Sox3 expression is maintained by FGF signaling and restricted to the neural plate by Vent proteins in the Xenopus embryo

Nodal and churchill1 position the expression of a notch ligand during Xenopus germ layer segregation

Churchill and Nodal signaling, which participate in vertebrates’ germ layer induction, position a domain of Delta/Notch activity, which refines germ layer boundaries during frog gastrulation.

In vertebrates, Nodal signaling plays a major role in endomesoderm induction, but germ layer delimitation is poorly understood. In avian embryos, the neural/mesoderm boundary is controlled by the transcription factor CHURCHILL1, presumably through the repressor ZEB2, but there is scarce knowledge about its role in other vertebrates. During amphibian gastrulation, Delta/Notch signaling refines germ layer boundaries in the marginal zone, but it is unknown the place this pathway occupies in the network comprising Churchill1 and Nodal. Here, we show that Xenopus churchill1 is expressed in the presumptive neuroectoderm at mid-blastula transition and during gastrulation, upregulates zeb2, prevents dll1 expression in the neuroectoderm, and favors neuroectoderm over endomesoderm development. Nodal signaling prevents dll1 expression in the endoderm but induces it in the presumptive mesoderm, from where it activates Notch1 and its target gene hes4 in the non-involuting marginal zone. We propose a model where Nodal and Churchill1 position Dll1/Notch1/Hes4 domains in the marginal zone, ensuring the delimitation between mesoderm and neuroectoderm.

Regulation of Oct4 in stem cells and neural crest cells

During embryonic development, cells gradually restrict their developmental potential as they exit pluripotency and differentiate into various cell types. The POU transcription factor Oct4 (encoded by Pou5f1) lies at the center of the pluripotency machinery that regulates stemness and differentiation in stem cells, and is required for reprogramming of somatic cells into induced pluripotent stem cells (iPSCs). Several studies have revealed that Oct4 and other stemness genes are also expressed in multipotent cell populations such as neural crest cells (NCCs), and are required to expand the NCC developmental potential. Transcriptional regulation of Oct4 has been studied extensively in stem cells during early embryonic development and reprogramming, but not in NCCs. Here, we review how Oct4 is regulated in pluripotent stem cells, and address some of the gaps in knowledge about regulation of the pluripotency network in NCCs.

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.

Mesoderm induction and patterning: Insights from neuromesodermal progenitors

The discovery of mesoderm inducing signals helped usher in the era of molecular developmental biology, and today the mechanisms of mesoderm induction and patterning are still intensely studied. Mesoderm induction begins during gastrulation, but recent evidence in vertebrates shows that this process continues after gastrulation in a group of posteriorly localized cells called neuromesodermal progenitors (NMPs). NMPs reside within the post-gastrulation embryonic structure called the tailbud, where they make a lineage decision between ectoderm (spinal cord) and mesoderm. The majority of NMP-derived mesoderm generates somites, but also contributes to lateral mesoderm fates such as endothelium. The discovery of NMPs provides a new paradigm in which to study vertebrate mesoderm induction. This review will discuss mechanisms of mesoderm induction within NMPs, and how they have informed our understanding of mesoderm induction more broadly within vertebrates as well as animal species outside of the vertebrate lineage. Special focus will be given to the signaling networks underlying NMP-derived mesoderm induction and patterning, as well as emerging work on the significance of partial epithelial-mesenchymal states in coordinating cell fate and morphogenesis.

The Organizer and Its Signaling in Embryonic Development

Germ layer specification and axis formation are crucial events in embryonic development. The Spemann organizer regulates the early developmental processes by multiple regulatory mechanisms. This review focuses on the responsive signaling in organizer formation and how the organizer orchestrates the germ layer specification in vertebrates. Accumulated evidence indicates that the organizer influences embryonic development by dual signaling. Two parallel processes, the migration of the organizer’s cells, followed by the transcriptional activation/deactivation of target genes, and the diffusion of secreting molecules, collectively direct the early development. Finally, we take an in-depth look at active signaling that originates from the organizer and involves germ layer specification and patterning.

Involvement of Oct4-type transcription factor Pou5f3 in posterior spinal cord formation in zebrafish embryos

In vertebrate embryogenesis, elongation of the posterior body is driven by de novo production of the axial and paraxial mesoderm as well as the neural tube at the posterior end. This process is presumed to depend on the stem cell-like population in the tail bud region, but the details of the gene regulatory network involved are unknown. Previous studies suggested the involvement of pou5f3, an Oct4-type POU gene in zebrafish, in axial elongation. In the present study, we first found that pou5f3 is expressed mainly in the dorsal region of the tail bud immediately after gastrulation, and that this expression is restricted to the posterior-most region of the elongating neural tube during somitogenesis. This pou5f3 expression was complementary to the broad expression of sox3 in the neural tube, and formed a sharp boundary with specific expression of tbxta (orthologue of mammalian T/Brachyury) in the tail bud, implicating pou5f3 in the specification of tail bud-derived cells toward neural differentiation in the spinal cord. When pou5f3 was functionally impaired after gastrulation by induction of a dominant-interfering pou5f3 mutant gene (en-pou5f3), trunk and tail elongation were markedly disturbed at distinct positions along the axis depending on the stage. This finding showed involvement of pou5f3 in de novo generation of the body from the tail bud. Conditional functional abrogation also showed that pou5f3 downregulates mesoderm-forming genes but promotes neural development by activating neurogenesis genes around the tail bud. These results suggest that pou5f3 is involved in formation of the posterior spinal cord.

Early Xenopus gene regulatory programs, chromatin states, and the role of maternal transcription factors

For decades, the early development of the Xenopus embryo has been an essential model system to study the gene regulatory mechanisms that govern cellular specification. At the top of the hierarchy of gene regulatory networks, maternally deposited transcription factors initiate this process and regulate the expression of zygotic genes that give rise to three distinctive germ layer cell types (ectoderm, mesoderm, and endoderm), and subsequent generation of organ precursors. The onset of germ layer specification is also closely coupled with changes associated with chromatin modifications. This review will examine the timing of maternal transcription factors initiating the zygotic genome activation, the epigenetic landscape of embryonic chromatin, and the network structure that governs the process.

Derivation of neural crest stem cells from human epidermal keratinocytes requires FGF-2, IGF-1, and inhibition of TGF-β1

Neural crest (NC) cells play a central role in forming the peripheral nervous system, the craniofacial skeleton, and the pigmentation of the skin during development due to their broad multilineage differentiation potential into neurons, Schwann cells, melanocytes, and mesenchymal stem cells. Recently, we identified an easily accessible source of pluripotent NC stem cells from human inter‐follicular keratinocyte (KC) cultures (KC‐NC). In this work, we examined specific conditions for the derivation of NC from KC cultures. More specifically, we examined the role of two growth factors, FGF2 and IGF1, in NC proliferation and in expression of two potent NC transcription factors, Sox10 and FoxD3. Using specific chemical inhibitors, we uncovered that the downstream regulatory pathways AKT/PI3K, MEK/ERK, and JNK/cJun may be critical in Sox10 and FoxD3 regulation in KC‐NC. The TGF‐β1 pathway was also implicated in suppressing Sox10 expression and NC proliferation. In summary, our study shed light into the role of FGF2, IGF1, and TGF‐β1 on the induction of NC from KC cultures and the pathways that regulate Sox10 and FoxD3. We also established culture conditions for sustaining KC‐NC multipotency and, therefore, the potential of these cells for regenerative medicine and cellular therapies.

A catenin-dependent balance between N-cadherin and E-cadherin controls neuroectodermal cell fate choices

Characterizing endogenous protein expression, interaction and function, this study identifies in vivo interactions and competitive balance between N-cadherin and E-cadherin in developing avian (Gallus gallus) neural and neural crest cells. Numerous cadherin proteins, including neural cadherin (Ncad) and epithelial cadherin (Ecad), are expressed in the developing neural plate as well as in neural crest cells as they delaminate from the newly closed neural tube. To clarify independent or coordinate function during development, we examined their expression in the cranial region. The results revealed surprising overlap and distinct localization of Ecad and Ncad in the neural tube. Using a proximity ligation assay and co-immunoprecipitation, we found that Ncad and Ecad formed heterotypic complexes in the developing neural tube, and that modulation of Ncad levels led to reciprocal gain or reduction of Ecad protein, which then alters ectodermal cell fate. Here, we demonstrate that the balance of Ecad and Ncad is dependent upon the availability of β-catenin proteins, and that alteration of either classical cadherin modifies the proportions of the neural crest and neuroectodermal cells that are specified.

Xbra and Smad-1 cooperate to activate the transcription of neural repressor ventx1.1 in Xenopus embryos

Crosstalk of signaling pathways play crucial roles in cell proliferation, cell differentiation, and cell fate determination for development. In the case of ventx1.1 in Xenopus embryos, both BMP-4/Smad-1 and FGF/Xbra signaling induce the expression of neural repressor ventx1.1. However, the details of how these two pathways interact and lead to neural inhibition by ventx1.1 remain largely unknown. In the present study, Xbra directly bound to the ventx1.1 promoter region and inhibited neurogenesis in a Ventx1.1-dependent manner. Furthermore, Smad-1 and Xbra physically interacted and regulated ventx1.1 transcription in a synergistic fashion. Xbra and Smad-1 interaction cooperatively enhanced the binding of an interacting partner within the ventx1.1 promoter and maximum cooperation was achieved in presence of intact DNA binding sites for both Smad-1 and Xbra. Collectively, BMP-4/Smad-1 and FGF/Xbra signal crosstalk cooperate to activate the transcription of neural repressor ventx1.1 in Xenopus embryos. This suggests that the crosstalk between BMP-4 and FGF signaling negatively regulates early neurogenesis by synergistic activation of ventx1.1 in Xenopus embryos.

A gene regulatory network underlying the formation of pre-placodal ectoderm in Xenopus laevis

BACKGROUND

The neural plate border ectoderm gives rise to key developmental structures during embryogenesis, including the neural crest and the preplacodal ectoderm. Many sensory organs and ganglia of vertebrates develop from cranial placodes, which themselves arise from preplacodal ectoderm, defined by expression of transcription factor Six1 and its coactivator Eya1. Here we elucidate the gene regulatory network underlying the specification of the preplacodal ectoderm in Xenopus, and the functional interactions among transcription factors that give rise to this structure.

RESULTS

To elucidate the gene regulatory network upstream of preplacodal ectoderm formation, we use gain- and loss-of-function studies to explore the role of early ectodermal transcription factors for establishing the preplacodal ectoderm and adjacent ectodermal territories, and the role of Six1 and Eya1 in feedback regulation of these transcription factors. Our findings suggest that transcription factors with expression restricted to ventral (non-neural) ectoderm (AP2, Msx1, FoxI1, Vent2, Dlx3, GATA2) and those restricted to dorsal (neural) ectoderm (Pax3, Hairy2b, Zic1) are required for specification of both preplacodal ectoderm and neural crest in a context-dependent fashion and are cross-regulated by Eya1 and Six1.

CONCLUSION

These findings allow us to elucidate a detailed gene regulatory network at the neural plate border upstream of preplacodal ectoderm formation based on functional interactions between ectodermal transcription factors. We propose a new model to explain the formation of immediately juxtaposed preplacodal ectoderm and neural crest territories at the neural plate border, uniting previous models.

Specifying neural crest cells: From chromatin to morphogens and factors in between

Neural crest (NC) cells are a stem-like multipotent population of progenitor cells that are present in vertebrate embryos, traveling to various regions in the developing organism. Known as the "fourth germ layer," these cells originate in the ectoderm between the neural plate (NP), which will become the brain and spinal cord, and nonneural tissues that will become the skin and the sensory organs. NC cells can differentiate into more than 30 different derivatives in response to the appropriate signals including, but not limited to, craniofacial bone and cartilage, sensory nerves and ganglia, pigment cells, and connective tissue. The molecular and cellular mechanisms that control the induction and specification of NC cells include epigenetic control, multiple interactive and redundant transcriptional pathways, secreted signaling molecules, and adhesion molecules. NC cells are important not only because they transform into a wide variety of tissue types, but also because their ability to detach from their epithelial neighbors and migrate throughout developing embryos utilizes mechanisms similar to those used by metastatic cancer cells. In this review, we discuss the mechanisms required for the induction and specification of NC cells in various vertebrate species, focusing on the roles of early morphogenesis, cell adhesion, signaling from adjacent tissues, and the massive transcriptional network that controls the formation of these amazing cells. This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Signaling Pathways > Cell Fate Signaling.

Gene regulatory networks in neural cell fate acquisition from genome-wide chromatin association of Geminin and Zic1

Neural cell fate acquisition is mediated by transcription factors expressed in nascent neuroectoderm, including Geminin and members of the Zic transcription factor family. However, regulatory networks through which this occurs are not well defined. Here, we identified Geminin-associated chromatin locations in embryonic stem cells and Geminin- and Zic1-associated locations during neural fate acquisition at a genome-wide level. We determined how Geminin deficiency affected histone acetylation at gene promoters during this process. We integrated these data to demonstrate that Geminin associates with and promotes histone acetylation at neurodevelopmental genes, while Geminin and Zic1 bind a shared gene subset. Geminin- and Zic1-associated genes exhibit embryonic nervous system-enriched expression and encode other regulators of neural development. Both Geminin and Zic1-associated peaks are enriched for Zic1 consensus binding motifs, while Zic1-bound peaks are also enriched for Sox3 motifs, suggesting co-regulatory potential. Accordingly, we found that Geminin and Zic1 could cooperatively activate the expression of several shared targets encoding transcription factors that control neurogenesis, neural plate patterning, and neuronal differentiation. We used these data to construct gene regulatory networks underlying neural fate acquisition. Establishment of this molecular program in nascent neuroectoderm directly links early neural cell fate acquisition with regulatory control of later neurodevelopment.

Assessing Primary Neurogenesis in Xenopus Embryos Using Immunostaining

Primary neurogenesis is a dynamic and complex process during embryonic development that sets up the initial layout of the central nervous system. During this process, a portion of neural stem cells undergo differentiation and give rise to the first populations of differentiated primary neurons within the nascent central nervous system. Several vertebrate model organisms have been used to explore the mechanisms of neural cell fate specification, patterning, and differentiation. Among these is the African clawed frog, Xenopus, which provides a powerful system for investigating the molecular and cellular mechanisms responsible for primary neurogenesis due to its rapid and accessible development and ease of embryological and molecular manipulations. Here, we present a convenient and rapid method to observe the different populations of neuronal cells within Xenopus central nervous system. Using antibody staining and immunofluorescence on sections of Xenopus embryos, we are able to observe the locations of neural stem cells and differentiated primary neurons during primary neurogenesis.

Analysis of neural progenitors from embryogenesis to juvenile adult in Xenopus laevis reveals biphasic neurogenesis and continuous lengthening of the cell cycle

Xenopus laevis is a prominent model system for studying neural development, but our understanding of the long-term temporal dynamics of neurogenesis remains incomplete. Here, we present the first continuous description of neurogenesis in X. laevis, covering the entire period of development from the specification of neural ectoderm during gastrulation to juvenile frog. We have used molecular markers to identify progenitors and neurons, short-term bromodeoxyuridine (BrdU) incorporation to map the generation of newborn neurons and dual pulse S-phase labelling to characterise changes in their cell cycle length. Our study revealed the persistence of Sox3-positive progenitor cells from the earliest stages of neural development through to the juvenile adult. Two periods of intense neuronal generation were observed, confirming the existence of primary and secondary waves of neurogenesis, punctuated by a period of quiescence before metamorphosis and culminating in another period of quiescence in the young adult. Analysis of multiple parameters indicates that neural progenitors alternate between global phases of differentiation and amplification and that, regardless of their behaviour, their cell cycle lengthens monotonically during development, at least at the population level.

Molecular Cloning, Promoter Analysis and Expression Profiles of the sox3 Gene in Japanese Flounder, Paralichthys olivaceus

Sox3, which belongs to the SoxB1 subgroup, plays major roles in neural and gonadal development. In the present study, Japanese flounder Paralichthys olivaceus sox3 gene (Posox3) and its promoter sequence were isolated and characterized. The deduced PoSox3 protein contained 298 amino acids with a characteristic HMG-box domain. Alignment and phylogenetic analyses indicated that PoSox3 shares highly identical sequence with Sox3 homologues from different species. The promoter region of Posox3 has many potential transcription factor (TF) binding sites. The expression profiles of Posox3 in different developmental stages and diverse adult tissues were analyzed by quantitative real-time RT-PCR (qRT-PCR). Posox3 mRNA was maternally inherited, and maintained at a considerably high expression level between the blastula stage and the hatching stage during embryonic development. Posox3 was abundantly expressed in the adult brain and showed sexually dimorphic expression pattern. In situ hybridization (ISH) was carried out to investigate the cellular distribution of Posox3 in the ovary, and results showed the uniform distribution of Posox3 throughout the cytoplasm of oogonia and stage I–III oocytes. These results indicate that Posox3 has potentially vital roles in embryonic and neural development and may be involved in the oogenesis process. Our work provides a fundamental understanding of the structure and potential functions of Sox3 in Paralichthys olivaceus.

NEURODEVELOPMENT. Shared regulatory programs suggest retention of blastula-stage potential in neural crest cells

Neural crest cells, which are specific to vertebrates, arise in the ectoderm but can generate cell types that are typically categorized as mesodermal. This broad developmental potential persists past the time when most ectoderm-derived cells become lineage-restricted. The ability of neural crest to contribute mesodermal derivatives to the bauplan has raised questions about how this apparent gain in potential is achieved. Here, we describe shared molecular underpinnings of potency in neural crest and blastula cells. We show that in Xenopus, key neural crest regulatory factors are also expressed in blastula animal pole cells and promote pluripotency in both cell types. We suggest that neural crest cells may have evolved as a consequence of a subset of blastula cells retaining activity of the regulatory network underlying pluripotency.

Regeneration of Xenopus laevis spinal cord requires Sox2/3 expressing cells

Spinal cord regeneration is very inefficient in humans, causing paraplegia and quadriplegia. Studying model organisms that can regenerate the spinal cord in response to injury could be useful for understanding the cellular and molecular mechanisms that explain why this process fails in humans. Here, we use Xenopus laevis as a model organism to study spinal cord repair. Histological and functional analyses showed that larvae at pre-metamorphic stages restore anatomical continuity of the spinal cord and recover swimming after complete spinal cord transection. These regenerative capabilities decrease with onset of metamorphosis. The ability to study regenerative and non-regenerative stages in Xenopus laevis makes it a unique model system to study regeneration. We studied the response of Sox2(/)3 expressing cells to spinal cord injury and their function in the regenerative process. We found that cells expressing Sox2 and/or Sox3 are present in the ventricular zone of regenerative animals and decrease in non-regenerative froglets. Bromodeoxyuridine (BrdU) experiments and in vivo time-lapse imaging studies using green fluorescent protein (GFP) expression driven by the Sox3 promoter showed a rapid, transient and massive proliferation of Sox2(/)3(+) cells in response to injury in the regenerative stages. The in vivo imaging also demonstrated that Sox2(/)3(+) neural progenitor cells generate neurons in response to injury. In contrast, these cells showed a delayed and very limited response in non-regenerative froglets. Sox2 knockdown and overexpression of a dominant negative form of Sox2 disrupts locomotor and anatomical-histological recovery. We also found that neurogenesis markers increase in response to injury in regenerative but not in non-regenerative animals. We conclude that Sox2 is necessary for spinal cord regeneration and suggest a model whereby spinal cord injury activates proliferation of Sox2/3 expressing cells and their differentiation into neurons, a mechanism that is lost in non-regenerative froglets.

Sox21 regulates the progression of neuronal differentiation in a dose-dependent manner

Members of the SoxB transcription factor family play critical roles in the regulation of neurogenesis. The SoxB1 proteins are required for the induction and maintenance of a proliferating neural progenitor population in numerous vertebrates, however the role of the SoxB2 protein, Sox21, is less clear due to conflicting results. To clarify the role of Sox21 in neurogenesis, we examined its function in the Xenopus neural plate. Here we report that misexpression of Sox21 expands the neural progenitor domain, and represses neuron formation by binding to Neurogenin (Ngn2) and blocking its function. Conversely, we found that Sox21 is also required for neuron formation, as cells lacking Sox21 undergo cell death and thus are unable to differentiate. Together our data indicate that Sox21 plays more than one role in neurogenesis, where a threshold level is required for cell viability and normal differentiation of neurons, but a higher concentration of Sox21 inhibits neuron formation and instead promotes progenitor maintenance.

Neural transcription factors: from embryos to neural stem cells

The early steps of neural development in the vertebrate embryo are regulated by sets of transcription factors that control the induction of proliferative, pluripotent neural precursors, the expansion of neural plate stem cells, and their transition to differentiating neural progenitors. These early events are critical for producing a pool of multipotent cells capable of giving rise to the multitude of neurons and glia that form the central nervous system. In this review we summarize findings from gain- and loss-of-function studies in embryos that detail the gene regulatory network responsible for these early events. We discuss whether this information is likely to be similar in mammalian embryonic and induced pluripotent stem cells that are cultured according to protocols designed to produce neurons. The similarities and differences between the embryo and stem cells may provide important guidance to stem cell protocols designed to create immature neural cells for therapeutic uses.

Early embryonic specification of vertebrate cranial placodes

Cranial placodes contribute to many sensory organs and ganglia of the vertebrate head. The olfactory, otic, and lateral line placodes form the sensory receptor cells and neurons of the nose, ear, and lateral line system; the lens placode develops into the lens of the eye; epibranchial, profundal, and trigeminal placodes contribute sensory neurons to cranial nerve ganglia; and the adenohypophyseal placode gives rise to the anterior pituitary, a major endocrine control organ. Despite these differences in fate, all placodes are now known to originate from a common precursor, the preplacodal ectoderm (PPE). The latter is a horseshoe-shaped domain of ectoderm surrounding the anterior neural plate and neural crest and is defined by expression of transcription factor Six1, its cofactor Eya1, and other members of the Six and Eya families. Studies in zebrafish, Xenopus, and chick reveal that the PPE is specified together with other ectodermal territories (epidermis, neural crest, and neural plate) during early embryogenesis. During gastrulation, domains of ventrally (e.g., Dlx3/Dlx5, GATA2/GATA3, AP2, Msx1, FoxI1, and Vent1/Vent2) and dorsally (e.g., Zic1, Sox3, and Geminin) restricted transcription factors are established in response to a gradient of BMP and help to define non-neural and neural competence territories, respectively. At neural plate stages, the PPE is then induced in the non-neural competence territory by signals from the adjacent neural plate and mesoderm including FGF, BMP inhibitors, and Wnt inhibitors. Subsequently, signals from more localized signaling centers induce restricted expression domains of various transcription factors within the PPE, which specify multiplacodal areas and ultimately individual placodes. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The author has declared no conflicts of interest for this article.

The role of targeted protein degradation in early neural development

As neural stem cells differentiate into neurons during neurogenesis, the proteome of the cells is restructured by de novo expression and selective removal of regulatory proteins. The control of neurogenesis at the level of gene regulation is well documented and the regulation of protein abundance through protein degradation via the Ubiquitin/26S proteasome pathway is a rapidly developing field. This review describes our current understanding of the role of the proteasome pathway in neurogenesis. Collectively, the studies show that targeted protein degradation is an important regulatory mechanism in the generation of new neurons.

PV.1 suppresses the expression of FoxD5b during neural induction in Xenopus embryos

Suppression of bone morphogenetic protein (BMP) signaling induces neural induction in the ectoderm of developing embryos. BMP signaling inhibits eural induction via the expression of various neural suppressors. Previous research has demonstrated that the ectopic expression of dominant negative BMP receptors (DNBR) reduces the expression of target genes down-stream of BMP and leads to neural induction. Additionally, gain-of-function experiments have shown that BMP downstream target genes such as MSX1, GATA1b and Vent are involved in the suppression of neural induction. For example, the Vent1/2 genes are involved in the suppression of Geminin and Sox3 expression in the neural ectodermal region of embryos. In this paper, we investigated whether PV.1, a BMP downstream target gene, negatively regulates the expression of FoxD5b, which plays a role in maintaining a neural progenitor population. A promoter assay and a cyclohexamide experiment demonstrated that PV.1 negatively regulates FoxD5b expression.

On becoming neural: what the embryo can tell us about differentiating neural stem cells

THE EARLIEST STEPS OF EMBRYONIC NEURAL DEVELOPMENT ARE ORCHESTRATED BY SETS OF TRANSCRIPTION FACTORS THAT CONTROL AT LEAST THREE PROCESSES: the maintenance of proliferative, pluripotent precursors that expand the neural ectoderm; their transition to neurally committed stem cells comprising the neural plate; and the onset of differentiation of neural progenitors. The transition from one step to the next requires the sequential activation of each gene set and then its down-regulation at the correct developmental times. Herein, we review how these gene sets interact in a transcriptional network to regulate these early steps in neural development. A key gene in this regulatory network is FoxD4L1, a member of the forkhead box (Fox) family of transcription factors. Knock-down experiments in Xenopus embryos show that FoxD4L1 is required for the expression of the other neural transcription factors, whereas increased FoxD4L1 levels have three different effects on these genes: up-regulation of neural ectoderm precursor genes; transient down-regulation of neural plate stem cell genes; and down-regulation of neural progenitor differentiation genes. These different effects indicate that FoxD4L1 maintains neural ectodermal precursors in an immature, proliferative state, and counteracts premature neural stem cell and neural progenitor differentiation. Because it both up-regulates and down-regulates genes, we characterized the regions of the FoxD4L1 protein that are specifically involved in these transcriptional functions. We identified a transcriptional activation domain in the N-terminus and at least two domains in the C-terminus that are required for transcriptional repression. These functional domains are highly conserved in the mouse and human homologues. Preliminary studies of the related FoxD4 gene in cultured mouse embryonic stem cells indicate that it has a similar role in promoting immature neural ectodermal precursors and delaying neural progenitor differentiation. These studies in Xenopus embryos and mouse embryonic stem cells indicate that FoxD4L1/FoxD4 has the important function of regulating the balance between the genes that expand neural ectodermal precursors and those that promote neural stem/progenitor differentiation. Thus, regulating the level of expression of FoxD4 may be important in stem cell protocols designed to create immature neural cells for therapeutic uses.

The neurogenic factor NeuroD1 is expressed in post-mitotic cells during juvenile and adult Xenopus neurogenesis and not in progenitor or radial glial cells

In contrast to mammals that have limited proliferation and neurogenesis capacities, the Xenopus frog exhibit a great potential regarding proliferation and production of new cells in the adult brain. This ability makes Xenopus a useful model for understanding the molecular programs required for adult neurogenesis. Transcriptional factors that control adult neurogenesis in vertebrate species undergoing widespread neurogenesis are unknown. NeuroD1 is a member of the family of proneural genes, which function during embryonic neurogenesis as a potent neuronal differentiation factor. Here, we study in detail the expression of NeuroD1 gene in the juvenile and adult Xenopus brains by in situ hybridization combined with immunodetections for proliferation markers (PCNA, BrdU) or in situ hybridizations for cell type markers (Vimentin, Sox2). We found NeuroD1 gene activity in many brain regions, including olfactory bulbs, pallial regions of cerebral hemispheres, preoptic area, habenula, hypothalamus, cerebellum and medulla oblongata. We also demonstrated by double staining NeuroD1/BrdU experiments, after long post-BrdU administration survival times, that NeuroD1 gene activity was turned on in new born neurons during post-metamorphic neurogenesis. Importantly, we provided evidence that NeuroD1-expressing cells at this brain developmental stage were post-mitotic (PCNA-) cells and not radial glial (Vimentin+) or progenitors (Sox2+) cells.

A Systematic Survey and Characterization of Enhancers that Regulate Sox3 in Neuro-Sensory Development in Comparison with Sox2 Enhancers

Development of neural and sensory primordia at the early stages of embryogenesis depends on the activity of two B1 Sox transcription factors, Sox2 and Sox3. The embryonic expression patterns of the Sox2 and Sox3 genes are similar, yet they show gene-unique features. We screened for enhancers of the 231-kb genomic region encompassing Sox3 of chicken, and identified 13 new enhancers that showed activity in different domains of the neuro-sensory primordia. Combined with the three Sox3-proximal enhancers determined previously, at least 16 enhancers were involved in Sox3 regulation. Starting from the NP1 enhancer, more enhancers with different specificities are activated in sequence, resulting in complex overlapping patterns of enhancer activities. NP1 was activated in the caudal lateral epiblast adjacent to the posterior growing end of neural plate, and by the combined action of Wnt and Fgf signaling, similar to the Sox2 N1 enhancer involved in neural/mesodermal dichotomous cell lineage segregation. The Sox3 D5 enhancer and Sox2 N3 enhancer were also activated similarly in the diencephalon, optic vesicle and lens placode, suggesting analogies in their regulation. In general, however, the specificities of the enhancers were not identical between Sox3 and Sox2, including the cases of the NP1 and D5 enhancers.

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.

Neural crest induction at the neural plate border in vertebrates

The neural crest is a transient and multipotent cell population arising at the edge of the neural plate in vertebrates. Recent findings highlight that neural crest patterning is initiated during gastrulation, i.e. earlier than classically described, in a progenitor domain named the neural border. This chapter reviews the dynamic and complex molecular interactions underlying neural border formation and neural crest emergence.

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.

FGF-dependent midline-derived progenitor cells in hypothalamic infundibular development

The infundibulum links the nervous and endocrine systems, serving as a crucial integrating centre for body homeostasis. Here we describe that the chick infundibulum derives from two subsets of anterior ventral midline cells. One set remains at the ventral midline and forms the posterior-ventral infundibulum. A second set migrates laterally, forming a collar around the midline. We show that collar cells are composed of Fgf3(+) SOX3(+) proliferating progenitors, the induction of which is SHH dependent, but the maintenance of which requires FGF signalling. Collar cells proliferate late into embryogenesis, can generate neurospheres that passage extensively, and differentiate to distinct fates, including hypothalamic neuronal fates and Fgf10(+) anterior-dorsal infundibular cells. Together, our study shows that a subset of anterior floor plate-like cells gives rise to Fgf3(+) SOX3(+) progenitor cells, demonstrates a dual origin of infundibular cells and reveals a crucial role for FGF signalling in governing extended infundibular growth.

Regulation of the SOX3 gene expression by retinoid receptors

Sox3/SOX3 gene is considered to be one of the earliest neural markers in vertebrates. Despite the mounting evidence that Sox3/SOX3 is one of the key players in the development of the nervous system, limited data are available regarding the transcriptional regulation of its expression. This review is focused on the retinoic acid induced regulation of SOX3 gene expression, with particular emphasis on the involvement of retinoid receptors. Experiments with human embryonal carcinoma cells identified two response elements involved in retinoic acid/retinoid X receptor-dependent activation of the SOX3 gene expression: distal atypical retinoic acid-response element, consisting of two unique G-rich boxes separated by 49 bp, and proximal element comprising DR-3-like motif, composed of two imperfect hexameric half-sites. Importantly, the retinoic acid-induced SOX3 gene expression could be significantly down-regulated by a synthetic antagonist of retinoid receptors. This cell model provides a solid base for further studies on mechanism(s) underlying regulation of expression of SOX3 gene, which could improve the understanding of molecular signals that induce neurogenesis in the stem/progenitor cells both during development and in adulthood.

Interaction of Sox1, Sox2, Sox3 and Oct4 during primary neurogenesis

Sox1, Sox2 and Sox3, the three members of the SoxB1 subgroup of transcription factors, have similar sequences, expression patterns and overexpression phenotypes. Thus, it has been suggested that they have redundant roles in the maintenance of neural stem cells in development. However, the long-term effect of overexpression or their function in combination with their putative co-factor Oct4 has not been tested. Here, we show that overexpression of sox1, sox2, sox3 or oct91, the Xenopus homologue of Oct4, results in the same phenotype: an expanded neural plate at the expense of epidermis and delayed neurogenesis. However, each of these proteins induced a unique profile of neural markers and the combination of Oct91 with each SoxB1 protein had different effects, as did continuous misexpression of the proteins. Overexpression studies indicate that Oct91 preferentially cooperates with Sox2 to maintain neural progenitor marker expression, while knockdown of Oct91 inhibits neural induction driven by either Sox2 or Sox3. Continuous expression of Sox1 and Sox2 in transgenic embryos represses neuron differentiation and inhibits anterior development while increasing cell proliferation. Constitutively active Sox3, however, leads to increased apoptosis suggesting that it functions as a tumor suppressor. While the SoxB1s have overlapping functions, they are not strictly redundant as they induce different sets of genes and are likely to partner with different proteins to maintain progenitor identity.

Mesodermal Wnt signaling organizes the neural plate via Meis3

In vertebrates, canonical Wnt signaling controls posterior neural cell lineage specification. Although Wnt signaling to the neural plate is sufficient for posterior identity, the source and timing of this activity remain uncertain. Furthermore, crucial molecular targets of this activity have not been defined. Here, we identify the endogenous Wnt activity and its role in controlling an essential downstream transcription factor, Meis3. Wnt3a is expressed in a specialized mesodermal domain, the paraxial dorsolateral mesoderm, which signals to overlying neuroectoderm. Loss of zygotic Wnt3a in this region does not alter mesoderm cell fates, but blocks Meis3 expression in the neuroectoderm, triggering the loss of posterior neural fates. Ectopic Meis3 protein expression is sufficient to rescue this phenotype. Moreover, Wnt3a induction of the posterior nervous system requires functional Meis3 in the neural plate. Using ChIP and promoter analysis, we show that Meis3 is a direct target of Wnt/beta-catenin signaling. This suggests a new model for neural anteroposterior patterning, in which Wnt3a from the paraxial mesoderm induces posterior cell fates via direct activation of a crucial transcription factor in the overlying neural plate.

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.

Reprogramming retinal pigment epithelium to differentiate toward retinal neurons with Sox2

Guiding non-neural, retinal pigment epithelium (RPE) to produce retinal neurons may offer a source of developing neurons for cell-replacement. Sox2 plays important roles in maintaining neural progenitor/stem cell properties and in converting fibroblasts into pluripotent stem cells. This study tests the possibility of using Sox2 to reprogram RPE to differentiate toward retinal neurons in vivo and in vitro. Expression of Sox2 in the chick retina was detected in progenitor cells, in cells at a discrete location in the layers of amacrine and ganglion cells, and in Muller glia. Overexpression of Sox2 in the developing eye resulted in hypopigmentation of the RPE. In the affected regions, expression of retinal ganglion cell markers became apparent in the RPE layer. In RPE cell culture, Sox2 promoted the expression of retinal ganglion and amacrine markers, and suppressed the expression of genes associated with RPE properties. Mechanistic investigation using the developing retina revealed a coexpression of Sox2 and basic fibroblast growth factor (bFGF), a growth factor commonly used in stem cell culture and capable of inducing RPE-to-retina transdifferentiation (or reprogramming) during early development. Similar patterns of changes in Sox2 expression and in bFGF expression were observed in atrophic retina and in injured retina. In RPE cell culture, Sox2 and bFGF mutually enhanced one another's expression. Upregulation of bFGF expression by Sox2 also occurred in the retina. These results suggest that Sox2 can initiate a reprogramming of RPE cells to differentiate toward retinal neurons and may engage bFGF during the process.

Neural induction and factors that stabilize a neural fate

The neural ectoderm of vertebrates forms when the bone morphogenetic protein (BMP) signaling pathway is suppressed. Herein, we review the molecules that directly antagonize extracellular BMP and the signaling pathways that further contribute to reduce BMP activity in the neural ectoderm. Downstream of neural induction, a large number of "neural fate stabilizing" (NFS) transcription factors are expressed in the presumptive neural ectoderm, developing neural tube and ultimately in neural stem cells. Herein, we review what is known about their activities during normal development to maintain a neural fate and regulate neural differentiation. Further elucidation of how the NFS genes interact to regulate neural specification and differentiation should ultimately prove useful for regulating the expansion and differentiation of neural stem and progenitor cells.

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

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

A role for Syndecan-4 in neural induction involving ERK- and PKC-dependent pathways

Syndecan-4 (Syn4) is a heparan sulphate proteoglycan that is able to bind to some growth factors, including FGF, and can control cell migration. Here we describe a new role for Syn4 in neural induction in Xenopus. Syn4 is expressed in dorsal ectoderm and becomes restricted to the neural plate. Knockdown with antisense morpholino oligonucleotides reveals that Syn4 is required for the expression of neural markers in the neural plate and in neuralised animal caps. Injection of Syn4 mRNA induces the cell-autonomous expression of neural, but not mesodermal, markers. We show that two parallel pathways are involved in the neuralising activity of Syn4: FGF/ERK, which is sensitive to dominant-negative FGF receptor and to the inhibitors SU5402 and U0126, and a PKC pathway, which is dependent on the intracellular domain of Syn4. Neural induction by Syn4 through the PKC pathway requires inhibition of PKCdelta and activation of PKCalpha. We show that PKCalpha inhibits Rac GTPase and that c-Jun is a target of Rac. These findings might account for previous reports implicating PKC in neural induction and allow us to propose a link between FGF and PKC signalling pathways during neural induction.

Epigenetic background of neuronal fate determination

The development of the central nervous system (CNS) starts from neural stem cells (NSCs). During this process, NSCs are specified in space- and time-related fashions, becoming spatially heterogeneous and generating a progressively restricted repertoire of cell types: neurons, astrocytes and oligodendrocytes. The processes of neurodevelopment are determined reciprocally by intrinsic and external factors which interface to program and re-program the profiling of fate-determination gene expression. Multiple signaling pathways act in a dynamic web mode to determine the fate of NSCs through modulating the activity of a distinct set of transcription factors which in turn trigger the transcription of neural fate-determination genes. Accumulating evidence reveals that during CNS development, multiple epigenetic factors regulate the activities of extracellular signaling and corresponding transcription factors in a coordinative manner, leading to the formation of a system with sophisticated structure and magic functions. This review aims to introduce recent advances in the epigenetic background of neural cell fate determination.

Xenopus Sox3 activates sox2 and geminin and indirectly represses Xvent2 expression to induce neural progenitor formation at the expense of non-neural ectodermal derivatives

The SRY-related, HMG box SoxB1 transcription factors are highly homologous, evolutionarily conserved proteins that are expressed in neuroepithelial cells throughout neural development. SoxB1 genes are down-regulated as cells exit the cell-cycle to differentiate and are considered functionally redundant in maintaining neural precursor populations. However, little is known about Sox3 function and its mode of action during primary neurogenesis. Using gain and loss-of-function studies, we analyzed Sox3 function in detail in Xenopus early neural development and compared it to that of Sox2. Through these studies we identified the first targets of a SoxB1 protein during primary neurogenesis. Sox3 functions as an activator to induce expression of the early neural genes, sox2 and geminin in the absence of protein synthesis and to indirectly inhibit the Bmp target Xvent2. As a result, Sox3 increases cell proliferation, delays neurogenesis and inhibits epidermal and neural crest formation to expand the neural plate. Our studies indicate that Sox3 and 2 have many similar functions in this process including the ability to activate expression of geminin in naïve ectodermal explants. However, there are some differences; Sox3 activates the expression of sox2, while Sox2 does not activate expression of sox3 and sox3 is uniquely expressed throughout the ectoderm prior to neural induction suggesting a role in neural competence. With morpholino-mediated knockdown of Sox3, we demonstrate that it is required for induction of neural tissue by BMP inhibition. Together these data indicate that Sox3 has multiple roles in early neural development including as a factor required for nogginmediated neural induction.