EpoR stimulates rapid cycling and larger red cells during mouse and human erythropoiesis

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

Erythroid differentiation in mouse erythroleukemia cells is driven via actin filament-tropomodulin3-tropomyosin networks

Erythroid differentiation (ED) is a complex cellular process entailing morphologically distinct maturation stages of erythroblasts during terminal differentiation. Studies of actin filament assembly and organization during terminal ED have revealed essential roles for the pointed-end actin filament capping proteins, tropomodulins (Tmod1 and Tmod3). Additionally, tropomyosin (Tpm) binding to Tmods is a key feature promoting Tmod-mediated actin filament capping. Global deletion of Tmod3 leads to embryonic lethality in mice with impaired ED. To test a cell autonomous function for Tmod3 and further decipher its biochemical function during ED, we generated a Tmod3 knockout in a mouse erythroleukemia cell line (Mel ds19). Tmod3 knockout cells appeared normal prior to ED, but showed defects during progression of ED, characterized by a marked failure to reduce cell and nuclear size, reduced viability and increased apoptosis. In Mel ds19 cells, both Tpms and actin were preferentially associated with the Triton-X 100 insoluble cytoskeleton during ED, indicating Tpm-coated actin filament assembly during ED. While loss of Tmod3 did not lead to a change in total actin levels, it led to a severe reduction in the proportion of Tpms and actin associated with the Triton-X 100 insoluble cytoskeleton during ED. We conclude that Tmod3-regulation of actin cytoskeleton assembly via Tpms is integral to morphological maturation and cell survival during normal erythroid terminal differentiation.

The Erythropoietin Receptor Stimulates Rapid Cycling and Formation of Larger Red Cells During Mouse and Human Erythropoiesis

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

CLPX regulates erythroid heme synthesis by control of mitochondrial heme synthesis enzymes and iron utilization

Heme is a prosthetic group that plays a critical role in catalyzing life-essential redox reactions in all cells, including critical metabolic processes. Heme synthesis must be tightly co-regulated with cellular requirements in order to maximize utilization and minimize toxicity. Terminally differentiating erythroid cells have an extremely high demand for heme for hemoglobin synthesis. While the enzymatic reactions of heme synthesis are extremely well studied, the mechanisms by which the mitochondrial homeostatic machinery interacts with and regulates heme synthesis are poorly understood. Knowledge of these regulatory mechanisms are key to understanding how red cells couple heme production with heme demand. Heme synthesis is tightly regulated by the mitochondrial AAA+ unfoldase CLPX, which has been reported to promote heme synthesis by activation of yeast δ-aminolevulinate synthase (ALAS/Hem1). CLPX was also reported to mediate heme-induced turnover of ALAS1 in human cells. However, a mutation in the ATP binding domain of CLPX that abrogated ATP binding caused an increase in ALAS activity, contrary to previous predictions that CLPX activated ALAS. Using loss-of-function assays in murine cells and zebrafish, we interrogated the mechanisms by which CLPX regulates erythroid heme synthesis. We found that consistent with previous studies, CLPX is required for erythroid heme synthesis. We show that ALAS2 stability and activity were both increased in the absence of CLPX, suggesting that CLPX primarily regulates ALAS2 by control of its turnover. However, we also showed that CLPX is required for PPOX activity and maintenance of FECH levels, likely accounting for the heme deficiency in the absence of CLPX. Lastly, CLPX is required for iron metabolism during erythroid terminal differentiation. Our results show that the role of CLPX in heme synthesis is not conserved across eukaryotes. Our studies reveal a potential mechanism for the role of CLPX in anemia and porphyria, and reveal multiple nodes at which heme synthesis is regulated by the mitochondrial housekeeping machinery.

Thyroid hormone receptor beta and NCOA4 regulate terminal erythrocyte differentiation

An effect of thyroid hormone (TH) on erythropoiesis has been known for more than a century but the molecular mechanism(s) by which TH affects red cell formation is still elusive. Here we demonstrate an essential role of TH during terminal human erythroid cell differentiation; specific depletion of TH from the culture medium completely blocked terminal erythroid differentiation and enucleation. Treatment with TRβ agonists stimulated premature erythroblast differentiation in vivo and alleviated anemic symptoms in a chronic anemia mouse model by regulating erythroid gene expression. To identify factors that cooperate with TRβ during human erythroid terminal differentiation, we conducted RNA-seq in human reticulocytes and identified nuclear receptor coactivator 4 (NCOA4) as a critical regulator of terminal differentiation. Furthermore,Ncoa4−/−mice are anemic in perinatal periods and fail to respond to TH by enhanced erythropoiesis. Genome-wide analysis suggests that TH promotes NCOA4 recruitment to chromatin regions that are in proximity to Pol II and are highly associated with transcripts abundant during terminal differentiation. Collectively, our results reveal the molecular mechanism by which TH functions during red blood cell formation, results that are potentially useful to treat certain anemias.

Tropomodulin3-null mice are embryonic lethal with anemia due to impaired erythroid terminal differentiation in the fetal liver

Key Points Tmod3 deletion leads to reduced erythroid progenitors and impaired erythroblast survival, cell-cycle exit, and enucleation. Erythroblast-macrophage islands are reduced in the absence of Tmod3, which is required in both cell types for island formation.

Systems Biology and Epigenetic Mechanisms in Erythropoiesis

Irreversible rapid cellular decisions are often controlled by network motifs known as bistable switches. We identified a cell-cycle regulated bistable switch that controls activation of the erythroid transcriptional program during early S phase of the last generation of erythroid colony-forming-unit progenitors (CFUe). This switch drives a rapid, multi-layered commitment event that activates GATA-1 transcription, renders the cells dependent on erythropoietin, and brings about chromatin reconfiguration at erythroid gene loci. In addition, it triggers an unusual process of genome-wide DNA demethylation, the first known example of such a process in somatic cell development. Approximately 25 to 30 percent of all methylation marks are lost from essentially all genomic elements during erythroid terminal differentiation. The bistable switch activating erythroid transcription consists of two linked double-negative feedback interactions of the erythroid transcriptional repressor PU.1, which antagonizes both S phase progression, and the erythroid master transcriptional regulator GATA-1. During operation of the switch, a rapid S phase-dependent decline in PU.1 activates GATA-1 transcription. The dependence of this switch on S phase progression coincides with a dramatic change in the nature of S phase itself, which becomes shorter and 50 percent faster. The accelerated intra-S phase DNA synthesis rate is essential for the loss of genome-wide DNA methylation, which in turn is required for the rapid induction of erythroid genes. Disclosures: No relevant conflicts of interest to declare.

Global DNA Demethylation During Physiological Erythropoiesis In Vivo

Abstract 2083 In the mammalian genome cytosine residues that are followed by guanine (5’-CpG-3’ dinucleotides) are frequently methylated, a modification that is associated with transcriptional silencing. Two genome-wide waves of demethylation, in primordial germ cells and in the early pre-implantation embryo, erase methylation marks and are each followed by de novo methylation, setting up a pattern subsequently inherited throughout development [1]. While no global methylation changes are thought to occur during further somatic development, methylation does alter at gene-specific loci, contributing to tissue-specific patterns of gene expression. We set out to study dynamic changes in DNA methylation during erythropoiesis. We used flow cytometry and the cell surface markers CD71 and Ter119 to subdivide freshly isolated fetal liver cells into a developmental sequence of six subsets, from the least mature Subset 0 (S0), to the most mature Subset 5 (S5) [2]. We measured DNA methylation in genomic DNA prepared from freshly sorted S0 to S5 cells. Surprisingly, we found that demethylation at the erythroid-specific β-globin locus control region (LCR) was coincident with progressive genome-wide methylation loss. Both global demethylation as well as demethylation at the β-globin LCR began with the upregulation of CD71 at the onset of erythroid terminal differentiation, and continued with erythroid maturation, with global hypomethylation persisting during enucleation. We employed several distinct methodologies to measure global DNA methylation level. Using Enzyme-Linked Immunosorbent Assay (ELISA), we found that genomic DNA isolated from increasingly mature erythroblasts had progressively reduced binding to a 5-methylcytosine-specific antibody. We also used the LUminometric Methylation Assay (LUMA) to compare the genome-wide cleavage of CCGG sites by each of the isoschizomers HpaII and MspI, which are methylation sensitive and insensitive, respectively. Both the ELISA and LUMA assays showed a global, progressive and significant loss of DNA methylation with erythroid differentiation: 70% of CpG dinucleotides genome-wide were methylated in S0, decreasing to 40–50% by S4/5 (p<0.01). Further, using pyrosequencing of bisulfite-converted DNA, we found a similar decrease in CpG methylation in the promoters of genes whose transcription is silenced with erythroid maturation, notably PU.1 and Fas. To characterize the global loss in methylation further, we examined the status of imprinted genes and of repetitive transposable elements, since both represent genetic loci that are usually stably and highly methylated in somatic cells. We found loss of methylation in imprinted loci, including PEG3 and the H19 Differentially Methylated Region (DMR). We also found a significant loss of methylation at the Long Interspersed Nuclear Element (LINE-1), a repetitive retrotransposon, whose methylation level decreased from over 90% in S0 cells, to 70% in S4/5. Mechanistically, global demethylation was associated with a rapid decline in the DNA methyltransferases DNMT3a and DNMT3b. However, exogenous re-expression of these enzymes in vitro was not sufficient to reverse the process. Both global and erythroid-specific demethylation required rapid DNA replication, triggered with the onset of erythroid terminal differentiation. We were able to slow down demethylation quantitatively by slowing down the rate of DNA replication with aphidicolin, an inhibitor of DNA polymerase α. Global loss of DNA methylation was not associated with a global increase in transcription, as determined by GeneChip analysis, nor was it associated with increased transcription of the LINE-1 retrotransposon. We propose that global demethylation is a consequence of global cellular mechanisms required for the rapid demethylation and induction of β-globin and other erythroid genes. Our findings suggest mechanisms of global demethylation in development and disease, and show that contrary to previously held dogma, DNA demethylation occurs globally during physiological somatic cell differentiation. References: 1. Reik W, Dean W, Walter J (2001) Epigenetic reprogramming in mammalian development. Science 293: 1089–1093. 2. Socolovsky M, Murrell M, Liu Y, Pop R, Porpiglia E, et al. (2007) Negative Autoregulation by FAS Mediates Robust Fetal Erythropoiesis. PLoS Biol 5: e252. Disclosures: No relevant conflicts of interest to declare.