Late Erythroid Differentiation Research Articles

Characteristics and Distribution of Erythroid Progenitors and Precursors Correlate with Erythropoiesis Stimulating Agents (ESAs) Response in Lower Risk Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are an heterogenous group of clonal myeloid disorders characterized by dysplastic ineffective hematopoiesis, peripheral blood cytopenias and a variable risk of progression to acute myeloid leukemia (AML) evaluated by IPSS-R score. Early stage erythropoiesis is dependent on growth factors like EPO, whereas late erythroid differentiation is iron-dependent. The entire process requires a fine regulation between apoptosis and cell survival. Anemia is the most common cytopenia in IPSS-R lower risk MDS (LR-MDS) and the mechanism underlying dyserythropoiesis is multifactorial, leading to apoptosis of erythroid progenitors and precursors. LR-MDS anemia is mainly treated with ESAs (e.g., Erythropoietin or EPO), but response is not optimal and eventually patients will suffer from chronic anemia and transfusion dependence. Moreover, the only strong predictor of ESAs treatment is the endogenous concentration of serum EPO (sEPO). The primary aim of this study was to detect quantitative and qualitative flow cytometry (FC) alterations of erythroid populations in the bone marrow (BM) of LR-MDS patients associated with resistance or response to ESAs and compared them with age-matched non cytopenic BM controls (CTRLs, 6). Secondly, we wanted to investigate if different WHO diagnostic MDS subgroups displayed specific pattern of erythroid maturation and/or FC abnormalities. We selected 97 LR-MDS anemic patients treated with ESAs at MDS Unit, Hematology dpt. AOU Careggi (Florence) and divided them according to type of ESAs response (e.g., primary refractory, PR; long responders, LR, with DOR longer than 24 months and secondary refractory, SR, with DOR shorter than 24 months). Baseline BM FC analysis was performed using an erythroid panel containing antibodies for CD33, CD34, CD36, HLA-DR, CD45, CD71, CD105, CD117. We also measured the coefficient of variance (CV) of CD71 and CD36 to assess qualitative FC abnormalities within the erythroid populations. We then identified all stages of erythroid maturation from A) CD34+ committed erythroid cells (CD34+, CD45dim, CD117+, CD71+, CD33+, HLA-DR +), or erythroid progenitors (COM-E), expressed both as fraction of all CD34+ cells and all 117+ progenitor cells, to B) CD34- erythroid precursors (from proerythroblasts, PROE, to orthochromatic erythroblasts). CV CD71 was increased in PR patients compared to CTRLs (p=0,009) and to responders (p=0,008). COM-E/tot.CD34+ cells were increased in all response categories, but there was a significant trend of higher values in LR vs. PR (p=0,055). Finally, (COM-E + PROE)/tot.117+ progenitors were only enriched in LR compared to CTRLs (p=0,009). Regarding WHO subgroups, MDS 5q- displayed a decrease in COM-E/CD34+ cells and in (COM-E + PROE)/tot.117+ cells compared to MDS-RS (p=0,00094 and p=0,009 respectively) and to CTRLs for the latter population (p=0,03). Moreover, total erythroid precursors were significantly reduced compared to MDS-SLD, -MLD and -RS. MDS-RS showed a marked erythroid hyperplasia and elevation of CV CD71 compared to CTRLs (p=0,0079 and p=0,024 respectively). Taken together, these results show that in LR-MDS, the increase of CV CD71 in erythroid cells is associated with resistance to ESAs, whereas response to ESAs is present in cases with enriched EPO sensitive COM-E and PROE populations compared to CTRLs, especially for LR vs PR (p=0,055). Finally, MDS-5q- and MDS-RS were the WHO subgroups with the highest rate of PR in our cohort (46% and 59% respectively), irrespective of sEPO. These 2 subgroups of MDS exhibited different repartition of erythroid subpopulations with characteristics associated with ESAs resistance. In 5q- COM-E and PROE were depauperate and, consequently, erythroid precursors reduced, while MDS-RS displayed an expansion of erythroid precursors and higher CV of CD71 compared to CTRLs. Finally, these FC parameters, easily and rapidly assessed at baseline BM evaluation in MDS patients, correlate with response to ESAs and with different WHO subgroups. FC erythroid abnormalities could partly explain the complex pathobiology underlying dyserythropoiesis and its treatment. The different repartition of erythroid progenitors observed could lead to molecular and transcriptome analyses focused on these altered subpopulations in order to investigate the exact mechanism of anemia in LR-MDS(s). Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

RASA3 Regulates Stage-Specific AKT Signaling and Cell Cycle Progression in Mammalian Erythropoiesis

Inherited bone marrow failure syndromes (IBMFS) are a heterogenous group of disorders characterized by dysregulated hematopoiesis across various lineages, predisposition to malignancy, and diverse syndromic features. The scat (severe combined anemia and thrombocytopenia) mouse model has been characterized as a unique model of IBMFS. Scat carries an autosomal recessive missense mutation in the Rasa3 gene that results in RASA3 mislocalization and loss of function. RASA3 functions as a Ras-GTPase activating protein, and its loss of function in scat results in increased erythroid Ras activity, increased reactive oxygen species, and altered cell cycle progression, all culminating in delayed terminal erythroid differentiation. However, the precise mechanism of RASA3 regulation of erythroid differentiation remains undefined, and elucidation of this mechanism is crucial to identifying new therapeutic targets in inherited anemia. Considering the role of RASA3 as regulator of Ras signaling and cell cycle progression, and the importance of these processes to erythroid differentiation, we sought to first characterize the coordination of Ras signaling pathways and cell cycle progression in normal murine erythropoiesis, then observe how loss of RASA3 function alters this regulatory axis. We observed that wild type (WT) erythroblasts demonstrate population-specific influence of ERK and PI3K/AKT signaling in regulating cell cycle progression. Inhibition of both pathways with increasing doses of U0126 and LY294002, respectively, induced accumulation in G0/G1 from the proerythroblast stage until the late basophilic/polychromatic stage (U0126 vehicle vs. 1uM p= 0.0023; LY294002 vehicle vs 1uM p=0.0389). At these later stages, ERK and PI3K/AKT inhibition led to a decrease in G0/G1 percentages, suggesting a stage-specific switch in signaling mediated cell cycle regulation, with PI3K inhibition demonstrating more potent and consistent effects (U0126 vehicle vs 1uM p=0.0406, 0.0481; LY294002 vehicle vs 1uM p=0.0003, 0.0197, 0.0086, n=3). These patterns suggest that ERK and AKT may facilitate cell cycle progression past the G0/G1 checkpoint in early erythropoiesis while inducing cell cycle exit or accumulation in G0/G1 in late erythropoiesis. In scat, we previously characterized increased active Ras in erythroid cells and a delay in terminal erythroid differentiation with accumulation at the polychromatic stage. We therefore next sought to examine the potential mechanistic contribution of altered PI3K/AKT signaling and cell cycle progression to the differentiation delay seen in scat. Phospho-flow analyses demonstrate that scat bone marrow-derived basophilic and polychromatic erythroblasts have increased AKT activation compared to WT (p=0.0402, p=0.0559, respectively; n=4), with similar trends evident in scat spleen basophilic erythroblasts (p=0.064; n=4). These results are consistent with increased Ras activation in scat. Ex vivo EdU/PI analyses revealed that scat bone marrow-derived polychromatic erythroblasts demonstrate G0/G1 accumulation (p=0.0466) and decreased progression to S-phase (0.0414; n=6), also with similar trends in scat spleen basophilic erythroblasts (p=0.004; n=6). These results correlate with the observed differentiation delay in scat and indicate that RASA3 regulates stage-specific signaling and cell cycle progression during erythropoiesis. To study if cell cycle dysregulation in scat begins at an earlier stage of erythroid differentiation, we analyzed murine hematopoietic and erythroid progenitors as Ter119-, cKit+ cells expressing increasing levels of CD71 and found that both CD71lo and CD71med bone marrow-derived scat progenitors present with G0/G1 accumulation (p=0.0015, p=0.0073, respectively) and decreased progression to S-phase (p=0.0014, p=0.0241; n=7). This suggests a dynamic relationship between RASA3, Ras signaling, and cell cycle progression throughout early and late erythroid differentiation. Together, these findings support the role of RASA3 as a regulator of the signaling networks governing erythropoiesis and reveal a new targetable axis in a model of inherited bone marrow failure. Disclosures No relevant conflicts of interest to declare.

Gfi1aa and Gfi1b set the pace for primitive erythroblast differentiation from hemangioblasts in the zebrafish embryo

AbstractThe transcriptional repressors Gfi1(a) and Gfi1b are epigenetic regulators with unique and overlapping roles in hematopoiesis. In different contexts, Gfi1 and Gfi1b restrict or promote cell proliferation, prevent apoptosis, influence cell fate decisions, and are essential for terminal differentiation. Here, we show in primitive red blood cells (prRBCs) that they can also set the pace for cellular differentiation. In zebrafish, prRBCs express 2 of 3 zebrafish Gfi1/1b paralogs, Gfi1aa and Gfi1b. The recently identified zebrafish gfi1aa gene trap allele qmc551 drives erythroid green fluorescent protein (GFP) instead of Gfi1aa expression, yet homozygous carriers have normal prRBCs. prRBCs display a maturation defect only after splice morpholino-mediated knockdown of Gfi1b in gfi1aaqmc551 homozygous embryos. To study the transcriptome of the Gfi1aa/1b double-depleted cells, we performed an RNA-Seq experiment on GFP-positive prRBCs sorted from 20-hour-old embryos that were heterozygous or homozygous for gfi1aaqmc551, as well as wt or morphant for gfi1b. We subsequently confirmed and extended these data in whole-mount in situ hybridization experiments on newly generated single- and double-mutant embryos. Combined, the data showed that in the absence of Gfi1aa, the synchronously developing prRBCs were delayed in activating late erythroid differentiation, as they struggled to suppress early erythroid and endothelial transcription programs. The latter highlighted the bipotent nature of the progenitors from which prRBCs arise. In the absence of Gfi1aa, Gfi1b promoted erythroid differentiation as stepwise loss of wt gfi1b copies progressively delayed Gfi1aa-depleted prRBCs even further, showing that Gfi1aa and Gfi1b together set the pace for prRBC differentiation from hemangioblasts.

Notch1 regulates progenitor cell proliferation and differentiation during mouse yolk sac hematopoiesis

Loss-of-function studies have demonstrated the essential role of Notch in definitive embryonic mouse hematopoiesis. We report here the consequences of Notch gain-of-function in mouse embryo hematopoiesis, achieved by constitutive expression of Notch1 intracellular domain (N1ICD) in angiopoietin receptor tyrosine kinase receptor-2 (Tie2)-derived enhanced green fluorescence protein (EGFP(+)) hematovascular progenitors. At E9.5, N1ICD expression led to the absence of the dorsal aorta hematopoietic clusters and of definitive hematopoiesis. The EGFP(+) transient multipotent progenitors, purified from E9.5 to 10.5 Tie2-Cre;N1ICD yolk sac (YS) cells, had strongly reduced hematopoietic potential, whereas they had increased numbers of hemogenic endothelial cells. Late erythroid cell differentiation stages and mature myeloid cells (Gr1(+), MPO(+)) were also strongly decreased. In contrast, EGFP(+) erythro-myeloid progenitors, immature and intermediate differentiation stages of YS erythroid and myeloid cell lineages, were expanded. Tie2-Cre;N1ICD YS had reduced numbers of CD41(++) megakaryocytes, and these produced reduced below-normal numbers of immature colonies in vitro and their terminal differentiation was blocked. Cells from Tie2-Cre;N1ICD YS had a higher proliferation rate and lower apoptosis than wild-type (WT) YS cells. Quantitative gene expression analysis of FACS-purified EGFP(+) YS progenitors revealed upregulation of Notch1-related genes and alterations in genes involved in hematopoietic differentiation. These results represent the first in vivo evidence of a role for Notch signaling in YS transient definitive hematopoiesis. Our results show that constitutive Notch1 activation in Tie2(+) cells hampers YS hematopoiesis of E9.5 embryos and demonstrate that Notch signaling regulates this process by balancing the proliferation and differentiation dynamics of lineage-restricted intermediate progenitors.

MiR-144 and MiR-451 Regulate Human Erythropoiesis By Targeting RAB14

MicroRNAs (miRs) are small noncoding RNAs that regulate gene expression by inhibiting translation and/or by degrading of target mRNAs. MiR-144 and miR-451 are expressed from a single precursor transcript, encoded by the miR-144∼451 cluster on human chromosome 17. Expression levels of both miR-144 and miR-451 are upregulated during zebrafish, mouse, and human erythroid differentiation. Several mouse and zebrafish studies have knocked down miR-451 expression and observed decreased late erythroid differentiation. In contrast, miR-144 knockdown had no or smaller effects in zebrafish or mouse erythropoiesis, and there is no study of the role of both members of this cluster in primary human erythropoiesis. Therefore, we investigated the effects of lentiviral-mediated loss-of-function of miR-144 and/or miR-451 in human erythropoiesis. We first utilized the TF1 human erythroleukemia cell line as a model, and then tested primary human CD34+ hematopoietic stem-progenitor cells (HSPCs) from normal human donors. When either TF1 or CD34+ cells are cultured with erythropoietin (EPO), the expression of the CD71 (transferrin receptor) and CD235a (glycophorin A) erythroid markers increases, and the expression of the CD34 HSPC marker decreases. In TF1 or primary CD34+ cells transduced with either a miR-144 trap or a miR-451 trap (using a lentivirus containing GFP and a series of 8 tandem complementary miR binding sequences to knock down function of either miR [Ebert, Nature Method, 2007]), as compared to control vector, erythropoiesis was decreased, as evidenced by lower numbers of erythroid (CD34-CD71hiCD235ahi) cells generated during EPO-induced erythroid differentiation.RAB14 was predicted to be a target of both miR-144 and miR-451 by the TargetScan 6.2 target prediction algorithm, with 2 predicted miR-144 binding sites and 1 predicted miR-451 binding site in its 3'UTR. RAB14 was validated to be a target of miR-451 in human non-small cell lung cancer cells [Wang, Oncogene, 2011]. RAB proteins have been reported to have important roles in vesicle trafficking, signal transduction, and receptor recycling. Recently, RAB12 was implicated in constitutive degradation of the transferrin receptor (CD71) in mouse embryonic fibroblast (MEF) cells [Matsui, Traffic, 2011], but the role of RAB14 in human erythropoiesis is unknown. We found that RAB14 protein expression decreases early during EPO-induced erythroid differentiation of TF1 cells. Then we showed that RAB14 is a direct target of miR-144, using luciferase assays and Western blots. We confirmed that shRNA-mediated RAB14 knockdown increased human erythropoiesis, as compared to control vector-transduced cells. Finally, shRNA-mediated RAB14 knockdown protected cells from miR-144/miR-451 trap-mediated erythropoietic inhibition.In summary, miR-144 and miR-451 regulate human erythropoiesis by decreasing RAB14 expression, which in turn might have a role in regulating function of receptors involved in erythropoiesis, such as the transferrin receptor. Disclosures:No relevant conflicts of interest to declare.

The RNA Binding Protein RBM38 (RNPC1) Regulates Splicing during Late Erythroid Differentiation

Alternative pre-mRNA splicing is a prevalent mechanism in mammals that promotes proteomic diversity, including expression of cell-type specific protein isoforms. We characterized a role for RBM38 (RNPC1) in regulation of alternative splicing during late erythroid differentiation. We used an Affymetrix human exon junction (HJAY) splicing microarray to identify a panel of RBM38-regulated alternatively spliced transcripts. Using microarray databases, we noted high RBM38 expression levels in CD71+ erythroid cells and thus chose to examine RBM38 expression during erythroid differentiation of human hematopoietic stem cells, detecting enhanced RBM38 expression during late erythroid differentiation. In differentiated erythroid cells, we validated a subset of RBM38-regulated splicing events and determined that RBM38 regulates activation of Protein 4.1R (EPB41) exon 16 during late erythroid differentiation. Using Epb41 minigenes, Rbm38 was found to be a robust activator of exon 16 splicing. To further address the mechanism of RBM38-regulated alternative splicing, a novel mammalian protein expression system, followed by SELEX-Seq, was used to identify a GU-rich RBM38 binding motif. Lastly, using a tethering assay, we determined that RBM38 can directly activate splicing when recruited to a downstream intron. Together, our data support the role of RBM38 in regulating alternative splicing during erythroid differentiation.

Combined inhibition of PI3K and activation of MAPK p38 signaling pathways trigger erythroid alternative splicing switch of 4.1R pre-mRNA in DMSO-induced erythroleukemia cells

There is increasing evidence showing that many extracellular cues modulate pre-mRNA alternative splicing, through different signaling pathways. We here show that 4.1R exon 16 splicing is altered in response to specific signals. The switch from erythroblastic isoform lacking exon 16 to mature erythrocytic isoform containing this exon is tightly regulated during late erythroid differentiation, and blocage of this splicing switch in erythroleukemia cells is seen as a consequence of the deregulation of important regulatory pathways. We support that combined inhibition of PI3K and activation of p38 signaling pathways impinge on erythroid 4.1R pre-mRNA alternative splicing switch, and on cell differentiation as witnessed by hemoglobin production. By contrast, MEK/ERK signaling appeared not to affect neither cell hemoglobin production nor erythroid 4.1R pre-mRNA splicing. We also found that the signal-induced alternative splicing is not typically distinctive of EPO-non-responsive cells, but operates in EPO-responsive cells as well. Pre-mRNA splicing is a major regulatory mechanism at the crossroad between transcription and translation. We here provide evidence that inhibition of PI3K activates the splicing switch in a promoter-dependent manner, whereas p38 activation induces this event in a promoter-independent fashion. Our data further support that constitutive activation of EPO-R by the viral protein gp55 and the short form of the tyrosine kinase receptor Stk, transduces PI3K proliferation signal, but not MAPK p38 differentiation signal. Concurrently, this work lend credence to the concept that DMSO triggers transient activation of p38 signaling and irreversible inhibition of PI3K/AKT signaling pathway, hence uncovering an old conundrum regarding the mechanism by which DMSO induces erythroleukemia cell differentiation.

Proteasome-Mediated Proteolysis of SRSF5 Splicing Factor Intriguingly Co-occurs with SRSF5 mRNA Upregulation during Late Erythroid Differentiation

SR proteins exhibit diverse functions ranging from their role in constitutive and alternative splicing, to virtually all aspects of mRNA metabolism. These findings have attracted growing interest in deciphering the regulatory mechanisms that control the tissue-specific expression of these SR proteins. In this study, we show that SRSF5 protein decreases drastically during erythroid cell differentiation, contrasting with a concomitant upregulation of SRSF5 mRNA level. Proteasome chemical inhibition provided strong evidence that endogenous SRSF5 protein, as well as protein deriving from stably transfected SRSF5 cDNA, are both targeted to proteolysis as the cells undergo terminal differentiation. Consistently, functional experiments show that overexpression of SRSF5 enhances a specific endogenous pre-mRNA splicing event in proliferating cells, but not in differentiating cells, due to proteasome-mediated targeting of both endogenous and transfection-derived SRSF5. Further investigation of the relationship between SRSF5 structure and its post-translation regulation and function, suggested that the RNA recognition motifs of SRSF5 are sufficient to activate pre-mRNA splicing, whereas proteasome-mediated proteolysis of SRSF5 requires the presence of the C-terminal RS domain of the protein. Phosphorylation of SR proteins is a key post-translation regulation that promotes their activity and subcellular availability. We here show that inhibition of the CDC2-like kinase (CLK) family and mutation of the AKT phosphorylation site Ser86 on SRSF5, have no effect on SRSF5 stability. We reasoned that at least AKT and CLK signaling pathways are not involved in proteasome-induced turnover of SRSF5 during late erythroid development.

ISG15 modulates development of the erythroid lineage.

Activation of erythropoietin receptor allows erythroblasts to generate erythrocytes. In a search for genes that are up-regulated during this differentiation process, we have identified ISG15 as being induced during late erythroid differentiation. ISG15 belongs to the ubiquitin-like protein family and is covalently linked to target proteins by the enzymes of the ISGylation machinery. Using both in vivo and in vitro differentiating erythroblasts, we show that expression of ISG15 as well as the ISGylation process related enzymes Ube1L, UbcM8 and Herc6 are induced during erythroid differentiation. Loss of ISG15 in mice results in decreased number of BFU-E/CFU-E in bone marrow, concomitant with an increased number of these cells in the spleen of these animals. ISG15-/- bone marrow and spleen-derived erythroblasts show a less differentiated phenotype both in vivo and in vitro, and over-expression of ISG15 in erythroblasts is found to facilitate erythroid differentiation. Furthermore, we have shown that important players of erythroid development, such as STAT5, Globin, PLC γ and ERK2 are ISGylated in erythroid cells. This establishes a new role for ISG15, besides its well-characterized anti-viral functions, during erythroid differentiation.

Growth differentiation factor 15 in erythroid health and disease

Growth differentiation factor 15 (GDF15) was identified as a hepcidin-suppression factor that is expressed at high levels in patients with ineffective erythropoiesis. This review addresses the regulation, expression and potential functions of GDF15 in the context of erythroid biology. GDF15 expression during late erythroid differentiation was discovered as part of an erythroblast transcriptome project. As GDF15 expression is associated with cellular stress or apoptosis, further investigation of the cytokine was focused upon its involvement in ineffective erythropoiesis. Remarkably high serum levels were detected in patients with thalassemia syndromes, congenital dyserythropoiesis and some acquired sideroblastic anemias. High-level GDF15 expression is not a feature of normal erythropoiesis, or erythroid recovery after bone-marrow transplantation. As GDF15 is a transforming growth factor-beta superfamily member, it was investigated as an effector of ineffective erythropoiesis that suppresses hepcidin expression despite iron overloading. In contrast to the low levels of GDF15 expressed during normal erythropoiesis, ineffective erythropoiesis causes high-level expression of GDF15. In patients with thalassemia and related anemias, GDF15 expression may contribute to iron overloading or other features of the disease phenotype.

Fox-2A Modulates Protein 4.1R Exon 16 5' Splice Site Activation.

Abstract 778Erythroid differentiation-induced Fox-2A is an important regulator for the differentiation-specific exon 16 splicing switch in protein 4.1R. Up-regulation of Fox-2A in late erythroid differentiation is critical for inclusion of exon 16, which encodes a portion of the spectrin/actin binding domain vital for maintaining the mechanical stability of red blood cell membranes. Fox-2A exerts its splicing enhancing activity in a motif-dependent manner, binding to UGCAUGs located downstream of exon 16. In this study, we investigated the mechanism by which Fox-2A modulates the expression of exon 16. The excision of introns and the joining of exons depends on the recognition and usage of 5' and 3' splice sites (5' ss and 3' ss, respectively) by the splicing machinery. Exon 16 possesses a relatively strong 3' ss but a weak 5' ss. Mutation of the weak 5' ss (GAGIGTTTGT) to a strong consensus 5' ss (GAGIGTAAGT) led to nearly total exon 16 inclusion. While mutations impairing Fox-2A binding drastically reduced exon 16 inclusion in the presence of the weak 5' ss, no effect on exon 16 inclusion was observed when the strong 5' ss was presented with these mutations. These results suggest that Fox-2A facilitates exon 16 splicing by supporting the weak 5' ss. Early recognition of the 5' ss involves base-pairing interaction with the 5' end of U1 snRNA and stabilization by U1 snRNP. Psoralen-mediated UV cross-linking assays revealed a reduction in U1 snRNA recruitment to the weak 5' ss when Fox-2A binding sites were impaired, suggesting that binding of Fox-2A could promote recruitment and stabilization of U1 snRNP to the weak 5' ss. In support for a role of Fox-2A in modulating the activation of the weak 5' ss by recruiting U1 snRNP, we demonstrated that Fox-2A directly interacts with U1 specific protein U1C in an RNA-independent manner. The C-terminal domain of Fox-2A is responsible for its association with U1C. These data suggest a novel mode for exon 16 5' ss activation in which the binding of Fox-2A to an intronic splicing enhancer element UGCAUG may stabilize the pre-mRNA-U1 snRNP complex through interactions with U1C. These could then result in spliceosome commitment complex formation and exon 16 inclusion. Disclosures:No relevant conflicts of interest to declare.

Alternative pre-mRNA splicing switches modulate gene expression in late erythropoiesis

AbstractDifferentiating erythroid cells execute a unique gene expression program that insures synthesis of the appropriate proteome at each stage of maturation. Standard expression microarrays provide important insight into erythroid gene expression but cannot detect qualitative changes in transcript structure, mediated by RNA processing, that alter structure and function of encoded proteins. We analyzed stage-specific changes in the late erythroid transcriptome via use of high-resolution microarrays that detect altered expression of individual exons. Ten differentiation-associated changes in erythroblast splicing patterns were identified, including the previously known activation of protein 4.1R exon 16 splicing. Six new alternative splicing switches involving enhanced inclusion of internal cassette exons were discovered, as well as 3 changes in use of alternative first exons. All of these erythroid stage-specific splicing events represent activated inclusion of authentic annotated exons, suggesting they represent an active regulatory process rather than a general loss of splicing fidelity. The observation that 3 of the regulated transcripts encode RNA binding proteins (SNRP70, HNRPLL, MBNL2) may indicate significant changes in the RNA processing machinery of late erythroblasts. Together, these results support the existence of a regulated alternative pre-mRNA splicing program that is critical for late erythroid differentiation.

Stage-Specific Switches in Alternative Pre-mRNA Splicing during Late Erythropoiesis Are Conserved from Mouse to Human

Spatial and temporal regulation of alternative pre-mRNA splicing determines which exons are incorporated into mature mRNA, modulating mRNA coding capacity to ensure synthesis of appropriate protein isoforms throughout normal differentiation and development. During erythropoiesis, a stage-specific switch in pre-mRNA splicing activates incorporation of protein 4.1R exon 16, thereby increasing 4.1R affinity for spectrin and actin and mechanically strengthening red blood cell membranes. We are exploring the hypothesis that stage-specific changes in pre-mRNA splicing regulate expression of other critical genes during terminal erythropoiesis. Last year we described exon microarray and RT-PCR studies that revealed several novel pre-mRNA splicing switches in terminally differentiating human erythroid progenitors. These alternative splicing events involved well-annotated exons with consensus exon-intron boundaries, supporting a model in which these events represent a regulated alternative splicing program rather than a breakdown of splicing integrity in late erythropoiesis. Here we report additional evidence for this model by showing that several erythroid stage-specific switches in alternative pre-mRNA splicing are conserved between human and mouse. Primary mouse splenic erythroblasts from FVA-infected mice were cultured in vitro under differentiation conditions and used as the source of RNA for analysis of murine erythroid splicing events. From a total of seven internal cassette exons whose splicing was activated in late human erythroblasts, five exhibited an analogous splicing switch in murine erythroblasts. Comparative genomic analysis showed that these alternative exons are embedded in regions of unusually high sequence conservation among vertebrate species, suggesting that important regulatory signals are contained within the adjacent introns. Indeed, the flanking introns for several of these exons contain binding motifs for Fox2, an RNA binding protein and known splicing regulator for many tissue-specific splicing events. Further analysis of the conserved erythroid splicing events revealed the following:three splicing switches occur in transcripts encoding RNA binding proteins (MBNL2, HNRPLL, and SNRP70), suggesting significant changes in the RNA processing machinery of late erythroblasts; andthree of these alternative exons encode premature stop codons that could induce nonsense mediated decay (NMD) and contribute to down-regulation of these genes during terminal erythropoiesis.Consistent with the latter hypothesis, inhibition of NMD in murine erythroblast cultures led to increased accumulation of mRNA isoforms containing the premature stop codons. Together these results suggest the existence of a highly regulated alternative splicing program that is critical for late erythroid differentiation.

Downregulation of the Spi-1/PU.1 oncogene induces the expression of TRIM10/HERF1, a key factor required for terminal erythroid cell differentiation and survival

Sustained expression of the Spi-1/PU.1 and Fli-1 oncoproteins blocks globin gene activation in mouse erythroleukemia cells; however, only Spi-1/PU.1 expression inhibits the inclusion of exon 16 in the mature 4.1R mRNA. This splicing event is crucial for a functional 4.1R protein and, therefore, for red blood cell membrane integrity. This report demonstrates that Spi-1/PU.1 downregulation induces the activation of TRIM10/hematopoietic RING finger 1 (HERF1), a member of the tripartite motif (TRIM)/RBCC protein family needed for globin gene transcription. Additionally, we demonstrate that TRIM10/HERF1 is required for the regulated splicing of exon 16 during late erythroid differentiation. Using inducible overexpression and silencing approaches, we found that: (1) TRIM10/HERF1 knockdown inhibits hemoglobin production and exon splicing and triggers cell apoptosis in dimethylsulfoxide (DMSO)-induced cells; (2) TRIM10/HERF1 upregulation is required but is insufficient on its own to activate exon retention; (3) Fli-1 has no effect on TRIM10/HERF1 expression, whereas either DMSO-induced downregulation or shRNA-knockdown of Spi-1/PU.1 expression is sufficient to activate TRIM10/HERF1 expression; and (4) Spi-1/PU.1 knockdown triggers both the transcription and the splicing events independently of the chemical induction. Altogether, these data indicate that primary Spi-1/PU.1 downregulation acts on late erythroid differentiation through at least two pathways, one of which requires TRIM10/HERF1 upregulation and parallels the Spi-1/PU.1-induced Fli-1 shutoff regulatory cascade.

Regulated Fox-2 isoform expression mediates protein 4.1R splicing during erythroid differentiation

A regulated splicing event in protein 4.1R pre-mRNA-the inclusion of exon 16-encoding peptides for spectrin-actin binding-occurs in late erythroid differentiation. We defined the functional significance of an intronic splicing enhancer, UGCAUG, and its cognate splicing factor, mFox2A, on exon 16 splicing during differentiation. UGCAUG displays cell-type-specific splicing regulation in a test neutral reporter and has a dose-dependent enhancing effect. Erythroid cells express 2 UGCAUG-binding mFox-2 isoforms, an erythroid differentiation-inducible mFox-2A and a commonly expressed mFox-2F. When overexpressed, both enhanced internal exon splicing in an UGCAUG-dependent manner, with mFox-2A exerting a much stronger effect than mFox-2F. A significant reciprocal increase in mFox-2A and decrease in mFox-2F occurred during erythroid differentiation and correlated with exon 16 inclusion. Furthermore, isoform-specific expression reduction reversed mFox-2A-enhancing activity, but not that of mFox-2F on exon 16 inclusion. Our results suggest that an erythroid differentiation-inducible mFox-2A isoform is a critical regulator of the differentiation-specific exon 16 splicing switch, and that its up-regulation in late erythroid differentiation is vital for exon 16 splicing.

Excessive Isolated Erythrocytosis Linked to Induction of SCL/Tal1 and Erythropoietin Receptor Expression.

The JAK2 V617F mutation has been detected in more than 90% polycythemia vera patients and is associated with increased erythropoietin (EPO) sensitivity or EPO independence of erythroid progenitor cells during early erythropoiesis. To determine if other molecular lesions give rise to increased erythrocytosis and realizing that the JAK2 V617F mutation may not be the initiating event in polycythemia vera, we identified an individual with isolated, excessive erythrocytosis who showed erythroid precursor hypersensitivity to EPO; no mutations on sequencing genes for erythropoietin receptor (EPO-R), VHL, or HIF-1 IRE; normal serum EPO level; normal hemoglobin function and p50; normal cardiac, hepatic and pulmonary function; no JAK2 V617F mutation and no EPO independent BFU-E. While it was not certain that this individual's erythrocytosis was lifelong, other relatives had increased hemoglobin levels. The cause of this individual's increased erythropoiesis was investigated using EPO-stimulated cultures of hematopoietic progenitor cells isolated from peripheral blood. As with normal controls, cell numbers were not increased after 12 days of incubation with low levels of EPO (<0.1 U/ml). With 1U/ml of EPO, cell proliferation was similar to control early in erythropoiesis but increased markedly after 8 days so that by day 12 cell numbers were 3-fold greater than in control cultures with 95% benzidine positive cells compared with 76% for control and 82–87% for JAK2 V617F positive erythroid progenitor cells. Analyses of transcription factor expression revealed that induction of GATA-1 and down regulation of GATA-2 were similar to control. However, SCL/Tal1 and EKLF markedly increased beginning at day 8 to 10 and continued to rise during late erythropoiesis in contrast to control and JAK2 V617F positive cultures in which SCL/Tal1 and EKLF peaked at day 10 and decreased with late erythroid differentiation. Since increased beta-globin expression is concomitant with induction of EKLF, the rise in EKLF may account for the marked increase in benzidine positive cells from our subject. Moreover, expression of EPO-R followed the continuing rise in SCL/Tal1, increasing by 3.5 fold at day 12 compared with control cultures. This high EPO-R expression is consistent with the dramatic increase in cell proliferation during late erythropoiesis. Using forced expression of SCL/Tal1, reporter gene assay and gel mobility shift analysis in erythroid cells, we determined that SCL/Tal1 is able to bind to E-boxes located 3′ of the EPO-R proximal promoter and to activate transcription. Although EPO stimulation of erythroid progenitor cells activates EPO-R, we found that forced expression of EPO-R in primary erythroid progenitor cells in the presence of EPO increases expression of GATA-1 as well as SCL/Tal1 and EKLF, providing further evidence that specific increase in SCL/Tal1 but not GATA-1 must precede induction of EPO-R in this case of excessive erythrocytosis. These data demonstrate a link between high level induction of SCL/Tal1 expression and increased erythrocytosis and suggest a mechanism that contributes to increased erythropoietin sensitivity during late erythropoiesis.

Protein 4.1R Exon 16 Splicing Mediated by Regulated Expression of RBM9 and PTB during Erythroid Differentiation.

Protein 4.1R (4.1R), a vital component of the red cell membrane cytoskeleton, stabilizes the spectrin-actin lattice and attaches it to the embedded membrane proteins. The inclusion of exon 16, which encodes peptides critical for spectrin/actin binding, occurs via an intricate interplay between the auxiliary cis-elements and transacting factors. An intronic splicing enhancer, UGCAUG, is present in triplicate and is situated between two polypyrimidinetract-binding (PTB) sites, TCTT, in the intron downstream of exon 16. In addition, PTB binding sites are also present in triplicate in the upstream intron of exon 16. In this study, we characterized the splicing factors that orchestrate the erythroid differentiation stage-specific switch in exon 16 splicing through these cis-elements using two cell systems: mouse erythroleukemia cells (MELC) that can be induced to erythroid differentiation and G1E-ER cells that undergo synchronous erythroid maturation after induced GATA-1 expression. We identified two RBM9 isoforms (RBM9-1A and RBM9-1F) with distinct amino-termini that interact with the intronic splicing enhancer UGCAUG. The expression of RBM9-1A is erythroid-specific while RBM9-1F can be detected in a wide variety of cell types. Real-time PCR and Western blot analyses showed that RBM9-1A expression is significantly increased while RBM9-1F is reduced during induced erythroid differentiation in both MELC and G1E-ER4 cells. The up-regulation of RBM9-1A correlated with exon 16 inclusion in differentiated cells. Furthermore, the inhibition of RBM9 expression by isoform specific-shRNA reversed 1A enhancing activity, but not that of 1F on exon 16 inclusion in differentiated cells. Thus, exon 16 splicing is mediated by a cell type-specific RBM9 isoform and its up-regulation in late erythroid differentiation is vital for exon 16 splicing. However, over-expression of PTB completely diminished the enhancing effect of RBM9-1A on exon 16 splicing in both differentiated MELC and G1E-ER4 cells, suggesting that PTB plays a role in exon 16 splicing. We analyzed PTB expression and its effect on the exon 16 splicing switch during erythroid differentiation. PTB, a repressive regulator of alternative splicing, binds to the exon 16 upstream and downstream intronic silencers. Its over-expression reduced exon 16 inclusion in both endogenous 4.1R and transfected exon 16 minigenes. Moreover, PTB expression was down-regulated and coincided with increased exon 16 splicing during erythroid differentiation suggesting that regulated expression of repressor PTB mediates exon 16 splicing. Our results further suggest that the differentiation-specific exon 16 splicing switch is achieved by varying the amount of either ubiquitously expressed or cell-type specific activators and inhibitors, and hence the relative efficiency of spliceosome recruitment in the exon inclusion pathway.

A Novel Splicing Factor RBM-9 Regulates Protein 4.1R Exon 16 Splicing.

Protein 4.1R (4.1R), a vital component of the red cell membrane cytoskeleton, stabilizes the spectrin-actin lattice and attaches it to embedded membrane proteins. A regulated splicing event, the inclusion of exon 16 that encodes for peptides critical for spectrin/actin binding, occurs during late erythroid differentiation. We showed earlier that an intricate combination of enhancer and silencer elements direct exon 16 splicing. Regulated expression of splicing factors, SF2/ASF and hnRNP A/B, has also been implicated in mediating exon 16 splicing. In this study, we attempted to characterize the mechanism involved in exon 16 splicing through UGCAUG, an intronic splicing enhancer present in three copies in the intron downstream of exon 16. We first used a wild-type minigene construct consisting of exons 13, 16, 17 and their respective flanking introns that mimics endogenous exon 16 splicing during the induced differentiation of mouse erythroleukemia cells (MELC). Mutational analysis showed a dose-dependent effect of UGCAUG on exon 16 splicing: the presence of all three copies had the most effect. These results were recapitulated with an internal chimeric exon in a test neutral reporter system “DUP4-1”, suggesting that the enhancing effect could be attributed directly to UGCAUG. Furthermore, we identified a novel splicing factor, RBM-9, from MELC that enhanced the internal exon splicing in an UGCAUG-dependent manner in both the exon 16 minigene and DUP4-1 reporter systems. Our characterization of RBM-9 revealed that diverse isoforms of RBM-9 are generated by the utilization of alternative translation initiation sites and tissue-specific alternative splicing; different isoforms from various tissues exhibited differential exon 16 splicing enhancing activities. MELC-RBM-9 enhanced exon 16 splicing the most among all RBM-9 isoforms tested. Inhibition of RBM-9 expression by RBM-9-shRNA reversed its enhancing activity on exon 16 inclusion in MELC. RBM-9-shRNA also reduced exon 16 splicing in a dose-dependent manner in HeLa cells. Furthermore, purified RBM-9 specifically binds to the UGCAUG sequence in a gel-mobility shift assay. Finally, expression of RBM-9 is upregulated and correlates with exon 16 inclusion during MELC differentiation. These results suggest that a novel splicing factor, RBM-9, enhances erythroid differentiation stage-specific exon 16 splicing by interacting with the splicing enhancer UGCAUG.

Transcriptional Profile of Carbohydrate Blood Group Genes in Erythroid Versus Neutrophil Differentiation of Human CD34+ Cells In Vitro.

Carbohydrate blood groups and their corresponding antibodies are clinically important, known to be involved in conditions such as hemolytic transfusion reactions, hemolytic disease of the newborn and spontaneous abortion. However, little is understood about the developmental changes in expression of carbohydrate blood groups during hematopoiesis, with preceding studies mainly focusing on protein-based blood group molecules. We have previously identified the carbohydrate blood group A antigen as the earliest, specific marker for definitive erythroid commitment, in preference to other suggested candidates such as Kell or Glycophorin C [Br J Haematol 2004; 127:451–63]. With regard to this lineage-restriction point we mapped the gene expression of some clinically important carbohydrate blood group systems during erythroid versus neutrophil differentiation in vitro. Human bone marrow CD34+ cells from healthy donors, carrying the blood group A1 allele and functional secretor (FUT2) and Lewis (FUT3) genes, were cultured in vitro towards erythroid or neutrophil development and sorted on a flow cytometer into subpopulations according to their surface expression of blood group A antigen/CD117 and CD15/CD33. Sorted cells were cultured in clonogenic assays in methylcellulose or analyzed by TaqMan real-time reverse transcriptase PCR for gene expression of a number of carbohydrate blood group glycosyltransferases. Surface expression of the blood group A antigen coincided with commitment to erythroid differentiation and the expression of CD15 with neutrophil/monocytic differentiation. In gene expression studies the ABO, H (FUT1), I (IGnT) and Pk (A4GALT) genes were all expressed in freshly isolated and sorted CD34+ cells. The ABO and the H genes were up-regulated in erythroid differentiation and silenced in neutrophil differentiation. The ABO gene expression was markedly decreased in late stages of erythroid maturation. The I gene was expressed both during erythroid and neutrophil development with an increased expression in late erythroid differentiation. The Pk gene retained a low expression throughout neutrophil differentiation and was up-regulated several-fold during erythroid differentiation. There was no detectable expression of FUT3 and the gene suggested being responsible for biosynthesis of the Sda antigen, GALGT2, in either erythroid or neutrophil differentiation.Our data support the identification of the blood group A antigen as an early and specific marker for definitive erythroid differentiation. In mature cells of the myeloid lineage, the results of the gene expression studies are compatible with previous findings of gene and/or surface expression of the I and Pk blood groups but not of ABO and H. The marked increase in expression of the Pk gene during erythroid differentiation may well agree with the fact that the Pk antigen is the precursor structure of globoside, the most abundant neutral glycolipid in the erythrocyte membrane. The absence of hematopoietic FUT3 expression in Lewis gene positive individuals was expected whilst the relevance of undetectable GALGT2 expression in hematopoietic differentiation is uncertain. The role of the GALGT2 gene in surface expression of Sda has not been definitively proven and the molecular basis of different Sda phenotype variants is unknown.In conclusion, our data extend previous findings of carbohydrate blood group distribution, primarily obtained from mature blood cells and leukemic cell lines, to normal human hematopoiesis.

GFI-1B Is Increased during Erythroid Differentiation and Appears to Regulate Beta Globin Expression and Differentiation.

We identified GFI-1B in DNA arrays as a gene that correlates with erythroid differentiation induced by controlled expression of JunB in the HCD57 erythroid cell line, a novel model for erythroid differentiation we recently developed. JunB expression induces the HCD57 cells to turn bright red from hemoglobin synthesis. Beta Globin, Glycophorin (TER-119), alpha Spectrin and other erythroid marker are also induced by Jun B expression, suggesting the usefulness of the HCD57-JunB cell in studying normal erythroid differentiation. Our initial experiment was designed to identify potential genes that control erythroid differentiation so we analyzed gene expression in HCD57 cells induced by JunB in the first 24-hour period. Whereas the DNA array data found that the expression of all known transcription factors either was unchanged or decreased by induction of JunB and differentiation, GFI-1B RNA initially declined in the first 8 hours but then increased above control at 24 hours. In additional experiments, nuclear GFI-1B protein, and DNA binding activity were found to markedly increase during the late erythroid differentiation, 48 to 72 after induction of JunB to trigger the erythroid differentiation in HCD57 cells. GFI-1B expression and activity also increased when F-MEL cells were induced to differentiate with DMSO. Hemin treatment (to induce globin synthesis) also induced GFI-1B expression and GFI-1B DNA binding in murine HCD57 cells and human erythroid cell lines, UT7 and K562. When we ectopically expressed GFI-1B in human UT7 erythroleukemia cells, we found that elevation of GFI-1B expression reduced the rate of proliferation and markedly enhanced beta-globin synthesis when these cells were treated with hemin. Beta globin mRNA was increased 24 hours earlier in UT7 cell transfected with GFI-1B compared to control UT7 cells, suggesting that up-regulation of GFI-1B is needed for maximum beta globin expression. GFI-1B is selectively expressed in erythroid progenitors and megakaryocytes and was cloned by others using low stringency hybridization with a probe for Growth Factor Independence-1, GFI-1, a gene identified previously as a proto-oncogene induced in IL-2 independent T-cell lymphomas. GFI-1 and GFI-1B both have 5 zinc finger domains in the COOH terminal but are believed to act as transcriptional repressors through a novel 20 amino acid NH terminal domain. The sparse literature on GFI-1B and its homologue, the proto-oncogene GFI-1, suggests these SNAG/zinc finger proteins are repressors of transcription and act in general to prevent differentiation and promote proliferation of hematopoietic progenitor cells independent of normally required growth factors. Our current finding, however, suggests that GFI-1B function is distinct from the hyper-proliferation and anti-differentiation functions of the related GFI-1. Instead, our data suggests a clear role of GFI-1B in erythroid differentiation in agreement with loss of defintive erythropoiesis in GFI-1B-/- mice (recently reported from the Orkin laboratory).