Identification and characterization of 2 types of erythroid progenitors that express GATA-1 at distinct levels

GATA-1 Promotes Erythroid Differentiation Through the Upregulation of miR-451a and miR-210-3p Expressions in CD34+ Cells in High-Altitude Polycythemia

Liu, Fang, Caiyan Hu, Jin Ding, Chengbing Fu, Shuqiong Wang, and Tiantian Li. GATA-1 promotes erythroid differentiation through the upregulation of miR-451a and miR-210-3p expression in CD34+ cells in high-altitude polycythemia. High Alt Med Biol. 24:59-67, 2023. Background: The clinical manifestations of high-altitude polycythemia (HAPC) include excessive accumulation of erythrocytes, and its pathogenesis is not yet clear. Methods: Peripheral blood was collected from 10 HAPC patients (HAPC group) and normal individuals (control group) each. CD34+ cells were sorted using immunomagnetic beads and differentiated into erythroid cells for 7, 11, and 15 days. Changes in GATA-binding protein 1 (GATA-1), miR-451a, and miR-210-3p expression and their possible regulatory relationships were investigated. Results: Under hypoxia, GATA-1 expression on day 15 was about 2.4 times that on day 7 in the control group and about 1.3 times that on day 7 in the HAPC group, which was significantly lower compared with the control group. miR-451a and miR-210-3p expressions in the HAPC group were 2.6 and 1.8 times that in the control group, respectively, and were significantly increased. When GATA-1 was inhibited, miR-451a and miR-210-3p expressions were significantly decreased by 0.43 and 0.39 times, respectively, compared with those in the control group. Conclusions: Hypoxia stimulated the upregulation of GATA-1 level and accelerated the change of expression, which promoted miR-451a and miR-210-3p expressions and shortened the time taken by cells to enter end-stage differentiation, so as to enhance erythroid differentiation, which may be a pathogenetic mechanism underlying HAPC polycytosis.

Erythropoietin-driven dynamic proteome adaptations during erythropoiesis prevent iron overload in the developing embryo

Erythropoietin (Epo) ensures survival and proliferation of colony-forming unit erythroid (CFU-E) progenitor cells and their differentiation to hemoglobin-containing mature erythrocytes. A lack of Epo-induced responses causes embryonic lethality, but mechanisms regulating the dynamic communication of cellular alterations to the organismal level remain unresolved. By time-resolved transcriptomics and proteomics, we show that Epo induces in CFU-E cells a gradual transition from proliferation signature proteins to proteins indicative for differentiation, including heme-synthesis enzymes. In the absence of the Epo receptor (EpoR) in embryos, we observe a lack of hemoglobin in CFU-E cells and massive iron overload of the fetal liver pointing to a miscommunication between liver and placenta. A reduction of iron-sulfur cluster-containing proteins involved in oxidative phosphorylation in these embryos leads to a metabolic shift toward glycolysis. This link connecting erythropoiesis with the regulation of iron homeostasis and metabolic reprogramming suggests that balancing these interactions is crucial for protection from iron intoxication and for survival.

Integrating transcription-factor abundance with chromatin accessibility in human erythroid lineage commitment

Master transcription factors (TFs) directly regulate present and future cell states by binding DNA regulatory elements and driving gene-expression programs. Their abundance influences epigenetic priming to different cell fates at the chromatin level, especially in the context of differentiation. In order to link TF protein abundance to changes in TF motif accessibility and open chromatin, we developed InTAC-seq, a method for simultaneous quantification of genome-wide chromatin accessibility and intracellular protein abundance in fixed cells. Our method produces high-quality data and is a cost-effective alternative to single-cell techniques. We showcase our method by purifying bone marrow (BM) progenitor cells based on GATA-1 protein levels and establish high GATA-1-expressing BM cells as both epigenetically and functionally similar to erythroid-committed progenitors.

Single-cell profiling of human bone marrow progenitors reveals mechanisms of failing erythropoiesis in Diamond-Blackfan anemia

Ribosome dysfunction underlies the pathogenesis of many cancers and heritable ribosomopathies. Here, we investigate how mutations in either ribosomal protein large (RPL) or ribosomal protein small (RPS) subunit genes selectively affect erythroid progenitor development and clinical phenotypes in Diamond-Blackfan anemia (DBA), a rare ribosomopathy with limited therapeutic options. Using single-cell assays of patient-derived bone marrow, we delineated two distinct cellular trajectories segregating with ribosomal protein genotypes. Almost complete loss of erythroid specification was observed in RPS-DBA. In contrast, we observed relative preservation of qualitatively abnormal erythroid progenitors and precursors in RPL-DBA. Although both DBA genotypes exhibited a proinflammatory bone marrow milieu, RPS-DBA was characterized by erythroid differentiation arrest, whereas RPL-DBA was characterized by preserved GATA1 expression and activity. Compensatory stress erythropoiesis in RPL-DBA exhibited disordered differentiation underpinned by an altered glucocorticoid molecular signature, including reduced ZFP36L2 expression, leading to milder anemia and improved corticosteroid response. This integrative analysis approach identified distinct pathways of erythroid failure and defined genotype-phenotype correlations in DBA. These findings may help facilitate therapeutic target discovery.

The role of iron in mediating testosterone's effects on erythropoiesis in mice

Testosterone stimulates iron-dependent erythropoiesis and suppresses hepcidin. To clarify the role of iron in mediating testosterone's effects on erythropoiesis, we induced iron deficiency in mice by feeding low iron diet. Iron-replete and iron-deficient mice were treated weekly with testosterone propionate or vehicle for 3 weeks. Testosterone treatment increased red cell count in iron-replete mice, but, surprisingly, testosterone reduced red cell count in iron-deficient mice. Splenic stress erythropoiesis was stimulated in iron-deficient mice relative to iron-replete mice, and further increased by testosterone treatment, as indicated by the increase in red pulp area, the number of nucleated erythroblasts, and expression levels of TfR1, GATA1, and other erythroid genes. Testosterone treatment of iron-deficient mice increased the ratio of early-to-late erythroblasts in the spleen and bone marrow, and serum LDH level, consistent with ineffective erythropoiesis. In iron-deficient mice, erythropoietin levels were higher but erythropoietin-regulated genes were generally downregulated relative to iron-replete mice, suggesting erythropoietin resistance. Conclusion: Testosterone treatment stimulates splenic stress erythropoiesis in iron-replete as well as iron-deficient mice. However, testosterone worsens anemia in iron-deficient mice because of ineffective erythropoiesis possibly due to erythropoietin resistance associated with iron deficiency. Iron plays an important role in mediating testosterone's effects on erythropoiesis.

Physicochemical and biological evaluation of JR-131 as a biosimilar to a long-acting erythropoiesis-stimulating agent darbepoetin alfa

Renal anemia is predominantly caused by a relative deficiency in erythropoietin (EPO). Conventional treatment for renal anemia includes the use of recombinant human EPO (rhEPO) or a long-acting erythropoiesis-activating agent named darbepoetin alfa, which is a modified rhEPO with a carbohydrate chain structure that differs from native hEPO. We have developed a biosimilar to darbepoetin alfa designated JR-131. Here, we comprehensively compare the physicochemical and biological characteristics of JR-131 to darbepoetin alfa. JR-131 demonstrated similar protein structure to the originator, darbepoetin alfa, by peptide mapping and circular dichroism spectroscopy. Additionally, mass spectroscopic analyses and capillary zone electrophoresis revealed similar glycosylation patterns between the two products. Human bone marrow-derived erythroblasts differentiated and proliferated to form colonies with JR-131 to a similar degree as darbepoetin alfa. Finally, JR-131 stimulated erythropoiesis and improved anemia in rats similarly to darbepoetin alfa. Our data show the similarity in physicochemical and biological properties of JR-131 to those of darbepoetin alfa, and JR-131 therefore represents a biosimilar for use in the treatment of renal anemia.

Regulation of GATA1 levels in erythropoiesis

GATA1 is considered as the "master" transcription factor in erythropoiesis. It regulates at the transcriptional level all aspects of erythroid maturation and function, as revealed by gene knockout studies in mice and by genome-wide occupancies in erythroid cells. The GATA1 protein contains two zinc finger domains and an N-terminal transactivation domain. GATA1 translation results in the production of the full-length protein and of a shorter variant (GATA1s) lacking the N-terminal transactivation domain, which is functionally deficient in supporting erythropoiesis. GATA1 protein abundance is highly regulated in erythroid cells at different levels, including transcription, mRNA translation, posttranslational modifications, and protein degradation, in a differentiation-stage-specific manner. Maintaining high GATA1 protein levels is essential in the early stages of erythroid maturation, whereas downregulating GATA1 protein levels is a necessary step in terminal erythroid differentiation. The importance of maintaining proper GATA1 protein homeostasis in erythropoiesis is demonstrated by the fact that both GATA1 loss and its overexpression result in lethal anemia. Importantly, alterations in any of those GATA1 regulatory checkpoints have been recognized as an important cause of hematological disorders such as dyserythropoiesis (with or without thrombocytopenia), β-thalassemia, Diamond-Blackfan anemia, myelodysplasia, or leukemia. In this review, we provide an overview of the multilevel regulation of GATA1 protein homeostasis in erythropoiesis and of its deregulation in hematological disease.

The Neural Crest as the First Production Site of the Erythroid Growth Factor Erythropoietin

While the neural crest is considered the fourth germ layer that originates a variety of tissues during mammalian development, we recently discovered that some neural crest cells and neuroepithelial cells play a unique role in secreting a vital hematopoietic hormone, erythropoietin (EPO), in mouse embryos. EPO production by the neural crest is transient in mid-stage embryos but essential for the first erythropoiesis in the yolk sac and for sufficient oxygen supply in the whole embryo growing in utero. The site of EPO production shifts from the neural crest to the liver in late embryonic stages, followed by interstitial fibroblasts of the kidneys in adults. Interestingly, the transition of EPO production sites synchronizes with the transition of erythropoietic sites during mouse development from the yolk sac and the fetal liver to the bone marrow. EPO produced by the neural crest and the neuroepithelium is first stored around the production sites and delivered to the yolk sac as an endocrine hormone for erythropoiesis after heartbeat activation. The fact that EPO is produced by some human cell lines derived from neuroblastoma, which mainly originates from the neural crest, provides evidence that the neural crest secretes EPO for primitive erythropoiesis not only in mouse but also in human embryos. Here, we introduce and discuss recent progress in studies on the mechanism of EPO production by the neural crest and its roles in erythropoietic development and in the fate of EPO-producing neural crest cells.

Excessive Iron Availability Caused by Disorders of Interleukin-10 and Interleukin-22 Contributes to High Altitude Polycythemia

Background: Because the pathogenesis of high altitude polycythemia (HAPC) is unclear, the aim of the present study was to explore whether abnormal iron metabolism is involved in the pathogenesis of HAPC and the possible cause. Methods: We examined the serum levels of iron, total iron binding capacity, soluble transferrin receptor (sTfR), ferritin, and hepcidin as well as erythropoietin (EPO) and inflammation-related cytokines in 20 healthy volunteers at sea level, 36 healthy high-altitude migrants, and 33 patients with HAPC. Mice that were exposed to a simulated hypoxic environment at an altitude of 5,000 m for 4 weeks received exogenous iron or intervention on cytokines, and the iron-related and hematological indices of peripheral blood and bone marrow were detected. The in vitro effects of some cytokines on hematopoietic cells were also observed. Results: Iron mobilization and utilization were enhanced in people who had lived at high altitudes for a long time. Notably, both the iron storage in ferritin and the available iron in the blood were elevated in patients with HAPC compared with the healthy high-altitude migrants. The correlation analysis indicated that the decreased hepcidin may have contributed to enhanced iron availability in HAPC, and decreased interleukin (IL)-10 and IL-22 were significantly associated with decreased hepcidin. The results of the animal experiments confirmed that a certain degree of iron redundancy may promote bone marrow erythropoiesis and peripheral red blood cell production in hypoxic mice and that decreased IL-10 and IL-22 stimulated iron mobilization during hypoxia by affecting hepcidin expression. Conclusion: These data demonstrated, for the first time, that an excess of obtainable iron caused by disordered IL-10 and IL-22 was involved in the pathogenesis of some HAPC patients. The potential benefits of iron removal and immunoregulation for the prevention and treatment of HAPC deserve further research.

GATA1 Is a Sensitive and Specific Nuclear Marker for Erythroid and Megakaryocytic Lineages

OBJECTIVES

GATA binding factor 1 (GATA1) is a transcription factor essential for erythromegakaryocytic differentiation. Given its function in lineage specification, we sought to evaluate the immunohistochemical profile of GATA1 in normal marrow and acute leukemia and assess the use of GATA1 as a specific erythromegakaryocytic immunohistochemical marker.

METHODS

Immunohistochemical studies for GATA1 expression were performed on bone marrow biopsy specimens to define its role in the evaluation of acute leukemia and other hematologic disorders.

RESULTS

In normal marrows, intense nuclear reactivity is seen in immature erythroid precursors and megakaryocytes. Weak to moderate nuclear positivity is seen in eosinophils and mast cells. In marrows involved by acute leukemia, blasts of pure erythroleukemia and acute megakaryoblastic leukemia exhibit intense nuclear GATA1 positivity, while blasts of acute myeloid leukemia of other categories are negative. GATA1 is also absent in the blasts of acute lymphoblastic leukemia/lymphoma and in the neoplastic cells of metastatic carcinoma and plasma cell neoplasms.

CONCLUSIONS

Intense GATA1 nuclear expression is a sensitive and specific marker for cells of erythroid and megakaryocytic lineages and is an excellent marker for neoplastic cells of pure erythroleukemia and acute megakaryoblastic leukemia.

Derepression of the DNA Methylation Machinery of the Gata1 Gene Triggers the Differentiation Cue for Erythropoiesis

GATA1 is a critical regulator of erythropoiesis. While the mechanisms underlying the high-level expression of GATA1 in maturing erythroid cells have been studied extensively, the initial activation of the Gata1 gene in early hematopoietic progenitors remains to be elucidated. We previously identified a hematopoietic stem and progenitor cell (HSPC)-specific silencer element (the Gata1 methylation-determining region [G1MDR]) that recruits DNA methyltransferase 1 (Dnmt1) and provokes methylation of the Gata1 gene enhancer. In the present study, we hypothesized that removal of the G1MDR-mediated silencing machinery is the molecular basis of the initial activation of the Gata1 gene and erythropoiesis. To address this hypothesis, we generated transgenic mouse lines harboring a Gata1 bacterial artificial chromosome in which the G1MDR was deleted. The mice exhibited abundant GATA1 expression in HSPCs, in a GATA2-dependent manner. The ectopic GATA1 expression repressed Gata2 transcription and induced erythropoiesis and apoptosis of HSPCs. Furthermore, genetic deletion of Dnmt1 in HSPCs activated Gata1 expression and depleted HSPCs, thus recapitulating the HSC phenotype associated with GATA1 gain of function. These results demonstrate that the G1MDR holds the key to HSPC maintenance and suggest that release from this suppressive mechanism is a fundamental requirement for subsequent initiation of erythroid differentiation.

Protein-based therapeutic for anemia caused by dyserythropoiesis

INTRODUCTION

Major advances have been recently made in understanding the molecular determinants of dyserythropoiesis, particularly due to recent works in β-thalassemia. The purpose of this review is devoted to underline the role of some proteins recently evidenced in the field, that may be new alternative therapeutic targets in the near future to alleviate different types of anemia. Areas covered: This review covers the contemporary aspects of some proteins involved in various types of dyserythropoiesis, including the transcriptional factor GATA-1 and its protective chaperone HSP70, but also cytokines of the transforming growth factor beta (TFG-β) family, TGF-β1 and GDF-11, and hormones as erythroferrone. It will be not exhaustive, but based on major recent published works from the literature in the past three years. Expert commentary: Sotatercept and lustatercept, two activin receptor II ligand traps that block GDF-11, are candidate drugs providing therapeutic hope in different types of ineffective erythropoiesis, including myelodysplastic syndromes (MDS) and β-thalassemia. Furthermore, a new concept emerges to consider erythroid lineage in the bone marrow as an endocrine gland.

Early Development of Definitive Erythroblasts from Human Pluripotent Stem Cells Defined by Expression of Glycophorin A/CD235a, CD34, and CD36

The development of human erythroid cells has been mostly examined in models of adult hematopoiesis, while their early derivation during embryonic and fetal stages is largely unknown. We observed the development and maturation of erythroblasts derived from human pluripotent stem cells (hPSCs) by an efficient co-culture system. These hPSC-derived early erythroblasts initially showed definitive characteristics with a glycophorin A⁺ (GPA⁺) CD34^lowCD36⁻ phenotype and were distinct from adult CD34⁺ cell-derived ones. After losing CD34 expression, early GPA⁺CD36⁻ erythroblasts matured into GPA⁺CD36^low/+ stage as the latter expressed higher levels of β-globin along with a gradual loss of mesodermal and endothelial properties, and terminally suppressed CD36. We establish a unique in vitro model to trace the early development of hPSC-derived erythroblasts by serial expression of CD34, GPA, and CD36. Our findings may provide insight into the understanding of human early erythropoiesis and, ultimately, therapeutic potential.

•

The hPSC/AGM-S3 co-culture system generates considerable definitive erythroblasts

•

hPSC-derived erythroblasts initiate from a unique GPA⁺CD34^lowCD36⁻ fraction

•

Human early erythropoiesis can be traced by serial expression of CD34, GPA, and CD36

Endothelial Cell-Selective Adhesion Molecule Expression in Hematopoietic Stem/Progenitor Cells Is Essential for Erythropoiesis Recovery after Bone Marrow Injury

Numerous red blood cells are generated every second from proliferative progenitor cells under a homeostatic state. Increased erythropoietic activity is required after myelo-suppression as a result of chemo-radio therapies. Our previous study revealed that the endothelial cell-selective adhesion molecule (ESAM), an authentic hematopoietic stem cell marker, plays essential roles in stress-induced hematopoiesis. To determine the physiological importance of ESAM in erythroid recovery, ESAM-knockout (KO) mice were treated with the anti-cancer drug, 5-fluorouracil (5-FU). ESAM-KO mice experienced severe and prolonged anemia after 5-FU treatment compared to wild-type (WT) mice. Eight days after the 5-FU injection, compared to WT mice, ESAM-KO mice showed reduced numbers of erythroid progenitors in bone marrow (BM) and spleen, and reticulocytes in peripheral blood. Megakaryocyte-erythrocyte progenitors (MEPs) from the BM of 5-FU-treated ESAM-KO mice showed reduced burst forming unit-erythrocyte (BFU-E) capacities than those from WT mice. BM transplantation revealed that hematopoietic stem/progenitor cells from ESAM-KO donors were more sensitive to 5-FU treatment than that from WT donors in the WT host mice. However, hematopoietic cells from WT donors transplanted into ESAM-KO host mice could normally reconstitute the erythroid lineage after a BM injury. These results suggested that ESAM expression in hematopoietic cells, but not environmental cells, is critical for hematopoietic recovery. We also found that 5-FU treatment induces the up-regulation of ESAM in primitive erythroid progenitors and macrophages that do not express ESAM under homeostatic conditions. The phenotypic change seen in macrophages might be functionally involved in the interaction between erythroid progenitors and their niche components during stress-induced acute erythropoiesis. Microarray analyses of primitive erythroid progenitors from 5-FU-treated WT and ESAM-KO mice revealed that various signaling pathways, including the GATA1 system, were impaired in ESAM-KO mice. Thus, our data demonstrate that ESAM expression in hematopoietic progenitors is essential for erythroid recovery after a BM injury.

Roles of renal erythropoietin-producing (REP) cells in the maintenance of systemic oxygen homeostasis

Erythropoietic induction is critical for enhancing the efficiency of oxygen delivery during the chronic phase of the systemic hypoxia response. The erythroid growth factor erythropoietin (Epo) triggers the erythropoietic induction through the activation of erythroid genes related to cell survival, differentiation, and iron metabolism. Because Epo is produced in renal Epo-producing (REP) cells in a hypoxia-inducible manner, REP cells serve as a control center for the systemic hypoxia response. In fact, the loss of Epo production in REP cells causes chronic severe anemia in genetically modified mice, and REP cell-specific inactivation of PHD2 (prolyl-hydroxylase domain enzyme 2) results in erythrocytosis via overexpression of the Epo gene due to the constitutive activation of HIF2α (hypoxia-inducible transcription factor 2α). REP cells are located in the interstitial spaces between renal tubules and capillaries, where the oxygen supply is low but oxygen consumption is high, for the highly sensitive detection of decreased oxygen supplies to the body. Under disease conditions, REP cells transform to myofibroblasts and lose their Epo-producing ability. Therefore, elucidation of Epo gene regulation and REP cell features directly contributes to understanding the pathology of chronic kidney disease. To further analyze REP cells, we introduce a newly established mouse line in which REP cells are efficiently labeled with fluorescent protein.

Renal erythropoietin-producing cells in health and disease

Erythropoietin (Epo) is an indispensable erythropoietic hormone primarily produced from renal Epo-producing cells (REPs). Epo production in REPs is tightly regulated in a hypoxia-inducible manner to maintain tissue oxygen homeostasis. Insufficient Epo production by REPs causes renal anemia and anemia associated with chronic disorders. Recent studies have broadened our understanding of REPs from prototypic hypoxia-responsive cells to dynamic fibrogenic cells. In chronic kidney disease, REPs are the major source of scar-forming myofibroblasts and actively produce fibrogenic molecules, including inflammatory cytokines. Notably, myofibroblast-transformed REPs (MF-REPs) recover their original physiological properties after resolution of the disease insults, suggesting that renal anemia and fibrosis could be reversible to some extent. Therefore, understanding the plasticity of REPs will lead to the development of novel targeted therapeutics for both renal fibrosis and anemia. This review summarizes the regulatory mechanisms how hypoxia-inducible Epo gene expression is attained in health and disease conditions.

Hypoxia Signaling Cascade for Erythropoietin Production in Hepatocytes

Erythropoietin (Epo) is produced in the kidney and liver in a hypoxia-inducible manner via the activation of hypoxia-inducible transcription factors (HIFs) to maintain oxygen homeostasis. Accelerating Epo production in hepatocytes is one plausible therapeutic strategy for treating anemia caused by kidney diseases. To elucidate the regulatory mechanisms of hepatic Epo production, we analyzed mouse lines harboring liver-specific deletions of genes encoding HIF-prolyl-hydroxylase isoforms (PHD1, PHD2, and PHD3) that mediate the inactivation of HIF1α and HIF2α under normal oxygen conditions. The loss of all PHD isoforms results in both polycythemia, which is caused by Epo overproduction, and fatty livers. We found that deleting any combination of two PHD isoforms induces polycythemia without steatosis complications, whereas the deletion of a single isoform induces no apparent phenotype. Polycythemia is prevented by the loss of either HIF2α or the hepatocyte-specific Epo gene enhancer (EpoHE). Chromatin analyses show that the histones around EpoHE dissociate from the nucleosome structure after HIF2α activation. HIF2α also induces the expression of HIF3α, which is involved in the attenuation of Epo production. These results demonstrate that the total amount of PHD activity is more important than the specific function of each isoform for hepatic Epo expression regulated by a PHD-HIF2α-EpoHE cascade in vivo.

In vivo regulation of erythropoiesis by chemically inducible dimerization of the erythropoietin receptor intracellular domain

Erythropoietin (Epo) and its receptor (EpoR) are required for the regulation of erythropoiesis. Epo binds to the EpoR homodimer on the surface of erythroid progenitors and erythroblasts, and positions the intracellular domains of the homodimer to be in close proximity with each other. This conformational change is sufficient for the initiation of Epo-EpoR signal transduction. Here, we established a system of chemically regulated erythropoiesis in transgenic mice expressing a modified EpoR intracellular domain (amino acids 247-406) in which dimerization is induced using a specific compound (chemical inducer of dimerization, CID). Erythropoiesis is reversibly induced by oral administration of the CID to the transgenic mice. Because transgene expression is limited to hematopoietic cells by the Gata1 gene regulatory region, the effect of the CID is limited to erythropoiesis without adverse effects. Additionally, we show that the 160 amino acid sequence is the minimal essential domain of EpoR for intracellular signaling of chemically inducible erythropoiesis in vivo. We propose that the CID-dependent dimerization system combined with the EpoR intracellular domain and the Gata1 gene regulatory region generates a novel peroral strategy for the treatment of anemia.

The Human GATA1 Gene Retains a 5' Insulator That Maintains Chromosomal Architecture and GATA1 Expression Levels in Splenic Erythroblasts

GATA1 is a key transcription factor for erythropoiesis. GATA1 gene expression is strictly regulated at the transcriptional level. While the regulatory mechanisms governing mouse Gata1 (mGata1) gene expression have been studied extensively, how expression of the human GATA1 (hGATA1) gene is regulated remains to be elucidated. To address this issue, we generated hGATA1 bacterial artificial chromosome (BAC) transgenic mouse lines harboring a 183-kb hGATA1 locus covering the hGATA1 exons and distal flanking sequences. Transgenic hGATA1 expression coincides with endogenous mGata1 expression and fully rescues hematopoietic deficiency in mGata1 knockdown mice. The transgene exhibited copy number-dependent and integration position-independent expression of hGATA1, indicating the presence of chromatin insulator activity within the transgene. We found a novel insulator element at 29 kb 5' to the hGATA1 gene and refer to this element as the 5' CCCTC-binding factor (CTCF) site. Substitution mutation of the 5' CTCF site in the hGATA1 BAC disrupted the chromatin architecture and led to a reduction of hGATA1 expression in splenic erythroblasts under conditions of stress erythropoiesis. Our results demonstrate that expression of the hGATA1 gene is regulated through the chromatin architecture organized by 5' CTCF site-mediated intrachromosomal interactions in the hGATA1 locus.

Elucidation of the EP defect in Diamond-Blackfan anemia by characterization and prospective isolation of human EPs

Diamond-Blackfan anemia (DBA) is a disorder characterized by a selective defect in erythropoiesis. Delineation of the precise defect is hampered by a lack of markers that define cells giving rise to erythroid burst- and erythroid colony-forming unit (BFU-E and CFU-E) colonies, the clonogenic assays that quantify early and late erythroid progenitor (EEP and LEP) potential, respectively. By combining flow cytometry, cell-sorting, and single-cell clonogenic assays, we identified Lin(-)CD34(+)CD38(+)CD45RA(-)CD123(-)CD71(+)CD41a(-)CD105(-)CD36(-) bone marrow cells as EEP giving rise to BFU-E, and Lin(-)CD34(+/-)CD38(+)CD45RA(-)CD123(-)CD71(+)CD41a(-)CD105(+)CD36(+) cells as LEP giving rise to CFU-E, in a hierarchical fashion. We then applied these definitions to DBA and identified that, compared with controls, frequency, and clonogenicity of DBA, EEP and LEP are significantly decreased in transfusion-dependent but restored in corticosteroid-responsive patients. Thus, both quantitative and qualitative defects in erythroid progenitor (EP) contribute to defective erythropoiesis in DBA. Prospective isolation of defined EPs will facilitate more incisive study of normal and aberrant erythropoiesis.

Progenitor stage-specific activity of a cis-acting double GATA motif for Gata1 gene expression

GATA1 is a master regulator of erythropoiesis, expression of which is regulated by multiple discrete cis-acting elements. In this study, we examine the activity of a promoter-proximal double GATA (dbGATA) motif, using a Gata1 bacterial artificial chromosome (BAC)-transgenic green fluorescent protein (GFP) reporter (G1BAC-GFP) mouse system. Deletion of the dbGATA motif led to significant reductions in GFP expression in hematopoietic progenitors, while GFP expression was maintained in erythroblasts. Consistently, in mice with a germ line deletion of the dbGATA motif (Gata1(ΔdbGATA) mice), GATA1 expression in progenitors was significantly decreased. The suppressed GATA1 expression was associated with a compensatory increase in GATA2 levels in progenitors. When we crossed Gata1(ΔdbGATA) mice with Gata2 hypomorphic mutant mice (Gata2(fGN/fGN) mice), the Gata1(ΔdbGATA)::Gata2(fGN/fGN) compound mutant mice succumbed to a significant decrease in the progenitor population, whereas both groups of single mutant mice maintained progenitors and survived to adulthood, indicating the functional redundancy between GATA1 and GATA2 in progenitors. Meanwhile, the effects of the dbGATA site deletion on Gata1 expression were subtle in erythroblasts, which showed increased GATA1 binding and enhanced accumulation of active histone marks around the 1st-intron GATA motif of the ΔdbGATA locus. These results thus reveal a novel role of the dbGATA motif in the maintenance of Gata1 expression in hematopoietic progenitors and a functional compensation between the dbGATA site and the 1st-intron GATA motif in erythroblasts.

Isolation and transcriptome analyses of human erythroid progenitors: BFU-E and CFU-E

Burst-forming unit-erythroid (BFU-E) and colony-forming unit-erythroid (CFU-E) cells are erythroid progenitors traditionally defined by colony assays. We developed a flow cytometry-based strategy for isolating human BFU-E and CFU-E cells based on the changes in expression of cell surface markers during in vitro erythroid cell culture. BFU-E and CFU-E are characterized by CD45(+)GPA(-)IL-3R(-)CD34(+)CD36(-)CD71(low) and CD45(+)GPA(-)IL-3R(-)CD34(-)CD36(+)CD71(high) phenotypes, respectively. Colony assays validated phenotypic assignment giving rise to BFU-E and CFU-E colonies, both at a purity of ∼90%. The BFU-E colony forming ability of CD45(+)GPA(-)IL-3R(-)CD34(+)CD36(-)CD71(low) cells required stem cell factor and erythropoietin, while the CFU-E colony forming ability of CD45(+)GPA(-)IL-3R(-)CD34(-)CD36(+)CD71(high) cells required only erythropoietin. Bioinformatic analysis of the RNA-sequencing data revealed unique transcriptomes at each differentiation stage. The sorting strategy was validated in uncultured primary cells isolated from bone marrow, cord blood, and peripheral blood, indicating that marker expression is not an artifact of in vitro cell culture, but represents an in vivo characteristic of erythroid progenitor populations. The ability to isolate highly pure human BFU-E and CFU-E progenitors will enable detailed cellular and molecular characterization of these distinct progenitor populations and define their contribution to disordered erythropoiesis in inherited and acquired hematologic disease. Our data provides an important resource for future studies of human erythropoiesis.

Drl.3 governs primitive hematopoiesis in zebrafish

The molecular program controlling hematopoietic differentiation is not fully understood. Here, we describe a family of zebrafish genes that includes a novel hematopoietic regulator, draculin-like 3 (drl.3). We found that drl.3 is expressed in mesoderm-derived hematopoietic cells and is retained during erythroid maturation. Moreover, drl.3 expression correlated with erythroid development in gata1a- and spi1b-depleted embryos. Loss-of-function analysis indicated that drl.3 plays an essential role in primitive erythropoiesis and, to a lesser extent, myelopoiesis that is independent of effects on vasculature, emergence of primitive and definitive progenitor cells and cell viability. While drl.3 depletion reduced gata1a expression and inhibited erythroid development, enforced expression of gata1a was not sufficient to rescue erythropoiesis, indicating that the regulation of hematopoiesis by drl.3 extends beyond control of gata1a expression. Knockdown of drl.3 increased the proportion of less differentiated, primitive hematopoietic cells without affecting proliferation, establishing drl.3 as an important regulator of primitive hematopoietic cell differentiation.

Erythropoietin production in neuroepithelial and neural crest cells during primitive erythropoiesis

Erythropoietin (Epo) supports both primitive erythropoiesis in the yolk sac and definitive erythropoiesis in the fetal liver and bone marrow. Although definitive erythropoiesis requires kidney- and liver-secreted Epo, it is unclear which cells produce Epo for primitive erythropoiesis. Here we find neural Epo-producing (NEP) cells in mid-gestational stage embryos using mouse lines that express green fluorescent protein (GFP) under the Epo gene regulation. In these mice, GFP is expressed exclusively in a subpopulation of neural and neural crest cells at embryonic day 9.0 when Epo-deficient embryos exhibit abnormalities in primitive erythropoiesis. The GFP-positive NEP cells express Epo mRNA and the ex vivo culture of embryonic day 8.5 neural tubes results in the secretion of Epo, which is able to induce the proliferation and differentiation of yolk sac-derived erythroid cells. These results thus suggest that NEP cells secrete Epo and might support the development of primitive erythropoiesis.

Concise review: erythroid versus myeloid lineage commitment: regulating the master regulators

Developmental processes, like blood formation, are orchestrated by transcriptional networks. Those transcriptional networks are highly responsive to various environmental stimuli and affect common precursors resulting in increased production of cells of the erythroid lineage or myeloid lineage (granulocytes, neutrophils, and macrophages). A significant body of knowledge has accumulated describing transcription factors that drive differentiation of these two major cellular pathways, in particular the antagonistic master regulators such as GATA-1 and PU.1. However, little is known about factors that work upstream of master regulators to enhance differentiation toward one lineage. These functions become especially important under various stress conditions like sudden loss of red blood cells or pathogen infection. This review describes recent studies that begin to provide evidence for such factors. An increased understanding of factors regulating cellular commitment will advance our understanding of the etiology of diseases like anemia, cancer, and possibly other blood related disorders.

A remote GATA2 hematopoietic enhancer drives leukemogenesis in inv(3)(q21;q26) by activating EVI1 expression

Chromosomal inversion between 3q21 and 3q26 results in high-risk acute myeloid leukemia (AML). In this study, we identified a mechanism whereby a GATA2 distal hematopoietic enhancer (G2DHE or -77-kb enhancer) is brought into close proximity to the EVI1 gene in inv(3)(q21;q26) inversions, leading to leukemogenesis. We examined the contribution of G2DHE to leukemogenesis by creating a bacterial artificial chromosome (BAC) transgenic model that recapitulates the inv(3)(q21;q26) allele. Transgenic mice harboring a linked BAC developed leukemia accompanied by EVI1 overexpression-neoplasia that was not detected in mice bearing the same transgene but that was missing the GATA2 enhancer. These results establish the mechanistic basis underlying the pathogenesis of a severe form of leukemia through aberrant expression of the EVI1 proto-oncogene.

A regulatory network governing Gata1 and Gata2 gene transcription orchestrates erythroid lineage differentiation

GATA transcription factor family members GATA1 and GATA2 play crucial roles in the regulation of lineage-restricted genes during erythroid differentiation. GATA1 is indispensable for survival and terminal differentiation of erythroid, megakaryocytic and eosinophilic progenitors, whereas GATA2 regulates proliferation and maintenance of hematopoietic stem and progenitor cells. Expression levels of GATA1 and GATA2 are primarily regulated at the transcriptional level through auto- and reciprocal regulatory networks formed by these GATA factors. The dynamic and strictly controlled change of expression from GATA2 to GATA1 during erythropoiesis has been referred to as GATA factor switching, which plays a crucial role in erythropoiesis. The regulatory network comprising GATA1 and GATA2 gives rise to the stage-specific changes in Gata1 and Gata2 gene expression during erythroid differentiation, which ensures specific expression of early and late erythroid genes at each stage. Recent studies have also shed light on the genome-wide binding profiles of GATA1 and GATA2, and the significance of epigenetic modification of Gata1 gene during erythroid differentiation. This review summarizes the current understanding of network regulation underlying stage-dependent Gata1 and Gata2 gene expressions and the functional contribution of these GATA factors in erythroid differentiation.

A mouse model of adult-onset anaemia due to erythropoietin deficiency

Erythropoietin regulates erythropoiesis in a hypoxia-inducible manner. Here we generate inherited super-anaemic mice (ISAM) as a mouse model of adult-onset anaemia caused by erythropoietin deficiency. ISAM express erythropoietin in the liver but lack erythropoietin production in the kidney. Around weaning age, when the major erythropoietin-producing organ switches from the liver to the kidney, ISAM develop anaemia due to erythropoietin deficiency, which is curable by administration of recombinant erythropoietin. In ISAM severe chronic anaemia enhances transgenic green fluorescent protein and Cre expression driven by the complete erythropoietin-gene regulatory regions, which facilitates efficient labelling of renal erythropoietin-producing cells. We show that the majority of cortical and outer medullary fibroblasts have the innate potential to produce erythropoietin, and also reveal a new set of erythropoietin target genes. ISAM are a useful tool for the evaluation of erythropoiesis-stimulating agents and to trace the dynamics of erythropoietin-producing cells.

The Gata1 5' region harbors distinct cis-regulatory modules that direct gene activation in erythroid cells and gene inactivation in HSCs

GATA1 is a master regulator of hematopoietic differentiation, but Gata1 expression is inactivated in hematopoietic stem cells (HSCs). Using a bacterial artificial chromosome containing the Gata1 gene modified with green fluorescent protein (GFP) reporter, we explored the function of the 3.7-kb Gata1 upstream region (GdC region) that harbors 3 core cis-elements: Gata1 hematopoietic enhancer, double GATA-motif, and CACCC-motif. Transgenic GFP expression directed by the Gata1-BAC faithfully recapitulated the endogenous Gata1 expression pattern. However, deletion of the GdC-region eliminated reporter expression in all hematopoietic cells. To test whether the combination of the core cis-elements represents the regulatory function of the GdC-region, we replaced the region with a 659-bp minigene that linked the three cis-elements (MG-GFP). The GFP reporter expression directed by the MG-GFP BAC fully recapitulated the erythroid-megakaryocytic Gata1 expression. However, the GFP expression was aberrantly increased in the HSCs and was associated with decreases in DNA methylation and abundant GATA2 binding to the transgenic MG-GFP allele. The 3.2-kb sequences interspaced between the Gata1 hematopoietic enhancer and the double GATA-motif were able to recruit DNA methyltransferase 1, thereby exerting a cis-repressive function in the HSC-like cell line. These results indicate that the 3.2-kb interspacing sequences inactivate Gata1 by maintaining DNA-methylation in the HSCs.

GATA factor switching from GATA2 to GATA1 contributes to erythroid differentiation

Transcription factor GATA2 is highly expressed in hematopoietic stem cells and progenitors, whereas its expression declines after erythroid commitment of progenitors. In contrast, the start of GATA1 expression coincides with the erythroid commitment and increases along with the erythroid differentiation. We refer this dynamic transition of GATA factor expression to as the 'GATA factor switching'. Here, we examined contribution of the GATA factor switching to the erythroid differentiation. In Gata1-knockdown embryos that concomitantly express Gata2-GFP reporter, high-level expression of GFP reporter was detected in accumulated immature hematopoietic cells with impaired differentiation, demonstrating that GATA1 represses Gata2 gene expression in hematopoietic progenitors in vivo. We have conducted chromatin immunoprecipitation (ChIP) on microarray analyses of GATA2 and GATA1, and results indicate that the GATA1-binding sites widely overlap with the sites pre-occupied by GATA2 before the GATA1 expression. Importantly, erythroid genes harboring GATA boxes bound by both GATA1 and GATA2 tend to be expressed in immature erythroid cells, whereas those harboring GATA boxes to which GATA1 binds highly but GATA2 binds only weakly are important for the mature erythroid cell function. Our results thus support the contention that preceding binding of GATA2 helps the following binding of GATA1 and thereby secures smooth expression of the transient-phase genes.

An in silico erythropoiesis model rationalizing synergism between stem cell factor and erythropoietin

Stem cell factor (SCF) and erythropoietin (EPO) are two most recognized growth factors that play in concert to control in vitro erythropoiesis. However, exact mechanisms underlying the interplay of these growth factors in vitro remain unclear. We developed a mathematical model to study co-signaling effects of SCF and EPO utilizing the ERK1/2 and GATA-1 pathways (activated by SCF and EPO) that drive the proliferation and differentiation of erythroid progenitors. The model was simplified and formulated based on three key features: synergistic contribution of SCF and EPO on ERK1/2 activation, positive feedback effects on proliferation and differentiation, and cross-inhibition effects of activated ERK1/2 and GATA-1. The model characteristics were developed to correspond with biological observations made known thus far. Our simulation suggested that activated GATA-1 has a more dominant cross-inhibition effect and stronger positive feedback response on differentiation than the proliferation pathway, while SCF contributed more to the activation of ERK1/2 than EPO. A sensitivity analysis performed to gauge the dynamics of the system was able to identify the most sensitive model parameters and illustrated a contribution of transient activity in EPO ligand to growth factor synergism. Based on theoretical arguments, we have successfully developed a model that can simulate growth factor synergism observed in vitro for erythropoiesis. This hypothesized model can be applied to further computational studies in biological systems where synergistic effects of two ligands are seen.

Changes in the Expression of Transcription Factors Involved in Modulating the Expression of EPO-R in Activated Human CD4-Positive Lymphocytes

We have recently described the presence of the erythropoietin receptor (EPO-R) on CD4⁺ lymphocytes and demonstrated that its expression increases during their activation, reaching a level reported to be typical for erythroid progenitors. This observation suggests that EPO-R expression is modulated during lymphocyte activation, which may be important for the cells’ function. Here we investigated whether the expression of GATA1, GATA3 and Sp1 transcription factors is correlated with the expression of EPO-R in human CD4⁺ lymphocytes stimulated with monoclonal anti-CD3 antibody. The expression of GATA1, GATA3 and Sp1 transcription factors in CD4⁺ cells was estimated before and after stimulation with anti-CD3 antibody by Western Blot and flow cytometry. The expression of EPO-R was measured using real-time PCR and flow cytometry. There was no change in the expression of GATA1 and GATA3 in CD4⁺ lymphocytes after stimulation with anti-CD3 antibody. However, stimulation resulted in the significantly increased expression of the Sp1 factor. CD4⁺ lymphocytes stimulated with anti-CD3 antibody exhibited an increase in both the expression level of EPOR gene and the number of EPO-R molecules on the cells’ surface, the latter being significantly correlated with the increased expression of Sp1. Sp1 is noted to be the single transcription factor among the ones studied whose level changes as a result of CD4⁺ lymphocyte stimulation. It seems that Sp1 may significantly affect the number of EPO-R molecules present on the surface of activated CD4⁺ lymphocytes.

TGF-beta-1 up-regulates extra-cellular matrix production in mouse hepatoblasts

Fetal liver is the major embryonic hematopoietic organ and is extrinsically colonized by circulating hematopoietic stem cells (HSCs). Integrin beta-1 expression on HSCs is crucial for colonization, suggesting that interaction of Integrin beta-1 with extra-cellular matrix (ECM) factors promotes HSC adherence to fetal liver. However, little is known about how ECM production is regulated in fetal liver. Here we used flow cytometry to sort fetal liver compartments and detected ECM gene and protein expression predominantly in sorted hepatoblasts. mRNA and protein analysis suggested that TGF-beta-1 expressed by hepatoblasts, sinusoid endothelial cells and hematopoietic cells, binds to the TGF-beta receptor type-2 expressed on hepatoblasts to stimulate ECM production. Intra-cardiac injection of TGF-inhibitors into mouse embryos dramatically decreased fetal liver ECM gene expression. Taken together, our observations suggest that hepatoblasts predominantly produce ECM factors under control of TGF-beta-1 in fetal liver.

The GATA1s isoform is normally down-regulated during terminal haematopoietic differentiation and over-expression leads to failure to repress MYB, CCND2 and SKI during erythroid differentiation of K562 cells

Background

Although GATA1 is one of the most extensively studied haematopoietic transcription factors little is currently known about the physiological functions of its naturally occurring isoforms GATA1s and GATA1FL in humans—particularly whether the isoforms have distinct roles in different lineages and whether they have non-redundant roles in haematopoietic differentiation. As well as being of general interest to understanding of haematopoiesis, GATA1 isoform biology is important for children with Down syndrome associated acute megakaryoblastic leukaemia (DS-AMKL) where GATA1FL mutations are an essential driver for disease pathogenesis.

Methods

Human primary cells and cell lines were analyzed using GATA1 isoform specific PCR. K562 cells expressing GATA1s or GATA1FL transgenes were used to model the effects of the two isoforms on in vitro haematopoietic differentiation.

Results

We found no evidence for lineage specific use of GATA1 isoforms; however GATA1s transcripts, but not GATA1FL transcripts, are down-regulated during in vitro induction of terminal megakaryocytic and erythroid differentiation in the cell line K562. In addition, transgenic K562-GATA1s and K562-GATA1FL cells have distinct gene expression profiles both in steady state and during terminal erythroid differentiation, with GATA1s expression characterised by lack of repression of MYB, CCND2 and SKI.

Conclusions

These findings support the theory that the GATA1s isoform plays a role in the maintenance of proliferative multipotent megakaryocyte-erythroid precursor cells and must be down-regulated prior to terminal differentiation. In addition our data suggest that SKI may be a potential therapeutic target for the treatment of children with DS-AMKL.

The effect of erythropoietin on normal and neoplastic cells

Erythropoietin (Epo) is an essential hormone that binds and activates the Epo receptor (EpoR) resident on the surface of erythroid progenitor cells, thereby promoting erythropoiesis. Recombinant human erythropoietin has been used successfully for over 20 years to treat anemia in millions of patients. In addition to erythropoiesis, Epo has also been reported to have other effects, such as tissue protection and promotion of tumor cell growth or survival. This became of significant concern in 2003, when some clinical trials in cancer patients reported increased tumor progression and worse survival outcomes in patients treated with erythropoiesis-stimulating agents (ESAs). One of the potential mechanisms proffered to explain the observed safety issues was that functional EpoR was expressed in tumors and/or endothelial cells, and that ESAs directly stimulated tumor growth and/or antagonized tumor ablative therapies. Since then, numerous groups have performed further research evaluating this potential mechanism with conflicting data and conclusions. Here, we review the biology of endogenous Epo and EpoR expression and function in erythropoiesis, and evaluate the evidence pertaining to the expression of EpoR on normal nonhematopoietic and tumor cells.

Isolation of siRNA target by biotinylated siRNA reveals that human CCDC12 promotes early erythroid differentiation

Erythroid differentiation is a tightly regulated multi-step process that has not been fully elucidated. We previously reported that a siRNA screened from random siRNA library, siRNA clone-67, induced erythroid differentiation in human erythroleukemia K-562cell line. Here we identified that human CCDC12 (coiled-coil domain containing 12) is a target of siRNA clone-67, by target capture with biotinylated siRNA. Over-expression of CCDC12 in K-562cell up-regulated the expression of CD235, ε-globin and γ-globin, accelerated cell growth, and slightly down-regulated the expression of GATA-2. Knockdown of CCDC12 slowed down the cell growth. These data indicate that CCDC12 is a new participant that promotes early erythroid differentiation.

Abnormal differentiation of erythroid precursors in p45 NF-E2(-/-) mice

The transcription factor p45 nuclear factor-erythroid-derived 2 (NF-E2) plays major roles in erythroid and megakaryocytic lineages. Here, we investigated the role of p45 NF-E2 in erythroid differentiation in vivo. Absence of p45 NF-E2 in mice leads to a twofold increase in serum erythropoietin levels. In the bone marrow of these animals, we found a different distribution of precursor populations compared to wild-type mice, suggesting abnormal differentiation. Loss of p45 NF-E2 was also associated with an increase in splenic erythropoiesis, as evidenced by an accumulation of early precursors, namely, late basophilic and polychromatic erythroblasts. These observations are consistent with a stress erythropoiesis phenotype and indicate that the spleen is likely compensating for ineffective erythropoiesis in the bone marrow. Analysis of bone marrow samples revealed increased GATA1 levels, as well as an increased proportion of erythroid cells arrested at the G(1) stage of cell cycle in p45 NF-E2-deficient mice. These results suggest that p45 NF-E2 is required for the differentiation of erythroid precursors.

Isolation and characterization of renal erythropoietin-producing cells from genetically produced anemia mice

Understanding the nature of renal erythropoietin-producing cells (REPs) remains a central challenge for elucidating the mechanisms involved in hypoxia and/or anemia-induced erythropoietin (Epo) production in adult mammals. Previous studies have shown that REPs are renal peritubular cells, but further details are lacking. Here, we describe an approach to isolate and characterize REPs. We bred mice bearing an Epo gene allele to which green fluorescent protein (GFP) reporter cDNA was knocked-in (Epo^GFP) with mice bearing an Epo gene allele lacking the 3′ enhancer (Epo^Δ3′E). Mice harboring the mutant Epo^GFP/Δ3′E gene exhibited anemia (average Hematocrit 18% at 4 to 6 days after birth), and this perinatal anemia enabled us to identify and purify REPs based on GFP expression from the kidney. Light and confocal microscopy revealed that GFP immunostaining was confined to fibroblastic cells that reside in the peritubular interstitial space, confirming our previous observation in Epo-GFP transgenic reporter assays. Flow cytometry analyses revealed that the GFP fraction constitutes approximately 0.2% of the whole kidney cells and 63% of GFP-positive cells co-express CD73 (a marker for cortical fibroblasts and Epo-expressing cells in the kidney). Quantitative RT-PCR analyses confirmed that Epo expression was increased by approximately 100-fold in the purified population of REPs compared with that of the unsorted cells or CD73-positive fraction. Gene expression analyses showed enrichment of Hif2α and Hif3α mRNA in the purified population of REPs. The genetic approach described here provides a means to isolate a pure population of REPs, allowing the analysis of gene expression of a defined population of cells essential for Epo production in the kidney. This has provided evidence that positive regulation by HIF2α and negative regulation by HIF3α might be necessary for correct renal Epo induction. (282 words)

Specific contribution of the erythropoietin gene 3' enhancer to hepatic erythropoiesis after late embryonic stages

Erythropoietin (Epo) is secreted from the liver and kidney, where Epo production is strictly regulated at the transcriptional level in a hypoxia- and/or anemia-inducible manner. Here, we examined the in vivo function of the enhancer located 3' to the Epo gene (EpoE-3'). Reporter transgenic-mouse analyses revealed that the EpoE-3' enhancer is necessary and sufficient for the liver-specific and hypoxia-responsive expression of the gene after embryonic day 14.5 (E14.5). However, the enhancer is dispensable for Epo gene expression in the kidney and early-stage embryonic liver. Genetic removal of EpoE-3' from the endogenous Epo gene resulted in mice with severe anemia at late embryonic and neonatal stages due to defects in hepatic erythropoiesis, but early hepatic and splenic erythropoiesis was not affected. The mutant mice recover from the anemia in the juvenile period when major Epo production switches from the liver to the kidney. These results demonstrate that EpoE-3' is necessary for late hepatic erythropoiesis by specifically supporting paracrine production of Epo in the liver. In contrast, Epo production in the kidney utilizes distinct regulatory machinery and supports erythropoiesis in the bone marrow and spleen in adult animals.

Transcriptional regulation by GATA1 and GATA2 during erythropoiesis

The transcription factor GATA1 regulates multiple genes in erythroid lineage cells. However, the means by which GATA1 regulates the expression of target genes during erythropoiesis remains to be elucidated. Three mechanisms have been postulated for the regulation of genes by GATA1. First, individual target genes may have multiple discrete thresholds for cellular GATA1. GATA1 has a dynamic expression profile during erythropoiesis, thus the expression of a set of GATA1 target genes may be triggered at a given stage of differentiation by cellular GATA1. Second, the expression of GATA1 target genes may be modified, at least in part, by GATA2 occupying the GATA-binding motifs. GATA2 is expressed earlier in erythropoiesis than GATA1, and prior GATA2 binding may afford GATA1 access to GATA motifs through epigenetic remodeling and thus facilitate target gene expression. Third, other regulatory molecules specific to each target gene may function cooperatively with GATA1. If GATA1 is required for the expression of such cofactors, a regulatory network will be formed and relevant gene expression will be delayed. We propose that the stage-specific regulation of erythroid genes by GATA1 is tightly controlled through a combination of these mechanisms in vivo.

Regulation of bone marrow hematopoietic stem cell is involved in high-altitude erythrocytosis

OBJECTIVE

Hypoxia at high altitudes can lead to increased production of red blood cells through the hormone erythropoietin (EPO). In this study, we observed how the EPO-unresponsive hematopoietic stem cell (HSC) compartment responds to high-altitude hypoxic environments and contributes to erythropoiesis.

MATERIALS AND METHODS

Using a mouse model at simulated high altitude, the bone marrow (BM) and spleen lineage marker(-)Sca-1(+)c-Kit(+) (LSK) HSC compartment were observed in detail. Normal LSK cells were then cultured under different conditions (varying EPO levels, oxygen concentrations, and BM supernatants) to investigate the causes of the HSC responses.

RESULTS

Hypoxic mice exhibited a marked expansion in BM and spleen LSK compartments, which were associated with enhanced proliferation. BM HSCs seemed to play a more important role in erythropoiesis at high altitude than spleen HSCs. There was also a lineage fate change of BM HSCs in hypoxic mice that was manifested in increased megakaryocyte-erythrocyte progenitors and periodically reduced granulocyte-macrophage progenitors in the BM. The LSK cells in hypoxic mice displayed upregulated erythroid-specific GATA-1 and downregulated granulocyte-macrophage-specific PU.1 messenger RNA expression, as well as the capacity to differentiate into more erythroid precursors after culture. BM culture supernatant from hypoxic mice (but not elevated EPO or varying O(2) tension) could induce expansion and erythroid-priority differentiation of the HSC population, a phenomenon partially caused by increasing interleukin-3 and interleukin-6 secretion in the BM.

CONCLUSIONS

The present study suggests a new EPO-independent HSC mechanism of high-altitude erythrocytosis.

Fractionation of mature eosinophils in GATA-reporter transgenic mice

Eosinophils contribute to the pathophysiology of allergic and infectious diseases, albeit their molecular functions remain unknown. Mature eosinophils are identified by their unique morphology and staining characteristics. However, it is difficult to fractionate living eosinophils by flow cytometry because these granulocytes express multiple cell surface markers that are shared by other cells of hematopoietic or non-hematopoietic origin. In this study, we describe a flow cytometry-based method to enumerate and fractionate eosinophils by developmental stages. To fractionate these cell types, we used transgenic mouse lines that express fluorescent proteins under control of the Gata1 gene hematopoietic regulatory region (Gata1-HRD), which is exclusively active in Gata1-expressing hematopoietic cells, including eosinophils. As expected, mature eosinophils were highly enriched in the fluorescent reporter-expressing subfraction of bone marrow myeloid cells that were negatively selected by using multiple antibodies against B220, CD4, CD8, Ter119, c-Kit and CD71. Cytochemical analyses of flow-sorted cells identified the cells in this fraction as eosinophils harboring eosinophilic granules. Additionally, expression of eosinophil-specific genes, for instance eosinophil enzymes and the IL-5 receptor alpha gene, were specifically detected in this fraction. The expression of these eosinophil-specific genes increased as the cells differentiated. This method for enrichment of bone marrow eosinophils is applicable to fractionation of eosinophils and bronchoalveolar lavage fluid from transgenic mice with atopic asthma. Thus, both pathological and developmental stages of eosinophils are efficiently fractionated by this flow cytometry-based method using Gata1-HRD transgenic reporter mice. This study, therefore, proposes a useful means to study the experimental allergic and inflammatory systems.

Genetic analysis of hierarchical regulation for Gata1 and NF-E2 p45 gene expression in megakaryopoiesis

GATA1 and NF-E2 p45 are two important regulators of megakaryopoiesis. Whereas GATA1 is known to regulate the p45 gene, details of the GATA1 contribution to the spatiotemporal expression of the p45 gene remain to be elucidated. To clarify the relationship between GATA1 and p45, we performed genetic complementation rescue analysis of p45 function in megakaryocytes utilizing the hematopoietic regulatory domain of the Gata1 gene (G1HRD). We established transgenic mouse lines expressing p45 under G1HRD regulation and crossed the mice with p45-null mice. Compound mutant mice displayed normal platelet counts and no sign of hemorrhage, indicating that G1HRD has the ability to express p45 in a spatiotemporally correct manner. However, deletion of 38 amino acids from the N-terminal region of p45 abrogated the p45 rescue function, suggesting the presence of an essential transactivation activity in the region. We then crossed the G1HRD-p45 transgenic mice with megakaryocyte-specific Gata1 gene knockdown (Gata1(Delta)(neo)(Delta)(HS)) mice. The G1HRD-p45 transgene was insufficient for complete rescue of the Gata1(Delta)(neo)(Delta)(HS) megakaryocytes, suggesting that GATA1 or other factors regulated by GATA1 are required to cooperate with p45 for normal megakaryopoiesis. This study thus provides a unique in vivo validation of the hierarchical relationship between GATA1 and p45 in megakaryocytes.

LRF is an essential downstream target of GATA1 in erythroid development and regulates BIM-dependent apoptosis

GATA-1-dependent transcription is essential for erythroid differentiation and maturation. Suppression of programmed cell death is also thought to be critical for this process; however, the link between these two features of erythropoiesis has remained elusive. Here, we show that the POZ-Krüppel family transcription factor, LRF (also known as Zbtb7a/Pokemon), is a direct target of GATA1 and plays an essential antiapoptotic role during terminal erythroid differentiation. We find that loss of Lrf leads to lethal anemia in embryos, due to increased apoptosis of late-stage erythroblasts. This programmed cell death is Arf and p53 independent and is instead mediated by upregulation of the proapoptotic factor Bim. We identify Lrf as a direct repressor of Bim transcription. In strong support of this mechanism, genetic Bim loss delays the lethality of Lrf-deficient embryos and rescues their anemia phenotype. Thus, our data define a key transcriptional cascade for effective erythropoiesis, whereby GATA-1 suppresses BIM-mediated apoptosis via LRF.

Characterization of a functional ZBP-89 binding site that mediates Gata1 gene expression during hematopoietic development

GATA-1 is a lineage-restricted transcription factor that plays essential roles in hematopoietic development. The Gata1 gene hematopoietic enhancer allowed Gata1 reporter expression in erythroid cells and megakaryocytes of transgenic mice. The Gata1 hematopoietic enhancer activity is strictly dependent on a GATA site located in the 5' region of the enhancer. However, the importance of the GC-rich region adjacent to the 3'-end of this GATA site has been also suggested. In this study, we show that this GC-rich region contains five contiguous deoxyguanosine residues (G(5) string) that are bound by multiple nuclear proteins. Interestingly, deletion of one deoxyguanosine residue from the G(5) string (G(4) mutant) specifically eliminates binding to ZBP-89, a Krüppel-like transcription factor, but not to Sp3 and other binding factors. We demonstrate that GATA-1 and ZBP-89 occupy chromatin regions of the Gata1 enhancer and physically associate in vitro through zinc finger domains. Gel mobility shift assays and DNA affinity precipitation assays suggest that binding of ZBP-89 to this region is reduced in the absence of GATA-1 binding to the G1HE. Luciferase reporter assays demonstrate that ZBP-89 activates the Gata1 enhancer depending on the G(5) string sequence. Finally, transgenic mouse studies reveal that the G(4) mutation significantly reduced the reporter activity of the Gata1 hematopoietic regulatory domain encompassing an 8.5-kbp region of the Gata1 gene. These data provide compelling evidence that the G(5) string is necessary for Gata1 gene expression in vivo and ZBP-89 is the functional trans-acting factor for this cis-acting region.

GATA2-dependent and region-specific regulation of Gata2 transcription in the mouse midbrain

Transcription factor GATA2 is expressed in numerous mammalian tissues, including neural, hematopoietic, cardiovascular and urogenital systems, and yet it plays important roles in the regulation of tissue-restricted gene expression. The Gata2 gene itself is also under stringent tissue-specific control and multiple cis-regulatory domains have been identified in the Gata2 locus. In this study we sought out and then examined in detail the domains that regulate Gata2 in the midbrain. We identified two discrete domains in the Gata2 promoter that direct midbrain expression; these distal 5H and proximal 2H regulatory domains are located 3.0 and 1.9 kbp, respectively, upstream of the transcriptional initiation site. Importantly, both domains contain GATA factor binding sites. Our analyses further revealed that GATA2 is essential for Gata2 gene expression in the midbrain, whereas GATA3 is not. Both the 2H and 5H domains have the independent ability to activate Gata2 gene expression in the midbrain superior colliculus, whereas the distal-5H domain is additionally capable of activating Gata2 transcription in the inferior colliculus. These results demonstrate that two distinct regulatory domains contribute to the Gata2 gene expression in the mouse midbrain and that Gata2 midbrain transcription is under positive autoregulation.

Leukemia-related transcription factor TEL/ETV6 expands erythroid precursors and stimulates hemoglobin synthesis

TEL/ETV6 located at chromosome 12p13 encodes a member of the E26 transformation-specific family of transcription factors. TEL is known to be rearranged in a variety of leukemias and solid tumors resulting in the formation of oncogenic chimeric protein. Tel is essential for maintaining hematopoietic stem cells in the bone marrow. To understand the role of TEL in erythropoiesis, we generated transgenic mice expressing human TEL under the control of Gata1 promoter that is activated during the course of the erythroid-lineage differentiation (GATA1-TEL transgenic mice). Although GATA1-TEL transgenic mice appeared healthy up to 18 months of age, the level of hemoglobin was higher in transgenic mice compared to non-transgenic littermates. In addition, CD71+/TER119+ and c-kit+/CD41+ populations proliferated with a higher frequency in transgenic mice when bone marrow cells were cultured in the presence of erythropoietin and thrombopoietin, respectively. In transgenic mice, enhanced expression of Alas-e and beta-major globin genes was observed in erythroid-committed cells. When embryonic stem cells expressing human TEL under the same Gata1 promoter were differentiated into hematopoietic cells, immature erythroid precursor increased better compared to controls as judged from the numbers of burst-forming unit of erythrocytes. Our findings suggest some roles of TEL in expanding erythroid precursors and accumulating hemoglobin.