Sox2 and inner ear development

Abstract

Sox2, a HMG box transcription factor, is well known for its role in stem cell maintenance, iPS (induced pluripotent stem cell) induction, and development of neural tissues such as central nervous system and sensory organs. Sox2 has been demonstrated to be essential for the development of inner ear sensory patches. It has been shown that Sox2 is under the regulation of multiple regulatory elements to obtain a tissue specific manner.

Two allelic mouse mutants, yellow submarine (Ysb) and Light coat and circling (Lcc) show hearing and balance impairments with different severity. They were made by random insertions of a transgene (pAA2) and X-ray irradiation respectively. Ysb and Lcc are both localized to chromosome 3 and involve complex chromosomal rearrangements. The Sox2 coding region is intact in the mutants, while the Sox2 expression in the otocyst is greatly reduced in Ysb and totally lost in Lcc, which indicates the tissue specific reduction of Sox2 may be due to the rearrangement of Sox2 regulatory element(s). Since Sox2 null mutants die before implantation, the two Sox2 inner ear mutants are valuable models for studying Sox2 knock down (Ysb) and Sox2 knock out (Lcc) condition in the inner ear. To understand the molecular basis behind Sox2 regulation in the inner ear, this project aims to identify the Sox2 otic regulatory elements, and potential Sox2 downstream targets involved in the development of inner ear.

Previous work has indicated that Nop1 and Nop2 are the otic specific regulatory elements of Sox2 in chicken ear. In this project, transgenic mice were generated using Nop1-Nop2, and the result showed Nop1-Nop2 could drive Sox2 expression to the dorsal side of the otiv vesicle, which is different from the endogenous Sox2 expression pattern. Therefore, Nop1 and Nop2 may require other regulatory element(s) to gain a correct regulatory pattern. BAC(RP23-335P23), which contained the DNA sequences close to Ysb integration site 1 was also been tested in transgenic mice. Interestingly, the result showed that BAC(RP23-335P23) could drive Sox2 expression to the ventral side of the otic vesicle, indicating that this BAC may contain the Sox2 otic regulatory element(s).

In this project, the binding relationship between Sox2 protein and Math1 enhancer has also been identified using chromatin immunoprecipitation (Ch-IP). Results showed that Sox2 could bind to Math1 enhancer A in the inner ear cochlea. So Sox2 may regulate Math1 through binding to Math1 enhancer A in inner ear development.

Using a bioinformatics approach, potential Sox2 target genes in inner ear development have been identified from public microarray data on E9 to E15 inner ear tissue by the presence of conserved Sox2 binding sites. Among these potential targets, 4 genes (Itga6, Erbb3, Sox10 and Mycn) have been selected based on their known functions. Their expression patterns in the cochlea of wild type, Ysb and Lcc were verified. The identification of Sox2 downstream target genes using a bioinformatics approach will help us to understand the molecular basis of Sox2 regulation, and also understand the role of Sox2 in the inner ear development.