Twisted gastrulation enhances BMP signaling through chordin dependent and independent mechanisms

Small-Molecule-Mediated Suppression of BMP Signaling by Selective Inhibition of BMP1-Dependent Chordin Cleavage

BMP signaling is critical for many biological processes. Therefore, small molecules that modulate BMP signaling are useful for elucidating the function of BMP signaling and treating BMP signaling-related diseases. Here, we performed a phenotypic screening in zebrafish to examine the in vivo effects of N-substituted-2-amino-benzoic acid analogs NPL1010 and NPL3008 and found that they affect BMP signaling-dependent dorsal-ventral (D-V) patterning and bone formation in zebrafish embryos. Furthermore, NPL1010 and NPL3008 suppressed BMP signaling upstream of BMP receptors. BMP1 cleaves Chordin, an antagonist of BMP, and negatively regulates BMP signaling. Docking simulations demonstrated that NPL1010 and NPL3008 bind BMP1. We found that NPL1010 and NPL3008 partially rescued the disruptions in the D-V phenotype caused by bmp1 overexpression and selectively inhibited BMP1-dependent Chordin cleavage. Therefore, NPL1010 and NPL3008 are potentially valuable inhibitors of BMP signaling that act through selective inhibition of Chordin cleavage.

Multifaceted Functions of TWSG1: From Embryogenesis to Cancer Development

Bone morphogenetic proteins (BMPs) play an important role in development. Twisted gastrulation BMP signaling modulator 1 (TWSG1) was initially identified as a regulator of the dorsoventral axis formation in Drosophila. The mechanism of BMP signaling modulation by TWSG1 is complex. TWSG1 inhibits BMP signaling by binding to BMP ligands including BMP4, whereas it enhances signaling by interacting with Chordin, a BMP antagonist. Therefore, TWSG1 can act as both a BMP agonist and antagonist. TWSG1 has various functions ranging from embryogenesis to cancer progression. TWSG1 knockout mice showed neural, craniofacial, and mammary defects. TWSG1 also regulated erythropoiesis and thymocyte development. Furthermore, the relationship between TWSG1 and cancer has been elucidated. Allelic loss of TWSG1 was detected in colorectal cancer. TWSG1 expression was upregulated in papillary thyroid carcinoma and glioblastoma but downregulated in gastric and endometrial cancers. TWSG1 suppressed BMP7-enhanced sphere formation and migration in endometrial cancer cells, indicating its tumor-suppressive role. Further studies are required to clarify the TWSG1 function and its association with BMP signaling in cancer development. Finally, TWSG1 is abundantly expressed in human and mouse ovaries and sustains follicular growth in rodent ovaries. Thus, TWSG1 has various functions ranging from fertility to cancer. Therefore, TWSG1 signaling modulation may be beneficial in treating specific diseases such as cancer.

Proteolytic Restriction of Chordin Range Underlies BMP Gradient Formation

A fundamental question in developmental biology is how morphogens, such as bone morphogenetic protein (BMP), form precise signaling gradients to impart positional and functional identity to the cells of the early embryo. We combine rigorous mutant analyses with quantitative immunofluorescence to determine that the proteases Bmp1a and Tolloid spatially restrict the BMP antagonist Chordin in dorsoventral (DV) axial patterning of the early zebrafish gastrula. We show that maternally deposited Bmp1a plays an unexpected and non-redundant role in establishing the BMP signaling gradient, while the Bmp1a/Tolloid antagonist Sizzled is surprisingly dispensable. Combining computational modeling and in vivo analyses with an immobile Chordin construct, we demonstrate that long-range Chordin diffusion is not necessary for BMP gradient formation and DV patterning. Our data do not support a counter-gradient of Chordin and instead favor a Chordin sink, established by Bmp1a and Tolloid, as the primary mechanism that drives BMP gradient formation.

The BMP morphogen generates a precise signaling gradient during axial patterning. In the zebrafish embryo, Tuazon et al. find that proteases Bmp1a/Tolloid are key to this process, preventing the long-range diffusion of the BMP antagonist, Chordin. By regionally restricting Chordin, Bmp1a/Tolloid establish the signaling sink that drives BMP gradient formation.

Bone Morphogenetic Protein-2 in Development and Bone Homeostasis

Bone morphogenetic proteins (BMPs) are multi-functional growth factors belonging to the Transforming Growth Factor-Beta (TGF-β) superfamily. These proteins are essential to many developmental processes, including cardiogenesis, neurogenesis, and osteogenesis. Specifically, within the BMP family, Bone Morphogenetic Protein-2 (BMP-2) was the first BMP to be characterized and has been well-studied. BMP-2 has important roles during embryonic development, as well as bone remodeling and homeostasis in adulthood. Some of its specific functions include digit formation and activating osteogenic genes, such as Runt-Related Transcription Factor 2 (RUNX2). Because of its diverse functions and osteogenic potential, the Food and Drug Administration (FDA) approved usage of recombinant human BMP-2 (rhBMP-2) during spinal fusion surgery, tibial shaft repair, and maxillary sinus reconstructive surgery. However, shortly after initial injections of rhBMP-2, several adverse complications were reported, and alternative therapeutics have been developed to limit these side-effects. As the clinical application of BMP-2 is largely implicated in bone, we focus primarily on its role in bone. However, we also describe briefly the role of BMP-2 in development. We then focus on the structure of BMP-2, its activation and regulation signaling pathways, BMP-2 clinical applications, and limitations of using BMP-2 as a therapeutic. Further, this review explores other potential treatments that may be useful in treating bone disorders.

Setting up for gastrulation in zebrafish

Soon after fertilization the zebrafish embryo generates the pool of cells that will give rise to the germline and the three somatic germ layers of the embryo (ectoderm, mesoderm and endoderm). As the basic body plan of the vertebrate embryo emerges, evolutionarily conserved developmental signaling pathways, including Bmp, Nodal, Wnt, and Fgf, direct the nearly totipotent cells of the early embryo to adopt gene expression profiles and patterns of cell behavior specific to their eventual fates. Several decades of molecular genetics research in zebrafish has yielded significant insight into the maternal and zygotic contributions and mechanisms that pattern this vertebrate embryo. This new understanding is the product of advances in genetic manipulations and imaging technologies that have allowed the field to probe the cellular, molecular and biophysical aspects underlying early patterning. The current state of the field indicates that patterning is governed by the integration of key signaling pathways and physical interactions between cells, rather than a patterning system in which distinct pathways are deployed to specify a particular cell fate. This chapter focuses on recent advances in our understanding of the genetic and molecular control of the events that impart cell identity and initiate the patterning of tissues that are prerequisites for or concurrent with movements of gastrulation.

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.

Extracellular bone morphogenetic protein modulator BMPER and twisted gastrulation homolog 1 preserve arterial-venous specification in zebrafish blood vessel development and regulate Notch signaling in endothelial cells

The bone morphogenetic protein (BMP) signaling pathway plays a central role during vasculature development. Mutations or dysregulation of the BMP pathway members have been linked to arteriovenous malformations. In the present study, we investigated the effect of the BMP modulators bone morphogenetic protein endothelial precursor-derived regulator (BMPER) and twisted gastrulation protein homolog 1 (TWSG1) on arteriovenous specification during zebrafish development and analyzed downstream Notch signaling pathway in human endothelial cells. Silencing of bmper and twsg1b in zebrafish embryos by morpholinos resulted in a pronounced enhancement of venous ephrinB4a marker expression and concomitant dysregulated arterial ephrinb2a marker expression detected by in situ hybridization. As arteriovenous specification was disturbed, we assessed the impact of BMPER and TWSG1 protein stimulation on the Notch signaling pathway on endothelial cells from different origin. Quantitative real-time PCR (qRT-PCR) and western blot analysis showed increased expression of Notch target gene hairy and enhancer of split, HEY1/2 and EPHRINB2. Consistently, silencing of BMPER in endothelial cells by siRNAs decreased Notch signaling and downstream effectors. BMP receptor antagonist DMH1 abolished BMPER and BMP4 induced Notch signaling pathway activation. In conclusion, we found that in endothelial cells, BMPER and TWSG1 are necessary for regular Notch signaling activity and in zebrafish embryos BMPER and TWSG1 preserve arteriovenous specification to prevent malformations.

The maternal control in the embryonic development of zebrafish

The maternal control directing the very first hours of life is of pivotal importance for ensuring proper development to the growing embryo. Thanks to the finely regulated inheritance of maternal factors including mRNAs and proteins produced during oogenesis and stored into the mature oocyte, the embryo is sustained throughout the so-called maternal-to-zygotic transition, a period in development characterized by a species-specific length in time, during which critical biological changes regarding cell cycle and zygotic transcriptional activation occur. In order not to provoke any kind of persistent damage, the process must be delicately balanced. Surprisingly, our knowledge as to the possible effects of beneficial bacteria regarding the modulation of the quality and/or quantity of both maternally-supplied and zygotically-transcribed mRNAs, is very limited. To date, only one group has investigated the consequences of the parentally-supplied Lactobacillus rhamnosus on the storage of mRNAs into mature oocytes, leading to an altered maternal control process in the F1 generation. Particular attention was called on the monitoring of several biomarkers involved in autophagy, apoptosis and axis patterning, while data on miRNA generation and pluripotency maintenance are herein presented for the first time, and can assist in laying the ground for further investigations in this field. In this review, the reader is supplied with the current knowledge on the above-mentioned biological process, first by drawing the general background and then by emphasizing the most important findings that have highlighted their focal role in normal animal development.

Structural characterization of twisted gastrulation provides insights into opposing functions on the BMP signalling pathway

Twisted gastrulation (Tsg) and chordin are secreted glycoproteins that function together as BMP (bone morphogenetic protein) antagonists to regulate BMP growth factor signalling. Chordin binds to BMPs, preventing them from interacting with their receptors and Tsg is known to strengthen this inhibitory complex. Tsg also acts as a BMP agonist by promoting cleavage of chordin by tolloid-family proteinases. Here we explore the structural mechanism through which Tsg exerts this dual activity. We have characterized the nanoscale structure of human Tsg using in-solution biomolecular analysis and show that Tsg is a globular monomer with a flattened cross shape. Tsg has a high proportion of N-linked glycans, in relation to its molecular weight, which supports a role in solubilising BMPs. Tsg binds with high affinity to the C-terminal region of chordin and was also able to inhibit BMP-7 signalling directly but did not have an effect on BMP-4 signalling. Although both Tsg and mammalian tolloid are involved in chordin cleavage, no interaction could be detected between them using surface plasmon resonance. Together these data suggest that Tsg functions as a BMP-agonist by inducing conformational change in chordin making it more susceptible to tolloid cleavage and as a BMP-antagonist either independently or via a chordin-mediated mechanism. Following single cleavage of chordin by tolloids, Tsg continues to strengthen the inhibitory complex, supporting a role for partially cleaved chordin in BMP regulation.

The role of chordin fragments generated by partial tolloid cleavage in regulating BMP activity

Chordin-mediated regulation of bone morphogenetic protein (BMP) family growth factors is essential in early embryogenesis and adult homoeostasis. Chordin binds to BMPs through cysteine-rich von Willebrand factor type C (vWC) homology domains and blocks them from interacting with their cell surface receptors. These domains also self-associate and enable chordin to target related proteins to fine-tune BMP regulation. The chordin–BMP inhibitory complex is strengthened by the secreted glycoprotein twisted gastrulation (Tsg); however, inhibition is relieved by cleavage of chordin at two specific sites by tolloid family metalloproteases. As Tsg enhances this cleavage process, it serves a dual role as both promoter and inhibitor of BMP signalling. Recent developments in chordin research suggest that rather than simply being by-products, the cleavage fragments of chordin continue to play a role in BMP regulation. In particular, chordin cleavage at the C-terminus potentiates its anti-BMP activity in a type-specific manner.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Mammalian tolloid proteinases: role in growth factor signalling

Tolloid proteinases are essential for tissue patterning and extracellular matrix assembly. The members of the family differ in their substrate specificity and activity, despite sharing similar domain organization. The mechanisms underlying substrate specificity and activity are complex, with variation between family members, and depend on both multimerization and substrate interaction. In addition, enhancers, such as Twisted gastrulation (Tsg), promote cleavage of tolloid substrate, chordin, to regulate growth factor signalling. Although Tsg and mammalian tolloid (mTLD) are involved in chordin cleavage, no interaction has been detected between them, suggesting Tsg induces a change in chordin to increase susceptibility to cleavage. All members of the tolloid family bind the N terminus of latent TGFβ‐binding protein‐1, providing support for their role in TGFβ signalling.

The Bone Morphogenetic Proteins and Their Antagonists

The bone morphogenetic proteins (BMPs) and the growth and differentiation factors comprise a single family of some 20 homologous, dimeric cytokines which share the cystine-knot domain typical of the TGF-β superfamily. They control the differentiation and activity of a range of cell types, including many outside bone and cartilage. They serve as developmental morphogens, but are also important in chronic pathologies, including tissue fibrosis and cancer. One mechanism for enabling tight spatiotemporal control of their activities is through a number of antagonist proteins, including Noggin, Follistatin, Chordin, Twisted gastrulation (TSG), and the seven members of the Cerberus and Dan family. These antagonists are secreted proteins that bind selectively to particular BMPs with high affinity, thereby blocking receptor engagement and signaling. Most of these antagonists also possess a TGF-β cystine-knot domain. Here, we discuss current knowledge and understanding of the structures and activities of the BMPs and their antagonists, with a particular focus on the latter proteins. Recent advances in structural biology of BMP antagonists have begun the process of elucidating the molecular basis of their activity, displaying a surprising variety between the modes of action of these closely related proteins. We also discuss the interactions of the antagonists with the glycosaminoglycan heparan sulfate, which is found ubiquitously on cell surfaces and in the extracellular matrix.

Temporally coordinated signals progressively pattern the anteroposterior and dorsoventral body axes

The vertebrate body plan is established through the precise spatiotemporal coordination of morphogen signaling pathways that pattern the anteroposterior (AP) and dorsoventral (DV) axes. Patterning along the AP axis is directed by posteriorizing signals Wnt, fibroblast growth factor (FGF), Nodal, and retinoic acid (RA), while patterning along the DV axis is directed by bone morphogenetic proteins (BMP) ventralizing signals. This review addresses the current understanding of how Wnt, FGF, RA and BMP pattern distinct AP and DV cell fates during early development and how their signaling mechanisms are coordinated to concomitantly pattern AP and DV tissues.

Secreted bone morphogenetic protein antagonists of the Chordin family

Chordin, Chordin-like 1, and Chordin-like 2 are secreted bone morphogenetic protein (BMP) antagonists with highly conserved Chordin-like cysteine-rich domains. Recently, Brorin and Brorin-like have been identified as new Chordin-like BMP antagonists. A Chordin ortholog, Short gastrulation, has been identified in Drosophila, a protostome, but not other orthologs. By contrast, Chordin, Chordin-like 1, and Chordin-like 2 have been identified in Ciona intestinalis, the closest living relatives of the vertebrates, but Brorin and Brorin-like have not. However, all these genes have been identified in most vertebrates. These results indicate that Chordin, Chordin-like 1, and Chordin-like 2 were generated early in the metazoan lineage. Later on, Brorin and Brorin-like were potentially generated by a genome duplication event in early vertebrate evolution. All four cysteine-rich domains of Chordin are essential for the regulation of its action. However, Chordin-like 1, Chordin-like 2, Brorin, and Brorin-like contain only two or three cysteine-rich domains. Although their mechanisms of action remain unclear, they might be distinct from that of Chordin. The expression profiles of these genes in mice and zebrafish indicate unique roles at embryonic and postnatal stages. Mutant/knockdown mouse and zebrafish phenotypes indicate roles in morphogenesis during gastrulation, dorsoventral axis formation, ear, pharyngeal, and neural development, and venous and arterial patterning. Aberrant Chordin expression might result in hereditary diseases and cancer. In addition, altered serum Chordin and Chordin-like 1 levels are also observed in non-hereditary diseases. Together, these results indicate pathophysiological roles.

Nanoscale structure of the BMP antagonist chordin supports cooperative BMP binding

Bone morphogenetic proteins (BMPs) orchestrate key cellular events, such as proliferation and differentiation, in development and homeostasis. Extracellular antagonists, such as chordin, are essential regulators of BMP signaling. Chordin binds to BMPs blocking interaction with receptors, and cleavage by tolloid proteinases is thought to relieve this inhibition. A model has been previously proposed where chordin adopts a horseshoe-like arrangement enabling BMP binding cooperatively by terminal domains (1). Here, we present the nanoscale structure of human chordin using electron microscopy, small angle X-ray scattering, and solution-based biophysical techniques, which together show that chordin indeed has a compact horseshoe-shaped structure. Chordin variants were used to map domain locations within the chordin molecule. The terminal BMP-binding domains protrude as prongs from the main body of the chordin structure, where they are well positioned to interact with the growth factor. The spacing provided by the chordin domains supports the principle of a cooperative BMP-binding arrangement that the original model implied in which growth factors bind to both an N- and C-terminal von Willebrand factor C domain of chordin. Using binding and bioactivity assays, we compared full-length chordin with two truncated chordin variants, such as those produced by partial tolloid cleavage. Cleavage of either terminal domain has little effect on the affinity of chordin for BMP-4 and BMP-7 but C-terminal cleavage increases the efficacy of chordin as a BMP-4 inhibitor. Together these data suggest that partial tolloid cleavage is insufficient to ablate BMP inhibition and the C-terminal chordin domains play an important role in BMP regulation.

Antagonism and synergy between extracellular BMP modulators Tsg and BMPER balance blood vessel formation

Growth and regeneration of blood vessels are crucial processes during embryonic development and in adult disease. Members of the bone morphogenetic protein (BMP) family are growth factors known to play a key role in vascular development. The BMP pathway is controlled by extracellular BMP modulators such as BMP endothelial cell precursor derived regulator (BMPER), which we reported previously acts proangiogenically on endothelial cells in a concentration-dependent manner. Here, we explore the function of other BMP modulators, especially Tsg, on endothelial cell behaviour and compare them to BMPER. In Matrigel assays, BMP modulators chordin and noggin had no stimulatory effect; however, gremlin and Tsg enhanced human umbilical vein endothelial cell (HUVEC) sprouting. As the activation dynamics of Tsg were similar to those of BMPER, we further investigated the proangiogenic effect of Tsg on endothelial cells. Tsg enhanced endothelial cell ingrowth in the mouse Matrigel plug assay as well as HUVEC sprouting, migration and proliferation in vitro, dependent on Akt, Erk and Smad signalling pathway activation in a concentration-dependent manner. Surprisingly, silencing of Tsg also increased HUVEC sprouting, migration and proliferation, which is again associated with Akt, Erk and Smad signalling pathway activation. Furthermore, we reveal that Tsg and BMPER interfere with each other to enhance proangiogenic events. However, in vivo the presence of Tsg as well as of BMPER is mandatory for regular development of the zebrafish vasculature. Taken together, our results suggest that BMPER and Tsg maintain a fine-tuned equilibrium that controls BMP pathway activity and is necessary for vascular cell homeostasis.

Dpp/BMP transport mechanism is required for wing venation in the sawfly Athalia rosae

The pattern of wing venation varies considerably among different groups of insects and has been used as a means of species-specific identification. However, little is known about how wing venation is established and diversified among insects. The decapentaplegic (Dpp)/bone morphogenetic protein (BMP) signaling pathway plays a critical role in wing vein formation during the pupal stages in Drosophila melanogaster. A key mechanism is BMP transport from the longitudinal veins (LVs) to the posterior crossvein (PCV) by the BMP-binding proteins, short gastrulation (Sog) and twisted gastrulation2/crossveinless (Tsg2/Cv). To investigate whether the BMP transport mechanism is utilized to specify insect wing vein patterns in other than Drosophila, we used the sawfly Athalia rosae as a model, which has distinct venation patterns in the fore- and hindwings. Here, we show that Ar-dpp is ubiquitously expressed in both the fore- and hindwings, but is required for localized BMP signaling that reflects distinct wing vein patterns between the fore- and hindwings. By isolating Ar-tsg/cv in the sawfly, we found that Ar-Tsg/Cv is also required for BMP signaling in wing vein formation and retains the ability to transport Dpp. These data suggest that the BMP transport system is widely used to redistribute Dpp to specify wing venation and may be a basal mechanism underlying diversified wing vein patterns among insects.

Sfrp5 modulates both Wnt and BMP signaling and regulates gastrointestinal organogenesis [corrected] in the zebrafish, Danio rerio

Sfrp5 belongs to the family of secreted frizzled related proteins (Sfrp), secreted inhibitors of Wingless-MMTV Integration Site (Wnt) signaling, which play an important role in cancer and development. We selected sfrp5 because of its compelling expression profile in the developing endoderm in zebrafish, Danio rerio. In this study, overexpression of sfrp5 in embryos results in defects in both convergent extension (CE) by inhibition of non-canonical Wnt signaling and defects in dorsoventral patterning by inhibition of Tolloid-mediated proteolysis of the BMP inhibitor Chordin. From 25 hours post fertilization (hpf) to 3 days post fertilization (dpf), both overexpression and knockdown of Sfrp5 decrease the size of the endoderm, significantly reducing liver cell number. At 3 dpf, insulin-positive endodermal cells fail to coalesce into a single pancreatic islet. We show that Sfrp5 inhibits both canonical and non-canonical Wnt signaling during embryonic and endodermal development, resulting in endodermal abnormalities.

Expression and functional study of extracellular BMP antagonists during the morphogenesis of the digits and their associated connective tissues

The purpose of this study is to gain insight into the role of BMP signaling in the diversification of the embryonic limb mesodermal progenitors destined to form cartilage, joints, and tendons. Given the importance of extracellular BMP modulators in in vivo systems, we performed a systematic search of those expressed in the developing autopod during the formation of the digits. Here, we monitored the expression of extracellular BMP modulators including: Noggin, Chordin, Chordin-like 1, Chordin-like 2, Twisted gastrulation, Dan, BMPER, Sost, Sostdc1, Follistatin, Follistatin-like 1, Follistatin-like 5 and Tolloid. These factors show differential expression domains in cartilage, joints and tendons. Furthermore, they are induced in specific temporal patterns during the formation of an ectopic extra digit, preceding the appearance of changes that are identifiable by conventional histology. The analysis of gene regulation, cell proliferation and cell death that are induced by these factors in high density cultures of digit progenitors provides evidence of functional specialization in the control of mesodermal differentiation but not in cell proliferation or apoptosis. We further show that the expression of these factors is differentially controlled by the distinct signaling pathways acting in the developing limb at the stages covered by this study. In addition, our results provide evidence suggesting that TWISTED GASTRULATION cooperates with CHORDINS, BMPER, and NOGGIN in the establishment of tendons or cartilage in a fashion that is dependent on the presence or absence of TOLLOID.

BMP-binding protein twisted gastrulation is required in mammary gland epithelium for normal ductal elongation and myoepithelial compartmentalization

Bone morphogenetic proteins (BMPs) are involved in embryonic mammary gland (MG) development and can be dysregulated in breast cancer. However, the role BMPs play in the postnatal MG remains virtually unknown. BMPs are potent morphogens that are involved in cell fate determination, proliferation, apoptosis and adult tissue homeostasis. Twisted gastrulation (TWSG1) is a secreted BMP binding protein that modulates BMP ligand availability in the extracellular space. Here we investigate the consequences of TWSG1 deletion on development of the postnatal MG. At puberty, Twsg1 is expressed in the myoepithelium and in a subset of body cells of the terminal end buds. In the mature duct, Twsg1 expression is primarily restricted to the myoepithelial layer. Global deletion of Twsg1 leads to a delay in ductal elongation, reduced secondary branching, enlarged terminal end buds, and occluded lumens. This is associated with an increase in luminal epithelial cell number and a decrease in apoptosis. In the MG, pSMAD1/5/8 level and the expression of BMP target genes are reduced, consistent with a decrease in BMP signaling. GATA-3, which is required for luminal identity, is reduced in Twsg1(-/-) MGs, which may explain why K14 positive cells, which are normally restricted to the myoepithelial layer, are found within the luminal compartment and shed into the lumen. In summary, regulation of BMP signaling by TWSG1 is required for normal ductal elongation, branching of the ductal tree, lumen formation, and myoepithelial compartmentalization in the postnatal MG.

Members of the DAN family are BMP antagonists that form highly stable noncovalent dimers

Signaling of bone morphogenetic protein (BMP) ligands is antagonized by a number of extracellular proteins, including noggin, follistatin and members of the DAN (differential screening selected gene abberative in neuroblastoma) family. Structural studies on the DAN family member sclerostin (a weak BMP antagonist) have previously revealed that the protein is monomeric and consists of an eight-membered cystine knot motif with a fold similar to transforming growth factor-β ligands. In contrast to sclerostin, certain DAN family antagonists, including protein related to DAN and cerberus (PRDC), have an unpaired cysteine that is thought to function in covalent dimer assembly (analogous to transforming growth factor-β ligands). Through a combination of biophysical and biochemical studies, we determined that PRDC forms biologically active dimers that potently inhibit BMP ligands. Furthermore, we showed that PRDC dimers, surprisingly, are not covalently linked, as mutation of the unpaired cysteine does not inhibit dimer formation or biological activity. We further demonstrated that the noncovalent PRDC dimers are highly stable under both denaturing and reducing conditions. This study was extended to the founding family member DAN, which also forms noncovalent dimers that are highly stable. These results demonstrate that certain DAN family members can form both monomers and noncovalent dimers, implying that biological activity of DAN family members might be linked to their oligomeric state.

Position matters: variability in the spatial pattern of BMP modulators generates functional diversity

Bone morphogenetic proteins (BMPs) perform a variety of functions during development. Considering a single BMP, what enables its multiple roles in tissues of varied sizes and shapes? What regulates the spatial distribution and activity patterns of the BMP in these different developmental contexts? Some BMP functions require controlling spread of the BMP morphogen, while others require formation of localized, high concentration peaks of BMP activity. Here we review work in Drosophila that describes spatial regulation of the BMP encoded by decapentaplegic (dpp) in different developmental contexts. We concentrate on extracellular modulation of BMP function and discuss the mechanisms that generate concentrated peaks of Dpp activity, subdivide territories of different activity levels or regulate spread of the Dpp morphogen from a point source. We compare these findings with data from vertebrates and non-model organisms to discuss how changes in the regulation of Dpp distribution by extracellular modulators may lead to variability in dpp function in different species.

Systems control of BMP morphogen flow in vertebrate embryos

Embryonic morphogenetic programs coordinate cell behavior to ensure robust pattern formation. Having identified components of those programs by molecular genetics, developmental biology is now borrowing concepts and tools from systems biology to decode their regulatory logic. Dorsal-ventral (D-V) patterning of the frog gastrula by Bone Morphogenetic Proteins (BMPs) is one of the best studied examples of a self-regulating embryonic patterning system. Embryological analyses and mathematical modeling are revealing that the BMP activity gradient is maintained by a directed flow of BMP ligands towards the ventral side. Pattern robustness is ensured through feedback control of the levels of extracellular BMP pathway modulators that adjust the flow to the dimensions of the embryonic field.

Maternal and zygotic control of zebrafish dorsoventral axial patterning

Vertebrate development begins with precise molecular, cellular, and morphogenetic controls to establish the basic body plan of the embryo. In zebrafish, these tightly regulated processes begin during oogenesis and proceed through gastrulation to establish and pattern the axes of the embryo. During oogenesis a maternal factor is localized to the vegetal pole of the oocyte that is a determinant of dorsal tissues. Following fertilization this vegetally localized dorsal determinant is asymmetrically translocated in the egg and initiates formation of the dorsoventral axis. Dorsoventral axis formation and patterning is then mediated by maternal and zygotic factors acting through Wnt, BMP (bone morphogenetic protein), Nodal, and FGF (fibroblast growth factor) signaling pathways, each of which is required to establish and/or pattern the dorsoventral axis. This review addresses recent advances in our understanding of the molecular factors and mechanisms that establish and pattern the dorsoventral axis of the zebrafish embryo, including establishment of the animal-vegetal axis as it relates to formation of the dorsoventral axis.

Binding between Crossveinless-2 and Chordin von Willebrand factor type C domains promotes BMP signaling by blocking Chordin activity

Background

Crossveinless-2 (CV2) is an extracellular BMP modulator protein of the Chordin family, which can either enhance or inhibit BMP activity. CV2 binds to BMP2 via subdomain 1 of the first of its five N-terminal von Willebrand factor type C domains (VWC1). Previous studies showed that this BMP binding is required for the anti-, but not for the pro-BMP effect of CV2. More recently, it was shown that CV2 can also bind to the BMP inhibitor Chordin. However, it remained unclear which domains mediate this binding, and whether it accounts for an anti- or pro-BMP effect.

Principal Findings

Here we report that a composite interface of CV2 consisting of subdomain 2 of VWC1 and of VWC2-4, which are dispensable for BMP binding, binds to the VWC2 domain of Chordin. Functional data obtained in zebrafish embryos indicate that this binding of Chordin is required for CV2's pro-BMP effect, which actually is an anti-Chordin effect and, at least to a large extent, independent of Tolloid-mediated Chordin degradation. We further demonstrate that CV2 mutant versions that per se are incapable of BMP binding can attenuate the Chordin/BMP interaction.

Conclusions

We have physically dissected the anti- and pro-BMP effects of CV2. Its anti-BMP effect is obtained by binding to BMP via subdomain1 of the VWC1 domain, a binding that occurs in competition with Chordin. In contrast, its pro-BMP effect is achieved by direct binding to Chordin via subdomain 2 of VWC1 and VWC2-4. This binding seems to induce conformational changes within the Chordin protein that weaken Chordin's affinity to BMP. We propose that in ternary Chordin-CV2-BMP complexes, both BMP and Chordin are directly associated with CV2, whereas Chordin is pushed away from BMP, ensuring that BMPs can be more easily delivered to their receptors.

A novel transgenic zebrafish model for blood-brain and blood-retinal barrier development

Background

Development and maintenance of the blood-brain and blood-retinal barrier is critical for the homeostasis of brain and retinal tissue. Despite decades of research our knowledge of the formation and maintenance of the blood-brain (BBB) and blood-retinal (BRB) barrier is very limited. We have established an in vivo model to study the development and maintenance of these barriers by generating a transgenic zebrafish line that expresses a vitamin D-binding protein fused with enhanced green fluorescent protein (DBP-EGFP) in blood plasma, as an endogenous tracer.

Results

The temporal establishment of the BBB and BRB was examined using this transgenic line and the results were compared with that obtained by injection of fluorescent dyes into the sinus venosus of embryos at various stages of development. We also examined the expression of claudin-5, a component of tight junctions during the first 4 days of development. We observed that the BBB of zebrafish starts to develop by 3 dpf, with expression of claudin-5 in the central arteries preceding it at 2 dpf. The hyaloid vasculature in the zebrafish retina develops a barrier function at 3 dpf, which endows the zebrafish with unique advantages for studying the BRB.

Conclusion

Zebrafish embryos develop BBB and BRB function simultaneously by 3 dpf, which is regulated by tight junction proteins. The Tg(l-fabp:DBP-EGFP) zebrafish will have great advantages in studying development and maintenance of the blood-neural barrier, which is a new application for the widely used vertebrate model.

Evolution of extracellular Dpp modulators in insects: The roles of tolloid and twisted-gastrulation in dorsoventral patterning of the Tribolium embryo

The formation of the BMP gradient which patterns the DV axis in flies and vertebrates requires several extracellular modulators like the inhibitory protein Sog/Chordin, the metalloprotease Tolloid (Tld), which cleaves Sog/Chordin, and the CR domain protein Twisted gastrulation (Tsg). While flies and vertebrates have only one sog/chordin gene they possess several paralogues of tld and tsg. A simpler and probably ancestral situation is observed in the short-germ beetle Tribolium castaneum (Tc), which possesses only one tld and one tsg gene. Here we show that in T. castaneum tld is required for early BMP signalling except in the head region and Tc-tld function is, as expected, dependent on Tc-sog. In contrast, Tc-tsg is required for all aspects of early BMP signalling and acts in a Tc-sog-independent manner. For comparison with Drosophila melanogaster we constructed fly embryos lacking all early Tsg activity (tsg;;srw double mutants) and show that they still establish a BMP signalling gradient. Thus, our results suggest that the role of Tsg proteins for BMP gradient formation has changed during insect evolution.

Genome-wide loss-of-function analysis of deubiquitylating enzymes for zebrafish development

Background

Deconjugation of ubiquitin and/or ubiquitin-like modified protein substrates is essential to modulate protein-protein interactions and, thus, signaling processes in cells. Although deubiquitylating (deubiquitinating) enzymes (DUBs) play a key role in this process, however, their function and regulation remain insufficiently understood. The "loss-of-function" phenotype studies can provide important information to elucidate the gene function, and zebrafish is an excellent model for this goal.

Results

From an in silico genome-wide search, we found more than 90 putative DUBs encoded in the zebrafish genome belonging to six different subclasses. Out of them, 85 from five classical subclasses have been tested with morpholino (MO) knockdown experiments and 57 of them were found to be important in early development of zebrafish. These DUB morphants resulted in a complex and pleiotropic phenotype that, regardless of gene target, always affected the notochord. Based on the huC neuronal marker expression, we grouped them into five sets (groups I to V). Group I DUBs (otud7b, uchl3 and bap1) appear to be involved in the Notch signaling pathway based on the neuronal hyperplasia, while group IV DUBs (otud4, usp5, usp15 and usp25) play a critical role in dorsoventral patterning through the BMP pathway.

Conclusion

We have identified an exhaustive list of genes in the zebrafish genome belonging to the five established classes of DUBs. Additionally, we performed the corresponding MO knockdown experiments in zebrafish as well as functional studies for a subset of the predicted DUB genes. The screen results in this work will stimulate functional follow-up studies of potential DUB genes using the zebrafish model system.

Spemann's organizer and the self-regulation of embryonic fields

Embryos and developing organs have the remarkable ability of self-regenerating after experimental manipulations. In the Xenopus blastula half-embryos can regenerate the missing part, producing identical twins. Studies on the molecular nature of Spemann's organizer have revealed that self-regulation results from the battle between two signaling centers under reciprocal transcriptional control. Long-range communication between the dorsal and ventral sides is mediated by the action of growth factor antagonists - such as the BMP antagonist Chordin - that regulate the flow of BMPs within the embryonic morphogenetic field. BMPs secreted by the dorsal Spemann organizer tissue are released by metalloproteinases of the Tolloid family, which cleave Chordin at a distance of where they were produced. The dorsal center secretes Chordin, Noggin, BMP2 and ADMP. The ventral center of the embryo secretes BMP4, BMP7, Sizzled, Crossveinless-2 and Tolloid-related. Crossveinless-2 binds Chordin/BMP complexes, facilitating their flow towards the ventral side, where BMPs are released by Tolloid allowing peak BMP signaling. Self-regulation occurs because transcription of ventral genes is induced by BMP while transcription of dorsal genes is repressed by BMP signals. This assures that for each action of Spemann's organizer there is a reaction in the ventral side of the embryo. Because both dorsal and ventral centers express proteins of similar biochemical activities, they can compensate for each other. A novel biochemical pathway of extracellular growth factor signaling regulation has emerged from these studies in Xenopus. This remarkable dorsal-ventral positional information network has been conserved in evolution and is ancestral to all bilateral animals.

Systems biology of the self-regulating morphogenetic gradient of the Xenopus gastrula

The morphogenetic field concept was proposed by experimental embryologists to account for the self-regulative behavior of embryos. Such fields have remained an abstract concept until the recent identification of their molecular components using a combination of genetics, biochemistry, and theoretical modeling. One of the best studied models of a morphogenetic field is the Dorsal-Ventral (D-V) patterning of the early frog embryo. This patterning system is regulated by the bone morphogenetic protein (BMP) signaling pathway and an intricate network of secreted protein antagonists. This biochemical pathway of interacting proteins functions in the extracellular space to generate a D-V gradient of BMP signaling, which is maintained during extensive morphogenetic movements of cell layers during gastrulation. The D-V field is divided into a dorsal and a ventral center, in regions of low and high BMP signaling respectively, under opposite transcriptional control by BMPs. The robustness of the patterning is assured at two different levels. First, in the extracellular space by secreted BMP antagonists that generate a directional flow of BMP ligands to the ventral side. The flow is driven by the regulated proteolysis of the Chordin inhibitor and by the presence of a molecular sink on the ventral side that concentrates BMP signals. The tolloid metalloproteinases and the Chordin-binding protein Crossveinless-2 (CV2) are key components of this ventral sink. Second, by transcriptional feedback at the cellular level: The dorsal and ventral signaling centers adjust their size and level of BMP signaling by transcriptional feedback. This allows cells on one side of a gastrula containing about 10,000 cells to communicate with cells in the opposite pole of the embryo.

Crossveinless-2 Is a BMP feedback inhibitor that binds Chordin/BMP to regulate Xenopus embryonic patterning

Vertebrate Crossveinless-2 (CV2) is a secreted protein that can potentiate or antagonize BMP signaling. Through embryological and biochemical experiments we find that: (1) CV2 functions as a BMP4 feedback inhibitor in ventral regions of the Xenopus embryo; (2) CV2 complexes with Twisted gastrulation and BMP4; (3) CV2 is not a substrate for tolloid proteinases; (4) CV2 binds to purified Chordin protein with high affinity (K(D) in the 1 nM range); (5) CV2 binds even more strongly to Chordin proteolytic fragments resulting from Tolloid digestion or to full-length Chordin/BMP complexes; (6) CV2 depletion causes the Xenopus embryo to become hypersensitive to the anti-BMP effects of Chordin overexpression or tolloid inhibition. We propose that the CV2/Chordin interaction may help coordinate BMP diffusion to the ventral side of the embryo, ensuring that BMPs liberated from Chordin inhibition by tolloid proteolysis cause peak signaling levels.

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.

crm-1 facilitates BMP signaling to control body size in Caenorhabditis elegans

We have identified in Caenorhabditis elegans a homologue of the vertebrate Crim1, crm-1, which encodes a putative transmembrane protein with multiple cysteine-rich (CR) domains known to have bone morphogenetic proteins (BMPs) binding activity. Using the body morphology of C. elegans as an indicator, we showed that attenuation of crm-1 activity leads to a small body phenotype reminiscent of that of BMP pathway mutants. We showed that the crm-1 loss-of-function phenotype can be rescued by constitutive supply of sma-4 activity. crm-1 can enhance BMP signaling and this activity is dependent on the presence of the DBL-1 ligand and its receptors. crm-1 is expressed in neurons at the ventral nerve cord, where the DBL-1 ligand is produced. However, ectopic expression experiments reveal that crm-1 gene products act outside the DBL-1 producing cells and function non-autonomously to facilitate dbl/sma pathway signaling to control body size.

von Willebrand Factor Type C Domain-containing Proteins Regulate Bone Morphogenetic Protein Signaling through Different Recognition Mechanisms

Bone morphogenetic protein (BMP) function is regulated in the extracellular space by many modulator proteins, including those containing a von Willebrand factor type C (VWC) domain. The function of the VWC domain-containing proteins in development and diseases has been extensively studied. The structural basis, however, for the mechanism by which BMP is regulated by these proteins is still poorly understood. By analyzing chordin, CHL2 (chordin-like 2), and CV2 (crossveinless 2) as well as their individual VWC domains, we show that the VWC domain is a versatile binding module that in its multiple forms and environments can expose a variety of binding specificities. Three of four, two of three, and one of five VWCs from chordin, CHL2, and CV2, respectively, can bind BMPs. Using an array of BMP-2 mutant proteins, it can be demonstrated that the binding-competent VWC domains all use a specific subset of BMP-2 binding determinants that overlap with the binding site for the type II receptors (knuckle epitope) or for the type I receptors (wrist epitope). This explains the competition between modulator proteins and receptors for BMP binding and therefore the inhibition of BMP signaling. A subset of VWC domains from CHL2 binds to the Tsg (twisted gastrulation) protein similar to chordin. A stable ternary complex consisting of BMP-2, CHL2, and Tsg can be formed, thus making CHL2 a more efficient BMP-2 inhibitor. The VWCs of CV2, however, do not interact with Tsg. The present results show that chordin, CHL2, and CV2 regulate BMP-2 signaling by different recognition mechanisms.

Twisted gastrulation (Tsg) is regulated by Tob and enhances TGF-beta signaling in activated T lymphocytes

Quiescent T cells express Tob, an APRO gene family member, which functions as a transcriptional regulator. Subtractive hybridization identified Twisted gastrulation (Tsg) as one of the genes suppressed by Tob. Tsg is a secreted protein that interacts with Drosophila decapentaplegic (Dpp) and its vertebrate orthologs BMP2/4 and regulates morphogenetic effects in embryos. Here, we report the expression and function of Tsg in human T cells. Tsg mRNA was almost undetectable in unstimulated T cells and was up-regulated after activation by TCR/CD3 and either CD28, IL-2, or PMA. Tsg protein had no effect on responses of primary T cells to TCR/CD3 stimulation but had a potent inhibitory effect on proliferation and cytokine production of primed alloreactive CD4+ cells. Surprisingly, Tsg did not affect phosphorylation of the BMP-specific Smad1 but induced phosphorylation of the TGF-beta-specific Smad2 and mediated DNA binding on Smad3/4 consensus-binding sites, suggesting that it acted downstream of TGF-beta. In vitro association assays revealed a direct interaction of Tsg and TGF-beta proteins. Thus, Tsg functions as an agonist synergizing with TGF-beta to inhibit T-cell activation. Modulation of Tsg signaling may represent a novel target for molecular intervention toward control of aberrant T-cell responses during ongoing graft-versus-host disease (GVHD) and autoimmune diseases.

Bone morphogenetic proteins in the early development of zebrafish

Bone morphogenetic proteins (BMPs) are known to be widely involved in various biological processes. Many of the members of the BMP family, as well as related factors, receptors and molecules in the BMP signaling pathway, have been isolated, but their precise functions are still unclear. In addition to the 'classical' model organism Xenopus, zebrafish, Danio rerio, is now considered to be a suitable model organism to study the roles of the BMP signaling pathway during embryogenesis. Mutagenesis screens have identified a number of mutants in the pathway. Although they do not cover the entire members of the BMP signaling cascade that are currently known, they serve as a powerful tool to broaden our understanding of BMP functions, in combination with other experimental techniques.

Manipulating mitotic recombination in the zebrafish embryo through RecQ helicases

RecQ DNA helicases resolve Rad-51-mediated recombination and suppress aberrant homologous recombination. RecQ gene loss is associated with cancer susceptibility and increased mitotic recombination. We have developed an in vivo assay based on a zebrafish pigment mutant for suppression of RecQ activity, and demonstrate that zebrafish RecQ genes have conserved function in suppressing mitotic recombination.

Proteolytic regulatory mechanisms in the formation of extracellular morphogen gradients

Growth factors are secreted into the extracellular space, where they encounter soluble inhibitors, extracellular matrix glycoproteins and proteoglycans, and proteolytic enzymes that can each modulate the spatial distribution, activity state, and receptor interactions of these signaling molecules. During development, morphogenetic gradients of these growth factors pattern fields of cells responsive to different levels of signaling, creating such structures as the branched pattern of airways and vasculature, and the arrangement of digits in the hand. This review focuses specifically on the roles of proteolytic enzymes and their regulators in the generation of such activity gradients. Evidence from Drosophila developmental pathways provides a detailed understanding of general mechanisms underlying proteolytic control of morphogen gradients, while recent studies of several mammalian growth factors illustrate the relevance of this proteolytic control to human development and disease.

Twisted Gastrulation Modulates Bone Morphogenetic Protein-induced Collagen II and X Expression in Chondrocytesin Vitroandin Vivo

Twisted gastrulation (TSG) is an extracellular modulator of bone morphogenetic protein (BMP) activity and regulates dorsoventral axis formation in early Drosophila and Xenopus development. Studies on tsg-deficient mice also indicated a role of this protein in skeletal growth, but the mechanism of TSG activity in this process has not yet been investigated. Here we show for the first time by in situ hybridization and immunohistochemistry that TSG is strongly expressed in bovine and mouse growth plate cartilage as well as in fetal ribs, vertebral cartilage, and cartilage anlagen of the skull. Furthermore we provide evidence that TSG is directly involved in BMP-regulated chondrocyte differentiation and maturation. In vitro, TSG impaired the dose-dependent BMP-2 stimulation of collagen II and X expression in cultures of MC615 chondrocytes and primary mouse chondrocytes. In the presence of chordin, a BMP antagonist, the inhibitory effect of TSG was further enhanced. TSG also inhibited BMP-2-stimulated phosphorylation of Smad factors in chondrocytes, confirming the role of TSG as a modulator of BMP signaling. For analysis of TSG functions in cartilage development in vivo, the gene was overexpressed in transgenic mice under the control of the cartilage-specific Col2a1 promoter. As a result, Col10a1 expression was significantly reduced in the growth plates of transgenic embryos and newborns in comparison with wild type littermates as shown by in situ hybridization and by real time PCR analysis. The data suggest that TSG is an important modulator of BMP-regulated cartilage development and chondrocyte differentiation.

Recruitment and SNARE-mediated fusion of vesicles in furrow membrane remodeling during cytokinesis in zebrafish embryos

Cytokinesis is the final stage in cell division that serves to partition cytoplasm and daughter nuclei into separate cells. Membrane remodeling at the cleavage plane is a required feature of cytokinesis in many species. In animal cells, however, the precise mechanisms and molecular interactions that mediate this process are not yet fully understood. Using real-time imaging in live, early stage zebrafish embryos, we demonstrate that vesicles labeled with the v-SNARE, VAMP-2, are recruited to the cleavage furrow during deepening in a microtubule-dependent manner. These vesicles then fuse with, and transfer their VAMP-2 fluorescent label to, the plasma membrane during both furrow deepening and subsequent apposition. This observation indicates that new membrane is being inserted during these stages of cytokinesis. Inhibition of SNAP-25 (a cognate t-SNARE of VAMP-2), using a monoclonal antibody, blocked VAMP-2 vesicle fusion and furrow apposition. Transient expression of mutant forms of SNAP-25 also produced defects in furrow apposition. SNAP-25 inhibition by either method, however, did not have any significant effect on furrow deepening. Thus, our data clearly indicate that VAMP-2 and SNAP-25 play an essential role in daughter blastomere apposition, possibly via the delivery of components that promote the cell-to-cell adhesion required for the successful completion of cytokinesis. Our results also support the idea that new membrane addition, which occurs during late stage cytokinesis, is not required for furrow deepening that results from contractile band constriction.

Temporal and spatial action of Tolloid (Mini fin) and Chordin to pattern tail tissues

In vertebrates, a bone morphogenetic protein (BMP) signaling pathway patterns all ventral cell fates along the embryonic axis. BMP activity is positively regulated by Tolloid, a metalloprotease, that can eliminate the activity of the BMP antagonist Chordin. A tolloid mutant in zebrafish, mini fin (mfn), exhibits a specific loss of ventral tail tissues. Here, we investigate the spatial and temporal requirements for Tolloid (Mfn) in dorsoventral patterning of the tail. Through chimeric analyses, we found that Tolloid (Mfn) functions cell non-autonomously in the ventral-most vegetal cells of the gastrula or their derivatives. We generated a tolloid transgene under the control of the inducible hsp70 promoter and demonstrate that tolloid (mfn) is first required at the completion of gastrulation. Although tolloid is expressed during gastrulation and dorsally and ventrally within the tail bud, our results indicate that Tolloid (Mfn) acts specifically in the ventral tail bud during a approximately 4 h period extending from the completion of gastrulation to early somitogenesis stages to regulate BMP signaling. Examination of the temporal requirements of Chordin activity by overexpression of the hsp70-tolloid transgene indicates that Chordin is required both during and after gastrulation for proper patterning of the tail, contrasting Tld's requirement only during post-gastrula stages. We hypothesize that the gastrula role of Chordin in tail patterning is to generate the proper size domains of cells to enter the ventral and dorsal tail bud, whereas post-gastrula Chordin activity patterns the derivatives of the tail bud. Thus, fine modulation of BMP signaling levels through the negative and positive actions of Chordin and Tolloid, respectively, patterns tail tissues.

Spemann's organizer and self-regulation in amphibian embryos

In 1924, Spemann and Mangold demonstrated the induction of Siamese twins in transplantation experiments with salamander eggs. Recent work in amphibian embryos has followed their lead and uncovered that cells in signalling centres that are located at the dorsal and ventral poles of the gastrula embryo communicate with each other through a network of secreted growth-factor antagonists, a protease that degrades them, a protease inhibitor and bone-morphogenic-protein signals.

Shaping BMP morphogen gradients in the Drosophila embryo and pupal wing

In the early Drosophila embryo, BMP-type ligands act as morphogens to suppress neural induction and to specify the formation of dorsal ectoderm and amnioserosa. Likewise, during pupal wing development, BMPs help to specify vein versus intervein cell fate. Here, we review recent data suggesting that these two processes use a related set of extracellular factors, positive feedback, and BMP heterodimer formation to achieve peak levels of signaling in spatially restricted patterns. Because these signaling pathway components are all conserved, these observations should shed light on how BMP signaling is modulated in vertebrate development.

Sizzled controls dorso-ventral polarity by repressing cleavage of the Chordin protein

The Bone morphogenetic protein (Bmp) signalling gradient has a major function in the formation of the dorso-ventral axis. The zebrafish ventralized mutant, ogon, encodes Secreted Frizzled (Sizzled). sizzled is ventrally expressed in a Bmp-dependent manner and is required for the suppression of Bmp signalling on the ventral side of zebrafish embryos. However, it remains unclear how Sizzled inhibits Bmp signalling and controls ventro-lateral cell fate. We found that Sizzled stabilizes Chordin, a Bmp antagonist, by binding and inhibiting the Tolloid-family metalloproteinase, Bmp1a, which cleaves and inactivates Chordin. The cysteine-rich domain of Sizzled is required for inhibition of Bmp1a activity. Loss of both Bmp1a and Tolloid-like1 (Tll1; another Tolloid-family metalloproteinase) function leads to a complete suppression and reversal of the ogon mutant phenotype. These results indicate that Sizzled represses the activities of Tolloid-family proteins, thereby creating the Chordin-Bmp activity gradient along the dorso-ventral axis. Here, we describe a previously unrecognized role for a secreted Frizzled-related protein.

Crossveinless 2 is an essential positive feedback regulator of Bmp signaling during zebrafish gastrulation

Signaling by bone morphogenetic proteins (Bmps) plays a pivotal role in developmental and pathological processes, and is regulated by a complex interplay with secreted Bmp binding factors, including Crossveinless 2 (Cvl2). Although structurally related to the Bmp antagonist Chordin, Crossveinless 2 has been described to be both a Bmp agonist and antagonist. Here, we present the first loss-of-function study of a vertebrate cvl2 homologue, showing that zebrafish cvl2 is required in a positive feedback loop to promote Bmp signaling during embryonic dorsoventral patterning. In vivo, Cvl2 protein undergoes proteolytic cleavage and this cleavage converts Cvl2 from an anti- to a pro-Bmp factor. Embryonic epistasis analyses and protein interaction assays indicate that the pro-Bmp function of Cvl2 is partly accomplished by competing with Chordin for binding to Bmps. Studies in cell culture and embryos further suggest that the anti-Bmp effect of uncleaved Cvl2 is due to its association with the extracellular matrix, which is not found for cleaved Cvl2. Our data identify Cvl2 as an essential pro-Bmp factor during zebrafish embryogenesis, emphasizing the functional diversity of Bmp binding CR-domain proteins. Differential proteolytic processing as a mode of regulation might account for anti-Bmp effects in other contexts.