Fate selection in the neural crest lineage and its regulation

Sammanfattning: The neural crest is a transient embryonic progenitor cell population that generates a diverse array of cell and tissue types, including cartilage, bone, teeth, soft connective and adipose tissues, peripheral sensory and autonomic neurons, myelinating and non-myelinating Schwann cells, melanocytes, cardiac outflow tract, adrenal medulla and other derivatives. Neural crest cells start as neuroepithelial cells forming at the border of neural and non-neural ectoderm in a vertebrate embryo. After delaminating at the time of the early neural tube closure, they migrate towards their final destinations in the body while undergoing major changes of their transcriptional states during fate selection and differentiation processes. Changes in gene regulatory networks controlling neural crest states direct neural crest development. Sets of interacting molecular signaling pathways, transcriptional factors, epigenetic states and downstream effector programs confer features like multipotency, multilineage differentiation and directed migratory capacity of the neural crest. The main goal of this doctoral thesis was to shed light on how neural crest cells make their fate choices during migration, and more specifically, how the neural crest-derived Schwann cell precursors contribute to neuroendocrine chromaffin cells and sympathetic neurons in the body. I also explored the avenues of how the process of healthy transition from nerve-associated Schwann cell precursors into chromaffin neuroendocrine cells and sympathoblasts might influence the formation of embryonic and early pediatric malignancies, such as neuroblastoma and other neuroendocrine tumors, using one of the most efficient approaches nowadays – single cell transcriptomics. In paper I, we followed the development of the adrenal medulla from the embryonic to postnatal stages and generated a new single cell transcriptomics atlas of chromaffin and Schwann cell development. We discovered that Schwann cell precursors show high microheterogeneity corresponding to early biases towards either Schwann or chromaffin terminal fates. Moreover, we found and distinguished previously unrecognized states/clusters of developing chromaffin cells, considering that some subclusters might have unique developmental roles. In paper II, we analyzed the difference in development of murine and human sympathoadrenal and aorta-gonad-mesonephros anlagens to find out human-specific aspects of human development, which might be a key to understanding the origin of neuroblastoma. We found cell state transitions between intra-adrenal sympathoblasts and immature chromaffin cells, showing that in human and mice the transitions happen in various ways, although chromaffin cells and specific sympathoblasts in both species are derivatives of Schwann cell precursors. In humans, the process of recruitment of Schwann cell precursors from the nerves and transition into sympatho-adrenal fates in large intra-adrenal ganglia-like structures continues for weeks, which creates a potential long-lasting reservoir of cells potentially giving rise to neuroblastoma. In paper III, we focused on the generation of the first single cell transcriptomics and spatial atlas of migrating neural crest cells in murine embryos to understand the hierarchy of fate choice via advanced trajectory analysis and obtaining insights from specific investigation of cell populations at and prior to bifurcation points in the embeddings of transcriptional states. We discovered the hierarchy of cell fate choices, starting from a subdivision of sensory and sympatho-adrenal fates. This approach also helped us to pinpoint and validate the key difference in the fate selection dynamics in skeletogenic cranial and non-skeletogenic trunk neural crest. Altogether, my studies provided new insights into fate selection machinery during neural crest and Schwann cell precursor development into downstream cell populations, more specifically, into diverse cells building adrenal medulla and Organ of Zuckerkandl, which are entirely responsible for production and release of adrenaline and noradrenaline in our body.