Epigenetic and transcriptional regulation of early mouse development.

The development of a multicelluar organism requires tightly controlled activation and repression of genes, while pluripotent stem cells acquire a lineage-specific fate. Therefore, different mechanisms have evolved especially on the level of DNA and chromatin modification. To investigate whether active DNA demethylation mediated by the recently identified Elongator complex is essential for early embryonic development, the Elongator subunit Elp4 was deleted. This resulted in stunted development and embryonic lethality. Formation of the three principal germ layers indicates that differentiation is initiated imperfect and thus leads to growth retardation. In vitro analysis revealed impaired differentiation and reduced proliferative capacity of mutant embryonic stem cells. Therefore, decreased progenitor recruitment or expansion could result in stunted embryonic development. Transcriptome analysis suggests that the Elongator complex is involved in genome-wide chromatin organisation likely by regulation of DNA methylation of developmental regulators, such as differentiation-associated and imprinted genes. In a second part, the role of canonical Wnt signalling in embryonic pattering and endoderm formation was analysed. Conditional gene deletion of β-catenin in the Sox17+ visceral and definitive endoderm resulted in anterior head and posterior neural tube truncations. In the anterior and posterior visceral endoderm, β-catenin is required non-cell autonomously for head and tail organizer formation, respectively. In the definitive endoderm, β-catenin is required cell-autonomously for mid- and hindgut development. Deletion of β-catenin in the endoderm causes loss of Sox17 expression suggesting its regulation by canonical Wnt signalling. This is supported by accumulation of Tcf4/β-catenin transactivation complexes on binding elements in the Sox17 regulatory region upon endoderm differentiation. Taken together, these results indicate that Wnt/β-catenin activates Sox17 expression for visceral endoderm patterning and definitive endoderm formation. In a third part, we generated a hormone-inducible Sox17Cre mouse line. Analysis of recombination characteristics of Sox17CreERT2 demonstrated that Cre-mediated recombination can be selectively induced in extra-embryonic and embryonic endoderm, vascular endothelial cells, hemogenic endothelium and in the ventral pancreas. Thus, this new Sox17CreERT2 mouse line can be used for tissue-specific genetic lineage tracing as well as for lineage-restricted genetic manipulations to investigate the impact of signalling cascades or factors during later stages of endoderm development