Hoogenkamp M, Lichtinger M, Krysinska H, Lancrin C, Clarke D, Williamson A et al.. Early chromatin unfolding by RUNX1: a molecular explanation for differential requirements during specification versus maintenance of the hematopoietic gene expression program. Blood 114: 299-309

Leeds Institute for Molecular Medicine, University of Leeds, Leeds, United Kingdom.
Blood (Impact Factor: 10.45). 05/2009; 114(2):299-309. DOI: 10.1182/blood-2008-11-191890
Source: PubMed


At the cellular level, development progresses through successive regulatory states, each characterized by their specific gene expression profile. However, the molecular mechanisms regulating first the priming and then maintenance of gene expression within one developmental pathway are essentially unknown. The hematopoietic system represents a powerful experimental model to address these questions and here we have focused on a regulatory circuit playing a central role in myelopoiesis: the transcription factor PU.1, its target gene colony-stimulating-factor 1 receptor (Csf1r), and key upstream regulators such as RUNX1. We find that during ontogeny, chromatin unfolding precedes the establishment of active histone marks and the formation of stable transcription factor complexes at the Pu.1 locus and we show that chromatin remodeling is mediated by the transient binding of RUNX1 to Pu.1 cis-elements. By contrast, chromatin reorganization of Csf1r requires prior expression of PU.1 together with RUNX1 binding. Once the full hematopoietic program is established, stable transcription factor complexes and active chromatin can be maintained without RUNX1. Our experiments therefore demonstrate how individual transcription factors function in a differentiation stage-specific manner to differentially affect the initiation versus maintenance of a developmental program.

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Available from: Christophe Lancrin, Sep 23, 2014
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    • "However, it is likely that, in addition to PU.1, other TFs participate in the initiation of the establishment of pre-existing enhancers and the activation of de novo enhancers. Thus, C/EBPa sites also were enriched for the RUNX motif, in line with the finding that during myelopoiesis Runx1 binds transiently to the URE element of the Pu.1 gene to establish open chromatin, permitting the binding of PU.1 (Hoogenkamp et al., 2009). In addition, it is possible that the Fos gene acts as a downstream effector, as it is directly regulated by C/EBPa and we observed enrichment of AP-1 motifs in C/EBP-bound sites where it might co-operate with C/EBPs. "
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    ABSTRACT: Transcription-factor-induced somatic cell conversions are highly relevant for both basic and clinical research yet their mechanism is not fully understood and it is unclear whether they reflect normal differentiation processes. Here we show that during pre-B-cell-to-macrophage transdifferentiation, C/EBPα binds to two types of myeloid enhancers in B cells: pre-existing enhancers that are bound by PU.1, providing a platform for incoming C/EBPα; and de novo enhancers that are targeted by C/EBPα, acting as a pioneer factor for subsequent binding by PU.1. The order of factor binding dictates the upregulation kinetics of nearby genes. Pre-existing enhancers are broadly active throughout the hematopoietic lineage tree, including B cells. In contrast, de novo enhancers are silent in most cell types except in myeloid cells where they become activated by C/EBP factors. Our data suggest that C/EBPα recapitulates physiological developmental processes by short-circuiting two macrophage enhancer pathways in pre-B cells. Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.
    Stem Cell Reports 07/2015; 5(2). DOI:10.1016/j.stemcr.2015.06.007 · 5.37 Impact Factor
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    • "Interestingly, the chromatin at this enhancer harbors active marks already in progenitors and is bound by PU.1 and IRF factors before Pax5 transcription takes place in committed pro-B cells (55). It was also shown that the concerted action of PU.1 and Runx1 primes the activation of both promoter and enhancer elements of the c-fms gene in myeloid cell (92, 93). All together, these data clearly indicate the pioneering and priming abilities of the master hematopoietic regulator PU.1. "
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    Frontiers in Immunology 04/2014; 5:156. DOI:10.3389/fimmu.2014.00156
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    • "As we suggested previously in the " cocktail party " model of differentiation (Sieweke & Graf, 1998), transcription factor complex formation is dynamic and differentiation stage specific. Runx1 thus forms complexes and cooperates with PU.1 during the initiation of c-fms expression but is dispensable at later stages of differentiation (Hoogenkamp et al., 2009), where Egr-2 cooperates with PU.1 (Krysinska et al., 2007). Similarly, at later stages of differentiation , the transcription factor IRF8 can cooperatively interact with DNA-bound PU.1 without binding to DNA itself and thereby enhances macrophage differentiation through targeted gene induction (Sharf et al., 1997; Tamura, Thotakura, Tanaka, Ko, & Ozato, 2005). "
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    ABSTRACT: Macrophages not only are prominent effector cells of the immune system that are critical in inflammation and innate immune responses but also fulfill important functions in tissue homeostasis. Transcription factors can define macrophage identity and control their numbers and functions through the induction and maintenance of specific transcriptional programs. Here, we review the mechanisms employed by lineage-specific transcription factors to shape macrophage identity during the development from hematopoietic stem and progenitor cells. We also present current insight into how specific transcription factors control macrophage numbers, by regulating coordinated proliferation and differentiation of myeloid progenitor cells and self-renewal of mature macrophages. We finally discuss how functional specialization of mature macrophages in response to environmental stimuli can be induced through synergistic activity of lineage- and stimulus-specific transcription factors that plug into preexisting transcriptional programs. Understanding the mechanisms that define macrophage identity, numbers, and functions will provide important insights into the differential properties of macrophage populations under various physiological and pathological conditions.
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