This laboratory is studying the molecular basis of embryogenesis in Xenopus laevis. This project utilized DNA microarray technology and other methods for gene discovery in the early embryo, with the aim to obtain information on gene expression patterns and gene function in development, and through this to lead to improved understanding of the molecular basis of normal embryogenesis and abnormalities that can arise by loss of function or malfunction of various genes. Several genes with a role in vertebrate embryogenesis have been studied in the recent period. One of the projects conducted in the laboratory using DNA microarray technology concerns the formation of the notochord in Xenopus;the notochord is the defining structure of chordates, the phylum that includes both frogs and humans. The notochord has been known to contain vacuoles and to be surrounded by a sheath;both of these structures are required to give it the mechanical strength that is an important characteristic of this tissue. Both vacuole and sheath formation involves the secretion of proteins, and previous studies in several laboratories have shown that secreted proteins from different classes are required for notochord formation. In our DNA microarray studies we found that activation of the genes encoding secretory pathway components is a hallmark of notochord differentiation. The great majority of genes that are differentially expressed in the notochord as compared to the rest of the embryo belong to this functional class. These results constitute an unusually informative output of microarray studies, as in most comparisons of different tissues or different embryonic stages, a great variety of genes are identified that belong to different functional classes and pathways. The coordinate activation of secretory pathway genes in the developing notochord requires the function of two transcription factors named XBP1 and Creb3L2. These factors themselves are preferentially expressed in the early notochord, and their activation in notochord precursor cells is an important step in the specification of the notochord. These studies contribute to an understanding of the molecular basis of differentiation of one of the earliest-forming tissues characteristic of the vertebrate embryo. In following up on our longstanding interest in neural crest development we have studied the novel factor Kctd15 that restricts neural crest induction in both the zebrafish and Xenopus embryo. Kctd15 is a BTB-domain protein that is first expressed in the embryo at the neural plate border, and subsequently in pharyngeal arches and other regions. Overexpression of Kctd15 strongly inhibits neural crest specification in whole embryos and in animal explants, as studied in so-called animal caps from Xenopus embryos. We found that Kctd15 inhibits the output of the canonical Wnt signaling pathway upstream of or at the level of -catenin as Kctd15 inhibited Wnt-induced but not -catenin-induced induction of neural crest markers. Wnt signaling is known to be crucial for induction of the neural crest and for distinguishing neural crest precursors from precursors of anterior placodes that arise at the neural plate border. We propose that Kctd15 is involved in delineating the separate placode and neural crest domains during embryogenesis.
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