Pluripotent stem cells (PSCs) retain the ability to differentiate into a wide range of cell types while undergoing self-renewal. They also exhibit an unusual mode of cell cycle regulation, reflected ...by a cell cycle structure where G1 and G2 phases are truncated. When individual PSCs are exposed to specification cues, they activate developmental programs and remodel the cell cycle so that the length of G1 and overall cell division times increase. The response of individual stem cells to pro-differentiation signals is strikingly heterogeneous, resulting in asynchronous differentiation. Recent evidence indicates that this phenomenon is due to cell cycle-dependent mechanisms that restrict the initial activation of developmental genes to the G1 phase. This suggests a broad biological mechanism where multipotent cells are ‘primed’ to initiate cell fate decisions during their transition through G1. Here, I discuss mechanisms underpinning the commitment towards the differentiated state and its relation to the cell cycle.
Graphical abstract Highlights ► Signaling in pluripotent stem cells is a complex, dynamic process. ► Multiple signaling pathways are required for maintenance of the stem cell state: Fgf/MAPK, ...TGFβ/SMAD2,3 and insulin/PI3K. ► These pathways are highly interactive and form a finely-balanced signaling network. ► Thresholds, temporal changes and combinatorial effects make important contributions to cell fate outcomes.
A strong connection exists between the cell cycle and mechanisms required for executing cell fate decisions in a wide-range of developmental contexts. Terminal differentiation is often associated ...with cell cycle exit, whereas cell fate switches are frequently linked to cell cycle transitions in dividing cells. These phenomena have been investigated in the context of reprogramming, differentiation and trans-differentiation but the underpinning molecular mechanisms remain unclear. Most progress to address the connection between cell fate and the cell cycle has been made in pluripotent stem cells, in which the transition through mitosis and G1 phase is crucial for establishing a window of opportunity for pluripotency exit and the initiation of differentiation. This Review will summarize recent developments in this area and place them in a broader context that has implications for a wide range of developmental scenarios.
Pluripotent stem cells (PSCs) proliferate rapidly with a characteristic cell cycle structure consisting of short G1‐ and G2‐gap phases. This applies broadly to PSCs of peri‐implantation stage ...embryos, cultures of embryonic stem cells, induced pluripotent stem cells, and embryonal carcinoma cells. During the early stages of PSC differentiation however, cell division times increase as a consequence of cell cycle remodeling. Most notably, this is indicated by elongation of the G1‐phase. Observations linking changes in the cell cycle with exit from pluripotency have raised questions about the role of cell cycle control in maintenance of the pluripotent state. Until recently however, this has been a difficult question to address because of limitations associated with experimental tools. Recent studies now show that pluripotency and cell cycle regulatory networks are intertwined and that cell cycle control mechanisms are an integral, mechanistic part of the PSC state. Studies in embryonal carcinoma, some 30 years ago, first suggested that pluripotent cells initiate differentiation when in the G1‐phase. More recently, a molecular “priming” mechanism has been proposed to explain these observations in human embryonic stem cells. Complexity in this area has been increased by the realization that pluripotent cells exist in multiple developmental states and that in addition to each having their own characteristic gene expression and epigenetic signatures, they potentially have alternate modes of cell cycle regulation. This review will summarize current knowledge in these areas and will highlight important aspects of interconnections between the cell cycle, self‐renewal, pluripotency, and cell fate decisions. Stem Cells 2016;34:1427–1436
For many years, stem cell metabolism was viewed as a byproduct of cell fate status rather than an active regulatory mechanism; however, there is now a growing appreciation that metabolic pathways ...influence epigenetic changes associated with lineage commitment, specification, and self-renewal. Here we review how metabolites generated during glycolytic and oxidative processes are utilized in enzymatic reactions leading to epigenetic modifications and transcriptional regulation. We discuss how “metabolic reprogramming” contributes to global epigenetic changes in the context of naive and primed pluripotent states, somatic reprogramming, and hematopoietic and skeletal muscle tissue stem cells, and we discuss the implications for regenerative medicine.
Ryall et al. discuss how “metabolic reprogramming” contributes to global epigenetic changes in stem cells, focusing on how metabolites generated during glycolytic and oxidative processes are utilized in enzymatic reactions for epigenetic regulation. Their discussion centers on changes in pluripotent stem cells and hematopoietic and skeletal muscle stem cells.
•Regulation is highly-dynamic, cell-type specific and involved in cell fate determination.•Aerobic glycolysis is a feature of ‘primed’ pluripotent cells and widely seen in adult stem cells.•Elevated ...rates of aerobic glycolysis are required for maintenance of the pluripotent state.•Metabolic switching is required for cellular reprogramming and establishment of pluripotency.•Aerobic glycolysis in stem cells is dependent on epigenetic control, redox regulation and signal regulated transcription factor networks.
Cell fate decisions are closely linked to changes in metabolic activity. Over recent years this connection has been implicated in mechanisms underpinning embryonic development, reprogramming and disease pathogenesis. In addition to being important for supporting the energy demands of different cell types, metabolic switching from aerobic glycolysis to oxidative phosphorylation plays a critical role in controlling biosynthetic processes, intracellular redox state, epigenetic status and reactive oxygen species levels. These processes extend beyond ATP synthesis by impacting cell proliferation, differentiation, enzymatic activity, ageing and genomic integrity. This review will focus on how metabolic switching impacts decisions made by multipotent cells and discusses mechanisms by which this occurs.
There exists a worldwide shortage of donor livers available for orthotropic liver transplantation and hepatocyte transplantation therapies. In addition to their therapeutic potential, primary human ...hepatocytes facilitate the study of molecular and genetic aspects of human hepatic disease and development and provide a platform for drug toxicity screens and identification of novel pharmaceuticals with potential to treat a wide array of metabolic diseases. The demand for human hepatocytes, therefore, heavily outweighs their availability. As an alternative to using donor livers as a source of primary hepatocytes, we explored the possibility of generating patient‐specific human hepatocytes from induced pluripotent stem (iPS) cells. Conclusion: We demonstrate that mouse iPS cells retain full potential for fetal liver development and describe a procedure that facilitates the efficient generation of highly differentiated human hepatocyte‐like cells from iPS cells that display key liver functions and can integrate into the hepatic parenchyma in vivo. (HEPATOLOGY 2010.)
Neural crest stem cells can be isolated from differentiated cultures of human pluripotent stem cells, but the process is inefficient and requires cell sorting to obtain a highly enriched population. ...No specific method for directed differentiation of human pluripotent cells toward neural crest stem cells has yet been reported. This severely restricts the utility of these cells as a model for disease and development and for more applied purposes such as cell therapy and tissue engineering. In this report, we use small-molecule compounds in a single-step method for the efficient generation of self-renewing neural crest-like stem cells in chemically defined media. This approach is accomplished directly from human pluripotent cells without the need for coculture on feeder layers or cell sorting to obtain a highly enriched population. Critical to this approach is the activation of canonical Wnt signaling and concurrent suppression of the Activin A/Nodal pathway. Over 12–14 d, pluripotent cells are efficiently specified along the neuroectoderm lineage toward p75+ Hnk1+ Ap2+ neural crest-like cells with little or no contamination by Pax6+ neural progenitors. This cell population can be clonally amplified and maintained for >25 passages (>100 d) while retaining the capacity to differentiate into peripheral neurons, smooth muscle cells, and mesenchymal precursor cells. Neural crest-like stem cell-derived mesenchymal precursors have the capacity for differentiation into osteocytes, chondrocytes, and adipocytes. In sum, we have developed methods for the efficient generation of self-renewing neural crest stem cells that greatly enhance their potential utility in disease modeling and regenerative medicine.
Physical interactions between distal regulatory elements have a key role in regulating gene expression, but the extent to which these interactions vary between cell types and contribute to ...cell-type-specific gene expression remains unclear. Here, to address these questions as part of phase III of the Encyclopedia of DNA Elements (ENCODE), we mapped cohesin-mediated chromatin loops, using chromatin interaction analysis by paired-end tag sequencing (ChIA-PET), and analysed gene expression in 24 diverse human cell types, including core ENCODE cell lines. Twenty-eight per cent of all chromatin loops vary across cell types; these variations modestly correlate with changes in gene expression and are effective at grouping cell types according to their tissue of origin. The connectivity of genes corresponds to different functional classes, with housekeeping genes having few contacts, and dosage-sensitive genes being more connected to enhancer elements. This atlas of chromatin loops complements the diverse maps of regulatory architecture that comprise the ENCODE Encyclopedia, and will help to support emerging analyses of genome structure and function.
Mouse embryonic stem cells (ESCs) cultured in serum are characterized by hyper-phosphorylated RB protein, lack of G1 control, and rapid progression through the cell cycle. Here, we show that ESCs ...grown in the presence of two small-molecule inhibitors (2i ESCs) have a longer G1-phase with hypo-phosphorylated RB, implying that they have a functional G1 checkpoint. Deletion of RB, P107, and P130 in 2i ESCs results in a G1-phase similar to that of serum ESCs. Inhibition of the ERK signaling pathway in serum ESCs results in the appearance of hypo-phosphorylated RB and the reinstatement of a G1 checkpoint. In addition, induction of a dormant state by the inhibition of MYC, resembling diapause, requires the presence of the RB family proteins. Collectively, our data show that RB-dependent G1 restriction point signaling is active in mouse ESCs grown in 2i but abrogated in serum by ERK-dependent phosphorylation.
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•2i ESCs have a longer G1-phase and a shorter G2-phase than serum-grown cells•RB family proteins control the restriction point and elongate the G1-phase in 2i ESCs•ERK pathway signaling abrogates the restriction point in serum-grown ESCs•Diapause-like quiescence of ESCs also depends on RB-mediated G1 regulation
It is widely thought that embryonic stem cells have an unusual and very rapid cell cycle, lacking normal G1 regulation. Stunnenberg and colleagues show that, in mouse ESCs, G1 regulation is, in fact, intact but abrogated in serum by ERK signaling and RB phosphorylation.
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