Erythroid Progenitor Cells in the Murine Bone Marrow: Parallels with Human Counterparts and Response to Acute Anemia

In mammals, the erythron consists of lineage-specific progenitor cells that transition to morphologically identifiable precursors that ultimately enucleate to become mature red blood cells. Early- and late-stage erythroid progenitors, BFU-E and CFU-E respectively, were originally defined by their capacity to generate colonies of erythroid cells in semisolid media. We have refined a flow cytometry-based strategy based on Pronk, et al. (2007) and Tusi, et al. (2018) that parallels similar strategies developed for human erythroid progenitor cells (Yan et al., 2021) to facilitate identification and isolation of progressive stages of primary murine erythroid progenitors, termed EP1 to EP4, in the bone marrow. Sorted erythroid progenitor (EP1 to EP4) populations were cultured in colony-forming assays. EP1 consists almost entirely of BFU-E, while EP4 was highly enriched in CFU-E, and the intermediate EP2 and EP3 populations progress from immature to more mature erythroid colony-forming progenitors. Global RNA-Seq studies were performed on three replicates of primary bone marrow EP1-4 from 9-12 week old outbred mice. EP1/2 and EP3/4 each shared the most similarities, consistent with the enrichment of BFU-E in the former and CFU-E in the latter. We also analyzed the global transcriptome of EP1-4 derived from in vitro culture of human CD34+ hematopoietic stem and progenitor cells to compare with the murine counterparts. Approximately two-thirds of differentially expressed genes displayed similar patterns as murine and human EP1 transition progressively to EP4. The largest transition in gene expression occurred between EP2 and EP3. Analysis of genes differentially expressed between EP2 and EP3 both in human and in mouse (adj. p-value <0.05, 361 down and 242 up) revealed marked downregulation of transcription factors expected from previous studies of erythroid progenitors (Gata2, Cd34), as well as those associated with multipotential, myeloid, and megakaryocyte lineages (Sp1 and Runx1). Upregulated transcription factors included prototypical factors associated with erythropoiesis (Klf1, Gata1 and Tal1), as well as Srebf1, a central regulator of cholesterol homeostasis. In both species the transition from EP2 to EP3 was characterized by downregulation of the gene ontology terms associated with myeloid and megakaryocyte development, and the upregulation of gene ontology terms associated with erythrocyte development, cell cycle, heme metabolism, and VLDL particle assembly. Interestingly, cholesterol content increases as EP1/2 transition to EP3/4 in both primary murine and human bone marrow. These data indicate a shared progression of erythroid progenitors can be proactively delineated in human and mouse systems.

To better define the response of erythroid progenitors to stress, we established an acute anemia model in mice, reproducibly lowering the hematocrit by half via phlebotomy. Induction of acute anemia leads to a marked expansion of EP3 and EP4 at 48 hours both in the bone marrow and in the spleen. Additionally, these expanded EP3/4 populations were characterized by an increase percentage of the cells in S-phase. Principal component analysis of global transcriptomic studies of EP1-4 isolated from the marrow 48 hours after the induction of anemia revealed parallel differentiation programs when compared to their steady-state murine counterparts. 70% of the shared genes upregulated in the transition from EP2 to EP3 were upregulated to a greater extent in response to anemia. Upregulated genes, particularly in EP3/4, included known EPO-responsive genes (Pim1, Socs2, Tfrc, and Cited4), as well as hypoxia-associated genes. Interestingly, key cholesterol biosynthesis genes (Hmgcr, Hmgcs1, and Sqle) were also further upregulated in anemic versus steady-state EP3/4. This latter finding suggests that cholesterol homeostasis may play a key role in the erythroid progenitor response to stress. We conclude that late-stage erythroid progenitor (EP3/4) expansion is a specific and integral component of regenerative erythropoiesis. The identification and isolation of stage-specific murine erythroid progenitor cells that are highly analogous to their human counterparts will facilitate an improved understanding of normal and disordered erythropoiesis.