Simon R. Tomlinson

The transcriptional foundation of pluripotency

A fundamental goal in biology is to understand the molecular basis of cell identity. Pluripotent embryonic stem (ES) cell identity is governed by a set of transcription factors centred on the triumvirate of Oct4, Sox2 and Nanog. These proteins often bind to closely localised genomic sites. Recent studies have identified additional transcriptional modulators that bind to chromatin near sites occupied by Oct4, Sox2 and Nanog. This suggests that the combinatorial control of gene transcription might be fundamental to the ES cell state. Here we discuss how these observations advance our understanding of the transcription factor network that controls pluripotent identity and highlight unresolved issues that arise from these studies.

Oct4 and LIF/Stat3 Additively Induce Krüppel Factors to Sustain Embryonic Stem Cell Self-Renewal

Embryonic stem cell (ESC) pluripotency is dependent on an intrinsic gene regulatory network centered on Oct4. Propagation of the pluripotent state is stimulated by the cytokine leukemia inhibitory factor (LIF) acting through the transcriptional regulator Stat3. Here, we show that this extrinsic stimulus converges with the intrinsic circuitry in Krüppel-factor activation. Oct4 primarily induces Klf2 while LIF/Stat3 selectively enhances Klf4 expression. Overexpression of either factor reduces LIF dependence, but with quantitative and qualitative differences. Unlike Klf4, Klf2 increases ESC clonogenicity, maintains undifferentiated ESCs in the genetic absence of Stat3, and confers resistance to BMP-induced differentiation. ESCs expanded with Klf2 remain capable of contributing to adult chimeras. Postimplantation-embryo-derived EpiSCs lack both Klf2 and Klf4 and expression of either can reinstate naive pluripotency. These findings indicate that Oct4 and Stat3 intersect in directing expression of Klf transcriptional regulators with overlapping properties that additively reinforce ground-state ESC pluripotency, identity, and self-renewal.

Esrrb is a direct Nanog target gene that can substitute for Nanog function in pluripotent cells.

Summary Embryonic stem cell (ESC) self-renewal efficiency is determined by the level of Nanog expression. However, the mechanisms by which Nanog functions remain unclear, and in particular, direct Nanog target genes are uncharacterized. Here we investigate ESCs expressing different Nanog levels and Nanog−/− cells with distinct functionally inducible Nanog proteins to identify Nanog-responsive genes. Surprisingly, these constitute a minor fraction of genes that Nanog binds. Prominent among Nanog-reponsive genes is Estrogen-related receptor b (Esrrb). Nanog binds directly to Esrrb, enhances binding of RNAPolII, and stimulates Esrrb transcription. Overexpression of Esrrb in ESCs maintains cytokine-independent self-renewal and pluripotency. Remarkably, this activity is retained in Nanog−/− ESCs. Moreover, Esrrb can reprogram Nanog−/− EpiSCs and can rescue stalled reprogramming in Nanog−/− pre-iPSCs. Finally, Esrrb deletion abolishes the defining ability of Nanog to confer LIF-independent ESC self-renewal. These findings are consistent with the functional placement of Esrrb downstream of Nanog.

Reduced Oct4 Expression Directs a Robust Pluripotent State with Distinct Signaling Activity and Increased Enhancer Occupancy by Oct4 and Nanog

Summary Embryonic stem cell (ESC) pluripotency is governed by a gene regulatory network centered on the transcription factors Oct4 and Nanog. To date, robust self-renewing ESC states have only been obtained through the chemical inhibition of signaling pathways or enforced transgene expression. Here, we show that ESCs with reduced Oct4 expression resulting from heterozygosity also exhibit a stabilized pluripotent state. Despite having reduced Oct4 expression, Oct4+/− ESCs show increased genome-wide binding of Oct4, particularly at pluripotency-associated enhancers, homogeneous expression of pluripotency transcription factors, enhanced self-renewal efficiency, and delayed differentiation kinetics. Cells also exhibit increased Wnt expression, enhanced leukemia inhibitory factor (LIF) sensitivity, and reduced responsiveness to fibroblast growth factor. Although they are able to maintain pluripotency in the absence of bone morphogenetic protein, removal of LIF destabilizes pluripotency. Our findings suggest that cells with a reduced Oct4 concentration range are maintained in a robust pluripotent state and that the wild-type Oct4 concentration range enables effective differentiation.

Cultured cambial meristematic cells as a source of plant natural products

A plethora of important, chemically diverse natural products are derived from plants. In principle, plant cell culture offers an attractive option for producing many of these compounds. However, it is often not commercially viable because of difficulties associated with culturing dedifferentiated plant cells (DDCs) on an industrial scale. To bypass the dedifferentiation step, we isolated and cultured innately undifferentiated cambial meristematic cells (CMCs). Using a combination of deep sequencing technologies, we identified marker genes and transcriptional programs consistent with a stem cell identity. This notion was further supported by the morphology of CMCs, their hypersensitivity to γ-irradiation and radiomimetic drugs and their ability to differentiate at high frequency. Suspension culture of CMCs derived from Taxus cuspidata, the source of the key anticancer drug, paclitaxel (Taxol), circumvented obstacles routinely associated with the commercial growth of DDCs. These cells may provide a cost-effective and environmentally friendly platform for sustainable production of a variety of important plant natural products.

High-resolution analysis with novel cell-surface markers identifies routes to iPS cells

The generation of induced pluripotent stem (iPS) cells presents a challenge to normal developmental processes. The low efficiency and heterogeneity of most methods have hindered understanding of the precise molecular mechanisms promoting, and roadblocks preventing, efficient reprogramming. Although several intermediate populations have been described, it has proved difficult to characterize the rare, asynchronous transition from these intermediate stages to iPS cells. The rapid expansion of minor reprogrammed cells in the heterogeneous population can also obscure investigation of relevant transition processes. Understanding the biological mechanisms essential for successful iPS cell generation requires both accurate capture of cells undergoing the reprogramming process and identification of the associated global gene expression changes. Here we demonstrate that in mouse embryonic fibroblasts, reprogramming follows an orderly sequence of stage transitions, marked by changes in the cell-surface markers CD44 and ICAM1, and a Nanog–enhanced green fluorescent protein (Nanog–eGFP) reporter. RNA-sequencing analysis of these populations demonstrates two waves of pluripotency gene upregulation, and unexpectedly, transient upregulation of several epidermis-related genes, demonstrating that reprogramming is not simply the reversal of the normal developmental processes. This novel high-resolution analysis enables the construction of a detailed reprogramming route map, and the improved understanding of the reprogramming process will lead to new reprogramming strategies.

GeneProf: analysis of high-throughput sequencing experiments

Existing tools are hence not sufficient to make highthroughput sequencing data fully accessible to the entire research community. To address these challenges, we developed GeneProf, which combines (i) an easy-to-use data analysis suite that automates the analysis process with (ii) a comprehensive resource of integrated, readily interpretable and reusable analysis results. GeneProf ’s user interface is web-based, obviating the need to install specialized software (Supplementary Figs. 1 and 2). The system uses a dedicated, remote compute cluster to carry out large-scale genomic analyses, simultaneously dealing with many computationally demanding tasks. GeneProf is available online (http://www.geneprof.org/) and is free for academic researchers. Technical details are available in Supplementary Discussion and Supplementary Figure 3. GeneProf simplifies the construction of complex workflows by providing assistive web forms (‘wizards’) that conceal the underlying complexities of workflow programming. These wizards reconceive common, best-practice analysis steps as a series of logical stages, which researchers can customize easily by answering only a few basic questions. Users may change wizard-generated workflows to suit specialized requirements, maintaining full methodological flexibility. GeneProf: analysis of high-throughput sequencing experiments

A direct physical interaction between Nanog and Sox2 regulates embryonic stem cell self-renewal

Embryonic stem (ES) cell self‐renewal efficiency is determined by the Nanog protein level. However, the protein partners of Nanog that function to direct self‐renewal are unclear. Here, we identify a Nanog interactome of over 130 proteins including transcription factors, chromatin modifying complexes, phosphorylation and ubiquitination enzymes, basal transcriptional machinery members, and RNA processing factors. Sox2 was identified as a robust interacting partner of Nanog. The purified Nanog–Sox2 complex identified a DNA recognition sequence present in multiple overlapping Nanog/Sox2 ChIP‐Seq data sets. The Nanog tryptophan repeat region is necessary and sufficient for interaction with Sox2, with tryptophan residues required. In Sox2, tyrosine to alanine mutations within a triple‐repeat motif (S X T/S Y) abrogates the Nanog–Sox2 interaction, alters expression of genes associated with the Nanog‐Sox2 cognate sequence, and reduces the ability of Sox2 to rescue ES cell differentiation induced by endogenous Sox2 deletion. Substitution of the tyrosines with phenylalanine rescues both the Sox2–Nanog interaction and efficient self‐renewal. These results suggest that aromatic stacking of Nanog tryptophans and Sox2 tyrosines mediates an interaction central to ES cell self‐renewal.

Identification of Plet-1 as a specific marker of early thymic epithelial progenitor cells.

The thymus is essential for a functional immune system, because the thymic stroma uniquely supports T lymphocyte development. We have previously identified the epithelial progenitor population from which the thymus arises and demonstrated its ability to generate an organized functional thymus upon transplantation. These thymic epithelial progenitor cells (TEPC) are defined by surface determinants recognized by the mAbs MTS20 and MTS24, which were also recently shown to identify keratinocyte progenitor cells in the skin. However, the biochemical nature of the MTS20 and MTS24 determinants has remained unknown. Here we show, via expression profiling of fetal mouse TEPC and their differentiated progeny and subsequent analyses, that both MTS20 and MTS24 specifically bind an orphan protein of unknown function, Placenta-expressed transcript (Plet)-1. In the postgastrulation embryo, Plet-1 expression is highly restricted to the developing pharyngeal endoderm and mesonephros until day 11.5 of embryogenesis, consistent with the MTS20 and MTS24 staining pattern; both MTS20 and MTS24 specifically bind cell lines transfected with Plet-1; and antibodies to Plet-1 recapitulate MTS20/24 staining. In adult tissues, we demonstrate expression in a number of sites, including mammary and prostate epithelia and in the pancreas, where Plet-1 is specifically expressed by the major duct epithelium, providing a specific cell surface marker for this putative reservoir of pancreatic progenitor/stem cells. Plet-1 will thus provide an invaluable tool for genetic analysis of the lineage relationships and molecular mechanisms operating in the development, homeostasis, and injury in several organ/tissue systems.

Fibroblast growth factor induces a neural stem cell phenotype in foetal forebrain progenitors and during embryonic stem cell differentiation

Neural stem (NS) cell lines may be derived via differentiation of pluripotent embryonic stem (ES) cells or from foetal forebrain. However, because NS cells arise in vitro from heterogeneous populations their immediate cellular origin remains unclear. We used microarray-based expression profiling to identify a set of markers expressed by mouse NS cells but not ES cells. One differentially expressed gene encodes the cell surface protein, CD44. CD44 expression is activated by FGF-2 in a subset of cells in both differentiating ES cells and foetal forebrain cultures. Following isolation by flow cytometry the CD44+ population was found to be highly enriched for NS cell founders. We found that other NS cell marker genes are also induced by FGF in culture, including: Adam12, Cadherin20, Cx3cl1, EGFR, Frizzled9, Kitl, Olig1, Olig2 and Vav3. We speculate that the self-renewing NS cell state may be generated in vitro following transcriptional resetting induced by FGF.