Molecular mechanisms regulating embryogenesis of the amphibian Xenopus laevis and the zebrafish have been studied. Pattern formation is controlled in the embryo by the action of cell signalling molecules that elicit the expression of transcription factor-encoding genes in a spatially and temporally regulated way. The focus of this laboratory has been the study of the earliest establishment of dorsoventral polarity and the function of LIM class of homeobox genes whose members are instrumental in pattern formation and tissue differentiation. The dorsoventral axis is established in the Xenopus embryo in very early development through a signalling pathway that can be initiated by certain Wnt factors. We have explored the signal transduction cascade of these factors by studying GSK-3, a kinase believed to be involved in the process. By expression of wild type and dominant negative GSK-3 constructs it was shown that inhibition of GSK-3 activity dorsalizes while activation of GSK-3 ventralizes the embryo. These reciprocal effects strongly support the involvment of GSK-3 and of the entire Wnt signalling pathway in dorsal axis formation. The Xlim-1 gene is expressed in the organizer region of the embryo. Expression of mutant forms of Xlim-1 can turn animal (ectodermal) cells into organizer-like cells in that they secrete factors that induce neural tissue in ectoderm and muscle in ventral type mesoderm. One of the factors whose production is elicited by activated Xlim-1 is the recently discovered signalling protein chordin; chordin may be a major signalling factor elaborated by the organizer. The nature of the induced neural tissue is anterior, as seen by the expression of several marker genes. The transcription factor Xbra acts to convert the anterior neural tissue induced by Xlim-1 into more posterior tissue without itself being a neuralizing agent. One of the key issue in studying early development is the transcriptional control of activation and maintenance of regulatory genes that are important at this time. Both the regulation of transcription of Xlim-1 and its activation of a target gene have been studied. A major form of regulation of gene activity in development is mediated by growth factors, and Xlim-1 is activated by the TGF-beta-related factor activin. This activation has been shown to depend on a constitutive promoter in the upstream region of the gene together with an activin-responsive silencer located in the first intron. The goosecoid gene is a target for Xlim-1 regulation. A region in the goosecoid upstream region has been shown to be required for a response to Xlim-1, and binding sites for the Xlim-1 homeodomain have been identified in this region. This work begins to establish a network of regulatory interrelationships during the gastrula period of embryogenesis in a vertebrate embryo.
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