The POU factor Oct-25 regulates the Xvent-2B gene and counteracts terminal differentiation in Xenopus embryos

Vertebrate Cell Differentiation, Evolution, and Diseases: The Vertebrate-Specific Developmental Potential Guardians VENTX/NANOG and POU5/OCT4 Enter the Stage

During vertebrate development, embryonic cells pass through a continuum of transitory pluripotent states that precede multi-lineage commitment and morphogenesis. Such states are referred to as “refractory/naïve” and “competent/formative” pluripotency. The molecular mechanisms maintaining refractory pluripotency or driving the transition to competent pluripotency, as well as the cues regulating multi-lineage commitment, are evolutionarily conserved. Vertebrate-specific “Developmental Potential Guardians” (vsDPGs; i.e., VENTX/NANOG, POU5/OCT4), together with MEK1 (MAP2K1), coordinate the pluripotency continuum, competence for multi-lineage commitment and morphogenesis in vivo. During neurulation, vsDPGs empower ectodermal cells of the neuro-epithelial border (NEB) with multipotency and ectomesenchyme potential through an “endogenous reprogramming” process, giving rise to the neural crest cells (NCCs). Furthermore, vsDPGs are expressed in undifferentiated-bipotent neuro-mesodermal progenitor cells (NMPs), which participate in posterior axis elongation and growth. Finally, vsDPGs are involved in carcinogenesis, whereby they confer selective advantage to cancer stem cells (CSCs) and therapeutic resistance. Intriguingly, the heterogenous distribution of vsDPGs in these cell types impact on cellular potential and features. Here, we summarize the findings about the role of vsDPGs during vertebrate development and their selective advantage in evolution. Our aim to present a holistic view regarding vsDPGs as facilitators of both cell plasticity/adaptability and morphological innovation/variation. Moreover, vsDPGs may also be at the heart of carcinogenesis by allowing malignant cells to escape from physiological constraints and surveillance mechanisms.

The functional diversity of the POUV-class proteins across vertebrates

POUV is a relatively newly emerged class of POU transcription factors present in jawed vertebrates (Gnathostomata). The function of POUV-class proteins is inextricably linked to zygotic genome activation (ZGA). A large body of evidence now extends the role of these proteins to subsequent developmental stages. While some functions resemble those of other POU-class proteins and are related to neuroectoderm development, others have emerged de novo. The most notable of the latter functions is pluripotency control by Oct4 in mammals. In this review, we focus on these de novo functions in the best-studied species harbouring POUV proteins-zebrafish, Xenopus (anamniotes) and mammals (amniotes). Despite the broad diversity of their biological functions in vertebrates, POUV proteins exert a common feature related to their role in safeguarding the undifferentiated state of cells. Here we summarize numerous pieces of evidence for these specific functions of the POUV-class proteins and recap available loss-of-function data.

Involvement of Oct4-type transcription factor Pou5f3 in posterior spinal cord formation in zebrafish embryos

In vertebrate embryogenesis, elongation of the posterior body is driven by de novo production of the axial and paraxial mesoderm as well as the neural tube at the posterior end. This process is presumed to depend on the stem cell-like population in the tail bud region, but the details of the gene regulatory network involved are unknown. Previous studies suggested the involvement of pou5f3, an Oct4-type POU gene in zebrafish, in axial elongation. In the present study, we first found that pou5f3 is expressed mainly in the dorsal region of the tail bud immediately after gastrulation, and that this expression is restricted to the posterior-most region of the elongating neural tube during somitogenesis. This pou5f3 expression was complementary to the broad expression of sox3 in the neural tube, and formed a sharp boundary with specific expression of tbxta (orthologue of mammalian T/Brachyury) in the tail bud, implicating pou5f3 in the specification of tail bud-derived cells toward neural differentiation in the spinal cord. When pou5f3 was functionally impaired after gastrulation by induction of a dominant-interfering pou5f3 mutant gene (en-pou5f3), trunk and tail elongation were markedly disturbed at distinct positions along the axis depending on the stage. This finding showed involvement of pou5f3 in de novo generation of the body from the tail bud. Conditional functional abrogation also showed that pou5f3 downregulates mesoderm-forming genes but promotes neural development by activating neurogenesis genes around the tail bud. These results suggest that pou5f3 is involved in formation of the posterior spinal cord.

Cytoskeletal Protein Zyxin Inhibits the Activity of Genes Responsible for Embryonic Stem Cell Status

Zyxin is a cytoskeletal LIM-domain protein that regulates actin cytoskeleton assembly and gene expression. In the present work, we find that zyxin downregulation in Xenopus laevis embryos reduces the expression of numerous genes that regulate cell differentiation, but it enhances that of several genes responsible for embryonic stem cell status, specifically klf4, pou5f3.1, pou5f3.2, pou5f3.3, and vent2.1/2. For pou5f3 family genes (mammalian POU5F1/OCT4 homologs), we show that this effect is the result of mRNA stabilization due to complex formation with the Y-box protein Ybx1. When bound to Ybx1, zyxin interferes with the formation of these complexes, thereby stimulating pou5f3 mRNA degradation. In addition, in zebrafish embryos and human HEK293 cells, zyxin downregulation increases mRNA levels of the pluripotency genes KLF4, NANOG, and POU5F1/OCT4. Our findings indicate that zyxin may play a role as a switch among morphogenetic cell movement, differentiation, and embryonic stem cell status.

Combination Effects of Organic and Inorganic Chromium on Production Performance, Reproductive Response, Immune Status, and Maternal Antibody Transmission in Breeder Quails Under Heat Stress

This experiment was carried out to investigate the combination effects of organic and inorganic chromium (Cr) on egg production, egg quality, reproductive response, and immune status of breeder quails and their offspring under heat stress. A total of 140 7-week-old Japanese breeder quails (120 females and 20 males) according to a completely randomized design were used in four treatment groups (five replicate and seven birds per each) lasted for 8 weeks. Quails exposed to 35 °C for 8 h/day for induction of cyclic heat stress treatments consisted of diets supplemented with (1) 1 mg CrCl₃ per kilogram of diet as control (CNT); (2) 1 mg Cr-L-Met per kilogram of diet as organic Cr (OCr); (3) 0.5 mg CrCl₃ plus 0.5 mg Cr-L-Met per kilogram of diet (ISO); (4) 1 mg CrCl₃ plus 0.5 mg Cr-L-Met per kilogram of diet (On-top). Productive performance and egg quality parameters were determined weekly. Fertility, hatchability, and embryonic mortality were measured at the end of experiment. Humoral immunity was assessed by primary and secondary antibody titer in sheep red blood cells (SRBC) and Newcastle disease (ND) tests. Cell-mediated immunity was assessed by the cutaneous basophil hypersensitivity (CBH) test to phytohemagglutinin (PHA) at days 20 and 45 of age. White blood cell count and immunoglobulin Y (IgY) content in serum and yolk of breeders and in serum and yolk residues of offspring were also measured. Results showed that maximum egg production, egg shell thickness, and Haugh unit were observed in birds fed ISO and On-top diets (P ≤ 0.05). The highest (P ≤ 0.05) antibody levels in ND test were observed in birds fed with OCr, ISO, and On-top diets. The highest cellular response (P ≤ 0.05) was in 12 h after primary PHA injection and 12 and 24 h after secondary PHA injection in birds fed with On-top diets. The highest count of heterophil and (H/L) were gained in breeder quails fed with CNT diet, and the lowest of them were reached with On-top diet (P ≤ 0.05). Results showed that the highest IgY level in serum of breeder and their offspring and that of yolk suck and egg yolk were observed in birds fed with On-top diet (P ≤ 0.05). These results suggest that extra supplemental organic Cr in combination with CrCl₃ could lead to higher egg production, egg quality, and immune status of breeder quails and their offspring.

OCT4B regulates p53 and p16 pathway genes to prevent apoptosis of breast cancer cells

Octamer-binding transcription factor 4 (OCT4) is a transcription factor with a well-defined role in stem cell pluripotency. Two OCT4 isoforms, OCT4A and OCT4B, tend to be downregulated as normal cells differentiate. However, OCT4, particularly OCT4B, may become reactivated in cancer cells. Despite this observation, the exact function of OCT4B re-expression in cancer is unclear. In the present study, the role of OCT4 in breast cancer cells was determined. In particular, the ability of OCT4 to regulate key genes involved in cellular proliferation and apoptosis, two pathways that are frequently deregulated in cancer, was examined. The cyclin-dependent kinase inhibitor 2A locus encodes p16INK4a and p14ARF, two important cell cycle inhibitors. The tumor suppressor p53 also has well characterized roles in suppressing proliferation and promoting apoptosis. The present study demonstrated, via overexpression and genetic knockdown techniques, that OCT4B regulates the expression of several of these genes and ultimately regulates the rate of apoptosis of MCF-7 breast cancer cells. It was also observed that, while OCT4B and OCT4A regulate one another, it is OCT4B that serves a more prominent role in regulating the transcription of downstream genes. Taken together, the present results suggest that OCT4B is re-expressed in a number of breast cancer cell lines, where it affects both the transcription of cell cycle genes and the rate of apoptosis. These properties of OCT4B may depend on, at least in part, the co-function of OCT4A.

Lineage commitment of embryonic cells involves MEK1-dependent clearance of pluripotency regulator Ventx2

During early embryogenesis, cells must exit pluripotency and commit to multiple lineages in all germ-layers. How this transition is operated in vivo is poorly understood. Here, we report that MEK1 and the Nanog-related transcription factor Ventx2 coordinate this transition. MEK1 was required to make Xenopus pluripotent cells competent to respond to all cell fate inducers tested. Importantly, MEK1 activity was necessary to clear the pluripotency protein Ventx2 at the onset of gastrulation. Thus, concomitant MEK1 and Ventx2 knockdown restored the competence of embryonic cells to differentiate. Strikingly, MEK1 appeared to control the asymmetric inheritance of Ventx2 protein following cell division. Consistently, when Ventx2 lacked a functional PEST-destruction motif, it was stabilized, displayed symmetric distribution during cell division and could efficiently maintain pluripotency gene expression over time. We suggest that asymmetric clearance of pluripotency regulators may represent an important mechanism to ensure the progressive assembly of primitive embryonic tissues.

DOI:http://dx.doi.org/10.7554/eLife.21526.001

Zebrafish Znfl1 proteins control the expression of hoxb1b gene in the posterior neuroectoderm by acting upstream of pou5f3 and sall4 genes

Transcription factors play crucial roles in patterning posterior neuroectoderm. Previously, zinc finger transcription factor znfl1 was reported to be expressed in the posterior neuroectoderm of zebrafish embryos. However, its roles remain unknown. Here, we report that there are 13 copies of znfl1 in the zebrafish genome, and all the paralogues share highly identical protein sequences and cDNA sequences. When znfl1s are knocked down using a morpholino to inhibit their translation or dCas9-Eve to inhibit their transcription, the zebrafish gastrula displays reduced expression of hoxb1b, the marker gene for the posterior neuroectoderm. Further analyses reveal that diminishing znfl1s produces the decreased expressions of pou5f3, whereas overexpression of pou5f3 effectively rescues the reduced expression of hoxb1b in the posterior neuroectoderm. Additionally, knocking down znfl1s causes the reduced expression of sall4, a direct regulator of pou5f3, in the posterior neuroectoderm, and overexpression of sall4 rescues the expression of pou5f3 in the knockdown embryos. In contrast, knocking down either pou5f3 or sall4 does not affect the expressions of znfl1s Taken together, our results demonstrate that zebrafish znfl1s control the expression of hoxb1b in the posterior neuroectoderm by acting upstream of pou5f3 and sall4.

Mechanisms of Vertebrate Germ Cell Determination

Two unique characteristics of the germ line are the ability to persist from generation to generation and to retain full developmental potential while differentiating into gametes. How the germ line is specified that allows it to retain these characteristics within the context of a developing embryo remains unknown and is one focus of current research. Germ cell specification proceeds through one of two basic mechanisms: cell autonomous or inductive. Here, we discuss how germ plasm driven germ cell specification (cell autonomous) occurs in both zebrafish and the frog Xenopus. We describe the segregation of germ cells during embryonic development of solitary and colonial ascidians to provide an evolutionary context to both mechanisms. We conclude with a discussion of the inductive mechanism as exemplified by both the mouse and axolotl model systems. Regardless of mechanism, several general themes can be recognized including the essential role of repression and posttranscriptional regulation of gene expression.

Evolution and functions of Oct4 homologs in non-mammalian vertebrates

PouV class transcription factor Oct4/Pou5f1 is a central regulator of indefinite pluripotency in mammalian embryonic stem cells (ESCs) but also participates in cell lineage specification in mouse embryos and in differentiating cell cultures. The molecular basis for this versatility, which is shared between Oct4 and its non-mammalian homologs Pou5f1 and Pou5f3, is not yet completely understood. Here, I review the current understanding of the evolution of PouV class transcription factors and discuss equivalent and diverse roles of Oct4 homologs in pluripotency, differentiation, and cell behavior in different vertebrate embryos. This article is part of a Special Issue entitled: The Oct Transcription Factor Family, edited by Dr. Dean Tantin.

Zygotic Genome Activators, Developmental Timing, and Pluripotency

The transcription factors Pou5f1, Sox2, and Nanog are central regulators of pluripotency in mammalian ES and iPS cells. In vertebrate embryos, Pou5f1/3, SoxB1, and Nanog control zygotic genome activation and participate in lineage decisions. We review the current knowledge of the roles of these genes in developing vertebrate embryos from fish to mammals and suggest a model for pluripotency gene regulatory network functions in early development.

The Xenopus Maternal-to-Zygotic Transition from the Perspective of the Germline

In Xenopus, the germline is specified by the inheritance of germ-plasm components synthesized at the beginning of oogenesis. Only the cells in the early embryo that receive germ plasm, the primordial germ cells (PGCs), are competent to give rise to the gametes. Thus, germ-plasm components continue the totipotent potential exhibited by the oocyte into the developing embryo at a time when most cells are preprogrammed for somatic differentiation as dictated by localized maternal determinants. When zygotic transcription begins at the mid-blastula transition, the maternally set program for somatic differentiation is realized. At this time, genetic control is ceded to the zygotic genome, and developmental potential gradually becomes more restricted within the primary germ layers. PGCs are a notable exception to this paradigm and remain transcriptionally silent until the late gastrula. How the germ-cell lineage retains full potential while somatic cells become fate restricted is a tale of translational repression, selective degradation of somatic maternal determinants, and delayed activation of zygotic transcription.

Vertical signalling involves transmission of Hox information from gastrula mesoderm to neurectoderm

Development and patterning of neural tissue in the vertebrate embryo involves a set of molecules and processes whose relationships are not fully understood. Classical embryology revealed a remarkable phenomenon known as vertical signalling, a gastrulation stage mechanism that copies anterior-posterior positional information from mesoderm to prospective neural tissue. Vertical signalling mediates unambiguous copying of complex information from one tissue layer to another. In this study, we report an investigation of this process in recombinates of mesoderm and ectoderm from gastrulae of Xenopus laevis. Our results show that copying of positional information involves non cell autonomous autoregulation of particular Hox genes whose expression is copied from mesoderm to neurectoderm in the gastrula. Furthermore, this information sharing mechanism involves unconventional translocation of the homeoproteins themselves. This conserved primitive mechanism has been known for three decades but has only recently been put into any developmental context. It provides a simple, robust way to pattern the neurectoderm using the Hox pattern already present in the mesoderm during gastrulation. We suggest that this mechanism was selected during evolution to enable unambiguous copying of rather complex information from cell to cell and that it is a key part of the original ancestral mechanism mediating axial patterning by the highly conserved Hox genes.

Spalt-like 4 promotes posterior neural fates via repression of pou5f3 family members in Xenopus

Amphibian neural development occurs as a two-step process: (1) induction specifies a neural fate in undifferentiated ectoderm; and (2) transformation induces posterior spinal cord and hindbrain. Signaling through the Fgf, retinoic acid (RA) and Wnt/β-catenin pathways is necessary and sufficient to induce posterior fates in the neural plate, yet a mechanistic understanding of the process is lacking. Here, we screened for factors enriched in posterior neural tissue and identify spalt-like 4 (sall4), which is induced by Fgf. Knockdown of Sall4 results in loss of spinal cord marker expression and increased expression of pou5f3.2 (oct25), pou5f3.3 (oct60) and pou5f3.1 (oct91) (collectively, pou5f3 genes), the closest Xenopus homologs of mammalian stem cell factor Pou5f1 (Oct4). Overexpression of the pou5f3 genes results in the loss of spinal cord identity and knockdown of pou5f3 function restores spinal cord marker expression in Sall4 morphants. Finally, knockdown of Sall4 blocks the posteriorizing effects of Fgf and RA signaling in the neurectoderm. These results suggest that Sall4, activated by posteriorizing signals, represses the pou5f3 genes to provide a permissive environment allowing for additional Wnt/Fgf/RA signals to posteriorize the neural plate.

[Pluripotency and induced nuclear reprogramming in vertebrates: new perspectives]

Pluripotency is a transitory state during vertebrate development. A pluripotent cell can theoretically acquire all cell fates of the organism. During ontogenetic dynamics, loss of pluripotency is associated with a progressive acquisition of a specific genetic program, which is determined both by instructions received and by cell position in the whole organism. Pluripotent embryonic stem cells can be isolated and cultured in vitro indefinitely. Using mammalian embryonic stem cells (ESCs), it has been possible to identify the factors involved in the establishment and maintenance of pluripotency state. In this review, we will describe recent scientific advances in the understanding of pluripotency, the molecular actors involved in such a regulation and their functional conservation during evolution. We shall focus on new concepts, obtained from the study of vertebrate model organisms, to shed light on the cell transition from pluripotency to differentiated state, and shall recapitulate fundamental and clinical applications of pluripotent cells, of "somatic cell nuclear transfer" (SCNT), of induced nuclear reprogramming in vitro and future perspectives of in vivo applications. Our results, in the xenopus, concerning the first in vivo induced nuclear reprogramming might open new perspectives about the understanding of cell plasticity in an integrated context. Our analyses sought to encourage new and alternative clinical approaches to achieve in situ tissue regeneration.

Regulation of germ layer formation by pluripotency factors during embryogenesis

The classical pluripotency factors Oct4, Klf4, Sox2, and Nanog are required for the maintenance of pluripotency and self-renewal of embryonic stem (ES) cells and can reprogram terminally differentiated cells into a pluripotent state. Alteration in the levels of these factors in ES cells will cause differentiation into different lineages, suggesting that they are critical determinants of cell fates. These factors show dynamic expression patterns during embryogenesis, in particular in the pluripotent or multipotent cells of an early stage embryo, implying that they are involved in the cell fate decision during early embryonic development. Functions and the underlying molecular mechanisms have been extensively studied for these factors in ES cells under cultured conditions. However, this does not mean that the results also hold true for intact embryos. In the review, I have summarized and discussed the findings on the functions and the underlying mechanisms of the classical pluripotency factors during early embryogenesis, in particular during germ layer formation.

Suv4-20h histone methyltransferases promote neuroectodermal differentiation by silencing the pluripotency-associated Oct-25 gene

Post-translational modifications (PTMs) of histones exert fundamental roles in regulating gene expression. During development, groups of PTMs are constrained by unknown mechanisms into combinatorial patterns, which facilitate transitions from uncommitted embryonic cells into differentiated somatic cell lineages. Repressive histone modifications such as H3K9me3 or H3K27me3 have been investigated in detail, but the role of H4K20me3 in development is currently unknown. Here we show that Xenopus laevis Suv4-20h1 and h2 histone methyltransferases (HMTases) are essential for induction and differentiation of the neuroectoderm. Morpholino-mediated knockdown of the two HMTases leads to a selective and specific downregulation of genes controlling neural induction, thereby effectively blocking differentiation of the neuroectoderm. Global transcriptome analysis supports the notion that these effects arise from the transcriptional deregulation of specific genes rather than widespread, pleiotropic effects. Interestingly, morphant embryos fail to repress the Oct4-related Xenopus gene Oct-25. We validate Oct-25 as a direct target of xSu4-20h enzyme mediated gene repression, showing by chromatin immunoprecipitaton that it is decorated with the H4K20me3 mark downstream of the promoter in normal, but not in double-morphant, embryos. Since knockdown of Oct-25 protein significantly rescues the neural differentiation defect in xSuv4-20h double-morphant embryos, we conclude that the epistatic relationship between Suv4-20h enzymes and Oct-25 controls the transit from pluripotent to differentiation-competent neural cells. Consistent with these results in Xenopus, murine Suv4-20h1/h2 double-knockout embryonic stem (DKO ES) cells exhibit increased Oct4 protein levels before and during EB formation, and reveal a compromised and biased capacity for in vitro differentiation, when compared to normal ES cells. Together, these results suggest a regulatory mechanism, conserved between amphibians and mammals, in which H4K20me3-dependent restriction of specific POU-V genes directs cell fate decisions, when embryonic cells exit the pluripotent state.

The quest of modern developmental biology is a detailed molecular description of the process that leads from the fertilized egg to the complex and highly differentiated adult organism. This process is controlled largely on the level of gene expression. While early embryonic cells are pluripotent and capable of transcribing most of their genome, older cells have become committed to the germ layer and differentiation programs during gastrulation. They express then a subset of genes compatible with their future physiological function. Young, pluripotent cells and post-gastrula, committed cells express different networks of transcription factors and contain chromatin of different structure and composition. How these two regulatory layers are interconnected during development is incompletely understood. We describe a novel and unexpected link between the pluripotency-associated POU-V gene Oct-25 and xSuv4-20h histone methyltransferases. XSuv4-20h enzymes are required to repress the Oct-25 gene, a homolog of mammalian Oct4, in the neuroectoderm of frog embryos as a prerequisite for neural differentiation. Consistently, murine Suv4-20h double-null ES cells show increased Oct4 protein levels before and during in vitro differentiation and display compromised differentiation in comparison to wild-type ES cells. Thus, Suv4-20h enzymes control specific POU-V genes and are involved in germ-layer specific differentiation.

Possible regulation of Oct60 transcription by a positive feedback loop in Xenopus oocytes

The POU family subclass V (POU-V) proteins have important roles in maintaining cells in an undifferentiated state. In Xenopus, expression of the POU-V protein Oct60 was detected in oocytes and was found to decrease in blastula- to gastrula-stage embryos. In addition, Oct60 overexpression inhibits some signals in early embryogenesis, including Activin/Nodal, BMP, and Wnt signalling. In this report, we analysed mechanisms of Oct60 promoter activation and discovered that Oct60 transcription was activated ectopically in somatic nuclei by oocyte extract treatment. Promoter assays demonstrated that Oct60 transcription was activated in oocytes specifically and that this activation was dependent on an Octamer-Sox binding motif. ChIP assays showed that the Oct60 protein binds the motif. These results suggest that Oct60 transcription is regulated by a positive-feedback loop in Xenopus oocytes.

Ventx factors function as Nanog-like guardians of developmental potential in Xenopus

Vertebrate development requires progressive commitment of embryonic cells into specific lineages through a continuum of signals that play off differentiation versus multipotency. In mammals, Nanog is a key transcription factor that maintains cellular pluripotency by controlling competence to respond to differentiation cues. Nanog orthologs are known in most vertebrates examined to date, but absent from the Anuran amphibian Xenopus. Interestingly, in silico analyses and literature scanning reveal that basal vertebrate ventral homeobox (ventxs) and mammalian Nanog factors share extensive structural, evolutionary and functional properties. Here, we reassess the role of ventx activity in Xenopus laevis embryos and demonstrate that they play an unanticipated role as guardians of high developmental potential during early development. Joint over-expression of Xenopus ventx1.2 and ventx2.1-b (ventx1/2) counteracts lineage commitment towards both dorsal and ventral fates and prevents msx1-induced ventralization. Furthermore, ventx1/2 inactivation leads to down-regulation of the multipotency marker oct91 and to premature differentiation of blastula cells. Finally, supporting the key role of ventx1/2 in the control of developmental potential during development, mouse Nanog (mNanog) expression specifically rescues embryonic axis formation in ventx1/2 deficient embryos. We conclude that during Xenopus development ventx1/2 activity, reminiscent of that of Nanog in mammalian embryos, controls the switch of early embryonic cells from uncommitted to committed states.

Non-viral expression of mouse Oct4, Sox2, and Klf4 transcription factors efficiently reprograms tadpole muscle fibers in vivo

Adult mammalian cells can be reprogrammed into induced pluripotent stem cells (iPSCs) by a limited combination of transcription factors. To date, most current iPSC generation protocols rely on viral vector usage in vitro, using cells removed from their physiological context. Such protocols are hindered by low derivation efficiency and risks associated with genome modifications of reprogrammed cells. Here, we reprogrammed cells in an in vivo context using non-viral somatic transgenesis in Xenopus tadpole tail muscle, a setting that provides long term expression of non-integrated transgenes in vivo. Expression of mouse mOct4, mSox2, and mKlf4 (OSK) led rapidly and reliably to formation of proliferating cell clusters. These clusters displayed the principal hallmarks of pluripotency: alkaline phosphatase activity, up-regulation of key epigenetic and chromatin remodeling markers, and reexpression of endogenous pluripotent markers. Furthermore, these clusters were capable of differentiating into derivatives of the three germ layers in vitro and into neurons and muscle fibers in vivo. As in situ reprogramming occurs along with muscle tissue repair, the data provide a link between these two processes and suggest that they act synergistically. Notably, every OSK injection resulted in cluster formation. We conclude that reprogramming is achievable in an anamniote model and propose that in vivo approaches could provide rapid and efficient alternative for non-viral iPSC production. The work opens new perspectives in basic stem cell research and in the longer term prospect of regenerative medicine protocols development.

Pou5f1/oct4 in pluripotency control: insights from zebrafish

Gastrulation in vertebrates is a conserved process, which involves transition from cellular pluripotency to early precursors of ectoderm, mesoderm, and endoderm. Pluripotency control during this stage is far from being understood. Recent genetic and transcriptomic studies in zebrafish suggest that the core pluripotency transcription factors (TFs) Pou5f1 and TFs of the SoxB1 group are critically involved in large-scale temporal coordination of gene expression during gastrulation. A significant number of evolutionary conserved target genes of Pou5f1 in zebrafish are also involved in stem-cell circuit in mammalian ES cell cultures. Here, I will review the roles of Pou5f1 in development and discuss the evolutionary conservation of Pou5f1 functions and their relation to pluripotency control.

Pescadillo homologue 1 and Peter Pan function during Xenopus laevis pronephros development

BACKGROUND INFORMATION

pes1 (pescadillo homologue 1) and ppan (Peter Pan) are multifunctional proteins involved in ribosome biogenesis, cell proliferation, apoptosis, cell migration and regulation of gene expression. Both proteins are required for early neural development in Xenopus laevis, as previously demonstrated.

RESULTS

We show that the expression of both genes in the developing pronephros depends on wnt4 and fzd3 (frizzled homologue 3) function. Loss of pes1 or ppan by MO (morpholino oligonucleotide)-based knockdown approaches resulted in strong malformations during pronephric tubule formation. Defects were already notable during specification of pronephric progenitor cells, as shown by lhx1 expression. Moreover, we demonstrated that Xenopus pes1 and ppan interact physically and functionally and that pes1 and ppan can cross-rescue the loss of function phenotype of one another. Interference with rRNA synthesis, however, did not result in a similar early pronephros phenotype.

CONCLUSION

These results demonstrate that pes1 and ppan are required for Xenopus pronephros development and indicate that their function in the pronephros is independent of their role in ribosome biosynthesis.

The forkhead transcription factor FoxB1 regulates the dorsal-ventral and anterior-posterior patterning of the ectoderm during early Xenopus embryogenesis

The formation of the dorsal-ventral (DV) and anterior-posterior (AP) axes, fundamental to the body plan of animals, is regulated by several groups of polypeptide growth factors including the TGF-β, FGF, and Wnt families. In order to ensure the establishment of the body plan, the processes of DV and AP axis formation must be linked and coordinately regulated. However, the molecular mechanisms responsible for these interactions remain unclear. Here, we demonstrate that the forkhead box transcription factor FoxB1, which is upregulated by the neuralizing factor Oct-25, plays an important role in the formation of the DV and AP axes. Overexpression of FoxB1 promoted neural induction and inhibited BMP-dependent epidermal differentiation in ectodermal explants, thereby regulating the DV patterning of the ectoderm. In addition, FoxB1 was also found to promote the formation of posterior neural tissue in both ectodermal explants and whole embryos, suggesting its involvement in embryonic AP patterning. Using knockdown analysis, we found that FoxB1 is required for the formation of posterior neural tissues, acting in concert with the Wnt and FGF pathways. Consistent with this, FoxB1 suppressed the formation of anterior structures via a process requiring the function of XWnt-8 and eFGF. Interestingly, while downregulation of FoxB1 had little effect on neural induction, we found that it functionally interacted with its upstream factor Oct-25 and plays a supportive role in the induction and/or maintenance of neural tissue. Our results suggest that FoxB1 is part of a mechanism that fine-tunes, and leads to the coordinated formation of, the DV and AP axes during early development.

Pou5f1 contributes to dorsoventral patterning by positive regulation of vox and modulation of fgf8a expression

Pou5f1/Oct-4 in mice is required for maintenance of embryonic pluripotent cell populations. Zebrafish pou5f1 maternal-zygotic mutant embryos (spiel ohne grenzen; MZspg) lack endoderm and have gastrulation and dorsoventral patterning defects. A contribution of Pou5f1 to the control of bmp2b, bmp4 and vox expression has been suggested, however the mechanisms remained unclear and are investigated in detail here. Low-level overexpression of a Pou5f1-VP16 activator fusion protein can rescue dorsalization in MZspg mutants, indicating that Pou5f1 acts as a transcriptional activator during dorsoventral patterning. Overexpression of larger quantities of Pou5f1-VP16 can ventralize wild-type embryos, while overexpression of a Pou5f1-En repressor fusion protein can dorsalize embryos. Lack of Pou5f1 causes a transient upregulation of fgf8a expression after mid-blastula transition, providing a mechanism for delayed activation of bmp2b in MZspg embryos. Overexpression of the Pou5f1-En repressor induces fgf8, suggesting an indirect mechanism of Pou5f1 control of fgf8a expression. Transcription of vox is strongly activated by Pou5f1-VP16 even when translation of zygotically expressed transcripts is experimentally inhibited by cycloheximide. In contrast, bmp2b and bmp4 are not activated under these conditions. We show that Pou5f1 binds to phylogenetically conserved Oct/Pou5f1 sites in the vox promoter, both in vivo (ChIP) and in vitro. Our data reveals a set of direct and indirect interactions of Pou5f1 with the BMP dorsoventral patterning network that serve to fine-tune dorsoventral patterning mechanisms and coordinate patterning with developmental timing.

Reversal of Xenopus Oct25 function by disruption of the POU domain structure

Xenopus Oct25 is a POU family subclass V (POU-V) transcription factor with a distinct domain structure. To investigate the contribution of different domains to the function of Oct25, we have performed gain of function analyses. Deletions of the N- or C-terminal regions and of the Hox domain (except its nuclear localization signal) result in mutants being indistinguishable from the wild type protein in the suppression of genes promoting germ layer formation. Deletion of the complete POU domain generates a mutant that has no effect on embryogenesis. However, disruption of the alpha-helical structures in the POU domain, even by a single amino acid mutation, causes reversal of protein function. Overexpression of such mutants leads to dorsalization of embryos and formation of secondary axial structures. The underlying mechanism is an enhanced transcription of genes coding for antagonists of the ligands for ventralizing bone morphogenetic protein and Wnt pathways. Corresponding deletion mutants of Xenopus Oct60, Oct91, or mouse Oct4 also exhibit such a dominant-negative effect. Therefore, our results reveal that the integrity of the POU domain is crucial for the function of POU-V transcription factors in the regulation of genes that promote germ layer formation.

An integrative ChIP-chip and gene expression profiling to model SMAD regulatory modules

Background

The TGF-β/SMAD pathway is part of a broader signaling network in which crosstalk between pathways occurs. While the molecular mechanisms of TGF-β/SMAD signaling pathway have been studied in detail, the global networks downstream of SMAD remain largely unknown. The regulatory effect of SMAD complex likely depends on transcriptional modules, in which the SMAD binding elements and partner transcription factor binding sites (SMAD modules) are present in specific context.

Results

To address this question and develop a computational model for SMAD modules, we simultaneously performed chromatin immunoprecipitation followed by microarray analysis (ChIP-chip) and mRNA expression profiling to identify TGF-β/SMAD regulated and synchronously coexpressed gene sets in ovarian surface epithelium. Intersecting the ChIP-chip and gene expression data yielded 150 direct targets, of which 141 were grouped into 3 co-expressed gene sets (sustained up-regulated, transient up-regulated and down-regulated), based on their temporal changes in expression after TGF-β activation. We developed a data-mining method driven by the Random Forest algorithm to model SMAD transcriptional modules in the target sequences. The predicted SMAD modules contain SMAD binding element and up to 2 of 7 other transcription factor binding sites (E2F, P53, LEF1, ELK1, COUPTF, PAX4 and DR1).

Conclusion

Together, the computational results further the understanding of the interactions between SMAD and other transcription factors at specific target promoters, and provide the basis for more targeted experimental verification of the co-regulatory modules.

Oct25 represses transcription of nodal/activin target genes by interaction with signal transducers during Xenopus gastrulation

The balance between differentiation signals and signals maintaining the undifferentiated state of embryonic cells ensures proper formation of germ layers. The nodal/activin pathway represents one of the major signaling chains responsible for the differentiation of embryonic cells into mesodermal and endodermal germ layers, while Oct4 is one of the major players in the maintenance of an undifferentiated state. Here we show that Oct25, an Oct4 homologue in Xenopus, antagonizes the activity of nodal/activin signaling by inhibiting the transcription of its target genes, Gsc and Mix2. The inhibitory effect is achieved by forming repression complexes on the promoters of Gsc and Mix2 between Oct25 and the signal transducers of the nodal/activin pathway, WBSCR11, FAST1, and Smad2. We have analyzed the significance of the Oct binding site for its inhibitory effect within the Gsc promoter. Albeit VP16-Oct25 fusion protein demonstrated a stimulating effect and EVE-Oct25 revealed a repression effect on an artificial reporter that is composed of eight repeats of Oct binding motifs, both fusions, like wild-type Oct25, inhibited mesendoderm formation and the activity of Gsc and Mix2 promoters. These results suggest that the regulatory effect of Oct25 on the expression of Gsc and Mix2 is mediated by specific protein/protein interactions. Furthermore, we demonstrate that histone deacetylase activities are not required for the inhibitory effect of Oct25. Our results provide a novel view in that Oct25 controls the nodal/activin pathway and thus maintains the undifferentiated state of embryonic cells in preventing them from premature differentiation.

POU-V factors antagonize maternal VegT activity and beta-Catenin signaling in Xenopus embryos

VegT and beta-Catenin are key players in the hierarchy of factors that are required for induction and patterning of mesendoderm in Xenopus embryogenesis. By descending the genetic cascades, cells lose their pluripotent status and are determined to differentiate into distinct tissues. Mammalian Oct-3/4, a POU factor of subclass V (POU-V), is required for the maintenance of pluripotency of embryonic stem cells. However, its molecular function within the early embryo is yet poorly understood. We here show that the two maternal Xenopus POU-V factors, Oct-60 and Oct-25, inhibit transcription of genes activated by VegT and beta-Catenin. Maternal POU-V factors and maternal VegT show an opposite distribution along the animal/vegetal axis. Oct-25, VegT and Tcf3 interact with each other and form repression complexes on promoters of VegT and beta-Catenin target genes. We suggest that POU-V factors antagonize primary inducers to allow germ layer specification in a temporally and spatially coordinated manner.

Xenopus Suppressor of Hairless 2 is involved in the cell fate decision during gastrulation through the transcriptional regulation of Xoct25/91

We have previously indicated that Xenopus Suppressor of Hairless 2, XSu(H)2, is involved in the morphogenesis of gastrula embryos in a different manner from the XESR-1-mediated Notch signaling pathway. To address the downstream factors of XSu(H)2, we investigated the effect of XSu(H)2 on XenopusPOU-V genes, Xoct25 and Xoct91. Knockdown of XSu(H)2 caused the downregulation of Xoct25, Xoct91 and Xbrachyury. Dominant-negative Xoct25 caused the delay of blastopore closure with a decrease of Xbrachyury expression. Overexpression of Xoct25 or Xoct91 could rescue the decrease of Xbrachyury expression caused by XSu(H)2 depletion. XSu(H)2EnR inhibited Xoct25 and Xoct91 expressions in CHX-treated animal caps. Promoter analysis of the Xoct25 showed that the upstream region of Xoct25 contains the prospective Su(H) binding motif, which is essential for the transcription of Xoct25. These results suggest that XSu(H)2 is engaged in the cell fate decision during gastrulation through the gene expression of the Xoct25/91-mediated pathway.

Xenopus POU factors of subclass V inhibit activin/nodal signaling during gastrulation

Three POU factors of subclass V, Oct-25, Oct-60 and Oct-91 are expressed in Xenopus oocytes and early embryos. We here demonstrate that vegetal overexpression of Oct-25, Oct-60, Oct-91 or mammalian Oct-3/4 suppresses mesendoderm formation in Xenopus embryos. Oct-25 and Oct-60 are shown to inhibit activin/nodal and FGF signaling pathways. Loss of Oct-25 and Oct-60 function results in elevated transcription of mesendodermal marker genes and ectopic formation of endoderm in the equatorial region of gastrula stage embryos. Within the ectoderm, Oct-25 promotes neural fate by upregulating neuroectodermal genes, such as Xsox2, which prevent differentiation of neural progenitors into neurons. We also show that mouse Oct-3/4 and Xenopus Oct-25 or Oct-60 behave as functional homologues. We conclude that Xenopus Oct proteins are required to control the levels of embryonic signaling pathways, thereby ensuring the correct specification of germ layers.

Xenopus laevis POU91 protein, an Oct3/4 homologue, regulates competence transitions from mesoderm to neural cell fates

Cellular competence is defined as a cell's ability to respond to signaling cues as a function of time. In Xenopus laevis, cellular responsiveness to fibroblast growth factor (FGF) changes during development. At blastula stages, FGF induces mesoderm, but at gastrula stages FGF regulates neuroectoderm formation. A Xenopus Oct3/4 homologue gene, XLPOU91, regulates mesoderm to neuroectoderm transitions. Ectopic XLPOU91 expression in Xenopus embryos inhibits FGF induction of Brachyury (Xbra), eliminating mesoderm, whereas neural induction is unaffected. XLPOU91 knockdown induces high levels of Xbra expression, with blastopore closure being delayed to later neurula stages. In morphant ectoderm explants, mesoderm responsiveness to FGF is extended from blastula to gastrula stages. The initial expression of mesoderm and endoderm markers is normal, but neural induction is abolished. Churchill (chch) and Sip1, two genes regulating neural competence, are not expressed in XLPOU91 morphant embryos. Ectopic Sip1 or chch expression rescues the morphant phenotype. Thus, XLPOU91 epistatically lies upstream of chch/Sip1 gene expression, regulating the competence transition that is critical for neural induction. In the absence of XLPOU91 activity, the cues driving proper embryonic cell fates are lost.

Conserved roles for Oct4 homologues in maintaining multipotency during early vertebrate development

All vertebrate embryos have multipotent cells until gastrulation but, to date, derivation of embryonic stem (ES) cell lines has been achieved only for mouse and primates. ES cells are derived from mammalian inner cell mass (ICM) tissue that express the Class V POU domain (PouV) protein Oct4. Loss of Oct4 in mice results in a failure to maintain ICM and consequently an inability to derive ES cells. Here, we show that Oct4 homologues also function in early amphibian development where they act as suppressors of commitment during germ layer specification. Antisense morpholino mediated PouV knockdown in Xenopus embryos resulted in severe posterior truncations and anterior neural defects. Gastrulation stage embryos showed reduced expression of genes associated with uncommitted marginal zone cells, while the expression of markers associated with more mature cell states was expanded. Importantly, we have tested PouV proteins from a number of vertebrate species for the ability to substitute Oct4 in mouse ES cells. PouV domain proteins from both Xenopus and axolotl could support murine ES cell self-renewal but the only identified zebrafish protein in this family could not. Moreover, we found that PouV proteins regulated similar genes in ES cells and Xenopus embryos, and that PouV proteins capable of supporting ES cell self-renewal could also rescue the Xenopus PouV knockdown phenotype. We conclude that the unique ability of Oct4 to maintain ES cell pluripotency is derived from an ancestral function of this class of proteins to maintain multipotency.

Maternal control of vertebrate dorsoventral axis formation and epiboly by the POU domain protein Spg/Pou2/Oct4

Dorsoventral (DV) axis formation of the vertebrate embryo is controlled by the maternal genome and is subsequently refined zygotically. In the zygote, repression of ventralizing Bmp activity on the dorsal side through chordin and noggin is crucial for establishment of a dorsally located organizer. This interplay generates a zygotic Bmp activity gradient that defines distinct positional values along the DV axis. The maternal processes that control expression of the zygotic genes implicated in DV patterning are largely unknown. spiel-ohne-grenzen (spg/pou2) is a maternally and zygotically expressed zebrafish gene that encodes the POU domain transcription factor Pou2, an ortholog of mammalian Oct4/Pou5f1. We show that embryos that are genetically depleted of both maternal and zygotic pou2 function (MZspg) exhibit extreme DV patterning defects and, independently, a blastoderm-specific arrest of epiboly. Dorsal tissues expand to the ventral side at the expense of ventrolateral tissue in MZspg embryos. Dorsally expressed Bmp-antagonists, such as Chd and Nog1, and Gsc are ectopically activated at ventral levels in MZspg. Lack of ventral specification is apparent very early, suggesting that maternal processes are affected in MZspg. Indeed, maternal pou2 function is necessary to initiate zygotic expression of ventrally expressed genes such as bmp2b and bmp4, and for proper activation of bmp7, vox, vent and eve1. A constitutively active Alk8-TGFbeta-receptor can ectopically induce bmp2b and bmp4 and rescues the dorsalization of MZspg. This indicates that pou2 acts upstream of Alk8, a maternally provided receptor implicated in the activation of zygotic bmp2b and bmp4 transcription. Consistent with this possibility, Bmp gene misexpression can rescue MZspg embryos, indicating that TGFbeta-mediated signal transduction itself is intact in absence of Pou2. Inhibition of Fgf signaling, another pathway with early dorsalizing activity, can also restore and even ventralize MZspg embryos. The requirement for pou2 to initiate bmp2b expression can therefore be bypassed by releasing the repressive function of Fgf signaling upon bmp2b transcription. In transplantation experiments, we find that dorsalized cells from prospective ventrolateral regions of MZspg embryos are non cell-autonomously respecified to a ventral fate within wild-type host embryos. Analysis of pou2 mRNA injected MZspg embryos shows that pou2 is required on the ventral side of cleavage stage embryos. Based on the maternal requirement for pou2 in ventral specification, we propose that ventral specification employs an active, pou2-dependent maternal induction step, rather than a default ventralizing program.

Transcriptional regulation of mesendoderm formation in Xenopus

Mesoderm and endoderm formation in Xenopus involves the coordinated efforts of maternally and zygotically expressed transcription factors together with growth factor signalling, including members of the TGFbeta and wnt families. In this review we discuss our current state of knowledge of these pathways, and describe in more detail some of the transcription factor-DNA interactions that are involved in mesendoderm formation.

XBP1 forms a regulatory loop with BMP-4 and suppresses mesodermal and neural differentiation in Xenopus embryos

The active form of the Xenopus X-box binding protein 1 (xXBP1) partially synergizes and partially antagonizes with BMP-4 signaling. xXBP1 overexpression inhibits mesoderm differentiation and formation of neural tissues. A functional knockdown promotes differentiation of lateral and dorsal mesoderm but not of ventral mesoderm and of neuroectoderm. We show that the active form of xXBP1 in gastrula and early neurula stage embryos is generated by removal of exon 4 and not by an endoribonuclease activity in the endoplasmic reticulum. The N-terminal region of xXBP1 which contains the basic leucine-zipper also contains a nuclear localization signal and both, the N-terminal as well as the C-terminal regions are required for xXBP1 function. The effects of xXBP1 are in part correlated to a regulatory loop between xXBP1 and BMP-4. xXBP1 and BMP-4 stimulate mutually the transcription of each other, but xXBP1 inhibits the BMP-4 target gene, Xvent-2. Both, in vitro and in vivo assays demonstrate that xXBP1 interacts with BMP-4 and Xvent-2B promoters. GST-pulldown assays reveal that xXBP1 can interact with c-Jun, the transcriptional co-activator p300 and with the BMP-4 responsive Smad1. On the other hand, xXBP1 also binds to the inhibitory Smads, Smad6 and Smad7, that can act as transcriptional co-repressors. Based on these data, we conclude that xXBP1 might function as an inhibitor of mesodermal and neural tissue formation by acting either as transcriptional activator or as repressor. This dual activity depends upon binding of co-factors being involved in the formation of distinct transcription complexes.

A consensus Oct1 binding site is required for the activity of the Xenopus Cdx4 promoter

Cdx homeodomain transcription factors have multiple roles in early vertebrate development. Furthermore, mis-regulation of Cdx expression has been demonstrated in metaplasias and cancers of the gut epithelium. Given the importance of Cdx genes in development and disease, the mechanisms underlying their expression are of considerable interest. We report an analysis of the upstream regulatory regions from the amphibian Xenopus laevis Cdx4 gene. We show that a GFP reporter containing 2.8 kb upstream of the transcription start site is expressed in the posterior of transgenic embryos. Deletion analysis of the upstream sequence reveals that a 247-bp proximal promoter fragment will drive posterior expression in transgenic embryos. We show that 63 bp of upstream sequence, that includes a consensus site for POU-domain octamer-binding proteins, retains significant promoter activity. Co-expression of the octamer-binding protein Oct1 induces expression from a Cdx4 reporter and mutation of the octamer site abolishes activity of the same reporter. We show that the octamer site is highly conserved in the promoters of the human, mouse, chicken, and zebrafish Cdx4 genes and within the promoters of amphibian Cdx1 and Cdx2. These data suggest a conserved function for octamer-binding proteins in the regulation of Cdx family members.

An Oct-1 binding site mediates activation of the gata2 promoter by BMP signaling

The gata2 gene encodes a transcription factor implicated in regulating early patterning of ectoderm and mesoderm, and later in numerous cell-specific gene expression programs. Activation of the gata2 gene during embryogenesis is dependent on the bone morphogenetic protein (BMP) signaling pathway, but the mechanism for how signaling controls gene activity has not been defined. We developed an assay in Xenopus embryos to analyze regulatory sequences of the zebrafish gata2 promoter that are necessary to mediate the response to BMP signaling during embryogenesis. We show that activation is Smad dependent, since it is blocked by expression of the inhibitory Smad6. Deletion analysis identified an octamer binding site that is necessary for BMP-mediated induction, and that interacts with the POU homeodomain protein Oct-1. However, this element is not sufficient to transfer a BMP response to a heterologous promoter, requiring an additional more proximal cooperating element. Based on recent studies with other BMP-dependent promoters (Drosophila vestigial and Xenopus Xvent-2), our studies of the gata2 gene suggest that POU-domain proteins comprise a common component of the BMP signaling pathway, cooperating with Smad proteins and other transcriptional activators.