Bone morphogenetic protein 4: a ventralizing factor in early Xenopus development

Negative control of Xenopus GATA-2 by activin and noggin with eventual expression in precursors of the ventral blood islands

To increase our understanding of haematopoiesis during early vertebrate development, we have studied the expression pattern of the transcription factor GATA-2 in Xenopus embryos, and asked how this is regulated. We show that the blood island precursors of the ventral mesoderm express GATA-2 RNA at neural tube stages, some 5 hours before globin RNA is detected in their derivatives. Prior to this however, GATA-2 is expressed much more widely within the embryo. Maternal transcripts are uniformly distributed, and zygotic transcription is activated during gastrulation throughout ventral and lateral regions of the embryo, with expression highest in the sensorial ectoderm and only weak in the ventral mesoderm. The domain of GATA-2 expression in neurulae outlines the region of the neural plate and suggests a possible wider role in dorsoventral patterning. To identify the signals involved in regulating this pattern of expression, we performed experiments with embryo explants. GATA-2 is activated autonomously in isolated animal caps and this activation is suppressed by the mesoderm-inducing factor activin, but not by FGF. Thus, the down-regulation of GATA-2 observed in the region of the Spemann organiser may be a response to an activin-like signal emanating from the dorsal-vegetal region or Nieuwkoop centre. GATA-2 activation in animal caps and ventral marginal zones was suppressed by co-culturing with dorsal marginal zones, suggesting that a signal from the Spemann organiser is involved in suppression of GATA-2 in the dorsal region of the embryo. Expression of a candidate for this signal, noggin, had the same effect. Taken together, the observations presented here suggest that GATA-2 activation occurs by default in the absence of signals, that the restriction of its expression within the early embryo is controlled by negative signals emanating from the Nieuwkoop centre and the organiser, and that noggin and activin-like molecules play a role in these signalling pathways.

Induction of mesoderm in Xenopus laevis embryos by translation initiation factor 4E

The microinjection of messenger RNA encoding the eukaryotic translation initiation factor 4E (eIF-4E) into early embryos of Xenopus laevis leads to the induction of mesoderm in ectodermal explants. This induction occurs without a stimulation of overall protein synthesis and is blocked by the co-expression of a dominant negative mutant of the proto-oncogene ras or a truncated activin type II receptor. Although other translation factors have been studied in vertebrate and invertebrate embryos, none have been shown to play a direct role in development. The results here suggest a mechanism for relaying and amplifying signals for mesoderm induction.

Slow emergence of a multithreshold response to activin requires cell-contact-dependent sharpening but not prepattern

The growth factor activin elicits mesodermal fates when applied to prospective ectodermal cells of the Xenopus blastula stage embryo. Previous experiments with dissociated cells showed that there are at least five different responses separated by closely spaced, sharp dose thresholds. Here we investigate this multithreshold activin response further using probes for genes expressed at early gastrula stages, namely Xbra, goosecoid, noggin, Xwnt-8 and Mix.1. We show that initial dose-response profiles are broad and smooth in contrast to the later threshold-bound patterns. For Xbra, goosecoid and noggin, the later expression ranges are subsets of earlier ones. Unexpectedly, Xwnt-8 is initially induced at high doses only, but later appears only in cells that have received a low dose of activin. Keeping the cells dissociated after activin treatment, rather than allowing them to reaggregate, prevents sustained expression of Xbra and Xwnt-8 but allows that of goosecoid and noggin. However, cell contact is required for sharpening the dose-response threshold of goosecoid. Finally, we show that a previously reported dorsoventral prepattern in the animal cap is also cell-contact dependent and it is not required for the multi-threshold response to activin.

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.

TGF-beta related genes in development

Embryonic induction is the process by which signals from one cell population change the developmental fate of another. Polypeptides related to growth factors are one group of molecules mediating many inductive events. Recent data on the embryonic expression and function of signaling proteins related to transforming growth factor beta, in both vertebrate and invertebrate systems, have shown that these molecules play important roles in both pattern formation and tissue specification during embryogenesis.

Periodontal regeneration: potential role of bone morphogenetic proteins

Initiation of osteogenesis and cementogenesis is a problem central to periodontal regeneration. A major advance in the understanding of bone formation has been the identification of an entirely new family of protein initiators, the bone morphogenetic proteins, that regulate cartilage and bone differentiation in vivo. The purification, genetic cloning and expression of recombinant human bone morphogenetic proteins (BMPs) have laid the foundation for the cellular and molecular dissection of bone development and regeneration. The striking evolutionary conservation of the BMP genes indicates that they are critical in the normal development and function of animals. In addition to postfetal osteogenesis, the BMPs may play multiple roles in embryonic development and organogenesis, including skeletogenesis and the development of craniofacial and dental tissues. The availability of recombinant human BMPs provides several challenges and opportunities to gain insights into the mechanisms regulating the regeneration of bone and cementum for optimal outcome in the periodontal patient.

Disruption of mesoderm and axis formation in fish by ectopic expression of activin variants: the role of maternal activin

Formation of mesoderm in Xenopus embryos is the result of an induction event in which peptides such as FGF or activins have been implicated. It was recently demonstrated, by the ectopic expression of a truncated activin receptor, that activin receptor signaling pathways are involved in the processes of mesoderm and axis formation in vivo. However, this approach does not directly address the role of activin itself nor the involvement of activins in the formation of mesoderm in embryos from other vertebrates. In addition, activins are expressed maternally as a protein component of the egg as well as transcribed zygotically, and it is not clear which of the two forms is involved in mesoderm formation. To address those three issues, we analyzed the role of activins in the development of fish embryos by generating two activin dominant-negative variants. One of the variants behaves as an inhibitor of activin protein. The second variant was found to deplete the activin pool when cotranslated with wild-type activin. Injection of RNA encoding these variants into the two-cell embryo of the small teleost fish Oryzias latipes (Japanese medaka) demonstrates that only the maternally provided activin protein is required for mesoderm and axis formation in fish in vivo.

Induction of erythropoiesis in the amphibian embryo

Germ-layer formation and differentiation of specialized tissues in vertebrate embryogenesis is a multistep mechanism that is mediated by different growth factors (or growth factor-related molecules). We have investigated various differentiation factors that contribute to mesoderm formation in amphibian embryos and analyzed the formation of blood islands during embryogenesis and in ectodermal explants that have been incubated with mesoderm inducing factors. Erythropoiesis in these explants is demonstrated by whole mount in situ hybridization using an embryonic alpha-globin probe. Furthermore, we have isolated several transcription factors of the fork head family and analyzed whether they are involved in the pathway leading to hematopoietic cells. One of these factors, termed Xenopus fork head (XFD)-2, is transcribed in blastula stage embryos for a limited time period in dorsal and ventral mesoderm. Moreover, the target sequence of this factor is found to be present within the upstream sequences of many genes that are expressed in mesoderm-derived tissues.

Activin/inhibin beta B subunit gene disruption leads to defects in eyelid development and female reproduction

Inhibins and activins are dimeric growth factors of the transforming growth factor-beta superfamily, a class of peptides that can regulate the growth and differentiation of a variety of cell types. Recently, activins have been implicated in early vertebrate development through their ability to evoke, in Xenopus embryo explants, both morphological and molecular changes characteristic of mesoderm induction. To understand these processes further, we have used homologous recombination in embryonic stem cells to create mouse strains carrying mutations in the gene encoding the activin/inhibin beta B subunit. These mice are expected to be deficient in activin B (beta B:beta B), activin AB (beta A:beta B), and inhibin B (alpha:beta B). Viable mutant animals were generated, indicating that the beta B subunit is not essential for mesoderm formation in the mouse. Mutant animals suffered, however, from distinct developmental and reproductive defects. An apparent failure of eyelid fusion during late embryonic development led to eye lesions in mutant animals. Whereas beta B-deficient males bred normally, mutant females manifested a profoundly impaired reproductive ability, characterized by perinatal lethality of their offspring. The phenotype of mutant mice suggests that activin beta B (1) plays a role in late fetal development and (2) is critical for female fecundity. In addition, we have found that expression of the related beta A subunit of activin is highly upregulated in ovaries of mutant females. Altered regulation of beta A activin in beta B-deficient mice may contribute to the mutant phenotype.

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.

Mesoderm induction by activin requires FGF-mediated intracellular signals

We have examined the role of FGF signaling during activin-mediated mesoderm induction in Xenopus. Using dominant inhibitory mutants of FGF signal transducers to disrupt the FGF-signaling pathway at the plasma membrane or in the cytosol prevents animal cap blastomeres from expressing several mesodermal markers in response to exogenous activin. Dominant inhibitory mutants of the FGF receptor, c-ras or c-raf inhibit the ability of activin to induce molecular markers of both dorsal and ventral mesoderm including Xbra, Mix1 and Xnot. Some transcriptional responses to activin such as goosecoid and Xwnt8 are inhibited less effectively than others, however, suggesting that there may differing requirements for an FGF signal in the responses of mesoderm-specific genes to activin induction. Despite the requirement for this signaling pathway during activin induction, downstream components of this pathway are not activated in response to activin, suggesting that activin does not signal directly through this pathway.

Inducing factors in Xenopus early embryos

Recent results make it possible to postulate credible candidates for each of the known inducing signals that act to determine cell fate during Xenopus early development. Experiments on biological activity, expression patterns and inhibition of function suggest that Vg-1 and Wnt-11 may act as the primary mesoderm-inducing signals, FGF and activin may serve to relay their effects, and noggin may be a major component of the dorsalizing and neural-inducing signals from the organizer.

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.

Bone morphogenetic proteins and a signalling pathway that controls patterning in the developing chick limb

We show here that bone morphogenetic protein 2 (BMP-2) is involved in patterning the developing chick limb. During early stages of limb development, mesenchymal expression of the Bmp-2 gene is restricted to the posterior part of the bud, in a domain that colocalizes with the polarizing region. The polarizing region is a group of cells at the posterior margin of the limb bud that can respecify the anteroposterior axis of the limb when grafted anteriorly and can activate expression of genes of the HoxD complex. We dissect possible roles of BMP-2 in the polarizing region signalling pathway by manipulating the developing wing bud. Retinoic acid application, which mimics the effects of polarizing region grafts, activates Bmp-2 gene expression in anterior cells. This shows that changes in anteroposterior pattern are correlated with changes in Bmp-2 expression. When polarizing region grafts are placed at the anterior margin of the wing bud, the grafts continue to express the Bmp-2 gene and also activate Bmp-2 expression in the adjacent anterior host mesenchyme. These data suggest that BMP-2 is part of the response pathway to the polarizing signal, rather than being the signal itself. In support of this, BMP-2 protein does not appear to have any detectable polarizing activity when applied to the wing bud. The pattern of Bmp-4 gene expression in the developing wing bud raises the possibility that BMP-2 and BMP-4 could act in concert. There is a close relationship, both temporal and spatial, between the activation of the Bmp-2 and Hoxd-13 genes in response to retinoic acid and polarizing region grafts, suggesting that expression of the two genes might be linked.

Expression of the Brachyury gene during mesoderm development in differentiating embryonal carcinoma cell cultures

When aggregated and treated with dimethyl sulfoxide (DMSO), P19 embryonal carcinoma cells differentiate into cell types normally derived from the mesoderm and endoderm including epithelium and cardiac and skeletal muscle. The Brachyury gene is expressed transiently in these differentiating cultures several days before the appearance of markers of the differentiated cell types. The expression of Brachyury is not affected by DMSO but is induced by cell aggregation, which requires extracellular calcium. Expression of Brachyury is also induced by various members of the TGF beta family such as activin and bone morphogenetic proteins. D3 is a mutant clone of P19 cells selected for its failure to differentiate when aggregated in DMSO. Aggregated D3 cells express Brachyury mRNA suggesting that the mutation(s) responsible for the phenotype of D3 cells is downstream of the chain of events initiated by Brachyury expression.

The TGF-beta superfamily: new members, new receptors, and new genetic tests of function in different organisms

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Cloning and expression of cDNA encoding Xenopus laevis bone morphogenetic protein-1 during early embryonic development

The Xenopus laevis DNA fragment encoding a protein homologous with human bone morphogenetic protein-1 (BMP-1) was amplified by polymerase chain reaction (PCR) from cDNA derived from stage 26 (st.26) embryonic RNA. Subsequently this fragment was used as a probe to isolate cDNA clones by screening of a X. laevis st.24 embryonic cDNA library. Two partial clones (22 and 63) were obtained and the missing 5'-end of the clone 22 was extended by the anchored PCR technique. The nucleotide sequence of the resulting clone (22AN) contained an open reading frame coding for a protein with 707 deduced amino acids. Three sizes of mRNA (2.9, 5.2 and 6.6 kb) were detected in blastula (st.9) and early gastrula (st.10) embryos, and in hatched tadpole (st.40), but little or no expression was observed in morula (st.7) and late gastrula (st.12) embryos, suggesting a physiological role(s) of X.laevis BMP-1 in normal embryonic development.

Xwnt-11: a maternally expressed Xenopus wnt gene

We have isolated and characterized a novel Xenopus wnt gene, Xwnt-11, whose expression pattern and overexpression phenotype suggest that it may be important for dorsal-ventral axis formation. Xwnt-11 mRNA is present during oogenesis and embryonic development through swimming tadpole stages. Xwnt-11 mRNA is ubiquitous in early oocytes and is localized during mid-oogenesis. By late oocyte stages, Xwnt-11 mRNA is localized to the vegetal cortex, with some mRNA in the vegetal cytoplasm. After egg maturation, Xwnt-11 mRNA is released from the vegetal cortex and is found in the vegetal cytoplasm. This early pattern of Xwnt-11 mRNA localization is similar to another vegetally localized maternal mRNA, Vg1 (D. A. Melton (1987) Nature 328, 80–82). In the late blastula, Xwnt-11 mRNA is found at high levels in the dorsal marginal zone. As gastrulation proceeds, Xwnt-11 mRNA appears in the lateral and ventral marginal zone and, during tadpole stages, it is found in the somites and first branchial arch. Injection of Xwnt-11 mRNA into UV-ventralized embryos can substantially rescue the UV defect by inducing the formation of dorsal tissues. The rescued embryos develop somitic muscle and neural tube; however, they lack notochord and anterior head structures.

The daf-4 gene encodes a bone morphogenetic protein receptor controlling C. elegans dauer larva development

The bone morphogenetic protein (BMP) family is a conserved group of signalling molecules within the transforming growth factor-beta (TGF-beta) superfamily. This group, including the Drosophila decapentaplegic (dpp) protein and the mammalian BMPs, mediates cellular interactions and tissue differentiation during development. Here we show that a homologue of human BMPs controls a developmental switch in the life cycle of the free-living soil nematode Caenorhabditis elegans. Starvation and overcrowding induce C. elegans to form a developmentally arrested, third-stage dauer larva. The daf-4 gene, which acts to inhibit dauer larva formation and promote growth, encodes a receptor protein kinase similar to the daf-1, activin and TGF-beta receptor serine/threonine kinases. When expressed in monkey COS cells, the daf-4 receptor binds human BMP-2 and BMP-4. The daf-4 receptor is the first to be identified for any growth factor in the BMP family.

Suramin prevents transcription of dorsal marker genes in Xenopus laevis embryos, isolated dorsal blastopore lips and activin A induced animal caps

Suramin, a polyanionic compound which is known to interact with the receptors of growth factors inhibits the expression of dorsal marker genes in whole embryos and isolated dorsal blastopore lips. Suramin also prevents activin A induced dorsalization of animal cap explants from blastula stage embryos, but it simultaneously evokes a shift of the differentiation pattern from dorsal mesodermal structures (notochord, somites) to ventral mesodermal derivatives (mesothelium and erythroid precursor cells). The results are consistent with the assumption that the dorsal vegetal zone (Nieuwkoop center) primarily releases more general/ventral mesodermalization signals. They further suggest a dual role of activin A in early embryogenesis. While the maternal component may contribute to a more general/ventral type of induction, increasing concentrations of the zygotic component along with the activation of primary response genes may contribute to the dorsalization of the organizer.

Induction and axial patterning of the neural plate: planar and vertical signals

In this review I summarize recent findings on the contributions of different cell groups to the formation of the basic plan of the nervous system of vertebrate embryos. Midline cells of the mesoderm--the organizer, notochord, and prechordal plate--and midline cells of the neural ectoderm--the notoplate and floor plate--appear to have a fundamental role in the induction and patterning of the neural plate. Vertical signals acting across tissue layers and planar signals acting through the neural epithelium have distinct roles and cooperate in induction and pattern formation. Whereas the prechordal plate and notochord have distinct vertical signaling properties, the initial anteroposterior (A-P) pattern of the neural plate may be induced by planar signals originating from the organizer region. Planar signals from the notoplate may also contribute to the mediolateral (M-L) patterning of the neural plate. These and other findings suggest a general view of neural induction and axial patterning.

Planar and vertical induction of anteroposterior pattern during the development of the amphibian central nervous system

In amphibians and other vertebrates, neural development is induced in the ectoderm by signals coming from the dorsal mesoderm during gastrulation. Classical embryological results indicated that these signals follow a "vertical" path, from the involuted dorsal mesoderm to the overlying ectoderm. Recent work with the frog Xenopus laevis, however, has revealed the existence of "planar" neural-inducing signals, which pass within the continuous sheet or plane of tissue formed by the dorsal mesoderm and presumptive neurectoderm. Much of this work has made use of Keller explants, in which dorsal mesoderm and ectoderm are cultured in a planar configuration with contact along only a single edge, and vertical contact is prevented. Planar signals can induce the full anteroposterior (A-P) extent of neural pattern, as evidenced in Keller explants by the expression of genes that mark specific positions along the A-P axis. In this review, classical and modern molecular work on vertical and planar induction will be discussed. This will be followed by a discussion of various models for vertical induction and planar induction. It has been proposed that the A-P pattern in the nervous system is derived from a parallel pattern of inducers in the dorsal mesoderm which is "imprinted" vertically onto the overlying ectoderm. Since it is now known that planar signals can also induce A-P neural pattern, this kind of model must be reassessed. The study of planar induction of A-P pattern in Xenopus embryos provides a simple, manipulable, two-dimensional system in which to investigate pattern formation.

Analysis of gastrulation: different types of gastrulation movement are induced by different mesoderm-inducing factors in Xenopus laevis

In this paper we analyze the control of gastrulation in Xenopus laevis. Our approach takes advantage of the observation that mesoderm-inducing factors such as activin, FGF and BMP-4 induce presumptive ectodermal cells to undergo gastrulation-like movements. Activin, for example, makes intact animal pole regions undergo convergent extension and causes individual cells to spread and migrate on a fibronectin-coated substrate. By varying the concentrations of the growth factors to which animal pole cells are exposed, and by applying them in different combinations, we show how graded distributions of a combination of factors could establish the correct spatial and temporal patterns of gastrulation in the Xenopus embryo. The distributions we propose support and develop the model previously suggested by Green et al. (1992) to account for the spatial patterns of gene activation in the early embryo.

Xwnt-5A: a maternal Wnt that affects morphogenetic movements after overexpression in embryos of Xenopus laevis

To contribute to an understanding of the roles and mechanisms of action of Wnts in early vertebrate development, we have characterized the normal expression of Xenopus laevis Wnt-5A, and investigated the consequences of misexpression of this putative signalling factor. Xwnt-5A transcripts are expressed throughout development, and are enriched in both the anterior and posterior regions of embryos at late stages of development, where they are found primarily in ectoderm, with lower levels of expression in mesoderm. Overexpression of Xwnt-5A in Xenopus embryos leads to complex malformations distinct from those achieved by ectopic expression of Xwnts −1, −3A, or −8. This phenotype is unlikely to result from Xwnt-5A acting as an inducing agent, as overexpression of Xwnt-5A does not rescue dorsal structures in UV-irradiated embryos, does not induce mesoderm in blastula caps, and Xwnt-5A does not alter the endogenous patterns of expression of goosecoid, Xbra, or Xwnt-8. To pursue whether Xwnt-5A has the capacity to affect morphogenetic movements, we investigated whether overexpression of Xwnt-5A alters the normal elongation of blastula cap explants induced by activin. Intriguingly, Xwnt-5A blocks the elongation of blastula caps in response to activin, without blocking the differentiation of either dorsal or ventral mesoderm within these explants. The data are consistent with Xwnt-5A having the potential activity of modifying the morphogenetic movements of tissues.

Processed Vg1 protein is an axial mesoderm inducer in Xenopus

Vg1 is a TGF beta-related growth factor encoded by a maternal mRNA localized to vegetal blastomeres in Xenopus embryos. Vg1 precursor protein is abundant in vegetal cells, but the processed mature form has not been readily detected and no activity has been demonstrated for the putative Vg1 mature protein. We have engineered a BMP2-Vg1 fusion (BVg1) that promotes formation of mature Vg1 protein in vivo. Injection of BVg1 mRNA induces dorsal mesoderm in animal cap cells, and BVg1 expression in ultraviolet-ventralized embryos fully restores a normal dorsal axis. Blastomeres expressing BVg1 act as a Nieuwkoop center, the region that induces the Spemann organizer. our results lead us to suggest that localized posttranslational processing of Vg1 precursor protein on the future dorsal side of the embryo is a key step in generating dorsal mesoderm and the body axis in Xenopus.

Osteoinductive proteins

Cartilage and bone tissues are rich in different polypeptide factors (growth factors) which participate in the regulation of skeletal development and growth. Parallels between the embryonal endochondral ossification, callus formation during fracture repair, and ectopic bone induction in postnatal life have encouraged scientists to search for common mechanisms underlying these processes. A set of polypeptide factors belonging to the TGF-beta superfamily called the bone morphogenetic proteins (BMPs), have been found to be of fundamental importance both in bone formation and mesenchymal-epithelial interactions in early embryogenesis. Thus, this group of proteins is a common denominator in all the above-mentioned processes involving osteoinduction and there is great potential for their clinical application as bone-inducing factors.

Control of vertebrate gastrulation: inducing signals and responding genes

Recently, genes with similar expression patterns in the early gastrulae of several different vertebrate species have been identified. The remarkable conservation of these expression patterns suggests that fundamental similarities exist within the vertebrates at remarkably early stages. It has yet to be established exactly how these genes are activated in the correct spatial patterns and what their functions might be.

Later embryogenesis: regulatory circuitry in morphogenetic fields

The subject of this review is the nature of regulatory processes underlying the spatial subdivision of morphogenetic regions in later embryogenesis. I have applied a non-classical definition of morphogenetic field, the progenitor field, which is a region of an embryo composed of cells whose progeny will constitute a given morphological structure. An important feature of such fields is that they have sharp spatial boundaries, across which lie cells whose progeny will express different fates. Two examples of the embryonic specification and development of such fields are considered. These are the formation of the archenteron in the sea urchin embryo and the formation of dorsal axial mesoderm in the Xenopus embryo. From these and a number of additional examples, from vertebrate, Drosophila, Caenorhabditis elegans and sea urchin embryos, it is concluded that the initial formation of the boundaries of morphogenetic progenitor fields depends on both positive and negative transcription control functions. Specification of morphogenetic progenitor fields, organization of the boundaries and their subsequent regionalization or subdivision are mediated by intercellular signaling. Genes encoding regionally expressed transcription factors that are activated in response to intercell signaling, and that in turn mediate signaling changes downstream, appear as fundamental regulatory circuit elements. Such [signal-->transcription factor gene-->signal] circuit elements appear to be utilized, often repetitively, in many different morphogenetic processes.

Intercellular signalling in mesoderm formation during amphibian development

The mesoderm of amphibian embryos arises through an inductive interaction in which a signal from the vegetal hemisphere of the blastula-stage embryo acts on overlying equatorial cells. Strong candidates for endogenous mesoderm-inducing signals include members of the fibroblast growth factor (FGF) and activin families. In this paper we show that cells form different mesodermal cell types in response to different concentrations of these factors, and that graded distributions of activin and FGF can, in principle, provide sufficient positional information to generate the body plan of the Xenopus embryo.

FGF signalling in the early specification of mesoderm in Xenopus

We have examined the role of FGF signalling in the development of muscle and notochord and in the expression of early mesodermal markers in Xenopus embryos. Disruption of the FGF signalling pathway by expression of a dominant negative construct of the FGF receptor (XFD) generally results in gastrulation defects that are later evident in the formation of the trunk and tail, though head structures are formed nearly normally. These defects are reflected in the loss of notochord and muscle. Even in embryos that show mild defects and gastrulate properly, muscle formation is impaired, suggesting that morphogenesis and tissue differentiation each depend on FGF. The XFD protein inhibits the expression of the immediate early gene brachyury throughout the marginal zone, including the dorsal side; it does not, however, inhibit the dorsal lip marker goosecoid, which is expressed in the first involuting mesoderm at the dorsal side that will underlie the head. The XFD protein also inhibits Xpo expression, an immediate early marker of ventral and lateral mesoderm. These results suggest that FGF is involved in the earliest events of most mesoderm induction that occur before gastrulation and that the early dorsal mesoderm is already composed of two cell populations that differ in their requirements for FGF.

Genetic control of gastrulation in the mouse

In the past, understanding of the process of gastrulation in the mouse has primarily been based on morphological analyses. Recently, a number of molecules have been implicated in mesoderm induction and axis formation in Xenopus, and several of these exhibit unique patterns of expression during mouse gastrulation. These gene-expression data, together with fate mapping, ectopic expression experiments and mutational analysis, will now facilitate studies on the functional aspects of gastrulation in the mouse.

Embryonic induction

The current understanding of the mechanism of embryonic induction is reviewed. The embryological data which are necessary to establish the existence of an inductive process are described and the criteria for the identification of inducing factors are discussed. These criteria comprise: a demonstration that the factor has the appropriate biological activity, that it is expressed in biologically available form at the correct time and place in the embryo, and that when it is inhibited in vivo, the interaction should fail. Current understanding of the molecular basis of competence and threshold responses is discussed. Four case studies are examined in further detail: the dorsoventral patterning in Drosophila is controlled by a gradient of the decapentaplegic gene product, a member of the TGF beta superfamily. Mesoderm induction in Xenopus embryos is thought to be controlled by several factors acting in concert: activins, fibroblast growth factors (FGFs), Wnt proteins and bone morphogenetic proteins (BMPs). The formation of the kidney in higher vertebrates involves a permissive interaction and some molecules are known to be necessary for the process but the identity of the primary inducing signal remains elusive. The anteroposterior pattern in the chick limb is controlled by a morphogen gradient emitted by the zone of polarising activity (ZPA). Although closely mimicked by retinoic acid (RA), this substance is probably not itself the morphogen. In general, the technical advances of recent years have enabled dramatic progress to be made in understanding the molecular basis of embryonic induction. Although much remains to be done, the methods of investigation are now well established.

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.

Activins are expressed in preimplantation mouse embryos and in ES and EC cells and are regulated on their differentiation

Members of the activin family have been suggested to act as mesoderm-inducing factors during early amphibian development. Little is known, however, about mesoderm formation in the mammalian embryo, and as one approach to investigating this we have studied activin expression during early mouse development. Activins are homo- or heterodimers of the beta A or beta B subunits of inhibin, itself a heterodimer consisting of one of the beta subunits together with an alpha subunit. Our results indicate that the oocyte contains mRNA encoding all three subunits, and antibody staining demonstrates the presence of both alpha and beta protein chains. From the fertilized egg stage onwards, alpha subunit protein cannot be detected, so the presence of beta subunits reflects the presence of activin rather than inhibin. Maternal levels of activin protein decline during early cleavage stages but increase, presumably due to zygotic transcription (see below), in the compacted morula. By 3.5 days, only the inner cell mass (ICM) cells of the blastocyst express activin, but at 4.5 days the situation is reversed; activin expression is confined to the trophectoderm. Using reverse transcription-PCR, neither beta A nor beta B mRNA was detectable at the two-cell stage but transcripts encoding both subunits were detectable at the morula stage, with beta B mRNA persisting into the blastocyst. We have also analyzed activin and inhibin expression in ES and EC cells. Consistent with the observation that activins are expressed in the ICM of 3.5-day blastocysts, we find high levels of beta A and beta B mRNA in all eight ES cell lines tested. F9 EC cells express only activin beta B, together with low levels of the inhibin alpha chain. When ES and EC cells are induced to differentiate, levels of activin fall dramatically. These results are consistent with a role for activins in mesoderm formation and other steps of early mouse development.

Transgenic mice as a model to study the role of TGF-beta-related molecules in hair follicles

There is increasing evidence that members of the TGF-beta superfamily are important regulators of epithelial growth and differentiation in vivo. Here, transgenic mice have been used to study the role of the TGF-beta-related growth factors BMP-2 and BMP-4 in hair and whisker development. In the mature hair follicle, BMP-2 transcripts are normally seen only in precortex cells at the base of the hair shaft. In the transgenic mice reported here, BMP-4, a closely related molecule, has been ectopically expressed in the outer root sheath of hair and whisker follicles using an expression vector based on the bovine cytokeratin IV* promoter. In response to transgene expression, both outer root sheath cells below the stem cell compartment and hair matrix cells around the dermal papilla cease proliferation. In addition, the expression pattern of cytokeratin markers is disturbed in some transgenic hair follicles. These results support a model in which members of the TGF-beta superfamily play an active role in the inhibiton of cell proliferation and the onset of expression of trichocyte-specific genes that take place when cells leave the matrix of the follicle and differentiate into shaft cells.

Interactions between Xwnt-8 and Spemann organizer signaling pathways generate dorsoventral pattern in the embryonic mesoderm of Xenopus

This study analyzes the hierarchy of signals that spatially restrict expression of Xenopus Xwnt-8 to mesodermal cells outside of the Spemann organizer field and examines the potential role that endogenous Xwnt-8 may play in dorsoventral patterning of the embryonic mesoderm. The effects of ectopic introduction of a Nieuwkoop center-like activity or of ectopic expression of goosecoid, on the distribution of endogenous Xwnt-8 transcripts were analyzed. The results of these studies are consistent with the hypothesis that maternally derived signals from the Nieuwkoop center function to positively regulate expression of the homeo box gene goosecoid in Spemann organizer cells, leading to a subsequent repression of Xwnt-8 expression in these cells. This exclusion of Xwnt-8 from cells of the organizer field may be important for normal dorsal development, in that ectopic expression of Xwnt-8 in organizer cells after the midblastula stage, by injection of plasmid DNA, ventralizes the fate of these cells. This is distinct from the previously observed dorsalizing effect of Xwnt-8 when expressed prior to the midblastula stage by injection of RNA. The effects of plasmid-derived Xwnt-8 on isolated blastula animal cap ectoderm were also analyzed. Expression of Xwnt-8 in animal pole ectoderm after the midblastula stage ventralizes the response of dorsal animal pole cells to activin and allows naive ectodermal cells to differentiate as ventral mesoderm in the absence of added growth factors. Collectively, these data are consistent with the hypothesis that Xwnt-8 plays a role in the mesodermal differentiation of ventral marginal zone cells during normal development. Furthermore, endogenous Xwnt-8 may ventralize the response of lateral mesodermal cells to dorsalizing signals from the organizer, thus contributing to the graded nature of the final body pattern.

Immunodetection of Xenopus bone morphogenetic protein-4 in early embryos

Specific antibodies to Xenopus laevis bone morphogenetic protein-4 (xBMP-4) were raised by immunizing rabbits with a fusion protein of bacterial beta-galactosidase and xBMP-4. The antibodies were used to detect xBMPs expressed in mammalian cells by Western blotting. The antibodies were found to recognize xBMP-4 specifically and not to cross-react with either xBMP-2 or xBMP-7 which are similar to xBMP-4. In addition, the antibodies recognized dimeric xBMP-4 whereas our previous antibodies recognized the reduced form only. The present antibodies detected an immunoreactive 27 kDa protein in extracts of developing Xenopus embryos from oocyte to tailbud embryo. The xBMP-4 peptide appeared to be monomeric in structure because the molecular weight did not shift upon reduction of disulfide bond(s).

The frog prince-ss: a molecular formula for dorsoventral patterning in Xenopus

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Mox-1 and Mox-2 define a novel homeobox gene subfamily and are differentially expressed during early mesodermal patterning in mouse embryos

We have isolated two mouse genes, Mox-1 and Mox-2 that, by sequence, genomic structure and expression pattern, define a novel homeobox gene family probably involved in mesodermal regionalization and somitic differentiation. Mox-1 is genetically linked to the keratin and Hox-2 genes of chromosome 11, while Mox-2 maps to chromosome 12. At primitive streak stages (approximately 7.0 days post coitum), Mox-1 is expressed in mesoderm lying posterior of the future primordial head and heart. It is not expressed in neural tissue, ectoderm, or endoderm. Mox-1 expression may therefore define an extensive ‘posterior’ domain of embryonic mesoderm before, or at the earliest stages of, patterning of the mesoderm and neuroectoderm by the Hox cluster genes. Between 7.5 and 9.5 days post coitum, Mox-1 is expressed in presomitic mesoderm, epithelial and differentiating somites (dermatome, myotome and sclerotome) and in lateral plate mesoderm. In the body of midgestation embryos, Mox-1 signal is restricted to loose undifferentiated mesenchyme. Mox-1 signal is also prominent over the mesenchyme of the heart cushions and truncus arteriosus, which arises from epithelial-mesenchymal transformation and over a limited number of craniofacial foci of neural crest-derived mesenchyme that are associated with muscle attachment sites. The expression profile of Mox-2 is similar to, but different from, that of Mox-1. For example, Mox-2 is apparently not expressed before somites form, is then expressed over the entire epithelial somite, but during somitic differentiation, Mox-2 signal rapidly becomes restricted to sclerotomal derivatives. The expression patterns of these genes suggest regulatory roles for Mox-1 and Mox-2 in the initial anterior-posterior regionalization of vertebrate embryonic mesoderm and, in addition, in somite specification and differentiation.

Responses of embryonic Xenopus cells to activin and FGF are separated by multiple dose thresholds and correspond to distinct axes of the mesoderm

The potent mesoderm-inducing factors activin and FGF are present as maternally synthesized proteins in embryos of X. laevis. We show that activin can act on explanted blastomeres to induce at least five different cell states ranging from posterolateral mesoderm to dorsoanterior organizer mesoderm. Each state is induced in a narrow dose range bounded by sharp thresholds. By contrast, FGF induces only posterolateral markers and does so over relatively broad dose ranges. FGF can modulate the actions of activin, potentiating them and broadening the threshold-bounded dose windows. Our results indicate that orthogonal gradients of activin and FGF would be sufficient to specify the main elements of the body plan.