Human embryo polarization requires PLC signaling to mediate trophectoderm specification

Abstract

Apico-basal polarization of cells within the embryo is critical for the segregation of distinct lineages during mammalian development. Polarized cells become the trophectoderm (TE), which forms the placenta, and apolar cells become the inner cell mass (ICM), the founding population of the fetus. The cellular and molecular mechanisms leading to polarization of the human embryo and its timing during embryogenesis have remained unknown. Here, we show that human embryo polarization occurs in two steps: it begins with the apical enrichment of F-actin and is followed by the apical accumulation of the PAR complex. This two-step polarization process leads to the formation of an apical domain at the 8-16 cell stage. Using RNA interference, we show that apical domain formation requires Phospholipase C (PLC) signaling, specifically the enzymes PLCB1 and PLCE1, from the 8-cell stage onwards. Finally, we show that although expression of the critical TE differentiation marker GATA3 can be initiated independently of embryo polarization, downregulation of PLCB1 and PLCE1 decreases GATA3 expression through a reduction in the number of polarized cells. Therefore, apical domain formation reinforces a TE fate. The results we present here demonstrate how polarization is triggered to regulate the first lineage segregation in human embryos.

Early human trophoblast development: from morphology to function

“…A recent study with supernumerary IVF human embryos at day 4 has determined the sequence of developmental events of human embryo polarization in relation to compaction [ 199 ]. Accordingly, compacted blastomeres display an apical enrichment of cortical F-actin at the cell-contact free surface, along with a slightly time-delayed polarization of the PAR complex components PARD6 and atypical Protein Kinase C (aPKC).…”

A conserved role of Hippo signaling in initiation of the first lineage specification event across mammals

“…We have previously shown that morula stage mouse, cow, and human embryos, acquire an apical-basal cell polarity in outer cells, with expression of aPKC localized to the contact-free domain, nuclear expression of the Hippo pathway downstream effectors YAP1 and WWTR1, and restricted expression of TE-associated factors such as GATA3, suggesting conservation of the initiation of a TE program across these species (Gerri et al, 2020). Furthermore, inhibition of aPKC activity impairs TE initiation at the morula stage in cow and human embryos (Gerri et al, 2020), and downregulation of the polarity regulators Phospholipase CB1 (PLC) and PLCE1 leads to decreased GATA3 expression in human embryos (Zhu et al, 2021).…”

Early human embryonic development: Blastocyst formation to gastrulation

“…Compaction and establishment of apical polarity marks the initial separation of outer polarized cells that will go on to form the TE and takes place between days 3 and 4 after fertilization in the human (Iwata et al, 2014). A recent study has detailed the dynamics of compaction in the human embryo at the 8-16 cell stage and showed that transmembrane phosphoinositide-specific phospholipase C (PLC) signaling is required for apical domain formation (Zhu et al, 2021). In final form and function, the human blastocyst at embryonic day (E) 6-7, with its three distinct cell lineages, is very similar to the mouse blastocyst, but the detailed mechanisms of lineage commitment may differ.…”

“…OCT4, which is a key transcription factor (TF) for ICM formation in mice (Nichols et al, 1998), is broadly expressed in all cells, including both ICM and TE, right up to the expanded blastocyst stage, while CDX2, which is a key regulator of TE initiation in the mouse (Strumpf et al, 2005), is not expressed until after blastocyst formation (Niakan and Eggan, 2013). Other genes involved in TE formation in the mouse, such as Elf5 and Eomes, are not expressed at the blastocyst stage in humans, while GATA3, which plays a subsidiary role in mouse TE formation (Ralston et al, 2010), is expressed in TE precursors in the human embryo (Gerri et al, 2020a;Petropoulos et al, 2016;Zhu et al, 2021).…”