Spatial positioning of preimplantation mouse embryo cells is regulated by mTORC1 and m ⁷ G-cap-dependent translation at the 8- to 16-cell transition

Abstract

Preimplantation mouse embryo development involves temporal–spatial specification and segregation of three blastocyst cell lineages: trophectoderm, primitive endoderm and epiblast. Spatial separation of the outer-trophectoderm lineage from the two other inner-cell-mass (ICM) lineages starts with the 8- to 16-cell transition and concludes at the 32-cell stages. Accordingly, the ICM is derived from primary and secondary contributed cells; with debated relative EPI versus PrE potencies. We report generation of primary but not secondary ICM populations is highly dependent on temporal activation of mammalian target of Rapamycin (mTOR) during 8-cell stage M-phase entry, mediated via regulation of the 7-methylguanosine-cap (m 7 G-cap)-binding initiation complex (EIF4F) and linked to translation of mRNAs containing 5′ UTR terminal oligopyrimidine (TOP-) sequence motifs, as knockdown of identified TOP-like motif transcripts impairs generation of primary ICM founders. However, mTOR inhibition-induced ICM cell number deficits in early blastocysts can be compensated by the late blastocyst stage, after inhibitor withdrawal; compensation likely initiated at the 32-cell stage when supernumerary outer cells exhibit molecular characteristics of inner cells. These data identify a novel mechanism specifically governing initial spatial segregation of mouse embryo blastomeres, that is distinct from those directing subsequent inner cell formation, contributing to germane segregation of late blastocyst lineages.

The translational oscillation in oocyte and early embryo development

“…This is consistent with our finding that eEF2 is dephosphorylated and active during the M-phases of meiosis and mitosis, while the mTOR-eIF4F translation pathway is highly activated during NEBD ( 14 ). Such activation disappears post-fertilization and reactivates in the 8- to 16-cell stage mouse embryo ( 41 ). Moreover, the dominant activity of mTOR1, RPS6, eIF4E, and ERK1/2 occurs in the oocyte, which correlates with the highest observed levels of global translation.…”