Nuclear export of circular RNA.

Circular RNAs (circRNAs), which are increasingly being implicated in a variety of functions in normal and cancerous cells1-5, are formed by back-splicing of precursor mRNAs in the nucleus6-10. circRNAs are predominantly localized in the cytoplasm, indicating that they must be exported from the nucleus. Here we identify a pathway that is specific for the nuclear export of circular RNA. This pathway requires Ran-GTP, exportin-2 and IGF2BP1. Enhancing the nuclear Ran-GTP gradient by depletion or chemical inhibition of the major protein exporter CRM1 selectively increases the nuclear export of circRNAs, while reducing the nuclear Ran-GTP gradient selectively blocks circRNA export. Depletion or knockout of exportin-2 specifically inhibits nuclear export of circRNA. Analysis of nuclear circRNA-binding proteins reveals that interaction between IGF2BP1 and circRNA is enhanced by Ran-GTP. The formation of circRNA export complexes in the nucleus is promoted by Ran-GTP through its interactions with exportin-2, circRNA and IGF2BP1. Our findings demonstrate that adaptors such as IGF2BP1 that bind directly to circular RNAs recruit Ran-GTP and exportin-2 to export circRNAs in a mechanism that is analogous to protein export, rather than mRNA export.

A bipartite function of ESRRB can integrate signaling over time to balance self-renewal and differentiation.

Cooperative DNA binding of transcription factors (TFs) integrates the cellular context to support cell specification during development. Naive mouse embryonic stem cells are derived from early development and can sustain their pluripotent identity indefinitely. Here, we ask whether TFs associated with pluripotency evolved to directly support this state or if the state emerges from their combinatorial action. NANOG and ESRRB are key pluripotency factors that co-bind DNA. We find that when both factors are expressed, ESRRB supports pluripotency. However, when NANOG is absent, ESRRB supports a bistable culture of cells with an embryo-like primitive endoderm identity ancillary to pluripotency. The stoichiometry between NANOG and ESRRB allows quantitative titration of this differentiation, and in silico modeling of bipartite ESRRB activity suggests it safeguards plasticity in differentiation. Thus, the concerted activity of cooperative TFs can transform their effect to sustain intermediate cell identities and allow ex vivo expansion of immortal stem cells. A record of this paper's transparent peer review process is included in the supplemental information.

Enhancers are activated by p300/CBP activity-dependent PIC assembly, RNAPII recruitment, and pause release.

The metazoan-specific acetyltransferase p300/CBP is involved in activating signal-induced, enhancer-mediated transcription of cell-type-specific genes. However, the global kinetics and mechanisms of p300/CBP activity-dependent transcription activation remain poorly understood. We performed genome-wide, time-resolved analyses to show that enhancers and super-enhancers are dynamically activated through p300/CBP-catalyzed acetylation, deactivated by the opposing deacetylase activity, and kinetic acetylation directly contributes to maintaining cell identity at very rapid (minutes) timescales. The acetyltransferase activity is dispensable for the recruitment of p300/CBP and transcription factors but essential for promoting the recruitment of TFIID and RNAPII at virtually all enhancers and enhancer-regulated genes. This identifies pre-initiation complex assembly as a dynamically controlled step in the transcription cycle and reveals p300/CBP-catalyzed acetylation as the signal that specifically promotes transcription initiation at enhancer-regulated genes. We propose that p300/CBP activity uses a "recruit-and-release" mechanism to simultaneously promote RNAPII recruitment and pause release and thereby enables kinetic activation of enhancer-mediated transcription.

Dynamic lineage priming is driven via direct enhancer regulation by ERK.

Central to understanding cellular behaviour in multi-cellular organisms is the question of how a cell exits one transcriptional state to adopt and eventually become committed to another. Fibroblast growth factor-extracellular signal-regulated kinase (FGF -ERK) signalling drives differentiation of mouse embryonic stem cells (ES cells) and pre-implantation embryos towards primitive endoderm, and inhibiting ERK supports ES cell self-renewal1. Paracrine FGF-ERK signalling induces heterogeneity, whereby cells reversibly progress from pluripotency towards primitive endoderm while retaining their capacity to re-enter self-renewal2. Here we find that ERK reversibly regulates transcription in ES cells by directly affecting enhancer activity without requiring a change in transcription factor binding. ERK triggers the reversible association and disassociation of RNA polymerase II and associated co-factors from genes and enhancers with the mediator component MED24 having an essential role in ERK-dependent transcriptional regulation. Though the binding of mediator components responds directly to signalling, the persistent binding of pluripotency factors to both induced and repressed genes marks them for activation and/or reactivation in response to fluctuations in ERK activity. Among the repressed genes are several core components of the pluripotency network that act to drive their own expression and maintain the ES cell state; if their binding is lost, the ability to reactivate transcription is compromised. Thus, as long as transcription factor occupancy is maintained, so is plasticity, enabling cells to distinguish between transient and sustained signals. If ERK signalling persists, pluripotency transcription factor levels are reduced by protein turnover and irreversible gene silencing and commitment can occur.

An automated microfluidic device for time-lapse imaging of mouse embryonic stem cells.

Long-term, time-lapse imaging studies of embryonic stem cells (ESCs) require a controlled and stable culturing environment for high-resolution imaging. Microfluidics is well-suited for such studies, especially when the media composition needs to be rapidly and accurately altered without disrupting the imaging. Current studies in plates, which can only add molecules at the start of an experiment without any information on the levels of endogenous signaling before the exposure, are incompatible with continuous high-resolution imaging and cell-tracking. Here, we present a custom designed, fully automated microfluidic chip to overcome these challenges. A unique feature of our chip includes three-dimensional ports that can connect completely sealed on-chip valves for fluid control to individually addressable cell culture chambers with thin glass bottoms for high-resolution imaging. We developed a robust protocol for on-chip culturing of mouse ESCs for minimum of 3 days, to carry out experiments reliably and repeatedly. The on-chip ESC growth rate was similar to that on standard culture plates with same initial cell density. We tested the chips for high-resolution, time-lapse imaging of a sensitive reporter of ESC lineage priming, Nanog-GFP, and HHex-Venus with an H2B-mCherry nuclear marker for cell-tracking. Two color imaging of cells was possible over a 24-hr period while maintaining cell viability. Importantly, changing the media did not affect our ability to track individual cells. This system now enables long-term fluorescence imaging studies in a reliable and automated manner in a fully controlled microenvironment.

Time-Resolved Analysis Reveals Rapid Dynamics and Broad Scope of the CBP/p300 Acetylome.

The acetyltransferases CBP and p300 are multifunctional transcriptional co-activators. Here, we combined quantitative proteomics with CBP/p300-specific catalytic inhibitors, bromodomain inhibitor, and gene knockout to reveal a comprehensive map of regulated acetylation sites and their dynamic turnover rates. CBP/p300 acetylates thousands of sites, including signature histone sites and a multitude of sites on signaling effectors and enhancer-associated transcriptional regulators. Time-resolved acetylome analyses identified a subset of CBP/p300-regulated sites with very rapid (<30 min) acetylation turnover, revealing a dynamic balance between acetylation and deacetylation. Quantification of acetylation, mRNA, and protein abundance after CBP/p300 inhibition reveals a kinetically competent network of gene expression that strictly depends on CBP/p300-catalyzed rapid acetylation. Collectively, our in-depth acetylome analyses reveal systems attributes of CBP/p300 targets, and the resource dataset provides a framework for investigating CBP/p300 functions and for understanding the impact of small-molecule inhibitors targeting its catalytic and bromodomain activities.

Insulin fine-tunes self-renewal pathways governing naive pluripotency and extra-embryonic endoderm.

Signalling downstream of Activin/Nodal (ActA) and Wnt can induce endoderm differentiation and also support self-renewal in pluripotent cells. Here we find that these apparently contradictory activities are fine-tuned by insulin. In the absence of insulin, the combination of these cytokines supports endoderm in a context-dependent manner. When applied to naive pluripotent cells that resemble peri-implantation embryos, ActA and Wnt induce extra-embryonic primitive endoderm (PrE), whereas when applied to primed pluripotent epiblast stem cells (EpiSC), these cytokines induce gastrulation-stage embryonic definitive endoderm. In naive embryonic stem cell culture, we find that insulin complements LIF signalling to support self-renewal; however, when it is removed, LIF, ActA and Wnt signalling not only induce PrE differentiation, but also support its expansion. Self-renewal of these PrE cultures is robust and, on the basis of gene expression, these cells resemble early blastocyst-stage PrE, a naive endoderm state able to make both visceral and parietal endoderm.

Surveillance for Secure Differentiation.

The precise place and time where embryonic differentiation begins is regulated by regionalized signaling. In this issue of Cell Stem Cell, Wang et al. (2017) investigate how converging Wnt and Nodal signals promote mesendoderm through a p53, Wnt3 feed-forward loop, pointing to a mechanism by which cells gauge time and fitness during differentiation.

Erk signaling suppresses embryonic stem cell self-renewal to specify endoderm.

Fgf signaling via Erk activation has been associated with both neural induction and the generation of a primed state for the differentiation of embryonic stem cells (ESCs) to all somatic lineages. To dissect the role of Erk in both ESC self-renewal and lineage specification, we explored the requirements for this pathway in various in vitro differentiation settings. A combination of pharmacological inhibition of Erk signaling and genetic loss of function reveal a role for Erk signaling in endodermal, but not neural differentiation. Neural differentiation occurs normally despite a complete block to Erk phosphorylation. In support of this, Erk activation in ESCs derepresses primitive endoderm (PrE) gene expression as a consequence of inhibiting the pluripotent/epiblast network. The early response to Erk activation correlates with functional PrE priming, whereas sustained Erk activity results in PrE differentiation. Taken together, our results suggest that Erk signaling suppresses pluripotent gene expression to enable endodermal differentiation.

ERK2 suppresses self-renewal capacity of embryonic stem cells, but is not required for multi-lineage commitment.

Activation of the FGF-ERK pathway is necessary for naïve mouse embryonic stem (ES) cells to exit self-renewal and commit to early differentiated lineages. Here we show that genetic ablation of Erk2, the predominant ERK isozyme expressed in ES cells, results in hyper-phosphorylation of ERK1, but an overall decrease in total ERK activity as judged by substrate phosphorylation and immediate-early gene (IEG) induction. Normal induction of this subset of canonical ERK targets, as well as p90RSK phosphorylation, was rescued by transgenic expression of either ERK1 or ERK2 indicating a degree of functional redundancy. In contrast to previously published work, Erk2-null ES cells exhibited no detectable defect in lineage specification to any of the three germ layers when induced to differentiate in either embryoid bodies or in defined neural induction conditions. However, under self-renewing conditions Erk2-null ES cells express increased levels of the pluripotency-associated transcripts, Nanog and Tbx3, a decrease in Nanog-GFP heterogeneity, and exhibit enhanced self-renewal in colony forming assays. Transgenic add-back of ERK2 is capable of restoring normal pluripotent gene expression and self-renewal capacity. We show that ERK2 contributes to the destabilization of ES cell self-renewal by reducing expression of pluripotency genes, such as Nanog, but is not specifically required for the early stages of germ layer specification.