A mesodermal factor, T (Brachyury), specifies mouse germ cell fate by directly activating germline determinants. Mus musculus

Germ cells ensure reproduction and heredity in metazoans. Primordial germ cells (PGCs) in mice are induced in pluripotent epiblasts by BMP4 and WNT3, yet their mechanism of action remains unclear. Here, using in vitro PGC specification system, we show that WNT3 induces many transcription factors associated with mesoderm in epiblast-like cells (EpiLCs) through b-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 b-CATENIN/TCF1, binds distinct regulatory elements of both Blimp1 and Prdm14, and without BMP4 and WNT3, directly up-regulates these genes, consequently delineating downstream PGC program. 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 transcription factor in PGC specification, a potentially evolutionarily conserved mechanism across metazoans. Overall design: Genome target search for transcription factors, SMAD1, T, and TCF1 on PGC-like cells