Detection of BMP Signaling in Preimplantation Mouse Embryos

Mammalian development begins with fertilization, followed by multiple cell cleavage events and the transition from maternal to zygotic transcription.  All these events work in concert to establish the first and thus highly significant developmental event, lineage differentiation.  The first step in establishment of different lineages in the preimplantation embryo gives rise to the trophectoderm (TE), the precursor for extraembryonic structures such as the placenta and yolk sac, and the Inner Cell Mass (ICM), which becomes the embryo proper.  While the regulatory interactions and morphological contributions of transcription factors such as Oct3/4, Nanog, Sox2, Tead4, and Cdx2 have been extensively studied, surprisingly little is known about the roles of secreted growth factors during these early stages of development.  The presence of bone morphogenetic proteins (BMPs) in these early mouse embryonic stages suggests likely functional roles for BMP signaling in lineage differentiation.  In order to determine the significance of BMP signaling, particularly in pre-implantation stage embryos, we developed a highly sensitive BRE-gal transgenic mouse line that specifically responds to BMP signaling. The BRE-gal reporter has a multimerized copy of BRE, a highly conserved BMP-Responsive Element (BRE) found from Drosophila to mammals.  Using the reporter mice, we find that the spatial and temporal activities of BMP signaling change dynamically in both pre- and post-implantation mouse embryos.  In developing blastocysts, the presence of active BMP signaling was detected in the ICM and TE cells as revealed by the presence of BRE-gal activity.  Confocal microscopy analysis also confirmed the presence of phosphorylated forms of Smad1/5 in developing blastocysts.