Dissecting the function of Wnt signalling in zebrafish neural crest development

Abstract

Wnt proteins are secreted ligands that activate signalling pathways which regulate many developmental processes. The Wnt/β-catenin signalling pathway activates gene regulatory networks during development to enable cell fate transitions in precise locations in the embryo. Wnt/β-catenin signalling is essential for the development of the neural crest (NC) lineage. The NC is a multipotent cell population with extraordinary migratory capacity. The NC is crucial for vertebrate development and forms a myriad of cell derivatives throughout the body, including pigment cells, neuronal and glial cells of the peripheral nervous system, cardiomyocytes and skeletogenic cells in craniofacial tissue. NC induction occurs at the end of gastrulation when multipotent NC progenitors emerge in the ectoderm, in the neural plate border (NPB) region. NC cells delaminate from the NPB and migrate throughout the embryo until they differentiate into various cell derivatives. NC specification is the process whereby fate-specific markers are expressed in multipotent progenitors, which subsequently adopt a specific fate. Wnt/β-catenin signalling promotes the specification of pigmented melanocytes from multipotent NC cells. In this thesis, I investigate the mechanisms of Wnt transport to migratory NC cells. Traditional paradigms of cell signalling suggest that ligands are secreted by source cells and diffuse through extracellular space to bind to receptors on target cells. However, this mechanism fails to explain how specific cells within a migratory population, like the NC, are targeted. I suggest an alternative mechanism of Wnt dispersal which involves transport of ligands along specialised cell protrusions called cytonemes. I focus on the midbrain-hindbrain boundary as a signalling centre to the NC. I demonstrate that neuroepithelial cells form cytonemes carrying Wnt3a, which activates melanocyte specification in NC cells. While further research is necessary to determine a requirement for cytonemes in NC development, this thesis presents a novel mechanism to enable precise targeting of Wnt ligands to NC cells.