Examining the regulation of the first lineage decision in the bovine pre-implantation embryo

Early embryogenesis is the crucial stage of development that results in the distinction of cell lineages with varying developmental potentials. The first lineage decision involves the transition from totipotent cells into two tissues: the inner cell mass and the trophoblast. Knowledge of how lineage is regulated is important for the agricultural, clinical and therapeutic industries that rely on artificial reproductive technology to progress. However, our current understanding of pre-implantation embryonic development is heavily based on the mouse model. Species-specific regulation of early embryogenesis is an important consideration that has not yet been thoroughly investigated. Cattle provide an opportunity for a new model of embryonic development that will not only serve the agricultural industry but provide new insights that will help establish bovine embryonic stem cells.

In this study, I have established a timeline of protein expression during bovine embryonic development during the first 7 days in in-vitro culture. The results show that compaction and polarisation events in cattle are not only delayed compared to the mouse but asynchronous. The establishment of distinct transcription profiles is also delayed in the bovine embryo, suggesting a lack of lineage commitment at the early blastocyst stages. Contrary to the mouse, pluripotency regulator, SOX2, and trophoblast marker GATA3 are not regulated by the YAP-TEAD4 transcriptional complex in cattle, shown by manipulating YAP activity using Verteporfin and LPA. It has also been demonstrated through further functional analyses using Y27632 and Go6983 that the apical domain, responsible for regulating Hippo pathway activity in mice, is not necessary for lineage specification in cattle.

Instead, we propose that transduction of mechanical cues via YAP operates in parallel to the Hippo pathway to establish lineage-specific transcription profiles. The urgency of the mouse blastocyst to implant has meant the species have evolved ways to utilise available signaling machinery to rapidly commit cells to trophoblast lineage. Cattle, who have a much longer pre-implantation period, coordinate information from various signaling inputs to progressively establish lineage commitment. This coordination has created a functional redundancy of the Hippo pathway in the first lineage decision.