The role of Sox2 in inner ear development

Abstract

The vertebrate inner ear is a structurally complex sensory organ responsible for detecting sound and maintaining balance. These functions are mediated by specialized sensory epithelia comprised of a mosaic of mechano-transducing hair cells and supporting cells. The sensory hair cells are innervated by neurons of the cochleo-vestibular ganglion (CVG, the VIIIth cranial nerve). Both neuronal and sensory lineages are thought to be specified early in the neural-sensory competent domain (NSD) of the ear rudiment. First, neuroblasts delaminate from the NSD to form neurons of the CVG. Then, cells remaining in the NSD adopt a sensory fate and develop into various sensory organs. The molecular mechanisms that specify neuronal and sensory cell fates are unclear. The aim of this dissertation is to provide a better understanding by examining the roles of the HMG (high mobility group)-box containing transcription factors Sox2 and Sox3 in developing chicken inner ears using gain and loss-of-function approaches. Over-expression of Sox2 in ovo readily induces Neurogenin 1(Ngn1) expression, an important gene required for the neurogenic fate. Nevertheless, neurogenesis fails to proceed based on the lack of Neurod1 up-regulation and consequently the size of the CVG is reduced. In contrast, over-expression of Ngn1 is capable of up-regulating Neurod1 and causes increased neuroblast formation, as well as Sox2 down-regulation. Similar increases in neurogenesis are obtained with over-expression of Neurod1. I provide evidence that Ngn1 and Neurod1 inhibit Sox2 transcription via the E-box of the nasal-otic placode specific enhancer 1 (NOP-1) within the Sox2 promoter. On the other hand, loss of Sox2 function paradigms did not result in loss of Ngn1 expression, suggesting that other factors may be required to induce Ngn1 normally. Furthermore, while Sox3 has been proposed to be up-stream of Sox2, it does not induce Ngn1 in a similar manner as Sox2. Taken together, my results suggest that Sox2 and likely other factors are involved in initiating neurogenesis by up-regulating Ngn1. The up-regulated Ngn1, in turn, down-regulates Sox2 expression and up-regulates Neurod1 to mediate progression of neurogenesis. Finally, I show that Sox2 and the Notch signaling pathway interact to specify neuronal and sensory cell fate choices.