XBMPRII, a novel Xenopus type II receptor mediating BMP signaling in embryonic tissues

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.

Controlling the Messenger: Regulated Translation of Maternal mRNAs in Xenopus laevis Development

The selective translation of maternal mRNAs encoding cell-fate determinants drives the earliest decisions of embryogenesis that establish the vertebrate body plan. This chapter will discuss studies in Xenopus laevis that provide insights into mechanisms underlying this translational control. Xenopus has been a powerful model organism for many discoveries relevant to the translational control of maternal mRNAs because of the large size of its oocytes and eggs that allow for microinjection of molecules and the relative ease of manipulating the oocyte to egg transition (maturation) and fertilization in culture. Consequently, many key studies have focused on the expression of maternal mRNAs during the oocyte to egg transition (the meiotic cell cycle) and the rapid cell divisions immediately following fertilization. This research has made seminal contributions to our understanding of translational regulatory mechanisms, but while some of the mRNAs under consideration at these stages encode cell-fate determinants, many encode cell cycle regulatory proteins that drive these early cell cycles. In contrast, while maternal mRNAs encoding key developmental (i.e., cell-fate) regulators that function after the first cleavage stages may exploit aspects of these foundational mechanisms, studies reveal that these mRNAs must also rely on distinct and, as of yet, incompletely understood mechanisms. These findings are logical because the functions of such developmental regulatory proteins have requirements distinct from cell cycle regulators, including becoming relevant only after fertilization and then only in specific cells of the embryo. Indeed, key maternal cell-fate determinants must be made available in exquisitely precise amounts (usually low), only at specific times and in specific cells during embryogenesis. To provide an appreciation for the regulation of maternal cell-fate determinant expression, an overview of the maternal phase of Xenopus embryogenesis will be presented. This section will be followed by a review of translational mechanisms operating in oocytes, eggs, and early cleavage-stage embryos and conclude with a discussion of how the regulation of key maternal cell-fate determinants at the level of translation functions in Xenopus embryogenesis. A key theme is that the molecular asymmetries critical for forming the body axes are established and further elaborated upon by the selective temporal and spatial regulation of maternal mRNA translation.

Differential requirement of bone morphogenetic protein receptors Ia (ALK3) and Ib (ALK6) in early embryonic patterning and neural crest development

Background

Bone morphogenetic proteins regulate multiple processes in embryonic development, including early dorso-ventral patterning and neural crest development. BMPs activate heteromeric receptor complexes consisting of type I and type II receptor-serine/threonine kinases. BMP receptors Ia and Ib, also known as ALK3 and ALK6 respectively, are the most common type I receptors that likely mediate most BMP signaling events. Since early expression patterns and functions in Xenopus laevis development have not been described, we have addressed these questions in the present study.

Results

Here we have analyzed the temporal and spatial expression patterns of ALK3 and ALK6; we have also carried out loss-of-function studies to define the function of these receptors in early Xenopus development. We detected both redundant and non-redundant roles of ALK3 and ALK6 in dorso-ventral patterning. From late gastrula stages onwards, their expression patterns diverged, which correlated with a specific, non-redundant requirement of ALK6 in post-gastrula neural crest cells. ALK6 was essential for induction of neural crest cell fate and further development of the neural crest and its derivatives.

Conclusions

ALK3 and ALK6 both contribute to the gene regulatory network that regulates dorso-ventral patterning; they play partially overlapping and partially non-redundant roles in this process. ALK3 and ALK6 are independently required for the spatially restricted activation of BMP signaling and msx2 upregulation at the neural plate border, whereas in post-gastrula development ALK6 exerts a highly specific, conserved function in neural crest development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-016-0101-5) contains supplementary material, which is available to authorized users.

Efficient retina formation requires suppression of both Activin and BMP signaling pathways in pluripotent cells

Retina formation requires the correct spatiotemporal patterning of key regulatory factors. While it is known that repression of several signaling pathways lead to specification of retinal fates, addition of only Noggin, a known BMP antagonist, can convert pluripotent Xenopus laevis animal cap cells to functional retinal cells. The aim of this study is to determine the intracellular molecular events that occur during this conversion. Surprisingly, blocking BMP signaling alone failed to mimic Noggin treatment. Overexpressing Noggin in pluripotent cells resulted in a concentration-dependent suppression of both Smad1 and Smad2 phosphorylation, which act downstream of BMP and Activin signaling, respectively. This caused a decrease in downstream targets: endothelial marker, xk81, and mesodermal marker, xbra. We treated pluripotent cells with dominant-negative receptors or the chemical inhibitors, dorsomorphin and SB431542, which each target either the BMP or Activin signaling pathway. We determined the effect of these treatments on retina formation using the Animal Cap Transplant (ACT) assay; in which treated pluripotent cells were transplanted into the eye field of host embryos. We found that inhibition of Activin signaling, in the presence of BMP signaling inhibition, promotes efficient retinal specification in Xenopus tissue, mimicking the affect of adding Noggin alone. In whole embryos, we found that the eye field marker, rax, expanded when adding both dominant-negative Smad1 and Smad2, as did treating the cells with both dorsomorphin and SB431542. Future studies could translate these findings to a mammalian culture assay, in order to more efficiently produce retinal cells in culture.

Nodal·Gdf1 heterodimers with bound prodomains enable serum-independent nodal signaling and endoderm differentiation

The TGFβ family member Nodal is central to control pluripotent stem cell fate, but its use as a stem cell differentiation factor is limited by low specific activity. During development, Nodal depends on growth and differentiation factor (Gdf)-1 and on the shared co-receptor Cryptic to specify visceral left-right axis asymmetry. We therefore asked whether the functionality of Nodal can be augmented by Gdf1. Because Nodal and Gdf1 coimmunoprecipitate each other, they were predicted to form heterodimers, possibly to facilitate diffusion or to increase the affinity for signaling receptors. Here, we report that Gdf1 suppresses an unexpected dependence of Nodal on serum proteins and that it is critically required for non-autonomous signaling in cells expressing Cryptic. Nodal, Gdf1, and their cleaved propeptides copurified as a heterodimeric low molecular weight complex that stimulated Activin receptor (Acvr) signaling far more potently than Nodal alone. Although heterodimerization with Gdf1 did not increase binding of Nodal to Fc fusions of co-receptors or Acvr extracellular domains, it was essential for soluble Acvr2 to inhibit Nodal signaling. This implies that Gdf1 potentiates Nodal activity by stabilizing a low molecular weight fraction that is susceptible to neutralization by soluble Acvr2. Finally, in differentiating human ES cells, endodermal markers were more efficiently induced by Nodal·Gdf1 than by Nodal, suggesting that Nodal·Gdf1 is an attractive new reagent to direct stem cell differentiation.

Systems proteomics of cardiac chromatin identifies nucleolin as a regulator of growth and cellular plasticity in cardiomyocytes

Myocyte hypertrophy antecedent to heart failure involves changes in global gene expression, although the preceding mechanisms to coordinate DNA accessibility on a genomic scale are unknown. Chromatin-associated proteins alter chromatin structure by changing their association with DNA, thereby altering the gene expression profile. Little is known about the global changes in chromatin subproteomes that accompany heart failure, and the mechanisms by which these proteins alter chromatin structure. The present study tests the fundamental hypothesis that cardiac growth and plasticity in the setting of disease recapitulates conserved developmental chromatin remodeling events. We used quantitative proteomics to identify chromatin-associated proteins extracted via detergent and to quantify changes in their abundance during disease. Our study identified 321 proteins in this subproteome, demonstrating it to have modest conservation (37%) with that revealed using strong acid. Of these proteins, 176 exhibited altered expression during cardiac hypertrophy and failure; we conducted extensive functional characterization of one of these proteins, Nucleolin. Morpholino-based knockdown of nucleolin nearly abolished protein expression but surprisingly had little impact on gross morphological development. However, hearts of fish lacking Nucleolin displayed severe developmental impairment, abnormal chamber patterning and functional deficits, ostensibly due to defects in cardiac looping and myocyte differentiation. The mechanisms underlying these defects involve perturbed bone morphogenetic protein 4 expression, decreased rRNA transcription, and a shift to more heterochromatic chromatin. This study reports the quantitative analysis of a new chromatin subproteome in the normal and diseased mouse heart. Validation studies in the complementary model system of zebrafish examine the role of Nucleolin to orchestrate genomic reprogramming events shared between development and disease.

p21-Activated kinase (PAK) is required for Bone Morphogenetic Protein (BMP)-induced dendritogenesis in cortical neurons

Bone Morphogenetic Proteins (BMPs) are crucial for many aspects of the development and differentiation of the nervous system and are important in controlling cytoskeletal remodeling during neuronal morphogenesis. BMPs are TGFβ superfamily members that signal through a heteromeric complex of type I and type II BMP receptors. The BMPRII receptor is particularly important in mediating remodeling of the neuronal cytoskeleton through the activation of BMPRII-bound cytoskeletal regulators, such as LIM Kinase (LIMK). Here, we show that PAK1, a key regulator of diverse neuronal processes and an upstream activator of LIMK, binds to the BMP type I receptor, ALK2. Although, PAK1 is dispensable for activation of the Smad transcriptional mediators, abrogation of PAK1 expression or inhibition of PAK1 activity prevents BMP-induced neurite outgrowth in cultured neuroblastoma cell lines. Moreover, in primary murine embryonic cortical neurons, inhibition of PAK activity blocks BMP7-induced cofilin phosphorylation, prevents remodeling of the actin cytoskeleton and thereby blocks BMP7-induced dendrite formation. Thus, we propose a model in which BMP7 signaling leads to the recruitment of ALK2-bound PAK1 to BMPRII, which binds a downstream regulator of the actin cytoskeleton, LIMK1, and that the BMP receptor complex thereby acts as a scaffold to localize and coordinate actin cytoskeletal remodeling. We propose that this scaffold plays a key role in mediating BMP7-dependent dendritogenesis in primary cortical neurons.

Myo/Nog cell regulation of bone morphogenetic protein signaling in the blastocyst is essential for normal morphogenesis and striated muscle lineage specification

Cells that express MyoD mRNA, the G8 antigen and the bone morphogenetic protein (BMP) inhibitor noggin (Nog) are present in the epiblast before gastrulation. Ablation of "Myo/Nog" cells in the blastocyst results in an expansion of canonical BMP signaling and prevents the expression of noggin and follistatin before and after the onset of gastrulation. Once eliminated in the epiblast, they are neither replaced nor compensated for as development progresses. Older embryos lacking Myo/Nog cells exhibit severe axial malformations. Although Wnts and Sonic hedgehog are expressed in ablated embryos, skeletal muscle progenitors expressing Pax3 are missing in the somites. Pax3+ cells do emerge adjacent to Wnt3a+ cells in vitro; however, few undergo skeletal myogenesis. Ablation of Myo/Nog cells also results in ectopically placed cardiac progenitors and cardiomyocytes in the somites. Reintroduction of Myo/Nog cells into the epiblast of ablated embryos restores normal patterns of BMP signaling, morphogenesis and skeletal myogenesis, and inhibits the expression of cardiac markers in the somites. This study demonstrates that Myo/Nog cells are essential regulators of BMP signaling in the early epiblast and are indispensable for normal morphogenesis and striated muscle lineage specification.

Reversal of Xenopus Oct25 function by disruption of the POU domain structure

Xenopus Oct25 is a POU family subclass V (POU-V) transcription factor with a distinct domain structure. To investigate the contribution of different domains to the function of Oct25, we have performed gain of function analyses. Deletions of the N- or C-terminal regions and of the Hox domain (except its nuclear localization signal) result in mutants being indistinguishable from the wild type protein in the suppression of genes promoting germ layer formation. Deletion of the complete POU domain generates a mutant that has no effect on embryogenesis. However, disruption of the alpha-helical structures in the POU domain, even by a single amino acid mutation, causes reversal of protein function. Overexpression of such mutants leads to dorsalization of embryos and formation of secondary axial structures. The underlying mechanism is an enhanced transcription of genes coding for antagonists of the ligands for ventralizing bone morphogenetic protein and Wnt pathways. Corresponding deletion mutants of Xenopus Oct60, Oct91, or mouse Oct4 also exhibit such a dominant-negative effect. Therefore, our results reveal that the integrity of the POU domain is crucial for the function of POU-V transcription factors in the regulation of genes that promote germ layer formation.

Structural and functional characterizations of an Activin type II receptor orthologue from the pacific oyster Crassostrea gigas

Members of the Transforming Growth factor beta (TGF-beta) superfamily of cell signalling polypeptides are known to play important roles in cell proliferation and differentiation during development and in various physiological processes of most animal clades. Recent findings in the mollusc Crassostrea gigas demonstrate the occurrence of a diversity of TGF-beta signalling components including various ligands, three type I receptors but only a single type II receptor. This report describes the characterization of Cg-ActRII, a new type II receptor displaying homology with vertebrate and Drosophila Activin type II receptors. The use of zebrafish embryo as a reporter organism revealed that, in a way similar to its zebrafish counterpart, overexpression of Cg-ActRII or its dominant negative acting truncated form resulted in a dose dependent range of dorsoventral defects coupled with anterior disorders. Expression pattern of Cg-ActRII transcripts examined by real time PCR and in situ PCR in C. gigas showed high levels of Cg-ActRII transcripts in early embryonic stages and in the developing larval central nervous system. Except for a high expression in the visceral ganglia, most oyster adult tissues displayed rather low levels of transcripts. Altogether, the data suggest a high degree of conservation at both the structural and functional levels during evolution for this class of receptors.

Two novel type II receptors mediate BMP signalling and are required to establish left-right asymmetry in zebrafish

Ligands of the transforming growth factor beta (TGFbeta) superfamily, like Nodal and bone morphogenetic protein (BMP), are pivotal to establish left-right (LR) asymmetry in vertebrates. However, the receptors mediating this process are unknown. Here we identified two new type II receptors for BMPs in zebrafish termed bmpr2a and bmpr2b that induce a classical Smad1/5/8 response to BMP binding. Morpholino-mediated knockdown of bmpr2a and bmpr2b showed that they are required for the establishment of concomitant cardiac and visceral LR asymmetry. Expression of early laterality markers in morphants indicated that bmpr2a and bmpr2b act upstream of pitx2 and the nodal-related southpaw (spaw), which are expressed asymmetrically in the lateral plate mesoderm (LPM), and subsequently regulate lefty2 and bmp4 in the left heart field. We demonstrated that bmpr2a is required for lefty1 expression in the midline at early segmentation while bmpr2a/bmpr2b heteromers mediate left-sided spaw expression in the LPM. We propose a mechanism whereby this differential interpretation of BMP signalling through bmpr2a and bmpr2b is essential for the establishment of LR asymmetry in the zebrafish embryo.

TGFbeta ligands promote the initiation of retinal ganglion cell dendrites in vitro and in vivo

Each type of neuron develops a unique morphology critical to its function, but almost all start with the basic plan of one long axon and multiple short, branched dendrites. Though extrinsic signals are known to direct many steps in the development of neuronal structure, little is understood about the initiation of processes, particularly dendrites. We find that Xenopus retinal ganglion cells (RGCs) explanted early will extend axons and not dendrites in dissociated cultures. If RGCs develop longer in vivo prior to culturing, many now extend dendrite-like processes in vitro, suggesting that an extrinsic factor is required to stimulate dendrite initiation. Members of the transforming growth factor beta (TGFbeta) superfamily, bone morphogenetic protein 2 (BMP2), and growth and differentiation factor 11 (GDF11), can signal cultured RGCs to form dendrites. Furthermore, TGFbeta ligands have an endogenous role: blocking BMP/GDF signaling with a secreted antagonist or inhibitory receptors reduces the number of primary dendrites extended in vivo.

BMP gradients steer nerve growth cones by a balancing act of LIM kinase and Slingshot phosphatase on ADF/cofilin

Bone morphogenic proteins (BMPs) are involved in axon pathfinding, but how they guide growth cones remains elusive. In this study, we report that a BMP7 gradient elicits bidirectional turning responses from nerve growth cones by acting through LIM kinase (LIMK) and Slingshot (SSH) phosphatase to regulate actin-depolymerizing factor (ADF)/cofilin-mediated actin dynamics. Xenopus laevis growth cones from 4–8-h cultured neurons are attracted to BMP7 gradients but become repelled by BMP7 after overnight culture. The attraction and repulsion are mediated by LIMK and SSH, respectively, which oppositely regulate the phosphorylation-dependent asymmetric activity of ADF/cofilin to control the actin dynamics and growth cone steering. The attraction to repulsion switching requires the expression of a transient receptor potential (TRP) channel TRPC1 and involves Ca²⁺ signaling through calcineurin phosphatase for SSH activation and growth cone repulsion. Together, we show that spatial regulation of ADF/cofilin activity controls the directional responses of the growth cone to BMP7, and Ca²⁺ influx through TRPC tilts the LIMK-SSH balance toward SSH-mediated repulsion.

Xenopus hairy2 functions in neural crest formation by maintaining cells in a mitotic and undifferentiated state

The neural crest is a population of mitotically active, multipotent progenitor cells that arise at the neural plate border. Neural crest progenitors must be maintained in a multipotent state until after neural tube closure. However, the molecular underpinnings of this process have yet to be fully elucidated. Here we show that the basic helix-loop-helix (bHLH) transcriptional repressor gene, Xenopus hairy2 (Xhairy2), is an essential early regulator of neural crest formation in Xenopus. During gastrulation, Xhairy2 is localized at the presumptive neural crest prior to the expression of such neural crest markers as Slug and FoxD3. Morpholino-mediated knockdown of Xhairy2 results in the repression of neural crest marker gene expression while inducing the ectopic expression of the cell cycle inhibitor p27(xic1) in the presumptive neural crest. We also found that ectopic p27(xic1) disturbs neural crest formation. Furthermore, the depletion of Xhairy2 leads to the apoptosis of mitotic cells. Our results suggest that Xhairy2 functions in neural crest specification by maintaining cells in the mitotic and undifferentiated state.

A silent H-bond can be mutationally activated for high-affinity interaction of BMP-2 and activin type IIB receptor

Background

Bone morphogenetic proteins (BMPs) are key regulators in the embryonic development and postnatal tissue homeostasis in all animals. Loss of function or dysregulation of BMPs results in severe diseases or even lethality. Like transforming growth factors β (TGF-βs), activins, growth and differentiation factors (GDFs) and other members of the TGF-β superfamily, BMPs signal by assembling two types of serine/threonine-kinase receptor chains to form a hetero-oligomeric ligand-receptor complex. BMP ligand receptor interaction is highly promiscuous, i.e. BMPs bind more than one receptor of each subtype, and a receptor bind various ligands. The activin type II receptors are of particular interest, since they bind a large number of diverse ligands. In addition they act as high-affinity receptors for activins but are also low-affinity receptors for BMPs. ActR-II and ActR-IIB therefore represent an interesting example how affinity and specificity might be generated in a promiscuous background.

Results

Here we present the high-resolution structures of the ternary complexes of wildtype and a variant BMP-2 bound to its high-affinity type I receptor BMPR-IA and its low-affinity type II receptor ActR-IIB and compare them with the known structures of binary and ternary ligand-receptor complexes of BMP-2. In contrast to activin or TGF-β3 no changes in the dimer architecture of the BMP-2 ligand occur upon complex formation. Functional analysis of the ActR-IIB binding epitope shows that hydrophobic interactions dominate in low-affinity binding of BMPs; polar interactions contribute only little to binding affinity. However, a conserved H-bond in the center of the type II ligand-receptor interface, which does not contribute to binding in the BMP-2 – ActR-IIB interaction can be mutationally activated resulting in a BMP-2 variant with high-affinity for ActR-IIB. Further mutagenesis studies were performed to elucidate the binding mechanism allowing us to construct BMP-2 variants with defined type II receptor binding properties.

Conclusion

Binding specificity of BMP-2 for its three type II receptors BMPR-II, Act-RII and ActR-IIB is encoded on single amino acid level. Exchange of only one or two residues results in BMP-2 variants with a dramatically altered type II receptor specificity profile, possibly allowing construction of BMP-2 variants that address a single type II receptor. The structure-/function studies presented here revealed a new mechanism, in which the energy contribution of a conserved H-bond is modulated by surrounding intramolecular interactions to achieve a switch between low- and high-affinity binding.

Dullard promotes degradation and dephosphorylation of BMP receptors and is required for neural induction

Bone morphogenetic proteins (BMPs) regulate multiple biological processes, including cellular proliferation, adhesion, differentiation, and early development. In Xenopus development, inhibition of the BMP pathway is essential for neural induction. Here, we report that dullard, a gene involved in neural development, functions as a negative regulator of BMP signaling. We show that Dullard promotes the ubiquitin-mediated proteosomal degradation of BMP receptors (BMPRs). Dullard preferentially complexes with the BMP type II receptor (BMPRII) and partially colocalizes with the caveolin-1-positive compartment, suggesting that Dullard promotes BMPR degradation via the lipid raft-caveolar pathway. Dullard also associates with BMP type I receptors and represses the BMP-dependent phosphorylation of the BMP type I receptor. The phosphatase activity of Dullard is essential for the degradation of BMP receptors and neural induction in Xenopus. Together, these observations suggest that Dullard is an essential inhibitor of BMP receptor activation during Xenopus neuralization.

BMP-3 is a novel inhibitor of both activin and BMP-4 signaling in Xenopus embryos

In Xenopus, the biological effects of BMP-3 oppose those of ventralizing BMPs, but the mechanism for this antagonism remains unclear. Here, we demonstrate that BMP-3 is a dorso-anteriorizing factor in Xenopus embryos that interferes with both activin and BMP signaling. BMP-3 acts by binding to ActRIIB, the common type II receptor for these proteins. Once BMP-3 binds to ActRIIB, it cannot be competed off by excess ligand making a receptor complex that is unable to activate R-Smads and transduce signal. Consistent with a model where BMP-3 interferes with activin and BMPs through a shared receptor, we show that overexpression of BMP-3 can only be rescued by co-injection of xActRIIB. Our results identify BMP-3 as a novel antagonist of both activin and BMPs and uncover how some of the diverse developmental processes that are regulated by both activin and BMP signaling can be modulated during embryogenesis.

Temporal analysis of the early BMP functions identifies distinct anti-organizer and mesoderm patterning phases

BMP signaling performs multiple important roles during early embryogenesis. Signaling through the BMP pathway is mediated by different BMP ligands expressed in partially overlapping temporal and spatial patterns. Assignment of different BMP-dependent activities to the individual ligands has relied on the patterns of expression of the various BMP genes. Temporal analysis of BMP signaling prior to and during gastrulation was performed using glucocorticoid-controlled Smad proteins. Overexpression of the BMP-specific Smad1 and Smad5 revealed that suppression of Spemann's organizer formation in Xenopus embryos can only take place by activating the BMP pathway prior to the onset of gastrulation. Blocking BMP signaling with the inhibitory Smad, Smad6, results in dorsalized embryos or secondary axis induction, only when activated up to early gastrula stages. BMP2 efficiently represses organizer-specific transcription from the midblastula transition onwards while BMP4 is unable to prevent the early activation of organizer-specific genes. Manipulation of the BMP pathway during mid/late gastrula affects mesodermal patterning with no external phenotypic effects. These observations suggest that the malformations resulting from inhibition or promotion of organizer formation, ventralized or dorsalized, respectively, are the result of a very early BMP function, through its antagonism of organizer formation. This function is apparently fulfilled by BMP2 and only at its latest phase by BMP4. Subsequently, BMP functions in the patterning of the mesoderm with no apparent phenotypic effects.

Early specification of ascidian larval motor neurons

In the tadpole larvae of the ascidian Halocynthia roretzi, six motor neurons, Moto-A, -B, and -C (a pair of each), are localized proximal to the caudal neural tube and show distinct morphology and innervation patterns. To gain insights into early mechanisms underlying differentiation of individual motor neurons, we have isolated an ascidian homologue of Islet, a LIM type homeobox gene. Earliest expression of Islet was detected in a pair of bilateral blastomeres on the dorsal edge of the late gastrula. At the neurula stage, this expression began to disappear and more posterior cells started to express Islet. Compared to expression of a series of motor neuron genes, it was confirmed that early Islet-positive blastomeres are the common precursors of Moto-A and -B, and late Islet-positive cells in the posterior neural tube are the precursors of Moto-C. Overexpression of Islet induced ectopic expression of motor neuron markers, suggesting that Islet is capable of regulating motor neuron differentiation. Since early expression of Islet colocalizes with that of HrBMPb, the ascidian homologue of BMP2/4, we tested a role of BMP in specification of the motor neuron fate. Overexpression of HrBMPb led to expansion of Lim and Islet expression toward the central area of the neural plate, and microinjection of mRNA coding for a dominant-negative BMP receptor weakened the expression of these genes. Our results suggest that determination of the ascidian motor neuron fate takes place at late gastrula stage and local BMP signaling may play a role in this step.

Bone morphogenetic protein receptor-II mutation Arg491Trp causes malignant phenotype of familial primary pulmonary hypertension

A four-generation pedigree of familial primary pulmonary hypertension (FPPH) with 14 alive members was collected. In the family, three of the 14 alive familial members were diagnosed as FPPH. Mutations in bone morphogenetic protein receptor-II (BMPR-II) gene were screened by using sequencing analysis. A C-to-T transition at position 1471 in exon 11 of the BMPR-II gene was identified, resulting in an Arg491Trp mutation. We confirmed segregation of the mutation within the family and excluded the presence of the mutations in a panel of 240 chromosomes from normal individuals. No mutations were found in BMPR-II gene in other 10 patients with sporadic primary pulmonary hypertension. The Arg491Trp mutation is located in the kinase domain and predicted to disturb the kinase activity of BMPR-II. Total 7 familial members died at age 8-45 years with various symptoms, indicating other genetic or environmental modifiers involved in the modification of the clinical phenotype.

The evolution of signalling pathways in animal development

Despite the bewildering number of cell types and patterns found in the animal kingdom, only a few signalling pathways are required to generate them. Most cell-cell interactions during embryonic development involve the Hedgehog, Wnt, transforming growth factor-beta, receptor tyrosine kinase, Notch, JAK/STAT and nuclear hormone pathways. Looking at how these pathways evolved might provide insights into how a few signalling pathways can generate so much cellular and morphological diversity during the development of individual organisms and the evolution of animal body plans.

Bone morphogenetic protein receptor signaling is necessary for normal murine postnatal bone formation

Functions of bone morphogenetic proteins (BMPs) are initiated by signaling through specific type I and type II serine/threonine kinase receptors. In previous studies, we have demonstrated that the type IB BMP receptor (BMPR-IB) plays an essential and specific role in osteoblast commitment and differentiation. To determine the role of BMP receptor signaling in bone formation in vivo, we generated transgenic mice, which express a truncated dominant-negative BMPR-IB targeted to osteoblasts using the type I collagen promoter. The mice are viable and fertile. Tissue-specific expression of the truncated BMPR-IB was demonstrated. Characterization of the phenotype of these transgenic mice showed impairment of postnatal bone formation in 1-mo-old homozygous transgenic mice. Bone mineral density, bone volume, and bone formation rates were severely reduced, but osteoblast and osteoclast numbers were not significantly changed in the transgenic mice. To determine whether osteoblast differentiation is impaired, we used primary osteoblasts isolated from the transgenic mice and showed that BMP signaling is blocked and BMP2-induced mineralized bone matrix formation was inhibited. These studies show the effects of alterations in BMP receptor function targeted to the osteoblast lineage and demonstrate a necessary role of BMP receptor signaling in postnatal bone growth and bone formation in vivo.

Endogenous patterns of BMP signaling during early chick development

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta superfamily signaling molecules that play important roles in a wide variety of developmental processes. In this study, we have used an antibody specific for the phosphorylated and activated form of Smad1 to examine endogenous patterns of BMP signaling in chick embryos during early development. We find complex spatial and temporal distributions of BMP signaling that elucidate how BMPs may function in multiple patterning events in the early chick embryo. In the pregastrula embryo, we find that BMP signaling is initially ubiquitous and is extinguished in the epiblast at the onset of primitive streak formation. At the head process stage, BMP signaling is inactivated in prospective neural plate, while it is strongly activated at the neural plate border, a region which is populated by cells that will give rise to neural crest. During later development, we find a dynamic spatiotemporal activation of BMP signaling along the rostrocaudal axis, in the dorsal neural tube, in the notochord, and in the somites during their maturation process. We discuss the implication of our results for endogenous functions of BMP signaling during chick development.

The Drosophila BMP type II receptor Wishful Thinking regulates neuromuscular synapse morphology and function

Proper synaptic development is critical for establishing all aspects of neural function including learning, memory, and locomotion. Here, we describe the phenotypic consequences of mutations in the wishful thinking (wit) gene, the Drosophila homolog of the vertebrate BMP type II receptor. Mutations in wit result in pharate lethality that can be rescued by expression of a wit transgene in motor neurons but not in muscles. Mutant larvae exhibit small synapses, severe defects in evoked junctional potentials, a lower frequency of spontaneous vesicle release, and an alteration in the ultrastructure of synaptic active zones. These results reveal a novel role for BMP signaling in regulating Drosophila neuromuscular junction synapse assembly and activity and may indicate that similar pathways could govern vertebrate synapse development.

TGF-beta signalling pathways in early Xenopus development

Many different ligands of the TGF-beta superfamily signal in the early Xenopus embryo and are required for the specification and patterning of the three germ layers as well as for gastrulation. Recent advances in the field are helping us understand how ligand activity is regulated both spatially and temporally, the mechanism by which the signals are transduced to the nucleus and how essentially the same signalling pathway can activate completely different sets of genes in different regions of the embryo.

Swift is a novel BRCT domain coactivator of Smad2 in transforming growth factor beta signaling

Transforming growth factor beta (TGFbeta) signaling is transduced via Smad2-Smad4-DNA-binding protein complexes which bind to responsive elements in the promoters of target genes. However, the mechanism of how the complexes activate the target genes is unclear. Here we identify Xenopus Swift, a novel nuclear BRCT (BRCA1 C-terminal) domain protein that physically interacts with Smad2 via its BRCT domains. We examine the activity of Swift in relation to gene activation in Xenopus embryos. Swift mRNA has an expression pattern similar to that of Smad2. Swift has intrinsic transactivation activity and activates target gene transcription in a TGFbeta-Smad2-dependent manner. Inhibition of Swift activity results in the suppression of TGFbeta-induced gene transcription and defective mesendoderm development. Blocking Swift function affects neither bone morphogenic protein nor fibroblast growth factor signaling during early development. We conclude that Swift is a novel coactivator of Smad2 and that Swift has a critical role in embryonic TGFbeta-induced gene transcription. Our results suggest that Swift may be a general component of TGFbeta signaling.

In synergy with noggin and follistatin, Xenopus nodal-related gene induces sonic hedgehog on notochord and floor plate

In early development of vertebrates, sonic hedgehog functions in dorsal-ventral patterning of dorsal tissue (nervous system and somites). In Xenopus, sonic hedgehog (Xshh) is first expressed in the Spemann organizer/notochord and floor plate. We report here the mechanism governing Xshh mRNA induction in these regions. In animal cap assays, the antagonizing BMPs signal was not sufficient to induce Xshh mRNA expression; however, it could induce Xshh mRNA expression in the presence of Xnr-1. In whole embryos, when secondary axes were induced by coexpressing noggin and Xnr-1 or follistatin and Xnr-1, Xshh mRNA expression was observed in the notochord and floor plate within the induced axes. It seems apparent that spatially restricted Xshh mRNA expression is determined as intersection of the two signals.

The ventralizing activity of Radar, a maternally expressed bone morphogenetic protein, reveals complex bone morphogenetic protein interactions controlling dorso-ventral patterning in zebrafish

In Xenopus and zebrafish, BMP2, 4 and 7 have been implicated, after the onset of zygotic expression, in inducing and maintaining ventro-lateral cell fate during early development. We provide evidence here that a maternally expressed bone morphogenetic protein (BMP), Radar, may control early ventral specification in zebrafish. We show that Radar ventralizes zebrafish embryos and induces the early expression of bmp2b and bmp4. The analysis of Radar overexpression in both swirl/bmp2b mutants and embryos expressing truncated BMP receptors shows that Radar-induced ventralization is dependent on functional BMP2/4 pathways, and may initially rely on an Alk6-related signaling pathway. Finally, we show that while radar-injected swirl embryos still exhibit a strongly dorsalized phenotype, the overexpression of Radar into swirl/bmp2b mutant embryos restores ventral marker expression, including bmp4 expression. Our results suggest that a complex regulation of different BMP pathways controls dorso-ventral (DV) patterning from early cleavage stages until somitogenesis.

BMP signaling is required for heart formation in vertebrates

In these studies, we have taken advantage of a transient transgenic strategy in Xenopus embryos to demonstrate that BMP signaling is required in vivo for heart formation in vertebrates. Ectopic expression of dominant negative Type I (tALK3) or Type II (tBMPRII) BMP receptors in developing Xenopus embryos results in reduction or absence of heart formation. Additionally, blocking BMP signaling in this manner downregulates expression of XNkx2-5, a homeobox gene required for cardiac specification, prior to differentiation. Notably, however, initial expression of XNkx2-5 is not affected. Mutant phenotypes can be rescued by co-injection of mutant with wild-type receptors or co-injection of mutant receptors with XSmad1, a downstream mediator of BMP signaling. Whole-mount in situ analyses indicate that ALK3 and XSmad1 are coexpressed in cardiogenic regions. Together, our results demonstrate that BMP signaling is required for maintenance of XNkx2-5 expression and heart formation and suggest that ALK3, BMPRII, and XSmad1 may mediate this signaling.

Endogenous patterns of TGFbeta superfamily signaling during early Xenopus development

Transforming growth factor beta (TGFbeta) superfamily signaling has been implicated in patterning of the early Xenopus embryo. Upon ligand stimulation, TGFbeta receptors phosphorylate Smad proteins at carboxy-terminal SS(V/M)S consensus motifs. Smads 1/5/8, activated by bone morphogenetic protein (BMP) signaling, induce ventral mesoderm whereas Smad2, activated by activin-like ligands, induces dorsal mesoderm. Although ectopic expression studies are consistent with roles for TGFbeta signals in early Xenopus embryogenesis, when and where BMP and activin-like signaling pathways are active endogenously has not been directly examined. In this study, we investigate the temporal and spatial activation of TGFbeta superfamily signaling in early Xenopus development by using antibodies specific for the type I receptor-phosphorylated forms of Smad1/5/8 and Smad2. We find that Smad1/5/8 and two distinct isoforms of Smad2, full-length Smad2 and Smad2(delta)exon3, are phosphorylated in early embryos. Both Smad1/5/8 and Smad2/Smad2(delta)exon3 are activated after, but not before, the mid-blastula transition (MBT). Endogenous activation of Smad2/Smad2(delta)exon3 requires zygotic transcription, while Smad1/5/8 activation at MBT appears to involve transcription-independent regulation. We also find that the competence of embryonic cells to respond to TGF(delta) superfamily ligands is temporally regulated and may be a determinant of early patterning. Levels of phospho-Smad1/5/8 and of phospho-Smad2/Smad2(delta)exon3 are asymmetrically distributed across both the animal-vegetal and dorsoventral axes. The timing of the development of these asymmetries differs for phospho-Smad1/5/8 and for phospho-Smad2/Smad2(delta)exon3, and the spatial distribution of phosphorylation of each Smad changes dramatically as gastrulation begins. We discuss the implications of our results for endogenous functions of BMP and activin-like signals as candidate morphogens regulating primary germ layer formation and dorsoventral patterning of the early Xenopus embryo.

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-beta 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.

Synexpression groups in eukaryotes

In 1960, Jacob and Monod described the bacterial operon, a cluster of functionally interacting genes whose expression is tightly coordinated. Global expression analysis has shown that the highly coordinate expression of genes functioning in common processes is also a widespread phenomenon in eukaryotes. These sets of co-regulated genes, or 'synexpression groups', show a striking parallel to the operon, and may be a key determinant facilitating evolutionary change leading to animal diversity.

Maternal and zygotic activity of the zebrafish ogon locus antagonizes BMP signaling

The dorsal-ventral axis of vertebrate embryos is thought to be specified by a gradient of bone morphogenetic protein (BMP) activity, which, in part, arises through the interaction of dorsally expressed antagonists Chordin and Noggin with the ventralizing BMPs. The zebrafish mercedes(tm305), ogon(m60), and short tail(b180) mutations produce ventralized phenotypes, including expanded bmp2b/4 expression domains. We find that the three mutations are allelic and that the locus they define, renamed ogon (ogo), maps to linkage group 25. The ogo(m60) and ogo(b180) mutations are deficiencies and thus represent null alleles, whereas the ENU-induced allele ogo(tm305) retains partial function. Aspects of the ogo(m60) and ogo(tm305) mutant phenotypes are fully suppressed by overexpression of BMP antagonists. Moreover, swirl(tc300), a null mutation in bmp2b, is epistatic to ogo(m60) mutation, providing further evidence that ogo normally functions in a BMP-dependent manner. Embryonic patterning is highly sensitive to maternal and zygotic ogo gene dosage, especially when the level of zygotic chordin activity is also reduced. However, elimination of the zygotic activity of both genes does not result in a completely ventralized embryo. Thus, while ogo and chordin are required to limit activity of BMPs, additional mechanisms must exist to block these ventralizing signals. We have ruled out zebrafish noggin homologues as candidates for the ogo gene, including a newly identified gene, nog1, which is specifically expressed in the gastrula organizer. The results suggest that ogo encodes an as yet unidentified dorsalizing factor that mediates dorsoventral patterning by directly or indirectly antagonizing BMP activity.

A role for the homeobox gene Xvex-1 as part of the BMP-4 ventral signaling pathway

BMP-4 is believed to play a central role in the patterning of the mesoderm by providing a strong ventral signal. As part of this ventral patterning signal, BMP-4 has to activate a number of transcription factors to fulfill this role. Among the transcription factors regulated by BMP-4 are the Xvent and the GATA genes. A novel homeobox gene has been isolated termed Xvex-1 which represents a new class of homeobox genes. Transcription of Xvex-1 initiates soon after the midblastula transition. Xvex-1 transcripts undergo spatial restriction from the onset of gastrulation to the ventral marginal zone, and the transcripts will remain in this localization including at the tailbud stage in the proctodeum. Expression of Xvex-1 during gastrula stages requires normal BMP-4 activity as evidenced from the injection of BMP-4, Smad1, Smad5 and Smad6 mRNA and antisense BMP-4 RNA. Xvex-1 overexpression ventralizes the Xenopus embryo in a dose dependent manner. Partial loss of Xvex-1 activity induced by antisense RNA injection results in the dorsalization of embryos and the induction of secondary axis formation. Xvex-1 can rescue the effects of overexpressing the dominant negative BMP receptor. These results place Xvex-1 downstream of BMP-4 during gastrulation and suggest that it represents a novel homeobox family in Xenopus which is part of the ventral signaling pathway.

Molecular evolution of a developmental pathway: phylogenetic analyses of transforming growth factor-beta family ligands, receptors and Smad signal transducers

Intercellular signaling by transforming growth factor-beta (TGF-beta) proteins coordinates developmental decisions in many organisms. A receptor complex and Smad signal transducers are required for proper responses to TGF-beta signals. We have taken a phylogenetic approach to understanding the developmental evolutionary history of TGF-beta signaling pathways. We were interested in detecting evolutionary influences among the physically interacting multigene families encoding TGF-beta ligands, receptors, and Smads. Our analyses included new ligands and Smads identified from genomic sequence as well as the newest published family members. From an evolutionary perspective we find that (1) TGF-beta pathways do not predate the divergence of animals, plants, and fungi; (2) ligands of the TGF-beta/activin subfamily likely originated after the divergence of nematodes and arthropods; (3) type I receptors from Caenorhabditis elegans are distinct from other receptors and may reflect an ancestral transitional state between type I and type II receptors; and (4) the Smad family appears to be evolving faster than, and independently of, ligands and receptors. From a developmental perspective we find (1) numerous phylogenetic associations not previously detected in each multigene family; (2) that there are unidentified pathway components that discriminate between type I and type II receptors; (3) that there are more Smads to be discovered in Drosophila and mammals; and (4) that the number of C-terminal serines is the best predictor of a Smad's role in TGF-beta signal transduction. We discuss these findings with respect to the coevolution of physically interacting genes.

In vivo analysis using variants of zebrafish BMPR-IA: range of action and involvement of BMP in ectoderm patterning

It has been an intriguing problem whether the polypeptide growth factors belonging to the transforming growth factor-beta (TGF-beta) superfamily function as direct and long-range signaling molecules in pattern formation of the early embryo. In this study, we examined the mechanism of signal propagation of bone morphogenetic protein (BMP) in the ectodermal patterning of zebrafish embryos, in which BMP functions as an epidermal inducer and a neural inhibitor. To estimate the effective range of zbmp-2, we first performed whole-mount in situ hybridization analysis. The zbmp-2-expressing domain and the neuroectoderm, marked by otx-2 expression, were complementary, suggesting that BMP has a short-range effect in vivo. Moreover, mosaic experiments using a constitutively active form of a zebrafish BMP type I receptor (CA-BRIA) demonstrated that the cell-fate conversion, revealed by ectopic expression of gata-3 and repression of otx-2, occurred in a cell-autonomous manner, denying the involvement of the relay mechanism. We also found that zbmp-2 was induced cell autonomously within the transplanted cells in the host ectoderm, suggesting that BMP cannot influence even the neighboring cells. This result is consistent with the observation that there is no gap between the expression domains of zbmp-2 and otx-2. Taken together, we propose that, in ectodermal patterning, BMP exerts a direct and cell-autonomous effect to fate uncommitted ectodermal cells to become epidermis.

Specific activation of Smad1 signaling pathways by the BMP7 type I receptor, ALK2

BMP7 and activin are members of the transforming growth factor beta superfamily. Here we characterize endogenous activin and BMP7 signaling pathways in P19 embryonic carcinoma cells. We show that BMP7 and activin bind to the same type II receptors, ActRII and IIB, but recruit distinct type I receptors into heteromeric receptor complexes. The major BMP7 type I receptor observed was ALK2, while activin bound exclusively to ALK4 (ActRIB). BMP7 and activin elicited distinct biological responses and activated different Smad pathways. BMP7 stimulated phosphorylation of endogenous Smad1 and 5, formation of complexes with Smad4 and induced the promoter for the homeobox gene, Tlx2. In contrast, activin induced phosphorylation of Smad2, association with Smad4, and induction of the activin response element from the Xenopus Mix.2 gene. Biochemical analysis revealed that constitutively active ALK2 associated with and phosphorylated Smad1 on the COOH-terminal SSXS motif, and also regulated Smad5 and Smad8 phosphorylation. Activated ALK2 also induced the Tlx2 promoter in the absence of BMP7. Furthermore, we show that ALK1 (TSRI), an orphan receptor that is closely related to ALK2 also mediates Smad1 signaling. Thus, ALK1 and ALK2 induce Smad1-dependent pathways and ALK2 functions to mediate BMP7 but not activin signaling.

Gene expression screening in Xenopus identifies molecular pathways, predicts gene function and provides a global view of embryonic patterning

In a large-scale gene expression screen 1765 randomly picked cDNAs were analyzed by whole-mount in situ hybridization in Xenopus embryos. Two hundred and seventy three unique, differentially expressed genes were identified, 204 of which are novel in Xenopus. Partial DNA sequences and expression patterns were documented and assembled into a database, 'AXelDB'. Approximately 30% of cDNAs analyzed represent differentially expressed genes and about 5% show highly regionalized expression. Novel marker genes and potential developmental regulators were found. Differential expression of mitochondrial genes was observed. Marker genes were used to study regionalization of the entire gastrula as well as the tail forming region and the epidermis of the tailbud embryo. Four 'synexpression' groups representing genes with shared, complex expression pattern that predict molecular pathways involved in patterning and differentiation were identified. According to their probable functional significance these groups are designated as Delta1, Bmp4, ER-import and Chromatin group. Within synexpression groups, a likely function of genes without sequence similarity can be predicted. The results indicate that synexpression groups have strong prognostic value. A cluster analysis was made by comparing gene expression patterns to derive a novel parameter, 'tissue relatedness'. In conclusion, this study describes a semi-functional approach to investigate genes expressed during early development and provides global insight into embryonic patterning.

DAN is a secreted glycoprotein related to Xenopus cerberus

We report that DAN, a potential cell cycle regulator and tumour suppressor, is a secreted glycoprotein related to Xenopus cerberus. DAN, cerberus, its mouse relative Cer-1/cer-l/Cerberus-like/Cerr1, and the recently described factor DRM/Gremlin, appear to be members of the cystine knot superfamily, which includes TGFbetas and BMPs. Like cerberus and mCer-1, DAN-induced cement glands as well as markers of anterior neural tissue and endoderm in Xenopus animal cap assays, features of BMP signalling blockade. During mouse embryogenesis, Dan was expressed from E8.5 in cranial mesenchyme and somites, then later in limb and facial mesenchyme. The pattern in somites was highly dynamic, with transcripts initially localized to the caudal half of the nascent epithelial somite, then, after maturation, to sclerotomal cells adjacent to the neural tube. Dan was also expressed in the developing myotome. The expression domains include sites in which BMP inhibition is known to be important for development. Thus, DAN appears to be a secreted factor belonging to the cystine knot superfamily, and one of a growing number of antagonists acting to modulate BMP signalling during development.