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Concentration- and co-factor-dependent Runx DNA binding site choices during early T-development

ABSTRACT: Runx1 and Runx3 function redundantly in early T-development, and together drive T-lineage developmental progression by regulating distinct sets of genes in different stages. Context-specific Runx target genes are particularly enriched near Runx binding sites that dynamically shift from pre-T-commitment stages (Phase 1) to post-T-commitment stages (Phase 2). As total Runx activities (Runx1+Runx3) are maintained stably throughout early T-development stages, yet Runx factors physically interact with multiple collaborators, we hypothesized that different Runx-collaborating transcription factors compete to recruit a limited pool of Runx transcription factors. To test whether increasing Runx availability can alter Runx DNA-binding site choices, Runx1 overexpression vectors were introduced to two different systems. First, we increased Runx1 expression in a DN3-like cell line (representing a post-commitment, Phase 2 stage) in the presence or absence of exogenous Phase 1 co-factor, PU.1. Second, we increased Runx1 expression in Phase 1 primary cells which naturally express PU.1 and other Phase 1 collaborators. Then, Runx1 binding behaviors were measured using ChIP-seq or CUT&RUN (C&R). A modest increase of Runx1 levels (about 2-3 fold increase) substantially increased the number of Runx binding sites seen and the intensity of occupancy in both systems. When Runx expression was at physiological levels, PU.1 dominated Runx1 site choice before T commitment by recruiting Runx1 to PU.1 sites. The Phase 2 cell line system showed that it did this while depleting Runx1 from alternative high quality Runx motif sites. However, when Runx1 was overexpressed, Runx1 was still recruited to PU.1 sites, but this recruitment did not evacuate Runx1 occupancy from default preferred sites. Notably, Runx1 overexpression in primary Phase 1 cells caused precocious occupancy of post-commitment, Phase 2-specific sites. We found that these are often co-occupied with TCF1, E2A, and HEB, but have minimal co-binding with PU.1. In addition, Runx1 overexpression resulted in new binding sites that were not normally observed in pro-T cells, which are mostly sequestered by closed chromatin normally although they harbor more numerous Runx motifs. Thus, these data suggest that Runx DNA binding site choices are sensitive to Runx concentration and co-factors during early T-development.

ORGANISM(S): Mus musculus

PROVIDER: GSE218147 | GEO | 2023/07/09

REPOSITORIES: GEO

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Project description:Runx transcription factors play pivotal roles in the development of hematopoietic cells, including T cells. However, the Runx-target genes in early stages of thymic T cell development have been only partially elucidated. Moreover, the relationships of different Runx paralogs, whether they compete or collaborate with each other, are unknown. We have performed CRISPR-Cas9 mediated acute deletion of Runx1 and Runx3, alone and in combination, to define the functions of Runx factors and their target genes. We focused on two distinct stages of early T cell development; before lineage commitment (Phase1) and after commitment (Phase2). Our data suggest that Runx1 and Runx3 are functionally redundant in Phase1, hence the absence of one factor was compensated by the other factor. In Phase2, Runx1 becomes a major contributor, while low levels of Runx3 still strengthened the Runx1-mediated target gene regulation and showed an additive effect. Of importance, we found that Runx1 and Runx3 upregulate T cell programs and inhibit alternative lineage genes. In order to capture the developmental status of Runx-perturbed cells, we performed supervised principal component analysis (PCA) using a fixed frame of PC loadings from single cell RNA-seq analysis of normal in vivo-derived thymocytes. Unlike the reference in vitro and in vivo pro-T cell population or sgControl introduced cells, Runx knockout cells either failed to progress fully (Phase1) or deflected from the normal developmental trajectory (Phase2), suggesting essential roles of Runx factors in early stages of T development. In addition, we analyzed the correlation between stage-specific genomic occupancy of Runx1 and Runx3 (using previously published data for Runx1, GSE103953) and Runx-mediated gene regulation responses at each stage. Interestingly, genes with dynamic, stage-sensitive expression showed a very significant association with phase-specific genomic interactions of Runx factors, and an especially strong correlation with Runx activated target genes. In summary, our data suggest a pivotal role for Runx transcription factors in thymic T cell development by imposing T lineage specification in Phase1 and instructing the lineage-committed progenitor cells to progress through a normal T developmental trajectory.

Project description:A major problem with linking transcription factor binding to function is that many factors bind to a large number of, at least in any given cell type, seemingly irrelevant regions. This makes it hard to filter out which binding sites are responsible for the regulation of a given gene. PU.1 (Spi1, Sfpi1) is an excellent example of a transcription factor that works both to mediate developmental choices and to serve the alternative developmental fates that emerge from these choices. Its role in T-cell development is confined to the early stages where high PU.1 expression persists through multiple cell divisions and is sharply downregulated over the DN2a-to-DN2b T-cell commitment stage. Even though it is known that PU.1 is necessary for the survival of the earliest T-cell progenitors, where it is needed for optimal proliferation, control of alternative lineage genes, and correct timing of the access to T-lineage genes, it has not been well-studied how PU.1 finds it targets and exerts its functions within this context. Here, we show in detail how PU.1 selects its binding sites and subsequently regulate gene expression. We show that PU.1 has two effective affinity thresholds for occupancy depending on current chromatin openness as measured by ATAC-sequencing, but that it is easily capable of binding sites in closed chromatin. Unexpectedly, although its binding to promoters is least constrained, promoters are the only major class of sites where it exerts a predominantly negative effect; otherwise it works locally as an activator, mainly mediated through binding to sites in closed or dynamically closing distal enhancer elements, where it can rapidly open chromatin and induce histone acetylation. However, its ability to open the chromatin depends not only on its own affinity but also on the presence of collaborating factors, because we show that PU.1 introduction into PU.1-negative cells triggers a massive reorganization of occupancy patterns of at least three other factors: Runx1, Satb1, and Gata3. Most strikingly, PU.1’s “theft” of Runx1 and Satb1 from many sites where they were binding in the absence of PU.1 enables PU.1 to exert a novel form of repression even on genes where it has no binding sites itself. We show here that PU.1 requires domains outside of its DNA binding domain to properly open chromatin, and this structural requirement is directly connected with its ability to bind to Runx1 and to Satb1, and moreover, Runx1, in particular, is an important collaborator to activate many of its target genes in the context of early T-cell development. Thus, PU.1 regulates gene expression via two distinct mechanisms. First, PU.1 steals Satb1 and Runx1 from many genomic sites, thereby repressing T cell gene expression indirectly. Second, PU.1, opens chromatin, recruits Satb1 and Runx1 to new sites, and directly activates its target gene expression. In summary, we here present a model where a transcription factor can work through redeployment of other factors and not only through sites that it binds itself.

Project description:Runx transcription factors are essential for generating functional T cells starting from the earliest stages in thymic T-development. To evaluate how Runx factors instruct T cell development, we perturbed Runx activities using 1) CRISPR-Cas9, simultaneously deleting two predominant, redundant Runx factors, Runx1 and Runx3, and 2) Runx1-overepxression at a stage before pro-T cells commit to a T cell fate. Then single-cell transcriptome analysis was performed. Runx1/Runx3 knockout (KO) vs. Runx1 overexpression (OE) resulted in contrasting deviations from the normal developmental trajectory. The separation of both perturbation conditions from the controls implied that Runx factors regulate gene networks in individual cells to control separate pathways instead of changing distributions of cells among different stages along the control pathway. Notably, a common core set of genes responded to both KO and OE, in opposite directions. Runx activation target genes were selectively enriched for T-identity program and innate lymphoid cell program genes, whereas inhibition targets were significantly associated with stem/progenitor program and myeloid lineage pathways. Pseudotime inference analysis suggested that Runx perturbations also substantially regulate developmental speed, as Runx KO significantly slowed down developmental progression, whereas Runx1 OE resulted in pronounced acceleration. This interpretation was largely supported by Runx activities regulating key transcription factors involved in establishing distinct gene network modules during T cell development. Indeed, further analysis showed that Runx target genes get combinatorial inputs from these Runx-regulated transcription factors as well as from Runx factors themselves. Together, our data suggest that Runx transcription factors drive early T developmental progression by launching gene expression modules associated with T-identity and innate lymphoid cells through mediating other transcription factor cooperativities.

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Concentration- and co-factor-dependent Runx DNA binding site choices during early T-development

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