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F Stennard

Publications and source records attributed to F Stennard.

8 recordsLinked to original sources
PubMed2000

The Xenopus eomesodermin promoter and its concentration-dependent response to activin.

Eomesodermin is an essential early gene in Xenopus mesoderm formation and shows a morphogen-like response to activin. Here we define the regions of the Eomesodermin promoter required for mesodermal expression and for concentration-dependent response to activin. We find an activin response element (ARE) located between -5.6 and -5.0 kb which contains two critical FAST2 binding sites. The ARE alone is necessary and sufficient for concentration-dependent response to activin. A 5.6 kb promoter recapitulates Eomes expression in normal mesoderm cells. A repressor element extinguishes Eomes expression in the endoderm. We relate our results to mesoderm patterning in early Xenopus development and to a mechanism of morphogen gradient response.

Activins↗
PubMed1999

Differential expression of VegT and Antipodean protein isoforms in Xenopus.

The VegT/Antipodean (Apod) gene is important for germ layer formation in Xenopus. To investigate the role of this gene at the protein level, as opposed to the RNA level, we have generated affinity purified polyclonal antibodies to Apod, and for comparison, to the other early T-box proteins Xbrachyury and Eomesodermin. An anti-VegT/Apod antibody reveals that there are two protein isoforms in Xenopus, one that we refer to as VegT and a smaller molecular weight isoform that we refer to as Apod. These isoforms have different N-terminal domains resulting from developmentally regulated alternative splicing of a primary transcript arising from a single VegT/Apod gene. VegT is maternally expressed. Its translation is blocked during oogenesis but the protein is present from the egg until gastrulation in the presumptive endoderm. There is no evidence for zygotic expression of this isoform. Conversely, the Apod protein isoform is expressed only after the onset of zygotic transcription in the presumptive mesoderm and is inducible by activin. We conclude that the developmental role of VegT/Apod is mediated by two different proteins, with entirely different patterns of expression and response to growth factors.

Activins↗
PubMed1999

Single cells can sense their position in a morphogen gradient.

Xenopus blastula cells show a morphogen-like response to activin by expressing different genes according to the concentration of activin to which they are exposed. To understand how cells recognize their position in a concentration gradient, it is essential to know whether each cell responds individually to activin concentration. An alternative idea, proposed by previous work, is that cells need to interact with their neighbours to generate a concentration-related response. To distinguish between these ideas, we have cultured blastula cells under conditions which provide different degrees of contact with other cells, allowing nil to maximum communication with their neighbours. The cultures include cells attached to fibronectin and cells resting unattached on an agarose base. The cultures also include cells that have no contact with any cell except their clonal progeny, cells that have lateral contact to neighbouring cells, and cells that are completely enveloped by other cells in a reaggregate. We have used RNase protection and in situ hybridization to assay the expression of the activin-responsive Xenopus genes Xbra, Xgsc, Xeomes, Xapod, Xchordin, Mix1, Xlim1 and Cerberus. We find no difference in gene expression between cells attached to fibronectin and those unattached on agarose. Most importantly, we find that cells respond to activin in a concentration-related way irrespective of their degree of contact with other cells. Therefore interaction among cells is not required for the interpretation of morphogen concentration, at least in the case of the early genes studied here. We conclude that isolated blastula cells can sense and respond individually to activin by expressing genes in a concentration-dependent way.

Activins↗
PubMed1997

Markers of vertebrate mesoderm induction.

Mesoderm formation is the first major differentiative event in vertebrate development. Many new mesoderm-specific genes have recently been described in the mouse, chick, frog and fish and belong to classes comprising T-domain genes, homeobox genes and those encoding secreted proteins. The T-domain genes have different but overlapping expression patterns and, in Xenopus, can ectopically activate nearly all other mesodermal genes. Several new homebox genes seem to mediate the ventralising activity of bone morphogenetic protein. New genes encoding secreted proteins induce dorsal mesoderm, in some cases by antagonizing ventralising factors.

Animals↗
PubMed1996

The Xenopus T-box gene, Antipodean, encodes a vegetally localised maternal mRNA and can trigger mesoderm formation.

We have used differential display to identify genes inducible by activin and isolated a novel member of the T-box gene family that includes the Xenopus genes Xbrachyury and Eomesodermin. Here we show that this novel gene is unique within the T-box family because it is maternally expressed at a high level. Furthermore, it belongs to a rare class of maternal mRNAs in Xenopus that are localised to the vegetal hemisphere of the egg and we have therefore named it Antipodean. We show here that low amounts of Antipodean injected into ectoderm (animal cap cells) strongly induce pan mesodermal genes such as Xbrachyury and ventral mesodermal genes such as Xwnt-8. Overexpression of Antipodean generates mesoderm of ventral character, and induces muscle only weakly. This property is consistent with the observed late zygotic Antipodean mRNA expression in the posterior paraxial mesoderm and ventral blastopore, and its exclusion from the most dorsal mesodermal structure, the notochord. Antipodean is induced by several molecules of the TGF-beta class, but in contrast to Xbrachyury, not by bFGF. This result suggests that the expression of these T-box genes may be under the control of different regulatory pathways. Finally, we demonstrate that Antipodean and Eomesodermin induce each other and both are able to induce Xbrachyury. The early zygotic expression of Antipodean is not induced by Xbrachyury, though later it is to some extent. Considering its maternal content, Antipodean could initiate a cascade of T-box gene activations. The expression of these genes may, in turn, sustain each other's expression to define and maintain the mesoderm identity in Xenopus.

Amino Acid Sequence↗
PubMedDate not supplied

Xenopus differentiation: VegT gets specific.

Depletion of the maternal store of the localised mRNA encoding the T-box transcription factor VegT in Xenopus embryos has recently been shown to dramatically block endoderm formation and change the normal position of the mesodermal and ectodermal germ layers.

Animals↗