Polarity-dependent distribution of angiomotin localizes Hippo signaling in preimplantation embryos

Geometric, cell cycle and maternal-to-zygotic transition-associated YAP dynamics during preimplantation embryo development

During the first cell fate decision in mammalian embryos, the inner cell mass cells, which will give rise to the embryo proper and other extraembryonic tissues, segregate from the trophectoderm cells, the precursors of the placenta. Cell fate segregation proceeds in a gradual manner encompassing two rounds of cell division, as well as cell positional and morphological changes. While it is known that the activity of the Hippo signaling pathway and the subcellular localization of its downstream effector YAP dictate lineage specific gene expression, the response of YAP to these dynamic cellular changes remains incompletely understood. Here we address these questions by quantitative live imaging of endogenously tagged YAP while simultaneously monitoring geometric cellular features and cell cycle progression throughout cell fate segregation. We apply a probabilistic model to our dynamic data, providing a quantitative characterization of the mutual effects of YAP and cellular relative exposed area, which has previously been shown to correlate with subcellular YAP localization in fixed samples. Additionally, we study how nuclear YAP levels are influenced by other factors, such as the decreasing pool of maternally provided YAP that is partitioned to daughter cells through cleavage divisions, cell cycle-associated nuclear volume changes, and a delay after divisions in adjusting YAP levels to new cell positions. Interestingly, we find that establishing low nuclear YAP levels required for the inner cell mass fate is largely achieved by passive cell cycle-associated mechanisms. Moreover, contrary to expectations, we find that mechanical perturbations that result in cell and nuclear shape changes do not influence YAP localization in the embryo. Together our work identifies how various inputs are integrated over a dynamic developmental time course to shape the levels of a key molecular determinant of the first cell fate choice.

Forces Shaping the Blastocyst

The blastocyst forms during the first days of mammalian development. The structure of the blastocyst is conserved among placental mammals and is paramount to the establishment of the first mammalian lineages. The blastocyst is composed of an extraembryonic epithelium, the trophectoderm (TE), that envelopes a fluid-filled lumen and the inner cell mass (ICM). To shape the blastocyst, embryos transit through three stages driven by forces that have been characterized in the mouse embryo over the past decade. The morphogenetically quiescent cleavage stages mask dynamic cytoskeletal remodeling. Then, during the formation of the morula, cells pull themselves together and the strongest ones internalize. Finally, the blastocyst forms after the pressurized lumen breaks the radial symmetry of the embryo before expanding in cycles of collapses and regrowth. In this review, we delineate the force patterns sculpting the blastocyst, based on our knowledge on the mouse and, to some extent, human embryos.

Polarization-independent regulation of the subcellular localization of Yes-associated protein 1 during preimplantation development

Cell polarization is a crucial developmental process that determines cell differentiation in mouse embryos. During this process, an extensively expressed transcriptional regulator, Yes-associated protein 1 (YAP1), is localized either to the cytoplasm or to the nucleus via HIPPO signaling. In mouse premorula embryos, YAP1 is present in the nuclei of all cells. Thereafter, YAP1 is distributed to the nuclei of outer cells or cytoplasm of inner cells, depending on the establishment of cell polarity and morula formation. However, the dynamics of YAP1 localization in other species, including ruminants, remain unclear. To gain an in-depth understanding of cell differentiation in mammalian embryos, we investigated YAP1 localization changes in bovine embryos. Unlike in mouse morulae, YAP1 displayed cytoplasmic localization in most cells, including the outer cells of bovine morulae, after the 32-cell stage. Next, we analyzed the relationship between cell polarity and nuclear localization of YAP1. Polarization of outer cells in the bovine morula began at the late 16-cell stage and was established by the late 32-cell stage, indicating that polarization preceded the nuclear localization of YAP1 in bovine embryos. To explore the regulation of YAP1 localization in bovine morula, we analyzed zona-free embryos and found that the presence of the zona pellucida significantly enhanced YAP1 cytoplasmic localization. Moreover, we observed ectopic expression of SRY-box transcription factor 2 in zona-free blastocysts, which indicated that cytoplasmic localization of YAP1 was associated with the suppression of pluripotency in the trophectoderm. These findings provide valuable insights into the molecular mechanisms underlying the first cell differentiation in mammalian embryos.

Origin, fate and function of extraembryonic tissues during mammalian development

Extraembryonic tissues have pivotal roles in morphogenesis and patterning of the early mammalian embryo. Developmental programmes mediated through signalling pathways and gene regulatory networks determine the sequence in which fate determination and lineage commitment of extraembryonic tissues take place, and epigenetic processes allow the memory of cell identity and state to be sustained throughout and beyond embryo development, even extending across generations. In this Review, we discuss the molecular and cellular mechanisms necessary for the different extraembryonic tissues to develop and function, from their initial specification up until the end of gastrulation, when the body plan of the embryo and the anatomical organization of its supporting extraembryonic structures are established. We examine the interaction between extraembryonic and embryonic tissues during early patterning and morphogenesis, and outline how epigenetic memory supports extraembryonic tissue development.

Characterization of Hippo signaling components in the early dorsal pancreatic bud

All pancreatic lineages originate from a transitory structure known as the multipotent progenitor epithelium (MPE), which is an endodermal placode formed via epithelial stratification. Cells within the MPE undergo de novo lumenogenesis to give rise to an epithelial plexus, which serves as a progenitor niche for subsequent development of endocrine, ductal and acinar cell types. Recent evidence suggests that Hippo signaling is required for pancreatic cell differentiation, but little is known about the function of Hippo signaling in the development of the MPE. Here, we characterize the expression of YAP1, TAZ, and the Hippo regulators LATS1/2 kinases and MERLIN in early murine pancreatic epithelium, during epithelial stratification, plexus development and emergence of endocrine cells. We find that YAP1 expression is relatively low in the pancreas bud during stratification but increases by E11.5. Intriguingly, we find differing patterns of TAZ and YAP1 immunoreactivty throughout pancreatic development. We further find that MERLIN and LATS1/2 kinases are expressed during the period of rapid stratification and become markedly apical at nascent lumens. To gain a better understanding of how Hippo signaling and lumen formation are connected, we analyzed the subcellular localization of Hippo signaling components during varying stages of lumen formation and found that they are dynamically localized during lumenogenesis. Together, our results point to a previously unsuspected relationship between Hippo signaling and lumen formation during pancreatic development.

The Hippo pathway: Organ size control and beyond

The Hippo signaling pathway is a highly conserved signaling network for controlling organ size, tissue homeostasis, and regeneration. It integrates a wide range of intracellular and extracellular signals, such as cellular energy status, cell density, hormonal signals, and mechanical cues, to modulate the activity of YAP/TAZ transcriptional coactivators. A key aspect of Hippo pathway regulation involves its spatial organization at the plasma membrane, where upstream regulators localize to specific membrane subdomains to regulate the assembly and activation of the pathway components. This spatial organization is critical for the precise control of Hippo signaling, as it dictates the dynamic interactions between pathway components and their regulators. Recent studies have also uncovered the role of biomolecular condensation in regulating Hippo signaling, adding complexity to its control mechanisms. Dysregulation of the Hippo pathway is implicated in various pathological conditions, particularly cancer, where alterations in YAP/TAZ activity contribute to tumorigenesis and drug resistance. Therapeutic strategies targeting the Hippo pathway have shown promise in both cancer treatment, by inhibiting YAP/TAZ signaling, and regenerative medicine, by enhancing YAP/TAZ activity to promote tissue repair. The development of small molecule inhibitors targeting the YAP-TEAD interaction and other upstream regulators offers new avenues for therapeutic intervention. SIGNIFICANCE STATEMENT: The Hippo signaling pathway is a key regulator of organ size, tissue homeostasis, and regeneration, with its dysregulation linked to diseases such as cancer. Understanding this pathway opens new possibilities for therapeutic approaches in regenerative medicine and oncology, with the potential to translate basic research into improved clinical outcomes.

Coupling of cell shape, matrix and tissue dynamics ensures embryonic patterning robustness

Tissue patterning coordinates morphogenesis, cell dynamics and fate specification. Understanding how precision in patterning is robustly achieved despite inherent developmental variability during mammalian embryogenesis remains a challenge. Here, based on cell dynamics quantification and simulation, we show how salt-and-pepper epiblast and primitive endoderm (PrE) cells pattern the inner cell mass of mouse blastocysts. Coupling cell fate and dynamics, PrE cells form apical polarity-dependent actin protrusions required for RAC1-dependent migration towards the surface of the fluid cavity, where PrE cells are trapped due to decreased tension. Concomitantly, PrE cells deposit an extracellular matrix gradient, presumably breaking the tissue-level symmetry and collectively guiding their own migration. Tissue size perturbations of mouse embryos and their comparison with monkey and human blastocysts further demonstrate that the fixed proportion of PrE/epiblast cells is optimal with respect to embryo size and tissue geometry and, despite variability, ensures patterning robustness during early mammalian development.

Geometric, cell cycle and maternal-to-zygotic transition-associated YAP dynamics during preimplantation embryo development

During the first cell fate decision in mammalian embryos the inner cell mass cells, which will give rise to the embryo proper and other extraembryonic tissues, segregate from the trophectoderm cells, the precursors of the placenta. Cell fate segregation proceeds in a gradual manner encompassing two rounds of cell division, as well as cell positional and morphological changes. While it is known that the activity of the Hippo signaling pathway and the subcellular localization of its downstream effector YAP dictate lineage specific gene expression, the response of YAP to these dynamic cellular changes remains incompletely understood. Here we address these questions by quantitative live imaging of endogenously tagged YAP while simultaneously monitoring geometric cellular features and cell cycle progression throughout cell fate segregation. We apply a probabilistic model to our dynamic data, providing a quantitative characterization of the mutual effects of YAP and cellular relative exposed area, which has previously been shown to correlate with subcellular YAP localization in fixed samples. Additionally, we study how nuclear YAP levels are influenced by other factors, such as the decreasing pool of maternally provided YAP that is partitioned to daughter cells through cleavage divisions, cell cycle-associated nuclear volume changes, and a delay after divisions in adjusting YAP levels to new cell positions. Interestingly, we find that establishing low nuclear YAP levels required for the inner cell mass fate is largely achieved by passive cell cycle-associated mechanisms. Moreover, contrary to expectations, we find that mechanical perturbations that result in cell shape changes do not influence YAP localization in the embryo. Together our work identifies how various inputs are integrated over a dynamic developmental time course to shape the levels of a key molecular determinant of the first cell fate choice.

Generative model for the first cell fate bifurcation in mammalian development

The first cell fate bifurcation in mammalian development directs cells toward either the trophectoderm (TE) or inner cell mass (ICM) compartments in preimplantation embryos. This decision is regulated by the subcellular localization of a transcriptional co-activator YAP and takes place over several progressively asynchronous cleavage divisions. As a result of this asynchrony and variable arrangement of blastomeres, reconstructing the dynamics of the TE/ICM cell specification from fixed embryos is extremely challenging. To address this, we developed a live imaging approach and applied it to measure pairwise dynamics of nuclear YAP and its direct target genes, CDX2 and SOX2, key transcription factors of TE and ICM, respectively. Using these datasets, we constructed a generative model of the first cell fate bifurcation, which reveals the time-dependent statistics of the TE and ICM cell allocation. In addition to making testable predictions for the joint dynamics of the full YAP/CDX2/SOX2 motif, the model revealed the stochastic nature of the induction timing of the key cell fate determinants and identified the features of YAP dynamics that are necessary or sufficient for this induction. Notably, temporal heterogeneity was particularly prominent for SOX2 expression among ICM cells. As heterogeneities within the ICM have been linked to the initiation of the second cell fate decision in the embryo, understanding the origins of this variability is of key significance. The presented approach reveals the dynamics of the first cell fate choice and lays the groundwork for dissecting the next cell fate bifurcations in mouse development.

WDR36 Regulates Trophectoderm Differentiation During Human Preimplantation Embryonic Development Through Glycolytic Metabolism

Mammalian pre-implantation development is a complex process involving sophisticated regulatory dynamics. WD repeat domain 36 (WDR36) is known to play a critical role in mouse early embryonic development, but its regulatory function in human embryogenesis is still elusive due to limited access to human embryos. The human pluripotent stem cell-derived blastocyst-like structure, termed a blastoid, offers an alternative means to study human development in a dish. In this study, after verifying that WDR36 inhibition disrupted polarization in mouse early embryos, it is further demonstrated that WDR36 interference can block human blastoid formation, dominantly hindering the trophectoderm lineage commitment. Both transcriptomics and targeted metabolomics analyses revealed that WDR36 interference downregulated glucose metabolism. WDR36 can interact with glycolytic metabolic protein lactate dehydrogenase A (LDHA), thereby positively regulating glycolysis during the late stage of human blastoid formation. Taken together, the study has established a mechanistic connection between WDR36, glucose metabolism, and cell fate determination during early embryonic lineage commitment, which may provide potential insights into novel therapeutic targets for early adverse pregnancy interventions.

Long-lived cytokinetic bridges coordinate sister-cell elimination in mouse embryos

Apoptosis is a key feature of preimplantation development, but whether it occurs in a cell-autonomous or coordinated manner was unknown. Here, we report that plasma membrane abscission, the final step of cell division, is profoundly delayed in early mouse embryos such that a cytokinetic bridge is maintained for the vast majority of the following interphase. Early embryos thus consist of many pairs of sister cells connected by stable cytokinetic bridges that allow them to share diffusible molecules. We show that apoptotic regulators are shared through cytokinetic bridges and that these bridges ensure that if one cell enters apoptosis, its sister cell does as well. Long-lived cytokinetic bridges are thus a previously unappreciated form of cell-cell communication within the mouse embryo that coordinate the clearance of pairs of cells with similar developmental histories.

Blastocoel expansion and AMOT degradation cooperatively promote YAP nuclear localization during epiblast formation

The epiblast is a pluripotent cell population formed in the late blastocyst stage of preimplantation embryos. During the process of epiblast formation from the inner cell mass (ICM) of the early blastocyst, activation of the Hippo pathway transcription factor TEAD by the nuclear translocation of the coactivator protein YAP is required for the robust expression of pluripotency factors. However, the mechanisms that alter YAP localization during epiblast formation remain unknown. Here, we reveal two such mechanisms. Expansion of the blastocoel promotes nuclear YAP localization by increasing cytoplasmic F-actin and reducing YAP phosphorylation. Additionally, cell differentiation regulates YAP. Expression of the junctional Hippo component, AMOT, gradually decreases during epiblast formation through a tankyrase-mediated degradation. SOX2 expression in the ICM is necessary for the reduction of AMOT and YAP phosphorylation. These two mechanisms function in parallel. Thus, the blastocoel-F-actin and SOX2-AMOT axes cooperatively suppress YAP phosphorylation and promote YAP nuclear localization during epiblast formation. The cooperation of these two distinct mechanisms likely contributes to the robustness of epiblast cell differentiation.

Regulation of trophectoderm morphogenesis by small GTPase RHOA through HIPPO signaling-dependent and -independent mechanisms in mouse preimplantation development

The trophectoderm (TE) is the first tissue to differentiate during the preimplantation development of the mammalian embryo. It forms the outer layer of the blastocyst and is responsible for generating the blastocoel, a fluid-filled cavity whose expansion is essential for successful hatching and implantation. Here, we investigated the role of the small GTPase RHOA in the morphogenesis of the TE, particularly its relationship with HIPPO signaling, using mouse embryos as a model. Inhibition of RHOA resulted in the failure to form a blastocoel and significantly altered the expression of numerous genes. Transcriptomic analysis revealed that 330 genes were down-regulated and 168 genes were up-regulated by more than two-fold. Notably, 98.4% of these transcriptional changes were reversed by simultaneous inhibition of LATS kinases, indicating that the transcriptional influence of RHOA is primarily mediated through HIPPO signaling. Many of the down-regulated genes are involved in critical processes of TE morphogenesis, such as apical-basal cell polarization, tight junction formation, and sodium and water transport, suggesting that RHOA supports TE development by enhancing the expression of morphogenesis-related genes through HIPPO signaling, specifically via TEAD transcription factors. However, RHOA inhibition also disrupted apical-basal polarity and tight junctions, effects that were not restored by LATS inhibition, pointing to additional HIPPO signaling-independent mechanisms by which RHOA controls TE morphogenesis. Furthermore, RHOA inhibition impaired cell viability at the late blastocyst stage, with partial rescue observed upon LATS inhibition, suggesting that RHOA maintains cell survival through both HIPPO signaling-dependent and -independent pathways. A deeper knowledge of the molecular mechanisms governing TE morphogenesis, including blastocoel expansion and cell viability, could significantly advance assisted reproductive technologies aimed at producing healthy blastocysts.

Signaling pathway regulators in preimplantation embryos

Embryonic development during the preimplantation stages is highly sensitive and critically dependent on the reception of signaling cues. The precise coordination of diverse pathways and signaling factors is essential for successful embryonic progression. Even minor disruptions in these factors can result in physiological dysfunction, fetal malformations, or embryonic arrest. This issue is particularly evident in assisted reproductive technologies, such as in vitro fertilization, where embryonic arrest is frequently observed. A detailed understanding of these pathways enhances insight into the fundamental mechanisms underlying cellular processes and their contributions to embryonic development. The significance of elucidating signaling pathways and their regulatory factors in preimplantation development cannot be overstated. The application of this knowledge in laboratory settings has the potential to support strategies for modeling developmental stages and diseases, drug screening, therapeutic discovery, and reducing embryonic arrest. Furthermore, using various factors, small molecules, and pharmacological agents can enable the development or optimization of culture media for enhanced embryonic viability. While numerous pathways influence preimplantation development, this study examines several critical signaling pathways in this contex.

Nuclear receptor-SINE B1 network modulates expanded pluripotency in blastoids and blastocysts

Embryonic stem cells possess the remarkable ability to self-organize into blastocyst-like structures upon induction. These stem cell-based embryo models serve as invaluable platforms for studying embryogenesis and therapeutic developments. Nevertheless, the specific intrinsic regulators that govern this potential for blastoid formation remain unknown. Here we demonstrate an intrinsic program that plays a crucial role in both blastoids and blastocysts across multiple species. We first establish metrics for grading the resemblance of blastoids to mouse blastocysts, and identify the differential activation of gene regulons involved in lineage specification among various blastoid grades. Notably, abrogation of nuclear receptor subfamily 1, group H, member 2 (Nr1h2) drastically reduces blastoid formation. Nr1h2 activation alone is sufficient to rewire conventional ESC into a distinct pluripotency state, enabling them to form blastoids with enhanced implantation capacity in the uterus and contribute to both embryonic and extraembryonic lineages in vivo. Through integrative multi-omics analyses, we uncover the broad regulatory role of Nr1h2 in the transcriptome, chromatin accessibility and epigenome, targeting genes associated with embryonic lineage and the transposable element SINE-B1. The Nr1h2-centred intrinsic program governs and drives the development of both blastoids and early embryos.

Hepatocyte differentiation requires anisotropic expansion of bile canaliculi

During liver development, bipotential progenitor cells called hepatoblasts differentiate into hepatocytes or cholangiocytes. Hepatocyte differentiation is uniquely associated with multi-axial polarity, enabling the anisotropic expansion of apical lumina between adjacent cells and formation of a three-dimensional network of bile canaliculi. Cholangiocytes, the cells forming the bile ducts, exhibit the vectorial polarity characteristic of epithelial cells. Whether cell polarization feeds back on the gene regulatory pathways governing hepatoblast differentiation is unknown. Here, we used primary mouse hepatoblasts to investigate the contribution of anisotropic apical expansion to hepatocyte differentiation. Silencing of the small GTPase Rab35 caused isotropic lumen expansion and formation of multicellular cysts with the vectorial polarity of cholangiocytes. Gene expression profiling revealed that these cells express reduced levels of hepatocyte markers and upregulate genes associated with cholangiocyte identity. Timecourse RNA sequencing demonstrated that loss of lumen anisotropy precedes these transcriptional changes. Independent alterations in apical lumen morphology induced either by modulation of the subapical actomyosin cortex or by increased intraluminal pressure caused similar transcriptional changes. These findings suggest that cell polarity and lumen morphogenesis feed back to hepatoblast-to-hepatocyte differentiation.

New Horizons in Cancer Progression and Metastasis: Hippo Signaling Pathway

The Hippo pathway is highly evolved to maintain tissue homeostasis in diverse species by regulating cell proliferation, differentiation, and apoptosis. In tumor biology, the Hippo pathway is a prime example of signaling molecules involved in cancer progression and metastasis. Hippo core elements LATS1, LATS2, MST1, YAP, and TAZ have critical roles in the maintenance of traditional tissue architecture and cell homeostasis. However, in cancer development, dysregulation of Hippo signaling results in tumor progression and the formation secondary cancers. Hippo components not only transmit biochemical signals but also act as mediators of mechanotransduction pathways during malignant neoplasm development and metastatic disease. This review confers knowledge of Hippo pathway core components and their role in cancer progression and metastasis and highlights the clinical role of Hippo pathway in cancer treatment. The Hippo signaling pathway and its unresolved mechanisms hold great promise as potential therapeutic targets in the emerging field of metastatic cancer research.

Characterization of Hippo Signaling Components in the Early Dorsal Pancreatic Bud

All pancreatic lineages originate from a transitory structure known as the multipotent progenitor epithelium (MPE), which is a placode formed via epithelial stratification. Cells within the MPE undergo de novo lumenogenesis to give rise to an epithelial plexus, which serves as a progenitor niche for subsequent development of endocrine, ductal and acinar cell types. Recent evidence suggests that Hippo signaling is required for pancreatic cell differentiation, but little is known about the function of Hippo signaling in the development of the MPE. Here, we characterize the expression of YAP1, TAZ, and the Hippo regulators LATS1/2 kinases and MERLIN in early murine pancreatic epithelium, during epithelial stratification, plexus development and emergence of endocrine cells. We find that YAP1 expression is relatively low in the pancreas bud during stratification, but increases by E11.5. Intriguingly, we find that TAZ, but not YAP1, is expressed in early endocrine cells. We further find that MERLIN and LATS1/2 kinases are robustly expressed during the period of rapid stratification and become markedly apical at nascent lumens. To gain a better understanding of how Hippo signaling and lumen formation are connected, we analyzed the expression of Hippo signaling components in an in vitro model of lumen formation and found that they are dynamically regulated during lumenogenesis. Together, our results point to a relationship between Hippo signaling and lumen formation during pancreatic development.

HIGHLIGHTS

YAP1 expression in the early mouse pancreatic anlagen is low until approximately E11.5, when it becomes localized to cell nuclei in multipotent progenitor cells. At E14.5, we find nuclear YAP1 in ductal cells.YAP1 is not expressed in early and midgestation endocrine cells. By contrast, TAZ is expressed in first transition endocrine cells.Hippo regulators MERLIN and LATS1/2 kinases are robustly expressed in the early pancreatic bud by E10.5. Both MERLIN and LATS1/2 exhibit strong apical localization in epithelial cells at nascent microlumens. Using in vitro models of de novo pancreas lumen formation, we show that YAP1 nuclear localization is high in early phases of lumen formation and gradually decreases as lumens matures.

The cellular localization and oncogenic or tumor suppressive effects of angiomiotin-like protein 2 in tumor and normal cells

Angiomiotin (AMOT) family comprises three members: AMOT, AMOT-like protein 1 (AMOTL1), and AMOT-like protein 2 (AMOTL2). AMOTL2 is widely expressed in endothelial cells, epithelial cells, and various cancer cells. Specifically, AMOTL2 predominantly localizes in the cytoplasm and nucleus in human normal cells, whereas associates with cell-cell junctions and actin cytoskeleton in non-human cells, and locates at cell junctions or within the recycling endosomes in cancer cells. AMOTL2 is implicated in regulation of tube formation, cell polarity, and shape, although the specific impact on tumorigenesis remains to be conclusively determined. It has been shown that AMOTL2 enhances tumor growth and metastasis in pancreatic, breast, and colon cancer, however inhibits cell proliferation and migration in lung, hepatocellular cancer, and glioblastoma. In addition to its role in cell shape and cytoskeletal dynamics through co-localization with F-actin, AMOTL2 modulates the transcription of Yes-associated protein (YAP) by binding to it, thereby affecting its phosphorylation and cellular sequestration. Furthermore, the stability and cellular localization of AMOTL2, influenced by its phosphorylation and ubiquitination mediated by specific proteins, affects its cellular function. Additionally, we observe that AMOTL2 is predominantly downregulated in some tumors, but significantly elevated in colorectal adenocarcinoma (COAD). Moreover, overall analysis, GSEA and ROC curve analysis indicate that AMOTL2 exerts as an oncogenic protein in COAD by modulating Wnt pathway, participating in synthesis of collagen formation, and interacting with extracellular matrix receptor. In addition, AMOTL2 potentially regulates the distribution of immune cells infiltration in COAD. In summary, AMOTL2 probably functions as an oncogene in COAD. Consequently, further in-depth mechanistic research is required to elucidate the precise roles of AMOTL2 in various cancers.

Exploring Mechanical Forces Shaping Self-Organization and Morphogenesis During Early Embryo Development

Embryonic development is a dynamic process orchestrated by a delicate interplay of biochemical and biophysical factors. While the role of genetics and biochemistry in embryogenesis has been extensively studied, recent research has highlighted the significance of mechanical regulation in shaping and guiding this intricate process. Here, we provide an overview of the current understanding of the mechanical regulation of embryo development. We explore how mechanical forces generated by cells and tissues play a crucial role in driving the development of different stages. We examine key morphogenetic processes such as compaction, blastocyst formation, implantation, and egg cylinder formation, and discuss the mechanical mechanisms and cues involved. By synthesizing the current body of literature, we highlight the emerging concepts and open questions in the field of mechanical regulation. We aim to provide an overview of the field, inspiring future investigations and fostering a deeper understanding of the mechanical aspects of embryo development.

The first lineage determination in mammals

This review describes in detail the morphological, cytoskeletal and gene expression events leading to the gene regulatory network bifurcation point of trophoblast and inner cell mass cells in a variety of mammalian preimplantation embryos. The interrelated processes of compaction and polarity establishment are discussed in terms of how they affect YAP/WWTR activity and the location and fate of cells. Comparisons between mouse, human, cattle, pig and rabbit embryos suggest a conserved role for YAP/WWTR signalling in trophoblast induction in eutherian animals though the mechanisms for, and timing of, YAP/WWTR activation differs among species. Downstream targets show further differences, with the trophoblast marker GATA3 being a direct target in all examined mammals, while CDX2-positive and SOX2-negative regulation varies.

Revisiting trophectoderm-inner cell mass lineage segregation in the mammalian preimplantation embryo

In the first days of life, cells of the mammalian embryo segregate into two distinct lineages, trophectoderm and inner cell mass. Unlike nonmammalian species, mammalian development does not proceed from predetermined factors in the oocyte. Rather, asymmetries arise de novo in the early embryo incorporating cues from cell position, contractility, polarity, and cell-cell contacts. Molecular heterogeneities, including transcripts and non-coding RNAs, have now been characterized as early as the 2-cell stage. However, it's debated whether these early heterogeneities bias cells toward one fate or the other or whether lineage identity arises stochastically at the 16-cell stage. This review summarizes what is known about early blastomere asymmetries and our understanding of lineage allocation in the context of historical models. Preimplantation development is reviewed coupled with what is known about changes in morphology, contractility, and transcription factor networks. The addition of single-cell atlases of human embryos has begun to reveal key differences between human and mouse, including the timing of events and core transcription factors. Furthermore, the recent generation of blastoid models will provide valuable tools to test and understand fate determinants. Lastly, new techniques are reviewed, which may better synthesize existing knowledge with emerging data sets and reconcile models with the regulative capacity unique to the mammalian embryo.

Angiomotin family proteins in the Hippo signaling pathway

The Motin family proteins (Motins) are a class of scaffolding proteins consisting of Angiomotin (AMOT), AMOT-like protein 1 (AMOTL1), and AMOT-like protein 2 (AMOTL2). Motins play a pivotal role in angiogenesis, tumorigenesis, and neurogenesis by modulating multiple cellular signaling pathways. Recent findings indicate that Motins are components of the Hippo pathway, a signaling cascade involved in development and cancer. This review discusses how Motins are integrated into the Hippo signaling network, as either upstream regulators or downstream effectors, to modulate cell proliferation and migration. The repression of YAP/TAZ by Motins contributes to growth inhibition, whereas subcellular localization of Motins and their interactions with actin fibers are critical in regulating cell migration. The net effect of Motins on cell proliferation and migration may contribute to their diverse biological functions.

Hippo pathway inactivation through subcellular localization of NF2/merlin in outer cells of mouse embryos

The Hippo pathway plays a crucial role in cell proliferation and differentiation during tumorigenesis, tissue homeostasis and early embryogenesis. Scaffold proteins from the ezrin-radixin-moesin (ERM) family, including neurofibromin 2 (NF2; Merlin), regulate the Hippo pathway through cell polarity. However, the mechanisms underlying Hippo pathway regulation via cell polarity in establishing outer cells remain unclear. In this study, we generated artificial Nf2 mutants in the N-terminal FERM domain (L64P) and examined Hippo pathway activity by assessing the subcellular localization of YAP1 in early embryos expressing these mutant mRNAs. The L64P-Nf2 mutant inhibited NF2 localization around the cell membrane, resulting in YAP1 cytoplasmic translocation in the polar cells. L64P-Nf2 expression also disrupted the apical centralization of both large tumor suppressor 2 (LATS2) and ezrin in the polar cells. Furthermore, Lats2 mutants in the FERM binding domain (L83K) inhibited YAP1 nuclear translocation. These findings demonstrate that NF2 subcellular localization mediates cell polarity establishment involving ezrin centralization. This study provides previously unreported insights into how the orchestration of the cell-surface components, including NF2, LATS2 and ezrin, modulates the Hippo pathway during cell polarization.

Comparison of pregnancy outcomes between 4th day morula and 5th day blastocyst after embryo transfer: a retrospective cohort study

BACKGROUND

This study was designed to evaluate pregnancy outcomes between morulae transferred on day 4 (D4) and blastocysts transferred on day 5 (D5).

METHODS

From September 2017 to September 2020, 1963 fresh transfer cycles underwent early follicular phase extra-long protocol for assisted conception in our fertility center were divided into D4 (324 cases) and D5 (1639 cases) groups, and the general situation and other differences of patients in both groups were compared. To compare the differences in pregnancy outcomes, the D4 and D5 groups were further divided into groups A and B based on single and double embryo transfers. Furthermore, the cohort was divided into two groups: those with live births (1116 cases) and those without (847 cases), enabling a deeper evaluation of the effects of D4 or D5 transplantation on assisted reproductive outcomes.

RESULTS

In single embryo transfer, there was no significant difference between groups D4A and D5A (P > 0.05). In double embryo transfer, group D4B had a lower newborn birthweight and a larger proportion of low birthweight infants (P < 0.05). The preterm delivery rate, twin delivery rate, cesarean delivery rate, and percentage of low birthweight infants were lower in the D5A group than in the D5B group (P < 0.05). Analysis of factors influencing live birth outcomes further confirmed the absence of a significant difference between D4 and D5 transplantation in achieving live birth (P > 0.05).

CONCLUSION

When factors such as working life and hospital holidays are being considered, D4 morula transfer may be a good alternative to D5 blastocyst transfer. Given the in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) success rate and risk of twin pregnancy, D4 morula transfer requires an adapted decision between single and double embryo transfer, although a single blastocyst transfer is recommended for the D5 transfer in order to decrease the twin pregnancy rate. In addition, age, endometrial thickness and other factors need to be taken into account to personalize the IVF program and optimize pregnancy outcomes.

Genome-Wide CRISPR Screen Identifies an NF2-Adherens Junction Mechanistic Dependency for Cardiac Lineage

BACKGROUND

Cardiomyocyte differentiation involves a stepwise clearance of repressors and fate-restricting regulators through the modulation of BMP (bone morphogenic protein)/Wnt-signaling pathways. However, the mechanisms and how regulatory roadblocks are removed with specific developmental signaling pathways remain unclear.

METHODS

We conducted a genome-wide CRISPR screen to uncover essential regulators of cardiomyocyte specification in human embryonic stem cells using a myosin heavy chain 6 (MYH6)-GFP (green fluorescence protein) reporter system. After an independent secondary single guide ribonucleic acid validation of 25 candidates, we identified NF2 (neurofibromin 2), a moesin-ezrin-radixin like (MERLIN) tumor suppressor, as an upstream driver of early cardiomyocyte lineage specification. Independent monoclonal NF2 knockouts were generated using CRISPR-Cas9, and cell states were inferred through bulk RNA sequencing and protein expression analysis across differentiation time points. Terminal lineage differentiation was assessed by using an in vitro 2-dimensional-micropatterned gastruloid model, trilineage differentiation, and cardiomyocyte differentiation. Protein interaction and post-translation modification of NF2 with its interacting partners were assessed using site-directed mutagenesis, coimmunoprecipitation, and proximity ligation assays.

RESULTS

Transcriptional regulation and trajectory inference from NF2-null cells reveal the loss of cardiomyocyte identity and the acquisition of nonmesodermal identity. Sustained elevation of early mesoderm lineage repressor SOX2 and upregulation of late anticardiac regulators CDX2 and MSX1 in NF2 knockout cells reflect a necessary role for NF2 in removing regulatory roadblocks. Furthermore, we found that NF2 and AMOT (angiomotin) cooperatively bind to YAP (yes-associated protein) during mesendoderm formation, thereby preventing YAP activation, independent of canonical MST (mammalian sterile 20-like serine-threonine protein kinase)-LATS (large tumor suppressor serine-threonine protein kinase) signaling. Mechanistically, cardiomyocyte lineage identity was rescued by wild-type and NF2 serine-518 phosphomutants, but not NF2 FERM (ezrin-radixin-meosin homology protein) domain blue-box mutants, demonstrating that the critical FERM domain-dependent formation of the AMOT-NF2-YAP scaffold complex at the adherens junction is required for early cardiomyocyte lineage differentiation.

CONCLUSIONS

These results provide mechanistic insight into the essential role of NF2 during early epithelial-mesenchymal transition by sequestering the repressive effect of YAP and relieving regulatory roadblocks en route to cardiomyocytes.

Determining the minimal amount of DMSO necessary to stabilize the Angiomotin lipid binding domain

Angiomotins (Amots) are a family of adaptor proteins with important roles in cell growth, migration, and proliferation. The Amot coiled-coil homology (ACCH) domain has a high affinity for non-phosphorylated and mono-phosphorylated phosphatidylinositol which provides specificity in the membrane association. The membrane specificity is linked with targeting and recycling of the membrane protein to maintain normal cell phenotypes and function. Therefore, we endeavored to find the minimal amount of DMSO to stabilize the Amot lipid binding domain to eventually understand the protein function by studying its atomic structure. Our laboratory looked to determine the structure using nuclear magnetic resonance (NMR), which requires higher protein concentrations than those possible in our current buffered solutions. Based on literature reported on other proteins, DMSO can be used as a stabilizing agent up to 33-70%. Therefore, this work shows our preliminary findings for the minimal amount of dimethyl sulfoxide (DMSO) needed to stabilize the domain at higher concentrations without disrupting its native structure. To that end, we determined DMSO related changes in protein structure by analyzing shifts in the melting point determined by dynamic scanning fluorescence measurements. As a result, we found that the ACCH domain is denatured in solutions >10% DMSO.

Tead4 and Tfap2c generate bipotency and a bistable switch in totipotent embryos to promote robust lineage diversification

The mouse and human embryo gradually loses totipotency before diversifying into the inner cell mass (ICM, future organism) and trophectoderm (TE, future placenta). The transcription factors TFAP2C and TEAD4 with activated RHOA accelerate embryo polarization. Here we show that these factors also accelerate the loss of totipotency. TFAP2C and TEAD4 paradoxically promote and inhibit Hippo signaling before lineage diversification: they drive expression of multiple Hippo regulators while also promoting apical domain formation, which inactivates Hippo. Each factor activates TE specifiers in bipotent cells, while TFAP2C also activates specifiers of the ICM fate. Asymmetric segregation of the apical domain reconciles the opposing regulation of Hippo signaling into Hippo OFF and the TE fate, or Hippo ON and the ICM fate. We propose that the bistable switch established by TFAP2C and TEAD4 is exploited to trigger robust lineage diversification in the developing embryo.

Role of angiomotin family members in human diseases (Review)

Angiomotin (Amot) family members, including Amot, Amot-like protein 1 (Amotl1) and Amot-like protein 2 (Amotl2), have been found to interact with angiostatins. In addition, Amot family members are involved in various physiological and pathological functions such as embryonic development, angiogenesis and tumorigenesis. Some studies have also demonstrated its regulation in signaling pathways such as the Hippo signaling pathway, AMPK signaling pathway and mTOR signaling pathways. Amot family members play an important role in neural stem cell differentiation, dendritic formation and synaptic maturation. In addition, an increasing number of studies have focused on their function in promoting and/or suppressing cancer, but the underlying mechanisms remain to be elucidated. The present review integrated relevant studies on upstream regulation and downstream signals of Amot family members, as well as the latest progress in physiological and pathological functions and clinical applications, hoping to offer important ideas for further research.

Temporally distinct roles of Aurora A in polarization of the C. elegans zygote

During asymmetric cell division, cell polarity is coordinated with the cell cycle to allow proper inheritance of cell fate determinants and the generation of cellular diversity. In the Caenorhabditis elegans zygote, polarity is governed by evolutionarily conserved Partitioning-defective (PAR) proteins that segregate to opposing cortical domains to specify asymmetric cell fates. Timely establishment of PAR domains requires a cell cycle kinase, Aurora A (AIR-1 in C. elegans). Aurora A depletion by RNAi causes a spectrum of phenotypes including reversed polarity, excess posterior domains and no posterior domain. How depletion of a single kinase can cause seemingly opposite phenotypes remains obscure. Using an auxin-inducible degradation system and drug treatments, we found that AIR-1 regulates polarity differently at different times of the cell cycle. During meiosis I, AIR-1 acts to prevent later formation of bipolar domains, whereas in meiosis II, AIR-1 is necessary to recruit PAR-2 onto the membrane. Together, these data clarify the origin of multiple polarization phenotypes in RNAi experiments and reveal multiple roles of AIR-1 in coordinating PAR protein localization with cell cycle progression.

Effects of Regulating Hippo and Wnt on the Development and Fate Differentiation of Bovine Embryo

The improvement of in vitro embryo development is a gateway to enhance the output of assisted reproductive technologies. The Wnt and Hippo signaling pathways are crucial for the early development of bovine embryos. This study investigated the development of bovine embryos under the influence of a Hippo signaling agonist (LPA) and a Wnt signaling inhibitor (DKK1). In this current study, embryos produced in vitro were cultured in media supplemented with LPA and DKK1. We comprehensively analyzed the impact of LPA and DKK1 on various developmental parameters of the bovine embryo, such as blastocyst formation, differential cell counts, YAP fluorescence intensity and apoptosis rate. Furthermore, single-cell RNA sequencing (scRNA-seq) was employed to elucidate the in vitro embryonic development. Our results revealed that LPA and DKK1 improved the blastocyst developmental potential, total cells, trophectoderm (TE) cells and YAP fluorescence intensity and decreased the apoptosis rate of bovine embryos. A total of 1203 genes exhibited differential expression between the control and LPA/DKK1-treated (LD) groups, with 577 genes upregulated and 626 genes downregulated. KEGG pathway analysis revealed significant enrichment of differentially expressed genes (DEGs) associated with TGF-beta signaling, Wnt signaling, apoptosis, Hippo signaling and other critical developmental pathways. Our study shows the role of LPA and DKK1 in embryonic differentiation and embryo establishment of pregnancy. These findings should be helpful for further unraveling the precise contributions of the Hippo and Wnt pathways in bovine trophoblast formation, thus advancing our comprehension of early bovine embryo development.

The Hippo signaling pathway in development and regeneration

The Hippo signaling pathway is a central growth control mechanism in multicellular organisms. By integrating diverse mechanical, biochemical, and stress cues, the Hippo pathway orchestrates proliferation, survival, differentiation, and mechanics of cells, which in turn regulate organ development, homeostasis, and regeneration. A deep understanding of the regulation and function of the Hippo pathway therefore holds great promise for developing novel therapeutics in regenerative medicine. Here, we provide updates on the molecular organization of the mammalian Hippo signaling network, review the regulatory signals and functional outputs of the pathway, and discuss the roles of Hippo signaling in development and regeneration.

Inverted apicobasal polarity in health and disease

Apicobasal epithelial polarity controls the functional properties of most organs. Thus, there has been extensive research on the molecular intricacies governing the establishment and maintenance of cell polarity. Whereas loss of apicobasal polarity is a well-documented phenomenon associated with multiple diseases, less is known regarding another type of apicobasal polarity alteration - the inversion of polarity. In this Review, we provide a unifying definition of inverted polarity and discuss multiple scenarios in mammalian systems and human health and disease in which apical and basolateral membrane domains are interchanged. This includes mammalian embryo implantation, monogenic diseases and dissemination of cancer cell clusters. For each example, the functional consequences of polarity inversion are assessed, revealing shared outcomes, including modifications in immune surveillance, altered drug sensitivity and changes in adhesions to neighboring cells. Finally, we highlight the molecular alterations associated with inverted apicobasal polarity and provide a molecular framework to connect these changes with the core cell polarity machinery and to explain roles of polarity inversion in health and disease. Based on the current state of the field, failure to respond to extracellular matrix (ECM) cues, increased cellular contractility and membrane trafficking defects are likely to account for most cases of inverted apicobasal polarity.

Origin and development of primary animal epithelia

Epithelia are the first organized tissues that appear during development. In many animal embryos, early divisions give rise to a polarized monolayer, the primary epithelium, rather than a random aggregate of cells. Here, we review the mechanisms by which cells organize into primary epithelia in various developmental contexts. We discuss how cells acquire polarity while undergoing early divisions. We describe cases where oriented divisions constrain cell arrangement to monolayers including organization on top of yolk surfaces. We finally discuss how epithelia emerge in embryos from animals that branched early during evolution and provide examples of epithelia-like arrangements encountered in single-celled eukaryotes. Although divergent and context-dependent mechanisms give rise to primary epithelia, here we trace the unifying principles underlying their formation.

Efficient Reprogramming of Mouse Embryonic Stem Cells into Trophoblast Stem-like Cells via Lats Kinase Inhibition

Mouse zygotes undergo multiple rounds of cell division, resulting in the formation of preimplantation blastocysts comprising three lineages: trophectoderm (TE), epiblast (EPI), and primitive endoderm (PrE). Cell fate determination plays a crucial role in establishing a healthy pregnancy. The initial separation of lineages gives rise to TE and inner cell mass (ICM), from which trophoblast stem cells (TSC) and embryonic stem cells (ESC) can be derived in vitro. Studying lineage differentiation is greatly facilitated by the clear functional distinction between TSC and ESC. However, transitioning between these two types of cells naturally poses challenges. In this study, we demonstrate that inhibiting LATS kinase promotes the conversion of ICM to TE and also effectively reprograms ESC into stable, self-renewing TS-like cells (TSLC). Compared to TSC, TSLC exhibits similar molecular properties, including the high expression of marker genes such as Cdx2, Eomes, and Tfap2c, as well as hypomethylation of their promoters. Importantly, TSLC not only displays the ability to differentiate into mature trophoblast cells in vitro but also participates in placenta formation in vivo. These findings highlight the efficient reprogramming of ESCs into TSLCs using a small molecular inducer, which provides a new reference for understanding the regulatory network between ESCs and TSCs.

Developmental perturbation in human embryos: Clinical and biological significance learned from time-lapse images

Background

Time-lapse technology (TLT) has gained widespread adoption worldwide. In addition to facilitating the undisturbed culture of embryos, TLT offers the unique capability of continuously monitoring embryos to detect spatiotemporal changes. Although these observed phenomena play a role in optimal embryo selection/deselection, the clinical advantages of introducing TLT remain unclear. However, manual annotation of embryo perturbation could facilitate a comprehensive assessment of developmental competence. This process requires a thorough understanding of embryo observation and the biological significance associated with developmental dogma and variation. This review elucidates the typical behavior and variation of each phenomenon, exploring their clinical significance and research perspectives.

Methods

The MEDLINE database was searched using PubMed for peer-reviewed English-language original articles concerning human embryo development.

Main findings

TLT allows the observation of consecutive changes in embryo morphology, serving as potential biomarkers for embryo assessment. In assisted reproductive technology laboratories, several phenomena have not revealed their mechanism, posing difficulties such as fertilization deficiency and morula arrest.

Conclusion

A profound understanding of the biological mechanisms and significance of each phenomenon is crucial. Further collaborative efforts between the clinical and molecular fields following translational studies are required to advance embryonic outcomes and assessment.

Control of stem cell renewal and fate by YAP and TAZ

Complex physiological processes control whether stem cells self-renew, differentiate or remain quiescent. Two decades of research have placed the Hippo pathway, a highly conserved kinase signalling cascade, and its downstream molecular effectors YAP and TAZ at the nexus of this decision. YAP and TAZ translate complex biological cues acting on stem cells - from mechanical forces to cellular metabolism - into genome-wide effects to mediate stem cell functions. While aberrant YAP/TAZ activity drives stem cell dysfunction in ageing, tumorigenesis and disease, therapeutic targeting of Hippo signalling and YAP/TAZ can boost stem cell activity to enhance regeneration. In this Review, we discuss how YAP/TAZ control the self-renewal, fate and plasticity of stem cells in different contexts, how dysregulation of YAP/TAZ in stem cells leads to disease, and how therapeutic modalities targeting YAP/TAZ may benefit regenerative medicine and cancer therapy.

Effect of microgravity on mammalian embryo development evaluated at the International Space Station

Mammalian embryos differentiate into the inner cell mass (ICM) and trophectoderm at the 8-16 cell stage. The ICM forms a single cluster that develops into a single fetus. However, the factors that determine differentiation and single cluster formation are unknown. Here we investigated whether embryos could develop normally without gravity. As the embryos cannot be handled by an untrained astronaut, a new device was developed for this purpose. Using this device, two-cell frozen mouse embryos launched to the International Space Station were thawed and cultured by the astronauts under microgravity for 4 days. The embryos cultured under microgravity conditions developed into blastocysts with normal cell numbers, ICM, trophectoderm, and gene expression profiles similar to those cultured under artificial-1 g control on the International Space Station and ground-1 g control, which clearly demonstrated that gravity had no significant effect on the blastocyst formation and initial differentiation of mammalian embryos.

Tetraploid embryo aggregation produces high-quality blastocysts with an increased trophectoderm in pigs

Tetraploid complementation is an ideal method for demonstrating the differentiation potential of pluripotent stem cells. In this study, we selected the most efficient tetraploid production method for porcine embryos and investigated whether tetraploid blastomere aggregation could enhance the quality of tetraploid embryos. Three methods were investigated to produce tetraploid embryos: First, tetraploid embryos were produced using electro-fusion of two-cell stage parthenogenetic blastomere (FUTP). Second, somatic cell was injected into the mature oocyte and fused to produce tetraploid embryos. Third, oocytes were matured with Cytochalasin B (CB) for the late 22 h of in vitro maturation to inhibit the first polar body (PB1). Following that, non-PB1 oocytes were treated with CB for 4 h after parthenogenetic activation. There was no significant difference in the blastocyst development rate and tetraploid production rate of the embryos produced through the three methods. However, FUTP-derived blastocysts had a significantly lower percentage of apoptotic cells compared to other methods. The developmental competence of embryos, expression of trophectoderm cell marker genes, and distribution of YAP1 protein were investigated in tetraploid embryos produced using the FUTP method. The FUTP method most effectively prevented apoptosis during porcine tetraploid embryo formation. Tetraploid aggregation-derived blastocysts have a high proportion of trophectoderm with increased expression of the CDX2 mRNA and high YAP1 intensity. High-quality blastocysts derived from a tetraploid embryo aggregation can serve as suitable source material for testing the differentiation potential of pluripotent stem cells for blastocyst complementation in pigs.

Temporally distinct roles of Aurora A in polarization of the C. elegans zygote

During asymmetric cell division, coordination of cell polarity and the cell cycle is critical for proper inheritance of cell fate determinants and generation of cellular diversity. In Caenorhabditis elegans (C. elegans), polarity is established in the zygote and is governed by evolutionarily conserved Partitioning defective (PAR) proteins that localize to distinct cortical domains. At the time of polarity establishment, anterior and posterior PARs segregate to opposing cortical domains that specify asymmetric cell fates. Timely establishment of these PAR domains requires a cell cycle kinase, Aurora A (AIR-1 in C.elegans). Aurora A depletion by RNAi causes a spectrum of phenotypes including no posterior domain, reversed polarity, and excess posterior domains. How depletion of a single kinase can cause seemingly opposite phenotypes remains obscure. Using an auxin-inducible degradation system, drug treatments, and high-resolution microscopy, we found that AIR-1 regulates polarity via distinct mechanisms at different times of the cell cycle. During meiosis I, AIR-1 acts to prevent the formation of bipolar domains, while in meiosis II, AIR-1 is necessary to recruit PAR-2 onto the membrane. Together these data clarify the origin of the multiple polarization phenotypes observed in RNAi experiments and reveal multiple roles of AIR-1 in coordinating PAR protein localization with the progression of the cell cycle.

Omics Views of Mechanisms for Cell Fate Determination in Early Mammalian Development

During mammalian preimplantation development, a totipotent zygote undergoes several cell cleavages and two rounds of cell fate determination, ultimately forming a mature blastocyst. Along with compaction, the establishment of apicobasal cell polarity breaks the symmetry of an embryo and guides subsequent cell fate choice. Although the lineage segregation of the inner cell mass (ICM) and trophectoderm (TE) is the first symbol of cell differentiation, several molecules have been shown to bias the early cell fate through their inter-cellular variations at much earlier stages, including the 2- and 4-cell stages. The underlying mechanisms of early cell fate determination have long been an important research topic. In this review, we summarize the molecular events that occur during early embryogenesis, as well as the current understanding of their regulatory roles in cell fate decisions. Moreover, as powerful tools for early embryogenesis research, single-cell omics techniques have been applied to both mouse and human preimplantation embryos and have contributed to the discovery of cell fate regulators. Here, we summarize their applications in the research of preimplantation embryos, and provide new insights and perspectives on cell fate regulation.

Inflammation and oxidative stress impair preimplantation embryonic morphogenesis in allergic asthma model

The incidence of allergic asthma has been increasing worldwide in recent decades. Also, an increasing number of women are suffering from poor pregnancy outcome. However, the causal relationship between allergic asthma and embryonic growth in terms of cell morphogenesis has not been well elucidated. Here, we investigated the impact of allergic asthma on the morphogenesis of preimplantation embryos. Twenty-four female BALB/c were randomly divided into control (PBS), 50-μg (OVA1), 100-μg (OVA2) and 150-μg (OVA3). On Days-0 and -14, mice were induced intraperitoneally (i.p) with ovalbumin (OVA). On Days-21 until -23, mice were challenged with OVA via intranasal instillation (i.n). Control animals were sensitized and challenged with PBS. At the end of treatment (Day-25), 2-cell embryos were retrieved and cultured in vitro until the blastocysts hatched. Results showed reduced number of preimplantation embryos at all developing stages in all treated groups (p ≤ 0.0001). Uneven blastomere size, partial compaction- and cavitation-activity, low formation of trophectoderm (TE), as well as cell fragmentation were noted in all the treated groups. Maternal serum interleukin (IL)-4, immunoglobulin (Ig)-E and 8-hydroxydeoxyguanosine (8-OHdG) were notably high (p ≤ 0.0001, p ≤ 0.01) in contrast with low total antioxidant capacity (TAOC) (p ≤ 0.0001). Our findings indicated that OVA-induced allergic asthma had compromised cell morphogenesis through reduced blastomere cleavage division, partial compaction and cavitation-activity, impairment of TE production, and cell fragmentation leading to embryonic cell death via OS mechanism.

Updates on preimplantation embryo research

Preimplantation development is the only stage of human development that can be studied outside the body in real time, as human embryos can be produced by in vitro fertilization and cultured in the laboratory as self-contained structures until the blastocyst stage. Here, we focus some of the key cellular and morphogenetic processes by which the 1-cell embryo is transformed gradually into a blastocyst ready for implantation. Although most of our knowledge about the dynamic series of events patterning preimplantation human development derives from work in mouse embryos, we discuss key differences that could exist with humans. Furthermore, we highlight how new approaches may enable to reveal many of the unknown processes driving human preimplantation development, particularly using noninvasive imaging and genetic technologies.

CircKDM5B sponges miR-128 to regulate porcine blastocyst development by modulating trophectoderm barrier function

Circular RNAs (circRNAs), which exert critical functions in the regulation of transcriptional and post-transcriptional gene expression, are found in mammalian cells but their functions in mammalian preimplantation embryo development remain poorly understood. Here, we showed that circKDM5B mediated miRNA-128 (miR-128) to regulate porcine early embryo development. We screened circRNAs potentially expressed in porcine embryos through an integrated analysis of sequencing data from mouse and human embryos, as well as porcine oocytes. An authentic circRNA originating from histone demethylase KDM5B (referred to as circKDM5B) was abundantly expressed in porcine embryos. Functional studies revealed that circKDM5B knockdown not only significantly reduced blastocyst formation but also decreased the number of total cells and trophectoderm (TE) cells. Moreover, the knockdown of circKDM5B resulted in the disturbance of tight junction assembly and impaired paracellular sealing within the TE epithelium. Mechanistically, miR-128 inhibitor injection could rescue the early development of circKDM5B knockdown embryos. Taken together, the findings revealed that circKDM5B functions as a miR-128 sponge, thereby facilitating early embryonic development in pigs through the modulation of gene expression linked to tight junction assembly.

Daughter cells inherit YAP localization from mother cells in early preimplantation embryos

The first stage of cell differentiation during mouse development is the differentiation into the trophectoderm and inner cell mass, which occurs during the 8-32-cell stages of preimplantation embryos. This differentiation is regulated by the Hippo signaling pathway. At the 32-cell stage, embryos establish a position-dependent distribution of the Hippo pathway coactivator, Yes-associated protein 1 (YAP, encoded by Yap1). The outer and inner cells showed nuclear and cytoplasmic localization of YAP, respectively. However, the process by which embryos establish position-dependent YAP localization remains elusive. Here, we established a YAP-reporter mouse line, Yap1mScarlet , and examined YAP-mScarlet protein dynamics during the 8-32-cell stages using live imaging. During mitosis, YAP-mScarlet diffused throughout the cells. YAP-mScarlet dynamics in daughter cells varied depending on the cell division patterns. YAP-mScarlet localization in daughter cells at the completion of cell division coincided with that in mother cells. Experimental manipulation of YAP-mScarlet localization in mother cells also altered its localization in daughter cells upon completion of cell division. In daughter cells, YAP-mScarlet localization gradually changed to the final pattern. In some divisions during the 8-16-cell stages, the cytoplasmic YAP-mScarlet localization preceded cell internalization. These results suggest that cell position is not a primary determinant of YAP localization and that the Hippo signaling status of the mother cell is inherited by the daughter cells, which likely contributes to the stabilization of the cell fate specification process beyond cell division.

WWC1/2 regulate spinogenesis and cognition in mice by stabilizing AMOT

WWC1 regulates episodic learning and memory, and genetic nucleotide polymorphism of WWC1 is associated with neurodegenerative diseases such as Alzheimer's disease. However, the molecular mechanism through which WWC1 regulates neuronal function has not been fully elucidated. Here, we show that WWC1 and its paralogs (WWC2/3) bind directly to angiomotin (AMOT) family proteins (Motins), and recruit USP9X to deubiquitinate and stabilize Motins. Deletion of WWC genes in different cell types leads to reduced protein levels of Motins. In mice, neuron-specific deletion of Wwc1 and Wwc2 results in reduced expression of Motins and lower density of dendritic spines in the cortex and hippocampus, in association with impaired cognitive functions such as memory and learning. Interestingly, ectopic expression of AMOT partially rescues the neuronal phenotypes associated with Wwc1/2 deletion. Thus, WWC proteins modulate spinogenesis and cognition, at least in part, by regulating the protein stability of Motins.

Spatial positioning of preimplantation mouse embryo cells is regulated by mTORC1 and m7G-cap-dependent translation at the 8- to 16-cell transition

Preimplantation mouse embryo development involves temporal-spatial specification and segregation of three blastocyst cell lineages: trophectoderm, primitive endoderm and epiblast. Spatial separation of the outer-trophectoderm lineage from the two other inner-cell-mass (ICM) lineages starts with the 8- to 16-cell transition and concludes at the 32-cell stages. Accordingly, the ICM is derived from primary and secondary contributed cells; with debated relative EPI versus PrE potencies. We report generation of primary but not secondary ICM populations is highly dependent on temporal activation of mammalian target of Rapamycin (mTOR) during 8-cell stage M-phase entry, mediated via regulation of the 7-methylguanosine-cap (m7G-cap)-binding initiation complex (EIF4F) and linked to translation of mRNAs containing 5' UTR terminal oligopyrimidine (TOP-) sequence motifs, as knockdown of identified TOP-like motif transcripts impairs generation of primary ICM founders. However, mTOR inhibition-induced ICM cell number deficits in early blastocysts can be compensated by the late blastocyst stage, after inhibitor withdrawal; compensation likely initiated at the 32-cell stage when supernumerary outer cells exhibit molecular characteristics of inner cells. These data identify a novel mechanism specifically governing initial spatial segregation of mouse embryo blastomeres, that is distinct from those directing subsequent inner cell formation, contributing to germane segregation of late blastocyst lineages.

Neurotrophin-4 promotes the specification of trophectoderm lineage after parthenogenetic activation and enhances porcine early embryonic development

Neurotrophin-4 (NT-4), a neurotrophic factor, appears to affect early embryonic development because it is secreted not only by neurons but also by oviductal and uterine epithelial cells. However, no studies have characterized the effects of NT-4 on early embryonic development in pigs. In this study, we applied the experimental model of parthenogenetic-activation (PA)-derived embryos. Herein, we investigated the effect of NT-4 supplementation during the in vitro culture (IVC) of embryos, analyzed the transcription levels of specific genes, and outlined the first cell lineage specification for porcine PA-derived blastocysts. We confirmed that NT-4 and its receptor proteins were localized in both the inner cell mass (ICM) and trophectoderm (TE) in porcine blastocysts. Across different concentrations (0, 1, 10, and 100 ng/mL) of NT-4 supplementation, the optimal concentration of NT-4 to improve the developmental competence of porcine parthenotes was 10 ng/mL. NT-4 supplementation during porcine IVC significantly (p < 0.05) increased the proportion of TE cells by inducing the transcription of TE lineage markers (CDX2, PPAG3, and GATA3 transcripts). NT-4 also reduced blastocyst apoptosis by regulating the transcription of apoptosis-related genes (BAX and BCL2L1 transcripts) and improved blastocyst quality via the interaction of neurotrophin-, Hippo-yes-associated protein (Hippo-YAP) and mitogen-activated protein kinase/extracellular regulated kinase (MAPK/ERK) pathway. Additionally, NT-4 supplementation during IVC significantly (p < 0.05) increased YAP1 transcript levels and significantly (p < 0.01) decreased LATS2 transcript levels, respectively, in the porcine PA-derived blastocysts. We also confirmed through fluorescence intensity that the YAP1 protein was significantly (p < 0.001) increased in the NT-4-treated blastocysts compared with that in the control. NT-4 also promoted differentiation into the TE lineage rather than into the ICM lineage during porcine early embryonic development. In conclusion, 10 ng/mL NT-4 supplementation enhanced blastocyst quality by regulating the apoptosis- and TE lineage specification-related genes and interacting with neurotrophin-, Hippo-YAP-, and MAPK/ERK signaling pathway during porcine in vitro embryo development.

Lineage segregation in human pre-implantation embryos is specified by YAP1 and TEAD1

STUDY QUESTION

Which processes and transcription factors specify the first and second lineage segregation events during human preimplantation development?

SUMMARY ANSWER

Differentiation into trophectoderm (TE) cells can be initiated independently of polarity; moreover, TEAD1 and YAP1 co-localize in (precursor) TE and primitive endoderm (PrE) cells, suggesting a role in both the first and the second lineage segregation events.

WHAT IS KNOWN ALREADY

We know that polarity, YAP1/GATA3 signalling and phospholipase C signalling play a key role in TE initiation in compacted human embryos, however, little is known about the TEAD family of transcription factors that become activated by YAP1 and, especially, whether they play a role during epiblast (EPI) and PrE formation. In mouse embryos, polarized outer cells show nuclear TEAD4/YAP1 activity that upregulates Cdx2 and Gata3 expression while inner cells exclude YAP1 which upregulates Sox2 expression. The second lineage segregation event in mouse embryos is orchestrated by FGF4/FGFR2 signalling which could not be confirmed in human embryos; TEAD1/YAP1 signalling also plays a role during the establishment of mouse EPI cells.

STUDY DESIGN, SIZE, DURATION

Based on morphology, we set up a development timeline of 188 human preimplantation embryos between Day 4 and 6 post-fertilization (dpf). The compaction process was divided into three subgroups: embryos at the start (C0), during (C1), and at the end (C2) of, compaction. Inner cells were identified as cells that were entirely separated from the perivitelline space and enclosed by cellular contacts on all sides. The blastulation process was divided into six subgroups, starting with early blastocysts with sickle-cell shaped outer cells (B0) and further on, blastocysts with a cavity (B1). Full blastocysts (B2) showed a visible ICM and outer cells referred to as TE. Further expanded blastocysts (B3) had accumulated fluid and started to expand due to TE cell proliferation and zona pellucida (ZP) thinning. The blastocysts then significantly expanded further (B4) and started to hatch out of the ZP (B5) until they were fully hatched (B6).

PARTICIPANTS/MATERIALS, SETTING, METHODS

After informed consent and the expiration of the 5-year cryopreservation duration, 188 vitrified high quality eight-cell stage human embryos (3 dpf) were warmed and cultured until the required stages were reached. We also cultured 14 embryos that were created for research until the four- and eight-cell stage. The embryos were scored according to their developmental stage (C0-B6) displaying morphological key differences, rather than defining them according to their chronological age. They were fixed and immunostained for different combinations of cytoskeleton (F-actin), polarization (p-ERM), TE (GATA3), EPI (NANOG), PrE (GATA4 and SOX17), and members of the Hippo signalling pathway (YAP1, TEAD1 and TEAD4). We choose these markers based on previous observations in mouse embryos and single cell RNA-sequencing data of human embryos. After confocal imaging (LSM800, Zeiss), we analysed cell numbers within each lineage, different co-localization patterns and nuclear enrichment.

MAIN RESULTS AND THE ROLE OF CHANCE

We found that in human preimplantation embryos compaction is a heterogeneous process that takes place between the eight-cell to the 16-cell stages. Inner and outer cells are established at the end of the compaction process (C2) when the embryos contain up to six inner cells. Full apical p-ERM polarity is present in all outer cells of compacted C2 embryos. Co-localization of p-ERM and F-actin increases steadily from 42.2% to 100% of the outer cells, between C2 and B1 stages, while p-ERM polarizes before F-actin (P < 0.00001). Next, we sought to determine which factors specify the first lineage segregation event. We found that 19.5% of the nuclei stain positive for YAP1 at the start of compaction (C0) which increases to 56.1% during compaction (C1). At the C2 stage, 84.6% of polarized outer cells display high levels of nuclear YAP1 while it is absent in 75% of non-polarized inner cells. In general, throughout the B0-B3 blastocyst stages, polarized outer/TE cells are mainly positive for YAP1 and non-polarized inner/ICM cells are negative for YAP1. From the C1 stage onwards, before polarity is established, the TE marker GATA3 is detectable in YAP1 positive cells (11.6%), indicating that differentiation into TE cells can be initiated independently of polarity. Co-localization of YAP1 and GATA3 increases steadily in outer/TE cells (21.8% in C2 up to 97.3% in B3). Transcription factor TEAD4 is ubiquitously present throughout preimplantation development from the compacted stage onwards (C2-B6). TEAD1 displays a distinct pattern that coincides with YAP1/GATA3 co-localization in the outer cells. Most outer/TE cells throughout the B0-B3 blastocyst stages are positive for TEAD1 and YAP1. However, TEAD1 proteins are also detected in most nuclei of the inner/ICM cells of the blastocysts from cavitation onwards, but at visibly lower levels as compared to that in TE cells. In the ICM of B3 blastocysts, we found one main population of cells with NANOG+/SOX17-/GATA4- nuclei (89.1%), but exceptionally we found NANOG+/SOX17+/GATA4+ cells (0.8%). In seven out of nine B3 blastocysts, nuclear NANOG was found in all the ICM cells, supporting the previously reported hypothesis that PrE cells arise from EPI cells. Finally, to determine which factors specify the second lineage segregation event, we co-stained for TEAD1, YAP1, and GATA4. We identified two main ICM cell populations in B4-6 blastocysts: the EPI (negative for the three markers, 46.5%) and the PrE (positive for the three markers, 28.1%) cells. We conclude that TEAD1 and YAP1 co-localise in (precursor) TE and PrE cells, indicating that TEAD1/YAP1 signalling plays a role in the first and the second lineage segregation events.

LIMITATIONS, REASONS FOR CAUTION

In this descriptive study, we did not perform functional studies to investigate the role of TEAD1/YAP1 signalling during the first and second lineage segregation events.

WIDER IMPLICATIONS OF THE FINDINGS

Our detailed roadmap on polarization, compaction, position and lineage segregation events during human preimplantation development paves the way for further functional studies. Understanding the gene regulatory networks and signalling pathways involved in early embryogenesis could ultimately provide insights into why embryonic development is sometimes impaired and facilitate the establishment of guidelines for good practice in the IVF lab.

STUDY FUNDING/COMPETING INTERESTS

This work was financially supported by Wetenschappelijk Fonds Willy Gepts (WFWG) of the University Hospital UZ Brussel (WFWG142) and the Fonds Wetenschappelijk Onderzoek-Vlaanderen (FWO, G034514N). M.R. is doctoral fellow at the FWO. The authors have no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.