The Hippo Signaling Pathway Components Lats and Yap Pattern Tead4 Activity to Distinguish Mouse Trophectoderm from Inner Cell Mass

Interpreting the stress response of early mammalian embryos and their stem cells

This review analyzes and interprets the normal, pathogenic, and pathophysiological roles of stress and stress enzymes in mammalian development. Emerging data suggest that stem cells from early embryos are induced by stress to perform stress-enzyme-mediated responses that use the strategies of compensatory, prioritized, and reversible differentiation. These strategies have been optimized during evolution and in turn have aspects of energy conservation during stress that optimize and maximize the efficacy of the stress response. It is likely that different types of stem cells have varying degrees of flexibility in mediating compensatory and prioritized differentiation. The significance of this analysis and interpretation is that it will serve as a foundation for yielding tools for diagnosing, understanding normal and pathophysiological mechanisms, and providing methods for managing stress enzymes to improve short- and long-term reproductive outcomes.

Factors regulating pluripotency and differentiation in early mammalian embryos and embryo-derived stem cells

Mammalian development relies on the cellular proliferation and precisely orchestrated differentiation processes. In preimplantation embryos preservation of the pluripotent state and timely onset of differentiation are secured by specific mechanisms involving such factors as OCT₄, NANOG, SOX₂, or SALL₄. The pluripotency-sustaining cellular machinery is operational not only in the cells of preimplantation embryos but also in embryo-derived embryonic stem cells and epiblast stem cells. However, certain variations in the execution of pluripotency exist and result in the differences not only between embryonic cells and stem cells of the same mammalian species, but also between those of different mammalian species, such as mouse, rat, bank vole, or humans. In this review we describe the involvement of exogenous stimuli (e.g., LIF, WNT, BMP, FGF, and Activin) and function of intrinsic factors (e.g., OCT₄, NANOG, SOX₂, SALL₄) in the regulation of pluripotency in mammalian preimplantation embryos and pluripotent stem cells derived from them. We also focus at the existence of species-specific differences at the level of growth factor requirements, signaling pathways, and transcription factors. Thus, we discuss differences in mechanisms which understanding is one of the necessary steps allowing establishment of methods of efficient derivation, defined in vitro culture conditions, and possible future therapeutic applications of pluripotent stem cells.

Hippo signaling: growth control and beyond

The Hippo pathway has emerged as a conserved signaling pathway that is essential for the proper regulation of organ growth in Drosophila and vertebrates. Although the mechanisms of signal transduction of the core kinases Hippo/Mst and Warts/Lats are relatively well understood, less is known about the upstream inputs of the pathway and about the downstream cellular and developmental outputs. Here, we review recently discovered mechanisms that contribute to the dynamic regulation of Hippo signaling during Drosophila and vertebrate development. We also discuss the expanding diversity of Hippo signaling functions during development, discoveries that shed light on a complex regulatory system and provide exciting new insights into the elusive mechanisms that regulate organ growth and regeneration.

Initiation of trophectoderm lineage specification in mouse embryos is independent of Cdx2

The separation of the first two lineages - trophectoderm (TE) and inner cell mass (ICM) - is a crucial event in the development of the early embryo. The ICM, which constitutes the pluripotent founder cell population, develops into the embryo proper, whereas the TE, which comprises the surrounding outer layer, supports the development of the ICM before and after implantation. Cdx2, the first transcription factor expressed specifically in the developing TE, is crucial for the differentiation of cells into the TE, as lack of zygotic Cdx2 expression leads to a failure of embryos to hatch and implant into the uterus. However, speculation exists as to whether maternal Cdx2 is required for initiation of TE lineage separation. Here, we show that effective elimination of both maternal and zygotic Cdx2 transcripts by an RNA interference approach resulted in failure of embryo hatching and implantation, but the developing blastocysts exhibited normal gross morphology, indicating that TE differentiation had been initiated. Expression of keratin 8, a marker for differentiated TE, further confirmed the identity of the TE lineage in Cdx2-deficient embryos. However, these embryos exhibited low mitochondrial activity and abnormal ultrastructure, indicating that Cdx2 plays a key role in the regulation of TE function. Furthermore, we found that embryonic compaction does not act as a 'switch' regulator to turn on Cdx2 expression. Our results clearly demonstrate that neither maternal nor zygotic Cdx2 transcripts direct the initiation of ICM/TE lineage separation.

Trophoblast stem cells

Trophoblast stem cells (TSC) are the precursors of the differentiated cells of the placenta. In the mouse, TSC can be derived from outgrowths of either blastocyst polar trophectoderm (TE) or extraembryonic ectoderm (ExE), which originates from polar TE after implantation. The mouse TSC niche appears to be located within the ExE adjacent to the epiblast, on which it depends for essential growth factors, but whether this cellular architecture is the same in other species remains to be determined. Mouse TSC self-renewal can be sustained by culture on mitotically inactivated feeder cells, which provide one or more factors related to the NODAL pathway, and a medium supplemented with FGF4, heparin, and fetal bovine serum. Repression of the gene network that maintains pluripotency and emergence of the transcription factor pathways that specify a trophoblast (TR) fate enables TSC derivation in vitro and placental formation in vivo. Disrupting the pluripotent network of embryonic stem cells (ESC) causes them to default to a TR ground state. Pluripotent cells that have acquired sublethal chromosomal alterations may be sequestered into TR for similar reasons. The transition from ESC to TSC, which appears to be unidirectional, reveals important aspects of initial fate decisions in mice. TSC have yet to be derived from domestic species in which remarkable TR growth precedes embryogenesis. Recent derivation of TSC from blastocysts of the rhesus monkey suggests that isolation of the human equivalents may be possible and will reveal the extent to which mechanisms uncovered by using animal models are true in our own species.

The hippo signaling pathway in development and cancer

First discovered in Drosophila, the Hippo signaling pathway is a conserved regulator of organ size. Central to this pathway is a kinase cascade leading from the tumor suppressor Hippo (Mst1 and Mst2 in mammals) to the oncoprotein Yki (YAP and TAZ in mammals), a transcriptional coactivator of target genes involved in cell proliferation and survival. Here, I review recent progress in elucidating the molecular mechanism and physiological function of Hippo signaling in Drosophila and mammals. These studies suggest that the core Hippo kinase cascade integrates multiple upstream inputs, enabling dynamic regulation of tissue homeostasis in animal development and physiology.

Expression patterns of ABCG2, Bmi-1, Oct-3/4, and Yap in the developing mouse incisor

Recent studies have demonstrated the existence of dental stem cells in the continuously growing tooth. However, much remains to be learned about the complex mechanism involving stem cells during tooth development. We determined the expression patterns of four stem cell markers ABCG2, Bmi-1, Oct-3/4, and Yap in the developing mouse incisors between embryonic day (E) 11 and postnatal day (PN) 20. ABCG2 was localized strongly in the perivascular region of the incisor mesenchyme from E11 to PN20, and in the odontoblasts from E18 to PN20. Bmi-1 was expressed in both the dental epithelium and mesenchyme from E11 to E14. The expression of Bmi-1 was noticeably reduced at E16, and was restricted to the apical bud from E16 to PN20. Oct-3/4 was localized in the nucleus of the cells in the superficial layer and stellate reticulum within the dental epithelium from E11 to E14 and in the apical bud from E16 to PN20. Meanwhile, once the ameloblasts and odontoblasts began to appear at E16, they expressed Oct-3/4 in the cytoplasm. Yap was expressed in most of the basal cells of the incisor dental epithelium from E11 to E14, but was expressed mainly in the transit-amplifying (TA) cells within the basal cell layer from E16 to PN20. The unique and overlapping expression patterns of ABCG2, Bmi-1, Oct-3/4, and Yap suggest the independent and interactive functions of the four stem cell markers in the developing mouse incisor.

Klf5 regulates lineage formation in the pre-implantation mouse embryo

Kruppel-like transcription factors (Klfs) are essential for the induction and maintenance of pluripotency of embryonic stem cells (ESCs), yet little is known about their roles in establishing the three lineages of the pre-implantation embryo. Here, we show that Klf5 is required for the formation of the trophectoderm (TE) and the inner cell mass (ICM), and for repressing primitive endoderm (PE) development. Although cell polarity appeared normal, Klf5 mutant embryos arrested at the blastocyst stage and failed to hatch due to defective TE development. Klf5 acted cell-autonomously in the TE, downstream of Fgf4 and upstream of Cdx2, Eomes and Krt8. In the ICM, loss of Klf5 resulted in reduced expression of pluripotency markers Oct4 and Nanog, but led to increased Sox17 expression in the PE, suggesting that Klf5 suppresses the PE lineage. Consistent with this, overexpression of Klf5 in transgenic embryos was sufficient to suppress the Sox17(+) PE lineage in the ICM. Klf5 overexpression led to a dose-dependent decrease in Sox17 promoter activity in reporter assays in cultured cells. Moreover, in chimeric embryos, Klf5(-/-) cells preferentially contributed to the Sox17(+) PE lineage and Cdx2 expression was not rescued in Klf5(-/-) outer cells. Finally, outgrowths from Klf5(-/-) embryos failed to form an ICM/pluripotent colony, had very few Oct4(+) or Cdx2(+) cells, but showed an increase in the percentage of Sox17(+) PE cells. These findings demonstrate that Klf5 is a dynamic regulator of all three lineages in the pre-implantation embryo by promoting the TE and epiblast lineages while suppressing the PE lineage.

Molecular mechanisms of pluripotency and reprogramming

Pluripotent stem cells are able to form any terminally differentiated cell. They have opened new doors for experimental and therapeutic studies to understand early development and to cure degenerative diseases in a way not previously possible. Nevertheless, it remains important to resolve and define the mechanisms underlying pluripotent stem cells, as that understanding will impact strongly on future medical applications. The capture of pluripotent stem cells in a dish is bound to several landmark discoveries, from the initial culture and phenotyping of pluripotent embryonal carcinoma cells to the recent induction of pluripotency in somatic cells. On this developmental time line, key transcription factors, such as Oct4, Sox2 or Nanog, have been revealed not only to regulate but also to functionally induce pluripotency. These early master regulators of development control developmental signalling pathways that affect the cell cycle, regulate gene expression, modulate the epigenetic state and repair DNA damage. Besides transcription factors, microRNAs have recently been shown to play important roles in gene expression and are embedded into the regulatory network orchestrating cellular development. However, there are species-specific differences in pluripotent cells, such as surface marker expression and growth factor requirements. Such differences and their underlying developmental pathways require clear definition and have major impacts on the preclinical test bed of pluripotent cells.

Efficient derivation of pluripotent stem cells from siRNA-mediated Cdx2-deficient mouse embryos

In the early mammalian embryo, lineage separation of and subsequent crosstalk between the trophectoderm (TE) and inner cell mass (ICM) are required to support further development. Previous studies have shown that the homeobox transcription factor Cdx2 is required for TE differentiation and that lack of Cdx2 expression causes death of embryos at the peri-implantation stage. In this study, we effectively eliminated Cdx2 transcripts by microinjection of siRNA into embryos and evaluated the effect on efficiency of deriving embryonic stem cells (ESCs). By this approach, we successfully created nonviable embryos similar to reported knockout embryos. Accordingly, the efficiency of ESC derivation dropped from 19.1% in control blastocysts to 2% in Cdx2-deficient blastocysts, indicating loss of pluripotency in the ICM. Strikingly, when 8-cell stage embryos were cultured under ESC culture conditions before lineage separation, fully functional pluripotent stem cell lines were obtained, with efficiency even greater than that for control embryos. These results demonstrate that Cdx2 plays an essential role within the microenvironment created by the TE to support ICM pluripotency but that the ESC culture system, with mouse embryonic fibroblasts, could rescue the pluripotent cell population for efficient ESC derivation.

A role for borg5 during trophectoderm differentiation

Stem cell differentiation is accompanied by a gradual cellular morphogenesis and transcriptional changes. Identification of morphological regulators that control cell behavior during differentiation could shed light on how cell morphogenesis is coupled to transcriptional changes during development. By analyzing cellular behavior during differentiation of mouse embryonic stem cells (ESCs), we uncover a role of Borg5 (binder of Rho guanosine 5′-triphosphatase 5) in regulating trophectoderm (TE) cell morphogenesis. We report that differentiation of ESCs toward TE is accompanied by enhanced actin protrusion and cell motility that require upregulation of Borg5. Borg5 interacts with both Cdc42 and atypical protein kinase C (aPKC) and functions downstream of Cdc42 to enhance TE cell motility. Borg5 is required for the sorting of differentiating TE to the outside of ESCs in vitro. In developing embryos, Borg5 protein localizes to cell–cell contacts and the cytoplasm after compaction. It exhibits higher levels of expression in outer cells than in inner cells in morula and blastocysts. Reduction of Borg5 disrupts aPKC localization and inhibits blastocyst formation. Since Cdx2 and Borg5 facilitate each other's expression as ESCs differentiate toward TE, we propose that cell morphogenesis is coupled with transcriptional changes to regulate TE differentiation. Our studies also demonstrate the utility of ESCs in identifying morphological regulators important for development.

Disorganized epithelial polarity and excess trophectoderm cell fate in preimplantation embryos lacking E-cadherin

The first two cell lineages in the mouse, the surface trophectoderm (TE) and inner cell mass (ICM), are morphologically distinguishable by E3.5, with the outer TE forming a polarized epithelial layer enclosing the apolar ICM. We show here that in mouse embryos completely lacking both maternal and zygotic E-cadherin (cadherin 1), the normal epithelial morphology of outside cells is disrupted, but individual cells still initiate TE- and ICM-like fates. A larger proportion of cells than normal showed expression of TE markers such as Cdx2, suggesting that formation of an organized epithelium is not necessary for TE-specific gene expression. Individual cells in such embryos still generated an apical domain that correlated with elevated Cdx2 expression. We also show that repolarization can occur in isolated early ICMs from both wild-type and Cdx2 mutant embryos, indicating that Cdx2 is not required for initiating polarity. The results demonstrate that epithelial integrity mediated by E-cadherin is not required for Cdx2 expression, but is essential for the normal allocation of TE and ICM cells. They also show that Cdx2 expression is strongly linked to apical membrane polarization.

Regulation of early trophoblast differentiation - lessons from the mouse

The earliest stages of trophoblast differentiation are of tremendous importance to mediate implantation and to lay the anatomical foundations for normal placental development and function throughout gestation. Yet our molecular insights into these early developmental processes in humans have been limited by the inaccessibility of material and the unavailability of trophoblast cell lines that fully recapitulate the behaviour of early placental trophoblast. In this review we highlight recent advances that have come from the study of distinct stem cell types representative of the embryonic and extraembryonic lineages in the mouse, and from the study of mouse mutants. These models have revealed the presence of intricate transcriptional networks that are set up by signalling pathways, translating extracellular growth factor and cell positional information into distinct lineage-specific transcriptional programmes. The trophoblast specificity of these networks is ensured by epigenetic mechanisms including DNA methylation and histone modifications that complement each other to define trophoblast cell fate and differentiation. Despite the anatomical differences between mouse and human placentas, it seems that important aspects of early trophoblast specification are conserved between both species. Thus we may be able to build on our insights from the mouse to better understand early trophoblast differentiation in the human conceptus which is important for improving assisted reproductive technologies and may enable us in the future to derive human trophoblast stem cell lines. These advances will facilitate the investigation of genetic, epigenetic and environmental influences on early trophoblast differentiation in normal as well as in pathological conditions.

Trophoblast cell lineage in cloned mouse embryos

Most conceptuses derived by somatic cell nuclear transfer (SCNT) in mice undergo developmental arrest as a result of embryonic or extraembryonic defects. Even when fetuses survive to term, prominent placental overgrowth or placentomegaly is often present, indicating that SCNT affects the development of trophoblast cell lineage. The trophoblast cell lineage is established at the blastocyst stage when the stem cell population of the trophoblast cell lineage resides in the polar trophectoderm. Therefore, it is possible that the developmental arrest and placentomegaly that accompany SCNT are induced by insufficient reprogramming of the donor somatic nucleus to enable the cells to acquire full potency as stem cells of the trophoblast cell lineage. Despite the abnormalities of the extraembryonic tissues of SCNT embryos, trophoblast stem (TS) cell lines have been successfully isolated from SCNT blastocysts and their properties appear to be indistinguishable from those of TS cells derived from native blastocysts. This suggests that SCNT does not affect the emergence and autonomous properties of TS cells. In this review, we discuss specification of cell lineage and the extent of reprogramming of TS cells in SCNT blastocysts.

Epigenetic modification affecting expression of cell polarity and cell fate genes to regulate lineage specification in the early mouse embryo

Formation of inner and outer cells of the mouse embryo distinguishes pluripotent inner cell mass (ICM) from differentiating trophectoderm (TE). Carm1, which methylates histone H3R17 and R26, directs cells to ICM rather that TE. To understand the mechanism by which this epigenetic modification directs cell fate, we generated embryos with in vivo-labeled cells of different Carm1 levels, using time-lapse imaging to reveal dynamics of their behavior, and related this to cell polarization. This shows that Carm1 affects cell fate by promoting asymmetric divisions, that direct one daughter cell inside, and cell engulfment, where neighboring cells with lower Carm1 levels compete for outside positions. This is associated with changes to the expression pattern and spatial distribution of cell polarity proteins: Cells with higher Carm1 levels show reduced expression and apical localization of Par3 and a dramatic increase in expression of PKCII, antagonist of the apical protein aPKC. Expression and basolateral localization of the mouse Par1 homologue, EMK1, increases concomitantly. Increased Carm1 also reduces Cdx2 expression, a transcription factor key for TE differentiation. These results demonstrate how the extent of a specific epigenetic modification could affect expression of cell polarity and fate-determining genes to ensure lineage allocation in the mouse embryo.

Breaking ground on translational stem cell research

Sponsored by the New York Stem Cell Foundation (NYSCF), the "Fourth Annual Translational Stem Cell Research Conference: Breaking Ground" convened October 13-14, 2009 at The Rockefeller University in New York City to discuss translational stem cell research. Attracting over 400 scientists, patient advocates, and stem cell research supporters from fifteen countries, the two-day conference featured an afternoon of panel discussions, intended for a broad audience, followed by a second day of scientific talks and poster presentations. This report summarizes both days of this exciting conference.

Developmental control of the early mammalian embryo: competition among heterogeneous cells that biases cell fate

The temporal and spatial segregation of the two extra-embryonic cell lineages, trophectoderm and primitive endoderm (TE and PE respectively), from the pluripotent epiblast (EPI) during mammalian pre-implantation development are prerequisites for the successful implantation of the blastocyst. The mechanisms underlying these earliest stages of development remain a fertile topic for research and informed debate. In recent years novel roles for various transcription factors, polarity factors and signalling cascades have been uncovered. This mini-review seeks to summarise some of this work and to put it into the context of the regulative nature of early mammalian development and to highlight how the increasing evidence of naturally occurring asymmetries and heterogeneity in the embryo can bias specification of the distinct cell types of the blastocyst.

The role of YAP transcription coactivator in regulating stem cell self-renewal and differentiation

Yes-associated protein (YAP) is a potent transcription coactivator acting via binding to the TEAD transcription factor, and plays a critical role in organ size regulation. YAP is phosphorylated and inhibited by the Lats kinase, a key component of the Hippo tumor suppressor pathway. Elevated YAP protein levels and gene amplification have been implicated in human cancer. In this study, we report that YAP is inactivated during embryonic stem (ES) cell differentiation, as indicated by decreased protein levels and increased phosphorylation. Consistently, YAP is elevated during induced pluripotent stem (iPS) cell reprogramming. YAP knockdown leads to a loss of ES cell pluripotency, while ectopic expression of YAP prevents ES cell differentiation in vitro and maintains stem cell phenotypes even under differentiation conditions. Moreover, YAP binds directly to promoters of a large number of genes known to be important for stem cells and stimulates their expression. Our observations establish a critical role of YAP in maintaining stem cell pluripotency.

A feed-forward circuit linking wingless, fat-dachsous signaling, and the warts-hippo pathway to Drosophila wing growth

The secreted morphogen Wingless promotes Drosophila wing growth by fueling a wave front of Fat-Dachsous signaling that recruits new cells into the wing primordium.

During development, the Drosophila wing primordium undergoes a dramatic increase in cell number and mass under the control of the long-range morphogens Wingless (Wg, a Wnt) and Decapentaplegic (Dpp, a BMP). This process depends in part on the capacity of wing cells to recruit neighboring, non-wing cells into the wing primordium. Wing cells are defined by activity of the selector gene vestigial (vg) and recruitment entails the production of a vg-dependent “feed-forward signal” that acts together with morphogen to induce vg expression in neighboring non-wing cells. Here, we identify the protocadherins Fat (Ft) and Dachsous (Ds), the Warts-Hippo tumor suppressor pathway, and the transcriptional co-activator Yorkie (Yki, a YES associated protein, or YAP) as components of the feed-forward signaling mechanism, and we show how this mechanism promotes wing growth in response to Wg. We find that vg generates the feed-forward signal by creating a steep differential in Ft-Ds signaling between wing and non-wing cells. This differential down-regulates Warts-Hippo pathway activity in non-wing cells, leading to a burst of Yki activity and the induction of vg in response to Wg. We posit that Wg propels wing growth at least in part by fueling a wave front of Ft-Ds signaling that propagates vg expression from one cell to the next.

Under normal conditions, animals and their various body parts grow until they achieve a genetically predetermined size and shape—a process governed by secreted organizer proteins called morphogens. How morphogens control growth remains unknown. In Drosophila, wings develop at the larval stage from wing primordia. Recently, we discovered that the morphogen Wingless promotes growth of the Drosophila wing by inducing the recruitment of neighboring cells into the wing primordium. Wing cells are defined by the expression of the “selector” gene vestigial. Recruitment depends on the capacity of wing cells to send a short-range, feed-forward signal that allows Wingless to activate vestigial in adjacent non-wing cells. Here, we identify the molecular components and circuitry of the recruitment process. We define the protocadherins Fat and Dachsous as a bidirectional ligand-receptor system that is controlled by vestigial to generate the feed-forward signal. Further, we show that the signal is transduced by the conserved Warts-Hippo tumor suppressor pathway via activation of its transcriptional effector Yorkie. Finally, we propose that Wingless propels wing growth by fueling a wave front of Fat-Dachsous signaling and Yorkie activity that propagates vestigial expression from one cell to the next.

Cell polarity regulator PARD6B is essential for trophectoderm formation in the preimplantation mouse embryo

In preimplantation mouse development, the first cell lineages to be established are the trophectoderm (TE) and inner cell mass. TE possesses epithelial features, including apical-basal cell polarity and intercellular junctions, which are crucial to generate a fluid-filled cavity in the blastocyst. Homologs of the partitioning defective (par) genes in Caenorhabditis elegans are critical regulators of cell polarity. However, their roles in regulating TE differentiation and blastocyst formation remain unclear. Here, the role of mouse Pard6b, a homolog of par-6 gene and a component of the PAR-atypical protein kinase C (aPKC) complex, was investigated. Pard6b expression was knocked down by microinjecting RNA interference construct into zygotes. Pard6b-knockdown embryos cleaved and compacted normally but failed to form the blastocyst cavity. The cavitation failure is likely the result of defective intercellular junctions, because Pard6b knockdown caused abnormal distribution of actin filaments and TJP1 (ZO-1) tight junction (TJ) protein and interfered with cavitation in chimeras containing cells from normal embryos. Defective TJ formation may be caused by abnormal cell polarization, because the apical localization of PRKCZ (aPKCzeta) was absent in Pard6b-knockdown embryos. Pard6b knockdown also diminished the expression of CDX2, a TE-lineage transcription factor, in the outer cells. TEAD4, a transcriptional activator that is required for Cdx2 expression and cavity formation, was not essential for the transcription of Pard6b. Taken together, Pard6b is necessary for blastocyst morphogenesis, particularly the development of TE-specific features-namely, the apical-basal cell polarity, formation of TJ, paracellular permeability sealing, and up-regulated expression of Cdx2.

The Hippo-YAP pathway in organ size control and tumorigenesis: an updated version

The Hippo signaling pathway is gaining recognition as an important player in both organ size control and tumorigenesis, which are physiological and pathological processes that share common cellular signaling mechanisms. Upon activation by stimuli such as high cell density in cell culture, the Hippo pathway kinase cascade phosphorylates and inhibits the Yes-associated protein (YAP)/TAZ transcription coactivators representing the major signaling output of the pathway. Altered gene expression resulting from YAP/TAZ inhibition affects cell number by repressing cell proliferation and promoting apoptosis, thereby limiting organ size. Recent studies have provided new insights into the Hippo signaling pathway, elucidating novel phosphorylation-dependent and independent mechanisms of YAP/Yki inhibition by the Hippo pathway, new Hippo pathway components, novel YAP target transcription factors and target genes, and the three-dimensional structure of the YAP-TEAD complex, and providing further evidence for the involvement of YAP and the Hippo pathway in tumorigenesis.

Brg1 is required for Cdx2-mediated repression of Oct4 expression in mouse blastocysts

During blastocyst formation the segregation of the inner cell mass (ICM) and trophectoderm is governed by the mutually antagonistic effects of the transcription factors Oct4 and Cdx2. Evidence indicates that suppression of Oct4 expression in the trophectoderm is mediated by Cdx2. Nonetheless, the underlying epigenetic modifiers required for Cdx2-dependent repression of Oct4 are largely unknown. Here we show that the chromatin remodeling protein Brg1 is required for Cdx2-mediated repression of Oct4 expression in mouse blastocysts. By employing a combination of RNA interference (RNAi) and gene expression analysis we found that both Brg1 Knockdown (KD) and Cdx2 KD blastocysts exhibit widespread expression of Oct4 in the trophectoderm. Interestingly, in Brg1 KD blastocysts and Cdx2 KD blastocysts, the expression of Cdx2 and Brg1 is unchanged, respectively. To address whether Brg1 cooperates with Cdx2 to repress Oct4 transcription in the developing trophectoderm, we utilized preimplantation embryos, trophoblast stem (TS) cells and Cdx2-inducible embryonic stem (ES) cells as model systems. We found that: (1) combined knockdown (KD) of Brg1 and Cdx2 levels in blastocysts resulted in increased levels of Oct4 transcripts compared to KD of Brg1 or Cdx2 alone, (2) endogenous Brg1 co-immunoprecipitated with Cdx2 in TS cell extracts, (3) in blastocysts Brg1 and Cdx2 co-localize in trophectoderm nuclei and (4) in Cdx2-induced ES cells Brg1 and Cdx2 are recruited to the Oct4 promoter. Lastly, to determine how Brg1 may induce epigenetic silencing of the Oct4 gene, we evaluated CpG methylation at the Oct4 promoter in the trophectoderm of Brg1 KD blastocysts. This analysis revealed that Brg1-dependent repression of Oct4 expression is independent of DNA methylation at the blastocyst stage. In toto, these results demonstrate that Brg1 cooperates with Cdx2 to repress Oct4 expression in the developing trophectoderm to ensure normal development.

Yorkie: the final destination of Hippo signaling

The Hippo signaling pathway is a key regulator of growth during animal development, whereas loss of normal Hippo pathway activity is associated with a wide range of cancers. Hippo signaling represses growth by inhibiting the activity of a transcriptional co-activator protein, known as Yorkie in Drosophila and Yap in vertebrates. In the 5 years since the first report linking Yorkie to Hippo signaling, intense interest in this pathway has led to rapid increases in our understanding of the action and regulation of Yorkie/Yap, which we review here. These studies have also emphasized the complexity of Yorkie/Yap regulation, including multiple, distinct mechanisms for repressing its transcriptional activity, and multiple DNA-binding partner proteins that can direct Yorkie to distinct downstream target genes.

Resolution of cell fate decisions revealed by single-cell gene expression analysis from zygote to blastocyst

Three distinct cell types are present within the 64-cell stage mouse blastocyst. We have investigated cellular development up to this stage using single-cell expression analysis of more than 500 cells. The 48 genes analyzed were selected in part based on a whole-embryo analysis of more than 800 transcription factors. We show that in the morula, blastomeres coexpress transcription factors specific to different lineages, but by the 64-cell stage three cell types can be clearly distinguished according to their quantitative expression profiles. We identify Id2 and Sox2 as the earliest markers of outer and inner cells, respectively. This is followed by an inverse correlation in expression for the receptor-ligand pair Fgfr2/Fgf4 in the early inner cell mass. Position and signaling events appear to precede the maturation of the transcriptional program. These results illustrate the power of single-cell expression analysis to provide insight into developmental mechanisms. The technique should be widely applicable to other biological systems.

Maternally and zygotically provided Cdx2 have novel and critical roles for early development of the mouse embryo

Divisions of polarised blastomeres that allocate polar cells to outer and apolar cells to inner positions initiate the first cell fate decision in the mouse embryo. Subsequently, outer cells differentiate into trophectoderm while inner cells retain pluripotency to become inner cell mass (ICM) of the blastocyst. Elimination of zygotic expression of trophectoderm-specific transcription factor Cdx2 leads to defects in the maintenance of the blastocyst cavity, suggesting that it participates only in the late stage of trophectoderm formation. However, we now find that mouse embryos also have a maternally provided pool of Cdx2 mRNA. Moreover, depletion of both maternal and zygotic Cdx2 from immediately after fertilization by three independent approaches, dsRNAi, siRNAi and morpholino oligonucleotides, leads to developmental arrest at much earlier stages than expected from elimination of only zygotic Cdx2. This developmental arrest is associated with defects in cell polarisation, reflected by expression and localisation of cell polarity molecules such as Par3 and aPKC and cell compaction at the 8- and 16-cell stages. Cells deprived of Cdx2 show delayed development with increased cell cycle length, irregular cell division and increased incidence of apoptosis. Although some Cdx2-depleted embryos initiate cavitation, the cavity cannot be maintained. Furthermore, expression of trophectoderm-specific genes, Gata3 and Eomes, and also the trophectoderm-specific cytokeratin intermediate filament, recognised by Troma1, are greatly reduced or undetectable. Taken together, our results indicate that Cdx2 participates in two steps leading to trophectoderm specification: appropriate polarisation of blastomeres at the 8- and 16-cell stage and then the maintenance of trophectoderm lineage-specific differentiation.

Governing cell lineage formation in cloned mouse embryos

Blastomeres of the pre-implantation mouse embryo form trophectoderm and inner cell mass via a process that requires the transcription factors Tead4, Cdx2, Oct4 and Nanog. In mouse morulae cloned by somatic cell nuclear transfer, we observed that the trophectoderm transcription factor Cdx2 is expressed very differently at the protein level compared to time- and stage-matched fertilized counterparts. Protein levels of Cdx2 in cloned embryos appear 'erratic,' i.e. are widely distributed, when plotted as histograms. In contrast to Cdx2, protein levels of the upstream factor Tead4 and of inner cell mass transcription factors Oct4 and Nanog are similar in cloned and fertilized embryos. These observations suggest that trophectoderm formation is initiated but not maintained correctly in cloned mouse morulae, which is consistent with cloned blastocysts' limited implantation and post-implantation success. Because a cell's ability to differentiate is greatly enhanced if it is surrounded by more cells differentiating the same way, a concept designated community effect by Gurdon, we reasoned that the insufficient cell numbers often observed in cloned embryos might lead to premature Cdx2 expression and differentiation of blastomeres into trophectoderm. Therefore, we created larger cloned embryos by aggregating them at the 4-cell stage. Homologous aggregation stimulates expression of multiple signaling pathways' components and results in cloned embryos with levels of Cdx2 similar to fertilized embryos. Most of the resultant morulae and blastocysts consist of cells of all three founders, indicating that aggregation increases stability of all of the individual components. We conclude that the induction of pluripotency in cloned embryos is more efficient than previously assumed, and we propose that a minimum cell number is necessary to stabilize pluripotency and inhibit premature expression of Cdx2 in cloned mouse embryos.

The transcription factor TCFAP2C/AP-2gamma cooperates with CDX2 to maintain trophectoderm formation

In mammals, cell lineage specification is established at the blastocyst stage. At this stage, transcription factor Cdx2 represses pluripotency genes, thus promoting extraembryonic trophoblast fate. Recently, transcription factor Gata3 was shown to act in a parallel pathway in promoting trophoblast cell fate, suggesting that there are more factors working in the trophoblast lineage. Here, we report that the transcription factor Tcfap2c is expressed at a high level in the trophectoderm and is able to induce trophoblast fate in embryonic stem cells. Trophoblast fate induced by Tcfap2c does not require Cdx2 and vice versa, suggesting that the molecules act in alternative pathways. However, both Tcfap2c and Cdx2 are required for the upregulation of Elf5, a marker of trophoblast stem cell maintenance, suggesting that both factors are required for stable trophoblast induction. Tcfap2c-induced trophoblast-like cells are stable in long-term culture, indicating that they are capable of self-renewal. Tcfap2c-controlled trophoblast maintenance involves the induction of Cdx2 and the repression of the pluripotency factor Nanog. Tcfap2c-induced trophoblast-like cells differentiate to trophoblast derivatives in vitro and contribute to the trophectoderm in blastocysts in vivo. Taken together, these observations suggest that Tcfap2c and Cdx2 cooperate to override the pluripotency program and establish the extraembryonic trophoblast maintenance program in murine embryos.

Making the blastocyst: lessons from the mouse

Mammalian preimplantation development, which is the period extending from fertilization to implantation, results in the formation of a blastocyst with three distinct cell lineages. Only one of these lineages, the epiblast, contributes to the embryo itself, while the other two lineages, the trophectoderm and the primitive endoderm, become extra-embryonic tissues. Significant gains have been made in our understanding of the major events of mouse preimplantation development, and recent discoveries have shed new light on the establishment of the three blastocyst lineages. What is less clear, however, is how closely human preimplantation development mimics that in the mouse. A greater understanding of the similarities and differences between mouse and human preimplantation development has implications for improving assisted reproductive technologies and for deriving human embryonic stem cells.

The placenta: transcriptional, epigenetic, and physiological integration during development

The placenta provides critical transport functions between the maternal and fetal circulations during intrauterine development. Formation of this interface relies on coordinated interactions among transcriptional, epigenetic, and environmental factors. Here we describe these mechanisms in the context of the differentiation of placental cells (trophoblasts) and synthesize current knowledge about how they interact to generate a functional placenta. Developing an understanding of these pathways contributes to an improvement of our models for studying trophoblast biology and sheds light on the etiology of pregnancy complications and the in utero programming of adult diseases.

Gata3 regulates trophoblast development downstream of Tead4 and in parallel to Cdx2

The mouse blastocyst and stem cells derived from its tissue lineages provide a unique genetic system for examining the establishment and loss of pluripotency. The transcription factor Cdx2 plays a central role by repressing pluripotency genes, such as Oct4, and promoting extraembryonic trophoblast fate at the blastocyst stage. However, genetic evidence has suggested that Cdx2 does not work alone in the trophoblast lineage. We have used bioinformatic and functional genomic strategies to identify the transcription factor Gata3 as a trophoblast factor. We show Gata3 to be capable of inducing trophoblast fate in embryonic stem cells and driving trophoblast differentiation in trophoblast stem cells. In addition, Cdx2 is not required for Gata3-induced expression of a subset of trophoblast genes in embryonic stem cells. We show that Gata3 is coexpressed with Cdx2 in the blastocyst, but this does not depend on Cdx2. In the embryo, expression of Gata3, like that of Cdx2, depends on Tead4, and the expression of both factors becomes restricted to trophoblast by a mechanism that does not initially rely on Oct4. These observations suggest that Gata3 and Cdx2 can act in parallel pathways downstream of Tead4 to induce the expression of common and independent targets in the trophoblast lineage, whereas Oct4 is required for continued repression of trophoblast fate in the embryonic lineage.

Origin and formation of the first two distinct cell types of the inner cell mass in the mouse embryo

A crucial question in mammalian development is how cells of the early embryo differentiate into distinct cell types. The first decision is taken when cells undertake waves of asymmetric division that generate one daughter on the inside and one on the outside of the embryo. After this division, some cells on the inside remain pluripotent and give rise to the epiblast, and hence the future body, whereas others develop into the primitive endoderm, an extraembryonic tissue. How the fate of these inside cells is decided is unknown: Is the process random, or is it related to their developmental origins? To address this question, we traced all cells by live-cell imaging in intact, unmanipulated embryos until the epiblast and primitive endoderm became distinct. This analysis revealed that inner cell mass (ICM) cells have unrestricted developmental potential. However, cells internalized by the first wave of asymmetric divisions are biased toward forming pluripotent epiblast, whereas cells internalized in the next two waves of divisions are strongly biased toward forming primitive endoderm. Moreover, we show that cells internalized by the second wave up-regulate expression of Gata6 and Sox17, and changing the expression of these genes determines whether the cells become primitive endoderm. Finally, with our ability to determine the origin of cells, we find that inside cells that are mispositioned when they are born can sort into the correct layer. In conclusion, we propose a model in which the timing of cell internalization, cell position, and cell sorting combine to determine distinct lineages of the preimplantation mouse embryo.

FGF signal-dependent segregation of primitive endoderm and epiblast in the mouse blastocyst

Primitive endoderm (PE) and epiblast (EPI) are two lineages derived from the inner cell mass (ICM) of the E3.5 blastocyst. Recent studies showed that EPI and PE progenitors expressing the lineage-specific transcriptional factors Nanog and Gata6, respectively, arise progressively as the ICM develops. Subsequent sorting of the two progenitors during blastocyst maturation results in the ormation of morphologically distinct EPI and PE layers at E4.5. It is, however, unknown how the initial differences between the two populations become established in the E3.5 blastocyst. Because the ICM cells are derived from two distinct rounds of polarized cell divisions during cleavage, a possible role for cell lineage history in promoting EPI versus PE fate has been proposed. We followed cell lineage from the eight-cell stage by live cell tracing and could find no clear linkage between developmental history of individual ICM cells and later cell fate. However, modulating FGF signaling levels by inhibition of the receptor/MAP kinase pathway or by addition of exogenous FGF shifted the fate of ICM cells to become either EPI or PE, respectively. Nanog- or Gata6-expressing progenitors could still be shifted towards the alternative fate by modulating FGF signaling during blastocyst maturation, suggesting that the ICM progenitors are not fully committed to their final fate at the time that initial segregation of gene expression occurs. In conclusion, we propose a model in which stochastic and progressive specification of EPI and PE lineages occurs during maturation of the blastocyst in an FGF/MAP kinase signal-dependent manner.

Preimplantation development of mouse: a view from cellular behavior

A mature animal body contains a variety of different cell types, and these cells are distributed in a well-organized fashion along the body axes. One of the major questions in developmental biology is how cells acquire different characteristics. In addition, it is important to understand how the embryo forms the body axes and how cells are allocated along these axes during development. Among mammalian species, the molecular mechanisms that regulate embryonic development have been well analyzed and characterized in mice. Here, mouse preimplantation embryonic development is briefly summarized and our current understanding of this complex process based on recent observations is reviewed.

Structural insights into the YAP and TEAD complex

The Yes-associated protein (YAP) transcriptional coactivator is a key regulator of organ size and a candidate human oncogene inhibited by the Hippo tumor suppressor pathway. The TEAD family of transcription factors binds directly to and mediates YAP-induced gene expression. Here we report the three-dimensional structure of the YAP (residues 50-171)-TEAD1 (residues 194-411) complex, in which YAP wraps around the globular structure of TEAD1 and forms extensive interactions via three highly conserved interfaces. Interface 3, including YAP residues 86-100, is most critical for complex formation. Our study reveals the biochemical nature of the YAP-TEAD interaction, and provides a basis for pharmacological intervention of YAP-TEAD hyperactivation in human diseases.

Mouse ES cell culture system as a model of development

Mouse embryonic stem (mES) cells are pluripotent stem cells derived from pre-implantation embryos. They are regarded as an essential tool for studying mouse development, as they provide a means for generating knock-out mouse lines. This, however, is not the sole utility of the mES cell system. They undergo differentiation in culture, mimicking the morphological differentiation of peri-implantation embryos from epiblast to egg-cylinder stage. Moreover, they retain the capacity to respond to triggers of differentiation toward trophectoderm and primitive endoderm by forced activation. For these reasons, mES cells can be regarded as a useful tool for analyzing molecular mechanisms underlying early mouse development.

Mechanisms of trophectoderm fate specification in preimplantation mouse development

During preimplantation mouse development, embryos establish two distinct cell lineages by the time of blastocyst formation: trophectoderm (TE) and inner cell mass (ICM). To explain the mechanism of this cell fate specification, two classical models, namely the inside-outside model and polarity model have been proposed based on experimental manipulation studies on embryos. This review summarizes recent findings on the molecular mechanisms of fate specification, and discusses how these findings fit into the classical models. TE development is regulated by a transcription factor cascade, the core transcription factors of which are Tead4 and Cdx2. The transcriptional activity of Tead4 is regulated by the position-dependent Hippo signaling pathway, thus supporting the inside-outside model. In contrast, several findings support the polarity model; some other findings suggest different mechanisms. We also discuss how the two classical models could be further developed in the light of recent molecular findings.

Establishment of trophectoderm and inner cell mass lineages in the mouse embryo

The first cell lineage specification in mouse embryo development is the formation of trophectoderm (TE) and inner cell mass (ICM) of the blastocyst. This article is to review and discuss the current knowledge on the cellular and molecular mechanisms of this particular event. Several transcription factors have been identified as the critical regulators of the formation or maintenance of the two cell lineages. The establishment of TE manifests as the formation of epithelium, and is dependent on many structural and regulatory components that are commonly found and that function in many epithelial tissues. Distinct epithelial features start to emerge at the late 8-cell stage, but the fates of blastomeres are not fixed as TE or ICM until around 32-cell stage. The location of blastomeres at this stage, that is, external or internal of the embryo, in effect defines the commitment towards the TE or ICM lineage, respectively. Some studies implicate the presence of a developmental bias among blastomeres at 2- or 4-cell stage, although it is unlikely to play a decisive role in the establishment of TE and ICM. The unique mode of cell lineage specification in the mouse embryo is further discussed in comparison with the formation of initial cell lineages, namely the three germ layers, in non-mammalian embryos.

From mouse egg to mouse embryo: polarities, axes, and tissues

This review describes the three classical models (mosaic, positional, and polarization) proposed to explain blastocyst formation and summarizes the evidence concerning them. It concludes that the polarization model incorporates elements of the other two models and best explains most known information. I discuss key requirements of a molecular basis for the generation and stabilization of polarity and identify ezrin/E-cadherin, PAR proteins, and Cdx2 as plausible key molecular players. I also discuss the idea of a network process operating to build cell allocations progressively into committed differences. Finally, this review critically considers the possibility of developmental information being encoded within the oocyte and zygote. No final decision can be reached on a mechanism of action underlying any encoded information, but a cell interaction process model is preferred over one that relies solely on differential inheritance.

Early embryonic cell fate decisions in the mouse

During development, initially totipotent cells of the embryo specialize to form discrete tissue lineages. The first lineages to form in the mouse are the extraembryonic tissues. Meanwhile, cells that do not become extraembryonic retain a pluripotent fate since they can give rise to all the germ layers of the fetus. Pluripotent stem cell lines have been derived from the fetal lineage at several stages of development. Interestingly, multipotent stem cell lines have been derived from the extraembryonic lineages around the same time. Examining the regulation of early embryonic cell fate decisions is therefore a rare opportunity to examine establishment of stem cell progenitors. Classical studies have provided considerable insight into specification of the first three lineages and use of modern molecular and imaging techniques has advanced this field further. Here we describe current understanding of the diverse molecular mechanisms that lead to establishment and maintenance of the first three lineages during mouse development.

BMP4 induction of trophoblast from mouse embryonic stem cells in defined culture conditions on laminin

Because mouse embryonic stem cells (mESCs) do not contribute to the formation of extraembryonic placenta when they are injected into blastocysts, it is believed that mESCs do not differentiate into trophoblast whereas human embryonic stem cells (hESCs) can express trophoblast markers when exposed to bone morphogenetic protein 4 (BMP4) in vitro. To test whether mESCs have the potential to differentiate into trophoblast, we assessed the effect of BMP4 on mESCs in a defined monolayer culture condition. The expression of trophoblast-specific transcription factors such as Cdx2, Dlx3, Esx1, Gata3, Hand1, Mash2, and Plx1 was specifically upregulated in the BMP4-treated differentiated cells, and these cells expressed trophoblast markers. These results suggest that BMP4 treatment in defined culture conditions enabled mESCs to differentiate into trophoblast. This differentiation was inhibited by serum or leukemia inhibitory factor, which are generally used for mESC culture. In addition, we studied the mechanism underlying BMP4-directed mESC differentiation into trophoblast. Our results showed that BMP4 activates the Smad pathway in mESCs inducing Cdx2 expression, which plays a crucial role in trophoblast differentiation, through the binding of Smad protein to the Cdx2 genomic enhancer sequence. Our findings imply that there is a common molecular mechanism underlying hESC and mESC differentiation into trophoblast.

Herding Hippos: regulating growth in flies and man

Control of cell number requires the coordinate regulation of cell proliferation and cell death. Studies in both the fly and mouse have identified the Hippo kinase pathway as a key signaling pathway that controls cell proliferation and apoptosis. Several studies have implicated the Hippo pathway in a variety of cancers. Recent studies have also revealed a role for the Hippo pathway in the control of cell fate decisions during development. In this review, we will cover the current model of Hippo signaling in development. We will explore the differences between the Hippo pathway in invertebrates and mammals, and focus on recent advances in understanding how this conserved pathway is regulated.

Crucial role for Mst1 and Mst2 kinases in early embryonic development of the mouse

Mammalian sterile 20-like kinases 1 and 2 (Mst1 and Mst2, respectively) are potent serine/threonine kinases that are involved in cell proliferation and cell death. To investigate the physiological functions of Mst1 and Mst2, we generated Mst1 and Mst2 mutant mice. Mst1(-/-) and Mst2(-/-) mice were viable and fertile and developed normally, suggesting possible functional overlaps between the two genes. A characterization of heterozygous and homozygous combinations of Mst1 and Mst2 mutant mice showed that mice containing a single copy of either gene underwent normal organ development; however, Mst1(-/-); Mst2(-/-) mice lacking both Mst1 and Mst2 genes started dying in utero at approximately embryonic day 8.5. Mst1(-/-); Mst2(-/-) mice exhibited severe growth retardation, failed placental development, impaired yolk sac/embryo vascular patterning and primitive hematopoiesis, increased apoptosis in placentas and embryos, and disorganized proliferating cells in the embryo proper. These findings indicate that both Mst1 and Mst2 kinases play essential roles in early mouse development, regulating placental development, vascular patterning, primitive hematopoiesis, and cell proliferation and survival.

Suppression of Erk signalling promotes ground state pluripotency in the mouse embryo

Embryonic stem (ES) cells can be derived and propagated from multiple strains of mouse and rat through application of small-molecule inhibitors of the fibroblast growth factor (FGF)/Erk pathway and of glycogen synthase kinase 3. These conditions shield pluripotent cells from differentiation-inducing stimuli. We investigate the effect of these inhibitors on the development of pluripotent epiblast in intact pre-implantation embryos. We find that blockade of Erk signalling from the 8-cell stage does not impede blastocyst formation but suppresses development of the hypoblast. The size of the inner cell mass (ICM) compartment is not reduced, however. Throughout the ICM, the epiblast-specific marker Nanog is expressed, and in XX embryos epigenetic silencing of the paternal X chromosome is erased. Epiblast identity and pluripotency were confirmed by contribution to chimaeras with germline transmission. These observations indicate that segregation of hypoblast from the bipotent ICM is dependent on FGF/Erk signalling and that in the absence of this signal, the entire ICM can acquire pluripotency. Furthermore, the epiblast does not require paracrine support from the hypoblast. Thus, naïve epiblast and ES cells are in a similar ground state, with an autonomous capacity for survival and replication, and high vulnerability to Erk signalling. We probed directly the relationship between naïve epiblast and ES cells. Dissociated ICM cells from freshly harvested late blastocysts gave rise to up to 12 ES cell clones per embryo when plated in the presence of inhibitors. We propose that ES cells are not a tissue culture creation, but are essentially identical to pre-implantation epiblast cells.

Epigenetic dynamics of stem cells and cell lineage commitment: digging Waddington's canal

Cells of the early mammalian embryo, including pluripotent embryonic stem (ES) cells and primordial germ cells (PGCs), are epigenetically dynamic and heterogeneous. During early development, this heterogeneity of epigenetic states is associated with stochastic expression of lineage-determining transcription factors that establish an intimate crosstalk with epigenetic modifiers. Lineage-specific epigenetic modification of crucial transcription factor loci (for example, methylation of the Elf5 promoter) leads to the restriction of transcriptional circuits and the fixation of lineage fate. The intersection of major epigenetic reprogramming and programming events in the early embryo creates plasticity followed by commitment to the principal cell lineages of the early conceptus.