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Editorials
Finding erythroid stress progenitors: cell surface markers revealed
Peng Ji
Department of Pathology, Feinberg School of Medicine, Northwestern University, Chicago, IL and Robert H. Lurie Comprehensive Cancer Center, Northwestern University, Chicago, IL,USA
E-mail: PENG JI - peng-ji@fsm.northwestern.edu doi:10.3324/haematol.2020.262493
The constant production of red blood cells maintains the hematologic homeostasis during steady state in
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mammals. Under stress conditions, such as bleed-
ing or hemolysis, the increased perfusion pressure enables the switch of steady erythropoiesis to stress ery- thropoiesis. Research over the past few decades led to our increased understanding of the molecular mechanisms in stress erythropoiesis, especially in murine models.2 However, finding the stress erythroid progenitor cells for detailed mechanistic investigation remains a demanding task. In this issue of Haematologica, Singbrant et al.3 discov- ered cell surface markers that could aid the identification of these progenitors.
Bone marrow is the major organ that produces red blood cells in adult mammals under steady state. During stress erythropoiesis in mouse models, red cell produc- tion switches to the spleen and liver. Accumulating evi- dence demonstrated that bone morphogenetic protein 4 (BMP4) signaling, along with erythropoietin (Epo), Hedgehog, stem cell factor (SCF), and hypoxia, play criti- cal roles during stress erythropoiesis.4-8 These signaling pathways ensure a rapid response of the hematopoietic system to produce a large quantity of erythrocytes for tis- sue hypoxia. The erythroid progenitors responsive for stress erythropoiesis are burst-forming unit-erythroid progenitors (BFU-E) found in murine spleens. Previous reports have shown that these splenic stress BFU-E express immature cell surface marker c-Kit and low levels of erythroid markers CD71 and Ter119.9 Further efforts were applied to expand the stress BFU-E population in vitro using additional markers such as CD34 and CD133.10 However, this progenitor population is rather heteroge- nous with a low percentage of cells being stress BFU-E.
To identify and enrich the stress erythroid progenitors, Singbrant et al.3 used an irradiation-induced anemia mouse model to mimic stress erythropoiesis. The authors previously showed that fetal erythroid BFU-E can be identified with high purity as lineage-cKit+ and CD71/CD24alowSca1-CD34- in mouse fetal liver.11 Since adult stress erythropoiesis in murine models closely resembles fetal erythropoiesis, the authors determined whether splenic stress erythroid progenitors could also be enriched using these and additional markers. This approach led to the discovery that stress BFU-E could be further enriched from Lineage-cKit+CD71/CD24alow cells as CD150+CD9+Sca1-. More than 20% of these cells pro- duced large BFU-E colonies, which represented over 100- fold improvement of purity compared to previous meth- ods. In addition to the identification of high purity stress BFU-E, multi-potent stress progenitors (stress-MPP) that give rise to stress BFU-E, and stress colony-forming unit- erythroid progenitors (stress-CFU-E) were also identified as CD150+CD9+Sca1+ and CD150+CD9-, respectively.
Using an elegant in vivo tracing technology with Kusabira orange (KuO) mice, the authors further demonstrated that stress-BFU-E and stress-CFU-E harbor a short-term radio-protective capacity by providing a transient wave of reconstitution in the peripheral blood and spleen. Stress-MPP follow the short-term wave and provide the multi-lineage reconstitution in the peripheral blood, spleen, and bone marrow.
Gene expressing profiling analyses further demonstrat- ed that stress MPP and BFU-E in murine spleen express target genes of BMP, which is consistent with its role in stress erythropoiesis. The authors extended these find- ings by showing that mice transplanted with BMP recep- tor II deficient donor bone marrow cells had smaller spleens and a significant reduction in spleen cells. BMP receptor II deficient bone marrow cells also showed sig- nificantly decreased potential to form stress BFU-E from lineage-cKit+ progenitors. These studies confirmed the critical role of BMP signaling in stress erythropoiesis in murine models from a genetic approach.
Interestingly, the authors also found CD150+CD9+ BFU- E progenitors in steady-state bone marrow but not the spleen, demonstrating that these markers are useful to identify BFU-E progenitors in both steady-state and stress conditions. Previous studies indicated that steady BFU-E migrate from the bone marrow to the spleen during stress erythropoiesis to become stress BFU-E,12 while more recent findings suggest that endogenous splenic stress BFU-E during stress erythropoiesis are distinctive from steady BFU-E.7 The CD150+CD9+ BFU-E progenitors Singbrant et al.3 discovered in both steady state and stress conditions would be helpful to resolve this contradiction in future studies.
Using RNA sequencing and gene set enrichment analy- ses, the authors further revealed that steady- and stress- BFU-E exhibited a large overlap in the transcriptome. The differences are mainly in genes associated with BMP and glucocorticoid signaling, proliferation, maturation block, and erythropoiesis that are highly expressed in stress BFU-E in the spleen. The most downregulated genes in stress BFU-E include those associated with myeloid cell development and immune response. While the transcrip- tional landscapes of steady- and stress-BFU-E are similar, another interesting finding in this study is that there is differential chromatin accessibility in the distal elements in stress erythroid progenitors. Transcriptional regulators CTCF and ERG are significantly enriched in open chro- matin regions in stress-BFU-E, indicating their potential roles in regulating stress erythropoiesis.
The findings by Singbrant et al.3 provide the field with an important tool to isolate stress-BFU-E in mice. This allows researchers to investigate stress erythropoiesis in a variety of mouse model systems that mimic disease con-
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