Neural crest (NC) cell development is a complex, multistep process that begins with induction and specification in the region between the neural and non-neural plate, termed the neural plate border (NPB). Once specified, NC cells undergo an epithelial-to-mesenchymal transition and migrate extensively throughout the body to the sites where they then differentiate. The various steps of NC cell development are orchestrated by cell-extrinsic signals and gene-regulatory interactions that can be described by a gene regulatory network (GRN). To date, the interrogation of the genetic mechanisms that govern NC cell development has largely focused on cells originating in the cranial region, despite our knowledge that the NC is regionalized into distinct subpopulations along the anteroposterior (AP) axis. In fact, it has been proposed that differences in the underlying genetic circuits account for the unique properties of cranial and trunk NC cells, namely their distinct differentiation potential and cellular behaviors during migration. Thus, a more detailed understanding of the mechanisms that govern the development of NC cells in the posterior body remains an important unaddressed question in the field. In this thesis, I investigated the role of cdx4 in regulating the development of NC cells in the posterior body of zebrafish embryos. Cdx factors are homeodomain-containing transcription factors that play a conserved and well-established function in the formation and patterning of the posterior body across vertebrate model systems. Here, I demonstrate that cdx4 is expressed in NC cell progenitors in the trunk and tail and is therefore a promising candidate for mediating the formation of NC cells in the posterior body. I employed transgenic and genetic approaches in zebrafish embryos, together with high-resolution microscopy and bioinformatics analysis to uncover the function of cdx4 within the NC GRN and its role in establishing the trunk-specific behaviors during NC cell migration. In Chapter 2, I explore how cdx4 links NC cell specification with the outgrowth of the posterior body. I demonstrate that Cdx4 binds near many of the genes that drive the early steps of NC cell specification, termed the NPB module. Cdx4 is necessary for the expression of the NC specifier gene foxd3, in the posterior neural tube and the developing tailbud. Additionally, my analysis of available ChIP-seq and microarray data suggests that Cdx4 may regulate the signaling pathways and genetic circuits that drive posterior body outgrowth. In Chapter 3, I show that cdx4 is necessary for the migration of trunk NC cells along segmental chains. Using in vivo single-plane illumination microscopy, I reveal that cdx4 regulates the establishment of leader and follower identities that drive the directed migration of trunk NC cells. Finally, because cdx4 expression and function is not limited to the NC, I performed cell transplantation experiments that demonstrated the impaired trunk NC migration is not due to loss of Cdx4 in the adjacent somitic mesoderm. Together, these findings have made significant progress towards establishing the genetic mechanisms that govern the development of NC cells in the posterior body of the zebrafish.



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