Metastable primordial germ cell-like state induced from mouse embryonic stem cells by Akt activation

Specification to primordial germ cells (PGCs) occurs under the mesoderm induction signals during gastrulation. Here, we found that Akt activation in embryonic stem (ES) cells generated self-renewing spheres during mesodermal differentiation induction and that the differentiation status of the sphere cells was in between ES cells and PGCs. Essential regulators for PGC specification and their downstream germ cell-specific genes were expressed in the spheres, showing that the cells of the sphere commenced the differentiation to germ lineage. However, the spheres could not proceed to spermatogenesis after transplantation to testes. Meanwhile, the transfer of the spheres to the original feeder-free ES cell culture conditions induced chaotic differentiation. In contrast, when the spheres were cultured on mouse embryonic fibroblasts or in the presence of ERK-cascade and GSK3 inhibitors, the reversion to the ES cell-like cell states was induced. These results indicate that the Akt signaling brings about a novel metastable and pluripotent state between ES cells and PGCs. Five samples were analyzed, which included the Akt-Mer-expressing ES cell (ESC) line #21 treated with or without 4OHT (4-hydroxytamoxifen), the #21 ESC-derived primordial germ cell (PGC)-like sphere cells and the ESC-like cells reverted from #21 PGC-like sphere cells. The PGC-like sphere cells derived from another Akt-Mer ESC line #42 was also examined.