Advances in understanding the mechanisms of erythropoiesis in homeostasis and disease

Abstract

Anaemia or decreased blood haemoglobin is the most common blood disorder often characterized by reduced red blood cell (RBC) numbers. RBCs are produced from differentiation and commitment of haematopoietic stem cells to the erythroid lineage by a process called erythropoiesis. Coordination of erythropoietin receptor signalling with several erythroid transcription factors including GATA1 is essential for this process. A number of additional players that are critical for RBC production have been identified in recent years. Major technological advances, such as the development of RNA interference, genetically modified animals, including zebrafish, and imaging flow cytometry have led to these discoveries; the emergence of -omics approaches in combination with the optimization of ex vivo erythroid cultures have also produced a more comprehensive understanding of erythropoiesis. Here we summarize studies describing novel regulators of erythropoiesis that modulate erythroid cell production in the context of human erythroid disorders involving hypoxia, iron regulation, immune-related molecules, and the transcription factor FOXO3.

Keywords: erythropoiesis; homeostasis; mechanisms.

Figure 1. Systemic regulation of erythropoiesis through EPO

Under hypoxic conditions resulting from low red blood cell (RBC) numbers, the kidney will produce erythropoietin (EPO), which is carried through the blood to the bone marrow (BM) and spleen to enhance proliferation and survival of erythroid progenitors, including burst-forming unit-erythroid (BFU-E) and colony-forming unit-erythroid (CFU-E) cells. Expansion of the erythroid-committed progenitor pool leads to increased RBC numbers, ablates the hypoxic conditions and reduces EPO production back to normal levels.

Figure 2. Systemic cross-talk between iron regulation and erythropoiesis

During iron-replete conditions, hepatocytes can directly sense transferrin-bound iron, which stimulates production of the hormone hepcidin to limit any further release of iron. Hepcidin directly binds and induces the endocytosis and proteolysis of Ferroportin, a transmembrane exporter of iron found mainly on enterocytes, splenic macrophages, and hepatocytes, which are the main reservoirs of iron in the body. Alternatively, when the body is in need of iron during increased erythropoiesis rate, differentiating erythroblasts produce the hormone erythroferrone, which suppresses hepcidin production. In turn, ferroportin accumulates on iron reservoir cells leading to increased plasma iron concentrations to feed the production of haemoglobin in erythroblasts.

Figure 3. Fine-tune regulation of HIF2α

The translation and degradation of hypoxia inducible factor 2α (HIF2α, also termed EPAS1)are highly regulated by intracellular iron and oxygen level. Under iron-replete conditions, iron regulatory protein 1 (IRP1) is suppressed from binding the iron response element (IRE) of Epas1 (also termed Hif2a) mRNA. This allows for normal rates of HIF2α translation and the induction of iron transport genes and Epo by HIF2α, but the protein levels of HIF2α are still kept in check by prolyl hydroxylase domain (PHD)-mediated degradation under normoxic conditions. When iron is limited, IRP1 is able to bind Epas1 mRNA and inhibit translation, preventing release of limited iron stores and the restricting the erythropoietic demand. Under hypoxic conditions, IRP1 and PHD become suppressed, allowing for rapid induction and accumulation of HIF2α. Subsequently, HIF2α translocates to the nucleus and binds HIF1β, which transactivates a gene programme to adapt to hypoxia and also restore oxygen tension levels. HRE or hypoxia-response element, is the consensus sequence found within promoter regions of HIF target genes, which are transcribed when bound by HIF.

Figure 4. Mechanisms of ineffective erythropoiesis in β-Thalassaemia

A) In β-thalassaemia patients and mice, the erythroid-committed progenitor pool is abnormally high with a significant increase in the frequency of apoptosis compared to normal cells. Growth differentiation factor 11 (GDF11) feeds the cycle of aberrant erythroid progenitor expansion and also inhibits terminal differentiation, which leads to an overall decrease in mature red blood cells (RBCs). The lower RBC numbers result in lower tissue oxygenation, subsequent induction of erythropoietin (EPO) expression and suppression of hepcidin production. Together, high levels of EPO and low levels of hepcidin further contribute to stimulating proliferation of erythroid progenitors and the iron overload phenotype, respectively. B) Due to the imbalance between α and β globin levels, α-globin misfolds and forms harmful α-globin aggregates. Normally HSP70 localizes within the nucleus of erythroid committed progenitors and early erythroblasts to protect GATA1 from caspase 3 cleavage. However, in β-thalassaemic erythroid cells, α-globin aggregates sequester HSP70 to the cytoplasm, leading to degradation of GATA1 and a blockade to terminal erythroblast maturation. α-globin aggregates also produce increased levels of reactive oxygen species (ROS), which can also induce GDF11 production in erythroid-committed progenitors. (Red arrows denote significant differences in β-thalassaemia relative to normal erythropoiesis.) EPO, erythropoietin; RBC, red blood cell; GDF11, growth differentiation factor 11; ROS, reactive oxygen species; GATA1, GATA binding protein 1; HSP70, heat shock protein 70.

Figure 5. FOXO3 regulation of terminal erythroblast maturation

The transcription factor forkhead box O3 (FOXO3) serves diverse functions dependent on the stage of murine erythroblast maturation. FOXO3 is responsible for regulation of a third of genes that are differentially expressed during normal post progenitor cell maturation. In early stage of bone marrow erythroblasts, FOXO3 regulates cell cycle progression and potentially has a role in modulating apoptosis. Furthermore, in late stage erythroblasts, FOXO3 regulates genes responsible for processes of terminal erythropoiesis including erythroblast maturation, enucleation, mitophagy and antioxidant defence. BFU-E, burst-forming unit-erythroid cells; CFU-E colony-forming unit-erythroid cells.