CCAAT/enhancer-binding protein α (C/EBPα) is one of the key transcription factors that mediate lineage specification and differentiation of multipotent myeloid progenitors into mature granulocytes. Although C/EBPα is known to induce granulopoiesis while suppressing monocyte differentiation, it is unclear how C/EBPα regulates this cell fate choice at the mechanistic level. Here we report that inducers of monocyte differentiation inhibit the alternate cell fate choice, that of granulopoiesis, through inhibition of C/EBPα. This inhibition is mediated by extracellular signal-regulated kinases 1 and/or 2 (ERK1/2), which interact with C/EBPα through an FXFP docking site and phosphorylate serine 21. As a consequence of C/EBPα phosphorylation, induction of granulocyte differentiation by C/EBPα or retinoic acid is inhibited. Our analysis of C/EBPα by fluorescent resonance energy transfer revealed that phosphorylation induces conformational changes in C/EBPα, increasing the distance between the amino termini of C/EBPα dimers. Thus, myeloid development is partly regulated by an ERK1/2-mediated change in the conformation of C/EBPα that favors monocyte differentiation by blocking granulopoiesis.
C/EBPα is required for the formation of granulocyte-monocyte progenitors; however, its role in subsequent myeloid lineage specification remains uncertain. Transduction of murine marrow with either of two Cebpa shRNAs markedly increases monocyte and reduces granulocyte colonies in methylcellulose or the monocyte to neutrophil ratio in liquid culture. Similar findings were found after marrow shRNA transduction and transplantation and with CEBPA knockdown in human marrow CD34+ cells. These results apparently reflect altered myeloid lineage specification, as similar knockdown allowed nearly complete 32Dcl3 granulocytic maturation. Cebpa knockdown also generated lineage-negative blasts with increased colony replating capacity but unchanged cell cycle parameters, likely reflecting complete differentiation block. The shRNA having the greatest effect on lineage skewing reduced Cebpa 3-fold in differentiating cells but 6-fold in accumulating blasts. Indicating that Cebpa is the relevant shRNA target, shRNA-resistant C/EBPα-ER rescued marrow myelopoiesis. Cebpa knockdown in murine marrow cells also increased in vitro erythropoiesis, perhaps reflecting 1.6-fold reduction in PU.1 leading to GATA-1 derepression. Global gene expression analysis of lineage-negative blasts that accumulate after Cebpa knockdown demonstrated reduction in Cebpe and Gfi1, known transcriptional regulators of granulopoiesis, and also reduced Ets1 and Klf5. Populations enriched for immature granulocyte or monocyte progenitor/precursors were isolated by sorting Lin−Sca-1−c-Kit+ cells into GCSFR+MCSFR− or GCSFR−MCSFR+ subsets. Cebpa, Cebpe, Gfi1, Ets1, and Klf5 RNAs were increased in the c-Kit+GCSFR+ and Klf4 and Irf8 in the c-Kit+MCSFR+ populations, with PU.1 levels similar in both. In summary, higher levels of C/EBPα are required for granulocyte and lower levels for monocyte lineage specification, and this myeloid bifurcation may be facilitated by increased Cebpa gene expression in granulocyte compared with monocyte progenitors.
Eosinophil lineage–committed progenitors (EoPs) are phenotypically isolatable in the steady-state murine bone marrow. Purified granulocyte/monocyte progenitors (GMPs) gave rise to eosinophils as well as neutrophils and monocytes at the single cell level. Within the short-term culture of GMPs, the eosinophil potential was found exclusively in cells activating the transgenic reporter for GATA-1, a transcription factor capable of instructing eosinophil lineage commitment. These GATA-1–activating cells possessed an IL-5Rα+CD34+c-Kitlo phenotype. Normal bone marrow cells also contained IL-5Rα+CD34+c-Kitlo EoPs that gave rise exclusively to eosinophils. EoPs significantly increased in number in response to helminth infection, suggesting that the EoP stage is physiologically involved in eosinophil production in vivo. EoPs expressed eosinophil-related genes, such as the eosinophil peroxidase and the major basic protein, but did not express basophil/mast cell–related mast cell proteases. The enforced retroviral expression of IL-5Rα in GMPs did not enhance the frequency of eosinophil lineage read-outs, whereas IL-5Rα+ GMPs displayed normal neutrophil/monocyte differentiation in the presence of IL-5 alone. Thus, IL-5Rα might be expressed specifically at the EoP stage as a result of commitment into the eosinophil lineage. The newly identified EoPs could be the cellular target in the treatment of a variety of disorders mediated by eosinophils.
The zinc finger transcription factor GATA-1 requires direct physical interaction with the cofactor friend of GATA-1 (FOG-1) for its essential role in erythroid and megakaryocytic development. We show that in the mast cell lineage, GATA-1 functions completely independent of FOG proteins. Moreover, we demonstrate that FOG-1 antagonizes the fate choice of multipotential progenitor cells for the mast cell lineage, and that its down-regulation is a prerequisite for mast cell development. Remarkably, ectopic expression of FOG-1 in committed mast cell progenitors redirects them into the erythroid, megakaryocytic, and granulocytic lineages. These lineage switches correlate with transcriptional down-regulation of GATA-2, an essential mast cell GATA factor, via switching of GATA-1 for GATA-2 at a key enhancer element upstream of the GATA-2 gene. These findings illustrate combinatorial control of cell fate identity by a transcription factor and its cofactor, and highlight the role of transcriptional networks in lineage determination. They also provide evidence for lineage instability during early stages of hematopoietic lineage commitment.
NF-κB is a key transcriptional inducer of the inflammatory response in mature myeloid cells, and also stimulates cell survival, but its role in immature myeloid cell development has not been well characterized. C/EBPα is required for the development of monocytic and granulocytic myeloid cells from early progenitors, and NF-κB and C/EBPβ cooperatively induce several inflammatory mediators. Having found that C/EBPα binds NF-κB p50 preferentially compared with NF-κB p65, we have now investigated myelopoiesis in nfkb1−/− mice lacking NF-κB p50. Absence of p50 leads to a significant reduction in the number of granulocytic progenitors, CFU-G, obtained with G-CSF or GM-CSF in vitro and reduces neutrophil production in vivo in response to G-CSF, with preservation of monopoiesis in vitro in response to cytokines or LPS. To gain insight into the mechanism underlying reduced granulopoiesis in the absence of NF-κB p50, we assessed the expression of several myeloid regulatory proteins in lineage-negative, immature myeloid cells. Although PU.1, C/EBPβ, and STAT3 levels were unchanged, C/EBPα protein and RNA levels were reduced approximately 3-fold in the absence of NF-κB p50. In addition, NF-κB p50 and C/EBPα bound the endogenous C/EBPα promoter in a chromatin immunoprecipitation assay, and NF-κB p50 trans-activated the C/EBPα promoter, alone or in cooperation with C/EBPα. Despite reduction of C/EBPα, GCSFR and MCSFR levels were maintained or total marrow and in lineage-negative cells. Together, these data indicate that acute inflammation not only activates mature myeloid cells but also stimulates neutrophil production via NF-κB p50 induction of C/EBPα transcription.
neutrophils; inflammation; cell differentiation; hematopoiesis; gene regulation
In contrast to the definitive role of the transcription factor, CCAAT/Enhancer binding protein α (C/EBPα), in steady-state granulopoiesis, previous findings have suggested that granulopoiesis during emergency situations, such as infection, is dependent on C/EBPβ. In this study, a novel lentivirus-based reporter system was developed to elucidate the molecular switch required for C/EBPβ-dependency. The results demonstrated that two cyclic AMP responsive elements (CREs) in the proximal promoter region of C/EBPβ were involved in the positive regulation of C/EBPβ transcription during granulocyte-macrophage colony-stimulating factor (GM-CSF)–induced differentiation of bone marrow cells. In addition, the transcripts of CRE binding (CREB) family proteins were readily detected in hematopoietic stem/progenitor cells. CREB was upregulated, phosphorylated and bound to the CREs in response to GM-CSF stimulation. Retroviral transduction of a dominant negative CREB mutant reduced C/EBPβ mRNA levels and significantly impaired the proliferation/differentiation of granulocyte precursors, while a constitutively active form of CREB facilitated C/EBPβ transcription. These data suggest that CREB proteins are involved in the regulation of granulopoiesis via C/EBPβ upregulation.
The transcription factor CCAAT/enhancer binding protein α (C/EBPα) regulates a number of myeloid cell-specific genes. To delineate the role of C/EBPα in human granulopoiesis, we studied its expression and function in human primary cells and bipotential (granulocytic/monocytic) myeloid cell lines. We show that the expression of C/EBPα initiates with the commitment of multipotential precursors to the myeloid lineage, is specifically upregulated during granulocytic differentiation, and is rapidly downregulated during the alternative monocytic pathway. Conditional expression of C/EBPα alone in stably transfected bipotential cells triggers neutrophilic differentiation, concomitant with upregulation of the granulocyte-specific granulocyte colony-stimulating factor receptor and secondary granule protein genes. Moreover, induced expression of C/EBPα in bipotential precursors blocks their monocytic differentiation program. These results indicate that C/EBPα serves as a myeloid differentiation switch acting on bipotential precursors and directing them to mature to granulocytes.
Cytokines stimulate granulopoiesis through signaling via receptors whose expression is controlled by lineage-specific transcription factors. Previously, we demonstrated that granulocyte colony-stimulating factor (G-CSF) receptor mRNA was undetectable and granulocyte maturation blocked in CCAAT enhancer binding protein α (C/EBPα)-deficient mice. This phenotype is distinct from that of G-CSF receptor−/− mice, suggesting that other genes are likely to be adversely affected by loss of C/EBPα. Here we demonstrate loss of interleukin 6 (IL-6) receptor and IL-6–responsive colony-forming units (CFU-IL6) in C/EBPα−/− mice. The observed failure of granulopoiesis could be rescued by the addition of soluble IL-6 receptor and IL-6 or by retroviral transduction of G-CSF receptors, demonstrating that loss of both of these receptors contributes to the absolute block in granulocyte maturation observed in C/EBPα-deficient hematopoietic cells. The results of these and other studies suggest that additional C/EBPα target genes, possibly other cytokine receptors, are also important for the block in granulocyte differentiation observed in vivo in C/EBPα-deficient mice.
CCAAT enhancer binding protein; knockout mice; colony-forming unit; hematopoiesis; myelopoiesis
We investigated the role of CCAAT enhancer-binding protein-α (C/EBPα) during zebrafish embryonic blood development.
Whole-mount mRNA in situ hybridization was performed to determine the spatio-temporal expression pattern of zebrafish cebpa in developing hematopoietic progenitors. A deletion mutation of cebpa (zD420), which mimics the human dominant-negative mutations of C/EBPα, was transfected into CV1 cell line to evaluate its transcriptional activity in vitro and injected into zebrafish embryos at the one- to two-cell stage to examine its effects on primitive hematopoiesis during early zebrafish development.
Zebrafish cebpa is expressed in the anterior and posterior lateral plate mesoderm at 12 hours postfertilization, along with scl, pu.1 and gata1 in developing hematopoietic progenitors. In vitro, the deletion mutation of cebpa (zD420) prevents expression of the full-length protein, allowing the expression of truncated isoforms from internal translational initiation sites. As in the human, the truncated zebrafish C/ebpα proteins did not activate the expression of known target granulocytic genes, and in fact, suppressed transactivation that was induced in vitro by the full-length protein. Forced expression of the zD420 mRNA in zebrafish embryos led to an expansion of primitive erythropoiesis, without a discernible effect on granulopoiesis.
Expression of the truncated isoforms of cebpa alters the developmental pattern of hematopoietic progenitor cells during embryogenesis.
Zebrafish; C/ebpα; erythropoiesis
Infection with Anaplasma phagocytophilum, a gram-negative, lipopolysaccharide (LPS)-negative, obligate intracellular bacterium, results in multiple peripheral blood cytopenias. We hypothesized that infection with this organism would result in decreased bone marrow (BM) function and shifts in hematopoietic progenitor cells (HPCs) and lineage-committed cells in a well-established murine model of infection. HPCs and lineage-committed progenitors were enumerated in the BM and spleen during acute infection. BM cytokine production and BM CXCL12 expression were determined. Infection resulted in peripheral blood bicytopenia, marked decreases in the number of lineage-committed HPCs in the BM along with concurrent increases in the number of lineage-committed HPCs in the spleen, and a mixed, predominantly myelosuppressive BM cytokine environment. There was significant downregulation of CXCL12 in BM cells that may have been partially responsible for changes in HPC trafficking observed. Changes occurred in the absence of direct pathogen infection of BM cells. Hematopoietic lineage assessment demonstrated that there was loss of erythrocytes and B lymphocytes from the BM along with increased granulopoiesis. These changes were accompanied by splenomegaly due to lymphoid hyperplasia and increased hematopoiesis, most notably erythropoiesis. These changes largely mimic well-described inflammation and endotoxin-mediated effects on the BM and spleen; however, the numbers of peripheral blood neutrophils appear to be independently modulated as granulocytic hyperplasia does not result in neutrophilia. Our findings highlight a well-conserved series of events that we demonstrate can be instigated by an LPS-negative pathogen in the absence of an endotoxin-mediated acute proinflammatory response.
Basic helix-loop-helix (bHLH) transcription factors play critical roles in lymphoid and erythroid development; however, little is known about their role in myeloid lineage development. In this study, we identify the bHLH transcription factor Twist-2 as a key negative regulator of myeloid lineage development, as manifested by marked increases in mature myeloid populations of macrophages, neutrophils, and basophils in Twist-2–deficient mice. Mechanistic studies demonstrate that Twist-2 inhibits the proliferation as well as differentiation of granulocyte macrophage progenitors (GMP) by interacting with and inhibiting the transcription factors Runx1 and C/EBPα. Moreover, Twist-2 was found to have a contrasting effect on cytokine production: inhibiting the production of proinflammatory cytokines such as interleukin-12 (IL-12) and interferon-γ (IFNγ) while promoting the regulatory cytokine IL-10 by myeloid cells. The data from further analyses suggest that Twist-2 activates the transcription factor c-Maf, leading to IL-10 expression. In addition, Twist-2 was found to be essential for endotoxin tolerance. Thus, this study reveals the critical role of Twist-2 in regulating the development of myeloid lineages, as well as the function and inflammatory responses of mature myeloid cells.
Hematopoiesis is coordinated by transcription factors that regulate proliferation, differentiation, and cell fate determinations. Myelopoiesis refers to the development of all white blood cells, excluding lymphocytes (B and T cells); however, the molecular regulation of this developmental process is still incompletely understood. In this study using mice that lack expression of Twist-2, we establish a novel role for this basic helix-loop-helix transcription factor as regulator of myeloid progenitors and fully differentiated myeloid cells. Specifically, Twist-2 acts to inhibit proliferation as well as differentiation of progenitors that give rise to macrophages, neutrophils, and basophils by inhibiting the important transcription factors Runx1 and C/EBPα. In mature myeloid cells, Twist-2 negatively regulates the production of proinflammatory cytokines while positively promoting the production of regulatory cytokine IL-10 by these cells. These findings provide significant insight into regulation of myeloid lineage development and function.
The transcription factor Twist-2 is a new regulator that inhibits the proliferation and differentiation of granulocyte macrophage progenitors. Twist-2 also inhibits proinflammatory cytokine production, while stimulating IL-10 by myeloid cells.
Transcription factors play a key role in lineage commitment and differentiation of stem cells into distinct mature cells. In hematopoiesis, they regulate lineage-specific gene expression in a stage-specific manner through various physical and functional interactions with regulatory proteins that are simultanously recruited and activated to ensure timely gene expression. The transcription factor CCAAT/enhancer binding protein α (C/EBPα) is such a factor and is essential for the development of granulocytic/monocytic cells. The activity of C/EBPα is regulated on several levels including gene expression, alternative translation, protein interactions and posttranslational modifications, such as phosphorylation. In particular, the phosphorylation of serine 248 of the transactivation domain has been shown to be of crucial importance for granulocytic differentiation of 32Dcl3 cells in vitro.
Here, we use mouse genetics to investigate the significance of C/EBPα serine 248 in vivo through the construction and analysis of CebpaS248A/S248A knock-in mice. Surprisingly, 8-week old CebpaS248A/S248A mice display normal steady-state hematopoiesis including unaltered development of mature myeloid cells. However, over time some of the animals develop a hematopoietic disorder with accumulation of multipotent, megakaryocytic and erythroid progenitor cells and a mild impairment of differentiation along the granulocytic-monocytic lineage. Furthermore, BM cells from CebpaS248A/S248A animals display a competitive advantage compared to wild type cells in a transplantation assay.
Taken together, our data shows that the substitution of C/EBPα serine 248 to alanine favors the selection of the megakaryocytic/erythroid lineage over the monocytic/granulocytic compartment in old mice and suggests that S248 phosphorylation may be required to maintain proper hematopoietic homeostasis in response to changes in the wiring of cellular signalling networks. More broadly, the marked differences between the phenotype of the S248A variant in vivo and in vitro highlight the need to exert caution when extending in vitro phenotypes to the more appropriate in vivo context.
Hematopoietic stem cell lineage choices are decided by genetic networks that are turned ON/OFF in a switch-like manner. However, prior to lineage commitment, genes are primed at low expression levels. Understanding the underlying molecular circuitry in terms of how it governs both a primed state and, at the other extreme, a committed state is of relevance not only to hematopoiesis but also to developmental systems in general. We develop a computational model for the hematopoietic erythroid-myeloid lineage decision, which is determined by a genetic switch involving the genes PU.1 and GATA-1. Dynamical models based upon known interactions between these master genes, such as mutual antagonism and autoregulation, fail to make the system bistable, a desired feature for robust lineage determination. We therefore suggest a new mechanism involving a cofactor that is regulated as well as recruited by one of the master genes to bind to the antagonistic partner that is necessary for bistability and hence switch-like behavior. An interesting fallout from this architecture is that suppression of the cofactor through external means can lead to a loss of cooperativity, and hence to a primed state for PU.1 and GATA-1. The PU.1–GATA-1 switch also interacts with another mutually antagonistic pair, C/EBPα–FOG-1. The latter pair inherits the state of its upstream master genes and further reinforces the decision due to several feedback loops, thereby leading to irreversible commitment. The genetic switch, which handles the erythroid-myeloid lineage decision, is an example of a network that implements both a primed and a committed state by regulating cooperativity through recruitment of cofactors. Perturbing the feedback between the master regulators and downstream targets suggests potential reprogramming strategies. The approach points to a framework for lineage commitment studies in general and could aid the search for lineage-determining genes.
An important question in developmental biology is how different lineage choices are regulated at the genetic level. Robust lineage decisions are implemented by genetic switches, whereby one set of master genes are ON and another set are OFF, leading to a specific expression pattern of genes for a particular lineage. We develop a computational model to illustrate these principles as applied to the hematopoietic erythroid-myeloid lineage choice, where two master regulator genes, PU.1 and GATA-1, function as a genetic switch. The model, which is based upon known interactions, suggests missing interactions between the master genes, which we hypothesize, so as to reproduce the desired dynamics. Furthermore, there exist feedback interactions between the master genes and their downstream targets. When these are included in the model, the dynamics imply that the feedback is responsible for irreversible commitment. Our results suggest the search for missing interactions between the master genes in terms of a coregulated cofactor. The second important result of the model is that reprogramming irreversible cell fate may be possible by perturbing feedback regulation between the master genes and their downstream targets. Hence, dynamical modeling provides prediction of novel mechanisms and also strategies for reprogramming the fates of cells.
Emergency granulopoiesis is a component of the innate immune response that is induced in response to infectious or inflammatory challenge. It is characterized by the rapid expansion and differentiation of granulocyte/monocyte progenitor (GMP) populations, which is due in part to a shortened S-phase of the cell cycle. We found that IRF8 (also known as ICSBP), an interferon regulatory transcription factor that activates phagocyte effector genes during the innate immune response, activates the gene encoding Fanconi C (Fancc) in murine myeloid progenitor cells. Moreover, IRF8-induced Fancc transcription was augmented by treatment with IL-1β, an essential cytokine for emergency granulopoiesis. The Fanconi pathway participates in repair of stalled or collapsed replication forks during DNA replication, leading us to hypothesize that the Fanconi pathway contributes to genomic stability during emergency granulopoiesis. In support of this hypothesis, Fancc–/– mice developed anemia and neutropenia during repeated, failed episodes of emergency granulopoiesis. Failed emergency granulopoiesis in Fancc–/– mice was associated with excess apoptosis of HSCs and progenitor cells in the bone marrow and impaired HSC function. These studies have implications for understanding the pathogenesis of bone marrow failure in Fanconi anemia and suggest possible therapeutic approaches.
Although STAT5 promotes survival of hematopoietic progenitors, STAT5−/− mice develop mild neutrophilia.
Here, we show that in STAT5−/− mice, liver endothelial cells (LECs) autonomously secrete high amounts of G-CSF, allowing myeloid progenitors to overcompensate for their intrinsic survival defect. However, when injected with pro-inflammatory cytokines, mutant mice cannot further increase neutrophil production, display a severe deficiency in peripheral neutrophil survival, and are therefore unable to maintain neutrophil homeostasis. In wild-type mice, inflammatory stimulation induces rapid STAT5 degradation in LECs, G-CSF production by LECs and other cell types, and then sustained mobilization and expansion of long-lived neutrophils.
We conclude that STAT5 is an ambivalent factor. In cells of the granulocytic lineage, it exerts an antiapoptotic function that is required for maintenance of neutrophil homeostasis, especially during the inflammatory response. In LECs, STAT5 negatively regulates granulopoiesis by directly or indirectly repressing G-CSF expression. Removal of this STAT5-imposed brake contributes to induction of emergency granulopoiesis.
The Signal Transducer and Activator of Transcription 5 (Stat5) plays a significant role in normal hematopoiesis and a variety of hematopoietic malignancies. Deficiency in Stat5 causes impaired cytokine-mediated proliferation and survival of progenitors and their differentiated descendants along major hematopoietic lineages such as erythroid, lymphoid, and myeloid cells. Overexpression and persistent activation of Stat5 are sufficient for neoplastic transformation and development of multi-lineage leukemia in a transplant model. Little is known, however, whether a continuous activation of this signal transducer is essential for the maintenance of hematopoietic malignancies. To address this issue, we developed transgenic mice that express a hyperactive mutant of Stat5 in hematopoietic progenitors and derived lineages in a ligand-controlled manner. In contrast to the transplant model, expression of mutant Stat5 did not adversely affect normal hematopoiesis in the presence of endogenous wildtype Stat5 alleles. However, the gain-of-function of this signal transducer in mice that carry Stat5a/b hypomorphic alleles resulted in abnormally high numbers of circulating granulocytes that caused severe airway obstruction. Downregulation of hyperactive Stat5 in diseased animals restored normal granulopoiesis, which also resulted in a swift clearance of granulocytes from the lung. Moreover, we demonstrate that Stat5 promotes the initiation and maintenance of severe granulophilia in a cell autonomous manner. The results of this study show that the gain-of-function of Stat5 causes excessive granulopoiesis and prolonged survival of granulocytes in circulation. Collectively, our findings underline the critical importance of Stat5 in maintaining a normal balance between myeloid and lymphoid cells during hematopoiesis, and we provide direct evidence for a function of Stat5 in granulophilia–associated pulmonary dysfunction.
Normally, neutrophil pools are maintained by homeostatic mechanisms that require
the transcription factor C/EBPα. Inflammation, however, induces neutrophilia
through a distinct pathway of “emergency” granulopoiesis that is
dependent on C/EBPβ. Here, we show in mice that alum triggers emergency
granulopoiesis through the IL-1RI-dependent induction of G-CSF. G-CSF/G-CSF-R
neutralization impairs proliferative responses of hematopoietic stem and
progenitor cells (HSPC) to alum, but also abrogates the acute mobilization of BM
neutrophils, raising the possibility that HSPC responses to inflammation are an
indirect result of the exhaustion of BM neutrophil stores. The induction of
neutropenia, via depletion with Gr-1 mAb or myeloid-specific ablation of Mcl-1,
elicits G-CSF via an IL-1RI-independent pathway, stimulating granulopoietic
responses indistinguishable from those induced by adjuvant. Notably, C/EBPβ,
thought to be necessary for enhanced generative capacity of BM, is dispensable
for increased proliferation of HSPC to alum or neutropenia, but plays a role in
terminal neutrophil differentiation during granulopoietic recovery. We conclude
that alum elicits a transient increase in G-CSF production via IL-1RI for the
mobilization of BM neutrophils, but density-dependent feedback sustains G-CSF
for accelerated granulopoiesis.
Transcription factors are key regulators of hematopoietic stem cells (HSCs) and act through their ability to bind DNA and impact on gene transcription. Their functions are interpreted in the complex landscape of chromatin, but current knowledge on how this is achieved is very limited. C/EBPα is an important transcriptional regulator of hematopoiesis, but its potential functions in HSCs have remained elusive. Here we report that C/EBPα serves to protect adult HSCs from apoptosis and to maintain their quiescent state. Consequently, deletion of Cebpa is associated with loss of self-renewal and HSC exhaustion. By combining gene expression analysis with genome-wide assessment of C/EBPα binding and epigenetic configurations, we show that C/EBPα acts to modulate the epigenetic states of genes belonging to molecular pathways important for HSC function. Moreover, our data suggest that C/EBPα acts as a priming factor at the HSC level where it actively promotes myeloid differentiation and counteracts lymphoid lineage choice. Taken together, our results show that C/EBPα is a key regulator of HSC biology, which influences the epigenetic landscape of HSCs in order to balance different cell fate options.
Hematopoietic stem cells (HSCs) are required for the lifelong generation of blood cells. To fulfill this requirement HSCs carefully balance cell fate decisions such as self-renewal, differentiation, quiescence, proliferation and death. These features are regulated in part by transcription factors, which act by controlling the expression of genes important for the functional properties of HSCs. C/EBPα is a well-known inducer of myeloid differentiation. It is lowly expressed in HSCs and its potential function in these cells has been extensively debated. Here, we demonstrate that Cebpa deletion impacts on HSC self-renewal, differentiation, quiescence and survival. Through gene expression and ChIP-seq analyses of stem and progenitor cell-enriched cell populations, we further show that C/EBPα binds to regulatory regions of genes that are induced during granulocytic differentiation, suggesting that C/EBPα acts to prime HSCs for differentiation along the myeloid lineage. Finally, we demonstrate that C/EBPα loss leads to epigenetic changes at genes central to HSC biology, which implies that it may act to recruit chromatin writers/erasers through mechanisms that remain to be characterized. In conclusion, our work identifies C/EBPα as a central hub for HSC function and highlights how a single transcription factor may coordinate several HSC fate options.
CCAAT enhancer binding protein alpha (C/EBPα) is the founding member of a family of basic region/leucine zipper (bzip) transcription factors and is a master regulator of granulopoiesis. It is expressed at high levels throughout myeloid differentiation and binds to the promoters of multiple myeloid- specific genes at different stages of myeloid maturation. Profound hematopoietic abnormalities occur in mice nullizygous for C/EBPα̤ including a selective early block in the differentiation of granulocytes. Mutations in C/EBPα are present in a subset of patients with AML presenting with a normal karyotype. These mutations can result in the expression of a 30kD dominant negative C/EBPα isoform, which contributes to loss of C/EBPα function. The molecular basis for this observation remains unknown. In addition to phoshorylation, C/EBPα is modified, post-translationally by a small ubiquitin-related modifier (SUMO) at a lysine residue (K159), which lies within the growth inhibitory region of the C/EBPα protein. Sumoylation at K159 in the C/EBPα protein prevents association of the SWI/SNF chromatin remodeling complex with C/EBPα, thereby hampering transactivation. In this review, the functional implications of post-translational modification, particularly sumoylation, of C/EBPα in normal granulopoiesis and leukemia are considered.
transcription factor; myeloid leukemia; post-translational modification
We used the paradigmatic GATA-PU.1 axis to explore, at the systems level, dynamic relationships between transcription factor (TF) binding and global gene expression programs as multipotent cells differentiate. We combined global ChIP-seq of GATA1, GATA2, and PU.1 with expression profiling during differentiation to erythroid and neutrophil lineages. Our analysis reveals (1) differential complexity of sequence motifs bound by GATA1, GATA2, and PU.1; (2) the scope and interplay of GATA1 and GATA2 programs within, and during transitions between, different cell compartments, and the extent of their hard-wiring by DNA motifs; (3) the potential to predict gene expression trajectories based on global associations between TF-binding data and target gene expression; and (4) how dynamic modeling of DNA-binding and gene expression data can be used to infer regulatory logic of TF circuitry. This rubric exemplifies the utility of this cross-platform resource for deconvoluting the complexity of transcriptional programs controlling stem/progenitor cell fate in hematopoiesis.
•Cross-platform resource for TF-network regulation of multipotent blood cell fate•DNA motif dependence and changing specificity of GATA factors in lineage choice•Modeling-based inference identifies GATA2 repression of PU.1 in multipotent cells•Priming, recruitment, and switching modes of GATA interplay during differentiation
A systems-level resource of the GATA-PU.1 axis provides insight into the dynamics of transcriptional programs during hematopoietic lineage commitment and differentiation.
In response to severe bacterial infection, bone marrow hematopoietic activity shifts toward promoting granulopoiesis. The underlying cell signaling mechanisms remain obscure. To study the role of Toll-like receptor 4 (TLR4)/stem cell antigen-1 (Sca-1) signaling in this process, bacteremia was induced in mice by intravenous injection of Escherichia coli. A subgroup of animals also received intravenous 5-bromo-2-deoxyuridine (BrdU). In a separate set of experiments, bone marrow lineage-negative (lin−) stem cell growth factor receptor-positive (c-kit+) Sca-1− cells containing primarily common myeloid progenitors were cultured in vitro without or with E. coli lipopolysaccharide (LPS). In genotypic background control mice, bacteremia significantly upregulated Sca-1 expression by lin− c-kit+ cells, as reflected by a marked increase in BrdU-negative lin− c-kit+ Sca-1+ cells in the bone marrow. In mice with the TLR4 gene deletion, this bacteremia-evoked Sca-1 response was blocked. In vitro, LPS induced a dose-dependent increase in Sca-1 expression by cultured marrow lin− c-kit+ Sca-1− cells. LPS-induced upregulation of Sca-1 expression was regulated at the transcriptional level. Inhibition of c-Jun N-terminal kinase/stress-activated protein kinase (JNK) activity with the specific inhibitor SP600125 suppressed LPS-induced upregulation of Sca-1 expression by marrow lin− c-kit+ Sca-1− cells. Engagement of Sca-1 with anti-Sca-1 antibodies enhanced the expression of Sfpi1 spleen focus-forming virus (SFFV) proviral integration 1 (PU.1) in marrow lin− c-kit+ Sca-1− cells cultured with LPS. Sca-1 null mice failed to maintain the marrow pool of granulopoietic cells following bacteremia. These results demonstrate that TLR4/Sca-1 signaling plays an important role in the regulation of hematopoietic precursor cell programming and their enhancement of granulocyte lineage commitment in response to E. coli bacteremia.
The granulocyte colony-stimulating factor receptor (G-CSFR) plays a critical role in maintaining homeostatic levels of circulating neutrophils (PMN). The mechanisms modulating G-CSFR surface expression to prevent chronic neutrophilia are poorly understood. Here, we report that neutrophil elastase (NE) proteolytically cleaves the G-CSFR on human PMN and blocks G-CSFR-mediated granulopoiesis in vitro.
Human peripheral blood PMN isolated from healthy donors were incubated with NE. Expression of the G-CSFR was analyzed by flow cytometry and western blot analyses. Detection of G-CSFR cleavage products from the culture supernatants was also performed. Human bone marrow mononuclear cells were also cultured in the presence or absence of NE to determine its effects on the proliferation of granulocyte-macrophage colony forming units (CFU-GM).
Treatment of PMN with NE induced a time-dependent decrease in G-CSFR expression that correlated with its degradation and the appearance of proteolytic cleavage fragments in conditioned media. Immunoblot analysis confirmed the G-CSFR was cleaved at its amino-terminus. Treatment of progenitor cells with NE prior to culture inhibited the growth of granulocyte-macrophage colony forming units.
These findings indicate that in addition to transcriptional controls and ligand-induced internalization, direct proteolytic cleavage of the G-CSFR by NE also downregulates G-CSFR expression and inhibits G-CSFR-mediated granulopoiesis in vitro. Our results suggest that NE negatively regulates granulopoiesis through a novel negative feedback loop.
During bacterial infection, the bone marrow hematopoietic activity shifts toward granulocyte production, which is critical for host defenses. Along with this enhancement of granulopoiesis, the bone marrow also increases its release of hematopoietic precursors. At the present time, little is known about the commitment of hematopoietic precursor cells including hematopoietic stem cells and progenitors in this response. To investigate the hematopoietic precursor cell response to bacterial infection, bacteremia was established in Balb/c mice by intravenous injection of Escherichia coli. Bacteremia caused a 10-fold increase in the number of lineage (lin)-c-kit+Sca-1+ cells in the bone marrow. This dramatic expansion of the lin-c-kit+Sca-1+ cell pool resulted from both increased mitosis of these cells and inversion from lin-c-kit+Sca-1- cell phenotype. Lipopolysaccharide, tumor necrosis factor-α, and interleukin-6 were potent factors capable of mediating phenotypic inversion of lin-c-kit+Sca-1- cells. Cells in the expanded lin-c-kit+Sca-1+ cell pool contained an increased number of colony-forming unit-granulocyte/macrophage (CFU-GM). Mobilization of lin-c-kit+Sca-1+ cells into the circulation was significantly enhanced following bacteremia. These results demonstrate that the lin-c-kit+Sca-1+ cell population in the bone marrow constitutes a key component of the host defense response to bacteremia. Functional modifications of these primitive hematopoietic precursors are critical for enhancing granulocyte production following bacterial infection.
Mouse; Stem/progenitor Cells; Granulopoiesis; Hematopoiesis; Bone Marrow; Immunity
CCAAT/enhancer binding protein (C/EBP)α is a myeloid-specific transcription factor that couples lineage commitment to terminal differentiation and cell cycle arrest, and is found mutated in 9% of patients who have acute myeloid leukemia (AML). We previously showed that mutations which dissociate the ability of C/EBPα to block cell cycle progression through E2F inhibition from its function as a transcriptional activator impair the in vivo development of the neutrophil granulocyte and adipose lineages. We now show that such mutations increase the capacity of bone marrow (BM) myeloid progenitors to proliferate, and predispose mice to a granulocytic myeloproliferative disorder and transformation of the myeloid compartment of the BM. Both of these phenotypes were transplantable into lethally irradiated recipients. BM transformation was characterized by a block in granulocyte differentiation, accumulation of myeloblasts and promyelocytes, and expansion of myeloid progenitor populations—all characteristics of AML. Circulating myeloblasts and hepatic leukocyte infiltration were observed, but thrombocytopenia, anemia, and elevated leukocyte count—normally associated with AML—were absent. These results show that disrupting the cell cycle regulatory function of C/EBPα is sufficient to initiate AML-like transformation of the granulocytic lineage, but only partially the peripheral pathology of AML.
Although carcinoembryonic antigen-related cell adhesion moclecule-1 (CEACAM1) is an activation marker for neutrophils and delays neutrophil apoptosis, the role of CEACAM1 in granulopoiesis and neutrophil dependent host immune responses has not been investigated. CEACAM1 expression correlated with granulocytic differentiation, and Ceacam1−/− mice developed neutrophilia due to loss of the Src- homology-phosphatase-1 (SHP-1) dependent inhibition of granulocyte-colony stimulating factor receptor (G-CSFR)-signal transducer and activator of transcription (Stat3) pathway provided by CEACAM1. Moreover, Ceacam1−/− mice were hypersensitive to Listeria Monocytogenes (LM) infection with an accelerated mortality. Reintroduction of CEACAM1 into Ceacam1−/− bone marrow restored normal granulopoiesis and host sensitivity to LM infection, while mutation of its immunoreceptor tyrosine-based inhibitory motifs (ITIMs) abrogated this restoration. shRNA mediated reduction of Stat3 amounts rescued normal granulopoiesis attenuating host sensitivity to LM infection in Ceacam1−/− mice. Thus, CEACAM1 acted as a co-inhibitory receptor for G-CSFR regulating granulopoiesis and host innate immune response to bacterial infections.