The Roles of Sox2 and Sox3 During Otic and Epibranchial Development in Zebrafish
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Sox2 and Sox3 are SoxB1 transcription factors that act redundantly in the neural ectoderm, but also are uniquely required in other systems. To address functions of sox2 and sox3 in otic and epibranchial development, we generated knockouts and heat shock-inducible transgenes in zebrafish. Expression of sox3 is one of the earliest markers of Fgf-dependent otic/epibranchial placode induction. We found that sox2 is also expressed in the early placode. Mutant analysis, and low-level misexpression, showed that sox2 and sox3 act redundantly to establish a full complement of otic/epibranchial cells. Disruption of pax8, another early regulator, caused similar placodal deficiencies to sox3 mutants or pax8-sox3 double mutants, suggesting that sox3 and pax8 operate in the same pathway. High-level misexpression of sox2 or sox3 during early stages cell-autonomously blocked placode induction, whereas misexpression several hours later could not reverse placodal differentiation. In an assay for ectopic placode-induction, partial knockdown of sox3 significantly enhanced ectopic induction of pax8, whereas full knockdown of sox3 inhibited this process. These findings show that sox2 and sox3 are together required for proper otic and epibranchial induction, but the level of expression must be tightly regulated to avoid suppression of differentiation and maintenance of pluripotency. Mutant analysis at later stages of otic development showed that sox2 and sox3 are uniquely required for sensory development and otic neurogenesis respectively. Moderate misexpression of sox2 during placodal stages led to development of otic vesicles with expanded sensory and reduced neurogenic domains. However, high-level misexpression of sox2 or sox3 expanded both sensory and neurogenic domains, filling the medial and lateral halves of the otic vesicle, respectively. Disruption of medial factor pax2a eliminated the ability of sox2/3 misexpression to expand sensory but not neurogenic domains. Additionally, mild misexpression of fgf8 during placodal development was sufficient to specifically expand the zone of prosensory competence. Later, cross-repression between atoh1a and neurog1 helps maintain the sensory-neural boundary. We also show that sox2 and sox3 exhibit intrinsic differences in promoting sensory vs. neural competence, but at high levels they can mimic each other to enhance both states. Regional cofactors like pax2a and fgf8 also modify sox2/3 functions.
Gou, Yunzi (2019). The Roles of Sox2 and Sox3 During Otic and Epibranchial Development in Zebrafish. Doctoral dissertation, Texas A&M University. Available electronically from