A Mesodermal Factor, T (BRACHYURY), specifies mouse germ cell fate by directly activating germline determinants.

The germ cell lineage ensures reproduction and heredity in metazoans. Primordial germ cells (PGCs) in mice are induced in pluripotent epiblast cells by BMP4 and WNT3, yet their mechanism of action remains elusive. Here, using in vitro PGC specification system, we show that WNT3, but not BMP4, induces many transcription factors associated with mesoderm in epiblast-like cells (EpiLCs) through beta-CATENIN. Among these, T (BRACHYURY), a classical and conserved mesodermal factor, was essential for robust activation of Blimp1 and Prdm14, two of the germline determinants. T, but not SMAD1 or beta-CATENIN/TCF1, binds distinct regulatory elements of both Blimp1 and Prdm14, and directly up-regulates these genes without BMP4 and WNT3. Without BMP4, a program induced by WNT3 prevents T from activating Blimp1 and Prdm14, demonstrating that BMP4 is permissive for PGC specification. These findings establish a fundamental role of a mesodermal gene in PGC specification, a potentially evolutionarily conserved mechanism across metazoans. Wnt3 (+/+) and Wnt3 (-/-) mouse embryonic stem cells (mESCs) bearing the BV transgenes expressing membrane-targeted Venus under the control of Blimp1 regulatory elements (Blimp1-mVenus: BV; PMID 18583473) were established and maintained. Epiblast-like cells (EpiLCs) were then induced from the established ES lines, followed by the cytokine stimulation (No cytokines [negative control], BMP4, Wnt3a, BMP4+Wnt3a) for different periods (0h, 12h, 24h, 36h).