Xmsx-1 modifies mesodermal tissue pattern along dorsoventral axis in Xenopus laevis embryo

BMP controls dorsoventral and neural patterning in indirect-developing hemichordates providing insight into a possible origin of chordates

A defining feature of chordates is the unique presence of a dorsal hollow neural tube that forms by internalization of the ectodermal neural plate specified via inhibition of BMP signaling during gastrulation. While BMP controls dorsoventral (DV) patterning across diverse bilaterians, the BMP-active side is ventral in chordates and dorsal in many other bilaterians. How this phylum-specific DV inversion occurs and whether it is coupled to the emergence of the dorsal neural plate are unknown. Here we explore these questions by investigating an indirect-developing enteropneust from the hemichordate phylum, which together with echinoderms form a sister group of the chordates. We found that in the hemichordate larva, BMP signaling is required for DV patterning and is sufficient to repress neurogenesis. We also found that transient overactivation of BMP signaling during gastrulation concomitantly blocked mouth formation and centralized the nervous system to the ventral ectoderm in both hemichordate and sea urchin larvae. Moreover, this mouthless, neurogenic ventral ectoderm displayed a medial-to-lateral organization similar to that of the chordate neural plate. Thus, indirect-developing deuterostomes use BMP signaling in DV and neural patterning, and an elevated BMP level during gastrulation drives pronounced morphological changes reminiscent of a DV inversion. These findings provide a mechanistic basis to support the hypothesis that an inverse chordate body plan emerged from an indirect-developing ancestor by tinkering with BMP signaling.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Roles for the TGFβ superfamily in the development and survival of midbrain dopaminergic neurons

The adult midbrain contains 75% of all dopaminergic neurons in the CNS. Within the midbrain, these neurons are divided into three anatomically and functionally distinct clusters termed A8, A9 and A10. The A9 group plays a functionally non-redundant role in the control of voluntary movement, which is highlighted by the motor syndrome that results from their progressive degeneration in the neurodegenerative disorder, Parkinson's disease. Despite 50 years of investigation, treatment for Parkinson's disease remains symptomatic, but an intensive research effort has proposed delivering neurotrophic factors to the brain to protect the remaining dopaminergic neurons, or using these neurotrophic factors to differentiate dopaminergic neurons from stem cell sources for cell transplantation. Most neurotrophic factors studied in this context have been members of the transforming growth factor β (TGFβ) superfamily. In recent years, an intensive research effort has focused on understanding the function of these proteins in midbrain dopaminergic neuron development and their role in the molecular architecture that regulates the development of this brain region, with the goal of applying this knowledge to develop novel therapies for Parkinson's disease. In this review, the current evidence showing that TGFβ superfamily members play critical roles in the regulation of midbrain dopaminergic neuron induction, differentiation, target innervation and survival during embryonic and postnatal development is analysed, and the implications of these findings are discussed.

Role of Sp5 as an essential early regulator of neural crest specification in xenopus

BACKGROUND

The neural crest (NC) is a multipotent embryonic cell population, which is induced by an integration of secreted signals including BMP, Wnt, and FGF and, subsequently, NC cell fates are specified by a regulatory network of specific transcription factors. This study was undertaken to identify a role of Sp5 transcription factor in vertebrates.

RESULTS

Xenopus Sp5 is expressed in the prospective neural crest regions from gastrulation through the tadpole stages in early development. Knockdown of Sp5 caused severe defects in craniofacial cartilage, pigmentation, and dorsal fin. Gain- and loss-of-function of Sp5 led to up- and down-regulation of the expression of NC markers in the neural fold, respectively. In contrast, Sp5 had no effect on neural induction and patterning. Sp5 regulated the expression of neural plate border (NPB) specifiers, Msx1 and Pax3, and these regulatory factors recovered the expression of NC marker in the Sp5-deficient embryos. Depletion of Sp5 impaired NC induction by Wnt/β-catenin or FGF signal, whereas its co-expression rescued NC markers in embryos in which either signal was blocked.

CONCLUSIONS

These results suggest that Sp5 functions as a critical early factor in the genetic cascade to regulate NC induction downstream of Wnt and FGF pathways.

Tbx2 controls lung growth by direct repression of the cell cycle inhibitor genes Cdkn1a and Cdkn1b

Vertebrate organ development relies on the precise spatiotemporal orchestration of proliferation rates and differentiation patterns in adjacent tissue compartments. The underlying integration of patterning and cell cycle control during organogenesis is insufficiently understood. Here, we have investigated the function of the patterning T-box transcription factor gene Tbx2 in lung development. We show that lungs of Tbx2-deficient mice are markedly hypoplastic and exhibit reduced branching morphogenesis. Mesenchymal proliferation was severely decreased, while mesenchymal differentiation into fibrocytes was prematurely induced. In the epithelial compartment, proliferation was reduced and differentiation of alveolar epithelial cells type 1 was compromised. Prior to the observed cellular changes, canonical Wnt signaling was downregulated, and Cdkn1a (p21) and Cdkn1b (p27) (two members of the Cip/Kip family of cell cycle inhibitors) were strongly induced in the Tbx2-deficient lung mesenchyme. Deletion of both Cdkn1a and Cdkn1b rescued, to a large degree, the growth deficits of Tbx2-deficient lungs. Prolongation of Tbx2 expression into adulthood led to hyperproliferation and maintenance of mesenchymal progenitor cells, with branching morphogenesis remaining unaffected. Expression of Cdkn1a and Cdkn1b was ablated from the lung mesenchyme in this gain-of-function setting. We further show by ChIP experiments that Tbx2 directly binds to Cdkn1a and Cdkn1b loci in vivo, defining these two genes as direct targets of Tbx2 repressive activity in the lung mesenchyme. We conclude that Tbx2-mediated regulation of Cdkn1a and Cdkn1b represents a crucial node in the network integrating patterning information and cell cycle regulation that underlies growth, differentiation, and branching morphogenesis of this organ.

During organ formation, proliferation rates and differentiation patterns vary widely between different stages and tissue compartments. It is poorly understood how cell cycle progression is locally controlled and integrated with patterning processes in these developmental programs. Here, we used the mouse lung as a model to study how growth and differentiation are controlled on a transcriptional level. Combining genetic loss- and gain-of-function approaches, we show that the T-box transcription factor gene Tbx2 is required and sufficient to direct appropriate lung growth by maintaining proliferation and inhibiting differentiation in the mesenchymal compartment of the lung. We found that expression of the cell cycle inhibitor genes Cdkn1a (p21) and Cdkn1b (p27) inversely correlates with expression of Tbx2 and that deletion of both genes rescues, to a large degree, the growth deficits of Tbx2-mutant lungs. We further show by biochemical assays that Tbx2 directly binds to Cdkn1a and Cdkn1b loci in vivo, defining these two genes as direct targets of Tbx2 repressive activity in the lung mesenchyme. We conclude that Tbx2-mediated regulation of Cdkn1a and Cdkn1b represents a crucial module for the tissue-specific control of cell cycle progression that underlies growth, differentiation, and branching morphogenesis of this organ.

Suppression of Bmp4 signaling by the zinc-finger repressors Osr1 and Osr2 is required for Wnt/β-catenin-mediated lung specification in Xenopus

Embryonic development of the respiratory system is regulated by a series of mesenchymal-epithelial interactions that are only partially understood. Mesenchymal FGF and Wnt2/Wnt2b signaling are implicated in specification of mammalian pulmonary progenitors from the ventral foregut endoderm, but their epistatic relationship and downstream targets are largely unknown. In addition, how wnt2 and wnt2b are regulated in the developing foregut mesenchyme is unknown. We show that the Odd-skipped-related (Osr) zinc-finger transcriptional repressors Osr1 and Osr2 are redundantly required for Xenopus lung specification in a molecular pathway linking foregut pattering by FGFs to Wnt-mediated lung specification and RA-regulated lung bud growth. FGF and RA signals are required for robust osr1 and osr2 expression in the foregut endoderm and surrounding lateral plate mesoderm (lpm) prior to respiratory specification. Depletion of both Osr1 and Osr2 (Osr1/Osr2) results in agenesis of the lungs, trachea and esophagus. The foregut lpm of Osr1/Osr2-depleted embryos fails to express wnt2, wnt2b and raldh2, and consequently Nkx2.1(+) progenitors are not specified. Our data suggest that Osr1/Osr2 normally repress bmp4 expression in the lpm, and that BMP signaling negatively regulates the wnt2b domain. These results significantly advance our understanding of early lung development and may impact strategies to differentiate respiratory tissue from stem cells.

The peripheral sensory nervous system in the vertebrate head: a gene regulatory perspective

In the vertebrate head, crucial parts of the sense organs and sensory ganglia develop from special regions, the cranial placodes. Despite their cellular and functional diversity, they arise from a common field of multipotent progenitors and acquire distinct identity later under the influence of local signalling. Here we present the gene regulatory network that summarises our current understanding of how sensory cells are specified, how they become different from other ectodermal derivatives and how they begin to diversify to generate placodes with different identities. This analysis reveals how sequential activation of sets of transcription factors subdivides the ectoderm over time into smaller domains of progenitors for the central nervous system, neural crest, epidermis and sensory placodes. Within this hierarchy the timing of signalling and developmental history of each cell population is of critical importance to determine the ultimate outcome. A reoccurring theme is that local signals set up broad gene expression domains, which are further refined by mutual repression between different transcription factors. The Six and Eya network lies at the heart of sensory progenitor specification. In a positive feedback loop these factors perpetuate their own expression thus stabilising pre-placodal fate, while simultaneously repressing neural and neural crest specific factors. Downstream of the Six and Eya cassette, Pax genes in combination with other factors begin to impart regional identity to placode progenitors. While our review highlights the wealth of information available, it also points to the lack information on the cis-regulatory mechanisms that control placode specification and of how the repeated use of signalling input is integrated.

Ventx factors function as Nanog-like guardians of developmental potential in Xenopus

Vertebrate development requires progressive commitment of embryonic cells into specific lineages through a continuum of signals that play off differentiation versus multipotency. In mammals, Nanog is a key transcription factor that maintains cellular pluripotency by controlling competence to respond to differentiation cues. Nanog orthologs are known in most vertebrates examined to date, but absent from the Anuran amphibian Xenopus. Interestingly, in silico analyses and literature scanning reveal that basal vertebrate ventral homeobox (ventxs) and mammalian Nanog factors share extensive structural, evolutionary and functional properties. Here, we reassess the role of ventx activity in Xenopus laevis embryos and demonstrate that they play an unanticipated role as guardians of high developmental potential during early development. Joint over-expression of Xenopus ventx1.2 and ventx2.1-b (ventx1/2) counteracts lineage commitment towards both dorsal and ventral fates and prevents msx1-induced ventralization. Furthermore, ventx1/2 inactivation leads to down-regulation of the multipotency marker oct91 and to premature differentiation of blastula cells. Finally, supporting the key role of ventx1/2 in the control of developmental potential during development, mouse Nanog (mNanog) expression specifically rescues embryonic axis formation in ventx1/2 deficient embryos. We conclude that during Xenopus development ventx1/2 activity, reminiscent of that of Nanog in mammalian embryos, controls the switch of early embryonic cells from uncommitted to committed states.

Early neural crest induction requires an initial inhibition of Wnt signals

Neural crest (NC) induction is a long process that continues through gastrula and neurula stages. In order to reveal additional stages of NC induction we performed a series of explants where different known inducing tissues were taken along with the prospective NC. Interestingly the dorso-lateral marginal zone (DLMZ) is only able to promote the expression of a subset of neural plate border (NPB) makers without the presence of specific NC markers. We then analysed the temporal requirement for BMP and Wnt signals for the NPB genes Hairy2a and Dlx5, compared to the expression of neural plate (NP) and NC genes. Although the NP is sensitive to BMP levels at early gastrula stages, Hairy2a/Dlx5 expression is unaffected. Later, the NP becomes insensitive to BMP levels at late gastrulation when NC markers require an inhibition. The NP requires an inhibition of Wnt signals prior to gastrulation, but becomes insensitive during early gastrula stages when Hairy2a/Dlx5 requires an inhibition of Wnt signalling. An increase in Wnt signalling is then important for the switch from NPB to NC at late gastrula stages. In addition to revealing an additional distinct signalling event in NC induction, this work emphasizes the importance of integrating both timing and levels of signalling activity during the patterning of complex tissues such as the vertebrate ectoderm.

The response of early neural genes to FGF signaling or inhibition of BMP indicate the absence of a conserved neural induction module

Background

The molecular mechanism that initiates the formation of the vertebrate central nervous system has long been debated. Studies in Xenopus and mouse demonstrate that inhibition of BMP signaling is sufficient to induce neural tissue in explants or ES cells respectively, whereas studies in chick argue that instructive FGF signaling is also required for the expression of neural genes. Although additional signals may be involved in neural induction and patterning, here we focus on the roles of BMP inhibition and FGF8a.

Results

To address the question of necessity and sufficiency of BMP inhibition and FGF signaling, we compared the temporal expression of the five earliest genes expressed in the neuroectoderm and determined their requirements for induction at the onset of neural plate formation in Xenopus. Our results demonstrate that the onset and peak of expression of the genes vary and that they have different regulatory requirements and are therefore unlikely to share a conserved neural induction regulatory module. Even though all require inhibition of BMP for expression, some also require FGF signaling; expression of the early-onset pan-neural genes sox2 and foxd5α requires FGF signaling while other early genes, sox3, geminin and zicr1 are induced by BMP inhibition alone.

Conclusions

We demonstrate that BMP inhibition and FGF signaling induce neural genes independently of each other. Together our data indicate that although the spatiotemporal expression patterns of early neural genes are similar, the mechanisms involved in their expression are distinct and there are different signaling requirements for the expression of each gene.

Direct response elements of BMP within the PV.1A promoter are essential for its transcriptional regulation during early Xenopus development

Xvent homeobox genes encode transcription factors that repress organizer genes and are essential for dorsoventral specification during early embryogenesis in Xenopus. In contrast to the Xvent-2 gene subfamily, Xvent-1 subfamily members, including PV.1A, have been proposed as indirect targets of Bone Morphogenetic Protein-4 (BMP-4) signaling. Because PV.1A is a critical downstream mediator of, and tightly regulated by, BMP-4 signaling, we hypothesized that its promoter contains a direct BMP-4 response element to effect this transcriptional regulation. We demonstrate that direct regulation by BMP-4 is necessary for transcription of PV.1A: its proximal promoter contains cis-acting binding elements for Smads and Oaz crucial to induction in response to BMP-4 signaling. In addition to these direct cis-acting BMP-4 responsive elements, an indirect Xvent-2 response element and several repressive elements exist in the PV.1A promoter to regulate its transcription. In summary, PV.1A undergoes combinatorial regulation during early Xenopus development as both the direct target of BMP-4 signaling and as the direct and indirect target of positive and negative regulatory factors.

Characterization of Cer-1 cis-regulatory region during early Xenopus development

Cerberus-related molecules are well-known Wnt, Nodal, and BMP inhibitors that have been implicated in different processes including anterior–posterior patterning and left–right asymmetry. In both mouse and frog, two Cerberus-related genes have been isolated, mCer-1 and mCer-2, and Xcer and Xcoco, respectively. Until now, little is known about the mechanisms involved in their transcriptional regulation. Here, we report a heterologous analysis of the mouse Cerberus-1 gene upstream regulatory regions, responsible for its expression in the visceral endodermal cells. Our analysis showed that the consensus sequences for a TATA, CAAT, or GC boxes were absent but a TGTGG sequence was present at position -172 to -168 bp, relative to the ATG. Using a series of deletion constructs and transient expression in Xenopus embryos, we found that a fragment of 1.4 kb of Cer-1 promoter sequence could reproduce the endogenous expression pattern of Xenopus cerberus. A 0.7-kb mcer-1 upstream region was able to drive reporter expression to the involuting mesendodermal cells, while further deletions abolished reporter gene expression. Our results suggest that although no sequence similarity was found between mouse and Xenopus cerberus cis-regulatory regions, the signaling cascades regulating cerberus expression, during gastrulation, is conserved.

Origin of muscle satellite cells in the Xenopus embryo

We have studied the origin of muscle satellite cells in embryos of Xenopus laevis. Fate mapping at the open neural plate stage was carried out using orthotopic grafts from transgenic embryos expressing GFP. This shows that most satellite cells originate from the dorsolateral plate rather than from the paraxial mesoderm. Specification studies were made by isolation of explants from the paraxial and dorsolateral regions of neurulae and these also indicated that the satellite cell progenitors arise from the dorsolateral plate. Muscle satellite cells express Pax7, but overexpression of Pax7 in blastomeres of whole embryos that populate the myogenic areas does not induce the formation of additional satellite cells. Moreover, a dominant-negative construct, Pax7EnR, does not reduce satellite cell formation. Neither Pax7 nor other myogenic transcription factor genes will induce satellite cell formation in animal caps treated with FGF. However, BMP RNA or protein will do so, both for FGF-treated animal caps and for paraxial neurula explants. Conversely, the induction of Noggin in dorsolateral explants from HGEM-Noggin transgenic neurulae will block formation of satellite cells, showing that BMP signaling is required in vivo for satellite cell formation. We conclude that satellite cell progenitors are initially specified in the dorsal part of the lateral plate mesoderm and later become incorporated into the myotomes. The initial specification occurs at the neurula stage and depends on the ventral-to-dorsal BMP gradient in the early embryo.

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.

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.

Initiation of limb regeneration: the critical steps for regenerative capacity

While urodele amphibians (newts and salamanders) can regenerate limbs as adults, other tetrapods (reptiles, birds and mammals) cannot and just undergo wound healing. In adult mammals such as mice and humans, the wound heals and a scar is formed after injury, while wound healing is completed without scarring in an embryonic mouse. Completion of regeneration and wound healing takes a long time in regenerative and non-regenerative limbs, respectively. However, it is the early steps that are critical for determining the extent of regenerative response after limb amputation, ranging from wound healing with scar formation, scar-free wound healing, hypomorphic limb regeneration to complete limb regeneration. In addition to the accumulation of information on gene expression during limb regeneration, functional analysis of signaling molecules has recently shown important roles of fibroblast growth factor (FGF), Wnt/beta-catenin and bone morphogenic protein (BMP)/Msx signaling. Here, the routine steps of wound healing/limb regeneration and signaling molecules specifically involved in limb regeneration are summarized. Regeneration of embryonic mouse digit tips and anuran amphibian (Xenopus) limbs shows intermediate regenerative responses between the two extremes, those of adult mammals (least regenerative) and urodele amphibians (more regenerative), providing a range of models to study the various abilities of limbs to regenerate.

Role of Bone morphogenetic protein 4 in zebrafish semicircular canal development

Bone morphogenetic proteins (BMPs) are known to play roles in inner ear development of higher vertebrates. In zebrafish, there are several reports showing that members of the BMP family are expressed in the otic vesicle. We have isolated a novel zebrafish mutant gallery, which affects the development of the semicircular canal. Gallery merely forms the lateral and the immature anterior protrusion, and does not form posterior and ventral protrusions. We found that the expression of bmp2b and bmp4, both expressed in the normal optic vesicle at the protrusion stage, are extremely upregulated in the otic vesicle of gallery. To elucidate the role of BMPs in the development of the inner ear of zebrafish, we have applied excess BMP to the wild-type otic vesicle. The formation of protrusions was severely affected, and in some cases, they were completely lost in BMP4-treated embryos. Furthermore, the protrusions in gallery treated with Noggin were partially rescued. These data indicate that BMP4 plays an important role in the development of protrusions to form semicircular canals.

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.

Zebrafish msxB, msxC and msxE function together to refine the neural-nonneural border and regulate cranial placodes and neural crest development

The zebrafish muscle segment homeobox genes msxB, msxC and msxE are expressed in partially overlapping domains in the neural crest and preplacodal ectoderm. We examined the roles of these msx genes in early development. Disrupting individual msx genes causes modest variable defects, whereas disrupting all three produces a reproducible severe phenotype, suggesting functional redundancy. Neural crest differentiation is blocked at an early stage. Preplacodal development begins normally, but placodes arising from the msx expression domain later show elevated apoptosis and are reduced in size. Cell proliferation is normal in these tissues. Unexpectedly, Msx-deficient embryos become ventralized by late gastrulation whereas misexpression of msxB dorsalizes the embryo. These effects appear to involve Distal-less (Dlx) protein activity, as loss of dlx3b and dlx4b suppresses ventralization in Msx-depleted embryos. At the same time, Msx-depletion restores normal preplacodal gene expression to dlx3b-dlx4b mutants. These data suggest that mutual antagonism between Msx and Dlx proteins achieves a balance of function required for normal preplacodal differentiation and placement of the neural-nonneural border.

Development and evolution of lateral line placodes in amphibians I. Development

Lateral line placodes are specialized regions of the ectoderm that give rise to the receptor organs of the lateral line system as well as to the sensory neurons innervating them. The development of lateral line placodes has been studied in amphibians since the early 1900s. This paper reviews these older studies and tries to integrate them with more recent findings. Lateral line placodes are probably induced in a multistep process from a panplacodal area surrounding the neural plate. The time schedule of these inductive processes has begun to be unravelled, but little is known yet about their molecular basis. Subsequent pattern formation, morphogenesis and differentiation of lateral line placodes proceeds in most respects relatively autonomously: Onset and polarity of migration of lateral line primordia, the type, spacing, size and number of receptor organs formed, as well as the patterned differentiation of different cell types occur normally even in ectopic locations. Only the pathways for migration of lateral line primordia depend on external cues. Thus, lateral line placodes act as integrated and relatively context-insensitive developmental modules.

Identification of novel genes affecting mesoderm formation and morphogenesis through an enhanced large scale functional screen in Xenopus

The formation of mesoderm is an important developmental process of vertebrate embryos, which can be broken down into several steps; mesoderm induction, patterning, morphogenesis and differentiation. Although mesoderm formation in Xenopus has been intensively studied, much remains to be learned about the molecular events responsible for each of these steps. Furthermore, the interplay between mesoderm induction, patterning and morphogenesis remains obscure. Here, we describe an enhanced functional screen in Xenopus designed for large-scale identification of genes controlling mesoderm formation. In order to improve the efficiency of the screen, we used a Xenopus tropicalis unique set of cDNAs, highly enriched in full-length clones. The screening strategy incorporates two mesodermal markers, Xbra and Xmyf-5, to assay for cell fate specification and patterning, respectively. In addition we looked for phenotypes that would suggest effects in morphogenesis, such as gastrulation defects and shortened anterior-posterior axis. Out of 1728 full-length clones we isolated 82 for their ability to alter the phenotype of tadpoles and/or the expression of Xbra and Xmyf-5. Many of the clones gave rise to similar misexpression phenotypes (synphenotypes) and many of the genes within each synphenotype group appeared to be involved in similar pathways. We determined the expression pattern of the 82 genes and found that most of the genes were regionalized and expressed in mesoderm. We expect that many of the genes identified in this screen will be important in mesoderm formation.

A balance between the anti-apoptotic activity of Slug and the apoptotic activity of msx1 is required for the proper development of the neural crest

We have studied the pattern of programmed cell death in the neural crest and analyzed how it is controlled by the activity of the transcription factors Slug and msx1. Our results indicate that apoptosis is more prevalent in the neural folds than in the rest of the neural ectoderm. Through gain- and loss-of-function experiments with inducible forms of both Slug and msx1 genes, we showed that Slug acts as an anti-apoptotic factor whereas msx1 promotes cell death, either in the neural folds of the whole embryos, in isolated or induced neural crest and in animal cap assays. The protective effect of expressing Slug can be reversed by expressing the apoptotic factor Bax, while the apoptosis promoted by msx1 can be abolished by expressing the Xenopus homologue of Bcl2 (XR11). Furthermore, we show that Slug and msx1 control the transcription of XR11 and several caspases required for programmed cell death. In addition, expression of Bax or Bcl2, produced similar effects on the survival of the neural crest and on the development of its derivatives to those produced by altering the activity of Slug or msx1. Finally, we show that in the neural crest, the region of the neural folds where Slug is expressed, cells undergo less apoptosis, than in the region where the msx1 gene is expressed, which correspond to cells adjacent to the neural crest. We show that the expression of Slug and msx1 controls cell death in certain areas of the neural folds, and we discuss how this equilibrium is necessary to generate sharp boundaries in the neural crest territory, and to precisely control cell number among neural crest derivatives.

Neptune is involved in posterior axis and tail formation inXenopus embryogenesis

In order to elucidate the molecular mechanisms underlying the posterior axis and tail formation in embryogenesis, the function of Neptune, a zinc-finger transcription factor, in Xenopus laevis embryos was investigated. Injection of neptune mRNA into the animal pole area of embryos resulted in the formation of an additional tail structure that included a neural tube and muscle tissue. This activity required FGF signaling since coinjection of a dominant-negative FGF receptor RNA (XFD) completely blocked the formation of a tail structure. A loss-of-function experiment using a fusion construct of neptune and Drosophila engrailed (en-neptune) RNA showed that endogenous Neptune is necessary for formation of the posterior trunk and tail. Furthermore, activity of Neptune was necessary for the endogenous expression of brachyury and fgf-8 at the late gastrula stage. These findings demonstrate a novel function of Neptune in the process of anterior-posterior axis formation through the FGF and brachyury signaling cascades. An experiment using a combination explant with ventral and dorsal marginal tissues showed that cooperation of these two distinct tissues is important for the tail formation and that expression of Neptune in prospective ventral cells may be involved in the activation of the process of tail formation.

Signals derived from the underlying mesoderm are dispensable for zebrafish neural crest induction

Signals from the non-neural ectoderm, the neural ectoderm, and the underlying mesoderm have all been implicated in the induction of neural crest. Bone morphogenetic protein (BMP) signaling in particular has an important role in this process; however, it is unclear whether this activity of BMP is due to its effects on patterning the underlying mesoderm, to its ability to establish a competent neural plate boundary zone, or to the direct specification of neural crest at intermediate levels of activity within a BMP gradient. We show neural crest induction occurs in zebrafish in the absence of involuted mesoderm, indicating that this tissue and signals derived from it are dispensable for the formation of neural crest. Dorsal-involuted mesoderm is a major source of secreted BMP antagonists, and the activity of BMP signaling is thought to depend on the presence of the opposing activity of these antagonists. We find that the three BMP antagonists known to be expressed during gastrulation in zebrafish, noggin1, follistatin, and chordin, are dispensable for neural crest induction. These results suggest that mechanisms for restricting the spatio-temporal pattern of BMP expression may compensate for the loss of secreted BMP antagonist activity in establishing dorso-ventral patterning, neural induction, and the neural crest.

Roof plate and dorsal spinal cord dl1 interneuron development in the dreher mutant mouse

The establishment of neural circuits in the spinal cord depends on the differentiation of functionally distinct types of neurons in the embryonic neural tube. A number of genes have recently been shown to control the generation of dorsal interneurons through inductive signals provided by the roof plate. The roof plate is a transient signaling center on the dorsal midline of the neural tube that coordinates dorsal CNS development through the action of local peptide signals, primarily the bone morphogenic proteins (BMPs) and the Wingless-related genes (Wnts). The role of the roof plate has become evident through studies of mutations of genes in these gene families, and through several spontaneously occurring mouse mutants, including dreher(J) (dr(J)), all of which cause dorsal neural tube defects. We previously demonstrated that the roof plate is missing in the dreher mouse. Positional cloning of the dreher locus demonstrated that an inactivating point mutation in the LIM homeodomain (HD) transcription factor encoded by the Lmx1a gene, is responsible for the dreher(J) phenotype [Nature, 403 (2000) 764]. Here we report that Lmx1a is first expressed at E8.5 in a small number of cells in the lateral neural plate. As the neural tube closes, Lmx1a expression is restricted to the roof plate. In dr(J)/dr(J), although non-functional Lmx1a is correctly expressed at E8.5-E9.5, its expression is lost in the spinal cord roof plate by E10.5. Coincident with the loss of Lmx1a expression, Bmp expression fails, and the generation and differentiation of the dorsal-most spinal cord neurons, the dl1 interneurons, is abnormal. In dr(J)/dr(J) embryos, defects are evident in the number of dl1 progenitors, as well as in their migration to form the lateral and medial nuclei, and axon patterning, through mechanisms that apparently involve defects in early steps of neuronal polarity. Consistent with the general hypothesis that a failure of roof plate formation and function results in deficits in dorsal patterning of the neural tube, the dreher affects the generation and differentiation of the dl1 interneuron population.

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.

Xenopus tropicalis nodal-related gene 3 regulates BMP signaling: an essential role for the pro-region

In vertebrates, nodal-related genes are crucial for specifying mesendodermal cell fates. Six nodal-related genes have been identified in Xenopus, but only one, nodal, has been identified in the mouse. The Xenopus nodal-related gene 3 (Xnr3), however, lacks the mesoderm-inducing activity of the other five nodal-related genes in Xenopus, and can directly induce neural tissue in animal caps by antagonizing BMP signals. In this study, we isolated three clones of the Xenopus (Silurana) tropicalis nodal-related gene 3 (Xtnr3) and analyzed their function. The Xtnr3 genes show high homology to Xnr3 and have the same activity. Southern blot and genomic PCR analyses indicate that the X. tropicalis genome has duplications in the Xtnr3 gene sequences and our three clones represent separate gene loci. We also found a partial clone of Xtnr3 that coded for the N-terminal part of its pro-region. Surprisingly, this sequence also induced neural tissue by antagonizing BMP signals, and its coded protein physically associated with BMP4 mature protein. Furthermore, we showed that the pro-region of Xnr5 has the same activity. Together, these findings indicate that the pro-region of nodal-related genes acts antagonistically towards BMP signals, which identifies a novel mechanism for the inhibition of BMP signaling.

A BMP-4-dependent transcriptional control element in the 5' flanking region of Xenopus SCL gene

We isolated 5.5kb genomic DNA fragment of Xenopus stem cell leukemia (SCL) that contains approximately 1.5kb of the 5' flanking region and 4.0kb of the first intron between a non-coding exon (exon 1) and a coding exon (exon 2). Sequencing result of the 5' flanking region has shown that there is a portion that shares 85% and 69% with the sequences of avian and mammalian genomes of SCL promoter region (-64 to +73). The 1.5kb 5' flanking region of SCL genome and various deletion constructs were inserted at the upstream of luciferase (luc) gene and used for the reporter assay. The reporter activity was first detected at the neurula stage in the embryos injected with -167+157/luc at the 2-cell stage and the values increased as the stages advanced. The experiments using dominant-negative constructs revealed that the activation of SCL transcription via the 5' flanking region requires the BMP-4 and GATA factors. Taken together with the in situ hybridization analysis indicating that expression of SCL was downregulated in the central nervous system in BMP-depleted embryos, the proximal sequence of SCL consists of a stage-dependent and BMP signaling-dependent control element.

Active repression of organizer genes by C-terminal domain of PV.1

PV.1, a homeotic protein, ventralizes dorsal mesoderm and inhibits neuralization by mediating BMP-4 signaling in Xenopus embryo. In our previous report antimorphic PV.1 causes a secondary axis by inducing the ectopic organizer. We analyzed the structure of this transcription factor through domain level assessment. In a phenotype-inducing test, half of the N-terminus at the N-terminal side was unessential for inducing ventralization of embryos. We examined the transacting activity of several regions of PV.1 utilizing GAL4 hybrid system. The C-terminal region/GAL4DBD (DNA binding domain) exhibited strong repressive activity on a reporter gene (operator/promoter/reporter; Gal4-TK-luc) as much as the whole polypeptide/GAL4DBD, whereas the N-terminal region/GAL4DBD showed only modest repression. The results suggest that PV.1 functions as a transcriptional repressor and this repressive activity is localized mostly to the C-terminal region. Additional characterizations of N- and C-terminus with respect to the effects on the expression of other genes are described.

Hensen's node gives rise to the ventral midline of the foregut: implications for organizing head and heart development

Patterning of the ventral head has been attributed to various cell populations, including endoderm, mesoderm, and neural crest. Here, we provide evidence that head and heart development may be influenced by a ventral midline endodermal cell population. We show that the ventral midline endoderm of the foregut is generated directly from the extreme rostral portion of Hensen's node, the avian equivalent of the Spemann organizer. The endodermal cells extend caudally in the ventral midline from the prechordal plate during development of the foregut pocket. Thus, the prechordal plate appears as a mesendodermal pivot between the notochord and the ventral foregut midline. The elongating ventral midline endoderm delimits the right and left sides of the ventral foregut endoderm. Cells derived from the midline endoderm are incorporated into the endocardium and myocardium during closure of the foregut pocket and fusion of the bilateral heart primordia. Bilateral ablation of the endoderm flanking the midline at the level of the anterior intestinal portal leads to randomization of heart looping, suggesting that this endoderm is partitioned into right and left domains by the midline endoderm, thus performing a function similar to that of the notochord in maintaining left-right asymmetry. Because of its derivation from the dorsal organizer, its extent from the forebrain through the midline of the developing face and pharynx, and its participation in formation of a single midline heart tube, we propose that the ventral midline endoderm is ideally situated to function as a ventral organizer of the head and heart.

Transcriptional regulation of Xbr-1a/Xvent-2 homeobox gene: analysis of its promoter region

Xvent homeobox proteins are induced by BMP-4 signaling and have been known to mediate many BMP-4 activities as key downstream transcriptional factors. In order to investigate the regulatory mode of Xvent transcription, we isolated genomic DNA of the Xbr-1a/Xvent-2 containing the promoter region responsive to BMP-4 signaling. The cis-acting elements located within the Xbr-1a/Xvent-2 promoter and the regulation modes by BMP-4 signaling were analyzed by serial deletion and site-directed mutagenesis experiments. The upstream -235bp of the promoter retained the full transcriptional activity and BMP-4-response when compared with the longest promoter construct. Further analysis indicated that two separated 15bp regions contained a strong positive element and BMP-4-response element. Site-directed mutagenesis of those regions suggests that those two regions cooperate for the promoter activity and BMP-4-response. Moreover, we found that the transcription factors, Oaz and PEBP2alphaA, were able to elicit additive effects with BMP-4 signaling on Xbr-1a/Xvent-2 reporter activities. These results indicate that transcriptional regulation of the Xbr-1a/Xvent-2 gene occurs in a complex mode through the cooperation of various transcription factors.

Common and distinct signals specify the distribution of blood and vascular cell lineages in Xenopus laevis embryos

In an effort to elucidate the regulatory mechanisms that determine the fate of blood cells and vascular cells in the ventral blood island mesoderm, the embryonic expression of Xtie-2, a Xenopus homolog of the tie-2 receptor tyrosine kinase, was examined. Whole-mount in situ hybridization analysis revealed that Xtie-2 mRNA is expressed at the late tailbud stage within the regions where endothelial precursor cells exist. On the ventral side of embryos, Xtie-2-positive cells are predominantly present just outside the boundary of alpha-globin-positive cells, thus the expression pattern of these two markers seems mutually exclusive. Further experiments revealed that there is a consistent and strong correlation between the induction of Xtie-2 and alpha-globin expression in embryos and explant tissues. First, these two markers displayed overlapping expression in embryos ventralized by the removal of a "dorsal determinant" from the vegetal cytoplasm at the 1-cell stage. Second, expression of both Xtie-2 and alpha-globin were markedly induced in ectodermal explants (animal caps) from embryos co-injected with activin and bone morphogenetic protein (BMP)-4 RNA. Furthermore, both Xtie-2 and alpha-globin messages were strongly positive in dorsal marginal zone explants that had been injected with BMP-4 RNA. In contrast, however, there was a clear distinction in the localization of these two transcripts in embryos dorsalized by LiCl treatment. Distinct localization was also found in the ventral marginal zone (VMZ) explants. Using the VMZ explant system, we demonstrate a role of fibroblast growth factor (FGF) signaling in enhancing the vascular cell marker and reducing the blood cell marker. The present study suggests that the early steps of blood and vascular cell differentiation are regulated by a common BMP-4-dependent signaling; however, distinct factor(s) such as FGF are involved in different distribution of these two cell lineages.

Antimorphic PV.1 causes secondary axis by inducing ectopic organizer

Xenopus homeobox gene, PV.1 ventralizes activin-induced dorsal mesoderm and inhibits neuralization of ectoderm in animal cap when overexpressed. Here we generated PV.1/engrailed fusion construct (N-PV1-EnR) to perform loss-of-function study for this transcription factor. N-PV1-EnR showed an extremely antimorphic effect, causing a partial secondary embryonic axis when expressed at ventral marginal zone of blastula. In ventral marginal zone cells, this chimeric protein induced organizer genes and suppressed ventral markers mimicking those effects reported for dominant negative BMP-4 receptor (DNBR). Moreover, N-PV1-EnR rescued the ventralized embryos caused by the ectopic dorsal expression of PV.1 but not by that of Xvent-2. These results suggested that PV.1 functions at downstream of BMP-4 as a ventralizing effector which acts separately from Xvent-2 and the dominant negative effect gained by this specific mutant is applicable for the further studies of BMP-4 downstream pathway.

Transgenic Xenopus embryos reveal that anterior neural development requires continued suppression of BMP signaling after gastrulation

In vertebrates, BMP signaling before gastrulation suppresses neural development. Later in development, BMP signaling specifies a dorsal and ventral fate in the forebrain and dorsal fate in the spinal cord. It is therefore possible that a change in the competence of the ectoderm to respond to BMP signaling occurs at some point in development. We report that exposure of the anterior neural plate to BMP4 before gastrulation causes suppression of all neural markers tested. To determine the effects of BMP4 after gastrulation, we misexpressed BMP4 using a Pax-6 promoter fragment in transgenic frog embryos and implanted beads soaked in BMP4 in the anterior neural plate. Suppression of most anterior neural markers was observed. We conclude that most neural genes continue to require suppression of BMP signaling into the neurula stages. Additionally, we report that BMP4 and BMP7 are abundantly expressed in the prechordal mesoderm of the neurula stage embryo. This poses the paradox of how the expression of most neural genes is maintained if they can be inhibited by BMP signaling. We show that at least one gene in the anterior neural plate suppresses the response of the ectoderm to BMP signaling. We propose that the suppressive effect of BMP signaling on the expression of neural genes coupled with localized suppressors of BMP signaling result in the fine-tuning of gene expression in the anterior neural plate.

Xmeis1, a protooncogene involved in specifying neural crest cell fate in Xenopus embryos

Meis1 (Myeloid Ecotropic viral Integration Site 1) is a homeobox gene that was originally isolated as a common site of viral integration in myeloid tumors of the BXH-2 recombinant inbred mice strain. We previously isolated a Xenopus homolog of Meis1 (Xmeis1). Here we show that Xmeis1 may play a significant role in neural crest development. In developing Xenopus embryos, Xmeis1 displays a broad expression pattern, but strong expression is observed in tissue of neural cell fate, such as midbrain, hindbrain, the dorsal portion of the neural tube, and neural crest derived branchial arches. In animal cap explants, overexpression of Xmeis1b, an alternatively spliced form of Xmeis1, induces expression of neural crest marker genes in the absence of mesoderm. Moreover, Xmeis1b induces XGli-3 and XZic3, pre-pattern genes involved at the earliest stages of neural crest development, and like these two genes, can induce ectopic pigmented cell masses when overexpressed in developing embryos. Misexpression of Xmeis1b also induces ectopic expression of neural crest markers along the antero-posterior axis of the neural tube in developing Xenopus embryos. In contrast, Xmeis1a, another splice variant, is much less effective at inducing these effects. These data suggest that Xmeis1b is involved in neural crest cell fate specification during embryogenesis, and can functionally intersect with the Gli/Zic signal transduction pathway.

Patterning the early zebrafish by the opposing actions of bozozok and vox/vent

Fish and frog embryos are patterned along the dorsal-ventral axis during the gastrula stage by opposing gradients of Bmps and Bmp inhibitory proteins. Three transcriptional repressors with partially overlapping expression domains have been proposed to be important mediators of Bmp function in Xenopus. We find that two related factors are expressed in the early zebrafish embryo. Although these factors are considerably divergent from the related Xenopus genes, they are expressed in domains similar to those of their Xenopus relatives throughout embryogenesis. Both of the zebrafish genes, which we have named vox and vent, are potent ventralizing factors in both zebrafish and Xenopus embryos. Using mutants in the Bmp pathway, we find that there are Bmp-dependent and Bmp-independent domains of vox expression, whereas vent is mostly dependent upon Bmp signaling. We show that ectopic vox or vent negatively regulates expression of the early dorsal gene bozozok (boz) and that ectopic boz eliminates vox and vent expression. Moreover, the normal exclusion of vox and vent from the organizer region is lost in boz mutant embryos. Our results show that boz and vox/vent are mutually antagonistic and indicate that the early establishment of the size of the organizer domain is dependent on an interplay between these early expressed transcriptional repressors.

Involvement of BMP-4/msx-1 and FGF pathways in neural induction in the Xenopus embryo

The msx homeodomain protein is a downstream transcription factor of the bone morphogenetic protein (BMP)-4 signal and a key regulator for neural tissue differentiation. Xmsx-1 antagonizes the dorsal expression of noggin and cerberus, as revealed by in situ hybridization and reverse transcription-polymerase chain reaction assays. In animal cap explants, Xmsx-1 and BMP-4 inhibit the neural tissue differentiation induced by noggin or cerberus. A loss-of-function study using the Xmsx-1/VP-16 fusion construct indicated that neural tissue formation was directly induced by the injection of fusion ribonucleic acid, although the expression of neural cell adhesion molecule (N-CAM) in the cap was less than that in the cap injected with tBR or noggin. In contrast to the single cap assay, unexpectedly, both BMP-4 and Xmsx-1 failed to inhibit neurulation in the ectodermal explants to which the organizer mesoderm was attached. The results of cell-lineage tracing experiments indicated that the neural cells were differentiated from the animal pole tissue where the excess RNA of either BMP-4 or Xmsx-1 was injected, whereas notochord was differentiated from the organizer mesoderm. Neural tissue differentiated from BMP-4-injected ectodermal cells strongly expressed posterior neural markers, such as hoxB9 and krox20, suggesting that the posterior neural cells differentiated regardless of the existence of the BMP signal. The introduction of a dominant-negative form of the fibroblast growth factor (FGF) receptor (XFD) into the ectodermal cells drastically reduced the expression of pan and posterior neural markers (N-CAM and hoxB-9) if co-injected with BMP-4 RNA, although XFD alone at the same dose did not shut down the expression of N-CAM in the combination explants. Therefore, it is proposed that an FGF-related molecule was involved in the direct induction of posterior neural tissue in the inducing signals from the organizer mesoderm in vivo.

Xenopus msx-1 regulates dorso-ventral axis formation by suppressing the expression of organizer genes

We demonstrated previously that Xmsx-1 is involved in mesoderm patterning along the dorso-ventral axis, under the regulation of BMP-4 signaling. When Xmsx-1 RNA was injected into the dorsal blastomeres, a mass of muscle tissue formed instead of notochord. This activity was similar to that of Xwnt-8 reported previously. In this study, we investigated whether the activity of Xmsx-1 is related to the ventralizing signal and myogenesis promoting factor, Xwnt-8. Whole-mount in situ hybridization showed that Xmsx-1, Xwnt-8, and XmyoD were expressed in overlapping areas, including the ventro-lateral marginal zone at mid-gastrula stage. The expression of XmyoD was induced by the ectopic expression of either Xmsx-1 or Xwnt-8 in dorsal blastomeres, and Xwnt-8 was induced by the ectopic expression of Xmsx-1. On the other hand, the expression of Xmsx-1 was not affected by the loading of pCSKA-Xwnt-8 or dominant-negative Xwnt-8 (DN-Xwnt-8) RNA. In addition, Xmsx-1 RNA did not abrogate the formation of notochord if coinjected with DN-Xwnt-8 RNA. These results suggest that Xmsx-1 functions upstream of the Xwnt-8 signal. Furthermore, the antagonistic function of Xmsx-1 to the expression of organizer genes, such as Xlim-1 and goosecoid, was shown by in situ hybridization analysis and luciferase reporter assay using the goosecoid promoter construct. Finally if Xmsx-1/VP-16 fusion RNA, which was expected to function as a dominant-negative Xmsx-1, was injected into ventral blastomeres, a partial secondary axis formed in a significant number of embryos. In such embryos, the activity of luciferase, under the control of goosecoid promoter sequence, was significantly elevated at gastrula stage. These results led us to conclude that Xmsx-1 plays a central role in establishing dorso-ventral axis in gastrulating embryo, by suppressing the expression of organizer genes.

Roles for Msx and Dlx homeoproteins in vertebrate development

This review provides a comparative analysis of the expression patterns, functions, and biochemical properties of Msx and Dlx homeobox genes. These comprise multi-gene families that are closely related with respect to sequence features as well as expression patterns during vertebrate development. Thus, members of the Msx and Dlx families are expressed in overlapping, but distinct, patterns and display complementary or antagonistic functions, depending upon the context. A common theme shared among Msx and Dlx genes is that they are required during early, middle, and late phases of development where their differential expression mediates patterning, morphogenesis, and histogenesis of tissues in which they are expressed. With respect to their biochemical properties, Msx proteins function as transcriptional repressors, while Dlx proteins are transcriptional activators. Moreover, their ability to oppose each other's transcriptional actions implies a mechanism underlying their complementary or antagonistic functions during development.

Requirement of Xmsx-1 in the BMP-triggered ventralization of Xenopus embryos

Signaling triggered by polypeptide growth factors leads to the activation of their target genes. Several homeobox genes are known to be induced in response to polypeptide growth factors in early Xenopus development. In particular, Xmsx-1, an amphibian homologue of vertebrate Msx-1, is well characterized as a target gene of bone morphogenetic protein (BMP). Here, using a dominant-negative form of Xmsx-1 (VP-Xmsx-1), which is a fusion protein made with the virus-derived VP16 activation domain, we have examined whether Xmsx-1 activity is required in the endogenous ventralizing pathway. VP-Xmsx-1 induced a secondary body axis, complete with muscle and neural tissues, when overexpressed in ventral blastomeres, suggesting that Xmsx-1 activity is necessary for both mesoderm and ectoderm to be ventralized. We have also examined the epistatic relationship between Xmsx-1 and another ventralizing homeobox protein, Xvent-1, and show that Xmsx-1 is likely to be acting upstream of Xvent-1. We propose that Xmsx-1 is required in the BMP-stimulated ventralization pathway that involves the downstream activation of Xvent-1.

FAST-1 is a key maternal effector of mesoderm inducers in the early Xenopus embryo

We have examined the role of the maternally encoded transcription factor FAST-1 in the establishment of the mesodermal transcriptional program in Xenopus embryos. FAST-1 has been shown to associate with Smad2 and Smad4, transducers of TGFbeta superfamily signals, in response to stimulation by several TGFbeta superfamily ligands. The FAST-1/Smad2/Smad4 complex binds and activates a 50 bp activin responsive element identified in the promoter of the meso-endodermal marker Mix.2. We have now used three complementary approaches to demonstrate that FAST-1 is a central regulator of mesoderm induction by ectopic TGFbeta superfamily ligands and during endogenous patterning: ectopic expression of mutationally activated FAST-1, ectopic expression of dominant inhibitory FAST-1, and injection of a blocking antibody specific for FAST-1. Expression of constitutively transcriptionally active FAST-1 fusion protein (FAST-VP16(A)) in prospective ectoderm can directly induce the same set of general and dorsal mesodermal genes, as well as some endodermal genes, as are induced by activin or Vg1. In intact embryos, this construct can induce secondary axes similar to those induced by activin or Vg1. Conversely, expression of a FAST-1-repressor fusion (FAST-En(R)) in prospective ectoderm blocks induction of mesodermal genes by activin, while expression of FAST-En(R) in intact embryos prevents general/dorsal mesodermal gene expression and axial development. Injection of a blocking antibody specific for FAST-1 prevents induction of mesodermal response genes by activin or Vg1, but not by FGF. In intact embryos, this antibody can prevent the expression of early mesodermal markers and inhibit axis formation, demonstrating that FAST-1 is a necessary component of the first steps in the specification of mesoderm.

DNA-binding specificity and embryological function of Xom (Xvent-2)

Xom (also known as Xvent-2) is a homeobox-containing gene expressed throughout the early gastrula of the Xenopus embryo with the exception of the organizer. Activation of Xom is an immediate-early response to BMP signaling, and overexpression of Xom, like overexpression of BMP family members, causes ventralization of the embryo. In this paper we first show that Xom is a transcriptional repressor and we then define its preferred DNA-binding site. Overexpression of wild-type Xom and a dominant-negative form suggests that Xom functions by repressing transcription of goosecoid, and analysis of the goosecoid promoter reveals a site which is required for Xom-mediated repression of goosecoid promoter reporter constructs. Together, these results suggest that Xom causes down-regulation of goosecoid in a direct fashion and that this accounts, at least in part, for the ability of Xom to cause ventralization of the Xenopus embryo.

Patterning the zebrafish axial skeleton requires early chordin function

Members of the bone morphogenetic protein (BMP) family actively promote ventral cell fates, such as epidermis and blood, in the vertebrate gastrula. More dorsally, the organizer region counteracts BMP signalling through secretion of BMP-binding antagonists chordin and noggin, allowing dorsally derived tissues such as neurectoderm and somitic muscle to develop. BMPs also function in skeletal development and regeneration of bone following injury. Noggin antagonism is thought to prevent osteogenesis at sites of joint formation, whereas chordin has not yet been implicated in skeletogenesis. Analyses of zebrafish mutants have confirmed the action of chordin (chd) in opposing ventralizing signals at gastrulation. Some ventralized mutants recover and develop into fertile adults, thereby revealing a requirement for chd function for the later processes of fin and caudal skeletal patterning. We observe in mutants the misexpression of genes encoding BMPs and putative downstream genes, and ectopic sclerotomal cells. Through injections of chd mRNA into the early embryo, we restored wild-type gene expression patterns, and the resultant fish, although genotypically mutant, developed normal axial skeletons and fins. Our results demonstrate that chordin function during gastrulation is important for the correct morphogenesis of the adult zebrafish skeleton.

A statistical analysis of the overexpression of the msx2 RNA in Xenopus laevis

The msx family of genes are important during development, and implicated in the development of various craniofacial structures. Here the frog, Xenopus laevis, was used to perform gain-of-function experiments to obtain further insight into the role of the msx2 gene. mRNAs of wild-type and mutated forms of the msx2 gene were injected into developing Xenopus embryos. Phenotypic changes in these embryos were noted, scored, and subjected to statistical analyses. Overexpression of the wild-type form of the msx2 gene resulted in embryos that were ventralized, i.e. with loss of anterior structures including head and eyes. A surprising finding was the statistically significant difference in phenotypic changes (p < 0.001 when compared to a buffer-injected group) of embryos microinjected with the mRNA of a mutated form of the msx2 gene (without the homeobox region). It is proposed that the msx2 overexpression system can be used as a consistent and reliable bioassay to map and study the functions of the msx2 gene during development, especially of the craniofacial region.

Inhibitory patterning of the anterior neural plate in Xenopus by homeodomain factors Dlx3 and Msx1

Patterning of the embryonic ectoderm is dependent upon the action of negative (antineural) and positive (neurogenic) transcriptional regulators. Msx1 and Dlx3 are two antineural genes for which the anterior epidermal-neural boundaries of expression differ, probably due to differential sensitivity to BMP signaling in the ectoderm. In the extreme anterior neural plate, Dlx3 is strongly expressed while Msx1 is silent. While both of these factors prevent the activation of genes specific to the nascent central nervous system, Msx1 inhibits anterior markers, including Otx2 and cement gland-specific genes. Dlx3 has little, if any, effect on these anterior neural plate genes, instead providing a permissive environment for their expression while repressing more panneural markers, including prepattern genes belonging to the Zic family and BF-1. These properties define a molecular mechanism for translating the organizer-dependent morphogenic gradient of BMP activity into spatially restricted gene expression in the prospective anterior neural plate.

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

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

Opposite effects of FGF and BMP-4 on embryonic blood formation: roles of PV.1 and GATA-2

In adult vertebrates, fibroblast growth factor (FGF) synergizes with many hematopoietic cytokines to stimulate the proliferation of hematopoietic progenitors. In vertebrate development, the FGF signaling pathway is important in the formation of some derivatives of ventroposterior mesoderm. However, the function of FGF in the specification of the embryonic erythropoietic lineage has remained unclear. Here we address the role of FGF in the specification of the erythropoietic lineage in the Xenopus embryo. We report that ventral injection of embryonic FGF (eFGF) mRNA at as little as 10 pg at the four-cell stage suppresses ventral blood island (VBI) formation, whereas expression of the dominant negative form of the FGF receptor in the lateral mesoderm, where physiologically no blood tissue is formed, results in a dramatic expansion of the VBI. Similar results were observed in isolated ventral marginal zones and animal caps. Bone morphogenetic protein-4 (BMP-4) is known to induce erythropoiesis in the Xenopus embryo. Therefore, we examined how the BMP-4 and FGF signaling pathways might interact in the decision of ventral mesoderm to form blood. We observed that eFGF inhibits BMP-4-induced erythropoiesis by differentially regulating expression of the BMP-4 downstream effectors GATA-2 and PV.1. GATA-2, which stimulates erythropoiesis, is suppressed by FGF. PV.1, which we demonstrate to inhibit blood development, is enhanced by FGF. Additionally, PV.1 and GATA-2 negatively regulate transcription of each other. Thus, BMP-4 induces two transcription factors which have opposing effects on blood development. The FGF and BMP-4 signaling pathways interact to regulate the specification of the erythropoietic lineage.

Xenopus brain factor-2 controls mesoderm, forebrain and neural crest development

The forkhead type Brain Factor 2 from mouse and chicken help pattern the forebrain, optic vesicle and kidney. We have isolated a Xenopus homolog (Xbf2) and found that during gastrulation it is expressed in the dorsolateral mesoderm, where it helps specify this territory by downregulating BMP-4 and its downstream genes. Indeed, Xbf2 overexpression caused partial axis duplication. Interference with BMP-4 signaling also occurs in isolated animal caps, since Xbf2 induces neural tissue. Within the neurula forebrain, Xbf2 and the related Xbf1 gene are expressed in the contiguous diencephalic and telencephalic territories, respectively, and each gene represses the other. Finally, Xbf2 seems to participate in the control of neural crest migration. Our data suggest that XBF2 interferes with BMP-4 signaling, both in mesoderm and ectoderm.