Developmental clock and mechanism of de novo polarization of the mouse embryo

Timing and trigger of cell polarization During mammalian embryo development, the first cell fate decision separates the progenitors of the trophectoderm (destined to form the placenta) from the inner cell mass (which forms all tissues of the embryonic and yolk sac). A key event for this first lineage segregation is the establishment of apicobasal cell polarity. This event is set to occur at a fixed developmental stage. The factors that trigger the establishment of cell polarity as well as its temporal regulation have remained unknown. Zhu et al. show in mouse embryos that three molecular regulators—Tfap2c, Tead4, and RhoA—are sufficient to advance the timing of cell polarization with subsequent cell fate specification and morphogenesis. Science, this issue p. eabd2703 The timing of the initial polarization event in mouse embryo development involves specific transcription factors and cytoskeletal components. INTRODUCTION During preimplantation development, the establishment of apicobasal cell polarity is key for the transition from totipotency to pluripotency, which induces cell differentiation toward trophectoderm (TE). In the mouse embryo, this event is programmed to occur at the eight-cell stage, and this timing follows an intrinsic developmental clock that is independent of embryo size or cell cycle progression. Despite the importance of apical domain formation, the molecular mechanisms to establish cell polarization and the temporal regulation of this event in mouse and human embryos have remained largely elusive. RATIONALE In different mammalian species, zygotic genome activation (ZGA) is evolutionarily conserved to occur before the establishment of cell polarization. We therefore hypothesized that zygotic transcription regulates the timing of polarization. To test this, we deployed assays to alter the cellular concentration of zygotic transcripts and to assess the consequences of these changes for the timing of embryo polarization. We also performed an RNA interference (RNAi) screen on 124 zygotically expressed transcripts to determine the molecular identity of zygotic transcripts crucial to cell polarization. Finally, we combined cutting-edge imaging methods with biophysical modeling to account for how the factors we identified regulate the de novo establishment of cell polarization. RESULTS Cell polarity in the mouse embryo is marked by the appearance of a cap-shaped apical domain. Consistent with our hypothesis, an increase or decrease of zygotic transcripts respectively accelerated or inhibited apical domain formation. Our RNAi screen identified two transcription factors—transcription factor AP-2 gamma (Tfap2c) and TEA domain transcription factor 4 (Tead4), which play a redundant role in regulating cell polarization timing. Both Tfap2c and Tead4 proteins accumulate after ZGA, and elevation of their expression allows polarity proteins to anchor to the apical surface prematurely at the four-cell stage. However, these apical proteins failed to organize into an expanded apical domain, instead becoming hypercentralized to form membrane protrusions. This indicates that an additional condition is required for the apical domain formation. We have previously characterized that Rho guanosine triphosphatase (GTPase) signaling, which regulates the actomyosin apical localization and becomes activated around the eight-cell stage, is important for cell polarity. In this study, we found that premature activation of Rho GTPase with expression of Tfap2c and Tead4 allows a complete, precocious induction of the apical domain, leading to the premature expression of TE transcription factors and to morphogenesis events downstream of cell polarization. By combining quantitative imaging measurements and mathematical modeling, we show that apical domain formation is driven by the dynamic interplay between two key processes: (i) the cooperative recruitment of ezrin via the actin network and (ii) the lateral mobility of ezrin on the membrane. On the basis of the experimental evidence and biophysical simulations of these interactions, we show that Tfap2c and Tead4 control the cooperative recruitment of ezrin, whereas RhoA promotes membrane mobility. CONCLUSION The timing and mechanisms for cell polarization have remained largely unknown. We now identify molecules that are necessary and sufficient for the de novo establishment of cell polarization in the mouse embryo. Our results indicate a direct role of ZGA in regulating the timing of cell polarization. Beyond identifying the key molecules sufficient to establish cell polarization, we also provide biophysical understanding of the mechanism by which these molecules act to build cell polarization in the mammalian embryo. Molecular mechanism and temporal regulation for the apical domain formation. Zygotic genome activation enables the expression of Tfap2c and Tead4, which regulates the cooperative membrane recruitment of apical proteins. Cooperative recruitment interacts with Rho GTPase–regulated apical protein lateral mobility to establish the apical domain. Embryo polarization is critical for mouse development; however, neither the regulatory clock nor the molecular trigger that it activates is known. Here, we show that the embryo polarization clock reflects the onset of zygotic genome activation, and we identify three factors required to trigger polarization. Advancing the timing of transcription factor AP-2 gamma (Tfap2c) and TEA domain transcription factor 4 (Tead4) expression in the presence of activated Ras homolog family member A (RhoA) induces precocious polarization as well as subsequent cell fate specification and morphogenesis. Tfap2c and Tead4 induce expression of actin regulators that control the recruitment of apical proteins on the membrane, whereas RhoA regulates their lateral mobility, allowing the emergence of the apical domain. Thus, Tfap2c, Tead4, and RhoA are regulators for the onset of polarization and cell fate segregation in the mouse.