A Tail of Two Signaling Pathways; How Xenopus Patterns its Posterior

The development of multicellular organisms relies on extracellular signaling pathways regulating the patterning and specification of tissues, with the same pathway often being deployed to serve many different functions. The development of Xenopus laevis has served as a classic model for identifying context specific transcriptional targets of signaling pathways and the regulatory loci responsible for their precise expression. The focus of this dissertation was to understand the regulation of the Wnt/ -catenin transcriptional target genes such that the correct patterns of expression are formed during gastrulation. During gastrulation Wnt, along with FGF, function to form the anterior to posterior axis of the embryo by inducing expression of target genes necessary for posterior identity. Target genes of the Wnt signaling pathway during gastrulation are not the same as those induced at early and later times and in addition, form different patterns. I begin in Chapter I by summarizing the early inductive events during Xenopus development that form the proper tissues for Wnt to act on as well as provide an overview of our current understanding of gene regulation and methods used to discover gene regulatory networks. In Chapter II, I explain the methods used in my dissertation. Then in Chapter III, I identify direct transcriptional targets of the Wnt signaling pathway. I found that this set of target genes are expressed around the blastopore during gastrulation, contain -catenin bound regions within 50kb of the gene, and are reduced in expression as a result of Wnt signaling knockdown. Next, in Chapter IV, I focus on two transcription factors, Sox2 and Zic1/2/3, to explore whether they are co-regulating a subset of Wnt target genes to provide context dependent activation. Lastly, in Chapter V, I aim to understand if FGF directly posteriorizes the embryos by activating either all or a subset of Wnt target genes identified in Chapter III. I found that FGF directly regulates all Wnt target genes during gastrulation. In addition, I found -catenin preferentially binds at the promoter of a subset of genes and that negative regulation in the absence of Wnt signaling may be responsible for the expression patterns of this subset of genes. Together, these results reveal the complex mechanisms of gene regulation that are necessary to insure the precise expression of Wnt and FGF target genes such that they form the anterior to posterior axis