Maternal Med12 safeguards trophoblast pluripotency and placental development
For a brief but critical period post-fertilization, the mammalian embryo is entirely dependent on maternal products inherited from the oocyte. Previous research showed that oocyte-specific loss of Med12, an X-linked gene and Mediator complex subunit, leads to female sterility despite normal folliculogenesis and ovulation. Here, we show that loss of maternal Med12 has minimal effect on the oocyte transcriptome and does not manifest in embryonic lethality until post-implantation. Implants derived from Med12­-null oocytes demonstrate abnormal placentation at E9.5, with an overabundance of trophoblast giant cells (TGC). This phenotype associates with downregulation of trophoblast pluripotency markers (e.g. Cdx2) and activation of drivers of TGC identity (e.g. Stra13) in the E7.5 extraembryonic ectoderm, revealing a previously undescribed role for Med12 in trophoblast pluripotency maintenance. Notably, we find consistently low Med12 expression in embryos derived from Med12-null oocytes, potentially due to programmed paternal X chromosome inactivation (XCI). To isolate the consequences of maternal Med12 depletion, we introduced an autosomal Med12 transgene and show that embryonic expression of the transgene rescues development of Med12-null oocytes. We conclude that oocyte-specific deletion of Med12 produces a maternal-zygotic double knock-out in extraembryonic tissues due to paternal XCI, leading to loss of pluripotency in the trophoblast, placental malformation, and embryonic death. Overall design: To investigate the role of oocyte-derived Med12 in mouse oogenesis and embryogenesis, we used a conditional knock-out strategy via Zp3-Cre-mediated recombination to ablate Med12 from oocytes. We then performed comparative gen expression profiling using data obtained from RNA-seq of growing oocytes from 12-day-old females (D12), germinal vesicle (GV) oocytes, metaphase-II stage (MII) oocytes, and E7.5 embryonic tissues from individual implants (ectoplacental cone - EPC, extraembryonic ectoderm - ExE, epiblast - EPI) collected from either (1) Zp3-Cre;Med12fl/fl dams, in which Med12 is ablated from oocytes ("Med12KO"); or  (2)  Med12fl/fl control ("WT") females. Embryonic implants were collected after timed-mating of experimental dams with wild-type male studs of proven fertility.