The primitive endoderm lineage of the mouse blastocyst: Sequential transcription factor activation and regulation of differentiation by Sox17

Abstract

Cells of the primitive endoderm (PrE) and the pluripotent epiblast (EPI), the two lineages specified within the inner cell mass (ICM) of the mouse blastocyst stage embryo, are segregated into adjacent tissue layers by the end of the preimplantation period. The PrE layer which emerges as a polarized epithelium adjacent to the blastocoel, with a basement membrane separating it from the EPI, has two derivatives, the visceral and parietal endoderm. In this study we have investigated the localization of two transcriptional regulators of the SOX family, SOX17 and SOX7, within the PrE and its derivatives. We noted that SOX17 was first detected in a salt-and-pepper distribution within the ICM, subsequently becoming restricted to the nascent PrE epithelium. This dynamic distribution of SOX17 resembled the localization of GATA6 and GATA4, two other PrE lineage-specific transcription factors. By contrast, SOX7 was only detected in PrE cells positioned in contact with the blastocoel, raising the possibility that these cells are molecularly distinct. Our observations support a model of sequential GATA6 > SOX17 > GATA4 > SOX7 transcription factor activation within the PrE lineage, perhaps correlating with the consecutive periods of cell lineage ‘naïvete’, commitment and sorting. Furthermore our data suggest that co-expression of SOX17 and SOX7 within sorted PrE cells could account for the absence of a detectable phenotype of Sox17 mutant blastocysts. However, analysis of implantation-delayed blastocysts, revealed a role for SOX17 in the maintenance of PrE epithelial integrity, with the absence of SOX17 leading to premature delamination and migration of parietal endoderm.

Pluripotency in the Embryo and in Culture

“…Hypoblast markers, such as platelet-derived growth factor receptor a (PDGFRa) and Sox17, also initially show heterogeneous expression in the early blastocyst. Cells expressing these markers appear to sort out from Nanog-positive cells and if they end up located adjacent to the cavity will up-regulate additional transcriptional determinants Gata4 and Sox7 and become committed to hypoblast differentiation (Artus et al 2011).…”

Conversion from mouse embryonic to extra-embryonic endoderm stem cells reveals distinct differentiation capacities of pluripotent stem cell states

“…In all, this demonstrates that cXEN cells are functionally and molecularly equivalent to embryo-derived XEN cells and are distinct from the mESCs from which they were derived. cXEN derivation requires expression of Gata6, Gata4, Sox17 and Pdgfra and downregulation of Nanog In PrE development, Gata6 is downstream of Grb2, an effector of the FGF-signalling pathway, and upstream of Gata4, Pdgfra and Sox17 (Chazaud et al, 2006;Artus et al, 2011;Plusa et al, 2008;Artus et al, 2010;Niakan et al, 2010). Although PrE cells are established in Gata6 mutant embryos, their further ExEn differentiation is compromised (Morrisey et al, 1998; Koutsourakis…”

Destabilization of pluripotency in the absence of Mad2l2

“…2A). Since the expression of Gata6 transcription factor, as a marker for primitive endoderm precursors, precedes the expression of Sox17 and Gata4, [30][31][32][33] we further focused on these 2 last markers to monitor commitment to the primitive endoderm lineage. Western blot analysis showed prominent levels of Sox17 and Gata4 proteins in Mad2l2 ¡/¡ ESC cultures (Fig.…”