Defining an essential transcription factor program for naïve pluripotency

N (6)-Methyladenosine (m(6)A) Methylation in mRNA with A Dynamic and Reversible Epigenetic Modification

N (6)-methyladenosine (m(6)A) is the most abundant and reversible internal modification ubiquitously occurring in eukaryotic mRNA, albeit the significant biological roles of m(6)A methylation have remained largely unclear. The well-known DNA and histone methylations play crucial roles in epigenetic modification of biologic processes in eukaryotes. Analogously, the dynamic and reversible m(6)A RNA modification, which is installed by methyltransferase (METTL3, METTL14, and WTAP), reversed by demethylases (FTO, ALKBH5) and mediated by m(6)A-binding proteins (YTHDF1-3, YTHDC1), may also have a profound impact on gene expression regulation. Recent discoveries of the distributions, functions, and mechanisms of m(6)A modification suggest that this methylation functionally modulates the eukaryotic transcriptome to influence mRNA transcription, splicing, nuclear export, localization, translation, and stability. This reversible mRNA methylation shed light on a new dimension of post-transcriptional regulation of gene expression at the RNA level. m(6)A methylation also plays significant and broad roles in various physiological processes, such as development, fertility, carcinogenesis, stemness, early mortality, meiosis and circadian cycle, and links to obesity, cancer, and other human diseases. This review mainly describes the current knowledge of m(6)A and perspectives on future investigations.

G1 cyclins link proliferation, pluripotency and differentiation of embryonic stem cells

Progression of mammalian cells through the G1 and S phases of the cell cycle is driven by the D-type and E-type cyclins. According to the current models, at least one of these cyclin families must be present to allow cell proliferation. Here, we show that several cell types can proliferate in the absence of all G1 cyclins. However, following ablation of G1 cyclins, embryonic stem (ES) cells attenuated their pluripotent characteristics, with the majority of cells acquiring the trophectodermal cell fate. We established that G1 cyclins, together with their associated cyclin-dependent kinases (CDKs), phosphorylate and stabilize the core pluripotency factors Nanog, Sox2 and Oct4. Treatment of murine ES cells, patient-derived glioblastoma tumour-initiating cells, or triple-negative breast cancer cells with a CDK inhibitor strongly decreased Sox2 and Oct4 levels. Our findings suggest that CDK inhibition might represent an attractive therapeutic strategy by targeting glioblastoma tumour-initiating cells, which depend on Sox2 to maintain their tumorigenic potential.

Tracking the embryonic stem cell transition from ground state pluripotency

Mouse embryonic stem (ES) cells are locked into self-renewal by shielding from inductive cues. Release from this ground state in minimal conditions offers a system for delineating developmental progression from naïve pluripotency. Here, we examine the initial transition process. The ES cell population behaves asynchronously. We therefore exploited a short-half-life Rex1::GFP reporter to isolate cells either side of exit from naïve status. Extinction of ES cell identity in single cells is acute. It occurs only after near-complete elimination of naïve pluripotency factors, but precedes appearance of lineage specification markers. Cells newly departed from the ES cell state display features of early post-implantation epiblast and are distinct from primed epiblast. They also exhibit a genome-wide increase in DNA methylation, intermediate between early and late epiblast. These findings are consistent with the proposition that naïve cells transition to a distinct formative phase of pluripotency preparatory to lineage priming.

Reprogramming of somatic cells: iPS and iN cells

Limited access to human neurons has posed a significant barrier to progress in biological and preclinical studies of the human nervous system. The advent of cell reprogramming technologies has widely disclosed unprecedented opportunities to generate renewable sources of human neural cells for disease modeling, drug discovery, and cell therapeutics. Both somatic reprogramming into induced pluripotent stem cells (iPSCs) and directly induced Neurons (iNeurons) rely on transcription factor-based cellular conversion processes. Nevertheless, they rely on very distinct mechanisms, biological barriers, technical limitations, different levels of efficiency, and generate neural cells with distinctive properties. Human iPSCs represent a long-term renewable source of neural cells, but over time genomic aberrations might erode the quality of the cultures and the in vitro differentiation process requires extensive time. Conversely, direct neuronal reprogramming ensures a fast and straightforward generation of iNeurons endowed with functional properties. However, in this last case, conversion efficiency is reduced when starting from adult human cells, and the molecular and functional fidelity of iNeurons with respect to their corresponding native neuronal subtype is yet to be fully ascertained in many cases. For any biomedical research application, it should be carefully pondered the reprogramming method to use for generating reprogrammed human neuronal subtypes that best fit with the following analysis considering the existing limitations and gap of knowledge still present in this young field of investigation.

Pluripotency and the endogenous retrovirus HERVH: Conflict or serendipity?

Remnants of ancient retroviral infections during evolution litter all mammalian genomes. In modern humans, such endogenous retroviral (ERV) sequences comprise at least 8% of the genome. While ERVs and other types of transposable elements undoubtedly contribute to the genomic "junk yard", functions for some ERV sequences have been demonstrated, with growing evidence that ERVs can be important players in gene regulatory processes. Here we focus on one particular large family of human ERVs, termed HERVH, which several recent studies suggest has a key regulatory role in human pluripotent stem cells. Remarkably, this is not the first instance of an ERV controlling pluripotency. We speculate as to why this convergent evolution might have come about, suggesting that it may reflect selection on the virus to extend the time available for transposition. Alternatively it may reflect serendipity alone.

Charting Developmental Dissolution of Pluripotency

The formation of tissues and organs during metazoan development begs fundamental questions of cellular plasticity: How can the very same genome program have diverse cell types? How do cell identity programs unfold during development in space and time? How can defects in these mechanisms cause disease and also provide opportunities for therapeutic intervention? And ultimately, can developmental programs be exploited for bioengineering tissues and organs? Understanding principle designs of cellular identity and developmental progression is crucial for providing answers. Here, I will discuss how the capture of embryonic pluripotency in murine embryonic stem cells (ESCs) in vitro has allowed fundamental insights into the molecular underpinnings of a developmental cell state and how its ordered disassembly during differentiation prepares for lineage specification.

A role for mitotic bookmarking of SOX2 in pluripotency and differentiation

Mitotic bookmarking transcription factors remain bound to chromosomes during mitosis and were proposed to regulate phenotypic maintenance of stem and progenitor cells at the mitosis-to-G1 (M-G1) transition. However, mitotic bookmarking remains largely unexplored in most stem cell types, and its functional relevance for cell fate decisions remains unclear. Here we screened for mitotic chromosome binding within the pluripotency network of embryonic stem (ES) cells and show that SOX2 and OCT4 remain bound to mitotic chromatin through their respective DNA-binding domains. Dynamic characterization using photobleaching-based methods and single-molecule imaging revealed quantitatively similar specific DNA interactions, but different nonspecific DNA interactions, of SOX2 and OCT4 with mitotic chromatin. Using ChIP-seq (chromatin immunoprecipitation [ChIP] combined with high-throughput sequencing) to assess the genome-wide distribution of SOX2 on mitotic chromatin, we demonstrate the bookmarking activity of SOX2 on a small set of genes. Finally, we investigated the function of SOX2 mitotic bookmarking in cell fate decisions and show that its absence at the M-G1 transition impairs pluripotency maintenance and abrogates its ability to induce neuroectodermal differentiation but does not affect reprogramming efficiency toward induced pluripotent stem cells. Our study demonstrates the mitotic bookmarking property of SOX2 and reveals its functional importance in pluripotency maintenance and ES cell differentiation.

Linking Telomere Regulation to Stem Cell Pluripotency

Embryonic stem cells (ESCs), somatic cell nuclear transfer ESCs, and induced pluripotent stem cells (iPSCs) represent the most studied group of PSCs. Unlimited self-renewal without incurring chromosomal instability and pluripotency are essential for the potential use of PSCs in regenerative therapy. Telomere length maintenance is critical for the unlimited self-renewal, pluripotency, and chromosomal stability of PSCs. While telomerase has a primary role in telomere maintenance, alternative lengthening of telomere pathways involving recombination and epigenetic modifications are also required for telomere length regulation, notably in mouse PSCs. Telomere rejuvenation is part of epigenetic reprogramming to pluripotency. Insights into telomere reprogramming and maintenance in PSCs may have implications for understanding of aging and tumorigenesis. Here, I discuss the link between telomere elongation and homeostasis to the acquisition and maintenance of stem cell pluripotency, and their regulatory mechanisms by epigenetic modifications.

Describing the Stem Cell Potency: The Various Methods of Functional Assessment and In silico Diagnostics

Stem cells are defined by their capabilities to self-renew and give rise to various types of differentiated cells depending on their potency. They are classified as pluripotent, multipotent, and unipotent as demonstrated through their potential to generate the variety of cell lineages. While pluripotent stem cells may give rise to all types of cells in an organism, Multipotent and Unipotent stem cells remain restricted to the particular tissue or lineages. The potency of these stem cells can be defined by using a number of functional assays along with the evaluation of various molecular markers. These molecular markers include diagnosis of transcriptional, epigenetic, and metabolic states of stem cells. Many reports are defining the particular set of different functional assays, and molecular marker used to demonstrate the developmental states and functional capacities of stem cells. The careful evaluation of all these methods could help in generating standard identifying procedures/markers for them.

Quantitative stem cell biology: the threat and the glory

Major technological innovations over the past decade have transformed our ability to extract quantitative data from biological systems at an unprecedented scale and resolution. These quantitative methods and associated large datasets should lead to an exciting new phase of discovery across many areas of biology. However, there is a clear threat: will we drown in these rivers of data? On 18th July 2016, stem cell biologists gathered in Cambridge for the 5th annual Cambridge Stem Cell Symposium to discuss 'Quantitative stem cell biology: from molecules to models'. This Meeting Review provides a summary of the data presented by each speaker, with a focus on quantitative techniques and the new biological insights that are emerging.

A light-colored region of caudal fin: a niche of melanocyte progenitors in crucian carp (Cyprinus carpio L.)

Melanocyte stem cells are a population of immature cells which sustain the self-renewal and replenish the differentiated melanocytes. In this research, a light-colored region (LCR) is observed at the heel of caudal fin in juvenile crucian carp. By cutting off the caudal fin, the operated caudal fin can regenerate in accordance with the original pigment pattern from the retained LCR. As markers of stem cells, Oct4 and Sox2 have been found to be highly expressed in the LCR as well as Mitfa, a label of the melanoblasts. In vitro, Mitfa⁺ melanoblasts are observed in the cells which are derived from the LCR and transfected with Mitfa-EGFP reporter by using Tol2 transposon system. Furthermore, by real-time qPCR, it is shown that the level of sox2 mRNA is gradually decreased from the LCR to proximal and distal caudal fin, and that of mitfa mRNA in the proximal caudal fin (PCF) is higher than that in the LCR, while it is the lowest in the distal caudal fin. Hence, we propose that the LCR is a pigment progenitor niche, sending melanocytes to the distal of caudal fin, which gradually emerges as caudal fin grow. We reveal that the LCR of caudal fin might be a niche of pigment progenitors, and contribute to pigment-producing stem cells in crucian carp.

Foxd3 Promotes Exit from Naive Pluripotency through Enhancer Decommissioning and Inhibits Germline Specification

Following implantation, mouse epiblast cells transit from a naive to a primed state in which they are competent for both somatic and primordial germ cell (PGC) specification. Using mouse embryonic stem cells as an in vitro model to study the transcriptional regulatory principles orchestrating peri-implantation development, here we show that the transcription factor Foxd3 is necessary for exit from naive pluripotency and progression to a primed pluripotent state. During this transition, Foxd3 acts as a repressor that dismantles a significant fraction of the naive pluripotency expression program through decommissioning of active enhancers associated with key naive pluripotency and early germline genes. Subsequently, Foxd3 needs to be silenced in primed pluripotent cells to allow re-activation of relevant genes required for proper PGC specification. Our findings therefore uncover a cycle of activation and deactivation of Foxd3 required for exit from naive pluripotency and subsequent PGC specification.

Mathematical Models of Pluripotent Stem Cells: At the Dawn of Predictive Regenerative Medicine

Regenerative medicine, ranging from stem cell therapy to organ regeneration, is promising to revolutionize treatments of diseases and aging. These approaches require a perfect understanding of cell reprogramming and differentiation. Predictive modeling of cellular systems has the potential to provide insights about the dynamics of cellular processes, and guide their control. Moreover in many cases, it provides alternative to experimental tests, difficult to perform for practical or ethical reasons. The variety and accuracy of biological processes represented in mathematical models grew in-line with the discovery of underlying molecular mechanisms. High-throughput data generation led to the development of models based on data analysis, as an alternative to more established modeling based on prior mechanistic knowledge. In this chapter, we give an overview of existing mathematical models of pluripotency and cell fate, to illustrate the variety of methods and questions. We conclude that current approaches are yet to overcome a number of limitations: Most of the computational models have so far focused solely on understanding the regulation of pluripotency, and the differentiation of selected cell lineages. In addition, models generally interrogate only a few biological processes. However, a better understanding of the reprogramming process leading to ESCs and iPSCs is required to improve stem-cell therapies. One also needs to understand the links between signaling, metabolism, regulation of gene expression, and the epigenetics machinery.

A Dynamic Role of TBX3 in the Pluripotency Circuitry

Pluripotency represents a cell state comprising a fine-tuned pattern of transcription factor activity required for embryonic stem cell (ESC) self-renewal. TBX3 is the earliest expressed member of the T-box transcription factor family and is involved in maintenance and induction of pluripotency. Hence, TBX3 is believed to be a key member of the pluripotency circuitry, with loss of TBX3 coinciding with loss of pluripotency. We report a dynamic expression of TBX3 in vitro and in vivo using genetic reporter tools tracking TBX3 expression in mouse ESCs (mESCs). Low TBX3 levels are associated with reduced pluripotency, resembling the more mature epiblast. Notably, TBX3-low cells maintain the intrinsic capability to switch to a TBX3-high state and vice versa. Additionally, we show TBX3 to be dispensable for induction and maintenance of naive pluripotency as well as for germ cell development. These data highlight novel facets of TBX3 action in mESCs.

Boolean regulatory network reconstruction using literature based knowledge with a genetic algorithm optimization method

Background

Prior knowledge networks (PKNs) provide a framework for the development of computational biological models, including Boolean models of regulatory networks which are the focus of this work. PKNs are created by a painstaking process of literature curation, and generally describe all relevant regulatory interactions identified using a variety of experimental conditions and systems, such as specific cell types or tissues. Certain of these regulatory interactions may not occur in all biological contexts of interest, and their presence may dramatically change the dynamical behaviour of the resulting computational model, hindering the elucidation of the underlying mechanisms and reducing the usefulness of model predictions. Methods are therefore required to generate optimized contextual network models from generic PKNs.

Results

We developed a new approach to generate and optimize Boolean networks, based on a given PKN. Using a genetic algorithm, a model network is built as a sub-network of the PKN and trained against experimental data to reproduce the experimentally observed behaviour in terms of attractors and the transitions that occur between them under specific perturbations. The resulting model network is therefore contextualized to the experimental conditions and constitutes a dynamical Boolean model closer to the observed biological process used to train the model than the original PKN. Such a model can then be interrogated to simulate response under perturbation, to detect stable states and their properties, to get insights into the underlying mechanisms and to generate new testable hypotheses.

Conclusions

Generic PKNs attempt to synthesize knowledge of all interactions occurring in a biological process of interest, irrespective of the specific biological context. This limits their usefulness as a basis for the development of context-specific, predictive dynamical Boolean models. The optimization method presented in this article produces specific, contextualized models from generic PKNs. These contextualized models have improved utility for hypothesis generation and experimental design. The general applicability of this methodological approach makes it suitable for a variety of biological systems and of general interest for biological and medical research. Our method was implemented in the software optimusqual, available online at http://www.vital-it.ch/software/optimusqual/.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-1287-z) contains supplementary material, which is available to authorized users.

BTR: training asynchronous Boolean models using single-cell expression data

Background

Rapid technological innovation for the generation of single-cell genomics data presents new challenges and opportunities for bioinformatics analysis. One such area lies in the development of new ways to train gene regulatory networks. The use of single-cell expression profiling technique allows the profiling of the expression states of hundreds of cells, but these expression states are typically noisier due to the presence of technical artefacts such as drop-outs. While many algorithms exist to infer a gene regulatory network, very few of them are able to harness the extra expression states present in single-cell expression data without getting adversely affected by the substantial technical noise present.

Results

Here we introduce BTR, an algorithm for training asynchronous Boolean models with single-cell expression data using a novel Boolean state space scoring function. BTR is capable of refining existing Boolean models and reconstructing new Boolean models by improving the match between model prediction and expression data. We demonstrate that the Boolean scoring function performed favourably against the BIC scoring function for Bayesian networks. In addition, we show that BTR outperforms many other network inference algorithms in both bulk and single-cell synthetic expression data. Lastly, we introduce two case studies, in which we use BTR to improve published Boolean models in order to generate potentially new biological insights.

Conclusions

BTR provides a novel way to refine or reconstruct Boolean models using single-cell expression data. Boolean model is particularly useful for network reconstruction using single-cell data because it is more robust to the effect of drop-outs. In addition, BTR does not assume any relationship in the expression states among cells, it is useful for reconstructing a gene regulatory network with as few assumptions as possible. Given the simplicity of Boolean models and the rapid adoption of single-cell genomics by biologists, BTR has the potential to make an impact across many fields of biomedical research.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-1235-y) contains supplementary material, which is available to authorized users.

Single-cell pluripotency regulatory networks

Pluripotent stem cells (PSCs) are a popular model system for investigating development, tissue regeneration, and repair. Although much is known about the molecular mechanisms that regulate the balance between self-renewal and lineage commitment in PSCs, the spatiotemporal integration of responsive signaling pathways with core transcriptional regulatory networks are complex and only partially understood. Moreover, measurements made on populations of cells reveal only average properties of the underlying regulatory networks, obscuring their fine detail. Here, we discuss the reconstruction of regulatory networks in individual cells using novel single-cell transcriptomics and proteomics, in order to expand our understanding of the molecular basis of pluripotency, including the role of cell-cell variability within PSC populations, and ways in which networks may be controlled in order to reliably manipulate cell behavior.

Looking to the future following 10 years of induced pluripotent stem cell technologies

The development of induced pluripotent stem cells (iPSCs) has fundamentally changed our view on developmental cell-fate determination and led to a cascade of technological innovations in regenerative medicine. Here we provide an overview of the progress in the field over the past decade, as well as our perspective on future directions and clinical implications of iPSC technology.

Automated Synthesis and Analysis of Switching Gene Regulatory Networks

Studying the gene regulatory networks (GRNs) that govern how cells change into specific cell types with unique roles throughout development is an active area of experimental research. The fate specification process can be viewed as a biological program prescribing the system dynamics, governed by a network of genetic interactions. To investigate the possibility that GRNs are not fixed but rather change their topology, for example as cells progress through commitment, we introduce the concept of Switching Gene Regulatory Networks (SGRNs) to enable the modelling and analysis of network reconfiguration. We define the synthesis problem of constructing SGRNs that are guaranteed to satisfy a set of constraints representing experimental observations of cell behaviour. We propose a solution to this problem that employs methods based upon Satisfiability Modulo Theories (SMT) solvers, and evaluate the feasibility and scalability of our approach by considering a set of synthetic benchmarks exhibiting possible biological behaviour of cell development. We outline how our approach is applied to a more realistic biological system, by considering a simplified network involved in the processes of neuron maturation and fate specification in the mammalian cortex.

Sall4 controls differentiation of pluripotent cells independently of the Nucleosome Remodelling and Deacetylation (NuRD) complex

Sall4 is an essential transcription factor for early mammalian development and is frequently overexpressed in cancer. Although it is reported to play an important role in embryonic stem cell (ESC) self-renewal, whether it is an essential pluripotency factor has been disputed. Here, we show that Sall4 is dispensable for mouse ESC pluripotency. Sall4 is an enhancer-binding protein that prevents precocious activation of the neural gene expression programme in ESCs but is not required for maintenance of the pluripotency gene regulatory network. Although a proportion of Sall4 protein physically associates with the Nucleosome Remodelling and Deacetylase (NuRD) complex, Sall4 neither recruits NuRD to chromatin nor influences transcription via NuRD; rather, free Sall4 protein regulates transcription independently of NuRD. We propose a model whereby enhancer binding by Sall4 and other pluripotency-associated transcription factors is responsible for maintaining the balance between transcriptional programmes in pluripotent cells.

Highlighted article: Sall4 and Sall1 inhibit neural differentiation in ESCs by acting at enhancer sequences independently of the NuRD complex, and are dispensable for the maintenance of pluripotency.

mRNA Cap Methylation in Pluripotency and Differentiation

The mRNA cap recruits factors essential for transcript processing and translation initiation. We report that regulated mRNA cap methylation is a feature of embryonic stem cell (ESC) differentiation. Expression of the mRNA cap methyltransferase activating subunit RAM is elevated in ESCs, resulting in high levels of mRNA cap methylation and expression of a cohort of pluripotency-associated genes. During neural differentiation, RAM is suppressed, resulting in repression of pluripotency-associated factors and expression of a cohort of neural-associated genes. An established requirement of differentiation is increased ERK1/2 activity, which suppresses pluripotency-associated genes. During differentiation, ERK1/2 phosphorylates RAM serine-36, targeting it for ubiquitination and proteasomal degradation, ultimately resulting in changes in gene expression associated with loss of pluripotency. Elevated RAM expression also increases the efficiency of fibroblast reprogramming. Thus, the mRNA cap emerges as a dynamic mark that instructs change in gene expression profiles during differentiation and reprogramming.

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The mRNA cap methyltransferase RNMT-RAM is highly expressed in embryonic stem cells

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RNMT-RAM is important for the expression of pluripotency-associated genes

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During neural differentiation, the cap methyltransferase activator RAM is repressed

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Repression of RAM contributes to upregulation of neural genes and neural morphology

A Method to Identify and Analyze Biological Programs through Automated Reasoning

Predictive biology is elusive because rigorous, data-constrained, mechanistic models of complex biological systems are difficult to derive and validate. Current approaches tend to construct and examine static interaction network models, which are descriptively rich, but often lack explanatory and predictive power, or dynamic models that can be simulated to reproduce known behavior. However, in such approaches implicit assumptions are introduced as typically only one mechanism is considered, and exhaustively investigating all scenarios is impractical using simulation. To address these limitations, we present a methodology based on automated formal reasoning, which permits the synthesis and analysis of the complete set of logical models consistent with experimental observations. We test hypotheses against all candidate models, and remove the need for simulation by characterizing and simultaneously analyzing all mechanistic explanations of observed behavior. Our methodology transforms knowledge of complex biological processes from sets of possible interactions and experimental observations to precise, predictive biological programs governing cell function.

A Myc-driven self-reinforcing regulatory network maintains mouse embryonic stem cell identity

Stem cell identity depends on the integration of extrinsic and intrinsic signals, which directly influence the maintenance of their epigenetic state. Although Myc transcription factors play a major role in stem cell self-renewal and pluripotency, their integration with signalling pathways and epigenetic regulators remains poorly defined. We addressed this point by profiling the gene expression and epigenetic pattern in ESCs whose growth depends on conditional Myc activity. Here we show that Myc potentiates the Wnt/β-catenin signalling pathway, which cooperates with the transcriptional regulatory network in sustaining ESC self-renewal. Myc activation results in the transcriptional repression of Wnt antagonists through the direct recruitment of PRC2 on these targets. The consequent potentiation of the autocrine Wnt/β-catenin signalling induces the transcriptional activation of the endogenous Myc family members, which in turn activates a Myc-driven self-reinforcing circuit. Thus, our data unravel a Myc-dependent self-propagating epigenetic memory in the maintenance of ESC self-renewal capacity.

The Myc transcription factor is a major regulator of stem cell (SC) self-renewal and pluripotency but how this integrates signals from other pathways is unclear. Here, the authors show that Myc activation triggers epigenetic memory in self renewing embryonic SCs via PRC2-mediated potentiation of the Wnt pathway.

Transposable elements in the mammalian embryo: pioneers surviving through stealth and service

Transposable elements (TEs) are notable drivers of genetic innovation. Over evolutionary time, TE insertions can supply new promoter, enhancer, and insulator elements to protein-coding genes and establish novel, species-specific gene regulatory networks. Conversely, ongoing TE-driven insertional mutagenesis, nonhomologous recombination, and other potentially deleterious processes can cause sporadic disease by disrupting genome integrity or inducing abrupt gene expression changes. Here, we discuss recent evidence suggesting that TEs may contribute regulatory innovation to mammalian embryonic and pluripotent states as a means to ward off complete repression by their host genome.

Capturing the ephemeral human pluripotent state

During human development, pluripotency is present only in early stages of development. This ephemeral cell potential can be captured in vitro by obtaining pluripotent stem cells (PSC) with self-renewal properties, the human embryonic stem cells (hESC). However, diverse studies suggest the existence of a plethora of human PSC (hPSC) that can be derived from both embryonic and somatic sources, depending on defined culture conditions, their spatial origin, and the genetic engineering used for reprogramming. This review will focus on hPSC, covering the conventional primed hESC, naïve-like hPSC that resemble the ground-state of development, region-selective PSC, and human induced PSC (hiPSC). We will analyze differences and similarities in their differentiation potential as well as in the molecular circuitry of pluripotency. Finally, we describe the need for human feeder cells to derive and maintain hPSC, because they could emulate the interaction of in vivo pluripotent cells with extraembryonic structures that support development. Developmental Dynamics 245:762-773, 2016. © 2016 Wiley Periodicals, Inc.

RISC-mediated control of selected chromatin regulators stabilizes ground state pluripotency of mouse embryonic stem cells

BACKGROUND

Embryonic stem cells are intrinsically unstable and differentiate spontaneously if they are not shielded from external stimuli. Although the nature of such instability is still controversial, growing evidence suggests that protein translation control may play a crucial role.

RESULTS

We performed an integrated analysis of RNA and proteins at the transition between naïve embryonic stem cells and cells primed to differentiate. During this transition, mRNAs coding for chromatin regulators are specifically released from translational inhibition mediated by RNA-induced silencing complex (RISC). This suggests that, prior to differentiation, the propensity of embryonic stem cells to change their epigenetic status is hampered by RNA interference. The expression of these chromatin regulators is reinstated following acute inactivation of RISC and it correlates with loss of stemness markers and activation of early cell differentiation markers in treated embryonic stem cells.

CONCLUSIONS

We propose that RISC-mediated inhibition of specific sets of chromatin regulators is a primary mechanism for preserving embryonic stem cell pluripotency while inhibiting the onset of embryonic developmental programs.

Stat3 promotes mitochondrial transcription and oxidative respiration during maintenance and induction of naive pluripotency

Transcription factor Stat3 directs self-renewal of pluripotent mouse embryonic stem (ES) cells downstream of the cytokine leukemia inhibitory factor (LIF). Stat3 upregulates pivotal transcription factors in the ES cell gene regulatory network to sustain naïve identity. Stat3 also contributes to the rapid proliferation of ES cells. Here, we show that Stat3 increases the expression of mitochondrial-encoded transcripts and enhances oxidative metabolism. Chromatin immunoprecipitation reveals that Stat3 binds to the mitochondrial genome, consistent with direct transcriptional regulation. An engineered form of Stat3 that localizes predominantly to mitochondria is sufficient to support enhanced proliferation of ES cells, but not to maintain their undifferentiated phenotype. Furthermore, during reprogramming from primed to naïve states of pluripotency, Stat3 similarly upregulates mitochondrial transcripts and facilitates metabolic resetting. These findings suggest that the potent stimulation of naïve pluripotency by LIF/Stat3 is attributable to parallel and synergistic induction of both mitochondrial respiration and nuclear transcription factors.

Naive Pluripotent Stem Cells Derived Directly from Isolated Cells of the Human Inner Cell Mass

Conventional generation of stem cells from human blastocysts produces a developmentally advanced, or primed, stage of pluripotency. In vitro resetting to a more naive phenotype has been reported. However, whether the reset culture conditions of selective kinase inhibition can enable capture of naive epiblast cells directly from the embryo has not been determined. Here, we show that in these specific conditions individual inner cell mass cells grow into colonies that may then be expanded over multiple passages while retaining a diploid karyotype and naive properties. The cells express hallmark naive pluripotency factors and additionally display features of mitochondrial respiration, global gene expression, and genome-wide hypomethylation distinct from primed cells. They transition through primed pluripotency into somatic lineage differentiation. Collectively these attributes suggest classification as human naive embryonic stem cells. Human counterparts of canonical mouse embryonic stem cells would argue for conservation in the phased progression of pluripotency in mammals.

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Karyotypically normal pluripotent stem cells derived from single human ICM cells

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Expanded lines retain anticipated molecular features of naive embryonic stem cells

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Consistency between ICM-derived and in vitro reset naive human pluripotent stem cells

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Presence of KLF17 protein in human ICM and naive stem cells

An experimentally validated network of nine haematopoietic transcription factors reveals mechanisms of cell state stability

Transcription factor (TF) networks determine cell-type identity by establishing and maintaining lineage-specific expression profiles, yet reconstruction of mammalian regulatory network models has been hampered by a lack of comprehensive functional validation of regulatory interactions. Here, we report comprehensive ChIP-Seq, transgenic and reporter gene experimental data that have allowed us to construct an experimentally validated regulatory network model for haematopoietic stem/progenitor cells (HSPCs). Model simulation coupled with subsequent experimental validation using single cell expression profiling revealed potential mechanisms for cell state stabilisation, and also how a leukaemogenic TF fusion protein perturbs key HSPC regulators. The approach presented here should help to improve our understanding of both normal physiological and disease processes.

Co-expression networks in generation of induced pluripotent stem cells

We developed an adenoviral vector, in which Yamanaka's four reprogramming factors (RFs) were controlled by individual CMV promoters in a single cassette (Ad-SOcMK). This permitted coordinated expression of RFs (SOX2, OCT3/4, c-MYC and KLF4) in a cell for a transient period of time, synchronizing the reprogramming process with the majority of transduced cells assuming induced pluripotent stem cell (iPSC)-like characteristics as early as three days post-transduction. These reprogrammed cells resembled human embryonic stem cells (ESCs) with regard to morphology, biomarker expression, and could be differentiated into cells of the germ layers in vitro and in vivo. These iPSC-like cells, however, failed to expand into larger iPSC colonies. The short and synchronized reprogramming process allowed us to study global transcription changes within short time intervals. Weighted gene co-expression network analysis (WGCNA) identified sixteen large gene co-expression modules, each including members of gene ontology categories involved in cell differentiation and development. In particular, the brown module contained a significant number of ESC marker genes, whereas the turquoise module contained cell-cycle-related genes that were downregulated in contrast to upregulation in human ESCs. Strong coordinated expression of all four RFs via adenoviral transduction may constrain stochastic processes and lead to silencing of genes important for cellular proliferation.

Summary: We developed a novel adenoviral iPSC reprogramming vector integrating Yamanaka's four factors in a single cassette, allowing for the identification of biologically relevant co-expression networks.

A decade of transcription factor-mediated reprogramming to pluripotency

The past 10 years have seen great advances in our ability to manipulate cell fate, including the induction of pluripotency in vitro to generate induced pluripotent stem cells (iPSCs). This process proved to be remarkably simple from a technical perspective, only needing the host cell and a defined cocktail of transcription factors, with four factors - octamer-binding protein 3/4 (OCT3/4), SOX2, Krüppel-like factor 4 (KLF4) and MYC (collectively referred to as OSKM) - initially used. The mechanisms underlying transcription factor-mediated reprogramming are still poorly understood; however, several mechanistic insights have recently been obtained. Recent years have also brought significant progress in increasing the efficiency of this technique, making it more amenable to applications in the fields of regenerative medicine, disease modelling and drug discovery.

Emerging Modeling Concepts and Solutions in Stem Cell Research

Modern stem cell research, as well as other fields of contemporary biology involves quantitative sciences in many ways. Identifying candidates for key differentiation or reprogramming factors, tracing global transcriptome changes, or finding drugs is now broadly involves bioinformatics and biostatistics. However, the next key step, understanding the underlying reasons and establishing causal links leading to differentiation or reprogramming requires qualitative and quantitative biological models describing complex biological systems. Currently, quantitative modeling is a challenging science, capable to deliver rather modest results or predictions. What model types are the most popular and what features of stem cell behavior they are capturing? What new insights do we expect from the computational modeling of stem cells in the foreseeable future? Current review attempts to approach these essential questions by considering published quantitative models and solutions emerging in the area of stem cell research.

The role of pluripotency factors to drive stemness in gastrointestinal cancer

A better molecular understanding of gastrointestinal cancers arising either from the stomach, the pancreas, the intestine, or the liver has led to the identification of a variety of potential new molecular therapeutic targets. However, in most cases surgery remains the only curative option. The intratumoral cellular heterogeneity of cancer stem cells, bulk tumor cells, and stromal cells further limits straightforward targeting approaches. Accumulating evidence reveals an intimate link between embryonic development, stem cells, and cancer formation. In line, a growing number of oncofetal proteins are found to play common roles within these processes. Cancer stem cells share features with true stem cells by having the capacity to self-renew in a de-differentiated state, to generate heterogeneous types of differentiated progeny, and to give rise to the bulk tumor. Further, various studies identified genes in cancer stem cells, which were previously shown to regulate the pluripotency circuitry, particularly the so-called "Yamanaka-Factors" (OCT4, KLF4, SOX2, and c-MYC). However, the true stemness potential of cancer stem cells and the role and expression pattern of such pluripotency genes in various tumor cell types remain to be explored. Here, we summarize recent findings and discuss the potential mechanisms involved, and link them to clinical significance with a particular focus on gastrointestinal cancers.

Kernel Architecture of the Genetic Circuitry of the Arabidopsis Circadian System

A wide range of organisms features molecular machines, circadian clocks, which generate endogenous oscillations with ~24 h periodicity and thereby synchronize biological processes to diurnal environmental fluctuations. Recently, it has become clear that plants harbor more complex gene regulatory circuits within the core circadian clocks than other organisms, inspiring a fundamental question: are all these regulatory interactions between clock genes equally crucial for the establishment and maintenance of circadian rhythms? Our mechanistic simulation for Arabidopsis thaliana demonstrates that at least half of the total regulatory interactions must be present to express the circadian molecular profiles observed in wild-type plants. A set of those essential interactions is called herein a kernel of the circadian system. The kernel structure unbiasedly reveals four interlocked negative feedback loops contributing to circadian rhythms, and three feedback loops among them drive the autonomous oscillation itself. Strikingly, the kernel structure, as well as the whole clock circuitry, is overwhelmingly composed of inhibitory, rather than activating, interactions between genes. We found that this tendency underlies plant circadian molecular profiles which often exhibit sharply-shaped, cuspidate waveforms. Through the generation of these cuspidate profiles, inhibitory interactions may facilitate the global coordination of temporally-distant clock events that are markedly peaked at very specific times of day. Our systematic approach resulting in experimentally-testable predictions provides insights into a design principle of biological clockwork, with implications for synthetic biology.

Isolation and cultivation of naive-like human pluripotent stem cells based on HERVH expression

The ability to derive and stably maintain ground-state human pluripotent stem cells (hPSCs) that resemble the cells seen in vivo in the inner cell mass has the potential to be an invaluable tool for researchers developing stem cell-based therapies. To date, derivation of human naive-like pluripotent stem cell lines has been limited to a small number of lineages, and their long-term culturing remains problematic. We describe a protocol for genetic and phenotypic tagging, selecting and maintaining naive-like hPSCs. We tag hPSCs by GFP, expressed by the long terminal repeat (LTR7) of HERVH endogenous retrovirus. This simple and efficient protocol has been reproduced with multiple hPSC lines, including embryonic and induced pluripotent stem cells, and it takes ∼6 weeks. By using the reporter, homogeneous hPSC cultures can be derived, characterized and maintained for the long term by repeated re-sorting and re-plating steps. The HERVH-expressing cells have a similar, but nonidentical, expression pattern to other naive-like cells, suggesting that alternative pluripotent states might exist.

Nac1 Coordinates a Sub-network of Pluripotency Factors to Regulate Embryonic Stem Cell Differentiation

Pluripotent cells give rise to distinct cell types during development and are regulated by often self-reinforcing molecular networks. How such networks allow cells to differentiate is less well understood. Here, we use integrative methods to show that external signals induce reorganization of the mouse embryonic stem cell pluripotency network and that a sub-network of four factors, Nac1, Oct4, Tcf3, and Sox2, regulates their differentiation into the alternative mesendodermal and neuroectodermal fates. In the mesendodermal fate, Nac1 and Oct4 were constrained within quantitative windows, whereas Sox2 and Tcf3 were repressed. In contrast, in the neuroectodermal fate, Sox2 and Tcf3 were constrained while Nac1 and Oct4 were repressed. In addition, we show that Nac1 coordinates differentiation by activating Oct4 and inhibiting both Sox2 and Tcf3. Reorganization of progenitor cell networks around shared factors might be a common differentiation strategy and our integrative approach provides a general methodology for delineating such networks.

A dual role of Erk signaling in embryonic stem cells

Erk signaling plays a critical role in maintaining the pluripotency of mouse embryonic stem cells (ESCs). Inhibition of Mek/Erk signaling by pharmacologic Mek inhibitor promotes self-renewal and pluripotency of mouse ESCs. However, knockout of Erk1/2 genes compromises the self-renewal and genomic stability of mouse ESCs. In this review, we summarize recent progress in understanding the role of Erk signaling in pluripotency maintenance, discuss the dual role of Erk in mouse ESCs, and provide explanations for the conflicting data regarding Mek inhibition and Erk knockout. Remaining questions and the prospects of Erk signaling in pluripotency maintenance are also discussed.

Monitoring Long Interspersed Nuclear Element 1 Expression During Mouse Embryonic Stem Cell Differentiation

Long Interspersed Elements-1 (LINE-1 or L1) are a class of transposable elements which account for almost 19 % of the mouse genome. This represents around 600,000 L1 fragments, among which it is estimated that 3000 intact copies still remain capable to retrotranspose and to generate deleterious mutation by insertion into genomic coding region. In differentiated cells, full length L1 are transcriptionally repressed by DNA methylation. However at the blastocyst stage, L1 elements are subject to a demethylation wave and able to be expressed and to be inserted into new genomic locations. Mouse Embryonic Stem Cells (mESCs) are pluripotent stem cells derived from the inner cell mass of blastocysts. Mouse ESCs can be maintained undifferentiated under controlled culture conditions or induced into the three primary germ layers, therefore they represent a suitable model to follow mechanisms involved in L1 repression during the process of differentiation of mESCs. This protocol presents how to maintain culture of undifferentiated mESCs, induce their differentiation, and monitor L1 expression at the transcriptional and translational levels. L1 transcriptional levels are assessed by real-time qRT-PCR performed on total RNA extracts using specific L1 primers and translation levels are measured by Western blot analysis of L1 protein ORF1 using a specific L1 antibody.

Maximizing Clonal Embryonic Stem Cell Derivation by ERK Pathway Inhibition

Since the development of inhibitor-based defined culture conditions (known as "2i"), multiple clonal embryonic stem cell (ESC) lines can be readily derived from single cells isolated directly from mouse embryos. In addition to providing an efficient means to generate ES cells from compound transgenic or murine disease models on any genetic background, this technology can be used to investigate the process of ESC derivation at both a functional and molecular level. Here, we provide details of the procedure for both maximizing the number of cells in the donor tissue and subsequent effective derivation of multiple clonal ES cell lines.

Direct lineage reprogramming: strategies, mechanisms, and applications

The direct lineage reprogramming of one specialized cell type into another using defined factors has fundamentally re-shaped traditional concepts regarding the epigenetic stability of differentiated cells. With the rapid increase in cell types generated through direct conversion in recent years, this strategy has become a promising approach for producing functional cells. Here, we review recent advances in lineage reprogramming, including the identification of novel reprogramming factors, underlying molecular mechanisms, strategies for generating functionally mature cells, and assays for characterizing induced cells. We also discuss progress toward the application of lineage reprogramming and the major future challenges for this strategy.

Set1 and MLL1/2 Target Distinct Sets of Functionally Different Genomic Loci In Vivo

Histone H3 lysine 4 trimethylation (H3K4me3) is known to correlate with both active and poised genomic loci, yet many questions remain regarding its functional roles in vivo. We identify functional genomic targets of two H3K4 methyltransferases, Set1 and MLL1/2, in both the stem cells and differentiated tissue of the planarian flatworm Schmidtea mediterranea. We show that, despite their common substrate, these enzymes target distinct genomic loci in vivo, which are distinguishable by the pattern each enzyme leaves on the chromatin template, i.e., the breadth of the H3K4me3 peak. Whereas Set1 targets are largely associated with the maintenance of the stem cell population, MLL1/2 targets are specifically enriched for genes involved in ciliogenesis. These data not only confirm that chromatin regulation is fundamental to planarian stem cell function but also provide evidence for post-embryonic functional specificity of H3K4me3 methyltransferases in vivo.

Pluripotency Factors on Their Lineage Move

Pluripotent stem cells are characterised by continuous self-renewal while maintaining the potential to differentiate into cells of all three germ layers. Regulatory networks of maintaining pluripotency have been described in great detail and, similarly, there is great knowledge on key players that regulate their differentiation. Interestingly, pluripotency has various shades with distinct developmental potential, an observation that coined the term of a ground state of pluripotency. A precise interplay of signalling axes regulates ground state conditions and acts in concert with a combination of key transcription factors. The balance between these transcription factors greatly influences the integrity of the pluripotency network and latest research suggests that minute changes in their expression can strengthen but also collapse the network. Moreover, recent studies reveal different facets of these core factors in balancing a controlled and directed exit from pluripotency. Thereby, subsets of pluripotency-maintaining factors have been shown to adopt new roles during lineage specification and have been globally defined towards neuroectodermal and mesendodermal sets of embryonic stem cell genes. However, detailed underlying insights into how these transcription factors orchestrate cell fate decisions remain largely elusive. Our group and others unravelled complex interactions in the regulation of this controlled exit. Herein, we summarise recent findings and discuss the potential mechanisms involved.

Lineage-Specific Profiling Delineates the Emergence and Progression of Naive Pluripotency in Mammalian Embryogenesis

Naive pluripotency is manifest in the preimplantation mammalian embryo. Here we determine transcriptome dynamics of mouse development from the eight-cell stage to postimplantation using lineage-specific RNA sequencing. This method combines high sensitivity and reporter-based fate assignment to acquire the full spectrum of gene expression from discrete embryonic cell types. We define expression modules indicative of developmental state and temporal regulatory patterns marking the establishment and dissolution of naive pluripotency in vivo. Analysis of embryonic stem cells and diapaused embryos reveals near-complete conservation of the core transcriptional circuitry operative in the preimplantation epiblast. Comparison to inner cell masses of marmoset primate blastocysts identifies a similar complement of pluripotency factors but use of alternative signaling pathways. Embryo culture experiments further indicate that marmoset embryos utilize WNT signaling during early lineage segregation, unlike rodents. These findings support a conserved transcription factor foundation for naive pluripotency while revealing species-specific regulatory features of lineage segregation.

The Current State of Naïve Human Pluripotency

Naïve or ground state pluripotency is a cellular state in vitro which resembles cells of the preimplantation epiblast in vivo. This state was first observed in mouse embryonic stem cells and is characterized by high rates of proliferation, the ability to differentiate widely, and global hypomethylation. Human pluripotent stem cells (hPSCs) correspond to a later or "primed" stage of embryonic development. The conversion of hPSCs to a naïve state is desirable as their features should facilitate techniques such as gene editing and more efficient differentiation. Here we review protocols which now allow derivation of naïve human pluripotent stem cells by transgene expression or the use of media formulations containing inhibitors and growth factors and correlate this with pathways involved. Maintenance of these ground state cells is possible using a combination of basic fibroblast growth factor and human leukemia inhibitory factor together with dual inhibition of glycogen synthase kinase 3 beta, and mitogen-activated protein kinase kinase (MEK). Close similarity between the ground state hPSC and the in vivo preimplantation epiblast have been shown both by demonstrating similar upregulation of endogenous retroviruses and correlation of global RNA-seq data. This suggests that the human naïve state is not an in vitro artifact.