NANOG and CDX2 pattern distinct subtypes of human mesoderm during exit from pluripotency.

Mesoderm is induced at the primitive streak (PS) and patterns subsequently into mesodermal subtypes and organ precursors. It is unclear whether mesoderm induction generates a multipotent PS progenitor or several distinct ones with restricted subtype potentials. We induced mesoderm in human pluripotent stem cells with ACTIVIN and BMP or with GSK3-ß inhibition. Both approaches induced BRACHYURY(+) mesoderm of distinct PS-like identities, which had differing patterning potential. ACTIVIN and BMP-induced mesoderm patterned into cardiac but not somitic subtypes. Conversely, PS precursors induced by GSK3-ß inhibition did not generate lateral plate and cardiac mesoderm and favored instead somitic differentiation. The mechanism of these cell fate decisions involved mutual repression of NANOG and CDX2. Although NANOG was required for cardiac specification but blocked somitic subtypes, CDX2 was required for somitic mesoderm but blocked cardiac differentiation. In sum, rather than forming a common PS progenitor, separate induction mechanisms distinguish human mesoderm subtypes.

Distinct activities of the anaphase-promoting complex/cyclosome (APC/C) in mouse embryonic cells.

The first differentiation event in mammalian development gives rise to the blastocyst, consisting of two cell lineages that have also segregated in how the cell cycle is structured. Pluripotent cells of the inner cell mass divide mitotically to retain a diploid DNA content, but the outer trophoblast cells can amplify their genomes more than 500-fold by undergoing multiple rounds of DNA replication, completely bypassing mitosis. Central to this striking divergence in cell cycle control is the E3 ubiquitin-ligase activity of the anaphase-promoting complex or cyclosome (APC/C). Extended suppression of APC/C activity during interphase of mouse pluripotent cells promotes rapid cell cycle progression by allowing stabilization of cyclins, whereas unopposed APC/C activity during S phase of mouse trophoblast cells triggers proteasomal-mediated degradation of geminin and giant cell formation. While differential APC/C activity might govern the atypical cell cycles observed in pre-implantation mouse embryos, geminin is a critical APC/C substrate that: (1) escapes degradation in pluripotent cells to maintain expression of Oct4, Sox2 and Nanog; and (2) mediates specification and endoreduplication when targeted for ectopic destruction in trophoblast. Thus, in contrast to trophoblast giant cells that lack geminin, geminin is preserved in both mouse pluripotent cells and non-endoreduplicating human cytotrophoblast cells.