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 geneswith a role in vertebrate embryogenesis have been studied in the recent period. One of th eprojects conducted in the laboratory concerns the identification of a protein in the family of guanine nucleotide exchange factors named WGEF that has a role in the Wnt-PCP pathway which regulates convergent extension movements in Xenopus. These cell movements are required in the establishment of body shape, and deficits in these movements can lead to malformations such a neural tube closure defects (NTDs). We have characterized the Rho-GEF we identified as a component of the Wnt-PCP signaling pathway that interacts with the known pathway components Dishevelled and Daam-1. The role and specific position of WGEF in the genetic hierarchy of the control of gastrulation movements has been determined. DNA microarray technology also led to the identification of a leucine-rich transmembrane protein named Lrig3 that we showed to be required for the formation of the neural crest. Experiments in the whole embryo and in explants induced to differentiate into neural crest have shown that the novel protein affects multiple signaling pathways in the embryo. In particular, Lrig3 modulates the function of the Fibroblast Growth Factor pathway to maintain the activity of this pathway within an optimal range. Epistasis experiments have been carried out to identify the position of the leucine-rich factor in the hierarchy of control of neural crest specification. The third project 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.
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