Synergistic effects of Vg1 and Wnt signals in the specification of dorsal mesoderm and endoderm

Insights into bone morphogenetic proteins in cardiovascular diseases

Bone morphogenetic proteins (BMPs) are secretory proteins belonging to the transforming growth factor-β (TGF-β) superfamily. These proteins play important roles in embryogenesis, bone morphogenesis, blood vessel remodeling and the development of various organs. In recent years, as research has progressed, BMPs have been found to be closely related to cardiovascular diseases, especially atherosclerosis, vascular calcification, cardiac remodeling, pulmonary arterial hypertension (PAH) and hereditary hemorrhagic telangiectasia (HHT). In this review, we summarized the potential roles and related mechanisms of the BMP family in the cardiovascular system and focused on atherosclerosis and PAH.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Signaling Cross Talk between TGF-β/Smad and Other Signaling Pathways

Cytokines of the transforming growth factor β (TGF-β) family, including TGF-βs, bone morphogenic proteins (BMPs), activins, and Nodal, play crucial roles in embryonic development and adult tissue homeostasis by regulating cell proliferation, survival, and differentiation, as well as stem-cell self-renewal and lineage-specific differentiation. Smad proteins are critical downstream mediators of these signaling activities. In addition to regulating the transcription of direct target genes of TGF-β, BMP, activin, or Nodal, Smad proteins also participate in extensive cross talk with other signaling pathways, often in a cell-type- or developmental stage-specific manner. These combinatorial signals often produce context-, time-, and location-dependent biological outcomes that are critical for development. This review discusses recent progress in our understanding of the cross talk between Smad proteins and signaling pathways of Wnt, Notch, Hippo, Hedgehog (Hh), mitogen-activated protein (MAP), kinase, phosphoinositide 3-kinase (PI3K)-Akt, nuclear factor κB (NF-κB), and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

Signaling Cross Talk between TGF-β/Smad and Other Signaling Pathways

Cytokines of the transforming growth factor β (TGF-β) family, including TGF-βs, bone morphogenic proteins (BMPs), activins, and Nodal, play crucial roles in embryonic development and adult tissue homeostasis by regulating cell proliferation, survival, and differentiation, as well as stem-cell self-renewal and lineage-specific differentiation. Smad proteins are critical downstream mediators of these signaling activities. In addition to regulating the transcription of direct target genes of TGF-β, BMP, activin, or Nodal, Smad proteins also participate in extensive cross talk with other signaling pathways, often in a cell-type- or developmental stage-specific manner. These combinatorial signals often produce context-, time-, and location-dependent biological outcomes that are critical for development. This review discusses recent progress in our understanding of the cross talk between Smad proteins and signaling pathways of Wnt, Notch, Hippo, Hedgehog (Hh), mitogen-activated protein (MAP), kinase, phosphoinositide 3-kinase (PI3K)-Akt, nuclear factor κB (NF-κB), and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

Molecular specification of germ layers in vertebrate embryos

In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood. Classic experiments in amphibian embryos revealed the tissue interactions involved in germ layer formation and provided the groundwork for the identification of secreted and intracellular factors involved in this process. We will begin this review by summarising the key findings of those studies. We will then evaluate them in the light of more recent genetic studies that helped clarify which of the previously identified factors are required for germ layer formation in vivo, and to what extent the mechanisms identified in amphibians are conserved across other vertebrate species. Collectively, these studies have started to reveal the gene regulatory network (GRN) underlying vertebrate germ layer specification and we will conclude our review by providing examples how our understanding of this GRN can be employed to differentiate stem cells in a targeted fashion for therapeutic purposes.

Temporal and contextual orchestration of cardiac fate by WNT-BMP synergy and threshold

Cardiomyogenic development proceeds with a cascade of intricate signalling events that operate in a temporo-spatial fashion to specify cardiac cell fate during early embryogenesis. In fact, conflicting reports exist regarding the role of Wnt/β-catenin signalling during cardiomyogenesis. Here, we describe a dose-dependent and temporal effect of Wnt/β-catenin signalling on in vitro cardiomyogenesis using embryonic stem cells (ESCs) as a model system. We could demonstrate that canonical Wnt activation during early stage of differentiation either through ligand or by GSK3β inhibition helped in maintaining Oct4 and Nanog expressions, and in parallel, it promoted mesoderm and endoderm inductions. In contrast, it led to attenuation in cardiomyogenesis that was reversed by moderate concentration of DKK1, but not soluble Fz8. However, higher DKK1 could also block cardiomyogenesis, suggesting thereby governance of a particular signalling threshold underlying this developmental event. Interestingly, Wnt signalling activation at early stage modulated BMP4 expression in a stage-specific manner. Wnt activation, synchronized with BMP4 and brachyury up-regulation at early stage, correlated well with mesoderm induction. Conversely, Wnt activation led to BMP4 and Wnt5a down-regulation at late stage culminating in cardiomyogenic attenuation. Our findings suggested the existence of precise regulatory machinery with context-dependent role of Wnt for fine tuning mesoderm induction and its derivatives, through establishment of Wnt gradient during ESCs’ differentiation. Moreover, contrary to mere activation/inhibition, a specific threshold of Wnt and BMP and their synergy seemed necessary for providing the guiding cues in orchestrating mesoderm induction and subsequent cardiomyogenesis.

The Xenopus Nieuwkoop center and Spemann-Mangold organizer share molecular components and a requirement for maternal Wnt activity

In Xenopus embryos, the dorso-ventral and antero-posterior axes are established by the Spemann-Mangold organizer. According to the prevalent model of early development, the organizer is induced by the dorsalizing Nieuwkoop signal, which is secreted by the Nieuwkoop center. Formation of the center requires the maternal Wnt pathway, which is active on the dorsal side of embryos. Nevertheless, the molecular nature of the Nieuwkoop signal remains unclear. Since the Nieuwkoop center and the organizer both produce dorsalizing signals in vitro, we asked if they might share molecular components. We find that vegetal explants, the source of Nieuwkoop signal in recombination assays, express a number of organizer genes. The product of one of these genes, chordin, is required for signaling, suggesting that the organizer and the center share at least some molecular components. Furthermore, experiments with whole embryos show that maternal Wnt activity is required in the organizer just as it is needed in the Nieuwkoop center in vitro. We conclude that the maternal Wnt pathway generates the Nieuwkoop center in vitro and the organizer in vivo by activating a common set of genes, without the need of an intermediary signaling step.

Vg1 has specific processing requirements that restrict its action to body axis patterning centers

Unlike most transforming growth factor-beta (TGF-beta) superfamily members, Vg1 has been shown not to produce gross phenotypic alterations in Xenopus embryos when overexpressed by mRNA injection. Experiments with artificial chimeric constructs and a recently identified second allele of Vg1 suggest that this may be due to unusually stringent requirements for proteolytic processing. We provide biological and biochemical evidence that cleavage by two distinct proteolytic enzymes is required for effective activation of Vg1. We demonstrate a tightly restricted overlap in expression patterns of Vg1 with the proteases required to release the mature peptide. The data presented may account for the long-standing observation that the vast majority of Vg1 protein, in vivo, is present in its unprocessed form. Taken together, these observations provide a plausible mechanism for local action of Vg1 consistent with requirements imposed by current models of pattern formation in the developing body axis.

Beta-catenin/LEF-1 signalling in breast cancer--central players activated by a plethora of inputs

Although the role of Wnt signalling in breast cancer is far from being fully understood, in the last years its importance has been reported frequently. Besides stimulation by canonical Wnt signalling, the downstream effectors beta-catenin and the transcriptional modulators of the T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) family can also be activated by other inputs including the TGF-beta pathway. Wnt and TGF-beta signalling are both major signal transduction pathways, which provide important cues during development and tumor progression. However, particularly TGF-beta has a complicated influence on oncogenesis, which ranges from suppressive to promoting activity. Signalling pathways activated in parallel with TGF-beta might determine the oncogenic influence, and therefore place signals cooperating with TGF-beta into the limelight. During early development Wnt and TGF-beta signalling collaborate extensively. Here we provide an overview of the known interactions of Wnt with TGF-beta signalling in development and metastasis, particularly in breast cancer. We want to focus on the Wnt-activated transcription factor complex beta-catenin/LEF-1, its upstream activators, its downstream targets and consequences on the cellular level in response to beta-catenin/LEF-1 activation.

Ascidian embryonic development: An emerging model system for the study of cell fate specification in chordates

The ascidian tadpole larva represents the basic body plan of all chordates in a relatively small number of cells and tissue types. Although it had been considered that ascidians develop largely in a determinative way, whereas vertebrates develop in an inductive way, recent studies at the molecular and cellular levels have uncovered several similarities in the way developmental fates are specified. In this review, we describe ascidian embryogenesis and its cell lineages, introduce several characteristics of ascidian embryos, describe recent advances in understanding of the mechanisms of cell fate specification, and discuss them in the context of what is known in vertebrates and other organisms.

Regulation of Wnt gene expression

Members of the Wnt gene family play important roles in the regulation of a number of basic developmental processes. Because Wnt is such a potent morphogen, its expression must be controlled tightly and precisely. While many review papers focused on Wnt signaling downstream of the receptor, this review addresses regulations of Wnt itself on several levels, including the transcriptional level, RNA splicing, the post-transcriptional level, the translational level, and the post-translational level. It is these multiple, precise and tight regulations that guarantee that Wnts function correctly both temporally and spatially.

Xenopus aristaless-related homeobox (xARX) gene product functions as both a transcriptional activator and repressor in forebrain development

Mutations in the aristaless-related homeobox (ARX) gene have been found in patients with a variety of X-linked mental retardation syndromes with forebrain abnormalities, including lissencephaly. Arx is expressed in the developing mouse, Xenopus, and zebrafish forebrain. We have used whole-mount in situ hybridization, overexpression, and loss-of-function studies to investigate the involvement of xArx in Xenopus brain development. We verified that xArx is expressed in the prospective diencephalon, as the forebrain is patterned and specified during neural plate stages. Expression spreads into the ventral and medial telencephalon as development proceeds through neural tube and tadpole stages. Overexpression of xArx resulted in morphological abnormalities in forebrain development, including loss of rostral midline structures, syn- or anophthalmia, dorsal displacement of the nasal organ, and ventral neural tube hyperplasia. Additionally, there is a delay in expression of many molecular markers of brain and retinal development. However, expression of some markers, dlx5 and wnt8b, was enhanced in xArx-injected embryos. Loss-of-function experiments indicated that xArx was necessary for normal forebrain development. Expansion of wnt8b expression depended on xArx function as a transcriptional repressor, whereas ectopic expression of dlx5, accompanied by development of ectopic otic structures, depended on function of Arx as a transcriptional activator. These results suggest that Arx acts as a bifunctional transcriptional regulator in brain development.

The roles of three signaling pathways in the formation and function of the Spemann Organizer

Since the three main pathways (the Wnt, VegT and BMP pathways) involved in organizer and axis formation in the Xenopus embryo are now characterized, the challenge is to understand their interactions. Here three comparisons were made. Firstly, we made a systematic comparison of the expression of zygotic genes in sibling wild-type, VegT-depleted (VegT–), β-catenin-depleted (β-catenin–) and double depleted (VegT–/β-catenin–) embryos and placed early zygotic genes into specific groups. In the first group some organizer genes, including chordin, noggin and cerberus, required the activity of both the Wnt pathway and the VegT pathway to be expressed. A second group including Xnr1, 2, 4 and Xlim1 were initiated by the VegT pathway but their dorsoventral pattern and amount of their expression was regulated by the Wnt pathway. Secondly, we compared the roles of the Wnt and VegT pathways in producing dorsal signals. Explant co-culture experiments showed that the Wnt pathway did not cause the release of a dorsal signal from the vegetal mass independent from the VegT pathway. Finally we compared the extent to which inhibiting Smad 1 phosphorylation in one area of VegT–, or β-catenin– embryos would rescue organizer and axis formation. We found that BMP inhibition with cm-BMP7 mRNA had no rescuing effects on VegT– embryos, while cm-BMP7 and noggin mRNA caused a complete rescue of the trunk, but not of the anterior pattern in β-catenin– embryos.

Identification and characterization of bone morphogenetic protein 2/4 gene from the starfish Archaster typicus

A bone morphogenetic protein 2/4 (BMP2/4) gene has been cloned from the starfish, Archaster typicus, for the purpose of investigating the expression pattern of the BMP4 gene in echinoderm embryos which do not produce micromeres. The isolated gene (named AtBMP2/4) contained two exons that encoded the entire coding region. The deduced AtBMP2/4 protein sequence contained 509 amino acids. Sequence comparison showed that it shared high amino acid similarity with sea urchin BMP2/4 and Xenopus BMP2 and BMP4. Northern blot analyses indicated that AtBMP2/4 mRNA initially appears at the blastula stage and has a maximal expression level at the gastrula stage. Whole-mount in situ hybridization revealed that AtBMP2/4 mRNA is expressed in the archenteron, coelomic vesicles, and ectodermal cells of gastrula stage embryos. The observed spatial distribution pattern vastly differs from that of sea urchin SpBMP2/4, which is expressed mainly in the oral ectoderm region of the mesenchyme blastula and early gastrula embryos.

Timing of endogenous activin-like signals and regional specification of theXenopusembryo

Signaling by activin-like ligands is important for induction and patterning of mesoderm and endoderm. We have used an antibody that specifically recognizes the phosphorylated and activated form of Smad2, an intracellular transducer of activin-like ligands, to examine how this signaling pathway patterns the early mesendoderm. In contrast to the simple expectation that activin-like signaling should be highest on the dorsal side of the gastrula stage embryo, we have found that while Smad2 phosphorylation is highest dorsally before gastrulation, signaling is attenuated dorsally and is highest on the ventral side by mid-gastrulation. Early dorsal initiation of Smad2 phosphorylation results from cooperation between the vegetally localized maternal transcription factor VegT and dorsally localized β-catenin. The subsequent ventral appearance of Smad2 phosphorylation is dependent on VegT, but not on signaling from the dorsal side. Dorsal attenuation of Smad2 phosphorylation during gastrulation is mediated by early dorsal expression of feedback inhibitors of activin-like signals.

In addition to regulation of Smad2 phosphorylation by the expression of activin-like ligands and their antagonists, the responsiveness of embryonic cells to activin-like ligands is also temporally regulated. Ectopic Vg1, Xnr1 and derrière all fail to activate Smad2 phosphorylation until after the midblastula transition, and the onset of responsiveness to these ligands is independent of transcription. Furthermore, the timing of cellular responsiveness differs for Xnr1 and derrière, and these distinct temporal patterns of responsiveness can be correlated with their distinctive phenotypic effects. These observations suggest that the timing of endogenous activin-like signaling is a determinant of patterning in the early Xenopus embryo.

Patterning and lineage specification in the amphibian embryo

Xenopus has been widely used to study early embryogenesis because the embryos allow for efficient functional assays of gene products by the overexpression of RNA. The first asymmetry of the embryo is initiated during oogenesis and is manifested by the darkly pigmented animal hemisphere and lightly pigmented vegetal hemisphere. Upon fertilization a second asymmetry, the dorsal-ventral asymmetry, is established, with the sperm entry site defining the prospective ventral region. During the cleavage stage, a vegetal cortical cytoplasm (VCC)/beta-catenin signaling pathway is differentially activated on the prospective dorsal side of the embryo. The overlapping of the VCC/beta-catenin and transforming growth factor beta (TGF-beta) pathways in the dorsal vegetal quadrant specifies dorsal-vental axis formation by regulating formation of the Spemann organizer, including the anterior endomesoderm. The organizer initiates gastrulation to form a triploblastic embryo in which the mesoderm layer is located between the ectoderm layer and the endoderm layer. The interplay between maternal and zygotic TGF-beta s and the T-box transcription factors in the vegetal hemisphere initiates the specification of germ-layer lineages. TGF-beta signaling originating from the vegetal region induces mesoderm in the equatorial region, and initiates endoderm differentiation directly in the vegetal region. The ectoderm develops from the animal region, which does not come into contact with the vegetal TGF-beta signals. A large number of the downstream components and transcriptional targets of early developmental pathways have been identified and characterized. This review gives an overview of recent advances in the understanding of the functional roles and interactions of the molecular players important for axis determination and germ-layer specification during early Xenopus embryogenesis.

foxD5a, a Xenopus winged helix gene, maintains an immature neural ectoderm via transcriptional repression that is dependent on the C-terminal domain

Xenopus foxD5a, the full-length fork head gene previously described as a PCR fragment (XFLIP), is first detectable at stage II of oogenesis. Low-abundance maternal transcripts are localized to the animal hemisphere of the cleavage embryo, and protein can be translocated to the nucleus prior to the onset of zygotic transcription. Zygotic expression is strongest in the presumptive neural ectoderm at gastrula and neural plate stages, but there is minor paraxial mesodermal expression during primary gastrulation that becomes significant in the tail bud during secondary gastrulation. Expression of foxD5a in animal cap explants induces elongation and expression of mesodermal, neural-inducing, and early neural-specifying genes, indicating a role in dorsal axis formation. Zygotic foxD5a expression is induced strongly by siamois, moderately by cerberus, weakly by Wnt8 and noggin, and not by chordin in animal cap explants. Expression of foxD5a in whole embryos has differential dorsal and ventral effects. Ventral mRNA injection induces partial secondary axes composed of expanded mesodermal and epidermal tissues, but does not induce ectopic neural tissues. Dorsal mRNA injection causes hypertrophy of the neural plate and expansion of early neural genes (sox3 and otx2), but this is not the result of increased proliferation or expanded neural-inducing mesoderm. The neural plate appears to be maintained in an immature state because otx2 expression is expanded and expression of en2, Krox20, proneural genes (Xnrgn1, neuroD) and a neural differentiation gene (n-tubulin) is repressed in foxD5a-expressing cells. These results indicate that foxD5a maintains an undifferentiated neural ectoderm after neural induction. Expression of foxD5a constructs fused with the engrailed repressor domain or with the VP16 activation domain demonstrates that FoxD5a acts as a transcriptional repressor in axis formation and neural plate expansion. Deletion constructs indicate that this activity requires the C-terminal domain of the protein.

Wnt signalling: a theme with nuclear variations

Wnt proteins are involved in a large number of events during development and disease. The crucial element in the transduction of the signal elicited by Wnt is the state and activity of beta-catenin. There are two pools of beta-catenin, one associated with cadherins at the cell surface and a soluble one in the cytolasm, whose state and concentration are critical for Wnt signalling. In the absence of Wnt, the cytoplasmic pool is low due to targetted degradation of beta-catenin. Upon Wnt signalling, beta-catenin is stabilized. As a consequence, it can access the nucleus where it interacts with members of the Tcf family of transcription factors to modulate the expression of defined targets. Recent reports indicate that, in addition to Tcfs, beta-catenin can interact with other nuclear proteins raising the possibility that Wnt signalling has a wider modulatory effect on transcription than is mediated by its interactions with Tcfs. BioEssays 23:311-318, 2001.

Bone morphogenetic protein function is required for terminal differentiation of the heart but not for early expression of cardiac marker genes

To examine potential roles for bone morphogenetic proteins (BMPs) in cardiogenesis, we used intracellular BMP inhibitors to disrupt this signaling cascade in Xenopus embryos. BMP-deficient embryos showed endodermal defects, a reduction in cardiac muscle-specific gene expression, a decrease in the number of cardiomyocytes and cardia bifida. Early expression of markers of endodermal and precardiac fate, however, was not perturbed. Heart defects were observed even when BMP signal transduction was blocked only in cells that contribute primarily to endodermal, and not cardiac fates, suggesting a non-cell autonomous function. Our results suggest that BMPs are not required for expression of early transcriptional regulators of cardiac fate but are essential for migration and/or fusion of the heart primordia and cardiomyocyte differentiation.

Mesendoderm induction and reversal of left-right pattern by mouse Gdf1, a Vg1-related gene

TGFbeta signals play important roles in establishing the body axes and germ layers in the vertebrate embryo. Vg1 is a TGFbeta-related gene that, due to its maternal expression and vegetal localization in Xenopus, has received close examination as a potential regulator of development in Xenopus, zebrafish, and chick. However, a mammalian Vg1 ortholog has not been identified. To isolate mammalian Vg1 we screened a mouse expression library with a Vg1-specific monoclonal antibody and identified a single cross-reactive clone encoding mouse Gdf1. Gdf1 is expressed uniformly throughout the embryonic region at 5.5-6.5 days postcoitum and later in the node, midbrain, spinal cord, paraxial mesoderm, lateral plate mesoderm, and limb bud. When expressed in Xenopus embryos, native GDF1 is not processed, similar to Vg1. In contrast, a chimeric protein containing the prodomain of Xenopus BMP2 fused to the GDF1 mature domain is efficiently processed and signals via Smad2 to induce mesendoderm and axial duplication. Finally, right-sided expression of chimeric GDF1, but not native GDF1, reverses laterality and results in right-sided Xnr1 expression and reversal of intestinal and heart looping. Therefore, GDF1 can regulate left-right patterning, consistent with the Gdf1 loss-of-function analysis in the mouse (C. T. Rankin, T. Bunton, A. M. Lawler, and S. J. Lee, 2000, Nature Genet. 24, 262-265) and a proposed role for Vg1 in Xenopus. Our results establish that Gdf1 is posttranslationally regulated, that mature GDF1 activates a Smad2-dependent signaling pathway, and that mature GDF1 is sufficient to reverse the left-right axis. Moreover, these findings demonstrate that GDF1 and Vg1 are equivalent in biochemical and functional assays, suggesting that Gdf1 provides a Vg1-like function in the mammalian embryo.

CaM kinase IV regulates lineage commitment and survival of erythroid progenitors in a non-cell-autonomous manner

Developmental functions of calmodulin-dependent protein kinase IV (CaM KIV) have not been previously investigated. Here, we show that CaM KIV transcripts are widely distributed during embryogenesis and that strict regulation of CaM KIV activity is essential for normal primitive erythropoiesis. Xenopus embryos in which CaM KIV activity is either upregulated or inhibited show that hematopoietic precursors are properly specified, but few mature erythrocytes are generated. Distinct cellular defects underlie this loss of erythrocytes: inhibition of CaM KIV activity causes commitment of hematopoietic precursors to myeloid differentiation at the expense of erythroid differentiation, on the other hand, constitutive activation of CaM KIV induces erythroid precursors to undergo apoptotic cell death. These blood defects are observed even when CaM KIV activity is misregulated only in cells that do not contribute to the erythroid lineage. Thus, proper regulation of CaM KIV activity in nonhematopoietic tissues is essential for the generation of extrinsic signals that enable hematopoietic stem cell commitment to erythroid differentiation and that support the survival of erythroid precursors.

The informational content of gradients of Wnt proteins

This perspective tackles the issues facing developmental biologists and cell biologists regarding how the molecular mechanisms for specifying cell fate are defined. This perspective focuses on members of the Wnt family. The author proposes that Wnt proteins may act as stabilizing signals for earlier inductive events in certain systems, for example, in Caenorhabditis elegans during the migration of two neurons and in Drosophila melanogaster during the patterning of the wing.

Molecular interactions continuously define the organizer during the cell movements of gastrulation

The organizer is a unique region in the gastrulating embryo that induces and patterns the body axis. It arises before gastrulation under the influence of the Nieuwkoop center. We show that during gastrulation, cell movements bring cells into and out of the chick organizer, Hensen's node. During these movements, cells acquire and lose organizer properties according to their position. A "node inducing center," which emits Vg1 and Wnt8C, is located in the middle of the primitive streak. Its activity is inhibited by ADMP produced by the node and by BMPs at the periphery. These interactions define the organizer as a position in the embryo, whose cellular makeup is constantly changing, and explain the phenomenon of organizer regeneration.

Wnt signalling: pathway or network?

Members of the Wnt family of secreted glycoproteins participate in many signalling events during development. Recent findings suggest that Wnt signals can sometimes play a permissive role during cell-fate assignment. Wnt proteins have been shown to interact with a number of extracellular and cell-surface proteins, whereas many intracellular components of the Wnt-signalling pathway are also involved in other cellular functions. The consequences of Wnt signalling can be affected by members of the MAP kinase family. These observations suggest that the future understanding of Wnt signalling may require models that are based on a signalling network rather than a single linear pathway.

Post-transcriptional regulation of Xwnt-8 expression is required for normal myogenesis during vertebrate embryonic development

The Xenopus Wnt-8 gene is transiently expressed in ventral and lateral mesoderm during gastrulation and plays a critical role in patterning these tissues. In the current study, we show that the spatial and temporal pattern of expression of endogenous Xwnt-8 is regulated, in part, at a post-transcriptional level. We have identified a novel sequence element in the 3′ untranslated region of the Xwnt-8 RNA that controls the polyadenylation status of reporter and endogenous Xwnt-8 RNAs, directs rapid RNA degradation beginning precisely at the early gastrula stage, and represses translation of transcripts throughout development. Expression of endogenous Xwnt-8 is normally downregulated within lateral (presomitic) mesoderm following gastrulation. We demonstrate that rapid degradation of Xwnt-8 transcripts, mediated by these regulatory elements in the 3′ untranslated region, is essential to this process and that downregulation is required to prevent overcommitment of somitic cells to a myogenic fate. These studies demonstrate a role for post-transcriptional regulation of zygotic gene expression in vertebrate embryonic patterning.

Mesoderm patterning and somite formation during node regression: differential effects of chordin and noggin

In Xenopus, one of the properties defining Spemann's organizer is its ability to dorsalise the mesoderm. When placed ajacent to prospective lateral/ventral mesoderm (blood, mesenchyme), the organizer causes these cells to adopt a more axial/dorsal fate (muscle). It seems likely that a similar property patterns the primitive streak of higher vertebrate embryos, but this has not yet been demonstrated clearly. Using quail/chick chimaeras and a panel of molecular markers, we show that Hensen's node (the amniote organizer) can induce posterior primitive streak (prospective lateral plate) to form somites (but not notochord) at the early neurula stage. We tested two BMP antagonists, noggin and chordin (both of which are expressed in the organizer), for their ability to generate somites and intermediate mesoderm from posterior streak, and find that noggin, but not chordin, can do this. Conversely, earlier in development, chordin can induce an ectopic primitive streak much more effectively than noggin, while neither BMP antagonist can induce neural tissue from extraembryonic epiblast. Neurulation is accompanied by regression of the node, which brings the prospective somite territory into a region expressing BMP-2, -4 and -7. One function of noggin at this stage may be to protect the prospective somite cells from the inhibitory action of BMPs. Our results suggest that the two BMP antagonists, noggin and chordin, may serve different functions during early stages of amniote development.

Pattern formation in zebrafish--fruitful liaisons between embryology and genetics

Vertebrate embryos, despite quite diverse early morphologies, appear to employ similar cellular strategies and conserved biochemical pathways in their development (Eyal-Giladi, 1997). In the past decade, a small tropical teleost, zebrafish (Danio rerio), became an important model system in which to study development (Streisinger et al., 1981). By combining embryology with molecular and classical genetic methods, our understanding of early inductive and morphogenetic events during vertebrate embryogenesis significantly advanced. In zebrafish, dorsal-ventral polarity is established during early cleavage and is dependent on microtubular transport of determinants from the vegetal pole to the blastomeres positioned on top of the yolk cell. The syncytium forming from these marginal blastomeres in the early blastula exhibits dorsal-ventral asymmetry with beta-catenin localized to the nuclei on the presumptive dorsal side of the syncytium. The yolk cell is a source of signals that induce and pattern overlying blastoderm. Therefore, the dorsal yolk syncytial layer is equivalent to the Nieuwkoop center of the amphibian embryo. The embryonic shield, a thickening of the dorsal blastoderm margin, exhibits properties similar to the amphibian Spemann organizer. However, certain inductive and patterning signals from the organizer might be produced before the shield forms or might originate outside of the shield. Similar to the amphibian embryo, the key patterning functions of the fish dorsal organizer (i.e., dorsalization of mesoderm, ectoderm, and coordination of gastrulation movements) are performed by secreted molecules that antagonize the ventralizing activity of the swil (zbmp-2) and zbmp-4 gene products expressed on the ventral side of the embryo. These functions of the dorsal organizer require the activity of the chordino gene (a zebrafish homologue of chordin), bozozok, mercedes and ogon loci.

BMP-4 is proteolytically activated by furin and/or PC6 during vertebrate embryonic development

Bone morphogenetic protein-4 (BMP-4) is a multifunctional developmental regulator. BMP-4 is synthesized as an inactive precursor that is proteolytically activated by cleavage following the amino acid motif -Arg-Ser-Lys-Arg-. Very little is known about processing and secretion of BMPs. The proprotein convertases (PCs) are a family of seven structurally related serine endoproteases, at least one of which, furin, cleaves after the amino acid motif -Arg-X-Arg/Lys-Arg-. To examine potential roles of PCs during embryonic development we have misexpressed a potent protein inhibitor of furin, alpha1-antitrypsin Portland (alpha1-PDX) in early Xenopus embryos. Ectopic expression of alpha1-PDX phenocopies the effect of blocking endogenous BMP activity, leading to dorsalization of mesoderm and direct neural induction. alpha1-PDX-mediated neural induction can be reversed by co-expression of downstream components of the BMP-4 signaling pathway. Thus, alpha1-PDX can block BMP activity upstream of receptor binding, suggesting that it inhibits an endogenous BMP-4 convertase(s). Consistent with this hypothesis, alpha1-PDX prevents cleavage of BMP-4 in an oocyte translation assay. Using an in vitro digestion assay, we demonstrate that four members of the PC family have the ability to cleave BMP-4, but of these, only furin and PC6B are sensitive to alpha1-PDX. These studies provide the first in vivo evidence that furin and/or PC6 proteolytically activate BMP-4 during vertebrate embryogenesis.

Mutant Vg1 ligands disrupt endoderm and mesoderm formation in Xenopus embryos

The Xenopus Vg1 gene, a TGFbeta superfamily member, is expressed as a maternal mRNA localized to prospective endoderm, and mature Vg1 protein can induce both endodermal and mesodermal markers in embryonic cells. Most previous work on embryonic inducers, including activin, BMPs and Vg1, has relied on ectopic expression to assay for gene function. Here we employ a mutant ligand approach to block Vg1 signaling in developing embryos. The results indicate that Vg1 expression is essential for normal endodermal development and the induction of dorsal mesoderm in vivo.

From cortical rotation to organizer gene expression: toward a molecular explanation of axis specification in Xenopus

After fertilization of Xenopus eggs, the cortex rotates relative to the cytoplasm, resulting in the formation of a cytoplasmic and transplantable dorsal-determining activity opposite the sperm entry point. This activity induces the dorsal expression of regulatory genes, which in turn establishes the Spemann organizer at the start of gastrulation. There has been considerable debate as to whether Vg1, or components of the Wnt-1 signaling pathway, normally function as this early dorsal determinant. Experiments now support the hypothesis that beta-catenin, a component of the Wnt pathway, provides the initial dorsoventral polarity to the embryo, and that Vg1 functions at a subsequent step in development. Specifically, beta-catenin is required for formation of the endogenous axes, and it is expressed at greater levels in dorsal cells during the early cleavage stages. Moreover, on the dorsal side of the embryo, complexes of beta-catenin and Tcf-3 directly bind the promoter of the dorsal regulatory genes siamois and twin and facilitate their expression, thereby contributing to the subsequent formation of the Spemann organizer. On the ventral side of the embryo, Tcf-3 likely works in the absence of beta-catenin as a transcriptional repressor of siamois. These and other data are considered in the context of how the initial polarization of the fertilized egg by the localized accumulation of beta-catenin establishes a range of subsequent dorsoventral asymmetries in the embryo.

Cooperation between the activin and Wnt pathways in the spatial control of organizer gene expression

The normal expression pattern of the Wnt responsive homeobox gene Siamois is restricted to the dorso-vegetal region of the Xenopus embryo. Because the Wnt signaling pathway (via beta-catenin) is active on the entire dorsal side of the early embryo, we have asked why Siamois expression is not seen in the dorsal ectoderm. Only Wnt signaling, via activation of beta-catenin, can induce directly Siamois, and signaling via the SMAD1 (BMP2/4) or SMAD2 (activin/Vg-1) pathways cannot. We now directly show that the SMAD2 pathway can cooperate with the Wnt pathway to induce expression of Siamois much more strongly than the Wnt pathway alone, in normal embryos. We demonstrate the significance of this cooperation in normal embryos by blocking the SMAD2 signaling pathway with a dominant negative activin receptor. The activin dominant negative receptor blocks this cooperative effect and reduces the expression of Siamois by threefold in early embryos. Furthermore, we find that this cooperative relationship between the SMAD2 and Wnt pathways is reciprocal. Thus, in normal embryos, the Wnt pathway can enhance induction, by the SMAD 2 pathway, of the organizer genes Gsc and Chd but not the pan-mesodermal marker genes Xbra and Eomes. We conclude that the Wnt and SMAD2 signaling pathways cooperate to induce the expression of Spemann-organizer specific genes and so help to localize their spatial expression.

Xenopus Smad8 acts downstream of BMP-4 to modulate its activity during vertebrate embryonic patterning

Bone morphogenetic proteins (BMPs) participate in the development of nearly all organs and tissues. BMP signaling is mediated by specific Smad proteins, Smad1 and/or Smad5, which undergo serine phosphorylation in response to BMP-receptor activation and are then translocated to the nucleus where they modulate transcription of target genes. We have identified a distantly related member of the Xenopus Smad family, Smad8, which lacks the C-terminal SSXS phosphorylation motif present in other Smads, and which appears to function in the BMP signaling pathway. During embryonic development, the spatial pattern of expression of Smad8 mirrors that of BMP-4. We show that an intact BMP signaling pathway is required for its expression. Overexpression of Smad8 in Xenopus embryos phenocopies the effect of blocking BMP-4 signaling, leading to induction of a secondary axis on the ventral side of intact embryos and to direct neural induction in ectodermal explants. Furthermore, Smad8 can block BMP-4-mediated induction of ventral mesoderm-specific gene expression in ectodermal explants. Overexpression of Smad8 within dorsal cells, however, causes patterning defects that are distinct from those reported in BMP-4-deficient embryos, suggesting that Smad8 may interact with additional signaling pathways. Indeed, overexpression of Smad8 blocks expression of Xbra in whole animals, and partially blocks activin signaling in animal caps. In addition, Smad8 inhibits involution of mesodermal cells during gastrulation, a phenotype that is not observed following blockade of activin or BMPs in Xenopus. Together, these results are consistent with the hypothesis that Smad8 participates in a negative feedback loop in which BMP signaling induces the expression of Smad8, which then functions to negatively modulate the amplitude or duration of signaling downstream of BMPs and, possibly, downstream of other transforming growth factor-beta (TGF-beta) family ligands.

Dorsal determinants in the Xenopus egg are firmly associated with the vegetal cortex and behave like activators of the Wnt pathway

The Xenopus egg contains maternal dorsal determinants that are specifically located at the vegetal cortex. To study physical and functional properties of the dorsal determinants, we took advantage of the animal-vegetal reversed embryo. The animal-vegetal reversed embryo is produced by inversion of the fertilized egg, which results in formation of ectoderm and endoderm from the unpigmented and the pigmented halves, respectively [Neff et al. (1983). Dev. Biol. 97, 103-112; Black and Gerhart (1985). Dev. Biol. 108, 310-324]. We demonstrated by cytoplasmic transplantation that the dorsal activity was specifically localized to the unpigmented cortical cytoplasm of the inverted egg, which is segregated into the future ectodermal lineage. This result suggests that the dorsal determinants are associated with the unpigmented cortex and are not dislodged by the inversion. In addition, we found that two vegetally localized transcripts, Xcat2 and Vg1 mRNAs, were present in the reversed animal pole of the inverted egg, suggesting their association with the unpigmented cortex. In order to compare the dorsal determinant activity with known dorsalizing molecules, we examined the expression pattern of Xnr3 and Siamois in the reversed embryo because these two genes are activated by the Wnt-pathway activators (Xwnt-8, beta-catenin, etc.) but not by other dorsalizing molecules (noggin, BVg1, etc.). Animal cap of the reversed embryo, which received the unpigmented cortex of the egg, expressed Xnr3 and Siamois. However, Mix.1, a marker expressed in endoderm and mesoderm in the normal embryo in response to mesodermal inducers, was not detected in the animal cap of the reversed embryo. In addition, we found that beta-catenin protein accumulated in nuclei of unpigmented animal pole cells of the reversed embryo. These results suggest that the maternal dorsal determinants behave more similarly to the Wnt-pathway activators than noggin or BVg1.

Mouse gastrulation: the formation of a mammalian body plan

The process of gastrulation is a pivotal step in the formation of the vertebrate body plan. The primary function of gastrulation is the correct placement of precursor tissues for subsequent morphogenesis. There is now mounting evidence that the body plan is established through inductive interactions between germ layer tissues and by the global patterning activity emanating from embryonic organizers. An increasing number of mouse mutants have been described that have gastrulation defects, providing important insights into the molecular mechanisms that regulate this complex process. In this review, we explore the mouse embryo before and during gastrulation, highlighting its similarities with other vertebrate embryos and its unique characteristics.

Animal and vegetal pole cells of early Xenopus embryos respond differently to maternal dorsal determinants: implications for the patterning of the organiser

The maternal dorsal determinants required for the specification of the dorsal territories of Xenopus early gastrulae are located at the vegetal pole of unfertilised eggs and are moved towards the prospective dorsal region of the fertilised egg during cortical rotation. While the molecular identity of the determinants is unknown, there are dorsal factors in the vegetal cortical cytoplasm (VCC). Here, we show that the VCC factors, when injected into animal cells activate the zygotic genes Siamois and Xnr3, suggesting that they act along the Wnt/beta-catenin pathway. In addition, Siamois and Xnr3 are activated at the vegetal pole of UV-irradiated embryos, indicating that these two genes are targets of the VCC factors in all embryonic cells. However, the consequences of their activation in cells that occupy different positions along the animal-vegetal axis differ. Dorsal vegetal cells of normal embryos or VCC-treated injected animal cells are able to dorsalise ventral mesoderm in conjugate experiments but UV-treated vegetal caps do not have this property. This difference is unlikely to reflect different levels of activation of FGF or activin-like signal transduction pathways but may reflect the activation of different targets of Siamois. Chordin, a marker of the head and axial mesoderm, is activated by the VCC/Siamois pathway in animal cells but not in vegetal cells whereas cerberus, a marker of the anterior mesendoderm which lacks dorsalising activity, can only be activated by the VCC/Siamois pathway in vegetal cells. We propose that the regionalisation of the organiser during gastrulation proceeds from the differential interpretation along the animal-vegetal axis of the activation of the VCC/beta-catenin/Siamois pathway.

Identification of Smad7, a TGFbeta-inducible antagonist of TGF-beta signalling

TGF-beta signals from the membrane to the nucleus through serine/threonine kinase receptors and their downstream effectors, termed SMAD proteins. The activated TGF-beta receptor induces phosphorylation of two such proteins, Smad2 and Smad3, which form hetero-oligomeric complex(es) with Smad4/DPC4 that translocate to the nucleus, where they then regulate transcriptional responses. However, the mechanisms by which the intracellular signals of TGF-beta are switched off are unclear. Here we report the identification of Smad7, which is related to Smad6. Transfection of Smad7 blocks responses mediated by TGF-beta in mammalian cells, and injection of Smad7 RNA into Xenopus embryos blocks activin/TGF-beta signalling. Smad7 associates stably with the TGF-beta receptor complex, but is not phosphorylated upon TGF-beta stimulation. TGFbeta-mediated phosphorylation of Smad2 and Smad3 is inhibited by Smad7, indicating that the antagonistic effect of Smad7 is exerted at this important regulatory step. TGF-beta rapidly induces expression of Smad7 mRNA, suggesting that Smad7 may participate in a negative feedback loop to control TGF-beta responses.

Daughters against dpp modulates dpp organizing activity in Drosophila wing development

The family of TGF-beta signalling molecules play inductive roles in various developmental contexts. One member of this family, Drosophila Decapentaplegic (Dpp) serves as a morphogen that patterns both the embryo and adult. We have now isolated a gene, Daughters against dpp (Dad), whose transcription is induced by Dpp. Dad shares weak homology with Drosophila Mad (Mothers against dpp), a protein required for transduction of Dpp signals. In contrast to Mad or the activated Dpp receptor, whose overexpression hyperactivates the Dpp signalling pathway, overexpression of Dad blocks Dpp activity. Expression of Dad together with either Mad or the activated receptor rescues phenotypic defects induced by each protein alone. Dad can also antagonize the activity of a vertebrate homologue of Dpp, bone morphogenetic protein, as evidenced by induction of dorsal or neural fate following overexpression in Xenopus embryos. We conclude that the pattern-organizing mechanism governed by Dpp involves a negative-feedback circuit in which Dpp induces expression of its own antagonist, Dad. This feedback loop appears to be conserved in vertebrate development.

Vertebrate head induction by anterior primitive endoderm

In vertebrates the antero-posterior organization of the embryonic body axis is thought to result from the activity of two separate centers, the head organizer and the trunk organizer, as operationally defined by Spemann in the 1920s. Current molecular studies have supported the existence of a trunk organizer activity while the presence of a distinct head inducing center has remained elusive. Mainly based on analyses of headless mutants in mice, it has been proposed that the anterior axial mesoderm plays a determining role in head induction. Recent gain- and loss-of-function studies in various organisms, however, provide compelling evidence that a largely ignored region, the anterior primitive endoderm, specifies rostral identity. In this review we discuss the emerging concept that the anterior primitive endoderm, rather than the prechordal plate mesoderm, induces head development in the vertebrate embryo.

Xwnt-8 and lithium can act upon either dorsal mesodermal or neurectodermal cells to cause a loss of forebrain in Xenopus embryos

When Xenopus gastrulae are made to misexpress Xwnt-8, or are exposed to lithium ions, they develop with a loss of anterior structures. In the current study, we have characterized the neural defects produced by either Xwnt-8 or lithium and have examined potential cellular mechanisms underlying this anterior truncation. We find that the primary defect in embryos exposed to lithium at successively earlier stages during gastrulation is a progressive rostral to caudal deletion of the forebrain, while hindbrain and spinal regions of the CNS remain intact. Misexpression of Xwnt-8 during gastrulation produces an identical loss of forebrain. Our results demonstrate that lithium and Wnts can act upon either prospective neural ectodermal cells, or upon dorsal mesodermal cells, to cause a loss of anterior pattern. Specifically, ectodermal cells isolated from lithium- or Wnt-exposed embryos are unable to form anterior neural tissue in response to inductive signals from normal dorsal mesoderm. In addition, although dorsal mesodermal cells from lithium- or Wnt-exposed embryos are specified properly, and produce normal levels of the anterior neural inducing molecules noggin and chordin, they show a greatly reduced capacity to induce anterior neural tissue in conjugated ectoderm. Taken together, our results are consistent with a model in which Wnt- or lithium-mediated signals can induce either mesodermal or ectodermal cells to produce a dominant posteriorizing morphogen which respecifies anterior neural tissue as posterior.