Structural and functional analysis of the BMP-4 promoter in early embryos of Xenopus laevis

Regulation of long-range BMP gradients and embryonic polarity by propagation of local calcium-firing activity

Many amniote vertebrate species including humans can form identical twins from a single embryo, but this only occurs rarely. It has been suggested that the primitive-streak-forming embryonic region emits signals that inhibit streak formation elsewhere but the signals involved, how they are transmitted and how they act has not been elucidated. Here we show that short tracks of calcium firing activity propagate through extraembryonic tissue via gap junctions and prevent ectopic primitive streak formation in chick embryos. Cross-regulation of calcium activity and an inhibitor of primitive streak formation (Bone Morphogenetic Protein, BMP) via NF-κB and NFAT establishes a long-range BMP gradient spanning the embryo. This mechanism explains how embryos of widely different sizes can maintain positional information that determines embryo polarity. We provide evidence for similar mechanisms in two different human embryo models and in Drosophila, suggesting an ancient evolutionary origin.

'Neighbourhood watch' model: embryonic epiblast cells assess positional information in relation to their neighbours

In many developing and regenerating systems, tissue pattern is established through gradients of informative morphogens, but we know little about how cells interpret these. Using experimental manipulation of early chick embryos, including misexpression of an inducer (VG1 or ACTIVIN) and an inhibitor (BMP4), we test two alternative models for their ability to explain how the site of primitive streak formation is positioned relative to the rest of the embryo. In one model, cells read morphogen concentrations cell-autonomously. In the other, cells sense changes in morphogen status relative to their neighbourhood. We find that only the latter model can account for the experimental results, including some counter-intuitive predictions. This mechanism (which we name the ‘neighbourhood watch’ model) illuminates the classic ‘French Flag Problem’ and how positional information is interpreted by a sheet of cells in a large developing system.

Summary: In a large developing system, the chick embryo before gastrulation, cells may interpret gradients of positional signals relative to their neighbours to position the primitive streak, establishing bilateral symmetry.

Foxd4l1.1 negatively regulates transcription of neural repressor ventx1.1 during neuroectoderm formation in Xenopus embryos

Neuroectoderm formation is the first step in development of a proper nervous system for vertebrates. The developmental decision to form a non-neural ectoderm versus a neural one involves the regulation of BMP signaling, first reported many decades ago. However, the precise regulatory mechanism by which this is accomplished has not been fully elucidated, particularly for transcriptional regulation of certain key transcription factors. BMP4 inhibition is a required step in eliciting neuroectoderm from ectoderm and Foxd4l1.1 is one of the earliest neural genes highly expressed in the neuroectoderm and conserved across vertebrates, including humans. In this work, we focused on how Foxd4l1.1 downregulates the neural repressive pathway. Foxd4l1.1 inhibited BMP4/Smad1 signaling and triggered neuroectoderm formation in animal cap explants of Xenopus embryos. Foxd4l1.1 directly bound within the promoter of endogenous neural repressor ventx1.1 and inhibited ventx1.1 transcription. Foxd4l1.1 also physically interacted with Xbra in the nucleus and inhibited Xbra-induced ventx1.1 transcription. In addition, Foxd4l1.1 also reduced nuclear localization of Smad1 to inhibit Smad1-mediated ventx1.1 transcription. Foxd4l1.1 reduced the direct binding of Xbra and Smad1 on ventx1.1 promoter regions to block Xbra/Smad1-induced synergistic activation of ventx1.1 transcription. Collectively, Foxd4l1.1 negatively regulates transcription of a neural repressor ventx1.1 by multiple mechanisms in its exclusively occupied territory of neuroectoderm, and thus leading to primary neurogenesis. In conjunction with the results of our previous findings that ventx1.1 directly represses foxd4l1.1, the reciprocal repression of ventx1.1 and foxd4l1.1 is significant in at least in part specifying the mechanism for the non-neural versus neural ectoderm fate determination in Xenopus embryos.

Bone morphogenetic proteins: New insights into their roles and mechanisms in CNS development, pathology and repair

Bone morphogenetic proteins (BMPs) are a highly conserved and diverse family of proteins that play essential roles in various stages of development including the formation and patterning of the central nervous system (CNS). Bioavailability and function of BMPs are regulated by input from a plethora of transcription factors and signaling pathways. Intriguingly, recent literature has uncovered novel roles for BMPs in regulating homeostatic and pathological responses in the adult CNS. Basal levels of BMP ligands and receptors are widely expressed in the adult brain and spinal cord with differential expression patterns across CNS regions, cell types and subcellular locations. Recent evidence indicates that several BMP isoforms are transiently or chronically upregulated in the aged or pathological CNS. Genetic knockout and pharmacological studies have elucidated that BMPs regulate several aspects of CNS injury and repair including cell survival and differentiation, reactive astrogliosis and glial scar formation, axon regeneration, and myelin preservation and repair. Several BMP isoforms can be upregulated in the injured or diseased CNS simultaneously yet exert complementary or opposing effects on the endogenous cell responses after injury. Emerging studies also show that dysregulation of BMPs is associated with various CNS pathologies. Interestingly, modulation of BMPs can lead to beneficial or detrimental effects on CNS injury and repair mechanisms in a ligand, temporally or spatially specific manner, which reflect the complexity of BMP signaling. Given the significance of BMPs in neurodevelopment, a better understanding of their role in the context of injury may provide new therapeutic targets for the pathologic CNS. This review will provide a timely overview on the foundation and recent advancements in knowledge regarding the role and mechanisms of BMP signaling in the developing and adult CNS, and their implications in pathological responses and repair processes after injury or diseases.

Rejuvenation of three germ layers tissues by exchanging old blood plasma with saline-albumin

Heterochronic blood sharing rejuvenates old tissues, and most of the studies on how this works focus on young plasma, its fractions, and a few youthful systemic candidates. However, it was not formally established that young blood is necessary for this multi-tissue rejuvenation. Here, using our recently developed small animal blood exchange process, we replaced half of the plasma in mice with saline containing 5% albumin (terming it a "neutral" age blood exchange, NBE) thus diluting the plasma factors and replenishing the albumin that would be diminished if only saline was used. Our data demonstrate that a single NBE suffices to meet or exceed the rejuvenative effects of enhancing muscle repair, reducing liver adiposity and fibrosis, and increasing hippocampal neurogenesis in old mice, all the key outcomes seen after blood heterochronicity. Comparative proteomic analysis on serum from NBE, and from a similar human clinical procedure of therapeutic plasma exchange (TPE), revealed a molecular re-setting of the systemic signaling milieu, interestingly, elevating the levels of some proteins, which broadly coordinate tissue maintenance and repair and promote immune responses. Moreover, a single TPE yielded functional blood rejuvenation, abrogating the typical old serum inhibition of progenitor cell proliferation. Ectopically added albumin does not seem to be the sole determinant of such rejuvenation, and levels of albumin do not decrease with age nor are increased by NBE/TPE. A model of action (supported by a large body of published data) is that significant dilution of autoregulatory proteins that crosstalk to multiple signaling pathways (with their own feedback loops) would, through changes in gene expression, have long-lasting molecular and functional effects that are consistent with our observations. This work improves our understanding of the systemic paradigms of multi-tissue rejuvenation and suggest a novel and immediate use of the FDA approved TPE for improving the health and resilience of older people.

Cooperative interaction of BMP signalling and Foxn1 gene dosage determines the size of the functionally active thymic epithelial compartment

Thymopoiesis strictly depends on the function of the Foxn1 transcription factor that is expressed in the thymic epithelium. During embryonic development, initial expression of the Foxn1 gene is induced in the pharyngeal endoderm by mesenchyme-derived BMP4 signals. Here, by engineering a time-delayed feedback system of BMP inhibition in mouse embryos, we demonstrate that thymopoiesis irreversibly fails if Foxn1 gene expression does not occur during a defining time span in mid-gestation. We also reveal an epistatic interaction between the extent of BMP signalling and the gene dosage of Foxn1. Our findings illustrate the complexities of the early steps of thymopoiesis and indicate that sporadic forms of thymic hypoplasia in humans may result from the interaction of genes affecting the magnitude of BMP signalling and Foxn1 expression.

A novel TGFβ modulator that uncouples R-Smad/I-Smad-mediated negative feedback from R-Smad/ligand-driven positive feedback

As some of the most widely utilised intercellular signalling molecules, transforming growth factor β (TGFβ) superfamily members play critical roles in normal development and become disrupted in human disease. Establishing appropriate levels of TGFβ signalling involves positive and negative feedback, which are coupled and driven by the same signal transduction components (R-Smad transcription factor complexes), but whether and how the regulation of the two can be distinguished are unknown. Genome-wide comparison of published ChIP-seq datasets suggests that LIM domain binding proteins (Ldbs) co-localise with R-Smads at a substantial subset of R-Smad target genes including the locus of inhibitory Smad7 (I-Smad7), which mediates negative feedback for TGFβ signalling. We present evidence suggesting that zebrafish Ldb2a binds and directly activates the I-Smad7 gene, whereas it binds and represses the ligand gene, Squint (Sqt), which drives positive feedback. Thus, the fine tuning of TGFβ signalling derives from positive and negative control by Ldb2a. Expression of ldb2a is itself activated by TGFβ signals, suggesting potential feed-forward loops that might delay the negative input of Ldb2a to the positive feedback, as well as the positive input of Ldb2a to the negative feedback. In this way, precise gene expression control by Ldb2a enables an initial build-up of signalling via a fully active positive feedback in the absence of buffering by the negative feedback. In Ldb2a-deficient zebrafish embryos, homeostasis of TGFβ signalling is perturbed and signalling is stably enhanced, giving rise to excess mesoderm and endoderm, an effect that can be rescued by reducing signalling by the TGFβ family members, Nodal and BMP. Thus, Ldb2a is critical to the homeostatic control of TGFβ signalling and thereby embryonic patterning.

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.

Pescadillo homologue 1 and Peter Pan function during Xenopus laevis pronephros development

BACKGROUND INFORMATION

pes1 (pescadillo homologue 1) and ppan (Peter Pan) are multifunctional proteins involved in ribosome biogenesis, cell proliferation, apoptosis, cell migration and regulation of gene expression. Both proteins are required for early neural development in Xenopus laevis, as previously demonstrated.

RESULTS

We show that the expression of both genes in the developing pronephros depends on wnt4 and fzd3 (frizzled homologue 3) function. Loss of pes1 or ppan by MO (morpholino oligonucleotide)-based knockdown approaches resulted in strong malformations during pronephric tubule formation. Defects were already notable during specification of pronephric progenitor cells, as shown by lhx1 expression. Moreover, we demonstrated that Xenopus pes1 and ppan interact physically and functionally and that pes1 and ppan can cross-rescue the loss of function phenotype of one another. Interference with rRNA synthesis, however, did not result in a similar early pronephros phenotype.

CONCLUSION

These results demonstrate that pes1 and ppan are required for Xenopus pronephros development and indicate that their function in the pronephros is independent of their role in ribosome biosynthesis.

XSip1 neuralizing activity involves the co-repressor CtBP and occurs through BMP dependent and independent mechanisms

The DNA-binding transcription factor Smad-interacting protein-1 (Sip1) (also named Zfhx1b/ZEB2) plays essential roles in vertebrate embryogenesis. In Xenopus, XSip1 is essential at the gastrula stage for neural tissue formation, but the precise molecular mechanisms that underlie this process have not been fully identified yet. Here we show that XSip1 functions as a transcriptional repressor during neural induction. We observed that constitutive activation of BMP signaling prevents neural induction by XSip1 but not the inhibition of several epidermal genes. We provide evidence that XSip1 binds directly to the BMP4 proximal promoter and modulates its activity. Finally, by deletion and mutational analysis, we show that XSip1 possesses multiple repression domains and that CtBPs contribute to its repression activity. Consistent with this, interference with XCtBP function reduced XSip1 neuralizing activity. These results suggest that Sip1 acts in neural tissue formation through direct repression of BMP4 but that BMP-independent mechanisms are involved as well. Our data also provide the first demonstration of the importance of CtBP binding in Sip1 transcriptional activity in vivo.

Extracellular modulation of BMP activity in patterning the dorsoventral axis

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

Noggin antagonism of BMP4 signaling controls development of the axial skeleton in the mouse

The interaction between bone morphogenetic proteins (BMPs) and their antagonist, Noggin, is critical for normal development. Noggin null mice die at birth with a severely malformed skeleton that is postulated to reflect the activity of unopposed BMP signaling. However, the widespread expression and redundancy of different BMPs have made it difficult to identify a specific role for individual BMPs during mammalian skeletal morphogenesis. Here, we report the effects of modifying Bmp4 dosage on the skeletal development of Noggin mutant mice. The reduction of Bmp4 dosage results in an extensive rescue of the axial skeleton of Noggin mutant embryos. In contrast, the appendicular skeletal phenotype of Noggin mutants was unchanged. Analysis of molecular markers of somite formation and somite patterning suggests that the loss of Noggin results in the formation of small mispatterned somites. Mis-specification and growth retardation rather than cell death most likely account for the subsequent reduction or loss of axial skeletal structures. The severe Noggin phenotype correlates with Bmp4-dependent ectopic expression of Bmp4 in the paraxial mesoderm consistent with Noggin antagonizing an auto-inductive feed-forward mechanism. Thus, specific interactions between Bmp4 and Noggin in the early embryo are critical for establishment and patterning of the somite and subsequent axial skeletal morphogenesis.

Coactivator recruitment is essential for liganded thyroid hormone receptor to initiate amphibian metamorphosis

Thyroid hormone receptors (TRs) can repress or activate target genes depending on the absence or presence of thyroid hormone (T3), respectively. This hormone-dependent gene regulation is mediated by recruitment of co-repressors in the absence of T3 and coactivators in its presence. Many TR-interacting coactivators have been characterized in vitro. In comparison, few studies have addressed the developmental roles of these cofactors in vivo. We have investigated the role of coactivators in transcriptional activation by TR during postembryonic tissue remodeling by using amphibian metamorphosis as a model system. We have previously shown that steroid receptor coactivator 3 (SRC3) is expressed and upregulated during metamorphosis, suggesting a role in gene regulation by liganded TR. Here, we have generated transgenic tadpoles expressing a dominant negative form of SRC3 (F-dnSRC3). The transgenic tadpoles exhibited normal growth and development throughout embryogenesis and premetamorphic stages. However, transgenic expression of F-dnSRC3 inhibits essentially all aspects of T3-induced metamorphosis, as well as natural metamorphosis, leading to delayed or arrested metamorphosis or the formation of tailed frogs. Molecular analysis revealed that F-dnSRC3 functioned by blocking the recruitment of endogenous coactivators to T3 target genes without affecting corepressor release, thereby preventing the T3-dependent gene regulation program responsible for tissue transformations during metamorphosis. Our studies thus demonstrate that coactivator recruitment, aside from corepressor release, is required for T3 function in development and further provide the first example where a specific coactivator-dependent gene regulation pathway by a nuclear receptor has been shown to underlie specific developmental events.

Tissue- and Gene-specific Recruitment of Steroid Receptor Coactivator-3 by Thyroid Hormone Receptor during Development

Numerous coactivators that bind nuclear hormone receptors have been isolated and characterized in vitro. Relatively few studies have addressed the developmental roles of these cofactors in vivo. By using the total dependence of amphibian metamorphosis on thyroid hormone (T3) as a model, we have investigated the role of steroid receptor coactivator 3 (SRC3) in gene activation by thyroid hormone receptor (TR) in vivo. First, expression analysis showed that SRC3 was expressed in all tadpole organs analyzed. In addition, during natural as well as T3-induced metamorphosis, SRC3 was up-regulated in both the tail and intestine, two organs that undergo extensive transformations during metamorphosis and the focus of the current study. We then performed chromatin immunoprecipitation assays to investigate whether SRC3 is recruited to endogenous T3 target genes in vivo in developing tadpoles. Surprisingly, we found that SRC3 was recruited in a gene- and tissue-dependent manner to target genes by TR, both upon T3 treatment of premetamorphic tadpoles and during natural metamorphosis. In particular, in the tail, SRC3 was not recruited in a T3-dependent manner to the target TRbetaA promoter, suggesting either no recruitment or constitutive association. Finally, by using transgenic tadpoles expressing a dominant negative SRC3 (F-dnSRC3), we demonstrated that F-dnSRC3 was recruited in a T3-dependent manner in both the intestine and tail, blocking the recruitment of endogenous coactivators and histone acetylation. These results suggest that SRC3 is utilized in a gene- and tissue-specific manner by TR during development.

The POU factor Oct-25 regulates the Xvent-2B gene and counteracts terminal differentiation in Xenopus embryos

The Xvent-2B promoter is regulated by a BMP-2/4-induced transcription complex comprising Smad signal transducers and specific transcription factors. Using a yeast one-hybrid screen we have found that Oct-25, a Xenopus POU domain protein related to mammalian Oct-3/4, binds as an additional factor to the Xvent-2B promoter. This interaction was further confirmed by both in vitro and in vivo analyses. The Oct-25 gene is mainly transcribed during blastula and gastrula stages in the newly forming ectodermal and mesodermal germ layers. Luciferase reporter gene assay demonstrated that Oct-25 stimulates transcription of the Xvent-2B gene. This stimulation depends on the Oct-25 binding site and the bone morphogenetic protein-responsive element. Furthermore, Oct-25 interacts in vitro with components of the Xvent-2B transcription complex, like Smad1/4 and Xvent-2. Overexpression of Oct-25 results in anterior/posterior truncations and lack of differentiation for neuroectoderm- and mesoderm-derived tissues including blood cells. This effect is consistent with an evolutionarily conserved role of class V POU factors in the maintenance of an undifferentiated cell state. In Xenopus, the molecular mechanism underlying this process might be coupled to the expression of Xvent proteins.

Fgf signalling controls the dorsoventral patterning of the zebrafish embryo

The establishment of dorsoventral (DV) patterning in vertebrate embryos depends on the morphogenic activity of a group of Tgfβ superfamily members, the bone morphogenetic proteins (Bmps) (which specify ventral cell fates), and on their interaction with their dorsally secreted cognate inhibitors chordin and noggin. In the zebrafish, genetic analysis has revealed that Bmp2b and Bmp7, as well as their antagonist chordin, are required for proper DV patterning. The expression of Bmp genes is initially activated in the whole blastula. Well before the beginning of gastrulation, Bmp gene expression progressively disappears from the dorsal side to become restricted to the ventral part of the embryo. We show that this early restriction of Bmp gene expression, which occurs independently of noggin and chordin, is an essential step in the establishment of DV patterning. The progressive ventral restriction of Bmp gene transcripts is coincident with the spreading of Fgf activity from the dorsal side of the embryo, suggesting that Fgf signalling is implicated in dorsal downregulation of Bmp gene expression. In accordance with this, activation of the Fgf/Ras/Mapk-signalling pathway inhibits ventral Bmp gene expression, thereby causing a dorsalisation of the embryo. Conversely,inhibition of Fgf signalling causes Bmp gene expression to expand dorsally,leading to an expansion of ventral cell fates. In accordance with an important role of Fgf signalling in the DV patterning of the zebrafish, we show that loss of Fgf8 function enhances the ventralisation of chordin-deficient embryos. Our results thereby demonstrate that pre-gastrula stage Fgf-signalling is essential to delimit the expression domain of the genes encoding the functional morphogen of the dorsoventral axis of the early zebrafish embryo.

A dominant-negative thyroid hormone receptor blocks amphibian metamorphosis by retaining corepressors at target genes

The total dependence of amphibian metamorphosis on thyroid hormone (T(3)) provides a unique vertebrate model for studying the molecular mechanism of T(3) receptor (TR) function in vivo. In vitro transcription and developmental expression studies have led to a dual function model for TR in amphibian development, i.e., TRs act as transcriptional repressors in premetamorphic tadpoles and as activators during metamorphosis. We examined molecular mechanisms of TR action in T3-induced metamorphosis by using dominant-negative receptors (dnTR) ubiquitously expressed in transgenic Xenopus laevis. We showed that T(3)-induced activation of T(3) target genes and morphological changes are blocked in dnTR transgenic animals. By using chromatin immunoprecipitation, we show that dnTR bound to target promoters, which led to retention of corepressors and continued histone deacetylation in the presence of T(3). These results thus provide direct in vivo evidence for the first time for a molecular mechanism of altering gene expression by a dnTR. The correlation between dnTR-mediated gene repression and inhibition of metamorphosis also supports a key aspect of the dual function model for TR in development: during T(3)-induced metamorphosis, TR functions as an activator via release of corepressors and promotion of histone acetylation and gene activation.

Pitx1 in vivo promoter activity and mechanisms of positive autoregulation

During early mouse embryogenesis, Pitx1 (pituitary homeobox 1), a member of the bicoid subgroup of PAIRED homeobox-containing transcription factors, marks the stomodeum, oral ectoderm, pituitary and first branchial arch in the anterior part of the embryo and lateral plate mesoderm only in the posterior half of the embryo. We have now defined PITX1 promoter fragments that mimic the anterior but not posterior expression of PITX1 in transgenic mice. In addition, we show positive regulation of this promoter in transfection studies by three members of the Pitx1 family (Pitx1, Pitx1b, Pitx2), as well as by a related factor, Otx1. PITX1 autoregulation depends on DNA-binding and trans-activation domains of Pitx1 and it may be responsible for establishment and/or maintenance of the Pitx1 expression domain.

Transcriptional activation of BMP-4 and regulation of mammalian organogenesis by GATA-4 and -6

Transcription factors GATA-4, -5, and -6 constitute an evolutionary conserved subfamily of vertebrate zinc finger regulators highly expressed in the developing heart and gut. Genetic evidence suggests that each protein is essential for embryonic development, but their exact functions are not fully elucidated. Moreover, because all three proteins share similar transcriptional properties in vitro, and because transcripts for two or more GATA genes are present in similar tissues, the molecular basis underlying in vivo specificity of GATA factors remains undefined. Knowledge of the exact cell types expressing each protein and identification of downstream targets would greatly help define their function. We have used high-resolution immunohistochemistry to precisely determine the cellular distribution of the GATA-4, -5, and -6 proteins in murine embryogenesis. The results reveal novel sites of expression in mesodermal and ectodermal cells. In particular, GATA-4 and -6 expression was closely associated with yolk sac vasculogenesis and early endoderm-mesoderm signaling. Additionally, GATA-6 was strongly expressed in the embryonic ectoderm, neural tube, and neural crest-derived cells. This pattern of expression closely paralled that of BMP-4, and the BMP-4 gene was identified as a direct downstream target for GATA-4 and -6. These findings offer new insight into the function of GATA-4 and -6 during early stages of embryogenesis and reveal the existence of a positive cross-regulatory loop between BMP-4 and GATA-4. They also raise the possibility that part of the early defects in GATA-4 and/or GATA-6 null embryos may be due to impaired BMP-4 signaling.

Xolloid-related: a novel BMP1/Tolloid-related metalloprotease is expressed during early Xenopus development

We have identified a novel Tolloid-like metalloprotease, called Xolloid-related (Xlr), that is expressed during early Xenopus development. Transcripts for xlr are localized to the marginal zone of mid-gastrulae and are most abundant in ventral and lateral sectors. At neurula stages xlr is strongly expressed around the blastopore and in the pharyngeal endoderm, and more weakly expressed throughout the ventral half of the embryo. Transcripts are detected in the nervous system, particularly the hindbrain and spinal cord, and tailbud of tailbud stage embryos, with weaker expression in the anterior nervous system, otic vesicle, heart, and pronephric duct. Transcription of xlr is increased by BMP4 and decreased by Noggin and tBR, indicating that xlr is regulated by BMP signalling. Injection of xlr mRNA inhibits dorsoanterior development and the dorsal axis inducing ability of coinjected chordin, but not noggin or tBR, mRNA. Xlr conditioned media cleaves Chordin in vitro, indicating that this protease may regulate the availability of Chordin in vivo.

Structure and expression of Xenopus tropicalis BMP-2 and BMP-4 genes

The Xenopus tropicalis BMP-2 and BMP-4 genes have been cloned and sequenced. A comparison with the corresponding genes from X. laevis reveals that the BMP-4 genes are conserved at a higher extent than the BMP-2 genes. This is especially evident for the intron sequences, but is also reflected by the exon sequences. While the amino acids of X. tropicalis and X. laevis BMP-4 proteins diverge at about 4%, those of BMP-2 proteins diverge at about 7%. By reverse transcriptase polymerase chain reaction analyses and whole mount in situ hybridizations, we demonstrate the temporal and spatial expression patterns of X. tropicalis BMP-2 and BMP-4 genes. BMP-2 is found to be expressed maternally, and later in development, in migrating heart progenitor cells as well as in the final heart, within the pineal gland and the olfactory placodes. BMP-4 is zygotically activated within the ventral marginal zone and later found in the eye, the otic vesicle, the heart and within blood islands. Although the overall patterns are very similar to those found in X. laevis, there is some distinct difference which might result from the accelerated development in X. tropicalis.

Pluripotent cells (stem cells) and their determination and differentiation in early vertebrate embryogenesis

Mammalian embryonic stem cells can be obtained from the inner cell mass of blastocysts or from primordial germ cells. These stem cells are pluripotent and can develop into all three germ cell layers of the embryo. Somatic mammalian stem cells, derived from adult or fetal tissues, are more restricted in their developmental potency. Amphibian ectodermal and endodermal cells lose their pluripotency at the early gastrula stage. The dorsal mesoderm of the marginal zone is determined before the mid-blastula transition by factors located after cortical rotation in the marginal zone, without induction by the endoderm. Secreted maternal factors (BMP, FGF and activins), maternal receptors and maternal nuclear factors (beta-catenin, Smad and Fast proteins), which form multiprotein transcriptional complexes, act together to initiate pattern formation. Following mid-blastula transition in Xenopus laevis (Daudin) embryos, secreted nodal-related (Xnr) factors become important for endoderm and mesoderm differentiation to maintain and enhance mesoderm induction. Endoderm can be induced by high concentrations of activin (vegetalizing factor) or nodal-related factors, especially Xnr5 and Xnr6, which depend on Wnt/beta-catenin signaling and on VegT, a vegetal maternal transcription factor. Together, these and other factors regulate the equilibrium between endoderm and mesoderm development. Many genes are activated and/or repressed by more than one signaling pathway and by regulatory loops to refine the tuning of gene expression. The nodal related factors, BMP, activins and Vg1 belong to the TGF-beta superfamily. The homeogenetic neural induction by the neural plate probably reinforces neural induction and differentiation. Medical and ethical problems of future stem cell therapy are briefly discussed.

Goosecoid promotes head organizer activity by direct repression of Xwnt8 in Spemann’s organizer

Formation of the vertebrate body plan is controlled by discrete head and trunk organizers that establish the anteroposterior pattern of the body axis. The Goosecoid (Gsc) homeodomain protein is expressed in all vertebrate organizers and has been implicated in the activity of Spemann’s organizer in Xenopus. The role of Gsc in organizer function was examined by fusing defined transcriptional regulatory domains to the Gsc homeodomain. Like native Gsc, ventral injection of an Engrailed repressor fusion (Eng-Gsc) induced a partial axis, while a VP16 activator fusion (VP16-Gsc) did not, indicating that Gsc functions as a transcriptional repressor in axis induction. Dorsal injection of VP16-Gsc resulted in loss of head structures anterior to the hindbrain, while axial structures were unaffected, suggesting a requirement for Gsc function in head formation. The anterior truncation caused by VP16-Gsc was fully rescued by Frzb, a secreted Wnt inhibitor, indicating that activation of ectopic Wnt signaling was responsible, at least in part, for the anterior defects. Supporting this idea, Xwnt8 expression was activated by VP16-Gsc in animal explants and the dorsal marginal zone, and repressed by Gsc in Activin-treated animal explants and the ventral marginal zone. Furthermore, expression of Gsc throughout the marginal zone inhibited trunk formation, identical to the effects of Frzb and other Xwnt8 inhibitors. A region of the Xwnt8 promoter containing four consensus homeodomain-binding sites was identified and this region mediated repression by Gsc and activation by VP16-Gsc, consistent with direct transcriptional regulation of Xwnt8 by Gsc. Therefore, Gsc promotes head organizer activity by direct repression of Xwnt8 in Spemann’s organizer and this activity is essential for anterior development.

Primitive erythropoiesis in theXenopusembryo: the synergistic role of LMO-2, SCL and GATA-binding proteins

Hematopoietic stem cells are derived from ventral mesoderm during vertebrate development. Gene targeting experiments in the mouse have demonstrated key roles for the basic helix-loop-helix transcription factor SCL and the GATA-binding protein GATA-1 in hematopoiesis. When overexpressed in Xenopus animal cap explants, SCL and GATA-1 are each capable of specifying mesoderm to become blood. Forced expression of either factor in whole embryos, however, does not lead to ectopic blood formation. This apparent paradox between animal cap assays and whole embryo phenotype has led to the hypothesis that additional factors are involved in specifying hematopoietic mesoderm. SCL and GATA-1 interact in a transcriptional complex with the LIM domain protein LMO-2. We have cloned the Xenopus homolog of LMO-2 and show that it is expressed in a similar pattern to SCL during development. LMO-2 can specify hematopoietic mesoderm in animal cap assays. SCL and LMO-2 act synergistically to expand the blood island when overexpressed in whole embryos. Furthermore, co-expression of GATA-1 with SCL and LMO-2 leads to embryos that are ventralized and have blood throughout the dorsal-ventral axis. The synergistic effect of SCL, LMO-2 and GATA-1, taken together with the findings that these factors can form a complex in vitro, suggests that this complex specifies mesoderm to become blood during embryogenesis.

The Wnt-activated Xiro1 gene encodes a repressor that is essential for neural development and downregulates Bmp4

In the early Xenopus embryo, the Xiro homeodomain proteins of the Iroquois (Iro) family control the expression of proneural genes and the size of the neural plate. We report that Xiro1 functions as a repressor that is strictly required for neural differentiation, even when the BMP4 pathway is impaired. We also show that Xiro1 and Bmp4 repress each other. Consistently, Xiro1 and Bmp4 have complementary patterns of expression during gastrulation. The expression of Xiro1 requires Wnt signaling. Thus, Xiro1 is probably a mediator of the known downregulation of Bmp4 by Wnt signaling.

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

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

The homeodomain transcription factor Xvent-2 mediates autocatalytic regulation of BMP-4 expression in Xenopus embryos

Like other genes of the transforming growth factor-beta family, the BMP-4 gene is regulated by an autocatalytic loop. In Xenopus embryos this loop can be ectopically induced by injection of BMP-2 RNA. However, cycloheximide treatment subsequent to BMP-2 overexpression revealed that BMP signaling is not direct but requires additional factor(s). As putative mediator we have identified Xvent-2 which is activated by BMP-2/4 signaling and, in turn, activates BMP-4 transcription. Using promoter/reporter assays we have delineated Xvent-2 responsive elements within the BMP-4 gene. We further demonstrate that Xvent-2 which has recently been characterized as a transcriptional repressor can also act, context dependent, as an activator binding two copies of a 5'-CTAATT-3' motif in the second intron of the BMP-4 gene. Replacement of Xvent-2 target sites within the goosecoid (gsc) promoter by the BMP-4 enhancer converts Xvent-2 caused repression of gsc to strong activation. This switch is obviously due to adjacent nucleotides probably binding a transcriptional co-activator interacting with Xvent-2. A model is presented describing the mechanism of BMP-4 gene activation in Xenopus embryos at the early gastrula stage.

c-Jun (AP-1) activates BMP-4 transcription in Xenopus embryos

Zygotic expression of the BMP-4 gene in Xenopus embryos is regulated by an auto-regulatory loop. Since AP-1 is known as a mediator of auto-regulatory loops both in the case of the Drosophila dpp and the mammalian TGF-beta genes, we have analysed the potential of Xenopus c-Jun (AP-1) as a mediator of BMP-4 expression during Xenopus development. RNA injection experiments revealed that both heteromeric c-Fos/c-Jun and homodimeric c-Jun/c-Jun strongly activate BMP-4 transcription, whereas BMP signaling was found to activate the Xenopus c-Jun gene only at a rather low extent. In addition, the lack of zygotic c-Jun transcripts until the end of gastrulation should exclude a role of AP-1 in the activation and the early expression of BMP-4 during gastrulation in vivo. However, at later stages of Xenopus development, we find a spatial overlap of c-Jun and BMP-4 transcripts which suggests that AP-1 might serve as an additional activatory component for the auto-regulation of BMP-4. Promoter/reporter and gel mobility shift assays demonstrate multiple responsive sites for AP-1 in the 5' flanking region and two in the second intron of the BMP-4 gene. We further demonstrate that AP-1 acts independently of Xvent-2 which has recently been shown to mediate the early expression of BMP-4 in gastrula stage embryos.

Regulation of the promoters for the human bone morphogenetic protein 2 and 4 genes

The bone morphogenetic proteins 2 and 4 are known to be important in bone formation and are expressed in both the developing and adult mammalian bone. Understanding the regulation of these genes in osteoblasts may yield methods by which we can control expression to induce bone formation. We have isolated and characterized the human BMP-2 and BMP-4 promoters and report substantially more upstream sequence information than that which has been published. Human osteoblasts were found to have a single transcript initiation site that is conserved across species, rather than multiple start sites, as has previously been reported (Feng, J.Q., Harris, M.A., Ghosh-Choudhury, N., Feng, M., Mundy, G.R., Harris, S.E., 1994. Structure and sequence of mouse morphogenetic protein-2 gene (BMP-2): comparison of the structures and promoter regions of BMP-2 and BMP-4 genes. Biochim. Biophys. Acta 1218, 221-224; Heller, L.C., Li, Y., Abrams, K.L., Rogers, M.B., 1999. Transcriptional regulation of the Bmp2 gene. J. Biol. Chem. 274, 1394-1400; Sugiura, T., 1999. Cloning and functional characterization of the 5'-flanking region of the human bone morphogenetic protein-2 gene. Biochem. J. 338, 433-440). A series of promoter deletions for both human BMP-2 and BMP-4 fused to the luciferase reporter gene were analyzed thoroughly in human and murine osteoblastic cell lines. Several compounds and growth factors that stimulate general or osteogenic pathways were used to treat cells transfected with the promoter constructs. Retinoic acid compounds and the phorbol ester, PMA were found to stimulate BMP-2 and, to a lesser degree, BMP-4. The combination of all trans-RA and PMA caused a synergistic increase in BMP-2 promoter activity and endogenous mRNA. The RA stimulation appears to be an indirect effect on the BMP-2 promoter, as the most highly conserved RRE in the BMP-2 promoter was unable to functionally bind or compete for protein binding. Potential binding sites in both promoters for the bone-specific transcription factor, Cbfa-1, were found to specifically bind Cbfa-1 protein in osteoblast nuclear extracts; however, deletion of these sites did not significantly affect transcriptional activity of the promoters in osteoblasts. These data thus present new sequence and regulatory information for the human BMP-2 and BMP-4 promoters and clarify the human BMP-2 gene transcriptional start site in osteoblasts.

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

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

Structure and expression of mGDF, a new member of the transforming growth factor-beta superfamily in the bivalve mollusc Crassostrea gigas

To gain insight into the evolution of the structure and functions of transforming growth factor (TGF)-beta superfamily members, a cDNA encoding a new member from the bivalve mollusc Crassostrea gigas named mGDF (molluscan growth and differentiation factor) was identified by PCR using degenerate primers. The mGDF precursor exhibits characteristic features of the TGF-beta superfamily and shows highest homology with human BMP2 and Drosophila DPP. Conversely, the mgdf gene displays a distinct pattern of expression during development. Indeed mgdf transcripts were not detected early in development but increased markedly before metamorphosis. These findings raise the possibility that mGDF could play a central role in the biological processes that allow larvae to become competent to metamorphose.

Equivalent genetic roles for bmp7/snailhouse and bmp2b/swirl in dorsoventral pattern formation

A bone morphogenetic protein (BMP) signaling pathway acts in the establishment of the dorsoventral axis of the vertebrate embryo. Here we demonstrate the genetic requirement for two different Bmp ligand subclass genes for dorsoventral pattern formation of the zebrafish embryo. From the relative efficiencies observed in Bmp ligand rescue experiments, conserved chromosomal synteny, and isolation of the zebrafish bmp7 gene, we determined that the strongly dorsalized snailhouse mutant phenotype is caused by a mutation in the bmp7 gene. We show that the original snailhouse allele is a hypomorphic mutation and we identify a snailhouse/bmp7 null mutant. We demonstrate that the snailhouse/bmp7 null mutant phenotype is identical to the presumptive null mutant phenotype of the strongest dorsalized zebrafish mutant swirl/bmp2b, revealing equivalent genetic roles for these two Bmp ligands. Double mutant snailhouse/bmp7; swirl/bmp2b embryos do not exhibit additional or stronger dorsalized phenotypes, indicating that these Bmp ligands do not function redundantly in early embryonic development. Furthermore, overexpression experiments reveal that Bmp2b and Bmp7 synergize in the ventralization of wild-type embryos through a cell-autonomous mechanism, suggesting that Bmp2b/Bmp7 heterodimers may act in vivo to specify ventral cell fates in the zebrafish embryo.

Bone morphogenetic protein-4 regulates its own expression in cultured osteoblasts

During development, bone morphogenetic proteins (BMPs) induce the differentiation of mesenchymal progenitor cells to enter into the osteoblastic lineage, and BMPs enhance osteoblastic function. BMPs and noggin, a specific binding protein that blocks BMP actions, are expressed by osteoblastic cells but there is limited information about regulation of BMP synthesis in skeletal cells. We tested for the expression and regulation of BMP-4 in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). BMP-4 caused a short-lived increase in BMP-4 mRNA followed by a marked inhibition of BMP-4 expression. The stimulatory effect was transcriptional, as determined by nuclear run-on assays, whereas the inhibitory effect was transcriptional and posttranscriptional, because longer BMP-4 exposure decreased its rate of transcription and shortened the half-life of BMP-4 mRNA in transcriptionally arrested Ob cells. BMP-2 and BMP-6 also inhibited BMP-4 mRNA levels. Transforming growth factor beta1 increased, whereas fibroblast growth factor-2, platelet-derived growth factor BB, and insulin-like growth factor I decreased BMP-4 mRNA in Ob cells. BMP-2 also was expressed by Ob cells and it was downregulated by BMP-2, BMP-4, and BMP-6. Noggin increased BMP-4 transcripts, suggesting autocrine control of BMP-4 expression. In conclusion, BMP-4 inhibits its own expression in Ob cells, a mechanism to limit BMP availability to osteoblasts.

The role of tolloid/mini fin in dorsoventral pattern formation of the zebrafish embryo

A highly conserved TGF-β signaling pathway is involved in the establishment of the dorsoventral axis of the vertebrate embryo. Specifically, Bone Morphogenetic Proteins (Bmps) pattern ventral tissues of the embryo while inhibitors of Bmps, such as Chordin, Noggin and Follistatin, are implicated in dorsal mesodermal and neural development. We investigated the role of Tolloid, a metalloprotease that can cleave Chordin and increase Bmp activity, in patterning the dorsoventral axis of the zebrafish embryo. Injection of tolloid mRNA into six dorsalized mutants rescued only one of these mutants, mini fin. Through chromosomal mapping, linkage and cDNA sequence analysis of several mini fin alleles, we demonstrate that mini fin encodes the tolloid gene. Characterization of the mini fin mutant phenotype reveals that Mini fin/Tolloid activity is required for patterning ventral tissues of the tail: the ventral fin, and the ventroposterior somites and vasculature. Gene expression studies show that mfn mutants exhibit reduced expression of ventrally restricted markers at the end of gastrulation, suggesting that the loss of ventral tail tissues is caused by a dorsalization occurring at the end of gastrulation. Based on the mini fin mutant phenotype and the expression of tolloid, we propose that Mini fin/Tolloid modifes the Bmp activity gradient at the end of gastrulation, when the ventralmost marginal cells of the embryo are in close proximity to the dorsal Chordin-expressing cells. At this time, unimpeded Chordin may diffuse to the most ventral marginal regions and inhibit high Bmp activity levels. In the presence of Mini fin/Tolloid, however, Chordin activity would be negatively modulated through proteolytic cleavage, thereby increasing Bmp signaling activity. This extracellular mechanism is amplified by an autoregulatory loop for bmp gene expression.

Regulation of dorsal gene expression in Xenopus by the ventralizing homeodomain gene Vox

Patterning in the vertebrate embryo is controlled by an interplay between signals from the dorsal organizer and the ventrally expressed BMPs. Here we examine the function of Vox, a homeodomain-containing gene that is activated by the ventralizing signal BMP-4. Inhibition of BMP signaling using a dominant negative BMP receptor (DeltaBMPR) leads to the ectopic activation of dorsal genes in the ventral marginal zone, and this activation is prevented by co-injection of Vox. chordin is the most strongly activated of those genes that are up-regulated by DeltaBMPR and is the gene most strongly inhibited by Vox expression. We demonstrate that Vox acts as a transcriptional repressor, showing that the activity of native Vox is mimicked by a Vox-repressor fusion (VoxEnR) and that a Vox-activator fusion (VoxG4A) acts as an antimorph, causing the formation of a partial secondary axis when expressed on the ventral side of the embryo. Although Vox can ectopically activate BMP-4 expression in whole embryos, we see no activation of BMP-4 by VoxG4A, demonstrating that this activation is indirect. Using a hormone-inducible version of VoxG4A, we find that a critical time window for Vox function is during the late blastula period. Using this construct, we demonstrate that only a subset of dorsal genes is directly repressed by Vox, revealing that there are different modes of regulation for organizer genes. Since the major direct target for Vox repression is chordin, we propose that Vox acts in establishing a BMP-4 morphogen gradient by restricting the expression domain of chordin.

Transcriptional regulation of Xvent homeobox genes

The Xvent homeobox multigene family is essential for the patterning of the ventral mesoderm in Xenopus embryos. We have identified two novel members of this family, Xvent-1B and Xvent-2B, and have characterized their genomic structures. These two genes show a clustered organization and have probably arisen by gene duplication with subsequent inversion. Cis-regulatory elements within the promoters of both genes have been identified which contribute to their spatial activation. Xvent-2B is activated by BMP-2/4 in the absence of de novo protein synthesis, suggesting that this gene is a direct target of BMP-signalling. In contrast, Xvent-1B does not directly respond to BMP-2/4, but is activated by Xvent-2B. This activation is documented by Xvent-1B promoter/reporter studies, Xvent-2B overexpression and loss-of-function analysis using a dominant-negative Xvent-2 mutant. However, cycloheximide experiments reveal that Xvent-2B by itself is not sufficient to activate transcription of the Xvent-1B gene, but that there is a requirement for additional factor(s) being synthesized after midblastula transition.