Review
Position- and polarity-dependent Hippo signaling regulates cell fates in preimplantation mouse embryos

https://doi.org/10.1016/j.semcdb.2015.05.003Get rights and content

Highlights

  • Hippo signaling plays a critical role in cell fate specification.

  • Hippo signaling does not regulate cell proliferation in preimplantation embryos.

  • Hippo signaling is controlled in a spatially regulated manner.

  • Cell–cell adhesion and cell polarity control Hippo signaling.

  • Hippo signaling cooperates with other regulatory mechanisms to control cell fates.

Abstract

During the preimplantation stage, mouse embryos establish two cell lineages by the time of early blastocyst formation: the trophectoderm (TE) and the inner cell mass (ICM). Historical models have proposed that the establishment of these two lineages depends on the cell position within the embryo (e.g., the positional model) or cell polarization along the apicobasal axis (e.g., the polarity model). Recent findings have revealed that the Hippo signaling pathway plays a central role in the cell fate-specification process: active and inactive Hippo signaling in the inner and outer cells promote ICM and TE fates, respectively. Intercellular adhesion activates, while apicobasal polarization suppresses Hippo signaling, and a combination of these processes determines the spatially regulated activation of the Hippo pathway in 32-cell-stage embryos. Therefore, there is experimental evidence in favor of both positional and polarity models. At the molecular level, phosphorylation of the Hippo-pathway component angiomotin at adherens junctions (AJs) in the inner (apolar) cells activates the Lats protein kinase and triggers Hippo signaling. In the outer cells, however, cell polarization sequesters Amot from basolateral AJs and suppresses activation of the Hippo pathway. Other mechanisms, including asymmetric cell division and Notch signaling, also play important roles in the regulation of embryonic development. In this review, I discuss how these mechanisms cooperate with the Hippo signaling pathway during cell fate-specification processes.

Introduction

Mouse embryogenesis occurs under powerful regulatory control. Embryos of other oviparous vertebrates such as zebrafish and Xenopus also employ regulatory mechanisms during embryogenesis, but axis formation depends on maternal determinants localized in unfertilized eggs and zygotes (see reviews [1], [2], [3], [4], [5]). In contrast, the development of mouse embryos does not critically depend on such factors. For example, manipulations such as removal of portions of the zygote or destruction of a single blastomere in embryos at the 8-cell stage do not affect mouse embryonic development. Furthermore, isolated blastomeres at the 4- or 8-cell stage embryos show totipotency when aggregated with host embryos [6], [7], [8], [9], [10], [11].

The ability of mouse embryos to develop properly without using localized information has been a hotly debated topic in developmental biology. Preimplantation mouse development has been under intense scientific scrutiny for many years and several models have been proposed. Recent molecular biology-based insights have revealed Hippo signaling as one of the earliest mechanisms influencing cell fate specification. In this review, I will summarize the role and regulation of the Hippo signaling pathway during the first cell-fate specification of mouse embryo development and discuss their relationships with the historical models.

Section snippets

Preimplantation mouse development

During preimplantation development, mouse embryos form a cyst-like structure called a blastocyst by 3.5 days post-coitus (dpc) (Fig. 1A). The early blastocysts consist of two cell types. The outer epithelial cells constitute the trophectoderm (TE) that is required for implantation into the uterus. At later developmental stages, the TE forms extraembryonic tissues, including the embryonic part of the placenta. The inner cells attached to one end of the TE form the inner cell mass (ICM). The ICM

Hippo signaling pathway

Recently, the Hippo signaling pathway was shown to play an important role in regulating cell fates during embryonic development. The Hippo signaling pathway was originally identified as a tumor suppressor-signaling pathway in Drosophila, but it is also conserved in mice and humans (see reviews [47], [48]). The core components of this signaling pathway (Drosophila counterparts are indicated in parentheses) are the protein kinases Mst1/2 (Hippo) and Lats1/2 (Warts), their respective cofactors

A combination of cell–cell adhesion and cell polarization establishes position-dependent Hippo signaling at the 32-cell stage

Establishment of position-dependent Hippo signaling involves differences in cell polarization: the outer and inner cells are polar and apolar, respectively. Polarization is required for TE development because disruptions of the Par–aPKC system, a key regulator of apicobasal cell polarity [65], prompt cells to take the inner position in mosaic embryos, although TE-specific gene expression has not been examined [66]. Furthermore, disruption of the Par–aPKC system in the entire embryo after

Asymmetric cell division initiates differential Hippo signaling during the 8-to-16-cell stage transition

Although the inner cells are first formed at the 16-cell stage, the mechanisms that establish position-dependent Hippo signaling have been mainly studied at around the 32-cell stage. However, slightly different mechanisms appear to regulate Hippo signaling at the 16- and 32-cell stages. During the 8-to-16-cell transition, some of the blastomeres undergo asymmetric cell division forming one polar and one apolar daughter cell [16], and some of the apolar cells transiently take the outer position

Notch signaling cooperates with Hippo signaling to regulate Cdx2 expression

Recently, Manzanares's group demonstrated the involvement of Notch signaling in regulating Cdx2 expression levels [57]. Analysis of cis-regulatory elements of the Cdx2 gene identified a TE-specific enhancer that reproduced the expression pattern of endogenous Cdx2. The enhancer contains two binding sites for Tead and four binding sites for RBPJ, a transcription factor that functions downstream of the Notch-signaling pathway [57]. Experiments with transgenic Notch-reporter embryos showed that

Roles of Hippo signaling in cell-fate specification during preimplantation development

As reviewed above, Hippo signaling is critically important for cell-fate specification of the ICM in preimplantation mouse embryos. The roles of Hippo signaling may differ depending on the developmental stage. Cooperation of the Hippo-signaling pathway with other regulatory mechanisms in controlling cell fate is summarized by the model depicted in Fig. 4. At the 4-cell stage, blastomeres already differ in terms of their developmental potential, epigenetic modifications, and nuclear retention of

Conclusions

Studies on the Hippo-signaling pathway have revealed that cells integrate information regarding cell–cell adhesion and cell polarization to interpret cell positions. Using this mechanism, embryos can properly establish two cell lineages and form blastocysts. However, several unresolved questions remain. First, the mechanism by which the Par–aPKC system restricts Amot expression is unknown. Second, Hippo signaling is not the only mechanism that controls cell fates, and its relationships with

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research (KAKENHI) from MEXT (21116003, 26112715, 15H01495) and JSPS (23247036) to H.S.

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