Function of Oct91 and SoxB1 proteins during neural development in Xenopus laevis
Archer, Tenley C.
Thesis (Ph.D.)--Georgetown University, 2009.; Includes bibliographical references. Neural progenitor identity in vertebrates is regulated in part by the three members of the SoxB1 subgroup (Sox1, Sox2 and Sox3). In this work we compare and contrast the regulation and function of three SoxB1 proteins and a putative partner, Oct91, and how the regulation and function of these genes compares or contrasts to higher vertebrates.; Sox3's zygotic expression begins pan-ectodermally and becomes restricted to the neuroectoderm via the interplay of three cis-regulatory modules: an enhancer module containing two unidentified forkhead transcription factor binding sites is necessary for induction of sox3; a repressor module containing Vent transcription factor binding sites inhibits sox3 expression in the non-neural ectoderm; and a second enhancer module containing binding sites for mediators of FGF signaling is necessary for the maintenance of sox3 in the neuroectoderm.; Since Sox1, Sox2 and Sox3, the three members of the SoxB1 subgroup, have similar expression patterns, protein structures and functions, it has been suggested that they have redundant roles in neural development. Here we show that Sox1, Sox2, Sox3, and Oct91, the Xenopus homolgue of Oct4, have overlapping and distinct roles in inducing neural progenitor identity. In naive ectoderm using ectodermal explant assays, Sox1, Sox2, Sox3 each induce a unique array neural genes markers but only Sox1 is sufficient for neuronal differentiation. Sox2 and Sox3 function independently of each other, yet Sox1 induces expression of both sox2 and sox3. While Sox2 induces limited neural progenitor markers, Sox3 induces a broad profile of neural progenitor and pro-neural markers and this may be due to its unique induction of FGF8 expression. Sox3-VP16, a dominant activator form of Sox3, does induce neuronal differentiation and sox2 expression, which indicates that Sox3 requires the presence of a co-factor for neuronal differentiation. Only a few co-factors of the Sox proteins have been identified and therefore their function in combination with a partner has not been tested in early development of the nervous system. In naive ectoderm, Oct91 induce sox1-3 expression and induces neuronal differentiation. Sox2 co-injected with Oct91 did not interfere with its ability to induce neural markers. However, the addition of Sox1 or Sox3 abolished the ability of Oct91 to induce any neural markers even though epidermis was still repressed. Together these data indicate that Sox2 and Oct91 function to maintain neural progenitor identity.; Transgenic embryos were generated using a bicistronic expression vector to determine if transient overexpression of the SoxB1s delays neuronal differentiation due to loss of the mRNA over time. Constitutively-active expression of Sox1 or Sox2 transgenic embryos represses neuronal differentiation indefinitely while also increasing cell proliferation. Constitutively-active Sox3 leads to cell death, which may explain why Sox3 is not expressed in neural cancers but Sox2 is found in gliomas.; Identifying conserved regulatory and functional elements of SoxB1 and Oct91 proteins in different organisms gives evidence for a neural transcription code. Alternatively, from the differences we can learn how the central nervous systems of different species changed over time and may have evolved. I propose a model for mammalian evolution in which the signals required to induce and maintain neural progenitor populations in lower vertebrates were co-opted in mammals to balance against imprinting of the paternal genome, which drives formation of the placental tissues thereby allowing the embryo to imbed internally.
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