The DSmurf ubiquitin-protein ligase restricts BMP signaling spatially and temporally during Drosophila embryogenesis.

We identified Drosophila Smurf (DSmurf) as a negative regulator of signaling by the BMP2/4 ortholog DPP during embryonic dorsal-ventral patterning. DSmurf encodes a HECT domain ubiquitin-protein ligase, homologous to vertebrate Smurf1 and Smurf2, that binds the Smad1/5 ortholog MAD and likely promotes its proteolysis. The essential function of DSmurf is restricted to its action on the DPP pathway. DSmurf has two distinct, possibly mechanistically separate, functions in controlling DPP signaling. Prior to gastrulation, DSmurf mutations cause a spatial increase in the DPP gradient, as evidenced by ventrolateral expansion in expression domains of target genes representing all known signaling thresholds. After gastrulation, DSmurf mutations cause a temporal delay in downregulation of earlier DPP signals, resulting in a lethal defect in hindgut organogenesis.     

BMP signaling regulates the dorsal planarian midline and is needed for asymmetric regeneration

Planarians can be cut into irregularly shaped fragments capable of regenerating new and complete organisms. Such regenerative capacities involve a robust ability to restore bilateral symmetry. We have identified three genes needed for bilaterally asymmetric fragments to regenerate missing body parts. These genes are candidate components of a signaling pathway that controls the dorsal-ventral patterning of many animal embryos: a BMP1/Tolloid-like gene (smedolloid-1), a SMAD4-like gene (smedsmad4-1), and a BMP2/4/DPP-like gene (smedbmp4-1). BMP signaling was involved in the formation of new tissues at the midline of regeneration, the dorsal-ventral patterning of new tissues, and the maintenance of the dorsal-ventral pattern of existing adult tissue in homeostasis. smedbmp4-1 was normally expressed at the dorsal midline. Asymmetric fragments lacking a midline displayed new smedbmp4-1 expression prior to formation of a regenerative outgrowth (blastema). Asymmetric fragments containing the midline displayed expanded smedbmp4-1 expression towards the wound. We suggest injured animals that lack left-right symmetry reset their midline through modulation of BMP activity as an early and necessary event in regeneration.     

Expression cloning of Xenopus Os4, an evolutionarily conserved gene, which induces mesoderm and dorsal axis.

Multiple factors, including members of the FGF, TGF beta, and Wnt family of proteins, are important mediators in the regulation of dorsal-ventral pattern formation during vertebrate development. By using an expression cloning approach to identify novel factors that could regulate dorsal-ventral patterning in the Xenopus embryo, we isolated the Xenopus homologue of the human Os4 gene by virtue of its ability to induce a secondary dorsal axis. While Os4 homologues have been identified in a variety of species, and human Os4 is overexpressed in human tumors, the biological function of Os4 is unknown. To explore the mechanism by which Xenopus Os4 (XOs4) induces a secondary dorsal axis, we used Xenopus explant and whole-embryo assays. The secondary axis induced by XOs4 is distinct from that induced by activation of Wnt or FGF pathways but similar to that induced by inhibition of BMP signaling or activation of an Activin pathway. However, XOs4 did not inhibit BMP signaling in dissociated animal cap explants, indicating that XOs4 does not inhibit BMP signaling. Similar to activation of an Activin-like pathway, expression of XOs4 induces molecular markers for mesoderm in animal cap explants, although expression of gastrula-stage mesodermal markers was very weak and substantially delayed. Yet, XOs4 does not require activity of the Activin signal-transduction pathway for mesoderm induction as dominant-negative components of the Activin/Nodal/Vg1 pathway did not prevent XOs4-mediated induction of mesodermal derivatives. Finally, like Activin/Nodal/Vg1 pathways, XOs4 requires FGF signaling for expression of mesoderm markers. Results presented in this study demonstrate that XOs4 can induce mesoderm and dorsalize ventral mesoderm resulting in ectopic dorsal axis formation, suggesting a role for this large evolutionarily conserved gene family in early development.     

Evolution of BMP signaling in Drosophila oogenesis: a receptor-based mechanism

The bone morphogenetic protein (BMP) signaling pathway is a conserved regulator of cellular and developmental processes in animals. The mechanisms underlying BMP signaling activation differ among tissues and mostly reflect changes in the expression of pathway components. BMP signaling is one of the major pathways responsible for the patterning of the Drosophila eggshell, a complex structure derived from a layer of follicle cells (FCs) surrounding the developing oocyte. Activation of BMP signaling in the FCs is dynamic. Initially, signaling is along the anterior-posterior (A/P) axis; later, signaling acquires dorsal-ventral (D/V) polarity. These dynamics are regulated by changes in the expression pattern of the type I BMP receptor thickveins (tkv). We recently found that signaling dynamics and TKV patterning are highly correlated in the FCs of multiple Drosophila species. In addition, we showed that signaling patterns are spatially different among species. Here, we use a mathematical model to simulate the dynamics and differences of BMP signaling in numerous species. This model predicts that qualitative and quantitative changes in receptor expression can lead to differences in the spatial pattern of BMP signaling. We tested these predications experimentally in three different Drosophila species and through genetic perturbations of BMP signaling in D. melanogaster. On the basis of our results, we concluded that changes in tkv patterning can account for the experimentally observed differences in the patterns of BMP signaling in multiple Drosophila species.     

BMPR-IA signaling is required for the formation of the apical ectodermal ridge and dorsal-ventral patterning of the limb.

We demonstrate that signaling via the bone morphogenetic protein receptor IA (BMPR-IA) is required to establish two of the three cardinal axes of the limb: the proximal-distal axis and the dorsal-ventral axis. We generated a conditional knockout of the gene encoding BMPR-IA (Bmpr) that disrupted BMP signaling in the limb ectoderm. In the most severely affected embryos, this conditional mutation resulted in gross malformations of the limbs with complete agenesis of the hindlimbs. The proximal-distal axis is specified by the apical ectodermal ridge (AER), which forms from limb ectoderm at the distal tip of the embryonic limb bud. Analyses of the expression of molecular markers, such as Fgf8, demonstrate that formation of the AER was disrupted in the Bmpr mutants. Along the dorsal/ventral axis, loss of engrailed 1 (En1) expression in the non-ridge ectoderm of the mutants resulted in a dorsal transformation of the ventral limb structures. The expression pattern of Bmp4 and Bmp7 suggest that these growth factors play an instructive role in specifying dorsoventral pattern in the limb. This study demonstrates that BMPR-IA signaling plays a crucial role in AER formation and in the establishment of the dorsal/ventral patterning during limb development.     

Ter94/VCP Is a Novel Component Involved in BMP Signaling

Bone morphogenetic proteins (BMPs), a subgroup of the transforming growth factor (TGF)-β family, transduce their signal through multiple components downstream of their receptors. Even though the components involved in the BMP signaling pathway have been intensely studied, many molecules mediating BMP signaling remain to be addressed. To identify novel components that participate in BMP signaling, RNA interference (RNAi)-based screening was established by detecting phosphorylated Mad (pMad) in Drosophila S2 cells. Ter94, a member of the family of AAA ATPases, was identified as a novel mediator of BMP signaling, which is required for the phosphorylation of Mad in Drosophila S2 cells. Moreover, the mammalian orthlog of Ter94 valosin-containing protein (VCP) plays a critical role in the BMP-Smad1/5/8 signaling pathway in mammalian cells. Genetic evidence suggests that Ter94 is involved in the dorsal-ventral patterning of the Drosophila early embryo through regulating decapentaplegic (Dpp)/BMP signals. Taken together, our data suggest that Ter94/VCP appears to be an evolutionarily conserved component that regulates BMP-Smad1/5/8 signaling.     

zen and the art of phenotypic maintenance: Canalization of embryonic dorsal-ventral patterning in Drosophila

We recently uncovered a novel genetic mechanism that generates the phenotypic uniformity, or canalization, of BMP signaling and cell fate specification during patterning of the dorsal-ventral (D/V) axis in D. melanogaster embryos. We went on to show that other wild-type Drosophila species lack this canalizing genetic circuitry and, consequently, have non-robust D/V patterning. In this review, we propose molecular mechanisms that may give rise to stereotyped BMP signaling, and we identify an additional species that could have decanalized D/V patterning. Extension of these analyses could in turn help explain why canalization is not a universal necessity for species survival.     

zen and the art of phenotypic maintenance: Canalization of embryonic dorsal-ventral patterning in Drosophila

We recently uncovered a novel genetic mechanism that generates the phenotypic uniformity, or canalization, of BMP signaling and cell fate specification during patterning of the dorsal-ventral (D/V) axis in D. melanogaster embryos. We went on to show that other wild-type Drosophila species lack this canalizing genetic circuitry and, consequently, have non-robust D/V patterning. In this review, we propose molecular mechanisms that may give rise to stereotyped BMP signaling, and we identify an additional species that could have decanalized D/V patterning. Extension of these analyses could in turn help explain why canalization is not a universal necessity for species survival.     

Extracellular modulation of BMP activity in patterning the dorsoventral axis

Signaling via bone morphogenetic proteins (BMPs) regulates a vast array of diverse biological processes in the developing embryo and in postembryonic life. Many insights into BMP signaling derive from studies of the BMP signaling gradients that pattern cell fates along the embryonic dorsal-ventral (DV) axis of both vertebrates and invertebrates. This review examines recent developments in the field of DV patterning by BMP signaling, focusing on extracellular modulation as a key mechanism in the formation of BMP signaling gradients in Drosophila, Xenopus, and zebrafish.     

Molluskan Dorsal–Ventral Patterning Relying on BMP2/4 and Chordin Provides Insights into Spiralian Development and Evolution

Although a conserved mechanism relying on BMP2/4 and Chordin is suggested for animal dorsal–ventral (DV) patterning, this mechanism has not been reported in spiralians, one of the three major clades of bilaterians. Studies on limited spiralian representatives have suggested markedly diverse DV patterning mechanisms, a considerable number of which no longer deploy BMP signaling. Here, we showed that BMP2/4 and Chordin regulate DV patterning in the mollusk Lottia goshimai, which was predicted in spiralians but not previously reported. In the context of the diverse reports in spiralians, it conversely represents a relatively unusual case. We showed that BMP2/4 and Chordin coordinate to mediate signaling from the D-quadrant organizer to induce the DV axis, and Chordin relays the symmetry-breaking information from the organizer. Further investigations on L. goshimai embryos with impaired DV patterning suggested roles of BMP signaling in regulating the behavior of the blastopore and the organization of the nervous system. These findings provide insights into the evolution of animal DV patterning and the unique development mode of spiralians driven by the D-quadrant organizer.     

The bone morphogenetic protein 1/Tolloid-like metalloproteinases.

A decade ago, bone morphogenetic protein 1 (BMP1) was shown to provide the activity necessary for proteolytic removal of the C-propeptides of procollagens I-III: precursors of the major fibrillar collagens. Subsequent studies have shown BMP1 to be the prototype of a small group of extracellular metalloproteinases that play manifold roles in regulating formation of the extracellular matrix (ECM). Soon after initial cloning of BMP1, genetic studies showed the related Drosophila proteinase Tolloid (TLD) to be necessary for the formation of the dorsal-ventral axis in early embryogenesis. It is now clear that the BMP1/TLD-like proteinases, conserved in species ranging from Drosophila to humans, act in dorsal-ventral patterning via activation of transforming growth factor beta (TGFbeta)-like proteins BMP2, BMP4 (vertebrates) and decapentaplegic (arthropods). More recently, it has become apparent that the BMP1/TLD-like proteinases are activators of a broader subset of the TGFbeta superfamily of proteins, with implications that these proteinases may be key in orchestrating the formation of ECM with growth factor activation and BMP signaling in morphogenetic processes.     

BMP signaling in skeletal development

Development of the vertebrate skeleton, a complex biological event that includes diverse processes such as formation of mesenchymal condensations at the sites of future skeletal elements, osteoblast and chondrocyte differentiation, and three dimensional patterning, is regulated by many growth factors. Bone morphogenetic proteins (BMPs), members of the TGF-beta superfamily, play a pivotal role in the signaling network and are involved in nearly all processes associated with skeletal morphogenesis. BMP signals are transduced from the plasma membrane receptors to the nucleus through both Smad pathway and non-Smad pathways, and regulated by many extracellular and intercellular proteins that interact with BMPs or components of the BMP signaling pathways. To gain a better understanding of the molecular mechanisms underlying the role of BMP in early skeletal development, it is necessary to elucidate the BMP signaling transduction pathways in chondrocytes and osteoblasts. The major objective of this review was to summarize BMP signaling pathways in the context of craniofacial, axial, and limb development. In particular, this discourse will focus on recent advances of the role of different ligands, receptors, Smads, and BMP regulators in osteoblast and chondrocyte differentiation during embryonic development.     

Xenopus staufen2 is required for anterior endodermal organ formation

Defining the regulatory molecular networks involved in patterning the developing anterior endoderm is essential to understand how the pancreas, liver, stomach, and duodenum are discretely specified from each other. In this study, we analyzed the expression and function of the double-stranded RNA-binding protein Staufen2 in Xenopus laevis endoderm. We found that staufen2 was broadly expressed within the developing endoderm beginning at gastrulation becoming localized to the anterior endoderm at later stages. Through morpholino-mediated knockdown, we demonstrate that Staufen2 function is required for proper formation of the stomach, liver, and pancreas. We define that its function is required during gastrulation for proper patterning of the dorsal-ventral axis and that it acts to regulate expression of BMP signaling components.     

Zili antagonizes Bmp signaling to regulate dorsal-ventral patterning during zebrafish early embryogenesis

Bone morphogenetic protein (Bmp) signaling plays a pivotal role in dorsal-ventral (DV) patterning in vertebrate embryos. Piwi proteins are required for germline and stem cell development. Our previous study demonstrated that Zili, zebrafish Piwil2, inhibits transforming growth factor (TGF)-βsignaling by interacting with Smad4, suggesting a role for zili in Bmp signaling. In the present study, zili-MO or zili mRNA was microinjected into one-cell embryos to knock down or elevate the expression of zili to study the role of zili during early zebrafish embryogenesis. Knockdown of zili inhibited the expression of dorsal marker genes, and enhanced that of ventral marker genes. In contrast, overexpression of zili promoted expression of dorsal marker genes, while it inhibited ventral marker genes. These results suggest that zili regulates DV patterning. The influence of zili on the Bmp pathway was further explored. Knockdown of zili resulted in higher expression levels of bmp2b, and bmp4, the Bmp signaling ligands, and reduced expression of chordin (chd), noggin (nog1), and follistatin (fst), which encode BMP antagonists. Meanwhile, overexpression of zili produced opposite effects. In conclusion, our results indicate that zili regulates dorsal-ventral patterning by antagonizing Bmp signaling during early embryogenesis in zebrafish.     

Is chordin a long-range- or short-range-acting factor? Roles for BMP1-related metalloproteases in chordin and BMP4 autofeedback loop regulation

Diffusible morphogen models have been used widely to explain regional specification of tissues and body axes during animal development. The three-signal model for patterning the dorsal-ventral axis of the amphibian embryo proposes, in part, that a factor(s) secreted from Spemann's organizer is responsible for converting lateral marginal zone into more dorsal cell fates. We examine the possibility that chordin, a secreted inhibitor of bone morphogenetic protein (BMP) signaling and candidate "dorsalizing signal," is a long-range-acting factor. We show that chordin can, when overexpressed, act directly over distances of at least 450 microm in the early Xenopus embryo to create a gradient of BMP signaling. However, since lower levels of chordin can still induce secondary axes and these amounts of chordin act only locally to inhibit a BMP target gene, we suggest that chordin likely acts as a short-range signal in vivo. Furthermore, BMP1, a secreted metalloprotease that cleaves chordin protein in vitro, inhibits chordin's axis-inducing effects, suggesting that BMP1 functions to negatively regulate chordin's action in vivo. A dominant-negative mutant BMP1 blocks the in vitro cleavage of chordin protein by wild-type BMP1 and induces secondary axes when injected ventrally. We argue that BMP1 and Xolloid are probably functionally redundant metalloproteases and may have two roles in the early Xenopus embryo. One role may be to inhibit the action of low-level chordin protein expressed throughout the entire embryo and a possible second role may be to inhibit activation of a juxtacrine cell relay, thereby confining chordin's action to the organizer region preventing chordin from functioning as a long-range-acting factor.