"Maturational" globin switching in primary primitive erythroid cells

The Glut1 and Glut4 glucose transporters are differentially expressed during perinatal and postnatal erythropoiesis

Glucose is a major source of energy for living organisms, and its transport in vertebrates is a universally conserved property. Of all cell lineages, human erythrocytes express the highest level of the Glut1 glucose transporter with more than 200 000 molecules per cell. However, we recently reported that erythrocyte Glut1 expression is a specific trait of vitamin C–deficient mammalian species, comprising only higher primates, guinea pigs, and fruit bats. Here, we show that in all other tested mammalian species, Glut1 was transiently expressed in erythrocytes during the neonatal period. Glut1 was up-regulated during the erythroblast stage of erythroid differentiation and was present on the vast majority of murine red blood cells (RBCs) at birth. Notably though, Glut1 was not induced in adult mice undergoing anemia-induced erythropoiesis, and under these conditions, the up-regulation of a distinct transporter, Glut4, was responsible for an increased glucose transport. Sp3 and Sp1 transcriptions factors have been proposed to regulate Glut1 transcription, and we find that the concomitant repression of Glut1 and induction of Glut4 was associated with a significantly augmented Sp3/Sp1 ratio. Glucose transporter expression patterns in mice and human erythrocytes are therefore distinct. In mice, there is a postnatal switch from Glut1 to Glut4, with Glut4 further up-regulated under anemic conditions.

Epigenetics of beta-globin gene regulation

It is widely recognized that the next great challenge in the post-genomic period is to understand how the genome establishes the cell and tissue specific patterns of gene expression that underlie development. The beta-globin genes are among the most extensively studied tissue specific and developmentally regulated genes. The onset of erythropoiesis in precursor cells and the progressive expression of different members of the beta-globin family during development are accompanied by dramatic epigenetic changes in the locus. In this review, we will consider the relationship between histone and DNA modifications and the transcriptional activity of the beta-globin genes, the dynamic changes in epigenetic modifications observed during erythroid development, and the potential these changes hold as new targets for therapy in human disease.

Ontogeny of erythropoiesis

PURPOSE OF REVIEW

The present study review examines the current understanding of the ontogeny of erythropoiesis with a focus on the emergence of the embryonic (primitive) erythroid lineage and on the similarities and differences between the primitive and the fetal/adult (definitive) forms of erythroid cell maturation.

RECENT FINDINGS

Primitive erythroid precursors in the mouse embryo and cultured in vitro from human embryonic stem cells undergo 'maturational' globin switching as they differentiate terminally. The appearance of a transient population of primitive 'pyrenocytes' (extruded nuclei) in the fetal bloodstream indicates that primitive erythroblasts enucleate by nuclear extrusion. In-vitro differentiation of human embryonic stem cells recapitulates hematopoietic ontogeny reminiscent of the murine yolk sac, including overlapping waves of hemangioblast, primitive, erythroid, and definitive erythroid progenitors. Definitive erythroid potential in zebrafish embryos, like that in mice, initially arises prior to, and independent of, hematopoietic stem cell emergence in the region of the aorta. Maturation of definitive erythroid cells within macrophage islands promotes erythroblast-erythroblast and erythroblast-stromal interactions that regulate red cell output.

SUMMARY

The study of embryonic development in several different model systems, as well as in cultured human embryonic stem cells, continues to provide important insights into the ontogeny of erythropoiesis. Contrasting the similarities and differences between primitive and definitive erythropoiesis will lead to an improved understanding of erythroblast maturation and the terminal steps of erythroid differentiation.

Primitive erythropoiesis and megakaryopoiesis in the yolk sac are independent of c-myb

Hematopoiesis initiates within the yolk sac of mammalian embryos in overlapping primitive and definitive waves, each containing erythroid and megakaryocyte progenitors. c-myb-null mouse fetuses lack definitive erythrocytes but contain primitive erythroblasts and hepatic megakaryocytes. However, it is unclear if c-myb-null embryos harbor definitive erythroid or any megakaryocyte progenitors. We determined that c-myb was not expressed in primitive erythroid precursors and that c-myb-null embryos had normal primitive erythroid and megakaryocyte progenitor numbers and kinetics between embryonic day (E) 7.0 and E9.0. While primitive hematopoiesis is c-myb-independent, no definitive erythroid potential was detected in c-myb-null embryos, confirming that definitive erythropoiesis, beginning at E8.25 in the yolk sac, is completely c-myb-dependent. In contrast, reduced numbers of megakaryocyte progenitors with restricted proliferative capacity persist in E10.5 yolk sac and E11.5 liver. Despite this impaired megakaryocyte potential, c-myb-null fetuses had normal platelet numbers at E12.5 but became thrombocytopenic by E15.5, suggesting that c-myb is required for sustained thrombopoiesis.

The chromatin-remodeling enzyme BRG1 plays an essential role in primitive erythropoiesis and vascular development

ATP-dependent chromatin-remodeling complexes contribute to the proper temporal and spatial patterns of gene expression in mammalian embryos and therefore play important roles in a number of developmental processes. SWI/SNF-like chromatin-remodeling complexes use one of two different ATPases as their catalytic subunit: brahma (BRM, also known as SMARCA2) and brahma-related gene 1 (BRG1, also known as SMARCA4). We have conditionally deleted a floxed Brg1 allele with a Tie2-Cre transgene, which is expressed in developing hematopoietic and endothelial cells. Brg1(fl/fl):Tie2-Cre(+) embryos die at midgestation from anemia, as mutant primitive erythrocytes fail to transcribe embryonic alpha- and beta-globins, and subsequently undergo apoptosis. Additionally, vascular remodeling of the extraembryonic yolk sac is abnormal in Brg1(fl/fl):Tie2-Cre(+) embryos. Importantly, Brm deficiency does not exacerbate the erythropoietic or vascular abnormalities found in Brg1(fl/fl):Tie2-Cre(+) embryos, implying that Brg1-containing SWI/SNF-like complexes, rather than Brm-containing complexes, play a crucial role in primitive erythropoiesis and in early vascular development.

Globin switches in yolk sac-like primitive and fetal-like definitive red blood cells produced from human embryonic stem cells

We have previously shown that coculture of human embryonic stem cells (hESCs) for 14 days with immortalized fetal hepatocytes yields CD34(+) cells that can be expanded in serum-free liquid culture into large numbers of megaloblastic nucleated erythroblasts resembling yolk sac-derived cells. We show here that these primitive erythroblasts undergo a switch in hemoglobin (Hb) composition during late terminal erythroid maturation with the basophilic erythroblasts expressing predominantly Hb Gower I (zeta(2)epsilon(2)) and the orthochromatic erythroblasts hemoglobin Gower II (alpha(2)epsilon(2)). This suggests that the switch from Hb Gower I to Hb Gower II, the first hemoglobin switch in humans is a maturation switch not a lineage switch. We also show that extending the coculture of the hESCs with immortalized fetal hepatocytes to 35 days yields CD34(+) cells that differentiate into more developmentally mature, fetal liver-like erythroblasts, that are smaller, express mostly fetal hemoglobin, and can enucleate. We conclude that hESC-derived erythropoiesis closely mimics early human development because the first 2 human hemoglobin switches are recapitulated, and because yolk sac-like and fetal liver-like cells are sequentially produced. Development of a method that yields erythroid cells with an adult phenotype remains necessary, because the most mature cells that can be produced with current systems express less than 2% adult beta-globin mRNA.

mNotch1 signaling and erythropoietin cooperate in erythroid differentiation of multipotent progenitor cells and upregulate beta-globin

OBJECTIVE

In many developing tissues, signaling mediated by activation of the transmembrane receptor Notch influences cell-fate decisions, differentiation, proliferation, and cell survival. Notch receptors are expressed on hematopoietic cells and cognate ligands on bone marrow stromal cells. Here, we investigate the role of mNotch1 signaling in the control of erythroid differentiation of multipotent progenitor cells.

MATERIALS AND METHODS

Multipotent FDCP-mix cell lines engineered to permit the conditional induction of the constitutively active intracellular domain of mNotch1 (mN1(IC)) by the 4-hydroxytamoxifen (OHT)-inducible system were used to analyze the effects of activated mNotch1 on erythroid differentiation and on expression of Gata1, Fog1, Eklf, NF-E2, and beta-globin. Expression was analyzed by Northern blotting and real-time polymerase chain reaction. Enhancer activity of reporter constructs was determined with the dual luciferase system in transient transfection assays.

RESULTS

Induction of mN1(IC) by OHT resulted in increased and accelerated differentiation of FDCP-mix cells along the erythroid lineage. Erythroid maturation was induced by activated Notch1 also under conditions that normally promote self-renewal, but required the presence of erythropoietin for differentiation to proceed. While induction of Notch signaling rapidly upregulated Hes1 and Hey1 expression, the expression of Gata1, Fog1, Eklf, and NF-E2 remained unchanged. Concomitantly with erythroid differentiation, activated mNotch1 upregulated beta-globin RNA. Notch signaling transactivated a reporter construct harboring a conserved RBP-J (CBF1) binding site in the hypersensitive site 2 (HS2) of human beta-globin. Transactivation by activated Notch was completely abolished when this RBP-J site was mutated to prevent RBP-J binding.

CONCLUSIONS

Our results show that activation of mNotch1 induces erythroid differentiation in cooperation with erythropoietin and upregulates beta-globin expression.

CpG hypomethylation in a large domain encompassing the embryonic beta-like globin genes in primitive erythrocytes

There is little evidence addressing the role of CpG methylation in transcriptional control of genes that do not contain CpG islands. This is reflected in the ongoing debate about whether CpG methylation merely suppresses retroelements or if it also plays a role in developmental and tissue-specific gene regulation. The genes of the beta-globin locus are an important model of mammalian developmental gene regulation and do not contain CpG islands. We have analyzed the methylation status of regions in the murine beta-like globin locus in uncultured primitive and definitive erythroblasts and other cultured primary and transformed cell types. A large ( approximately 20-kb) domain is hypomethylated only in primitive erythroid cells; it extends from the region just past the locus control region to before beta-major and encompasses the embryonic genes Ey, beta h1, and beta h0. Even retrotransposons in this region are hypomethylated in primitive erythroid cells. The existence of this large developmentally regulated domain of hypomethylation supports a mechanistic role for DNA methylation in developmental regulation of globin genes.

Dissecting molecular steps in chromatin domain activation during hematopoietic differentiation

GATA factors orchestrate hematopoiesis via multistep transcriptional mechanisms, but the interrelationships and importance of individual steps are poorly understood. Using complementation analysis with GATA-1-null cells and mice containing a hypomorphic allele of the chromatin remodeler BRG1, we dissected the pathway from GATA-1 binding to cofactor recruitment, chromatin loop formation, and transcriptional activation. Analysis of GATA-1-mediated activation of the beta-globin locus, in which GATA-1 assembles dispersed complexes at the promoters and the distal locus control region (LCR), revealed molecular intermediates, including GATA-1-independent and GATA-1-containing LCR subcomplexes, both defective in promoting loop formation. An additional intermediate consisted of an apparently normal LCR complex and a promoter complex with reduced levels of total RNA polymerase II (Pol II) and Pol II phosphorylated at serine 5 of the carboxy-terminal domain. Reduced BRG1 activity solely compromised Pol II and serine 5-phosphorylated Pol II occupancy at the promoter, phenocopying the LCR-deleted mouse. These studies defined a hierarchical order of GATA-1-triggered events at a complex locus and establish a novel mechanism of long-range gene regulation.

The megakaryocyte lineage originates from hemangioblast precursors and is an integral component both of primitive and of definitive hematopoiesis

In the adult, platelets are derived from unipotential megakaryocyte colony-forming cells (Meg-CFCs) that arise from bipotential megakaryocyte/erythroid progenitors (MEPs). To better define the developmental origin of the megakaryocyte lineage, several aspects of megakaryopoiesis, including progenitors, maturing megakaryocytes, and circulating platelets, were examined in the murine embryo. We found that a majority of hemangioblast precursors during early gastrulation contains megakaryocyte potential. Combining progenitor assays with immunohistochemical analysis, we identified 2 waves of MEPs in the yolk sac associated with the primitive and definitive erythroid lineages. Primitive MEPs emerge at E7.25 along with megakaryocyte and primitive erythroid progenitors, indicating that primitive hematopoiesis is bilineage in nature. Subsequently, definitive MEPs expand in the yolk sac with Meg-CFCs and definitive erythroid progenitors. The first GP1bbeta-positive cells in the conceptus were identified in the yolk sac at E9.5, while large, highly reticulated platelets were detected in the embryonic bloodstream beginning at E10.5. At this time, the number of megakaryocyte progenitors begins to decline in the yolk sac and expand in the fetal liver. We conclude that the megakaryocyte lineage initially originates from hemangioblast precursors during early gastrulation and is closely associated both with primitive and with definitive erythroid lineages in the yolk sac prior to the transition of hematopoiesis to intraembryonic sites.

Subcellular transport of EKLF and switch-on of murine adult beta maj globin gene transcription

Erythroid Krüppel-like factor (EKLF) is an essential transcription factor for mammalian beta-like globin gene switching, and it specifically activates transcription of the adult beta globin gene through binding of its zinc fingers to the promoter. It has been a puzzle that in the mouse, despite its expression throughout the erythroid development, EKLF activates the adult beta(maj) globin promoter only in erythroid cells beyond the stage of embryonic day 10.5 (E10.5) but not before. We show here that expression of the mouse beta(maj) globin gene in the aorta-gonad-mesonephros region of E10.5 embryos and in the E14.5 fetal liver is accompanied by predominantly nuclear localization of EKLF. In contrast, EKLF is mainly cytoplasmic in the erythroid cells of E9.5 blood islands in which beta(maj) is silenced. Remarkably, in a cultured mouse adult erythroleukemic (MEL) cell line, the activation of the beta(maj) globin gene by dimethyl sulfoxide (DMSO) or hexamethylene-bis-acetamide (HMBA) induction is also paralleled by a shift of the subcellular location of EKLF from the cytoplasm to the nucleus. Blockage of the nuclear import of EKLF in DMSO-induced MEL cells with a nuclear export inhibitor repressed the transcription of the beta(maj) globin gene. Transient transfection experiments further indicated that the full-sequence context of EKLF was required for the regulation of its subcellular locations in MEL cells during DMSO induction. Finally, in both the E14.5 fetal liver cells and induced MEL cells, the beta-like globin locus is colocalized the PML oncogene domain nuclear body, and concentrated with EKLF, RNA polymerase II, and the splicing factor SC35. These data together provide the first evidence that developmental stage- and differentiation state-specific regulation of the nuclear transport of EKLF might be one of the steps necessary for the switch-on of the mammalian adult beta globin gene transcription.

The journey of developing hematopoietic stem cells

Hematopoietic stem cells (HSCs) develop during embryogenesis in a complex process that involves multiple anatomical sites. Once HSC precursors have been specified from mesoderm, they have to mature into functional HSCs and undergo self-renewing divisions to generate a pool of HSCs. During this process,developing HSCs migrate through various embryonic niches, which provide signals for their establishment and the conservation of their self-renewal ability. These processes have to be recapitulated to generate HSCs from embryonic stem cells. Elucidating the interactions between developing HSCs and their niches should facilitate the generation and expansion of HSCs in vitro to exploit their clinical potential.

Dynamic posttranscriptional regulation of epsilon-globin gene expression in vivo

Functional studies of embryonic epsilon-globin indicate that individuals with beta thalassemia or sickle cell disease are likely to benefit from therapeutic, transcriptional derepression of its encoding gene. The success of epsilon-globin gene-reactivation strategies, however, will be tempered by the stability that epsilon-globin mRNA exhibits in developmental stage-discordant definitive erythroid progenitors. Using cell culture and transgenic mouse model systems, we demonstrate that epsilon-globin mRNA is modestly unstable in immature, transcriptionally active erythroid cells, but that this characteristic has relatively little impact on the accumulation of epsilon-globin mRNA at subsequent stages of terminal differentiation. Importantly, the constitutive stability of epsilon-globin mRNA increases in transgenic mouse models of beta thalassemia, suggesting that epsilon- and beta-globin mRNAs are coregulated through a shared posttranscriptional mechanism. As anticipated, relevant cis-acting determinants of epsilon-globin mRNA stability map to its 3' UTR, consistent with the positioning of functionally related elements in other globin mRNAs. These studies demonstrate that posttranscriptional processes do not pose a significant practical barrier to epsilon-globin gene reactivation and, moreover, indicate that related therapeutic strategies may be particularly effective in individuals carrying beta-thalassemic gene defects.

Maturation and enucleation of primitive erythroblasts during mouse embryogenesis is accompanied by changes in cell-surface antigen expression

Primitive erythroblasts (EryPs) are the first hematopoietic cell type to form during mammalian embryogenesis and emerge within the blood islands of the yolk sac. Large, nucleated EryPs begin to circulate around midgestation, when connections between yolk sac and embryonic vasculature mature. Two to 3 days later, small cells of the definitive erythroid lineage (EryD) begin to differentiate within the fetal liver and rapidly outnumber EryPs in the circulation. The development and maturation of EryPs remain poorly defined. Our analysis of embryonic blood at different stages reveals a stepwise developmental progression within the EryP lineage from E9.5 to E12.5. Thereafter, EryDs are also present in the bloodstream, and the 2 lineages are not easily distinguished. We have generated a transgenic mouse line in which the human epsilon-globin gene promoter drives expression of green fluorescent protein exclusively within the EryP lineage. Here, we have used this line to characterize changes in cell morphology and surface-marker expression as EryPs mature and to track EryP numbers and enucleation throughout gestation. This study identifies previously unrecognized synchronous developmental stages leading to the maturation of EryPs in the mouse embryo. Unexpectedly, we find that EryPs are a stable cell population that persists through the end of gestation.

Chromatin-binding in vivo of the erythroid kruppel-like factor, EKLF, in the murine globin loci

EKLF is an erythroid-specific, zinc finger-containing transcription factor essential for the activation of the mammalian beta globin gene in erythroid cells of definitive lineage. We have prepared a polyclonal anti-mouse EKLF antibody suitable for Western blotting and immunoprecipitation (IP) qualities, and used it to define the expression patterns of the EKLF protein during mouse erythroid development. We have also used this antibody for the chromatin-immunoprecipitation (ChIP) assay. EKLF was found to bind in vivo at both the mouse beta-major-globin promoter and the HS2 site of beta-LCR in the mouse erythroleukemia cells (MEL) in a DMSO-inducible manner. The DMSO-induced bindings of EKLF as well as three other proteins, namely, RNA polymerase II, acetylated histone H3, and methylated histone H3, were not abolished but significantly lowered in CB3, a MEL-derived cell line with null-expression of p45/NF-E2, an erythroid-enriched factor needed for activation of the mammalian globin loci. Interestingly, binding of EKLF in vivo was also detected in the mouse alpha-like globin locus, at the adult alpha globin promoter and its far upstream regulatory element alpha-MRE (HS26). This study provides direct evidence for EKLF-binding in vivo at the major regulatory elements of the mouse beta-like globin gene clusters the data also have interesting implications with respect to the role of EKLF-chromatin interaction in mammalian globin gene regulation.

Definitive-like erythroid cells derived from human embryonic stem cells coexpress high levels of embryonic and fetal globins with little or no adult globin

Human embryonic stem cells are a promising tool to study events associated with the earliest ontogenetic stages of hematopoiesis. We describe the generation of erythroid cells from hES (H1) by subsequent processing of cells present at early and late stages of embryoid body (EB) differentiation. Kinetics of hematopoietic marker emergence suggest that CD45+ hematopoiesis peaks at late D14EB differentiation stages, although low-level CD45- erythroid differentiation can be seen before that stage. By morphologic criteria, hES-derived erythroid cells were of definitive type, but these cells both at mRNA and protein levels coexpressed high levels of embryonic (epsilon) and fetal (gamma) globins, with little or no adult globin (beta). This globin expression pattern was not altered by the presence or absence of fetal bovine serum, vascular endothelial growth factor, Flt3-L, or coculture with OP-9 during erythroid differentiation and was not culture time dependent. The coexpression of both embryonic and fetal globins by definitive-type erythroid cells does not faithfully mimic either yolk sac embryonic or their fetal liver counterparts. Nevertheless, the high frequency of erythroid cells coexpressing embryonic and fetal globin generated from embryonic stem cells can serve as an invaluable tool to further explore molecular mechanisms.