Requirement for anti-dorsalizing morphogenetic protein in organizer patterning

The amphibian Spemann organizer is subdivided in trunk and head organizer and it is unclear how this division is regulated. The Xenopus trunk organizer expresses anti-dorsalizing morphogenetic protein (ADMP), a potent organizer antagonist. We show that ADMP represses head formation during gastrulation and that its expression is activated by BMP antagonists. A specifically acting dominant-negative ADMP anteriorizes embryos and its coexpression with BMP antagonists induces secondary embryonic axes with heads as well as expression of head inducers. Unlike other BMPs, ADMP is not inhibited by a dominant-negative BMP type I receptor, Noggin, Cerberus and Chordin but by Follistatin, suggesting that it utilizes a distinct TGF-β receptor pathway and displays differential sensitivity to BMP antagonists. The results indicate that ADMP functions in the trunk organizer to antagonize head formation, thereby regulating organizer patterning.     

Requirement for anti-dorsalizing morphogenetic protein in organizer patterning

The amphibian Spemann organizer is subdivided in trunk and head organizer and it is unclear how this division is regulated. The Xenopus trunk organizer expresses anti-dorsalizing morphogenetic protein (ADMP), a potent organizer antagonist. We show that ADMP represses head formation during gastrulation and that its expression is activated by BMP antagonists. A specifically acting dominant-negative ADMP anteriorizes embryos and its coexpression with BMP antagonists induces secondary embryonic axes with heads as well as expression of head inducers. Unlike other BMPs, ADMP is not inhibited by a dominant-negative BMP type I receptor, Noggin, Cerberus and Chordin but by Follistatin, suggesting that it utilizes a distinct TGF-β receptor pathway and displays differential sensitivity to BMP antagonists. The results indicate that ADMP functions in the trunk organizer to antagonize head formation, thereby regulating organizer patterning.     

Noggin and noggin-like genes control dorsoventral axis regeneration in planarians

Planarians regenerate a whole animal from a small body piece within a few days. Recent studies have shown that the bone morphogenetic protein (BMP) pathway is required to reestablish the dorsoventral (DV) axis. In vertebrates, the specification of the DV axis depends on the coordinated action of a dual organizer defined by BMP and antidorsalizing morphogenetic protein (ADMP) under the control of several factors, including the inhibitors chordin and noggin. Planarians have an expanded noggin family (up to ten members), which have been classified as canonical noggin (nog) and noggin-like (nlg) genes, the latter carrying an insertion within the noggin domain. Here we show that a BMP/ADMP organizer governs DV axis reestablishment during planarian regeneration, highlighting a greater-than-thought conservation of the mechanisms that establish this axis in protostomes and deuterostomes. Also, we report that whereas noggin genes function as canonical BMP inhibitors, the silencing of planarian nlg8 induces ectopic neurogenesis and enhances ventralizing bmp(RNAi) phenotypes. Finally, we show that noggin-like genes are conserved from cnidarian to vertebrates and that both planarian nlg8 and Xenopus nlg ventralize Xenopus embryos when overexpressed. Remarkably, this ventralization is not associated with an increase in SMAD1/5/8 phosphorylation.     

Characterization of follistatin isoforms in early Xenopus embryogenesis

Follistatin is expressed in Spemann's organizer in the Xenopus gastrula and mimics the activity of the organizer, inducing a neural fate directly in the ectoderm. We have previously shown that follistatin inhibits BMP activity through a direct interaction. In this study, we have characterized the localization and function of two follistatin isoforms to examine the functional differences between them. One notable difference, previously described, is that the shorter form (xFSS or xFS319) but not the C-terminally extended long form (xFSL) associates with cell-surface matrices. Here, we show that the spatial-temporal expression pattern of xFSL and xFSS is indistinguishable. Interestingly, however, xFSS was found to have a more potent inhibitory activity against BMP-4 than xFSL. Furthermore, using a surface plasmon resonance biosensor, xFSS was shown to have a higher binding capacity for BMP subtypes. The diffusion rates of xFSS and xFSL ectopically expressed in Xenopus embryos were similar. Taken together, our results suggest that the difference in BMP-inhibiting activity of the two follistatin isoforms is mainly attributable to a difference in their BMP binding properties rather than to their diffusion rates.     

Depletion of Bmp2, Bmp4, Bmp7 and Spemann organizer signals induces massive brain formation in Xenopus embryos

To address the patterning function of the Bmp2, Bmp4 and Bmp7 growth factors, we designed antisense morpholino oligomers (MO) that block their activity in Xenopus laevis. Bmp4 knockdown was sufficient to rescue the ventralizing effects caused by loss of Chordin activity. Double Bmp4 and Bmp7 knockdown inhibited tail development. Triple Bmp2/Bmp4/Bmp7 depletion further compromised trunk development but did not eliminate dorsoventral patterning. Unexpectedly, we found that blocking Spemann organizer formation by UV treatment or beta-Catenin depletion caused BMP inhibition to have much more potent effects, abolishing all ventral development and resulting in embryos having radial central nervous system (CNS) structures. Surprisingly, dorsal signaling molecules such as Chordin, Noggin, Xnr6 and Cerberus were not re-expressed in these embryos. We conclude that BMP inhibition is sufficient for neural induction in vivo, and that in the absence of ventral BMPs, Spemann organizer signals are not required for brain formation.     

Robust stability of the embryonic axial pattern requires a secreted scaffold for chordin degradation

Dorsal axial formation during vertebrate embryogenesis exhibits robust resistance to perturbations in patterning signals. However, how such stability is supported at the molecular level remains largely elusive. Here we show that Xenopus ONT1, an Olfactomedin-class secreted protein, stabilizes axial formation by restricting Chordin activity on the dorsal side. When ONT1 function is attenuated, the embryo becomes hyperdorsalized by a normally subeffective dose of Chordin. ONT1 binds Chordin and BMP1/Tolloid-class proteinases (B1TP) via distinct domains and acts as a secreted scaffold that enhances B1TP-mediated Chordin degradation by facilitating enzyme-substrate association. ONT1 is indispensable for fine-tuning BMP signaling in the axial tissue, and a similar role has been suggested for dorsally expressed BMPs such as ADMP. Simultaneous inhibition of ONT1 and dorsally expressed BMPs (ADMP and BMP2) synergistically caused drastic dorsalization. These results indicate that stable axial formation depends on two compensatory regulatory pathways involving ONT1/B1TP and dorsally expressed BMPs.     

The homeobox gene bozozok promotes anterior neuroectoderm formation in zebrafish through negative regulation of BMP2/4 and Wnt pathways

The neuroectoderm of the vertebrate gastrula was proposed by Nieuwkoop to be regionalized into forebrain, midbrain, hindbrain and spinal cord by a two-step process. In the activation step, the Spemann gastrula organizer induces neuroectoderm with anterior character, followed by posteriorization by a transforming signal. Recently, simultaneous inhibition of BMP and Wnt signaling was shown to induce head formation in frog embryos. However, how the inhibition of BMP and Wnt signaling pathways specify a properly patterned head, and how they are regulated in vivo, is not understood. Here we demonstrate that the loss of anterior neural fates observed in zebrafish bozozok (boz) mutants occurs during gastrulation due to a reduction and subsequent posteriorization of neuroectoderm. The neural induction defect was correlated with decreased chordino expression and consequent increases in bmp2b/4 expression, and was suppressed by overexpression of BMP antagonists. Whereas expression of anterior neural markers was restored by ectopic BMP inhibition in early boz gastrulae, it was not maintained during later gastrulation. The posteriorization of neuroectoderm in boz was correlated with ectopic dorsal wnt8 expression. Overexpression of a Wnt antagonist rescued formation of the organizer and anterior neural fates in boz mutants. We propose that boz specifies formation of anterior neuroectoderm by regulating BMP and Wnt pathways in a fashion consistent with Nieuwkoop's two-step neural patterning model. boz promotes neural induction by positively regulating organizer-derived chordino and limiting the antineuralizing activity of BMP2b/4 morphogens. In addition, by negative regulation of Wnt signaling, boz promotes organizer formation and limits posteriorization of neuroectoderm in the late gastrula.     

Depletion of three BMP antagonists from Spemann's organizer leads to a catastrophic loss of dorsal structures

Transplanted Spemann's organizer induces dorsal embryonic cell fates such as the nervous system and somites, but in normal development, elimination of individual organizer signals (such as the bone morphogenetic protein [BMP] antagonists) has surprisingly modest effects on these tissues. Thus, the role of BMP antagonists may be limited to fine tuning the size of the dorsal domain. However, at least five BMP antagonists are specifically expressed in the organizer, and all can mimic aspects of organizer function, suggesting overlapping functions. Here, we deplete the function of three BMP antagonists, chordin, noggin, and follistatin, in Xenopus tropicalis. We demonstrate that this results in catastrophic failure of dorsal development and expansion of ventral and posterior fates. We conclude that BMP antagonists are required for formation of the neural plate and dorsal mesoderm. In addition, our results show that neural specification requires the continuous activity of BMP antagonists from blastula through gastrula stages.     

ADMP2 is essential for primitive blood and heart development in Xenopus

We describe here the cloning of a new member of the TGF-beta family with similarity to the anti-dorsalizing morphogenetic proteins (ADMPs). This new gene, ADMP2, is expressed in a broad band of mesendoderm cells that appear to include the progenitors of the endoderm and the ventral mesoderm. Antisense morpholino oligonucleotide knockdown of ADMP2 results in near-complete disruption of primitive blood and heart development, while the development of other mesoderm derivatives, including pronephros, muscle and lateral plate is not disrupted. Moreover, the development of the primitive blood in ADMP2 knockdown embryos cannot be rescued by BMP. These results suggests that ADMP2 plays an early role in specifying presumptive ventral mesoderm in the leading edge mesoderm, and that ADMP2 activity may be necessary to respond to BMP signaling in the context of ventral mesoderm induction.     

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(-)), beta-catenin-depleted (beta-catenin(-)) and double depleted (VegT(-)/beta-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 beta-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 beta-catenin(-) embryos.     

Lost-a-fin encodes a type I BMP receptor, Alk8, acting maternally and zygotically in dorsoventral pattern formation

TGFbeta signaling pathways of the bone morphogenetic protein (BMP) subclass are essential for dorsoventral pattern formation of both vertebrate and invertebrate embryos. Here we determine by chromosomal mapping, linkage analysis, cDNA sequencing and mRNA rescue that the dorsalized zebrafish mutant lost-a-fin (laf) is defective in the gene activin receptor-like kinase 8 (alk8), which encodes a novel type I TGFbeta receptor. The alk8 mRNA is expressed both maternally and zygotically. Embyros that lack zygotic, but retain maternal Laf/Alk8 activity, display a weak dorsalization restricted to the tail and die by 3 days postfertilization. We rescued the laf dorsalized mutant phenotype by alk8 mRNA injection and generated homozygous laf/alk8 mothers to investigate the maternal role of Laf/Alk8 activity. Adult fish lacking Laf/Alk8 activity are fertile, exhibit a growth defect and are significantly smaller than their siblings. Embryos derived from homozygous females, which lack both maternal and zygotic Laf/Alk8 activity, display a strongly dorsalized mutant phenotype, no longer limited to the tail. These mutant embryos lack almost all gastrula ventral cell fates, with a concomitant expansion of dorsal cell types. During later stages, most of the somitic mesoderm and neural tissue circumscribe the dorsoventral axis of the embryo. Zygotic laf/alk8 mutants can be rescued by overexpression of the BMP signal transducer Smad5, but not the Bmp2b or Bmp7 ligands, consistent with the Laf/Alk8 receptor acting within a BMP signaling pathway, downstream of a Bmp2b/Bmp7 signal. Antibodies specific for the phosphorylated, activated form of Smad1/5, show that BMP signaling is nearly absent in gastrula lacking both maternal and zygotic Laf/Alk8 activity, providing further evidence that Laf/Alk8 transduces a BMP signal. In total, our work strongly supports the role of Laf/Alk8 as a type I BMP receptor required for the specification of ventral cell fates.     

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.     

The Integrator Complex Subunit 6 (Ints6) Confines the Dorsal Organizer in Vertebrate Embryogenesis

Dorsoventral patterning of the embryonic axis relies upon the mutual antagonism of competing signaling pathways to establish a balance between ventralizing BMP signaling and dorsal cell fate specification mediated by the organizer. In zebrafish, the initial embryo-wide domain of BMP signaling is refined into a morphogenetic gradient following activation dorsally of a maternal Wnt pathway. The accumulation of β-catenin in nuclei on the dorsal side of the embryo then leads to repression of BMP signaling dorsally and the induction of dorsal cell fates mediated by Nodal and FGF signaling. A separate Wnt pathway operates zygotically via Wnt8a to limit dorsal cell fate specification and maintain the expression of ventralizing genes in ventrolateral domains. We have isolated a recessive dorsalizing maternal-effect mutation disrupting the gene encoding Integrator Complex Subunit 6 (Ints6). Due to widespread de-repression of dorsal organizer genes, embryos from mutant mothers fail to maintain expression of BMP ligands, fail to fully express vox and ved, two mediators of Wnt8a, display delayed cell movements during gastrulation, and severe dorsalization. Consistent with radial dorsalization, affected embryos display multiple independent axial domains along with ectopic dorsal forerunner cells. Limiting Nodal signaling or restoring BMP signaling restores wild-type patterning to affected embryos. Our results are consistent with a novel role for Ints6 in restricting the vertebrate organizer to a dorsal domain in embryonic patterning.     

A Bmp/Admp regulatory circuit controls maintenance and regeneration of dorsal-ventral polarity in planarians

Animal embryos have diverse anatomy and vary greatly in size. It is therefore remarkable that a common signaling pathway, BMP signaling, controls development of the dorsoventral (DV) axis throughout the Bilateria. In vertebrates, spatially opposed expression of the BMP family proteins Bmp4 and Admp (antidorsalizing morphogenetic protein) can promote restoration of DV pattern following tissue removal. bmp4 orthologs have been identified in all three groups of the Bilateria (deuterostomes, ecdysozoans, and lophotrochozoans). By contrast, the absence of admp orthologs in ecdysozoans such as Drosophila and C. elegans has suggested that a regulatory circuit of oppositely expressed bmp4 and admp genes represents a deuterostome-specific innovation. Here we describe the existence of spatially opposed bmp and admp expression in a protostome. An admp ortholog (Smed-admp) is expressed ventrally and laterally in adult Schmidtea mediterranea planarians, opposing the dorsal-pole expression of Smed-bmp4. Smed-admp is required for regeneration following parasagittal amputation. Furthermore, Smed-admp promotes Smed-bmp4 expression and Smed-bmp4 inhibits Smed-admp expression, generating a regulatory circuit that buffers against perturbations of Bmp signaling. These results suggest that a Bmp/Admp regulatory circuit is a central feature of the Bilateria, used broadly for the establishment, maintenance, and regeneration of the DV axis.     

The role of the Spemann organizer in anterior-posterior patterning of the trunk

The formation of the vertebrate body axis during gastrulation strongly depends on a dorsal signaling centre, the Spemann organizer as it is called in amphibians. This organizer affects embryonic development by self-differentiation, regulation of morphogenesis and secretion of inducing signals. Whereas many molecular signals and mechanisms of the organizer have been clarified, its function in anterior-posterior pattern formation remains unclear. We dissected the organizer functions by generally blocking organizer formation and then restoring a single function. In experiments using a dominant inhibitory BMP receptor construct (tBr) we find evidence that neural activation by antagonism of the BMP pathway is the organizer function that enables the establishment of a detailed anterior-posterior pattern along the trunk. Conversely, the exclusive inhibition of neural activation by expressing a constitutive active BMP receptor (hAlk-6) in the ectoderm prohibits the establishment of an anterior-posterior pattern, even though the organizer itself is still intact. Thus, apart from the formerly described separation into a head and a trunk/tail organizer, the organizer does not deliver positional information for anterior-posterior patterning. Rather, by inducing neurectoderm, it makes ectodermal cells competent to receive patterning signals from the non-organizer mesoderm and thereby enable the formation of a complete and stable AP pattern along the trunk.     

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.