Sox9 and Sox10: Comparative role in  neural crest and placode development

Neural crest (NC) cells and cranial placodes play essential roles in the development and evolution of the vertebrate head. NC cells contribute to the cranial skeleton, cranial sensory ganglia and multiple other structures. Placodes form the olfactory, otic, lateral line organs and many sensory neurons. The SoxE proteins Sox9 and Sox10 are transcription factors acting as neural crest specifiers. They are also expressed in the otic placode, which gives rise to the inner ear. However, little is known about their different functions during NC and placode development. This study identified their candidate direct target genes in each tissue during two developmental stages in Xenopus laevis: neurula and organogenesis. After overexpression of hormone inducible forms of Sox9 and Sox10, NC or placodes were explanted from GFP labelled embryos or were grafted orthotopically from GFP-labelled donor embryos into unlabelled hosts and nuclear translocation of Sox9 or Sox10 was induced. Cells were then FACS-sorted, followed by bulk RNA sequencing to identify candidate target genes in these two tissues. To confirm the direct binding sites of Sox9/10, CUT&RUN sequencing was then performed using NC and placodal explants overexpressing Sox9 and Sox10. The data obtained show that during neurula stages, Sox9 activates NC specifier genes specifically in the NC, whilst repressing them in the placodal region. Only a handful of placode-specific genes were activated by Sox9 or Sox10 in the placodal region. However, both Sox9 and Sox10 repressed certain NC specifiers and neural plate border genes in the placodal region, while activating genes that promote non-neural ectodermal (NNE) competence. During organogenesis stages, Sox9 suppresses numerous NC, neural plate (NP) and placodal genes in the NC while activating certain placodal genes in the otic placode. In contrast, Sox10 suppresses some but activates other neural ectodermal (NE) and NP genes in the NC, while activating NNE genes in the otic placode. In addition, Sox9 and, to some extent, Sox10 also activate transcription factors, which promote neurogenesis in the NC at the neurula stage and in the otic placode at the organogenesis stage. Overall, the data presented in this thesis demonstrate that during neurulation, Sox9 acts as a transcriptional activator of NC genes in the NC, and it represses the same in placodes. In contrast, Sox10 functions primarily by suppressing non-neural, placodal, and neural plate-specific genes in the NC while activating PPR genes in the placode. This has important implications for our understanding of syndromic neurocristopathies with deficits in tissues derived from both NC and otic placode.

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University of Galway

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University of Galway Theses (PhD Theses)