Context-specific function of the LIM homeobox 1 transcription factor in head formation of the mouse embryo

Lhx1 encodes a LIM homeobox transcription factor that is expressed in the primitive streak, mesoderm and anterior mesendoderm of the mouse embryo. Using a conditional Lhx1 flox mutation and three different Cre deleters, we demonstrated that LHX1 is required in the anterior mesendoderm, but not in the mesoderm, for formation of the head. LHX1 enables the morphogenetic movement of cells that accompanies the formation of the anterior mesendoderm, in part through regulation of Pcdh7 expression. LHX1 also regulates, in the anterior mesendoderm, the transcription of genes encoding negative regulators of WNT signalling, such as Dkk1, Hesx1, Cer1 and Gsc. Embryos carrying mutations in Pcdh7, generated using CRISPR-Cas9 technology, and embryos without Lhx1 function specifically in the anterior mesendoderm displayed head defects that partially phenocopied the truncation defects of Lhx1-null mutants. Therefore, disruption of Lhx1-dependent movement of the anterior mesendoderm cells and failure to modulate WNT signalling both resulted in the truncation of head structures. Compound mutants of Lhx1, Dkk1 and Ctnnb1 show an enhanced head truncation phenotype, pointing to a functional link between LHX1 transcriptional activity and the regulation of WNT signalling. Collectively, these results provide comprehensive insight into the context-specific function of LHX1 in head formation: LHX1 enables the formation of the anterior mesendoderm that is instrumental for mediating the inductive interaction with the anterior neuroectoderm and LHX1 also regulates the expression of factors in the signalling cascade that modulate the level of WNT activity.

Histone deacetylation promotes mouse neural induction by restricting Nodal-dependent mesendoderm fate

Cell fate determination requires the cooperation between extrinsic signals and intrinsic molecules including transcription factors as well as epigenetic regulators. Nevertheless, how neural fate commitment is regulated by epigenetic modifications remains largely unclear. Here we show that transient histone deacetylation at epiblast stage promotes neural differentiation of mouse embryonic stem cells (mESCs). Histone deacetylase 1 (HDAC1) deficiency in mESCs partially phenocopies the inhibition of histone deacetylation in vitro, and displays reduced incorporation into neural tissues in chimeric mouse embryos in vivo. Mechanistic studies show that Nodal, which is repressed by histone deacetylation, is a direct target of HDAC1. Furthermore, the inhibition of histone deacetylation in the anterior explant of mouse embryos at E7.0 leads to Nodal activation and neural development repression. Thus, our study reveals an intrinsic mechanism that epigenetic histone deacetylation ensures neural fate commitment by restricting Nodal signalling in murine anterior epiblast ex vivo and mESC in vitro.

Intrinsic regulations in neural fate commitment

Neural fate commitment is an early embryonic event that a group of cells in ectoderm, which do not ingress through primitive streak, acquire a neural fate but not epidermal or mesodermal lineages. Several extracellular signaling pathways initiated by the secreted proteins bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), wingless/int class proteins (WNTs) and Nodal play essential roles in the specification of the neural plate. Accumulating evidence from the studies on mouse and pluripotent embryonic stem cells reveals that except for the extracellular signals, the intracellular molecules, including both transcriptional and epigenetic factors, participate in the modulation of neural fate commitment as well. In the review, we mainly focus on recent findings that the initiation of the nervous system is elaborately regulated by the intrinsic programs, which are mediated by transcriptional factors such as Sox2, Zfp521, Sip1 and Pou3f1, as well as epigenetic modifications, including histone methylation/demethylation, histone acetylation/deacetylation, and DNA methylation/demethylation. The discovery of the intrinsic regulatory machineries provides better understanding of the mechanisms by which the neural fate commitment is ensured by the cooperation between extracellular factors and intracellular molecules.

Identification of side population cells in chicken embryonic gonads

The side population (SP) phenotype, defined by the ability of a cell to efflux fluorescent dyes such as Hoechst, is common to several stem/progenitor cell types. In avian species, SP phenotype has been identified in pubertal and adult testes, but nothing is known about its expression during prenatal development of a male gonad. In this study, we characterized the Hoechst SP phenotype via the cytofluorimetric analysis of disaggregated testes on different days of chicken embryonic development. Male prenatal gonads contained a fraction of SP cells at each stage analyzed. At least two main SP fractions, named P3 and P4, were identified. The percentage of P3 fraction decreased as development proceeds, whereas P4 cell number was not affected by gonad growth. Functional inhibition of BCRP1 channel membrane using Verapamil and/or Ko143 showed that P3, but not P4 phenotype, was dependent on BCRP1 activity. Molecular analysis of both P3- and P4-sorted fractions revealed a differential RNA expression pattern, indicating that P3 cells mainly contained germinal stem cell markers, whereas P4 was preferentially composed of both Sertoli and Leydig cell progenitor markers. Finally, these findings provided evidence that the SP phenotype is a common feature of both germ and somatic cells detected in chicken developing testis.

Head formation: OTX2 regulates Dkk1 and Lhx1 activity in the anterior mesendoderm

The Otx2 gene encodes a paired-type homeobox transcription factor that is essential for the induction and the patterning of the anterior structures in the mouse embryo. Otx2 knockout embryos fail to form a head. Whereas previous studies have shown that Otx2 is required in the anterior visceral endoderm and the anterior neuroectoderm for head formation, its role in the anterior mesendoderm (AME) has not been assessed specifically. Here, we show that tissue-specific ablation of Otx2 in the AME phenocopies the truncation of the embryonic head of the Otx2 null mutant. Expression of Dkk1 and Lhx1, two genes that are also essential for head formation, is disrupted in the AME of the conditional Otx2-deficient embryos. Consistent with the fact that Dkk1 is a direct target of OTX2, we showed that OTX2 can interact with the H1 regulatory region of Dkk1 to activate its expression. Cross-species comparative analysis, RT-qPCR, ChIP-qPCR and luciferase assays have revealed two conserved regions in the Lhx1 locus to which OTX2 can bind to activate Lhx1 expression. Abnormal development of the embryonic head in Otx2;Lhx1 and Otx2;Dkk1 compound mutant embryos highlights the functional intersection of Otx2, Dkk1 and Lhx1 in the AME for head formation.

The transcription factor Pou3f1 promotes neural fate commitment via activation of neural lineage genes and inhibition of external signaling pathways

The neural fate commitment of pluripotent stem cells requires the repression of extrinsic inhibitory signals and the activation of intrinsic positive transcription factors. However, how these two events are integrated to ensure appropriate neural conversion remains unclear. In this study, we showed that Pou3f1 is essential for the neural differentiation of mouse embryonic stem cells (ESCs), specifically during the transition from epiblast stem cells (EpiSCs) to neural progenitor cells (NPCs). Chimeric analysis showed that Pou3f1 knockdown leads to a markedly decreased incorporation of ESCs in the neuroectoderm. By contrast, Pou3f1-overexpressing ESC derivatives preferentially contribute to the neuroectoderm. Genome-wide ChIP-seq and RNA-seq analyses indicated that Pou3f1 is an upstream activator of neural lineage genes, and also is a repressor of BMP and Wnt signaling. Our results established that Pou3f1 promotes the neural fate commitment of pluripotent stem cells through a dual role, activating internal neural induction programs and antagonizing extrinsic neural inhibitory signals.

Dkk1-dependent inhibition of Wnt signaling activates Hesx1 expression through its 5' enhancer and directs forebrain precursor development

Development of the anterior forebrain precursor (AFBP) in the anterior neural plate (ANP) depends on the activation of the Hesx1 transcription factor gene. The Hesx1-expression domain of the ANP is underlain by Dkk1-expressing tissues, initially proximal-most anterior visceral endoderm (AVE), and later anterior mesendoderm (AME). As Dkk1-null embryos fail to develop the Hesx1-expressing domain, it is likely that Wnt signal inhibition in the ANP is required for the Hesx1 activation. To investigate the regulation of the AFBP development, we took advantage of epiblast stem cells (EpiSCs), which develop into the ANP in the absence of activin signaling. Expression of Hesx1 and Six3, both involved in the AFBP development, was strongly activated 2 days after activin removal and concomitant addition of Wnt signal inhibitors, Dkk1 or XAV939. Furthermore, we showed that activation of the 720-bp Hesx1 5' enhancer is responsible for Hesx1 expression in the AFBP and depends on Wnt signal inhibition. In addition, we showed that Wnt inhibition during the first day has larger impact on the activation of Hesx1 and Six3 than the second day, suggesting that in embryos Wnt inhibition caused by the AVE-derived Dkk1, rather than the AME-derived Dkk1, contributes greatly in the establishment of the AFBP.

Distinct Wnt-driven primitive streak-like populations reflect in vivo lineage precursors

During gastrulation, epiblast cells are pluripotent and their fate is thought to be constrained principally by their position. Cell fate is progressively restricted by localised signalling cues from areas including the primitive streak. However, it is unknown whether this restriction accompanies, at the individual cell level, a reduction in potency. Investigation of these early transition events in vitro is possible via the use of epiblast stem cells (EpiSCs), self-renewing pluripotent cell lines equivalent to the postimplantation epiblast. Strikingly, mouse EpiSCs express gastrulation stage regional markers in self-renewing conditions. Here, we examined the differentiation potential of cells expressing such lineage markers. We show that undifferentiated EpiSC cultures contain a major subfraction of cells with reversible early primitive streak characteristics, which is mutually exclusive to a neural-like fraction. Using in vitro differentiation assays and embryo grafting we demonstrate that primitive streak-like EpiSCs are biased towards mesoderm and endoderm fates while retaining pluripotency. The acquisition of primitive streak characteristics by self-renewing EpiSCs is mediated by endogenous Wnt signalling. Elevation of Wnt activity promotes restriction towards primitive streak-associated lineages with mesendodermal and neuromesodermal characteristics. Collectively, our data suggest that EpiSC pluripotency encompasses a range of reversible lineage-biased states reflecting the birth of pioneer lineage precursors from a pool of uncommitted EpiSCs similar to the earliest cell fate restriction events taking place in the gastrula stage epiblast.

The neural stem cell lineage reveals novel relationships among spermatogonial germ stem cells and other pluripotent stem cells

The embryonic stem cell (ESC) derived from the inner cell mass is viewed as the core pluripotent cell (PC) type from which all other cell types emanate. This familiar perspective derives from an embryological time line in which PCs are ordered according to their time of appearance. However, this schema does not take into account their potential for interconversion, thereby excluding this critical quality of PCs. The persistence of bona fide pluripotent adult stem cells has garnered increasing attention in recent years. Adult pluripotent spermatogonial germ stem cells (aSGSCs) arise from primordial germ cells (pGCs) that emerge from the epiblast during gastrulation. Adult definitive neural stem cells (dNSCs) arise clonally from pluripotent embryonic primitive neural stem cells (pNSCs), which can also be derived clonally from ESCs. To test for stem cell-type convertibility, we employed differentiation in the clonal lineage from ESCs to pNSCs to dNSCs, and revealed the relationships and lineage positioning among various PC populations, including spermatogonial germ cells (aSGSCs), epiblast-derived stem cells (Epi-SCs) and the bFGF, Activin, and BIO-derived stem cell (FAB-SC). Adult, murine aSGSCs assumed a 'pseudo-ESC' state in vitro, and then differentiated into dNSCs, but not pNSCs. Similarly, Epi-SCs and FAB-SCs only gave rise to dNSCs and not to pNSCs. The results of these experiments suggest a new pluripotency lineage model describing the relationship(s) among PCs that better reflects the transitions between these cell types in vitro.

Otx2 expression in anterior neuroectoderm and forebrain/midbrain is directed by more than six enhancers

Otx2 plays essential roles in each site at each step of head development. We previously identified the AN1 enhancer at 91kb 5' upstream for the Otx2 expressions in anterior neuroectoderm (AN) at neural plate stage before E8.5, and the FM1 enhancer at 75kb 5' upstream and the FM2 enhancer at 122kb 3' downstream for the expression in forebrain/midbrain (FM) at brain vesicle stage after E8.5. The present study identified a second AN enhancer (AN2) at 88kb 5' upstream; the AN2 enhancer also recapitulates the endogenous Otx2 expression in choroid plexus, cortical hem and choroidal roof. However, the enhancer mutants indicated the presence of another AN enhancer. The study also identified a third FM enhancer (FM3) at 153kb 5' upstream. Thus, the Otx2 expressions in anterior neuroectoderm and forebrain/midbrain are regulated by more than six enhancers located far from the coding region. The enhancers identified are differentially conserved among vertebrates; none of the AN enhancers has activities in caudal forebrain and midbrain at brain vesicle stage after E8.5, nor do any of the FM enhancers in anterior neuroectoderm at neural plate stage before E8.5.

Sox proteins: regulators of cell fate specification and differentiation

Sox transcription factors play widespread roles during development; however, their versatile funtions have a relatively simple basis: the binding of a Sox protein alone to DNA does not elicit transcriptional activation or repression, but requires binding of a partner transcription factor to an adjacent site on the DNA. Thus, the activity of a Sox protein is dependent upon the identity of its partner factor and the context of the DNA sequence to which it binds. In this Primer, we provide an mechanistic overview of how Sox family proteins function, as a paradigm for transcriptional regulation of development involving multi-transcription factor complexes, and we discuss how Sox factors can thus regulate diverse processes during development.

Location of transient ectodermal progenitor potential in mouse development

Ectoderm is one of the three classic germ layers in the early mouse embryo, with the capacity to develop into both the central nervous system and epidermis. Because it is a transient phase of development with few molecular markers, the early ectoderm is the least understood germ layer in mouse embryonic development. In this work, we studied the differentiation potential of isolated ectoderm tissue in response to BMP signaling at various developmental stages (E6.5, E7.0 and E7.5), and identified a transient region in the anterior-proximal side of the embryo at E7.0 that possesses the ability to become neural or epidermal ectoderm in response to the absence or presence of BMP4, respectively. Furthermore, we demonstrated that inhibition of Nodal signaling could direct the pluripotent E6.5 epiblast cells towards ectoderm lineages during differentiation in explants in vitro. Our work not only improves our understanding of ectodermal layer development in early embryos, but also provides a framework for regenerative differentiation towards ectodermal tissues.

A competitive protein interaction network buffers Oct4-mediated differentiation to promote pluripotency in embryonic stem cells

The dynamic competition for complex formation between the pluripotency network components Oct4, Nanog, Tcf3, and β-catenin prevents embryonic stem cell differentiation by controlling the levels of free Oct4.

Pluripotency is defined by the ratios between the levels of pluripotency factors rather than by their absolute levels.

Competition between different protein complexes involving Nanog, Oct4, Tcf3, and β-catenin can account for the ratios associated with pluripotency.

The unstable pluripotency of Nanog mutant cells was shown to depend on the interactions between Oct4 and β-catenin.

The function of the protein competition network is to control the levels of free Oct4, which are balanced by Nanog and β-catenin in embryonic stem cells.

Pluripotency in embryonic stem cells is maintained through the activity of a small set of transcription factors centred around Oct4 and Nanog, which control the expression of ‘self-renewal' and ‘differentiation' genes. Here, we combine single-cell quantitative immunofluorescence microscopy and gene expression analysis, together with theoretical modelling, to investigate how the activity of those factors is regulated. We uncover a key role for post-translational regulation in the maintenance of pluripotency, which complements the well-established transcriptional regulatory layer. Specifically, we find that the activity of a network of protein complexes involving Nanog, Oct4, Tcf3, and β-catenin suffices to account for the behavior of ES cells under different conditions. Our results suggest that the function of the network is to buffer the transcriptional activity of Oct4, which appears to be the main determinant to exit pluripotency. The protein network explains the mechanisms underlying the gain and loss of function in different mutants, and brings us closer to a full understanding of the molecular basis of pluripotency.

A Systematic Survey and Characterization of Enhancers that Regulate Sox3 in Neuro-Sensory Development in Comparison with Sox2 Enhancers

Development of neural and sensory primordia at the early stages of embryogenesis depends on the activity of two B1 Sox transcription factors, Sox2 and Sox3. The embryonic expression patterns of the Sox2 and Sox3 genes are similar, yet they show gene-unique features. We screened for enhancers of the 231-kb genomic region encompassing Sox3 of chicken, and identified 13 new enhancers that showed activity in different domains of the neuro-sensory primordia. Combined with the three Sox3-proximal enhancers determined previously, at least 16 enhancers were involved in Sox3 regulation. Starting from the NP1 enhancer, more enhancers with different specificities are activated in sequence, resulting in complex overlapping patterns of enhancer activities. NP1 was activated in the caudal lateral epiblast adjacent to the posterior growing end of neural plate, and by the combined action of Wnt and Fgf signaling, similar to the Sox2 N1 enhancer involved in neural/mesodermal dichotomous cell lineage segregation. The Sox3 D5 enhancer and Sox2 N3 enhancer were also activated similarly in the diencephalon, optic vesicle and lens placode, suggesting analogies in their regulation. In general, however, the specificities of the enhancers were not identical between Sox3 and Sox2, including the cases of the NP1 and D5 enhancers.