WNT signalling supported by MEK/ERK inhibition is essential to maintain pluripotency in bovine preimplantation embryo

Abstract

Capturing stable embryonic stem cell (ESC) lines from domesticated animals still remains one of the challenges of non-rodent embryology. The stake is high, as stable ESCs derived from species such as cattle present high economic and scientific value. Understanding of the processes leading to the embryonic lineage segregation is crucial to provide species-orientated molecular environment capable of supporting self-renewal and pluripotency. Therefore, the aim of this study was to validate the action of the two core regulatory pathways (WNT and MEK/ERK) during bovine embryo development. In vitro produced bovine embryos were obtained in the presence of inhibitors (i), which enable activation of the WNT pathway (via GSK3i, CHIR99021) and suppression of MEK signalling by PD0325901 in the 2i system and PD184325 and SU5402 in the 3i system. We have followed the changes in the distribution of the key lineage specific markers both at the transcript and protein level. Our results showed that WNT signalling promotes the expression of key inner cell mass (ICM) specific markers in bovine embryos, regardless of the MEK/ERK inhibitor cocktail used. MEK/ERK downregulation is crucial to maintain OCT4 and NANOG expression within the ICM and to prevent their exclusion from the trophectoderm (TE). At the same time, the classical TE marker (CDX2) was downregulated at the mRNA and protein level. As a follow up for the observed pluripotency stimulating effect of the inhibitors, we have tested the potential of the 2i and the 3i culture conditions (supported by LIF) to derive primary bovine ESC lines. As a result, we propose a model in which all of the primary signalling pathways determining embryonic cell fate are active in bovine embryos, yet the requirement for pluripotency maintenance in cattle may differ from the described standards. WNT activation leads to the formation (and stabilisation of the ICM) and MEK/ERK signalling is maintained at low levels. Unlike in the mouse, GATA6 is expressed in both ICM and TE. MEK/ERK signalling affects HP formation in cattle, but this process is activated at the post-blastocyst stage. With regard to self-renewal, 2i is preferable, as 3i also blocks the FGF receptor, what may prevent PI3K signalling, important for pluripotency and self-renewal.

Keywords: Bovine embryo; Lineage specification; MEK/ERK; Pluripotency; WNT.