Here we provide evidence that BMP and Wnt pathways play a dynamic role in hematopoietic regeneration through co-occupancy of regulatory elements with lineage regulators at cell-type specific genes for each lineage. The co-occupancy of Smad1 and Gata2 was observed on blood targets in hematopoietic progenitors during in vivo regeneration. Lineage-restricted co-occupancy of TCF7L2 or SMAD1 with GATA1 and GATA2 in erythroid cells, C/EBPα in myeloid cells, and GATA2 in progenitors occur genome-wide. This binding is selective for cell-specific enhancers bound by master regulators. BMP and Wnt signaling cooperate with lineage regulators to enhance transcription of cell-type specific target genes. Lastly, ectopic expression of a lineage transcription factor was sufficient to direct the genomic localization of signaling-specific factors.
Master regulators direct the site selection of signaling transcription factors in every step during the differentiation process. Recent genome wide studies have shown that signaling pathway transcription factors localize in binding sites adjacent to embryonic stem cell master regulators (Chen et al., 2008
; Cole et al., 2008
; Young, 2011
). Our data show that this mechanism occurs in many cell types, and establishes an order by which the lineage regulators recruit the signaling factors to sites of active genes of biological relevance throughout the whole genome. Alterations of the expression and binding of hematopoietic lineage regulators to target genes during regeneration or differentiation can dictate the binding of signaling factors.
Signaling factors selectively co-localize with master regulators, but our studies hint that other factors may help fine-tune signaling factor binding. For example, the transcriptional status of a gene can modify SMAD1 binding. In CD34 progenitor cells, SMAD1 co-binds with GATA2 on genes expressed in progenitors, but is mostly absent from erythroid genes that are not expressed. In erythroid cells, SMAD1 is able to bind these same targets with GATA1, which helps activate these genes (Bresnick et al., 2010
; Grass et al., 2003
). Additionally, It has been suggested that GATA2 works in different complexes to target progenitor vs
. erythroid genes (Wilson et al., 2010
), and GATA2 is expressed at different levels in these two cell populations. We speculate that it is not only the function of an individual master regulator, but also the combinations and the levels of each lineage regulator in a cell along with the transcriptional state of the target that help dictate the genomic location of signaling factors.
Here we provide a model for how this mechanism could be utilized to help orchestrate hematopoietic differentiation during a stress response. During hematopoietic regeneration when these pathways are required, the activation of BMP and Wnt signaling results in the co-localization of SMAD and TCF with the master regulators on genes defining progenitor cell fate. As regeneration continues and progenitor cells begin to differentiate, different master regulators activate the cell-specific genes of more mature hematopoietic lineages and again redefine the binding of signaling transcription factors. Co-localization of lineage and signaling factors has as a result the co-localization of signaling factors themselves. This fact may explain some of the synergistic effects observed between many signaling pathways (Schier and Talbot, 2005
). As the BMP and Wnt pathways appear to have a selective function during regeneration, throughout this stress response, coupling transcriptional regulation to the transcription factors expressed highly in a cell lineage explains how BMP and Wnt signaling pathways can have cell-context dependent effects.
Regeneration is a process of tissue self-renewal. Our data imply that an underlying mechanism of self-renewal is the control of the entire hematopoietic program through the collaboration of master and signaling transcription factors, which is similar to the mechanism by which core self-renewal factors in embryonic stem cells perform key roles in embryonic stem cell fate. Our observation that the BMP and Wnt signaling transcription factors are associated with master regulators of multiple hematopoietic cell types provide insight into the effects of BMP and Wnt signaling that occur during development, regeneration, and disease.