Ventral expression of GATA-1 and GATA-2 in the Xenopus embryo defines induction of hematopoietic mesoderm

Normal Table of Xenopus development: a new graphical resource

Normal tables of development are essential for studies of embryogenesis, serving as an important resource for model organisms, including the frog Xenopus laevis. Xenopus has long been used to study developmental and cell biology, and is an increasingly important model for human birth defects and disease, genomics, proteomics and toxicology. Scientists utilize Nieuwkoop and Faber's classic 'Normal Table of Xenopus laevis (Daudin)' and accompanying illustrations to enable experimental reproducibility and reuse the illustrations in new publications and teaching. However, it is no longer possible to obtain permission for these copyrighted illustrations. We present 133 new, high-quality illustrations of X. laevis development from fertilization to metamorphosis, with additional views that were not available in the original collection. All the images are available on Xenbase, the Xenopus knowledgebase (http://www.xenbase.org/entry/zahn.do), for download and reuse under an attributable, non-commercial creative commons license. Additionally, we have compiled a 'Landmarks Table' of key morphological features and marker gene expression that can be used to distinguish stages quickly and reliably (https://www.xenbase.org/entry/landmarks-table.do). This new open-access resource will facilitate Xenopus research and teaching in the decades to come.

Ventx Family and Its Functional Similarities with Nanog: Involvement in Embryonic Development and Cancer Progression

The Ventx family is one of the subfamilies of the ANTP (antennapedia) superfamily and belongs to the NK-like (NKL) subclass. Ventx is a homeobox transcription factor and has a DNA-interacting domain that is evolutionarily conserved throughout vertebrates. It has been extensively studied in Xenopus, zebrafish, and humans. The Ventx family contains transcriptional repressors widely involved in embryonic development and tumorigenesis in vertebrates. Several studies have documented that the Ventx family inhibited dorsal mesodermal formation, neural induction, and head formation in Xenopus and zebrafish. Moreover, Ventx2.2 showed functional similarities to Nanog and Barx1, leading to pluripotency and neural-crest migration in vertebrates. Among them, Ventx protein is an orthologue of the Ventx family in humans. Studies have demonstrated that human Ventx was strongly associated with myeloid-cell differentiation and acute myeloid leukemia. The therapeutic potential of Ventx family inhibition in combating cancer progression in humans is discussed. Additionally, we briefly discuss genome evolution, gene duplication, pseudo-allotetraploidy, and the homeobox family in Xenopus.

Comparing Sensory Organs to Define the Path for Hair Cell Regeneration

Deafness or hearing deficits are debilitating conditions. They are often caused by loss of sensory hair cells or defects in their function. In contrast to mammals, nonmammalian vertebrates robustly regenerate hair cells after injury. Studying the molecular and cellular basis of nonmammalian vertebrate hair cell regeneration provides valuable insights into developing cures for human deafness. In this review, we discuss the current literature on hair cell regeneration in the context of other models for sensory cell regeneration, such as the retina and the olfactory epithelium. This comparison reveals commonalities with, as well as differences between, the different regenerating systems, which begin to define a cellular and molecular blueprint of regeneration. In addition, we propose how new technical advances can address outstanding questions in the field.

Expression of Bone Morphogenetic Proteins in Stromal Cells from Human Bone Marrow Long-term Culture

Highly purified primitive hemopoietic stem cells express BMP receptors but do not synthesize bone morphogenetic proteins (BMPs). However, exogenously added BMPs regulate their proliferation, differentiation, and survival. To further explore the mechanism by which BMPs might be involved in hemopoietic differentiation, we tested whether stromal cells from long-term culture (LTC) of normal human bone marrow produce BMPs, BMP receptors, and SMAD signaling molecules. Stromal cells were immunohistochemically characterized by the presence of lyzozyme, CD 31, factor VIII, CD 68, S100, alkaline phosphatase, and vimentin. Gene expression was analyzed by RT-PCR and the presence of BMP protein was confirmed by immunohistochemistry (IHC). The supportive role of the stromal cell layer in hemopoiesis in vitro was confirmed by a colony assay of clonogenic progenitors. Bone marrow stromal cells express mRNA and protein for BMP-3, -4, and -7 but not for BMP-2, -5, and -6 from the first to the eighth week of culture. Furthermore, stromal cells express the BMP type I receptors, activin-like kinase-3 (ALK-3), ALK-6, and the downstream transducers SMAD-1, -4, and -5. Thus, human bone marrow stromal cells synthesize BMPs, which might exert their effects on hemopoietic stem cells in a paracrine manner through specific BMP receptors.

Expression pattern of bcar3, a downstream target of Gata2, and its binding partner, bcar1, during Xenopus development

Primitive hematopoiesis generates red blood cells that deliver oxygen to the developing embryo. Mesodermal cells commit to a primitive blood cell fate during gastrulation and, in order to do so the mesoderm must receive non-cell autonomous signals transmitted from other germ layers. In Xenopus, the transcription factor Gata2 functions in ectodermal cells to generate or transmit the non-cell autonomous signals. Here we have identified Breast Cancer Antiestrogen Resistance 3 (bcar3) as a gene that is induced in ectodermal cells downstream of Gata2. Bcar3 and its binding partner Bcar1 function to transduce integrin signaling, leading to changes in cellular morphology, motility and adhesion. We show that gata2, bcar3 and bcar1 are co-expressed in ventral ectoderm from early gastrula to early tailbud stages. At later stages of development, bcar3 and bcar1 are co-expressed in the spinal cord, notochord, fin mesenchyme and pronephros but each shows additional unique sites of expression. These co-expression and unique expression patterns suggest that Bcar3 and Bcar1 may function together but also independently during Xenopus development.

Expression and localization of Rdd proteins in Xenopus embryo

The previous study has shown that repeated D domain-like (Rdd) proteins, a group of novel secretory proteins consisting of repeated domains of a cysteine-rich sequence, are involved in the process of blood vessel formation in Xenopus embryo. We performed further experiments to examine the localization of Rdd proteins in embryogenesis. Detection of tagged Rdd proteins expressed in blastomeres showed that Rdd proteins formed a high molecular weight complex and existed in the extracellular space. A rabbit antibody against the Rdd synthetic peptide identified a single band of 28 kD in embryonic tissue extract. By whole-mount immunostaining analysis, signal was detected in the regions of inter-somites, vitelline veins, and branchial arches at the tailbud stage. Staining of Rdd was remarkably reduced in the embryos injected with vascular endothelial growth factor Morpholino. We suggest that Rdd proteins interact with a molecule(s) associated with vascular precursor cells.

Embryonic Expression and Function of the Xenopus Ink4d Cyclin D-Dependent Kinase Inhibitor

Here we report the cloning and functional characterization of the cyclin D-dependent kinase 4 and 6 (Cdk4/6) inhibitory protein Cdkn2d/p19^Ink4d of Xenopuslaevis (Xl-Ink4d). Xl-Ink4d is the only Ink4 family gene highly expressed during Xenopus development and its transcripts were detected maternally and during neurulation. The Xl-Ink4d protein has 63% identity to mouse and human Cdkn2d/p19^Ink4d and its function as a negative regulator of cell cycle traverse is evolutionary conserved. Indeed, Xl-lnk4d can functionally substitute for mouse Cdkn2d in binding to mouse Cdk4 and inhibiting cyclin-D1-dependent CDK4 kinase activity. Further, enforced expression of Xl-lnk4d arrests mouse fibroblasts in the G1 phase of the cell cycle. These findings indicate that CDKN2d/p19^Ink4d is conserved through vertebrate evolution and suggest Xl-lnk4d may contribute to the development of Xenopuslaevis.

Combinatorial regulation of tissue specification by GATA and FOG factors

The development of complex organisms requires the formation of diverse cell types from common stem and progenitor cells. GATA family transcriptional regulators and their dedicated co-factors, termed Friend of GATA (FOG) proteins, control cell fate and differentiation in multiple tissue types from Drosophila to man. FOGs can both facilitate and antagonize GATA factor transcriptional regulation depending on the factor, cell, and even the specific gene target. In this review, we highlight recent studies that have elucidated mechanisms by which FOGs regulate GATA factor function and discuss how these factors use these diverse modes of gene regulation to control cell lineage specification throughout metazoans.

Differential distribution of competence for panplacodal and neural crest induction to non-neural and neural ectoderm

It is still controversial whether cranial placodes and neural crest cells arise from a common precursor at the neural plate border or whether placodes arise from non-neural ectoderm and neural crest from neural ectoderm. Using tissue grafting in embryos of Xenopus laevis, we show here that the competence for induction of neural plate, neural plate border and neural crest markers is confined to neural ectoderm, whereas competence for induction of panplacodal markers is confined to non-neural ectoderm. This differential distribution of competence is established during gastrulation paralleling the dorsal restriction of neural competence. We further show that Dlx3 and GATA2 are required cell-autonomously for panplacodal and epidermal marker expression in the non-neural ectoderm, while ectopic expression of Dlx3 or GATA2 in the neural plate suppresses neural plate, border and crest markers. Overexpression of Dlx3 (but not GATA2) in the neural plate is sufficient to induce different non-neural markers in a signaling-dependent manner, with epidermal markers being induced in the presence, and panplacodal markers in the absence, of BMP signaling. Taken together, these findings demonstrate a non-neural versus neural origin of placodes and neural crest, respectively, strongly implicate Dlx3 in the regulation of non-neural competence, and show that GATA2 contributes to non-neural competence but is not sufficient to promote it ectopically.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

CDK9/cyclin complexes modulate endoderm induction by direct interaction with Mix.3/mixer

Mix-related homeodomain proteins are involved in endoderm formation in the early vertebrate embryo. We used a yeast two-hybrid screen to identify proteins that interact with Mix.3/mixer to regulate endoderm induction. We demonstrate that cyclin-dependent kinase 9 (CDK9) interacts with the carboxyl terminal domain of Mix.3. CDK9 is the catalytic subunit of the PTEF-b transcription elongation complex that phosphorylates the C-terminal domain of RNA polymerase II to promote efficient elongation of nascent transcripts. Using whole embryo transcription reporter and animal pole explant assays, we show that Mix.3 activity is regulated by CDK9/cyclin complexes. Co-expression of cyclin T2 and cyclin K had different effects on Mix.3 transcriptional activity and endoderm induction. Our data suggest that binding of CDK9, and the recruitment of different cyclin partners, can modulate the endoderm-inducing activity of Mix.3 during embryonic development. Developmental Dynamics 238:1346-1357, 2009. (c) 2009 Wiley-Liss, Inc.

A p38 MAPK-CREB pathway functions to pattern mesoderm in Xenopus

Dorsal-ventral patterning is specified by signaling centers secreting antagonizing morphogens that form a signaling gradient. Yet, how morphogen gradient is translated intracellularly into fate decisions remains largely unknown. Here, we report that p38 MAPK and CREB function along the dorsal-ventral axis in mesoderm patterning. We find that the phosphorylated form of CREB (S133) is distributed in a gradient along the dorsal-ventral mesoderm axis and that the p38 MAPK pathway mediates the phosphorylation of CREB. Knockdown of CREB prevents chordin expression and mesoderm dorsalization by the Spemann organizer, whereas ectopic expression of activated CREB-VP16 chimera induces chordin expression and dorsalizes mesoderm. Expression of high levels of p38 activator, MKK6E or CREB-VP16 in embryos converts ventral mesoderm into a dorsal organizing center. p38 MAPK and CREB function downstream of maternal Wnt/beta-catenin and the organizer-specific genes siamois and goosecoid. At low expression levels, MKK6E induces expression of lateral genes without inducing the expression of dorsal genes. Loss of CREB or p38 MAPK activity enables the expansion of the ventral homeobox gene vent1 into the dorsal marginal region, preventing the lateral expression of Xmyf5. Overall, these data indicate that dorsal-ventral mesoderm patterning is regulated by differential p38/CREB activities along the axis.

Essential role for PDGF signaling in ophthalmic trigeminal placode induction

Much of the peripheral nervous system of the head is derived from ectodermal thickenings, called placodes, that delaminate or invaginate to form cranial ganglia and sense organs. The trigeminal ganglion, which arises lateral to the midbrain, forms via interactions between the neural tube and adjacent ectoderm. This induction triggers expression of Pax3, ingression of placode cells and their differentiation into neurons. However, the molecular nature of the underlying signals remains unknown. Here, we investigate the role of PDGF signaling in ophthalmic trigeminal placode induction. By in situ hybridization, PDGF receptor beta is expressed in the cranial ectoderm at the time of trigeminal placode formation, with the ligand PDGFD expressed in the midbrain neural folds. Blocking PDGF signaling in vitro results in a dose-dependent abrogation of Pax3 expression in recombinants of quail ectoderm with chick neural tube that recapitulate placode induction. In ovo microinjection of PDGF inhibitor causes a similar loss of Pax3 as well as the later placodal marker, CD151, and failure of neuronal differentiation. Conversely, microinjection of exogenous PDGFD increases the number of Pax3+ cells in the trigeminal placode and neurons in the condensing ganglia. Our results provide the first evidence for a signaling pathway involved in ophthalmic (opV) trigeminal placode induction.

A role of D domain-related proteins in differentiation and migration of embryonic cells in Xenopus laevis

We have characterized a cDNA clone, rdd (repeated D domain-like), that encodes for a secretory protein consisting of repeated domains of cysteine-rich sequence. Whole-mount in situ hybridization analysis revealed that rdd2, rdd3 and rdd4 are transiently expressed in the ventral and lateral mesoderm and the overlying ectoderm at the late gastrula and tailbud stages. Morpholino oligonucleotide (MO) was used to inhibit the translation of endogenous rdd3 and rdd4, and we found that the circulation of red blood cells completely disappears in the MO-injected tadpoles. Histological analysis showed that formation of the ventral aorta, dorsal aorta and posterior cardinal vein in the trunk region was severely disorganized in these animals. Injection of MO affected the expression of alpha-globin, a terminal differentiation marker of red blood cells, but did not affect the expression of scl, flk-1 or tie-2, suggesting that angiopoietic and hematopoietic precursor cells differentiate normally in the rdd-depleted embryo. The transplantation of labeled tissues followed by tracing of the donor cells revealed a role of rdds in migration of the embryonic angioblasts and myeloid cells. These observations first demonstrate the role of the novel cysteine-rich proteins in migration of the embryonic cells.

GATA-2 functions downstream of BMPs and CaM KIV in ectodermal cells during primitive hematopoiesis

In Xenopus, primitive blood originates from the mesoderm, but extrinsic signals from the ectoderm are required during gastrulation to enable these cells to differentiate as erythrocytes. The nature of these signals, and how they are transcriptionally regulated, is not well understood. We have previously shown that bone morphogenetic proteins (BMPs) are required to signal to ectodermal cells to generate secondary non-cell-autonomous signal(s) necessary for primitive erythropoiesis, and that calmodulin-dependent protein kinase IV (CaM KIV) antagonizes BMP signaling. The current studies demonstrate that Gata-2 functions downstream of BMP receptor activation in these same cells, and is a direct target for antagonism by CaM KIV. We show, using loss of function analysis in whole embryos and in explants, that ectodermal Gata-2 is required for primitive erythropoiesis, and that BMP signals cannot rescue blood defects caused by ectoderm removal or loss of ectodermal GATA-2. Furthermore, we provide evidence that acetylation of GATA-2 is required for its function in primitive blood formation in vivo. Our data support a model in which Gata-2 is a transcriptional target downstream of BMPs within ectodermal cells, while activation of the CaM KIV signaling pathway alters GATA-2 function posttranslationally, by inhibiting its acetylation.

The molecular basis of ageing in stem cells

Ageing is often defined in the context of telomerase activity and telomere length regulation. Most somatic cells have limited replication ability and undergo senescence eventually. Stem cells are unique as they possess more abundant telomerase activity and are able to maintain telomere lengths for a longer period. Embryonic stem cells are particularly resistant to ageing and can be propagated indefinitely. Remarkably, adult somatic cells can be reprogrammed to an ESC-like state by various means including cell fusion, exposure to ESC cell-free extracts, enforced expression of specific molecules, and somatic cell nuclear transfer. Thus, the rejuvenation of an 'aged' state can be effected by the activation of specific key molecules in the cell. Here, we argue that cellular ageing is a reversible process, and this is determined by the balance of biological molecules which directly or indirectly control telomere length and telomerase activity, either through altering gene expression and/or modulating the epigenetic state of the chromatin.

A role for GATA factors in Xenopus gastrulation movements

Gastrulation movements in Xenopus laevis are becoming increasingly well characterised, however the molecular mechanisms involved are less clear. Active migration of the leading edge mesendoderm across the fibronectin-coated blastocoel roof is necessary for further development of tissues such as head mesoderm, heart, blood and liver. The zinc finger transcription factors GATA4 and GATA6 are expressed in this migratory tissue during gastrulation, but their role here is unknown. This study further characterises the expression of GATA4 and 6 during gastrulation, and investigates their function in migratory behaviour. Gain-of-function experiments with these GATA factors induce cell spreading, polarisation and migration in non-motile presumptive ectoderm cells. Expression of a dominant-interfering form of GATA6, which inhibits transactivation of GATA targets, severely impairs the ability of dorsal leading edge mesendoderm to spread and translocate on fibronectin. Mosaic inhibition of GATA activity indicates that GATA factors function cell autonomously to induce cell spreading and movement in dorsal mesendoderm. Knockdown of specific GATA factors using anti-sense morpholinos indicates that GATA4 and GATA6 both contribute to dorsal mesendoderm migration in vitro. GATA4 and GATA6 are known to be involved in cell-specification of mesoderm and endoderm-derived tissues, but this is the first description of an additional role for these factors in cell migration.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

The effect of VEGF on blood vessels and blood cells during Xenopus development

Vascular endothelial growth factor (VEGF) is known to play an essential role in vascular development. We have overexpressed VEGF122 or VEGF170, which are equivalent to mouse VEGF120 and VEGF164, in developing Xenopus embryos. Overexpression of VEGF170 but not VEGF122 demonstrated an absence of expression of hematopoietic markers alpha-globin and GATA-1 but only in the posterior portion of the blood island. Interestingly, strong signals of endothelial markers, msr, fli-1, and tie-2, were detectable in those regions, instead of hematopoietic markers. These results suggested both that injection of VEGF170 resulted in disturbance of vasculogenesis in the posterior portion of the blood island, with excessive production of endothelial cells at the expense of blood cells, and that the anterior and posterior portions of the VBI may have distinct characteristics.

Ectodermally derived steel/stem cell factor functions non-cell autonomously during primitive erythropoiesis in Xenopus

Signals derived from nonhematopoietic tissues are essential for normal primitive erythropoiesis in vertebrates, but little is known about the nature of these signals. In Xenopus, unidentified factors secreted by ectodermal cells during gastrulation are required to enable the underlying ventral mesoderm to form blood. Steel is expressed in the ectoderm of early Xenopus embryos and is known to regulate definitive erythroid progenitor survival and differentiation in other organisms, making it an excellent candidate regulator of primitive erythropoiesis. In this study, we tested whether steel signaling is required for primitive red blood cell differentiation in mice and frogs. We show that Xsl is expressed in the ectoderm in Xenopus gastrulae and that c-kit homologs are expressed in the underlying mesoderm at the same stages of development. We present loss of function data in whole Xenopus embryos and explants that demonstrate a requirement for ectodermally derived steel to signal through c-kit in the mesoderm to support early steps in the differentiation of primitive erythroid but not myeloid cells. Finally, we show that primitive erythropoiesis is not disrupted in mouse embryos that lack c-kit function. Our data suggest a previously unrecognized and unique function of steel/c-kit during primitive erythropoiesis in Xenopus.

Determination of the minimal domains of Mix.3/Mixer required for endoderm development

The Mix/Bix family of Pax-like homeodomain transcription factors is expressed early in vertebrate development and play important roles in endoderm and mesoderm formation. Like other Pax-related homeodomain proteins, the Mix/Bix family binds DNA as monomers or dimers and dimerization is mediated by the homeodomain. While the Mix/Bix family shares extensive sequence homology within the DNA-binding homeodomain, ectopic expression of these proteins has profoundly different outcomes. Expression of Xenopus Mix.3/Mixer in explanted ectoderm results in endoderm differentiation, whereas Mix.1 expression does not. In this study we sought to define the domains of Mix.3/Mixer that are responsible for this endoderm inducing activity. We generated domain swap mutants between Mix.3/Mixer and Mix.1 and tested their ability to induce endoderm in explanted ectoderm. We demonstrate that the homeodomain and sixty-two amino acids in the carboxyl terminus are required to induce endoderm and that these domains must be on the same polypeptide and can not act in trans as a heterodimer. A Smad2 interaction motif in Mix.3/Mixer is involved in endoderm differentiation but is not essential. Thus, we have defined the regions of Mix.3/Mixer that confer endoderm-inducing activity. These studies reveal a novel co-operation between the homeodomain and a small domain in the carboxyl terminal region that is essential for Mix.3/Mixer function.

Cloning and expression pattern of the Xenopus erythropoietin receptor

Cytokine signaling plays an important role in the survival and differentiation of vertebrate hematopoietic cells. In red blood cells, erythropoietin is a key component of the differentiation program and maintains the homeostasis of the erythroid compartment. In the adult, anemia stimulates high levels of circulating erythropoietin that drives erythropoiesis to restore normal levels of red blood cells in circulation. Erythropoietin activates the erythropoietin receptor on immature red blood cell precursors to promote their survival and differentiation. Although extensively studied in mammalian systems, a complete understanding of the function of the erythropoietin receptor during primitive erythropoiesis has been lacking. To address this problem, we have cloned the Xenopus laevis erythropoietin receptor in order to further understand the development of primitive erythropoiesis. The amphibian erythropoietin receptor shares 33% amino acid sequence identity with the mammalian erythropoietin receptors and contains the conserved extracellular ligand binding and fibronectin domains, the WSXWS motif common to cytokine receptors, and several tyrosine phosphorylation sites located on the intracellular domain of the receptor. Expression of the erythropoietin receptor is first detected by in situ hybridization in the ventral blood island during tailbud stages.

Expression of Xenopus suppressor of cytokine signaling 3 (xSOCS3) is induced by epithelial wounding

The suppressor of cytokine signaling (SOCS) family of proteins are intracellular mediators of cytokine signaling. These proteins are induced rapidly by cytokine stimulation and act in a classic negative-feedback loop to attenuate the cellular response to the cytokine signal. In this study, we present the cloning and initial characterization of the Xenopus SOCS3 gene. We show that xSOCS3 is rapidly induced in response to epithelial wounding in the tadpole. The induction of xSOCS3 in response to trauma is transient with maximal expression being reached 1 hr after the injury and diminishing after that. Unlike other genes known to be responsive to wound-induced activation of the mitogen-activated protein (MAP) kinase pathway, such as Egr1, SOCS3 expression in response to trauma is unaffected by blockade of the MAP kinase pathway by chemical inhibitors.

An Oct-1 binding site mediates activation of the gata2 promoter by BMP signaling

The gata2 gene encodes a transcription factor implicated in regulating early patterning of ectoderm and mesoderm, and later in numerous cell-specific gene expression programs. Activation of the gata2 gene during embryogenesis is dependent on the bone morphogenetic protein (BMP) signaling pathway, but the mechanism for how signaling controls gene activity has not been defined. We developed an assay in Xenopus embryos to analyze regulatory sequences of the zebrafish gata2 promoter that are necessary to mediate the response to BMP signaling during embryogenesis. We show that activation is Smad dependent, since it is blocked by expression of the inhibitory Smad6. Deletion analysis identified an octamer binding site that is necessary for BMP-mediated induction, and that interacts with the POU homeodomain protein Oct-1. However, this element is not sufficient to transfer a BMP response to a heterologous promoter, requiring an additional more proximal cooperating element. Based on recent studies with other BMP-dependent promoters (Drosophila vestigial and Xenopus Xvent-2), our studies of the gata2 gene suggest that POU-domain proteins comprise a common component of the BMP signaling pathway, cooperating with Smad proteins and other transcriptional activators.

BMP4-dependent expression of Xenopus Grainyhead-like 1 is essential for epidermal differentiation

Morphogen-dependent epidermal-specific transacting factors have not been defined in vertebrates. We demonstrate that a member of the grainyhead transcription factor family, Grainyhead-like 1 (XGrhl1) is essential for ectodermal ontogeny in Xenopus laevis. Expression of this factor is restricted to epidermal cells. Moreover, XGrhl1 is regulated by the BMP4 signaling cascade. Disruption of XGrhl1 activity in vivo results in a severe defect in terminal epidermal differentiation, with inhibition of XK81A1 epidermal keratin gene expression, a key target of BMP4 signaling. Furthermore, transcription of the XK81A1 gene is modulated directly by binding of XGRHL1 to a promoter-localized binding motif that is essential for high-level expression. These results establish a novel developmental role for XGrhl1 as a crucial tissue-specific regulator of vertebrate epidermal differentiation.

Zygotic nucleosome assembly protein-like 1 has a specific, non-cell autonomous role in hematopoiesis

Nucleosome assembly proteins (NAPs) bind core histones, facilitate chromatin remodeling, and can act as transcriptional coactivators. We previously described the isolation of a Xenopus NAP1-like (xNAP1L) cDNA, which encodes a member of this protein family. Its zygotic expression is restricted to neural cells, the outer cells of the ventral blood island (VBIs), and the ectoderm overlying the blood precursors. Here, we report that depletion of zygotic xNAP1L in embryos produces no obvious morphologic phenotype, but ablates alpha-globin mRNA expression in the VBIs. Transcript levels of the hematopoietic precursor genes SCL and Xaml (Runx-1) are also reduced in the VBIs. SCL expression can be rescued by injection of xNAP1L mRNA into the ectoderm, showing that the effect of xNAP1L can be non-cell autonomous. Fli1 and Hex, genes expressed in hemangioblasts but subsequently endothelial markers, were unaffected, suggesting that xNAP1L is required for the hematopoietic lineage specifically. Our data are consistent with a requirement for xNAP1L upstream of SCL, and injection of SCL mRNA into xNAP1L-depleted embryos rescues alpha-globin expression. Thus, xNAP1L, which belongs to a family of proteins previously believed to have general roles, has a specific function in hematopoiesis.

Hematopoietic capacity of adult human skeletal muscle is negligible

Using experimental mouse models, hematopoietic potential has been shown to exist within skeletal muscle. In humans, the clinical utility of using muscle-derived hematopoietic progenitors remains uncertain. Here, we evaluate the hematopoietic potential of human skeletal muscle. De novo adult muscle contained markedly reduced levels of hematopoietic colony-forming units (hCFU) and negligible responsiveness to hematopoietic ex vivo culture conditions that augment hematopoietic activity of fetal muscle. Neither fetal nor adult muscle yielded significant engraftment in transplanted immune-deficient mice. Although adult muscle possessed 1.5+/-0.9 hCFU/g, similar hematopoietic activity (2.3+/-0.17 hCFU) could also be demonstrated from as little as 3-10 microl of contaminating peripheral blood. We suggest that the clinical utility of adult skeletal muscle as an alternative source of hematopoietic cells in humans appears limited due to the low yield of blood-forming precursors and their lack of responsiveness to ex vivo expansion.

Pur alpha and Sp8 as opposing regulators of neural gata2 expression

Gata2 is an essential hematopoietic transcriptional factor that is also expressed prominently in the nervous system. The early lethality of knockout mice due to severe anemia has largely precluded studies of gata2 neural regulation and function. In this report, we describe the identification of zebrafish Pur alpha and Sp8 orthologs as two factors that function to regulate neuronal expression of gata2. During embryogenesis, Pur alpha is expressed widely, whereas Sp8 has an overlapping pattern of expression with gata2 in the nervous system. Knockdown and ectopic expressions of Pur alpha and Sp8 indicate that these factors function, respectively, as a repressor and an activator of gata2 gene expression in the nervous system. With consideration given to the previously established roles for these factors, we propose a model for how the transcriptional regulation of neural gata2 expression may be involved in controlling cellular proliferation in the nervous system.

BMP4: Its Role in Development of the Hematopoietic System and Potential as a Hematopoietic Growth Factor

Blood formation occurs throughout the life of an individual in a process driven by hematopoietic stem cells (HSCs). The ability of bone marrow (BM) and cord blood (CB) HSC to undergo self-renewal and develop into multiple blood lineages has made these cells an important clinical resource. Transplantation with BM- and CB-derived HSCs is now used extensively for treatment of hematological disorders, malignancies, and immunodeficiencies. An understanding of the embryonic origin of HSC and the factors regulating their generation and expansion in vivo will provide important information for the manipulation of these cells ex vivo. This is critical for the further development of CB transplantation, the potential of which is limited by small numbers of HSC in the donor population. Although the origins of HSCs have become clearer and progress has been made in identifying genes that are critical for the formation and maintenance of HSCs, less is known about the signals that commit specific populations of mesodermal precursors to hematopoietic cell fate. Critical signals acting on these precursor cells are likely to be derived from visceral endoderm in yolk sac and from underlying stroma in the aorta-gonad-mesonephros region. Here we summarize briefly the origin of yolk sac and embryonic HSCs before detailing evidence that bone morphogenic protein-4 (BMP4) has a crucial role in Xenopus and mammalian HSC development. We discuss evidence that BMP4 acts as a hematopoietic growth factor and review its potential to modulate HSC in ex vivo expansion cultures from cord blood.

Essential role of HGF (hepatocyte growth factor) in blood formation in Xenopus

In this study, we investigated the role of hepatocyte growth factor (HGF) in blood formation during Xenopus development. First, we examined the gene expression of HGF and its receptor, c-met, by whole-mount in situ hybridization during development. Strong signals of HGF as well as c-met were detected early in the developing ventral mesoderm, which later gives rise to the ventral blood island. Furthermore, to study the role of HGF, we blocked the HGF signaling pathway in Xenopus embryos by using truncated c-met lacking the tyrosine kinase domain. Injection of truncated c-met mRNA resulted in a marked decrease in the number of circulating blood cells. Similar results were obtained using morpholino antisense HGF oligonucleotides. Moreover, we also analyzed the expression of several early primitive blood markers in the blood island of these embryos. RNA in situ analysis revealed a significant reduction (or absence) of stem cell leukemia (SCL), alpha-globin, and GATA-1 expression, but not GATA-2 expression. In contrast, no significant difference was observed in the levels of expression of early definitive blood markers, SCL, GATA-2, and GATA-3 in the dorsolateral plate, as analyzed by in situ hybridization. Overall, the present study demonstrated that HGF is necessary for primitive hematopoiesis by regulating the expression of SCL.

Xenopus Xlmo4 is a GATA cofactor during ventral mesoderm formation and regulates Ldb1 availability at the dorsal mesoderm and the neural plate

We have identified and functionally characterized the Xenopus Xlmo4 gene, which encodes a member of the LIM-domain-only protein family. Xlmo4 is activated at gastrula stages in the mesodermal marginal zone probably in response to BMP4 signaling. Soon after, Xlmo4 is downregulated in the dorsal region of the mesoderm. This repression seems to be mediated by organizer-expressed repressors, such as Gsc. Xlmo4 downregulation is necessary for the proper formation of this territory. Increasing Xlmo4 function in this region downregulates Spemman Organizer genes and suppresses dorsal-anterior structures. By binding to Ldb1, Xlmo4 may restrict the availability of this cofactor for transcription factors expressed at the Spemman Organizer. In the ventral mesoderm, Xlmo4 is required to establish the identity of this territory by acting as a positive cofactor of GATA factors. In the neural ectoderm, Xlmo4 expression depends on Xiro homeoprotein activity. In this region, Xlmo4 suppresses differentiation of primary neurons and interferes with gene expression at the Isthmic Organizer, most likely by displacing Ldb1 from active transcription factor complexes required for these processes. Together, our data suggest that Xlmo4 uses distinct mechanisms to participate in different processes during development.

Primitive erythropoiesis is regulated by Smad-dependent signaling in postgastrulation mesoderm

The bone morphogenetic proteins (BMPs) are required for the development of ventral mesoderm, which contributes to the ventral blood island and primitive (yolk sac stage) hematopoiesis. Primitive erythropoiesis is defective when BMP signaling is blocked during gastrulation of Xenopus embryos. This phenotype might be attributed to changes in mesoderm patterning leading indirectly to altered erythropoiesis. We developed an inducible system in order to block BMP signaling in a controlled fashion at later time points in development. For this purpose, an inhibitory Smad, xSmad6, was fused to the estrogen receptor ligand-binding domain. We show that ER-xSmad6 is inactive when expressed in developing embryos, but its activity is induced by estradiol. When induced early in development, ER-xSmad6 causes a dorsalized phenotype, equivalent to overexpression of native xSmad6. When ER-xSmad6 is induced after gastrulation, there is a specific defect in primitive erythropoiesis without any apparent effect on axial patterning. Our results identify an embryonic signal that is Smad-dependent, is required for maintaining expression of GATA-1, and functions within mesoderm and not the overlying ectoderm. Thus, BMP signaling is necessary both during mesoderm patterning and also following early specification events for proper regulation of the primitive erythroid lineage.

Neural Tissue in Ascidian Embryos Is Induced by FGF9/16/20, Acting via a Combination of Maternal GATA and Ets Transcription Factors

In chordates, formation of neural tissue from ectodermal cells requires an induction. The molecular nature of the inducer remains controversial in vertebrates. Here, using the early neural marker Otx as an entry point, we dissected the neural induction pathway in the simple embryos of Ciona intestinalis. We first isolated the regulatory element driving Otx expression in the prospective neural tissue, showed that this element directly responds to FGF signaling and that FGF9/16/20 acts as an endogenous neural inducer. Binding site analysis and gene loss of function established that FGF9/16/20 induces neural tissue in the ectoderm via a synergy between two maternal response factors. Ets1/2 mediates general FGF responsiveness, while the restricted activity of GATAa targets the neural program to the ectoderm. Thus, our study identifies an endogenous FGF neural inducer and its early downstream gene cascade. It also reveals a role for GATA factors in FGF signaling.

From mesoderm to blood islands: patterns of key molecules during yolk sac erythropoiesis

Several identified genes play key roles in the specification of the blood-forming system, from commitment of mesoderm to differentiation of hemopoietic and endothelial cells. We have thoroughly analyzed the expression dynamics of some of these genes during yolk sac erythropoiesis in the chick embryo. The study includes transcription factors which are known to participate in multimeric complexes: GATA-1, -2, SCL/tal-1 and Lmo2 (whose avian orthologue we have cloned), VEGF-R2, a critical regulator of hemopoietic and endothelial commitment, and hemoglobin used as a marker of the last step in erythroid differentiation. Several findings were unexpected. (1) Two distinct patterns were revealed for GATA-2, first: low expression, ubiquitous in all mesodermal cells, as soon as cells ingress through the primitive streak; secondly: high, blood island-specific expression. (2) VEGF-R2 is coexpressed with GATA-2 at the level of the primitive streak. (3) SCL and Lmo2 expression is restricted to presumptive hemangioblasts. (4) The up-regulation of GATA-2 in newly formed blood islands is shortly followed by GATA-1 expression. (5) Lmo2 is up-regulated in blood island angioblasts thus appearing as one of the earliest markers for endothelial cell commitment. VEGF-R2 is down-regulated in hemopoietic cells prior to GATA-2, SCL/tal-1, Lmo2 and GATA-1 in erythroblasts.

The secreted Frizzled-related protein Sizzled functions as a negative feedback regulator of extreme ventral mesoderm

The prevailing model of dorsal ventral patterning of the amphibian embryo predicts that the prospective mesoderm is regionalized at gastrulation in response to a gradient of signals. This gradient is established by diffusible BMP and Wnt inhibitors secreted dorsally in the Spemann organizer. An interesting question is whether ventrolateral tissue passively reads graded levels of ventralizing signals, or whether local self-organizing regulatory circuits may exist on the ventral side to control cell behavior and differentiation at a distance from the Organizer. We provide evidence that sizzled, a secreted Frizzled-related protein expressed ventrally during and after gastrulation, functions in a negative feedback loop that limits allocation of mesodermal cells to the extreme ventral fate, with direct consequences for morphogenesis and formation of the blood islands. Morpholino-mediated knockdown of Sizzled protein results in expansion of ventral posterior mesoderm and the ventral blood islands, indicating that this negative regulation is required for proper patterning of the ventral mesoderm. The biochemical activity of sizzled is apparently very different from that of other secreted Frizzled-related proteins, and does not involve inhibition of Wnt8. Our data are consistent with the existence of some limited self-organizing properties of the extreme ventral mesoderm.

Revisions to the Xenopus gastrula fate map: implications for mesoderm induction and patterning

A revised fate map of the gastrula Xenopus embryo predicts the existence of patterning mechanisms that operate within the animal/vegetal axis of the mesoderm-forming marginal zone. We review here molecular and embryologic data that demonstrate that such mechanisms are present and that they operate independently of the Spemann organizer. Evidence suggests that polarized fibroblast growth factor activity in the animal/vegetal axis patterns this axis. We present a model of mesoderm induction and patterning that integrates the new data on Spemann organizer-independent animal/vegetal patterning with data on other inductive pathways known to act on the gastrula marginal zone.

Emergence of muscle and neural hematopoiesis in humans

During human development, hematopoiesis is thought to be compartmentalized to the fetal circulation, liver, and bone marrow. Here, we show that combinations of cytokines together with bone morphogenetic protein-4 and erythropoietin could induce multiple blood lineages from human skeletal muscle or neural tissue. Under defined serum-free conditions, the growth factors requirements, proliferation, and differentiation capacity of muscle and neural hematopoiesis were distinct to that derived from committed hematopoietic sites and were uniquely restricted to CD45(-)CD34(-) cells expressing the prominin AC133. Our study defines epigenetic factors required for the emergence of hematopoiesis from unexpected tissue origins and illustrates that embyronically specified microenvironments do not limit cell fate in humans.

Primitive and definitive blood share a common origin in Xenopus: a comparison of lineage techniques used to construct fate maps

Primitive blood constitutes the ventralmost mesoderm in amphibians, and its cleavage-stage origin reveals important clues about the orientation of the dorsal/ventral axis in the embryo. In recent years, investigators employing various lineage-labeling strategies have reported disparate results for the origin of primitive blood in Xenopus [W. D. Tracey, Jr., M. E. Pepling, G. H. Thomsen, and J. P. Gergen (1998). Development 125, 1371-1380; M. C. Lane W. C. Smith (1999). Development 126, 423-434; K. R. Mills, D. Kruep, and M. S. Saha (1999). Dev. Biol. 209, 352-368; A. Ciau-Uitz, M. Walmsley, and R. Patient (2000). Cell 102, 787-796]. These discrepancies must be resolved in order to elucidate early embryonic patterning mechanisms in vivo. We directly compared two of the techniques used to determine the origin of the ventral blood islands and primitive blood, injection of either beta-galactosidase mRNA or conjugated dextrans, by coinjecting both tracers simultaneously into individual blastomeres in cleavage-stage embryos. We find that dextrans label progeny efficiently, while beta-galactosidase activity is not present in many of the progeny of an injected blastomere, suggesting that mRNA fails to diffuse throughout a blastomere. This result demonstrates that beta-galactosidase mRNA fails to meet the criterion for a true lineage label, namely efficient detection of the progeny of a blastomere, and raises questions about interpretations based on mapping the ventral blood islands using Lac Z mRNA as a tracer. We examined the origins of the ventral blood islands and primitive blood from the vegetal region of the marginal zone in regularly cleaving embryos by coinjecting both reporters into C-tier blastomeres. Our results demonstrate that both the ventral blood islands and primitive blood routinely arise from all C-tier blastomeres. Our data, in combination with published mapping results for the dorsal aorta, demonstrate that primitive and definitive blood do not have separate origins at the 32-cell stage in Xenopus. In addition, these results support a proposal to align the dorsal/ventral axis of the mesendoderm with the animal/vegetal axis in pregastrula Xenopus.

Cooperative interaction of Xvent-2 and GATA-2 in the activation of the ventral homeobox gene Xvent-1B

The Xvent family of homeobox transcription factors is essential for the establishment of the dorsal-ventral body axis during Xenopus embryogenesis. In contrast to Xvent-2B and other members of the Xvent-2 subfamily, Xvent-1B is not a direct response gene of bone morphogenetic protein-4 signaling. Xvent-1B is activated by Xvent-2, but CHX experiments revealed the requirement of additional factors. In this study, we report on the cooperative effect of Xvent-2 and the zinc finger transcription factor GATA-2 on the promoter of the Xvent-1B gene. We show that GATA-2 is a direct target gene of bone morphogenetic protein-4 and that GATA-2 interacts with Xvent-2 to activate transcription of Xvent-1B. Both transcription factors bind to distinct elements within the Xvent-1B promoter, and GATA-2 physically interacts with the C-terminal domain of Xvent-2. Promoter/reporter studies in Xenopus embryos revealed that full activation of Xvent-1B requires both Xvent-2 and GATA-2. Moreover, the two factors are sufficient to direct transcription of Xvent-1B in the presence of CHX at the ventral side of the embryo. The failure of both factors to activate Xvent-1B on the dorsal side suggests the existence of a dorsal inhibitor. This inhibitor is likely a component of the dorsal Wnt signaling pathway because nuclear translocation of beta-catenin before midblastula transition results in a suppression of Xvent-1B transcription.

Zebrafish DeltaNp63 is a direct target of Bmp signaling and encodes a transcriptional repressor blocking neural specification in the ventral ectoderm

Bone morphogenetic proteins (Bmps) promote ventral specification in both the mesoderm and the ectoderm of vertebrate embryos. Here we identify zebrafish DeltaNp63, encoding an isoform of the p53-related protein p63, as an ectoderm-specific direct transcriptional target of Bmp signaling. DeltaNp63 itself acts as a transcriptional repressor required for ventral specification in the ectoderm of gastrulating embryos. Loss of DeltaNp63 function leads to reduced nonneural ectoderm followed by defects in epidermal development during skin and fin bud formation. In contrast, forced DeltaNp63 expression blocks neural development and promotes nonneural development, even in the absence of Bmp signaling. Together, DeltaNp63 fulfills the criteria to be the neural repressor postulated by the "neural default model."

Calmodulin-dependent protein kinase IV mediated antagonism of BMP signaling regulates lineage and survival of hematopoietic progenitors

In the current study, we show that bone morphogenetic proteins (BMPs) play a role in hematopoiesis that is independent of their function in specifying ventral mesodermal fate. When BMP activity is upregulated or inhibited in Xenopus embryos hematopoietic precursors are specified properly but few mature erythrocytes are generated. Distinct cellular defects underlie this loss of erythrocytes: inhibition of BMP activity induces erythroid precursors to undergo apoptotic cell death, whereas constitutive activation of BMPs causes an increase in commitment of hematopoietic progenitors to myeloid differentiation and a concomitant decrease in erythrocytes that is not due to enhanced apoptosis. These blood defects are observed even when BMP activity is misregulated solely in non-hematopoietic (ectodermal) cells, demonstrating that BMPs generate extrinsic signals that regulate hematopoiesis independent of mesodermal patterning. Further analysis revealed that endogenous calmodulin-dependent protein kinase IV (CaM KIV) is required to negatively modulate hematopoietic functions of BMPs downstream of receptor activation. Our data are consistent with a model in which CaM KIV inhibits BMP signals by activating a substrate, possibly cAMP-response element-binding protein (CREB), that recruits limiting amounts of CREB binding protein (CBP) away from transcriptional complexes functioning downstream of BMPs.

Role of the thrombopoietin (TPO)/Mpl system: c-Mpl-like molecule/TPO signaling enhances early hematopoiesis in Xenopus laevis

Multiple organs are induced in the primitive embryonic ectoderm excised from blastula stage Xenopus laevis embryos, under the strict control of mesoderm inducing factors. This in vitro system is useful for exploring the mechanisms of development. In this study, the function of thrombopoietin (TPO)/c-Mpl signaling in the development of hematopoietic cells was investigated. An optimal hematopoietic cell induction system was established to evaluate the influence of growth factors on hematopoiesis. It was found that exogenous TPO enhanced hematopoiesis in explants induced by activin and bone morphogenetic protein (BMP)-4 and increased the number of both erythrocytes and leukocytes in a dose-dependent manner. Addition of anti-c-Mpl antibody completely inhibited the expansion of hematopoietic cells stimulated by TPO, and the antibody specifically recognized blood-like cells. These results demonstrate that TPO acts on hematopoietic progenitors induced in explants and the c-Mpl-like molecule in Xenopus mediates the cellular function of TPO. We also found that forced expression of TPO in embryos promoted hematopoiesis in the ventral blood island and the dorsal-- lateral plate mesoderm. These results suggest that hematopoietic stem and progenitor cells are regulated by TPO/c-Mpl signaling from when they appear in their ontogeny. They also suggest that TPO/c-Mpl signaling play a crucial role in the formation of hematopoietic cells in Xenopus.

Non-cell autonomous requirement for thebloodlessgene in primitive hematopoiesis of zebrafish

Vertebrate hematopoiesis occurs in two distinct phases, primitive (embryonic) and definitive (adult). Genes that are required specifically for the definitive program, or for both phases of hematopoiesis, have been described. However, a specific regulator of primitive hematopoiesis has yet to be reported. The zebrafish bloodless (bls) mutation causes absence of embryonic erythrocytes in a dominant but incompletely penetrant manner. Primitive macrophages appear to develop normally in bls mutants. Although the thymic epithelium forms normally in bls mutants, lymphoid precursors are absent. Nonetheless, the bloodless mutants can progress through embryogenesis, where red cells begin to accumulate after 5 days post-fertilization (dpf). Lymphocytes also begin to populate the thymic organs by 7.5 dpf. Expression analysis of hematopoietic genes suggests that formation of primitive hematopoietic precursors is deficient in bls mutants and those few blood precursors that are specified fail to differentiate and undergo apoptosis. Overexpression of scl, but not bmp4 or gata1, can lead to partial rescue of embryonic blood cells in bls. Cell transplantation experiments show that cells derived from bls mutant donors can differentiate into blood cells in a wild-type host, but wild-type donor cells fail to form blood in the mutant host. These observations demonstrate that the bls gene product is uniquely required in a non-cell autonomous manner for primitive hematopoiesis, potentially acting via regulation of scl.

otx2 expression in the ectoderm activates anterior neural determination and is required for Xenopus cement gland formation

We previously showed that otx2 regulates Xenopus cement gland formation in the ectoderm. Here, we show that otx2 is sufficient to direct anterior neural gene expression, and that its activity is required for cement gland and anterior neural determination. otx2 activity at midgastrula activates anterior and prevents expression of posterior and ventral gene expression in whole embryos and ectodermal explants. These data suggest that part of the mechanism by which otx2 promotes anterior determination involves repression of posterior and ventral fates. A dominant negative otx2-engrailed repressor fusion protein (otx2-En) ablates endogenous cement gland formation, and inhibits expression of the mid/hindbrain boundary marker engrailed-2. Ectoderm expressing otx2-En is not able to respond to signals from the mesoderm to form cement gland, and is impaired in its ability to form anterior neural tissue. These results compliment analyses in otx2 mutant mice, indicating a role for otx2 in the ectoderm during anterior neural patterning.

neptune, a Krüppel-like transcription factor that participates in primitive erythropoiesis in Xenopus

The specification of the erythroid lineage from hematopoietic stem cells requires the expression and activity of lineage-specific transcription factors. One transcription factor family that has several members involved in hematopoiesis is the Krüppel-like factor (KLF) family [1]. For example, erythroid KLF (EKLF) regulates beta-globin expression during erythroid differentiation [2-6]. KLFs share a highly conserved zinc finger-based DNA binding domain (DBD) that mediates binding to CACCC-box and GC-rich sites, both of which are frequently found in the promoters of hematopoietic genes. Here, we identified a novel Xenopus KLF gene, neptune, which is highly expressed in the ventral blood island (VBI), cranial ganglia, and hatching and cement glands. neptune expression is induced in response to components of the BMP-4 signaling pathway in injected animal cap explants. Similar to its family member, EKLF, Neptune can bind CACCC-box and GC-rich DNA elements. We show that Neptune cooperates with the hematopoietic transcription factor XGATA-1 to enhance globin induction in animal cap explants. A fusion protein comprised of Neptune's DBD and the Drosophila engrailed repressor domain suppresses the induction of globin in ventral marginal zones and in animal caps. These studies demonstrate that Neptune is a positive regulator of primitive erythropoiesis in Xenopus.

The neural plate specifies somite size in the Xenopus laevis gastrula

The organizer has traditionally been considered the major source of somite-inducing signals. We show here that signaling from the neural plate specifies somite tissue and regulates somite size in the Xenopus gastrula. Ectopic undifferentiated neural tissue induces massive somite expansion at the expense of intermediate and lateral plate mesoderm. Although the early expanded somite expresses muscle-specific markers, only a portion terminally differentiates, suggesting that myotome development requires additional signals. Explant assays demonstrate that neural tissue induces somite-specific marker expression even in the absence of the organizer. Finally, we demonstrate that neural tissue is required for proper somite development because elimination of neural precursors results in pronounced somite reduction. Thus, an important reciprocal interaction exists between somite and neural tissue that is mutually reinforcing and critical for normal embryonic patterning.