Isolation and functional characterization of human erythroblasts at distinct stages: implications for understanding of normal and disordered erythropoiesis in vivo

Complete absence of GLUT1 does not impair human terminal erythroid differentiation

ABSTRACT

The glucose transporter 1 (GLUT1) is 1 of the most abundant proteins within the erythrocyte membrane and is required for glucose and dehydroascorbic acid (vitamin C precursor) transport. It is widely recognized as a key protein for red cell structure, function, and metabolism. Previous reports highlighted the importance of GLUT1 activity within these uniquely glycolysis-dependent cells, in particular for increasing antioxidant capacity needed to avoid irreversible damage from oxidative stress in humans. However, studies of glucose transporter roles in erythroid cells are complicated by species-specific differences between humans and mice. Here, using CRISPR-mediated gene editing of immortalized erythroblasts and adult CD34+ hematopoietic progenitor cells, we generate committed human erythroid cells completely deficient in expression of GLUT1. We show that absence of GLUT1 does not impede human erythroblast proliferation, differentiation, or enucleation. This work demonstrates, to our knowledge, for the first time, generation of enucleated human reticulocytes lacking GLUT1. The GLUT1-deficient reticulocytes possess no tangible alterations to membrane composition or deformability in reticulocytes. Metabolomic analyses of GLUT1-deficient reticulocytes reveal hallmarks of reduced glucose import, downregulated metabolic processes and upregulated AMP-activated protein kinase signaling, alongside alterations in antioxidant metabolism, resulting in increased osmotic fragility and metabolic shifts indicative of higher oxidant stress. Despite detectable metabolic changes in GLUT1-deficient reticulocytes, the absence of developmental phenotype, detectable proteomic compensation, or impaired deformability comprehensively alters our understanding of the role of GLUT1 in red blood cell structure, function, and metabolism. It also provides cell biological evidence supporting clinical consensus that reduced GLUT1 expression does not cause anemia in GLUT1-deficiency syndrome.

Enhancing terminal erythroid differentiation in human embryonic stem cells through TRIB3 overexpression

Tribbles pseudokinase 3 (TRIB3) expression significantly increases during terminal erythropoiesis in vivo. However, we found that TRIB3 expression remained relatively low during human embryonic stem cell (hESC) erythropoiesis, particularly in the late stage, where it is typically active. TRIB3 was expressed in megakaryocyte-erythrocyte progenitor cells and its low expression was necessary for megakaryocyte differentiation. Thus, we proposed that the high expression during late stage of erythropoiesis could be the clue for promotion of maturation of hESC-derived erythroid cells. To our knowledge, the role of TRIB3 in the late stage of erythropoiesis remains ambiguous. To address this, we generated inducible TRIB3 overexpression hESCs, named TRIB3tet-on OE H9, based on a Tet-On system. Then, we analyzed hemoglobin expression, condensed chromosomes, organelle clearance, and enucleation with or without doxycycline treatment. TRIB3tet-on OE H9 cells generated erythrocytes with a high proportion of orthochromatic erythroblast in flow cytometry, enhanced hemoglobin and related protein expression in Western blot, decreased nuclear area size, promoted enucleation rate, decreased lysosome and mitochondria number, more colocalization of LC3 with LAMP1 (lysosome marker) and TOM20 (mitochondria marker) and up-regulated mitophagy-related protein expression after treatment with 2 μg/mL doxycycline. Our results showed that TRIB3 overexpression during terminal erythropoiesis may promote the maturation of erythroid cells. Therefore, our study delineates the role of TRIB3 in terminal erythropoiesis, and reveals TRIB3 as a key regulator of UPS and downstream mitophagy by ensuring appropriate mitochondrial clearance during the compaction of chromatin.

Erythropoietin-dependent Acquisition of CD71 hi CD105 hi Phenotype within CD235a - Early Erythroid Progenitors

The development of committed erythroid progenitors and their continued maturation into mature erythrocytes requires the cytokine erythropoietin (Epo). Here, we describe the immunophenotypic identification of a unique Epo-dependent colony-forming unit-erythroid (CFU-E) cell subtype that forms during early erythropoiesis (EE). This previously undescribed CFU-E subtype, termed late-CFU-E (lateC), lacks surface expression of the characteristic erythroid marker CD235a (glycophorin A) but has high levels of CD71 and CD105. LateCs could be prospectively detected in human bone marrow (BM) cells and, upon isolation and reculture, exhibited the potential to form CFU-E colonies in medium containing only Epo (no other cytokines) and continued differentiation along the erythroid trajectory. Analysis of ex vivo cultures of BM CD34 + cells showed that acquisition of the CD7 hi CD105 hi phenotype in lateCs is gradual and occurs through the formation of four EE cell subtypes. Of these, two are CD34 + burst-forming unit-erythroid (BFU-E) cells, distinguishable as CD7 lo CD105 lo early BFU-E and CD7 hi CD105 lo late BFU-E, and two are CD34 - CFU-Es, also distinguishable as CD71 lo CD105 lo early CFU-E and CD7 hi CD105 lo mid-CFU-E. The transition of these EE populations is accompanied by a rise in CD36 expression, such that all lateCs are CD36 + . Single cell RNA-sequencing analysis confirmed Epo-dependent formation of a CFU-E cluster that exhibits high coexpression of CD71, CD105, and CD36 transcripts. Gene set enrichment analysis revealed the involvement of genes specific to fatty acid and cholesterol metabolism in lateC formation. Overall, in addition to identifying a key Epo-dependent EE cell stage, this study provides a framework for investigation into mechanisms underlying other erythropoiesis-stimulating agents.

Creg1 Regulates Erythroid Development via TGF-β/Smad2-Klf1 Axis in Zebrafish

Understanding the regulation of normal erythroid development will help to develop new potential therapeutic strategies for disorders of the erythroid lineage. Cellular repressor of E1A-stimulated genes 1 (CREG1) is a glycoprotein that has been implicated in the regulation of tissue homeostasis. However, its role in erythropoiesis remains largely undefined. In this study, it is found that CREG1 expression increases progressively during erythroid differentiation. In zebrafish, creg1 mRNA is preferentially expressed within the intermediate cell mass (ICM)/peripheral blood island (PBI) region where primitive erythropoiesis occurs. Loss of creg1 leads to anemia caused by defective erythroid differentiation and excessive apoptosis of erythroid progenitors. Mechanistically, creg1 deficiency results in reduced activation of TGF-β/Smad2 signaling pathway. Treatment with an agonist of the Smad2 pathway (IDE2) could significantly restore the defective erythroid development in creg1-/- mutants. Further, Klf1, identified as a key target gene downstream of the TGF-β/Smad2 signaling pathway, is involved in creg1 deficiency-induced aberrant erythropoiesis. Thus, this study reveals a previously unrecognized role for Creg1 as a critical regulator of erythropoiesis, mediated at least in part by the TGF-β/Smad2-Klf1 axis. This finding may contribute to the understanding of normal erythropoiesis and the pathogenesis of erythroid disorders.

Predicting the Long-Term Effects of Therapeutic Neutralization of Oncostatin M on Human Hematopoiesis

Therapeutic neutralization of Oncostatin M (OSM) causes mechanism-driven anemia and thrombocytopenia, which narrows the therapeutic window complicating the selection of doses (and dosing intervals) that optimize efficacy and safety. We utilized clinical data from studies of an anti-OSM monoclonal antibody (GSK2330811) in healthy volunteers (n = 49) and systemic sclerosis patients (n = 35), to quantitatively determine the link between OSM and alterations in red blood cell (RBC) and platelet production. Longitudinal changes in hematopoietic variables (including RBCs, reticulocytes, platelets, erythropoietin, and thrombopoietin) were linked in a physiology-based model, to capture the long-term effects and variability of therapeutic OSM neutralization on human hematopoiesis. Free serum OSM stimulated precursor cell production through sigmoidal relations, with higher maximum suppression (Imax) and OSM concentration for 50% suppression (IC50) for platelets (89.1% [95% confidence interval: 83.4-93.0], 6.03 pg/mL [4.41-8.26]) than RBCs (57.0% [49.7-64.0], 2.93 pg/mL [2.55-3.36]). Reduction in hemoglobin and platelets increased erythro- and thrombopoietin, respectively, prompting reticulocytosis and (partially) alleviating OSM-restricted hematopoiesis. The physiology-based model was substantiated by preclinical data and utilized in exploration of once-weekly or every other week dosing regimens. Predictions revealed an (for the indication) unacceptable occurrence of grade 2 (67% [58-76], 29% [20-38]) and grade 3 (17% [10-25], 3% [0-7]) anemias, with limited thrombocytopenia. Individual extent of RBC precursor modulation was moderately correlated to skin mRNA gene expression changes. The physiological basis and consideration of interplay among hematopoietic variables makes the model generalizable to other drug and nondrug scenarios, with adaptations for patient populations, diseases, and therapeutics that modulate hematopoiesis or exhibit risk of anemia and/or thrombocytopenia.

PGC-1α agonism induces fetal hemoglobin and exerts antisickling effects in sickle cell disease

Sickle cell disease is a growing health burden afflicting millions around the world. Clinical observation and laboratory studies have shown that the severity of sickle cell disease is ameliorated in individuals who have elevated levels of fetal hemoglobin. Additional pharmacologic agents to induce sufficient fetal hemoglobin to diminish clinical severity is an unmet medical need. We recently found that up-regulation of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) can induce fetal hemoglobin synthesis in human primary erythroblasts. Here, we report that a small molecule, SR-18292, increases PGC-1α leading to enhanced fetal hemoglobin expression in human erythroid cells, β-globin yeast artificial chromosome mice, and sickle cell disease mice. In SR-18292-treated sickle mice, sickled red blood cells are significantly reduced, and disease complications are alleviated. SR-18292, or agents in its class, could be a promising additional therapeutic for sickle cell disease.

DNMT1 regulates human erythropoiesis by modulating cell cycle and endoplasmic reticulum stress in a stage-specific manner

The dynamic balance of DNA methylation and demethylation is required for erythropoiesis. Our previous transcriptomic analyses revealed that DNA methyltransferase 1 (DNMT1) is abundantly expressed in erythroid cells at all developmental stages. However, the role and molecular mechanisms of DNMT1 in human erythropoiesis remain unknown. Here we found that DNMT1 deficiency led to cell cycle arrest of erythroid progenitors which was partially rescued by treatment with a p21 inhibitor UC2288. Mechanically, this is due to decreased DNA methylation of p21 promoter, leading to upregulation of p21 expression. In contrast, DNMT1 deficiency led to increased apoptosis during terminal stage by inducing endoplasmic reticulum (ER) stress in a p21 independent manner. ER stress was attributed to the upregulation of RPL15 expression due to the decreased DNA methylation at RPL15 promoter. The upregulated RPL15 expression subsequently caused a significant upregulation of core ribosomal proteins (RPs) and thus ultimately activated all branches of unfolded protein response (UPR) leading to the excessive ER stress, suggesting a role of DNMT1 in maintaining protein homeostasis during terminal erythroid differentiation. Furthermore, the increased apoptosis was significantly rescued by the treatment of ER stress inhibitor TUDCA. Our findings demonstrate the stage-specific role of DNMT1 in regulating human erythropoiesis and provide new insights into regulation of human erythropoiesis.

An atypical form of 60S ribosomal subunit in Diamond-Blackfan anemia linked to RPL17 variants

Diamond-Blackfan anemia syndrome (DBA) is a ribosomopathy associated with loss-of-function variants in more than 20 ribosomal protein (RP) genes. Here, we report the genetic, functional, and biochemical dissection of 2 multigenerational pedigrees with variants in RPL17, a large ribosomal subunit protein-encoding gene. Affected individuals had clinical features and erythroid proliferation defects consistent with DBA. Further, RPL17/uL22 depletion resulted in anemia and micrognathia in zebrafish larvae, and in vivo complementation studies indicated that RPL17 variants were pathogenic. Lymphoblastoid cell lines (LCLs) derived from patients displayed a ribosomal RNA maturation defect reflecting haploinsufficiency of RPL17. The proteins encoded by RPL17 variants were not incorporated into ribosomes, but 10%-20% of 60S ribosomal subunits contained a short form of 5.8S rRNA (5.8SC), a species that is marginal in normal cells. These atypical 60S subunits were actively engaged in translation. Ribosome profiling showed changes of the translational profile, but those are similar to LCLs bearing RPS19 variants. These results link an additional RP gene to DBA. They show that ribosomes can be modified substantially by RPL17 haploinsufficiency but support the paradigm that translation alterations in DBA are primarily related to insufficient ribosome production rather than to changes in ribosome structure or composition.

Analyzing the role of ferroptosis in ribosome-related bone marrow failure disorders: From pathophysiology to potential pharmacological exploitation

Within the last decade, the scientific community has witnessed the importance of ferroptosis as a novel cascade of molecular events leading to cellular decisions of death distinct from apoptosis and other known forms of cell death. Notably, such non- apoptotic and iron-dependent regulated cell death has been found to be intricately linked to several physiological processes as well as to the pathogenesis of various diseases. To this end, recent data support the notion that a potential molecular connection between ferroptosis and inherited bone marrow failure (IBMF) in individuals with ribosomopathies may exist. In this review, we suggest that in ribosome-related IBMFs the identified mutations in ribosomal proteins lead to changes in the ribosome composition of the hematopoietic progenitors, changes that seem to affect ribosomal function, thus enhancing the expression of some mRNAs subgroups while reducing the expression of others. These events lead to an imbalance inside the cell as some molecular pathways are promoted while others are inhibited. This disturbance is accompanied by ROS production and lipid peroxidation, while an additional finding in most of them is iron accumulation. Once lipid peroxidation and iron accumulation are the two main characteristics of ferroptosis, it is possible that this mechanism plays a key role in the manifestation of IBMF in this type of disease. If this molecular mechanism is further confirmed, new pharmacological targets such as ferroptosis inhibitors that are already exploited for the treatment of other diseases, could be utilized to improve the treatment of ribosomopathies.

Decoding human in vitro terminal erythropoiesis originating from umbilical cord blood mononuclear cells and pluripotent stem cells

Ex vivo red blood cell (RBC) production generates unsatisfactory erythroid cells. A deep exploration into terminally differentiated cells is required to understand the impairments for RBC generation and the underlying mechanisms. Here, we mapped an atlas of terminally differentiated cells from umbilical cord blood mononuclear cells (UCBMN) and pluripotent stem cells (PSC) and observed their dynamic regulation of erythropoiesis at single-cell resolution. Interestingly, we detected a few progenitor cells and non-erythroid cells from both origins. In PSC-derived erythropoiesis (PSCE), the expression of haemoglobin switch regulators (BCL11A and ZBTB7A) were significantly absent, which could be the restraint for its adult globin expression. We also found that PSCE were less active in stress erythropoiesis than in UCBMN-derived erythropoiesis (UCBE), and explored an agonist of stress erythropoiesis gene, TRIB3, could enhance the expression of adult globin in PSCE. Compared with UCBE, there was a lower expression of epigenetic-related proteins (e.g., CASPASE 3 and UBE2O) and transcription factors (e.g., FOXO3 and TAL1) in PSCE, which might restrict PSCE's enucleation. Moreover, we characterized a subpopulation with high proliferation capacity marked by CD99high in colony-forming unit-erythroid cells. Inhibition of CD99 reduced the proliferation of PSC-derived cells and facilitated erythroid maturation. Furthermore, CD99-CD99 mediated the interaction between macrophages and erythroid cells, illustrating a mechanism by which macrophages participate in erythropoiesis. This study provided a reference for improving ex vivo RBC generation.

Enhancement of erythropoietic output by Cas9-mediated insertion of a natural variant in haematopoietic stem and progenitor cells

Some gene polymorphisms can lead to monogenic diseases, whereas other polymorphisms may confer beneficial traits. A well-characterized example is congenital erythrocytosis-the non-pathogenic hyper-production of red blood cells-that is caused by a truncated erythropoietin receptor. Here we show that Cas9-mediated genome editing in CD34+ human haematopoietic stem and progenitor cells (HSPCs) can recreate the truncated form of the erythropoietin receptor, leading to substantial increases in erythropoietic output. We also show that combining the expression of the cDNA of a truncated erythropoietin receptor with a previously reported genome-editing strategy to fully replace the HBA1 gene with an HBB transgene in HSPCs (to restore normal haemoglobin production in cells with a β-thalassaemia phenotype) gives the edited HSPCs and the healthy red blood cell phenotype a proliferative advantage. Combining knowledge of human genetics with precise genome editing to insert natural human variants into therapeutic cells may facilitate safer and more effective genome-editing therapies for patients with genetic diseases.

An Optimized Human Erythroblast Differentiation System Reveals Cholesterol-Dependency of Robust Production of Cultured Red Blood Cells Ex Vivo

The generation of cultured red blood cells (cRBCs) ex vivo represents a potentially unlimited source for RBC transfusion and other cell therapies. Human cRBCs can be generated from the terminal differentiation of proliferating erythroblasts derived from hematopoietic stem/progenitor cells or erythroid precursors in peripheral blood mononuclear cells. Efficient differentiation and maturation into cRBCs highly depend on replenishing human plasma, which exhibits variable potency across donors or batches and complicates the consistent cRBC production required for clinical translation. Hence, the role of human plasma in erythroblast terminal maturation is investigated and uncovered that 1) a newly developed cell culture basal medium mimicking the metabolic profile of human plasma enhances cell growth and increases cRBC yield upon erythroblast terminal differentiation and 2) LDL-carried cholesterol, as a substitute for human plasma, is sufficient to support erythroid survival and terminal differentiation ex vivo. Consequently, a chemically-defined optimized medium (COM) is developed, enabling robust generation of cRBCs from erythroblasts of multiple origins, with improved enucleation efficiency and higher reticulocyte yield, without the need for supplementing human plasma or serum. In addition, the results reveal the crucial role of lipid metabolism during human terminal erythropoiesis.

Disruption of mitochondrial energy metabolism is a putative pathogenesis of Diamond-Blackfan anemia

Energy metabolism in the context of erythropoiesis and related diseases remains largely unexplored. Here, we developed a primary cell model by differentiating hematopoietic stem progenitor cells toward the erythroid lineage and suppressing the mitochondrial oxidative phosphorylation (OXPHOS) pathway. OXPHOS suppression led to differentiation failure of erythroid progenitors and defects in ribosome biogenesis. Ran GTPase-activating protein 1 (RanGAP1) was identified as a target of mitochondrial OXPHOS for ribosomal defects during erythropoiesis. Overexpression of RanGAP1 largely alleviated erythroid defects resulting from OXPHOS suppression. Coenzyme Q10, an activator of OXPHOS, largely rescued erythroid defects and increased RanGAP1 expression. Patients with Diamond-Blackfan anemia (DBA) exhibited OXPHOS suppression and a concomitant suppression of ribosome biogenesis. RNA-seq analysis implied that the substantial mutation (approximately 10%) in OXPHOS genes accounts for OXPHOS suppression in these patients. Conclusively, OXPHOS disruption and the associated disruptive mitochondrial energy metabolism are linked to the pathogenesis of DBA.

Hemolysis-driven IFNα production impairs erythropoiesis by negatively regulating EPO signaling in sickle cell disease

ABSTRACT

Disordered erythropoiesis is a feature of many hematologic diseases, including sickle cell disease (SCD). However, very little is known about erythropoiesis in SCD. Here, we show that although bone marrow (BM) erythroid progenitors and erythroblasts in Hbbth3/+ thalassemia mice were increased more than twofold, they were expanded by only ∼40% in Townes sickle mice (SS). We further show that the colony-forming ability of SS erythroid progenitors was decreased and erythropoietin (EPO)/EPO receptor (EPOR) signaling was impaired in SS erythroid cells. Furthermore, SS mice exhibited reduced responses to EPO. Injection of mice with red cell lysates or hemin, mimicking hemolysis in SCD, led to suppression of erythropoiesis and reduced EPO/EPOR signaling, indicating hemolysis, a hallmark of SCD, and could contribute to the impaired erythropoiesis in SCD. In vitro hemin treatment did not affect Stat5 phosphorylation, suggesting that hemin-induced erythropoiesis suppression in vivo is via an indirect mechanism. Treatment with interferon α (IFNα), which is upregulated by hemolysis and elevated in SCD, led to suppression of mouse BM erythropoiesis in vivo and human erythropoiesis in vitro, along with inhibition of Stat5 phosphorylation. Notably, in sickle erythroid cells, IFN-1 signaling was activated and the expression of cytokine inducible SH2-containing protein (CISH), a negative regulator of EPO/EPOR signaling, was increased. CISH deletion in human erythroblasts partially rescued IFNα-mediated impairment of cell growth and EPOR signaling. Knocking out Ifnar1 in SS mice rescued the defective BM erythropoiesis and improved EPO/EPOR signaling. Our findings identify an unexpected role of hemolysis on the impaired erythropoiesis in SCD through inhibition of EPO/EPOR signaling via a heme-IFNα-CISH axis.

BMI1 regulates human erythroid self-renewal through both gene repression and gene activation

The limited proliferative capacity of erythroid precursors is a major obstacle to generate sufficient numbers of in vitro-derived red blood cells (RBC) for clinical purposes. We and others have determined that BMI1, a member of the polycomb repressive complex 1 (PRC1), is both necessary and sufficient to drive extensive proliferation of self-renewing erythroblasts (SREs). However, the mechanisms of BMI1 action remain poorly understood. BMI1 overexpression led to 10 billion-fold increase BMI1-induced (i)SRE self-renewal. Despite prolonged culture and BMI1 overexpression, human iSREs can terminally mature and agglutinate with typing reagent monoclonal antibodies against conventional RBC antigens. BMI1 and RING1B occupancy, along with repressive histone marks, were identified at known BMI1 target genes, including the INK-ARF locus, consistent with an altered cell cycle following BMI1 inhibition. We also identified upregulated BMI1 target genes with low repressive histone modifications, including key regulator of cholesterol homeostasis. Functional studies suggest that both cholesterol import and synthesis are essential for BMI1-associated self-renewal. These findings support the hypothesis that BMI1 regulates erythroid self-renewal not only through gene repression but also through gene activation and offer a strategy to expand the pool of immature erythroid precursors for eventual clinical uses.

Erythroid Differentiation Enhances RNA Mis-Splicing in SF3B1-Mutant Myelodysplastic Syndromes with Ring Sideroblasts

Myelodysplastic syndromes with ring sideroblasts (MDS-RS) commonly develop from hematopoietic stem cells (HSC) bearing mutations in the splicing factor SF3B1 (SF3B1mt). Direct studies into MDS-RS pathobiology have been limited by a lack of model systems that fully recapitulate erythroid biology and RS development and the inability to isolate viable human RS. Here, we combined successful direct RS isolation from patient samples, high-throughput multiomics analysis of cells encompassing the SF3B1mt stem-erythroid continuum, and functional assays to investigate the impact of SF3B1mt on erythropoiesis and RS accumulation. The isolated RS differentiated, egressed into the blood, escaped traditional nonsense-mediated decay (NMD) mechanisms, and leveraged stress-survival pathways that hinder wild-type hematopoiesis through pathogenic GDF15 overexpression. Importantly, RS constituted a contaminant of magnetically enriched CD34+ cells, skewing bulk transcriptomic data. Mis-splicing in SF3B1mt cells was intensified by erythroid differentiation through accelerated RNA splicing and decreased NMD activity, and SF3B1mt led to truncations in several MDS-implicated genes. Finally, RNA mis-splicing induced an uncoupling of RNA and protein expression, leading to critical abnormalities in proapoptotic p53 pathway genes. Overall, this characterization of erythropoiesis in SF3B1mt RS provides a resource for studying MDS-RS and uncovers insights into the unexpectedly active biology of the "dead-end" RS.

SIGNIFICANCE

Ring sideroblast isolation combined with state-of-the-art multiomics identifies survival mechanisms underlying SF3B1-mutant erythropoiesis and establishes an active role for erythroid differentiation and ring sideroblasts themselves in SF3B1-mutant myelodysplastic syndrome pathogenesis.

Complete absence of GLUT1 does not impair human terminal erythroid differentiation

The Glucose transporter 1 (GLUT1) is one of the most abundant proteins within the erythrocyte membrane and is required for glucose and dehydroascorbic acid (Vitamin C precursor) transport. It is widely recognized as a key protein for red cell structure, function, and metabolism. Previous reports highlighted the importance of GLUT1 activity within these uniquely glycolysis-dependent cells, in particular for increasing antioxidant capacity needed to avoid irreversible damage from oxidative stress in humans. However, studies of glucose transporter roles in erythroid cells are complicated by species-specific differences between humans and mice. Here, using CRISPR-mediated gene editing of immortalized erythroblasts and adult CD34+ hematopoietic progenitor cells, we generate committed human erythroid cells completely deficient in expression of GLUT1. We show that absence of GLUT1 does not impede human erythroblast proliferation, differentiation, or enucleation. This work demonstrates for the first-time generation of enucleated human reticulocytes lacking GLUT1. The GLUT1-deficient reticulocytes possess no tangible alterations to membrane composition or deformability in reticulocytes. Metabolomic analyses of GLUT1-deficient reticulocytes reveal hallmarks of reduced glucose import, downregulated metabolic processes and upregulated AMPK-signalling, alongside alterations in antioxidant metabolism, resulting in increased osmotic fragility and metabolic shifts indicative of higher oxidant stress. Despite detectable metabolic changes in GLUT1 deficient reticulocytes, the absence of developmental phenotype, detectable proteomic compensation or impaired deformability comprehensively alters our understanding of the role of GLUT1 in red blood cell structure, function and metabolism. It also provides cell biological evidence supporting clinical consensus that reduced GLUT1 expression does not cause anaemia in GLUT1 deficiency syndrome.

Unveiling P. vivax invasion pathways in Duffy-negative individuals

Vivax malaria has long been thought to be absent from sub-Saharan Africa owing to the high proportion of individuals lacking the Duffy antigen receptor for chemokines (DARC) in their erythrocytes. The interaction between P. vivax Duffy-binding protein (PvDBP) and DARC is assumed to be the main pathway used by merozoites to invade reticulocytes. However, the increasing number of reports of vivax malaria cases in genotypically Duffy-negative (DN) individuals has raised questions regarding the P. vivax invasion pathway(s). Here, we show that a subset of DN erythroblasts transiently express DARC during terminal erythroid differentiation and that P. vivax merozoites, irrespective of their origin, can invade DARC+ DN erythroblasts. These findings reveal that a large number of DN individuals may represent a silent reservoir of deep P. vivax infections at the sites of active erythropoiesis with low or no parasitemia, and it may represent an underestimated biological problem with potential clinical consequences in sub-Saharan Africa.

Duffy antigen is expressed during erythropoiesis in Duffy-negative individuals

The erythrocyte silent Duffy blood group phenotype in Africans is thought to confer resistance to Plasmodium vivax blood-stage infection. However, recent studies report P. vivax infections across Africa in Fy-negative individuals. This suggests that the globin transcription factor 1 (GATA-1) SNP underlying Fy negativity does not entirely abolish Fy expression or that P. vivax has developed a Fy-independent red blood cell (RBC) invasion pathway. We show that RBCs and erythroid progenitors from in vitro differentiated CD34 cells and from bone marrow aspirates from Fy-negative samples express a functional Fy on their surface. This suggests that the GATA-1 SNP does not entirely abolish Fy expression. Given these results, we developed an in vitro culture system for P. vivax and show P. vivax can invade erythrocytes from Duffy-negative individuals. This study provides evidence that Fy is expressed in Fy-negative individuals and explains their susceptibility to P. vivax with major implications and challenges for P. vivax malaria eradication.

Comprehensive Characterization and Global Transcriptome Analysis of Human Fetal Liver Terminal Erythropoiesis

The fetal liver (FL) is the key erythropoietic organ during fetal development, but knowledge on human FL erythropoiesis is very limited. In this study, we sorted primary erythroblasts from FL cells and performed RNA sequencing (RNA-seq) analyses. We found that temporal gene expression patterns reflected changes in function during primary human FL terminal erythropoiesis. Notably, the expression of genes enriched in proteolysis and autophagy was up-regulated in orthochromatic erythroblasts (OrthoEs), suggesting the involvement of these pathways in enucleation. We also performed RNA-seq of in vitro cultured erythroblasts derived from FL CD34+ cells. Comparison of transcriptomes between the primary and cultured erythroblasts revealed significant differences, indicating impacts of the culture system on gene expression. Notably, the expression of lipid metabolism-related genes was increased in cultured erythroblasts. We further immortalized erythroid cell lines from FL and cord blood (CB) CD34+ cells (FL-iEry and CB-iEry, respectively). FL-iEry and CB-iEry were immortalized at the proerythroblast stage and can be induced to differentiate into OrthoEs, but their enucleation ability was very low. Comparison of the transcriptomes between OrthoEs with and without enucleation capability revealed the down-regulation of pathways involved in chromatin organization and mitophagy in OrthoEs without enucleation capacity, indicating that defects in chromatin organization and mitophagy contribute to the inability of OrthoEs to enucleate. Additionally, the expression of HBE1, HBZ, and HBG2 was up-regulated in FL-iEry compared with CB-iEry, and such up-regulation was accompanied by down-regulated expression of BCL11A and up-regulated expression of LIN28B and IGF2BP1. Our study provides new insights into human FL erythropoiesis and rich resources for future studies.

Elevated CDKN1A (P21) mediates β-thalassemia erythroid apoptosis, but its loss does not improve β-thalassemic erythropoiesis

β-thalassemias are common hemoglobinopathies due to mutations in the β-globin gene that lead to hemolytic anemias. Premature death of β-thalassemic erythroid precursors results in ineffective erythroid maturation, increased production of erythropoietin (EPO), expansion of erythroid progenitor compartment, extramedullary erythropoiesis, and splenomegaly. However, the molecular mechanism of erythroid apoptosis in β-thalassemia is not well understood. Using a mouse model of β-thalassemia (Hbbth3/+), we show that dysregulated expression of the FOXO3 transcription factor is implicated in β-thalassemia erythroid apoptosis. In Foxo3-/-/Hbbth3/+ mice, erythroid apoptosis is significantly reduced, whereas erythroid cell maturation, and red blood cell and hemoglobin production are substantially improved even with elevated reactive oxygen species in double-mutant erythroblasts. However, persistence of elevated reticulocytes and splenomegaly suggests that ineffective erythropoiesis is not resolved in Foxo3-/-/Hbbth3/+. We found the cell cycle inhibitor Cdkn1a (cyclin-dependent kinase inhibitor p21), a FOXO3 target gene, is markedly upregulated in both mouse and patient-derived β-thalassemic erythroid precursors. Double-mutant p21/Hbbth3/+ mice exhibited embryonic lethality with only a fraction of mice surviving to weaning. Notably, studies in adult mice displayed greatly reduced apoptosis and circulating Epo in erythroid compartments of surviving p21-/-/Hbbth3/+ mice relative to Hbbth3/+ mice, whereas ineffective erythroid cell maturation, extramedullary erythropoiesis, and splenomegaly were not modified. These combined results suggest that mechanisms that control β-thalassemic erythroid cell survival and differentiation are uncoupled from ineffective erythropoiesis and involve a molecular network including FOXO3 and P21. Overall, these studies provide a new framework for investigating ineffective erythropoiesis in β-thalassemia.

The impact of erythroblast enucleation efficiency on the severity of anemia in patients with myelodysplastic syndrome

Anemia is the most common manifestation in myelodysplastic syndrome (MDS) patients, but the cause of ineffective hematopoiesis is not fully understood. Enucleation is an important event in the maturation process of erythroblasts. According to a series of morphological phenotypes of the pathological development of MDS erythroblasts, we speculate that there may be enucleation disorders. To verify this hypothesis, we cultured MDS bone marrow CD34+ cells in vitro and induced erythroblast development. The results showed that erythroblast enucleation in MDS was significantly lower than that in the normal group, and the rate of enucleation was positively correlated with hemoglobin concentration. Risk stratification of MDS was performed to further analyze the differences in enucleation among the normal group, low-middle risk group and high-risk group. The results showed that the enucleation rate of the high risk group was higher than that of the low-middle risk group but still lower than that of the normal group. Moreover, the expression of pERK and pAKT in MDS erythroblasts in the high risk group was higher than that in the normal group, while the expression of pERK and pAKT in the low-middle risk group was lower than that in the normal group. Furthermore, the enucleation of MDS was positively correlated with the phosphorylation degree of ERK and AKT. In conclusion, this study reveals that the enucleation of erythroblasts is one of the possible causes of anemia in MDS. Video Abstract.

Accessory-cell-free differentiation of hematopoietic stem and progenitor cells into mature red blood cells

BACKGROUND AIMS

The culture and ex vivo engineering of red blood cells (RBCs) can help characterize genetic variants, model diseases, and may eventually spur the development of applications in transfusion medicine. In the last decade, improvements to the in vitro production of RBCs have enabled efficient erythroid progenitor proliferation and high enucleation levels from several sources of hematopoietic stem and progenitor cells (HSPCs). Despite these advances, there remains a need for refining the terminal step of in vitro human erythropoiesis, i.e., the terminal maturation of reticulocytes into erythrocytes, so that it can occur without feeder or accessory cells and animal-derived components.

METHODS

Here, we describe the near-complete erythroid differentiation of cultured RBCs (cRBCs) from adult HSPCs in accessory-cell-free and xeno-free conditions.

RESULTS

The approach improves post-enucleation cell integrity and cell survival, and it enables subsequent storage of cRBCs for up to 42 days in classical additive solution conditions without any specialized equipment.

CONCLUSIONS

We foresee that these improvements will facilitate the characterization of RBCs derived from gene-edited HSPCs.

Plasmodium falciparum infection of human erythroblasts induces transcriptional changes associated with dyserythropoiesis

During development down the erythroid lineage, hematopoietic stem cells undergo dramatic changes to cellular morphology and function in response to a complex and tightly regulated program of gene expression. In malaria infection, Plasmodium spp parasites accumulate in the bone marrow parenchyma, and emerging evidence suggests erythroblastic islands are a protective site for parasite development into gametocytes. Although it has been observed that Plasmodium falciparum infection in late-stage erythroblasts can delay terminal erythroid differentiation and enucleation, the mechanism(s) underlying this phenomenon are unknown. Here, we apply RNA sequencing after fluorescence-activated cell sorting of infected erythroblasts to identify transcriptional responses to direct and indirect interaction with P falciparum. Four developmental stages of erythroid cells were analyzed: proerythroblast, basophilic erythroblast, polychromatic erythroblast, and orthochromatic erythroblast. We found extensive transcriptional changes in infected erythroblasts compared with that in uninfected cells in the same culture, including dysregulation of genes involved in erythroid proliferation and developmental processes. Although some indicators of cellular oxidative and proteotoxic stress were common across all stages of erythropoiesis, many responses were specific to cellular processes associated with developmental stage. Together, our results evidence multiple possible avenues by which parasite infection can induce dyserythropoiesis at specific points along the erythroid continuum, advancing our understanding of the molecular determinants of malaria anemia.

The transcriptomic landscape of normal and ineffective erythropoiesis at single-cell resolution

The anemias of myelodysplastic syndrome (MDS) and Diamond Blackfan anemia (DBA) are generally macrocytic and always reflect ineffective erythropoiesis yet result from diverse genetic mutations. To delineate shared mechanisms that lead to cell death, we studied the fate of single erythroid marrow cells from individuals with DBA or MDS-5q. We defined an unhealthy (vs healthy) differentiation trajectory using transcriptional pseudotime and cell surface proteins. The pseudotime trajectories diverge immediately after cells upregulate transferrin receptor (CD71), import iron, and initiate heme synthesis, although cell death occurs much later. Cells destined to die express high levels of heme-responsive genes, including ribosomal protein and globin genes, whereas surviving cells downregulate heme synthesis and upregulate DNA damage response, hypoxia, and HIF1 pathways. Surprisingly, 24% ± 12% of cells from control subjects follow the unhealthy trajectory, implying that heme might serve as a rheostat directing cells to live or die. When heme synthesis was inhibited with succinylacetone, more DBA cells followed the healthy trajectory and survived. We also noted high numbers of messages with retained introns that increased as erythroid cells matured, confirmed the rapid cycling of colony forming unit-erythroid, and demonstrated that cell cycle timing is an invariant property of differentiation stage. Including unspliced RNA in pseudotime determinations allowed us to reliably align independent data sets and accurately query stage-specific transcriptomic changes. MDS-5q (unlike DBA) results from somatic mutation, so many normal (unmutated) erythroid cells persist. By independently tracking erythroid differentiation of cells with and without chromosome 5q deletions, we gained insight into why 5q+ cells cannot expand to prevent anemia.

Targeting of Calbindin 1 rescues erythropoiesis in a human model of Diamond Blackfan anemia

Diamond Blackfan anemia (DBA) is an inherited bone marrow failure syndrome characterized by congenital anomalies, cancer predisposition and a severe hypo-proliferative anemia. It was the first disease linked to ribosomal dysfunction and >70 % of patients have been identified to have a haploinsufficiency of a ribosomal protein (RP) gene, with RPS19 being the most common mutation. There is significant variability within the disease in terms of phenotype as well as response to therapy suggesting that other genes contribute to the pathophysiology and potential management of this disease. To explore these questions, we performed a genome-wide CRISPR screen in a cellular model of DBA and identified Calbindin 1 (CALB1), a member of the calcium-binding superfamily, as a potential modifier of the disordered erythropoiesis in DBA. We used human derived CD34+ cells cultured in erythroid stimulating media with knockdown of RPS19 as a model for DBA to study the effects of CALB1. We found that knockdown of CALB1 in this DBA model promoted erythroid maturation. We also noted effects of CALB1 knockdown on cell cycle. Taken together, our results reveal CALB1 is a novel regulator of human erythropoiesis and has implications for using CALB1 as a novel therapeutic target in DBA.

Stage-specific dual function: EZH2 regulates human erythropoiesis by eliciting histone and non-histone methylation

Enhancer of zeste homolog 2 (EZH2) is the lysine methyltransferase of polycomb repressive complex 2 (PRC2) that catalyzes H3K27 tri-methylation. Aberrant expression and loss-of-function mutations of EZH2 have been demonstrated to be tightly associated with the pathogenesis of various myeloid malignancies characterized by ineffective erythropoiesis, such as myelodysplastic syndrome (MDS). However, the function and mechanism of EZH2 in human erythropoiesis still remains largely unknown. Here, we demonstrated that EZH2 regulates human erythropoiesis in a stage-specific, dual-function manner by catalyzing histone and non-histone methylation. During the early erythropoiesis, EZH2 deficiency caused cell cycle arrest in the G1 phase, which impaired cell growth and differentiation. Chromatin immunoprecipitation sequencing and RNA sequencing discovered that EZH2 knockdown caused a reduction of H3K27me3 and upregulation of cell cycle proteindependent kinase inhibitors. In contrast, EZH2 deficiency led to the generation of abnormal nuclear cells and impaired enucleation during the terminal erythropoiesis. Interestingly, EZH2 deficiency downregulated the methylation of HSP70 by directly interacting with HSP70. RNA-sequencing analysis revealed that the expression of AURKB was significantly downregulated in response to EZH2 deficiency. Furthermore, treatment with an AURKB inhibitor and small hairpin RNAmediated AURKB knockdown also led to nuclear malformation and decreased enucleation efficiency. These findings strongly suggest that EZH2 regulates terminal erythropoiesis through a HSP70 methylation-AURKB axis. Our findings have implications for improved understanding of ineffective erythropoiesis with EZH2 dysfunction.

Isolation and evaluation of erythroid progenitors in the livers of larval, froglet, and adult Xenopus tropicalis

Xenopus liver maintains erythropoietic activity from the larval to the adult stage. During metamorphosis, thyroid hormone mediates apoptosis of larval-type erythroid progenitors and proliferation of adult-type erythroid progenitors, and a globin switch occurs during this time. In addition, the whole-body mass and the liver also change; however, whether there is a change in the absolute number of erythroid progenitors is unclear. To isolate and evaluate erythroid progenitors in the Xenopus liver, we developed monoclonal ER9 antibodies against the erythropoietin receptor (EPOR) of Xenopus. ER9 recognized erythrocytes, but not white blood cells or thrombocytes. The specificity of ER9 for EPOR manifested as its inhibitory effect on the proliferation of a Xenopus EPOR-expressing cell line. Furthermore, ER9 recognition was consistent with epor gene expression. ER9 staining with Acridine orange (AO) allowed erythrocyte fractionation through fluorescence-activated cell sorting. The ER9+ and AO-red (AOr)high fractions were highly enriched in erythroid progenitors and primarily localized to the liver. The method developed using ER9 and AO was also applied to larvae and froglets with different progenitor populations from adult frogs. The liver to body weight and the number of ER9+ AOrhigh cells per unit body weight were significantly higher in adults than in larvae and froglets, and the number of ER9+ AOrhigh cells per unit liver weight was the highest in froglets. Collectively, our results show increased erythropoiesis in the froglet liver and demonstrate growth-dependent changes in erythropoiesis patterns in specific organs of Xenopus.

Normal and dysregulated crosstalk between iron metabolism and erythropoiesis

Erythroblasts possess unique characteristics as they undergo differentiation from hematopoietic stem cells. During terminal erythropoiesis, these cells incorporate large amounts of iron in order to generate hemoglobin and ultimately undergo enucleation to become mature red blood cells, ultimately delivering oxygen in the circulation. Thus, erythropoiesis is a finely tuned, multifaceted process requiring numerous properly timed physiological events to maintain efficient production of 2 million red blood cells per second in steady state. Iron is required for normal functioning in all human cells, the erythropoietic compartment consuming the majority in light of the high iron requirements for hemoglobin synthesis. Recent evidence regarding the crosstalk between erythropoiesis and iron metabolism sheds light on the regulation of iron availability by erythroblasts and the consequences of insufficient as well as excess iron on erythroid lineage proliferation and differentiation. In addition, significant progress has been made in our understanding of dysregulated iron metabolism in various congenital and acquired malignant and non-malignant diseases. Finally, we report several actual as well as theoretical opportunities for translating the recently acquired robust mechanistic understanding of iron metabolism regulation to improve management of patients with disordered erythropoiesis, such as anemia of chronic inflammation, β-thalassemia, polycythemia vera, and myelodysplastic syndromes.

Nrf2 sensitizes ferroptosis through l-2-hydroxyglutarate-mediated chromatin modifications in sickle cell disease

Sickle cell disease (SCD) is a chronic hemolytic and systemic hypoxia condition with constant oxidative stress and significant metabolic alterations. However, little is known about the correlation between metabolic alterations and the pathophysiological symptoms. Here, we report that Nrf2, a master regulator of cellular antioxidant responses, regulates the production of the metabolite l-2-hydroxyglutarate (L2HG) to mediate epigenetic histone hypermethylation for gene expression involved in metabolic, oxidative, and ferroptotic stress responses in SCD. Mechanistically, Nrf2 was found to regulate the expression of L2HG dehydrogenase (L2hgdh) to mediate L2HG production under hypoxia. Gene expression profile analysis indicated that reactive oxygen species (ROS) and ferroptosis responses were the most significantly affected signaling pathways after Nrf2 ablation in SCD. Nrf2 silencing and L2HG supplementation sensitize human sickle erythroid cells to ROS and ferroptosis stress. The absence of Nrf2 and accumulation of L2HG significantly affect histone methylation for chromatin structure modification and reduce the assembly of transcription complexes on downstream target genes to regulate ROS and ferroptosis responses. Furthermore, pharmacological activation of Nrf2 was found to have protective effects against ROS and ferroptosis stress in SCD mice. Our data suggest a novel mechanism by which Nrf2 regulates L2HG levels to mediate SCD severity through ROS and ferroptosis stress responses, suggesting that targeting Nrf2 is a viable therapeutic strategy for ameliorating SCD symptoms.

Dynamics of human hematopoietic stem and progenitor cell differentiation to the erythroid lineage

Erythropoiesis, the development of erythrocytes from hematopoietic stem cells, occurs through four phases: erythroid progenitor (EP) development, early erythropoiesis, terminal erythroid differentiation (TED), and maturation. According to the classical model that is based on immunophenotypic profiles of cell populations, each of these phases comprises multiple differentiation states that arise in a hierarchical manner. After segregation of lymphoid potential, erythroid priming begins during progenitor development and progresses through progenitor cell types that have multilineage potential. Complete separation of the erythroid lineage is achieved during early erythropoiesis with the formation of unipotent EPs: burst-forming unit-erythroid and colony-forming unit-erythroid. These erythroid-committed progenitors undergo TED and maturation, which involves expulsion of the nucleus and remodeling to form functional biconcave, hemoglobin-filled erythrocytes. In the last decade or so, many studies employing advanced techniques such as single-cell RNA-sequencing (scRNA-seq) as well as the conventional methods, including colony-forming cell assays and immunophenotyping, have revealed heterogeneity within the stem, progenitor, and erythroblast stages, and uncovered alternate paths for segregation of erythroid lineage potential. In this review, we provide an in-depth account of immunophenotypic profiles of all cell types within erythropoiesis, highlight studies that demonstrate heterogeneous erythroid stages, and describe deviations to the classical model of erythropoiesis. Overall, although scRNA-seq approaches have provided new insights, flow cytometry remains relevant and is the primary method for validation of novel immunophenotypes.

Post-transcriptional regulation of erythropoiesis

Erythropoiesis is a complex, precise, and lifelong process that is essential for maintaining normal body functions. Its strict regulation is necessary to prevent a variety of blood diseases. Normal erythropoiesis is precisely regulated by an intricate network that involves transcription levels, signal transduction, and various epigenetic modifications. In recent years, research on post-transcriptional levels in erythropoiesis has expanded significantly. The dynamic regulation of splicing transitions is responsible for changes in protein isoform expression that add new functions beneficial for erythropoiesis. RNA-binding proteins adapt the translation of transcripts to the protein requirements of the cell, yielding mRNA with dynamic translation efficiency. Noncoding RNAs, such as microRNAs and lncRNAs, are indispensable for changing the translational efficiency and/or stability of targeted mRNAs to maintain the normal expression of genes related to erythropoiesis. N6-methyladenosine-dependent regulation of mRNA translation plays an important role in maintaining the expression programs of erythroid-related genes and promoting erythroid lineage determination. This review aims to describe our current understanding of the role of post-transcriptional regulation in erythropoiesis and erythroid-associated diseases, and to shed light on the physiological and pathological implications of the post-transcriptional regulation machinery in erythropoiesis. These may help to further enrich our understanding of the regulatory network of erythropoiesis and provide new strategies for the diagnosis and treatment of erythroid-related diseases.

Potent and uniform fetal hemoglobin induction via base editing

Inducing fetal hemoglobin (HbF) in red blood cells can alleviate β-thalassemia and sickle cell disease. We compared five strategies in CD34+ hematopoietic stem and progenitor cells, using either Cas9 nuclease or adenine base editors. The most potent modification was adenine base editor generation of γ-globin -175A>G. Homozygous -175A>G edited erythroid colonies expressed 81 ± 7% HbF versus 17 ± 11% in unedited controls, whereas HbF levels were lower and more variable for two Cas9 strategies targeting a BCL11A binding motif in the γ-globin promoter or a BCL11A erythroid enhancer. The -175A>G base edit also induced HbF more potently than a Cas9 approach in red blood cells generated after transplantation of CD34+ hematopoietic stem and progenitor cells into mice. Our data suggest a strategy for potent, uniform induction of HbF and provide insights into γ-globin gene regulation. More generally, we demonstrate that diverse indels generated by Cas9 can cause unexpected phenotypic variation that can be circumvented by base editing.

Diagnosis of erythroid dysplasia by flow cytometry: a review

INTRODUCTION

The diagnosis of myelodysplastic syndrome (MDS) is complex. Flow cytometric analysis of the myelomonocytic compartment can be helpful, but it is highly subjective and reproducibility by non-specialized groups is unclear. Analysis of the erythroid lineage by flow cytometry is emerging as potentially more reproducible and easier to conduct, while keeping a high diagnostic performance.

AREAS COVERED

We review the evidence in this area, including 1) the use of well-established markers - CD71 and CD36 - and other less well-established markers and parameters; 2) the use of flow cytometric scores for the erythroid lineage; and 3) additional aspects, including the emergence of computational tools and the roles of flow cytometry beyond diagnosis. Finally, we discuss the limitations with the current evidence, including 1) the impact of the sample processing protocol and reagents on the results, 2) the lack of a standard gating strategy, and 3) conceptualization and design issues in the available publications.

EXPERT OPINION

We end by offering our recommendations for the current use - and our personal take on the value - of the analysis of erythroid lineage by flow cytometry.

Single-cell multi-omics of mitochondrial DNA disorders reveals dynamics of purifying selection across human immune cells

Pathogenic mutations in mitochondrial DNA (mtDNA) compromise cellular metabolism, contributing to cellular heterogeneity and disease. Diverse mutations are associated with diverse clinical phenotypes, suggesting distinct organ- and cell-type-specific metabolic vulnerabilities. Here we establish a multi-omics approach to quantify deletions in mtDNA alongside cell state features in single cells derived from six patients across the phenotypic spectrum of single large-scale mtDNA deletions (SLSMDs). By profiling 206,663 cells, we reveal the dynamics of pathogenic mtDNA deletion heteroplasmy consistent with purifying selection and distinct metabolic vulnerabilities across T-cell states in vivo and validate these observations in vitro. By extending analyses to hematopoietic and erythroid progenitors, we reveal mtDNA dynamics and cell-type-specific gene regulatory adaptations, demonstrating the context-dependence of perturbing mitochondrial genomic integrity. Collectively, we report pathogenic mtDNA heteroplasmy dynamics of individual blood and immune cells across lineages, demonstrating the power of single-cell multi-omics for revealing fundamental properties of mitochondrial genetics.

Immunoregulatory properties of erythroid nucleated cells induced from CD34+ progenitors from bone marrow

CD 71+ erythroid nucleated cells have pronounced immunoregulatory properties in normal and pathological conditions. Many populations of cells with immunoregulatory properties are considered candidates for cellular immunotherapy for various pathologies. This study characterized the immunoregulatory properties of CD71+ erythroid cells derived from CD34-positive bone marrow cells under the influence of growth factors that stimulate differentiation into erythroid cells. CD34-negative bone marrow cells were used to isolate CD71+ erythroid nuclear cells. The resulting cells were used to assess the phenotype, determine the mRNA spectrum of the genes responsible for the main pathways and processes of the immune response, and obtain culture supernatants for the analysis of immunoregulatory factors. It was found that CD71+ erythroid cells derived from CD34+ cells carry the main markers of erythroid cells, but differ markedly from natural bone marrow CD71+ erythroid cells. The main differences are in the presence of the CD45+ subpopulation, distribution of terminal differentiation stages, transcriptional profile, secretion of certain cytokines, and immunosuppressive activity. The properties of induced CD71+ erythroid cells are closer to the cells of extramedullary erythropoiesis foci than to natural bone marrow CD71+ erythroid cells. Thus, when cultivating CD71+ erythroid cells for clinical experimental studies, it is necessary to take into account their pronounced immunoregulatory activity.

MBD2a-NuRD binds to the methylated γ-globin gene promoter and uniquely forms a complex required for silencing of HbF expression

During human development, there is a switch in the erythroid compartment at birth that results in silencing of expression of fetal hemoglobin (HbF). Reversal of this silencing has been shown to be effective in overcoming the pathophysiologic defect in sickle cell anemia. Among the many transcription factors and epigenetic effectors that are known to mediate HbF silencing, two of the most potent are BCL11A and MBD2-NuRD. In this report, we present direct evidence that MBD2-NuRD occupies the γ-globin gene promoter in adult erythroid cells and positions a nucleosome there that results in a closed chromatin conformation that prevents binding of the transcriptional activator, NF-Y. We show that the specific isoform, MBD2a, is required for the formation and stable occupancy of this repressor complex that includes BCL11A, MBD2a-NuRD, and the arginine methyltransferase, PRMT5. The methyl cytosine binding preference and the arginine-rich (GR) domain of MBD2a are required for high affinity binding to methylated γ-globin gene proximal promoter DNA sequences. Mutation of the methyl cytosine-binding domain (MBD) of MBD2 results in a variable but consistent loss of γ-globin gene silencing, in support of the importance of promoter methylation. The GR domain of MBD2a is also required for recruitment of PRMT5, which in turn results in placement of the repressive chromatin mark H3K8me2s at the promoter. These findings support a unified model that integrates the respective roles of BCL11A, MBD2a-NuRD, PRMT5, and DNA methylation in HbF silencing.

The novel GATA1-interacting protein HES6 is an essential transcriptional cofactor for human erythropoiesis

Normal erythropoiesis requires the precise regulation of gene expression patterns, and transcription cofactors play a vital role in this process. Deregulation of cofactors has emerged as a key mechanism contributing to erythroid disorders. Through gene expression profiling, we found HES6 as an abundant cofactor expressed at gene level during human erythropoiesis. HES6 physically interacted with GATA1 and influenced the interaction of GATA1 with FOG1. Knockdown of HES6 impaired human erythropoiesis by decreasing GATA1 expression. Chromatin immunoprecipitation and RNA sequencing revealed a rich set of HES6- and GATA1-co-regulated genes involved in erythroid-related pathways. We also discovered a positive feedback loop composed of HES6, GATA1 and STAT1 in the regulation of erythropoiesis. Notably, erythropoietin (EPO) stimulation led to up-regulation of these loop components. Increased expression levels of loop components were observed in CD34+ cells of polycythemia vera patients. Interference by either HES6 knockdown or inhibition of STAT1 activity suppressed proliferation of erythroid cells with the JAK2V617F mutation. We further explored the impact of HES6 on polycythemia vera phenotypes in mice. The identification of the HES6-GATA1 regulatory loop and its regulation by EPO provides novel insights into human erythropoiesis regulated by EPO/EPOR and a potential therapeutic target for the management of polycythemia vera.

Plasmodium falciparum infection of human erythroblasts induces transcriptional changes associated with dyserythropoiesis

During development down the erythroid lineage, hematopoietic stem cells undergo dramatic changes to cellular morphology and function in response to a complex and tightly regulated program of gene expression. In malaria infection, Plasmodium spp . parasites accumulate in the bone marrow parenchyma, and emerging evidence suggests erythroblastic islands are a protective site for parasite development into gametocytes. While it has been observed that Plasmodium falciparum infection of late-stage erythroblasts can delay terminal erythroid differentiation and enucleation, the mechanism(s) underlying this phenomenon are unknown. Here, we apply RNA-seq after fluorescence-activated cell sorting (FACS) of infected erythroblasts to identify transcriptional responses to direct and indirect interaction with Plasmodium falciparum . Four developmental stages of erythroid cells were analyzed: proerythroblast, basophilic erythroblast, polychromatic erythroblast, and orthochromatic erythroblast. We found extensive transcriptional changes in infected erythroblasts compared to uninfected cells in the same culture, including dysregulation of genes involved in erythroid proliferation and developmental processes. Whereas some indicators of cellular oxidative and proteotoxic stress were common across all stages of erythropoiesis, many responses were specific to cellular processes associated with developmental stage. Together, our results evidence multiple possible avenues by which parasite infection can induce dyserythropoiesis at specific points along the erythroid continuum, advancing our understanding of the molecular determinants of malaria anemia.

Key Points

Erythroblasts at different stages of differentiation have distinct responses to infection by Plasmodium falciparum . P. falciparum infection of erythroblasts alters expression of genes related to oxidative and proteotoxic stress and erythroid development.

Arginine metabolism regulates human erythroid differentiation through hypusination of eIF5A

Metabolic programs contribute to hematopoietic stem and progenitor cell (HSPC) fate, but it is not known whether the metabolic regulation of protein synthesis controls HSPC differentiation. Here, we show that SLC7A1/cationic amino acid transporter 1-dependent arginine uptake and its catabolism to the polyamine spermidine control human erythroid specification of HSPCs via the activation of the eukaryotic translation initiation factor 5A (eIF5A). eIF5A activity is dependent on its hypusination, a posttranslational modification resulting from the conjugation of the aminobutyl moiety of spermidine to lysine. Notably, attenuation of hypusine synthesis in erythroid progenitors, by the inhibition of deoxyhypusine synthase, abrogates erythropoiesis but not myeloid cell differentiation. Proteomic profiling reveals mitochondrial translation to be a critical target of hypusinated eIF5A, and accordingly, progenitors with decreased hypusine activity exhibit diminished oxidative phosphorylation. This affected pathway is critical for eIF5A-regulated erythropoiesis, as interventions augmenting mitochondrial function partially rescue human erythropoiesis under conditions of attenuated hypusination. Levels of mitochondrial ribosomal proteins (RPs) were especially sensitive to the loss of hypusine, and we find that the ineffective erythropoiesis linked to haploinsufficiency of RPS14 in chromosome 5q deletions in myelodysplastic syndrome is associated with a diminished pool of hypusinated eIF5A. Moreover, patients with RPL11-haploinsufficient Diamond-Blackfan anemia as well as CD34+ progenitors with downregulated RPL11 exhibit a markedly decreased hypusination in erythroid progenitors, concomitant with a loss of mitochondrial metabolism. Thus, eIF5A-dependent protein synthesis regulates human erythropoiesis, and our data reveal a novel role for RPs in controlling eIF5A hypusination in HSPCs, synchronizing mitochondrial metabolism with erythroid differentiation.

Analysis of Immunophenotypic Changes during Ex Vivo Human Erythropoiesis and Its Application in the Study of Normal and Defective Erythropoiesis

Erythropoiesis is a highly regulated process and undergoes several genotypic and phenotypic changes during differentiation. The phenotypic changes can be evaluated using a combination of cell surface markers expressed at different cellular stages of erythropoiesis using FACS. However, limited studies are available on the in-depth phenotypic characterization of progenitors from human adult hematopoietic stem and progenitor cells (HSPCs) to red blood cells. Therefore, using a set of designed marker panels, in the current study we have kinetically characterized the hematopoietic, erythroid progenitors, and terminally differentiated erythroblasts ex vivo. Furthermore, the progenitor stages were explored for expression of CD117, CD31, CD41a, CD133, and CD45, along with known key markers CD36, CD71, CD105, and GPA. Additionally, we used these marker panels to study the stage-specific phenotypic changes regulated by the epigenetic regulator; Nuclear receptor binding SET Domain protein 1 (NSD1) during erythropoiesis and to study ineffective erythropoiesis in myelodysplastic syndrome (MDS) and pure red cell aplasia (PRCA) patients. Our immunophenotyping strategy can be used to sort and study erythroid-primed hematopoietic and erythroid precursors at specified time points and to study diseases resulting from erythroid dyspoiesis. Overall, the current study explores the in-depth kinetics of phenotypic changes occurring during human erythropoiesis and applies this strategy to study normal and defective erythropoiesis.

Ex vivo prime editing of patient haematopoietic stem cells rescues sickle-cell disease phenotypes after engraftment in mice

Sickle-cell disease (SCD) is caused by an A·T-to-T·A transversion mutation in the β-globin gene (HBB). Here we show that prime editing can correct the SCD allele (HBBS) to wild type (HBBA) at frequencies of 15%-41% in haematopoietic stem and progenitor cells (HSPCs) from patients with SCD. Seventeen weeks after transplantation into immunodeficient mice, prime-edited SCD HSPCs maintained HBBA levels and displayed engraftment frequencies, haematopoietic differentiation and lineage maturation similar to those of unedited HSPCs from healthy donors. An average of 42% of human erythroblasts and reticulocytes isolated 17 weeks after transplantation of prime-edited HSPCs from four SCD patient donors expressed HBBA, exceeding the levels predicted for therapeutic benefit. HSPC-derived erythrocytes carried less sickle haemoglobin, contained HBBA-derived adult haemoglobin at 28%-43% of normal levels and resisted hypoxia-induced sickling. Minimal off-target editing was detected at over 100 sites nominated experimentally via unbiased genome-wide analysis. Our findings support the feasibility of a one-time prime editing SCD treatment that corrects HBBS to HBBA, does not require any viral or non-viral DNA template and minimizes undesired consequences of DNA double-strand breaks.

Iron-loaded deferiprone can support full hemoglobinization of cultured red blood cells

Iron, supplemented as iron-loaded transferrin (holotransferrin), is an essential nutrient in mammalian cell cultures, particularly for erythroid cultures. The high cost of human transferrin represents a challenge for large scale production of red blood cells (RBCs) and for cell therapies in general. We evaluated the use of deferiprone, a cell membrane-permeable drug for iron chelation therapy, as an iron carrier for erythroid cultures. Iron-loaded deferiprone (Def₃·Fe³⁺, at 52 µmol/L) could eliminate the need for holotransferrin supplementation during in vitro expansion and differentiation of erythroblast cultures to produce large numbers of enucleated RBC. Only the first stage, when hematopoietic stem cells committed to erythroblasts, required holotransferrin supplementation. RBCs cultured in presence of Def₃·Fe³⁺ or holotransferrin (1000 µg/mL) were similar with respect to differentiation kinetics, expression of cell-surface markers CD235a and CD49d, hemoglobin content, and oxygen association/dissociation. Replacement of holotransferrin supplementation by Def₃·Fe³⁺ was also successful in cultures of myeloid cell lines (MOLM13, NB4, EOL1, K562, HL60, ML2). Thus, iron-loaded deferiprone can partially replace holotransferrin as a supplement in chemically defined cell culture medium. This holds promise for a significant decrease in medium cost and improved economic perspectives of the large scale production of red blood cells for transfusion purposes.

Aptamer-Mediated Enrichment of Rare Circulating Fetal Nucleated Red Blood Cells for Noninvasive Prenatal Diagnosis

Isolation of circulating fetal nucleated red blood cells (cfNRBCs) from maternal peripheral blood provides a superior strategy for noninvasive prenatal genetic diagnosis. Recent technical advances in single-cell isolation and genetic analyses have promoted the clinical application of circulating fetal cell-based noninvasive prenatal diagnosis. However, the lack of highly specific ligands for rare circulating fetal cell enrichment from massive maternal cells significantly impedes the clinical transformation progress. In this work, aptamers specific to NRBCs were developed through clinical sample-based cell-SELEX. Herein, the complex clinical system provides natural selection stringency through binding competition between target and background cells, and it empowers aptamers with high specificity. An aptamer-based strategy was also established to isolate cfNRBCs from maternal peripheral blood. Results show the remarkable selectivity and affinity of developed aptamers, enabling efficient enrichment of cfNRBCs from abundant maternal cells. Moreover, screening for fetal sex and trisomy syndrome achieved high accuracy through chromosome analysis of enriched cfNRBCs. To the best of our knowledge, this is the first report to develop aptamer ligands for cfNRBC enrichment, providing an efficient strategy to screen cfNRBC-specific ligands and demonstrating broad application potential for cfNRBC-based noninvasive prenatal diagnosis.

Heterochromatin rewiring and domain disruption-mediated chromatin compaction during erythropoiesis

Mammalian erythropoiesis involves progressive chromatin compaction and subsequent enucleation in terminal differentiation, but the mechanisms underlying the three-dimensional chromatin reorganization remain obscure. Here, we systematically analyze the higher-order chromatin in purified populations of primary human erythroblasts. Our results reveal that heterochromatin regions undergo substantial compression, with H3K9me3 markers relocalizing to the nuclear periphery and forming a significant number of long-range interactions, and that ~58% of the topologically associating domain (TAD) boundaries are disrupted, while certain TADs enriched for markers of the active transcription state and erythroid master regulators, GATA1 and KLF1, are selectively maintained during terminal erythropoiesis. Finally, we demonstrate that GATA1 is involved in safeguarding selected essential chromatin domains during terminal erythropoiesis. Our study therefore delineates the molecular characteristics of a development-driven chromatin compaction process, which reveals transcription competence as a key indicator of the selected domain maintenance to ensure appropriate gene expression during the extreme compaction of chromatin.

Automated Good Manufacturing Practice-Compatible CRISPR-Cas9 Editing of Hematopoietic Stem and Progenitor Cells for Clinical Treatment of β-Hemoglobinopathies

Cellular therapies hold enormous potential for the cure of severe hematological and oncological disorders. The forefront of innovative gene therapy approaches including therapeutic gene editing and hematopoietic stem cell transplantation needs to be processed by good manufacturing practice to ensure safe application in patients. In the present study, an effective transfection protocol for automated clinical-scale production of genetically modified hematopoietic stem and progenitor cells (HSPCs) using the CliniMACS Prodigy^® system including the CliniMACS Electroporator (Miltenyi Biotec) was established. As a proof-of-concept, the enhancer of the BCL11A gene, clustered regularly interspaced short palindromic repeat (CRISPR) target in ongoing clinical trials for β-thalassemia and sickle-cell disease treatment, was disrupted by the CRISPR-Cas9 system simulating a large-scale clinical scenario, yielding 100 million HSPCs with high editing efficiency. In vitro erythroid differentiation and high-performance liquid chromatography analyses corroborated fetal hemoglobin resurgence in edited samples, supporting the feasibility of running the complete process of HSPC gene editing in an automated closed system.

Immunophenotypical profiling of myeloid neoplasms with erythroid predominance using mass cytometry (CyTOF)

Acute erythroid leukemia (AEL) is a disease continuum between Myelodysplastic syndrome (MDS) and Acute myeloid leukemia (AML) with the cellular hallmark of uncontrolled proliferation and impaired differentiation of erythroid progenitor cells. First described by Giovanni di Guglielmo in 1917 AEL accounts for less than 5% of all de novo AML cases. There have been efforts to characterize AEL at a molecular level, describing recurrent alterations in TP53, NPM1 and FLT3 genes. A genomic analysis of AEL cases confirmed its complexity. Despite these advances, the biology underlying erythroid proliferations remains unclear and the prognosis is dismal with a median survival of only 3 months for pure erythroid leukemia (PEL). Marker combinations suitable for the identification and characterization of leukemic stem cell (LSC) candidates, monitoring measurable residual disease (MRD) during chemotherapy treatment and the development of innovative targeted therapies are missing. Here, we developed a mass cytometry panel for an in-depth characterization of erythroid and myeloid blast cell populations from human AEL bone marrow samples in comparison to other AML subtypes and healthy counterparts. A total of 8 AEL samples were analyzed and compared to 28 AML samples from different molecular subtypes, healthy bone marrow counterparts (n = 5) and umbilical cord blood (n = 6) using high-dimensional mass cytometry. Identification of erythroid and myeloid blast populations in high-dimensional mass cytometry data enabled a refined view on erythroblast differentiation stages present in AEL erythroid blasts and revealed immunophenotypical profiles specific to AEL. Profiling of phenotypic LSCs revealed aberrant erythroid marker expression in the CD34⁺ CD38^- stem cell compartment. In addition, the identification of novel candidate surface marker combinations and aberrancies might enhance clinical diagnostics of AEL. We present a high-parameter mass cytometry approach feasible for immunophenotypical analysis of blast and stem cell populations in myeloid neoplasms with erythroid predominance laying the foundation for more precise experimental approaches in the future.

Role of Caspase-10-P13tBID axis in erythropoiesis regulation

Red blood cell production is negatively controlled by the rate of apoptosis at the stage of CFU-E/pro-erythroblast differentiation, depending on the balance between erythropoietin (EPO) levels and activation of the Fas/FasL pathway. At this stage, activation of transient caspases through depolarization via mitochondrial outer membrane permeabilization (MOMP) is also required for terminal erythroid differentiation. Molecular mechanisms regulating the differential levels of MOMP during differentiation and apoptosis, however, remain poorly understood. Here we show a novel and essential role for the caspase-10-P13-tBID axis in erythroid terminal differentiation. Caspase-10 (but not caspase-8, which is activated during apoptosis) is activated at the early stages of erythroid terminal differentiation leading to the cleavage of P22-BID into P18-tBID, and later into P13-tBID. Erythropoietin (EPO) by inducing casein kinase I alpha (CKIα) expression, which in turn phosphorylates P18-tBID, prevents the generation of MYR-P15-tBID (leading to apoptosis) and allows the generation of P13-tBID by caspase-10. Unlike P15-tBID, P13-tBID is not myristoylated and as such, does not irreversibly anchor the mitochondrial membrane resulting in a transient MOMP. Likewise, transduction of a P13-tBID fragment induces rapid and strong erythroid terminal differentiation. Thus, EPO modulates the pattern of BID cleavage to control the level of MOMP and determines the fate of erythroblasts between apoptosis and differentiation. This pathway is impaired in 5q- myelodysplastic syndromes because of CK1α haplo-insufficiency and may contribute to erythroid differentiation arrest and high sensitivity of this disease to lenalidomide (LEN).

Regulation of eIF4E guides a unique translational program to control erythroid maturation

Translation control is essential in balancing hematopoietic precursors and differentiation; however, the mechanisms underlying this program are poorly understood. We found that the activity of the major cap-binding protein eIF4E is unexpectedly regulated in a dynamic manner throughout erythropoiesis that is uncoupled from global protein synthesis rates. Moreover, eIF4E activity directs erythroid maturation, and increased eIF4E expression maintains cells in an early erythroid state associated with a translation program driving the expression of PTPN6 and Igf2bp1. A cytosine-enriched motif in the 5' untranslated region is important for eIF4E-mediated translation specificity. Therefore, selective translation of key target genes necessary for the maintenance of early erythroid states by eIF4E highlights a unique mechanism used by hematopoietic precursors to rapidly elicit erythropoietic maturation upon need.