Endogenous patterns of TGFbeta superfamily signaling during early Xenopus development

The function of heterodimeric AP-1 comprised of c-Jun and c-Fos in activin mediated Spemann organizer gene expression

BACKGROUND

Activator protein-1 (AP-1) is a mediator of BMP or FGF signaling during Xenopus embryogenesis. However, specific role of AP-1 in activin signaling has not been elucidated during vertebrate development.

METHODOLOGY/PRINCIPAL FINDINGS

We provide new evidence showing that overexpression of heterodimeric AP-1 comprised of c-jun and c-fos (AP-1(c-Jun/c-Fos)) induces the expression of BMP-antagonizing organizer genes (noggin, chordin and goosecoid) that were normally expressed by high dose of activin. AP-1(c-Jun/c-Fos) enhanced the promoter activities of organizer genes but reduced that of PV.1, a BMP4-response gene. A loss of function study clearly demonstrated that AP-1(c-Jun/c-Fos) is required for the activin-induced organizer and neural gene expression. Moreover, physical interaction of AP-1(c-Jun/c-Fos) and Smad3 cooperatively enhanced the transcriptional activity of goosecoid via direct binding on this promoter. Interestingly, Smad3 mutants at c-Jun binding site failed in regulation of organizer genes, indicating that these physical interactions are specifically necessary for the expression of organizer genes.

CONCLUSIONS/SIGNIFICANCE

AP-1(c-Jun/c-Fos) plays a specific role in organizer gene expression in downstream of activin signal during early Xenopus embryogenesis.

Zebrafish cardiac development requires a conserved secondary heart field

The secondary heart field is a conserved developmental domain in avian and mammalian embryos that contributes myocardium and smooth muscle to the definitive cardiac arterial pole. This field is part of the overall heart field and its myocardial component has been fate mapped from the epiblast to the heart in both mammals and birds. In this study we show that the population that gives rise to the arterial pole of the zebrafish can be traced from the epiblast, is a discrete part of the mesodermal heart field, and contributes myocardium after initial heart tube formation, giving rise to both smooth muscle and myocardium. We also show that Isl1, a transcription factor associated with undifferentiated cells in the secondary heart field in other species, is active in this field. Furthermore, Bmp signaling promotes myocardial differentiation from the arterial pole progenitor population, whereas inhibiting Smad1/5/8 phosphorylation leads to reduced myocardial differentiation with subsequent increased smooth muscle differentiation. Molecular pathways required for secondary heart field development are conserved in teleosts, as we demonstrate that the transcription factor Tbx1 and the Sonic hedgehog pathway are necessary for normal development of the zebrafish arterial pole.

Intercellular signaling pathways active during and after growth and differentiation of the lumbar vertebral growth plate

STUDY DESIGN

Vertebral growth plates at different postnatal ages were assessed for active intercellular signaling pathways.

OBJECTIVE

To generate a spatial and temporal map of the major signaling pathways active in the postnatal mouse lumbar vertebral growth plate.

SUMMARY OF BACKGROUND DATA

The growth of all long bones is known to occur by cartilaginous growth plates. The growth plate is composed of layers of chondrocyets that actively proliferate, differentiate, die and, are replaced by bone. The role of major cell signaling pathways has been suggested for regulation of the fetal long bones. But not much is known about the molecular or cellular signals that control the postnatal vertebral growth plate and hence postnatal vertebral bone growth. Understanding such molecular mechanisms will help design therapeutic treatments for vertebral growth disorders such as scoliosis.

METHODS

Antibodies against activated downstream intermediates were used to identify cells in the growth plate responding to BMP, TGFβ, and FGF in cryosections of lumbar vertebrae from different postnatal age mice to identify the zones that were responding to these signals. Reporter mice were used to identify the chondrocytes responding to hedgehog (Ihh), and Wnt signaling.

RESULTS

We present a spatial/temporal map of these signaling pathways during growth, and differentiation of the mouse lumbar vertebral growth plate.

CONCLUSION

During growth and differentiation of the vertebral growth plate, its different components respond at different times to different intercellular signaling ligands. Response to most of these signals is dramatically downregulated at the end of vertebral growth.

Activity of the RhoU/Wrch1 GTPase is critical for cranial neural crest cell migration

The neural crest (NC) is a stem cell-like population that arises at the border of neural and non-neural ectoderm. During development, NC undergoes an epithelio-mesenchymal transition (EMT), i.e. loss of epithelial junctions and acquisition of pro-migratory properties, invades the entire embryo and differentiates into a wide diversity of terminal tissues. We have studied the implication of Rho pathways in NC development and previously showed that RhoV is required for cranial neural crest (CNC) cell specification. We show here that the non-canonical Wnt response rhoU/wrch1 gene, closely related to rhoV, is also expressed in CNC cells but at later stages. Using both gain- and loss-of-function experiments, we demonstrate that the level of RhoU expression is critical for CNC cell migration and subsequent differentiation into craniofacial cartilages. In in vitro cultures, RhoU activates pathways that cooperate with PAK1 and Rac1 in epithelial adhesion, cell spreading and directional cell migration. These data support the conclusion that RhoU is an essential regulator of CNC cell migration.

Geminin cooperates with Polycomb to restrain multi-lineage commitment in the early embryo

Transient maintenance of a pluripotent embryonic cell population followed by the onset of multi-lineage commitment is a fundamental aspect of development. However, molecular regulation of this transition is not well characterized in vivo. Here, we demonstrate that the nuclear protein Geminin is required to restrain commitment and spatially restrict mesoderm, endoderm and non-neural ectoderm to their proper locations in the Xenopus embryo. We used microarray analyses to demonstrate that Geminin overexpression represses many genes associated with cell commitment and differentiation, while elevating expression levels of genes that maintain pluripotent early and immature neurectodermal cell states. We characterized the relationship of Geminin to cell signaling and found that Geminin broadly represses Activin-, FGF- and BMP-mediated cell commitment. Conversely, Geminin knockdown enhances commitment responses to growth factor signaling and causes ectopic mesodermal, endodermal and epidermal fate commitment in the embryo. We also characterized the functional relationship of Geminin with transcription factors that had similar activities and found that Geminin represses commitment independent of Oct 4 ortholog (Oct25/60) activities, but depends upon intact Polycomb repressor function. Consistent with this, chromatin immunoprecipitation assays directed at mesodermal genes demonstrate that Geminin promotes Polycomb binding and Polycomb-mediated repressive histone modifications, while inhibiting modifications associated with gene activation. This work defines Geminin as an essential regulator of the embryonic transition from pluripotency through early multi-lineage commitment, and demonstrates that functional cooperativity between Geminin and Polycomb contributes to this process.

The role and regulation of GDF11 in Smad2 activation during tailbud formation in the Xenopus embryo

A key role for phosphorylation of Smad2 by TGFβ superfamily ligands in the axial patterning of early embryos is well established. The regulation and role of Smad2 signaling in post-neurula embryonic patterning, however, is less well understood. While a variety of TGFβ superfamily ligands are implicated in various stages of anterior-posterior patterning, the ligand GDF11 has been shown to have a particular role in post-gastrula patterning in the mouse. Mouse GDF11 is specifically localized to the developing tail and is essential for normal posterior axial patterning. Mature GDF11 ligand is inhibited by its own prodomain, and extracellular proteolysis of this prodomain is thought to be necessary for GDF11 activity. The contribution of this proteolytic regulatory mechanism to Smad activation during embryogenesis in vivo, and to the development of posterior pattern, has not been characterized. We investigate here the role of Xenopus GDF11 in the activation of Smad2 during the development of tailbud-stage embryos, and the role of this activation in larval development. We also demonstrate that the activity of BMP-1/Tolloid-like proteases is necessary for the normal GDF11-dependent activation of Smad2 phosphorylation during post-gastrula development. These data demonstrate that GDF11 has a central role in the activation of Smad2 phosphorylation in tailbud stage Xenopus embryos, and provide the first evidence that BMP-1/Tolloid-mediated prodomain cleavage is important for activation of GDF11 in vivo.

Role of BMPs in controlling the spatial and temporal origin of GFAP astrocytes in the embryonic spinal cord

In the vertebrate central nervous system (CNS), astrocytes are the most abundant and functionally diverse glial cell population. However, the mechanisms underlying their specification and differentiation are still poorly understood. In this study, we have defined spatially and temporally the origin of astrocytes and studied the role of BMPs in astrocyte development in the embryonic chick spinal cord. Using explant cultures, we show that astrocyte precursors started migrating out of the neuroepithelium in the mantle layer from E5, and that the dorsal-most level of the neuroepithelium, from the roof plate to the dl3 level, did not generate GFAP-positive astrocytes. Using a variety of early astrocyte markers together with functional analyses, we show that dorsal-most progenitors displayed a potential for astrocyte production but that dorsally-derived BMP signalling, possibly mediated through BMP receptor 1B, promoted neuronal specification instead. BMP treatment completely prevented astrocyte development from intermediate spinal cord explants at E5, whereas it promoted it at E6. Such an abrupt change in the response of this tissue to BMP signalling could be correlated to the onset of new foci of BMP activity and enhanced expression of BMP receptor 1A, suggesting that BMP signalling could promote astrocyte development in this region.

Scaling of BMP gradients in Xenopus embryos

Metazoan organisms can 'scale', that is, maintain similar proportions regardless of size. Ben-Zvi et al. use experiments in Xenopus to support a quantitative model that explains morphological scaling as the result of scaling of a gradient of bone morphogenetic protein (BMP) signals. We believe that the evidence for scaling in Xenopus is misinterpreted, and that their model for embryonic patterning disagrees with prior data. The experiments they present supporting their model admit alternative interpretations.

Activin signaling: effects on body composition and mitochondrial energy metabolism

Activin-betaA and activin-betaB (encoded by Inhba and Inhbb genes, respectively) are closely related TGF-beta superfamily members that participate in a variety of biological processes. We previously generated mice with an insertion allele at the Inhba locus, Inhba(BK). In this allele, the sequence encoding the Inhba mature domain is replaced with that of Inhbb, rendering the gene product functionally hypomorphic. Homozygous (Inhba(BK/BK)) and hemizygous (Inhba(BK/-)) mice are smaller and leaner than their wild-type littermates, and many tissues are disproportionately small relative to total body weight. To determine the mechanisms that contribute to these phenomena, we investigated the metabolic consequences of the mutation. Although the growth of Inhba(BK) mice is improved by providing a calorie-rich diet, diet-induced obesity, fatty liver, and insulin resistance (hallmarks of chronic caloric excess) do not develop, despite greater caloric intake than wild-type controls. Physiological, molecular, and biochemical analyses all revealed characteristics that are commonly associated with increased mitochondrial energy metabolism, with a corresponding up-regulation of several genes that reflect enhanced mitochondrial biogenesis and function. Oxygen consumption, an indirect measure of the metabolic rate, was markedly increased in Inhba(BK/BK) mice, and polarographic analysis of liver mitochondria revealed an increase in ADP-independent oxygen consumption, consistent with constitutive uncoupling of the inner mitochondrial membrane. These findings establish a functional relationship between activin signaling and mitochondrial energy metabolism and further support the rationale to target this signaling pathway for the medical treatment of cachexia, obesity, and diabetes.

MicroRNA in cell differentiation and development

The regulation of gene expression by microRNAs (miRNAs) is a recently discovered pattern of gene regulation in animals and plants. MiRNAs have been implicated in various aspects of animal development and cell differentiation, such as early embryonic development, neuronal development, muscle development, and lymphocyte development, by the analysis of genetic deletions of individual miRNAs in mammals. These studies show that miRNAs are key regulators in animal development and are potential causes of human diseases. Here we review some recent discoveries about the functions of miRNAs in cell differentiation and development.

Intercellular signaling pathways active during intervertebral disc growth, differentiation, and aging

STUDY DESIGN

Intervertebral discs at different postnatal ages were assessed for active intercellular signaling pathways.

OBJECTIVE

To generate a spatial and temporal map of the signaling pathways active in the postnatal intervertebral disc (IVD).

SUMMARY OF BACKGROUND DATA

The postnatal IVD is a complex structure, consisting of 3 histologically distinct components, the nucleus pulposus, fibrous anulus fibrosus, and endplate. These differentiate and grow during the first 9 weeks of age in the mouse. Identification of the major signaling pathways active during and after the growth and differentiation period will allow functional analysis using mouse genetics and identify targets for therapy for individual components of the disc.

METHODS

Antibodies specific for individual cell signaling pathways were used on cryostat sections of IVD at different postnatal ages to identify which components of the IVD were responding to major classes of intercellular signal, including sonic hedgehog, Wnt, TGFbeta, FGF, and BMPs.

RESULTS

We present a spatial/temporal map of these signaling pathways during growth, differentiation, and aging of the disc.

CONCLUSION

During growth and differentiation of the disc, its different components respond at different times to different intercellular signaling ligands. Most of these are dramatically downregulated at the end of disc growth.

foxD5 plays a critical upstream role in regulating neural ectodermal fate and the onset of neural differentiation

foxD5 is expressed in the nascent neural ectoderm concomitant with several other neural-fate specifying transcription factors. We used loss-of-function and gain-of-function approaches to analyze the functional position of foxD5 amongst these other factors. Loss of FoxD5 reduces the expression of sox2, sox11, soxD, zic1, zic3 and Xiro1-3 at the onset of gastrulation, and of geminin, sox3 and zic2, which are maternally expressed, by late gastrulation. At neural plate stages most of these genes remain reduced, but the domains of zic1 and zic3 are expanded. Increased FoxD5 induces geminin and zic2, weakly represses sox11 at early gastrula but later (st12) induces it; weakly represses sox2 and sox3 transiently and strongly represses soxD, zic1, zic3 and Xiro1-3. The foxD5 effects on zic1, zic3 and Xiro1-3 involve transcriptional repression, whereas those on geminin and zic2 involve transcriptional activation. foxD5's effects on geminin, sox11 and zic2 occur at the onset of gastrulation, whereas the other genes require earlier foxD5 activity. geminin, sox11 and zic2, each of which is up-regulated directly by foxD5, are all required to account for foxD5 phenotypes, indicating that this triad constitutes a transcriptional network rather than linear path that coordinately up-regulates genes that promote an immature neural fate and inhibits genes that promote the onset of neural differentiation. We also show that foxD5 promotes an ectopic neural fate in the epidermis by reducing BMP signaling. Several of the genes that are repressed by foxD5 in turn reduce foxD5 expression, contributing to the medial-lateral patterning of the neural plate.

[The pharmacogenetics of response to antiretroviral therapy]

The plasma concentrations of most antiretroviral drugs show wide interindividual variability, which may lead to differences in response rate and toxicity. Among the factors determining this variable exposure to antiviral drugs are differences in metabolism, interactions due to the use of concomitant medication, problems in treatment adherence, underlying diseases, and host genetic factors. Pharmacogenetics analyzes the genetic bases of interindividual variation in the bioavailability and response to drugs. The aim is to establish the foundations for individualized therapy. Among the genetic factors that are in some way involved in antiretroviral treatment response are those that directly or indirectly affect antiviral plasma concentrations, as is the case of drug transport proteins and metabolizing enzymes. Some host factors also influence antiviral response, such as interleukins and major histocompatibility complex. The present article analyzes the most important genetic markers associated with antiretroviral treatment response.

Identification and functional characterization of NODAL rare variants in heterotaxy and isolated cardiovascular malformations

NODAL and its signaling pathway are known to play a key role in specification and patterning of vertebrate embryos. Mutations in several genes encoding components of the NODAL signaling pathway have previously been implicated in the pathogenesis of human left-right (LR) patterning defects. Therefore, NODAL, a member of TGF-beta superfamily of developmental regulators, is a strong candidate to be functionally involved in congenital LR axis patterning defects or heterotaxy. Here we have investigated whether variants in NODAL are present in patients with heterotaxy and/or isolated cardiovascular malformations (CVM) thought to be caused by abnormal heart tube looping. Analysis of a large cohort of cases (n = 269) affected with either classic heterotaxy or looping CVM revealed four different missense variants, one in-frame insertion/deletion and two conserved splice site variants in 14 unrelated subjects (14/269, 5.2%). Although similar with regard to other associated defects, individuals with the NODAL mutations had a significantly higher occurrence of pulmonary valve atresia (P = 0.001) compared with cases without a detectable NODAL mutation. Functional analyses demonstrate that the missense variant forms of NODAL exhibit significant impairment of signaling as measured by decreased Cripto (TDGF-1) co-receptor-mediated activation of artificial reporters. Expression of these NODAL proteins also led to reduced induction of Smad2 phosphorylation and impaired Smad2 nuclear import. Taken together, these results support a role for mutations and rare deleterious variants in NODAL as a cause for sporadic human LR patterning defects.

Bmp signaling is necessary and sufficient for ventrolateral endoderm specification in Xenopus

Here we show that Bmp signaling is necessary and sufficient for the specification of ventral endoderm in Xenopus embryos. Overexpression of Bmp4 in ectoderm induces markers of endoderm, including Sox17beta, Mixer, and VegT, but cannot induce the expression of the dorsoanterior markers, Xhex and Cerberus. Furthermore, knockdown approaches using overexpression of Bmp antagonists and morpholinos designed against Bmp4, Bmp2, and Bmp7 demonstrate that Bmp signaling is critical for ventral, but not dorsoanterior endoderm formation. This activity is not simply a result of embryonic dorsalization as markers for dorsal endoderm are not expanded. We further show that endodermal cells of either ventral or dorsal character do not form when both Wnt and Bmp signals are abolished. Overall, this report strongly suggests that Bmp plays an essential role in ventral endoderm specification.

Retinoid signaling can repress blastula Wnt signaling and impair dorsal development in Xenopus embryo

Vitamin A derivatives (retinoids) are actively involved during vertebrate embryogenesis. However, exogenous retinoids have also long been known as potent teratogens. The defects caused by retinoid treatment are complex. Here, we provided evidence that RAR-mediated retinoid signaling can repress Xenopus blastula Wnt signaling and impair dorsal development. Exogenous retinoic acid (RA) could antagonize the dorsalizing effects of lithium chloride-mediated Wnt activation in blastula embryos. The Wnt-responsive reporter gene transgenesis and luciferase assay showed that excess RA can repress the Wnt signaling in blastula embryos. In addition, the downstream target genes of the Wnt signaling that direct embryonic dorsal development, were also down-regulated in the RA-treated embryos. Mechanically, RA did not interfere with the stability of beta-catenin, but promoted its nuclear accumulation. The inverse agonist of retinoic acid receptors (RAR) rescued the Wnt signaling repression by RA and relieved the RA-induced nuclear accumulation of beta-catenin. Our results explain one of the reasons for the complicated teratogenic effects of retinoids and shed light on the endogenous way of interactions between two developmentally important signaling pathways.

An in vivo reporter of BMP signaling in organogenesis reveals targets in the developing kidney

Background

Bone morphogenetic proteins (BMPs) regulate essential processes during organogenesis, and a functional understanding of these secreted proteins depends on identification of their target cells. In this study, we generate a transgenic reporter for organogenesis studies that we use to define BMP pathway activation in the developing kidney.

Results

Mouse strains reporting on BMP pathway activation were generated by transgenically expressing β-galactosidase under the control of BMP responsive elements from Id1. Reporter expression corresponds well with immunoassays for pathway activation in all organs studied, validating the model. Using these reporters we have generated a detailed map of cellular targets of BMP signaling in the developing kidney. We find that SMAD dependent BMP signaling is active in collecting duct trunks, but not tips. Furthermore, glomerular endothelial cells, and proximal nephron tubules from the renal vesicle stage onward show pathway activation. Surprisingly, little activation is detected in the nephrogenic zone of the kidney, and in organ culture BMP treatment fails to activate SMAD dependent BMP signaling in nephron progenitor cells. In contrast, signaling is efficiently induced in collecting duct tips.

Conclusion

Transgenic reporters driven by control elements from BMP responsive genes such as Id1 offer significant advantages in sensitivity and consistency over immunostaining for studies of BMP pathway activation. They also provide opportunities for analysis of BMP signaling in organ and primary cell cultures subjected to experimental manipulation. Using such a reporter, we made the surprising finding that SMAD dependent BMP signaling is inactive in nephron progenitors, and that these cells are refractory to activation by applied growth factors. Furthermore, we find that the BMP pathway is not normally active in collecting duct tips, but that it can be ectopically activated by BMP treatment, offering a possible explanation for the inhibitory effects of BMP treatment on collecting duct growth and branching.

A new model for growth factor activation: type II receptors compete with the prodomain for BMP-7

Bone morphogenetic proteins (BMPs) are morphogens with long-range signaling activities. BMP-7 is secreted as a stable complex consisting of a growth factor noncovalently associated with two propeptides. In other transforming growth factor-beta-like growth factor complexes, the prodomain (pd) confers latency to the complex. However, we detected no difference in signaling capabilities between the growth factor and the BMP-7 complex in multiple in vitro bioactivity assays. Biochemical and biophysical methods elucidated the interaction between the BMP-7 complex and the extracellular domains of its type I and type II receptors. Results showed that type II receptors, such as BMP receptor II, activin receptor IIA, and activin receptor IIB, competed with the pd for binding to the growth factor and displaced the pd from the complex. In contrast, type I receptors interacted with the complex without displacing the pd. These studies suggest a new model for growth factor activation in which proteases or other extracellular molecules are not required and provide a molecular mechanism consistent with a role for BMP receptors in the establishment of early morphogen gradients.

Patterning the embryonic kidney: BMP signaling mediates the differentiation of the pronephric tubules and duct in Xenopus laevis

The Bone morphogenetic proteins (BMPs) mediate a wide range of diverse cellular behaviors throughout development. Previous studies implicated an important role for BMP signaling during the differentiation of the definitive mammalian kidney, the metanephros. In order to examine whether BMP signaling also plays an important role during the patterning of earlier renal systems, we examined the development of the earliest nephric system, the pronephros. Using the amphibian model system Xenopus laevis, in combination with reagents designed to inhibit BMP signaling during specific stages of nephric development, we revealed an evolutionarily conserved role for this signaling pathway during renal morphogenesis. Our results demonstrate that conditional BMP inhibition after specification of the pronephric anlagen is completed, but prior to the onset of morphogenesis and differentiation of renal tissues, results in the severe malformation of both the pronephric duct and tubules. Importantly, the effects of BMP signaling on the developing nephron during this developmental window are specific, only affecting the developing duct and tubules, but not the glomus. These data, combined with previous studies examining metanephric development in mice, provide further support that BMP functions to mediate morphogenesis of the specified renal field during vertebrate embryogenesis. Specifically, BMP signaling is required for the differentiation of two types of nephric structures, the pronephric tubules and duct.

Activin/nodal signaling modulates XPAPC expression during Xenopus gastrulation

Gastrulation is the first obligatory morphogenesis during vertebrate development, by which the body plan is established. Nodal signaling is a key player in many developmental processes, including gastrulation. XPAPC has been found to exert its biological function through modifying the adhesion property of cells and interacting with other several important molecules in embryos. In this report, we show that nodal signaling is necessary and sufficient for XPAPC expression during Xenopus gastrulation. Furthermore, we isolated 4.8 kb upstream DNA sequence of Xenopus XPAPC, and proved that this 4.8-kb genomic contig is sufficient to recapitulate the expression pattern of XPAPC from gastrula to tail bud stage. Transgene and ChIP assays indicate that Activin/nodal signaling participates in regulation of XPAPC expression through a Smad binding element within the XPAPC promoter. Concomitant investigation suggests that the canonical Wnt pathway-activated XPAPC expression requires nodal signaling.

Calcium signalling during neural induction in Xenopus laevis embryos

In Xenopus, experiments performed with isolated ectoderm suggest that neural determination is a 'by default' mechanism, which occurs when bone morphogenetic proteins (BMPs) are antagonized by extracellular antagonists, BMP being responsible for the determination of epidermis. However, Ca(2+) imaging of intact Xenopus embryos reveals patterns of Ca(2+) transients which are generated via the activation of dihydropyridine-sensitive Ca(2+) channels in the dorsal ectoderm but not in the ventral ectoderm. These increases in the concentration of intracellular Ca(2+)([Ca(2+)]i) appear to be necessary and sufficient to orient the ectodermal cells towards a neural fate as increasing the [Ca(2+)]i artificially results in neuralization of the ectoderm. We constructed a subtractive cDNA library between untreated and caffeine-treated ectoderms (to increase [Ca(2+)]i) and then identified early Ca(2+)-sensitive target genes expressed in the neural territories. One of these genes, an arginine methyltransferase, controls the expression of the early proneural gene, Zic3. Here, we discuss the evidence for the existence of an alternative model to the 'by default' mechanism, where Ca(2+) plays a central regulatory role in the expression of Zic3, an early proneural gene, and in epidermal determination which only occurs when the Ca(2+)-dependent signalling pathways are inactive.

The BMP signaling gradient patterns dorsoventral tissues in a temporally progressive manner along the anteroposterior axis

Patterning of the vertebrate anteroposterior (AP) axis proceeds temporally from anterior to posterior. How dorsoventral (DV) axial patterning relates to AP temporal patterning is unknown. We examined the temporal activity of BMP signaling in patterning ventrolateral cell fates along the AP axis, using transgenes that rapidly turn "off" or "on" BMP signaling. We show that BMP signaling patterns rostral DV cell fates at the onset of gastrulation, whereas progressively more caudal DV cell fates are patterned at progressively later intervals during gastrulation. Increased BMP signal duration is not required to pattern more caudal DV cell fates; rather, distinct temporal intervals of signaling are required. This progressive action is regulated downstream of, or in parallel to, BMP signal transduction at the level of Smad1/5 phosphorylation. We propose that a temporal cue regulates a cell's competence to respond to BMP signaling, allowing the acquisition of a cell's DV and AP identity simultaneously.

Small non-coding RNAs in animal development

The modulation of gene expression by small non-coding RNAs is a recently discovered level of gene regulation in animals and plants. In particular, microRNAs (miRNAs) and Piwi-interacting RNAs (piRNAs) have been implicated in various aspects of animal development, such as neuronal, muscle and germline development. During the past year, an improved understanding of the biological functions of small non-coding RNAs has been fostered by the analysis of genetic deletions of individual miRNAs in mammals. These studies show that miRNAs are key regulators of animal development and are potential human disease loci.

Identification of a novel pool of extracellular pro-myostatin in skeletal muscle

Myostatin, a transforming growth factor-beta superfamily ligand, negatively regulates skeletal muscle growth. Generation of the mature signaling peptide requires cleavage of pro-myostatin by a proprotein convertase, which is thought to occur constitutively in the Golgi apparatus. In serum, mature myostatin is found in an inactive, non-covalent complex with its prodomain. We find that in skeletal muscle, unlike serum, myostatin is present extracellularly as uncleaved pro-myostatin. In cultured cells, co-expression of pro-myostatin and latent transforming growth factor-beta-binding protein-3 (LTBP-3) sequesters pro-myostatin in the extracellular matrix, and secreted pro-myostatin can be cleaved extracellularly by the proprotein convertase furin. Co-expression of LTBP-3 with myostatin reduces phosphorylation of Smad2, and ectopic expression of LTBP-3 in mature mouse skeletal muscle increases fiber area, consistent with reduction of myostatin activity. We propose that extracellular pro-myostatin constitutes the major pool of latent myostatin in muscle. Post-secretion activation of this pool by furin family proprotein convertases may therefore represent a major control point for activation of myostatin in skeletal muscle.

TGF-beta signaling is required for multiple processes during Xenopus tail regeneration

Xenopus tadpoles can fully regenerate all major tissue types following tail amputation. TGF-beta signaling plays essential roles in growth, repair, specification, and differentiation of tissues throughout development and adulthood. We examined the localization of key components of the TGF-beta signaling pathway during regeneration and characterized the effects of loss of TGF-beta signaling on multiple regenerative events. Phosphorylated Smad2 (p-Smad2) is initially restricted to the p63+ basal layer of the regenerative epithelium shortly after amputation, and is later found in multiple tissue types in the regeneration bud. TGF-beta ligands are also upregulated throughout regeneration. Treatment of amputated tails with SB-431542, a specific and reversible inhibitor of TGF-beta signaling, blocks tail regeneration at multiple points. Inhibition of TGF-beta signaling immediately following tail amputation reversibly prevents formation of a wound epithelium over the future regeneration bud. Even brief inhibition immediately following amputation is sufficient, however, to irreversibly block the establishment of structures and cell types that characterize regenerating tissue and to prevent the proper activation of BMP and ERK signaling pathways. Inhibition of TGF-beta signaling after regeneration has already commenced blocks cell proliferation in the regeneration bud. These data reveal several spatially and temporally distinct roles for TGF-beta signaling during regeneration: (1) wound epithelium formation, (2) establishment of regeneration bud structures and signaling cascades, and (3) regulation of cell proliferation.

Detection of nuclear beta-catenin in Xenopus embryos

Immunodetection of beta-catenin accumulation in the nucleus is the most direct and reliable method to determine the intensity and the spatial/temporal patterns of Wnt-dependent signaling activity. Due to the large size of the Xenopus embryo, staining must be done on sections. We present here a simple protocol to prepare cryosections and produce high-quality images of the early embryo using immunofluorescence. We also provide comments on various conceptual and technical issues from fixation to image collection, which may assist in optimizing immunodetection in embryos and tissues beyond the specific scope of beta-catenin localization.

The small GTPase RhoV is an essential regulator of neural crest induction in Xenopus

In vertebrates, the Rho family of GTPases is made of 20 members which regulate a variety of cellular functions, including actin cytoskeleton dynamics, cell adhesion and motility, cell growth and survival, gene transcription and membrane trafficking. To get a comprehensive view of Rho implication in physiological epithelial-mesenchymal transition, we carried out an in situ hybridization-based screen to identify Rho members expressed in Xenopus neural crest cells, in which we previously reported RhoB expression at the migrating stage. In the present study, we identify RhoV as an early expressed neural crest marker and provide evidence that its activity is essential for neural crest cell induction. RhoV mRNA is maternally expressed and accumulates shortly after gastrulation in the neural crest forming region. Using antisense morpholino injection, we show that at neurula stages, RhoV depletion impairs expression of the neural crest markers Sox9, Slug or Twist but has no effect on Snail induction. At the tailbud stage, RhoV knockdown causes a dramatic loss of cranial neural crest derived structures. All these defects are rescued by ectopic wild-type RhoV, whose overexpression on its own expands the neural crest territory. Our findings disclose an unprecedented Rho function in pathways that control neural crest cells specification.

MicroRNA control of Nodal signalling

MicroRNAs are crucial modulators of gene expression, yet their involvement as effectors of growth factor signalling is largely unknown. Ligands of the transforming growth factor-beta superfamily are essential for development and adult tissue homeostasis. In early Xenopus embryos, signalling by the transforming growth factor-beta ligand Nodal is crucial for the dorsal induction of the Spemann's organizer. Here we report that Xenopus laevis microRNAs miR-15 and miR-16 restrict the size of the organizer by targeting the Nodal type II receptor Acvr2a. Endogenous miR-15 and miR-16 are ventrally enriched as they are negatively regulated by the dorsal Wnt/beta-catenin pathway. These findings exemplify the relevance of microRNAs as regulators of early embryonic patterning acting at the crossroads of fundamental signalling cascades.

BMP and FGF regulatory pathways in semilunar valve precursor cells

In the developing atrioventricular (AV) valve, limb bud, and somites, cartilage cell lineage differentiation is regulated by bone morphogenetic protein (BMP), while fibroblast growth factor (FGF) controls tendon cell fate. We observed aggrecan and sox9, characteristic of cartilage cell types, and scleraxis and tenascin, characteristic of tendon cell types, in developing avian semilunar valves. Addition of BMP4 to outflow tract (OFT) precursor cells of young (E4.5) but not older (E6) chick embryos activated Smad1/5/8 and induced sox9 and aggrecan expression, while FGF4 treatment increased phosphorylated MAPK (dpERK) signaling and promoted expression of scleraxis and tenascin. These results identify BMP and FGF pathways that promote expression of cartilage- or tendon-like characteristics in semilunar valve precursor cells. In contrast to AV valve precursor cells, which diversify into leaflets (cartilage-like) or chordae tendineae (tendon-like), semilunar valve cells exhibit both cartilage- and tendon-like characteristics in the developing and mature valve cusp.

Intracellular mediators of transforming growth factor beta superfamily signaling localize to endosomes in chicken embryo and mouse lenses in vivo

Background

Endocytosis is a key regulator of growth factor signaling pathways. Recent studies showed that the localization to endosomes of intracellular mediators of growth factor signaling may be required for their function. Although there is substantial evidence linking endocytosis and growth factor signaling in cultured cells, there has been little study of the endosomal localization of signaling components in intact tissues or organs.

Results

Proteins that are downstream of the transforming growth factor-β superfamily signaling pathway were found on endosomes in chicken embryo and postnatal mouse lenses, which depend on signaling by members of the TGFβ superfamily for their normal development. Phosphorylated Smad1 (pSmad1), pSmad2, Smad4, Smad7, the transcriptional repressors c-Ski and TGIF and the adapter molecules Smad anchor for receptor activation (SARA) and C184M, localized to EEA-1- and Rab5-positive vesicles in chicken embryo and/or postnatal mouse lenses. pSmad1 and pSmad2 also localized to Rab7-positive late endosomes. Smad7 was found associated with endosomes, but not caveolae. Bmpr1a conditional knock-out lenses showed decreased nuclear and endosomal localization of pSmad1. Many of the effectors in this pathway were distributed differently in vivo from their reported distribution in cultured cells.

Conclusion

Based on the findings reported here and data from other signaling systems, we suggest that the localization of activated intracellular mediators of the transforming growth factor-β superfamily to endosomes is important for the regulation of growth factor signaling.

Interpretation of BMP signaling in early Xenopus development

Very little is known about how the extracellular binding of a morphogen is transduced to the nucleus of a cell in a concentration-related way, enabling cells to interpret their position in a concentration gradient. Here, we have analyzed when and how Xenopus embryo cells perceive and interpret a BMP signal. Dissociated embryo cells are exposed for short times to different concentrations of BMP4. We find that cells are already competent to receive a BMP4 signal at the blastula stage. They phosphorylate Smad1 very rapidly and express downstream genes less than half an hour after exposure to BMP. However, Smad1 is present in the nucleus even in the absence of BMP. To quantitate intracellular signaling after BMP exposure, we have constructed a chimeric type I receptor that registers BMP signaling as the intranuclear migration of Smad2, and as the transcription of Smad2 downstream genes. The combination of the chimeric receptor and GFP-Smad2 makes it possible to follow the transduction of BMP signaling to the nucleus. From our results, we conclude that an extracellular BMP concentration is interpreted by the steady state nuclear concentration of phosphorylated Smad1.

Kinesin-mediated transport of Smad2 is required for signaling in response to TGF-beta ligands

During vertebrate development, Activin/Nodal-related ligands signal through Smad2, leading to its activation and accumulation in the nucleus. Here, we demonstrate that Smad2 constantly shuttles between the cytoplasm and nucleus both in early Xenopus embryo explants and in living zebrafish embryos, providing a mechanism whereby the intracellular components of the pathway constantly monitor receptor activity. We have gone on to demonstrate that an intact microtubule network and kinesin ATPase activity are required for Smad2 phosphorylation and nuclear accumulation in response to Activin/Nodal in early vertebrate embryos and TGF-beta in mammalian cells. The kinesin involved is kinesin-1, and Smad2 interacts with the kinesin-1 light chain subunit. Interfering with kinesin activity in Xenopus and zebrafish embryos phenocopies loss of Nodal signaling. Our results reveal that kinesin-mediated transport of Smad2 along microtubules to the receptors is an essential step in ligand-induced Smad2 activation.

The bone morphogenetic protein antagonist noggin regulates mammalian cardiac morphogenesis

Bone morphogenetic proteins (BMPs) play many roles in mammalian cardiac development. Here we address the functions of Noggin, a dedicated BMP antagonist, in the developing mouse heart. In early cardiac tissues, the Noggin gene is mainly expressed in the myocardial cells of the outflow tract, atrioventricular canal, and future right ventricle. The major heart phenotypes of Noggin mutant embryos are thicker myocardium and larger endocardial cushions. Both defects result from increased cell number. Cell proliferation is increased and cell cycle exit is decreased in the myocardium. Although we find evidence of increased BMP signal transduction in the myocardium and endocardium, we show that the cardiac defects of Noggin mutants are rescued by halving the gene dosage of Bmp4. In culture, BMP increases the epithelial-to-mesenchymal transformation (EMT) of endocardial explant cells. Increased EMT likely accounts for the enlarged atrioventricular cushion. In the outflow tract cushion, we observed an increased contribution of cardiac neural crest cells to the mutant cushion mesenchyme, although many cells of the cushion were not derived from neural crest. Thus the enlarged outflow tract cushion of Noggin mutants likely arises by increased contributions both of endocardial cells that have undergone EMT as well as cells that have migrated from the neural crest. These data indicate that antagonism of BMP signaling by Noggin plays a critical role in ensuring proper levels of cell proliferation and EMT during cardiac morphogenesis in the mouse.

Molecular Basis of Vertebrate Endoderm Development

The embryonic endoderm gives rise to the epithelial lining of the digestive and respiratory systems and organs such as the thyroid, lungs, liver, gallbladder, and pancreas. Studies in Xenopus, zebrafish, and mice have revealed a conserved molecular pathway controlling vertebrate endoderm development. The TGFbeta/Nodal signaling pathway is at the top of this molecular hierarchy and controls the expression of a number of key transcription factors including Mix-like homeodomain proteins, Gata zinc finger factors, Sox HMG domain proteins, and Fox forkhead factors. Here we review the function of these molecules comparing and contrasting their roles in each model organism. Finally, we will describe how our understanding of the molecular pathway governing endoderm development in embryos is being used to differentiate embryonic stem cells in vitro along endodermal lineages, with the ultimate goal of making therapeutically useful tissue.

Calcium transients and calcium signalling during early neurogenesis in the amphibian embryo Xenopus laevis

Development of the vertebrate embryonic nervous system is characterized by a cascade of signalling events. In Xenopus, the initial step in this cascade results from signals emanating from the dorsal mesoderm that divert the fate of the ectoderm from an epidermal to a neural lineage. These signals include extracellular antagonists of the bone morphogenetic protein (BMP). Experiments performed with isolated ectoderm suggest that epidermis is induced by BMP, whereas neural fates arise by default following BMP inhibition; however, we show that this mechanism is not sufficient for neural determination. Ca2+ imaging of intact Xenopus embryos reveals patterns of Ca2+ transients in the dorsal ectoderm but not in the ventral ectoderm. These increases in intracellular calcium concentration ([Ca2+](i)), which occur via the activation of dihydropyridine (DHP)-sensitive Ca2+ channels, are necessary and sufficient to orientate the ectodermal cells toward a neural fate. On the one hand, the treatments that antagonize the increase in [Ca2+](i), inhibit neuralization, while on the other hand, an artificial increase in [Ca2+](i), whatever its origin, neuralizes the ectoderm. Using these properties, we have constructed a subtractive cDNA library between untreated ectoderm and caffeine-treated ectoderm. The caffeine stimulates an increase in [Ca2+](i) and thus orientates the cells towards the neural pathway. We have identified early Ca2+ target genes expressed in neural territories. One of these genes, an arginine methyl transferase, controls the expression of the early proneural gene, Zic3. Here, we discuss an alternative model where Ca2+ plays a central regulatory role in early neurogenesis. This model integrates the activation of a Ca2+ -dependent signalling pathway due to an influx of Ca2+ through DHP-Ca2+ channels. While Ca2+ is required for neural determination, epidermal determination occurs when Ca2+ -dependent signalling pathways are inactive.

Multiple functions of Cerberus cooperate to induce heart downstream of Nodal

The TGFbeta family member Nodal has been implicated in heart induction through misexpression of a dominant negative version of the type I Nodal receptor (Alk4) and targeted deletion of the co-receptor Cripto in murine ESCs and mouse embryos; however, whether Nodal acts directly or indirectly to induce heart tissue or interacts with other signaling molecules or pathways remained unclear. Here we present Xenopus embryological studies demonstrating an unforeseen role for the DAN family protein Cerberus within presumptive foregut endoderm as essential for differentiation of cardiac mesoderm in response to Nodal. Ectopic activation of Nodal signaling in non-cardiogenic ventroposterior mesendoderm, either by misexpression of the Nodal homologue XNr1 together with Cripto or by a constitutively active Alk4 (caAlk4), induced both cardiac markers and Cerberus. Mosaic lineage tracing studies revealed that Nodal/Cripto and caAlk4 induced cardiac markers cell non-autonomously, thus supporting the idea that Cerberus or another diffusible factor is an essential mediator of Nodal-induced cardiogenesis. Cerberus alone was found sufficient to initiate cardiogenesis at a distance from its site of synthesis. Conversely, morpholino-mediated specific knockdown of Cerberus reduced both endogenous cardiomyogenesis and ectopic heart induction resulting from misactivation of Nodal/Cripto signaling. Since the specific knockdown of Cerberus did not abrogate heart induction by the Wnt antagonist Dkk1, Nodal/Cripto and Wnt antagonists appear to initiate cardiogenesis through distinct pathways. This idea was further supported by the combinatorial effect of morpholino-medicated knockdown of Cerberus and Hex, which is required for Dkk1-induced cardiogenesis, and the differential roles of essential downstream effectors: Nodal pathway activation did not induce the transcriptional repressor Hex while Dkk-1 did not induce Cerberus. These studies demonstrated that cardiogenesis in mesoderm depends on Nodal-mediated induction of Cerberus in underlying endoderm, and that this pathway functions in a pathway parallel to cardiogenesis initiated through the induction of Hex by Wnt antagonists. Both pathways operate in endoderm to initiate cardiogenesis in overlying mesoderm.

Sp-Smad2/3 mediates patterning of neurogenic ectoderm by nodal in the sea urchin embryo

Nodal functions in axis and tissue specification during embryogenesis. In sea urchin embryos, Nodal is crucial for specification of oral ectoderm and is thought to pattern neurogenesis in the animal plate. To determine if Nodal functions directly in suppressing neuron differentiation we have prepared mutant forms of Sp-Smad2/3. Expressing an activated form produces embryos similar to embryos overexpressing Nodal, but with fewer neurons. In chimeras in which Nodal is suppressed, cells expressing activated Sp-Smad2/3 form oral ectoderm, but not neurons. In embryos with vegetal signaling blocked, neurons do not form if activated Smad2/3 is co-expressed. Expression of dominant negative mutants produces embryos identical to those resulting from blocking Nodal expression. In chimeras overexpressing Nodal, cells expressing dominant negative Sp-Smad2/3 form aboral ectoderm and give rise to neurons. In permanent blastula chimeras dominant negative Sp-Smad2/3 is able to suppress the effects of Nodal permitting neuron differentiation. In these chimeras Nodal expression in one half suppresses neural differentiation across the interface. Anti-phospho-Smad3 reveals that the cells adjacent to cells expressing Nodal have nuclear immunoreactivity. We conclude Sp-Smad2/3 is a component of the Nodal signaling pathway in sea urchins and that Nodal diffuses short distances to suppress neural differentiation.

Expression of RhoB in the developing Xenopus laevis embryo

Rho GTPases are signaling components that participate to the control of cell morphology, adhesion and motility through the regulation of F-actin cytoskeleton dynamics. In this paper, we report the identification of RhoB in Xenopus laevis (XRhoB) and its expression pattern during early development. Whole-mount in situ hybridization analysis indicated that XrhoB is expressed at high levels in the dorsal marginal zone early in gastrula and in the dorsal midline at later stages. At mid-neurula stages, XrhoB expression extends to the central nervous system, presomitic mesoderm and somites. Later during development, rhoB mRNA is detected in the eyes, the migrating neural crest cells as well as the dorso-lateral part of the somites.

Smurf1 regulates neural patterning and folding in Xenopus embryos by antagonizing the BMP/Smad1 pathway

The ubiquitin ligase Smurf1 can target a handful of signaling proteins for ubiquitin-mediated proteasomal destruction or functional modification, including TGF-beta receptors, Smads, transcription factors, RhoA and MEKK2. Smurf1 was initially implicated in BMP pathway regulation in embryonic development, but its potential role in vertebrate embryogenesis has yet to be clarified. Here we demonstrate that inhibition of Smurf1 in Xenopus laevis embryos with an antisense morpholino oligonucleotide or a dominant-negative protein disrupts early development, with the nervous system being the principal target. Smurf1 is enriched on the dorsal side of gastrula stage embryos, and blocking Smurf1 disturbs neural folding and neural, but not mesoderm differentiation, enhances BMP/Smad1 signaling, and elevates phospho-Smad1 levels in the dorsal ectoderm. We conclude that in Xenopus embryos, the BMP pathway is a major physiological target of Smurf1, and we propose that in normal development Smurf1 cooperates with secreted BMP antagonists to limit BMP signaling in dorsal ectoderm. Our data also reveal a novel role for Smurf1 and Smad1 in neural plate morphogenesis.

Unique players in the BMP pathway: small C-terminal domain phosphatases dephosphorylate Smad1 to attenuate BMP signaling

Smad transcription factors are key signal transducers for the TGF-beta/bone morphogenetic protein (BMP) family of cytokines and morphogens. C-terminal serine phosphorylation by TGF-beta and BMP membrane receptors drives Smads into the nucleus as transcriptional regulators. Dephosphorylation and recycling of activated Smads is an integral part of this process, which is critical for agonist sensing by the cell. However, the nuclear phosphatases involved have remained unknown. Here we provide functional, biochemical, and embryological evidence identifying the SCP (small C-terminal domain phosphatase) family of nuclear phosphatases as mediators of Smad1 dephosphorylation in the BMP signaling pathway in vertebrates. Xenopus SCP2/Os4 inhibits BMP activity in the presumptive ectoderm and leads to neuralization. In Xenopus embryos, SCP2/Os4 and human SCP1, 2, and 3 cause selective dephosphorylation of Smad1 compared with Smad2, inhibiting BMP- and Smad1-dependent transcription and leading to the induction of the secondary dorsal axis. In human cells, RNAi-mediated depletion of SCP1 and SCP2 increases the extent and duration of Smad1 phosphorylation in response to BMP, the transcriptional action of Smad1, and the strength of endogenous BMP gene responses. The present identification of the SCP family as Smad C-terminal phosphatases sheds light on the events that attenuate Smad signaling and reveals unexpected links to the essential phosphatases that control RNA polymerase II in eukaryotes.

STAT5 acts as a repressor to regulate early embryonic erythropoiesis

STAT5 regulates definitive (adult stage) erythropoiesis through its ability to transduce signals from the erythropoietin receptor. A function for STAT-dependent signaling during primitive (embryonic) erythropoiesis has not been analyzed. We tested this in the Xenopus system, because STAT5 is expressed at the right time and place to regulate development of the embryonic primitive ventral blood island. Depletion of STAT5 activity results in delayed accumulation of the first globin-expressing cells, indicating that the gene does regulate primitive erythropoiesis. Our results suggest that in this context STAT5 functions as a repressor, since forced expression of an activator isoform blocks erythropoiesis, while embryos expressing a repressor isoform develop normally. The erythroid phenotype caused by the activator isoform of STAT5 resembles that caused by overexpression of fibroblast growth factor (FGF). We show that STAT5 isoforms can function epistatic to FGF and can be phosphorylated in response to hyperactivated FGF signaling in Xenopus embryos. Therefore, our data indicate that STAT5 functions in both primitive and definitive erythropoiesis, but by different mechanisms.

Mesoderm induction: from caps to chips

Vertebrate mesoderm induction is one of the classical problems in developmental biology. Various developmental biology approaches, particularly in Xenopus and zebrafish, have identified many of the key factors that are involved in this process and have provided major insights into how these factors interact as part of a signalling and transcription-factor network. These data are beginning to be refined by high-throughput approaches such as microarray assays. Future challenges include understanding how the prospective mesodermal cells integrate the various signals they receive and how they resolve this information to regulate their morphogenetic behaviours and cell-fate decisions.

Roles of organizer factors and BMP antagonism in mammalian forebrain establishment

A critical question in mammalian development is how the forebrain is established. In amphibians, bone morphogenetic protein (BMP) antagonism emanating from the gastrula organizer is key. Roles of BMP antagonism and the organizer in mammals remain unclear. Anterior visceral endoderm (AVE) promotes early mouse head development, but its function is controversial. Here, we explore the timing and regulation of forebrain establishment in the mouse. Forebrain specification requires tissue interaction through the late streak stage of gastrulation. Foxa2(-/-) embryos lack both the organizer and its BMP antagonists, yet about 25% show weak forebrain gene expression. A similar percentage shows ectopic AVE gene expression distally. The distal VE may thus be a source of forebrain promoting signals in these embryos. In wild-type ectoderm explants, AVE promoted forebrain specification, while anterior mesendoderm provided maintenance signals. Embryological and molecular data suggest that the AVE is a source of active BMP antagonism in vivo. In prespecification ectoderm explants, exogenous BMP antagonists triggered forebrain gene expression and inhibited posterior gene expression. Conversely, BMP inhibited forebrain gene expression, an effect that could be antagonized by anterior mesendoderm, and promoted expression of some posterior genes. These results lead to a model in which BMP antagonism supplied by exogenous tissues promotes forebrain establishment and maintenance in the murine ectoderm.

BMP and FGF regulatory pathways control cell lineage diversification of heart valve precursor cells

The atrioventricular heart valve leaflets and chordae tendineae are composed of diverse cell lineages and highly organized extracellular matrices that share characteristics with cartilage and tendon cell types in the limb buds and somites. During embryonic chicken valvulogenesis, aggrecan and sox9, characteristic of cartilage cells, are observed in the AV valve leaflets, in contrast to tendon-associated genes scleraxis and tenascin, present in the chordae tendineae. In the limb buds and somites, cartilage cell lineage differentiation is regulated by BMP2, while FGF4 controls tendon cell fate. The ability of BMP2 and FGF4 to induce similar patterns of gene expression in heart valve precursor cells was examined. In multiple assays of cells from prefused endocardial cushions, BMP2 is sufficient to activate Smad1/5/8 phosphorylation and induce sox9 and aggrecan expression, while FGF4 treatment increases phosphorylated MAPK (dpERK) signaling and promotes expression of scleraxis and tenascin. However, these treatments do not alter differentiated lineage gene expression in valve progenitors from fused cushions of older embryos. Together, these studies define regulatory pathways of AV valve progenitor cell diversification into leaflets and chordae tendineae that share inductive interactions and differentiation phenotypes with cartilage and tendon cell lineages.

Xenopus ADAMTS1 negatively modulates FGF signaling independent of its metalloprotease activity

We have isolated the Xenopus ortholog of ADAMTS1 (a disintegrin and metalloprotease with thrombospondin motifs), XADAMTS1, which is expressed in the presumptive ectoderm, then the Spemann organizer, and later in the trunk organizer region and posterior ectoderm in the Xenopus embryo. We show that, when overexpressed in the dorsal marginal zone or in the anterior ectoderm by mRNA injection, XADAMTS1 inhibits gastrulation or generates embryos with an enlarged cement gland, respectively. XADAMTS1 also reduces the expression of Xbra in both whole embryos and FGF-treated animal caps. These effects of XADAMTS1 are likely to be due to its inhibition of the Ras-MAPK cascade because XADAMTS1 inhibits the phosphorylation of ERK by FGF4 in animal caps. Deletion analysis of XADAMTS1 revealed that a combination of the signal peptide and the C-terminal region containing the thrombospondin type 1 repeats is necessary and sufficient for this function, whereas the metalloprotease domain is dispensable. In addition, loss-of-function analysis with antisense morpholino oligos showed that knockdown of XADAMTS1 sensitizes animal caps to Xbra induction by FGF2. These data suggest that secreted XADAMTS1 negatively modulates FGF signaling in the Xenopus embryo.

Inhibitor-resistant type I receptors reveal specific requirements for TGF-beta signaling in vivo

Activin/nodal-like TGF-beta superfamily ligands signal through the type I receptors Alk4, Alk5, and Alk7, and are responsible for mediating a number of essential processes in development. SB-431542, a chemical inhibitor of activin/nodal signaling, acts by specifically interfering with type I receptors. Here, we use inhibitor-resistant mutant receptors to examine the efficacy and specificity of SB-431542 in Xenopus and zebrafish embryos. Treatment with SB-431542 eliminates Smad2 phosphorylation in vivo and generates a phenotype very similar to those observed in genetic mutants in the nodal signaling pathway. Inhibitor-resistant Alk4 efficiently rescues Smad2 signaling, developmental phenotype, and marker gene expression after inhibitor treatment. This system was used to examine type I receptor specificity for several activin/nodal ligands. We find that Alk4 can efficiently rescue signaling by a wide range of ligands, while Alk7 can only weakly rescue signaling by the same ligands. In whole embryos, nodal signaling during gastrulation can be rescued with Alk4, but not Alk7, while Alk5 can only mediate signaling by ligands expressed later in development. The combination of the ALK inhibitor SB-431542 with inhibitor-resistant ALKs provides a powerful set of tools for examining nodal/activin signaling during embryogenesis.