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

Smad4 is essential for epiblast scaling and morphogenesis after implantation, but nonessential before implantation

Bone morphogenic protein (BMP) signaling plays an essential and highly conserved role in embryo axial patterning in animal species. However, in mammalian embryos, which develop inside the mother, early development includes a preimplantation stage, which does not occur in externally developing embryos. During preimplantation, the epiblast is segregated from extra-embryonic lineages that enable implantation and development in utero. Yet, the requirement for BMP signaling is imprecisely defined in mouse early embryos. Here, we show that, in contrast to previous reports, BMP signaling (SMAD1/5/9 phosphorylation) is not detectable until implantation when it is detected in the primitive endoderm - an extra-embryonic lineage. Moreover, preimplantation development appears to be normal following deletion of maternal and zygotic Smad4, an essential effector of canonical BMP signaling. In fact, mice lacking maternal Smad4 are viable. Finally, we uncover a new requirement for zygotic Smad4 in epiblast scaling and cavitation immediately after implantation, via a mechanism involving FGFR/ERK attenuation. Altogether, our results demonstrate no role for BMP4/SMAD4 in the first lineage decisions during mouse development. Rather, multi-pathway signaling among embryonic and extra-embryonic cell types drives epiblast morphogenesis postimplantation.

Smad4 is essential for epiblast scaling and morphogenesis after implantation, but nonessential prior to implantation in the mouse

Bone Morphogenic Protein (BMP) signaling plays an essential and highly conserved role in axial patterning in embryos of many externally developing animal species. However, in mammalian embryos, which develop inside the mother, early development includes an additional stage known as preimplantation. During preimplantation, the epiblast lineage is segregated from the extraembryonic lineages that enable implantation and development in utero. Yet, the requirement for BMP signaling in mouse preimplantation is imprecisely defined. We show that, in contrast to prior reports, BMP signaling (as reported by SMAD1/5/9 phosphorylation) is not detectable until implantation, when it is detected in the primitive endoderm - an extraembryonic lineage. Moreover, preimplantation development appears normal following deletion of maternal and zygotic Smad4, an essential effector of BMP signaling. In fact, mice lacking maternal Smad4 are viable. Finally, we uncover a new requirement for zygotic Smad4 in epiblast scaling and cavitation immediately after implantation, via a mechanism involving FGFR/ERK attenuation. Altogether, our results demonstrate no role for BMP4/SMAD4 in the first lineage decisions during mouse development. Rather, multi-pathway signaling among embryonic and extraembryonic cell types drives epiblast morphogenesis post-implantation.

From "self-differentiation" to organoids-the quest for the units of development

As proposed by Wilhelm Roux in 1885, the key goal of experimental embryology ("Entwicklungsmechanik") was to elucidate whether organisms or their parts develop autonomously ("self-differentiation") or require interactions with other parts or the environment. However, experimental embryologists soon realized that concepts like "self-differentiation" only make sense when applied to particular parts or units of the developing embryo as defined both in time and space. Whereas the formation of tissues or organs may initially depend on interactions with surrounding tissues, they later become independent of such interactions or "determined." Moreover, the determination of a particular tissue or organ primordium has to be distinguished from the spatially coordinated determination of its parts-what we now refer to as "patterning." While some primordia depend on extrinsic influences (e.g., signals from adjacent tissues) for proper patterning, others rely on intrinsic mechanisms. Such intrinsically patterned units may behave as "morphogenetic fields" that can compensate for lost parts and regulate their size and proper patterning. While these insights were won by experimental embryologists more than 100 years ago, they retain their relevance today. To enable the generation of more life-like organoids in vitro for studying developmental processes and diseases in a dish, questions about the spatiotemporal units of development (when and how tissues and organs are determined and patterned) need to be increasingly considered. This review briefly sketches this conceptual history and its continued relevance by focusing on the determination and patterning of the inner ear with a specific emphasis on some studies published in this journal.

Targeted search for scaling genes reveals matrixmetalloproteinase 3 as a scaler of the dorsal-ventral pattern in Xenopus laevis embryos

How embryos scale patterning according to size is still not fully understood. Through in silico screening and analysis of reaction-diffusion systems that could be responsible for scaling, we predicted the existence of genes whose expression is sensitive to embryo size and which regulate the scaling of embryonic patterning. To find these scalers, we identified genes with strongly altered expression in half-size Xenopus laevis embryos compared with full-size siblings at the gastrula stage. Among found genes, we investigated the role of matrix metalloproteinase-3 (mmp3), which was most strongly downregulated in half-size embryos. We show that Mmp3 scales dorsal-ventral patterning by degrading the slowly diffusing embryonic inducers Noggin1 and Noggin2 but preventing cleavage of the more rapidly diffusing inducer Chordin via degradation of a Tolloid-type proteinase. In addition to unraveling the mechanism underlying the scaling of dorsal-ventral patterning, this work provides proof of principal for scalers identification in embryos of other species.

Nodal and BMP dispersal during early zebrafish development

The secreted TGF-β superfamily signals Nodal and BMP coordinate the patterning of vertebrate embryos. Nodal specifies endoderm and mesoderm during germ layer formation, and BMP specifies ventral fates and patterns the dorsal/ventral axis. Five major models have been proposed to explain how the correct distributions of Nodal and BMP are achieved within tissues to orchestrate embryogenesis: source/sink, transcriptional determination, relay, self-regulation, and shuttling. Here, we discuss recent experiments probing these signal dispersal models, focusing on early zebrafish development.

Dynamics of BMP signaling and distribution during zebrafish dorsal-ventral patterning

During vertebrate embryogenesis, dorsal-ventral patterning is controlled by the BMP/Chordin activator/inhibitor system. BMP induces ventral fates, whereas Chordin inhibits BMP signaling on the dorsal side. Several theories can explain how the distributions of BMP and Chordin are regulated to achieve patterning, but the assumptions regarding activator/inhibitor diffusion and stability differ between models. Notably, ‘shuttling’ models in which the BMP distribution is modulated by a Chordin-mediated increase in BMP diffusivity have gained recent prominence. Here, we directly test five major models by measuring the biophysical properties of fluorescently tagged BMP2b and Chordin in zebrafish embryos. We found that BMP2b and Chordin diffuse and rapidly form extracellular protein gradients, Chordin does not modulate the diffusivity or distribution of BMP2b, and Chordin is not required to establish peak levels of BMP signaling. Our findings challenge current self-regulating reaction-diffusion and shuttling models and provide support for a graded source-sink mechanism underlying zebrafish dorsal-ventral patterning.

Animals start life as clumps of cells that ultimately give rise to complex structures and organs. Over a century of research has revealed a small number of proteins that are crucial for complex structures to form from these clumps, including one protein called BMP. Different levels of BMP instruct cells to give rise to different tissues. In zebrafish, BMP is more abundant on one side of the embryo than the other. This gradient in BMP levels causes different tissues to form at distinct positions and helps coordinate embryo development.

Several theories have been proposed to explain how the BMP gradient is established. They all suggest that a second protein – Chordin – plays an important role in influencing how cells sense the BMP gradient by blocking BMP’s activity. However, the exact role of Chordin in the formation of the BMP gradient is disputed. To address this, Pomreinke, Soh, Rogers et al. directly tested five theories of how BMP and Chordin molecules spread through embryos.

The experiments used microscopy to track the movements of fluorescent versions of both molecules in zebrafish embryos. The measurements contradict one theory stating that BMP does not move, and another in which Chordin increases the mobility of BMP. Pomreinke, Soh, Rogers et al. also found that embryos that lack Chordin have increased BMP signaling levels only on the side where Chordin is normally made but not on the opposite side where BMP is made, ruling out several of the theories. The findings are most consistent with the idea that the BMP gradient forms mainly as a result of higher production of BMP on one side of the embryo combined with movement of BMP away from where it is made. Chordin produced at the opposite end of the embryo helps to ensure that only the correct cells receive instructions from BMP.

In the future, two approaches could further clarify how the BMP gradient is formed. First, better techniques to directly observe the BMP gradient in normally developing embryos would be useful. Second, new theories that take into account additional players other than BMP and Chordin might help explain some features of development that current theories cannot address. Uncovering the mechanisms that control the formation of BMP gradients will improve our understanding of how clumps of cells can develop into animals.

Affinity of the heparin binding motif of Noggin1 to heparan sulfate and its visualization in the embryonic tissues

Heparin binding motifs were found in many secreted proteins and it was suggested that they are responsible for retardation of the protein diffusion within the intercellular space due to the binding to heparan sulfate proteoglycanes (HSPG). Here we used synthetic FITC labeled heparin binding motif (HBM peptide) of the Xenopus laevis secreted BMP inhibitor Noggin1 to study its diffusion along the surface of the heparin beads by FRAP method. As a result, we have found out that diffusivity of HBM-labeled FITC was indeed much lesser than those predicted by theoretical calculations even for whole protein of the Noggin size. We also compared by isothermal titration calorimetry the binding affinity of HBM and the control oligolysine peptide to several natural polyanions including heparan sulfate (HS), heparin, the bacterial dextran sulfate and salmon sperm DNA, and demonstrated that HBM significantly exceeds oligolysine peptide in the affinity to HS, heparin and DNA. By contrast, oligolysine peptide bound with higher affinity to dextran sulfate. We speculate that such a difference may ensure specificity of the morphogen binding to HSPG and could be explained by steric constrains imposed by different distribution of the negative charges along a given polymeric molecule. Finally, by using EGFP-HBM recombinant protein we have visualized the natural pattern of the Noggin1 binding sites within the X. laevis gastrula and demonstrated that these sites forms a dorsal-ventral concentration gradient, with a maximum in the dorsal blastopore lip. In sum, our data provide a quantitative basis for modeling the process of Noggin1 diffusion in embryonic tissues, considering its interaction with HSPG.

The significance and scope of evolutionary developmental biology: a vision for the 21st century

Evolutionary developmental biology (evo-devo) has undergone dramatic transformations since its emergence as a distinct discipline. This paper aims to highlight the scope, power, and future promise of evo-devo to transform and unify diverse aspects of biology. We articulate key questions at the core of eleven biological disciplines-from Evolution, Development, Paleontology, and Neurobiology to Cellular and Molecular Biology, Quantitative Genetics, Human Diseases, Ecology, Agriculture and Science Education, and lastly, Evolutionary Developmental Biology itself-and discuss why evo-devo is uniquely situated to substantially improve our ability to find meaningful answers to these fundamental questions. We posit that the tools, concepts, and ways of thinking developed by evo-devo have profound potential to advance, integrate, and unify biological sciences as well as inform policy decisions and illuminate science education. We look to the next generation of evolutionary developmental biologists to help shape this process as we confront the scientific challenges of the 21st century.

A gradient of Bmp7 specifies the tonotopic axis in the developing inner ear

The auditory systems of animals that perceive sounds in air are organized to separate sound stimuli into their component frequencies. Individual tones then stimulate mechanosensory hair cells located at different positions on an elongated frequency (tonotopic) axis. During development, immature hair cells located along the axis must determine their tonotopic position in order to generate frequency-specific characteristics. Expression profiling along the developing tonotopic axis of the chick basilar papilla (BP) identified a gradient of Bmp7. Disruption of that gradient in vitro or in ovo induces changes in hair cell morphologies consistent with a loss of tonotopic organization and the formation of an organ with uniform frequency characteristics. Further, the effects of Bmp7 in determination of positional identity are shown to be mediated through activation of the Mapk, Tak1. These results indicate that graded, Bmp7-dependent, activation of Tak1 signalling controls the determination of frequency-specific hair cell characteristics along the tonotopic axis.

Organizer-derived Bmp2 is required for the formation of a correct Bmp activity gradient during embryonic development

Bone morphogenetic proteins (Bmps) control dorsoventral patterning of vertebrate embryos through the establishment of a ventrodorsal gradient of the activated downstream cytoplasmic effectors Smad1/5/8. Some Bmp ligands are expressed in the ventral and lateral regions and in the organizer during gastrulation of the embryo, but it remains unclear how organizer-derived Bmps contribute to total Bmp ligand levels and to the establishment of the correct phospho-Smad1/5/8 gradient along the ventrodorsal axis. Here we demonstrate that interference with organizer-specific Bmp2b signalling in zebrafish embryos alters the phospho-Smad1/5/8 gradient throughout the ventrodorsal axis, elevates the levels of the Bmp antagonist Chordin and dorsalizes the embryos. Moreover, we show that organizer-derived Bmp2b represses chordin transcription in the organizer and contributes to the control of the Chordin gradient. Combining these experimental results with simulations of Bmp’s reaction-diffusion dynamics, our data indicate that organizer-produced Bmp2b is required for the establishment and maintenance of a Bmp activity gradient and for appropriate embryonic dorsoventral patterning during gastrulation.

The morphogen, Bmp, regulates differentiation of cell fates along the ventral to dorsal axis during vertebrate embryonic development. Here, Xue et al. show that Bmp2b produced by the organizer during early gastrulation in zebrafish embryos has a role in the establishment of an appropriate Bmp morphogen activity gradient and the correct dorsoventral patterning of the embryos.

Development: scaling to size by protease inhibition

The dorsal half of bisected Xenopus laevis embryos can regenerate a well-proportioned organism on a smaller scale. A new study indicates that the removal of ventral tissue generates a steeper Chordin gradient by reducing Sizzled, a secreted inhibitor of Tolloid chordinases.

Mechanisms of scaling in pattern formation

Many organisms and their constituent tissues and organs vary substantially in size but differ little in morphology; they appear to be scaled versions of a common template or pattern. Such scaling involves adjusting the intrinsic scale of spatial patterns of gene expression that are set up during development to the size of the system. Identifying the mechanisms that regulate scaling of patterns at the tissue, organ and organism level during development is a longstanding challenge in biology, but recent molecular-level data and mathematical modeling have shed light on scaling mechanisms in several systems, including Drosophila and Xenopus. Here, we investigate the underlying principles needed for understanding the mechanisms that can produce scale invariance in spatial pattern formation and discuss examples of systems that scale during development.

Chordin forms a self-organizing morphogen gradient in the extracellular space between ectoderm and mesoderm in the Xenopus embryo

The vertebrate body plan follows stereotypical dorsal-ventral (D-V) tissue differentiation controlled by bone morphogenetic proteins (BMPs) and secreted BMP antagonists, such as Chordin. The three germ layers--ectoderm, mesoderm, and endoderm--are affected coordinately by the Chordin-BMP morphogen system. However, extracellular morphogen gradients of endogenous proteins have not been directly visualized in vertebrate embryos to date. In this study, we improved immunolocalization methods in Xenopus embryos and analyzed the distribution of endogenous Chordin using a specific antibody. Chordin protein secreted by the dorsal Spemann organizer was found to diffuse along a narrow region that separates the ectoderm from the anterior endoderm and mesoderm. This Fibronectin-rich extracellular matrix is called "Brachet's cleft" in the Xenopus gastrula and is present in all vertebrate embryos. Chordin protein formed a smooth gradient that encircled the embryo, reaching the ventral-most Brachet cleft. Depletion with morpholino oligos showed that this extracellular gradient was regulated by the Chordin protease Tolloid and its inhibitor Sizzled. The Chordin gradient, as well as the BMP signaling gradient, was self-regulating and, importantly, was able to rescale in dorsal half-embryos. Transplantation of Spemann organizer tissue showed that Chordin diffused over long distances along this signaling highway between the ectoderm and mesoderm. Chordin protein must reach very high concentrations in this narrow region. We suggest that as ectoderm and mesoderm undergo morphogenetic movements during gastrulation, cells in both germ layers read their positional information coordinately from a single morphogen gradient located in Brachet's cleft.

Scaling of dorsal-ventral patterning in the Xenopus laevis embryo

Scaling of pattern with size has been described and studied for over a century, yet its molecular basis is understood in only a few cases. In a recent, elegant study, Inomata and colleagues proposed a new model explaining how bone morphogenic protein (BMP) activity gradient scales with embryo size in the early Xenopus laevis embryo. We discuss their results in conjunction with an alternative model we proposed previously. The expansion-repression mechanism (ExR) provides a conceptual framework unifying both mechanisms. Results of Inomata and colleagues implicate the chordin-stabilizing protein sizzled as the expander molecule enabling scaling, while we attributed this role to the BMP ligand Admp. The two expanders may work in concert, as suggested by the mathematical model of Inomata et al. We discuss approaches for differentiating the contribution of sizzled and Admp to pattern scaling.

Spatial regulation of BMP activity

The bone morphogenetic protein (BMP) signalling pathway is critical for embryonic development and tissue homeostasis, and impaired BMP signalling has been implicated in multiple diseases. Molecular tools have been developed to visualise BMP activity in vivo and these have allowed a better understanding of the intricate ways in which BMP activity is regulated spatially. In particular, generation and interpretation of BMP activity gradients during development result from the complex interplay between core BMP signalling components and specific regulators. In this essay we discuss the mechanisms by which spatial regulation of BMP activity is achieved and its functional consequences.

Morphogen gradients in development: from form to function

Morphogens are substances that establish a graded distribution and elicit distinct cellular responses in a dose-dependent manner. They function to provide individual cells within a field with positional information, which is interpreted to give rise to spatial patterns. Morphogens can consist of intracellular factors that set up a concentration gradient by diffusion in the cytoplasm. More commonly, morphogens comprise secreted proteins that form an extracellular gradient across a field of cells. Experimental studies and computational analyses have provided support for a number of diverse strategies by which extracellular morphogen gradients are formed. These include free diffusion in the extracellular space, restricted diffusion aided by interactions with heparan sulfate proteoglycans, transport on lipid-containing carriers or transport aided by soluble binding partners. More specialized modes of transport have also been postulated such as transcytosis, in which repeated rounds of secretion, endocytosis, and intracellular trafficking move morphogens through cells rather than around them, or cytonemes, which consist of filopodial extensions from signal-receiving cells that are hypothesized to reach out to morphogen-sending cells. Once the gradient has formed, cells must distinguish small differences in morphogen concentration and store this information even after the gradient has dissipated. This is often achieved by translating ligand concentration into a proportional increase in numbers of activated cell surface receptors that are internalized and continue to signal from endosomal compartments. Ultimately, this leads to activation of one or a few transcription factors that transduce this information into qualitatively distinct gene responses inside the nucleus.

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.

Fez function is required to maintain the size of the animal plate in the sea urchin embryo

Partitioning ectoderm precisely into neurogenic and non-neurogenic regions is an essential step for neurogenesis of almost all bilaterian embryos. Although it is widely accepted that antagonism between BMP and its inhibitors primarily sets up the border between these two types of ectoderm, it is unclear how such extracellular, diffusible molecules create a sharp and precise border at the single-cell level. Here, we show that Fez, a zinc finger protein, functions as an intracellular factor attenuating BMP signaling specifically within the neurogenic region at the anterior end of sea urchin embryos, termed the animal plate. When Fez function is blocked, the size of this neurogenic ectoderm becomes smaller than normal. However, this reduction is rescued in Fez morphants simply by blocking BMP2/4 translation, indicating that Fez maintains the size of the animal plate by attenuating BMP2/4 function. Consistent with this, the gradient of BMP activity along the aboral side of the animal plate, as measured by pSmad1/5/8 levels, drops significantly in cells expressing Fez and this steep decline requires Fez function. Our data reveal that this neurogenic ectoderm produces an intrinsic system that attenuates BMP signaling to ensure the establishment of a stable, well-defined neural territory, the animal plate.

Organizing the DV axis during planarian regeneration

During regeneration, lost structures are rebuilt and perfectly integrated within the remaining non-injured tissues. This fascinating process captured the attention of one of the founders of modern genetics, T.H. Morgan. He was particularly interested in understanding regeneration in freshwater planarians, which can regenerate a whole animal from a small piece of their bodies. He performed numerous experiments to understand how polarity is re-established such that an anterior-facing wound regenerates a head whereas a posterior-facing wound regenerates a tail. However, it has not been until more than 100 years later that the molecules required to determine axial polarity have been identified. Several studies have now shown that the Wnt/β-catenin and Hedgehog pathways are required for anteroposterior axis specification, whereas the establishment of the planarian dorsoventral (DV) axis relies on the Bone Morphogenetic Protein (BMP) pathway. Two recent papers have now uncovered additional conserved (anti-dorsalizing morphogenetic protein) and novel (noggin-like genes) elements that regulate planarian DV axis regeneration. Here, we summarize those results and present new data and hypotheses to explain the role that noggin-like genes might play.

Dorsal-ventral patterning: Crescent is a dorsally secreted Frizzled-related protein that competitively inhibits Tolloid proteases

In Xenopus, dorsal-ventral (D-V) patterning can self-regulate after embryo bisection. This is mediated by an extracellular network of proteins secreted by the dorsal and ventral centers of the gastrula. Different proteins of similar activity can be secreted at these two poles, but under opposite transcriptional control. Here we show that Crescent, a dorsal protein, can compensate for the loss of Sizzled, a ventral protein. Crescent is a secreted Frizzled-Related Protein (sFRP) known to regulate Wnt8 and Wnt11 activity. We now find that Crescent also regulates the BMP pathway. Crescent expression was increased by the BMP antagonist Chordin and repressed by BMP4, while the opposite was true for Sizzled. Crescent knock-down increased the expression of BMP target genes, and synergized with Sizzled morpholinos. Thus, Crescent loss-of-function is compensated by increased expression of its ventral counterpart Sizzled. Crescent overexpression dorsalized whole embryos but not ventral half-embryos, indicating that Crescent requires a dorsal component to exert its anti-BMP activity. Crescent protein lost its dorsalizing activity in Chordin-depleted embryos. When co-injected, Crescent and Chordin proteins greatly synergized in the dorsalization of Xenopus embryos. The molecular mechanism of these phenotypes is explained by the ability of Crescent to inhibit Tolloid metalloproteinases, which normally degrade Chordin. Enzyme kinetic studies showed that Crescent was a competitive inhibitor of Tolloid activity, which bound to Tolloid/BMP1 with a K(D) of 11 nM. In sum, Crescent is a new component of the D-V pathway, which functions as the dorsal counterpart of Sizzled, through the regulation of chordinases of the Tolloid family.

Conservation and diversification of an ancestral chordate gene regulatory network for dorsoventral patterning

Formation of a dorsoventral axis is a key event in the early development of most animal embryos. It is well established that bone morphogenetic proteins (Bmps) and Wnts are key mediators of dorsoventral patterning in vertebrates. In the cephalochordate amphioxus, genes encoding Bmps and transcription factors downstream of Bmp signaling such as Vent are expressed in patterns reminiscent of those of their vertebrate orthologues. However, the key question is whether the conservation of expression patterns of network constituents implies conservation of functional network interactions, and if so, how an increased functional complexity can evolve. Using heterologous systems, namely by reporter gene assays in mammalian cell lines and by transgenesis in medaka fish, we have compared the gene regulatory network implicated in dorsoventral patterning of the basal chordate amphioxus and vertebrates. We found that Bmp but not canonical Wnt signaling regulates promoters of genes encoding homeodomain proteins AmphiVent1 and AmphiVent2. Furthermore, AmphiVent1 and AmphiVent2 promoters appear to be correctly regulated in the context of a vertebrate embryo. Finally, we show that AmphiVent1 is able to directly repress promoters of AmphiGoosecoid and AmphiChordin genes. Repression of genes encoding dorsal-specific signaling molecule Chordin and transcription factor Goosecoid by Xenopus and zebrafish Vent genes represents a key regulatory interaction during vertebrate axis formation. Our data indicate high evolutionary conservation of a core Bmp-triggered gene regulatory network for dorsoventral patterning in chordates and suggest that co-option of the canonical Wnt signaling pathway for dorsoventral patterning in vertebrates represents one of the innovations through which an increased morphological complexity of vertebrate embryo is achieved.

Positional information and patterning revisited

The concept of positional information proposes that cells acquire positional values as in a coordinate system, which they interpret by developing in particular ways to give rise to spatial patterns. Some of the best evidence for positional information comes from regeneration experiments, and the patterning of the leg and antenna in Drosophila and the vertebrate limb. Central problems are how positional information is set up, how it is recorded, and then how it is interpreted by the cells. A number of models have been proposed for the setting up of positional gradients, and most are based on diffusion of a morphogen and its interactions with extracellular molecules. It is argued that diffusion may not be reliable mechanism. There are also mechanisms based on timing. There is no good evidence for the quantitative aspects of any of the gradients and details how they are set up. The way in which a signalling gradient regulates differential gene expression in a concentration-dependent manner also raises several mechanistic issues.

Mechanisms driving neural crest induction and migration in the zebrafish and Xenopus laevis

The neural crest is an evolutionary adaptation, with roots in the formation of mesoderm. Modification of neural crest behavior has been is critical for the evolutionary diversification of the vertebrates and defects in neural crest underlie a range of human birth defects. There has been a tremendous increase in our knowledge of the molecular, cellular, and inductive interactions that converge on defining the neural crest and determining its behavior. While there is a temptation to look for simple models to explain neural crest behavior, the reality is that the system is complex in its circuitry. In this review, our goal is to identify the broad features of neural crest origins (developmentally) and migration (cellularly) using data from the zebrafish (teleost) and Xenopus laevis (tetrapod amphibian) in order to illuminate where general mechanisms appear to be in play, and equally importantly, where disparities in experimental results suggest areas of profitable study.

Crossveinless-2 is required for the relocalization of Chordin protein within the vertebral field in mouse embryos

Bone morphogenetic proteins (BMPs), as well as the BMP-binding molecules Chordin (Chd), Crossveinless-2 (CV2) and Twisted Gastrulation (Tsg), are essential for axial skeletal development in the mouse embryo. We previously reported a strong genetic interaction between CV2 and Tsg and proposed a role for this interaction in the shaping of the BMP morphogenetic field during vertebral development. In the present study we investigated the roles of CV2 and Chd in the formation of the vertebral morphogenetic field. We performed immunostainings for CV2 and Chd protein on wild-type, CV2(-/-) or Chd(-/-) mouse embryo sections at the stage of onset of the vertebral phenotypes. By comparing mRNA and protein localizations we found that CV2 does not diffuse away from its place of synthesis, the vertebral body. The most interesting finding of this study was that Chd synthesized in the intervertebral disc accumulates in the vertebral body. This relocalization does not take place in CV2(-/-) mutants. Instead, Chd was found to accumulate at its site of synthesis in CV2(-/-) embryos. These results indicate a CV2-dependent flow of Chd protein from the intervertebral disc to the vertebral body. Smad1/5/8 phosphorylation was decreased in CV2(-/-)vertebral bodies. This impaired BMP signaling may result from the decreased levels of Chd/BMP complexes diffusing from the intervertebral region. The data indicate a role for CV2 and Chd in the establishment of the vertebral morphogenetic field through the long-range relocalization of Chd/BMP complexes. The results may have general implications for the formation of embryonic organ-forming morphogenetic fields.

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.

Enzymatic regulation of pattern: BMP4 binds CUB domains of Tolloids and inhibits proteinase activity

In Xenopus embryos, a dorsal-ventral patterning gradient is generated by diffusing Chordin/bone morphogenetic protein (BMP) complexes cleaved by BMP1/Tolloid metalloproteinases in the ventral side. We developed a new BMP1/Tolloid assay using a fluorogenic Chordin peptide substrate and identified an unexpected negative feedback loop for BMP4, in which BMP4 inhibits Tolloid enzyme activity noncompetitively. BMP4 binds directly to the CUB (Complement 1r/s, Uegf [a sea urchin embryonic protein] and BMP1) domains of BMP1 and Drosophila Tolloid with high affinity. Binding to CUB domains inhibits BMP4 signaling. These findings provide a molecular explanation for a long-standing genetical puzzle in which antimorphic Drosophila tolloid mutant alleles displayed anti-BMP effects. The extensive Drosophila genetics available supports the relevance of the interaction described here at endogenous physiological levels. Many extracellular proteins contain CUB domains; the binding of CUB domains to BMP4 suggests a possible general function in binding transforming growth factor-beta (TGF-beta) superfamily members. Mathematical modeling indicates that feedback inhibition by BMP ligands acts on the ventral side, while on the dorsal side the main regulator of BMP1/Tolloid enzymatic activity is the binding to its substrate, Chordin.

Models for the generation and interpretation of gradients

Source regions for morphogen gradients-organizing regions-can be generated if a local self-enhancing reaction is coupled with a long-ranging reaction that acts antagonistically. Resulting gradients can be translated into patterns of stable gene activities using genes whose products have a positive feedback on the activation on themselves. If several autoregulatory genes compete with each other for activity, cells make an unequivocal choice. Although the signal consists of a smoothly graded distribution, the all-or-nothing response of the cells leads to regions of differently determined cells that are delimited by sharp borders. In some systems, it is not the absolute but the relative level of a gradient that matters. The sequence of head, tentacles, and foot formation in hydra is controlled by a head activation gradient and is an example of this widely used but conceptually rather neglected mode. For subpatterns such as legs and wings, different "compartments" cooperate to produce new signaling substances. Here, morphogen production is restricted to the common borders or where they intersect. The model accounts for the formation of substructures in pairs at the correct positions within the embryo and for the correct orientation and handedness with respect to the main body axes.