Smad4is essential for epiblast scaling and morphogenesis after implantation, but nonessential prior to implantation in the mouse

Abstract

AbstractBone Morphogenic Protein (BMP) signaling plays an essential and highly conserved role in axial patterning in embryos of many externally developing animal species. However, in mammalian embryos, which develop inside the mother, early development includes an additional stage known as preimplantation. During preimplantation, the epiblast lineage is segregated from the extraembryonic lineages that enable implantation and developmentin utero. Yet, the requirement for BMP signaling in mouse preimplantation is imprecisely defined. We show that, in contrast to prior reports, BMP signaling (as reported by SMAD1/5/9 phosphorylation) is not detectable until implantation, when it is detected in the primitive endoderm – an extraembryonic lineage. Moreover, preimplantation development appears normal following deletion of maternal and zygoticSmad4,an essential effector of BMP signaling. In fact, mice lacking maternalSmad4are viable. Finally, we uncover a new requirement for zygoticSmad4in epiblast scaling and cavitation immediately after implantation, via a mechanism involving FGFR/ERK attenuation. Altogether, our results demonstrate no role for BMP4/SMAD4 in the first lineage decisions during mouse development. Rather, multi-pathway signaling among embryonic and extraembryonic cell types drives epiblast morphogenesis post-implantation.Summary StatementGene expression, gene deletion, and pathway visualization evidence show thatSmad4-dependent signaling is first active after mouse embryo implantation, when it promotes epiblast morphogenesis non-cell autonomously.