Haematopoietic cell-fate decisions, chromatin regulation and ikaros

Epigenetic Flexibility Underlying Lineage Choices in the Adaptive Immune System

Although fundamental models have emerged in recent years describing how chromatin and transcription regulation interface with one another in the developing immune system, the order of events and their biological impact are still being resolved. Recent advances have provided a flexible, rather than static, view of chromatin regulation to reveal how both positive and negative forces work concomitantly to establish specific chromatin structures and regulate gene expression. The challenge will now be to explore new epigenetic models and validate them during lymphocyte development, with the ultimate goal of unraveling the long-sought mechanisms that support the emerging complexity of the adaptive immune response.

Genomic expression during human myelopoiesis

Background

Human myelopoiesis is an exciting biological model for cellular differentiation since it represents a plastic process where multipotent stem cells gradually limit their differentiation potential, generating different precursor cells which finally evolve into distinct terminally differentiated cells. This study aimed at investigating the genomic expression during myeloid differentiation through a computational approach that integrates gene expression profiles with functional information and genome organization.

Results

Gene expression data from 24 experiments for 8 different cell types of the human myelopoietic lineage were used to generate an integrated myelopoiesis dataset of 9,425 genes, each reliably associated to a unique genomic position and chromosomal coordinate. Lists of genes constitutively expressed or silent during myelopoiesis and of genes differentially expressed in commitment phase of myelopoiesis were first identified using a classical data analysis procedure. Then, the genomic distribution of myelopoiesis genes was investigated integrating transcriptional and functional characteristics of genes. This approach allowed identifying specific chromosomal regions significantly highly or weakly expressed, and clusters of differentially expressed genes and of transcripts related to specific functional modules.

Conclusion

The analysis of genomic expression during human myelopoiesis using an integrative computational approach allowed discovering important relationships between genomic position, biological function and expression patterns and highlighting chromatin domains, including genes with coordinated expression and lineage-specific functions.

Functional sequestration of transcription factor activity by repetitive DNA

Higher eukaryote genomes contain repetitive DNAs, often concentrated in transcriptionally inactive heterochromatin. Although repetitive DNAs are not typically considered as regulatory elements that directly affect transcription, they can contain binding sites for some transcription factors. Here, we demonstrate that binding of the transcription factor CCAAT/enhancer-binding protein alpha (C/EBPalpha) to the mouse major alpha-satellite repetitive DNA sequesters C/EBPalpha in the transcriptionally inert pericentromeric heterochromatin. We find that this sequestration reduces the transcriptional capacity of C/EBPalpha. Functional sequestration of C/EBPalpha was demonstrated by experimentally reducing C/EBPalpha binding to the major alpha-satellite DNA, which elevated the concentration of C/EBPalpha in the non-heterochromatic subcompartment of the cell nucleus. The reduction in C/EBPalpha binding to alpha-satellite DNA was induced by the co-expression of the transcription factor Pit-1, which removes C/EBPalpha from the heterochromatic compartment, and by the introduction of an altered-specificity mutation into C/EBPalpha that reduces binding to alpha-satellite DNA but permits normal binding to sites in some gene promoters. In both cases the loss of alpha-satellite DNA binding coincided with an elevation in the binding of C/EBPalpha to a promoter and an increased transcriptional output from that promoter. Thus, the binding of C/EBPalpha to this highly repetitive DNA reduced the amount of C/EBPalpha available for binding to and regulation of this promoter. The functional sequestration of some transcription factors through binding to repetitive DNAs may represent an underappreciated mechanism controlling transcription output.

Interleukin 2 gene transcription is regulated by Ikaros-induced changes in histone acetylation in anergic T cells

In T cells anergy may be evoked by an unbalanced stimulation of the T-cell receptor in the absence of costimulation. Anergic T cells are unresponsive to new antigen receptor engagement and do not produce interleukin 2. We present evidence that anergizing stimuli induce changes in histone acetylation, which mediates transcriptional repression of interleukin 2 expression. In response to calcium signaling, anergic T cells up-regulate the expression of Ikaros, a zinc finger transcription factor essential for lymphoid lineage determination. Ikaros binds to the interleukin 2 promoter where it induces histone deacetylation. Confirming the role of Ikaros in the induction of T-cell anergy, cells with reduced Ikaros activity show defective inactivation in response to an anergizing stimulus. We propose a model in which tolerizing stimuli induce epigenetic changes on the interleukin 2 locus that are responsible for the stable inhibition of the expression of this cytokine in anergic T cells.

Ikaros is expressed in developing striatal neurons and involved in enkephalinergic differentiation

The Ikaros (Ik) gene encodes alternatively spliced zinc-finger proteins originally identified in developing hematopoietic organs and acts as master regulator of lymphoid development. During our search for transcription factors that control the developmental expression of the enkephalin (ENK) gene we found that Ik-1 and Ik-2 isoforms are specifically expressed in the embryonic striatum and bind the Ik-like cis-regulatory DNA element present on the ENK gene. Ik proteins are expressed by both proliferating (BrdU+/nestin+) and by post-mitotic differentiating (MAP2+) cells in the developing striatum between embryonic day 12 and post-natal day 2 and mRNAs encoding for the Ik and ENK genes are co-expressed by a subset of differentiating striatal neurons. Blocking the DNA binding of Ik proteins in differentiating embryonic striatal neuronal cultures resulted in decreased ENK expression and mutant animals lacking the DNA-binding domain of Ik had a deficit in the number of ENK but not in dynorphin or substance P mRNA+ cells. Animals lacking the protein interaction domain of Ik showed no deficit. These results demonstrate that Ik-1 and Ik-2 proteins through their DNA binding act as positive regulators of ENK gene expression in the developing striatum and participate in regulating enkephalinergic differentiation.

C. elegans SIN-3 and its associated HDAC corepressor complex act as mediators of male sensory ray development

Mab21 gene family members are required for embryonic development and sensory organ formation in both invertebrates and vertebrates. However, their mechanistic role on differentiation is largely unexplored. We report here the isolation of SIN-3 as a MAB-21 interacting molecule. sin-3 is co-expressed with mab-21 in the ray structural cells and genetically interacts with mab-21 to control sensory organ development. Using pharmacological and RNAi approaches, we demonstrated that histone deacetylase and conserved SIN-3-associated components are required for ray patterning. Conserved physical interactions between these components were also observed, implicating the recruitment of HDAC complex by MAB-21/SIN-3 may occur to determine ray identity in males.

Ikaros Is Regulated through Multiple Histone Modifications and Deoxyribonucleic Acid Methylation in the Pituitary

The transcription factor Ikaros (Ik) is at the center of a functionally diverse chromatin-remodeling network that is critical for the development and regulation of both the immune and endocrine systems. Dominant negative forms of Ik result in neoplastic growth in mouse genetic studies and have been identified in human tumors. Ik modulates chromatin accessibility through associations with members of the NURD complex including histone deacetylase complexes. We show here that Ik expression in mouse pituitary corticotroph cells is itself regulated through histone modifications as well as DNA methylation. Examination of primary human pituitary specimens also identified a correlation of loss of Ik expression with the presence of DNA methylation in the untranslated exon 1 CpG island. These findings have important implications for the understanding of Ikaros’ role in epigenetic functions and suggest a potential role for demethylating agents in the treatment of related disorders.

High-resolution genomic profiling of childhood ALL reveals novel recurrent genetic lesions affecting pathways involved in lymphocyte differentiation and cell cycle progression

Gross cytogenetic anomalies are traditionally being used as diagnostic, prognostic and therapeutic markers in the clinical management of cancer, including childhood acute lymphoblastic leukemia (ALL). Recently, it has become increasingly clear that genetic lesions driving tumorigenesis frequently occur at the submicroscopic level and, consequently, escape standard cytogenetic observations. Therefore, we profiled the genomes of 40 childhood ALLs at high resolution. We detected multiple de novo genetic lesions, including gross aneuploidies and segmental gains and losses, some of which were subtle and affected single genes. Many of these lesions involved recurrent (partially) overlapping deletions and duplications, containing various established leukemia-associated genes, such as ETV6, RUNX1 and MLL. Importantly, the most frequently affected genes were those controlling G1/S cell cycle progression (e.g. CDKN2A, CDKN1B and RB1), followed by genes associated with B-cell development. The latter group includes microdeletions of the B-lineage transcription factors PAX5, EBF, E2-2 and IKZF1 (Ikaros), as well as genes with other established roles in B-cell development, that is RAG1 and RAG2, FYN, PBEF1 or CBP/PAG. The fact that we frequently encountered multiple lesions affecting genes involved in cell cycle regulation and B-cell differentiation strongly suggests that both these processes need to be targeted independently and simultaneously to trigger ALL development.

The chromatin remodeler Mi-2β is required for establishment of the basal epidermis and normal differentiation of its progeny

Using conditional gene targeting in mice, we show that the chromatin remodeler Mi-2β is crucial for different aspects of skin development. Early (E10.5) depletion of Mi-2β in the developing ventral epidermis results in the delayed reduction of its suprabasal layers in late embryogenesis and to the ultimate depletion of its basal layer. Later (E13.5)loss of Mi-2β in the dorsal epidermis does not interfere with suprabasal layer differentiation or maintenance of the basal layer, but induction of hair follicles is blocked. After initiation of the follicle, some subsequent morphogenesis of the hair peg may proceed in the absence of Mi-2β, but production of the progenitors that give rise to the inner layers of the hair follicle and hair shaft is impaired. These results suggest that the extended self-renewal capacity of epidermal precursors arises early during embryogenesis by a process that is critically dependent on Mi-2β. Once this process is complete, Mi-2β is apparently dispensable for the maintenance of established repopulating epidermal stem cells and for the differentiation of their progeny into interfollicular epidermis for the remainder of gestation. Mi-2β is however essential for the reprogramming of basal cells to the follicular and, subsequently, hair matrix fates.

Transcriptional regulation of T cell tolerance

Self-reactive T cells that escape negative selection in the thymus must be kept under control in the periphery. Mechanisms of peripheral tolerance include deletion or functional inactivation of self-reactive T cells and mechanisms of dominant tolerance mediated by regulatory T cells. In the absence of costimulation, T cell receptor (TCR) engagement results in unopposed calcium signaling that leads to the activation of a cell-intrinsic program of inactivation, which makes T cells hyporesponsive to subsequent stimulations. The activation of this program in anergic T cells is a consequence of the induction of a nuclear factor of activated T cells (NFAT)-dependent program of gene expression. Recent studies have offered new insights into the mechanisms responsible for the implementation and maintenance of T cell anergy and have provided evidence that the proteins encoded by the genes upregulated in anergic T cells are responsible for the implementation of anergy by interfering with TCR signaling or directly inhibiting cytokine gene transcription.

Critical function of Ikaros in controlling Aiolos gene expression

To characterize the regulation of lymphoid Aiolos transcription factor, we have cloned its promoter. Full promoter and nested deletions were expressed in lymphoid and non-lymphoid cell lines. The minimal promoter activity could be considered as a 172bp upstream from the ATG for Jurkat and HEK293 cells and as a 370bp fragment for U937 cells. Moreover, we have mapped the transcription initiation site. Retardation gels showed binding activity for Ikaros, NFkappaB and AP4 transcription factors and mutations in their binding sites abolish Aiolos promoter activity. Chromatin immunoprecipitation assay revealed that Ikaros, NFkappaB and AP4 are bound to Aiolos promoter. The important function of Ikaros and NFkappaB is underlined by their over expression, which results in the trans-activation of the promoter and drives Aiolos expression in cell lines and in freshly isolated B and T cells, while over expression of a dominant negative Ikaros isoform is able to block Aiolos expression.

LymphTF-DB: a database of transcription factors involved in lymphocyte development

B and T cells develop following a similar early stepwise progression to later stages where multiple developmental options are available. These developmental regimes necessitate differential gene expression regulated by a large number of transcription factors (TFs). The resultant burgeoning amount of information has opened a knowledge gap between TF activities during lymphocyte development and a researcher's experiments. We have created the LymphTF database (DB) to fill this gap. This DB holds interactions between individual TFs and their specific targets at a given developmental time. By storing such interactions as a function of developmental progression, we hope to advance the elucidation of regulatory networks that guide lymphocyte development. Besides queries for TF-target gene interactions in developmental stages, the DB provides a graphical representation of downloadable target gene regulatory sequences with locations of the transcriptional start sites and TF-binding sites. The LymphTF-DB can be accessed freely on the web at http://www.iupui.edu/~tfinterx/.

The chromatin remodeling factor Mi-2alpha acts as a novel co-activator for human c-Myb

The c-Myb protein belongs to a group of early hematopoietic transcription factors that are important for progenitor generation and proliferation. These factors have been hypothesized to participate in establishing chromatin patterns specific for hematopoietic genes. In a two-hybrid screening we identified the chromatin remodeling factor Mi-2alpha as an interaction partner for human c-Myb. The main interacting domains were mapped to the N-terminal region of Mi-2alpha and the DNA-binding domain of c-Myb. Surprisingly, functional analysis revealed that Mi-2alpha, previously studied as a subunit in the NuRD co-repressor complex, enhanced c-Myb-dependent reporter activation. Consistently, knock-down of endogenous Mi-2alpha in c-Myb-expressing K562 cells, led to down-regulation of the c-Myb target genes NMU and ADA. When wild-type and helicase-dead Mi-2alpha were compared, the Myb-Mi-2alpha co-activation appeared to be independent of the ATPase/DNA helicase activity of Mi-2alpha. The rationale for the unexpected co-activator function seems to lie in a dual function of Mi-2alpha, by which this factor is able to repress transcription in a helicase-dependent and activate in a helicase-independent fashion, as revealed by Gal4-tethering experiments. Interestingly, desumoylation of c-Myb potentiated the Myb-Mi-2alpha transactivational co-operation, as did co-transfection with p300.

Bifurcation dynamics in lineage-commitment in bipotent progenitor cells

Lineage specification of multipotent progenitor cells is governed by a balance of lineage-affiliated transcription factors, such as GATA1 and PU.1, which regulate the choice between erythroid and myelomonocytic fates. But how ratios of lineage-determining transcription factors stabilize progenitor cells and resolve their indeterminacy to commit them to discrete, mutually exclusive fates remains unexplained. We used a simple model and experimental measurements to analyze the dynamics of a binary fate decision governed by a gene-circuit containing auto-stimulation and cross-inhibition, as embodied by the GATA1-PU.1 paradigm. This circuit generates stable attractors corresponding to erythroid and myelomonocytic fates, as well as an uncommitted metastable state characterized by coexpression of both regulators, explaining the phenomenon of "multilineage priming". GATA1 and PU.1 mRNA and transcriptome dynamics of differentiating progenitor cells confirm that commitment occurs in two stages, as suggested by the model: first, the progenitor state is destabilized in an almost symmetrical bifurcation event, resulting in a poised state at the boundary between the two lineage-specific attractors; second, the cell is driven to the respective, now accessible attractors. This minimal model captures fundamental features of binary cell fate decisions, uniting the concepts of stochastic (selective) and deterministic (instructive) regulation, and hence, may apply to a wider range of binary fate decision points.

Ikaros DNA-Binding Proteins as Integral Components of B Cell Developmental-Stage-Specific Regulatory Circuits

Ikaros DNA-binding proteins are critical for the development of lymphocytes and other hematopoietic lineages, but it remains unclear how they cooperate with other regulators of signaling and transcription to achieve ordered gene expression during development. Here, we show that Ikaros proteins regulate the pre-BCR component lambda5 in a stage-specific manner. In pre-BI cells, Ikaros modulated lambda5 expression in competition with the transcriptional activator EBF. This required Ikaros binding to the Igll1 (lambda5) promoter and was abolished either by mutation of the Ikaros DNA-binding domain or by deletion of a single Ikaros site from the Igll1 promoter. At the transition from the pre-BI to pre-BII stage, the expression of the Ikaros family member Aiolos was upregulated and required for the efficient silencing of Igll1. Aiolos expression was controlled by pre-BCR signals via the adaptor protein SLP-65. Thus, pre-BCR signaling regulates Aiolos and the silencing of Igll1 via a developmental-stage-specific feedback loop.

Early decisions in lymphoid development

Recent research suggests that lymphoid progenitors in the bone marrow comprise a heterogeneous cell population. This population first loses megakaryocyte/erythroid, and then granulocyte/macrophage, potential before committing to lymphoid lineages. B and T cells can originate by way of different pathways that appear to be used with varying frequencies in the animal. In the bone marrow, B cell specification and commitment is driven by the concerted action of transcription factors and IL-7 signaling. In the thymus, multipotent progenitors become committed to the T-cell lineage through the action of Notch1. The activated intracellular form of Notch1 suppresses transcription factors that can instruct myeloid cell fates, thereby directly coupling extracellular signaling with changes in transcriptional networks. In conclusion, although a lot is known about B and T cell commitment, more work needs to be done to clarify the earliest steps in lymphoid specification.

Regulation of mouseHeparanasegene expression in T lymphocytes and tumor cells

Heparanase (HPSE) is an endoglycosidase that cleaves heparan sulfate (HS) and plays an important role in tumor metastasis, angiogenesis and inflammation. The regulation of HPSE expression and function is tightly controlled and the increasing use of the mouse as an animal model to define the role of HPSE in many physiological and pathological settings, makes understanding the regulatory mechanisms of HPSE in this species of fundamental importance. However, the expression distribution of the mouse Hpse gene and the mechanisms that regulate its transcription are poorly defined. In this study, the mouse Hpse gene was determined to encode for two mRNA transcripts of 1.9 and 3.2 kb in length with identical open reading frames that showed similar tissue expression distribution to the human HPSE. The mouse Hpse promoter was cloned and a 478-bp minimal promoter was identified that contained regulatory elements responsible for both basal promoter activity in mouse tumor cells as well as inducible activity in T cells. Mutagenesis and transactivation studies identified a functional site in the minimal promoter region for the transcription factor Early growth response gene 1 (Egr1). Interestingly, Egr1 acted differentially in mouse tumor cells, functioning in an activating or repressive manner in breast carcinoma or melanoma cells, respectively. Furthermore, the proximal region of the promoter, identified as important in the regulation of Hpse transcription, was shown to become accessible in T cells upon cell activation. Significantly, the maximal accessibility of the promoter occurred at 16 h post-stimulation, which correlated with the induction kinetics of Hpse mRNA expression. In summary, this study demonstrates that mouse Hpse is expressed and regulated in a similar manner to human HPSE and also provides some novel insights into mechanisms of Hpse gene regulation that are likely to be relevant to control of the human gene.

Fate decisions regulating bone marrow and peripheral B lymphocyte development

In adult mammals, bone marrow pluripotent hematopoietic stem cells generate B lymphoid-specified progeny that progress through a series of well-characterized stages before generating B-cell receptor expressing B lymphocytes. These functionally immature B lymphocytes then migrate to the spleen wherein they differentiate through transitional stages into follicular or marginal zone B lymphocytes capable of responding to T-dependent and -independent antigens, respectively. During the terminal stages of B lymphocyte development in the bone marrow, as well as immediately following egress into the peripheral compartments, B lymphocytes are counterselected to eliminate B lymphocytes with potentially dangerous self-reactivity. These developmental and selection events in the bone marrow and periphery are dependent on the integration of intrinsic genetic programs with extrinsic microenvironmental signals that drive progenitors toward increasing B lineage commitment and maturation. This chapter provides a comprehensive overview of the various stages of primary and secondary B lymphocyte development with an emphasis on the selection processes that affect decisions at critical checkpoints. Our intent is to stress the concept that at many steps in the developmental process leading to a mature immunocompetent B lymphocyte, B lineage cells are integrating multiple and different signaling inputs that are translated into specific and appropriate cell fate decisions.

IL-33, the IL-1-like cytokine ligand for ST2 receptor, is a chromatin-associated nuclear factor in vivo

Recent studies indicate that IL-1alpha functions intracellularly in pathways independent of its cell surface receptors by translocating to the nucleus and regulating transcription. Similarly, the chromatin-associated protein HMGB1 acts as both a nuclear factor and a secreted proinflammatory cytokine. Here, we show that IL-33, an IL-1-like cytokine that signals via the IL-1 receptor-related protein ST2 and induces T helper type 2-associated cytokines, is an endothelium-derived, chromatin-associated nuclear factor with transcriptional repressor properties. We found that IL-33 is identical to NF-HEV, a nuclear factor preferentially expressed in high endothelial venules (HEV), that we previously characterized. Accordingly, in situ hybridization demonstrated that endothelial cells constitute a major source of IL-33 mRNA in chronically inflamed tissues from patients with rheumatoid arthritis and Crohn's disease. Immunostaining with three distinct antisera, directed against the N-terminal part and IL-1-like C-terminal domain, revealed that IL-33 is a heterochromatin-associated nuclear factor in HEV endothelial cells in vivo. Association of IL-33 with heterochromatin was also observed in human and mouse cells under living conditions. In addition, colocalization of IL-33 with mitotic chromatin was noted. Nuclear localization, heterochromatin-association, and targeting to mitotic chromosomes were all found to be mediated by an evolutionarily conserved homeodomain-like helix-turn-helix motif within the IL-33 N-terminal part. Finally, IL-33 was found to possess transcriptional repressor properties, associated with the homeodomain-like helix-turn-helix motif. Together, these data suggest that, similarly to IL1alpha and HMGB1, IL-33 is a dual function protein that may function as both a proinflammatory cytokine and an intracellular nuclear factor with transcriptional regulatory properties.

Expression of a non-DNA-binding isoform of Helios induces T-cell lymphoma in mice

Helios is a zinc-finger protein belonging to the Ikaros family of transcriptional regulators. It is expressed, along with Ikaros, throughout early stages of thymocyte development where it quantitatively associates with Ikaros through C-terminal zinc-finger domains that mediate heterodimerization between Ikaros family members. To understand the role of Helios in T-cell development, we used a retroviral vector to express full-length Helios or a Helios isoform that lacked the N-terminal DNA-binding domain in hematopoietic progenitor cells of reconstituted mice. Constitutive expression of full-length Helios resulted in an inhibition of T-cell development at the double-negative stage within the thymus. Although expression of the DNA-binding mutant of Helios did not contribute to developmental abnormalities at early times after transplantation, 60% of animals that expressed the Helios DNA-binding mutant developed an aggressive and transplantable T-cell lymphoma 4 to 10 months after transplantation. These results demonstrate a vital function for Helios in maintaining normal homeostasis of developing T cells and formally show that non-DNA-binding isoforms of Helios are lymphomagenic if aberrantly expressed within the T-cell lineage.

Conserved functions of Ikaros in vertebrate lymphocyte development: genetic evidence for distinct larval and adult phases of T cell development and two lineages of B cells in zebrafish

Zebrafish has been advocated as an alternative animal model to study lymphocyte development, although the similarities in the genetic requirements of lymphopoiesis between fish and mammals have not yet been investigated. In this study, we examine the role of the transcription factor Ikaros in zebrafish lymphopoiesis. In fish larvae homozygous for an ikaros allele predicted to lack the C-terminal zinc fingers, T lymphopoiesis is absent; the presence of V(H)DmuJmu rearrangements in adolescent fish is delayed in mutants. In adolescent mutant fish, T cells expressing tcrb and tcrd and B cells expressing igm are formed with low efficiency and display an oligoclonal Ag receptor repertoire. By contrast, B cells expressing the igz isotype do not develop, providing genetic evidence for two separate B cell lineages in zebrafish. Thus, Ikaros appears to play similar roles in fish and mammalian lymphopoiesis.

Determinants of lymphoid-myeloid lineage diversification

In recent years, investigators have made great progress in delineating developmental pathways of several lymphoid and myeloid lineages and in identifying transcription factors that establish and maintain their fate. However, the developmental branching points between these two large cell compartments are still controversial, and little is known about how their diversification is induced. Here, we give an overview of determinants that play a role at lymphoid-myeloid junctures, in particular transcription factors and cytokine receptors. Experiments showing that myeloid lineages can be reversibly reprogrammed into one another by transcription factor network perturbations are used to highlight key principles of lineage commitment. We also discuss experiments showing that lymphoid-to-myeloid but not myeloid-to-lymphoid conversions can be induced by the enforced expression of a single transcription factor. We close by proposing that this asymmetry is related to a higher complexity of transcription factor networks in lymphoid cells compared with myeloid cells, and we suggest that this feature must be considered when searching for mechanisms by which hematopoietic stem cells become committed to lymphoid lineages.

CtIP activates its own and cyclin D1 promoters via the E2F/RB pathway during G1/S progression

Cell cycle progression from G(1) to S phase is mainly controlled by E2F transcription factors and RB family proteins. Previously we showed that the presence of CtIP is essential for G(1)/S transition in primary mouse blastocysts, as well as in NIH 3T3 cells. However, how CtIP executes this function remains to be elucidated. Here we show that in NIH 3T3 cells the expression of CtIP is regulated by the E2F/RB pathway during late G(1) and S phases. The presence of wild-type CtIP, but not the E157K mutant form, which failed to interact with RB, enhanced its own promoter activity. Chromatin immunoprecipitation analysis indicated that the recruitment of CtIP to its promoter occurs concomitantly with TFIIB, a component of the RNA polymerase II complex, and with dissociation of RB from the promoter during late G(1) and G(1)/S transition. Similar positive regulation of cyclin D1 expression by CtIP was also observed. Consistently, cells expressing the CtIP(E157K) protein alone exhibited growth retardation, an increase in the G(1) population, and a decrease in the S-phase population. Taken together, these results suggest that, contrary to the postulated universal corepressor role, CtIP activates a subset of E2F-responsive promoters by releasing RB-imposed repression and therefore promotes G(1)/S progression.

Ikaros has a crucial role in regulation of B cell receptor signaling

The transcription factor Ikaros, a key regulator of hematopoiesis, has an essential role in lymphocyte development. In mice, fetal lymphoid differentiation is blocked in the absence of Ikaros, and whereas T cells develop postnatally, B cells are totally absent. The significance of Ikaros in the B cell development is evident, but how Ikaros regulates B cell function has neither been established nor previously been studied with B cells that lack Ikaros expression. Here we show that disruption of Ikaros in the chicken B cell line DT40 induces a B cell receptor (BCR) signaling defect with reduced phospholipase Cgamma2 phosphorylation and impaired intracellular calcium mobilization, which is restored by Ikaros reintroduction. Furthermore, we show that lack of Ikaros induces hyperphosphorylation of Casitas B lymphoma protein subsequent to BCR activation. These results indicate that the absolute need of Ikaros for development, cell fate decisions and maintenance of B cells is due to the enhancement of BCR signaling.

Isolation and characterization of SATB2, a novel AT-rich DNA binding protein expressed in development- and cell-specific manner in the rat brain

AT-rich DNA elements play an important role in regulating cell-specific gene expression. One of the AT-rich DNA binding proteins, SATB1 is a novel type of transcription factor that regulates gene expression in the hematopoietic lineage through chromatin modification. Using DNA-affinity purification followed by mass spectrometry we identified and isolated a related protein, SATB2 from the developing rat cerebral cortex. SATB2 shows homology to SATB1 and the rat protein is practically identical to the mouse and human SATB2. Using competitive EMSA, we show that recombinant SATB2 protein binds with high affinity and specificity to AT-rich dsDNA. Using RT-PCR, Western analysis and immunohistochemistry we demonstrate that SATB2 expression is restricted to a subset of postmitotic, differentiating neurons in the rat neocortex at ages E16 and P4. We suggest that similar to its homologue SATB1, SATB2 is also involved in regulating gene expression through altering chromatin structure in differentiating cortical neurons.

Mutations of the Drosophila zinc finger-encoding gene vielfältig impair mitotic cell divisions and cause improper chromosome segregation

We describe the molecular characterization and function of vielfältig (vfl), a X-chromosomal gene that encodes a nuclear protein with six Krüppel-like C2H2 zinc finger motifs. vfl transcripts are maternally contributed and ubiquitously distributed in eggs and preblastoderm embryos, excluding the germline precursor cells. Zygotically, vfl is expressed strongly in the developing nervous system, the brain, and in other mitotically active tissues. Vfl protein shows dynamic subcellular patterns during the cell cycle. In interphase nuclei, Vfl is associated with chromatin, whereas during mitosis, Vfl separates from chromatin and becomes distributed in a granular pattern in the nucleoplasm. Functional gain-of-function and lack-of-function studies show that vfl activity is necessary for normal mitotic cell divisions. Loss of vfl activity disrupts the pattern of mitotic waves in preblastoderm embryos, elicits asynchronous DNA replication, and causes improper chromosome segregation during mitosis.

Multistable and multistep dynamics in neutrophil differentiation

Background

Cell differentiation has long been theorized to represent a switch in a bistable system, and recent experimental work in micro-organisms has revealed bistable dynamics in small gene regulatory circuits. However, the dynamics of mammalian cell differentiation has not been analyzed with respect to bistability.

Results

Here we studied how HL60 promyelocytic precursor cells transition to the neutrophil cell lineage after stimulation with the differentiation inducer, dimethyl sulfoxide (DMSO). Single cell analysis of the expression kinetics of the differentiation marker CD11b (Mac-1) revealed all-or-none switch-like behavior, in contrast to the seemingly graduated change of expression when measured as a population average. Progression from the precursor to the differentiated state was detected as a discrete transition between low (CD11b^Low) and high (CD11b^High) expressor subpopulations distinguishable in a bimodal distribution. Hysteresis in the dependence of CD11b expression on DMSO dose suggests that this bimodality may reflect a bistable dynamic. But when an "unswitched" (CD11b^Low) subpopulation of cells in the bistable/bimodal regime was isolated and cultured, these cells were found to differ from undifferentiated precursor cells in that they were "primed" to differentiate.

Conclusion

These findings indicate that differentiation of human HL60 cells into neutrophils does not result from a simple state transition of a bistable switch as traditionally modeled. Instead, mammalian differentiation appears to be a multi-step process in a high-dimensional system, a result which is consistent with the high connectivity of the cells' complex underlying gene regulatory network.

Commitment issues: linking positive selection signals and lineage diversification in the thymus

The thymus is responsible for the production of CD4+ helper and CD8+ cytotoxic T cells, which constitute the cellular arm of the immune system. These cell types derive from common precursors that interact with thymic stroma in a T-cell receptor (TCR)-specific fashion, generating intracellular signals that are translated into function-specific changes in gene expression. This overall process is termed positive selection, but it encompasses a number of temporally distinct and possibly mechanistically distinct cellular changes, including rescue from apoptosis, initiation of cell differentiation, and commitment to the CD4+ or CD8+ T-cell lineage. One of the puzzling features of positive selection is how specificity of the TCR controls lineage commitment, as both helper and cytolytic T cells utilize the same antigen-receptor components, with the exception of the CD4 or CD8 coreceptors themselves. In this review, we focus on the signals required for positive selection, particularly as they relate to lineage commitment. Identification of genes encoding transcriptional regulators that play a role in T-cell development has led to significant recent advances in the field. We also provide an overview of nuclear factors in this context and, where known, how their regulation is linked to the same TCR signals that have been implicated in initiating and regulating positive selection.

Role of transcription factors in commitment and differentiation of early B lymphoid cells

B lymphopoiesis is a differentiation process in which hematopoietic stem cells are converted into antibody-producing plasma cells. B cell differentiation involves multiple steps, including cell specification, commitment to the B cell lineage, immunoglobulin rearrangements, maturation of B cells and terminal differentiation into plasma cells. Each of these steps is controlled by signaling pathways and transcription factors that act in synergy, feedback-loops or cross-antagonism to generate complex regulatory networks that allow for plasticity and stability of B cell differentiation.

Ikaros increases normal apoptosis in adult erythroid cells

Ikaros is a critical transcriptional regulator of hematopoietic cell differentiation. In addition to its effects on the lymphoid system and hematopoietic stem-cell compartment, we have previously shown that Ikaros is also required for normal erythroid development. In this report, we compare Ikaros-dependent gene expression in erythroid cells of mice lacking the Ikaros protein with that of normal mice in purified adult bone-marrow erythroid cells (BMRC). Gene expression, measured by Affymetrix microarray analysis, indicates that in the BMRC of Ikaros-null mice, there is significant up-regulation of SMADs 6 and 7, serine protease inhibitor 3, and immediate-early protein 3 (IER3), all proteins that play a modulating role in apoptosis. We investigate the role of Ikaros in oxidative stress-induced apoptosis using Annexin-V staining and FACS analysis. We find a decrease in apoptosis in the BMRC of Ikaros-null mice compared to normal mice. This effect is also seen in nonerythroid cells but is stronger in BMRC. We conclude that normal Ikaros function increases normal apoptosis in erythroid cells. The data also suggest that Ikaros plays a role in apoptosis-mediated events in other normal hematopoietic cell lineages.

BCL11B functionally associates with the NuRD complex in T lymphocytes to repress targeted promoter

BCL11 genes play crucial roles in lymphopoiesis and have been associated with hematopoietic malignancies. Specifically, disruption of the BCL11B (B-cell chronic lymphocytic leukemia/lymphoma 11B) locus is linked to T-cell acute lymphoblastic leukemia, and the loss of heterozygosity in mice results in T-cell lymphoma. BCL11 proteins are related C2H2 zinc-finger transcription factors that act as transcriptional repressors. Here, we demonstrate the association of the endogenous BCL11B with the nucleosome remodeling and histone deacetylase (NuRD) complex, one of the major transcriptional corepressor complexes in mammalian cells. BCL11B complexes from T lymphocytes possess trichostatin A-sensitive histone deacetylase activity, confirming the functionality of the complexes. Analysis of the BCL11B-NuRD association demonstrated that BCL11B directly interacted with the metastasis-associated proteins MTA1 and MTA2 through the amino-terminal region. We provide evidence for the functional requirement of MTA1 in transcriptional repression mediated by BCL11B through the following: (1) overexpression of MTA1 enhanced the transcriptional repression mediated by BCL11B, (2) knockdown of MTA1 expression by small interfering RNA inhibited BCL11B transcriptional repression activity and (3) MTA1 was specifically recruited to a BCL11B targeted promoter. Taken together, these data support the hypothesis that the NuRD complex mediates transcriptional repression function of BCL11B.

Transgenic expression of Helios in B lineage cells alters B cell properties and promotes lymphomagenesis

Helios, a member of the Ikaros family of DNA-binding proteins, is expressed in multipotential lymphoid progenitors and throughout the T lineage. However, in most B lineage cells, Helios is not expressed, suggesting that its absence may be critical for B cell development and function. To test this possibility, transgenic mice were generated that express Helios under the control of an Ig mu enhancer. Commitment to the B cell lineage was unaltered in Helios transgenic mice, and numbers of surface IgM(+) B cells were normal in the bone marrow and spleen. However, both bone marrow and splenic B cells exhibited prolonged survival and enhanced proliferation. B cells in Helios transgenic mice were also hyperresponsive to Ag stimulation. These alterations were observed even though the concentration of ectopic Helios in B lineage cells, like that of endogenous Helios in thymocytes, was well below the concentration of Ikaros. Further evidence that ectopic Helios expression contributes to B cell abnormalities was provided by the observation that Helios transgenic mice developed metastatic lymphoma as they aged. Taken together, these results demonstrate that silencing of Helios is critical for normal B cell function.

The pre-B-cell receptor induces silencing of VpreB and lambda5 transcription

The pre-B-cell receptor (pre-BCR), composed of Ig heavy and surrogate light chain (SLC), signals pre-BII-cell proliferative expansion. We have investigated whether the pre-BCR also signals downregulation of the SLC genes (VpreB and lambda5), thereby limiting this expansion. We demonstrate that, as BM cells progress from the pre-BI to large pre-BII-cell stage, there is a shift from bi- to mono-allelic lambda5 transcription, while the second allele is silenced in small pre-BII cells. A VpreB1-promoter-driven transgene shows the same pattern, therefore suggesting that VpreB1 is similarly regulated and thereby defines the promoter as a target for transcriptional silencing. Analyses of pre-BCR-deficient mice show a temporal delay in lambda5 downregulation, thereby demonstrating that the pre-BCR is essential for monoallelic silencing at the large pre-BII-cell stage. Our data also suggest that SLP-65 is one of the signaling components important for this process. Furthermore, the VpreB1/lambda5 alleles undergo dynamic changes with respect to nuclear positioning and heterochromatin association, thereby providing a possible mechanism for their transcriptional silencing.

Leukomogenic factors downregulate heparanase expression in acute myeloid leukemia cells

Heparanase is a heparan sulfate-degrading endoglycosidase expressed by mature monocytes and myeloid cells, but not by immature hematopoietic progenitors. Heparanase gene expression is upregulated during differentiation of immature myeloid cells. PML-RARalpha and PLZF-RARalpha fusion gene products associated with acute promyelocytic leukemia abrogate myeloid differentiation and heparanase expression. AML-Eto, a translocation product associated with AML FAB M2, also downregulates heparanase gene expression. The common mechanism that underlines the activity of these three fusion gene products involves the recruitment of histone deacetylase complexes to specific locations within the DNA. We found that retinoic acid that dissociates PML-RARalpha from the DNA, and which is used to treat acute promyelocytic leukemia patients, restores heparanase expression to normal levels in an acute promyelocytic leukemia cell line. The retinoic acid effects were also observed in primary acute promyelocytic leukemia cells and in a retinoic acid-treated acute promyelocytic leukemia patient. Histone deacetylase inhibitor reverses the downregulation of heparanase expression induced by the AML-Eto fusion gene product in M2 type AML. In summary, we have characterized a link between leukomogenic factors and the downregulation of heparanase in myeloid leukemic cells.

Regulation of temporal identities during Drosophila neuroblast lineage development

One of the major goals of neurobiology is to describe, in molecular terms, how a neural progenitor cell can generate an ordered series of uniquely fated neurons and glia. It has become clear that many, or all, neural-subtype identities can be linked to sequentially changing regulatory programs within neural precursors. Recent studies shed light on regulatory inputs and timing mechanisms that generate temporally defined cell identities, and new contributions are beginning to establish a link between the temporal network and cell function.

Stem cells in postnatal myogenesis: molecular mechanisms of satellite cell quiescence, activation and replenishment

Satellite cells are the primary stem cells in adult skeletal muscle, and are responsible for postnatal muscle growth, hypertrophy and regeneration. In mature muscle, most satellite cells are in a quiescent state, but they activate and begin proliferating in response to extrinsic signals. Following activation, a subset of satellite cell progeny returns to the quiescent state during the process of self-renewal. Here, we review recent studies of satellite cell biology and focus on the key transitions from the quiescent state to the state of proliferative activation and myogenic lineage progression and back to the quiescent state. The molecular mechanisms of these transitions are considered in the context of the biology of the satellite cell niche, changes with age, and interactions with established pathways of myogenic commitment and differentiation.

Differential regulation of the IL-10 gene in Th1 and Th2 T cells

Interleukin-10 (IL-10), an immunoregulatory cytokine, modulates the function of various immune and nonimmune cells, yet little information is available on the molecular mechanism of transcriptional regulation at the chromatin level. During T cell differentiation from naive T cells into Th1 and Th2 cells, the expression of IL-10 in Th1 cells slowly disappears, whereas Th2 cells produce more IL-10. We examined the chromatin structural changes associated with IL-10 gene transcription by naive and differentiated murine Th1 and Th2 cells. Naive T cells lack DNase I hypersensitivity (HS) sites in the vicinity of the IL-10 gene, whereas differentiated T cells display a strong 3' constitutive HS site as well as several inducible sites. In committed Th1 cells, the mechanism of IL-10 gene silencing is associated with a closed chromatin structure, the lack of an HS site at the promoter region, and the development of repressive histone modification near the IL-10 promoter and introns 3 and 4. We confirm that the majority of HS sites coincide with conserved noncoding sequences (CNSs) identified by comparative genomic sequence alignment between human and mouse genomes. Potential transcription factor binding sites were located by comparing CNSs with the TRANSFAC database. Predicted in vivo binding of specific factors on the CNS locus were confirmed by chromatin immunoprecipitation assays. Our results suggest that the combination of HS site and comparative genomic approaches allows identification of regulatory elements involved in differential IL-10 gene expression between Th1 and Th2 cells during T cell differentiation.

Dwindling competition with constant demand: Can homeostatic adjustments explain age-associated changes in peripheral B cell selection?

The close relationship between specificity-based selection and homeostatic processes in maintaining peripheral B cell pools has become increasingly evident. Thus, age-associated changes observed within these pools may reflect homeostatic responses to proximal primary lesions. Marked shifts in the size and dynamics of most B lymphocyte subsets and their progenitors occur with age: perturbations in B lineage precursors result in reduced production of immature B lymphocytes in the bone marrow and transitional pools in the periphery, but these effects appear to be offset by compensatory homeostatic processes at the marrow-periphery interface. We propose a model whereby these "distal" homeostatic adjustments relax the stringency of specificity based selection, affording a potential explanation for the increased frequency of autoreactive specificities with age.

STAT4 is a target of the hematopoietic zinc-finger transcription factor Ikaros in T cells

STAT4 is a transcription factor activated in response to IL-12, and is involved in Th1 cell development. The molecular mechanisms controlling the transcription of the STAT4 gene are however, unclear. Sequence comparison of the 5' flanking regions of human, mouse and pufferfish (Fugu rubripes) Stat4 genes revealed a high frequency of Ikaros (Ik) binding elements in all three species. We then investigated the role of Ik binding elements in the human STAT4 promoter using Jurkat T cells. Transactivation, electrophoretic mobility shift assay and RNA interference-mediated gene knockdown experiments revealed that Ik is involved in the regulation of STAT4 in human T cells.

The mSin3A chromatin-modifying complex is essential for embryogenesis and T-cell development

The corepressor mSin3A is the core component of a chromatin-modifying complex that is recruited by multiple gene-specific transcriptional repressors. In order to understand the role of mSin3A during development, we generated constitutive germ line as well as conditional msin3A deletions. msin3A deletion in the developing mouse embryo results in lethality at the postimplantation stage, demonstrating that it is an essential gene. Blastocysts derived from preimplantation msin3A null embryos and mouse embryo fibroblasts (MEFs) lacking msin3A display a significant reduction in cell division. msin3A null MEFs also show mislocalization of the heterochromatin protein, HP1alpha, without alterations in global histone acetylation. Heterozygous msin3A(+/-) mice with a systemic twofold decrease in mSin3A protein develop splenomegaly as well as kidney disease indicative of a disruption of lymphocyte homeostasis. Conditional deletion of msin3A from developing T cells results in reduced thymic cellularity and a fivefold decrease in the number of cytotoxic (CD8) T cells, while helper (CD4) T cells are unaffected. We show that CD8 development is dependent on mSin3A at a step downstream of T-cell receptor signaling and that loss of mSin3A specifically decreases survival of double-positive and CD8 T cells. Thus, msin3A is a pleiotropic gene which, in addition to its role in cell cycle progression, is required for the development and homeostasis of cells in the lymphoid lineage.

Ikaros-family proteins: in search of molecular functions during lymphocyte development

The regulatory steps that lead to the differentiation of hematopoietic cells from a multipotential stem cell remain largely unknown. A beginning to the understanding of these steps has come from the study of DNA-binding proteins that are thought to regulate the expression of genes required for specific developmental events. Ikaros is the founding member of a small family of DNA-binding proteins required for lymphocyte development, but the members of this family differ from other key regulators of lymphopoiesis in that direct target genes have not been conclusively identified, and reasonable support has been presented for only a few potential targets. Therefore, the molecular mechanisms that Ikaros uses for regulating lymphocyte development remain largely unknown. Current data suggest that, in some instances, Ikaros may function as a typical transcription factor. However, recent results suggest that it may function more broadly, perhaps in the formation of silent and active chromatin structures. In this review, our current knowledge of the molecular functions of Ikaros will be discussed.

Early B-cell factor ‘pioneers’ the way for B-cell development

Early B-cell factor (EBF) is a DNA-binding protein required for B-cell lymphopoiesis. The lack of EBF results in an early developmental blockade, including the lack of functional B cells and Igs. Recent studies have elucidated a central role for EBF in the specification of B-lineage cells. EBF directs progenitor cells to undergo B lymphopoiesis and activates transcription of B cell-specific genes in the absence of upstream regulators. How EBF mediates these effects has yet to be thoroughly explored, however, it initiates epigenetic modifications necessary for gene activation and the function of other transcriptional regulators, including Pax5. Together, these observations suggest a molecular basis for the role of EBF in the hierarchical network of factors that control B lymphopoiesis.

Ikaros integrates endocrine and immune system development

Ikaros transcription factors are essential regulators of lymphopoiesis and the development of the immune system. We now show that Ikaros is expressed in hormone-producing pituitary corticomelanotroph cells, where it binds the proopiomelanocortin promoter and regulates endogenous gene expression. Loss of Ikaros in vivo results in contraction of the pituitary corticomelanotroph population, reduced circulating adrenocorticotrophic hormone levels, and adrenal glucocorticoid insufficiency. While hemopoietic reconstitution failed to correct this hormonal deficit, the phenotype of reduced body weight and diminished survival was rescued by systemic glucocorticoid-hormone administration. Given the established immunomodulatory properties of glucocorticoid hormones, these findings reveal a novel role for Ikaros in orchestrating immune-endocrine development and function.

Correlation between differentiation plasticity and mRNA expression profiling of CD34+-derived CD14− and CD14+ human normal myeloid precursors

In spite of their apparently restricted differentiation potentiality, hematopoietic precursors are plastic cells able to trans-differentiate from a maturation lineage to another. To better characterize this differentiation plasticity, we purified CD14- and CD14+ myeloid precursors generated by 'in vitro' culture of human CD34+ hematopoietic progenitors. Morphological analysis of the investigated cell populations indicated that, as expected, they consisted of granulocyte and monocyte precursors, respectively. Treatment with differentiation inducers revealed that CD14- cells were bipotent granulo-monocyte precursors, while CD14+ cells appeared univocally committed to a terminal macrophage maturation. Flow cytometry analysis demonstrated that the conversion of granulocyte precursors to the mono-macrophage maturation lineage occurs through a differentiation transition in which the granulocyte-related myeloperoxidase enzyme and the monocyte-specific CD14 antigen are co-expressed. Expression profiling evidenced that the observed trans-differentiation process was accompanied by a remarkable upregulation of the monocyte-related MafB transcription factor.

Transcriptome profiling of adult zebrafish at the late stage of chronic tuberculosis due to Mycobacterium marinum infection

The Mycobacterium marinum-zebrafish infection model was used in this study for analysis of a host transcriptome response to mycobacterium infection at the organismal level. RNA isolated from adult zebrafish that showed typical signs of fish tuberculosis due to a chronic progressive infection with M. marinum was compared with RNA from healthy fish in microarray analyses. Spotted oligonucleotide sets (designed by Sigma-Compugen and MWG) and Affymetrix GeneChips were used, in total comprising 45,465 zebrafish transcript annotations. Based on a detailed comparative analysis and quantitative reverse transcriptase-PCR analysis, we present a validated reference set of 159 genes whose regulation is strongly affected by mycobacterial infection in the three types of microarrays analyzed. Furthermore, we analyzed the separate datasets of the microarrays with special emphasis on the expression profiles of immune-related genes. Upregulated genes include many known components of the inflammatory response and several genes that have previously been implicated in the response to mycobacterial infections in cell cultures of other organisms. Different marker genes of the myeloid lineage that have been characterized in zebrafish also showed increased expression. Furthermore, the zebrafish homologs of many signal transduction genes with relationship to the immune response were induced by M. marinum infection. Future functional analysis of these genes may contribute to understanding the mechanisms of mycobacterial pathogenesis. Since a large group of genes linked to immune responses did not show altered expression in the infected animals, these results suggest specific responses in mycobacterium-induced disease.

Histone dynamics on the interleukin-2 gene in response to T-cell activation

Several models have been proposed for the mechanism of chromatin remodelling across the promoters of inducible genes in mammalian cells. The most commonly held model is one of cooccupation where histone proteins are modified by acetylation or phosphorylation and nucleosomes are remodelled, allowing the assembly of transcription factor complexes. Using chromatin immunoprecipitation, we observed an apparent decrease of histone acetylation and phosphorylation signals at the proximal promoter region of the inducible interleukin-2 and granulocyte-macrophage colony-stimulating factor genes in response to T-cell activation. We showed that this apparent decrease was due to a loss of histone H3 and H4 proteins corresponding to a decrease in nucleosome occupation of the promoter. This histone loss is reversible; it is dependent on the continual presence of appropriate activating signals and transcription factors and is not dependent on the acetylation status of the histone proteins. These data show for the first time that histone proteins are lost from a mammalian promoter upon activation of transcription and support a model of activation-dependent disassembly and reassembly of nucleosomes.

IL-3 Induces Inhibitor of DNA-Binding Protein-1 in Hemopoietic Progenitor Cells and Promotes Myeloid Cell Development

Hemopoiesis depends on the expression and regulation of transcription factors, which control the maturation of specific cell lineages. We found that the helix-loop-helix transcription factor inhibitor of DNA-binding protein 1 (Id1) is not expressed in hemopoietic stem cells (HSC), but is increased in more committed myeloid progenitors. Id1 levels decrease during neutrophil differentiation, but remain high in differentiated macrophages. Id1 is expressed at low levels or is absent in developing lymphoid or erythroid cells. Id1 expression can be induced by IL-3 in HSC during myeloid differentiation, but not by growth factors that promote erythroid and B cell development. HSC were transduced with retroviral vectors that express Id1 and were transplanted in vivo to evaluate their developmental potential. Overexpression of Id1 in HSC promotes myeloid but impairs B and erythroid cell development. Enforced expression of Id1 in committed myeloid progenitor cells inhibits granulocyte but not macrophage differentiation. Therefore, Id1 may be part of the mechanism regulating myeloid vs lymphoid/erythroid cell fates, and macrophage vs neutrophil maturation.