Highlights d Optimal transport analysis recovers trajectories from 315,000 scRNA-seq profiles d Induced pluripotent stem cell reprogramming produces diverse developmental programs d Regulatory analysis identifies a series of TFs predictive of specific cell fates d Transcription factor Obox6 and cytokine GDF9 increase reprogramming efficiency

Concomitant activation of the Wnt pathway and suppression of Mapk signalling by two small molecule inhibitors (2i) in the presence of leukaemia inhibitory factor (LIF) (hereafter termed 2i/L) induces a naive state in mouse embryonic stem (ES) cells that resembles the inner cell mass (ICM) of the pre-implantation embryo. Since the ICM exists only transiently in vivo, it remains unclear how sustained propagation of naive ES cells in vitro affects their stability and functionality. Here we show that prolonged culture of male mouse ES cells in 2i/L results in irreversible epigenetic and genomic changes that impair their developmental potential. Furthermore, we find that female ES cells cultured in conventional serum plus LIF medium phenocopy male ES cells cultured in 2i/L. Mechanistically, we demonstrate that the inhibition of Mek1/2 is predominantly responsible for these effects, in part through the downregulation of DNA methyltransferases and their cofactors. Finally, we show that replacement of the Mek1/2 inhibitor with a Src inhibitor preserves the epigenetic and genomic integrity as well as the developmental potential of ES cells. Taken together, our data suggest that, although short-term suppression of Mek1/2 in ES cells helps to maintain an ICM-like epigenetic state, prolonged suppression results in irreversible changes that compromise their developmental potential

Combining high mass accuracy mass spectrometry, isobaric tagging, and novel software for multiplexed, large-scale protein quantification, we report deep proteomic coverage across multiple biological replicates and cell lines. We applied this method to study four human embryonic stem cell and four induced pluripotent stem cell lines in biological triplicate, a 24-sample comparison resulting in the largest set of identified proteins and phosphorylation sites in pluripotent cells to date. The statistical analysis afforded by this approach revealed, for the first time, subtle but reproducible differences in protein and protein phosphorylation between embryonic stem cells and induced pluripotent cells. Merging these results with RNA-seq analyses, we found functionally related differences across each tier of regulation. Finally, we introduce the Stem Cell–Omics Repository (SCOR), a resource that collates and displays quantitative information across multiple planes of measurement, including mRNA, protein, and post-translational modifications.

Background:The mechanisms by which human PUF proteins repress target mRNAs remain unknown. Results: PUM1 and PUM2 reduce protein and mRNA levels of targets by recruiting the CNOT deadenylase complex and by a poly(A)-independent mechanism. Conclusion: PUMs employ deadenylation-dependent and -independent mechanisms of repression. Significance: Deadenylation is a conserved means of PUF repression but additional mechanism(s) contribute to mRNA regulation.

Cellular differentiation involves profound remodeling of chromatic landscapes, yet the mechanisms by which somatic cell identity is subsequently maintained remain incompletely understood. To further elucidate regulatory pathways that safeguard the somatic state, we performed two comprehensive RNAi screens targeting chromatin factors during transcription factor-mediated reprogramming of mouse fibroblasts to induced pluripotent stem cells (iPSCs). Remarkably, subunits of the chromatin assembly factor-1 (CAF-1) complex emerged as the most prominent hits from both screens, followed by modulators of lysine sumoylation and heterochromatin maintenance. Optimal modulation of both CAF-1 and transcription factor levels increased reprogramming efficiency by several orders of magnitude and facilitated iPSC formation in as little as 4 days. Mechanistically, CAF-1 suppression led to a more accessible chromatin structure at enhancer elements early during reprogramming. These changes were accompanied by a decrease in somatic heterochromatin domains, increased binding of Sox2 to pluripotency-specific targets and activation of associated genes. Notably, suppression of CAF-1 also enhanced the direct conversion of B cells into macrophages and fibroblasts into neurons. Together, our findings reveal the histone chaperone CAF-1 as a novel regulator of somatic cell identity during transcription factor-induced cell fate transitions and provide a potential strategy to modulate cellular plasticity in a regenerative setting.

Epigenetic regulation through chromatin is thought to play a critical role in the establishment and maintenance of pluripotency. Traditionally, antibody-based technologies were used to probe for specific posttranslational modifications (PTMs) present on histone tails, but these methods do not generally reveal the presence of multiple modifications on a single-histone tail (combinatorial codes). Here, we describe technology for the discovery and quantification of histone combinatorial codes that is based on chromatography and mass spectrometry. We applied this methodology to decipher 74 discrete combinatorial codes on the tail of histone H4 from human embryonic stem (ES) cells. Finally, we quantified the abundances of these codes as human ES cells undergo differentiation to reveal striking changes in methylation and acetylation patterns. For example, H4R3 methylation was observed only in the presence of H4K20 dimethylation; such context-specific patterning exemplifies the power of this technique.electron transfer dissociation ͉ epigenetics ͉ posttranslational modification ͉ histone code ͉ pluripotency P luripotency-the ability to differentiate into any specialized lineage-is the hallmark of embryonic stem (ES) cells and the basis for their experimental and therapeutic potential. The precise molecular mechanisms that define pluripotency remain elusive; however, a number of recent works suggest a central role for epigenetic regulation through chromatin (1-6). Either by recruiting or shielding certain factors, modifications on histone proteins modulate a gene's local environment and thereby regulate expression (7-12). Concerted changes in histone modification states occur during differentiation (13). For example, high levels of histone H3 and H4 acetylation are characteristic of pluripotent cells in mice and the abundance of these marks decreases during differentiation (14, 15). Methylation of R2, R17, and R26 of histone H3 by CARM1 also correlates with cell fate and potency (6). Cells with higher levels of methylation, at these residues, were enriched in the embryonic part of the blastocyst. Next, specific patterns of histone H3K4me3 and H3K27me3 are observed at promoter regions of genes that are regulated during differentiation (1, 2). Finally, demethylation of H3K27me3 is required for activation of certain HOX genes essential for proper development (16,17). The demethylase responsible interacts directly with MLL 2/3 complexes, which methylate histone H3K4 (18). Taken together these experiments have shed new light on the power of epigenetic regulation within ES cells; however, the precise details and role(s) of such combinatorial PTM patterns remain largely unknown.Technological limitations in our ability to discover and quantify combinatorial histone PTMs has, and continues to, present a major obstacle. Most of our knowledge of epigenetics has been derived by antibody-based approaches. Antibodies require a priori knowledge of individual modifications, are subject to epitope occlusion, and have difficulty distingu...

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SUMMARY Blastocyst-derived embryonic stem cells (ESCs) and gonad-derived embryonic germ cells (EGCs) represent two classic types of pluripotent cell lines, yet their molecular equivalence remains incompletely understood. Here, we compare genome-wide methylation patterns between isogenic ESC and EGC lines to define epigenetic similarities and differences. Surprisingly, we find that sex rather than cell type drives methylation patterns in ESCs and EGCs. Cell fusion experiments further reveal that the ratio of X-chromosomes to autosomes dictates methylation levels, with female hybrids being hypomethylated and male hybrids being hypermethylated. We show that the X-linked MAPK phosphatase DUSP9 is upregulated in female compared to male ESCs, and its heterozygous loss in female ESCs leads to male-like methylation levels. Notably, male and female blastocysts are similarly hypomethylated, indicating that sex-specific methylation differences arise in culture. Collectively, our data demonstrate the epigenetic similarity of sex-matched ESCs and EGCs and identify DUSP9 as a regulator of female-specific hypomethylation.