The first round of embryo differentiation is accompanied by changes in the patterns of transcription in the resulting trophectoderm (TE) and inner cell mass (ICM). Two epigenetic modifiers show highly reciprocal patterns of gene expression: Prdm14 (high in ICM, low in TE) and Dnmt3b (high in TE, low in ICM), leading to a hypothesis that PRDM14 acts as a negative regulator of DNMT3B 1. Analysis of DNMT3B protein levels show it follows its gene expression pattern 2. In this study, we examined: (1) PRDM14 across preimplantation development; and (2) the effect of PRDM14 knock-down with morpholino oligonucleotides on embryo development.
The pro-nuclei of zygotes showed a low level of PRDM14. Staining increased ~ 340-fold in 2-cell embryos. It was present throughout the cytoplasm but accumulated within the nuclei. From the 4-cell to blastocyst stages cytoplasmic staining decreased and antigen became predominantly localized to the nuclei. Staining was present in the nuclei of both the TE and ICM, contrary to gene expression studies1. Treatment with anti-PRDM14 morpholino oligonucleotides for 72h resulted in > 80% loss of PRDM14. Despite this extensive knockdown, embryos still developed into the blastocysts without any notable phenotypical difference from controls.
Our results show that the reciprocal expression of Prdm14 and Dnmt3b in blastocysts is not recapitulated at the protein level. The work does not provide evidence for PRDM14 being differentially allocated to the ICM, and by inference does not support it having a direct role in the differential expression of Dnmt3b. The failure of PRDM14 knockdown to affect blastocyst formation is inconsistent with reports of this epigenetic modifier having a primary role in committing cells to the ICM lineage.
1 Burton, A. et al. Cell Reports 5, 687-701 (2013).
2 Li, Y., Seah, M. K. Y. & O'Neill, C. Reproduction 151, 83-95 (2016).