Biomedical subjects

H Yasuo

Publications and source records attributed to H Yasuo.

At least 19 recordsLinked to original sources

PubMed2001

Phosphatidylinositol-3 kinase acts in parallel to the ERK MAP kinase in the FGF pathway during Xenopus mesoderm induction.

Phosphoinositide 3-kinases (PI3Ks) are lipid kinases that can phosphorylate phosphaditylinositides leading to the cell type-specific regulation of intracellular protein kinases. PI3Ks are involved in a wide variety of cellular events including mitogenic signalling, regulation of growth and survival, vesicular trafficking, and control of the cytoskeleton. Some of these enzymes also act downstream of receptor tyrosine kinases or G-protein-coupled receptors. Using two strategies to inhibit PI3K signalling in embryos, we have analysed the role of PI3Ks during early Xenopus development. We find that a class 1A PI3K catalytic activity is required for the definition of trunk mesoderm during the blastula stages, but is less important for endoderm and prechordal plate mesoderm induction or for organiser formation. It is required in the FGF signalling pathway downstream of Ras and in parallel to the extracellular signal-regulated kinase (ERK) MAP kinases. In addition, our results show that ERKs and PI3Ks can synergise to convert ectoderm into mesoderm. These data provide the first evidence that class 1 PI3Ks are required for a specific set of patterning events in vertebrate embryos. Furthermore, they bring new insight into the FGF signalling cascade in Xenopus.

Animals↗

PubMed2001

Role of Goosecoid, Xnot and Wnt antagonists in the maintenance of the notochord genetic programme in Xenopus gastrulae.

The Xenopus trunk organiser recruits neighbouring tissues into secondary trunk axial and paraxial structures and itself differentiates into notochord. The inductive properties of the trunk organiser are thought to be mediated by the secretion of bone morphogenetic protein (BMP) antagonists. Ectopic repression of BMP signals on the ventral side is sufficient to mimic the inductive properties of the trunk organiser. Resultant secondary trunks contain somite and neural tube, but no notochord. We show that inhibition of BMP signalling is sufficient for the initiation of the trunk organiser genetic programme at the onset of gastrulation. During late gastrulation, however, this programme is lost, due to an invasion of secreted Wnts from neighbouring tissues. Maintenance of this programme requires co-repression of BMP and Wnt signalling within the presumptive notochord region. To shed light on the molecular cascade that leads to the repression of the Wnt pathway, we looked for individual organiser genes whose overexpression could complement the inhibition of BMP signalling to promote notochord formation in the secondary trunks. Two genes, gsc and Xnot, were thus identified and shown to act in different ways. Xnot acts as a transcriptional repressor within the mesodermal region. Gsc acts in deeper vegetal cells, where it regulates Frzb expression to maintain Xnot expression in the neighbouring notochord territory. These results suggest that, during gastrulation, the necessary repression of Wnt/beta-catenin signalling in notochord precursors is achieved by the action of secreted inhibitors, such as Frzb, emitted by gsc-expressing dorsal vegetal cells.

Amino Acid Sequence↗

PubMed2001

Generation of the germ layers along the animal-vegetal axis in Xenopus laevis.

After completion of gastrulation, typical vertebrate embryos consist of three cell sheets, called germ layers. The outer layer, the ectoderm, which produces the cells of the epidermis and the nervous system; the inner layer, the endoderm, producing the lining of the digestive tube and its associated organs (pancreas, liver, lungs etc.) and the middle layer, the mesoderm, which gives rise to several organs (heart, kidney, gonads), connective tissues (bone, muscles, tendons, blood vessels), and blood cells. The formation of the germ layers is one of the earliest embryonic events to subdivide multicellular embryos into a few compartments. In Xenopus laevis, the spatial domains of three germ layers are largely separated along the animal-vegetal axis even before gastrulation; ectoderm in the animal pole region; mesoderm in the equatorial region and endoderm in the vegetal pole region. In this review, we summarise the recent advances in our understanding of the formation of the germ layers in Xenopus laevis.

Animals↗

PubMed1999

A two-step model for the fate determination of presumptive endodermal blastomeres in Xenopus embryos.

BACKGROUND: In Xenopus, the endoderm germ layer is derived from the vegetal blastomeres of cleavage-stage embryos. Cell transplantation experiments have revealed that the endodermal fate becomes gradually fixed during the late blastula stages. Sox17alpha, Mix.1, Mixer and GATA-4 encode vegetal zygotic transcription factors with endoderm-inducing activity. The accumulation of their transcripts during the late blastula stages may cause determination of the endodermal fate. VegT, a T-box transcription factor, the maternal transcripts of which are vegetally localised, is also required for endoderm formation. RESULTS: We analysed the events leading to the progressive accumulation of the transcripts for Sox17alpha, Mix.1, Mixer and GATA-4. Two phases could be distinguished in the endodermal programme. In phase 1, Sox17alpha, Mix.1, and the genes encoding transforming growth factor beta-related signalling molecules Xnr1, Xnr2 and Derrière were activated cell-autonomously at around the mid-blastula transition (MBT) by maternal determinants. In phase 2, TGFbeta signalling, possibly involving Xnr1, Xnr2 and Derrière, led to the activation of Mixer and GATA-4 in late blastula stages and to the reinforcement of the expression of Sox17alpha and Mix.1. Overexpression of VegT in animal caps triggered a developmental programme qualitatively similar to that observed in vegetal blastomeres, except that Xnr1 and GATA-4 were not activated by the early gastrula stage. CONCLUSIONS: Our results support a two-step model for endoderm determination between fertilisation and the onset of gastrulation. The initial cell-autonomous activation of early endodermal genes by maternal determinants including, but not limited to, VegT is relayed by the action of zygotic TGFbetas such as Xnr1, Xnr2 and Derrière.

Animals↗

PubMed1998

Conservation of the developmental role of Brachyury in notochord formation in a urochordate, the ascidian Balocynthia roretzi.

The notochord is one of the characteristic features of the phylum Chordata. The vertebrate Brachyury gene is known to be essential for the terminal differentiation of chordamesoderm into notochord. In the ascidian, which belongs to the subphylum Urochordata, differentiation of notochord cells is induced at the late phase of the 32-cell stage through cellular interaction with adjacent endoderm cells as well as neighboring notochord cells. The ascidian Brachyury gene (As-T) is expressed exclusively in the notochord-lineage blastomeres, and the timing of gene expression at the 64-cell stage precisely coincides with that of the developmental fate restriction of the blastomeres. In addition, experimental studies have demonstrated a close relationship between the inductive events and As-T expression. In the present study, we show that overexpression of As-T by microinjection of the synthesized As-T RNA results in the occurrence, without the induction, of notochord-specific features in the A-line presumptive notochord blastomeres. We also show that overexpression of As-T RNA leads to ectopic expression of notochord-specific features in non-notochord lineages, including those of spinal cord and endoderm. These results strongly suggest that the developmental role of the Brachyury is conserved throughout chordates in notochord formation.

Animals↗

PubMed1998

Developmental signalling: a careful balancing act.

Embryos of arthropods and chordates are patterned along the dorso-ventral axis by a gradient of secreted morphogens of the Bmp4/Dpp family. This gradient now appears to be shaped by the opposing activities of Bmp-sequestering proteins, on the one hand, and Bmp-releasing metalloproteases, on the other.

Animals↗

PubMed1997

Autonomy of ascidian fork head/HNF-3 gene expression.

We have characterized the expression pattern of a class I fork head/HNF-3 gene (HrHNF3-1) of the ascidian Halocynthia roretzi. Zygotic HrHNF3-1 expression was detectable as early as the 16-cell stage, and the transcript was evident in blastomeres of the endoderm, notochord and mesenchyme lineages of the early embryos. After the late gastrula stage, HrHNF3-1 was also expressed in the presumptive spinal cord cells and some brain cells. The spinal cord of the ascidian tadpole consists of four layers of cells; the dorsal layer, two lateral layers and the ventral layer, the latter of which simply lies on the notochord. Cross-sections of in situ hybridized specimens showed that HrHNF3-1 was expressed in cells of the ventral layer, reminiscent of the floor plate of vertebrate embryos. In addition, we found autonomy in the initiation of early HrHNF3-1 expression, because the gene was expressed in blastomeres continuously dissociated from the first cleavage until the 16-cell stage.

Amino Acid Sequence↗

PubMed1996

The ascidian genome contains another T-domain gene that is expressed in differentiating muscle and the tip of the tail of the embryo.

The vertebrate Brachyury (T) gene is transiently expressed in nascent and migrating mesoderm, in the differentiating notochord, and in the tail bud, reflecting its independent functions. In contrast, the expression of an ascidian Brachyury gene (As-T) is restricted to differentiating notochord. The present study revealed that the genome of Halocynthia roretzi contains another T-domain gene (As-T2) which encodes a divergent T-domain protein. The transient expression of As-T2 was detected in the endoderm- and muscle-lineage blastomeres of the early embryo and the transcript was retained by involuting and differentiating muscle cells until it became undetectable by the mid-tailbud stage. In addition, As-T2 was expressed transiently in cells that form the tip of the newly forming tail. Interestingly, the combined pattern of spatial expression of As-T and As-T2 appears to correspond to that of a single vertebrate Brachyury gene.

Amino Acid Sequence↗

PubMed1996

Basic fibroblast growth factor induces notochord formation and the expression of As-T, a Brachyury homolog, during ascidian embryogenesis.

The tadpole larva of an ascidian develops 40 notochord cells in the center of its tail. Most of the notochord cells originate from the A-line precursors, among which inductive interactions are required for the subsequent differentiation of notochord. The presumptive-endoderm blastomeres or presumptive-notochord blastomeres themselves are inducers of notochord formation. Notochord induction takes place during the 32-cell stage. In amphibia, mesoderm induction is thought to be mediated by several growth factors, for example, activins and basic fibroblast growth factor (bFGF). In the ascidian, Halocynthia roretzi, treatment with bFGF of presumptive-notochord blastomeres that had been isolated at the early 32-cell stage promoted the formation of notochord at a low concentration of bFGF (0.02 ng/ml), while activin failed to induce notochord differentiation. The effect of bFGF reached a maximum at the end of the 32-cell stage and rapidly faded at the beginning of the subsequent cleavage, the time for full induction of notochord being at least 20 minutes. The expression of As-T, a previously isolated ascidian homolog of the mouse Brachyury (T) gene, starts at the 64-cell stage and is detectable exclusively in the presumptive-notochord blastomeres. The present study showed that presumptive-notochord blastomeres, isolated at the early 32-cell stage, neither differentiated into notochord nor expressed the As-T gene. However, when the presumptive-notochord blastomeres were coisolated or recombined with inducer blastomeres, transcripts of As-T were detected. When presumptive-notochord blastomeres were treated with bFGF, the expression of the As-T gene was also detected. These results suggest that inductive interaction is required for the expression of the As-T gene and that the expression of the As-T gene is closely correlated with the determined state of the notochord-precursor cells.

Activins↗

PubMed1995

A sea urchin homologue of the chordate Brachyury (T) gene is expressed in the secondary mesenchyme founder cells.

Chordates are thought to have emerged from some common ancestor of deuterostomes by organizing shared anatomical and embryological features including a notochord, a dorsal nerve cord and pharyngeal gill slits. Because the notochord is the most prominent feature of chordates and because the Brachyury (T) gene is essential for notochord formation, the T gene is a key molecular probe with which to explore the origin and evolution of chordates. We investigated whether the sea urchin (echinoderm) conserves the T gene and, if so, where the sea urchin T gene is expressed. A cDNA clone for the sea urchin T (HpTa) gene contained a long open reading frame that encodes a polypeptide of 434 amino acids. Although the overall degree of amino acid identity was not very high (52%, sea urchin/mouse), in the T domain of the N terminus the amino acid identity was 73% (sea urchin/mouse). The HpTa gene is present as a single copy per haploid genome. As with the chordate T gene, the expression of HpTa is transient, being first detected in the swimming blastula, maximally transcribed in the gastrula, decreasing at the prism larval stage and barely detectable at the pluteus larval stage. HpTa transcripts were found in the secondary mesenchyme founder cells, vegetal plate of the mesenchyme blastula, extending tip of the invaginating archenteron and, finally, the secondary mesenchyme cells at the late-gastrula stage.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1994

IgM and IgA antibodies generated against hepatitis C virus core antigen in patients with acute and chronic HCV infection.

Antibody subclasses directed against the core protein (HCc) of hepatitis C virus (HCV) were measured in 27 patients with acute non-A, non-B (NANB) hepatitis, and 99 patients with chronic HCV-associated liver disease. IgM, IgA, and IgG anti-HCc responses were observed in 11 (40.7%), 7 (25.9%), and 18 (67%) patients with acute NANB hepatitis, respectively. Twenty-four (24.2%) and 40 (40.4%) patients with chronic HCV infection also had detectable IgM and IgA, respectively. IgM anti-HCc inconsistently detected acute infection, and HCV ribonucleic acid (RNA) could be detected preceding the rise in anti-HCc antibodies in five consecutive patients with acute hepatitis. IgM anti-HCc also could not distinguish acute from chronic infection and did not correlate with histologic progression. However, the form of IgA present (polymeric vs monomeric) did discriminate acute from chronic infection and the IgA anti-HCc titer correlated with histologic evidence of liver disease in patients with chronic HCV infection.

Acute Disease↗

PubMed1993