The order of expression of transcription factors directs hierarchical specification of hematopoietic lineages

The Poly(C) Binding Protein Pcbp2 and Its Retrotransposed Derivative Pcbp1 Are Independently Essential to Mouse Development

RNA-binding proteins participate in a complex array of posttranscriptional controls essential to cell type specification and somatic development. Despite their detailed biochemical characterizations, the degree to which each RNA-binding protein impacts mammalian embryonic development remains incompletely defined, and the level of functional redundancy among subsets of these proteins remains open to question. The poly(C) binding proteins, PCBPs (αCPs and hnRNP E proteins), are encoded by a highly conserved and broadly expressed gene family. The two major Pcbp isoforms, Pcbp2 and Pcbp1, are robustly expressed in a wide range of tissues and exert both nuclear and cytoplasmic controls over gene expression. Here, we report that Pcbp1-null embryos are rendered nonviable in the peri-implantation stage. In contrast, Pcbp2-null embryos undergo normal development until midgestation (12.5 to 13.5 days postcoitum), at which time they undergo a dramatic loss in viability associated with combined cardiovascular and hematopoietic abnormalities. Mice heterozygous for either Pcbp1 or Pcbp2 null alleles display a mild and nondisruptive defect in initial postpartum weight gain. These data reveal that Pcbp1 and Pcbp2 are individually essential for mouse embryonic development and have distinct impacts on embryonic viability and that Pcpb2 has a nonredundant in vivo role in hematopoiesis. These data further provide direct evidence that Pcbp1, a retrotransposed derivative of Pcpb2, has evolved an essential function(s) in the mammalian genome.

Minimal PU.1 reduction induces a preleukemic state and promotes development of acute myeloid leukemia

Modest transcriptional changes caused by genetic or epigenetic mechanisms are frequent in human cancer. Although loss or near-complete loss of the hematopoietic transcription factor PU.1 induces acute myeloid leukemia (AML) in mice, a similar degree of PU.1 impairment is exceedingly rare in human AML; yet, moderate PU.1 inhibition is common in AML patients. We assessed functional consequences of modest reductions in PU.1 expression on leukemia development in mice harboring DNA lesions resembling those acquired during human stem cell aging. Heterozygous deletion of an enhancer of PU.1, which resulted in a 35% reduction of PU.1 expression, was sufficient to induce myeloid-biased preleukemic stem cells and their subsequent transformation to AML in a DNA mismatch repair-deficient background. AML progression was mediated by inhibition of expression of a PU.1-cooperating transcription factor, Irf8. Notably, we found marked molecular similarities between the disease in these mice and human myelodysplastic syndrome and AML. This study demonstrates that minimal reduction of a key lineage-specific transcription factor, which commonly occurs in human disease, is sufficient to initiate cancer development, and it provides mechanistic insight into the formation and progression of preleukemic stem cells in AML.

Ctr2 Regulates Mast Cell Maturation by Affecting the Storage and Expression of Tryptase and Proteoglycans

Copper (Cu) is essential for multiple cellular functions. Cellular uptake of Cu(+) is carried out by the Ctr1 high-affinity Cu transporter. The mobilization of endosomal Cu pools is regulated by a protein structurally similar to Ctr1, called Ctr2. It was recently shown that ablation of Ctr2 caused an increase in the concentration of Cu localized to endolysosomes. However, the biological significance of excess endolysosomal Cu accumulation has not been assessed. In this study, we addressed this issue by investigating the impact of Ctr2 deficiency on mast cells, a cell type unusually rich in endolysosomal organelles (secretory granules). We show that Ctr2(-/-) mast cells have increased intracellular Cu concentrations and that the absence of Ctr2 results in increased metachromatic staining, the latter indicating an impact of Ctr2 on the storage of proteoglycans in the secretory granules. In agreement with this, the absence of Ctr2 caused a skewed ratio between proteoglycans of heparin and chondroitin sulfate type, with increased amounts of heparin accompanied by a reduction of chondroitin sulfate. Moreover, transmission electron microscopy analysis revealed a higher number of electron-dense granules in Ctr2(-/-) mast cells than in wild-type cells. The increase in granular staining and heparin content is compatible with an impact of Ctr2 on mast cell maturation and, in support of this, the absence of Ctr2 resulted in markedly increased mRNA expression, storage, and enzymatic activity of tryptase. Taken together, the present study introduces Ctr2 and Cu as novel actors in the regulation of mast cell maturation and granule homeostasis.

Transcription Factor Repertoire of Homeostatic Eosinophilopoiesis

The production of mature eosinophils (Eos) is a tightly orchestrated process with the aim to sustain normal Eos levels in tissues while also maintaining low numbers of these complex and sensitive cells in the blood. To identify regulators of homeostatic eosinophilopoiesis in mice, we took a global approach to identify genome-wide transcriptome and epigenome changes that occur during homeostasis at critical developmental stages, including Eos-lineage commitment and lineage maturation. Our analyses revealed a markedly greater number of transcriptome alterations associated with Eos maturation (1199 genes) than with Eos-lineage commitment (490 genes), highlighting the greater transcriptional investment necessary for differentiation. Eos-lineage-committed progenitors (EoPs) were noted to express high levels of granule proteins and contain granules with an ultrastructure distinct from that of mature resting Eos. Our analyses also delineated a 976-gene Eos-lineage transcriptome that included a repertoire of 56 transcription factors, many of which have never previously been associated with Eos. EoPs and Eos, but not granulocyte-monocyte progenitors or neutrophils, expressed Helios and Aiolos, members of the Ikaros family of transcription factors, which regulate gene expression via modulation of chromatin structure and DNA accessibility. Epigenetic studies revealed a distinct distribution of active chromatin marks between genes induced with lineage commitment and genes induced with cell maturation during Eos development. In addition, Aiolos and Helios binding sites were significantly enriched in genes expressed by EoPs and Eos with active chromatin, highlighting a potential novel role for Helios and Aiolos in regulating gene expression during Eos development.

Prospective isolation of human erythroid lineage-committed progenitors

Determining the developmental pathway leading to erythrocytes and being able to isolate their progenitors are crucial to understanding and treating disorders of red cell imbalance such as anemia, myelodysplastic syndrome, and polycythemia vera. Here we show that the human erythrocyte progenitor (hEP) can be prospectively isolated from adult bone marrow. We found three subfractions that possessed different expression patterns of CD105 and CD71 within the previously defined human megakaryocyte/erythrocyte progenitor (hMEP; Lineage(-) CD34(+) CD38(+) IL-3Rα(-) CD45RA(-)) population. Both CD71(-) CD105(-) and CD71(+) CD105(-) MEPs, at least in vitro, still retained bipotency for the megakaryocyte (MegK) and erythrocyte (E) lineages, although the latter subpopulation is skewed in differentiation toward the erythroid lineage. Notably, the proliferative and differentiation output of the CD71(intermediate(int)/+) CD105(+) subset of cells within the MEP population was completely restricted to the erythroid lineage with the loss of MegK potential. CD71(+) CD105(-) MEPs are erythrocyte-biased MEPs (E-MEPs) and CD71(int/+) CD105(+) cells are EPs. These previously unclassified populations may facilitate further understanding of the molecular mechanisms governing human erythroid development and serve as potential therapeutic targets in disorders of the erythroid lineage.

Murine and human mast cell progenitors

The development of mature mast cells (MCs) from hematopoietic progenitor cells as well as the identification and characterization of committed progenitor cells are a current focus of mast cell research. Most published reports in this area are on the origin and differentiation of MCs in mice. Evidence for the human system, i.e. derived from primary human MCs, is widely lacking. Based on the published data, MCs develop either from a committed progenitor or from a common basophil/mast cell precursor. This review summarizes the current knowledge on MC development and MC differentiation.

The transcription factor XBP1 is selectively required for eosinophil differentiation

The transcription factor XBP1 has been linked to the development of highly secretory tissues such as plasma cells and Paneth cells, yet its function in granulocyte maturation has remained unknown. Here we discovered an unexpectedly selective and absolute requirement for XBP1 in eosinophil differentiation without an effect on the survival of basophils or neutrophils. Progenitors of myeloid cells and eosinophils selectively activated the endoribonuclease IRE1α and spliced Xbp1 mRNA without inducing parallel endoplasmic reticulum (ER) stress signaling pathways. Without XBP1, nascent eosinophils exhibited massive defects in the post-translational maturation of key granule proteins required for survival, and these unresolvable structural defects fed back to suppress critical aspects of the transcriptional developmental program. Hence, we present evidence that granulocyte subsets can be distinguished by their differential reliance on secretory-pathway homeostasis.

Distinct Sources of Hematopoietic Progenitors Emerge before HSCs and Provide Functional Blood Cells in the Mammalian Embryo

Hematopoietic potential arises in mammalian embryos before adult-repopulating hematopoietic stem cells (HSCs). At embryonic day 9.5 (E9.5), we show the first murine definitive erythro-myeloid progenitors (EMPs) have an immunophenotype distinct from primitive hematopoietic progenitors, maturing megakaryocytes and macrophages, and rare B cell potential. EMPs emerge in the yolk sac with erythroid and broad myeloid, but not lymphoid, potential. EMPs migrate to the fetal liver and rapidly differentiate, including production of circulating neutrophils by E11.5. Although the surface markers, transcription factors, and lineage potential associated with EMPs overlap with those found in adult definitive hematopoiesis, they are present in unique combinations or proportions that result in a specialized definitive embryonic progenitor. Furthermore, we find that embryonic stem cell (ESC)-derived hematopoiesis recapitulates early yolk sac hematopoiesis, including primitive, EMP, and rare B cell potential. EMPs do not have long-term potential when transplanted in immunocompromised adults, but they can provide transient adult-like RBC reconstitution.

The +37 kb Cebpa Enhancer Is Critical for Cebpa Myeloid Gene Expression and Contains Functional Sites that Bind SCL, GATA2, C/EBPα, PU.1, and Additional Ets Factors

The murine Cebpa gene contains an evolutionarily conserved 453 bp enhancer located at +37 kb that, together with its promoter, directs expression to myeloid progenitors and to long-term hematopoietic stem cells in transgenic mice. In human acute myeloid leukemia cases, the enhancer lacks point mutations but binds the RUNX1-ETO oncoprotein. The enhancer contains the H3K4me1 and H3K27Ac histone modifications, denoting an active enhancer, at progressively increasing levels as long-term hematopoietic stem cells transition to granulocyte-monocyte progenitors. We previously identified four enhancer sites that bind RUNX1 and demonstrated that their integrity is required for maximal enhancer activity in 32Dcl3 myeloid cells. The +37 kb Cebpa enhancer also contains C/EBP, Ets factor, Myb, GATA, and E-box consensus sites conserved in the human +42 kb CEBPA enhancer. Mutation of the two C/EBP, seven Ets, one Myb, two GATA, or two E-box sites reduces activity of an enhancer-promoter reporter in 32Dcl3 cells. In 293T gel shift assays, exogenous C/EBPα binds both C/EBP sites, c-Myb binds the Myb site, PU.1 binds the second Ets site, PU.1, Fli-1, ERG, and Ets1 bind the sixth Ets site, GATA2 binds both GATA sites, and SCL binds the second E-box. Endogenous hematopoietic RUNX1, PU.1, Fli-1, ERG, C/EBPα, GATA2, and SCL were previously shown to bind the enhancer, and we find that endogenous PU.1 binds the second Ets site in 32Dcl3 cells. Using CRISPR/Cas9, we developed 32Dcl3 lines in which the wild-type enhancer alleles are replaced with a variant mutant in the seven Ets sites. These lines have 20-fold reduced Cebpa mRNA when cultured in IL-3 or G-CSF, demonstrating a critical requirement for enhancer integrity for optimal Cebpa expression. In addition, these results indicate that the +37 kb Cebpa enhancer is the focus of multiple regulatory transcriptional pathways that impact its expression during normal hematopoiesis and potentially during myeloid transformation.

GATA2 is critical for the maintenance of cellular identity in differentiated mast cells derived from mouse bone marrow

GATA2 plays a crucial role for the mast cell fate decision. We herein demonstrate that GATA2 is also required for the maintenance of the cellular identity in committed mast cells derived from mouse bone marrow (BMMCs). The deletion of the GATA2 DNA binding domain (GATA2ΔCF) in BMMCs resulted in a loss of the mast cell phenotype and an increase in the number of CD11b- and/or Ly6G/C-positive cells. These cells showed the ability to differentiate into macrophage- and neutrophil-like cells but not into eosinophils. Although the mRNA levels of basophil-specific genes were elevated, CD49b, a representative basophil marker, never appeared on these cells. GATA2 ablation led to a significant upregulation of C/EBPα, and forced expression of C/EBPα in wild-type BMMCs phenocopied the GATA2ΔCF cells. Interestingly, simultaneous deletion of the Gata2 and Cebpa genes in BMMCs restored the aberrant increases of CD11b and Ly6G/C while retaining the reduced c-Kit expression. Chromatin immunoprecipitation assays indicated that GATA2 directly binds to the +37-kb region of the Cebpa gene and thereby inhibits the RUNX1 and PU.1 binding to the neighboring region. Upregulation of C/EBPα following the loss of GATA2 was not observed in cultured mast cells derived from peritoneal fluid, whereas the repression of c-Kit and other mast cell-specific genes were observed in these cells. Collectively, these results indicate that GATA2 maintains cellular identity by preventing Cebpa gene activation in a subpopulation of mast cells, whereas it plays a fundamental role as a positive regulator of mast cell-specific genes throughout development of this cell lineage.

The STAT5-GATA2 pathway is critical in basophil and mast cell differentiation and maintenance

Transcription factor GATA binding protein 2 (GATA2) plays critical roles in hematopoietic stem cell survival and proliferation, granulocyte-monocyte progenitor differentiation, and basophil and mast cell differentiation. However, precise roles of GATA2 in basophil and mast cell differentiation and maintenance have not been delineated. We have identified GATA2 as an essential transcription factor in differentiation of newly identified common basophil and mast cell progenitors into basophils and mast cells. We observed Gata2 haploinsufficiency for mast cell differentiation, but not for basophil differentiation. We examined the precise role of GATA2 in maintaining the expression of a wide range of genes that are important for performing basophil or mast cell functions. The effects of GATA2 on gene expression were broadly based. We demonstrated that GATA2 was required for maintaining Fcer1a mRNA and FcεRIα protein expression on both basophils and mast cells, as well as for maintaining Kit mRNA and c-Kit protein expression on mast cells. GATA2 was required for histamine synthesis and was also critical for Il4 mRNA expression in basophils and Il13 mRNA expression in mast cells. We demonstrate a STAT5-GATA2 connection, showing that the STAT5 transcription factor directly bound to the promoter and an intronic region of the Gata2 gene. Overexpression of the Gata2 gene was sufficient to direct basophil and mast cell differentiation in the absence of the Stat5 gene. Our study reveals that the STAT5-GATA2 pathway is critical for basophil and mast cell differentiation and maintenance.

C/EBPα in normal and malignant myelopoiesis

CCAAT/enhancer binding protein α (C/EBPα) dimerizes via its leucine zipper (LZ) domain to bind DNA via its basic region and activate transcription via N-terminal trans-activation domains. The activity of C/EBPα is modulated by several serine/threonine kinases and via sumoylation, its gene is activated by RUNX1 and additional transcription factors, its mRNA stability is modified by miRNAs, and its mRNA is subject to translation control that affects AUG selection. In addition to inducing differentiation, C/EBPα inhibits cell cycle progression and apoptosis. Within hematopoiesis, C/EBPα levels increase as long-term stem cells progress to granulocyte-monocyte progenitors (GMP). Absence of C/EBPα prevents GMP formation, and higher levels are required for granulopoiesis compared to monopoiesis. C/EBPα interacts with AP-1 proteins to bind hybrid DNA elements during monopoiesis, and induction of Gfi-1, C/EBPε, KLF5, and miR-223 by C/EBPα enables granulopoiesis. The CEBPA ORF is mutated in approximately 10 % of acute myeloid leukemias (AML), leading to expression of N-terminally truncated C/EBPαp30 and C-terminal, in-frame C/EBPαLZ variants, which inhibit C/EBPα activities but also play additional roles during myeloid transformation. RUNX1 mutation, CEBPA promoter methylation, Trib1 or Trib2-mediated C/EBPαp42 degradation, and signaling pathways leading to C/EBPα serine 21 phosphorylation reduce C/EBPα expression or activity in additional AML cases.

Transcription factor-mediated reprogramming toward hematopoietic stem cells

De novo generation of human hematopoietic stem cells (HSCs) from renewable cell types has been a long sought-after but elusive goal in regenerative medicine. Paralleling efforts to guide pluripotent stem cell differentiation by manipulating developmental cues, substantial progress has been made recently toward HSC generation via combinatorial transcription factor (TF)-mediated fate conversion, a paradigm established by Yamanaka's induction of pluripotency in somatic cells by mere four TFs. This review will integrate the recently reported strategies to directly convert a variety of starting cell types toward HSCs in the context of hematopoietic transcriptional regulation and discuss how these findings could be further developed toward the ultimate generation of therapeutic human HSCs.

Mast cell plasticity and sphingosine-1-phosphate in immunity, inflammation and cancer

Mast cells (MC) are found in all vascularized tissues at homeostasis and, until recently, were viewed only as effector cells of allergic reactions via degranulation, the canonical process through which MC release mediators, including histamine and pre-formed proteases and cytokines such as TNF. Cross-linking of IgE bound to surface high affinity receptors for IgE (FcɛRI) by a specific antigen (Ag) triggers signaling events leading to degranulation. We and others have reported the concomitant production and export of an influential multifaceted sphingolipid mediator, sphingosine-1-phosphate (S1P) transported outside of MC by ATP-binding cassettes (ABC) transporters, i.e., independently of degranulation. Indeed, the MC horizon expanded by the discovery of their unique ability to selectively release mediators depending upon the stimulus and receptors involved. Aside from degranulation and transporter usage, MC are also endowed with piecemeal degranulation, a slower process during which mediator release occurs with minor morphological changes. The broad spectrum of pro- and anti-inflammatory bioactive substances MC produce and release, their amounts and delivery pace render these cells bona fide fine-tuners of the immune response. In this viewpoint article, MC developmental, phenotypic and functional plasticity, its modulation by microRNAs and its relevance to immunity, inflammation and cancer will be discussed.

Transcription factor IRF8 plays a critical role in the development of murine basophils and mast cells

Basophils and mast cells play critical roles in host defense against pathogens and allergic disorders. However, the molecular mechanism by which these cells are generated is not completely understood. Here we demonstrate that interferon regulatory factor-8 (IRF8), a transcription factor essential for the development of several myeloid lineages, also regulates basophil and mast cell development. Irf8(-/-) mice displayed a severe reduction in basophil counts, which was accounted for by the absence of pre-basophil and mast cell progenitors (pre-BMPs). Although Irf8(-/-) mice retained peripheral tissue mast cells, remaining progenitors from Irf8(-/-) mice including granulocyte progenitors (GPs) were unable to efficiently generate either basophils or mast cells, indicating that IRF8 also contributes to the development of mast cells. IRF8 appeared to function at the GP stage, because IRF8 was expressed in GPs, but not in basophils, mast cells, and basophil/mast cell-restricted progenitor cells. Furthermore, we demonstrate that GATA2, a transcription factor known to promote basophil and mast cell differentiation, acts downstream of IRF8. These results shed light on the pathways and mechanism underlying the development of basophils and mast cells.

C/EBPα controls mast cell function

CCAAT/enhancer binding protein alpha (C/EBPα) is a transcription factor that influences immune cell fate and differentiation. However, the effect of C/EBPα on mast cells is not fully understood. In this study, we showed that C/EBPα suppressed granule formation in mast cells and increased macrophage inflammatory protein (MIP)-2 production from mast cells upon bacterial stimulation. These results indicate that C/EBPα regulates the balance between the allergic response and the innate immune response of mast cells. Furthermore, we showed that stimulation of mast cells with the Lactobacillus casei JCM1134(T) strain during late differentiation up-regulated C/EBPα expression in differentiated mast cells. This suggests that intestinal commensal bacteria modulate C/EBPα expression and thereby regulate mast cell function.

Transcriptional diversity during lineage commitment of human blood progenitors

Blood cells derive from hematopoietic stem cells through stepwise fating events. To characterize gene expression programs driving lineage choice, we sequenced RNA from eight primary human hematopoietic progenitor populations representing the major myeloid commitment stages and the main lymphoid stage. We identified extensive cell type-specific expression changes: 6711 genes and 10,724 transcripts, enriched in non-protein-coding elements at early stages of differentiation. In addition, we found 7881 novel splice junctions and 2301 differentially used alternative splicing events, enriched in genes involved in regulatory processes. We demonstrated experimentally cell-specific isoform usage, identifying nuclear factor I/B (NFIB) as a regulator of megakaryocyte maturation-the platelet precursor. Our data highlight the complexity of fating events in closely related progenitor populations, the understanding of which is essential for the advancement of transplantation and regenerative medicine.

Mechanisms controlling mast cell and basophil lineage decisions

Basophils and mast cells have long been known to play critical roles in allergic disease and host defense against parasitic infections. Recent recognition of these effector cells in immune regulations, host defense against bacteria and virus, and autoimmune diseases entices increased interest in studying these cells. However, origin and molecular regulation of basophil and mast cell differentiation remain incompletely understood. In this review, we focus on recent advances of the understanding the origin and molecular regulation of mouse basophil and mast cell development. We also summarize progress in the understanding of the origin and molecular regulation of human basophil and mast cell development. A more complete understanding of molecular regulation of basophils and mast cells will lead to the development of interventions that are more effective in achieving long-term success.

Basophils have emerged as a key player in immunity

Basophils had long been neglected in immunological studies, because of their paucity and phenotypic similarity with tissue-resident mast cells. However, recent development of analytical tools has cast new light on this neglected minority, and revealed previously unappreciated roles of basophils, distinct from those of mast cells, in various immune responses. Primary function of basophils appears to be the protection against infections with parasites, including ticks and helminths. This is why basophils are evolutionally conserved well in many animal species, albeit a small number. Nevertheless, basophils sometimes exert host-deleterious functions in immunological disorders such as allergy. Here we summarize recent advance in our understanding of basophil ontogeny and their in vivo roles under physiological and pathological conditions.

Reprogramming committed murine blood cells to induced hematopoietic stem cells with defined factors

Hematopoietic stem cells (HSCs) sustain blood formation throughout life and are the functional units of bone marrow transplantation. We show that transient expression of six transcription factors Run1t1, Hlf, Lmo2, Prdm5, Pbx1, and Zfp37 imparts multilineage transplantation potential onto otherwise committed lymphoid and myeloid progenitors and myeloid effector cells. Inclusion of Mycn and Meis1 and use of polycistronic viruses increase reprogramming efficacy. The reprogrammed cells, designated induced-HSCs (iHSCs), possess clonal multilineage differentiation potential, reconstitute stem/progenitor compartments, and are serially transplantable. Single-cell analysis revealed that iHSCs derived under optimal conditions exhibit a gene expression profile that is highly similar to endogenous HSCs. These findings demonstrate that expression of a set of defined factors is sufficient to activate the gene networks governing HSC functional identity in committed blood cells. Our results raise the prospect that blood cell reprogramming may be a strategy for derivation of transplantable stem cells for clinical application.

Cytokine signaling in the differentiation of innate effector cells

Innate effector cells, including innate effector cells of myeloid and lymphoid lineages, are crucial components of various types of immune responses. Bone marrow progenitors differentiate into many subsets of innate effector cells after receiving instructional signals often provided by cytokines. Signal transducer and activator of transcription (STATs) have been shown to be essential in the differentiation of various types of innate effector cells. In this review, we focus specifically on the differentiation of innate effector cells, particularly the role of cytokine signaling in the differentiation of innate effector cells.

Aging-like phenotype and defective lineage specification in SIRT1-deleted hematopoietic stem and progenitor cells

Aging hematopoietic stem cells (HSCs) exhibit defective lineage specification that is thought to be central to increased incidence of myeloid malignancies and compromised immune competence in the elderly. Mechanisms underlying these age-related defects remain largely unknown. We show that the deacetylase Sirtuin (SIRT)1 is required for homeostatic HSC maintenance. Differentiation of young SIRT1-deleted HSCs is skewed toward myeloid lineage associated with a significant decline in the lymphoid compartment, anemia, and altered expression of associated genes. Combined with HSC accumulation of damaged DNA and expression patterns of age-linked molecules, these have striking overlaps with aged HSCs. We further show that SIRT1 controls HSC homeostasis via the longevity transcription factor FOXO3. These findings suggest that SIRT1 is essential for HSC homeostasis and lineage specification. They also indicate that SIRT1 might contribute to delaying HSC aging.

The Cebpa +37-kb enhancer directs transgene expression to myeloid progenitors and to long-term hematopoietic stem cells

C/EBPα is expressed preferentially in myeloid compared with lymphoid or erythroid cells and directs myeloid lineage specification. C/EBPα is also expressed at lower levels in HSCs and in several nonhematopoietic tissues. The Cebpa gene has a conserved, 450-bp segment at +37 kb that harbors enhancer-specific epigenetic marks and is activate in a myeloid cell line. Herein, we characterize transgenic C57BL/6 mice, in which the Cebpa enhancer and 845-bp promoter regulate a hCD4 reporter. FACS analysis, in vitro colony assays, and in vivo competitive and secondary transplantation revealed that myeloid but not MEPs or lymphoid progenitors and also functional LT-HSCs are found almost exclusively in the Cebpa-hCD4(+) compared with hCD4(-) marrow population. hCD4(+) CMP yielded predominantly myeloid, whereas hCD4(-) CMP generated mainly Meg/E colonies. Providing insight into control of CMP maturation, Cebpa and Pu.1 RNAs were preferentially expressed in hCD4(+) CMP, Scl, Gata2, Gata1, Klf1, Ets1, and Fli1 predominated in hCD4(-) CMP, and Runx1, Myb, HoxA9, and Erg levels were similar in both. Cebpa-hCD4 transgene expression was lacking in multiple nonhematopoietic tissues. In summary, the +37-kb Cebpa enhancer and promoter are sufficient for marrow myeloid progenitor and LT-HSC-specific expression.

Identification of microRNAs regulating the developmental pathways of bone marrow derived mast cells

Background

MicroRNAs (miRNAs) play important roles in leukocyte differentiation, although those utilised for specific programs and key functions remain incompletely characterised. As a global approach to gain insights into the potential regulatory role of miRNA in mast cell differentiation we characterised expression in BM cultures from the initiation of differentiation. In cultures enriched in differentiating mast cells we characterised miRNA expression and identified miRNA targeting the mRNA of putative factors involved in differentiation pathways and cellular identity. Detailed pathway analysis identified a unique miRNA network that is intimately linked to the mast cell differentiation program.

Methodology/Principal Findings

We identified 86 unique miRNAs with expression patterns that were up- or down- regulated at 5-fold or more during bone marrow derived mast cells (BMMC) development. By employing TargetScan and MeSH databases, we identified 524 transcripts involved in 30 canonical pathways as potentially regulated by these specific 86 miRNAs. Furthermore, by applying miRanda and IPA analyses, we predict that 7 specific miRNAs of this group are directly associated with the expression of c-Kit and FcεRIα and likewise, that 18 miRNAs promote expression of Mitf, GATA1 and c/EBPα three core transcription factors that direct mast cell differentiation. Furthermore, we have identified 11 miRNAs that may regulate the expression of STATs-3, -5a/b, GATA2 and GATA3 during differentiation, along with 13 miRNAs that target transcripts encoding Ndst2, mMCP4 and mMCP6 and thus may regulate biosynthesis of mast cell secretory mediators.

Conclusions/Significance

This investigation characterises changes in miRNA expression in whole BM cultures during the differentiation of mast cells and predicts functional links between miRNAs and their target mRNAs for the regulation of development. This information provides an important resource for further investigations of the contributions of miRNAs to mast cell differentiation and function.

Expression profiling of differentiating eosinophils in bone marrow cultures predicts functional links between microRNAs and their target mRNAs

BACKGROUND

MicroRNAs (miRNAs) are small non-coding RNAs that regulate complex transcriptional networks underpin immune responses. However, little is known about the specific miRNA networks that control differentiation of specific leukocyte subsets. In this study, we profiled miRNA expression during differentiation of eosinophils from bone marrow (BM) progenitors (bmEos), and correlated expression with potential mRNA targets involved in crucial regulatory functions. Profiling was performed on whole BM cultures to document the dynamic changes in miRNA expression in the BM microenvironment over the differentiation period. miRNA for network analysis were identified in BM cultures enriched in differentiating eosinophils, and chosen for their potential ability to target mRNA of factors that are known to play critical roles in eosinophil differentiation pathways or cell identify.

METHODOLOGY/PRINCIPAL FINDINGS

We identified 68 miRNAs with expression patterns that were up- or down- regulated 5-fold or more during bmEos differentiation. By employing TargetScan and MeSH databases, we identified 348 transcripts involved in 30 canonical pathways as potentially regulated by these miRNAs. Furthermore, by applying miRanda and Ingenuity Pathways Analysis (IPA), we identified 13 specific miRNAs that are temporally associated with the expression of IL-5Rα and CCR3 and 14 miRNAs associated with the transcription factors GATA-1/2, PU.1 and C/EBPε. We have also identified 17 miRNAs that may regulate the expression of TLRs 4 and 13 during eosinophil differentiation, although we could identify no miRNAs targeting the prominent secretory effector, eosinophil major basic protein.

CONCLUSIONS/SIGNIFICANCE

This is the first study to map changes in miRNA expression in whole BM cultures during the differentiation of eosinophils, and to predict functional links between miRNAs and their target mRNAs for the regulation of eosinophilopoiesis. Our findings provide an important resource that will promote the platform for further understanding of the role of these non-coding RNAs in the regulation of eosinophil differentiation and function.

Concise review: erythroid versus myeloid lineage commitment: regulating the master regulators

Developmental processes, like blood formation, are orchestrated by transcriptional networks. Those transcriptional networks are highly responsive to various environmental stimuli and affect common precursors resulting in increased production of cells of the erythroid lineage or myeloid lineage (granulocytes, neutrophils, and macrophages). A significant body of knowledge has accumulated describing transcription factors that drive differentiation of these two major cellular pathways, in particular the antagonistic master regulators such as GATA-1 and PU.1. However, little is known about factors that work upstream of master regulators to enhance differentiation toward one lineage. These functions become especially important under various stress conditions like sudden loss of red blood cells or pathogen infection. This review describes recent studies that begin to provide evidence for such factors. An increased understanding of factors regulating cellular commitment will advance our understanding of the etiology of diseases like anemia, cancer, and possibly other blood related disorders.

Mast cell progenitors: origin, development and migration to tissues

Mast cells in tissues are developed from mast cell progenitors emerging from the bone marrow in a process highly regulated by transcription factors. Through the advancement of the multicolor flow cytometry technique, the mast cell progenitor population in the mouse has been characterized in terms of surface markers. However, only cell populations with enriched mast cell capability have been described in human. In naïve mice, the peripheral tissues have a constitutive pool of mast cell progenitors. Upon infections in the gut and in allergic inflammation in the lung, the local mast cell progenitor numbers increase tremendously. This review focuses on the origin and development of mast cell progenitors. Furthermore, the evidences for cells and molecules that govern the migration of these cells in mice in vivo are described.

Epigenetic choreography of stem cells: the DNA demethylation episode of development

Reversible DNA methylation is a fundamental epigenetic manipulator of the genomic information in eukaryotes. DNA demethylation plays a very significant role during embryonic development and stands out for its contribution in molecular reconfiguration during cellular differentiation for determining stem cell fate. DNA demethylation arbitrated extensive make-over of the genome via reprogramming in the early embryo results in stem cell plasticity followed by commitment to the principal cell lineages. This article attempts to highlight the sequential phases and hierarchical mode of DNA demethylation events during enactment of the molecular strategy for developmental transition. A comprehensive knowledge regarding the pattern of DNA demethylation during embryogenesis and organogenesis and study of the related lacunae will offer exciting avenues for future biomedical research and stem cell-based regenerative therapy.

High c-Kit expression identifies hematopoietic stem cells with impaired self-renewal and megakaryocytic bias

c-Kit^lo HSCs exhibit enhanced self-renewal and long-term reconstitution potential and give rise to c-Kit^hi HSCs that have a megakaryocytic bias.

Hematopoietic stem cells (HSCs) are heterogeneous with respect to their self-renewal, lineage, and reconstitution potentials. Although c-Kit is required for HSC function, gain and loss-of-function c-Kit mutants suggest that even small changes in c-Kit signaling profoundly affect HSC function. Herein, we demonstrate that even the most rigorously defined HSCs can be separated into functionally distinct subsets based on c-Kit activity. Functional and transcriptome studies show HSCs with low levels of surface c-Kit expression (c-Kit^lo) and signaling exhibit enhanced self-renewal and long-term reconstitution potential compared with c-Kit^hi HSCs. Furthermore, c-Kit^lo and c-Kit^hi HSCs are hierarchically organized, with c-Kit^hi HSCs arising from c-Kit^lo HSCs. In addition, whereas c-Kit^hi HSCs give rise to long-term lymphomyeloid grafts, they exhibit an intrinsic megakaryocytic lineage bias. These functional differences between c-Kit^lo and c-Kit^hi HSCs persist even under conditions of stress hematopoiesis induced by 5-fluorouracil. Finally, our studies show that the transition from c-Kit^lo to c-Kit^hi HSC is negatively regulated by c-Cbl. Overall, these studies demonstrate that HSCs exhibiting enhanced self-renewal potential can be isolated based on c-Kit expression during both steady state and stress hematopoiesis. Moreover, they provide further evidence that the intrinsic functional heterogeneity previously described for HSCs extends to the megakaryocytic lineage.

GATA-1 regulates the generation and function of basophils

Developmental processes of hematopoietic cells are orchestrated by transcriptional networks. GATA-1, the founding member of the GATA family of transcription factors, has been demonstrated to play crucial roles in the differentiation of erythroid cells, magakaryocytes, eosinophils, and mast cells. However, the role of GATA-1 in basophils remains elusive. Here we show that basophils abundantly express Gata1 mRNAs, and that siRNA-mediated knockdown of Gata1 resulted in impaired production of IL-4 by basophils in response to the stimulation with IgE plus antigens. ΔdblGATA mice that carry the mutated Gata1 promoter and are widely used for functional analysis of eosinophils owing to their selective loss of eosinophils showed a decreased number of basophils with reduced expression of Gata1 mRNAs. The number of basophil progenitors in bone marrow was reduced in these mice, and the generation of basophils from their bone marrow cells in culture with IL-3 or thymic stromal lymphopoietin was impaired. ΔdblGATA basophils responded poorly ex vivo to stimulation with IgE plus antigens compared with wild-type basophils as assessed by degranulation and production of IL-4 and IL-6. Moreover, ΔdblGATA mice showed impaired responses in basophil-mediated protective immunity against intestinal helminth infection. Thus, ΔdblGATA mice showed numerical and functional aberrancy in basophils in addition to the known deficiency of eosinophils. Our findings demonstrate that GATA-1 plays a key role in the generation and function of basophils and underscore the need for careful distinction of the cell lineage responsible for each phenotype observed in ΔdblGATA mice.

COP1 targets C/EBPα for degradation and induces acute myeloid leukemia via Trib1

The ubiquitin ligase constitutively photomorphogenic 1 (COP1) is involved in many biological responses in mammalian cells, but its role in tumorigenesis remains unclear. Here we show that COP1 is a ubiquitin ligase for the tumor suppressor CCAAT/enhancer-binding protein (C/EBPα) and promotes its degradation in vivo, thereby blocking myeloid differentiation of hematopoietic cells for tumorigenesis. In this process, mammalian homolog of Tribbles, Trib1, which contains a COP1-binding motif, is essential for down-regulation of C/EBPα expression. Murine bone marrow transplantation experiments showed that coexpression of COP1 accelerates development of acute myeloid leukemia induced by Trib1, which pathologically resembles that of p42C/EBPα-deficient mice. Interestingly, coexpression of ligase activity-deficient COP1 mutant abrogated Trib1-induced leukemogenesis. These results indicate that COP1 and Trib1 act as an oncoprotein complex functioning upstream of C/EBPα, and its ligase activity is crucial for leukemogenesis.

Antagonistic regulation by the transcription factors C/EBPα and MITF specifies basophil and mast cell fates

It remains unclear whether basophils and mast cells are derived from a common progenitor. Furthermore, how basophil versus mast cell fate is specified has not been investigated. Here, we have identified a population of granulocyte-macrophage progenitors (GMPs) that were highly enriched in the capacity to differentiate into basophils and mast cells while retaining a limited capacity to differentiate into myeloid cells. We have designated these progenitor cells "pre-basophil and mast cell progenitors" (pre-BMPs). STAT5 signaling was required for the differentiation of pre-BMPs into both basophils and mast cells and was critical for inducing two downstream molecules: C/EBPα and MITF. We have identified C/EBPα as the critical basophil transcription factor for specifying basophil cell fate and MITF as the crucial transcription factor for specifying mast cell fate. C/EBPα and MITF silenced each other's transcription in a directly antagonistic fashion. Our study reveals how basophil and mast cell fate is specified.

Foundational concepts of cell fate conversion to the hematopoietic lineage

Direct conversion of cellular fate provides a potential approach to generate cells of the hematopoietic lineage without the requisite reversion to a pluripotent state via somatic cell reprogramming. The utilization of this technology has enabled transcription factor-mediated conversion of somatic cell types to primitive and mature hematopoietic cells. Recent studies demonstrate that the direct conversion of somatic cells to the hematopoietic lineage likely requires the use of pioneer transcription factors to establish an accessible chromatin state that is responsive to enforced expression of hematopoietic-specific transcription factors, in combination with appropriate culture conditions that facilitate reprogramming. Developing adaptable, experimental strategies that incorporate these parameters should enable the efficient generation of human hematopoietic cells with translational potential.

Revision of the human hematopoietic tree: granulocyte subtypes derive from distinct hematopoietic lineages

The classical model of hematopoiesis predicts a dichotomous lineage restriction of multipotent hematopoietic progenitors (MPPs) into common lymphoid progenitors (CLPs) and common myeloid progenitors (CMPs). However, this idea has been challenged by the identification of lymphoid progenitors retaining partial myeloid potential (e.g., LMPPs), implying that granulocytes can arise within both the classical lymphoid and the myeloid branches. Here, we resolve this issue by using cell-surface CD133 expression to discriminate functional progenitor populations. We show that eosinophilic and basophilic granulocytes as well as erythrocytes and megakaryocytes derive from a common erythro-myeloid progenitor (EMP), whereas neutrophilic granulocytes arise independently within a lympho-myeloid branch with long-term progenitor function. These findings challenge the concept of a CMP and restore dichotomy to the classical hematopoietic model.

Different GATA factors dictate CCR3 transcription in allergic inflammatory cells in a cell type-specific manner

The chemokine receptor CCR3 is expressed in prominent allergic inflammatory cells, including eosinophils, mast cells, and Th2 cells. We previously identified a functional GATA element within exon 1 of the CCR3 gene that is responsible for GATA-1-mediated CCR3 transcription. Because allergic inflammatory cells exhibit distinct expression patterns of different GATA factors, we investigated whether different GATA factors dictate CCR3 transcription in a cell type-specific manner. GATA-2 was expressed in EoL-1 eosinophilic cells, GATA-1 and GATA-2 were expressed in HMC-1 mast cells, and GATA-3 was preferentially expressed in Jurkat cells. Unlike a wild-type CCR3 reporter, reporters lacking the functional GATA element were not active in any of the three cell types, implying the involvement of different GATA factors in CCR3 transcription. RNA interference assays showed that small interfering RNAs specific for different GATA factors reduced CCR3 reporter activity in a cell type-specific fashion. Consistent with these findings, chromatin immunoprecipitation and EMSA analyses demonstrated cell type-specific binding of GATA factors to the functional GATA site. More importantly, specific inhibition of the CCR3 reporter activity by different GATA small interfering RNAs was well preserved in respective cell types differentiated from cord blood; in particular, GATA-3 was entirely responsible for reporter activity in Th2 cells and replaced the role predominantly played by GATA-1 and GATA-2. These results highlight a mechanistic role of GATA factors in which cell type-specific expression is the primary determinant of transcription of the CCR3 gene in major allergic inflammatory cells.

Protective and pathological roles of mast cells and basophils

Mast cells and basophils are potent effector cells of the innate immune system, and they have both beneficial and detrimental functions for the host. They are mainly implicated in pro-inflammatory responses to allergens but can also contribute to protection against pathogens. Although both cell types were identified more than 130 years ago by Paul Ehrlich, their in vivo functions remain poorly understood. The precursor cell populations that give rise to mast cells and basophils have recently been characterized and isolated. Furthermore, new genetically modified mouse strains have been developed, which enable more specific targeting of mast cells and basophils. Such advances offer new opportunities to uncover the true in vivo activities of these cells and to revisit their previously proposed effector functions.

Leveling Waddington: the emergence of direct programming and the loss of cell fate hierarchies

For decades, Waddington's concept of the 'epigenetic landscape' has served as an educative hierarchical model to illustrate the progressive restriction of cell differentiation potential during normal development. While still being highly valuable in the context of normal development, the Waddington model falls short of accommodating recent breakthroughs in cell programming. The advent of induced pluripotent stem (iPS) cells and advances in direct cell fate conversion (also known as transdifferentiation) suggest that somatic and pluripotent cell fates can be interconverted without transiting through distinct hierarchies. We propose a non-hierarchical 'epigenetic disc' model to explain such cell fate transitions, which provides an alternative landscape for modelling cell programming and reprogramming.

Basophil modulation by cytokine instruction

Basophils have recently been recognized as critical effector cells in allergic reactions and protective immunity against helminths. Precise characterization of basophil biology could help to develop specific therapies that interfere with differentiation, tissue recruitment, or induction of effector functions and thereby ameliorate allergic disorders. The development, homeostasis, and effector functions of basophils are tightly regulated by extrinsic signals and in particular by cytokines. IL-3, GM-CSF, and thymic stromal lymphopoietin activate the STAT5 pathway that promotes proliferation, activation, and cytokine secretion but also induces a negative feedback loop via Pim-1 and SOCS proteins. Basophils further express receptors for IL-18 and IL-33, which are associated with the signaling adaptor MyD88 and activate the NF-κB and MAP kinase pathways. This review focuses on positive and negative regulation of basophils by these cytokines.

Microenvironment-evoked cell lineage conversion: Shifting the focus from internal reprogramming to external forcing

Seeking possible ways to create replacement cells for the faded ones with deficits in functionality or quantity inspires comprehensive needs for cell lineage conversion. To fulfill this promise, reprogramming and microenvironment direction have been used to manipulate abundant cell fates. We briefly describe the evolution and fundamental insights of these two major strategies applied for lineage specification, comment generally on their current limitations, and analyze the orchestral interplay between them. We also present several future directions and discuss the potential clinical uses. Based on the relatively slight safety and technical issues, we conclude that microenvironment-evoked cell lineage conversion, instead of reprogramming, will be the shifting focus in regenerative medicine.

Hematopoiesis

Enormous numbers of adult blood cells are constantly regenerated throughout life from hematopoietic stem cells through a series of progenitor stages. Accessibility, robust functional assays, well-established prospective isolation, and successful clinical application made hematopoiesis the classical mammalian stem cell system. Most of the basic concepts of stem cell biology have been defined in this system. At the same time, many long-standing disputes in hematopoiesis research illustrate our still limited understanding. Here we discuss the embryonic development and lifelong maintenance of the hematopoietic system, its cellular components, and some of the hypotheses about the molecular mechanisms involved in controlling hematopoietic cell fates.

Transcription factor-mediated epigenetic reprogramming

Input from various signaling pathways in conjunction with specific transcription factors (TFs), noncoding RNAs, and epigenetic modifiers governs the maintenance of cellular identity. Endogenous or exogenous TFs operate within certain boundaries, which are set, in part, by the cell type-specific epigenetic landscape. Ectopic expression of selected TFs can override the cellular identity and induce reprogramming to alternative fates. In this minireview, we summarize many of the classic examples and a large number of recent studies that have taken advantage of TF-mediated reprogramming to produce cell types of biomedical relevance.

Lineage switching in acute leukemias: a consequence of stem cell plasticity?

Acute leukemias are the most common cancer in childhood and characterized by the uncontrolled production of hematopoietic precursor cells of the lymphoid or myeloid series within the bone marrow. Even when a relatively high efficiency of therapeutic agents has increased the overall survival rates in the last years, factors such as cell lineage switching and the rise of mixed lineages at relapses often change the prognosis of the illness. During lineage switching, conversions from lymphoblastic leukemia to myeloid leukemia, or vice versa, are recorded. The central mechanisms involved in these phenomena remain undefined, but recent studies suggest that lineage commitment of plastic hematopoietic progenitors may be multidirectional and reversible upon specific signals provided by both intrinsic and environmental cues. In this paper, we focus on the current knowledge about cell heterogeneity and the lineage switch resulting from leukemic cells plasticity. A number of hypothetical mechanisms that may inspire changes in cell fate decisions are highlighted. Understanding the plasticity of leukemia initiating cells might be fundamental to unravel the pathogenesis of lineage switch in acute leukemias and will illuminate the importance of a flexible hematopoietic development.

Definition of genetic events directing the development of distinct types of brain tumors from postnatal neural stem/progenitor cells

Although brain tumors are classified and treated based upon their histology, the molecular factors involved in the development of various tumor types remain unknown. In this study, we show that the type and order of genetic events directs the development of gliomas, central nervous system primitive neuroectodermal tumors, and atypical teratoid/rhabdoid-like tumors from postnatal mouse neural stem/progenitor cells (NSC/NPC). We found that the overexpression of specific genes led to the development of these three different brain tumors from NSC/NPCs, and manipulation of the order of genetic events was able to convert one established tumor type into another. In addition, loss of the nuclear chromatin-remodeling factor SMARCB1 in rhabdoid tumors led to increased phosphorylation of eIF2α, a central cytoplasmic unfolded protein response (UPR) component, suggesting a role for the UPR in these tumors. Consistent with this, application of the proteasome inhibitor bortezomib led to an increase in apoptosis of human cells with reduced SMARCB1 levels. Taken together, our findings indicate that the order of genetic events determines the phenotypes of brain tumors derived from a common precursor cell pool, and suggest that the UPR may represent a therapeutic target in atypical teratoid/rhabdoid tumors.

Role of misfolded N-CoR mediated transcriptional deregulation of Flt3 in acute monocytic leukemia (AML)-M5 subtype

The nuclear receptor co-repressor (N-CoR) is a key component of the generic multi-protein complex involved in transcriptional control. Flt3, a key regulator of hematopoietic cell growth, is frequently deregulated in AML (acute myeloid leukemia). Here, we report that loss of N-CoR-mediated transcriptional control of Flt3 due to misfolding, contributes to malignant growth in AML of the M5 subtype (AML-M5). An analysis of hematopoietic genes in AML cells led to the identification of Flt3 as a transcriptional target of N-CoR. Flt3 level was inversely related to N-CoR status in various leukemia cells. N-CoR was associated with the Flt3 promoter in-vivo, and a reporter driven by the Flt3 promoter was effectively repressed by N-CoR. Blocking N-CoR loss with Genistein; an inhibitor of N-CoR misfolding, significantly down-regulated Flt3 levels regardless of the Flt3 receptor mutational status and promoted the differentiation of AML-M5 cells. While stimulation of the Flt3 receptor with the Flt3 ligand triggered N-CoR loss, Flt3 antibody mediated blockade of Flt3 ligand-receptor binding led to N-CoR stabilization. Genetic ablation of N-CoR potentiated Flt3 ligand induced proliferation of BA/F3 cells. These findings suggest that N-CoR-induced repression of Flt3 might be crucial for limiting the contribution of the Flt3 signaling pathway on the growth potential of leukemic cells and its deregulation due to N-CoR loss in AML-M5, could contribute to malignant growth by conferring a proliferative advantage to the leukemic blasts. Therapeutic restoration of N-CoR function could thus be a useful approach in restricting the contribution of the Flt3 signaling pathway in AML-M5 pathogenesis.

Delineating nuclear reprogramming

Nuclear reprogramming is described as a molecular switch, triggered by the conversion of one cell type to another. Several key experiments in the past century have provided insight into the field of nuclear reprogramming. Previously deemed impossible, this research area is now brimming with new findings and developments. In this review, we aim to give a historical perspective on how the notion of nuclear reprogramming was established, describing main experiments that were performed, including (1) somatic cell nuclear transfer, (2) exposure to cell extracts and cell fusion, and (3) transcription factor induced lineage switch. Ultimately, we focus on (4) transcription factor induced pluripotency, as initiated by a landmark discovery in 2006, where the process of converting somatic cells to a pluripotent state was narrowed down to four transcription factors. The conception that somatic cells possess the capacity to revert to an immature status brings about huge clinical implications including personalized therapy, drug screening and disease modeling. Although this technology has potential to revolutionize the medical field, it is still impeded by technical and biological obstacles. This review describes the effervescent changes in this field, addresses bottlenecks hindering its advancement and in conclusion, applies the latest findings to overcome these issues.