Vegetal cell fate specification and anterior neuroectoderm formation by Hroth, the ascidian homologue of orthodenticle/otx

Endodermal Maternal Transcription Factors Establish Super-Enhancers during Zygotic Genome Activation

Elucidation of the sequence of events underlying the dynamic interaction between transcription factors and chromatin states is essential. Maternal transcription factors function at the top of the regulatory hierarchy to specify the primary germ layers at the onset of zygotic genome activation (ZGA). We focus on the formation of endoderm progenitor cells and examine the interactions between maternal transcription factors and chromatin state changes underlying the cell specification process. Endoderm-specific factors Otx1 and Vegt together with Foxh1 orchestrate endoderm formation by coordinated binding to select regulatory regions. These interactions occur before the deposition of enhancer histone marks around the regulatory regions, and these TFs recruit RNA polymerase II, regulate enhancer activity, and establish super-enhancers associated with important endodermal genes. Therefore, maternal transcription factors Otx1, Vegt, and Foxh1 combinatorially regulate the activity of super-enhancers, which in turn activate key lineage-specifying genes during ZGA.

How do maternal transcription factors interact with chromatin regions to coordinate the endodermal gene regulatory program? Paraiso et al. demonstrate that combinatorial binding of maternal Otx1, Vegt, and Foxh1 to select enhancers and super-enhancers in the genome controls endodermal cell fate specification during zygotic gene activation.

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.

Transcription regulatory mechanism of Pitx in the papilla-forming region in the ascidian, Halocynthia roretzi, implies conserved involvement of Otx as the upstream gene in the adhesive organ development of chordates

Pitx genes play important roles in a variety of developmental processes in vertebrates. In an ascidian species, Halocynthia roretzi, Hr-Pitx, the only Pitx gene of this species, has been reported to be expressed in the left epidermis at the tailbud stage. In the present study, first, we have shown that Hr-Pitx is also expressed in the papilla-forming region at the neurula to tailbud stages, and then we addressed transcription regulatory mechanisms for the expression of Hr-Pitx in the papilla-forming region. We have identified the genomic region ranging from 850 to 1211 bp upstream from the translation start site of the Hr-Pitx gene as an enhancer region that drives the transcription of Hr-Pitx in the papilla-forming region. Within the enhancer region, putative transcriptional factor binding sites for Otx as well as Fox were shown to be required for its activity. Finally, we carried out knocking down experiments of Hr-Otx function using an antisense morpholino oligonucleotide, in which the knocking down of Hr-Otx function resulted in reduction of the enhancer activity and loss of the expression of Hr-Pitx in the papilla-forming region. In Xenopus laevis, it has been reported that Pitx genes are expressed downstream of Otx function during development of the cement gland, an adhesive organ of its larva. Taken together, it is suggested that the expression regulatory mechanism of Pitx, involving Otx as the upstream gene, in the developing adhesive organ is conserved between ascidians and vertebrates.

Diversity in insect axis formation: two orthodenticle genes and hunchback act in anterior patterning and influence dorsoventral organization in the honeybee (Apis mellifera)

Axis formation is a key step in development, but studies indicate that genes involved in insect axis formation are relatively fast evolving. Orthodenticle genes have conserved roles, often with hunchback, in maternal anterior patterning in several insect species. We show that two orthodenticle genes, otd1 and otd2, and hunchback act as maternal anterior patterning genes in the honeybee (Apis mellifera) but, unlike other insects, act to pattern the majority of the anteroposterior axis. These genes regulate the expression domains of anterior, central and posterior gap genes and may directly regulate the anterior gap gene giant. We show otd1 and hunchback also influence dorsoventral patterning by regulating zerknült (zen) as they do in Tribolium, but that zen does not regulate the expression of honeybee gap genes. This suggests that interactions between anteroposterior and dorsal-ventral patterning are ancestral in holometabolous insects. Honeybee axis formation, and the function of the conserved anterior patterning gene orthodenticle, displays unique characters that indicate that, even when conserved genes pattern the axis, their regulatory interactions differ within orders of insects, consistent with relatively fast evolution in axis formation pathways.

doublesex/mab3 related-1 (dmrt1) is essential for development of anterior neural plate derivatives in Ciona

Ascidian larvae have a hollow, dorsal central nervous system that shares many morphological features with vertebrate nervous systems yet is composed of very few cells. We show here that a null mutation in the gene dmrt1 in the ascidian Ciona savignyi results in profound abnormalities in the development of the sensory vesicle (brain), as well as other anterior ectodermal derivatives, including the palps and oral siphon primordium (OSP). Although the phenotype of the mutant embryos is variable, the majority have a complete loss of the most anterior structures (palps and OSP) and extensive disruption of sensory structures, such as the light-sensitive ocellus, in the sensory vesicle. dmrt1 is expressed early in the blastula embryo in a small group of presumptive ectodermal cells as they become restricted to anterior neural, OSP and palp fates. Despite the early and restricted expression of dmrt1, we were unable, using several independent criteria, to observe a defect in the mutant embryos until the early tailbud stage. We speculate that the variability and late onset in the phenotype may be due to partially overlapping activities of other gene products.

Genomic organization, gene structure, and developmental expression of three clustered otx genes in the sea anemone Nematostella vectensis

Otx homeodomain transcription factors have been studied in a variety of eumetazoan animals where they have roles in anterior neural development, endomesoderm formation, and the formation of larval ciliated fields. Here, we describe the gene structure and developmental expression of three Otx loci in the starlet sea anemone, Nematostella vectensis (phylum Cnidaria; class Anthozoa). Nematostella's three Otx genes (OtxA, OtxB, and OtxC) are located in a compact genomic cluster spanning 63.6 kb. The homeodomains of all three Otx genes are highly similar to their bilaterian counterparts, but only OtxB exhibits the conserved WSP motif that is located downstream of the homeodomain in many Otx proteins. The genomic organization, in concert with phylogenetic analyses, indicates that two tandem duplications occurred in the lineage leading to Nematostella some time after the Cnidaria diverged from the Bilateria. In situ hybridization reveals that otx is initially expressed by invaginating mesendodermal cells in the gastrula. Later, each of the three otx paralogs is expressed in three discrete larval body regions: in the endoderm of the foot or physa, in an endodermal ring surrounding the pharynx, and in the ectoderm of the tentacles. These data suggest that a single otx locus had already acquired diverse developmental functions in the cnidarian-bilaterian ancestor. Furthermore, following two gene duplications in the line leading to Nematostella, there have been only minor alterations in the spatiotemporal expression of the three Otx paralogs. However, the absence of a conserved protein domain in OtxA and OtxC suggests functional evolution of the protein itself.

Developmental expression and transcriptional regulation of Ci-Pans, a novel neural marker gene of the ascidian, Ciona intestinalis

A novel gene, named Ci-Pans, was isolated and characterized from the ascidian Ciona intestinalis. It is an 885-bp cDNA, is thought to encode a protein with no sequence similarities to known proteins and shows a spatial and temporal specific expression pattern. In fact, besides a transient early localization in the muscle precursors, it is expressed in a dynamic fashion in the nervous system, during C. intestinalis embryogenesis, reaching very high level of expression as the development proceeds. To study Ci-Pans transcriptional control, we isolated the predicted promoter region of C. intestinalis Ci-Pans using databases for this species. Analysis of transgenic embryos, with a green fluorescence protein (GFP) reporter, showed that approximately 1 kb of the 5'-flanking sequence of the Ci-Pans gene was implicated in its specific expression in the CNS. The data on the expression pattern of Ci-Pans together with the strong activity exhibited by the 1 kb promoter region we have identified, indicate that a more deeply investigation on Ci-Pans could provide clues for exploring the complex network of nervous system-specific genes.

Regionalization of the Tail-tip Epidermis Requires Inductive Influence from Vegetal Cells and FGF Signaling in the Development of an Ascidian, Halocynthia roretzi

The epidermis of an ascidian larva derived from animal-hemisphere cells is regionalized along the anterior-posterior (AP) axis through inductive signals emanating from vegetal-hemisphere cells in early stages of the development. Previously, we showed by blastomere isolation and ablation experiments that the contact between the animal and vegetal hemispheres until the 32-cell stage is necessary for the proper AP patterning of the epidermis in the tailbud-stage embryo. We here addressed the patterning mechanism of the posteriormost epidermis using a tail-tip epidermis marker, HrTT-1. Employing blastomere isolation and ablation experiments along with knockdown of a master regulator gene for posterior mesoderm, we have demonstrated that presence of the posterior vegetal cells after the 32-cell stage is necessary for the expression of HrTT-1. To explore the timing and nature of the influence of the posterior vegetal cells, we treated the embryos with FGF signaling inhibitors at various developmental stages and found that HrTT-1 expression was lost from embryos treated with the inhibitors from stages earlier than the late neurula stage, just prior to the onset of HrTT-1 expression but not after the initial tailbud stage, at which the expression of HrTT-1 had started. In embryos lacking HrTT-1 expression, the expression domain of Hrcad, which would otherwise be localized anterior to that of HrTT-1, expanded to the tail-tip. These results suggest that FGF signaling from the neurula to initial tailbud stages is necessary for the initiation but not maintenance of HrTT-1 expression in the tail-tip epidermis. The contact with posterior vegetal cells until and after the 32-cell stage may be required for FGF signaling to occur in the posterior tail, which in turn regionalizes the tail-tip epidermal territory.

HeOtx expression in an indirectly developing polychaete correlates with gastrulation by invagination

The expression of an Otx homolog in the indirectly developing polychaete Hydroides elegans was characterized during embryo, trochophore, and feeding-larva stages. In the animal hemisphere, HeOtx is first expressed in 1q(12) blastomeres and their immediate descendants. Such discrete embryonic animal hemisphere Otx expression perhaps relates to cell-type specification functions of the larva. During feeding stages, transcripts are detected in adult cerebral ganglia precursors and putative adult eye precursors, where it may have adult brain regionalization functions. HeOtx is not expressed in primary trochoblast precursors, but it is expressed in cells adjacent to the ciliary band. HeOtx is also expressed in a group of cells in the dorsal midline of the early trochophore larva in putative posterior sensory organ precursors. The vegetal hemisphere expression starts in oral and lateral sides of the blastopore and later expands to central blastomeres that lead the gastrulation movements. During late gastrulation stages, the expression declines in foregut precursors, but it is maintained in midgut precursors, suggesting its involvement in tripartite gut subdivision functions. HeOtx broader and earlier endoderm expression correlates with gastrulation by invagination associated with the formation of the feeding trochophore, in contrast with a later and orally restricted Otx expression found in a polychaete that gastrulates by epiboly and forms a non-feeding trochophore. The endoderm expression and functional roles in other bilaterians suggest an ancestral role of Otx related to gastrulation by invagination.

Brain induction in ascidian embryos is dependent on juxtaposition of FGF9/16/20-producing and -receiving cells

Coordinated regulation of inductive events, both spatially and temporally, during animal development ensures that tissues are induced at their specific positions within the embryo. The ascidian brain is induced in cells at the anterior edge of the animal hemisphere by fibroblast growth factor (FGF) signals secreted from vegetal cells. To clarify how this process is spatially regulated, we first identified the sources of the FGF signal by examining the expression of brain markers Hr-Otx and Hr-ETR-1 in embryos in which FGF signaling is locally inhibited by injecting individual blastomeres with morpholino oligonucleotide against Hr-FGF9/16/20, which encodes an endogenous brain inducer. The blastomeres identified as the inducing sources are A5.1 and A5.2 at the 16-cell stage and A6.2 and A6.4 at the 24-cell stage, which are juxtaposed with brain precursors at the anterior periphery of the embryo at the respective stages. We also showed that all the cells of the animal hemisphere are capable of expressing Hr-Otx in response to the FGF signal. These results suggest that the position of inducers, rather than competence, plays an important role in determining which animal cells are induced to become brain tissues during ascidian embryogenesis. This situation in brain induction contrasts with that in mesoderm induction, where the positions at which the notochord and mesenchyme are induced are determined mainly by intrinsic competence factors that are inherited by signal-receiving cells.

Otx genes in the evolution of the vertebrate brain

Only until a decade ago, animal phylogeny was traditionally based on the assumption that evolution of bilaterians went from simple to complex through gradual steps in which the extant species would represent grades of intermediate complexity that reflect the organizational levels of their ancestors. The advent of more sophisticated molecular biology techniques combined to an increasing variety of functional experiments has provided new tools, which lead us to consider evolutionary studies under a brand new light. An ancestral versus derived low-complexity of a given organism has now to be carefully re-assessed and also the molecular data so far accumulated needs to be re-evaluated. Conserved gene families expressed in the nervous system of all the species have been extensively used to reconstruct evolutionary steps, which may lead to identify the morphological as well as molecular features of the last common ancestor of bilaterians (Urbilateria). The Otx gene family is among these and will be here reviewed.

Decoding cis-regulatory systems in ascidians

Ascidians, or sea squirts, are lower chordates, and share basic gene repertoires and many characteristics, both developmental and physiological, with vertebrates. Therefore, decoding cis-regulatory systems in ascidians will contribute toward elucidating the genetic regulatory systems underlying the developmental and physiological processes of vertebrates. cis-Regulatory DNAs can also be used for tissue-specific genetic manipulation, a powerful tool for studying ascidian development and physiology. Because the ascidian genome is compact compared with vertebrate genomes, both intergenic regions and introns are relatively small in ascidians. Short upstream intergenic regions contain a complete set of cis-regulatory elements for spatially regulated expression of a majority of ascidian genes. These features of the ascidian genome are a great advantage in identifying cis-regulatory sequences and in analyzing their functions. Function of cis-regulatory DNAs has been analyzed for a number of tissue-specific and developmentally regulated genes of ascidians by introducing promoter-reporter fusion constructs into ascidian embryos. The availability of the whole genome sequences of the two Ciona species, Ciona intestinalis and Ciona savignyi, facilitates comparative genomics approaches to identify cis-regulatory DNAs. Recent studies demonstrate that computational methods can help identify cis-regulatory elements in the ascidian genome. This review presents a comprehensive list of ascidian genes whose cis-regulatory regions have been subjected to functional analysis, and highlights the recent advances in bioinformatics and comparative genomics approaches to cis-regulatory systems in ascidians.

Patterning across the ascidian neural plate by lateral Nodal signalling sources

Ascidians are invertebrate chordates with a simple larval tadpole form containing a notochord and an overlying dorsal neural tube. As in vertebrates,the neural tube of ascidian larvae displays positional differences along the rostral-caudal and dorsal-ventral axes in terms of neuronal cell types generated, morphology and gene expression. However, how these differences are established in this simple chordate remains largely unknown. In this study, we show that a single blastomere named b6.5, which is situated in a lateral position in the 32-cell-stage embryo, is a source of signal(s) required for patterning across the medial-lateral axis (future ventral-dorsal axis) of the neural plate. We identify this signal as a Ciona homologue of Nodal, Ci-Nodal. Transcriptional activation of Ci-Nodal in b6.5 depends upon vegetally derived Ci-FGF9/16/20. Using three distinct reagents to inhibit Nodal signals, we show that Nodal signalling is required for neural plate patterning across the medial-lateral axis and that, in the absence of this signal, the caudal-lateral part of the neural plate adopts a medial-like fate. Secondary muscle fate is similarly affected. We conclude that specification of the lateral neural plate is initiated by signalling sources laterally flanking the neural plate and involves a cell-fate choice between lateral and medial neural fates, with Nodal signalling promoting lateral fate. This role for Nodal signalling during ascidian neural plate patterning contrasts with that in vertebrates, where it is implicated in promoting a medial neural fate, the floor plate.

Roles of Hroth, the ascidian otx gene, in the differentiation of the brain (sensory vesicle) and anterior trunk epidermis in the larval development of Halocynthia roretzi

Otx genes are expressed in the anterior neural tube and endoderm in all of the chordates so far examined. In mouse embryos, important roles of otx genes in the brain development have been well documented. However, roles of otx genes in other chordate species have been less characterized. To advance our understanding about roles of otx genes in chordates, we have studied Hroth, otx of the ascidian, Halocynthia roretzi. Hroth is expressed in the anterior part of the neural tube (the sensory vesicle), the endoderm and anterior epidermis in the development. In this study, we investigated roles of Hroth in the larval development through an antisense morpholino oligonucleotides (MOs) approach. Embryos injected with Hroth-targeting MO (Hroth knockdown embryos) developed into larvae without the adhesive organ, sensory pigment cells and cavity of the sensory vesicle. The tissues, in which defects were observed, are derived from anterior-animal cells of the embryo in early cleavage stages. During cleavage stages, Hroth is also expressed in the endoderm precursors of the vegetal hemisphere. However, Hroth expression in the anterior endoderm precursors do not seem to be essential for the above defects, since MO injection into the anterior-animal but not anterior-vegetal pair cells at the 8-cell stage gave the defects. Analysis of marker gene expression demonstrated that the fate choice of the sensory vesicle precursors and the specification of the sensory vesicle territory occurred normally, but the subsequent differentiation of the sensory vesicle was severely affected in Hroth knockdown embryos. The anterior trunk epidermis including the adhesive organ-forming region was also affected, indicating that anterior epidermal patterning requires Hroth function. Based on these findings, similarities and differences in the roles of otx genes between ascidians and mice are discussed.

Expression and function of a starfish Otx ortholog, AmOtx: a conserved role for Otx proteins in endoderm development that predates divergence of the eleutherozoa

The sea urchin orthodenticle (Otx)-related transcription factor is an early activator of other endomesodermally expressed transcription factors. Its normal function is required for the development of the archenteron and to lock cells into endomesodermal fate. To determine if this is a basal Otx function in echinoderms we have studied the role of an Otx ortholog in a starfish, Asterina miniata. The patterns of AmOtx expression are found to be similar, in many details, to those reported for other indirectly developing echinoderms and hemichordates, suggestive of a conserved function both in endoderm development and ciliary band formation in these two phyla. When downstream targets of the AmOtx protein are suppressed using a dominant engrailed repressor strategy, embryos fail to develop the endodermal component of the archenteron, though initial phases of mesoderm development proceed normally. The function of Otx proteins in endodermal development at least predated the evolution of the free-living echinoderms (Eleutherozoa).

Ets-mediated brain induction in embryos of the ascidian Halocynthia roretzi

The larval ascidian brain (sensory vesicle) is located on the dorsal side of the trunk region and forms part of the anterior central nervous system. Sensory organs such as the otolith, ocellus, and hydrostatic-pressure organ reside in the brain. The brain coordinates the core roles of the larval nervous system. The brain is derived from anterior animal a-line blastomeres. The default fate of these blastomeres is epidermis, and the inductive signals from anterior vegetal blastomeres convert the fate into brain. It remains unclear, however, when these inductive interactions take place. To determine when, we examined whether partial embryos derived from brain-lineage blastomeres isolated at various stages express neural and epidermal marker genes. Partial embryos derived from brain-lineage blastomeres isolated after the 32-cell stage expressed all the neural marker genes examined. The expression of the epidermal marker gene was first reduced in partial embryos when blastomeres were isolated at the 64-cell stage. Moreover, the process for brain specification seemed to continue after the 110-cell stage. We also investigated the function of HrEts, an ascidian homolog of Ets transcription factors, to elucidate the molecular mechanism of brain induction. HrEts functions were inhibited by the use of antisense morpholino oligonucleotides. Loss of Ets functions resulted in loss of the expression of some of the neural marker genes and the ectopic expression of the epidermal marker gene in brain precursor cells. These results suggest that HrEts is an essential transcription factor that mediates ascidian brain induction.

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

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

An Ets transcription factor, HrEts, is target of FGF signaling and involved in induction of notochord, mesenchyme, and brain in ascidian embryos

In ascidian embryos, a fibroblast growth factor (FGF) signal induces notochord, mesenchyme, and brain formation. Although a conserved Ras/MAPK pathway is known to be involved in this signaling, the target transcription factor of this signaling cascade has remained unknown. We have isolated HrEts, an ascidian homolog of vertebrate Ets1 and Ets2, to elucidate the transcription factor involved in the FGF signaling pathway in embryos of the ascidian Halocynthia roretzi. Maternal mRNA of HrEts was detected throughout the entire egg cytoplasm and early embryos. Its zygotic expression started in several tissues, including the notochord and neural plate. Overexpression of HrEts mRNA did not affect the general organization of the tadpoles, but resulted in formation of excess sensory pigment cells. In contrast, suppression of HrEts function by morpholino antisense oligonucleotide resulted in severe abnormalities, similar to those of embryos in which the FGF signaling pathway was inhibited. Notochord-specific Brachyury expression at cleavage stage and notochord differentiation at the tailbud stage were abrogated. Formation of mesenchyme cells was also suppressed, and the mesenchyme precursors assumed muscle fate. In addition, expression of Otx in brain-lineage blastomeres was specifically suppressed. These results suggest that an Ets transcription factor, HrEts, is involved in signal transduction of FGF commonly in notochord, mesenchyme, and brain induction in ascidian embryos.

Early steps in the formation of neural tissue in ascidian embryos

Ascidians are simple invertebrate chordates whose lineage diverged from that of vertebrates at the base of the chordate tree. Their larvae display a typical chordate body plan, but are composed of a remarkably small number of cells. Ascidians develop with an invariant cell lineage, and their embryos can be easily experimentally manipulated during the cleavage stages. Their larval nervous system is organised in a similar way as in vertebrates but is composed of less than 130 neurones and around 230 glial cells. This remarkable simplicity offers an opportunity to understand, at the cellular and molecular levels, the ontogeny and function of each neural cell. Here, we first review the organisation of the ascidian nervous system and its lineage. We then focus on the current understanding of the processes of neural specification and patterning before and during gastrulation. We discuss these advances in the context of what is currently known in vertebrates.

Specification of developmental fates in ascidian embryos: molecular approach to maternal determinants and signaling molecules

Tadpole larvae of ascidians represent the basic body plan of chordates with a relatively small number and few types of cells. Because of their simplicity, ascidians have been intensively studied. More than a century of research on ascidian embryogenesis has uncovered many cellular and molecular mechanisms responsible for cell fate specification in the early embryo. This review describes recent advances in our understanding of the molecular mechanisms of fate specification mainly uncovered in model ascidian species--Halocynthia roretzi, Ciona intestinalis, and Ciona savignyi. One category of developmentally important molecules represents maternal localized mRNAs that are involved in cell-autonomous processes. In the second category, signaling molecules and downstream transcription factors are involved in inductive cell interactions. Together with genome-wide information, there is a renewed interest in studying ascidian embryos as a fascinating model system for understanding how single-celled eggs develop a highly organized chordate body plan.

HrzicN, a new Zic family gene of ascidians, plays essential roles in the neural tube and notochord development

Two axial structures, a neural tube and a notochord, are key structures in the chordate body plan and in understanding the origin of chordates. To expand our knowledge on mechanisms of development of the neural tube in lower chordates, we have undertaken isolation and characterization of HrzicN, a new member of the Zic family gene of the ascidian, Halocynthia roretzi. HrzicN expression was detected by whole-mount in situ hybridization in all neural tube precursors, all notochord precursors, anterior mesenchyme precursors and a part of the primary muscle precursors. Expression of HrzicN in a- and b-line neural tube precursors was detected from early gastrula stage to the neural plate stage, while expression in other lineages was observed between the 32-cell and the 110-cell stages. HrzicN function was investigated by disturbing translation using a morpholino antisense oligonucleotide. Embryos injected with HrzicN morpholino ('HrzicN knockdown embryos') exhibited failure of neurulation and tail elongation, and developed into larvae without a neural tube and notochord. Analysis of neural marker gene expression in HrzicN knockdown embryos revealed that HrzicN plays critical roles in distinct steps of neural tube formation in the a-line- and A-line precursors. In particular HrzicN is required for early specification of the neural tube fate in A-line precursors. Involvement of HrzicN in the neural tube development was also suggested by an overexpression experiment. However, analysis of mesodermal marker gene expression in HrzicN knockdown embryos revealed unexpected roles of this gene in the development of mesodermal tissues. HrzicN knockdown led to loss of HrBra (Halocynthia roretzi Brachyury) expression in all of the notochord precursors, which may be the cause for notochord deficiency. Hrsna (Halocynthia roretzi snail) expression was also lost from all the notochord and anterior mesenchyme precurosrs. By contrast, expression of Hrsna and the actin gene was unchanged in the primary muscle precursors. These results suggest that HrzicN is responsible for specification of the notochord and anterior mesenchyme. Finally, regulation of HrzicN expression by FGF-like signaling was investigated, which has been shown to be involved in induction of the a- and b-line neural tube, the notochord and the mesenchyme cells in Halocynthia embryos. Using an inhibitor of FGF-like signaling, we showed that HrzicN expression in the a- and b-line neural tube, but not in the A-line lineage and mesodermal lineage, depends on FGF-like signaling. Based on these data, we discussed roles of HrzicN as a key gene in the development of the neural tube and the notochord.

Ascidian otx gene Hroth activates transcription of the brain-specific gene HrTRP

The brain (sensory vesicle) of the ascidian larvae is thought to be homologous to the vertebrate forebrain and midbrain and, thus, is proposed as a simplified model to investigate mechanisms of brain formation in vertebrates. However, the genetic circuitry that governs formation of the sensory vesicle is largely unknown. To address this issue, we investigated the transcriptional regulation of the sensory vesicle-specific gene HrTRP by Hroth, the otx gene of the ascidian Halocynthia roretzi. A 133-bp 5'-flanking region of HrTRP, identified as a promoter that can drive expression of the reporter gene in the sensory vesicle, contains two otx binding consensus sites. When the two otx sites were deleted or mutated, the promoter activity of this region was decreased. Hroth overexpression can transactivate this promoter in an otx site-dependent manner. Transactivation of HrTRP promoter by Hroth overexpression was mimicked by overexpression of Hroth/VP16, which encodes a fusion protein of Hroth and the activator domain of VP16, and is suppressed by coexpression with Hroth/En, which encodes a fusion protein of Hroth and the Engrailed repressor domain. Finally, translational interference of Hroth by a morpholino oligonucleotide resulted in the reduction of HrTRP expression in the ascidian embryos. These results suggest that Hroth acts as a direct activator of HrTRP transcription during sensory vesicle development.

Origins of anteroposterior patterning and Hox gene regulation during chordate evolution

All chordates share a basic body plan and many common features of early development. Anteroposterior (AP) regions of the vertebrate neural tube are specified by a combinatorial pattern of Hox gene expression that is conserved in urochordates and cephalochordates. Another primitive feature of Hox gene regulation in all chordates is a sensitivity to retinoic acid during embryogenesis, and recent developmental genetic studies have demonstrated the essential role for retinoid signalling in vertebrates. Two AP regions develop within the chordate neural tube during gastrulation: an anterior 'forebrain-midbrain' region specified by Otx genes and a posterior 'hindbrain-spinal cord' region specified by Hox genes. A third, intermediate region corresponding to the midbrain or midbrain-hindbrain boundary develops at around the same time in vertebrates, and comparative data suggest that this was also present in the chordate ancestor. Within the anterior part of the Hox-expressing domain, however, vertebrates appear to have evolved unique roles for segmentation genes, such as Krox-20, in patterning the hindbrain. Genetic approaches in mammals and zebrafish, coupled with molecular phylogenetic studies in ascidians, amphioxus and lampreys, promise to reveal how the complex mechanisms that specify the vertebrate body plan may have arisen from a relatively simple set of ancestral developmental components.

Reconstructing the eyes of Urbilateria

The shared roles of Pax6 and Six homologues in the eye development of various bilaterians suggest that Urbilateria, the common ancestors of all Bilateria, already possessed some simple form of eyes. Here, we re-address the homology of bilaterian cerebral eyes at the level of eye anatomy, of eye-constituting cell types and of phototransductory molecules. The most widespread eye type found in Bilateria are the larval pigment-cup eyes located to the left and right of the apical organ in primary, ciliary larvae of Protostomia and Deuterostomia. They can be as simple as comprising a single pigment cell and a single photoreceptor cell in inverse orientation. Another more elaborate type of cerebral pigment-cup eyes with an everse arrangement of photoreceptor cells is found in adult Protostomia. Both inverse larval and everse adult eyes employ rhabdomeric photoreceptor cells and thus differ from the chordate cerebral eyes with ciliary photoreceptors. This is highly significant because on the molecular level we find that for phototransduction rhabdomeric versus ciliary photoreceptor cells employ divergent rhodopsins and non-orthologous G-proteins, rhodopsin kinases and arrestins. Our comparison supports homology of cerebral eyes in Protostomia; it challenges, however, homology of chordate and non-chordate cerebral eyes that employ photoreceptor cells with non-orthologous phototransductory cascades.

Ascidian Wnt-5 gene is involved in the morphogenetic movement of notochord cells

Wnt proteins play important roles in many developmental events. Wnts are divided into two groups according to biological function. The Wnt-5a class proteins function in morphogenetic movement during embryogenesis. Previously, a Wnt-5 homolog has been isolated from the ascidian, Halocynthia roretzi. HrWnt-5 is expressed in the notochord until the tail-bud stage, implying a role in the notochord. In this study, the function of HrWnt-5 was investigated. When HrWnt-5 mRNA was injected into fertilized eggs, the embryos showed morphologic defects at around the neurula stage. The anterior-posterior axis was shorter than in control embryos. These defects were caused by the abnormal movement of notochord cells. However, the overexpression of HrWnt-5 mRNA did not affect the differentiation of tissues, suggesting that HrWnt-5 solely regulates the morphogenetic movement. Although endogenous HrWnt-5 is expressed in the notochord, the overexpression of HrWnt-5 mRNA caused the defects, suggesting that the amount of HrWnt-5 mRNA in the notochord is strictly regulated. These results suggest that HrWnt-5 regulates the morphogenetic movement of notochord cells during ascidian embryogenesis.

Early embryonic expression of a LIM-homeobox geneCs-lhx3is downstream of β-catenin and responsible for the endoderm differentiation inCiona savignyiembryos

In early Ciona embryos, nuclear accumulation of β-catenin is most probably the first step of endodermal cell specification. If β-catenin is mis- and/or overexpressed, presumptive notochord cells and epidermal cells change their fates into endodermal cells, whereas if β-catenin nuclear localization is downregulated by the overexpression of cadherin, the endoderm differentiation is suppressed, accompanied with the differentiation of extra epidermal cells (Imai, K., Takada, N., Satoh, N. and Satou, Y. (2000) Development127, 3009-3020). Subtractive hybridization screens of mRNAs between β-catenin overexpressed embryos and cadherin overexpressed embryos were conducted to identify potential β-catenin target genes that are responsible for endoderm differentiation in Ciona savignyi embryos. We found that a LIM-homeobox gene (Cs-lhx3), an otx homolog (Cs-otx) and an NK-2 class gene (Cs-ttf1) were among β-catenin downstream genes. In situ hybridization signals for early zygotic expression of Cs-lhx3 were evident only in the presumptive endodermal cells as early as the 32-cell stage, those of Cs-otx in the mesoendodermal cells at the 32-cell stage and those of Cs-ttf1 in the endodermal cells at the 64-cell stage. Later, Cs-lhx3 was expressed again in a set of neuronal cells in the tailbud embryo, while Cs-otx was expressed in the anterior nervous system of the embryo. Expression of all three genes was upregulated in β-catenin overexpressed embryos and downregulated in cadherin overexpressed embryos. Injection of morpholino oligonucleotides against Cs-otx did not affect the embryonic endoderm differentiation, although the formation of the central nervous system was suppressed. Injection of Cs-ttf1 morpholino oligonucleotides also failed to suppress the endoderm differentiation, although injection of its synthetic mRNAs resulted in ectopic development of endoderm differentiation marker alkaline phosphatase. By contrast, injection of Cs-lhx3 morpholino oligo suppressed the endodermal cell differentiation and this suppression was rescued by injection of Cs-lhx3 mRNA into eggs. In addition, although injection of delE-Ci-cadherin mRNA into eggs resulted in the suppression of alkaline phosphatase development, injection of delE-Ci-cadherin mRNA with Cs-lhx3 mRNA rescued the alkaline phosphatase development. These results strongly suggest that a LIM-homeobox gene Cs-lhx3 is one of the β-catenin downstream genes and that its early expression in embryonic endodermal cells is responsible for their differentiation.

Otx genes are required for tissue specification in the developing eye

Patterning of the vertebrate eye appears to be controlled by the mutual regulation and the progressive restriction of the expression domains of a number of genes initially co-expressed within the eye anlage. Previous data suggest that both Otx1 and Otx2 might contribute to the establishment of the different eye territories. Here, we have analysed the ocular phenotype of mice carrying different functional copies of Otx1 and Otx2 and we show that these genes are required in a dose-dependent manner for the normal development of the eye. Thus, all Otx1(-/-); Otx2(+/-) and 30% of Otx1(+/-); Otx2(+/-) genotypes presented consistent and profound ocular malformation, including lens, pigment epithelium, neural retina and optic stalk defects. During embryonic development, optic vesicle infolding was severely altered and the expression of pigment epithelium-specific genes, such as Mitf or tyrosinase, was lost. Lack of pigment epithelium specification was associated with an expansion of the prospective neural retina and optic stalk territories, as determined by the expression of Pax6, Six3 and Pax2. Later in development the presumptive pigment epithelium region acquired features of mature neural retina, including the generation of Islet1-positive neurones. Furthermore, in Otx1(-/-); Otx2(+/-) mice neural retina cell proliferation, cell differentiation and apoptotic cell death were also severely affected. Based on these findings we propose a model in which Otx gene products are required for the determination and differentiation of the pigment epithelium, co-operating with other eye patterning genes in the determination of the specialised tissues that will constitute the mature vertebrate eye.

A gene encoding a new ONECUT class homeodomain protein in the ascidian Halocynthia roretzi functions in the differentiation and specification of neural cells in ascidian embryogenesis

Genes encoding a novel group of homeodomain transcription factors, ONECUT class homeodomain proteins, have previously been isolated from vertebrate and insect. Among them, vertebrate HNF-6 is expressed in hepatocytes and the central nervous system during embryogenesis. Although the functions of HNF-6 in hepatocytes have been well studied, the functions of HNF-6 in the central nervous system remain unknown. In this study, we isolated HrHNF-6, which encodes a new ONECUT class homeodomain protein, from an ascidian, Halocynthia roretzi. HrHNF-6 mRNA was expressed exclusively in neural cells, just posterior to the expression of Hroth during embryogenesis. One of the functions of HrHNF-6 in neural cells is the activation of the expression of HrTBB2, the ascidian beta-tubulin gene. Another is the restriction of the expression of HrPax-258 (which is expressed in the neural tube), suggesting that HrHNF-6 functions in the specification of the neural tube. These results indicate that HrHNF-6 functions in the differentiation and regional specification of neural cells during ascidian embryogenesis.

Otx genes in brain morphogenesis

Most of the gene candidates for the control of developmental programmes that underlie brain morphogenesis in vertebrates are the homologues of Drosophila genes coding for signalling molecules or transcription factors. Among these, the orthodenticle group includes the Drosophila orthodenticle (otd) and the vertebrate Otx1 and Otx2 genes, which are mostly involved in fundamental processes of anterior neural patterning. These genes encode transcription factors that recognise specific target sequences through the DNA binding properties of the homeodomain. In Drosophila, mutations of otd cause the loss of the anteriormost head neuromere where the gene is transcribed, suggesting that it may act as a segmentation "gap" gene. In mouse embryos, the expression patterns of Otx1 and Otx2 have shown a remarkable similarity with the Drosophila counterpart. This suggested that they could be part of a conserved control system operating in the brain and different from that coded by the HOX complexes controlling the hindbrain and spinal cord. To verify this hypothesis a series of mouse models have been generated in which the functions of the murine genes were: (i) fully inactivated, (ii) replaced with each others, (iii) replaced with the Drosophila otd gene. Otx1-/- mutants suffer from epilepsy and are affected by neurological, hormonal, and sense organ defects. Otx2-/- mice are embryonically lethal, they show gastrulation impairments and fail in specifying anterior neural plate. Analysis of the Otx1-/-; Otx2+/- double mutants has shown that a minimal threshold level of the proteins they encode is required for the correct positioning of the midbrain-hindbrain boundary (MHB). In vivo otd/Otx reciprocal gene replacement experiments have provided evidence of a general functional equivalence among otd, Otx1 and Otx2 in fly and mouse. Altogether these data highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system (CNS) and lead to hypothesize that modification of their regulatory control may have influenced morphogenesis and evolution of the brain.

The homeobox gene Otx of the jellyfish Podocoryne carnea: role of a head gene in striated muscle and evolution

In many bilaterian animals members of the Otx gene family are expressed in head or brain structures. Cnidarians, however, have no clearly homologous head and no distinct brain; but an Otx homolog from the jellyfish Podocoryne carnea is highly conserved in sequence and domain structure. Sequence similarities extend well beyond the homeodomain and Podocoryne Otx can be aligned over its entire length to human OTX1, OTX2, and CRX. The overall structure of Otx is better conserved from Podocoryne to deuterostomes while protostomes appear to be more derived. In contrast, functions seem to be conserved from protostomes to vertebrates but not in Podocoryne or echinoderms. Podocoryne Otx is expressed only during medusa bud formation and becomes restricted to the striated muscle of medusae. Cnidaria are the most basal animals with striated muscle. Podocoryne polyps have no striated muscle and no Otx expression; both appear only during the asexual medusa budding process. The common ancestor of all animals that gave rise to cnidarians, protostomes, and deuterostomes already had an Otx gene more similar to today's Podocoryne and human homologs than to Drosophila otd, while the head-specific function appears to have evolved only later.

Anteroposterior patterning of the epidermis by inductive influences from the vegetal hemisphere cells in the ascidian embryo

Patterning along the anteroposterior axis is a critical step during animal embryogenesis. Although mechanisms of anteroposterior patterning in the neural tube have been studied in various chordates, little is known about those of the epidermis. To approach this issue, we investigated patterning mechanisms of the epidermis in the ascidian embryo. First we examined expression of homeobox genes (Hrdll-1, Hroth, HrHox-1 and Hrcad) in the epidermis. Hrdll-1 is expressed in the anterior tip of the epidermis that later forms the adhesive papillae, while Hroth is expressed in the anterior part of the trunk epidermis. HrHox-1 and Hrcad are expressed in middle and posterior parts of the epidermis, respectively. These data suggested that the epidermis of the ascidian embryo is patterned anteroposteriorly. In ascidian embryogenesis, the epidermis is exclusively derived from animal hemisphere cells. To investigate regulation of expression of the four homeobox genes in the epidermis by vegetal hemisphere cells, we next performed hemisphere isolation and cell ablation experiments. We showed that removal of the vegetal cells before the late 16-cell stage results in loss of expression of these homeobox genes in the animal hemisphere cells. Expression of Hrdll-1 and Hroth depends on contact with the anterior-vegetal (the A-line) cells, while expression of HrHox-1 and Hrcad requires contact with the posterior-vegetal (the B-line) cells. We also demonstrated that contact with the vegetal cells until the late 32-cell stage is sufficient for animal cells to express Hrdll-1, Hroth and Hrcad, while longer contact is necessary for HrHox-1 expression. Contact with the A-line cells until the late 32-cell stage is also sufficient for formation of the adhesive papillae. Our data indicate that the epidermis of the ascidian embryo is patterned along the anteroposterior axis by multiple inductive influences from the vegetal hemisphere cells and provide the first insight into mechanisms of epidermis patterning in the chordate embryos.