A role for notochord in axial vascular development revealed by analysis of phenotype and the expression of VEGR-2 in zebrafish flh and ntl mutant embryos

The notochord is required for the differentiation of nearby tissues, including the neural tube and the floor plate. Because the dorsal aorta and axial vein are midline structures, their development might also be influenced by the notochord. To investigate this possibility, we cloned zebrafish VEGR-2, homologous to the earliest known marker of endothelial cells in mammals. In flh and ntl mutant embryos, which lack a notochord, we found a defect in axial blood vessel formation, and a delay in the fusion of VEGR-2 positive endothelial progenitor cells into the primary vascular plexus and a block in the establishment of mature vessels. Differences in the vascular phenotype between the two mutations correlated with the severity of their axial mesodermal defects. These observations support a role for the notochord in vasculogenesis.

Establishment of the dorso-ventral axis in Xenopus embryos is presaged by early asymmetries in beta-catenin that are modulated by the Wnt signaling pathway

Eggs of Xenopus laevis undergo a postfertilization cortical rotation that specifies the position of the dorso-ventral axis and activates a transplantable dorsal-determining activity in dorsal blastomeres by the 32-cell stage. There have heretofore been no reported dorso-ventral asymmetries in endogenous signaling proteins that may be involved in this dorsal-determining activity during early cleavage stages. We focused on beta-catenin as a candidate for an asymmetrically localized dorsal-determining factor since it is both necessary and sufficient for dorsal axis formation. We report that beta-catenin displays greater cytoplasmic accumulation on the future dorsal side of the Xenopus embryo by the two-cell stage. This asymmetry persists and increases through early cleavage stages, with beta-catenin accumulating in dorsal but not ventral nuclei by the 16- to 32-cell stages. We then investigated which potential signaling factors and pathways are capable of modulating the steady-state levels of endogenous beta-catenin. Steady-state levels and nuclear accumulation of beta-catenin increased in response to ectopic Xenopus Wnt-8 (Xwnt-8) and to the inhibition of glycogen synthase kinase-3, whereas neither Xwnt-5A, BVg1, nor noggin increased beta-catenin levels before the mid-blastula stage. As greater levels and nuclear accumulation of beta-catenin on the future dorsal side of the embryo correlate with the induction of specific dorsal genes, our data suggest that early asymmetries in beta-catenin presage and may specify dorso-ventral differences in gene expression and cell fate. Our data further support the hypothesis that these dorso-ventral differences in beta-catenin arise in response to the postfertilization activation of a signaling pathway that involves Xenopus glycogen synthase kinase-3.

Activation of Siamois by the Wnt pathway

The Wnt pathway and the recently described Xenopus homeobox gene Siamois share the ability to mimic Nieuwkoop organizer signaling activity when ectopically expressed, prompting us to ask whether the Wnt pathway can activate Siamois expression. We show that activators of the Wnt pathway, and the dorsalizing substance LiCl, activate Siamois expression in Xenopus animal caps. Furthermore, Siamois expression is shown to be activated by beta-catenin cell-autonomously. Although certain TGF-beta family members, such as Vg1, are potent dorsal mesoderm inducers, the TGF-beta pathway only weakly activated Siamois. Finally, we demonstrate that Siamois expression is eliminated from the marginal zone of UV-irradiated Xenopus embryos, supporting the hypothesis that cortical rotation moves a Wnt-like activity into the dorsal region. These results suggest that formation of the Nieuwkoop organizer is dependent on activation of Siamois by a localized Wnt-like signaling activity, which may act synergistically with a vegetally localized TGF-beta signal.

The axis-inducing activity, stability, and subcellular distribution of beta-catenin is regulated in Xenopus embryos by glycogen synthase kinase 3

The serine/threonine kinase Xgsk-3 and the intracellular protein beta-catenin are necessary for the establishment of the dorsal-ventral axis in Xenopus. Although genetic evidence from Drosophila indicates that Xgsk-3 is upstream of beta-catenin, direct interactions between these proteins have not been demonstrated. We demonstrate that phosphorylation of beta-catenin in vivo requires an in vitro amino-terminal Xgsk-3 phosphorylation site, which is conserved in the Drosophila protein armadillo. beta-catenin mutants lacking this site are more active in inducing an ectopic axis in Xenopus embryos and are more stable than wild-type beta-catenin in the presence of Xgsk-3 activity, supporting the hypothesis that Xgsk-3 is a negative regulator of beta-catenin that acts through the amino-terminal site. Inhibition of endogenous Xgsk-3 function with a dominant-negative mutant leads to an increase in the steady-state levels of ectopic beta-catenin, indicating that Xgsk-3 functions to destabilize beta-catenin and thus decrease the amount of beta-catenin available for signaling. The levels of endogenous beta-catenin in the nucleus increases in the presence of the dominant-negative Xgsk-3 mutant, suggesting that a role of Xgsk-3 is to regulate the steady-state levels of beta-catenin within specific subcellular compartments. These studies provide a basis for understanding the interaction between Xgsk-3 and beta-catenin in the establishment of the dorsal-ventral axis in early Xenopus embryos.

Regulation of dorsal-ventral patterning: the ventralizing effects of the novel Xenopus homeobox gene Vox

The formation of the dorsal-ventral axis in Xenopus laevis is elicited by a signaling cascade on the dorsal side of the embryo initiated by cortical rotation. These early developmental events impart an initial axial polarity to the embryo. By the time gastrulation occurs, the embryo has established opposing dorsal and ventral regulatory regions. Through a dynamic process, the embryo acquires a definitive pattern that reflects the distribution of future cell fates. Here we present a novel homeobox gene, Vox, whose expression reflects this dynamic process. Vox is first expressed throughout the embryo and subsequently eliminated from the notochord and neural plate. Ectopic expression of Vox demonstrates that the normal function of this gene may be to suppress dorsal genes such as Xnot and chordin, and induce ventral and paraxial genes such as Bmp-4 and MyoD. Ectopic expression of BMP-4 ventralizes embryos and positively regulates the expression of Vox, suggesting that these genes are components of a reciprocal regulatory network.

Overexpression of Xgsk-3 disrupts anterior ectodermal patterning in Xenopus

The Xenopus homolog of glycogen synthase kinase-3, Xgsk-3, plays a major role in regulating the formation of the dorsal-ventral axis, most likely through effects on the mesoderm. To determine whether Xgsk-3 is involved in ectodermal patterning, Xgsk-3 was ectopically overexpressed in the presumptive ectoderm. This approach resulted in a dramatically expanded cement gland, which is due to early changes in cement gland specification at the anterior end of the embryo. Explant experiments were used to show that Xgsk-3 overexpression enhances the response of ectoderm to cement-gland-inducing signals from the mesoderm and to the intercellular signaling factor noggin. Expression of two other noggin-inducible genes, Xotx2 and XANF-2, was also expanded in whole embryos, while the expression of the epidermal marker, Xgbx-2, was eliminated. These results suggest that Xgsk-3 may play a role in anterior ectodermal patterning as a component of an intracellular pathway that regulates the ectodermal responsiveness to endogenous inducing signals.

The Xenopus laevis homeobox gene Xgbx-2 is an early marker of anteroposterior patterning in the ectoderm

In a search for homeobox genes expressed during early Xenopus development, we have isolated a gene which appears to be the Xenopus cognate of the mouse Gbx-2 gene. Expression of Xgbx-2 is first detectable by in situ hybridization at the midgastrula stage when it is predominantly expressed in the dorsolateral ectoderm, with a gap in expression at the dorsal midline. By the end of gastrulation and during neurulation, Xgbx-2 is expressed dorsolaterally in the neural ectoderm and laterally and ventrally in the epidermis with sharp anterior expression borders in both tissues. The anteriormost expression in the neural ectoderm persists throughout the early stages of development, and was mapped to the region of rhombomere 1, with an anterior expression border in the region of the midbrain-hindbrain boundary. Thus Xgbx-2 is expressed anterior to the Hox genes. Xgbx-2 expression is induced by retinoic acid (RA) in animal caps, and RA treatment of whole embryos expands and enhances Xgbx-2 expression in the ectoderm. We suggest a role for Xgbx-2 in establishing the midbrain-hindbrain boundary, which appears to separate early neurectodermal regions expressing genes that are positively and negatively regulated by RA.

Drosophila short gastrulation induces an ectopic axis in Xenopus: evidence for conserved mechanisms of dorsal-ventral patterning

The Spemann organizer has long been recognized as a major source of patterning signals during the gastrula stage of amphibian embryogenesis. More recent evidence has suggested that the ventral side of the embryo also plays an important role in dorsal-ventral patterning during gastrulation through the action of signaling factors such as BMP-4. Bmp-4 is closely related to the Drosophila decapentaplegic (dpp) gene, and like Bmp-4, dpp is excluded from the neurogenic region. Recently we showed that Bmp-4 functions in an analogous role to that of dpp in Drosophila, suggesting that the mechanism of dorsal-ventral patterning in Xenopus and Drosophila embryos may be conserved. To further test this hypothesis, RNA of the Drosophila short gastrulation (sog) gene was injected into Xenopus embryos, since sog has been shown genetically to be an antagonist of dpp function. Overexpression of sog RNA in Xenopus dorsalizes the embryo by expanding neurogenic and dorsal paraxial tissue. When ectopically expressed on the ventral side of the embryo, sog induces a partial secondary axis. In addition, sog partially rescues embryos ventralized by ultraviolet irradiation. Since sog induces many similar changes in gene expression to that caused by truncated BMP receptors, we suggest that sog functions in part by opposing BMP-4 signaling. The recent identification of a possible Xenopus sog homolog, chordin, in conjunction with these results supports the hypothesis that dorsal-ventral patterning mechanisms are conserved between these two species.

A homeobox gene essential for zebrafish notochord development

The notochord is a midline mesodermal structure with an essential patterning function in all vertebrate embryos. Zebrafish floating head (flh) mutants lack a notochord, but develop with prechordal plate and other mesodermal derivatives, indicating that flh functions specifically in notochord development. We show that floating head is the zebrafish homologue of Xnot, a homeobox gene expressed in the amphibian organizer and notochord. We propose that flh regulates notochord precursor cell fate.

BMP-4 regulates the dorsal-ventral differences in FGF/MAPKK-mediated mesoderm induction in Xenopus

Recent studies on Xenopus development have revealed an increasingly complex array of inductive, prepatterning, and competence signals that are necessary for proper mesoderm formation. In this study, we establish that fibroblast growth factor (FGF) signals through mitogen-activated protein kinase kinase (MAPKK) to induce mesodermal gene expression. We demonstrate that a partially activated form of MAPKK restores expression of the mesodermal genes Xcad-3 and Xbra, eliminated by the dominant-negative FGF receptor (delta FGFR). Similar to the results reported earlier with delta FGFR, expression of a dominant-negative form of MAPKK (MAPKKD) preferentially eliminates the dorsal expression of Xcad-3 and Xbra. We tested whether the regional localization of bone morphogenetic protein-4 (BMP-4) could explain why both MAPKKD and delta FGFR eliminate the dorsal and not the ventral expression of Xcad-3 and Xbra. We show that ectopic expression of BMP-4 is sufficient to maintain the dorsal expression of Xcad-3 and Xbra in embryos containing delta FGFR and that expression of a dominant-negative BMP receptor reduces the dorsal-ventral differences in delta FGFR embryos. These results indicate that regional localization of BMP-4 is responsible for the dorsal-ventral asymmetry in FGF/MAPKK-mediated mesoderm induction.

FGF is a prospective competence factor for early activin-type signals in Xenopus mesoderm induction

Normal pattern formation during embryonic development requires the regulation of cellular competence to respond to inductive signals. In the Xenopus blastula, vegetal cells release mesoderm-inducing factors but themselves become endoderm, suggesting that vegetal cells may be prevented from expressing mesodermal genes in response to the signals that they secrete. We show here that addition of low levels of basic fibroblast growth factor (bFGF) induces the ectopic expression of the mesodermal markers Xbra, MyoD and muscle actin in vegetal explants, even though vegetal cells express low levels of the FGF receptor. Activin, a potent mesoderm-inducing agent in explanted ectoderm (animal explants), does not induce ectopic expression of these markers in vegetal explants. However, activin-type signaling is present in vegetal cells, since the vegetal expression of Mix.1 and goosecoid is inhibited by the truncated activin receptor. These results, together with the observation that FGF is required for mesoderm induction by activin, support our proposal that a maternal FGF acts at the equator as a competence factor, permitting equatorial cells to express mesoderm in response to an activin-type signal. The overlap of FGF and activin-type signaling is proposed to restrict mesoderm to the equatorial region.

Localized BMP-4 mediates dorsal/ventral patterning in the early Xenopus embryo

Mesoderm is initially induced in the amphibian embryo by events that occur in the early cleavage stages prior to the midblastula transition (MBT) and morphogenesis. These inductive interactions establish the mesoderm at the equator and create a distinction between the dorsal and the ventral regions. After the MBT, zygotic factors pattern the mesoderm and induce the neuroectoderm on the dorsal side of the embryo. Most previous studies have focused on the effects of signals originating in the dorsal mesoderm. We show that BMP-4 transcripts are localized to the ventral side of the gastrula embryo and provide evidence that localized expression of BMP-4 is important for regulating the expression of mesodermal and neural genes. We show that ectopic expression of BMP-4 inhibits the formation of dorsal and lateral mesoderm and reduces the size of the neural plate. Elimination of BMP-4 signaling with a dominant-negative BMP receptor expands the lateral mesoderm and neural plate without expanding the expression of genes along the dorsal midline. These results suggest that BMP-4 may act to oppose the action of dorsalizing signals and neural-inducing signals that originate in the dorsal organizer region. We suggest that BMP-4 may have an analogous role to the Drosophila gene, dpp, in dorsal/ventral pattern formation.

COUP-TFI is a potential regulator of retinoic acid-modulated development in Xenopus embryos

Isomers of retinoic acid are considered likely regulators of developmental pattern formation in vertebrate embryos. The orphan receptor COUP-TFI, which can alter cellular responses to retinoic acid in cultured cells, is expressed in distinct regions of the developing zebrafish and mouse anterior central nervous system. We asked if COUP-TFI can modulate retinoic acid signaling and anterior neural development in a vertebrate embryo by examining: (1) whether COUP-TFI could alter transcriptional responses to retinoic acid in Xenopus embryonic explants, and (2) whether misexpression of COUP-TFI could regulate anterior neural gene expression during early Xenopus development. The results from these studies show that COUP-TFI is a potent regulator of retinoic acid-induced gene expression in Xenopus embryonic cells, and that misexpression of COUP-TFI causes deficiencies in anterior neural structures and head development in Xenopus embryos with a concomitant change in anterior neural gene expression. These results support the proposition that COUP-TFI has a role in the elaboration and patterning of anterior neural gene expression in vertebrates, possibly via effects on the retinoic acid signaling pathways.

Regulation of Spemann organizer formation by the intracellular kinase Xgsk-3

Dorsal axis formation in the Xenopus embryo can be induced by the ectopic expression of several Wnt family members. In Drosophila, the protein encoded by the Wnt family gene, wingless, signals through a pathway that antagonizes the effects of the serine/threonine kinase zeste-white 3/shaggy. We describe the isolation and characterization of a Xenopus homolog of zeste-white 3/shaggy, Xgsk-3. A kinase-dead mutant of Xgsk-3, Xgsk-3K-->R, has a dominant negative effect and mimics the ability of Wnt to induce a secondary axis by induction of an ectopic Spemann organizer. Xgsk-3K-->R, like Wnt, induces dorsal axis formation when expressed in the deep vegetal cells, which do not contribute to the axis. These results indicate that the dorsal fate is actively repressed by Xgsk-3, which must be inactivated for dorsal axis formation to occur. Furthermore, our work suggests that the effects of Xgsk-3K-->R are mediated by an additional intercellular signal.

Xenopus laevis: a model system for the study of embryonic retinoid metabolism. II. Embryonic metabolism of all-trans-3,4-didehydroretinol to all-trans-3,4-didehydroretinoic acid

This study demonstrates early embryonic metabolism of exogenous all-trans-3,4-didehydroretinol (vitamin A2) to all-trans-3,4-didehydroretinal and to all-trans-3,4-didehydroretinoic acid in Xenopus embryos during neurulation. The latter metabolite was recently shown to bind with high affinity and to activate various retinoic acid receptors. Embryos treated with all-trans-3,4-didehydroretinol during early or late gastrulation exhibited abnormalities along the anteroposterior axis. The abnormalities were primarily in the posterior regions of the embryo, with only minor defects anteriorally. Eye malformations, typical for early exposure to 9-cis- and all-trans-retinols and retinals (companion paper), were not observed. We also present evidence that all-trans-3,4-didehydroretinoic acid is present endogenously during early neurulation and is evenly distributed along the anteroposterior axis. After treatment with all-trans-3,4-didehydroretinol, embryonic levels of all-trans-3,4-didehydroretinoic acid exceeded endogenous levels of this metabolite during early and late neurulation. We hypothesize that the dysmorphogenic effects produced by treatment of Xenopus embryos with the alcohol precursor, all-trans-3,4-didehydroretinol, are the result of its embryonic conversion to its corresponding acid ligand.

Xenopus laevis: a model system for the study of embryonic retinoid metabolism. I. Embryonic metabolism of 9-cis- and all-trans-retinals and retinols to their corresponding acid forms

Recently, the temporal and spatial distribution patterns of two established, endogenous retinoid receptor ligands, 9-cis-retinoic acid and all-trans-retinoic acid and various precursor retinoids were described in Xenopus embryos during early development (Creech Kraft et al., Proc. Natl. Acad. Sci. U.S.A. 1994; Biochem. J. 1994). Each of these two receptor ligands is a metabolite of vitamin A (all-trans-retinol), and each is also a potent dysmorphogen in Xenopus embryos as well as in embryos of several other vertebrate species. This study demonstrates early embryonic metabolism of exogenous all-trans-retinol, 9-cis-retinol, all-trans-retinal, and 9-cis-retinal to 9-cis-retinoic acid, all-trans-retinoic acid, and other metabolites in Xenopus embryos during neurulation, a specific stage of development that spans a time period of approximately 8 hr. Our results demonstrate that the Xenopus embryo provides a suitable model system for studying the embryonic bioconversion of retinoids and dysmorphogenic effects within a single time window of development.

Combinatorial signaling in development

Intercellular signaling plays a major role in the development of vertebrate and invertebrate embryos. In several cases, including the induction of mesoderm and neural ectoderm induction in Xenopus and the induction of the vulva in C. elegans, multiple intercellular signals are utilized. This review examines a number of examples of signaling in development wherein two signals combine to affect the fate of a cell. The examples are placed in distinct categories, based on whether the signals synergize with or antagonize one another, and on the inductive potential of the individual signals. These types of combinatorial signaling events are suggested to be a general feature of embryonic development.

Temporal distribution, localization and metabolism of all-trans-retinol, didehydroretinol and all-trans-retinal during Xenopus development

Recently, the temporal and spatial distribution patterns of the retinoid receptor ligands 9-cis-retinoic acid and all-trans-retinoic acid were described in Xenopus embryos during early development [Creech Kraft, Schuh, Juchau and Kimelman (1994) Proc. Natl. Acad. Sci. U.S.A., in the press]. The present study demonstrates the presence and distribution of their likely precursors, all-trans-retinol, didehydroretinol, didehydroretinal and all-trans-retinal, as well as the occurrence of 4-oxo metabolites, in Xenopus embryos. The temporal and spatial distribution patterns of all-trans-retinol, didehydroretinol and all-trans-retinal did not coincide with that observed for 9-cis-retinoic acid but, in certain regards, were similar to the patterns delineated for all-trans-retinoic acid and all-trans-retinoyl beta-glucuronide. Evidence is presented that 9-cis-retinoic acid can be synthesized from both all-trans-retinoic acid and all-trans-retinol in Xenopus embryos, suggesting that the difference between the distributions of 9-cis-retinoic acid and the other retinoids may be caused by selective synthesis and/or protein binding of the 9-cis isomer.

The retinoid X receptor ligand, 9-cis-retinoic acid, is a potential regulator of early Xenopus development

Endogenous retinoids are potential regulators of vertebrate embryogenesis that have been implicated in early anterior-posterior patterning and limb-bud development. We have characterized the temporal and spatial distribution of 9-cis-retinoic acid in the Xenopus embryo and compared it to two other retinoids, all-trans-retinoic acid and all-trans-retinoyl-beta-glucuronide. 9-cis-Retinoic acid is first detected after the midblastula transition and by the end of gastrulation is localized primarily within the anterior and posterior dorsal regions of the embryo. Since 9-cis-retinoic acid is a 6-fold more potent dysmorphogen than trans-retinoic acid, we suggest that it is involved in the early specification of the Xenopus anterior-posterior axis.

Fibroblast growth factor, but not activin, is a potent activator of mitogen-activated protein kinase in Xenopus explants

Isolated explants from the animal hemisphere of Xenopus embryos were incubated with Xenopus basic fibroblast growth factor (XbFGF) or human activin A. XbFGF incubation resulted in the rapid activation of mitogen-activated protein kinase (MAPK) and ribosomal S6 protein kinase (pp90rsk) in a dose-dependent manner with the highest levels of activation occurring at 50 ng/ml. Maximal activation occurred within 6-10 min after the addition of growth factor, and the activity of both kinases declined to unstimulated levels after 30 min. Activin was unable to activate either MAPK or pp90rsk in the Xenopus explants to a substantial level, although it induced dorsal mesoderm better than XbFGF under the same experimental conditions. The regulatory protein Xwnt-8 did not activate MAPK, nor did it enhance the activation of MAPK by XbFGF. XbFGF was able to activate MAPK through at least the midgastrula stage, suggesting that this family of growth factors may have a role in gastrula-stage events.

Activin-mediated mesoderm induction requires FGF

The early patterning of mesoderm in the Xenopus embryo requires signals from several intercellular factors, including mesoderm-inducing agents that belong to the fibroblast growth factor (FGF) and TGF-beta families. In animal hemisphere explants (animal caps), basic FGF and the TGF-beta family member activin are capable of converting pre-ectodermal cells to a mesodermal fate, although activin is much more effective at inducing dorsal and anterior mesoderm than is basic FGF. Using a dominant-negative form of the Xenopus type 1 FGF receptor, we show that an FGF signal is required for the full induction of mesoderm by activin. Animal caps isolated from embryos that have been injected with the truncated FGF receptor and cultured with activin do not extend and the induction of some genes, including cardiac actin and Xbra, is greatly diminished, while the induction of other genes, including the head organizer-specific genes gsc and Xlim-1, is less sensitive. These results are consistent with the phenotype of the truncated FGF receptor-injected embryo and imply that the activin induction of mesoderm depends on FGF, with some genes requiring a higher level of FGF signaling than others.

Dorsal-ventral differences in Xcad-3 expression in response to FGF-mediated induction in Xenopus

We have identified a dorsal-ventral difference in the specification of mesoderm in vivo by examining the effect of the dominant-negative FGF receptor on a new member of the Xenopus caudal gene family, Xcad-3. Xcad-3 is expressed throughout the marginal zone during the gastrula stages and serves as a useful marker for events occurring within the mesoderm. Disruption of the FGF signaling pathway by the dominant-negative FGF receptor, disrupts the Xcad-3 expression pattern, eliminating expression preferentially from the dorsal regions of the embryo. We also find that the expression of the Xenopus brachyury homolog, Xbra, is more readily eliminated from the dorsal than the ventral region of the embryo by the dominant-negative FGF receptor, indicating that the observed dorsal-ventral differences are not unique to Xcad-3. These results demonstrate the importance of regional effects on FGF-mediated induction in vivo and suggest that FGF-dependent expression of mesodermal genes depends upon the localization of other factors which establish dorsal-ventral differences within the embryo.

v-erbA and citral reduce the teratogenic effects of all-trans retinoic acid and retinol, respectively, in Xenopus embryogenesis

Treatment of late blastula/early gastrula stage Xenopus embryos with all-trans retinoic acid results in disruption of the primary body axis through effects on both mesoderm and neuroectoderm. This effect of retinoic acid, coupled with the known presence of retinoic acid in Xenopus embryos has led to the proposal that retinoic acid may be an endogenous morphogen providing positional information in early development. To further elucidate the role of retinoic acid in early Xenopus development, we have attempted to interfere with the retinoic acid signalling pathway both at the level of retinoic acid formation, by treatment with citral (3,7-dimethy-2,6-octadienal), and at the level of nuclear retinoic acid receptor function, by microinjection of v-erbA mRNA. The feasibility of this approach was demonstrated by the ability of citral treatment and v-erbA mRNA injection to reduce the teratogenic effects of exogenous retinol and retinoic acid, respectively, in early Xenopus development. Interestingly, v-erbA mRNA injection and citral treatment of gastrula stage embryos resulted in tadpoles with a similar set of developmental defects. The defects were chiefly found in tissues that received a contribution of cells from the neural crest, suggesting that at least a subset of neural crest cells may be sensitive to the endogenous level of retinoic acid. In accord with this proposal, it was found that the expression patterns of two early markers of cranial neural crest cells, Xtwi and XAP-2, were altered in embryos injected with v-erbA mRNA. These results indicate that structures in addition to the primary axis are regulated by retinoic acid signalling during early Xenopus development.

Induction of the Xenopus organizer: expression and regulation of Xnot, a novel FGF and activin-regulated homeo box gene

We have searched for homeo box-containing genes expressed during gastrulation in Xenopus embryos with the expectation that analysis of the spatial and temporal expression of these genes will lead to greater understanding of the regionalization of the mesoderm. We describe the cloning and expression of Xnot, a novel homeo box-containing gene expressed primarily in the gastrula organizing region. We have studied the regulation of Xnot by signaling molecules involved in mesoderm induction and regionalization. Surprisingly, we found that FGF signaling is required for expression of Xnot in the gastrula organizing region, clearly implicating FGF in the induction of dorsal mesoderm. Furthermore, we found that Xnot is initially expressed throughout the embryo and that progressive translation of an unknown protein restricts expression of Xnot to the organizing region. Our results provide experimental evidence supporting the proposed division of Spemann's organizer into independently regulated organizing centers.

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 embryogical 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.

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.

Induction of dorsal and ventral mesoderm by ectopically expressed Xenopus basic fibroblast growth factor

Peptide growth factors from the fibroblast growth factor (FGF) and transforming growth factor-beta families are likely regulators of mesoderm formation in the early Xenopus embryo. Although basic FGF is found in the Xenopus embryo at the correct time and at sufficient concentrations to suggest that it is the FGF-type inducer, the lack of a secretory signal sequence in the basic FGF peptide has raised questions as to its role in the inductive process. We show here that Xenopus basic FGF can ectopically induce mesoderm when translated from injected synthetic RNA within the cells of a Xenopus embryo. Basic FGF produced in this manner is able to induce the formation of both dorsal and ventral mesoderm with the type of mesoderm formed dependent on the inherent dorsal-ventral polarity of the animal hemisphere. Surprisingly, although Xenopus basic FGF produced from the injected mRNA has a potent mesodermalizing effect on animal hemisphere cells, virtually no phenotypic effect is observed with intact embryos. These results suggest that the role of Xenopus basic FGF is to specify the size of the marginal zone, and synergistically with a dorsally localized prepatterning signal, to initially establish the dorsal-ventral axis of the mesoderm.

An antisense mRNA directs the covalent modification of the transcript encoding fibroblast growth factor in Xenopus oocytes

The Xenopus oocyte contains three maternal basic fibroblast growth factor (bFGF) transcripts, the largest of which encodes the complete bFGF polypeptide. We show that the smallest transcript is transcribed in the opposite direction of the largest transcript and overlaps part of the coding sequence of bFGF. This antisense transcript encodes a highly conserved 25 kd polypeptide. In addition, the antisense transcript causes modification of the mRNA encoding bFGF during maturation of the oocyte, converting half of the adenine residues to inosine in the region of overlap between the sense and antisense transcripts. As this activity acts only on double-stranded RNA, the two mRNAs must be hybridized in the oocyte. We discuss a possible role for the antisense transcript in regulating the stability of the bFGF mRNA.

The presence of fibroblast growth factor in the frog egg: its role as a natural mesoderm inducer

A complementary DNA clone corresponding to a 4.2-kilobase transcript that is present in the Xenopus oocyte and newly transcribed in the neurula stages of development has been isolated. This messenger RNA encodes a 155-amino acid protein that is 84% identical to the human basic fibroblast growth factor (bFGF). When expressed in Escherichia coli and purified, the Xenopus FGF induced mesoderm in animal cell blastomeres as measured by muscle actin expression. Immunoblots with an antibody to a Xenopus FGF peptide show that the oocyte and early embryo contain a store of the FGF polypeptide at high enough concentrations to induce mesoderm. The presence of FGF in the oocyte, together with the apparent lack of a secretory signal sequence in the protein, suggest that the regulation of mesoderm induction may involve novel mechanisms that occur after the translation of FGF.

Synergistic induction of mesoderm by FGF and TGF-beta and the identification of an mRNA coding for FGF in the early Xenopus embryo

The primary patterning event in early vertebrate development is the formation of the mesoderm and its subsequent induction of the neural tube. Classic experiments suggest that the vegetal region signals the animal hemisphere to diverge from the pathway of forming ectoderm to form mesoderm such as muscle. Here we show that bovine basic FGF has a limited capacity to induce muscle actin expression in animal hemisphere cells. This level of expression can be raised to levels normally induced in the embryo by another mammalian growth factor, TGF-beta, which by itself will not induce actin expression. We show that the Xenopus embryo contains an mRNA encoding a protein highly homologous to basic FGF. These results together with the identification of a maternal mRNA with strong homology to TGF-beta, suggest that molecules closely related to FGF and TGF-beta are the natural inducers of mesoderm in vertebrate development.

The events of the midblastula transition in Xenopus are regulated by changes in the cell cycle

The midblastula transition (MBT) in Xenopus can be initiated prematurely by blocking the fundamental cell-cycle oscillator with cycloheximide, in which case motility and transcription are quickly initiated. Using various inhibitors of specific events of the cell cycle that do not inhibit the autonomous oscillator, we have shown that transcription is activated when DNA synthesis is interrupted and motility is activated when cell cleavage is inhibited. Furthermore, very low levels of transcription are found to occur before the MBT. These results demonstrate that the pre-MBT egg is fully competent for transcription and motility and suggest that different features of the rapid early cell cycle normally suppress these events.

A novel general approach to eucaryotic mutagenesis functionally identifies conserved regions within the adenovirus 13S E1A polypeptide

A new approach to the isolation of mutations in mammalian genes was developed which permits both the selection of infrequently occurring mutants that alter the cellular morphology of recipient cells and the rapid reisolation of the mutant gene. The adenovirus type 5 13S early region 1a (E1a) gene was mutagenized in vitro with sodium bisulfite and then efficiently transferred into cells with a retrovirus shuttle vector. Three classes of mutants of the 13S E1a gene product were isolated, each of which induced a distinct morphological alteration. The mutant E1a gene was reisolated from each cell line, and the precise nucleotide changes were determined. The E1a-induced morphological alterations were further examined by the construction of single and double point mutations within different regions of the polypeptides by utilizing the amino acid substitutions obtained from the original mutants. The results suggest that each of the three regions of highly conserved amino acids within the E1a 13S polypeptide has a distinct role in the alteration of cellular morphology and the activation of gene expression.

Individual adenovirus type 5 early region 1A gene products elicit distinct alterations of cellular morphology and gene expression

Adenovirus early region 1A (E1A), which gives rise to three overlapping transcripts, was inserted into a murine leukemia virus-derived vector, and recombinant viruses were used to prepare permanent cell lines of NIH 3T3 cells containing DNA copies of the individual 13S, 12S, and 9S mRNAs. Integrated proviral copies of the recombinant genomes were rescued as bacterial plasmids from each of the cell lines, and the DNA sequence of E1A was demonstrated to be a precise copy of the individual transcripts. The DNA copies were shown to be expressed as part of the full-length retroviral transcript by S1 nuclease analysis, and the synthesis of their encoded polypeptides was demonstrated by immunoprecipitation. Those cell lines expressing the polypeptide encoded by the 13S transcript were shown to contain that function required for regulating the accumulation of mRNAs from adenovirus early genes by their ability to complement the adenovirus type 5 E1A deletion mutant dl312. Cell lines expressing polypeptides encoded by the 13S, 12S, and 9S transcripts showed characteristic alterations in morphology. Two-dimensional gel electrophoresis of total cellular protein derived from the three cell lines demonstrated that each E1A gene product elicits specific alterations in the patterns of proteins expressed. Studies of the expression of two specific genes, those encoding fibronectin and collagen type 1, indicated that the observed alteration in levels of the two proteins results from a reduction in RNA levels induced by E1A functions.

E1a regions of the human adenoviruses and of the highly oncogenic simian adenovirus 7 are closely related

Simian adenovirus 7 (SA7) is a highly oncogenic virus, capable of causing tumors in hamsters upon the direct injection of viral DNA. We determined the transcriptional organization of the transforming region and compared it with that of the human adenoviruses. This analysis demonstrated that there are two independently promoted transcription units similar to the E1a and E1b regions of the human adenoviruses. The nucleotide sequence of the SA7 E1a region demonstrated considerable homology with the human adenoviruses, both in the sequences that regulate E1a expression and in the encoded polypeptides. The amino acid homology was reflected in the ability of SA7 to complement the growth of human adenoviruses mutant in the E1a region. Furthermore, we found two regions of amino acid homology unique to SA7 and the highly oncogenic human adenovirus 12.

Synthesis in Escherichia coli of human adenovirus type 12 transforming proteins encoded by early region 1A 13S mRNA and 12S mRNA

Human adenovirus (Ad)-encoded early region 1A (E1A) tumor (T) antigens have been implicated in the positive regulation of viral early genes, the positive and negative regulation of some cellular genes, and cell immortalization and transformation. To further study the Ad E1A T antigens and to facilitate their purification, we have cloned cDNA copies of the Ad12 E1A 13S mRNA and 12S mRNA downstream of a hybrid Escherichia coli trp-lac (tac) promoter. Up to 8% of the protein synthesized in E. coli cells transformed by each of the two different Ad12 E1A cDNA constructs were immunoprecipitated as a Mr 47,000 protein by antibody to a synthetic peptide encoded in the Ad12 E1A DNA sequence. Both proteins produced in E. coli appear to be authentic and complete Ad12 E1A T antigens because they possess (i) the Ad12 E1A NH2-terminal amino acid sequence predicted from the DNA sequence; (ii) the Ad12 E1A COOH-terminal sequence, as shown by immunoprecipitation with anti-peptide antibody; and (iii) a molecular weight and an acidic isoelectric point similar to that of the E1A T antigens synthesized in Ad12-infected and transformed mammalian cells. The T antigens were purified to near homogeneity in yields of 100-200 micrograms per g wet weight of transformed E. coli cells.

Identification of adenovirus 12-encoded E1A tumor antigens synthesized in infected and transformed mammalian cells and in Escherichia coli

A 16-amino acid peptide, H2N-Arg-Glu-Gln-Thr-Val-Pro-Val-Asp-Leu-Ser-Val-Lys-Arg-Pro-Arg-Cys-COOH (peptide 204), targeted to the common C-terminus of human adenovirus 12 (Ad12) tumor antigens encoded by the E1A 13S mRNA and 12S mRNA, has been synthesized. Antibody prepared in rabbits against peptide 204 immunoprecipitated two proteins of apparent Mr 47,000 and 45,000 from extracts of [35S]methionine-labeled Ad12-early infected KB cells and a 47,000 protein from extracts of the Ad12-transformed hamster cell line, HE C19. Immunoprecipitation analysis of infected and transformed cells labeled with 32Pi showed that both major Ad12 E1A T antigens are phosphoproteins. Immunofluorescence microscopy of Ad12-early infected KB cells with antipeptide antibody showed the site of E1A protein concentration to be predominantly nuclear. E1A proteins were detected by immunofluorescence at 4 to 6 h postinfection and continued to increase until at least 18 h postinfection. Antipeptide 204 antibody was used to analyze the proteins synthesized in Escherichia coli cells transformed by plasmids containing cDNA copies of the Ad12 E1A 13S mRNA or 12S mRNA under the control of the tac promoter (D. Kimelman, L. A. Lucher, M. Green, K. H. Brackmann, J. S. Symington, and M. Ptashne, Proc. Natl. Acad. Sci. U.S.A., in press). A major protein of ca. 47,000 was immunoprecipitated from extracts of each transformed E. coli cell clone. Two-dimensional gel electrophoretic analysis of immunoprecipitates revealed that the T antigens synthesized in infected KB cells, transformed hamster cells, and transformed E. coli cells possess very similar molecular weights and acidic isoelectric points of 5.2 to 5.4.

Production of a monospecific antiserum against the early region 1A proteins of adenovirus 12 and adenovirus 5 by an adenovirus 12 early region 1A-beta-galactosidase fusion protein antigen expressed in bacteria

Antisera were prepared against the amino acid sequences encoded within the N-terminal half of the adenovirus 12 (Ad12) early region 1A (E1A) gene. This was accomplished by construction of a plasmid vector which encoded the N-terminal 131 amino acids of Ad12 E1A joined in frame to the coding sequence of beta-galactosidase. After induced synthesis in Escherichia coli, the Ad12 E1A-beta-galactosidase fusion protein (12-1A-FP) was extracted with urea and used to raise antibodies in rabbits. The 12-1A-FP antisera immunoprecipitated major phosphoproteins of 39,000 and 37,000 apparent molecular weights from Ad12-transformed and infected cells. The 12-1A-FP antisera also immunoprecipitated E1A phosphoproteins from Ad5-transformed and infected cells. Immunospecificity of the 12-1A-FP antisera was demonstrated by the ability of 12-1A-FP antigen to block immunoprecipitation of E1A proteins. Furthermore, E1A proteins immunoprecipitated from in vivo-labeled cells comigrated with those translated in vitro by RNA that had been hybridization selected to E1A DNA.

Expression of the human fibroblast interferon gene in Escherichia coli

We applied the method of Guarente et al. [Guarente, L., Lauer, G., Roberts, T.M. & Ptashne, M. (1980) Cell 20, 543-553] to construct plasmids that direct expression in Escherichia coli of the human fibroblast interferon (F-IF) gene. Two plasmids were recovered. One directs efficient synthesis of a protein whose primary sequence is that of pre-F-IF and the other, that of mature F-IF. Extracts of bacteria synthesizing mature F-IF display antiviral activity characteristic of human F-IF. This activity is lower than that expected from the differential rate of synthesis of the protein. We have detected no such activity in extracts of bacteria synthesizing pre-F-IF.