In this thesis, I describe the transcriptional regulation of neural crest throughout neurulation of Xenopus embryos. This work focuses on the global expression dynamics of genes involved in the specification of the ectodermal territories, i.e., neural plate, pre-placodal, neural border, and epidermis. In particular, I study the function of tfap2e, a member of the tfap2 family of transcription factors that is expressed in the neural crest of Xenopus.
In the first chapter, I provide an overview of the morphogenetic movements and signaling pathways that contribute to adoption of the ectodermal fates in Xenopus embryos, and in particular, to the process of specification of the neural crest. Furthermore, I describe what is currently known about the role of Tfap2 transcription factors in vertebrate neural crest development.
In the second chapter, I characterize the functional role of tfap2e in Xenopus development. I identified tfap2e in a screen for genes that are expressed exclusively in the neural crest. Through gain and loss of function analyses, I concluded that tfap2e is required for the early processes of neural crest specification, and it is also necessary and sufficient to promote neural crest delamination, a mesenchymal property essential for neural crest cell behaviors. To better understand why tfap2e is important for these processes, I developed a transcriptomic analysis strategy to identify genes whose expression is either positively or negatively correlated with Tfap2e activity. These genes are likely to be regulated directly or indirectly by Tfap2e. The results of this study contribute to a further understanding of the roles of Tfap2 factors within the neural crest module of the gene regulatory network.
The third chapter summarizes the results of collaborative work between the Harland and Eisen labs, at U.C. Berkeley, and the Monsoro-Burq lab at the Curie Institute in Orsay, France. I describe the expression dynamics of genes known to specify the different ectodermal fates in Xenopus embryos. In addition, I describe how we obtained and analyzed the expression profiles of manually dissected ectodermal tissues throughout neurulation. The dissection dataset was utilized as a reference to evaluate how tfap2e knockdown altered the fate of the neural plate border. In agreement with the analyses from the second chapter, we conclude that loss of tfap2e delays neural crest specification, and consequently, neural plate border cells seem to adopt an expression profile that is similar to the anterior placodes.
In the fourth and final chapter, I describe the syntenic relationships between the different tfap2 clusters. The analysis provides insights into the genomic rearrangements that have occurred during the evolution of vertebrates, which may have contributed to alterations in gene expression patterns after the divergence between the common ancestor of vertebrates and tunicates. Finally, I summarize the results of chromatin immunoprecipitation to identify putative cis-regulatory elements of genes that are downstream of Tfap2e. This protein may have both activator and repressor functions that help maintain the neural crest fate over other neural plate border fates.
The work described here has contributed to our understanding of how the development the neural crest is regulated, both through focusing on a single transcription factor, tfap2e, and more broadly by characterizing gene expression dynamics in this unique cell population and important vertebrate innovation.