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Key Points Repressive epigenetic marks (such as DNA methylation, dimethylation of histone H3 at lysine 9 (H3K9me2) and H3K27me3) are dispensable for pre-implantation embryonic development and naive pluripotency. DNA methylation patterns change through development: naive pluripotency, pre-implantation epiblasts and primordial germ cells are associated with global DNA demethylation, whereas post-implantation epiblasts and epiblast-derived stem cells (EpiSCs) have high levels of DNA methylation. Dynamic chromatin interactions occur during development and stem cell differentiation. Topologically associated domains (TADs) are largely stable during stem cell differentiation but sub-TAD interactions might change. Enhancer activation is initiated by pioneer transcription factors and is often followed by H3K4me1 deposition (enhancer priming) and H3K27 acetylation (enhancer activation). H3K27me3 and H3K4me3 promoter bivalency is observed in stem cells and in differentiated cells. Bivalency has a prominent role in post-implantation embryonic development. DNA methylation and H3K27me3 are mutually exclusive at CpG-rich promoters: DNA methylation prevents deposition of the H3K27me3 mark. Exit from naive pluripotency and embryonic stem cell differentiation is accompanied by progressive restriction of chromatin accessibility and histone acetylation. Cell state transitions during embryonic development are associated with epigenetic changes that alter chromatin structure and gene expression. Interplay between epigenetic regulatory layers can be studied using genomic technologies and embryonic stem cell cultures that reflect in vivo cell states. Chromatin, the template for epigenetic regulation, is a highly dynamic entity that is constantly reshaped during early development and differentiation. Epigenetic modification of chromatin provides the necessary plasticity for cells to respond to environmental and positional cues, and enables the maintenance of acquired information without changing the DNA sequence. The mechanisms involve, among others, chemical modifications of chromatin, changes in chromatin constituents and reconfiguration of chromatin interactions and 3D structure. New advances in genome-wide technologies have paved the way towards an integrative view of epigenome dynamics during cell state transitions, and recent findings in embryonic stem cells highlight how the interplay between different epigenetic layers reshapes the transcriptional landscape.

The pluripotency of mammalian early and late epiblast could be recapitulated by naïve embryonic stem cells (ESCs) and primed epiblast stem cells (EpiSCs), respectively. However, these two states of pluripotency may not be sufficient to reflect the full complexity and developmental potency of the epiblast during mammalian early development. Here we report the establishment of self-renewing formative pluripotent stem cells (fPSCs) which manifest features of epiblast cells poised for gastrulation. fPSCs can be established from different mouse ESCs, pre-/early-gastrula epiblasts and induced PSCs. Similar to pre-/early-gastrula epiblasts, fPSCs show the transcriptomic features of formative pluripotency, which are distinct from naïve ESCs and primed EpiSCs. fPSCs show the unique epigenetic states of E6.5 epiblast, including the super-bivalency of a large set of developmental genes. Just like epiblast cells immediately before gastrulation, fPSCs can efficiently differentiate into three germ layers and primordial germ cells (PGCs) in vitro. Thus, fPSCs highlight the feasibility of using PSCs to explore the development of mammalian epiblast.

Combinatorial screening of developmental factors improves directed differentiation of human pluripotent stem cells into neuronal subtypes. Specification of cell identity during development depends on exposure of cells to sequences of extrinsic cues delivered at precise times and concentrations. Identification of combinations of patterning molecules that control cell fate is essential for the effective use of human pluripotent stem cells (hPSCs) for basic and translational studies. Here we describe a scalable, automated approach to systematically test the combinatorial actions of small molecules for the targeted differentiation of hPSCs. Applied to the generation of neuronal subtypes, this analysis revealed an unappreciated role for canonical Wnt signaling in specifying motor neuron diversity from hPSCs and allowed us to define rapid (14 days), efficient procedures to generate spinal and cranial motor neurons as well as spinal interneurons and sensory neurons. Our systematic approach to improving hPSC-targeted differentiation should facilitate disease modeling studies and drug screening assays.

Single-cell measurement of chromatin states, including histone modifications and non-histone protein binding, remains challenging. Here, we present a low-cost, efficient, simultaneous indexing and tagmentation-based ChIP-seq (itChIP-seq) method, compatible with both low cellular input and single cells for profiling chromatin states. itChIP combines chromatin opening, simultaneous cellular indexing and chromatin tagmentation within a single tube, enabling the processing of samples from tens of single cells to, more commonly, thousands of single cells per assay. We demonstrate that single-cell itChIP-seq (sc-itChIP-seq) yields ~9,000 unique reads per cell. Using sc-itChIP-seq to profile H3K27ac, we sufficiently capture the earliest epigenetic priming event during the cell fate transition from naive to primed pluripotency, and reveal the basis for cell-type specific enhancer usage during the differentiation of bipotent cardiac progenitor cells into endothelial cells and cardiomyocytes. Our results demonstrate that itChIP is a widely applicable technology for single-cell chromatin profiling of epigenetically heterogeneous cell populations in many biological processes. He and colleagues develop itChIP-seq based on simultaneous cellular indexing and chromatin tagmentation. itChIP-seq is applicable to both low-input and single-cell analyses of chromatin states.

Following implantation, the naive pluripotent epiblast of the mouse blastocyst generates a rosette, undergoes lumenogenesis and forms the primed pluripotent egg cylinder, which is able to generate the embryonic tissues. How pluripotency progression and morphogenesis are linked and whether intermediate pluripotent states exist remain controversial. We identify here a rosette pluripotent state defined by the co-expression of naive factors with the transcription factor OTX2. Downregulation of blastocyst WNT signals drives the transition into rosette pluripotency by inducing OTX2. The rosette then activates MEK signals that induce lumenogenesis and drive progression to primed pluripotency. Consequently, combined WNT and MEK inhibition supports rosette-like stem cells, a self-renewing naive-primed intermediate. Rosette-like stem cells erase constitutive heterochromatin marks and display a primed chromatin landscape, with bivalently marked primed pluripotency genes. Nonetheless, WNT induces reversion to naive pluripotency. The rosette is therefore a reversible pluripotent intermediate whereby control over both pluripotency progression and morphogenesis pivots from WNT to MEK signals.Neagu, van Genderen and Escudero et al. show that simultaneous inhibition of WNT and MEK signalling maintains a naive-primed intermediate pluripotency state in vitro, which displays features of the mouse embryonic rosette.

A new strategy for derivation of human midbrain dopamine neurons from pluripotent cells was developed; transplantation of the neurons in mice, rats and parkinsonian monkeys show they are a promising source of cells for applications in regenerative medicine. Repairing parkinsonian tissue Lorenz Studer and colleagues have developed a new strategy for the efficient derivation of human midbrain dopamine (DA) neurons from pluripotent stem cells. The DA neurons showed functionality in vivo and achieved long-term engraftment in three Parkinson's disease model systems (6-OHDA-lesioned mice and rats, and transplantation into parkinsonian monkeys). The DA neurons are a promising source of cells for applications in regenerative medicine. Human pluripotent stem cells (PSCs) are a promising source of cells for applications in regenerative medicine. Directed differentiation of PSCs into specialized cells such as spinal motoneurons 1 or midbrain dopamine (DA) neurons 2 has been achieved. However, the effective use of PSCs for cell therapy has lagged behind. Whereas mouse PSC-derived DA neurons have shown efficacy in models of Parkinson’s disease 3 , 4 , DA neurons from human PSCs generally show poor in vivo performance 5 . There are also considerable safety concerns for PSCs related to their potential for teratoma formation or neural overgrowth 6 , 7 . Here we present a novel floor-plate-based strategy for the derivation of human DA neurons that efficiently engraft in vivo , suggesting that past failures were due to incomplete specification rather than a specific vulnerability of the cells. Midbrain floor-plate precursors are derived from PSCs 11 days after exposure to small molecule activators of sonic hedgehog (SHH) and canonical WNT signalling. Engraftable midbrain DA neurons are obtained by day 25 and can be maintained in vitro for several months. Extensive molecular profiling, biochemical and electrophysiological data define developmental progression and confirm identity of PSC-derived midbrain DA neurons. In vivo survival and function is demonstrated in Parkinson’s disease models using three host species. Long-term engraftment in 6-hydroxy-dopamine-lesioned mice and rats demonstrates robust survival of midbrain DA neurons derived from human embryonic stem (ES) cells, complete restoration of amphetamine-induced rotation behaviour and improvements in tests of forelimb use and akinesia. Finally, scalability is demonstrated by transplantation into parkinsonian monkeys. Excellent DA neuron survival, function and lack of neural overgrowth in the three animal models indicate promise for the development of cell-based therapies in Parkinson’s disease.

Early enhancers revealed Identifying the genomic regulatory sequences, such as enhancers, that control early embryonic development remains a difficult challenge. Profiling of histone modifications and chromatin regulators in human embryonic stem cells now reveals unique signatures that are used to identify more than 2,000 putative enhancers. These enhancers are either active in the embryonic stem cells or are associated with early developmental genes. Identifying the genomic regulatory sequences, such as enhancers, that control early embryonic development remains a difficult challenge. Here, profiling of histone modifications and chromatin regulators in human embryonic stem cells (hESCs) reveals unique signatures that are used to identify over 2,000 putative enhancers. These enhancers are either active in the h ESCs or associated with early developmental genes. Cell-fate transitions involve the integration of genomic information encoded by regulatory elements, such as enhancers, with the cellular environment 1 , 2 . However, identification of genomic sequences that control human embryonic development represents a formidable challenge 3 . Here we show that in human embryonic stem cells (hESCs), unique chromatin signatures identify two distinct classes of genomic elements, both of which are marked by the presence of chromatin regulators p300 and BRG1, monomethylation of histone H3 at lysine 4 (H3K4me1), and low nucleosomal density. In addition, elements of the first class are distinguished by the acetylation of histone H3 at lysine 27 (H3K27ac), overlap with previously characterized hESC enhancers, and are located proximally to genes expressed in hESCs and the epiblast. In contrast, elements of the second class, which we term ‘poised enhancers’, are distinguished by the absence of H3K27ac, enrichment of histone H3 lysine 27 trimethylation (H3K27me3), and are linked to genes inactive in hESCs and instead are involved in orchestrating early steps in embryogenesis, such as gastrulation, mesoderm formation and neurulation. Consistent with the poised identity, during differentiation of hESCs to neuroepithelium, a neuroectoderm-specific subset of poised enhancers acquires a chromatin signature associated with active enhancers. When assayed in zebrafish embryos, poised enhancers are able to direct cell-type and stage-specific expression characteristic of their proximal developmental gene, even in the absence of sequence conservation in the fish genome. Our data demonstrate that early developmental enhancers are epigenetically pre-marked in hESCs and indicate an unappreciated role of H3K27me3 at distal regulatory elements. Moreover, the wealth of new regulatory sequences identified here provides an invaluable resource for studies and isolation of transient, rare cell populations representing early stages of human embryogenesis.

Dynamic changes in mechanical microenvironments, such as cell crowding, regulate lineage fates as well as cell proliferation. Although regulatory mechanisms for contact inhibition of proliferation have been extensively studied, it remains unclear how cell crowding induces lineage specification. Here we found that a well-known oncogene, ETS variant transcription factor 4 (ETV4), serves as a molecular transducer that links mechanical microenvironments and gene expression. In a growing epithelium of human embryonic stem cells, cell crowding dynamics is translated into ETV4 expression, serving as a pre-pattern for future lineage fates. A switch-like ETV4 inactivation by cell crowding derepresses the potential for neuroectoderm differentiation in human embryonic stem cell epithelia. Mechanistically, cell crowding inactivates the integrin–actomyosin pathway and blocks the endocytosis of fibroblast growth factor receptors (FGFRs). The disrupted FGFR endocytosis induces a marked decrease in ETV4 protein stability through ERK inactivation. Mathematical modelling demonstrates that the dynamics of cell density in a growing human embryonic stem cell epithelium precisely determines the spatiotemporal ETV4 expression pattern and, consequently, the timing and geometry of lineage development. Our findings suggest that cell crowding dynamics in a stem cell epithelium drives spatiotemporal lineage specification using ETV4 as a key mechanical transducer. Yang, Golkaram et al. reported that in human embryonic stem cells, cellular crowding leads to the blockade of FGFR1 endocytosis, resulting in a decrease in ETV4 expression. This, in turn, derepresses the neuroectoderm fate.

Mutations in CHD7 cause CHARGE syndrome, affecting multiple organs including the inner ear in humans. We investigate how CHD7 mutations affect inner ear development using human pluripotent stem cell-derived organoids as a model system. We find that loss of CHD7 or its chromatin remodeling activity leads to complete absence of hair cells and supporting cells, which can be explained by dysregulation of key otic development-associated genes in mutant otic progenitors. Further analysis of the mutant otic progenitors suggests that CHD7 can regulate otic genes through a chromatin remodeling-independent mechanism. Results from transcriptome profiling of hair cells reveal disruption of deafness gene expression as a potential underlying mechanism of CHARGE-associated sensorineural hearing loss. Notably, co-differentiating CHD7 knockout and wild-type cells in chimeric organoids partially rescues mutant phenotypes by restoring otherwise severely dysregulated otic genes. Taken together, our results suggest that CHD7 plays a critical role in regulating human otic lineage specification and hair cell differentiation. Mutations in the chromatin remodeler CHD7 cause CHARGE syndrome, affecting development of several organs including the inner ear. Here, the authors recapitulated pathogenesis of this disease with human inner ear organoids and found that CHD7 is indispensable for proper otic lineage specification and hair cell differentiation.