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PubMed1994

A MAP kinase homolog, mpk-1, is involved in ras-mediated induction of vulval cell fates in Caenorhabditis elegans.

During development of the Caenorhabditis elegans hermaphrodite, the gonadal anchor cell induces nearby Pn.p cells to adopt vulval fates. The response to this signal is mediated by a receptor tyrosine kinase signal transduction pathway that has been remarkably well conserved during metazoan evolution. Because mitogen-activated protein (MAP) kinases are activated by receptor tyrosine kinase pathways in vertebrate cells, we hypothesized that C. elegans MAP kinase homologs may play a role in vulval induction. Two C. elegans MAP kinase genes, mpk-1 and mpk-2 (mpk, MAP kinase), were cloned using degenerate oligonucleotide primers and PCR amplification; in parallel, genes involved in vulval induction were identified by screening for mutations that suppress the vulval defects caused by an activated let-60 ras gene. One such suppressor mutation is an allele of mpk-1. We used a new type of mosaic analysis to show that mpk-1 acts cell autonomously in the Pn.p cells. Our results show that mpk-1 plays an important functional role as an activator in ras-mediated cell signaling in vivo.

Amino Acid Sequence↗
PubMed1979

Fate maps and cell differentiation in the amphibian embryo--an experimental study.

The aim of this paper is to test two different fate maps for the amphibian blastula with respect to their predictions concerning the process of cell differentiation. The first of these fate maps is the one proposed by Vogt, according to which all three germ layers can be projected on to the surface of the embryo. The second is a revision which claims that only endoderm and ectoderm are located in the surface, while the mesoderm is represented by free cells in the interior of the embryo. The testing has been performed by observing the differentiation of small explants of cells taken from various regions of the embryo. It was found that the spontaneous cell differentiation comprises three patterns: undifferentiated cells (free interior cells and circumpolar endodermal cells), fibroblast-like cells (the remaining endodermal cells) and epidermis (ectodermal cells). Further differentiation occurs only through induction, exerted either by the fibroblast-like endodermal cells or by heparan sulphate. When induced, the equatorial ectodermal cells give rise to swollen, hyaline cells (chordocytes), while the remaining ectodermal cells form a sequence of cell differentiation patterns, mesenchyme cells, nerve cells, melanophores and xanthophores. The free interior cells differentiate into striated muscle cells and elongated collagen-producing fibroblasts. Our results thus confirm the revised fate map, and they also give an insight into the mechanisms of the initial cell differentiations in the amphibian embryo.

Ambystoma↗
PubMed1983

Control of tyrosinase gene expression and its relationship to neural crest induction in Rana pipiens. II. Measurement of tyrosinase mRNA accumulation during early embryogenesis using a specific cDNA probe.

The relationship between the process of neural induction and the control of tyrosinase gene expression in the cells that derive from the neural crest of amphibians has been examined at the molecular level. [3H] Tyrosinase cDNA was utilized as a probe to measure the levels of tyrosinase RNA transcripts present in Rana pipiens embryos from the time of fertilization through Stage 25 of cleavage (operculum complete, 240 h) and to correlate these levels with those published for the rate of tyrosinase protein synthesis. R. pipiens [3H]tyrosinase cDNA was synthesized from a purified tyrosinase mRNA template using avian myeloblastosis virus reverse transcriptase and was enriched for full length copies by preparative polyacrylamide gel electrophoresis. This cDNA product was estimated to represent greater than 90% by length of tyrosinase mRNA and hybridized to tyrosinase mRNA to greater than 97% within two orders of magnitude of R0t values. The extent of hybridization of [3H]tyrosinase cDNA to total embryonic RNA throughout early development paralleled the rate of synthesis of tyrosinase protein. Tyrosinase RNA transcripts were first detected at Stage 12-13 (0.0032% of total RNA) and rose to maximal levels by Stage 19 (0.011%). This represents a 50-fold increase from preinduction levels. These results are consistent with a model which predicts that one of the early events in the development of neural crest derivatives is the transcriptionally dependent accumulation of functionally mature tyrosinase mRNA.

Animals↗
PubMed2003

The evolution of developmental mechanisms.

Over the past two to three decades, developmental biology has demonstrated that all multicellular organisms in the animal kingdom share many of the same molecular building blocks and many of the same regulatory genetic pathways. Yet we still do not understand how the various organisms use these molecules and pathways to assume all the forms we know today. Evolutionary developmental biology tackles this problem by comparing the development of one organism to another and comparing the genes involved and gene functions to understand what makes one organism different from another. In this review, we revisit a set of seven concepts defined by Lewis Wolpert (fate maps, asymmetric division, induction, competence, positional information, determination, and lateral inhibition) that describe the characters of many developmental systems and supplement them with three additional concepts (developmental genomics, genetic redundancy, and genetic networks). We will discuss examples of comparative developmental studies where these concepts have guided observations on the advent of a developmental novelty. Finally, we identify a set of evolutionary frameworks, such as developmental constraints, cooption, duplication, parallel and convergent evolution, and homoplasy, to adequately describe the evolutionary properties of developmental systems.

Animals↗
PubMed1998

Chordin regulates primitive streak development and the stability of induced neural cells, but is not sufficient for neural induction in the chick embryo.

We have investigated the role of Bone Morphogenetic Protein 4 (BMP-4) and a BMP antagonist, chordin, in primitive streak formation and neural induction in amniote embryos. We show that both BMP-4 and chordin are expressed before primitive streak formation, and that BMP-4 expression is downregulated as the streak starts to form. When BMP-4 is misexpressed in the posterior area pellucida, primitive streak formation is inhibited. Misexpression of BMP-4 also arrests further development of Hensen's node and axial structures. In contrast, misexpression of chordin in the anterior area pellucida generates an ectopic primitive streak that expresses mesoderm and organizer markers. We also provide evidence that chordin is not sufficient to induce neural tissue in the chick. Misexpression of chordin in regions outside the future neural plate does not induce the early neural markers L5, Sox-3 or Sox-2. Furthermore, neither BMP-4 nor BMP-7 interfere with neural induction when misexpressed in the presumptive neural plate before or after primitive streak formation. However, chordin can stabilise the expression of early neural markers in cells that have already received neural inducing signals. These results suggest that the regulation of BMP signalling by chordin plays a role in primitive streak formation and that chordin is not sufficient to induce neural tissue.

Amino Acid Sequence↗
PubMed1995

Restrictions to floor plate induction by hedgehog and winged-helix genes in the neural tube of frog embryos.

Intercellular signaling molecules of the vertebrate hedgehog family and transcription factors of the winged-helix family have been implicated in floor plate development. We have examined the consequences of misexpressing the vertebrate hedgehog gene vhh-1 (sonic hedgehog, shh) and the winged-helix gene HNF-3 beta in the neural plate and neural tube of frog embryos. Misexpression of either of these genes induces floor plate differentiation at ectopic locations. However, ectopic floor plate induction in response to both vhh-1 and HNF-3 beta was temporally and spatially restricted. At neural plate stages, ectopic floor plate differentiation was not detected. After neural tube closure, ectopic floor plate differentiation was detected but was restricted predominantly to the dorsal region of the neural tube. The ability of winged-helix and vertebrate hedgehog genes to induce floor plate differentiation in vivo may, therefore, be constrained by additional signals that specify the time and position of floor plate differentiation.

Animals↗
PubMed2000

Dorsal induction from dorsal vegetal cells in Xenopus occurs after mid-blastula transition.

We performed some experiments to investigate the temporal and spatial details of the dorsal induction exerted by dorsal vegetal cells in Xenopus embryo. Two dorsal vegetal cells (D1 cells) were transplanted into the ventral vegetal region of a recipient at the 32-cell stage. At various times after transplantation, the ventral animal-equatorial part was explanted and cultured. The explants isolated 5.5 h after transplantation (time 5.5) elongated and formed somites. In RT-PCR analysis, the expression of dorsal gene, chordin was activated in the explants isolated after time 4.0 (about the 4000-cell stage which corresponds to the mid blastula transition (MBT)) at control stage 10. In another series of experiments, ventral animal-equatorial and dorsal vegetal parts were isolated from the 4000-cell stage embryos and they were combined for 2.0-2.5 h. These ventral animal-equatorial explants elongated and formed somites. The chordin expression was also observed in the explants. But the 32- and 256-cell stage dorsal vegetal cells failed to exert the dorsalizing activity within the 2.0-2.5 h of the conjugation. These results suggest that 2 h contact after MBT is necessary and sufficient for the dorsal induction from the dorsal vegetal cells and it occurs as a result of the zygotic gene expression. Consistent with this idea, the zygotic dorsal genes, siamois and chordin were expressed on the upper regions of the transplanted D1 descendants at stage 10. Furthermore, this region began to gastrulate when the D1 cell was transplanted with upside-down orientation. Our data indicate that the upper region of the D1 descendants by itself act as the Spemann organizer rather than the Nieuwkoop center.

Animals↗
PubMed1998

A gonad-derived survival signal for vulval precursor cells in two nematode species.

Intercellular cell-survival signals play a major role in animal development [1]. In the nematode Caenorhabditis elegans, however, the stereotyped cell deaths that occur reproducibly during development are regulated in a cell-autonomous fashion (or, in a few cases, by a death-inducing signal) [2]. We show here the existence of a cell-survival signal acting on the vulval precursor cells in two nematodes, Turbatrix aceti and Halicephalobus sp. JB128. In C. elegans [3], as in many other nematode species [4] [5-7], ablation of the gonad causes all vulval precursor cells to adopt a default epidermal fate: a gonadal signal is required for the induction of vulval fates. In the nematodes T. aceti and Halicephalobus sp. JB128, however, we found that ablation of the gonad in the L1 larval stage caused all vulval precursor cells to undergo programed cell death. Thus, in intact Turbatrix and Halicephalobus, a survival signal from the gonad prevents activation of the cell-death program in vulval precursor cells. Our results demonstrate the existence of intercellular cell-survival signals in nematodes and uncover an evolutionary variation in the role of the gonad in nematode vulval development.

Animals↗
PubMed1993

XASH-3, a novel Xenopus achaete-scute homolog, provides an early marker of planar neural induction and position along the mediolateral axis of the neural plate.

We have isolated a novel Xenopus homolog of the Drosophila achaete-scute genes, called XASH-3. XASH-3 expression is neural specific and is detected as early as stage 11 1/2, making it one of the earliest markers of neural induction so far described. Moreover, XASH-3 expression within the neural plate is regionally restricted. Transverse bands of XASH-3 mRNA mark discrete positions along the anteroposterior axis, while longitudinal bands mark a discrete position along the mediolateral axis. This latter site of XASH-3 expression appears to demarcate the prospective sulcus limitans, a boundary zone that later separates the functionally distinct dorsal (alar) and ventral (basal) regions of the spinal cord. In sandwich explants lacking any underlying mesoderm, XASH-3 is expressed in longitudinal stripes located lateral to the midline. This provides the first indication that planar or midline-derived inductive signals are sufficient to establish at least some aspects of positional identity along the mediolateral axis of the neural plate. By contrast, the transverse stripes of XASH-3 expression are not detected, suggesting that this aspect of anteroposterior neural pattern is lost or delayed in the absence of vertically passed signals. The restricted mediolateral expression of XASH-3 suggests that mediolateral patterning of the neural plate is an early event, and that this regionalization can be achieved in the absence of inducing signals derived from underlying mesoderm.

Animals↗
PubMed1996

Induction of seminal vesicle morphogenesis from dog epididymal epithelium.

A number of carnivores, including the dog, do not have seminal vesicles (SVs). They have been induced experimentally by heterotypic recombination of adult dog epididymal epithelium with mesenchyme of neonatal rat SV and allowed to grow for 4 weeks under the renal capsule of male athymic nude mice. The induced SVs bore a striking similarity to rat SV both histologically and ultrastructurally. However, they secreted proteins which were different from those in the rat SV and reacted negatively to antibodies specific to rat SV secretory proteins and dog prostate secretory proteins. The results strongly suggested that the glands induced from dog epididymal epithelium were likely to be "dog" SVs.

Animals↗
PubMed1995

Target-induced neurogenesis in the leech CNS involves efferent projections to the target.

During a critical period in leech embryogenesis, the sex nerves that connect the 5th and 6th midbody ganglia (MG5 and MG6) to the primordium of the male sexual organ carry a spatially localized signal that induces the birth of several hundred neurons specific to these ganglia. We examined particular cellular elements (afferents, efferents, non-neuronal components) within these nerves as potential conveyors of the inductive signal. We show that axons of peripheral sensory neurons in the male genitalia travel along the sex nerves and into MG5 and MG6, but reach the CNS after the critical period has elapsed and cannot, therefore, be involved in the induction. Of the six sex nerves, four contain non-neuronal cells that span the entire distance between the male genitalia and the sex ganglia. However, when male genitalia were transplanted to ectopic locations close to MG6, induction occurred frequently but only in MG6, mediated by ectopic nerves that do not contain these cells. Thus, non-neuronal cells specific to the normal sex nerves are not necessary for induction. In addition, dye injections into the target during the critical period failed to reveal migrating cells in the sex nerves that could convey the inductive signal to the CNS. Finally, we show that 11 pairs of central neurons in each ganglion project to the male organ early during the critical period. In the adult, at least 3 additional pairs of neurons in MG6 also innervate this target. We conclude that the only components of the sex nerves that connect the sex ganglia to the target during the critical period that could be associated with induced central mitogenesis are the axons of central neurons that innervate the male genitalia.

Animals↗
PubMed2000

dackel acts in the ectoderm of the zebrafish pectoral fin bud to maintain AER signaling.

Classical embryological studies have implied the existence of an apical ectodermal maintenance factor (AEMF) that sustains signaling from the apical ectodermal ridge (AER) during vertebrate limb development. Recent evidence suggests that AEMF activity is composed of different signals involving both a sonic hedgehog (Shh) signal and a fibroblast growth factor 10 (Fgf10) signal from the mesenchyme. In this study we show that the product of the dackel (dak) gene is one of the components that acts in the epidermis of the zebrafish pectoral fin bud to maintain signaling from the apical fold, which is homologous to the AER of tetrapods. dak acts synergistically with Shh to induce fgf4 and fgf8 expression but independently of Shh in promoting apical fold morphogenesis. The failure of dak mutant fin buds to progress from the initial fin induction phase to the autonomous outgrowth phase causes loss of both AER and Shh activity, and subsequently results in a proximodistal truncation of the fin, similar to the result obtained by ridge ablation experiments in the chicken. Further analysis of the dak mutant phenotype indicates that the activity of the transcription factor engrailed 1 (En1) in the ventral non-ridge ectoderm also depends on a maintenance signal probably provided by the ridge. This result uncovers a new interaction between the AER and the dorsoventral organizer in the zebrafish pectoral fin bud.

Animals↗
PubMed2002

The mouse polydactylous mutation, luxate (lx), causes anterior shift of the anteroposterior border in the developing hindlimb bud.

Pattern formation along the anterior-posterior axis of the vertebrate limb is established upon activation of Sonic Hedgehog (SHH) in the zone of polarizing activity (ZPA). Since many mouse mutants with preaxial polydactyly show ectopic expression of Shh at the anterior margin of the limb buds, it has been thought to be a primary defect caused by these mutations. We show here that the mouse mutation luxate (lx) exhibits dose-dependent reduction in the size of the Fgf8 expression domain in the ectoderm from the initial stage of limb development. This aberration was independent of Fgf10 expression in the limb mesenchyme. Shh was induced in the mesenchyme underlying the posterior end of the Fgf8 expression domain, indicating an anterior shift of Shh expression in lx hindlimb buds. Prior to the ectopic induction of Shh, the expression domains of genes downstream from Shh, namely dHAND, Gli1, Ptc and Gre, which are normally expressed in posterior mesenchyme of limb buds, expanded anteriorly on the lx hindlimb buds. Conversely, the expression domains of anterior mesenchymal markers such as Gli3and Alx4 decreased in size. Thus, ectopic Shh is not a primary defect of the lx mutation. Rather, our results indicate that the lx mutation affects the positioning of the anteroposterior border in developing hindlimb buds.

Animals↗
PubMed1990

Mesodermal control of neural cell identity: floor plate induction by the notochord.

The floor plate is a specialized group of midline neuroepithelial cells that appears to regulate cell differentiation and axonal growth in the developing vertebrate nervous system. A floor plate-specific chemoattractant was used as a marker to examine the role of the notochord in avian floor plate development. Expression of this chemoattractant in lateral cells of the neural plate and neural tube was induced by an ectopic notochord, and midline neural tube cells did not express the chemoattractant after removal of the notochord early in development. These results provide evidence that a local signal from the notochord induces the functional properties of the floor plate.

Animals↗
PubMed2006

Mesoderm induction: from caps to chips.

Vertebrate mesoderm induction is one of the classical problems in developmental biology. Various developmental biology approaches, particularly in Xenopus and zebrafish, have identified many of the key factors that are involved in this process and have provided major insights into how these factors interact as part of a signalling and transcription-factor network. These data are beginning to be refined by high-throughput approaches such as microarray assays. Future challenges include understanding how the prospective mesodermal cells integrate the various signals they receive and how they resolve this information to regulate their morphogenetic behaviours and cell-fate decisions.

Animals↗
PubMed2003

Maternal induction of ventral fate by zebrafish radar.

In vertebrate embryos, maternal determinants are thought to preestablish the dorsoventral axis by locally activating zygotic ventral- and dorsal-specifying genes, e.g., genes encoding bone morphogenetic proteins (BMPs) and BMP inhibitors, respectively. Whereas the canonical Wntbeta-catenin pathway fulfills this role dorsally, the existence of a reciprocal maternal ventralizing signal remains hypothetical. Maternal noncanonical WntCa(2+) signaling may promote ventral fates by suppressing Wntbeta-catenin dorsalizing signals; however, whether any maternal determinant is directly required for the activation of zygotic ventral-specifying genes is unknown. Here, we show that such a function is achieved, in part, in the zebrafish embryo by the maternally encoded transforming growth factor beta (TGF-beta) signaling molecule, Radar. Loss-of-function experiments, together with epistasis analyses, identify maternal Radar as an upstream activator of bmps expression. Maternal induction of bmps by Radar is essential for zebrafish development as its removal results in larval-lethal dorsalized phenotypes. Double-morphant analyses further suggest that Radar functions through the TGF-beta receptor Alk8 to initiate the expression of bmp genes. Our results support the existence of a previously uncharacterized maternal ventralizing pathway. They might further indicate that maternal TGF-betaRdr and WntCa(2+) pathways complementarily specify ventral cell fates, with the former triggering bmps expression and the latter indirectly repressing genes encoding BMP antagonists.

Animals↗
PubMed1999

A PP2A regulatory subunit positively regulates Ras-mediated signaling during Caenorhabditis elegans vulval induction.

We describe evidence that a regulatory B subunit of protein phosphatase 2A (PP2A) positively regulates an RTK-Ras-MAP kinase signaling cascade during Caenorhabditis elegans vulval induction. Although reduction of sur-6 PP2A-B function causes few vulval induction defects in an otherwise wild-type background, sur-6 PP2A-B mutations suppress the Multivulva phenotype of an activated ras mutation and enhance the Vulvaless phenotype of mutations in lin-45 raf, sur-8, or mpk-1. Double mutant analysis suggests that sur-6 PP2A-B acts downstream or in parallel to ras, but likely upstream of raf, and functions with ksr-1 in a common pathway to positively regulate Ras signaling.

Adaptor Proteins, Signal Transducing↗
PubMed2003

Wise, a context-dependent activator and inhibitor of Wnt signalling.

We have isolated a novel secreted molecule, Wise, by a functional screen for activities that alter the anteroposterior character of neuralised Xenopus animal caps. Wise encodes a secreted protein capable of inducing posterior neural markers at a distance. Phenotypes arising from ectopic expression or depletion of Wise resemble those obtained when Wnt signalling is altered. In animal cap assays, posterior neural markers can be induced by Wnt family members, and induction of these markers by Wise requires components of the canonical Wnt pathway. This indicates that in this context Wise activates the Wnt signalling cascade by mimicking some of the effects of Wnt ligands. Activation of the pathway was further confirmed by nuclear accumulation of beta-catenin driven by Wise. By contrast, in an assay for secondary axis induction, extracellularly Wise antagonises the axis-inducing ability of Wnt8. Thus, Wise can activate or inhibit Wnt signalling in a context-dependent manner. The Wise protein physically interacts with the Wnt co-receptor, lipoprotein receptor-related protein 6 (LRP6), and is able to compete with Wnt8 for binding to LRP6. These activities of Wise provide a new mechanism for integrating inputs through the Wnt coreceptor complex to modulate the balance of Wnt signalling.

Activins↗