Evolution of the mammalian embryonic pluripotency gene regulatory network

Abstract

Embryonic pluripotency in the mouse is established and maintained by a gene-regulatory network under the control of a core set of transcription factors that include octamer-binding protein 4 (Oct4; official name POU domain, class 5, transcription factor 1, Pou5f1), sex-determining region Y (SRY)-box containing gene 2 (Sox2), and homeobox protein Nanog. Although this network is largely conserved in eutherian mammals, very little information is available regarding its evolutionary conservation in other vertebrates. We have compared the embryonic pluripotency networks in mouse and chick by means of expression analysis in the pregastrulation chicken embryo, genomic comparisons, and functional assays of pluripotencyrelated regulatory elements in ES cells and blastocysts. We find that multiple components of the network are either novel to mammals or have acquired novel expression domains in early developmental stages of the mouse. We also find that the downstream action of the mouse core pluripotency factors is mediated largely by genomic sequence elements nonconserved with chick. In the case of Sox2 and Fgf4, we find that elements driving expression in embryonic pluripotent cells have evolved by a small number of nucleotide changes that create novel binding sites for core factors. Our results show that the network in charge of embryonic pluripotency is an evolutionary novelty of mammals that is related to the comparatively extended period during which mammalian embryonic cells need to be maintained in an undetermined state before engaging in early differentiation events. E mbryonic pluripotency is an essential property of a small group of cells of the mammalian blastocyst which transiently keeps them in an indeterminate, uncommitted state. This condition is a consequence of the earliest differentiation events taking place in the preimplantation embryo. The first lineage decision produces the inner cell mass (ICM) and the trophectoderm (TE). The TE produces most of the extraembryonic structures, mainly the placenta. In the second lineage choice, the ICM gives rise to the epiblast (EPI), which will generate the embryo proper, and the primitive endoderm (PE), another extraembryonic tissue (1).Cells in the ICM and the EPI retain the full potential to develop into all embryonic tissues and germ layers and are the source of ES cells, which have the capacity of indefinite self-renewal and maintenance of pluripotency in tissue culture. Pluripotency results from the expression of a small network of transcription factors that actively maintain the undetermined state and at the same time repress the differentiation program (2). The core members of this network are the products of the Oct4; official name, Pou5f1, Nanog, and Sox2 genes. These factors act together through autoand cross-regulatory interactions and also through direct and overlapping binding to multiple locations throughout the genome, where they regulate downstream target genes (3-5).Thus far, little is known about the evolutionary conservation of the embryonic plu...

Pluripotency and lineages in the mammalian blastocyst: An evolutionary view

“…3,4 Using the mouse embryo, with the chick as a non-mammalian amniote model for comparison, we found that most of the core regulators of ICM and TE fates are present in all amniotes but their discrepant expression patterns make it difficult to connect them to the establishment of blastocyst lineages and embryonic pluripotency. Furthermore, sequence comparisons and transgenic assays revealed that genomic regions bound by core pluripotency factors are not conserved between chick and mammals, suggesting that the evolution of embryonic pluripotency was driven at least in part by the appearance of new regulatory elements in mammals.…”

“…38,39 Expression analysis of orthologs of other embryonic pluripotency GRN components showed that they were not expressed until post-gastrulation stages in the chick. 3 Thus the orthologs of genes that maintain embryonic pluripotency and sustain the early lineages in mouse are not expressed in equivalent territories of the pre-gastrulation chick embryo (Fig. 3).…”

“…Since, as we have seen, homologs of the core pluripotency factors are present in non-mammalian vertebrates, we examined a set of downstream targets of these factors in mouse 3 to determine if they show enrichment in "younger" genes ( Fig. 1).…”

“…Comparison with orthology maps for other pluripotency genes (such as Nanog; Fig. 2), even those for which local synteny is not preserved (Dppa1, Rex1), 3 indicates that there has not been a general loss of embryonic pluripotency GRN components in avians. This analysis thus shows that the true Oct4 ortholog was lost in avians, and that the duplication of the ancestral Pou5 gene did not coincide with the emergence of mammals.…”

Embryonic Stem Cells: Introducing Exogenous Regulators into Embryonic Stem Cells

“…On the other hand, the cells isolated within the blastoceol become inner cell mass (ICM) (Johnson and Mcconnell, 2004). ICM is a mass of cells having pluripotency and its cells can differentiate into all cell types of adult and extraembryonic membranes (Fernanez-Tresquerres et al, 2010). In specific in vitro conditions, ICM cells can keep their potency and maintain their proliferation ability (Yamanakaet al, 2006).…”