Sox2 in Embryonic Stem Cells and Lung Development

Abstract

Sox2 is a fascinating transcription factor with multiple roles during embryonic development. In early embryonic development, Sox2 is one of the key transcription factors in the maintenance of the pluripotent status of the cells of the inner cell mass (ICM). Sox2 is also in the transcription factor network that controls the self-renewal and pluripotency of embryonic stem (ES) cells. Later during development, the expression pattern of Sox2 in the developing lung epithelium may suggest a role of Sox2 in lung development. In this thesis we focus on three aspects of Sox2, its function in lung development, its nuclear import and its interacting partners in embryonic stem cells. Chapter 1 gives an overall introduction of the basics of lung development, which is helpful to understand the study presented in the chapter 3 of this thesis. I start with a description of the mature respiratory system, the phases of lung development and the different cell types of the respiratory epithelia. Subsequently, the molecular control underlying the specification of the foregut, lung bud formation and branching morphogenesis is discussed in more detail. The chapter finishes with a description of some of the factors involved in the molecular control of lung epithelium differentiation. Chapter 2 describes the characteristics of the Sox family of transcription factors, their classification within the HMG superfamily, their proteins-protein interactions and their nuclear transport properties. I also describe the expression pattern of Sox2 and its function during mouse development. The second part of this chapter gives an overview of the role of Sox2 in ES cells and in the generation of iPS cells. Chapter 3 describes the effect of Sox2 ectopic expression on lung development. Transgenic mice were generated that express Sox2 in the airway epithelium, in a doxycycline-inducible manner. The results showed that continuous expression of Sox2 disrupts the normal branching morphogenesis, suggesting that downregulation of Sox2 is necessary for the airway epithelium to respond to branch inducing signals. Furthermore, Sox2 ectopic expression does not lead to a complete abrogation of the epithelial differentiation program but prematurely drives cells into committed progenitors. Chapter 4 reports the identification of Exportin 4 (Exp4) as the nuclear import factor for Sox2 and Sry. It also presents two alternative nuclear import pathways for Sox2, mediated by Imp9 and the Impβ/7 heterodimer. These data suggest that several parallel import pathways facilitate the nuclear import of Sox proteins, which makes import regulation at the level of the import receptor unlikely. Chapter 5 presents a biochemical approach to identify Sox2 interacting proteins in mouse ES cells. Affinity-purified FLAG-tagged Sox2 and its interacting proteins were analyzed by mass spectrometry. The list of putative Sox2 partners contains ES cell self-renewal factors, chromatin remodelling factors, lineage-specific transcriptions factors and Exportin 4. These results are discussed in relation to recent studies performed in ES cells that reported the target genes of Sox2, the promoter co-occupancy with other pluripotency factors, including Oct4 and Nanog, and the effect on gene expression of Sox2 depletion. Chapter 6 is a general discussion of the studies carried out in this thesis and the possible future directions.