Assessing the Function of SOX21 in Xenopus laevis Neurogenesis: Exploring Homeolog Sub-Functionalization

Abstract

Neurogenesis is a tightly controlled developmental process through which neural progenitor cells progress to committed neurons. While the SoxB1 transcription factors are well characterized to have functions in the induction and maintenance of neural progenitors, the involvement of SoxB2 proteins in neurogenesis is not as well understood. SoxB2 protein Sox21 has been shown to repress SoxB1 function and promote differentiation, yet also be required for maintenance of progenitors, depending on the context of the analysis. Previous work from our lab looked to Xenopus to assess the role of Sox21 in neurogenesis and resolved that like many Sox proteins, the role of Sox21 is a function of the spatiotemporal context and expression levels. Sox21 has been revealed to be one of many genes which were retained following the polyploidization event in Xenopus laevis evolution, which led us to investigate differences between Sox21.S and Sox21.L in gastrula and neurula embryos. Our gain-of-function studies showed that Sox21.L is more effective at promoting differentiation in gastrula and has regional sub-functionalization in neurula embryos. These analyses also demonstrate that sox21 expression levels are finely controlled via a novel homeolog-specific autonomous feedback loop. Given all Sox proteins recognize and bind the same DNA sequence, it is accepted that cooperation with partner proteins help to dictate their DNA-binding specificity at target gene loci. Here we generated constructs toward building a better understanding of how Sox21 physically interacts with various partners, along with how post-translational modifications of Sox proteins affect their capacity to interact. With these data, we present additional information regarding Sox21 function in neurogenesis, including alloallelic spatiotemporal specificity. Additionally, this work presents one of the first cases of homeolog-specific semi-autonomous gene regulation within Xenopus laevis. Furthermore, we developed additional tools to further investigate the homeolog-specific functions of Sox21 across development.