Accentuated response to phenylhydrazine and erythropoietin in mice genetically impaired for their GATA-1 expression (GATA-1(low) mice)

Novel insights into the ontogeny of basophils

Basophils are the least common granulocytes, accounting for <1% of peripheral blood leukocytes. In the last 20 years, analytical tools for mouse basophils have been developed, and we now recognize that basophils play critical roles in various immune reactions, including the development of allergic inflammation and protective immunity against parasites. Moreover, the combined use of flow cytometric analyses and knockout mice has uncovered several progenitor cells committed to basophils in mice. Recently, advancements in single-cell RNA sequencing (scRNA-seq) technologies have challenged the classical view of the differentiation of various hematopoietic cell lineages. This is also true for basophil differentiation, and studies using scRNA-seq analysis have provided novel insights into basophil differentiation, including the association of basophil differentiation with that of erythrocyte/megakaryocyte and the discovery of novel basophil progenitor cells in the mouse bone marrow. In this review, we summarize the recent findings of basophil ontogeny in both mice and humans, mainly focusing on studies using scRNA-seq analyses.

hGATA1 Under the Control of a μLCR/β-Globin Promoter Rescues the Erythroid but Not the Megakaryocytic Phenotype Induced by the Gata1 low Mutation in Mice

The phenotype of mice carrying the Gata1^low mutation that decreases expression of Gata1 in erythroid cells and megakaryocytes, includes anemia, thrombocytopenia, hematopoietic failure in bone marrow and development of extramedullary hematopoiesis in spleen. With age, these mice develop myelofibrosis, a disease sustained by alterations in stem/progenitor cells and megakaryocytes. This study analyzed the capacity of hGATA1 driven by a μLCR/β-globin promoter to rescue the phenotype induced by the Gata1^low mutation in mice. Double hGATA1/Gata1^low/0 mice were viable at birth with hematocrits greater than those of their Gata1^low/0 littermates but platelet counts remained lower than normal. hGATA1 mRNA was expressed by progenitor and erythroid cells from double mutant mice but not by megakaryocytes analyzed in parallel. The erythroid cells from hGATA1/Gata1^low/0 mice expressed greater levels of GATA1 protein and of α- and β-globin mRNA than cells from Gata1^low/0 littermates and a reduced number of them was in apoptosis. By contrast, hGATA1/Gata1^low/0 megakaryocytes expressed barely detectable levels of GATA1 and their expression of acetylcholinesterase, Von Willebrand factor and platelet factor 4 as well as their morphology remained altered. In comparison with Gata1^+/0 littermates, Gata1^low/0 mice contained significantly lower total and progenitor cell numbers in bone marrow while the number of these cells in spleen was greater than normal. The presence of hGATA1 greatly increased the total cell number in the bone marrow of Gata1^low/0 mice and, although did not affect the total cell number of the spleen which remained greater than normal, it reduced the frequency of progenitor cells in this organ. The ability of hGATA1 to rescue the hematopoietic functions of the bone marrow of the double mutants was confirmed by the observation that these mice survive well splenectomy and did not develop myelofibrosis with age. These results indicate that hGATA1 under the control of µLCR/β-globin promoter is expressed in adult progenitors and erythroid cells but not in megakaryocytes rescuing the erythroid but not the megakaryocyte defect induced by the Gata1^low/0 mutation.

The Role of miRNAs as Therapeutic Tools in Sickle Cell Disease

Background and Objectives: Sickle cell disorder (SCD) is a paradigmatic example of a complex monogenic disorder. SCD is characterized by the production of abnormal hemoglobin, primarily in the deoxygenated state, which makes erythrocytes susceptible to intracellular hemoglobin polymerization. Functional studies have affirmed that the dysregulation of miRNAs enhances clinical severity or has an ameliorating effect in SCD. miRNAs can be effectively regulated to reduce the pace of cell cycle progression, to reduce iron levels, to influence hemolysis and oxidative stress, and most importantly, to increase γ-globin gene expression and enhance the effectiveness of hydroxyurea. Results: This review highlights the roles played by some key miRNAs in hemoglobinopathies, especially in hematopoiesis, erythroid differentiation, and severity of anemia, which make miRNAs attractive molecular tools for innovative therapeutic approaches. Conclusions: In this era of targeted medicine, miRNAs mimics and antagomirs may be promising inducers of HbF synthesis which could ameliorate the clinical severity of SCD.

BCL-XL expression is essential for human erythropoiesis and engraftment of hematopoietic stem cells

The anti-apoptotic BCL-2 proteins (BCL-2, BCL-XL, MCL-1, A1, BCL-W) counteract apoptotic signals emerging during development and under stress conditions, and are thus essential for the survival of every cell. While the “BCL-2 addiction” of different cell types is well described in mouse models, there is only limited information available on the role of different anti-apoptotic BCL-2 proteins in a given human cell type. Here we characterize the role of BCL-XL for survival and function of human hematopoietic cells, with the aim to predict hematological side effects of novel BCL-XL-inhibiting BH3-mimetics and to identify hematological malignancies potentially responsive to such inhibitors. Earlier clinical studies have shown that the combined BCL-2/BCL-XL/BCL-W inhibitor, Navitoclax (ABT-263) induces severe thrombocytopenia caused by direct platelet demise and counteracted by increased megakaryopoiesis. In contrast, murine studies have reported important contribution of BCL-XL to survival of late erythroid cells and megakaryocytes. Using lentiviral knockdown, we show that the roles of BCL-XL for human hematopoietic cells are much more pronounced than expected from murine data and clinical trials. Efficient genetic or chemical BCL-XL inhibition resulted in significant loss of human erythroid cells beginning from very early stages of erythropoiesis, and in a reduction of megakaryocytes. Most importantly, BCL-XL deficient human hematopoietic stem cells and multipotent progenitors were reduced in numbers, and they showed a severely impaired capacity to engraft in mice during xenotransplantation. BCL-XL deficiency was fully compensated by BCL-2 overexpression, however, loss of its antagonist BIM did not result in any rescue of human erythroid or stem and progenitor cells. We thus conclude that novel and specific BCL-XL inhibitors might be efficient to treat malignancies of erythroid or megakaryocytic origin, such as polycythemia vera, acute erythroid leukemia, essential thrombocytosis or acute megakaryocytic leukemia. At the same time, it can be expected that they will have more severe hematological side effects than Navitoclax.

Novel targets to cure primary myelofibrosis from studies on Gata1low mice

In 2002, we discovered that mice carrying the hypomorphic Gata1^low mutation that reduces expression of the transcription factor GATA1 in megakaryocytes (Gata1^low mice) develop myelofibrosis, a phenotype that recapitulates the features of primary myelofibrosis (PMF), the most severe of the Philadelphia-negative myeloproliferative neoplasms (MPNs). At that time, this discovery had a great impact on the field because mutations driving the development of PMF had yet to be discovered. Later studies identified that PMF, as the others MPNs, is associated with mutations activating the thrombopoietin/JAK2 axis raising great hope that JAK inhibitors may be effective to treat the disease. Unfortunately, ruxolitinib, the JAK1/2 inhibitor approved by FDA and EMEA for PMF, ameliorates symptoms but does not improve the natural course of the disease, and the cure of PMF is still an unmet clinical need. Although GATA1 is not mutated in PMF, reduced GATA1 content in megakaryocytes as a consequence of ribosomal deficiency is a hallmark of myelofibrosis (both in humans and mouse models) and, in fact, a driving event in the disease. Conversely, mice carrying the hypomorphic Gata1^low mutation express an activated TPO/JAK2 pathway and partially respond to JAK inhibitors in a fashion similar to PMF patients (reduction of spleen size but limited improvement of the natural history of the disease). These observations cross-validated Gata1^low mice as a bona fide animal model for PMF and prompted the use of this model to identify abnormalities that might be targeted to cure the disease. We will summarize here data generated in Gata1^low mice indicating that the TGF-β/P-selectin axis is abnormal in PMF and represents a novel target for its treatment.

GATA1 insufficiencies in primary myelofibrosis and other hematopoietic disorders: consequences for therapy

INTRODUCTION

GATA1, the founding member of a family of transcription factors, plays important roles in the development of hematopoietic cells of several lineages. Although loss of GATA1 has been known to impair hematopoiesis in animal models for nearly 25 years, the link between GATA1 defects and human blood diseases has only recently been realized. Areas covered: Here the current understanding of the functions of GATA1 in normal hematopoiesis and how it is altered in disease is reviewed. GATA1 is indispensable mainly for erythroid and megakaryocyte differentiation. In erythroid cells, GATA1 regulates early stages of differentiation, and its deficiency results in apoptosis. In megakaryocytes, GATA1 controls terminal maturation and its deficiency induces proliferation. GATA1 alterations are often found in diseases involving these two lineages, such as congenital erythroid and/or megakaryocyte deficiencies, including Diamond Blackfan Anemia (DBA), and acquired neoplasms, such as acute megakaryocytic leukemia (AMKL) and the myeloproliferative neoplasms (MPNs). Expert commentary: Since the first discovery of GATA1 mutations in AMKL, the number of diseases that are associated with impaired GATA1 function has increased to include DBA and MPNs. With respect to the latter, we are only just now appreciating the link between enhanced JAK/STAT signaling, GATA1 deficiency and disease pathogenesis.

Hypoxia inducible factors are dispensable for myeloid cell migration into the inflamed mouse eye

Hypoxia inducible factors (HIFs) are ubiquitously expressed transcription factors important for cell homeostasis during dynamic oxygen levels. Myeloid specific HIFs are crucial for aspects of myeloid cell function, including their ability to migrate into inflamed tissues during autoimmune disease. This contrasts with the concept that accumulation of myeloid cells at ischemic and hypoxic sites results from a lack of chemotactic responsiveness. Here we seek to address the role of HIFs in myeloid trafficking during inflammation in a mouse model of human uveitis. We show using mice with myeloid-specific Cre-deletion of HIFs that myeloid HIFs are dispensable for leukocyte migration into the inflamed eye. Myeloid-specific deletion of Hif1a, Epas1, or both together, had no impact on the number of myeloid cells migrating into the eye. Additionally, stabilization of HIF pathways via deletion of Vhl in myeloid cells had no impact on myeloid trafficking into the inflamed eye. Finally, we chemically induce hypoxemia via hemolytic anemia resulting in HIF stabilization within circulating leukocytes to demonstrate the dispensable role of HIFs in myeloid cell migration into the inflamed eye. These data suggest, contrary to previous reports, that HIF pathways in myeloid cells during inflammation and hypoxia are dispensable for myeloid cell tissue trafficking.

A novel role for nuclear factor-erythroid 2 in erythroid maturation by modulation of mitochondrial autophagy

We have recently demonstrated that the transcription factor nuclear factor-erythroid 2, which is critical for erythroid maturation and globin gene expression, plays an important role in the pathophysiology of myeloproliferative neoplasms. Myeloproliferative neoplasm patients display elevated levels of nuclear factor-erythroid 2 and transgenic mice overexpressing the transcription factor develop myeloproliferative neoplasm, albeit, surprisingly without erythrocytosis. Nuclear factor-erythroid 2 transgenic mice show both a reticulocytosis and a concomitant increase in iron deposits in the spleen, suggesting both enhanced erythrocyte production and increased red blood cell destruction. We therefore hypothesized that elevated nuclear factor-erythroid 2 levels may lead to increased erythrocyte destruction by interfering with organelle clearance during erythroid maturation. We have previously shown that nuclear factor-erythroid 2 overexpression delays erythroid maturation of human hematopoietic stem cells. Here we report that increased nuclear factor-erythroid 2 levels also impede murine maturation by retarding mitochondrial depolarization and delaying mitochondrial elimination. In addition, ribosome autophagy is delayed in transgenics. We demonstrate that the autophagy genes NIX and ULK1 are direct novel nuclear factor-erythroid 2 target genes, as these loci are bound by nuclear factor-erythroid 2 in chromatin immunoprecipitation assays. Moreover, Nix and Ulk1 expression is increased in transgenic mice and in granulocytes from polycythemia vera patients. This is the first report implying a role for nuclear factor-erythroid 2 in erythroid maturation by affecting autophagy.

Polycythemia and high levels of erythropoietin in blood and brain blunt the hypercapnic ventilatory response in adult mice

Changes in arterial Po2, Pco2, and pH are the strongest stimuli sensed by peripheral and central chemoreceptors to adjust ventilation to the metabolic demand. Erythropoietin (Epo), the main regulator of red blood cell production, increases the hypoxic ventilatory response, an effect attributed to the presence of Epo receptors in both carotid bodies and key brainstem structures involved in integration of peripheral inputs and control of breathing. However, it is not known whether Epo also has an effect on the hypercapnic chemoreflex. In a first attempt to answer this question, we tested the hypothesis that Epo alters the ventilatory response to increased CO2 levels. Basal ventilation and hypercapnic ventilatory response (HCVR) were recorded from control mice and from two transgenic mouse lines constitutively expressing high levels of human Epo in brain only (Tg21) or in brain and plasma (Tg6), the latter leading to polycythemia. To tease apart the potential effects of polycythemia and levels of plasma Epo in the HCVR, control animals were injected with an Epo analog (Aranesp), and Tg6 mice were treated with the hemolytic agent phenylhydrazine after splenectomy. Ventilatory parameters measured by plethysmography in conscious mice were consistent with data from electrophysiological recordings in anesthetized animals and revealed a blunted HCVR in Tg6 mice. Polycythemia alone and increased levels of plasma Epo blunt the HCVR. In addition, Tg21 mice with an augmented level of cerebral Epo also had a decreased HCVR. We discuss the potential implications of these findings in several physiopathological conditions.

Pathways of retinoid synthesis in mouse macrophages and bone marrow cells

In vivo pathways of natural retinoid metabolism and elimination have not been well characterized in primary myeloid cells, even though retinoids and retinoid receptors have been strongly implicated in regulating myeloid maturation. With the use of a upstream activation sequence-GFP reporter transgene and retrovirally expressed Gal4-retinoic acid receptor α in primary mouse bone marrow cells, we identified 2 distinct enzymatic pathways used by mouse myeloid cells ex vivo to synthesize retinoic acid receptor α ligands from free vitamin A metabolites (retinyl acetate, retinol, and retinal). Bulk Kit(+) bone marrow progenitor cells use diethylaminobenzaldehyde-sensitive enzymes, whereas bone marrow-derived macrophages use diethylaminobenzaldehyde-insensitive enzymes to synthesize natural retinoic acid receptor α-activating retinoids (all-trans retinoic acid). Bone marrow-derived macrophages do not express the diethylaminobenzaldehyde-sensitive enzymes Aldh1a1, Aldh1a2, or Aldh1a3 but instead, express Aldh3b1, which we found is capable of diethylaminobenzaldehyde-insensitive synthesis of all trans-retinoic acid. However, under steady-state and stimulated conditions in vivo, diverse bone marrow cells and peritoneal macrophages showed no evidence of intracellular retinoic acid receptor α-activating retinoids, despite expression of these enzymes and a vitamin A-sufficient diet, suggesting that the enzymatic conversion of retinal is not the rate-limiting step in the synthesis of intracellular retinoic acid receptor α-activating retinoids in myeloid bone marrow cells and that retinoic acid receptor α remains in an unliganded configuration during adult hematopoiesis.

Decreased "ineffective erythropoiesis" preserves polycythemia in mice under long-term hypoxia

Hypoxia induces innumerable changes in humans and other animals, including an increase in peripheral red blood cells (polycythemia) caused by the activation of erythropoiesis mediated by increased erythropoietin (EPO) production. However, the elevation of EPO is limited and levels return to normal ranges under normoxia within 5-7 days of exposure to hypoxia, whereas polycythemia continues for as long as hypoxia persists. We investigated erythropoiesis in bone marrow and spleens from mouse models of long-term normobaric hypoxia (10 % O2) to clarify the mechanism of prolonged polycythemia in chronic hypoxia. The numbers of erythroid colony-forming units (CFU-E) in the spleen remarkably increased along with elevated serum EPO levels indicating the activation of erythropoiesis during the first 7 days of hypoxia. After 14 days of hypoxia, the numbers of CFU-E returned to normoxic levels, whereas polycythemia persisted for >140 days. Flow cytometry revealed a prolonged increase in the numbers of TER119-positive cells (erythroid cells derived from pro-erythroblasts through mature erythrocyte stages), especially the TER119 (high) CD71 (high) population, in bone marrow. The numbers of annexin-V-positive cells among the TER119-positive cells particularly declined under chronic hypoxia, suggesting that the numbers of apoptotic cells decrease during erythroid cell maturation. Furthermore, RT-PCR analysis showed that the RNA expression of BMP-4 and stem cell factor that reduces apoptotic changes during erythroid cell proliferation and maturation was increased in bone marrow under hypoxia. These findings indicated that decreased apoptosis of erythroid cells during erythropoiesis contributes to polycythemia in mice during chronic exposure to long-term hypoxia.

Abnormal P-selectin localization during megakaryocyte development determines thrombosis in the gata1low model of myelofibrosis

Patients with primary myelofibrosis have increased risk for bleeding and thrombosis. It is debated whether propensity to thrombosis is due to increased numbers of platelet microparticles and/or to pathological platelet-neutrophil interactions. Platelet neutrophil interactions are mediated by P-selectin and even though the megakaryocytes of myelofibrosis patients express normal levels of P-selectin, it remains abnormally localized to the demarcation membrane system rather than being assembled into the α-granules in platelets. Mice carrying the hypomorphic Gata1(low) mutation express the same megakaryocyte abnormalities presented by primary myelofibrosis patients, including abnormal P-selectin localization to the DMS and develop with age myelofibrosis, a disease that closely resembles human primary myelofibrosis. Whether these mice would also develop thrombosis has not been investigated as yet. The aim of this study was to determine whether Gata1(low) mice would develop thrombosis with age and, in this case, the role played by P-selectin in the development of the trait. To this aim, Gata1(low) mice were crossed with P-sel(null) mice according to standard genetic protocols and Gata1(low)P-sel(wt), Gata1(low)P-sel(null) and Gata1(WT)P-sel(null) or Gata1(wt)P-sel(wt) (as controls) littermates obtained. It was shown that platelet counts, but not hematocrit, are reduced in Gata1(low) mice. Moreover, platelet microparticles are reduced in Gata1(low) mice and P-selectin positive platelet microparticles were not found. To determine the phenotypic implications of the different mutations, bleeding time was estimated by a tail cut procedure. Mutant mice were sacrificed and presence of thrombosis was determined by immunohistological staining of organs. Gata1(low) mice with or without the P-selectin null trait had a prolonged bleeding time compared to wild type mice. However, in Gata1(low) mice significantly higher frequency of thrombotic events was seen in adult and old Gata1(low) mice compared to Gata1(low)P-sel(null) mice. Thus, presence of the P-selectin null trait rescued Gata1(low) mice from the thrombotic phenotype, but did not change the level of platelet microparticles. Taken together these data indicate that abnormal localization of P-selectin, induced by the Gata1(low) mutation, and thus, increased pathological interactions with leucocytes, is responsible for the increased presence of thrombosis seen in these mice.

Megakaryocyte pathology and bone marrow fibrosis: the lysyl oxidase connection

Megakaryocytes (MKs), the platelet precursors, are capable of accumulating DNA greater than a diploid content as part of their cell cycle. MKs have been recognized as mediating fibrosis in a subset of hematologic malignancies, including acute megakaryoblastic leukemia and a subset of myeloproliferative neoplasms. The mechanisms responsible for fibrosis remain only partially understood. Past studies highlighted the role of growth factors in such pathologies, and recently, the protein lysyl oxidase (LOX) has been implicated in proliferation of MKs, ploidy and deposition of fibers. LOX was initially characterized as a protein responsible for the intermolecular cross-linking of elastin and collagen, and in recent years it has been identified as regulator of various pathologies, such as cancer and inflammation. Here, we review recent advances in the understanding of the contribution of MKs to the progression of myelofibrosis, highlighting the newly identified role of LOX.

Overexpression of microRNA-16-2 contributes to the abnormal erythropoiesis in polycythemia vera

Deregulated expression of microRNAs is associated with neoplasia. Here, we show that mature miR-16 levels are abnormally increased in CD34+ cells of patients with polycythemia vera as a consequence of preferential expression of miR-16-2 on chromosome 3 rather than of miR-16-1 on chromosome 13. Forced expression of miRNA-16 in normal CD34+ cells stimulated erythroid cell proliferation and maturation. Conversely, exposure of polycythemia vera CD34+ cells to small interfering RNA against pre-miR-16-2 reduced erythroid colonies and largely prevented formation of erythropoietin-independent colonies; myeloid progenitors remained unaffected. Experiments with knock down of JAK2 indicated that overexpression of miR-16 was independent of JAK/STAT pathway activation. Mice injected with an miR-16 antagomir showed a blunted erythroid response to exogenous erythropoietin, which indicates a role of miR-16 in normal erythropoiesis. These data suggest that deregulation of miR-16-2 contributes to abnormal expansion of erythroid lineage in polycythemia vera. However, the mechanisms for miR-16-2 overexpression remain to be elucidated, because no genetic abnormalities at the miR-16-2 locus were discovered.

LYL-1 deficiency induces a stress erythropoiesis

OBJECTIVE

LYL-1 is a transcription factor containing a basic helix-loop-helix motif closely related to SCL/TAL-1, a regulator of erythroid differentiation. Because LYL-1 is expressed in erythroid cell populations, we addressed its role in erythropoiesis using knockin mice.

MATERIALS AND METHODS

Erythropoiesis of LYL-1(-/-) mice was studied by progenitor assays, flow cytometry, reconstitution assays, and functional tests. Expression of LYL-1, SCL, and GATA-1 was assessed at messenger RNA level by quantitative reverse transcription polymerase chain reaction.

RESULTS

LYL-1(-/-) mice displayed decreased erythropoiesis with a partial arrest in differentiation, and enhanced apoptosis associated with decreased Bcl-x(L) expression in the bone marrow (BM). In addition, LYL-1(-/-) BM cells were severely impaired in their abilities to reconstitute the erythroid lineage in competitive assays, suggesting a cell autonomous abnormality of erythropoiesis. In parallel, erythroid progenitor and precursor cells were significantly increased in the spleen of LYL-1(-/-) mice. Expression of LYL-1 was differentially regulated during maturation of erythroblasts and strikingly different between spleen- and BM-derived erythroblasts. Expression of LYL-1 decreased during erythroid differentiation in the spleen whereas it increased in the BM to reach the same level in mature erythroblasts as in the soleen. Loss of Lyl-1 expression was accompanied with an increase of SCL/TAL-1 and GATA-1 transcripts in spleen but not in BM-derived erythroblasts. Furthermore, phenylhydrazine-induced stress erythropoiesis was elevated in LYL-1(-/-) mice and mutant BM and spleen erythroid progenitors were hypersensitive to erythropoietin.

CONCLUSIONS

Taken together, these results suggest that LYL-1 plays a definite role in erythropoiesis, albeit with different effects in BM specifically regulating basal erythropoiesis, and spleen, controlling stress-induced erythropoiesis.

Differential localization of P-selectin and von Willebrand factor during megakaryocyte maturation

An important step in megakaryocyte maturation is the appropriate assembly of at least two distinct subsets of alpha-granules. The mechanism that sorts the alpha-granule components into distinct structures and mediates their release in response to specific stimuli is now emerging. P-selectin and von Willebrand factor are two proteins present in the alpha-granules that recognize P-selectin glycoprotein ligand on neutrophils and collagen in the subendothelial matrix. These proteins may play an important role in determining the differential release of the alpha-granule contents in response to external stimuli. If P-selectin and von Willebrand factor are localized in the same or different alpha-granules is not known. To clarify this question, we analyzed by immunoelectron microscopy the localization of von Willebrand factor and P-selectin during the maturation of wild-type and Gata1(low) megakaryocytes induced in vivo by treating animals with thrombopoietin. Gata1(low) is a hypomorphic mutation that blocks megakaryocyte maturation, reduces the levels of von Willebrand factor expression and displaces P-selectin on the demarcation membrane system. The maturation block induced by this mutation is partially rescued by treatment in vivo with thrombopoietin. In immature megakaryocytes, both wild-type and Gata1(low), the two receptors were co-localized in the same cytoplasmic structures. By contrast, the two proteins were segregated to separate alpha-granule subsets as the megakaryocytes matured. These observations support the hypothesis that P-selectin and von Willebrand factor may ensure differential release of the alpha-granule content in response to external stimuli.

microRNA miR-144 modulates oxidative stress tolerance and associates with anemia severity in sickle cell disease

Although individuals with homozygous sickle cell disease (HbSS) share the same genetic mutation, the severity and manifestations of this disease are extremely heterogeneous. We have previously shown that the microRNA expression in normal and HbSS erythrocytes exhibit dramatic differences. In this study, we identify a subset of HbSS patients with higher erythrocytic miR-144 expression and more severe anemia. HbSS erythrocytes are known to have reduced tolerance for oxidative stress, yet the basis for this phenotype remains unknown. This study reveals that miR-144 directly regulates nuclear factor-erythroid 2-related factor 2, a central regulator of cellular response to oxidative stress, and modulates the oxidative stress response in K562 cell line and primary erythroid progenitor cells. We further demonstrate that increased miR-144 is associated with reduced NRF2 levels in HbSS reticulocytes and with decreased glutathione regeneration and attenuated antioxidant capacity in HbSS erythrocytes, thereby providing a possible mechanism for the reduced oxidative stress tolerance and increased anemia severity seen in HbSS patients. Taken together, our findings suggest that erythroid microRNAs can serve as genetic modifiers of HbS-related anemia and can provide novel insights into the clinical heterogeneity and pathobiology of sickle cell disease.

Potent and selective inhibition of polycythemia by the quinoxaline JAK2 inhibitor NVP-BSK805

The recent discovery of an acquired activating point mutation in JAK2, substituting valine at amino acid position 617 for phenylalanine, has greatly improved our understanding of the molecular mechanism underlying chronic myeloproliferative neoplasms. Strikingly, the JAK2(V617F) mutation is found in nearly all patients suffering from polycythemia vera and in roughly every second patient suffering from essential thrombocythemia and primary myelofibrosis. Thus, JAK2 represents a promising target for the treatment of myeloproliferative neoplasms and considerable efforts are ongoing to discover and develop inhibitors of the kinase. Here, we report potent inhibition of JAK2(V617F) and JAK2 wild-type enzymes by a novel substituted quinoxaline, NVP-BSK805, which acts in an ATP-competitive manner. Within the JAK family, NVP-BSK805 displays more than 20-fold selectivity towards JAK2 in vitro, as well as excellent selectivity in broader kinase profiling. The compound blunts constitutive STAT5 phosphorylation in JAK2(V617F)-bearing cells, with concomitant suppression of cell proliferation and induction of apoptosis. In vivo, NVP-BSK805 exhibited good oral bioavailability and a long half-life. The inhibitor was efficacious in suppressing leukemic cell spreading and splenomegaly in a Ba/F3 JAK2(V617F) cell-driven mouse mechanistic model. Furthermore, NVP-BSK805 potently suppressed recombinant human erythropoietin-induced polycythemia and extramedullary erythropoiesis in mice and rats.

Evidence for organ-specific stem cell microenvironments

The X-linked Gata1(low) mutation in mice induces strain-restricted myeloproliferative disorders characterized by extramedullary hematopoiesis in spleen (CD1 and DBA/2) and liver (CD1 only). To assess the role of the microenvironment in establishing this myeloproliferative trait, progenitor cell compartments of spleen and marrow from wild-type and Gata1(low) mice were compared. Phenotype and clonal assay of non-fractionated cells indicated that Gata1(low) mice contain progenitor cell numbers 4-fold lower and 10-fold higher than normal in marrow and spleen, respectively. However, progenitor cells prospectively isolated from spleen, but not from marrow, of Gata1(low) mice expressed colony-forming function in vitro. Therefore, calculation of cloning activity of purified cells demonstrated that the total number of Gata1(low) progenitor cells was 10- to 100-fold lower than normal in marrow and >1,000 times higher than normal in spleen. This observation indicates that Gata1(low) hematopoiesis is favored by the spleen and is in agreement with our previous report that removal of this organ induces wild-type hematopoiesis in heterozygous Gata1(low/+) females (Migliaccio et al., 2009, Blood 114:2107). To clarify if rescue of wild-type hematopoiesis by splenectomy prevented extramedullary hematopoiesis in liver, marrow cytokine expression profile and liver histopathology of splenectomized Gata1(low/+) females were investigated. After splenectomy, the marrow expression levels of TGF-beta, VEGF, osteocalcin, PDGF-alpha, and SDF-1 remained abnormally high while Gata1(low) hematopoiesis was detectable in liver of both CD1 and DBA/2 mutants. Therefore, in the absence of the spleen, Gata1(low) hematopoiesis is supported by the liver suggesting that treatment of myelofibrosis in these animals requires the rescue of both stem cell and microenvironmental functions.

Nix is a selective autophagy receptor for mitochondrial clearance

Autophagy is the cellular homeostatic pathway that delivers large cytosolic materials for degradation in the lysosome. Recent evidence indicates that autophagy mediates selective removal of protein aggregates, organelles and microbes in cells. Yet, the specificity in targeting a particular substrate to the autophagy pathway remains poorly understood. Here, we show that the mitochondrial protein Nix is a selective autophagy receptor by binding to LC3/GABARAP proteins, ubiquitin-like modifiers that are required for the growth of autophagosomal membranes. In cultured cells, Nix recruits GABARAP-L1 to damaged mitochondria through its amino-terminal LC3-interacting region. Furthermore, ablation of the Nix:LC3/GABARAP interaction retards mitochondrial clearance in maturing murine reticulocytes. Thus, Nix functions as an autophagy receptor, which mediates mitochondrial clearance after mitochondrial damage and during erythrocyte differentiation.

Loss of the Gata1 gene IE exon leads to variant transcript expression and the production of a GATA1 protein lacking the N-terminal domain

GATA1 is essential for the differentiation of erythroid cells and megakaryocytes. The Gata1 gene is composed of multiple untranslated first exons and five common coding exons. The erythroid first exon (IE exon) is important for Gata1 gene expression in hematopoietic lineages. Because previous IE exon knockdown analyses resulted in embryonic lethality, less is understood about the contribution of the IE exon to adult hematopoiesis. Here, we achieved specific deletion of the floxed IE exon in adulthood using an inducible Cre expression system. In this conditional knock-out mouse line, the Gata1 mRNA level was significantly down-regulated in the megakaryocyte lineage, resulting in thrombocytopenia with a marked proliferation of megakaryocytes. By contrast, in the erythroid lineage, Gata1 mRNA was expressed abundantly utilizing alternative first exons. Especially, the IEb/c and newly identified IEd exons were transcribed at a level comparable with that of the IE exon in control mice. Surprisingly, in the IE-null mouse, these transcripts failed to produce full-length GATA1 protein, but instead yielded GATA1 lacking the N-terminal domain inefficiently. With low level expression of the short form of GATA1, IE-null mice showed severe anemia with skewed erythroid maturation. Notably, the hematological phenotypes of adult IE-null mice substantially differ from those observed in mice harboring conditional ablation of the entire Gata1 gene. The present study demonstrates that the IE exon is instrumental to adult erythropoiesis by regulating the proper level of transcription and selecting the correct transcription start site of the Gata1 gene.

Gata1 expression driven by the alternative HS2 enhancer in the spleen rescues the hematopoietic failure induced by the hypomorphic Gata1low mutation

Rigorously defined reconstitution assays developed in recent years have allowed recognition of the delicate relationship that exists between hematopoietic stem cells and their niches. This balance ensures that hematopoiesis occurs in the marrow under steady-state conditions. However, during development, recovery from hematopoietic stress and in myeloproliferative disorders, hematopoiesis occurs in extramedullary sites whose microenvironments are still poorly defined. The hypomorphic Gata1(low) mutation deletes the regulatory sequences of the gene necessary for its expression in hematopoietic cells generated in the marrow. By analyzing the mechanism that rescues hematopoiesis in mice carrying this mutation, we provide evidence that extramedullary microenvironments sustain maturation of stem cells that would be otherwise incapable of maturing in the marrow.

Interaction between the glucocorticoid and erythropoietin receptors in human erythroid cells

OBJECTIVE

The aim of this study was to identify whether the rapid membrane-associated pathway of the glucocorticoid receptor (GR) is active in erythroid cells and plays any role in determining the reversible inhibition on erythroid maturation exerted by GR.

MATERIALS AND METHODS

First we determined the biological effects (inhibition of apoptosis and induction of beta-globin expression) induced in primary erythroblasts by erythropoietin (EPO) and the GR agonist dexamethasone (DXM), alone and in combination. Next, by biochemical analysis, we determined the association between GR and EPO receptor in proerythroblasts generated in vitro from 10 normal adult donors. These studies also analyzed the levels of signal transducers and activators of transcription-5 (STAT-5) phosphorylation induced when the cells were stimulated with DXM alone or in combination with EPO.

RESULTS

DXM antagonized the beta-globin messenger RNA increases, but not the inhibition of apoptosis induced by EPO in primary cells. DXM also antagonized the ability of EPO to induce STAT-5 phosphorylation in these cells. In fact, EPO and DXM alone, but not in combination, induced phosphorylation and nuclear translocation of STAT-5. The inhibition likely occurred through an interaction between the two receptors because GR became associated with the EPO receptor and STAT-5 in cells stimulated with EPO and DXM.

CONCLUSION

These data suggest that glucocorticoids inhibit erythroid maturation not only through a transcriptional mechanism, but also through a rapid membrane-associated pathway that interferes with EPO receptor signaling.

Targeting erythroblast-specific apoptosis in experimental anemia

Erythrocyte production is regulated by balancing precursor cell apoptosis and survival signaling. Previously, we found that BH3-only proapoptotic factor, Nix, opposed erythroblast-survival signaling by erythropoietin-induced Bcl-xl during normal erythrocyte formation. Since erythropoietin treatment of human anemia has limitations, we explored the therapeutic potential of abrogating Nix-mediated erythroblast apoptosis to enhance erythrocyte production. Nix gene ablation blunted the phenylhydrazine-induced fall in blood count, enhanced hematocrit recovery, and reduced erythroblast apoptosis, despite lower endogenous erythropoietin levels. Similar to erythropoietin, Nix ablation increased early splenic erythroblasts and circulating reticulocytes, while maintaining a pool of mature erythroblasts as erythropoietic reserve. Erythrocytes in Nix-deficient mice showed morphological abnormalities, suggesting that apoptosis during erythropoiesis not only controls red blood cell number, but also serves a "triage" function, preferentially eliminating abnormal erythrocytes. These results support the concept of targeting erythroblast apoptosis to maximize erythrocyte production in acute anemia, which may be of value in erythropoietin resistance.

Essential role for Nix in autophagic maturation of erythroid cells

Erythroid cells undergo enucleation and the removal of organelles during terminal differentiation. Although autophagy has been suggested to mediate the elimination of organelles for erythroid maturation, the molecular mechanisms underlying this process remain undefined. Here we report a role for a Bcl-2 family member, Nix (also called Bnip3L), in the regulation of erythroid maturation through mitochondrial autophagy. Nix(-/-) mice developed anaemia with reduced mature erythrocytes and compensatory expansion of erythroid precursors. Erythrocytes in the peripheral blood of Nix(-/-) mice exhibited mitochondrial retention and reduced lifespan in vivo. Although the clearance of ribosomes proceeded normally in the absence of Nix, the entry of mitochondria into autophagosomes for clearance was defective. Deficiency in Nix inhibited the loss of mitochondrial membrane potential (DeltaPsi(m)), and treatment with uncoupling chemicals or a BH3 mimetic induced the loss of DeltaPsi(m) and restored the sequestration of mitochondria into autophagosomes in Nix(-/-) erythroid cells. These results suggest that Nix-dependent loss of DeltaPsi(m) is important for targeting the mitochondria into autophagosomes for clearance during erythroid maturation, and interference with this function impairs erythroid maturation and results in anaemia. Our study may also provide insights into molecular mechanisms underlying mitochondrial quality control involving mitochondrial autophagy.

H3 K79 dimethylation marks developmental activation of the beta-globin gene but is reduced upon LCR-mediated high-level transcription

Genome-wide analyses of the relationship between H3 K79 dimethylation and transcription have revealed contradictory results. To clarify this relationship at a single locus, we analyzed expression and H3 K79 modification levels of wild-type (WT) and transcriptionally impaired beta-globin mutant genes during erythroid differentiation. Analysis of fractionated erythroid cells derived from WT/Delta locus control region (LCR) heterozygous mice reveals no significant H3 K79 dimethylation of the beta-globin gene on either allele prior to activation of transcription. Upon transcriptional activation, H3 K79 di-methylation is observed along both WT and DeltaLCR alleles, and both alleles are located in proximity to H3 K79 dimethylation nuclear foci. However, H3 K79 di-methylation is significantly increased along the DeltaLCR allele compared with the WT allele. In addition, analysis of a partial LCR deletion mutant reveals that H3 K79 dimethylation is inversely correlated with beta-globin gene expression levels. Thus, while our results support a link between H3 K79 dimethylation and gene expression, high levels of this mark are not essential for high level beta-globin gene transcription. We propose that H3 K79 dimethylation is destabilized on a highly transcribed template.

Mast cell transcriptional networks

Unregulated activation of mast cells can contribute to the pathogenesis of inflammatory and allergic diseases, including asthma, rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis. Absence of mast cells in animal models can lead to impairment in the innate immune response to parasites and bacterial infections. Aberrant clonal accumulation and proliferation of mast cells can result in a variety of diseases ranging from benign cutaneous mastocytosis to systemic mastocytosis or mast cell leukemia. Understanding mast cell differentiation provides important insights into mechanisms of lineage selection during hematopoiesis and can provide targets for new drug development to treat mast cell disorders. In this review, we discuss controversies related to development, sites of origin, and the transcriptional program of mast cells.

Ablation of Gata1 in adult mice results in aplastic crisis, revealing its essential role in steady-state and stress erythropoiesis

The transcription factor Gata1 is expressed in several hematopoietic lineages and plays essential roles in normal hematopoietic development during embryonic stages. The lethality of Gata1-null embryos has precluded determination of its role in adult erythropoiesis. Here we have examined the effects of Gata1 loss in adult erythropoiesis using conditional Gata1 knockout mice expressing either interferon- or tamoxifen-inducible Cre recombinase (Mx-Cre and Tx-Cre, respectively). Mx-Cre-mediated Gata1 recombination, although incomplete, resulted in maturation arrest of Gata1-null erythroid cells at the proerythroblast stage, thrombocytopenia, and excessive proliferation of megakaryocytes in the spleen. Tx-Cre-mediated Gata1 recombination resulted in depletion of the erythroid compartment in bone marrow and spleen. Formation of the early and late erythroid progenitors in bone marrow was significantly reduced in the absence of Gata1. Furthermore, on treatment with a hemolytic agent, these mice failed to activate a stress erythropoietic response, despite the rising erythropoietin levels. These results indicate that, in addition to the requirement of Gata1 in adult megakaryopoiesis, Gata1 is necessary for steady-state erythropoiesis and for erythroid expansion in response to anemia. Thus, ablation of Gata1 in adult mice results in a condition resembling aplastic crisis in human.

Vascular endothelium as a contributor of plasma sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P), an abundant lipid mediator in plasma, regulates vascular and immune cells by activating S1P receptors. In this report, we investigated the mechanisms by which high plasma S1P levels are maintained in mice. We found that plasma S1P turns over rapidly with a half-life of approximately 15 minutes, suggesting the existence of a high-capacity biosynthetic source(s). Transplantation of bone marrow from wild-type to Sphk1(-/-)Sphk2(+/-) mice restored plasma S1P levels, suggesting that hematopoietic cells are capable of secreting S1P into plasma. However, plasma S1P levels were not appreciably altered in mice that were thrombocytopenic, anemic, or leukopenic. Surprisingly, reconstitution of Sphk1(-/-)Sphk2(+/-) bone marrow cells into wild-type hosts failed to reduce plasma S1P, suggesting the existence of an additional, nonhematopoietic source for plasma S1P. Adenoviral expression of Sphk1 in the liver of Sphk1(-/-) mice restored plasma S1P levels. In vitro, vascular endothelial cells, but not hepatocytes, secreted S1P in a constitutive manner. Interestingly, laminar shear stress downregulated the expression of S1P lyase (Sgpl) and S1P phosphatase-1 (Sgpp1) while concomitantly stimulating S1P release from endothelial cells in vitro. Modulation of expression of endothelial S1P lyase with small interfering RNA and adenoviral expression altered S1P secretion, suggesting an important role played by this enzyme. These data suggest that the vascular endothelium, in addition to the hematopoietic system, is a major contributor of plasma S1P.

Role of thrombopoietin in mast cell differentiation

Mast cells are important elements of the body response to foreign antigens, being those represented either by small molecules (allergic response) or harbored by foreign microorganisms (response to parasite infection). These cells derive from hematopoietic stem/progenitor cells present in the marrow. However, in contrast with most of the other hematopoietic lineages, mast cells do not differentiate in the marrow but in highly vascularized extramedullary sites, such as the skin or the gut. Mast cell differentiation in the marrow is activated as part of the body response to parasites. We will review here the mast cell differentiation pathway and what is known of its major intrinsic and extrinsic control mechanisms. It will also be described that thrombopoietin, the ligand for the Mpl receptor, in addition to its pivotal rule in the control of thrombocytopoiesis and of hematopoietic stem/progenitor cell proliferation, exerts a regulatory function in mast cell differentiation. Some of the possible implications of this newly described biological activity of thrombopoietin will be discussed.

Role of GATA-1 in normal and neoplastic hemopoiesis

GATA-1 exerts a concentration-dependent control on the differentiation of erythroid, megakaryocytic, mast, and eosinophilic cells. The concentration of GATA-1 is, in turn, regulated by specific sequences within the GATA-1 locus. On the basis of its levels of expression, the GATA-1 protein becomes associated with suitable partners forming transcription complexes that, by binding to lineage-specific enhancers, activate the expression of the corresponding target genes. Instrumental to our understanding of the role of GATA-1 in hemopoietic differentiation has been the generation of genetically engineered mutant mice and the discovery of naturally occurring mutations associated with either inherited or acquired human pathologies. We review our current understanding of the role of GATA-1 in normal and neoplastic hematopoiesis as emerging from these genetic approaches.

Real-time monitoring of stress erythropoiesis in vivo using Gata1 and beta-globin LCR luciferase transgenic mice

Erythroid progenitors have the potential to proliferate rapidly in response to environmental stimuli. This process is referred to as stress erythropoiesis, with erythropoietin (EPO) playing central roles in its promotion. In this study, we wanted to elucidate the molecular mechanisms governing the regulation of stress erythropoiesis and the maintenance of red-cell homeostasis. This was achieved by our development of a noninvasive real-time monitoring system for erythropoiesis using transgenic mouse lines expressing luciferase under the control of the mouse Gata1 hematopoietic regulatory domain (G1-HRD-luc) or human beta-globin locus control region (Hbb-LCR-luc). Optical bioluminescence images revealed that the luciferase was specifically expressed in spleen and bone marrow and was induced rapidly in response to anemia and hypoxia stimuli. The G1-HRD-luc activity tracked the emergence and disappearance of proerythroblast-stage progenitors, whereas the Hbb-LCR-luc activity tracked erythroblasts and later stage erythroid cells. Increased plasma EPO concentration preceded an increase in G1-HRD-luc, supporting our contention that EPO acts as the key upstream signal in stress erythropoiesis. Hence, we conclude that G1-HRD-luc and Hbb-LCR-luc reporters are differentially activated during stress erythropoiesis and that the transgenic mouse lines used serve as an important means for understanding the homeostatic regulation of erythropoiesis.

Differential Amplification of Murine Bipotent Megakaryocytic/Erythroid Progenitor and Precursor Cells During Recovery from Acute and Chronic Erythroid Stress

Two murine bipotent erythroid/megakaryocytic cells, the progenitor (MEP) and precursor (PEM) cells, recently have been identified on the basis of the phenotypes of linnegc-kitposSca-1neg CD16/CD32lowCD34low and TER119pos4A5pos or 2D5pos, respectively. However, the functional relationship between these two subpopulations and their placement in the hemopoietic hierarchy is incompletely understood. We compared the biological properties of these subpopulations in marrow and spleen of mice with and without acute or chronic erythroid stress. MEP cells, but not PEM cells, express c-kit, respond to stem cell factor in vitro, and form spleen colonies in vivo. PEM cells comprise up to 50%-70% of the cells in BFU-E-derived colonies but are not present among the progeny of purified MEP cells cultured under erythroid and megakaryocytic permissive conditions. PEM cells increase 10- to 20-fold under acute and chronic stress, whereas MEP cell increases (21%-84%) are observed only in acutely stressed animals. These data suggest that MEP and PEM cells represent distinct cell populations that may exist in an upstream-downstream differentiation relationship under conditions of stress. Whereas the dynamics of both populations are altered by stress induction, the differential response to acute and chronic stress suggests different regulatory mechanisms. A model describing the relationship between MEP, PEM, and common myeloid progenitor cells is presented.

Early block to erythromegakaryocytic development conferred by loss of transcription factor GATA-1

Transcription factor GATA-1 is essential at multiple stages of hematopoiesis. Murine gene targeting and analysis of naturally occurring human mutations demonstrate that GATA-1 drives the maturation of committed erythroid precursors and megakaryocytes. Prior studies also suggest additional, poorly defined, roles for GATA-1 at earlier stages of erythromegakaryocytic differentiation. To investigate these functions further, we stimulated Gata1- murine embryonic stem-cell-derived hematopoietic cultures with thrombopoietin, a multistage cytokine. Initially, the cultures generated a wave of mutant megakaryocytes. However, these were rapidly overgrown by a unique population of thrombopoietin-dependent blasts that express immature markers and proliferate indefinitely. Importantly, on restoration of GATA-1 function, these cells differentiated into both erythroid and megakaryocytic lineages, suggesting that they represent bipotential progenitors. Identical cells are also present in vivo, as indicated by flow cytometry and culture analysis of fetal livers from Gata1- chimeric mice. Our findings indicate that loss of GATA-1 impairs the maturation of megakaryocyte-erythroid progenitors. This defines a new role for GATA-1 at a relatively early stage of hematopoiesis and provides potential insight into recent discoveries that human GATA1 mutations promote acute megakaryoblastic leukemia, a clonal malignancy with features of both erythroid and megakaryocyte maturation.

Variegation of the phenotype induced by the Gata1low mutation in mice of different genetic backgrounds

All mice harboring the X-linked Gata1low mutation in a predominantly CD1 background are born anemic and thrombocytopenic. They recover from anemia at 1 month of age but remain thrombocytopenic all their life and develop myelofibrosis, a syndrome similar to human idiopathic myelofibrosis, at 12 months. The effects of the genetic background on the myelofibrosis developed by Gata1low mice was assessed by introducing the mutation, by standard genetic approaches, in the C57BL/6 and DBA/2 backgrounds and by analyzing the phenotype of the different mutants at 12 to 13 (by histology) and 16 to 20 (by cytofluorimetry) months of age. Although all the Gata1low mice developed fibrosis at 12 to 13 months, variegations were observed in the severity of the phenotype expressed by mutants of different backgrounds. In C57BL/6 mice, the mutation was no longer inherited in a Mendelian fashion, and fibrosis was associated with massive osteosclerosis. Instead, DBA/2 mutants, although severely anemic, expressed limited fibrosis and osteosclerosis and did not present tear-drop poikilocytes in blood or extramedullary hemopoiesis in liver up to 20 months of age. We propose that the variegation in myelofibrosis expressed by Gata1low mutants of different strains might represent a model to study the variability of the clinical picture of the human disease.

Erythropoietin stimulates phosphorylation and activation of GATA-1 via the PI3-kinase/AKT signaling pathway

Erythropoietin (Epo) stimulation of its receptor's downstream signaling pathways and optimum function of GATA-1 transcription factor are both essential for normal erythroid cell development. Epo-receptor (EpoR) signaling and GATA-1 regulate proliferation, survival, differentiation, and maturation of erythroid cells. Whether any signal that is generated by EpoR targets GATA-1 or affects GATA-1 transcriptional activity is not known. Here, we demonstrate that stimulation of EpoR results in phosphorylation of GATA-1 at serine 310 (S310) in primary fetal liver erythroid progenitors and in cultured erythroid cells. We show that phosphorylation of GATA-1 is important for Epo-induced maturation of fetal liver erythroid progenitor cells. The PI3-kinase/AKT signaling pathway is identified as a mediator of Epo-induced phosphorylation of GATA-1. AKT serine threonine kinase phosphorylates GATA-1S310 in vitro and in erythroid cells and enhances GATA-1 transcriptional activity. These data demonstrate that EpoR signaling phosphorylates GATA-1 and modulates its activity via the PI3-kinase/AKT signaling pathway.

Repression of c-kit and its downstream substrates by GATA-1 inhibits cell proliferation during erythroid maturation

Stem cell factor (SCF), erythropoietin (Epo), and GATA-1 play an essential role(s) in erythroid development. We examined how these proteins interact functionally in G1E cells, a GATA-1(-) erythroblast line that proliferates in an SCF-dependent fashion and, upon restoration of GATA-1 function, undergoes GATA-1 proliferation arrest and Epo-dependent terminal maturation. We show that SCF-induced cell cycle progression is mediated via activation of the Src kinase/c-Myc pathway. Restoration of GATA-1 activity induced G1 cell cycle arrest coincident with repression of c-Kit and its downstream effectors Vav1, Rac1, and Akt. Sustained expression of each of these individual signaling components inhibited GATA-1-induced cell cycle arrest to various degrees but had no effects on the expression of GATA-1-regulated erythroid maturation markers. Chromatin immunoprecipitation analysis revealed that GATA-1 occupies a defined Kit gene regulatory element in vivo, suggesting a direct mechanism for gene repression. Hence, in addition to its well-established function as an activator of erythroid genes, GATA-1 also participates in a distinct genetic program that inhibits cell proliferation by repressing the expression of multiple components of the c-Kit signaling axis. Our findings reveal a novel aspect of molecular cross talk between essential transcriptional and cytokine signaling components of hematopoietic development.

Functional but abnormal adult erythropoiesis in the absence of the stem cell leukemia gene

Previous studies have indicated that the stem cell leukemia gene (SCL) is essential for both embryonic and adult erythropoiesis. We have examined erythropoiesis in conditional SCL knockout mice for at least 6 months after loss of SCL function and report that SCL was important but not essential for the generation of mature red blood cells. Although SCL-deleted mice were mildly anemic with increased splenic erythropoiesis, they responded appropriately to endogenous erythropoietin and hemolytic stress, a measure of late erythroid progenitors. However, SCL was more important for the proliferation of early erythroid progenitors because the predominant defects in SCL-deleted erythropoiesis were loss of in vitro growth of the burst-forming erythroid unit and an in vivo growth defect revealed by transplant assays. With respect to erythroid maturation, SCL-deleted proerythroblasts could generate more mature erythroblasts and circulating red blood cells. However, SCL was required for normal expression of TER119, one of the few proposed target genes of SCL. The unexpected finding that SCL-independent erythropoiesis can proceed in the adult suggests that alternate factors can replace the essential functions of SCL and raises the possibility that similar mechanisms also explain the relatively minor defects previously observed in SCL-null hematopoietic stem cells.

Isolation of TPO-dependent subclones from the multipotent 32D cell line

Using thrombopoietin (TPO), as selective pressure, several TPO-dependent clones were isolated from the murine multipotential IL-3-dependent cell line 32D. Four of them were fully characterized. They depended on TPO for survival and proliferation and, although retaining the capacity to grow in IL-3, did not respond to either EPO, G-CSF or GM-CSF. 32D TPO cells were heterogeneous in morphology and ranged from small cells, with a DNA content nearly tetraploid and a modal chromosome no. 66, to cells 50-75 microm in diameter containing multiple (up to 5-6) interconnected nuclei with a clear megakaryocyte (Mk) morphology by electron microscopy. Cell sorter isolation and single cell cloning experiments indicated that the small cells were those capable to proliferate in TPO and to generate the larger ones over time. 32D TPO cells expressed Mk-specific markers by FACS (CD41, CD61 and 2D5) and RT-PCR (acetyl cholinesterase E and platelet factor 4) and their unique profile, by gene array analysis, included expression of urokinase plasminogen activator surface receptor (CD87 or uPAR), plasminogen activator inhibitor and coagulation factor II (thrombin) receptor (Cf2r). In addition, by quantitative RT-PCR, 32D TPO clones expressed levels of Gata1 similar to those expressed by freshly isolated Mks (DeltaCt approximately 4.7 in both cases). In conclusion, the 32D TPO subclones described here are among the few pure Mk cell lines isolated so far and, for their unique properties, may prove themselves as a useful model to study Mk differentiation.

Transgenic over-expression of GATA-1 mutant lacking N-finger domain causes hemolytic syndrome in mouse erythroid cells

Transcription factor GATA-1 is essential for erythroid cell differentiation. GATA-binding motifs have been found in the regulatory regions of various erythroid-specific genes, suggesting that GATA-1 contributes to gene regulation during the entire process of erythropoiesis. A GATA-1 germ-line mutation results in embryonic lethality due to defective primitive erythropoiesis and GATA-1-null embryonic stem cells fails to differentiate beyond the proerythroblast stage. Therefore, the precise roles of GATA-1 in the later stages of erythropoiesis could not be clarified. Under the control of a GATA-1 gene hematopoietic regulatory domain, a GATA-1 mutant lacking the N-finger domain (DeltaNF mutant) was over-expressed in mice. These mice exhibited abnormal morphology in peripheral red blood cells (RBCs), reticulocytosis, splenomegaly, and erythroid hyperplasia, indicating compensated hemolysis. These mice were extremely sensitive to phenylhydrazine (PHZ), an agent that induces hemolysis, and their RBCs were osmotically fragile. Importantly, the hemolytic response to PHZ was partially restored by the simultaneous expression of wild-type GATA-1 with the DeltaNF mutant, supporting our contention that DeltaNF protein competitively inhibits the function of endogenous GATA-1. These data provide the first in vivo evidence that the NF domain contributes to the gene regulation that is critical for differentiation and survival of mature RBCs in postnatal erythropoiesis.

Lyn deficiency reduces GATA-1, EKLF and STAT5, and induces extramedullary stress erythropoiesis

In vitro studies have implicated the Lyn tyrosine kinase in erythropoietin signaling. In this study, we show that J2E erythroid cells lacking Lyn have impaired signaling and reduced levels of transcription factors STAT5a, EKLF and GATA-1. Since mice lacking STAT5, EKLF or GATA-1 have red cell abnormalities, this study also examined the erythroid compartment of Lyn(-/-) mice. Significantly, STAT5, EKLF and GATA-1 levels were appreciably lower in Lyn(-/-) erythroblasts, and the phenotype of Lyn(-/-) animals was remarkably similar to GATA-1(low) animals. Although young adult Lyn-deficient mice had normal hematocrits, older mice developed anemia. Grossly enlarged erythroblasts and florid erythrophagocytosis were detected in the bone marrow of mice lacking Lyn. Markedly elevated erythroid progenitors and precursor levels were observed in the spleens, but not bone marrow, of Lyn(-/-) animals indicating that extramedullary erythropoiesis was occurring. These data indicate that Lyn(-/-) mice display extramedullary stress erythropoiesis to compensate for intrinsic and extrinsic erythroid defects.

A pathobiologic pathway linking thrombopoietin, GATA-1, and TGF-beta1 in the development of myelofibrosis

Idiopathic myelofibrosis (IM) is a disease characterized by marrow fibrosis, abnormal stem/progenitor cell trafficking, and extramedullary hematopoiesis frequently associated with alterations in megakaryocytes (Mks). Mice harboring genetic alterations in either the extrinsic (ectopic thrombopoietin expression, TPO(high) mice) or intrinsic (hypomorphic GATA-1 mutation, GATA-1(low) mice) control of Mk differentiation develop myelofibrosis, a syndrome similar to IM. The relationship, if any, between the pathobiologic mechanism leading to the development of myelofibrosis in the 2 animal models is not understood. Here we show that plasma from GATA-1(low) mice contained normal levels of TPO. On the other hand, Mks from TPO-treated wild-type animals (TPO(high) mice), as those from GATA-1(low) animals, had similar morphologic abnormalities and contained low GATA-1. In both animal models, development of myelofibrosis was associated with high transforming growth factor beta1 (TGF-beta1) content in extracellular fluids of marrow and spleen. Surprisingly, TPO treatment of GATA-1(low) mice restored the GATA-1 content in Mks and halted both defective thrombocytopoiesis and fibrosis. These data indicate that the TPO(high) and GATA-1(low) alterations are linked in an upstream-downstream relationship along a pathobiologic pathway leading to development of myelofibrosis in mice and, possibly, of IM in humans.

Enhanced recovery from thrombocytopenia and neutropenia in mice constitutively expressing a placental hematopoietic cytokine

Expression of the placental hormone, prolactin-like protein E (PLP-E), a potent cytokine that acts on multiple myeloid lineages, is normally restricted to pregnancy and certain hematopoietic disease states. We hypothesized that the restricted pattern of PLP-E expression is necessary to avoid hyperstimulation of myelopoiesis. To test this idea, we have produced PLP-E transgenic mice and analyzed their steady-state blood cell levels. We find that blood cell levels remain in the normal range, and thus the constitutive expression of a cytokine of pregnancy fails to overcome the tight control of hematopoietic set points for blood cell levels. In contrast, an effect of constitutive PLP-E expression is detected during the recovery from low blood platelet levels (acute thrombocytopenia) and from low granulocyte levels (acute neutropenia) but not from anemia. Mice producing high circulating concentrations of PLP-E recover more rapidly from both thrombocytopenia and neutropenia, as seen both by an earlier increase of progenitor numbers in the bone marrow and the earlier return to normal circulating blood cell levels.

Activated Fps/Fes tyrosine kinase regulates erythroid differentiation and survival

OBJECTIVE

A substantial body of evidence implicates the cytoplasmic protein tyrosine kinase Fps/Fes in regulation of myeloid differentiation and survival. In this study we wished to determine if Fps/Fes also plays a role in the regulation of erythropoiesis.

METHODS

Mice tissue-specifically expressing a "gain-of-function" mutant fps/fes transgene (fps(MF)) encoding an activated variant of Fps/Fes (MFps), were used to explore the in vivo biological role of Fps/Fes. Erythropoiesis in these mice was assessed by hematological analysis, lineage marker analysis, bone-marrow colony assays, and biochemical approaches.

RESULTS

fps(MF) mice displayed reductions in peripheral red cell counts. However, there was an accumulation of immature erythroid precursors, which displayed increased survival. Fps/Fes and the related Fer kinase were both detected in early erythroid progenitors/blasts and in mature red cells. Fps/Fes was also activated in response to erythropoietin (EPO) and stem cell factor (SCF), two critical factors in erythroid development. In addition, increased Stat5A/B activation and reduced Erk1/2 phosphorylation was observed in fps(MF) primary erythroid cells in response to EPO or SCF, respectively.

CONCLUSIONS

These data support a role for Fps/Fes in regulating the survival and differentiation of erythroid cells through modulation of Stat5A/B and Erk kinase pathways induced by EPO and SCF. The increased numbers and survival of erythroid progenitors from fps(MF) mice, and their differential responsiveness to SCF and EPO, implicates Fps/Fes in the commitment of multilineage progenitors to the erythroid lineage. The anemic phenotype in fps(MF) mice suggests that downregulation of Fps/Fes activity might be required for terminal erythroid differentiation.

Oral administration of K-11706 inhibits GATA binding activity, enhances hypoxia-inducible factor 1 binding activity, and restores indicators in an in vivo mouse model of anemia of chronic disease

Erythropoietin (Epo) gene expression is under the control of hypoxia-inducible factor 1 (HIF-1), and is negatively regulated by GATA. Interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α), which increase the binding activity of GATA and inhibit Epo promoter activity, are increased in patients with anemia of chronic disease (ACD). We previously demonstrated the ability of K-7174 (a GATA-specific inhibitor), when injected intraperitoneally, to improve Epo production that had been inhibited by IL-1β or TNF-α treatment. In the present study, we examined the ability of both K-11706, which inhibits GATA and enhances HIF-1 binding activity, and K-13144, which has no effect on GATA or HIF-1 binding activity, to improve Epo production following inhibition by IL-1β or TNF-α in Hep3B cells in vitro and in an in vivo mouse assay. Oral administration of K-11706 reversed the decreases in hemoglobin and serum Epo concentrations, reticulocyte counts, and numbers of erythroid colony-forming units (CFU-Es) induced by IL-1β or TNF-α. These results raise the possibility of using orally administered K-11706 for treating patients with ACD.

GATA1 in normal and malignant hematopoiesis

In the late 1980s, several research groups independently discovered the founding member of the GATA family of transcription factors, GATA-1. Each group had evidence that GATA-1 played an important role in erythroid gene expression, but little did they know that it would turn out to be a key regulator of development of not only red blood cells, but of several other hematopoietic cell types as well. Furthermore, few would have guessed that missense mutations in GATA1 would cause inherited blood disorders, while acquired mutations would be found associated with essentially all cases of acute megakaryoblastic leukemia (AMKL) in children with Down syndrome (DS). With respect to the latter disorder, the presence of a GATA1 mutation is now arguably the defining feature of this leukemia. In this review, I will summarize our current knowledge of the role of GATA-1 in normal development, and discuss how mutations in GATA1 lead to abnormal and malignant hematopoiesis.

Increased and pathologic emperipolesis of neutrophils within megakaryocytes associated with marrow fibrosis in GATA-1low mice

Deletion of megakaryocytic-specific regulatory sequences of GATA-1 (Gata1tm2Sho or GATA-1low mutation) results in severe thrombocytopenia, because of defective thrombocytopoiesis, and myelofibrosis. As documented here, the GATA-1low mutation blocks megakaryocytic maturation between stage I and II, resulting in accumulation of defective megakaryocytes (MKs) in the tissues of GATA-1low mice. The block in maturation includes failure to properly organize α granules because von Willebrand factor is barely detectable in mutant MKs, and P-selectin, although normally expressed, is found frequently associated with the demarcation membrane system (DMS) instead of within granules. Conversely, both von Willebrand factor and P-selectin are barely detectable in GATA-1low platelets. Mutant MKs are surrounded by numerous myeloperoxidase-positive neutrophils, some of which appear in the process to establish contact with MKs by fusing their membrane with those of the DMS. As a result, 16% (in spleen) to 34% (in marrow) of GATA-1low MKs contain 1 to 3 neutrophils embedded in a vacuolated cytoplasm. The neutrophil-embedded GATA-1low MKs have morphologic features (high electron density and negativity to TUNEL staining) compatible with those of cells dying from para-apoptosis. We suggest that such an increased and pathologic neutrophil emperipolesis may represent one of the mechanisms leading to myelofibrosis by releasing fibrogenic MK cytokines and neutrophil proteases in the microenvironment.

Lentivectors for regulated and reversible cutaneous gene delivery

Systemic administration of therapeutic proteins has value in treating a wide variety of disorders, including erythropoietin (Epo)-responsive anemias. Recombinant proteins, however, are costly and require repeated injections, while gene delivery approaches have suffered from inefficiency and difficulties with regulation. The skin effectively delivers polypeptides to the circulation, and improved approaches would support sustainable, topically regulated protein expression after a single vector injection. Toward this goal, we generated lentivectors in which both gene delivery and persistence in skin are regulated by administration of distinct steroid ligands. Following a single injection of regulated lentivector into human skin regenerated on immunodeficient mice, topical glucocorticoid ligands regulated Epo levels and hematocrit over time. Abrogation of gene delivery was achieved by both glucocorticoid cessation and proviral excision via a 4-hydroxytamoxifen-inducible Cre recombinase. These findings establish an approach to durable, topically controlled systemic delivery of therapeutic proteins from human skin tissue.