Dickkopf (Dkk) 1 promotes the differentiation of mouse embryonic stem cells toward neuroectoderm

Redox heterogeneity in mouse embryonic stem cells individualizes cell fate decisions

Pluripotent embryonic stem cells (ESCs) can develop into any cell type in the body. Yet, the regulatory mechanisms that govern cell fate decisions during embryogenesis remain largely unknown. We now demonstrate that mouse ESCs (mESCs) display large natural variations in mitochondrial reactive oxygen species (mitoROS) levels that individualize their nuclear redox state, H3K4me3 landscape, and cell fate. While mESCs with high mitoROS levels (mitoROSHIGH) differentiate toward mesendoderm and form the primitive streak during gastrulation, mESCs, which generate less ROS, choose the alternative neuroectodermal fate. Temporal studies demonstrated that mesendodermal (ME) specification of mitoROSHIGH mESCs is mediated by a Nrf2-controlled switch in the nuclear redox state, triggered by the accumulation of redox-sensitive H3K4me3 marks, and executed by a hitherto unknown ROS-dependent activation process of the Wnt signaling pathway. In summary, our study explains how ESC heterogeneity is generated and used by individual cells to decide between distinct cellular fates.

Wnt pathway: A mechanism worth considering in endocrine disrupting chemical action

Endocrine disrupting chemicals (EDCs) are defined as exogenous substances that can alter the development and functioning of the endocrine system. The Wnt signaling pathway is an evolutionarily conserved pathway consisting of proteins that transmit cell-to-cell receptors through cell surface receptors, regulating important aspects of cell migration, polarity, neural formation, and organogenesis, which determines the fate of the cell during embryonic development. Although the effects of EDCs have been studied in terms of many molecular mechanisms; because of its critical role in embryogenesis, the Wnt pathway is of special interest in EDC exposure. This review provides information about the effects of EDC exposure on the Wnt/β-catenin pathway focusing on studies on bisphenol A, di-(2-ethylhexyl) phthalate, diethylstilbestrol, cadmium, and 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Modeling Mammalian Commitment to the Neural Lineage Using Embryos and Embryonic Stem Cells

Early mammalian embryogenesis relies on a large range of cellular and molecular mechanisms to guide cell fate. In this highly complex interacting system, molecular circuitry tightly controls emergent properties, including cell differentiation, proliferation, morphology, migration, and communication. These molecular circuits include those responsible for the control of gene and protein expression, as well as metabolism and epigenetics. Due to the complexity of this circuitry and the relative inaccessibility of the mammalian embryo in utero, mammalian neural commitment remains one of the most challenging and poorly understood areas of developmental biology. In order to generate the nervous system, the embryo first produces two pluripotent populations, the inner cell mass and then the primitive ectoderm. The latter is the cellular substrate for gastrulation from which the three multipotent germ layers form. The germ layer definitive ectoderm, in turn, is the substrate for multipotent neurectoderm (neural plate and neural tube) formation, representing the first morphological signs of nervous system development. Subsequent patterning of the neural tube is then responsible for the formation of most of the central and peripheral nervous systems. While a large number of studies have assessed how a competent neurectoderm produces mature neural cells, less is known about the molecular signatures of definitive ectoderm and neurectoderm and the key molecular mechanisms driving their formation. Using pluripotent stem cells as a model, we will discuss the current understanding of how the pluripotent inner cell mass transitions to pluripotent primitive ectoderm and sequentially to the multipotent definitive ectoderm and neurectoderm. We will focus on the integration of cell signaling, gene activation, and epigenetic control that govern these developmental steps, and provide insight into the novel growth factor-like role that specific amino acids, such as L-proline, play in this process.

The depalmitoylase APT1 directs the asymmetric partitioning of Notch and Wnt signaling during cell division

Asymmetric cell division results in two distinctly fated daughter cells. A molecular hallmark of asymmetric division is the unequal partitioning of cell fate determinants. We have previously established that growth factor signaling promotes protein depalmitoylation to foster polarized protein localization, which, in turn, drives migration and metastasis. We report protein palmitoylation as a key mechanism for the asymmetric partitioning of the cell fate determinants Numb and β-catenin through the activity of the depalmitoylating enzyme APT1. Using point mutations, we showed that specific palmitoylated residues on Numb were required for its asymmetric localization. By live-cell imaging, we showed that reciprocal interactions between APT1 and the Rho family GTPase CDC42 promoted the asymmetric localization of Numb and β-catenin to the plasma membrane. This, in turn, restricted Notch- or Wnt-responsive transcriptional activity to one daughter cell. Moreover, we showed that altering APT1 abundance changed the transcriptional signatures of MDA-MB-231 triple receptor-negative breast cancer cells, similar to changes in Notch and β-catenin-mediated Wnt signaling. We also showed that loss of APT1 depleted a specific subpopulation of tumorigenic cells in colony formation assays. Together, our findings suggest that APT1-mediated depalmitoylation is a major mechanism of asymmetric cell division that maintains Notch- and Wnt-associated protein dynamics, gene expression, and cellular functions.

Effects of Wnt/β-catenin signaling on bisphenol A exposure in male mouse reproductive cells

Bisphenol A (BPA) is a chemical used in numerous consumer products that is able to interfere with the mammalian endocrine system. The aim of the present study was to investigate the effects of BPA on male mouse reproductive cells following prenatal to postnatal exposure. In addition, the influence of BPA was detected on the expression levels of β‑catenin and dickkopf WNT signaling pathway inhibitor 1 (DKK‑1) during the differentiation of spermatogenic cells in the mouse testes. β‑catenin and DKK‑1 are two important proteins of the Wnt/β‑catenin signaling pathway. On gestational day 1, pregnant ICR mice were randomly divided into four groups: A dimethyl sulfoxide group, and three groups treated with various concentrations of BPA (0.5, 10, and 50 µg/kg). BPA was administered from gestational day 1 to weaning on postnatal day (PND) 42. The number of murine pups and the male:female ratio was recorded for each group. On PND 42, the male pups were sacrificed and their wet weights and testicular coefficients were measured. Immunohistochemical and western blot analyses were used to detect the protein expression of β‑catenin and DKK‑1 in the testicular tissue samples of the six‑week‑old male mice. The results indicated that the number of murine pups, as well as the testicular viscera coefficient of the male mice, significantly decreased in the BPA‑treated groups, as compared with the control group (P<0.05, P<0.01); however, no significant difference was observed in the male/female ratio in the BPA‑treated groups, as compared with the control group (P>0.05). The results from the immunohistochemical and western blot analyses indicated that the protein expression of β‑catenin and DKK‑1 were significantly increased in the BPA‑treated groups, as compared with the control group, and the distribution of spermospore and Leydig cells also increased in the testes. These results suggest that high expression levels of β‑catenin and DKK‑1 may participate in BPA‑induced pathogenesis in male mouse reproductive cells.

Directed Differentiation of Human Embryonic Stem Cells into Neural Progenitors

A variety of protocols have been used to produce neural progenitors from human embryonic stem cells. We have focused on a monolayer culture approach that generates neural rosettes. To initiate differentiation, cells are plated in a serum-free nutrient-poor medium in the presence of a BMP inhibitor. Depending on the cell line used, additional growth factor inhibitors may be required to promote neural differentiation. Long-term culture and addition of the Notch inhibitor DAPT can promote terminal neuronal differentiation. Extent of differentiation is monitored using immunocytochemistry for cell type-specific markers.

Roles of brain and muscle ARNT-like 1 and Wnt antagonist Dkk1 during osteogenesis of bone marrow stromal cells

OBJECTIVES

Many studies have demonstrated that the clock gene, brain and muscle ARNT-like 1 (Bmal1), is directly related to bone ageing by affecting age-related changes to mesenchymal stem cells (MSCs). As a main developmental signal, Wnt may play an important role in this process. Here, we have aimed to elucidate whether Bmal1 positively regulates osteogenesi via Wnt pathways.

MATERIALS AND METHODS

Bone marrow stromal cells were cultured in basic and in osteo-induction medium with Wnt signalling inhibitor Dkk1 and Bmal1 transfection. Proliferation and osteogenesis of MSCs, expression of Bmal1 and activation of Wnt signalling were investigated by flow cytometry, senescence-associated β-galactosidase (SA-β-gal) staining, real-time quantitative PCR and western blot analysis.

RESULTS

Expression of Bmal1 (specially after 7 days osteo-induction), activation of Wnt signalling and osteo-related factors fell significantly during osteo-induction after Dkk1 addition. When cellular Bmal1 was increased by transfection, osteogenesis inhibition by Dkk1 was rescued to a certain extent with activation of Wnt signalling. However, Dkk1 did not significantly affect proliferation or senescence of MSCs during early periods of culture.

CONCLUSION

These findings demonstrated that Bmal1 and Wnt signalling may have a synergistic effect at a particular stage of osteogenesis. Inhibition of Wnt signalling did not greatly affect ageing of MSCs through early passages.

Transcription factor Zic2 inhibits Wnt/β-catenin protein signaling

The Zic transcription factors play critical roles during embryonic development. Mutations in the ZIC2 gene are associated with human holoprosencephaly, but the etiology is still unclear. Here, we report a novel function for ZIC2 as a regulator of β-catenin·TCF4-mediated transcription. We show that ZIC2 can bind directly to the DNA-binding high mobility group box of TCF4 via its zinc finger domain and inhibit the transcriptional activity of the β-catenin·TCF4 complex. However, the binding of TCF4 to DNA was not affected by ZIC2. Zic2 RNA injection completely inhibited β-catenin-induced axis duplication in Xenopus embryos and strongly blocked the ability of β-catenin to induce expression of known Wnt targets in animal caps. Moreover, Zic2 knockdown in transgenic Xenopus Wnt reporter embryos led to ectopic Wnt signaling activity mainly at the midbrain-hindbrain boundary. Together, our results demonstrate a previously unknown role for ZIC2 as a transcriptional regulator of the β-catenin·TCF4 complex.

Dynamic changes in Wnt signaling are required for neuronal differentiation of mouse embryonic stem cells

Embryonic stem cells (ESC) and the epiblast share a similar gene expression profile and an attenuated cell cycle, making them an accessible and tractable model system to study lineage choice at gastrulation. Differentiation of the epiblast and ESC to the mesendodermal lineage has been shown to rely on Wnt/β-catenin signaling; which counterintuitively, is also required to inhibit differentiation and maintain pluripotency. To examine these seemingly contradictory roles, we developed a mouse ESC (ESC) line that inducibly expresses a dominant negative Tcf4 (dnTcf4) protein to block canonical Wnt signaling. Cells expressing the dnTcf4 protein differentiated largely to Sox3 positive neural precursors but were unable to progress to βIII tubulin positive neurons unless Wnt signaling was derepressed, demonstrating a sequential requirement for Wnt signaling in lineage differentiation. To determine if Wnt/β-catenin signaling is similarily required at sequential stages of neural differentiation in the intact embryo, we delivered shRNA targeting β-catenin to pregnant mice on E5.5 of development. Blocking canonical Wnt signaling during post-implantation development increased the number of neural precursors which failed to differentiate to mature neurons, and produced defects of embryonic axis elongation, neurulation and neural tube closure that phenocopy the β-catenin null embryo. These results demonstrate that lineage differentiation relies on sequential repression and derepression of critical signaling pathways involved in maintaining pluripotency versus differentiation.

Dkk1 regulates ventral midbrain dopaminergic differentiation and morphogenesis

Dickkopf1 (Dkk1) is a Wnt/β-catenin inhibitor that participates in many processes during embryonic development. One of its roles during embryogenesis is to induce head formation, since Dkk1-null mice lack head structures anterior to midbrain. The Wnt/β-catenin pathway is also known to regulate different aspects of ventral midbrain (VM) dopaminergic (DA) neuron development and, in vitro, Dkk1-mediated inhibition of the Wnt/β-catenin pathway improves the DA differentiation in mouse embryonic stem cells (mESC). However, the in vivo function of Dkk1 on the development of midbrain DA neurons remains to be elucidated. Here we examined Dkk1^+/− embryos and found that Dkk1 is required for the differentiation of DA precursors/neuroblasts into DA neurons at E13.5. This deficit persisted until E17.5, when a defect in the number and distribution of VM DA neurons was detected. Furthermore, analysis of the few Dkk1^−/− embryos that survived until E17.5 revealed a more severe loss of midbrain DA neurons and morphogenesis defects. Our results thus show that Dkk1 is required for midbrain DA differentiation and morphogenesis.

Developmental changes in thermoprotective actions of insulin-like growth factor-1 on the preimplantation bovine embryo

Insulin-like growth factor 1 (IGF1) is an important endocrine signal for regulation of early embryonic development. It increases the proportion of preimplantation embryos becoming blastocysts, alters blastocyst gene expression, improves resistance of embryos to various stresses and can enhance survival of embryos after transfer to recipients. The present study had two objectives. The first was to determine whether the thermoprotective actions of IGF1 on the preimplantation bovine embryo was developmentally regulated, with the two-cell embryo being refractory to IGF1. The second was to determine the molecular basis for the improved competence of embryos treated with IGF1 to establish pregnancy after transfer to heat-stressed recipients. Treatment of embryos with 100 ng/ml IGF1 reduced the effects of heat shock on embryos ≥16 cells at day 5 after insemination but did not provide thermoprotection to two-cell embryos. Failure of IGF1 to alter embryo survival after heat shock was not associated with reduced expression of genes involved in IGF1 signaling (IGF1R, RAF1, PI3K, and MAPK) or immunoreactive IGF1R protein. Treatment with IGF1 had little effect on the transcriptome at the blastocyst stage of development, with a total of 102 differentially expressed genes identified. Among the differentially expressed genes were several involved in apoptosis, protection against free radicals and development. Changes in gene expression were consistent with IGF1 acting to induce an anti-apoptotic state and inhibit neurulation. In conclusion, thermoprotective actions of IGF1 are developmentally regulated. Failure of IGF1 to protect the two-cell embryo from heat shock could reflect the fact that these embryos are maximally sensitive to damage caused by heat shock or reflect the quiescence of the embryonic genome at this stage of development. Changes in gene expression at the blastocyst stage induced by IGF1 could contribute to the increased survival of IGF1-treated embryos when transferred during periods of heat stress.

Wip1-expressing feeder cells retain pluripotency of co-cultured mouse embryonic stem cells under leukemia inhibitory factor-deprivated condition

The optimization of in vitro culture conditions for embryonic stem cells (ESCs) is a matter of critical importance; a prompt supply of a sufficient population of cells that retain their pluripotency capabilities must be secured in order to make possible future cell therapies. Despite a number of reports asserting that a variety of cytokines, signaling ligands, and small molecules can help in maintaining the pluripotency of ESCs, mammalian feeder cells continue to be broadly accepted as the method of choice for ESC cultures. This appears to be because mammalian feeder cells seem to produce some as-yet-unidentified factor that makes them very effective as feeder cells. In this study, we investigated wild-type p53 inducible phosphatase (Wip1), the knockdown of which increases Wnt inhibitory factor-1 expression, in its feeder functions toward mouse embryonic stem cells, lowering the effect of Wnt, one of key signaling in maintaining stemness of ESCs. For this purpose, Wip1 was stably expressed in mouse embryonic fibroblast cell line (STO) using retro-viral gene delivery system and then the function as a feeder cell was monitored either with or without leukemia inhibitory factor (LIF) in culture medium. We demonstrated that mouse embryonic stem cells grown with Wip1 expressing STO showed higher alkaline phosphatase activity and sustained Oct-4 expression level even under LIF deprivation condition compared to both control and Wip1 phosphatase activity dead mutant expressing STO. These results imply that Wip1 phosphatase activity in feeder cells is important to retain pluripotency of mouse embryonic stem cells under LIF deprivation conditions. These results indicate that genetically engineered feeder cells such as Wip1 expressing cell lines, are alternative strategy for the optimization of maintenance and expansion of mouse embryonic stem cells.