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Erythroid terminal differentiation entails cell divisions that are coupled to progressive decreases in cell size. EpoR signaling is essential for the survival of erythroid precursors, but it is unclear whether it has other functions in these cells. Here we endowed mouse precursors that lack the EpoR with survival signaling, finding that this was sufficient to support their differentiation into enucleated red cells, but that the process was abnormal. Precursors underwent fewer and slower cell cycles and yet differentiated into smaller red cells. Surprisingly, EpoR further accelerated cycling of early erythroblasts, the fastest cycling cells in the bone marrow, while simultaneously increasing their cell size. EpoR-mediated formation of larger red cells was independent of the established pathway regulating red cell size by iron through Heme-regulated eIF2α kinase (HRI). We confirmed the effect of Epo on red cell size in human volunteers, whose mean corpuscular volume (MCV) increased following Epo administration. This increase persisted after Epo declined and was not the result of increased reticulocytes. Our work reveals a unique effect of EpoR signaling on the interaction between the cell cycle and cell growth. Further, it suggests new diagnostic interpretations for increased red cell volume, as reflecting high Epo and erythropoietic stress.

The erythroid terminal differentiation program couples sequential cell divisions with progressive reductions in cell size. The erythropoietin receptor (EpoR) is essential for erythroblast survival, but its other functions are not well characterized. Here we use Epor−/− mouse erythroblasts endowed with survival signaling to identify novel non-redundant EpoR functions. We find that, paradoxically, EpoR signaling increases red cell size while also increasing the number and speed of erythroblast cell cycles. EpoR-regulation of cell size is independent of established red cell size regulation by iron. High erythropoietin (Epo) increases red cell size in wild-type mice and in human volunteers. The increase in mean corpuscular volume (MCV) outlasts the duration of Epo treatment and is not the result of increased reticulocyte number. Our work shows that EpoR signaling alters the relationship between cycling and cell size. Further, diagnostic interpretations of increased MCV should now include high Epo levels and hypoxic stress., Maturing erythroblasts become smaller with every cell division. Here, the authors show that Epo stimulation promotes cell division and also generates larger red cells, and that this occurs in mouse and human cells, suggesting that red cell size could be a diagnostic marker for hypoxic stress.

The hormone erythropoietin (Epo) is required for erythropoiesis, yet its molecular mechanism of action remains poorly understood, particularly with respect to chromatin dynamics. To investigate how Epo modulates the erythroid epigenome, we performed epigenetic profiling using an ex vivo murine cell system that undergoes synchronous erythroid maturation in response to Epo stimulation. Our findings define the repertoire of Epo-modulated enhancers, illuminating a new facet of Epo signaling. First, a large number of enhancers rapidly responded to Epo stimulation, revealing a cis-regulatory network of Epo-responsive enhancers. In contrast, most of the other identified enhancers remained in an active acetylated state during Epo signaling, suggesting that most erythroid enhancers are established at an earlier precursor stage. Second, we identified several hundred super-enhancers that were linked to key erythroid genes, such as Tal1, Bcl11a, and Mir144/451. Third, experimental and computational validation revealed that many predicted enhancer regions were occupied by TAL1 and enriched with DNA-binding motifs for GATA1, KLF1, TAL1/E-box, and STAT5. Additionally, many of these cis-regulatory regions were conserved evolutionarily and displayed correlated enhancer:promoter acetylation. Together, these findings define a cis-regulatory enhancer network for Epo signaling during erythropoiesis, and provide the framework for future studies involving the interplay of epigenetics and Epo signaling.

The paper is devoted to multi-scale modelling of erythropoiesis and hemoglobin production. Red blood cells, which carry oxygen from the lungs to the other body tissues, are produced in the bone marrow of adult humans in cell units called erythroblastic islands. Erythroblastic islands are composed by a central macrophage surrounded by erythroid cells in different stages of maturation. Immature cells, the colony-forming units-erythroid, make a choice between self-renewal, differentiation and apoptosis determined by the intracellular proteins and extracellular substances. Moreover, this choice is regulated by erythropoietin and other hormones. Erythropoietin is produced in the kidney in response to hypoxia from decreased numbers of red blood cells, and it is delivered in the plasma to the bone marrow. Erythropoietin stimulates differentiation of erythroid cells and increases their proliferation by downregulating apoptosis. The rate of erythropoietin production depends on the level of hemoglobin in blood which is function of the number of circulating red blood cells. Hemoglobin is produced in the erythroid cells within the bone marrow in the process of their terminal differentiation. Thus, there is a feedback between production of red blood cells by the bone marrow, the level of hemoglobin contained in these cells and the level of erythropoietin. The multi-scale model developed in this work includes erythroid cells in the bone marrow, their intracellular and extracellular regulations, hemoglobin production, and the feedback by erythropoietin. This model describes normal functioning of erythropoiesis and its response to anemia resulting from the loss of red blood cells.

As they mature into erythrocytes during normal erythropoiesis, reticulocytes lose surface transferrin receptors before or concurrently with reticulin. Exosome release accounts for most of the loss of transferrin receptors from reticulocytes. During erythropoietic stress, reticulocytes are released early from hematopoietic tissues and have increased reticulin staining and transferrin receptors. Flow cytometry of dually stained erythrocytes of mice recovering from phlebotomy demonstrated delayed loss of reticulin and transferrin receptors during in vitro maturation compared to in vivo maturation, indicating that an in vivo process extrinsic to the reticulocytes facilitates their maturation. Splenectomy or macrophage depletion by liposomal clodronate inhibited in vivo maturation of reticulocytes and increased the numbers of reticulin-negative, transferrin receptor-positive cells during and after recovery from phlebotomy. This reticulin-negative, transferrin receptor-positive population was rarely found in normal mice. Transmission electron microscopy demonstrated that the reticulin-negative, transferrin receptor-positive cells were elongated and discoid erythrocytes, but they had intracellular and surface structures that appeared to be partially degraded organelles. The results indicate that maturation of circulating stress reticulocytes is enhanced by an extrinsic process that occurs in the spleen and involves macrophage activity. Complete loss of reticulin with incomplete loss of surface transferrin receptors in this process produces a reticulin-negative, transferrin receptor-positive erythrocyte population that has potential utility for detecting prior erythropoietic stresses including bleeding, hemolysis and erythropoietin administration, even after recovery has been completed. Am. J. Hematol. 91:875-882, 2016. © 2016 Wiley Periodicals, Inc.

EpoR signaling is essential for the survival of erythroid precursors during terminal differentiation, when these cells undergo 3-5 cell divisions that are coupled to a rapid loss in cell size. The absolute dependence of erythroblasts on EpoR signaling for survival makes it challenging to identify other essential functions of EpoR in these cells. Key open questions include a role for EpoR in cell cycle regulation; and a potential role for EpoR in the induction of erythroid genes. To address these gaps, we developed a genetic system that allowed us to examine essential non-survival functions of EpoR signaling in erythroid differentiation. We rescued mouse Epor-/- fetal liver erythroid progenitors from apoptosis by transduction with the anti-apoptotic protein bcl-xL, comparing their ensuing differentiation with that of Epor-/- progenitors that were rescued by re-introduction of the EpoR. We found that the bcl-xL survival signal, in the absence of any EpoR signaling, was sufficient to allow erythroid terminal differentiation and formation of anucleated red cells. However, key features were abnormal. First, erythroblasts underwent slower and fewer cell cycles, generating fewer than half the number of red cells, showing an essential role for EpoR signaling in the regulation of cell cycle speed and number. Second, we found that EpoR signaling unexpectedly increased red cell size, in spite of the concurrent increase in the number of cell divisions. Using mice doubly deleted for both EpoR and HRI, we found that EpoR regulation of red cell size is independent of the well-described red cell size regulation by iron and heme. Healthy human volunteers that were administered Epo showed an increase in mean corpuscular volume (MCV) that persisted long after Epo and reticulocyte levels returned to baseline, supporting a role for the regulation of red cell size by Epo and suggesting new diagnostic interpretations for this common clinical test. Together, our findings reveal novel functions for the EpoR in erythroid differentiation including an unexpected role in the regulation of red cell size.

International audience; Multiple myeloma (MM) is a genetically complex hematological cancer that is characterized by proliferation of malignant plasma cells in the bone marrow. MM evolves from the clonal premalignant disorder monoclonal gammopathy of unknown significance (MGUS) by sequential genetic changes involving many different genes, resulting in dysregulated growth of multiple clones of plasma cells. The migration, survival, and proliferation of these clones require the direct and indirect interactions with the non-hematopoietic cells of the bone marrow. We develop a hybrid discrete-continuous model of MM development from the MGUS stage. The discrete aspect of the model is observed at the cellular level: cells are represented as individual objects which move, interact, divide, and die by apoptosis. Each of these actions is regulated by intracellular and extracellular processes as described by continuous models. The hybrid model consists of the following submodels that have been simplified from the much more complex state of evolving MM: cell motion due to chemotaxis, intracellular regulation of plasma cells, extracellular regulation in the bone marrow, and acquisition of mutations upon cell division. By extending a previous, simpler model in which the extracellular matrix was considered to be uniformly distributed, the new hybrid model provides a more accurate description in which cytokines are produced by the marrow microenvironment and consumed by the myeloma cells. The complex multiple genetic changes in MM cells and the numerous cell-cell and cytokine-mediated interactions between myeloma cells and their marrow microenviroment are simplified in the model such that four related but evolving MM clones can be studied as they compete for dominance in the setting of intraclonal heterogeneity.

The functional unit of definitive mammalian erythropoiesis, the erythroblastic island, consists of a central macrophage surrounded by adherent erythroid progenitor cells at the colony-forming unit/proerythroblast (CFU-E/Pro-EB) stages of differentiation and their differentiating progeny, the erythroblasts. Central macrophages display on their surface or secrete various growth or inhibitory factors that influence the fate of the surrounding erythroid cells. CFU-E/Pro-EBs have three possible fates: a) expansion of their numbers without differentiation, b) differentiation through the erythroblast stages into reticulocytes that are released into the blood, c) death by apoptosis. CFU-E/Pro-EB fate is under the control of a complex intracellular molecular network that is highly dependent upon environmental conditions in the erythroblastic island. Direct examination of erythroblastic island function in vivo has been limited in mice and unfeasible in humans. In order to assess the functional role of spatial organization coupled with the complex network behavior in erythroblastic islands, we developed hybrid discrete-continuous models of erythropoiesis. A mathematical model was developed in which the cells of the erythroblastic island are considered as individual physical objects, intracellular regulatory networks are modeled with ordinary differential equations, and extracellular concentrations of cytokines or hormones are modeled by partial differential equations. The concentrations of the cytokines Fas-ligand and bone morphogenetic protein-4, which are produced locally in the erythroblastic island, and the hormones erythropoietin and glucocorticosteroid hormone, which are produced at remote locations in the body, are included in the model. We used the model in simulations that investigated the impact of an important difference between humans and mice in which mature late-stage erythroblasts produce the most Fas-ligand in humans, and early-stage erythroblasts produce the most Fas-ligand in mice. Although the global behaviors of the erythroblastic islands in both species were similar, differences were found, including a relatively slower recovery time of hematocrits and erythrocyte numbers to their baselines following the development of acute anemia in humans as compared to mice. These simulation results with the model were consistent with the more rapid recovery to baseline in mice that were bled to about one-half of their normal hematocrit compared to two patients who had acute blood loss to about one-half of their respective baseline hematocrits and recovered without erythrocyte transfusions. Our modeling approach was also very consistent with the previously reported results of in vitro cultures, where the central macrophages in reconstituted erythroblastic islands of mice had a strong impact on the dynamics of erythroid cell proliferation. The spatial organization of cells in erythroblastic islands is important for the normal, stable functioning of mammalian erythropoiesis, both in vitro and in vivo. Our model of a simplified molecular network controlling erythroid progenitor cell decision and fate provides a realistic functional unit of mammalian erythropoiesis that integrates factors within the microenvironment of the erythroblastic island with those of circulating regulators of erythropoiesis. Our model highlights the need for proper inclusion of the spatial relationships of erythropoietic cells and allowing decisions to be made at the level of individual erythroid cells in the modeling process. Disclosures: Koury: Keryx Biopharmaceuticals, Inc.: Consultancy; TG Therapeutics, Inc.: Consultancy.

The TAL1 (or SCL) gene, originally discovered through its involvement by a chromosomal translocation in T-cell acute lymphoblastic leukemia, encodes a basic helix-loop-helix (bHLH) transcription factor essential for hematopoietic and vascular development. To identify its interaction partners, we expressed a tandem epitope-tagged protein in murine erythroleukemia (MEL) cells and characterized affinity-purified Tal1-containing complexes by liquid chromatography-tandem mass spectrometry analysis. In addition to known interacting proteins, two proteins related to the Eight-Twenty-One (ETO) corepressor, Eto2/Mtg16 and Mtgr1, were identified from the peptide fragments analyzed. Tal1 interaction with Eto2 and Mtgr1 was verified by coimmunoprecipitation analysis in Tal1, Eto2-, and Mtgr1-transfected COS-7 cells, MEL cells expressing V5 epitope-tagged Tal1 protein, and non-transfected MEL cells. Mapping analysis with Gal4 fusion proteins demonstrated a requirement for the bHLH domain of Tal1 and TAF110 domain of Eto2 for their interaction, and transient transfection and glutathione S-transferase pull-down analysis showed that Mtgr1 and Eto2 enhanced the other’s association with Tal1. Enforced expression of Eto2 in differentiating MEL cells inhibited the promoter of the Protein 4.2 (P4.2) gene, a direct target of TAL1 in erythroid progenitors, and transduction of Eto2 and Mtgr1 augmented Tal1-mediated gene repression. Finally, chromatin immunoprecipitation analysis revealed that Eto2 occupancy of the P4.2 promoter in MEL cells decreased with differentiation, in parallel with a decline in Eto2 protein abundance. These results identify Eto2 and Mtgr1 as authentic interaction partners of Tal1 and suggest they act as heteromeric corepressors of this bHLH transcription factor during erythroid differentiation.

Erythrocytes have the same intracellular concentration of ascorbate as plasma, which is much lower than that of nucleated cells. To determine why erythrocytes are unable to concentrate ascorbate, we tested for the presence of ascorbate transporters in these cells. Human erythrocytes had very low rates of uptake of radiolabeled ascorbate, which was accounted for by the lack of ascorbate transporter SVCT2 in immunoblots. Using a cell culture model of Friend virus-infected mouse erythroblasts, immunoblots showed that the SVCT2 was present in the erythroblast stages, but was lost following extrusion of the nucleus in the formation of the reticulocyte stage. Rates of specific ascorbate transport correlated with the presence of the SVCT2. These results show that mature erythrocytes fail to concentrate ascorbate due to the loss of SVCT2 during maturation in the bone marrow.

Red blood cell production rates increase rapidly following blood loss or hemolysis, but the expansion of erythropoiesis in these anemic states is tightly regulated such that rebound polycythemia does not occur. The erythroid cells that respond to erythropoietic stimulation or suppression are the progenitor stages of burst-forming units–erythroid (BFU-Es) and colony-forming units–erythroid (CFU-Es). Results from an early study of the changes in the size, location, and cell cycling status of BFU-E and CFU-E populations in mice under normal conditions, erythropoietic stimulation, and erythropoietic suppression are used as reference points to review subsequent developments related to erythroid progenitor populations and regulation of their size. The review concerns development of erythroid progenitor populations mainly in mice and humans, with a focus on the mechanisms related to the rapid but highly regulated expansion of erythropoiesis in spleens of erythropoietically stimulated mice. Current knowledge is used as a model of erythroid progenitor populations in mice under normal, erythropoietically suppressed, and erythropoietically stimulated conditions. Clinical applications of information learned from studies of erythropoietic expansion, in terms of current therapies for anemia, are reviewed.

Enucleation, a rare feature of mammalian differentiation, occurs in 3 cell types: erythroblasts, lens epithelium, and keratinocytes. Previous investigations suggest that caspase activation functions in lens epithelial and keratinocyte enucleation, as well as in early erythropoiesis encompassing erythroid burst-forming unit (BFU-E) differentiation to proerythroblast. To determine whether caspase activation contributes to later erythropoiesis and whether nuclear substructures other than chromatin reorganize, we analyzed distributions of nuclear subcompartment proteins and assayed for caspase-induced cleavage of subcompartmental target proteins in mouse erythroblasts. We found that patterns of lamin B in the filamentous network interacting with both the nuclear envelope and DNA, nuclear matrix protein NuMA (Nuclear mitotic apparatus), and splicing factors Sm and SC35 persisted during nuclear condensation, consistent with effective transcription of genes expressed late in differentiation. Thus, nuclear reorganization prior to enucleation is selective, allowing maintenance of critical transcriptional processes independent of extensive chromosomal reorganization. Consistent with these data, we found no evidence for caspase-induced cleavage of major nuclear subcompartment proteins during late erythropoiesis, in contrast to what has been observed in early erythropoiesis and in lens epithelial and keratinocyte differentiation. These findings imply that nuclear condensation and extrusion during terminal erythroid differentiation involve novel mechanisms that do not entail major activation of apoptotic machinery. (Blood. 2005;106:2200-2205)

Most studies of mammalian reticulocyte maturation have used blood reticulocytes.Nascent reticulocytes, as found in bone marrow, have not been available in developmentally synchronized populations. Nascent murine reticulocytes formed in vitro by enucleation of Friend virus–infected erythroblasts were purified and recultured for 110 hours. At 0 hours, all recultured cells were lobulated and contained dense, centralized reticulin. By 110 hours, about 20% to 25% of the cells became biconcave erythrocytes. Most ribosomes and cellular RNAs were degraded within 20 hours, and during that period, heme synthesis declined from a rate equal to that of late erythroblasts to less than 10% of that rate. Many mitochondria appeared normal until they showed outer membrane swelling, degradation, and apparent fusion with intracellular vacuoles at 40 hours of culture. During the period of mitochondrial loss, Bcl-XL, an antiapoptotic protein that accumulates during erythroblast differentiation and maintains mitochondrial membrane integrity, demonstrated progressive decreases and changes consistent with deamidation. Nevertheless, the reticulocytes did not undergo apoptosis, because their apoptotic machinery was degraded. This experimental system that provides a developmentally synchronized population of nascent murine reticulocytes that mature into biconcave erythrocytes in vitro should be useful in further investigations of the cellular events involved in reticulocyte maturation.

During erythroblast enucleation, nuclei surrounded by plasma membrane separate from erythroblast cytoplasm. A key aspect of this process is sorting of erythroblast plasma membrane components to reticulocytes and expelled nuclei. Although it is known that cytoskeletal elements actin and spectrin partition to reticulocytes, little is understood about molecular mechanisms governing plasma membrane protein sorting. We chose glycophorin A (GPA) as a model integral protein to begin investigating protein-sorting mechanisms. Using immunofluorescence microscopy and Western blotting we found that GPA sorted predominantly to reticulocytes. We hypothesized that the degree of skeletal linkage might control the sorting pattern of transmembrane proteins. To explore this hypothesis, we quantified the extent of GPA association to the cytoskeleton in erythroblasts, young reticulocytes, and mature erythrocytes using fluorescence imaged microdeformation (FIMD) and observed that GPA underwent dramatic reorganization during terminal differentiation. We discovered that GPA was more connected to the membrane cytoskeleton, either directly or indirectly, in erythroblasts and young reticulocytes than in mature cells. We conclude that skeletal protein association can regulate protein sorting during enucleation. Further, we suggest that the enhanced rigidity of reticulocyte membranes observed in earlier investigations results, at least in part, from increased connectivity of GPA with the spectrin-based skeleton.

Among the alternative pre-mRNA splicing events that characterize protein 4.1R gene expression, one involving exon 2′ plays a critical role in regulating translation initiation and N-terminal protein structure. Exon 2′ encompasses translation initiation site AUG1 and is located between alternative splice acceptor sites at the 5′ end of exon 2; its inclusion or exclusion from mature 4.1R mRNA regulates expression of longer or shorter isoforms of 4.1R protein, respectively. The current study reports unexpected complexity in the 5′ region of the 4.1R gene that directly affects alternative splicing of exon 2′. Identified far upstream of exon 2 in both mouse and human genomes were 3 mutually exclusive alternative 5′ exons, designated 1A, 1B, and 1C; all 3 are associated with strong transcriptional promoters in the flanking genomic sequence. Importantly, exons 1A and 1B splice differentially with respect to exon 2′, generating transcripts with different 5′ ends and distinct N-terminal protein coding capacity. Exon 1A-type transcripts splice so as to exclude exon 2′ and therefore utilize the downstream AUG2 for translation of 80-kDa 4.1R protein, whereas exon 1B transcripts include exon 2′ and initiate at AUG1 to synthesize 135-kDa isoforms. RNA blot analyses revealed that 1A transcripts increase in abundance in late erythroblasts, consistent with the previously demonstrated up-regulation of 80-kDa 4.1R during terminal erythroid differentiation. Together, these results suggest that synthesis of structurally distinct 4.1R protein isoforms in various cell types is regulated by a novel mechanism requiring coordination between upstream transcription initiation events and downstream alternative splicing events.

In the hematopoietic hierarchy, only stem cells are thought to be capable of long-term self-renewal. Erythroid progenitors derived from fetal or adult mammalian hematopoietic tissues are capable of short-term, or restricted (102- to 105-fold), ex vivo expansion in the presence of erythropoietin, stem cell factor, and dexamethasone. Here, we report that primary erythroid precursors derived from early mouse embryos are capable of extensive (106- to 1060-fold) ex vivo proliferation. These cells morphologically, immunophenotypically, and functionally resemble proerythroblasts, maintaining both cytokine dependence and the potential, despite prolonged culture, to generate enucleated erythrocytes after 3-4 maturational cell divisions. This capacity for extensive erythroblast self-renewal is temporally associated with the emergence of definitive erythropoiesis in the yolk sac and its transition to the fetal liver. In contrast, hematopoietic stem cell-derived definitive erythropoiesis in the adult is associated almost exclusively with restricted ex vivo self-renewal. Primary primitive erythroid precursors, which lack significant expression of Kit and glucocorticoid receptors, lack ex vivo self-renewal capacity. Extensively self-renewing erythroblasts, despite their near complete maturity within the hematopoietic hierarchy, may ultimately serve as a renewable source of red cells for transfusion therapy.

International audience; Erythropoiesis is the process of red blood cell production in the bone marrow. Terminal stages of human erythropoiesis occur in multicellular structures called erythroblastic islands (EBIs). EBIs contain up to several dozen erythroid cells of varying maturities organized around a central macrophage. Immature erythroid cells, burst forming units (BFU-E) circulate in blood and home to bone marrow, where they can have limited but random movement. When BFU-Es approach a macrophage, they divide producing colony forming units-erythroid (CFU-E), which are the next stage of erythroid differentiation. CFU-Es and their immediate progeny, proerythroblasts, can self-renew, differentiate into more mature cells or die by apoptosis. The BFU-E, CFU-E, and the subsequent erythroblast stages provide normal functioning of erythropoiesis. In this work we develop a hybrid discrete-continuous model in order to describe normal erythropoiesis in the bone marrow. Cells are represented as individual objects that move, divide, differentiate, die and interact with each other. We show how BFU-E cells initiate EBIs.

Deficiency of folate or vitamin B12 (cobalamin) causes megaloblastic anemia, a disease characterized by pancytopenia due to the excessive apoptosis of hematopoietic progenitor cells. Clinical and experimental studies of megaloblastic anemia have demonstrated an impairment of DNA synthesis and repair in hematopoietic cells that is manifested by an increased percentage of cells in the DNA synthesis phase (S phase) of the cell cycle, compared with normal hematopoietic cells. Both folate and cobalamin are required for normal de novo synthesis of thymidylate and purines. However, previous studies of impaired DNA synthesis and repair in megaloblastic anemia have concerned mainly the decreased intracellular levels of thymidylate and its effects on nucleotide pools and misincorporation of uracil into DNA. An in vitro model of folate-deficient erythropoiesis was used to study the relationship between the S-phase accumulation and apoptosis in megaloblastic anemia. The results indicate that folate-deficient erythroblasts accumulate in and undergo apoptosis in the S phase when compared with control erythroblasts. Both the S-phase accumulation and the apoptosis were induced by folate deficiency in erythroblasts fromp53 null mice. The complete reversal of the S-phase accumulation and apoptosis in folate-deficient erythroblasts required the exogenous provision of specific purines or purine nucleosides as well as thymidine. These results indicate that decreased de novo synthesis of purines plays as important a role as decreased de novo synthesis of thymidylate in the pathogenesis of megaloblastic anemia.

Ectopic expression of the basic helix-loop-helix transcription factor TAL1 (or SCL) is the most frequent gain-of-function mutation in T-cell acute lymphoblastic leukaemia. Gene-knockout studies in mice have demonstrated that TAL1 is required for embryonic and adult haematopoiesis, and considerable evidence suggests it also has important functions in terminal erythroid differentiation. We reported previously that TAL1 phosphorylation is stimulated by erythropoietin in splenic proerythroblasts isolated from mice infected with the anaemia-inducing strain of Friend virus and show here the signalling pathway responsible. Erythropoietin was found to stimulate nuclear mitogen-activated protein kinase activity in addition to TAL1 protein phosphorylation, both of which were quantitatively inhibited by the mitogen-activated protein kinase kinase inhibitor PD 098059 and the phosphatidylinositol 3-kinase inhibitor wortmannin. Tryptic phosphopeptide analysis of radiolabelled TAL1 immunoprecipitated from nuclear extracts of Friend virus-induced proerythroblasts revealed that phosphorylation of Ser122, shown previously to be a substrate for the mitogen-activated protein kinase ERK1 (extracellular signal-regulated protein kinase) in vitro, was specifically, although not exclusively, increased by erythropoietin and inhibited by wortmannin and PD 098059. These results are consistent with an erythropoietin-stimulated signalling pathway in which there is direct activation of a mitogen-activated protein kinase kinase by phosphatidylinositol 3-kinase and identify TAL1 as one of its nuclear targets. These data suggest, in addition, a specific mechanism by which the principal regulator of erythroid differentiation could enhance TAL1 function, in addition to increasing its expression.

Gata1 is expressed from either one of two alternative promoters, the erythroid (proximal to the AUG) and the testis (distal to the AUG) promoter, both used by hemopoietic cells. To clarify the role of the distal and proximal Gata1 transcripts in erythroid differentiation, we determined by specific reverse transcriptase-polymerase chain reactions their relative levels of expression during the differentiation of erythroid precursors purified from the spleen of mice treated with phenylhydrazine (PHZ) or infected with the anemia-inducing strain of the Friend virus (FVA cells). PHZ cells are erythroid precursors that progress in vivo to erythroblasts in 3 days. Both PHZ and FVA cells synchronously proliferate and differentiate in vitro in the presence of erythropoietin (EPO). The levels of total and of distal, but not of proximal, Gata1 transcripts increased by five- to eightfold during in vivo and in vitro differentiation of FVA and PHZ cells. The increase in expression was temporally associated with an increase in the expression of Eklf, Scl, and Nfe2, three genes required for erythroid differentiation, and preceded by 24 h the repression of Gata2 and Myb expression. The day 1 PHZ cells that survived 18 h in the absence of EPO do not express globin genes and express detectable levels of distal but not of proximal Gata1 transcripts. These cells activate the expression of the globin genes within 2 h when exposed to EPO. Therefore, during erythroid differentiation of primary cells, increased expression of distal Gata1 transcripts underlies the increase in the expression of total Gata1 associated with the establishment of the erythroid differentiation program.

Chemotherapeutic agents exert their antitumor effects by inducing apoptosis. The microculture kinetic (MiCK) assay provides an automated, continuous means of monitoring apoptosis in a cell population. We used the MiCK assay to determine the chemosensitivities of the human promyelocytic HL-60 and lymphoblastic CEM cell lines and leukemia cells freshly isolated from patients with acute nonlymphocytic (ANLL) or acute lymphocytic (ALL) leukemias. Continuous monitoring of apoptosis in the MiCK assay permits determination of the time to the maximum apoptosis (Tm) and its two components which are initiation time (Ti) and development time (Td). Duration of the three timing components of apoptosis varies from hours to days depending on the drug, drug concentration, and type of target cells. In the MiCK assay, the extent of apoptosis is reported in kinetic units of apoptosis. Kinetic units are determined by the slope of the curve created when optical density caused by cell blebbing is plotted as a function of time. Using the leukemia cell lines, we define the relationship between kinetic units determined by the MiCK assay and the percentage of morphologically apoptotic cells in the culture. Flow cytometry analysis of apoptosis in Annexin-V-fluorescein isothiocyanate–labeled preparations of HL-60 and CEM cells was also used to compare with data obtained by the MiCK assay. The feasibility of the MiCK assay of apoptosis as a chemosensitivity test was confirmed by its comparison with a 3H-thymidine incorporation assay. We show that samples from 10 ANLL and ALL patients patients tested for sensitivity to various doses of idarubicin (IDR), daunorubicin (DNR), or mitoxantrone (MTA) gave the same percentages of apoptotic cells when calculated by the MiCK assay as when determined by morphological analysis. The MiCK assay was used for dose-response analyses of the sensitivities to IDR, DNR, and MTA of leukemia cells from 4 other patients (2 ANLL and 2 ALL). The results from both cell lines and patient samples indicate that ANLL cells are more sensitive than ALL cells to all three of these chemotherapeutic agents. However, for individual patients the chemosensitivities varied significantly among the three chemotherapeutic agents. These varying responses to IDR, DNR, and MTA indicate that the MiCK assay results can be of potential use in designing a treatment regimen for a specific patient with acute leukemia. Among several drugs of presumed similar efficacy, the MiCK assay can permit the selection of the specific chemotherapeutic agent that causes the most apoptosis in the patient's leukemic cells.© 1998 by The American Society of Hematology.

Clinical studies have indicated that folate deficiency may enhance the development of various malignancies. In animal studies that examined the effect of folate deficiency on malignancies, conflicting results have been reported. In some studies, folate deficiency increased the development and growth of malignant tumors; in others, it decreased the development and growth of malignancies. We examined the effect of transient folate deficiency on the development of leukemia in mice infected with the anemia-inducing strain of Friend leukemia virus. Friend virus disease can be considered as a model for human acute leukemias that are preceded by a preleukemic period. These include leukemias that develop in patients who received previous chemotherapy and/or radiation therapy, as well as patients with chronic granulocytic leukemia or myelodysplasia. Folate deficiency around the time of Friend virus-infection delayed the onset but increased the incidence of leukemia. The rates of rearrangement of the Spi-1 (PU.1 ) oncogene by provirus integration and alteration of the p53 tumor-suppressor gene were the same in leukemia cell lines derived from folate-deficient mice as they were in cell lines from control mice. These results indicate that folate deficiency did not exert its enhancement of leukemogenesis through changes in either Spi-1 or p53, even though these two genes have been found to be the most frequently altered ones in Friend virus-induced leukemias. Our results suggest that folate deficiency may enhance the development of acute leukemia in patients who are at high risk for this disease.

An in vitro model of folate-deficient erythropoiesis has been developed using proerythroblasts isolated from the spleens of Friend virus-infected mice fed an amino acid-based, folate-free diet. Control proerythroblasts were obtained from Friend virus-infected mice fed the same diet plus 2 mg folic acid/kg diet. Our previous studies showed that, after 20 to 32 hours of culture in folate-deficient medium with 4 U/mL of erythropoietin, the folate-deficient proerythroblasts underwent apoptosis, whereas control erythroblasts survived and differentiated into reticulocytes over a period of 48 hours. The addition of folic acid or thymidine to the folate-deficient medium prevented the apoptosis of the folate-deficient erythroblasts, thereby implicating decreased thymidylate synthesis as the main cause of apoptosis in the folate-deficient erythroblasts. In the study reported here, we examined intracellular folate levels, uracil misincorporation into DNA, p53 and p21 proteins, and reticulocyte formation in erythroblasts cultured in folate-deficient or control medium. In all experiments, the folate-deficient erythroblasts cultured in folate-deficient medium gave results that varied significantly from folate-deficient erythroblasts cultured in control medium or control erythroblasts cultured in either folate-deficient or control media. Folate-deficient erythroblasts cultured in folate-deficient medium had marked decreases in all coenzyme forms of folate that persisted throughout culture, increased uracil misincorporation into DNA, persistent accumulations of p53 and p21, and decreased reticulocyte production but increased size of individual reticulocytes. A model of folate-deficient erythropoiesis based on apoptosis of late stage erythroblasts is presented. This model provides explanations for the clinical findings in megaloblastic anemia.

The expression of c-myc was analyzed in murine and human erythroblasts throughout their differentiation in vitro into reticulocytes. The murine cells were splenic erythroblasts from animals infected with the anemia strain of Friend virus (FVA cells). In FVA cells cultured without EPO, the c-myc mRNA and protein levels decrease sharply within 3 to 4 h, showing that continual EPO stimulation is required to maintain c-myc expression. When cultured with EPO, the c-myc mRNA level of FVA cells is raised within 30 min of exposure. The c-myc mRNA and protein reach maxima at 1 to 3 h, then decline slowly to very low levels by 18 h. In contrast, c-fos and c-jun mRNA levels are not regulated by EPO in FVA cells. The human cells analyzed were colony-forming units-erythroid, CFU-E, derived in vitro by the culture of peripheral blood burst-forming units-erythroid (BFU-E). When grown in EPO and insulin-like growth factor 1 (IGF-1) these cells differentiate into reticulocytes over 6 days rather than the 2 days required for murine cells, but the c-myc mRNA kinetics and response to EPO parallel those of mouse cells at similar stages of differentiation. Both IGF-1 and c-kit ligand (SCF) cause an additive increase in c-myc mRNA in human CFU-E in conjunction with EPO. These additive effects suggest that EPO, IGF-1, and SCF affect c-myc mRNA accumulation by distinct mechanisms. Addition of an antisense oligonucleotide to c-myc in cultures of human CFU-E specifically inhibited cell proliferation but did not affect erythroid cell differentiation or apoptosis. When human cells were grown in high SCF concentrations, an environment which enhances proliferation and retards differentiation, antisense oligonucleotide to c-myc strongly inhibited proliferation, but such inhibition did not induce differentiation. This latter result indicates that differentiation requires signals other than depression of c-Myc and resultant depression of proliferation.

Erythropoietin (Epo) inhibits apoptosis in murine proerythroblasts infected with the anemia-inducing strain of Friend virus (FVA cells). We have shown that the apoptotic process in FVA cell populations deprived of Epo is asynchronous as a result of a heterogeneity in Epo dependence among individual cells. Here we investigated whether apoptosis in FVA cells correlated with cell cycle phase or stabilization of p53 tumor suppressor protein. DNA analysis in nonapoptotic FVA cell subpopulations cultured without Epo demonstrated little change in the percentages of cells in G1,S, and G2/M phases over time. Analysis of the apoptotic subpopulation revealed high percentages of cells in G1 and S, with few cells in G2/M at any time. When cells were sorted from G1 and S phases prior to culture without Epo, apoptotic cells appeared at the same rate in both populations, indicating that no prior commitment step had occurred in either G1 or S phase. Steady-state wild-type p53 protein levels were very low in FVA cells compared with control cell lines and did not accumulate in Epo-deprived cultures; however, p53 protein did accumulate when FVA cells were treated with the DNA-damaging agent actinomycin D. These data indicate that erythroblast apoptosis caused by Epo deprivation (i) occurs throughout G1 and S phases and does not require cell cycle arrest, (ii) does not have a commitment event related to cell cycle phase, and (iii) is not associated with conformational changes or stabilization of wild-type p53 protein.

Murine erythroid progenitors infected with the anemia-inducing strain of Friend virus (FVA cells) undergo apoptosis when deprived of erythropoietin (EPO). When cultured with EPO, they survive and complete terminal differentiation. Although cell volume is decreased and nuclear chromatin is condensed during both apoptosis and terminal differentiation, morphologic and biochemical distinctions between these two processes were observed. In apoptosis, homogeneous nuclear condensation with nuclear envelope loss occurred in cells that had not reached the stage of hemoglobin synthesis. In terminal erythroid differentiation, nuclear condensation with heterochromatin, euchromatin, and nuclear envelope preservation occurred simultaneously with hemoglobin synthesis. Cells with apoptotic morphology appeared asynchronously in EPO-deprived cultures, indicating that only a portion of the cells were undergoing apoptosis at any given time. The percentages of apoptotic cells and cleaved DNA increased with time in EPO-deprived cultures. Inhibition of DNA cleavage was directly proportional to EPO concentration over a wide physiologic range, demonstrating a heterogeneity in susceptibility to apoptosis based on variability in the EPO sensitivity of individual cells. A subpopulation of FVA cells with increased EPO sensitivity (decreased EPO requirement) was isolated from EPO-deprived cultures. This increased EPO sensitivity did not result from differences in EPO receptor number, affinity, or structure, suggesting that the differences are in the signal transduction pathway. These results indicate that control of red blood cell production involves both prevention of apoptosis by EPO and heterogeneity in the EPO requirement of individual progenitor cells.

Terminal erythroid differentiation in vertebrates is characterized by progressive heterochromatin formation and chromatin condensation and, in mammals, culminates in nuclear extrusion. To date, although mechanisms regulating avian erythroid chromatin condensation have been identified, little is known regarding this process during mammalian erythropoiesis. To elucidate the molecular basis for mammalian erythroblast chromatin condensation, we used Friend virus-infected murine spleen erythroblasts that undergo terminal differentiation in vitro. Chromatin isolated from early and late-stage erythroblasts had similar levels of linker and core histones, only a slight difference in nucleosome repeats, and no significant accumulation of known developmentally regulated architectural chromatin proteins. However, histone H3(K9) dimethylation markedly increased while histone H4(K12) acetylation dramatically decreased and became segregated from the histone methylation as chromatin condensed. One histone deacetylase, HDAC5, was significantly upregulated during the terminal stages of Friend virus-infected erythroblast differentiation. Treatment with histone deacetylase inhibitor, trichostatin A, blocked both chromatin condensation and nuclear extrusion. Based on our data, we propose a model for a unique mechanism in which extensive histone deacetylation at pericentromeric heterochromatin mediates heterochromatin condensation in vertebrate erythroblasts that would otherwise be mediated by developmentally-regulated architectural proteins in nucleated blood cells.

In situ hybridization using antisense RNA probes was used to localize cells that produce erythropoietin (EPO) in the livers of anemic transgenic mice expressing the human EPO gene and in livers of anemic nontransgenic mice. In transgenic mice bled from a hematocrit of 55% to one of 10%, hepatocytes surrounding central veins synthesized large amounts of human EPO mRNA. EPO-producing cells were very rare in the area of portal triads. In transgenic mice bled to a hematocrit of 20%, a similar number and distribution of cells contained human EPO mRNA as was found with a 10% hematocrit, but the cells were less heavily labeled, indicating increased EPO production per cell at 10% hematocrit as compared with 20% hematocrit. No human EPO mRNA was detected in the kidneys of anemic transgenic mice, although endogenous murine EPO mRNA was strongly expressed in cortical interstitial cells. In sections of livers from nontransgenic mice bled from a hematocrit of 45% to one of 10%, only isolated cells produced EPO. When the types of cells could clearly be identified, approximately 80% of these cells were hepatocytes, while 20% had a nonepithelial morphology and were located in or adjacent to the sinusoidal spaces. When the sense strand was used as the RNA probe for in situ hybridization, no labeled cells were seen in normal or anemic livers. These results demonstrate that hepatocytes are responsible for production of EPO in both transgenic and nontransgenic mice and that a second cell type that is similar in morphology to EPO-producing interstitial cells in the kidney also produces EPO in the livers of nontransgenic mice.

Metabolic development was examined in erythroid precursor cells, which were isolated from the spleens of mice infected with the anemia- inducing strain of Friend virus (FVA cells). FVA cells undergo differentiation in vitro from the proerythroblast stage through the reticulocyte stage over a 48-hour period in the presence of erythropoietin. Concomitant with marked decreases in cellular size and energy demand, metabolic capacities of both glycolysis and oxygen consumption diminish after 48 hours in culture by 7- and 18-fold, respectively. Because the oxidative capacity decreases more than glycolytic ability does, the metabolic machinery increasingly shifts toward anaerobic metabolism. During the 48-hour period of differentiation, the 2,3-diphosphoglyceric acid (DPG) content per cell and 2,3-DPG mutase activity per cell increased eightfold and threefold, respectively. Freshly harvested FVA cells have adenosine triphosphate (ATP) levels of 7.23 +/- 2.52 mumol/10(10) cells or 3.76 +/- 1.31 mumol/mL cell water which are 12- or 2.3-fold higher, respectively, than the ATP levels of mature red blood cells. In the course of FVA cell differentiation, ATP content per cell decreases by fourfold, but ATP concentration in cell water remains unchanged because of a corresponding decrease in cellular size and water content during differentiation. These studies show that in the face of dramatic decreases in cell size and cellular energy demand, terminally differentiating erythroid cells maintain a constant ATP level by undergoing an involution of their glycolytic machinery as well as by losing their aerobic metabolic capacity.

Erythroblasts adhere to central macrophages forming erythroblastic islands in hematopoietic tissues, but the function of these islands is not understood. Murine erythroblastic islands were reconstituted in vitro with macrophages and developmentally synchronous proerythroblasts. Erythroblasts cocultured with macrophages proliferated 3-fold greater than erythroblasts cultured alone. Direct contact with the macrophages was necessary for this enhanced erythroblast proliferation, which resulted from decreased transit time in the G0/G1 phase of cell cycle. Increased erythroblast proliferation in erythroblastic islands occurred over a wide range of erythropoietin concentrations and was the result of a mechanism different from the antiapoptotic effect of erythropoietin. Erythroblasts adherent to macrophages had slightly delayed enucleation, but otherwise differentiation was similar to erythroblasts cultured alone or those that became nonadherent in cocultures. These results suggest a mechanism for the development of anemias associated with abnormal macrophage function and for reduced responsiveness of those anemias to erythropoietin therapy.

Diseases that infiltrate the bone marrow disrupt erythropoiesis leading to anemia. In multiple myeloma (MM), anemia severity can be correlated with degree of marrow infiltration by myeloma cells. Infiltrating MM may impair the function and structure of erythroblastic islands (EBIs), the marrow erythropoietic niches. An EBI consists of a central macrophage surrounded by colony-forming units-erythroid/proerythroblasts (CFU-E/pro-EBs) and their progeny, the differentiating erythroblasts. Cytokines produced by MM cells, such as Fas ligand (FL), tumor necrosis factor (TNF), and TNF-related apoptosis-inducing ligand (TRAIL), can induce erythroid cell apoptosis. Physical displacement of the erythroid cells away from central macrophages by MM can destroy the EBIs. Non-erythrotoxic therapies that kill MM cells while sparing erythropoietic cells allow quantification of erythropoiesis and marrow MM infiltration before and after treatment of newly diagnosed MM patients. Marrow biopsies from 15 newly diagnosed MM patients were obtained before and after 4 courses of non-erythrotoxic induction therapy with bortezomib, dexamethasone, and lenalidomide (Richardson et al, Blood 2010). CBCs and serum MM paraprotein quantifications were obtained with the marrow biopsies and before each course of therapy. No patient had renal insufficiency, iron or cobalamin deficiency, erythropoietin (EPO) therapy, or RBC transfusion. At diagnosis, percentages of marrow space occupied by MM and erythroid cells were negatively correlated. Percentages of marrow space infiltrated by MM (range = 2.3 - 72.3%) were also negatively correlated with hemoglobin (Hb), hematocrit (Hct) and RBCs. One patient had a partial response: marrow myeloma decreasing from 27.5% to 5.3%. All other patients had reductions in marrow myeloma to < 2.2%. The 8 patients with < 30% MM infiltration at diagnosis had no change (-1.4% to 1.8%) in marrow space occupied by erythroid cells following therapy, whereas 7 patients with > 35% MM infiltration at diagnosis increased marrow space occupied by erythroid cells following therapy (3.4 to 19.2%). Hb, Hct, and RBCs did not change during therapy in patients with < 30% MM infiltration, but those with > 35% myeloma infiltration at diagnosis had progressive increases in Hb, Hct, and RBCs during therapy. These clinical data were used to study the relationship between marrow infiltration by MM and erythropoiesis. Mathematical models of MM infiltration effects on marrow EBI structure/function were developed and tested in simulations. A previously developed hybrid discrete-continuous model of erythropoiesis based on EBI (Eymard et al, J Math Biol 2015) was extended to a larger area of marrow containing multiple EBIs. In the model, CFU-E/proEBs have 3 fates-- self-renewal, differentiation, and apoptosis--that depend upon factors produced systemically, such as glucocorticoids and EPO, and locally, such as stem cell factor and bone morphogenetic protein 4 by central macrophages and FL by mature erythroblasts. Intracellular regulatory networks were modeled with ordinary differential equations and extracellular concentrations by partial differential equations. Under normal conditions, EBIs achieve a steady-state that produces new RBCs at rates which maintain normal Hb, Hct and RBCs. At early times after the section of bone marrow is infiltrated by small foci of proliferating MM cells, EBI function is not affected. With further proliferation, infiltrating MM cells occupy more marrow space, inducing erythroid cell apoptosis by producing FL, TNF or TRAIL and by displacing erythroid cells from central macrophages, thereby destroying EBIs. However, central macrophages of destroyed islands persist or are replaced by differentiation of monocyte-macrophage precursors. After MM cells are killed by therapy, the residual macrophages can interact with burst-forming units-erythroid (BFU-E), thereby reestablishing EBIs. If the MM infiltrate is not sufficiently reduced after a course of therapy, it can physically interfere with the macrophage-BFU-E interaction, preventing the reestablishment of an EBI and full recovery of RBC production until a subsequent therapy reduces the infiltrate sufficiently that the EBI is reestablished. The model is consistent with the clinical data and may apply to other marrow infiltrative diseases including myelofibrosis, systemic infections, or other malignancies. Disclosures No relevant conflicts of interest to declare.

Human erythrocytes contain an unidentified plasma membrane redox system that can reduce extracellular monodehydroascorbate by using intracellular ascorbate (Asc) as an electron donor. Here we show that human erythrocyte membranes contain a cytochrome b(561) (Cyt b(561)) and hypothesize that it may be responsible for this activity. Of three evolutionarily closely related Cyts b(561), immunoblots of human erythrocyte membranes showed only the duodenal cytochrome b(561) (DCytb) isoform. DCytb was also found in guinea pig erythrocyte membranes but not in erythrocyte membranes from the mouse or rat. Mouse erythrocytes lost a majority of the DCytb in the late erythroblast stage during erythropoiesis. Absorption spectroscopy showed that human erythrocyte membranes contain an Asc-reducible b-type Cyt having the same spectral characteristics as recombinant DCytb and biphasic reduction kinetics, similar to those of the chromaffin granule Cyt b(561). In contrast, mouse erythrocytes did not exhibit Asc-reducible b-type Cyt activity. Furthermore, in contrast to mouse erythrocytes, human erythrocytes much more effectively preserved extracellular Asc and transferred electrons from intracellular Asc to extracellular ferricyanide. These results suggest that the DCytb present in human erythrocytes may contribute to their ability to reduce extracellular monodehydroascorbate.

The long form of B-cell lymphoma-x (Bcl-xL), an outer mitochondrial membrane protein, has been proposed to mediate the antiapoptotic action of erythropoietin on erythroid progenitor cells and to be necessary for heme synthesis in erythroblasts. Mice with conditional knockout of Bcl-xL (conditional bcl-x-/- mice) develop severe anemia that has been attributed to hemolysis and is accompanied by splenomegaly. We characterized further the anemia of conditional bcl-x-/- mice and investigated the role of Bcl-xL in the action of erythropoietin and in heme synthesis. We analyzed peripheral blood cells and cultured splenic erythroblasts of conditional bcl-x-/- mice and littermates that were rendered anemic by bleeding. Although they had massive splenic erythroblastosis, conditional bcl-x-/- mice had decreased circulating reticulocytes compared to littermates even prior to bleeding the littermates. Compared to erythroblasts of bled littermates, bcl-x-/- erythroblasts cultured with erythropoietin underwent apoptosis during the later, hemoglobin-synthesizing stages of differentiation. The bcl-x-/- erythroblasts synthesized heme, but at reduced rates compared to bled littermate erythroblasts. When cultured without erythropoietin, bcl-x-/- erythroblasts underwent apoptosis at early stages of differentiation, prior to hemoglobin synthesis. Bcl-xL is not required for heme synthesis and does not mediate the antiapoptotic effects of erythropoietin, but it prevents ineffective erythropoiesis due to apoptosis in late-stage, hemoglobin-synthesizing erythroblasts. (Blood. 2005;106:1857-1863)

The dramatic increase in knowledge during the last half century about the hormone erythropoietin is reviewed. The description of these events has been separated into two parts. The first part describes how the rapid changes in response to tissue oxygenation in the kidneys changes the rate of erythropoietin production. The second part describes how changes in erythropoietin concentrations act on erythroid progenitor cells, resulting in prompt changes in rates of erythrocyte production. Together these two aspects of erythropoietin biology provide an explanation for the tight physiological regulation of the numbers of circulating erythrocytes and, in a more general manner, provide a model for the control of the numbers of other specific blood cells by their respective hematopoietic growth factors.

Intercellular adhesion molecule-4 (ICAM-4), a newly characterized adhesion molecule, is expressed early in human erythropoiesis and functions as a ligand for binding alpha4beta1 and alphaV integrin-expressing cells. Within the bone marrow, erythroblasts surround central macrophages forming erythroblastic islands. Evidence suggests that these islands are highly specialized subcompartments where cell adhesion events, in concert with cytokines, play critical roles in regulating erythropoiesis and apoptosis. Since erythroblasts express alpha4beta1 and ICAM-4 and macrophages exhibit alphaV, ICAM-4 is an attractive candidate for mediating cellular interactions within erythroblastic islands. To determine whether ICAM-4 binding properties are conserved across species, we first cloned and sequenced the murine homologue. The translated amino acid sequence showed 68% overall identity with human ICAM-4. Using recombinant murine ICAM-4 extracellular domains, we discovered that hematopoietic alpha4beta1- expressing HEL cells and nonhematopoietic alphaV-expressing FLY cells adhered to mouse ICAM-4. Cell adhesion studies showed that FLY and HEL cells bound to mouse and human proteins with similar avidity. These data strongly suggest conservation of integrin-binding properties across species. Importantly, we characterized a novel second splice cDNA that would be predicted to encode an ICAM-4 isoform, lacking the membrane-spanning domain. Erythroblasts express both isoforms of ICAM-4. COS-7 cells transfected with green flourescent protein constructs of prototypic or novel ICAM-4 cDNA showed different cellular localization patterns. Moreover, analysis of tissue culture medium revealed that the novel ICAM-4 cDNA encodes a secreted protein. We postulate that secretion of this newly described isoform, ICAM-4S, may modulate binding of membrane-associated ICAM-4 and could thus play a critical regulatory role in erythroblast molecular attachments.

MicroRNAs are small non-coding RNAs that regulate gene expression through mRNA degradation or translational inhibition. MicroRNAs are emerging as key regulators of normal hematopoiesis and hematologic malignancies. Several miRNAs are differentially expressed during hematopoiesis and their specific expression regulates key functional proteins involved in hematopoietic lineage differentiation. This study focused on the functional role of microRNA-223 (miR-223) on erythroid differentiation.Purified cord blood CD34+ hematopoietic progenitor cells were grown in strictly controlled conditions in the presence of saturating dosage of erythropoietin to selectively induce erythroid differentiation. The effects of enforced expression of miR-223 in unilin-eage erythroid cultures were evaluated in liquid phase culture experiments and clonogenic studies.In unilineage erythroid culture of cord blood CD34+ hematopoietic progenitor cells miR-223 is down-regulated, whereas LMO2, an essential protein for erythroid differentiation, is up-regulated. Functional studies showed that enforced expression of miR-223 reduces the mRNA and protein levels of LMO2, by binding to LMO2 3' UTR, and impairs differentiation of erythroid cells. Accordingly, knockdown of LMO2 by short interfering RNA mimics the action of miR-223. Furthermore, hematopoietic progenitor cells transduced with miR-223 showed a significant reduction of their erythroid clonogenic capacity, suggesting that downmodulation of this miRNA is required for erythroid progenitor recruitment and commitment.These results show that the decline of miR-223 is an important event for erythroid differentiation that leads to the expansion of erythroblast cells at least partially mediated by unblocking LMO2 protein expression.

In this issue of Blood , Fabriek and colleagues identify CD163, a scavenger receptor expressed on macrophages, as a mediator of erythroblast-macrophage adhesion in erythroblastic islands. Given that CD163 is also the macrophage receptor for circulating hemoglobin-haptoglobin complexes and its

The anemia of chronic disease (ACD) is commonly encountered in medical practice and can be the first sign of disease. Inflammatory cytokines such as tumor necrosis factor α (TNFα), interleukin-6, and interferon γ are produced in the bone marrow or other organs of patients with chronic diseases.

Erythropoietin (EPO) is the glycoprotein hormone that controls the production of erythrocytes in mammals. If one administers EPO to an animal, shortly thereafter, new erythrocytes (RBCs) appear in the circulation. The manner by which new RBCs are formed has been surmised by in vitro culture of bone marrow or spleen cells in semisolid medium in the presence of EPO. In such cultures, colonies of erythroblasts arise from committed erythroid progenitor cells that divide and mature simultaneously. The committed erythroid progenitor cells constitute continuous stages of differentiation that have been defined by the time in vitro required for them to form colonies of mature erythroblasts and by the number of cells in the colonies (1). Burst-forming units-erythroid (BFU-E) are the most immature committed erythroid progenitor cells; they give rise to colonies of thousands of mature erythroblasts in 7 days for murine BFU-E and 14 days for human BFU-E. How the BFU-E arise by “commitment” to erythroid differentiation of pluripotent hematopoietic cells is not understood. Colony-forming units-erythroid (CFU-E) are intermediate in their level of differentiation, giving rise to colonies of 8–32 erythroblasts after 2 days of culture for murine CFU-E and 7 days for human CFU-E. The immediate descendants of CFU-E are the proerythroblasts that are the earliest morphologically recognizable, committed erythroid cells and are capable of very limited proliferation.

In a normal person, the number of circulating erythrocytes remains within an extremely narrow range, despite a turnover of about 1%, or 200 billion cells, per day. Erythrocyte production is tightly regulated by the plasma erythropoietin (EPO) level, which in turn is controlled by oxygen delivery to

Erythropoietin (EPO) retards DNA breakdown characteristic of programmed cell death (apoptosis) and promotes survival in erythroid progenitor cells. The mechanism by which EPO inhibits programmed death is unknown. In the well-characterized model of glucocorticoid-treated thymocytes, activation of a Ca2+/Mg(2+)-sensitive endonuclease and new protein and RNA syntheses have been found necessary for apoptosis. We examined the effects of EPO on the free intracellular calcium ion concentration ([Ca2+]i), and the roles of Ca2+ and RNA and protein syntheses on DNA cleavage in erythroid progenitor cells. The murine model of erythroid differentiation using Friend leukemia virus-infected proerythroblasts (FVA cells) was used. EPO did not affect the [Ca2+]i in FVA cells. Decreasing [Ca2+]i by extracellular Ca2+ chelation with EGTA facilitated DNA breakdown. Increasing [Ca2+]i with the calcium ionophore 4-bromo-A23187 increased DNA cleavage; however, DNA fragments generated by high [Ca2+]i were much larger than those seen in the absence of EPO or presence of EGTA. Increased [Ca2+]i also inhibited DNA breakdown to small oligonucleosomal fragments characteristic of cells cultured without EPO. However, no concentration of ionophore protected the high molecular weight DNA as did EPO. Cycloheximide inhibited DNA breakdown in a dose dependent manner in cultures lacking EPO, but two other protein synthesis inhibitors, pactamycin and puromycin, did not prevent DNA breakdown. Inhibition of RNA synthesis with actinomycin D did not prevent DNA breakdown. Cells with morphological characteristics similar to those reported in other cells undergoing programmed death accumulated in EPO-derived cultures. These studies demonstrate that although DNA cleavage and morphological changes are common to apoptotic cells, the roles for Ca2+ and protein and RNA syntheses are not universal and suggest that apoptosis can be regulated by different biochemical mechanisms in different cell types.

Reticulocytes (Retics) are immature RBCs that are detected by staining of RNA in the internal organelles, mainly the ribosomes, which remain in mammalian erythroid cells following enucleation. Along with other internal organelles, ribosomes are degraded during maturation of Retics, resulting in complete loss of Retic staining in mature erythrocytes. At the same time as internal organelle degradation, plasma membranes of Retics mature with loss of specific proteins including transferrin receptors (TfR) and the alpha-4 component of several integrins (α4). Retic plasma membrane maturation involves an intrinsic mechanism, exocytosis, as well as an extrinsic mechanism that occurs, at least partially, in the spleen. TfR and α4 expressions on Retic plasma membranes and thiazole orange (TO) staining of RNA were examined by flow cytometry during murine Retic maturation under normal and stress conditions. During their 4 days of maturation in vitro, nascent reticulocytes derived from cultured erythroblasts stably expressed TfR in nearly 80% of cells and α4 in 40% of cells on their plasma membranes, while TO staining was completely lost over the 4 days. To compare in vivo and in vitro Retic maturation, mice were bled causing an anemia with reticulocytosis. Retic maturation in vitro was examined in cultures of Retic-rich blood removed from these bled mice, while Retic maturation in vivo was examined after the bled mice were hypertransfused to cease further erythroblast production. Surface TfR and α4 expressions disappeared during Retic maturation in vivo, but not in vitro. As the Retics matured in vivo, a population of erythrocytes with surface TfR expression, but without TO staining, accumulated to a maximum of 6% of the RBCs. These TfR+/TO− RBCs then disappeared gradually in vivo over several days indicating that they were developmentally between Retics and mature erythrocytes. These TfR+/TO− RBCs were not found in unbled, control mice (