The organ of Corti arises from a population of cells, referred to as prosensory cells, that are located within the inner ear. Based on existing data, prosensory cells are believed to be uniquely competent to develop as both hair cells and associated supporting cells. However, the factors that specify the prosensory domain remain unknown. Recent data has suggested that the notch signaling pathway could play a role in prosensory specification. The notch pathway is a highly conserved signaling cascade that is utilized in multiple tissues and in multiple species including flies, worms, mice and humans. The mammalian genome contains four notch genes, each of which can partially compensate for the loss of any other notch gene. Therefore, to determine the complete role of notch signaling in prosensory formation, we used a cre-lox approach to specifically delete Rbp-j (a key component of all notch signaling) in the inner ear. Results indicated that deletion of notch signaling has a profound effect on prosensory formation that includes the loss of nearly all inner ear sensory cells. However, a thorough examination of the inner ears from these mice indicated that some prosensory cells still form in the absence of notch, but that these cells fail to maintain a prosensory identity and subsequently lose the ability to develop as hair cells and supporting cells. As part of the study described above, we examined overall changes in gene expression in normal and Rbp-j deleted inner ears. Among the genes that were down-regulated under these circumstances were members of the insulin-like growth factor signaling pathway. To determine whether these genes play a role in the regulation of prosensory identity by notch signaling, we characterized the expression patterns for both the insulin-like growth factors (Igf1 and Igf2), the insulin-like growth factor receptors (Igfr1 and igfr2) and the insulin-like growth factor binding proteins (Igfbp1-5). Each of these genes was found to be expressed in a specific region of the developing inner ear. Based on the patterns of expression, we used Rnai to down-regulate Igfbp5 expression in explant cultures of the inner ear. Results indicated specific activation of the notch pathway, suggesting that a reciprocal signaling loop exists between notch and Ifgbp5. The specific effects of Igfb5 are in the process of being determined. Finally, previous work from our laboratory had demonstrated that Sox2, a transcription factor involved in prosensory specification also had the ability to induce some non-sensory cells within the cochlea to develop as neurons. This finding lead us to examine whether other transcription factors that are known to be expressed in developing inner ear neurons might have a similar ability. In particular, two transcription factors, Neurogenin1 and NeuroD1, were expressed in non-neuronal cells within the inner ear using gene transfer. Both of these factors were able to induce the expression of neuronal markers and a neuronal phenotype. Moreover, electrophysiological assessments of these cells indicated that they expressed ion channels and electrical characteristics consistent with neuronal phenotypes. These results suggest that it may be possible to induce the formation of inner ear neurons using gene transfer. Such an approach could be used to develop therapies for the induction of new inner ear neurons in individuals in which these cells have been lost as a result of genetic mutation or trauma.
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