ankAT-1 is a novel gene mediating the apical tuft formation in the sea urchin embryo

In sea urchin embryos, the apical tuft forms within the neurogenic animal plate. When FoxQ2, one of the earliest factors expressed specifically in the animal plate by early blastula stage, is knocked down, the structure of the apical tuft is altered. To determine the basis of this phenotype, we identified FoxQ2-dependent genes using microarray analysis. The most strongly down-regulated gene in FoxQ2 morphants encodes a protein with ankyrin repeats region in its N-terminal domain. We named this gene ankAT-1, Ankyrin-containing gene specific for Apical Tuft. Initially its expression in the animal pole region of very early blastula stage embryos is FoxQ2-independent but becomes FoxQ2-dependent beginning at mesenchyme blastula stage and continuing in the animal plate of 3-day larvae. Furthermore, like FoxQ2, this gene is expressed throughout the expanded apical tuft region that forms in embryos lacking nuclear β-catenin. When AnkAT-1 is knocked-down by injecting a morpholino, the cilia at the animal plate in the resulting embryos are much shorter and their motility is less than that of motile cilia in other ectoderm cells, and remains similar to that of long apical tuft cilia. We conclude that AnkAT-1 is involved in regulating the length of apical tuft cilia.

Cloning and pattern of expression of the shiro-uo vasa gene during embryogenesis and its roles in PGC development

The vasa genes are expressed in the germ cell lineage in many organisms, but their expression patterns show large variations. Recent studies suggest that vasa transcripts are involved in germ cell lineage development. In this paper, we isolated the vasa cDNA clone from a teleost, shiro-uo, Leucopsarion petersii and examined its expression pattern during embryogenesis. Then, we examined the functional significance of vasa mRNA during the formation of primordial germ cells (PGCs). The amino acid sequence of shiro-uo VASA is 61.1% identical to that of zebrafish. In whole-mount in situ hybridization, vasa transcripts appeared at the 4- and 8-cell stages as four spots at both ends of two cleavage planes between the lower tier of blastomeres and the yolk cell mass. At the 16-cell stage, eight spots were observed. After the blastula stage, shiro-uo vasa transcripts showed similar localization as in the zebrafish. Ultrastructural analysis of 4-cell stage embryos revealed the presence of a subcellular organelle that resembled 'nuage' in the germ cell lineage observed in the embryos of various organisms. We carried out micromanipulation of 4- or 8-cell stage embryos to remove the vasa mRNA-containing spots and then measured the number of the vasa-expressing PGCs in the genital ridge of the manipulated embryos. The numbers decreased when all of the four spots were removed, indicating that the vasa-containing spots at early cleavage stages have important functions in the development of PGCs.

Histopathological effects induced by paraquat during Xenopus laevis primary myogenesis

The oxidative agent paraquat induced tail abnormalities during Xenopus laevis development. Specimens exposed from blastula to the tadpole stage revealed pear-shaped myocytes and irregular intersomitic boundaries. The histological feature of the axial musculature was evaluated in embryos sampled at significant stages of the primary myogenesis. During the somitogenesis PQ-treated embryos showed normal appearing myotomes, but reduced PAS activity in the post-rotating myotomal cells, and myoblasts with slight vacuolations. Once etched from the vitelline envelope, embryos showed severely altered myoblasts with irregular cellular apexes, heavy sarcoplasmic vacuolations, pyknotic nuclei and disorganizing intersomitic boundaries. Myotomes with many necrotic myocytes containing disorganized contractile material and heavily malformed intersomitic boundaries characterized the late myogenic stages. Our results evidence the heaviest PQ histopathological effects to affect myogenesis of post-etched embryos, suggesting a possible linkage between the swimming activity and the oxidative damage to muscle tissue.

[The ultrastructural patterns of germinal and yolk granules in oocytes and embryonic cells of the holothurian Apostichopus japonicus]

The yolk germinal granules in oocytes and embryonic cells of Apostichopus japonicus were studied by transmission electron microscopy. Analysis of the features of synthesis and utilization of yolk granules made it possible to reveal ultrastructural criteria to distinguish between granules of the forming and utilized yolk, and germinal granules. Based on these findings, the authors suppose that identification of germ plasm elements in oocytes and embryonic cells of A. japonicus is quite possible with ultrastructural analysis only, and does not require utilizing molecular markers.

[The ultrastructural patterns of germinal and yolk granules in oocytes and embryonic cells of the holothurian Apostichopus japonicus]

The yolk germinal granules in oocytes and embryonic cells of Apostichopus japonicus were studied by transmission electron microscopy. Analysis of the features of synthesis and utilization of yolk granules made it possible to reveal ultrastructural criteria to distinguish between granules of the forming and utilized yolk, and germinal granules. Based on these findings, the authors suppose that identification of germ plasm elements in oocytes and embryonic cells of A. japonicus is quite possible with ultrastructural analysis only, and does not require utilizing molecular markers.

PM-2: an ECM epitope necessary for morphogenesis in embryos of the starfish, Pisaster ochraceus

Anti-PM-2 is a monoclonal antibody that has been developed against the ECM of embryo/larvae of the starfish Pisaster ochraceus. Immunofluorescent staining shows that the PM-2 epitope is present in the cortical granules of unfertilized eggs and is released into the perivitelline space on fertilization. At the blastula stage, staining is very faint and limited to the blastocoel and a few granules within the cells. Strong staining appears in the embryonic/larval body cavity shortly after gastrulation and continues to increase in both the embryonic/larval body cavity and lumen of the gut at least until the bipinnaria stage. The presence of PM-2 in the Golgi apparatus, its susceptibility to enzymes that attack carbohydrates, and inhibition of PM-2 synthesis by tunicamycin, a drug that inhibits the linkage of carbohydrate moieties to protein backbone chains, suggest that the PM-2 epitope is or contains carbohydrate. Western blots of the whole embryo homogenates show bands at molecular weights of 130, 122, 100, 70, and 50 kDa. As embryos grow, two other high molecular weight (greater than 200 kDa) bands also appear. This suggests that the epitope is present on a series of molecules and that some of the lower MW molecules are precursors of the higher MW ones. A single 24-h exposure to the antibody just posthatching appears to inhibit normal mesenchymal migration at the gastrula stage, and if development of these treated embryos/larvae is allowed to continue to the bipinnaria stage, the embryos are stunted and have a smaller oral hood and esophagus. Long-term exposure results in stunted animals with distorted shapes. Such animals develop a very small embryonic/larval body cavity or none at all and differentiation of the larval GI tract fails to occur. The results suggest that molecules exhibiting the PM-2 epitope are necessary for the proper formation of the blastocoel, for mesenchyme cell movement and for proper development of the larvae GI tract.

Xenopus Cdc14 alpha/beta are localized to the nucleolus and centrosome and are required for embryonic cell division

Background

The dual specificity phosphatase Cdc14 has been shown to be a critical regulator of late mitotic events in several eukaryotes, including S. cerevisiae, S. pombe. C. elegans and H. sapiens. However, Cdc14 homologs have clearly evolved to regulate distinct cellular processes and to respond to regulatory signals important for these processes. The human paralogs hCdc14A and B are the only vertebrate Cdc14 homologues studied to date, but their functions are not well understood. Therefore, it is of great interest to examine the function Cdc14 homologs in other vertebrate species.

Results

We identified two open reading frames from Xenopus laevis closely related to human Cdc14A, called XCdc14α and XCdc14β, although no obvious paralog of the hCdc14B was found. To begin a functional characterization of Xcdc14α and XCdc14β, we raised polyclonal antibodies against a conserved region. These antibodies stained both the nucleolus and centrosome in interphase Xenopus tissue culture cells, and the mitotic centrosomes. GFP-tagged version of XCdc14α localized to the nucleulus and GFP-XCdc14β localized to the centrosome, although not exclusively. XCdc14α was also both meiotically and mitotically phosphorylated. Injection of antibodies raised against a conserved region of XCdc14/β into Xenopus embryos at the two-cell stage blocked division of the injected blastomeres, suggesting that activities of XCdc14α/β are required for normal cell division.

Conclusion

These results provide evidence that XCdc14α/β are required for normal cellular division and are regulated by at least two mechanisms, subcellular localization and possibly phosphorylation. Due to the high sequence conservation between Xcdc14α and hCdc14A, it seems likely that both mechanisms will contribute to regulation of Cdc14 homologs in vertebrates.

[Structurization of cortical layer of loach yolk cell after wounding as a "minimal" model of morphogenesis]

Structural rearrangements of the yolk cell surface were studied in loach embryos using SEM and TEM, which take place within 30 min after a point-like puncture at the late blastula stage. The effects of sucking off or addition of a part of yolk, lowered temperature, and absence of Ca2+ on structurization were studied. Around the area of puncture, the yolk granules were submerged, the number of vesicles increased, and numerous membrane folds were formed. The folds were aggregated to form two sharply distinct types of structures: a group of rounded evaginations around the site of puncture and a system of radial folds in the periphery. Small radial folds are aggregated in radial strands, several dozens folds in each. Sucking off a part of yolk accelerated the above processes, while addition of yolk, cooling, and absence of Ca2+ in the incubation medium slowed down or suppressed these processes. The observed structurization can be considered as self-organization at the level of the yolk cell cortical level, largely similar to that during normal morphogenesis at the level of multicellular sheets. Hence, the membrane dynamics in the yolk cell wall after its damage can be considered as one of simplified ("minimal") models of morphogenesis. A study of this model makes it possible to narrow down the circle of factors essential for self-organization of morphogenetic processes.

Overexpression of S-adenosylmethionine decarboxylase (SAMDC) in Xenopus embryos activates maternal program of apoptosis as a "fail-safe" mechanism of early embryogenesis

In Xenopus, injection of S-adenosylmethionine decarboxylase (SAMDC) mRNA into fertilized eggs or 2-cell stage embryos induces massive cell dissociation and embryo-lysis at the early gastrula stage due to activation of the maternal program of apoptosis. We injected SAMDC mRNA into only one of the animal side blastomeres of embryos at different stages of cleavage, and examined the timing of the onset of the apoptotic reaction. In the injection at 4- and 8-cell stages, a considerable number of embryos developed into tadpoles and in the injection at 16- and 32-cell stages, all the embryos became tadpoles, although tadpoles obtained were sometimes abnormal. However, using GFP as a lineage tracer, we found that descendant cells of the blastomere injected with SAMDC mRNA at 8- to 32-cell stages are confined within the blastocoel at the early gastrula stage and undergo apoptotic cell death within the blastocoel, in spite of the continued development of the injected embryos. These results indicate that cells overexpressed with SAMDC undergo apoptotic cell death consistently at the early gastrula stage, irrespective of the timing of the mRNA injection. We assume that apoptosis is executed in Xenopus early gastrulae as a "fail-safe" mechanism to eliminate physiologically-severely damaged cells to save the rest of the embryo.

Application of a sea urchin micronucleus assay to monitoring aquatic pollution: influence of sample osmolality

We have improved our sea urchin micronucleus assay for aquatic samples and used it to evaluate marine pollution. We found that the water samples we had collected for 2 years from the Tokyo bay coast near Tokyo, an industrial megalopolis, were positive due to the water samples being hypo-osmotic rather than to chemical pollutants. The evidence was as follows: (i) the osmolality and salinity of the samples were about half that of sea water; (ii) the micronucleus frequency induced in the water sample decreased to the control level when the osmolality was increased to that of sea water; (iii) artificial sea water diluted with distilled water induced micronuclei dilution-dependently. Since micronucleus induction in the sea urchin assay is influenced by sample osmolality, the osmolality must be adjusted to that of sea water for the assay and osmotic pressure must be considered when evaluating water pollution.

EphA4 catalytic activity causes inhibition of RhoA GTPase in Xenopus laevis embryos

The Eph family of receptor tyrosine kinases is involved in limiting cell and tissue interactions via a repulsive mechanism. The mechanism of repulsion involves reorganizing the actin cytoskeleton, but little is known of the molecular components that connect the receptor to the actin cytoskeleton. Recent studies in retinal ganglion cells have demonstrated that EphA4 activates the small GTPase Rho. We have investigated the involvement of Rho in signaling downstream from EphA4. As a model system, we have used a chimeric receptor called EPP that we express and activate in early Xenopus embryos. Previous studies demonstrated that EPP activation leads to loss of cell-cell adhesion and change in cell shape, plus loss of aspects of cell polarity in epithelial cells, such as apical microvilli and the apical/basolateral boundary. In this study, we show that injecting inhibitors of Rho GTPases into early Xenopus embryos produces a phenotype very similar to that resulting from EPP activation. More importantly, expression of a constitutively active form of Xenopus RhoA (XRhoA) concurrent with activated EPP rescued embryos from the loss of cell-cell adhesion and change in cell shape associated with EPP. These data argue that, in contrast to the case in retinal ganglion cells, EphA4 in early Xenopus embryos acts to inhibit RhoA, suggesting that this receptor may regulate Rho differently (and therefore affect the cytoskeleton differently) in neuronal and non-neuronal cells. Furthermore, overexpression of ephexin, a novel guanine nucleotide exchange factor for Rho family GTPases, also blocks EPP-induced dissociation. This suggests that EphA4, which has been demonstrated to activate ephexin in cultured neuronal cells, may also target Rho GTPase via an ephexin-independent pathway.