Intestinal lineage commitment of embryonic stem cells

Generating lineage-committed intestinal stem cells from embryonic stem cells (ESCs) could provide a tractable experimental system for understanding intestinal differentiation pathways and may ultimately provide cells for regenerating damaged intestinal tissue. We tested a two-step differentiation procedure in which ESCs were first cultured with activin A to favor formation of definitive endoderm, and then treated with fibroblast-conditioned medium with or without Wnt3A. The definitive endoderm expressed a number of genes associated with gut-tube development through mouse embryonic day 8.5 (Sox17, Foxa2, and Gata4 expressed and Id2 silent). The intestinal stem cell marker Lgr5 gene was also activated in the endodermal cells, whereas the Msi1, Ephb2, and Dcamkl1 intestinal stem cell markers were not. Exposure of the endoderm to fibroblast-conditioned medium with Wnt3A resulted in the activation of Id2, the remaining intestinal stem cell markers and the later gut markers Cdx2, Fabp2, and Muc2. Interestingly, genes associated with distal gut-associated mesoderm (Foxf2, Hlx, and Hoxd8) were also simulated by Wnt3A. The two-step differentiation protocol generated gut bodies with crypt-like structures that included regions of Lgr5-expressing proliferating cells and regions of cell differentiation. These gut bodies also had a smooth muscle component and some underwent peristaltic movement. The ability of the definitive endoderm to differentiate into intestinal epithelium was supported by the vivo engraftment of these cells into mouse colonic mucosa. These findings demonstrate that definitive endoderm derived from ESCs can carry out intestinal cell differentiation pathways and may provide cells to restore damaged intestinal tissue.     

Transcriptional analysis of early lineage commitment in human embryonic stem cells

Background  

The mechanisms responsible for the maintenance of pluripotency in human embryonic stem cells, and those that drive their commitment into particular differentiation lineages, are poorly understood. In fact, even our knowledge of the phenotype of hESC is limited, because the immunological and molecular criteria presently used to define this phenotype describe the properties of a heterogeneous population of cells.     

Beta-catenin signaling is required for neural differentiation of embryonic stem cells

Culture of embryonic stem (ES) cells at high density inhibits both beta-catenin signaling and neural differentiation. ES cell density does not influence beta-catenin expression, but a greater proportion of beta-catenin is targeted for degradation in high-density cultures. Moreover, in high-density cultures, beta-catenin is preferentially localized to the membrane further reducing beta-catenin signaling. Increasing beta-catenin signaling by treatment with Wnt3a-conditioned medium, by overexpression of beta-catenin, or by overexpression of a dominant-negative form of E-cadherin promotes neurogenesis. Furthermore, beta-catenin signaling is sufficient to induce neurogenesis in high-density cultures even in the absence of retinoic acid (RA), although RA potentiates the effects of beta-catenin. By contrast, RA does not induce neurogenesis in high-density cultures in the absence of beta-catenin signaling. Truncation of the armadillo domain of beta-catenin, but not the C terminus or the N terminus, eliminates its proneural effects. The proneural effects of beta-catenin reflect enhanced lineage commitment rather than proliferation of neural progenitor cells. Neurons induced by beta-catenin overexpression either alone or in association with RA express the caudal neuronal marker Hoxc4. However, RA treatment inhibits the beta-catenin-mediated generation of tyrosine hydroxylase-positive neurons, suggesting that not all of the effects of RA are dependent upon beta-catenin signaling. These observations suggest that beta-catenin signaling promotes neural lineage commitment by ES cells, and that beta-catenin signaling may be a necessary co-factor for RA-mediated neuronal differentiation. Further, enhancement of beta-catenin signaling with RA treatment significantly increases the numbers of neurons generated from ES cells, thus suggesting a method for obtaining large numbers of neural species for possible use in for ES cell transplantation.     

Phospholipase Cdelta1 is required for skin stem cell lineage commitment

Phosphoinositide-specific phospholipase C (PLC) is a key enzyme in phosphoinositide turnover and is involved in a variety of physiological functions. Here we report that PLCdelta(1)-deficient mice undergo progressive hair loss in the first postnatal hair cycle. Epidermal hyperplasia was observed, and many hairs in the skin of PLCdelta(1)-deficient mice failed to penetrate the epidermis and became zigzagged owing to occlusion of the hair canal. Two major downstream signals of PLC, calcium elevation and protein kinase C activation, were impaired in the keratinocytes and skin of PLCdelta(1)-deficient mice. In addition, many cysts that had remarkable similarities to interfollicular epidermis, as well as hyperplasia of sebaceous glands, were observed. Furthermore, PLCdelta(1)-deficient mice developed spontaneous skin tumors that had characteristics of both interfollicular epidermis and sebaceous glands. From these results, we conclude that PLCdelta(1) is required for skin stem cell lineage commitment.     

Hedgehog signaling is required for commitment but not initial induction of slow muscle precursors

In the embryonic mouse retina, retinoic acid (RA) is unevenly distributed along the dorsoventral axis: RA-rich zones in dorsal and ventral retina are separated by a horizontal RA-poor stripe that contains the RA-inactivating enzyme CYP26A1. To explore the developmental role of this arrangement, we studied formation of the retina and its projections in Cyp26a1 null-mutant mice. Expression of several dorsoventral markers was not affected, indicating that CYP26A1 is not required for establishing the dorsoventral retina axis. Analysis of the mutation on a RA-reporter mouse background confirmed, as expected, that the RA-poor stripe was missing in the retina and its projections at the time when the optic axons first grow over the diencephalon. A day later, however, a gap appeared both in retina and retinofugal projections. As explanation, we found that CYP26C1, another RA-degrading enzyme, had emerged centrally in a narrower domain within the RA-poor stripe. While RA applications increased retinal Cyp26a1 expression, they slightly reduced Cyp26c1. These observations indicate that the two enzymes function independently. The safeguard of the RA-poor stripe by two distinct enzymes during later development points to a role in maturation of a significant functional feature like an area of higher visual acuity that develops at its location.     

Fourier transform infrared microspectroscopy identifies early lineage commitment in differentiating human embryonic stem cells

Human ESCs (hESCs) are a valuable tool for the study of early human development and represent a source of normal differentiated cells for pharmaceutical and biotechnology applications and ultimately for cell replacement therapies. For all applications, it will be necessary to develop assays to validate the efficacy of hESC differentiation. We explored the capacity for FTIR spectroscopy, a technique that rapidly characterises cellular macromolecular composition, to discriminate mesendoderm or ectoderm committed cells from undifferentiated hESCs. Distinct infrared spectroscopic “signatures” readily distinguished hESCs from these early differentiated progeny, with bioinformatic models able to correctly classify over 97% of spectra. These data identify a role for FTIR spectroscopy as a new modality to complement conventional analyses of hESCs and their derivatives. FTIR spectroscopy has the potential to provide low-cost, automatable measurements for the quality control of stem and differentiated cells to be used in industry and regenerative medicine.     

ERG is required for the differentiation of embryonic stem cells along the endothelial lineage

Background

During skeletogenesis, protein levels of β-catenin in the canonical Wnt signaling pathway determine lineage commitment of skeletal precursor cells to osteoblasts and chondrocytes. Adenomatous polyposis coli (Apc) is a key controller of β-catenin turnover by down-regulating intracellular levels of β-catenin.

Results

To investigate whether Apc is involved in lineage commitment of skeletal precursor cells, we generated conditional knockout mice lacking functional Apc in Col2a1-expressing cells. In contrast to other models in which an oncogenic variant of β-catenin was used, our approach resulted in the accumulation of wild type β-catenin protein due to functional loss of Apc. Conditional homozygous Apc mutant mice died perinatally showing greatly impaired skeletogenesis. All endochondral bones were misshaped and lacked structural integrity. Lack of functional Apc resulted in a pleiotropic skeletal cell phenotype. The majority of the precursor cells lacking Apc failed to differentiate into chondrocytes or osteoblasts. However, skeletal precursor cells in the proximal ribs were able to escape the noxious effect of functional loss of Apc resulting in formation of highly active osteoblasts. Inactivation of Apc in chondrocytes was associated with dedifferentiation of these cells.

Conclusion

Our data indicate that a tight Apc-mediated control of β-catenin levels is essential for differentiation of skeletal precursors as well as for the maintenance of a chondrocytic phenotype in a spatio-temporal regulated manner.     

Notch signaling is required for arterial-venous differentiation during embryonic vascular development

Recent evidence indicates that acquisition of artery or vein identity during vascular development is governed, in part, by genetic mechanisms. The artery-specific expression of a number of Notch signaling genes in mouse and zebrafish suggests that this pathway may play a role in arterial-venous cell fate determination during vascular development. We show that loss of Notch signaling in zebrafish embryos leads to molecular defects in arterial-venous differentiation, including loss of artery-specific markers and ectopic expression of venous markers within the dorsal aorta. Conversely, we find that ectopic activation of Notch signaling leads to repression of venous cell fate. Finally, embryos lacking Notch function exhibit defects in blood vessel formation similar to those associated with improper arterial-venous specification. Our results suggest that Notch signaling is required for the proper development of arterial and venous blood vessels, and that a major role of Notch signaling in blood vessels is to repress venous differentiation within developing arteries. Movies available on-line     

Notch promotes neural lineage entry by pluripotent embryonic stem cells

A central challenge in embryonic stem (ES) cell biology is to understand how to impose direction on primary lineage commitment. In basal culture conditions, the majority of ES cells convert asynchronously into neural cells. However, many cells resist differentiation and others adopt nonneural fates. Mosaic activation of the neural reporter Sox-green fluorescent protein suggests regulation by cell-cell interactions. We detected expression of Notch receptors and ligands in mouse ES cells and investigated the role of this pathway. Genetic manipulation to activate Notch constitutively does not alter the stem cell phenotype. However, upon withdrawal of self-renewal stimuli, differentiation is directed rapidly and exclusively into the neural lineage. Conversely, pharmacological or genetic interference with Notch signalling suppresses the neural fate choice. Notch promotion of neural commitment requires parallel signalling through the fibroblast growth factor receptor. Stromal cells expressing Notch ligand stimulate neural specification of human ES cells, indicating that this is a conserved pathway in pluripotent stem cells. These findings define an unexpected and decisive role for Notch in ES cell fate determination. Limiting activation of endogenous Notch results in heterogeneous lineage commitment. Manipulation of Notch signalling is therefore likely to be a key factor in taking command of ES cell lineage choice.     

Regulation of developmental competence and commitment towards the definitive endoderm lineage in human embryonic stem cells

Human embryonic stem cells (hESCs) can self-renew and become all three germ layers. Nodal/Activin signaling specifies developmental status in hESCs: moderate Nodal/Activin signaling maintains pluripotency, while enhancement and inhibition promote definitive endoderm (DE) and neuroectoderm (NE) development, respectively. However, how modulation of Nodal/Activin signaling influences developmental competence and commitment toward specific lineages is still unclear. Here, we showed that enhancement of Nodal/Activin signaling for 4 days was necessary and sufficient to upregulate DE markers, while it diminished the upregulation of NE markers by inhibition of Nodal/Activin signaling. This suggests that after 4 days of enhanced Nodal/Activin signaling, hESCs are committed to the DE lineage and have lost competence toward the NE lineage. In contrast, inhibition of Nodal/Activin signaling using LY364947 for 2 days was sufficient to impair competence toward the DE lineage, although cells were still able to activate LEFTY1 and NODAL, direct targets of Nodal/Activin signaling. Expression analyses indicated that the levels of pluripotency regulators NANOG and POU5F1 were significantly diminished by 2 days of LY364947 treatment, although the expression of NANOG, but not POU5F1, was restored immediately upon Activin A treatment. Thus, downregulation of POU5F1 coincided with the abrogation of DE competence caused by inhibition of Nodal/Activin signaling.     

Notch2, but not Notch1, is required for proximal fate acquisition in the mammalian nephron

The Notch pathway regulates cell fate determination in numerous developmental processes. Here we report that Notch2 acts non-redundantly to control the processes of nephron segmentation through an Rbp-J-dependent process. Notch1 and Notch2 are detected in the early renal vesicle. Genetic analysis reveals that only Notch2 is required for the differentiation of proximal nephron structures (podocytes and proximal convoluted tubules) despite the presence of activated Notch1 in the nuclei of putative proximal progenitors. The inability of endogenous Notch1 to compensate for Notch2 deficiency may reflect sub-threshold Notch1 levels in the nucleus. In line with this view, forced expression of a gamma-secretase-independent form of Notch1 intracellular domain drives the specification of proximal fates where all endogenous, ligand-dependent Notch signaling is blocked by a gamma-secretase inhibitor. These results establish distinct (non-redundant), instructive roles for Notch receptors in nephron segmentation.     

p53 is required for etoposide-induced apoptosis of human embryonic stem cells

The molecular mechanisms controlling DNA-damage-induced apoptosis of human embryonic stem cells (hESC) are poorly understood. Here we investigate the role of p53 in etoposide-induced apoptosis. We show that p53 is constitutively expressed at high levels in the cytoplasm of hESC. Etoposide treatment results in a rapid and extensive induction of apoptosis and leads to a further increase in p53 and PUMA expression as well as Bax processing. p53 both translocates to the nucleus and associates with the mitochondria, accompanied by colocalization of Bax with Mcl1. hESC stably transduced with p53 shRNA display 80% reduction of endogenous p53 and exhibit an 80% reduction in etoposide-induced apoptosis accompanied by constitutive downregulation of Bax and an attenuated upregulation of PUMA. Our data further show that undifferentiated hESC that express Oct4 are much more sensitive to etoposide-induced apoptosis than their more differentiated progeny. Our study demonstrates that p53 is required for etoposide-induced apoptosis of hESC and reveals, at least in part, the molecular mechanism of DNA-damage-induced apoptosis in hESC.     

Securin is not required for cellular viability, but is required for normal growth of mouse embryonic fibroblasts

Sister chromatid separation depends on the release of cohesion by the activity of Esp1, a member of the caspase family [1, 2]. In budding yeast, Esp1p is kept inactive by its association with Pds1p, until the onset of anaphase, when Pds1p is ubiquitinated by the APC/Cdc20 complex [3--5] and subsequently degraded by the 26S proteasome. Pds1 is not an essential gene in budding yeast, but is required for cell cycle arrest prior to anaphase in response to the disruption of spindle structures [6, 7]. Thus, Pds1 mutant yeast cells display precocious sister chromatid separation in the presence of nocodazole [6]. Mammalian orthologs of yeast Esp1 and Pds1, separin and securin, have been identified [8], and, as anticipated, a nondegradable mutant form of securin inhibits sister separation when added to mitotic Xenopus egg extracts [8]. Securin was also independently identified as PTTG (pituitary tumor transforming gene), a gene overexpressed in pituitary tumors [9]. The relationship between its overexpression in tumors and its control of sister chromatid cohesion remains ill defined. To explore securin function in mammals, we took a targeted gene disruption approach in mice. Here, we report that securin is neither essential for cell viability nor required for spindle checkpoint function, and mice lacking securin are viable and apparently normal, but mouse embryonic fibroblasts lacking securin grow abnormally in culture.     

Transferrin receptor induction is required for human B-lymphocyte activation but not for immunoglobulin secretion

Transferrin receptors are expressed on proliferating cells and are required for their growth. Transferrin receptors can be detected after, but not before, mitogenic stimulation of normal peripheral blood T and B cells. In the experiments reported here we have examined the regulation of transferrin receptor expression on activated human B cells and whether or not these receptors are necessary for activation to occur. Activation was assessed by studying both proliferation and immunoglobulin secretion. We have determined that transferrin receptor expression on B cells is regulated by a factor contained in supernatants of mitogen-stimulated T cells (probably B-cell growth factor). This expression is required for proliferation to occur, since antibody to transferrin receptor (42/6) blocks B-cell proliferation. Induction of immunoglobulin secretion, however, although dependent on PHA-treated T-cell supernatant, is not dependent on transferrin receptor expression and can occur in mitogen-stimulated cells whose proliferation has been blocked by antitransferrin receptor antibody. In addition, we have demonstrated that IgM messenger RNA induction following mitogen stimulation is unaffected by antitransferrin receptor antibody. These findings support a model for B-cell activation in which mitogen (or antigen) delivers two concurrent but distinct signals to B cells: one, dependent on B-cell growth factor and transferrin receptor expression, for proliferation, and a second, dependent on T cell-derived factors and not requiring transferrin receptors, which leads to immunoglobulin secretion.     

Activation of retinoic acid receptor signaling coordinates lineage commitment of spontaneously differentiating mouse embryonic stem cells in embryoid bodies

Retinoid signaling has been implicated in embryonic stem cell differentiation. Here we present a systematic analysis of gene expression changes in mouse embryonic stem cells (mESCs), during their spontaneous differentiation into embryoid bodies and the effect of all-trans retinoic acid (ATRA) on this process. We show that retinoic acid is present in the serum and is sufficient to activate retinoid signaling at a basal level in undifferentiated mESCs. This signal disappears during embryoid body formation. However exogenously added ATRA resets the spontaneous differentiation programs in embryoid bodies and initiates a distinct genetic program. These data suggest that retinoid signaling not only promotes a particular pathway but also acts as a context dependent general coordinator of the differentiation states in embryonic stem cells.     

DeltaNp63 is essential for epidermal commitment of embryonic stem cells

In vivo studies have demonstrated that p63 plays complex and pivotal roles in pluristratified squamous epithelial development, but its precise function and the nature of the isoform involved remain controversial. Here, we investigate the role of p63 in epithelial differentiation, using an in vitro ES cell model that mimics the early embryonic steps of epidermal development. We show that the DeltaNp63 isoform is activated soon after treatment with BMP-4, a morphogen required to commit differentiating ES cells from a neuroectodermal to an ectodermal cell fate. DeltaNp63 gene expression remains high during epithelial development. P63 loss of function drastically prevents ectodermal cells to commit to the K5/K14-positive stratified epithelial pathway while gain of function experiments show that DeltaNp63 allows this commitment. Interestingly, other epithelial cell fates are not affected, allowing the production of K5/K18-positive epithelial cells. Therefore, our results demonstrate that DeltaNp63 may be dispensable for some epithelial differentiation, but is necessary for the commitment of ES cells into K5/K14-positive squamous stratified epithelial cells.     

Canonical Wnt signaling is required for development of embryonic stem cell-derived mesoderm

Formation of mesoderm from the pluripotent epiblast depends upon canonical Wnt/beta-catenin signaling, although a precise molecular basis for this requirement has not been established. To develop a robust model of this developmental transition, we examined the role of Wnt signaling during the analogous stage of embryonic stem cell differentiation. We show that the kinetics of Wnt ligand expression and pathway activity in vitro mirror those found in vivo. Furthermore, inhibition of this endogenous Wnt signaling abrogates the functional competence of differentiating ES cells, reflected by their failure to generate Flk1(+) mesodermal precursors and subsequent mature mesodermal lineages. Microarray analysis at various times during early differentiation reveal that mesoderm- and endoderm-associated genes fail to be induced in the absence of Wnt signaling, indicating a lack of germ layer induction that normally occurs during gastrulation in vivo. The earliest genes displaying Wnt-dependent expression, however, were those expressed in vivo in the primitive streak. Using an inducible form of stabilized beta-catenin, we find that Wnt activity, although required, does not autonomously promote primitive streak-associated gene expression in vitro. Our results suggest that Wnt signaling functions in this model system to regulate the thresholds or stability of responses to other effector pathways and demonstrate that differentiating ES cells represent a useful model system for defining complex regulatory interactions underlying primary germ layer induction.