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

Outstanding questions in developmental ERK signaling

The extracellular signal-regulated kinase (ERK) pathway leads to activation of the effector molecule ERK, which controls downstream responses by phosphorylating a variety of substrates, including transcription factors. Crucial insights into the regulation and function of this pathway came from studying embryos in which specific phenotypes arise from aberrant ERK activation. Despite decades of research, several important questions remain to be addressed for deeper understanding of this highly conserved signaling system and its function. Answering these questions will require quantifying the first steps of pathway activation, elucidating the mechanisms of transcriptional interpretation and measuring the quantitative limits of ERK signaling within which the system must operate to avoid developmental defects.

Microinjection of Exogenous DNA into Eggs of Halocynthia roretzi

Exogenous gene expression assays during development, including reporters under the control of 5' upstream enhancer regions of genes, constitute a powerful technique for understanding the mechanisms of tissue-specific gene expression regulation and determining the characteristics, behaviors, and functions of cells that express these genes. The simple marine chordate Halocynthia roretzi has been used for these transgenic analyses for a long time and is an excellent model system for such studies, especially in comparative analyses with other ascidians. In this study, I describe simple methods for microinjecting H. roretzi eggs with exogenous DNA, such as a promoter construct consisting of a 5' upstream region and a reporter gene, which are prerequisites for transgenic analyses. I also describe basic knowledge regarding this ascidian species, providing reasons why it is an ideal subject for developmental biology studies.

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.

Eye evolution: common use and independent recruitment of genetic components

Animal eyes can vary in complexity ranging from a single photoreceptor cell shaded by a pigment cell to elaborate arrays of these basic units, which allow image formation in compound eyes of insects or camera-type eyes of vertebrates. The evolution of the eye requires involvement of several distinct components-photoreceptors, screening pigment and genes orchestrating their proper temporal and spatial organization. Analysis of particular genetic and biochemical components shows that many evolutionary processes have participated in eye evolution. Multiple examples of co-option of crystallins, Galpha protein subunits and screening pigments contrast with the conserved role of opsins and a set of transcription factors governing eye development in distantly related animal phyla. The direct regulation of essential photoreceptor genes by these factors suggests that this regulatory relationship might have been already established in the ancestral photoreceptor cell.

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.

A genomic view of the sea urchin nervous system

The sequencing of the Strongylocentrotus purpuratus genome provides a unique opportunity to investigate the function and evolution of neural genes. The neurobiology of sea urchins is of particular interest because they have a close phylogenetic relationship with chordates, yet a distinctive pentaradiate body plan and unusual neural organization. Orthologues of transcription factors that regulate neurogenesis in other animals have been identified and several are expressed in neurogenic domains before gastrulation indicating that they may operate near the top of a conserved neural gene regulatory network. A family of genes encoding voltage-gated ion channels is present but, surprisingly, genes encoding gap junction proteins (connexins and pannexins) appear to be absent. Genes required for synapse formation and function have been identified and genes for synthesis and transport of neurotransmitters are present. There is a large family of G-protein-coupled receptors, including 874 rhodopsin-type receptors, 28 metabotropic glutamate-like receptors and a remarkably expanded group of 161 secretin receptor-like proteins. Absence of cannabinoid, lysophospholipid and melanocortin receptors indicates that this group may be unique to chordates. There are at least 37 putative G-protein-coupled peptide receptors and precursors for several neuropeptides and peptide hormones have been identified, including SALMFamides, NGFFFamide, a vasotocin-like peptide, glycoprotein hormones and insulin/insulin-like growth factors. Identification of a neurotrophin-like gene and Trk receptor in sea urchin indicates that this neural signaling system is not unique to chordates. Several hundred chemoreceptor genes have been predicted using several approaches, a number similar to that for other animals. Intriguingly, genes encoding homologues of rhodopsin, Pax6 and several other key mammalian retinal transcription factors are expressed in tube feet, suggesting tube feet function as photosensory organs. Analysis of the sea urchin genome presents a unique perspective on the evolutionary history of deuterostome nervous systems and reveals new approaches to investigate the development and neurobiology of sea urchins.

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.

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.

Non-neural ectoderm is really neural: evolution of developmental patterning mechanisms in the non-neural ectoderm of chordates and the problem of sensory cell homologies

In chordates, the ectoderm is divided into the neuroectoderm and the so-called non-neural ectoderm. In spite of its name, however, the non-neural ectoderm contains numerous sensory cells. Therefore, the term "non-neural" ectoderm should be replaced by "general ectoderm." At least in amphioxus and tunicates and possibly in vertebrates as well, both the neuroectoderm and the general ectoderm are patterned anterior/posteriorly by mechanisms involving retinoic acid and Hox genes. In amphioxus and tunicates the ectodermal sensory cells, which have a wide range of ciliary and microvillar configurations, are mostly primary neurons sending axons to the CNS, although a minority lack axons. In contrast, vertebrate mechanosensory cells, called hair cells, are all secondary neurons that lack axons and have a characteristic eccentric cilium adjacent to a group of microvilli of graded lengths. It has been highly controversial whether the ectodermal sensory cells in the oral siphons of adult tunicates are homologous to vertebrate hair cells. In some species of tunicates, these cells appear to be secondary neurons, and microvillar and ciliary configurations of some of these cells approach those of vertebrate hair cells. However, none of the tunicate cells has all the characteristics of a hair cell, and there is a high degree of variation among ectodermal sensory cells within and between different species. Thus, similarities between the ectodermal sensory cells of any one species of tunicate and craniate hair cells may well represent convergent evolution rather than homology.

The neurobiology of the ascidian tadpole larva: recent developments in an ancient chordate

With little more than 330 cells, two thirds within the sensory vesicle, the CNS of the tadpole larva of the ascidian Ciona intestinalis provides us with a chordate nervous system in miniature. Neurulation, neurogenesis and its genetic bases, as well as the gene expression territories of this tiny constituency of cells all follow a chordate plan, giving rise in some cases to frank structural homologies with the vertebrate brain. Recent advances are fueled by the release of the genome and EST expression databases and by the development of methods to transfect embryos by electroporation. Immediate prospects to test the function of neural genes are based on the isolation of mutants by classical genetics and insertional mutagenesis, as well as by the disruption of gene function by morpholino antisense oligo-nucleotides. Coupled with high-speed video analysis of larval swimming, optophysiological methods offer the prospect to analyze at single-cell level the function of a CNS built on a vertebrate plan.

Pigment cell lineage-specific expression activity of the ascidian tyrosinase-related gene

Solitary ascidian tadpole larvae develop two types of black pigment cells in the major sensory organs of the brain. Such pigment cells have been demonstrated to express the melanogenic genes, tyrosinase and Tyrp/TRP (tyrosinase-related protein). To understand the genetic and developmental mechanisms underlying the differentiation of chordate pigment cells, we examined the function of the promoter region of Tyrp/TRP gene, an ascidian (Halocynthia roretzi) tyrosinase family gene. The expression of the gene in pigment cell lineage starts at the early-mid gastrula stages. To identify the transcriptional regulatory region of the gene allowing cell-type-specific expression, a deletion series of the HrTyrp 5' flanking region fused to a lacZ reporter gene was constructed and microinjected into ascidian fertilized eggs. The region of 73 bp in HrTyrp was identified as sufficient for expression in pigment cell-precursors of tailbud stage embryos. It is noteworthy that there is no M-box element highly conserved in the promoters for vertebrate tyrosinase family genes such as tyrosinase, Tyrp1/TRP-1 and Tyrp2/TRP-2 (Dct). Although the regulatory system of ascidian pigment-cell development is likely to contain most factors critical to vertebrate pigment-cell development, there might be critical differences in the mode of regulation, such as the developmental timing of interactions of factors, proteins and genes, involved in pigment cell differentiation and pigmentation.

Eye development: a view from the retina pigmented epithelium

The retina pigment epithelium (RPE) is a highly specialised epithelium that serves as a multifunctional and indispensable component of the vertebrate eye. Although a great deal of attention has been paid to its transdifferentiation capabilities and its ancillary functions in neural retina development, little is known about the molecular mechanisms that specify the RPE itself. Recent advances in our understanding of the genetic network that controls the progressive specification of the eye anlage in vertebrates have provided some of the initial cues to the mechanisms responsible for RPE patterning. Here, we have outlined many recent findings that suggest that a limited number of transcription factors, including Otx2, Mitf and Pax6 and a few signalling cascades, are the elements required for the onset of RPE specification in vertebrates. Furthermore, using this information and the data available on the specification of the pigmented cells of primitive chordates, we have ventured some hypotheses on the origin of RPE cells during evolution.

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.

OTX2 activates the molecular network underlying retina pigment epithelium differentiation

The retina pigment epithelium (RPE) is fundamental for the development and function of the vertebrate eye. Molecularly, the presumptive RPE can be identified by the early expression of two transcription factors, Mitf and Otx. In mice deficient for either gene, RPE development is impaired with loss of melanogenic gene expression, raising the possibility that in the eye OTX proteins operate either in a feedback loop or in cooperation with MITF for the control of RPE-specific gene expression. Here we show that Otx2 induces a pigmented phenotype when overexpressed in avian neural retina cells. In addition, OTX2 binds specifically to a bicoid motif present in the promoter regions of three Mitf target genes, QNR71, TRP-1, and tyrosinase, leading to their transactivation. OTX2 and MITF co-localize in the nuclei of RPE cells and physically interact, and their co-expression results in a cooperative activation of QNR71 and tyrosinase promoters. Collectively, these data suggest that both transcription factors operate at the same hierarchical level to establish the identity of the RPE.