Finding the way: Transcriptional regulation of migratory neuroblast lineages in C. elegans

In Chapter 1, the bilateral pair of post-embryonic Q neuroblast lineages, the in vivo model system that is used in all the experimental work presented in this thesis, is introduced. A detailed explanation of the hierarchy and temporal progression of these lineages is provided as well as an overview of the genetic mechanisms underlying their biology, mainly focused on the regulation of their migration. Chapter 2 describes an improved protocol developed to allow efficient fluorescent-activated cell sorting of large numbers of rare types of cells, such as the Q lineage cells, that is the basis of the Q lineage-specific RNA-seq that is presented in the subsequent chapters. In Chapter 3, a comprehensive resource for the discovery of novel genes involved in the regulation of QR lineage fate is presented. Using FACS-based isolation of this lineage combined with RNA-seq, we generated a genome-wide transcriptional profile that we further validated through the more systematic analysis of mRNA and protein dynamics of a selected set of transcription factors. In Chapter 4, we focus on the role of canonical Wnt/β-catenin signaling in the termination of migration of QR descendants. Using Q lineage RNA-seq and functional genetic studies we discover that this pathway activates a specific transcriptional program, and that two of its direct targets, the Slt - Robo pathway component EVA-1/EVA1C and the Rho GTPase activating protein RGA-9b/ARHGAP, are necessary and sufficient for migration inhibition. Furthermore, we propose a new model where the crosstalk between canonical and non-canonical Wnt signaling occurs through antagonistic regulation of Rho GTPases to regulate migration of these cells. Chapter 5 addresses the mechanisms by which the canonical Wnt/β-catenin signaling target gene mab-5 regulates posterior migration of QL descendants. Starting from a Q lineage-specific transcriptional profiling, we unveil a novel list of targets regulated by this transcription factor, and demonstrate that vab-8, eva-1, and ebax-1 are involved in this migratory process. Moreover, we show that the E3 ligase PLR-1/ZNRF3/RNF43 integrates the negative feedback that results in robust mab-5 expression, which is an essential mechanism for the correct migration of these cells. Finally, in Chapter 6 the results presented in this thesis are summarized and discussed.