Bone marrow sinusoidal endothelium controls terminal erythroid differentiation and reticulocyte maturation

Within the bone marrow microenvironment, endothelial cells (EC) exert important functions. Arterial EC support hematopoiesis while H-type capillaries induce bone formation. Here, we show that BM sinusoidal EC (BM-SEC) actively control erythropoiesis. Mice with stabilized β-catenin in BM-SEC (Ctnnb1OE-SEC) generated by using a BM-SEC-restricted Cre mouse line (Stab2-iCreF3) develop fatal anemia. While activation of Wnt-signaling in BM-SEC causes an increase in erythroblast subsets (PII–PIV), mature erythroid cells (PV) are reduced indicating impairment of terminal erythroid differentiation/reticulocyte maturation. Transplantation of Ctnnb1OE-SEC hematopoietic stem cells into wildtype recipients confirms lethal anemia to be caused by cell-extrinsic, endothelial-mediated effects. Ctnnb1OE-SEC BM-SEC reveal aberrant sinusoidal differentiation with altered EC gene expression and perisinusoidal ECM deposition and angiocrine dysregulation with de novo endothelial expression of FGF23 and DKK2, elevated in anemia and involved in vascular stabilization, respectively. Our study demonstrates that BM-SEC play an important role in the bone marrow microenvironment in health and disease.

Dissecting GATA1 Protein Interactions in Normal and Malignant Human Erythroblasts

Acute erythroid leukemia (AEL) is characterized by uncontrolled accumulation of transformed erythroblasts. Previous analysis of murine and human AEL revealed aberrant regulation of the master regulator GATA1, which controls terminal erythroid differentiation in multi-protein complexes acting as activators or repressors of gene expression. Although most malignant erythroblasts constitutively express abundant GATA1 protein, terminal erythroid differentiation is impaired. Notably, overexpression of GATA1 significantly induced partial or complete terminal erythroid differentiation of the human AEL cell line K562 or immortalised HUDEP2 human erythroblasts, respectively. These observations led us to hypothesize that blocked terminal erythroid differentiation in AEL might be the consequence of titratable dose-dependent aberrant GATA1 protein interactions. We comparatively analysed nuclear extracts from three human AEL cell lines (F36P, K562, KMOE2) and primary cells from an AEL patient. In addition, we analysed HUDEP2 and primary human erythroblasts (hEBST) from healthy donors that retain the potential for complete in vitro terminal erythroid differentiation. We quantified protein expression using a tandem mass tag (TMT) based approach (n=3/cell type) and we compared putative GATA1 interactions by immunoprecipitation (IP) followed by liquid chromatography mass spectrometry (MS) (n=3/cell type). Quantitative proteomics identified 6774 commonly expressed proteins in AEL and "normal" erythroblasts with a high reproducibility (mean coefficients of variation <10%) for all six different cell types. Unsupervised hierarchical clustering displayed a clear separation of the AEL cells from the "normal erythroblasts" (hEBST, HUDEP2). 386 proteins were higher expressed in the AEL group (logFC>=2; q<0.05), whereas 623 were more abundant in normal erythroblasts (logFC>=2; q<0.05). IP-MS analysis of nuclear lysates from the AEL cell lines, the AEL primary sample, HUDEP2 and hEBST resulted in a matrix containing 1616 proteins from which 126 proteins seem to significantly differentially interact with GATA1. 54 proteins were more enriched in the AEL group, whereas 72 proteins were more enriched in "normal" erythroblasts (q<0.5). Principal component analysis (PCA) showed for all cell lines a similar clustering pattern, accounting for 24% and 32% of the variance. Pulled-down proteins in hEBST and HUDEP2 clustered together and were closer to F36P and KMOE, than LAM49 and K562. Notably, we found significant enrichment (validated by immunoblotting) of the SKI protooncogene in AEL cells (logFC=1.82; q=0.013), a finding which not only confirmed previous findings in murine AEL models (MEL cells, erythroblasts from Nsd1 -/- mice) but also speaks for the functionality of our approach. Similarly, the LRPPRC leucine-rich PPR-motif-containing protein overexpressed in several cancers, as well the lactate dehydrogenases A and B (LDHA, LDHB) were significantly enriched in malignant erythroblasts (logFC>2; q<0.05). Furthermore, the ZEB2 zinc finger E-box-binding homeobox 2 protein, was significantly enriched in AEL cells (logFC=2.02; q=0.005). In contrast, the hematopoietic master transcription factor Runt-related transcription factor 1 (RUNX1) (logFC=2.48; q=0.0018) as well as DNA binding protein Ikaros (IKZF1) (logFC=1.71; q=0.13) were significantly enriched (validated by immunoblotting) in HUDEP2 and hEBST. Moreover, the MCM6 DNA binding mini-chromosome maintenance complex component 6 critical for proper DNA replication was enriched in normal erythroblasts (logFC=1.99; q=0.0001). Interestingly, one of the most strongly enriched (and validated by immunoblotting) proteins in normal erythroblasts was the nuclear pore complex protein NUP155 (logFC=6.1; q=0.0000001). Integration of the quantitative proteomics and the IP-MS analysis identified 118 proteins differentially expressed and differentially pulled-down by GATA1-IP, of which 49 were enriched in malignant and 69 proteins in normal erythroblasts (q<0.5). This shows that we reproducibly identified proteins that are differentially associated with GATA1 which are also differentially expressed in AEL cells versus normal erythroblasts. A targeted CRISPR/Cas9 screen is under way to identify GATA1-interacting proteins responsible for impaired erythroid differentiation of AEL cells. Disclosures Valent: Novartis: Honoraria; Pfizer: Honoraria, Research Funding; Celgene/BMS: Honoraria, Research Funding; Incyte: Honoraria, Research Funding; OAP Orphan Pharmaceuticals: Honoraria.

Regulatory association of long noncoding RNAs and chromatin accessibility facilitates erythroid differentiation

Erythroid differentiation is a dynamic process regulated by multiple factors, while the interaction between long non-coding RNAs and chromatin accessibility and its influence on erythroid differentiation remains unclear. To elucidate this interaction, we employed hematopoietic stem cells, multipotent progenitor cells, common myeloid progenitor cells, megakaryocyte-erythroid progenitor cells, and erythroblasts from human cord blood as an erythroid differentiation model to explore the coordinated regulatory functions of lncRNAs and chromatin accessibility by integrating RNA-Seq and ATAC-Seq data. We revealed that the integrated network of chromatin accessibility and lncRNAs exhibits stage-specific changes throughout the erythroid differentiation process, and that the changes at the EB stage of maturation are dramatic. We identified a subset of stage-specific lncRNAs and transcription factors (TFs) that associate with chromatin accessibility during erythroid differentiation, in which lncRNAs are key regulators of terminal erythroid differentiation via a lncRNA-TF-gene network. LncRNA PCED1B-AS1 was revealed to regulate terminal erythroid differentiation by coordinating GATA1 dynamically binding to the chromatin and interacting with cytoskeleton network during erythroid differentiation. DANCR, another lncRNA that is highly expressed at the MEP stage, was verified to promote erythroid differentiation by compromising megakaryocyte differentiation and coordinating with chromatin accessibility and TFs, such as RUNX1. Overall, our results identified the associated network of lncRNAs and chromatin accessibility in erythropoiesis and provide novel insights into erythroid differentiation and abundant resources for further study.

Vesicular formation regulated by ERK/MAPK pathway mediates human erythroblast enucleation

Enucleation is a key event in mammalian erythropoiesis responsible for generation of enucleated reticulocytes. While progress is being made in developing mechanistic understanding of enucleation, our understanding of mechanisms for enucleation is still incomplete. Mitogen-activated protein kinase (MAPK) pathway plays diverse roles in biological processes but its role in erythropoiesis is yet to be fully defined. Analysis of RNA-seq data revealed that MAPK pathway is significantly up regulated during human terminal erythroid differentiation. MAPK pathway consists of three major signaling cassettes, MEK/ERK, p38 and c-Jun N-terminal Kinases (JNK). In the present study, we show that amongst these three cassettes, only ERK was significantly up regulated in late stage human erythroblasts. The increased expression of ERK along with its increased phosphorylation suggests a potential role of ERK activation in enucleation. To explore this hypothesis, we treated sorted populations of human orthochromatic erythroblasts with MEK/ERK inhibitor U0126 and found that U0126 inhibited enucleation. In contrast, inhibitors of either p38 or JNK had no effect on enucleation. Mechanistically, U0126 selectively inhibited formation/accumulation of cytoplasmic vesicles and endocytosis of the transferrin receptor without affecting chromatin condensation, nuclear polarization and enucleosome formation. Treatment with vacuolin-1 that induces vacuole formation partially rescued the blockage of enucleation by U0126. Moreover, phosphoproteomic analysis revealed that inactivation of the ERK pathway led to down regulation of endocytic recycling pathway. Collectively, our findings uncovered a novel role of ERK activation in human erythroblast enucleation by modulating vesicle formation and have implications for understanding anemia associated with defective enucleation.

Decreased PGC1β expression results in disrupted human erythroid differentiation, impaired hemoglobinization and cell cycle exit

Production of red blood cells relies on proper mitochondrial function, both for their increased energy demands during differentiation and for proper heme and iron homeostasis. Mutations in genes regulating mitochondrial function have been reported in patients with anemia, yet their pathophysiological role often remains unclear. PGC1β is a critical coactivator of mitochondrial biogenesis, with increased expression during terminal erythroid differentiation. The role of PGC1β has however mainly been studied in skeletal muscle, adipose and hepatic tissues, and its function in erythropoiesis remains largely unknown. Here we show that perturbed PGC1β expression in human hematopoietic stem/progenitor cells from both bone marrow and cord blood results in impaired formation of early erythroid progenitors and delayed terminal erythroid differentiation in vitro, with accumulations of polychromatic erythroblasts, similar to MDS-related refractory anemia. Reduced levels of PGC1β resulted in deregulated expression of iron, heme and globin related genes in polychromatic erythroblasts, and reduced hemoglobin content in the more mature bone marrow derived reticulocytes. Furthermore, PGC1β knock-down resulted in disturbed cell cycle exit with accumulation of erythroblasts in S-phase and enhanced expression of G1-S regulating genes, with smaller reticulocytes as a result. Taken together, we demonstrate that PGC1β is directly involved in production of hemoglobin and regulation of G1-S transition and is ultimately required for proper terminal erythroid differentiation.

Impairment of human terminal erythroid differentiation by histone deacetylase 5 deficiency

Histone deacetylases (HDACs) are a group of enzymes catalyzing the removal of acetyl groups from histone and non-histone proteins. HDACs have been shown to play diverse functions in a wide range of biological processes. However, their roles in mammalian erythropoiesis remain to be fully defined. We show here that of the eleven classic HDAC family members, six of them (HDAC 1,2,3 and HDAC 5,6,7) are expressed in human erythroid cells with HDAC5 most significantly up regulated during terminal erythroid differentiation. Knockdown of HDAC5 by either shRNA or siRNA in human CD34+ cells followed by erythroid cell culture led to increased apoptosis, decreased chromatin condensation, and impaired enucleation of erythroblasts. Biochemical analyses revealed that HDAC5 deficiency resulted in activation of p53 in association with increased acetylation of p53. Furthermore, while acetylation of histone 4 (H4) is decreased during normal terminal erythroid differentiation, HDAC5 deficiency led to increased acetylation of H4 (K12) in late stage erythroblasts. This increased acetylation was accompanied by decreased chromatin condensation, implying a role for H4 (K12) deacetylation in chromatin condensation. ATAC-seq and RNA-seq analyses revealed that HDAC5 knockdown leads to increased chromatin accessibility genome wide and global changes in gene expression. Moreover, pharmacological inhibition of HDAC5 by the inhibitor LMK235 also led to increased H4 acetylation, impaired chromatin condensation and enucleation. Taken together, our findings have uncovered previously unrecognized roles and molecular mechanisms of action for HDAC5 in human erythropoiesis. These results may provide insights into understanding the anemia associated with HDAC inhibitor treatment.

The dynamic interactive network of long non-coding RNAs and chromatin accessibility facilitates erythroid differentiation

Erythroid differentiation is a dynamic process regulated by multiple factors, while the interaction between long non-coding RNAs and chromatin accessibility and its influence on erythroid differentiation remains unclear. To elucidate this interaction, we employed hematopoietic stem cells, multipotent progenitor cells, common myeloid progenitor cells, megakaryocyte-erythroid progenitor cells, and erythroblasts from human cord blood as an erythroid differentiation model to explore the coordinated regulatory functions of lncRNAs and chromatin accessibility in erythropoiesis by integrating RNA-Seq and ATAC-Seq data. We revealed that the integrated network of chromatin accessibility and LncRNAs exhibits stage-specific changes throughout the erythroid differentiation process, and that the changes at the EB stage of maturation are dramatic. We identified a subset of stage-specific lncRNAs and transcription factors (TFs) that coordinate with chromatin accessibility during erythroid differentiation, in which lncRNAs are key regulators of terminal erythroid differentiation via a lncRNA-TF-gene network. LncRNA PCED1B-AS1 was revealed to regulate terminal erythroid differentiation by coordinating GATA1 dynamically binding to the chromatin during erythroid differentiation. DANCR, another lncRNA that is highly expressed at the MEP stage, was verified to promote erythroid differentiation by compromising megakaryocyte differentiation and coordinating with chromatin accessibility and TFs, such as RUNX1. Overall, our results identified the interactive network of lncRNAs and chromatin accessibility in erythropoiesis and provide novel insights into erythroid differentiation and abundant resources for further study.Key PointsLncRNAs regulate erythroid differentiation through coordinating with chromatin accessibility.The integrative multi-omics analysis reveals stage-specific interaction network of LncRNAs and chromatin accessibility in erythropoiesis.

Luspatercept in the treatment of lower-risk myelodysplastic syndromes

Transforming growth factor beta (TGF-β) signaling pathway is key to hematopoiesis regulation. Increased activation of this pathway contributes to ineffective terminal erythroid differentiation in myelodysplastic syndromes (MDS). Luspatercept is a novel fusion protein that traps TGF-β ligands preventing them from binding to Type II TGF-β receptors, thereby decreasing phosphorylated SMAD2/3 resulting in the downstream effect of promoting erythropoiesis. Seminal clinical trials using luspatercept, PACE-MD and MEDALIST, demonstrated impressive efficacy in the treatment of transfusion-dependent anemia in intermediate risk or lower MDS had led to the US FDA approval for this indication. This review summarizes luspatercept mechanisms of action, efficacy/safety data supporting its use and ongoing clinical trials in MDS.

Recapitulation of erythropoiesis in congenital dyserythropoietic anaemia type I (CDA-I) identifies defects in differentiation and nucleolar abnormalities

The investigation of inherited disorders of erythropoiesis has elucidated many of the principles underlying the production of normal red blood cells and how this is perturbed in human disease. Congenital Dyserythropoietic Anaemia type 1 (CDA-I) is a rare form of anaemia caused by mutations in two genes of unknown function: CDAN1 and CDIN1 (previously called C15orf41), whilst in some cases, the underlying genetic abnormality is completely unknown. Consequently, the pathways affected in CDA-I remain to be discovered. To enable detailed analysis of this rare disorder we have validated a culture system which recapitulates all of the cardinal haematological features of CDA-I, including the formation of the pathognomonic ‘spongy’ heterochromatin seen by electron microscopy. Using a variety of cell and molecular biological approaches we discovered that erythroid cells in this condition show a delay during terminal erythroid differentiation, associated with increased proliferation and widespread changes in chromatin accessibility. We also show that the proteins encoded by CDAN1 and CDIN1 are enriched in nucleoli which are structurally and functionally abnormal in CDA-I. Together these findings provide important pointers to the pathways affected in CDA-I which for the first time can now be pursued in the tractable culture system utilised here.

Putative regulators for the continuum of erythroid differentiation revealed by single-cell transcriptome of human BM and UCB cells

Fine-resolution differentiation trajectories of adult human hematopoietic stem cells (HSCs) involved in the generation of red cells is critical for understanding dynamic developmental changes that accompany human erythropoiesis. Using single-cell RNA sequencing (scRNA-seq) of primary human terminal erythroid cells (CD34−CD235a+) isolated directly from adult bone marrow (BM) and umbilical cord blood (UCB), we documented the transcriptome of terminally differentiated human erythroblasts at unprecedented resolution. The insights enabled us to distinguish polychromatic erythroblasts (PolyEs) at the early and late stages of development as well as the different development stages of orthochromatic erythroblasts (OrthoEs). We further identified a set of putative regulators of terminal erythroid differentiation and functionally validated three of the identified genes,AKAP8L,TERF2IP, andRNF10, by monitoring cell differentiation and apoptosis. We documented that knockdown ofAKAP8Lsuppressed the commitment of HSCs to erythroid lineage and cell proliferation and delayed differentiation of colony-forming unit-erythroid (CFU-E) to the proerythroblast stage (ProE). In contrast, the knockdown ofTERF2IPandRNF10delayed differentiation of PolyE to OrthoE stage. Taken together, the convergence and divergence of the transcriptional continuums at single-cell resolution underscore the transcriptional regulatory networks that underlie human fetal and adult terminal erythroid differentiation.