Project description:Previous unbiased screening for the nucleosome binding affinity of transcription factors predicts that POU domain transcription factors have strong nucleosome binding potential. Here using reprogramming strategy we found that POU4F3 has pioneer factor activity at ATOH1 target elements that require POU4F3 for their accessibility.
Project description:During embryonic development, hierarchical cascades of transcription factors interact with lineage specific chromatin structures to control the sequential steps in the differentiation of specialized cell types. While examples of transcription factor cascades have been well documented, the mechanisms underlying developmental changes in accessibility of cell type-specific enhancers remain poorly understood. Here we show that the transcriptional ‘master regulator’, ATOH1 – which is necessary for the differentiation of two distinct mechanoreceptor cell types, hair cells in the inner ear, and Merkel cells of the epidermis – is unable to access much of its target enhancer network in the progenitor populations of either cell type when it first appears, imposing a block to further differentiation. This block is overcome by a feed-forward mechanism in which ATOH1 first stimulates expression of POU4F3, which subsequently acts as a pioneer factor to provide access to closed ATOH1 enhancers, allowing hair cell and Merkel cell differentiation to proceed. Our analysis also indicates the presence of both shared and divergent ATOH1/POU4F3-dependent, enhancer networks in hair cells and Merkel cells. These cells share a deep developmental lineage relationship, deriving from their common epidermal origin, and suggesting that this feed-forward mechanism preceded the evolutionary divergence of these very different mechanoreceptive cell types.
Project description:The role of POU4F3 in Merkel cell maturation, but not induction, is similar to its role in hair cells. This prompted us to investigate whether POU4F3 also regulates the accessibility of ATOH1 targets in a feed-forward manner during the maturation of Merkel cells. Our results suggest that, just as in hair cells, the pioneer factor activity of POU4F3 is also required for ATOH1 to correctly coordinate mechanosensory differentiation in Merkel cells.
Project description:Hearing and balance are mediated in vertebrates by inner ear mechanosensory hair cells. Hair cell development, maturation and survival require POU4F3, a class IV POU domain transcription factor that is expressed in hair cells shortly after they start to differentiate. Here, we show that POU4F3 has pioneer factor activity, and is necessary to promote chromatin accessibility at approximately half of the distal regulatory elements that appear as hair cells differentiate. Pou4f3 expression in the inner ear is initiated by ATOH1, a bHLH transcription factor that is also necessary for hair cell differentiation and survival. We show that ATOH1 also binds a large number of POU4F3-dependent distal regulatory elements, suggesting a synergistic feed-forward model in which ATOH1 induces POU4F3, which then acts to promote ATOH1-dependent hair cell differentiation.
Project description:GATA4 is a pioneer transcription factor. The mechanisms by which pioneer factors select and occupy specific loci, open chromatin, and activate enhancers is not well understood. To address the influence of partner, non-pioneer factors on pioneer factor activity, we analyzed the effect of NKX2-5 or ETS1 on GATA4 pioneer activity. NIH3T3 cells were transduced with lentivirus to obtain stable cell lines that express GFP (control), GATA4 (G), NKX2-5 (N), ETS1 (E), GATA4+NKX2-5 (GN), and GATA4+ETS1 (GE). We measured chromatin accessibility using ATAC-seq, and enhancer activation using H3K27ac. From these measurements we determined GATA4, GATA4+ETS, and GATA4+NKX2-5 pioneer binding, pioneer opening, and pioneer enhancer activation. Our results show that ETS1 or NKX2-5 alter GATA4 pioneer binding, opening, and enhancer activation.
Project description:We report application of RNA-seq for transcriptomic analyses of cochlear tissues from wildtype and DFNA15 deafness mouse model Pou4f3(Δ/+) to reveal gene expressions altered by Pou4f3 mutation
Project description:We and others have suggested that pioneer activity–a transcription factor’s (TF’s) ability to bind and open inaccessible loci–is not a qualitative trait limited to a select class of pioneer TFs. We hypothesize that most TFs display pioneering activity that depends on the TF concentration and the motif content at their target loci. Here we present a quantitative measure of pioneer activity that captures the relative difference in a TF’s ability to bind accessible versus inaccessible DNA. The metric is based on experiments that use CUT&Tag to measure binding of doxycycline (dox) inducible TFs. For each location across the genome we determine a “dox50,” the concentration of dox required for a TF to reach half-maximal occupancy. We propose that the ratio of a TF’s average dox50 between ATAC-seq labeled inaccessible and accessible binding sites, its Δdox50, is a measure of its pioneer activity. We measured Δdox50’s for the endodermal TFs FOXA1 and HNF4A and show that HNF4A has a smaller Δdox50 than FOXA1, suggesting that HNF4A has stronger pioneer activity than FOXA1. We further show that FOXA1 binding sites with more copies of its motif have a lower Δdox50, suggesting that strong motif content may compensate for weak pioneer activity. Our results suggest that Δdox50s, or other similar measures that assess the difference in TF affinity for inaccessible and accessible DNA, are reasonable measures of pioneer activity.