Biomedical subjects

M Asashima

Publications and source records attributed to M Asashima.

At least 73 records · Page 4Linked to original sources

PubMed1998

Activin-treated ectoderm has complete organizing center activity in Cynops embryos.

The differentiation and organizer activity of newt ectoderm treated with activin A was studied in explantation and transplantation experiments. In the explantation experiments, ectoderm dissected from late morulae-early gastrulae stage embryos treated with a high concentration of activin A (100 ng/mL) formed only yolk-rich endodermal cells. Mesodermal tissues, such as notochord and muscle, were seldom found in these explants. When they were transplanted into the blastocoele of other early gastrulae, they formed part of the endoderm of the host embryo and induced a secondary axis with only posterior characters (including axial mesoderm and neural tissues). In contrast, whole secondary axes were induced when activin-treated ectoderm was transplanted into the ventral marginal zone (VMZ) of early blastulae. The transplanted pieces invaginated by themselves and differentiated into foregut structures including pharynx, stomach, and liver. These phenomena were also observed in experiments in which presumptive foregut was used instead of activin-treated ectoderm. These findings show that activin-treated ectoderm can act as the complete organizing center in Cynops.

Activins↗

PubMed1998

Neural induction in embryos.

Neural differentiation of the ectoderm is inhibited by bone morphogenetic protein 4 (BMP-4) in amphibia as well as mammalia. This inhibition is released by neural inducing factor(s), which are secreted from the dorsal mesoderm. Masked neuralizing factor(s) are already present in the ectoderm before induction. In homogenates from Xenopus oocytes and embryos neural inducing factors were found in the supernatant (centrifuged at 105000 g), in small vesicles and a ribonucleoprotein fraction. A neuralizing factor, which is a protein of small size, has been partially purified from Xenopus gastrulae. Genes that are expressed in the dorsal mesoderm and involved in the de novo synthesis of neuralizing factor(s) have been cloned. The differentiation of cells with a neuronal fate starts in the neural plate immediately after neural induction. Genes homologous to the Notch and Delta genes of lateral inhibition in insects are involved in this process.

Animals↗

PubMed1998

Patterns of gene expression in the core of Spemann's organizer and activin-treated ectoderm in Cynops pyrrhogaster.

The presumptive pharyngeal endoderm region of the Cynops early gastrula induces head or trunk-tail structures in sandwich culture. Activin-treated ectoderm can mimic this phenomenon at least at the histological level. The patterns of expression of organizer-specific genes were examined to compare these two inductive materials at the molecular level. A chordin cDNA clone from Cynops pyrrhogaster (Cychd) was isolated by reverse transcription-polymerase chain reaction (RT-PCR). Cychd mRNA was first detected in the presumptive pharyngeal endoderm and prechordal plate regions of stage 11 embryos, and was expressed continuously until stage 20. The spatiotemporal expression pattern of Cychd was similar to that of Xenopus chordin. The patterns of expression of organizer-related genes in the pharyngeal endoderm and activin-treated ectoderm were compared by RT-PCR analysis. Expression of Cychd in these two materials peaked at the time when they can induce head structures in sandwich culture. Expression of fork head and goosecoid did not change in the presumptive pharyngeal endoderm over this period. Cychd may play a key role in head formation in the Cynops embryo.

Activins↗

PubMed1998

Cloning and expression pattern of Xenopus prx-1 (Xprx-1) during embryonic development.

Homeobox genes are expressed both temporally and spatially during vertebrate development, and regulate the tissue-specific expression of other genes. A Xenopus paired-related homeobox- 1 (Xprx-1) cDNA was cloned. Xprx-1 had a paired-related homeodomain, but did not contain a paired-box. The sequence of Xprx-1 had a high level of homology with K-2(mouse) and Prx-1 (chicken), thus Xprx-1 is assumed to be the Xenopus homolog of these genes. Xprx-1 transcripts were maternally restricted, in Xenopus embryos, and a decrease in the late blastula stage was followed by an increase in zygotic transcripts after gastrulation. The transcripts were localized to the animal hemisphere of the late blastula and were concentrated in the branchial arches of the tail-bud stage embryo. In animal cap experiments, Activin A dose-dependently induced Xprx-1 gene expression. These results suggest that Xprx-1 plays a role in early Xenopus development similar to other species.

Activins↗

PubMed1998

Midkine counteracts the activin signal in mesoderm induction and promotes neural formation.

Midkine (MK) is a heparin-binding growth factor that has been implicated in neural survival and differentiation, fibrinolysis, and carcinogenesis. It is expressed in the nervous system during early Xenopus development. In the present study, we demonstrated that injection of vegetal blastomeres with Xenopus MK at the 8-cell stage results in incomplete invagination. In the case of dorsal vegetal injection, hypertrophic neural tissue is produced. Animal caps isolated from embryos that have been injected with Xenopus MK and cultured with activin do not elongate, and all mesoderm markers examined, including both head and trunk/tail ones, are greatly diminished. In contrast, head-specific neural markers, XANF-1 and Xotx2, are induced, while trunk/tail neural markers, XlHbox6 and F-spondin, are decreased. Moreover, MK showes the same effects in animal caps injected with Xenopus Smad2 mRNA.

Activins↗

PubMed1998

Axil, a member of the Axin family, interacts with both glycogen synthase kinase 3beta and beta-catenin and inhibits axis formation of Xenopus embryos.

Using a yeast two-hybrid method, we identified a novel protein which interacts with glycogen synthase kinase 3beta (GSK-3beta). This protein had 44% amino acid identity with Axin, a negative regulator of the Wnt signaling pathway. We designated this protein Axil for Axin like. Like Axin, Axil ventralized Xenopus embryos and inhibited Xwnt8-induced Xenopus axis duplication. Axil was phosphorylated by GSK-3beta. Axil bound not only to GSK-3beta but also to beta-catenin, and the GSK-3beta-binding site of Axil was distinct from the beta-catenin-binding site. Furthermore, Axil enhanced GSK-3beta-dependent phosphorylation of beta-catenin. These results indicate that Axil negatively regulates the Wnt signaling pathway by mediating GSK-3beta-dependent phosphorylation of beta-catenin, thereby inhibiting axis formation.

Adaptor Proteins, Signal Transducing↗

PubMed1998

Evidence that far infrared radiation promotes growth of Xenopus laevis.

In most ectotherms, environmental temperature has differential effects on growth and differentiation. For example, amphibian size at maturity decreases with increasing temperature. To address how radiant heat in the form of far-infrared radiation (FIR) may affect development of the aquatic ectotherm Xenopus laevis, we continuously irradiated swimming larvae as they developed into young adults. Here we report evidence that FIR promotes growth of these organisms in an aqueous environment.

Animals↗

PubMed1997

An interferon regulatory factor-related gene (xIRF-6) is expressed in the posterior mesoderm during the early development of Xenopus laevis.

Out of a Xenopus neurula cDNA library, we isolated a clone which encodes a 52.4-kDa protein highly similar to the mouse interferon regulatory factor, IRF-6, whose function is unknown. The mRNA of this gene, named xIRF-6, seems to be maternally transmitted, but its amount rapidly decreases after the tailbud stage. Whole-mount in situ hybridization showed that xIRF-6 mRNA is expressed in the presumptive somitic mesoderm in the late gastrula, and then confined to a segment of posterior somite during the neurula through the tailbud stage. The temporally and spatially limited expression of the xIRF-6 gene product may contribute to the transcriptional regulation of specific genes which are necessary for the development of the posterior somites.

Amino Acid Sequence↗

PubMed1997

Xenopus FK 506-binding protein homolog induces a secondary axis in frog embryos, which is inhibited by coexisting BMP 4 signaling.

FK 506-binding protein (FKBP) is an immunosuppressant mediator in mammals, but its endogenous physiological function has yet to be determined. Here we report a Xenopus homolog of FKBP, which is expressed at early stages of development. Injection of synthesized Xenopus FKBP mRNA, as well as murine constitutively active calcineurin, induced a secondary axis in Xenopus embryos, while an FKBP mutant which does not bind to calcineurin did not. This secondary-axis-inducing effect was inhibited when FKBP was coinjected with Xmad 1 or XBMP 4 mRNA. These results suggest that FKBP modifies BMP 4 signalling by recruiting calcineurin and may have an important role in axis formation during Xenopus development.

Amino Acid Sequence↗

PubMed1997

A novel gene encoding a ferredoxin reductase-like protein expressed in the neuroectoderm in Xenopus neurula.

In an attempt to elucidate the molecular mechanisms of early neural development in Xenopus laevis, we identified, using a differential display method, several genes that are induced after Concanavalin A treatment in the animal caps prepared from stage 9 blastula. One such gene was found to encode a possible type IIIa membrane protein of 66.2 kDa sharing similarities with several prokaryotic and eukaryotic redox enzymes, hence the putative product was named Nfrl, neurula-specific ferredoxin reductase-like protein. Northern blot analysis confirmed that the expression of the Nfrl gene is up-regulated around the neurula stage, and is much lower in embryos of earlier stages and in adult tissues. The temporally limited expression of this gene implies neurula- and early larva-specific redox reactions of certain substrates, the nature of which remains to be elucidated.

Animals↗

PubMed1997

Conversion of ectoderm into a neural fate by ATH-3, a vertebrate basic helix-loop-helix gene homologous to Drosophila proneural gene atonal.

We have isolated a novel basic helix-loop-helix (bHLH) gene homologous to the Drosophila proneural gene atonal, termed ATH-3, from Xenopus and mouse. ATH-3 is expressed in the developing nervous system, with high levels of expression in the brain, retina and cranial ganglions. Injection of ATH-3 RNA into Xenopus embryos dramatically expands the neural tube and induces ectopic neural tissues in the epidermis but inhibits non-neural development. This ATH-3-induced neural hyperplasia does not require cell division, indicating that surrounding cells which are normally non-neural types adopt a neural fate. In a Xenopus animal cap assay, ATH-3 is able to convert ectodermal cells into neurons expressing anterior markers without inducing mesoderm. Interestingly, a single amino acid change from Ser to Asp in the basic region, which mimics phosphorylation of Ser, severely impairs the anterior marker-inducing ability without affecting general neurogenic activities. These results provide evidence that ATH-3 can directly convert non-neural or undetermined cells into a neural fate, and suggest that the Ser residue in the basic region may be critical for the regulation of ATH-3 activity by phosphorylation.

Amino Acid Sequence↗

PubMed1997

An essay on the similarities and differences between inductive interactions in anuran and urodele embryos.

As a first step towards providing a conceptual approach to understanding similarities and differences in the mechanisms which guide inductive interactions among related organisms (e.g. various amphibia), a set of five principles is offered here. These principles were formulated by analyzing literature examples of classical embryological phenomena and by performing experiments with activin, a peptide growth factor which is currently suspected to play for a role in mesoderm induction. Mechanisms which account, at least in part, for the observed differences between anuran and urodele inductive processes can be derived from these principles.

Animals↗

PubMed1997

The Na+,K+-ATPase alpha subunit requires gastrulation in the Xenopus embryo.

Na+,K+-ATPase participates in reabsorption of ions and water and produces an electrochemical gradient between the intra- and extracellular spaces across the cell membrane. It also plays an important role in many developmental phenomena such as a blastocoele formation and neural formation. To elucidate the expression pattern of Na+,K+-ATPase in the Xenopus embryo, the spatial expression patterns of the Na+,K+-ATPase alpha subunit were studied in a normal embryo by whole-mount in situ hybridization. These transcripts were localized around the dorsal blastopore at the gastrula stage, in the neural tube at the neurula stage, and then in the pronephros and cloaca at the tail-bud stage. To study the function of Na+,K+-ATPase in embryogenesis after mid-blastula transition, the expression of the Na+,K+-ATPase alpha subunit was inhibited by the injection of specific antisense RNA. Embryos injected with Na+,K+-ATPase antisense RNA showed inhibition of gastrulation. When antisense RNA was injected into the dorsal blastomeres, head differentiation was markedly inhibited. These results suggest that this transcript plays an important role during gastrulation and head differentiation.

Activins↗

PubMed1996

cDNA cloning and expression of the Xenopus laevis vitellogenin receptor.

A Xenopus laevis oocyte cDNA library was screened with a PCR-generated X. laevis vitellogenin (VTG) receptor probe and a 3.6 kb cDNA clone containing the entire open reading frame, and 5' and 3' noncoding regions were isolated. The deduced amino acid sequence was 72% homologous to the chicken VLDL/VTG receptor, and the characteristic domains were highly conserved. Ligand binding analysis confirmed that the cloned receptor was Xenopus VTG-specific. Although Northern blotting analysis revealed that this gene was expressed as a major transcript of 3.6 kb in Xenopus ovary, weak but significant expression was observed in other tissues by RT-PCR analysis. The fact that major expression of the gene occurs in the ovary suggests that it has an important function in this organ.

Amino Acid Sequence↗

PubMed1996

Occurrence of immunoreactive activin/inhibin beta(B) in gonadotrophs, thyrotrophs, and somatotrophs of the Xenopus pituitary.

An antibody against the Xenopus activin/inhibin beta(B) subunit (94-107) was raised in a rabbit. Using this antibody, the distribution of activin/inhibin beta(B) immunoreactivity in the pituitary of adult X. laevis was studied. Beta(B) immunoreactivity was detected in gonadotrophs, thyrotrophs, and somatotrophs under light microscopy. Electron microscopy revealed that a beta(B)-immunoreactive substance exists in LH, TSH, and GH granules, in contrast to findings in the rat and goldfish. These results indicate that the expression of activin/inhibin beta(B) in pituitary cells is not consistent among vertebrate species.

Activins↗

PubMed1996

AstroNewt: early development of newt in space.

AstroNewt experiment explores the effects of earth gravity on the early development of Japanese red-bellied newt, Cynops pyrrhogaster. Since female newts keep spermatophore in cloaca, fertilized eggs could be obtained without mating. Fertilization of newt's egg occurs just prior to spawning, so that gonadotrophic cues applied to females in orbit leads to laying eggs fertilized just in space. A property of newt being kept in hibernation at low temperature may be of great help for the space experiment carried out with much limited resources. A general outline of the AstroNewt project is shown here in addition to some technical advances for the development of the project. Experimental schemes of two space experiments (IML-2 in summer 1994 and unmanned SFU at the beginning of 1995) are also shown.

Animals↗

PubMed1996

Activin treated urodele ectoderm: a model experimental system for cardiogenesis.

The tissue interactions which comprise the inductive phenomena associated with urodele heart morphogenesis are relatively well understood. In order to take full advantage of the experimental potential of this system formulation of an in vitro tissue culture system would be very helpful. Herein are described conditions for culturing Cynops pyrrhogaster early gastrula ectoderm tissue in the presence of the peptide growth factor activin. Two-week old explant cultures frequently displayed beating heart-like rudiments within. The beating frequency was measured and the extent to which cytodifferentiation mimicked normal heart differentiation assessed. Both measurements provided optimistic assessments which should encourage further exploitation of this model system.

Activins↗

PubMed1996

[Embryonic induction and role of activins during early amphibian development].

Growth factors are known to act for the formation of the animal tissues and organs. We showed that activins, members of the TGF-beta family growth factors, exist in early amphibian embryos and induce mesoderm tissues and organs in undifferentiated blastomeres (animal cap). Activins are characteristic in that they can establish embryonic body axis, by inducing different mesoderm and endoderm organs depending on their dose. The action of activin seems to be a primary important phenomenon which commonly create the body pattern of vertebrate embryos.

Activins↗