A Review of OCT4 Functions and Applications to Equine Embryos

OCT4 is a core transcription factor involved in pluripotency maintenance in the early mammalian embryo. The POU5F1 gene that encodes the OCT4 protein is highly conserved across species, suggesting conserved function. However, studies in several species including mice, cattle, and pigs, suggest that there are differences in where and when OCT4 is expressed. Specifically, in the horse, several studies have shown that exposure to the uterine environment may be necessary to induce OCT4 expression restriction to the inner cell mass (ICM) of the developing embryo, suggesting that there may be equine-specific extrinsic regulators of OCT4 expression that have not yet been investigated. However, an alternative hypothesis is that this restriction may not be evident in equine embryos because of our inability to culture them to the epiblast stage, preventing the observation of this restriction. In vitro studies have identified that OCT4 is expressed in the immature equine oocyte and in the early equine embryo, but OCT4 expression has not been studied after the formation of the ICM in the equine embryo. Despite the gaps in knowledge about equine-specific functions of OCT4, this factor has been used in studies assessing equine embryonic stem cells and to induce pluripotency in equine somatic cells. This review describes the role of OCT4 in the equine embryo and its applications in equine stem cell research.