of the anterior central nervous system of the zebrafish embryo also depends on a small group of ectodermal cells located in the prospective head region. Removal of these ectodermal cells during gastrulation perturbs subsequent neural patterning and results in widespread cell death. Transplantation of these cells shows that they can induce forebrain-specific gene expression in more posterior regions of the neural plate. Our results indicate that an early step in neural patterning is the establishment of a small population of signalling cells within the most anterior region of the embryo. These cells are required for patterning and survival of the anterior brain.

Animals↗
PubMed2006

Germ-line induction of the Caenorhabditis elegans vulva.

Development of the Caenorhabditis elegans vulva serves as a paradigm for intercellular signaling during animal development. In wild-type animals, the somatic gonadal anchor cell generates the LIN-3/EGF ligand to induce vulval fates in the underlying hypodermis, whereas FBF, FOG-1, and FOG-3 control germ-line development. Here we report that FBF functions redundantly with FOG-1 and FOG-3 to control vulval induction: animals lacking FBF and either FOG-1 or FOG-3 have multiple vulvae, the Muv phenotype. The fog; fbf Muv phenotype is generated by aberrant induction of vulval precursor cells (VPCs): in wild-type animals, three VPCs are induced to form a single vulva, but, in fog; fbf mutants, four or five VPCs are typically induced, resulting in ectopic vulvae. Laser ablation experiments and mosaic analyses demonstrate that the germ line is critical for the fog; fbf Muv phenotype. Consistent with that site of action, we detect FBF and FOG-1 in the germ line but not in the VPCs. The simplest interpretation is that FOG-1, FOG-3, and FBF act in the germ line to influence vulval fates. The LIN-3/EGF ligand may be the germ-line signal to the VPCs: the fog; fbf Muv phenotype depends on LIN-3 activity, and the lin-3 3' UTR possesses an FBF binding element. Our findings reveal new insights into germ line-to-soma signals and the role of PUF proteins in animal development.

Animals↗
PubMed2005

Monorail/Foxa2 regulates floorplate differentiation and specification of oligodendrocytes, serotonergic raphé neurones and cranial motoneurones.

In this study, we elucidate the roles of the winged-helix transcription factor Foxa2 in ventral CNS development in zebrafish. Through cloning of monorail (mol), which we find encodes the transcription factor Foxa2, and phenotypic analysis of mol-/- embryos, we show that floorplate is induced in the absence of Foxa2 function but fails to further differentiate. In mol-/- mutants, expression of Foxa and Hh family genes is not maintained in floorplate cells and lateral expansion of the floorplate fails to occur. Our results suggest that this is due to defects both in the regulation of Hh activity in medial floorplate cells as well as cell-autonomous requirements for Foxa2 in the prospective laterally positioned floorplate cells themselves. Foxa2 is also required for induction and/or patterning of several distinct cell types in the ventral CNS. Serotonergic neurones of the raphenucleus and the trochlear motor nucleus are absent in mol-/- embryos, and oculomotor and facial motoneurones ectopically occupy ventral CNS midline positions in the midbrain and hindbrain. There is also a severe reduction of prospective oligodendrocytes in the midbrain and hindbrain. Finally, in the absence of Foxa2, at least two likely Hh pathway target genes are ectopically expressed in more dorsal regions of the midbrain and hindbrain ventricular neuroepithelium, raising the possibility that Foxa2 activity may normally be required to limit the range of action of secreted Hh proteins.

Animals↗
PubMed1992

Retinoic acid and the late phase of neural induction.

Regional neural gene expression in Xenopus is the result of a number of processes that continue well beyond the end of gastrulation. By considering two of the basic features of neural induction, the duration of contact between mesoderm and ectoderm and the timing of neural competence, it has been possible to distinguish two phases in neural tissue formation. The late phase includes the period following gastrulation. A factor in determining regional neural gene expression is the difference in inducing ability of the mesoderm that develops during gastrulation along the anterior-posterior axis. The resulting ability to express regional neural genes is subsequently refined during the late phase by a signal that progresses from the posterior part of the embryo. Using a dorsal explant system, it is shown that this progressive signal can be mimicked by the addition of retinoic acid (RA). However, the observation that regions along the anterior-posterior axis respond in different ways to the addition of RA suggests that additional factors are also important in defining regional neural gene expression. One possibility is that the expression of retinoic acid receptors along the axis may demarcate regions that respond to RA in particular ways.

Animals↗
PubMed1995

Induction of epidermis and inhibition of neural fate by Bmp-4.

During gastrulation in vertebrates, ectodermal cells choose between two fates, neural and epidermal. The nervous system forms in response to signals from the Spemann organizer; ectoderm that does not receive these signals becomes epidermis. Unexpectedly, however, in Xenopus, neural tissue also forms when cell-cell communication within the ectoderm is disrupted by cell dissociation or by antagonists of the growth factor activin. These observations suggest that epidermal specification depends on local signalling, by activin or a close relative, and that neural tissue forms when this communication is blocked. Here we report that bone morphogenesis protein 4 (Bmp-4), a relative of activin that is expressed in the embryo at the time of ectodermal fate determination, is a potent epidermal inducer and neural inhibitor, the first reported in any vertebrate. Activin can inhibit neuralization by inducing mesoderm, but does not induce epidermis. Moreover, the dominant-negative activin receptor, which stimulates neuralization when expressed in the embryo, blocks Bmp-4 in our assay. Our findings demonstrate that epidermal fate can be induced, and thus provide further evidence that neural specification is under inhibitory control in vertebrates.

Activin Receptors↗
PubMed2005

Notch in the pathway: the roles of Notch signaling in neural crest development.

Here, we review recent studies that suggest that Notch signaling has two roles during neural crest development: first in establishing the neural crest domain within the ectoderm via lateral induction and subsequently in diversifying the fates of cells that arise from the neural crest via lateral inhibition. The first of these roles, specification of neural crest via lateral induction, has been explored primarily in the cranial neural folds from which the cranial neural crest arises. Evidence for such a role has thus far only been obtained from chick and frog; results from these two species differ, but share the feature that Notch signaling regulates genes that are expressed by cranial neural crest through effects on expression of Bmp family members. The second of these roles, diversification of neural crest progeny via lateral inhibition, has been identified thus far only in trunk neural crest. Evidence from several species suggests that Notch-mediated lateral inhibition functions in multiple episodes in this context, in each case inhibiting neurogenesis. In the 'standard' mode of lateral inhibition, Notch promotes proliferation and in the 'instructive' mode, it promotes specific secondary fates, including cell death or glial differentiation. We raise the possibility that a single molecular mechanism, inhibition of so-called proneural bHLH genes, underlies both modes of lateral inhibition mediated by Notch signaling.

Animals↗
PubMed1995

MEK-2, a Caenorhabditis elegans MAP kinase kinase, functions in Ras-mediated vulval induction and other developmental events.

Activated Ras initiates a cascade of sequential phosphorylation events, including the protein kinases Raf, MEK, and MAP kinase. The Let-60 Ras-mediated signal transduction pathway controls vulval induction in Caenorhabditis elegans. Both Lin-45 Raf and Sur-1 MAP kinase have been determined to be essential factors during vulval induction; however, the C. elegans mek gene has not been identified. In this paper, we have cloned a C. elegans mek gene, mek-2, and demonstrated that the MEK-2 protein possesses the biochemical properties of MAP kinase kinases: The C. elegans MEK-2 protein can phosphorylate and activate a human MAP kinase (ERK1), and MEK-2 itself can be phosphorylated and activated by immunoprecipitated mammalian Raf. The mek-2 gene plays a key role in the let-60 ras-mediated vulval induction pathway, as loss-of-function mutations in the gene (ku114 and h294) significantly reduce the signal transmitted through Ras. mek-2(ku114) completely suppressed the Multivulva (Muv) phenotype of a hyperactive let-60 ras mutation, and animals homozygous for mek-2(ku114) also displayed a partial larval lethal phenotype. Animals homozygous for mek-2(h294) exhibited a highly penetrant sterile and Vulvaless phenotype. Microinjection of a gain-of-function mek-2 mutation resulted in Muv and other mutant phenotypes, whereas microinjection of a dominant-negative mutation not only suppressed the Muv phenotype of an activated let-60 ras mutation but also caused an egg-laying defective phenotype in otherwise wild type animals. Our results demonstrate that mek-2 acts between lin-45 raf and sur-1/mpk-1 in a signal transduction pathway used in the control of vulval differentiation and other developmental events.

Amino Acid Sequence↗
PubMed2003

Hedgehog signalling is required for correct anteroposterior patterning of the zebrafish otic vesicle.

Currently, few factors have been identified that provide the inductive signals necessary to transform the simple otic placode into the complex asymmetric structure of the adult vertebrate inner ear. We provide evidence that Hedgehog signalling from ventral midline structures acts directly on the zebrafish otic vesicle to induce posterior otic identity. We demonstrate that two strong Hedgehog pathway mutants, chameleon (con(tf18b)) and slow muscle omitted (smu(b641)) exhibit a striking partial mirror image duplication of anterior otic structures, concomitant with a loss of posterior otic domains. These effects can be phenocopied by overexpression of patched1 mRNA to reduce Hedgehog signalling. Ectopic activation of the Hedgehog pathway, by injection of sonic hedgehog or dominant-negative protein kinase A RNA, has the reverse effect: ears lose anterior otic structures and show a mirror image duplication of posterior regions. By using double mutants and antisense morpholino analysis, we also show that both Sonic hedgehog and Tiggy-winkle hedgehog are involved in anteroposterior patterning of the zebrafish otic vesicle.

Animals↗
PubMed1996

Epithelial-mesenchymal transformations in early avian heart development.

Cardiac morphogenesis proceeds from a sequential series of epithelial-mesenchymal transitions which begins by establishing bipotential heart-forming cells and later their segregation into endocardial and myocardial lineages. Cells within each lineage integrate to form two concentric epithelia which inductively interact to transform cells of the inner epithelium, the endocardium, into mesenchymal or 'cushion' cells. Noncardiogenic epithelia (dorsal mesocardium, epicardium, neural ectoderm and coelomic mesothelium) undergo transition into populations of extracardiac mesenchyme that combine over time with cushion tissue to remodel the simple tubular heart into a four-chambered organ. Model systems are described for studying the mechanisms of cardiac-related transformations including primary cultures of precardiac epithelia and a differentiation-inducible, avian stem cell line called QCE-6. Focus is centered on the molecular mechanism by which endocardial epithelium transforms into cushion mesenchyme. Experimental findings are reviewed and interpreted in the context of a hypothetical model that seeks to answer why only some cells within an epithelium transform and whether the transformation process is regulated by intrinsic or extrinsic mechanisms. The model proposes that epithelial cells competent to transform to mesenchyme express characteristic markers including receptors for extrinsic signals secreted by stimulator cells (e.g. myocardium). Candidate extrinsic signals include multicomponent complexes called adherons. If applied directly to cultured endocardium, myocardial adherons but not those secreted by L6 myoblasts, induce changes in gene expression within target endocardial cells for proteases and cell:cell and cell:matrix adhesion molecules that accompanied transformation to mesenchyme. A main component of myocardial adherons has been identified as ES antigens, one of which, ES/130, has been cloned, found to have a novel sequence and in culture assays shown to be required for endocardium to transform to mesenchyme. The spatiotemporal pattern of ES protein expression within the embryo suggests that common mechanisms may exist for embryonic epithelial-mesenchymal transformations.

Animals↗
PubMed1991

Fate maps of the first quartet micromeres in the gastropod Ilyanassa obsoleta.

Cell fate specification in the gastropod mollusc Ilyanassa obsoleta involves both cell autonomous and inductive mechanisms, which depend on determinants localized first in the polar lobe and then in the D quadrant of the embryo. A complete cell lineage is lacking for this embryo and is essential for a critical interpretation of previous experimental results and an analysis of the mechanisms at the molecular level. Lineages of the first quartet micromeres were followed using Lucifer Yellow dextran as a tracer. The tracer was injected into individual first quartet micromeres using iontophoresis and patterns of fluorescence were analyzed in the larva after 8 days of development. Fluorescence was limited to head structures, including eyes, tentacles and velum. Structures on the left side were derived from 1a and 1d micromeres; 1a gave rise to the left eye, including the lens. Right side structures were derived from the 1c micromere and 1b contributed to the apical plate between the eyes and symmetrically to both sides of the velum. First quartet lineage data are compared with results from previous cell ablation experiments and with lineage data from other species.

Animals↗
PubMed1996

Cloning and early dorsal axial expression of Flik, a chick follistatin-related gene: evidence for involvement in dorsalization/neural induction.

We have cloned and sequenced a chick gene, Flik (follistatin-like) that appears to be the homolog of the mammalian TSC36. The ORF encodes a secreted protein of approx 38 kDa, containing a single cysteine-rich domain that shows a strong relationship with the second of the four from which Follistatin is constructed. The remainder of the Flik protein shows no strong family affinities. We describe here the normal expression pattern of the gene during primitive streak and neurula stages. We also give revised data for early neurectodermal expression of chick follistatin (Connolly et al., 1995, Developmental Genetics 17(1), 65-77) and follow the new ectopic expression of both genes during the induction of a second neural axis, after grafting of Hensen's node into a peripheral position in a host blastoderm. Both genes mark the organizer (node and/or mesodermal head process) and early neural plate, and could thus be involved in intercellular signaling during mesodermal dorsalization and neural induction. Flik expression appears earlier than that of follistatin however, and unlike that of follistatin, it is maintained strongly in the dorsal midline with intensity smoothly declining into presumptive lateral regions. We show that both genes are upregulated in host tissue in the neighborhood of node grafts, but whereas follistatin is transcribed after 8-10 hr in host epiblast that has formed new neural plate, Flik is expressed within 4 hr in this region, sometimes detectable before the first structural changes (columnarization) of neuralization. Thus, although ectodermal Flik expression is later confined within neural plate, and mesodermal expression concentrated in dorsal axial tissue, its early distribution is consistent with the idea that the encoded protein may first be involved in generating a graded system that positions the boundaries of both neural and dorsal axial mesodermal territories. The results are discussed in relation to this hypothesis and to other recent findings regarding control of vertebrate dorsoventral patterning.

Amino Acid Sequence↗
PubMed1994

Suramin and heparin: aspecific inhibitors of mesoderm induction in the Xenopus laevis embryo.

Xenopus embryos in solutions containing suramin show a dose-dependent decrease in the formation of dorsoanterior structures. Continuous treatment with 1 mM suramin produces embryos without mesodermal derivatives but with mesenchymal cells. Brief immersions of 20 min were used to determine the most sensitive stages and to establish dose-effect curves: a 20 min treatment with 3 mM suramin at stages 7-8.5 produces blastula-like embryos, never classified before, with atypical epidermis, cells full of yolk and mesenchyme in between. The lack of dorsal mesoderm was confirmed by an RNase protection assay with alpha-cardiac actin probe. Heparin also causes a reduction in dorsal structures, but its action is weaker and and there are also strong toxic effects such as superficial cell dissociation. The effect of heparin is dose-dependent and brief immersions show a very sensitive period around stage 6.5. The lowest DAI obtained is 1.5, an extremely microcephalic embryo with forked tail codes, a stocky notochord, and abnormally shaped, abundant neural tissue. Immunofluorescence shows that the distribution of fibronectin-containing fibrils is normal in heparin-treated embryos, whereas there are no such fibrils in suramin-treated embryos at control stage 12.

Animals↗