Patterning across the ascidian neural plate by lateral Nodal signalling sources

Mechanism of DNA replication-dependent transcriptional activation of the acetylcholinesterase gene in the Ciona intestinalis embryo

The acetylcholinesterase-encoding gene in the ascidian Ciona intestinalis (Ci-AChE) is expressed in tail muscle cells from the gastrula stage. When the embryo was continuously treated with aphidicolin from the 32-cell stage, Ci-AChE was not expressed even when control embryos reached the tailbud stage. This result suggests that Ci-AChE acquires the competence to be transcribed after passing through a certain number of DNA replication cycles. A lacZ reporter gene containing the 5' flanking region of Ci-AChE was expressed in the tail muscle cells. Aphidicolin treatment from the 32-cell stage affected, but did not completely suppress, the expression of lacZ. A bisulfite sequencing analysis was carried out to examine the methylation status of four regions within the 5' flanking sequence and the first exon. However, all of these regions remained unmethylated from the 16-cell to 110-cell stages. The results suggested that the DNA of the Ci-AChE locus is not responsible for counting the rounds of replication. We examined the expression of the C. intestinalis MyoD (Ci-MyoD), a transcription factor that activates Ci-AChE. Aphidicolin treatment from the 32-cell stage suppressed the expression of Ci-MyoD, even when control embryos reached the gastrula stage. These results suggest that a lack of Ci-MyoD is critical to the suppression of Ci-AChE in aphidicolin-treated embryos.

Transcriptional regulation of the retinoic acid receptor in the dorsal midline epidermis in the Ciona intestinalis embryo

Retinoic acid regulates the spatial pattern of gene expression mainly in the epidermis in the protochordate ascidian, Ciona intestinalis. Our previous study characterized the enhancer element responsible for the activation of the retinoic acid receptor (RAR) in the dorsal midline epidermis (DME). In the present study, detailed analysis identified two small sequence elements required for the transcriptional activation in the DME. Deletion of either one of these sequences resulted in suppression of the DME enhancer, suggesting that both sites are necessary. The nucleotide sequences of these two sites were similar to the consensus recognition sequences for the Msx and Sox transcription factors, respectively. These transcription factors are expressed in the DME lineage blastomeres (b7.9 and b7.10 blastomere pairs of bilaterally symmetrical embryos of this species) at the 64-cell stage. Electrophoretic mobility shift assays revealed that recombinant Msxb and SoxB1 proteins specifically bound to these sequences. These results suggest that Msxb and SoxB1 synergistically activate the enhancer in the DME.

Gene-regulatory networks in the Ciona embryos

Ascidians belong to the subphylum Urochordata or Tunicata, which is the sister group of the vertebrates. The simple architecture of the ascidian larva represents the basic chordate body plan. Recent analyses have shown many instances of developmental mechanisms conserved during evolution, while these studies have also revealed a much larger number of instances of divergence. However, to precisely determine the degree of conservation and divergence, that is, how many ways are used to make tadpole-like larvae, we need a systems-level understanding of development. Because animal development is organized by the genome and the minimal functional unit of development is a cell, comprehensiveness and single-cell resolution are necessary for a systems-biological understanding of the development. In the ascidian Ciona intestinalis, gene-regulatory networks responsible for the embryonic development have been studied on a genome-wide scale and at single-cell resolution. The simplicity and compactness of the genome facilitates genome-wide studies. In the Ciona genome, only approximately 670 transcription factor genes are encoded, and their expression profiles during the embryonic development have been analyzed. Gene-knockdown analyses of the transcription factor genes expressed during the embryonic development have been performed. The simplicity of the embryo permits these analyses to be done at single-cell resolution. Actually, these simple embryos are now being modeled in the computer, which allows us to understand the gene-regulatory networks very precisely in three dimensions.

Gene regulatory networks underlying the compartmentalization of the Ciona central nervous system

The tripartite organization of the central nervous system (CNS) may be an ancient character of the bilaterians. However, the elaboration of the more complex vertebrate brain depends on the midbrain-hindbrain boundary (MHB) organizer, which is absent in invertebrates such as Drosophila. The Fgf8 signaling molecule expressed in the MHB organizer plays a key role in delineating separate mesencephalon and metencephalon compartments in the vertebrate CNS. Here, we present evidence that an Fgf8 ortholog establishes sequential patterns of regulatory gene expression in the developing posterior sensory vesicle, and the interleaved ;neck' region located between the sensory vesicle and visceral ganglion of the simple chordate Ciona intestinalis. The detailed characterization of gene networks in the developing CNS led to new insights into the mechanisms by which Fgf8/17/18 patterns the chordate brain. The precise positioning of this Fgf signaling activity depends on an unusual AND/OR network motif that regulates Snail, which encodes a threshold repressor of Fgf8 expression. Nodal is sufficient to activate low levels of the Snail repressor within the neural plate, while the combination of Nodal and Neurogenin produces high levels of Snail in neighboring domains of the CNS. The loss of Fgf8 patterning activity results in the transformation of hindbrain structures into an expanded mesencephalon in both ascidians and vertebrates, suggesting that the primitive MHB-like activity predates the vertebrate CNS.

Ascidians and the plasticity of the chordate developmental program

Little is known about the ancient chordates that gave rise to the first vertebrates, but the descendants of other invertebrate chordates extant at the time still flourish in the ocean. These invertebrates include the cephalochordates and tunicates, whose larvae share with vertebrate embryos a common body plan with a central notochord and a dorsal nerve cord. Tunicates are now thought to be the sister group of vertebrates. However, research based on several species of ascidians, a diverse and wide-spread class of tunicates, revealed that the molecular strategies underlying their development appear to diverge greatly from those found in vertebrates. Furthermore, the adult body plan of most tunicates, which arises following an extensive post-larval metamorphosis, shows little resemblance to the body plan of any other chordate. In this review, we compare the developmental strategies of ascidians and vertebrates and argue that the very divergence of these strategies reveals the surprising level of plasticity of the chordate developmental program and is a rich resource to identify core regulatory mechanisms that are evolutionarily conserved in chordates. Further, we propose that the comparative analysis of the architecture of ascidian and vertebrate gene regulatory networks may provide critical insight into the origin of the chordate body plan.

The pro-domain of the zebrafish Nodal-related protein Cyclops regulates its signaling activities

Nodal proteins are secreted signaling factors of the transforming growth factor β (TGFβ) family with essential roles in embryonic development in vertebrates. Mutations affecting the Nodal factors have severe consequences in mammals and fish. Furthermore, increased Nodal levels have been associated with melanoma tumor progression. Like other TGFβ-related proteins, Nodal factors consist of a pro-domain and a mature domain. The pro-domain of mouse Nodal protein stabilizes its precursor. However, the mechanisms by which the pro-domains exert their activities are unknown. Here, we characterize the zebrafish Nodal-related factor Cyclops (Cyc) and find unexpected functions for the pro-domain in regulating Cyc activity. We identified a lysosome-targeting region in the Cyc pro-domain that destabilizes the precursor and restricts Cyc activity, revealing the molecular basis for the short-range signaling activities of Cyc. We show that both the pro- and mature-domains of Cyc regulate its stability. We also characterize a mutation in the pro-domain of human NODAL (hNODAL) that underlies congenital heterotaxia. Heterologous expression of mutant hNODAL increases expression of Nodal-response genes. Our studies reveal unexpected roles for the pro-domain of the Nodal factors and provide a possible mechanism for familial heterotaxia.

Ephrin signaling establishes asymmetric cell fates in an endomesoderm lineage of the Ciona embryo

Mesodermal tissues arise from diverse cell lineages and molecular strategies in the Ciona embryo. For example, the notochord and mesenchyme are induced by FGF/MAPK signaling, whereas the tail muscles are specified autonomously by the localized determinant, Macho-1. A unique mesoderm lineage, the trunk lateral cells, develop from a single pair of endomesoderm cells, the A6.3 blastomeres, which form part of the anterior endoderm, hematopoietic mesoderm and muscle derivatives. MAPK signaling is active in the endoderm descendants of A6.3, but is absent from the mesoderm lineage. Inhibition of MAPK signaling results in expanded expression of mesoderm marker genes and loss of endoderm markers, whereas ectopic MAPK activation produces the opposite phenotype: the transformation of mesoderm into endoderm. Evidence is presented that a specific Ephrin signaling molecule, Ci-ephrin-Ad, is required to establish asymmetric MAPK signaling in the endomesoderm. Reducing Ci-ephrin-Ad activity via morpholino injection results in ectopic MAPK signaling and conversion of the mesoderm lineage into endoderm. Conversely, misexpression of Ci-ephrin-Ad in the endoderm induces ectopic activation of mesodermal marker genes. These results extend recent observations regarding the role of Ephrin signaling in the establishment of asymmetric cell fates in the Ciona notochord and neural tube.

Sequential and combinatorial inputs from Nodal, Delta2/Notch and FGF/MEK/ERK signalling pathways establish a grid-like organisation of distinct cell identities in the ascidian neural plate

The ascidian neural plate has a grid-like organisation, with six rows and eight columns of aligned cells, generated by a series of stereotypical cell divisions. We have defined unique molecular signatures for each of the eight cells in the posterior-most two rows of the neural plate - rows I and II. Using a combination of morpholino gene knockdown, dominant-negative forms and pharmacological inhibitors, we tested the role of three signalling pathways in defining these distinct cell identities. Nodal signalling at the 64-cell stage was found to be required to define two different neural plate domains - medial and lateral - with Nodal inducing lateral and repressing medial identities. Delta2, an early Nodal target, was found to then subdivide each of the lateral and medial domains to generate four columns. Finally, a separate signalling system along the anteroposterior axis, involving restricted ERK1/2 activation, was found to promote row I fates and repress row II fates. Our results reveal how the sequential integration of three signalling pathways - Nodal, Delta2/Notch and FGF/MEK/ERK - defines eight different sub-domains that characterise the ascidian caudal neural plate. Most remarkably, the distinct fates of the eight neural precursors are each determined by a unique combination of inputs from these three signalling pathways.

Nodal regulates neural tube formation in the Ciona intestinalis embryo

Overexpression of a lefty orthologue, Ci-lefty, caused a failure of neural tube closure in the protochordate ascidian Ciona intestinalis. The body bent dorsally, and anterior-posterior elongation was inhibited. A similar phenotype was observed in embryos treated with SB431542, an inhibitor of Nodal receptors, suggesting that Ci-Lefty antagonized Nodal signaling as reported in other deuterostome species. Overexpression of Ci-nodal also resulted in a similar phenotype, suggesting that a correct quantity and/or a spatial restriction of Nodal signaling are important for the neural tube to form. In addition to known Ci-Nodal target genes, orthologues of Zic (Ci-ZicL) and cdx (Ci-cdx) were activated by Ci-Nodal. Expression of a dominant negative Ci-cdx caused defects in neural tube formation similar to those obtained on treatment with SB431542 or overexpression of Ci-lefty. A regulatory cascade composed of Ci-Nodal, Ci-ZicL, and Ci-Cdx may play an important role in neural tube formation in the Ciona embryo.

FGF9/16/20 and Wnt-5α signals are involved in specification of secondary muscle fate in embryos of the ascidian, Halocynthia roretzi

The tail muscle cells of the ascidian tadpole larva originate from two different lineages, the B- (primary) and A- and b- (secondary) line blastomeres of the eight-cell stage embryo. The primary muscle cells assume muscle fate cell-autonomously with the involvement of a localized muscle determinant, macho-1. On the other hand, fate determination of secondary muscle cells is a non-cell-autonomous process that depends on cellular interactions. In this paper, we investigated the mechanisms underlying fate specification of secondary muscle cells in Halocynthia roretzi. We found that FGF and Wnt5 signals were required. In contrast, the Nodal signal, which is required for specification of A-line muscle cells in another ascidian, Ciona intestinalis, was not necessary for the formation of any secondary muscle cells in Halocynthia embryo. Therefore, Halocynthia and Ciona show distinctly different mechanisms for generation of the secondary lineages, despite the fact that embryogenesis appears very similar between these species. We also found that the mechanisms involved in specification of A- and b-line muscle cells were distinct in that the required timing of the FGF signal for the A-line muscle cells preceded that for the b-line. Moreover, the inducer blastomeres for specification of these two lineages were different.

Cilia-driven leftward flow determines laterality in Xenopus

Determination of the vertebrate left-right body axis during embryogenesis results in asymmetric development and placement of most inner organs. Although the asymmetric Nodal cascade is conserved in all vertebrates, the mechanism of symmetry breakage has remained controversial. In mammalian and fish embryos, a cilia-driven leftward flow of extracellular fluid is required for initiation of the Nodal cascade. This flow is localized at the posterior notochord ("node") and Kupffer's vesicle, respectively. In frog and chick embryos, however, molecular asymmetries are required earlier, from cleavage stages through gastrulation. The validity of a cilia-based mechanism for all vertebrates therefore has been questioned. Here we show that a cilia-driven leftward flow precedes asymmetric nodal expression in the frog Xenopus. Motile monocilia emerged on the gastrocoel roof plate during neurulation and lengthened and polarized from an initially central position to the posterior pole of cells. Concomitantly, a robust leftward fluid flow developed from stage 15 onward, significantly before asymmetric nodal transcription started in the left-lateral-plate mesoderm at stage 19. Injection of 1.5% methylcellulose into the archenteron prevented leftward flow and resulted in laterality defects, demonstrating that the flow itself was required for asymmetric gene expression and organ placement.

The multidimensionality of cell behaviors underlying morphogenesis: a case study in ascidians

Databases where different types of information from different sources can be integrated, cross-referenced and interactively accessed are necessary for building a quantitative understanding of the molecular and cell biology intrinsic to the morphogenesis of an embryo. Tassy and colleagues recently reported the development of software tailor-made to perform such a task, along with the generation and integration of three-dimensional anatomical models of embryos. They convincingly illustrated the utility of their approach by applying it to the early ascidian embryo.

Muscle development in Ciona intestinalis requires the b-HLH myogenic regulatory factor gene Ci-MRF

The activity of myogenic regulatory factor (MRF) genes is essential for vertebrate muscle development, whereas invertebrate muscle development is largely independent of MRF function. This difference indicates that myogenesis is controlled by distinct regulatory mechanisms in these two groups of animals. Here we used overexpression and gene knockdown to investigate the role in embryonic myogenesis of the single MRF gene of the invertebrate chordate Ciona intestinalis (Ci-MRF). Injection of Ci-MRF mRNA into eggs resulted in increased embryonic muscle-specific gene activity and revealed the myogenic activity of Ci-MRF by inducing the expression of four muscle marker genes, Acetylcholinesterase, Actin, Troponin I, and Myosin Light Chain in non-muscle lineages. Conversely, inhibiting Ci-MRF activity with antisense morpholinos down-regulated the expression of these genes. Consistent with the effects of morpholinos on muscle gene activity, larvae resulting from morpholino injection were paralyzed and their "muscle" cells lacked myofibrils. We conclude that Ci-MRF is required for larval tail muscle development and thus that an MRF-dependent myogenic regulatory network probably existed in the ancestor of tunicates and vertebrates. This possibility raises the question of whether the earliest myogenic regulatory networks were MRF-dependent or MRF-independent.

Anteriorward shifting of asymmetric Xnr1 expression and contralateral communication in left-right specification in Xenopus

Transient asymmetric Nodal signaling in the left lateral plate mesoderm (L LPM) during tailbud/early somitogenesis stages is associated in all vertebrates examined with the development of stereotypical left-right (L-R) organ asymmetry. In Xenopus, asymmetric expression of Nodal-related 1 (Xnr1) begins in the posterior L LPM shortly after the initiation of bilateral perinotochordal expression in the posterior tailbud. The L LPM expression domain rapidly shifts forward to cover much of the flank of the embryo before being progressively downregulated, also in a posterior-to-anterior direction. The mechanisms underlying the initiation and propagation of Nodal/Xnr1 expression in the L LPM, and its transient nature, are not well understood. Removing the posterior tailbud domain prevents Xnr1 expression in the L LPM, consistent with the idea that normal embryos respond to a posteriorly derived asymmetrically acting positive inductive signal. The forward propagation of asymmetric Xnr1 expression occurs LPM-autonomously via planar tissue communication. The shifting is prevented by Nodal signaling inhibitors, implicating an underlying requirement for Xnr1-to-Xnr1 induction. It is also unclear how asymmetric Nodal signals are modulated during L-R patterning. Small LPM grafts overexpressing Xnr1 placed into the R LPM of tailbud embryos induced the expression of the normally L-sided genes Xnr1, Xlefty, and XPitx2, and inverted body situs, demonstrating the late-stage plasticity of the LPM. Orthogonal Xnr1 signaling from the LPM strongly induced Xlefty expression in the midline, consistent with recent findings in the mouse and demonstrating for the first time in another species conservation in the mechanism that induces and maintains the midline barrier. Our findings suggest that there is long-range contralateral communication between L and R LPM, involving Xlefty in the midline, over a substantial period of tailbud embryogenesis, and therefore lend further insight into how, and for how long, the midline maintains a L versus R status in the LPM.

A conserved role for the nodal signaling pathway in the establishment of dorso-ventral and left–right axes in deuterostomes

Nodal factors play crucial roles during embryogenesis of chordates. They have been implicated in a number of developmental processes, including mesoderm and endoderm formation and patterning of the embryo along the anterior-posterior and left-right axes. We have analyzed the function of the Nodal signaling pathway during the embryogenesis of the sea urchin, a non-chordate organism. We found that Nodal signaling plays a central role in axis specification in the sea urchin, but surprisingly, its first main role appears to be in ectoderm patterning and not in specification of the endoderm and mesoderm germ layers as in vertebrates. Starting at the early blastula stage, sea urchin nodal is expressed in the presumptive oral ectoderm where it controls the formation of the oral-aboral axis. A second conserved role for nodal signaling during vertebrate evolution is its involvement in the establishment of left-right asymmetries. Sea urchin larvae exhibit profound left-right asymmetry with the formation of the adult rudiment occurring only on the left side. We found that a nodal/lefty/pitx2 gene cassette regulates left-right asymmetry in the sea urchin but that intriguingly, the expression of these genes is reversed compared to vertebrates. We have shown that Nodal signals emitted from the right ectoderm of the larva regulate the asymmetrical morphogenesis of the coelomic pouches by inhibiting rudiment formation on the right side of the larva. This result shows that the mechanisms responsible for patterning the left-right axis are conserved in echinoderms and that this role for nodal is conserved among the deuterostomes. We will discuss the implications regarding the reference axes of the sea urchin and the ancestral function of the nodal gene in the last section of this review.

Ascidian embryonic development: An emerging model system for the study of cell fate specification in chordates

The ascidian tadpole larva represents the basic body plan of all chordates in a relatively small number of cells and tissue types. Although it had been considered that ascidians develop largely in a determinative way, whereas vertebrates develop in an inductive way, recent studies at the molecular and cellular levels have uncovered several similarities in the way developmental fates are specified. In this review, we describe ascidian embryogenesis and its cell lineages, introduce several characteristics of ascidian embryos, describe recent advances in understanding of the mechanisms of cell fate specification, and discuss them in the context of what is known in vertebrates and other organisms.

Formation of the ascidian epidermal sensory neurons: insights into the origin of the chordate peripheral nervous system

The vertebrate peripheral nervous system (PNS) originates from neural crest and placodes. While its developmental origin is the object of intense studies, little is known concerning its evolutionary history. To address this question, we analyzed the formation of the larval tail PNS in the ascidian Ciona intestinalis. The tail PNS of Ciona is made of sensory neurons located within the epidermis midlines and extending processes in the overlying tunic median fin. We show that each midline corresponds to a single longitudinal row of epidermal cells and neurons sharing common progenitors. This simple organization is observed throughout the tail epidermis, which is made of only eight single-cell rows, each expressing a specific genetic program. We next demonstrate that the epidermal neurons are specified in two consecutive steps. During cleavage and gastrula stages, the dorsal and ventral midlines are independently induced by FGF9/16/20 and the BMP ligand ADMP, respectively. Subsequently, Delta/Notch-mediated lateral inhibition controls the number of neurons formed within these neurogenic regions. These results provide a comprehensive overview of PNS formation in ascidian and uncover surprising similarities between the fate maps and embryological mechanisms underlying formation of ascidian neurogenic epidermis midlines and the vertebrate median fin.

A signalling relay involving Nodal and Delta ligands acts during secondary notochord induction in Ciona embryos

The notochord is one of the defining features of chordates. The ascidian notochord is a rod like structure consisting of a single row of 40 cells. The anterior 32 ;primary' notochord cells arise from the A-line (anterior vegetal) blastomeres of the eight-cell stage embryo, whereas the posterior 8 ;secondary' notochord cells arise from the B-line (posterior vegetal) blastomeres of the eight-cell stage embryo. Specification of notochord precursors within these two lineages occurs in a spatially and temporally distinct manner. We show that specification of the secondary but not the primary notochord in Ciona intestinalis requires a relay mechanism involving two signalling pathways. First, we show evidence that acquisition of secondary notochord fate is dependent upon lateral Nodal signalling sources, situated in the adjacent b-line animal cells. Expression of the notochord specific gene Ci-Brachyury in the secondary notochord precursor was downregulated following selective inhibition of Nodal signal reception in B-line derivatives and also, strikingly, following selective inhibition of Nodal signal reception in A-line cell derivatives. Within the A-line, Nodal signals are required for localised expression of Delta2, which encodes a divergent form of Delta ligand. Using four distinct reagents to inhibit Delta2/Notch signals, we showed that Delta2 signalling from A-line cells, which activates the Notch/Su(H) pathway in adjacent B-line cells, is required for specification of the secondary notochord precursor. We propose a model whereby laterally produced Nodal acts to specify the secondary notochord precursor both directly in the B-line cells and via Delta2 induction in adjacent A-line cells.

Ci-FoxA-a is the earliest zygotic determinant of the ascidian anterior ectoderm and directly activates Ci-sFRP1/5

This work focuses on the anteroposterior patterning of the ectoderm in the invertebrate chordate Ciona intestinalis. Previous work indicated that, by the eight-cell stage, the anterior and posterior animal blastomeres have acquired different properties, including a differential responsiveness to inducing signals from the underlying mesendoderm. Here, we investigated the molecular basis of this distinction. For this, we studied the regulation of the earliest marker specific for the anterior ectoderm, Ci-sFRP1/5, which is activated at the 64-cell stage. We first found that the activation of this marker in the anterior ectoderm does not involve communication with other lineages. We then identified, by phylogenetic footprinting and deletion analysis, a short conserved minimal enhancer driving the onset of expression of Ci-sFRP1/5. We showed that this enhancer was a direct target of the Ci-FoxA-a gene, a FoxA/HNF3 orthologue expressed in anterior ectodermal and mesendodermal lineages from the eight-cell stage. Gain- and loss-of-function experiments revealed that Ci-FoxA-a is necessary and sufficient within the ectoderm to impose an ectodermal anterior identity, and to repress the posterior programme. Thus, Ci-FoxA-a constitutes a major early zygotic anterior determinant for the ascidian ectoderm, acting autonomously in this territory, prior to the onset of vegetal inductions. Interestingly, while vertebrate FoxA2 are also involved in the regionalization of the ectoderm, they are thought to act during gastrulation to control, in the mesendoderm, the expression of organizer signals. We discuss the evolution of chordate ectodermal patterning in light of our findings.

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

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

FGF8/17/18 functions together with FGF9/16/20 during formation of the notochord in Ciona embryos

Fibroblast growth factor (FGF) signalling has been implicated in the generation of mesoderm and neural fates in chordate embryos including ascidians and vertebrates. In Ciona, FGF9/16/20 has been implicated in both of these processes. However, in FGF9/16/20 knockdown embryos, notochord fate recovers during later development. It is thus not clear if FGF signalling is an essential requirement for notochord specification in Ciona embryos. We show that FGF-MEK-ERK signals act during two distinct phases to establish notochord fate. During the first phase, FGF signalling is required during an asymmetric cell division to promote notochord at the expense of neural identity. Consistently, ERK1/2 is specifically activated in the notochord precursors following this cell division. Sustained activation of ERK1/2 is then required to maintain notochord fate. We demonstrate that FGF9/16/20 acts solely during the initial induction step and that, subsequently, FGF8/17/18 together with FGF9/16/20 is involved in the following maintenance step. These results together with others' show that the formation of a large part of the mesoderm cell types in ascidian larvae is dependent on signalling events involving FGF ligands.

Regulatory blueprint for a chordate embryo

Ciona is an emerging model system for elucidating gene networks in development. Comprehensive in situ hybridization assays have identified 76 regulatory genes with localized expression patterns in the early embryo, at the time when naïve blastomeres are determined to follow specific cell fates. Systematic gene disruption assays provided more than 3000 combinations of gene expression profiles in mutant backgrounds. Deduced gene circuit diagrams describing the formation of larval tissues were computationally visualized. These diagrams constitute a blueprint for the Ciona embryo and provide a foundation for understanding the evolutionary origins of the chordate body plan.

Specification of ectoderm restricts the size of the animal plate and patterns neurogenesis in sea urchin embryos

The animal plate of the sea urchin embryo becomes the apical organ, a sensory structure of the larva. In the absence of vegetal signaling, an expanded and unpatterned apical organ forms. To investigate the signaling that restricts the size of the animal plate and patterns neurogenesis, we have expressed molecules that regulate specification of ectoderm in embryos and chimeras. Enhancing oral ectoderm suppresses serotonergic neuron differentiation, whereas enhancing aboral or ciliary band ectoderm increases differentiation of serotonergic neurons. In embryos in which vegetal signaling is blocked, Nodal expression does not reduce the size of the thickened animal plate; however, almost no neurons form. Expression of BMP in the absence of vegetal signaling also does not restrict the size of the animal plate, but abundant serotonergic neurons form. In chimeras in which vegetal signaling is blocked in the entire embryo, and one half of the embryo expresses Nodal, serotonergic neuron formation is suppressed in both halves. In similar chimeras in which vegetal signaling is blocked and one half of the embryo expresses Goosecoid (Gsc), serotonergic neurons form only in the half of the embryo not expressing Gsc. We propose that neurogenesis is specified by a maternal program that is restricted to the animal pole by signaling that is dependent on nuclearization of beta-catenin and specifies ciliary band ectoderm. Subsequently, neurogenesis in the animal plate is patterned by suppression of serotonergic neuron formation by Nodal. Like other metazoans, echinoderms appear to have a phase of neural development during which the specification of ectoderm restricts and patterns neurogenesis.

Mechanisms of ventral patterning in the vertebrate nervous system

Dorsoventral patterning of the neural tube has a crucial role in shaping the functional organization of the CNS. It is well established that hedgehog signalling plays a key role in specifying ventral cell types throughout the neuroectoderm, and major progress has been made in elucidating how hedgehog signalling works in this ventral specification. In addition, other molecular pathways, including nodal, retinoic acid and fibroblast growth factor signalling, have been identified as important molecular cues for ventral patterning of the spinal cord, telencephalon and eye. Here, we discuss recent advances in this field, highlighting the emerging interplay of these signalling pathways in the molecular specification of ventral patterning at different rostrocaudal levels of the CNS.