Tcf3: a transcriptional regulator of axis induction in the early embryo

The roles of Lef/Tcf proteins in determining cell fate characteristics have been described in many contexts during vertebrate embryogenesis, organ and tissue homeostasis, and cancer formation. Although much of the accumulated work on these proteins involves their ability to transactivate target genes when stimulated by beta-catenin, Lef/Tcf proteins can repress target genes in the absence of stabilized beta-catenin. By ablating Tcf3 function, we have uncovered an important requirement for a repressor function of Lef/Tcf proteins during early mouse development. Tcf3-/- embryos proceed through gastrulation to form mesoderm, but they develop expanded and often duplicated axial mesoderm structures, including nodes and notochords. These duplications are preceded by ectopic expression of Foxa2, an axial mesoderm gene involved in node specification, with a concomitant reduction in Lefty2, a marker for lateral mesoderm. By contrast, expression of a beta-catenin-dependent, Lef/Tcf reporter (TOPGal), is not ectopically activated but is faithfully maintained in the primitive streak. Taken together, these data reveal a unique requirement for Tcf3 repressor function in restricting induction of the anterior-posterior axis.     

Identification of ELF3 as an early transcriptional regulator of human urothelium

Despite major advances in high-throughput and computational modelling techniques, understanding of the mechanisms regulating tissue specification and differentiation in higher eukaryotes, particularly man, remains limited. Microarray technology has been explored exhaustively in recent years and several standard approaches have been established to analyse the resultant datasets on a genome-wide scale. Gene expression time series offer a valuable opportunity to define temporal hierarchies and gain insight into the regulatory relationships of biological processes. However, unless datasets are exactly synchronous, time points cannot be compared directly.     

Fibroblast growth factor during mesoderm induction in the early chick embryo

A chick genomic clone that reveals a high degree of homology to the mammalian and Xenopus bFGF gene has been isolated. The pattern of expression of bFGF has been examined during early chick embryogenesis. RNA blot analysis revealed that chick bFGF is already transcribed at pregastrula stages. Immunolabeling analysis indicated that bFGF protein is present at these early developmental stages and is distributed evenly in the epiblast, hypoblast and marginal zone of the chick blastula. Substances that can inhibit FGF action were applied to early chick blastoderms grown in vitro under defined culture conditions (DCM). Both heparin and suramin were capable of blocking the formation of mesodermal structures in a dose-dependent manner. Our results indicate that FGF-like substances may need to be present for axial structures to develop although they may be acting earlier during the induction of non-axial mesoderm.     

Identification of distinct genes with restricted expression in the somitic mesoderm in Xenopus embryo

We have used whole-mount in situ hybridisation to identify genes expressed in the somitic mesoderm during Xenopus early development. We report here the analysis of eight genes whose expression pattern has not been described previously. They include the Xenopus homologues of eukaryotic initiation factor 2beta, methionine adenosyltransferase II, serine dehydratase, alpha-adducin, oxoglutarate dehydrogenase, fragile X mental retardation syndrome related protein 1, monocarboxylate transporter and voltage-dependent anion channel 1. Interestingly, these genes exhibit very dynamic expression pattern during early development. At early gastrula stages several genes do not show localised expression pattern, while other genes are expressed in the marginal mesoderm or in ectoderm. As development proceeds, the expression of these genes is gradually restricted to different compartments of somite. This study thus reveals an unexpected dynamic expression pattern for various genes with distinct function in vertebrates.     

Nidogen-1. Expression and ultrastructural localization during the onset of mesoderm formation in the early mouse embryo.

Nidogen-1, a key component of basement membranes, is considered to function as a link between laminin and collagen Type IV networks and is expressed by mesenchymal cells during embryonic and fetal development. It is not clear which cells produce nidogen-1 in early developmental stages when no mesenchyme is present. We therefore localized nidogen-1 and its corresponding mRNA at the light and electron microscopic level in Day 7 mouse embryos during the onset of mesoderm formation by in situ hybridization, light microscopic immunostaining, and immunogold histochemistry. Nidogen-1 mRNA was found not only in the cells of the ectoderm-derived mesoderm but also in the cytoplasm of the endoderm and ectoderm, indicating that all three germ layers express it. Nidogen-1 was localized only in fully developed basement membranes of the ectoderm and was not seen in the developing endodermal basement membrane or in membranes disrupted during mesoderm formation. In contrast, laminin-1 and collagen Type IV were present in all basement membrane types at this developmental stage. The results indicate that, in the early embryo, nidogen-1 may be expressed by epithelial and mesenchymal cells, that both cell types contribute to embryonic basement membrane formation, and that nidogen-1 might serve to stabilize basement membranes in vivo. (J Histochem Cytochem 48:229-237, 2000)     

Ultrabithorax is a regulator of beta 3 tubulin expression in the Drosophila visceral mesoderm.

beta 3 tubulin expression accompanies the specification and differentiation of the Drosophila mesoderm. The genetic programs involved in these processes are largely unknown. Our previous studies on the regulation of the beta 3 tubulin gene have shown that upstream sequences guide the expression in the somatic musculature, while regulatory elements in the first intron are necessary for expression in the visceral musculature. To further analyse this mode of regulation, which reflects an early embryonic specification program, we undertook a more detailed analysis of the regulatory capabilities of the intron. The results reveal not only a certain degree of redundancy in the cis-acting elements, which act at different developmental stages in the same mesodermal derivatives, but they also demonstrate in the visceral mesoderm, which forms a continuous epithelium along the body axis of the embryo, an early action of regulators guiding gene expression along the anterior-posterior axis of the embryo: an enhancer element in the intron leads to expression in a subdomain restricted along the anterior-posterior axis. This pattern is altered in mutants in the homeotic gene Ultrabithorax (Ubx), whereas ectopic Ubx expression leads to activity of the enhancer in the entire visceral mesoderm. So this element is likely to be a target of homeotic genes, which would define the beta 3 tubulin gene as a realisator gene under the control of selector genes.