Pioneer and PRDM transcription factors coordinate bivalent epigenetic states to safeguard cell fate.

Pioneer transcription factors (TFs) regulate cell fate by establishing transcriptionally primed and active states. However, cell fate control requires the coordination of both lineage-specific gene activation and repression of alternative-lineage programs, a process that is poorly understood. Here, we demonstrate that the pioneer TF FOXA coordinates with PRDM1 TF to recruit nucleosome remodeling and deacetylation (NuRD) complexes and Polycomb repressive complexes (PRCs), which establish highly occupied, accessible nucleosome conformation with bivalent epigenetic states, thereby preventing precocious and alternative-lineage gene expression during human endoderm differentiation. Similarly, the pioneer TF OCT4 coordinates with PRDM14 to form bivalent enhancers and repress cell differentiation programs in human pluripotent stem cells, suggesting that this may be a common and critical function of pioneer TFs. We propose that pioneer and PRDM TFs coordinate to safeguard cell fate through epigenetic repression mechanisms.

Profiling Accessible Chromatin and Nucleosomes in the Mammalian Genome.

Genomic DNA wraps around core histones to form nucleosomes, which provides steric constraints on how transcription factors (TFs) can interact with gene regulatory sequences. It is increasingly apparent that well-positioned, accessible nucleosomes are an inherent feature of active enhancers and can facilitate cooperative TF binding, referred to as nucleosome-mediated cooperativity. Thus, profiling chromatin and nucleosome properties (accessibility, positioning, and occupancy) on the genome is crucial to understand cell-type-specific gene regulation. Here we describe a simplified protocol to profile accessible nucleosomes in the mammalian genome using low-level and high-level micrococcal nuclease (MNase) digestion followed by genome-wide sequencing.

SOX transcription factors direct TCF-independent WNT/ß-catenin responsive transcription to govern cell fate in human pluripotent stem cells.

WNT/ß-catenin signaling controls gene expression across biological contexts from development and stem cell homeostasis to diseases including cancer. How ß-catenin is recruited to distinct enhancers to activate context-specific transcription is unclear, given that most WNT/ß-catenin-responsive transcription is thought to be mediated by TCF/LEF transcription factors (TFs). With time-resolved multi-omic analyses, we show that SOX TFs can direct lineage-specific WNT-responsive transcription during the differentiation of human pluripotent stem cells (hPSCs) into definitive endoderm and neuromesodermal progenitors. We demonstrate that SOX17 and SOX2 are required to recruit ß-catenin to lineage-specific WNT-responsive enhancers, many of which are not occupied by TCFs. At TCF-independent enhancers, SOX TFs establish a permissive chromatin landscape and recruit a WNT-enhanceosome complex to activate SOX/ß-catenin-dependent transcription. Given that SOX TFs and the WNT pathway are critical for specification of most cell types, these results have broad mechanistic implications for the specificity of WNT responses across developmental and disease contexts.

Gene network transitions in embryos depend upon interactions between a pioneer transcription factor and core histones.

Gene network transitions in embryos and other fate-changing contexts involve combinations of transcription factors. A subset of fate-changing transcription factors act as pioneers; they scan and target nucleosomal DNA and initiate cooperative events that can open the local chromatin. However, a gap has remained in understanding how molecular interactions with the nucleosome contribute to the chromatin-opening phenomenon. Here we identified a short α-helical region, conserved among FOXA pioneer factors, that interacts with core histones and contributes to chromatin opening in vitro. The same domain is involved in chromatin opening in early mouse embryos for normal development. Thus, local opening of chromatin by interactions between pioneer factors and core histones promotes genetic programming.

In vitro effects of bisphenol A on developing hypothalamic neurons.

Estradiol plays an essential role in sexual differentiation of the rodent hypothalamus. Endocrine disruptors with estrogenic activity such as bisphenol A (BPA) are reported to disturb sexual differentiation of the hypothalamus. The purpose of the present study was to examine in vitro effects of BPA on developing hypothalamic neurons by focusing on a presynaptic protein synapsin I and microtubule-associated protein 2 (MAP2). In cultured hypothalamic cells from fetal rats, treatment with BPA enhanced both dendritic and synaptic development, as evidenced by increases in the area of dot-like staining of synapsin I and MAP2-positive area. An estrogen receptor (ER) antagonist, ICI 182,780, only partially blocked BPA-induced increase in the synapsin I-area, while it suppressed the MAP2-area increased by BPA. A specific ERK inhibitor, U0126, reduced the synapsin I-area without affecting the MAP2-area. BPA significantly decreased protein levels of synapsin I phosphorylated at Ser-9 and Ser-603. These findings indicate that BPA-inducing effects on dendritic and synaptic development are mediated by different molecular pathways.

Effects of estrogen on synapsin I distribution in developing hypothalamic neurons.

Estradiol (17beta-estradiol, E(2)) plays an essential role in sexual differentiation of the rodent brain. The purpose of the present study was to investigate the effects of E(2) on developing hypothalamic neurons by focusing on a presynaptic protein, synapsin I. We applied E(2) to cultured hypothalamic cells removed from fetal rats and investigated resultant effects upon synapsin I. Our immunocytochemical study revealed that administration of E(2) increased the dendritic area ('MAP2-area') and synaptic area detected as dot-like staining of synapsin I ('synapsin I-area'). However, immunoblotting and real-time PCR showed that E(2) did not increase both protein and mRNA expression levels of synapsin I. Studies with cyclohexamide (CHX), membrane impermeable E(2) (E(2)-BSA), and an estrogen receptor (ER) antagonist ICI 182,780 indicated that E(2) affected the synapsin I-area mainly via a non-genomic pathway mediated by membrane ER. Immunoblotting showed that E(2) suppressed phosphorylation of synapsin I at residues Ser-9, Ser-553, and Ser-603. On the other hand, E(2) did not affect phosphorylation of synapsin I at Ser-62, Ser-67 and Ser-549. The present study suggests that E(2) affects localization of synapsin I in hypothalamic neurons by altering site-specific phosphorylation of synapsin I, which is likely mediated by membrane ER.

Evolution of non-coding regulatory sequences involved in the developmental process: reflection of differential employment of paralogous genes as highlighted by Sox2 and group B1 Sox genes.

In higher vertebrates, the expression of Sox2, a group B1 Sox gene, is the hallmark of neural primordial cell state during the developmental processes from embryo to adult. Sox2 is regulated by the combined action of many enhancers with distinct spatio-temporal specificities. DNA sequences for these enhancers are conserved in a wide range of vertebrate species, corresponding to a majority of highly conserved non-coding sequences surrounding the Sox2 gene, corroborating the notion that the conservation of non-coding sequences mirrors their functional importance. Among the Sox2 enhancers, N-1 and N-2 are activated the earliest in embryogenesis and regulate Sox2 in posterior and anterior neural plates, respectively. These enhancers differ in their evolutionary history: the sequence and activity of enhancer N-2 is conserved in all vertebrate species, while enhancer N-1 is fully conserved only in amniotes. In teleost embryos, Sox19a/b play the major pan-neural role among the group B1 Sox paralogues, while strong Sox2 expression is limited to the anterior neural plate, reflecting the absence of posterior CNS-dedicated enhancers, including N-1. In Xenopus, neurally expressed SoxD is the orthologue of Sox19, but Sox3 appears to dominate other B1 paralogues. In amniotes, however, Sox19 has lost its group B1 Sox function and transforms into group G Sox15 (neofunctionalization), and Sox2 assumes the dominant position by gaining enhancer N-1 and other enhancers for posterior CNS. Thus, the gain and loss of specific enhancer elements during the evolutionary process reflects the change in functional assignment of particular paralogous genes, while overall regulatory functions attributed to the gene family are maintained.