An extended transcriptional network for pluripotency of embryonic stem cells

Nanog is the gateway to the pluripotent ground state

Pluripotency is generated naturally during mammalian development through formation of the epiblast, founder tissue of the embryo proper. Pluripotency can be recreated by somatic cell reprogramming. Here we present evidence that the homeodomain protein Nanog mediates acquisition of both embryonic and induced pluripotency. Production of pluripotent hybrids by cell fusion is promoted by and dependent on Nanog. In transcription factor-induced molecular reprogramming, Nanog is initially dispensable but becomes essential for dedifferentiated intermediates to transit to ground state pluripotency. In the embryo, Nanog specifically demarcates the nascent epiblast, coincident with the domain of X chromosome reprogramming. Without Nanog, pluripotency does not develop, and the inner cell mass is trapped in a pre-pluripotent, indeterminate state that is ultimately nonviable. These findings suggest that Nanog choreographs synthesis of the naive epiblast ground state in the embryo and that this function is recapitulated in the culmination of somatic cell reprogramming.

Acting locally and globally: Myc's ever-expanding roles on chromatin

Myc regulates key cellular processes including cell cycle, differentiation, and apoptosis. It has long been thought to direct these functions by acting solely as a classic transcription factor regulating expression of a small number of key target genes through discrete chromatin events in their promoters. A recent wave of genomics studies together directly challenge the narrowness of this model. For example, Myc binds to tens of thousands of sites in the human genome. It also regulates histone acetylation at and transcription of a remarkable number of genes, far beyond that expected of a classical transcription factor. The influence of Myc on chromatin also surprisingly extends to both genic and expansive intergenic regions. These studies support an evolving model in which Myc activity on chromatin is far more complex than previously imagined. The ability of Myc to act both locally and globally on chromatin may be responsible for its wide-ranging effects on the biology of stem and tumor cells.

Pluripotency genes overexpressed in primate embryonic stem cells are localized on homologues of human chromosomes 16, 17, 19, and X

While human embryonic stem cells (hESCs) are predisposed toward chromosomal aneploidities on 12, 17, 20, and X, rendering them susceptible to transformation, the specific genes expressed are not yet known. Here, by identifying the genes overexpressed in pluripotent rhesus ESCs (nhpESCs) and comparing them both to their genetically identical differentiated progeny (teratoma fibroblasts) and to genetically related differentiated parental cells (parental skin fibroblasts from whom gametes were used for ESC derivation), we find that some of those overexpressed genes in nhpESCs cluster preferentially on rhesus chromosomes 16, 19, 20, and X, homologues of human chromosomes 17, 19, 16, and X, respectively. Differentiated parental skin fibroblasts display gene expression profiles closer to nhpESC profiles than to teratoma cells, which are genetically identical to the pluripotent nhpESCs. Twenty over- and underexpressed pluripotency modulators, some implicated in neurogenesis, have been identified. The overexpression of some of these genes discovered using pedigreed nhpESCs derived from prime embryos generated by fertile primates, which is impossible to perform with the anonymously donated clinically discarded embryos from which hESCs are derived, independently confirms the importance of chromosome 17 and X regions in pluripotency and suggests specific candidates for targeting differentiation and transformation decisions.

Immortality and the base of multicellular life: Lessons from cnidarian stem cells

Cnidarians are phylogenetically basal members of the animal kingdom (>600 million years old). Together with plants they share some remarkable features that cannot be found in higher animals. Cnidarians and plants exhibit an almost unlimited regeneration capacity and immortality. Immortality can be ascribed to the asexual mode of reproduction that requires cells with an unlimited self-renewal capacity. We propose that the basic properties of animal stem cells are tightly linked to this archaic mode of reproduction. The cnidarian stem cells can give rise to a number of differentiated cell types including neuronal and germ cells. The genomes of Hydra and Nematostella, representatives of two major cnidarian classes indicate a surprising complexity of both genomes, which is in the range of vertebrates. Recent work indicates that highly conserved signalling pathways control Hydra stem cell differentiation. Furthermore, the availability of genomic resources and novel technologies provide approaches to analyse these cells in vivo. Studies of stem cells in cnidarians will therefore open important insights into the basic mechanisms of stem cell biology. Their critical phylogenetic position at the base of the metazoan branch in the tree of life makes them an important link in unravelling the common mechanisms of stem cell biology between animals and plants.

Myc-regulated microRNAs attenuate embryonic stem cell differentiation

Myc proteins are known to have an important function in stem cell maintenance. As Myc has been shown earlier to regulate microRNAs (miRNAs) involved in proliferation, we sought to determine whether c-Myc also affects embryonic stem (ES) cell maintenance and differentiation through miRNAs. Using a quantitative primer-extension PCR assay we identified miRNAs, including, miR-141, miR-200, and miR-429 whose expression is regulated by c-Myc in ES cells, but not in the differentiated and tumourigenic derivatives of ES cells. Chromatin immunoprecipitation analyses indicate that in ES cells c-Myc binds proximal to genomic regions encoding the induced miRNAs. We used expression profiling and seed homology to identify genes specifically downregulated both by these miRNAs and by c-Myc. We further show that the introduction of c-Myc-induced miRNAs into murine ES cells significantly attenuates the downregulation of pluripotency markers on induction of differentiation after withdrawal of the ES cell maintenance factor LIF. In contrast, knockdown of the endogenous miRNAs accelerate differentiation. Our data show that in ES cells c-Myc acts, in part, through a subset of miRNAs to attenuate differentiation.

Systems biology of stem cell fate and cellular reprogramming

Stem cell differentiation and the maintenance of self-renewal are intrinsically complex processes requiring the coordinated dynamic expression of hundreds of genes and proteins in precise response to external signalling cues. Numerous recent reports have used both experimental and computational techniques to dissect this complexity. These reports suggest that the control of cell fate has both deterministic and stochastic elements: complex underlying regulatory networks define stable molecular 'attractor' states towards which individual cells are drawn over time, whereas stochastic fluctuations in gene and protein expression levels drive transitions between coexisting attractors, ensuring robustness at the population level.

Zfp206, Oct4, and Sox2 are integrated components of a transcriptional regulatory network in embryonic stem cells

Zfp206 (recently renamed Zscan10) encodes a zinc finger transcription factor specifically expressed in human and mouse embryonic stem cells (ESC). It has been shown that Zfp206 is required to maintain ESC in an undifferentiated, pluripotent state. Presented here are data showing that Zfp206 works together with two other transcription factors, Oct4 and Sox2, which are also essential regulators of ESC pluripotency. We show that Zfp206 binds to the Oct4 promoter and directly regulates Oct4 expression. Genome-wide mapping of Zfp206-binding sites in ESC identifies more than 3000 target genes, many of which encode transcription factors that are also targeted for regulation by Oct4 and Sox2. In addition, we show that Zfp206 physically interacts with both Oct4 and Sox2. These data demonstrate that Zfp206 is a key component of the core transcriptional regulatory network and together with Oct4 and Sox2 regulates differentiation of ESC.

Ly-1 antibody reactive clone is an important nucleolar protein for control of self-renewal and differentiation in embryonic stem cells

Embryonic stem cells (ESCs) possess the capacity to self-renew and differentiate into all cell types of an organism. It is essential to understand how these properties are controlled for the potential usage of their derivatives in clinical settings and reprogramming of differentiated somatic cells. Although transcriptional factors, such as Oct4, Sox2, and Nanog, have been considered as a part of the core regulatory circuitry, a growing body of evidence suggests that additional factors exist and contribute to the control of ESC self-renewal and differentiation. Here, we report that Ly-1 antibody reactive clone (LYAR), a zinc finger nucleolar protein highly expressed in undifferentiated ESCs, plays a critical role in maintaining ESC identity. Its downregulation significantly reduces the rate of ESC growth and increases their apoptosis. Moreover, reduced expression of LYAR in ESCs impairs their differentiation capacity, failing to rapidly silence pluripotency markers and to activate differentiation genes upon differentiation. Mechanistically, LYAR forms a complex with another nucleolar protein, nucleolin, and prevents its self-cleavage, maintaining a normal steady-state level of nucleolin protein in undifferentiated ESCs. Interestingly, the downregulation of nucleolin is detrimental to the growth of ESCs and increases the rate of apoptosis, similarly to the knockdown of LYAR. Thus, our data emphasize the fact that other genes besides Oct4 and Nanog are uniquely required for ESC self-renewal and differentiation and demonstrate that LYAR functions to control the stability of nucleolin protein, which in turn is essential for maintaining the self-renewal of ESCs.

Stem cells, stress, metabolism and cancer: a drama in two Octs

It is a classic story of two related transcription factors. Oct4 is a potent regulator of pluripotency during early mammalian embryonic development, and is notable for its ability to convert adult somatic cells to pluripotency. The widely expressed Oct1 protein shares significant homology with Oct4, binds to the same sequences, regulates common target genes, and shares common modes of upstream regulation, including the ability to respond to cellular stress. Both proteins are also associated with malignancy, yet Oct1 cannot substitute for Oct4 in the generation of pluripotency. The molecular underpinnings of these phenomena are emerging, as are the consequences for adult stem cells and cancer, and thereby hangs a tale.

The FunGenES database: a genomics resource for mouse embryonic stem cell differentiation

Embryonic stem (ES) cells have high self-renewal capacity and the potential to differentiate into a large variety of cell types. To investigate gene networks operating in pluripotent ES cells and their derivatives, the “Functional Genomics in Embryonic Stem Cells” consortium (FunGenES) has analyzed the transcriptome of mouse ES cells in eleven diverse settings representing sixty-seven experimental conditions. To better illustrate gene expression profiles in mouse ES cells, we have organized the results in an interactive database with a number of features and tools. Specifically, we have generated clusters of transcripts that behave the same way under the entire spectrum of the sixty-seven experimental conditions; we have assembled genes in groups according to their time of expression during successive days of ES cell differentiation; we have included expression profiles of specific gene classes such as transcription regulatory factors and Expressed Sequence Tags; transcripts have been arranged in “Expression Waves” and juxtaposed to genes with opposite or complementary expression patterns; we have designed search engines to display the expression profile of any transcript during ES cell differentiation; gene expression data have been organized in animated graphs of KEGG signaling and metabolic pathways; and finally, we have incorporated advanced functional annotations for individual genes or gene clusters of interest and links to microarray and genomic resources. The FunGenES database provides a comprehensive resource for studies into the biology of ES cells.

Incorporating existing network information into gene network inference

One methodology that has met success to infer gene networks from gene expression data is based upon ordinary differential equations (ODE). However new types of data continue to be produced, so it is worthwhile to investigate how to integrate these new data types into the inference procedure. One such data is physical interactions between transcription factors and the genes they regulate as measured by ChIP-chip or ChIP-seq experiments. These interactions can be incorporated into the gene network inference procedure as a priori network information. In this article, we extend the ODE methodology into a general optimization framework that incorporates existing network information in combination with regularization parameters that encourage network sparsity. We provide theoretical results proving convergence of the estimator for our method and show the corresponding probabilistic interpretation also converges. We demonstrate our method on simulated network data and show that existing network information improves performance, overcomes the lack of observations, and performs well even when some of the existing network information is incorrect. We further apply our method to the core regulatory network of embryonic stem cells utilizing predicted interactions from two studies as existing network information. We show that including the prior network information constructs a more closely representative regulatory network versus when no information is provided.

Using small molecules to great effect in stem cell differentiation

Several recent reports, including two Cell Stem Cell papers (Zhu et al., 2009 [this issue]; Borowiak et al., 2009), screened small molecule libraries for compounds that promote embryonic stem cell differentiation. Their combined success helps bypass challenges associated with using natural protein factors and has revealed insights into controlling stem cell differentiation.

A core MYC gene expression signature is prominent in basal-like breast cancer but only partially overlaps the core serum response

BACKGROUND

The MYC oncogene contributes to induction and growth of many cancers but the full spectrum of the MYC transcriptional response remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Using microarrays, we conducted a detailed kinetic study of genes that respond to MYCN or MYCNDeltaMBII induction in primary human fibroblasts. In parallel, we determined the response to steady state overexpression of MYCN and MYCNDeltaMBII in the same cell type. An overlapping set of 398 genes from the two protocols was designated a 'Core MYC Signature' and used for further analysis. Comparison of the Core MYC Signature to a published study of the genes induced by serum stimulation revealed that only 7.4% of the Core MYC Signature genes are in the Core Serum Response and display similar expression changes to both MYC and serum. Furthermore, more than 50% of the Core MYC Signature genes were not influenced by serum stimulation. In contrast, comparison to a panel of breast cancers revealed a strong concordance in gene expression between the Core MYC Signature and the basal-like breast tumor subtype, which is a subtype with poor prognosis. This concordance was supported by the higher average level of MYC expression in the same tumor samples.

CONCLUSIONS/SIGNIFICANCE

The Core MYC Signature has clinical relevance as this profile can be used to deduce an underlying genetic program that is likely to contribute to a clinical phenotype. Therefore, the presence of the Core MYC Signature may predict clinical responsiveness to therapeutics that are designed to disrupt MYC-mediated phenotypes.

Transcriptomics: unravelling the biology of transcription factors and chromatin remodelers during development and differentiation

Mammalian development is a highly complex and tightly regulated process. Transcription factors and chromatin remodelers, acting downstream of cell signalling pathways, are the key intrinsic factors which control gene expression. Recent advances in transcriptomics are allowing biologists to begin to unravel the complex biological roles played by these factors. This review focuses on how genome-wide gene expression and chromatin immunoprecipitation studies are expanding our understanding of the roles played by transcription factors and chromatin remodelers during cell fate decisions in development and differentiation.

Deciphering the stem cell machinery as a basis for understanding the molecular mechanism underlying reprogramming

Stem cells provide fascinating prospects for biomedical applications by combining the ability to renew themselves and to differentiate into specialized cell types. Since the first isolation of embryonic stem (ES) cells about 30 years ago, there has been a series of groundbreaking discoveries that have the potential to revolutionize modern life science. For a long time, embryos or germ cell-derived cells were thought to be the only source of pluripotency--a dogma that has been challenged during the last decade. Several findings revealed that cell differentiation from (stem) cells to mature cells is not in fact an irreversible process. The molecular mechanism underlying cellular reprogramming is poorly understood thus far. Identifying how pluripotency maintenance takes place in ES cells can help us to understand how pluripotency induction is regulated. Here, we review recent advances in the field of stem cell regulation focusing on key transcription factors and their functional interplay with non-coding RNAs.

The Integrated Genome Browser: free software for distribution and exploration of genome-scale datasets

Summary: Experimental techniques that survey an entire genome demand flexible, highly interactive visualization tools that can display new data alongside foundation datasets, such as reference gene annotations. The Integrated Genome Browser (IGB) aims to meet this need. IGB is an open source, desktop graphical display tool implemented in Java that supports real-time zooming and panning through a genome; layout of genomic features and datasets in moveable, adjustable tiers; incremental or genome-scale data loading from remote web servers or local files; and dynamic manipulation of quantitative data via genome graphs.

Availability: The application and source code are available from http://igb.bioviz.org and http://genoviz.sourceforge.net.

Contact:aloraine@uncc.edu

Current progress and prospects of induced pluripotent stem cells

Induced pluripotent stem (iPS) cells are derived from somatic cells by ectopic expression of few transcription factors. Like embryonic stem (ES) cells, iPS cells are able to self-renew indefinitely and to differentiate into all types of cells in the body. iPS cells hold great promise for regenerative medicine, because iPS cells circumvent not only immunological rejection but also ethical issues. Since the first report on the derivation of iPS cells in 2006, many laboratories all over the world started research on iPS cells and have made significant progress. This paper reviews recent progress in iPS cell research, including the methods to generate iPS cells, the molecular mechanism of reprogramming in the formation of iPS cells, and the potential applications of iPS cells in cell replacement therapy. Current problems that need to be addressed and the prospects for iPS research are also discussed.

Signed weighted gene co-expression network analysis of transcriptional regulation in murine embryonic stem cells

Background

Recent work has revealed that a core group of transcription factors (TFs) regulates the key characteristics of embryonic stem (ES) cells: pluripotency and self-renewal. Current efforts focus on identifying genes that play important roles in maintaining pluripotency and self-renewal in ES cells and aim to understand the interactions among these genes. To that end, we investigated the use of unsigned and signed network analysis to identify pluripotency and differentiation related genes.

Results

We show that signed networks provide a better systems level understanding of the regulatory mechanisms of ES cells than unsigned networks, using two independent murine ES cell expression data sets. Specifically, using signed weighted gene co-expression network analysis (WGCNA), we found a pluripotency module and a differentiation module, which are not identified in unsigned networks. We confirmed the importance of these modules by incorporating genome-wide TF binding data for key ES cell regulators. Interestingly, we find that the pluripotency module is enriched with genes related to DNA damage repair and mitochondrial function in addition to transcriptional regulation. Using a connectivity measure of module membership, we not only identify known regulators of ES cells but also show that Mrpl15, Msh6, Nrf1, Nup133, Ppif, Rbpj, Sh3gl2, and Zfp39, among other genes, have important roles in maintaining ES cell pluripotency and self-renewal. We also report highly significant relationships between module membership and epigenetic modifications (histone modifications and promoter CpG methylation status), which are known to play a role in controlling gene expression during ES cell self-renewal and differentiation.

Conclusion

Our systems biologic re-analysis of gene expression, transcription factor binding, epigenetic and gene ontology data provides a novel integrative view of ES cell biology.

Experimental advances in the characterization of metazoan gene regulatory networks

Gene regulatory networks (GRNs) play a vital role in metazoan development and function, and deregulation of these networks is often implicated in disease. GRNs depict the dynamic interactions between genomic and regulatory state components. The genomic components comprise genes and their associated cis-regulatory elements. The regulatory state components consist primarily of transcriptional complexes that bind the latter elements. With the availability of complete genome sequences, several approaches have recently been developed which promise to significantly enhance our ability to identify either the genomic or regulatory state components, or the interactions between these two. In this review, I provide an in-depth overview of these approaches and detail how each contributes to a more comprehensive understanding of GRN composition and function.

Molecules that promote or enhance reprogramming of somatic cells to induced pluripotent stem cells

Reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) can be achieved by viral-mediated transduction of defined transcription factors. Moving toward the eventual goal of clinical application, it is necessary to overcome limitations such as low reprogramming efficiency and genomic alterations due to viral integration. Here, we review recent progress made in the usage of genetic factors, chemical inhibitors, and signaling molecules that can either replace core reprogramming factors or enhance reprogramming efficiency. Current iPSC studies will provide a paradigm for the combinatorial use of genetic factors and chemicals for the broader applications to alter cellular states of potency.

From genes to cells to tissues--modelling the haematopoietic system

Haematopoiesis (or blood formation) in general and haematopoietic stem cells more specifically represent some of the best studied mammalian developmental systems. Sophisticated purification protocols coupled with powerful biological assays permit functional analysis of highly purified cell populations both in vitro and in vivo. However, despite several decades of intensive research, the sheer complexity of the haematopoietic system means that many important questions remain unanswered or even unanswerable with current experimental tools. Scientists have therefore increasingly turned to modelling to tackle complexity at multiple levels ranging from networks of genes to the behaviour of cells and tissues. Early modelling attempts of gene regulatory networks have focused on core regulatory circuits but have more recently been extended to genome-wide datasets such as expression profiling and ChIP-sequencing data. Modelling of haematopoietic cells and tissues has provided insight into the importance of phenotypic heterogeneity for the differentiation of normal progenitor cells as well as a greater understanding of treatment response for particular pathologies such as chronic myeloid leukaemia. Here we will review recent progress in attempts to reconstruct segments of the haematopoietic system. A variety of modelling strategies will be covered from small-scale, protein-DNA or protein-protein interactions to large scale reconstructions. Also discussed will be examples of how stochastic modelling may be applied to multi cell systems such as those seen in normal and malignant haematopoiesis.

Novel sequential ChIP and simplified basic ChIP protocols for promoter co-occupancy and target gene identification in human embryonic stem cells

Background

The investigation of molecular mechanisms underlying transcriptional regulation, particularly in embryonic stem cells, has received increasing attention and involves the systematic identification of target genes and the analysis of promoter co-occupancy. High-throughput approaches based on chromatin immunoprecipitation (ChIP) have been widely used for this purpose. However, these approaches remain time-consuming, expensive, labor-intensive, involve multiple steps, and require complex statistical analysis. Advances in this field will greatly benefit from the development and use of simple, fast, sensitive and straightforward ChIP assay and analysis methodologies.

Results

We initially developed a simplified, basic ChIP protocol that combines simplicity, speed and sensitivity. ChIP analysis by real-time PCR was compared to analysis by densitometry with the ImageJ software. This protocol allowed the rapid identification of known target genes for SOX2, NANOG, OCT3/4, SOX17, KLF4, RUNX2, OLIG2, SMAD2/3, BMI-1, and c-MYC in a human embryonic stem cell line. We then developed a novel Sequential ChIP protocol to investigate in vivo promoter co-occupancy, which is basically characterized by the absence of antibody-antigen disruption during the assay. It combines centrifugation of agarose beads and magnetic separation. Using this Sequential ChIP protocol we found that c-MYC associates with the SOX2/NANOG/OCT3/4 complex and identified a novel RUNX2/BMI-1/SMAD2/3 complex in BG01V cells. These two TF complexes associate with two distinct sets of target genes. The RUNX2/BMI-1/SMAD2/3 complex is associated predominantly with genes not expressed in undifferentiated BG01V cells, consistent with the reported role of those TFs as transcriptional repressors.

Conclusion

These simplified basic ChIP and novel Sequential ChIP protocols were successfully tested with a variety of antibodies with human embryonic stem cells, generated a number of novel observations for future studies and might be useful for high-throughput ChIP-based assays.

SOX2 silencing in glioblastoma tumor-initiating cells causes stop of proliferation and loss of tumorigenicity

Glioblastoma, the most aggressive cerebral tumor, is invariably lethal. Glioblastoma cells express several genes typical of normal neural stem cells. One of them, SOX2, is a master gene involved in sustaining self-renewal of several stem cells, in particular neural stem cells. To investigate its role in the aberrant growth of glioblastoma, we silenced SOX2 in freshly derived glioblastoma tumor-initiating cells (TICs). Our results indicate that SOX2 silenced glioblastoma TICs, despite the many mutations they have accumulated, stop proliferating and lose tumorigenicity in immunodeficient mice. SOX2 is then also fundamental for maintenance of the self-renewal capacity of neural stem cells when they have acquired cancer properties. SOX2, or its immediate downstream effectors, would then be an ideal target for glioblastoma therapy.

Characterization of the phosphoinositide 3-kinase-dependent transcriptome in murine embryonic stem cells: identification of novel regulators of pluripotency

Phosphoinositide 3-kinase (PI3K)-dependent signaling has been implicated in the regulation of embryonic stem (ES) cell fate. To gain further insight into the mechanisms regulated by PI3Ks in murine ES cells, we have performed expression profiling using Affymetrix GeneChips to characterize the transcriptional changes that arise as a result of inhibition of PI3K-dependent signaling. Using filtering of greater than 1.5-fold change in expression and an analysis of variance significance level of p < .05, we have defined a dataset comprising 646 probe sets that detect changes in transcript expression (469 down and 177 up) on inhibition of PI3Ks. Changes in expression of selected genes have been validated by quantitative reverse transcription polymerase chain reaction. Gene ontology analyses reveal significant over-representation of transcriptional regulators within our dataset. In addition, several known regulators of ES cell pluripotency, for example, Nanog, Esrrb, Tbx3, and Tcl-1, are among the downregulated genes. To evaluate the functional involvement of selected genes in regulation of ES cell self-renewal, we have used short interfering RNA-mediated knockdown. These studies identify genes not previously associated with control of ES cell fate that are involved in regulating ES cell pluripotency, including the protein tyrosine phosphatase Shp-1 and the Zscan4 family of zinc finger proteins. Further gain-of-function analyses demonstrate the importance of Zscan4c in regulation of ES cell pluripotency.

Molecular characterization of the human NANOG protein

NANOG is a key transcriptional regulator of pluripotent stem cell (PSC) self-renewal. NANOG occupies promoters that are active and others that are repressed during self-renewal; however, the mechanisms by which NANOG regulates transcriptional repression and activation are unknown. We hypothesized that individual protein domains of NANOG control its interactions with both the promoters and its coregulators. We performed a detailed characterization of the functional domains in the human (h) NANOG protein, using a panel of deletion-mutant and point-mutant constructs. We determined that six amino acids in the homeodomain ((136)YKQVKT(141)) are sufficient for the nuclear localization of hNANOG. We also determined that the tryptophan-rich region (W) of hNANOG contains a CRM1-independent signal for nuclear export, suggesting a possible cellular shuttling behavior that has not been reported for hNANOG. We also show that at least four tryptophans are required for nuclear export. We also determined that similar to murine (m) NANOG, the W region of hNANOG contains a homodimerization domain. Finally, in vitro transactivation analyses identified distinct regions that enhance or diminish activity at gene promoters that are active during self-renewal. Specifically, the N-terminal region interferes with transcription and removal of this region that produced a "super-active" hNANOG with enhanced transcriptional activity. We also confirmed that the transcriptional activator in hNANOG is contained in the C-terminal region, similar to murine NANOG. In summary, this study has characterized the structure and function of hNANOG protein leading to an increased understanding of the mechanism by which hNANOG regulates both transcriptional activation and repression during PSC self-renewal.

Pluripotency rush! Molecular cues for pluripotency, genetic reprogramming of adult stem cells, and widely multipotent adult cells

In the last few years, pluripotent stem cells have been the objective of intense investigation efforts. These cells are of paramount therapeutic interest, since they could be utilized: as in vitro models of disease, for pharmaceutical screening purposes, and for the regeneration of damaged organs. Over the years, pluripotent cells have been cultured from teratomas, the inner cell mass, and primordial germ cells. Accumulating informations have partially decrypted the molecular machinery responsible for the maintenance of a very primitive state, permitting the reprogramming of differentiated cells. Although the debate is still open, an extreme excitement is arising from two strictly related possibilities: pluripotent cells could be obtained from adult tissues with minimal manipulations or very rare pluripotent cells could be identified in adult tissues. This intriguing option will trigger new researches aimed both at identifying the possible biological role of pluripotent adult stem cells and at exploiting their potential clinical use. The present review article will summarize current knowledge of the molecular cues for pluripotency but also discusses whether pluripotent stem cells could be obtained from adult tissues.

Gene regulatory networks in embryonic stem cells and brain development

Embryonic stem cells (ESCs) are endowed with the ability to generate multiple cell lineages and carry great therapeutic potentials in regenerative medicine. Future application of ESCs in human health and diseases will embark on the delineation of molecular mechanisms that define the biology of ESCs. Here, we discuss how the finite ESC components mediate the intriguing task of brain development and exhibit biomedical potentials to cure diverse neurological disorders.

N-Myc regulates expression of pluripotency genes in neuroblastoma including lif, klf2, klf4, and lin28b

myc genes are best known for causing tumors when overexpressed, but recent studies suggest endogenous myc regulates pluripotency and self-renewal of stem cells. For example, N-myc is associated with a number of tumors including neuroblastoma, but also plays a central role in the function of normal neural stem and precursor cells (NSC). Both c- and N-myc also enhance the production of induced pluripotent stem cells (iPSC) and are linked to neural tumor stem cells. The mechanisms by which myc regulates normal and neoplastic stem-related functions remain largely open questions. Here from a global, unbiased search for N-Myc bound genes using ChIP-chip assays in neuroblastoma, we found lif as a putative N-Myc bound gene with a number of strong N-Myc binding peaks in the promoter region enriched for E-boxes. Amongst putative N-Myc target genes in expression microarray studies in neuroblastoma we also found lif and three additional important embryonic stem cell (ESC)-related factors that are linked to production of iPSC: klf2, klf4, and lin28b. To examine the regulation of these genes by N-Myc, we measured their expression using neuroblastoma cells that contain a Tet-regulatable N-myc transgene (TET21N) as well as NSC with a nestin-cre driven N-myc knockout. N-myc levels closely correlated with the expression of all of these genes in neuroblastoma and all but lif in NSC. Direct ChIP assays also indicate that N-Myc directly binds the lif promoter. N-Myc regulates trimethylation of lysine 4 of histone H3 in the promoter of lif and possibly in the promoters of several other stem-related genes. Together these findings indicate that N-Myc regulates overlapping stem-related gene expression programs in neuroblastoma and NSC, supporting a novel model by which amplification of the N-myc gene may drive formation of neuroblastoma. They also suggest mechanisms by which Myc proteins more generally contribute to maintenance of pluripotency and self-renewal of ESC as well as to iPSC formation.

Genetic p53 deficiency partially rescues the adrenocortical dysplasia phenotype at the expense of increased tumorigenesis

Telomere dysfunction and shortening induce chromosomal instability and tumorigenesis. In this study, we analyze the adrenocortical dysplasia (acd) mouse, harboring a mutation in Tpp1/Acd. Additional loss of p53 dramatically rescues the acd phenotype in an organ-specific manner, including skin hyperpigmentation and adrenal morphology, but not germ cell atrophy. Survival to weaning age is significantly increased in Acd(acd/acd) p53(-/-) mice. On the contrary, p53(-/-) and p53(+/-) mice with the Acd(acd/acd) genotype show a decreased tumor-free survival, compared with Acd(+/+) mice. Tumors from Acd(acd/acd) p53(+/-) mice show a striking switch from the classic spectrum of p53(-/-) mice toward carcinomas. The acd mouse model provides further support for an in vivo role of telomere deprotection in tumorigenesis.

The transcriptional program controlled by the stem cell leukemia gene Scl/Tal1 during early embryonic hematopoietic development

The basic helix-loop-helix transcription factor Scl/Tal1 controls the development and subsequent differentiation of hematopoietic stem cells (HSCs). However, because few Scl target genes have been validated to date, the underlying mechanisms have remained largely unknown. In this study, we have used ChIP-Seq technology (coupling chromatin immunoprecipitation with deep sequencing) to generate a genome-wide catalog of Scl-binding events in a stem/progenitor cell line, followed by validation using primary fetal liver cells and comprehensive transgenic mouse assays. Transgenic analysis provided in vivo validation of multiple new direct Scl target genes and allowed us to reconstruct an in vivo validated network consisting of 17 factors and their respective regulatory elements. By coupling ChIP-Seq in model cell lines with in vivo transgenic validation and sophisticated bioinformatic analysis, we have identified a widely applicable strategy for the reconstruction of stem cell regulatory networks in which biologic material is otherwise limiting. Moreover, in addition to revealing multiple previously unrecognized links to known HSC regulators, as well as novel links to genes not previously implicated in HSC function, comprehensive transgenic analysis of regulatory elements provided substantial new insights into the transcriptional control of several important hematopoietic regulators, including Cbfa2t3h/Eto2, Cebpe, Nfe2, Zfpm1/Fog1, Erg, Mafk, Gfi1b, and Myb.

Tracing the stemness of porcine skin-derived progenitors (pSKP) back to specific marker gene expression

Multipotent skin-derived progenitors (SKP) can produce both neural and mesodermal progeny in vitro, sharing the characteristics of embryonic neural crest stem cells. However, the molecular basis for the property of multiple lineage potential and neural crest origin of SKPs is still elusive. Here we report the cooperative expression of pluripotency related genes (POU5F1, SOX2, NANOG, STAT3) and neural crest marker genes (p75NTR, TWIST1, PAX3, SNAI2, SOX9, SOX10) in GFP-transgenic porcine skin-derived progenitors (pSKP). The proportion of cells positive for POU5F1, nestin, fibronectin, and vimentin were 12.3%, 15.1%, 67.9% and 53.7%, showing the heterogeneity of pSKP spheres. Moreover, pSKP cells can generate both neural (neurons and glia) and mesodermal cell types (smooth muscle cells and adipocytes) in vitro, indicating the multiple lineage potency. Four transcription factors (POU5F1, SNAI2, SOX9, and PAX3) were identified that were sensitive to mitogen (FBS) and/or growth factors (EGF and bFGF). We infer that POU5F1, SNAI2, SOX9, and PAX3 may be the key players for maintaining the neural crest derived multipotency of SKP cells in vitro. This study has provided new insight into the molecular mechanism of stemness for somatic-derived stem cells at the level of transcriptional regulation.

Reprogramming of murine fibroblasts to induced pluripotent stem cells with chemical complementation of Klf4

Ectopic expression of defined transcription factors can reprogram somatic cells to induced pluripotent stem (iPS) cells, but the utility of iPS cells is hampered by the use of viral delivery systems. Small molecules offer an alternative to replace virally transduced transcription factors with chemical signaling cues responsible for reprogramming. In this report we describe a small-molecule screening platform applied to identify compounds that functionally replace the reprogramming factor Klf4. A series of small-molecule scaffolds were identified that activate Nanog expression in mouse fibroblasts transduced with a subset of reprogramming factors lacking Klf4. Application of one such molecule, kenpaullone, in lieu of Klf4 gave rise to iPS cells that are indistinguishable from murine embryonic stem cells. This experimental platform can be used to screen large chemical libraries in search of novel compounds to replace the reprogramming factors that induce pluripotency. Ultimately, such compounds may provide mechanistic insight into the reprogramming process.

REST selectively represses a subset of RE1-containing neuronal genes in mouse embryonic stem cells

REST is a transcriptional repressor that targets a group of neuronal genes in non-neuronal cells. In embryonic stem (ES) cells, REST has been implicated in controlling the expression of transcription factor genes that are crucial for lineage determination and for maintaining ES cell potential. Here, we asked whether REST directly regulates neural-specifying genes in mouse ES cells using siRNA-mediated REST knockdown and ES cells that lack functional REST protein as a result of gene targeting. Loss of REST did not affect the expression of any of ten transcription factor genes known to promote neural commitment and did not affect the expression of several microRNAs, including miR-21, a putative REST target in ES cells. REST-deficient ES cells retained the ability to self-renew and to undergo appropriate differentiation towards mesoderm, endoderm and ectoderm lineages upon LIF withdrawal. Genome-wide expression profiling showed that genes that were deregulated in the absence of REST were preferentially expressed in the brain and highly enriched for the presence of canonical REST binding sites (RE1). Chromatin immunoprecipitation studies confirmed these genes as direct targets of REST in ES cells. Collectively, these data show that REST selectively silences a cohort of neuronal genes in ES cells.

NAC-1, a potential stem cell pluripotency factor, contributes to paclitaxel resistance in ovarian cancer through inactivating Gadd45 pathway

Nucleus accumbens-1 (Nac1 or NAC-1) belongs to the BTB/POZ (Pox virus and Zinc finger/Bric-a-brac Tramtrack Broad complex) transcription factor family and is a novel protein that potentially participates in self-renewal and pluripotency in embryonic stem cells. In human cancer, NAC-1 is upregulated in several types of neoplasms, but particularly in recurrent chemoresistant ovarian carcinomas, suggesting a biological role for NAC-1 in the development of drug resistance in ovarian cancer. We have assessed this possibility and shown a correlation between NAC-1 expression and ex vivo paclitaxel resistance in ovarian serous carcinoma tissues and cell lines. We found that expression of Gadd45-gamma-interacting protein 1 (Gadd45gip1), a downstream target negatively regulated by NAC-1, was reduced in paclitaxel-resistant cells. Ectopic expression of NAC-1 or knockdown of Gadd45gip1 conferred paclitaxel resistance, whereas NAC-1 knockdown or ectopic expression of Gadd45gip1 increased paclitaxel sensitivity. Furthermore, silencing NAC-1 expression or disrupting NAC-1 homodimerization by a dominant negative NAC-1 protein that contained only the BTB/POZ domain induced the expression of Gadd45gamma, which interacted with Gadd45gip1. Reducing Gadd45gamma expression by small hairpin RNAs partially enhanced paclitaxel resistance. Thus, this study provides new evidence that NAC-1 upregulation and homodimerization contribute to tumor recurrence by equipping ovarian cancer cells with the paclitaxel-resistant phenotype through negative regulation of the Gadd45 pathway.

Structure of the human Nac1 POZ domain

Nac1 is a POZ-domain transcription factor that is involved in the self-renewal of embryonic stem cells. It is overexpressed in ovarian serous carcinoma and targeting the interactions of its POZ domain is a potential therapeutic strategy. Nac1 lacks a zinc-finger DNA-binding domain and thereby differs from most other POZ-domain transcription factors. Here, the crystal structure of the Nac1 POZ domain at 2.1 A resolution is reported. The Nac1 POZ domain crystallized as a dimer in which the interaction interfaces between subunits resemble those found in the POZ-zinc finger transcription factors. The organization of the Nac1 POZ-domain core resembles reported POZ-domain structures, whereas the C-terminus differs markedly. The C-terminal alpha-helix of the Nac1 POZ domain is shorter than that observed in most other POZ-domain transcription factors; variation in the organization of this region may be a general feature of POZ-domain structures.

REST and the RESTless: in stem cells and beyond

Since its original discovery as a negative regulator of neuronal differentiation, the repressor element (RE)-1 silencing transcription factor (REST), also known as the neuron-restrictive silencer factor, has been implicated in novel processes such as maintenance of embryonic stem cell pluripotency and self-renewal and regulation of mitotic fidelity in non-neural cells. REST expression and activity is tightly controlled by transcriptional and post-transcriptional mechanisms in a cell and developmental stage-specific manner and perturbations in its levels or function are associated with various pathological states. REST differentially influences target-gene expression through interaction with a wide variety of cellular cofactors in a context-dependent manner. However, the influence of the microenvironment on REST-mediated regulation of gene expression is poorly understood. This review will present our current understanding of REST signaling with a greater focus on its emerging ties with noncoding RNAs and novel interacting partners, as well as its roles in embryonic stem cell self-renewal, cellular plasticity and oncogenesis/tumor suppression.

Genome-wide association of hypoxia-inducible factor (HIF)-1alpha and HIF-2alpha DNA binding with expression profiling of hypoxia-inducible transcripts

Hypoxia-inducible factor (HIF) controls an extensive range of adaptive responses to hypoxia. To better understand this transcriptional cascade we performed genome-wide chromatin immunoprecipitation using antibodies to two major HIF-α subunits, and correlated the results with genome-wide transcript profiling. Within a tiled promoter array we identified 546 and 143 sequences that bound, respectively, to HIF-1α or HIF-2α at high stringency. Analysis of these sequences confirmed an identical core binding motif for HIF-1α and HIF-2α (RCGTG) but demonstrated that binding to this motif was highly selective, with binding enriched at distinct regions both upstream and downstream of the transcriptional start. Comparison of HIF-promoter binding data with bidirectional HIF-dependent changes in transcript expression indicated that whereas a substantial proportion of positive responses (>20% across all significantly regulated genes) are direct, HIF-dependent gene suppression is almost entirely indirect. Comparison of HIF-1α- versus HIF-2α-binding sites revealed that whereas some loci bound HIF-1α in isolation, many bound both isoforms with similar affinity. Despite high-affinity binding to multiple promoters, HIF-2α contributed to few, if any, of the transcriptional responses to acute hypoxia at these loci. Given emerging evidence for biologically distinct functions of HIF-1α versus HIF-2α understanding the mechanisms restricting HIF-2α activity will be of interest.

Epigenetic reprogramming and induced pluripotency

The cloning of animals from adult cells has demonstrated that the developmental state of adult cells can be reprogrammed into that of embryonic cells by uncharacterized factors within the oocyte. More recently, transcription factors have been identified that can induce pluripotency in somatic cells without the use of oocytes, generating induced pluripotent stem (iPS) cells. iPS cells provide a unique platform to dissect the molecular mechanisms that underlie epigenetic reprogramming. Moreover, iPS cells can teach us about principles of normal development and disease, and might ultimately facilitate the treatment of patients by custom-tailored cell therapy.

A genome-wide RNAi screen identifies a new transcriptional module required for self-renewal

We performed a genome-wide siRNA screen in mouse embryonic stem (ES) cells to identify genes essential for self-renewal, and found 148 genes whose down-regulation caused differentiation. Many of the identified genes function in gene regulation and/or development, and are highly expressed in ES cells and embryonic tissues. We further identified target genes of two transcription regulators Cnot3 and Trim28. We discovered that Cnot3 and Trim28 co-occupy many putative gene promoters with c-Myc and Zfx, but not other pluripotency-associated transcription factors. They form a unique module in the self-renewal transcription network, separate from the core module formed by Nanog, Oct4, and Sox2. The transcriptional targets of this module are enriched for genes involved in cell cycle, cell death, and cancer. This supports the idea that regulatory networks controlling self-renewal in stem cells may also be active in certain cancers and may represent novel anti-cancer targets. Our screen has implicated over 100 new genes in ES cell self-renewal, and illustrates the power of RNAi and forward genetics for the systematic study of self-renewal.

The C-terminal pentapeptide of Nanog tryptophan repeat domain interacts with Nac1 and regulates stem cell proliferation but not pluripotency

Overexpression of Nanog in mouse embryonic stem (ES) cells has been shown to abrogate the requirement of leukemia inhibitory factor for self-renewal in culture. Little is known about the molecular mechanism of Nanog function. Here we describe the role of the tryptophan repeat (WR) domain, one of the two transactivators at its C terminus, in regulating stem cell proliferation as well as pluripotency. We first created a supertransactivator, W2W3x10, by duplicating repeats W2W3 10 times and discovered that it can functionally substitute for wild type WR at sustaining pluripotency, albeit with a significantly slower cell cycle, phenocopying Nanog(9W) with the C-terminal pentapeptide (WNAAP) of WR deleted. ES cells carrying both W2W3x10 and Nanog(9W) have a longer G1 phase, a shorter S phase in cell cycle distribution and progression analysis, and a lower level of pAkt(Ser473) compared with wild type Nanog, suggesting that both mutants impact the cell cycle machinery via the phosphatidylinositol 3-kinase/Akt pathway. Both mutants remain competent in dimerizing with Nanog but cannot form a complex with Nac1 efficiently, suggesting that WNAAP may be involved in Nac1 binding. By tagging Gal4DBD with WNAAP, we demonstrated that this pentapeptide is sufficient to confer Nac1 binding. Furthermore, we can rescue W2W3x10 by placing WNAAP at the corresponding locations. Finally, we found that Nanog and Nac1 synergistically up-regulate ERas expression and promote the proliferation of ES cells. These results suggest that Nanog interacts with Nac1 through WNAAP to regulate the cell cycle of ES cells via the ERas/phosphatidylinositol 3-kinase/Akt pathway, but not pluripotency, thus decoupling cell cycle control from pluripotency.

Is REST required for ESC pluripotency?

The DNA-binding protein REST (also called NRSF) is a transcriptional repressor that targets many neuronal genes and is abundant in human and mouse pluripotent embryonic stem cells (ESCs). In a recent Letter to Nature, Singh et al. suggested that REST controls the self-renewal and pluripotency of ESCs, because they found that ESCs in which a single REST allele was disrupted (Fig. 1a, beta-geo-stop insertion) had reduced alkaline phosphatase activity and expressed lower levels of several pluripotency-associated genes. Here we show that partial or complete loss of functional REST protein does not abrogate ESC potential as reflected by marker gene expression. These data are consistent with earlier reports, and argue that REST is not required for maintaining ESC pluripotency.

Histone h3 lysine 56 acetylation is linked to the core transcriptional network in human embryonic stem cells

Lysine 56 acetylation in the helical core of histone H3 opens yeast chromatin and enables histone gene transcription, DNA replication, and DNA repair and prevents epigenetic silencing. While K56Ac is globally abundant in yeast and flies, its presence has been uncertain in mammals. We show here using mass spectrometry and genome-wide analyses that K56Ac is present in human embryonic stem cells (hESCs), overlapping strongly at active and inactive promoters with the binding of the key regulators of pluripotency, NANOG, SOX2, and OCT4. This includes also the canonical histone gene promoters and those for the hESC-specific microRNAs. K56Ac then relocates to developmental genes upon cellular differentiation. Thus the K56Ac state more accurately reflects the epigenetic differences between hESCs and somatic cells than other active histone marks such as H3 K4 trimethylation and K9 acetylation. These results suggest that K56Ac is involved in the human core transcriptional network of pluripotency.

Toward stem cell systems biology: from molecules to networks and landscapes

The last few years have seen significant advances in our understanding of the molecular mechanisms of stem-cell-fate specification. New and emerging high-throughput techniques, as well as increasingly accurate loss-of-function perturbation techniques, are allowing us to dissect the interplay among genetic, epigenetic, proteomic, and signaling mechanisms in stem-cell-fate determination with ever-increasing fidelity (Boyer et al. 2005, 2006; Ivanova et al. 2006; Loh et al. 2006; Cole et al. 2008; Jiang et al. 2008; Johnson et al. 2008; Kim et al. 2008; Liu et al. 2008; Marson et al. 2008; Mathur et al. 2008). Taken together, recent reports using these new techniques demonstrate that stem-cell-fate specification is an extremely complex process, regulated by multiple mutually interacting molecular mechanisms involving multiple regulatory feedback loops. Given this complexity and the sensitive dependence of stem cell differentiation on signaling cues from the extracellular environment, how are we best to develop a coherent quantitative understanding of stem cell fate at the systems level? One approach that we and other researchers have begun to investigate is the application of techniques derived in the computational disciplines (mathematics, physics, computer science, etc.) to problems in stem cell biology. Here, we briefly sketch a few pertinent results from the literature in this area and discuss future potential applications of computational techniques to stem cell systems biology.

Use of in vivo biotinylation to study protein-protein and protein-DNA interactions in mouse embryonic stem cells

In gene regulation, proteins function as members of protein complexes to recognize chromosomal target DNA loci. In dissecting the pluripotent state in mouse embryonic stem (mES) cells, we have used in vivo biotinylation of critical transcription factors for affinity purification of protein complexes and chromatin immunoprecipitation (ChIP)-on-chip for target identification, respectively. Here, we describe detailed procedures for such studies to dissect protein-protein and protein-DNA interactions in mES cells. Specifically, the following three procedures will be described: (i) in vivo biotinylation system setup in mES cells; (ii) affinity purification of multiprotein complexes by one-step streptavidin capture and tandem anti-FLAG/streptavidin affinity purification; (iii) biotin-mediated ChIP (bioChIP). The system setup takes approximately 50 d to complete, and it takes another approximately 15 d and approximately 3 d to perform affinity purification of protein complexes and bioChIP, respectively.