Regeneration alters open chromatin and<i>cis</i>-regulatory landscape of erythroid precursors

Stress erythropoiesis elevates the rate of red blood cell (RBC) production as a physiological response to stressors such as anemia or hypoxia. In acute anemia, RBC progenitors and precursors temporarily rewire their transcriptome, up- and downregulating hundreds of genes to accelerate the production of mature RBCs. Effective regeneration requires communication between critical cytokine signals (e.g., BMP4) and <i>cis</i>-regulatory elements on chromatin which coordinate transcriptional changes. To identify <i>cis</i>-regulatory changes that underlie anemia-specific gene expression and cellular responses, we analyzed chromatin accessibility in populations of cells enriched for red blood cell precursors isolated from mice at a range of time points after anemia induction. Early in the anemia response, chromatin is transiently open at AP-1-containing regions, correlated with increased <i>Jun</i> and <i>Fos</i> transcript/protein levels. <i>Jun</i> knockdown ex vivo decreases the percentage of KIT⁺ erythroid precursors after anemia induction. We observe a second rewiring event at time points consistent with anemia resolution, involving repression of GATA factor-accessible regions and activation of ETS factor-accessible regions. In both mouse in vivo models and human CD34⁺ cells stimulated with BMP4, accessibility changes at regions with prior associations to human blood phenotypes. Dozens of BMP4- and anemia-activated loci are sensitive to natural human variation. The representation of red blood cell trait-associated loci in ATAC-seq data remains durably elevated more than 1 month after anemia resolution. Together, these findings provide a framework to understand the early establishment and late resolution of a regeneration-dependent transcriptome in RBC precursors.