Initiation of Hippo signaling is linked to polarity rather than to cell position in the pre-implantation mouse embryo

Abstract

In the mouse embryo, asymmetric divisions during the 8-16 cell division generate two cell types, polar and apolar cells, that are allocated to outer and inner positions, respectively. This outer/inner configuration is the first sign of the formation of the first two cell lineages: trophectoderm (TE) and inner cell mass (ICM). Outer polar cells become TE and give rise to the placenta, whereas inner apolar cells become ICM and give rise to the embryo proper and yolk sac. Here, we analyze the frequency of asymmetric divisions during the 8-16 cell division and assess the relationships between cell polarity, cell and nuclear position, and Hippo signaling activation, the pathway that initiates lineage-specific gene expression in 16-cell embryos. Although the frequency of asymmetric divisions varied in each embryo, we found that more than six blastomeres divided asymmetrically in most embryos. Interestingly, many apolar cells in 16-cell embryos were located at outer positions, whereas only one or two apolar cells were located at inner positions. Live imaging analysis showed that outer apolar cells were eventually internalized by surrounding polar cells. Using isolated 8-cell blastomeres, we carefully analyzed the internalization process of apolar cells and found indications of higher cortical tension in apolar cells than in polar cells. Last, we found that apolar cells activate Hippo signaling prior to taking inner positions. Our results suggest that polar and apolar cells have intrinsic differences that establish outer/inner configuration and differentially regulate Hippo signaling to activate lineage-specific gene expression programs.

Integrin signalling regulates YAP and TAZ to control skin homeostasis

“…Mechanical force has been proposed to modulate signalling by both the apical Crb-Mer/Ex-Kibra-Sav system in Drosophila (Fletcher et al, 2015;Rauskolb et al, 2014) as well as the basal integrin-Src system in mammalian cell culture (reviewed in Humphrey et al, 2014;Lawson and Burridge, 2014). In the early mouse pre-implantation embryo, cortical tension is higher in outer cells than inner cells, leading to nuclear YAP in outer cells despite the presence of an apical domain (Anani et al, 2014;Kono et al, 2014;Nishioka et al, 2009). Consistently, reducing cortical tension with a ROCK inhibitor abolishes nuclear localisation of YAP in early mouse embryos (Anani et al, 2014;Kono et al, 2014;Nishioka et al, 2009).…”

Lineage specification in the mouse preimplantation embryo

“…Blastomeres from 8-cell embryos can polarize without cell-cell contacts, which is in clear contrast to tissue culture cells (Johnson and Ziomek, 1981b;Stephenson et al, 2010;Anani et al, 2014). The apical domain is enriched for microvilli, F-actin and an evolutionarily conserved apical protein complex containing PAR3 (PARD3), PAR6 (PARD6) and atypical protein kinase C (aPKC) .…”

“…More recently, two studies demonstrated that this internalization is mediated by active cellular behaviours (Anani et al, 2014;Samarage et al, 2015). One study focused on inheritance of the apical domain after the 8-cell division and demonstrated that outer apolar cells are actually internalized instead of adopting polarity in intact embryos or in isolated blastomeres (Anani et al, 2014).…”

“…There is wide variation in the frequency of asymmetric divisions during the 8-to 16-cell transition (Fleming, 1987;Anani et al, 2014). In some 8-cell embryos, all blastomeres divide asymmetrically, generating 8 polar and 8 apolar cells at the 16-cell stage (Anani et al, 2014), whereas other embryos may only exhibit one or two asymmetric divisions.…”

Venus trap in the mouse embryo reveals distinct molecular dynamics underlying specification of first embryonic lineages

“…Venus expression images are shown using a fire LUT, and mT and mCherry are shown using a gray scale LUT. [32,[44][45][46]. Finally, all the extracted lineage relationships, gene expression dynamics and binary (inside or outside) cell-positional information were integrated into a comprehensive lineage map starting from the beginning of 8-cell until the late 32-or late 64-cell stage (Fig EV5).…”