Vertebrate head formation encompasses successful neural tube closure and neural crest development, which ultimately give rise to the functional brain and craniofacial structures, respectively. These two processes take place during early stages of embryonic development and involve distinct molecular events. However, to some extent, neural tube closure and neural crest formation are interdependent on each other since the signalling pathways involved in their regulation largely overlap. So far, the precise mechanisms by which these processes intersect remain poorly understood, and a better understanding is needed to improve our current knowledge of how congenital disorders arise. Neural crest cells (NCCs) are multipotent, transient population of cells that originate from the neural plate border (NPB). Subsequently, these cells delaminate and migrate extensively throughout the embryo to populate different targeted destinations where they give rise to multiple cell lineages. Among the different subtypes of NCCs, cranial NCCs (CNCCs) contribute to the majority of the craniofacial features within a living organism. The low-density lipoprotein receptor-related protein 2 (LRP2) is an endocytic receptor that plays a pivotal role during the early stages of embryonic development by mediating the internalisation of various ligands, including folate and several growth factors. Additionally, LRP2 is crucial in regulating the subapical scaffolding complex within the neuroepithelial cells, where it helps to ensure a proper cellular integrity during embryonic morphogenesis. While our previous studies have highlighted the critical role of LRP2 in regulating proper neural tube closure and subsequent forebrain development, the craniofacial abnormalities observed in Lrp2 null mutants have yet to be addressed. Herein, this dissertation provides ample evidence that LRP2 is a key factor that helps in ensuring proper neural crest development during early mouse cranial morphogenesis, thereby providing a reasonable explanation for the craniofacial deformities associated with its loss of function. Most importantly, the results of this dissertation demonstrate that LRP2 serves an important role in maintaining an adequate neural crest fate during its initial phases of development. Loss of LRP2 resulted in a folate-deficient state that impaired cell fate decision making during NPB specification, leading to an abnormal expansion of the NPB without diminishing the neural plate fate. Since the NPB harbours a mixture of intermingled progenitors that exist in the primed multipotent states, the expanded NPB opened up opportunities for a greater number of cells to adopt neural crest fate, as represented by the ectopic expansion of the pre-migratory and migratory NCC territories within Lrp2-/- embryos. Besides, the inappropriate fate adopted by the presumptive NCCs residing within the expanded NPB of Lrp2-/- embryos also resulted in the subsequent loss of the classical features required for proper NCC delamination and migration. Through time-lapse imaging on the cranial neural tube explant cultures, our results demonstrated that murine NCCs exhibited contact inhibition of locomotion (CIL) behaviour during directed collective migration ex vivo. In particular, CIL behaviour during directed migration of CNCCs ex vivo required the function of LRP2. As observed in the Lrp2-/- explants, migratory NCCs failed to exhibit CIL behaviour and showed disorganised actomyosin cytoskeleton, leading to the loss of directional persistence in their migration. Finally, this study has successfully identified a number of potential candidates that inform the possible molecular mechanisms in which LRP2 governs neural crest development using spatial proteomics approach. In summary, this study proposes LRP2 as a central regulator of vertebrate cranial morphogenesis, essential not only for embryonic brain development but also for shaping craniofacial frameworks through the regulation of NCC formation. This study, for the first time, unmasks the potential role of an endocytic machinery in governing murine neural crest development, starting from cell fate specification to the downstream migration. Altogether, these findings expand our current understanding of the complexity of embryonic cranial morphogenesis and can have broader implications that benefit not only the field of developmental biology but also cancer biology, given the great deal of similarities shared between the process of neural crest development and cancer progression.
