Downregulation of Pecam-1 is insufficient to redefine embryonic stem cells' fate toward PrE in chimaeric mouse blastocysts.

In this work, we aimed to investigate whether Pecam-1 (platelet endothelial cell adhesion molecule 1) surface protein of ICM cells is involved in primitive endoderm (PrE) differentiation. For this purpose, we used embryonic stem cells (ESCs) as an in vitro model for ICM cells, and induced differentiation of ESCs into PrE cells by retinoic acid (RA). Using immunostaining, we observed that at the protein level Pecam-1 diminishes in the early stages of ESC differentiation towards PrE. It is known that in early blastocysts Pecam-1 is detected in all ICM cells, but after the segregation of the epiblast (EPI) and PrE layers, its presence is restricted to EPI cells. Moreover, it was previously reported that a population of ESCs with reduced SSEA-1 and Pecam-1 levels differentiates into PrE. Thus, we examined whether the absence of Pecam-1 in ESCs is sufficient to induce differentiation of these cells into the PrE. To this end, ESCs in which Pecam-1 expression had been downregulated by siRNA were microinjected into 8-cell and blastocyst stage embryos. We found that not only the modified ESCs failed to differentiate into PrE, but similarly to the unmodified control cells, integrated within the EPI. This indicates that the decrease in Pecam-1 expression is not sufficient to promote the differentiation of ESCs into PrE in chimaeric embryos.