Ascidian tyrosinase gene: its unique structure and expression in the developing brain

Molecular characterization, spatiotemporal expression, and background adaptation regulation of tyrosinase in loach (Misgurnus anguillicaudatus)

The Poyang Lake region is home to large-blackspot loaches (LBL), small-blackspot loaches (SBL), and non-blackspot loaches (NBL), Misgurnus anguillicaudatus. To investigate the impact of tyrosinase on spot development, the complementary DNAs (cDNA) of tyrosinase in M. anguillicaudatus (designated as Matyr) were cloned using the rapid amplification of cDNA ends (RACE)-PCR method. The full-length cDNA for Matyr was 2020 bp, and the open-reading frame comprised 1617 bp, encoding a predicted protein with 538 amino acids. Phylogenetic studies revealed that MaTyr was first grouped with Tyr of Triplophysa tibetana and Leptobotia taeniops, and then Tyr of other cyprinid fish. The quantitative reverse-transcription-PCR results show that Matyr was highly expressed in the muscle, caudal fin, and dorsal skin. The Matyr gene's messenger RNA expression pattern steadily increased from the fertilized ovum period to the somitogenesis period, and from the muscle effect stage to 6 days after fertilization, it considerably increased (p < 0.01). The Matyr hybridization signals with similar location could be found in all developmental stages of three kinds of loaches using whole-mount in situ hybridization (WISH) technology and were the strongest during the organ development period and melanin formation period. Dot hybridization signals in LBLs rapidly spread to the back of the body beginning at the period when the eyes first formed melanin, and their dimensions were larger than those of NBLs during the same time period. The body color of loaches could change reversibly with black/white background adaptation. The α-msh, mitfa, and tyr are mainly expressed in loaches adapted with a black background. Tyr gene could be involved in the development of blackspots and body color polymorphism, and contribute to organ development in the loach.

New putative phenol oxidase in ascidian blood cells

The phenol oxidase system is ancient and ubiquitously distributed in all living organisms. In various groups it serves for the biosynthesis of pigments and neurotransmitters (dopamine), defence reactions and tissue hardening. Ascidians belong to subphylum Tunicata, which is considered the closest living relative to Vertebrates. Two phenol oxidases previously described for ascidians are vertebrate-like and arthropod-like phenol oxidases. In our present study, we described a new ascidian protein, Tuphoxin, with putative phenol oxidase function, which bears no sequence similarity with two enzymes described previously. The closest related proteins to Tuphoxin are mollusc haemocyanins. Unlike haemocyanins, which are oxygen transporting plasma proteins, Tuphoxin is synthesised in ascidian blood cells and secreted in the extracellular matrix of the tunic—ascidian outer coverings. Single mature transcript coding for this phenol oxidase can give several protein products of different sizes. Thus limited proteolysis of the initial protein is suggested. A unique feature of Tuphoxins and their homologues among Tunicata is the presence of thrombospondin first type repeats (TSP1) domain in their sequence which is supposed to provide interaction with extracellular matrix. The finding of TSP1 in the structure of phenol oxidases is new and we consider this to be an innovation of Tunicata evolutionary lineage.

Production of a mutant of large-scale loach Paramisgurnus dabryanus with skin pigmentation loss by genome editing with CRISPR/Cas9 system

CRISPR/Cas9 system has been developed as a highly efficient genome editing technology to specifically induce mutations in a few aquaculture species. In this study, we described induction of targeted gene (namely tyrosinase, tyr) mutations in large-scale loach Paramisgurnus dabryanus, an important aquaculture fish species and a potential model organism for studies of intestinal air-breathing function, using the CRISPR/Cas9 system. Tyr gene in large-scale loach was firstly cloned and then its expressions were investigated. Two guide RNAs (gRNAs) were designed and separately transformed with Cas9 in the loach. 89.4% and 96.1% of injected loach juveniles respectively displayed a graded loss of pigmentation for the two gRNAs, in other words, for target 1 and target 2. We classified the injected loach juveniles into five groups according to their skin color phenotypes, including four albino groups and one wild-type-like group. And one of them was clear albino group, which was of high ornamental and commercial value. More than 50 clones for each albino transformant with a visible phenotype in each target were randomly selected and sequenced. Results obtained here showed that along with the increase of pigmentation, wild-type alleles appeared in the injected loach juveniles more often and insertion/deletion alleles less frequently. This study demonstrated that CRISPR/Cas9 system could be practically performed to modify large-scale loach tyr to produce an albino mutant of high ornamental and commercial value, and for the first time showed successful use of the CRISPR/Cas9 system for genome editing in a Cobitidae species.

Evolutionary loss of melanogenesis in the tunicate Molgula occulta

BACKGROUND

Analyzing close species with diverse developmental modes is instrumental for investigating the evolutionary significance of physiological, anatomical and behavioral features at a molecular level. Many examples of trait loss are known in metazoan populations living in dark environments. Tunicates are the closest living relatives of vertebrates and typically present a lifecycle with distinct motile larval and sessile adult stages. The nervous system of the motile larva contains melanized cells associated with geotactic and light-sensing organs. It has been suggested that these are homologous to vertebrate neural crest-derived melanocytes. Probably due to ecological adaptation to distinct habitats, several species of tunicates in the Molgulidae family have tailless (anural) larvae that fail to develop sensory organ-associated melanocytes. Here we studied the evolution of Tyrosinase family genes, indispensible for melanogenesis, in the anural, unpigmented Molgula occulta and in the tailed, pigmented Molgula oculata by using phylogenetic, developmental and molecular approaches.

RESULTS

We performed an evolutionary reconstruction of the tunicate Tyrosinase gene family: in particular, we found that M. oculata possesses genes predicted to encode one Tyrosinase (Tyr) and three Tyrosinase-related proteins (Tyrps) while M. occulta has only Tyr and Tyrp.a pseudogenes that are not likely to encode functional proteins. Analysis of Tyr sequences from various M. occulta individuals indicates that different alleles independently acquired frameshifting short indels and/or larger mobile genetic element insertions, resulting in pseudogenization of the Tyr locus. In M. oculata, Tyr is expressed in presumptive pigment cell precursors as in the model tunicate Ciona robusta. Furthermore, a M. oculata Tyr reporter gene construct was active in the pigment cell precursors of C. robusta embryos, hinting at conservation of the regulatory network underlying Tyr expression in tunicates. In contrast, we did not observe any expression of the Tyr pseudogene in M. occulta embryos. Similarly, M. occulta Tyr allele expression was not rescued in pigmented interspecific M. occulta × M. oculata hybrid embryos, suggesting deleterious mutations also to its cis-regulatory sequences. However, in situ hybridization for transcripts from the M. occulta Tyrp.a pseudogene revealed its expression in vestigial pigment cell precursors in this species.

CONCLUSIONS

We reveal a complex evolutionary history of the melanogenesis pathway in tunicates, characterized by distinct gene duplication and loss events. Our expression and molecular data support a tight correlation between pseudogenization of Tyrosinase family members and the absence of pigmentation in the immotile larvae of M. occulta. These results suggest that relaxation of purifying selection has resulted in the loss of sensory organ-associated melanocytes and core genes in the melanogenesis biosynthetic pathway in M. occulta.

Structure, biosynthesis and possible function of tunichromes and related compounds

Several species of ascidians (phylum Chordata, subphylum Urochordata) contain a group of oligopeptides called "tunichromes" in their blood cells. These peptides have been implicated in (a) metal chelation and accumulation/sequestration of vanadium or iron; (b) crosslinking of structural fibers in tunic formation, (c) wound healing and (d) defense reactions. However, their biosynthesis, metabolism, and biological function remain largely un-elucidated due to their extreme instability and high reactivity. Tunichromes and related compounds uniquely possess dehydrodopamine moieties, all originating from post-translational modification of peptidyl tyrosine. It is conceivable that the presence of such novel post-translationally modified groups provide attributes that are crucial for their biological roles. Therefore, we examined the chemistry and reactivity of tunichromes in light of the available knowledge of the biochemistry of simple monomeric dehydro-N-acyldopamine units. Based on the reactivity of such simple compounds, the potential biological activities of tunichromes are predicted. Their possible biosynthetic route from peptidyl tyrosine is critically evaluated to provide a better basis for unraveling their biological functions. Prevalence of dehydro-N-acyldopamine units in different tunichromes, some marine antibiotic compounds, insect cuticular sclerotizing precursors and some bioadhesive marine proteins may aid in the de novo design of unique biomaterials with potential antibiotic/adhesive properties.

New insights into the evolution of metazoan tyrosinase gene family

Tyrosinases, widely distributed among animals, plants and fungi, are involved in the biosynthesis of melanin, a pigment that has been exploited, in the course of evolution, to serve different functions. We conducted a deep evolutionary analysis of tyrosinase family amongst metazoa, thanks to the availability of new sequenced genomes, assessing that tyrosinases (tyr) represent a distinctive feature of all the organisms included in our study and, interestingly, they show an independent expansion in most of the analyzed phyla. Tyrosinase-related proteins (tyrp), which derive from tyr but show distinct key residues in the catalytic domain, constitute an invention of chordate lineage. In addition we here reported a detailed study of the expression territories of the ascidian Ciona intestinalis tyr and tyrps. Furthermore, we put efforts in the identification of the regulatory sequences responsible for their expression in pigment cell lineage. Collectively, the results reported here enlarge our knowledge about the tyrosinase gene family as valuable resource for understanding the genetic components involved in pigment cells evolution and development.

FGF/MAPK/Ets signaling renders pigment cell precursors competent to respond to Wnt signal by directly controlling Ci-Tcf transcription

FGF and Wnt pathways constitute two fundamental signaling cascades, which appear to crosstalk in cooperative or antagonistic fashions in several developmental processes. In vertebrates, both cascades are involved in pigment cell development, but the possible interplay between FGF and Wnt remains to be elucidated. In this study, we have investigated the role of FGF and Wnt signaling in development of the pigment cells in the sensory organs of C. intestinalis. This species possesses the basic features of an ancestral chordate, thus sharing conserved molecular developmental mechanisms with vertebrates. Chemical and targeted perturbation approaches revealed that a FGF signal, spreading in time from early gastrulation to neural tube closure, is responsible for pigment cell precursor induction. This signal is transmitted via the MAPK pathway, which activates the Ci-Ets1/2 transcription factor. Targeted perturbation of Ci-TCF, a downstream factor of the canonical Wnt pathway, indicated its contribution to pigment cell differentiation Furthermore, analyses of the Ci-Tcf regulatory region revealed the involvement of the FGF effector, Ci-Ets1/2, in Ci-Tcf transcriptional regulation in pigment cell precursors. Our results indicate that both FGF and the canonical Wnt pathways are involved in C. intestinalis pigment cell induction and differentiation. Moreover, we present a case of direct transcriptional regulation exerted by the FGF signaling cascade, via the MAPK-ERK-Ets1/2, on the Wnt downstream gene Ci-Tcf. Several examples of FGF/Wnt signaling crosstalk have been described in different developmental processes; however, to our knowledge, FGF-Wnt cross-interaction at the transcriptional level has never been previously reported. These findings further contribute to clarifying the multitude of FGF-Wnt pathway interactions.

The evolutionarily conserved leprecan gene: its regulation by Brachyury and its role in the developing Ciona notochord

In Ciona intestinalis, leprecan was identified as a target of the notochord-specific transcription factor Ciona Brachyury (Ci-Bra) (Takahashi, H., Hotta, K., Erives, A., Di Gregorio, A., Zeller, R.W., Levine, M., Satoh, N., 1999. Brachyury downstream notochord differentiation in the ascidian embryo. Genes Dev. 13, 1519-1523). By screening approximately 14 kb of the Ci-leprecan locus for cis-regulatory activity, we have identified a 581-bp minimal notochord-specific cis-regulatory module (CRM) whose activity depends upon T-box binding sites located at the 3'-end of its sequence. These sites are specifically bound in vitro by a GST-Ci-Bra fusion protein, and mutations that abolish binding in vitro result in loss or decrease of regulatory activity in vivo. Serial deletions of the 581-bp notochord CRM revealed that this sequence is also able to direct expression in muscle cells through the same T-box sites that are utilized by Ci-Bra in the notochord, which are also bound in vitro by the muscle-specific T-box activators Ci-Tbx6b and Ci-Tbx6c. Additionally, we created plasmids aimed to interfere with the function of Ci-leprecan and categorized the resulting phenotypes, which consist of variable dislocations of notochord cells along the anterior-posterior axis. Together, these observations provide mechanistic insights generally applicable to T-box transcription factors and their target sequences, as well as a first set of clues on the function of Leprecan in early chordate development.

RNA interference by expressing short hairpin RNA in the Ciona intestinalis embryo

We carried out RNA interference by expressing short hairpin RNA (shRNA) in the Ciona intestinalis embryo. For this purpose, we identified a gene encoding U6 small nuclear RNA (snRNA) in the C. intestinalis genome. The 1-kb sequence upstream of the U6 snRNA gene was sufficient for directing transcription of short RNA as revealed by Northern blot hybridization. An shRNA-expressing plasmid vector was constructed, in which shRNA-encoding oligonucleotides are inserted downstream of the U6 promoter. An shRNA that contained a sequence homologous to the C. intestinalis tyrosinase gene (Ci-tyrosinase) suppressed melanization of pigment cells in the brain of morphologically normal tailbud embryos. An shRNA that perfectly matched the translated sequence of enhanced green fluorescent protein (EGFP) (a mutant type of Aequorea victoria green fluorescent protein) suppressed the expression of the coelectroporated EGFP transgene. These results suggest that the expression of shRNA interferes with functions of both endogenous and exogenous genes. The shRNA-expressing plasmid constructed in the present study provides an easy and inexpensive alternative for the functional analysis of genes in ascidian embryos.

Novel expression patterns of Pax3/Pax7 in early trunk neural crest and its melanocyte and non-melanocyte lineages in amniote embryos

Neural crest cells are considered a key vertebrate feature that is studied intensively because of their relevance to development and evolution. Here we report the expression of Pax7 in the dorsal non-neural ectoderm and in the trunk neural crest of the early chick embryo. Pax7 is expressed in the trunk neural crest migrating along the ventral and dorsolateral routes. Pax7 is first downregulated in the neural crest-derived neuronal precursors, secondly in the glial, and finally in the melanocyte precursors. Conserved developmental expression in the melanocyte lineage of both Pax3 and Pax7 was evidenced in chick and quail, but only Pax3 in mouse and rat.

A Genome-Wide Survey of Genes for Enzymes Involved in Pigment Synthesis in an Ascidian, Ciona intestinalis

The draft genome sequence and a large quantity of EST and cDNA information are now available for the ascidian Ciona intestinalis. In the present study, genes involved in pigment synthesis pathways were identified in the decoded genome of Ciona, and information about these genes was obtained from available EST and cDNA sequences. It was found that the Ciona genome contains orthologous genes for each enzyme of the melanin, pteridine, ommochrome, papiliochrome, and heme synthesis pathways. Several appear as independent duplications in the Ciona genome. Because cDNA clones for all but two of these genes have already been isolated by the cDNA project, C. intestinalis will provide an experimental system to explore molecular mechanisms underlying color patterns, through future genome-wide studies.

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.

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.

Cloning and functional analysis of ascidian Mitf in vivo: insights into the origin of vertebrate pigment cells

The microphthalmia-associated transcription factor (Mitf) is a basic-helix-loop-helix-leucine zipper (bHLH-ZIP) transcription factor essential for the development and function of all melanin-producing pigment cells in vertebrates. To elucidate the evolutionary history of Mitf and the antiquity of its association with pigment cells, we have isolated and characterized HrMitf, a sole member of the Mitf-TFE bHLH-ZIP subfamily in the ascidian Halocynthia roretzi. Maternal HrMitf mRNA is detected in the fertilized egg and in the animal hemisphere from 4-cell stage through the gastrula stage. From the neurula through the early tailbud stage, HrMitf is preferentially expressed in the pigment-lineage cells that express the lineage-specific melanogenesis genes tyrosinase (HrTyr) and Tyrp. Overexpression of HrMitf induced ectopic expression of HrTyr enzyme activity in mesenchymal cells where the same enzyme activity was induced by overexpression of HrPax3/7, suggesting that a part(s) of the Pax3-Mitf-tyrosinase gene regulatory cascade seen in vertebrate melanocytes is operative during ascidian embryogenesis. We also show HrMitf and mouse Mitf-A, a Mitf isoform abundantly expressed in pigmented epithelial cells, share similar functional characteristics. These results suggest antiquity of the association of the Mitf-TFE subfamily with pigment cells and may support the idea that acquisition of multiple promoters (isoforms) by an ancestral Mitf gene has allowed the evolution of multiple pigment cell types.

Specific gene silencing using small interfering RNAs in fish embryos

Recently, small interfering RNAs (siRNAs) have been used for gene knockdown in mammalian cultured cells, but their utility in fish has remained unexplored. Here we demonstrate a siRNA-mediated gene silencing technique in rainbow trout embryos. We found that siRNAs effectively suppressed the transient expression of episomally located foreign GFP genes at an early developmental stage and inhibited the expression of GFP genes in stable transgenic trout embryos. Similar gene silencing was observed with an siRNA against the endogenous tyrosinase A gene. siRNAs interfered with the expression of maternally inherited mRNA. siRNAs did not affect non-relevant gene expression and siRNAs with a 4 base mismatch did not affect target gene expression. siRNA gene silencing is therefore highly sequence-specific. Our findings are the first evidence that siRNA-mediated gene silencing is effective in fish. This technique could be a powerful tool for studying gene function during embryonic development in aquacultural fish species, zebrafish, and medaka.

Isolation and developmental expression of tyrosinase family genes in Xenopus laevis

The tyrosinase family of genes in vertebrates consists of three related members encoding melanogenic enzymes, tyrosinase (Tyr), tyrosinase-related protein-1 (TRP-1, Tyrp1) and tyrosinase-related protein-2 (Dct, TRP-2, Tyrp2). These proteins catalyze melanin production in pigment cells and play important roles in determining vertebrate coloration. This is the first report examining melanogenic gene expression in pigment cells during embryonic development of amphibians. Xenopus provides a useful experimental system for analyzing molecular mechanisms of pigment cells. However, in this animal little information is available not only about the developmental expression but also about the isolation of pigmentation genes. In this study, we isolated homologues of Tyr, Tyrp1 and Dct in Xenopus laevis (XlTyr, XlTyrp1, and XlDct). We studied their expression during development using in situ hybridization and found that all of them are expressed in neural crest-derived melanophores, most of which migrate through the medial pathway, and in the developing diencephalon-derived retinal pigment epithelium (RPE). Further, XlDct was expressed earlier than XlTyr and XlTyrp1, which suggests that XlDct is the most suitable marker gene for melanin-producing cells among them. XlDct expression was detected in migratory melanoblasts and in the unpigmented RPE. In addition, the expression of XlDct was detected in the pineal organ. The sum of these studies suggests that expression of the tyrosinase family of genes is conserved in pigment cells of amphibians and that using XlDct as a marker gene for pigment cells will allow further study of the developmental mechanisms of pigment cell differentiation using Xenopus.

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.

Functional conservation of the promoter regions of vertebrate tyrosinase genes

Tyrosinase is the key enzyme for synthesizing melanin pigments, which primarily determine mammalian skin coloration. Considering the important roles of pigments in the evolution and the adaptation of vertebrates, phylogenetic changes in the coding and flanking regulatory sequences of the tyrosinase gene are particularly intriguing. We have now cloned cDNA encoding tyrosinase from Japanese quail and snapping turtle. These nonmammalian cDNA are highly homologous to those of the mouse and human tyrosinases, whereas the 5' flanking sequences are far less conserved except for a few short sequence motifs. Nevertheless, we demonstrate that the 5' flanking sequences from the quail or turtle tyrosinase genes are capable of directing the expression of a fused mouse tyrosinase cDNA when introduced into cultured mouse albino melanocytes. This experimental method, which reveals the functional conservation of regulatory sequences in one cell type (the melanocyte), may be utilized to evaluate phylogenetic differences in mechanisms controlling specific gene expression in many other types of cells. We also provide evidence that the 5' flanking sequences from these nonmammalian genes are functional in vivo by producing transgenic mice. Phylogenetic changes of vertebrate tyrosinase promoters and the possible involvement of conserved sequence motifs in melanocyte-specific expression of tyrosinase are discussed.

The BMP/CHORDIN antagonism controls sensory pigment cell specification and differentiation in the ascidian embryo

We have investigated the role of the bone morphogenetic protein (BMP) pathway during neural tissue formation in the ascidian embryo. The orthologue of the BMP antagonist, chordin, was isolated from the ascidian Halocynthia roretzi. While both the expression pattern and the phenotype observed by overexpressing chordin or BMPb (the dpp-subclass BMP) do not suggest a role for these factors in neural induction, BMP/CHORDIN antagonism was found to affect neural patterning. Overexpression of BMPb induced ectopic sensory pigment cells in the brain lineages that do not normally form pigment cells and suppressed pressure organ formation within the brain. Reciprocally, overexpressing chordin suppressed pigment cell formation and induced ectopic pressure organ. We show that pigment cell formation occurs in three steps. (1) During cleavage stages ectodermal cells are neuralized by a vegetal signal that can be substituted by bFGF. (2) At the early gastrula stage, BMPb secreted from the lateral nerve cord blastomeres induces those neuralized blastomeres in close proximity to adopt a pigment cell fate. (3) At the tailbud stage, among these pigment cell precursors, BMPb induces the differentiation of specifically the anterior type of pigment cell, the otolith; while posteriorly, CHORDIN suppresses BMP activity and allows ocellus differentiation.

Diverse roles of conserved asparagine-linked glycan sites on tyrosinase family glycoproteins

The tyrosinase family of genes has been conserved throughout vertebrate evolution. The role of conserved N-glycan sites in sorting, stability, and activity of tyrosinase family proteins was investigated using two family members from two different species, mouse gp75/tyrosinase-related protein (TRP)-1/Tyrp1 and human tyrosinase. Potential N-linked glycosylation sites on the lumenal domains of mouse gp75/TRP-1/Tyrp1 and human tyrosinase were eliminated by site-directed mutagenesis (Asn to Gln substitutions). Our results show that selected conserved N-glycan sites on tyrosinase family members are crucial for stability in the secretory pathway and endocytic compartment and for enzymatic activity. Different glycan sites on the same tyrosinase family polypeptide can perform distinct functions, and conserved sites on tyrosinase family paralogues can perform different functions.

Pigment cell-specific expression of the tyrosinase gene in ascidians has a different regulatory mechanism from vertebrates

Tyrosinase is the key enzyme required for the synthesis of melanin pigments. Sequence comparison and functional analysis of the 5' upstream regions of vertebrate tyrosinase genes have revealed the importance of conserved E-box motifs in regulating their specific expression in pigment cells, optic cup-derived retinal pigment epithelium (RPE) and neural crest-derived melanocytes. In ascidians (more basal protochordates), two pigment cells that resemble vertebrate RPE cells are formed and specifically express the orthologous tyrosinase gene (HrTyr) in the cerebral vesicle located at the anterior end of the neural tube. To define regulatory sequences required for pigment cell-lineage-specific expression of HrTyr during embryogenesis, a series of mutations of the 5' upstream region of HrTyr were fused to the lacZ reporter gene and were microinjected into fertilized eggs. We found that the -152bp upstream of the translational start site is essential for expression in pigment cell precursors of tailbud-stage embryos. Further, additional positive and unique restriction elements were identified in the region up to -1.8kb. Surprisingly, in the -152bp minimal promoter or in other regions with regulatory activities, there are no E-box motifs or sequences correlating with other conserved elements regulating vertebrate tyrosinase promoters. The possibility that Pax proteins regulate HrTyr expression is also discussed.

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.

Developmental gene activities in ascidian embryos

The fertilized egg of ascidians develops quickly into a tadpole-type larva consisting of several distinct types of tissues including epidermis, central nervous system, endoderm, mesenchyme, notochord, and muscle. This architecture of the ascidian larva represents the most simplified chordate body plan. Taking advantage of simple, well-defined cell lineages, the expression of developmental genes is analyzed at single-cell level. Advances in the methodology promote the ascidian embryo as a useful system for studying transcriptional control involved in the specification of embryonic cells and pattern formation of the embryo.

An L1 element intronic insertion in the black-eyed white (Mitf[mi-bw]) gene: the loss of a single Mitf isoform responsible for the pigmentary defect and inner ear deafness

Waardenburg syndrome type 2 (WS2) is an autosomal dominant disorder characterized by a combination of pigmentary and auditory abnormalities. Approximately 20% of WS2 cases are associated with mutations in the gene encoding microphthalmia-associated transcription factor (MITF). MITF plays a critical role in the development of both neural-crest-derived melanocytes and optic cup-derived retinal pigmented epithelium (RPE); the loss of a functional Mitf in mice results in complete absence of all pigment cells, which in turn induces microphthalmia and inner ear deafness. The black-eyed white Mitf mi-bw homozygous mouse normally has a pigmented RPE but lacks melanocytes essential for the pigmentation of the body and hearing. We show here that Mitf mi-bw is caused by an insertion into intron 3 of a 7.2 kb novel L1 element, L1bw, which belongs to an actively retrotransposing TF subfamily. The L1bw insertion reduces the amount of mRNAs for two Mitf isoforms, Mitf-A and Mitf-H, by affecting their overall expression levels and pre-mRNA splicing patterns, while it abolishes mRNA expression of another isoform, Mitf-M, which is specifically expressed in neural-crest-derived melanocytes. The consequence of the L1 insertion in the black-eyed white Mitf mi-bw mouse is that the developmental programme for RPE cells proceeds normally, most likely because of the presence of residual, full-length Mitf-A and Mitf-H proteins, whereas the lack of Mitf-M results in loss of the melanocyte population. The results suggest that melanocyte development depends critically on a single Mitf isoform, Mitf-M, and raise the possibility that specific mutations affecting MITF-M, the human equivalent of Mitf-M, may be responsible for a subset of WS2 conditions.

Structure and developmental expression of the ascidian TRP gene: insights into the evolution of pigment cell-specific gene expression

The tyrosinase family in vertebrates consists of three related melanogenic enzymes: tyrosinase, tyrosinase-related protein-1 (TRP-1), and TRP-2. These proteins control melanin production in pigment cells and play a crucial role in determining vertebrate coloration. We have isolated a gene from the ascidian Halocynthia roretzi which encodes a tyrosinase-related protein (HrTRP) with 45-49% identity with vertebrate TRP-1 and TRP-2. The expression of the HrTRP gene in pigment lineage a8.25 cells starts at the early-mid gastrula stage, which coincides with the stage when these cells are determined as pigment precursor cells; therefore, it provides the earliest pigment lineage-specific marker, which enables us to trace the complete cell lineage leading to two pigment cells in the larval brain. In addition, the expression pattern of the HrTRP gene appears to share similar characteristics with the mouse TRP-2 gene although structurally the HrTRP gene is more closely related to mammalian TRP-1 genes. Based on these observations and on results from molecular phylogenetic and hybridization analyses, we suggest that triplication of the tyrosinase family occurred during the early radiation of chordates. Initially, duplication of an ancestral tyrosinase gene produced a single TRP gene before the urochordate and cephalochordate-vertebrate divergence, and a subsequent duplication of the ancestral TRP gene in the vertebrate lineage gave rise to two TRP genes before the emergence of teleost fishes. Evolution of the melanin synthetic pathway and possible phylogenetic relationships among chordate pigment cells that accommodate the metabolic process are discussed. Dev Dyn 1999;215:225-237.

The structural organization of the ascidian, Halocynthia roretzi, calmodulin genes. The vicissitude of introns during the evolution of calmodulin genes

Two distinct calmodulin (CaM) genes are isolated from the ascidian, Halocynthia roretzi, (Hr-CaM A and Hr-CaM B) and those structures are determined. There are three nucleotide substitutions, producing two amino acid differences between Hr-CaM A and Hr-CaM B, and those are corresponding to two of the known eight variable residues among metazoan CaMs. Both Hr-CaM A and Hr-CaM B are constructed from six exons and five introns, and the positions of introns are identical. The positions of introns of Hr-CaMs are also identical with those of vertebrate CaMs, except third introns. The third introns of Hr-CaMs are inserted at 28bp upstream when compared with vertebrate CaMs. Thus, sliding of the third intron might have occurred in only the ascidian lineage prior to the gene duplication that also occurred only in that lineage. In addition, with the comparison of the intron positions, we attempt to investigate the vicissitude of introns during the evolution of metazoan CaMs.

Role of the prophenoloxidase-activating system in invertebrate immunity

The melanization reaction, which is a common response to parasite entry in invertebrate animals, especially arthropods, is due to the activity of an oxidoreductase, phenoloxidase. This enzyme is part of a complex system of proteinases, pattern recognition proteins and proteinase inhibitors constituting the so-called prophenoloxidase-activating system. It is proposed to be a non-self recognition system because conversion of prophenoloxidase to active enzyme can be brought about by minuscule amounts of molecules such as lipopolysaccharide, peptidoglycan and beta-1, 3-glucans from micro-organisms. Several components of this system recently have been isolated and their structure determined.

Cell fate specification by localized cytoplasmic determinants and cell interactions in ascidian embryos

Tadpole larvae of ascidians show the basic body plan of chordates. An ascidian larva consists of only a few types of cells and has a relatively small number of cells. Cell lineages are invariant among individuals and have been described in detail. These advantages facilitate the analysis of how the fate of each blastomere becomes specified during development. Over a century of research on ascidian embryogenesis has uncovered many interesting features concerning cellular mechanisms responsible for the fate specification. During embryogenesis, the developmental fate of a blastomere is specified by one of three different mechanisms: localized maternal cytoplasmic determinants, inductive interactions, or lateral inhibition in an equivalence cell group.

Albinism due to transposable element insertion in fish

The i locus of the medaka fish, Oryzias latipes, is responsible for tyrosinase expression, and several mutant alleles have been identified. The genotype i1/i1 exhibits a complete albino phenotype, having pale orange-red skin and red eyes. This mutant lacks in vivo tyrosinase activity. The genotype i4/i4, on the other hand, shows a quasi-albino phenotype with skin as bright as that of i1/i1 but with red-wine-colored eyes. At the light microscope level, reduced pigmentation is observed both in the skin and eyes of this mutant. The tyrosinase genes for the i1 and the i4 alleles were cloned and sequenced, and compared with that of the wild-type tyrosinase gene. The i1 allele was found to contain a 1.9-kb transposable element in the 1st exon, and the i4 allele was found to contain a 4.7-kb transposable element in the 5th exon. Both i1 and i4 are alleles that were found in a commercial breeding population. The insertion of a transposable element thus appears to constitute a natural cause of mutations that cause albinism in this organism.

The origins of the neural crest. Part II: an evolutionary perspective

The neural crest and cranial ectodermal placodes are traditionally thought to be unique to vertebrates; however, they must have had evolutionary precursors. Here, we review recent evidence suggesting that such ancestral cell types can be identified in modern non-vertebrate chordates, such as amphioxus (a cephalochordate) and ascidians (urochordates). Hence, migratory neuroectodermal cells may well have been present in the common ancestor of the chordates, such that the possibility of their existence in non-chordate deuterostomes (hemichordates and echinoderms) must also be considered. Finally, we discuss the various non-neuronal cell types produced by the neural crest in order to demonstrate that it is plausible that these different cell types evolved from an ancestral population that was neuronal in nature.

Neural tube is partially dorsalized by overexpression of HrPax-37: the ascidian homologue of Pax-3 and Pax-7

The origin and elaboration of the central nervous system played an important role in chordate and vertebrate history. All chordates possess a dorsal tubular central nervous system, but elaboration of dorsoventral and segmental pattern is far more pronounced in cephalochordates and vertebrates than in the more basal urochordates. Analysis of the urochordates, therefore, should allow deduction of the neural organization and neuronal patterning mechanisms that predated overt dorsoventral and segmental complexity. Here we report functional studies of the ascidian Pax gene (HrPax-37). The spatiotemporal expression pattern of HrPax-37 has suggested involvement in two distinct developmental processes: specification of dorsal cell fates of ectoderm during neurulation, and regional differentiation of the neural tube in later stages. Here we show that HrPax-37 is descendent from the precursor of the Pax-3 and Pax-7 genes implicated in specification of dorsal fate in the vertebrate neural tube. We also demonstrate that injection of HrPax-37 RNA into fertilized eggs causes ectopic expression of the dorsal neural marker tyrosinase gene in neurulae, confirming a regulatory role in dorsal patterning of the neural tube comparable to its vertebrate homologues. These results suggest that dorsal specification in the neural tube by Pax-3/7 subfamily genes was established in the ancestors of extant chordates during emergence of the dorsal tubular nervous system.